,transcriber ,note ,text emphasis denoted by ,italics and ,bold4 ,whole and fractional parts of numbers as #abc-d_/e4 ,,the ,,ancient ,,volcanoes ,,of ,,great ,,britain ,illustration ,,the ,,ancient ,,volcanoes ,,of ,,great ,,britain ,,by ,,sir ,,archibald ,,geikie1 ,f4,r4,s4 ,d4,c4,l4 ,oxf41 ,d4,sc4 ,camb41 ,dubl42 ,,ll4,d4 ,st4 ,and4 ,edinb4 ,,director-,,general ,,of ,,the ,,geological ,,survey ,,of ,,great ,,britain ,,and ,,ireland2 ,,correspondent ,,of ,,the ,,institute ,,of ,,france2 ,,of ,,the ,,academies ,,of ,,berlin1 ,,vienna1 ,,munich1 ,,turin1 ,,belgium1 ,,stockholm1 ,g,,ttingen1 ,,new ,,york2 ,,of ,,the ,,imperial ,,mineralogical ,,society ,,and ,,society ,,of ,,naturalists ,,st4 ,,petersburg2 ,,natural ,,history ,,society1 ,,moscow2 ,,scientific ,,society1 ,,christiania2 ,,american ,,philosophical ,,society2 ,,of ,,the ,,geological ,,societies ,,of ,,london1 ,,france1 ,,belgium1 ,,stockholm1 ,,etc4 ,,with ,,seven ,,maps ,,and ,,numerous ,,illustrations ,,in ,,two ,,volumes ,,vol4 ,,ii ,london ,,macmillan ,,and ,,co41 ,limited ,,new ,,york3 ,,the ,,macmillan ,,company #ahig ,all rights reserved ,,contents ,,chapter ,,xxix ,the ,carboniferous ,volcanoes of ,england ,,page ,the ,north of ,england3 ,dykes1 ,great ,whin ,sill--,the ,derbyshire ,toadstones--,the ,isle of ,man--,east ,somerset--,devonshire #a ,,chapter ,,xxx ,the ,carboniferous ,volcanoes of ,ireland ,king's ,county--,the ,limerick ,basin--,the ,volcanic ,breccias of ,doubtful ,age in ,county ,cork #cg ,,book ,,vii ,,the ,,permian ,,volcanoes ,,chapter ,,xxxi ,the ,permian ,volcanoes of ,scotland ,geographical ,changes at the ,close of the ,carboniferous ,period--,land and ,inland-,seas of ,permian time--,general ,characteristics and ,nature of the ,materials erupted--,structure of the several ,volcanic ,districts3 #a4 ,ayrshire1 ,nithsdale1 ,annandale2 #b4 ,basin of the ,firth of ,forth #ec ,,chapter ,,xxxii ,permian ,volcanoes of ,england ,the ,devonshire ,centre--,eruptive ,rocks of the ,midland ,coal-fields #id ,,book ,,viii ,,the ,,volcanoes ,,of ,,tertiary ,,time ,,chapter ,,xxxiii ,vast lapse of time between the close of the ,palozoic and beginning of the ,tertiary ,volcanic ,eruptions--,prolonged ,volcanic ,quiescence--,progress of ,investigation among the ,tertiary ,volcanic ,series of ,britain #ajg ,,chapter ,,xxxiv ,the ,system of ,dykes in the ,tertiary ,volcanic ,series ,geographical ,distribution--,two ,types of ,protrusion--,nature of component ,rocks--,hade--,breadth--,interruptions of ,lateral ,continuity--,length--,persistence of ,mineral ,characters #aah ,,chapter ,,xxxv ,the ,system of ,dykes--continued ,direction--,termination upward--,known vertical extension--,evidence as to the movement of the ,molten ,rock in the ,fissures--,branches and ,veins--,connection of ,dykes with ,intrusive ,sheets--,intersection of ,dykes--,dykes of more than one infilling--,contact metamorphism of the ,dykes--,relation of the ,dykes to the ,geological ,structure of the ,districts which they traverse--,data for estimating the ,geological ,age of the ,dykes--,origin and ,history of the ,dykes #ade ,,chapter ,,xxxvi ,the ,plateaux ,nature and ,arrangement of the ,rocks3 #a4 ,lavas4--,basalts1 ,dolerites1 ,andesites--,structure of the ,lavas in the ,field--#b4 ,fragmental ,rocks4--,agglomerates1 ,conglomerates1 and ,breccias--,tuffs and their accompaniments #aha ,,chapter ,,xxxvii ,the ,several ,basalt-,plateaux and their ,geological ,history--,antrim1 ,mull1 ,morven and ,ardnamurchan #aii ,,chapter ,,xxxviii ,the ,basalt-,plateau of the ,parish of ,small ,isles--,rivers of the ,volcanic ,period #bae ,,chapter ,,xxxix ,the ,basalt-,plateaux of ,skye and of the ,faroe ,isles #bdi ,,chapter ,,xl ,the ,modern ,volcanoes of ,iceland as illustrative of the ,tertiary ,volcanic ,history of ,north-,western ,europe #bfj ,,chapter ,,xli ,the ,eruptive ,vents of the ,basalt-,plateaux ,vents filled with ,basalt or other ,lava-form ,rock--,vents filled with ,agglomerate #bgj ,,chapter ,,xlii ,the ,basic ,sills of the ,basalt-,plateaux #bih ,,chapter ,,xliii ,the ,bosses and ,sheets of ,gabbro ,petrography of the ,rocks--,relations of the ,gabbros to the other members of the ,volcanic series--,description of the ,gabbro districts--,skye #cbg ,,chapter ,,xliv ,the ,bosses and ,sheets of ,gabbro in the ,districts of ,rum1 ,ardnamurchan1 ,mull1 ,st4 ,kilda and ,north-,east ,ireland4 ,history of the ,gabbro ,intrusions #cdi ,,chapter ,,xlv ,the ,acid ,rocks ,their ,petrography--,their ,stratigraphical ,position and its ,analogies in ,central ,france #cfd ,,chapter ,,xlvi ,types of ,structure in the ,acid ,rocks--,bosses #cgh ,,chapter ,,xlvii ,the ,acid ,bosses of ,mull1 ,small ,isles1 ,st4 ,kilda1 ,arran1 and the ,north-,east of ,ireland #cie ,,chapter ,,xlviii ,the ,acid ,sills1 ,dykes and ,veins #dcj ,,chapter ,,xlix ,the ,subsidences and ,dislocations of the ,plateaux #ddg ,,chapter ,l ,effects of ,denudation #dee ,,chapter ,,li ,summary and ,general ,deductions #dff ,,list ,,of ,,illustrations ,,fig4 ,,page #agf4 ,section from the great ,limestone escarpment on the west to the ,millstone ,grit hills east of ,teesdale #d #agg4 ,sections of the ,carboniferous ,limestone series of ,northumberland showing the variations in the position of the ,whin ,sill4 ,by ,messrs4 ,topley and ,lebour #f #agh4 ,view of two volcanic necks in the ,carboniferous ,limestone series1 at ,grange ,mill1 five miles west of ,matlock ,bath1 from the north #ad #agi4 ,plan of necks and bedded tuff at ,grange ,mill1 five miles west of ,matlock ,bath #ae #ahj4 ,section across the smaller volcanic neck and the stratified tuff in ,carboniferous ,limestone1 ,grange ,mill #ae #aha4 ,section of vesicular and amygdaloidal diabase resting on ,carboniferous ,limestone1 ,peak ,forest ,limeworks1 ,great ,rocks ,quarry #ai #ahb4 ,view of the superposition of ,carboniferous ,limestone upon toadstone1 ,raven's ,tor1 ,millersdale 7length about #ajj feet7 #ai #ahc4 ,section at lime-kiln1 south of ,viaduct1 ,millersdale ,station #bj #ahd4 ,limestones passing under stratified tuffs1 ,poyll ,vaaish1 ,isle of ,man #bd #ahe4 ,section of tuff1 showing intercalations of black impure chert1 west of ,closenychollagh ,point1 near ,castletown1 ,isle of ,man #be #ahf4 ,section of intercalated dark limestone1 shale and chert in the tuff south of ,poyll ,vaaish ,bay1 ,isle of ,man #bf #ahg4 ,section of part of a volcanic neck on shore to the south-east of ,poyll ,vaaish ,bay1 ,isle of ,man #bi #ahh4 ,section of successive discharges and disturbances within a volcanic vent4 ,scarlet ,point1 ,isle of ,man #bi #ahi4 ,section of dyke and sill in the tuffs west of ,scarlet ,point1 ,isle of ,man #cj #aij4 ,section on south side of vesicular sill west of ,scarlet ,point #ca #aia4 ,bands of vesicles in the same sill #ca #aib4 ,croghan ,hill1 ,king's ,county1 from ,s4,s4,w4 #ch #aic4 ,section in quarry on roadside east of ,limerick1 close to viaduct of the ,limerick and ,erris ,railway #dd #aid4 ,section of the volcanic escarpment1 east of ,shehan's ,cross-roads1 south of ,limerick #de #aie4 ,view of ,derk ,hill1 a volcanic neck on the south side of the ,limerick basin #dg #aif4 ,section across the ,limerick volcanic basin #dh #aig4 ,section of a bed of volcanic breccia in the ,carboniferous ,slate2 ,white ,bull ,head1 ,county ,cork #ej #aih4 ,volcanic breccia invading and enclosing ,carboniferous slate1 ,white ,bull ,head #ej #aii4 ,general section across the ,permian basin of ,ayrshire #ei #bjj4 ,section of lavas1 east side of ,mauchline ,hill #fj #bja4 ,section of the top of the volcanic series near ,eastside ,cottage1 ,carron ,water1 ,nithsdale #fj #bjb4 ,section of two outliers of the ,permian volcanic series at the foot of ,windyhill ,burn1 ,water of ,ae1 ,dumfriesshire #fa #bjc4 ,the ,green ,hill1 ,waterside1 ,dalmellington1 from the south2 a tuff-neck of ,permian age #fb #bjd4 ,patna ,hill from the ,doon ,bridge1 ,ayrshire2 a tuff-neck of ,permian age #fc #bje4 ,ground plans of ,permian volcanic vents from the ,ayrshire ,coal-field4 ,on the scale of six inches to a mile #fd #bjf4 ,section of sills traversing the ,permian volcanic series4 ,river ,ayr1 ,ballochmyle #ff #bjg4 ,section showing the relations of the later rocks of ,arthur ,seat #fh #bjh4 ,section in brooks between ,bonny town and ,baldastard1 ,largo #gj #bji4 ,view of ,largo ,law from the east #ga #baj4 ,view of small neck in ,calciferous ,sandstones1 on the shore1 three miles east from ,st4 ,andrews #gb #baa4 ,ground-plan of ,permian volcanic vents #gc #bab4 ,small neck in ,calciferous ,sandstones a little east from the ,rock and ,spindle1 two and a half miles east from ,st4 ,andrews #gd #bac4 ,plan of volcanic necks at ,kellie ,law1 ,east of ,fife1 on the scale of three inches to one mile #ge #bad4 ,plan of the craters in ,volcanello1 ,lipari ,islands #ge #bae4 ,section of the strata at the edge of the volcanic vent on the east side of ,elie ,harbour #gf #baf4 ,agglomerate of neck on shore at ,ardross1 two miles east from ,elie #gg #bag4 ,ground-plan of volcanic neck1 ,elie ,harbour1 showing circular disposition of the stratification #hj #bah4 ,section across the great vent of ,kincraig1 ,elie1 on a true scale1 vertical and horizontal1 of six inches to a mile #ha #bai4 ,dyke in volcanic neck1 on the beach1 ,st4 ,monans #hb #bbj4 ,section of part of crater rim1 ,island of ,volcano #hc #bba4 ,dyke rising through the agglomerate of a volcanic vent2 ,kincraig1 ,elie #hd #bbb4 ,radiating columnar dyke in the tuff of a volcanic vent4 ,rock and ,spindle1 two and a half miles east from ,st4 ,andrews #hf #bbc4 ,view of part of the shore front of the great vent at ,kincraig1 looking westward1 with the columnar basalt in front #hh #bbd4 ,plan of volcanic neck on beach near ,st4 ,monans #hi #bbe4 ,columnar basalt in the neck of ,kincraig1 ,elie1 seen from the west #ij #bbf4 ,section across ,largo ,law #ia #bbg4 ,vein of white-trap cutting black carbonaceous shales1 a little west from ,st4 ,monans ,church #ib #bbh4 ,section at ,belvedere1 ,s4 ,w4 of ,exeter #ig #bbi4 ,diagram to show the unconformability and overlap of the ,permian rocks in the ,crediton ,valley #ig #bcj4 ,section of the volcanic series at ,kellerton1 ,devonshire #ih #bca4 ,section of agglomerate overlain with sandstone and andesite1 ,posbury1 ,crediton #ii #bcb4 ,diagrammatic section across ,titterstone ,clee ,hill #ajb #bcc4 ,dyke on the south-east coast of the ,island of ,mull #aai #bcd4 ,fissure left by the weathering out of a dyke #abj #bce4 ,plan of basalt-veins with selvages of black basalt-glass1 east side of ,beinn ,tighe1 ,isle of ,eigg #abf #bcf4 ,arrangement of lines of amygdales in a dyke1 ,strathmore1 ,skye #acj #bcg4 ,systems of joints in the dykes #acb #bch4 ,section of cylindrical vein or dyke1 cutting the bedded lavas1 east side of ,fugl1 ,faroe ,islands #acc #bci4 ,joint-structures in the central vitreous portion of the ,eskdale ,dyke 7,b4 ,n4 ,peach7 #acc #bdj4 ,microscopic structure of the vitreous part of the ,eskdale ,dyke #acf #bda4 ,section along the line of the ,cleveland ,dyke at ,cliff ,ridge1 ,guisbrough 7,g4 ,barrow71 scale1 #ab inches to #a mile #adg #bdb4 ,section along the course of the ,cleveland ,dyke1 at the head of ,lonsdale1 ,yorkshire 7,g4 ,barrow1 in the ,memoirs of the ,geol4 ,survey1 ,geology of ,cleveland1 p4 #fa7 #adh #bdc4 ,section across the extreme upper limit of ,cleveland ,dyke1 on the scale of #bj feet to one inch 7,mr4 ,g4 ,barrow7 #adi #bdd4 ,upper limit of ,cleveland ,dyke in quarry near ,cockfield 7after ,mr4 ,teall7 #adi #bde4 ,section along the course of the ,cleveland ,dyke across the ,cross ,fell escarpment 7scale of one inch to one mile7 #aej #bdf4 ,branching portion of the great dyke near ,hawick 7length about one mile7 #aec #bdg4 ,branching dyke at foot of ,glen ,artney 7length about four miles7 #aec #bdh4 ,basic veins traversing ,secondary limestone and sandstone on the coast cliffs1 ,aidnamurchan #aee #bdi4 ,section showing the connection of a ,dyke with an ,intrusive ,sheet1 ,point of ,suisnish1 ,skye #aef #bej4 ,section to show the connection of a ,dyke with an ,intrusive ,sheet1 ,stirlingshire ,coal-field #aeg #bea4 ,intersection of dykes in bedded basalt1 ,calliach ,point1 ,mull #aeh #beb4 ,basalt veins traversing bedded dolerites1 ,kildonan1 ,eigg #aei #bec4 ,ground-plan of intersecting dykes in ,lias limestone1 ,shore1 ,harrabol1 ,east of ,broadford1 ,skye #aei #bed4 ,compound dyke1 ,market ,stance1 ,broadford1 ,skye #afb #bee4 ,section of coal rendered columnar by intrusive basalt1 shore1 ,saltcoats1 ,ayrshire #afd #bef4 ,dolerite dyke with marginal bands of white trap1 in black shale1 ,lower ,lias1 ,pabba #aff #beg4 ,map of the chief dykes between ,lochs ,ridden and ,striven 7,c4 ,t4 ,clough1 ,geological ,survey1 ,sheet #bi7 #agj #beh4 ,basalt-veins traversing granophyre1 ,st4 ,kilda #agc #bei4 ,section of scoriaceous and prismatic basalt1 ,camas ,tharbernish1 north shore of ,canna ,island #ahg #bfj4 ,banded amygdaloidal basalt showing layers of elongated and steeply inclined vesicles1 ,macleod's ,maidens1 ,skye #aia #bfa4 ,termination of basalt-beds1 ,carsaig1 ,mull #aic #bfb4 ,breccia and blocks of mica-schist1 quartzite1 etc41 lying between bedded basalts1 ,isle of ,mull #aig #bfc4 ,section of ,knocklayd1 an outlier of the ,antrim basalt-plateau lying on ,chalk #bjb #bfd4 ,diagram-,section of the ,antrim ,plateau #bjc #bfe4 ,view ,of ,basalt escarpment1 ,giant's ,causeway1 with the ,amphitheatre and ,chimneys4 7,from a photograph by ,mr4 ,r4 ,welch7 #bjg #bff4 ,basalt-capping on the top of ,ben ,iadain1 ,morven #bji #bff;a4 ,view of the south side of ,staffa1 showing the bedded and columnar structure of the basalt #baj #bfg4 ,view of ,rum from the harbour of ,canna #baf #bfh4 ,section of the cliffs below ,compass ,hill1 ,isle of ,canna #bah #bfi4 ,lava cutting out conglomerate and shale4 ,shore below ,canna ,house #bbd #bgj4 ,section of shales and tuffs1 with a coniferous stump lying between two basalt-sheets1 ,cl nam ,marbh1 ,canna #bbe #bga4 ,dn ,mr1 ,sanday4 7,from a photograph by ,miss ,thom7 #bbf #bgb4 ,view of the ,dn ,beag1 ,sanday1 seen from the south4 7,from a ,photograph by ,miss ,thom7 #bcj #bgc4 ,view of ,dn ,beag1 ,sanday1 from the north4 ,the island of ,rum in the distance4 7,from a ,photograph by ,miss ,thom7 #bca #bgd4 ,section of eastern front of ,dn ,beag #bcb #bge4 ,enlarged section on the western side of ,dn ,beag #bcc #bgf4 ,geological map of the ,island of ,eigg #bce #bgg4 ,section of the geological structure of the ,island of ,eigg #bcf #bgh4 ,view of the ,scuir of ,eigg from the east #bcg #bgi4 ,natural section at the cliff of ,bideann ,boidheach1 north-west end of the ,scuir of ,eigg #bci #bhj4 ,view of the ,scuir of ,eigg from the south #bdb #bha4 ,view of the ,scuir of ,eigg from the south-west of the ,loch a ,bhealaich1 showing the bedded character of the mass #bdc #bhb4 ,section at the base of the ,scuir of ,eigg 7east end7 #bdd #bhc4 ,terraced hills of basalt plateau 7,macleod's ,tables71 ,skye #bej #bhd4 ,macleod's ,maidens and part of basalt cliffs of ,skye #bea #bhe4 ,intercalated group of strata between ,basalts1 ,an ,ceannaich1 western side of ,skye #beb #bhf4 ,escarpment of ,plateau-basalts1 ,cliffs of ,talisker1 ,skye #bec #bhg4 ,section of the largest of ,macleod's ,maidens #bed #bhh4 ,dying out of lava-beds1 east side of ,sand1 ,faroe ,isles #beg #bhi4 ,lenticular lavas1 western front of ,hest1 ,faroe ,isles #beg #bij4 ,lenticular lavas1 east side of ,svin1 ,faroe ,isles #beh #bia4 ,section at ,frodbonyp1 ,suder1 ,faroe #beh #bib4 ,fissure 7gj7 in a lava-field1 ,iceland4 7,from a photograph by ,dr4 ,tempest ,anderson7 #bfb #bic4 ,cones on the great ,laki fissure1 ,iceland4 7,from a photograph by ,dr4 ,tempest ,anderson7 #bfc #bic;a4 ,plan of small craters along the line of great ,laki fissure1 ,iceland4 7,after ,mr4 ,helland1 reduced7 #bfd #bid4 ,slemish1 a volcanic neck or vent on the ,antrim plateau1 seen from the north #bgb #bie4 ,section of volcanic vent at ,carnmony ,hill 7,e4 ,hull7 #bgb #bif4 ,section of the east side of ,scawt ,hill1 near ,glenarm #bgc #big4 ,section of ,neck of basalt1 ,bendoo1 ,ballintoy #bgc #bih4 ,volcanic ,neck of dolerite near ,cushendall #bgd #bii4 ,section of ,volcanic ,neck at ',s ,airde ,beinne1 near ,tobermory1 ,mull #bgd #cjj4 ,interior of the ,volcanic ,neck of ',s ,airde ,beinne1 near ,tobermory1 ,mull #bge #cja4 ,diagram to show the probable relation of the ,neck at ,carrick-a-raide1 ,antrim1 to an adjacent group of tuffs #bgg #cjb4 ,section of agglomerate ,neck at ,maclean's ,nose1 ,ardnamurchan #bgi #cjc4 ,diagram to show the probable relations of the rocks on the southern flank of ,beinn ,dearg ,bheag #bhb #cjd4 ,section of ,volcanic ,vent and connected lavas and tuffs1 ,scorr1 ,camas ,garbh1 ,portree ,bay1 ,skye #bhd #cje4 ,section of the ,volcanic ,series at ,ach na ,hannait1 south of ,portree1 ,skye #bhh #cjf4 ,view of part of a ,volcanic ,neck at the eastern end of the island of ,canna4 7,from a photograph by ,miss ,thom7 #bhi #cjg4 ,columnar ,basalt invading agglomerate of ,volcanic ,vent1 ,coroghon ,mr1 ,isle of ,canna4 7,height above #bj feet7 #bia #cjh4 ,columnar ,basalt invading ,volcanic conglomerate1 north side of ,alman ,islet1 ,canna #bia #cji4 ,view of neck-like mass of breccia1 ,brochel1 ,raasay #bib #caj4 ,view of ,volcanic ,neck piercing and overlain by the ,plateau-,basalts1 ,strom1 entrance of ,vaagfjord1 ,faroe ,islands4 7,from a photograph by ,colonel ,evans7 #bid #caa4 ,section of the same ,neck as that shown in ,fig4 #caj #bie #cab4 ,volcanic ,neck close to that shown in ,figs4 #caj and #caa #bif #cac4 ,section of wall of another ,neck of agglomerate in the same group with those represented in ,figs4 #caj1 #caa1 and #cab #bif #cad4 ,view of ,segregation-,veins in a dolerite sill1 ,portrush1 ,antrim #cjj #cae4 ,view of ,fair ,head1 from the east1 showing the main upper sill and a thinner sheet cropping out along the talus #cja #caf4 ,view of ,fair ,head from the shore4 7,from a photograph by ,mr4 ,r4 ,welch7 #cjb #cag4 ,section at ,farragandoo cliff1 west end of ,fair ,head1 showing the rapid splitting up and dying out of an ,intrusive ,sheet slope #cjd #cah4 ,view of the ,trotternish ,coast1 showing the position of the band of ,sills #cje #cai4 ,columnar ,sill intrusive in ,jurassic ,strata east of ,kilmartin1 ,trotternish1 ,skye #cjf #cbj4 ,view of the northern precipice 7#ejj feet high7 of the largest of the ,shiant ,isles4 7,from a photograph by ,colonel ,evans7 #cjh #cba4 ,section of thin ,intrusive ,sheets and ,veins in carbonaceous shales lying among the ,plateau-basalts1 cliffs north of ,ach na ,hannait1 between ,portree ,bay and ,loch ,sligachan #caa #cbb4 ,upper part of ,sill1 ,moonen ,bay1 ,waternish1 ,skye1 showing the divergence of veins #cac #cbc4 ,section of the base of the ,basalt-plateau with sill and dykes1 ,sound of ,soa1 ,skye #cad #cbd4 ,section of ,dolerite ,sill cut by another sill1 both being traversed by dykes1 ,rudh' an ,iasgaich1 western side of ,sleat1 ,skye #caf #cbe4 ,section to show ,bedded and ,intrusive ,sheets1 ,eigg #cah #cbf4 ,ground plan of ,sills at ,ben ,hiant1 ,ardnamurchan4 #cba #cbg4 ,section of two ,sills in schistose grits1 west end of ,beinn na h-,urchrach1 ,ardnamurchan #cbb #cbh4 ,sill traversing bedded ,basalts1 cliffs of ,strom1 at entrance of ,vaagfjord #cbc #cbi4 ,view of the same ,sill seen from the channel opposite the island of ,kolter #cbd #ccj4 ,granulitic and coarsely foliated ,gabbro traversed by later veins of felspathic ,gabbro1 ,druim an ,eidhne1 ,cuillin ,hills1 ,skye #cca #cca4 ,scuir na ,gillean1 ,cuillin ,hills1 showing the characteristic craggy forms of the ,gabbro4 7,from a photograph by ,mr4 ,abraham1 ,keswick7 #cce #ccb4 ,section across ,glen ,brittle1 to show the general relations of the ,bedded ,basalts and the ,gabbros #ccf #ccc4 ,view of the crest of the ,cuillin ,hills1 showing the weathering of the ,gabbro along its joints and of a compound basic dyke which rises through it4 7,from a photograph by ,mr4 ,abraham1 ,keswick7 #cch #ccd4 ,section across the ,coire ,uaigneich1 ,skye #cda #cce4 ,banded and puckered gabbro1 ,druim an ,eidhne1 ,glen ,sligachan1 ,skye #cdb #ccf4 ,banded structure in the ,gabbro1 from the ridge of ,druim an ,eidhne1 between ,loch ,coruisk and ,glen ,sligachan #cdc #ccg4 ,banded and doubly folded ,gabbro1 ,druim an ,eidhne1 #aj feet broad #cde #cch4 ,sketch of banded structure in the ,gabbros of the hills at the head of ,loch ,scavaig #cdg #cci4 ,outline of the hills of the ,island of ,rum1 sketched from near the ,isle of ,eigg #cej #cdj4 ,view of ,allival1 ,rum1 sketched from the base of the north-east side of the cone #ceb #cda4 ,section of foliated ,gabbros in the ,tertiary volcanic series of ,allival1 ,rum #cec #cdb4 ,altered ,plateau-,basalts invaded by ,gabbro1 and with a ,dyke of prismatic ,basalt cutting both rocks1 north slope of ,ben ,buy1 ,mull #ceg #cdc4 ,theoretical representation of the structure of one of the ,gabbro bosses of the ,inner ,hebrides #cfb #cdd4 ,section through the ,puy de la ,goutte and ,puy de ,chopine #cgd #cde4 ,view of the ,huche ,pointue and ,huche ,platte west of ,le ,pertuis #cgf #cdf4 ,view of ,glamich1 #becg feet1 ,glen ,sligachan4 7,from a photograph by ,r4 ,j4 ,a4 ,berry1 ,m4,d41 lent by the ,scottish ,mountaineering ,club7 #chj #cdg4 ,section across the north slope of ,beinn an ,dubhaich1 ,skye #chc #cdh4 ,section from ,beinn ,dearg to ,beinn an ,dubhaich1 ,skye #che #cdi4 ,section at north end of ,beinn na ,cro1 ,skye #chh #cej4 ,ground-plan of basic dyke in ,cambrian limestones truncated by granophyre which encloses large blocks of the dyke1 ,torrin1 ,skye #cic #cea4 ,section on south side of ,beinn an ,dubhaich1 ,skye1 showing the truncation of a basalt-dyke #cid #ceb4 ,view of the hills on the south side of the head of ,loch na ,keal1 showing the junction of the ,granophyre and the bedded basalts #cif #cec4 ,section on south side of ,cruach ,trr an ,lochain1 ,mull #cih #ced4 ,section at head of ,allt na ,searmoin1 ,mull #cih #cee4 ,section on south side of ,beinn ,fhada1 ,mull #cii #cef4 ,section to south of ,loch na ,diridh1 ,mull #djj #ceg4 ,section of junction of south side of ,loch ,ba' ,granophyre boss1 with the bedded basalts1 ,mull #dja #ceh4 ,mass of dark gabbro about two feet in diameter traversed by pale veins of ,granophyre1 lying on north slope of ,creag na h-,iolaire1 ,mull #djb #cei4 ,section at ,creag na h-,iolaire1 ,glen ,more1 ,mull1 showing basalts and gabbros resting on and pierced by ,granophyre #djb #cfj4 ,section on north side of ,orval1 ,rum #djd #cfa4 ,junction of ,quartz-porphyry 7,microgranite7 and basic rocks1 south-east side of ,orval1 ,rum #djd #cfb4 ,junction of ,granophyre and gabbro1 north side of ,st4 ,kilda #daj #cfc4 ,veins of ,granophyre traversing gabbro and splitting up into thin threads1 north side of ,st4 ,kilda #daa #cfd4 ,pale ,granophyre injected into dark basalt1 ,south ,bay1 ,st4 ,kilda #dab #cfe4 ,veins of ,granophyre traversing a fine-grained gabbro and scarcely entering a coarse-grained sheet1 west side of ,rueval1 ,st4 ,kilda #dac #cff4 ,view of sills and veins of pale ,granophyre traversing sheets of gabbro1 west side of ,st4 ,kilda4 7,from a photograph by ,colonel ,evans7 #dad #cfg4 ,section of the sea-cliff below ,conacher1 ,st4 ,kilda1 showing basic dykes in ,granophyre #dag #cfh4 ,triple basic dyke1 sea-cliff1 east side of ,st4 ,kilda #dag #cfi4 ,jointed structure of the ,granite near the top of ,goatfell1 ,arran4 7,from a photograph by ,mr4 ,w4 ,douglas1 lent by the ,scottish ,mountaineering ,club7 #dai #cgj4 ,intrusive ,rhyolite in the lower basalt group of ,antrim1 ,templepatrick #dbg #cga4 ,section across the southern slope of ,carnearny ,hill1 ,antrim #dbg #cgb4 ,section across the ,granophyre ,sills at ,loch a' ,mhullaich1 above ,skulamus1 ,skye #dcc #cgc4 ,section to show the connection of a sill of ,granophyre with its probable funnel of supply1 ,raasay #dcf #cgd4 ,granophyre sill resting on ,lower ,lias shales with a dyke of basalt passing laterally into a sill1 ,suisnish ,point1 ,isle of ,raasay #dcf #cge4 ,weathered surface of spherulitic ,granophyre from dyke in banded gabbros1 ,druim an ,eidhne1 ,meall ,dearg1 ,glen ,sligachan1 ,skye4 ,natural size #dch #cgf4 ,plan of portion of the ridge north of ,druim an ,eidhne1 ,glen ,sligachan1 ,skye1 showing three dykes issuing from a mass of ,granophyre #dci #cgg4 ,weathered surface of spherulitic ,granophyre1 from dyke in banded gabbros1 ,druim an ,eidhne1 ,meall ,dearg1 ,glen ,sligachan1 ,skye4 ,natural size #ddj #cgh4 ,plan of pale ,granophyric dyke1 with spherulitic and flow-structure1 cutting and enclosing dark gabbro1 ,druim an ,eidhne #dda #cgi4 ,dyke 7six to ten feet broad7 proceeding from a large body of ,granophyre and traversing gabbro1 from the same locality as ,figs4 #cge and #cgg #ddb #chj4 ,section of intruded veins of various acid rocks above ,river ,clachaig1 ,mull #ddc #cha4 ,pitchstone vein traversing the bedded basalts1 ,rudh an ,tangairt1 ,eigg #dde #chb4 ,reversed fault on the eastern side of ,svin1 ,faroe ,isles #ded #chc4 ,reversed fault on the north-east headland of ,sand1 ,faroe ,isle #ded ,,maps ,v4 ,map of the ,permian volcanic districts of ,scotland ,to face p4 #ajf ,,vi4 ,map of the ,tertiary volcanic region of the ,west of ,scotland ,to face p4 #bif ,,vii4 ,map of the ,tertiary volcanic district of the ,north-,east of ,ireland ,to face p4 #ddf ,,chapter ,,xxix ,,the ,,carboniferous ,,volcanoes ,,of ,,england ,the ,north of ,england3 ,dykes1 ,the ,great ,whin ,sill--,the ,derbyshire ,toadstones--,the ,isle of ,man--,east ,somerset--,devonshire #a4 ,,the ,,north ,,of ,,england ,the volcanic intercalations which diversify the ,lower ,carboniferous formations of ,southern ,scotland extend but a short way across the ,english ,border1 and although1 over the moors and hills of the north of ,cumberland and ,northumberland1 the ,carboniferous sandstones1 limestones and shales are well exposed1 they present no continuation of either the plateau or puy-eruptions which play so prominent a part in the geology of ,roxburghshire and ,dumfriesshire4 ,this deficiency is all the more noticeable seeing that the ,carboniferous system is exposed down to its very base1 in the deep dales of the ,north of ,england4 ,had any truly interstratified volcanic material existed in the system there1 it could hardly fail to have been detected4 ,but while contemporaneous volcanic rocks are absent1 the northern ,english counties contain many intrusive masses of dolerite1 diabase1 andesite or other eruptive rocks1 which may be found traversing all the subdivisions of the ,carboniferous system4 ,these eruptive materials have taken two forms3 in some cases they rise as ,dykes1 in others they appear as ,sills4 ,dykes4--,with regard to the dykes1 some are probably much later than the ,carboniferous period1 and consequently will be more appropriately considered in ,chapters xxxiv4 and xxxv4 ,the great ,cleveland dyke1 for example1 which runs across the ,carboniferous1 ,permian1 ,triassic and ,jurassic formations1 is probably referable to the ,older ,tertiary volcanic period4 ,one dyke known as the ,hett ,dyke1 has been plausibly claimed as possibly of ,carboniferous age4 ,it runs in a ,w4,s4,w4 direction from the ,magnesian ,limestone escarpment at ,quarrington ,hill1 a few miles to the east of ,durham1 through the great ,coal-field1 across the ,millstone ,grit and ,carboniferous ,limestone1 disappearing near ,middleton in ,teesdale4 ,its total length is thus about #bc miles4 ,it varies in breadth from about #f to about #ae feet1 and appears to increase in dimensions as it goes westward4#a ,footnote #a3 ,sedgwick1 ,trans4 ,geol4 ,soc4 #bnd series1 iii4 part #a 7#ahbf-bh71 p4 #fc2 ,trans4 ,cambridge ,phil4 ,soc4 ii4 7#ahbb71 p4 #ba4 ,sir ,j4 ,lowthian ,bell1 ,proc4 ,roy4 ,soc4 xxiii4 7#ahge71 p4 #edc4 ,the age of this dyke cannot at present be satisfactorily fixed4 ,it must be later than the ,coal-measures through which it rises4 ,sedgwick long ago pointed out that though it reaches the escarpment of the ,magnesian ,limestone1 it does not cut it2 yet it is found in coal-mining to traverse the ,coal-measures underlying the ,limestone4 ,he was accordingly inclined to believe it to be of older date than the ,magnesian ,limestone4 ,at its western extremity it approaches close to the ,great ,whin ,sill of ,teesdale1 though no absolute connection between the two has been established4 ,mr4 ,teall1 however1 has called attention to the similarity between the microscopic structure of the rock forming the ,hett ,dyke and that of the mass of the ,whin ,sill1 and he is strongly inclined to regard them as belonging to the same period of intrusion4#b ,footnote #b3 ,quart4 ,journ4 ,geol4 ,soc4 xl4 7#ahhd71 p4 #bcj4 ,it is especially worthy of remark that in the course of its nearly rectilinear course across the ,durham ,coal-field1 the ,hett ,dyke1 where it crosses the ,wear1 is flanked on the north at a distance of a little more than two miles by a second parallel dyke of nearly identical composition4 ,between the two dykes1 during mining operations1 a sill about #bj feet thick has been met with1 lying between two well-known coal-seams at a depth of about #fj fathoms from the surface1 and extending over an area of at least #ae acres4#c ,microscopic examination of this sill by ,mr4 ,teall proved that the rock presents the closest resemblance to that of the ,hett ,dyke4#d ,in this case1 it may be regarded as probable that the two dykes and the intermediate sill form one related series of intrusions1 and the conjecture that the ,hett ,dyke may be connected with the ,whin ,sill thus receives corroboration4 ,the age of the ,whin ,sill itself will be discussed a few pages further on4 ,footnote #c3 ,sir ,lowthian ,bell1 ,proc4 ,roy4 ,soc4 xxiii4 7#ahge71 p4 #edd4 ,footnote #d3 ,quart4 ,journ4 ,geol4 ,soc4 xl4 7#ahhd71 p4 #bcj4 ,of the other dykes which may possibly be coeval with the ,hett ,dyke we may specially note those which follow the same ,w4,s4,w4 trend1 for that strike differs from the general ,w4,n4,w4 direction of most of the dykes4 ,two conspicuous examples of the south-westerly trend may be seen1 one near ,morpeth1 the other north of ,bellingham4 ,the former dyke1 as regards microscopic structure1 is more nearly related to the majority of the series in the ,north of ,england4 ,but that north of ,bellingham 7,high ,green7 presents affinities both in structure and composition with the ,hett ,dyke1#e and may perhaps belong to the same period of intrusion4 ,footnote #e3 ,mr4 ,teall1 op4 cit4 p4 #bdd4 ,quart4 ,journ4 ,geol4 ,soc4 xxxix4 7#ahhd71 p4 #fef1 and ,proc4 ,geol4 ,assoc4 7#ahhf74 ,see also ,prof4 ,lebour1 ,geology of ,northumberland and ,durham1 chap4 xi4 ,the ,great ,whin ,sill4--,the geologist who1 after making himself acquainted with the abundant sills among the ,carboniferous rocks in the centre of ,scotland1 finds his way into ,northumberland1 meets there with geological features that have become familiar to him further north4 ,the sea-cliffs of ,bamborough and ,dunstanborough1 the rocky islets of ,farne1 the long lines of brown crag and green slope that strike inland through the ,kyloe ,hills and wind across the cultivated lowlands and the moorlands beyond1 remind him at every turn of the scenery in the basin of the ,forth4 ,but not until he has traced these ridges for many miles southwards and found their component rocks to form there an almost continuous sheet does he realize that nothing of the kind among the ,scottish ,carboniferous rocks can be compared for extent to this display in the ,north of ,england4#f ,footnote #f3 ,the ,whin ,sill has been the subject of much discussion1 and a good deal of geological literature has been devoted to its consideration4 ,the writings of ,trevelyan1 ,sedgwick1 ,w4 ,hutton1 ,phillips and ,tate are especially deserving of recognition4 ,the intrusive character of the ,sill1 maintained by some of these writers1 was finally established by the mapping of the ,geological ,survey1 and was discussed and illustrated by ,messrs4 ,w4 ,topley and ,g4 ,a4 ,lebour in a paper in the #ccrd volume of the ,quart4 ,journ4 ,geol4 ,soc4 7#ahgg71 in which references to the earlier observers will be found4 ,see also ,prof4 ,lebour's ,outlines of the ,geology of ,northumberland1 #bnd edit4 7#ahhf71 p4 #ib4 ,the petrography of the ,whin ,sill is fully treated by ,mr4 ,teall in ,quart4 ,journ4 ,geol4 ,soc4 xl4 7#ahhd71 p4 #fdj1 where a bibliography of the subject is also given4 ,from the furthest skerries of the ,farne ,islands southwards to ,burton ,fell on the great ,pennine escarpment1 a distance in a straight line of about #hj miles1 this intrusive sheet may be traced in the ,carboniferous ,limestone series 7,map ,i474 ,there are intervals where its continuity cannot be actually followed at the surface1 but that it really runs unbroken from one end to the other underground cannot be doubted by any one who has examined the region4 ,this singular feature in the geology and scenery of the ,north of ,england is known locally as the ,great ,whin ,sill4#g ,from the rocky islets and castle-crowned crags of the coast-line it maintains its characteristic topography1 structure and composition throughout its long course in the interior4 ,so regularly parallel with the sedimentary strata does it appear to lie1 that it was formerly regarded by many observers as a true lava-sheet1 poured out upon the sea-floor over which the limestones and shales were laid down4 ,but its really intrusive character has now been clearly demonstrated4 ,not a vestige of any tuff has been detected associated with it1 nor does it ever present the usual characters of a true lava-stream4#h ,its internal structure and the wonderful uniformity in its character mark it out as a typical intrusive sheet4 ,footnote #g3 ,whin is a common term in ,scotland and the ,north of ,england for any hard kind of stone1 especially such as can be used for making and mending roads4 ,sill denotes a flat course or bed of stone1 and was evidently applied to this intrusive sheet from its persistent flat-bedded position and its prominence among the other gently inclined strata among which it lies4 ,it is from this example in the ,north of ,england that the word sill has passed into geological literature4 ,footnote #h3 ,on the coast at ,bamborough and the ,harkess ,rocks the usual petrographical characters of the ,whin ,sill are exchanged for those of fine-grained amygdaloidal diabases arranged in distinct sheets1 which in their upper parts are highly vesicular and show ropy surfaces--peculiarities suggestive of true lava-streams4 ,but according to ,professor ,lebour the rocks are intrusive into limestone and shale 7,geology of ,northumberland and ,durham1 p4 #ih74 ,mr4 ,teall has expressed the suspicion that these rocks must have consolidated under conditions somewhat different from those which characterized the normal ,whin ,sill 7,quart4 ,journ4 ,geol4 ,soc4 xl4 p4 #fdc74 ,they seem to be the only parts of the sill which present features that might possibly indicate superficial outflow4 ,among the manifestations of the subterranean intrusion of igneous rocks in the ,british ,isles the ,great ,whin ,sill1 next after the ,dalradian sills of ,scotland1 is the most extensive4 ,its striking continuity for so great a distance1 and the absence around it of any other trace of igneous action1 save a few dykes1 place it in marked contrast to the ordinary type of ,carboniferous sills4 ,the occasional gaps on its line of outcrop in the northern part of its course do not really affect our impression of the persistence of the sheet4 ,they not improbably indicate merely that in its protrusion it had a wavy irregular limit1 which in the progress of denudation has occasionally been not yet reached4 ,for mile after mile the sill has been mapped by the ,geological ,survey in lines of crag across the moorlands1 and as a conspicuous band among the limestones and shales that form the steep front of the ,pennine escarpment1 where it has long been known in the fine sections exposed among the gullies by which that noble rock-face has been furrowed4 ,illustration3 ,fig4 #agf4--,section from the great ,limestone escarpment on the west to the ,millstone ,grit hills east of ,teesdale4 #a4 ,silurian strata2 #b4 ,carboniferous ,limestone series2 #c4 ,the ,great ,whin ,sill1 which gradually rises to higher stratigraphical position as it goes westward2 #d4 ,millstone ,grit4 ,along its main outcrop1 the sill dips gently eastwards below the portion of the ,carboniferous ,limestone series which overlies it4 ,but so slight are the inclinations1 so gentle the undulations of the rocks in this part of the country1 that far to the east of that outcrop the sill has been laid bare by the streams which in the larger dales have cut their way through the overlying cake of ,carboniferous strata down to the ,silurian platform on which they rest 7,fig4 #agf74 ,among these inland revelations of the eastward continuation of the sill under ,carboniferous ,limestone strata1 the most striking and best known are those which have been made by the ,river ,tees1 and of which the famous waterfalls of the ,high ,force and ,cauldron ,snout are the most picturesque features4 ,the distance of the remotest of these denuded outcrops or inliers from the main escarpment is not less than #bj miles4 ,it is not possible to form an accurate estimate of the total underground area of the ,whin ,sill4 ,in the southern half of the district1 south of the line of the ,roman ,wall1 where1 the inclination of the strata being generally low1 the same stratigraphical horizons are exposed by denudation far to the east of the main outcrops of the rocks1 we know that the sill must have a subterranean extent of more than #djj square miles4 ,yet this is probably only a small part of the total area over which the molten material was injected4 ,in the northern part of the district1 the ,carboniferous ,limestone series is not exposed over so broad a stretch of country1 and denudation has not there revealed the eastward extension of the sill4 ,but there is no reason to suppose the sheet to be less continuous and massive there4 ,we must remember also that the present escarpment has been produced by denudation1 and that the intrusive sheet must have once extended westwards beyond its present limits at the surface4 ,if1 therefore1 we were to state broadly that the ,great ,whin ,sill has been intruded into the ,carboniferous ,limestone series over an area of #ajjj square miles we should probably be still below the truth4 ,the rock composing this vast intrusive sheet is a dolerite or diabase1 which maintains throughout its wide extent a remarkable uniformity of petrographical characters4 ,in this and other respects it illustrates the typical features of sills4 ,thus it is coarsest in texture where it is thickest1 and somewhat finer in grain towards its upper and lower surfaces than in the centre4 ,among the coarser varieties the component crystals of augite are not infrequently an inch in length and occur in irregular patches4#i ,occasional amygdaloidal portions are observable1 but these are not more marked than those to be found in the whin-dykes of the same region4#aj ,the amygdaloidal and vesicular fine-grained rocks of the ,bamborough district may possibly be quite distinct from the main body of the ,whin ,sill4 ,footnote #i3 ,sedgwick1 ,cambridge ,phil4 ,trans4 ii4 p4 #aff4 ,mr4 ,teall1 ,quart4 ,journ4 ,geol4 ,soc4 xl4 p4 #fdc4 ,footnote #aj3 ,messrs4 ,topley and ,lebour1 ,quart4 ,journ4 ,geol4 ,soc4 xxxiii4 p4 #dah4 ,under the microscope the rock is seen to consist essentially of the usual minerals--plagioclase1 augite and titaniferous magnetic iron-ore4 ,an ophitic intergrowth of the augite and felspar is observable1 likewise a certain quantity of micropegmatite which plays the part of groundmass between the interstices of the lath-shaped felspars4 ,full details of the characteristics of the component minerals and their arrangement are given by ,mr4 ,teall in the paper already cited4 ,the main body of the sill is a sheet which sometimes diminishes to less than #bj feet in thickness and sometimes expands to #aej feet1 but averages from #hj to #ajj feet4 ,it occasionally divides1 as near ,great ,bavington1 where it appears at the surface in two distinct beds separated by an intervening group of limestones and shales4 ,occasionally1 as at ,elf's ,hill ,quarry1 it gives out branches which send strings into the adjacent limestone4#aa ,footnote #aa3 ,messrs4 ,topley and ,lebour1 op4 cit4 p4 #dac4 ,although in most natural sections it seems to lie quite parallel with the strata above and below1 yet a number of examples of its actual intrusion have been observed4 ,when traced across the country1 it is found not to remain on a definite horizon1 but to pass transgressively across considerable thicknesses of strata4 ,its variations in this respect are well shown in the accompanying table of comparative sections constructed by ,messrs4 ,topley and ,lebour4#ab ,it will be seen that while at ,harlow ,hill the sill is found overlying the ,great ,limestone of ,alston ,moor1 at ,rugley1 five miles off it lies about #ajjj feet lower down1 far below the position of the ,tyne-bottom ,limestone4 ,still farther north1 however1 the sill west of ,holy ,island is said to lie #hjj feet above the ,great ,limestone and to come among the higher beds of the ,carboniferous ,limestone series4#ac ,footnote #ab3 ,op4 cit4 plate xviii4 ,footnote #ac3 ,op4 cit4 p4 #dad4 ,the ,whin ,sill appears generally to thicken in an easterly or north-easterly direction4 ,there are further indications that it was intruded from east to west4 ,thus1 at ,shepherd's ,gap1 on the ,great ,roman ,wall1 the dolerite1 coming evidently from an easterly quarter1 has broken up and thrust itself beneath a bed of limestone4 ,again1 when the sill bifurcates the branches unite towards the east or north-east4#ad ,the sill can be proved to thin away to the west from ,teesdale to the ,pennine escarpment1 and in ,weardale the ,little ,whin ,sill diminishes from #bj feet1 till in three miles it disappears4#ae ,footnote #ad3 ,op4 cit4 p4 #dae4 ,footnote #ae3 ,op4 cit4 p4 #dai4 ,illustration3 ,walker ,bontall sc4 ,fig4 #agg4--,sections of the ,carboniferous ,limestone series of ,northumberland showing the variations in the position of the ,whin ,sill4 ,by ,messrs4 ,topley and ,lebour4 ,the strata in contact with the ,whin ,sill1 both above and below1 have been more or less altered4 ,sandstones have been least affected2 shales have suffered most1 passing into a kind of porcellanite1 with development of garnet and other minerals4#af ,limestone often shows only slight traces of change1 though here and there it has become crystalline4 ,footnote #af3 ,mr4 ,teall1 op4 cit4 xxxix4 7#ahhd71 p4 #fdb1 and authors cited by him4 ,no trace of any boss or neck has been detected in the whole region which might be supposed to mark a funnel of ascent for the material of the ,whin ,sill4 ,the ,hett ,dyke and the ,high ,green ,dyke1 already noticed1 may1 however1 have been possibly connected with the injection of this great intrusive sheet4 ,no other visible mass of igneous rock in the region has been even plausibly conjectured to indicate a point or line of emission for the sill4 ,it is certainly singular that in so wide a territory1 where the whole succession of strata has been so admirably laid bare by denudation in thousands of natural sections1 and where1 moreover1 much additional information has been obtained from lead-mining as to the nature of the rocks below ground1 not a single vestige of tuff1 agglomerate or interstratified lava has been up to the present time recorded1 unless the ,harkess rocks already alluded to can be so regarded4 ,judging1 however1 from the analogy of the other districts of igneous rocks in ,britain1 we can hardly resist the conclusion that the ,great ,whin ,sill is essentially a manifestation of volcanic action1 that it was connected with the uprise of basic lava in volcanic orifices1 and that the subterranean energy may quite probably have succeeded in reaching the surface and ejecting there both lavas and tuffs4 ,it appears to be certain that any vents which existed cannot have lain to the west of the present escarpment of the sill1 for no trace of them can be found there piercing the ,carboniferous or older formations4 ,they must have lain somewhere to the east in the area now overspread with ,millstone ,grit and ,coal-measures1 or still farther east in the tract now concealed under the ,north ,sea4 ,the evidence of the sill itself1 as we have seen1 corroborates this view of the probable situation of the centre of disturbance4 ,the question of the geological age of the sill is one of considerable difficulty1 to which no confident answer can be given4#ag ,the injection of the diabase must obviously be considerably later than the highest strata through which it has risen2 that is1 it must be younger than some of the higher members of the ,carboniferous ,limestone series4 ,but here our positive evidence fails4 ,footnote #ag3 ,see ,messrs4 ,topley and ,lebour1 op4 cit4 p4 #dah4 ,the ,sill is traversed by the same faults which disrupt the surrounding ,carboniferous rocks4 ,it is therefore of older date than these dislocations4 ,its striking general parallelism with the shales and limestones probably proves that it was intruded before the rocks were much disturbed from their original horizontal position4 ,but the manner in which the intrusive rock has been thrust into and has involved the shales and limestones seems to indicate that these strata had already become consolidated and lay under the pressure of a great thickness of superincumbent ,carboniferous strata4 ,in the absence of all certainty on the subject it seems most natural to place the ,whin ,sill provisionally among the ,carboniferous volcanic series with which petrographically and structurally it has so much in common4 ,in ,scotland the puy-eruptions continued till the time of the ,coal-measures4 ,if1 before the close of the ,carboniferous period1 volcanic vents were opened somewhere to the east of the coal-fields of ,northumberland and ,durham1 they might be accompanied with basic sills injected into the ,carboniferous ,limestone series1 which was then lying still approximately horizontal under a thickness of from #cejj to #ejjj feet of ,carboniferous sedimentary deposits4 ,these still undiscovered volcanoes seem to have been endowed with even more energy than those of ,central and ,southern ,scotland1 at least nowhere else among the ,carboniferous records of ,britain is there such a colossal manifestation of subterranean intrusion as the ,great ,whin ,sill4 #b4 ,,the ,,derbyshire ,,toadstones ,in the absence of any certain evidence that the ,whin ,sill belongs to the ,carboniferous period1 we must advance southward into the very heart of ,england before any clear vestiges can be found of contemporaneous volcanic eruptions among the members of the ,carboniferous system4 ,after quitting the lavas and tuffs of ,roxburghshire and their brief continuations across the ,english border1 we do not again meet with any truly bedded volcanic rocks in that system until we reach the middle of ,derbyshire4 ,in this picturesque district1 famous for its lead-mines and its mineral waters1 a feebly developed but interesting group of intercalated lavas1 locally called toadstones1 has long been known4 ,there is thus a space of some #aej miles across which1 though the formations are there so fully developed and so abundantly trenched by valleys from the top to the bottom of the system1 no volcanic vents nor any trace of ,carboniferous volcanic ejections has yet been found4 ,on the other hand1 after the district of the toadstones is passed1 the ,carboniferous rocks are again destitute of any volcanic intercalations across the centre and south-west of ,england and over ,wales1 until after a space of about #aej miles they reappear in ,somerset4 ,the volcanic group of ,derbyshire thus stands out entirely isolated4 ,lying in the ,carboniferous ,limestone1 where that formation is typically developed1 it presents an admirable example of a thoroughly marine phase of volcanic action 7,map ,i474 ,one of the most prominent features in the geology of the centre of ,england is the broad anticlinal fold which brings up the lower portion of the ,carboniferous system to form the long ridge of the ,pennine chain that runs from ,yorkshire to the ,midland plain1 and separates the eastern from the western coal-fields4 ,this fold widens southwards until not only the ,millstone ,grit and ,yoredale rocks1 but the underlying ,mountain ,limestone is laid bare4 ,a broad limestone district is thus exposed in the very heart of the country1 ranging as a green fertile undulating tableland1 deeply cut by winding valleys1 which expose admirable sections of the strata1 but nowhere reach the base of the system4 ,the total visible depth of the limestone series is computed to be about #aejj feet2 the ,yoredale shales and limestones may be #ejj feet more2 so that the calcareous formations in which the volcanic phenomena are exhibited reach a thickness of at least #bjjj feet4 ,it is not yet definitely known through what vertical extent of this thickness of sedimentary material the volcanic platforms extend1 but where most fully developed they perhaps range through #ajjj feet1 lying chiefly in the ,carboniferous ,limestone1 but apparently in at least one locality extending up into the lower division of the ,yoredale group4 ,the area within which they can be studied corresponds nearly with that in which the limestone forms the surface of the country1 or a district measuring about #bj miles from north to south1 with an extreme breadth of #aj miles in an east and west direction4 ,a special historical interest belongs to the ,derbyshire toadstones4#ah ,they furnished ,whitehurst with material for his speculations1 and were believed by him to be as truly igneous rocks as the lava which flows from ,hecla1 ,vesuvius or ,etna4 ,but he thought that they had been introduced among the strata and did not overflow the surface of the earth1 according to the usual operations of volcanoes4#ai ,footnote #ah3 ,this word has by some writers been supposed to be corrupted from tod-stein1 dead-stone1 in allusion to the dying out of the lead veins there2 by others the name has been thought to be derived from the peculiar green speckled aspect of much of the rock1 resembling the back of a toad4 ,footnote #ai3 ,an ,enquiry into the ,original ,state and ,formation of the ,earth1 #aggh1 ,appendix1 pp4 #adi1 et seq4 ,his views were published as far back as #aggh1 three years after ,hutton read the first outline of his theory of the earth and made known his observations regarding the igneous origin of whinstones4#bj ,the first detailed account of the ,derbyshire eruptive rocks was that given by ,fairey1#ba which has served as the basis of all subsequent descriptions4 ,conybeare1 in particular1 prepared a succinct narrative from ,fairey's more diffuse statements1 and thus placed clearly before geologists the nature and distribution of these volcanic intercalations4#bb ,subsequently the district was mapped by ,de la ,beche and the officers of the ,geological ,survey1 and the areas occupied by the several outcrops of igneous rock could then be readily seen4#bc ,footnote #bj3 ,trans4 ,roy4 ,soc4 ,edin4 i4 p4 #bge1 et seq4 ,hutton specially mentions the toadstone of ,derbyshire as one of the rocks produced by fusion1 p4 #bgg4 ,footnote #ba3 ,general ,view of the ,agriculture and ,minerals of ,derbyshire 7#ahaa74 ,footnote #bb3 ,outlines of the ,geology of ,england and ,wales 7#ahbb71 p4 #ddh4 ,footnote #bc3 ,see ,sheets #ga ,n4,w41 #gb ,n4,e41 #ha ,n4,e4 and ,s4,e4 and #hb ,s4,w4 of the ,geological ,survey of ,england and ,wales4 ,though the toadstones were believed to form definite platforms among the limestone strata1 and thus to be capable of being used as reliable horizons in the mineral fields of ,derbyshire1 they appear to have been generally regarded as intrusive sheets like the ,whin ,sill of the north4 ,thus ,de la ,beche in his ,manual of ,geology1 giving a summary of what was known at the time regarding intercalated igneous rocks1 remarks with regard to the ,derbyshire toadstones that they may from all analogy be considered to have been injected among the limestones which would be easily separated by the force of the intruded igneous material4#bd ,but the same observer1 after his experience among the ancient volcanic rocks of ,devonshire1 came fully to recognize the proofs of contemporaneous outflow among the ,derbyshire toadstones4 ,in his subsequently published ,geological ,observer1 he described the toadstones as submarine lavas that had been poured out over the floor of the sea in which the ,carboniferous ,limestone was deposited1 and had been afterwards covered up under fresh deposits of limestone4#be ,it is remarkable1 however1 that he specially comments on the absence1 as he believed1 of any contemporaneously ejected ashes and lapilli1 such as occur in ,devonshire4 ,that true tuffs or volcanic ashes are associated with the toadstones was noticed by ,jukes in #ahfa1bf and afterwards by the ,geological ,survey4#bg ,since that time geologists have generally recognized these ,derbyshire igneous rocks as truly contemporaneous intercalations4 ,but very little has recently been written on the structure of the district1 our information regarding it being still based mainly on the early observations of ,fairey and the mapping of the ,geological ,survey4 ,footnote #bd3 ,manual1 #crd edit4 #ahcc1 p4 #dfb4 ,footnote #be3 ,geological ,observer 7#ahea71 pp4 #fdb-fde4 ,footnote #bf3 ,student's ,manual of ,geology1 #bnd edit4 7#ahfc71 p4 #ebc4 ,for a general rsum of the proofs of contemporaneity furnished by the toadstones1 see ,the ,geology of ,north ,derbyshire1 by ,messrs4 ,a4 ,h4 ,green and ,a4 ,strahan 7,memoirs of the ,geological ,survey1 #bnd edit4 7#ahhg71 p4 #abc74 ,footnote #bg3 ,in the first edition of the ,memoir on the ,geology of ,north ,derbyshire1 published in #ahei1 the authors of which were ,messrs4 ,a4 ,h4 ,green1 ,c4 le ,neve ,foster and ,j4 ,r4 ,dakyns4 ,the subject1 however1 has now been resumed by ,mr4 ,h4 ,arnold ,bemrose1 who in #ahid1 after a prolonged study of the petrography of the rocks1 communicated the results of his researches to the ,geological ,society4#bh ,in his excellent paper1 to which ,i shall immediately make fuller reference1 he mentions the localities at which lava-form and fragmental rocks may be observed1 but does not enter on the discussion of the geological structure of the region or of the history of the volcanic eruptions4 ,before the announcement of his paper1 hearing that ,i proposed to make for the first time a rapid traverse of the toadstone district1 for the purpose of acquainting myself with the rocks on the ground1 he kindly offered to conduct me over it4 ,my chief object1 besides that of seeing the general nature of the volcanic phenomena of the region1 was to examine more particularly the areas of the volcanic fragmental rocks1 with the view of discovering whether among them some remains might not be found of the actual vents of discharge4 ,in this search ,i was entirely successful4 ,aided by ,mr4 ,bemrose's intimate knowledge of the ground1 ,i was enabled to visit in rapid succession those tracts which seemed most likely to furnish the required evidence1 and in a few days was fortunate enough to obtain proofs of six or seven distinct vents1 ranging from the extreme northern to the furthest southern boundary of the volcanic district4 ,mr4 ,bemrose has undertaken to continue the investigation1 and will1 ,i trust1 work out the detailed stratigraphy of the ,carboniferous ,limestone so as eventually to furnish an exhaustive narrative of the whole volcanic history of ,derbyshire4 ,meanwhile no adequate account of the area can be given4 ,but ,i will here state all the essential facts which up to the present time have been ascertained4 ,footnote #bh3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 l4 7#ahid71 p4 #fjc4 #a4 ,,the ,,rocks ,,erupted4--,mr4 ,allport has described the microscopic character of some of the toadstones1#bi and further details have been supplied by ,mr4 ,teall4#cj ,the fullest account of the subject1 however1 is that given by ,mr4 ,bemrose in the paper above referred to4 ,this observer distinguishes the lava-form from the fragmental rocks1 and gives the minute characters of each series4 ,he does not1 however1 separate true interstratified lavas from injected sills1 nor the bedded tuffs from the coarse agglomerates which fill up the vents4 ,these distinctions are obviously required in order that the true nature and sequence of the materials in the volcanic eruptions may be traced1 and that the phenomena exhibited in ,derbyshire may be brought into comparison with those found in other ,carboniferous districts4 ,but to establish them satisfactorily the whole region must be carefully re-examined and even to some extent re-mapped4 ,footnote #bi3 ,quart4 ,journ4 ,geol4 ,soc4 xxx4 7#ahgd71 p4 #ebi4 ,footnote #cj3 ,british ,petrography1 p4 #bji4 ,the lavas 7including1 in the meantime1 sheets which there can be little doubt are sills7 show three main types of minute structure and composition1 which are discriminated by ,mr4 ,bemrose as--7a7 ,olivine-dolerites2 these1 the most abundant of the series1 consist of augite in grains1 olivine in idiomorphic crystals1 plagioclase giving lath-shaped and tabular sections1 and magnetite or ilmenite in rods and grains2 7b7 ,ophitic olivine-dolerites1 consisting of augite in ophitic plates forming the groundmass1 in which are imbedded idiomorphic olivine1 plagioclase 7often giving large lath-shaped sections and magnetite or ilmenite72 7c7 ,olivine-basalts2 these rocks are distinguished by containing crystals of augite and olivine in a groundmass of small felspar-laths1 granular augite and magnetite or ilmenite1 with very little interstitial matter4 ,they have been noticed only in two of the outcrops of toadstone4 ,the fragmental rocks have been shown by ,mr4 ,bemrose to cover a much more extensive space than had been previously supposed4 ,he has found them to be distinguished by an abundance of lapilli varying from minute fragments up to pieces about the size of a pea1 and composed of a material that differs in structure from the dolerites and basalts with which the tuffs are associated4 ,these lapilli consist largely of a glassy base more or less altered1 which is generally finely vesicular and encloses abundant skeleton crystals and crystallites4 ,the tuffs thus very closely resemble some of the ,carboniferous basic tuffs of ,fife1 already referred to 7vol4 i4 p4 #dbb71 and like these they include abundant blocks of dolerite and basalt4 #b4 ,,geological ,,structure ,,of ,,the ,,toadstone ,,district4--,as the volcanic rocks of ,derbyshire lie among the ,carboniferous ,limestones of a broad anticlinal dome1 they are only exposed where these limestones have been sufficiently denuded1 and as the base of the limestones is nowhere laid bare1 the lowest parts of the volcanic series may be concealed4 ,over the tract where the toadstones can be examined they appear as bands regularly intercalated with the limestones1 but varying in thickness in the course of their outcrops4 ,as they are prone to decay1 they usually form smooth grassy slopes between the limestone scarps1 though isolated blocks of the dull brown igneous rocks may often be seen protruding from the surface4 ,now and then a harder bed of toadstone caps a hill1 and thus forms a prominent feature in the landscape1 but as a rule these igneous bands play no distinguishing part in the scenery1 and are indeed less conspicuous than the white escarpments of limestone which overlie them4 ,it was the opinion of the older geologists that three distinct platforms of toadstone extend without break throughout the district1 and subdivide the limestones into four portions4 ,but this opinion does not seem to have been based on good evidence either of sequence or of continuity4 ,various facts were brought forward by the officers of the ,geological ,survey to show that the supposed persistence of the three platforms of toadstone did not really exist1 but that these sheets of igneous material are found at different spots on very different horizons1 and are of limited horizontal range4#ca ,so far as my own limited observations go1 they entirely corroborate this view4 ,there can be little doubt1 ,i think1 that the identity of certain outcrops of toadstone has been assumed1 and the assumption has been carried throughout the district4 ,the truth is that the number of successive platforms on which igneous materials appear will never be satisfactorily determined until the stratigraphy of the ,derbyshire ,carboniferous ,limestone is worked out in detail4 ,when the successive members of this great calcareous formation have been identified by lithological and palontological characters over the district1 it will be easy to allocate each outcrop of toadstone to its true geological horizon4 ,when this labour has been completed1 it will probably be found that instead of three1 there have been many discharges of volcanic material during the deposition of the limestone series2 that these have proceeded from numerous small vents1 and that they are all of comparatively restricted horizontal extent4 ,such a detailed examination will also determine how far the toadstones include veritable sills1 and on what horizons these intrusive sheets have been injected4 ,footnote #ca3 ,geol4 ,surv4 ,mem4 on ,north ,derbyshire1 by ,messrs4 ,green and ,strahan 7#ahhg71 p4 #ajd4 ,in the meantime1 we know that the lowest visible bands of toadstone are underlain by several hundred feet of limestone1 thus proving that the earliest known volcanic explosions took place over the floor of the ,carboniferous ,limestone sea1 after at least #gjj or #hjj feet of calcareous sediment had accumulated there4 ,the latest traces of volcanic activity are found in a part of the ,yoredale group of shales and limestones which form the uppermost member of the ,carboniferous ,limestone of this region4 ,but it is not quite clear whether the vesicular diabase found there is interstratified or intrusive4 ,certainly no contemporaneous tuffs have yet been found among the ,yoredale rocks1 nor in any higher subdivision of the ,carboniferous system1 though coarse agglomerates marking the position of vents do traverse the ,yoredale group at ,kniveton4 ,it may be remarked that in the district over which the toadstones can be seen1 two areas are recognizable1 in each of which the exposures of the igneous rocks are numerous1 while between them lies an intervening tract wherein there is hardly any visible outcrop of these rocks4 ,the northern and much the more extensive area stretches from ,castleton to ,sheldon1 while the southern spreads from ,winster to ,kniveton4 ,this distribution not improbably points to the original position of the vents1 and indicates a northern more numerous group of volcanic orifices1 and a southern tract where the vents were fewer1 or at least spread their discharges over a more limited space4 #c4 ,,the ,,vents4--,it had always appeared to me singular that1 in ground so deeply trenched by valleys as the toadstone district of ,derbyshire1 no trace had been recognized of any bosses or necks from which these volcanic sheets might have been erupted4 ,it is true that in mining operations masses of toadstone had been penetrated to a considerable depth without their bottom being reached1 and the suggestion had been made that in such cases a shaft may actually have been sunk on one of the vents through which the toadstone came up4#cb ,one instance in particular was cited where1 at ,black ,hillock1 on ,tideswell ,moor1 close to ,peak ,forest ,village1 a mass of toadstone was not cut through1 though pierced to a depth of #ajj fathoms4 ,in that neighbourhood1 however1 several of the sheets of eruptive material are probably sills1 and the shaft at ,black ,hillock may have been sunk upon the pipe or vein that supplied one or more of these intrusive sheets4 ,footnote #cb3 ,geol4 ,surv4 ,mem4 on ,north ,derbyshire1 p4 #acd4 ,it was therefore with no little interest that ,i detected a series of vents at four separate localities1 viz4 ,castleton1 ,grange ,mill1 ,hopton1 and ,kniveton ,wood4 ,i have no doubt that a more extended search will bring others to light4 ,those observed by me are all filled with coarse agglomerate1 the blocks in which are mostly composed of different lavas1 sometimes with the addition of blocks of limestone1 while the matrix consists mainly of lapilli of basic devitrified glass4 ,the most typical examples form a group of two1 possibly three1 vents which rise into two isolated1 smooth1 grassy dome-shaped hills at ,grange ,mill1 five miles west from ,matlock ,bath4#cc ,in external form and colour1 these eminences present a contrast to the scarped slopes of limestone around them4 ,they at once recall the contours of many of the volcanic necks in ,central ,scotland4 ,on examination it is found that the material composing them is a dull green agglomerate1 the matrix of which is a compact substance weathering spheroidally1 and full of small lapilli of minutely vesicular diabase4 ,the larger stones consist1 for the most part1 of various vesicular dolerites or diabases1 together with some pieces of limestone and occasionally large blocks of the latter rock1 altered into a saccharoid condition4 ,two dykes of dolerite or basalt traverse the margin of the larger vent4 ,footnote #cc3 ,this is ,mr4 ,bemrose's outcrop1 ,no4 #df1 op4 cit4 p4 #fcc4 ,the steep sides of these agglomerate domes rise from the low ground around them to a height of #ajj to #ahj feet1 their summits being a little more than #ijj feet above the sea4 ,the smaller neck is nearly circular1 and measures about #ajjj feet in diameter4 ,the larger mass is less regular in shape1 and is prolonged into such a bulge on the south-east as to suggest that its prolongation in that direction may really mark the position of a third and much smaller vent contiguous to it4 ,the longer diameter of the larger mass is #bcjj and the shorter #acjj feet4 ,illustration3 ,fig4 #agh4--,view of two volcanic necks in the ,carboniferous ,limestone series1 at ,grange ,mill1 five miles west of ,matlock ,bath1 from the north4 ,on the south and west sides1 the surrounding limestone can be traced up to within a few feet of the edge of the agglomerate1 and its strata are there found to be much jumbled and broken1 while their texture is rather more crystalline than usual1 though not saccharoid4 ,the two necks are separated by a narrow valley in which no rock is visible4 ,their opposite declivities meet at the bottom of this hollow1 and are so definitely marked off that1 even in the absence of proof that they are disjoined by intervening limestone1 there can be little hesitation in regarding each hill as marking a distinct vent4 ,a wider valley extends along the eastern base of the necks1 and slopes upward on its east side until it is crowned by a long escarpment of limestone1 which reaches a height of #ajjj feet above the sea1 or about #ajj feet above the valley from which it rises4 ,unfortunately1 the bottom and slopes of this depression are thickly covered with soil1 but at one or two places debris of fine tuff may be observed1 and at the northern and southern ends of the hollow well-bedded green and reddish tuff appears1 dipping gently below the limestone escarpment4 ,this band of volcanic detritus evidently underlies the limestone1 and forms most of the gentle slope on the east side of the valley4 ,it may be from #gj to #ajj feet thick4 ,that it was discharged from one or both of the necks seems tolerably clear4 ,its material resembles that forming the matrix of the agglomerate4 ,the general arrangement of the rocks at this interesting locality is represented in ,fig4 #agi1 which is reduced from my survey on the scale of six inches to a mile4 ,a section across the smaller vent would show the structure represented in ,fig4 #ahj4 ,illustration3 ,fig4 #agi4--,plan of necks and bedded tuff at ,grange ,mill1 five miles west of ,matlock ,bath4 ,illustration3 ,fig4 #ahj4--,section across the smaller volcanic neck and the stratified tuff in ,carboniferous ,limestone1 ,grange ,mill4 #a4 ,limestone2 #b4 ,stratified tuff intercalated among the limestones2 #c4 ,agglomerate4 ,this group of vents lies in the southern of the two tracts of the volcanic district4 ,in the northern tract a mass of agglomerate pierces the base of the limestone escarpment about a quarter of a mile west from the entrance to the ,peak ,cavern at ,castleton4#cd ,it is rudely semicircular in area1 stretching down the slope until its northern extension is lost under the lower ground4 ,the agglomerate is not well exposed1 but it can be seen to be a green1 granular crumbling rock1 made up in great part of minutely vesicular lapilli1 enclosing blocks of various diabases two feet long or more4 ,from the abrupt way in which this agglomerate rises through the limestone1 there can be little doubt that it marks the position of one of the volcanic vents of the time4 ,as it stands on the extreme northern verge of the limestone area1 the ground further north being covered with the ,yoredale rocks and ,millstone ,grit1 it is the most northerly of the whole volcanic district4 ,footnote #cd3 ,this is outcrop ,no4 #a of ,mr4 ,bemrose's paper1 p4 #fbe4 ,along the southern margin of the limestone country a group of agglomerate masses probably marks another chain of vents4 ,these are specially interesting1 inasmuch as they abut on the ,yoredale series1 and may thus be looked upon as among the latest of the volcanic chimneys4 ,one of them is seen at ,hopton1#ce where along the side of the road a good section is exposed of coarse tumultuous agglomerate1 having a dull green matrix1 composed of green1 brown1 and black1 minutely cellular1 basic1 devitrified1 glassy lapilli1 showing under the microscope abundant microlites and crystals or calcareous pseudomorphs of olivine1 augite1 and felspar1 and much magnetite dust4 ,through this matrix are distributed blocks of slaggy basalt and dolerite4 ,an interesting feature of this mass is the occurrence in it of some veins1 two or three inches broad1 of a compact black porphyritic basalt4 ,i did not trace the relations of this agglomerate to the stratified rocks around it4 ,but its internal structure and composition mark it out as a true neck4 ,it extends1 according to the ,geological ,survey map1 for about half a mile along the edge of the limestone1 and is represented as being separated by two faults from the ,yoredale series immediately to the south4 ,so long as the belief is entertained that the toadstones are contemporaneous outflows of lava lying on certain definite horizons1 far below the summit of the limestones1 the position of the ,hopton agglomerate is only explicable on the assumption of some dislocation by which the ,yoredale shales have been brought down against it4 ,but when we realize that the rock is an unstratified agglomerate1 probably marking the place of a volcanic vent1 and therefore rising transgressively through the surrounding strata1 the necessity for a fault is removed1 or if a fault is inserted its existence should be justified on other evidence than the relations of the igneous rock to the surrounding strata4 ,footnote #ce3 ,geol4 ,surv4 ,mem4 ,north ,derbyshire1 p4 #bd4 ,this is outcrop ,no4 #ec of ,mr4 ,bemrose's paper1 p4 #fce4 ,four miles to the south-west of ,hopton1 on the slope of the hill at ,kniveton ,wood1 another remarkable mass of agglomerate forms a rounded ridge between the two forks of a small stream4#cf ,its granular matrix1 like that of the other necks1 consists of lapilli of minutely vesicular basic glassy lava or pumice1 and encloses large and small rounded blocks of finely cellular basalt and pieces of limestone4 ,the rock is unstratified1 and in all respects resembles that of ordinary ,carboniferous necks in ,scotland4 ,its relations to the ,yoredale rocks are laid bare in the channels of the streamlets4 ,there the shales and thin limestones may be seen much broken and plicated1 their curved and fractured ends striking directly at the agglomerate4 ,they may be traced to within a yard of the agglomerate4 ,on the ,geological ,survey map the igneous rock is represented as bounded by two parallel faults4 ,but ,i hardly think that this explanation suffices to account for the relations of the rocks and their remarkable boundary-line1 which seems to me to be undoubtedly the wall of a volcanic vent4 ,to the east of the streams1 another mass of agglomerate may mark another neck1 while to the north1 a third detached area of the same kind of rock1 rising among the limestones1 may be regarded as likewise a distinct mass4 ,at this locality1 therefore1 there are two1 possibly three1 vents4 ,one of these1 from the way in which it cuts across the ,yoredale shales and limestones1 is to be assigned to a time later than the older part of the ,yoredale series1 and thus1 like the ,hopton mass1 it indicates that in the south of the volcanic area eruptions did not cease with the close of the deposition of the thick limestones1 but were prolonged even into the time of the ,yoredale rocks4 ,footnote #cf3 ,outcrop ,no4 #ef1 p4 #fch of ,mr4 ,bemrose's paper4 ,a further proof of the late age of these southern patches of volcanic material is shown by two bands of vesicular toadstone in the ,yoredale series1 a little south from the village of ,kniveton4 ,these rocks are traced on the ,survey ,map1 and are shown in a diagram in the ,memoir1 where their position is sought to be explained by a system of parallel faulting4#cg ,i was able to trace the actual contact of the western band with the strata underneath it1 and satisfied myself that there is no fault at the junction4 ,the igneous material is regularly bedded with the ,yoredale shales and limestones4 ,either1 therefore1 these bands are intercalated lava-streams or intrusive sills4 ,if mere vesicular structure were enough to distinguish true outflowing lavas1 then there could be no doubt about these ,kniveton rocks4 ,but this structure is found in so many ,carboniferous sills1 particularly in those thin sheets which have been injected into coals and black shales1 that its presence is far from decisive4 ,the vesicles in the ,kniveton rocks are small and pea-like1 tolerably uniform in size and shape1 and crowded together4 ,they are thus not at all like the irregular cavities in the ordinary cellular and scoriaceous lavas of the toadstone series4 ,footnote #cg3 ,op4 cit4 p4 #hg4 ,whether or not the question of their true relations be ever satisfactorily settled1 these ,kniveton bands are certainly younger than the lower portion of the ,yoredale group4 ,their evidence thus agrees with that of the southern agglomerates in showing that the volcanic activity of this region was continued even after the thick calcareous masses of the ,carboniferous ,limestone series had ceased to be deposited4 ,besides the six necks to which ,i have referred1 a rock in ,ember ,lane1 above ,bonsall1 probably belongs to another vent4#ch ,it is particularly interesting from the great preponderance of limestone fragments in it4 ,the volcanic explosions at this locality broke up the already solidified limestones on the floor of the ,carboniferous ,limestone sea1 and strewed them around1 mingled with volcanic blocks and dust of the prevailing type4 ,footnote #ch3 ,this is outcrop ,no4 #ci of ,mr4 ,bemrose's paper1 p4 #fcb4 ,when the district has been more carefully searched1 other centres of eruption will no doubt be discovered4 ,it may then be possible to depict the distribution of the active vents1 and to connect with them the outflow of the bedded lavas4 ,so far as ,i have been able to ascertain1 there are no necks of dolerite or basalt1 though1 as ,i have shown1 dykes or veins of molten rock are occasionally to be found in the agglomerates of the necks4 #d4 ,,the ,,lavas ,,and ,,tuffs4--,i have referred to the opinion of ,de la ,beche that the toadstones of ,derbyshire were poured out as lava-streams without any accompanying fragmentary discharges1 and to the correction of this opinion by the subsequent observations of ,jukes and of the ,geological ,survey4 ,but though the existence of interbedded tuffs has long been known1 it was not until ,mr4 ,bemrose's more careful scrutiny that the relative importance of the tuffs among the lavas was first indicated4 ,he has shown that a number of the bands mapped as toadstone are tuffs1 and he has discovered other bands of tuff which have not yet been placed on any published map4 ,in examining the outcrops of the various toadstones of ,derbyshire we learn that some of them are lavas without tuffs1 probably including a number of bands1 which are really sills2 that others are formed of both lavas and tuffs1 and that a third type shows only bedded tuff4 ,each of these developments will deserve separate description4 ,but before entering into details1 we may take note of the varying thicknesses of the different toadstones which have been determined by observation at the surface or by measurement underneath in mining operations4 ,in some cases a distinct band of toadstone1 separated by many feet or yards of limestone from the next band1 and therefore serving to mark a separate volcanic discharge1 may not exceed a yard or two in total thickness1 and from that minimum may swell out to #ajj feet4 ,the majority of the bands probably range between #ej and #ajj feet in thickness4 ,in one exceptional case at ,snitterton1 a mass of blackstone is said to have been proved to be #bdj feet thick1 but this rock may not improbably have been a sill4#ci ,the true contemporaneous intercalations seem to be generally less than #ajj feet in thickness4 ,footnote #ci3 ,a difference is made by the mining community between toadstone and what is called blackstone4 ,the former name appears to be restricted to the amygdaloidal green and generally more or less decayed lavas2 the latter1 so far as ,i can learn1 is applied to the dark1 more solid and crystalline rocks4 ,if this distinction be well founded the one name may perhaps serve to mark the open cellular lavas1 the other the more compact1 dark1 and heavy intrusive sheets4 7a7 ,lavas without ,tuffs4--,examples occur of sheets of toadstone which consist entirely of contemporaneously ejected diabase1 basalt or dolerite4 ,this rock is then dull green or brown in colour1 more or less earthy in texture1 and irregularly amygdaloidal4 ,the vesicles are extremely varied in size1 form and distribution1 sometimes expanding until the rock becomes a slaggy mass4 ,a central more solid portion between a scoriaceous bottom and top may sometimes be observed1 as at the ,great ,rocks ,quarry1 ,peak ,forest ,limeworks 7,fig4 #aha74 ,in this1 as in other examples1 a remarkably hummocky and uneven surface of limestone lies below the igneous band1 the calcareous rock presenting knobs and ridges1 separated by cauldron-shaped cavities and clefts1 some of which are several yards deep4 ,these inequalities are filled in and covered over with a soft yellow and brown clay1 varying up to three or four feet thickness1 and passing upwards into the more solid toadstone4 ,there can hardly be any doubt that this singularly uneven limestone surface is due to the solvent action of water lying between the limestone and the somewhat impervious toadstone above1 and that the clay represents partly the insoluble residue of the calcareous rock1 but chiefly the result of the action of the infiltrating water on the bottom of the igneous band4#dj ,footnote #dj3 ,geological ,survey ,memoir on ,north ,derbyshire1 p4 #bj and footnote4 ,illustration3 ,fig4 #aha4--,section of vesicular and amygdaloidal diabase resting on ,carboniferous limestone1 ,peak ,forest ,limeworks1 ,great ,rocks ,quarry4 #a4 ,limestone with a surface dissolved into cauldron-like hollows2 #b4 ,rotten yellow and brown clay resulting from decomposition of toadstone and white clay from the solution of the limestone--sometimes three or four feet thick2 #c4 ,toadstone1 a diabase with highly slaggy base4 ,illustration3 ,fig4 #ahb4--,view of the superposition of ,carboniferous limestone upon toadstone1 ,raven's ,tor1 ,millersdale 7length about #ajj feet74 #a4 ,toadstone2 #b4 ,limestone2 f1 ,fault4 ,junctions of the upper surfaces of the lava-sheets with the overlying limestone show that the igneous material sometimes assumed hummocky forms1 which the calcareous deposits gradually overspread and covered4#da ,a good example of this kind may be observed by the roadside at the foot of ,raven's ,tor1 ,millersdale4 ,as shown in the subjoined figure1 the limestone has here been worn into a cave1 the floor of which is formed by the toadstone4 ,the latter rock1 of the usual dull green1 slaggy and amygdaloidal character1 is covered immediately by the limestone1 but ,i did not observe any fragments of the toadstone1 nor any trace of ashy materials in the overlying calcareous strata4 ,this section shows that after the outflow of the lava1 the sedimentation of the limestone was quietly resumed1 and the igneous interruption was entirely buried4 ,footnote #da3 ,compare ,de la ,beche1 ,geological ,observer1 pp4 #eei1 #efj1 and ,north ,derbyshire ,memoir1 p4 #abc4 ,in some cases there is evidence of more than one outflow of lava in the same band of toadstone4 ,jukes believed that each band was the result1 not of one simultaneous ejection of igneous matter1 but of several1 proceeding from different foci uniting together to form one band1 and he found that near ,buxton1 two solid beds of toadstone could be seen to have proceeded from opposite quarters towards each other without overlapping4#db ,footnote #db3 ,student's ,manual of ,geology1 #b;d edit4 7#ahfb71 p4 #ebc4 ,in ,millersdale the authors of the ,geological ,survey ,memoir on ,north ,derbyshire observed that a band of toadstone about #ajj feet thick showed six distinct divisions1 which they were disposed to regard as marking so many separate beds4#dc ,in ,tideswell ,dale1 on the west side of the valley1 immediately to the south of the old toadstone quarry1 two bands of toadstone are seen to be separated by a few yards of limestone4 ,footnote #dc3 ,op4 cit4 p4 #ai4 7b7 ,lavas with ,tuffs4--,it will probably be found that in many1 if not in most cases1 the outflow of lava was preceded1 accompanied or followed by fragmental discharges4 ,as far back as #ahfa1 ,jukes noticed that a toadstone band1 about #ej feet thick1 near ,buxton consisted of two solid beds of lava with beds of purple and green ash1 greatly decomposed into clay1 both above and below each bed and between the two4#dd ,footnote #dd3 ,op4 cit4 p4 #ebc4 ,illustration3 ,fig4 #ahc4--,section at lime-kiln1 south of ,viaduct1 ,millersdale ,station4 ,an interesting section1 showing this intercalation of the two kinds of material is exposed at the lime-kilns beyond the southern end of the railway viaduct at ,millersdale ,station4 ,over a mass of solid blue limestone 7#a in ,fig4 #ahc7 lies a band of bright yellow and brown clay 7#b71 varying from six inches to two feet in thickness4 ,this may be compared with the clay found above the limestone at ,peak ,forest 7,fig4 #aha74 ,but it is probably a layer of highly decomposed tuff4 ,it is succeeded by a thin band of greenish limestone 7#c7 containing an admixture of fine volcanic detritus1 and partially cut out by an irregular bed1 four to eight feet thick1 of a highly slaggy1 greenish1 decomposing1 spheroidal and amygdaloidal diabase 7#d74 ,this unmistakable lava-sheet is followed by a bed of green granular tuff 7#e71 which in some places reaches a thickness of three feet1 but rapidly dies out4 ,over a space several yards in breadth1 the succeeding strata are concealed1 and the next visible rock is a dark1 compact dolerite which weathers spheroidally 7#f74 7c7 ,tuffs without ,lavas4--,mr4 ,bemrose has shown that some of the bands of toadstone consist entirely of bedded tuff4 ,in these cases1 so far as the present visible outcrops allow us to judge1 no outflow of lava accompanied the eruption of fragmentary materials4 ,but that the ejection of these materials was not the result of a sudden spasmodic explosion1 but of a continued series of discharges varying in duration and intensity1 is indicated by the well-bedded character of the tuff and the alternation of finer and coarser layers4 ,large blocks of lava1 two feet or more in diameter1 may mark some of the more vigorous paroxysms of the vents1 while the usual fine granular nature of the tuff may point to the prevailing uniformity and less violent character of the eruptions4 ,bands of tuff #gj feet or more in thickness1 without the intercalation of any limestone or other non-volcanic intercalation1 point to episodes of such continued volcanic activity that the ordinary sedimentation of the sea-bottom was interrupted1 or at least masked1 by the abundant fall of dust and stones4 ,one of the best exposures of such intercalations of bedded tuffs was pointed out to me by ,mr4 ,bemrose1 immediately to the east of the village of ,litton4 ,the matrix is crowded with the usual minutely vesicular glassy lapilli1 and encloses fragments of diabase of all sizes1 up to blocks more than a foot in diameter4 ,the rock is well stratified1 and the layers of coarse and fine detritus pass beneath a group of limestone beds4 ,the actual junction is concealed under the roadway1 but only two or three feet of rock cannot be seen4 ,the lowest visible layer of limestone is nodular and contains decayed bluish fragments which may be volcanic lapilli4 ,immediately above the lower limestones the calcareous bands become richly fossiliferous4 ,some of their layers consist mainly of large bunches of coral2 others are crowded with cup-corals1 or are made up mainly of crinoids with abundant brachiopods1 polyzoa1 lamellibranchs1 gasteropods and occasional fish-teeth4 ,this remarkable profusion of marine life is interesting inasmuch as it succeeds immediately the band of volcanic ash4 ,another well-marked zone of tuff1 with no traceable accompaniment of lava1 has already been referred to as connected with the ,grangemill vents4 ,in this case also1 the limestone that lies directly upon the volcanic material is rather impure and nodular in character4 ,the tuff itself is well bedded1 perhaps from #gj to #ajj feet thick and dips underneath an overlying series of marine limestones4 ,i did not observe thin partings of tuff and disseminated volcanic lapilli among the limestones1 such as are so marked in the ,lower ,carboniferous formations of ,west ,lothian1 and in the ,limerick basin1 to be described in the following chapter4 ,but a diligent search might discover examples of them1 and thus prove that1 besides the more prolonged and continuous eruptions that produced the thick bands of tuff1 there were occasional feeble and intermittent explosions during the accumulation of the thick sheets of limestone4 ,some of the layers of red clay observed in shafts sunk for mining purposes may perhaps represent such spasmodic discharges of fine fragmental material4 #e4 ,,the ,,sills4--,no attempt has yet been made to determine whether and to what extent the toadstone bands include true intrusive sheets4 ,my own brief examination of the ground does not warrant me in making any positive statement on this subject4 ,i can hardly doubt1 however1 that some1 perhaps not a few1 of the toadstone bands are really sills4 ,in the accounts of these rocks contained in the mining records a distinction1 as already remarked1 appears to have been generally drawn between toadstone and blackstone4 ,the latter term is applied to the black1 fresh1 more coarsely crystalline1 and generally non-amygdaloidal rocks1 which1 so far as ,i have been able to examine them1 have the general external and many of the internal characters of the ,carboniferous sills of ,central ,scotland4 ,at ,snitterton near ,matlock one of these blackstones1 as already mentioned1 is said to have been found to be #bdj feet thick4#de ,footnote #de3 ,north ,derbyshire ,memoir1 p4 #bc4 ,it is stated that the toadstones1 though subject to great variations in thickness1 are never seen to cut across the limestones4#df ,but ,i suspect that proofs of intrusion and transgression will be found when diligently sought for4 ,it appeared to me that the dark1 compact1 crystalline dolerite1 which was formerly quarried in the middle of ,tideswell ,dale1 may be separated from the vesicular toadstone of that valley1 which is undoubtedly a true lava-flow1 and that it does not always occupy the same horizon there1 being sometimes below and sometimes above the amygdaloid4 ,where it rests on a band of red clay the latter rock has been made columnar to a depth of nine feet4#dg ,alteration of this kind is very rare among the ,carboniferous bedded lavas1 but is by no means infrequent in the case of sills4 ,but the most important proof of alteration which ,i have myself observed occurs at ,dale ,farm near the village of ,peak ,forest1 where the limestone above a coarsely crystalline dolerite has been converted into a white saccharoid marble for about two yards from the junction4 ,footnote #df3 ,op4 cit4 p4 #abc4 ,footnote #dg3 ,j4 ,m4 ,mello1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xxvi4 7#ahga71 p4 #gja4 #c4 ,,the ,,isle ,,of ,,man ,rising from the middle of the ,irish ,sea1 within sight of each of the three kingdoms1 with a history and associations so distinct1 yet so intimately linked with those of the rest of ,britain1 this interesting island presents in its geological structure features that connect it alike with ,england1 ,scotland and ,ireland1 while at the same time it retains a marked individuality in regard to some of the rocks that form its framework4 ,its great central ridge of grits and slates1 which still rises #bjjj feet above the sea in the summit of ,snaefell1 must have formed a tract of dry land in ,carboniferous time1 until it sank under sea-level1 and was buried beneath the ,carboniferous and later formations4 ,along the southern margin of this ancient land1 a relic of the floor of the ,carboniferous sea has been preserved in a small basin of ,carboniferous ,limestone which covers about seven or eight square miles4 ,this remnant has a special interest in geological history1 for it has preserved the records of a series of volcanic eruptions which took place contemporaneously with the deposition of the ,carboniferous ,limestone4 ,the geology of the ,isle of ,man was sketched in outline by ,j4 ,f4 ,berger1#dh ,j4 ,macculloch1#di and ,j4 ,s4 ,henslow1#ej and was afterwards more fully illustrated by ,j4 ,g4 ,cumming4#ea ,to the last-named observer we owe the recognition of true intercalated volcanic rocks among the calcareous formations of the southern end of the island4 ,these rocks have subsequently been studied in greater detail by a number of geologists4 ,an excellent general account of them was published in #ahgd by ,mr4 ,john ,horne1 of the ,geological ,survey4#eb ,a few years later some further observations on them were prepared by ,j4 ,clifton ,ward4#ec ,more recently their petrography has been studied by ,messrs4 ,e4 ,dickson1 ,p4 ,holland and ,f4 ,rutley1#ed and in more detail by ,mr4 ,b4 ,hobson4#ee ,to some of the observations of these writers reference will be made in the succeeding pages4 ,during the progress of the ,geological ,survey in the ,isle of ,man1 the rocks in question have been mapped in detail by ,mr4 ,a4 ,strahan and ,mr4 ,g4 ,w4 ,lamplugh1 and ,i have had an opportunity of examining the coast-sections with the last-named geologist4 ,the following description of these sections is taken mainly from my field note-book4 ,the full details will appear in the official ,memoirs4 ,footnote #dh3 ,trans4 ,geol4 ,soc4 #ast ser4 vol4 ii4 7#ahad71 p4 #bi4 ,footnote #di3 ,western ,islands of ,scotland 7#ahai71 vol4 ii4 p4 #ega4 ,footnote #ej3 ,trans4 ,geol4 ,soc4 #ast ser4 vol4 v4 7#ahba71 p4 #dhb4 ,footnote #ea3 ,the ,isle of ,man 7#ahdh71 chap4 x4 ,footnote #eb3 ,trans4 ,geol4 ,soc4 ,edin4 ii4 7#ahgd71 p4 #ccb4 ,footnote #ec3 ,geol4 ,mag4 #ahhj1 p4 #d4 ,footnote #ed3 ,proc4 ,liverpool ,geol4 ,soc4 vol4 vi4 7#ahhh-hi71 p4 #abc4 ,footnote #ee3 ,quart4 ,journ4 ,geol4 ,soc4 xlvii4 7#ahia71 p4 #dcb4 ,this paper was reprinted with additions and corrections in ,yn ,lioar ,manninagh1 ,douglas1 ,isle of ,man1 vol4 i4 ,no4 #aj1 ,april #ahib4 ,it may be remarked at the outset that the last outcrop of the plateau-lavas of the ,solway basin occurs only #fj miles from the south end of the ,isle of ,man1 at the foot of the hills of ,galloway1 the blue outline of which can be seen from that island4 ,the distance from the ,manx volcanoes to the nearest of the puys of ,liddesdale is about #ajj miles4 ,though the fragment which has been left of the ejections is too small to warrant any confident parallelism1 there appears to be reason to believe that1 alike in geological age and in manner of activity1 the ,manx volcanoes may be classed with the type of the puys4 ,the ,carboniferous strata of the ,isle of ,man lie in a small trough at the south end of the island4 ,the lowest members of the series consist of red conglomerates and sandstones1 which pass upward into dark limestones full of the characteristic fossils of the ,carboniferous ,limestone4 ,as the bottom of the basin is on the whole inclined seawards1 the highest strata occur along the extreme southern coast4 ,it is there that the volcanic rocks are displayed4 ,they occupy a narrow strip less than two miles in length1 which is almost entirely confined to the range of cliffs and the ledges of the foreshore4 ,yet though thus extremely limited in area1 they have been so admirably dissected along the coast1 that they furnish a singularly ample body of evidence bearing on the history of ,carboniferous volcanic action4 ,unfortunately the bottom of the volcanic group is nowhere visible4 ,at the east or lower end of the series1 exposed on the shore1 an agglomerate with its dykes appears to truncate the ,castletown ,limestones4 ,no trace of any tuff has been noticed among these lower limestones4 ,we may infer that the volcanic activity began after they were deposited4 ,the highest accessible portions of the volcanic group1 as ,mr4 ,horne showed1 are clearly exposed on the coast at ,poyll ,vaaish1 intercalated in and overlying the dark limestones of that locality 7,fig4 #ahd71 which have been assigned1 from their fossil contents1 to the upper part of the ,carboniferous ,limestone series4#ef ,the ,manx volcanoes may therefore be regarded as having probably been in eruption during the later portion of the ,carboniferous ,limestone period4 ,footnote #ef3 ,r4 ,etheridge jun41 in ,mr4 ,horne's paper above cited4 ,illustration3 ,fig4 #ahd4--,limestones passing under stratified tuffs1 ,poyll ,vaaish1 ,isle of ,man4 ,owing to irregularities of inclination1 the thickness of the volcanic group can only be approximately estimated4 ,it is probably not less than #bjj or #cjj feet4 ,but as merely the edge of the group lies on the land1 the volcanic rocks may reach a considerably greater extent and thickness under the sea4 ,the volcanic materials consist mainly of bedded tuffs1 but include also several necks of agglomerate and a number of dykes and sills4 ,so far as ,i have observed1 they comprise no true lava-streams4#eg ,these ,manx tuffs present many of the familiar features of those belonging to the puy-eruptions of ,central ,scotland1 but with some peculiarities worthy of attention4 ,they are on the whole distinctly bedded1 and as their inclination is generally in a westerly direction1 an ascending order can be traced in them from the eastern end of the section to the highest parts of the group associated with the ,poyll ,vaaish limestones4 ,their colour is the usual dull yellowish-green1 varying slightly in tint with changes in the texture of the materials1 the palest bands consisting of the finest dust or volcanic mud4 ,great differences in the size of their fragmentary constituents may be observed in successive beds1 coarse and fine bands rapidly alternating1 with no admixture of non-volcanic sediment1 though occasional layers of fine ash or mudstone1 showing distinct current-bedding1 may be noticed4 ,footnote #eg3 ,the occurrence of intercalated lavas has been described in this series1 but1 as ,i shall show in the sequel1 they are probably intrusive masses4 ,pauses in the succession of eruptions are marked by the intercalation of seams of limestone or groups of limestone1 shale and black impure chert4 ,such interstratifications are sometimes curiously local and interrupted4 ,they may be observed to die out rapidly1 thereby allowing the tuff above and below them to unite into one continuous mass4 ,they seem to have been accumulated in hollows of the tuff during somewhat prolonged intervals of volcanic quiescence1 and to have been suddenly brought to an end by a renewal of the eruptions4 ,there are some four or five such intercalated groups of calcareous strata in the thick series of tuffs1 and we may regard them as marking the chief pauses in the continuity or energy of the volcanic explosions4 ,an attentive examination of these interpolated sedimentary deposits affords some interesting information as to the submarine conditions in which the eruptions took place4 ,the intercalations1 sometimes #ab feet or more in thickness1 consist mainly of dark limestones1 enclosing the usual ,carboniferous ,limestone fossils2 black shales1 sometimes showing very fragmentary and much macerated remains of ferns and other land-plants2 and black impure argillaceous chert or flint1 arranged in bands interposed between the other strata1 and also in detached lumps and strings4 ,the dark flaggy limestones and black shales may be paralleled lithologically with those of ,castletown and ,poyll ,vaaish4 ,indeed1 there seems to be little doubt that they represent the contemporaneous type of marine sediment that was gathering on the sea-floor outside the volcanic area1 and which during intervals of quiescence or feeble eruptivity spread more or less continuously into that area4 ,the thick mass of tuff must thus have been strictly contemporaneous with a group of calcareous muddy and siliceous deposits which gathered over the bottom beyond the limits of the showers of ashes4 ,illustration3 ,fig4 #ahe4--,section of tuff1 showing intercalations of black impure chert1 west of ,closenychollagh ,point1 near ,castletown1 ,isle of ,man4 ,one of the most singular features of these sedimentary intercalations is the occurrence of the black cherty material4 ,it may generally be observed best developed at the bottom and top of each group of included strata4 ,looking at the lumps of this substance scattered through the adjoining tuffs1 we might at first take them for ejected fragments1 and such no doubt may have been the derivation of some of them4 ,but further examination will show that1 as a rule1 they are of a concretionary nature1 and were formed in situ contemporaneously with or subsequent to the deposition of the tuffs4 ,the accompanying section 7,fig4 #ahe7 represents the manner in which the chert is distributed through two or three square yards of tuff overlying one of the calcareous groups4 ,the material has been segregated not only into lumps1 but into veins and bands1 which1 though on the whole parallel with the general stratification-planes of the deposits1 sometimes run irregularly in tongues or strings across these planes1 as shown in ,fig4 #ahf1 where the dark chert band which overlies the limestones and shales sends a tongue upwards for several inches into the overlying tuff4 ,that these interstratified calcareous and muddy strata were laid down in water of some considerable depth may be inferred from their general lithological characters4 ,the dark carbonaceous aspect of the limestones points to the probable intermingling of much decayed vegetation with the remains of the calcareous organisms of which these strata chiefly consist4 ,the thin unimportant bands or partings of dark shale show that only the finest muddy sediment reached the quiet depths in which the strata were deposited1 while the macerated fern-fragments suggest a long flotation and ultimate entombment of terrestrial vegetation borne seawards from some neighbouring land4 ,illustration3 ,fig4 #ahf4--,section of intercalated dark limestone1 shale and chert in the tuff south of ,poyll ,vaaish ,bay1 ,isle of ,man4 #a4 ,limestones and shales2 #b4 ,chert2 #c4 ,tuff4 ,the cherty bands and nodules1 like the flints of the chalk1 bear their testimony to the quiet character of the sedimentation in rather deep water beyond the limits within which the sediment from the land was mainly accumulated on the sea-bottom4 ,the origin of these siliceous parts of the series of deposits has still to be investigated4 ,whether or not they are to be referred to organic causes like chalk-flints1 and the radiolarian cherts of the ,lower ,silurian system1 they furnish a fresh example of the remarkable association of such siliceous material with volcanic phenomena1 which has now been observed in many widely separated areas all over the world4 ,if we next turn to the stratification of the tuffs1 we obtain further evidence of undisturbed conditions of deposition on the sea-floor4 ,the bedding of these volcanic masses1 though distinct1 appears for the most part to be due rather to the eruption and settlement of alternately finer and coarser detritus than to any marked drifting and rearrangement of these materials by current-action into different layers4 ,throughout the series of tuffs1 indeed1 there is1 on the whole1 a notable absence of any structure suggestive of strong currents or of wave-action in the dispersal and reassortment of the volcanic detritus4 ,the ashes and stones were discharged in such a way as to gather irregularly over the sea-floor into ridges and hollows4 ,there does not seem to have been sufficient movement in the bottom water to level down these inequalities of surface1 for we find that they remained long enough to allow twelve feet or more of calcareous and siliceous ooze to gather in the hollows1 while the intervening ridges still stood uneffaced until buried under the next fall of ashes4 ,at rare intervals some transient current or deeper wave may have reached the bottom and spread out the volcanic detritus lying there4 ,such exceptional disturbances of the still water are not improbably indicated by occasional well-defined stratification1 and even by distinct false-bedding1 in certain finer layers of tuff4 ,the materials of the tuffs are remarkably uniform in character and conspicuously volcanic in origin4 ,with the exception of occasional blocks of limestone1 which range up to masses several feet1 and occasionally several yards1 in diameter1 the dust1 lapilli and included stones consist entirely of fragmentary basic lava1 so persistent in its lithological features that we may regard its slightly different varieties as merely marking different conditions of the same rock4 ,the accumulation of pumiceous ash in this southern coast of the ,isle of ,man is one of the most remarkable in ,britain4 ,as ,mr4 ,hobson has well shown1 the matrix of this tuff consists of irregular lapilli1 representing what may have been various conditions of solidification in one original volcanic magma4 ,this magma he has described as an augite-porphyrite or olivine-basalt4 ,some of the lapilli1 as he noted1 consist of a pumice crowded with vesicles which occupy more space than the solid part2 others show nearly as many vesicles1 but the glass is made brown by the number of its fine dust-like inclusions2 a third type presents the cells and cell-walls in nearly equal proportions4 ,the same observer found that where the substance is most cellular the vesicles1 fairly uniform in size1 measure about a tenth of a millimetre in longest diameter4 ,an interesting feature of the tuffs is the abundant occurrence of loose felspar crystals throughout the whole group up to the highest visible strata4 ,these crystals1 sometimes nearly an inch in length1 appear conspicuously as white spots on weathered surfaces of the rock4 ,they are so much decayed1 however1 that it is difficult to extract them entire4 ,on the most cursory inspection they are observed to enclose blebs of a greenish substance like the material that fills up the vesicles in the pumiceous fragments and in the pieces of cellular lava4 ,i have not ascertained the original source of these scattered felspars4 ,in one of the dykes on the north side of the agglomerate at ,scarlet ,point1 as was pointed out by ,mr4 ,hobson1 large crystals of plagioclase occur in the melaphyre1 but the felspars in the tuffs and agglomerates differ so much from these that we cannot suppose them to have come from the explosion of such a rock4 ,i failed to detect any other mineral in detached crystals in the tuffs1 but a more diligent search might reveal such1 and afford some grounds for speculating on the probable nature of the magma from the explosion of which the scattered crystals were derived4 ,it is at least certain that this magma must have included a large proportion of plagioclase crystals4 ,between the lapilli and the minute pumice-dust that constitute the matrix of this tuff much calcite may be detected4 ,though this mineral may have been partly derived from the decay of the felspar in the lava-fragments1 ,i believe that it is mainly to be attributed to the intermingling of fine calcareous ooze with the ash accumulated on the sea-floor4 ,a more remarkable association of the same kind will be described in later pages from ,king's ,county in ,ireland4 ,that abundant calcareous organisms peopled the sea in which the ,manx ,carboniferous volcanoes were active is shown by the contemporaneously deposited limestones4 ,the tuffs themselves are occasionally fossiliferous4 ,species of ,spirifer1 ,productus and other brachiopods1 together with broken stems of encrinites1 may be found in them1 and doubtless the diffused calcite1 though now crystalline1 as in the limestones1 and showing no organic structure1 owes its presence to the detritus of once living organisms4 ,the stones imbedded in the tuff consist almost exclusively of slightly different varieties of the same pale1 always vesicular rock1 and sometimes pass into a coarse slag4 ,they vary up to six feet or more in length4 ,in many cases1 they appear to have been derived from the disruption of already solidified lava1 for their vesicles are not elongated or arranged with reference to the form of the block1 but have been broken across and appear in section on the outer surface4 ,in other instances1 however1 the cavities are large and irregular in the centre of the block1 while on the outside they are smaller and are drawn out round the rudely spherical shape of the mass1 as in true volcanic bombs4 ,the limestone fragments enclosed in the tuff include pieces of the dark carbonaceous and of the pale encrinal varieties4 ,in no case did ,i observe any sensible alteration of these fragments4 ,they seem to have been derived from material disrupted and ejected during the opening of successive vents1 and not to have been exposed for any considerable time to the metamorphic influence of volcanic heat and vapours4 ,narrow though the strip of volcanic material is along the south coast of the ,isle of ,man1 it has fortunately preserved for us some of the vents from which the tuffs were ejected4 ,a group of these vents1 three or four in number1 may be traced along the shore in a general ,w4,n4,w4 and ,e4,s4,e4 line from ,scarlet ,point for rather more than a mile4 ,their margins are in some places exceedingly well defined4 ,the most striking example of this feature occurs in the most westerly vent1 where a neck of remarkably coarse volcanic agglomerate rises vertically through well-bedded1 westerly-dipping tuff 7,fig4 #ahg74 ,in other portions of their boundaries no sharp line can be drawn between the material filling the vent and that of the surrounding tuffs4 ,hence it is difficult to define precisely the form and size of the vents4 ,i am inclined to believe from this indefiniteness of outline1 and from the remarkable structure of the dykes1 to which ,i shall afterwards refer1 that the presently visible parts of these necks must lie close to the mouths of the original vents1 if indeed they do not actually contain parts of the craters and of their surrounding walls4 ,the materials that have filled up the eruptive vents consist chiefly of agglomerate1 but partly also of intrusive portions of vesicular lava4 ,the agglomerate is composed of similar materials to the tuffs4 ,its matrix shows the same extraordinarily abundant fine greenish-grey basic pumiceous lapilli1 with the same kind of plentiful loose felspar-crystals4 ,the large blocks of lava1 too1 resemble in composition and structure those of the bedded tuffs1 but greatly exceed them in size and abundance4 ,besides the fragments of vesicular lava1 there occur also occasional blocks of limestone4 ,some of these are several yards in length4 ,messrs4 ,strahan and ,lamplugh have mapped a large mass of limestone at the ,scarlet vent1 which1 so far as can be observed1 lies in the agglomerate--a large cake of white limestone with pebbles of quartz1 which has probably been broken off from some underlying bed and carried up in the chimney of the volcano4 ,as a rule the agglomerate is a tumultuous1 unstratified mass4 ,but in many places it shows lines of bedding and1 as already stated1 passes outward into ordinary bedded tuff1 the number and size of the ejected blocks rapidly diminishing4 ,where this transition occurs we seem to see a remnant of the base of the actual volcanic cone4 ,thus1 in the most westerly vent already cited1 while the wall of the vent has been laid bare on the side next the sea1 so that the agglomerate on the beach descends vertically through the surrounding bedded tuffs1 on the western side the cliffs have preserved a portion of the material that accumulated outside the orifice 7,fig4 #ahg74 ,in this section we observe that the coarse agglomerate which fills up the main part of the vent has been left with a hummocky1 uneven surface1 and that a subsequent and perhaps feebler eruption of finer material has covered over these inequalities1 and has extended to the left above the fine tuffs through which the agglomerate has been drilled4 ,illustration3 ,fig4 #ahg4--,section of part of a volcanic neck on shore to the south-east of ,poyll ,vaaish ,bay1 ,isle of ,man4 ,illustration3 ,fig4 #ahh4--,section of successive discharges and disturbances within a volcanic vent4 ,scarlet ,point1 ,isle of ,man4 ,again1 in the largest of the vents1 that near ,scarlet ,point1 still clearer proof of successive eruptions and dislocations within a volcanic chimney may be noticed4 ,at one point the accompanying section 7,fig4 #ahh7 has been laid bare by the waves4 ,the oldest accumulation is a fine green granular tuff 7a71 rudely and faintly arranged in layers inclined at high angles1 like the fine materials in many of the vents of the basin of the ,firth of ,forth4 ,this peculiar stratification1 due not to the assortment of materials in water1 but to the deposition of coarser and finer detritus by successive explosions1 and to subsequent slipping or tilting1 is a characteristic feature of the detritus which has filled up ancient volcanic funnels4 ,a later explosion from some adjacent part of the same vent has given rise to the discharge of a coarse agglomerate 7b71 which with blocks sometimes six feet long1 overspreads the earlier material4 ,a third detrital accumulation in the same vent1 consisting of a firm brecciated tuff 7c7 with much calcite in its matrix1 has been brought down by a slip 7f7 which cuts across both of the previous deposits4 ,a broad dyke 7d7 of vesicular diabase 7augite-porphyry7 traverses the vent1 and is probably later than any of the other rocks in the section4 ,i will conclude this account of the ,manx ,carboniferous volcanic rocks with a brief reference to the intrusive masses which form a prominent feature of the coast-line4 ,from the picturesque headland of ,scarlet ,point the broad dyke which forms that promontory may be traced for some distance westwards4 ,several other parallel dykes run in the same direction which1 it will be observed1 is also that of the chain of vents4 ,it might be said that the vents are1 as it were1 strung together by a line of dykes4 ,these eruptive masses traverse both the agglomerates and the bedded tuffs4 ,they probably belong1 therefore1 to a comparatively late part of the volcanic history4 ,that they are truly intrusive and not lava-flows is1 ,i think1 clearly shown by their vertical walls which descend through the surrounding rocks1 and by the greater closeness of their texture1 as well as the diminution in the size of their vesicles along the contact surfaces4 ,but it must be admitted that in their remarkably developed vesicular structure they look more like streams of lava than ordinary dykes4 ,it is this structure which gives to these dykes their peculiar interest4 ,bands of vesicles1 from an inch or less to several inches in breadth1 run along the dykes parallel to the outer walls4 ,unlike the familiar rows of little amygdaloidal cells in ordinary basalt dykes1 such as those of the ,tertiary series in ,scotland1 these vesicles1 though small and pea-like in the narrower bands towards the margins of the dykes1 became so large1 numerous1 and irregular in the broader and more central bands1 that the rock passes there into a rough slag4 ,illustration3 ,fig4 #ahi4--,section of dyke and sill in the tuffs west of ,scarlet ,point1 ,isle of ,man4 ,while the intrusive material has for the most part risen in the form of dykes1 in one part of the coast-section1 a little to the west of ,scarlet ,point1 it has been injected as a sill among the bedded tuffs4#eh ,a section taken at this locality gives the structure represented in ,fig4 #ahi4 ,on the north side of the great dyke1 the strata of tuff which dip under it1 roll over and support an outlying sheet of the same material4 ,the slaggy structure of parts of this sill give it some resemblance to a true lava-flow4 ,but it is the same structure which can be seen in the dykes1 while the closer grain along the contact-surface further connects it with these intrusions4 ,footnote #eh3 ,it is this sheet which has been described as a lava-stream4 ,illustration3 ,fig4 #aij4--,section on south side of vesicular sill west of ,scarlet ,point4 ,illustration3 ,fig4 #aia4--;,bands of vesicles in the same sill4 ,there is1 however1 a peculiarity about the development of the vesicular structure in this sill which ,i have not observed anywhere else4 ,if we examine the southern side of the crag near its eastern end we observe that the successive bands of vesicles are arranged in the same direction as the surface of contact with the underlying tuffs1 precisely as they are ranged in dykes parallel to the bounding walls4 ,so far the structure is quite normal4 ,but1 moving a few yards westwards1 we find that the bands begin to curve1 and1 instead of following the contact surface1 strike it first obliquely and then at right angles1 until we have the structure shown in ,fig4 #aia4 ,the bands here vary from less than an inch to more than a foot in breadth1 and where broadest assume a slaggy texture4 ,i sought in vain for any evidence of subsequent disturbance such as might have truncated these parallel rows of vesicles and pushed the rock bodily over the tuffs4 ,the perfect parallelism of the bands with the surface of the tuff at the east end1 and the absence of all trace of a thrust-plane at the base of the sill1 seem to show that1 though the rows of vesicles were undoubtedly at first arranged parallel to the surfaces between which the intrusion took place1 the mass1 before completely consolidating and coming to rest1 was ruptured1 and a portion of it was driven onwards at right angles to its previous line of movement4 ,a consideration of the singularly slag-like structure of the injected masses in the tuffs and agglomerates leads to the conclusion that though what we now see of these rocks did not actually flow out at the sea-bottom in streams of lava1 it was intruded so close to the surface that the imprisoned vapours had opportunity to expand1 as in superficial outflows4#ei ,this inference is in accord with that derived from an examination of the necks1 wherein we find evidence of the probable survival of parts of the actual craters and volcanic cones4 ,footnote #ei3 ,as illustrative of the occurrence of the vesicular structure in superficial intrusions1 ,i may again cite the dyke which cuts the ash of the outer crater-wall of the ,puy de ,pariou in ,auvergne4 ,the andesite of this dyke is in places as vesicular as the lava-stream with which it was doubtless connected1 but the vesicles have been flattened and drawn out parallel to the walls of the dyke4 ,in this instance it is quite certain that there could never have been any great depth of detrital material above the fissure into which the material of the dyke was injected 7see vol4 i4 p4 #ff74 ,as the records of the earliest eruptions during the ,carboniferous ,limestone period in the district of the ,isle of ,man are concealed1 so also those of the last of the series lie under the sea4 ,where the highest visible tuffs overlie the ,poyll ,vaaish limestones they show no change in the nature of the materials ejected1 or in the energy of eruption4 ,they lie so abruptly on the dark calcareous deposits as to show that a considerable pause in volcanic activity was followed by a violent explosion4 ,the same abundant grey-green pumice1 the same kind of loose crystals of felspar1 the same type of lava-blocks and bombs as had characterized the foregoing eruptions remained as marked at the end4 ,but the further volcanic records cannot be perused1 and we are left to speculate whether the coast-sections reveal almost the whole chronicle1 or if they merely lay before us the early chapters of a great volcanic history of which the main records lie buried under the waves of the ,irish ,sea4 #d4 ,,east ,,somerset ,various limited outcrops of igneous rocks have long been known to occur in the eastern part of ,somerset4 ,the largest of these lies in the midst of the ,old ,red ,sandstone1 on the crest of the axis of the ,mendip ,hills1 between ,downhead and ,beacon ,hill4 ,smaller patches occur in the ,carboniferous ,limestone near ,wrington ,warren1 on the north side of ,middle ,hope1 on ,worle ,hill and at ,uphill4 ,these rocks have been mapped as intrusive1 though some of them have been described as conglomeratic or as volcanic breccias4 ,while some of the masses are probably intrusive1 others appear to be truly contemporaneous with the deposition of the ,carboniferous ,limestone4 ,the highly vesicular basalt of ,middle ,hope looks much more like a superficial lava than an intrusion4 ,mr4 ,aveline gave a section showing three alternations of limestone and igneous rock at ,middle ,hope4 ,a recent examination of that coast-line by ,mr4 ,a4 ,strahan shows that there are undoubted tuffs interstratified with the calcareous strata4 ,there is thus proof that one or more small volcanic vents were in eruption on the floor of the ,carboniferous ,limestone sea in the neighbourhood of ,weston-super-,mare4#fj ,footnote #fj3 ,see ,geological ,survey ,memoir ,on ,east ,somerset1 by ,h4 ,b4 ,woodward1 #ahgf1 and authorities there cited4 ,mr4 ,aveline's section above referred to will be found on p4 #bb4 #e4 ,,devonshire ,the change from the typical ,old ,red ,sandstone of ,south ,wales to the ,devonian system of ,devonshire1 to which ,i have already referred1 is hardly more striking than the contrast between the ,carboniferous formations of these two areas4#fa ,the well-marked threefold subdivisions of ,carboniferous ,limestone1 ,millstone ,grit and ,coal-measures1 so persistent throughout ,britain1 and nowhere more typically developed than in ,south ,wales1 are replaced in a distance of less than forty miles by the peculiar ,culm-measures of ,devonshire--a series of black shales1 grey sandstones and thin limestones and lenticular seams of impure coal 7culm71 which are not only singularly unlike in original characters to the ordinary ,carboniferous formations1 but have been made still more unlike by the extensive and severe cleavage to which the ,palozoic rocks of ,devon and ,cornwall have been subjected4 ,that these ,culm-measures are truly ,carboniferous is made abundantly clear by their fossil contents1 though it has not yet been possible to determine how far they include representatives of the great stratigraphical subdivisions in other parts of the country4 ,footnote #fa3 ,in the centre of ,england numerous outlying areas of igneous rocks are found in the ,carboniferous ,limestone1 ,millstone ,grit and ,coal-measures4 ,these will be considered by themselves in ,chap4 xxxii4 ,it is to ,de la ,beche that geology owes the first intimation of the occurrence of interstratified igneous rocks in the ,carboniferous series of ,devonshire4 ,as far back as the year #ahcd1 in his singularly suggestive treatise1 ,researches in ,theoretical ,geology1 this eminent geologist expressed his opinion that not only were the trappean bands regularly intercalated in the sedimentary series and continuously traceable with the general stratification1 but that they occurred at various localities in such a manner as to raise the suspicion that these points may mark some of the centres of eruption4 ,he particularly cited the example of ,brent ,tor as a remarkable volcanic-looking hill1 composed in part of a conglomerate having every appearance of volcanic cinders4#fb ,footnote #fb3 ,op4 ,cit4 p4 #chd4 ,in his subsequently published ,report on the ,geology of ,cornwall1 ,devonshire and ,west ,somerset1 ,de la ,beche dwelt in more detail on the results of his study of these rocks1 which he had traced out on the ground and expressed upon the maps of the ,ordnance ,geological ,survey4#fc ,hardly any additions have since been made to our knowledge of the field-relations of the rocks4 ,it is to the maps and ,report of ,de la ,beche that we must turn for nearly all the published information on the subject4 ,i shall therefore give here a summary of what can be gathered from these publications4 ,footnote #fc3 ,sheets #bb1 #bc1 #bd1 #be1 #cj1 #ca1 #cb and #cc4 ,in tracing the limits of the ,culm-measures1 ,de la ,beche found that no well-defined line could be drawn between these strata and the grauwacke or ,devonian formations underneath4 ,the ,carboniferous series lies in a great trough1 of which the axis runs nearly east and west1 so that the lowest members of the series rise along the northern and southern margins4 ,but ,de la ,beche was struck with one remarkable contrast between the two opposite sides of the trough--a contrast which marks the ,devonian as well as the ,carboniferous formations of this region4 ,on the south side an abundant and persistent group of intercalated bands of igneous1 or as he called them1 trappean1 materials can be followed along the whole line of boundary1 while no such group occurs on the north side4 ,he found these bands to be lenticular1 traceable sometimes for a number of miles1 then dying out and reappearing on the same or other horizons4 ,he mapped them the whole way from ,boscastle on the west to near ,exeter on the east1 and found that though the individual sheets might be short1 the trappean zone was continuous as far as the southern margin of the ,carboniferous series could be seen1 except where it had been broken through by the great granitic mass of ,dartmoor4 ,he ascertained that the intercalated trappean rocks are not confined to the ,culm-measures1 but occur also in the contiguous portions of the grauwacke or ,devonian system4 ,but further1 he clearly recognized that the bands of igneous material which he mapped included both greenstones1 together with other varieties of massive eruptive rocks1 and also volcanic ash or tuff1 though he did not attempt to separate these out upon the maps1 but contented himself with representing them all under the same colour4 ,he admitted that some doubt might be entertained as to the age of the greenstones1 for some of them might be intrusive and therefore later than the sedimentary deposits between which they lie4 ,but he contended that there could be no uncertainty with regard to the trappean ash or tuff1 which being regularly interstratified in the ,carboniferous series1 must be contemporaneous with it4 ,he pointed out that many of the greenstones1 as well as fragments in the conglomerates or ashes1 were highly vesicular and must originally have been in the condition of pumice4 ,as an illustration of the centres of eruption from which these materials were ejected1 ,de la ,beche drew special attention once more to the conspicuous eminence of ,brent ,tor and the rocks in its neighbourhood4 ,his remarks on this subject are well worthy of being quoted--,the idea that in the vicinity of ,brent ,tor a volcano has been in action1 producing effects similar to those produced by active volcanoes1 forcibly presents itself4 ,that this volcano projected ashes1 which1 falling into adjacent water1 became interstratified with the mud1 silt and sand there depositing1 seems probable4 ,that greenstones and other solid trappean rocks constituted the lavas of that period and locality1 here and there intermingled with the ash1 appears also a reasonable hypothesis4 ,upon the whole there seems as good evidence as could be expected that to the north and north-west of ,tavistock1 ash1 cinders and liquid melted rocks were ejected and became intermingled with mud1 silt and sand during this ancient geological epoch1 corresponding with the phenomena exhibited in connection with volcanoes of the present day1 more particularly when they adjoin or are situated in the sea1 or other waters where ejected ashes1 cinders and lava can be intermingled with ordinary mud1 silt and sand4#fd ,footnote #fd3 ,op4 cit4 p4 #abb4 ,it remains for some future observer to fill up the outlines thus sketched by ,de la ,beche1 by tracing the respective areas of lavas and tuffs1 distinguishing the various petrographical types1 separating the intrusive from the interstratified sheets1 identifying the necks and bosses that may mark centres of eruption1 and expressing these various details upon maps on a sufficiently large scale4 ,a serious difficulty in this research arises from the effect of the profound alteration which has been produced on the igneous rocks by the cleavage of the region4 ,many of the greenstones have been so cleaved as to become slaty or almost schistose4 ,de la ,beche recognized this change and wrote of the schistose trappean ash4 ,a result of this metamorphism has been to impart to rocks originally massive the same fissile structure as the adjacent slates possess2 and in this condition it is often hardly possible to distinguish between greenstone and fine-grained ash4 ,there can indeed be little doubt that among these ,carboniferous volcanic rocks1 as we have seen to be the case with those of the ,devonian system in the same region1 many lavas or sills have been mapped as tuffs4 ,the chief additions to our knowledge of the ,carboniferous volcanic group of ,devonshire since the time of ,de la ,beche have been made by ,mr4 ,f4 ,rutley1 ,mr4 ,w4 ,a4 ,ussher and ,general ,m',mahon4 ,mr4 ,rutley#fe has endeavoured to trace the respective areas occupied by the different varieties of volcanic rocks in the district around ,brent ,tor1 near ,tavistock1 and to show the probable connection of the successive bands of lavas and tuffs with a central vent of discharge situated at that hill4 ,he believes that these bands occur on four different horizons in the sedimentary series4 ,he has studied the microscopic structure of the rocks1 which in his view include amphibolites1 gabbros1 basalts1 pitchstones and schistose ashes1 or clastic rocks of a doubtful nature4#ff ,footnote #fe3 ,the ,eruptive ,rocks of ,brent ,tor and its ,neighbourhood1 ,mem4 ,geol4 ,surv4 #ahgh4 ,on the ,schistose ,volcanic ,rocks occurring on the west of ,dartmoor1 with some ,notes on the ,structure of the ,brent ,tor ,volcano1 ,quart4 ,journ4 ,geol4 ,soc4 xxxvi4 7#ahhj71 p4 #bhf4 ,footnote #ff3 ,the ,eruptive ,rocks of ,brent ,tor1 p4 #de4 ,mr4 ,ussher has re-mapped the tract of ,culm-measures on the east side of the ,dartmoor granite1 besides visiting some of the other areas outside of the granite mass4 ,while confirming the general accuracy of ,de la ,beche's survey1 he has been able to improve the mapping by inserting more detail1 separating especially the tuffs from the greenstones4 ,the latter have been found by him to be mostly dolerites1 some of which1 from their parallelism the bands of tuff1 may be in his opinion contemporaneous lavas1 though the majority of them are evidently intrusive4 ,the tuffs are regularly interstratified among the ,culm-measures1 their most important band in this district having an average breadth of about #ajj yards1 and being traceable for at least two miles1 possibly considerably further4#fg ,in going over this tract with ,mr4 ,ussher ,i was led to regard many of the sheets of diabase 7dolerite7 or gabbro as true sills and bosses4 ,most of them occur as short lenticular or oval patches tolerably numerous1 but not traceable for more than a short distance1 though a connection may often exist which cannot be detected by the scanty evidence on the surface4 ,one sheet which has been followed by ,mr4 ,ussher from ,combe to beyond ,ashton1 a distance of nearly two miles1 presents in the centre a somewhat coarsely crystalline texture which rapidly gives way to a much closer grain1 and the rock then becomes highly vesicular4 ,it is overlain with dark ,culm-shales and bands of fine shaly tuff1 passing upward into a granular tuff4 ,some layers of this tuff assume a finely foliated appearance by the development of pale leek-green folia1 which show slickensided surfaces parallel with the bedding4 ,the rock then presents one of the usual appearances of schalstein4 ,this structure seems obviously due to mechanical movement along the planes of stratification4 ,footnote #fg3 ,the ,british ,culm-measures1 ,proc4 ,somerset ,archol4 and ,nat4 ,his4 ,soc4 xxxviii4 7#ahib71 p4 #afa4 ,bands of black chert and cherty shale are interpolated among the tuffs1 which also contain here and there nodular lumps of similar black impure earthy chert--an interesting association like that alluded to as occurring in the ,carboniferous volcanic series of the ,isle of ,man1 and like the occurrence of the radiolarian cherts with the ,lower ,silurian volcanic series already described4#fh ,footnote #fh3 ,cherts containing numerous species of radiolaria have recently been found by ,dr4 ,hinde and ,mr4 ,howard ,fox to form an important part of the ,lower ,culm-measures of ,devonshire1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 li4 7#ahie71 p4 #fji4 ,the volcanic belt in the valley of the ,teign can be followed for about two miles4 ,it is undoubtedly interstratified among the dark ,culm-measures1 which are distinctly seen dipping under and overlying it4 ,general ,m',mahon has recently shown what may be done by careful and detailed examination of the ground broadly sketched in by ,de la ,beche4 ,he chose for study a strip of greenstone shown on the ,geological ,survey ,map to extend for about three and a half miles along the north-west margin of the ,dartmoor granite4 ,he has found that what is represented under one wash of colour on that map includes both tuffs and lavas4 ,the tuffs1 in spite of the alteration which they appear to have undergone from the proximity of the great granite mass1 are found by microscopic investigation to be made up of fine volcanic dust containing minute lapilli of various lavas4 ,sometimes as many as six or seven different kinds of lava may be represented in the same microscopic slide4 ,these include felsitic or rhyolitic and trachytic rocks together with fragments of dark glassy lava full of magnetite dust4 ,with the tuffs are intercalated sheets of felsite and trachyte4 ,in the same district coarse volcanic agglomerate occur1 made up of blocks of different lavas and pieces of different sedimentary rocks4#fi ,footnote #fi3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 l4 7#ahid71 p4 #cch4 ,these observations are of special interest1 inasmuch as they point to the eruption of a much more acid series of volcanic lavas and tuffs than had previously been known to exist in the ,culm-measures4 ,until the ground has been more accurately mapped1 it is impossible to say whether these rocks are older or younger than those that lie around ,brent ,tor1 a few miles to the south-west4 ,general ,m',mahon has noted the presence of more basic eruptive rocks in the same district4 ,he specially cites the occurrence of mica-diorite1 of basaltic lavas altered into a serpentinous mass1 and of a dolerite which may possibly mark the actual vent of the old ,brent ,tor volcano4 ,his observations on the influence of the ,dartmoor granite in inducing new mineral rearrangements in the igneous rocks of the ,culm-measure series are full of interest4 ,,chapter ,,xxx ,,the ,,carboniferous ,,volcanoes ,,of ,,ireland ,king's ,county--,the ,limerick ,basin--,the ,volcanic ,breccias of ,doubtful ,age in ,county ,cork4 ,although the ,carboniferous system spreads over by far the larger part of the surface of ,ireland1 and is laid bare in many thousands of natural and artificial sections1 it displays undoubtedly contemporaneous igneous rocks1 so far as at present known1 at only one locality--the region around ,limerick4 ,a second district1 however1 lies in ,king's ,county1 where some vents occur which may be of ,carboniferous age1 and of which a description will be given in the following pages4 ,that the relics of volcanic action should be so few1 while the exposures of the ,carboniferous formations are so numerous and so completely disclose the geological history of the whole system1 must be regarded as good evidence that while volcanoes abounded and continued long active in ,scotland and in parts of the ,centre and ,south-west of ,england1 they hardly appeared at all in ,ireland4 ,it is worthy of remark1 also1 that the ,irish eruptions belong to the time of the ,carboniferous ,limestone--a period distinguished by volcanic activity in ,scotland and ,england--that the nature of the materials erupted bears a close resemblance to that of the lavas and tuffs of the sister island1 and that the manner of their eruption finds a close counterpart in the ,puy-eruptions1 already described4 #a4 ,,king',s ,,county ,in the progress of the ,geological ,survey several small tracts of greenstone ash and greenstone were mapped within an area of a few square miles lying to the north of ,philipstown4 ,these igneous rocks were shown to form ,croghan ,hill1 which1 rising into a conical eminence #gfi feet above the sea1 and some #dej feet above the general level of the great limestone plain around it1 forms the only conspicuous feature in the landscape for many miles4 ,in the maps and their accompanying ,explanations1 the greenstones are treated as intrusive masses1 but the greenstone ash or breccia appears to have been regarded as interstratified in the ,carboniferous ,limestone1 though the admission is made that from the scanty exposures of the rocks and the total absence of any connected section1 it has been found impossible to arrive at any definite conclusion as to the relations existing between these traps and ashes with regard to each other or to the surrounding limestone4#gj ,footnote #gj3 ,see ,sheets #aji and #aaj of the ,geological ,survey of ,ireland and ,explanation to accompany ,sheets #ih1 #ii1 #ajh and #aji1 by ,f4 ,j4 ,foote and ,j4 ,o',kelly 7#ahfe71 pp4 #g-ah4 ,in the course of a brief visit to this locality ,i did not succeed in obtaining any certain proof of the age of the igneous rocks1 but ,i found their structures to be more varied and interesting than would be inferred from the way in which they have been mapped1 and ,i came to the conclusion that the strong balance of probability was in favour of regarding them as of the age of the ,carboniferous ,limestone4 ,illustration3 ,fig4 #aib4--;,croghan ,hill1 ,king's ,county1 from ,s4,s4,w4 ,the first and most important fact to be announced regarding the district is that it includes a group of volcanic necks which rise through the ,carboniferous ,limestones4 ,the chief of these forms ,croghan ,hill4 ,it is nearly circular in ground-plan1 and measures about #djjj feet in diameter from the limestone on one side to that on the other4 ,it rises with steep grassy slopes out of the plain1 the naked rock projecting here and there in crags and low cliffs4 ,its general outward resemblance to the ,carboniferous necks of ,scotland strikes the eye of the geologist as he approaches it 7,fig4 #aib74 ,but ,croghan ,hill1 though the chief1 is not the only vent of the district4 ,it forms the centre round which a group of subsidiary vents has been opened4 ,these form smaller and lower eminences1 the most distant being one and a half miles ,e4,s4,e4 from the summit of ,croghan ,hill1 and measuring approximately #abjj feet in its longest and #hjj feet in its shortest diameter4 ,that the igneous materials of these necks really break through the limestones may be clearly seen in several sections4 ,thus by the roadside at ,gorteen1 on the south-western side of ,croghan ,hill1 the limestones have been thrown into a highly inclined position1 dipping towards the east at #fj or more1 and their truncated ends abut against the side of the neck4 ,again1 on the eastern side of the same hill the limestones have been much disturbed close to the margin of the neck1 sometimes dipping towards the volcanic centre1 and sometimes striking at it4 ,among these strata a small neck of breccia1 of which only a few square yards are visible1 rises close to the edge of the bog that covers the adjacent part of the great plain4 ,the material which chiefly forms these necks is one of the most remarkable breccias anywhere to be found in the volcanic records of the ,british ,isles4 ,the first feature noticeable in it is the pumiceous character of its component fragments4 ,these consist of a pale bluish-grey basic pumice1 and are generally about the size of a hazel-nut1 but descend to mere microscopic dust1 while sometimes exceeding a foot in length4 ,they are angular1 subangular and rounded4 ,occasionally they stand out as hollow shells on weathered surfaces1 and in one instance ,i noted that the vesicles were flattened and drawn out parallel to the surfaces of the shell1 as if deformed by gyration1 like a true bomb4 ,the breccia remains singularly uniform in character throughout all the necks4 ,its basic pumice presents much resemblance to that so characteristic of the ,carboniferous necks of ,scotland1 ,derbyshire and the ,isle of ,man4 ,the abundant vesicles are generally spherical1 and as they have been filled with calcite or chlorite1 they look like small seeds scattered through a grey paste4 ,though ,i broke hundreds of the lapilli1 ,i did not notice among them any volcanic rock other than this pumice4 ,i am not aware of any other neck so homogeneously filled up with one type of pyroclastic material1 and certainly there is no other example known in the ,british ,isles of so large and uniform a mass of fragmentary pumice4 ,limestone fragments are not uncommon in this breccia4 ,they resemble the strata around the vents4 ,pieces of the adjacent cherts may also be observed4 ,in one or two cases1 the limestone fragments were found by me to have an exceptionally crystalline texture1 which may possibly indicate a certain degree of marmarosis1 but on the whole there is little trace of alteration4 ,the fragments of pumice in the breccia are bound together by a cement of calcite4 ,in fact the rock is1 so to speak1 saturated with calcareous material1 which1 besides filling up the interstices between the lapilli1 has permeated the pumice and filled up such of its vesicles as are not occupied by some chloritic infiltration4 ,i did not observe unmistakable evidence that any part of the breccia is stratified and intercalated among the limestones1 nor any vestige of ashy material in these limestones4 ,but it is possible that traces of such interstratification may occur in the low ground to the north-west of ,croghan ,hill1 which ,i did not examine4 ,in only two places did ,i notice even a semblance of the intercalation of limestone in the breccia4 ,one of these is at ,gorteen1 where a band of limestone strata a few feet thick is underlain and overlain by breccia4 ,but though the superposition of the layers of finely stratified dark limestone and chert on the breccia is well seen and thoroughly defined1 no lapilli or ashy material are to be seen in the limestone4 ,detached pieces of similar limestone and chert occur in the breccia4 ,the band of stratified rock1 if in situ1 may be a tongue projecting from the wall into the body of the neck1 like some instances already cited from ,scotland1 but more probably it is really a large included mass lying within the vent itself4 ,the breccia here as elsewhere is entirely without any trace of stratification4 ,the second locality occurs at the most easterly neck north of ,coole ,house1 where the limestones1 rapidly undulating1 seem at last to plunge below the breccia1 which shows a series of parallel divisional planes suggestive of bedding4 ,but these may be only joint-structures1 for there is no stratification of the component materials of the rock4 ,in the necks1 and also through the limestone surrounding them1 masses of eruptive rock have been intruded as irregular bosses and veins4 ,the material of these intrusions presents little variety1 and1 so far as ,i could note1 gives no indication of the successive protrusion of progressively different lava4 ,it varies from a deep blue-black fine-grained basalt to a dolerite where the plagioclase is distinct4 ,some portions1 however1 are more basic and pass into limburgite4 ,externally there is nothing worthy of special remark in these rocks unless it be their prevalent amygdaloidal structure4 ,the amygdales1 generally of calcite1 vary from small pea-like forms in the basalts up to kernels half an inch long or more in the dolerites4 ,from a microscopic examination ,mr4 ,watts found that some of the basalts have a base of felspar and augite rich in brown mica1 and that their porphyritic felspars enclose idiomorphic crystals of augite4 ,perhaps the most noticeable feature in these later parts of the volcanic series is the occurrence in them at one locality in ,croghan ,demesne of lumps of a highly crystalline material quite distinct from the surrounding rock4 ,these enclosures vary from an inch or two to a foot or more in diameter4 ,they must be regarded as blocks which have been carried up in the ascent of the basic lava4 ,their composition has been ascertained by ,mr4 ,watts from microscopic examination to be somewhat singular4 ,one specimen contains relics of garnets1 surrounded by rings of kelyphite1 imbedded in a mosaic of felspar1 with a mineral which may possibly be idocrase4 ,another specimen from the same locality 7south-east from ,gorteen7 contains the relics of garnets preserved as kelyphite1 set in a matrix of quartz-grains1 much strained1 and containing a profusion of crystals of greenish-yellow or red sillimanite4 ,this appears to be a metamorphic rock1 and may be a fragment of some sediment enclosed in the igneous rocks4#ga ,footnote #ga3 ,guide to the ,collections of ,rocks and ,fossils belonging to the ,geological ,survey1 in the ,museum of ,science and ,art1 ,dublin 7#ahie71 pp4 #ch1 #ci4 ,as regards the history of volcanic action in ,britain one of the chief points of interest connected with these ,irish breccias and lavas relates to their geological age4 ,as no proof has been produced that any portion of them is contemporaneously interstratified in the ,carboniferous ,limestone which surrounds them1 we cannot definitely affirm that the volcanic eruptions which they record took place during the accumulation of that formation4 ,the vents must1 of course1 be later than that portion of the limestone which they pierce4 ,but the evidence seems to me to be on the whole most favourable to the view that they are of ,carboniferous ,limestone age1 for the following reasons3-- #a4 ,the breccias of ,croghan ,hill do not present a resemblance to any of those belonging to the ,tertiary volcanic series in ,antrim or the ,inner ,hebrides4 ,the possibility of their being of ,tertiary age may therefore be dismissed from consideration4 #b4 ,there are no known ,permian volcanic rocks in ,ireland4 ,nor does the ,croghan ,hill breccia show any resemblance to the ordinary material of the breccias in the ,permian necks of ,scotland4 ,it is thus not likely to be of ,permian age4 #c4 ,the peculiar basic pumice of these ,croghan ,hill vents has many points in common with the palagonite fragments so abundant among the volcanic breccias and tuffs of ,carboniferous age in ,scotland1 ,derbyshire1 and the ,isle of ,man1 and which occurs also among the ,carboniferous tuffs of the ,limerick basin4 ,it differs from the general type of the material in its pale colour1 in its uniformity of character1 in its calcareous cement1 and above all in its vast preponderance over all the other materials in the breccia4 #d4 ,the saturation of the ,croghan ,hill breccia with calcite is a singular feature in the composition of the rock4 ,had the vents been opened long subsequent to the deposition of the ,carboniferous ,limestone1 it is difficult to understand how this calcite could have been introduced4 ,mere percolation of meteoric water from the adjacent limestone does not seem adequate to account for the scale and thoroughness of the permeation4 ,but if the vents were opened on the floor of the ,carboniferous ,limestone sea1 it is intelligible that much fine calcareous silt should have found its way down among the interstices of the breccia and into the pores of the pumice which1 being caked together within the vent1 did not all float away when the sea gained access to the volcanic funnel4 ,the effect of subsequent percolation would doubtless be to carry the lime into still unfilled crevices1 and to impart to the cement a crystalline structure similar to that which has been developed in the ordinary limestones4 #b4 ,,the ,,limerick ,,basin ,about #gj miles to the south-west of the area just described lies the most compact1 and1 for its size1 one of the most varied and complete1 of all the ,carboniferous volcanic districts of ,britain 7,map ,i474 ,it takes the form of an oval basin in the ,carboniferous ,limestone series near the town of ,limerick1 about twelve miles long from east to west and six miles broad from north to south4 ,round this basin the volcanic rocks extend as a rim about a mile broad4 ,a portion of a second or inner rim1 marking a second and higher volcanic group1 partially encloses a patch of ,millstone ,grit or ,coal-measures1 which lies in the heart of the limestone basin4 7,see the section in ,fig4 #aif47 ,but it is evident that1 as the denuded edges of the volcanic sheets emerge at the surface all round the basin1 the present area over which these rocks extend must be considerably less than that which they originally covered4 ,some indication of their greater extension is supplied by outliers of the bedded lavas and tuffs1 as well as by bosses which doubtless indicate the position of some of the eruptive vents4 ,the distance between the furthest remaining patches is #bd miles4 ,the original tract over which the volcanic materials were spread cannot have been less than #bd miles long by #aj miles broad4 ,if we assume its area to have been between #bej and #cjj square miles we shall probably be under the truth4 ,this volcanic centre made its appearance on the floor of the ,carboniferous ,sea in the same district which had witnessed the eruptions of ,upper ,old ,red ,sandstone time4 ,the two visible vents that crown the ,knockfeerina and ,ballinleeny anticlines 7,chapter xxii471 are only some ten miles distant1 and there may be others of the same age even under the ,limerick basin4 ,this district thus supplies another instance of that recurrence of volcanic energy in the same area1 after a longer or shorter geological interval1 which stands out as a conspicuous feature in the history of volcanic action in ,britain4 ,that a prolonged interval elapsed between the extinction of the ,old ,red ,sandstone volcanoes and the outbreak of their successors during the accumulation of the ,carboniferous limestone series1 may be inferred from the thickness of strata which separate their respective tuffs4 ,from the published sections of the ,geological ,survey there would appear to be about #ejj feet of ,old ,red ,sandstone above the volcanic series of that formation4 ,then comes the ,lower ,limestone shale1 which is computed to be about the same thickness4 ,from the scarcity of observable dip among the ,lower ,limestones and their variable inclination1 it is not easy to form any satisfactory estimate of the depth of this group up to the base of the volcanic series4 ,it may be as much as #hjj feet1#gb and if so there would thus intervene a mass of sedimentary material nearly #bjjj feet in thickness between the two volcanic platforms4 ,throughout this thick accumulation of stratified deposits no trace of contemporaneous volcanic activity has been detected4 ,from the descriptions published more than thirty years ago by ,jukes and his colleagues in the ,geological ,survey of ,ireland1 geologists learnt how full and interesting are the proofs of great volcanic activity contemporaneous with the deposition of the ,carboniferous ,limestone series in the ,limerick district4#gc ,nowhere1 indeed1 is the evidence more complete for the occurrence of a long succession of volcanic eruptions during a definite period of geological time4 ,the officers of the ,survey showed that two epochs of activity during the older part of the ,carboniferous period were each marked by a group of tuffs and lavas1 while the interval of quiescence between them is represented by a thousand feet of limestone4 ,the same observers likewise mapped outside the volcanic ring a number of eruptive bosses1 which they regarded as probably marking some of the actual vents of that time4 ,footnote #gb3 ,this is the thickness given in the ,explanation to ,sheet #add of the ,geological ,survey of ,ireland1 p4 #h4 ,a still greater thickness is claimed in ,explanation to ,sheet #aed1 p4 #h4 ,footnote #gc3 ,see especially ,explanations of ,sheets #adc1 #add1 #aec and #aed1 ,geol4 ,surv4 ,ireland 7#ahfj1 #ahfa74 ,the geology of the district had been previously noticed by earlier observers1 to whose writings reference is made on p4 #bf of the ,explanation of ,sheet #add4 ,see also ,jas4 ,apjohn1 ,journ4 ,geol4 ,soc4 ,dublin1 vol4 i4 7#ahcb71 p4 #bd2 ,prof4 ,hull1 ,geol4 ,mag4 for #ahgd1 p4 #bje4 ,jukes 7,student's ,manual of ,geology1 #bnd edit4 #ahfb1 p4 #cbe7 gave subsequently an excellent epitome of the volcanic history4 ,the microscopic structure of some of the ,limerick volcanic rocks has been described by ,mr4 ,allport1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xxx4 7#ahgd71 p4 #eeb1 and by ,prof4 ,hull1 ,geol4 ,mag4 for #ahgc1 p4 #aec4 ,see also ,mr4 ,watts' account of these rocks in the ,guide to the ,collections of ,rocks and ,fossils 7,dublin1 #ahie71 p4 #ic4 ,the lower volcanic group1 which forms a complete ring round the ,upper ,limestones of the ,limerick basin1 is estimated to reach a thickness of #ajjj feet in some parts of its course4#gd ,its base appears to coincide generally with the upward termination of the ,lower ,limestone group of this district1 though here and there small patches of volcanic rocks in that group have been regarded as interstratified and contemporaneous bands4#ge ,it consists of a series of lavas and tuffs1 the alternations and rapid incoming and dying out of which were well made out by the ,geological ,survey4 ,footnote #gd3 ,explanation of ,sheet #add1 p4 #bg4 ,footnote #ge3 ,some of them1 however1 have characters that rather seem to place them with the intrusive materials of the district1 and therefore not necessarily earlier than the bedded lavas and tuffs4 ,the boundary line of the volcanic series is not consistently followed along the same horizon on the ,survey maps4 ,thus to the east of ,caherconlish1 a strip of the ,upper ,limestone is inserted below the base of the tuffs for a distance of about four miles4 ,unless a different horizon has been in some places taken for the boundary between the two groups of limestones1 it would appear that the eruptions had not extended over the north and north-east of the district until some time after the deposition of the ,upper ,limestone had begun4 ,the division between the two limestone groups is taken at a set of chert-bands1 but as these are not constant it is sometimes difficult to draw a satisfactory line of division4 ,tuffs4--,the base of the volcanic series is generally formed by a band of tuff sometimes as much as #cej feet thick1#gf which may be traced nearly continuously round the basin as well as in detached outliers even as far as ,carrigogunnel overlooking the alluvial plain of the ,shannon4 ,the manner in which the bottom of this tuff is interstratified with the limestone below it may be instructively examined in many quarries around the town of ,limerick4 ,striking evidence is there supplied that the first eruptions were comparatively feeble and spasmodic1 and were separated by intervals of longer and shorter duration1 during which the limestone with its fragmentary organisms was deposited1 little or no volcanic detritus falling at that time4 ,yet even in some of the limestones the microscope reveals fine broken needles of felspar1 representing doubtless the finest ejected dust4#gg ,footnote #gf3 ,explanation of ,sheet #aed1 p4 #ba4 ,footnote #gg3 ,for the details of the microscopic structure of the ,limerick volcanic rocks ,i am mainly indebted to the examination of them made for me by my ,survey colleague1 ,mr4 ,w4 ,w4 ,watts4 ,as an illustration of the way in which the volcanic and organic detritus alternated over the sea-floor1 the following section from a quarry in the townland of ,loch ,gur on the southern side of the basin is here given3#gh-- ,footnote #gh3 ,explanation of ,sheet #aed1 pp4 #ba1 #bb4 ,cherty limestone more than #bj feet #j in4 ,decomposed green tuff #b #f ,bluish-green1 calcareous laminated tuff #d #j ,limestone1 slightly ashy #a #h ,green tuff #j #b ,fine-grained decomposed tuff #j #d ,green tuff1 obliquely laminated #a #g ,fine laminated tuff #j #h ,green compact tuff #a #h ,obliquely laminated shaly tuff #j #aj ,concretionary ashy limestone #a #d ,compact ashy limestone #b #j ,green shaly tuff1 much weathered #j #e ,ashy limestone #j #g ,compact green tuff more than #d #j -------------- #da feet #i in4 ,the tuffs which in the southern part of the basin underlie the less basic lavas differ in some respects from those which further north are associated with the ,upper ,limestones4 ,they are green1 sometimes dull purplish-red1 finely granular rocks1 made up in large part of andesitic debris4 ,they are full of loose felspar crystals1 minute1 somewhat rounded and subangular lapilli of andesite or some less basic lava1 together with bits of grit and baked shale4 ,though generally much decomposed1 they are sometimes compact enough to be used for building-stone4 ,under the microscope these tuffs are seen to abound in andesite-lapilli1 with a few pieces of felsitic rocks enclosed in an opaque base1 through which are scattered broken felspars and occasional vesicular lapilli4 ,illustration3 ,fig4 #aic4--,section in quarry on roadside east of ,limerick close to viaduct of the ,limerick and ,erris ,railway4 #a4 ,limestone2 #b4 ,calcareous tuff2 #c4 ,ashy limestone or calcareous tuff4 ,the tuffs around ,limerick1 interbedded with the ,black 7,upper7 ,limestone1 are distinguished by a scarcity of andesitic debris1 by their persistent dull greenish-grey colour1 and more particularly by the abundance of minute lapilli and larger fragments of an epidote-green1 finely vesicular1 easily sectile basic pumice4 ,under the microscope much of this material is found to be an altered basic glass of the nature of palagonite4 ,these tuffs are in evident relation with the more basic lavas that accompany them4 ,the manner in which they alternate with the black limestone shows that the conditions for the eruption of this more basic detritus continued to be very similar to those that existed when the andesitic tuffs were ejected4 ,as a good illustration of this feature the accompanying section 7,fig4 #aic7 is given from a quarry on the side of the high-road between ,limerick and ,annacotty4 ,the total depth of strata here represented is about #ae feet4 ,the black limestone at the bottom is a tolerably pure calcareous rock4 ,it is divided into bands by thin partings of a fine greenish calcareous tuff1 each marking a brief discharge of ashes from some neighbouring vent4 ,half-way up the succession of strata1 the ashy material rapidly increases until it usurps the place of the limestone1 though its calcareous composition shows that the accumulation of calcareous sediment had not been entirely suspended during the eruption of ash4 ,among these tuffs ,i have noticed fragments of fine1 dark1 flinty felsite1 grit and other rocks4 ,the stones are for the most part small1 but vary up to blocks occasionally a foot in diameter4 ,lavas4--,the lavas occur in numerous sheets1 sometimes separated by thin partings or thicker beds of tuff and volcanic conglomerate4 ,on the northern rim of the basin ,mr4 ,g4 ,h4 ,kinahan has described the volcanic series east of ,shehan's ,cross-roads as composed of six zones of tuff1 each bed varying from about #ej to #bej feet in thickness1 alternating with as many sheets of lava ranging from #bg to #ahj feet in thickness1 the total depth of tuff being estimated at nearly #ejj feet and that of the lavas at about #hjj feet4#gi ,some of these tuffs are coarse conglomerates or agglomerates1 with blocks of lava occasionally #aj feet long4 ,footnote #gi3 ,explanation of ,sheet #add1 p4 #bh4 ,some of the lavas in the lower volcanic group are andesites quite like those of the plateau series in the ,carboniferous system of ,scotland4 ,externally they appear as dull reddish-brown or purplish-red compact rocks1 with abundant porphyritic felspars scattered through the fine-grained base4 ,they are generally much decomposed1 showing on a fresh fracture pseudomorphs of chlorite1 hmatite and calcite after some of the minerals1 with abundant hmatitic staining through the body of the rock4 ,amygdaloidal structure is commonly developed4 ,these andesites1 when examined microscopically1 were found by ,mr4 ,watts to present the characteristic base of minute felspar-laths with magnetite and enstatite1 and with porphyritic crystals1 often large1 of zoned plagioclase1 as well as of ilmenite and hmatite4 ,illustration3 ,fig4 #aid4--,section of the volcanic escarpment1 east of ,shehan's ,cross-roads1 south of ,limerick4 #a4 ,limestone2 #b #b4 ,tuffs2 #c #c4 ,lavas4 ,but besides the andesites there occur also1 and1 so far as ,i have observed1 in larger number1 sheets of true basalt4 ,this rock is typically black1 exceedingly close-grained in the central portion of each sheet1 but becoming highly slaggy and vesicular along the upper and lower parts4 ,under the microscope it is found to contain granular augite and magnetite1 set in a more or less devitrified glass1 with microlites of felspar1 porphyritic plagioclase1 serpentinized olivine1 and some well-marked augite4 ,these rocks form distinct escarpments along the northern rim of the basin as in the foregoing section east from ,shehan's ,cross-roads 7,fig4 #aid74 ,from the summit of this ridge1 which is about #fjj feet above the sea1 the eye looks northward over the plain1 across which low outliers of the volcanic series are scattered1 and southwards across the basin to the corresponding line of volcanic heights forming the southern rim4 ,the upper volcanic group has been estimated by the officers of the ,geological ,survey to lie about #ajjj feet higher in the ,carboniferous system than the lower1 the intervening strata consisting of the ,upper ,limestone4#hj ,it is possible that the interval is greater in some parts of the district than in others1 and if so1 the difference may be due either to greater local accumulation of volcanic materials1 or to local prolongation of the eruptions into higher stratigraphical horizons4 ,the outcrop of the upper volcanic band forms about half of a ring round the little cup of ,millstone ,grit or ,coal-measures which lies within the volcanic basin4 ,on the north-west side of the cup the volcanic rocks disappear4 ,hence the upper band has a much more restricted area than the lower4 ,but if the tuffs immediately around ,limerick are assigned to the upper group1 its extent will be proportionately increased4 ,there can be little doubt1 however1 that neither in thickness nor in superficial area did the lavas and tuffs of the second group equal those of the first4 ,the volcanic energy was gradually dying out4 ,footnote #hj3 ,explanation of ,sheet #aed1 p4 #bd4 ,the lavas of the second period are characteristic dull1 black1 compact basalts1 like those of the first period1 becoming here and there strongly amygdaloidal1 and being occasionally separated by slaggy or conglomeratic partings4 ,but they include also certain rocks wherein the felspar diminishes in quantity1 while augite and olivine become conspicuous1 together with a little enstatite4 ,the augite occurs in large porphyritic forms1 as well as of medium size and in small prisms4 ,the olivine1 as usual1 is now in the condition of serpentine4 ,these rocks are more basic than the ordinary basalts1 containing only #chff per cent of silica1 and thus approaching the limburgites4 ,with these basic lavas are associated dull green tuffs and conglomerates1 made up largely of basalt-debris1 together with abundant pieces of finely vesicular basic pumice and lapilli of a palagonitic material4 ,the manner in which the lavas and tuffs have alternated with each other1 and also with the limestones1 is well seen on ,nicker ,hill above ,pallas ,grean4#ha ,the ,survey sections show eight sheets of lava1 separated by six bands of tuff and eight intercalations of limestone1 the whole passing under the ,coal-measures4 ,footnote #ha3 ,see ,explanation of ,sheet #add1 p4 #cj1 where a description with detailed map and sections of this ground will be found4 ,the upper volcanic group may be as much as #fjj or #hjj feet thick4 ,it appears to have been left1 at the close of the eruptions1 with a very uneven surface1 some portions being so low as to be overspread with the ,upper ,limestones1 other parts so high as not to be covered until the ,coal-measure shales and flagstones came to be deposited4#hb ,footnote #hb3 ,explanation of ,sheet #aed1 pp4 #bd1 #ce4 ,vents4--,all round the edges of the ,limerick basin1 where the escarpments of the volcanic groups1 rising abruptly above the plain1 show that these rocks once extended beyond their present limits1 the progress of denudation has revealed a number of bosses which1 as above stated1 ,jukes and his associates looked upon as marking some of the vents from which the lavas and tuffs were erupted4 ,especially striking is the line of these vents along the southern margin4 ,the rocks now filling them present some unusual and rather anomalous features4 ,they are decidedly more acid than the lavas of the basin1 some of them even containing free quartz4 ,mr4 ,watts remarks that though they have a good deal in common with the trachytes1 they are crystalline throughout4 ,they are red granite-looking rocks1 which are made up chiefly of stumpy idiomorphic prisms of felspar which is mainly orthoclase4 ,some plagioclase also occurs1 and the two felspars are imbedded in interstitial quartz4 ,a trace of hornblende or mica is frequently present1 and the rocks contain about #fe per cent of silica4 ,these characters are specially observable in the necks furthest removed from the basin1 which may possibly have been connected with the andesitic outflows4 ,nearer to the basin the necks contain about #fj per cent of silica1 seldom show any interstitial quartz1 and stand between trachytes and porphyrites1 some perhaps being bostonites4#hc ,footnote #hc3 ,guide to the ,collections of ,rocks1 etc41 ,geol4 ,survey1 ,ireland1 p4 #ic1 ,dublin #ahie4 ,illustration3 ,fig4 #aie4--,view of ,derk ,hill1 a volcanic neck on the south side of the ,limerick basin4 ,a geologist1 familiar with the ,carboniferous and ,permian necks of ,scotland1 has no hesitation in confirming the surmise of ,jukes and his colleagues that the cones and domes around the ,limerick basin mark the sites of eruptive vents4 ,on the south side of the basin1 at least nine such necks rise into view1 partly from among the lavas and tuffs1 but chiefly through the limestones that emerge from below these volcanic sheets4 ,one of the most conspicuous of them1 ,derk ,hill 7,fig4 #aie71 rises to a height of #gha feet above the sea1 and comes through the bedded andesites1 as represented in ,fig4 #aif1 which gives1 in diagrammatic form1 the general structure of the ,limerick volcanic basin4 ,around the northern side of the basin a smaller number of necks has been observed1 consisting of similar acid rocks4 ,illustration3 ,fig4 #aif4--,section across the ,limerick volcanic basin4 #a4 ,lower limestone2 #b4 ,lower series of lavas and tuffs2 #c4 ,middle and ,upper ,limestone2 #d4 ,upper series of lavas and tuffs2 #e #e4 ,two volcanic necks2 #f4 ,millstone ,grit series4 ,a few of the necks appear to be filled with volcanic agglomerate4 ,here and there detached patches of fragmental volcanic material have been shown on the ,survey maps1 and referred to in the ,explanations1 as if they were outliers of the bedded tuffs2 though in some cases the coarseness of their materials and the want of any distinct bedding1 together with the absence of any indication of their relation to the nearest limestones1 have evidently offered considerable difficulty in their mapping4 ,one of the best examples occurs about two miles to the south-east of the village of ,oola4 ,the boundaries of this patch1 as put on the map1 are confessed to be entirely speculative4 ,it was only seen on the side of the railway where it appeared as a very coarse brecciated purple ash4#hd ,footnote #hd3 ,explanation of ,sheet #aed1 p4 #be4 ,on comparing the maps of the ,limerick basin with those of the ,carboniferous districts of ,scotland1 the main difference will probably be acknowledged to be the absence of any recognizable sills in the ,irish ground4 ,that no sills actually occur1 ,i am not prepared to affirm4 ,indeed some of the more acid rocks1 both outside the basin and among the rocks of the older volcanic group1 appeared to me during my traverses of the ground to have much of the character of sills4 ,a more critical examination of the area would not improbably detect some truly intrusive sheets which have hitherto been mapped among the interstratified lavas4 ,some appear to exist among the surrounding ,lower ,limestones4 ,an intrusive mass1 like a sill or dyke1 is represented on the ,geological ,survey ,map as traversing the ,coal-measures in the inner basin south of ,ballybrood4 ,but as the strata are on end along its southern margin1 it may possibly be only a portion of the upper volcanic series which has been thrown into its present position by one or more faults4#he ,footnote #he3 ,sheet #aed and ,explanation to the same1 p4 #bd4 #c4 ,,the ,,volcanic ,,breccias ,,of ,,doubtful ,,age ,,in ,,county ,,cork ,in the south-western headlands of ,ireland1 from ,bear ,island to ,dursey ,island1 various igneous rocks have been traced on the maps of the ,geological ,survey4 ,they have been described as consisting of greenstone1 felstone1 and ash or breccia1 and as including both interstratified and intrusive masses4#hf ,if contemporaneous with the strata in which they occur1 they would prove the existence of a group of volcanic rocks in the ,carboniferous slate1 or lowest division of the ,carboniferous system4 ,after an examination of the coast-line ,i came to the conclusion that while there is undoubtedly evidence of former volcanic activity in this part of ,ireland1 no proof has been obtained that the eruptions occurred in the ,carboniferous period4 ,the felsites and dolerites appeared to me to be all intrusive1 the former having certainly been injected before the terrestrial movements that have disturbed the rocks1 for some of them share very markedly in the cleavage of the region4 ,the dolerites and diabases1 on the other hand1 so far as ,i observed1 are not cleaved1 and are thus probably of later date4 ,footnote #hf3 ,see ,sheets #aig and #aih of the ,geological ,survey of ,ireland1 and the ,explanation of these ,sheets by ,messrs4 ,jukes1 ,kinahan1 ,wilson1 and ,o',kelly1 #ahfj4 ,the most interesting rocks are undoubtedly the ash and breccia1 for they are obviously of volcanic as distinguished from plutonic origin4 ,on the coast north of ,white ,bull ,head1 a bed of volcanic breccia may be seen made up of rounded and angular fragments of different sandstones1 shales and limestones1 with pieces of felsite and andesite wrapped up in a dull-grey fine-grained sandy felspathic matrix4 ,the rock weathers with a rough or rugged surface1 owing to the dropping out of the more decomposable stones4 ,this bed1 about five feet thick1 runs with the bedding of the strata around it1 and like these dips ,s4,s4,w4 at an angle of #gj4 ,if no other evidence were obtainable1 this breccia would be naturally set down as a truly interstratified deposit of volcanic detritus4 ,a short distance from it1 a second1 rather thicker band of similar material occurs1 specially distinguished by its abundant worn crystals of hornblende1 sometimes three inches in diameter1 as well as large crystals of muscovite4 ,these minerals are not unknown elsewhere in volcanic agglomerates4 ,the occurrence of lumps of augite in the vents of ,upper ,old ,red ,sandstone age in ,caithness has been already alluded to1 and a still larger series of ejected minerals will be shown in a later chapter to characterize the younger necks of ,central ,scotland4 ,in parts of its course1 this second band appears to run so perfectly parallel with the bedding of the strata between which it lies that the observer would readily believe it to be a part of the same series of deposits1 and might therefore regard it as affording good evidence of volcanic action contemporaneous with the formation of these deposits4 ,a transverse section of the bed1 where thus apparently conformable1 is shown in ,fig4 #aig4 ,illustration3 ,fig4 #aig4--,section of a bed of ,volcanic ,breccia in the ,carboniferous ,slate2 ,white ,bull ,head1 ,county ,cork4 #a #a4 ,sandstones and shales2 #b4 ,breccia4 ,illustration3 ,fig4 #aih4--,volcanic ,breccia invading and enclosing ,carboniferous ,slate1 ,white ,bull ,head4 ,further examination1 however1reveals that this seemingly regular sequence is entirely deceptive4 ,at various points the breccia abruptly truncates the sandstones1 and involves large pieces of them1 as shown in ,fig4 #aih ,a4 ,at other places1 the lower side of the breccia1 or what would be its base if it were a regular bed1 cuts out the strata and sends veins into them 7,b74 ,and the same structure is visible1 on its upper side1 or what would be its top 7,c74 ,it is clear that these highly-inclined bands of breccia are not contemporaneous with the deposition of the ,carboniferous ,slate1 but have been introduced into their position at some time subsequent not only to the deposition1 but to the disturbance and elevation of the strata4 ,the peninsula of ,white ,bull ,head is crossed by several other similar bands4 ,on ,black ,bull ,head1 also1 together with abundant felsitic and doleritic intrusions1 a similar breccia or agglomerate is to be seen4 ,in some parts it is compact in texture with spheroidal flinty lumps1 and weathers somewhat like a nodular felsite4 ,this variety ends off rather abruptly to the north1 but swells out southward1 and then runs out into a high1 narrow headland1 in which it contains asbestos1 as well as rounded crystals of hornblende4 ,it has here disrupted the shales and sandstones1 and near the junction is largely composed of fragments of them1 the strata themselves being jumbled1 bent1 and broken up4 ,the only semblance of a neck-like mass of this volcanic fragmental material occurs on ,white ,bull ,head1 where one of the bands expands about the centre of the ridge1 and is there full of large blocks of grey sandstone4 ,the breccia appears to have filled fissures which have been opened between the bedding planes of the highly tilted strata1 giving rise to long narrow dyke-like intercalations4 ,we have seen that among the ,carboniferous volcanic phenomena such dyke-like masses of agglomerate occasionally present themselves in the vents both of the plateaux and the puys4 ,in one or two places ,i noticed what may be traces of cleavage in the breccia4 ,the rock is not one that would yield easily to the rearrangements required for the production of this structure1 and the doubtful cleavage may be deceptive4 ,if we are justified in regarding the introduction of this volcanic material as having necessarily taken place after the tilting of the strata1 we may not unreasonably infer further that the eruptions could only have been effected at no great distance from the surface4 ,but the ,carboniferous ,slate in which these agglomerates lie is the lowest member of the ,carboniferous system4 ,as there is no known unconformability throughout this system in the south of ,ireland1 the whole of the rest of the pile of ,carboniferous strata1 amounting to a depth of several thousand feet1 once probably extended over this region4 ,it must1 therefore1 have been not only after the plication1 but after extensive denudation of the formations that the fissures were filled with agglomerate4 ,these geological changes no doubt occupied a vast period of time4 ,while1 therefore1 no positive evidence has yet been gathered to fix the age of these volcanic eruptions of the south-west of ,ireland1 it is tolerably clear that they cannot be assigned to the ,carboniferous period1 but must belong to some later volcanic epoch4 ,they may be of ,permian age1 perhaps even as late as the ,tertiary volcanic series4 ,,book ,,vii ,,the ,,permian ,,volcanoes ,,chapter ,,xxxi ,,the ,,permian ,,volcanoes ,,of ,,scotland ,geographical ,changes at the ,close of the ,carboniferous ,period--,land- and ,inland-,seas of ,permian time--,general ,characteristics and ,nature of the ,materials erupted--,structure of the several ,volcanic ,districts3 #a4 ,ayrshire1 ,nithsdale1 ,annandale2 #b4 ,basin of the ,firth of ,forth4 ,the close of the ,carboniferous portion of the geological record in ,britain is marked by another of those great gaps which so seriously affect the continuity of geological history4 ,no transitional formation1 such as in other countries marks the gradation from the ,carboniferous into the succeeding period1 has been definitely recognized in this country4 ,the highest ,carboniferous strata are here separated from all younger deposits by an unconformability1 indicating the lapse of vast periods of time whereof1 within the ,british area1 no chronicle has been preserved4 ,when we pass from the ,carboniferous system to that which comes next to it in order of time1 we soon become sensible that great changes in geography1 betokening an immense interval1 took place between them4 ,the prolonged subsidence during which the ,coal-measures were accumulated1 not only carried down below sea-level all the tracts over which the ,carboniferous system was deposited1 but possibly submerged the last of the islets1 which1 like those of ,charnwood ,forest1 had survived so many geological changes4 ,eventually1 however1 and after what may have been a vast period of quiescence1 underground movements began anew1 and the tracts of ,coal-measures were unequally ridged up into land4 ,the topography thus produced appears to have resulted in the formation of a series of inland seas somewhat like those of the ,old ,red ,sandstone1 but probably less in area and in depth4 ,in these basins the water seems to have been on the whole unfavourable to life1 for the red sand and mud deposited in them are generally unfossiliferous1 though1 when the conditions became more suitable1 calcareous or dolomitic sediment accumulated on the bottom1 to form what is now known as the ,magnesian ,limestone1 and muddy sediment was deposited which is now the ,marl ,slate4 ,in these less ferruginous strata1 betokening a less noxious condition of water1 various marine organisms are met with4#hg ,footnote #hg3 ,in some recent borings around ,hartlepool the ,magnesian ,limestone has been found to be interstratified with thick bands of gypsum and anhydrite1 and to be overlain by more than #bej feet of the latter substance4 ,nothing could show more forcibly the exceedingly saline and insalubrious character of the ,permian lakes or inland seas4 ,the vegetation of the land surrounding these basins was still essentially ,palozoic in character4 ,it presented a general resemblance to that of ,carboniferous time1 but with some notable differences4 ,the jungles of ,sigillaria seem to have disappeared1 while on the other hand1 conifers increased in number and variety4 ,the sediments of the water-basins have handed down only a scanty remnant of the animal life of the time4 ,along the sandy shores walked various amphibians which have left their footprints on the sand4 ,a few genera of ganoid fishes have been found in some of the shales1 and a comparatively poor assemblage of crinoids and molluscs has been obtained from the ,magnesian ,limestone4 ,to the geological period distinguished by these geographical and biological characters the name of ,permian is assigned4 ,in his survey of the progress of volcanic history in the area of ,britain1 the geologist finds that the long period of quiescence indicated by the deposition of the ,coal-measures1 and probably also by the unconformability between the ,coal-measures and the ,permian formations1 was at length terminated by a renewed volcanic outbreak1 but on a singularly diminished scale and for a comparatively brief period of time4 ,whether1 had the ,permo-,carboniferous strata which connect the ,coal-measures with the ,permian formations on the ,continent been found in this country1 they would have filled up the gap in the geological record1 and would have supplied any trace of contemporaneous volcanic action1 cannot even be surmised4 ,all that we know is that1 after a vast interval1 and during the deposition of the breccias and red sandstones which unconformably overlie the ,coal-measures1 a few scattered groups of little volcanoes appeared in the area of the ,british ,isles4 ,it is unfortunate that in those districts where these volcanic relics have been preserved1 the stratigraphical record is singularly imperfect1 and that on the eastern side of ,england1 where this record is tolerably complete1 there are no intercalated volcanic rocks4 ,the latter occur in tracts where the strata are almost wholly destitute of fossils1 and where therefore no palontological evidence is available definitely to fix the geological age of the eruptions4 ,nevertheless there is usually ample proof that the strata in question are much later than the ,coal-measures1 while their geological position and lithological characters link them with the undoubted ,permian series of the north-east of ,england4 ,they may1 however1 belong to a comparatively late part of the ,permian period1 if indeed some of them may not be referable to the succeeding or ,triassic period4 ,the comparatively feeble and short-lived volcanoes now to be described are found in two regions wide apart from each other4 ,the more important of these lies in the south-west and centre of ,scotland4 ,a second group rose in ,devonshire4 ,it is possible that a third group appeared between these two regions1 somewhere in the midlands4 ,the evidence for the history of each area will be given in a separate section in the following pages4 i4 ,,general ,,characteristics--,,nature ,,of ,,materials ,,erupted ,the chief district for the display of volcanic eruptions that may be assigned to the ,permian period lies in the centre of ,ayrshire and the valleys of the ,nith and ,annan4 ,but1 for reasons stated below1 ,i shall include within the same volcanic province a large part of the eastern half of the basin of the ,firth of ,forth 7see ,map ,v474 ,unfortunately the interesting volcanic rocks now to be considered have suffered severely from the effects of denudation4 ,they have been entirely removed from wide tracts over which they almost certainly once extended4 ,but this enormous waste has not been wholly without compensations4 ,the lavas and tuffs ejected at the surface1 and once widely spread over it1 during the deposition of the red sandstones1 have been reduced to merely a few detached fragments4 ,but1 on the other hand1 their removal as a superficial covering has revealed the vents of discharge to an extent unequalled in any older geological system1 even among the puys of the ,carboniferous period4 ,the ,permian rocks1 escaping the effects of those great earth-movements which dislocated1 plicated and buried the older ,palozoic systems of deposits1 still remain for the most part approximately horizontal or only gently inclined4 ,they have thus been more liable to complete removal from wide tracts of country than older formations which have been protected by having large portions of their mass carried down by extensive faults and synclinal folds1 and by being buried under later sedimentary accumulations4 ,we ought not1 therefore1 to judge of the extent of the volcanic discharges during ,permian time merely from the small patches of lava and tuff which have survived in one or two districts1 but rather from the number1 size and distribution of the vents which the work of denudation has laid bare4 ,the evidence for the geological age of the volcanic series now to be described is less direct and obvious than most of that with which ,i have been hitherto dealing4 ,it consists of two kinds4 7a7 ,in the first of these comes the series of lavas and tuffs just referred to as regularly interstratified with the red sandstones1 which1 on the grounds given in the next paragraph1 it is agreed to regard as ,permian4 7b7 ,connected with these rocks are necks which obviously served as vents for the discharge of the volcanic materials4 ,they pierce not only the ,coal-measures1 but even parts of the overlying bedded lavas4 ,so far there is not much room for difference of opinion2 but as we recede northward from ,ayrshire and ,nithsdale1 where the intercalation of the volcanic series in the red sandstones is well displayed1 we enter extensive tracts where these interstratified rocks have disappeared and only the necks remain4 ,all that can be positively asserted regarding the age of these necks is that they must be later than the rocks which they pierce4 ,but we may inferentially connect them with the interstratified lavas and tuffs by showing that they can be followed continuously outward from the latter as one prolonged group1 having the same distribution1 structure and composition1 and that here and there they rise through the very highest part of the ,coal-measures4 ,it is by reasoning of this kind that ,i include1 as not improbably relics of ,permian volcanoes1 a large number of vents scattered over the centre of ,scotland1 in the ,east of ,fife4 ,the red sandstones among which the volcanic series is intercalated cover several detached areas in ,ayrshire and ,dumfriesshire4 ,lithologically they present a close resemblance to the ,penrith sandstone and breccias of ,cumberland1 the ,permian age of which is generally admitted4 ,they lie unconformably sometimes on ,lower and ,upper ,silurian rocks1 sometimes on the lower parts of the ,carboniferous system1 and sometimes on the red sandstones which form the highest subdivision of that system4 ,they are thus not only younger than the latest ,carboniferous strata1 but are separated from them by the interval represented by the unconformability4 ,on these grounds they are naturally looked upon as not older than the ,permian period4 ,the only palontological evidence yet obtained from them in ,scotland is that furnished by the well-known footprints of ,annandale1 which indicate the existence of early forms of amphibians or reptiles during the time of the deposition of the red sand4 ,the precise zoological grade of these animals1 however1 has never yet been determined1 so that they furnish little help towards fixing the stratigraphical position of the red rocks in which the footprints occur4 ,the stratigraphical relations of the red sandstones of ,ayrshire and ,nithsdale were discussed by ,murchison1 ,binney and ,harkness4#hh ,these observers noticed certain igneous rocks near the base of the sandstones1 to which1 however1 as being supposed intrusive masses1 they did not attach importance4 ,they regarded the volcanic tuffs of the same district as ordinary breccias1 which they classed with those of ,dumfries and ,cumberland1 though ,binney noticed the resemblance of their cementing paste to that of volcanic tuff1 and in the end was doubtful whether to regard the igneous rocks as intrusive or interstratified4 ,footnote #hh3 ,see ,murchison's ,siluria1 #dth edit4 p4 #cca2 ,quart4 ,journ4 ,geol4 ,soc4 vol4 vii4 7#ahea71 p4 #afc1 note2 vol4 xii4 7#ahef71 p4 #bfg2 ,binney1 ibid4 vol4 xii4 7#ahef71 p4 #ach2 vol4 xviii4 7#ahfb71 p4 #dcg2 ,harkness1 ibid4 vol4 xii4 7#ahef71 p4 #bfb4 ,in the year #ahfb1 on visiting the sections in the ,river ,ayr1 ,i recognized the breccia as a true volcanic tuff4 ,during the following years1 while mapping the district for the ,geological ,survey1 ,i established the existence of a series of contemporaneous lavas and tuffs at the base of the ,permian basin of ,ayrshire1 and of numerous necks marking the vents from which these materials had been erupted4 ,an account of these observations was published in the year #ahff4hi ,since that time the progress of the ,survey has extended the detailed mapping into ,nithsdale and ,annandale1 but without adding any new facts of importance to the evidence furnished by the ,ayrshire tract4#ij ,footnote #hi3 ,geol4 ,mag4 for #ahff1 p4 #bdc2 and ,murchison's ,siluria1 #dth edit4 7#ahfg71 p4 #ccb4 ,footnote #ij3 ,the rocks are shown in ,sheets #i1 #ad and #ae of the ,geological ,survey of ,scotland1 to which1 and their accompanying ,explanations1 reference is made4 ,the ,ayrshire basin was mapped by me1 the necks in the ,dalmellington ground by ,mr4 ,james ,geikie1 the ,nithsdale area by ,mr4 ,r4 ,l4 ,jack1 ,mr4 ,h4 ,skae and myself4 ,the materials erupted by the ,scottish ,permian volcanoes display a very limited petrographical range1 contrasting strongly in this respect with the ejections of all the previous geological periods4 ,they consist of lavas generally more or less basic1 and often much decayed at the surface2 and of agglomerates and tuffs derived from the explosion of the same lavas4 ,the lavas are dull reddish or purplish-grey to brown or almost black rocks2 sometimes compact and porphyritic1 but more usually strongly amygdaloidal1 the vesicles have been filled up with calcite1 zeolites or other infiltration4 ,the porphyritic minerals are in large measure dull red earthy pseudomorphs of hmatite1 in many cases after olivine4 ,these rocks have not yet been fully studied in regard to their composition and microscopic structure4 ,a few slides1 prepared from specimens collected in ,ayrshire and ,nithsdale1 examined by ,dr4 ,hatch1 were found to present remarkably basic characters4 ,one from ,mauchline ,hill is a picrite1 composed chiefly of olivine and augite1 with a little striped felspar4 ,others from the ,thornhill basin in ,dumfriesshire show an absence of olivine1 and sometimes even of augite4 ,the rock of ,morton ,castle consists of large crystals of augite and numerous grains of magnetite in a felspathic groundmass full of magnetite4 ,around ,thornhill are magnetite-felspar rocks1 composed sometimes of granular magnetite with interstitial felspar4 ,throughout all the rocks there has been a prevalent oxidation of the magnetite1 with a consequent reddening of the masses4 ,the pyroclastic materials consist of unstratified agglomerates and tuffs1 generally found in necks1 and of stratified tuffs1 which more or less mingled with non-volcanic material1 especially red sandstone1 are intercalated among the bedded lavas or overlie them1 and pass upward into the ordinary ,permian red sandstones4 ,the agglomerates1 though sometimes coarse1 never contain such large blocks as are to be seen among the older ,palozoic volcanic groups4 ,their composition bears reference to that of the bedded lavas associated with them1 pieces of the various basalts1 andesites1 etc41 which constitute these lavas being recognizable1 together with others1 especially a green1 finely-vesicular1 palagonitic substance1 which has not been detected among the sheets of lava4 ,in general the agglomerates contain more matrix than blocks1 and pass readily into gravelly tuffs4 ,a series of specimens collected by me from necks which pierce the ,dalmellington coal-field has been sliced and examined under the microscope by ,mr4 ,watts1 who finds it to consist of basic tuffs1 in which the lapilli include various types of olivine-basalt1 sometimes glassy1 sometimes palagonitic1 and occasionally holocrystalline1 also pieces of grit1 shale and limestone4 ,in one case a crinoid joint detached from its matrix was noticed4 ,a specimen from ,patna ,hill consists of a clear irregularly cracked aggregate of carbonates and quartz with hornblende1 and its structure reminds one of that of olivine4 ,the hornblende is in small irregular patches surrounded by the clear mineral1 and is probably a replacement of a pyroxene1 perhaps diallage4 ,if this stone was once an olivine nodule1 the agglomerate might in this respect be compared with some of the tuffs of the ,eifel so well known for their lumps of olivine4 ,the stratified tuffs are generally more or less gravelly deposits1 composed of lapilli varying in size from mere grains up to pea-like fragments1 but with numerous larger stones and occasional blocks of still greater dimensions4 ,they often pass into a tough dull compact mudstone4 ,in colour they are greenish or reddish4 ,they have been largely derived from the explosion of lavas generally similar to those of which fragments occur in the agglomerates4 ,they often contain non-volcanic detritus1 derived from the blowing up of the rocks through which the vents were opened4 ,occasionally they include also various minerals such as pyrope1 black mica1 sanidine1 augite1 and others which appear to have been ejected as loose and often broken crystals4 ,this character is more fully described in regard to its occurrence among the necks of the east part of ,fife4 ,the intrusive rocks1 probably referable to the same volcanic period1 consist chiefly of dolerites and basalts which occur as dykes1 sills and bosses1 and are more particularly developed in the south-west of ,ayrshire4 ii4 ,,geological ,,structure ,,of ,,the ,,volcanic ,,districts #a4 ,ayrshire1 ,nithsdale and ,annandale 7#a7 ,interstratified ,lavas and ,tuffs4--,it will be convenient to consider first the volcanic chronicle as it has been preserved in the south-west and south of ,scotland1 where the existence of ,permian volcanoes in ,britain was first recognized4 ,the volcanic rocks in the middle of the ,ayrshire coal-field rise from under a central basin of red sandstone1 which they completely enclose4 ,their outcrop at the surface varies up to about a mile or rather more in breadth1 and forms a pear-shaped ring1 measuring about nine miles across at its greatest width 7,map ,v474#ia ,footnote #ia3 ,mr4 ,gunn has recently detected among the newest red sandstones of ,arran a small patch of volcanic rocks which may be of this age4 ,mr4 ,a4 ,macconochie has also found what may be traces of a similar volcanic band below the ,permian sandstones of ,loch ,ryan1 in ,wigtonshire4 ,this volcanic ring runs as a tract of higher ground encircling the hollow in which the ,permian red sandstones lie1 and forming a marked chain of heights above the ,carboniferous country around4 ,it is built up of a succession of sheets of different lavas1 with occasional partings of tuff or volcanic breccia1 which present their escarpments towards the coal-field outside1 and dip gently into the basin under the inner trough of brick-red sandstones4 ,good sections of the rocks are exposed in the ravines of the ,river ,ayr1 particularly at ,ballochmyle1 in the ,dippol ,burn near ,auchinleck ,house1 and in the railway cutting near ,mossgiel4 ,that these are true lava-flows1 and not intrusive sills1 is sufficiently obvious from their general outward lithological aspect1 some of them being essentially sheets of slag and scori4 ,their upper surfaces may be found with a fine indurated red sand wrapping round the scoriform lumps and protuberances1 and filling in the rents and interspaces1 as in the case of the ,old ,red ,sandstone lavas already referred to4 ,as an example of these characteristics1 ,i may cite the section represented in ,fig4 #bjj4 ,at the bottom lies a red highly ferruginous and coarsely amygdaloidal basalt 7a74 ,over it comes a volcanic conglomerate three feet thick1 made up of balls of vesicular lava like that below1 wrapped in a brick-red sandy matrix 7b74 ,lenticular bands of sandstone without blocks occur in the conglomerate1 and others lie in hollows of its upper surface 7c74 ,this intercalation of detrital material is followed by another basic lava 7d71 about six feet thick1 highly amygdaloidal in its lower and upper parts1 more compact in the centre4 ,the amygdales and joints are largely filled with calcite4 ,the slaggy bottom has caught up and now encloses some of the red sand of the deposit below4 ,another lava from three to six feet thick next appears 7e71 which is remarkable for its slaggy structure1 and is so decomposed that it crumbles away4 ,like the others it is dull-red and ferruginous and full of calcite4 ,it must have been at the time of its outflow a sheet of rough slag that cracked into open fissures4 ,that it was poured out under water is again shown in the same interesting way just referred to1 by the red sand which has been washed into the interspaces between the clinkers and has filled up the fissures1 in which it is stratified horizontally between the walls4 ,above this band1 and perhaps passing into it as its slaggy base1 lies another more compact lava 7f7 like the lower sheets4 ,illustration3 ,fig4 #aii4--;,general section across the ,permian basin of ,ayrshire4 #a4 ,highest group of the ,coal-measures2 #b4 ,volcanic tuffs and ashy brick-red sandstones2 #c4 ,lavas with interstratified tuffs and brick-red sandstones2 #d4 ,brick-red ,permian sandstones2 #e1 #e4 ,necks of volcanic agglomerate2 #f4 ,boss of dolerite4 ,throughout the series of lavas1 as indicated in the foregoing section1 traces of the pauses that elapsed between the separate outflows may be seen in the form either of layers of red sandstone or of tuff and volcanic breccia4 ,here and there1 under the platform of bedded lavas1 the brick-red sandstone is full of fragments of slag and fine volcanic dust4 ,but the most abundant accumulation of such detritus is to be seen at the top of the volcanic series1 where it contains the records of the closing phases of eruption4 ,thick beds of tuff and volcanic breccia occur there1 interleaved with seams of red sandstone1 like the chief mass of that rock1 into which they gradually pass upward4 ,yet1 even among the sandstones above the main body of tuff1 occasional nests of volcanic lapilli1 and even large bomb-like lumps of slag1 point to intermittent explosions before the volcanoes became finally extinct and were buried under the thick mass of red ,permian sandstone4 ,illustration3 ,fig4 #bjj4--,section of lavas east side of ,mauchline ,hill4 ,illustration3 ,fig4 #bja4--,section of the top of the volcanic series near ,eastside ,cottage1 ,carron ,water1 ,nithsdale4 ,there is good reason to believe that both the volcanic sheets and the red sandstones overlying them1 instead of being restricted to an area of only about #cj square miles1 once stretched over the lowlands of ,ayrshire2 and not only so1 but that they ran down ,nithsdale1 and extended into several of its tributary valleys1 if indeed1 they were not continuous across into the valley of the ,annan4#ib ,traces of the lavas and tuffs are to be found at intervals over the area here indicated4 ,the most important display of them1 next to their development in ,ayrshire1 occurs in the vale of the ,nith at ,thornhill1 whence they extend continuously up the floor of the ,carron ,valley for six miles4 ,they form here1 as in ,ayrshire1 a band at the base of the brick-red sandstones1 and consist mainly of bedded lavas with the basic characters above referred to4 ,these lavas1 however1 are followed here by a much thicker development of fragmental volcanic materials4 ,abundant volcanic detritus is diffused through the overlying sandstones1 sometimes as a gravelly intermixture1 sometimes in large slaggy blocks or bombs1 and sometimes in intercalated layers of tuff1 while an occasional sheet of one of the dull red lavas may also be detected4 ,the final dying-out of the volcanic energy in a series of intermittent explosions1 while the ordinary red sandy sediment was accumulating1 is here also admirably chronicled4 ,as an illustration of these features the accompanying section is given 7,fig4 #bja74 ,the last of the lavas 7a7 presents an uneven surface against which the various kinds of detritus have been laid down4 ,first comes a coarse volcanic breccia 7b7 made up of angular and subangular blocks of different lavas imbedded in a matrix of red ashy sand4 ,this deposit is succeeded by a band of dull red tufaceous sandstone1 evidently formed of ordinary red sandy sediment1 into which a quantity of volcanic dust and lapilli fell at the time of its accumulation4 ,some of the ejected blocks which lie inclosed in the finer sediment are upwards of a foot in length4 ,a more vigorous discharge of fragmental material is shown by the next bed 7d71 which consists of a coarse nodular tuff1 mingled with a little red sandstone and crowded with blocks of the usual lavas4 ,beyond the locality of this section these tuffs are found to pass up insensibly into the ordinary ,permian sandstone4 ,footnote #ib3 ,see ,memoirs of ,geol4 ,surv4 ,scotland1 ,sheet #ae 7#ahga71 p4 #ce2 ,sheet #i 7#ahgg71 p4 #ca4 ,illustration3 ,fig4 #bjb4--,section of two outliers of the ,permian volcanic series at the foot of ,windyhill ,burn1 ,water of ,ae1 ,dumfriesshire4 ,but we can detect the edges of yet more distant streams of lava emerging from under the red sandstones and breccias to the east of the ,nith4 ,on the farther side of the ,silurian ridge that forms the eastern boundary of the ,nith valley1 above which it rises some #gjj or #hjj feet1 there is preserved at the bottom of the valley of the ,capel ,water1 which flows into ,annandale1 another small outlier of a similar volcanic band4 ,three miles to the south-east of it two little fragments of the volcanic group lie on the sides of a small tributary of the ,water of ,ae4 ,since these may serve as a good illustration of the extent to which denudation has reduced the area of the ,permian volcanic series1 a section of the locality is here given 7,fig4 #bjb74 ,the general foundation rocks of the country are the ,silurian greywackes and shales in highly inclined and contorted positions 7a74 ,each outlier has1 as its basement material1 a volcanic breccia 7bb7 in which1 together with the usual lava-fragments1 are mingled pieces of the surrounding ,silurian strata4 ,in the smaller outlier lying to the north-east1 this detrital layer is only about one foot thick4 ,it is overlain by a slaggy amygdaloid of the usual character 7cc71 which in the lower outlier is covered with boulder clay 7d74 ,there can be little doubt that these detached fragments were once united in a continuous sheet of lava which filled the valley of the ,water of ,ae and that of its tributary4 ,that the lava stretched down the ,ae valley for some distance is proved by the occurrence of another outlier of it two miles below4 ,but there is still additional evidence for the wide extension of these volcanic sheets4 ,it appears to be certain that they stretch far to the eastward1 under the ,permian sandstones of the ,lochmaben basin of ,annandale1 for breccias largely made up of pieces of the bedded lavas are found close to the northern edge of the basin on the west side of the ,river ,annan4 ,to this remarkable adherence of the lavas and tuffs to the bottom of the ,permian valleys ,i shall afterwards more specially refer4 ,the thickness of the whole volcanic group cannot be very accurately determined4 ,it reaches a maximum in the ,ayrshire basin1 where1 at its greatest1 it probably does not exceed #ejj feet1 but is generally much less2 while in the ,nithsdale and ,annandale ground the detached and much denuded areas show a still thinner development4 ,illustration3 ,fig4 #bjc4--,the ,green ,hill1 ,waterside1 ,dalmellington1 from the south2 a tuff-neck of ,permian age4 7#b7 ,vents4--,one of the most interesting features in this south-western district of ,scotland is the admirable way in which the volcanic vents of ,permian time have been preserved4 ,their connection with the lavas and tuffs can there be so clearly traced that they serve as a guide in the interpretation of other groups of vents in districts where no such connection now remains4 ,in ,ayrshire1 the lower part of the ,permian volcanic band is pierced by several small necks of agglomerate4 ,there cannot1 ,i think1 be any doubt that these necks mark the positions of some of the vents from which the later eruptions took place4 ,immediately beyond them necks of precisely similar character rise through the upper division of the ,coal-measures4 ,there can be as little hesitation in placing these also among the ,permian vents4 ,and thus step by step we are led away from the central lavas1 through groups of necks preserving still the same features1 external and internal1 and rising indifferently through rocks of any geological age from the ,coal-measures backward4 ,thus1 although if we began the investigation at the outer limits of the chain of necks1 we might well hesitate as to their age1 yet1 when we can fix their geological position in one central area1 we are1 ,i think1 justified in classing1 as parts of one geologically synchronous series1 all the connected groups that retain the same general characteristics4 ,it is to denudation that we owe their having been laid bare to view2 but at the same time1 denudation has removed the sheet of ejected materials which may have originally connected most of these vents together4 ,in this regard1 it is most instructive to follow the vents south-eastwards from the ,ayrshire basin into ,nithsdale for a distance of some eighteen miles4 ,if we traced them down that valley to ,sanquhar1 without meeting with any vestige of superficial outflows to mark their stratigraphical position1 we might possibly hesitate whether the age of those which are so far removed from the evidence that would fix it should not be left in doubt4 ,but if we continued our traverse only a few hundred yards farther1 we should find some fragmentary outliers of the ,permian lavas capping the ,upper ,coal-measures2 and if we merely crossed from the ,nith into the tributary valley of the ,carron ,water1 we should see preserved in that deep hollow a great series of ,permian lavas1 tuffs and agglomerates4 ,it is only by a happy accident that here and there these superficial volcanic accumulations have not been swept away4 ,there was probably never any great thickness of them1 but they no doubt covered most1 if not all1 of the district within which the vents are found4 ,the ,permian necks are1 on the whole1 smaller than those of the ,carboniferous period4 ,the largest of them in the ,ayrshire and ,nithsdale region do not exceed #djjj feet in longest diameter2 the great majority are much less in size1 while the smallest measure #bj yards1 or even less4 ,those of ,fife1 to be afterwards described1 exhibit a wider range of dimensions1 and have the special advantage of being exposed in plan along the shore4 ,illustration3 ,fig4 #bjd4--,patna ,hill from the ,doon ,bridge1 ,ayrshire2 a tuff-neck of ,permian age4 ,these necks1 from their number and shapes1 form a marked feature in the scenery4 ,they generally rise as prominent1 rounded1 dome-shaped1 or conical hills1 which1 as the rock comes close to the surface1 remain permanently covered with grass 7,figs4 #bjc and #bjd74 ,such smooth green puys are conspicuous in the heart of ,ayrshire1 and likewise further south in the ,dalmellington coal-field1 where some of them are locally known as ,green ,hill1 from their verdant slopes in contrast to the browner vegetation of the poorer soil around them 7,fig4 #bjc74 ,as in those of older geological periods1 the necks of this series are1 for the most part1 irregularly circular or oval in ground-plan1 but sometimes1 like those of the ,carboniferous system1 they take curious oblong shapes1 and occasionally look as if two vents had coalesced 7,fig4 #bje74 ,here and there also the material of the vents has consolidated between the walls of a fissure or the planes of the strata1 so as to appear rather as a dyke than as a neck4 ,descending1 as usual1 vertically through the rocks which they pierce1 the necks have the form of vertical columns of volcanic material1 ending at the surface in grassy rounded hillocks or hills4 ,in almost all cases1 the necks of the ,ayrshire region consist of a gravelly tuff or agglomerate1 reddish or greenish in colour1 made up of blocks of such lavas as form the bedded sheets1 together with fragments of the stratified rocks through which the chimneys have been blown out4 ,thus1 in some of the necks1 pieces of black shale are abundant1 as at ,patna4 ,in other cases1 there are proofs of the derivation of the stones from much greater depths1 as in the ,green ,hill of ,waterside1 where fragments of fine greywacke are not infrequent1 probably derived from the ,silurian formations which lie deep beneath the ,carboniferous and ,old ,red ,sandstone series4 ,the fragmentary material of the necks is generally unstratified1 but a rude stratification may sometimes be noticed1 the dip being irregularly inward at high angles towards the middle of the vent4 ,this structure1 best seen in the vents of the ,fife coast1 as will be shown in the sequel1 may be detected in some of the necks of the ,dalmellington district4 ,illustration3 ,fig4 #bje4--;,ground plans of ,permian volcanic vents from the ,ayrshire ,coal-field4 ,on the scale of six inches to a mile4 #a4 ,neck half a mile north-west from ,dalmellington2 #b4 ,neck at ,auchengee1 four miles north-east from ,patna2 #c4 ,neck at head of ,drumbowie ,burn1 five and a half miles due north from ,dalmellington2 #d4 ,patna ,hill1 #hec feet above sea-level 7for outline of this hill see the preceding ,fig472 #e4 ,neck on ,kiers ,hill 7#ajje feet above the sea71 two miles south from ,patna1 with lava adhering to part of the wall4 ,occasionally some form of molten rock has risen in the funnel1 and has partially or wholly removed or concealed the agglomerate4 ,this feature is especially noticeable among the necks that pierce the ,dalmellington coal-field4 ,portions of basic lavas traverse the agglomerate or intervene between it and the surrounding strata4 ,these have probably in most cases been forced up the wall of the funnel1 while here and there sills run outward from the necks into the surrounding ,coal-measures4 ,sometimes a thin sheet of lava1 adhering to the wall of a funnel1 may be the remnant of a mass of rock that once filled up the orifice4 ,in one of the necks of the ,muirkirk ,coal-field1 which was pierced by a mine driven through it from side to side1 fingers and sheets of white trap1 or highly altered basalt1 were found to run out from the neck into the surrounding strata4#ic ,dark heavy basalt1 or some still more basic rock1 has here and there filled up a vent4 ,as so many of the necks rise through the coal-fields1 opportunities are afforded of studying the effects of volcanic action upon the coal-seams1 which for some distance from them have been destroyed4 ,footnote #ic3 ,explanation of ,sheet #bc1 ,geol4 ,surv4 ,scotland1 p4 #ci4 ,another feature1 which can be recognized from the information obtained in mining operations1 is that1 in the great majority of instances1 no connection is traceable between the positions of the vents and such lines of dislocation as can be detected at the surface or in the underground workings4 ,some vents1 indeed1 have evidently had their positions determined by lines of fault1 as1 for instance1 that of the ,green ,hill below ,dalmellington4 ,yet in the same neighbourhood a number of other examples may be found where the volcanic funnels seem to have avoided faults1 though these exist close to them4 ,in this south-western district of ,scotland upwards of sixty distinct vents have been mapped in the course of the ,geological ,survey4 ,they run from the north of ,ayrshire to the foot of the ,southern ,uplands1 and descend for some distance the vale of the ,nith4 ,the area over which they are distributed measures roughly about forty miles from north-west to south-east1 and at its greatest breadth twenty miles from south-west to north-east4 ,within this tract the vents are scattered somewhat sporadically in groups1 sometimes numbering twenty necks in a space of sixteen square miles1 as in the remarkable district of ,dalmellington4 ,in considering their distribution we cannot but be impressed by the striking manner in which these necks keep to the valleys and low grounds4 ,i have already alluded to this characteristic1 as shown by the volcanoes of the ,old ,red ,sandstone and ,carboniferous periods4 ,but it is displayed by the ,permian volcanoes in a still more astonishing way4 ,beginning at the northern end of the long chain of necks in the ,west of ,scotland1 we find a row of them on the plains fronting the volcanic plateau of the ,ardrossan1 ,dunlop and ,stewarton ,hills4 ,thence we may follow them1 as single individuals or in small groups1 across the broad lowland of ,ayrshire1 southward to the very base of the great chain of the ,southern ,uplands4 ,there1 a cluster of some two dozen of them may be seen rising out of the ,carboniferous rocks on the low grounds1 but they abruptly cease close to the base of the hills2 not one has been detected on the adjacent ,silurian heights4 ,moreover1 if we turn into the valleys that lead away from the great ,ayrshire plain to the interior1 we find necks of the same character in these depressions4 ,they ascend the valley of ,muirkirk1 and may be met with even at its very head1 near the base of the ,hagshaw ,hills4 ,again1 on the floor of the remarkable transverse valley trenched by the ,nith across the ,southern ,uplands1 ,permian necks pierce the ,coal-measures1 while the outlying fragments of bedded lava show that these vents flooded the bottom of that valley with molten rock4 ,turning out of ,nithsdale into the long narrow glen of the ,carron ,water1 we observe its floor and sides to be covered with the sheets of lava and tuff already noticed4 ,and so travelling onward from the vale of the ,nith into that of the ,capel ,water1 thence into the ,water of ,ae and across into the great strath of ,annandale1 we may detect1 if not actual vents1 at least the beds of lava and layers of volcanic detritus that were ejected from them4 ,all along these valleys1 which were already valleys in ,carboniferous time1 traces of the volcanic activity of this epoch may be detected4 ,but1 so far as ,i am aware1 in not a single case has any vent been observed to have been opened on the high surrounding ridges4 ,there has obviously been a determining cause why the volcanic orifices should have kept to the plains and the main valleys with their tributaries1 and should have avoided the hills which rise now to heights of #ejj to #ajjj feet or more above the bottoms of the valleys that traverse them4 ,it might be said that the valleys follow lines of fracture1 and that the vents have been opened along these lines4 ,but my colleagues in the ,geological ,survey1 as well as myself1 have failed1 in most cases1 to find any evidence of such dislocations among the rocks that form the surface of the country1 while it is sometimes possible to prove that they really do not exist there4 ,though only a few scattered patches of the ,permian bedded lavas and tuffs have been preserved1 enough is left to indicate that the vents were active only in the early part of the period represented by the ,scottish ,permian red sandstones1 for it is entirely in the lower part of these strata that volcanic rocks occur4 ,the eruptions gradually ceased1 and the sheets of ejected material1 probably also the volcanic cones1 were buried under at least several hundred feet of red sandstone4 ,whether or not any portion of the erupted material was for a time built up above the level of the water1 there seems to be no question that the vents were1 on the whole1 subaqueous4 ,illustration3 ,fig4 #bjf4--,section of sills traversing the ,permian volcanic series4 ,river ,ayr1 ,ballochmyle4 a1 ,coal-measures2 b b1 ,basic lavas2 c c1 ,brick-red sandstones with tuff2 d1 ,red tuff and volcanic breccia2 e e1 ,dolerite sills4 #c4 ,sills4--,the phenomena of sills and dykes are less clearly developed among the ,permian volcanic rocks of the ,ayrshire basin than among those of older formations4 ,in the section exposed in the course of the ,river ,ayr at ,howford ,bridge1 a coarsely crystalline dolerite which extends for nearly #cjj yards up the stream1 cuts the ,permian lavas1 of which it encloses patches as well as pieces of sandstone4 ,at the contact1 the rock becomes fine-grained 7,fig4 #bjf74 ,through the coarsely crystalline material run long parallel segregation veins of a paler1 more acid substance1 as among the ,carboniferous sills4 ,similar rocks are well seen in the ,dippol ,burn near ,auchinleck ,house4 ,passing outward into the ,coal-measures1 we encounter a much larger display of similar intrusive sheets4 ,the best district for the study of these sills lies around ,dalmellington4 ,the ,coal-measures are there traversed by many intrusions1 which have produced great destruction among the coal-seams4 ,some of the rocks are extremely basic1 including a beautiful picrite like that of ,inchcolm 7,letham ,hill1 near ,waterside74 ,the age of these sills must be later than the ,coal-measures into which they have been injected4 ,some of them are obviously connected with the agglomerate-necks1 and the whole or the greater number should thus probably be assigned to the ,permian period4#id ,the phenomena of intrusion presented by these rocks reproduce the appearances already described in connection with the basic intrusive sheets of ,carboniferous age4 ,footnote #id3 ,explanation of ,sheet #ad1 ,geol4 ,surv4 ,scotland1 p4 #bb4 #b4 ,basin of the ,firth of ,forth ,the other district of ,southern ,scotland1 where traces of volcanic action later in age than the ,coal-measures may be observed1 lies in the basin of the ,firth of ,forth 7,map ,v474 ,they include no bedded lavas1 and only at one locality do any relics of a covering of stratified tuffs overspread the ,carboniferous formations4 ,the evidence for the old volcanoes consists almost entirely of necks of tuff1 which mark the position of vents of eruption4 7#a7 ,vents4--,on the south side of the estuary of the ,forth there is only one neck which may be plausibly placed in this series4 ,it forms the upper part of ,arthur ,seat1 at ,edinburgh4 ,this hill has already been cited as consisting of two distinct portions4 ,the lower1 built up of bedded tuffs1 basalts and andesites1 forms part of the ,midlothian volcanic plateau of ,carboniferous time4 ,the vent from which these materials were ejected must lie at some little distance1 and its site has not been certainly ascertained4 ,the upper part of the hill is formed of a distinct group of rocks which has now to be described4 ,the geological structure of ,arthur ,seat has long been well known4 ,it served as a theme for discussion in the ,neptunist and ,plutonist controversy1 and was often referred to in the various mineralogical or geognostical writings of the time4 ,the first thorough examination of it as a relic of ancient volcanic action was that of ,charles ,maclaren1 published in #ahci4ie ,this author clearly recognized the later age and unconformable position of the coarse mass of agglomerate pierced by the basalt of the apex1 and pointed out the evidence of the upheaval and denudation of the older volcanic series during a long interval of repose before the latest eruptions took place4 ,subsequently ,edward ,forbes suggested that the upper part of the hill might be of ,tertiary age4#if ,thereafter ,i mapped the ground in detail for the ,geological ,survey1 entirely confirming the observations of ,maclaren4#ig ,in the end it seemed to me that the interval between the two epochs of volcanic activity might not be so great as ,forbes had supposed2 and after tracing the ,permian vents of ,ayrshire1 ,i came to the conclusion that the younger unconformable agglomerate of ,arthur ,seat was not improbably ,permian4 ,footnote #ie3 ,geology of ,fife and the ,lothians1 p4 #cd4 ,in a reprint of this work1 published in #ahff1 the venerable author briefly remarked in a footnote that he no longer believed in the second period of volcanic activity4 ,this view was adopted in #ahge by ,professor ,judd1 ,quart4 ,journ4 ,geol4 ,soc4 xxxi4 p4 #aca4 ,for the reasons stated in the text ,i believe ,maclaren's original explanation of the structure of the hill to be correct4 ,footnote #if3 ,forbes never published his views regarding ,arthur ,seat1 but expounded them to his class1 and explained them in diagrams1 some of which are preserved in the ,edinburgh ,museum of ,science and ,art1 in association with the specimens which he collected from the hill4 ,footnote #ig3 ,sheet #cb1 ,geol4 ,survey of ,scotland and descriptive ,memoir4 ,see also ,rep4 ,brit4 ,assoc4 #ahfg1 address ,geol4 ,sect41 and ,murchison's ,siluria1 #dth edit4 p4 #cca4 ,the older volcanic series of this hill has been broken through by the agglomerate which occupies a true neck1 and is abruptly marked off from all the rocks older than itself4 ,there is no trace of any of the older lavas or tuffs thickening towards this vent4 ,on the contrary they are completely truncated by it1 and their outcrops on the north side reappear from under the agglomerate on the south side4 ,their escarpments are wrapped round by the agglomerate which likewise fills the head of the hollow that had been previously worn by denudation out of the stratified deposits between the oldest lavas4 ,there is thus a violent unconformability between the later and the older volcanic rocks of ,arthur ,seat4 ,the length of time indicated by this stratigraphical break must be great4 ,there is no known discordance in the ,carboniferous system of the ,lothians1 yet the ,coal-measures1 ,millstone ,grit1 ,carboniferous ,limestone series and much of the ,calciferous ,sandstones were stripped from this hill before the eruption of the agglomerate4 ,it will be shown in the sequel that a nearly similar amount of denudation preceded some of the probably ,permian eruptions of ,fife4 ,the agglomerate contains abundant fragments of the older volcanic series4 ,its matrix is a dull red gravelly detritus1 crowded with blocks of all sizes up to a yard or more in diameter4 ,it is pierced by a column or plug of basalt1 which sends veins into it1 and rises to the apex of the hill4 ,a beautiful olivine-basalt forms the lateral mass of the ,lion's ,haunch1 which rests on the agglomerate4 ,illustration3 ,fig4 #bjg4--,section showing the relations of the later rocks of ,arthur ,seat4 #a4 ,grey and reddish sandstones and shales 7,calciferous ,sandstones72 #b4 ,the lava of the ,long ,row3 the oldest of the ,carboniferous volcanic series2 #c4 ,tuffs of the ,dry ,dam2 #d4 ,columnar basalts overlying the tuffs2 #e4 ,andesite lavas of the eastern half of ,arthur ,seat2 #f4 ,sill of ,heriot ,mount2 #g4 ,sill of ,salisbury ,crags2 #h4 ,sill of the ,dasses4 ,these complete the ,lower ,carboniferous volcanic series 7compare ,fig4 #aab74 #i4 ,white sandstones and black shales1 upper division of the ,calciferous ,sandstones2 #aj4 ,younger volcanic agglomerate resting on the denuded ends of the older volcanic series2 #aa4 ,basalt of the summit sending veins into the agglomerate2 #ab4 ,basalt of the ,lion's ,haunch4 ,in general characters the agglomerate of ,arthur ,seat resembles that of some of the younger vents of ,fife which pierce the ,coal-measures and are connected with tuffs that lie unconformably on the ,carboniferous ,limestone4 ,on these various grounds ,i think that it may be reasonably assigned to the same geological period4 ,that a new vent should be opened1 after the lapse of one or more geological periods1 on or near the site of more ancient volcanic orifices is an incident of which1 as we have seen1 the geological history of the ,british ,isles furnishes a number of examples4 ,it will be remembered that little more than a mile to the south of ,arthur ,seat lies the great vent of the ,braid ,hills1 which in the time of the ,lower ,old ,red ,sandstone gave forth such a huge pile of lavas and tuffs4 ,volcanic energy thereafter entirely died away1 and in this district was succeeded by a prolonged period of quiescence1 during which the ,lower ,old ,red ,sandstone was upraised and extensively denuded1 while the ,upper ,old ,red ,sandstone was deposited4 ,at length1 in the immediate neighbourhood1 from one or more vents1 the exact site of which is not certainly known1 the older lavas and tuffs of ,arthur ,seat1 ,calton ,hill and ,craiglockhart ,hill were erupted4 ,again1 after another vast interval1 a new volcano appeared1 and the agglomerate and younger basalts of ,arthur ,seat were ejected from it4 ,this is one of the most striking examples in this country of the remarkable persistence of volcanic energy in the same locality4 ,there is no evidence at ,arthur ,seat itself to fix the geological date of the last volcanic activity of the hill4 ,if the group of younger rocks stood alone1 with no other trace of post-,carboniferous eruptions in the surrounding district1 a plausible conjecture as to its age would not be easily offered4 ,but in reality it is not a solitary example of such rocks2 for within sight1 on the opposite side of the ,firth of ,forth1 its counterparts may be seen4 ,to the description of these numerous and clearer illustrations ,i now proceed4 ,the ,east of ,fife is remarkable for a large assemblage of volcanic vents1 which1 unlike those in ,ayrshire and ,nithsdale1 stand alone1 their superficial ejections having been removed by denudation1 and no connection being traceable between them and any ,permian sandstones4 ,the vents filled up with agglomerate and pierced with plugs and veins of basalt1 rise through the ,carboniferous rocks1 but have left no record for precisely defining their geological age4 ,on the one hand1 it is quite certain that in this district volcanic eruptions took place during the earlier half of the ,carboniferous period4 ,to the north of ,largo1 and still more distinctly to the north-east of ,leven1 sections occur to show the contemporaneous outpouring of volcanic rocks during the time of the ,carboniferous ,limestone4 ,the ,leven section1 seen in a ravine a little to the north-east of the town1 is specially important4 ,it presents a succession of red and green fine sandy tuffs1 interstratified with fire-clays and sandstones1 and containing a zone of basalt in the centre4 ,these rocks lie not far from the top of the ,carboniferous ,limestone series4 ,on the other hand1 there is equally clear proof of far later eruptions4 ,from ,st4 ,andrews to ,elie a chain of necks may be traced1 having the same general characters1 and piercing alike the ,calciferous ,sandstones1 and the older part of the ,carboniferous ,limestone series4 ,that these vents must in many cases be long posterior to the rocks among which they rise1 is indicated by some curious and interesting kinds of evidence4 ,they are often replete with angular fragments of shale1 sandstone and limestone1 of precisely the same mineral characters as the surrounding strata1 and containing the same organic remains in an identical state of fossilization4 ,it is clear that these strata must have had very much their present lithological aspect before the vents were opened through them4 ,again1 the necks may often be observed to rise among much contorted strata1 as1 for example1 along the crest of a sharp anticlinal arch1 or across a synclinal basin4 ,the ,carboniferous rocks must thus have been considerably plicated before the time of the volcanic eruptions4 ,in the next place1 the vents often occur on lines of dislocation without being affected thereby4 ,they must be posterior1 however1 not only to these dislocations1 but also to much subsequent denudation1 inasmuch as their materials overspread the rocks on each side of a fault without displacement4 ,hence we conclude with confidence1 that a great deal of volcanic activity in the ,east of ,fife must have been posterior to most1 if not all1 of the ,carboniferous period4 ,illustration3 ,fig4 #bjh4--,section in brooks between ,bonnytown and ,baldastard1 ,largo4 a1 ,sandstone shales and coals of ,carboniferous ,limestone series2 b1 unconformable tuff4 ,in the neighbourhood of ,largo1 further important evidence is presented1 confirming and extending this conclusion4 ,the highest member of the ,upper ,coal-measures1 consisting of various red sandstones1 with red and purple clays1 shales1 thin coals and ironstones1 is prolonged from the ,fife coal-field in a tongue which extends eastward beyond the village of ,lower ,largo4 ,it is well displayed on the shore1 where every bed may be followed in succession along the beach for a space of nearly two miles4 ,two volcanic necks1 presenting the same features as those which pierce the older portions of the ,carboniferous system to the east1 rise through these red rocks4 ,we are thus carried not only beyond the time of the ,carboniferous ,limestone1 but beyond the close of the very latest stage of the ,carboniferous period in ,central ,scotland4 ,connected with these and other vents farther north1 there is a large area of tuff which has been thrown out upon the faulted and greatly denuded ,carboniferous rocks4 ,it may be traced passing from the red ,upper ,coal-measures across the large fault which here separates that formation from the ,carboniferous ,limestone1 and extending inland athwart different horizons of the latter series4 ,outlying fragmentary cakes of it may be seen resting on the upturned edges of the sandstones1 shales and coal-seams1 even at a distance of some miles towards the north-west1 proving that the fragmentary materials discharged from the vents spread over a considerable area4 ,the accompanying section 7,fig4 #bjh7 may serve as an illustration of the relation between this sheet of bedded tuff and the underlying rocks4 ,though interstratified volcanic rocks occur in the ,carboniferous system of the ,east of ,fife1 no connection has been traced between them and any of the vents now referred to4 ,while none of these vents can be proved to be of ,carboniferous age1 it is of course possible that such may be the true date of some of them4 ,others1 nevertheless1 and probably much the largest number1 judged from the data just given1 may be regarded as probably post-,carboniferous4 ,those which happen to rise through the uppermost ,coal-measures do not appear to be distinguishable by any essential characters from those which pierce indifferently the ,carboniferous ,limestone series and ,calciferous ,sandstones of the ,east of ,fife4 ,they seem to be all one connected aggregate1 resembling each other alike in their external characters1 internal structure and component materials1 and the limit of their age must be determined by the geological horizon of the youngest formation which they traverse4 ,by this process of reasoning ,i reach the conclusion that this remarkable series of old volcanoes in the ,east of ,scotland not improbably dates from the same time as that of ,ayrshire and ,nithsdale1 already described4 ,illustration3 ,fig4 #bji4--,view of ,largo ,law from the east 7the crag on the left1 at the base of the cone1 is a portion of a basalt-stream4 ,see ,fig4 #bbf74 ,some idea of the importance and interest of the volcanic area of ,eastern ,fife may be gathered from the fact that in a space of about #gj square miles no fewer than #fj necks may be counted1 and others are probably concealed below the drift-deposits which cover so much of the interior of the country4 ,the area of this remarkable display extends from ,st4 ,andrews ,bay and the ,vale of the ,eden southwards to the coast of the ,firth of ,forth between ,lundin ,links and ,st4 ,monans4 ,all over the inland tract the necks form more or less marked eminences1 of which the largest are conspicuous landmarks from the southern side of the ,firth4 ,but the distinguishing characteristic of the area is the display of the necks along the coast1 where1 in a series of natural dissections1 their form1 composition1 internal structure and relations to the surrounding rocks have been laid open in such clearness and variety as have been met with in the volcanic records of no other geological period within the compass of these islands4 ,as this district thus possesses a singular interest and value for the study of volcanic vents1 ,i shall enter in some detail into the description of the sections so admirably laid bare4 ,illustration3 ,fig4 #baj4--,view of small neck in ,calciferous ,sandstones1 on the shore1 three miles east from ,st4 ,andrews4 7,this illustration1 likewise ,figs4 #bab1 #baf1 #bai1 #bba1 #bbb1 #bbe and #bbg are from photographs taken for the ,geological ,survey by ,mr4 ,r4 ,lunn47 ,as in ,ayrshire1 the necks in the ,east of ,fife generally rise as isolated conical or dome-shaped hills1 with smooth grassy slopes1 but where a dyke or boss of basalt occurs in them1 it usually stands out as a crag or knoll4 ,largo ,law 7,fig4 #bji7 may be taken as a singularly perfect example of the cone-shaped neck4 ,this hill1 however1 comprises more than one vent4 ,the mass of tuff of which it consists probably includes at least three distinct funnels of discharge1 and surrounding it there still remains a good deal of the fragmental material that gathered around these vents and is now seen to lie unconformably upon the ,carboniferous formations 7,fig4 #bjh74 ,there must be a total area of not much less than four square miles over which tuff occupies the surface of the ground4 ,while the ,fife necks possess the great advantage of having been laid bare by the sea1 their frequent small size on the coast allows their whole area to be examined4 ,as illustrations of these little vents1 two plates are here given from the coast-line to the east of ,st4 ,andrews1 where a number of small necks of agglomerate have been planted among the plicated ,calciferous ,sandstones4 ,in ,fig4 #baj the abrupt truncation of the sandstones by the volcanic rock is well shown4 ,the strata on the right have been broken through1 and the sea has indented a small gully along the wall of the old volcanic funnel4 ,the sandstones in front1 however1 still adhere firmly to the agglomerate1 which rises above them as a rugged mass of rock4 ,in ,fig4 #bab the edge of the vent can be traced partly in section and partly in plan for about half of its circumference4 ,on the right hand1 the actual wall of the funnel is visible where the false-bedded sandstones are sharply cut off by the agglomerate4 ,in front the strata appear in plan on the beach1 and their ledges can be seen to the left striking at the margin of the neck4 ,illustration3 ,fig4 #baa4--;,ground-plan of ,permian volcanic vents4 ,illustration3 ,fig4 #bab4--,small neck in ,calciferous ,sandstones a little east from the ,rock and ,spindle1 two and a half miles east from ,st4 ,andrews4 ,the shape of the ,fife vents is1 as usual1 generally circular or oval2 but is subject to considerable irregularity4 ,the coast-section between ,largo and ,st4 ,monans exposes many ground-plans of them1 and permits their irregularities to be closely examined4 ,the accompanying figure 7,fig4 #baa7 exhibits some characteristic forms4 ,eccentricities of outline no doubt arose from the irregular way in which the rocks yielded to the forces of explosion during the piercing of a volcanic orifice4 ,this is often well shown by the veins and nests of tuff or agglomerate which have been forced into the rents or sinuosities of the orifices4 ,in other cases1 however1 it is probable that1 as among the ,ayrshire necks1 and those of ,carboniferous age already cited1 what appears now as one volcanic neck was the result of a shifting of the actual funnel of discharge1 so that the neck really represents several closely adjacent vents4 ,the case of ,largo ,law has been already noticed4 ,the necks at ,kellie ,law 7,fig4 #bac7 show clearly the same structure1 the ,law itself 7#a7 probably consisting of two contiguous vents1 while a third 7#b7 forms a smaller cone immediately to the east4 ,such a slight lateral displacement of the vent has been noticed at many ,tertiary and recent volcanic orifices4 ,in the island or peninsula of ,volcanello1 for example1 three craters indicate successive shiftings of the vent1 the most perfect of them marking the latest and diminishing phase of volcanic activity 7,fig4 #bad1 compare ,fig4 #bi1 vol4 i41 p4 #gj74 ,illustration3 ,fig4 #bac4--,plan of volcanic necks at ,kellie ,law1 east of ,fife1 on the scale of three inches to one mile4 #a1 ,kellie ,law 7tuff72 #b1 ,carnbee ,law 7tuff72 #c1 #d1 #e1 small tuff necks2 ,b ,b1 basalt dykes and bosses2 c c1 coal-seams2 l1 limestone2 f1 fault4 ,the arrows mark the dip of the strata through which the necks have been drilled4 ,illustration3 ,fig4 #bad4--,plan of the craters in ,volcanello1 ,lipari ,islands4 ,the ,fife necks vary from only a few yards up to perhaps #djjj feet in diameter4 ,one of the smallest and most completely exposed occurs on the shore at ,newark ,castle1 near ,st4 ,monans4 ,it measures only #fj yards in length by about #cg yards in breadth4 ,a ground-plan of it is given in ,fig #bbd4 ,still smaller is the neck at ,buddo ,ness1 on the coast east of ,st4 ,andrews1 which measures only #bj yards across4 ,from the way in which the vents have been dissected by the sea along the ,fife coast1 the geologist is enabled to study in minute detail the effects of the volcanic operations upon the strata through which the funnels have been drilled4 ,considerable variation may be observed in the nature and amount of change4 ,sometimes the orifice has been made without any noticeable alteration of the sandstones1 shales and limestones1 which retain their dip and strike up to the very wall of the chimney4 ,usually there is more or less jumbling and crushing of the stratification1 and often a considerable amount of induration4 ,as a typical example of these effects ,i give a section from the margin of the neck of tuff on the east side of ,elie ,harbour 7,fig4 #bae74 ,here the sandstones and shales 7a7 have been doubled over and dragged down against the tuff 7b74 ,they have likewise been hardened into a kind of quartzite1 and this alteration extends for about #bj to #cj feet from the edge of the neck4 ,illustration3 ,fig4 #bae4--,section of the strata at the edge of the volcanic vent on the east side of ,elie ,harbour4 ,the material which has filled up the vents is almost entirely fragmental1 varying from a coarse agglomerate to a fine volcanic tuff4 ,some minor necks have been completely or in great part filled with angular debris of the ordinary rocks of the neighbourhood4 ,in the western neck on the ,largo shore1 for example1 which rises through the red rocks of the ,upper ,coal-measures1 the material consists largely of fragments of red sandstone1 clay and shale4 ,between ,elie and ,st4 ,monans1 some of the necks are filled almost wholly with debris of black shale and encrinal limestone4 ,there does not appear to be any relation between the diameter of a funnel and the size of the blocks that now fill it4 ,some of the larger necks1 for example1 consist of comparatively fine tuff4 ,the ,buddo ,ness1 on the other hand1 though so small a vent1 is packed with blocks of shale six feet long1 while the sandstone through which the orifice has been drilled passes1 as usual1 into quartzite for several yards away from the edge4 ,as an example of the general aspect presented by one of the coarse agglomerates in the necks of the ,fife coast1 a view is given in ,fig4 #baf of a portion of the neck at ,ardross1 about two miles east from ,elie4 ,this thoroughly volcanic accumulation is here shown to consist of blocks of all sizes heaped together without any definite arrangement4 ,illustration3 ,fig4 #baf4--;,agglomerate of neck on shore at ,ardross1 two miles east from ,elie4 ,since the first stage in the history of the vents has been the perforation of the solid crust by explosion1 and the consequent production of debris from the disrupted rocks1 we may hope to detect underneath the pile of thoroughly volcanic ejections1 traces of the first explosions4 ,i have been much struck with the fact that in the ,east of ,fife such traces may frequently be found here and there within the outer border of the vents4 ,at ,largo1 and again between ,elie and ,st4 ,monans1 it may be noticed that the mass of material adhering to the wall of a neck1 exposed in ground-plan upon the beach1 often consists largely1 or even wholly1 of debris of sandstone1 shale and limestone1 while the central and chief mass is made up of green tuff or agglomerate1 with occasional pieces of the surrounding stratified rocks scattered through it4 ,it seems probable1 therefore1 that the sections of these ,fife necks1 laid bare on the present shore1 do not lie far below the original crater-bottoms4 ,some light might be expected to be thrown upon the phenomena in an active volcanic chimney by the condition of the fragments of recognizable sedimentary rocks imbedded in the ejected debris which has filled up the orifice4 ,but the assistance from this source is neither so full nor so reliable as could be wished4 ,in some of the ,fife vents1 indeed1 the fragments of shale1 sandstone and other sedimentary strata are so unchanged that they cannot on a fresh fracture be distinguished from the adjacent parent strata4 ,the ,spirifers1 ,lingul1 crinoids1 cyprid-cases1 ganoid scales and other fossils are often as fresh and perfect in the fragments of rock imbedded in tuff as they are in the rock in situ4 ,in some cases1 however1 distinct1 and occasionally even extreme1 metamorphism may be detected1 varying in intensity from mere induration to the production of a crystalline texture4 ,the amount of alteration has depended not merely upon the heat of the volcanic vent1 but also in great measure upon the susceptibility of the fragments to undergo change and the duration of their exposure to it4 ,dr4 ,heddle has computed the temperature to which fragments of shale1 etc41 in tuff-necks of the ,fife coast have been subjected4 ,he found that the bituminous shales have lost all their illuminants1 and of organic matter have retained only some black carbonaceous particles2 that the encrinal limestones have become granular and crystalline2 that the sandstones present themselves as quartzite1 and that black carbonaceous clays show every stage of a passage into ,lydian-stone4 ,he inferred from the slight depth to which the alteration has penetrated the larger calcareous fragments1 that the heat to which they were exposed must have been but of short continuance4 ,as the result of his experiments1 he concluded that the temperature at which the fragments were finally ejected from the volcanic vents probably lay between #ffj and #ijj ,fahr4#ih ,footnote #ih3 ,trans4 ,roy4 ,soc4 ,edin4 vol4 xxviii4 p4 #dhg4 ,it may be perhaps legitimate to infer that1 while the fragments that fell back into the volcanic funnel1 or which were detached from the sides of the vent1 after having been exposed for some time to intense heat under considerable pressure1 would suffer more or less metamorphism1 those1 on the other hand1 which were discharged by the riform explosions from the cool upper crust1 on the first outburst of a vent1 would not exhibit any trace of such a change4 ,where1 therefore1 we meet with a neck full of fragments of unaltered stratified rocks1 we may suppose it to have been that of a short-lived volcano2 where1 on the other hand1 the fragments are few and much altered1 they may mark the site of a vent which continued longer active4 ,the metamorphism of the fragmentary contents of a volcanic funnel by the action of ascending vapours has already been described in the case of one of the vents of the ,carboniferous plateaux 7vol4 i4 p4 #djd74 ,one of the most curious and puzzling features in the contents of the tuff necks of the ,fife coast is the occurrence there of crystals and fragments of minerals1 often of considerable size1 which do not bear evidence of having-been formed in situ1 but have undoubtedly been ejected with the other detritus4 ,dr4 ,heddle has noticed the fact1 and has described some of the minerals which occur in this way4 ,the following list comprises the species which he and ,i have found3-- ,hornblende1 in rounded fragments of a glassy black cleavable variety4 ,augite1 sometimes in small crystals1 elsewhere in rounded fragments of an augitic glass4 ,orthoclase 7,sanidine71 abundant in worn twin crystals in the tuffs of the east of ,fife4 ,plagioclase4 ,biotite4 ,pyrope1 in the tuffs 7and more rarely in the basalts7 of ,elie4 ,nigrine1 common in some of the dykes1 more rarely in the tuffs of ,elie4 ,saponite1 ,delessite and other decomposition products4 ,semi-opal1 one specimen found in the later 7,permian87 agglomerate of ,arthur's ,seat4 ,asphalt1 abundant at ,kincraig1 near ,elie4 ,fragments of wood1 with structure well preserved1 may be included here4 ,dr4 ,heddle has described from the neck of tuff at ,kinkell1 near ,st4 ,andrews1 large twin crystals of a glassy orthoclase1 which are invariably much worn1 and preserve only rudely the form of crystals4 ,he justly remarks that they have no connection with drusy cavity1 exfiltration vein1 or with any other mineral1 and look as if a portion of their substance has been dissolved away4 ,internally1 however1 they are quite fresh and brilliant in lustre1 though sometimes much fissured4#ii ,footnote #ii3 ,trans4 ,roy4 ,soc4 ,edin4 vol4 xxviii4 p4 #bbc4 ,the tuffs at ,elie are full of similar crystals4 ,i obtained from one of the necks east of that village a specimen which measures #d inches in length1 #c-a_/b in breadth1 and #b-a_/d in thickness1 and weighs about #b lbs4 ,it is1 however1 a well-striated felspar4 ,from the same tuff ,i procured an orthoclase twin in the ,carlsbad form4 ,all the felspar pieces1 though fresh and brilliant internally1 have the same rounded and abraded external appearance4 ,the fragments of hornblende form a characteristic feature in several of the ,elie dykes 7to be afterwards described71 and in the neighbourhood of these intrusive rocks occur more sparingly in the tuff4 ,it is a glossy-black cleavable mineral1 in rounded pieces of all sizes1 up to that of a small egg4 ,dr4 ,heddle obtained a cleavage angle of #abd #ai'1 and found on analysis that the mineral was hornblende4#ajj ,footnote #ajj3 ,op4 cit4 xxviii4 p4 #ebb4 ,augite occurs sparingly in two forms among the rocks4 ,i have obtained small crystals from the red agglomerate on the south side of ,arthur ,seat1 recalling in their general appearance those of ,somma4 ,lumps of an augitic glass have been found by ,dr4 ,heddle1 sometimes as large as a pigeon's egg1 in two of the dykes at ,elie1 and in the tuff at the ,kinkell neck1 near ,st4 ,andrews4 ,he observed the same substance at the ,giant's ,causeway1 both in the basalt and scattered through one of the interstratified beds of red bole4 ,much larger rounded masses of a similar augitic glass1 but with a distinct trace of cleavage1 have already been referred to as occurring in a volcanic vent of ,upper ,old ,red ,sandstone age at ,john o' ,groat's ,house4#aja ,footnote #aja3 ,op4 cit4 xxviii4 pp4 #dha et seq41 and ante1 vol4 i4 p4 #ceb4 ,biotite is not a rare mineral in some tuffs4 ,it may be obtained in ,lower ,carboniferous tuffs of ,dunbar1 in plates nearly an inch broad2 but the largest specimen ,i have obtained is one from the same ,elie vent which yielded the large felspar fragment4 ,it measures #b-a_/b #b #a_/b inches4 ,these mica tables1 like the other minerals1 are abraded specimens4 ,that these various minerals were ejected as fragments1 and have not been formed in situ1 is the conclusion forced upon the observer who examines carefully their mode of occurrence4 ,some of them were carried up to the surface by liquid volcanic mud1 and appear in dykes of that material like plums in a cake4 ,but even there they present the same evidence of attrition4 ,they assuredly have not been formed in the dykes any more than in the surrounding tuff4 ,in both cases they are extraneous objects which have been accidentally involved in the volcanic rocks4 ,dr4 ,heddle remarks that the occurrence of the worn pieces of orthoclase in the tuff is an enigma to him4 ,i have been as unable to frame any satisfactory explanation of it4#ajb ,footnote #ajb3 ,occasionally the crystals can be matched in some lava-form rock of the same volcanic area2 but many of them have no such counterparts4 ,see vol4 i4 p4 #fb and note4 ,illustration3 ,fig4 #bag4--;,ground-plan of volcanic neck1 ,elie ,harbour1 showing circular deposition of the stratification4 ,t1 ,tuff of the neck1 the arrows showing its inward dip2 ,b ,b1 ,dykes2 ,s1 ,sandstones and shales1 through which the neck has been opened4 ,it might have been thought that within the throat of a volcano1 if in any circumstances1 loose materials should have taken an indefinite amorphous aggregation4 ,and1 as has been shown in the foregoing chapters1 this is usually the case where the materials are coarse and the vent small4 ,oblong blocks are found stuck on end1 while small and large are all mixed confusedly together4 ,but in numerous cases where the tuff is more gravelly in texture1 and sometimes even where it is coarse1 traces of stratification may be observed4 ,layers of coarse and fine material succeed each other1 as they are seen to do among the ordinary interstratified tuffs4 ,the stratification is usually at high angles of inclination1 often vertical4 ,so distinctly do the lines of deposit appear amid the confused and jumbled masses1 that an observer may be tempted to explain the problem by supposing the tuff to belong not to a neck1 but to an interbedded deposit which has somehow been broken up by dislocations4 ,that the stratification1 however1 belongs to the original volcanic vents themselves is made exceedingly clear by some of the coast-sections in the ,east of ,fife4 ,on both sides of ,elie1 examples occur in which a distinct circular disposition of the bedding can be traced corresponding to the general form of the neck4 ,the accompanying ground-plan 7,fig4 #bag7 represents this structure as seen in the neck which forms the headland at ,elie harbour4 ,alternations of coarse and fine tuff with bands of coarse agglomerate1 dipping at angles of #fj and upwards1 may be traced round about half of the circle4 ,the incomplete part may have been destroyed by the formation of another contiguous neck immediately to the east4 ,to the west of ,earlsferry another large1 but also imperfect1 circle may be traced in one of the shore necks4 ,a quarter of a mile farther west rises the great cliff-line of ,kincraig1 where a large neck has been cut open into a range of precipices #bjj feet high1 as well as into a tide-washed platform more than half a mile long4 ,the inward dip and high angles of the tuff are admirably laid bare along that portion of the coast-line4 ,the section in which almost every bed can be seen1 and where1 therefore1 there is no need for hypothetical restoration1 is as shown in ,fig4 #bah4 ,i have already referred to the frequently abundant pieces of stratified tuff1 found as ejected blocks in vents filled with tuff1 and to the derivation of these blocks from tuff originally deposited within the crater4 ,there can1 ,i think1 be little hesitation in regarding the stratification of these ,fife vents as a record of successive deposits of volcanic detritus inside the vents4 ,the general dip inwards from the outer rim of the vent strikingly recalls that of some modern volcanoes4 ,by way of illustration1 ,i give here a section of part of the outer rim of the crater of the ,island of ,volcano1 sketched by myself in the year #ahgj while ascending the mountain from the north side 7,fig4 #bbj74 ,the crater wall at this point consists of two distinct parts--an older tuff 7a71 which may have been in great measure cleared out of the crater before the ejection of the newer tuff 7b74 ,the latter lies on the outer slope of the cone at the usual angle of #cj4 ,it folds over the crest of the rim1 and dips down to the flat tuff-covered crater bottom1 at an angle of #cg4 ,these are its natural angles of repose4 ,illustration3 ,fig4 #bah4--,section across the great vent of ,kincraig1 ,elie1 on a true scale1 vertical and horizontal1 of six inches to a mile4 #a1 ,sandstones1 shale1 etc41 of ,lower ,carboniferous age1 plunging down toward the neck ,t2 ,b1 columnar basalt1 shown also in ,figs4 #bbc and #bbe4 ,illustration3 ,fig4 #bai4--;,dyke in volcanic neck1 on the beach1 ,st4 ,monans4 ,illustration3 ,fig4 #bbj4--,section of part of crater rim1 ,island of ,volcano4 ,applying modern analogies of this kind1 ,i have been led to conclude that the stratification so conspicuous in the tuff of the vents in the east of ,fife and in the ,carboniferous series of the ,lothians belongs to the interior of the crater and the upper part of the volcanic funnel4#ajc ,these stratified tuffs1 on this view of their origin1 must be regarded as remains of the beds of dust and stones which gathered within the crater and volcanic orifice1 and which1 on the cessation of volcanic action1 sometimes remained in their original position1 or were dislocated and slipped down into the cavity beneath4 ,that the tuffs consolidated on slopes1 perhaps quite as steep as those of ,volcano1 is now and then indicated by an interesting structure4 ,the larger stones imbedded in the layers of tuff may be observed to have on their fronts in one direction a small heap of coarse gravelly debris1 while fine tuff is heaped up against their opposite side4 ,this arrangement doubtless points to deposit on a slope of loose debris1 from which the larger blocks protruded so as to arrest the smaller stones1 and allow the fine dust to gather behind4 ,footnote #ajc3 ,further illustrations of this characteristic structure of some vents will be found in the account of the ,tertiary vents of the ,faroe ,isles in ,chapter xli4 ,see also the remarks in the introductory chapters1 vol4 i4 p4 #fc4 ,if the inference be correct1 that the stratification here described belongs to the old craters or the upper parts of the funnels1 it furnishes additional evidence of the wide interval of time that elapsed between the deposition of the ,carboniferous strata and the outbreak of these vents4 ,during that interval prolonged denudation reduced the upturned ,carboniferous ,limestone series to nearly its present form of surface1 and any materials discharged from the vents over the surrounding ground would obviously lie with a violent unconformability on the rocks below4 ,the frequent great disturbance in the bedding of the tuff within the vents may be connected with some kind of collapse1 subsidence or shrinkage of the materials in the funnel below4 ,that a movement of this nature did take place is shown by the remarkable bending down of the strata round the margins of the vents1 which has been already described4 ,the minor vents for the most part contain only fragmentary materials2 but those of larger size usually present masses of lava in some characteristic forms4 ,in not a few cases1 the lava has risen in the central pipe and has hardened there into a column of solid rock4 ,subsequent denudation1 by removing most of the cone1 has left the top of this thick column projecting as a round knoll upon the hill-top4 ,arthur ,seat presents a good example of this structure4 ,where the denudation has not proceeded so far1 we may still meet with a remnant of the cake of lava which sometimes overflowed the bottom of a crater4 ,the summit of ,largo ,law affords indications of this arrangement1 the cone of tuff being there capped with basalt1 evidently the product of successive streams1 which welling out irregularly covered the crater bottom with hummocks and hollows 7,fig4 #bbf74 ,the knolls are beautifully columnar1 and sometimes show a divergent arrangement of the prisms4 ,illustration3 ,fig4 #bba4--;,dyke rising through the agglomerate of a volcanic vent2 ,kincraig1 ,elie4 ,but the most frequent form assumed by the lava in the necks is that of veins or dykes running as wall-like bands through the tuff or agglomerate4 ,many admirable examples may be cited from the shore between ,largo and ,st4 ,monans4 ,two illustrations of them are given in ,figs4 #bai and #bba4 ,in ,fig4 #bai the dyke is about four feet broad1 and is seen to present the common transverse jointing as it pursues its way through the tuff4 ,white veins of calcite along its margin serve to define its limits4 ,its position in reference to the general body of the neck is shown in the ground-plan ,fig4 #bbd4 ,the second instance 7,fig4 #bba7 is that of a dyke of basalt only one foot wide1 which runs like a wall up the agglomerate of the ,kincraig neck near ,elie4 ,it is seen at the bottom of the cliff projecting from the agglomerate2 but higher up it has decayed1 leaving its fissure as a gaping chasm4 ,here the general character of the pyroclastic material is well brought out4 ,one or two large blocks may be seen imbedded in it1 the largest lying above where the dyke bends away to the left4 ,the intruded masses vary in breadth from mere threadlike veins up to dykes several yards in breadth1 which sometimes expand into large irregular lumps4 ,they generally consist of some form of basalt2 now and then1 as at ,ruddon ,point1 near ,elie1 they are amygdaloidal2 and it may be observed among them1 as among dykes in general1 that where the amygdaloidal texture is developed1 it is apt to occur most markedly in the central part of the vein1 the amygdales running there in one or more lines parallel with the general trend of the mass4 ,that the basalt of these veins and dykes was sometimes injected in an extremely liquid condition is shown by its frequently exceedingly close homogeneous texture4 ,within the neck on the shore to the west of ,largo1 the basalt assumes in places an almost flinty character1 which here and there passes into a thin external varnish of basalt-glass4 ,a farther indication of the liquidity of the original rock seems to be furnished by the great number of included extraneous fragments here and there to be observed in the basalt4 ,but besides basalt1 other materials may more rarely be detected assuming the form of dykes or veins within the necks4 ,thus1 at the ,largo neck just referred to1 strings of an exceedingly horny quartz-felsite accompany the basalt--a remarkable conjunction of acid and basic rock within the same volcanic chimney4 ,to the east of ,elie some dykes1 which stand out prominently on the beach from a platform worn by the sea in a neck1 consist of an extremely compact volcanic mudstone1 stuck full of the worn twin crystals of orthoclase and pieces of hornblende and biotite already noticed4 ,so like is this rock to one of the decomposing basalts that its true fragmental nature may easily escape notice1 and it might be classed confidently as a somewhat decayed basalt4 ,a considerable amount of a similar fine compact mudstone is to be seen round the edges of some of the ,elie vents4 ,this material must have been injected into open fissures1 where it solidified4 ,there is further evidence of the presence of mud-lava in some of the vents of ,east ,fife1 where these orifices contain a remarkable compact volcanic sandstone1 composed of the usual detritus1 but weathering into spheroidal crusts1 so as externally to be readily mistaken for some form of basalt4 ,illustration3 ,fig4 #bbb4--,radiating columnar dyke in the tuff of a volcanic vent1 ,rock and ,spindle1 two and a half miles east from ,st4 ,andrews4 ,a columnar arrangement may often be observed among the basalt dykes4 ,when the vein or dyke is vertical1 the columns of course seem piled in horizontal layers one above the other4 ,the exposed side of the dyke then reveals a wall of rock1 seemingly built up of hexagonal or polygonal1 neatly fitting blocks of masonry1 as in the ,lower ,carboniferous vent of the ,binn of ,burntisland 7,figs4 #aff1 #afh74 ,an inclination of the dyke from the vertical throws up the columns to a proportional departure from the horizontal4 ,sometimes a beautiful fan-shaped grouping of the prisms has taken place4 ,of this structure the ,rock and ,spindle1 near ,st4 ,andrews1 presents a familiar example 7,fig4 #bbb74 ,much more striking1 however1 though less known1 is the magnificent basalt mass of ,kincraig1 to the west of ,elie1 where the columns sweep from summit to base of the cliff1 a height of fully #aej feet1 like the ,orgues d',expailly1 near ,le ,puy in ,auvergne4 ,the general position of this basalt in the vent is represented in the section 7,b1 ,fig4 #bah74 ,the curvature of the basalt is shown in ,fig4 #bbc1 which is taken from the ,elie side looking westward1 beyond the intrusions1 to the picturesque cliffs of tuff4 ,the details of the cliff are given in ,fig4 #bbe4 ,that many of the dykes served as lines of escape for the basalt to the outer slopes of the cones is highly probable1 though denudation has usually destroyed the proofs of such an outflow4 ,a distinct radiation of the dykes from the centre of a neck is still sometimes traceable4 ,this structure is most marked on the south cone of ,largo ,law1 where a number of hard ribs of basalt project from the slopes of the hill4 ,their general trend is such that if prolonged they would meet somewhere in the centre of the cone4 ,on the south-east side of the hill a minor eminence1 termed the ,craig ,rock1 stands out prominently 7,fig4 #bji74 ,it is oblong in shape1 and1 like the dykes1 points towards the centre of the cone4 ,it consists of a compact columnar basalt1 the columns converging from the sides towards the top of the ridge4 ,it looks like the fragment of a lava-current which flowed down a gully on the outer slope of the cone 7,b' in ,fig4 #bbf74 ,veins of basalt are not confined to the necks1 but may be seen running across the surrounding rocks4 ,the shore at ,st4 ,monans furnishes some instructive examples of this character4 ,as the veins thin away from the main mass of basalt they become more close-grained and lighter in colour1 and when they enter dark shales or other carbonaceous rocks they pass1 as usual1 into the white earthy clay-like white-trap4 ,the influence of carbonaceous strata in thus altering basic dykes and sills may be instructively studied along the shore of the ,east of ,fife4 ,a good instance occurs near ,st4 ,monans ,church 7,fig4 #bbg71 where a vein of white-trap traverses black shales which have been somewhat jumbled4 ,illustration3 ,fig4 #bbc4--,view of part of the shore front of the great vent at ,kincraig1 looking westward1 with the columnar basalt in front4 ,in a modern volcano no opportunity is afforded of examining the contact of the erupted material with the rocks through which the vent has been opened4 ,but in the basin of the ,firth of ,forth1 within the area now under description1 a numerous series of coast-sections lays bare this relation in the most satisfactory manner4 ,the superincumbent cones of tuff have been swept away1 and we can examine1 as it were1 the very roots of the old volcanoes4 ,the margin of a neck or volcanic vent is thus found to be almost always sharply defined4 ,the rocks through which the funnel has been drilled have been cut across1 as if a huge auger had been sunk through them4 ,this is well displayed in the beautifully perfect neck already cited at ,newark ,castle1 near ,st4 ,monans 7,fig4 #bbd74 ,the strata through which this neck rises consist of shales1 sandstones1 thin coal and encrinal limestones1 dipping in a westerly direction at angles ranging from #be to #fj4 ,at the south end of the neck they are sharply truncated1 as if by a fault4 ,elsewhere they are much jumbled1 slender vein-like portions of the tuff being insinuated among the projecting strata4 ,a large vertical bed of sandstone1 #bd yards long by #g yards broad1 stands up as a sinuous reef on the east side of the vent 7s74 ,it is a portion of some of the surrounding strata1 but1 so far as can be seen at the surface1 is entirely surrounded with agglomerate4 ,here and there the shales have been excessively crumpled1 and at the north end have been invaded by a vein of basalt which1 where it runs through them1 assumes the usual clay-like character4 ,the strata have been blown out1 and their place has been occupied by a corresponding mass of volcanic agglomerate4 ,but their remaining truncated edges round the margin of the orifice have undergone comparatively little alteration4 ,in some places they have been hardened1 but their usual texture and structure remain unaffected4 ,illustration3 ,fig4 #bbd4--,plan of volcanic neck on beach near ,st4 ,monans4 ,t1 ,neck of tuff enclosing a mass of sandstone 7s71 and piercing sandstones and shales ,with beds of limestone1 7l l71 and a thin seam of coal 7c72 ,b1 ,basalt white-trap dyke4 ,the arrows show the dip of the strata4 ,in a few examples1 the progress of denudation has not advanced so far that the cone cannot still be partially made out amidst its surrounding masses of tuff4 ,one of the most interesting of these is ,largo ,law1 of which an outline has been given in ,fig4 #bji4 ,the accompanying section 7,fig4 #bbf7 represents what appears to me to be the structure of this hill4 ,each of the two now conjoined cones was probably in succession the vent of the volcano4 ,the southern and rather lower eminence1 as already mentioned1 is traversed by rib-like dykes of basalt1 which point towards its top1 where there is a bed of the same rock underlying a capping of tuff4 ,on its eastern declivity lies the basalt stream already described 7p4 #hg74 ,the higher cone is surmounted by a cake of basalt which1 as ,i have above suggested1 may have solidified at the bottom of the latest crater4 ,of course all trace of the crater has disappeared1 but the general conical form of the volcanic mass remains4 ,doubtless1 still more of the old volcano would have been removed by denudation but for the protection afforded to the tuff by the intrusion of the basalt4 ,the upper dotted lines in the figure are inserted merely to indicate hypothetically how the cone may originally have stood4 ,on the west side the sheets of tuff which were thrown out over the surrounding country have been almost entirely removed1 but on the east and south they still cover an extensive area4 7,see ,fig4 #bjh74 ,illustration3 ,fig4 #bbe4--;,columnar basalt in the neck of ,kincraig1 ,elie1 seen from the west4 ,illustration3 ,fig4 #bbf4--,section across ,largo ,law4 l l1 ,lower ,carboniferous strata2 t1 tuff of cones2 t'1 tuff of plain beyond the cones2 ,b ,b1 basalt ascending vents and sending out veins3 ,b'1 basalt which has probably flowed out at the surface4 ,the dotted lines are suggestive of the original outline of the hill4 7#b7 ,sills4--,in the ,clyde coal-field and in the basin of the ,firth of ,forth1 among the vast number of sills which there traverse the ,carboniferous formations1 it is possible that some belong to the ,permian volcanic period 7see vol4 i4 p4 #dgd74 ,where the sheets have been intruded along horizons that lie below the upper stratigraphical limit of the puy eruptions1 they may not unnaturally be held to belong to these manifestations of volcanic energy1 though it is obviously quite conceivable that some of them may be of much later date4 ,but where they lie above the highest platforms of ,carboniferous lavas and tuffs1 they may be assigned to a younger volcanic period4 ,we know as yet of only two such periods after the deposition of the ,carboniferous ,limestone series in ,scotland--,permian and older ,tertiary4 ,unless1 therefore1 these higher sills formed part of some other display of subterranean activity which is not known to have culminated in eruptions at the surface1 they must be looked upon as probably either ,permian or ,tertiary4 ,in the great coal-field of ,stirlingshire and ,lanarkshire1 among the large sills that break into the ,millstone ,grit and the ,coal-measures1 one lies entirely in the ,coal-measures1 and covers about six square miles of ground1 stretching from near ,caldercruix ,station1 a little east of ,airdrie1 to near ,kirk of ,shotts1 a distance of about four miles4 ,a group of smaller sheets1 possibly connected with the larger mass1 runs for four miles further west to beyond ,new ,monkland4 ,another chain of sills1 which may also be part of the same great intrusion1 extends from the ,cant ,hills1 near the ,kirk of ,shotts1 for more than eight miles in a north-easterly direction4 ,the largest mass in this chain stretches from ,blackridge1 west of ,bathgate1 for upwards of three miles1 covering an area of about three square miles and terminating on the north at the line of dislocation which has been followed by one of the east and west dykes4 ,another large sill1 which appears nearly two miles further east on the north side of that dyke1 lies on a lower stratigraphical horizon1 for it cuts the ,carboniferous ,limestone series1 and does not reach the top of the ,millstone ,grit4 ,this sill is cut through by two of the later dykes4 ,illustration3 ,fig4 #bbg4--,vein of white-trap cutting black carbonaceous shales1 a little west from ,st4 ,monans ,church4 ,that these great intrusions took place later than the deposition of the ,coal-measures is obvious4 ,there is no satisfactory evidence to enable us to decide to which of the two post-,carboniferous volcanic periods they may with most probability be assigned4 ,as one of them is distinctly cut by dykes that have been referred to the ,tertiary series1 it might be plausibly argued that it at least is of pre-,tertiary date1 and therefore probably ,permian4 ,on the other hand1 as will be shown in a later chapter1 some portion of the sills appears to be connected with the younger or ,tertiary dykes4 ,this problem must for the present remain unsolved4 ,in ,ayrshire where1 as already described1 basic sills traverse the ,permian volcanic series1 other large intrusive sheets are found around the ,permian basin4 ,on the north side an important group of them1 passing through the ,coal-measures into the ,carboniferous ,limestone series1 runs from ,troon eastward for more than eight miles to beyond ,craigie4 ,on the south side a much more extensive series may be traced from the ,river ,ayr southwards into the ,dalmellington coal-field1 and thence north-eastwards in a wide semicircular sweep into the coal-field of ,new ,cumnock and ,airds ,moss4 ,that some of these sills proceed from the ,permian necks has been definitely ascertained1 and this fact has been already alluded to in connection with the vents4 ,i have little doubt that the great majority1 if not the whole1 of these intrusive sheets are to be referred to the ,permian period4 ,some of the sills must be later than a part of the ,permian volcanic eruptions1 for they are found in at least three places intercalated in the zone of lavas and tuffs4 ,but no instance has been observed of their traversing the basin of ,permian sandstone which overlies that zone1 though a few dykes1 possibly of ,tertiary age1 do cut this sandstone4 ,,chapter ,,xxxii ,,permian ,,volcanoes ,,of ,,england ,the ,devonshire ,centre--,eruptive ,rocks of the ,midland ,coal-fields4 ,from the south of ,scotland we need to pass to the extreme south-west of ,england before we again encounter a group of volcanic rocks which may be referred with some confidence to the ,permian period4 ,an interesting group of lavas and tuffs has been preserved in some of the valleys over a limited area in the east of ,devonshire4 ,the ,midland coal-fields1 however1 are traversed by a series of basic eruptive rocks which are younger than the ,coal-measures1 and may possibly be ,permian4 ,their mode of occurrence1 and the arguments regarding their geological age1 will be given in the present chapter4 #a4 ,,devonshire ,the counties of ,devon and ,cornwall furnish one of the most striking examples to be met with in ,britain of the persistence of volcanic action over a limited area through a long succession of geological periods4 ,the extensive eruptions in ,devonian time were followed after a long interval by a diminished series in the ,carboniferous period4 ,but the subterranean energy was not then wholly exhausted1 for it showed itself on a feeble scale in at least one limited tract of the same region during the ,permian period4 ,thus throughout the later half of ,palozoic time the extreme south-west of ,england continued to be a theatre of volcanic action4 ,the geological age of the igneous rocks now to be referred to depends upon the particular place in the geological record to which we assign the remarkable breccias and sandstones with which they are associated4 ,by many geologists who have been unable to recognize any true break in the red rocks from their base up to the bottom of the ,lias1 these strata have been grouped as one great series referable to the ,new ,red ,sandstone or ,trias4 ,this is the classification adopted on the one-inch maps of the ,geological ,survey4 ,on the other hand1 various able observers have pointed out the close resemblance of the coarse and fine breccias at the bottom of the series to recognized ,permian deposits in the centre of ,england and to parts of the typical ,rothliegende of ,germany4 ,i need only refer to the strongly expressed views of ,murchison1 in which1 as he stated in his ,siluria1 he entirely agreed with ,conybeare and ,buckland1 who1 after a journey in ,germany in #ahaf1 distinctly identified the ,heavytree conglomerate1 near ,exeter1 with the ,rothliegende of the ,germans4#ajd ,in the absence of any fossil evidence1 we have only lithological characters and sequence to guide us1 and though the known facts hardly warrant a very positive opinion1 my inclination is to regard these red ,devonshire breccias as probably ,permian1 and to follow ,murchison in looking upon their associated igneous masses as furnishing additional reason for assigning them to that particular geological platform4#aje ,footnote #ajd3 ,siluria1 #dth edit4 7#ahfg71 p4 #ccc4 ,see also ,berger1 ,trans4 ,geol4 ,soc4 vol4 i4 7#ahaa71 pp4 #ih-ajb2 ,conybeare and ,phillips1 ,geology of ,england and ,wales1 p4 #cac1 footnote2 ,de la ,beche1 ,report on the ,geology of ,cornwall1 ,devon and ,west ,somerset 7#ahci71 chap4 vii4 p4 #aic4 ,messrs4 ,hull and ,irving 7,quart4 ,journ4 ,geol4 ,soc4 vol4 xlviii4 #ahib1 pp4 #fj1 #fh7 have more recently discussed the subject1 and follow the view of ,murchison4 ,footnote #aje3 ,murchison cogently argued that as no signs of volcanic activity were known in the ,trias1 but were abundant in the ,permian system1 the ,devonshire rocks might be regarded as appertaining to the older series1 op4 cit4 ,triassic volcanic rocks1 however1 are now well known on the ,continent4 ,no proper account has yet been written of the volcanic group which ,i now propose to describe4#ajf ,de la ,beche was1 ,i think1 the first to recognize the true volcanic nature of the rocks and their contemporaneous interstratification in the red sandstone series4#ajg ,as traced by him on the ,geological ,survey maps1 these rocks lie at or near the base of the red sedimentary deposits1 resting sometimes directly on the ,culm-measures1 sometimes on an intervening layer of red strata4 ,he found them in three separate districts in the neighbourhood of ,exeter1 the most northerly lying near ,tiverton1 the central extending from ,kellerton for a few miles up the ,yeo ,valley1 beyond ,crediton1 and the third stretching from the ,city of ,exeter to ,pen ,hill1 about five miles to the south-west4 ,he recognized the amygdaloids as slaggy lavas1 and saw that the volcanic breccias and tuffs are interleaved with the sandstones4 ,with regard to the probable vents from which these materials were ejected1 he thought that the chief centre of activity lay at ,kellerton ,park1 while in other localities he believed the bosses of igneous rock to descend in mass downwards1 as if filling up some crater or fissure through which these rocks had been vomited4#ajh ,he speaks also of quartziferous porphyries occurring among them1 a statement which1 if petrographically accurate1 would suggest the uprise of a later more acid lava in some of the vents4 ,footnote #ajf3 ,an outline of some of their characters will be found in a paper by ,mr4 ,w4 ,vicary in ,trans4 ,devonshire ,assoc4 #ahfe1 vol4 i4 part iv4 p4 #dc4 ,footnote #ajg3 ,see his ,report cited in the note above4 ,de la ,beche quotes ,j4 ,j4 ,conybeare as pointing out the intimate connection of these igneous and stratified rocks 7,annals of ,philosophy1 #bnd series1 vol4 ii4 7#ahba7 p4 #afe72 but this author wrote at the time of the ,plutonist and ,neptunist controversy1 and does not commit himself to any distinct expression of opinion on the subject4 ,footnote #ajh3 ,report1 p4 #bja4 ,more recently the ground has been revised by ,mr4 ,w4 ,a4 ,e4 ,ussher of the ,geological ,survey1 who has ascertained that the volcanic rocks appear in many more places than those where they were noted on the older maps1 and likewise extend for some miles further to the north and west4 ,it now appears that in the central and chief district the lavas can be followed westward from ,spray ,down near ,kellerton to ,greenslade near ,north ,tawton1 a distance of about twenty-one miles4 ,their most northerly outcrop is at ,thorn above ,loxbere in the ,tiverton district1 and their most southerly visible portion passes under the ,cretaceous rocks of ,pen ,hill4 ,the distance between these extreme points is likewise about twenty-one miles4 ,the whole display of volcanic phenomena is comprised within an area of less than #djj square miles4 ,one of the most obvious features in this volcanic tract is the way in which the erupted materials lie along the lines of hollow or valley in which the red rocks were deposited4 ,this is most distinctly exhibited in the central district4 ,here a belt of breccias and sandstones1 varying from one to three and a half miles in breadth1 runs for about five and twenty miles westward in a depression of the ,culm-measures4 ,at intervals1 the lavas which lie near the base of the red rocks crop out along the margin of the belt throughout most of its extent4 ,but they do not spread out over the older rocks1 and they have evidently been erupted from orifices situated along the line of the valley4 ,it is another example of the relation between the trend of hollows and the outbreak of volcanic vents1 which ,i have referred to as so strikingly displayed in the distribution of the ,permian volcanic rocks of south-western ,scotland4 ,the volcanic materials of the ,devonshire ,permian district consist mainly of lavas1 but include also red sandy and gravelly tuffs4 ,the whole volcanic group is remarkably thin1 never attaining even the limited development of the ,ayrshire series4 ,no adequate petrographical investigation of these rocks has yet been made4 ,externally1 as seen in the quarries and lanes1 the lavas present the closest resemblance to those of the ,permian basins of ,ayrshire and ,nithsdale4 ,they show considerable differences of texture even within the same mass1 some portions being dull1 fine-grained purplish-red rocks1 with scattered pseudomorphs of hmatite and a few porphyritic felspars1 other parts passing into an exceedingly coarse amygdaloid or slaggy pumice4 ,dr4 ,hatch1 after a microscopical examination of a small collection of specimens1 found that while most are olivine-basalts1 containing ferruginous pseudomorphs after olivine 7,raddon ,court1 ,pocombe1 and near ,budlake71 others are true andesites 7,ide1 ,kellerton ,park7 and even mica-trachytes 7,copplestone1 near ,knowle ,hill74#aji ,as already remarked1 some of the older writers mention the existence of quartz-porphyries4#aaj ,footnote #aji3 ,geol4 ,mag4 #ahib1 p4 #bej4 ,the rocks have been more recently described by ,mr4 ,b4 ,hobson1 ,quart4 ,jour4 ,geol4 ,soc4 vol4 xlviii4 7#ahib71 p4 #ejb4 ,the rock of ,kellerton ,park is called by ,mr4 ,hobson mica-augite-andesite1 and he gives a chemical analysis of it by ,mr4 ,e4 ,haworth1 op4 cit4 p4 #ejg4 ,mr4 ,watts has lately found one of the orthoclase rocks to be rich in olivine4 ,footnote #aaj3 ,see ,de la ,beche1 ,report1 pp4 #bjc1 #bjd4 ,my colleague1 ,mr4 ,ussher1 found close to the ,thurlestone outlier of conglomerate near ,kingsbridge1 ,devonshire1 a small boss of quartz-porphyry which rises through and alters the ,devonian rocks4 ,the actual junction of this mass with the conglomerate is not seen1 nor have any fragments of the porphyry been noticed among the pebbles4 ,mr4 ,ussher informs me that in the quarry the visible exposure of the acid rock is surrounded an covered by mica-porphyrite1 probably andesite4 ,the geographical conditions in which the red rocks of ,devonshire accumulated were those so characteristic of the ,permian and ,trias formations throughout ,britain4 ,the red sandstones and sandy marls gathered in inland basins1 where the water seems to have become too saline and bitter to support animal life4 ,the strata are consequently singularly devoid of organic remains4 ,the climate was probably arid1 and the absence or scarcity of traces of terrestrial vegetation indicates that the land around the water-basins stretched in wide sandy and rocky wastes4 ,in the dry atmosphere and under the influence of rapid radiation the cliffs and crags of ,culm-measures would disintegrate into angular rubbish1 and this material1 slipping into the lakes or washed down by occasional rain-storms1 forms now the breccias that constitute so typical a feature in the ,permian system4 ,illustration3 ,fig4 #bbh4--,section at ,belvedere1 ,s4,w4 of ,exeter4 a1 ,culm-measures2 b1 breccia and marls2 c1 lavas2 d1 red pebbly sandstones4 ,illustration3 ,fig4 #bbi4--;,diagram to show the unconformability and overlap of the ,permian rocks in the ,crediton ,valley4 a1 ,culm-measures2 b1 breccias and sandstones2 c1 lava-group2 d1 breccias with fragments of lava passing up into sandstones and marls 7e74 ,it was while this geographical type continued in the ,south-west of ,england that the volcanic eruptions took place which we are now considering4 ,de la ,beche correctly referred these eruptions to the early part of the red sandstone series4 ,a brief examination of the ground suffices to show that although1 as he pointed out1 the volcanic rocks lie towards the base of that series1 as shown in ,fig4 #bbh1 they do not all occupy the same platform4 ,that in some cases the lavas lie directly on the ,culm-measures1 while in others they are separated from these strata by #ajj feet or more of red sandstones and breccias 7,fig4 #bbi71 would not in itself be proof of any difference of age or stratigraphical position in the igneous rocks1 for the floor on which the ,permian formations were here laid down can be shown to have been singularly uneven4 ,prominent hills of ,culm grit1 several hundred feet high1 rose above the basins in which the earliest ,permian sediments were deposited1 and these eminences were gradually submerged and buried under the detritus4 ,but that the volcanic zone includes in some places more than one outflow of lava with layers of sandstone1 breccia and tuff between the successive sheets may be proved in different parts of the district4 ,thus the two conspicuous hills at ,kellerton are composed of several sheets of highly slaggy lava1 separated by breccia1 and a third much thinner sheet lies above these1 intercalated in a mass of breccia1 sandstone and sandy tuff 7,fig4 #bcj74 ,again1 at ,budlake the sandstones and fine breccias include a thin band of vesicular lava1 while farther to the east they are interrupted by a higher and thicker zone of similar material4 ,illustration3 ,north ,hill ,south ,hill ,fig4 #bcj4--,section of the volcanic series at ,kellerton1 ,devonshire4 a1 ,breccias and sandstones2 b1 lavas4 ,these igneous sheets can be shown by many interesting sections to have been poured out contemporaneously with the deposit of the sedimentary material among which they occur4 ,at ,crabtree1 for instance1 near ,kellerton1 the uppermost lava is a thin sheet of highly slaggy texture1 which rests immediately on the gravelly red sandstone and catches up parts of it1 while the pebbles include fragments of some of the andesites below4 ,the dark lavas are occasionally traversed by veins of fine hard sandstone1 which descending from above1 like those in the ,old ,red ,sandstone and ,permian lavas of ,scotland1 have been produced by the silting or drifting of fine sand into cracks in the lava1 before the igneous material was entirely buried4 ,these features are well exposed in the high ridge of the ,belvedere near ,exeter 7,fig4 #bbh71 where1 over a thin and inconstant band of red breccia and marl which rests on the upturned ends of the ,culm-measures1 a band of dull-red andesite may be seen4 ,this rock1 partly compact and partly highly amygdaloidal1 is in some portions full of irregular fissures and cavities filled with sandstone4 ,nowhere among the ,palozoic volcanic rocks of ,britain are more remarkable examples of the slaggy structure to be found than in these ,devonshire lavas of probably ,permian age4 ,i would especially cite the rock of ,knowle ,farm1 a few miles to the west of ,crediton1 as in part a mere spongy pumice1 blocks of which would originally have floated in water4 ,one of the best sections in the district for the exemplification of the internal structures of these lavas is that in the large quarry at the top of ,posbury ,hill4 ,on the west side of this quarry the rock is tolerably compact1 but contains vesicles and irregular steam-holes4 ,on the east side it passes upward and laterally into a coarse agglomerate of its own fragments1 and in its mass it encloses similar agglomerate4 ,no sharp passage can be traced between the two rocks4 ,so far as ,i could judge1 it seemed to me that the lava had broken up as it moved along1 possibly shattered by coming in contact with water4 ,the agglomerate is overlain by some reddish ashy sandstone1 which fills up the interstices between the slags1 and is immediately covered by a bed of lilac andesite1 marking another distinct outflow4 ,illustration3 ,fig4 #bca4--,section of agglomerate overlain with sandstone and andesite1 ,posbury1 ,crediton4 ,as in ,ayrshire1 the lavas of ,devonshire are not accompanied by any thick accumulation of tuff4 ,the fragmentary discharges consisted in both areas of fine dust and gravelly detritus of small lapilli1 which were not ejected in such quantities as entirely to conceal the ordinary non-volcanic sediment of the water-basin4 ,the dust and cinders mingled with the red sand and angular scree-material1 so that we now see a group of red1 somewhat ashy sandstones and breccias4 ,among the component fragments of the breccias1 a considerable variety of igneous material may be observed4 ,while the most of the non-volcanic stones may have been derived by ordinary processes of weathering from rocks exposed at the surface1 it is by no means improbable that some of them1 including even pieces of ,culm grit1 killas and baked slate1 may have been ejected from volcanic vents4#aaa ,footnote #aaa3 ,on the composition of the ,devonshire breccias see ,mr4 ,r4 ,n4 ,worth1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xlvi4 7#ahij71 p4 #fi4 ,this author has adopted the view that the granite of ,dartmoor represents the neck of a great volcano from which these later volcanic materials were ejected4 ,but all the evidence seems to me in favour of numerous small vents situated not far from the outcrops of the lavas1 as stated in the text4 ,see ,mr4 ,b4 ,hobson1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xlviii4 7#ahib71 p4 #dih4 ,the ,dartmoor granite is later than the surrounding ,carboniferous rocks1 but no good evidence has been obtained to connect it with the ,permian volcanic phenomena of ,devonshire4 ,taking the volcanic rocks of this district as a whole1 ,i regard them as the mere edges of sheets that have flowed from vents which not improbably lie concealed somewhere along the centres of these old ,permian valleys4 ,no visible necks have been described from any part of the area1 and though ,i have not examined the whole of it1 nothing of that nature was detected by me either in the ,crediton ,valley or between ,silverton and the ,exeter neighbourhood4 ,the ,tiverton district1 which has not yet been searched1 appears to be the only tract where any chance remains of finding some of the vents4 ,no indication of any sills has been met with among the ,devonshire ,permian rocks4 ,none of the lavas which ,i have seen have the internal characters of true sills1 while in the field their association with the sandstones and breccias in no observed case points to intrusion4 ,though much remains to be done in this region before an adequate account can be given of the interesting series of eruptions which concludes the long volcanic history of the ,south-west of ,england1 enough is known to indicate the general character of the phenomena4 ,the eruptions were on even a feebler scale than those of the ,permian period in ,scotland1 but they seem to have resembled them in their general character4 ,small puy-like vents were opened1 from which dark scoriaceous lavas and showers of gravelly tuff and stones were discharged over the floor of the inland sea or lake-basin in which the red sandstones and breccias were accumulated4 ,these outflows and explosions took place too1 as in ,scotland1 towards the beginning of the deposition of the red strata1 and entirely ceased long before that deposition came to an end4 ,in each area the eruptions mark the close of ,palozoic volcanic activity in ,britain4 ,the varied and recurrent volcanic episodes which distinguished each successive geological period from the ,archan onwards now definitely terminate1 not to be resumed until after the passing of the whole of the vast cycle of ,mesozoic ages4 #b4 ,,eruptive ,,rocks ,,in ,,the ,,midland ,,coal-,,fields ,between the thick and thoroughly marine development of the ,carboniferous ,limestone in ,derbyshire and in ,south ,wales1 there lies the region1 already referred to1 wherein both the ,carboniferous ,limestone and ,millstone ,grit die out against what must have been a ridge of land or group of islands that stretched in a general east and west direction from the high grounds of ,wales through ,shropshire1 ,staffordshire and ,leicestershire4 ,on the slopes of this ridge the limestone is gradually overlapped by the ,millstone ,grit1 and both are in turn overlapped by the ,coal-measures1 which are then found lying immediately on the more ancient rocks of the region--,cambrian or pre-,cambrian1 ,silurian and ,old ,red ,sandstone4 ,the gradual subsidence that led to the deposit of several thousand feet of ,carboniferous strata over the regions to north and south1 before the beginning of the ,coal-measure period1 does not seem to have sensibly affected the persistence of this old terrestrial surface1 which probably lay on an axis of upward movement1 so that1 amidst the surrounding depression1 its position above water was on the whole maintained4 ,but there are indications that the inequality of movement in this part of the earth's crust was of much older date than the ,carboniferous period4 ,the ,old ,red ,sandstone is conformably continuous below the base of the ,carboniferous system1 and in ,wales is estimated to be some #aj1jjj feet thick4 ,no break has yet been detected in this vast accumulation of sedimentary material1 though it is highly probable that some such unconformability must exist in it as that between the ,scottish ,lower ,old ,red ,sandstone1 which passes down into the ,upper ,silurian shales1 and ,upper ,old ,red ,sandstone1 which graduates upward into the base of the ,carboniferous formations4 ,but even if such a break should be discovered1 it will not account for the position of the ,coal-measures on ,cambrian or even perhaps older rocks4 ,it is hardly conceivable that1 had these rocks been covered with a full development of ,old ,red ,sandstone1 they could have been stripped of it by denudation before the deposition of the ,coal-measures4 ,it seems much more probable that the discrepancy in the terrestrial movements had commenced in ,old ,red ,sandstone time1 and that these ridges of ancient ,palozoic rocks never sank below the waters in which the vast thickness of red sandstones1 marls and conglomerates was laid down4#aab ,footnote #aab3 ,see a discussion of this subject in ,jukes' ,preface to his ,south ,staffordshire ,coal-field4 ,but apart from the question of its antiquity1 this tract of persistent land has a special interest in the history of volcanic action in ,britain1 for it was the scene of some remarkable protrusions of eruptive material which took place after a part1 and possibly after the whole1 of the ,coal-measures were accumulated4 ,the date of these protrusions cannot be fixed with greater precision2 but there can be no doubt that they belong to one of the later volcanic periods in the geological history of ,britain1 and the account of them is therefore included in the present ,chapter of this work4 ,in the ,english ,midlands south of ,stafford1 over a tract of country about #gjj square miles in extent1 stretching from ,birmingham on the east1 across the vale of the ,severn1 to the uplands of ,shropshire on the west1 the ,coal-measures1 partly isolated into outliers by denudation and partly separated by overlying younger formations1 are pierced by masses of intrusive igneous rocks4 ,many of these masses have long been familiar to geologists4 ,those1 for example1 of the ,clee ,hills of ,shropshire1 and the ,rowley1 ,barrow and ,pouk ,hills of ,staffordshire and ,worcestershire1 have been frequently described1 their relations to the surrounding strata have been minutely sought out1 their composition has been chemically determined1 and their microscopic structure has been investigated4 ,but they have been studied rather as individual masses of local importance4 ,no attempt has yet been made to ascertain how far they are capable of being grouped together as one connected series1 linked with each other in chemical and mineralogical characters1 and containing a definite record in the volcanic history of the country4 ,this is a task which1 it is to be hoped1 some competent inquirer will before long undertake4 ,in the meantime it is only possible to review here the already published information1 and to gather from it what may at present be surmised to have been the history of these later eruptions of the ,midlands4 ,the areas where the igneous rocks now to be described are exhibited may be conveniently placed in the following five groups3--#ast1 ,titterstone ,clee ,hill2 #bnd1 ,brown ,clee ,hill2 #crd1 ,the ,forest of ,wyre ,coal-field2 #dth1 ,the ,coalbrookdale ,coal-field2 and #eth1 ,the ,south ,staffordshire ,coal-field4 #a4 ,the ,titterstone ,clee ,hill forms a ridge about seven miles long and a mile and a quarter broad1 running in a north-easterly direction over the ,old ,red ,sandstone uplands of the south of ,shropshire4 ,the ground rises gradually towards the south-west1 until it reaches there a height of #aged feet 7,fig4 #bcb74 ,on the north-western side of the ridge1 the last vanishing representative of the ,carboniferous ,limestone can be seen to be overlapped the ,millstone ,grit1 which1 as it is traced towards the south-west1 is in turn overlapped by the ,coal-measures1 and these1 about #djj feet thick1 then rest immediately on the ,old ,red ,sandstone4 ,two sheets of columnar olivine-dolerite1 possibly originally connected1 lie as cakes on the summit and eastern slope of the ridge1 and cover in all a space of about a square mile and a half4 ,the larger sheet1 which varies from #fj to #ahj feet in thickness1 overlies the ,coal-measures1 and the coals of the ,cornbrook coal-field have been worked underneath it4 ,the smaller mass1 which may be #cjj feet in thickness1 forms the summit of the ridge4 ,on its eastern side it reposes on ,coal-measures1 which are there much disturbed2 but on the west side1 where it forms a bold capping to the escarpment1 it is underlain at once by the ,old ,red ,sandstone4 ,there cannot be any doubt that these masses of eruptive material are sills1 which have been injected into the ,carboniferous strata1 and partly between these strata and the ,old ,red ,sandstone4 ,one or more dykes of eruptive rock have been met with in mining1 and the coal on approaching them undergoes alteration4#aac ,footnote #aac3 ,see ,j4 ,r4 ,wright1 ,trans4 ,geol4 ,soc4 7#bnd ser47 iii4 7#ahcb71 p4 #dhg4 ,titterstone ,clee ,hill is shown on ,sheet #ee ,n4,e4 and ,n4,w4 of the ,geological ,survey1 and in ,horizontal ,sections1 ,sheets #cc and #cf1 from which ,fig4 #bcb is reduced4 ,the microscopic structure of the dolerite has been described by ,mr4 ,allport1 ,geol4 ,mag4 #ahgj1 p4 #aei2 ,quart4 ,journ4 ,geol4 ,soc4 xxx4 7#ahgd71 p4 #eej4 ,illustration3 ,fig4 #bcb4--;,diagrammatic section across ,titterstone ,clee ,hill4 #a4 ,old ,red ,sandstone2 #b4 ,carboniferous ,limestone2 #c4 ,millstone ,grit2 #d4 ,coal-measures2 #e #e4 ,columnar olivine-dolerite4 #b4 ,brown ,clee ,hill consists of two outliers of ,coal-measures1 each about a mile long1 placed on the summit of a broad ridge of ,old ,red ,sandstone1 and rising to a height of #ahjj feet above the sea4 ,both of the outliers is capped with a cake of dolerite1 and a third smaller patch of the same material lies on the southern outlier between the cappings4 ,neither at this locality nor around ,titterstone ,clee have any eruptive rocks been observed rising through the older strata4 ,it is evident that in both cases the orifices or fissures up which the molten material rose lie concealed under the surviving cakes of dolerite4#aad ,footnote #aad3 ,brown ,clee ,hill is mapped in ,sheet #fa ,s4,w4 of the ,geological ,survey1 and its structure is shown in ,sheet #cf of the ,horizontal ,sections4 #c4 ,forest of ,wyre ,coal-field4--,on both sides of this extensive tract of ,coal-measures1 the strata near the base of the series are traversed by sills or dykes of olivine-dolerite like that of the ,clee ,hills4 ,the sandstones in contact with the eruptive rock have been indurated4 ,in this district1 also1 the evidence shows that the sheets are intrusive1 and later than the portion of the ,coal-measures there visible4#aae ,footnote #aae3 ,this district is represented in ,sheets #ee ,n4,e4 and #fa ,s4,e4 of the ,geological ,survey4 ,the microscopic structure of the larger mass on the west side of the coal-field1 and the variations in the minute structure of the intrusion which forms a long ridge on the east side1 are described by ,mr4 ,allport1 ,quart4 ,journ4 ,geol4 ,soc4 xxx4 pp4 #eej1 #eea4 #d4 ,coalbrookdale ,coal-field4--,in this interesting district a sill of rather finely crystalline olivine-dolerite1 which is estimated to be nearly #bjj feet thick1 is traceable from near ,little ,wenlock for three miles to the north1 intercalated between the ,carboniferous ,limestone and the ,silurian rocks underneath4 ,it appears to underlie the western part of the ,coal-field1 for it is exposed by denudation in several valleys between ,little ,wenlock and ,great ,dawley4 ,owing to the thinning out of the ,carboniferous ,limestone in an easterly direction1 the sill gradually comes to have the ,millstone ,grit on its upper surface1 and at one point is represented on the ,geological ,survey map as even intruded into the ,coal-measures4 ,here again we have an intrusive sheet of later date than at least the earlier part of the ,coal-measures1 and no evidence of any superficial outflow of volcanic material4#aaf ,footnote #aaf3 ,the ,coalbrookdale coal-field has been described by ,sir ,joseph ,prestwich1 ,trans4 ,geol4 ,soc4 7#b7 v4 p4 #dbh2 and ,prof4 ,e4 ,hull1 ,quart4 ,jour4 ,geol4 ,soc4 xxxiii4 7#ahgg71 p4 #fbi4 ,the minute structure of the sill at ,little ,wenlock is referred to by ,mr4 ,allport1 op4 cit4 p4 #eej4 ,the ground is mapped on ,sheet #fa ,n4,e4 of the ,geological ,survey1 and its structure is shown on ,sheet #ed of the ,horizontal ,sections4 #e4 ,south ,staffordshire ,coal-field4--,this district1 in respect to its igneous intercalations1 has been much more fully examined and described than any of the others4 ,it forms the subject of an exceedingly able memoir by ,jukes1 who carefully studied its geology and delineated it on the maps and sections of the ,geological ,survey4 ,since his time the rocks have been studied microscopically1 but no material facts regarding the stratigraphy have been obtained in addition to those which he patiently collected and generalized upon4#aag ,footnote #aag3 ,jukes1 ,south ,staffordshire ,coal-field1 ,mem4 ,geol4 ,surv4 #bnd edit4 7#ahei74 ,the area is embraced in ,sheet #fb ,n4,w4 and ,s4,w4 of the ,geological ,survey1 and is illustrated in ,sheets #bc1 #bd and #be of the ,horizontal ,sections4 ,this coal-field is above #bj miles long and #e miles broad4 ,its strata rest unconformably on ,upper ,silurian strata1 which1 as part of the ancient ridge or island already referred to1 project here and there from amidst the ,coal-measures4 ,the boundaries of the field on the east and west sides are chiefly made by faults which bring down ,permian and ,triassic formations against the ,carboniferous strata4 ,throughout this coal-field sheets of igneous rock are abundant4 ,in the detailed account of them given by ,jukes in his admirable essay on the ,south ,staffordshire ,coal-field1#aah he distinguished two kinds of igneous material--basalt1 which comes out at the surface1 and sometimes overlies the ,coal-measures in large cakes like that of the ,rowley ,hills1 which extends for two miles in one direction and more than a mile in another2 and greenstone1 which burrows among the coal-bearing strata1 and gives off dykes and veins of white rock-trap4 ,there does not appear1 however1 to be any essential difference in composition1 age or origin between these contrasted kinds of igneous material4 ,they not improbably all belong to one series of extrusions1 their distinctions being due rather to the conditions under which they were erupted1 and in particular to their comparative thickness1 and the influence of adjacent coals and carbonaceous shales upon them4 ,footnote #aah3 ,op4 cit4 p4 #aag4 ,the igneous rocks seen at the surface in this district form a series of well-marked eminences4 ,of these the largest extends as a ridge from ,dudley to beyond ,rowley ,regis1 a distance of more than two miles4 ,to the west of this tract1 a number of small patches of the same material crop out at the surface1 the most important forming ,barrow ,hill4 ,six miles farther north another group of similar patches may be seen4 ,of these the largest occurs at ,wednesfield1 but the most noted forms the ,pouk ,hill1 which has long been noted for the beauty of its columnar structure4 ,the sheets of greenstone met with in the coal-field are more numerous and extensive than the detached areas of more compact rock visible above ground1 a single sheet being sometimes traceable in the coal-workings for two miles in one direction4 ,the eruptive rocks of this district1 when examined in their freshest form1 consist of well-preserved olivine-dolerite4 ,an examination of the greenstone and the white rock-trap1 which runs in fingers and threads through the coal1 shows that these are really the same dolerite which has undergone alteration1 the ferruginous silicates having especially been decomposed4#aai ,footnote #aai3 ,allport1 ,quart4 ,journ4 ,geol4 ,soc4 xxx4 7#ahgd71 p4 #edg4 ,chemical analysis also shows the identity of the rocks and the nature of the alteration of the white rock4 ,see ,jukes1 ,south ,staffordshire ,coal-field1 pp4 #aag1 #aah4 ,the sills of greenish decomposed material that have been injected amongst and alter the coals1 vary from #ae feet to #hj or #ij feet in thickness4 ,the largest of the dolerite cakes on the surface1 that of the ,rowley ,hills1 is somewhat irregular in its thickness1 but may reach as much as #ajj feet4 ,that nearly the whole of the igneous material is intrusive is admitted by all observers who have studied the ground4 ,the manner in which the basalts and greenstones send out veins into the ,coal-measures shows conclusively that they have been injected into the strata4 ,the only rock about which some doubt has been expressed is that of the ,rowley ,hills1 which ,jukes was disposed1 though not without some hesitation1 to consider as part of an actual lava-stream4 ,he based this inference chiefly on the occurrence1 immediately under the dolerite1 of what he looked upon as a trappean breccia or brecciated ash1 containing rounded and angular fragments of igneous rock lying in a brown rather ferruginous paste1 that looks like the debris of a basaltic rock4#abj ,this breccia he regarded as belonging to and passing into the ,coal-measures1 and he was thus inclined to regard the dolerite as a lava of ,coal-measure age4 ,footnote #abj3 ,op4 cit4 p4 #aai4 ,it is possible1 however1 that the trappean breccia may be of the same nature as the uncompressed balls of basalt bedded in a mass of decomposed basalt or basaltic 'wacke' or clay#aba--that is1 a decayed contact layer of the eruptive rock4 ,but if it be regarded as the fragmental accompaniment of a lava-stream1 it can hardly belong to the ,coal-measures4 ,if the dolerite had been a lava of that age1 it ought to be found lying conformably on the ,coal-measures4 ,but this it does not appear to do4 ,making every allowance for the way in which an advancing current of lava might plough up soft sediment on the bottom of the sea or of a lake1 we can hardly thus account for the very uneven surface of ,coal-measures on which the sheet of igneous rock rests4 ,if the ,rowley rock be looked upon as a lava which flowed out at the surface1 it must1 ,i think1 be assigned to a time subsequent to that of the ,coal-measures1 when these strata had been upraised and had suffered some amount of denudation4 ,i confess1 however1 that the petrographical characters of the rock1 the alteration of the coals which have been worked underneath it1 and the abundant veins of white rock which there traverse the seams1 induce me to regard this rock as forming no exception to the general rule in the ,midlands1 but as having been intruded as a sill1 now laid bare by denudation4 ,its fresher condition may arise from its thickness1 or from some other circumstance which has not been ascertained4 ,footnote #aba3 ,op4 cit4 p4 #abf4 ,we have now to consider the probable geological date of the various intrusions of basic igneous material which can be traced over so wide an area in the centre of ,england4 ,in discussing the subject1 ,jukes pointed out that in the surrounding district no igneous rocks of any kind are found in any formation newer than the ,coal-measures4#abb ,this statement is1 with the exception of one locality1 undoubtedly true4#abc ,but on any view there must have been a long interval of time between the formation of the highest strata of the ,south ,staffordshire coal-field and that of the lowest ,permian deposits of the district4 ,it is quite conceivable1 though at present incapable of proof1 that the extravasation of eruptive material took place after the close of the ,carboniferous period and during the earlier part of the ,permian period4 ,footnote #abb3 ,op4 cit4 p4 #aca4 ,footnote #abc3 ,see note on next page4 ,jukes further shows that at whatever period these igneous rocks were produced1 they were all existent before the production of the faults and dislocations that have traversed the ,coal-measures1 and before any great denudation had been effected on the country4 ,this argument may be readily granted4 ,but1 so far as we know1 many1 if not most1 of the faults traverse also the surrounding ,permian and ,triassic rocks1 so that igneous masses protruded during those periods would be affected by the same dislocations4 ,when we consider the history of ,palozoic time in this country1 and especially the proof1 obtainable everywhere else in ,britain1 that volcanic energy became quiescent during the accumulation of the ,coal-measures1 we may well demand better evidence than has hitherto been forthcoming that any portion of the dolerites of the ,midlands is of ,carboniferous age4 ,it is important to notice that though the dolerite sills and veins are so abundant in the ,south ,staffordshire coal-field1 coming even in many places up to the present surface of the ground1 no single case has been observed where they rise into the ,permian rocks that overlie the ,coal-measures unconformably4 ,it is difficult to believe that1 had these intrusions taken place after the deposition of the younger formation1 they should not be found penetrating it4#abd ,it seems almost certain that they must be of an age intermediate between the ,coal-measures of ,south ,staffordshire and the surrounding breccias and sandstones of the ,permian series4 ,and as there is clear evidence of contemporaneous volcanic action in the lowest part of the ,permian system to the north in ,scotland and to the south in ,devonshire1 the inference seems not unreasonable that these intrusive basalts of the ,midlands are most probably of ,permian age4 ,footnote #abd3 ,only one instance is known where in ,staffordshire any igneous rock has been intruded into rocks younger than the ,coal-measures 7,allport1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xxx4 p4 #eea2 ,sheet #gb ,s4 ,w4 of the ,geological ,survey1 and ,horizontal ,sections1 ,sheet #eg74 ,it forms a dyke which has been traced near ,norton ,bridge1 ,swinnerton and ,butterton1 running for #h miles in a ,n4,n4,w4 direction1 and rising through ,permian1 ,bunter and ,keuper strata4 ,it is a highly basic olivine-basalt1 and is unquestionably a dyke4 ,mr4 ,j4 ,kirkby1 who has recently mapped and described it 7,trans4 ,north ,staffordshire ,naturalists' ,field-,club1 xxviii4 7#ahid71 p4 #abi71 suggests that it may be connected with the igneous rocks of the ,south ,staffordshire coal-field4 ,but of this idea there is no evidence4 ,the last point to which the dyke has been traced is some five-and-twenty miles from the nearest known portion of the dolerites of the coal-field4 ,i have little doubt that this dyke is really an outlying member of the great system of ,tertiary dykes described in ,book ,,viii4 of the present work4 ,no trace of vents has been met with in the ,coal-measures of the ,midland district or among the surrounding older rocks1 nor any proof that the abundant sills and veins were connected with the eruption of volcanic materials at the surface4 ,nevertheless1 from the analogy of the structure of these intrusive sheets to that of the sills in such volcanic districts as the southern half of ,scotland1 we may well believe that they were connected here and there with eruptive vents1 and thus that besides the northern and southern districts of ,permian volcanoes1 there rose a central group among the lagoons of the heart of ,england4 ,though no vestige of any such group has been detected1 we must remember that a large portion of the ,midlands is overspread with ,permian and ,triassic deposits1 and that much more igneous rock may be concealed than appears at the surface4 ,possibly there may be buried under these younger sheets of red sandstone and marl1 lavas and tuffs with their connected vents1 such as may be seen where the ,permian volcanic series has been laid bare by denudation in ,ayrshire and ,devonshire4 ,in this respect it would be interesting to make a thorough examination of the ,permian breccias of the district1 with the view of discovering whether1 though the volcanic rocks in situ may still lie covered up1 fragments of them may not be found in these deposits4 ,illustration3 ,,to ,,accompany ,,sir ,,archibald ,,geikie',s ,,ancient ,,volcanoes ,,of ,,britain ,map ,v4 ,,map ,,of ,,the ,,permian ,,volcanic ,,districts ,,of ,,scotland ,the ,edinburgh ,geographical ,institute ,copyright ,j4 ,g4 ,bartholomew4 ,,book ,,viii ,,the ,,volcanoes ,,of ,,tertiary ,,time ,,chapter ,,xxxiii ,vast lapse of time between the close of the ,palozoic and beginning of the ,tertiary ,volcanic ,eruptions--,prolonged ,volcanic ,quiescence--,progress of ,investigation among the ,tertiary ,volcanic ,series of ,britain4 ,from the evidence which has been led in the foregoing chapters it is clear that during the later stages of the ,palozoic period there was a gradual enfeeblement of volcanic vigour over the area of the ,british ,isles4 ,when the last puys of the ,permian series became extinct a remarkable volcanic quiescence settled down on the region4 ,this interval of rest lasted throughout the whole of the long succession of the ,mesozoic ages4 ,though the geological record of this section of geological time is singularly complete in ,britain1 not a single vestige has yet been found in it of any contemporaneous eruption4 ,and what is true of this country is1 on the whole1 true of the entire ,european continent4 ,with some trifling exceptions there were no volcanoes in ,europe1 so far as we know1 during the enormous lapse of time between the last of the ,palozoic and the earliest of the ,tertiary eruptions4 ,when the geologist attempts to form an estimate of the chronological value of this interval of time he is soon lost in bewilderment over its obvious vastness1 and the impossibility of discovering any standards of measurement by which to reckon its duration4 ,on the one hand1 he sees that it lasted long enough to admit of the gradual elaboration of many thousands of feet of various sedimentary deposits1 which1 from their remarkable diversities of character1 were evidently accumulated1 on the whole1 with extreme slowness and amidst many geographical vicissitudes4 ,on the other hand1 he perceives that the interval sufficed to bring about an entire change in the fauna and flora of the globe4 ,indeed1 the more he investigates the details of this biological transformation1 the more he is impressed with the length of time that it must have required4 ,for it is not merely one complete change1 but a multifold succession of changes4 ,the stratigraphical records of the long array of geological periods over which it was spread show that the biological evolution advanced through a vast series of species1 genera and orders which one by one appeared and disappeared4 ,the ages that elapsed between the final dying out of the ,palozoic volcanoes and the outburst of those of ,tertiary time were so protracted that many revolutions of the geography of ,europe were comprised within them4 ,land and sea changed places again and again4 ,first came the singular topography of the ,trias1 which prolonged and accentuated the characteristics of the closing ,palozoic ages4 ,next arose the more genial climate and more varied geography of the ,jurassic period1 when comparatively shallow seas overspread the site of most of the ,european continent1 and tracts of old land stretched away to the west and north4 ,another crowded succession of changes in the disposition of land and sea filled the long ,cretaceous period1 at the close of which a more rapid and complete transformation in ,european geography took place4 ,yet during all these transitions and vicissitudes1 so far as we know1 volcanic energy remained quiescent throughout ,western ,europe4 ,it was not until some time after the great terrestrial movements that raised so much of the ,cretaceous sea-floor into land1 and laid the foundations of the modern continent1 that the subterranean fires once more awoke to vigorous action4 ,the renewal of eruptions in the early ages of ,tertiary time was as widespread as it was energetic4 ,over many regions of the ,european continent volcanoes broke out either in new areas or on old sites4 ,for the most part they appeared as scattered puys or as ,vesuvian vents1 generally not of the first magnitude1 like those of ,central ,france1 ,hungary1 ,wrtemberg and ,italy4 ,but in the north-west they assumed more colossal proportions1 and took the form of fissure-eruptions by which many thousands of square miles of country were deluged with lava4 ,from the ,south of ,antrim all along the ,west of ,scotland to the north of the ,inner ,hebrides remains of these basalt-floods form striking features in the existing scenery4 ,the same kind of rocks reappear in the ,faroe ,islands and in ,iceland1 so that an enormous tract of ,north-western ,europe1 much of it now submerged under the sea1 was the scene of activity of the ,tertiary volcanoes4 ,in entering1 therefore1 upon a consideration of the ,british ,tertiary volcanic rocks1 we are brought face to face with the records of the most stupendous succession of volcanic phenomena in the whole geological history of ,europe4 ,fortunately these records have been fully preserved in the ,british ,isles1 so that ample materials remain there for the elucidation of this last and most marvellous of all the volcanic epochs in the evolution of the continent4 ,as the remains of the ,tertiary series of volcanic eruptions are the youngest of all the volcanic records of ,britain1 they are naturally the freshest and most abundantly preserved4 ,they consequently reveal with singular clearness multitudes of volcanic phenomena that are less distinctly recognizable1 or not to be found at all1 among the ,palozoic systems4 ,hence they will be discussed in greater detail in the following chapters4 ,as a consequence of their greater freshness and wider extent1 and largely also because of the way in which they have been exposed along many leagues of picturesque sea-cliffs in the ,north of ,ireland and the ,west of ,scotland1 they attracted attention at an earlier time than the less obvious volcanic memorials of older ages4 ,the gradual development of opinion regarding the nature and history of volcanic rocks is thus in no small measure bound up with the progress of observation and inference in regard to the ,tertiary volcanic series4 ,i shall therefore begin this narrative by offering a rapid sketch of the history of inquiry respecting the ,tertiary volcanic areas of the ,british ,isles4 ,the basaltic cliffs of ,antrim and the ,inner ,hebrides had attracted the notice of passing travellers1 and their striking scenery had become more or less familiar to the reading public1 before any attention was paid to their remarkable geological structure and history4 ,in particular1 the wonders of the ,giant's ,causeway and the ,antrim coast had already begun to draw pilgrims1 even from distant countries1 at a time when geology had not come into existence4 ,the scientific tourist of those days who might care to look at rocks was1 in most cases1 a mineralogist1 for whom their structural relations and origin were subjects that lay outside of the range of his knowledge or habits of thought4 ,in the year #aggb ,sir ,joseph ,banks1 together with ,solander and a party1 visited ,staffa and brought back the earliest account of the marvels of that isle as they appeared to the sober eyes of science4 ,his narrative was communicated to ,pennant1 together with a number of drawings of the cliffs and of ,fingal's ,cave4 ,these were inserted by that geographer in his ,second ,tour1 published in #aggd1 and from their careful measurements of the basaltic pillars and their delineation of the basaltic structure1 are of special interest in the history of volcanic geology4 ,an intelligent appreciation of some of the geological interest of the region is to be found in the writings of ,whitehurst1#abe who gave a good account of the basalt-cliffs of ,antrim1 and regarded the basaltic rocks as the results of successive outflows of lava from some centre now submerged beneath the ,atlantic4 ,more important are the observations contained in two letters of ,abraham ,mills4#abf ,this writer had been struck with the dykes on the north coast of ,ireland1 and was led to examine also those in some of the nearer ,scottish islands4 ,he believed them to be of truly volcanic origin1 and spoke of them as veins of lava4 ,a few years later1 ,faujas ,st4 ,fond made his well-known pilgrimage to the ,western ,isles4 ,familiar with the volcanic rocks of ,central ,france1 he at once recognized the volcanic origin of the basalts of ,mull1 ,staffa and the adjoining islands4#abg ,his account of the journey1 published in ,paris in #agig1 may be taken as the beginning of the voluminous geological literature which has since gathered round the subject4 ,three years afterwards 7#ahjj7 appeared ,jameson's ,outline of the ,mineralogy of the ,scottish ,isles4 ,fresh from the teaching of ,werner at ,freiberg1 the future distinguished ,professor of ,natural ,history in the ,edinburgh ,university naturally saw everything in the peculiar ,wernerian light4 ,he gave the first detailed enumeration of some of the eruptive rocks of the ,hebrides1 but of course ridiculed the idea of their igneous origin4 ,having heard of a reported crater of a volcano near ,portree1 he ironically expressed a hope that there may be still sufficient heat to revive the spirits of some forlorn fire-philosopher1 as he wanders through this cold1 bleak country4#abh ,footnote #abe3 ,inquiry into the ,original ,state and ,formation of the ,earth1 #bnd edit4 #aghf4 ,footnote #abf3 ,philosophical ,transactions for #agij4 ,footnote #abg3 ,voyage en ,angleterre1 en cosse et aux les ,hbrides4 ,paris1 #agig4 ,footnote #abh3 ,it will be shown in a later chapter that there is a remarkably perfect volcanic vent near ,portree1 but the supposed crater referred to by ,jameson was probably some little corry among the sheets of basalt4 ,the advent of ,jameson to ,edinburgh gave a fresh impetus to the warfare of the ,plutonists and ,neptunists1 for he brought to the ranks of the latter a mineralogical skill such as none of their ,scottish opponents could boast4 ,the igneous origin of basalt1 which the ,plutonists stoutly maintained1 was as strongly denied by the other side4 ,for some years one of the most telling arguments against the followers of ,hutton was derived from the alleged occurrence of fossil shells in the basalt of the north coast of ,ireland4 ,kirwan#abi quoted with evident satisfaction ,richardson's observation of shells in the basalts of ,ballycastle1 and ,richardson#acj himself1 though the true explanation1 that the supposed basalt is only ,lias shale altered by basalt1 had been stated in #ahjb by ,playfair1#aca continued for ten years afterwards to reiterate his belief in the aqueous origin of basalt4 ,thus the ,tertiary volcanic rocks furnished effective weapons to the combatants on both sides4 ,the dispute regarding the black fossiliferous rocks of ,portrush had the effect of drawing special attention to the geology of the ,north of ,ireland4 ,among the more noted geologists who were led to examine them1 particular reference must be made to ,conybeare and ,buckland1 who1 in the year #ahac1 studied the interesting coast-sections of ,antrim4 ,the report of their observations gives an excellent summary of the arguments for the truly igneous origin of basalt1 and a statement of opinion in favour of the view that the bedded basalts are the products of submarine volcanoes4 ,berger also about the same time described in fuller detail the geology of the ,antrim district1 and showed the rocks of the basalt-plateau to be younger than the ,chalk4 ,he likewise made a study of the basalt-dykes of the ,north of ,ireland1 and was the first to point out their prevalent north-westerly direction4 ,the memoirs of these geologists#acb may justly be regarded1 to quote the words of ,portlock1 as the first effectual step made in ,irish geology4 ,portlock's own description is still the most complete summary of the geology of that interesting region4#acc ,footnote #abi3 ,geological ,essays1 #agii1 p4 #beb1 footnote4 ,footnote #acj3 ,richardson lived on the ,antrim coast1 and had daily opportunities of examining the admirable rock-sections there exposed4 ,it was he who found the shells in supposed basalt1 and led the geologists of his day astray on this subject4 ,he made a clever but irrelevant reply to ,playfair's plain statement of facts 7,trans4 ,roy4 ,irish ,acad4 vol4 ix4 #ahjc1 p4 #dha74 ,his elaborate attack on the ,volcanic ,theory will be found in ,trans4 ,roy4 ,irish ,acad4 vol4 x4 7#ahjf71 pp4 #ce-ajg4 ,though lively enough as a specimen of controversial writing1 it forms1 when seriously considered1 rather a melancholy chapter in geological literature4 ,footnote #aca3 ,illustrations of the ,huttonian ,theory1 #beb4 ,footnote #acb3 ,they are contained in the third volume of the ,transactions of the ,geological ,society4 ,footnote #acc3 ,report on the ,geology of the ,county of ,londonderry and parts of ,tyrone and ,fermanagh1 ,mem4 ,geol4 ,survey1 #ahdc4 ,while such advances were being made in the knowledge of the structure of the volcanic rocks of the ,north of ,ireland1 the geologist had already appeared who was the first to attempt a systematic examination of the ,western ,islands1 and whose published descriptions are still a chief source of information regarding the geology of this extensive region4 ,dr4 ,macculloch seems to have made his first explorations among the ,hebrides some time previous to the year #ahad1 for in that year he published some remarks on specimens from that district transmitted to the ,geological ,society4#acd ,for several years in succession he devoted himself with great energy and enthusiasm to the self-imposed task of geologically examining and mapping in a generalized way all the islands that lie to the westward of ,scotland1 from the remote ,st4 ,kilda even as far as the ,isle of ,man4 ,from time to time1 notices of parts of his work were given in the ,transactions of the ,geological ,society4 ,but eventually in #ahai he embodied the whole in his ,description of the ,western ,islands of ,scotland1 including the ,isle of ,man4 ,footnote #acd3 ,trans4 ,geol4 ,soc4 vol4 ii4 #ahad4 ,this great classic marks a notable epoch in ,british geology4 ,properly to estimate its value1 we should try to realize what was the state of the science in this country at the time of its appearance4 ,so laborious a collection of facts1 and so courageous a resolution to avoid theorizing about them1 gave to his volumes an altogether unique character4 ,his descriptions were at once adopted as part of the familiar literature of geology4 ,his sections and sketches were reproduced in endless treatises and text-books4 ,few single works of descriptive geology have ever done so much to advance the progress of the science in this country4 ,with regard to the special subject of the present memoir1 ,macculloch showed that the basalts and other eruptive rocks of the ,inner ,hebrides pierce and overlie the ,secondary strata of these islands1 and must therefore be of younger date4 ,but though he distinguished the three great series of trap-rocks1 syenites and hypersthene-rocks or augite-rocks1 and indicated approximately their respective areas1 he did not attempt to unravel their relations to each other4 ,nor did he venture upon any speculations as to the probable conditions under which these rocks were produced4 ,he claimed that those who might follow him would find a great deal which he had not described1 but little that he had not examined4 ,subsequent observers have noted many important facts1 of which1 had he observed them1 he would at once have seen the meaning1 and which he certainly would not have passed over in silence4 ,but as a first broad outline of the subject1 ,macculloch's work possesses a great value1 which is not lessened by the subsequent discovery of details that escaped his notice1 and of important geological relations which he failed to detect4 ,it has already been pointed out that some of the earliest and ablest observations among the volcanic rocks of this country1 especially in ,scotland1 were made by foreigners4 ,students who had repaired from abroad to ,edinburgh for education sometimes caught the geological enthusiasm1 then so marked in that city1 and made numerous journeys through the country in search of further knowledge of ,scottish rocks and minerals4 ,in other instances1 geologists of established reputation1 attracted by the interest which the published accounts of the geology of ,scotland had excited1 were led to visit the country and to record their impressions of its rock-structure4 ,of the first class of observers the two most noted were ,ami ,bou and ,l4 ,a4 ,necker2 of the second1 special acknowledgment is due to ,faujas ,st4 ,fond and to ,von ,oyenhausen and ,von ,dechen4 ,the labours of ,bou#ace have already been referred to in connection with the literature of the ,scottish ,old ,red ,sandstone 7vol4 i4 p4 #bfi74 ,in his treatment of the ,tertiary ,volcanic series of ,scotland he appears to have relied mainly on the then recently published volumes of ,macculloch4 ,footnote #ace3 ,essai gologique sur l'cosse4 ,paris1 #ahbj4 ,l4 ,a4 ,necker1 as the grandson of the illustrious ,de ,saussure1 had strong claims on the friendly assistance of the ,school of ,geology at ,edinburgh when he went thither in #ahjf1 at the age of twenty1 to prosecute his studies4 ,he was equally well received by the ,plutonists and ,neptunists1 and devoted some time to the exploration of the geology not only of the ,lowlands1 but of the ,highlands and the ,inner ,hebrides4 ,most of his observations appear to have been made in the year #ahjg1 but it was not until fourteen years afterwards that he published the account of them4#acf ,the geological part of this work must be admitted to be somewhat disappointing4 ,the author's caution not to commit himself to either side of the geological controversy then waging makes his descriptions and explanations rather colourless4 ,he adds little to what was previously known4 ,even as regards the origin of the basalts of the ,western ,islands1 he could not make up his mind whether or not to regard them as volcanic1 but contented himself by referring them to the trappean formation4 ,yet these islands had so fascinated him that eventually he returned to them as his adopted home1 passed the last twenty years of his life among them1 and died and was buried there4 ,besides his ,voyage1 he published in ,french an account of the dykes of the ,island of ,arran4#acg ,footnote #acf3 ,voyage en cosse et aux les ,hbrides4 ,see also biographical notice of ,l4 ,a4 ,necker1 by ,principal ,j4 ,d4 ,forbes1 ,proc4 ,roy4 ,soc4 ,edin4 v4 7#ahfb71 p4 #ec4 ,footnote #acg3 ,trans4 ,roy4 ,soc4 ,edin4 vol4 xiv4 7#ahdj71 p4 #ffg4 ,among the foreign geologists who have been drawn to the ,scottish mountains and islands by the interest of their ,tertiary volcanic rocks1 ,i have already spoken of ,faujas ,st4 ,fond4 ,much more important1 however1 were the observations made some thirty years later by two ,german men of science1 ,von ,oyenhausen and ,von ,dechen4 ,their careful descriptions of the geology of ,skye1 ,eigg and ,arran added new materials to the knowledge already acquired by native geologists4#ach ,to some of the more interesting parts of their work reference will be made in later pages4 ,footnote #ach3 ,karsten's ,archiv 7#ahbi71 vol4 i4 p4 #ef4 ,the numerous trap-dykes of ,northumberland1 ,durham and ,northern ,yorkshire at an early date attracted the attention of geologists4 ,as far back as #ahag1 they had been the subject of a memoir by ,n4 ,j4 ,winch1#aci who gave an account of their effects on the adjacent rocks4 ,more important were the subsequent papers on the same subject by ,sedgwick1 who1 discussing the lithological characters1 probable origin and geological age of the dykes1 pointed out that while the ,cleveland dyke was undoubtedly younger than a large part of the ,jurassic rocks1 there was no direct evidence to determine whether dykes farther north were earlier or later than the time of the ,magnesian ,limestone4#adj ,subsequent accounts of the dykes of the same region were given by ,buddle1#ada ,m4 ,forster1#adb ,n4 ,wood1#adc ,h4 ,t4 ,m4 ,witham1#add ,tate #ade and others1 while in more recent years important additions to our knowledge of these dykes and of their effects have been made by ,sir ,j4 ,lowthian ,bell#adf and ,mr4 ,j4 ,j4 ,h4 ,teall4#adg ,footnote #aci3 ,trans4 ,geol4 ,soc4 vol4 iv4 7#ahag71 p4 #ba4 ,see also ,tilloch's ,phil4 ,mag4 vols4 xlix4 and l4 ,footnote #adj3 ,cambridge ,phil4 ,trans4 vol4 ii4 7#ahbg71 pp4 #ba1 #aci4 ,footnote #ada3 ,trans4 ,nat4 ,hist4 ,soc4 ,northumberland1 i4 7#ahca71 p4 #i4 ,footnote #adb3 ,op4 cit4 i4 p4 #dd4 ,footnote #adc3 ,op4 cit4 i4 pp4 #cje1 #cjf1 #cjh1 #cji4 ,footnote #add3 ,op4 cit4 ii4 7#ahch71 p4 #cdc4 ,footnote #ade3 ,trans4 ,northumberland and ,durham1 ii4 7#ahfh71 p4 #cj4 ,footnote #adf3 ,proc4 ,roy4 ,soc4 xxiii4 7#ahge71 p4 #edc4 ,footnote #adg3 ,quart4 ,journ4 ,geol4 ,soc4 xl4 7#ahhd71 p4 #bji4 ,the geological age of the great series of ,tertiary volcanic rocks has only been determined district by district1 and at wide intervals4 ,that some part of the ,antrim basalts is younger than the ,chalk of that region was clearly shown by ,berger1 ,conybeare and ,buckland4 ,portlock1 however1 referred to the occurrence of detached blocks of basalt which he supposed to be immersed in the ,chalk near ,portrush1 and which inclined him to believe that the basaltic flows commenced at a remote period of the ,cretaceous system4#adh ,macculloch showed that the corresponding basaltic plateaux of the ,inner ,hebrides were certainly younger than the ,oolitic rocks of that region4 ,but no nearer approximation to their date had yet been made when in the year #ahej the ,duke of ,argyll announced the discovery of strata containing fossiliferous chalk-flints and dicotyledonous leaves1 lying between the bedded basalts of ,ardtun ,head1 in the ,isle of ,mull4#adi ,in the following year these fossil leaves were described by ,edward ,forbes1 who regarded them as decidedly ,tertiary1 and most probably ,miocene4 ,this was the first palontological evidence for the determination of the geological age of any portion of the basalt-plateaux1 and it indicated that the basalts of the south-west of ,mull were of older ,tertiary date4 ,taken also in connection with the occurrence of lignite-beds between the basalts of ,antrim1 it suggested that these volcanic plateaux were not due to submarine eruptions1 as the earlier geologists had supposed1 but were rather the result of the subrial outpouring of lava at successive intervals1 during which terrestrial vegetation sprang up upon the older outflows4 ,footnote #adh3 ,report on the ,geology of ,londonderry1 p4 #ic4 ,there can be no doubt that this was an error of observation4 ,the ,antrim basalts are all certainly younger than the ,chalk4 ,the supposed lumps of basalt were probably the ends of veins intruded into the ,chalk1 and perhaps partially disconnected from the main parts of the veins4 ,such apparently detached masses of intrusive rock are not infrequent occurrence in connection with the ,tertiary intrusive sills4 ,an example will be found represented in ,fig4 #cba4 ,footnote #adi3 ,brit4 ,assoc4 ,report1 #ahej1 ,sections1 p4 #gj2 and ,quart4 ,jour4 ,geol4 ,soc4 vii4 7#ahea71 p4 #hg4 ,while ,forbes brought forward palontological proofs of the ,tertiary age of the volcanic rocks of the south-west of ,mull1 he at the same time laid before the ,geological ,society a paper on the ,estuary ,beds and the ,oxford ,clay of ,loch ,staffin1 in ,skye1 wherein1 while admitting the existence of appearances which might be regarded as favourable to the view that the intercalated basalts of that region were of much later date than the ,oolitic strata between which they might have been intrusively injected1 he stated his own belief that they were really contemporaneous with the associated stratified rocks1 and thus marked an outbreak of volcanic energy at the close of the ,middle ,oolitic period4#aej ,the ,duke of ,argyll1 in the paper which he on the same occasion communicated to the ,geological ,society1 adopted this view of the probable age of most of the basalts of the ,western ,islands4 ,he looked upon the ,tertiary volcanic rocks of ,mull as occupying a restricted area1 the great mass of the basalt of that island1 like that of ,skye1 being regarded by him as probably not later than some part of the ,secondary period4 ,footnote #aej3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 vii4 7#ahea71 p4 #ajd4 ,it must be granted that the appearances of contemporaneous intercalation of the basalt among the ,secondary strata are singularly deceptive4 ,when1 several years after the announcement of the ,tertiary age of the basalts of ,ardtun1 ,i began my geological work in the ,inner ,hebrides1 ,i was led to the same conclusion as ,edward ,forbes1 and expressed it in an early paper4#aea ,all over the north of ,skye ,i traced what appeared to be evidence of the contemporaneous interstratification of basalts with the ,jurassic rocks and ,i concluded 7though with some reservation7 that the whole of the vast basaltic plateaux of that island were not younger than some late part of the ,jurassic period4 ,in that same paper the attention of geologists was called to the probable connection of the great system of east-and-west dykes traversing ,scotland and the ,north of ,england1 with the basalt-plateaux of the ,inner ,hebrides1 and as ,i believed the latter to be probably of the age of the ,oolitic rocks1 ,i assigned the dykes to the same period in geological history4 ,but subsequent explorations enabled me to correct the mistake into which1 with other geologists1 ,i had fallen regarding the age of the volcanic phenomena of the ,western ,islands4 ,in #ahfg ,i showed that instead of being confined to a mere corner of ,mull1 the ,tertiary basalts1 with younger associated trachytic or granitic rocks1 covered nearly the whole of that island1 and that in all likelihood the long chain of basaltic masses1 extending from the ,north of ,ireland along the west coast of ,scotland to the ,faroe ,islands1 and beyond these to ,iceland1 was all erupted during the ,tertiary period4 ,at the same time ,i drew special attention to the system of east-and-west dykes as proofs of the vigour of volcanic action at that period1 and ,i furnished evidence that this action was prolonged through a vast interval of time1 during which great subrial denudation of the older lavas took place before the outflow of the younger4#aeb ,later in the same year1 in an address to the ,geological ,section of the ,british ,association1 ,i reiterated these views1 and more particularly emphasized the importance of the system of dykes1 which in my opinion was possibly the most striking manifestation of the vigour of ,tertiary volcanic action4#aec ,in #ahga1 after further explorations in the field1 ,i gave a detailed account of the structure which had led to the mistake as to the age of the ,tertiary volcanic rocks of the ,western ,islands2 and in a description of the island of ,eigg1 ,i brought forward data to show the enormous duration of the ,tertiary volcanic period in the west of ,britain4#aed ,footnote #aea3 ,on the ,chronology of the ,trap-rocks of ,scotland1 ,trans4 ,roy4 ,soc4 ,edin4 xxii4 7#ahfa71 p4 #fdi4 ,footnote #aeb3 ,proc4 ,roy4 ,soc4 ,edin4 vi4 7#ahfg71 p4 #ga4 ,footnote #aec3 ,brit4 ,assoc4 ,report 7,dundee71 #ahfg1 ,sections1 p4 #di4 ,footnote #aed3 ,quart4 ,journ4 ,geol4 ,soc4 xxvii4 7#ahga71 p4 #bgi4 ,three years later ,mr4 ,j4 ,w4 ,judd read before the ,geological ,society a paper ,on the ,ancient ,volcanoes of the ,highlands4#aee ,the most novel feature of this paper was the announcement that the author had recognized the basal wrecks of five great central volcanoes in the ,western ,islands1 among which that of ,mull was inferred by him to have been at least #ad1ejj feet high4 ,he was led to the conclusion that the volcanic period in these regions was divisible into three sections--the first marked by the outburst of acid rocks 7felspathic lavas and ashes1 connected with deeper and more central granitic masses72 the second by the extrusion of basic lavas and tuffs 7the basaltic plateaux72 the third by the appearance of small sporadic volcanic cones 7felspathic1 basaltic1 or intermediate in composition7 after the great central cones had become extinct4 ,it will be seen in the following pages that these conclusions of ,professor ,judd are not supported by a more detailed study of the region4 ,footnote #aee3 ,quart4 ,journ4 ,geol4 ,soc4 xxx4 7#ahgd71 p4 #bbj4 ,in the year #ahgi1 during a traverse of some portions of the volcanic region of ,wyoming1 ,montana and ,utah1 ,i was vividly impressed by the identity of structure between the basaltic plateaux of these territories and the youngest volcanic areas of ,britain4 ,it then appeared to me that some of the puzzling features in the ,tertiary volcanic series of the ,inner ,hebrides might be explained by the structures so admirably displayed in these lava-fields of the ,far ,west4#aef ,riding over the great basalt-plains of the ,snake ,river and looking at the sections cut by the river through the thick series of horizontal basalt-beds1 ,i appreciated for the first time the significance of ,baron von ,richthofen's views regarding massive or fissure eruptions1 as contradistinguished from those of great central cones of the type of ,etna or ,vesuvius1 and ,i gathered so many suggestions from my examination of these ,american regions that ,i renewed with increased interest the investigation of the ,tertiary volcanic tracts of ,britain4 ,at last1 after another interval of nine years1 during which my weeks of leisure were given to the task1 ,i was able to complete a discussion of the whole history of ,tertiary volcanic action in this country1 which was communicated to the ,royal ,society of ,edinburgh in the early summer of #ahhh4aeg ,since that time ,i have continued the research1 and have from time to time communicated my results to the ,geological ,society4 ,these various memoirs are combined with hitherto unpublished details in the following account of the ,british ,tertiary ,volcanic ,rocks4 ,footnote #aef3 ,geological ,essays at ,home and ,abroad 7#ahhb71 pp4 #bga1 #bgd2 ,nature1 ,november #ahhj4 ,footnote #aeg3 ,trans4 ,roy4 ,soc4 ,edin4 vol4 xxxv4 part ii4 7#ahhh71 pp4 #bc-ahd4 ,professor ,judd has also prosecuted the investigation of the petrography of the rocks1 and has published his observations in the ,quarterly ,journal of the ,geological ,society4#aeh ,to these papers by him more detailed reference will be made in later ,chapters4 ,footnote #aeh3 ,quart4 ,journ4 ,geol4 ,soc4 vols4 xlv4 7#ahhi71 xlvi4 7#ahij71 xlix4 7#ahic74 ,in the first of these volumes ,professor ,judd offered a detailed criticism of my views as to the order of succession and history of the volcanic rocks of the ,inner ,hebrides4 ,subsequent investigation having entirely confirmed my main conclusions1 it is not necessary to enter here upon matters of controversy4 ,reference1 however1 will be made in subsequent ,chapters to some of the points in dispute4 ,in describing the geological history of a great series of rocks1 chronological order is usually the most convenient method of treatment4 ,where1 however1 the rocks are of volcanic origin1 and do not always precisely indicate their relative age1 and where moreover the same kinds of rock may appear on widely-separated geological horizons1 it is not always possible or desirable to adhere to the strict order of sequence4 ,with this necessary latitude1 ,i propose to follow the chronological succession from the older to the newer portions of the series4 ,i shall treat first of the system of dykes1 by which so large a part of ,scotland and of the north of ,england and ,ireland is traversed4 ,many of the dykes are undoubtedly among the youngest members of the volcanic series1 and in no case has their age been as yet determined except relatively to the antiquity of the rocks which they traverse4 ,they must1 of course1 be posterior to these rocks1 and hence it would be quite logical to reserve them for discussion at the very end of the whole volcanic phenomena4 ,my reason for taking them at the beginning will be apparent in the sequel4 ,after the dykes1 ,i shall describe the great volcanic plateaux which1 in spite of vast denudation1 still survive in extensive fragments in ,antrim1 the ,inner ,hebrides and the ,faroe ,islands4 ,the eruptive bosses of basic rocks that have broken through the plateaux will next be discussed4 ,an account will then be given of the protrusions of acid rocks which have disrupted these basic bosses4 ,the last chapters will contain a sketch of the subsidences and dislocations which the basalt-plateaux have suffered1 and of the denudation to which they have been subjected4 ,as has been explained in ,chapter iii41 the volcanic cycle of any district1 during a given geological period1 embraces the whole range of erupted products from the beginning to the end of a complete series of eruptions4 ,reference was made in ,book ,i4 to the remarkable variation in the character of the lavas successively poured out from the same volcanic reservoir during the continuance of a single cycle1 and it was pointed out that ,richthofen's law generally holds good that while the first eruptions may be of a basic or average and intermediate nature1 those of succeeding intervals become progressively more acid1 but are often found to return again at the close to thoroughly basic compounds4 ,this law is well illustrated by the volcanic history of ,tertiary time in ,britain4 ,we shall find that the earliest eruptions of which the relative date is known consisted generally of basic lavas 7dolerites and basalts71 but including also more sparingly andesites1 trachytes and rhyolites2 that the oldest intrusive masses consisted of bosses1 sills and dykes of dolerite and gabbro2 that these intrusions were followed by others of a much more acid character--felsites1 pitchstones1 quartz-porphyries or rhyolites1 granophyres and granites2 that the latest lava is a somewhat acid rock1 being a vitreous form of dacite2 and that the most recent volcanic products of all are dykes of a thoroughly basic nature1 like some of the earlier intruded masses4 ,,chapter ,,xxxiv ,,the ,,system ,,of ,,dykes ,,in ,,the ,,tertiary ,,volcanic ,,series ,geographical ,distribution--,two ,types of ,protrusion--,nature of ,component ,rocks--,hade--,breadth--,interruptions of ,lateral ,continuity--,length--,persistence of ,mineral ,characters4 ,if a geologist were asked to select that feature in the volcanic geology of the ,british ,isles which1 more than any other1 marks this region off from the rest of the ,european area1 he would probably choose the remarkable system of wall-like masses of erupted igneous rock1 to which the old ,saxon word dykes has been affixed4 ,from the moors of eastern ,yorkshire to the ,perthshire ,highlands1 and from the basins of the ,forth and ,tay to the west of ,donegal and the far headlands of the ,hebrides1 the country is ribbed across with these singular protrusions to such an extent that it may be regarded as a typical region for the study of the phenomena of dykes4 ,that all the dykes in this wide tract of country are of ,tertiary age cannot be maintained4 ,it has been shown in previous ,chapters that each of the great volcanic periods has had its system of dykes1 even as far back as the time of the ,lewisian ,gneiss4 ,but when all the dykes which can reasonably be referred to older geological periods are excluded1 there remains a large series which cannot be so referred1 but which are connected together by various kinds of evidence into one great system that must be of late geological date1 and can be assigned to no other than the ,tertiary period in the volcanic history of ,britain4 ,as far back as the year #ahfa1 when ,i first drew attention to this great system of dykes in connection with the progress of volcanic action in the country1 ,i pointed out the grounds on which it seemed to me that these rocks belong to a comparatively recent geological period4#aei ,my own subsequent experience and the full details of structure collected by my colleagues of the ,geological ,survey in all parts of the country1 have amply confirmed this view4 ,the characters which link this great series of dykes together as one connected system of late geological date are briefly enumerated in the following list1 and will be more fully discussed in later pages4 ,footnote #aei3 ,trans4 ,roy4 ,soc4 ,edin4 vol4 xxii4 7#ahfa71 p4 #fej4 #a4 ,the prevalent tendency of the dykes to take a north-westerly course4 ,there are exceptions to this normal trend1 especially where the dykes are small and locally numerous2 but it remains singularly characteristic over the whole region4 #b4 ,the increasing abundance of the dykes as they are traced to the west coast and the line of the great ,tertiary volcanic plateaux of ,antrim and the ,inner ,hebrides4 #c4 ,the rectilinear direction so characteristic of them and so different from the tortuous course of local groups of dykes4 ,the exceptions to this normal feature are as a rule confined to the same localities where departures from the prevalent westerly trend occur4 ,illustration3 ,fig4 #bcc4--;,dyke on the south-east coast of the ,island of ,mull4 #d4 ,the great breadth of the larger dykes of the system and their persistence for long distances4 ,this is one of their most remarkable and distinctive characters4 #e4 ,the posteriority of the dykes to the rest of the geological structure of the regions which they traverse4 ,they are not only younger than the other rocks1 but younger than nearly all the folds and faults by which the rocks are affected4 #f4 ,the manner in which they cut the ,jurassic1 ,cretaceous and older ,tertiary rocks in the districts through which they run4 ,at the south-eastern end of the region they rise through the ,lias and ,oolite formations1 in the west they intersect the ,chalk and also the ,tertiary volcanic plateaux together with their later eruptive bosses4 #g4 ,their petrographical characters1 among which perhaps the most distinctive is the frequent appearance of the original glass of the plagioclase-pyroxene-magnetite 7olivine7 rock1 of which they mostly consist4 ,this glass1 or its more or less completely devitrified representative1 often still recognizable with the microscope among the individualized microlites and crystals throughout the body of a dyke1 is also not infrequent as a black vitreous varnish-like coating on the outer walls1 and occasionally appears in strings and veins even in the centre4 ,it is the assemblage of dykes presenting these features which ,i propose to describe4 ,obviously1 the age of each particular dyke can only be fixed relatively for itself4 ,but when this remarkable community of characters is considered1 and when the post-,mesozoic age of at least a very large number of the dykes can be demonstrated1 the inference is reasonable that one great system of dykes was extravasated during a time of marked volcanic disturbance1 which could not have been earlier than the beginning of the ,tertiary period4 ,and this inference may be maintained even when we frankly admit that every dyke within the region is by no means claimed as belonging to the ,tertiary series4 ,illustration3 ,fig4 #bcd4--;,fissure left by the weathering out of a dyke4 ,in spite of their number and the extraordinary volcanic activity to which they bear witness1 the dykes form a much less prominent feature in the landscape than might have been anticipated4 ,in the lowlands of the interior1 they have for the most part been concealed under a cover of superficial accumulations1 though in the water-courses they not infrequently project as hard rocky barriers across the channels1 and occasionally form picturesque waterfalls4 ,on the barer uplands1 they protrude in lines of broken crag and scattered boulders1 which by their decay give rise to a better soil covered by a greener vegetation than that of the surrounding brown moorland4 ,among the ,highland hills1 they are often traceable from a distance as long black ribs that project from the naked faces of crag and corry4 ,along the sea-coast1 their peculiarities of scenery are effectively displayed4 ,where they consist of a close-grained rock1 they often rise from the beach as straight walls which1 with a strangely artificial look1 mount into the face of the cliffs on the one side1 and project in long black reefs into the sea on the other 7,fig4 #bcc74 ,every visitor to the islands of the ,clyde will remember how conspicuous such features are there4 ,but it is among the ,inner ,hebrides that this kind of scenery is to be found in greatest perfection4 ,the soft dark ,lias shales of the island of ,pabba1 for example1 are ribbed across with scores of dykes which strike boldly out to sea4 ,where1 on the other hand1 the material of the dykes is coarse in grain1 or is otherwise more susceptible to the disintegrating influences of the weather1 it has often rotted away and left yawning clefts behind1 the vertical walls of which are those of the fissures up which the molten rock ascended 7,fig4 #bcd74 ,some good instances of this kind are well known to summer visitors on the eastern shores of ,arran4 ,others1 on a large scale1 may be seen in the interior of the same island along the crests of the granite ridges1 and still more conspicuously on the jagged summits of ,blath ,beinn and the ,cuillin ,hills 7,fig4 #ccc71 and intersecting the ,jurassic strata along the cliffs of ,strathaird in ,skye4 #a4 ,,geographical ,,distribution ,the limits of the region within which the dykes occur cannot be very precisely fixed4 ,there can be no doubt1 however1 that on their southern side they reach to the ,cleveland ,hills of ,yorkshire and the southern borders of ,lancashire1 perhaps even as far as ,north ,staffordshire 7p4 #ajf71 and on the northern side to the farther shores of the island of ,lewis--a direct distance of #cfj miles4 ,they stretch across the basin of the ,irish ,sea1 including the ,isle of ,man1 and appear in ,ireland north of a line drawn from ,dundalk ,bay to the ,bays of ,sligo and ,donegal4 ,dykes are of frequent occurrence over the north of ,england and south of ,scotland1 at least as far north as a line drawn from the coast of ,kincardineshire along the southern flank of the ,grampian ,hills1 by the head of ,glen ,shee and ,loch ,tay1 to the north-western coast of ,argyleshire4 ,they abound all along the line of the ,inner ,hebrides and on parts of the adjacent coasts of the mainland1 from the remoter headlands of ,skye to the shores of ,county ,louth4 ,they traverse also the chain of the ,long ,island in the ,outer ,hebrides4 ,so far as ,i am aware1 they are either absent or extremely rare in the ,highlands north of the line ,i have indicated4 ,but a good many have been found by my colleagues in the course of the ,geological ,survey of the northern lowlands of ,aberdeenshire and ,banffshire4 ,the longest of these has been traced by ,mr4 ,l4 ,hinxman for rather more than two miles running in a nearly east and west direction through the ,old ,red ,sandstone of ,strathbogie1 with an average width of about #ce feet4 ,another in the same district has a width of from #de to #ij feet1 and has been followed for a third of a mile4 ,but far beyond these northern examples1 ,i have found a number of narrow basalt-veins traversing the ,old ,red flagstones of the ,mainland of ,orkney1 which ,i have little doubt are also a prolongation of the same late series4 ,taking1 however1 only those western and southern districts in which the younger dykes form a notable feature in the geology1 we find that the dyke-region embraces an area of upwards of #dj1jjj square miles--that is1 a territory greater than either ,scotland or ,ireland1 and equal to more than a third of the total land-surface of the ,british ,isles 7,map ,i474 ,of this extensive region the greater portion has now been mapped in detail by the ,geological ,survey4 ,every known dyke has been traced1 and the appearances it presents at the surface have been recorded4 ,we are accordingly now in possession of a larger body of evidence than has ever before been available for the discussion of this remarkable feature in the geology of the ,british ,isles4 ,i have made use of this detailed information1 and besides the data accumulated in my own note-books1 ,i have availed myself of those of my colleagues in the ,survey1 for which due acknowledgment is made where they are cited4 ,the ,tertiary basalt-plateaux of ,britain have their counterpart in the ,faroe ,islands and in ,iceland1 and whether or not the lava-fields stretched throughout ,north-western ,europe from ,antrim to the farthest headlands of ,ultima ,thule1 there can hardly be any doubt that1 if not continuous1 these volcanic areas were at least geologically contemporaneous in their activity4 ,their characteristic scenery and structure are prolonged throughout the whole region1 reappearing with all their familiar aspects alike in ,faroe and in ,iceland4 ,i have not seen the latter island1 but in the ,faroe archipelago ,i have found the dykes to be sufficiently common1 and to cut the basalt-plateaux there in the same way as they do those of the ,inner ,hebrides4 ,on the whole1 however1 dykes do not play1 in these northern isles1 the important part which they take in the geology and scenery of the ,west of ,scotland4 ,i have not had sufficient opportunity to ascertain whether there is a general direction or system among the ,faroe dykes4 ,in the fjords north of ,thorshaven1 and again along the west side of ,strom1 many of them show an ,e4 and ,w4 strike or one from ,e4,n4,e4 to ,w4,s4,w4 #b4 ,,two ,,types ,,of ,,protrusion ,the dykes are far from being equally distributed over the wide region within which they occur4 ,in certain limited areas they are crowded together1 sometimes touching each other to the almost entire exclusion of the rocks through which they ascend1 while elsewhere they appear only at intervals of several miles4 ,viewed in a broad way1 they may be conveniently grouped in two types1 which1 though no hard line can be drawn between them1 nevertheless probably point to two more or less distinct phases of volcanic action and to more than one period of intrusion4 ,in the first1 which for the sake of distinction we may term the ,solitary type1 there is either a single dyke separated from its nearest neighbours by miles of intervening and entirely dykeless ground1 or a group of two or more running parallel to each other1 but sometimes a mile or more apart4 ,the rock of which they consist is1 on the whole1 less basic than in the second type2 it includes the andesitic varieties4 ,it is to this type that the great dykes of the north of ,england and the south and centre of ,scotland belong4 ,the ,cleveland dyke1 for example1 at its eastern end has no known dyke near it for many miles4 ,the coal-field of ,scotland is traversed by five main dykes1 which run in a general sense parallel to each other1 with intervals of from half a mile to nearly five miles between them4 ,dykes of this type display most conspicuously the essential characters of the dyke-structure1 in particular the vertical marginal walls1 the parallelism of their sides1 their great length1 and their persistence in the same line4 ,in the second1 or what for brevity may be called the ,gregarious type1 the dykes occur in great abundance within a particular district4 ,they are on the whole narrower1 shorter1 less strikingly rectilinear1 more frequently tortuous and vein-like1 and generally more basic in composition than those of the first type4 ,they include the true basalts and dolerites4 ,illustrative districts for dykes of this class are the islands of ,arran1 ,mull1 ,eigg and ,skye4 ,the great single or solitary dykes may be observed to increase in number1 though very irregularly1 from south to north1 and also in ,central ,scotland from east to west4 ,they are specially abundant in the tract stretching from the ,firth of ,clyde along a belt of country some thirty miles broad on either side of the ,highland line1 as far at least as the valley of the ,tay4 ,they form also a prominent feature in the islands of ,jura and ,islay4 ,dykes of the gregarious type are abundantly and characteristically displayed in the basin of the ,firth of ,clyde4 ,their development in ,arran formed the subject of the interesting paper by ,necker1 already mentioned1 who catalogued and described #adi of them1 and estimated their total number in the whole island to be about #aejj4afj ,as the area of ,arran is #afe square miles1 there would be1 according to this computation1 about nine dykes to every square mile4 ,but they are far from being uniformly distributed4 ,while appearing only rarely in many inland tracts1 they are crowded together along the shore1 particularly at the south end of the island1 where the number in each square mile must far exceed the average just given4 ,the portion of ,argyleshire1 between the hollow of ,loch ,long and the ,firth of ,clyde on the east and ,loch ,fyne on the west1 has been found by my colleague1 ,mr4 ,c4 ,t4 ,clough1 to contain an extraordinary number of dykes 7see ,fig4 #beg74 ,the coast line of ,renfrewshire and ,ayrshire shows that the same feature is prolonged into the eastern side of the basin of the ,clyde estuary4 ,but immediately to the westward of this area the crowded dykes disappear from the basin of ,loch ,fyne4 ,in ,cantire their scarcity is as remarkable as their abundance in ,cowal4 ,footnote #afj3 ,trans4 ,roy4 ,soc4 ,edin4 xiv4 7#ahdj71 p4 #fgg4 ,both in the ,north of ,ireland and through the ,inner ,hebrides1 dykes are singularly abundant in and around1 but particularly beneath1 the great plateaux of basalt4 ,their profusion in ,skye was described early in this century by ,macculloch1 who called attention more especially to their extraordinary development in the district of ,strathaird4 ,they nearly equal in some places1 he says1 when collectively measured1 the stratified rock through which they pass4 ,i have counted six or eight in the space of fifty yards1 of which the collective dimensions could not be less than sixty or seventy feet4 ,he supposed that it would not be an excessive estimate to regard the igneous rock as amounting to one-tenth of the breadth of the strata which it cuts4#afa ,this estimate1 however1 falls much short of the truth in some parts of ,strathaird1 where the dykes are almost or quite contiguous1 and the ,jurassic strata1 through which they rise1 are hardly to be seen at all4 ,footnote #afa3 ,trans4 ,geol4 ,soc4 iii4 7#ahae71 p4 #gi4 ,this locality is further noticed on p4 #afd4 ,among the districts where dykes of the gregarious type abound at a distance from any of the basalt-plateaux1 reference should be made to the curious isolated tract of the central granite core of ,western ,donegal4 ,in that area a considerable number of dykes rises through the granite1 to which they are almost wholly confined4 ,again1 far to the east another limited district1 where dykes are crowded together1 lies among the ,mourne ,mountains4 ,these granite hills are probably to be classed with those of ,arran1 as portions of a series of granite protrusions belonging to a late part of the ,tertiary volcanic period which will be treated of in ,chapter xlvii4 ,though the dykes may be conveniently grouped in two series or types1 which on the whole are tolerably well marked1 it is not always practicable to draw any line between them1 or to say to which group a particular dyke should be assigned4 ,in some districts1 however1 in which they are both developed1 we can separate them without difficulty4 ,in the ,argyleshire region above referred to1 for example1 which ,mr4 ,clough has mapped1 he finds that the abundant dykes belonging to the gregarious type run in a general ,n4,w4 or ,n4,n4,w4 direction1 and distinctly intersect the much scarcer and less basic dykes of the solitary type1 which here run nearly ,e4 and ,w4 7,fig4 #beg74 ,hence1 besides their composition1 distinction in number1 breadth1 rectilinearity and persistence1 the two series in that region demonstrably belong to distinct periods of eruption4#afb ,footnote #afb3 ,mr4 ,clough is inclined to suspect that the ,e4 and ,w4 dykes are older than the ,tertiary series and may be later ,palozoic4 ,the characteristic habit in gregarious dykes of occurring in crowded groups which are separated from each other by intervals of variable dimensions1 marked by the presence of comparatively few dykes1 is well illustrated in the district of ,strath in ,skye1 which indeed may be taken as a typical area for this peculiarity of distribution4 ,while the dykes are there singularly abundant in the ,cambrian ,limestone and the ,liassic strata1 they have been found by ,mr4 ,clough and ,mr4 ,harker to be comparatively infrequent in the tracts of ,torridon ,sandstone4 ,it is not easy to understand this peculiar arrangement4 ,as the ,torridon ,sandstone is the most ancient rock of the district1 it probably underlies all the ,cambrian and ,jurassic formations1 so that the dykes which penetrate these younger strata must also rise through the ,torridonian rocks4 ,some formations appear to have been fissured more readily than others1 and thus to have provided more abundant openings for the uprise of the basaltic magma from below4 ,to the effect of such local differences in the structure of the terrestrial crust we have to add the concentration of the volcanic foci in certain areas1 though there seems no means of ascertaining what part each of these causes has played in the distribution of the dykes of any particular district4 #c4 ,,nature ,,of ,,component ,,rocks ,the ,tertiary dykes of ,britain include representatives of four distinct groups of igneous rocks4 #ast1 ,the vast majority of them consist of plagioclase-pyroxene-magnetite rocks with or without olivine4 ,these are the normal basalts and dolerites4 #bnd1 ,a number of large dykes have a rather more acid composition and are classed as andesites4 #crd1 ,a few dykes of trachyte have been observed in ,cowal and in ,skye cutting the dykes of basalt 7p4 #ach74 #dth1 ,in some districts large numbers of still more acid dykes occur4 ,these are sometimes crystalline in structure 7granophyre71 more frequently felsitic 7felsite1 spherulitic quartz-porphyry71 and often glassy 7pitchstone74 ,in some exceptional cases the basic and acid materials are conjoined in the same dyke4 ,such compound varieties are described at p4 #afa4 ,the acid dykes1 connected as they so generally are with the large bodies of granophyre or granite1 are doubtless younger than the great majority of the basic dykes4 ,they will be treated in connection with the acid intrusions in ,chapter xlviii4 ,by far the greater number of the dykes of the ,tertiary volcanic series belong to the first group1 and it is these more especially which will be discussed in the present and the following ,chapter4 ,as1 however1 the andesitic group is intimately linked with the basaltic it will be here included with them4 #a4 ,basalt1 ,dolerite and ,andesite ,dykes4--,to the field-geologist1 who regards merely their external features1 the ,tertiary dykes present a striking uniformity in general petrographical character4 ,they vary indeed in fineness or coarseness of texture1 in the presence or absence of porphyritic crystals1 amygdales1 glassy portions and other points of structure4 ,but there is seldom any difficulty in perceiving that they generally belong to one or other of the types of the basalts1 dolerites1 diabases or andesites4 ,this sameness of composition1 traceable from ,yorkshire to ,skye and from ,donegal to ,perthshire1 is one of the strongest arguments for referring this system of dykes to one geological period4 ,at the same time1 there are enough of minor variations and local peculiarities to afford abundant exercise for the observing faculties alike in the field and in the study1 and to offer materials for arriving at some positive conclusions regarding the geological processes involved in the uprise of the dykes4 ,there appears to be reason to believe that1 when the petrography of the dykes is more minutely studied1 marked differences of material will be found to denote distinct periods of eruption4 ,already ,mr4 ,a4 ,harker of the ,geological ,survey1 who is engaged in mapping the interesting and complicated district of ,strath in ,skye1 has observed that the dykes which are older than the great granophyre bosses of that tract may be distinguished from those which are later than these protrusions4 ,the older basic dykes are not conspicuously porphyritic1 are frequently marked by a close-grained margin or even with a veneer of basalt-glass1 sometimes have an inclination of as much as #de1 are occasionally discontinuous1 and not infrequently branch or send out veins4 ,the younger dykes1 on the other hand1 as will be more particularly noticed in the following chapter1 are distinguished by the frequent and remarkable character of their porphyritic inclusions1 by the presence of foreign fragments in them1 by the greater perfection of their jointing1 and by their seldom departing much from the vertical4#afc ,they are likewise often markedly acid in composition1 including such rocks as granophyre1 felsite and pitchstone4 ,footnote #afc3 ,in the ,blath ,bheinn group of gabbro-hills1 however1 it is the youngest dykes which have been found by ,mr4 ,harker to possess the lowest hade4 7#a7 ,external ,characters4--,as regards the grain of the rock1 every gradation may be found1 from a coarsely crystalline mass1 in which the component minerals are distinctly traceable with the naked eye1 to a black lustrous basalt-glass4 ,each dyke generally preserves the same character throughout its extent4 ,as a rule1 broad and long dykes are coarser in grain than narrow and short ones4 ,for the most part1 there runs along each side of a dyke a selvage of finer grain than the rest of the mass4 ,this marginal strip varies in breadth from an inch or less up to a foot or more1 and obviously owes its origin to the more rapid chilling of the molten rock along the walls of the fissure4 ,it usually shades away inperceptibly into the larger-grained inner portion4 ,even with the naked eye its component materials can be seen to be more finely crystalline than the rest of the dyke1 though where dispersed porphyritic felspars occur they are as large in the marginal strip as in any other part of a dyke1 for they belong to an earlier period of crystallization than the smaller felspars of the groundmass and were already floating in the magma while it was still in a molten state4 ,this finer-grained external band1 so distinctive of an eruptive and injected rock1 is of great service in enabling us to trace dykes when they traverse other dykes or masses of igneous rock of similar characters to their own4 ,when one dyke crosses another1 that which has its marginal band of finer grain unbroken must obviously be the younger of the two4 ,illustration3 ,fig4 #bce4--,plan of basalt-veins with selvages of black basalt-glass1 east side of ,beinn ,tighe1 ,isle of ,eigg4 ,but in many examples in the south of ,scotland1 ,argyleshire and the ,inner ,hebrides1 the fineness of grain of the outer band culminates in a perfect volcanic glass4 ,where this occurs1 the glass is usually jet black1 more rarely greenish or bluish black in tint1 and varies in thickness from about a couple of inches to a mere varnish-like film on the outer face of the dyke1 the average width being probably less than a quarter of an inch 7,fig4 #bce74 ,on their weathered surface these external glassy layers generally present a pattern of rounded or polygonal prominences1 varying up to four or five lines or even more in diameter1 and separated by depressions or narrow ribs4 ,the transition from the glass to the crystalline part of the marginal fine-grained strip is usually somewhat abrupt1 insomuch that on weathered faces it is often difficult to get good specimens1 owing to the tendency of the vitreous portion to fly off when struck with the hammer4 ,the glass doubtless represents the original condition of the rock of the dyke4 ,it was suddenly chilled and solidified by contact with the cold walls of the fissure4 ,inside this external glassy coating1 the molten material could probably still move1 and had time to assume a more or less completely crystalline condition before solidification4 ,not infrequently the glass shows spherulitic forms1 visible to the naked eye1 and likewise a more or less distinctly developed perlitic structure4 ,these features1 however1 are best studied in thin sections of the rock with the aid of the microscope1 as will be subsequently referred to4 ,in some dykes1 the glass is not confined to the edges1 but runs in strings or broader bands along the central portions1 or has been squeezed into little cavities like steam-holes or into minute fissures4 ,one of the most remarkable examples of this peculiarity occurs in the well-known dyke of ,eskdale1 which runs for so many miles across the southern uplands of ,scotland4#afd ,this dyke throughout most of its course is a crystalline rock of the andesitic type4 ,at ,wat ,carrick1 in ,eskdale1 it presents an arrangement into three parallel bands4 ,on either side1 a zone about eight feet broad consists of the usual crystalline material4 ,between these two marginal portions lies an intercalated mass #af to #ah feet broad1 of a very compact and more or less vitreous rock4 ,the demarcation between this central band and the more crystalline zones of the outside is quite sharp1 and the two kinds of rock show a totally distinct system of jointing4 ,there can1 therefore1 be little doubt that the glassy centre belongs to a later uprise than the outer portions1 though possibly it may still have been included in the long process of solidification of one original injected mass of molten material4 ,if the marginal parts adhered firmly to the walls1 the centre1 which with its band of vesicles seems often to have been a line of weakness1 might be ruptured and subsequent intrusions would find their way along the rent4 ,examples of this splitting of dykes with the intrusion of later eruptive ,material will be cited in later pages4 ,footnote #afd3 ,see ,proc4 ,roy4 ,phys4 ,soc4 ,edin4 v4 7#ahhj71 p4 #bda4 ,mr4 ,clough1 while mapping for the ,geological ,survey the extraordinarily numerous dykes in the eastern part of ,argyleshire between the ,firth of ,clyde and ,upper ,loch ,fyne1 observed six or seven examples of dykes showing glassy bands in their centres1 with characters similar to those of the ,eskdale dyke4 ,he found an absence of definite and regular joints in the central glassy band1 and on the other hand1 an irregular set of divisional planes by which the rock is traversed1 and which he compared to those seen in true perlitic structure4 ,while1 as a general rule1 the external portions of a dyke are closer-grained than the centre1 rare cases occur where the middle is the most finely crystalline part4 ,i am disposed to regard these cases and the glassy centres as forming in reality no true exceptions to the rule1 that the outer portions of a dyke consolidated first1 and are therefore finest in texture4 ,for the most part1 each dyke appears to be due to a single uprise of molten matter1 though considerable movements may have taken place within its mass before the whole stiffened into stone4 ,some particulars regarding these movements will be given in section #ab of the next ,chapter4 ,it has already been mentioned that in large dykes which have served as volcanic pipes1 it is conceivable that while the material next the outside consolidated and adhered to the walls1 the central portion may have remained liquid1 and may even have been propelled upward and have been succeeded by a different kind of magma1 as has been suggested by ,mr4 ,iddings4 ,in such cases1 which1 if they occur1 are probably excessively rare1 we may expect that the earlier and later material will not be sharply marked off from each other1 unless we suppose that the whole of the earlier liquid magma was so entirely ejected that only its congealed marginal selvage was left as bounding walls for the newer injection4 ,where1 after more or less complete consolidation had taken place1 the fissure opened again1 or from any other cause the dyke was split along its centre1 any lava which rose up the rent would tend to take a finer grain than the material of the rest of the dyke1 and might even solidify as glass4 ,large scattered crystals of felspar1 of an earlier consolidation than that of the minuter forms of the same mineral in the general groundmass of the rock1 give a porphyritic structure and andesitic character to many dykes4 ,occasionally such crystals attain a considerable size4 ,mr4 ,clough has observed them in some of the ,argyleshire dykes reaching a length of between three and four inches1 with a thickness of two inches4 ,sometimes they are distributed with tolerable uniformity through the substance of the dyke4 ,but not infrequently they may be observed in more or less definite bands parallel with the boundary walls4 ,unlike the younger lath-shaped and much smaller felspars of the groundmass1 they show no diminution either in size or abundance towards the edge of the dyke4 ,on the contrary1 as already mentioned1 they are often conspicuous in the close-grained marginal strip1 and may be found even in the glassy selvage1 or touching the very wall of the fissure4 ,indeed1 they are sometimes more abundant in the outer than in the inner portions of a dyke1 having travelled outwards to the surfaces of earliest cooling and crystallization4 ,mr4 ,clough has given me the details of an interesting case of this kind observed by him in ,glen ,tarsan1 ,eastern ,argyleshire3--,for an inch or so from the edge of this dyke1 he remarks1 porphyritic felspars giving squarish sections1 and ranging up to one-third of an inch in length1 are so abundant as nearly to equal in bulk the surrounding groundmass4 ,for the next inch and a half1 they are decidedly fewer1 occupying perhaps hardly an eighth of the area exposed4 ,then for a breadth of three inches they come in again nearly as abundantly as at the sides2 after which they diminish through a band #bg inches broad1 where they may form from #a_/h to #a_/ab of the rock4 ,he found another case where1 in a dyke several yards wide1 porphyritic felspars1 sometimes an inch long1 are common along the eastern margin of the dyke in a band about two inches broad1 but nearly absent from the rest of the rock4 ,elsewhere the crystals are grouped rather in patches than in bands4 ,among the dykes south of ,oban some similar instances of coarsely porphyritic felspars may be observed4 ,not only are these porphyritic felspars apt to occur in bands parallel with the outer margins of the dykes1 but they tend to range themselves with their longer axis in the same direction1 thus even on a large scale1 visible at some distance1 showing the flow-structure1 which is so often erroneously regarded as essentially a microscopic arrangement1 and as specially characteristic of superficial lava-streams4 ,mr4 ,harker in his survey of ,strath1 ,skye1 has met with some remarkable examples of the enclosure and incorporation of foreign materials in the younger group of dykes which in that district traverse the granophyres and gabbros4 ,he remarks that the great majority of these dykes are basic1 and he has found them to be capable of convenient division into two groups4 #ast1 ,non-porphyritic basic dykes with a specific gravity between #bhg and #big1 and an amygdaloidal structure affording clear indication of flowing movement1 either at the sides or along a central band4 ,these dykes do not greatly differ from those of pre-granophyre eruption4 #bnd1 ,porphyritic basic dykes which present features of peculiar interest4 ,the porphyritic 7or pseudo-porphyritic7 elements1 according to ,mr4 ,harker's observations1 are constantly felspar1 frequently subordinate augite1 and exceptionally quartz4 ,the felspars have for the most part rounded outlines with a bordering zone of glass cavities apparently of secondary origin4 ,the augite1 in rounded composite crystal-grains1 differs from that of the groundmass and resembles the augite of the gabbros4 ,the quartz-grains are likewise rounded1 and show sometimes a distinct corroded border4 ,these characters1 ,mr4 ,harker observes1 are those of crystals derived from some foreign source1 and it can scarcely be doubted that this is the explanation of their presence4 ,he noticed that the dykes in question frequently enclose fragments1 varying up to several inches in diameter1 of gabbro1 granite or granophyre1 bedded lava1 quartzite1 etc41 which show clear evidence of having been rounded and corroded by an enveloping magma1 and recognizable crystals from some of the fragments may be observed in the surrounding parts of the matrix of the dykes4 ,most of the felspar and augite crystals disseminated through these porphyritic basic dykes may be referred to the partial reabsorption of enclosed fragments of gabbro4 ,the same observer has found that many of the dykes which rise through the basalt-plateau of ,strathaird are crowded with gabbro fragments4 ,another megascopic character of the material composing the dykes is the frequent presence of amygdales4 ,it has sometimes been supposed that amygdaloidal structure may be relied upon as a test to distinguish a mass of molten rock which has reached the surface from one which has consolidated under considerable pressure below ground4 ,that this supposition1 however1 is erroneous is demonstrated by hundreds of dykes in the great system which ,i am now describing4 ,but the amygdales of a dyke offer certain peculiarities which serve in a general way to mark them off from those of an outflowing lava4 ,they are usually smaller and more uniform in size than in the latter rock4 ,they are also more regularly spherical and less frequently elongated in the direction of flow4 ,moreover1 they are not usually distributed through the whole breadth of a dyke1 but tend to arrange themselves in lines especially towards its centre 7,fig4 #bcf74 ,in these central bands the cavities are largest and depart farthest from the regular spherical form1 so that for short spaces they may equal in bulk the mass of enclosing rock4 ,in some rare instances1 a whole dyke is composed of cellular basalt1 like one of the lava-sheets in the plateaux1 as may be seen on the north flank of ,beinn ,suardal1 ,skye4 ,mr4 ,harker has observed that an amygdaloidal structure is more common among the earlier than among the later dykes of that district4 ,illustration3 ,fig4 #bcf4--;,arrangement of lines of amygdales in a dyke1 ,strathmore1 ,skye4 ,besides the common arrangement of fine-grained edges and a more coarsely crystalline centre1 instances are found where one of the contrasted portions of a dyke traverses the other in the form of veins4 ,of these1 ,i think1 there are two distinct kinds1 probably originating in entirely different conditions4 ,in the first place1 they may be of coarser grain than the rest of the rock2 but such a structure appears to be of extremely rare occurrence4 ,i have noticed some examples on the coast of ,renfrewshire1 where strings of a more coarsely crystalline texture traverse the finer-grained body of the rock4 ,veins of this kind are probably of the same nature as the so-called segregation-veins1 to be afterwards referred to as of frequent occurrence among the thicker ,tertiary sills4 ,they consist of the same minerals as the rest of the rock1 but in a different and more developed crystalline arrangement1 and they contain no glassy or devitrified material1 except such portions of that of the surrounding groundmass as may have been caught between their crystalline constituents4 ,the second kind of veins1 which1 though not common1 is of much more frequent occurrence than the first1 is more particularly to be met with among the broader dykes1 and is distinguished by a remarkable fineness of grain1 sometimes approaching the texture of felsite or jasper1 and occasionally taking the form of actual glass4 ,such veins vary from half an inch or less1 up to four or five inches in breadth4 ,they run sometimes parallel with the walls of the dyke1 but often irregularly in all directions1 and for the most part avoid the marginal portions1 though now and then coming up to the edge4 ,they never extend beyond the body of the dyke itself into the surrounding rock4 ,though they have obviously been injected after the solidification of the rock which they traverse1 they may quite possibly be extrusions of a deeper unconsolidated portion of the same rock into rents of the already stiffened overlying parts4 ,the field-geologist cannot fail to be struck with the much greater hardness of these fine-grained veins and strings that ramify through the coarsely crystalline dolerite1 andesite or other variety of the broader dykes4 ,he can readily perceive in many cases their more siliceous composition1 and the inferences he deduces from the rough observations he can make in the field are confirmed by the results of chemical analysis 7see p4 #acg74 ,in connection with veins of finer material1 that may belong to a late stage of the consolidation of the general body of a dyke1 reference may be made here to the occasional occurrence of patches of an exceedingly compact or homogeneous texture immersed in the usual finely crystalline marginal material4 ,they look like angular and subangular portions of the more rapidly cooled outer edge1 which have been broken off and carried upward by the still moving mass in the fissure4#afe ,footnote #afe3 ,see ,mr4 ,j4 ,j4 ,h4 ,teall1 ,quart4 ,journ4 ,geol4 ,soc4 xl4 7#ahhd71 p4 #bad4 ,in general1 each dyke is composed of one kind of rock1 and retains its chemical and mineralogical characters with singular persistence4 ,the difference of texture between the fine-grained chilled margin1 with its occasional glassy coating1 and the more coarsely crystalline centre is due to cooling and crystalline segregation in what was no doubt originally one tolerably uniform molten mass4 ,the glassy central bands1 too1 though they indicate a rupture of the dyke up the middle1 may at the same time quite conceivably be1 as ,i have said1 extrusions from a lower portion of the dyke before the final solidification of the whole4 ,the ramifying veins of finer grain that now and then traverse one of the large dykes are likewise explicable as parts of a stage towards entire consolidation4 ,all these vitreous portions1 whether still remaining as glass or having undergone devitrification1 are more acid than the surrounding crystalline parts of the rock4 ,they represent the siliceous mother-liquor1 so to speak1 which was left after the separation from it of the crystallized minerals1 and which1 perhaps1 entangled here and there in vesicles of the slowly cooling and consolidating rock1 was ready to be forced up into cracks of the overlying mass during any renewal of terrestrial disturbance4 ,but examples occur where a dyke1 instead of consisting of one rock1 is made up of two or more bands of rock which1 even if they resemble each other closely1 can be shown to be the results of separate eruptions4 ,these1 which are obviously not exceptions to the general rule of the homogeneity of dykes1 ,i will consider in the next ,chapter4 ,among the petrographical varieties observable in the field is the occasional envelopment of portions of the surrounding rocks in the body of a dyke4 ,angular fragments torn off from the fissure-walls have been carried upwards in the ascending lava1 and now appear more or less metamorphosed1 the amount of alteration seeming to depend chiefly upon the susceptibility of the enclosed rock to change from the effects of heat4 ,cases of such entanglement1 however1 are of less common occurrence than those already referred to1 where pieces of some deep-seated rock1 such as the gabbros of ,skye1 have been carried up in the ascending magma4 ,occasionally1 where the enclosed fragments are oblong1 they are arranged with their longer axes parallel to the walls of the dyke1 showing flow-structure on a large scale4 ,mr4 ,clough has found some dykes near ,dunoon which enclose fragments of schist nearly three feet in length4 ,one of the most interesting of the megascopic features of the dykes is the joints by which they are traversed4 ,these divisional planes are no doubt to be regarded as consequences of the contraction of the original molten rock during cooling and consolidation between its fissure-walls4 ,they are of considerable interest and importance1 inasmuch as they furnish a ready means of tracing a dyke when it runs through rock of the same nature as itself1 and also help to throw some light on the stages in the consolidation of the material of the dyke4 ,illustration3 ,fig4 #bcg4--,systems of joints in the dykes4 a1 parallel2 b1 transverse4 ,two distinct systems of joints are recognizable 7,fig #bcg74 ,though sometimes combined in the same dyke1 they are most conspicuously displayed when each occurs1 as it generally does1 by itself4 ,the first and less frequent system of joints 7a7 has been determined by lines of retreat1 which are parallel to the walls of the dyke4 ,the joints are then closest together at the margin1 and may be few or altogether absent in the centre4 ,they are sometimes so numerous1 parallel and defined towards the borders of the dyke1 as to split the rock up into thin flags4 ,where transverse joints are also present these flags are divided into irregular tesser4 ,in the second or transverse system of joints 7b71 which is the more usual1 the divisional lines pass across the breadth of the dyke1 either completely from side to side1 or from one wall for a longer or shorter distance towards the other4 ,where this series of joints is most completely developed the dyke appears to be built up of prisms piled horizontally1 or nearly so1 one above another4 ,these prisms1 in rare instances1 are as regular as the columns of a basalt-sheet 7see ,fig4 #aff74 ,usually1 however1 they have irregularly defined faces1 and merge into each other4 ,where the prismatic structure is not displayed1 the joints1 starting sharply at the wall of the dyke1 strike inwards in irregular curving lines4 ,it is such transverse joints that enable the eye1 even from a distance1 to distinguish readily the course of a dyke up the face of a cliff of basalt-beds1 for they belong to the dyke itself1 are often at right angles to those of the adjacent basalt1 and by their alternate projecting and re-entering angles seam the dyke with parallel bars of light and shade 7see the double dyke in ,fig4 #ccc74 ,where they traverse not only the general mass of a dyke1 but also the contemporaneous veins which cross it1 it may be inferred that these veins were injected before the final solidification and contraction of the whole dyke4 ,illustration3 ,fig4 #bch4--,section of cylindrical vein or dyke1 cutting the bedded lavas1 east side of ,fugl1 ,faroe ,islands4 ,an interesting modification of the transverse joints may sometimes be observed1 where1 as in the case of the ,palozoic ,rock and ,spindle1 at ,st4 ,andrews 7,fig4 #bbb71 the molten material has solidified in a tubular or spherical cavity4 ,the joints then radiate inwards from the outer curved surface4 ,the most remarkable instance of this structure which ,i have found among the ,tertiary volcanic plateaux occurs on the east side of the island ,fugl1 the most north-easterly of the group of the ,faroes4 ,it is cut in section by the face of the precipice1 where it appears as a round mass about #dj or #ej feet in diameter piercing the plateau-basalts4 ,a selvage of finer material round its outer edge shows the effect of rapid chilling1 while the joints diverge from the periphery and extend in fan-shape towards the centre 7,fig4 #bch74 ,illustration3 ,fig4 #bci4--;,joint-structures in the central vitreous portion of the ,eskdale ,dyke 7,b4 ,n4 ,peach74 ,a1 ,view of a square yard of the outer wall of the vitreous central band1 showing the polygonal arrangement of the prisms and their investing sheath of ribs4 ,b1 ,view of a smaller portion of the same wall to show the detailed structure of the ribs 7a a7 and their vitreous cores 7b b74 ,c1 ,profile of a part of the weathered face of the wall1 showing the way in which the hard ribs or sheaths project at the surface4 ,one of the most remarkable exhibitions of joint-structure hitherto noticed among the ,tertiary dykes is that which occurs in the central vitreous band of the ,eskdale dyke already referred to4 ,the rock is divided into nearly horizontal prisms1 each of which consists of an inner more vitreous core and an outer more lithoid sheath4 ,by the coherence of their polygonal and irregular faces1 and the greater durability of their material1 these sheaths project on the weathered wall of the vitreous centre of the dyke in a curiously reticulated grouping of prominent ribs each about two inches broad 7,fig4 #bci1 ,a71 while the vitreous cores1 being more readily acted on by the weather1 are hollowed out into little cup-shaped depressions4 ,each rib is thus composed of the sheaths or outer lithoid portions of two prisms1 the line of separation being marked by a suture along the centre 7,b74 ,between this median suture and the inner glassy core the rib is further cut into small segments by a set of close joints1 which are placed generally at right angles to the course of the rib 7,c74 ,examined with a lens1 the lithoid substance of these sheaths has a dull finely granular aspect1 like that of felsitic rocks1 with scattered felspars4 ,it is obviously a more devitrified condition of the material which forms the core of each prism4 ,this material presents on a fresh fracture a deep iron-black colour1 dull resinous lustre and vitreous texture4 ,it at once recalls the aspect of many acid pitchstones1 and in the early days of petrography was naturally mistaken for one of these rocks4 ,through its substance numerous kernels of more glassy lustre are dispersed1 each of which usually contains one or more amygdales of dull white chalcedony1 but sometimes only an empty black cavity4 ,these black glistening kernels of glass1 of all sizes up to that of a small bean1 scattered through the dull resinous matrix1 form with the white amygdales the most prominent feature in the cores2 but crystals of felspars may also be observed4 ,some details of the microscopic characters of this remarkable structure will be given in a subsequent page4 ,the relation of the cores and sheaths to the prismatic jointing of the rock seems to show that devitrification had not been completed when these joints were established1 and that it proceeded from the faces of each prism inwards4 7#b7 ,microscopic ,characters4--,much information has now been obtained regarding the microscopic structure of the basaltic1 doleritic and andesitic dykes4 ,the crystalline characters of those in the ,north of ,england have been studied by ,mr4 ,teall1#aff and some of those from the ,west of ,scotland have been investigated by ,professors ,judd and ,cole4#afg ,taken as a whole1 the rocks composing the dykes are found1 when examined microscopically1 to consist essentially of mixtures of a plagioclase felspar1 pyroxene and iron oxide1 with or without olivine1 and usually with more or less interstitial matter4 ,footnote #aff3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xl4 7#ahhd74 ,footnote #afg3 ,op4 cit4 vol4 xxxix4 7#ahhc7 p4 #ddd 7basalt-glass72 xlii4 7#ahhf7 p4 #di1 where ,professor ,judd discusses the gabbros1 dolerites and basalts as a whole4 ,the felspar appears to be in some cases labradorite1 in others anorthite1 but there may be a mingling of several species in many of the dykes1 as in the augite-andesite of the ,santorin eruption in #ahff1 wherein ,professor ,fouqu found that the larger porphyritic felspars were mainly labradorite1 but partly anorthite1 while those of the groundmass were microlites of albite and oligoclase4#afh ,the large felspars scattered porphyritically through the groundmass are evidently the result of an early consolidation1 unless where they are survivals from fragments of older porphyritic rocks which have been enveloped and partially dissolved in the dykes4 ,they are often cracked1 penetrated by the groundmass1 or even broken into fragments1 and have corroded borders4 ,they sometimes include portions of the groundmass1 and present the zonal growth structure in great perfection4 ,the small felspars of the groundmass1 on the other hand1 are as obviously the result of a later crystallization1 for they vary in size and crystallographic development according to their position in the dyke4 ,those from the centre are often in well-formed crystals1 which sometimes pass round their borders into acicular microlites4 ,those in the marginal parts of the dyke occur chiefly in the form of these microlites1 forming the felted aggregate so characteristic of the andesites4 ,curious skeleton forms1 composed of aggregates of microlites1 connect the latter with the more completely developed crystals1 and illustrate the mode of crystallization of the felspathic constituents of the dykes4#afi ,footnote #afh3 ,santorin et ses ruptions1 #ahgi1 p4 #bjc4 ,footnote #afi3 ,see ,mr4 ,teall's excellent description of the ,cleveland dyke1 in the paper above cited4 ,the pyroxene is probably in most cases monoclinic 7black or common augite71 but is sometimes rhombic 7usually enstatite1 less frequently perhaps hypersthene74 ,it occurs in 7a7 well-developed crystals1 7b7 crystalline masses with some of the faces of the crystals developed1 7c7 granular aggregates which polarise in one plane1 7d7 separate granules and microscopic microlites1 which may be spherical 7globulites7 or oblong 7longulites74 ,the black iron-oxide is sometimes magnetite1 sometimes ilmenite1 or other titaniferous ore4 ,apatite not infrequently occurs among the original constituents4 ,olivine is entirely absent from most of the large solitary dykes1 especially at a distance from the great volcanic centres1 and no serpentinous matter remains to indicate that it was ever present in them4 ,but it is to be met with in numerous basalt-dykes in the volcanic areas1 either in sparsely scattered or in tolerably abundant crystals4 ,biotite occasionally appears4 ,among the secondary products1 calcite and pyrites are doubtless the most common4 ,to these must be added quartz1 chalcedony and various zeolitic substances1 besides the aggregates which result from the decomposition of the ferro-magnesian constituents and the oxidation of the ferrous oxides4 ,in many dykes there is little or no interstitial matter between the crystalline constituents of the groundmass4 ,in others this matter amounts to a half or more of the whole composition1 and from such cases a series of gradations may be traced into a complete glass containing only the rudimentary forms of crystals 7globulites1 longulites1 etc471 with scattered porphyritic crystals of an earlier consolidation4 ,the process of the disappearance of this original glass may be admirably studied in many dykes4 ,at the outer wall1 the glass remains nearly as it was when contact with the cold walls of the fissure solidified it4 ,from that external vitreous layer the successive devitrification products and crystalline growths may be followed inwards until in the central parts of a broad dyke little or no trace of the interstitial matter may be left4 ,illustration3 ,fig4 #bdj4--,microscopic structure of the vitreous part of the ,eskdale ,dyke4 ,this section shows a crystal of augite1 enclosing magnetite and surrounded with microlites1 each of which consists of a central pale yellow rod crusted with pale yellow isotropic globulites4 ,the glass around this aggregation is clear1 but at a little distance globulites 7many of them elongated and dichotomous7 abound1 with here and there scattered microlites1 some of which are curved and spiral4 7#hjj diameters47#agj ,footnote #agj3 ,proc4 ,roy4 ,phys4 ,soc4 ,edin4 v4 7#ahhj71 p4 #bee4 ,the most instructive example of the process of devitrification which has come under my observation occurs in the ,eskdale dyke4 ,the central cores already referred to present a true glass1 which in thin sections is perfectly transparent and almost colourless1 but by streaks and curving lines of darker tint shows beautiful flow-structure4 ,the devitrification of this glass has been accomplished by the development of crystallites and crystals1 which increase in number until all the vitreous part of the rock disappears4 ,what seems under a low power to be a structureless or slightly dusty glass can be resolved with a higher objective into an aggregate of minute globules or granules 7globulites71 which average perhaps #a_/bj1jjj of an inch in diameter4 ,some of these bodies are elongated and even dichotomous at the ends4 ,these granules are especially crowded upon clear yellow dart-shaped rods1 which in turn are especially prominent upon crystals and crystalline grains of augite that bristle with them1 while the immediately surrounding glass has become clear4 ,there can be little doubt that these rudimentary bodies are stages in the arrested development of augite crystals4 ,there occur also opaque grains1 rods and trichites1 which no doubt consist in whole of magnetite 7or other iron oxide71 or are crusted over with that mineral4 ,at least two broad types of microscopic structure may be recognized among the basic and intermediate dykes4 7#a7 ,holocrystalline1 or with only a trifling proportion of interstitial matter4 ,this type includes the dolerites and basalts1 as well as rocks which ,german petrographers would class as diabases or diabase-porphyrites4 ,the rocks are very generally characterized by ophitic structure1 where the lath-shaped felspars penetrate the augite1 and are therefore of an earlier consolidation4 ,in such cases there is a general absence of any true interstitial matter4 ,the rocks of this type are often rich in olivine1 and appear to be on the whole considerably more basic than those of the second group4 ,it is observable that they increase in numbers from the centre of ,scotland westwards1 and throughout the region of the basalt-plateaux they form the prevailing type4 7#b7 ,in this type there is a marked proportion of interstitial substance1 which is inserted in wedge-shaped portions among the crystallised constituents 7intersertal structure of ,rosenbusch74 ,the ophitic structure appears to be absent1 and olivine is either extremely rare or does not occur at all4 ,the rocks of this group are obviously less basic than those of the other4 ,they form the large dykes that rise so conspicuously through the ,south of ,scotland and ,north of ,england1 and their general characters are well described by ,mr4 ,teall in the paper already cited4 ,in some instances they enclose abundant porphyritic felspars of earlier consolidation1 and then present most of the characters of andesites4 ,professor ,rosenbusch has extended the name of ,tholeiites to rocks of this group in the ,north of ,england4#aga ,the vitreous condition is found in both types1 but is perhaps more frequent in the second4 ,the glass of the basalts1 however1 even in thin slices1 is characteristically opaque from its crowded inclusions2 while that of the andesitic forms1 though black in hand specimens1 appears perfectly transparent and sometimes even colourless in thin slices4 ,footnote #aga3 ,mikroskopische ,physiographie1 #crd edit4 #ajga et seq4 7#c7 ,chemical ,characters4--,the only one of these to which reference will be made here is the varying proportion of silica4 ,while the dykes as a whole are either intermediate or basic1 some of them contain so high a percentage of silica as to link them with the acid rocks4 ,the average proportions of this ingredient range from less than #ej to nearly #fj per cent4 ,the rocks with the lower percentage of acid are richer in the heavy bases1 and have a specific gravity which sometimes rises above #cj4 ,they include the true dolerites and basalts4 ,those1 on the other hand1 with the higher ratio of silica1 are poorer in the heavy bases1 and have a specific gravity from #bgf to #bif4 ,they comprise the tholeiites1 andesites and other more coarsely crystalline rocks of the great eastern and south-eastern dykes4#agb ,footnote #agb3 ,for analyses of dykes1 see ,sir ,i4 ,l4 ,bell1 ,proc4 ,roy4 ,soc4 xxiii4 p4 #edf2 ,mr4 ,j4 ,s4 ,grant ,wilson1 ,proc4 ,roy4 ,phys4 ,soc4 ,edin4 v4 p4 #bec2 ,mr4 ,teall1 ,quart4 ,journ4 ,geol4 ,soc4 xl4 p4 #bji2 ,professors ,judd and ,cole1 ,quart4 ,jour4 ,geol4 ,soc4 xxxix4 p4 #ddd4 ,not only do the dykes differ considerably from each other in their relative proportions of silica1 but even the same dyke may sometimes be found to present a similar diversity in different parts of its mass4 ,it has long been a familiar fact that the glassy parts of such rocks are more acid than the surrounding crystalline portions4 ,the original magma may be regarded as a natural glass or fused silicate1 in which all the elements of the rock were dissolved1 and which necessarily became more acid as the various basic minerals crystallised out of it4#agc ,in the ,eskdale dyke the silica percentage of this glassy portion is #ehfg1 that of the little kernels of black glass dispersed through the rock as much as #fedi4agd ,in the ,dunoon dyke observed by ,mr4 ,clough the siliceous finer-grained veins contain no less than #fhje per cent of silica1 while the mass of the dyke itself shows on analysis only #dgcf per cent4#age ,similar red strings have been noticed by the same careful observer in an east and west dyke near ,lochgoilhead4 ,from ,mr4 ,teall's examination a large part of the felspar in these veins is probably orthoclase4 ,it forms a much larger percentage of the entire rock than the felspar does in normal dolerites4 ,footnote #agc3 ,on this subject see a paper by ,dr4 ,a4 ,lagorio1 ber die ,natur der ,glasbasis sowie der ,krystallisationsvorgnge im eruptiven ,magma1 ,tschermak's ,mineralog4 ,mittheil4 viii4 7#ahhg71 p4 #dba4 ,footnote #agd3 ,mr4 ,j4 ,s4 ,grant ,wilson1 ,proc4 ,roy4 ,soc4 ,phys4 ,edin4 v4 7#ahhj7 p4 #bec4 ,footnote #age3 ,unpublished analyses made by the late ,professor ,dittmar of ,glasgow1 and communicated to me by ,mr4 ,clough4 #b4 ,trachyte ,dykes4--,in the ,cowal ,district of ,argyleshire1 and in the south of ,skye1 ,mr4 ,clough has encountered a limited number of dykes of trachyte4 ,on a hasty inspection these are not always readily distinguished from the basalt-dykes with which they agree in general external aspect and in direction4 ,where their relation to these dykes1 however1 can be determined they are found to traverse them1 and thus to be on the whole later1 though one case has been observed where a trachytic dyke is in turn traversed by one of the basic series4 ,mr4 ,clough has supplied me with the following notes of his observations regarding the trachytic dykes4 ,they are all characterized by the possession of spherulitic structures near their margins4 ,these features1 easily perceptible to the naked eye1 afford the readiest means of distinguishing the dykes of this group4 ,so abundant are the spherulites that they not infrequently impinge on each other in long parallel rows forming rod-like aggregates4 ,thus in a dyke near ,craigendavie1 at the head of ,loch ,striven1 numerous planes about a quarter of an inch apart1 and composed of such close-set rods1 may be observed running parallel to the marginal wall for a distance of several inches from the edge4 ,most of these planes show on their surfaces that the rods are always parallel to each other1 but may run in different directions in the different layers1 being sometimes horizontal1 sometimes vertical1 or at any angle between4 ,on examination1 each rod is found to be made up of polygonal bodies1 the angles of which are quite sharp1 but with their sides often slightly curved1 as if they had assumed their forms from the mutual pressure of original spherical bulbs4 ,further scrutiny shows that the polygonal bodies often exhibit an internal radiate structure4 ,in the central parts of the dyke the spherulitic arrangement is not traceable4 ,about a foot from the margin it begins to be recognizable4 ,at a distance of three or four inches the spherulites are about the size of peas1 and gradually diminish towards the edge until they can no longer be seen4 ,another characteristic of the trachyte dykes has been found by ,mr4 ,clough to be a useful guide in discriminating them from the basalt-group4 ,while the amygdales in the latter are generally rudely spherical1 those in the trachytes are commonly elongated in the direction of the length of the dyke1 and are frequently three quarters of an inch1 sometimes even an inch and a half1 in length1 though less than a quarter of an inch in breadth4 ,a good example of these trachytic dykes1 which occurs at ,dunans1 about the head of ,glendaruel1 has been examined microscopically and chemically4 ,the central better crystallised portion was found by ,mr4 ,teall to be composed mainly of small lath-shaped crystals of orthoclase1 together with scales of brown biotite1 a few prismatic crystals of pale somewhat altered pyroxene and scattered granules of magnetite4 ,the chemical analysis of this rock by ,mr4 ,j4 ,h4 ,player gave the following composition3-- ,silica #efd ,alumina #aij ,ferric oxide #ce ,ferrous oxide #dh ,lime #bf ,magnesia #ae ,soda #de ,potash #ej ,loss on ignition #bf ---- #iii #d4 ,,hade ,in the majority of cases1 especially among the great single dykes1 the intrusive rock has assumed a position nearly or quite vertical4 ,but occasionally1 where one of these solitary examples crosses a deep valley1 a slight hade is perceptible by the deviation of the line of the dyke from its normal course4 ,sedgwick long ago noticed that the ,cleveland dyke has1 in places1 an inclination of at least #hj to its ,n4,e4 side4#agf ,in the coal-workings1 also1 a trifling deviation from the vertical is sometimes perceptible1 especially where a dyke has found its way along a previously existing line of fault1 as in several examples in ,stirlingshire4 ,but in those districts where the dykes are gregarious1 departures from the vertical position are not infrequent1 more particularly near the great basalt-plateaux4 ,it was noticed by ,necker1 that even in such a dyke-filled region as ,arran1 almost all of the dykes are vertical1 though sometimes deviating from that position to the extent of #bj4agg ,berger found that the angle of deviation among those of the north of ,ireland ranges from #i to #bj1 with a mean of #ac4agh ,the most oblique examples are probably those which occur in the basalt-plateaux of the ,inner ,hebrides1 where the same dyke in some parts of its course runs horizontally between two beds1 across which it also descends vertically 7see ,figs4 #bea1 #beb1 #cgd74 ,but with these minor exceptions1 the verticality of the great system of dykes1 pointing to the perpendicular fissure-walls between which the molten rock ascended1 is one of the most notable features in their geological structure4 ,in the ,strath district of ,skye ,mr4 ,harker has noticed that while the earlier dykes have sometimes a hade of #de1 those younger than the granophyre are generally vertical or nearly so4 ,in the ,blath ,bheinn group of hills1 however1 as already alluded to1 he has observed that it is the youngest dykes which are inclined in a north-westerly direction1 with a hade of as much as #dj from the horizon4 ,footnote #agf3 ,cambridge ,phil4 ,trans4 ii4 p4 #bh4 ,footnote #agg3 ,trans4 ,roy4 ,soc4 ,edin4 xiv4 p4 #fgg4 ,footnote #agh3 ,trans4 ,geol4 ,soc4 iii4 p4 #bbg4 #e4 ,,breadth ,an obvious characteristic of most dykes is the apparent uniformity of their breadth4 ,many of them1 as exposed along shore-sections1 vary as little in dimensions as well-built walls of masonry do4 ,departures from such uniformity may often indeed be noted1 whether a dyke is followed laterally or vertically4 ,the largest amount of variation is1 of course1 to be found among the dykes of the gregarious type1 the thinner examples of which may diminish to a width of only one inch or less1 while their average breadth is much smaller than in the case of the great solitary dykes4 ,in the district of ,strathaird1 in ,skye1 ,macculloch estimated that the remarkably abundant dykes there developed vary from #e to #bj feet in breadth1 but with an average breadth of not more than #aj feet4#agi ,in the isle of ,arran1 according to ,necker's careful measurements1 most of the dykes range from #b or #c to #aj or #ae feet1 but some diminish to a few inches1 while others reach a width of #bj1 #cj1 or even #ej feet4#ahj ,in the ,north of ,ireland1 ,berger observed that the average breadth of thirty-eight dykes traversing primitive rocks 7schist1 granites1 etc47 was #i feet2 and of twenty-four in ,secondary rocks1 #bd feet4#aha ,footnote #agi3 ,trans4 ,geol4 ,soc4 iii4 p4 #hj4 ,footnote #ahj3 ,trans4 ,roy4 ,soc4 ,edin4 xiv4 p4 #fij et seq4 ,footnote #aha3 ,trans4 ,geol4 ,soc4 iii4 p4 #bbf4 ,he believed that dykes in ,secondary rocks reach a much greater thickness than in other formations4 ,my own observations do not confirm this generalisation4 ,but when we pass to the great solitary dykes1 that run so far and so continuously across the country1 we encounter much thicker masses of igneous rock4 ,most of the measurements of these dykes have been made at the surface1 and the variations noted in their breadth occur along their horizontal extension4 ,the ,cleveland dyke1 which is the longest in ,britain1 varies from #ae feet to more than #ajj feet1 with perhaps an average width of between #gj and #ij feet4#ahb ,some of the great dykes that cross ,scotland are of larger dimensions4 ,most of them1 however1 like that of ,cleveland1 are liable to considerable variations in breadth when followed along their length4 ,the dyke which runs from the eastern coast across the ,cheviot ,hills and ,teviotdale to the head of the ,ale ,water1 is in some places only #aj feet broad1 but at its widest parts is probably about #ajj feet4 ,the ,eskdale and ,moffat dyke is in parts of its course #ahj feet wide1 but elsewhere it diminishes to not more than #dj feet4 ,these variations are repeated at irregular intervals1 so that the dyke alternately widens and contracts as its course is traced across the hills4 ,some of the dykes further to the north and west attain yet more gigantic proportions4 ,that which crosses ,cantyre opposite ,ardlamont ,point has been measured by ,mr4 ,j4 ,b4 ,hill1 of the ,geological ,survey1 who finds it to be from #aej to #ahj feet broad on the shore of ,loch ,fyne1 and to swell out beyond the west side of ,loch ,tarbert to a breadth of #bdj to #bgj feet4 ,a dyke near ,strathmiglo1 in ,fife1 is about #djj feet wide4 ,the broadest dyke known to me is one which ,i traced near ,beith1 in ,ayrshire1 traversing the ,carboniferous ,limestone4 ,its maximum width is #fdj feet4 ,footnote #ahb3 ,at ,cockfield1 where it has long been quarried1 it varies from #ae to #ff feet2 at ,armathwatie1 in the vale of the ,eden1 it is about #ed feet 7,mr4 ,teall1 ,quart4 ,journ4 ,geol4 ,soc4 xl4 p4 #baa74 ,unfortunately1 it is much less easy to get evidence of the width of dykes at different levels in their vertical extension4 ,yet this is obviously an important point in the theoretical discussion of their origin4 ,two means are available of obtaining information on the subject--7a7 from mining operations1 and 7b7 from observations at precipices and between hill-crests and valley-bottoms4 7a7 ,in the ,central ,scottish coal-field and in that of ,ayrshire1 some large dykes have been cut through at depths of two or three hundred feet beneath the surface4 ,but there does not appear to be any well-ascertained variation between their width so far below ground and at the surface4 ,in not a few cases1 indeed1 dykes are met with in the lower workings of the coal-pits which do not reach the surface or even the workings in the higher coals4 ,such upward terminations of dykes will be afterwards considered1 and it will be shown that towards its upper limit a dyke may rapidly diminish in width4 7b7 ,more definite information1 and often from a wider vertical range1 is to be gathered on coast-cliffs and in hilly districts1 where the same dyke can be followed through a vertical range of many hundred feet4 ,but so far as my own observations go1 no general rule can be established that dykes sensibly vary in width as they are traced upward4 ,every one who has visited the basalt-precipices of ,antrim or the ,inner ,hebrides1 where dykes are so numerous1 will remember how uniform is their breadth as they run like ribbons up the faces of the escarpments4#ahc ,now and then one of them may be observed to die out1 but in such cases 7which are far from common7 the normal width is usually maintained up to within a few feet of the termination4 ,footnote #ahc3 ,this point did not escape the attention of that excellent observer1 ,berger1 in his examination of the dykes in the ,north of ,ireland4 ,we find him expressing himself thus3--,the depth to which the dykes descend is unknown2 and after having observed the sections of a great many along the coast in cliffs from #ej to #djj feet in height1 ,i have not been able to ascertain 7except in one or two cases7 that their sides converge or have a wedgeform tendency 7,trans4 ,geol4 ,soc4 iii4 p4 #bbg74 ,all over the southern half of ,scotland1 where the dykes run along the crests of the hills and also cross the valleys1 a difference of level amounting to several hundred feet may often be obtained between adjacent parts of the same dyke4 ,but the breadth of igneous rock is not perceptibly greater in the valleys than on the ridges4 ,the depth of boulder clay and other superficial deposits on the valley bottoms1 however1 too frequently conceals the dykes at their lowest levels4 ,perhaps the best sections in the country for the study of this interesting part of dyke-structure are to be found among the higher hills of the ,inner ,hebrides1 such as the quartzites of ,jura and the granophyres and gabbros of ,skye4 ,on these bare rocky declivities1 numerous dykes may be followed from almost the sea-level up to the rugged and splintered crests1 a vertical distance of between #bjjj and #cjjj feet4 ,the dykes are certainly not as a rule sensibly less in width on the hill-tops than in the glens4 ,so far1 therefore1 as ,i have been able to gather the evidence1 there does not appear to me to be1 as a general rule1 any appreciable variation in the width of dykes for at least #bjjj or #cjjj feet of their descent4 ,the fissures which they filled must obviously have had nearly parallel walls for a long way down4 #f4 ,,interruptions ,,of ,,lateral ,,continuity ,in tracing the great solitary dykes across the country1 the geologist is often surprised to meet with gaps1 varying in extent from a few hundred feet to several miles1 in which no trace whatever of the igneous rock can be detected at the surface4 ,this disappearance is not always explicable by the depth of the cover of superficial accumulations2 for it may be observed over ground where the naked rocks come almost everywhere to the surface1 and where1 therefore1 if the conspicuous material of the dykes existed1 it could not fail to be found4 ,no dyke supplies better illustrations of this discontinuity than that of ,cleveland4 ,traced north-westward across the ,carboniferous tracts that lie between the mouth of the ,tees and the ,yale of the ,eden1 this dyke disappears sometimes for a distance of six or eight miles4 ,in the mining ground round the head of the ,south ,tyne the rocks are bare1 so that the absence of the dyke among them can only be accounted for by its not reaching the surface4 ,yet there can be no doubt that the various separated exposures1 which have the same distinctive lithological characters and occur on the same persistent line1 are all portions of one dyke which is continuous at some depth below ground4 ,we have thus an indication of the exceedingly irregular upward limit of the dykes1 as will be more particularly discussed further on4 ,but there are also instances where the continuity is interrupted and then resumed on a different line4 ,one of the best illustrations of this character is supplied by the large dyke which rises through the hills about a mile south of ,linlithgow and runs westward across the coal-field4 ,at ,blackbraes it ends off in a point1 and is not found again to the westward in any of the coal-workings4 ,but little more than a quarter of a mile to the south a precisely similar dyke begins1 and strikes westward parallel to the line of the first one4 ,the two separated strips of igneous rock overlap each other for about three-quarters of a mile4 ,but that they are merely interrupted portions of what is really a single dyke can hardly be questioned4 ,a second example is furnished by another of the great dykes of the same district1 which after running for about twelve miles in a nearly east and west direction suddenly stops at ,chryston1 and begins again in the same direction1 but on a line about a third of a mile further north4 ,such examples serve to mark out irregularities in the great fissures up which the materials of the dykes rose4 #g4 ,,length ,in those districts where the small and crowded dykes of the gregarious type are developed1 one cannot usually trace them for more than a short distance4 ,the longest examples known to me are those which have been mapped with much patience and skill by ,mr4 ,clough in ,eastern ,argyleshire4 ,some of them he has been able to track over hill and valley for four or five miles1 though the great majority are much shorter4 ,in ,arran and in the ,inner ,hebrides1 it is seldom possible to follow what we can be sure is the same dyke for more than a few hundred yards4 ,this difficulty arises partly1 no doubt1 from the frequent spread of peat or other superficial accumulation which conceals the rocks1 and partly also from the great number of dykes and the want of sufficiently distinct lithological characters for the identification of any particular one4 ,but making every allowance for these obstacles1 we are compelled1 ,i think1 to regard the gregarious dykes as essentially short as well as relatively irregular4 ,in striking contrast to these1 come the great solitary dykes4 ,in estimating their length1 as ,i have already remarked1 we must bear in mind the fact that they occasionally undergo interruptions of continuity owing to the local failure of the igneous material to rise to the level of what is now the surface of the ground4 ,a narrow wall-like mass of andesite or dolerite1 which sinks beneath the surface for a few hundred yards1 or for several miles1 and reappears on the same line with the same petrographical characters1 while there may be no similar rock for miles to right and left1 can only be one dyke prolonged underneath in the same great line of fissure4 ,but even if we restrict our measurements of length to those dykes or parts of dykes where no serious interruption of continuity takes place1 we cannot fail to be astonished at the persistence of these strips of igneous rock through the most diverse kinds of geological structure4 ,a few illustrative examples of this feature may be selected4 ,it will be observed that the longest and broadest dykes are found furthest from the basalt-plateaux1 while the shortest and narrowest are most abundant near these plateaux4 ,not far from what ,i have taken provisionally as the northern boundary of the dyke region1 two dykes occur which have been mapped from the head of ,loch ,goil by ,arrochar across ,lochs ,lomond and ,katrine by ,ben ,ledi to ,glen ,artney1 whence they strike into the ,old ,red ,sandstone of ,strathmore1 and run on to the ,tay near ,perth--a total distance of about #fj miles4 ,if the dyke which continues in the same line on the other side of the estuary of the ,tay beyond ,newburgh1 is a prolongation of one of these1 then its entire length exceeds #gj miles4 ,a few miles further south1 one of a group of dykes can be followed from the heart of ,dumbartonshire by ,callander across the ,braes of ,doune to ,auchterarder--a distance of #dg miles1 with an average breadth of more than #ajj feet4 ,in the district between the ,forth and ,clyde a number of long parallel dykes can be traced for many miles across hill and plain1 and through the coal-fields4 ,one of these is continuous for #be miles from the heart of ,linlithgowshire into ,lanarkshire4 ,still longer is the dyke which runs from the ,firth of ,forth at ,grangemouth westward to the ,clyde1 opposite ,greenock--a distance of about #cf miles4 ,coming southward1 we encounter a striking series of single dykes on the uplands between the counties of ,lanark and ,ayr1 whence they strike into the ,silurian hills of the southern counties4 ,one of these runs across the crest of the ,haughshaw ,hills1 and can be followed for some #cj miles4 ,but if1 as is probable1 it is prolonged in one of the dykes that traverse the moorlands of the north of ,ayrshire and south of ,renfrewshire to the ,clyde1 its actual length must be at least twice that distance4 ,the great ,moffat and ,eskdale dyke strikes for more than #ej miles across the ,south of ,scotland and ,north of ,england4 ,the ,hawick and ,cheviot dyke runs for #bf miles in ,scotland and for #cb miles in ,northumberland4 ,but the most remarkable instance of persistence is furnished by the ,cleveland dyke4 ,from where it is first seen near the coast-cliffs of ,yorkshire the strip of igneous rock can be followed1 with frequent interruptions1 during which for sometimes several miles no trace of it appears at the surface1 across the ,north of ,england as far as ,dalston ,hall south of ,carlisle1 beyond which the ground onwards to the ,solway ,firth is deeply covered with superficial deposits4 ,the total distance through which this dyke can be recognized is thus about #aaj miles4 ,but it probably goes further still4 ,on the opposite side of the ,solway1 a dyke which runs in the same line1 rises through the ,permian strata a little to the east of the mouth of the ,nith4 ,some miles further to the north-west1 near ,moniaive1 ,mr4 ,j4 ,horne1 in the progress of the ,geological ,survey1 traced a dark compact dyke with kernels of basalt-glass near its margin1 running in the same north-westerly direction4 ,still further on in the same line1 another similar rock is found high on the flanks of the lofty hill known as ,windy ,standard4 ,and lastly1 in the ,ayrshire coal-field1 a dyke still continuing the same trend1 runs for several miles1 and strikes out to sea near ,prestwick4 ,it cannot1 of course1 be proved that these detached ,scottish protrusions belong to one great dyke1 or that if such a continuous dyke exists1 it is a prolongation of that from ,cleveland4 ,at the same time1 ,i am on the whole inclined to connect the various outcrops together as those of one prolonged subterranean wall of igneous rock4 ,the distance from the last visible portion of the ,cleveland dyke near ,carlisle to the dyke that runs out into the ,firth of ,clyde near ,prestwick1 is about #hj miles4 ,if we consider this extension as a part of the great ,north of ,england dyke1 then the total length of this remarkable geological feature will be about #aij miles4 #h4 ,,persistence ,,of ,,mineral ,,characters ,not less remarkable than their length is the preservation of their normal petrographical characters by some dykes for long distances4 ,in this respect the ,cleveland dyke may again be cited as a typical example4 ,the megascopic and microscopic structures of the rock of this dyke distinguish it among the other eruptive rocks of the ,north of ,england4 ,and these peculiarities it maintains throughout its course4#ahd ,similar though less prominent uniformity may be traced among the long solitary dykes of the ,south of ,scotland1 the chief variations in these arising from the greater or less extent to which the original glassy magma has been retained4 ,the same dyke will at one part of its course show abundant glassy matter even to the naked eye1 while at a short distance the vitreous groundmass has been devitrified1 and its former presence can only be detected with the aid of the microscope4 ,where a dyke has caught up and absorbed abundant foreign materials its composition naturally varies considerably from point to point4 ,mr4 ,harker has observed some good examples of this variation in ,skye4 ,footnote #ahd3 ,see the careful examination of this dyke by ,mr4 ,teall1 ,quart4 ,journ4 ,geol4 ,soc4 xl4 p4 #bji4 ,,chapter ,,xxxv ,,the ,,system ,,of ,,dykes--continued ,direction--,termination upward--,known vertical ,extension--,evidence as to the movement of the ,molten ,rock in the ,fissures--,branches and ,veins--,connection of ,dykes with ,intrusive ,sheets--,intersection of ,dykes--,dykes of more than one infilling--,contact ,metamorphism of the ,dykes--,relation of the ,dykes to the ,geological ,structure of the ,districts which they traverse--,data for estimating the ,geological ,age of the ,dykes--,origin and ,history of the ,dykes4 #i4 ,,direction ,another characteristic feature of the dykes is their generally rectilinear course4 ,so true are the solitary dykes to their normal trend that1 in spite of varying inequalities of surface and wide diversities of geological structure in the districts which they traverse1 they run over hill and dale almost with the straightness of lines of ,roman road4 ,in the districts where the gregarious type prevails1 the dykes depart most widely from the character of the great solitary series1 but still tend to run in straight or approximately straight lines1 or1 if wavy in their course1 to preserve a general parallelism of direction4 ,yet even among the great persistent dykes instances may be cited where the rectilinear trend is exchanged for a succession of zig-zags1 though the normal direction is on the whole maintained4 ,in such cases1 it is evident that the fissures were not long straight dislocations1 like the larger lines of fault in the earth's crust1 but were rather notched rents or cracks which1 though keeping1 on the whole1 one dominant direction1 were continually being deflected for short distances to either side4 ,as a good illustration of this character1 reference may be made to the ,cheviot and ,hawick dyke4 ,in ,teviotdale1 this dyke can be followed continuously among the rocky knolls1 so that its deviations can be seen and mapped4 ,from the median line of average trend the salient angles sometimes retire fully a quarter of a mile on either side4 ,some examples of the same feature may be noticed in the ,eskdale dyke4 ,the large dyke which runs westward from ,dunoon has been observed by ,mr4 ,clough to change sharply in direction three times in four miles1 running occasionally for a short distance at a right angle to its general direction 7see ,fig4 #beg74 ,among these solitary dykes also1 though the persistence of their trend is so predominant1 there occur instances where the general direction undergoes great change4 ,some of the most remarkable cases of this kind have been mapped by ,mr4 ,b4 ,n4 ,peach and ,mr4 ,r4 ,l4 ,jack1 in the course of the ,geological ,survey of ,perthshire4 ,several important dykes strike across the ,old ,red ,sandstone plain for many miles in a direction slightly south of west4 ,but when they approach the rocks of the ,highland border in ,glen ,artney1 they bend round to south-west1 and continue their course along that new line4 ,many years ago ,i called attention to the dominant trend of the dykes from north-west to south-east4#ahe ,subsequent research has shown this to be on the whole the prevalent direction throughout the whole region of dykes4 ,but the detailed mapping1 carried on by my colleagues and myself in the ,geological ,survey1 has brought to light some curious and interesting variations from the normal trend4 ,in the districts where dykes of the gregarious type abound there is sometimes no one prevalent direction1 but the dykes strike to almost all points of the compass4 ,of the ,arran dykes1 so carefully catalogued by ,necker1 only about a third have a general north-westerly course4 ,but in ,eastern ,argyleshire the abundant dykes mapped by ,mr4 ,clough trend almost without exception towards ,n4,n4,w4 ,in the ,north of ,ireland1 ,berger found the direction of thirty-one dykes to vary from #ag to #ga ,w4 of ,n41 giving a mean of ,n4 #cf ,w4#ahf ,in ,islay1 ,jura1 ,eigg1 ,mull1 and ,skye the mean of several hundred observations has given me similar results4 ,among the ,inner ,hebrides1 however1 though the general north-westerly trend is characteristic1 many of the later dykes show marked departures from it4 ,thus in ,strath1 ,skye1 some of the youngest follow a nearly north and south direction 7,fig4 #bec74 ,in the ,blath ,bhein hill-range1 ,mr4 ,harker has found that the latest dykes cut the gabbro at right angles to the prevalent trend and are further distinguished by their low hade4 ,footnote #ahe3 ,trans4 ,roy4 ,soc4 ,edin4 xxii4 7#ahfa71 p4 #fej4 ,footnote #ahf3 ,trans4 ,geol4 ,soc4 iii4 p4 #bbe4 ,it appears1 therefore1 that though there is sometimes extraordinary local diversity in the direction of the dykes in those districts where they present the gregarious type1 the general north-westerly trend can usually still be recognized4 ,but when we turn to the long massive solitary dykes1 we soon perceive a remarkable change in their direction as we follow them northward into ,scotland4 ,i formerly pointed out how the general north-westerly trend becomes east and west in the ,lothians1 with a tendency to veer a little to the south of west and north of east4#ahg ,this departure from the normal direction is now seen to be part of a remarkable radial arrangement of the dykes4 ,beginning at the southern margin of the dyke-region1 we have the notable example of the ,cleveland dyke1 which in its course from ,cleveland to ,carlisle runs nearly ,w4 #ae ,n4 ,the ,eskdale dyke has an average trend of ,w4 #cb ,n41 and the same general direction is maintained by the group of dykes which run from the ,southern ,uplands across the south-west of ,lanarkshire and north-east of ,ayrshire4 ,but proceeding northwards we observe the trend to turn gradually round towards the west4 ,the dyke that runs from near the mouth of the ,coquet across the ,cheviot ,hills to beyond ,hawick has a general course of ,w4 #h ,n4 ,in the great central coal-field of ,scotland the average direction may be taken to be nearly east and west1 the same dyke running sometimes to the north1 and sometimes to the south of that line4 ,but immediately to the north a decided tendency to veer round southwards makes its appearance4 ,thus the long dyke which runs from the ,carse of ,stirling through the ,campsie ,fells to the ,clyde west of ,leven1 has a mean direction of ,w4 #e ,s4 ,this continues to be the prevalent trend of the remarkable series of dykes which crosses the ,old ,red ,sandstone plains1 though some of these revert in whole or in part to the more usual direction by keeping a little to the north of west4 ,even as far as ,loch ,tay and the head of ,strathardle1 the course of the dykes continues to be to the south of west4 ,tracing these lines upon a map of the country we perceive that they radiate from an area lying along the eastern part of ,argyleshire and the head of the ,firth of ,clyde 7see ,map ,i474 ,footnote #ahg3 ,trans4 ,roy4 ,soc4 ,edin4 xxii4 p4 #fea4 ,illustration3 ,fig4 #bda4--,section along the line of the ,cleveland ,dyke at ,cliff ,ridge1 ,guisbrough 7,g4 ,barrow74 ,scale1 #ab inches to #a mile4 #aj4 ,,termination ,,upwards ,it was pointed out many years ago by ,winch that some of the dykes which traverse the ,northumberland coal-field do not cut the overlying ,magnesian ,limestone4 ,the ,hett dyke1 south of ,durham1 is said to end off abruptly against the floor of the limestone4#ahh ,here and there1 among the precipices of the ,inner ,hebrides1 a dyke may be seen to die out before it reaches the top of the cliff4 ,but in the vast majority of cases1 no evidence remains as to how the dykes terminated upwards4 ,i have referred to the occasional interruptions of the continuity of a dyke1 where1 though the rock does not reach the surface1 it must be present in the fissure underneath4 ,such interruptions show that1 in some places at least1 there was no rise of the rock even up to the level of what is now the surface of the ground1 and that the upward limit of the dykes must have been exceedingly irregular4 ,footnote #ahh3 ,this is expressed in the ,geological ,survey ,map1 ,sheet #ic1 ,n4,e4 ,excellent illustrations of this feature are supplied by sections on the line of the ,cleveland dyke4 ,towards its south-easterly extremity1 this great band of igneous rock ascends from the low ,triassic plain of the ,tees into the high uplands of ,cleveland4 ,its course across the ridges and valleys there has been carefully traced for the ,geological ,survey by ,mr4 ,g4 ,barrow1 who has shown that over certain parts of its course it does not reach the surface1 but remains concealed under the ,jurassic rocks1 which it never succeeded in penetrating4 ,but that in places it comes within a few feet of the soil is shown by the baked shale at the surface1 for the alteration which it has induced on the surrounding rocks only extends a few feet from its margin4 ,these interruptions of continuity show how uneven is the upper limit of the dyke4 ,the characteristic porphyritic rock may be observed running up one side of a hill to the crest1 but never reaching the surface on the other side4 ,at ,cliff ,ridge1 for example1 about three miles south-west of ,guisbrough1 ,mr4 ,barrow has followed it up to the summit on the west side2 but has found that on the east side it does not pierce the shales1 which there form the declivity4 ,this structure is represented in ,fig #bda4 ,the vertical distance between the summit to the left1 where the dyke 7b7 disappears1 and the point to the right1 where the ,lias shale 7a7 of the hillside is concealed by drift 7c71 amounts to #bej feet1 the horizontal distance being a little more than #ijj feet4 ,but as the shale when last seen at the foot of the slope is quite unaltered1 the dyke must there be still some little distance beneath the surface1 so that the vertical extension of this upward tongue of the dyke must be more than #bej feet4 ,mr4 ,barrow1 to whom ,i am indebted for these particulars1 has also drawn the accompanying section 7,fig4 #bdb7 along the course of the dyke for a distance of nearly #aa miles eastward from the locality represented in ,fig4 #bda4 ,from this section it will be observed that in that space there are at least three tongues or upward projections of the upper limit of the dyke4 ,several additional examples of the same structure are to be seen further east towards the last visible outcrop of the dyke4 ,illustration3 ,fig4 #bdb4--,section along the course of the ,cleveland ,dyke1 at the head of ,lonsdale1 ,yorkshire 7,g4 ,barrow1 in the ,memoirs of the ,geol4 ,surrey1 ,geology of ,cleveland1 p4 #fa74 a1 ,liassic shales1 sandstones and ironstones2 b1 the dyke4 ,another feature connected with the upward termination of the dyke is well seen in some parts of the ground through which the two foregoing sections are taken4 ,mr4 ,barrow informs me that at ,ayton a level course has been driven into the hill for mining operations1 at a height of #djj feet above sea-level1 and the dyke has there been ascertained to be #hj feet broad4 ,higher on the hill1 close to the #gej feet contour--line1 its breadth is only #bj feet1 so that it narrows upward as much as #fj feet in a vertical height of #cej feet4 ,its contraction in width during the last twenty feet is still more rapid1 and in the last few yards it diminishes to two or three feet1 and has a rounded top over which the strata are bent upward4 ,the accompanying section 7,fig4 #bdc7 across the upper part of the dyke will make these features clear4 ,illustration3 ,fig4 #bdc4--,section across the extreme upper limit of ,cleveland ,dyke1 on the scale of #bj feet to one inch 7,mr4 ,g4 ,barrow74 a a1 ,jurassic shales1 etc42 b1 ,dyke4 ,illustration3 ,fig4 #bdd4--,upper limit of ,cleveland ,dyke in quarry near ,cockfield 7after ,mr4 ,teall74 a a1 ,carboniferous shales2 b1 dyke4 ,further to the west an exposure of the upper limit of the dyke has been described and figured by ,mr4 ,teall4 ,in #ahhb1 at one of the ,cockfield quarries 7,fig4 #bdd71 the dyke was seen to terminate upwards very abruptly in the form of a low and somewhat irregular dome1 over which the ,coal-measure shales passed without any fracture1 and only with a slight upward arching4#ahi ,footnote #ahi3 ,quart4 ,jour4 ,geol4 ,soc4 xl4 p4 #baj4 ,near the other or north-western termination of this great dyke1 similar evidence is found of an uneven upper limit4 ,after an interrupted course through the ,alston moors1 the dyke reaches the ground that slopes eastward from the edge of the ,cross ,fell escarpment4 ,its highest visible outcrop is at a height of #agjj feet4 ,but westwards from that point the dyke disappears under the ,carboniferous rocks1 and does not emerge along the front of the great escarpment that descends upon the valley of the ,eden1 where among the naked scarps of rock it would unquestionably be visible if it reached the surface4 ,its upper edge must rapidly descend somewhere behind the face of the escarpment1 for the igneous rock crops out a little to the west of the foot of the cliff1 about #ajjj feet below the point where it is last seen on the hills above4 ,here the top of the dyke has a vertical drop of not less than #ajjj feet1 in a horizontal distance of five miles1 as shown in ,fig4 #bde1 which has been drawn for me by ,mr4 ,j4 ,g4 ,goodchild4 ,it will be observed that in these sections 7,figs4 #bda1 #bdb and #bde7 there is a curiously approximate coincidence between the inequalities in the upper surface of the dyke and those in the form of the overlying ground4 ,the coincidence is too marked and too often repeated to be merely accidental4 ,whether the ancient topographical features had any influence in determining1 by cooling or otherwise1 the limit of the upward rise of the lava1 or whether the dyke1 even though concealed1 has affected the progress of the denudation of the ground overlying it1 is a question worthy of fuller investigation4 ,illustration3 ,fig #bde4--,section along the course of the ,cleveland ,dyke across the ,cross ,fell escarpment4 ,the shaded part shows the position of the dyke1 the unshaded part overlying it marks where the dyke does not reach the surface4 ,scale of one inch to one mile4 #aa4 ,,known ,,vertical ,,extension ,closely connected with the determination of the upper limit reached by the dykes1 is the total vertical distance to which they can be traced4 ,of course1 the depth of the original reservoir of molten rock which supplied them remains unknown1 and probably undiscoverable4 ,but it is possible1 in many cases1 to determine at least the inferior limit of the thickness of rock through which the molten material of the dykes has ascended4 ,along the great basalt-escarpments of ,mull and ,skye1 the ascent of dykes from base to summit may often be observed4 ,thus1 on the cliffs of ,dunvegan ,head1 on the west coast of ,skye1 which rise out of the sea to a height of about #ajjj feet1 several dykes may be observed rising through the whole series of basalts up to the crest of the precipice4 ,in the dark gabbro hills of the same island1 numerous dykes may be seen climbing from the glens right up the steep rugged acclivities and over the crests1 through a vertical thickness of more than #cjjj feet of rock 7,fig4 #ccc74 ,the dykes which cross ,loch ,lomond1 and ascend the hills on either side of that deep depression1 must rise through at least as great a thickness4 ,but where a knowledge of the geological structure of the ground enables us to estimate the bulk of the successive rock-formations which underlie the surface1 it can be shown that the lava ascended through a much greater depth of rock4 ,measurements of this kind can best be made towards the eastern end of the ,cleveland dyke1 where the different sedimentary groups have not been seriously disturbed1 and where1 from natural sections and artificial borings1 their thicknesses are capable of satisfactory computation4 ,the highest bed of the ,jurassic series anywhere touched by the dyke is the ,cornbrash4 ,it is certain1 therefore1 that the igneous rock rises through all the subjacent members of the ,jurassic series up to that horizon4 ,there can be no doubt also that the ,trias and ,magnesian ,limestone continue in their normal thickness underneath the ,jurassic strata4 ,to what extent the ,coal-measures exist under ,cleveland has not been ascertained2 possibly they have been entirely denuded from that area1 as from the ground to the west4 ,but the ,millstone ,grit and ,carboniferous ,limestone probably extend over the district in full development2 and below them there must lie a vast depth of ,upper and ,lower ,silurian strata1 probably also of still older ,palozoic rocks and beneath all the thick ,archan platform4 ,tabulating these successive geological formations1 and taking only the ascertained thickness of each in the district1 we find that they give the results shown in the subjoined table4#aij ,footnote #aij3 ,drawn up for me by ,mr4 ,g4 ,barrow4 ,,strata ,,cut ,,by ,,the ,,cleveland ,,dyke ,cornbrash-- ,feet4 ,lower ,oolite and ,upper ,lias1 as proved by bore-hole on ,gerrick ,moor1 #iej ,middle and ,lower ,lias1 ascertained from measurement of cliff-sections and from mining operations to be more than #hej ,new ,red ,sandstone and ,marl1 found by boring close to the ,tees to exceed #a1fjj ,magnesian ,limestone1 at least #ejj ,coal-measures1 possibly absent #j ,millstone ,grit1 not less than #ejj ,carboniferous ,limestone series at least #c1jjj ,silurian rocks1 probably not less than #aj1jjj ------- #ag1djj ,there is thus evidence that this dyke has risen through probably more than three miles of stratified rocks4 ,how much deeper still lay the original reservoir of molten material that supplied the dyke1 we have at present no means of computing4 #ab4 ,,evidence ,,as ,,to ,,movement ,,of ,,the ,,molten ,,rock ,,in ,,the ,,fissures ,it is usual to speak of the molten material of the dykes as having risen vertically within the fissures4 ,and doubtless1 on the whole1 the expression is sufficiently accurate4 ,in the case of such long dykes as those of ,central ,scotland and the ,north of ,england1 where the petrographical character of the material remains so uniform throughout1 it is obvious that the andesite or dolerite cannot have come from a mere single pipe like a volcanic orifice4 ,nor can we easily understand how it could have been supplied even from a series of such pipes4 ,the general aspect and structure of the dykes suggest that the fissures were rent so profoundly in the crust of the earth as to reach down to a reservoir of molten rock which straightway rose in them4 ,the roof of such a reservoir1 however1 may have been irregular and uneven1 so that a fissure need not have traversed it continuously1 but may have only touched its upward projecting vaults4 ,hence gaps would arise in the continuity of the dyke-material4 ,the ascent of lava from a line of such separate openings along a fissure would necessarily involve lateral as well as vertical movements in the molten mass which would be forced along the open rent until the several streams united and filled it up4 ,we might therefore expect somewhere to find instances of flow-structure in the dykes pointing to these movements4 ,i have already referred to the lines of amygdales frequently noticed in dykes1 especially towards the centre4 ,occasionally these steam-vesicles may be observed to be drawn out in one general direction indicative of the trend of motion of the molten rock4 ,some of the best examples of this feature which have come under my observation occur among the trachytic dykes of the south-east coast of ,skye between ,kyle ,rhea and ,loch na ,daal1 where they have been mapped and carefully investigated by ,mr4 ,clough1 who has conducted me over the sections4 ,in some of these dykes1 as already narrated1 the marginal portions display a finely spherulitic structure1 the small pea-like spherulites being grouped into fine ribs or rods4 ,it is also observable that the steam-vesicles which may retain their spherical forms in the centre are elongated in the same direction as the rows of spherulites4 ,where this lineation is developed vertically1 it no doubt points to the vertical ascent of the lava between the two walls of the fissure4 ,but in other examples1 the elongation is nearly horizontal1 and between the two positions ,mr4 ,clough has registered many intermediate trends4 ,it would thus appear that in some places the lava has certainly flowed laterally between the fissure-walls4 ,moreover1 the trend of the spherulitic rods and of the amygdales is found to vary in closely adjoining planes at different distances from the margin1 as if after the outer portions of the dyke had consolidated into position1 there was still movement enough to drag the rows of spherulites and vesicles up or down along the trend of the fissure4 ,mr4 ,clough has observed that in some dykes1 while the amygdaloidal vesicles are large and undeformed in the centre1 they become elongated and inclined downward in the direction of the margin1 as if the central portions had not only remained fluid longer than the rest1 but had a tendency to rise upwards in the fissure1 though there was obviously less motion after these central vesicles appeared than in the marginal parts where the vesicles are so much drawn out4 #ac4 ,,branching ,,dykes ,,and ,,veins ,it might have been anticipated that the uprise of such abundant masses of molten rock1 in so many long and wide fissures1 would generally be attended with the intrusion of the same material into lateral rents and irregular openings1 so that each dyke would have a kind of fringe of offshoots or processes striking from it into the surrounding ground4 ,it might have been expected also that dykes would often branch1 and that the arms would come together again and enclose portions of the rocks through which they rise4 ,but in reality such excrescences and bifurcations are of comparatively rare occurrence4 ,as a rule1 each dyke is a mere wall of igneous rock1 with little more projection or ramification than may be seen in a stone field-fence4 ,among the short1 narrow and irregular dykes of the gregarious type branchings are occasionally seen1 and in some districts are extraordinarily abundant4 ,but among the great single dykes such irregularities are far less common than might have been looked for4 ,a few characteristic examples from each type of dyke may here be given4 ,illustration3 ,fig4 #bdf4--;,branching portion of the great ,dyke near ,hawick 7length about one mile74 ,illustration3 ,fig4 #bdg4--;,branching ,dyke at foot of ,glen ,artney 7length about four miles74 ,the ,cleveland dyke1 which in so many respects is typical of the great solitary dykes of the country1 has been traced for many miles without the appearance of a single offshoot of any kind4 ,yet here and there along its course1 it departs from its usual regularity4 ,as it crosses the ,carboniferous tracts of ,durham and ,cumberland1 there appear near its course lateral masses of eruptive rock1 most of which doubtless belong to the much older ,whin ,sill4 ,but there is at least one locality1 at ,bolam near ,cockfield1 in the county of ,durham1 where the dyke1 crossing the ,millstone ,grit1 suddenly expands into a boss1 and immediately contracts to its usual dimensions4 ,around this knot several short dykes or veins seem to radiate from it4 ,the dyke has been quarried here1 and its relations to the surrounding strata have been laid bare1 as will be again referred to a little further on4#aia ,footnote #aia3 ,this locality was well described by ,sedgwick1 in his early paper on ,trap-,dykes in ,yorkshire and ,durham1 ,trans4 ,cambridge ,phil4 ,soc4 ii4 p4 #bg4 ,among the great persistent dykes of ,scotland the absence of bifurcation and lateral offshoots offers a striking contrast to the behaviour of the dykes in those districts where they are small in size and many in number4 ,but exceptions to the general rule may be gathered4 ,thus the ,eskdale dyke is flanked at ,wat ,carrick with a large lateral vein1 which is almost certainly connected with the main fissure4 ,the ,hawick and ,cheviot dyke splits up on the hill immediately to the east of the town of ,hawick1 sends off some branches1 and then resumes its normal course 7,fig4 #bdf74 ,again1 one of the two nearly parallel dykes which run from ,lochgoilhead across ,ben ,ledi into ,glen ,artney bifurcates at the foot of that valley1 its northern limb 7about two miles long7 speedily dying out1 and its southern branch throwing off another lateral vein1 and then continuing eastward as the main dyke 7,fig4 #bdg74 ,in the districts of gregarious dykes1 however1 abundant instances may be found of dykes that branch1 and of others that lose the parallelism of their walls1 become irregular in breadth1 direction1 and inclination1 so as to pass into those intrusive forms that are more properly classed as veins4 ,excellent illustrations of bifurcating dykes may be observed along the shores of the ,firth of ,clyde1 particularly on the eastern coast-line of the isle of ,arran4 ,the venous character has become familiar to geologists from the sketches given by ,macculloch from the lower parts of the cliffs of ,trotternish in ,skye4#aib ,still more striking examples are to be seen in the breaker-beaten cliffs of ,ardnamurchan4 ,the pale ,secondary limestones and calcareous sandstones of that locality are traversed by a series of dark basic veins1 and the contrast of tint between the two kinds of rock is so marked as even to catch the eye of casual tourists in the passing steamboats4 ,the veins vary in width from less than an inch to several feet or yards4 ,they run in all directions and intersect each other1 forming such a confused medley as requires some patience on the part of the geologist who would follow out each independent ribbon of injected material in its course up the cliffs1 or still more1 would sketch their ramifications in his note-book4 ,a good1 though perhaps somewhat exaggerated1 illustration of their general character was given by ,macculloch4#aic ,the accompanying figure 7,fig4 #bdh7 is less sensational1 but represents with as much accuracy as ,i could reach1 the network of veins near the foot of the cliffs4 ,one conspicuous group of veins1 which1 seen from a distance1 looks like a rude sketch of a lug-sail traced in black outline upon a pale ground1 is known to the boatmen as ,m',niven's ,sail4 ,another admirable locality for the study of dykes and tortuous veins is the northern coast of the ,sound of ,soa1 where an extraordinary number of injections traverse the ,torridon ,sandstones on which the plateau-basalts rest 7,fig4 #cbc74 ,footnote #aib3 ,western ,islands1 plate xvii4 ,footnote #aic3 ,op4 cit4 plate xxxiii4 ,fig4 #a4 ,as a general rule1 the narrower the vein the finer in grain is the rock of which it consists4 ,this compact dark homogeneous material has commonly passed by the name of basalt4 ,its minuteness of texture probably in most cases arises from local rapidity of cooling1 and it is doubtless the same substance which1 where in larger mass in the immediate neighbourhood1 has solidified as one of the other pyroxene-plagioclase-magnetite rocks4 ,with regard to the places where such abundant tortuous veins are more especially developed1 ,i may remark that they are particularly prominent under a thick overlying mass of erupted rock1 such as a great intrusive sheet1 or the bedded basalts of the plateaux1 or where there is good reason to believe that such a deep cover1 though now removed by denudation1 once overspread the area in which they appear4 ,it will be shown in the sequel that such horizons have been peculiarly liable to intrusions of igneous material of various kinds1 and at many different intervals1 during the volcanic period4 ,a thick cake of crystalline rock seems to have offered such resistance to the uprise of molten material through it1 that when the subterranean energy was not sufficient to rend it open by great fissures1 and thus give rise to dykes1 the lavas were either forced into such irregular cracks as were made partly in the softer rocks underneath and partly in the cake itself1 or found escape along pre-existing divisional planes4 ,in ,ardnamurchan1 round the ,cuillin ,hills of ,skye1 and in ,rum1 the overlying resisting cover now consists mainly of gabbro sheets4 ,in the east of ,skye1 in ,eigg1 and in ,antrim1 it is made up of the thick mass of the plateau-basalts4 ,illustration3 ,fig4 #bdh4--;,basic veins traversing secondary limestone and sandstone on the coast cliffs1 ,ardnamurchan4 #ad4 ,,connection ,,of ,,dykes ,,with ,,sills ,every field-geologist is aware how seldom he can actually find the vent or pipe up which rose the igneous rock that supplied the material of sills and laccolites4 ,he might well be pardoned were he to anticipate that1 in a district much traversed by dykes1 there should be many examples of intrusive sheets and frequent opportunities of tracing the connection of such sheets with the fissures from which their material might be supposed to have been supplied4 ,but such an expectation is singularly disappointed by an actual examination of the ,tertiary volcanic region of ,britain4 ,that there are many intrusive sheets belonging to the great volcanic period with which ,i am now dealing1 ,i shall endeavour to show in the sequel4 ,but it is quite certain that though these sheets have of course each had its subterranean pipe or fissure of supply1 they can only in rare instances be directly traced to the system of dykes4 ,on the other hand1 the districts where great single dykes are most conspicuous1 are for the most part free from intrusive sheets1 except those of much older date1 like the ,carboniferous ,whin ,sill of ,durham and those of ,linlithgowshire1 ,stirlingshire and ,fife4 ,yet a few interesting examples of the relation of dykes to sheets have been noticed among ,british ,tertiary volcanic rocks4 ,the earliest observed instances were those figured and described by ,macculloch4 ,among them one has been familiar to geologists from having done duty in text-books of the science for more than half a century4 ,i allude to the diagram of ,trap and ,sandstone near ,suishnish4#aid ,in that drawing seven dykes are shown as rising vertically through the horizontal sandstone1 and merging into a thick overlying mass of trap4 ,the author in his explanation leaves it an open question whether the intruding material has ascended from below and overflowed the strata1 or has descended from the mass1 though from the language he uses in his text we may infer that he was inclined to regard the overlying body as the source of the veins below it4#aie ,footnote #aid3 ,western ,islands of ,scotland1 pl4 xiv4 ,fig4 #d4 ,footnote #aie3 ,op4 cit4 vol4 i4 pp4 #chd1 #che4 ,illustration3 ,fig4 #bdi4--,section showing the connection of a ,dyke with an ,intrusive ,sheet1 ,point of ,suisnish1 ,skye4 g1 ,granophyre of ,carn ,dearg2 f1 similar rock1 which appears eastward under the sill 7d72 e1 intrusive sheet of fine-grained basalt2 d1 intrusive sheet or sill of coarse dolerite1 #bjj feet thick at its maximum1 and rapidly thinning out2 c1 dyke or pipe of finer grain than d2 b1 yellowish-brown shaly sandstones1 and a1 dark sandy shales 7,lias74 ,the section given by ,macculloch1 however1 does not quite accurately represent the facts4 ,the narrow dykes there drawn have no connection with the overlying sheet1 but are part of the abundant series of basaltic dykes found all over ,skye4 ,the feeder of the gabbro sill was presumably the broad dyke which descends the steep bank immediately on the southern front of ,carn ,dearg 7#fcf feet high74 ,the accompanying figure 7,fig4 #bdi7 shows what seemed to me to be the structure of the locality1 but the actual junction of the dyke and sheet is concealed under the talus of the slope4#aif ,i shall have occasion in a later ,chapter to refer again to this section in connection with the history of intrusive sheets1 and also to cite from the neighbouring island of ,raasay another good example of the same relation between dyke and sill4 ,footnote #aif3 ,in more recently surveying this ground1 ,mr4 ,harker has been led to regard the coarse sill as independent of the other intrusions1 and as almost certainly later than the basalt-sheets of the same locality4 ,when it reaches the base of these sills it turns so as to pass beneath them as a gabbro-sill1 which is conspicuous near the summit of ,carn ,dearg4 ,it runs westward for some distance1 almost immediately breaking across the bedding so as to leave the basalt1 and rapidly tapering until it dies out4 ,sedgwick1 in the paper above quoted1 gave an account and figure of the expansion of the ,cleveland dyke at ,bolam1 to which allusion has already been made4 ,he showed that from a part of the dyke which is unusually contracted a great lateral extension of the igneous rock takes place on either side over beds of shale and coal4 ,while in the dyke the prisms are as usual directed horizontally inward from the two walls1 those in the connected sheet are vertical1 and descend upon the surface of highly indurated strata on which the sheet rests4 ,the most important examples known to me are those which occur in the coal-field of ,stirlingshire4 ,in that part of the country1 the remarkable group of dykes already referred to1 lying nearly parallel to each other and from half a mile to about three miles apart1 runs in a general east and west direction4 ,from one of these dykes no fewer than four sills strike off into the surrounding ,coal-measures4 ,the largest of them stretches southwards for three miles1 but the same rock is probably continued in a succession of detached areas which spread westwards through the coal-field and circle round to near the two western sheets that proceeded from the same dyke4 ,another thick mass of similar rock extends on the north side of the dyke for two and a half miles down the valley of the river ,avon4 ,these various processes1 attached to or diverging from the dyke1 are unquestionably intrusive sheets1 which occupy different horizons in the ,carboniferous series4 ,the one on the north side has inserted itself a little above the top of the ,carboniferous ,limestone series4 ,those on the south side lie on different levels in the ,coal-measures1 or1 rather1 they pass transgressively from one platform to another in that group of strata4 ,illustration3 ,fig4 #bej4--,section to show the connection of a ,dyke with an ,intrusive ,sheet1 ,stirlingshire ,coal-field4 a1 ,dyke in line of fault2 b1 ,sill traversing and altering the coal2 i1 ,slaty-band ,ironstone4 ,no essential difference can be detected by the naked eye between the material of the dyke and that of the sheets4 ,if a series of specimens from the different exposures were mixed up1 it would be impossible to separate those of the dyke from those of the sheets4 ,a microscopical examination of the specimens likewise shows that they are perfectly identical in composition and structure1 being chiefly referable to rocks of the dolerite1 but partly of the tholeiite type4 ,i have therefore little doubt that these remarkable appendages to this dyke are truly offshoots from it1 and are not to be classed with the general mass of the sills of ,central ,scotland1 which are of ,carboniferous1 partly of ,permian1 age4 ,the accompanying diagrammatic section 7,fig4 #bej7 explains the geological structure of the ground4 ,an interesting and important fact remains to be stated in connection with these sheets4 ,they are traversed by some of the other east and west dykes4 ,this is particularly observable in the case of the sheet which extends northwards from the dyke through the parish of ,torphichen4 ,two well-marked dykes can be seen running westwards among the ridges of the sheet4 ,it is obvious1 therefore that these particular dykes are younger than the sheet4 ,but1 as will be shown in the sequel1 there is abundant evidence that all the dykes of a district are not of one eruption4 ,the intersection of one eruptive mass by another does not necessarily imply any long interval of time between them4 ,they mark successive1 but it may be rapidly successive1 manifestations of volcanic action4 ,hence the cutting of the sheets by other dykes does not invalidate the identification of these sheets as extravasations from the great dyke by which they are bounded4 #ae4 ,,intersection ,,of ,,dykes ,illustration3 ,fig4 #bea4--;,intersection of ,dykes in bedded basalt1 ,calliach ,point1 ,mull4 ,innumerable instances may be cited1 where one dyke1 or one set of dykes1 cuts across another4 ,to some of these ,i shall refer in discussing the data for estimating the relative ages of dykes4 ,in considering the intersection from the point of view of geological structure1 we are struck with the clean sharp way in which it so generally takes place4 ,the rents into which the younger dykes have been injected seem1 as a rule1 not to have been sensibly influenced in width and direction by the older dykes1 but go right across them4 ,hence the younger dykes retain their usual breadth and trend 7,fig4 #bea74 ,in trying to ascertain the relative ages of such dykes we obtain a valuable clue in studying the respective chilled edges of the two intersecting masses1 as has already been pointed out4 ,not only do dykes cross each other1 but still more is this the case among the narrower tortuous intrusions known as ,veins 7,fig4 #beb74 ,among the illustrations which the dykes of the ,inner ,hebrides supply of these features one further characteristic example may be culled from the shore of ,skye1 near ,broadford1 where the gently-inclined sheets of ,lias limestone are traversed by three systems of dykes 7,fig4 #bec74 ,one of these systems runs in a ,n4,w4 or ,n4,n4,w4 direction1 a second follows a more nearly easterly trend1 while the third and youngest runs nearly north and south4 ,illustration3 ,fig4 #beb4--;,basalt ,veins traversing bedded dolerites1 ,kildonan1 ,eigg4 ,illustration3 ,fig4 #bec4--;,ground-plan of intersecting ,dykes in ,lias limestone1 ,shore1 ,harrabol1 ,east of ,broadford1 ,skye4 #af4 ,,dykes ,,of ,,more ,,than ,,one ,,in-,,filling ,the intersections of dykes prove that the process of fissuring in the earth's crust took place at more than one period1 and prepare us for the reception of evidence that the same line of fissure might be again re-opened1 even after it had been filled with molten material4 ,numerous instances have now been accumulated in which dykes are not single or simple intrusions1 but where the original dyke-fissure has been re-opened and has been invaded by successive uprisings of lava4#aig ,compound dykes have thus been formed1 consisting of two or more parallel bands of similar or dissimilar rock4 ,footnote #aig3 ,see an example figured by ,macculloch1 ,western ,isles1 plate xviii4 ,fig4 #a4 ,while it is not difficult to conceive of the re-opening of a vertical fissure during terrestrial strain1 and the injection into it of later intrusions of a volcanic magma1 it is not so easy to understand the mechanism where the line of weakness has been slightly inclined or horizontal1 and where1 consequently1 there has been the enormous superincumbent pressure of the overlying part of the earth's crust to overcome4 ,yet gently inclined compound dykes exhibit their parallel bands with hardly less regularity than do those that are vertical4 ,the difficulty of explanation is felt most strongly in the attempt to realize the origin of the compound sills described in ,chapter xlviii4 ,in the re-opening of dyke-fissures the later intrusions have generally taken place along the walls1 or where the dykes were already compound1 between some of the component bands4 ,less frequently the first dyke has been split open along the middle1 and a second injection has forced its way along the rent4 ,of the first of these two types1 numerous instances have now been observed in the ,west of ,scotland4 ,if the portion of a compound dyke exposed at the surface be limited in extent1 we may be unable to determine which is the older of two parallel bands of igneous rock1 though the fact that they present to each other the usual fine-grained edge due to more rapid cooling1 shows that they are not one but two dykes1 belonging to distinct eruptions4 ,so far as ,i have noticed1 where one of the dykes can be continuously traced for a considerable distance1 the other is comparatively short4 ,i infer that the shorter one is the younger of the two4 ,in the ,strath district of ,skye1 ,mr4 ,harker has recently observed that many of the basic dykes1 both those older and those younger than the granophyre protrusions1 are double1 triple or multiple4 ,thus in a conspicuous dyke1 more than #ajj feet wide1 to the south-east of ,loch ,kilchrist1 belonging to the older series1 he has detected at least six contiguous dykes which as they are traced south-eastward1 in spite of their interruption by the ,beinn an ,dubhaich granite1 can be seen to separate and take different courses1 or successively die out4 ,he remarks1 further1 that many cases of apparent bifurcation of dykes are really due to the separation of distinct dykes which have run for some distance in one fissure4 ,sometimes apparent variations in the width of a dyke are to be explained by this dying out of one member of a double dyke4 ,these multiple dykes are less easily detected in the newer than the older set1 owing to greater uniformity of lithological type in the prevalent kinds and to the frequent absence of chilled selvages4#aih ,an example of a compound basic dyke cutting the crest of the gabbro-mass of the ,cuillin ,hills is shown in ,fig4 #ccc4 ,footnote #aih3 ,,ms4 notes supplied by ,mr4 ,harker4 ,instances of the second type of compound dykes are less common4 ,here1 instead of being re-opened along one of the walls1 the fissure has been ruptured along the centre of the dyke1 and a second injection of molten material has then taken place4 ,this structure may be observed where the materials of the compound dyke are on the whole similar1 such as varieties of dolerite1 basalt1 diabase or andesite4 ,in these cases the rock of the central dyke is generally rather fine-grained1 sometimes decidedly porphyritic1 and often a true basalt4 ,where broad enough to show the difference of texture between margin and centre1 it exhibits the usual close grain along its edges1 indicative of quicker cooling4 ,the older dyke presenting no such change at its junction with the younger1 was obviously already cooled and consolidated before its rupture4 ,whilst the centre of a dyke has occasionally proved to be a line of weakness which has given way under intense strains in the terrestrial crust1 this rupture and the accompanying or subsequent ascent of molten material in the re-opened fissure may sometimes have been included as phases of one connected volcanic episode4 ,in those instances1 for example1 which have been above described1 where a central vitreous band has risen along the heart of a dyke1 the petrographical affinities of the rocks may be so close as to suggest that although the main dyke had consolidated and had subsequently been ruptured along its centre by powerful earth-movements1 these changes all belonged to the same period of dyke-making1 and the subsequent uprise of glassy material was merely a later phase in the movements of the same subterranean magma4 ,but where1 as probably happens in the large majority of compound dykes1 there is a strongly marked difference between the respective bands of rock1 we must either infer that two essentially different magmas co-existed in the volcanic reservoirs underneath1 and were successively injected into the same fissures1 or that a sufficient lapse of time occurred to permit a total renewal of the nature of the magma1 and an uprise of this changed material into fissures which sometimes coincided with older dykes4 ,if any interlocking of the crystals of the several bands of a compound dyke could be detected1 we might suppose that the first-injected material had not become consolidated and cold before the uprise of the newer rock4 ,but in general it would seem that so sharp a line of demarcation can be drawn between the two rocks as to indicate that their protrusion was due to two distinct and perhaps widely-separated volcanic paroxysms4 ,compound dykes of basic material occur not only among the ordinary straight north-westerly series1 but also among the less regular gregarious dykes and veins1 such as abundantly intersect the gabbro bosses4 ,moreover they are to be found among the youngest intrusions1 for they traverse the masses of granophyre4 ,conspicuous examples of such late compound dykes are displayed along the cliffs of ,st4 ,kilda1 as will be more particularly described in a later ,chapter4 ,these ,st4 ,kilda dykes often occupy not vertical fissures but parallel rents with a gentle inclination 7see ,figs4 #cfg1 #cfh74 ,the ,tertiary volcanic series of ,scotland furnishes many examples of compound dykes of a much more complex character where parallel bands of some acid 7granophyre1 felsite1 quartz-porphyry7 or intermediate 7andesite7 rock is associated with others of the more usual basic material 7dolerite1 basalt1 diabase74 ,as the acid intrusions belong to a comparatively late part of the volcanic history1 their modes of occurrence will be discussed in ,chapters xlvi41 xlvii4 and xlviii4 ,but no account of the general system of dykes would be complete without some reference to these compound examples1 which will therefore be briefly described in the present section of this work4 ,early in this century some striking illustrations of the association of acid and more basic rocks within the same fissure were noticed by ,jameson in the island of ,arran4 ,he described and figured instances at ,tormore1 on the west side of that island1 where a group of pitchstones and basalts or andesites have been successively protruded into the same fissures in the 7probably ,permian7 red sandstones of that district4#aii ,footnote #aii3 ,mineralogy of the ,scottish ,isles1 #ahjj4 ,in some instances the more basic rock has been first injected1 and has subsequently been disrupted1 by the more acid pitchstone4 ,in other cases the order has been the reverse4 ,the successive ruptures have taken place sometimes along the centre1 sometimes at the margins1 and sometimes irregularly along the breadth of the dykes4 ,professor ,judd has recently studied these rocks1 and has given descriptions of their chemical composition and microscopic characters4 ,he regards them as having been successively injected into the fissures from the same subterranean reservoir1 in which two magmas of very different chemical constitution were simultaneously present4#bjj ,footnote #bjj3 ,quart4 ,jour4 ,geol4 ,soc4 vol4 xlix4 7#ahic71 p4 #ecf4 ,full details of the compound dykes of ,tormore and ,cir ,mhor in ,arran1 and references to previous writers will be found in this paper4 ,the probable age of the youngest eruptive rocks of this island will be discussed in ,chapter xlvii4 p4 #dah4 ,nowhere in the ,tertiary volcanic regions of ,britain do compound dykes appear to be so abundant as in the centre and southern part of the island of ,skye4 ,during the progress of the ,geological ,survey in that district1 ,mr4 ,clough and ,mr4 ,harker have mapped a large number in the ground between the ,sound of ,sleat and the ,red ,hills4 ,with regard to these dykes ,mr4 ,harker observes that the several members are generally petrographically different1 some being basic1 others intermediate1 and others acid4 ,there is usually1 he remarks1 a symmetrical disposition1 two similar and more basic dykes being divided by a more acid one2 for example1 two andesites separated by a pitchstone4 ,thus at the mouth of the little stream which runs from ,torran into the bay east from ,dn ,beag a dyke1 apparently #ah feet wide1 is found on examination to consist of a central dyke 7specific gravity #bhf7 flanked by two more basic dykes 7specific gravity #cjb74 ,in the great majority of examples hitherto observed in ,skye the two lateral dykes consist of some basic rock 7diabase or basalt71 while the central and thickest band is of some acid material 7granophyre or quartz-felsite74 ,this triple arrangement occurs both in dykes and sills4 ,illustration3 ,fig4 #bed4--;,compound dyke1 ,market ,stance1 ,broadford1 ,skye4 a1 ,granophyre2 b b1 ,basalt2 c c1 ,torridon sandstone4 ,as an illustration of the association of the two kinds of rock in dykes ,i may cite an example which appears on the southern edge of the ,market ,stance of ,broadford 7,fig4 #bed74 ,here the characteristic triple arrangement is typically developed4 ,a central light-coloured band1 about eight to ten feet broad1 consists of a spherulitic granophyre in which the spherulites are crowded together and project from the weathered surface like peas1 though they do not here show the curious rod-like aggregation so marked in some other dykes4 ,on either side of this acid centre a narrow basalt dyke intervenes as a wall next to the ,torridon sandstone which here forms the country-rock4 ,such compound dykes have sometimes a total width of #ajj feet or more4 ,in this instance1 and generally throughout the district1 there is nothing to indicate that the different bands of the dyke have any relation to each other as connected uprises of material from the same original magma which was either heterogeneous or was undergoing a process of differentiation beneath the terrestrial crust4 ,on the contrary1 the several parts of each dyke are as distinctly marked off from each other as they could have been had they been injected at widely separated intervals of volcanic activity4 ,mr4 ,harker1 in the course of his survey of this ,skye ground1 has observed that where evidence is available1 the central acid dyke is found to be newer than the basic ones4 ,it has not split a single basic dyke1 but has insinuated itself between the two members of a double dyke4 ,this is more clearly seen when the acid magma has been forced into a triple or multiple basic dyke2 the perfect symmetry of arrangement may in this case be lost4 ,for instance1 on the shore north-east of ,corry1 ,broadford1 a #ac feet dyke of granophyre occurs in a multiple dyke of basalt1 but it has taken its line so as to leave only a one-foot dyke on one side1 and a group with a total width of #ab feet on the other4 ,also it has not accurately kept its course1 but has cut obliquely across one of the group of dykes alluded to4 ,in some cases it is certain that the acid magma has to some extent dissolved a portion of the wall of a basic dyke with which it has come in contact4 ,this may account for the magma finding its easiest path along1 and especially between1 pre-existing more basic dykes4 ,this subject will be again referred to in ,chapter xlviii41 when the phenomena of compound sills are discussed4 ,before closing this account of compound dykes1 ,i may remark that no examples have yet been observed among the ordinary ,tertiary dykes of ,britain where1 by a process of differentiation between the walls of a fissure1 successive zones have been developed in the dyke1 differing from each other in structure and composition1 but becoming progressively and insensibly more acid towards the centre1 such as have been described from the older rocks of ,norway and ,canada4 ,among the ,tertiary gabbro bosses1 indeed1 there occur sheets or dykes which present a remarkably banded structure1 to which full reference will be made in later pages4 ,but ,i have never seen anything at all resembling such a structure among the dykes of andesite1 dolerite1 or basalt4 #ag4 ,,contact-,,metamorphism ,,of ,,the ,,dykes ,a geologist might naturally expect that such abundant intrusions of igneous rock as those of the dykes should be accompanied with plentiful proofs of contact-metamorphism4 ,but in actual fact1 evidence of any serious amount of alteration is singularly scarce4 ,a slight induration of the rocks on either side of a dyke is generally all the change that can be detected4 ,illustration3 ,fig4 #bee4--,section of coal rendered columnar by intrusive basalt1 shore1 ,saltcoats1 ,ayrshire4 a1 ,fireclay2 b1 ,coal rendered prismatic near the basalt2 c1 ,dark shale2 d1 ,basalt-rock4 ,some of the larger dykes1 however1 show more marked metamorphism1 the nature of which appears in many cases to be chiefly determined by the chemical composition of the rock affected4 ,thus a considerable alteration has been superinduced on carbonaceous strata1 particularly on seams of coal4 ,in the ,ayrshire coal-field the alteration of the coal extends sometimes #aej feet from the dyke1 the extent of the change depending not merely on the mass of the igneous rock1 but on the nature of the coal1 and possibly on other causes4 ,close to a dyke1 coal passes into a kind of soot or cinder1 sometimes assumes the form of a finely columnar coke 7,fig4 #bee71 and occasionally has become vesicular after being fused4#bja ,shales are converted into a hard flinty substance that breaks with a conchoidal fracture and rings under the hammer4 ,fireclay is baked into a porcelain-like material4 ,limestone is changed for a few inches into marble4 ,as an illustration of this alteration1 ,i may cite a dyke ten feet broad which cuts through the chalk in the ,templepatrick ,quarry1 ,antrim4 ,for about six inches from the igneous rock the chalk has passed into a finely saccharoid condition1 and its organisms are effaced4 ,but beyond that distance the crystalline structure rapidly dies away1 the micro-organisms begin to make their appearance1 and within a space of one foot from the dyke the chalk assumes its ordinary character4 ,footnote #bja3 ,explanation of ,sheet #bb1 ,geological ,survey of ,scotland1 p4 #bf4 ,sandstones are indurated by dykes into a kind of quartzite1 sometimes assume a columnar structure 7the columns being directed away from the dyke-walls71 and for several feet or yards have their yellow or red colours bleached out of them4 ,the granite of ,ben ,cruachan where quarried on ,loch ,awe1 as ,i am informed by ,mr4 ,j4 ,s4 ,grant ,wilson1 is traversed by a basic dyke1 and for a distance of about #bj feet is rendered darker in colour1 becomes granular1 and cannot be polished and made saleable4 ,where many dykes have been crowded together1 their collective effects in the alteration of the strata traversed by them have sometimes been strongly developed4 ,one of the most remarkable illustrations of this influence is presented by the district of ,strathaird1 which was cited by ,macculloch for the abundance of its dykes4 ,in recently mapping this ground for the ,geological ,survey1 ,mr4 ,harker has observed in some places a score or more dykes in actual juxtaposition1 while over considerable distances he found it difficult to detect any trace of the ,jurassic strata1 through which the igneous rocks have ascended4 ,as might be expected under these circumstances1 such portions of the strata as can be seen display an altogether exceptional amount of contact-metamorphism4 ,mr4 ,harker has noticed some limestones at ,camasunary which have been changed into very remarkable lime-silicate rocks1 with singular bunches of diopside crystals4 ,these1 however1 are the extremes of contact-metamorphism by the ,tertiary basic dykes4 ,a geologist visiting the ,liassic shores of ,strath in ,skye will not fail to be surprised at the very slight degree of alteration in circumstances where he would have expected to find it strongly pronounced4 ,the dark shales1 though ribbed across with dykes1 are sometimes hardly even hardened1 and at the most are only indurated from an inch or two to about two feet4 ,these baked bands project above the rest of the more easily denuded shales1 and so adhere to the dykes as almost to seem part of them4 ,again the limestones1 where traversed by dykes some distance apart1 are not rendered in any appreciable degree more crystalline even up to the very margin of the intrusive rock4 ,where the igneous material has been thrust between the strata in sills1 it has produced far more general and serious metamorphism than when it occurs in the form of single dykes4 ,the famous rock of ,portrush1 already referred to as having been once gravely cited as an example of fossiliferous basalt1 is a good illustration of the way in which ,lias shale is porcellanized when the intruded igneous material has been thrust between the planes of bedding4 ,in the ,west of ,scotland1 where dykes are so abundantly developed1 considerable differences can be observed between the amount of metamorphism superinduced by adjacent dykes which may be of the same thickness1 and cut through the same kind of strata4 ,such variations have not probably arisen from differences in the temperature of the original molten rock4 ,perhaps they are rather to be assigned to the length of time occupied by the ascent of the lava in the fissure4 ,if1 for instance1 the fissure opened to the surface and discharged lava there1 the rocks of its walls would be exposed to a continuous stream of molten rock as long as the outflow lasted4 ,they would thus have their temperature more highly raised1 and maintained at such an elevation for a longer time than where the magma1 at once arrested within the fissure1 immediately proceeded to cool and consolidate there4 ,it would be an interesting and important conclusion if we could1 from the nature or amount of their contact-metamorphism1 distinguish those dykes which for some time served as channels for the discharge of lava above ground4 ,some dykes which have caught up fragments of older rocks in their ascent have exercised a considerable solvent action on these inclusions4 ,examples of this feature have already been cited from ,skye1 where they have been studied by ,mr4 ,harker 7pp4 #abi1 #afc74 ,in connection with the metamorphism superinduced by dykes1 reference may again be made to the alteration which they themselves undergo where they have invaded a carbonaceous shale or coal4 ,the igneous rock1 as we have seen1 loses its dark colour and obviously crystalline structure1 and becomes a pale yellow or white1 dull1 earthy substance1 or white trap4 ,the chemical changes involved in this alteration have been described by ,sir ,j4 ,lowthian ,bell4#bjb ,dr4 ,stecher has also discussed the alterations traceable by the aid of the microscope4#bjc ,though most of the instances of such transformation in ,britain occur in the ,carboniferous system1 and have taken place in intrusive rocks of probably1 for the most part1 ,carboniferous or ,permian age1 yet they are not unknown in the ,tertiary volcanic series4 ,some of the white trap of the ,coal-measures may indeed belong to the ,tertiary period1 but the coals and carbonaceous shales interstratified in the ,tertiary basalt-plateaux have reacted on both the superficial lavas and the sills1 and have given rise to the same kind of alteration as in the ,carboniferous system1 as will be shown in a later ,chapter4 ,footnote #bjb3 ,proc4 ,roy4 ,soc4 xxiii4 7#ahge71 p4 #edc4 ,footnote #bjc3 ,tschermak's ,mineralogische ,mittheilungen1 ix4 7#ahhg71 p4 #ade1 and ,proc4 ,roy4 ,soc4 ,edin4 #ahhh4 ,illustration3 ,fig4 #bef4--;,dolerite dyke with marginal bands of white trap1 in black shale1 ,lower ,lias1 ,pabba4 a1 ,black carbonaceous ,lower ,lias ,shale2 b b1 bands of indurated shale from #ae inches to #b feet broad2 c1 dolerite dyke #c feet #c inches broad2 d d1 bands of altered dolerite or white trap1 #c to #e inches broad4 ,some marked examples of this alteration of intrusive igneous material are to be observed among the basalt dykes which cut the ,lower ,lias ,shales of ,skye4 ,these shales1 where black and carbonaceous1 as in the island of ,pabba1 have exercised an unmistakable influence on the abundant dykes which intersect them4 ,the chilled selvage of each dyke has assumed the dull earthy pale-grey or yellowish aspect1 which extends for a few inches from the wall into the interior1 where it rapidly passes into the ordinary black crystalline basalt4 ,these features will be readily understood from the accompanying diagram 7,fig4 #bef74 ,where the dykes give off narrow veins a few inches broad1 these consist entirely of the white trap4 ,the shales are often traversed with strong joints parallel to the walls of the dykes1 and the transverse joints of the dykes are sometimes prolonged into the bands of indurated shale4 #ah4 ,,relation ,,of ,,dykes ,,to ,,the ,,geological ,,structure ,,of ,,the ,,districts ,,which ,,they ,,traverse4 ,in no respect do the ,tertiary dykes of ,britain stand more distinguished from all the other rocks of the country than in their extraordinary independence of geological structure4 ,the successive groups of ,palozoic and ,mesozoic strata have been so tilted as to follow each other in approximately parallel bands1 which run obliquely across the island from south-west to north-east4 ,the most important lines of fault take the same general line4 ,the contemporaneously included igneous rocks follow1 of course1 the trend of the stratified deposits among which they lie1 and even the intrusive sills group themselves along the general strike of the whole country4 ,but the ,tertiary dykes have their own independent direction1 to which they adhere amid the extremest diversities of geological arrangement4 ,in the first place1 the dykes intersect nearly the whole range of the geological formations of the ,british ,islands4 ,in the ,outer ,hebrides and north-west ,highlands1 they rise through the most ancient 7,lewisian7 gneisses1 through the red pre-,cambrian 7,torridon7 sandstones1 and through the oldest members of the ,cambrian system4 ,in the southern ,highlands1 they pursue their course across the gnarled and twisted schists of the younger crystalline 7,dalradian7 series4 ,in the ,south of ,scotland and ,north of ,england1 they traverse the various subdivisions of the ,lower and ,upper ,silurian rocks4 ,in the basins of the ,tay1 ,forth1 and ,clyde they cross the plains and ridges of the ,old ,red ,sandstone1 with its deep pile of intercalated volcanic material4 ,in ,central ,scotland1 and the northern ,english counties1 they occur abundantly in the ,carboniferous system1 and have destroyed the seams of coal4 ,in ,cumberland and ,durham1 they traverse the ,permian and ,trias groups4 ,in ,yorkshire1 and along the ,west of ,scotland1 they are found running through ,jurassic strata4 ,in ,antrim1 they intersect the ,chalk4 ,both in the ,north of ,ireland1 and all through the chain of the ,inner ,hebrides1 they abound in the great sheets and bosses of ,tertiary volcanic rocks4 ,these are the youngest formations through which they rise4 ,but it is deserving of note1 that they intersect every great group of these ,tertiary volcanic products1 so that they include in their number the latest known manifestations of eruptive action in the geological history of ,britain4#bjd ,footnote #bjd3 ,they have not been found cutting the pitchstone-lava of the ,scuir of ,eigg4 ,in the second place1 in ranging across groups of rock belonging to such widely diverse periods1 the dykes must necessarily often pass abruptly from one kind of material and geological structure to another4 ,but1 as a rule1 they do so without any sensible deviation from their usual trend1 or any alteration of their average width4 ,here and there1 indeed1 we may observe a dyke to follow a more wavy or more rapidly sinuous or zig-zag course in one group of rocks than in another4 ,yet1 so far as ,i have myself been able to observe1 such sinuosities may occur in almost any kind of material1 and are not satisfactorily explicable by any difference of texture or arrangement in the rocks at the surface4 ,no dyke traverses a greater variety of sedimentary formations than that of ,cleveland4 ,in the eastern part of its course1 it rises through all the ,mesozoic groups up to the ,cornbrash4 ,further west it cuts across each of the different subdivisions of the ,carboniferous system2 and1 of course1 it must traverse all the older formations which underlie these4 ,but the occasional rapid changes noticeable in its width and direction do not seem to be referable to any corresponding structure in the surrounding rocks4 ,the ,cheviot dyke crosses from the ,carboniferous area of ,northumberland into the ,upper ,silurian rocks and ,lower ,old ,red ,sandstone volcanic tract of the ,cheviot ,hills4 ,it then strikes across the ,upper ,old ,red ,sandstone of ,roxburghshire1 and still maintaining the same persistent trend1 sweeps westward into the intensely plicated ,silurian rocks of the ,southern ,uplands4 ,its occasional deviations have no obvious reference to any visible change of structure in the adjacent formations4 ,again1 some of the great dykes at the head of ,clydesdale furnish striking illustrations of entire indifference to the nature of the rock through which they run4 ,quitting the ,silurian uplands1 they keep their line across ,old ,red ,sandstone and ,carboniferous rocks1 and through large masses of eruptive material4 ,in the third place1 not only are the dykes not deflected by great diversities in the lithological character of the rocks which they traverse1 they even cross without deviation some of the most important geological features in the general framework of the country4 ,some of the ,scottish examples are singularly impressive in this respect4 ,those which strike north-westward from the uplands of ,clydesdale cross without deflection the great boundary-fault which1 by a throw of several thousand feet1 brings the ,lower ,old ,red ,sandstone against ,silurian rocks4 ,they traverse some large faults in the valley of the ,douglas coal-field1 pass completely across the axis of the ,haughshaw ,hills1 where the ,upper ,silurian rocks are once more brought up to the surface1 and also the long felsite ridge of ,priesthill4 ,the dykes in the centre of the kingdom maintain their line across some of the large masses of igneous rock that protrude through the ,carboniferous system4 ,further north1 the dykes of ,perthshire cut across the great sheets of volcanic material that form the ,ochil ,hills1 as well as through the piles of sandstone and conglomerate of the ,lower ,old ,red ,sandstone1 and then go right across the boundary-fault of the ,highlands1 to pursue their way in the same independent manner through grit1 quartzite1 or mica-schist1 and across glen and lake1 moor and mountain4 ,no one can contemplate these repeated examples of an entire want of connection between the dykes and the nature and arrangement of the rocks which they traverse without being convinced that the lines of rent up which the material of the dykes rose were not1 as a rule1 old fractures in the earth's crust1 but were fresh fissures1 opened across the course of the older dislocations and strike of the country by the same series of subterranean operations to which the uprise of the molten material of the dykes was also due4 ,in the fourth place1 the dykes for the most part are not coincident with visible lines of fault4 ,after the examination of hundreds of dykes in all parts of the country1 and with all the help which bare hillsides and well-exposed coast-sections can afford1 the number of instances which have been met with where dykes have availed themselves of lines of fault is surprisingly small4 ,some of these cases will be immediately cited4 ,to whatever cause we may ascribe the rupture of the solid crust of the earth1 which admitted the rise of molten rock to form the dykes1 there can be no doubt that it was not generally attended with that displacement of level on one or both sides of the dislocation1 which we associate with the idea of a fault4 ,nowhere can this important part of dyke-structure be more clearly illustrated than along the ,cleveland dyke1 where the igneous rock rises through almost horizontal ,jurassic strata and gently inclined ,coal-measures 7,figs4 #bda1 #bdb1 #bdc1 #bdd74 ,besides the localities already cited1 mining operations both for coal and for the ,liassic ironstone have proved over a wide area that the dyke has not risen along a line of fault4 ,again1 in ,skye1 ,raasay1 ,eigg1 and other parts of the west coast1 where ,jurassic strata and the horizontal basalts of the plateaux are plentifully cut through by dykes1 the same beds may generally be seen at the same level on either side of them4 ,in the fifth place1 while complete indifference to geological structure is the general rule among the dykes1 instances do occur in which the molten material has found its way upward along old lines of rupture4 ,most of such instances are to be found in districts where previously existing faults happened to run in the same general direction as that followed by the dykes4 ,these lines of fracture might naturally be re-opened by any great earth-movements acting in their direction1 and would afford ready channels for the ascent of the lava1 as we have seen to have not infrequently happened in the case of dyke-fissures1 which are shown by compound dykes to have sometimes been re-opened several times in succession even after having been filled up with basalt4 ,yet it is curious that1 even when their trend would have suited the line of the dykes1 faults have not been more largely made use of for the purpose of relief4 ,some of the best examples of the coincidence of dykes with pre-existing faults in the same direction are to be found in the ,stirlingshire coal-field4 ,the dyke that runs from ,torphichen for #bc miles to ,cadder occupies a line of fault which at ,slamannan has a down-throw of more than #gj fathoms4 ,the next dyke further south has also risen along an east and west fault4 ,but other examples may be observed where pre-existing fissures have served to deflect dykes from their usual line of trend4 ,thus the ,cleveland dyke1 after crossing several faults in the ,coal-measures1 at last encounters one near ,cockfield ,fell1 which lies obliquely across its path4 ,instead of crossing this fault it bends sharply round a few points south of west1 and after keeping along the southern flank of the fault for about a mile1 sinks out of reach4 ,some of the ,scottish examples are more remarkable4 ,one of the best of them occurs in the ,sanquhar coal-field1 where a dyke runs for two miles and a half along the large fault that here brings down the ,coal-measures against the ,lower ,silurian rocks4 ,at the north-western end of the basin1 this fault makes an abrupt bend of #fj to ,w4,s4,w41 and the dyke turns round with it1 keeping this altered course for a mile and a half1 when it strikes away from the fault1 crosses a narrow belt of ,lower ,silurian rocks1 and finds its way into the parallel boundary fault which defines the north-western margin of the ,southern ,uplands4 ,illustration3 ,fig4 #beg4--,map of the chief dykes between ,lochs ,riddon and ,striven 7,c4 ,t4 ,clough1 ,geological ,survey1 ,sheet #bi74 ,the large ,e4 and ,w4 dyke is a continuation of that which reaches the shore of the ,firth of ,clyde at ,dunoon4 ,some of the ,perthshire dykes1 where they reach the great boundary-fault of the ,highlands1 present specially interesting features4 ,there can be no doubt that this dislocation is one of the most important in the general framework of the ,british ,isles1 though no definite estimate has yet been formed of how much rock has been actually displaced by it4 ,the fact that in one place the beds of ,old ,red ,sandstone are thrown on end for some two miles back from it1 shows that it must be a very powerful fracture4 ,here1 therefore1 if anywhere1 either an entire cessation of the dykes1 or at least a complete deflection of their course might be anticipated4 ,it would require1 we might suppose1 a singularly potent dislocation to open a way for the ascent of the lava through such crushed and compressed rocks1 and still more to prolong the general line of a fracture across the old fault4 ,two great dykes1 about half a mile apart1 run in a direction a little south of west across the plain of ,strathearn4 ,passing to the south of the village of ,crieff1 they hold on their way until they reach the highly-inclined beds of sandstone and conglomerate which here lean against the ,highland fault in ,glen ,artney4 ,they then turn round towards south-west1 and run up the glen along the strike of the beds1 keeping approximately parallel to the fault for about three miles1 when they both strike across the fault1 and pursue a ,w4,s4,w4 line through the contorted crystalline rocks of the ,highlands4 ,about two miles further south1 another dyke continues its normal course across the belt of upturned ,old ,red ,sandstone2 but when it reaches the fault it bends round and follows the line of dislocation1 sometimes coinciding with1 sometimes crossing or running parallel with that line1 at a short distance 7see ,fig4 #bdg74 ,some remarkable examples have been mapped by ,mr4 ,clough in ,eastern ,argyleshire1 where broad bands of basalt or other allied rock run in a ,n4 and ,s4 direction1 and are formed by the confluence of ,n4,w and ,s4,e4 or ,n4,n4,w4 and ,s4,s4,e4 dykes1 where these are drawn into a line of fault 7,fig4 #beg74 ,these broad bands1 he has found to be not usually traceable for more than a mile or so1 for the dykes of which they are made up will not be diverted from their regular paths for more than a certain distance1 so that one by one the dykes leave the compound band to pursue their normal course4 ,he has observed that the occasional great thickness of these compound bands depends partly on the size and partly on the number of separate dykes that are diverted into the line of transverse fissure2 for1 where the fissure crosses an area with fewer north-west dykes1 the band becomes thinner or ceases altogether4 ,in some rare cases1 the dykes have been shifted by more recent faults4 ,i shall have occasion to show that faults of more than #ajjj feet have taken place since the ,tertiary basalt-plateaux were formed4 ,there is therefore no reason why here and there a fault with a low hade should not have shifted the outcrop of a dyke4 ,but the fact remains1 that1 as a general rule1 the dykes run independently of faults even where they approach close to them4 ,mr4 ,clough has observed some interesting cases in ,south-eastern ,argyleshire1 where the apparent shifting of a dyke by faults proves to be deceptive1 and where the dyke has for short distances merely availed itself of old lines of fracture4 ,one of the most remarkable of these is presented by the large dyke which runs westward from ,dunoon4 ,no fewer than three times1 in the course of four miles between ,lochs ,striven and ,riddon1 does this dyke make sharp changes of trend nearly at right angles to its usual direction1 where it encounters north and south faults 7,fig4 #beg74 ,it would be natural to conclude that these changes are actual dislocations due to the faults4 ,but the careful observer just cited has been able to trace the dyke in a very attenuated and uncrushed form along some of these cross faults1 and thus to prove that the faults are of older date1 but that they have modified the line of the long east and west fissure up which the material of the dyke ascended4 #ai4 ,,data ,,for ,,estimating ,,the ,,geological ,,age ,,of ,,the ,,dykes ,i have already assigned reasons for regarding the system of north-west and south-east or east and west dykes as belonging to the ,tertiary volcanic period in the geographical history of the ,british ,islands4 ,but ,i have no evidence that they were restricted to any part of that period4 ,on the contrary1 there is every reason to consider the uprise of the earliest and latest dykes to have been separated by a protracted interval4 ,that they do not all belong to one epoch has been already indicated1 and may now be more specially proved4 ,the intersection of one dyke by another furnishes an obvious criterion of relative age4 ,macculloch drew attention to this test1 and stated that it had enabled him to make out two distinct sets of dykes in ,skye and ,rum4 ,but he confessed that it failed to afford any information as to the length of the interval of time between them4#bje ,it is not always so easy as might be thought to make sure which of two intersecting dykes is the older4 ,as was explained in ,chapter vi4 7vol4 i4 p4 #ha71 we have to look for the finer-grained marginal strip at the edge of a dyke1 which1 where traceable across another dyke1 marks at once their relative age4 ,the cross joints of the two dykes also run in different directions4 ,reference may again be made to the illustration given in ,fig4 #bec where three distinct groups of dykes intersect each other as they traverse the ,lias limestones of ,skye4 ,the chilled edges and the different arrangement of joints mark these dykes out from each other1 while the order in which they cross each other furnishes a clue to their relative age4 ,if from such sections1 repeated in different parts of a district1 certain persistent petrographical characters can be ascertained to distinguish each particular system of dykes1 a guide may thereby be obtained for the chronological grouping of the intrusions even where evidence of actual intersection is not visible4 ,in the case just cited from ,skye1 the later north and south dykes are characterized by their lines of vesicular cavities and by the large porphyritic felspars which they contain4 ,footnote #bje3 ,trans4 ,geol4 ,soc4 iii4 p4 #ge4 ,it is obvious1 however1 that although sections of this kind suffice to prove the dykes to belong to distinct periods of intrusion1 no longer interval need have elapsed between their successive production than was required for the solidification and assumption of a joint-structure by an older dyke before a younger broke through it4 ,they may both belong to one brief period of volcanic activity4 ,but when we pass to a series of dykes traversing a considerable district of country1 and find that those which run in one direction are invariably cut by those which run in another1 the inference can hardly be resisted that they do not belong to the same period of eruption1 but mark successive epochs of volcanic energy4 ,an excellent example of this kind of evidence is furnished by ,mr4 ,clough from ,eastern ,argyleshire4 ,the east and west dykes in that district are undoubtedly older than those which run in a ,n4,n4,w4 direction 7,fig4 #beg74bjf ,the latter are by far the most abundant1 and are on the whole much narrower1 less persistent1 and finer in grain4 ,on the opposite coast of the ,clyde1 a similar double set of dykes may be traced through ,renfrewshire1 those in an east and west direction being comparatively few1 while the younger ,n4,n4,w4 series is well developed4 ,the great sheets or sills connected with one of the ,stirlingshire dykes1 already described1 appear to me to furnish similar evidence in the younger dykes which run through them4 ,and this evidence is peculiarly valuable1 for it shows a succession even among adjacent dykes which all run in the same general direction4 ,footnote #bjf3 ,as already stated1 ,mr4 ,clough and also ,mr4 ,gunn are inclined to separate these older east and west dykes from the ,tertiary series and to regard them as probably of late ,palozoic age4 ,but in all these cases it is obvious that we have little indication of the length of time that intervened between the successive injections of the dykes4 ,in ,skye1 however1 more definite evidence presents itself that the interval must have been in some cases a protracted one4 ,as far back as the year #aheg1bjg ,i showed that the basic dykes of ,strath in ,skye are of two ages2 that one set was erupted before the appearance of the syenite 7granophyre7 of that district1 and was cut off by the latter rock2 and that the other arose after the syenite which it intersected4 ,recent re-examination has enabled me to confirm and extend this observation4 ,the younger series which traverses the granophyre is much less numerous than the older series in the same districts4 ,in ,chapter xlvi41 where the relations of the granophyres to other members of the volcanic series will be discussed1 further details will be given from that region of ,skye to demonstrate that there is a pre-granophyre and a post-granophyre series of basic dykes4 ,as a good illustration of the younger series ,i may refer to the way in which these rocks make their appearance in the island group of ,st4 ,kilda1 where both the gabbros and granophyres of the ,tertiary volcanic series are characteristically developed4 ,numerous dykes traverse both these rocks4 ,those in the gabbro are more abundant than those in the granophyre--a circumstance which is exactly paralleled among the basic and acid bosses of ,skye4 ,it is not improbable that in these remote islands a similar difference in age and in petrographical character may be made out between two series of dykes1 one older and the other younger than the granophyre4 ,there is ample proof1 at all events1 of a post-granophyre series4 ,footnote #bjg3 ,quart4 ,jour4 ,geol4 ,soc4 vol4 xiv4 p4 #af4 ,illustration3 ,fig4 #beh4--;,basalt-veins traversing granophyre1 ,st4 ,kilda4 ,the pale colour of the precipices in which the ,st4 ,kilda granophyre plunges into the sea gives special prominence to the dark ribbon-like streaks which mark the course of basalt-dykes through that rock4 ,moreover the greater liability of the material of the dykes to decay causes them to weather into long lines of notch or recess4 ,four or five such dykes follow each other in nearly parallel bands1 which slant upward from the sea-level on the eastern face of the hill ,conacher to a height of several hundred feet4#bjh 7,fig4 #beh1 see also ,fig4 #cfg47 ,footnote #bjh3 ,this relation of the later dykes to the granophyre was observed here by ,macculloch 7,western ,isles1 vol4 ii4 p4 #ee74 ,the acid eruptions of the ,inner ,hebrides are marked by so varied a series of rocks1 and so complex a geological structure1 that they may1 with some confidence1 be regarded as having occupied a considerable interval of geological time4 ,yet we find that this prolonged episode in the volcanic history was both preceded and followed by the extravasation of basic dykes4 ,reference has already been made to recent observations by ,mr4 ,harker1 who1 in mapping the ,strath district of ,skye for the ,geological ,survey1 has not only confirmed the generalization as to the existence of a series of dykes earlier1 and another later1 than the great granophyre protrusions of the ,inner ,hebrides1 but has made some progress towards the detection of a means of distinguishing the two series even where no direct test of their relative age may be available4 ,he thinks that the general habit and petrographical characters of the dykes may on further investigation be found to afford a sufficiently reliable basis for discrimination4 ,he finds that where the relative ages of the dykes with reference to the granophyre can be fixed1 the earlier or pre-granophyre series is without exception basic4 ,it consists of fine-textured basalts or diabases1 without any conspicuous porphyritic crystals4 ,its dykes are less regular and persistent in their bearing than those of the later series2 have frequently a considerable hade1 even as much as #de1 and often show chilled edges with tachylitic selvages4 ,in ,skye many of these earlier dykes may be connected with the gabbro4 ,they appear to be more basic and to have a higher specific gravity than those of the later series which most resemble them4 ,the later or post-granophyre dykes include several types1 the relative ages of which are not yet definitely fixed4 ,they run in straight parallel lines1 and thus seldom intersect each other4 ,they are generally vertical or highly inclined1 and are much more frequently characterized by amygdaloiclal structure than the earlier series4 ,mr4 ,harker distinguishes the following varieties among them3 7a7 ,quartz-felsites and other acid rocks2 these are not very common4 7b7 ,pitchstones and various spherulitic and variolitic rocks3 the actual pitchstones observed are comparatively few in number1 but it is certain that some of spherulitic varieties are devitrified pitchstones4 7c7 ,basic rocks1 not conspicuously porphyritic and less decidedly basic than the dykes of the pre-granophyre series2 most of the later groups come into this or the next group1 7d7 ,porphyritic basic dykes not infrequently carrying inclusions of gabbro1 granophyre or other rocks4 ,the porphyritic felspars seem to be in great part of foreign derivation1 and the same is certainly true of the augite which occasionally accompanies them and of the quartz that appears in some examples4#bji ,footnote #bji3 ,annual ,report of the ,director-,general of the ,geological ,survey in ,report of ,science and ,art ,department for #ahie4 ,in the ,carlingford district of the ,north-east of ,ireland1 similar evidence has been obtained that one series of dykes preceded and another followed the protrusion of the granites and granophyre which are in all probability geologically coeval with the acid bosses of the ,inner ,hebrides4 ,the distinction was observed and mapped by ,mr4 ,traill for the ,geological ,survey4 ,professor ,sollas in recently confirming these observations has not noticed any striking difference between the pre-granite and post-granite dykes1 the whole appearing to consist of the same coarsely porphyritic material4#baj ,footnote #baj3 ,see ,sheets #ei1 #fj1 and #ga of the ,geological ,survey ,map of ,ireland2 ,professor ,sollas1 ,trans4 ,roy4 ,irish ,acad4 vol4 xxx4 7#ahid71 p4 #dgg2 and ,annual ,report of the ,director-,general of the ,geological ,survey for #ahie4 ,while the eruption of the granophyre bosses furnishes proof that the dykes are not all of the same age1 other evidence may be gathered to show how much older some of the dykes are than the youngest lava-streams in the volcanic history of ,tertiary time in ,britain4 ,the ,scuir of ,eigg1 to which fuller reference will be made in ,chapter xxxviii41 is formed of a mass of pitchstone1 which has filled up an ancient valley eroded out of the terraced basalts of the plateaux4 ,at both ends of the ridge1 these basalts are seen to be traversed by dykes that are abruptly cut off by the shingle of the old river-bed which the pitchstone has occupied 7,figs4 #bgi1 #bhb74 ,it is thus evident that1 though these dykes are younger than the plateau-basalts1 they are much older than the excavation of the valley out of these basalts1 and still older than the eruption of pitchstone4 ,the latter rock probably belongs to the close of the period of lava-eruptions4 ,the enormous denudation of the basalt-plateaux after the injection of the dykes and before the outflow of the pitchstone affords a convincing proof of the vastness of the interval between the eruption of the two kinds of rock4#baa ,footnote #baa3 ,quart4 ,jour4 ,geol4 ,soc4 xiv4 p4 #a4 ,it is thus demonstrable that the dykes which in ,britain form part of the great ,tertiary volcanic series1 were not all produced at one epoch1 but belong to at least two 7and probably to many more7 episodes in one long volcanic history4 ,as they rise through every member of that series of rocks 7save the pitchstones71 some of them must be among the latest records of the prolonged volcanic activity4 ,but1 on the other hand1 some probably go back to the very beginning of the ,tertiary volcanic period4 #bj4 ,,origin ,,and ,,history ,,of ,,the ,,dykes ,reference has already been made to the doubt expressed by ,macculloch whether the dykes in ,skye had been filled in from above or from below4 ,that the dykes of the country as a whole were supplied from above1 was the view entertained and enforced by ,bou4 ,he introduces the subject with the following remarks3--,scotland is renowned for the number of its basaltic veins1 which gave ,hutton his ideas regarding the injection of lava from below2 but1 as the greatest genius is not infallible1 and as volcanic countries present us with examples of such veins arising evidently from accidental fissures that were filled up by currents of lava which moved over them1 and as the ,scottish instances are of the same kind1 we regard it as infinitely probable that all these veins have been formed in the same way notwithstanding the enormous denudation which this supposition involves2 and that only rarely do cases occur where they have been filled laterally or in some other irregular manner4#bab ,i need not say that this view1 which1 except among ,wernerians1 had never many supporters1 has long ago been abandoned and forgotten4 ,there is no further question that the molten material came from below4 ,footnote #bab3 ,essai ,gologique sur l'cosse1 p4 #bgb4 #a4 ,in discussing the history of the dykes1 we are first confronted with the problem of the formation of the fissures up which the molten material rose4 ,from what has been said above regarding the usual want of relation between dykes and the nature and arrangements of the rocks which they traverse1 it is1 ,i think1 manifest that the fissures could not have been caused by any superficial action1 such as that which produces cracks of the ground during earthquake-shocks4 ,the fact that they traverse rocks of the most extreme diversities of elasticity1 structure1 and resistance1 and yet maintain the same persistent trend through them all1 shows that they originated far below the limits to which the known rocks of the surface descend4 ,we have seen that in the case of the ,cleveland dyke1 the fissure can be proved to be at least some three miles deep4 ,but the seat of the origin of the rents no doubt lay much deeper down within the earth's crust4 ,it is also evident that the cause which gave rise to these abundant fissures must have been quite distinct from the movements that produced the prevalent strike and the main faults of this country4 ,from early geological time1 as is well known1 the movements of the earth's crust beneath the area of ,britain1 have been directed in such a manner as to give the different stratified formations a general north-east and south-west strike1 and to dislocate them by great faults with the same average trend4 ,but the fissures of the ,tertiary dykes run obliquely and even at a right angle across this prevalent older series of lines and are distinct from any other architectonic feature in the geology of the country4 ,they did not arise therefore by a mere renewal of some previous order of disturbances1 but were brought about by a new set of movements to which it is difficult to find any parallel in the earlier records of the region4#bac ,footnote #bac3 ,the only other known example of such a dyke-structure in ,britain is that of the ,pre-,cambrian series of dykes in the ,lewisian gneiss of ,sutherland1 described in ,chapter viii4 ,we have further to remember that the fissures were not produced merely by one great disturbance4 ,the evidence of the dykes proves beyond question that some of them are earlier than others1 and hence that the cause to which the fissures owed their origin came into operation repeatedly during the protracted ,tertiary volcanic period4 ,one of the most instructive lessons in this respect is furnished by the huge eruptive masses of gabbro and granitoid rocks in ,skye4 ,these materials have been erupted through the plateau-basalts4 ,the granitoid bosses are the younger protrusions1 for they send veins into the gabbros2 but their appearance was later than that of some of the dykes and older than that of others4 ,nevertheless1 the youngest dykes generally maintain the usual north-westerly trend across the thickest masses of the granophyre4 ,thus we perceive that1 even after the extrusion of thousands of feet of such solid crystalline igneous rocks1 covering areas of many square miles1 the fissuring of the ground was renewed1 and rents were opened through these new piles of material4 ,from the evidence of the dykes also we learn that some fissures were repeatedly re-opened and admitted a new ascent of molten magma between their walls4 ,the general direction of the fissures remained from first to last tolerably uniform4 ,here and there indeed1 where one set of dykes traverses another1 as in ,skye and the basin of the ,clyde1 we meet with proofs of a deviation from the normal trend4 ,but it is remarkable that dykes which pierce the latest eruptive bosses of the ,inner ,hebrides rose in fissures that were opened in the normal north-westerly line through these great protrusions of basic and acid rock4 ,such a gigantic system of parallel fissures points to great horizontal tension of the terrestrial crust over the area in which they are developed4 ,hopkins1 many years ago1 discussed from the mathematical side the cause of the production of such fissures4#bad ,he assumed the existence of some elevatory force acting under considerable areas of the earth's crust at any assignable depth1 either with uniform intensity at every point or with a somewhat greater intensity at particular points4 ,he did not assign to this force any definite origin1 but supposed it to act upon the lower surface of the uplifted mass through the medium of some fluid1 which may be conceived to be an elastic vapour1 or1 in other cases1 a mass of matter in a state of fusion from heat4#bae ,he showed that such an upheaving force would produce in the affected territory a system of parallel longitudinal fissures1 which1 when not far distant from each other1 could only have been formed simultaneously1 and not successively2 that each fissure would begin not at the surface but at some depth below it1 and would be propagated with great velocity2 that there would be more fissures at greater than at lesser depths1 many of them never reaching the surface2 that they would be of approximately uniform width1 the mean width tending to increase downwards2 that continued elevation might increase these fissures1 but that new fissures in the same direction would not arise in the separated blocks which would now be more or less independent of each other2 that subsequent subsidences would give rise to transverse fissures1 and by allowing the separated blocks to settle down would cause irregularities in the width of the great parallel fissures4 ,he considered also the problem presented by those cases where the ruptures of the terrestrial crust have been filled with igneous matter1 and now appear as dykes4 ,the results above obtained1 he says1 will manifestly hold equally1 whether we suppose the uplifted mass acted upon immediately through the medium of an elastic vapour or by matter in a state of fusion in immediate contact with its lower surface4 ,in the latter case1 however1 this fused matter will necessarily ascend into the fissures1 and if maintained there till it cools and solidifies1 will present such phenomena as we now recognize in dykes and veins of trap4 ,footnote #bad3 ,cambridge ,phil4 ,trans4 vi4 7#ahce71 p4 #a4 ,footnote #bae3 ,ibid4 p4 #aj4 ,the existence of a vast lake or reservoir of molten rock under the fissure-region of ,britain is demonstrated by the dykes4 ,but1 if we inquire further what terrestrial operation led to the uprise of so vast a body of lava towards the surface in older ,tertiary time1 we find that as yet no satisfactory answer can be given4 #b4 ,in some districts the dykes can be connected with the gabbros which occur as intrusive sills and irregular bosses in the basalt-plateaux and among older rocks4 ,the gabbros1 however1 are traversed by still later dykes1 which must then be independent of any visible mass of these rocks4 ,the connection of dykes with the gabbros is what we might naturally expect to find1 if the more coarsely crystalline rock represents portions of the basic magma which consolidated at some depth below the surface4 ,if we could penetrate deep enough1 it is not improbable that the dykes might be found in large measure to shade downward into vast bodies of gabbro4 ,such a relation has been observed in the ,yellowstone district1 where ,mr4 ,iddings has noticed that the centre toward which the dykes of the ,old ,crandale volcano converge is a large mass of granular gabbro1 passing into diorite1 the dykes becoming rapidly coarser in grain as they approach the gabbro-core4#baf ,footnote #baf3 ,journ4 ,geol4 i4 7#ahic71 p4 #fjh4 #c4 ,the rise of molten rock in thousands of fissures over so wide a region is to my mind by far the most wonderful feature in the history of volcanic action in ,britain4 ,the great plateaux of basalt1 and the mountainous bosses of rock by which they have been disrupted1 are undoubtedly the most obvious memorials of ,tertiary volcanism4 ,but1 after all1 they are merely fragments restricted to limited districts4 ,the dykes1 however1 reveal to us the extraordinary fact that1 at a period so recent as older ,tertiary time1 there lay underneath the area of ,britain a reservoir or series of reservoirs of lava1 the united extent of which must have exceeded #dj1jjj square miles4 ,that the material of the dykes rose in general directly from below1 and was not1 except locally1 injected laterally along the open fissures1 may be inferred1 although proof of such lateral injection on a small scale may here and there be detected4 ,the narrowness of the rents1 and their enormous relative length1 make it physically impossible that molten rock could have moved along them for more than short distances4 ,the usual homogeneous character of the dyke-rocks1 the remarkable scarcity of any broken-up consolidated fragments of them immersed in a matrix of different grain1 the general uniformity of composition and structure from one end of a long dyke to another1 the spherical form of the amygdales1 the usual paucity of fragments from the fissure walls--all point to a quiet welling of the lava upward4 ,over the whole of the region traversed by the dykes1 from the hills of ,yorkshire and ,lancashire to the remotest ,hebrides1 molten rock must have lain at a depth1 which1 in one case1 we know to have exceeded three miles1 and which was probably everywhere considerably greater than that limit4 ,forced upwards1 partly perhaps by pressure due to terrestrial contraction and partly by the enormous expansive force of the gases and vapours absorbed within it1 the lava rose in thousands of fissures that had been opened for it in the solid overlying crust4 ,that in most cases its ascent terminated short of the surface of the ground may reasonably be inferred4 ,at least1 we know1 that many dykes do not reach the present surface1 and that those which do have shared in the enormous denudation of the surrounding country4 ,that even in the same dyke the lava rose hundreds of feet higher at one place than at another is abundantly proved4 ,when1 however1 we consider the vast number of dykes that now come to the light of day1 and reflect that the visible portions of some of them differ more than #cjjj feet from each other in altitude1 we can hardly escape the conviction that it would be incredible that nowhere should the lava have flowed out at the surface4 ,subsequent denudation has undoubtedly removed a great thickness of rock from what was the surface of the ground during older ,tertiary time1 and hundreds of dykes are now exposed that doubtless originally lay deeply buried beneath the overlying part of the earth's crust through which they failed to rise4 ,but some relics1 at least1 of the outflow of lava might be expected to have survived4 ,i believe that such relics remain to us in the great basalt-plateaux of ,antrim and the ,inner ,hebrides4 ,these deep piles of almost horizontal sheets of basalt1 emanating from no great central volcanoes1 but with evidence of many local vents1 appear to me to have proceeded in large measure from dykes which1 communicating with the surface of the ground1 allowed the molten material to flow out in successive streams with occasional accompaniments of fragmentary ejections4#bag ,the structure of the basalt-plateaux1 and their mode of origin1 will form the subject of the next division of this volume4 ,footnote #bag3 ,it is interesting to note that in the great paper on ,physical ,geology already cited1 ,hopkins considered the question of the outflow of lava from the fissures which he discussed4 ,if the quantity of fluid matter forced into these fissures1 he says1 be more than they can contain1 it will1 of course1 be ejected over the surface2 and if this ejection take place from a considerable number of fissures1 and over a tolerably even surface1 it is easy to conceive the formation of a bed of the ejected matter of moderate and tolerably uniform thickness1 and of any extent 7op4 cit4 p4 #ga74 ,we can hardly suppose that the lava flowed out only in the western region of the existing plateaux4 ,probably it was most frequently emitted and accumulated to the greatest depth in that area4 ,but over the centre of ,scotland and ,north of ,england there may well have been many places where dykes actually communicated with the outer air1 and allowed their molten material to stream over the surrounding country1 either from open fissures or from vents that rose along these4 ,the disappearance of such outflows need cause no surprise1 when we consider the extent of the denudation which many dykes demonstrate4 ,i have elsewhere shown that all over ,scotland there is abundant proof that hundreds and even thousands of feet of rock have been removed from parts of the surface of the land since the time of the uprise of the dykes4#bah ,the evidence of this denudation is singularly striking in such districts as that of ,loch ,lomond1 where the difference of level between the outcrop of the dykes on the crest of the ridges and in the bottom of the valleys exceeds #cjjj feet4 ,it is quite obvious1 for example1 that had the deep hollow of ,loch ,lomond lain1 as it now does1 in the pathway of these dykes1 the molten rock1 instead of ascending to the summits of the hills1 would have burst out on the floor of the valley4 ,we are1 therefore1 forced to admit that a deep glen and lake-basin have been in great measure hollowed out since the time of the dykes4 ,if a depth of many hundreds of feet of hard crystalline schists could have been removed in the interval1 there need be no difficulty in understanding that by the same process of waste1 many sheets of solid basalt may have been gradually stripped off the face of ,central ,scotland and ,northern ,england4 ,footnote #bah3 ,scenery of ,scotland1 #bnd edit4 7#ahhg71 p4 #adi4 ,but see the remarks already made 7p4 #aej7 on the curious coincidence sometimes observable between the upper limit of a dyke and the overlying inequalities of surface4 ,the association of fissures and dykes with the accumulation of thick and extensive volcanic plateaux1 over so wide a region of ,north-western ,europe as from ,antrim to the ,north of ,iceland1 finds its parallel in different parts of the world4 ,one of the closest analogies presents itself among the ,ghauts of the ,bombay ,presidency1 where vast basaltic sheets1 probably of ,cretaceous age1 display topographical and structural features closely similar to those of the ,tertiary volcanic plateaux of the ,british ,isles4 ,the dykes connected with these ,indian basaltic outflows correspond almost exactly in their general character and stratigraphical relations to those of this country4 ,they occur in great numbers1 rising through every rock in the district up to the crests of the ,ghauts1 #djjj feet above the sea4 ,they vary from #a or #b to #aj1 #bj1 #dj1 and even occasionally #ajj or #aej feet in width1 and are often many miles in length4 ,they observe a general parallelism in one average direction1 and show no perceptible difference in character even when traced up to elevations of #cjjj and #djjj feet4#bai ,footnote #bai3 ,mr4 ,g4 ,t4 ,clark1 ,quart4 ,journ4 ,geol4 ,soc4 xxv4 7#ahfi7 p4 #afc4 ,for remarks on the connection of dykes with superficial lavas1 see postea1 p4 #bfh4 ,thousands of square miles in the ,western ,states and ,territories of the ,american ,union have been similarly flooded with basic lavas4 ,denudation has not yet advanced far enough to lay bare much of the platform on which these lavas rest4 ,but the dykes that traverse the rocks outside of the lava-deserts afford an example of the structure which will ultimately be revealed when the wide and continuous basalt-plains shall have been trenched by innumerable valleys and reduced to fragmentary plateaux with lofty escarpments 7p4 #bfg74 ,it is to the modern eruptions of ,iceland1 however1 that we turn for the completest illustration of the phenomena connected with dykes and fissures4 ,an account of these eruptions will therefore be given in ,chapter xl4 as an explanation of the history of the ,tertiary basalt-plateaux of ,britain4 ,,chapter ,,xxxvi ,,the ,,plateaux ,nature and ,arrangement of the ,rocks3 #a4 ,lavas4--,basalts1 ,dolerites1 ,andesites--,structure of the ,lavas in the ,field--#b4 ,fragmental ,rocks4--,agglomerates1 ,conglomerates1 and ,breccias--,tuffs and their accompaniments4 ,we have now to consider the structure and history of those volcanic masses which1 during ,tertiary time1 were ejected to the surface within the area of the ,british ,islands1 and now remain as extensive plateaux4 ,short though the interval has been in a geological sense since these rocks were erupted1 it has been long enough to allow of very considerable movements of the ground and of enormous denudation1 as will be more fully discussed in ,chapters xlviii4 and xlix4 ,hence the superficial records of ,tertiary volcanic action have been reduced to a series of broken and isolated fragments4 ,i have already stated that no evidence now remains to show to what extent there were actual superficial outbursts of volcanic material over much of the dyke-region of ,britain4 ,the subsequent waste of the surface has been so enormous that various lava-fields may quite possibly have stretched across parts of ,england and ,scotland1 whence they have since been wholly stripped off1 leaving behind them only that wonderful system of dykes from which their molten materials were supplied4 ,there can be little doubt1 however1 that whether or not other ,phlegrean fields extended over portions of the country whence they have since been worn away1 the chief volcanic tract lay in a broad and long hollow that stretched from the south of ,antrim to the ,minch4 ,from the southern to the northern limit of the fragmentary lava-fields that remain in this depression is a distance of some #bej miles1 and the average breadth of ground within which these lava-fields are preserved may be taken to range from #bj to #ej miles4 ,if1 therefore1 the sheets of basalt and layers of tuff extended over the whole of this strip of country1 they covered a space of some #gjjj or #hjjj square miles4 ,but they were not confined to the area of the ,british ,islands4 ,similar rocks rise into an extensive plateau in the ,faroe ,islands1 and it may reasonably be conjectured that the remarkable submarine ridge which extends thence to the ,north-west of ,scotland1 and separates the basin of the ,atlantic from that of the ,arctic ,ocean1 is partly at least of volcanic origin4 ,still further north come the extensive ,tertiary basaltic plateaux of ,iceland1 while others of like aspect and age cover a vast area in ,southern ,greenland4 ,without contending that one continuous belt of lava-streams stretched from ,ireland to ,iceland and ,greenland1 we can have no doubt that in older ,tertiary time the north-west of ,europe was the scene of more widely-extended volcanic activity than had shown itself at any previous period in the geological history of the whole continent4 ,the present active vents of ,iceland and ,jan ,mayen are not improbably the descendants in uninterrupted succession of those that supplied the materials of the ,tertiary basaltic plateaux1 the volcanic fires slowly dying out from south to north4 ,but so continuous and stupendous has been the work of denudation in these northern regions1 where winds and waves1 rain and frost1 floe-ice and glaciers reach their highest level of energy1 that the present extensive sheets of igneous rock can be regarded only as magnificent relics1 the grandeur of which furnishes some measure of the magnitude of the last episode in the extended volcanic history of ,britain4 ,the long and wide western valley in which the basalt-plateaux of this country were accumulated seems1 from a remote antiquity1 to have been a theatre of considerable geological activity4 ,there are traces of some such valley or depression even back in the period of the ,torridon ,sandstone of the north-west4 ,this formation1 as we have seen1 was laid down between the great ridge of the ,outer ,hebrides and some other land to the east1 of which a few of the higher mountains1 once buried under the sandstone1 are now being revealed by denudation between ,loch ,maree and ,loch ,broom1 and also in ,assynt4 ,the conglomerates and volcanic rocks of ,lorne may represent the site of one of the older water-basins of this ancient hollow4 ,the ,carboniferous rocks1 which run through the ,north of ,ireland1 cross into ,cantyre1 and are found even as far north as the ,sound of ,mull1 mark how1 in later ,palozoic time1 the same strip of country was a region of subsidence and sedimentation4 ,during the ,mesozoic ages1 similar operations were continued2 the hollow sank several thousand feet1 and ,jurassic strata to that depth filled it up4 ,before the ,cretaceous period1 underground movements had disrupted and irregularly upheaved the ,jurassic deposits1 and prolonged denudation had worn them away1 so that when the ,cretaceous formations came to be laid down on the once more subsiding depression1 they were spread out with a strong unconformability on everything older than themselves1 resting on many successive horizons of the ,jurassic system1 and passing from these over to the submerged hillsides of the crystalline schists4 ,yet again1 after the accumulation of the ,chalk1 the sea-floor along the same line was ridged up into land1 and the ,chalk1 exposed to denudation1 was deeply trenched by valleys1 and entirely removed from wide tracts which it once covered4 ,it was in this long broad hollow1 with its memorials of repeated subsidences and upheavals1 sedimentation and denudation1 that the vigour of subterranean energy at last showed itself in volcanic outbreaks1 and in the gradual piling up of the materials of the basalt-plateaux4 ,so far as we know1 these outbursts were subrial4 ,at least no trace of any marine deposit has yet been found even at the base of the pile of volcanic rocks4 ,sheet after sheet of lava was poured out1 until several thousand feet had accumulated1 so as perhaps to fill up the whole depression1 and once more to change entirely the aspect of the region4 ,but the volcanic period1 long and important as it was in the geological history of the country1 came to an end4 ,it1 too1 was merely an episode during which denudation still continued active1 and since which subterranean disturbance and superficial erosion have again transformed the topography4 ,in wandering over these ancient lava-fields1 we see on every hand the most stupendous evidence of change4 ,they have been dislocated by faults1 sometimes with a displacement of hundreds of feet1 and have been hollowed out into deep and wide valleys and arms of the sea4 ,their piles of solid rock1 hundreds of feet thick1 have been totally stripped off from wide tracts of ground which were once undoubtedly buried under them4 ,hence1 late though the volcanic events are in the long history of the land1 they are already separated from us by so vast an interval that there has been time for cutting down the wide plateaux of basalt into a series of mere scattered fragments4 ,but the process of land-sculpture has been of the utmost service to geology1 for1 by laying bare the inner structure of these plateaux1 it has provided materials of almost unequalled value and extent for the study of one type of volcanic action4 ,i4 ,,nature ,,and ,,arrangement ,,of ,,the ,,rocks ,,of ,,the ,,plateaux ,the superficial outbursts of volcanic action during ,tertiary time in ,britain are represented by a comparatively small variety of rocks4 ,these consist almost wholly of basalts1 but include a number of less basic rocks which may be classed as andesites4 ,many andesitic sheets1 like the andesitic dykes1 have been intruded into the basalts1 and are really sills4 ,besides the lavas of the basaltic-plateaux there are intercalated deposits of tuffs and breccias and large masses of agglomerate4 ,a brief notice of the general petrography of the various constituents of the plateaux and their mode of occurrence will here be given4 ,the intrusive bosses which have disrupted the superficial lavas will be discussed in subsequent chapters4 i4 ,,lavas #a4 ,petrographical ,characters 7a7 ,basalts and ,dolerites4--,in external characters these rocks range from coarsely crystalline varieties1 in which the constituent minerals may be more or less readily detected with the naked eye or a field-lens1 to dense black compounds in which only a few porphyritic crystals may be megascopically visible4 ,one of their characteristic features is the presence of the ophitic structure1 sometimes only feebly developed1 sometimes showing itself in great perfection4 ,many of the rocks are holocrystalline1 but usually show more or less interstitial matter2 in others the texture is finer1 and the interstitial matter more developed2 in no case1 as far as ,i have observed1 are there any glassy varieties1 which are restricted to the dykes and sills1 though in some of the basalts the proportion of glassy or incompletely devitrified substance is considerable4 ,the felspars are generally of the characteristic lath-shaped forms1 and are usually quite clear and fresh4 ,the augite resembles that of the dykes1 occurring sometimes in large plates that enclose the felspars1 at other times in a finely granular form4 ,olivine is frequently not to be detected1 even by green alteration products4 ,magnetite is sometimes present in such quantity as to affect the compass of the field-geologist4 ,porphyritic varieties occur with large felspar phenocrysts2 but such varieties are1 ,i think1 less frequent among the plateau-rocks than among the dykes4 ,they are well developed in the west part of the island of ,canna1 and have been described from the ,faroe islands4 ,occasionally the plateau lavas are full of enclosed fragments of other rocks which have been carried up in the ascending magma4 7b7 ,andesites and ,trachytes4--,probably the majority of these rocks where they occur intercalated between the basalts of the plateaux are1 as already remarked1 intrusive sheets rather than true lavas4 ,but they have also been poured out intermittently among the basalts and dolerites4 ,the most extensive development of lavas which are readily distinguishable from the group of plateau-basalts1 and must be placed in the present series1 occurs in the island of ,mull4 ,these rocks form part of a group of pale lavas which overlie the main mass of the plateau-basalts1 and cap the mountain ,ben ,more1 together with several of its lofty neighbours4 ,they are interstratified with true ophitic dolerites1 and basalts showing characteristic granular augite4 ,they are not so heavy as the ordinary plateau-lavas1 their specific gravity ranging from #bee to #bgd4 ,externally they are light grey in colour and dull in texture1 sometimes strongly amygdaloidal1 sometimes with a remarkable platy structure1 which1 in the process of weathering1 causes them to split up like stratified rocks4 ,in some of their amygdaloidal varieties the cells are filled with epidote1 which also appears in the fissures1 and sometimes even as a constituent of the rock4 ,specimens from this pale group of ,ben ,more1 when examined in thin slices under the microscope1 were found by ,dr4 ,hatch to consist almost wholly of felspar in minute laths or microlites1 but in no instance sufficiently definite for satisfactory determination4 ,in one of them he observed that each lath of felspar passed imperceptibly into those adjacent to it2 the double refraction being very weak1 and the twin-striation1 if present1 not being traceable4#bbj ,more recently my colleague1 ,mr4 ,w4 ,w4 ,watts1 has looked at some of the same slides4 ,he is disposed to class the rocks rather with the trachytes than the andesites4 ,he remarks that in the apparent holocrystalline character1 the size and shape of the felspars1 the sort of damascened appearance in polarized light1 the finely scattered iron-ores and the presence of a pale green hornblende1 possibly augite1 in small1 often complex1 grains1 these rocks much resemble the ,carboniferous trachytes of the ,garlton ,hills in ,scotland4 ,footnote #bbj3 ,in the course of my investigations ,i have had many hundreds of thin slices cut from the ,tertiary volcanic rocks for microscopic determination4 ,these ,i have myself studied in so far as their microscopic structure appeared likely to aid in the investigation of those larger questions of geological structure in which ,i was more especially interested4 ,but for further and more detailed study ,i placed them with ,dr4 ,hatch1 who submitted to me the results of his preliminary examination1 and where these offered points of geological import ,i availed myself of them in the memoir published in #ahhh in the ,transactions of the ,royal ,society of ,edinburgh4 ,i have retained most of these citations in their place in the present volume1 and have supplemented them by notes supplied to me from fresh observations by ,mr4 ,watts and ,mr4 ,harker4 ,professor ,judd1 in a series of valuable papers1 has discussed the general petrography of the ,tertiary volcanic rocks 7,quart4 ,jour4 ,geog4 ,soc4 vols4 xxxix4 xli4 xlii4 xlvi4 xlix47 ,one of the most interesting lavas of the ,tertiary volcanic series is the pitchstone-porphyry of the ,scuir of ,eigg4 ,this rock1 the latest known outflow of lava in any of the volcanic areas of ,britain1 was formerly classed with the acid series4 ,microscopical and chemical analyses prove it1 however1 to be of intermediate composition1 and to be referable to the andesites or dacites4 ,it is more particularly described in ,chapter xxxviii4 ,professor ,judd1 collecting the andesitic rocks as a whole 7both lavas and sills71 has grouped them into amphibole and mica-andesites1 and pyroxene-andesites4#bba ,the thick lumpy and non-persistent sheets of these rocks sometimes found near the centres of protrusion of the gabbros and granophyres are probably sills4 ,footnote #bba3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xlvi4 7#ahij71 p4 #cef4 ,professor ,judd has there described under the name of propylites various members of the volcanic series which he believes to have undergone alteration from solfataric action4 ,i have not been able to discover any trace of such action1 but ,i have found that the lavas of the plateaux assume a peculiar condition where they have been affected by large intrusive masses of granophyre or gabbro4 7,see postea1 ,chapter xlvi47 7c7 ,rhyolites4--,in the ,antrim plateau a group of rhyolite bosses occurs1 some of which have been claimed as superficial lavas4 ,in some cases it can be demonstrated that they are intrusive1 and in no instance can they be decisively shown to have escaped in streams at the surface4 ,it is probable1 however1 that some of these bosses did actually communicate with the outer air1 for between the lower and upper group of basalts in this plateau1 bands of rhyolitic conglomerate occur which may indicate the degradation of exposed masses of rhyolite4 ,the description of these ,antrim bosses will be given in ,chapter xlvii41 in connection with the acid eruptive rocks of the ,tertiary volcanic series4 #b4 ,structure in the ,field ,passing now to the consideration of the lavas as they are built up into the plateaux1 we have to note their distinctive characters as individual sheets of rock1 and their influence on the topography of the regions in which they occur4 ,every tourist who has sailed along the cliffs of ,antrim1 ,mull1 ,skye1 or the ,faroe ,islands is familiar with the singular terraced structure of the great volcanic escarpments which stretch as mural precipices along these picturesque shores4 ,successive sheets of lava1 either horizontal or only gently inclined1 rise above each other from base to summit of the cliffs as parallel bars of brown rock with intervening strips of bright green grassy slope4 ,the geologist who for the first time visits these coast-lines is impressed by the persistence of the same lithological characters giving rise to the same topographical features4 ,he soon realises that the plateaux1 so imposingly truncated by the great escarpments that spring from the edge of the sea1 are built up essentially of dark lavas--basalts and dolerites--and that fragmental volcanic accompaniments1 though here and there well developed1 play1 on the whole1 a quite insignificant part in the structure and composition of these thick piles of volcanic material4 ,closer examination in the field enables him to ascertain that1 regarded as rock-masses1 the lavas include four distinct types3-- #ast4 ,thick1 massive1 prismatic or rudely-jointed sheets1 rather more coarsely crystalline and obviously more durable than the other types1 inasmuch as they project in tabular ledges and tend to retain perpendicular faces owing to the falling away of slices of the rock along lines of vertical joints4 ,many rocks of this type are undoubtedly intrusive sheets1 and as such will be further referred to in a later chapter4 ,but the type includes also true superficial lavas which show the characteristic slaggy or vesicular bands at their upper and lower surfaces4 ,the mere presence of such bands may not be enough1 indeed1 absolutely to establish that the rock possessing them flowed at the surface as a lava1 for they are occasionally1 though it must be confessed rarely1 exhibited by true sills4 ,but the rough scoriaceous top of a lava-stream1 and the presence of fragments of this surface in the overlying tuff1 or wrapped round by the next succeeding lava1 sufficiently attest the true superficial outflow of the mass4 #bnd4 ,prismatic or columnar basalts1 which1 as at the ,giant's ,causeway and ,staffa1 have long attracted notice as one of the most striking topographical elements of the plateaux4 ,columnar structures are typical of the more compact heavy basalts4 ,a considerable variety is observable in the degree of perfection of their development4 ,where they are least definite1 the rock is traversed by vertical joints1 somewhat more regular and close-set than those in the dolerites1 by the intersection of which it is separated into rude quadrangular or polygonal columns4 ,the true columnar structure is shown in two chief forms4 7a7 ,the rock is divided into close-fitting parallel1 usually six-sided columns2 the number of sides varying1 however1 from three up to nine4 ,the columns run the whole thickness of the bed1 and vary from #h or #aj to #dj or even #hj feet in length4 ,they are segmented by cross joints which sometimes1 as at ,giant's ,causeway1 take the ball-and-socket form4 ,occasionally they are curved1 as at the well-known ,clam-shell cave of ,staffa4 7b7 ,the prisms are much smaller1 and diverge in wavy groups crowded confusedly over each other1 but with a general tendency upwards4 ,this starch-like aggregation may be observed superposed directly upon the more regular columnar form as at the ,giant's ,causeway and also at ,staffa4 ,excellent illustrations of both these types may be seen at many points along the sea-cliffs of the ,inner ,hebrides2 the western coast of ,skye1 the south-west side of ,mull1 and the cliffs of the island of ,canna may be specially cited4 ,illustration3 ,fig4 #bei4--,section of scoriaceous and prismatic ,basalt1 ,camas ,tharbernish1 north shore of ,canna ,island4 ,though generally rather compact1 becoming indeed dense1 almost vitreous rocks in some sheets1 the columnar basalts are often more or less cellular throughout1 and highly slaggy along their upper and under surfaces4 ,in some cases1 as in that of a prismatic sheet which overlies the rough scoriaceous lava of ,camas ,tharbernish1 in the island of ,canna1 the rows of vesicles are disposed in lines parallel to the under surface of the sheet 7,fig4 #bei47 ,as already remarked with regard to the massive1 rudely-jointed sheets1 many of the most perfectly columnar rocks of the plateaux are not superficial lavas1 but intrusive sills1 bosses or dykes4 ,conspicuous examples of such sills are displayed on the coast of ,trotternish in ,skye1 and of the bosses and dykes at the eastern end of ,canna4 ,to these further reference will be made in the sequel4 ,it is not always possible to be certain that columnar sheets which appear to be regularly intercalated among the undoubted lavas of the volcanic series may not be really intrusive4 ,in some instances1 indeed1 we can demonstrate that they are so1 when after continuing perfectly parallel with the lavas above and below them1 they eventually break across them4 ,one of the most remarkable examples of this feature is supplied by the great sill of the south-west of ,strom1 in the ,faroe ,islands1 of which ,i shall give some account in ,chapter xlii4 7,figs4 #cab1 #cbh1 #cbi74 #crd4 ,slaggy or amygdaloidal lavas without any regular jointed structure1 but often with roughly scoriform upper and under layers1 and tending to decay into brown earthy debris4 ,some of the upper surfaces of such sheets among the ,tertiary basalt-plateaux must have resembled the so-called ,aa of the ,sandwich ,islands4 ,a striking example of the structure may be noticed at ,camas ,tharbernish1 on the north coast of the ,island of ,canna4 ,there the hummocks on the upper surface of a slaggy basalt measure about #ae feet in breadth1 and rise about three feet above the hollows between them1 like a succession of waves 7see ,fig4 #bei74 ,the steam-holes are disposed in a general direction parallel to the strike of the hummocks4 ,great variety obtains in the size and shape of the vesicles4 ,huge cavities a foot or more in diameter may occasionally be found1 and from such extremes every gradation may be traced down to minute pore-like vacuoles that can hardly be made out even with a strong lens4 ,in regard to the deformation of the vesicles1 it is a familiar general rule that they have been drawn out in the direction of the flow of the original lava4 ,occasionally they have become straight1 narrow1 sometimes bifurcating pipes1 several inches long1 and only an eighth of an inch or so in diameter4#bbb ,a number of such pipes1 parallel to each other1 resembles a row of worm-burrows 7see ,fig4 #b74 ,footnote #bbb3 ,some examples have been deposited by me in the ,museum of ,practical ,geology1 ,jermyn ,street1 in the case illustrating rock-structures4 ,the elongation of the vesicles into annelide-like tubes may also be observed among the stones in the volcanic agglomerates4 ,it may often be noticed that1 even where the basalt is most perfectly prismatic1 it presents a cellular and even slaggy structure at the bottom4 ,the rock that forms the ,giant's ,causeway1 for instance1 is distinctly vesicular1 the vesicles being drawn out in a general east and west direction4 ,the beautiful columnar bed of ,staffa is likewise slaggy and amygdaloidal for a foot or so upwards from its base1 and portions of this lower layer have here and there been caught up and involved in the more compact material above it4 ,even the bottom of the confusedly prismatic bed above the columnar one on that island also presents a cellular texture4 ,a similar rock at ,ardtun1 in ,mull1 passes upward into a rugged slag and confused mass of basalt blocks1 over which the leaf-beds lie4 ,amygdaloidal structure is more or less developed throughout the whole series of basalts4 ,but it is especially marked in certain abundant sheets1 which1 for the sake of distinction1 are called amygdaloids4 ,these beds1 which form a considerable proportion of the materials of every one of the plateaux1 are distinguished by the abundance and large size of their vesicles4 ,in some places1 the cavities occupy at least as much of the rock as the solid matrix in which they lie4 ,they have generally been filled up with some infiltrated mineral--calcite1 chalcedony1 zeolites1 etc4 ,the amygdales of the west of ,skye and of ,antrim have long been noted for their zeolites4 ,as a consequence of their cellular texture and the action of infiltrating water upon them1 these amygdaloidal sheets are always more or less decomposed4 ,their dull1 lumpy1 amorphous aspect contrasts well with the sharply-defined columnar sheets above and below them1 and as they crumble down they are apt to be covered over with vegetation4 ,hence1 on a sea-cliff or escarpment1 the green declivities between the prominent columnar basalts usually mark the place of such less durable bands4 ,exceedingly slag-like lavas are to be seen among the amygdaloids1 immediately preceded and followed by beds of compact black basalt with few or no vesicles4 ,from the manner in which such rocks yield to the weather1 they often assume a singularly deceptive resemblance to agglomerates4 ,one of the best examples of this resemblance which have come under my notice is that of the rock on which stands ,dunluce ,castle1 on the north coast of ,antrim4 ,huge rounded blocks of a harder consistency than the rest of the rock project from the surface of the cliffs1 like the bombs of a true volcanic agglomerate1 while the matrix in which they are wrapped has decayed from around them4 ,but an examination of this matrix will soon convince the observer that it is strongly amygdaloidal1 and that the apparent bombs are only harder and less cellular portions of it4 ,the contrast between the weathering of the two parts of the rocks seems to have arisen from an original variety in the relative abundance of steam-cavities4 ,the origin of such nodular or pillow-like blocks has been already referred to at pp4 #bf and #aic4 ,another singular instance occurs at the foot of the outlier of ,fionn ,chro 7,fig4 #cfj71 in the island of ,rum4 ,a conspicuous band underlying the basalts there might readily be taken for a basalt-conglomerate4 ,but in this case1 also1 the apparent matrix is found to be amygdaloidal1 and the rounded blocks are really amygdales1 sometimes a foot in length1 filled or lined with quartz1 chalcedony1 c4 ,a somewhat different structure1 in which1 however1 the appearance of volcanic breccia or agglomerate due to explosion from a vent is simulated1 may be alluded to here4 ,the best instance which ,i have observed of it occurs at the south end of ,loch-na-,mna1 in the island of ,eigg1 within a basalt which is remarkable for a streaky flow-structure4 ,on the weathered faces the streaky layers may be observed to have been broken up1 and their disconnected fragments have been involved in ordinary basalt wherein this flow-structure is not developed1 while large blocks and irregular masses are wrapped round in a more decomposing matrix4 ,there can be no doubt that in such cases we see the effects of the disruption of chilled crusts1 and the entanglement of the broken pieces in the still fluid lava4 ,it is a common belief that the filling in of the steam-cavities has taken place long subsequent to the volcanic period1 by the slow percolation of meteoric water through the rock4 ,i believe1 however1 that at least in some cases1 if not in all1 the conversion of the vesicular lavas into amygdaloids was effected during the volcanic period4 ,thus it can be shown that the basalts which have been disrupted by the gabbros and granophyres were already amygdaloids before these basic intrusions disturbed them1 for the kernels of calcite1 zeolite1 etc41 have shared in the general metamorphism induced in the enclosing rock4 ,again1 the blocks of amygdaloid contained in the agglomerates of the volcanic series are in every respect like the amygdaloidal lavas of the plateaux4 ,it would thus seem that the infilling of the cavities with mineral secretions was not merely a long secular process of infiltration from the cool atmosphere1 but was more rapidly completed by the operation of warmer water1 either supplied from volcanic sources or heated by the still high temperature of the cellular lavas into which it descended from the surface4#bbc ,footnote #bbc3 ,professor ,j4 ,d4 ,dana1 originally an advocate of infiltration from above1 subsequently supported the view that the kernels of amygdaloids were filled in by the action of moisture within the rocks during the time of cooling4--,amer4 ,journ4 ,sci4 ser4 #c1 vol4 xx4 7#ahhj71 p4 #cca4 ,messrs4 ,harker and ,marr have demonstrated that the ,lower ,silurian vesicular lavas of the ,lake district had already become amygdaloids before the uprise of the ,shap granite4--,quart4 ,journ4 ,geol4 ,soc4 vol4 xlix4 7#ahic74 #dth4 ,banded or stratiform lavas1 consisting of successive parallel layers or bands which weather into projecting ribs and flutings4 ,the deceptive resemblance to sedimentary rocks thus produced has no doubt frequently led to these lavas being mistaken for tuffs4 ,as ,i have recently found them to be much more plentiful than ,i had supposed1 a more detailed description of them seems to be required4 ,the banded character arises from marked distinctions in the texture of different layers of a lava-sheet4 ,in some cases 7a7 these distinctions arise from differences in the size of the crystals or in the disposition of the component minerals of the rock2 in others 7b7 from the varying number and size of the vesicles1 which may be large or abundantly crowded together in some layers1 and small or only sparsely developed in others4 ,the structure thus points to original conditions of the lava at the time of its emission and may be regarded as1 to some extent1 a kind of flow-structure on a large scale4 7a7 ,where the banding is due to differences of crystalline texture1 the constituent felspars1 augites1 and iron-ores may be seen even with the naked eye as well-defined minerals along the prominent surfaces of the harder ribs1 while the broader intervening flutings of finer material show the same minerals in minuter forms4 ,the alternating layers of coarser and finer crystallization lie1 on the whole1 parallel with the upper and under surfaces of the sheets in which they occur4 ,but they likewise undulate like the streaky lines in ordinary flow-structure4 ,banded structure of this type may be seen well developed in the lower parts of the basalt-plateaux throughout the ,inner ,hebrides and the ,faroe ,islands4 ,a specimen taken from the west end of the island of ,sanday1 near ,canna1 which showed the structure by a conspicuous parallel fluting on weathered surfaces1 was sliced for microscopical examination4 ,mr4 ,harker has been kind enough to supply me with the following observations regarding this slice3-- ,in the slice #fffjbbd the banding becomes less conspicuous under the microscope4 ,the rock is of basaltic composition1 and1 with reference to its micro-structure1 might be styled a fine-grained olivine-diabase or olivine-dolerite in some parts of the slice1 an olivine-basalt in others4 ,it consists of abundant grains of olivine1 imperfect octahedra and shapeless granules of magnetite1 little simple or twinned prisms of labradorite1 and a pale brown augite4 ,the last-named mineral is always the latest product of consolidation1 but it varies in habit1 being sometimes in ophitic patches moulded upon or enclosing the other minerals1 sometimes in small granules occupying the interstices between the felspars and other crystals4 ,the ophitic habit predominates in the slice1 while the granulitic comes in especially along certain bands4 ,if the former be taken as indicative of tranquil conditions1 the latter of a certain amount of movement in the rock during the latest stages of its consolidation1 the banding1 though not strictly a flow-structure1 may be ascribed in some degree to a flowing movement of the nearly solidified rock4 ,there is1 however1 more than this merely structural difference between the several bands4 ,they differ to some extent in the relative proportions of the minerals1 especially of olivine and augite2 which points to a considerable flowing movement at an early stage in a magma which was initially not homogeneous4 ,footnote #bbd3 ,the figures within square brackets throughout the following pages refer to the numbers of the microscopic slides in the ,geological ,survey collection1 where ,i have deposited all those prepared from my specimens4 7b7 ,where the banding arises from the distribution of the vesicles1 somewhat similar weathered surfaces are produced4 ,in some instances1 while the basalt is throughout finely cellular1 interposed bands of harder1 rather finer-grained and less thoroughly vesicular character serve to give the stratified appearance4 ,instances may be observed where the vesicles have been crowded together in certain bands1 which consequently weather out differently from the layers above and below them4 ,an excellent illustration of this arrangement occurs in the lowest lava but one of the largest of the three picturesque stacks known as ,macleod's ,maidens on the west coast of ,skye 7,figs4 #bfj1 #bhc1 #bhd and #bhg74 ,this lava is thoroughly amygdaloidal1 but the vesicles are specially crowded together in certain parallel bands from an inch to three or four inches thick4 ,some of these layers lie close to each other1 while elsewhere there may be a band of more close-grained1 less vesicular material between them4 ,but the most singular feature of the rock is to be seen in the shape and position of the vesicles that are crowded together in the cellular bands4 ,instead of being drawn out into flattened forms in the general direction of banding1 they are placed together at high angles4 ,each layer remains parallel to the general bedding1 but its vesicles are steeply inclined in one direction1 which was doubtless that of the flow of the still unconsolidated lava4#bbe ,weathering along these bands1 the lava might easily be mistaken at a little distance for a tuff or other stratified intercalation4 ,footnote #bbe3 ,this elongation of vesicles1 more or less perpendicular to the general bedding1 may be noticed sometimes even in sills1 as will be shown in a later ,chapter4 ,illustration3 ,fig4 #bfj4--;,banded amygdaloidal basalt showing layers of elongated and steeply inclined vesicles1 ,macleod's ,maidens1 ,skye4 ,banded lavas possessing the characters now described are of frequent occurrence among the ,inner ,hebrides4 ,many striking examples of them may be seen along the west coast of ,skye4 ,still more abundant in ,faroe1 they form one of the most conspicuous features in the geology of that group of islands4 ,along the whole of its western seaboard1 on island after island1 they are particularly prominent in the lower parts of the precipices1 while the upper parts consist largely of amorphous or prismatic sheets4 ,so much do they resemble stratified rocks that it was not until ,i had landed at various points that ,i could satisfy myself that they are really banded lavas4#bbf ,footnote #bbf3 ,for recent contributions to the ,geology of the ,faroe ,islands1 see ,prof4 ,james ,geikie1 ,trans4 ,roy4 ,soc4 ,edin4 vol4 xxx4 7#ahhj71 p4 #bag1 where the banding of the basalts is noticed2 ,prof4 ,a4 ,helland1 ,dansk4 ,geografisk4 ,tidskr4 7#ahha72 ,r4 ,bron1 ,notes pour servir l'tude de la ,gologie de l',islande et des ,isles ,faeroe 7#ahhd72 ,mr4 ,j4 ,lomas1 ,proc4 ,geol4 ,soc4 ,liverpool1 vol4 vii4 7#ahie71 p4 #bib4 ,various writers have treated of the petrography of ,faroe1 particularly ,a4 ,osann1 ,neues ,jahrb4 7#ahhd71 vol4 i4 p4 #de1 and ,m4 ,bron in the volume here cited4 #eth4 ,ordinary flow-structure1 save in these banded lavas1 is rather rare among the plateaux4 ,it may1 however1 be occasionally observed1 where there is no distinct banding4 ,on a weathered surface it appears in fine1 widely parallel streaks1 which are sometimes wavy1 puckered and broken up1 as in rhyolites and felsites1 while the porphyritic felspars are arranged with their long axes in the direction of flow4 ,a good example of these characters may be seen on the summit of the ,dn ,can--the remarkable truncated cone which forms the highest point on the ,island of ,raasay4 ,the rock is a black olivine-basalt1 partly amygdaloidal1 with zeolites filling up the cavities1 and its flow-lines are prominent on the weathered faces where they lie parallel to the general bedding of the lavas4 ,another illustration may be observed in the basalt already cited from ,loch-na-,mna1 in the island of ,eigg1 where the rock presents in places a remarkable streaky structure which1 though hardly visible on a fresh fracture1 reveals itself on a weathered face in thin nearly parallel ribs coincident in direction with the upper and under surfaces of the mass4 ,great variety is to be found in the thickness of different sheets of lava in the plateaux4 ,some of them are not more than #f or #h feet2 others reach to #hj or #ajj feet1 and sometimes1 though rarely1 to even greater dimensions4 ,in ,antrim1 the average thickness of the flows is probably from #ae to #bj feet4#bbg ,in the fine coast-sections at the ,giant's ,causeway1 however1 some bands may be seen far in excess of that measurement4 ,the bed that forms the ,causeway1 for instance1 is about #fj or #gj feet thick1 and seems to become even thicker further east4 ,along the great escarpment1 #gjj feet high1 which rises from the shores of ,gribon1 on the west coast of ,mull1 there are twenty separate beds1 which give an average of #ce feet for the thickness of each flow4 ,on the great range of sea-precipices along the west coast of ,skye1 which present the most stupendous section of the basalts anywhere to be seen within the limits of the ,british ,islands1 the average thickness of the beds can be conveniently measured4 ,at the ,talisker cliffs some of the flows are not more than #f or #h feet2 others are #cj or #dj feet4 ,the chief precipice1 #ieg feet high 7,fig4 #bhf71 contains at least #ah or #bj separate lava-sheets1 which thus average of from #dg to #ec feet in thickness4 ,in the cliffs that form the seaward margin of the tableland of ,macleod's ,tables 7,fig4 #bhc7 fourteen successive beds of basalt can be counted in a vertical section of #djj feet1 which is equal to an average thickness of about #bh feet4 ,but some of the basalts are only about #f feet thick1 while others are #ej or #fj4 ,the ,hoe of ,duirinish1 #gei feet high1 is composed of about sixteen distinct beds1 which thus have a mean thickness of #df feet4 ,the average thickness of the successive flows on ,dunvegan ,head1 which is #ajjj feet high and contains at least twenty-five separate sheets1 is about #dj feet4 ,still further north1 the cliffs1 #hjj feet high1 comprise sixteen successive flows1 which have thus an average of #ej feet each4 ,among the ,faroe ,islands the average thickness of the basalt-sheets seems to be nearly the same as in ,britain4 ,thus in the magnificent ranges of precipices of ,kals1 ,kun and ,bor1 forty or more sheets may be counted in the vast walls of rock some #bjjj feet high1 giving a mean of about #ej feet4 ,footnote #bbg3 ,see ,explanation of ,sheet #bj1 ,geol4 ,survey1 ,ireland1 p4 #aa4 ,each bed appears1 on a cursory inspection1 to retain its average thickness1 and to be continuous for a long distance4 ,but ,i believe that this persistence is in great measure deceptive4 ,we can seldom follow the same bed with absolutely unbroken continuity for more than a mile or two4 ,even in the most favourable conditions1 such as are afforded by a bare sea-cliff on which every sheet can be seen1 there occur small faults1 gullies where the rocks are for the time concealed1 slopes of debris1 and other failures of continuity2 while the rocks are generally so like each other1 that on the further side of any such interruption1 it is not always possible to make sure that we are still tracing the same bed of basalt which we may have been previously following4 ,on the other hand1 a careful examination of one of these great natural sections will usually supply us with proofs that1 while the bedded character may continue well marked1 the individual sheets die out1 and are replaced by others of similar character4 ,cases may not infrequently be observed where the basalt of one sheet abruptly wedges out1 and is replaced by that of another4 ,where both are of the same variety of rock1 it requires close inspection to make out the difference between them2 but where one is a green1 dull1 earthy1 amorphous amygdaloid1 and the other is a compact1 black1 prismatic basalt1 the contrast between the two beds can be recognized from a distance 7,fig4 #bfa74 ,in the basaltic cliffs of the west coast of ,skye1 the really lenticular character of the flows can be well seen4 ,i may especially cite the great headland south of ,talisker ,bay1 already referred to1 where1 in the pile of nearly horizontal sheets1 two beds may be seen to die out1 one towards the north1 the other towards the south4 ,further north1 in the cliff of the ,hoe of ,duirinish1 a similar structure presents itself4 ,along the coast-cliffs of ,mull1 ,morven and ,canna the same fact is clearly displayed4 ,thus on the west side of the ,sound of ,mull the slopes above ,fishnish ,bay show a group of basalts1 which die out southward1 and are overlapped by a younger group that has been poured over their ends4 ,such sections are best seen in the evening1 when the grass-covered lavas show their successive sheets by their respective shadows1 their individuality being lost in the full light of day4 ,a more striking example occurs beyond the west end of ,glen ,more in ,mull1 where one series of basalts has been tilted up1 probably during some volcanic episode1 and has had a younger series banked up against its edges4 ,illustration3 ,fig4 #bfa4--,termination of ,basalt-beds1 ,carsaig1 ,mull4 ,in ,antrim also1 remarkable evidence is presented of the rapid attenuation not of single beds only1 but of a whole series of basalts4 ,thus1 at ,ballycastle1 the group of lavas known as the ,lower ,basalts1 which underlie the well-known horizon of iron-ore1 are at least #cej feet thick4 ,but1 as we trace them westwards1 bed after bed thins out until1 a little to the west of ,ballintoy1 in a distance of only about #f miles1 the whole depth of the group has diminished to somewhere about #dj feet4 ,a decrease of more than #cjj feet in six miles or #ej feet per mile points to considerable inequalities in the accumulation of the lavas4 ,if the next series of flows came from another vent and accumulated against such a gentle slope1 it would be marked by a slight unconformability4 ,structures of this kind are much rarer than we should expect them to be1 considering the great extent to which the plateaux have been dissected and laid open in cliff-sections4 ,the basalt-plateau of the ,faroe ,islands exhibits with remarkable clearness the lenticular character of the basalt-sheets1 and a number of examples will be cited in the description of that region to be given in ,chapter xxxix4 ,in these northern climes vegetation spreads less widely over rock and slope than it does in the milder air of the ,inner ,hebrides4 ,hence the escarpments sweep in precipices of almost bare rock from the level of the sea up to the serrated crests of the islands1 some #bjjj feet in height4 ,each individual bed of basalt can thus be followed continuously along the fjords1 and its variation or disappearance can be readily observed4 ,coasting along these vast natural sections1 we readily perceive that1 as among the ,western ,isles1 the successive sheets of basalt have proceeded from no one common centre of eruption4 ,they die out now towards one quarter1 now towards another1 yet everywhere retain the universal regularity and gentle inclinations of the whole volcanic series4 ii4 ,,fragmental ,,rocks ,while the plateaux are built up mainly of successive flows of basaltic lavas1 they include various intercalations of fragmental materials1 which1 though of trifling thickness1 are of great interest and importance in regard to the light which they cast on the history of the different regions during the volcanic period4 ,i shall enumerate the chief varieties of these rocks here1 and afterwards give fuller details regarding their stratigraphical relations and mode of occurrence in connection with the succession of beds in each of the plateaux4 7a7 ,volcanic ,agglomerates4--,in the tumultuous unstratified masses of fragmentary materials which fill eruptive vents in and around the plateaux1 the stones1 which vary in size up to blocks several feet in diameter1 consist for the most part of basalts1 often highly slaggy and scoriaceous4 ,they include also fragments of different acid eruptive rocks 7generally felsitic or rhyolitic in texture71 with pieces of the non-volcanic rocks through which the volcanic pipes have been drilled4 ,the paste is granular1 dirty-green or brown in colour1 and seems generally to consist chiefly of comminuted basalt4 ,as in the ,carboniferous and ,permian necks1 the ,tertiary agglomerates contain abundant detritus of a basic minutely cellular pumice4 7b7 ,volcanic ,conglomerates and ,breccias in beds intercalated between the flows of ,basalt4--,these are of at least three kinds4 7a7 ,basalt-conglomerates1 composed mainly of rounded and subangular blocks of basalt 7or allied basic lava71 sometimes a yard or more in diameter1 not unfrequently in the form of pieces of rough slag or even of true bombs1 imbedded in a granular matrix of comminuted basalt-debris4 ,in some cases1 the stones form by far the most abundant constituents of the rock1 which then resembles some of the coarse agglomerates just described4 ,perhaps the most remarkable accumulations of this kind are those intercalated among the basalts in the islands of ,canna and ,sanday1 of which a detailed account will be given in ,chapter xxxviii4 ,these conglomerates1 besides their volcanic materials1 contain rounded blocks of ,torridon sandstone and other rocks1 which must have been carried from the east by some tolerably powerful river that flowed across the basalt-plains during the volcanic period4 ,again1 on the east side of ,mull1 the slaggy basalts of ,beinn ,chreagach ,mhor are occasionally separated by volcanic conglomerates4 ,as a rule1 however1 such intercalations are seldom more than a few feet or yards in thickness4 ,their coarseness and repetition on successive horizons indicate that they probably accumulated in the near neighbourhood of one or more small vents1 from which discharges of fragmentary materials took place at the beginning or at the close of an outflow of lava1 and that the stones were sometimes swept away from the cones and rolled about by streams before being buried under the succeeding lava-sheets4 ,more commonly the dirty-green or dark-brown granular matrix exceeds in bulk the stones embedded in it4 ,it has obviously been derived mainly from the trituration of already cooled basalt--masses1 and probably also from explosions of the still molten rock in the vents4 ,a striking illustration of this type of rock may be seen on the south side of ,portree ,harbour1 where a mass of dark-green basalt-conglomerate1 with a coaly layer above it1 lies near the base of the bedded basalts1 and attains at one part of its course a thickness of about #bjj feet4 ,this rock will be again referred to in connection with the vent from which its materials were probably derived4 ,as in the case of the agglomerates of the vents1 pieces of older acid lavas1 and still more of the non-volcanic rocks that underlie the plateaux1 are found in the bedded conglomerates and breccias4 ,in ,antrim and ,mull1 for instance1 fragments of flint and chalk are of common occurrence4 ,a characteristic example of this kind of rock forms the platform of the columnar bed out of which ,fingal's ,cave1 ,staffa1 has been excavated 7,fig4 #bff;a74 7,greek3 beta7 ,felsitic ,breccia4--,this variety1 though of rare occurrence1 is to be seen in a number of localities in the island of ,mull4 ,it is composed in great measure of angular fragments of close-grained flinty felsitic or rhyolitic rocks1 sometimes showing beautiful flow-structure1 together with pieces of quartzite and amygdaloidal basalt1 the dull dirty-green matrix appearing to be made up chiefly of basalt-dust4 7,greek3 gamma7 ,rhyolitic ,conglomerate4--,between the upper and lower group of basalts in the ,antrim plateau there occur bands of a pale fawn-coloured conglomerate largely made up of more or less rounded fragments of rhyolite1 like some of the varieties of the rock which occur in place on the plateau4 ,the rhyolitic debris is often mixed with pebbles of basalt4 ,sometimes it becomes so fine as to pass into pale clays4 7,greek3 delta7 ,breccias of non-volcanic materials4--,these1 the most exceptional of all the fragmentary intercalations in the plateaux1 consist almost wholly of angular blocks of rocks which are known to underlie the basalts1 but with a variable admixture of basalt fragments4 ,they are due to volcanic explosions which shattered the subjacent older crust of rocks1 and discharged fragments of these from the vents or allowed them to be borne upwards on an ascending column of lava4 ,pieces of the non-volcanic platform are of common occurrence among the fragmentary accumulations1 especially in the lower parts of the plateaux basalts4 ,but ,i have never seen so remarkable an example of a breccia of this kind as that which occurs near the summit of ,sgurr ,dearg1 in the south-east of ,mull4 ,the bedded basalt encloses a lenticular band of exceedingly coarse breccia1 consisting mainly of angular pieces of quartzite1 with fragments of amygdaloidal basalt4 ,in the midst of the breccia lies a huge mass or cake of erupted mica-schist1 at least #ajj yards long by #cj yards wide1 as measured across the strike up the slope of the hill4 ,to the west1 owing to the thinning out of the breccia1 this piece of schist comes to lie between two beds of basalt4 ,a little higher up1 other smaller but still large blocks of similar schist are involved in the basalt1 as shown in ,fig4 #bfb4 ,as the huge cake of mica-schist plunges into the hill1 its whole dimensions cannot be seen2 but there are visible1 at least1 #ae1jjj cubic yards1 which must weigh more than #cj1jjj tons4 ,blocks of quartzite of less dimensions occur in the basalts on ,loch ,spelve1 in the same district4 ,there can be no doubt1 ,i think1 that these enormous fragments were torn off from the underlying crystalline schists which form the framework of the ,western ,highlands1 and were floated upward in an ascending flow of molten basalt4 ,had the largest mass occurred at or near the base of the volcanic series1 its size and position would have been less remarkable4 ,but it lies more than #bjjj feet up in the basalts1 and hence must have been borne upward for more than that height4 ,a similar but less striking breccia occurs on the south coast of the same island1 near ,carsaig1 made up chiefly of pieces of quartzite and quartz4#bbh ,footnote #bbh3 ,this is noticed by ,mr4 ,starkie ,gardner1 ,quart4 ,journ4 ,geol4 ,soc4 xliii4 7#ahhg71 p4 #bhc1 note4 ,some remarkable agglomerates1 near ,forkhill1 ,armagh1 probably belonging to the ,tertiary volcanic series1 will be described in the account of the ,irish acid rocks 7,chapter xlvii474 ,they consist entirely of non-volcanic stones and dust and are traceable for some miles along the line of a fissure4 ,where they have been discharged through granite they consist entirely of the detritus of that rock1 but where they have been erupted in the ,silurian area they consist of fragments of grits and shales4 ,they seem to have been produced by riform discharges1 without the uprise of any volcanic magma1 though eventually andesite and rhyolite ascended the fissure and became full of granitic and ,silurian fragments4 ,some remarkable necks filled almost entirely with fragments of ,torridon ,sandstone have been observed in the west of ,applecross1 ,ross-shire1 and some curious plug-like masses of breccia1 also made up of fragments of ,torridonian strata1 occur in the island of ,raasay4 ,these examples will be more particularly described on later pages 7pp4 #bib1 #bic74 7c7 ,tuffs4--,the tuffs intercalated in the basalt-plateaux generally consist essentially of basic materials1 derived from the destruction of different varieties of basalts1 though also containing occasional fragments of older felsitic rocks1 as well as pieces of chalk1 flint1 quartz1 and other non-volcanic materials4 ,they are generally dull1 dirty-green in colour1 but become red1 lilac1 brown1 and yellow1 according to the amount and state of combination and oxidation of their ferruginous constituents4 ,they usually contain abundant fragments of amygdaloidal and other basalts4 ,as a rule1 they are distinctly stratified1 and occur in bands from a few inches to #ej feet or more in thickness4 ,the matrix being soft and much decomposed1 these bands crumble away under the action of the weather1 and contribute to the abruptness of the basalt-escarpments that overlie them4 ,illustration3 ,fig4 #bfb4--;,breccia and ,blocks of mica-schist1 quartzite1 etc41 lying between bedded ,basalts1 ,isle of ,mull4 a a1 ,bedded basalts2 b1 ,breccia2 d1 ,basic dyke4 ,in the group of strata between the two series of basalts in ,antrim1 some of the tuffs consist chiefly of rhyolitic detritus1 both glassy and lithoid4 ,where the tuffs become fine-grained and free from imbedded stones1 they pass into variously-coloured clays4 ,among these are the bauxite and lithomarge of ,antrim1 probably derived from pale rhyolitic tuffs and conglomerates 7p4 #bjd74 ,associated with these deposits in the same district1 is a pisolitic hmatite1 which has been proved to occur over a considerable area on the same horizon4 ,many of the clays are highly ferruginous4 ,the red streaks that intervene between successive sheets of basalt are of this nature 7bole1 plinthite1 etc474 ,the source of the iron-oxide is doubtless to be traced to the decomposition of the basic lavas during the volcanic period4 7d7 ,there occur also grey and black clays and shales1 of ordinary sedimentary materials1 containing leaves of terrestrial plants 7leaf-beds71 with occasional wing-cases of beetles1 sometimes associated with impure limestones1 but more frequently with sandstones and indurated gravels or conglomerates containing pieces of fossil wood4 ,these intercalated bands undoubtedly indicate the action of running water1 sometimes even of river-floods1 and the accumulation of sediment in hollows of the exposed flows of basalt at intervals during the piling up of the successive lava-sheets that form the plateaux4 ,the alternation of fluviatile gravels with volcanic tuffs1 fluviatile conglomerates1 and lava-streams1 is admirably displayed in the island of ,canna1 as will be narrated in detail in ,chapter xxxviii4 ,the vegetable matter has in some places gathered into lenticular seams of lignite1 and even occasionally of black glossy coal4 ,amber also has been found in the lignite4 ,where the vegetation has been exposed to the action of intrusive dykes or sheets1 it has sometimes passed into the state of graphite4 ,the remarkable terrestrial flora found in the leaf-beds1 and in association with the lignites1 was first made known by the descriptions of ,edward ,forbes already referred to1 and has subsequently been studied and described by ,heer1 ,w4 ,h4 ,baily1 and ,mr4 ,starkie ,gardner4#bbi ,it was regarded by ,forbes as of ,miocene age1 and this view has generally been adopted by geologists4 ,mr4 ,starkie ,gardner1 however1 contends that it indicates a much wider range of geological time4 ,he believes that a succession of floras may be recognised1 the oldest belonging to an early part of the ,eocene period4 ,terrestrial plants1 it must be admitted1 are not always a reliable test of geological age1 and ,i am not yet satisfied that in this instance they afford evidence of such a chronological sequence as ,mr4 ,gardner claims1 though ,i am convinced that the ,tertiary volcanic period was long enough to have allowed of the development of considerable changes in the character of the vegetation4 ,footnote #bbi3 ,on this subject consult ,duke of ,argyll1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 vii4 7#ahea71 p4 #hi2 ,e4 ,forbes1 ,ibid4 p4 #ajc2 ,w4 ,h4 ,baily1 op4 cit4 xxv4 7#ahfi71 pp4 #afb1 #ceg2 ,brit4 ,assoc4 ,rep4 7#ahgi7 p4 #afb2 7#ahhj7 p4 #ajg2 7#ahha7 p4 #aea2 7#ahhd7 p4 #bji2 ,mr4 ,j4 ,starkie ,gardner1 ,palontographical ,society1 vols4 xxxviii4 xxxix4 ,in the last of ,mr4 ,baily's papers he notices that the ,rev4 ,dr4 ,grainger found a portion of a fish 7,percid1 possibly ,lates74 ,the discovery of the remains of a fresh-water fish is an important additional testimony to the terrestrial conditions under which the lavas were erupted4 ,the genus ,lates now inhabits the ,nile and the ,ganges4 ,for the purpose of the present volume1 however1 the precise stage in the geological record1 which this flora indicates1 is of less consequence than the broad fact that the plants prove beyond all question that the basalts among which they lie were erupted on land during the older part of the long succession of ,tertiary periods4 ,their value in this respect cannot be overestimated4 ,stratigraphical evidence shows that the eruptions must be later than the ,upper ,chalk2 but the imbedded plants definitely limit them to the earlier half of ,tertiary time4 ,,chapter ,,xxxvii ,,the ,,several ,,basalt-,,plateaux ,,and ,,their ,,geological ,,history1 ,,antrim1 ,,mull1 ,,morven ,,and ,,ardnamurchan ,there are five districts in ,north-western ,europe where the original widespread ,tertiary lava-fields have been less extensively eroded than elsewhere1 or at least where they have survived in larger and thicker masses4 ,whether or not each of them was an isolated area of volcanic activity cannot now be determined4 ,their several outflows of lava within the area of the ,british ,isles may have united into one continuous volcanic tract1 and their present isolation there may be due entirely to subterranean movements and denudation4 ,there is a certain convenience1 however1 in treating the districts separately4 ,they are--#a4 ,antrim2 #b4 ,mull1 ,morven and ,ardnamurchan2 #c4 ,small ,isles2 #d4 ,skye2 #e4 ,the ,faroe ,islands4 i4 ,,antrim#bcj ,footnote #bcj3 ,the basalts of ,antrim are the subject of an abundant literature4 ,i may refer particularly to the papers of ,berger and ,conybeare 7,trans4 ,geol4 ,soc4 iii471 the ,geological ,report of ,portlock1 and the ,explanations of the ,sheets of the ,geological ,survey of ,ireland4 ,other papers will be afterwards cited4 ,the general features of the ,antrim plateau are shown on ,map ,,vii4 ,the largest of the basalt-plateaux of ,britain is that which forms so prominent a feature in the scenery and geology of the ,north of ,ireland1 stretching from ,lough ,foyle to ,belfast ,lough1 and from ,rathlin ,island to beyond the southern margin of ,lough ,neagh4 ,its area may be roughly computed at about #bjjj square miles4 ,but1 as its truncated strata rise high along its borders1 and look far over the surrounding low grounds1 it must be regarded as a mere fragment of the original volcanic plain4 ,it may be described as an undulating tableland1 which almost everywhere terminates in a range of bold cliffs1 but which1 towards the centre and south1 sinks gently into the basin of ,lough ,neagh4 ,the marginal line of escarpment1 however1 presents considerable irregularity both in height and form1 besides being liable to frequent local interruptions4 ,it is highest on the west side1 one of its crests reaching at ,mullaghmore1 in ,county ,londonderry1 a height of #ahbe feet4 ,it sinks down into the valley of the ,bann1 east of which it gradually ascends1 forming the well-known range of cliffs from the ,giant's ,causeway and ,bengore ,head to ,ballycastle4 ,it then strikes inland1 and making a wide curve in which it reaches a height of more than #acjj feet1 comes to the sea again at ,garron ,point4 ,from that headland the cliffs of basalt form a belt of picturesque ground southwards beyond ,belfast1 interrupted only by valleys that convey the drainage of the interior of the plateau to the ,north ,channel4 ,above the valley of the ,lagan the crest of the plateau rises to a height of more than #aejj feet4 ,throughout most of its extent the basalt-escarpment rests on the white limestone or ,chalk of ,antrim1 beneath which lie soft ,lias shales and ,triassic marls4 ,here and there1 where the substratum of ,chalk is thin1 the action of underground water on the crumbling shales and marls below it has given rise to landslips4 ,the slopes beneath the base of the basalt are strewn with slipped masses of that rock1 almost all the way from ,cushendall to ,larne1 some of the detached portions being so large as to be readily taken for parts of the unmoved rock4 ,on the west side also1 a group of huge landslips cumbers the declivities beneath the mural front of ,benevenagh4 ,i have found some difficulty in the attempt to ascertain what was the probable form of surface over which the volcanic rocks of this plateau began to be poured out4 ,the ,chalk sinks below the sea-level on the north coast1 but1 in the outlier of ,slieve ,gallion1 three miles beyond the western base of the escarpment1 it rises to a height of #aejj feet above the sea4 ,on the east side also1 it shows remarkable differences of level4 ,thus1 below the ,white ,head at the mouth of ,belfast ,lough1 it passes under the sea-level1 but only #af miles to the south1 where it crops out from under the basalt1 its surface is about #ajjj feet above that level4 ,if these variations in height existed at the time of the outpouring of the basalt1 the surface of the ground over which the eruptions took place was so irregular that some hundreds of feet of lava must have accumulated before the higher chalk hills were buried under the volcanic discharges4 ,but it seems to me that much of this inequality in the height of the upper surface of the ,chalk is to be attributed to unequal movements since the volcanic period1 which involved the basalt in their effects1 as well as the platform of ,chalk below it4 ,had the present undulations of that platform been older than the volcanic discharges1 it is obvious that upper portions of the basalt-series would have overlapped lower1 and would have come to rest directly on the ,chalk4 ,but this arrangement1 so far as ,i am aware1 never occurs1 except on a trifling scale4 ,wherever the ,chalk appears1 it is covered by sheets of the lower and not of the upper of the two groups into which the ,antrim basalts are divisible4 ,we have actual proof of considerable terrestrial disturbance1 subsequent to the date of the formation of the volcanic plateau4 ,thus1 near ,ballycastle1 a fault lets down the basalt and its ,chalk platform against the crystalline schists of that district4 ,on the east side of the fault1 the ,chalk is found far up the slope1 circling round the base of the beautiful cone of ,knocklayd--an outlier of the basalt which reaches a height of #afie feet 7,fig4 #bfc74 ,the amount of vertical displacement of the volcanic sheets is here #gjj feet4#bca ,many other displacements1 as shown by the mapping of my colleagues in the ,geological ,survey1 have shifted the base of the escarpment from a few inches up to several hundred feet4 ,besides actual dislocations1 the ,antrim plateau has undergone some marked subsidences of which the most notable is that of ,lough ,neagh4#bcb ,footnote #bca3 ,explanatory ,memoir of ,sheets #g and #h1 ,geological ,survey1 ,ireland1 by ,messrs4 ,symes1 ,egan1 and ,m',henry 7#ahhh71 p4 #cg4 ,footnote #bcb3 ,these inequalities in the level of the base of the ,antrim plateau will be more particularly discussed in ,chapter xlix41 in connection with the subsidences and dislocations which have affected the region since the close of the volcanic period4 ,it is evident1 therefore1 that the present position of the ,chalk platform is far from agreeing with that which it presented to the outflow of the sheets of basalt4 ,but1 on the other hand1 there can be no doubt that its surface at the beginning of the volcanic outbursts was not a level plain4 ,it was probably a rolling country of low bare chalk-downs1 like parts of the ,south-east of ,england4 ,the ,irish ,chalk attains its maximum thickness of perhaps #bej feet at ,ballintoy4 ,but it is liable to rapid diminution4 ,on the shore at ,ballycastle about #aej feet of it can be seen1 its base being concealed2 but only two and a half miles to the south1 on the outlier of ,knocklayd1 the thickness is not quite half so much4 ,on the west side of the plateau also1 there are rapid changes in the thickness of the ,chalk4 ,such variations appear to be mainly attributable to unequal erosion before the overflow of the basalts4 ,so great indeed had been the denudation of the ,cretaceous and underlying ,secondary formations previous to the beginning of the volcanic eruptions1 that in some places the whole of these strata had been stripped off the country1 so that the older platform of ,palozoic or still more ancient masses was laid bare4 ,thus1 on the west side of the escarpment1 the basalt steals across the ,chalk and comes to rest directly upon ,lower ,carboniferous rocks4 ,the authors who have described the junction of the ,chalk and basalts in ,antrim have generally referred to the uneven surface of the former rock as exposed in any given section4 ,the floor on which the basalt lies is remarkably irregular1 rising into ridges and sinking into hollows or trenches1 but almost everywhere presenting a layer of earthy rubbish made of brown ferruginous clays1 mixed with pieces of flint1 chalk1 and even basalt4#bcc ,the flints are generally reddened and shattery4 ,the chalk itself has been described as indurated1 and its flints as partially burned by the influence of the overlying basalt4 ,but ,i have not noticed1 at any locality1 evidence of alteration of the solid chalk1 except where dykes or intrusive sheets have penetrated it4#bcd ,there can be no doubt that the hardness of the rock is an original peculiarity1 due to the circumstances of its formation4 ,the irregular earthy rubble1 that almost always intervenes between the chalk and the base of the basalt1 like the clay with flints so general over the ,chalk of ,southern ,england1 no doubt represents long-continued subrial weathering previous to the outflow of the basalt4 ,even1 therefore1 if there were no other evidence1 we might infer with some confidence from this layer of rubble1 that the surface over which the lavas were poured was a terrestrial one4 ,here and there1 too1 we may detect traces of the subsidence of the basalt into swallow-holes dissolved in the chalk subsequent to the outflow of the basalt-sheets4 ,footnote #bcc3 ,portlock1 ,report on ,geology of ,londonderry1 etc4 7,geological ,survey71 p4 #aag4 ,footnote #bcd3 ,see ,portlock1 op4 cit4 p4 #aaf4 ,the ,antrim plateau is not only the largest in the ,british ,islands1 it is also the most continuous and regular4 ,it may be regarded1 indeed1 as one unbroken sheet of volcanic material1 not disrupted by any such mountainous masses of intrusive rock as in the other plateaux interrupt the continuity of the horizontal or gently inclined sheets of basalt4 ,around its margin1 indeed1 a few outliers tower above the plains1 and serve as impressive memorials of its losses by denudation4 ,of these1 by much the most picturesque and imposing1 though not the loftiest1 is ,knocklayd already referred to1 which forms so striking a feature in the north-east of ,antrim 7,fig4 #bfc74 ,illustration3 ,fig4 #bfc4--,section of ,knocklayd1 an outlier of the ,antrim basalt-plateau lying on ,chalk4 #a4 ,crystalline schists2 #b4 ,cretaceous strata2 #c4 ,lower basalts2 #d4 ,group of tuffs1 clays and iron-ore2 #e4 ,upper basalts2 f4 ,fault4 ,the total thickness of volcanic rocks in the ,antrim plateau exceeds #ajjj feet2 but1 as the upper part of the series has been removed by denudation1 the whole depth of lava originally poured out cannot now be told4 ,a well-marked group of tuffs and clays1 traceable throughout a large part of ,antrim1 forms a good horizon in the midst of the basalts1 which are thus divisible into a lower and upper group 7,fig #bfd74 ,the ,lower ,basalts have a thickness of from #djj to #ejj feet4 ,but1 as already mentioned 7p4 #aid71 they die out in about six miles to no more than #dj feet at ,ballintoy4 ,they are distinguished by their generally cellular and amygdaloidal character1 and less frequently columnar structure4 ,the successive flows1 each averaging perhaps above #ae feet in thickness1 are often separated by thin red ferruginous clayey partings1 sometimes by bands of green or brown fine gravelly tuff4 ,the most extensive of these tuff-bands occurs in the lower part of the group at ,ballintoy1 and can be traced along the coast for about five miles4 ,in the middle of its course1 near the picturesque ,carrick-a-raide1 it reaches a maximum thickness of about #ajj feet and gradually dies out to east and west4 ,the neck of coarse agglomerate at ,carrick-a-raide1 is doubtless the vent from which this mass of tuff was discharged 7see ,fig4 #cja74 ,owing to the thinning out of the sheets of basalts1 as they approach the vent1 the tuff comes to rest directly on the ,chalk1 and for some distance westwards forms the actual base of the volcanic series4#bce ,occasional seams of carbonaceous clays1 or of lignite1 appear in different horizons among the basalts4 ,beneath the whole mass of basalt1 indeed1 remains of terrestrial vegetation here and there occur4 ,thus1 near ,banbridge1 ,county ,down1 a patch of lignite1 four feet ten inches thick1 underlies the basalt1 and rests directly on ,silurian rocks4 ,such fragmentary records are an interesting memorial of the wooded land-surface over which the earliest outflows of basalt spread4 ,footnote #bce3 ,see ,explanation of ,sheets #g and #h of the ,geological ,survey of ,ireland 7#ahhh71 p4 #bc4 ,illustration3 ,fig4 #bfd4--;,diagram-,section of the ,antrim ,plateau4 #a4 ,triassic series2 #b1 #c4 ,rhaetic strata and ,lias2 #d4 ,greensand2 #e4 ,chalk2 #f4 ,gravel and soil2 #g4 ,lower group of basalts2 #h4 ,group of tuffs1 clays and iron-ore2 #i4 ,upper group of basalts4 ,in looking at the great basalt-escarpments of ,antrim1 the ,inner ,hebrides or the ,faroe ,islands1 and in following with the eye the successive sheets of lava in orderly sequence of level bands from the breaking waves at the base to the beetling crest above1 we are apt to take note only of the proofs of regularity and repetition in the outflows of molten rock and to miss the evidence that these outflows did not always rapidly follow each other1 but were separated by intervals of varying1 sometimes even of long duration4 ,one of the most frequent and conspicuous proofs of such intervals is to be found in the red layers or partings above referred to which1 throughout all the basalt-plateaux1 so commonly intervene between successive sheets of basalt4 ,these red streaks cannot fail to arrest the eye on the coast-precipices where by their brilliant contrast of colour1 they help to emphasize the bedded character of the whole volcanic series4 ,examined more closely1 they are found to consist of clay or bole which shades into the decomposed top of the bed whereon it lies1 and is usually somewhat sharply marked off from that which covers it4 ,this layer has long1 and ,i think correctly1 been regarded as due to the atmospheric disintegration of the surface of the basalt on which it rests1 before the eruption of the overlying flow4 ,it varies in thickness from a mere line up to a foot or more1 and it passes into the tuffs and clays which are sometimes interposed between the sheets of basalts4 ,it may be looked upon as probably furnishing evidence of the lapse of an interval sufficiently extended to permit a considerable subserial decay of the surface of a lava-sheet before the outflow of the next lava4 ,but an attentive study of the plateaux discloses other and even more remarkable indications that the pauses between the consecutive basalt-beds were frequently so prolonged as to allow extensive topographical changes to be made in a district4 ,nowhere is the long duration of some of these intervals more impressively taught than in the central zone of sedimentary strata in ,antrim4 ,this persistent group of tuffs1 clays1 and iron-ore is generally from #cj to #dj and sometimes as much as #gj feet thick4 ,from the occurrence of the ore in it1 it has been explored more diligently in recent years than any other group of rocks in the district1 and its outcrop is now known over most of the plateau4 ,the iron-ore bed varies from less than an inch up to #ah inches in thickness1 and consists of pisolitic concretions of hmatite1 from the size of a pea to that of a hazel nut1 wrapped up in a soft ochreous clayey matrix4#bcf ,where it is absent1 its place is sometimes taken by an aluminous clay1 worked as bauxite1 which has yielded stumps of trees and numerous leaves and cones4 ,beneath the iron-ore or its representative1 lies what is called the pavement1--a ferruginous tuff1 #h to #aj feet thick1 resting on lithomarge1--a lilac or violet mottled aluminous earth sometimes full of rounded blocks or bombs of basalt4 ,the well-known horizon for fossil plants at ,ballypallidy is a red tuff in this zone4 ,the section of strata between the two basalt-groups at this locality may serve as an illustration of the nature and arrangement of the deposits4#bcg ,footnote #bcf3 ,consult a good essay on the ,iron-ore and ,basalts of ,north-east ,ireland by ,messrs4 ,tate and ,holden1 ,quart4 ,journ4 ,geol4 ,soc4 xxvi4 7#ahgj71 p4 #aea4 ,in this paper the nature1 composition and modes of origin of the iron-ore and its associated strata are fully discussed4 ,footnote #bcg3 ,a4 ,m',henry1 ,geol4 ,mag4 7#ahie71 p4 #bfc4 ,upper ,basalt1 compact and often columnar sheets4 ,brown laminated tuff and volcanic clays4 ,laminated brown impure earthy lignite1 #b feet #c inches4 ,brown and red variegated clays1 tuffs and sandy layers1 with irregular seams of coarse conglomerate composed of rounded and subangular fragments of rhyolite and basalt1 #c feet #d inches4 ,brown1 red and yellowish laminated tuffs1 mudstones1 and bole1 with occasional layers of fine conglomerate 7rhyolitic and basaltic71 pisolitic iron-ore band and plant-beds1 #h feet #aj inches4 ,lower basalt1 amygdaloidal4 ,in some of the ,ballypallidy tuffs the most frequent lapilli are pieces of green and brown glass1 which ,mr4 ,watts compares with the pitchstone of ,sandy ,braes1 though rarely containing phenocrysts as that rock does4 ,he has found also in these strata a smaller proportion of lithoidal rhyolites and occasionally fragments of basic rock4 ,the pale and coloured clays that occur in this marked sedimentary intercalation have doubtless been produced by the decomposition of the volcanic rocks and the washing of their fine detritus by water4 ,possibly this decay may have been in part the result of solfataric action4 ,from true bauxite or aluminium-hydrate1 the sediments vary in composition and specific gravity and pass into aluminous silicates and iron-ores4 ,they seem to indicate a prolonged interval of volcanic quiescence when the lavas and tuffs already erupted were denuded and decomposed4#bch ,footnote #bch3 ,see a note on ,bauxite by ,professor ,g4 ,a4 ,cole1 ,scientif4 ,trans4 ,royal ,dublin ,soc4 vol4 vi4 series ii4 7#ahif71 p4 #aje4 ,the area over which this interesting series of stratified deposits now extends is obviously much less than it was originally4 ,it has indeed been so reduced by denudation into mere scattered patches that it probably does not exceed #agj square miles4 ,but the group can be traced from ,divis ,hill1 near ,belfast1 to ,rathlin ,island1 a distance of #ej miles1 and from the valley of the ,bann to the coast above ,glenarm1 more than #bj miles4 ,there can be little doubt that it was once continuous over all that area1 and that it probably extended some way further on each side4 ,if the so-called ,pliocene clays of ,lough ,neagh be regarded as parts of this group of strata1 its extent will be still further increased4 ,hence the original area over which the iron-ore and its accompanying tuffs and clays were laid down can hardly have been less than #ajjj square miles4 ,this extensive tract was evidently the site of a lake during the volcanic period1 formed by a subsidence of the floor of the lower basalts4 ,the salts of iron contained in solution in the water1 whether derived from the decay of the surrounding lavas or from the discharges of chalybeate springs1 were precipitated as peroxide in pisolitic form1 as similar ores are now being formed on lake-bottoms in ,sweden4 ,for a long interval1 quiet sedimentation went on in this lake1 the only sign of volcanic energy during that time being the dust and stones that were thrown out and fell over the water-basin1 or were washed into it by rains from the cones of the lava-slopes around4 ,it may here be remarked that the tendency to subsidence in the ,antrim plateau seems to have characterized this region since an early part of the volcanic period4 ,the lake in which the deposits now described accumulated was entirely effaced and overspread by the thick group of upper basalts4 ,but long after the eruptions had ceased1 a renewed sinking of the ground gave rise to the sheet of water which now forms ,lough ,neagh4#bci ,footnote #bci3 ,this subject will be discussed in ,chapter xlix4 ,nowhere else among the ,tertiary basalt-plateaux of ,britain has any trace been found of so marked and prolonged a pause in the volcanic activity as is indicated by the ,antrim zone of tuffs and clays4 ,throughout the ,inner ,hebrides1 indeed1 numerous intercalations of sedimentary material occur among the basalts1 but these consist mainly of tuffs and volcanic conglomerates with less frequent shales and coal-seams1 and they never suggest so distinct and lengthened an interval as is indicated by the ,antrim deposit4 ,it is not improbable that this interval was marked by the outbreak of rhyolitic eruptions somewhere in the region4 ,the abundance of rhyolite fragments in some of the tuffs is striking evidence that acid rocks were in one way or other brought to the surface at this time4 ,at ,glenarm one of the members of the stratified series is a marked rhyolitic conglomerate1 composed of rounded pebbles of a rock not unlike the well-known rhyolite of ,tardree and ,carnearny4 ,these fragments1 obviously of local origin1 must either have been derived from a surface of acid rock laid bare by denudation1 or from rhyolite ejected in lapilli or poured out in streams4 ,i formerly believed that all the ,antrim rhyolites had been injected into the basalts after the close of the plateau-period4 ,but the proved abundance and wide extent of the rhyolitic detritus among the sediments associated with the iron-ore point to a possible outflow of acid lavas with accompanying tuffs during the sedimentary interval between the two groups of basalt4 ,the characters of the ,antrim rhyolites1 however1 will be more particularly discussed in ,chapter xlvii41 in connection with the acid rocks of the ,tertiary volcanic series4 ,immediately above the iron-ore of ,antrim1 or separated from it in places by only a few inches of tuff1 comes the group of ,upper ,basalts1 which varies up to #fjj feet in thickness1 though as the upper portion has been everywhere removed by denudation1 no measure remains of what may have been the original depth of the group4 ,the general character of these basalts is more frequently columnar1 black and compact1 and with fewer examples of a strongly amygdaloidal structure than in the lower group4 ,but this distinction is less marked in the south than in the north of ,antrim1 so that where the intervening zone of tuffs and iron-ore disappears1 no satisfactory line of division can be traced between the two groups of basalt4 ,the occurrence of that zone1 however1 by giving rise to a hollow or slope1 from which the upper basalts rise as a steep bank or cliff1 furnishes a convenient topographical feature for mapping the boundary of these rocks4 ,among the upper basalts1 also1 there is perhaps a less frequent occurrence of those thin red partings of bole between successive flows1 so conspicuous in the lower group4 ,but the flows are not less distinctly marked off from each other4 ,nowhere can their characteristic features be better seen than along the magnificent range of cliffs from the ,giant's ,causeway eastwards4 ,the columnar bed that forms the ,causeway is the lowest sheet of the upper group1 and may be seen resting directly on the zone of grey and red tuffs4 ,it is about #fj or #gj feet thick2 and1 while perfectly regular in its columnar structure at the ,causeway and the ,organ1 assumes further eastward the confusedly starch-like arrangement of prisms already referred to4 ,but in the great cliff section of the ,amphitheatre1 the more regular structure is resumed1 the bed swells out to about #hj feet in thickness1 and columns of that length run up the face of the precipice1 weathering out at the top into separate pillars1 which1 perched on the crest of an outstanding ridge1 are known as the ,chimneys4 ,the basalt-beds that succeed the lowest one are each only about #aj to #ae feet thick 7,fig4 #bfe74 ,illustration3 ,fig4 #bfe4--,view of ,basalt escarpment1 ,giant's ,causeway1 with the ,amphitheatre and ,chimneys4 7,from a photograph by ,mr4 ,r4 ,welch47 ,between the successive sheets of the ,upper ,basalts thin seams of red ferruginous clay though1 as ,i have said1 less frequent perhaps than in the lower group1 continue to show that the intervals between successive eruptions were of sufficient duration to admit of some subrial decay of the surface of a lava before the outflow of the next bed4 ,occasional thin layers of tuff also1 and even of pisolitic iron-ore1 have been observed among these higher basalts4 ,but the most interesting and important intercalations are inconstant seams of lignite4 ,one of the most conspicuous of these lies immediately above the basalt of the ,causeway1 where it was long worked for fuel1 and was found to be more than six feet thick4 ,but it is quite local1 as may be seen at the ,organ over which it lies1 having a thickness of only #ab inches and rapidly dying out so as to allow the basalts above and below it to come together4 ,the removal of the upper portion of the basalts by denudation has destroyed the records of the latest part of the volcanic history of the ,irish plateaux4 ,it is obvious that nowhere in ,antrim does any trace exist of a central vent or cone from which the volcanic materials were discharged4 ,there is no perceptible thickening of the individual basalt-sheets1 nor of the whole series in one general direction1 in such a manner as to point to the site of some chief focus of eruption4 ,nor can we place reliance on the inclination of the several parts of the plateau4 ,i have pointed out that the varying dip of the beds must be attributed mainly to post-volcanic movements1 or at least to movements which1 if not later than all the phases of volcanic action1 must have succeeded the outpouring of the plateau-basalts4 ,there has been a general subsidence towards the central and southern tracts now occupied by the valley of the ,bann and ,lough ,neagh4 ,but nowhere in the depression is there any trace of the ruins of a central cone or focus of discharge4 ,the ,antrim plateau1 in these respects1 resembles the others4 ,but as has already been remarked1 it differs from them in one important particular4 ,it has nowhere been disrupted by huge bosses of younger rocks1 such as have broken up the continuity of the old lava-fields further north4 ,yet it also is not without its memorials of younger protrusions4 ,it contains not a few excellent examples of true volcanic vents1 and1 as above stated1 it includes some small acid bosses that may represent the great protrusions of the ,inner ,hebrides1 and may have been connected with superficial outflows of rhyolitic lava and showers of rhyolitic tuff4 ii4 ,,mull1 ,,morven ,,and ,,ardnamurchan ,this plateau covers nearly the whole of the island of ,mull1 embraces a portion of ,morven on the ,argyleshire mainland1 and1 stretching across ,loch ,sunart1 includes the western part of the peninsula of ,ardnamurchan 7,map ,,vi474 ,that these now disconnected areas were once united into a continuous lava-field which extended far beyond its present limits is impressively indicated by their margin of cliffs and fringe of scattered islands and outliers4 ,the plateau went west1 at least1 as far as the ,treshnish ,isles1 which are composed of basalt4 ,on its eastern border1 a capping of basalt on the top of ,beinn ,iadain 7#ahgc feet7 in ,morven1 and others further north1 prove that its volcanic sheets once spread into the interior of ,argyleshire 7,fig4 #bff74 ,on the south1 its fine range of lofty cliffs1 with their horizontal bars of basalt1 bear witness to the diminution which it has undergone on that side2 while1 on the north1 similar sea-walls tell the same tale4 ,not only has it suffered by waste along its margin1 it has also been deeply trenched by the excavation of glens and arms of the sea4 ,the ,sound of ,mull cuts it in two1 and the mainland portion is further bisected by ,loch ,sunart1 and again by ,loch ,aline4 ,the island of ,mull is so penetrated by sea-lochs and divided by deep valleys that a comparatively slight depression would turn it into a group of islands4 ,but1 besides its enormous denudation1 this plateau has been subjected to disruption1 and perhaps also to subsidence1 from subterranean movements4 ,in the southern portion of the island of ,mull it has been broken up by the intrusion of large bosses and sheets of gabbro1 and by masses as well as innumerable veins of various granitoid and felsitic rocks4 ,in ,ardnamurchan1 it has suffered so much disturbance from the same cause that its original structure has been almost obliterated over a considerable area4 ,moreover1 it has been dislocated by many faults1 by which different portions have been greatly shifted in level4 ,the most important of these breaks is one noticed by ,professor ,judd1 and visible to every tourist who sails up the ,sound of ,mull4 ,it traverses the cliffs on the ,morven side1 opposite ,craignure1 bringing the basalts against the crystalline schists1 and strikes thence inland1 wheeling round into the long valley in which ,lochs ,arienas and ,teacus lie4 ,on its western side1 the base of the basalt-series is almost at the sea-level2 on its eastern side1 that platform rises high into the outliers of ,beinn na h-,uamha 7#aeba feet7 and ,beinn ,iadain4 ,the amount of displacement is probably more than #ajjj feet4 ,many other minor faults in the same district show how much the crust of the earth has been fractured here since older ,tertiary time4 ,illustration3 ,fig4 #bff4--;,basalt-capping on top of ,beinn ,iadain1 ,morven4 ,the hummocky ground to the right consists of the ,highland schists against which the basalts are brought by lines of dislocation4#bdj ,footnote #bdj3 ,there are no fewer than three faults in the basalt-capping on the summit of ,beinn ,iadain4 ,by bringing the basalts and schists into juxtaposition1 they have given rise to topographical features that can be seen even from a distance4 ,a little to the west of ,mull1 and belonging originally to the same plateau1 lies the isle of ,staffa1 the famous columnar basalts of which first attracted the attention of travellers1 and gave to the ,tertiary volcanic rocks of ,scotland their celebrity 7,fig4 #bff;a74 ,illustration3 ,fig4 #bff;a4--,view of the south side of ,staffa1 showing the bedded and columnar structure of the basalt4 ,the rock in which the cave to the left hand has been eroded is a conglomeratic tuff underlying the basalt2 to the right is ,fingal's ,cave4 ,these caverns bear witness to the enormous erosive power of the ,atlantic breakers4 ,in spite of the extent to which it has suffered from denudation and subterranean disturbance1 and indeed in consequence thereof1 the ,mull plateau presents clear sections of many features in the history of the basalt-outflows and of the subsequent phases of ,tertiary volcanic action which cannot be seen in the more regular and continuous tableland of ,antrim4 ,moreover1 it still possesses in its highest mountain1 ,ben ,more 7#cafi feet71 a greater thickness1 and probably a higher series1 of lavas than can now be seen in any other of the plateaux4 ,the difficulties1 already referred to in regard to ,antrim1 of tracing the probable form of ground on which the volcanic eruptions began1 are even greater in the case of the ,mull plateau4 ,we can dimly perceive that the depression in the crystalline rocks of the ,highlands which had1 from at least the older part of the ,jurassic period1 stretched in a ,n4,n4,w4 direction along what is now the western margin of ,argyleshire1 lay beneath the sea in ,jurassic time1 and was then more or less filled up with sedimentary deposits4 ,the hollow appears thereafter to have become a land-valley1 whence the ,jurassic strata were to a large extent cleared out by denudation before its subsequent submergence under the sea in which the upper ,cretaceous deposits accumulated4 ,professor ,judd has shown that relics of these ,cretaceous strata appear on both sides of the plateau from under the protecting cover of basalt-sheets4 ,but1 before the volcanic eruptions began1 the area had once again been raised into land1 and the youngest ,secondary formations had been extensively eroded4 ,in their general aspect the basalts of ,mull agree with those of ,antrim1 and the circumstances under which they were erupted were no doubt essentially the same4 ,but considerable differences in detail are observable between the succession of rocks in the two areas4 ,when ,i first visited the island in #ahff1 the only available maps1 with any pretensions to accuracy1 were the ,admiralty charts2 but1 as these do not give the interior except in a generalized way1 it was difficult to plot sections from them1 and to arrive at satisfactory conclusions as to the thickness of different groups of rock4 ,accordingly1 as the successive nearly flat flows of basalt can be traced from the sea-level up to the top of ,ben ,more1 ,i contented myself with the fact that the total depth of lava-beds in ,mull was at least equal to the height of that mountain1 or #cafi feet4 ,the publication of the ,ordnance ,survey ,maps now enables us to make a nearer approximation to the truth4 ,from the western base of the magnificent headland of ,gribon1 the basalts in almost horizontal beds rise in one vast sweep of precipice and terraced slope to a height of over #afjj feet1 and then stretch eastwards to pass under the higher part of ,ben ,more1 at a distance of some eight miles4 ,they have a slight easterly inclination1 so that the basement sheets seen at the sea-level1 at the mouth of ,loch ,scridain1 gradually sink below that level as they go eastward4 ,it is not easy to get a measurement of dip among these basalts1 except from a distance4 ,if we take the inclination at only #a1 the beds which are at the base of the cliff on the west1 must be about #gjj feet below the sea on the line of ,ben ,more1 which would give a total thickness of nearly #cijj feet of bedded lava below the top of that mountain4 ,we shall not probably overestimate the thickness of the ,mull plateau if we put it at #cejj feet4 ,the base of the volcanic series of ,mull can best be seen on the south coast at ,carsaig1 and at the foot of the precipices of ,gribon4 ,as already stated1 it is there found resting above ,cretaceous and ,jurassic rocks4 ,the lowest beds are basalt-tuffs1 of the usual dull green colour4 ,they are in places much intermingled with sandy and gravelly sediment1 as if the volcanic debris had fallen into water where such sediment was in course of deposition4 ,one of the most interesting features1 indeed1 in this basement part of the series1 is the occurrence of bands of non-volcanic material which accumulated after the tuffs and some of the lavas had been erupted1 but before the main mass of basalts4 ,those at ,carsaig include a lenticular bed1 #be feet thick1 of rolled flints1 which1 with some associated sandy bands1 lies between sheets of basalt4 ,on the opposite side of the promontory is the well-known locality of ,ardtun1 from which the first land-plants in the volcanic series were determined4 ,the actual base of the basalts is not there seen1 being covered by the sea4 ,the leaf-beds1 with their accompanying sandstones1 gravels1 and limestone1 lie upon a sheet of basalt1 which in some parts is exceedingly slaggy on the top1 passing down into a black compact structure1 and assuming at the base of the cliff a columnar arrangement1 with the prisms curved and built up endways towards each other4 ,some of the gravels exceed #cj feet in thickness1 and consist of rolled flints1 bits of chalk1 and pieces of basalt and other basic igneous rocks4 ,but some of their most interesting ingredients are pebbles of sanidine lavas1 which have been recognized in them by ,prof4 ,g4 ,cole4#bda ,no known protrusions of such lavas occur anywhere beneath or interstratified with the plateau-basalts of this district4 ,as will be afterwards shown1 all the visible acid rocks1 the geological relations of which can be ascertained1 are here of younger date than these basalts4 ,i am disposed to regard the fragments found in the ,ardtun conglomerates as probably derived from some of the basalt-conglomerates of the plateau1 in which fragments of siliceous igneous rocks do occur4 ,though there is no evidence that any lavas of that nature were here poured out at the surface before or during the emission of the basalts1 the contents of these fragmental volcanic accumulations suggest that such lavas1 already consolidated1 lay at some depth beneath the surface1 and that fragments were torn off from them during the explosions that threw out the materials of the basalt-conglomerates to the surface4 ,footnote #bda3 ,quart4 ,jour4 ,geol4 ,soc4 xliii4 7#ahhg7 p4 #bgg4 ,the succession of strata at the ,ardtun headland varies considerably in a short distance1 some of the sedimentary deposits rapidly increasing or diminishing in thickness4 ,the section as measured by ,mr4 ,starkie ,gardner is as follows#bdb3-- ,columnar basalt1 #dj feet4 ,position of first leaf-bed1 obscured by grass1 about #b feet4 ,gravel varying from about #be feet to a maximum of nearly #dj feet4 ,black or second leaf-bed1 #b-a_/b feet4 ,gravel about #g feet4 ,grey clay1 #b feet4 ,laminated sandstone1 #f inches1 with #c inches of fine limestone1 containing leaves at the base4 ,clay1 with leaves at base1 #a foot4 ,clunch1 with rootlets1 #g inches4 ,amorphous basalt1 becoming columnar at base1 about #fj feet4 ,footnote #bdb3 ,op4 cit4 p4 #bhj4 ,mr4 ,starkie ,gardner has called attention to the extraordinarily fresh condition of the vegetation in some of the layers of the ,ardtun section4 ,one of the leaf-beds he has found to be made up for an inch or two of a pressed mass of leaves1 lying layer upon layer1 and retaining almost the colours of dead vegetation4 ,among the plants represented is a large purple ,ginkgo and a fine ,platanites1 one leaf measuring #ae-a_/b inches long by #aj-a_/b broad4 ,the characteristic dicotyledonous leaves at this locality possessed relatively large foliage4#bdc ,footnote #bdc3 ,for fuller local details regarding the ,ardtun leaf-beds1 ,i may refer to the original paper by the ,duke of ,argyll 7,quart4 ,jour4 ,geol4 ,soc4 vii4 p4 #hi71 and to the memoir by ,mr4 ,starkie ,gardner 7op4 cit4 xliii4 7#ahhg71 p4 #bgj74 ,to the early observations of ,macculloch we are indebted for the record of an interesting fact in connection with the vegetation of the land-surface over which the first lava-flows spread4 ,he figured a vertical tree trunk1 imbedded in prismatic basalt1 and rightly referred it to some species of fir4#bdd ,this relic may still be seen under the basalt precipices of ,gribon4 ,mr4 ,gardner found it to be a large trunk of a coniferous tree1 five feet in diameter1 perhaps ,podocarpus1 which has been enveloped1 as it stood1 in one of the flows of trap to the height of #dj feet4 ,its solidity and girth evidently enabled it to resist the fire1 but it had decayed before the next flow passed over it1 for its trunk is a hollow cylinder filled with debris1 and lined with the charred wood4 ,a limb of another1 or perhaps the same tree1 is in a fissure not far off4#bde ,footnote #bdd3 ,western ,islands1 vol4 i4 p4 #efh1 and plate xxi4 ,fig4 #a4 ,footnote #bde3 ,quart4 ,jour4 ,geol4 ,soc4 xliii4 p4 #bhc4 ,at different levels in the volcanic series of ,mull1 beds of lignite and even true coal are observable4 ,these seem to be always mere lenticular patches1 only a few square yards in extent4 ,the best example ,i have met with lies among the basalts near ,carsaig4 ,it is in part a black glossy coal1 and partly dull and shaly4 ,some years ago it was between two and three feet thick1 but now1 owing to its having been dug away by the shepherds1 only some six or eight inches are to be seen4 ,it lies between two basalt-flows1 and rapidly disappears on either side4 ,more frequent than these inconstant layers of fossil vegetation are the thin partings of tuff and layers of red clay1 sometimes containing iron-ore1 which occur at intervals throughout the series between different flows of basalt4 ,but even such intercalations are of trifling thickness1 and only of limited extent4 ,the magnificent precipices of ,m',gorry's ,head and ,gribon expose a succession of beds of columnar amorphous and amygdaloidal basalt1 which must attain a thickness of at least #bejj feet1 before they are overlain by the higher group of pale lavas in ,ben ,more4 ,on the east side of the island1 thin tuffs and bands of basalt-conglomerate occur on different horizons among the bedded basalts1 from near the sea-level up to the summit of the ridge which culminates in ,beinn ,meadhon 7#bjhg feet71 ,dn-da-,ghaoithe 7#beab feet71 and ,mainnir-nam-,fiadh 7#bdhc feet74 ,reference has already been made to the remarkably coarse character of some of the breccias intercalated among the basalts in this part of ,mull1 and to the enormous dimensions of some of the masses of mica-schist and quartzite which have been carried up from a depth of #bjjj feet or more by volcanic agency 7see ante1 p4 #aif1 and ,fig4 #bfb74 ,above the ordinary compact and amygdaloidal basalt comes the higher group of pale lavas already referred to as forming the uppermost part of ,ben ,more1 whence it stretches continuously along the pointed ridge of ,a',chioch1 and thence northwards into ,beinn ,fhada4 ,the same lavas are likewise found in two outliers1 capping ,beinn a' ,chraig1 a mile further north1 and ,i have found fragments of them on some of the loftier ridges to the south-east4 ,this highest and youngest group of lavas in the plateaux has been reduced to mere isolated patches1 and a little further denudation will remove it altogether4 ,yet it is not less than about #hjj feet thick1 and consists of bedded andesitic or trachytic lavas1 which alternate with and follow continuously and conformably upon the top of the ordinary plateau-basalts4 ,these dull1 finely crystalline or compact1 light-grey rocks weather with a characteristic platy form1 which has been mistaken for the bedding of tuffs4 ,the fissility1 however1 has none of the regularity or parallelism of true bedding1 and may be observed to run sometimes parallel with the bedding of the sheets1 sometimes obliquely or even at right angles to it4 ,even where this structure is best developed1 the truly crystalline nature of the rocks can readily be detected4 ,some of them are porphyritic and amygdaloidal1 the very topmost bed of the mountain being a coarse amygdaloid4 ,intercalated with these curious rocks there are others in which the ordinary characters of the dolerites and basalts of the plateaux can be recognised4 ,the amygdaloids are often full of delicate prisms of epidote4 ,in ,mull1 as in the other areas of terraced basalts1 we everywhere meet with gently inclined sheets1 which do not thicken out individually or collectively in any given direction1 except as the result of unequal denudations4 ,so far as ,i have been able to discover1 they afford no evidence of any great volcanic cone from which they proceeded4 ,their present inclinations are unquestionably due1 as in ,ireland1 to movements subsequent to the formation of the plateau4 ,in ,loch-na-keal they dip gently to the ,e4,n4,e42 in ,ulva and the north-west coast to ,n4,n4,e42 near ,salen to ,w4,s4,w4 on the one side1 and ,n4,w4 on the other4 ,round the southern and eastern margins of the mountainous tract of the island1 they dip generally inwards to the high grounds4 ,the ,mull plateau presents a striking contrast to that of ,antrim1 in the extraordinary extent to which it has been disrupted by later protrusions of massive basic and acid rocks over a rudely circular area1 extending from the head of ,loch ,scridain to the ,sound of ,mull1 and from ,loch-na-keal to ,loch ,buy4 ,the bedded basalts have been invaded by masses of dolerite1 gabbro1 and granophyre1 with various allied kinds of rock4 ,they have not only been disturbed in their continuity1 but have undergone considerable metamorphism4 ,again1 further to the north1 in the promontory of ,ardnamurchan1 the plateau has been disrupted in a similar way1 and only a few recognisable fragments of it have been left4 ,these changes will be more appropriately discussed in connection with similar phenomena in the other plateaux further north4 ,,chapter ,,xxxviii ,,the ,,basalt-,,plateau ,,of ,,the ,,parish ,,of ,,small ,,isles--,,rivers ,,of ,,the ,,volcanic ,,period iii4 ,,parish ,,of ,,small ,,isles ,,plateau ,the parish of ,small ,isles includes the islands of ,eigg1 ,rum1 ,canna1 ,sanday and ,muck 7,map ,,vi474 ,the fragmentary basalt-plateau which it contains1 although the smallest of the whole series1 is surpassed by none in the variety and interest of its geology4 ,it contains by far the most complete records of the rivers which1 during the volcanic period1 flowed across the lava plains4 ,and it alone has preserved a relic of the latest lava which1 after the basalt-plateau had been built up and had been greatly eroded1 flowed over the denuded surface in streams of volcanic-glass that found their way into a river-channel and sealed it up4 ,that the fragments of the basaltic plateau preserved in each member of the group of the ,small ,isles were once connected as a continuous volcanic plain can hardly be doubted4 ,indeed1 as already stated1 they were not improbably united with the plateau of ,skye on the north1 and with that of ,mull1 ,morven and ,ardnamurchan oh the south4 ,taking the whole space of land and sea within which the basalt of ,small ,isles is now confined1 we may compute it at not much less than #bjj square miles4 ,in ,eigg1 ,muck1 ,canna and ,sanday the basalts retain their almost horizontal position1 and from underneath them the ,jurassic strata emerge in the first of these islands4 ,the central part of the plateau in the island of ,rum has suffered greatly from denudation4 ,it now consists of four small outliers of basalt1 which lie at levels of #abjj feet and upwards1 on the western slope4 ,the basalt is underlain by a thick mass of red ,torridon ,sandstone1 which1 with some gneisses and schists1 forms the general underlying platform of this island4 ,these rocks are doubtless a continuation of the red sandstone and schists of ,sleat1 in ,skye1 and like them have been subjected to those post-,cambrian convolutions and metamorphism whereby the ,lewisian ,gneiss and ,torridon ,sandstone have been brought above younger rocks1 and have been crushed and rolled out so as to assume a new schistose arrangement4 ,before the time when volcanic action began1 a mass of high ground1 consisting of these ancient rocks1 stood where the island of ,rum is now situated4 ,the streams of basalt spread around it1 not only covering the surrounding low tracts of ,jurassic rocks1 but gradually accumulating against the hills1 and thus reducing them both in area and in height above the plain4#bdf ,viewed from ,canna the western coast of ,rum presents a striking picture of the general relations of the volcanic masses of the ,inner ,hebrides and of the enormous denudation which they have undergone 7,fig4 #bfg74 ,the ,torridon ,sandstones are there seen to mount into ranges of hills1 capped with outliers of the basalt-plateau1 while behind rise the great eruptive bosses of gabbro and granophyre4 ,the edges of the sheets that form the outliers would1 if prolonged1 cover the northern or lower half of the island1 where pre-,cambrian rocks form the surface4 ,in the southern half1 the continuity of the basalt has been partly obscured and partly destroyed by the protrusion of the great masses of gabbro that form the singularly picturesque mountain group to which this island owes its prominence as a landmark far and wide along the ,west ,coast of ,scotland4 ,footnote #bdf3 ,that the lava-fields did not completely bury this nucleus of older rocks has been supposed to be shown by the fragments of red sandstone found in the ancient river-bed of ,eigg1 which was scooped out of the basalt-plateau and sealed up under pitchstone4 ,but ,i am disposed to think that these fragments1 together with those of ,jurassic sandstone1 came1 not from ,rum1 but from some district more to the north and east1 as will be explained in a later page4 ,at ,canna1 a few miles to the west1 fragments of red sandstone not improbably derived from ,rum are abundant in the conglomerates between the basalts4 ,illustration3 ,fig4 #bfg4--,view of ,rum from the harbour of ,canna4 ,the ground indicated by single birds is the area of ,torridon ,sandstone2 two birds1 the plateau basalts2 three birds1 the gabbro just seen at one point above the granophyre hills2 four birds1 the granophyre4 ,the most varied and interesting of the fragments of the basaltic plateau in the area of the ,small ,isles is that which forms the island of ,canna1 with its appendage ,sanday4 ,canna measures five miles in length by from half a mile to a mile in breadth1 and consists entirely of the rocks of the plateau and their accompaniments4 ,the basalts are exposed along the north coast in a range of mural precipices rising to a height of about #fjj feet above the sea4 ,from the top of that escarpment the ground falls by successive rocky terraces and grassy slopes to the southern shore-line4 ,sanday1 connected with the large island by a shoal and foot-bridge1 is two miles long and #bbj to about #abjj yards broad4 ,its highest cliffs range along its southern shore to a height of #aic feet1 whence they slope gently northward into the hollow between the two islands4 ,this peculiar topography accounts for the manner in which the geological sections of most interest are distributed4 ,the first1 and still the best1 account of the geology of these islands is that of ,macculloch4 ,he showed that the rocks all belong to the series of the plateau-basalts1 and he described the presence among them of a trap-conglomerate4 ,he noticed the occurrence also of trap-tuff and the occasional appearance of carbonized wood in these deposits4 ,reasoning upon these observations in his characteristically vague and verbose manner1 bewildered in the regions of conjecture1 he concludes that the basalts instead of belonging to one general formation have been successively deposited on the same spot1 since lapse of time is evidently implied in the formation of a conglomerate4 ,he inclines to believe that they have been discharged by ancient volcanoes from which in the course of time all traces of their original outline have been more or less completely removed1 the existing basalts being merely fragments of once more extensive masses4#bdg ,footnote #bdg3 ,western ,isles1 vol4 i4 pp4 #ddh-dei1 and pl4 xix4 ,figs4 #b1 #c and #d4 ,see also ,jameson's ,mineralogy of the ,scottish ,isles4 ,macculloch regarded the intercalated-conglomerates as having been arranged under water and as marking pauses in the deposition of the sheets of trap4 ,he gave two diagrams in illustration of the relations of these detrital deposits1 but he expressed no definite opinion as to their origin1 though from one passage it would seem that he inclined towards the belief that they were formed in the sea4#bdh ,since his time1 so far as ,i am aware1 no fresh light has been thrown upon the subject4 ,footnote #bdh3 ,op4 cit4 pp4 #ddi1 #deg1 pl4 xix4 ,figs4 #b and #c4 ,during a yachting cruise in the summer of #ahid ,i visited ,canna for the first time and found so much that was new to me in regard to the history of ,tertiary volcanic action1 and which demanded a careful survey1 that ,i returned to the locality the following summer and remained in the island until ,i had mapped it and its dependencies upon the ,ordnance ,survey sheets on the scale of six inches to a mile4 ,the following narrative is the result of the observations then made4 ,as far back as the year #ahfe ,i published an account of an ancient river-channel which1 during the volcanic period1 had been eroded on the surface of the basalt-plateau1 and of which a small portion had been preserved under a stream of pitchstone-lava that had flowed into and buried it4#bdi ,this water-course1 now marked by the picturesque ridge of the ,scuir of ,eigg1 was shown to have been excavated by a stream which came from the north-east or east1 and to be younger1 not only than the plateau-basalts of the district1 but than even the dykes which cut these basalts4 ,yet that it belonged to the volcanic period was proved by the manner in which it had been sealed up and preserved under the black glassy lava of the ,scuir4 ,its history and the data from which this history is compiled will be narrated in a later part of this chapter4 ,footnote #bdi3 ,scenery of ,scotland 7#ahfe72 ,quart4 ,journ4 ,geo4 ,soc4 vol4 xxvii4 7#ahga71 p4 #cjc4 ,my examination of the islands of ,canna and ,sanday1 however1 brought to light other and more abundant evidence of river-action in the same region of the ,inner ,hebrides1 but belonging to an earlier part of the volcanic period4 ,this evidence reveals that a powerful river1 flowing westwards from the ,highland mountains1 swept over the volcanic plain1 while the sheets of basalt were still being poured forth1 and while volcanic eruptions were taking place from cones of slag4 ,the basalt-plateau of ,canna resembles in all essential particulars those of the other ,western ,isles4 ,its base is everywhere concealed under the sea1 but from the fragments of ,torridon ,sandstone in its agglomerates we may infer that it probably rests on that formation1 like the volcanic outliers in ,rum4 ,it is formed of successive sheets of different basalts including the usual banded1 amygdaloidal and columnar forms4 ,some of them towards the west are specially marked by the great abundance and large size of their porphyritic felspars4 ,the magnetic properties of the basalts at the east end of the island have long been known1 and have given rise to various modern myths regarding their influence on the compasses of passing vessels4 ,illustration3 ,fig4 #bfh4--,section of the cliffs below ,compass ,hill1 ,isle of ,canna4 ,but it is in its conglomerates1 tuffs and agglomerates and the light they cast on some aspects of the volcanic period1 elsewhere hardly recorded1 that the geology of ,canna possesses a special importance4 ,to these1 therefore1 we may at once turn4 ,the conglomerates are best developed at the eastern end of the island1 where the cliffs present the structure represented in ,fig4 #bfh4 ,at the base1 and passing under the level of the sea1 lies the agglomerate 7a7 of a vent which will be described in ,chapter xli41 together with other eruptive orifices of the various plateaux 7p4 #bhh74 ,this rock has a somewhat uneven upper surface which rises in places about #aej feet above high tide-mark4 ,here and there it shades off upward into the conglomerate that overlies it2 water-worn pebbles appear among its contents1 and rude traces of bedding begin to show themselves1 until1 within the course of a few feet1 we pass upward into an undoubted conglomerate4 ,elsewhere1 however1 and particularly along the precipices west of ,compass ,hill1 the two deposits are more distinctly marked off from each other4 ,the agglomerate has there a hummocky1 irregular upper surface1 as if it had been thrown down in heaps4 ,the hollows between these protuberances have been filled up with conglomerate and sandstone1 forming the base of the thick overlying deposit4 ,it is thus clear that the loose materials of the vent were directly exposed at the surface when the conglomerate was accumulated1 and1 indeed1 that these materials served to supply some of the detritus of which the conglomerate consists4 ,the absence of any trace of a cone and crater at the vent may perhaps be explicable on the supposition that their incoherent material was washed down by the currents that swept along and deposited the conglomerate4 ,the mass of sedimentary material 7b7 which overlies the agglomerate of the vent forms a conspicuous feature along the lower half of the precipices at the eastern end of ,canna4 ,it rises to a height of #bej to #cjj feet above sea-level1 and must reach a maximum thickness of probably not less than #ajj to #aej feet4 ,it gradually descends in a westward direction1 both along the northern cliffs and in the lower ground round ,canna ,harbour1 insomuch that in about a mile1 owing to the gentle westerly dip of the whole volcanic series1 combined with the effect of a number of small faults1 it passes under the level of the sea4 ,great variation in the character of the detritus composing this thick group of strata may be observed as it is followed westward4 ,on the cliffs below ,compass ,hill1 as represented in ,fig4 #bfh1 the coarse conglomerate with water-worn stones1 hardly to be distinguished from the volcanic agglomerate of the vent1 shows more or less distinct bedding1 or at least a succession of coarser and finer bands4 ,towards its base it encloses numerous pieces of ,torridon ,sandstone1 sometimes subangular1 but often so well and smoothly rounded as to show that they must have been long subjected to the action of moving water4 ,it is further observable that1 while in the agglomerate the volcanic stones have rough surfaces1 those in the conglomerate begin to show increasing evidence of attrition1 until1 as the deposit is traced upwards1 they become almost as well rounded and water-worn as the non-volcanic stones4 ,yet amidst and overlying these proofs of transport from some little distance lie abundant huge slags and blocks of amygdaloidal lava1 sometimes closely aggregated1 sometimes scattered through a volcanic tuff or ashy sandstone4 ,the composition and structure of these stones1 and the manner of their dispersion through the deposit1 leave little doubt that they were ejected from the vent4 ,we are thus confronted with the interesting fact that1 while the materials of the volcanic cone were being washed down by running water1 eruptions were still taking place4 ,but by degrees these indications of contemporaneous volcanic activity diminish4 ,the detrital materials become coarser and more distinctly water-rolled until they pass into greenish sandstones and fine conglomerates4 ,yet the matrix even of these higher sediments is largely composed of fine volcanic detritus1 and probably points to occasional discharges of dust and ashes4 ,various sills or intrusive sheets have been injected into this sedimentary group along the precipices at the east end of ,canna1 and form there lenticular bands4 ,one of these 7c7 is shown in ,fig4 #bfh4 ,immediately above the massive greenish pebbly sandstone 7d7 which caps the stratified series lies a group of basalts 7e71 composed of several distinct beds1 having a united thickness of from #hj to #ajj feet4 ,the lowest of these has a regular columnar structure1 while those overlying it exhibit the confused starch-like grouping of curved and rather indistinctly-formed prisms4 ,the next band in upward succession is one of conglomerate 7f71 which runs as a continuous and conspicuous feature along the upper part of the cliff4 ,this rock presents in many respects a strong contrast to the conglomerates underneath4 ,it is dull-green to yellow in colour1 and is well stratified1 being marked by the interstratification of finer layers1 and passing down into a band of pebbly sandstone1 which rests immediately on the basalt 7e74 ,its component stones are thoroughly water-worn1 ranging up to six inches or even more in length4 ,but its most distinctive character lies in the nature of its pebbles4 ,instead of consisting mainly of volcanic materials1 these stones have almost all been transported for some distance4 ,they include abundant fragments of ,torridon ,sandstone1 gneiss1 schists1 grits1 and other rocks like those in ,rum and ,western ,inverness-shire4 ,no such rocks exist in situ in ,canna4 ,the nearest tract of ,torridon ,sandstone is in ,rum1 about four miles to the eastward4 ,but the pieces of schist and epidotic grit like the rocks of the ,western ,highlands1 have probably travelled at least #cj miles4 ,it is important to observe that all these transported stones indicate a derivation from some source lying to the eastward of ,canna4 ,the evidence in this respect agrees with that furnished by the ancient river-gravel under the pitchstone of the ,scuir of ,eigg4 ,it is clear that the waters which found their way across the lava-fields of this part of the ,inner ,hebrides took their rise somewhere to the eastward1 probably among the mountains of ,inverness-shire4 ,the conglomerate now described is from #dj to #ej feet thick4 ,it can be followed along the face of the cliffs for more than a mile on the north side of ,canna4 ,less persistent on the south side1 its outcrop strikes from the edge of the precipice inland1 keeping to the south of the top of ,compass ,hill4 ,it is well seen in the ravine above the ,coroghon1 but cannot be followed further westward among the basalt-terraces4 ,yet1 though this stratified intercalation is not traceable far as a band of conglomerate1 the same stratigraphical horizon is probably indicated elsewhere by other kinds of sedimentary deposits1 to which further reference will be made in the sequel4 ,the section now described establishes the existence of at least two successive platforms of conglomerate in the volcanic series4 ,following these platforms along their outcrop1 we obtain additional light on their origin1 and on the topographical conditions under which they were deposited1 and we learn further that other prolonged intervals1 which were likewise marked by intercalations of sedimentary material1 occurred in the outpouring of the basalts4 ,taking first the lower conglomerate of ,compass ,hill and tracing it westward1 we find it to form the depression in which the sheltered inlet of ,canna ,harbour lies4 ,it is exposed along the shores and also in the islands enclosed within the same bay4 ,but it is not traceable further west1 possibly because it seems to sink beneath the level of the sea4 ,to the south-east1 though it is there likewise for the most part concealed under the waves1 it rises above them in one or two parts of the coast-line of ,sanday1 particularly at the ,uamh ,ruadh or ,red ,cave1 and likewise on a surf-beaten skerry off ,ceann an ,eilein1 the highest part of the ,sanday cliffs--a distance of about a mile and a half from ,compass ,hill4 ,throughout this space it retains its remarkably coarse character and is mainly made up of volcanic material4 ,the numerous sections exposed in ,canna ,harbour enable us to study the composition and local variations of this curious deposit4 ,on the north side of the basin1 while the lower part of the sedimentary series continues to be an exceedingly coarse volcanic conglomerate1 it passes upward into finer conglomerates1 tuffs1 and shales4 ,in front of ,canna ,house the imbedded blocks are of large size1 occasionally as much as three or four feet in diameter4 ,they are still more gigantic on the island of ,eilean a' ,bhaird1 where ,i found one to contain #aej cubic feet in the exposed part1 the rest being still concealed in the matrix4 ,as they are generally somewhat rounded1 here and there markedly so1 most of these stones have probably undergone a certain amount of attrition in water4 ,the great majority of them1 and certainly all those of larger size1 are pieces of basalt1 dolerite1 andesite1 etc4 ,among them huge blocks of amygdaloid and coarsely vesicular lava are specially abundant4 ,some of these look like pieces of slag torn from the upper surface of lava-streams4 ,others1 displaying a highly vesicular centre and a close-grained outer crust1 are suggestive of bombs4 ,it is interesting to note here again that the amygdaloidal blocks present their zeolitic infiltrations so precisely like those of the amygdaloids of the plateau that it seems reasonable to suppose the carbonate of lime1 zeolites1 etc4 to have been introduced before the blocks were imbedded in the conglomerate4 ,the whole aspect of this deposit is eminently volcanic4 ,it looks like a vast sheet of lava-fragments swept away from one or more cones of slags and cinders1 or from the scoriaceous surface of a lava-stream4 ,where the vesicles were still empty1 the large boulders could be more easily swept along by moving water4 ,but a powerful current must have been needed to transport and wear down into more or less rounded forms blocks of basic lava1 many of which must weigh several tons4 ,the large block on ,eilean a' ,bhaird probably exceeds #ab tons in weight4 ,besides the obviously volcanic contents of the conglomerate there occur here also1 as in the ,compass ,hill cliffs1 abundant pieces of ,torridon ,sandstone4 ,these stones are notably smaller in size and more perfectly water-worn and even polished than the blocks of lava4 ,obviously they have travelled further and have undergone more prolonged attrition4 ,the matrix of the rock consists essentially of the fine detritus of basic lavas1 probably mingled with true volcanic dust4 ,the coarser parts display only the feeblest indication of stratification2 indeed1 in a limited exposure the rock might be regarded as a tumultuous agglomerate4 ,but the manner in which the deposit is intercalated with1 and sometimes overlies1 green tuffs and shales1 together with the water-worn condition of its stones1 shows that it has not been accumulated in a volcanic chimney1 but has been thrown down by some powerful body of water1 with probably the co-operation of volcanic discharges4 ,while the composition of the conglomerate suffices to indicate that this deposit was formed at a time when some volcano was active in the immediate neighbourhood1 singularly convincing proofs of the work of this vent are to be seen in the form of intercalated sheets of lava4 ,thus on ,eilean a' ,bhaird the boulders of the conglomerate are overlain and wrapped round by a sheet of rudely prismatic basalt1 with lines of vesicles arranged in the direction of the bedding4 ,a similar relation can be traced along the beach between ,canna ,house and the wooden pier1 where successive sheets of basalt have flowed over the conglomerate 7,fig4 #bfi74 ,but1 besides coarse volcanic detritus1 the sedimentary platform represented by the lower conglomerate of ,compass ,hill includes other deposits of which good sections may be examined all round ,canna ,harbour4 ,beds of fine well-stratified dull-green tuff pass by an admixture of pebbles into fine ashy conglomerate or pebbly sandstone1 and by an increase in the proportion of their fine detritus into volcanic mudstone and fine shales4 ,the shales vary from a pale grey or white tone into blackish grey1 brown1 and black4 ,they are well stratified and are frequently interleaved with layers of fine tuff4 ,the darker bands are carbonaceous1 and are not infrequently full of ill-preserved vegetation4 ,indeed1 leaves and stems in a rather macerated condition are of common occurrence in all the shaly layers4 ,here and there1 especially in some ashy shales in front of ,canna ,house1 ,i observed a recognisable ,sequoia4 ,the mudstones are dull green1 close-grained shattery rocks composed of fine volcanic detritus1 and pass both laterally and vertically into shales1 tuffs1 and conglomerates4 ,they suggest showers of fine dust or streams of volcanic mud4 ,they1 too1 contain fragmentary plants4 ,it is a noteworthy fact that the sedimentary intercalations among the ,canna basalts generally end upward in carbonaceous shales or coaly layers4 ,the strong currents and overflows of water1 which rolled and spread out the coarse materials of the conglomerates1 gave way to quieter conditions that allowed silt and mud to gather over the water-bottom1 while leaves and other fragments of vegetation1 blown or washed into these quiet reaches1 were the last of the suspended materials to sink to the bottom4 ,good illustrations of this sequence in the case of the lower conglomerate zone of ,canna may be studied along the shores of ,sanday1 from the ,catholic ,chapel eastwards4 ,the fine pebbly sandstones1 tuffs1 and shales1 which there overlie the coarse conglomerate1 are surmounted by dark brown or black carbonaceous shale1 with lenticles of matted vegetation that pass into impure coal4 ,immediately overlying this coaly layer comes a sheet of prismatic vesicular basalt1 followed by another with an exceedingly slaggy texture4 ,lenticles of shale and mudstone likewise occur in the heart of the finer parts of the conglomerate1 especially towards the top1 as may be seen in the section exposed beneath the basalt behind the first cottage west from ,canna ,house4 ,one of the most interesting layers in this section is a seam of tuff1 varying up to about two inches in thickness1 which lies at the top of the lenticular band of tuffs and shales1 and immediately beneath the band of basalt-conglomerate1 on which a basalt1 carrying a vesicular band near its bottom1 rests4 ,traced laterally1 the dark brown tuff of this seam gradually passes into a series of rounded bodies and flattened shells composed of a colourless mineral which has evidently been developed in situ after the deposition of the tuff4 ,mr4 ,harker's notes on thin slices made from this band are as follows3-- ,this is a rusty-brown1 dull-looking rock1 rather soft and seemingly light1 but too absorbent to permit of its specific gravity being tested4 ,the dark brown mass is in great part studded with little spheroidal bodies1 #a_/ej to #a_/aj inch in diameter1 of paler colour1 but the larger ones having a dark nucleus4 ,in other parts larger flat bodies have been formed1 as if by the coalescence of the spheroids1 extending as inconstant bands in the direction of lamination for perhaps #a_/b inch1 with a thickness of #a_/aj inch or less4 ,the appearance is that of a spherulitic rather than an oolitic structure4 ,a slice #ffeh ,a shows the general mass of the rock to be of an extremely finely divided but coherent substance of brown colour1 which can scarcely be other than a fine volcanic dust1 composed of minute particles of basic glass or 'palagonite' compacted together4 ,scattered through this are fragments of crystals recognizable as triclinic and perhaps monoclinic felspars1 green hornblende1 augite1 olivine 7871 and magnetite1 usually quite fresh4 ,the curious spheroidal and elongated growths already mentioned are better seen in another slide #ffeh ,b1 where they occupy the larger part of the field1 leaving only an interstitial framework of the brown matrix4 ,the substance of the little spheroids is clear1 colourless1 and apparently structureless4 ,the centre is often occupied by an irregularly stellate patch of brown colour1 and sometimes cracks tend to run in radiating fashion1 but these are the only indications of radial structure4 ,the outer boundary is sharply defined1 and where the slice is shattered the spheroids have separated from the matrix4 ,the matrix is darker than in the normal rock1 being obscured by iron-oxide which we may conceive as having been expelled from the spaces occupied by the spheroids4 ,the little crystal-fragments are enclosed in the spheroids as well as in the matrix1 but there is no appearance of their having served as starting-points for radiate growths4 ,the flat elongated bodies are like the spheroids1 with merely the modifications implied in their different shape4 ,the identity of the clear colourless substance seems to be rather doubtful4 ,it is sensibly isotropic and of refractive power distinctly lower than that of felspar4 ,these characters would agree with analcime1 which is not unknown as a contact-mineral2 but it is difficult to understand how analcime1 even a lime-bearing variety like that of ,plas ,newydd1#bej could be formed in abundance from palagonitic material4 ,an alternative supposition1 perhaps more probable1 is that the clear substance is a glass1 modified from its former nature1 especially by the expulsion of the iron-oxide into the remaining matrix4 ,a comparison is at once suggested with certain types of ',knotenschiefer1' but respecting the thermal metamorphism of fine volcanic tuffs there seems to be little or no direct information4 ,footnote #bej3 ,henslow1 ,trans4 ,camb4 ,phil4 ,soc4 7#ahba71 vol4 i4 p4 #djh2 ,mr4 ,harker1 ,geol4 ,mag4 7#ahhg71 p4 #dad4 ,mr4 ,w4 ,w4 ,watts suggests a comparison with the hexagonal bodies figured by ,mr4 ,monckton in an altered limestone from ,stirlingshire3 ,quart4 ,journ4 ,geol4 ,soc41 vol4 li4 p4 #dhg4 ,lenticular interstratifications of shale and mudstone make their appearance even in the coarser parts of the conglomerate1 as may be observed on the beach below ,canna ,house where1 as shown in ,fig4 #bfi1 some shales and tuffs 7a7 full of ill-defined leaves are surmounted by a conglomerate 7b74 ,the deposition of this overlying bed of boulders has given rise to some scooping-out of the finer strata underneath4 ,subsequently both the conglomerate and shales have been overspread by a stream of dolerite 7c71 the slaggy bottom of which has ploughed its way through them4 ,illustration3 ,fig4 #bfi4--,lava cutting out conglomerate and shale4 ,shore below ,canna ,house4 ,before discussing the probable conditions under which the group of sedimentary deposits now described was formed1 we may conveniently follow the upper conglomerate band of the ,compass ,hill1 and note the variations in structure and composition which its outcrop presents4 ,this yellowish conglomerate can be traced along the cliffs for more than a mile1 when it descends below the sea-level at the solitary stack of ,bod an ,stl4 ,a few hundred yards further west1 what is probably the same band appears again at the base of the precipice overlain by prismatic basalts4 ,but the conglomerate1 here only #ab feet thick1 is made of much finer detritus which1 largely composed of volcanic material1 includes small well-rounded and polished pebbles of ,torridon ,sandstone4 ,beneath it lies a bed of dark shale1 with remains of plants1 resting immediately on a zeolitic amygdaloid which plunges into the sea4 ,the chief interest of this locality is to be found in the shale which1 instead of being at the top of the sedimentary group1 lies at the bottom4 ,i was informed by ,mr4 ,a4 ,thom that leaves had been obtained from this shale2 but ,i was not successful in my search for them4 ,the locality is only accessible by boat1 and1 as the coast is fully exposed to the ,atlantic swell1 landing at the place is usually difficult and often impossible4 ,about a mile and a half still further west1 where a foreshore fronts the precipice of ,earnagream at the ,camas ,tharbernish1 a band of intercalated sedimentary material underlies the great escarpment of basalts and rests upon the slaggy sheet with the singular surface already referred to 7p4 #ahg74 ,this band not improbably occupies the same platform as the upper conglomerate of ,compass ,hill4 ,it is only about seven feet thick1 the lower four feet consisting of a dull green pebbly tuff or ashy sandstone1 with small rounded pieces of ,torridon ,sandstone1 while the upper three feet are formed of dark shale with crowded but indistinct remains of plants4 ,here the more usual order in the sequence of deposition is restored4 ,the shale is indurated and shattery1 so that no slabs can be extracted without the use of quarrying tools4 ,illustration3 ,fig4 #bgj4--,section of shales and tuffs1 with a coniferous stump lying between two basalt-sheets1 ,cl nam ,marbh1 ,canna4 ,rather less than half a mile towards the south1 on the roadside at the gully of ,cl nam ,marbh1 the basalts enclose a sedimentary interstratification which not improbably lies on the same horizon as those just described along the northern shore4 ,the relations of the rocks at this locality are shown in ,fig4 #bgj4 ,a remarkable slaggy basalt 7a7 rises into a hummock1 against which have been deposited some fine granular tuffs 7b7 whereof only a few inches are visible1 that pass up into a thin band of dark shale 7c71 including a layer of pebbly ferruginous tuff1 with small rounded pea-like pieces of basalt1 basic pumice1 bole1 limonite1 etc4 ,at the top of this shale an irregular parting of coaly material 7d7 lies immediately under the slaggy base of the succeeding basalt 7e74 ,it will be observed that this upper lava cuts out the shale and thus comes to rest directly upon the lower sheet4 ,at the point where it begins to descend it has caught up and enclosed a small tree-stump 7d7 which stands upright on the coaly parting and shale4 ,this stump1 at the time of my visit1 measured five inches in height by three inches in breadth2 it had been thoroughly charred and was crumbling away on exposure1 but among the pieces which ,i took from it sufficient trace of structure can be detected with the microscope to show the tree to have been a conifer4 ,we have here another instance of the deposition of volcanic dust and fine mud in a pool that filled a hollow in the lava-field4 ,again we see that the closing act of sedimentation was the subsidence of vegetable matter in the pool1 which was finally buried under another outflow of basalt4 ,illustration3 ,fig4 #bga4--;,dn ,mr1 ,sanday4 7,from a photograph by ,miss ,thom47 ,it is on the southern coast of the isle of ,sanday that the higher intercalations of sedimentary material among the basalts are most instructively displayed4 ,at the eastern end of this island1 as already stated1 the lowest and coarsest conglomerate is visible on a skerry immediately to the south of the headland ,ceann an ,eilein4 ,it doubtless underlies the ,sanday cliffs1 but is not there visible1 for the basalts descend below sea-level4 ,these volcanic sheets have a slight inclination westward2 hence in that direction we gradually pass into higher parts of the series4 ,in the ,creag nam ,faoileann 7,seamews' ,crag7 and the gully that cuts its eastern end1 likewise in the two singularly picturesque stacks of ,dn ,mr and ,dn ,beag 7,big and ,little ,gull ,rocks71 which here rise from the foreshore1 two distinct platforms of detrital material may be noticed among the basalts4 ,both of these can be well seen on ,dn ,mr1 about #ajj feet high1 which is represented in ,fig4 #bga4 ,the lower band1 four or live feet thick1 is here a rather coarse conglomerate which lies upon a sheet of scoriaceous basalt that extends up to the base of the ,creag nam ,faoileann4 ,it is directly overlain by another basalt1 about #cj feet thick1 which dips seawards and forms a broad shelving platform1 whereon the tides rise and fall4 ,on this stack a second coarse conglomerate1 about #aj feet thick1 forms a conspicuous band about a third of the height from the bottom2 it is composed mainly of well-rounded blocks of various lavas up to #ah inches or more in diameter1 but it contains also pieces of ,torridon ,sandstone4 ,it is covered by about #fj feet of basalt1 which towards the base is somewhat regularly columnar1 but passes upward into the wavy1 starch-like1 prismatic structure4 ,if now we trace these two intercalated zones of conglomerate along the shore1 we find them both rapidly to change their characters and to disappear4 ,the lower1 though formed of coarse detritus under the ,dn ,mr1 passes on the opposite cliff in a space of not more than #fj yards1 into fine tuff and shale1 about six feet thick1 which become carbonaceous at the top where they are overlain by the next basalt4 ,a hundred yards to the east1 the band likewise consists of tuffs and ashy shales1 which underlie the basalts on the ,dn ,beag1 and again show the usual coaly layers at the top4 ,on the east side of the gully in the coast1 about #afj yards to the north-east of ,dn ,mr1 the same band is reduced to not more than three feet in thickness1 consisting chiefly of fine conglomerate1 wherein well water-worn pebbles of ,torridon ,sandstone and epidotic grit appear among the predominant volcanic detritus4 ,this conglomerate is surmounted by a few inches of dark carbonaceous mudstone or shale4 ,rough slaggy basalts lie above and below the band4 ,the upper conglomerate dies out1 both towards the east and the west1 in the cliff opposite to ,dn ,mr1 dwindling down at last to merely a few pebbles between the basalts4 ,it lies in a kind of channel or hollow among these lavas4 ,this depression1 in an east and west direction1 cannot be more than about #fe yards broad4 ,probably still higher in the series of basalts is another intercalation of sedimentary layers which may be seen in the little bay to the east of ,tallabric1 rather more than a mile to the west of the ,creag nam ,faoileann4 ,it rests upon a coarsely slaggy amygdaloid1 and is from six to ten feet in thickness4 ,the lower and larger part of the deposit consists of greenish pebbly sandstone and fine conglomerate1 largely composed of basaltic detritus1 but including abundant well-smoothed and polished pebbles of ,torridon ,sandstone1 green grit1 quartzite1 etc4 ,the stones vary from mere pea-like pebbles up to pieces two or three inches long1 the largest being generally fragments of slag and amygdaloid which are less water-worn than the sandstones and other foreign ingredients4 ,the uppermost two or three feet of the intercalation consist of dark carbonaceous mudstone or shale1 made up in large measure of volcanic detritus1 which may have been derived partly from eruptions of fine dust1 partly from subrial disintegration of the basalt-sheets4 ,some layers of these finer strata are full of remains of much macerated plants4 ,other thin coaly intercalations have been observed among the basalts of ,canna1 some of which may possibly mark still higher horizons than those now described4 ,but1 confining our attention to the regular sequence of intercalations exposed along the ,sanday coast1 we find at least four distinct platforms of interstratified sediment among the plateau-basalts of this district4 ,each of these marks a longer or shorter interval in the outflow of lava1 and points to the action of moving-water over the surface of the lava-fields4 ,we may now consider the probable conditions under which this intervention of aqueous action took place4 ,the idea that the sea had anything to do with these conglomerates1 sandstones1 and shales may be summarily dismissed from consideration4 ,the evidence that the basalt-eruptions took place on a terrestrial surface is entirely convincing1 and geologists are now agreed upon this question4 ,excluding marine action1 we have to choose among forms of fresh water--between lakes on the one hand and rivers on the other4 ,that the agency concerned in the transport and deposition of these strata was that of a river may be confidently concluded on the following grounds3-- #a4 ,the large size and rolled shape of the boulders in the conglomerates4 ,to move blocks several tons in weight1 and not only to move them but to wear them into more or less rounded forms1 must have required the operation of strong currents of water4 ,the coarse detritus intercalated among the basalts is quite comparable to the shingle of a modern river1 which descends with rapidity and in ample volume from a range of hills4 #b4 ,the evidence that the materials of the conglomerates are not entirely local1 but include a marked proportion of foreign stones4 ,the proofs of transport are admirably exhibited by pieces of ,torridon ,sandstone1 epidotic grit1 quartzite1 and other hard rocks none of which occur in situ except at some distance from ,canna4 ,these stones are often not merely rounded1 but so well smoothed and polished as to show that they must have been rolled along for some considerable time in water4 #c4 ,the lenticular character and rapid lithological variations of the strata1 both laterally and vertically4 ,the coarse conglomerates die out as they are followed along their outcrop and pass into finer sediment4 ,they seem to occur in irregular banks1 which may not be more than #bjj feet broad1 like the shingle-banks of a river4 ,the coarser sediment generally lies in the lower part of the sedimentary group4 ,but cases may be observed1 such as that shown in ,fig4 #bfi1 where fine sediment1 laid down upon the bottom conglomerate1 has subsequently been overspread by another inroad of coarse shingle4 ,such alternations are not difficult to understand if they are looked upon as indicating the successive floods and quieter intervals of a river4 ,for these reasons ,i regard the platforms of sedimentary materials intercalated among the basalts of ,canna and ,sanday as the successive flood-plains of a river which1 like the rivers that traverse the lava-deserts of ,iceland1 flowed perhaps in many separate channels across the basalt-fields of the ,inner ,hebrides1 and was liable to have its course shifted from time to time by fresh volcanic eruptions4 ,that this river came from the east or north-east and had its source among the ,western ,highlands of ,inverness-shire1 may be inferred from the nature of the stones which it has carried for #cj miles or more along its bed4 ,and that it crossed in its course the tract of ,torridon ,sandstone1 of which a portion still remains in ,rum1 is manifest from the abundance of the fragments of that formation in the conglomerates4 ,with the remarkable exception of the section on ,dn ,beag1 to be immediately referred to1 no trace of any eroded channel of this river through the lavas of the great volcanic plain has been preserved4 ,possibly frequent invasions of its bed by streams of basalt from different vents hindered it from remaining long enough in one course to erode anything like a gorge or canon4 ,but1 in any case1 the main channel of the river probably lay rather to the east of the present islands of ,canna and ,sanday1 on ground which is now covered by the sea4 ,the banks or sheets of boulder-conglomerate undoubtedly show where its current swept with great force over the lava-plain1 but the manner in which these coarser materials are so often covered with fine silt suggests that the sedimentary materials now visible were rather deposited on the low grounds over which the steam rushed in times of flood4 ,pools of water would often be left after such inundations1 and in these depressions silt would gradually accumulate1 partly carried in suspension by the river1 partly washed in by rain1 while drift-wood that found its way into these eddies1 and leaves blown into them from the trees and shrubs of the surrounding country1 would remain for some time afloat and would be the last of the detritus to sink to the bottom4 ,hence1 no doubt1 the carbonaceous character of the hardened silt in the upper part of each intercalation of sediment4 ,if we were to look upon the volcanic materials in the conglomerates as derived from the subrial disintegration of the fields of basalt1 we should be compelled to admit a very large amount of erosion of the surface of the volcanic plain during the period when the river flowed over that tract4 ,it would be necessary to suppose not only that there was a considerable rainfall1 but that the differences of temperature1 either from day to night1 or from summer to winter1 were so great as to split up the lavas at the surface1 in order to provide the river with the blocks which it has rolled into rounded boulders4 ,i do not think1 however1 that such a deduction would be sound4 ,if we compare the materials that have filled up the large eruptive vent at the east end of ,canna 7to be afterwards described7 with the great majority of the blocks in the coarse conglomerates1 we cannot fail to note their strong resemblance4 ,the abundance of lumps of slaggy lava in the river-shingle corresponds with their predominance in the agglomerate of the vent4 ,the boulders of basalt1 dolerite1 and andesite which crowd the conglomerates need not have been derived from the action of atmospheric waste on the lava-fields1 but might quite well have been mainly supplied by the demolition of volcanic cones of fragmental materials4 ,illustration3 ,fig4 #bgb4--,view of the ,dn ,beag1 ,sanday1 seen from the south4 7,from a ,photograph by ,miss ,thom47 ,that such has really been the chief source of the blocks in the conglomerates ,i cannot doubt4 ,at the east end of ,canna we actually detect a volcanic cone1 partly washed down and overlain by a pile of river-shingle4 ,there were probably many such mounds of slag and stones along lines of fissure all over the lava-fields4 ,the river in its winding course might come upon one cone after another1 and during times of flood1 or when its waters burst through any temporary barrier created by volcanic operations it would attack the slopes of loose material and sweep their detritus onward4 ,at the same time1 the current would carry forward its own natural burden of far-transported sediment1 and hence on its flood-plains1 buried and preserved under sheets of basalt1 we find abundant pebbles of the old ,highland rocks which it had borne across the whole breadth of the basaltic lowland4 ,but the destruction of volcanic cones was probably not the only source of the basaltic detritus in the conglomerates of ,canna and ,sanday4 ,i have shown that these conglomerates pass laterally into tuffs1 and are sometimes underlain1 sometimes overlain1 with similar material4 ,it is quite obvious that their deposition was contemporaneous with volcanic action in the immediate neighbourhood1 and that at least part of their finer sediment was obtained directly from volcanic explosions4 ,in wandering over the coast-sections of these coarse deposits1 ,i have been impressed with the enormous size of many of the stones1 their resemblance to the ejected blocks of the agglomerate1 and the distinction that may sometimes be made with more or less clearness between their rather angular forms and the more rounded and somewhat water-worn aspect of the other boulders4 ,it seems to me not improbable that some of the remarkably coarse masses of unstratified conglomerate in ,canna ,harbour consist largely of ejected blocks from the adjacent vent4 ,illustration3 ,fig4 #bgc4--,view of ,dn ,beag1 ,sanday1 from the north4 ,the island of ,rum in the distance4 7,from a ,photograph by ,miss ,thom47 ,the only instance which ,i have observed of erosion of the basalt contemporaneous with the operations of the river that spread out this conglomerate is to be found in the striking stack of ,dn ,beag already alluded to4#bea ,this extraordinary monument of geological history forms an outlying obelisk which rises from the platform of the shore to a height of about #gj feet4 ,seen from the south-west1 it appears to consist entirely of bedded basalt resting on some stratified tuff and shale which intervene between these lavas and that of the broad platform of basalt on which the obelisk stands4 ,on that side it presents no essential difference from the structure of the ,dn ,mr to the west1 save that the lower conglomerate of that outlier is here represented by fine sediment1 and the upper conglomerate is wanting4 ,the general aspect of this south-western front of the stack is shown in ,fig4 #bgb4 ,if1 however1 we approach the rock from the coast-gully to the north1 we form a very different impression of its structure4 ,it then appears to consist chiefly of conglomerate with a capping of basalt on the top 7,fig4 #bgc74 ,not until a close scrutiny is made of the eastern and western faces of the column do the true structure and history of this singular piece of topography become apparent4 ,footnote #bea3 ,this pinnacle of rock is referred to by ,macculloch in his account of ,canna1 and is figured in ,plate xix4 ,fig4 #c of his work already cited4 ,but neither his description nor his drawing conveys any idea of the real structure of the rock4 ,illustration3 ,fig4 #bgd4--,section of eastern front of ,dn ,beag4 a1 ,very shaggy amygdaloidal basalt2 b1 shales and tuff2 c1 slaggy and jointed basalts2 d1 conglomerate2 e1 prismatic basalt4 ,the dotted lines indicate the supposed form of the ravine4 ,on the eastern front1 the section represented in ,fig4 #bgd is exposed4 ,at the bottom1 forming the pediment of the column1 lies a sheet of slaggy and vesicular or amygdaloidal basalt 7a71 which shelves gently in a south-westerly direction into the sea4 ,the lowest band 7b7 in the structure of the stack is a thin group of lilac1 brown1 and green shale and volcanic mudstone or tuff1 which encloses pieces of coniferous wood1 and becomes markedly carbonaceous in its uppermost layers4 ,above these strata on the south front comes the pile of bedded basalts 7c7 with their slaggy lower and upper surfaces4 ,but as we follow them round the east side1 we find them to be abruptly cut off by a mass of conglomerate 7d74 ,that the vertical junction-line is not a fault is speedily ascertained4 ,the lower platform of slaggy basalt runs on unbroken under both shales and conglomerate4 ,moreover1 the line of meeting of this conglomerate with the basalts that overlie the shales is not a clean-cut straight wall1 but displays projections and recesses of the igneous rocks1 round and into which the materials of the conglomerate have been deposited4 ,the pebbles may be seen filling up little crevices1 passing under overhanging ledges of the basalts1 and sharply truncating lines of scoriaceous structure in these rocks4 ,the same relations may be observed on the west front of the stack4 ,there the ashy shales and tuffs are sharply cut out by the conglomerate1 which wraps round and underlies a projecting cornice of the slaggy bottom of the basalt that rests on the stratified band 7,fig4 #bge74 ,illustration3 ,fig4 #bge4--;,enlarged ,section on the western side of ,dn ,beag4 a1 amygdaloid2 b1 tuff2 c1 ashy shales2 d1 layer of coaly shale2 e1 amygdaloidal basalts conglomerate4 ,the conglomerate is rudely stratified horizontally1 its bedding being best shown by occasional partings of greenish sandstone4 ,it consists of well-rounded1 polished1 and water-worn stones1 chiefly of members of the volcanic series--basalts1 and dolerites1 both compact and amygdaloidal or slaggy--but with a conspicuous admixture of ,torridon ,sandstone1 gneiss1 grey granite1 grit and different schists4 ,the coarsest part of the deposit lies toward the bottom where the volcanic blocks1 some of them being six and eight feet in diameter1 may have originally fallen from the basalts against which the conglomerate now reposes4 ,the far-transported stones are also of considerable size1 pieces of granite and gneiss frequently exceeding a foot in length4 ,the well-rounded pebbles of foreign materials have been washed into the interstices between the large volcanic blocks4 ,it is1 ,i think1 tolerably clear that the wall of basalt against which this conglomerate has been laid down is one of erosion4 ,the beds of basalt have here been trenched by some agent which has likewise scooped out the soft underlying shales1 and even cut them away from under their protecting cover of basalt4 ,there can be little hesitation in regarding this agent as a water-course1 which for some considerable interval of time continued to dig its channel through the hard basalts4 ,there is not room enough between the basalt-wall of ,dn ,beag and the opposite cliffs of the shore 7where no trace of this conglomerate is to be seen7 for any large stream to have found its way4 ,i do not therefore seek to identify this relic of an ancient waterway with the channel of the main river which deposited the conglomerate bands of ,canna and ,sanday4 ,more probably it was either a mere torrential chasm1 or a tributary stream draining a certain part of the volcanic plateau and allowed to retain its channel long enough to be able to erode it to a depth of nearly #ej feet4 ,erosion had reached down through the underlying tuffs to the slaggy basalt below1 but before it had made any progress in that sheet its operations were brought to an end at this locality by the floods that swept in the coarse shingle1 and by the subsequent stream of basalt of which a mere outlying fragment now forms the upper third of the stack 7e1 ,fig4 #bgd74 ,that the ravine or gully of ,dn ,beag probably lay within the reach of the floods of the main river1 may be inferred from the number and size of the far-transported rocks in its conglomerate4 ,it was filled up gradually1 but the conditions of deposition remained little changed during the process1 except that the largest blocks of rock were swept into the chasm in the earlier part of its history1 while much smaller and more water-worn shingle were introduced towards the close4 ,denudation1 which has performed such marvels in the topography of the ,west of ,scotland since older ,tertiary time1 has here obliterated every trace of this ancient gully1 save the little fragment of one of the walls which survives in the stack of ,dn ,beag4 ,when in the course of centuries this picturesque obelisk shall have yielded to the action of the elements1 the last leaflet of one of the most interesting chapters in the geological history of the ,inner ,hebrides will have been destroyed4 ,the question naturally arises--,what was the subsequent history of the river which has left so many records of its floods entombed among the basalts of ,canna and ,sanday8 ,in particular1 can any connection be traced or plausibly conjectured between it and the river-bed preserved under the ,scuir of ,eigg8 ,to this question ,i shall return after the evidence for the existence and date of the latter stream has been laid before the reader4 ,in the chain of the ,inner ,hebrides1 broken as it is in outline and varied in its types of scenery1 there is no object more striking than the island of ,eigg4 ,though only about five miles long and from a mile and a half to three miles and a half broad1 and nowhere reaching a height of so much as #acjj feet1 this little island1 from the singularity of one feature of its surface1 forms a conspicuous and familiar landmark4 ,viewed in the simplest way1 ,eigg may be regarded as consisting of an isolated part of the basaltic plateau which1 instead of forming a rolling tableland or a chain of hills with terraced sides1 as in ,antrim1 ,mull and ,skye1 has been so tilted that1 while it caps a lofty cliff about #ajjj feet above the waves at the north end1 it slopes gently along the length of the island to the south end4 ,in its southern half1 however1 the ground rises1 owing to the preservation of an upper mass of lavas1 which denudation has removed from the northern half4 ,on this thicker part of the plateau stands the distinguishing feature of the island1 the strange fantastic ridge of the ,scuir1 which1 seen from the north or south1 looks like a long steep hill-crest1 ending in a sharp precipice on the east4 ,viewed from the east1 this precipice is seen to be the end of a huge mountain-wall1 which rises vertically above the basalt-plateau to a height of more than #cej feet4 ,the accompanying map 7,fig4 #bgf7 shows that the ridge of the ,scuir corresponds with the area occupied by a mass of pitchstone1 and that while the basaltic rocks cover the whole of the rest of the southern half of the island1 they gradually rise towards the north1 successive members of the ,jurassic series making their appearance until1 at the cliffs of ,dunan ,thalasgair1 the latter cover the greater part of the surface1 and leave the volcanic rocks as a mere stripe capping the cliffs4 ,in the section 7,fig4 #bgg7 the general structure of the island is represented4 ,illustration3 ,fig4 #bgf4--;,geological ,map of the ,island of ,eigg4 ,p1 ,pitchstone-lava of the ,scuir2 ,r1 old river gravel under pitchstone2 p p1 small veins of ,pitchstone2 b b1 dykes1 veins and sheets of intrusive basalt2 the short black lines running north-west and south-east are basalt dykes2 f f1 granophyre sills2 ,d1 bedded basalts with occasional tuffs2 ,f1 andesite2 #a1 #b1 #c1 #d1 clays1 shales1 sandstones1 limestones1 etc4 7,jurassic72 xx1 ,loch ,beinn ,tighe2 x1 ,loch a ,bhealaich4 -- ,general dip of the rocks4 ,illustration3 ,fig4 #bgg4--,section of the geological structure of the ,island of ,eigg4 ,p1 ,pitchstone-lava of ,scuir2 c1 ancient river-gravel2 p p1 pitchstone veins2 f f1 intrusive granophyre1 etc42 b b1 dolerite and basalt dykes and veins2 ,b1 intrusive dolerite and basalt-sheets2 ,d1 bedded dolerites and basalts2 ,f1 andesite bed2 #a-d1 ,jurassic rocks4 ,in ,eigg the fragment of the basalt-plateau which has been preserved1 rests unconformably on successive platforms of the ,jurassic formations4 ,its component sheets of lava rise in cliffs around the greater part of the island4 ,as they dip gently southwards their lower members are seen along the northern and eastern shores1 while on the south-west side their higher portions are exposed in the lofty precipices which there plunge vertically into the sea4 ,the total thickness of the volcanic series may here be about #aajj feet4 ,the rocks consist of the usual types--black1 fine-grained1 columnar and amorphous basalts1 more coarsely crystalline dolerites1 dull earthy amygdaloids with red partings1 and occasional thin bands of basalt-conglomerate or tuff4 ,the individual beds range in thickness from #bj to #ej or #fj feet4 ,though they seem quite continuous when looked at from the sea1 yet1 on closer examination1 they are found not unfrequently to die out1 the place of one bed being taken by another1 or even by more than one1 in continuation of the same horizon4 ,the only marked petrographical variety which occurs among them is a light-coloured band which stands out conspicuously among the darker ordinary sheets of the escarpment on the east side of the island4 ,the microscopic characters of this rock show it to belong to the same series of highly felspathic1 andesitic1 or trachitic lavas as the pale group of ,ben ,more1 in ,mull4 ,it is strongly vesicular1 and the cells are in some parts so flattened and elongated as to impart a kind of fissile texture to the rock4 ,there can be no doubt that this band is a true lava1 and that it was poured out during the accumulation of the basalt-plateau4 ,it supplies an interesting example of the intercalation of a lighter and less basic lava among the ordinary heavy basic basalts and dolerites4 ,that feature of the island of ,eigg which renders it so remarkable and conspicuous an object on the west coast is the long ridge of the ,scuir4 ,rising gently from the valley which crosses the island from ,laig ,bay to the ,harbour1 the basaltic plateau ascends south-westwards in a succession of terraces1 until along its upper part it forms a long crest1 from #ijj to #ajjj feet above the sea1 to which it descends on the other or south-west side1 first by a sharp slope1 and then by a range of precipices4 ,along the watershed of this crest runs1 in a graceful double curve1 the abrupt ridge of the ,scuir1 terminating on the north-west at the edge of the great sea-cliff 7#ige feet71 and ending off on the south-east in that strange well-known mountain-wall 7#abgb feet high7 which rises in a sheer cliff nearly #cjj feet above the basalt-plateau on the one side and more than #djj feet on the other 7,fig4 #bgh74 ,the total length of the ,scuir ridge is two miles and a quarter1 its greatest breadth #aebj1 its least breadth #cej feet4 ,its surface is very irregular1 rising into minor hills and sinking into rock-basins1 of which nine are small tarns1 besides still smaller pools1 while six others1 also filled with water1 lie partly on the ridge and partly on the basaltic plateau4 ,no one1 indeed1 who looks on the ,scuir from below1 and notes how evenly it rests upon the basalt-plateau1 would be prepared for so rugged a landscape as that which meets his eye everywhere along the top of the ridge4 ,two minor arms project from the east side of the ridge2 one of these forms the rounded hill called ,beinn ,tighe 7#ifh feet71 the other the hill of ,a chor ,bheinn4 ,illustration3 ,fig4 #bgh4--,view of the ,scuir of ,eigg from the east4 ,singular as the ,scuir of ,eigg is1 regarded merely as one of the landmarks of the ,hebrides1 its geological history is not less peculiar4 ,the natural impression which arises in the mind when this mountain comes into view for the first time is1 that the huge wall is part of a great dyke or intrusive mass which has been thrust through the older rocks4#beb ,it was not until after some time that the influence of this first impression passed off my own mind1 and the true structure of the mass became apparent4 ,footnote #beb3 ,hay ,cunningham remarks3--,in regard to the relations of the pitchstone-porphyry of the ,scuir and the trap-rocks with which it is connected1 it can1 after a most careful examination around the whole mass1 be confidently asserted that it exists as a great vein which has been erupted through the other ,plutonic rocks--thus agreeing in age with all the other pitchstones of the island4 ,macculloch leaves us to infer that he regarded the rock of the ,scuir to be regularly interstratified with the highest beds of the dolerite series 7,western ,isles1 i4 p4 #ebb74 ,hugh ,miller speaks of the ,scuir of ,eigg as resting on the remains of a prostrate forest4--,cruise of the ,betsy1 p4 #cb4 ,the ridge of the ,scuir1 presenting as it does so strong a topographical contrast to the green terraced slopes of the plateau-basalts on which it rests1 consists of some very distinct bands of black and grey lava1 long known as pitchstone-porphyry4 ,to the nature and history of these rocks ,i shall return after we have considered a remarkable bed of conglomerate which lies below them4 ,on the lower or southern side of the ridge the bottom of the pitchstone1 dipping into the hill1 is exposed on the roof of a small cave where the ends of its columns form a polygonal reticulation4 ,it is there seen to repose upon a bed of breccia or conglomerate1 having a pale-yellow or grey felspathic matrix like the more decomposing parts of the grey devitrified parts of the pitchstone4 ,through this deposit are dispersed great numbers of angular and subangular pieces of pitchstone1 some of which have a striped texture4 ,fragments of basalt1 red 7,torridon7 sandstone1 and other rocks are rare2 and the bed suggests the idea that it is a kind of brecciated base or floor of the main pitchstone mass4 ,a similar rock is found along the bottom of the pitchstone on both sides of the ridge 7c1 in ,fig4 #bgi74 ,here and there where this breccia is only a yard or two in thickness1 it consists of subangular fragments of the various dolerites and basalts of the neighbourhood1 together with pieces of red sandstone1 quartzite1 clay-slate1 etc4 ,the matrix is in some places a mass of hard basalt debris2 in others it becomes more calcareous1 passing into a sandstone or grit in which chips and angular or irregular-shaped pieces of coniferous wood are abundant4#bec ,a little further east1 beyond the base of the ,scuir1 a patch of similar breccia is seen1 but with the stones much more rounded and smoothed4 ,this outlier rests against the denuded ends of the basalt-beds forming the side of the hill4 ,its interest arises from the evidence it affords of the prolongation of the deposit eastward1 and consequently of the former extension of the precipice of the ,scuir considerably beyond its present front4 ,footnote #bec3 ,the microscopic structure of this wood was briefly described by ,witham 7,fossil ,vegetables1 p4 #cg71 and two magnified representations were given to show its coniferous character4 ,lindley and ,hutton further described it in their ,fossil ,flora1 naming it ,pinites eiggensis1 and regarding it as belonging to the ,oolitic series of the ,hebrides--an inference founded perhaps on the erroneous statement of ,witham to that effect4 ,william ,nicol corrected that statement by showing that the wood-fragments occurred1 not among the lias rocks1 but among the debris of the pitchstone 7,edin4 ,new ,phil4 ,journal1 xviii4 p4 #aed74 ,hay ,cunningham1 in the paper already cited1 states that the fossil wood really lies in the pitchstone itself6 ,the actual position of the wood1 however1 in the breccia and conglomerates underlying the pitchstone is beyond all dispute4 ,i have myself dug it out of the bed4 ,the geological horizon assigned to this conifer1 on account of its supposed occurrence among ,oolitic rocks1 being founded on error1 no greater weight can be attached to the identification of the plant with an ,oolitic species4 ,our knowledge of the specific varieties of the microscopic structure of ancient vegetation is hardly precise enough to warrant us in definitely fixing the horizon of a plant merely from the examination of the minute texture of a fragment of its wood4 ,from the internal organization of the ,eigg pine1 there is no evidence that the fossil is of ,jurassic age4 ,from the position of the wood above the dolerites and underneath the pitchstone of the ,scuir it is absolutely certain that the plant is not of ,jurassic but of ,tertiary date4 ,it is at the extreme north-western extremity of the pitchstone ridge1 however1 that the most remarkable exposure of this intercalated detrital band is now to be seen4 ,sweeping along the crest of the plateau the ridge reaches the edge of the great precipice of ,bideann ,boidheach1 by which its end is truncated1 so as to lay open a section of the gravelly deposit along which the pitchstone flowed4 ,the accompanying diagram 7,fig4 #bgi7 represents the natural section there exposed4 ,rising over each other in successive beds1 with a hardly perceptible southerly dip of #b1 the sheets of basalt form a mural cliff about #gjj feet high4 ,the bedded character of these rocks and their alternations of compact1 columnar1 amorphous and amygdaloidal beds are here strikingly seen4 ,they are traversed by veins and dykes of an exceedingly close-grained1 sometimes almost flinty1 basalt4 ,but the conspicuous feature of the cliff is the hollow which has been worn out of these rocks1 and which1 after being partially filled with coarse conglomerate1 has been buried under the huge pitchstone mass of the ,scuir4 ,the conglomerate consists of water-worn fragments1 chiefly of dolerite and basalt1 but with some also of the white ,jurassic sandstones1 imbedded in a compacted sand derived from the waste of the older volcanic rocks4 ,the grey devitrified bands in the pitchstone1 so conspicuous at the east end of the ,scuir1 here disappear and leave the conglomerate covered by one huge overlying mass of glassy pitchstone4 ,illustration3 ,fig4 #bgi4--,natural ,section at the ,cliff of ,bideann ,boidheach1 north-west end of the ,scuir of ,eigg4 a a1 ,bedded dolerites and basalts2 b1 basalt dykes and veins2 c1 ancient river-bed filled with conglomerate2 p1 pitchstone of the ,scuir4 ,if any doubt could arise as to the origin of the mass of detritus exposed under the pitchstone at the east end of the ,scuir it would be dispelled by the section at the west end1 which shows with unmistakable clearness that the conglomerate is a fluviatile deposit and lies in the actual channel of the ancient river which was eroded out of the basalt plateau1 and was subsequently sealed up by streams of pitchstone-lava4 ,an examination of the fragments of rock found in the conglomerate affords here1 as in ,canna and ,sanday1 some indication of the direction in which the river flowed4 ,the occurrence of pieces of red sandstone1 which no one who knows ,west-,highland geology can fail to recognize as of ,torridonian derivation1 at once makes it clear that the higher grounds from which they were borne probably lay to the north or north-east4 ,the fragments of white sandstone may also have been derived from the same quarter1 for the thick ,jurassic series of ,eigg once extended further in that direction4 ,the pieces of quartzite and clay-slate bear similar testimony to an eastern or north-eastern source4 ,in short1 there seems every probability that this old ,tertiary river flowed through a forest-clad region1 of which the red ,torridon mountains of ,ross-shire1 the white sandstone cliffs of ,raasay and ,skye1 and the quartzite and schist uplands of ,western ,inverness-shire are but fragments1 that it passed over a wide and long tract of the volcanic plateau1 and continued to flow long enough to be able to carve out for itself a channel on the surface of the basalt4 ,its course across what is now the island of ,eigg took a somewhat north-westerly direction1 probably guided by inequalities on the surface of the lava-plain4 ,it is there marked by the winding ridge of the ,scuir1 the pitchstone of which flowed into the river-bed and sealed it up4 ,several minor spurs1 which project from the eastern side of the main ridge1 show the positions of small tributary rivulets that entered the principal channel from the slopes of the basaltic tableland4 ,one of these1 on the south-east side of the hill called ,corven1 must have been a gully in the basalt with a rapid or waterfall4 ,the pitchstone has flowed into it1 and some of the rounded pebbles that lay in the channel of this vanished brook may still be gathered where the degradation of the pitchstone has once more exposed them to the light4 ,that the ,eigg river here flowed in a westerly direction may be inferred from the angle at which the beds of the small tributaries meet the main stream1 and also from the fact that the old river-bed at the east end of the ,scuir is considerably higher than at the west end4 ,several features in the geological structure of this locality serve to impress on the mind the great lapse of time represented by the erosion of the river-channel of ,eigg4 ,thus at the narrowest point of the pitchstone ridge1 near the little ,loch a' ,bhealaich1 the bottom of the glassy lava is about #bjj feet above its base on the south side1 so that the valley cut out of the plateau-basalts must have been more than #bjj feet deep4 ,even the little tributaries had cut ravines or caons in the basalts before the ground was buried under the floods of pitchstone4 ,in the most northerly spur of the ridge1 for example1 the hill of ,beinn ,tighe1 which represents one of these tributaries1 shows a considerable difference between the level of the bottom of the pitchstone on the east and west sides4 ,again1 all along the ridge of the ,scuir1 the basalt-dykes are abruptly cut off at the denuded surface on which the pitchstone rests4 ,this feature is conspicuously displayed on the great sea-wall at the west end 7,fig4 #bgi74 ,the truncation of the dykes demonstrates that a considerable mass of material must have been eroded before these lava-filled fissures could be laid bare at the surface4 ,and the removal of this material shows that the denudation must have been continued for a long period of time4 ,the river-channel of ,eigg1 since it was eroded long after the cessation of the outflows of basalt in the plateau of ,small ,isles1 must be much later in origin than those of ,canna and ,sanday which1 as we have seen1 were contemporaneous with the basalt-eruptions4 ,but the river that excavated the channels and deposited the gravels may have been the same in both areas4 ,in dealing with this subject1 though the evidence is admittedly scanty1 we are not left wholly to conjecture4 ,a consideration of the general topographical features of the wide region of the ,inner ,hebrides1 from the beginning of the volcanic period onward1 will convince us that1 in spite of the effects of prolonged basalt-eruptions1 the persistent flow of the drainage of the ,western ,highlands must have taken a westerly direction4 ,it was towards the west that the low grounds lay4 ,though the long and broad valley which stretched northwards from ,antrim1 between the line of the ,outer ,hebrides and the ,west of ,scotland1 was gradually buried under a depth of two or three thousand feet of lava1 the volcanic plain that overspread it probably remained even to the end lower than the mountainous ,western ,highlands4 ,hence the rivers1 no matter how constantly they may have had their beds filled up and may have been driven into new channels1 would nevertheless always seek their way westwards into the ,atlantic4 ,on ,canna and ,sanday the traces of a river are preserved which poured its flood-waters across the lava-fields in that part of the volcanic region1 while streams of basalt were still from time to time issuing from vents and fissures4 ,not more than fourteen miles to south-east stands the ,scuir of ,eigg1 with its buried river-channel and its striking evidence that there1 also1 a river flowed westwards1 but at a far later time1 when the basalt-eruptions had ceased and the volcanic plain had been already deeply trenched by erosion1 yet before the subterranean fires were finally quenched1 as the pitchstone of the ,scuir abundantly proves4 ,when one reflects upon the enormous denudation of this region1 to which more special reference will be made in the sequel1 one is not surprised that many connecting links should have been effaced4 ,the astonishment rather arises that so continuous a story can still be deciphered4 ,even1 however1 had the original record been left complete1 it would have been exceedingly difficult to trace the successive mutations of a river-channel during long ages of volcanic eruptions4 ,such a channel would have been concealed from view by each lava-stream that poured into it1 and would not have been again exposed save by the very process of erosion that destroys while it reveals4 ,while1 therefore1 there is not and can never be any positive proof that in the fluviatile records of ,canna1 ,sanday and ,eigg successive phases are registered in the history of one single stream1 ,i believe that this identity is highly probable4 ,it was a river which seems to have risen among the mountains of ,western ,inverness-shire1 and it had doubtless already taken its course to the sea before any volcanic eruptions began4 ,it continued to flow westwards across the lava-floor that gradually spread over the plains4 ,its channel was constantly being filled up by fresh streams of basalt or deflected by the uprise of new cinder-cones4 ,but1 fed by the ,atlantic rains1 it maintained its seaward flow until the general subsidence which carried so much of the volcanic plain below the sea4 ,yet the higher part of this ancient water-course is no doubt unsubmerged1 still traversing the schists of the ,western ,highlands as it has done since older ,tertiary time4 ,it may1 perhaps1 be recognized in one of the glens which carry seaward the drainage of the districts of ,morar1 ,arisaig1 or ,moidart4 ,illustration3 ,fig4 #bhj4--,view of the ,scuir of ,eigg from the ,south4 ,let us now turn to the remarkable lava which has sealed up the river-channel of ,eigg1 and of which the remaining fragment stands up as the great ridge of the ,scuir4 ,this rock presents characters that strongly distinguish it from the surrounding basalts4 ,it is not one single uniform mass1 but consists of a number of distinct varieties1 some of which are a volcanic glass1 while others are a grey porphyry1 or devitrified pitchstone4 ,these are arranged in somewhat irregular1 but well-marked1 and1 in a general sense1 horizontal sheets4 ,on the great eastern terminal gable of the ,scuir this bedded structure is not clearly displayed1 for the cliff seems there to be built up of one homogeneous mass1 save a markedly columnar band that runs obliquely up the base of the precipice 7,fig4 #bgh74 ,if1 however1 the ridge is looked at from the south1 the truly bedded character of its materials becomes a conspicuous feature4 ,along the cliffs on that side the two varieties of rock are strongly distinguished by their contrasting colour and mode of weathering1 the sombre-hued pitchstone standing up in a huge precipice striped with columns1 and barred horizontally with bands of the pale-grey porphyry1 which1 from its greater proneness to decay1 seems sunk into the face of the cliff4 ,at the south-east end of the ridge the bedding is especially distinct4 ,west of the precipices1 to the south of the ,loch a' ,bhealaich1 the dark pitchstone which forms the main mass is divided by two long parallel intercalations of grey rock1 and two other short lenticular seams of the same material 7see ,figs4 #bhj1 #bha74 ,it is clear from these features1 which are not seen by most travellers who pass ,eigg in the tourist-steamer that the ,scuir is in no sense of the word a dyke4 ,but although the ,scuir is thus a bedded mass1 the bedding is far different from the regularity and parallelism of that which obtains among the bedded basalt-rocks below4 ,even where no intervening porphyry occurs1 the pitchstone can be recognized as made up of many beds1 each marked by the different angle at which its columns lie4 ,and when the porphyry does occur and forms so striking a division in the pitchstone1 its beds die out rapidly1 appearing now on one horizon1 now on another1 along the face of the cliffs1 and thickening and thinning abruptly in short distances along the line of the same bed4 ,perhaps the best place for examining these features is at the ,bhealaich1 the only gully practicable for ascent or descent1 at the south-eastern face of the ridge4 ,illustration3 ,fig4 #bha4--,view of the ,scuir of ,eigg from the ,south-west of the ,loch a' ,bhealaich1 showing the bedded character of the mass4 ,by much the larger part of the mass of the ,scuir consists of vitreous material4 ,as a rule this rock is columnar1 the columns being much slimmer and shorter than those of the basalt-rocks4 ,they rise sometimes vertically1 and often obliquely1 or project even horizontally from the face of the cliff4 ,they are seldom quite straight1 but have a wavy outline2 and when grouped in knolls here and there along the top of the ridge they remind one of gigantic bunches of some of the ,palozoic corals1 such as ,lithostrotion4 ,in other cases they slope out from a common centre1 and show an arrangement not very unlike that of a ,highland peat-stack4 ,the pitchstone of the ,scuir differs considerably in petrographical character from other pitchstones of the island which occur in dykes and veins4 ,its base is of a velvet-black colour1 and is so much less vitreous in aspect than ordinary pitchstone as to have been described by ,jameson and later writers as intermediate between pitchstone and basalt4#bed ,a chemical analysis of the rock by ,mr4 ,barker ,north1#bee gave the following composition3-- ,silica #feha ,alumina #adja ,ferric oxide #ddc ,lime #bja ,magnesia #jhi ,soda #dae ,potash #fjh ,loss in ignition #bgj ------ #ajjjh ,footnote #bed3 ,mineralogy of the ,scottish ,isles1 vol4 ii4 p4 #dg4 ,see also ,macculloch1 ,western ,isles1 vol4 i4 p4 #eba1 and ,hay ,cunningham1 ,mem4 ,wern4 ,soc4 vol4 viii4 p4 #aee4 ,footnote #bee3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xlvi4 7#ahij71 p4 #cgi4 ,the grey devitrified bands1 which occur as a subordinate part of the mass of the ,scuir ridge1 are usually somewhat decomposed4 ,where a fresh fracture is obtained1 the material shows a fine-grained1 sometimes almost flinty1 grey felsitic base1 containing clear granules of quartz1 and facets of glassy felspar4 ,in some places the rock is strongly porphyritic4 ,examined under the microscope it presents a more thoroughly devitrified groundmass1 with the minutest depolarizing microlites1 large porphyritic crystals of plagioclase and sanidine1 grains of augite1 and sometimes exceedingly abundant particles of magnetite4#bef ,footnote #bef3 ,the microscopic structure of the identical pitchstone of ,hysgeir is given on p4 #bdg4 ,illustration3 ,fig4 #bhb4--,section at the base of the ,scuir of ,eigg 7east end74 ,although the line of separation between the grey dull felsitic sheets and the more ordinary glassy pitchstone is usually well defined1 the two rocks may be observed to shade into each other in such a manner as to show that the lithoid material is only a devitrified and somewhat decomposed condition of the glassy rock4 ,this connection is particularly to be observed under the precipice at the east end of the ,scuir4 ,at that locality the pitchstone is underlain by a very hard flinty band1 varying in colour from white through various shades of flesh-colour and brown into black1 containing a little free quartz and crystals of glassy felspar4 ,where it becomes black it passes into a rock like that of the main mass of the ,scuir4 ,such vitreous parts of the bed lie as kernels in the midst of the more lithoid and decomposed rock4 ,the lower six feet of the porphyry are white and still more decomposed4 ,the relations of this mass are represented in ,fig4 #bhb1 where the basalt-rocks of the plateau 7a7 are shown to be cut through by basalt dykes 7b b71 and overlain by the porphyry 7c7 and the pitchstone 7d74 ,in the porphyry are shown several pitchstone kernels 7p1 p74 ,it is deserving of remark also that in different parts of the ,scuir1 particularly along the north side1 the bottom of the pitchstone beds passes into a dull grey earthy lithoid substance1 like that now under description4 ,the bedded character of the rock of the ,scuir and the well-marked lithological distinction between its several component sheets show the lava to have been the product of a number of separate outflows that found their way one after another into the river-valley1 which was the lowest ground in the vicinity of the active vent4 ,there can be little doubt1 ,i think1 that the lava flowed down the valley4 ,its successive streams are still inclined from east to west4 ,the vent of eruption1 therefore1 ought to be looked for towards the east4 ,nowhere within the ,tertiary volcanic region is there any boss of pitchstone or any mass the shape or size of which is suggestive of this vent4 ,in the island of ,eigg no boss of any kind exists1 save those of granophyric porphyry to be afterwards referred to4 ,but none of these affords any satisfactory links of connection with the rock of the ,scuir4 ,more probably the vent lay somewhere to the east on ground now overflowed by the sea4 ,the pitchstone veins of ,eigg may represent some of the subterranean extrusions from the same volcanic pipe1 and if so1 its site could not be far off4 ,the rock of the ,scuir of ,eigg has a special importance in the history of the volcanic plateaux4 ,it is1 so far as we know1 the latest of all the superficial lavas of ,britain4#beg ,from the basalts on which it rests it was separated by an enormous interval of time1 during which these older lavas were traversed by dykes and were worn down into valleys4 ,its presence shows that long after the basalts of ,small ,isles had ceased to be erupted1 a new outbreak of volcanic activity took place in this district1 when lavas of a more acid composition flowed out at the surface4 ,whether this outburst was synchronous with the appearance of the great granophyric protrusions of the ,inner ,hebrides1 or with the still later extravasation of pitchstone dykes1 can only be surmised4 ,footnote #beg3 ,the rocks of ,beinn ,hiant in ,ardnamurchan have been claimed by ,professor ,judd as superficial lavas4 ,for reasons to be afterwards given 7p4 #cah7 ,i regard them as intrusive sheets4 ,professor ,cole believes the rhyolites and pitchstones of ,tardree to be probably evidence of a volcano later than the basalts of ,antrim4 ,as ,i have not been able to detect any actual proofs of superficial outflow there1 ,i relegate the description of the rocks to a future chapter1 in which the acid protrusions will be discussed 7p4 #dbf74 ,when one scans the great precipice on the west side of ,eigg1 with its transverse section of the pitchstone-lava1 buried river-bed and basalt-plateau underneath1 there seems no chance of any further westward trace of the pitchstone being ever found4 ,the truncated end of the ,scuir looks from the top of the cliff out to sea1 and the progress of denudation might have been supposed to have effectually destroyed all evidence of the continuation of the rock in a westerly direction4 ,some years ago1 however1 my friend ,prof4 ,heddle1 while cruising among the ,inner ,hebrides1 landed upon the little uninhabited islet of ,hysgeir1 which1 some eighteen miles to the westward of ,eigg1 rises out of the open sea4 ,he at once recognized the identity of the rock composing this islet with that of the ,scuir1 and in the year #ahib published a brief account of this interesting discovery4#beh ,footnote #beh3 ,appendix ,c to ,a ,vertebrate ,fauna of ,argyle and the ,inner ,hebrides1 by ,messrs4 ,j4 ,a4 ,harvie-,brown and ,thomas ,e4 ,buckley1 p4 #bdh4 ,i have myself been able to land on ,hysgeir in two successive summers1 and can entirely confirm ,prof4 ,heddle's identification4 ,the islet stands on the eastern edge of the submarine ridge which1 running in a north-easterly direction1 culminates in the island of ,canna4 ,hysgeir is a mere reef or skerry1 of which the top rises only #ch feet above the ,ordnance datum-level4 ,its surface is one of bare rock1 save where a short but luxuriant growth of grasses has found root on the higher parts of two or three of its ridges1 and on the old storm-beach of shingle which remains on the summit4 ,the rock undulates in long low swells1 that run in a general direction #bj to #de west of north1 and are separated by narrow channels or hollows4 ,the place is a favourite haunt of gulls1 terns1 eider-ducks and grey seals1 and is used by the proprietor of ,canna for the occasional pasturage of sheep or cattle4 ,so numerous are the sea-fowl during the breeding-season that the geologist1 intent upon his own pursuits1 may often tread on their nests unawares1 while he is the centre of a restless circle of white wings and anxious cries4 ,the pitchstone of ,hysgeir1 like that of ,eigg1 is columnar1 the columns being irregularly polygonal and varying from three to ten inches in diameter4 ,they are packed so close together that the domes of rock on which their ends appear look like rounded masses of honeycomb4 ,they may here and there be observed to be arranged radially with their ends at right angles to the curved exterior of the ridges1 as if this external surface represented the original form of the cooled pitchstone1 and were not due to mere denudation4 ,there can be no doubt1 however1 that the island has been well ice-worn4 ,at the north-west promontory a beautiful example of fan-shaped grouping of columns may be observed on a face of rock which descends vertically into the sea4 ,here1 too1 is almost the only section on which the sides of the columns may be examined1 for1 as a rule1 it is merely their ends on the rounded domes which are to be observed1 and which everywhere slip under the waves4 ,the columns in a cliff from #ae to #bj feet high show the slightly wavy1 starch-like arrangement so often to be met with among the plateau-basalts4 ,the rock presents a tolerably uniform texture throughout1 though in some parts it is blacker1 more resinous1 and less charged with porphyritic enclosures than in the general body of the rock4 ,large fresh felspars are generally scattered through it4 ,to the naked eye it reproduces every feature of the pitchstone of the ,scuir of ,eigg4 ,a microscopic examination completes our recognition of the identity of these two rocks4 ,mr4 ,harker has examined a thin slice prepared from the ,hysgeir pitchstone1 and remarks regarding it that the large felspars are not the only porphyritic element4 ,the microscope shows the presence also of smaller imperfect crystals of augite1 very faint green in the slice1 and small grains of magnetite4 ,the felspars have been deeply corroded by the enveloping magma1 and irregular included patches of the groundmass occupy nearly half the bulk of some of the crystals4 ,this latter feature is seen especially in some of the larger crystals1 which seem to be sanidine4 ,they are1 for the most part1 apparently simple crystals1 but in places there is a scarcely defined lamellar twinning1 or1 again1 small patches not extinguishing with the rest2 so that we are probably dealing with some perthitic intergrowth on a minute scale4#bei ,footnote #bei3 ,comp4 ,prof4 ,judd's remarks on the ,scuir of ,eigg rock1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xlvi4 7#ahij71 p4 #chj4 ,rather smaller felspar-crystals are rounded by corrosion1 but lack the inclusions of groundmass2 these have albite-and sometimes pericline-lamellation1 and may be referred to oligoclase-andesine4 ,the groundmass of the rock is a brown glass with perlitic cracks1 enclosing very numerous microlites of felspar about #jja inch in length #ffai4 ,the rock is probably to be regarded as a dacite rather than a rhyolite1 and thus agrees with ,mr4 ,barker ,north's analysis of the ,eigg pitchstone4#bfj ,footnote #bfj3 ,op4 cit4 p4 #cgi4 ,there is no trace of any conglomerate in situ like that under the ,scuir of ,eigg1 nor of any other rock1 aqueous or igneous4 ,as the pitchstone everywhere slips under the sea1 its geological relations are entirely concealed4 ,the great variety of materials met with in the form of boulders on the island is a testimony to the transport of erratics from the neighbouring islands and the mainland during the ,glacial ,period4 ,the most abundant rock in these boulders is ,torridon ,sandstone1 derived no doubt from the hills of ,rum1 but there occur also various kinds of schist1 gneisses1 quartzites1 granites1 porphyries1 probably from the west of ,inverness-shire1 as well as pieces of white sandstone1 probably ,jurassic1 which may have come from ,eigg4 ,that the pitchstone of ,hysgeir is a continuation of that of the ,scuir may be regarded as highly probable4 ,if not a continuation1 it must be another stream of the same kind1 and doubtless of the same date4 ,if it be regarded as probably a westward prolongation of the ,eigg rock1 and if it be about as thick as that mass at the west end of the ,scuir1 then its bottom lies #bjj or #cjj feet under the waves4 ,the river-channel occupied by the ,eigg pitchstone undoubtedly sloped from east to west4 ,the position of ,hysgeir1 #ah miles further west1 may indicate a further fall in the same direction at the rate of perhaps as much as #ce feet in the mile4#bfa ,unfortunately1 however1 as no trace of the river-bed can now be seen on this island1 any statement in regard to it must rest on mere conjecture4 ,footnote #bfa3 ,rep4 ,brit4 ,assoc4 #ahid1 p4 #fec4 ,although the question of the denudation of the basalt-plateaux since the close of the volcanic period will be the subject of a special chapter in a later part of this volume1 ,i cannot here refrain from calling attention to the pitchstone of ,eigg and ,hysgeir as one of the most impressive monuments of denudation to be found within the ,british ,isles4 ,though now so prominent an object in the ,west ,highlands1 this rock once occupied the bottom of a valley worn out of the basaltic tableland4 ,prolonged and stupendous denudation has destroyed the connection with its source1 has cut down its ends into beetling precipices1 has reduced the former surrounding hills into gentle slopes and undulating lowland1 and has turned the bottom of the ancient valley into a long1 narrow and high crest4 ,moreover1 we see that the erosion has not been uniform4 ,the great wall of the ,scuir does not stand fairly on the crest of the basalt-plateau but on the south side of it1 so that the southern half of the old valley1 with all its surrounding hills1 has been entirely cut away4 ,that subsidence has also come into play in the destruction of even the youngest parts of the volcanic plateaux will be more fully discussed in a later chapter4 ,i need only remark here that the submergence of ,hysgeir probably points to extensive depression of the land-surface on which the lavas were poured out4 ,,chapter ,,xxxix ,,the ,,basalt-,,plateaux ,,of ,,skye ,,and ,,of ,,the ,,faroe ,,isles iv4 ,,the ,,skye ,,plateau ,this largest and geologically most important of all the ,scottish plateaux comprises the island of ,skye1 at least as far south as ,loch ,eishort1 and the southern half of ,raasay1 but is shown by its sills to stretch as far as the ,shiant ,isles on the north1 and the ,point of ,sleat on the south 7see ,map ,,vi474 ,it may be reckoned to embrace an area of not less than #hjj square miles4 ,the evidence that its limits1 like those of the other plateaux1 are now greatly less than they originally were1 is abundant and impressive4 ,the truncated edges of its basalts1 rising here and there for a thousand feet as a great sea-wall above the breakers at their base1 and presenting everywhere their succession of level or gently inclined bars1 are among the most impressive monuments of denudation in this country4 ,but still more striking to the geologist is the proof1 furnished beyond the margins of the plateau1 that the ,jurassic and other older rocks there visible were originally buried deep under the basalt-sheets1 which have thus been entirely stripped off that part of the country4 ,throughout most of the district1 wherever the base of the basalts can be seen1 it is found to rest upon some member of the ,jurassic series1 but with a complete unconformability4 ,the underlying sedimentary strata had been dislocated and extensively denuded before the volcanic period began4 ,on the southern margin1 however1 the red 7,torridon7 sandstones emerge from under the basalts of ,loch ,scavaig1 and extending into the island of ,soay are prolonged under the sea into ,rum4 ,this ridge probably represents the range of the ancient high ground of the latter island already referred to4 ,nowhere are the distinctive topographical features and geological structure of the basalt-plateaux better displayed than in the northern half of the island of ,skye4 ,the green terraced slopes1 with their parallel bands of brown rock formed by the outcrop of the nearly flat basalt-beds1 rise from the bottoms of the valleys into flat-topped ridges and truncated cones 7,fig4 #bhc74 ,the hills everywhere present a curiously tabular form that bears witness to the horizontal sheets of rock of which they are composed4#bfb ,and along the sea-precipices1 each excessive sheet of basalt can be counted from base to summit1 and followed from promontory to promontory 7,figs4 #bhd1 #bhf74 ,in the district of ,trotternish1 the basalt hills reach a height of #bcfj feet4 ,further west1 the singular flat-topped eminences1 called ,macleod's ,tables 7,fig4 #bhc7 ascend to #afjj feet4 ,footnote #bfb3 ,these features are more fully described in my ,scenery of ,scotland1 #bnd edit 7#ahhg71 pp4 #gd1 #ade1 #baf4 ,illustration3 ,fig4 #bhc4--,terraced ,hills of ,basalt ,plateau 7,macleod's ,tables71 ,skye4 ,along the western side of ,skye1 the basalts descend beneath the level of the ,atlantic1 save at ,eist in ,duirinish1 where the ,secondary strata1 with their belt of intrusive sills1 rise from underneath them1 and at the ,sound of ,soa1 where they rest on the ,torridon ,sandstone4 ,along the eastern side1 their base runs on the top of the great ,jurassic escarpment1 whose white and yellow sandstones rise there1 and on the east side of ,raasay1 into long lines of pale cliffs4 ,to the south-east1 the regularity of the volcanic plateau is effaced1 as in ,mull and ,ardnamurchan1 by the protrusion of extensive masses of eruptive rocks constituting the ,cuillin and ,red ,hills1 east of which the basalts have been almost entirely removed by denudation1 so as to expose the older rocks which they once covered1 and through which the younger eruptive bosses made their way4 ,this is undoubtedly the most instructive district for the study of that late phase in the volcanic history of ,britain comprised in the eruptive bosses of basic and acid rocks4 ,the magnificent plateau of this island has been so profoundly cut down into glens and arms of the sea1 and its component layers are exposed along so many leagues of precipice1 that its structure is perhaps more completely laid open than that of any of the other ,tertiary volcanic areas in ,britain4 ,it is built up of a succession of basalts and dolerites of the usual types1 which still reach a thickness of more than #bjjj feet1 though in this instance1 also1 denudation has left only a portion of them1 without any evidence by which to reckon what their total original depth may have been4 ,in rambling over ,skye1 the geologist is more than ever struck with the remarkable scarcity and insignificance of the interstratifications of tuff or of any other kind of sedimentary deposit between the successive lava-sheets4 ,one of the thickest accumulations of volcanic tuff and conglomerate has already been referred to as occurring on the south side of ,portree ,harbour1 where it attains a depth of about #bjj feet4 ,as it is in immediate connection with its parent vent1 it will be more fully alluded to in ,chapter xli4 ,here1 as is so generally observable among the basalt-plateaux1 traces of vegetation are plentiful among the stratified intercalations1 even forming thin seams of lignite and coal1 one of which was formerly worked4 ,that volcanic eruptions1 though possibly of a feebler kind1 continued during the interval between the basalt-outflows at this locality1 is shown by the thick accumulation of tuff and by the occurrence of abundant lapilli of fine basic pumice among the shales1 even to a distance of several miles from the vent4 ,illustration3 ,fig4 #bhd4--,macleod's ,maidens and part of ,basalt ,cliffs of ,skye4 ,another conspicuous intercalation of sedimentary materials in the ,skye plateau occurs on the ,talisker cliffs at the mouth of ,loch ,bracadale1 where1 on the face of the great precipice of ,rudha nan ,clach1 some conspicuous bands of lilac and red are interspersed among the basalts4 ,these bands were noticed by ,macculloch1 who described them as varieties of iron-clay4#bfc ,i have not had an opportunity of examining them except from the sea at a little distance4 ,but they suggest a similarity to some of the variegated clays between the upper and lower basalt series of ,antrim4 ,footnote #bfc3 ,western ,islands1 vol4 i4 p4 #cgf4 ,though good coal is not well developed in the ,tertiary volcanic plateaux of the ,british ,isles1 it has already been pointed out that coaly layers are abundant1 and that as the vegetable matter may confidently be assumed always to indicate terrestrial vegetation1 the presence of the carbonaceous bands may be regarded as good evidence of some lapse of time between the eruption of the basalts which they separate4 ,i have also called attention to the fact that the vegetable material is more especially observable in the highest parts of a group of intercalated sediments between two sheets of basalt4 ,this relation1 so strikingly exhibited in the isle of ,canna1 as already observed1 is also to be remarked in the ,skye plateau4 ,i may here cite an interesting example which occurs at the base of the lofty sea-cliff of ,an ,ceannaich1 to the south of ,dunvegan ,head1 on the west coast of ,skye 7,fig4 #bhe74 ,at the base of the precipice1 ledges of a highly cellular basalt 7a7 show a singularly scoriaceous and amygdaloidal structure1 with abundant and beautiful zeolites1 the hollows of the upper surface of the sheet being filled in with dark brown carbonaceous shale1 forming a layer from one to fourteen inches thick1 marked by coaly streaks and lenticles 7b74 ,a band of green and yellow sandstone 7c7 next supervenes1 which1 from its pale colour1 attracts attention from a distance1 and led me1 while yachting along the coast1 to land at the locality in the hope that it might prove to be a plant-bearing limestone4 ,this sandy stratum is only some three or four inches thick at the north end of the section1 but increases rapidly southward to a thickness of as many feet or more1 when1 owing to the cessation of the underlying shale1 it comes to lie directly on the amygdaloid and to enclose slaggy portions of that rock4 ,immediately above the sandstone two or three feet of fissile shale1 black with plant-remains 7d71 include brown layers that yield to the knife like some oil-shales4 ,the next stratum is a seam of coal 7e7 about a foot thick1 of remarkable purity4 ,it is glossy1 hard1 and cubical1 including layers that break like jet4 ,it has been succeeded by a deposit of green sand 7f71 but while this material was in course of deposition another outpouring of lava 7g7 took place1 whereby the terrestrial pool or hollow of the lava-field1 in which the group of sedimentary materials accumulated1 was filled up and buried4 ,this lava is about #bj feet thick1 and consists of a coarsely-crystalline1 jointed dolerite with highly amygdaloidal upper and under surface4 ,its slaggy bottom has caught up or pushed aside the layer of green sand1 so as to lie directly on the coal1 and has there been converted into the earthy modification so familiar under the name of white trap among our coal-fields4 ,it is interesting to find that this kind of alteration1 where molten rock comes in contact with carbonaceous materials1 is not confined to subterranean sills1 but may show itself in lavas that have flowed over a terrestrial surface4 ,illustration3 ,fig4 #bhe4--;,intercalated group of strata between ,basalts1 ,an ,ceannaich1 western side of ,skye4 ,from the frequent intercalation of such local deposits of sedimentary material between the basalts1 we may reasonably infer that during older ,tertiary time the rainfall in ,north-,western ,europe was copious enough to supply many little lakes and streams of water4 ,as the surface of the lava-fields decayed into soil1 vegetation spread over it1 so that1 perhaps for long intervals1 some tracts remained green and forest-clad4 ,but volcanic action still continued to show itself1 now from one vent1 now from another4 ,these wooded tracts were buried under overflows of lava1 and1 the water-courses being filled up1 their streams were driven into new channels1 and other pools and lakes were formed4 ,illustration3 ,fig4 #bhf4--;,escarpment of ,plateau-basalts1 ,cliffs of ,talisker1 ,skye4 ,in no part of the ,tertiary volcanic area of ,britain can the characters of the lavas and the structure of the plateaux be better seen than along the west side of ,skye1 north of ,loch ,bracadale4 ,the precipices rise sheer out of the sea1 to heights of sometimes #ajjj feet1 and from base to summit every individual bed may be counted4 ,some particulars have already been given 7p4 #aib7 regarding the average thickness of the basalt-sheets on this coast-line4 ,the general aspect of these cliffs and the arrangement of their component lavas is shown in ,fig4 #bhf4 ,as a further detailed illustration of the general succession of the basalts in the ,skye plateau1 ,i give a diagrammatic view of the largest of ,macleod's ,maidens--the three weird sea-stalks that rise so grandly in front of the storm-swept precipice at the mouth of ,loch ,bracadale4 ,the height of the stack must be at least #aej feet 7,figs4 #bhd and #bhg74 ,about ten distinct sheets of igneous rock can be counted in it1 which gives an average thickness of #ae feet for the individual beds4 ,it will be observed that there is a kind of alternation between the compact1 prismatic basalts and the more earthy amygdaloids1 but that the former are generally thickest4#bfd ,footnote #bfd3 ,a striking and illustrative contrast between the relative thickness of the beds of the two kinds of rock is supplied by the fine sections of this district4 ,the amygdaloids range from perhaps #f or #h to #be or #cj feet2 but the prismatic basalts1 while never so thin as the others1 sometimes enormously exceed them in bulk4 ,in the island of ,wiay1 for example1 a bed of compact black basalt1 with the confused starch-like grouping of columns1 reaches a thickness of no less than #agj feet4 ,its bottom rests upon a red parting on the top of a dull greenish earthy amygdaloid4 ,it is possible1 however1 that some of these columnar sheets of basalt are really sills4 ,illustration3 ,fig4 #bhg4--,section of the largest of ,macleod's ,maidens4 ,these features1 which are repeated on cliff after cliff1 may be considered typical for all the plateaux4 ,another characteristic point1 well displayed here1 is the intervening red parting between the successive beds4 ,if the occurrence and thickness of this layer could be assumed as an indication of the relative lapse of time between the different flows of lava1 it would furnish us with a rude kind of chronometer for estimating the proportionate duration of the intervals between the eruptions4 ,it is to be noticed on the top both of the compact prismatic and of the earthy amygdaloidal sheets2 but it is more frequent and generally thicker on the latter than on the former1 which may only mean that the surfaces of the cellular lavas were more prone to subrial decay than those of the compact varieties4 ,nevertheless1 ,i am disposed to attach some value to it1 as an index of time4 ,in the present instance1 for example1 it seems to me probable that the lavas in the lower half of ,macleod's ,maiden1 where the red layers are very prominent1 were poured out at longer intervals than those that form the upper half4 ,the remarkable banded arrangement of the vesicles in one of the cellular lavas of this sea-stack has been already referred to 7p4 #aia74 ,another characteristic plateau-feature is admirably displayed in ,skye--the flatness of the basalts and the continuity of their level terraces 7though not of individual sheets7 from cliff to cliff and hillside to hillside4 ,this feature may be followed with almost tiresome monotony over the whole of the island1 north of a line drawn from ,loch ,brittle to ,loch ,sligachan4 ,throughout that wide region1 the regularity of the basalt-plateau is unbroken1 except by minor protrusions of eruptive rock1 which1 as far as ,i have noticed1 do not seriously affect the topography4 ,but south of the line just indicated1 the plateau undergoes the same remarkable change as in ,rum1 ,ardnamurchan and ,mull4 ,portions of it which have survived indicate with sufficient clearness that it once spread southwards and eastwards over the mountainous district1 and even farther south into the low parts of the island4 ,its removal from that tract has been of the utmost value to geological research1 for some of the subterranean aspects of volcanism have thereby been revealed1 which would otherwise have remained buried under the thick cover of basalt4 ,denudation has likewise cut deeply into the eruptive bosses1 and has carved out of them the groups of the ,red ,hills and the ,cuillins1 to whose picturesque forms ,skye owes so much of its charm4 ,in this1 as in each of the other plateaux1 there is no trace of any thickening of the basalts towards a supposed central vent of eruption4 ,the nearly level sheets may be followed up to the very edge of the great mountainous tract of eruptive rocks1 retaining all the way their usual characters2 they do not become thicker there either collectively or individually1 nor are they more abundantly interstratified with tuffs or volcanic conglomerates4 ,on the contrary1 their very base is exposed around the mountain ground1 and the thickest interstratifications of fragmentary materials are found at a distance from that area4 ,so far as regards the structure of the remaining part of the plateau1 the eruption of the gabbros and granitoid rocks might apparently have taken place as well anywhere further north4 v4 ,,the ,,faroe ,,islands#bfe ,footnote #bfe3 ,for references to the recent geological literature connected with these islands see the footnote ante1 p4 #aia4 ,though these islands lie beyond the limits of the region embraced by the present work1 ,i wish to cite them for the singular confirmation and extension they afford to observations made among ,british ,tertiary volcanic rocks4 ,over a united extent of coast-cliffs which may be roughly estimated at about #ejj ,english miles1 the nearly level sheets of basalts1 with their occasional tuffs1 conglomerates1 leaf-beds and coals1 can be followed with singular clearness4 ,although the ,faroe ,islands have been so frequently visited and so often described that their general structure is sufficiently well known1 they present in their details such a mass of new material for the illustration of volcanic action that they deserve a far more minute and patient survey than they have yet received4 ,they cannot be adequately mapped and understood by the traveller who merely sails round them4 ,they must be laboriously explored1 island by island and cliff by cliff4 ,while ,i cannot pretend to more than a mere general acquaintance with their structure1 ,i have learnt by experience that one may sail near their precipices and yet miss some essential features of their volcanic structure4 ,in the summer of the year #ahid ,i passed close to the noble range of precipices on the west side of ,strom1 at the mouth of the ,vaagfjord1 and sketched the sill which forms so striking a part of the geology of that district 7,figs4 #cab1 #cbh and #cbi74 ,but ,i failed to observe a much more remarkable and interesting feature at the base of the same sea-cliffs4 ,the following summer1 probably under better conditions of light1 ,i was fortunate enough to detect with my field-glass1 from the deck of the yacht1 what looked like a mass of agglomerate1 and found on closer examination the interesting group of volcanic vents described in ,chapter xli4 ,the magnificent precipices of ,faroe1 which in ,myling ,head reach a height of #bbfj feet1 present a series of natural sections altogether without a rival in the rest of ,europe4 ,they are less concealed with verdure than those of ,mull and ,skye1 and therefore display their geological details with even greater clearness than can be found either in ,scotland or in ,ireland4 ,i would especially refer to the bare precipitous sides of the long narrow islands of ,kals and ,kun1 as admirable sections wherein the characters of the plateau-basalts are revealed as in a series of gigantic diagrams4 ,the scarcity of vegetation1 and the steepness of the declivities which prevents the abundant accumulation of screes of detritus1 enable the observer to trace individual beds of basalt with the eye for several miles4 ,thus on the west side of ,kun1 one conspicuous dark sheet in the lower part of the section can be followed from opposite ,mygledahl in ,kals to the southern end of the island4 ,there is one concealed space at the mouth of the corrie behind ,kun village1 but the same1 or at least a similar band of rock at the same level1 emerges from the detritus on the further side1 and may possibly run into the opposite promontory of ,bod4 ,it extends in ,kun for at least six geographical miles4 ,illustration3 ,fig4 #bhh4--;,dying out of ,lava-beds1 east side of ,sand1 ,faroe ,isles4 ,these vast escarpments of naked rock show1 with even greater clearness than the precipices of the ,inner ,hebrides1 how frequently the basalts die out1 now in one direction now in another4 ,the two sides of the ,kalsfjord exhibit many examples of this structure1 and some striking instances of it are to be seen on the west side of ,haraldsfjord4 ,in these cliffs1 which must be about #bjjj feet high1 upwards of forty distinct flows can sometimes be traced from the sea-level to the crest4 ,the average thickness of each bed is thus somewhat less than #ej feet4 ,such vast escarpments1 with wide semicircular corries scooped out of their sides1 such serrated crests and dark rifts in the precipices1 such deep fjords winding through nearly horizontal basalts1 of which the parallel sheets can be followed by the eye from island to island1 fill the mind with a vivid conception at once of the enormous scale of the volcanic eruptions and of the stupendous denudation which this portion of ,north-,western ,europe has undergone since ,tertiary time4 ,as the lenticular character of the basalts1 and the evidence they supply of having been discharged from many small local vents are of great importance in the comprehension of the volcanic history of the plateaux1 some further illustrations of these features may with advantage be given here4 ,thus the traveller who skirts the western precipices of ,suder will notice some good examples to the north of the highest part of the cliffs4 ,on ,strom he will detect other cases of the same structure4 ,similar features will arrest his attention on the precipices of ,sand1 where1 though at first sight the basalts seem to be regular and continuous1 a nearer view of them reveals such sections as that shown in ,fig4 #bhh1 where a group of sheets rapidly dies out towards the north against a thicker band that thins away in the opposite direction4 ,further north he will come upon other examples in the range of low cliffs between ,kirkebonaes and ,thorshaven1 and more impressive still in the rugged precipices that front the ,atlantic on the western front of ,hest 7,fig4 #bhi71 where the disappearance is in a northerly direction4 ,illustration3 ,fig4 #bhi4--,lenticular lavas1 western front of ,hest1 ,faroe ,isles4 ,but it is in the northern part of the ,faroes1 where the basalt-plateau has been so deeply trenched by parallel fjords as to be broken up into a group of long1 narrow1 lofty1 and precipitous insular ridges1 that the really local and non-persistent character of the lavas can best be seen4 ,the eastern cliffs of ,svin present admirable examples1 where in the same vertical wall of rock some of the basalts die out to the south1 others to the north1 while occasionally a shorter sheet may be seen to disappear in both directions as if it were the end of a stream that flowed at right angles to the others 7,fig4 #bij74 ,illustration3 ,fig4 #bij4--,lenticular lavas east side of ,svin1 ,faroe ,isles4 ,the more the basalt-plateaux of ,britain and the ,faroe ,islands are studied1 the more certain does the conclusion become that these widespread sheets of lava never flowed from a few large central volcanoes of the type of ,etna or ,vesuvius1 but were emitted from innumerable minor vents or from open fissures4 ,in a later chapter an account will be given of the vents1 which may still be seen under the overlying sheets of basalt1 and1 in particular1 a remarkable group in the ,faroe ,islands will be described4 ,illustration3 ,fig4 #bia4--,section at ,frodbonyp1 ,suder1 ,faroe4 ,the occurrence of tuffs1 leaf-beds and thin coals between the plateau-basalts of the ,faroe ,islands has long been known4 ,these stratified deposits are well seen in the island of ,suder1 where they serve to divide two distinct series of basalts1 like the iron-ore and its accompaniments in ,antrim4 ,as a characteristic illustration of the same diversity of deposits observable between the lava-sheets of the basalt-plateaux of the ,british ,isles ,i give here a section exposed on the east side of this island--a locality often visited and described in connexion with its coal-seams 7,fig4 #bia74 ,at the base lies a sheet of basalt 7a7 with an irregularly lumpy upper surface4 ,it may be remarked that the lower group of basalts is marked by the occurrence of numerous columnar sheets1 some of them possibly sills1 and also more massive1 solid1 and durable basalts than the sheets above4 ,the lowest of the intercalated sediments are light-coloured clays1 passing down into dark nodular mudstone and dark shale1 the whole having a thickness of at least #bj feet 7b74 ,these strata are succeeded by 7c7 pale clays with black plant-remains1 about three feet thick4 ,immediately above this band comes the coal or coaly layer 7d71 here about six inches thick1 which improves in thickness and quality further inland1 where it has been occasionally worked for economic purposes4 ,a deposit of green and brown volcanic mudstone 7e71 twelve feet in thickness1 overlies the coal and passes under a well-bedded granular green tuff and mudstone three feet thick 7f74 ,the uppermost band is another volcanic mudstone 7g7 four feet in thickness1 dark green in colour1 and more or less distinctly stratified1 with irregular concretions1 and also pieces of wood4 ,above this layer comes another thick overlying group of basalts 7h7 distinguished by their abundantly amygdaloidal character1 and by their weathering into globular forms which at a little distance give them a resemblance to agglomerates4 ,we have here an intercalated group of strata upwards of #dj feet thick1 consisting partly of tuffs and partly of fine clays1 which may either have been derived from volcanic explosions or from the atmospheric disintegration of basaltic lavas4 ,through some of these strata abundant carbonaceous streaks and other traces of plants are distributed1 while among them lies a band almost wholly composed of compressed vegetation4 ,unfortunately none of the strata at this locality seem to have preserved the plant-remains with sufficient definiteness for identification4 ,there can be no doubt1 however1 that they were terrestrial forms like those of ,mull and ,antrim4 ,this coal1 with its accompanying sedimentary deposits1 has been traced through ,suder1 and another outcrop1 possibly of the same horizon1 occurs on ,myggenaes1 the extreme western member of the group of islands1 at a distance of some #dj miles4#bff ,footnote #bff3 ,see in particular ,prof4 ,j4 ,geikie1 ,trans4 ,roy4 ,soc4 ,edin4 vol4 xxx4 7#ahhj71 p4 #bbi4 ,,chapter ,,xl ,,the ,,modern ,,volcanoes ,,of ,,iceland ,,as ,,illustrative ,,of ,,the ,,tertiary ,,volcanic ,,history ,,of ,,north-,,western ,,europe ,from the facts stated in the foregoing chapters concerning the structure of the basalt-plateaux of ,north-,western ,europe1 it is evident that in none of these areas have the eruptions come from one great central volcano like ,etna or ,vesuvius4 ,on the contrary1 in every instance there is abundant evidence that the basalt has flowed from many scattered points of eruption4 ,the uniformity of the lava-sheets in petrographical characters1 their continuity when viewed in mass1 their general horizontality1 and their constant thinning away in different directions1 show that the eruptive vents must have been distributed over the whole plateau-areas4 ,the conditions under which such eruptions took place can be most readily understood by a comparison of the phenomena with those observable in modern volcanic tracts where extensive outflows of lava have taken place without the existence of any great central cones4 ,of these regions the most instructive is undoubtedly to be found among the recent lava-deserts of ,iceland4 ,there the parallels to the structures described from the ,british and ,faroe plateaux are so numerous and so close that an account of the ,icelandic region may appropriately be inserted here4 ,the evidence furnished by ,iceland is of special value in our present enquiry1 inasmuch as that island1 besides its modern eruptions1 includes vast basaltic plateaux of ,tertiary age4 ,these areas of nearly level sheets of basalt belong to the same geological period as those of the ,british and ,faroe ,islands1 and display the same internal structure and external features4 ,but they have this distinguishing peculiarity that the volcanic fires beneath them are not yet extinguished4 ,they have been broken through again and again in recent times by volcanic eruptions which have repeated many of the characteristics of their ,tertiary predecessors4 ,the old and the new development of the same volcanic type are thus visible side by side4 ,the ,tertiary volcanic series of ,iceland reaches a thickness of upwards of #cjjj metres1 or nearly #aj1jjj ,english feet1 but as its base is nowhere seen1 it may be still thicker4 ,its successive sheets1 piled over each other in parallel layers1 form terraced hills and bold escarpments along the coast1 whence they slope gently inland4 ,the plateau1 as in the ,faroe ,islands and in ,scotland1 has been extensively eroded1 and has been trenched by many long valleys and fjords ,the composition of the basalts remains remarkably uniform over the island4 ,the lava sheets are often decomposing1 amygdaloidal1 and filled with zeolites2 while higher in the series compact basalts abound1 the uppermost fine-grained sheets being especially constant in structure and composition4 ,numerous dykes traverse the plateau1 and some of them cut even its highest members4 ,the parallel with the geological structure of the ,inner ,hebrides is continued in ,iceland by the appearance of intrusive masses of gabbro and granophyre1 which represent the deeper parts of the ,tertiary volcanic series1 while the basalts were poured out at the surface4 ,thus1 at ,papafjord1 the gabbro rises into mountainous peaks and1 like the similar rock in ,mull and ,skye1 is intersected by dykes of a coarse-grained granitoid liparite or granophyre4 ,large dykes and ramifying veins of the same acid material1 often with a thoroughly granitic aspect1 extend into the basalts4#bfg ,footnote #bfg3 ,mr4 ,thoroddsen1 ,dansk4 ,geografisk ,tidsskrift1 vol4 xiii4 ,a long series of eruptions has taken place in ,iceland since the ,glacial ,period4 ,there were likewise pre-glacial eruptions4 ,the glaciated lava-streams are found underneath the modern lavas4 ,so far indeed as is known1 no evidence exists of any important cessation of subterranean activity there since ,tertiary time4#bfh ,the existing volcanic phenomena may with probability be regarded as the survival of those which were so widely manifested over the ,icelandic area and the north-west of ,europe in the older ,tertiary ages4 ,a careful study of them may therefore be expected to throw light on the history of the ,tertiary basaltic plateaux2 while1 on the other hand1 the thorough dissection of these plateaux by the denuding agencies will not improbably be found to explain some parts of the subterranean mechanism of the modern ,icelandic volcanoes4 ,footnote #bfh3 ,see ,dr4 ,johnston-,lavis1 ,scottish ,geographical ,magazine1 #ahie1 p4 #ddb4 ,in calling attention to some of the more obvious analogies which may be traced between the modern and the ancient volcanoes1 ,i am more particularly indebted to the excellent memoirs of the resident ,icelandic geologist1 ,mr4 ,th4 ,thoroddsen1 who has examined so large a part of the island4#bfi ,the account given by ,mr4 ,a4 ,holland of the ,laki craters has likewise been of much service to me4#bgj ,among other recent observers ,i may cite ,dr4 ,tempest ,anderson1#bga who has made himself familiar with extensive tracts of ,iceland4 ,he was accompanied one year by ,dr4 ,johnston-,lavis1 who has published a narrative of the journey4#bgb ,footnote #bfi3 ,see ,in particular his paper on the volcanoes of north-east ,iceland 7,bihang till4 k4 ,svensk4 ,vet4 ,akad4 ,handl4 xiv4 ii4 ,no4 #e1 #ahhh7 and that on ,snaefell and ,faxebugt in the south-west of the island 7op4 cit4 xvii4 ii4 ,no4 #b1 #ahia72 also papers in ,dansk4 ,geografisk ,tidsskrift1 vols4 xii4 xiii4 7#ahic-ie72 ,verhand4 ,gesellsch4 ,erdkunde zu ,berlin1 #ahid-ie4 ,footnote #bgj3 ,lakis ,kratere og ,lavastrmme1 ,universittsprogram1 ,christiania1 #ahhe4 ,see ,mr4 ,thoroddsen's remarks on this paper1 ,verhand4 ,gesell4 ,erdkunde1 #ahid1 p4 #bhi4 ,footnote #bga3 ,brit4 ,assoc4 ,rep4 #ahid1 p4 #fej4 ,footnote #bgb3 ,dr4 ,johnston-,lavis1 ,scottish ,geographical ,magazine1 ,september #ahie4 ,it is a mistake to suppose that the ,icelandic volcanoes are generally built on the plan of such mountains as ,vesuvius or ,etna4 ,mr4 ,thoroddsen can evidently hardly repress his impatience to find these two ,italian cones cited in almost every handbook of geology as types of modern volcanoes and their operations4 ,the regular volcanic cone1 composed of alternations of lavas and tuffs1 plays a very subordinate part in ,iceland4 ,illustration3 ,fig4 #bib4--;,fissure 7gj7 in a lava-field1 ,iceland4 7,from a photograph by ,dr4 ,tempest ,anderson47 ,the fundamental feature in the ,icelandic eruptions is the production of fissures which reach the surface and discharge streams of lava from many points4 ,two systems of such fissures appear to be specially marked1 one in southern ,iceland running from south-west to north-east1 the other1 in the north part of the island1 stretching from south to north4#bgc ,hekla and ,laki belong to the former4 ,the dislocations have often followed the boundaries of the horsts1 or solid blocks of country which have withstood terrestrial displacement4 ,the vast outbreaks of ,oddahraun and ,myvatn have almost all issued from fissures of that nature4 ,footnote #bgc3 ,in the ,snaefell promontory they run nearly east and west4 ,mr4 ,thoroddsen1 ,bihang4 ,svensk4 ,akad4 xvii4 7ii47 ,no4 #b1 p4 #ia4 ,the violent eruption of #ahge in ,askja found its exit at the intersection of two lines of fissures4 ,many large fissures were opened on the surface in a nearly north and south direction1 which could be followed for #hj kilometres or nearly #ej ,english miles4 ,some of them became the theatre of intense volcanic activity4#bgd ,footnote #bgd3 ,mr4 ,thoroddsen1 op4 cit4 xiv4 ii4 ,no4 #e1 p4 #fc4 ,many lines of fissure are traceable at the surface as clefts or gjs1 that run nearly straight for long distances1 with a width of one to three yards1 and sometimes of unknown depth4#bge ,the most stupendous example of the structure yet discovered is probably the ,eldgj found by ,dr4 ,thoroddsen in the year #ahic1 below the ,mrdalsjkull4 ,this gigantic chasm has a length of #cj kilometres 7more than #ah ,english miles71 and a depth of #acj to #bjj metres 7#dbf to #fef feet74 ,over its vertical walls lofty waterfalls plunge from the crest to the bottom4 ,footnote #bge3 ,on the various modes of origin of these chasms1 see ,dr4 ,tempest ,anderson1 ,brit4 ,assoc4 ,rep4 p4 #fej4 ,the gj shown in ,fig4 #bib is not an eruptive fissure4 ,for this and the following illustration ,i am indebted to the kindness of ,dr4 ,tempest ,anderson1 who himself photographed the scenes4 ,occasionally a fissure has not been continuously opened to the surface4 ,an interesting example of such intermittent chasms is supplied by the great rent which gave forth the enormous volume of lava in #aghc4 ,the mountain of ,laki1 composed of palagonite tuff1 stands on the line of this dislocation1 but has not been entirely ruptured4 ,the fissure has closed up beneath the mountain1 a short distance above the bottom of the slope1 as is shown by the position of a couple of small craters4#bgf ,footnote #bgf3 ,mr4 ,a4 ,helland1 op4 cit4 p4 #be4 ,some fissures have remained mere open chasms without any discharge of volcanic material2 others have served as passages for the escape of lava and the ejection of loose slags and cinders4#bgg ,footnote #bgg3 ,mr4 ,thoroddsen has observed that in the ,reykjanes peninsula in the south-west of ,iceland1 by the subsidence of one side of a fissure1 a row of four craters has been cut through1 leaving their segments perched upon the upper side4 ,globus4 vol4 lxix4 ,no4 #e4 ,illustration3 ,fig4 #bic4--;,cones on the great ,laki fissure1 ,iceland4 7,from a photograph by ,dr4 ,tempest ,anderson47 ,in some instances1 according to ,mr4 ,thoroddsen1 lava wells out from the whole length of a fissure without giving rise to the formation of cones1 the molten material issuing either from one or from both sides and flowing out tranquilly4 ,thus from three points on the great ,eldgj chasm lava spread out quietly without giving rise to any craters1 though at the southern prolongation of the fissure1 where it becomes narrower1 a row of low slag-cones was formed4 ,the three lava-streams flooded the low ground over an area of #fic square kilometres1 or #bgj ,english square miles4 ,in the great majority of cases1 however1 the lava as it ascends in the fissure gives rise to long ramparts of slags and blocks of lava piled up on either side1 or to a row of cones along the line of the open chasm4 ,thus1 on the ,laki fissure1 which runs for about #bj miles in a north-east direction1 the cones amount to some hundreds in number4 ,illustration3 ,fig4 #bic;a4--,plan of small craters along the line of great ,laki fissure1 ,iceland4 7,after ,mr4 ,helland1 reduced47 ,the cones consist generally of slags1 cinders1 and blocks of lava4 ,they are on the whole not quite circular but oblong1 their major axis coinciding with the line of the chasm on which they have been piled up1 as along the marvellous line of the ,laki fissure4 ,in many places they are exceedingly irregular in form1 changes in the direction of outflow of lava or of escape of steam having caused the cones partially to efface each other4 ,as regards their size1 the cones present a wide range4 ,some of them are only a few yards in diameter1 others several hundred yards4 ,generally they are comparatively low mounds4 ,on a fissure hardly #cj feet long1 ,mr4 ,thoroddsen found a row of twelve small cones built exactly like those of largest size1 but with craters less than three feet in diameter4 ,on the ,laki fissure some are only a couple of yards high2 the majority are much less than #ej yards in height1 and hardly one is as much as #ajj yards4#bgh ,and yet these little monticules1 as ,mr4 ,helland remarks1 represent the pipes from which milliards of cubic metres of lava have issued4 ,while other ,european volcanoes form conspicuous features in the landscape1 the ,icelandic volcanoes of the ,laki district1 from which the vastest floods of lava have issued in modern times1 are so low that they might escape notice unless they were actually sought for4#bgi ,footnote #bgh3 ,mr4 ,thoroddsen1 however1 states that there are about #ajj ranging between #bj and #ajj metres in height4 ,footnote #bgi3 ,op4 cit4 p4 #bg4 ,as they have generally arisen along lines of fissure1 the cones are1 for the most part1 grouped in rows4 ,the hundreds of cones that mark the line of the ,laki fissure present an extraordinary picture of volcanic energy of this type4 ,in other instances the cones occur in groups1 though this distribution may have arisen from the irregular uprise of scattered vents along a series of parallel fissures4 ,thus to the north-east of ,laki a series of old cones entirely surrounded by the lavas of #aghc lie in groups1 the most northerly of which consists of about #ajj exceedingly small craters that have sent out streams of lava towards the ,n4,n4,e4#bhj ,footnote #bhj3 ,op4 cit4 p4 #be4 ,the great lava-fields of ,iceland are likewise dotted over with secondary craters or hornitos which have no direct connection with the magma below1 but arise from local causes affecting the outflowing lava4 ,they are grouped in hundreds over a small space4 ,it would appear from ,mr4 ,helland's observations that the same fissure has sometimes been made use of at more than one period of eruption4 ,he describes some old craters on the line of the ,laki fissure1 which had been active long before the outbreak of #aghc4bha ,footnote #bha3 ,op4 cit4 p4 #bf4 ,when the lava issues from fissures it is in such a condition of plasticity that it can be drawn out into threads and spun into ropes4 ,when the slope over which it flows is steep it often splits up into blocks on the surface4 ,where the ground is flat the lava spreads out uniformly on all sides1 forming wide plains as level as a floor4 ,thus the vast lava-desert of ,oddahraun covers a plain #cfdj square kilometres in area1 or1 if the small-lava-streams north from ,vatnajkull be included1 #dcij square kilometres4 ,this vast flood of lava 7about #agjj ,english square miles in extent7 would1 according to ,mr4 ,thoroddsen1 cover ,denmark to a depth of #af feet4 ,the whole of this enormous discharge has been given forth from more than twenty vents situated for the most part on parallel fissures4 ,not less striking is the picture of fissure-eruption to be met with at ,laki--the scene of the great lava-floods of #aghc4 ,conceive now1 says ,mr4 ,helland1 these hundreds of craters1 or1 as they are called by the ,icelanders1 'borge1' lying one behind another in a long row2 every one of them having sent out two or more streams of lava1 now to the one side1 now to the other4 ,understand further that these streams merge into each other1 so as to flow wholly round the cones and form fields of lava miles in width1 which1 like vast frozen floods1 flow down to the country districts1 and you may form some idea of this remarkable region4#bhb ,footnote #bhb3 ,op4 cit4 p4 #bd4 ,mr4 ,helland allows an average thickness of #cj metres for the mass of lava which issued in two streams1 one #hj kilometres 7nearly #ej miles71 the other #de kilometres 7about #bh miles7 long4 ,he estimates the total volume of lava discharged in the #aghc eruption at #bg milliards of cubic metres1 equal to a block #aj kilometres 7#f miles #cgf yards7 long1 #e kilometres 7#c miles #ahh yards7 broad1 and #edj metres 7#agga feet7 high2 op4 cit4 p4 #ca4 ,mr4 ,thoroddsen remarks that the older estimates of the volume of lava discharged by this eruption have been greatly exaggerated4 ,he puts the area covered by lava at #efe square kilometres and the contents at #ab-a_/c cubic kilometres4 ,verhand4 ,gesell4 ,erdkunde ,berlin1 #ahid1 p4 #bif4 ,the basaltic lavas have issued in a comparatively liquid state1 form thin sheets and reach to great distances4 ,the western stream from the ,laki eruption of #aghc flowed for upwards of #dj miles2 a prehistoric lava from ,trlladyngj in ,oddahraun flowed for more than #fj miles4 ,in the course of time the successive streams of lava poured out upon one of these wide volcanic plains gradually increase the height of the ground1 while preserving its generally level aspect4 ,the loose slag-cones of earlier eruptions are effaced or swallowed up1 as one lava-stream follows another4 ,eventually1 when1 by the operation of running water or by fissure and subsidence1 transverse sections are cut through these lava-sheets1 the observer can generally notice only horizontal beds of lava piled one above another1 including the dykes connected with them and intercalated masses of loose slag1 that remain as relics of the old craters4 ,in some places the lava has gradually built up enormous domes1 like those of ,hawaii1 having a gentle inclination in every direction1 as may be seen especially in the district between ,floderne ,skjalfanafljot and ,jkuls ,most of the large volcanic piles of ,north ,iceland are of this nature4 ,the highest of them are #abji and #adia metres high by from #f to #ae kilometres in diameter4 ,the elliptical crater of the highest of these eminences measures #aajj by #chj metres4#bhc ,footnote #bhc3 ,mr4 ,thoroddsen1 op4 cit4 xiv4 ii4 ,no4 #e1 pp4 #aj1 #bc4 ,large conical volcanoes of the ,vesuvian type built up of alternating lavas and tuffs are not common in ,iceland1 but some occur and rise into lofty glacier-covered mountains1 such as rfajkull 7#fbda feet71 ,eyjafjallajkull 7#edcb71 and ,snaefellsjkull 7#degg74 ,hekla 7#difa7 also is similarly composed of sheets of lava and tuffs1 but has not been built as a cone4 ,it forms an oblong ridge which has been fissured in the direction of its length and bears a row of craters along the fissure4#bhd ,footnote #bhd3 ,mr4 ,thoroddsen1 ,dansk4 ,geograf4 ,tidsskrift1 vol4 xiii4 ,explosion-craters likewise occur among the modern volcanic phenomena of ,iceland4 ,one of these was formed by a violent explosion at ,askja on #bith ,march #ahge4 ,it has a diameter of only about #bhj feet1 yet so great was the vigour of the outburst that pumiceous stones were spread over an area of more than #ajj ,danish 7#dfh ,english7 square miles1 and the dust was carried as far as ,norway and ,sweden4 ,nine years later ,mr4 ,thoroddsen found the bottom of this crater filled with bluish-green boiling mud1 which will probably in the end become a sheet of still water4 ,the borders of these ,icelandic explosion-craters seem to be very little higher than the ground around them4 ,most of the ejected material is expelled with such force and to such a distance that only a small fraction of it falls down around the orifice of eruption4#bhe ,footnote #bhe3 ,mr4 ,thoroddsen1 op4 cit4 ,there is still another feature of the ,icelandic volcanic regions which may be cited as an interesting parallel to the sequence of eruptive discharges among the ,inner ,hebrides4 ,while the lavas are as a rule more or less basic--many of them being true basalts--they have been at different times pierced by much more acid liparites and obsidians4 ,examples of these rocks of post-,glacial age have recently been traced on the ground by ,mr4 ,thoroddsen1#bhf and their petrographical characters have been studied by ,mr4 ,bckstrm4#bhg ,the wide distribution of such rocks all over the island1 their occurrence in isolated bosses among the more basic lavas1 and their remarkable internal structures have been noted by several observers4#bhh ,the liparites and obsidians are contrasted with the basalt by the colours and forms of their streams4 ,some of them are so black as to look like heaps of coal1 though their surfaces pass into grey pumice4 ,they have flowed out in a much less liquid condition than the basalts1 and have consequently formed short1 thick and irregular sheets4 ,the liparites and basalts appear to have been nearly contemporaneous4 ,they certainly belong to the same volcanic cycle and their vents lie close to each other4 ,though none of the acid eruptions are known to have occurred in modern times1 some of the liparites are crusted with sulphur and from the connected fissures steam still rises4 ,footnote #bhf3 ,geol4 ,fren4 ,stockholm ,frhandl4 xiii4 7#ahia71 p4 #fji2 ,bihang4 ,svensk4 ,vet4 ,akad4 ,handl4 xvii4 ii4 p4 #ba 7#ahia72 ,dansk4 ,geograf4 ,tidsskrift1 xiii4 7#ahie74 ,footnote #bhg3 ,geol4 ,fren4 ,stockholm ,frhandl4 xiii4 7#ahia71 p4 #fcg4 ,footnote #bhh3 ,see in particular ,c4 ,w4 ,schmidt1 ,zeitsch4 ,deutsch4 ,geol4 ,gesellsch4 xxxvii4 7#ahhe71 p4 #gcg4 ,it will thus be seen how entirely the modern volcanic eruptions of ,iceland agree with the phenomena presented by our ,tertiary basalt-plateaux4 ,it is1 therefore1 to the ,icelandic type of fissure-eruptions1 and not to great central composite cones like ,vesuvius or ,etna that we must look for the modern analogies that will best serve as commentary and explanation for the latest chapter in the long volcanic history of the ,british ,isles4#bhi ,footnote #bhi3 ,in his memoir of #ahgd1 ,professor ,judd announced his conclusion that there were formerly five great volcanoes amongst the ,western ,isles1 and that the lavas of the plateaux had issued from these4 ,he subsequently reiterated this view 7,quart4 ,journ4 ,geol4 ,soc4 xlv41 #ahij1 p4 #ahg71 and ridiculed the explanation of fissure-eruptions4 ,the evidence adduced by me in a paper published in #ahif 7same journal1 vol4 lii4 p4 #cca7 and reprinted with additions in this chapter1 will1 ,i trust1 be regarded by geologists as having finally settled this question4 ,as a further but more ancient illustration of the type of volcanic action which appears to have been prevalent during the formation of the ,tertiary volcanic plateaux of ,britain1 ,i may again refer to the vast basalt-fields of ,western ,america4 ,the basalt of ,idaho stretches out as an apparently limitless plain4 ,along its northern boundary1 this sea of black lava runs up the valleys and round the promontories of the older trachytic hills with almost the flatness of a sheet of water4 ,it has been deeply trenched1 however1 by the streams that wind across it1 and especially by the ,snake ,river1 which has cut out a gorge some #gjj feet deep1 on the walls of which the successive beds of basalt lie horizontally one upon another1 winding along the curving face of the precipice exactly as those of ,antrim and the ,inner ,hebrides do along their sea-worn escarpments4 ,here and there1 a low cinder-cone on the surface of the plain marks the site of a late outflow4 ,one is struck1 however1 with the singular absence of tuffs and volcanic conglomerates4 ,the basalts appear to have flowed out stream after stream with few fragmentary discharges4 ,these characteristic features of one distinctive type of volcanic action have been repeated over a vast region1 or rather a whole series of regions1 in ,western ,america1 the united area of which must equal that of a considerable part of ,europe4 ,from ,idaho1 the basalt-fields may be followed southwards interruptedly into ,utah and ,nevada1 and across the great plateau-country of the caons into ,arizona and ,new ,mexico1 northwards into ,montana1 and westwards into ,oregon4 ,the tract which has as yet been most carefully traversed and described is probably that of the high plateaux of ,utah and ,arizona4 ,thus on the ,uinkaret plateau1 which measures some #de to #ej miles in length by #h to #ab in breadth1 a thick covering of basalt has been spread composed of many successive flows4 ,between #afj and #agj separate cones have been counted on this area1 most of them quite small1 mere low mounds of scori1 though a few reach a height of #gjj or #hjj feet1 with a diameter of a mile4 ,from three to seven or eight may be found in a row1 as if springing from a single line of fissure4 ,but generally the grouping is quite irregular4#bij ,my friend ,captain ,c4 ,e4 ,dutton1 from whose admirable memoir these details are quoted1 remarks further that among the ,utah plateaux no trace of a cone is to be found at or near some of the most recent basalt-fields1 and that the most extensive outpours are most frequently without cones4 ,the lavas1 he adds1 appear to have reached the surface and overflowed like water from a spring1 spreading out immediately and deluging a broad surface around the orifice4#bia ,the deep gorges cut by the rivers through these thick accumulations of horizontal or nearly horizontal basalts1 have here and there revealed parallel dykes that traverse the rocks1 and in at least one case have shown the dyke running for half a mile up a cliff and actually communicating with a crater of scori at the top4#bib ,again1 in ,new ,mexico1 ,captain ,dutton noticed vast tracts of younger basalt1 about which a striking fact is the entire absence of all distinguishable traces of the vents from which they came4 ,some of them1 however1 indicate unmistakably their sources in small depressed cones of very flat profiles4 ,no fragmental ejecta 7scori1 lapilli1 etc47 have been found in connection with these young eruptions4#bic ,such ,i believe to have been the general conditions under which the basalts of the ,tertiary plateaux of the ,british ,isles were also erupted4#bid ,footnote #bij3 ,captain ,c4 ,e4 ,dutton1 ,tertiary ,history of the ,grand ,caon ,district1 ,u4,s4 ,geol4 ,survey 7#ahhb71 p4 #ajd4 ,footnote #bia3 ,captain ,c4 ,e4 ,dutton1 ,geology of the ,high ,plateaux of ,utah1 ,u4,s4 ,geol4 ,survey of the ,rocky ,mountain ,region 7#ahhj71 pp4 #aih1 #bjj4 ,see also pp4 #bcb1 #bcd1 #bgf of the same ,monograph for additional examples4 ,footnote #bib3 ,tertiary ,history of the ,grand ,caon1 etc41 p4 #ie4 ,footnote #bic3 ,nature1 xxxi4 7#ahhd71 p4 #di4 ,footnote #bid3 ,i may again refer to ,hopkins's ,researches in ,physical ,geology1 where the conditions of the problem here discussed have been distinctly realized4 ,speaking of the ejection of lava from a number of fissures1 he remarks that the imperfect fluidity of the melted material would seem to require a number of points or lines of ejection as a necessary condition4 ,if there were only a single centre of eruption1 a bed of such matter approximating to uniformity of thickness1 could only be produced on a surface of a conical form4 ,where no such tendency to this conical structure can be traced1 it would probably be in vain to look for any single centre of eruption4 ,on the supposition1 too1 of ejection through continued fissures1 or from a number of points1 that minor unevenness of surface which must probably have existed under all circumstances during the formation of the earth's crust1 would not necessarily destroy the continuity of a comparatively thin extensive bed of the ejected matter1 in the same degree in which it would inevitably produce that effect in the case of central ejection 7,cambridge ,phil4 ,trans4 vi4 #ahce1 p4 #ga74 ,although we may be convinced1 from their general structure and relations1 that the stratified lavas of these plateaux have been poured out from fissures and not from great central cones1 it must obviously be difficult to obtain demonstrative evidence of this origin from any single section4 ,of the thousands of dykes which traverse the ,british plateaux and the ground around them1 ,i am not aware of a single one which can be actually seen to have ever communicated with the surface4 ,the very process of denudation which has revealed these dykes has at the same time removed all trace of any former connection they may have had with the surface4 ,the only places where we may hopefully search for the missing evidence are the fronts of the escarpments4 ,on these precipices dykes may sometimes be seen to end off at some particular platform among the basalt-sheets1 but ,i have never found a case which could be confidently cited as an example of lava rising in a fissure and spreading out as a superficial sheet4 ,that this connection may eventually be found when a more detailed survey is made of these great sea-walls ,i fully anticipate4 ,in recently mapping the basalt-plateau of ,strathaird in ,skye1 ,mr4 ,harker has made some interesting observations regarding the probable connection of the dykes with the plateau basalts4 ,he has noticed that the flanks of ,slat ,bheinn1 a portion of the plateau1 are abundantly traversed by dykes containing numerous enclosed pieces of gabbro1 while the basalt on the summit of the plateau is full of similar fragments--an occurrence not observed elsewhere4 ,it is conceivable that the gabbro-bearing basalt-sheets are sills1 but ,mr4 ,harker has found no proof that they are so1 the evidence so far as it has been collected being rather in favour of the view that these sheets are superficial lavas1 and that they have been supplied from the dyke-fissures4 ,various considerations suffice to assure us that actual instances of the outflow of the basalt from its parent fissures should be expected to be exceptional4 ,the absence or scarcity of beds of scori among the basalt-plateaux may be taken as an indication that the lava as a rule flowed out without the formation of cinder-cones1 and therefore that these conspicuous monuments of the eruptive vents were probably always rare in ,britain4 ,if the lava was poured out tranquilly from one or two points along a fissure which were subsequently buried under floods of similar lava issuing from other fissures1 the chances that such points of emission should be laid open along the front of any escarpment are small4 ,and1 even when so exposed1 it might be difficult to feel sure that the dyke below was really the feeder of the basalt above1 unless the cliff were accessible and the rocks could be scrutinized foot by foot4 ,these elements of uncertainty are happily removed where the volcanic energy has drilled well-marked funnels of discharge and left them filled with the erupted materials1 as will be narrated in the next chapter4 ,,chapter ,,xli ,,the ,,eruptive ,,vents ,,of ,,the ,,basalt-,,plateaux ,vents filled with ,basalt or other ,lava-form ,rock--,vents filled with ,agglomerate ,it is one of the most interesting points in the ,tertiary volcanic history that1 in spite of the enormous geological revolutions that have passed since they became extinct1 the sites of many scattered vents can still be recognized4 ,a far greater number must lie buried under the basalts1 and of others the positions are concealed by the sea1 which now covers so large an area of the old lava-fields4 ,nevertheless1 partly within the area of the plateaux1 but still more on the surrounding tracts from which the basalts have been removed by denudation1 the traces of unmistakable vents of discharge may be recognized amid the general wreck4 ,in ,britain and the ,faroe ,isles1 it is chiefly along the coast-line that the process of denudation has revealed the volcanic vents of ,tertiary time4 ,the interior of the country is often loaded with peat1 covered with herbage1 or strewn with glacial detritus3 and even where indications of the vents are to be detected1 it is not always possible to ascertain their true limits and connections4 ,but where the structure of the plateaux has been laid bare along ranges of rocky precipice1 the vents have sometimes been so admirably dissected by the sea that every feature of their arrangements can be satisfactorily determined4 ,as the actual physical connexion of these volcanic orifices with the plateaux has been in most cases removed by denudation1 we can usually only by inference place them in what was probably their true relation to the plateau-eruptions4 ,those which project from the surface of the plateaux must1 of course1 be younger than the basalts through which they rise2 how much younger we cannot tell4 ,they may possibly be later than any of the plateau-sheets2 they may even belong to a subsequent and waning condition of volcanic action4 ,on the other hand1 the vents which can now be traced outside of the present limits of the edges of the plateaux may1 like those just mentioned1 be younger than the basalt-sheets1 or1 on the contrary1 they may be records of a period of eruptivity anterior to the emission of any of the rocks of the plateaux1 and may have been deeply buried under a mass of basalt-beds subsequently removed4 ,positive demonstration is1 from the nature of the case1 impossible in these instances4 ,but examples will be cited from the ,western ,isles and from ,faroe1 where the vents can be proved to belong to the time of the plateau-eruptions1 for they are seen to have broken through some of the basalt-sheets and to have been buried under others4 ,with this clear evidence of relationship in some cases1 there need be little hesitation in believing that in other instances where no such positive connexion can be found1 but where the vents are obviously such as the general structure of the plateaux would have led us to expect1 they may be confidently regarded as part of the phenomena of the plateau-eruptions4 ,sometimes the vents can be linked with lines of fissures or dykes4 ,this is especially the case where they are small in size4 ,more usually1 however1 no such relation can be demonstrated4 ,it will be remembered that among the modern ,icelandic eruptions1 some eruptive vents1 like the later cinder-cones of ,laki1 are ranged in a linear direction along the great fissure1 while others1 of an older series in the same district1 almost engulphed amidst the more recent lavas1 are clustered irregularly in groups4 ,a similar diversity of arrangement has been observed among the volcanic cones of the ,velay in ,central ,france4 ,considering as a whole the volcanic necks or eruptive vents which rise from the older rocks around the ,tertiary basalt-plateaux1 and sometimes even from the surface of these plateaux themselves1 we may conveniently follow the same classification as was adopted in dealing with those of ,palozoic age1 and1 according to the nature of the material that now fills them1 arrange them in two series3 7#a7 ,those occupied by some form of crystalline eruptive rock1 and 7#b7 those filled with volcanic agglomerate4 i4 ,,vents ,,filled ,,with ,,dolerite1 ,,basalt1 ,,etc4 ,these1 as the composition of the plateaux would lead us to anticipate1 are numerous4 ,they perhaps attain their most conspicuous development in ,antrim1 either on the tableland or among the underlying rocks round its edges4 ,the finest example in that district is undoubtedly furnished by the lofty eminence called ,slemish1 which rises above the surrounding basalt-terrace1 to a height of #adcg feet above the sea 7,fig4 #bid74 ,it is elliptical in ground-plan1 measuring some #djjj feet in length by #ajjj in breadth4 ,seen from the north1 it appears as a nearly perfect cone4 ,the material of which it consists is a coarsely crystalline olivine-dolerite1 presenting under the microscope a nearly holocrystalline aggregate1 in which the lath-shaped felspars penetrate the augite1 with abundant fresh olivine1 and wedge-shaped patches of interstitial matter4 ,the rock is massive and amorphous1 except that it is divided by parallel joints into large quadrangular blocks like a granitic rock1 and wholly different from the character of the surrounding basalts4 ,the latter1 which possess the ordinary characters of the rocks of the plateaux1 can be followed to within #hj yards of this neck1 which rises steeply from them1 but their actual junction with it is concealed under the depth of talus4 ,illustration3 ,fig4 #bid4--,slemish1 a ,volcanic ,neck or ,vent on the ,antrim ,plateau1 seen from the north4 ,illustration3 ,fig4 #bie4--,section of ,volcanic ,vent at ,carnmony ,hill 7,e4 ,hull74 ,t1 ,lower basalt2 ,c1 ,cretaceous strata2 ,l1 ,lower ,lias2 ,m1 ,triassic marls2 ,v1 ,vent4 ,at the nearest point to which the two rocks are traceable1 the basalts appear somewhat indurated1 break with a peculiar splintery fracture1 and weather with a white crust4 ,these characters are still better shown on abundant fragments which may be picked up among the debris further up the slope4 ,there can be no doubt1 ,i think1 that a ring of flinty basalt1 differing considerably in texture from the usual aspect of that rock in the district1 surrounds the neck4 ,the meaning of this ring will be more clearly seen from the description of another example in ,mull4 ,about four miles to the north-east of ,slemish1 a smaller and less conspicuous neck rises out of the plateau-basalts4 ,the rock of which it consists is less coarsely crystalline than that of ,slemish1 but its relations to the surrounding volcanic rocks are obviously the same4 ,on the west side of ,belfast ,lough a boss of similar rock1 about #abjj feet in diameter1 rises at the very edge of the basalt escarpment into the eminence known as ,carnmony ,hill 7,fig4 #bie74 ,on its northern side it presents along its wall a mass of interposed volcanic agglomerate4#bie ,on visiting with ,mr4 ,m',henry the quarry opened on the eastern face of this vent1 ,i was much struck with the remarkable cellular structure of some parts of the dolerite4 ,many of the vesicles are lined with a thin pellicle of black glass1 and the same substance occurs in minute patches in the body of the rock4 ,a thin slice exhibiting this structure was found by ,mr4 ,watts to possess the following characters3--,the rock is an ophitic dolerite consisting of plagioclase1 augite1 and iron ores1 without olivine1 enclosing one or two patches of finer basalt4 ,the vesicles in the latter1 and certain angular spaces between the crystals of the former1 have been wholly or partially filled with brown glass1 the outer part of which has been converted into radiating crystals of a brown mineral4 ,the occurrence of patches of glass which seem to have been squeezed into vesicles or cracks in the body of a dolerite or andesite has been noticed in some of the ,tertiary dykes4 ,but in the present case the glass occurs as a mere coating on the walls of the larger spheroidal vesicles1 the interior of which generally remains empty4 ,footnote #bie3 ,this neck was recognised by ,du ,noyer in #ahfh as one of the great pipes or feeders of the basaltic flows4 ,see ,prof4 ,hull1 ,explanation of ,sheets #ba1 #bh and #bi1 ,geol4 ,survey of ,ireland 7#ahgf71 p4 #cj4 ,of the other doleritic necks scattered over the surface of the ,antrim plateau1 ,i will refer to only one which occurs on the hillslopes between ,glenarm and ,larne4 ,it forms a prominence known as the ,scawt ,hill1 and consists of a boss of basalt1 which1 in rising through a vent in the plateau-sheets1 has carried up with it and converted into marble a large mass of chalk which is now exposed along its eastern wall 7,fig4 #bif74 ,illustration3 ,fig4 #bif4--,section of the east side of ,scawt ,hill1 near ,glenarm4 a1 bedded basalt2 b1 mass of chalk2 c1 basalt neck4 ,illustration3 ,fig4 #big4--,section of ,neck of ,basalt1 ,bendoo1 ,ballintoy4 a a1 ,chalk2 b1 neck4 ,as examples of similar necks which have been exposed by denudation outside the present limits of the same plateau1 ,i may allude to those which rise through the ,cretaceous and other ,secondary strata on the northern coast near ,ballintoy4 ,one of the most striking of these may be seen at ,bendoo1 where a plug of basalt1 measuring about #adjj feet in one diameter and #hjj feet in another1 rises through the ,chalk1 and alters it around the line of contact 7,fig4 #big74 ,another remarkably picturesque example is to be seen near ,cushendall1 where a prominent doleritic cone rises out of the platform of ,old ,red ,sandstone1 some distance to the north of the present edge of the volcanic escarpment 7,fig4 #bih74 ,illustration3 ,fig4 #bih4--,volcanic ,neck of ,dolerite near ,cushendall4 ,the greater coarseness of grain of the material filling these pipes1 compared with that of the sheets in the terraces1 is only what the very different conditions of cooling and consolidation would lead us to expect4 ,there is no essential difference of composition between the two rocks4 ,where the erupted material has been poured out at the surface1 it has assumed a finely crystalline texture1 while1 where it has slowly solidified within a volcanic pipe at some depth beneath the surface1 and where consequently its component crystals have had more time for development1 the resulting structure is much more largely crystalline1 with a more or less complete development of the ophitic structure4 ,illustration3 ,fig4 #bii4--,section of ,volcanic ,neck at ',s ,airde ,beinne1 near ,tobermory1 ,mull4 a a1 bedded basalts2 b b1 bedded basalts altered along the side of vent2 c c1 dolerite4 ,in the island of ,mull1 another instance of the same kind of vent has been observed and described by ,professor ,judd4#bif ,it rises in the conspicuous hill1 ',s ,airde ,beinne 7,sarta ,beinn71 about two miles south-west from ,tobermory1 and consists of a coarsely crystalline dolerite1 which becomes finer in grain towards the outer margin 7,fig4 #bii74 ,no bedding1 or structure of any kind beyond jointing1 is perceptible in it4 ,examined in thin sections under the microscope1 this rock is found to be another typical ophitic dolerite1 consisting of lath-shaped felspars embedded in augite1 with here and there wedge-shaped portions of interstitial matter and grains of olivine4 ,dr4 ,hatch found the felspars to contain spherical inclusions of devitrified glass1 filled with black granules and trichites1 and he observed that1 under a high power1 the interstitial matter is seen to consist mainly of a greenish-brown isotropic substance1 in which are inclosed small crystals of augite1 skeleton-forms and microlites of felspar1 sometimes in stellate aggregates1 as well as club-shaped1 cruciform1 arrow-headed and often crested microlites of magnetite4 ,footnote #bif3 ,quart4 ,jour4 ,geol4 ,soc41 xxx4 7#ahgd71 p4 #bfd4 ,illustration3 ,fig4 #cjj4--;,interior of the ,volcanic ,neck of ',s ,airde ,beinne1 near ,tobermory1 ,mull4 ,towering prominently above the flat basalt sheets1 this neck has an oval form1 measuring about half a mile in length by a quarter of a mile in breadth4 ,its central portion1 however1 instead of rising into a rugged hill-top1 as is usually the case1 sinks into a deep hollow1 which is filled with water1 and reminds one of a true crater-lake 7,figs4 #bii1 #cjj74 ,the middle of the neck is thus concealed from view1 and we can only examine the hard prominent ring of dolerite that surrounds the tarn4 ,the material occupying the hollow may be softer than that of the ring1 and may have been scooped out by denudation4 ,what we now see may not be the original surface1 but may have been exposed after the removal of possibly hundreds of feet of overlying material4 ,on the other hand1 it is conceivable that the hollow is really a crater-lake which was filled up with detritus and may have been overspread with basalt1 since removed4 ,it may be suggestively compared with the crater-hollows revealed by denudation on the cliffs of ,strom and ,portree ,harbour1 which will be described in a later part of this chapter4 ,possibly some more easily removable agglomerate1 representing an eruption later than that of the dolerite1 may occupy the centre of the volcanic pipe4 ,one of the most interesting features of this vent is to be found in its relation to the surrounding basalts4 ,the marginal parts of the rock along the line of contact are much finer in grain than the rest1 and have obviously cooled more rapidly4 ,the contrast between them and the ordinary dolerite nearer the centre1 however1 cannot be properly understood1 except in thin sections under the microscope4 ,dr4 ,hatch1 to whom ,i submitted my specimens1 observed that1 in place of the structure above described1 the marginal parts show an absence of the ophitic grouping except in small isolated patches4 ,instead of occurring in large grains or plates enveloping the felspars1 the augite is found in numerous small roundish grains1 together with grains of magnetite1 in equal abundance and of similar size4 ,the felspars are speckled over with opaque particles2 olivine has not been detected4 ,for miles around the vent1 the plateau-rocks are of the usual type--black1 compact1 sometimes amygdaloidal1 alternating with more coarsely crystalline decomposing bands1 the separation between different sheets being often marked by the ordinary red ferruginous partings4 ,but around the margin of the neck1 they have undergone a remarkable metamorphism4 ,the portions of them which adhere to the outer wall of the neck have lost their distinct bedding1 and have been1 as it were1 welded together into an indurated compact1 black to dull-grey rock1 so shattery and jointed that fresh hand-specimens1 three or four inches in length1 are not easily obtainable4 ,especially marked is one set of joints which1 running approximately parallel1 cause the rock to split into plates or slabs4 ,these joints are sometimes curved4 ,yet1 in spite of the alteration from its normal character1 the basalt retains in places some of its more usual external features1 such1 for instance1 as its amygdaloidal structure1 the amygdales consisting of calcite1 finely acicular mesotype1 and other minerals4 ,examined under the microscope1 this altered basalt presents a confused aggregate of colourless microlites 7felspar87 and innumerable minute granules of magnetite1 these two constituents being very unequally distributed4 ,sometimes the colourless portions preponderate1 in other places the opaque granules are heaped together in black patches1 which may possibly mark the position of fused augites4#big ,footnote #big3 ,notes by ,dr4 ,hatch4 ,in the zone of contact-metamorphism around some of the volcanic pipes in the plateaux1 we see changes analogous to1 but less developed than1 those which have been superinduced on so large a scale round the great eruptive bosses of gabbro1 granophyre1 etc41 that have broken up the terraced basalts along the west coast of ,scotland4 ,i shall accordingly return to this subject in connection with phenomena presented by these younger rocks 7p4 #chf74 ii4 ,,vents ,,filled ,,with ,,agglomerate ,while the necks of dolerite or basalt cannot always be satisfactorily discriminated from bosses which may never have established a connection with the surface1 there is no room for any doubt in this respect in the case of those filled with fragmentary materials4 ,as has been already pointed out1 the occurrence of true volcanic agglomerate may be accepted as evidence of the existence of an eruptive vent communicating with the surface of the earth4 ,the agglomerate in the vents associated with the basalt-plateaux1 like that of the ,palozoic vents1 is generally exceedingly coarse1 and without any trace of structure4 ,blocks of all sizes up to masses some yards in length1 and of the most diversified materials1 both volcanic and non-volcanic1 are dispersed confusedly through a granular paste of similar miscellaneous composition4 ,illustration3 ,fig4 #cja4--;,diagram to show the probable relation of the ,neck at ,carrick-a-raide1 ,antrim1 to an adjacent group of tuffs4 a a1 ,chalk2 b b1 lower group of bedded basalts2 c1 vent of ,carrick-a-raide1 filled with coarse volcanic agglomerate2 d d1 bedded tuffs2 e e1 large veins of basalt traversing the agglomerate2 f f1 zone of tuffs and pisolitic iron ore2 g g1 upper group of bedded basalts4 ,an instructive example of the general characteristics of agglomerate-vents1 and of the relation of these vents to the surrounding tuffs and basalts1 is to be found at the island of ,carrick-a-raide1 on the north coast of ,antrim1 and on the opposite mainland4 ,the visible mass of this neck is about #ajjj feet in diameter1 but the boundaries1 except on the land side1 are concealed by the sea4 ,the material filling up the vent is a coarse agglomerate1 in which blocks and bombs of basalt1 with pieces of chalk and flint1 are stuck at all angles in a dull dirty-green granular tuff4 ,some large and small intrusions of basalt rise through it4 ,owing partly to these intrusions1 and partly to the grass-covered slope that separates it from the line of cliff1 the actual contact of this neck with the volcanic beds of the escarpment cannot be seen4 ,i have no doubt1 however1 that the tuff1 which has already been referred to as so conspicuous a member of the series here1 was discharged from this vent4#bih ,the materials are as usual coarser in the pipe than beyond it1 but the finer portion or matrix of the agglomerate is similar to many bands of the tuff4 ,the structure of the locality may be diagrammatically represented as in ,fig4 #cja4 ,the bedded tuff is thickest in the neighbourhood of the vent1 and gradually dies away on either side of it4 ,footnote #bih3 ,see ,explanation of ,sheets #g and #h1 ,geol4 ,survey of ,ireland 7#ahhh71 p4 #ca4 ,but another important inference may be drawn from this locality4 ,i have already pointed out that the lower basalts here reach their minimum thickness4 ,their basement beds thin away towards the vent as markedly as the tuff thickens4 ,obviously they cannot have proceeded from that point of eruption4 ,yet1 that they had begun to be poured out before the discharge of the tuff is shown by their underlying as well as overlying that rock1 though westward1 owing to the thinning away of the undermost basalts1 the tuff comes to lie directly on the ,chalk4 ,hence1 we may legitimately infer that in this neighbourhood one or more other vents supplied the sheets of the lower basalts4 ,in the island of ,mull a number of detached bosses or patches of agglomerate much obscured by invasions of granophyre probably mark the sites of volcanic vents4 ,they will be more particularly noticed in ,chapter xlvii4 ,one of their most interesting features is the large number of fragments of felsitic or rhyolitic rocks which they contain4 ,in the promontory of ,ardnamurchan1 where the basalt-plateau has been invaded and displaced by later intrusions of crystalline rocks1 and has likewise been reduced to such a fragmentary condition by denudation1 some interesting examples of agglomerate necks have been laid bare4 ,one of the largest of these occurs on the north shore at ,faskadale4 ,cut open by the sea for more than a quarter of a mile1 this neck is seen to be filled with a coarse agglomerate1 composed mainly of basalt-blocks and debris1 but crowded also with angular and subangular pieces of different close-grained andesitic1 felsitic and porphyritic rocks belonging to the acid series to be afterwards described4#bii ,some of these stones exhibit a very perfect flow-structure1 and closely resemble certain fine-grained1 flinty1 intrusive rocks in ,mull1 to which allusion will subsequently be made4 ,the matrix of the agglomerate is of the usual dull dirty-green colour1 but is so intensely indurated that on a fresh fracture it can hardly be distinguished from some of the crystalline rocks of the locality4 ,the neck is pierced in all directions with dykes and veins of basalt1 dolerite1 andesite1 gabbro1 and felsitic rocks4 ,similar intrusions continue and increase in numbers farther west until the cliffs become a labyrinth of dykes and veins running through a mass of rocks which appears to consist mainly of dull dolerites and fine gabbros4 ,though the relations of this vent to the plateau-basalts are not quite plain1 the agglomerate seemed to me to rise out of these rocks4 ,at least the basalts extend from ,achateny to ,faskadale1 but1 as they are followed westwards1 they are more and more invaded by eruptive sheets1 and assume the indurated character to which ,i have already referred4 ,footnote #bii3 ,one of these felsites when viewed under a high magnifying power is seen to present an abundant development of exceedingly minute micropegmatite arranged in patches and streaks parallel with the lines of flow-structure in the general cryptocrystalline groundmass4 ,the close relationship between the felsites1 quartz-porphyries1 and granophyres will be afterwards pointed out in the description of the acid rocks4 ,it is remarkable that1 though these rocks occur abundantly in fragments in the volcanic necks and agglomerates of the plateaux1 not a single instance has been observed of their intercalation as contemporaneous sheets among the basic lavas4 ,the analogous case of the interstratification of felsitic tuffs among basic lavas in the volcanic series of the ,old ,red ,sandstone of ,central ,scotland has been described 7vol4 i4 p4 #bgi74 ,it is interesting to note that liparitic pumice and dykes have been erupted by some of the basaltic craters of ,iceland1 for example at ,askja1 rfajkull and ,snaefellsjkull4 7,mr4 ,thoroddsen1 ,dansk4 ,geograf4 ,tidsskrift1 vol4 xiii4 #gth and #hth parts47 ,on the south side of the peninsula of ,ardnamurchan1 another agglomerate1 noticed by ,professor ,judd1#cjj rises into the bold headland of ,maclean's ,nose1 at the mouth of ,loch ,sunart1 and affords better evidence of its relation to the bedded basalts4 ,it measures about #ajjj yards in length by #cjj in breadth1 and its summit rises more that #ijj feet above the sea1 which washes the base of its southern front4 ,it is filled with an agglomerate even coarser than that on the northern coast4 ,the blocks are of all sizes1 up to eight or ten feet in diameter4 ,by far the largest proportion of them consists of varieties of basalt and andesite1 slaggy and vesicular structures being especially conspicuous4 ,there are also large blocks of different andesitic porphyries and felsitic rocks like those just referred to1 a porphyry with felspar crystals two inches long being particularly abundant4 ,all the stones are more or less rounded1 and are wrapped up in a dull-green compact matrix of basalt-debris4 ,there is no stratification or structure of any kind in the mass4 ,numerous dykes or veins of basalt1 of andesite1 and of a porphyry1 resembling that of ,craignure1 in ,mull1 traverse the agglomerate4 ,some of the narrow basalt-dykes cut through the others4 ,footnote #cjj3 ,quart4 ,journ4 ,geol4 ,soc4 xxx4 7#ahgd71 p4 #bfa4 ,professor ,judd has subsequently 7op4 cit4 xlvi4 #ahij1 pp4 #cgd et seq47 given a map1 section and description of what he believes to be the structure of this ground1 with numerous details as to the petrography of the rocks4 ,the geological structure of this area is more fully referred to on pp4 #cah et seq4 ,illustration3 ,fig4 #cjb4--,section of agglomerate ,neck at ,maclean's ,nose1 ,ardnamurchan4 a a1 quartzites and schists2 b1 bedded basalts lying partly on the schists and partly on patches of ,jurassic sandstones that occupy hollows of the older crystalline rocks2 c1 agglomerate2 d d1 dykes and veins traversing the agglomerate2 e1 dolerite sheets of ,ben ,hiant4 ,the position of the vent1 with reference to the surrounding rocks1 will be understood from the accompanying section 7,fig4 #cjb74 ,on the eastern side1 the agglomerate can be seen to abut against the truncated ends of the flat beds of the plateau-basalts1 which are of the usual bedded compact and amygdaloidal character4 ,there can be no doubt1 therefore1 that the vent has been opened through these basalts4 ,but it will be observed that the latter belong to the lower part of the volcanic series4 ,these lowest sheets are exposed on the slope1 resting upon yellowish and spotted grey sandstone1 with seams of jet and a reddish breccia1 which1 lying in hollows of the quartzites1 quartz-schists1 and mica-schists1 form no doubt the local base of the ,jurassic rocks of the district4 ,hence1 the vent1 though younger than the older sheets of the plateau1 may quite well be contemporaneous with some of the later sheets4#cja ,footnote #cja3 ,it may here be remarked that there is evidence of great differences in the level of the base of the ,jurassic series and the bottom of the volcanic plateau in this district4 ,on the south and west sides of ,ben ,hiant the ,jurassic conglomerates may be seen lying on the edges of the crystalline schists only a little above high-water mark1 while on the north side1 the schists1 with their overlying unconformable cake of limestones1 rise several hundred feet above sea-level4 ,the surface on which the basalts were poured out was probably very uneven1 but there may also have been some considerable displacements of these basalts either before or during the injection of the dolerite sills of ,ben ,hiant4 ,an interesting feature at this locality is the peculiar grouping of some of the large dykes in the area around the agglomerate4 ,they run in the direction of the vent1 and one or other of them may represent the fissure or fissures on which the volcanic orifice was blown open to the surface4 ,another notable element in the geological structure of the ground is the vast amount of intrusive material1 both in dykes and sheets1 which has been erupted4 ,the intrusive sheets of ,ben ,hiant form the most prominent eminence in this part of ,ardnamurchan4 ,reserving them for description in the following ,chapter 7p4 #cah71 ,i will only remark here that they partly overlie the agglomerate1 and are therefore1 to some extent at least1 younger than the vent4 ,they belong to that late stage in the history of the basalt-plateaux when the molten material1 no longer getting ready egress to the surface1 forced its way among the rocks about the base of the bedded basalts1 and more especially on the sites of older vents1 which were doubtless weak places1 where it could more easily find relief4 ,the large neck now described is only one of a group scattered around it in the ground to the north4 ,two of these may be seen rising through a detached area of ,jurassic limestones and shales at the northern base of ,ben ,hiant4 ,a third1 almost obliterated by the intrusive sheets1 may be traced at the western end of that mountain above ,coiremhuilinn4 ,two others rising through the schists on either side of ,beinn na h-,urchrach1 have been much invaded by the sills of that eminence 7,fig4 #cbf74 ,it is doubtless owing to the extensive denudation of the basalt-plateau1 and the consequent uncovering of the rocks underneath it1 that this series of vents has been laid bare4#cjb ,footnote #cjb3 ,professor ,judd has united these scattered vents into a continuous platform of volcanic agglomerates1 which he represents as underlying the supposed lavas of ,ben ,hiant4 ,since the publication of his map and description1 ,i have re-examined the ground without being able to discover any trace of this platform4 ,all the visible agglomerates are separate necks1 their actual walls being sometimes exposed1 as in the neck immediately north of the base of ,ben ,hiant1 where the limestone in contact is marmorised1 though twelve yards of it is an ordinary dull blue rock4 ,by far the largest mass of agglomerate in any of the ,tertiary volcanic areas of ,britain is that which occurs on the north side of the main valley of ,strath1 in ,skye4#cjc ,unfortunately1 it has been so seriously invaded by the eruptive rocks of the ,red ,hills1 that its original dimensions and its relations to the surrounding rocks1 especially to the bedded basalts1 are much obscured 7see ,fig4 #cdh74 ,it can be followed continuously from the lower end of ,loch ,kilchrist along the southern slopes of ,beinn ,dearg ,bheag round to the western roots of ,beinn ,dearg ,mhor--a distance of more than two miles in a straight line1 and from ,kilbride to the flank of ,beinn na ,caillich above ,coire-chat-achan--a direct distance of two miles and a quarter4 ,a similar rock1 possibly a portion of the same mass1 appears in ,creagan ,dubha1 on the north side of the ,red ,hills4 ,if the whole of this agglomerate forms part of one originally continuous mass1 it must have been upwards of two miles in diameter4 ,there may1 however1 have been two or three closely adjacent vents4 ,the ,beinn na ,caillich patch1 for example1 appears to belong to a different area1 and that of ,creagan ,dubha is also probably distinct4 ,but there seems no reason to doubt that the mass which forms ,cnoc nam ,fitheach1 and all the long declivity on the southern flank of ,beinn ,dearg ,bheag1 occupies part of the site of a single volcano4 ,owing to the absence of sufficient sections1 it is hardly possible to determine how much of this fragmentary material should be assigned to the actual chimney4 ,the diameter of the whole mass is almost two miles4 ,but possibly a considerable proportion of this accumulation belongs to the external cone which gathered round the vent1 so that the eruptive pipe might thus be of much smaller dimensions than the superficial area of the agglomerate4 ,the subsequent invasion of so much granophyre1 not only that of the ,red ,hills1 but that of numerous smaller intrusions1 has indurated the agglomerate and made the investigation of its structure somewhat unsatisfactory4 ,footnote #cjc3 ,this extensive mass was not separated from the syenite of the ,red ,hills by ,macculloch4 ,von ,oeynhausen and ,von ,dechen noticed it as a conglomerate with quartz pebbles1 but did not realise its volcanic nature 7,karsten's ,archiv1 i4 p4 #ij74 ,in my map of ,strath 7,quart4 ,jour4 ,geol4 ,soc4 xiv4 plate i47 ,i distinguished it from the rock of the ,red ,hills1 but no name for it appears in the legend of the map1 nor is it referred to in the text4 ,its character as a true volcanic agglomerate was recognised by ,professor ,judd1 op4 cit4 p4 #bee4 ,see postea1 pp4 #chd et seq4 ,it might be supposed that the mere existence of intrusive bosses and veins rather furnishes an argument in favour of considering the visible agglomerate to belong to a deeper-seated part of the erupted material than the external cone4 ,but1 as will be afterwards shown1 there is some reason to regard the present conical or dome-shaped outlines of the granophyre hills as not far from their original forms1 and to believe that1 like the trachytic ,puys of ,auvergne1 they were much more superficial than plutonic eruptions4 ,a study of the cinder cones of ,central ,france shows that even these superficial accumulations have been invaded not only by bosses but by dykes4#cjd ,footnote #cjd3 ,the existence of a small dyke of andesite on the northern rim of the well-known crater of the ,puy ,parion has already been noticed4 ,the agglomerate of the great ,strath vent is a coarse tumultuous assemblage of blocks and bombs1 imbedded in the usual dull1 dirty-green matrix4 ,among the stones1 grit and sandstone1 together with scoriaceous1 vesicular and amygdaloidal basalts are specially abundant2 also pieces of various quartz-porphyries and granophyres1 among which a black felsite like that of ,mull may often be recognised4 ,in some places1 large masses of altered limestone and quartzite 7,cambrian7 are included2 in others1 pieces of yellow sandstone and dark shale 7,jurassic71 or of the bedded lavas4 ,some of these masses may be #ajj yards or more in length4 ,occasionally a breccia1 mainly made up of acid materials--granophyre or granite1--has been noticed by ,mr4 ,harker along the north side of the ,red ,hills1 which he thinks may rather be of the nature of a crush-breccia than a part of the true agglomerate4 ,the agglomerate of this district is wholly without stratification or structure of any kind4 ,on the north-west side of ,loch ,kilchrist1 indeed1 it weathers into large tabular forms1 the parallel surfaces of which dip to south-west2 but this is probably due only to jointing4 ,here and there1 dykes of basalt cut the rock in a general north-westerly direction1 but their number is remarkably small when compared with the prodigious quantity of them in the limestone at the bottom and opposite side of the valley1 some of which may possibly mark the fissure on which the vent was placed4 ,more abundant and extensive are the masses of granophyre that rise particularly along the outer margin of the agglomerate near ,loch ,kilchrist4 ,these may be connected with the great boss that forms the ,red ,hills1 of which further details will be given in ,chapter xlvi4#cje ,footnote #cje3 ,the granophyre intrusions in this agglomerate have been found by ,mr4 ,harker to have taken up and dissolved a considerable proportion of fragments of gabbro1 ,chapter xlvi4 p4 #cib4 ,the important question of the relation of this agglomerate to the plateau-basalts does not admit of satisfactory treatment1 owing to destruction of the evidence by the intrusion of the granophyre1 and likewise to enormous denudation4 ,nevertheless1 some traces still remain to indicate that the basalts once stretched over the site of the vent1 which probably rose through them4 ,looking westward from the ,hanks of ,beinn ,dearg ,bheag to the other side of ,loch ,slapin1 the geologist sees the bold basalt-escarpment of ,strathaird presenting its truncated beds to him at a distance of only two miles4 ,that these lavas were once prolonged eastwards beyond their present limits is obvious1 and that they stretched at least over these two intervening miles can hardly be doubted4 ,but we can still detect relics of them on the flanks of ,beinn ,dearg4 ,as we follow the agglomerate round the margin of the granophyre that mounts steeply from it1 we lose it here and there under beds of amygdaloidal basalt4 ,the rocks next the great eruptive mass of the mountain are so indurated and shattered that it is difficult to separate them from each other and determine their relative positions4 ,but1 so far as ,i could ascertain1 these basalts are fragments of beds that overlie the agglomerate 7,fig4 #cjc74 ,this is not the only place along the flanks of the ,red ,hills where portions of the bedded basalts have survived4 ,other localities will be subsequently alluded to4 ,illustration3 ,fig4 #cjc4--;,diagram to show the probable relations of the rocks on the southern flank of ,beinn ,dearg ,bheag4 a1 agglomerate2 b1 amygdaloidal and compact basalt-rocks2 c1 granophyre4 ,the ,strath vent has been drilled through the ,cambrian limestone1 and as the result of protracted denudation it now towers steeply #ejj or #fjj feet above that formation on the floor of the valley4 ,of the material discharged from it over the surrounding country no certain trace now remains4 ,we may infer from the nature of the rock which fills it that towards the end1 if not from the beginning of its activity1 its discharges consisted mainly of dust and stones4 ,a cone1 of which the remains are two miles in diameter1 must surely have sent its fragmentary materials far and wide over the surrounding region4 ,but on the bare platform of older rocks to the south1 beyond the bottom of the agglomerate declivities1 not a vestige of these erupted materials can now be found4 ,westward the escarpment of ,strathaird remains to assure us that no thick showers of ashes fell at even so short a distance as two miles1 either before or during the outpouring of the successive basalt sheets still remaining there4 ,we may therefore conclude with some confidence that here1 as at ,ardnamurchan1 the vent is younger than at least the older parts of the basalt-plateau4 ,unfortunately the uprise of the large bosses of granophyre that stretch from the ,red ,hills to ,loch ,sligachan has entirely destroyed the vent and its connections in that direction4 ,there is no certain proof that any molten rock ever issued from this orifice1 unless we suppose the fragmentary patches of amygdaloid on the southern flank of ,beinn ,dearg ,bheag to be portions of flows that proceeded from this centre of eruption4 ,the basalt-plateau which still remains in ,strathaird no doubt formerly extended eastwards over ,strath and northwards across the site of the ,red ,hills and ,cuillins1 joining on to the continuous tableland north of ,lochs ,brittle and ,sligachan4 ,how much of the plateau had been built up here before the outburst of the vent cannot be ascertained4 ,the agglomerate may possibly1 of course1 belong to the very latest period of the plateau-eruptions1 or even to a still younger phase of ,tertiary volcanic history4 ,the impression1 however1 made on my mind by a study of the evidence from the ,western and ,faroe ,isles is that the necks of agglomerate1 like those of dolerite and basalt1 really belong to different epochs of the plateau period itself2 and mark some of the vents from which the materials of the plateaux were successively emitted4 ,the example of ,carrick-a-raide 7p4 #bgg7 is peculiarly suggestive when we regard it in connexion with the great ,strath vent4 ,already the progress of denudation has removed at least half of the layer of dust and stones which1 thrown out from that little orifice1 fell over the bare chalk-wolds and black basalt-fields of ,antrim4 ,the neck that marks the position of the volcanic funnel has been largely cut away by the waves1 and is almost entirely isolated among them4 ,the vents at ,canna1 ,portree and the ,faroe ,isles1 to be afterwards described1 unquestionably belong to the eruptions of the plateau-period1 for their connection with the basalts can be clearly established4 ,at the ,strath vent1 however1 the march of destruction has been greater4 ,the connexion between this vent and the materials ejected from it has been entirely removed1 and we can only guess from the size of the remaining neck what may have been the area covered by the discharges from this largest of all the volcanic cones of the ,inner ,hebrides4 ,other masses of similar agglomerate are observable in the same region of ,skye1 where they not improbably mark the sites of other vents4 ,unfortunately their original limits and relations to the rocks through which the eruptive orifices were drilled have been much obscured by the uprise of the great masses of gabbro and granophyre of the ,cuillin ,hills4 ,several of these isolated intrusions occur in the midst of the gabbro1 as in ,harta ,corry and on the west side of the ,blaven ridge4 ,another mass is interposed between the gabbro and granophyre on ,druim an ,eidhne and at the base of the lavas between ,druim an ,eidhne and the ,camasunary valley4 ,mr4 ,harker has found a huge mass of agglomerate underlying the bedded basalts to the north and west of ,belig1 one of the hills on the west side of the large valley that runs from the head of ,loch ,slapin to ,loch ,aynort4 ,this mass has its bottom concealed by the granophyre which underlies it2 but it reaches a maximum thickness of perhaps #ajjj feet1 rapidly thinning out and disappearing4 ,it generally resembles the ,strath agglomerate1 but is distinguished by including a large proportion of fragments of gabbro4 ,mr4 ,harker remarks that a study of these agglomerates points to the existence of both gabbros and granophyres older than the volcanic series1 and therefore distinct from the gabbros and granophyres now exposed at the surface4 ,it is a suggestive fact that so many detached masses of agglomerate should occur around and within the areas of the great eruptive bosses of gabbro and granophyre4 ,they seem to indicate the former existence of groups of volcanic vents in these tracts1 and may thus account for the uprise of such large bodies of intrusive material through what must have been a weakened part of the terrestrial crust4 ,further north in ,skye a much smaller but more perfectly preserved vent has been laid open by denudation on the south side of ,portree ,bay--a deep inlet which has been cut out of the plateau-basalts and their underlying platform of ,jurassic sandstones and shales4 ,the great escarpment of the basalts has1 at the recess of ,camas ,garbh1 been trenched by a small rivulet1 aided by the presence of two dykes4 ,the gully thus formed exposes a section of a neck of agglomerate that underlies the basalts of the upper half of the cliff4 ,this neck is connected with a thick deposit of volcanic conglomerate and tuff which1 lying between the basalts1 extends from the neck to a considerable distance on either hand4 ,the general relations of the rocks at this locality are represented in ,fig4 #cjd4 ,illustration3 ,fig4 #cjd4--,section of ,volcanic ,vent and connected lavas and tuffs1 ,scorr1 ,camas ,garbh1 ,portree ,bay1 ,skye4 a1 ,rudely-bedded dull green tuff2 b1 coarse agglomerate2 c1 prismatic basalt2 d1 massive jointed basalt2 e1 red banded decomposing rock1 probably of detrital origin2 f1 plateau-basalts1 prismatic and rudely columnar2 g1 dyke of dolerite1 somewhat vesicular1 five to six feet broad2 h1 basalt dyke two to three feet broad2 i1 dyke or sill of similar basalt to h1 and possibly connected with it4 ,the agglomerate 7b7 is quite tumultuous1 and here and there strikingly coarse4 ,some of its included blocks measure five feet in length4 ,these fragments represent most of the varieties of the lavas of the district4 ,large slaggy masses are abundant among them1 and sometimes exhibit the annelide-like elongation of the vesicles which ,i have referred to as occasionally displayed by the plateau-basalts4 ,more than #fj feet of agglomerate are visible in vertical height from where its base is concealed by debris and vegetation to where its upper surface passes under a banded rock to be afterwards described4 ,that this unstratified mass of volcanic detritus marks the site of a vent can hardly be doubted1 although denudation has not revealed the actual walls of the chimney4 ,the steep grassy slopes do not permit the relations of the rocks to be everywhere seen1 but the agglomerate appears to pass laterally into finer1 rudely-stratified material of a similar kind1 which extends towards east and west as a thick deposit between the bedded basalts4 ,possibly denudation has only advanced far enough to lay bare the crater and its surrounding sheets of fragmentary material1 while the chimney lies still buried underneath4 ,to the east or left of the agglomerate the detritus becomes less coarse1 and shows increasing indications of a bedded arrangement4 ,close to the agglomerate the dip of the coarse tuff is towards that rock at about #aj4 ,a few yards further east a sheet of very slaggy basalt is seen to lie against the tuff1 which it does not pierce4 ,the vesicles in this adhering cake of lava have been pulled out in the direction of the slope till they have become narrow tubes four or five inches long and parallel to each other4 ,some parts of this rock have a curved ropy surface1 like that of well-known ,vesuvian lavas1 suggestive of the molten rock having flowed in successive thin viscous sheets down the slope1 which has a declivity of about #cj4 ,this part of the section may possibly preserve a fragment of the actual inner slope of the crater formed of rudely-bedded tuffs4 ,continuing still eastward1 we find the feebly stratified tuff 7a7 to be perhaps #bjj feet thick4 ,it forms a grassy declivity that descends from the basalt-escarpment above to the grass-covered platform which overlies a lower group of basalts4 ,the visible portion of this tuff presents a thoroughly volcanic character1 being made up of the usual dull dirty-green granular paste1 through which are dispersed angular and rough lumps of slag and pieces of more solid basalt1 varying up to a foot or two feet in length4 ,these stones are generally disposed parallel to the indistinct bedding1 but are sometimes placed on end1 as if they had assumed that position on falling from an explosive shower4 ,among the smaller stones1 pieces of a finely vesicular basic pumice are frequent and are among the most strikingly volcanic products of the deposit4 ,from a characteristic sample of these stones1 a thin slice was prepared and placed in ,mr4 ,harker's hands4 ,the following are his observations on it3--,a very compact dark grey rock1 amygdaloidal on a minute scale4 ,the lighter grey crust is probably due merely to weathering1 and the specimen seems to be a distinct fragment1 not a true bomb4 ,the slice shows it to be essentially a brown glass with only occasional microscopic crystals of a basic plagioclase4 ,it has been highly vesicular1 and the vesicles are now filled by various secondary products1 including a chloritic mineral1 nearly colourless and singly refracting in thin section1 and a zeolite4 ,tracing now the tuff from the west or right side of the vent1 we can follow it to a greater distance4 ,no abrupt line can be detected here1 any more than on the other side1 between the agglomerate and the tuff4 ,the latter rock extends under the overlying plateau of basalt1 at least as far west as ,portree ,loch1 a distance of fully a mile1 but rapidly diminishes in thickness in that direction4 ,traces of what is probably the same tuff can be detected between the basalts at ,ach na ,hannait1 more than three miles to the south 7,fig4 #cje74 ,it is thus probable that from the ,portree vent fragmentary discharges took place over an area of several square miles4 ,above the agglomerate of this vent two lavas may be seen to start towards opposite directions4 ,one of these 7c71 already referred to1 is a dull prismatic basalt with a slaggy bottom1 its vesicles being pulled out in the direction of the general bedding of the section4 ,it descends by a twist or step1 and then lies on the inclined surface of the tuff which dips towards the agglomerate and seems to pass into that rock4 ,further east this basalt increases in thickness and forms the lowest of the basalt-sheets of the cliff4 ,the lava that commences on the west side of the agglomerate 7d7 is a massive jointed basalt1 which1 though not seen at the vent1 appears immediately to the west of it and rapidly swells out so as to become one of the thickest sheets of the locality4 ,it lies upon the rudely-bedded tuff1 and is covered by the other basalts of the cliff4 ,that these two basalts came out of this vent cannot be affirmed4 ,if they did so at different times1 their emission must have been followed by the explosion which cleared the funnel and left the central mass of agglomerate there4 ,but that some kind of saucer-shaped depression was still left above the site of the vent is indicated by the curious elliptical mass of rock 7e7 that lies immediately above the agglomerate1 from which it is sharply marked off4 ,this is one of the most puzzling rocks in the district1 probably in large measure owing to its advanced state of decay4 ,it is dull-red in colour1 and decomposes into roughly parallel layers1 so that at a short distance it looks like a bedded tuff1 or like some of the crumbling varieties of banded lavas4 ,i could not obtain specimens fresh enough to put its nature and origin beyond dispute4 ,whatever may have been its history1 this ferruginous rock rests in a flat basin-shaped hollow directly above the agglomerate of the vent4 ,the form of this depression corresponds fairly well with what we may suppose to have been the final position and shape of the crater of the little volcano4 ,the rock that occupies the bowl dies out towards the east on the face of the cliff1 and the prismatic basalt 7c7 is then immediately covered by the rest of the basalt-sheets of the plateau 7f74 ,on the west side its precise termination is concealed by grass4 ,but it must rapidly dwindle in that direction also1 for not many yards away it is found to have disappeared1 and the basalts 7d and f7 come together4 ,though the decayed state of this rock does not warrant any very confident opinion regarding its history1 ,i am inclined to look upon it as a deposit of much disintegrated volcanic detritus washed into the hollow of the old crater when it had become filled with water1 and had passed into the condition of a maar4 ,the peculiarly oxidized condition of its materials points probably to long atmospheric exposure1 and an examination of the surrounding parts of the district furnishes more or less distinct evidence that a considerable lapse of time did actually intervene between the cessation of the eruptions of the ,portree volcano and the next great basalt-floods of this part of ,skye4 ,that volcanic eruptions from other vents continued after the ,portree vent had become extinct is proved by the great sheets of basalt 7f7 that overspread it1 and still bury a large tract of the fragmentary material which it discharged4 ,at a later time a fissure that was opened across the vent1 allowed the uprise of a basalt dyke 7g71 and subsequently another injection of similar material took place along the same line of weakness 7h74 ,before leaving this interesting locality we may briefly take note of the distribution of the ashes and stones ejected by the volcano1 and the evidence for the relative length of the interval between the outflow of the lavas below and that of those above the tuff and volcanic conglomerate4 ,these deposits may be traced in clear sections along the base of the cliffs for a mile to the west of the vent4 ,they thin away so rapidly in that direction that at a distance of three-quarters of a mile they do not much exceed fifty feet in thickness4 ,at ,camas ,bn they consist mainly of a fine1 dull-green1 granular1 rudely-stratified basalt-tuff1 through which occasional angular pieces of different lavas and rough slags are irregularly dispersed4 ,these stones occur here and there in rows1 suggestive of more vigorous discharges1 the layers between the platforms of coarser detritus being occupied by fine tuff4 ,some of the ejected blocks are imbedded on end--an indication of the force with which they were projected so as to fall nearly a mile from the crater4 ,the upper parts of the tuff pass upward into fine yellow1 brown1 and black clays a few feet in thickness1 the darker layers being full of carbonaceous streaks4 ,on this horizon the coal of ,portree was formerly mined4 ,the workings1 however1 have long been abandoned1 and1 owing to the fall of large blocks from the basalt-cliff overhead1 the entrance to the mine is almost completely blocked up4 ,one wooden prop may still be seen keeping up the roof of the adit1 which is here a slaggy basalt4 ,to the east and south-east of the ,portree vent1 extensive landslips of the volcanic series and of the underlying ,jurassic formations make it hardly possible to trace the continuation of the tuff-zone in that direction4 ,to the south1 however1 at a distance of rather more than three miles1 what is probably the same stratigraphical horizon may be conveniently examined from ,ach na ,hannait for some way to the north of ,tianavaig ,bay4 ,at the former locality the calcareous sandstones of the ,inferior ,oolite are unconformably covered by the group of rocks represented in ,fig4 #cje4 ,at the bottom of the volcanic series lies a sheet of nodular dolerite with a slaggy upper surface 7a74 ,wrapping round the projections and filling up the depressions of this lava comes a thin group of sedimentary strata from an inch or two to eighteen inches or more in thickness 7b74 ,these deposits consist of hardened shale charged with macerated fragments of linear leaves and other plant-remains1 including and passing into streaks of coal1 which may be looked upon as probably occupying the same horizon with the coal of ,portree4 ,but here1 instead of reposing on a mass of stratified tuff1 the carbonaceous layers lie on one of the bedded lavas4 ,the tuff has died out in the intervening three miles1 yet that some of the discharges of volcanic detritus reached even to this distance1 and that they took place during the accumulation of these layers of mud and vegetation1 is shown by the occurrence in the shales of pieces of finely amygdaloidal basalt1 from less than an inch to six inches in length1 likewise lapilli of a fine minutely cellular basic pumice1 like some varieties of palagonite4 ,the overlying dolerite 7c7 becomes finely prismatic at its junction with the sedimentary layers and has probably indurated them4 ,illustration3 ,fig4 #cje4--,section of the ,volcanic ,series at ,ach na ,hannait1 south of ,portree1 ,skye4 ,this intercalation of a shaly and coaly band among the lavas can be followed northward along the coast4 ,in some places it has been invaded by dykes1 sills1 and threads of basalt on the most remarkably minute scale1 of which ,i shall give some account in ,chapter xlii4 7see ,fig4 #cba74 ,north of ,tianavaig ,bay--that is1 about three-quarters of a mile nearer to the ,portree vent--a perceptible increase in the amount of volcanic material is observable among the shales and leaf-beds4 ,not only are lapilli of basic pumice abundant1 but the volcanic detritus has accumulated here and there in sufficient amount to form a band of dull greenish-brown tuff4 ,these coast-sections in the neighbourhood of ,portree afford additional illustrations of the characteristic fact1 on which ,i have already insisted1 that the interstratifications of sedimentary material in the basalt-plateaux frequently terminate upward in leaf-beds1 thin coals1 or layers of shale1 full of indistinctly preserved remains of plants4 ,as ,i have endeavoured to show1 this vegetation1 which was undoubtedly terrestrial1 probably grew not far from the sites where its remains have been preserved4 ,leaves and seeds would naturally be blown or washed into pools on the lava-fields1 and would gather there among the mud and sand carried by rain from the surrounding ground4 ,such a topography and such a sequence of events point to intervals of longer or shorter duration between the successive outpourings of basalt4 ,it was probably during one of these intervals of quietude that the crater of the ,portree volcano became a maar and was finally silted up4 ,reference has already been made to a conspicuous mass of agglomerate which occurs at the east end of the island of ,canna1 and marks the site of an important volcanic vent belonging to the ,small ,isles plateau4 ,a portion of it projects from the grassy slopes1 and rises vertically above the beach as a picturesque crag1 in front of the precipice of ,compass ,hill 7,fig4 #cjf74 ,but the same rock may be traced southward to the ,coroghon ,mr1 and north-westward in the lower part of the cliffs to a little beyond the sea-stack of ,an ,stll4 ,it has thus a diameter of at least #cjjj feet4 ,westward it passes under the conglomerate described in ,chapter xxxviii4 ,its eastern extension has been concealed by the sea4 ,illustration3 ,fig4 #cjf4--,view of part of a ,volcanic ,neck at the eastern end of the island of ,canna4 7,from a photograph by ,miss ,thom47 ,the materials that fill this vent consist of a typical agglomerate composed entirely1 or almost entirely1 of volcanic detritus4 ,the embedded blocks vary up to eight feet in diameter or even more4 ,they are chiefly fragments of various basalts and andesites1 generally vesicular or amygdaloidal4 ,some of these1 which have evidently been broken off from already consolidated lavas1 are angular or subangular in shape1 and their steam-holes are cut across by the outer surfaces of the stones4 ,where they consist of calcite1 zeolite1 etc41 the amygdales so exactly resemble those of the bedded basalts of the plateaux that1 as already remarked1 we must believe them to have been already filled by infiltration before the disruption of the rocks by volcanic explosions4 ,other blocks are true bombs1 with a fine-grained crust outside and a more cellular texture inside1 the vesicles of the outer crust being sometimes dragged round the surface of the stone4 ,the variety of materials included among the ejected blocks and the abundance of pieces of the red bole which so generally separates the plateau-basalts indicate that a considerable thickness of bedded lavas has probably been broken through by the vent4 ,beside the volcanic materials1 occasional angular pieces of red 7,torridon7 sandstone may be observed in the agglomerate4 ,the paste is a comminuted mass of the same material as the blocks1 tolerably compact1 and entirely without any trace of stratification4 ,the actual margin of this vent has nowhere been detected by me4 ,we never reach here the base of the volcanic series1 for it is sunk under the sea-level4 ,on the other hand1 the upper limits of the agglomerate have been partially effaced or obscured by the conglomerates which overlie it4 ,from the breadth of ground across which the agglomerate can be followed along the shore1 the vent might be regarded as having been perhaps not less than three-quarters of a mile in diameter4 ,but there is the same difficulty here as at the ,strath vent in ,skye in determining the actual limits of the volcanic funnel4 ,possibly there may have been more than one vent in close proximity4 ,even if there was only one1 the existing agglomerate may include not only what filled the chimney1 but also a portion of what had accumulated round the orifice and formed the external cone4 ,that the volcano continued for some time in vigorous eruption may be judged from the amount of material ejected from it1 the large size of its blocks1 and the distance to which they were sometimes thrown4 ,the pieces of ,torridon ,sandstone were no doubt derived from the extension of that formation underneath ,canna4 ,on the opposite island of ,rum1 where these pre-,cambrian red sandstones are copiously developed1 they form the platform through which the ,tertiary volcanic series has been erupted4 ,the several remaining outliers of the bedded basalts1 referred to in a previous chapter 7p4 #bae and ,fig4 #bfg7 as visible on the west side of this island1 show that the basalt-plateau of ,small ,isles1 which once covered that area1 rested immediately on the inclined edges of the ,torridon ,sandstones4 ,probably the same structure stretches westward under ,canna and ,sanday4 ,no traces of any ,jurassic strata have been detected beneath the volcanic rocks of ,rum1 though they are so well developed a few miles to the east in the island of ,eigg4 ,either they were not deposited over the pre-,cambrian rocks of ,rum1 or they had been removed from that ancient ridge before the beginning of the ,tertiary volcanic period4 ,certainly ,i have not detected a single recognizable fragment of any ,jurassic sedimentary rock in the agglomerate of ,canna4 ,this ,canna vent exhibits1 better than is usually shown1 the occurrence of dykes and irregular injections of lava through the agglomerate4 ,a large mass of a finely columnar basalt runs up from the beach at ,garbh ,asgarnish4 ,a similar rock forms several detached crags a little further south1 particularly in the headland of ,coroghon ,mr and the island of ,alman4 ,here the basalt is beautifully columnar1 its slender prisms curving from a central line until their ends abut against the agglomerate4 ,the truly intrusive character of this basalt is well shown on the southern front of ,coroghon ,mr1 and on the northern face of ,alman1 as represented in the accompanying diagrams 7,figs4 #cjg and #cjh74 ,illustration3 ,fig4 #cjg4--;,columnar ,basalt invading ,agglomerate of ,volcanic ,vent1 ,coroghon ,mr1 ,isle of ,canna4 7,height above #bj feet47 ,illustration3 ,fig4 #cjh4--;,columnar ,basalt invading ,volcanic ,conglomerate1 north side of ,alman ,islet1 ,canna4 ,although there is no conclusive evidence that these intrusions belong to the time of the activity of the vent1 yet they differ so much from the ordinary dykes 7one of which also cuts the agglomerate and ascends through the conglomerates and basalts above71 are confined so markedly to the vent and its immediate proximity1 and resemble so closely the basalt-injections of other vents1 such as those of the ,carboniferous and ,permian necks of ,scotland1 that they may with probability be regarded as part of the mechanism of the ,canna volcano4 ,though the form and size of the vent of this volcano cannot be precisely defined1 the upper part of its agglomerate1 as we have seen 7ante1 p4 #bai71 is dovetailed in the most interesting way with the series of coarse conglomerates1 which indicate strong river-action in this part of the volcanic area during the time of the eruption of the plateau-basalts4 ,the agglomerate vents described in the foregoing pages as occurring in ,antrim and among the ,inner ,hebrides all appear either in the midst of the plateau-basalts or in close proximity to them4 ,before quitting the ,scottish examples1 ,i may refer to some that rise through much more ancient formations at a distance from any portion of the volcanic plateaux1 and yet may with probability be assigned to the ,tertiary volcanic period4 ,during the progress of the ,geological ,survey through the district of ,applecross1 in the western part of the mainland of ,ross-shire1 and far away from the basalt-plateau of ,skye1 ,mr4 ,john ,horne#cjf has found two small necks rising on each side of a line of fracture1 through gently inclined ,torridon ,sandstones4 ,they are conspicuous from a distance by the verdure of their slopes1 in contrast with the brown tints of the surrounding moorland4 ,the larger of the two necks measures about #ahj by #aej feet1 and abruptly truncates the beds of ,torridon ,sandstone1 which as they approach it assume a bleached aspect and become indurated4 ,the material filling this vent is an agglomerate made up mainly of pieces of ,torridon ,sandstone and grit which1 though generally small1 occasionally measure a foot across1 and in one case were found to reach a length of four feet4 ,they are not as a rule markedly altered1 but some of them have acquired a glazed or vitreous texture4 ,besides these fragments of the general rock of the district1 there occur abundant lapilli of a basic volcanic rock1 found by ,mr4 ,teall to consist of porphyritic felspar1 extremely minute acicular microlites of felspar1 somewhat irregular transparent spaces now occupied by a yellowish-green substance1 and interstitial matter4 ,at the south end of the vent a small mass of decayed basalt appears to pierce the agglomerate4 ,footnote #cjf3 ,trans4 ,geol4 ,soc4 ,edin4 vii4 7#ahid71 p4 #ce4 ,illustration3 ,fig4 #cji4--,view of neck-like mass of breccia1 ,brochel1 ,raasay4 ,though there is no indication of the age of these necks1 they agree so closely in general character with known vents of the ,tertiary volcanic plateaux that there cannot be much hesitation in regarding them as dating from the same great period of basalt-eruption4 ,but no relic now exists anywhere around of lavas or tuffs ejected from them4 ,they rise on the bare ,applecross hills1 #ajjj feet above sea-level1 two miles from the shore1 and about ten miles from the nearest outlier of the basalt-plateau in the ,dn ,can of ,raasay4 ,if they once discharged streams of lava that united with the rest of the plateau1 the total destruction of this lava affords another impressive picture of the waste which the volcanic rocks of the ,inner ,hebrides have undergone4 ,the large proportion of ,torridon ,sandstone blocks in these two ,applecross necks suggests1 however1 that the orifices never became active volcanic vents4 ,they may have been mere spiracles1 or blow-holes1 where the funnels drilled by explosive vapours were filled up with the debris of the rocks that were blown out4 ,but that lava did rise within them is shown by the basic lapilli in the agglomerate1 and by the basalt which in both vents has found its way up the chimney4 ,in the island of ,raasay ,mr4 ,teall1 during the summer of #ahid1 observed a group of curious neck-like masses of breccia which pierce the ,torridon ,sandstone near ,brochel 7,fig4 #cji74 ,the blocks in them are large angular unaltered pieces of the surrounding sandstones and shales1 sometimes ten feet or more in length1 and the matrix is sometimes pure crystalline calcite like ,iceland spar4 ,the breccia is generally coarsest towards the outer margin4 ,but though the ,lewisian gneiss exists immediately below the thin cake of ,torridonian strata1 not a fragment of it could either ,mr4 ,teall or ,i1 when ,i visited the locality with him1 find among the components of the breccia4 ,nor did we detect any trace of volcanic material4 ,the general ground-plan of these masses is elliptical1 the most northerly measuring #cj yards in diameter4 ,where the junction of the breccia with the ,torridon strata can be seen it is a nearly vertical one1 the sandstones and shales being much jumbled and broken1 but not sensibly indurated4 ,this little cluster of patches of breccia can hardly be due to local crushing of the rocks4 ,their definite outlines and composition seem rather to indicate spiracles of ,tertiary time1 which never became vents erupting lava or ashes4 ,the absence of fragments of the underlying gneiss may be accounted for if we suppose that the orifices were completely cleared out by the violence of the explosions and were afterwards filled up by the falling in of the walls of the higher parts now removed by denudation1 which consisted of ,torridon ,sandstone and shale4#cjg ,footnote #cjg3 ,it is on one of these neck-like patches of breccia that ,brochel ,castle stands1 of which ,macculloch gave so sensational a picture in one of the plates of his ,western ,isles4 ,further research may detect at still greater distances from the basalt-plateaux ancient volcanic necks that might1 with more or less probability1 be referred to the ,tertiary period4 ,as an instance of this kind1 ,i refer to the neck at ,bunowen1 ,county ,galway1 recently described by ,mr4 ,m',henry and ,professor ,sollas4 ,though so remote from the ,tertiary basalt-plateaux1 the rock of this boss is an olivine-basalt presenting a close resemblance to some of the rocks of ,antrim4#cjh ,footnote #cjh3 ,trans4 ,roy4 ,irish ,acad41 #ahif4 ,as a final illustration of ,tertiary volcanic vents ,i will now describe the ,faroe group already alluded to 7vol4 i4 p4 #fc1 vol4 ii4 p4 #bef74 ,it was almost by a kind of happy accident that these vents were discovered4 ,noticing at a distance of a mile or more from the deck of a steam-yacht that the base of the great basalt cliffs on the west side of ,strom were varied by what looked like agglomerate1 ,i steamed inshore1 and was delighted to find1 as the vessel drew near to the cliff1 that the agglomerate assumed definite boundaries and occurred in several distinct patches1 until at last it presented the unmistakable outlines of a group of vents underlying and overspread by the bedded basalts of the plateau4 ,favoured by an unusually calm sea1 ,i was enabled to boat into every nook and round every buttress and islet of this part of the coast-line4 ,illustration3 ,fig4 #caj4--,view of ,volcanic ,neck piercing and overlain by the ,plateau-,basalts1 ,strom1 entrance of ,vaagfjord1 ,faroe ,islands4 7,from a photograph by ,colonel ,evans47 ,the basalt-plateau here presents to the western ocean a nearly vertical escarpment which must reach a height of at least #ajjj feet 7see ,fig4 #cbh71 and displays a magnificent section of the bedded lavas4 ,the lower part of this section shows chiefly the banded structure already described1 the layers of different consistency being etched out by the weather in such a way as to give them the look of stratified rocks4 ,in the upper part of the precipice columnar and jointed or prismatic sheets are more common1 but the most prominent band is the great sill1 to which further reference will be made in the next ,chapter4 ,in the course of the gradual retreat of the cliff1 as the waves tunnel its base1 and slice after slice is detached from its vertical front1 a group of at least five small vents has been uncovered lying along a nearly north and south line4 ,of two of these a segment remains still on the cliff-wall and passes under the basalts2 the others have been dissected and half cut away from the cliff1 while groups of stacks and rocky islets of agglomerate may mark the position of others almost effaced4 ,the horizontal distance within which the vents are crowded is probably less than half a mile1 but the lofty proportions of the precipice tend to lead the eye to underestimate both heights and distances4 ,illustration3 ,fig4 #caa4--,section of the same ,neck as that shown in ,fig4 #caj4 ,the agglomerate is a thoroughly volcanic rock1 consisting of large and small blocks of various basalts1 among which large slags are specially conspicuous1 the whole being wrapped in a granular matrix of comminuted volcanic detritus4 ,the arrangement of this material is best seen in the fourth vent 7,figs4 #caj and #caa74 ,in this characteristic volcanic neck 7b in ,fig4 #caa7 the boundary walls1 as laid bare on the face of the precipice1 are vertical1 and are formed of the truncated ends of the banded lavas 7a a7 which have been blown out at the time of the formation of the orifice4 ,the visible diameter of the vent was roughly estimated by me to be about #ajj yards4 ,no appreciable alteration was observed in the ends of the lavas next the vent4 ,the agglomerate is coarsest in the centre1 where huge blocks of slaggy lava lie imbedded in the amorphous mass of compacted debris4 ,on either side of this structureless central portion the agglomerate is distinctly stratified from the walls towards the middle1 at angles of #cj to #ce4 ,even from a distance it can be observed that the upper limit of the agglomerate is saucer-shaped1 the sloping sides of the depression dipping towards the centre of the neck at about the same angle as the rudely-stratified agglomerate underneath4 ,from the bottom of this basin to the sea-level may be a vertical distance of some #cj yards4 ,the basin itself has been filled up by three successive flows of basalt1 of which the first 7c7 has merely overflowed the bottom1 the second 7d71 entering from the northern rim of the basin1 extends across to the southern slope1 while the third 7e71 also flowing from the north1 has filled up the remainder of the hollow and extended completely across it4 ,the next succeeding lava 7f7 stretched over the site in such a way as to bury it entirely1 and to provide a level floor for the piling up of the succeeding sheets of basalt4 ,illustration3 ,fig4 #cab4--,volcanic ,neck close to that shown in ,figs4 #caj and #caa4 ,illustration3 ,fig4 #cac4--,section of wall of another ,neck of agglomerate in the same group with those represented in ,figs4 #caj1 #caa1 and #cab4 ,the second vent1 which is represented in ,fig #cab1 exhibits the same features1 but with some additional points of interest4 ,it measures roughly about #bj yards in diameter at the sea-level1 rises through the same group of banded basalt 7a a71 and is filled with a similar agglomerate 7b74 ,its more northerly wall is now coincident with a line of fault 7h7 which ascends the cliff1 and probably marks some subsidence after the eruptions had ceased4 ,the southern wall shows that a dyke of basalt 7g7 has risen between the agglomerate and the banded basalts1 and that a second dyke 7g7 traverses the latter at a distance of a few feet4 ,in this instance1 also1 the upper surface of the agglomerate forms a cup-shaped depression which has been filled in by two successive streams of lava 7c1 d74 ,among the succeeding lavas 7e7 the prominent sill 7f7 has been intruded1 to which further allusion is made on p4 #cbc4 ,these necks are obviously volcanic vents belonging to the time of the basaltic eruptions4 ,they have been drilled through the basalts of the lower part of the cliff1 but have been buried under those of the central and higher parts4 ,the arrangement of their component materials in rude beds dipping towards the middle of each vent shows that the ejected dust and stones must have fallen back into the orifice so as to be rudely stratified towards the centre of the chimney1 which was finally closed by its own last discharges of coarse detritus4 ,the saucer-shaped upper limit of the agglomerate seems to indicate1 as has been suggested above in the case of the ,portree volcano1 that after the eruptions ceased each vent remained as a hollow or maar on the surface of the lava-fields4 ,and the manner in which they are filled with successive sheets of basalt shows that in course of time other eruptions from neighbouring orifices gave forth streams of lava which1 in flowing over the volcanic fields1 eventually buried and obliterated each of the vents4 ,illustration3 ,,to ,,accompany ,,sir ,,archibald ,,geikie',s ,,ancient ,,volcanoes ,,of ,,britain ,map ,,vi ,,map ,,of ,,the ,,tertiary ,,volcanic ,,region ,,of ,,the ,,inner ,,hebrides ,the ,edinburgh ,geographical ,institute ,copyright ,j4 ,g4 ,bartholomew ,in the destruction of the precipice some of the vents have been so much cut away that only a small part of the wall is left1 with a portion of the agglomerate adhering to it4 ,the third neck1 for instance1 affords the section represented in ,fig4 #cac1 where the horizontal sheets of basalt 7a7 have still a kind of thick pellicle of the volcanic detritus 7b7 adhering to what must have been part of the side of the orifice of eruption4 ,the waves have cut out a cave at the base1 so that we can1 by boat1 get behind the agglomerate and see the margin of the volcanic funnel in the roof overhead4 ,the fragment of geological history so picturesquely laid bare on the ,strom cliffs presents a significant illustration of what seems to have been a frequent1 if not the normal type of volcanic vent in the ,tertiary basalt-plateaux4 ,by the fortunate accident that denudation has not proceeded too far1 we are able to observe the original tops of at least two of the vents1 and to see how such volcanic orifices1 which were doubtless abundant all over these plateaux1 came to be entombed under the ever-increasing pile of accumulating basalt4 ,there is still one feature of interest in these cliff-sections which deserves notice here4 ,every geologist who has studied the composition of the basalt-plateaux has remarked the comparatively insignificant part played by tuffs in these volcanic accumulations4 ,hundreds of feet of successive basalt-sheets may often be examined without the discovery of any intercalation of fragmental materials1 and even where such intercalations do occur they are for the most part quite thin and extremely local4 ,i found it impossible to scale the precipice for the purpose of ascertaining whether around the ,strom vents1 and connected with them1 there might not be some beds of tuff interstratified between the basalts4 ,if such beds exist1 they can only be of trifling thickness and extent4 ,here1 then1 are examples of once active vents1 the funnels of which are still choked up with coarse fragmentary ejections1 yet from which little or no discharge of ashes and stones took place over the surrounding ground4 ,they seem to have been left as crater-like hollows on the bare surface of the lava-fields4 ,,chapter ,,xlii ,,the ,,basic ,,sills ,,of ,,the ,,basalt-,,plateaux ,we have now followed the distribution of the basalt-plateaux1 the arrangement of their component materials which were erupted at the surface1 and the character of the dyke-fissures and vents from which these materials were ejected4 ,but there remains to be considered an extensive series of rocks which display some of the underground phenomena of the ,tertiary volcanoes4 ,the injection of many basaltic sheets had been clearly enforced by ,macculloch4 ,in #ahga ,i pointed out that at different horizons in the plateau-basalts1 but especially at their base and among the stratified rocks underneath them1 sheets of basalt and dolerite occur which1 though lying parallel with the stratification of the volcanic series1 are not truly bedded1 but intrusive1 and therefore younger than the rocks between which they lie4#cji ,the non-recognition of their true nature had led to their being regarded as proofs of volcanic intercalations in the ,jurassic series of ,scotland4 ,there is1 however1 no trace of the true interstratification of a volcanic band in that series1 every apparent example being due to the way in which intrusive sheets simulate the characters of contemporaneous flows4 ,footnote #cji3 ,quart4 ,jour4 ,geol4 ,soc4 xxvii4 7#ahga71 p4 #bif4 ,if such sheets had been met with only at one or two localities1 we might regard them as due to some mere local accident of structure in the overlying crust through which the erupted material had to make its way4 ,but when we find them everywhere from the cliffs of ,antrim to the far headlands of ,skye and the ,shiant ,isles1 and see them reappear among the ,faroe ,islands1 it is obvious that1 like those of ,palozoic time1 they must be due to some general cause1 and that they contain the record of a special period or phase in the building up of the ,tertiary volcanic tablelands4 ,i will first describe some typical examples of them from different districts1 and then discuss their probable relations with the other portions of the plateaux4 i4 ,,antrim ,first to be examined1 and now most familiar to geologists1 are the remarkable sheets that underlie the plateau of ,antrim1 and project at various parts of the picturesque line of coast between ,portrush and ,fair ,head4 ,from the shore at ,portrush1 as ,i have already remarked1 came the evidence that was supposed to prove basalt to be a rock of aqueous origin1 inasmuch as shells were obtained there from what was believed to be basalt4 ,the long controversy to which this supposed discovery gave rise is one of the most curious in the history of geology4#caj ,it continued even after the illustrious ,playfair had shown that the pretended basalt was in reality highly indurated shale1 and hence that1 instead of furnishing proof of the aqueous formation of basalt1 the ,portrush sections only contributed another strong confirmation of the ,huttonian theory1 which claimed basalt to be a rock of igneous origin4 ,footnote #caj3 ,for an excellent summary of this controversy and an epitome of the descriptions of the ,portrush section1 see the ,report on the ,geology of ,londonderry1 etc4 7,mem4 ,geol4 ,survey71 by ,j4 ,e4 ,portlock 7#ahdc71 p4 #cg4 ,it is now well known that the rock which yielded the fossils is a ,liassic shale1 that it is traversed by several sheets of eruptive rock1 and that by contact-metamorphism it has been changed into a highly indurated substance1 breaking with a splintery1 conchoidal fracture1 but still retaining its ammonites and other fossils4 ,the eruptive material is a coarse1 distinctly crystalline dolerite1 in some parts of which the augite1 penetrated by lath-shaped crystals of plagioclase1 is remarkably fresh1 while the olivine has begun to show the serpentinous change along its cracks4#caa ,this rock has been thrust between the bedding planes of the shales1 but also breaks across them1 and occurs in several sheets1 though these may all be portions of one subterranean mass4 ,some of the sheets are only a few inches thick1 and might at first be mistaken for sedimentary alternations in the shale4 ,but their mode of weathering soon enables the observer readily to distinguish them4 ,it is to be noticed that these thin layers of eruptive material assume a fine grain1 and resemble the ordinary dykes of the district4 ,this closeness of texture1 as ,griffith long ago pointed out1#cab is also to be noticed along the marginal portions of the thicker sheets where they lie upon or are covered by the shales4 ,but away from the surfaces of contact1 the rock assumes a coarser grain1 insomuch that in its thickest mass it presents crystals measuring sometimes an inch in length1 and then externally resembles a gabbro4 ,a more curious structure is shown in one of these coarsely crystalline portions by the occurrence of a band a few inches broad which is strongly amygdaloidal1 the cells1 sometimes three inches or more in diameter1 being filled with zeolites4#cac ,the general dip of the shales and of the intrusive sheets which have been injected between them is towards the east4 ,from underneath them a thick mass of dolerite rises up to form the long promontory that here projects northwards from the coast-line1 and is prolonged seawards in the chain of the ,skerries4 ,footnote #caa3 ,dr4 ,f4 ,hatch1 ,explanation of ,sheets #g and #h1 ,geol4 ,survey of ,ireland1 p4 #dj4 ,footnote #cab3 ,address to ,geological ,society of ,dublin1 #ahce1 p4 #ac1 ,jour4 ,geol4 ,soc4 ,dublin1 vol4 i4 ,the varieties of the ,portrush rock were described by the late ,dr4 ,oldham1 in ,portlock's ,report on the ,geology of ,londonderry1 p4 #aej2 see also the same work for ,portlock's own remarks1 p4 #ig4 ,footnote #cac3 ,for a list of the minerals in this rock1 see ,oldham1 op4 cit4 p4 #aea4 ,an interesting feature of the ,portrush sections is the clear way in which they exhibit the phenomena of segregation-veins--so characteristic of the thicker and more coarsely crystalline sills4 ,these veins or seams here differ from the rest of the rock mainly in the much larger size and more definitely crystalline form of their component minerals4 ,though sharply defined1 when looked at from a little distance1 they are found on closer inspection to shade into the surrounding rock by a complete interlacing of crystals4 ,on the shore1 they can be seen to lie1 on the whole1 parallel with the bedding of the sheets in which they occur1 but without rigidly following it1 since they undulate and even ramify4 ,a good section across their dip has been exposed in a quarry near the end of the promontory1 and shows that they are considerably less regular than the plan of their outcrop on the shore would have led us to anticipate4 ,the accompanying drawing 7,fig4 #cad7 represents the veins laid bare on a face of rock nine feet in length by five feet in height4 ,it will be seen that while there is a general tendency to conform to the dip-slope1 which is here from right to left1 the seams or layers unite into a large rudely-bedded mass1 which sends out processes at different angles4 ,the peculiar aggregation of minerals which distinguishes such veins is perhaps best seen at ,fair ,head1 and ,i reserve for the description of that locality what ,i have to say on the subject1 only remarking with regard to the ,portrush rock that the felspar shows a disposition to collect in the centre of the veins with the augite and the other dark minerals at the outer margins4 ,illustration3 ,fig4 #cad4--,view of ,segregation-,veins in a dolerite sill1 ,portrush1 ,antrim4 ,the contact-metamorphism at this locality is of more historical interest in connection with the progress of geological theory than of scientific importance4 ,it consists mainly in an intense induration of the argillaceous strata4 ,these pass here from their usual condition of fissile1 laminar1 dull1 dark shales into an exceedingly compact1 black1 flinty substance1 which in its fracture1 colour and hardness reminds one of ,lydian stone4 ,yet the ammonites and other organic remains have not been destroyed4 ,they are preserved in pyrites4 ,illustration3 ,fig4 #cae4--,view of ,fair ,head1 from the east1 showing the main upper sill and a thinner sheet cropping out along the talus slope4 ,of all the examples of ,tertiary sills in ,britain few are more imposing than that of the noble range of precipices which form the promontory of ,fair ,head4 ,leaving out of account the minor masses of eruptive rock which occur underneath it1 we find the main sheet to extend along the coast for nearly four miles1 to rise to a height of #fcf feet above the sea1 and to attain a maximum thickness of #bej feet4 ,this enormous bed dies out rapidly both to the east and west1 and seems also to thin away inland4 ,seen from the north1 it stands upon a talus of blocks as a sheer vertical wall1 #bej feet high1 and the rude prisms into which it is divided are continuous from top to bottom 7,fig4 #cae74 ,so regular is this prismatic structure1 and so much does it recall the more minute columnar grouping of the bedded basalts1 that at a little distance we can hardly realize the true scale of the structure4 ,it is only when we stand at the base of the cliff or scramble down its one accessible gully1 the ,grey ,man's ,path1 that we appreciate how long and thick each of the prisms actually is 7,fig4 #caf74 ,it may here be remarked that this regular prismatic jointing is one of the distinguishing features of the large sills1 and serves to mark them off from the bedded basalts1 even when these have assumed a columnar structure4 ,the prisms are much larger than the basalt-columns1 and never display the irregular starch-like arrangement so common among the plateau-basalts4 ,illustration3 ,fig4 #caf4--,view of ,fair ,head from the shore4 7,from a ,photograph by ,mr4 ,r4 ,welch47 ,the rock composing this magnificent sheet is a coarsely crystalline1 ophitic1 olivine-dolerite4#cad ,the same diminution of the component crystals1 which is so marked along the margins of the eruptive masses at ,portrush1 is strikingly exhibited at the borders of the ,fair ,head sill4 ,for about #ah or #bj inches upward from the bottom1 where the bed rests on the black1 ,carboniferous shales1 the dolerite is dark and finely crystalline1 weathering spheroidally in the usual manner4 ,but immediately above that bottom layer of closer grain1 the normal coarsely crystalline texture rapidly supervenes4 ,a similar closeness of grain is observable at the surfaces of contact where the sheet splits up on its western border4 ,footnote #cad3 ,professor ,judd has described what he calls a glomero-porphyritic structure in this rock 7,quart4 ,journ4 ,geol4 ,soc4 xlii4 7#ahhf71 p4 #ga74 ,nowhere1 so far as ,i know1 can the phenomena of segregation-veins be so instructively studied as along the abundant exposures of this great sheet4 ,the veins are most conspicuous where the rock occurs in thickest mass4 ,they vary up to three or four feet in thickness1 and1 as at ,portrush and elsewhere1 lie on the whole parallel to the upper and under surfaces of the sheet4 ,an erroneous impression may be conveyed by the term veins applied to them4 ,they are quite as much layers1 parallel on the whole with the bedding of the sheet1 yet not adhering rigidly to one plane1 but passing across here and there from one horizon to another4 ,that they are not due to any long subsequent protrusion of younger material through the main sheet is made manifest by the thorough interlocking of their component crystals with those of the body of the rock in which they lie4 ,the material that fills these veins has obviously been introduced into them while there was still some freedom of movement among the crystals of the surrounding rock1 which must thus have been still not quite consolidated and therefore intensely hot4 ,both crystallized slowly1 and in so doing their component minerals dovetailed with each other4 ,the constituents of the veins consist of an exceedingly coarse aggregate of crystals1 or rather of crystalline lumps of the same minerals that constitute the general mass of the rock1 the felspar and augite showing the ophitic intergrowth of the main rock1 but on a far larger scale4 ,some of the pieces of augite measure two inches or more in diameter4 ,the conditions under which these veins were produced must have differed in some essential respects from those that prevailed during the formation of the fine-grained1 highly siliceous veins already described as occurring in some dykes and sills4 ,this great ,fair ,head sill lies upon ,carboniferous strata1 but that it is to be classed with the ,tertiary volcanic series is1 ,i think1 demonstrated by its relations to the ,chalk at its eastern end4 ,it has there broken through that rock1 and converted it for a short distance into a white1 granular marble4 ,but it is at the western side that the most interesting sections occur to show the truly intrusive nature of the mass4 ,the rock there splits up into about a dozen sheets1 which1 keeping generally parallel with each other1 have forced their way between and partly across the bedding planes of the ,carboniferous shales 7,fig4 #cag74 ,in this way the huge1 unbroken mass1 #bej feet thick1 subdivides itself and disappears in a few hundred yards1 though it continues a little further inland1 and approaches the shore again half a mile to the south-west4 ,further evidence of the intrusive nature of this rock may be observed along the base of the precipice1 where at least one sheet #gj feet thick diverges from the main mass and runs eastwards between the ,carboniferous shales 7,fig4 #cae74 ,at the contact with the eruptive rock the shales are everywhere much indurated4 ,illustration3 ,fig4 #cag4--,section at ,farragandoo ,cliff1 west end of ,fair ,head1 showing the rapid splitting up and dying out of an ,intrusive ,sheet4 a1 ,carboniferous sandstone2 b1 ,carboniferous shale2 c1 intrusive sheet4 ii4 ,,skye ,all through the ,inner ,hebrides the base of the basalt-plateaux presents abundant examples of sills4 ,the general parallelism of these intrusive sheets to the bedding of the ,jurassic strata among which they lie has been above referred to as having given rise to the erroneous conclusion that in ,skye and elsewhere the basalts are interstratified with ,jurassic rocks1 and are consequently of ,jurassic age4 ,it was ,macculloch who first described and figured in detail the proofs of their intrusive nature4 ,his well-known sections in plate xvii4 of the illustrations to his work on the ,western ,islands have been repeatedly copied1 and have served as typical figures of intrusive igneous rocks4 ,nowhere in ,north-,western ,europe can the phenomena of sills be studied so fully and with such exuberance and variety of detail as in the island of ,skye and its surrounding islets4 ,on the western coast the greater subsidence of the basaltic plateau has for the most part submerged the platform of intrusive sheets1 though wherever the base of the bedded lavas is brought up to the surface the accompanying sills are exposed to view4 ,the east coast of the island has been classic ground for this part of volcanic geology since it supplied the materials for ,macculloch's descriptions and diagrams4 ,from the mouth of ,loch ,sligachan to ,rudha ,hunish1 at the north end of ,skye1 a series of sills may be traced1 sometimes crowning the cliffs as a columnar mural escarpment1 sometimes burrowing in endless veins and threads through the ,jurassic rocks4 ,the horizontal distance to which this continuous band of sills extends in ,skye is not far short of #cj miles4 ,but it stretches beyond the limits of the island4 ,it forms the group of islets which prolongs the geological structure and topographical features of ,trotternish for #d miles further to the north-west4 ,it reappears #aj miles still further on in the ,shiant ,isles4 ,thus its total visible length is fully #dj miles1 or if we include some outlying sills near the ,point of ,sleat1 to be afterwards described1 it extends over a distance of not less than #fj miles4 ,from the last outlier in ,skye to the sills of the ,isle of ,eigg is a distance of only #h miles1 thence to those of ,ardnamurchan #ag miles1 and to those of the south coast of ,mull #be miles4 ,thus this platform of intrusive sheets of the ,inner ,hebrides can be interruptedly followed for a space of not less than #aaj miles4 ,illustration3 ,fig4 #cah4--,view of the ,trotternish ,coast1 showing the position of the band of ,sills4 ,the dark band crowning the first slope above sea-level marks a conspicuous band of sills which towards the right descends to the beach and is prolonged seaward in the group of islands4 ,the ,storr ,rock appears as a slanting obelisk of rock on the hill to the left4 ,though none of the sills in ,skye itself attain the dimensions of the ,fair ,head sheet1 they present a greater variety of rock and of geological structure than is to be found in ,antrim4 ,they are specially developed at the base of the thick1 overlying1 basalt-plateau--a platform on which such a prodigious quantity of eruptive material has been injected4 ,part of this material consists of basic rocks in the form of dykes1 veins1 or sills2 part of it is included in the intermediate and acid groups1 and comprises veins1 sheets1 and bosses of granitoid1 felsitic1 rhyolitic1 trachytic1 and pitchstone rocks4 ,one of the peculiarities of the ,skye sills is the occurrence among them of compound examples1 where sheets of basic and acid material have been injected along the same general platform4 ,these will be more specially referred to in ,chapter xlviii4 ,with regard to the basic sills 7dolerites1 basalts1 etc471 ,i would remark that while in ,western ,scotland the ,antrim type of short1 thick intrusions1 or laccolites1 is also found1 the vast majority of the sheets are much thinner1 more persistent1 and less easily distinguishable from the bedded basalts4 ,illustration3 ,fig4 #cai4--;,columnar ,sill intrusive in ,jurassic ,strata east of ,kilmartin1 ,trotternish1 ,skye4 ,the high ground to the left is a portion of the basalt-plateau to the north of the well-known ,quiraing4 ,in describing the sills of ,skye ,i shall take first those of the eastern and then those of the western side of the island4 ,along the east coast1 from ,loch ,sligachan to the most northerly headlands and islets the sills play a notable part in the scenery1 inasmuch as they cap the great sea-cliff of ,trotternish and run as a line of ridges parallel to the trend of the coast1 while the plateau-basalts rise above them further inland as a lofty escarpment1 which includes the picturesque landslips of the ,storr ,rock and ,quiraing 7,figs4 #cah1 #cai74 ,beneath the thick sills1 the ,jurassic sandstones form a range of pale yellow precipices1 along which many thinner sheets of eruptive material have been intruded4 ,as ,macculloch well showed1 many of these sheets1 if seen only at one point1 might readily be taken for regularly interstratified beds1 but perhaps only a few yards distant they may be found to break across the strata and to resume their course on a different level4 ,the sills of this ,trotternish coast may be distinguished even at some distance from the bedded basalts by the regular prismatic jointing1 already referred to1 and by their frequently greater thickness1 while on closer inspection they are characterized by their much coarser texture4 ,they are generally somewhat largely crystalline ophitic dolerites1 gabbros or diabases1 and exhibit the persistent uniformity of composition and structure so characteristic of intrusive sheets and dykes4 ,these characters are well exhibited in the ,kilt ,rock1 a columnar sill capping the cliffs to the south of ,loch ,staffin 7,fig4 #cai74 ,these massive sills are prolonged in a series of picturesque flat tabular islets beyond the most northerly headlands of ,skye4 ,they probably continue northwards under the sea at least #ab miles further1 for sills of the same type rise there in the singularly striking group of the ,shiant ,isles 7,fig4 #cbj74 ,these lonely islets1 extending in an east and west direction for about three miles1 display in great perfection most of the chief characters of the ,skye sills4 ,they are especially noteworthy for including the thickest intrusive sheet and the noblest columnar cliff in the whole of the ,tertiary volcanic series of ,britain4 ,the larger of the two chief islands consists of two masses of rock connected by a strip of shingle-beach1 and having a united length from north to south of about two miles4 ,the northern half1 or ,garbh ,eilean1 presents towards the north a sheer precipice #ejj feet high4 ,this magnificent face of rock consists of one single sill1 but as its original upper limit has been removed by denudation and its base1 where it is thickest1 is concealed under the sea1 the sill may exceed #ejj feet in thickness4 ,the rock has the usual prismatic structure1 which imparts to it an impressive appearance of regularity4 ,the columns retain their individuality to a great height1 and though none of them perhaps can be followed from base to crest of the cliff1 many of them are evidently at least #cjj or #djj feet long4 ,macculloch1 who gave the first geological description of the ,shiant ,isles1 showed the intrusive nature of the igneous rocks1 and described the remarkable globular or botryoidal structure of the ,jurassic shales between which they have been injected4#cae ,professor ,heddle has published a brief account of the geology of the islands4#caf ,professor ,judd visited the group and brought away a series of specimens of their eruptive rocks1 which he found to include basic and ultra-basic varieties4#cag ,footnote #cae3 ,western ,islands1 vol4 i4 p4 #dda4 ,footnote #caf3 ,trans4 ,norfolk ,nat4 ,hist4 ,soc4 vol4 iii4 7#ahhj7 p4 #fa4 ,footnote #cag3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xxxiv4 7#ahgh7 p4 #fgg1 and xli4 7#ahhe7 p4 #cic4 ,my description in the text is the result of three successive visits to the islands4 ,illustration3 ,fig4 #cbj4--,view of the northern precipice 7#ejj feet high7 of the largest of the ,shiant ,isles4 7,from a ,photograph by ,colonel ,evans47 ,in ,garbh ,eilean1 where the thickest mass of erupted material presents itself1 at least three sills may be observed4 ,some low reefs that run parallel with the northern coast of the island consist of coarse ophitic gabbro in two or more sheets which have been intruded between the ,jurassic shales4 ,above these strata comes the great columnar sill1 its base gradually sinking towards the west until it passes under the sea1 and the vertical columns then plunge abruptly into the water4 ,the rock of which this massive sill consists is another large-grained gabbro or dolerite1 with an ophitic structure4 ,owing to the form of the ground it cannot be so satisfactorily examined as the neighbouring island of ,eilean ,mhuire1 which1 though less lofty and rather smaller than ,garbh ,eilean1 affords a succession of admirable and easily examined sections along its precipitous shores4 ,professor ,judd found that while the rocks are mainly ophitic gabbros and dolerites1 they include such highly basic compounds as dunite4 ,an examination of the ,eilean ,mhuire cliffs enables the observer to ascertain that the sills display considerable variety in texture and in the character and arrangement of their component minerals4 ,they are marked by a persistent1 more or less distinct disposition in rude beds1 and these again often display a banding of their constituents in lines parallel with the general bedding4 ,some of these bands are largely felspathic1 and are thus paler in colour4 ,others1 where the ferro-magnesian minerals and ores are more specially aggregated1 are dark in colour4 ,in some layers the long black prisms of augite are ranged in a general parallelism with the banding4 ,a specimen selected as typical of the ordinary coarse-grained amorphous rock was sliced and placed in ,mr4 ,harker's for microscopic examination1 and he has supplied the following observations regarding it3 ,the gabbro from ,eilean ,mhuire #gaaj is a crystalline rock showing to the eye lustrous black augites1 half an inch long1 and 7predominating7 felspar4 ,the microscope reveals1 in addition1 irregular grains of black iron-ore and little hexagonal prisms of apatite4 ,no olivine is to be detected4 ,as regards structure1 the augite has tended to crystallise out in advance of the felspar1 but this relation is not constant4 ,the augite is of a light-brown tint in slices1 and has an unusual kind of pleochroism4 ,the colour for vibrations parallel to the ,greek3 b-axis is of the purplish-brown tone seen in some soda-bearing augites2 parallel to ,greek3 g and ,greek3 a it has a yellow or citron tint4 ,the colour and pleochroism are more marked in the interior of a crystal than towards the margin1 but some crystals pass at the margin into a slightly pleochroic1 pale-green1 recalling gerine-augite4 ,the felspar tends to build elongated crystals4 ,it is a rather finely lamellated labradorite1 sometimes showing pericline- as well as albite-lamellae4 ,another specimen from one of the black bands in the same island1 with a linear arrangement of its component minerals1 is thus described by the same petrographer3 ,this rock #gaaa is of darker appearance than the preceding1 and contains abundant black iron-ore1 besides some pyrites4 ,it also differs in having a marked parallel disposition of its crystals4 ,except for the greater prominence of large irregular grains of iron-ore1 this rock under the microscope closely resembles the last described1 the parallel structure not being conspicuous in the slice4 ,the augite has the peculiar colour and pleochroism already noted1 and the felspar is of the same kind as before4 ,i did not succeed in finding in place any bands of dunite1 but this basic material probably occurs at the base of some of the sills where it has segregated from the rest of the mass1 like the picrite at the bottom of the ,bathgate diabase4 ,the amount of contact-metamorphism effected even by such thick sills as those of ,trotternish and ,shiant is much less than might be expected4 ,it seldom goes beyond a mere induration of the strata for a few yards1 often only for a few inches from the surface of junction4 ,in the ,shiant ,isles1 however1 the shales between the sills have undergone a more remarkable alteration4 ,they have not only been greatly indurated1 but have acquired the globular or botryoidal structure so fully described by ,macculloch4 ,the spheroidal aggregates vary from not more than a line to more than half an inch in diameter1 and appear on the surface as dark1 irregularly grouped1 pea-like aggregates4 ,this structure is perhaps best developed immediately under the thick sill on the west side of ,eilean ,mhuire4 ,the massive sills are not the only evidence of the injection of igneous material on the ,shiant ,isles4 ,the sill1 or more probably group of sills1 forming ,eilean ,mhuire is traversed by a number of sheets of basalt varying from only two or three inches to #bj feet in thickness4 ,these black fine-grained rocks invariably present chilled selvages next the coarse gabbro1 and though they have been on the whole injected parallel to the general bedding or banding1 they here and there break across it as veins4 ,the most important of these later intrusions forms a columnar sill on the eastern side of the island1 and can be followed for several hundred yards4 ,it consists of a dark finely crystalline olivine-basalt1 which towards the margin assumes a dense black texture4 ,under the microscope ,mr4 ,harker found a thin slice of this rock to be an olivine-basalt of semi-ophitic1 semi-granulitic structure #gaab4 ,the olivine is mostly fresh1 but part of it is converted into a yellowish-brown pseudomorph like iddingsite4 ,magnetite occurs chiefly in imperfect octohedra4 ,the felspar is in little lath-shaped sections1 many of which are finely striated1 and give extinction-angles indicating a labradorite4 ,the augite1 light brown in the slice1 never has crystal-boundaries1 and often enwraps the felspars4 ,the narrow veins are composed of a much closer-grained basalt in which a few scattered felspars are visible4 ,mr4 ,harker remarks1 with regard to a thin slice of one of these rocks #gaac1 that the microscope shows this1 too1 to be an olivine-basalt4 ,the porphyritic felspars are twinned on the ,carlsbad and albite laws4 ,olivine and pseudomorphs after it are well represented4 ,magnetite is only sparingly present4 ,the general mass of the rock consists of very small striated prisms of labradorite1 granules of augite1 and interstitial matter which must be partly glassy4 ,this is perhaps the most striking of all the examples known to me where an older sill has been split open to receive a subsequent injection of molten material4 ,the ,eilean ,mhuire gabbro must be at least #bjj feet thick1 and it not impossibly passes under the still thicker pile of ,garbh ,eilean4 ,yet it has been horizontally ruptured near its base1 and into the rent thus produced another mass of molten matter has been thrust4 ,this subject will be again referred to in connection with another remarkable example on the west coast of ,skye4 ,illustration3 ,fig4 #cba4--,section of thin ,intrusive ,sheets and ,veins in carbonaceous shales lying among the ,plateau-basalts1 cliffs north of ,ach na ,hannait1 between ,portree ,bay and ,lock ,sligachan4 ,in contrast to such enormous thicknesses of intrusive material as those of ,trotternish and the ,shiant ,isles1 instances may be culled from the same belt of sills where the molten rock has been injected in thin leaves and mere threads into the ,jurassic sandstones and shales1 or into the shales and coals intercalated among the plateau-basalts4 ,thus1 on the cliff immediately to the north of ,ach na ,hannait1 between ,loch ,sligachan and ,portree ,bay1 the section may be seen which is represented in ,fig4 #cba4 ,at the base lies a vesicular dolerite with a slaggy upper surface 7a74 ,next comes a zone of sedimentary material about five or six feet thick1 the lower portion consisting of an impure coal1 which passes towards the right hand into brown and grey carbonaceous shale with plant-remains 7b74 ,this coaly layer has been already alluded to as probably lying on the same horizon with the coal of ,portree 7p4 #bhh74 ,traced northward1 it is found to have a bed of fine tuff beneath it1 and sometimes a volcanic breccia or conglomerate4 ,it fills up rents in the underlying slaggy lava1 and was undoubtedly deposited upon the cooled surface of that rock4 ,immediately above this lower band the black carbonaceous shale which follows has been invaded by an extraordinary number of thin cakes or sills and also by veins or threads of basalt4 ,for a thickness of two or three feet the band 7d7 consists mainly of these intrusions1 which1 in the form of a fine grey basalt1 vary from less than an inch to three or four inches in thickness4 ,they are separated by thin partings of coaly shale1 and as they tend to break up into detached nodule-like portions1 especially towards the right hand of the section1 they might1 on casual inspection1 be easily mistaken for nodules in the dark shales4 ,somewhat later in the time of intrusion are veins of basalt which1 as at c1 break across the nodular sills1 and sometimes expand into thicker beds 7c74 ,i have never seen such a congeries of minute sills among the ,tertiary basalt-plateaux as that here exhibited4 ,in a space of about three feet of vertical height there must be more than a dozen of roughly parallel leaves of intrusive rock4 ,veins 7e7 run up from the chief band of eruptive material into the overlying finely vesicular basalt 7f74 ,the dyke 7g7 is probably the youngest rock in the section4 ,the more general and extensive submergence of the base of the basalt-plateau on the west side of ,skye has for the most part carried the platform of sills below sea-level1 so that it is only exceptionally where1 owing to local irregularities1 that base has been brought up to the air1 that the intrusive sheets show themselves4 ,yet the persistence of the platform on that side is indicated by its extension even as far as the southern promontory of the island4 ,the ,trotternish type of sill extends down the west coast under the headlands of ,duirinish4 ,thus at the mouth of ,dunvegan ,loch1 where the underlying ,jurassic platform has been ridged up above the surface of the sea1 it has carried with it the marked sill which forms the islets of ,mingay and ,clett that lie as a protecting breakwater across the entrance of the inlet4 ,the intrusive rock rests on shell-limestones full of oysters 7,ostrea hebridica71 and referable to the ,loch ,staffin group of the ,great ,oolite ,series4 ,this sill1 when observed from a little distance1 presents the usual regularly prismatic or columnar structure so well developed among the ,trotternish examples1 but on a closer view shows this structure less distinctly4 ,it is an olivine-dolerite of medium and fine texture1 which in thin slices displays under the microscope a distinctly ophitic structure1 the abundant light-brown augite enclosing the striated felspars4 ,its lowest portion1 from three to seven or eight inches upward from the bottom1 is much closer-textured than the rest of the rock and is finely amygdaloidal4 ,its vesicles are in many cases drawn out to a length of three or four inches1 and the zeolites which now fill them look like parallel annelid tubes or stems of ,lithostrotion4 ,it is noteworthy also that the elongation of the vesicles has sometimes taken place at a right angle to the surface of contact with the underlying strata4 ,but the most remarkable feature in this sill is the surface which it presents to the oyster-beds on which it rests4 ,the fine-grained dark dolerite has there assumed the aspect of a sheet of iron-slag1 with a smooth or wrinkled1 twisted1 ropy surface1 which displays fine curving flow-lines4 ,no one looking at a detached specimen of this surface would be ready to admit that it could possibly have come from anything but a true lava-stream that flowed out at the surface4 ,the contours of a viscous lava are here precisely reproduced on the under surface of a massive sill4 ,a little further south1 the promontory of ,eist1 forming the western breakwater of ,moonen ,bay1 consists of an important sill or group of sills which has insinuated itself among shales1 shell-limestones1 and shaly sandstones1 full of ,ostrea hebridica1 ,cyrena aurata1 etc41 and belonging to the ,loch ,staffin group of the ,great ,oolite ,series4 ,the shore-cliff below the waterfall affords the section given in ,fig4 #cbb1 illustrating the manner in which a thick intrusive sheet may sometimes give off thin veins from its mass4 ,the rock attains on the ,eist promontory a thickness of probably at least #ajj feet1 where it is thickest and undivided4 ,but the two main sheets1 or branches of one great sheet1 on this peninsula have probably a united depth of more than #cjj feet4 ,landwards the rock splits up and encloses cakes of the ,jurassic strata4 ,it possesses the usual prismatic structure and doleritic composition4 ,in ,moonen ,bay1 as shown in ,fig4 #cbb1 it presents a banded structure1 marked especially by an alternation of lines of amygdales and layers of more compact and solid dolerite1 with occasional enclosed cakes of baked shale or sandstone4 ,its upper surface is somewhat uneven1 and from it are given off narrow1 wavy1 ribbon-like veins 7d71 from less than an inch to three inches or more in width1 which keep in a general sense parallel to the top of the sill1 but at a distance of a few inches or feet from it4 ,the sill becomes as usual fine-grained towards the contact1 the shales and sandstones being indurated and the limestone marmorized4 ,illustration3 ,fig4 #cbb4--,upper part of ,sill1 ,moonen ,bay1 ,waternish1 ,skye1 showing the divergence of veins4 a1 false-bedded shaly sandstone2 b1 shell-limestone2 c1 dolerite sill2 d1 veins proceeding from the sill4 ,length of section about five yards4 ,the next uprise of the base of the basalt-plateau on the west side of ,skye lies about #be miles to the south-east1 where it emerges from the sea in the ,sound of ,soa 7,fig4 #cbc74 ,a vast volcanic pile has there been heaped up on the ,torridon sandstone1 the whole of the thick ,jurassic series1 which is found in force only three miles distant in ,strathaird1 having been removed by denudation from this area before the beginning of the ,tertiary volcanic period4 ,the plateau-basalts rests on the upturned edges of the ,torridonian sandstones and shales1 and are accompanied as usual by their underlying network of intrusive rocks4 ,it is hardly possible to exaggerate the wild confusion of sills1 dykes and veins which have been injected among the rocks1 at and on both sides of the unconformability4 ,endless sheets of basalt and dolerite have forced their way between the bedded basalts and the sandstones1 while across the whole rise vast numbers of dykes and veins4 ,narrow1 black1 wavy ribbons of basic material cross many of these veins1 while the later north-west dykes cut sharply through everything older than themselves4 ,as a natural section for the study of the phenomena of intrusion in many of their most characteristic phases1 ,i know no locality equal to the northern coast-line of the ,sound of ,soa1 unless it be the cliffs of ,ardnamurchan4 ,but the ,skye cliffs1 though less imposing than those of the great ,argyllshire headland1 have this advantage1 that instead of being exposed to the full roll of the open ,atlantic1 they form the margin of a comparatively sheltered strait1 and can thus be conveniently examined4 ,illustration3 ,fig4 #cbc4--,section of the base of the ,basalt-plateau with sill and dykes1 ,sound of ,soa1 ,skye4 a a1 ,torridon ,sandstone2 b1 ,bedded basalts2 c1 ,sill2 d d1 ,dykes4 ,following still the western seaboard of ,skye1 we meet with other striking examples of sills at a distance of some eight miles in a straight line eastward where1 between ,lochs ,slapin and ,eishort1 the prominent headland of ,suisnish juts out into the sea4 ,this promontory has long been known to geologists from the section of it given by ,macculloch as an instance of the connection between overlying rocks and dykes4 ,i have already alluded to it in that relation1 and refer to it again as an example of one of the thicker intrusive sheets of the ,inner ,hebrides4 ,denudation has here also proceeded so far that the whole of the volcanic plateau has been stripped off1 only some of the underlying sills being left1 together with the platform of older rocks between which and the vanished basalts they were injected4 ,most of these sills consist of granophyres belonging to the acid group of rocks to be afterwards described4 ,but basic sheets occur not infrequently interposed between the granophyres and the subjacent ,lias1 and sometimes even intercalated in the former rock4 ,though at first sight it might be thought that these sills had insinuated themselves after the eruption of the granophyre1 and there are instances where this cannot be shown not to be the case1 ,i have obtained so many proofs of the invasion of the basic by the acid rock that ,i have no doubt the former is1 as a general rule1 the older of the two4 ,the ,suisnish headland exhibits the structure represented in ,fig4 #bdi4 ,for about #cjj feet above the sea-level the steep grassy slope shows outcrops of the dark1 sandy shales and yellowish brown1 shaly sandstones of the ,lias which form the range of cliffs to the eastward4 ,these gently inclined strata are cut through by many vertical basalt-dykes1 some of which intersect each other1 but among which by far the largest is the mass shown in the figure4 ,this broad dyke consists of a dolerite or gabbro the largely crystalline texture of which marks it off at once from the others1 which are of the usual dark1 heavy1 fine-grained type1 with an occasional less basic and porphyritic variety4 ,traced up from the sea-margin1 the dyke loses itself in a talus of blocks from the cliff above1 so that its actual junction with the mural front of the sill cannot be seen4 ,but that it joins that mass1 with which it agrees in petrographical characters1 hardly admits of question4 ,the cliff consists of a thick sheet of coarsely crystalline dolerite or gabbro 7d in ,fig4 #bdi71 which in its general aspect at once recalls the rock of ,fair ,head4 ,it varies considerably in texture1 some parts of the mass are exceedingly coarse1 like the ,skye gabbros1 and present a fibrous structure in their augite resembling that of the diallage in these rocks2 other portions assume the compactness of basalt4 ,a specimen of medium grain under the microscope shows the typical ophitic structure so generally found among the dolerites both of the plateaux and of the intrusive sheets4 ,this sill must be about #bjj feet thick1 and like the rock at ,fair ,head is traversed from top to bottom by joints that divide it into prisms4 ,it appears to bifurcate eastward1 one portion running with a tolerably uniform thickness of a few feet as a prominent band at the top of the shales and sandstones1 the other slanting upwards and gradually thinning away in the granophyre4 ,towards its base1 near the contact with the underlying shales1 the rock as usual becomes finer grained1 and the thin band just referred to resembles in texture one of the wider basalt-dykes4 ,westwards the rock can be followed round the top of the grassy slopes formed by the decay of the shales4 ,though concealed by intervals of moorland and peat1 it is visible in the stream sections1 and ,i think must be continuous1 as a band only a few yards thick1 round the northern side of the hills as far as ,beinn ,bhuidhe1 where a similar sill makes a prominent crag4 ,its total area measures a mile and a quarter in length by half a mile in breadth4 ,the granophyre which overlies it forms part of an interesting series of sheets which ,i have traced all the way from ,suisnish to the braes above ,skulamus4 ,whether or not the whole sheet of basic rock is continuous1 and whether it all proceeded from the great ,suisnish dyke1 cannot be confidently decided until the ground is mapped in detail1 though from the great thickness of the sill at the dyke1 its attenuation outwards from that centre and its uniformity of petrographical character1 ,i am disposed to answer affirmatively4 ,there is no other probable vent to be seen in the neighbourhood1 unless a massive dyke that runs from ,loch ,fada north-westwards into ,glen ,boreraig can be so regarded4 ,not far from the extreme southern point of ,skye a singularly interesting example of a sill remains as a detached survival of the basaltic plateau and its accompaniments4 ,in his map of ,skye1 ,macculloch showed the position of this outlier1 which he classed with the general trap formation of the island4 ,the locality was visited by ,professor ,judd1 who regarded the intrusive rock as a phonolite#cah ,in #ahid1 during an excursion with my colleague ,mr4 ,c4 ,t4 ,clough1 ,i had an opportunity of examining the rocks and collecting notes for the following account of them4 ,footnote #cah3 ,quart4 ,jour4 ,geol4 ,soc4 vol4 xxxiv4 7#ahgh7 p4 #fib4 ,at ,rudh' an ,iasgaich1 about two miles from the ,point of ,sleat1 a small outlier of conglomerate lies on the edges of the ,torridon ,sandstone4 ,this deposit has been correctly identified by ,professor ,judd with the similar strata which1 in ,skye and elsewhere on the west coast of ,scotland1 underlie the ,liassic series4 ,it is here about #aj or #ab feet thick1 reddish and yellowish in colour1 and distinctly calcareous4 ,its component pebbles consist largely of ,cambrian 7,durness7 limestone1 quartzite1 and ,torridon ,sandstone--rocks which all occur in situ in ,sleat4 ,it may be compared with the limestone conglomerates of ,strath and those which underlie the ,lias at ,heast on ,loch ,eishort4#cai ,that here1 as elsewhere in this region1 the basement conglomerate was followed by the rest of the ,lias and ,oolites may be inferred with some confidence from the copious development of the ,jurassic series a few miles off1 both to north and south4 ,but the whole of this overlying succession of formations has here been swept away1 and1 but for the protection afforded by the eruptive rocks of ,rudh' an ,iasgaich1 the conglomerate would likewise have disappeared4 ,footnote #cai3 ,op4 cit4 vol4 xiv4 7#aheg71 p4 #i2 vol4 xliv4 7#ahhh71 p4 #ga4 ,illustration3 ,fig4 #cbd4--,section of ,dolerite ,sill cut by another sill1 both being traversed by dykes1 ,rudh' an ,iasgaich1 western side of ,sleat1 ,skye4 ,above the conglomeratic band lies a sheet of intrusive rock1 which in one place has apparently cut it out1 so as to rest directly upon the ,torridon ,sandstone 7a1 ,fig4 #cbd74 ,the decay of the softer detrital rock underneath has caused the sill to break off in slices1 which have left behind them a bold mural escarpment 7b b74 ,the rock of this sill is a rather coarsely crystalline porphyritic olivine-dolerite1 which towards the north attains a thickness of about #gj feet4 ,it exhibits the usual prismatic jointing1 but less perfectly than some of the ,trotternish sills already referred to4 ,besides these vertical joints1 it is also traversed by a system of horizontal divisional planes which1 though somewhat irregular in their course1 run1 in a general sense1 parallel to the upper and under surfaces of the sill4 ,it seems to have been along this transverse series of joints that a second sill 7c71 five or six feet thick1 has been injected4 ,the material of this younger intrusion is a black1 finely crystalline dolerite or basalt1 with rudely prismatic jointing4 ,its most striking feature1 besides its regularity of position and persistency for several hundred yards as a platform along the shore1 is the basalt-glass which marks both its under and upper surfaces of contact1 and which is here developed upon a scale to which ,i have not met with an equal among the ,tertiary sills of this country4 ,the selvage of glass appears as a black tar-like layer1 varying from a mere film to two or three inches in thickness4 ,it is found not only on the upper and under surfaces1 but descends along abrupt step-like interruptions of the upper surface1 a foot or more in height1 as if the sill had been broken by a series of subsidences4 ,the apparent fracture1 however1 is probably due to the irregularities of the passage forced for itself by the molten rock as it passed from one line of horizontal joint to another through the heart of the older sheet4 ,the exposed surface of black glass on the top of the younger sill exhibits long parallel lines1 probably marking flow-structure1 which are made conspicuous by a pale yellow ferruginous weathered crust4 ,portions of the larger intrusive sheet have been broken off and involved in the later rock4 ,the younger sill disappears to the north1 and is not found in the cliff of ,rudha ,chrn nan ,cearc1 where the thick sill1 lying once more on the band of conglomerate1 forms a fine escarpment above the shore4 ,dykes of fine-grained basalt 7d d7 with compact chilled margins rise through both sills1 together with veins which pursue a wavy upward path like strips of black ribbon4 ,this example1 and that of the ,shiant ,isles already described1 cannot but impress the observer with the prodigious force with which the material of the sills was injected4 ,in these instances solid sheets of intrusive rock have subsequently been rent open1 doubtless under a superincumbent pressure of many hundreds of feet of the terrestrial crust1 and a new injection of molten magma has made its way into the rents thus caused4 ,in each case1 the position of the rents was obviously determined by structural lines in the older sills1 but we are lost in astonishment at the energy required to split open1 even along these lines1 such solid crystalline masses as the thick sills1 and to overcome the superincumbent pressure of so deep a pile of rock4 ,the isolation of a relic of the ,tertiary sills on the west side of the promontory of ,sleat presents some interesting problems to the mind of the geologist4 ,the locality lies about midway between the basalt-plateau of ,strathaird and that of ,eigg1 and some eight or nine miles in a direct line from either4 ,the basalts cannot be proved to have once stretched continuously between ,eigg and ,strathaird1 and to have covered this part of ,sleat2 but the position of the ,sleat sills makes it probable that this continuation did formerly exist4 ,the denudation of the ,west of ,scotland since early ,tertiary time has been so stupendous that ,i am prepared for almost any seemingly incredible evidence of its effects4 ,there can hardly be any doubt1 however1 that the sills here described belong to the great platform of intrusive sheets1 and that they were injected under a pile of ,secondary strata1 if not also of ,tertiary basalts1 which has here been entirely removed4 ,reference may be made1 in conclusion1 to a not infrequent feature of the ,skye sills4 ,like the dykes1 they are often double or multiple1 molten material having been successively injected along the same plane4 ,the example just cited from the west side of ,sleat illustrates one type of such compound sills4 ,more frequently1 however1 the subsequent injections have been made along the floor or roof of the first sheet4 ,mr4 ,harker has found numerous cases of this structure in the ,strath district4 ,they are recognizable even from a distance by their terraced contours when seen in profile4 ,they often vary considerably in thickness owing to the dying out or coming-in of their separate bands2 while1 on the other hand1 single sills tend to maintain a uniform thickness for long distances1 or taper away gradually4 ,the compound arrangement of the basic sills is well brought out where acid material has been injected between the sheets1 as will be more fully described in ,chapter xlviii4 iii4 ,,eigg1 ,,ardnamurchan ,the phenomena of the coasts of ,skye are repeated on the east side of ,raasay1 in ,eigg1 and still more magnificently along the south coast of ,mull4 ,a single example is here given 7,fig4 #cbe7 from the east side of ,eigg4 ,over the ,jurassic sandstones 7a a7 a sill of basalt 7#a7 four to six feet thick has been injected between the stratification1 and another 7#b7 two to four feet thick has forced its way across the middle of one of the bedded basalts 7b b7 in which it bifurcates1 and above which comes the thick series of lavas of the plateau 7c1 d74 ,in one of the streamlets1 which exposes a section of the ,jurassic strata below the volcanic escarpment1 more than twenty intrusive sheets may be counted among the shales and limestones4 ,they are sometimes not six inches thick1 and seldom exceed six or eight feet4 ,illustration3 ,fig4 #cbe4--,section to show ,bedded and ,intrusive ,sheets1 ,eigg4 ,i will conclude this account of the ,tertiary basic sills of ,britain by referring to one further district in the ,west of ,scotland1 where they are well displayed on bare hillslopes and also along a picturesque sea-coast4 ,in the promontory of ,ardnamurchan in the west of ,argyleshire1 one of the most conspicuous eminences1 known as ,ben ,hiant1 affords a striking mass of intrusive material1 which1 extending along a rugged shore for three-quarters of a mile1 mounts thence inland in a series of rocky knolls1 and in rather less than a mile culminates in a summit1 #agbi feet above sea-level4#cbj ,the rocks which cover this large space are disposed in numerous rude beds1 which have a seaward dip of perhaps #ae to #bj1 and are sometimes distinctly prismatic1 the prisms being not infrequently grouped in fan-shape4 ,they are evidently due to successive intrusions4 ,although generally coarsely crystalline in texture1 they include also intermediate and fine-grained sheets4 ,they are never1 so far as ,i have been able to discover1 amygdaloidal1#cba nor do they present the ordinary external characters of the beds of the plateaux1 though here and there they appear to have caught up portions of the plateau-series4 ,they distinctly overlie the bedded basalts on their eastern and southern margins2 but westwards they appear to lie transgressively across the edges of these rocks1 while to the north-west and north they rest on quartzites and schists and on ,jurassic limestones4 ,an outlier from the main mass forms the prominent hill of ,srn ,mhor1 and can be seen distinctly overlying the bedded basalts as well as the neck of agglomerate already described 7,fig4 #cjb74 ,footnote #cbj3 ,this locality has been described by ,professor ,judd 7,quart4 ,jour4 ,geol4 ,soc4 xxx4 7#ahgd71 p4 #bfa2 and xlvi4 7#ahij71 p4 #cgc74 ,footnote #cba3 ,as amygdaloidal structure is occasionally to be found among both dykes and sills its presence in the ,ben ,hiant rocks would not be inconsistent with their intrusive origin4 ,the prevalent rocks of ,ben ,hiant are well crystallized1 ophitic olivine-dolerites and gabbros4 ,a specimen taken from the shore on the west side of the mass was found by ,dr4 ,hatch to present under the microscope its augite in large plates1 which enclose narrow laths and needles of plagioclase felspar as well as grains of olivine4 ,all the felspars are in lath-shapes1 sometimes extremely long and narrow4 ,the iron-ore likewise assumes an ophitic character1 enclosing rectangular portions of felspar4 ,dr4 ,hatch observed in another specimen1 taken from the south-east side of the hill1 a curious intermixture of two different structures4 ,scattered portions which show the usual ophitic structure1 their felspar and augite occurring in large crystals1 are1 so to speak1 imbedded in a groundmass which presents rather a basaltic type1 its felspar1 augite1 and magnetite1 in long thin needles1 microlites1 and other skeleton forms1 being enclosed in a dark devitrified base4 ,a third specimen1 selected from one of the columnar sheets near the top of ,ben ,hiant1 is a fine-grained dolerite 7or gabbro7 showing little ophitic structure1 the augite occurring in roundish grains1 and only slightly intergrown with the felspars1 which are more or less lath-shaped4 ,the rock contains a considerable quantity of black iron-ore in irregular grains and some dirty-green viridite4 ,still another variety of structure occurs in a specimen which ,i broke from one of the shore crags on the south-west side of the hill4 ,under the microscope1 ,dr4 ,hatch found in it a beautiful aggregate of skeleton crystals and microlites of plagioclase1 with here and there a rectangular crystal1 long slender microlites of augite1 and short serrated microlites of magnetite1 the whole being confusedly imbedded in a dark glassy base powdered over with a fine magnetite dust4#cbb ,a sill of pitchstone lies among the bedded basalts on the east side of the hill4 ,footnote #cbb3 ,professor ,judd has called the rocks of ,beinn ,hiant augite-andesites1 and has given descriptions and figures of their structure1 and analyses of their chemical composition 7op4 cit474 ,from a number of specimens collected by me during a second visit to this district in the summer of #ahif1 ,i selected some for microscopic examination and submitted them to ,mr4 ,harker1 who has furnished me with the following descriptions of them3 ,the sill at the north end of ,camas na ,cloiche1 ,ben ,hiant #gaad is an olivine-gabbro of medium grain and fresh appearance4 ,olivine1 fresh or partly serpentinized1 is plentiful4 ,the felspar is a labradorite with ,carlsbad- and albite- 7rarely pericline-7 twinning1 and some of it has zonary banding4 ,it is for the most part in crystals giving rectangular sections1 but there are some of allotriomorphic form4 ,magnetite occurs chiefly in shapeless grains of later crystallization than the felspar1 but sometimes presenting crystal-faces to the augite4 ,the augite is light-brown in the slice1 without any true diallage-structure1 and tends to enwrap the earlier minerals in ophitic patches4 ,the sill south of ,uamh na ,creadha1 on the west side of ,ben ,hiant #gaae1 is a rock of different type1 having porphyritic crystals of felspar1 up to an inch or more in length1 in a rather finely-crystalline groundmass4 ,the microscope shows it to be a dolerite of granulitic structure1 the main mass of the rock consisting of little striated labradorite-crystals1 grains of pyroxene1 and rather abundant crystal-grains of magnetite4 ,the pyroxene seems to be chiefly augite1 but hypersthene is also present1 and builds rather larger and more idiomorphic crystals with characteristic pleochroism4 ,in rambling over this ,ardnamurchan district ,i have often been reminded of the great intrusive sheets of ,fair ,head4 ,one of the features in which the rocks of the two localities resemble each other is their tendency to assume a coarsely crystalline texture4 ,in some parts of ,ben ,hiant the individual crystals reach an inch or more in length4 ,these more largely crystalline portions1 however1 do not form distinct bands so much as patches in the midst of the general mass2 at least ,i have not noticed any examples of such veins of segregation as are so prominent in ,antrim4 ,no one familiar with the well-marked distinctions between the lavas of the plateaux and the sills which traverse them can hesitate in which series to place the rocks of ,ben ,hiant4 ,since1 however1 these rocks have been claimed by ,professor ,judd as the superficial lava-currents of a volcano which broke out after the time of the plateau-basalts1 like the ,scuir of ,eigg1 some further details in regard to the geological structure of the district1 which would otherwise be superfluous1 may here be given4 ,the number of sills and dykes in ,ardnamurchan is astonishingly great4 ,there must be hundreds of them visible1 and perhaps as many more concealed under superficial coverings4 ,they are well exposed on the shore traversing the ,jurassic strata and the schists4 ,the sills become especially large and abundant in the direction of ,ben ,hiant1 which has evidently been the principal centre from which their materials were injected4 ,the rocks composing these sills are quite similar to those of ,ben ,hiant1 save that1 as they occur in thinner sheets than in that mountain1 they do not attain the same coarseness of texture which the more massive beds there display4 ,they generally possess fine-grained chilled selvages along their upper and under surfaces4 ,illustration3 ,fig4 #cbf4--;,ground-plan of ,sills at ,ben ,hiant1 ,ardnamurchan4 a a1 crystalline schists2 b b1 necks of volcanic agglomerate2 c c1 numerous thin sills2 ,d1 massive sill of ,beinn na h-,urchrach2 ,e1 north side of ,ben ,hiant2 ,f1 sill proceeding from the series forming ,ben ,hiant and joining that of ,beinn na h-,urchrach4 ,the arrows mark the dip4 ,these abundant sills may be traced up into the mass of ,ben ,hiant from which they have issued1 and of the individual sheets of which they are a continuation4 ,one of the most striking and easily-followed examples of this connection is to be seen on the north side of the mountain4 ,a thick sheet in the middle of ,ben ,hiant descends from among its contiguous sheets and1 as a prominent rib1 runs down the scree-slope into the valley below1 where it forms a prominent feature4 ,crossing the streamlet in the middle of the valley1 where a section has been cut through its upper surface1 it gradually bends round towards the north-east1 mounts the side of ,beinn na h-,urchrach until it reaches the crest of the ridge and joins the other sills of which this eminence is built up4 ,the route of this band of rock will be understood from the annexed ground-plan 7,fig4 #cbf74 ,that this prolongation of one of the thick beds of ,ben ,hiant is in no respect a superficial lava-stream but a true sill1 is proved not only by its escarpment and dip-slope1 but by its actually passing under and indurating the schistose grits1 as may be seen in the stream-section4 ,again ,beinn na h-,urchrach1 which is mapped by ,professor ,judd as a northern expansion of ,ben ,hiant1 is likewise not a lava but a true sill4 ,not only does it dip northwards at an angle of about #bj1 having the schists immediately below its crest on the one side and descending with a long dip-slope on the other1 but dwindling down rapidly from a thickness of #ajj or #bjj feet in the centre to no more than a few feet in a south-westerly direction1 it there passes under schistose grits like those on which it lies4 ,the strata that adhere to its upper surface are as usual indurated4 ,a section drawn across this attenuated development of the ,beinn na h-,urchrach sill and that from ,ben ,hiant shows the structure represented in the accompanying diagram 7,fig4 #cbg71 which simply gives the facts as exposed on the ground4 ,the lower sill is that which issues from the main body of ,ben ,hiant1 massive at first but diminishing in thickness as it recedes from its source4 ,again1 among the sheets which descend from the northern face of the summit of ,ben ,hiant and strike into the ,jurassic outlier below1 intensely indurated shale may be seen lying between two of the dolerites1 which are unquestionably sills that have been injected into the ,jurassic series4 ,the ridge of ,ben ,hiant is thus found to consist of a thick and complex series of sills1 some of which are not traceable beyond the side of the mountain1 while others can be followed outwards among the surrounding rocks4 ,the specially marked dyke-like sills diverge from the main mass and run for some distance north-eastward1 one of them1 fully a mile long1 descending among the schists into the valley and ascending into the basalt-plateau on the opposite side4#cbc ,footnote #cbc3 ,the sills of ,ben ,hiant descend on the south-west side into the sea1 and can be examined along the slopes and the beach1 where ,professor ,judd has mapped a continuous platform of agglomerate4 ,the broad hollow between that mountain and ,beinn na h-,urchrach1 over which he has spread his augite-andesite lavas1 appears to be underlain mainly by the crystalline schists through which sills from ,ben ,hiant have been injected4 ,the northern eminence1 which he has united with ,ben ,hiant1 is entirely separate and1 as above shown1 is an obvious sill4 ,illustration3 ,fig4 #cbg4--,section of two ,sills in schistose grits1 west end of ,beinn na h-,urchrach1 ,ardnamurchan4 a a1 crystalline schists2 b1 neck of volcanic agglomerate2 c1 small sill2 ,d1 massive sill of ,beinn na h-,urchrach2 ,f1 sill proceeding from the series forming ,ben ,hiant and joining that of ,beinn na h-,urchrach4 ,on the south-east side of the mountain where the bedded basalts can be traced close up to the intrusive dolerites1 they are found to present the usual dull indurated aspect so characteristic of contact alteration among these rocks4 ,there cannot therefore be any doubt that ,ben ,hiant never was itself a volcano4 ,its rocks are characteristically those of subterranean intrusions4 ,they seem to have been injected from a line of fissure or from several such lines1 running in a general north-easterly direction1 at some late part of the volcanic period4 ,the group of agglomerate necks of older date shows that already the ground underneath had been drilled by a number of distinct volcanic funnels1 and discloses a weak part in the terrestrial crust4 iv4 ,,faroe ,,isles ,in the ,faroe ,islands the actual base of the volcanic series is nowhere visible4 ,hence1 the great lower platform of intrusive sheets being there concealed1 this feature of the basalt-plateaux is less conspicuous than it is in the ,inner ,hebrides4 ,a number of sills1 however1 have been noticed by previous observers1#cbd and ,i have observed others on the sides of ,strom1 ,kals1 ,kun and other islands4 ,in the lofty precipices of the ,haraldsfjord1 many of the massive light-coloured prismatic sheets are intrusive1 for though they preserve their parallelism with the bedded sheets for considerable distances1 they may be seen sometimes to break across these1 as is strikingly shown in one of the great corries on the east side of ,kun4 ,footnote #cbd3 ,see in particular ,prof4 ,james ,geikie and ,mr4 ,lomas1 in the papers already cited on p4 #aia4 ,illustration3 ,fig4 #cbh4--,sill traversing bedded ,basalts1 cliffs of ,strom1 at entrance of ,vaagfjord4 ,the caves and notches shown at the bottom of the precipice mark the position of the vents represented in ,figs4 #caa1 #cab1 #cac1 #cad4 ,one of the most remarkable sills in the ,faroe ,islands is probably that which forms so prominent an object on the western cliffs of ,strom1 at the entrance into the ,vaagfjord 7,figs4 #cbh1 #cbi74 ,it is prismatic in structure1 and where it runs along the face of the cliffs1 parallel to the bedded basalts among which it has been intruded1 presents the familiar characters of such sheets4 ,the precipice of which it forms a part is that which rises above the row of volcanic vents already described4 ,but it there begins to ascend the cliffs obliquely across the basalts until it reaches the crest of the great wall of volcanic rock at a height of probably about #ajjj feet above the waves4 ,from the crest of the precipice the upward course of the sill is continued into the interior of the island4 ,it pursues its way as a line of bold crag along the ridges of the plateau1 gradually ascending till it forms the summit of one of the most prominent hills in the district 7,fig4 #cbi74 ,some further idea of the enormous energy with which the sills were injected may be formed from this example1 where the eruptive materials followed neither the line of bedding nor a vertical fissure1 but took an oblique course through the plateau-basalts for a vertical distance of probably more than #aejj feet4 ,v4 ,,general ,,deductions ,,regarding ,,the ,,tertiary ,,basic ,,sills ,if we consider the facts which have now been adduced regarding the position and structure of the sills1 we are led1 ,i think1 to regard these masses as certainly belonging to the history of the basalt-plateaux1 but1 on the whole1 to a comparatively late part of it4 ,they consist of essentially the same materials as the lavas that form these plateaux1 though with the differences of structure which the conditions of their production would lead us naturally to expect4 ,where they occur in thick masses1 which must obviously have cooled much more slowly at some depth beneath the surface than the comparatively thin sheets could do that were poured out above ground1 they have assumed a far more largely crystalline texture than that of the superficial lavas4 ,as a rule1 we may say that the thicker the sill the coarser is its texture1 while the thinnest sheets are the most close-grained4 ,sills are especially abundant about the base of the basaltic-plateaux4 ,we may examine miles of the central and higher parts of the escarpments without detecting a single example of them1 but if the escarpment is cut down to the base we seldom need to search far to find them4 ,illustration3 ,fig4 #cbi4--,view of the same ,sill seen from the channel opposite the island of ,kolter4 ,that the efforts of the internal magma to establish an outlet towards the surface were accompanied by powerful disturbances of the terrestrial crust is shown by the abundant dykes which traverse all the volcanic districts from ,antrim to ,iceland1 and some of which ascend even to the very highest remaining lavas of the basalt-plateaux4 ,the parallel fissures filled by these dykes prove that even after the accumulation of more than #cjjj feet of basalt-sheets1 the movements continued to be so powerful as to disrupt these vast piles of volcanic material4 ,but undoubtedly the highest parts of the plateau-basalts are less cut by dykes than the lower parts4 ,there would no doubt come a time when the dislocations would more seldom reach the surface1 when dykes would not be formed so abundantly or up to such a high level1 and when the volcanic energies would more and more sparingly result in the opening of new vents or in the discharge of fresh eruptions from old ones4 ,it appears to me most probable that the injection of the sills was connected with the same terrestrial disturbances that produced the dykes which traverse the plateaux4 ,besides being dislocated by parallel fissures1 the earth's crust in ,north-,western ,europe seems to have been ruptured internally along lines more or less at right angles to the vertical fissures4 ,the deep accumulation of bedded basalts presented an increasing obstacle to the ascent of the magma to the surface4 ,unable to gain ample enough egress through such vertical fissures as might be formed in the volcanic pile1 the molten rock would find its lines of least resistance along the planes of the strata and the lower basalt-beds1 either by the aid of terrestrial ruptures there1 or in virtue of its own energy4 ,on these horizons1 accordingly1 the sills occur in extraordinary profusion throughout the volcanic regions4 ,they are no doubt of all ages in the progress of the building up of the volcanic plateaux1 but ,i am disposed to believe that a large number of them may belong to the very latest period of the uprise of basalt within the area of ,britain4 ,one of the most suggestive features of the abundant ,tertiary sills lies in the evidence they furnish of the enormous energy concerned in the ascent and intrusion of volcanic material4 ,the infilling of dykes or the outpouring of successive streams of lava at the surface hardly appeals to our imagination so strikingly as the proof that the sills have been impelled into their places with a vigour which1 even when guided and aided by gigantic terrestrial ruptures1 was capable of overcoming the vertical pressure of hundreds1 or even thousands of feet of overlying rock4 ,had these intrusive sheets been mere thin layers1 their horizontal extent and persistence would still have excited our astonishment1 but when we find them sometimes several hundred feet thick1 and to extend in a continuous series for horizontal distances of #ej miles or more1 we are lost in wonder at the prodigious expansive strength of the volcanic forces4 ,again1 the intrusions have not always taken place between the bedding-planes of the stratified or igneous rocks1 but1 as we have seen1 solid sheets of already deeply buried lavas have sometimes been split open and the intrusive material has forced itself between the disrupted portions4 ,such subterranean proofs of the vigour of volcanic energy teach some of the most impressive lessons in the chronicles of volcanic action in the ,british ,isles4 99 99 99 99 99 ,in closing this history of the accumulation of the great ,tertiary volcanic plateaux of ,north-,western ,europe1 ,i would remark that as the result of prolonged eruptions from innumerable vents1 the depression that extended from the south of ,antrim to the ,minch was gradually filled up with successive sheets of basalt to a depth of more than #cjjj feet4 ,a succession of lava-fields stretched from the ,north of ,ireland across the ,west of ,scotland1 and perhaps even to the ,faroe ,islands1 ,iceland and ,greenland4 ,that the lava spread round the base of the ,highland mountains and ran up the ,highland glens1 much as the sea now does1 is made clear from the position of the outliers of it which have been left perched on the ridges of ,morven and ,ardnamurchan4 ,so far as can now be surmised1 these wide ,phlegran fields were only varied by occasional volcanic cones scattered over their surface1 marking some of the last vents from which streams of basalt had flowed4 ,but the volcanic energy was still far from exhaustion4 ,after the accumulation of such a deep and far-extended sheet of lava1 those underground movements which produced the fissures that served as channels for the uprise of the earliest dykes continued to show their vigour4 ,the pile of bedded lavas was rent open by innumerable long parallel fissures in the prevalent north-westerly direction1 up which basic lavas rose to form dykes1 while vast numbers of sills were injected underneath4 ,whether the outflow of basalt at the surface had wholly ceased when the last of these dykes were injected into the plateaux cannot be told4 ,nor is there any evidence whether it had ended before the next great episode of the volcanic history--the extravasation of the gabbro bosses4 ,all that we can affirm with certainty is1 that the formation of north-west fissures and the uprise of basalt in them were again repeated1 for we find north-west dykes traversing even the crests of the later eruptive masses of basic and acid rocks4 ,it is difficult to suppose that none of these latest dykes communicated with the surface1 and gave rise to cones with the outpouring of basalt and the ejection of dust and stones4 ,but of such later outflows of basic material over the surface of the plateaux no undoubted trace has yet been recognised4 ,,chapter ,,xliii ,,the ,,bosses ,,and ,,sheets ,,of ,,gabbro ,petrography of the ,rocks--,relations of the ,gabbros to the other members of the ,volcanic series--,description of the ,gabbro districts--,skye ,in singular contrast to the nearly flat basalts of the plateaux1 another series of rocks rises high and abruptly above these tablelands into groups of dome-shaped1 conical1 spiry1 and rugged hills4 ,it is these heights which1 more than any other feature1 relieve the monotony of the wide areas of almost horizontal stratification so characteristic of the volcanic region of the north-west4 ,their geological structure and history are much less obvious than those of the bedded basalts4 ,their mountainous forms at once suggest a wholly different origin4 ,some portions of them have even been compared with the oldest or ,archan rocks4#cbe ,that they are really portions of the ,tertiary volcanic series1 and that they reveal a wholly distinct phase in the history of volcanic action1 is now frankly admitted4 ,whether we regard them from the petrographical or structural point of view1 they naturally arrange themselves into two well-defined groups4 ,of these one consists of highly basic compounds1 of which olivine-gabbro is the most prominent4 ,the other comprises numerous varieties--granite1 granophyre1 felsite1 quartz-porphyry1 pitchstone and others--all of them being more or less decidedly acid1 and some of them markedly so4 ,for reasons which will appear in the sequel1 the former group must be considered as the older of the two1 and it will therefore be described first4 ,footnote #cbe3 ,this was my own first impression1 when ,i began1 as a boy1 to ramble among them4 ,the remarkable resemblance of some parts of them to ancient gneisses will be afterwards dwelt upon4 ,macculloch had correctly grouped them with the other overlying rocks1 and this conclusion was afterwards confirmed by ,prof4 ,zirkel4 i4 ,,petrography ,,of ,,the ,,gabbro ,,areas ,since the publications of ,macculloch1 the occurrence of beautiful varieties of highly basic rocks among the igneous masses of the ,western ,isles has been familiar to geologists4 ,they were named by him hypersthene rock and augite rock1#cbf names which continued in use until #ahga1 when my friend ,professor ,zirkel published the results of his tour through the ,west of ,scotland1 and showed that the rocks in question were mostly true gabbros4#cbg ,since his observations were published some of these rocks have formed the subject of important papers by ,professor ,judd4#cbh ,footnote #cbf3 ,western ,islands1 vol4 i4 pp4 #che1 #dhd4 ,footnote #cbg3 ,zeitschrift4 ,deutsch4 ,geol4 ,gesellsch4 xxiii4 7#ahga71 p4 #a4 ,footnote #cbh3 ,quart4 ,jour4 ,geol4 ,soc4 xli4 7#ahhe71 p4 #ced2 xlii4 7#ahhf71 p4 #di4 ,the general petrographical characters of the gabbro areas of ,western ,scotland may be summarized as follows3--,a very considerable variety of petrological structure and chemical composition is observable among the rocks4 ,at the one end of the series are compounds of plagioclase and augite1 which1 though wanting in olivine1 have the general structure and habit of dolerites4 ,at the other end are mixtures wherein felspar is scarce or absent1 and where olivine becomes the chief constituent4 ,between these two extremes are many intermediate grades1 of which the most important are those containing the variety of augite known as diallage and also olivine4 ,these are the olivine-gabbros1 which form so marked a feature in the central parts of the great basic bosses4 ,that some of these varieties of rock pass into each other cannot be doubted4 ,their distinctive composition and structure appear to have been largely determined by their position in the eruptive mass4 ,the outer and thinner sheets are in great measure dolerites1 with little or no olivine4 ,coarse gabbros are abundant in the inner portions4 ,rocks rich in olivine1 however1 occur at the outer and especially the lower part of the gabbro masses of ,rum and in some parts of ,skye4 ,the following leading varieties may be enumerated3-- ,dolerite4--,this rock varies from an exceedingly close grain 7when it approaches and graduates into basalt7 up to a coarse granular crystalline texture1 in which the component minerals are distinctly visible to the naked eye4 ,an average sample is found to consist of plagioclase1 usually lath-shaped1 and crystals or grains of augite with or without olivine4 ,under the microscope1 the different varieties are distinguished by the presence of more or less distinct ophitic structure1 the felspar being enveloped in the augite4 ,for the most part they are holocrystalline1 but occasionally show traces of a glassy base4 ,ilmenite is not infrequent1 with its characteristic turbid decomposition product 7leucoxene74 ,in other cases1 the iron-ore is probably magnetite4 ,between the dolerites and gabbros no line of demarcation can be drawn in the field1 nor can a much more satisfactory limitation be made even with the aid of the microscope4 ,as a rule1 the thickest and largest intrusive masses or bosses are gabbro1 those of less size are dolerite1 while the smallest 7and sometimes the edges of the others7 assume externally the aspect of basalts4 ,gabbro4--,under this term ,i arrange1 as proposed by ,professor ,judd1 all the coarse-grained granitoid basic rocks of the region without reference to the variety of augite present in them4 ,under the microscope1 they are found to be holocrystalline1 but with a granitic or granulitic rather than an ophitic structure1 though traces of the latter are by no means rare4 ,to the naked eye their component minerals are usually recognizable4 ,professor ,zirkel1 from his examination of the ,mull gabbros1 believed them to consist of three parts of plagioclase1 two parts of olivine1 and one part of diallage4#cbi ,olivine1 however1 is not invariably present4#ccj ,the pyroxene also does not always show the peculiar fibrous structure of diallage4 ,professor ,judd1 indeed1 maintains that the diallagic form is due to a deep-seated process of alteration 7schillerization71 and that the same crystal may consist partly of ordinary augite and partly of diallage4#cca ,ilmenite 7with leucoxene71 magnetite1 apatite1 biotite1 and epidote are not infrequent constituents4 ,footnote #cbi3 ,zeitschr4 ,deutsch4 ,geol4 ,gesellsch4 xxiii4 7#ahga71 p4 #ei4 ,footnote #ccj3 ,professor ,judd 7,quart4 ,jour4 ,geol4 ,soc4 xlii4 p4 #fb7 believes that originally all the gabbros contained olivine1 and that where it is now absent1 it has been altered into magnetite or serpentine4 ,but in some coarse massive gabbros this mineral does not appear to have been an essential constituent4 ,see op4 cit4 vol4 l4 p4 #fed4 ,footnote #cca3 ,op4 cit4 xli4 ,in a later paper he insists on the gradation of the coarse granitoid varieties 7gabbros7 into holocrystalline compounds1 where the felspar appears in lath-shapes with crystals or rounded grains of augite and olivine 7dolerites71 and thence into true basalts1 magma-basalts1 and tachylytes 7op4 cit4 xlii4 p4 #fb74 ,in a recent study of the gabbros of the ,cuillin ,hills of ,skye by ,mr4 ,j4 ,j4 ,h4 ,teall and myself1 four characteristic types have been recognized4#ccb ,footnote #ccb3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 #a4 7#ahid71 pp4 #fde-fei1 and ,plates xiii4 xxvi4-xxviii4 ,see also ,prof4 ,judd's paper1 op4 cit4 7#ahhf71 p4 #di4 7#a7 ,granulitic ,gabbros4--,these are dark1 fine-grained rocks which externally resemble some of the altered basalts of the plateau-series4 ,they occur in bands or sheets which1 so far as can be made out1 are the oldest portions of the whole gabbro mass4 ,under the microscope they are found to possess a finely granulitic structure1 and to consist of grains of pyroxene 7augite1 but more usually with the inclusions characteristic of diallage and pseudo-hypersthene71 and of felspar allied to labradorite1 with green pseudomorphs agreeing in form and size with the pyroxene-grains1 but made of minute prisms and fibres of green hornblende and a little chlorite4 7#b7 ,banded ,gabbros4--,these are characterized by a remarkable arrangement in parallel bands of different mineral composition like the banding of ancient gneisses4 ,this structure will be more particularly described in later pages4 ,they are coarse-grained rocks composed of pyroxene1 plagioclase1 olivine and magnetite4 ,but these minerals are not distributed equally through the mass4 ,the pale bands contain much felspar2 the dark bands are largely composed of the ferro-magnesian minerals and magnetite4 ,the pyroxene1 occurring as ordinary augite1 not uncommonly shows a tendency to ophitic structure4 ,the felspar1 a variety closely allied to labradorite1 occurs as grains1 as irregular ophitic patches1 and also in forms that give broad rectangular sections4 ,olivine in an unaltered condition has been detected by ,mr4 ,teall in only one specimen1 and he thinks that this mineral probably never played an important part in the original constitution of these rocks4 ,its rounded grains may be observed to have the other minerals moulded round them1 whence it may be inferred to be of older consolidation4 ,magnetite is generally present1 either in rounded grains or in large irregular masses4 ,though it occurs also in strings traversing the other minerals as a secondary product1 it must undoubtedly have entered largely into the original composition of these rocks4 ,it is found enclosing the augite grains and behaving like a groundmass between the felspars4 ,among the dark bands there occur narrow lenticular black layers 7'schlieren'7 composed entirely of augite and iron-ore4 ,the extraordinary differences between the composition of the pale felspathic and the dark ultra-basic bands are well brought out in the following analyses by ,mr4 ,j4 ,hort ,player1 ,no4 #a being from a light-coloured band consisting mainly of labradorite with some augite1 uralitic hornblende and magnetite2 ,no4 #b from a dark band composed of augite1 magnetite and labradorite2 and ,no4 #c from a thin ultra-basic layer mainly formed of augite and magnetite4 ,all these specimens were taken from the ridge of ,druim an ,eidhne1 on the eastern side of the ,cuillin ,hills1 ,skye4#ccc ,i4 ,,ii4 ,,iii4 ,silica #ebh #djb #bie ,titanic acid #e #dg #ib ,alumina #agh #ie #ch ,ferric oxide #ab #ig #agh ,ferrous oxide #dh #abb #ahb ,ferric sulphide #d #d ,oxide of manganese #d #c ,lime #abi #aca #ajj ,magnesia #dh #hj #hg ,soda #cj #h #b ,potash #e #b #a ,loss by ignition #ab #e #aj ---- ---- ---- #iie #iig #iib ---- ---- ---- ,spec4 grav4 #bia #ccf #chg ,footnote #ccc3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 #a4 7#ahid71 p4 #fec4 ,banded structures have been recognized in many gabbros of different ages4 ,see the references in this paper2 also ,mr4 ,w4 ,s4 ,bayley1 ,journ4 ,geol4 ,chicago1 ii4 7#ahie71 p4 #had1 and vol4 iii4 p4 #a4 7#c7 ,coarse-grained massive ,gabbros4--,these rocks1 so abundant among the great basic bosses of the ,inner ,hebrides1 are characterized by their coarse granitic structure1 their component crystals being sometimes more than an inch long4 ,they occur as sheets1 veins and irregular masses traversing the varieties of gabbro already mentioned4 ,they consist of the same minerals as the banded forms1 and indeed are themselves sometimes banded4 ,they are more uniform in composition than the typical banded gabbros1 though showing also some variation in the relative proportions of their constituents4 ,the specific gravity of three specimens was found to be #bhb1 #big1 and #cjf4 ,illustration3 ,fig4 #ccj4--;,granulitic and coarsely foliated gabbro traversed by later veins of felspathic gabbro1 ,druim an ,eidhne1 ,cuillin ,hills1 ,skye4 7#d7 ,pale ,gabbros of the ,veins4--,these occur abundantly as irregular branching veins1 from less than an inch to several yards in width1 and cross all the other varieties 7,fig4 #ccjccd74 ,their whiteness on weathered surfaces makes them conspicuous by contrast with the dark brown or black hue of the rocks which they traverse1 and shows at once that they must be poorer in bases than these4 ,they are found on microscopic examination to consist of the same minerals as the more coarsely crystalline gabbros1 but with a much greater abundance of the felspar4 ,they contain also apatite1 and hornblende appears to predominate in them over augite4 ,they are to be distinguished from the pale veins that form apophyses from the intrusive granophyres4 ,footnote #ccd3 ,figs4 #ccj1 #ccf and #ccg are from photographs taken for the ,geological ,survey by ,mr4 ,r4 ,lunn4 ,troctolite 7,forellenstein74--,this beautiful variety of plagioclase-olivine rock occurs as a conspicuous feature on the east side of the gabbro-area of the island of ,rum4 ,it forms a sill on the side of the mountain ,allival1 in which the component minerals are drawn out parallel with the upper and under surfaces of the bed 7,fig4 #cda74 ,so marked is this flow-structure that hand-specimens might readily be taken at the first glance for ancient schistose limestone4 ,the felspathic ingredient 7probably labradorite or anorthite7 is white1 and its lath-shaped crystals have ranged themselves with their long axes parallel to the line of flow4 ,the olivine occurs in perfectly fresh grains1 which in hand-specimens have a delicate green tint4 ,under the microscope they appear colourless1 and are penetrated by the felspar prisms in ophitic intergrowth4 ,there is a small quantity of a pale brownish augite1 which not only occurs in wedge-shaped portions between the felspars1 but also as a narrow zone round the olivines4#cce ,considerable differences are visible in the development of the flow-structure1 and with these there appear to be accompanying variations in the microscopic structure4 ,dr4 ,hatch1 to whom ,i submitted my specimens1 informed me that in one of them1 where the flow-structure is so marked as to give a finely schistose aspect to the rock1 there is a larger proportion of augite1 some of which exhibits a distinct diallagic striping2 the olivine grains show no ophitic structure1 but are sometimes completely embedded in the augite4 ,to this remarkable flow-structure ,i shall again refer in connection with the light it throws on the bedded character of much of the gabbro bosses4 ,footnote #cce3 ,,ms4 of ,dr4 ,hatch4 ,between the different basic intrusive igneous rocks of the ,inner ,hebrides1 as ,professor ,judd has shown1 there are many gradations according to the varying proportions of the chief component minerals4 ,thus from the olivine-gabbros1 by the diminution or disappearance of the augite we get such rocks as troctolite2 where the plagioclase diminishes or vanishes1 we have different forms of picrite2 where the olivine is left out1 we come to compounds1 like eucrite2 while by the lessening or disappearance of the felspar and augite1 we are led to ultra basic compounds1 consisting in greatest part of olivine1 like lherzolite1 dunite and serpentine4 ,to some of the features and probable origin of these chemical and mineralogical diversities in the same great eruptive mass further reference will be made in later pages4 ii4 ,,relations ,,of ,,the ,,gabbros ,,to ,,the ,,other ,,members ,,of ,,the ,,volcanic ,,series ,various opinions have been expressed regarding the connection between the amorphous eruptive rocks of the hill-groups and the level basalt-sheets of the plateaux4 ,jameson1 though he landed at ,rudh' an ,dunain1 in ,skye1 where this connection can readily be found1 does not seem to have made any attempt to ascertain it4 ,he noticed that the lower grounds were formed of basalt1 and that the mountains appeared to be wholly composed of syenite and hornblende rock1 traversed by basalt veins4#ccf ,macculloch1 in many passages of his ,western ,islands1 alludes to the subject as one which he knew would interest geologists1 but about which he felt that he could give no satisfactory information1 and with characteristic verbiage he refers to the impossibility of determining boundaries1 to the transition from one rock into another1 to the inaccessible nature of the ground1 to the almost insuperable obstacles that impede examination1 to the distance from human habitation1 and to the stormy climate1--a formidable list of barriers1 in presence of which he leaves the relative position and age of the rocks unsettled4#ccg ,footnote #ccf3 ,mineralogical ,travels 7#ahac71 vol4 ii4 p4 #gb4 ,footnote #ccg3 ,see his ,western ,islands1 vol4 i4 pp4 #cfh1 #cgd1 #che1 #chf4 ,with much admiration for the insight and zeal1 amounting almost to genius1 which ,macculloch displayed in his work among the ,western ,islands1 at a time when1 with poor maps and inadequate means of locomotion1 geological surveying was a more difficult task than it is now1 ,i have found it impossible to follow in his footsteps with his descriptions in hand1 and not to wish that for his own fame he had been content to claim credit only for what he had seen4 ,his actual achievements were enough to make the reputation of half a dozen good geologists4 ,it was unfortunate that he did not realize how inexhaustible nature is1 how impossible it is for one man to see and understand every fact even in the little corner of nature which he may claim to have explored4 ,he seems to have had a morbid fear lest any one should afterwards discover something he had missed2 he writes as if with the object of dissuading men from travelling over his ground1 and he indeed tacitly lays claim to anything they may ascertain by averring that those who may follow him will find a great deal that is not here described1 although little that has not been examined 7p4 #cgc74 ,principal ,forbes long ago exposed this weak side of ,macculloch and his work 7,edin4 ,new ,phil4 ,journ4 xl4 #ahdf1 p4 #hb74 ,von ,oyenhausen and ,von ,dechen1 who wrote so excellent an account of their visit to ,skye1 and who traced much of the boundary-line between the gabbros and the other mountainous eruptive masses 7syenite71 seem to have made no attempt to work out the connection between the former and the rest of the volcanic rocks4#cch ,footnote #cch3 ,karsten's ,archiv1 i4 p4 #ii4 ,they frankly admit that the relation of the hypersthene rock to the other trap rocks was not ascertained4 ,j4 ,d4 ,forbes1 in his able sketch of the ,topography and ,geology of the ,cuchullin ,hills1 was the first to recognize the superposition of the hypersthene rock upon the common trap rocks--that is1 the plateau-basalts4 ,he was disposed to consider the hypersthene mass as a vast bed1 thinning out both ways1 and inclined at a moderate angle towards the ,s4,e4#cci ,footnote #cci3 ,edin4 ,new ,phil4 ,journ4 xl4 7#ahdf71 pp4 #he1 #hf4 ,professor ,judd regarded the bosses of basic and acid rocks that rise out of the bedded basalts as the basal cores of enormously denuded volcanic cones4 ,he believed the granitoid rocks to have been first erupted1 and that after a long interval the basic masses were forced through them1 partly consolidating underneath and partly appearing at the surface as the plateau-basalts4#cdj ,that the order of appearance of the several rocks has been exactly the reverse of this supposed sequence was fully established by me in the year #ahhh1 and has since been amply confirmed4#cda ,professor ,zirkel recognized that the gabbros are a dependence of the basalts1 that they overlie them1 and that on the naked flanks of the mountains they are regularly bedded with them4#cdb ,footnote #cdj3 ,quart4 ,journ4 ,geol4 ,soc4 xxx4 7#ahgd71 p4 #bdi4 ,footnote #cda3 ,trans4 ,roy4 ,soc4 ,edin4 xxxv4 7#ahhh71 pp4 #abb et seq42 ,quart4 ,journ4 ,geol4 ,soc4 vol4 #a4 7#ahid71 pp4 #baf1 #fde2 vol4 lii4 7#ahif71 p4 #chd1 and ,mr4 ,harker1 ibid4 p4 #cbj4 ,footnote #cdb3 ,zeitschrift4 ,deutsch4 ,geol4 ,gesellsch4 xxiii4 7#ahga71 pp4 #eh1 #ib4 ,up to the time of the publication of my memoir in #ahhh no one had traced out in more detail the actual boundaries of the several rocks on the ground1 so as to obtain evidence of their true relations to each other as regards structure and age4 ,some of the numerous impediments recorded by ,macculloch no doubt retarded the investigation4 ,but1 as ,forbes so well pointed out1 there is really no serious difficulty in determining the true structural connection of the amorphous rocks with each other and with the bedded basalts of the plateaux4 ,i have ascertained them in each of the districts1#cdc and have found that there cannot be the least doubt that the amorphous bosses1 both basic and acid1 are younger than the surrounding bedded basalts1 and that the acid protrusions are on the whole younger than the basic1 ,i shall now proceed to show how these conclusions are established by the evidence of each of the areas where the several kinds of rock occur4 ,footnote #cdc3 ,in two of my excursions in ,mull1 and once in ,skye1 ,i was accompanied by my former colleague ,mr4 ,h4 ,m4 ,cadell1 who rendered me great assistance in mapping those regions4 iii4 ,,descriptions ,,of ,,the ,,several ,,gabbro-,,districts #a4 ,the ,gabbro of ,skye ,the largest1 most picturesque1 and to the geologist most important area of ,tertiary gabbro in ,britain1 is that of ,skye 7,map4 ,,vi474 ,though1 like every other portion of the ,tertiary volcanic districts1 it has suffered enormous denudation1 and has thereby been trenched to the very core1 it reveals1 more conspicuously and clearly than can be seen anywhere else1 the relation of the gabbro to the bedded basalts on the one hand1 and to the acid protrusions on the other4 ,its chief portion is that which rises into the group of the ,cuillin ,hills1 which for blackness of hue1 ruggedness of surface1 jaggedness of crest1 and general grimness of aspect1 have certainly no rivals within the limits of the ,british ,isles 7,fig4 #cca74 ,it has long been known to extend eastwards into ,blath ,bheinn 7,blaven7 and its immediate northern neighbours4 ,there is1 indeed1 no break whatever between the rock of the ,cuillins and that of the hills on the east side of ,strath na ,creitheach4 ,in ,strath ,more the gabbro is interrupted by the granitoid mass of the ,red ,hills4 ,patches of it1 however1 occur further to the east1 even as far as the ,sound of ,scalpa4 ,illustration3 ,fig4 #cca4--,scuir na ,gillean1 ,cuillin ,hills1 shewing the characteristic craggy forms of the ,gabbro4 7,from a photograph by ,mr4 ,abraham1 ,keswick47 ,if we throw out of account the invading granitoid rocks1 and look upon the whole tract within which the gabbro occurs as originally one connected area1 we find that it covered an elliptical space measuring about nine miles from south-west to north-east and six miles from north-west to south-east1 and embracing at least #dj square miles4#cdd ,but that its original size was greater is strikingly shown more particularly on the western margin1 which like that of the basalt-escarpments1 has obviously been determined by denudation1 for its separate beds present their truncated ends to the horizon all along the flanks of the ,cuillins1 from the head of ,glen ,brittle round to ,loch ,scavaig 7,fig4 #ccb71 and from ,strath na ,creitheach round the southern flanks of ,blath ,bheinn to ,loch ,slapin and ,strath ,more4 ,footnote #cdd3 ,though this and the other bosses are here spoken of as consisting of gabbro1 it will be understood that this rock only constitutes the larger portion of their mass1 which includes also dolerites and other more basic compounds1 together with involved portions of the plateau-basalts and masses of agglomerate which probably mark the position of older vents4 ,illustration3 ,fig4 #ccb4--,section across ,glen ,brittle1 to show the general relations of the ,bedded ,basalts 7a7 and the ,gabbros 7b74 ,the first point to be ascertained in regard to the gabbro and associated basic rocks of the mountainous tract is their connection in geological structure and age with the bedded basalts of the plateau4 ,this initial and fundamental relation1 as ,forbes long ago said1 can be examined along the whole western and southern flank of the ,cuillin ,hills1 from the foot of ,glen ,sligachan round to the mouth of ,loch ,scavaig4 ,even from a distance1 the observer1 who is favoured with clear weather1 can readily trace the almost level sheets of basalt till they dip gently under the darker1 more massive rock of the hills4 ,tourists1 who approach ,skye by way of ,loch ,coruisk1 have an opportunity1 as the steamer nears the island of ,soay1 of following with the eye the basalt-terraces of the promontory of ,rudh' an ,dunain until they disappear under the gabbro of the last spur of the ,cuillins that guards the western entrance to ,loch ,scavaig4 ,what is so evident at a distance becomes still more striking when viewed from nearer ground4 ,nowhere can it be more impressively seen than at the head of ,glen ,brittle4 ,looking westwards1 the traveller sees in front of him only the familiar level terraces and green slopes of the basalt-plateau1 rising platform above platform to a height of nearly #aejj feet above the sea4 ,but turning to the east1 he beholds the dark1 gloomy1 cauldron-like ,corry na ,creiche1 from which rise some of the ruggedest and loftiest crests of the ,cuillins4 ,on the hills that project from either side of this recess and half enclose it1 the bedded basalts mount from the bottom of the valley1 with their lines of parallel terrace dipping gently inward below the black rugged gabbro that crowns them and sweeps round to form the back or head of the corry4 ,down the whole length of ,glen ,brittle the same structure conspicuously governs the topographical features4 ,on the right hand1 the ordinary terraced basalts form the slopes2 and they rise for some #ejj or #fjj feet up the eastern side1 until they pass under the darker1 more rugged1 and less distinctly bedded rocks of the mountains 7,fig4 #ccb74 ,the dip of the whole series is here at a gentle angle towards south-east1 that is1 into or under the main mass of the ,cuillin group4 ,when1 however1 we proceed to examine the junction between the two rocks we find it to be less simple than it appears4 ,it is not an instance of mere superposition4 ,the gabbro unquestionably overlies the basalts1 and is therefore of younger date4 ,but it overlies them1 not as they rest on each other1 in regular conformable sequence of eruption1 but intrusively1 as a sill does upon the rocks on which it appears to follow in the unbroken order of accumulation4 ,this important structure may be ascertained in almost any of the many sections cut by the torrents which have so deeply trenched with gullies the flanks of the hills4 ,starting from the ordinary bedded basalts1 we observe1 in mounting the slopes and approaching the gabbro1 that the rocks insensibly assume that indurated shattery character1 which has been referred to as characteristic of them round the margins of vents1 and which will be shown to be not less so in contact with large eruptive masses of basic or acid rock4#cde ,beds of dolerite make their appearance among the basalts1 so distinctly crystalline1 and so similar in character to the rocks of the sills1 that there can be little hesitation in regarding them as intrusive4 ,these sills increase in size and number as we ascend1 though hardened amygdaloidal basalts may still be observed4 ,true gabbros then supervene in massive beds1 and at last we find ourselves entirely within the gabbro area1 where1 however1 thin bands of highly altered basalt may still for some distance appear4 ,one further fact will generally be noticed1 viz4 that before reaching the main mass of gabbro1 veins and sills of basalt1 as well as of various felsitic and porphyritic members of the acid group1 come in abundantly1 crossing and recrossing each other in the most intricate network4 ,the base of the thick gabbro-sheets is thus another horizon on which1 as on that below the plateau-basalts1 intrusive masses have been especially developed4 ,through all these rocks numerous parallel basalt-dykes1 running in a general persistent ,n4,n4,w4 direction1 with a later ,n4,e4 series1 rise from below the sea-level up even to the very crests of the ,cuillins 7,fig4 #ccc74 ,footnote #cde3 ,this indurated1 altered character of the bedded basalts near the intrusive bosses and sills will be more particularly described in a later chapter in connection with the granophyre intrusions 7see p4 #chf74 ,the metamorphism induced by the basic rocks has generally been less pronounced than that effected by the acid masses4 ,the sections on the western side of the gabbro area of ,skye thus prove that this rock inosculates with the bedded basalts by sending into them1 between their bedding planes1 sheets which vary in texture from fine dolerites at the outside into coarse gabbros further towards the central mass1 and that this intrusion has been accompanied by a certain amount of induration of the older rocks4 ,illustration3 ,fig4 #ccc4--,view of the crest of the ,cuillin ,hills1 showing the weathering of the gabbro along its joints1 and of a compound basic dyke which rises through it4 7,from a photograph by ,mr4 ,abraham1 ,keswick47 ,on the eastern side1 the same structure can be even more distinctly seen1 for it is not only exposed in gullies and steep declivities1 but can be traced outward into the basalt-plateau4 ,in the promontory of ,strathaird1 ,jurassic sandstones and shales1 which form the coast-line and lower grounds1 are surmounted by the bedded basalts4 ,denudation has cut the plateau into two parts4 ,the smaller of these makes the outlier that rises into ,ben ,meabost 7#aabh feet74 ,the larger stretches continuously from ,glen ,scaladal and ,strathaird ,house northward into ,blath ,bheinn4 ,hence from the ordinary terraced basalts1 with their amygdaloids1 thin tuffs1 red partings1 and seams of lignite1 every step can be followed into the huge gabbro mountain4 ,starting from the black ,jurassic shales on which the lowest basalt lies1 we walk over the successive terraces up into the projecting ridge of ,an da ,bheinn4 ,but as we ascend1 sheets of dolerite and gabbro make their appearance between the basalts1 which gradually assume the altered aspect already noticed4 ,the dip of the whole series is at a low angle northwards1 and the beds can be followed round the head of the ,glen nan ,leac into the southern slopes of ,blath ,bheinn4 ,seen from the eastern side of this valley1 the bedded character of that mountain is remarkably distinct1 but it becomes less marked towards the upper part of the ridge where the gabbros preponderate4 ,one of the most striking features of the locality is the number and persistence of the dykes1 which strike across from the ordinary unaltered basalts of the plateau up into the highest gabbros of the range4 ,where less durable than the intractable gabbro1 they have weathered out on the face of the precipices1 thereby causing the vertical rifts and gashes and the deep notches on the crest that form so marked a feature in the scenery4 ,on the other hand1 they are often less destructible than the plateau-basalts1 and hence in the ,glen nan ,leac they may be seen projecting as low dams across the stream which throws itself over them in picturesque waterfalls4 ,the youngest dykes in the ,blath ,bheinn group of hills1 have been found by ,mr4 ,harker to have a north-easterly trend1 and a north-westerly hade of about #dj1 and to give a stratified appearance to the gabbro when viewed from a distance4 ,the deep dark hollow of the ,coire ,uaigneich has been cut out of the very core of ,blath ,bheinn1 and lays bare the structure of the east part of the mountain in the most impressive as well as instructive way 7,fig4 #ccd74 ,by ascending into this recess from ,loch ,slapin1 we pass over the whole series of rocks1 and can examine them in an almost continuous section in the bed of the stream and on the bare rocky slopes on either side4 ,sandstones and shales of the ,jurassic series extend up the ,allt na ,dunaiche for nearly a mile1 much veined with basalt and quartz-porphyry1 by which the sandstones are locally indurated into quartzite4 ,at last these strata are overlapped by the basalts of the ,strathaird plateau1 which with a marked inclination to ,n4,n4,w41 here dip towards the mountains4 ,but by the time these rocks have reached the valley1 they have already lost their usual brown colour and crumbling surfaces1 and have assumed the indurated splintery character1 though still showing their amygdaloidal structure4 ,they are much traversed by felsitic veins and strings which proceed from a broad band of fine-grained hornblende-granite that runs up the bottom of the ,coire ,uaigneich and1 ascending the col1 crosses it south-westwards into the ,glen nan ,leac4 ,on the left or south-eastern side of this intrusive mass1 a portion of ,lias shales and limestone 7here and there altered into white marble7 is traceable for several hundred yards up the stream4#cdf ,footnote #cdf3 ,this limestone was formerly identified by me with the ,cambrian strata of the district4 ,it was noticed by ,von ,oyenhausen and ,von ,dechen1 who1 as ,mr4 ,harker has recently ascertained1 correctly believed it to be a portion of the ,lias torn off and carried upward by the eruptive rocks 7,karsten's ,archiv1 i4 p4 #gi74 ,the bedded basalts of ,strathaird1 after dipping down towards the ,n4,n4,w41 bend up where they are interbanded with dolerites and gabbros1 and form the prominence called ,an ,stac1 which rises as the eastern boundary of the ,coire ,uaigneich4 ,their steep dip away from the mountain is well seen from the east side1 and their outward inclination is continued into the ridge to the southward4 ,similar rocks appear on the other flank of the band of granite1 and form the base of ,blath ,bheinn4 ,they are likewise continued in the mountains further north called ,sgurr nan ,each and ,belig1 where they dip in a northerly direction away from ,blath ,bheinn1 which seems to be the centre of uprise1 with the gabbro-sheets dipping away from it4 ,the bedded basalts have been traced by ,mr4 ,harker up to a height of well over #bjjj feet on the ,blath ,bheinn range4 ,they are of the usual altered1 indurated1 and splintery character4 ,the intrusive sheets interposed between them become thicker and more abundant higher up1 until they constitute the main mass of the mountain4 ,but that they are in separate sheets1 and not in one amorphous mass1 can be recognized by the parallel lines that mark their boundaries4 ,the junction of the gabbro sills and the lavas is a very irregular one1 portions of the latter rocks being enveloped in the intrusive sills4 ,the granite which sends out veins into the surrounding rocks is obviously the youngest protrusion of the locality1 except of course the basalt-dykes which cross it1 and which are nowhere seen in a more imposing display than round the flanks of ,blath ,bheinn4 ,a section across the corry shows the structure represented in ,fig4 #ccd4 ,it is thus demonstrable that when its line of junction with the surrounding plateau-basalts is traced in some detail1 the gabbro is found to overlie them as a whole1 but also to be intercalated with them in innumerable beds1 bands1 or veins which rapidly die out as they recede outwards from the main central mass2 that these interposed beds are intrusive sheets or sills from that mass which have cut off and enveloped portions of the basalts1 and that the contiguous bedded basalts show more or less marked metamorphism4 ,we have now to consider the structure of the interior of the gabbro area of the ,cuillin ,hills4 ,the first impression of the geologist who visits that wild district is that the main mass of rock is as thoroughly amorphous as a core of granite4 ,yet a little further examination will reveal to him many varieties of texture1 sometimes graduating into1 sometimes sharply marked off from1 each other1 and suggesting that the rock is not the product of one single protrusion4 ,he will notice further indications of successive discharges or extravasations of crystalline material during probably a protracted period of time1 and in the intricate network of veins crossing each other and the general body of the rock in every direction1 as well as in the system of basalt-dykes that traverse all the other rocks1 he will recognize the completion of the evidence of repeated renewals of subterranean energy4 ,illustration3 ,fig4 #ccd4--,section across the ,coire ,uaigneich1 ,skye4 a1 b1 ,jurassic sandstones and shales2 c c1 bedded basalts and dolerites2 d d1 gabbros and dolerites with indurated basalts2 e1 fine-grained hornblende-granite sending veins into surrounding rocks2 f f1 basalt-dykes running through all the other rocks4 ,but the observer will be struck with the absence of the more usual proofs of volcanic activity in such forms as vesicular lavas and abundant masses of slag1 bombs and tuffs1 which are commonly associated with the idea of the centre of a volcanic orifice1 though he will meet with isolated masses of coarse volcanic agglomerate within the gabbro area and along some parts of its junction with the granophyre4 ,the general characters of the rocks around him suggest that he stands1 as it were1 far beneath that upper part of the earth's crust which is familiar to us in the phenomena of modern volcanoes2 that he has been admitted into the heart of one of the deeper layers1 where he can study the operations that go on at the very roots of an active vent4 ,when the geologist begins a more leisurely and systematic examination of the interior of the gabbro area of ,skye he soon sees reason to modify the impression he may at first have received that this rugged region presents the characters of one single eruptive mass4 ,the more he climbs among the hills the more will he meet with evidence of long-continued and oft-repeated extravasation1 one portion having solidified before another broke through it1 and both having been subsequently disrupted by still later protrusions4 ,but if by chance he should begin his examination of the ground upon some of the more typically banded varieties of rock1 he may for a time almost refuse to admit that these can be either of volcanic origin or of ,tertiary age4#cdg ,he will find among them such startling counterparts of the structure of the ancient ,lewisian gneiss of the ,north-,west of ,scotland that he may well be pardoned if for a time he seeks for evidence that they really do belong to that primeval formation1 and have only been accidentally involved among the ,tertiary volcanic rocks4 ,if1 for instance1 he should land in ,loch ,scavaig1 and first set foot upon the gabbros as they appear around ,loch ,coruisk1 he would find himself upon masses of grey coarsely crystalline1 rudely banded rock1 like much of the old gneiss of ,sutherland and ,ross4 ,ascending over the ice-worn domes1 he would notice that the banding becomes here and there more definitely marked by strong differences in texture and colour1 while elsewhere it disappears and is replaced by a granitoid arrangement of the crystals1 which are often as large as walnuts4 ,footnote #cdg3 ,see ,quart4 ,journ4 ,geol4 ,soc4 vol4 l4 pp4 #bag1 #feg1 and a paper by the author1 ,sur la ,structure rubanne des plus anciens ,gneiss et des ,gabbros ,tertiaires1 ,compt4 rend4 ,cong4 ,gol4 ,internat4 #ahid1 p4 #aci4 ,illustration3 ,fig4 #cce4--;,banded and puckered gabbro1 ,druim an ,eidhne1 ,glen ,sligachan1 ,skye4 ,nowhere is the gneissoid banding more beautifully developed than on the east side of the ,cuillin group near the head of ,glen ,sligachan along the ridge of ,druim an ,eidhne4 ,it was at this locality that the four typical structures were observed which have already been referred to 7p4 #cbi74 ,the varieties of colour and composition depend upon the exceedingly irregular distribution of the component minerals4 ,the paler bands1 rich in felspar1 lie parallel with dark brown bands full of pyroxene1 olivine and magnetite1 in which1 moreover1 thin ribs of glistening black consist in large part of the iron ore4 ,these layers vary in thickness from mere pasteboard-like lamin to beds a yard or more in thickness4 ,within a space of a few square yards their parallelism reminds one of stratified deposits 7,fig4 #ccf71 but traced over a wider space they are found to be more or less irregular in thickness and lenticular in form4 ,illustration3 ,fig4 #ccf4--;,banded structure in the ,gabbro1 from the ridge of ,druim an ,eidhne between ,loch ,coruisk and ,glen ,sligachan4 ,the resemblance to gneisses1 and sometimes to the flow-structure of coarse rhyolites1 is still further sustained by occasional undulations or minute puckerings 7,fig4 #cce74 ,still more extraordinary are the examples of the actual plication of a group of successive bands1 as shown in ,fig4 #ccg1 wherein such a group about ten feet thick is shown to have been doubly folded between parallel bands above and below4 ,this structure is not due to any deformation of the gabbro long subsequent to the consolidation of the mass4 ,it belongs to the phenomena of protrusion and solidification4 ,an examination of thin slices of these rocks under the microscope reveals no evidence of crushing4 ,on the contrary1 the minerals of one band interlock with those of the band adjoining1 in such a manner as to prove that the differences of composition cannot be due to crushing and shearing or to successive intrusion1 but must have been present before the final consolidation of the whole rock4#cdh ,footnote #cdh3 ,mr4 ,j4 ,j4 ,h4 ,teall and ,a4 ,g41 ,quart4 ,journ4 ,geol4 ,soc4 vol4 #a4 7#ahid71 p4 #feb4 ,the conclusion which seems most consonant with the facts is that the magma which supplied the visible masses of gabbro in ,skye existed below in a heterogeneous condition1 that portions of it1 differing considerably from each other in composition1 were simultaneously intruded1 and that by the deformation of these portions during their intrusion their present plicated structures were produced4 ,a careful study of these banded gabbros offers many suggestive points of comparison with the gneisses and anorthosite 7,norian7 rocks of pre-,cambrian age4 ,it seems in the highest degree probable that the banded structures and peculiar mineral aggregation in these ancient rocks arose under conditions closely analogous to1 if not identical with1 those in which the ,tertiary gabbros of ,skye originated4#cdi ,footnote #cdi3 ,consult the ,memoirs cited in the footnote on p4 #cdb4 ,similar structures are found to be widely developed through the gabbros of the ,cuillin ,hills4 ,not only are these rocks disposed in distinct beds1 but many of the beds display the most perfect banding4 ,thus the mountains that surround the head of ,loch ,scavaig and sweep round ,loch ,coruisk up to the great splintered crests of ,sgurr na ,banachdich display on their bare black crags a distinct bedded structure4 ,on the east side of ,loch ,scavaig the rock presents a rudely-banded character1 the bands or beds being piled over each other from the sea-level up to the summits of the rugged precipices1 and dipping into the hill at angles of #be to #ce4 ,abundant dykes and veins of various basic1 intermediate and acid rocks cut this structure4 ,the individual layers here show sometimes the wavy and puckered condition already referred to4 ,even from a distance the alternating lighter and darker bands can readily be seen1 so that this structure1 with the variations in its inclination1 can be followed from hill to hill 7,fig4 #cch74 ,the regularity of the arrangement1 however1 is often less pronounced on closer inspection4 ,while the gabbro is rudely disposed in thick beds1 indicative of different intrusive sheets or sills1 with which the banding is generally parallel1 considerable irregularities may be observed in the arrangement of the structure of individual sheets4 ,these sheets may be parallel to each other1 and yet1 while in some the banding is tolerably regular in the direction of the planes of the sheets1 in others it is much twisted or inclined at various angles4 ,illustration3 ,fig4 #ccg4--;,banded and doubly-folded ,gabbro1 ,druim an ,eidhne1 #aj feet broad4 ,on the west side of the ,coruisk river the banding is vertical2 southward from that stream it inclines slightly towards the south1 but soon again becomes vertical1 and continues conspicuously so at the junction of the gabbro with the ,torridon sandstones and plateau-basalts on the west side of ,loch ,scavaig4 ,thus1 instead of being one great eruptive boss1 the gabbro of this district is in reality an exceedingly complicated network of sills1 veins and dykes4 ,while the general inclination of the bedding sometimes continues uniform in direction and amount from one ridge to another1 it is apt to change rapidly1 as if the complex assemblage of intruded masses had been disrupted and had subsided in different directions4 ,for example1 after overlying the bedded basalts of the plateau all the way from ,glen ,brittle to the west side of ,loch ,scavaig1 the gabbro descends abruptly across these basalts and also across the ,torridon sandstones1 on which they unconformably rest4 ,these two groups of rocks are not only truncated by the gabbro1 but are traversed by the intricate system of sills1 dykes and veins already referred to4 ,where it abuts against the sandstones and basalts in ,loch ,scavaig1 the gabbro is arranged in vertical bands of different mineral composition and texture4 ,much of it is remarkably coarse1 some bands displaying pyroxene crystals more than an inch in length4 ,there is no fine-grained selvage here1 indicative of more rapid cooling4 ,so coarse1 indeed1 is the rock close up against the sandstone1 that the junction-line can hardly be supposed to be the normal contact of the intrusive rock4 ,this inference is confirmed by the existence of a singular kind of breccia between the gabbro and the sandstones4 ,it is a tumultuous mass of fragments of coarse and fine gabbro1 ,torridon sandstone and shale1 and plateau-basalts1 embedded in a pale crystalline matrix of fine granular granophyre2 veins from this acid intrusion run off into the gabbro on the one side as well as into the ,torridon sandstones on the other4 ,it would seem that this junction-line has been one of great movement1 that the gabbro-sheets have subsided against a fault-wall of plateau-basalt and ,torridon sandstone1 and that subsequently an intrusion of finely granular granophyre has come up the fissure1 involving in its ascent fragments of all the materials around4 ,the rocks for a considerable distance to the south of the gabbro are intensely altered4 ,the ,torridon sandstone has been so indurated as to pass into a bleached white quartzite1 while the shales interstratified with it have been converted into a kind of porcellanite4 ,but the most interesting alterations are those to be observed in the plateau-basalts1 which at a height of about #cjj feet above the sea1 are to be seen in nearly horizontal sheets that lie immediately on the upturned edges of the ,torridon sandstones4 ,these lavas have suffered great metamorphism1 to which more particular reference will be made in ,chapter xlvi4 in connection with the action of the granophyre4 ,whether this alteration has been produced by the intrusion of the gabbro or of some concealed mass of granophyre underneath1 of which only projecting dykes and veins reach the surface1 must remain a matter of doubt4 ,on the whole1 as the gabbro is here undoubtedly thrown against the basalts and ,torridon sandstone by a fault1 it seems most probable that the change has been mainly due to the influence of the acid rock4 ,in the ,blath ,bheinn group of hills the relations of the gabbro to the bedded basalts have recently been mapped in detail by ,mr4 ,harker during the progress of the ,geological ,survey of ,skye4 ,he has observed that1 allowing for irregularities of form1 the mass of gabbro obliquely overlies the basalts as a great sheet1 not necessarily due to a single intrusion1 which dips towards the west4 ,he has found the rock to vary from a coarse gabbro to a diabasic type1 and to vary also in mineralogical constitution1 becoming in places very rich in olivine1 though the banded structure is here only exceptionally developed4 ,north of ,garbh ,bheinn the gabbro is much crushed and the outlying patch to the north of ,belig is in part a crush-breccia4 ,mr4 ,harker remarks that similar brecciated structures are common among the granophyres of the ,red ,hills1 and that it is sometimes difficult to distinguish their structure from that of the true volcanic agglomerates4 ,illustration3 ,fig4 #cch4--,sketch of ,banded ,structure in the ,gabbros of the hills at the head of ,loch ,scavaig4 ,besides the main area of gabbro in ,skye1 a great many small detached bosses1 sills and dykes lie further east on the flanks of the ,red ,hills4 ,one of the best marked of these detached areas forms a conspicuous crag on the east side of ,strath ,more1 immediately to the north of ,beinn na ,cro4 ,it consists of beds of coarse gabbro1 with others of dolerite intercalated in an outlier of the plateau-basalts1 and is traversed by veins from the granophyre of the glen1 as well as by the usual north-west basalt dykes 7,fig4 #cdi74 ,it appears to be a marginal portion of the main gabbro area separated by the intrusion of the great granitoid boss of the ,red ,hills4 ,on the north-eastern side of ,beinn na ,caillich numerous intrusive sheets of gabbro and dolerite traverse the quartzite and limestone1 and extend down to the sea-margin in the ,sound of ,scalpa4 ,there is an important feature in the main gabbro area of ,skye not yet clearly understood1 and which only a minute and patient survey can elucidate4 ,though ,i have found among the ,cuillin ,hills no distinct proof that the mass of gabbro ever gave rise to discharges of material1 either lava-form or fragmentary1 which reached the surface1 the gabbro area1 as already remarked1 contains unquestionable evidence of explosions and the production of pyroclastic masses4 ,among the moraine-mounds of ,harta ,corry1 blocks of basalt-agglomerate are strewn about1 full of angular fragments of altered basalt1 sometimes highly amygdaloidal1 and also boulders in which lumps of coarse gabbro are enveloped in a matrix of finer material4 ,i did not find the parent rocks from which these glacier-borne masses had been derived1 but there can be no doubt that they exist among the gabbro crags that surround that deep glen4 ,reference has already been made to the similar rock found in situ on the opposite side of the ,cuillin ridge at the head of the great cauldron of ,corry na ,creich2 likewise to the mass of coarse agglomerate which forms a group of knolls and crags on the east side of ,druim an ,eidhne above the head of ,glen ,sligachan4 ,this rock contains abundant blocks of various slaggy lavas like those of the basalt-plateau1 and runs for some distance along the eastern limit of the gabbro1 between that rock and the granophyre4 ,it is intersected by numerous basalt-veins4 ,mr4 ,harker1 as above mentioned1 has recently found some considerable strips of agglomerate which1 like that which ,i traced round the west side of ,beinn ,dearg1 are interposed between the gabbro and the bosses of granophyre1 or lie at the base of the volcanic series 7p4 #bhd74 ,there does not1 however1 appear to be any evidence to connect these isolated masses of agglomerate with the phenomena attending the uprise of the gabbro4 ,they seem to be more probably related to the plateau eruptions1 and may be compared with those of ,strath1 ,ardnamurchan and ,mull 7pp4 #bgh1 #bhj1 #chd74 ,that the huge gabbro mass of ,skye1 besides invading and altering the bedded basalts1 may have communicated eventually with the surface1 and have given rise to superficial discharges1 is not at all improbable1 but of any such outflows not a vestige appears now to remain4 ,we must remember1 however1 that the gabbro no doubt in many places found its readiest upward ascent in vents belonging to the plateau-period1 and that portions of the agglomerates of these earlier vents may be expected to be found involved in it1 as the agglomerate of the great vent of ,strath has been invaded by the granophyre4 ,,chapter ,,xliv ,,the ,,bosses ,,and ,,sheets ,,of ,,gabbro ,,in ,,the ,,districts ,,of ,,rum1 ,,ardnamurchan1 ,,mull1 ,,st4 ,,kilda ,,and ,,north-,,east ,,ireland4 ,,history ,,of ,,the ,,gabbro ,,intrusions #b4 ,the ,island of ,rum ,the mountains of the island of ,rum1 rising as they do from a wide expanse of open sea1 present one of the most prominent and picturesque outlines in the ,west ,highlands 7,map ,,vi474 ,more inaccessible than most of the other parts of the volcanic region1 they have been less visited by geologists4 ,they were described by ,macculloch as composed of varieties of augite rock4 ,he noticed in this rock a tendency to the same obscurely bedded disposition as is observed in other rocks of the trap family1 and found at one place that it assumed a regularly bedded form1 being disposed in thin horizontal strata1 among which are interposed equally thin beds of a rock resembling basalt in its general characters4#cej ,professor ,judd repeats ,macculloch's observation1 that the great masses of gabbro in ,rum often exhibit that pseudo-stratification so often observed in igneous rocks4 ,he regards these masses1 like those of ,skye and ,mull1 as representing the core of a volcano from which the superficial discharges have been entirely removed1 and he gives a section of the island in which the gabbro is represented as an amorphous boss sending veins into a surrounding mass of granite4#cea ,in a subsequent paper he gave an excellent detailed account of the mineralogical composition of some of the remarkably varied and beautiful basic rocks constituting the hills of ,rum1 but added no further information regarding the geological structure of the island4#ceb ,footnote #cej3 ,western ,islands1 i4 p4 #dhf4 ,footnote #cea3 ,quart4 ,journ4 ,geol4 ,soc4 xxx4 p4 #bec4 ,footnote #ceb3 ,op4 cit4 xli4 7#ahhe7 p4 #ced4 ,see also his paper in vol4 xlii4 of the same ,journal4 ,even from a distance of eight or ten miles1 the hills of ,rum are seen to be obviously built up of successive nearly horizontal tiers of rock4 ,as the summer tourist is carried past the island1 in that wonderful moving panorama revealed to him by the swift steamer of modern days1 these great dark cones remind him of colossal pyramids1 and as the ever-varying lights and shadows reveal more prominently the alternate nearly level bars of crag and stripes of slope1 the resemblance to architectural forms stamps these hills with an individuality which strikes his imagination and fixes itself in his memory4 ,if choice or chance should give him a nearer view of the scene1 he would not fail to notice that it is among the northern hills of the island that the bedded character is so conspicuous1 and that it ceases to be prominent in the southern heights1 though here and there1 as in the upper part of ,scuir na ,gillean1 it may in certain lights be detected even from a distance4 ,crossing over from ,eigg1 he would recognize each of the features represented in the sketch reproduced in ,fig4 #cci4 ,along the shore1 red sandstones rise in naked cliffs1 from the top of which the ground slopes upward in brown moors to the bare rocky declivities4 ,a deep valley 7,glen ,dibidil7 is seen to run into the heart of the hills1 between the bedded group to the north and the structureless group to the south4 ,if the weather is favourable1 some eight or more prominent parallel bars of rock may be counted on the two higher cones to the right4 ,these bars are not quite level1 but slope gently from right to left4 ,they remind one of the terraced basalts of the plateaux1 but present a massiveness and a breadth of intervening bare talus-slope such as are not usual among those rocks4 ,illustration3 ,fig4 #cci4--,outline of the ,hills of the ,island of ,rum1 sketched from near the ,isle of ,eigg4 ,nor is this impression of regularity and bedded arrangement lessened when we actually climb the slopes of the hills4 ,i had for years been familiar with the outlines of ,rum as seen from a distance1 and had sketched them from every side1 but ,i shall never forget the surprise and pleasure when my first ascent of the cones revealed to me the meaning of these parallel tiers of rock4 ,i found it to be the structure of the ,cuillin ,hills repeated1 but with some minor differences which are of interest1 inasmuch as they enlarge our conceptions of the process by which the gabbro-bosses were formed4 ,the northern half of the island of ,rum consists almost entirely of red sandstone1 which1 as already stated1 is a continuation of the same formation 7,torridonian7 so well developed in the south-east of ,skye1 ,applecross and ,loch ,torridon1 and traceable between the ,archan gneiss and the ,cambrian strata up as far as ,cape ,wrath4 ,the sandstones1 though full of false bedding1 show quite distinctly their true stratification1 which is inclined with singular persistence towards ,w4,n4,w41 at angles averaging from #ae to #bj4 ,if they are not repeated by folds or faults1 they must reach in this island a thickness of some #aj1jjj feet4 ,their red or rather pinkish tint seems mainly to arise from the pink felspar so abundant in them1 for in many places they really consist of a kind of arkose4 ,pebbly bands with rounded pieces of quartz are of common occurrence throughout the whole formation4 ,dykes and veins of basalt are profusely abundant4 ,sometimes these run with the bedding1 and might at a distance be taken for dark layers among the pink sandstones4 ,they often also strike obliquely up the face of the cliffs like ribbons4 ,but1 notwithstanding their apparent continuity1 there can be no doubt that these sandstones have suffered from those powerful terrestrial disturbances which have affected all the older rocks of the ,north-,west ,highlands4 ,on the west side1 where they plunge steeply into the sea1 they have undergone a change into fine laminated rocks1 which might at first be mistaken for shales1 but which owe their fissility to shearing movements4 ,along their southern border1 from a point on the east coast near ,bagh-na-h-,uamha1 south of ,loch ,scresort1 to the head of ,kilmory ,glen1 they are abruptly truncated against a group of dark1 flaggy and fissile schists and fine quartzites or grits1 which in some places are black and massive like basalt1 and in others are associated with coarse grey gneiss4 ,that some of these rocks are portions of the ,lewisian series can hardly be doubted1 and their structure and relations are probably repetitions of those between the ,lewisian gneiss and ,torridon sandstone of ,sleat in ,skye4 ,i found also on the northern slopes of ,glen ,dibidil a patch of much altered grey and white limestone or marble1 which reminded me of the ,cambrian limestone of ,skye4 ,the red sandstones in a more or less altered condition are prolonged to the south-east promontory of the island4 ,in passing over the zone of these more ancient rocks1 we find them to present increasing signs of alteration as they are traced up the slopes towards the great central mass of erupted material4 ,the pink sandstones gradually lose their characteristic tint1 and grow much harder and more compact1 while the veins and dykes of basalt and sheets of dolerite intersecting them increase in number4 ,the zone of black compact quartzite1 which lies to the south of the sandstones1 and which at one point reminds us of basalt1 at another of the flinty slate of the schistose series1 likewise displays increasing induration4 ,its bedding1 not always to be detected1 is often vertical and crumpled4 ,but the most remarkable point in its structure is the intercalation in it of bands of breccia4 ,these vary from less than an inch to several yards in diameter2 they run mostly with the bedding1 but occasionally across it4 ,the stones in them are fragments of the surrounding rock embedded in a matrix of the same material1 but also with pieces of a somewhat coarser grit or quartzite4 ,a band of coarse breccia forms the southern limit of this zone along the northern base of ,barkeval and ,allival4 ,in general character it resembles the thinner seams of the same material just referred to4 ,the matrix so closely agrees with the black flinty quartzite1 that but for the included stones it could hardly be distinguished2 so greatly has the mass been indurated that the stones seem to shade off into the rest of the rock4 ,but here and there its true brecciated nature is conspicuously revealed by prominent blocks of hardened sandstone4 ,this band of breccia must in some places be #aej or #bjj feet broad4 ,it has no distinct bedding1 but seems to lie as a highly inclined bed dipping into the hill4 ,it may possibly be a crush-breccia belonging to a period earlier than the volcanic eruptions4 ,it is at once succeeded by a black flinty felsite like that of ,mull4 ,the groundmass of this rock1 so thickly powdered with magnetite grains as to be almost opaque under the microscope1 displays good flow-structure round the turbid crystals of orthoclase and the clear granules of quartz4 ,further up the hill1 the rock becomes lighter in colour and less flinty in texture--a change which is found to arise from more complete devitrification1 the groundmass having become a crystalline granular aggregate of quartz and felspar with scattered porphyritic crystals of these minerals 7microgranite74 ,in some places1 the felsite incloses fragments of other rocks4 ,a specimen of this kind1 taken from the head of ,coire ,dubh1 shows under the microscope a brown micro-felsitic groundmass1 with crystals of felspar and augite1 inclosing a piece of basalt1 composed of fine laths of plagioclase1 abundant magnetite and a smaller proportion of granules of augite4 ,illustration3 ,fig4 #cdj4--,view of ,allival1 ,rum1 sketched from the base of the north-east side of the cone4 ,this band of felsite and microgranite may be traced continuously from ,loch ,gainmich along the base of ,barkeval and ,allival1 and similar rocks appear at intervals on the same line round the eastern base of the hills4 ,immediately above this belt of felsitic protrusions comes the great body of gabbro4 ,it will be observed that here1 as in ,skye1 the base of the gabbro mass presents a horizon on which injections of acid rocks have been particularly abundant4 ,whether the breccias be regarded as the result of earlier rock-crushings1 or as due to volcanic explosions during the ,tertiary period1 they are evidently older than the eruption of the gabbros4 ,in that respect they may be compared with the agglomerates through which the youngest eruptive bosses of ,skye have made their way2 but their component materials have been derived from the surrounding platform of ancient rocks1 and not from subterranean lavas4 ,illustration3 ,fig4 #cda4--,section of foliated gabbros in the ,tertiary volcanic series of ,allival1 ,rum4 a1 massive gabbro with rude lamination parallel to bedding1 only seen in some weathered surfaces2 b1 laminated troctolite2 c1 massive coarsely crystalline gabbro rudely laminated4 ,for my present purpose1 however1 the chief point of importance is the structure of the gabbro mass that springs from that platform into the great conical hills of ,rum4 ,the accompanying sketch 7,fig4 #cdj7 will convey a better idea of this structure than a mere description4 ,at the base1 immediately above the felsite just referred to1 bedded dolerites make their appearance1 much intersected with veins from the siliceous rock4 ,veins and dykes of basalt also cut all the rocks here1 the newest being those which run in a north-west direction4 ,the lowest sheets of dolerite are succeeded by overlying sills of coarser dolerites1 gabbros1 troctolites1 etc41 which are as regular in their thickness and continuity as the ordinary basalts of the plateaux4 ,the band of light-coloured troctolite1 in particular 7,fig4 #cda71 about #bj to #cj feet thick1 which has been already referred to for its remarkable laminar structure1 can be followed for some distance along the base of the hill as a marked projecting escarpment4 ,this rock at once arrests attention by its platy or fissile structure1 parallel to the bedding-surfaces of the sheet4 ,indeed hand-specimens of it1 as ,i have said1 might readily pass for pieces of schistose limestone1 especially if taken from the upper part4 ,it consists of successive layers1 which on the weathered surface divide it into beds almost as regular as those of a flagstone1 each bed being further separated into lamin marked off by the darker and lighter tints of their mineral constituents4 ,the darker layers consist of olivine1 and the lighter of plagioclase4 ,this segregation here and there takes the form of rounded masses1 where the minerals are more indefinitely gathered together4 ,the affinity of the rock with intrusive sheets is further displayed by the occurrence of abundant nut-like aggregates of pale green olivine4 ,examined under the microscope1 flow-structure is admirably seen1 the lath-shaped felspars being drawn out parallel to the planes of movement1 and giving thereby the peculiarly schistose structure which is so deceptive4 ,the massive and coarsely crystalline gabbros below and above this troctolite are all more or less affected by the same laminar structure4 ,some of those in higher parts of the mountain are quite massive in part1 but also include bands of lamination4 ,banding like that of the ,skye gabbros is generally developed among them1 the individual bands varying from less than an inch to a foot or more in thickness4 ,this structure1 like the lamination1 is parallel to the general bedding of the sheets4 ,as in the ,cuillin ,hills1 the bands differ from each other in the relative proportions of the constituent minerals1 especially the predominant pyroxene and olivine4 ,the crystals or crystalline aggregates are often from a quarter of an inch to an inch in diameter1 and in these large forms are crowded together in certain bands4 ,magnetite1 on the whole1 is rather less conspicuous than in the ,cuillin gabbro3 at least1 it is not so prominently aggregated in special layers4 ,in one or two instances ,i have observed curvature of the banding1 but no example so striking as that cited from the ,cuillin area 7,fig4 #ccg74 ,on weathered surfaces1 where the felspars decay into a creamy white and the ferro-magnesian minerals assume tints of green1 brown and red1 the resemblance of the rocks to schists is striking4 ,this external likeness is combined with a tendency to split into thin plates parallel to the lamination1 which still further increases their schistose appearance4 ,though less developed than in ,skye1 the banding appears to be of the same kind and origin2 but in ,rum it is combined with the remarkable lamination above mentioned1 produced by the arrangement of the component minerals with their longer axes parallel to the planes of bedding1 as in flow-structure--a combination which ,i have not yet observed in ,skye4 ,the bedded arrangement of the gabbros of ,rum1 so conspicuous in the great eastern cones 7,figs4 #cci and #cdj71 is emphasized by the fact that some sheets1 of a more durable kind1 stand out boldly as prominent ribs1 while the softer crumble into a kind of sand1 which forms talus-slopes between the others4 ,alternations of this nature are continued up to the very top of the mountains4 ,the beds are nearly flat1 but dip slightly into the interior or towards the south-west4 ,on the west side of the island also1 beyond ,loch ,sgathaig1 a distinct bedding may be traced1 the inclination being here once more inwards or to the east4 ,but from ,glen ,harris and the base of ,askival this structure becomes less marked1 and gradually disappears4 ,there is thus a central or southern more amorphous region1 while round the margin towards the north and east the rock appears in frequent alternating beds4 ,it is clear that in the broad features of their architecture the hills of ,rum follow closely the plan shown in the ,cuillin ,hills of ,skye4 ,but1 unfortunately1 in the former island denudation has gone so far that no connection can be traced on the ground between the gabbros and the plateau-basalts4 ,as already stated1 the latter rocks have been almost entirely stripped off from the platform of sandstones and schists which they undoubtedly at one time covered1 and the few outliers of them that remain lie at some little distance from the margin of the gabbro area 7ante1 p4 #baf74 ,nevertheless1 we are not without some indications of them underneath the gabbros4 ,i have alluded to the basalts that lie at the base of the eastern cones4 ,as we follow the bottom of the gabbro southward round the flanks of the hills1 dull compact black shattery basalts1 with a white crust1 appear from under the more crystalline sheets4 ,these at once remind one of the altered basalts of ,skye and ,mull4 ,on the west side also1 beds of basalt emerge from under the gabbro1 but they have been so veined and indurated by the granophyre of that district1 that their relations to the gabbro are somewhat obscured4 ,if we could restore the lost portions of the plateau1 ,i believe we should find the gabbros of ,rum resting on part of the volcanic plateau1 and some of the gabbro-beds prolonged as sills between the sheets of basalt4 #c4 ,the ,gabbro of ,ardnamurchan ,the promontory of ,ardnamurchan reveals as clearly as the flanks of the ,cuillin ,hills1 though in a less imposing way1 the relations of the gabbros to the plateau-basalts 7,map ,,vi474 ,from the southern shore at ,kilchoan to the northern shore at ,kilmory1 bedded basalts1 of the usual type1 amygdaloidal and compact1 weathering into brown soil1 may be followed along the eastern slopes of the hills1 resting upon the schists and ,jurassic series of western ,argyleshire4 ,these rocks are a continuation of those that cap the ridges further to the south-east and cross ,loch ,sunart into ,morven4 ,they dip westwards1 and followed upwards in that direction1 they soon present the usual marks of alteration4 ,they weather with a white crust and become indurated and splintery4 ,sheets of dolerite with many veins and dykes of basalt run between and across them4 ,bands of gabbro make their appearance1 and these1 as we advance westwards1 increase in number and in coarseness of grain until this rock1 in its rudely bedded form1 constitutes practically the whole of the promontory from ,meall nan ,con to the light-house4 ,many admirable sections may be seen on the coast-cliffs and in the rugged interior1 showing the irregular bedding of the gabbro1 and how prone this rock is to develop its component minerals in bands or ribbons1 sometimes made up of large crystals1 as in ,skye1 ,rum and ,mull4 #d4 ,the ,gabbro of ,mull ,in the island of ,mull1 the conclusions to which the geology of the other volcanic districts leads us as to the position of the gabbros in the series of volcanic phenomena1 are further confirmed4 ,the first geologist who appears to have observed the relation of these rocks in that island was ,jameson1 who classed them under the old name of greenstone1 including in the same designation rocks now termed dolerites and gabbros4 ,he ascended one of the hills above ,loch ,don1 probably ,mainnir nam ,fiadh 7#bdhc feet71 which he found to consist of strata of basalt and greenstone1 with some basalt-breccia or tuff and a capping of basalt4 ,he speaks of the singular scorified-like aspect of the weathered greenstone--a description which applies to some of the coarser gabbro bands of that locality4 ,but he appears to have recognized the general bedded arrangement of the rocks up even to the summit of the hill4#cec ,footnote #cec3 ,mineralogy of the ,scottish ,isles i4 p4 #bje4 ,it was not1 however1 until the visit of ,professor ,zirkel in #ahfh1 that the true petrographical characters of the gabbro of ,mull were recognized4 ,this observer remarked that the rock is regularly interstratified with the basalt4#ced ,professor ,judd1 as already stated1 has supposed the gabbros to be the deep-seated portion of the masses which when poured out at the surface became the plateau-basalts1 and he represents them in his map and sections of ,mull as ramifying through the granitic rocks4#cee ,footnote #ced3 ,zeitsch4 ,deutsch4 ,geol4 ,gesellsch4 xxiii4 7#ahga7 p4 #eh4 ,footnote #cee3 ,quart4 ,jour4 ,geol4 ,soc4 xxx4 7#ahgd74 ,in ,mull the disposition of the gabbro in beds1 sheets or sills is well displayed1 for there is here no great central complicated mass of interlacing banded and amorphous sheets4 ,we have seen that a higher group of plateau-basalts has survived in this island better than in the other plateaux1 and it would seem that denudation has not yet succeeded here in cutting down so deeply into the gabbro core as in ,skye1 ,rum and ,ardnamurchan4 ,only the upper or outer fringe of intrusive sheets among the bedded basalts has been laid bare4 ,the district within which this fringe may be observed is tolerably well-defined by the difference of contour between the long terraced uplands of the ordinary basalts and the more conical forms of the southern group of gabbro hills between ,loch na ,keal and ,loch ,spelve4 ,the number and thickness of the gabbro-sheets increase as we proceed inwards from the basalt-plateau4 ,these sheets are specially prominent along the higher parts of the ridge that runs northwards from the northern end of ,loch ,spelve1 and along the west side of ,glen ,forsa4 ,but they swell out into the thickest mass in the south-western part of the hilly ground1 where1 from above ,craig1 in ,glenmore1 they cross that valley1 and form the rugged ridge that rises into ,ben ,buy 7#bced feet71 and stretches eastward to near ,ardara 7,map ,,vi474 ,it is in this southern mass that the ,mull gabbro approaches nearest in general characters to that of ,skye4 ,but even here its true intercalation above a great mass of bedded basalt may readily be ascertained in any of the numerous ravines and rocky declivities4 ,one of the best lines of section for exhibiting the relations of the rocks is the declivity to the west of ,ben ,buy and ,loch ,fhuaran4 ,ascending from the west side1 we walk over successive low escarpments and terraces of the plateau-basalts with a gentle inclination towards north-east or east4 ,these rocks weather in the usual way1 some into a brown loam1 others into spheroidal exfoliating masses4 ,but as we advance uphill they gradually assume the peculiar indurated shattery character already referred to4 ,the soft earthy amygdaloids become dull splintery rocks1 in which the amygdales are no longer sharply defined from the matrix1 but rather seem to shade off into it1 sometimes with a border of interlacing fibres of epidote4 ,the compact basalts have undergone less change1 but they too have become indurated1 and generally assume a white or grey crust1 and none of them weather out into columnar forms4 ,strings and threads full of epidote run through much of these altered rocks4 ,abundant granophyric and felsitic veins traverse them4 ,sheets of dolerite likewise make their appearance between the basalts1 followed further up the slope by sheets of gabbro until the latter form the main body of the hill4 ,on the north side of the same ridge similar evidence is obtainable1 though somewhat complicated by the injections of granophyric and felsitic veins and bosses1 to which more detailed reference will afterwards be made4 ,but the altered basalts with their amygdaloidal bands and their intercalated basalt-tuffs and breccias1 can be followed from the bottom of the glen up to a height of some #agjj feet1 above which the main gabbro mass of ,ben ,buy sets in4 ,many minor sheets of dolerite and gabbro make their appearance along the side of the hill before the chief overlying body of the rock is reached4 ,some of these can be distinctly seen breaking across or ending off between the bedded basalts which here dip gently into the hill 7,fig4 #cdb74 ,a conspicuous band of coarse basalt-agglomerate1 containing blocks of compact and amygdaloidal basalt a yard or more in diameter1 shows by the excessive induration of its dull-green matrix the general alteration which the rocks of the basalt-plateau have here undergone4 ,an almost incredible number of veins of fine basalt1 porphyry and felsite has been injected into these rocks--a structure which is precisely a counterpart of what occurs under the main body of gabbro in ,skye1 ,ardnamurchan and ,rum4 ,illustration3 ,fig4 #cdb4--;,altered ,plateau-,basalts invaded by ,gabbro1 and with a ,dyke of prismatic ,basalt cutting both rocks1 north slope of ,ben ,buy1 ,mull4 a a1 amygdaloidal basalt1 much altered2 b1 gabbro2 c1 finely prismatic basalt4 ,the gabbro mass of the ,ben ,buy ridge is thus undoubtedly a huge overlying sheet1 which probably reaches a thickness of at least #hjj feet4 ,it seems to descend rather across the bedding into the hollow of ,glen ,more1 and possibly its main pipe of supply lay in that direction4 ,being enormously thicker than any other sheet in the island1 it exhibits the crystalline peculiarities which are so well developed in the central portions of the larger bosses of gabbro4 ,it presents more coarsely crystalline varieties than appear in the thinner sheets1 some portions showing crystals of diallage and felspar upwards of an inch in length4 ,it likewise contains admirable examples of banded structure1 which1 as in ,skye and elsewhere1 is best developed where the texture becomes especially coarse4 ,veins or bands1 in which the constituent minerals have crystallized out in more definite and conspicuous forms1 here and there succeed each other so quickly as to impart a bedded or foliated look to the body of rock1 recalling1 as in ,skye1 the aspect of some coarsely crystalline granitoid gneiss4 ,in these respects the ,mull gabbro closely resembles that of the ,cuillin ,hills4 ,occasionally1 on the exposed faces of crags1 portions of such bands or veins are seen to be detached and enveloped in a finer surrounding matrix4 ,the thick belts or bands of coarser and finer texture alternate1 and give an appearance of bedding to the mass4 ,nevertheless they are really intrusive sills1 which run generally parallel with beds of finer gabbro or with sheets of highly indurated basalt1 that may be detached portions of the ordinary rocks of the plateau4 ,the thick sheet of ,ben ,buy1 like the mass of the ,cuillin ,hills1 is thus the result not of one but of many uprises of gabbro4 ,of the thinner sheets of dolerite and gabbro in ,mull little need here be said4 ,i have referred to their great abundance in the range of eastern hills that rise from the ,sound of ,mull between ,loch ,spelve and ,fishnish ,bay4 ,though obviously intrusive1 they lie on the whole parallel to the bedding of the basalts4 ,the latter rocks exhibit the usual dull indurated shattery character which they assume where large bosses of gabbro have invaded them1 and which gradually disappears as we follow them down hill away from the intrusive sheets to the shores of the ,sound4 ,they dip towards the centre of the hill group1 that is1 to south-west in the ridge of ,mainnir nam ,fiadh1 ,dn da ,ghaoithe1 and ,beinn ,meadhon1 the angle increasing southwards to #ae-bj1 and at the south end reaching as much as #ce-dj4 ,some fine crags of gabbro and dolerite form a prominent spur on the east side of the ridge of ,ben ,talaidh1 in the upper part of ,glen ,forsa4 ,these consist of successive sheets bedded with the basalts1 and dipping south-west4 ,a large sheet stands out conspicuously on the north front of ,ben ,more1 lying at the base of the pale lavas1 and immediately above the ordinary basalts4 ,it circles round the fine corry between ,ben ,more and ,a',chioch1 some of its domes being there beautifully ice-worn4 ,this is the highest platform to which ,i have satisfactorily traced any of the intrusive sheets of ,mull4 ,another dyke-like mass emerges from beneath the talus slopes of ,a',chioch1 on the southern side1 and runs eastward across the col between the ,clachaig ,glen and ,loch ,scridain4 #e4 ,the ,gabbros of ,st4 ,kilda and ,north-east ,ireland ,sixty miles to the westward of the ,outer ,hebrides lies the lonely group of islets of which ,st4 ,kilda is the chief4 ,as the main feature of geological interest in this group is the relation of the acid protrusions to the other rocks1 the account of the geology will be more appropriately given as a whole in ,chapter xlvii4 ,i need only remark here that the predominant rocks of these islands are dark basic masses1 chiefly varieties of gabbro1 but including also dolerites and basalts4 ,reasons will be afterwards brought forward for regarding these rocks as parts of the ,tertiary volcanic series4 ,they present a close parallel to the gabbros and associated rocks of ,skye4 ,but in one important respect they stand alone4 ,no certain trace remains of any basalt-plateau at ,st4 ,kilda such as those through which the gabbros of ,skye1 ,mull and ,ardnamurchan have been injected4 ,in regard to their mode of production they have doubtless been intruded at some considerable depth beneath the surface4 ,but no relic appears to have survived of the overlying cover of rock under which they consolidated1 and into which they were injected4 ,in the remarkable volcanic district of the north-east of ,ireland a series of basic rocks appears1 which in its mode of occurrence and its relation to the other members of the series presents many points of resemblance to the gabbros of the ,inner ,hebrides4 ,the ,irish gabbros are well developed in the ,carlingford district1 where they form intrusive bosses and sheets which have been erupted through the ,palozoic rocks 7,map ,,vii474 ,they are themselves pierced by later masses of granophyre and other acid rocks4 ,further reference will be made to these gabbros in later pages1 where an account will be given of the granite masses of ,mourne1 ,barnavave and ,slieve ,gullion4 99 99 99 99 99 ,it is interesting to observe that1 while in ,st4 ,kilda no relic of any basaltic plateau has been preserved1 in the ,faroe ,islands1 on the other hand1 no sign has been revealed by denudation that the volcanic plateau of that region is pierced by any eruptive core of gabbro or of granophyre4 ,during my cruises round these islands and through their channels1 ,i was ever on the outlook for any difference in topography that might indicate the presence of some eruptive boss like the gabbro and granophyre masses of the ,inner ,hebrides4 ,but nothing of that nature could be discerned4 ,everywhere the long level lines of the bedded basalts were seen mounting up to the crests of the ridges and the tops of the highest peaks4 ,though ,i cannot assert that no intrusions of gabbro or of granophyre exist among the ,faroe ,islands1 ,i feel confident that any such masses which may appear at the surface must be of quite insignificant dimensions1 and do not make the important feature in geology and topography which they do among the ,inner ,hebrides4 ,it is1 of course1 possible that1 vast as the denudation of these islands has undoubtedly been1 it has not yet trenched the plateau deeply enough to expose any great intrusive bosses and sills which may underlie and invade the basalts4 iv4 ,,history ,,of ,,the ,,gabbro ,,intrusions ,we are now in a position to draw1 from the observations which have been given in this and the preceding chapter regarding the different areas of gabbro in the ,tertiary volcanic region of ,britain1 some general conclusions with respect to the type of geological structure and the phases of volcanic energy which they illustrate4 #a4 ,no evidence exists to show that the masses of gabbro ever communicated directly with the surface4 ,they never exhibit the cellular1 slaggy and other structures so characteristic of surface-flows4 ,they are1 on the whole1 free from included pyroclastic material1 though masses of agglomerate are enclosed in1 and have probably been invaded by1 the gabbro of the ,cuillin ,hills4 ,if the gabbro-bosses ever were continuous with sheets of rock emitted above ground1 all such upward continuations have been entirely removed4 ,in any case1 we may be quite certain that in an outburst at the surface1 the rock would not have appeared in the form of a coarsely crystalline or granitoid gabbro4 #b4 ,the crystalline structures of the gabbros point unmistakably to slow cooling and consolidation at some depth beneath the surface4 ,the most coarsely-crystalline varieties1 and those with the best developed banded structure1 occur in the largest bodies of rock1 where the cooling and consolidation would be most prolonged4#cef ,footnote #cef3 ,on this subject1 see the papers by ,professor ,judd already cited4 #c4 ,the remarkable differences in composition between the dark and pale layers in the banded gabbros cannot be accounted for by segregation or successive intrusion1 but seem to point to the existence of a heterogeneous magma from which these distinct varieties of material were simultaneously intruded4 #d4 ,from the prevalence of a bedded structure and the occurrence of bands and more irregular portions of considerably different texture and even mineralogical composition which intersect each other1 it may be confidently inferred that even what appears now as one continuous mass was produced by more than one intrusion4 #e4 ,in every case there would necessarily be one or more pipes up which the igneous material rose4 ,these channels might sometimes be wider parts of fissures1 such as those filled by the dykes4 ,in other places1 they may have been determined by older vents1 which had served for the emission of the plateau-basalts and their pyroclastic accompaniments4 ,there can be no doubt that some of these vents afforded egress for the subsequent eruption of granitoid rocks1 as will be pointed out in the following chapters4 ,in the case of the gabbros1 however1 the position of the vents seems to have been generally concealed by the tendency of these rocks to spread out laterally4 ,denudation has cut deeply into the gabbro-masses1 but apparently not deep enough to isolate any of the pipes from the larger bodies of material which issued from them1 and thus to leave solitary necks like those in and around the basalt-plateaux4 ,in ,skye1 where the central core of gabbro is largest and most completely encircled1 we cannot tell how much of it lies above the true pipe or pipes1 and has spread out on all sides from the centre of eruption4 ,the prevalence of rude bedding and a banded structure indicate that most of the visible rock occurs in the form of sills1 successively injected not only into the plateau-basalts1 but between and across each other4 ,round the margin of the gabbro we undoubtedly reach horizons below that rock1 and see that it lies as a cake or series of cakes upon the plateau-basalts4 ,the actual pipe or fissure of supply must in each case lie further inward1 away from the margin1 and may be of comparatively small diameter4 #f4 ,from the central pipe or group of pipes or fissures which rose from the platform of older rocks into the thick mass of the basalt-plateaux1 successive sheets of dolerite and gabbro were forced outward between the layers of basalt4 ,this took place all round the orifices of supply1 on many different horizons1 and doubtless at many different times4 ,in some cases1 the intrusive sheets were injected into the very bottom of the basalts1 and even between these rocks and the older surface on which they rested4 ,this is particularly the case in ,rum1 where the gabbro-cones spring almost directly from the ancient grits1 schists and sandstones on which they rest4 ,the intrusive sheets have likewise found egress at every higher platform in the basalt-series1 up at least to the base of the pale group in ,mull--that is1 through a continuous pile of more than #bjjj feet of bedded basalt4 ,but the intrusion did not proceed equally all round an orifice4 ,at all events1 the progress of denudation has revealed that on one side of a gabbro area the injected portions may occur on a lower stratigraphical level than they do on the opposite side4 ,at the ,cuillin ,hills1 for example1 the visible sheets of dolerite and gabbro to the north of ,coire na ,creiche begin about #afjj feet above the sea1 which must be much more than that distance above the bottom of the basalts4 ,on the south-east side1 however1 they come down to near the base of the basalts at ,loch ,scavaig2 that is to say1 their lowest members lie at least #afjj feet below those on the opposite margin4 #g4 ,the uprise of so much igneous material in one or more funnels1 and its injection between the beds of plateau-basalt1 would necessarily elevate the surface of the ground immediately above1 even if we believe that surface to have been eventually disrupted and superficial discharges to have been established4 ,if no disruption took place1 then the ground would probably be upraised into a smooth dome1 the older lavas being bent up over the cone of injected gabbro until the portion of the plateau so pushed upward had risen some hundreds of feet above the surrounding country4 ,the amount of elevation1 which would of course be greatest at the centre of the dome1 might be far from equable all round1 one side being pushed up further or with a steeper slope than another side4 ,but even in the case of the ,cuillin ,hill area1 it is conceivable that the total uplift produced at the surface a gentle inclination of no more than #h or #aj4 ,it is along the periphery of a gabbro area that we may most hopefully search for traces of this uplift4 ,but unfortunately it is just there that the work of denudation has been most destructive4 ,there appears also to have been a general tendency to sagging subsequent to the gabbro protrusions1 and the inward dip thereby produced has probably been instrumental in effacing at least the more gentle outward inclinations caused by the uprise of the eruptive rock4 ,in one striking locality1 however1 to which ,i have already referred1 the effects of both movements are1 ,i think1 preserved4 ,the basalt-plateau of ,strathaird1 which in its southern portion exhibits the ordinary nearly level bedding1 dips in its northern part at an unusually steep angle to the north-west1 towards the gabbro mass of ,blath ,bheinn4 ,but before reaching that mountain the basalts1 much interbanded with sheets of dolerite and gabbro1 suddenly bend up to form the prominent eminence of ,an ,stac1 where they dip rapidly towards south-east and south 7,fig4 #ccd74 ,this steep dip away from the central mass of gabbro1 is repeated in the hills to the north1 where the beds are inclined to north-east1 the angle gradually lessening northwards till they are truncated by the granophyre of ,strathmore4 ,the mass of ,blath ,bheinn thus occupies the centre of the dome or anticline4 ,the theoretical structure of one of the gabbro bosses is represented in ,fig4 #cdc4 ,it will be understood1 however1 that what for the sake of clearness is here represented in one uniform tint of black in reality consists of an exceedingly complex network of sheets and dykes differing from each other in texture and structure1 as well as in the relative dates of their intrusion4 #h4 ,the injection of so much igneous material among the bedded basalts has induced in these rocks a certain amount of contact metamorphism4 ,i have referred to it as showing itself in the field as a marked induration1 the rocks becoming closer grained1 dull1 splintery1 and weathering1 with a grey or white crust1 while their amygdales lose their definite outlines1 and epidote and calcite run in strings1 veins and patches through many parts of the rocks4 ,as already remarked1 it is difficult to determine how much of this change should be referred to the influence of the gabbro1 and how much to that of the numerous intrusions of granophyre which may be apophyses of much larger bodies of that rock lying not far underneath4 ,on account of this difficulty1 the more detailed description of the metamorphism of the plateau-basalts is reserved for ,chapter xlvi41 where it will find a place in connection with the effects produced by the intruded granophyres1 which have undoubtedly been more extensive than those effected by the gabbros4 ,illustration3 ,fig4 #cdc4--,theoretical representation of the structure of one of the ,gabbro ,bosses of the ,inner ,hebrides4 a a1 platform of older rock on which the bedded basalts 7b b7 have been poured out2 c1 gabbro4 ,the structure and history of the gabbro bosses of the ,inner ,hebrides find a close parallel in those of the ,henry ,mountains of ,southern ,utah1 so well described by ,mr4 ,g4 ,k4 ,gilbert of the ,united ,states ,geological ,survey4#ceg ,in that fine group of mountains1 rising to an extreme height of #ejjj feet above the surrounding plateau1 and #aa1jjj feet above the level of the sea1 masses of trachyte have been injected between sedimentary strata belonging to the ,jura-,triassic and ,cretaceous systems4 ,these masses1 thirty-six in number1 have consolidated in dome-shaped bodies1 termed by ,mr4 ,gilbert laccolites1 which have arched up the overlying strata1 sending sheets1 veins and dykes into them1 and producing in them the phenomena of contact metamorphism4 ,there is no proof that any of these protrusions communicated with the surface1 and there is positive evidence that most if not all of them did not4 ,the progress of denudation has laid bare the inner structure of this remarkable type of hill1 and yet has left records of every stage in its sculpture4 ,in one place are seen only arching strata1 the process of erosion not having yet cut down through the dome of stratified rocks into the trachyte that was the cause of their uprise4 ,in another place1 a few dykes pierce the arch2 in a third1 where a greater depth has been bared away1 a network of dykes and sheets is revealed2 in a fourth1 the surface of the underlying laccolite is exposed2 in a fifth1 the laccolite1 long uncovered1 has been carved into picturesque contours by the weather1 and its original form is more or less destroyed4#ceh ,footnote #ceg3 ,see the remarks and diagram1 ante1 p4 #hf4 ,footnote #ceh3 ,geology of the ,henry ,mountains1 by ,mr4 ,g4 ,k4 ,gilbert1 ,u4,s4 ,geographical and ,geological ,survey of the ,rocky ,mountain ,region1 #ahgg4 ,the gabbro laccolites of the ,west of ,scotland belong to an older geological period than those of ,utah1 and have1 therefore1 been longer subject to the processes of denudation4 ,they have been enormously eroded4 ,the overlying cover of basalt has been stripped off from them1 though from the escarpments beyond them it is not difficult in imagination to restore it4 ,in ,rum it has been so completely removed1 that only a few fragments remain at some distance from the core of gabbro1 which now stands isolated4 ,in ,ardnamurchan1 and still more in ,skye1 the surrounding plateau of basalt remains in contact with the gabbro bosses4 ,but in ,mull1 where the plateau-basalts reach now1 and perhaps attained originally a greater thickness than anywhere else1 they have protected the intrusive sheets1 which are therefore less deeply cut away than in any of the other districts1 and no great central core of gabbro has yet been uncovered4 ,,chapter ,,xlv ,,the ,,acid ,,rocks ,their ,petrography--,their ,stratigraphical ,position and its ,analogies in ,central ,france ,we now come to the examination of another distinct phase of volcanic action during ,tertiary time in ,britain4 ,the igneous rocks that have been under consideration in the foregoing chapters1 whether poured out at the surface or injected below ground1 have been chiefly of basic1 partly indeed1 like the peridotites1 of ultra-basic character4 ,some1 however1 have shown an andesitic or intermediate composition4 ,reference has also been made to the probable eruption of acid rhyolites in the long interval between the outflow of the lower and the upper basalts in ,antrim4 ,but we now encounter a great series1 decidedly acid in composition1 in the more largely crystalline members of which the excess of silica is visible to the eye in the form of free quartz4 ,while there is a strong contrast in chemical composition between this series and the rocks hitherto under discussion1 there are also marked differences in structure and mode of occurrence4 ,like the gabbros1 all the masses of acid rock now visible appear to be intrusive4 ,they have been injected beneath the surface1 and therefore record for us subterranean rather than superficial manifestations of volcanic action4 ,the existence of rocks of this class in the midst of the basic masses has long been recognized4 ,they were noticed by ,jameson1 who described the hills between ,loch ,sligachan and ,broadford as composed of a compound of felspar and quartz1 or what may be called a granitel1 with occasional veins of pitchstone4#cei ,macculloch gave a fuller account of the same region1 and classed the rocks as chiefly syenite and porphyry4#cfj ,in ,antrim1 also1 even in the midst of the basalt-tableland1 masses of pitchstone-porphyry pearlstone-porphyry1 clay-porphyry1 and greystone were observed and described4#cfa ,in more recent years ,professor ,zirkel has given a brief account of the so-called syenite and porphyry of ,mull and ,skye1#cfb and the late ,professor ,von ,lasaulx fully described the trachyte or rhyolite of ,antrim4#cfc ,footnote #cei3 ,mineralogical ,travels1 ii4 #ij4 ,footnote #cfj3 ,western ,isles1 see the descriptions of ,skye1 ,mull and ,rum4 ,footnote #cfa3 ,berger1 ,trans4 ,geol ,soc4 iii4 7#ahaf71 p4 #aij2 ,portlock1 ,journ4 ,geol4 ,soc4 ,ireland1 vol4 i4 7#ahcd71 p4 #i4 ,footnote #cfb3 ,zeitsch4 ,deutsch4 ,geol4 ,gesellsch4 xxiii4 7#ahga71 pp4 #ed1 #gg1 #hd1 #hh4 ,footnote #cfc3 ,tschermak's ,min4 und ,petrog4 ,mittheilungen1 #ahgh1 p4 #dab4 ,the chemical composition of this rock and its place among the rhyolites had already been determined by ,e4 ,t4 ,hardman from analysis1 ,journ4 ,geol4 ,soc4 ,ireland1 vol4 iii4 7#ahga71 p4 #cb4 ,this interesting series of rocks embraces a greater variety of petrographical characters than any other portion of the ,british ,tertiary volcanic rocks4 ,on the one hand1 it presents thoroughly vitreous masses1 some of which in their colour1 lustre and microscopic structure remind us of recent obsidians4 ,on the other hand1 it affords coarsely crystalline compounds1 to which no other name than granite can be assigned1 and which1 did we not know their geological position1 might almost be classed with some of the most ancient eruptive rocks4 ,between these two extremes abundant gradations may be found1 including beautiful spherulitic rocks1 felsites and rhyolites4 ,in dealing with such a series of intrusive rocks1 we again encounter the difficulty of reaching certainty as to their relative dates of eruption1 since in each case all that can usually be affirmed is that the intrusive mass is younger than that into which it is injected4 ,it is quite possible that protrusions of acid rocks occurred at intervals during the accumulation of the basic masses1 as may perhaps be inferred from the rhyolite-tuffs and conglomerates of ,antrim and from the occurrence of fragments of siliceous lavas in the gravels near the base of the basalt-plateau of ,mull1 and in the agglomerates of that island as well as of other districts4#cfd ,it is probable1 therefore1 that at the time when the basalts of the plateaux were emitted1 there existed1 within reach of volcanic explosions1 masses of granophyric1 felsitic or rhyolitic rocks1 fragments from which were shot up the funnels of discharge4 ,that portions of these rocks were actually intruded into the basalt-sheets before the building up of the plateaux was completed appears to be proved in ,antrim4 ,elsewhere1 however1 no evidence has yet been obtained of any such intrusion until after the close of the plateau-period4 ,on the contrary1 in every case where the relative ages of the rocks can be fixed1 the acid are younger than the basic protrusions4 ,footnote #cfd3 ,reference may also again be made to the agglomerates of ,strath1 ,skye1 which contain in some parts abundant fragments of acid rocks that closely resemble some of the masses of granophyre which disrupt these agglomerates4 ,the only known exceptions to this rule are the latest basalt-dykes4 ,hence1 while amid the large and varied series of acid rocks1 which no doubt represents a wide interval of time1 some may belong to comparatively early epochs in the protracted volcanic period1 the actual available evidence places the emission of these rocks1 as a whole1 towards the end of the volcanic history4 ,this evidence ,i shall bring1 forward in full detail1 since it necessitates an abandonment of what has been the general belief in regard to the relative ages of the rocks4 i4 ,,petrography ,,of ,,the ,,acid ,,rocks ,the classification of the rocks which best harmonizes the field-evidence and the detailed study of their mineralogical composition1 is one that arranges these volcanic protrusions into two series4 ,in the one1 the orthoclase is sanidine1 and the rocks range from the most vitreous pitchstone through perlitic and spherulitic varieties to rhyolite 7quartz-trachyte74 ,in the other series1 which embraces by far the largest proportion of the whole1 the orthoclase is always turbid1 and in this respect as well as in many others the rocks remind us rather of ancient eruptive masses than of those which have appeared in ,tertiary time4 ,they range from flinty felsitic varieties1 which are obviously devitrified glasses1 through different textures of quartz-porphyry into granophyre1 and finally into granite4 ,as ,i have been unable to recognize any essential difference of structure and composition between these acid ,tertiary rocks and those of far earlier geological time1 ,i give them the names which no petrographer would hesitate to apply to them if they were of ,palozoic age4 ,it has long appeared to me that these rocks furnish conclusive evidence of the misleading artificiality of any petrographical nomenclature in which relative antiquity is made an essential element of discrimination4 ,granite4--,that true granites form part of the ,tertiary volcanic series of the ,british ,isles has now been completely established4 ,they occur as bosses and sills which have been intruded into the gabbros and all older rocks4 ,they are thus proved not only to belong to the ,tertiary period1 but to one of the latest phases of its volcanic history4 ,but besides these granites1 the relative age of which can be definitely fixed1 there occur others which1 standing alone and at some distance from the basaltic plateaux1 can only be inferentially classed in the ,tertiary series4 ,to this group belong the granite masses of the ,isle of ,arran and the ,mourne ,mountains in north-eastern ,ireland4 ,taking first the unquestionably ,tertiary granites which occur as bosses and intrusive sheets1 we have to note that the more coarsely crystalline granophyres are hardly to be distinguished externally from granite4 ,as the dark ferro-magnesian constituent of these rocks was generally believed to be hornblende1 they were called by the older petrographers syenite2 that is1 granite with hornblende instead of mica4 ,the peculiar micropegmatitic groundmass1 which constitutes the distinguishing feature of the granophyres1 may occasionally be observed so reduced in amount as only to appear here and there between the other minerals1 which are grouped in a granitic structure4 ,from this condition1 one step further carries us into a true granite1 from which all trace of the granophyric character has disappeared4 ,such gradations may be traced even within short distances in the same boss of rock4 ,thus1 in the hornblende-biotite-granite boss of ,beinn-an-,dubhaich1 ,skye1 a thoroughly granitic arrangement of the component minerals is observable in the centre1 while a specimen taken from near the edge on the shore of ,camas ,malag shows the development of a granophyric groundmass4 ,but1 though the large bosses are usually somewhat coarsely crystalline in the centre1 and tend to assume finer felsitic textures around their borders1 as was observed long ago by ,oeynhausen and ,von ,dechen1#cfe the granitic structure is sometimes exhibited even at the very edge1 and not only so1 but in the dykes that protrude from the bosses into the surrounding rocks4 ,thus the ,beinn-an-,dubhaich mass1 at its margin in ,camas ,malag1 sends a vein into the surrounding limestone1 but though more close-grained than the main body of the rock1 this vein is neither felsitic nor granophyric1 but truly granitic in structure4 ,footnote #cfe3 ,karsten's ,archiv1 i4 p4 #hi4 ,so far as ,i have observed1 the true granites contain a brown mica and also a little hornblende1 both visible to the naked eye1 but generally somewhat decomposed4 ,these rocks are thus hornblende-biotite-granites 7amphibole-granitites of ,rosenbusch74 ,they may be defined as medium-grained aggregates of quartz1 orthoclase 7also plagioclase71 biotite and hornblende1 with sometimes magnetite1 apatite1 epidote and zircon4 ,dr4 ,hatch found that in some instances 7,beinn-an-,dubhaich7 the quartz contains minute inclusions 7glass871 bearing immovable bubbles with strongly-marked contours2 while in others 7,beinn-na-,chro1 ,skye7 this mineral is full of liquid inclusions with bubbles1 sometimes vibratile1 sometimes fixed4 ,he remarked that the quartz and felspar have consolidated almost simultaneously1 but that in some instances 7,marsco1 ,glen ,sligachan7 there are isolated roughly idiomorphic crystals1 of a white1 less turbid orthoclase1 which belong to a slightly earlier consolidation than that of the more kaolinized felspar of the rest of the rock4 ,the granite of the island of ,arran1 in the ,birth of ,clyde1 which is here included in the ,tertiary volcanic series1 has long been recognized as consisting of two distinct portions1 an eastern or coarse-grained1 and a western or fine-grained variety4 ,the latter sends veins into the former4 ,these granites contain orthoclase1 plagioclase1 quartz and dark mica1 the quartz being often idiomorphic with respect to the felspar1 and a tendency towards a micropegmatitic structure being sometimes observable4 ,a distinguishing characteristic of the ,arran granite is the cavernous or drusy structure which it presents1 the cavities being often lined with well-crystallized orthoclase and smoky quartz4#cff ,the granite of the ,mourne ,mountains in ,ireland closely resembles that of ,arran4 ,its druses1 with their beautifully terminated minerals1 have long been well known4 ,footnote #cff3 ,see ,mr4 ,teall's ,british ,petrography1 p4 #cbh4 ,microgranite4--,this term is applied to certain intrusive masses1 which megascopically may be classed with the quartz-porphyries and felsites1 but which microscopically are found to possess a holocrystalline granitic groundmass of quartz and orthoclase1 through which are scattered porphyritic crystals of the same two minerals1 sometimes also with plagioclase1 augite1 magnetite or apatite4 ,rocks of this type do not appear to be abundant4 ,they occur as dykes and bosses1 but occasionally also as sheets4 ,i have collected them from ,skye1 ,rum and ,ardnamurchan4 ,granophyre4--,under this name may be grouped the large majority of the acid rocks which play an important part in the geology of the ,west of ,scotland4 ,they are typically developed in the islands of ,mull and ,skye4 ,generally pale grey or buff in colour1 they range in texture from the true granites1 into which1 as above stated1 they graduate1 to exceedingly close-grained varieties like the felsites of ,palozoic formations4 ,in the great majority of them the micrographic intergrowth of quartz and felspar1 known as micropegmatite1 is their conspicuous structure1 and even constitutes most of their substance4 ,they may thus be classed generally as granophyres1 in the sense in which this term is employed by ,rosenbusch1 but without his limitation of it to pre-,tertiary rocks4 ,the specific gravity of these rocks has been determined from a series of specimens by ,mr4 ,a4 ,harker to range from about #bc among the felsites to #bg among the granites4 ,no chemical analyses of these rocks have yet been made1 but they have been subjected to microscopical examination1 and their general structure and composition are now known4 ,the typical granophyre of the ,inner ,hebrides outwardly closely resembles an ordinary granite of medium grain1 in which the component dull felspar and clear quartz can be readily distinguished by the naked eye4 ,throughout all the varieties of texture there is a strong tendency to the development of minute irregularly-shaped drusy 7miarolitic7 cavities1 which here and there give a carious aspect to the rock4 ,that these cavities1 however1 are part of the original structure of the rock1 and are not due to mere weathering1 is shown by the well-terminated crystals of quartz and felspar which project into them4 ,on a small scale1 it is the same structure so characteristic of the granite of the ,mourne ,mountains and of parts of that of ,arran4 ,examined under the microscope1 a normal specimen of the granophyre of the ,western ,isles presents a holocrystalline groundmass1 which fills all the interspaces between the crystals of earlier consolidation4 ,this groundmass consists of an aggregate of clear quartz and turbid orthoclase1 arranged as micropegmatite1 but also in more or less idiomorphic crystals4 ,in some parts1 the two dominant minerals are grouped in alternate parallel fibres1 diverging from the surface of the enclosed crystals1 which are thus more or less completely surrounded by a radially fibrous mass4 ,the felspathic portion of the micropegmatite which usually surrounds the orthoclase crystals1 when viewed between crossed ,nicols1 is found to extinguish simultaneously with the central crystal4#cfg ,in other parts1 the felspar forms a kind of network1 the meshes of which are filled up with quartz4 ,through the groundmass1 besides the clear quartz and dull orthoclase1 some ferro-magnesian or other additional constituent is generally distributed1 but usually somewhat decomposed4 ,in certain varieties ,dr4 ,hatch found an abundant brown mica1 as in the rock at ,camas ,malag1 ,skye4 ,in others1 a pyroxene occurs1 which he observed in minute greenish grains1 sometimes completely enclosed in the quartz4 ,in a third variety1 the dark constituent is hornblende1 the most remarkable example of which is one to be seen at ,ishriff1 in the ,glen ,more of ,mull1 where the ferro-magnesian mineral takes the form of long dirty-green needles1 conspicuous on a weathered surface of the rock4 ,a fourth variety is distinguished by containing plagioclase in addition to or instead of orthoclase4 ,in the rock of the sheet forming ,cnoc ,carnach1 near ,heast1 in ,skye1 ,dr4 ,hatch observed both orthoclase and plagioclase scattered through a fine micropegmatitic groundmass1 and in a part of the boss at ,ishriff he found the rock to be composed mainly of plagioclase1 in a micropegmatitic groundmass of quartz and felspar1 with a few scattered grains of a pale brown augite and grains of magnetite4 ,a fifth variety is marked by the prominence of the crystals of quartz and felspar of earlier consolidation1 and by the fineness of grain in the surrounding micropegmatitic groundmass1 whereby a distinct porphyritic structure is developed4 ,rocks of this kind are megascopically like ordinary quartz-porphyries4 ,still another variety has been detected by ,mr4 ,teall in the rock of ,meall ,dearg1 at the head of ,glen ,sligachan1 ,skye1 in which1 besides irregular patches which may represent decayed biotite1 and others which are possibly ilmenite1 the rare mineral riebeckite is present4#cfh ,footnote #cfg3 ,mr4 ,teall1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 #a4 7#ahid7 p4 #bai4 ,see also his ,british ,petrography1 p4 #cbg4 ,footnote #cfh3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 #a4 7#ahid71 p4 #bai4 ,felsite4--,the close-grained rocks into which the ordinary granophyres frequently graduate may be conveniently grouped under the general name of ,felsite4 ,they differ in no essential feature from the felsites of the ,palozoic formations4 ,they are more particularly developed1 as might be expected1 in those places where the conditions have been most favourable for rapid cooling1 while the more coarsely crystalline granophyres occur where the material may be supposed to have consolidated most slowly4 ,where the acid magma has been injected into chinks and fissures so as to take the form of veins or dykes1 it is sometimes felsitic1 sometimes granophyric1 in texture4 ,along the margin of large bosses1 like those of ,mull and ,skye1 it frequently though not invariably has assumed a fine texture1 with even spherulitic and flow-structures4 ,but in the centre of large bosses it usually appears as coarse granophyre or as granite4 ,the felsites vary in texture from flinty or horny to dull finely-granular1 and in colour from white through shades of grey1 buff and lilac1 to black1 generally with porphyritic felspars and blebs of quartz4 ,where these porphyritic enclosures increase in size and number1 the rocks cannot be distinguished externally from ancient quartz-porphyries4 ,in general the groundmass of these rocks has been completely devitrified4 ,but in some dykes enough of the glassy base remains to show their original vitreous condition4 ,a gradation can thus be traced from thoroughly glassy pitchstone into completely lithoid felsites and crystalline granophyres4 ,a characteristic feature of the felsitic varieties of acid rock is their flow-structure1 which they often display in great perfection4 ,sometimes1 indeed1 this structure has been so strongly developed as to cause the rock to weather along the planes of flow and to break up into thin slabs4 ,many of these rocks also present admirably developed spherulitic structures1 varying from microscopic minuteness up to large round or egg-shaped balls nearly two inches in diameter1 and often distributed in lines along those of flow-structure4 ,they likewise exhibit a frequent development of micropegmatite4 ,no line indeed can be drawn between these felsites and the granitoid varieties1 for the same characteristic granophyric intergrowth of felspar and quartz runs through them all4 ,pitchstone4--,this name is applied to the glassy varieties apart from their chemical composition1 and specially denotes the possession of a vitreous structure4 ,some of the rocks to which it has been applied are probably glassy varieties of andesite1 others are dacites1 while some may be as acid as the most acid felsites and granophyres4 ,the pitchstones are found in veins or dykes which traverse different geological formations up to and including the great granophyre bosses of the ,inner ,hebrides4 ,they vary in colour from a deep jet-black or raven-black to a pale bottle-green1 and in lustre from an almost glassy obsidian-like to a dull resinous aspect4 ,occasionally they assume a felsitic texture1 owing to devitrification1 and also a finely spherulitic structure4 ,some varieties appear to the naked eye to be perfectly homogeneous1 others become porphyritic by the appearance of abundant sanidine crystals4 ,the microscopic structure of the ,british pitchstones has not yet been fully worked out4 ,the beautiful feathery microlites of the ,arran dykes1 first made known by ,david ,forbes1 and subsequently described by ,zirkel1 ,allport and others1 are well known objects to geological collectors4 ,dr4 ,hatch1 in whose hands ,i placed my tolerably large collection of specimens and their thin slides1 furnished me with some preliminary notes on the slides1 from which the following generalized summary is compiled4 ,at the one end of the pitchstone group we have a nearly pure glass1 with no microlites1 and only a few scattered crystals of sanidine1 quartz1 augite or magetite4 ,the glass in thin slices is almost colourless1 but generally inclines to yellow1 sometimes to dark-grey4 ,some varieties of the rock are crowded with microlites1 in others these bodies are gathered into groups1 the glass between which is nearly free from them4 ,among the minerals that have been observed in this microlitic form are sanidine1 augite1 hornblende 7forming the beautiful green feathery or fern-like aggregates in the ,arran pitchstones1 ,fig4 #c7 and magnetite4 ,sometimes the rudimentary forms appear as globulites1 or as belonites1 but more commonly as dark trichites4 ,among the more definite mineral forms are grains of sanidine1 quartz and augite4 ,the porphyritic crystals are chiefly sanidine1 augite and magnetite1 but plagioclase occasionally occurs4 ,the development of spherulites is well seen in a few of the slides1 and occasionally perlitic structure makes its appearance4 ,the interesting rhyolitic areas of ,antrim include several varieties of pitchstone4 ,one of these is described by ,professor ,cole as a glassy pyroxene-rhyolite1 on the verge of the rhyolitic andesites4 ,another is a blue-black porphyritic obsidian4#cfi ,footnote #cfi3 ,scientif4 ,trans4 ,roy4 ,dublin ,soc4 vol4 vi4 7ser4 ii47 #ahif1 p4 #gg4 ,rhyolite 7,quartz-,trachyte74--,this rock has been abundantly erupted in north-east ,ireland1 where it rises in occasional bosses among the plateau-basalts4#cgj ,it is best exposed at the ,tardree and ,carnearny ,hills1 where it has long been quarried4 ,its petrographical characters at that locality were described by ,von ,lasaulx as those of a typical quartz-trachyte rich in tridymite1 and containing large crystals of glassy sanidine1 isolated narrow laths of plagioclase 7probably andesine71 grains of smoky-grey quartz1 partly bounded by dihexahedral faces1 and a few scattered flakes of a dark-coloured mica4 ,the groundmass is microgranitic1 and under a high power is resolvable into a confused aggregate of minute microlites of felspar1 with interstitial quartz-granules4#cga ,more recently a detailed investigation of the petrography of the ,antrim rhyolites has been conducted by ,professor ,cole1 who has called attention to their remarkable varieties of structure1 ranging from perfect volcanic glass to a thoroughly lithoidal texture1 and exhibiting flow1 perlitic and spherulitic structures4#cgb ,footnote #cgj3 ,fragments of acid rock were detected by ,prof4 ,cole in the gravel among the ,ardtun basalt of ,mull1 as already noticed on p4 #bab4 ,footnote #cga3 ,tschermak's ,min4 und ,pet4 ,mittheil4 #ahgh1 p4 #dab4 ,footnote #cgb3 ,scientif4 ,trans4 ,roy4 ,dublin ,soc4 vol4 vi4 7ser4 ii47 #ahif1 p4 #gg4 ,this paper gives an excellent account of the microscopical character and mineralogical and chemical compositions of these rocks4 ,intrusive masses of rhyolite are also found in the ,carlingford region4 ,one of these1 seen at ,forkhill1 is a velvet-black almost resinous rock with abundant quartz and felspar1 and sometimes displaying beautiful flow-structure4 ,it will be more particularly described in ,chapter xlvii4 ,some of the acid dykes and sills of the ,inner ,hebrides are varieties of rhyolite4 ,no undoubted example has yet been observed of a superficial rhyolite-lava1 though such not improbably appeared in the interval between the lower and upper basalts of ,antrim4 ii4 ,,stratigraphical ,,position4--,,analogies ,,from ,,central ,,france ,in the history of opinion regarding the relative position of the ,tertiary eruptive rocks1 no feature is so remarkable as the universal acceptance of the misconception regarding the place of the acid protrusions4 ,in tracing this mistake to its source1 we find that it probably arose from the fact that along their line of junction the granitoid masses generally underlie the basic4 ,this order of superposition1 which would usually suffice to fix the age of two groups of stratified rocks1 is obviously not of itself enough to settle the relative epochs of two groups of intrusive rocks4 ,yet it has been assumed as adequate for this purpose1 and hence what can be proved to be really the youngest has been placed as the oldest part of the ,tertiary volcanic series4 ,macculloch1 who showed that his syenites and porphyries had invaded the ,secondary strata of the ,inner ,hebrides1 and must therefore be of younger date than these1 left their relations to the other igneous rocks of the region in a curiously indefinite position4 ,he was disposed to regard them all as merely parts of one great series2 and seems to have thought that they graduate into each other1 and that any attempt to discriminate between them as to relative age is superfluous4 ,yet he evidently felt that the contrasts of topography which he described could hardly fail to raise the question of whether rocks so distinct in outward form did not differ also in relative antiquity4 ,but he dismissed the question without answering it1 remarking that if there is any difference of age between the two kinds of rock1 there appears no great prospect of discovering it4#cgc ,he records an instance of a vein of syenite traversing the hypersthene rock in the valley of ,coruisk4 ,if this vein1 he says1 could be traced to the mass of syenite1 it might be held a sufficient ground of judgment1 but under the present circumstances it is incapable of affording any assistance in solving the difficulty4#cgd ,instead1 however1 of being a solitary instance1 it is only one of hundreds of similar intrusions which can be connected with the general body of granitic and granophyric masses1 and which put the relative ages of the several groups of rock beyond any further doubt4 ,footnote #cgc3 ,western ,islands1 i4 p4 #cfh2 see also pp4 #dhh1 #ege1 #egh4 ,footnote #cgd3 ,op4 cit4 p4 #cgj4 ,bou1 who knew the geology of some of the extinct volcanic regions of ,europe1 recognized the similarity of the ,scottish masses to those of the ,continent1 and classed the acid rocks as trachytes4 ,he saw in each of the volcanic areas of the ,west of ,scotland a trachytic centre1 and supposed that the more granitoid parts might represent the centres in the ,european trachytic masses4 ,he traced in imagination the flow of the lava-streams from these foci of volcanic activity1 distinguishing them as products of different epochs of eruption1 among the last of which he thought that the trachytic porphyries might have been discharged4 ,he admitted1 however1 that his restoration could not be based on the few available data without recourse to theoretical notions drawn from the analogy of other regions4#cge ,footnote #cge3 ,essai ,gologique sur l'cosse1 pp4 #bia1 #cbb1 #cbg4 ,in the careful exploration of the central region of ,skye made by ,von ,oeynhausen and ,von ,dechen1 these able observers traced the boundary between the syenite and the hypersthene rock2 and as they found the former lying underneath the latter1 they seem naturally to have considered it to be the older protrusion of the two4#cgf ,principal ,forbes came to a similar conclusion from the fact that he found the dark gabbro always overlying the light-coloured felspathic masses4#cgg ,professor ,zirkel also observed the same relative position1 and adopted the same inference as to the relative age of the rocks4#cgh ,professor ,judd followed these writers in placing the acid rocks before the basic4 ,he supposed the granitoid masses to form the cores of volcanic piles probably of ,eocene age1 through and over which the protrusions of gabbro and the eruptions of the plateau-basalts took place4#cgi ,footnote #cgf3 ,karsten's ,archiv1 i4 p4 #hb4 ,it will be shown in later pages that the apparent infraposition of the granophyre is often deceptive1 the real junction being vertical4 ,footnote #cgg3 ,edin4 ,new ,phil4 ,jour4 xl4 7#ahdf7 p4 #hd4 ,footnote #cgh3 ,zeitsch4 ,deutsch4 ,geol4 ,gesellsch4 xxiii4 7#ahga7 pp4 #ij1 #ie4 ,he says that the gabbro seems to be the younger rock1 so far as their relations to each other can be seen4 ,footnote #cgi3 ,quart4 ,jour4 ,geol4 ,soc4 xxx4 7#ahgd7 p4 #bee4 ,the evidence for the posteriority of the acid rocks will be fully detailed in later pages4 ,before entering upon its consideration1 however1 ,i would remark that the uprise of the ,british granophyres presents so many points of resemblance to that of the trachytes and phonolites among the basalt-plateaux of ,auvergne and the ,velay in ,central ,france1 that a brief account of the acid protrusions of these regions may be suitably given here as an introduction to the account of those of the ,inner ,hebrides4 ,a succession of stages in the progress of denudation allows us to follow the gradual isolation and dissection of the ,french volcanic groups4 ,the youngest examples occur in the chain of cones and craters1 in the region of the ,puy de ,dme4 ,these may be of ,pleistocene1 or even of more recent date4 ,older and more deeply eroded than these are the numerous domes and cones in the territory of ,haute ,loire4 ,yet more ancient and still more stupendously denuded come the bosses1 sills and dykes of ,britain4 ,nevertheless1 the geologist1 by the methods so admirably devised by ,desmarest1 may follow the chain of relationship through these different regions and trace a remarkable continuity of structure4 ,the younger rocks serve to illustrate the original condition of the more ancient1 while the latter1 by their extensive denudation1 permit points of structure to be seen which in the former are still concealed4 ,no feature in the interesting volcanic district of ,auvergne has attracted more attention than the trachytic protrusions4#chj ,rising conspicuously along the chain of puys1 they claim notice even from a distance owing to the topographical contrast which their pale rounded domes offer to the truncated1 crater-bearing cones of dark cinders around them4 ,they consist of masses of a pale variety of trachyte 7domite71 which in ground-plan present a circular or somewhat elliptical outline4 ,they vary in size from the nearly circular dome of the ,grand ,sarcoui1 which measures about #djj yards in diameter1 to the largest mass of all--that of the ,puy de ,dme1 which extends for some #aejj yards from north to south with a breadth varying from #ejj to #hjj yards4 ,they are likewise prominent from their height2 in the ,puy de ,dme they form the highest elevation of the whole region 7#adfe metres71 and even in the less conspicuous hills they rise from #ejj to #fjj feet above the surrounding plateau4 ,footnote #chj3 ,the admirable ,map and ,memoirs of ,desmarest on ,auvergne are classics in geology4 ,scrope's work1 vol4 i4 p4 #de1 gives still the best published account of this district4 ,see also the work of ,lecoq 7ibid474 ,the results of more detailed petrographical research regarding the rocks will be found in the essays of ,m4 ,michel ,lvy 7,bull4 ,soc4 ,gol4 ,france1 #ahij1 p4 #fhh7 and in the ,clermont sheet of the ,geological ,survey ,map of ,france 7,feuille1 #aff74 ,a bibliography of the district up to the year #ahij is given in the volume of the ,bull4 ,soc4 ,gol4 ,france just cited1 p4 #fgd4 ,five such dome-shaped protrusions of trachyte have made their appearance among the cinder-cones in a space of about five ,english miles in length by about two miles in extreme breadth4 ,though opinions have varied as to the mode of formation of these domes1 there has been a general agreement that their present topographic contours cannot be far from the original outlines assumed by the masses at the time of their production4 ,the position of the trachyte bosses among the puys serves to show that they were not deep-seated masses which have been entirely uncovered by denudation1 but were essentially superficial1 and were protruded to the surface at various points along the plateau in the midst of already existing cinder-cones4 ,in some cases1 they have risen on or near the position of the vents of these cones4 ,thus the ,puy de ,chopine is half encircled by the crater of the ,puy de la ,goutte1 and the ,grand ,sarcoui stands in a similar relation to the fragmentary crater-wall of the ,petit ,sarcoui4 ,m4 ,michel ,lvy1 in pointing out the superficial character of the domitic protrusions1 has forcibly dwelt on the evidence that these rocks have undergone a comparatively trifling denudation1 and that they could never have extended much beyond their present limits4#cha ,as ,scrope pointed out1 they were obviously protruded in a pasty condition1 not flowing out in streams like the other lavas of the district1 but consolidating within their chimneys and rising from these in rounded domes4 ,footnote #cha3 ,op4 cit4 p4 #gaa4 ,illustration3 ,fig4 #cdd4--,section through the ,puy de la ,goutte and ,puy de ,chopine4 #a1 ,mica-schist2 #b #b1 ,granite2 #c #c1 ,tuffs2 #d1 ,trachyte2 #e1 ,basalt dyke4 ,undoubtedly denudation1 cannot have left them altogether unaffected1 but must have removed some amount of material from their surface4 ,there is reason to believe that the material so removed may have been in large part of a fragmental character1 and that it was under a covering of loose pyroclastic debris that the upward termination of the trachyte column assumed its typical dome-form4 ,thus in the crater-wall of the ,puy de la ,goutte1 layers of buff-coloured trachytic tuff dip gently away from the central domite mass of the ,puy de ,chopine4 ,that this material was thrown out from the vent previous to the uprise of the domite may be inferred from the way in which the latter rock has obliterated the northern half of the crater4 ,the relations of the rocks are somewhat obscured by talus and herbage1 but when ,i last visited the locality in the spring of #ahie the structure seemed to me to be as expressed in the accompanying diagram 7,fig4 #cdd74chb ,footnote #chb3 ,compare ,m4 ,michel ,lvy1 ibid4 ,the relative date of the protrusion of the trachytic domes cannot be very precisely defined4 ,there can1 indeed1 be no doubt that it belongs to a late phase of the volcanic history4 ,it came long after the outpouring of the older basaltic plateaux1 of which large fragments emerge from beyond the limits of the younger lavas on both sides of the great ridge of the puys1 and not only long after that outpouring1 but even after the widespread sheets of basalt had been deeply trenched by valleys and isolated into outliers capping the hill-tops4 ,yet there is good evidence also that the uprise of the comparatively acid trachytes was not the last volcanic episode of the district4 ,the abundance of dark slags and fragments of basalt lying on the domite hills shows that discharges of more basic detritus occurred after these hills had taken their place in the landscape4 ,since the latest eruptions1 a gradual alteration of the topographical features by denudation has been slowly but continuously going on4 ,the ,grand ,sarcoui1 possibly from having originally had a considerable covering of fragmentary material1 shows least the effects of this waste4 ,its remarkably regular form1 like that of an inverted cauldron 7the ,chaudron1 as it is called in the district71 presents1 in a distant view1 a smooth grassy surface which slopes steeply down into the great volcanic plain4 ,but on a nearer examination these declivities are found to be seamed with trenches which the rain-storms of centuries have dug out4 ,the covering of loose debris has been largely washed away1 though many fragments of dark slag are still strewn over the slopes1 and the scars are now being cut into the domite below4 ,a more advanced stage of decay may be seen on the ,puy de ,dme1 where1 from greater elevation and exposure1 the domite is already deeply gashed by gullies and ravines1 while the slopes below are strewn with its detritus4 ,the region of the ,velay displays on a far more extensive scale the protrusion of trachytic and phonolitic bosses1 but as its volcanic history goes back beyond the time of the ,puys of ,auvergne1 its volcanic monuments have consequently been more extensively affected by denudation4#chc ,a series of basaltic eruptions forming extensive sheets can there be traced1 the oldest dating from ,miocene time1 the youngest coming down to the age of the mammoth1 cave-bear and early man4 ,during this prolonged outpouring of basic lavas there were several intervals during which materials of a more acid nature--trachytes and phonolites--were erupted4 ,these rocks occur partly as extensive tracts1 covering five or six square miles1 like those of the ,mezenc1 the ,megal1 the ,pic de ,lizieux1 and the ,rand1 and partly in isolated conical or dome-shaped prominences1 sometimes only a few hundred feet in diameter4 ,upwards of one hundred distinct eruptions of phonolite have been observed in the ,velay4 ,even in the tracts where they cover the largest space1 several prominent eminences may usually be observed1 not unlike in general shape the isolated cones and domes of ,auvergne4 ,in these wider areas there appears to be evidence of the outcome of the lava from one or more vents1 either as superficial streams or as underground intrusive sheets4 ,m4 ,boule has expressed his opinion that most of the masses of trachyte and phonolite have been the result of local and limited eruptions1 the pasty rock having risen in and accumulated around its pipe1 without flowing far in any direction4 ,a section across one of these masses would present a somewhat mushroom-shaped form4#chd ,footnote #chc3 ,in addition to the work of ,scrope1 the student of this important volcanic district will find an invaluable guide in the ,le ,puy ,sheet 7,no4 #ahf7 of the ,geological ,survey ,map of ,france1 and in the ,bulletins of the ,survey1 particularly those by ,,mm4 ,termier and ,boule1 ,no4 #ac 7#ahij7 and ,no4 #bh 7#ahib74 ,footnote #chd3 ,bull4 ,carte4 ,gol4 ,france1 ,no4 #bh 7tome iv47 p4 #abe4 ,that fragmentary ejections accompanied the protrusion of these rocks1 though probably on a very limited scale1 is shown by the occasional survival of portions of trachyte tuff around them4 ,one of the most notable of these deposits occurs in the hollow between the ,suc du ,pertuis and the next dome to the south4 ,it consists of fine and coarse1 trachytic detritus1 which in one place is rudely bedded and appears to dip away from the phonolite dome behind it at an angle of #cj4 ,this material and its inclination are what might be expected to occur round an eruptive vent1 and may be compared with those of the crater-wall of the ,puy de la ,goutte in relation to the domite boss of the ,puy de ,chopine4 ,the denudation of ,velay has undoubtedly advanced considerably further than that of the ,puys of ,auvergne4 ,the pyroclastic material which may have originally covered the domes of trachyte and phonolite has been in great part swept away4 ,the surrounding rocks1 too1 both aqueous and igneous1 have been extensively removed from around the necks of more enduring material4 ,hence the trachyte and phonolite bosses stand out with so striking a prominence as to arrest the eye even for a distance of many miles4 ,illustration3 ,fig4 #cde4--,view of the ,huche ,pointue and ,huche ,platte west of ,le ,pertuis4 ,the cone is one of the trachytic domes1 while the flat plateau to the left is a denuded outlier of the basalt sheets4 ,there cannot be any doubt that these necks have pierced the older basalts1 and therefore belong to a later epoch in the volcanic history4 ,the approximately horizontal sheets of basalt have been deeply eroded and reduced to mere fragments1 and in some instances their existing portions owe their survival to the protection afforded to them by the immense protrusions of more acid material4 ,but there is here1 as well as in ,auvergne1 evidence of the uprise of a later more basic magma1 for sheets of basalt are found overlying some parts of the trachytes and phonolites4 ,while the external forms of these ,velay necks recall with singular vividness the features of many more ancient necks in ,britain1 an examination of the internal structure of some of them affords some further interesting points of resemblance4 ,the slabs into which1 by means of weathering along the joints1 the rock is apt to split up are sometimes arranged with a general dip outwards from the centre of the hill1 so that their flat surfaces roughly coincide with the hillslopes4 ,in other cases the peculiar platy structure1 so characteristic of phonolite1 is disposed vertically or dips at a steep angle into the hill1 so that the edges of the slabs are presented to the declivities1 which consequently become more abrupt and rugged4 ,though none of the volcanic series in ,auvergne or the ,velay is so acid in composition as the more acid members of the ,tertiary volcanic series of ,britain1 the manner in which the trachytes and phonolites of the ,french region make their appearance presents some suggestive analogies to that of the corresponding rocks in this country4 ,we see that they were erupted long after the outpouring of extensive basaltic plateaux1 that they belonged to successive epochs of volcanic activity1 that they were protruded in a pasty condition to the surface1 where1 more or less covered with fragmentary ejections1 they terminated in dome-shaped hills or spread out to a limited distance around the vents1 and lastly1 that they were succeeded by a still later series of more basic eruptions1 which completed the long volcanic history4 ,we shall see in the following pages how closely the various stages in this complex record of volcanic activity may be paralleled in the geological records of ,tertiary time in ,britain4#che ,footnote #che3 ,the phonolite necks of ,bohemia1 which form so prominent a feature in the ,tertiary geology of that country1 might likewise be cited here in illustration of the acid domes and bosses of the ,british ,isles4 ,,chapter ,,xlvi ,,types ,,of ,,structure ,,in ,,the ,,acid ,,rocks--,,bosses ,returning now to the consideration of the acid rocks as these manifest themselves in the volcanic areas of ,britain1 ,i would remark that three distinct types of structure may be noted among them1 viz4 7#a7 bosses1 7#b7 sills or intrusive sheets1 7#c7 veins and dykes4 ,these types1 as above remarked1 belong entirely to the underground operations of volcanism1 for though the rhyolitic fragments in the tuffs and agglomerates of the plateaux prove that acid lavas existed near the surface1 no undoubted case of superficial lava belonging to the acid series has yet been observed4#chf ,footnote #chf3 ,the rhyolites of ,tardree in ,antrim have recently been claimed by ,professor ,cole as true lavas grouped round an eruptive vent4 ,for reasons to be given in the next chapter ,i regard them as intrusive masses1 though they may not improbably have been connected with streams of lava now entirely removed4 ,the bosses of acid material in the ,british ,tertiary volcanic series are irregular protrusions1 varying in size from knobs only a few square yards in area up to huge masses many square miles in extent1 and comprising groups of lofty hills4 ,as a rule1 their outlines are markedly irregular4 ,beneath the surface they plunge down almost vertically through the rocks which they traverse1 but in not a few instances their boundaries are inclined to the horizon1 so that the contiguous rocks seem to rest against them1 and sometimes lie in outliers on their sides and summits4 ,from the margins of these bosses apophyses are given off into the surrounding rocks1 sometimes only rarely and at wide intervals1 in other places in prodigious numbers4 ,sometimes the acid material has been injected in thousands of veins and minute threads1 which completely enclose fragments of the surrounding rock4 ,the rock of which the bosses consist is generally granophyric in texture1 passing on the one hand1 particularly in the central parts1 into granite1 and on the other1 and especially towards the margin1 into various more compact felsitic varieties1 and sometimes exhibiting along the outer edge more or less developed spherulitic and flow-structures4 ,decided contact metamorphism is traceable round the bosses1 but is by no means uniform even in the same rock1 some parts being highly altered1 while others1 exposed apparently to the same influences1 have undergone little change4 ,the most marked examples of this metamorphism are those in which the ,cambrian limestone of ,skye has been converted into a pure white saccharoid marble4 ,but the most interesting to the student of volcanic action are those where the altered rocks are older parts of the volcanic series4 ,as the bosses of each volcanic area offer distinctive peculiarities they will here be described geographically4 i4 ,,the ,,acid ,,bosses ,,of ,,skye ,it is in the island of ,skye that the granophyre and granite bosses attain their largest dimensions and afford1 on the whole1 the most complete evidence of their structures and their relations to the other parts of the volcanic series 7,map ,,vi474 ,they cover there a total area of about #be square miles1 and form characteristic groups of hills from #bjjj to #bejj feet in height4 ,on the south-east side1 three conspicuous cones 7the ,red ,hills7 rise from the valley of ,strath 7,beinn ,dearg ,mhor1 ,beinn ,dearg ,bheag and ,beinn na ,caillich74 ,a solitary graceful pointed cone 7,beinn na ,cro7 stands between ,strathmore and ,strathbeg1 while to the north-west a continuous chain of connected cones runs from ,loch ,sligachan up into the heart of the ,cuillin ,hills4 ,their conical outlines1 their smooth declivities1 marked with long diverging lines of screes1 and their pale reddish or reddish-yellow hue1 that deepens after a shower into glowing orange1 mark off these hills from all the surrounding eminences1 and form in especial a singular contrast to the black1 spiry1 and rugged contours of the gabbro heights to the west of them4 ,besides this large continuous mass1 a number of minor bosses are scattered over the district4 ,of these the largest forms the ridge of ,beinn an ,dubhaich1 south of ,loch ,kilchrist4 ,several minor protrusions lie between that ridge and the flank of ,beinn ,dearg4 ,others protrude through the moory ground above ,corry2 several occur on the side of the ,sound of ,scalpa1 about ,strollamus2 and one1 already referred to1 lies at the eastern base of ,blath ,bheinn4 ,in the neighbouring island of ,raasay1 a large area of granophyre likewise occurs1 which will be described with the ,sills in later pages4 ,in so extensive a district there is room for considerable diversity of composition and texture among the rocks4 ,as already stated1 in some places1 more particularly in the central parts of the hills1 the acid material assumes the character of a granite1 being made up of a holocrystalline aggregate of quartz1 orthoclase1 plagioclase1 hornblende and biotite1 without granophyric structure1 and thus becomes a hornblende-biotite-granite 7quartz-syenite1 granite-syenite of ,zirkel1 or amphibole-granitite of ,rosenbusch74 ,by the development of the micropegmatitic structure and radiated spherical concretions1 it passes into granophyre4 ,by the appearance of a felsitic groundmass1 it shades off into different varieties of quartz-porphyry or rhyolite1 sometimes with distinct bi-pyramidal crystals of quartz4#chg ,this change1 which here and there is observable along the edge of a boss1 is sometimes accompanied with an ample development of spherulitic and flow-structures4 ,as it is convenient to adopt some general term to express the whole series of varieties1 ,i have used the word granophyre for this purpose4 ,footnote #chg3 ,the best account yet published of these varieties in ,skye is that by ,prof4 ,zirkel1 ,zeitsch4 ,deutsch4 ,geol4 ,gesellsch4 xxiii4 7#ahga7 p4 #hh4 ,illustration3 ,fig4 #cdf4--,view of ,glamich1 #becg feet1 ,glen ,sligachan4 7,from a photograph by ,r4 ,j4 ,a4 ,berry1 ,m4,d41 lent by the ,scottish ,mountaineering ,club74 ,that the large area of these rocks in ,skye was the result of many separate protrusions from distinct centres of emission may be inferred1 ,i think1 not only from the varieties of petrographical character in the material1 but also from the peculiar topography of the ground1 and perhaps from the curious relation which seems1 in some instances at least1 to be traceable between the external features and apparent internal structure of the hills4 ,it will be seen from the ,map 7,no4 ,,vi47 that in the area lying to the east of ,strath ,more the granophyre is broken up into nearly detached portions by intervening patches of older rocks4 ,there can be little doubt that the mass of ,beinn na ,caillich and the two ,beinn ,deargs is the product of a distinct orifice1 if not of more than one4 ,beinn na ,cro1 lying between its two deep bounding glens1 is another protrusion4 ,the western cones stand so closely together that their screes meet at the bottoms of the intervening valleys4 ,yet each group is not improbably the result of emission from an independent funnel1 like the separate domite puys of ,auvergne4 ,but1 though ,i believe this large area of granitoid rock to have proceeded not from one but from many orifices1 ,i have only here and there obtained1 from the individual hills themselves1 indications of an internal structure suggestive of distinct and successive protrusions of material from the same vent of discharge4 ,on the outer declivities of some of the cones we may detect a rudely bedded structure1 which will be subsequently referred to as well displayed in ,rum 7p4 #djc74 ,this structure is specially observable along the east side of ,glen ,sligachan4 ,down the northern slopes of ,marsco the granophyre 7here in part a hornblende-biotite-granite7 is disposed in massive sheets or beds that plunge outwards from the centre of the hill at angles of #cj to #dj4 ,on the southern front of the same graceful cone1 as well as on the flanks of its neighbour1 ,ruadh ,stac1 still plainer indications of a definite arrangement of the mass of the rock in irregular lenticular beds may be noticed4 ,these beds1 folding over the axis of the hill1 dip steeply down as concentric coats of rock4 ,the external resemblance of the red conical mountains of ,skye to the trachyte puys of ,auvergne was long ago remarked by ,j4 ,d4 ,forbes1#chh and in this internal arrangement of their materials1 indefinite though it may be1 there is a further resemblance to the onion-like coatings which ,von ,buch and ,scrope remarked in the structure of the interior of the ,grand ,sarcoui4#chi ,footnote #chh3 ,edin4 ,new ,phil4 ,jour4 xl4 p4 #gh4 ,footnote #chi3 ,von ,buch1 ,geognostische ,beobachtungen auf ,reisen durch ,deutschland und ,italien1 vol4 ii4 7#ahji7 p4 #bde2 ,scrope1 ,geology and ,extinct ,volcanoes of ,central ,france1 #bnd edit4 p4 #fh4 ,von ,buch regarded the external form of this ,puy as having been determined by its internal structure4 ,where the contour of the cones is regular1 and the declivities are not marked by prominent scars and ribs of rock1 this monotony of feature betokens a corresponding uniformity of petrographical character4 ,but where1 on the other hand1 the slopes are diversified by projecting crags and other varieties of outline1 a greater range of texture and composition in the material of the hills is indicated4 ,this relation is well brought out on the western front of ,marsco1 where numerous alternations of granitoid and felsitic textures occur4 ,on many declivities also1 which at a distance look quite smooth1 but which are really rough with angular blocks detached from the parent mass underneath1 an occasional basalt-dyke will be observed to rise as a prominent dark rib4 ,a good example of this structure is to be seen on the south front of ,beinn na ,caillich4 ,where a group of dark parallel dykes runs along the sides of one of these pale cones1 it sometimes produces a curiously deceptive appearance of bedding4 ,a conspicuous illustration may be noticed on the southern front of ,beinn ,dearg ,meadhonach1 north from ,marsco4 ,when ,i first saw that hillside ,i could not realize that the parallel bars were actually dykes until ,i had crossed the valley and climbed the slopes of the hill4#cij ,footnote #cij3 ,the difference of contour and colour between the ordinary reddish smooth-sloped syenite and the black craggy hypersthene rock and greenstone in the ,glamaig group of hills caught the eyes of ,von ,oeynhausen and ,von ,dechen 7,karsten's ,archiv1 i4 p4 #hc74 ,good evidence of successive protrusions of the acid rock within the great area of the ,red ,hills may be found on the south side of ,meall ,dearg at the head of ,glen ,sligachan1 where the granophyre is traversed by a younger band or dyke of fine-grained spherulitic material about ten feet broad4 ,the rock exhibits there the same beautiful flow-structure with rows of spherulites as is to be seen along the contact of the main granophyre mass with the gabbro on the same hill1 which will be afterwards described4 ,this dyke1 vein or band1 though possibly belonging to the same epoch of protrusion as the surrounding granophyre1 must obviously be later than the consolidation of the rock which it traverses4 ,occasionally round the margin of the granophyre a singular brecciated structure is to be seen4 ,i have found it well marked on weathered faces1 along the flanks of ,glamaig and of ,marsco1 and ,mr4 ,harker has observed many examples of it on the north side of the granophyre mass of the ,red ,hills4 ,when the rock is broken open1 it is less easy to detect the angular and subangular fragments from the surrounding matrix1 which is finely crystalline or felsitic4 ,the actual junction of the eruptive mass with the surrounding rocks through which it has ascended is generally a nearly vertical boundary1 but the granophyre sometimes plunges at a greater or less angle under the rocks that lie against or upon it4 ,on the north side of ,glamaig1 for instance1 the prophyritic and felsitic margin of the great body of eruptive rock descends as a steeply inclined wall1 against which the red sandstones and marls at the base of the ,secondary formations are sharply tilted4 ,on the south side of the area a similar steep face of fine-grained rock forms the edge of the granophyre of the great southern cones1 and plunges down behind ,lias limestone and shale1 ,cambrian limestone and quartzite1 or portions of the ,tertiary volcanic series4 ,where the granophyre cuts vertically through the gabbro1 the latter rock being more durable is apt to rise above the more decomposable granophyre as a crag or wall1 and thus the deceptive appearance arises of the basic overlying the acid rock4 ,as above mentioned1 there seems every reason to believe that this peculiarity of weathering has given rise to or confirmed the mistaken impression that the granophyre is older than the gabbro4 ,there can be no doubt1 however1 that along many parts of the boundary-line the acid eruptive mass extends underneath the surface far beyond the actual base of the cones1 for projecting knobs as well as veins and dykes of it rise up among the surrounding rocks4 ,this is well seen along the northern foot of ,beinn na ,caillich4 ,but of all the ,skye bosses none exhibits its line of junction with the surrounding rocks so well and continuously as ,beinn an ,dubhaich4 ,this isolated tract of eruptive material lies entirely within the area of the ,cambrian limestone1 and its actual contact with that rock1 and with the basalt-dykes that traverse it1 can be examined almost everywhere4 ,the junction is usually vertical or nearly so1 sometimes inclining outwards1 sometimes inwards4 ,it is notched and wavy1 the granite sending out projecting spurs or veins1 and retiring into little bays1 which are occupied by the limestone4 ,the subdivisions of the latter rock have recently been traced by ,mr4 ,harker up to one side of the granite and recognized again on the other side1 with no apparent displacement1 as if so much limestone had been punched out to make way for the uprise of the acid boss4 ,the older dykes1 too1 are continuous on either side of the ridge4 ,the granite is massive and jointed1 splitting up into great quadrangular blocks like an ancient granite1 and weathering into rounded boulders4 ,its granitic composition and texture are best seen where the mass is broadest1 south of ,kilbride4 ,towards its margin1 on the shore of ,camas ,malag1 the granophyric structure appears1 especially in narrow ribbons or veins that run through the more granitic parts of the rock4 ,these may be compared with the much larger dyke of spherulitic rock above noticed as traversing the granophyre of ,meall ,dearg4 ,illustration3 ,fig4 #cdg4--,section across the north slope of ,beinn an ,dubhaich1 ,skye4 a a1 ,cambrian limestone2 b b1 basalt dykes2 c1 granite4 ,immediately to the south of ,camas ,malag the junction with the limestone is well displayed1 and the eruptive rock1 which is there granitic in character1 sends out into the limestone a vein or dyke about two feet broad1 of closer grain than the main body of the boss1 but still distinctly granitic in structure4 ,the junction on the north side is equally well seen below the crofts of ,torran4 ,here the rock of the boss1 for a few yards from its margin1 assumes a fine-grained felsitic aspect1 and under the microscope presents a curious brecciated appearance1 suggestive of its having broken up at the margin before final consolidation4 ,portions of the already crystallized granite seem to be involved in a microgranitic base4 ,the rock has here truncated a number of basalt-dykes which intersect the ,cambrian limestone4 ,to one of these further reference will be made in the sequel4 ,on the surface of the mass of ,beinn an ,dubhaich1 a few little patches of limestone occur to the south of ,kilchrist ,loch4 ,considering the nearly vertical wall which the granophyre presents to the adjacent rock all round its margin1 we may perhaps reasonably infer that these outliers of limestone are remnants of a once continuous limestone sheet that overlay the eruptive rock1 and hence that1 with due allowance for considerable denudation1 the present surface of the boss represents approximately the upper limit to which the granophyre ascended through the limestone4 ,the actual facts are shown in ,fig4 #cdg4 ,all round the margin of this boss1 the limestone has been converted for a variable distance of a few feet or many yards into a granular crystalline marble4 ,the lighter portions of the limestone have become snowy white2 but some of the darker carbonaceous beds retain their dark tint4 ,the nodules of chert1 abundant in many of the limestones1 project from the weathered faces of the marble4 ,the dolomitic portions of the series have likewise undergone alteration into a thoroughly crystalline-granular or saccharoid rock4 ,the most thorough metamorphism is exhibited by portions of the limestone which are completely surrounded by and rest upon the granite4 ,the largest of these overlying patches was many years ago quarried for white marble above the old ,manse of ,kilchrist4 ,i have shown by lithological1 stratigraphical and palontological evidence that this limestone1 instead of belonging to the ,lias1 as was formerly believed1 forms a part of the ,cambrian or possibly the very lowest ,silurian series1 being a continuation of the fossiliferous limestone of western ,sutherland and ,ross-shire4#cia ,mr4 ,clough and ,mr4 ,harker1 in the progress of the ,geological ,survey in ,skye1 have ascertained that the distinctive characters of the three groups of strata into which the limestone can be divided may be recognized even through the midst of the metamorphism4#cib ,footnote #cia3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xliv4 7#ahhh7 p4 #fb4 ,footnote #cib3 ,annual ,report of ,director-,general of the ,geological ,survey for #ahie4 ,the generally vertical line of separation between the rock of ,beinn an ,dubhaich and the contiguous limestone has been taken advantage of for the segregation of mineral veins4 ,on the southern boundary at ,camas ,malag1 a greenish flinty layer1 from less than an inch to two or three inches in width1 consisting of a finely-granular aggregate of some nearly colourless mineral1 which polarizes brilliantly1 coats the wall of the granophyre1 and also both sides of the vein which proceeds from that rock into the limestone4 ,but the most abundant and interesting deposits are metalliferous4 ,fragments of a kind of gossan may be noticed all along the boundary-line of the boss1 and among these are pieces of magnetic iron-ore and sulphides of iron and copper4 ,the magnetite may be seen in place immediately to the south of ,kilbride4 ,a mass of this ore several feet in diameter sends strings and disseminated particles through the surrounding granophyre1 and is partially coated along its joints with green carbonate of copper4 ,from the ,skye area important evidence is obtainable in regard to the relation of the acid eruptions to 7#a7 earlier eruptive vents filled with agglomerate2 7#b7 the bedded basalts of the plateaux2 7#c7 the bosses1 sills and dykes of gabbro and dolerite2 and 7#d7 the great system of basic dykes4 7#a7 ,relation of the ,granophyre to older ,eruptive ,vents4--,the granophyre of ,beinn na ,caillich and the two ,beinn ,deargs has invaded on its north-eastern side the ,cambrian limestone and quartzite1 and has truncated the sheets of intrusive dolerite and gabbro that have there been injected into them4 ,but to the south-west it rises through the great ,strath agglomerate already described1 and continues in that rock round to the entrance into ,strath ,beg4 ,the eruptive mass is in great part surrounded with a ring of agglomerate1 as if it had risen up a huge volcanic chimney and solidified there1 though probably there were more than one vent in this agglomerate area4 ,again the thick mass of agglomerate north of ,belig is interposed between the bedded lavas and the great granophyre mass which extends northwards to ,loch ,sligachan4 ,on the west side of the ,blaven ridge1 a number of masses of agglomerate are found on both sides of ,glen ,sligachan1 along the border of the same great tract of acid rock4 ,illustration3 ,fig4 #cdh4--,section from ,beinn ,dearg to ,beinn an ,dubhaich1 ,skye4 a a1 ,cambrian limestone2 b b1 volcanic agglomerate2 c c c1 basalt-dykes older than granophyre2 d#a1 granophyre of ,beinn ,dearg2 d#b1 granophyre in the agglomerate neck2 d#c1 granite of ,beinn an ,dubhaich2 e1 basalt-dyke younger than granite4 ,with regard to the relation of the granophyre of the ,red ,hills to the great agglomerate of ,strath1 we may infer that the granophyre has not risen exactly in the centre of the old funnel1 but rather to the north of it1 unless we suppose1 as already suggested1 that some of the agglomerate belongs to the cone that gathered round the eruptive orifice4 ,it is interesting to observe1 however1 that granophyre1 from the same or from another centre of protrusion1 has likewise risen along the outer or southern margin of the agglomerate1 generally between that rock and the limestone1 but sometimes entirely within the agglomerate4 ,the distance between the nearest part of this ring of eruptive rock and the edge of the boss of ,beinn an ,dubhaich is under #djj yards1 the intervening space being occupied by limestone 7or marble71 much traversed by north-west basalt-dykes4 ,most of these dykes do not enter the rocks of the vent1 and are abruptly truncated by the mass of ,beinn an ,dubhaich4 ,the probable structure of this locality is shown in ,fig #cdh4 ,the masses of agglomerate which further westward so curiously follow the margin of the great granophyre bosses1 and those which are entangled in that rock and in the gabbro1 probably indicate1 as already suggested1 the position of a group of older volcanic funnels which provided facilities for the uprise of the basic and acid magmas4 ,the group of vents which1 as we have seen1 probably rose out of the plateau-basalts1 and first served for the rise of the masses of gabbro1 has by the subsequent protrusion of the granophyres been still further destroyed and concealed4 ,the granophyre intrusions in the great ,strath agglomerate have lately been mapped and described by ,mr4 ,harker4 ,as regards their internal structure and composition1 this observer remarks that compared with the normal granophyres of the ,red ,hills and other bosses of the district1 these smaller intrusive masses are darker and manifestly richer in the iron-bearing minerals1 and have a slightly higher specific gravity4 ,but in their general characters they agree with the other granophyres4 ,the most interesting feature in them is the evidence they afford that they have enclosed and partially dissolved fragments of basic rocks4 ,to this evidence further reference will be made on a later page 7see p4 #cib74 7#b7 ,relation of the ,granophyre to the ,bedded ,basalts of the ,plateaux4 ,metamorphism of the ,basalts4--,on the north-west side1 the granophyre of ,glamaig and ,glen ,sligachan mounts directly out of the bedded basalts4 ,these latter rocks1 which rise into characteristic terraced slopes on the north side of ,loch ,sligachan1 appear on the south side immediately to the west of ,sconser1 and stretch westwards round the roots of ,glamaig into the ,coire na ,sgairde4 ,as they approach that hill they assume the usual dull1 indurated1 splintery1 veined character of their contact metamorphism1 and weather with a pale crust4 ,some of them are highly amygdaloidal1 and between their successive beds thin bands of basalt-breccia1 also much hardened1 occasionally appear4 ,veins of granophyre become more numerous nearer the main mass of that rock4 ,the actual line of junction runs into the ,coire na ,sgairde and slants up the ,druim na ,ruaige1 ascending to within a few feet of the top of that ridge4 ,a dark basic rock lies on the granophyre1 the latter being here finer grained and greenish in colour1 and projecting up into the former4#cic ,there is so much detritus along the sides and floor of ,glen ,sligachan that the relations of the two groups of rock cannot be well examined there4 ,but the basalts1 which present their ordinary characters to the north of the ,inn1 are observed to become more and more indurated1 close-grained1 dull and splintery1 as they draw nearer to the granophyre of ,marsco4 ,this part of the district furnishes the clearest evidence of the posteriority of the great cones of ,glamaig and its neighbours to the plateau-basalts which come up to the very base of these hills4#cid ,footnote #cic3 ,i think it probable that some of the greenish portions of the granophyre along this part of the junction-line will be found to have had their structure and composition altered by having incorporated into their substance a proportion of the bedded basalts through which they have been disrupted4 ,footnote #cid3 ,the dykes of granophyre in these basalts are referred to at p4 #ddd4 ,round the eastern group of cones some interesting fragments of the once continuous sheet of plateau-basalts remain1 and show the same relation of the acid protrusions on that side4 ,one of these lies on the granophyre of the flanks of ,beinn na ,caillich1 a little to the west of the loch at the northern base of that hill4 ,another of larger size forms a prominent knob about three-quarters of a mile further west1 and is prolonged into the huge dark excrescence of ,creagan ,dubha1 which rises in such striking contrast to the smooth red declivities of the granophyre cones around it4 ,this prominence at its eastern and northern parts consists of highly indurated splintery basalt in distinct beds1 some of which are strongly amygdaloidal4 ,the bedding is nearly vertical1 but with an inclination inwards to the hill4 ,towards the south-west end a thin band of basalt-breccia makes its appearance between two beds of basalt4 ,its thickness rapidly increases southward until it is the only rock adhering to the granophyre4 ,beyond the foot of the hill1 limestone and quartzite occupy for some distance the bottom of ,strath ,beg1 much invaded by masses of quartz-porphyry4 ,at the summit of ,creagan ,dubha abundant veins run into the basic rocks from the granophyre1 which is here finer grained towards the margin2 and there are likewise veins of quartz-porphyry which1 though their actual connection with the main mass of granophyre cannot be seen1 are no doubt apophyses from it4 ,this outlier of altered basalt and breccia appears to me to be a fragment of the plateau-basalts which once overlay the ,cambrian and ,jurassic rocks of ,strath ,beg1 and were disrupted by the uprise of the granophyre4 ,it continues to adhere to the wall of the eruptive mass that broke up and baked its rocks4 ,its breccia1 passing southward into a coarse agglomerate1 may be a product of the same vent or group of vents that discharged the great agglomerate mass above ,kilbride and ,kilchrist4 ,i have already 7p4 #bhb7 referred to what appears to be another outlier of the basalts on the south side of ,beinn ,dearg4 ,on the northern and southern flanks of ,beinn na ,cro1 similar evidence may be observed of the posteriority of the granophyre to the basic rocks4 ,round the northern base of the hill a continuous tract of plateau-basalts1 dolerites and gabbros forms the ridge between ,strathmore and ,strathbeg4 ,there is an admirable section of the relation of the two groups of rock on the eastern side of the western glen4 ,along the lower part of the declivity1 coarsely-crystalline gabbros1 like some of those in the ,cuillin ,hills1 are succeeded by sheets of dolerite and basalt1 the whole forming an ascending succession of beds to the summit of the ridge4 ,the edges of these beds are obliquely truncated by the body of granophyre1 which slants up the hill across them and sends veins into them4 ,they are further traversed by basalt dykes1 which here1 as almost everywhere1 abound 7,fig4 #cdi74 ,on the south side of ,beinn na ,cro1 highly indurated black and grey ,lias shales and sandstones have been tilted up steeply and indurated by the eruptive rock of the hill2 and at one place some #hjj feet above the sea1 a little patch of altered basalt1 lying on the shale1 but close up against the steep declivity of granophyre1 forms a conspicuous prominence on the otherwise featureless slope4 ,reference has already been made to the mass of fine-grained hornblende-granite which runs for several miles at the base of the volcanic series on the eastern side of the ,blaven group of hills4 ,mr4 ,harker has traced a great development of granophyre on the west side of these hills1 where the acid rock sends apophyses both into the bedded basalts and into the gabbros4 ,illustration3 ,fig4 #cdi4--,section at north end of ,beinn na ,cro1 ,skye4 a1 basalt1 dolerite and gabbro2 b1 granophyre of ,beinn na ,cro2 b1 dyke of granophyre2 c c1 basalt dykes4 ,combining the results of observations made not only in ,skye but in ,mull1 ,rum and ,ardnamurchan1 ,i shall here give a fuller account of the metamorphism of the basalts1 to which frequent allusion has been made as one of the evidences of the posteriority of the eruptive bosses of rock round which it occurs4#cie ,the field-geologist observes that the basalts1 as they are traced towards these bosses1 lose their usual external characters4 ,they no longer weather into spheroidal blocks with a rich brown loam1 but project in much jointed crags1 and their hard rugged surface shows when broken a thin white crust1 beneath which the rock appears black or dark bluish-grey1 dull and splintery4 ,they are generally veined with minute threads or strings of calcite1 epidote and quartz1 which form a yellowish-brown network that projects above the rest of the weathered surface4 ,where they are amygdaloidal1 the kernels no longer decay away or drop out1 leaving the empty smooth-surfaced cells1 but remain as if they graduated into the surrounding rock by an interlacing of their crystalline constituents4 ,they then look at a distance more like spots of decoloration1 and even when seen close at hand would hardly at first betray their real nature4 ,footnote #cie3 ,many years ago ,i was much struck with the evidence of alteration in the igneous rocks of ,mull1 and referred to it in several papers1 ,proc4 ,roy4 ,soc4 ,edin4 7#ahff-fg7 vol4 vi4 p4 #gc2 ,quart4 ,journ4 ,geol4 ,soc4 xxvii4 7#ahga7 p4 #bhb1 note4 ,the subject was more fully discussed in my memoir in the ,trans4 ,roy4 ,soc4 ,edin4 vol4 xxxv4 7#ahhh7 p4 #afg1 from which the account in the text is taken4 ,prof4 ,judd has more recently referred the alteration to solfataric action 7,quart4 ,journ4 ,geol4 ,soc4 xlvi4 #ahij1 p4 #cda74 ,as already mentioned1 ,i have been unable to detect evidence of such action4 ,the alteration is always intimately connected with the presence of intrusive masses1 and it affects indifferently any part of the basalt-plateaux which may chance to lie next to these masses4 ,the bedded lavas can be traced step by step from their usual unaltered condition in the plateaux to their metamorphosed state next to the eruptive rocks4 ,the nature or degree of the metamorphism has doubtless somewhat varied with the composition and structure of the rocks affected1 and with the character and mass of the eruptive material2 but it is certainly not confined to the older parts of the plateaux1 nor to any supposed pre-basaltic group of andesites4 ,i have found no evidence that such a group anywhere preceded the plateau-basalts4 ,the andesites1 so far at least as my observations go1 were erupted at intervals during the plateau period1 and alternate with the true basalts4 ,the greatest accumulation of them lies not below but above the general body of the basalts1 in the pale group of ,mull4 ,nor even if the term propylite be adopted for these altered rocks1 can it be applied to any special horizon in the volcanic series4 ,the alteration of the basic rocks by the granophyre of ,st4 ,kilda will be described in the account of that island in ,chapter xlvii4 ,from the specimens collected by me among the ,inner ,hebrides up to the year #ahhh1 ,i selected two dozen which seemed to be fairly typical of these altered rocks1 and placed thin slices of them for microscopic examination in ,dr4 ,hatch's hands4 ,his notes may be condensed into the following summary4 ,one of the most frequent features in the slides is the tendency in the component minerals to assume granular forms4 ,in one specimen from ,loch ,spelve1 ,mull1 the rock1 probably originally a dolerite1 shows only a few isolated recognizable crystals of plagioclase and augite1 the whole of the rest of the rock consisting of roundish granules embedded in a felspathic matrix4 ,the felspar crystals are sometimes broken up into a mosaic1 though retaining their external contours4 ,besides the granules1 which are no doubt augite1 a few grains of magnetite are scattered through the rock1 aggregated here and there into little groups4 ,in another specimen1 taken from the junction with the granophyre in ,glenmore in the same island1 parts of the augite crystals are converted into granular aggregates associated with large grains and patches of magnetite4 ,the latter mineral also assumes in some of the rocks granular and even globular shapes suggestive of fusion4 ,the felspars1 which in most of the basic rocks are usually remarkably clear and fresh1 show marked kaolinization in some of these altered masses4 ,minute dusky scales of kaolin are developed1 sometimes also with the separation of minute grains of quartz4 ,the augite shows frequent alteration to hornblende1 proceeding as usual from the exterior inward4 ,in some cases only an envelope of uralite appears round the augite1 while in others only a kernel of the original mineral is left1 or the whole crystal has been changed4 ,in many cases the altered substance appears as minute needles1 blades and fibres of actinolite4 ,occasionally1 besides the green hornblende1 shred-like pieces of a strongly pleochroic brown hornblende make their appearance4 ,serpentinous and chloritic substances are not infrequent4 ,epidote is sometimes abundant4 ,the titaniferous iron has commonly passed more or less completely into leucoxene4 ,here and there a dark mica may be detected4 ,since the year #ahhh ,i have continued the investigation of this subject1 and have especially studied the metamorphism of the bedded basalts on the western shores of ,loch ,scavaig1 where1 as already described1 they are truncated by vertical beds of gabbro1 and are traversed by basalt-dykes and by abundant veins of fine-grained granophyre4 ,the alteration here effected affords excellent materials for study1 as the very same sheets of basalt can be followed from the normal conditions outside to the altered state within the influence of the metamorphic agent4 ,the alternations of amygdaloidal and more compact sheets can still be recognized1 although their enclosed amygdales have in places been almost effaced4 ,they show the dull1 indurated1 splintery character1 with the white weathered crust1 so distinctive of this type of contact-metamorphism4 ,they are traversed by numerous sills and veins of gabbro4 ,as has been already suggested1 although no large mass of granophyre appears here at the surface1 the alteration of the basalts is probably to be attributed not so much to the influence of the gabbro1 as to the abundant acid sills1 dykes and veins1 for there may be a considerable body of granophyre underneath the locality1 the dykes and veins being indications of its vicinity4 ,in the summer of #ahie ,i examined the locality with much care1 and collected some typical specimens illustrative of the conditions of metamorphism presented by different varieties of the bedded basalts4 ,thin slices cut from these specimens were placed in ,mr4 ,harker's hands for microscopical examination1 and he furnished me with the following notes regarding them4 ,in hand-specimens the bedded basalts from the neighbourhood of the gabbro of ,loch ,scavaig #ffac-ffah do not appear very different from the normal basalts of this region4 ,the most conspicuous secondary mineral is yellowish-green epidote in patches1 and especially in the amygdales4 ,the texture of the rocks varies1 and the slices show that the micro-structure also varies1 the augite occurring sometimes in small ophitic plates1 sometimes in small rounded granules4 ,the chief secondary change in the body of the rock is shown by the augite1 which is seen in various stages of conversion to greenish fibrous hornblende4 ,some round patches seem also to consist mainly of the latter mineral1 and are probably pseudomorphs after olivine4 ,here the little fibres are confusedly matted together1 without the parallelism proper to uralite derived from augite4 ,no fresh olivine has been observed4 ,the felspar and magnetite of the basalts show little or no sign of metamorphic processes1 unless a rather unusual degree of clearness in the felspar crystals is to be regarded in that light4 ,the contents of the metamorphosed amygdales are not always the same4 ,epidote is usually present in some abundance1 and in well-shaped crystals4 ,it has a pale citron tint in the slices1 with marked pleochroism2 but a given crystal is not always uniform in its optical characters4 ,frequently the interior is pale1 and has a quite low birefringence4 ,this is probably to be regarded as an intergrowth of zoisite in the epidote1 and there are a few distinct crystals of zoisite seen in some places4 ,in the slide which best exhibits these features #ffac the crystals of epidote are in part enwrapped and enclosed by what are doubtless zeolitic minerals4 ,at least two of these are to be distinguished4 ,one1 very nearly isotropic1 and with a pale-brownish tint1 is probably analcime4 ,associated with this is a colourless mineral with partial radiate arrangement and with twin lamellation2 the birefringence is somewhat higher than that of quartz1 and the -axis of optic elasticity makes a small angle with the twin-line4 ,these characters agree with those of epistilbite4 ,in other parts of the same large amygdale1 the epidote crystals are embedded in what seems to be a felspar4 ,this latter mineral is rather obscure1 and twin-lamellation is rarely to be detected2 but it seems highly probable that felspar has here been developed by metamorphic agency at the expense of zeolites which once occupied the amygdale4 ,i have observed undoubted examples of this in metamorphosed basalts from other parts of ,skye1 e4g4 from ,creagan ,dubha1 near the granophyre mass of ,beinn ,dearg4#cif ,the felspar occurs there in the same fashion1 and in the same relation to epidote #bgjj1 #bgja4 ,in the specimens now described the chief minerals in the metamorphosed amygdales are those already named3 others occur more sparingly1 associated with them4 ,in some cases there is a grass-green1 strongly pleochroic1 actinolitic hornblende1 accompanied by a little iron pyrites #ffae4 ,footnote #cif3 ,compare ,trans4 ,roy4 ,soc4 ,edin4 vol4 xxxv4 p4 #aff4 ,epidote and various hornblendic and augitic minerals are characteristic products in the metamorphism of amygdaloidal basalts in other regions3 felspar with this mode of occurrence ,i have not seen except in ,skye1 where it seems to connect itself naturally with the abundance of zeolites in the amygdales of the non-metamorphosed lavas4 ,it is to be observed that in these basalts from ,loch ,scavaig the alteration is shown especially in the amygdales1 the body of the rock not being greatly affected3 this indicates a not very advanced stage of metamorphism4 ,the production of uralitic hornblende1 rather than brown mica1 from the augite and its decomposition-products1 seems to be characteristic of the metamorphism of basaltic as distinguished from andesitic rocks1 and is well illustrated by a comparison of the two sets of lavas near the ,shap granite4#cig ,footnote #cig3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xlix4 7#ahic7 p4 #cfa4 ,mr4 ,harker1 who is at present engaged in mapping the central region of ,skye1 has had occasion to go over a number of the localities 7,creagan ,dubha1 etc47 originally cited by me1 and1 while corroborating my general conclusions regarding them1 has been able to obtain much fresh evidence regarding the nature and extent of the metamorphism which the bedded basalts have undergone4 ,the results of his investigations will be published when the ,geological ,survey of ,skye is further advanced4 7#c7 ,relation of the ,granophyre to the ,gabbros4--,that the granophyres invade the gabbros has been incidentally illustrated in the foregoing pages4 ,but as the mutual relations of the two rocks in the island of ,skye have been the subject of frequent reference in previous writings of geologists1 it is desirable to adduce some detailed evidence from a region which has been regarded as the typical one for this feature in the geological structure of the ,inner ,hebrides4 ,no geological boundary is more easily traced than that between the pale reddish granophyre and the dark gabbro4 ,it can be followed with the eye up a whole mountain side1 and can be examined so closely that again and again the observer can walk or climb for some distance with one foot on each rock4 ,that there should ever have been any doubt about the relations of the two eruptive masses is possibly explicable by the very facility with which their junction can be observed4 ,their contrasts of form and colour make their boundary over crag and ridge so clear that geologists do not seem to have taken the trouble to follow it out in detail4 ,and as the pale rock undoubtedly often underlies the dark1 they have assumed this infraposition to mark its earlier appearance4 ,i will only cite one part of the junction line1 which is easily accessible1 for it lies in ,glen ,sligachan immediately to the south of the mouth of ,harta ,corry4 ,the rounded eminence of ,meall ,dearg1 which rises to the south of the two ,black ,lochs1 belongs to the granophyre1 while the rugged ground to the west of it lies in the gabbro4 ,the actual contact between the two rocks can be followed from the side of ,harta ,corry over the ridge and down into ,strath na ,creitheach1 whence it sweeps northward between the red cone of ,ruadh ,stac and the black rugged declivities of ,garbh ,beinn4 ,there is no more singular scene in ,skye than the lonely tract on the south side of ,meall ,dearg4 ,the ground for some way is nearly level1 and strewn with red shingle from the decomposing granophyre underneath4 ,it reminds one of some parts of the desert ,bad lands of ,western ,america4 ,grim dark crags of gabbro1 with veins from the granophyre1 rise along its western border1 beyond which tower the black precipices of the ,cuillins1 while the flaming reddish-yellow cones of ,glen ,sligachan stand out against the northern sky4 ,having recently described in some detail the relations of the boss of granophyre at this interesting locality1 ,i will only here offer a brief summary of the chief features4#cih ,the granophyre of ,meall ,dearg forms a marginal portion of the great mass of the ,red ,hills4 ,it has broken across the banded gabbros1 and also cuts an isolated boss of agglomerate in the ridge of ,druim an ,eidhne4 ,its line of junction is nearly vertical1 but along part of its course the wall of gabbro rises higher than that of the more decomposable granophyre4 ,hence the origin of the black crags that crown the red slopes of granophyre debris4 ,seen from a distance the basic rock seems to rest as a great bed upon the acid mass4 ,footnote #cih3 ,see ,quart4 ,journ4 ,geol4 ,soc4 vol4 #a4 7#ahid7 p4 #bab4 ,the younger date and intrusive nature of the granophyre are well shown by the change in the texture of the mass as it approaches the rocks against which it has cooled4 ,the ordinary granophyric characters rapidly pass into a fine-grained felsitic texture1 and this change is accompanied with the development of a remarkably well-defined flow-structure and of rows of spherulites which run parallel to the boundary wall4 ,in a ravine on the west side of ,meall ,dearg1 the lines of flow-structure and rows of large spherulites are seen to be arranged vertically against the face of gabbro4 ,further proof of the later date of the protrusion of the granophyre is supplied by abundant felsitic dykes and veins which traverse the gabbro1 and some of which can be seen to proceed from the main body of granophyre4 ,these intrusions will be described in the next chapter1 in connection with the dykes and veins of the acid rocks4 ,additional evidence as to the posteriority of the granophyre to the gabbro has recently been obtained by ,mr4 ,harker from a study of the internal structure and composition of the masses of these rocks which have been intruded into the agglomerate above ,loch ,kilchrist in ,strath4 ,he has found that the granophyre has there caught up from some subterranean depth portions of gabbro1 and has partially dissolved them1 thereby undergoing a modification of its own composition4 ,the gabbro-debris1 he remarks1 has been for the most part completely disintegrated by the caustic or solvent action of the acid magma on some of its minerals4 ,those constituents which resisted such action have been set free and now figure as xenocrysts foreign crystals1 either intact or more or less perfectly transformed into other substances4 ,at the same time the material absorbed has modified the composition of the magma1 in the general sense of rendering it less acid4 ,mr4 ,harker has traced the fate of each of the minerals of the gabbro in the process of solution and isolation in the acid magma1 which1 where this process has been most developed1 is believed by him to have taken up foreign material amounting to fully one-fourth of its own bulk1 derived not from the rocks immediately around1 but from a gabbro probably at a considerable depth beneath4#cii ,footnote #cii3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 lii4 7#ahif7 p4 #cbj4 ,illustration3 ,fig4 #cej4--;,ground-plan of basic dyke in ,cambrian ,limestones truncated by granophyre which encloses large blocks of the dyke1 ,torrin1 ,skye4 7#d7 ,relation of the ,granophyre to the ,basic ,dykes and ,veins4--,reference has already been made to the fact that the syenite bosses of ,skye cut off most of the basalt-dykes1 but are themselves traversed by a few others4#djj ,the locality that furnished me with the evidence on which this statement was originally made nearly forty years ago affords in small compass a clearer presentation of the facts than ,i have elsewhere met with4 ,the sections described by me are visible at the eastern end of the boss of ,beinn an ,dubhaich1 ,strath2 but similar and even better examples may be cited from the whole northern and southern margins of that eruptive mass4 ,on the north side an extraordinary number of dykes may be traced in the ,cambrian limestone from the shores of ,loch ,slapin eastwards4 ,they have a general north-westerly trend1 but one after another1 as ,i have already remarked1 is abruptly cut off by the granophyre4 ,as an example of the way in which this truncation takes place1 ,i may site a single illustration from the northern margin of the eruptive mass1 near ,torrin4 ,it might perhaps be contended that the numerous dykes which traverse the limestone and stop short at the edge of the acid rock1 are not necessarily older than the granophyre1 but may actually be younger1 their sudden termination at the edge of the acid boss being due to their inability to traverse that rock4 ,that this explanation is untenable is readily proved by such sections as that given in ,fig4 #cej1 where a basic dyke 7b7 #i or #aj feet broad running through the ,cambrian ,limestone 7a a7 is abruptly cut off by the edge of the great granophyre boss4 ,not only is the dyke sharply truncated1 but numerous pieces of it1 from an inch to more than a foot in length1 are enclosed in the granophyre4 ,the latter is well exposed along the shore of ,loch ,slapin in an almost continuous section of nearly a mile in length4 ,the contrast therefore between the development of dykes within and beyond its area cannot but arrest the attention of the observer4 ,though ,i was on the outlook for dykes in the granophyre1 ,i found only one4 ,yet immediately beyond the eruptive boss they at once appear on either side up to its very edge1 where they suddenly cease4 ,the conclusion cannot be resisted that the protrusion of the acid rock took place after most of the dykes of the district had been formed1 but before the emission of the very latest dykes1 which pursue a north-west course across the boss 7,fig4 #cdh74 ,footnote #djj3 ,ante1 p4 #agc1 and ,quart4 ,journ4 ,geol4 ,soc4 vol4 xiv4 7#aheg7 p4 #af4 ,some sections on the southern margin of ,beinn an ,dubhaich complete the demonstration that such has been the order of appearance of the rocks4 ,near the head of the ,allt ,lth ,slighe 7or ,half-way ,burn71 where the granite has pushed a long tongue into the limestone1 a north-west basalt-dyke is abruptly cut off by the main body of the boss and by the protruded vein 7,fig4 #cea74 ,besides this truncation1 the acid rock sends out strings and threads of its own substance into and across the dyke1 these injected portions being as usual of an exceedingly fine felsitic texture4 ,illustration3 ,fig4 #cea4--,section on south side of ,beinn an ,dubhaich1 ,skye1 showing the truncation of a basalt-dyke 7b71 in ,cambrian ,limestone 7a71 by the granite 7c7 of ,beinn an ,dubhaich1 ,skye4 ,similar evidence may be gathered from the area of the great granophyre cones further north4 ,the profusion of basalt-dykes in the surrounding rocks stops short at the margin of that area4 ,the comparatively few dykes which cross the boundary pursue a general north-west course through the granophyre1 and1 as already remarked1 from their dark colour1 greater durability and straightness of direction1 stand out as prominent ribs on the flanks of the pale cones which they traverse4 ,,chapter ,,xlvii ,,the ,,acid ,,bosses ,,of ,,mull1 ,,small ,,isles1 ,,st4 ,,kilda1 ,,arran ,,and ,,the ,,north-,,east ,,of ,,ireland ii4 ,,the ,,acid ,,bosses ,,of ,,mull ,though of comparatively small extent1 the granophyre bosses of the island of ,mull afford to the geologist a large amount of instruction in regard to the relations of the different members of the volcanic series to each other4 ,especially important is the evidence which they contain of the connection between the acid and basic groups of rocks4 ,they have been laid bare in many natural sections1 some of which1 forming entire hillsides1 are among the most astonishing in the whole wonderful series which1 dissected by denudation1 reveal to us the structure of these volcanic regions4 ,they lie in two chief areas4 ,one of these extends along the northern flanks of the mountainous tract from the western side of ,beinn ,fhada across ,loch ,ba' to the west side of ,glen ,forsa4 ,the other occupies for over three miles the bottom of ,glen ,more1 the deep valley which1 skirting the southern side of the chief group of hills1 connects the east side of the island by road with the head of the great western inlet of ,loch ,scridain4 ,there are other minor areas4 ,one of these extends for about a mile along the declivities to the south of ,salen1 across the valley of the ,allt na ,searmoin2 another occurs at ,salen2 a third runs along the shore at ,craignure4 ,in the interior also1 many isolated areas of similar rocks1 besides thousands of veins1 occur in the central group of hills and valleys which form the basins of the ,glencannel and ,forsa rivers 7,map ,,vi474 ,the chief northern boss1 which for the sake of convenience of reference may be called that of ,loch ,ba'1 has a length of nearly six miles1 with a breadth varying from a quarter of a mile to about a mile and a quarter4 ,it descends to within #ej feet of the sea-level1 and is exposed along the crest of ,beinn ,fhada at a height of more than #ahjj feet4 ,it chiefly consists of a grey crystalline rock which might readily be identified as a granite1 but which when examined microscopically is found to possess the granophyric structure4 ,with this distinctly granular-crystalline rock are associated various porphyritic and felsitic masses1 which pass into it1 and are more specially observable along its border4 ,an exceedingly compact black quartz-felsite or rhyolite forms its southern boundary1 runs as a broad dyke-like ridge from the head of the ,scarrisdale ,water north-eastward across ,loch ,ba' 7,fig4 #ceb71 and spreads out eastward into a mass more than a mile broad on the heights above ,kilbeg in ,glen ,forsa4 ,the sharp line of demarcation of this felsite1 and its mass and extent1 point to a different period of extravasation4 ,illustration3 ,fig4 #ceb4--,view of the hills on the south side of the head of ,loch na ,keal1 showing the junction of the granophyre and the bedded basalts4 ,one bird1 the bedded basalts of the ,gribon plateau2 two birds1 the bedded dolerites and basalts of ,beinn a' ,chraig adhering to the northern slope and capping the hill2 three birds1 summit of ,ben ,more1 with ,a',chioch to the left and the top of ,beinn ,fhada appearing in the middle distance between them2 four birds1 the granophyre slopes of ,beinn a' ,chraig with the great dyke-like mass of felsite on the left4 ,the geologist1 who approaches this district from the north-east1 has his attention arrested1 even at a distance of several miles1 by the contrast between the outer and inner parts of the hills that lie to the south-west of ,loch ,ba'4 ,he can readily trace from afar the dark bedded basic rocks rising terrace above terrace1 from the shores of ,loch na ,keal1 to form the seaward faces of the hills along the southern side of that fjord4 ,but he observes that immediately behind these terraces the mass of the rising ground obviously consists of some amorphous rock1 which weathers into white debris4 ,nothing can be sharper than the contrast of colour and form between the two parts of the hills4 ,the bedded plateau-rocks lie as a kind of wall or veneer against a steep face of the structureless interior 7,fig4 #ceb74 ,seen from the other or hilly side1 the contrast is perhaps even more striking4 ,but the astonishment with which it is beheld at a distance becomes intensified when one climbs the slopes1 and finds that the sheets of dolerite and basalt 7which from some points of view look quite level1 yet dip towards the north-east at a gentle angle7 are immediately behind the declivity abruptly truncated by a mass of granophyre4 ,of all the junction-lines between the acid bosses and the lavas of the plateaux1 those exposed on these ,mull hillsides are certainly the most extraordinary4 ,so little disturbed are the lavas1 that one's first impulse is to search for pebbles of the granophyre between the basalts1 for it seems incredible that the inner rock should be anything but a central core of older eruptive material1 against and round which the younger basic rocks have flowed4 ,but1 though the granophyre is so decomposing and covers its slopes with such screes of debris1 that had the basalts been poured round it1 they must infallibly have had some of its fragments washed down between their successive flows1 not a single pebble of it is there to be found4 ,this might not be considered decisive evidence1 but it is extended and confirmed by the fact that the acid rock gives off veins which ramify through the basalts4 ,before examining the actual contact of the two rocks1 however1 the geologist will not fail to observe here an admirable example of the gradual change which was described in the foregoing chapter as coming over the bedded basalts near the acid bosses4 ,as he approaches the nucleus of white rock1 the basalts assume the usual hard indurated character1 not decaying into brown sand as on the plateaux1 but often standing out as massive crags with vertical clean-cut joint-faces4 ,this metamorphosed condition extends in some cases to a considerable distance from the main body of acid rock1 especially where knobs of that material1 protruding through the more basic lavas1 show that it must extend in some mass underneath4 ,thus along the shore at ,saline the bedded basalts succeed each other in well-defined sheets1 some being solid1 massive and non-amygdaloidal1 others quite vesicular1 and recalling the black scoriform surfaces of recent ,vesuvian lavas2 yet they are all more indurated than in the normal plateau-country1 and they break with a hard splintery fracture4 ,immense numbers of dykes cut these rocks1 and they are likewise pierced by occasional felsitic intrusions4 ,if we cross to the other side of the island and trace the bedded basalts away from the central masses of acid rock we meet with so gradual a diminution of the induration that no definite boundary-line for the metamorphism can be drawn4 ,as we recede from the centre of alteration1 the rocks insensibly begin to show brown weathered crusts1 with spheroidal exfoliation1 the reticulations of epidote and calcite become much less abundant1 the amygdaloids gradually assume their normal earthy character1 and eventually we find ourselves on the familiar types of the plateau4 ,this transition is well seen along the shores of ,loch na ,keal4#dja ,footnote #dja3 ,some of the thick massive sheets of basic rock along the south side of this inlet may possibly be altered sills4 ,these proofs of the alteration of the plateau-basalts are accompanied in ,mull as in ,skye by further abundant evidence that the acid rocks are of younger date than the basic4 ,in particular1 dykes and veins may be traced proceeding from the former and intersecting the latter4 ,thus1 in the bed of the south fork of the ,scarrisdale stream1 a separate mass of granophyre 7which under the microscope exhibits in perfection the characteristic structure of this rock7 protrudes through the basalts in advance of the main mass1 and a little higher up on the outskirts of that mass narrow ribbons of the granophyre run through the basic rocks4 ,the contrast of colour between the pale veins of the intrusive rock and the dark tint of the basalts is well shown in the channel of the water4 ,similar sections may be seen on the flanks of ,beinn ,fhada1 especially in the great corry north of ,ben ,more1 where the granophyre sends a tongue of finer grain between the beds of basalt4 ,on the east side of ,loch ,ba' numerous proofs of similar intrusion may be observed4 ,thus at the east end of ,loch na ,diridh1 where the granophyre has been intruded into the basalts1 hand-specimens may be obtained showing the two rocks welded together4 ,on the slopes of ,cruach ,trr an ,lochain1 where the granophyre has a felsitic selvage1 the bedded basalts are traversed by veins of the latter material 7,fig4 #cec74 ,a little further east1 at the head of the ,allt na ,searmoin1 the bedded basalts1 some of which are separated by slaggy scoriaceous surfaces1 are intersected by another protrusion from the compact felsitic porphyry 7,fig4 #ced74djb ,a mile lower down the same valley a separate mass of granophyre sends out veins into the basalt1 which as usual is dark bluish-grey in colour1 indurated and splintery4 ,footnote #djb3 ,this rock appears to the eye as a black finely crystalline-granular felsite4 ,under the microscope1 it was found by ,dr4 ,hatch to present a markedly granulitic structure1 consisting mainly of small rounded grains of dirty brown turbid felspar1 with isolated granules of colourless quartz4 ,scattered through the rock1 or accumulated in patches1 are small spherical or drop-like granules of a bright green augite 7coccolite74 ,illustration3 ,fig4 #cec4--,section on south side of ,cruach ,trr an ,lochain1 ,mull4 a1 bedded basalts and dolerites2 b1 granophyre2 c1 marginal finer-grained band2 d d1 veins from the granophyre traversing the basic rocks4 ,illustration3 ,fig4 #ced4--,section at head of ,allt na ,searmoin1 ,mull4 a1 basalts and dolerites1 with slaggy upper surfaces2 b1 felsite4 ,as the posteriority of the ,mull granophyre and felsites to the basalts is thus proved1 the further question remains as to their manner of intrusion4 ,here and there1 especially on the south-eastern side1 between the head of the ,scarrisdale river and ,loch ,ba'1 the line of junction between the two rocks is nearly vertical1 but a body of black felsite intervenes as a huge wall between the ordinary granophyre and the basalt4 ,on ,beinn ,fhada and ,beinn a' ,chraig the line of separation1 as ,i have above remarked1 is inclined outwards1 and plunges under the basalts at an angle of #cj to #dj4 ,the terraced basalts and dolerites are not sensibly disturbed1 but end off abruptly against the steep face of intrusive rock4 ,we might suppose that in this case the younger rock had merely carried upward the continuation of the beds that are truncated by it1 as if an orifice had been punched out for its ascent4 ,but on the top of the ridge of ,beinn a' ,chraig we find that the outliers which there remain are not portions of the lower basalts1 but of the upper pale group of ,ben ,more4 ,the same rocks are prolonged on the other side of the ,scarrisdale ,glen1 sweep over the summit of ,beinn ,fhada1 and run on continuously into the crest of ,a',chioch and the upper part of ,ben ,more4 ,the granophyre has usurped the place of the lower dolerites and basalts1 but has left the more felspathic lavas of the pale group in their proper position4 ,and to make this remarkable structure still more clear1 sections may be seen on the southern flanks of ,beinn ,fhada1 where the upper surface of the granophyre comes down obliquely across the edges of the lavas1 and allows the junction of the basalts and the pale group to be seen above it 7,fig4 #cee74 ,as in the case of ,beinn an ,dubhaich1 it is as if the granophyre had eaten its way upward and dissolved the rocks which it has replaced4 ,illustration3 ,fig4 #cee4--,section on south side of ,beinn ,fhada1 ,mull4 a1 bedded basalts and dolerites2 b1 pale group of ,ben ,more2 c1 granophyre4 ,the usual kind of contact-metamorphism has been produced around this intrusive boss4 ,it is most marked in the outliers that cap ,beinn a' ,chraig and on the two ridges to the south-west1 where it is seen to consist in a high degree of induration1 the production of a shattery1 irregularly-jointed structure1 and the effacement of the obvious bedding which characterizes the unaltered rocks4 ,the position of this eruptive mass1 quite a mile broad1 breaking through1 without violently tilting1 more than #ahjj feet of the bedded basalts1 and then stopping short about the base of the pale group1 presents a curious problem to the student of geological physics4 ,it at once reminds him of many sections among ,palozoic granites where an eruptive boss has ascended and taken the place of an equivalent volume of the surrounding rocks1 which1 though more or less metamorphosed1 are not made to dip away from it as from a solid wedge driven upwards through them4 ,in this ,mull case1 however1 there are some peculiar features that deserve consideration1 for they seem to show that here1 as elsewhere1 passages for the uprise of the intrusive rock were already provided by the presence of volcanic pipes1 which1 even if filled up with fragmentary materials1 would no doubt continue to be points of weakness4 ,round the flanks of the ,loch ,ba' boss1 and here and there on its surface1 patches of intensely indurated volcanic agglomerate may be detected4 ,a little to the south of the tarn called ,loch na ,diridh1 the granophyre is succeeded by the black1 flinty felsite or rhyolite already referred to4 ,this rock in some places exhibits a beautiful flow-structure1 with large porphyritic felspars1 and encloses a great many fragments of dolerite and gabbro1 varying from the size of a pea up to blocks several inches in diameter4 ,lying on its surface are detached knolls of much altered dolerite1 basalt1 and coarse breccia or agglomerate4 ,on its southern margin one of these patches of agglomerate contains abundant fragments of various felsitic rocks1 among which are pieces of a compact rock with flow-structure like that found in place immediately to the north2 also rounded pieces of quartzite1 and of compact and amygdaloidal basalt wrapped up in a very hard matrix which seems to consist largely of basalt-dust4 ,no bedding can be made out in this rock1 and the mass looks like part of a true neck4 ,further down the slope the bedded basalts appear4 ,the actual junctions of the different rocks cannot be satisfactorily traced1 but the structure of the ground appears to me to be as shown in ,fig4 #cef4 ,a patch of similar agglomerate appears a little to the south-west of the last section in front of a cliff of the felsite1 and seems to be enclosed in the latter rock1 and other exposures of agglomerate1 underlain and intensely indurated by the felsite1 may be noticed on the ground that slopes towards ,loch ,ba'4 ,illustration3 ,fig4 #cef4--,section to south of ,loch na ,diridh1 ,mull4 a1 basalts2 b1 dolerite2 c1 volcanic agglomerate2 d1 black felsite2 e1 granophyre4 ,that these agglomerates do not belong to the period of the eruption of the granophyre and felsite1 but to that of the bedded basalts1 may be inferred from their intense induration next the acid rocks1 and also from the fact that similar breccias are actually found here interposed between the bedded basalts4 ,this is well shown on the hill above the ,coille na ,srine1 where the accompanying section can be seen 7,fig4 #ceg74 ,the broad dyke-like mass of black flinty felsite already referred to runs as a prominent rib over the southern end of ,beinn a' ,chraig into the head of the ,scarrisdale glen 7see ,fig4 #ceb74 ,it cuts across the bedded basalts1 and immediately to the south of where these appear1 a thin intercalated bed of breccia crops out1 of the usual dull-green colour1 with abundant fragments of basalt and many of yellow and grey felsite4 ,from these various facts we may1 ,i think1 conclude that along the strip of ground now occupied by the ,loch ,ba' boss of granophyre and felsite1 there once stood a line or group of vents1 from which1 besides the usual basalt-debris1 there were ejected many pieces of different felsitic or rhyolitic rocks1 and that these eruptions of fragmentary material took place during the accumulation of the plateau-basalts4 ,these volcanic funnels occasioned a series of points or a line of weakness of which1 in a long subsequent episode of the protracted volcanic period1 the acid rocks took advantage1 forcing themselves upwards therein1 and leaving only slight traces of the vents which assisted their ascent4 ,the mingling of acid and basic fragments in the material ejected from these vents is another proof of the existence of acid rocks in the volcanic reservoirs before the advent of the great granophyre intrusions4 ,the evidence thus entirely confirms the conclusions deduced from the ,skye area4 ,illustration3 ,fig4 #ceg4--,section of junction of south side of ,loch ,ba' granophyre boss1 with the bedded basalts1 ,mull4 a1 bedded basalts2 b b1 basalt-tuff and breccia2 c1 granophyre2 d1 black felsite2 e1 coarse dolerite dyke1 #cj or #dj feet wide4 ,the second or ,glen ,more boss1 instead of rising into hilly ground1 is confined to the bottom of the main and tributary valleys1 and has only been revealed by the extensive denudation to which these hollows owe their origin4 ,it begins nearly a mile below ,torness and extends up to ,loch ,airdeglais--a distance of almost four miles4 ,though singularly devoid of topographical feature1 it exhibits with admirable clearness the relation of the granophyres to the gabbros1 and thus deserves an important place among the tracts of acid rocks in the ,western ,islands4 ,its petrographical characters change considerably from one part of its body to another4 ,for the most part1 it is a true granophyre1 sometimes with orthoclase1 sometimes with plagioclase as its predominant felspar4 ,at ,ishriff1 as already stated1 it is sprinkled with long acicular decayed crystals of hornblende2 but at the watershed the ferro-magnesian mineral is augite4 ,the surrounding rocks are mainly the plateau-basalts1 with their sills of dolerite and gabbro4 ,illustration3 ,fig4 #ceh4--,mass of dark gabbro about two feet in diameter traversed by pale veins of granophyre1 lying on north slope of ,creag na h-,iolaire1 ,mull4 ,illustration3 ,fig4 #cei4--,section at ,creag na h-,iolaire1 ,glen ,more1 ,mull1 showing basalts and gabbros resting on and pierced by granophyre4 a1 much indurated and altered basalts and dolerites2 b b1 gabbro2 c1 granophyre2 d d1 basalt dykes4 ,this strip of granophyre sends abundant apophyses from its mass into the dark basic rocks around it4 ,some of the best sections to show the nature of these offshoots are to be found on the steep hillslope which mounts from the watershed in ,glen ,more southward into the ,creag na h-,iolaire 7,eagle's ,crag71 and thence up into the great gabbro ridge of ,ben ,buy4 ,from the main body of granophyre a multitude of veins ascends through the basalts and gabbros from two feet or more in breadth down to mere filaments 7,fig4 #ceh74 ,even at a height of #cjj feet up the hill some of these veins are still three inches broad1 and present the usual granophyric structure1 though rather finer in grain than the general mass of the boss1 and sometimes assuming a compact felsitic and spherulitic texture at the immediate contact with the surrounding rock4 ,one of the most striking proofs of the posteriority of these veins is furnished by the perfect flow-structure they not infrequently exhibit along their margins1 their long felspar crystals being arranged parallel to the walls in lines that follow the sinuosities of the boundary between the two rocks4 ,patches of gabbro and of the indurated basalts may be seen lying on the granophyre1 from which veins and strings ramify through them 7,fig4 #cei74 ,similar veins can be traced upward into the main body of coarse gabbro1 forming the ridge of ,ben ,buy4 ,some of them are of the usual granular granophyric texture1 others are dull and fine-grained 7claystones of the older authors74 ,hence it is evident that the granophyres of ,mull have been protruded not only after the accumulation of the plateau-basalts1 but after these were traversed by the sheets and veins of gabbro4 ,the amount of acid rock injected into these older rocks over the mountainous part of the island is enormous2 but ,i reserve further reference to it for the section on acid ,dykes and ,veins1 for these are the forms in which it chiefly occurs in that region4 ,it should be added1 that in the localities here referred to basalt-veins and dykes are generally abundant1 cutting through all the other rocks 7,fig4 #cei74 ,so numerous are they that the geologist ceases to take note of them when his thoughts are engaged upon the problems presented by the masses through which they rise4 iii4 ,,the ,,acid ,,bosses ,,of ,,small ,,isles ,in the island of ,eigg three small bosses or sheets of acid rock occur4 ,that at the northern end rises through the ,jurassic sedimentary rocks1 and forms a bold cliff from #aej to #bjj feet high4 ,it is a light grey granophyric porphyry1 with rounded blebs of quartz in a micropegmatic base of quartz and felspar4 ,the other two masses1 of smaller size1 cut through the bedded basalts#djc 7,map ,,vi474 ,footnote #djc3 ,quart4 ,journ4 ,geol4 ,soc4 xxvii4 7#ahga7 p4 #bid4 ,in the opposite island of ,rum1 the acid protrusions play a much more important part4 ,on the east side of the hills1 they occur in sheets at the base of the gabbros2 on the west side1 they form a large tract of hilly ground1 which1 stretching along the coast-line for about three and a half miles from the headland of ,a' ,bhrideanach to ,harris1 forms there a range of shattered sea-cliffs1 that tower for #ajjj feet above the ,atlantic breakers that beat about their base4 ,the area extends inland to the slopes on the west side of ,loch ,sgathaig1 a distance of about three and a half miles1 descending in a range of precipices along its northern front1 and reaching in its culminating summit1 ,orval1 a height of #ahfh feet above the sea4 ,the rocks of which this triangular area consists resemble those of the ,mull bosses4 ,they are chiefly quartz-porphyries1 becoming felsitic in texture towards their contact with adjacent rocks4 ,in some places1 as was noticed by ,macculloch on the sea-cliffs1#djd they have a rudely bedded structure4 ,thus on the north-west front of ,orval1 this structure is shown by parallel planes that dip outwards or north-west at #cj to #dj1 and which are made still more distinct by an occasional intrusive dyke or sheet of basalt between their surfaces4 ,i have already alluded to indications of an internal arrangement in the granitoid bosses of ,skye 7p4 #cha74 ,footnote #djd3 ,western ,islands1 vol4 i4 p4 #dhg4 ,illustration3 ,fig4 #cfj4--,section on north side of ,orval1 ,rum4 a1 ,torridon sandstones2 b1 bedded basalts of ,fionn ,chro2 c1 dolerite2 d1 quartz-porphyry4 ,illustration3 ,fig4 #cfa4--;,junction of ,quartz-,porphyry 7,microgranite7 and ,basic ,rocks1 south-east side of ,orval1 ,rum4 a1 basalts and dolerites2 b1 dolerite and gabbro veins2 c1 quartz-porphyry cutting a and b4 ,as in the other islands1 the granophyres1 porphyries and felsites of ,rum have been intruded at the base of the volcanic series4 ,over much1 if not all1 of their area they lie directly on the red ,torridon sandstone4 ,that the bedded basalts once covered them is indicated by the position of the three outliers of the basalt-plateau already noticed4 ,but a fourth outlier still lies upon the porphyry of ,orval as a cake that dips gently northward4 ,it consists of a bedded1 dark1 finely-crystalline1 ophitic dolerite1 porphyritic in places1 with a rudely prismatic or columnar structure 7,fig4 #cfj74 ,it has undergone contact-metamorphism1 and tongues from the underlying rock project up into it4 ,on the south-eastern side of the same hill1 still more striking evidence is presented of the posteriority of the acid to the basic rocks4 ,the porphyry shows here the same tendency to assume a bedded structure1 the parallel beds again dipping outward or south-east at #dj4 ,they plunge under the body of gabbro1 dolerite and other intrusive masses which from this point stretch eastward into the great cones of ,allival and its neighbours4 ,the rock at the junction is a fine microgranite with traces of micropegmatite4 ,it is composed of a holocrystalline base of quartz and orthoclase1 with porphyritic crystals of microcline1 blebs of quartz and scattered granules of augite4 ,the rocks that rest immediately next it are basalt and dolerite1 into which it has sent an intricate network of veins 7,fig4 #cfa74dje ,it has also pushed long tongues down the slope into them1 which may be seen traversing the dolerite and gabbro veins that cut the basalts4 ,the basic rocks next the porphyry have been intensely altered4 ,they seem in places as if they have been shattered by some explosive force1 and had then been invaded by the mass that rushed into all the rents thus caused4 ,this remarkable structure is still better displayed on ,st4 ,kilda1 and is more fully described in the following account of the geology of that island4 ,footnote #dje3 ,in a thin slice cut from a specimen showing the junction1 there is a minute vein of the porphyry penetrating the basalt which is much altered1 while the porphyry becomes much finer in grain than at a distance from the contact4 iv4 ,,the ,,rocks ,,of ,,st4 ,,kilda ,brief allusions to ,st4 ,kilda and its rocks have already been made 7pp4 #agc1 #ceh74 ,we may now enter more fully upon the consideration of its geological structure and history4 ,when the weather is clear there may be seen from the western headlands of the ,outer ,hebrides a small blue cone rising above the ,atlantic horizon at a distance of about #fj miles4 ,as the voyager approaches this distant land it gradually shapes itself into a group of islets of which ,st4 ,kilda1 the largest and only inhabited1 has an extreme length of about four miles1 a breadth of less than two miles1 and a height of #abfb feet above the sea4 ,four miles to the north-east ,borrera1 about one square mile in extent1 rises with precipitous sides to a height of #ajjj feet4 ,off the north-western promontory of ,st4 ,kilda the huge rock of ,soay1 half a square mile in area1 towers from #fjj to #hjj feet above the waves4 ,borrera has two attendant rocks--,stack ,li and ,stack an ,armin--huge pyramidal masses several hundred feet high1 and the home of thousands of gannets4 ,st4 ,kilda possesses two less imposing islets between its north-western headland and ,soay1 and a third to the south-east known as ,levenish4 ,the scenery of this picturesque group affords a good indication of its geological structure4 ,it displays two distinct types of topographical form4 ,in ,borrera the marvellous combination of spiry ridges1 deep gullies and clefts1 notched crests and splintered pinnacles1 at once reminds the visitor of the outlines of the ,cuillin ,hills of ,skye4 ,the same features are repeated on a less magnificent scale in ,soay and along the whole of the south-western precipitous coast-line of ,st4 ,kilda4 ,in marked contrast to these varied outlines1 the eastern half of ,st4 ,kilda rises with a smooth green surface1 varied with sheets of grey screes1 up to the rounded summit of ,conagher1 the highest point in the island4 ,if the dark crags of the rest of the island group remind one of the ,cuillins1 this eastern tract recalls at once the form and colour of the ,red ,hills of ,skye4 ,a closer examination shows that in each case the topography arises from the influence of the very same rocks and geological structure as in that island4 ,there is1 however1 one aspect in which ,st4 ,kilda has no rival throughout the ,western ,isles4 ,its russet-coloured cone1 though rising on the west side with gentle green slopes from the central valley1 plunges on the eastern side in one vast precipice from a height of #ajjj feet or more into the surge at its base4 ,nowhere among the ,inner ,hebrides1 not even on the south-western side of ,rum1 is there any such display of the capacity of the youngest granite to assume the most rugged and picturesque forms4 ,it is hardly possible to exaggerate the variety of outline assumed by the rock as it yields along its system of joints to the influence of a tempestuous climate4 ,it has been carved into huge projecting buttresses and deep alcoves1 the naked stone glowing with tints of orange and fawn colour1 veiled here and there with patches of bright green slope1 or edged with fringes of sea-pink and camomile4 ,every outstanding bastion is rent with chasms and split into blocks1 which accumulate on the ledges like piles of ruined walls4 ,to one who boats underneath these cliffs the scene of ceaseless destruction which they present is vividly impressive4 ,the geology of ,st4 ,kilda was sketched by ,macculloch1 who recognized the close resemblance of its two groups of rock to the augite-rock 7gabbro7 and syenite 7granophyre7 of ,skye and other islands of the ,inner ,hebrides4 ,but he left the relations of the two groups to each other undetermined4#djf ,professor ,heddle has published a brief reference to the rocks of ,st4 ,kilda1 without1 however1 offering any definite opinion as to the geological structure of the islands4#djg ,the best account of the geology has been given by ,mr4 ,alexander ,ross1 who obtained evidence that the acid sends veins into the basic rock4 ,he brought away specimens clearly showing this relation1 but in his description left the question open for further inquiry4#djh ,to some of the observations in these papers reference will be made in the sequel4 ,the following account is based on the results of two visits paid by me to ,st4 ,kilda in the summers of #ahie and #ahif1 during which ,i was enabled to examine the rocks on land1 and to sail several times round the islands1 boating along those parts of the cliffs which presented features of special geological importance4 ,footnote #djf3 ,description of the ,western ,isles1 vol4 ii4 p4 #ed4 ,footnote #djg3 ,in an article on the general geological features of the ,outer ,hebrides contributed to ,a ,vertebrate ,fauna of the ,outer ,hebrides1 by ,j4 ,a4 ,harvie-,brown and ,t4 ,e4 ,buckley1 #ahhh4 ,footnote #djh3 ,british ,association ,report1 #ahhe1 p4 #ajdj1 and a much fuller paper in the ,proceedings of the ,inverness ,field ,club1 vol4 iii4 7#ahhd71 p4 #gb4 ,in the ,st4 ,kilda islets three groups of rock differing from each other in age may be recognized4 #ast1 ,a series of gabbros1 dolerites and basalts which have been intruded through and between each other as sills2 #bnd1 a mass of granophyre which invades these sills2 and #crd1 abundant dykes and veins of basalt which occur both in the basic and acid masses4 ,from the extension of the basalt-dykes across the ,outer ,hebrides it is clear that the ,tertiary volcanic region reached at least to within #fj miles of ,st4 ,kilda4 ,whether or not it stretched over the intervening space now overflowed by the ,atlantic must be matter for conjecture4 ,there can be no doubt that the intrusive rocks of ,st4 ,kilda are in age and origin the equivalents of those of the ,inner ,hebrides4 ,the remnants left of them were assuredly not superficial extrusions1 but are characteristic examples of the more deep-seated intrusions of the ,tertiary volcanic period4 ,down to the most minute details of structure they reproduce the features so well displayed by the gabbros and granophyres of ,skye1 ,rum and ,mull4 ,if it is demonstrable in the case of these islands that the intrusions have taken place under a deep cover of basalt-sheets1 now in large part removed1 the inference may legitimately be drawn that at ,st4 ,kilda a basalt-plateau once existed which has been more completely destroyed than in the other regions4 ,not a fragment of such a plateau has survived1 unless we may perhaps be allowed to recognize it in some of the basalts enclosed among the gabbro-sills4 ,placed far amid the melancholy main and exposed to the full fury of the ,atlantic gales1 these islets must be regarded as the mere fragmentary cores of a once much more extensive volcanic area4 ,the geologist who visits them is deeply impressed at every turn by the evidence of the active and unceasing destruction which their cliffs are undergoing4 ,nothing now remains save the deep-seated nucleus of intrusive sills1 bosses and dykes4 #a4 ,the ,gabbro ,sills4--,the rudely-bedded arrangement of these rocks is conspicuous along the west side of ,st4 ,kilda1 in ,soay and in ,borrera4 ,they consist of coarse and fine varieties disposed in successive sheets which dip at angles varying from as little as #ae up to as much as #fj or even more4 ,in ,st4 ,kilda they form the picturesque promontory of the ,dune1 and extend thence along the western side of the island to its extreme northern end4 ,their escarpments face the ocean1 and their dip-slopes descend towards the north-east in grassy declivities to the south bay and the long verdant glen which runs thence across to the north bay4 ,the same strike is prolonged into ,soay1 but further east in ,borrera the direction curves so as to present vast escarpments towards the west and shelving sheets of rock towards the east4 ,none of the gabbros seen by me are as coarse as the large-grained varieties of ,skye1 nor does there appear ever to be such a marked banded structure among them as that displayed by the ,cuillin rocks4 ,faint banding1 however1 may be noticed4 ,a series of specimens which ,i collected from the west side of the island has been sliced for microscopic examination1 and ,mr4 ,harker has furnished me with the following notes regarding them4 ,an olivine-gabbro from the west side of ,st4 ,kilda #gajg is a dark1 heavy1 medium-grained rock1 in which augite and felspar are conspicuous4 ,the microscope shows1 in addition1 plentiful grains of olivine1 with but little original iron-ore1 and some apatite-needles4 ,the structure is ophitic1 the plates of pale-brown augite enveloping both olivine and felspar4 ,a little brown hornblende and red-brown mica are probably original1 the rock showing little sign of alteration4 ,the felspar is labradorite1 with albite- and ,carlsbad-twinning1 and forms elongated rectangular crystals4 ,another specimen #gajh is a rock of similar appearance but somewhat coarser texture1 and structurally is a more typical gabbro than the preceding1 the felspar having little of the 'lath' shape1 while the augite1 though still moulded on the felspar1 scarcely assumes an ophitic habit4 ,a striking feature in this rock is the way in which the augite is crowded with 'schiller'-inclusions1 in places so closely as to be almost opaque4 ,a high magnification shows that these inclusions are dark1 linear in form1 and disposed along two directions intersecting at a high angle4 ,the labradorite has unusually close twin-lamellation on both albite and pericline laws1 and it is possible that this is a strain-effect4 ,a third specimen #gaji is from a rock in every respect identical with the preceding1 except that the olivine is rather more plentiful1 and in some grains is partially serpentinized4 ,while the gabbros of ,st4 ,kilda are not a mere uniform boss1 but a series of sills and irregular masses which have been successively injected into each other1 they have subsequently been cut through by many basalt-dykes and veins4 ,these1 which are sometimes as abundant as in the gabbro of the ,cuillin ,hills1 traverse the rocks both in the line of bedding and also at many different angles across it4 ,as they generally weather faster than the gabbros1 they give rise to deep narrow clefts which may be traced up the whole height of the precipices1 occasioning sea-caves below and sharp notches on the crests above4 ,these scenic features1 so indicative of the geological structure that causes them1 are specially well seen on the western face of the ,dune or south-western promontory of the island1 and likewise in the strangely rifted precipices further north and in ,soay4 ,they are1 however1 most impressively displayed around the naked walls of ,borrera1 which in their marvellous combination of spiry ridges1 deep straight gullies1 and splintered crests1 remind one at every turn of the scenery of ,blaven and the ,cuillin ,hills4 #b4 ,the ,granophyre ,boss and its ,apophyses4--,the eastern half of the island of ,st4 ,kilda consists of a pale rock which ,macculloch long ago identified with the granophyre of ,skye1 and which1 as he pointed out1 has much resemblance to parts of the granite of ,arran4#dji ,not only does it give rise to topographical forms like those of the ,red ,hills1 but it weathers1 like the ,skye granophyre and the ,arran granite1 into thick bed-like sheets divided by transverse joints into large quadrangular blocks4 ,on closer inspection it is found to resemble still more precisely the acid rocks of the ,inner ,hebrides4 ,it possesses the same drusy micropegmatitic structure as the granophyres of ,skye1 ,rum and ,mull4 ,the ferro-magnesian constituents are present in small quantity1 hence the pale hue of the stone4 ,the quartz and felspar project in well-terminated crystals into the drusy cavities1 which are sometimes further adorned with delicate tufts of clear crystallized epidote4 ,in these and other respects the rock displays the familiar external forms of the younger or ,tertiary granites of ,britain4 ,footnote #dji3 ,description1 vol4 ii4 p4 #ed4 ,mr4 ,harker's notes on the microscopic structure of this granophyre are as follows3--,the prevailing felspar is orthoclase1 often very turbid from secondary products4 ,even what appear to be distinct crystals are sometimes seen in the slices to be invaded on the margin by quartz in rough micrographic intergrowths1 and much of the finer intergrowth occurs as a fringe to the crystals4 ,in this case the felspar of the micropegmatite can often be verified to be in crystalline continuity with the crystal which has served as a nucleus #ffbd4 ,quartz occurs in distinct crystals and grains as well as in the micropegmatite4 ,there is a more granitoid variety of the rock1 in which only a very rude approach to micrographic intergrowths is seen #ffbc4 ,in both varieties there is but little trace of any ferro-magnesian mineral2 the more typical granophyre has what seems to be destroyed augite1 while the granitoid rock contains a little deep-brown biotite4 ,scattered crystal-grains of magnetite occur in both4 ,narrow ribbon-like veins of a finer material1 sometimes only an inch in breadth1 traverse the ordinary granophyre4 ,similar veins run through the rock of the ,red ,hills in ,skye2 they are sharply defined from the enclosing rock1 as if the latter had already solidified before their intrusion4 ,with regard to the microscopic structure of some thin slices prepared from these veins1 ,mr4 ,harker remarks that the material of the veins is of a type intermediate between granophyre and microgranite #ffbb1 #ffbc4 ,the chief bulk is a finely-granular aggregate of quartz and felspar1 the latter very turbid2 but in this aggregate are imbedded numerous patches of micropegmatite1 often of perfect and delicate structure4 ,these areas of micropegmatite show some approach to a radiate or rudely spherulitic structure1 and1 in some cases1 are clustered round a crystal of felspar or quartz4 ,some granules of magnetite and rare flakes of brown biotite are the only other constituents of the rock4 ,although they must be of somewhat later date1 there is evidently nothing in the petrographical characters of these fine-textured veins to separate them widely from the ordinary granophyres of the region4 ,these veins may be compared with the spherulitic dyke that traverses the granophyre of ,meall ,dearg at the head of ,glen ,sligachan 7described at p4 #cha71 which1 though undoubtedly somewhat younger than the rock that contains it1 yet presents the very same structures as are visible at the margin of that rock4#daj ,the material of this dyke and of the finer veins of ,st4 ,kilda and the ,red ,hills probably belongs to a later period of protrusion from a deeper unconsolidated portion of the same acid magma as at first supplied the general body of granophyre4 ,footnote #daj3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 #a4 7#ahid71 p4 #bbj4 ,undoubtedly the most interesting feature in the granophyre of ,st4 ,kilda is its junction with the mass of basic rock to the west of it4 ,this junction-line runs from about the middle of the chief or south bay 7where1 however1 its precise position is concealed under detritus7 across the island to the north shore1 where it descends the face of the precipice and plunges under the sea4 ,important as the actual contact of the two rocks obviously is in regard to their relative date1 it has not hitherto been observed or described4 ,macculloch noticed numerous fragments of trap penetrated by veins of syenite1 but he did not see these rocks in place1 and1 in spite of their apparent testimony to the posteriority of the acid intrusions1 he was inclined to believe that the veins were not real veins1 but that the trap and syenite had a common origin and would be found to pass into each other1 as he thought also occurred in ,mull and ,rum4 ,in recent years ,mr4 ,alexander ,ross1 during his visit to ,st4 ,kilda1 collected specimens illustrating the varieties of gabbro1 dolerite and basalt1 and showing the intrusion of the acid into the basic rocks4 ,as already stated1 he was disposed to regard the granite as of younger date than the gabbros1 but left the question undecided4#daa ,footnote #daa3 ,in his paper1 ,proceed4 ,inverness ,field ,club1 vol4 iii4 7#ahhd71 p4 #gh1 ,mr4 ,ross quotes a letter from ,prof4 ,judd1 who there states that the rock supposed to be granite is seen under the microscope to be a quite different rock--a quartz-diorite4 ,some of the specimens from ,st4 ,kilda collected by ,mr4 ,ross were exhibited at a meeting of the ,geological ,society on #beth ,january #ahic4 ,with regard to these ,prof4 ,judd1 in the course of the discussion on his paper on ,inclusions of ,tertiary ,granite in the ,gabbro of the ,cuillin ,hills1 remarked3--,they show a dark rock traversed by veins of a light one1 but the dark rock is not a gabbro and the light one is not a granite 7,quart4 ,journ4 ,geol4 ,soc4 vol4 xlix4 7#ahic71 p4 #aih74 ,the best locality for the examination of the junction of the main granophyre mass with the gabbros is inaccessible save by boat1 and only in the calmest weather4 ,it occurs in the great cliff on the northern side of the island between the north bay and the sea-stack known as the ,bragstack4 ,the line of contact emerges from below the sea-level1 and ascends the cliff with a westward inclination of from #fj to #hj4 ,here1 as in ,skye1 the acid rock underlies the basic masses1 which are rudely bedded and much jointed4 ,about #aej feet above the sea-level1 the nearly vertical cliff breaks up into an exceedingly rocky and rugged acclivity1 across which the junction seems to slope at a lower angle4 ,but the place is hardly reachable1 save perhaps by the intrepid1 barefooted cragsmen of ,st4 ,kilda4 ,illustration3 ,fig4 #cfb4--;,junction of granophyre and gabbro1 north side of ,st4 ,kilda4 ,along the sharply defined line of contact the granophyre is close-grained1 and sends a network of veins into the dark sheets of gabbro4 ,the general features of the junction are represented in ,fig4 #cfb4 ,the veins are narrow1 those nearest the main body of granophyre diverging from it at a still more acute angle than those from the mass of ,meall ,dearg 7,fig4 #cgf71 and then branching so as to enclose masses of the gabbro and to run across them in long parallel veins4 ,a characteristic feature of many of these veins1 besides their narrowness1 is their tendency to split up at the ends into mere fingers and threads as represented in ,fig4 #cfc4 ,owing to the depth of soil on the cultivated land1 and of boulders and sand on the beach1 the actual junction of the main body of granophyre with the gabbro is not seen on the southern shore4 ,but a few yards to the westward of where it must lie1 the beach is cumbered with large blocks of rock broken up from the mass1 which can be seen in situ a little further south forming a line of low cliff with a rugged foreshore4 ,these rocks consist of various gabbros and basalts of rather fine grain1 profusely traversed with veins of white granophyre4 ,some of these veins are two feet or more in breadth1 and1 when of that size1 show the distinctive granular texture and drusy structure of the main part of the acid rock4 ,but from these dimensions they can be traced through every stage of diminution until they become mere threads4 ,when they are only an inch or two broad1 they assume a finely granular texture like that of the veins that run through the body of the granophyre4 ,illustration3 ,fig4 #cfc4--,veins of granophyre traversing gabbro and splitting up into thin threads1 north side of ,st4 ,kilda4 ,the amount of injected material in the dark basic rocks is here and there so great as to form a kind of breccia 7,fig4 #cfd71 which1 from the contrast of tone between its two constituents1 makes a conspicuous object on the shore4 ,here1 as in the example already cited from ,rum1 the basic rocks seem to have been shattered into fragments1 and the acid material to have been injected into the minutest interstices between them4 ,the enclosed fragments are of all sizes from mere grains up to blocks a foot or more in length4 ,they are generally angular1 like rock-chips from a quarry4 ,moreover1 they are not all of the same kind of material4 ,while at this locality most of them consist of basalt1 they include also a few large and small pieces of rather coarse gabbro4 ,there has evidently been a certain amount of transport of material1 as well as an extensive disruption of the rocks in situ4 ,the granophyre here and there assumes a darker or greener tint1 as if it had dissolved and absorbed some portion of the older rock4 ,still more astonishing are the sections to be seen on the western cliffs and rocky declivities of the ridge to the north of the ,dune1 at a distance of perhaps #ejj or #fjj yards westwards from those of the ,south ,bay4 ,here the gabbro-sheets are traversed by a number of conspicuous white bands1 which on examination prove to be veins or dykes of granophyre4 ,as viewed from the sea1 the general disposition of the two groups of rocks is represented in ,fig4 #cff4 ,the broadest mass of granophyre breaks out towards the bottom of the precipice1 and slants upward as a sheet intercalated between the gabbro sills1 with a breadth of about #dj or #ej feet1 but rapidly thinning away in its ascent4 ,one of the bands below it has a breadth of about #ae feet4 ,the material of these intrusions is a pale fine-grained granophyre like that of the ,south ,bay1 ,i did not detect1 either here or anywhere else in ,st4 ,kilda1 a definite spherulitic structure such as is so common in the granophyre dykes of ,skye4 ,illustration3 ,fig4 #cfd4--,pale granophyre injected into dark basalt1 ,south ,bay1 ,st4 ,kilda4 ,the crags on the further side of the bay are the gabbro sheets of the ,dune4 7,from a photograph by ,colonel ,evans47 ,though the acid intrusions are somewhat irregular both in thickness and direction1 they lie generally parallel to each other in the line of strike of the bedding of the gabbros4 ,they are no doubt apophyses from the main body of granophyre1 which emerges to the surface about a third of a mile to the eastward1 but may of course be at no great depth underneath4 ,illustration3 ,fig4 #cfe4--,veins of granophyre traversing a line-grained gabbro and scarcely entering a coarse-grained sheet1 west side of ,rueval1 ,st4 ,kilda4 ,besides the broader bands of acid rock1 and diverging from them1 a complicated network of veins ramifies in all directions through the gabbros1 as at the ,south ,bay4 ,the extraordinary degree to which the basic rocks have been shattered into fragments is strikingly displayed here1 likewise the extreme liquidity of the acid magma1 whereby it was able to insinuate itself into every chink and cranny4 ,but the observer notices that this condition of excessive disruption is not shared by all the basic sills1 and is not attendant upon all the acid dykes4 ,as an example of this irregular distribution of the structure1 ,i give the accompanying sketch 7,fig4 #cfe71 where a fine-grained gabbro has been completely broken up and intersected with granophyre veins1 while the coarser sheet overlying it has almost entirely escaped4 ,the dark basalt-like sheets appear generally to have been much more disrupted than the more largely-crystalline varieties4 ,it is noticeable here1 also1 that the fragments entangled in the network of granophyre veinings do not entirely belong to the rock that has been shattered1 but sometimes include large and small lumps of different gabbros1 showing some transference of material with the inrush of the acid magma4 ,though closer in grain where it comes in contact with the gabbro1 the granophyre never assumes any vitreous texture along its margin4 ,a series of thin slices1 prepared from specimens collected by me in the ,south ,bay in the summer of #ahie1 was examined by ,mr4 ,harker1 who furnished the following notes regarding them3--,the basalt traversed by the granophyre is a fine-textured variety with small porphyritic felspars4 ,these latter seem to be usually unaltered1 retaining the glass cavities which in some of the crystals are abundant4 ,the groundmass1 however1 shows minerals of metamorphic origin which must be derived mainly from the original augite4 ,a brown mica is the most conspicuous2 but with it are associated some brownish-green hornblende and certain chloritic and perhaps serpentinous substances4 ,it is chiefly near the margin of a fragment of basalt that the mica gives place to these minerals4 ,the basalt still retains plenty of unaltered granules of augite in the central parts of a fragment4 ,it is not certain that the secondary minerals named come exclusively from the augite of the basalt2 from their form and mode of occurrence they may in part have replaced olivine or even rhombic pyroxene4 ,illustration3 ,fig4 #cff4--,view of sills and veins of pale granophyre traversing dark sheets of gabbro1 west side of ,st4 ,kilda4 7,from a photograph by ,colonel ,evans47 ,the acid rock1 though styled granophyre above1 belongs to a granitoid variety of that group of rocks1 and has but little indication of micrographic structures4 ,compared with the other granophyres from ,st4 ,kilda1 sliced and examined1 these examples show a less acid composition4 ,this is expressed mineralogically in the presence of a somewhat larger proportion of ferro-magnesian minerals and of soda-lime felspar4 ,these features might indeed be matched in many normal granophyres among the ,western ,isles1 but in the present case it can hardly be doubted that they are to be explained1 at least in some degree1 by the acid magma having taken up a certain amount of material from the basalt4 ,many of these ,tertiary granophyres have undoubtedly been modified by the incorporation of pieces of basalt and gabbro1 and a collection made in the ,strath district of ,skye will furnish examples for future study4 ,professor ,sollas's description of similar phenomena in the ,carlingford district has already proved the importance of this kind of action4#dab ,in the present instance1 both brown mica and hornblende occur plentifully in the granophyre1 and especially round the basalt fragments4 ,this latter point is conclusive as to the derivation of the basic material1 and further proves a certain degree of viscosity in the acid magma at the time of its intrusion4 ,footnote #dab3 ,trans4 ,roy4 ,irish ,acad4 vol4 xxx4 7#ahid71 pp4 #dgg-egb4 ,another series of specimens which ,i collected in the following year was submitted to ,mr4 ,harker for petrographical determination1 and his observations on two of the microscopic slices are as follow3 ,a breccia from the ,south ,bay1 ,st4 ,kilda #gaje1 consists of angular fragments up to two inches in diameter set in a matrix of grey granophyre of medium texture4 ,the fragments belong to two types--one of very close texture 7basalt71 the other more evidently crystalline 7diabase74 ,both are cut by the slice4 ,the basalt shows very evident metamorphism1 its augite being wholly transformed into greenish-brown hornblende4 ,the little felspar-laths and granules of iron-ore seem to be unaltered1 though the latter may perhaps have contributed to the formation of the hornblende4 ,another fragment of basalt has some larger crystal-grains of augite1 and these are not converted into hornblende4 ,the diabase shows a less marked boundary under the microscope1 but otherwise has similar characters to the preceding4 ,the striated felspar-crystals and grains of iron-ore have not been re-crystallized4 ,a considerable amount of pale augite remains1 but there is also plenty of deeply-coloured hornblende1 both fibrous and compact4 ,this diabase is certainly an intrusive rock1 but the basalt1 from its petrographic character1 might be from a lava-flow or from a dyke4 ,the granophyre is of somewhat coarse texture1 the micrographic structure being only of a rude type4 ,it is notably richer in the darker constituents than is usual in such rocks4 ,further1 the hornblende and magnetite tend to cluster in little patches which suggest destroyed fragments of basic rocks4 ,a grain or two of sphene occur1 a mineral foreign to the normal granophyres4 ,another similar specimen #gajf from the same locality shows a basic rock of coarser texture1 approaching some of the gabbros in appearance and with boundaries in places not very sharply defined4 ,the grey matrix is again relatively rich in the dark elements1 and the manner in which they occur in little patches1 like nearly obliterated 'xenoliths1' points unmistakably to a certain amount of absorption of basic material by the acid magma1 with consequent enrichment in the ferro-magnesian minerals4 ,the slice cuts only the acid rock1 which is seen to be of granitoid rather than granophyric structure1 though the tendency of the felspar to enclose quartz-grains is unlike a typical granite4 ,oligoclase1 with combined albite- and ,carlsbad-twinning1 is well represented in addition to orthoclase1 and some zoned crystals seem to be of albite with a border of oligoclase4 ,brown hornblende and a little brown mica are the coloured constituents4 ,magnetite and apatite are also observed4 ,the testimony of the rocks of ,st4 ,kilda to the posteriority of the granophyre to the gabbros and basalts is thus clear and emphatic4 ,it entirely confirms my previous observations regarding the order of sequence of these rocks in ,mull1 ,rum and ,skye4 ,but the ,st4 ,kilda sections display1 even more strikingly than can be usually seen in these islands1 the intricate network of veins which proceed from the granophyre1 the shattered condition of the basic rocks which these veins penetrate1 the remarkable liquidity of the acid magma at the time of its intrusion1 and the solvent action of this magma on the basic fragments which it enveloped4 #c4 ,the ,basic ,dykes4--,reference has already been made to the numerous dykes by which the gabbros of the ,st4 ,kilda group of islets is traversed4 ,similar dykes occur also1 though less plentifully1 in the granophyre4 ,it remains for future observation to determine whether there is one series older and another later than the intrusion of the acid rock4 ,in any case1 it is quite certain that the dykes in the gabbro do not all belong to one period of injection1 for frequent examples of intersection may be noticed1 especially on the cliffs of ,borrera1 and also cases of double and even treble dykes which have been formed by successive infillings within the same fissure4 ,the remarkably varied precipices of that island are marked by the long narrow rifts left by the weathering of vertical dykes1 which1 as above remarked1 may be followed with the eye from the sea-level to the sky-line1 ascending obliquely across the bedding of the gabbro sheets4 ,another group of dykes may be traced sloping upward at low angles along the face of the cliffs and affording admirable ledges with overarching roofs for innumerable gannets1 kittywakes and guillemots4 ,other dykes and ribbon-like veins may be seen traversing the gabbro in many different directions1 precisely as among the ,cuillin ,hills4 ,as no similar network of dykes and veins is to be observed in the granophyre1 ,i am disposed to regard a large number of these intrusions as older than that rock4 ,but ,i did not observe any actual example of a basic dyke truncated by the granophyre4 ,there can be no doubt1 however1 that an injection of similar dykes and veins took place after the invasion of the granophyre4 ,these later intrusions are conspicuously displayed along the cliffs that extend from the gabbro junction on the north side of ,st4 ,kilda round the eastern coast into the ,south ,bay4 ,they maintain a general parallelism and ascend from the sea-level at varying angles of inclination1 running up the pale sea-wall as dark bands4 ,they consist of basalt-rocks1 and may often be seen to branch and to die out4 ,like those in the gabbro1 they are not infrequently compound1 being made up of two or three or even more distinct dykes4 ,this is well seen on the great precipice below ,conacher1 where the section given in ,fig4 #cfg is displayed4 ,here in a vertical height of about #hjj or #ijj feet1 there must be at least seven dykes1 simple and compound4 ,a little further south a triple dyke may be seen to be composed of a thick central zone and two thinner marginal bands1 of which the lower strikes off from the others and maintains an independent course through the granophyre 7,fig4 #cfh74 ,illustration3 ,fig4 #cfg4--,section of the sea-cliff below ,conacher1 ,st4 ,kilda1 showing basic dykes in granophyre4 ,illustration3 ,fig4 #cfh4--,triple basic dyke1 sea-cliff1 east side of ,st4 ,kilda4 ,v4 ,,the ,,granite ,,of ,,arran ,the northern half of the island of ,arran is mainly occupied by one of the most compact and picturesque groups of granite mountains in ,scotland4#dac ,these heights1 rising out of the ,firth of ,clyde to a height of #bhff feet1 present1 in their spiry and serrated crests1 a contrast to the smoother contours of the older granitic elevations of this country4 ,the granite is surrounded by a ring of schistose rocks1 belonging to the metamorphic series of the ,southern ,highlands1 save for a short distance on the eastern margin1 where it comes in contact with and indurates the ,lower ,old ,red ,sandstone4 ,macculloch long ago pointed out that no pebbles of the granite are to be found in the surrounding conglomerates and red sandstones of ,carboniferous and younger age4#dad ,geologists accordingly came to the conclusion that the protrusion of the granite took place after ,carboniferous time1 and hence that it had no connection with the appearance of the far older granites of the ,highlands4 ,in the year #ahgc ,i gave reasons for believing the granite to be not only younger than the ,carboniferous formations1 but to be referable with most probability to the ,tertiary volcanic series4#dae ,the progress of inquiry has tended to confirm this inference1 though no direct proof of its correctness has been obtained4 ,two lines of investigation may be pursued1 and each leads to the conclusion of the probability of the ,tertiary age of the granite4 ,one of these proceeds on a comparison of the petrographical characters of the ,arran rocks with those of undoubted members of the ,tertiary series among the ,western ,isles4 ,the other inquiry deals with the relation of the rocks to each other in the general geological structure of ,arran itself4 ,footnote #dac3 ,the rocks of ,arran have often been described4 ,besides the work of ,macculloch above quoted1 reference may be made to the paper by ,sedgwick and ,murchison1 ,trans4 ,geol4 ,soc4 #bnd ,ser4 vol4 iii4 p4 #ba2 ,a4 ,c4 ,ramsay's ,geology of the ,island of ,arran1 #ahda1 the paper of ,necker de ,saussure quoted on p4 #dab2 ,j4 ,bryce's ,geology of ,clydesdale and ,arran1 #crd edit4 #ahfe4 ,the island is at present being surveyed for the ,geological ,survey by ,mr4 ,w4 ,gunn4 ,footnote #dad3 ,description of the ,western ,islands of ,scotland1 vol4 ii4 p4 #chh4 ,footnote #dae3 ,trans4 ,edin4 ,geol4 ,soc4 vol4 ii4 part iii4 ,macculloch first remarked the strong lithological resemblance of the ,arran granite to the syenite1 or granophyre1 of ,skye and ,st4 ,kilda4#daf ,more recent petrographical investigation1 as already stated1 has furnished additional proofs of the connection between the acid rocks of these islands4 ,so closely indeed are these rocks linked by megascopic and microscopic characters1 that the petrologist has no hesitation in placing them together as probably products of the same period of igneous activity4 ,footnote #daf3 ,description1 vol4 ii4 p4 #ceb4 ,from the general geological structure of ,arran1 a further strong argument may be deduced in favour of the late date of the eruptions of granite4 ,good reasons have been given for classing as ,permian the bright red sandstones which occupy much of the central and southern parts of this island1 and include the little volcanic group already referred to4 ,these sandstones have been invaded by a complex series of eruptive rocks which would thus be later than the ,permian period4 ,no igneous masses posterior to this period are certainly known in ,britain save those of ,tertiary age4 ,the larger body of granite in the northern half of the island nowhere comes into direct contact with the newer red sandstones1 but these strata are pierced by smaller bodies of granite4 ,hence1 both by the evidence of their internal structure and by the stratigraphy of the ground1 the later igneous rocks of ,arran may be reasonably grouped together as one important and consecutive series1 comparable in age and general characters with those of ,tertiary date in the ,inner ,hebrides4 ,illustration3 ,fig4 #cfi4--;,jointed structure of the granite near the top of ,goatfell ,arran4 7,from a photograph by ,mr4 ,w4 ,douglas1 lent by the ,scottish ,mountaineering ,club47 ,the igneous rocks of ,arran1 later than the probably ,permian sandstones1 range from acid to basic in composition4 ,besides the northern granite1 there are in the southern part of the island acid rocks that include granite1 coarse-grained quartz-porphyry and fine-grained felsite4 ,where the relations of these rocks to each other can be seen1 the felsite is found by ,mr4 ,gunn to be newer than the porphyry1 into which it sends sills and dykes4 ,a feature observed by the same geologist in ,arran offers a further point of resemblance to the acid sills and dykes of ,skye4 ,he has noticed that accompanying the quartz-porphyry of ,drumadoon and ,bennan1 a mass of basic rock forms a kind of fringe or selvage round it1 sometimes with what appears to be a rock of intermediate character between them4 ,basic sills are abundant south of ,glen ,ashdale1 though to the west of ,whiting ,bay most of the intrusive sheets are of acid material4 ,some of the quartz-porphyry sheets are markedly columnar4 ,one of them1 near ,corriegills1 displays a divergent grouping of the prisms1 not unlike parts of the pitchstone sheets of ,eigg and ,hysgeir1 and suggestive of the rock having flowed along a hollow like that of a valley4 ,no certain trace1 however1 has been found of any ,tertiary lava-stream in ,arran1 nor has evidence of tuffs been detected in any part of the younger igneous series4 ,all the rocks appear to be intrusive1 though so abundant and varied are they as to indicate that they belong to a vigorous eruptive centre1 which may have poured out at the surface lavas and ashes1 since entirely removed by denudation4 ,the numerous basic dykes for which the south end of ,arran has long been celebrated have a general northerly trend1 and appear to be all of the same or nearly the same age4 ,they undoubtedly cut through the quartz-porphyries and the coarse-grained basic sills1 but are less numerously visible in the finer-grained basic sills1 while in the felsitic sheets they are seldom to be seen4 ,in several places dykes running in an ,e4,n4,e4 direction cut the others1 and are therefore of later date4#dag ,the compound dykes of ,tormore on the west side of the island have been already noticed 7p4 #afa74 ,footnote #dag3 ,ann4 ,rep4 of ,geol4 ,surv4 for #ahid1 p4 #bhf4 ,,vi4 ,,the ,,north-,,east ,,of ,,ireland ,in the north-eastern counties of ,ireland there are two regions which afford ample material for discussion in connection with the protrusion of acid rocks during the ,tertiary volcanic period4 ,one of these1 which for distinction may be called the ,carlingford region1 embraces the tract of country which includes the ,mourne ,mountains on the north-east side of ,carlingford ,lough and the ranges of ,slieve ,foye and ,slieve ,gullion on the south-west side4 ,the other lies mainly within the basaltic plateau1 the largest of its scattered portions forming parts of the hills of ,carnearny and ,tardree in the county of ,antrim 7,map ,,vii474 #a4 ,the ,carlingford ,region a4 ,the ,mourne ,mountains4--,this compact and picturesque group of hills1 about twelve miles long and six miles broad1 and reaching a height of #bgih feet in ,slieve ,donard1 presents a comparatively simple geological structure1 since it consists almost entirely of granitic rocks which pierce1 overlie and underlie ,upper ,silurian grits and shales4 ,so far as regards the contact of these rocks with the disrupted sedimentary formations1 all that can be asserted is that the granite must be later than at least the older part of the ,upper ,silurian period4 ,but for at least two reasons1 the eruptive rocks may be regarded with some confidence as part of the ,tertiary series4 ,in the first place1 there is a strong petrographical resemblance between the ,mourne ,mountain granite and that of the ,island of ,arran and the granitic parts of the granophyre of the ,western ,isles4 ,and this resemblance is so close as to furnish a cogent argument in favour of grouping all these rocks together as parts of one geologically contemporaneous series4 ,in the second place1 the ,mourne ,mountain granite abruptly cuts off a large number of basic dykes which1 running in a general ,n4,n4,w4 direction1 may be looked upon as almost certainly members of the ,tertiary system of protrusions4 ,the manner in which the granite of the district behaves towards certain detached areas of ,silurian strata with their accompanying dykes is one of the most astonishing features in the whole assemblage of intrusive rocks in ,britain4 ,as has been excellently shown in the ,geological ,survey ,map and sections by ,mr4 ,w4 ,a4 ,traill1 the granite has carried up on its surface broad cakes of vertical ,silurian strata1 together with all their network of dykes4#dah ,a cake of this kind1 from #ej to about #bjj feet thick and nearly two miles broad1 has been bodily uplifted from the rest of the mass and carried upward by the granite1 so that the truncated ends of the beds of grit and shale with their system of dykes stand upon a platform of granite1 from which also numerous veins penetrate them4 ,there can be little doubt that the basic dykes thus broken through are parts of the great ,tertiary system1 and if so1 the granite which disrupts them cannot be older than ,tertiary time4 ,footnote #dah3 ,see ,sheets #fj1 #fa and #ga of the one-inch map of the ,geological ,survey of ,ireland1 and ,sheets #bb1 #bc and #bd of the ,horizontal ,sections4 ,the ,explanation to these ,sheets of the map was written by ,professor ,hull1 ,mr4 ,traill having previously retired from the service4 ,the ,mourne ,mountain area is now undergoing critical revision by ,prof4 ,sollas for the ,geological ,survey1 and important additional material for the elucidation of this district may be expected from him4 ,besides the older basic dykes disrupted by the granite1 a younger but much less abundant series traverses that rock1 and also follows a general north-westerly direction4 ,these later dykes in some cases cross more acid dykes which have risen through the granite4 ,there is no trace of any superficial discharge from the ,mourne ,mountain area4 ,but from the analogy of other districts we may easily conceive that the granite represents the underground parts of volcanic material which has now been entirely removed4 b4 ,slieve ,foye and ,barnavave ,district4--,this area embraces the mountainous ground lying between ,carlingford ,lough and ,dundalk ,bay1 and culminating in ,slieve ,foye 7#aice feet74 ,it measures roughly about six miles in extreme length and four miles in breadth4 ,the remarkable assemblage of basic and acid materials in this area has received considerable attention from geologists4 ,the relative order of the two groups of rocks was first clearly recognized by ,griffith1 who showed that the granite 7granophyre7 is intruded into the gabbro4#dai ,professor ,haughton subsequently confirmed this observation1 and proved the post-,carboniferous date of the intrusive materials1 which he compared with those of ,skye4#dbj ,the general distribution of the rocks was traced out in some detail by the ,geological ,survey1 and described in the official ,memoirs4#dba ,more recently the district has been examined by ,professor ,sollas1 who1 bringing the photographic camera and the microscope to the aid of field-geology1 has elucidated the structure and relations of the rocks1 and has obtained abundant evidence that the acid and basic rocks maintain there the same relative order as among the ,inner ,hebrides4#dbb ,footnote #dai3 ,journ4 ,geol4 ,soc4 ,ireland 7#ahdc71 p4 #aac4 ,footnote #dbj3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xii4 7#ahef71 p4 #aga2 xiv4 p4 #cjj2 and ,journ4 ,geol4 ,soc4 ,ireland 7#ahgf71 p4 #ia4 ,footnote #dba3 ,sheet #ga of the ,geol4 ,surv4 ,ireland1 and accompanying ,explanation4 ,these were the work of ,mr4 ,w4 ,a4 ,traill4 ,footnote #dbb3 ,trans4 ,roy4 ,irish ,acad4 vol4 xxx4 7#ahid71 p4 #dgg4 ,this is part i4 of what is intended to be a series of papers4 ,one of the first features in this tract of country to arrest the eye of the geologist is the situation of this centre of protrusion and that of ,slieve ,gullion along a north-west line1 coincident with the general direction of the numerous basic dykes of the region4 ,whether or not the successive intrusions took place contemporaneously in the two areas1 they have followed each other in the same order4 ,in the ,barnavave district the igneous rocks occupy an area of about #bj square miles4 ,they consist of a central and chief mass composed of acid materials1 which have risen through the basic rocks now found as an interrupted ring round them4 ,in his more recent examination1 ,prof4 ,sollas has devoted special attention to the influence of the solvent action of the acid magma upon the basic rocks and upon its own composition and structure4 ,besides confirming the work of previous observers as to the order of appearance of the two kinds of material1 he has obtained evidence that the gabbro had not only completely solidified1 but was traversed by contraction-joints1 possibly even fractured by earth-movements1 before the injection of the granophyric material4 ,he found that this material1 like that of the ,inner ,hebrides and ,st4 ,kilda1 must have been in a state of great fluidity at the time of its intrusion1 and made its way into the minutest cracks and crevices4 ,in observing the solvent action of the granophyre1 he ascertained that this action took place even in comparatively narrow dykes1 which probably consolidated at no great depth beneath the surface4#dbc ,footnote #dbc3 ,op4 cit4 c4 ,the ,slieve ,gullion ,district4--,this area is separated from that just described by a narrow strip of ,silurian strata1 so that its isolation as a separate igneous district is complete4 ,it will be observed from the map to continue the same north-westerly line as the ,slieve ,foye tract1 the two together running in that direction for a distance of some #af miles4 ,it is interesting to note the adoption of this predominant north-westerly trend even by eruptive masses which were mainly of acid material4 ,this district measures about ten miles in length by from one to five miles in breadth4 ,the rocks are1 on the whole1 similar to those in the area south of ,carlingford ,lough1 and bear the same relation to each other1 the acid being intrusive in the basic series4 ,it is worthy of remark that the ,tertiary eruptive rocks have made their appearance in the midst of the older granite of ,newry4 ,this granite has been already alluded to as disrupting ,upper ,silurian strata1 and being probably of the age of the ,lower ,old ,red ,sandstone 7vol4 i4 p4 #bij74 ,in long subsequent ages1 after protracted denudation1 during which its cover of ,silurian and ,carboniferous formations was stripped off and it was laid bare1 it was broken through by the whole series of basic and acid protrusions of ,slieve ,gullion4 ,this district is portrayed on ,sheets #ei1 #fj1 #gj and #ga of the ,geological ,survey of ,ireland1 which show a central core of basic and acid material piercing the ,newry granite4#dbd ,round this core and touching it at its north-western and south-eastern end1 but elsewhere separated from it by a space of several miles1 runs a curiously continuous band of igneous material which is marked as quartziferous porphyry and felstone-porphyry on the ,survey maps4 ,footnote #dbd3 ,the ground was chiefly mapped and described by ,mr4 ,joseph ,nolan and ,mr4 ,f4 ,w4 ,egan4 ,the south-western portion of this elliptical ring possesses a peculiar interest from its including certain remarkable masses of breccia or agglomerate4 ,these rocks have been mapped by ,mr4 ,nolan1 and are described by him in the official ,explanation1 but in more detail in two separate papers4#dbe ,having had an opportunity of paying a brief visit to the ground1 ,i can confirm the general accuracy of his mapping and description1 and am able to add a few further particulars to the facts enumerated by him4 ,footnote #dbe3 ,sheet #gj of the ,geol4 ,surv4 ,map of ,ireland and ,explanation thereto2 also ,journ4 ,roy4 ,geol4 ,soc4 ,ireland1 vol4 iv4 7#ahgg71 p4 #bcc2 ,geol4 ,mag4 #ahgh4 ,the tract of ground where these agglomerates appear forms a prominent ridge which rises several hundred feet above the lower country on either side1 and extends in a ,w4,n4,w4 direction for about seven miles1 nearly along the line of junction between the ,newry granite and the ,silurian strata4 ,the ridge has a breadth varying from a few hundred yards to upwards of a mile4 ,it is separated from the main igneous mass of the ,slieve ,gullion area by an intervening strip of lower ground from three-quarters of a mile to about a mile and a half in width1 which is occupied by the ,newry granite4 ,at the north-west end of the ridge the newer eruptive rocks lie within the area of that granite1 while at the south-east end they rise entirely amongst the ,silurian strata4 ,beginning at the south-eastern extremity1 we find the agglomerate occupying several detached eminences and surrounded by altered ,silurian grits and shales4 ,further west the rock occurs in larger and more continuous masses1 appearing at intervals1 especially along the southern borders of the quartz-porphyry which forms by much the greater part of the ridge4 ,actual junctions of the agglomerate with the older rocks around seem to be seldom visible4 ,i found one1 however1 above the gamekeeper's house on the southern flanks of the hill called ,tievecrom4 ,the ,upper ,silurian grits and shales1 in a much indurated and shattered condition1 are there traceable for several hundred feet up the slope1 until they are abruptly cut off by the agglomerate4 ,the line of separation appears to be nearly vertical1 the truncated ends of the strata being wrapped round by the mass of fragmental material4 ,the most remarkable features of this agglomerate1 which has been well described by ,mr4 ,nolan1 are the notable absence of truly volcanic stones in it1 and the derivation of its materials from the rocks around it4 ,i found only one piece of amygdaloid1 but not a single lump of slag1 no bombs1 no broken fragments of lava-crusts1 and no fine volcanic dust or enclosed lapilli4 ,the rock may be said to consist entirely of fragments of ,silurian grits and shales where it lies among these strata1 and of granite where it comes through that rock4 ,blocks of these materials1 of all sizes up to two feet in breadth1 are confusedly piled together in a matrix made of comminuted debris of the same ingredients4 ,the agglomerate on the ridge of ,carrickbroad has no definite boundary1 but seems to graduate into an andesitic rock1 and then into a quartz-felsite or rhyolite4 ,this apparent gradation is one of the most singular features of the ridge4 ,the andesite resembles some of the porphyrites of the ,old ,red ,sandstone4 ,it is close-grained1 with abundant minute felspar-laths1 and numerous large porphyritic felspars1 which latter are sometimes aggregated in patches1 as in the old porphyries of ,portraine1 ,lambay ,island and the ,chair of ,kildare4 ,this rock has undoubtedly been erupted at the time of the formation of the agglomerate1 or at least before the loose materials were compacted together2 for it is full of separate stones of the same materials1 and becomes so charged with them as to become itself a kind of agglomerate1 with a small proportion of andesitic matrix cementing the blocks4 ,a thin slice prepared from one of the specimens obtained by me from this hill has been studied by ,mr4 ,watts1 who reports that the fine-grained andesitic matrix in which the stones are imbedded has often been injected into their minute fissures1 and that the minute fragments enclosed in this matrix consist here of a trachyte-like porphyry1 felsite1 andesites1 basalts of various degrees of fineness and olivine-basalt1 together with isolated grains of felspar1 such as might have been derived from the breaking up of some of these fragments4 ,westward from ,carrickbroad1 the chief eruptive rock is a dark1 sometimes nearly velvet-black1 flinty1 occasionally almost resinous1 quartz-porphyry or rhyolite1 with abundant quartz and large felspars and occasional well-marked flow-structure4 ,this material1 near the much smaller protrusion of andesite1 is curiously mixed up with that rock1 as if the two had come up together4 ,sometimes they seem to pass into each other1 at least the separation between them cannot be sharply drawn4 ,there can be little doubt1 however1 that the acid magma continued to ascend after the other1 for it sends veins and strings into the more basic material1 and encloses blocks of it4 ,this thoroughly acid porphyry plays the same part as the andesite in regard to the stones of the agglomerate4 ,throughout its whole extent1 it is found to enclose these stones1 which here and there become so numerous as to form the main bulk of the mass1 leaving only a limited amount of quartz-porphyry 7rhyolite7 matrix to bind the whole into an exceedingly compact variety of breccia4 ,occasionally the acid rock cuts through the ordinary clastic agglomerate1 as may be well seen on the southern face of ,tievecrom4 ,a specimen of this porphyry with its enclosed fragments1 which was collected by me from above the old tower at ,glendovey1 ,carrickbroad1 has been sliced and examined by ,mr4 ,watts under the microscope1 and is thus described by him3 ,the large fragment in this slide consists of ophitic olivine-dolerite full of large phenocrysts of olivine4 ,it is broken up and penetrated by veins of quartz-porphyry1 rich in quartz1 which exhibits a beautiful flow-structure4 ,the felspars and augite of the dolerite do not appear to have suffered much alteration at the margin of the fragment1 but the olivines are much serpentinized1 the serpentine passing into a border of actinolite which runs in veins into the neighbouring rock and even passes out into the quartz-porphyry at the junction1 impregnating it with actinolite and chlorite for some distance4 ,a few particles of basalt also occur and a portion of a granite-fragment comes into the slide1 from the edge of which a piece of biotite has floated off into the quartz-porphyry4 ,the essentially non-volcanic material of the agglomerate shows1 as ,mr4 ,nolan pointed out1 that it was produced by riform explosions1 which blew out the ,silurian strata and granite in fragments and dust4 ,these discharges probably took place either from a series of vents placed along a line of fissure running in a north-westerly line1 or directly from the open fissure itself4 ,possibly both of these channels of escape were in use2 detached vents appearing at the east end and a more continuous discharge from the fissure further west4 ,after the earliest explosions had thrown out a large amount of granitic and ,silurian detritus1 andesitic lava rose in the fissure1 and solidifying there enclosed a great deal of the loose fragmentary material that fell back into the chasm4 ,subsequently1 and on a more extensive scale1 a much more acid magma ascended from below1 likewise involving and carrying up a vast quantity of loose stones1 among which are pieces of basalt and dolerite4 ,no evidence remains as to the extent of the material discharged over the surface from this fissure4 ,denudation has removed all the surrounding fragmental sheets as well as any lava that may have flowed out upon or become intercalated among them4 ,there remains now only the cores of the little necks at the east end1 and the indurated agglomerate and lava that consolidated along the mouth of the fissure or vents4 ,this is the only example of such a line of fissure-eruption which has yet been met with in the ,british ,isles4 ,its connection with the eruptive masses of ,slieve ,gullion and ,carlingford links it with the ,tertiary volcanic series4 ,but no evidence appears to remain regarding the epoch in the long volcanic period when the eruptions from it took place4 ,they may possibly date back to the time of the plateau-basalts2 but the abundant acid magma1 which constitutes one of their distinguishing characteristics1 suggests that they more probably belong to the later time when the main protrusions of acid material took place4 ,they suggest that coeval with the uprise of the great domes of ,slieve ,gullion1 ,carlingford and the ,mourne ,mountains there may have been many superficial eruptions of which1 after prolonged denudation1 all trace has now been effaced4 #b4 ,the ,antrim ,region ,reference was made in ,chapter xxxvii4 to the occurrence of rhyolitic conglomerate and tuff between the lower and upper series of basalts in the ,antrim plateau1 and to the evidence furnished by these detrital deposits either that masses of rhyolite appeared at the surface1 or that rhyolitic ashes were discharged from volcanic vents in the long interval that elapsed between the two groups of basalt4 ,the further consideration of this question1 and an account of the rhyolite bosses1 were reserved for the present chapter1 that they might be taken in connection with the other acid eruptions of ,tertiary time in ,britain4#dbf ,footnote #dbf3 ,for an early account of the ,antrim trachytic rocks1 see ,berger1 ,trans4 ,geol4 ,soc4 iii4 7#ahaf71 p4 #aij4 ,professor ,hull has described the ,tardree rock in the ,explanation to ,sheets #ba1 #bh and #bi1 ,geol4 ,survey of ,ireland 7#ahgf71 p4 #ag1 and has supposed it to be older than the basalts1 referring it to the ,eocene period 7,physical ,geology and ,geography of ,ireland1 #bnd edit4 7#ahia71 pp4 #hg1 #ie74 ,duffin 7quoted by ,mr4 ,kinahan7 believed that the trachytes occur at the centre of eruption1 and were probably poured out at the end of the outburst4 ,du ,noyer also 7quoted by the same writer7 thought them to be newer than the plateau-basalts1 and to have lifted up masses of these rocks4 ,mr4 ,kinahan himself 7,geology of ,ireland1 p4 #agb7 has pointed to the absence of any rhyolitic fragments between the basalts as an argument against the supposed antiquity of the acid protrusions4 ,a petrographical account of the ,tardree rock is given by ,von ,lasaulx in the paper already cited1 ,tschermak's ,min4 ,pet4 ,mittheil4 7#ahgh71 p4 #dab4 ,a more elaborate discussion of the petrography by ,prof4 ,cole will be found in the ,memoir above referred to 7,scientif4 ,trans4 ,roy4 ,dublin ,soc4 vol4 vi4 #ahif71 and the geological relations of the rocks are discussed by him in another shorter paper1 ,geol4 ,mag4 7#ahie71 p4 #cjc4 ,see also ,mr4 ,m',henry on the trachytic rocks of ,antrim1 ,geol4 ,mag4 7#ahie71 p4 #bfj1 and ,proc4 ,geol4 ,assoc4 vol4 xiv4 7#ahie71 p4 #adj4 ,with one exception1 all the known protrusions of acid material in the ,antrim area lie within the limits of the basalt-plateau 7see ,map4 ,no4 ,,vii474 ,they occur along a line at intervals for a distance of about #ag miles1 from ,templepatrick to a point four miles north of ,ballymena4 ,it is worthy of remark that here again the line of protrusion has a north-west trend4 ,it not improbably indicates the position of a fissure up which the acid material rose at various points4 ,the petrography of the rocks has been frequently discussed4 ,they include several varieties of rhyolite1 generally rather coarsely crystalline1 but sometimes becoming compact1 and even passing into dark obsidian4 ,no undoubted tuff occurs associated with them in any of the exposures1 nor do the rhyolites anywhere display structures that point to their having flowed out at the surface4#dbg ,that the masses now visible may have communicated with the surface is quite conceivable1 but what we now see appears in every case to be a subterranean and not a superficial part of the protrusion4 ,footnote #dbg3 ,at ,sandy ,braes an exposure is visible of what at first might be thought to be a volcanic conglomerate1 but closer examination shows the rock to consist of obsidian1 which decomposes into a clay1 leaving round sharply-defined glassy cores enclosed in the decayed material4 ,the banded rhyolites do not exhibit any kind of flow-structure that may not be met with in dykes and bosses4 ,nor have any satisfactory traces been found of vesicular or pumiceous bands such as might mark the upper surfaces of true lava-streams4 ,professor ,cole has described what he calls ,the ,volcanoe of ,tardree 7,geol4 ,mag4 ,july #ahie74 ,if the ,tardree mass ever was a volcano1 which is far from improbable1 its superficial ejections have long ago disappeared4 ,at least1 after the most diligent search1 ,i have been unable to discover any trace of them1 all that now remains appearing to me to be the neck or core of protruded material4 ,illustration3 ,fig4 #cgj4--;,intrusive rhyolite in the ,lower ,basalt group of ,antrim1 ,templepatrick4 #a #a1 ,chalk2 #b #b1 ,gravel2 #c #c1 ,bedded basalt2 #d #d1 ,rhyolite1 intrusive4 ,most of the rhyolitic exposures are extremely limited in area--mere little knobs1 sometimes rising in the middle of a bog1 and never forming conspicuous features in the landscape4 ,the relation of these rocks to the basalts are generally concealed1 but the isolation of the small rhyolitic patches leaves no doubt that they are intrusive as regards the surrounding basalts4 ,this relation is well seen at ,templepatrick1 where it was first observed by ,mr4 ,m',henry of the ,geological ,survey 7,fig4 #cgj74 ,the rhyolite there forms a sill which has been thrust between the basalts and the gravel that underlies them1 the basalts being bent back and underlain by the acid rock4#dbh ,footnote #dbh3 ,the progress of quarrying operations during the last eight years has somewhat destroyed the section as exposed in #ahhh4 ,but we now see that the basalt has not only been bent back but is underlain by the acid rock4 ,the largest and most interesting of the ,antrim rhyolite tracts covers a space of about ten square miles in the heart of the basalt-plateau to the north-east of the town of ,antrim4 ,it rises to about #ajjj feet above the sea1 and forms a few featureless hills1 some of which are capped with basalt4 ,the best known localities in this tract are ,tardree and ,carnearny4 ,the rock is chiefly a somewhat coarse-textured lithoidal rhyolite1 but includes also vitreous portions4 ,illustration3 ,fig4 #cga4--,section across the southern slope of ,carnearny ,hill1 ,antrim4 a a a1 bedded basalts2 b1 rhyolite4 ,owing to the cover of soil and turf1 the junction of this mass with the surrounding basalts cannot be so clearly seen as in the sections of the ,inner ,hebrides1 and hence the stratigraphical relations of the two groups are apt to be misunderstood4 ,what is actually seen is represented in ,fig4 #cga4 ,the lithoidal rhyolite emerges from underneath the basalts which abut against its sloping surface1 forming on the north side of ,carnearny ,hill a steep bank about #aej feet above the more gently inclined slope below4 ,the basalts consist of successive nearly level sheets of compact and amygdaloidal rock4 ,it is obvious that only two explanations of this section are possible4 ,either the rhyolite was in existence before the basalts which flowed round it and gradually covered it1 or it has been erupted through these rocks1 and is therefore of later date4 ,the former supposition has been the more usually received4 ,the rhyolite has been supposed to form the summit of an ancient volcanic dome1 perhaps of ,eocene age1 which had been worn down before the outflow of the plateau-basalts under which it was eventually entombed4 ,had this been the true history of the locality1 it is inconceivable that of a rock which decays so rapidly as this rhyolite1 and strews its slopes with such abundance of detritus1 not a single fragment should occur between the successive beds of basalt which are supposed to have surrounded and buried it4 ,though the several beds of basalt are well exposed all round1 ,i could not1 on my first visit1 find a trace of any rhyolitic fragments between them1 nor had ,mr4 ,symes1 who mapped the ground in detail for the ,geological ,survey1 been more successful4 ,i have since made a second search with ,mr4 ,m',henry1 but without detecting a single pebble of the acid rock among the basalts4 ,yet it is clear from the upper surfaces of some of these lavas that a considerable interval of time separated their successive outflows1 so that there was opportunity enough for the scattering of rhyolite-debris had any hill of that rock existed in the vicinity4 ,again1 little more than a mile to the east of ,carnearny ,hill1 an outlier of the basalts forming the prominent height of the ,brown ,dod lies upon and is completely surrounded by the rhyolite1 which along the east side of the hill can be traced as it passes under the level sheets of basalt4 ,the line of junction ascends and descends on that flank of the outlier1 so that successive flows of basalt are truncated by the acid rock4 ,but ,i could find no rhyolitic debris between them4 ,it appears to me1 therefore1 that the relations between the two groups of rock in this area are similar to those between the granophyres and bedded basalts on the south side of ,loch na ,keal in ,mull 7p4 #cif74 ,in other words1 the rhyolites have risen through the basalts1 and are therefore younger than these lavas4 ,this conclusion is corroborated by the actual proofs of the intrusion of rhyolite into the basalts at ,templepatrick4 ,all the known rhyolitic masses in ,antrim are confined to the ,lower group of basalts4#dbi ,and as they traverse some of its highest members1 they may be regarded as certainly younger than that group4 ,mr4 ,m',henry1 who first indicated this relation1 suggested that the rhyolites were erupted in the interval between the two basaltic series1 and he connected with their eruption the rhyolitic detritus found in association with the iron-ore at so many places in ,antrim4 ,it appears to me that this suggestion carries with it much probability4 ,the rhyolitic conglomerate of ,glenarm proves that1 in the long period represented by the iron-ore and its associated group of sedimentary deposits1 there were masses of rhyolite at the surface1 the waste of which could supply such detritus4 ,the resemblance between the material of that conglomerate and the rhyolites now visible at ,tardree and elsewhere is so close that we cannot doubt that1 if not derived from some of the known rhyolitic protrusions1 this material certainly came from exposed masses that had the same general petrographic characters4 ,footnote #dbi3 ,the only exception to this rule was believed to be that of the mass at ,eslerstown1 four miles east of ,ballymena1 which1 as originally mapped1 was shown as crossing from the ,lower into the ,upper basalts4 ,mr4 ,m',henry1 however1 has recently ascertained that the acid rock is entirely restricted to the area of the older group4 ,while the rhyolite pebbles in the ,glenarm conglomerate are distinctly rounded and water-worn1 showing that some prominences of acid rock were undergoing active denudation at the time when this conglomerate was laid down1 the finer rhyolitic detritus in the tuffs of ,ballypallidy rather suggests the actual discharge of rhyolitic ashes during the same period4 ,but it would appear that the superficial outbursts of rhyolitic material1 whether in the form of lava or of tuff1 were only of trifling extent1 or else that the interval between the eruption of the two basalt-groups was so prolonged that any such superficial material was then removed by denudation4 ,the varieties of lithological character to be met with among the acid protrusions of ,antrim suggests a succession of uprises of rhyolites differing from each other more or less in composition and structure4 ,unfortunately the ground is generally so covered with superficial accumulations1 and the exposures of rock are so poor and limited1 that no sequence has yet been determined among the several kinds of acid rock4 ,the only locality where ,i have observed clear evidence of such a sequence is on the old quarries half a mile west of ,shankerburn ,bridge1 and three miles north-west of ,dromore1 ,county ,down4 ,a small boss of rhyolite there rises through the ,silurian strata4 ,it consists partly of a coarse-grained lithoidal rhyolite1 with large smoky quartzes and felspars1 and partly of a much finer textured variety4 ,the latter1 on the south side of the small brook which separates the quarries1 can be seen to ascend vertically through the coarse-grained rock into which it sends a projecting vein4 ,its margin shows a streaky flow-structure parallel with its vertical wall and is in places spherulitic4 ,here the closer-grained rock is certainly later than the rest of the mass4 ,,chapter ,,xlviii ,,the ,,acid ,,sills1 ,,dykes ,,and ,,veins i4 ,,the ,,sills ,not only have the acid rocks been protruded in small and large bosses1 they have also been injected as sills between the bedding-planes of stratified rocks1 between the surfaces of the basalt-beds1 and between the bottom of the plateau-basalts or of the gabbros and the platform of older rock on which the volcanic series has been piled up4 ,every gradation of size may be observed1 from mere partings not more than an inch or two in thickness1 up to massive sheets1 which now1 owing to the removal of their original covering of rock by denudation1 form minor groups and ranges of hills4 ,where the sheets are numerous1 they are usually small in size2 where1 on the other hand1 they are few in number1 they reach their greatest dimensions4 ,it is not always possible to discriminate between bosses and large irregular sills4 ,a good illustration of the connection between these two forms of intrusion will be cited from the island of ,raasay1 where a widespread intrusive sheet is in part connected with a true boss4 ,in ,mull1 sills of acid eruptive rocks are profusely abundant throughout the central mountainous tract between ,loch na ,keal and ,loch ,spelve4 ,if we ascend the slopes from the ,sound of ,mull1 for instance1 we have not gone far before some of these sheets make their appearance4 ,they are usually dull granular quartz-porphyries1 or granophyres1 often only two or three feet in thickness1 and interposed between the beds of basalt that form the mass of the hills4 ,along the crest of the ridge that stretches through ,beinn ,chreagach ,mhor to ,mainnir nam ,fiadh they take a prominent place among the ledges of basalt1 basalt-conglomerate and dolerite4 ,the largest sheet in ,mull is probably that which has thrust itself between the base of the basalts and the underlying ,jurassic strata and crystalline-schists on the shore of the ,sound of ,mull at ,craignure4 ,the porphyry of this sheet is referred to by ,professor ,zirkel as only a finer-grained variety of the same quartziferous rock1 with hornblende and orthoclase crystals1 which in ,skye breaks through the ,lias4#dcj ,on the south coast also1 at the base of the thick basalt series1 similar porphyries have been injected into the underlying strata2 and under the great gabbro mass of ,ben ,buy similar protrusions occur4 ,but as we retire from the mountainous tract into the undisturbed basalts of the plateau1 these acid intercalations gradually disappear4 ,footnote #dcj3 ,zeitsch4 ,deutsch4 ,geol4 ,gesellsch4 xxiii4 p4 #ed4 ,in the islands of ,eigg and ,rum1 excellent examples occur of the tendency which the sheets of porphyry or granophyre manifest to appear at or about the base of the bedded basalts4 ,i have already alluded to the boss or sheet at the north end of the former island4 ,a still more striking illustration occurs in ,rum4 ,all along the base of the great mass of gabbro1 protrusions of various kinds of acid rock have taken place4 ,the great mass of ,orval1 already described1 is one of these4 ,below ,barkeval and round the foot of the hills to the south-east of that eminence an interrupted band of quartz-porphyry may be traced1 from which veins proceed into the gabbros and dolerites4 ,but it is in ,skye and ,raasay that the intrusive sheets of the acid group of rocks reach their chief development4 ,they have been most abundantly injected underneath the bedded basalts1 particularly among the ,jurassic strata4 ,a band or belt of them1 though not continuous1 can be traced round the east side of the main body of granophyre1 at a distance of from a mile and a half to about three miles4 ,beginning near the point of ,suisnish1 this belt curves through the hilly ground for some five miles1 until it dies out on the slopes above ,skulamus4 ,it may be found again on the west side of the ridge of ,beinn ,suardal1 and on the moors above ,corry1 till it reaches the shore at the ,rudh' an ,eireannich 7,irishman's ,point74 ,it skirts the west side of ,scalpa ,island1 and runs for some miles through ,raasay4 ,another series of sills occurs below the basalts and gabbros in the ,blaven group of hills4 ,over a large part of their course1 the rocks of the eastern belt rest in great overlying sheets upon the ,jurassic strata1 which may almost everywhere be seen dipping under them4 ,from the analogy of other districts1 we may1 ,i think1 infer that the position of these sills here points to their having been intruded at the base of the plateau-basalts which have since been removed from almost the whole tract4 ,fortunately1 a portion of the basalts remains in ,raasay1 and enables us to connect that island with the great plateau of ,skye of which it once formed a part4 ,there can be no doubt that the basalts of the ,dn ,caan ridge once extended westwards across the tract of granophyre which now forms most of the surface between that ridge and the ,sound of ,raasay4 ,a thin sheet of quartz-porphyry1 interposed among the ,oolitic strata1 may be seen a little inland from the top of the great eastern cliff and below the position of the bedded basalts4 ,the great sheet1 or rather series of sheets1 which stretches north-eastwards from ,suisnish at the mouth of ,loch ,eishort in ,skye1 consists of a rock which for the most part may readily be distinguished in the field from the granitoid material of the bosses4 ,it appears to the naked eye to be a rather close-grained or finely crystalline-granular quartz-porphyry1 with scattered blebs or bi-pyramidal crystals of quartz and crystals of orthoclase4 ,at the contact with adjacent rocks1 the texture becomes more felsitic1 sometimes distinctly spherulitic 7west side of ,carn ,nathragh1 next ,lias shale74 ,under the microscope the rock is seen to be a fine-grained granophyric porphyry or porphyritic granophyre4 ,it caps ,carn ,dearg 7#fcf feet7 above ,suisnish1 where it covers a space of nearly a square mile1 and reaches at its eastern extremity 7,beinn ,bhuidhe71 a height of #ijh feet above the sea 7,fig4 #bdi74 ,this rock rests upon a sill of dolerite1 and is apparently split up by it4 ,but1 as ,i have already stated1 the basic rock is probably the older of the two1 and the granophyre seems to have wedged itself between two earlier doleritic sheets4 ,to the north-west of ,carn ,dearg1 above the northern end of the crofts of ,suisnish1 the same sill1 or one occupying a similar position1 crops out between masses of granophyre1 and is intersected by narrow veins from that rock4 ,though severed by denudation1 the large sheets of granophyre to the east of ,beinn ,bhuidhe are no doubt continuations of the ,carn ,dearg mass1 or at least occupy a similar position4 ,that they are completely unconformable to the ,jurassic strata is shown by the fact1 that while at ,suisnish they lie on sandstones which must be fully #ajjj feet above the bottom of the ,lias1 only two miles to the east they are found resting on the very basement limestones1 within a few yards from the underlying quartzite and ,torridon sandstone4 ,i do not think that this transgression can be accounted for by intrusion obliquely across the stratification4 ,i regard it as arising from the eruptive rock having forced its way between the bottom of the now vanished basalt-plateau and the denuded surface of ,jurassic rocks1 over which the basalts were poured4 ,the platform underneath these granophyre sills thus represents1 in my opinion1 the terrestrial surface before the beginning of the volcanic period4 ,but there is abundant proof that though the intruded granophyre sills followed generally this plane of separation1 they did not rigidly adhere to it1 but burrowed1 as it were1 along lower horizons4 ,thus on the south-east front of ,beinn a' ,chirn1 which forms so fine an escarpment above the valley of ,heast1 the base of the granophyre1 after creeping upward across successive beds of limestone1 sends out a narrow tongue into these strata1 and continues its course a little higher up in the ,lias4 ,the same rock1 after spreading out into the broad flat tableland of ,beinn a' ,chirn 7#ihc feet71 rapidly contracts north-eastwards into a narrow strip which forms the crest of the ridge1 and at once suggests a much-weathered lava-stream4 ,the resemblance to a coule is heightened by the curious thinning off of the rocks where the two streams emerge from the ,heast lochs2 it looks as if the igneous mass were a mere superficial ridge which had been cut down by erosion1 so as to expose the shales beneath it4 ,but that the granophyre is really a sill becomes abundantly clear at its eastern end1 where we find that it consists of two separate sheets with intervening ,liassic shales4 ,the structure of this interesting locality is shown in ,fig4 #cgb4 ,in this instance also1 there is evidence that the acid sills are younger than the basic1 for the upper sheet of granophyre sends up into the overlying dark basaltic rock narrow vertical felsitic veins1 a quarter of an inch to an inch in width1 which being more durable1 stand out above the decomposable surface of the containing rock1 and show their quartz-blebs and felspar crystals on the weathered surface4 ,perhaps the most striking feature of the granophyre sills of ,skye is their general association with thinner basic intrusive sheets between which they have insinuated themselves4 ,this characteristic structure1 pointed out by me in #ahhh1 has recently been more minutely mapped in the progress of the ,geological ,survey4 ,mr4 ,harker has found the typical arrangement to be the occurrence of a thick sill of granophyre interposed between two sills of basalt1 each of which is usually not more than six or eight feet thick4 ,where the granophyre has been intruded independently among the ,lias formations1 it does not assume the regularity and persistence which mark it where it has followed the course of basic sills4 ,illustration3 ,fig4 #cgb4--,section across the ,granophyre ,sills at ,loch a' ,mhullaich1 above ,skulamus1 ,skye4 a1 ,jurassic sandstones and shales2 b1 ,jurassic dark brown sandy shales2 c1 sills of basalt1 some bands highly cellular2 c1 basalt-sill with veins of felsite rising into it from the granophyre below2 d d1 intrusive sheets or sills of granophyre4 ,the acid rock1 ,mr4 ,harker observes1 is invariably the later intrusion1 for it sends narrow veins into the basalts1 metamorphosing them to some extent and frequently enclosing fragments of them4 ,these fragments are always rounded by corrosion1 and show various stages of dissolution down to mere darker patches as seen by the naked eye4 ,such inclusions and patches are found in the marginal part of a granophyre1 where no continuous basalt occurs1 but where the acid magma has evidently in places completely destroyed the earlier basic sheets between which it was forced4 ,it seems probable that in all cases a certain amount of solution of the basalt by the granophyre magma took place at their contact1 facilitating the injection of the later intrusion and accounting for its persistent choice of the contact-plane of two basalt-sills as the surface offering least resistance to its injection4 ,these observations throw fresh light on the remarkable original regularity and persistence of the basic sills4 ,where one of these sills disappears above or below a granophyre sheet its probable former presence is often indicated by corroded fragments of the basic in the acid rock4 ,mr4 ,harker remarks that the acid magma seems to have been in itself less adapted than the basic to follow accurately a definite horizon and to maintain a uniform thickness in its intruded sheets1 but could do both when guided by a pre-existing basalt-sill1 or especially when insinuated between contiguous basalt-sills4 ,the corrosive action of the acid magma on the surface of the basalt1 which enabled it to force its way more readily between the basic sills1 might proceed so far as partially or wholly to destroy these sills4 ,this solvent action may serve to explain some of the irregularities of the granophyre intrusions4 ,according to the same observer1 such irregularities are found where the granophyre sheet and its encasing basalt-sills are not co-extensive1 or again where the two basalt-sills separate1 owing to one of them cutting obliquely across the bedding4 ,in the latter case1 which is not common1 the granophyre follows one of the basalt-sills1 necessarily parting from the other4 ,when one of the two guiding basalt-sills dies out1 the granophyre may still continue1 following the sill which persists4 ,if the latter also dies out1 while the granophyre is still in some force1 the acid magma seems to have been reluctant to travel beyond the limit of the basalt1 but has drawn towards it1 and the granophyre presents a blunt laccolitic form1 which contrasts with the acutely tapering edge of a granophyre which dies out before reaching the limit of its basalt-sills4 ,if1 on the other hand1 on reaching the limit of the basalt1 the acid magma has been in such force as to be driven further1 it is usually found to lose something of its regularity and to depart from the exact horizon which it has hitherto followed4 ,this seems to happen1 for instance1 in the ,beinn a' ,chirn sheet1 which1 when traced westward1 is found to behave as a 'boss' and is obviously transgressive1 having cut across the bedding of the strata so as to enter the limestones1 where it no longer behaves in any degree as a sill4 ,the district affords many examples of the tendency of intrusive masses in general to cut sharply across the beds when they enter a group of limestones4 ,more complex examples of acid sills are to be found where there have been three or more basic sheets together4 ,the great granophyre sheet already referred to at ,suisnish affords the best illustration of this structure4 ,mr4 ,harker has noticed that round most of its circumference there is seen merely a single basalt-sill passing under the granophyre4 ,probably there has been another similar sheet over the acid rock1 but if so1 it has been removed by erosion1 the granophyre itself forming everywhere the surface of the plateau4 ,on the southern side1 however1 we see that the original basalt must have been at least triple1 or counting the uppermost member1 now removed1 quadruple4 ,the granophyre has forced its way in between the several members of the multiple basalt-sill1 the intermediate ones being thus completely enveloped4 ,they are evidently metamorphosed as well as veined by the granophyre1 and when traced onward they give place to detached portions which1 floating as it were in the acid rock1 are soon lost4 ,it is seldom easy to determine where lay the vent or vents from which the granophyre sills proceeded4 ,those of the ,skye platform just described may be chiefly concealed under some of the larger areas of the rock1 such as the sheets of ,carn ,dearg or ,beinn a' ,chirn4 ,but in several places1 in close association with the compound sills of granophyre and basalt1 ,mr4 ,harker has found large dyke-like bodies of the acid rock1 which may with considerable probability be regarded as marking the position of the channels by which the material of the sills ascended4 ,these bodies1 he remarks1 either occur isolated by erosion1 the sills or the parts of the sills presumed to have been in connection with the dykes having been removed1 or are only very partially exhibited in direct connection with sills still remaining4 ,where they can be examined in detail they are seen to be dykes varying up to about #ajj feet in width1 but of no great longitudinal extent4 ,between ,suisnish and ,cnoc ,carnach they bear ,e4,n4,e41 that is1 at right angles to the ordinary basic dykes of the district and parallel to the general direction of the axes of folding1 though further north they change this trend1 but still remain parallel to the strike of the ,lias4 ,these dykes are composed essentially of granophyre1 identical with that of the sills4 ,in some cases1 they are flanked with basalt-dykes on one or both sides1 or the former existence of such lateral dykes is indicated by partly-destroyed inclusions of the basic rock in the granophyre4 ,the basalt found in these cases is identical with that of the basic sills1 and shows the same relation to the granophyre4 ,discontinuity and failure of the basalt are commoner1 however1 in the dykes than in the sills--a difference presumably attributable to more energetic destructive action of the acid magma when it was hotter and fresher4 ,these supposed feeders of the granophyre sills are certainly in some cases1 and have possibly been in all1 double or triple dykes4 ,the acid magma thus appears not only to have spread laterally along the same platforms as the earlier basalts1 but to have reached these levels by rising through the same fissures which had already given passage to the basic magma4#dca ,footnote #dca3 ,,ms4 notes supplied by ,mr4 ,harker4 ,the granophyre sills which1 as already stated1 can be followed as an interrupted band from ,suisnish ,point to the ,sound of ,scalpa1 emerge again beyond ,loch ,sligachan and also in the island of ,raasay1 where a great sheet of the acid rock covers an area of about five square miles4 ,this tract has recently been mapped for the ,geological ,survey by ,mr4 ,h4 ,b4 ,woodward1 who has found it to have been intruded across the ,jurassic series1 a large part of its mass coming in irregularly about the top of the thick white sandstones of the ,inferior ,oolite4 ,but it descends beneath the ,secondary rocks altogether1 and in some places intervenes between the base of the ,infra-liassic conglomerates and the ,torridon sandstone4 ,its irregular course transgressively across the ,mesozoic formations is probably to be regarded as another example of the intrusion of the acid material preferentially along the line of unconformability between the older rocks and the ,tertiary basalts1 now nearly all removed from ,raasay by denudation1 though the intrusion does not rigidly follow that line of division1 but sometimes descends below it4 ,the central portions of this ,raasay granophyre possess the ordinary structures of the corresponding rocks in ,skye4 ,they show a finely crystalline-granular1 micropegmatitic base1 through which large felspars and quartzes are dispersed4 ,but at the upper and under junction with the sedimentary rocks1 beautiful spherulitic structures are developed4 ,this is well seen on the shore near the ,point of ,suisnish 7,raasay71 where1 below the ,lias ,limestones1 the top of the granophyre appears1 and where its bottom is seen to lie on the ,torridon sandstone4 ,this granophyre sheet presents a further point of interest inasmuch as it appears to have preserved one of the dyke-like masses which may mark channels of escape from the general body of the acid magma below4 ,near the ,manse the section represented in ,fig4 #cgc may be observed4 ,owing to great denudation1 the massive sheet of granophyre has been cut into isolated outliers which cap the low hills1 and the rock may be seen descending through the ,jurassic sandstones1 which in places are much indurated4 ,it is observable that the amount of contact-metamorphism induced by the granophyre sills upon the rocks between which they have been injected is1 in general1 comparatively trifling4 ,it is for the most part a mere induration1 sometimes accompanied with distortion and fracture4 ,illustration3 ,fig4 #cgc4--,section to show the connection of a sill of ,granophyre with its probable funnel of supply1 ,raasay4 a a1 ,jurassic sandstones2 b1 granophyre4 ,illustration3 ,fig4 #cgd4--;,granophyre sill resting on ,lower ,lias shales with a dyke of basalt passing laterally into a sill1 ,suisnish ,point1 ,isle of ,raasay4 ,although the intrusion of the granophyre sills has been subsequent to that of the basalt-sheets with which they are so generally associated1 we may expect that as there is a series of post-granophyre basic dykes1 so there may be some basic sills later than the injections of the acid sheets4 ,the ,raasay granophyre appears to furnish an example of such a later basic intrusion4 ,at the ,point of ,suisnish on that island ,i have observed the relations shown in ,fig4 #cgd4 ,there the dark shales of the ,lower ,lias 7a a7 are immediately overlain by the granophyre sill 7b71 and are cut by a basalt-dyke which1 when it rises to the base of the granophyre1 turns abruptly to one side1 and then pursues its course as a sill 7c7 between the granophyre and the shales4 ,there can be little doubt that this intrusion is later than the granophyre4 ,here a basic sill is interposed at the bottom of the acid sheet2 and is visibly connected with the actual fissure up which its molten material was impelled4 ii4 ,,the ,,acid ,,dykes ,,and ,,veins ,besides bosses and sills1 the acid rocks of the ,inner ,hebrides take the form of ,dykes and ,veins which have invaded the other members of the volcanic series4 ,some of these have already been referred to2 but a more particular description of the venous development of the acid rocks as a whole is now required4 ,as regards their occurrence and distribution1 they present two phases1 which1 however1 cannot always be distinguished from each other4 ,on the one hand1 they are found abundantly either directly proceeding from the bosses 7more rarely from the sills71 or in such immediate proximity and close relationship to these as to indicate that they must be regarded as apophyses from the larger bodies of eruptive material4 ,on the other hand1 they present themselves as solitary individuals1 or in groups at a distance of sometimes several miles from any visible boss of granophyre4 ,in such cases1 it is of course obvious that though not exposed at the surface1 there may be a large mass of the acid magma at no great distance beneath1 and that these isolated dykes and veins do not essentially differ in origin from those of which the relations to eruptive bosses can be satisfactorily observed or inferred4 ,considered as a petrographical group1 these ,dykes and ,veins are marked by the following characters4 ,at the one extreme1 we have thoroughly vitreous rocks in the pitchstones4 ,from these1 through various degrees of devitrification1 we are led to completely lithoid felsites1 quartz-porphyries or rhyolites4 ,micropegmatitic structure is commonly present1 and as it increases in development1 the rocks assume the ordinary characters of granophyre4 ,occasionally the structure becomes microgranitic in the immediate periphery of a boss wherein a granitic character has been assumed4 ,viewed as a whole1 however1 it may be said that the dull lithoid rocks of the dykes and veins can generally be resolved under the microscope into some variety of granophyric porphyry or granophyre4 ,a characteristic feature in the granophyric1 felsitic or rhyolitic dykes and veins is the presence of spherulitic structure 7,figs4 #cge1 #cgg74 ,in some cases this structure is hardly traceable save with the aid of the microscope1 but from these minute proportions it may be followed up to such a strong development that the individual spherulites may be an inch or two in diameter1 and lie crowded together1 like the round pebbles of a conglomerate4 ,the structure is a contact phenomenon1 being specially marked along the margin of the dykes1 as it is on the edge of sills and bosses4 ,in the ,strath district of ,skye1 ,mr4 ,clough and ,mr4 ,harker have observed that the spherulites are apt to be grouped in parallel lines so as to form rod-like aggregates along the walls1 and that where the rock is fairly fresh the centre of the dyke sometimes consists of glassy pitchstone1 so that the spherulitic felsite or granophyre is probably devitrified pitchstone4 ,frequently flow-structure is admirably developed in these dykes1 the streaky layers of devitrification flowing round the spherulites and any enclosed fragments as perfectly as in any rhyolitic lava 7,fig4 #cgh74 ,illustration3 ,fig4 #cge4--,weathered surface of spherulitic granophyre from dyke in banded gabbros1 ,druim an ,eidhne1 ,meall ,dearg1 ,glen ,sligachan1 ,skye4 ,natural size4 ,in regard to their modes of occurrence1 the dykes of acid material differ in some important respects from those of basic composition4 ,more especially they are apt to assume the irregular venous form1 rather than the vertical wall-like character of ordinary dykes4 ,they take the form of dykes1 particularly where their material has been guided in its uprise by one or more already existent basic or intermediate dykes1 as in the compound dykes1 already described4 ,the conditions for their production must thus have been essentially different from those of the great body of the basic dykes4 ,their intrusion was not marked by any general and widespread fissuring of the earth's crust1 such as prepared rents for the reception of the basalt and andesite dykes4 ,they were rather accompaniments of the protrusion of large masses of acid magma into the terrestrial crust4 ,this magma1 as we have seen1 was often markedly liquid1 and was impelled1 sometimes with what might be called explosive violence1 into the irregular cracks of the shattered surrounding rocks or into pre-existing dyke-fissures4 ,hence long straight dykes of the acid rocks are much less common than short irregular tortuous veins and strings4 ,illustration3 ,fig4 #cgf4--,plan of portion of the ridge north of ,druim an ,eidhne1 ,glen ,sligachan1 ,skye1 showing three dykes issuing from a mass of granophyre4 a1 gabbros2 b1 granophyre2 ,i4 ,,ii4 ,,iii41 three dykes proceeding from the granophyre4 ,the arrows show the direction of dip of the bands of gabbro4 ,much difference may be noticed among the granophyre bosses in regard to their giving off a fringe of apophyses4 ,thus1 along the well-exposed boundary of ,beinn-an-,dubhaich in ,skye1 though the edge of the boss is remarkably notched1 hardly any veins deserving the name diverge from it4 ,on the other hand1 the ridge of ,meall ,dearg at the head of ,glen ,sligachan1 already referred to1 is distinguished by the number and variety of the dykes and veins which proceed from the granophyre and traverse the banded gabbros4 ,as this locality has been elsewhere fully described1 ,i will give here only the leading structural features which it presents4#dcb ,footnote #dcb3 ,professor ,judd 7,quart4 ,journ4 ,geol4 ,soc4 vol4 xlix4 7#ahic71 p4 #age7 described the granophyre dykes of this locality as inclusions of ,tertiary granite in the gabbro1 and cited them in proof of his contention that the acid eruptions of the ,western ,isles are older than the basic4 ,their true character was shown by me in a paper published in the ,quart4 ,journ4 ,geol4 ,soc4 vol4 #a4 7#ahid71 p4 #bab4 ,illustration3 ,fig4 #cgg4--,weathered surface of spherulitic granophyre from dyke in banded gabbros1 ,druim an ,eidhne1 ,meall ,dearg1 ,glen ,sligachan1 ,skye4 ,natural size4 ,within a horizontal distance of less than #ajj yards three well-marked dykes issue from the spherulitic edge of the ,meall ,dearg granophyre1 and run in a south-easterly direction in the handed gabbros 7,fig4 #cgf74 ,the most northerly of these is traceable in a nearly straight line for #hjj feet4 ,the central dyke1 which can be followed for #bjj feet or more1 rises as a band six to ten feet broad between the dark walls of gabbro as represented in ,fig4 #cgi4 ,these dykes are marked by the most perfectly developed spherulitic and flow-structures 7,figs4 #cge1 #cgg74 ,numerous detached portions of other dykes and also irregular veins are to be observed cutting the banded gabbros all over the ridge of ,druim an ,eidhne for a distance of a mile or more4 ,many of these exhibit the same exquisitely beautiful spherulitic and flow-structure displayed by the dykes which can actually be traced into the main body of granophyre4 ,the lines of flow conform to every sinuosity in the boundary-walls of gabbro1 and sometimes sweep round and enclose blocks of that rock4 ,the example of this structure1 given in ,fig4 #cgh1 shows how these lines1 curving round projections and bending into eddy-like swirls1 exhibit the motion of a viscous lava flowing in a cleft between two walls of solid rock4 ,sometimes the lamin of flow have been disrupted1 and broken portions of them have been carried onward and enveloped in the yet unconsolidated material4 ,certain portions of this dyke are richly spherulitic1 the spherulites varying from the size of small peas up to that of tennis-balls4 ,occasionally two large spherulites have coalesced into an #h-shaped concretion1 and it may be observed in some cases that the spherulites are hollow shells4 ,illustration3 ,fig4 #cgh4--,plan of pale granophyric dyke1 with spherulitic and flow-structure1 cutting and enclosing dark gabbro1 ,druim an ,eidhne4 ,illustration3 ,fig4 #cgi4--;,dyke 7six to ten feet broad7 proceeding from a large body of granophyre and traversing gabbro1 from the same locality as ,figs4 #cge and #cgg4 ,a remarkable feature has been recently observed by ,mr4 ,harker among the abundant granophyre dykes and veins which intersect the gabbros and older rocks1 along the eastern flanks of the ,red ,hills of ,skye between ,broadford and the ,sound of ,scalpa4 ,broad dykes of granophyre which traverse the ,cambrian limestone of that district might be supposed at first sight to be cut off by the intrusions of gabbro4 ,but closer examination proves that their apparent truncation arises from their suddenly breaking up into a network of small veins where they abut against the basic rock4 ,this structure evidently belongs to the same type as that of the ,st4 ,kilda granophyre4 ,illustration3 ,fig4 #chj4--,section of intruded veins of various acid rocks above ,river ,clachaig1 ,mull4 a a1 basalt1 dolerite1 etc42 b b1 granophyre4 ,compound dykes and sills1 where one or more of the injections has consisted of acid material1 have been already noticed as intimately associated together in ,skye 7p4 #afb74 ,dykes of this nature are more particularly abundant in ,strath1 especially along its eastern side4 ,in addition to the examples cited already from that district1 ,i may refer to other two which intersect the ,middle ,lias shales and limestones in the island of ,scalpa4 ,they are both compound dykes1 but the more basic marginal bands are not always continuous1 having possibly been here and there dissolved by the acid invasion4 ,though they do not show any distinct spherulitic forms1 the presence of flow-structure is indicated by the thin slabs into which the rocks weather parallel to the dyke-walls4 ,the rock in each case is a fine-grained felsitic mass1 with bi-pyramidal crystals of quartz4 ,it is observable that where these dykes come directly against the ,liassic strata1 the latter are more seriously indurated than where they are traversed by the ordinary basic dykes4 ,in the central mountainous tract of the island of ,mull veins of acid material are extraordinarily abundant4 ,they probably proceed from a much larger subterranean body of granophyre than any of the comparatively small bosses of this rock which appear at the present surface of the ground4 ,they show themselves partly at the margins of the visible bosses1 but much more profusely in that tract of altered basalt1 with intrusive sheets and dykes of basalt1 dolerite and gabbro1 which lies within the great ring of heights between ,loch na ,keal and ,loch ,spelve4 ,in some areas1 the amount of injected material appears to equal the mass of more basic rock into which it has been thrust4 ,pale grey and yellowish porphyries and granophyres1 varying from thick dykes down to the merest threads1 ramify in an intricate network through the dark rocks of the hills1 as shown in the accompanying illustration 7,fig4 #chj71 which represents a portion of the hillside between ,beinn ,fhada and the ,clachaig ,river4 ,such a profusion of veins probably indicates the existence here of some large mass of granophyre or granite1 at no great depth beneath the surface4 ,in ,mull1 as in the other islands of the ,inner ,hebrides1 two horizons on which protrusions of acid materials have been specially abundant1 are the base of the bedded basalts of the plateau and the bottom of the thick sheets of gabbro4 ,dykes and veins of granophyre1 quartz-porphyry1 felsite and other allied rocks are sometimes crowded together along these two horizons1 though they may be infrequent above or below them4 ,illustrations of solitary veins in the midst of unaltered plateau-basalts or in older rocks may be gathered from many parts of the ,western ,isles4 ,some remarkable instances are to be seen among the basalts that form the terraced slopes on the north side of ,loch ,sligachan4 ,several thick dykes of granophyre run up the declivity1 cutting across hundreds of feet of the nearly level basalt-beds4 ,some of them can be seen on the shore passing under the sea4 ,they trend in a ,s4,s4,e4 direction towards ,glamaig1 and they are not improbably apophyses from that huge boss1 the nearest edge of which is three-quarters of a mile distant4 ,another example may be cited from the basalt-outlier of ,strathaird1 where two veins of felsite1 one of them a pale flinty rock showing flow-structure parallel to the walls1 may be seen on the west front of ,ben ,meabost4 ,in this case1 the veins are three miles and a half from the granophyre mass of ,strath na ,creitheach to the north1 four miles from that of ,beinn an ,dubhaich to the north-east1 and nearly three miles from that of ,coire ,uaigneich at the foot of ,blath ,bheinn4 ,a special place must be reserved for the pitchstone-veins4 ,ever since the early explorations of ,jameson and ,macculloch1 the ,west of ,scotland has been noted as one of the chief ,european districts for these vitreous rocks4 ,from ,skye to ,arran1 and thence to ,antrim1 many localities have furnished examples of them1 but always within the limits of the ,tertiary volcanic region4 ,that all of the pitchstones are of ,tertiary age cannot1 of course1 be proved1 for some of them are found traversing only ,palozoic rocks1 and of these all that can be absolutely affirmed is that they must be younger than the ,carboniferous or even the ,permian system4 ,but1 as most of them are unquestionably parts of the ,tertiary volcanic series1 they are probably all referable to that series4 ,not only so1 but there is1 ,i think1 good reason to place them among its very youngest members4 ,it is a significant fact that they almost always occur either in or close to granophyre or granite bosses1 the comparatively late origin of which has now been proved4 ,illustration3 ,fig4 #cha4--,pitchstone vein traversing the bedded basalts1 ,rudh an ,tangairt1 ,eigg4 ,the first pitchstone observed in ,skye was found by ,jameson on the flanks of the great granophyre cone of ,glamaig4 ,another rises on the side of the porphyry mass of ,glas ,bheinn ,bheag1 in ,strath ,beg4 ,several occur at the foot of ,beinn na ,callich4 ,in ,rum1 ,i found a pitchstone vein traversing the western slopes of the wide granophyre boss of ,orval4 ,in ,eigg1 the well-known veins of this rock intersect the plateau-basalts 7,fig4 #cha71 but they are accompanied1 even within the same fissure1 with granophyre1 and in their near neighbourhood lie the masses of this rock already alluded to4#dcc ,in ,antrim1 pitchstone and obsidian occur in the midst of the rhyolite4 ,the only marked exceptions to the general rule1 with which ,i am acquainted1 are those of the island of ,arran4 ,most of the pitchstone-veins in that district traverse the red sandstones which may be ,permian4 ,but none of them are far removed from the great granite boss of the northern half of the island1 while large masses of quartz-porphyry1 which strikingly resemble some of those of ,skye and ,mull1 lie still nearer to them4 ,it is also worthy of notice that pitchstone-veins rise through the ,arran granite boss itself1 the probably ,tertiary date of which has been already discussed4 ,footnote #dcc3 ,for an account of the pitchstone veins of ,eigg1 see ,quart4 ,journ4 ,geol4 ,soc4 xxvii4 p4 #bii4 ,this common association of pitchstone-veins with the ,tertiary eruptive bosses of acid rocks can hardly be a mere accidental coincidence4 ,it seems to prove a renewed extravasation of acid material1 now in vitreous form1 from the same vents that had supplied the granitoid1 granophyric1 porphyritic and felsitic varieties of earlier protrusions4 ,we must remember that the pitchstone-veins are not mere local glassy parts of the larger bodies of granophyre or granite in which they lie4 ,their margins are sharply defined2 they are indeed in all respects as manifestly intruded1 and therefore later masses1 as are the basalt-dykes4 ,their occurrence1 therefore1 within the acid bosses proves them to be younger than these members of the ,tertiary volcanic series4 ,whether they are also later than the latest basalt-dykes cannot yet be decided1 for ,i have never succeeded in finding an example of the intersection of these two groups of veins and dykes4 ,but1 with this possible exception1 the pitchstones are the most recent of all the eruptive rocks of ,britain4 ,as a rule1 the intrusive pitchstones occur as veins which cannot be traced far1 and which vary from a few yards to less than an inch in width4 ,they generally show considerable irregularity in breadth and direction1 sometimes sending out strings into the surrounding rock 7,fig4 #cha74 ,the outer portions are not infrequently more glassy and obsidian-like than the interior4 ,occasionally the vitreous character disappears by devitrification1 and the rock assumes the texture of a compact felsite or of a spherulitic rock4 ,among the later movements of the acid magma account must be taken here of the pale fine-grained veins which have already been referred to as traversing the granophyre bosses4 ,these intrusions1 so well seen in the bosses of ,skye and ,st4 ,kilda1 are often so close in texture that they may be called quartz-felsites4 ,their sharply-defined edges and felsitic character suffice to separate them from what are termed veins of segregation4 ,in at least one instance1 that of ,meall ,dearg1 already cited1 a mass of typical granophyre which has developed spherulitic and flow-structures along its margin1 and which sends out dykes having the very same structures for a distance of several hundred feet across the banded gabbros1 is itself traversed by a dyke of precisely similar character4 ,here we see that after the intrusion of its apophyses1 and after its own consolidation in the upper parts1 the granophyric magma that rose into rents in the solidified portion retained the same tendency to produce large spherulites as it had shown at first4 ,the fine felsitic veins that traverse the granophyre of the ,red ,hills are now being mapped by ,mr4 ,harker during the progress of the ,geological ,survey4 ,he has not yet obtained evidence of the age of these veins in relation to the latest basic dykes4 ,he has observed that they appear to be on the whole rather less acid than the material of the surrounding bosses1 though they were probably all connected with the same underlying acid magma from which the bosses were protruded4 ,a somewhat similar relation has been noticed between older granites and their surrounding dykes1 as in ,cornwall and ,galloway4 ,illustration3 ,,to ,,accompany ,,sir ,,archibald ,,geikie',s ,,ancient ,,volcanoes ,,of ,,britain ,map ,,vii ,,map ,,of ,,the ,,tertiary ,,volcanic ,,district ,,of ,,north ,,east ,,ireland ,the ,edinburgh ,geographical ,institute ,copyright ,j4 ,g4 ,bartholomew ,,chapter ,,xlix ,,the ,,subsidences ,,and ,,dislocations ,,of ,,the ,,plateaux ,there can be no doubt that considerable alterations of level have taken place over the volcanic areas of ,north-,western ,europe since the eruptions that produced the basalt-plateaux1 ,these alterations embrace general and local subsidences1 and also dislocations by which considerable displacements of the crust either in a downward or upward direction have been effected4 i4 ,,subsidences ,the mere fact that in many places the lower members of the series of terrestrial lavas have been submerged under the sea may be taken to prove a subsidence since older ,tertiary time4 ,along the west coast of ,skye this depression is well shown by the almost entire concealment of the bottom of the plateau under the ,atlantic4 ,in the ,faroe ,isles the subsidence has advanced still further1 for not a trace of the underlying platform on which the basalts rest remains above water4 ,in ,iceland1 too1 the complete submergence of the base of the ,tertiary volcanic sheets points to a widespread subsidence of that region4 ,another strong argument in favour of considerable depression may be derived from a comparison of the submarine topography with that of the tracts above sea-level4 ,it is obvious that the same forms of contour which are conspicuous on the land are prolonged under the ,atlantic4 ,if we are correct in regarding the valleys as great lines of subrial erosion1 their prolongations as fjords and submarine troughs must be considered as having had a similar origin4 ,we can thus carry down the surface of erosion several hundred feet lower than the line along which it disappears under the waves4 ,i know no locality where this kind of reasoning is so impressively enforced upon the mind as the west end of the ,scuir of ,eigg4 ,the old river-bed and its pitchstone terminate abruptly at the top of a great precipice4 ,assuredly they must once have continued much further westward1 as well as the sheets of basalt that form the main part of the cliff4 ,yet the sea in front of this truncated face of rock rapidly deepens to fully #ejj feet in some places4 ,had any such hollow existed in the volcanic period it would have been filled up by the long-continued outflowings of basalt4 ,making every allowance for concealed faults and local subsidences1 we can only account for this submarine topography by regarding it as having been carved out1 together with the topography of the land1 at a time when the level of the latter was at least #ejj feet higher than it is now4 ,the subsidence which is thus indicated along the whole of the ,north-,west of ,europe probably varied in amount from one region to another4 ,we seem to have traces of such inequalities in the varying inclinations of different segments of the basalt-plateaux4 ,the angles of inclination are almost always gentle1 but they differ so much in direction from island to island1 and even among the several districts of the same island1 as to indicate that certain portions of the volcanic plain have sunk rather more than other portions4 ,thus in the ,faroe ,islands1 where the bare cliffs allow the varying angles of inclination to be easily determined1 a general gentle dip of the basalts in a south-easterly direction has been noted among the central and northern islands by previous observers4 ,this inclination1 however1 is replaced among the southern islands by an equally gentle dip towards the north-east4 ,the centre of depression would thus seem to lie somewhere about ,sand and ,sku4 ,the highest angle of inclination which ,i noticed anywhere was at ,myggenaes1 where the basalts dip ,e4,s4,e4 at about #ae4 ,among the ,western ,isles1 also1 where similar variations in the inclination of the basalt-sheets are observable1 it might be possible by careful survey to ascertain the probable position of the areas of maximum depression1 and to show to what extent differential movements have affected the originally nearly level volcanic floor4 ,it would doubtless be found that everywhere the dominant movement has been one of subsidence4 ,the vast outpourings of lava would tend to leave the overlying crust unsupported1 and to cause it to sink into the cavities thus produced4 ,perhaps the most extensive subsidence of this kind1 at least that which admits of most satisfactory investigation1 because it still remains above sea-level1 is displayed by the vast hollow in the ,antrim plateau1 which embraces the basin of ,lough ,neagh and the valley of the ,lower ,bann4 ,this depression measures about #fj miles in length by about #bj in breadth4 ,its axis follows the ,n4,n4,w4 trend so characteristic of the volcanic features of ,tertiary time4 ,the depression may be said to involve the entire basaltic plateau of ,antrim1 for with the exception of a few insignificant areas along the borders1 especially on the east side between ,larne and ,cushendall1 the whole region slopes inward from its marginal line of escarpments1 which reach heights of #ahjj feet and upwards1 towards the great hollow in its centre 7see ,map ,,vii474 ,lough ,neagh1 which occupies the deepest part of this hollow1 and covers about one-eighth of the whole area of subsidence1 is the largest sheet of fresh water in the ,british ,isles1 for it exceeds #aej square miles in extent of surface4 ,yet1 for its size1 it is one of the shallowest of our lakes1 its average depth being less than #dj feet4 ,its shallowness1 compared with its wide area1 marks it out in strong contrast to most of the larger ,british lakes4 ,its surface is only #dh feet above the level of the sea4 ,the origin of ,lough ,neagh1 the theme of various legends1 has been seriously discussed by different writers1 but most exhaustively by the late ,e4 ,t4 ,hardman of the ,geological ,survey4#dcd ,this author connected the formation of the lake-basin with a series of large faults which are found intersecting the rocks around the basin1 and passing under the water in a general north-easterly direction4 ,he showed that these faults have produced serious displacements of the strata1 amounting sometimes to as much as #bjjj feet1 and he believed that it was by the concurrent effect of such dislocations that the depression of ,lough ,neagh had been caused4 ,footnote #dcd3 ,on the ,age and ,formation of ,lough ,neagh1 ,journ4 ,roy4 ,geol4 ,soc4 ,ireland1 vol4 iv4 7#ahge-gf71 p4 #agj2 also ,explanation of ,sheet #ce of the ,geol4 ,surv4 ,ireland 7#ahgg71 p4 #gb4 ,it is possible that these displacements may have contributed to at least the earlier stages in the history of the ,antrim subsidence4 ,they have undoubtedly taken place after the outpouring of the basalts1 for these rocks are involved in their effects4 ,but in the hollow of the ,bann valley north of ,lough ,neagh the faults which have been detected in the basaltic plateau are few and trifling4 ,the bold and bare escarpments1 that so clearly display the relations of the rocks1 reveal few traces of any important transverse dislocations4 ,nor has any proof of large longitudinal faults parallel with the axis of depression been obtained within the area of the ,bann valley4 ,the earliest evidence for the existence of a lake on the site of the present ,lough ,neagh has been supposed to be furnished by certain fine clays1 sands1 seams of lignite and clay-ironstone1 which have been referred to the ,pliocene period4 ,these deposits have been regarded as indicating the accumulation of fine sediment with drift vegetation brought down into a quiet lake by streams entering from the south4 ,their fresh-water origin was believed to be further corroborated by the occurrence of shells belonging to the lacustrine or fluviatile genus1 ,unio4#dce ,footnote #dce3 ,these shells were regarded as forms of ,unio by the late ,w4 ,h4 ,baily2 but ,dr4 ,henry ,woodward assigned them to ,mytilus4 ,see ,prof4 ,hull's ,physical ,geology and ,geography of ,ireland1 #bnd edit4 p4 #aja4 ,the shells have been more recently dug out by ,mr4 ,clement ,reid1 who has found them to be the common ,mytilus edulis4 ,the thickness of this series of strata1 their position above sea-level1 and their distribution are important parts of the evidence for the geological history of the locality4 ,at one place the deposits are said to have been bored through to a depth of #bid feet1 and ,mr4 ,hardman believed them to be not less than #ejj feet deep4 ,the same observer found that they certainly reach a height of #abj feet above the sea1 and he was of opinion that in some places their height was not less than #adj feet4 ,the deposition of strata to the depth of #cjj feet below a level of #abj feet above the sea would1 of course1 entirely fill up ,lough ,neagh1 and spread over a large tract of low ground around it4 ,the pottery-clays and lignites1 however1 appear to be confined to the southern half of the lake1 from which they rise gently into the low country around4 ,the distribution of these deposits and their extraordinary variations in altitude1 as described by ,mr4 ,hardman1 present great difficulties in the attempt to regard them as the sediments of a ,pliocene lake4 ,a more recent examination of the ground by ,mr4 ,clement ,reid of the ,geological ,survey has led that able observer to believe that two totally different groups of strata at ,lough ,neagh have been confounded4 ,he noticed the ,mytilus-clay to be a dark blue mass full of derived boulder-clay stones1 and yielding ,mytilus edulis and seeds of a sedge4 ,this deposit cannot be ,pliocene1 but must be of ,glacial or post-,glacial age1 possibly contemporary with the ,clyde beds4 ,the junction of this clay with the pipe-clays is not at present seen1 but the lithological contrast between the two groups of strata is so strong as to indicate their independence of each other4 ,mr4 ,reid found the white1 red and mottled pipe-clays with their masses of lignite to present a strong resemblance to the ,bagshot group in the ,tertiary series4 ,it is possible1 as already suggested1 that the pipe-clays and lignites may belong to the sedimentary zone that separates the lower and upper basalts of ,antrim4 ,at all events they furnish no proof of any ,pliocene lake1 and may not indicate more than a deeper part of the depression in which the tuffs1 lignites and iron-ore were laid down4 ,the existence of the ,mytilus-clay shows that in ,glacial or post-,glacial times the valley of the ,bann was a strait or fjord into which the sea entered4 ,thick masses of drift have been laid down all round and over the depression now occupied by ,lough ,neagh1 insomuch that had any older lake existed here in ,glacial times1 it could hardly have escaped being filled up4 ,the observer1 who from one of the basalt-heights looks down upon the expanse of ,lough ,neagh and the broad peat-covered plain that continues the level platform of the lake-surface down the valley of the ,bann1 cannot but be impressed with the size of this wide hollow in the heart of the ,antrim plateau1 and with the evident continuity of the whole depression from the lake to the sea4 ,if he be a geologist1 he will be further struck by the fact that while the ,chalk and other older rocks appear from under the basalt-escarpments all round the plateau1 at heights of many hundred feet above the sea1 the floor of this wide hollow is entirely covered with basalt4 ,had the depression been merely due to denudation1 the rocks that underlie the volcanic series would have been exposed to view4 ,the base of the basalts which1 on either side of the depression1 is often more than #ajjj feet above the sea-level1 sinks below that level in the hollow of the ,bann and ,lough ,neagh4 ,this inequality of position may have been partially brought about by faults like those around ,lough ,neagh1 and may thus have been begun long before the ,glacial period4 ,but it appears to me to be mainly due to a wide subsidence1 of which the axis ran in a ,n4,n4,w4 and ,s4,s4,e4 direction from the present coast up the valley of the ,bann and the basin of ,lough ,neagh to beyond ,portadown4 ,we may conceive that after the cessation of the outflows of basalt1 the territory overlying the lava-reservoir that had been emptied would tend to subside1 partly by ruptures of the crust producing faults and partly by a downward movement of a more general kind4 ,in course of time1 these disturbances turned the drainage into the hollow now traversed by the ,bann4 ,denudation would necessarily accompany them1 and the surface of the country would be continually eroded and lowered4 ,lough ,neagh has been carefully sounded by the ,admiralty1 and its chart affords much suggestive material for the consideration of the geologist4#dcf ,from the soundings there given it has long been known that the lake deepens towards its northern end1 and attains a maximum depth of #ajb feet4 ,but it is not until we trace on the chart a series of contour-lines for successive depths1 as shown by the soundings1 that we realize the remarkable form of the lake bottom4 ,we then discover that below a depth of #ej feet a well-defined channel extends for rather more than half the length of the lake4 ,this channel begins to be distinctly perceptible between ,kiltagh ,point and ,langford ,lodge4 ,it first runs in a northerly course on the west side of the centre of the ,lough1 but when it comes into a line with ,saltera ,castle on the western shore1 it wheels round so as to conform to the curve of the ,antrim coast-line1 which it follows northward until1 about two miles from the exit of the lake1 its outline ceases to be traceable on the gently shelving bottom4 ,its total length is thus about #ab miles4 ,footnote #dcf3 ,lough ,neagh surveyed and sounded by ,lieut4 ,thomas ,graves1 ,r4,n4 ,there can hardly be any doubt that this channel is a former bed of the ,river ,bann4 ,it occupies exactly the position which that stream would take if the lake were drained1 and its depth and breadth correspond to those of the valley-bottom of the present river4 ,if this conclusion be accepted1 some important conclusions may be further deduced from it4 #a4 ,the presence of a former course of the ,bann on the bottom of ,lough ,neagh proves the lake to be much younger than the ,ice ,age4 ,the thick boulder-clays and ,glacial gravels which so encumber the country around and descend under the lake1 would assuredly have filled up the river-channel had it existed at the time of their deposition4 ,the channel has obviously been cut out of these drifts since the ,glacial period4 ,when the erosion took place1 the present ,lough ,neagh could not have existed1 but the ,bann followed a continuous course across the plain which the lake now covers4 ,the river probably maintained its place for a long period1 so as to be able to excavate so wide and deep a bed in the drifts1 if1 indeed1 it did not to some extent slowly carve its bed out of the underlying basalts4 ,it must be remembered that sediment is being continually poured into ,lough ,neagh1 and that some of the silt must have accumulated in the submerged river-course1 thus lessening its depth and width4 ,that the channel should still be so marked may be used as an argument for the comparatively late date of the subsidence4 #b4 ,the submerged river-course is a clear proof of subsidence4 ,the present ,lough ,neagh cannot be looked upon as a glacial lake formed by rock-erosion or by irregular deposition of drift4 ,its floor must have been a land surface when the ,bann cut out its bed upon it4 ,the whole area has sunk down1 the drainage has been arrested1 and some #bj miles of the course of the ,bann are now under a sheet of shallow water4 ,this subsidence was not brought about by faults4 ,it seems rather to have resulted from a general sinking of the ground4 ,the movement was probably comparatively rapid1 otherwise the river-course would hardly have survived so well4 #c4 ,these inferences1 based upon purely geological considerations1 have an interesting bearing upon the allusions to the origin of ,lough ,neagh contained in some ancient historical documents4 ,various legends have from an early period been handed down as to the first appearance of this sheet of water4 ,these myths1 though differing in details1 agree in describing such a sudden or rapid accumulation of water as destroyed human life1 in a district which had previously been inhabited by man4 ,the earliest records indicate that the alleged catastrophe took place in the first century of the ,christian era4#dcg ,it appears to me not improbable that the tradition1thus preserved in these legends1 may have had its basis in the actual disturbance which1 on geological grounds1 can be shown to have determined the existence of ,lough ,neagh4 ,though the event may go back far beyond the first century1 there can be no doubt that1 in a geological sense1 it was one of the most recent topographical changes which the ,british ,isles have undergone4 ,footnote #dcg3 ,for versions of the legends1 see ,dr4 ,todd's ,irish ,version of the ,historia ,britonum of ,nennius1 ,roy4 ,hist1 and ,archol4 ,assoc4 ,ireland2 ,dr4 ,reeves' ,ecclesiastical ,antiquities of ,down1 etc41 p4 #cgj2 ,mr4 ,j4 ,o',beirne ,crowe's ,ancient ,lake ,legends of ,ireland1 ,no4 #a in ,journ4 ,roy4 ,hist4 and ,archol4 ,assoc4 ,ireland1 vol4 i4 7#ahgj-ga71 p4 #id2 ,giraldus ,cambrensis1 vol4 v4 cap4 ix4 p4 #ia--;de lacu magno miram originem habente4 ,moore's well-known lines embody the popular belief that round towers and other buildings were submerged by the inundation4 ,thus the ,antrim basalt-plateau1 in addition to the high interest of its volcanic history1 has the additional claim to our attention that it has preserved1 more fully and clearly than any other of the plateaux1 the evidence for the latest subterranean movements that followed the long series of volcanic eruptions during ,tertiary time4 ,it contains the record of a post-,glacial subsidence that gave birth to the largest lake in ,britain4 ii4 ,,dislocations ,though ,i have not observed any features among the ,tertiary basalt-plateaux of the ,british ,isles that can be compared to the remarkable rifts and subsidences of ,iceland1 it can be shown that these piles of volcanic material have undoubtedly been fractured1 and that portions of them have subsided along the lines of dislocation4 ,careful examination of the basalt-escarpments of the ,inner ,hebrides discloses the existence of numerous faults which1 though generally of small displacement1 nevertheless completely break the continuity of all the rocks in a precipice of #gjj or #ajjj feet in height4 ,not infrequently such dislocations give rise to clefts in the cliffs4 ,some good illustrations of this feature may be noticed on the north side of the island of ,canna1 where the precipice has been fissured by a series of dislocations1 having a hade towards the west and a throw which may in some cases amount to about #bj or #be feet4 ,the cumulative effect of this system of faulting1 combined with a gentle westerly dip1 is to bring down to the sea-level the upper band of conglomerate which further to the east lies at the top of the cliff4 ,again1 the basalt-escarpment on the west side of ,skye1 from ,dunvegan ,head to ,loch ,eynort1 is traversed by a series of small faults4 ,on the east side of ,skye and in ,raasay1 a number of faults1 some of them having perhaps a throw of several hundred feet1 has been mapped by ,mr4 ,h4 ,b4 ,woodward4 ,the largest dislocation observed by me among the basalt-plateaux of the ,inner ,hebrides is that already referred to 7p4 #bji71 which runs at the back of the ,morven outlier1 in the west of ,argyllshire1 from the ,sound of ,mull by the head of ,loch ,aline to the mouth of ,loch ,sunart1 along the line of valley that contains the salt-water fjord ,loch ,teacus and the fresh-water lakes ,loch ,durinemast and ,loch ,arienas4 ,while the ,cretaceous deposits and the bottom of their overlying basalts rise but little above the sea-level on the south-west side of this line1 they are perched as outliers on hill-tops on the north-east side1 where they rise to #acjj feet above the sea4 ,the amount of vertical displacement here probably exceeds #ajjj feet4 ,the fault runs in a north-westerly direction1 and has obviously been the guiding influence in the erosion of the broad and deep valley which marks its course at the surface4 ,this dislocation is only the largest of a number by which the basalt-plateau has been broken in the district of ,morven4 ,their effects are well shown in the outlier of basalt which caps ,ben ,iadain1 where two parallel faults bring down the lavas against the platform of schists on which they lie 7see ,fig4 #bff74 ,many faults have been traced in the ,antrim plateau1 and are represented on the ,geological ,survey ,maps4 ,in general they are of comparatively trifling displacement4 ,occasionally1 however1 they amount to several hundred feet1 as in those already referred to as occurring near ,ballycastle and around the southern part of the basin of ,lough ,neagh4 ,to what extent the dislocations that traverse the ,british ,tertiary basalts are to be regarded as comparable to those which in ,iceland have been referred to subsidence caused by the tapping and outflow of the lower still liquid parts of lava-sheets must be matter for further inquiry4 ,so far as my own observations have yet gone1 the faults do not seem explicable by any mere superficial action of the kind supposed4 ,where they descend through many hundreds of feet of successive sheets of basalt1 and dislocate the ,secondary formations underneath1 they must obviously have been produced by much more general and deep-seated causes4 ,it is conceivable that1 if these dislocations took place during the volcanic period1 they broke up the lava-plains into sections1 some of which sank down so as to leave a vertical wall at the surface on one side of the rent1 or even to form open gjs1 like those of ,iceland4 ,but it is noteworthy that the fissures1 which have been filled with basalt and now appear as dykes1 comparatively seldom show any displacement in the relative levels of their two sides4 ,in ,iceland1 also1 the great lava-emitting fissures seem to be in general free from marked displacements of that kind4 ,illustration3 ,fig4 #chb4--,reversed fault on the eastern side of ,svin1 ,faroe ,isles4 ,the faults in the ,inner ,hebrides1 so far as ,i have observed1 are all normal1 and indicate nothing more than gentle subsidence4 ,but among the ,faroe ,islands ,i have come upon several instances of reversed faults1 which1 in spite of the usually gentle inclinations of the basalts1 probably point to more vigorous displacement within the terrestrial crust4 ,on the east side of ,svin a fault with a low hade runs from sea-level up to the top of the cliff1 a height of several hundred feet4 ,it has a down-throw of a few yards1 but is a reversed fault1 as will be seen from ,fig4 #chb4 ,another similar instance may be noticed on the north-east headland of ,sand1 where1 however1 on the upcast side1 the basalts appear as if they had been driven upward1 a portion of them having been pushed up into a low arch 7,fig4 #chc74 ,illustration3 ,fig4 #chc4--,reversed fault on the north-east headland of ,sand1 ,faroe ,isle4 ,when the ,tertiary basalt-plateaux of the ,hebrides and the ,faroe ,isles come to be worked out in detail1 many examples of dislocation will doubtless be discovered4 ,we shall then learn more of the amount and effects of the terrestrial disturbances which have affected ,north-,western ,europe since older ,tertiary time4 ,in the meantime evidence enough has been adduced to prepare us for proofs of very considerable recent displacements even among regions of crystalline schists1 like that which has been disrupted by the ,morven faults above alluded to4 ,while the study of the ,tertiary volcanic rocks demonstrates the vast general denudation of the country since older ,tertiary time1 the proofs that these rocks have been faulted acquire a special interest in relation to the origin and evolution of the topography of the region4 ,,chapter ,l ,,effects ,,of ,,denudation ,among the more impressive lessons which the basalt-plateaux of ,north-,western ,europe teach the geologist1 the enormous erosion of the surface of this part of the continental area since older ,tertiary time takes a foremost place4 ,he may be ready almost without question to accept the evidence adduced in favour of a vast amount of denudation among such soft and incoherent strata as those of the older ,tertiary formations of the south-east of ,england or the north-west of ,france4 ,but he is hardly prepared for the proofs which meet him among the north-western isles that such thick masses of solid volcanic rocks have been removed during the same geological interval4 ,to gain some idea of the amount of this waste we must1 in the first place1 picture to our minds the extent of ground over which the lavas were poured1 and the depth to which they were piled upon it4 ,though we may never be able to ascertain whether the now isolated basalt-plateaux of ,britain were once united into a continuous plain of lava1 we can be quite certain that every one of these plateaux was formerly more extensive than it is now1 for each of them presents1 as its terminal edge1 a line of wall formed by the truncated ends of horizontal basalt-sheets4 ,and there seems no improbability in the assumption that the whole of the great hollow from the centre of ,antrim up to the ,minch was flooded with lavas which flowed from many vents between the hills of ancient crystalline rocks forming the line of the ,outer ,hebrides on the west1 and those of the mainland of ,scotland on the east4 ,it is certain that the depth to which some parts of this long hollow were overflowed with lava exceeded #cjjj feet1 for more than that depth of rock can be shown to have been in some places removed4 ,the original inequalities of surface were buried under the volcanic materials which were spread out in a vast plain or series of plains1 like those that have been deluged by modern eruptions in ,iceland4 ,owing1 however1 to a general but unequal movement of subsidence1 the lava-fields sank down here and there to1 perhaps1 an extent of several hundred feet1 so that the old land-surface on which they began to be poured out now lies in those places below the level of the sea4 ,i have shown that even during the volcanic period1 while the lavas were still flowing from time to time1 erosion was in active progress over the surface of the volcanic plain4 ,the records of river-action in ,canna and ,sanday1 and the buried river-channel of the ,scuir of ,eigg1 prove that1 while eruptions still continued1 rivers descending from the mountains of the ,western ,highlands carried the detritus of these uplands for many miles across the lava-fields1 swept away the loose material of volcanic cones1 and cut channels for themselves out of the black rugged floor of basalt4 ,the erosion thus early begun has probably been carried on continuously ever since4 ,the present streams may be looked upon as practically the same as those which were flowing in the ,tertiary period4 ,there may have been slight changes of level1 oscillations both upward and downward in the relative positions of land and sea1 and shiftings of the water-courses to one side or other2 but there seems no reason to doubt that the existing basalt-plateaux1 which were built up as terrestrial areas1 have remained land-surfaces with little intermission ever since1 although their lower portions may have been in large measure submerged4 ,in the existing valleys1 fjords and sea-straits by which these plateaux have been so deeply and abundantly trenched1 we may recognize some of the drainage-lines traced out by the rivers which flowed across the volcanic plains4 ,the results achieved by this prolonged denudation are of the most stupendous kind4 ,the original lava-floor has been cut down into a fragmentary tableland4 ,hundreds of feet of solid rock have been removed from its general surface4 ,outliers of it may be seen scattered over the mountains of ,morven1 whence they look into the heart of the ,highlands4 ,others cap the hills of ,rum1 where they face the open ,atlantic4 ,several miles from the main body of the plateau in ,skye1 a solitary remnant1 perched on the highest summit of ,raasay1 bears eloquent witness that the basaltic tableland once stretched far to the east of its present limits4 ,two lines of observation and of argument may be followed in the effort to demonstrate how great the denudation has been since older ,tertiary time4 ,in the first place1 there is the evidence of the level or nearly level sheets of basalt that form the plateaux1 and1 in the second place1 there is the testimony of the dykes1 sills and bosses by which these lavas have been disrupted4 #a4 ,the study of the denudation of the ,tertiary volcanic rocks of ,north-,western ,europe is most satisfactorily begun by an attempt to measure the minimum amount of waste which in certain places the basalt-plateaux can be proved to have undergone4 ,for the purposes of this study1 the stratification of the lavas and their nearly horizontal1 or at least very slightly disturbed1 position afford exceptional facilities4 ,amorphous rocks1 such as granites and gabbros1 or even foliated masses like the old gneisses and schists1 may have been enormously denuded4 ,their mere presence at the existing surface may be taken as proof of such waste1 yet they furnish in themselves no criterion by which the amount of removed material may be estimated4 ,but in the case of the basalt-plateaux1 as in that of horizontal sedimentary formations1 the successive lines of superposition of the component beds of the whole stratigraphical series supply admirable datum-lines which1 on the one hand1 vividly impress the imagination by the demonstration which they afford of the reality and magnitude of the denudation1 and1 on the other hand1 furnish a measure by which the minimum amount of this denudation may be actually computed4 ,availing ourselves of this kind of evidence it is easy to show that valleys many miles long1 several miles broad1 and from crest to bottom several thousand feet deep1 have been excavated out of the basalt-plateaux since the close of the volcanic period4 ,and if this conclusion can be demonstrated for these plateaux1 it must obviously apply equally to the rest of the country4 ,we thus obtain a most important contribution to the investigation of the origin and relative age of the present topographical features of the surface of the land4 ,let me give a few illustrations of the nature of the investigation and of the results to which it leads4 ,throughout the ,western and ,faroe ,islands the level bars of basalt present their truncated ends in the great escarpment-cliffs which wind mile after mile along their picturesque coasts4 ,where they front the open sea1 it is obviously impossible to say how much further seaward they once extended4 ,but where they retire in fjords or sea-lochs1 and sweep inland into glens1 it is easy to measure the distance from the bottom of the eroded hollow to its bounding watersheds1 and to estimate the amount of material that has been worn out of it4 ,the only uncertainty in this computation arises from our inability to determine to what extent movements of subsidence may have come into play to aid in the disappearance of the basalts4 ,where the bottom of the lavas can be seen at the same level on either side of an inlet1 with no evidence of faulting1 or where a definite horizon in the volcanic series can be traced round the head of a glen or sea-loch1 the influence of underground movements may be eliminated4 ,the evidence of vast denudation is always visible1 the proofs of subsidence are much less frequently observable4 ,the island of ,mull supplies many striking examples of the enormous waste of the basalt-plateau4 ,the ,sound of ,mull1 for instance1 has been eroded out of the volcanic series for a distance of #bj miles1 with a mean breadth of about two miles4 ,from the deepest part of this fjord to the summit of the ,mull plateau is a vertical height of #cfjj feet4 ,the whole of this vast excavation has taken place since older ,tertiary time4 ,on the opposite side of ,mull the hollow of ,loch ,scridain has been eroded to a mean depth of at least #abjj feet below the average level of the surrounding plateau1 with a breadth of rather more than a mile4 ,the scattered islands which lie to the west of ,mull tell the same tale4 ,they are all outliers of the same basalt-plateau1 and have not only been greatly lowered by the removal of their upper lavas1 but have been separated by the erosion of long and deep hollows between them4 ,thus from the summit of the ,gribon cliffs in ,mull to the deepest part of the sea-floor between that precipice and the ,treshnish ,isles a vertical depth of at least #bjjj feet of rock has been removed since the basalts ceased to be erupted4 ,i have referred to the impressive evidence of denudation displayed on the west side of the island of ,eigg4 ,the vertical distance from the summit of the ,eigg plateau to the bottom of the submarine valley between this island and ,rum is about #aejj feet1 but as that summit lies below the original surface of the lava-field1 the depth of rock which has been removed must exceed #aejj feet4 ,we thus learn that since the close of the volcanic period the hollow between the islands of ,eigg and ,rum has been eroded to this great depth4 ,still more striking is the evidence of enormous waste presented by the ,faroe ,islands4 ,the cliffs there are loftier and barer1 and the fjords have been cut more deeply and precipitously out of the basalt-plateau4 ,i shall never forget the first impression made on my mind when the dense curtain of mist within which ,i had approached the southern end of the archipelago rapidly cleared away1 and the sunlit slopes and precipices of ,suder1 the two ,dimons1 ,sku and ,sand1 rose out of a deep blue sea4 ,each island showed its prolongation of the same long level lines of rock-terrace4 ,the eye at once seized on these features as the dominant element in the geology and the topography1 for they revealed at a glance the true structure of the islands1 and gave a measure of the amount and irregularity of the erosion of the original basalt-plateau4 ,and this first impression of stupendous degradation only deepened as one advanced further north into the more mountainous group of islands4 ,probably nowhere else in ,europe is the potency of denudation as a factor in the evolution of topographical features so marvellously and instructively displayed as among the north-eastern members of the ,faroe group4 ,availing ourselves of the datum-lines supplied by the nearly level bars of basalt1 we easily perceive that in many parts of the ,faroe ,isles the amount of volcanic material left behind1 stupendous though it be1 is less than the amount which has been removed4 ,thus the island of ,kals is merely a long narrow ridge separating two broad valleys which are now occupied by fjords4 ,the material carved out of these valleys would make several islands as large as ,kals4 ,again1 the lofty precipice of ,myling ,head1 #bbfj feet high1 built up of bedded basalts from the summit to below sea-level1 faces the north-western ,atlantic1 and the sea rapidly deepens in front of it to the surface of the submarine ridge #bjj to #cjj feet below4 ,the truncated ends of the vast pile of basalt-sheets which form that loftiest sea-wall of ,europe bears testimony to the colossal denudation which has swept away all of the volcanic plateau that once extended further towards the west4 ,nevertheless1 enormous as has been the waste of this plateau of the ,faroe ,islands1 we may still trace some of its terrestrial features that date back probably to the volcanic period4 ,even more distinctly1 perhaps1 than among the ,western ,isles of ,scotland1 we may recognize the position of the original valleys1 and trace some of the main drainage lines of the area when it formed a wide and continuous tract of land4 ,a line of watershed can be followed in a south-westerly direction from the east side of ,vider1 across ,bor to the centre of ,oster1 and thence by the ,sund across ,strom and ,vaag4 ,from this line the fjords and valleys diverge towards the north-west and south-east4 ,there can hardly be any doubt that on the whole this line corresponds with the general trend of the water-parting at the time when the ,tertiary streams were flowing over the still continuous volcanic plain4 ,considerable depression of the whole region has since then sent the sea up the lower and wider valleys1 converting them into fjords1 and isolating their intervening ridges into islands4 ,the topography of the ,faroe ,islands seems to me eminently deserving of careful study in the light of its geological origin4 ,there is assuredly no other region in ,europe where the interesting problems presented by this subject could be studied so easily1 where the geological structure is throughout so simple1 where the combined influences of the atmosphere and of the sea could be so admirably worked out and distinguished1 and where the imagination1 kindled to enthusiasm by the contemplation of noble scenery1 could be so constantly and imperiously controlled by the accurate observation of ascertainable fact4 #b4 ,impressive and easily comprehended as are the proofs of denudation supplied by the basalts of the plateaux1 they are perhaps to a geological eye less overwhelming than those furnished by the eruptive rocks which have been injected into these plateaux4 ,in the case of at least the basic intrusions1 we may reasonably infer that they assumed their present position under a greater or less depth of overlying rock which has since been removed4 ,when1 therefore1 they are found at or above the summits of the plateaux1 they demonstrate that a vast amount of material has been removed from these summits4 ,the argument from the position of the dykes has already been enforced4 ,it is absolutely certain that valleys several thousand feet deep must have been excavated since these dykes were erupted1 for had such valleys existed at the time when the dykes were injected across their site1 the molten rock1 instead of ascending to the tops of the surrounding mountains1 would obviously have rushed forth over the valley-bottoms4 ,i have shown that this reasoning applies not merely to the volcanic districts1 but to the whole surface of the country within the region of dykes4 ,thus the uplands of ,southern ,scotland1 and wide areas in the ,southern and ,western ,highlands1 can be proved to have had glens cut out of their mass to a depth of hundreds of feet since the ,tertiary volcanic period4 ,not less convincing is the evidence afforded by the great eruptive masses of gabbro4 ,we have seen that these complex accumulations of sills1 dykes1 and bosses include rocks so coarse in grain as to show that they must have consolidated at some considerable depth1 but that they now appear in hill-groups #bjjj to #cjjj feet in height1 the whole of the original basaltic cover having been stripped off from them4 ,but these gabbro hills have been in turn traversed up to the very crests by later basalt-dykes1 which thus supply additional proof that the erosion here has been stupendous4 ,the granophyre bosses tell the same tale4 ,though1 like the domite ,puys of ,auvergne1 they may still retain1 in their conical forms1 indications of the original shapes which their component material assumed at the time of its protrusion1 we may be confident that their existing surfaces have been reached after the removal of much rock which once lay above them4 ,this inference is confirmed by the fact that these eruptive bosses have been invaded by a younger system of dykes4 ,the black ribs of basalt which may be traced along their pale declivities1 which cross the glens that have been eroded in them and which mount up to their very crests1 prove that since the latest manifestations of volcanic energy in the ,west of ,scotland1 extensive changes in the topography of the land have been effected by the operation of the subrial agents of degradation4 ,so much for what can be demonstrated4 ,but how much more may1 with the highest probability1 be inferred6 ,the original limits of the plateaux are unknown4 ,the waves of the wide ,atlantic now roll over many a square league of the old lava-plains1 and wide tracts of the islands and the mainland from which the basalt has been entirely stripped1 or where it remains only in scattered outliers1 were once deeply buried under piles of lava-sheets4 ,it would probably be no exaggeration to affirm that over the ,british area1 as well as over the ,faroe ,isles1 the amount of ,tertiary volcanic rock that now remains1 large as it is1 falls short in amount of what has been removed4 ,the geologist who has made himself familiar with the effects of denudation in other ,tertiary volcanic districts1 such as ,central ,france1 ,saxony and ,bohemia1 will be prepared for almost any conceivable amount of erosion among the far older volcanic series of the north-west of ,europe4 ,to the student of the origin of the existing topography of the land there is a profound interest in the demonstration which these volcanic rocks supply of the vast changes which the terrestrial surface has undergone within a period geologically so recent as older ,tertiary time4 ,when1 on the one hand1 he finds himself more and more restricted in his demands for time by the confident assertions of the physicist that all the phenomena of geological history must have been comprised within a few millions of years1 and when1 on the other hand1 he watches the seemingly feeble and tardy operations of the forces of denudation and sedimentation which have played the chief parts in that history1 he may well be excused if sometimes he is apt to despair of ever reconciling the facts which he observes with the physical deductions that are somewhat dogmatically brought forward in opposition to his interpretation of them4 ,he may feel sure that his facts cannot be gainsaid1 and he may be unable to find any other way of comprehending them save by the admission that they necessitate a liberal allowance of time4 ,yet he may not feel himself to be in a position to offer any valid objections to the arguments from physical considerations that would so seriously abridge the length of time which geology requires4 ,in these circumstances it is some satisfaction to be provided with definite measurements of the amount of geological change which has been effected within a limited and relatively recent period of time4 ,this change has resulted from the operation of the same agents by which it is still being carried on4 ,no break in the history can be detected4 ,there is not the least reason to suppose that the agents of denudation and sedimentation have1 during the period in question1 differed in their rate of working4 ,their activity at the present time is probably neither greater nor less than it was then4 ,if1 therefore1 during so recent an interval such a stupendous amount of material has been worn away from the surface of the land and deposited on the sea-floor as the ,tertiary volcanic rocks demonstrate1 the geologist may surely contemplate without misgiving the lapse of time required for the completion of older geological revolutions4 ,he may oppose to the arguments of the physicist the measurements and computations which he himself makes from data which are at least as reliable as the postulates whereon these arguments are based4 ,the rate at which denudation and sedimentation are now taking place has been measured with tolerable accuracy1 and a fair average for it has been obtained4 ,whatever may be maintained as to this rate in early geological ages1 there can be no serious opposition to its being taken as fairly constant since older ,tertiary time4 ,we are thus provided with data for estimating the minimum amount of time that can have elapsed since the volcanic plateaux began to be denuded4 ,but as no relic remains of the original upper surface of those plateaux1 and as we are consequently ignorant of how much rock has been removed from their highest surviving outliers1 it is obvious that such estimates are more likely to err in understating than overstating the amount of time required4 ,it would be beyond the scope of the present volume to enter fully into the measurements and calculations required for the adequate treatment of this subject4 ,i will merely illustrate my argument by again taking a few data from the plateau of ,mull4 ,the original height of this plateau is shown by the outlier of ,ben ,more to have been at least #cbjj feet4 ,if to this figure we add the portion of the basalt-group submerged under the sea the height will probably be increased by several hundred feet4 ,but let us take #cjjj feet as a moderate computation for the average thickness of the volcanic series here at the close of the plateau-period4 ,until a number of sections have been carefully plotted from the ,ordnance ,maps1 in order to ascertain with approximate accuracy the average height of the present surface of the ,mull basaltic plateau1 making due allowance for the vast erosion of the ,sound of ,mull and the numerous glens and sea-lochs that traverse the island1 any estimate which may be offered as to this average must be merely provisional4 ,if1 in the meantime1 we suppose the present mean level of the plateau to be #ajjj feet above the sea1 the difference between this amount and the assumed original height will be #bjjj feet4 ,if1 further1 we take the present average rate of degradation of the ,mull plateau to be #a_/fjjj of a foot in a year1 which has been shown to be probably a fair estimate1 then the time required for the lowering of the ,mull plateau from its original to its present average level amounts to twelve millions of years4 ,yet this period1 vast though it be1 does not carry us back even as far as the beginning of ,tertiary time4 99 99 99 99 99 ,in concluding this lengthened discussion of the ,tertiary volcanic history of ,britain1 ,i may1 perhaps1 usefully add a brief summary of the leading features of the long record4 ,the region within which volcanic activity displayed itself during older ,tertiary time in the ,british ,isles1 if our estimate of its area is restricted to those parts of the country where igneous rocks1 probably of that age1 now appear at the surface1 embraces the ,north of ,england and of ,ireland1 the southern half and the west coast of ,scotland--a total area of more than #dj1jjj square miles4 ,over that extensive region volcanic phenomena were displayed during an enormously protracted interval of geological time4 ,the earliest beginnings of disturbance may possibly have started in the ,eocene1 and the final manifestations may not have ceased until the ,miocene period4 ,so prolonged was the duration of the eruptions1 that enormous topographical changes from denudation1 and probably also considerable variation in the fauna and flora1 alike of land and sea1 may have been effected4 ,owing to some cause which has not yet in this relation been investigated1 but which is probably referable to secular terrestrial contraction1 the volcanic region underwent elevation1 while1 at the same time1 a vast subterranean lake or sea of molten rock existed underneath it4 ,enormous horizontal tension thus arose1 and at last the stretched terrestrial crust gave way4 ,a system of approximately parallel fissures opened in it1 having a general direction towards north-west4 ,the rapid and simultaneous production of such a gigantic series of rents must have given rise to earthquakes of enormous magnitude and destructive force4 ,the great majority of the fractures1 doubtless1 did not reach to the surface of the ground1 though probably not a few did so4 ,such was the potency of this development of terrestrial energy1 that the fissures ran through the most varied kinds of rocks and the most complicated geological structures1 crossing even earlier lines of powerful dislocation1 and yet retaining their direction and parallelism for sometimes #ej or #ajj miles4 ,into the fissures thus formed the molten magma from underneath was forced for many hundreds or even thousands of feet above the surface of the subterranean lava-reservoir4 ,solidifying between the fissure walls1 it formed the crowd of basic dykes that stand out as the most widespread and distinctive feature of the volcanic region4 ,where the fissures reached the surface or near to it1 the molten rock would seek relief by egress in streams of lava4 ,this probably occurred in many places from which subsequent denudation has removed all vestige of superficial volcanic manifestations4 ,but1 in the great range of basalt-plateaux1 from ,antrim northwards through the chain of the ,inner ,hebrides1 there are still left abundant remains of the surface-outflows4 ,like the modern lavas of ,iceland1 the molten material probably flowed out sometimes from the open fissures1 sometimes from vents formed along the chasms4 ,after the convulsions ceased which produced the earliest dykes1 the communication that had been established between the magma-reservoir underneath and the air above would be maintained1 and repeated eruptions might take place1 either from the original fissures and vents or from others afterwards opened by the volcanic energy4 ,as in the modern eruptions of ,iceland1 new fissures are successively opened through the older lava-sheets1 so in the ,tertiary volcanic areas1 renewed ruptures of the earth's crust allowed later dykes to be formed4 ,the basalt-plateaux are traversed by such dykes1 even up to their highest sheets4 ,it is impossible to say how often the process of dyke-making may have been repeated4 ,not improbably it recurred again and again during the building of the basalt-plateaux1 and we know that it was renewed even after the protrusion of the granophyre bosses which mark one of the latest phases of volcanism in the region4 ,for a protracted geological period1 with long intervals of quiescence1 various basic lavas 7basalts1 dolerites1 etc471 with occasionally some of intermediate composition 7andesites1 trachytes71 and perhaps in ,antrim acid rhyolites1 flowed out from fissures and vents until they had filled up the hollows of the great valley1 which then stretched from the south of ,antrim northwards between the west coast of ,scotland and the chain of the ,outer ,hebrides4 ,in some places the accumulated pile of these ejections even now exceeds #cjjj feet in thickness1 but we cannot tell how much material has been bared away from its top by denudation4 ,the volcanic discharges consisted mostly of lava1 fragmentary materials being comparatively insignificant in amount and local in origin1 though layers of fine tuff and basalt-breccias occur in all the plateaux4 ,none of the erupted materials thicken towards any centres that might be taken to mark volcanoes of the type of ,vesuvius or ,etna4 ,on the contrary1 the persistent flatness and uniformity of the volcanic series1 and the thinning out of the separate beds in different directions1 show that the lavas issued from many points all over the region4 ,the positions of some of the actual vents can still be ascertained4 ,they are now filled sometimes with dolerite1 sometimes with coarse agglomerate4 ,the surface over which the lava flowed seems to have been mainly terrestrial4 ,here and there1 between the successive sheets of basalt1 the leaves1 stems1 and fruit of land-plants1 sometimes in most perfect preservation1 may be observed1 together with the remains of insects and fresh-water fish4 ,distinct relics of old river-channels can be recognized which have been buried under streams of lava4 ,among the deposits left by these streams the uppermost layers are commonly dark with decayed vegetation1 while layers of coal are found here and there between the basalts4 ,as the pile of erupted materials gradually thickened1 and the subterranean energy possibly grew feebler1 the ascending magma was forced between the layers of sedimentary strata underneath the basalts1 or between these strata and the overlying volcanic series1 or along any other plane of weakness in the terrestrial crust4 ,in this way arose the multitudinous sills or intrusive sheets4 ,when the great volcanic plateaux had been built up to a thickness of several thousand feet1 another remarkable episode in the history occurred4 ,at certain points large bodies of coarsely crystalline basic rocks were pushed into and through the plateaux-basalts1 upraising them in dome-shaped elevations1 and ultimately solidifying as dolerites1 gabbros1 troctolites1 picrites1 etc4 ,there is reason to believe that the points of extravasation of these materials were mainly determined by the positions of the larger or more closely clustered vents of the plateau-period1 where points of weakness consequently existed in the terrestrial crust4 ,rising as huge bosses through such weak places1 the gabbros and associated rocks raised up the overlying bedded basalts1 and forced themselves between them1 forming thus a fringe of finer-grained intrusive sills and veins around the central banded and amorphous masses of more coarsely crystalline material4 ,whether1 in any of these vast domes of upheaval1 the summit was disrupted1 so as to allow the basic intrusion to flow out as lava at the surface1 cannot now be told1 owing to enormous subsequent denudation4 ,the next chapter in the chronicle shows us that probably long after the eruption of the gabbros1 when possibly all outward symptom of volcanic action had ceased1 a renewed outbreak of subterranean activity gave rise to the protrusion of another and wholly different class of materials4 ,this time the rocks were of a markedly acid type4 ,they included varieties that range from obsidians1 pitchstones1 flinty felsites and rhyolites1 through porphyries and granophyres1 into compounds which cannot be classed under any other name than granite4 ,these masses likewise availed themselves of older vents in the plateaux1 and broke through them4 ,they now form huge conical hills1 which1 in their outer aspect1 and even to some extent in their inner structure1 recall the trachytic puys of ,auvergne4 ,but the granophyres not only ascended through the basalt-plateaux and the gabbro-bosses2 they sent into these rocks a network of veins1 pushed their way in huge sheets or sills between the strata below1 and actually incorporated a considerable proportion of the basic materials into their own substance4 ,around the bosses of gabbro and granophyre1 the bedded basalts have undergone considerable contact-metamorphism4 ,the gabbro and granophyre bosses of the ,inner ,hebrides demonstrate with singular force how unreliable petrographical characters are as a test of the relative age of rocks4 ,no one1 looking at hand-specimens of these rocks1 or even studying them in the field1 would at first suspect them to be of ,tertiary date4 ,they closely resemble rocks of similar kinds in ,palozoic and even ,archan formations4 ,yet1 of their late appearance in geological time1 there cannot be any possibility of doubt4 ,after the uprise of the granophyre1 and the injection of the network of felsitic veins1 there came once more a period of terrestrial convulsion1 like that of the earliest basic dykes1 but of less intensity4 ,again1 the crust of the earth over the volcanic region was pushed upward and rent open by another system of parallel fissures4 ,again1 from a reservoir or basin of basic lava underneath1 molten rock was forced upwards into the rents1 and thus another system of basic dykes was formed4 ,these dykes are found crossing those of earlier date1 and rising through the other volcanic rocks4 ,they traverse the plateau-basalts from bottom to top2 they climb to the summits of the gabbro mountains1 and they even pursue their undeviating course over the huge domes of granophyre4 ,no proof has yet been found that from any of these dykes there was a superficial outflow of lava4 ,but so great has been the subsequent denudation of the areas1 that such outflows might quite well have taken place1 and have subsequently been destroyed4 ,whether these basic dykes were the last manifestation of volcanic energy in our region cannot yet be decidedly affirmed4 ,so far as the evidence at present goes1 they are possibly older than another series of acid veins and dykes 7pitchstone1 felsite1 and granophyre71 which are found at many points from ,antrim to the far end of the ,inner ,hebrides4 ,these protrusions traverse every other member of the volcanic series1 except some of the youngest basic dykes1 and do not appear to be themselves cut by any4 ,since the close of the volcanic period considerable disturbance of the basalt-plateaux has taken place4 ,the whole volcanic region has subsided1 some districts having sunk more than others4 ,in ,britain the most striking evidence of such depression is supplied by the basin of ,lough ,neagh4 ,but throughout the ,inner ,hebrides much of the lower portion of the terrestrial lava-plateaux is now below sea-level4 ,in the ,faroe ,islands and in ,iceland the subsidence has been still more marked4 ,dislocations1 also1 sometimes amounting to more than a thousand feet of displacement1 have occurred among the volcanic masses4 ,the bedded basalts1 originally on the whole nearly flat1 have thus been broken up into large blocks of country wherein the sheets are now inclined in various directions4 ,one of the most important lessons taught by the ,tertiary volcanic series of the north-west of ,europe is the extent of the denudation of the land since the close of the volcanic period4 ,the horizontal or gently inclined layers of bedding among the basalts afford datum-lines from which the minimum amount of material removed may be measured4 ,as a reasonable estimate it may be inferred that in the case of the ,mull plateau1 for example1 the average amount by which its surface has been lowered since the close of the volcanic period cannot be less than #bjjj feet4 ,if the rate of lowering of the land-surface in western ,europe by subrial denudation be taken as #a_/fjjj of a foot in a year1 then the lapse of time required for the degradation of the ,mull plateau must amount to about twelve millions of years4 ,some such interval has therefore elapsed since the last ,tertiary volcanoes became extinct4 ,,chapter ,,li ,,summary ,,and ,,general ,,deductions ,the foregoing chapters comprise a connected narrative of the history of volcanic action in the area of the ,british ,isles during the vast succession of ages from the early ,archan dawn down to the latest eruptions of ,tertiary time4 ,in this final chapter ,i propose to present a brief summary of the facts of largest import and widest interest which this protracted history has placed before us1 together with a statement of deductions which may be drawn from them regarding the nature and progress of volcanism in the evolution of the globe4 #a4 ,among the broad features which soonest arrest attention in such a survey is the geographical position of the theatre of this volcanic activity4 ,in the distribution of volcanoes at the present time we are familiar with their tendency to range themselves along continental borders or in oceanic islands4 ,the volcanic energy so conspicuous in the geological history of ,britain has shown itself along the western or ,atlantic margin of the ,european continent4 ,when the eruptions have not been actually on the land itself1 they have taken place within the shallow tracts near the land1 where the lavas and tuffs have been interstratified with sediments derived from the adjacent coasts4 ,moreover the volcanic rocks in ,britain are ranged along the greatest length of the group of islands1 in a general north and south line1 from the south of ,devonshire to the far ,shetlands4 ,it is on the western side of the country that they occur4 ,east of a line drawn from ,berwick by ,leicester to ,exeter1 although the geological formations1 ranging from the ,carboniferous ,limestone to the latest ,pleistocene deposits1 are there abundantly exposed to view1 they include no contemporaneous volcanic rocks4 #b4 ,a second and still more remarkable feature in the geological history of ,western ,europe is the persistence of volcanic activity along the site of the ,british ,isles4 ,evidence has been brought forward in these volumes that from the primeval time vaguely termed ,archan1 onward to that of the older ,tertiary clays and sands of the south-east of ,england--that is to say1 through by far the largest part of geological history1 as chronicled in the stratified crust of the globe--this long strip of territory continued to be intermittently a theatre of volcanic action4 ,every great division of ,palozoic time was marked by volcanic eruptions1 sometimes over tracts hundreds of square miles in area and on a colossal scale4 ,after a long period of quiescence during the ,mesozoic ages1 the renewed outbreak of volcanic energy in older ,tertiary time1 so marked over the western half of ,europe1 reached its maximum of development along the ,atlantic border1 from the north of ,england and ,ireland through the chain of the ,inner ,hebrides to the ,faroe ,islands1 ,iceland and ,greenland4 #c4 ,not only has there been a remarkable persistence of volcanic activity over the comparatively limited area of the ,british ,isles1 viewed as a whole1 but if we examine the different parts of this area we perceive that many of them1 of relatively restricted extent1 have been the sites of a recrudescence of volcanic action1 again and again1 through a vast succession of geological periods4 ,while the whole region has been in different quarters and at different times affected1 there have been districts where the volcanic fires have been rekindled after long intervals of quiescence1 the new vents being opened among or near to the sites of earlier volcanoes4 ,in the south-west of ,england1 for example1 the ,middle ,devonian tuffs and diabases were succeeded in the ,carboniferous period by the eruptions of the ,culm-measures1 and in the very same tracts came last of all the lavas and tuffs of the ,permian conglomerates4 ,still more astonishing is the record of volcanic energy in the south of ,scotland1 where1 within a space of not many hundred square miles1 there are the chronicles of the ,arenig1 ,llandeilo and ,bala eruptions of the ,southern ,uplands1 the huge piles of lavas and tuffs of the ,lower ,old ,red ,sandstone1 the long succession of the plateaux and then of the puys of the ,carboniferous period1 the groups of tuff-cones of the ,permian period1 and1 lastly1 the numerous dykes connected with the ,tertiary volcanoes4 ,while some portions of the region have been specially liable to exhibitions of volcanic action1 others have continuously escaped4 ,some of these horsts1 or stationary and unaffected blocks of country1 have been surrounded by or have risen close to the borders of this volcanic district1 yet have maintained their immunity through a long series of ages4 ,thus the ,central ,highlands of ,scotland1 though they were flanked on the south and south-west by the active volcanoes of the ,old ,red ,sandstone1 and again on the south by those of ,carboniferous time1 had no vents opened on their surface after the metamorphism of their schists4 ,still more striking perhaps is the immunity of the ,southern ,uplands4 ,though they were in large measure surrounded by the volcanoes of the ,lower ,old ,red ,sandstone1 then by those of the ,calciferous ,sandstones and ,carboniferous ,limestone1 and though they looked down on the ,permian eruptions of ,ayrshire and ,nithsdale1 which spread streams of lava and showers of ash along their flanks1 these hills formed a solid block that seems to have resisted perforation by the volcanic funnels4 ,again1 the tracts covered with ,carboniferous ,limestone in ,england and ,ireland almost entirely escaped from invasion by volcanic eruptions4 ,we thus learn that even within comparatively restricted regions some portions of the terrestrial crust have been areas of weakness1 liable to serve again and again as lines of escape for volcanic energy1 while close to them other portions of greater solidity have been persistently left intact4 #d4 ,the sites of volcanic vents in all the geological systems wherein they occur in ,britain have not usually been determined by any obvious structure in the rocks now visible4 ,they comparatively seldom depend on ascertainable lines of fault1 even when faults1 probably already existent1 occur in their near neighbourhood4 ,this independence1 to which1 however1 there are occasional marked exceptions1 comes out more particularly in the coal-fields pierced by vents1 for mining operations have there revealed the positions of many more faults than can be traced at the surface4 ,if the sites of the vents have been fixed by dislocations or lines of weakness in the terrestrial crust1 these must generally lie below the formations now visible at the surface4 ,there is one striking connection between the sites of the vents and ancient topographical features to which frequent reference has been made in the foregoing chapters4 ,all through the long volcanic history1 as far back as such features can be traced1 we see that orifices of discharge for the erupted materials have been opened along low grounds and valleys rather than on ridges and hills4 ,the great central hollow of the ,scottish midlands was a depression even as long ago as the time of the ,lower ,old ,red ,sandstone1 and though it has probably been several times since then filled up1 and more or less completely effaced1 its ancient features have been partially revealed by extensive denudation4 ,this vast depression1 #dj miles broad1 between the ,highland mountains on the one side and the ,southern ,uplands on the other1 was the chief centre of volcanic activity in western ,europe during the latter half of ,palozoic time4 ,the vents of the ,old ,red ,sandstone1 ,carboniferous and ,permian series are scattered all over it1 but few or none of them are to be found on the high grounds that bound it4 ,again1 in ,tertiary time1 the great outpouring of lava took place in the hollow that lay between the ridge of the ,outer ,hebrides and the mainland of ,scotland4 ,this wide and long tract of low ground was buried under upwards of #cjjj feet of lava and tuff1 but these materials were erupted from fissures and vents within its own border and not from the mountains on either side4 ,but perhaps the most conspicuous example of any in which the vents keep to the valleys is that supplied by the ,permian necks of ,nithsdale and the neighbouring glens4 ,these depressions are as old as ,permian1 and even as ,carboniferous time1 but they appear to be entirely hollows of erosion2 at least they have yielded no evidence that their direction has been determined by lines of fault4 ,the chain of vents can be followed from the lowlands of ,ayrshire up to the base of the ,southern ,uplands1 down the wide valley cut by the ,nith in these hills and up some of the tributary valleys1 and though the volcanoes continued for some time in vigorous eruption1 not a trace of any contemporary vent has yet been met with on the surrounding hills4 ,while the position of volcanic vents in lines of valley may be generally due to guiding lines of fissure in the crust underneath1 either within or below the rocks visible at the surface1 there may sometimes be conditions in which other dominant causes come into play4 ,the curious coincidence between variations in the upper limit of dykes and inequalities in the configuration of the overlying ground1 suggest that where the subterranean magma has ascended to within a comparatively short distance from the surface1 a difference of a few hundreds or thousands of feet in the depth of overlying rock1 such as the difference of height between the bottom of a valley and the tops of the adjacent hills1 may determine the path of escape for the magma through the least thickness of overarching roof4 #e4 ,volcanic phenomena cannot be regarded as a mere isolated and incidental feature in the physics of the globe4 ,during the short time within which man has been observing the operations of existing volcanoes1 he has hardly yet had sufficient opportunity of watching how far they can be correlated with other terrestrial movements4 ,nor1 when he endeavours to trace some such connection among the records of the geological past1 has he yet collected materials enough to furnish a sufficiently broad and firm basis of comparison4 ,one formidable obstacle is presented by the difficulty in determining chronological equivalents in separated groups of rock4 ,geologists have tried to discover whether the volcanoes of some particular period or region were in any way connected with such geological changes as extensive plication1 dislocations of the crust1 or elevation of mountain-chains4 ,in regard to the volcanic history of ,britain1 various possible relations of this kind obviously suggest themselves4 ,thus the division of geological time comprised within the ,lower ,silurian period was undoubtedly an interval of considerable terrestrial disturbance in western ,europe4 ,the unconformabilities and overlaps in the series of formations belonging to that period1 the frequent conglomerates1 the great and often rapid changes in the thickness and lithological characters of the strata1 all point to instability of land-surface and sea-floor4 ,during these oscillations a prolonged and widespread series of volcanic eruptions took place4 ,the volcanic manifestations began in ,cambrian time and continued in intermittent activity till towards the close of the deposition of the ,lower ,silurian formations4 ,it is certainly a significant fact that the ,upper ,silurian deposits1 in their lithological characters1 present a strong contrast to those that preceded them4 ,they point1 on the whole1 to quiet sedimentation1 during an interval of comparative calm in the terrestrial crust4 ,with this evidence of tranquillity there is1 over almost the whole of the ,british ,isles1 an entire absence of any trace of renewed volcanic activity4 ,with the exception of the ,dingle lavas and tuffs1 in the extreme west of ,ireland1 not a single undoubted instance is yet known of an ,upper ,silurian volcano4 ,after the deposition of the ,upper ,silurian rocks an interval of great terrestrial disturbance ensued1 and these rocks over a large part of ,britain were intensely plicated and crushed4 ,the movements1 continued into the period of the ,lower ,old ,red ,sandstone1 were1 in their later stages1 accompanied or1 at least1 followed by the vast outpourings of lava which now cover so much of the tracts of ,old ,red ,sandstone in ,scotland and ,ireland4#dch ,footnote #dch3 ,trans4 ,geol4 ,soc4 ,edin4 vol4 ii4 part iii4 7#ahgd74 ,in proportion as the volcanic energy was vigorous1 widespread and long-continued1 we may expect it to have been connected with important terrestrial movements affecting extensive regions of the earth4 ,the ,tertiary volcanic history seems to afford a remarkable instance of this connection4 ,a wide area of the ,european continent is dotted over with old centres of volcanic activity which were in eruption at successive epochs throughout the ,tertiary period4 ,of all these centres the most important was that of the north-western basalt-plateaux1 where floods of lava were discharged over many thousand square miles from ,ireland to ,greenland4 ,the geological date of these outpourings probably coincides with the last great orographic movements that gave to the mountain-chains of ,europe their latest elevation and dimensions4 ,but without entering into what must be for the present a field of speculation1 we can be assured of one important fact in the connection of ancient volcanoes with movements of the terrestrial crust4 ,a study of the records of volcanic action in ,britain proves beyond dispute that the volcanoes of past time have been active on areas of the earth's surface that were sinking and not rising4 ,we usually associate volcanic action with elevation rather than subsidence1 and there are certainly abundant proofs of such elevation around active or recently extinct volcanoes4 ,many of the active vents of the present time1 like ,vesuvius and ,etna1 began with submarine eruptions and have been gradually upraised into land4 ,it may be1 however1 that such uprise is merely a temporary incident1 and that if we could survey the whole geological period of which human history chronicles so small a part1 we might find that subsidence1 and not upheaval1 is ultimately the rule over volcanic areas4 ,be this as it may1 there can be no question that with the one solitary exception of the ,tertiary volcanoes1 which were terrestrial and not submarine1 all the ,british vents were carried down and eventually buried under aqueous sediments4 ,even the ,tertiary lava-fields have in many places sunk down below sea-level since their eruptions ceased4 ,that there are any ,palozoic volcanic rocks now visible at the surface is obviously due to subsequent movements not immediately connected with their original conditions of eruption1 and to gigantic denudation4 ,the amount of subsidence which followed on a volcanic episode was sometimes enormous1 even within the same geological period1 as one may see by observing the prodigious piles of sedimentary material heaped over the lavas and tuffs of ,arenig time1 or over those of the ,lower ,old ,red ,sandstone4 ,i do not wish to maintain that the downward movement was necessarily a consequence of volcanic ejections1 for we know that it took place over tracts remote from centres of eruption4 ,but ,i have sometimes asked myself whether it was not possibly increased as a sequel to vigorous volcanic action2 whether1 for instance1 the great depth of the ,palozoic sedimentary rocks in some regions1 as compared with their feeble development in others1 may not have been due to an acceleration of subsidence consequent upon volcanic action4 #f4 ,a review of the geological history of ,britain cannot but impress the geologist with a conviction of the essential uniformity of volcanism in its manifestations since the early beginnings of geological time4 ,the composition and structure of the materials erupted from the interior have remained with but little change4 ,the manner in which these materials have been discharged has likewise persisted from the remotest periods4 ,the three modern types of ,vesuvian cones1 puys and fissure-eruptions can be seen to have played their parts in the past as they do to-day4 ,among the earliest igneous masses of which the relative geological date can be fixed are the dykes which form so striking a system among the ,archan rocks of the north-west1 and show how far back the modern type of volcanic fissures and dykes can be traced4 ,no relic1 indeed1 has survived of any lavas that may have flowed out from these ancient fissures1 but so far as regards underground structure1 the type is essentially the same as that of the ,tertiary and modern ,icelandic lava-fields4 ,the early ,palozoic volcanoes formed cones of lava and tuff comparable to those of such vents as ,vesuvius and ,etna4 ,in the ,lake ,district the pile of material ejected during ,lower ,silurian time was at least #hjjj or #ijjj feet thick4 ,in the ,old ,red ,sandstone basins of ,central ,scotland there were more than one mass of lavas and tuffs thicker than those of ,vesuvius4 ,the puys of the later half of ,palozoic time closely resembled their ,tertiary successors in ,central ,france1 the ,eifel1 and the ,phlegran ,fields4 ,nor1 as regards extent and vigour1 did the eruptions of the geological past differ in any important respect from those of the present time4 ,there is assuredly no evidence that volcanic energy has gradually waned since the dawn of geological history4 ,the latest eruptions of ,north-,western ,europe1 forming the ,tertiary basalt-plateaux1 far exceeded in area1 and possibly also in bulk of material discharged1 all the eruptions that had preceded them in the geological record4 #g4 ,nevertheless1 while the ,tertiary eruptions showed no diminution of vigour1 it is undoubtedly true that the volcanic energy has not manifested itself in a uniform way since the beginning of geological time4 ,there have been periods of maximum activity followed by others of lessened force4 ,thus if we take a broad view of the general features of volcanic action during the ,palozoic ages in ,britain1 we see clear evidence of a gradual diminution in its vigour4 ,the widespread outpourings of lava and tuff in the ,silurian period in ,england1 ,wales1 ,scotland and ,ireland were succeeded by the somewhat diminished1 though still important1 eruptions of the ,lower ,old ,red ,sandstone basins4 ,the latter were followed by the still lessened outflows of the ,carboniferous plateaux1 which in turn were succeeded by the yet feebler and more localized eruptions of the ,carboniferous puys1 the whole prolonged volcanic succession ending in the small scattered vents of the ,permian period4 ,there were of course oscillations of relative energy during this history1 some of the maxima and minima being of considerable moment4 ,but though progress towards extinction was not regular and uniform1 it was a dominant feature of the phenomena4 #h4 ,the ,permian volcanoes were the last of the long ,palozoic series1 and1 so far as we yet know1 the whole of the ,mesozoic periods within the area of ,britain were absolutely unbroken by a single volcanic eruption4 ,the chronological value of this enormous interval of quiescence may1 perhaps1 never be ascertainable1 but the interval must assuredly cover a large part of geological time4 ,it was an era of geological calm1 during which the ,triassic1 ,jurassic and ,cretaceous formations were slowly accumulated over the larger part of ,europe4 ,the stratigraphical quietude was not indeed unbroken4 ,the widespread subsidence of the sea-bottom was interrupted here and there by important upheavals1 and considerable geographical changes were in process of time accomplished4 ,but1 save in one or two widely separated areas of ,europe1 there were no active volcanoes over the whole continent4#dci ,here again the scarcity or absence of intercalated volcanic rocks is in harmony with the general stratigraphy of the formations4 ,footnote #dci3 ,the ,triassic eruptions of ,predazzo and ,monzoni were important1 and traces of others are said to occur in the ,cretaceous system in ,portugal and ,silesia4 #i4 ,after the prodigious interval represented by the whole of the ,mesozoic and the earlier part of the ,tertiary formations1 a time of disturbance arose once more1 and the great basalt-floods of the north-west were poured forth4 ,evidence has been adduced in the foregoing chapters that this latest volcanic period was one of vast duration2 that it was marked by long intervals of quiescence1 and by repeated renewals of volcanic energy4 ,yet over the area of ,britain the whole of its manifestations were probably comprised within the earlier 7,oligocene and perhaps early ,miocene7 part of older ,tertiary time4 ,since its eruptions ceased1 another interval of profound quiescence has succeeded1 which still continues4 ,but this interval is almost certainly of less duration than that which elapsed between the ,palozoic and ,tertiary outbursts4 ,in other words1 remote as the date of these ,tertiary volcanoes appears to be from our own day1 it comes much nearer to us than did the era of the last ,permian eruptions to the earliest of the ,tertiary series4 #aj4 ,by the dissection which prolonged denudation has effected among the old volcanic centres of ,britain1 materials are supplied for studying the sequence of events from the beginning to the end of a volcanic period4 ,these events have generally followed the same tolerably well-defined order4 ,in the case of fissure-eruptions1 rents formed in the crust of the earth and communicating with the surface have allowed lava to rise and flow out above ground1 either from the lips of the fissures or from vents opened along the lines of chasm4 ,the thousands of parallel dykes in ,britain remain as evidence of this mode of the ascent of the molten magma4 ,lines of large cones of the ,vesuvian type may be presumed to have risen along guiding fissures in the terrestrial crust4 ,but it is evident from a study of the ,british examples that the existence of a fissure in the visible part of the crust is not always necessary for the production of a volcanic vent4 ,in hundreds of instances1 communication from the internal magma to the surface was effected by successive explosions1 which finally blew out an orifice at the surface with no visible relation to any fissures or dykes4 ,of course1 beneath the formations that now form the surface1 and through which the necks rise1 there may be lines of fault or weakness in older rocks which we cannot see4 ,but1 in what can be actually examined1 vents have commonly been drilled through rocks independently of faults4 ,the discharge of explosive vapours was sometimes the first and only effort of volcanic energy4 ,generally1 however1 fragmentary volcanic materials were ejected1 or1 if the eruption was more vigorous1 lava was poured out4 ,in a vast number of cases1 especially in the later ages of ,palozoic time1 only ashes were projected1 and cones of tuff were formed4 ,in the earlier ages1 on the other hand1 there was a much larger proportion of lava expelled4 ,towards the close of a volcanic period1 the vents were gradually choked up with the fragmentary materials that were ejected from and fell back into them4 ,occasionally1 during the process of extinction1 an explosion might still occur and clear the chimney1 so as to allow of the uprise of a column of molten rock which solidified there2 or the sides of the crater1 as well as of the cavernous funnel underneath1 fell in and filled up the passage4 ,heated vapours sometimes continued to ascend through the debris in the vent1 and to produce on it a marked metamorphism4 ,there seems to have been commonly a contraction and subsidence of the materials in the vents1 with a consequent dragging down or sagging of the rocks immediately outside1 which are thus made to plunge steeply towards the necks4 ,when the vents were plugged up by the consolidation of fragmentary matter or the uprise of lava in them1 the final efforts of the volcanoes led to the intrusion of sills and dykes1 not only into the rocks beneath the volcanic sheets1 but also1 in many instances1 into at least the older parts of the sheets themselves4 ,these subterranean manifestations of volcanic action may be recognized in almost every district4 ,they vary greatly in the degree to which they are developed4 ,sometimes1 as in the ,cader ,idris1 ,arenig and ,snowdon regions1 they attain considerable importance1 alike as regards the number and thickness of the sheets4 ,in other cases1 they are exhibited on so small a scale that they might be overlooked1 as in the tract of ,carboniferous puy-eruptions in the north of ,ayrshire4 ,but they are so generally present as to form a remarkably characteristic feature of the volcanic activity of each geological period from the earliest time to the latest4 ,the basic sheets in the ,dalradian series of ,scotland display early and colossal examples4 ,all through the successive eruptive periods of ,palozoic time1 sills are found as accompaniments of superficial ejections4 ,the ,tertiary basalt-plateaux supply numerous and gigantic examples of intruded sheets4 ,tertiary cones of ,vesuvian type are not found in ,britain1 but where on the continent they have been sufficiently laid open by denudation1 they present sometimes an astonishing series of sills4 ,as a striking illustration of this structure reference may be made to the sheets of trachyte that have been injected between and have marmorized the ,cretaceous strata on which ,monte ,venda stands1 among the ,euganean ,hills4#ddj ,footnote #ddj3 ,g4 vom ,rath1 ,zeitsch4 ,deutsch4 ,geol4 ,gesellsch41 xvi4 7#ahfd71 p4 #dfa4 ,e4 ,suess1 ,sitzungsber4 k4 ,akad4 ,wien41 lxxi4 7#ahge71 p4 #g2 ,antlitz der ,erde1 vol4 i4 p4 #aic4 ,e4 ,reyer1 ,die ,euganeen1 #ahgg4 ,this volcano is further referred to1 postea1 p4 #dgg4 ,it is obvious that the time of intrusion of the sills cannot be precisely determined4 ,they were not likely to be injected at an epoch when the volcanic magma could find ready egress to the surface4 ,that they did not arise before such egress was obtained may be inferred from their petrographical characters1 which are usually those of the later and not of the earlier outflows of the magma2 and from the fact that they not only lie among the rocks below the volcanic series1 but intersect the lower parts of that series1 sometimes even the higher parts4 ,we may therefore1 with every probability1 regard the sills as among the closing phases of a volcanic period4 ,as the lavas and tuffs of each volcanic period are intercalated among the successive geological formations1 a definite beginning and end to the period are stratigraphically fixed4 ,we see exactly where in the sedimentary series the first showers of ashes fell1 and where the last mingled with the ordinary sand and mud of the sea-door4 ,the same record shows that the volcanic accumulations were finally washed down1 that they subsided with the rest of the ground around them1 and that usually they were buried under overlying conformable sedimentary deposits4 ,thus cones of ashes and lava which may have been several thousand feet high completely disappeared4 #aj4 ,a consideration of the distribution of the volcanic rocks in time shows not only how singularly uniform the course of volcanic activity has been1 but that there is no evidence of the cessation of any of the broader petrographical types during geological history4 ,quite as much variety may be observed among the erupted materials of ,tertiary time in ,britain as among those of the early ages1 when the earth was younger and its volcanic vigour might be supposed to have been greater and more varied than it is now4 ,the table on the following page will make these features at once apparent4 ,from this table it will be seen that while some of the acid rocks have not always been extruded1 the basic masses have played their part in every volcanic period4 #aa4 ,a study of the volcanic products of a long series of eruptions within the same geographical region may be expected to throw light on the changes that take place during the course of ages in the character of the internal molten magma4 ,in a former chapter 7vol4 i4 p4 #bg7 reference was made to the subject of volcanic cycles and to the sequence1 observed in various widely separated parts of the world1 among the materials erupted from below4 ,allusion was likewise made in a later chapter 7vol4 i4 p4 #ij7 to the remarkable differences in texture and composition noticeable within some large bodies of eruptive material1 and to the evidence which these differences furnish of a segregation or differentiation among the constituents of an eruptive mass after it has been injected into its position within the crust of the earth4 ,table of the ,periods of ,volcanic ,action in the ,british ,isles and of the ,chronological ,distribution of the ,volcanic ,products4 ,key to ,columns ,gr ,granites1 ,granophyres1 etc4 ,fe ,felsites1 ,rhyolites1 etc4 ,da ,dacite1 ,pitchstone of ,eigg4 ,tr ,trachytes4 ,an ,andesites 7,porphyrites74 ,ga ,gabbros4 ,do ,dolerites1 ,basalts 7,diabases74 ,pi ,picrites and highly basic lavas4 ,tu ,tuffs1 acid or basic4 ---------------------------------------- ------------------------- ,gr ,fe ,da ,tr ,an ,ga ,do ,pi ,tu ---------------------------------------- ------------------------- ,older ,tertiary 7,plateaux1 dykes1 necks1 bosses1 sills7 99 99 99 99 99 99 99 99 99 ,mesozoic ,no volcanic rocks4 ,permian 99 99 99 99 99 ,carboniferous 8 ,puy type 99 99 99 99 99 ,plateau type 99 99 99 99 99 99 ,devonian 99 99 ,old ,red ,sandstone ,upper 99 99 ,lower 99 99 99 99 99 99 ,silurian ,upper 99 99 ,lower1 ,bala 99 99 99 99 99 99 99 99 ,arenig 99 99 99 99 99 99 99 ,cambrian 99 99 99 99 ,uriconian 99 99 99 ,dalradian 99 8 ,torridonian ,lewisian 99 99 99 ---------------------------------------- ------------------------- ,from the history of volcanic action in the ,british ,isles it is clear that differentiation is effected under three distinct conditions4 ,in the first place1 a notable difference may be occasionally observed between two adjacent parts of the same mass of lava which has flowed out at the surface4 ,thus1 in the ,carboniferous picrite of ,blackburn1 there has been a separation of the heavy basic constituents1 which have in great part settled down into the lower part of the sheet1 while the lighter felspar has mainly come to the top4 ,in this case the gradual transition from top to bottom suggests that the separation occurred after the lava had reached the surface and taken the form of a stream or sheet4 ,in the second place1 segregation has taken place in the magma within the terrestrial crust after intrusion1 for it is frequently observable in large bosses and sometimes in sills1 the basic elements having tended to mass themselves towards the margins of the rock1 leaving more acid material in the centre4 ,the cases of ,garabol ,hill among the ,dalradian schists of ,scotland1 of ,carrock ,fell among the ,silurian strata of the ,lake ,district1 and of the ,cramond picrite among the ,carboniferous formations of ,midlothian1 with others that might be cited from various other regions and geological formations in ,britain1 prove to what a considerable extent a separation of ingredients may take place in a boss1 and even sometimes in a comparatively thin sill before the molten mass consolidates4 ,in the third place1 there is good evidence that already before the magma is either intruded or extruded1 and while it still lies within the internal reservoir1 it may not possess a general uniformity of composition1 but may have become more or less heterogeneous4 ,in regard to intrusive rocks1 the extraordinarily banded gabbros of the ,tertiary series of ,skye obviously proceeded from a magma in which the molten material consisted in some parts mainly of felspar1 and in others mainly of the ferro-magnesian minerals and iron-ores4 ,streams from these differently constituted parts of the magma were simultaneously or successively injected as sills into the older portions of the volcanic series1 while1 as the process of differentiation within the magma proceeded1 still more felspathic liquid was left behind1 to be thrust into cracks in the sills previously consolidated4 ,moreover1 the banded basalts of the ,tertiary plateaux show that this heterogeneity was not confined to internal intrusions1 but maintained its place even when the molten material was ejected to the surface4 ,the differentiation indeed is not so striking there as among the sills of gabbro2 but its presence1 even in a less degree1 proves that the separation of constituent minerals was not due to any general cooling of an erupted body of igneous rock1 but was already developed in the reservoir from which the molten material was propelled to the surface4 ,attention has been called to the remarkable similarity of structure between these banded intrusive rocks and some of the ancient gneisses4 ,the resemblance is so close that we may with every probability infer that the gneisses acquired their characteristic banding as intrusive masses of igneous rocks1 discharged from heterogeneous magmas1 like that which supplied the gabbros of the ,cuillin ,hills4 ,and as these gneisses belong to pre-,cambrian formations1 we are thus led to the interesting result that the tendency to develop heterogeneity was already as characteristic of the magma-basins of the earliest geological time as it has been of those of later periods4 ,the evidence of differentiation presented by superficial lavas1 and by intrusive sills and bosses1 acquires great interest when considered in connection with the changes which are seen to have occurred in the character of the materials erupted during the course of a definite volcanic period4 ,an attentive examination of the volcanic products of the various ages1 so fully recorded in the geological structure of the ,british ,isles1 shows that a recognizable sequence in the nature of the materials erupted during a single volcanic period can be traced from the earliest to the latest times1 and that1 in spite of occasional departures1 the normal order remains broadly uniform4 ,with the important exception of the ,snowdonian region and possibly others1 we find that the earlier eruptions of each period were generally most basic1 and that the later intrusions were most acid4 ,thus the diabase-lavas and tuffs at the base of the ,cambrian series of ,st4 ,david's are pierced by quartz-porphyry veins4 ,the andesites of the ,lower ,old ,red ,sandstone were succeeded by bosses1 sills1 and dykes of granite1 felsite1 and lamprophyre4 ,the eruptions of the ,carboniferous plateaux began with extremely basic lavas1 and ended with trachytes1 felsites1 and quartz-porphyries4 ,the basalts of the great lava-fields of the ,tertiary period are pierced by masses of granophyre and even granite4 ,there has evidently been1 on the whole1 a progressive diminution in the quantity of bases and a corresponding increase in the proportion of acid in the lavas erupted during the lapse of one volcanic period4 ,this sequence is so well marked and so common that it cannot be merely accidental4 ,the acid and basic rocks1 occurring as they do at each volcanic centre in the same relation to each other1 are obviously parts of one connected series of eruptions4 ,we seem to see in this sequence an indication of what was taking place within the subterranean magma4 ,there was first an extensive separation of the more basic constituents1 such as the ferro-magnesian minerals and ores1 and the lavas which came off at that time were heavy and basic basalts1 and even picrites4 ,the removal of these elements left the magma more acid1 and such rocks as andesites were poured out1 until at last the deeper intrusive sills1 dykes and bosses became thoroughly acid rocks1 such as felsite1 quartz-porphyry and granite1 while if any superficial outflow took place it was such a rock as dacite4 ,in the case of the ,tertiary volcanic series there is evidence that after the acid protrusions a final uprise of basic material occurred4 ,no satisfactory proof of any similar return to basic eruptions has been detected among the ,palozoic formations4 ,but it is possible that some of the basic sills and dykes1 the precise age of which cannot be fixed1 may really mark such a reversion1 even in the earlier volcanic periods4 ,some illustrative examples of volcanic cycles from other countries were cited in ,chapter iii4 ,to these ,i may add another instance which presents a close analogy to some of the phenomena characteristic of the ,british examples of ,palozoic as well as of ,tertiary age4 ,monte ,venda in the ,euganean ,hills1 already alluded to 7p4 #dgd71 may be cited as an interesting specimen of an older ,tertiary volcano1 which has been so dissected by denudation as to show not only the succession of its superficial discharges1 but the position and order of its subterranean intrusions4 ,the volcanic eruptions of this neighbourhood1 judging from the area which they still cover and the height they reach1 may have piled up a mountain rivalling or surpassing ,etna in dimensions4 ,in ,monte ,venda the lowest visible igneous rocks are sills of oligoclase-trachyte that have been thrust between and have highly altered ,cretaceous 7,tithonian7 limestones4 ,other intrusive sheets of trachyte follow in the overlying ,cretaceous strata 7,neocomian and ,scaglia74 ,it is not until the older ,tertiary formations are reached that undoubted tuffs and lavas occur1 indicative of truly interstratified volcanic materials4 ,these formations1 consisting of nummulitic limestones and other strata together with fossiliferous tuffs1 show that the volcano began as a submarine vent4 ,it discharged dark basic dolerites and tuffs4 ,the highest lava1 however1 crowning the summit of the mountain is a trachyte4 ,there appears to have been a rapid decrease of the bases in the magma1 for the later lavas were rhyolites1 accompanied with rhyolitic tuffs of ,oligocene age1 and followed in the end by the black vitreous trachyte of ,monte ,sieva4 #ab4 ,from the evidence detailed in these volumes1 it appears that the sequence from basic to acid discharges was on the whole characteristic of each eruptive period4 ,it is obvious1 however1 that as the protrusions of successive periods took place within the same limited geographical area1 the internal magma during the interval between two such periods must in some way have been renewed as regards its constitution1 for when1 after long quiescence1 eruptions began once more1 basic lavas appeared first and were eventually followed by acid kinds4 ,this cycle of transformation is admirably exhibited in ,central ,scotland1 where the andesites of the ,old ,red ,sandstone with their felsite sills are followed by the limburgites1 picrites and other highly basic lavas at the bottom of the ,carboniferous plateaux1 succeeded in turn by the andesites1 trachytes and acid sills of that series4 ,when the puy eruptions ensued1 the magma had once more become decidedly basic4 ,that the true explanation of these alterations is of a complex order may be inferred from the exceptions which occur to the general rule4 ,i have alluded to the ,snowdon region1 where the acid rhyolites are followed by more basic andesites1 and where the sills are also more basic than the superficial lavas4 ,in the ,arenig and ,cader ,idris country the sills are likewise more basic than the bedded lavas4 ,among the ,carboniferous puys of the basin of the ,firth of ,forth1 the sills are not sensibly more acid than many of the superficial basalts1 and they even include such rocks as picrite4 ,possibly in this last-named region we see an arrested sequence1 the volcanic protrusions having from some cause ceased before the general uprise of the more acid magma4 ,,index ,aa form of lava in the ,sandwich ,islands1 ii4 #ahg ,abereiddy ,bay1 i4 #bjf ,abich1 ,h41 i4 #cb ,acid igneous rocks1 silica percentage of1 i4 #ad2 devitrification of1 #ai2 flow-structure of1 #ba2 occur in thicker sheets than basic1 #bd2 alternations of1 with basic1 #bh1 #fa1 #aeb1 #aeg1 #afe1 #bjg1 #bac1 #bcc1 #bhd1 #cah2 ii4 #bcf1 #bff1 #bgh2 metamorphic action of1 i4 #ie1 #if2 connection with mountains1 ii4 #ih2 scenery of1 #ajb4 ,acids1 mineral1 at volcanoes1 i4 #gb ,acland1 ,mr4 ,h4 ,d41 i4 #acc ,aegean ,sea1 volcanoes of1 i4 #a ,agglomerates1 i4 #ca1 #eg1 #eh2 in dykes1 #gj2 ,archan1 #abj1 #acj1 #ace2 ,cambrian1 #adh1 #adi1 #afg2 ,silurian1 #agh1 #ahj1 #aha1 #ahd1 #ahe1 #aid1 #aii1 #bjf1 #bad1 #bcg1 #bda1 #bdd1 #bdg1 #bec1 #bee2 ,old ,red ,sandstone1 #bgi1 #bhe1 #bhi1 #cjj1 #cac1 #cbe1 #cch1 #cdi1 #ceb2 ,carboniferous1 #cha1 #cii1 #djb1 #djd1 #dbg1 #dbi1 #dci1 #ddj2 ii4 #ac1 #bd1 #bh1 #bi2 ,permian1 #fb1 #fd1 #ii2 ,tertiary1 #aid1 #bgg1 #bgh1 #bha1 #bhi1 #bib1 #bic1 #chd1 #djj1 #dbc ,allan1 ,t41 i4 #cfc ,allotriomorphic minerals1 i4 #ba ,allport1 ,mr41 i4 #ie1 #acj1 #aca1 #bfj1 #dea2 ii4 #aa1 #db1 #ajb1 #ajc1 #ajd1 #ajf1 #cgj ,amber in ,tertiary volcanic series1 ii4 #aih ,america1 ,western ,north1 volcanic rocks of1 i4 #aj1 #ajj2 ii1 #bfg ,amygdales1 origin of1 i4 #ae2 ii4 #ahi1 #bba1 #bhe1 #bij ,amygdaloidal structure1 i4 #ae1 #af1 #ag1 #ei1 #bgd1 #che2 ii4 #c1 #ca1 #eg1 #abi1 #ahh ,analyses of ,cambrian tuffs1 i4 #adh1 #adi2 of ,cambrian diabases1 #aec2 of ,old ,red ,sandstone diabases1 #bgd2 of ,old ,red ,sandstone andesites1 #bge2 of ,old ,red ,sandstone trachytes1 #bgf2 of ,old ,red ,sandstone felsites1 #bgh2 of ,carboniferous limburgite1 #cgg2 of ,carboniferous basalts1 #cgi2 of ,carboniferous trachytes1 #chj2 of ,carboniferous phonolite1 #cha2 of ,tertiary trachyte1 ii4 #aci2 of ,tertiary dacite1 #bdd ,anderson1 ,dr4 ,tempest1 ii #bfa1 #bfb1 #bfc ,andesite1 i4 #bd1 #aca1 #acf1 #afd1 #afe1 #afg1 #agh1 #ahj1 #ahd1 #ahi1 #aij1 #bjd1 #bab1 #bac1 #bad1 #bae1 #bbi1 #bcj1 #bde1 #bdf1 #bdg1 #beb1 #bgd1 #bge1 7analyses71 #bgg1 #bib1 #cjj1 #cjf1 #cji1 #cae1 #cah1 #cbe1 #ccj1 #ccc1 #cde1 #cgg1 #cgi1 #chf1 #djc1 #dba2 ii4 #de1 #eg1 #if1 #abe1 #acg1 #ahd1 #bcf1 #dbd ,anglesey1 gneisses and schists of1 i4 #abf2 volcanic rocks of1 #ahi1 #bai ,anhydrite deposits1 ii4 #ed ,annandale1 ,permian volcanic rocks of1 ii4 #ef1 #eh1 #fj1 #fa1 #ff ,antrim1 ,old ,red ,sandstone volcanic rocks of1 i4 #cad2 ,tertiary volcanic rocks of1 #dg1 #eb2 ii4 #aji1 #aaj1 #aac1 #aci1 #adj1 #aii2 basalts of1 #aib1 #aic1 #aii1 #bjb1 #bjf2 clays and iron-ore of1 #bjd2 rhyolites of1 #ahe1 #cfd1 #cgj1 #cga1 #dbf1 #dde2 deceptive agglomerate of1 #ahh2 rhyolitic conglomerate of1 #aie1 #bjf2 plateau of1 #aii2 tuffs of1 #bjb1 #bjd2 vents of1 #bga1 #bgg2 sills of1 #bih2 central subsidence of basalt-plateau of1 #ddh ,apatite1 ii4 #ace ,apjohn1 ,j41 ii4 #db ,applecross1 volcanic vents in1 ii4 #bib ,arans1 the1 i4 #age1 #agf1 #agi1 #ahd1 #ahf1 #bjg ,archan period1 i4 #aaj1 #aaa2 volcanic rocks of1 #abj ,ardnamurchan1 dykes and veins of1 ii4 #aed1 #cbj2 basalt-plateau of1 #bjh2 vents of1 #bhg2 sills of1 #cah2 gabbro of1 #cee ,arenig group1 i4 #age2 lower limit of1 #agg1 #ahe2 top of1 #agh1 #bbh1 #bdf ---- volcano of1 i4 #db1 #age1 #agf1 #agi1 #ahf1 #bjg ---- rocks in ,scottish ,highlands1 i4 #abc1 #abf2 in ,merionethshire1 #agf1 #agi2 of ,shropshire1 #ahi2 of ,ayrshire1 #aif2 of ,scottish ,highlands1 #bja2 of ,anglesey1 #bba2 of ,lake district1 #bbi2 of ,ireland1 #bci ,argyll1 ,duke of1 ii4 #aac1 #aad1 #aih ,argyllshire1 dykes of1 ii4 #abg1 #abh1 #ach1 #adb1 #adf1 #aga1 #agb2 vents of1 #bgh ,arizona1 explosion crater in1 i4 #eh2 laccolites in1 #hf ,arran1 ,old ,red ,sandstone volcanic rocks of1 i4 #bih1 #caa2 ,carboniferous volcanic rocks of1 #chf1 #cib2 possible ,permian volcanic rocks of1 ii4 #eh2 granite of1 i4 #ic2 ii4 #cff1 #cfg1 #dah2 pitchstone of1 i4 #ai2 ii4 #dde2 dykes of1 #abc1 #aci1 #adj1 #adb1 #adf1 #aed1 #afa ,arthur ,seat1 i4 #cfd1 #cgc1 #cgh1 #che1 #chf2 ii4 #fg ,arvonian1 i4 #ade1 #aef ,asbestos in volcanic breccia1 ii4 #ea ,ascension ,island1 cellular lava of1 i4 #ae ,ashes1 volcanic 7see ,tuffs7 ,ashprington volcanic series1 i4 #bfb ,asphalt1 ii4 #gi ,atherstone1 i4 #agj ,augite1 loose crystals of1 in volcanic vents1 i4 #fb1 #agh1 #aha2 ii4 #eh1 #gi2 lumps of1 in volcanic vents1 i4 #ceb ,augite-aphanites1 i4 #agh ,auvergne1 old volcanoes of1 i4 #bi1 #cb1 #ff1 #gj1 #ajj2 ii4 #cgc ,aveline1 ,mr4 ,w4 ,t41 i4 #bbg1 #bcj2 ii4 #cb ,ayrshire1 example of volcanic neck in1 i4 #ef2 ,silurian volcanic rocks of1 #aib2 ,old ,red ,sandstone volcanic rocks of1 #bge1 #bhb1 #bhc1 #bhe1 #bia1 #cca2 ,carboniferous volcanic plateau of1 #ajb1 #cfh1 #chh1 #cic1 #cih1 #daj2 ,carboniferous ,puys of1 #dae1 #daf1 #dcd1 #ddj1 #dgd2 ,permian volcanic rocks of1 ii4 #ee1 #eh1 #fb ,azoic period1 i4 #aji ,bckstrm1 ,mr41 ii4 #bff ,baily1 ,w4 ,h41 i4 #bea1 #beb2 ii4 #aih1 #ddi ,bala group1 i4 #age1 #aij1 #aif1 #bja1 #bjf1 #bjg1 #bbc1 #bdb2 limestone of1 #dg1 #age1 #bbi1 #bde1 #bea2 volcanic rocks of1 #ahf1 #aij1 #bjg1 #bac1 #bba1 #bda1 #bdh ,balbriggan1 igneous rocks of1 i4 #bdd ,ballagan beds 7,lower ,carboniferous71 i4 #chd1 #chg1 #cib1 #cic1 #dab1 #ddg ,ballantrae1 volcanic rocks at1 i4 #aib1 #aii ,ballypallidy1 tuffs and leaf-beds of1 ii4 #bjd1 #dbi ,bamborough1 ,whin ,sill at1 ii4 #b1 #c1 #e ,banding of igneous rocks1 i4 #hd1 #bjg2 ii4 #ahi1 #bid1 #cbi1 #ced1 #ceg1 #dgf ---- of gneiss1 i4 #aaf ,bangor group1 i4 #aff ,banks1 ,sir ,joseph1 ii4 #aji ,barnavave1 eruptive rocks of1 ii4 #dba ,barrow1 ,mr4 ,g41 i4 #bja1 #bbf1 #bgb1 #bgi1 #chj2 ii4 #adg1 #adh ,basalt1 columnar structure of1 i4 #bd1 #be2 relation to gabbro1 #gh2 altered by carbonaceous strata1 #ie2 shells supposed to occur in1 ii4 #aaj2 banded1 #ahi2 thickness of sheets of1 #aib2 meaning of red layer between sheets of1 #aig1 #bjc1 #bjf1 #bed2 metamorphism of1 #bgb1 #bgf1 #ccg1 #cci1 #cdj1 #cdg1 #cee1 #cef1 #ceg1 #ceh1 #cfb1 #cgh1 #chc1 #chf1 #cig1 #cii1 #djj1 #djd1 #dac ---- pre-,cambrian1 i4 #aai1 #aca2 ,silurian1 #bjf1 #bjg1 #bcj1 #bde2 ,carboniferous1 #cgh1 #djc1 #djg1 #dag2 ii4 #aa1 #de1 #df2 ,permian1 #eg1 #if2 ,tertiary1 #abe1 #acf1 #ahc1 #aii1 #bjh1 #bia ,basalt-conglomerate1 ii4 #aie ,basic volcanic rocks1 silica-percentage of1 i4 #ad2 devitrification of1 #bj2 flow-structure of1 #ba2 occur in thinner sheets than the acid1 #bd2 metamorphic action of1 #id2 erupted at low levels1 #ih2 scenery of1 #ajb2 converted into schists by deformation1 #ge1 #aad1 #aah1 #aai1 #abd1 #abi2 alternation with acid1 #bh1 #fa1 #aca1 #aeg1 #afe1 #bjg1 #bac1 #bcc1 #bhd1 #cah2 ii4 #bcf1 #bff1 #bgh ,bass ,rock1 i4 #cgb1 #cgc1 #djc ,bassenthwaite ,lake1 i4 #cce ,bathgate1 puy eruptions of1 i4 #ddj1 #ddb1 #dde1 #def1 #dfa ,bauer1 ,dr4 ,m41 i4 #fb ,bauxite1 ii4 #aig1 #bjd ,bayley1 ,mr4 ,w4 ,s41 ii4 #ccj ,bedding in lavas1 i4 #bd ,bell1 ,sir ,i4 ,lowthian1 ii4 #a1 #aac1 #acg1 #afe ,bemrose1 ,mr4 ,h4 ,a41 ii4 #aj1 #aa1 #ac1 #af1 #ag1 #ah1 #bj1 #ba ,ben ,cruachan1 alteration of granite at1 i4 #cdc ---- ,hiant1 basic sills of1 ii4 #cah ,benaun ,more1 felsite of1 i4 #cdg ,berger1 ,j4 ,f41 ii4 #bb1 #ie1 #aaj1 #aac1 #aci1 #adj1 #ada1 #ade1 #aii1 #cfd1 #dbf ,bertrand1 ,prof4 ,m41 i4 #bh ,berwickshire1 i4 #bgb1 #bij1 #cch1 #cge1 #che1 #dja1 #dac ,berwyn ,hills1 i4 #agf1 #ahf1 #bjh1 #bah ,biggar1 volcanic area1 i4 #bhg1 #cbe ,binney1 ,e41 ii4 #ef ,binny ,craig type of basalt1 i4 #dai1 #dba 7#ddd7 ,biotite 7see ,mica7 ,bitumen in intrusive rocks1 i4 #dba ,blackstone 7,derbyshire71 ii4 #ah1 #ba ,blair-,atholl ,limestone1 i4 #abb ,blake1 ,rev4 ,j4 ,f41 i4 #abf1 #acj1 #add1 #afj1 #afa1 #afb1 #afc1 #afe1 #aff1 #afh1 #bbj1 #bba1 #bbb ,blocks1 ejected1 i4 #cf1 #dbc1 #dch2 ii4 #aig1 #bba ,bole between lavas1 i4 #ddb2 ii4 #aig1 #bjc1 #bjf1 #bed ,bombay1 volcanic plateau of1 ii4 #ahj ,bombs1 volcanic1 i4 #fj2 ii4 #ci ,bonney1 ,prof41 i4 #ie1 #abf1 #acj1 #acf1 #add1 #afj1 #afb1 #afc1 #afd1 #afe1 #aff1 #afg1 #afh1 #aib1 #baj1 #bbg ,borrowdale ,volcanic ,series1 i4 #bbg ,bosses1 volcanic1 i4 #ef1 #gh1 #hh2 petrography of1 #hi2 differentiation in1 #ij2 ii4 #dgf2 granitic1 i4 #ic2 metamorphism around1 #id1 #ie2 conditions of their intrusion1 #ig1 #ih2 weathering of1 #ajb ---- ,silurian1 i4 #bae1 #bce2 ,old ,red ,sandstone1 #bgg1 #bhh2 ,carboniferous1 #djc1 #deh ---- ,tertiary1 ii4 #bga1 #bhd1 #cbg1 #cff1 #cgh1 #cie1 #djc2 boundaries of1 #chb2 relation to older eruptive vents1 #bhj1 #chd1 #cii2 relation to plateau basalts1 #chf1 #cif1 #djb1 #djd2 relation to gabbro intrusions1 #cia1 #djb1 #djd2 relation to the basic dykes1 #cie ,bostonite1 ii4 #dg ,bou1 ,ami1 i4 #bfh1 #cfc2 ii4 #aab1 #cgb ,boule1 ,m41 i4 #bg1 #bi1 #dd1 #de1 #df1 #fa2 ii4 #cge ,boutan1 ,m41 i4 #fb ,bowden ,hill1 type of doleritic basalt1 i4 #dah1 #dba ,braid ,hills1 great vent of1 i4 #bhi1 #bic1 #caa1 #cah1 #cbc ,branco1 ,prof4 ,w41 i4 #df1 #dag ,breccias1 volcanic1 i4 #ca1 #cb1 #abj1 #aca1 #ace1 #adg1 #afe1 #ahi1 #aij1 #aig1 #bac1 #bbd1 #bbe1 #bcc1 #bcd1 #bdf1 #beb1 #bee1 #bhi1 #cdg2 ii4 #ci1 #da1 #di1 #aie ---- of non-volcanic materials1 ii4 #aif1 #dbc ,brecciated structure1 i4 #afb1 #baa ,breidden ,hills1 i4 #agf1 #aij1 #bjh ,brent ,tor1 ii4 #cc1 #ce1 #cf ,bron1 ,m4 ,r41 ii4 #aia ,britain1 advantageous position of1 for the study of ancient volcanic action1 i4 #f2 completeness of the ,geological ,record in1 #f2 direction of folds and fractures in1 #aa2 chief lavas found in1 #ca2 ,vesuvian cones of1 #db2 volcanic plateaux of1 #dc2 puys of1 #df2 lacustrine volcanoes of1 #di2 fissure eruptions of1 #eb2 scenery of volcanic rocks of1 #ajj1 #aja2 pre-,cambrian rocks of1 #aaa2 in ,cambrian time1 #ada2 in ,silurian time1 #agc2 in ,devonian time1 #beh2 in ,old ,red ,sandstone time1 #bfc2 in ,carboniferous time1 #cee2 in ,permian time1 ii4 #ec2 in older ,tertiary time1 #ajh ,brgger1 ,prof41 i4 #bh1 #hh1 #ij1 #ia1 #ib ,bryce1 ,j41 i4 #cad1 #cfi ,buch1 ,l4 von1 i4 #bg2 ii4 #cha ,buckland1 ,w41 ii4 #ie1 #aaj1 #aac ,buddle1 ,j41 ii4 #aac ,builth1 i4 #agf1 #bjc ,burdiehouse ,limestone1 i4 #cfa1 #cgd1 #chh1 #dae1 #dfc ,burnt ,country of ,asia ,minor1 i4 #b ,burntisland1 ,binn of1 i4 #dbh1 #dbi1 #dcc1 #dce1 #deg1 #dei ,burntisland ,sill type of dolerite1 i4 #dah1 #dba ,busz1 ,mr4 ,k41 i4 #bfa ,bute1 ,isle of1 i4 #cfi1 #cgh1 #djg ,cadell1 ,mr4 ,h4 ,m41 i4 #aad1 #dbc2 ii4 #ccd ,cader ,idris1 volcanic rocks of1 i4 #db1 #age1 #agf1 #agg1 #agh1 #agi1 #ahj1 #aha1 #ahb1 #ahh1 #bjg ,caer ,caradoc1 i4 #aca1 #acb1 #agj ,caerfai group 7,cambrian71 i4 #aee ,caernarvonshire1 volcanic rocks of1 i4 #aei1 #bjg ,caithness ,flags1 i4 #cdc1 #ceb ---- volcanic vents in1 i4 #ceb ,calciferous ,sandstones1 i4 #cfa1 #cff1 #dae ,calcite as a matrix of tuffs1 ii4 #bg1 #ci1 #da ,caldecote volcanic rocks1 i4 #agj ,callaway1 ,dr4 ,c41 i4 #abf1 #acj1 #acb1 #acd1 #bbj1 #bba ,calton ,hill1 lavas and tuffs of1 i4 #cgc1 #cgh1 #che1 #chf1 #chi ,cambrian system1 i4 #aab1 #abc1 #acc1 #aci1 #adc1 #add2 volcanoes of1 #ade1 #aei ,campbeltown1 volcanic rocks of1 i4 #cab1 #chf ,campsie ,fells1 i4 #ajb1 #cfh1 #cfi1 #chd1 #chf1 #chi1 #cic1 #cig1 #cih1 #djj1 #djc1 #daj1 #dab1 #ddg ,canary ,islands1 i4 #bg ,canna1 basalts of1 ii4 #ahd1 #ahg1 #aij1 #bae1 #baf2 vent in1 #bhh ,cantyre1 volcanic rocks of1 i4 #caa1 #cfi1 #cgj1 #chf ,caradoc group1 i4 #age1 #aif ,carbonaceous rocks1 influence of1 on igneous masses1 i4 #ie1 #dbf1 #ddi1 #def2 ii4 #fe1 #hg1 #ajd1 #afe ,carboniferous ,limestone1 origin of1 i4 #ceg ---- system1 subdivisions of1 in ,britain1 i4 #ceh1 #cfj1 #cff2 ancient geography of1 #cee1 #cfa1 #cfb1 #dcb1 #dfb2 flora and fauna of1 #cef ---- volcanic plateaux1 distribution of1 i4 #cfd1 #cfg2 nature of materials constituting1 #cgg2 structure of1 #chc2 bedded lavas and tuffs of1 #chc2 vents of1 #ed1 #cid1 #cii ---- ,puys1 i4 #df1 #dg1 #cjh1 #cfd2 of ,scotland1 #dad2 nature of the materials erupted by1 #daf2 necks of1 #dbd2 bedded lavas and tuffs of1 #dag1 #dcf1 #ddj2 sills of1 #ddf1 #dgb2 bosses of1 #deh1 #dfe2 dykes of1 #dfj2 of ,derbyshire1 ii4 #h2 ,isle of ,man1 #bb2 of ,somerset1 #cb2 of ,devonshire1 #cb2 of ,king's ,county1 #cg2 of ,limerick1 #da ,carlingford1 igneous rocks of1 i4 #if2 ii4 #age1 #cga1 #dbj ,carnedd ,dafydd1 i4 #bji ,carnmony ,hill1 ii4 #bgb ,carrock ,fell1 differentiation in rocks of1 i4 #ia2 metamorphism at1 #id1 #if2 as a volcanic boss1 #bce1 #bcf ,cement-stone group1 i4 #cfb1 #cff1 #chg1 #dah1 #dfb ,cellular structure of volcanic rocks1 i4 #ae1 #cc ,chalk1 metamorphism of1 by a dyke1 ii4 #afd ,champernowne1 ,a41 i4 #bfj1 #bfb ,charnwood ,forest1 i4 #acd2 ii4 #ec ,cherts associated with volcanic rocks1 i4 #abc1 #afg1 #afi1 #agc1 #agd1 #ahd1 #aif1 #aig1 #bdj1 #bed2 ii4 #be1 #cf ,cheviot ,hills1 i4 #ajb1 #bga1 #bgb1 #bgd1 #bge1 #bgg1 #bgh1 #bij1 #bic1 #ccf ,chilled margin in intrusive rocks1 i4 #ha1 #hc2 ii4 #abf1 #aeh1 #afj1 #agb1 #bii1 #cjc1 #caj1 #cag1 #cba1 #cib1 #djb ,christiania1 eruptive rocks of1 i4 #bh ,chronology1 volcanic1 how determined1 i4 #df ,clark1 ,mr4 ,g4 ,t41 ii4 #ahj ,claystone1 i4 #bgg1 #bgi1 #cah1 #cbd1 #cbg2 ii4 #djc ,cleavage1 effects of1 on igneous rocks1 i4 #afb1 #afe1 #bbd1 #bca1 #bcb1 #bcd1 #bcg1 #bfj1 #bfa2 ii4 #cf ,clee ,hills1 ii4 #aja1 #ajb ,cleveland ,dyke1 ii4 #a1 #abb1 #aci1 #adj1 #adb1 #add1 #adf1 #adg1 #aej1 #aec1 #afg1 #afh1 #afi ,clough1 ,mr4 ,c4 ,t41 i4 #aad1 #bja1 #bcf1 #bgd1 #bij1 #ccg2 ii4 #abc1 #abd1 #abg1 #abh1 #acb1 #acg1 #ach1 #adb1 #ade1 #adf1 #aeb1 #afb1 #aga1 #agb1 #caf1 #chd1 #dcg ,clyde1 ,carboniferous volcanic plateau of1 i4 #cfh1 #chd1 #che1 #cic1 #djj1 #djg1 #daa ,coal interbedded among volcanic rocks1 i4 #cib1 #dbc2 ii4 #aih1 #bac1 #bea1 #bhg ---- alteration of1 at volcanic vents1 i4 #gb2 ii4 #fd ---- alteration of intrusive rocks by1 i4 #ie1 #dea ---- alteration of by sills1 dykes1 etc41 ii4 #fg1 #afd1 #aff ,coal-measures1 i4 #ceh1 #cfj1 #cff ,coalbrookdale ,coal-field1 ii4 #ajc ,cole1 ,prof4 ,g4 ,a41 i4 #agf1 #agg1 #agh1 #agi1 #ahj1 #aha1 #ahd1 #ahg1 #ahh1 #baj1 #baa2 ii4 #acd1 #bje1 #bab1 #bde1 #cgj1 #cga1 #cgh1 #dbf ,colorado1 ,grand ,caon of1 i4 #cj2 laccolites of1 #hf ,columnar structure1 i4 #be1 #bg1 #cdc1 #cgh1 #che1 #dei2 ii4 #afd1 #ahf1 #bjf1 #cja ,comley ,sandstone1 i4 #add ,cones1 volcanic1 connection of1 with necks1 i4 #gj2 contemporaneous denudation of1 #gc2 ii4 #bjb1 #bah1 #bcj2 entombment of1 i4 #dcc1 #dfc2 ii4 #ff ,conglomerates1 volcanic1 i4 #ca1 #cg1 #ahc1 #aij1 #bhf1 #cjj1 #cjg1 #cji1 #caj1 #cad1 #cae1 #ccj1 #cda2 ii4 #aie1 #aih1 #bah1 #bhd ,coniston ,limestone1 i4 #bbi1 #bca ,contemporaneity in ,geology1 #bja ,continents1 origin of1 i4 #aa ,contraction1 effects of terrestrial1 i4 #ab1 #ig1 #ih ,conybeare1 ,j4 ,j41 ii4 #ie ---- ,w41 i4 #aga2 ii4 #i1 #ie1 #aaj1 #aac1 #aii ,cooling1 effects of1 in inducing varieties of texture in igneous rocks1 i4 #gh1 #gi1 #ha2 ii4 #bgd1 #bge1 #bii1 #cjc1 #caj1 #cag1 #cib1 #djb ,coon ,butte1 ,arizona1 i4 #eh ,cork1 ,county1 volcanic breccias of1 ii4 #di ,corndon1 sill of1 i4 #agf1 #ahi1 #aij ,corston ,hill1 i4 #cgc1 #chf1 #chg ,craiglockhart type of dolerite and basalt1 i4 #dah ,crater1 consolidation of tuff within a1 i4 #dbi ,crater-lakes1 i4 #eh2 ii4 #bff1 #bge ,cretaceous period1 geography of the1 ii4 #ajh1 #ahb ,cross ,fell1 i4 #bbh1 #bbi1 #bch ,cross1 ,mr4 ,whitman1 i4 #hf ,crush-conglomerates or breccias1 i4 #cb1 #bbj1 #bbc1 #bbe1 #bdd2 ii4 #bha1 #cdg1 #ceb ,crushing1 mechanical effects of1 i4 #cae 7see ,schist7 ,crust1 contraction of the terrestrial1 i4 #ab1 #ig1 #ih2 oldest rocks of1 #aaj2 deformation of1 #aag1 #aba1 #bfd1 #bie1 #big ,cryptocrystalline type of basalt1 i4 #dai ,crystallites of volcanic rocks1 i4 #ah ,crystals1 different periods of formation of1 in volcanic rocks1 i4 #ai1 #bj1 #ba1 #dba2 ii4 #abh1 #aca1 #acd1 #ace2 ejected by volcanic vents1 i4 #fb1 #agh1 #ahj1 #aha1 #aie1 #bac1 #bcd1 #bde2 ii4 #bg1 #di1 #eh1 #gi ,cuillin ,hills1 scenery of1 i4 #ajf2 gabbro of1 ii4 #cbi1 #cfa2 acid rocks of1 #cia ,culots1 i4 #gh1 #hh ,culm-measures1 ii4 #cc ,cumbrae ,islands1 i4 #cfh1 #cfi1 #cgh1 #djg ,cumming1 ,j4 ,g41 ii4 #bb ,cycles1 volcanic1 i4 #bg1 #ib2 ii4 #aaf ,dacite1 i4 #bcj2 ii4 #ahe ,dakyns1 ,mr4 ,j4 ,r41 i4 #ij1 #bbi1 #bgb2 ii4 #aj ,dalmellington1 volcanic rocks at1 i4 #ccc2 ii4 #fb ,dalmeny type of dolerite and basalt1 i4 #dah1 #dbj ,dalradian rocks1 probable crushed necks of1 i4 #ge1 #abe2 lavas and sills of1 #aba2 green schists of1 #abd ,dalry1 ,ayrshire1 buried volcanoes of1 i4 #dcd ,dana1 ,j4 ,d41 ii4 #ahi ,darwin1 ,c41 i4 #bg ,daubre1 ,a41 i4 #gb1 #djd ,davies1 ,j41 i4 #aef1 #aeg ,dechen1 ,h4 von1 i4 #df2 ii4 #aab1 #bhj1 #ccc1 #cdj1 #cfg1 #cgb1 #cha ,deformation1 effects of1 on volcanic rocks1 i4 #ge1 #aae1 #aag1 #aai1 #aba1 #abg1 #abi1 #afb ,de la ,beche1 ,h41 i4 #adb1 #adc1 #agj1 #age1 #bjd1 #bje1 #bjg1 #bei2 ii4 #i1 #aj1 #ai1 #cc1 #ie1 #if1 #ig ,delessite1 ii4 #gi ,denudation1 influence of1 on volcanoes1 i4 #c1 #d1 #h1 #dj1 #dc1 #de1 #df1 #ed1 #eh1 #ga1 #gc1 #ge1 #gi1 #hg1 #ajj-ajg1 #cgj1 #dcc1 #dcd1 #dcf1 #dgf2 ii4 #ee1 #fa1 #fb1 #agi1 #aha1 #ahb1 #bda1 #bde1 #bdh1 #bdi1 #bee1 #beg1 #bhb1 #bhc1 #bib1 #caf1 #cag1 #cfc1 #cgc1 #djg1 #dee ,derbyshire1 toadstones of1 i4 #cei2 ii4 #h ,desmarest1 ii4 #cgc ,devitrification of volcanic rocks1 i4 #ah1 #ai1 #gh2 ii4 #dcg1 #ddf ,devonian system1 i4 #beg2 volcanoes of1 #bei ,devonshire1 volcanic scenery of1 i4 #ajc2 ,devonian volcanic rocks of1 #bei2 ,carboniferous volcanic rocks of1 ii4 #cb2 ,permian volcanic rocks of1 #id ,diabase1 i4 #aea1 #aec 7analyses71 #aef1 #aib1 #aid1 #bjd1 #bjf1 #bad1 #bag1 #bce1 #bdj1 #bdg1 #bdi1 #bgc 7analyses71 #bgh1 #bib1 #cah1 #cbj1 #ccj1 #cce1 #cdd1 #cde1 #cea1 #djc2 ii4 #e1 #acf1 #dae ,diabase-porphyrite1 i4 #aib1 #bjd ,diamond found in volcanic vents1 i4 #fb ,dick1 ,mr4 ,a41 jun41 i4 #chj ,dickson1 ,mr4 ,e41 ii4 #bc ,differentiation in igneous rocks1 i4 #bb1 #bg1 #hd1 #he1 #ij1 #ia1 #ddi2 ii4 #cjj1 #dge ,dimetian1 i4 #ade ,dingle-beds1 i4 #cdf ,dingle1 ,upper ,silurian nodular lavas of1 i4 #bj1 #bed ,diorite1 i4 #gh1 #bdg1 #bdi1 #bgg1 #bgh1 #bhh2 ii4 #cf ,dirrington ,law1 i4 #bij ,dittmar1 ,prof41 ii4 #acg ,dolerite1 i4 #aai1 #acd1 #agh1 #aij1 #bjf1 #bcj1 #bdg1 #bfa1 #cgh1 #djc1 #djg1 #dag1 #ddh2 ii4 #e1 #aa1 #ce1 #di1 #ajb1 #ajc1 #ajd1 #abe1 #acf1 #aeg1 #ahc1 #bga1 #bii1 #cjc1 #cjg1 #cai1 #cbh ,dolgelli1 i4 #afi1 #agh1 #ahh ,donegal1 ,dalradian rocks of1 i4 #abb2 dykes in1 ii4 #abd ,drogheda1 volcanic rocks near1 i4 #bdd ,duffin1 ,w4 le ,s41 ii4 #dbf ,dumbarton1 rocks near1 i4 #djb1 #djd ,dumfoyn1 a volcanic neck1 i4 #cie1 #cih1 #djj ,dumgoyn1 a volcanic neck1 i4 #cie1 #cig1 #cih1 #djj ,dundee1 sills and bosses near1 i4 #bib1 #cjf ,duneaton ,water1 volcanic rocks of1 i4 #cbi ,dunite1 ii4 #cji ,du ,noyer1 ,g4 ,v41 i4 #bde1 #bej1 #bed2 ii4 #bgb1 #dbf ,durham1 ,mr41 i4 #bge ,durness ,limestone1 i4 #aab1 #aba1 #abc1 #ada ,dust1 volcanic1 i4 #ac ,dutton1 ,capt4 ,c4 ,e41 i4 #fh2 ii4 #bfg ,dykes1 vitreous margins of1 i4 #ah2 formation of1 #ed1 #ih2 in necks1 #ff2 filled with agglomerate1 #gj2 grouping of1 among intrusive rocks1 #gg2 character of1 #gi2 extent of1 in ,britain1 #hj2 age of1 #ha2 compound1 #ha2 ii4 #ei2 expulsion of lava from1 i4 #hb2 ii4 #abh2 connected with the surface1 i4 #hb2 pre-,cambrian1 #aah2 flow-structure in1 #afa2 ,cambrian1 #aef2 ,silurian1 #ahg1 #baf1 #bce1 #bcg1 #bdh1 #bdi2 ,old ,red ,sandstone1 #bgg1 #bia1 #cch1 #cde2 ,carboniferous1 #djf1 #dbi1 #dfj2 ii4 #a1 #cj2 ,permian1 #hc2 amygdaloidal structure of1 #he ---- ,tertiary1 ii4 #aad2 arguments for their geological age1 #aah1 #abe1 #aga2 geographical distribution1 #aba2 two types of protrusion of1 #abb2 nature of component rocks of1 #abe2 external character of1 #abf2 classification of basic1 #abi2 enclosed fragments in1 #abi1 #aca1 #add2 porphyritic and amygdaloidal structures of1 #abh1 #abi1 #acj2 veins in1 #acj2 joints in1 #acb1 #aff2 microscopic characters of1 #acd2 chemical characters of1 #acg1 #aci2 hade of1 #aci2 breadth of1 #aci2 interruptions of1 #adb2 length of1 #adb2 persistence of mineral characters of1 #add2 direction of1 #ade1 #aei2 upward termination of1 #adg2 known vertical extension of1 #aej2 evidence of movement of molten rock of1 #aea2 branches and veins from1 #aeb2 connection with sills1 #aee2 intersecting1 #aeh2 compound or of more than one infilling1 #aei2 double1 treble1 and multiple1 #afj1 #cah1 #dag1 #dci2 compound1 with basic and acid bands1 #afa1 #dce2 contact metamorphism of1 #afc2 relation of1 to geological structure1 #aff2 origin and history of1 #age2 ,icelandic example of1 #bfa2 example communicating with cinder cone in ,utah1 #bfh2 connection of1 with surface1 #agi1 #bfi1 #bhj2 latest protrusions of1 #cha1 #djh1 #daf2 of granophyre1 #dce1 #dcf1 #dcg1 #dci ,earth1 condition of the interior of the1 i4 #aj2 fractures in crust of the1 #aa ,earthquakes1 influence of1 on early man1 i4 #a2 transient effects of1 #c1 #h ,east ,lothian1 trachyte lavas of1 i4 #bd2 ,carboniferous volcanic plateau of1 #cgj1 #chi1 #djc1 #dji ,edinburgh1 volcanic rocks near1 i4 #bd1 #ajb1 #ajd1 #bfi1 #bgc1 #bgf1 #bgi1 #bha1 #bhe1 #bhg1 #bhi1 #bia1 #bic1 #caa1 #cag1 #cah1 #cbc1 #cfd1 #cgj1 #cgc1 #che1 #chf1 #chg1 #chi1 #daj1 #dbj1 #dcf1 #ddi ,egan1 ,mr4 ,f4 ,w41 i4 #bdb2 ii4 #bja1 #dbc ,eifel1 i4 #d1 #df1 #eh1 #ajj ,eigg1 ,isle of1 ii4 #aae2 pitchstone of1 #ahe1 #bag1 #bdb1 #dde2 brecciated basalt in1 #ahi1 #aib2 basalt plateau of1 #bae1 #bcd2 ,scuir of1 #bag1 #bcd1 #ddg2 sills of1 #cah2 acid bosses of1 #djc2 acid sills of1 #dca2 proofs of subsidence at1 #ddg2 enormous denudation of1 #bci1 #ddg1 #deh ,eildon ,hills1 i4 #cge ,electric ,peak1 i4 #gi1 #hb1 #hd ,elvans1 i4 #bdi1 #bha ,engulphment craters1 i4 #eh ,ennerdale1 granite of1 i4 #bcf ,enniscorthy1 volcanic rocks near1 i4 #bde ,environment1 influence of1 on early man1 i4 #a ,eozoic period1 i4 #aaj ,epidiorite1 i4 #aah1 #abd1 #abi1 #ahd1 #bdg1 #bdi ,erosion1 laws of1 i4 #aja ,eruptions1 transient effects of1 i4 #c2 old submarine1 how ascertained1 #dh2 lacustrine1 #di2 fluviatile1 #di2 terrestrial1 #ej2 evidence of intervals between1 #bhc1 #bhg1 #cjj1 #ddb2 ii4 #db1 #ei1 #bjc1 #bje1 #bba1 #bea1 #bed1 #bhg ,erzeroum1 old volcanoes near1 i4 #cb ,eskdale ,dyke1 ii4 #abg1 #acc1 #acf1 #acg1 #adj1 #adc1 #ade1 #adf1 #aec ---- 7,lake ,district71 granite of1 i4 #bcf ,etheridge1 ,mr4 ,r1 jun41 ii4 #bd ,etna1 i4 #b1 #d1 #aj1 #ee2 ii4 #bfa ,eurite1 i4 #ahh ,europe1 basalt plateaux of north-western1 i4 #ea1 #eb2 ii4 #aha2 pre-,cambrian disturbances of north-western1 i4 #aag ,explosion-craters1 i4 #eh2 ii4 #bff ,explosions1 volcanic1 i4 #bdf2 ii4 #aif1 #bff1 #dbe1 #dgb ,extrusive rocks1 defined1 i4 #ad2 textures of1 #gh ,fair ,head1 sills of1 ii4 #cja ,farey1 ,j41 ii4 #i ,farne ,islands1 ii4 #b ,faroe ,isles1 basalt plateaux of1 i4 #eb1 #ajb2 ii4 #aia1 #aib1 #aid1 #bef2 vents in1 i4 #fc2 ii4 #bic2 dykes of1 #abb1 #acc2 tuffs and lignites of1 #beh2 sills of1 #cbb2 absence of gabbro bosses in1 #cee2 subsidence of1 #ddg2 dip of basalts in1 #ddh2 proofs of denudation in1 #deh ,faujas ,st4 ,fond1 ii4 #aji1 #aab ,faults1 connexion of volcanic vents with1 i4 #fi2 ii4 #fe2 boundary1 i4 #bid1 #cjc1 #cje1 #cfi2 ii4 #afi2 effects of1 i4 #ddf2 ii4 #bjj2 connection with1 dykes1 #afh ---- of ,tertiary basalt-plateau1 ii4 #deb ,felsite 7,felstone71 ,torridonian1 i4 #abj2 ,uriconian1 #acj1 #acc2 of ,malverns1 #acd2 ,cambrian1 #aea1 #afj1 #afa1 #afd1 #afe1 #afg1 #afh2 ,silurian1 #ahd1 #aii1 #bje1 #bjf1 #bjg1 #baj1 #bab1 #bah1 #bca1 #bcb1 #bdf1 #bdg1 #beb1 #bee2 ,old ,red ,sandstone1 #bgf1 #bgg1 #bia1 #bic1 #cba1 #cbg1 #cce1 #cdf2 ,carboniferous1 ii4 #cf1 #di2 ,permian1 #he2 ,tertiary1 #agd1 #cfi1 #dbd1 #ddf ,felsitic breccia1 ii4 #aie ---- type of devitrification1 i4 #ai ,felspar1 ejected crystals of1 i4 #aha2 ii4 #eh1 #gi2 large porphyritic crystals of1 in dykes1 #abi1 #ace ,fife1 ,old ,red ,sandstone volcanic rocks of1 i4 #cjg2 ,carboniferous volcanic rocks of1 #dbh1 #dbi1 #dcj1 #dcc1 #dcg1 #ddh2 ,permian volcanic rocks of1 ii4 #ef1 #fi ,fingal's ,cave1 i4 #be2 ii4 #baj ,fisher1 ,rev4 ,o41 i4 #ih ,fishguard1 volcanic rocks at1 i4 #bje ,fissure type of volcanoes1 i4 #db1 #eb2 ii4 #ajh1 #aae1 #bfg ,fissures1 volcanic1 i4 #db1 #eb1 #ec1 #ed1 #dbe2 ii4 #ada1 #ade1 #aei1 #agf2 filled with agglomerate1 i4 #gj2 filled with dykes1 #ha1 #aah2 compound1 #hb1 ii4 #aei2 pre-,cambrian1 i4 #aah1 #aai2 ,carboniferous1 #dbe2 ,tertiary1 ii4 #ada1 #aei1 #agf1 #dbe2 modern of ,iceland1 #bfb2 cause of1 #agg ,fleming1 ,john1 i4 #bfh ,flow-structure1 i4 #ba1 #aeg1 #afj1 #afa1 #afb1 #ahd1 #baj1 #bcb1 #bdf1 #bdh1 #bee1 #cae1 #cba1 #cbg1 #cdf2 ii4 #abi1 #aeb1 #aij1 #aia1 #ccb1 #cfi1 #cib1 #djb1 #dbd1 #dcg1 #dda ,foot1 ,f4 ,j41 i4 #caf ,forbes1 ,d41 ii4 #cgj ---- ,edward1 ii4 #ff1 #aac1 #aad1 #aih ---- ,j4 ,d41 ii4 #aab1 #ccc1 #cgb1 #cha ,forellenstein1 ii4 #ccb ,forest of ,wyre coal-field1 ii4 #ajb ,forfarshire1 volcanic rocks of1 i4 #bhe1 #bii2 flagstones of1 #bii ,forster1 ,m41 ii4 #aac ,forth-basin1 ,carboniferous system of1 i4 #cfa2 ,carboniferous plateaux of1 #cgj2 ,carboniferous puys of1 #daf1 #dbg1 #dbi1 #dcj1 #dcb1 #dcd1 #dcg1 #ddj1 #ddf1 #dfb2 ,permian volcanoes of1 ii4 #ee1 #fg ,foster1 ,mr4 ,c4 le ,neve1 ii4 #aj ,fouqu1 ,prof41 i4 #ah1 #ba2 ii4 #acd ,fox1 ,mr4 ,howard1 ii4 #cf ,fox ,strangways1 ,mr4 ,c41 i1 #ace ,fragmental volcanic rocks1 i4 #ad2 only arise from explosions which reach the surface1 #eg 7see ,agglomerates1 ,conglomerates1 ,tuffs7 ,france1 ,tertiary volcanoes of ,central1 i4 #d1 #aj1 #bi1 #da1 #de1 #di1 #eh1 #fj1 #gj2 ii4 #ca1 #bga1 #bha1 #cgc2 ,carboniferous volcanic action in1 i4 #ceg ,frankland1 ,prof4 ,e41 i4 #bgc1 #bgh ,fundamental complex of oldest gneiss1 i4 #aad1 #aae ---- gneiss1 i4 #aae ,gabbro1 granulitic1 ii4 #cbi2 banded structure of1 i4 #aaf2 ii4 #cbi1 #ced1 #ceg1 #dgf2 coarse-grained massive1 #ccj2 pale varieties in veins1 #ccj2 gneiss-like aspect of1 #cdb1 #bed1 #ceh ---- of ,carrock ,fell1 i4 #ia2 ,silurian1 #aie1 #bjf1 #bdg2 ,devonian1 #bfb2 ,tertiary1 #hd1 #ij1 #aaf2 ii4 #cjg1 #cjh1 #cji1 #cai1 #cbg1 #ccd1 #cdi1 #cee1 #ceh1 #cia1 #djf1 #djg ,gairloch1 peculiar pre-,cambrian rocks of1 i4 #aae1 #aag ,galapagos ,islands1 i4 #bg ,gallaston type of dolerite and basalt1 i4 #dah ,galloway1 granites of1 i4 #ic1 #ie1 #bgb1 #bgg1 #bij1 #cca ,garabol ,hill1 differentiation at1 i4 #ij ,gardiner1 ,miss1 i4 #ie ,gardiner1 ,mr4 ,c4 ,j41 i4 #bef ,gardner1 ,mr4 ,starkie1 ii4 #aif1 #aih1 #bab ,garlton ,hills1 i4 #ajb1 #cgj1 #cgg1 #cgh1 #cgi1 #chj1 #chf1 #cij1 #dje1 #dab ,garnet found in volcanic vents1 i4 #fb ,garth ,grit1 i4 #agg1 #ahe1 #bjh ,gases dissolved in the volcanic magma1 i4 #ac1 #ae1 #gb1 #ig1 #ii ,geikie1 ,prof4 ,j41 i4 #bgg1 #cjf1 #cjh1 #cca1 #ccf1 #cci1 #cdj1 #cfi1 #cge1 #dbf2 ii4 #eg1 #aia1 #bei1 #cbb ,genvre1 ,mont1 i4 #aid ,geological action1 supposed former greater intensity of1 i1 #aci ---- contrasts1 i4 #ajc ---- history1 i4 #aji1 #aac ---- ,survey of ,great ,britain1 i4 #aac1 #aae1 #aah1 #aai1 #aba1 #abb1 #abc1 #abd1 #abe1 #abf1 #abi1 #acj1 #acc1 #ace1 #adb1 #adc1 #add1 #ade1 #aei1 #afj1 #aff1 #agj1 #aga1 #age1 #agf1 #agi1 #aha1 #ahb1 #ahc1 #ahf1 #ahg1 #ahh1 #aij1 #aif1 #aih1 #bja1 #bjd1 #bje1 #bjg1 #bjh1 #bab1 #bad1 #bae1 #baf1 #bag1 #bah1 #bai1 #bbj1 #bba1 #bbe1 #bbg1 #bbh1 #bcb1 #bcc1 #bch1 #bci1 #bdj1 #bdb1 #bdc1 #bdd1 #bde1 #bej1 #bea1 #bed1 #bei1 #bgj1 #bge1 #bgh1 #bid1 #bii1 #cjf1 #cjg1 #cjh1 #cad1 #cae1 #cag1 #cah1 #cbe1 #cbi1 #cca1 #ccf1 #cci1 #cdj1 #cdd1 #cdf1 #cdi1 #cej1 #ceb1 #cfd1 #cfi1 #cgb1 #cgc1 #cge1 #cig1 #djc1 #djd1 #djf1 #djg1 #daa1 #dbc1 #dbe1 #dcd1 #ddi1 #dfb1 #dge1 #dgf2 ii4 #c1 #d1 #i1 #aj1 #ab1 #ac1 #af1 #ag1 #bj1 #bc1 #cc1 #cf1 #cg1 #db1 #dc1 #df1 #dh1 #di1 #ef1 #eh1 #fe1 #ff1 #fh1 #id1 #ie1 #ajb1 #ajc1 #aah1 #aba1 #abe1 #abg1 #add1 #ade1 #adh1 #afb1 #afd1 #agj1 #agd1 #age1 #aij1 #aib1 #aii1 #bja1 #bjc1 #bec1 #bgb1 #bgg1 #bib1 #cdg1 #chd1 #cia1 #dbj1 #dbb1 #dbc1 #dbf1 #dbh1 #dcc1 #dce1 #ddf1 #ddi ,giant's ,causeway1 ii4 #hj1 #aji1 #ahf1 #ahh1 #aib1 #bjf ,gilbert1 ,mr4 ,g4 ,k41 i4 #hg2 ii4 #cfb1 #cfc ,girvan1 i4 #aib1 #bjj ,glaciation1 absence of1 in ,devonshire1 i4 #bfa ,glass in volcanic rocks1 i4 #ah1 #cc1 #fj1 #gh1 #ahj1 #baa1 #baf1 #bcj1 #bcb1 #bce1 #caf2 ii4 #he1 #abj1 #abf1 #acc1 #ace1 #acg1 #ahd1 #bjd1 #bdg1 #bgb1 #bhe1 #caf1 #cag ,globulites1 ii4 #ace ,gloucestershire1 ,silurian volcanoes of1 i4 #bch ,gneiss1 analogies of1 with igneous rocks1 i4 #ic2 ii4 #dgf2 oldest1 i4 #aaj1 #aae ,godwin-,austen1 ,a4 ,c41 i4 #bei1 #bfb ,goodchild1 ,mr4 ,j4 ,g41 i4 #bbi1 #ddi2 ii4 #aej ,grainger1 ,rev4 ,dr41 ii4 #aih ,grand ,sarcoui1 ii4 #cgc1 #cgd1 #cha ,granite1 bosses of1 i4 #hh1 #ij1 #ic2 plutonic and volcanic1 #hi2 metamorphism by1 #ie2 altered by dykes1 ii4 #afd2 pre-,cambrian1 i4 #aai2 post-,arenig in ,highlands1 #abf1 #caj2 in ,cambrian rocks1 #aee2 in ,silurian rocks1 #bjj1 #bbi1 #bcf1 #bch1 #bdi2 of probably ,old ,red ,sandstone age1 #bgb1 #bgg1 #bij1 #cca1 #ccg2 ,tertiary1 ii4 #cff1 #dah1 #dbj ,granitite1 i4 #ahh1 #bgg1 #bij1 #ccg2 ii4 #cfg ,granophyre1 alteration of rocks by1 i4 #ie1 #if2 scenery of1 #aje2 solvent action of1 #hb1 #hd1 #he1 #if1 #ii2 ii4 #afc1 #cib1 #dae1 #dbb1 #dcc2 brecciated1 #chb2 spherulitic1 #cha2 bedded structure of1 #cha1 #djc1 #djd2 apt to be intruded at the base of a volcanic series1 #djc2 shattering of rocks invaded by1 #dje1 #daa1 #dac1 #daf1 #dci2 veins of1 #dji1 #daj1 #dcb1 #dcg2 ,silurian1 i4 #bad1 #bae ---- ,tertiary1 i4 #cci2 ii4 #cfh1 #cie1 #djh1 #dcj2 boundaries of1 #chb1 #dji2 relation to older vents1 #bhj1 #chd1 #cii2 relation to plateau-basalts1 #chf1 #cif1 #djb1 #djd2 relation to gabbro1 #cia1 #djb1 #djd1 #daj2 relation to basic dykes1 #cie2 proof of liquidity of1 #dac2 sills of1 #dcj1 #dcf1 #dcg2 dykes of1 #dce ,granophyric structure1 i4 #bj2 ii4 #cff ,graphite in ,tertiary volcanic series1 ii4 #aih ,graptolites1 i4 #agd1 #aif1 #aig ,grauwacke or ,devonian rocks1 ,de la ,beche on1 i4 #bei1 ii4 #cc ,graves1 ,lieut4 ,t41 ii4 #dea ,great ,glen of ,scotland1 i4 #aba ,greeks1 influence of volcanoes on1 i4 #a ,green1 ,a4 ,h41 i4 #acc1 #acd1 #afc2 ii4 #aj1 #ab ,greenland1 ,tertiary basalts of1 ii4 #ahb ,greenly1 ,mr4 ,e41 i4 #abi1 #bad ,greenock1 ,lord1 i4 #cfc ,green schists of the ,scottish ,highlands1 i4 #abb2 of ,anglesey1 #abi ,greenstone1 i4 #ahc1 #ahg1 #bjf1 #bag1 #bai1 #bdi1 #bei1 #bfa2 ii4 #cd1 #ce1 #cg1 #ajc1 #ajd1 #cee ,greenstone-ash1 i4 #bai ,griffith1 ,sir ,r41 ii4 #bii1 #dbb ,gunn1 ,mr4 ,w41 i4 #aad1 #bih1 #caa1 #ccf1 #cfi1 #djg1 #daj2 ii4 #eh1 #agb1 #dbj ,gypsum deposits1 ii4 #ed ,hade of dykes1 ii4 #aci ,hmatitic iron-ore1 ii4 #aig ,hall1 ,sir ,james1 i4 #gb1 #cfc ,hlleflinta1 i4 #aca1 #afg ,hardman1 ,e4 ,t41 ii4 #cfe1 #ddi ,harker1 ,mr4 ,a41 i4 #ij1 #ia1 #ic1 #ie1 #if1 #ii1 #ahh1 #bji1 #baj1 #baa1 #bab1 #bac1 #bad1 #bag1 #bah1 #bbb1 #bbg1 #bbh1 #bcj1 #bca1 #bcb1 #bce1 #bcf1 #bcg1 #bch1 #bij2 ii4 #abd1 #abe1 #abf1 #abi1 #acj1 #aci1 #add1 #adf1 #afj1 #afb1 #afc1 #afd1 #agd1 #ahe1 #ahi1 #aij1 #bbc1 #bbd1 #bdg1 #bfi1 #bha1 #bhd1 #bhe1 #cji1 #caj1 #cah1 #cbj1 #ccd1 #cci1 #cdg1 #cdh1 #cfh1 #chb1 #chd1 #che1 #chg1 #chi1 #cib1 #djg1 #djh1 #dji1 #dac1 #dae1 #dcc1 #dcd1 #dcg1 #dda1 #ddf ,harkness1 ,r41 i4 #bbh2 ii4 #ef ,harlech anticline1 i4 #aei1 #agi1 #ahg2 group1 #agf ,hatch1 ,dr4 ,f4 ,h41 i4 #ahc1 #ahd1 #ahg1 #ahh1 #bbi1 #bcj1 #bdf1 #bdg1 #bdh1 #bdi1 #bfa1 #bgg1 #bgh1 #cjf1 #cgg1 #chj1 #cha1 #dag1 #dai1 #dbj2 ii4 #eg1 #if1 #ahd1 #bgd1 #bgf1 #bii1 #cai1 #ccb1 #cfg1 #cfh1 #cfi1 #cgj1 #chh1 #cih ,haughton1 ,prof4 ,s41 i4 #cdf2 ii4 #dbb ,hawaii1 lava-fountains of1 i4 #ab2 differentiation in lavas of1 #bg2 lava-cauldron of1 #eh ,haworth1 ,mr4 ,e41 ii4 #if ,hay ,cunningham1 ,r4 ,i41 i4 #bfi1 #cag1 #cfc1 #cgb1 #cgc1 #ddi1 #dea2 ii4 #bcg1 #bch1 #bdd ,heaphy1 ,mr4 ,c41 i4 #dcb ,hebrides1 basalt-sheets of1 i4 #bd1 #dg1 #eb1 #ajb2 acid rocks of1 #ie1 #ajb2 gabbros of1 #hd1 #ij1 #ajb2 scenery of1 #aje2 pre-,cambrian rocks of1 #aab1 #aad1 #aag1 #aba2 ,cambrian land of1 #ada2 early observations on the ,tertiary volcanic rocks of1 ii4 #aji1 #aaj1 #aaa2 dykes of1 #aah1 #adf1 #aeh1 #agd2 basalts of1 #aha1 #ahf1 #bae2 pitchstone lava of1 #bch1 #bdf2 plateau-scenery of1 #bdi2 ,tertiary rivers and lakes of1 #bag1 #bbh1 #bca1 #bcd1 #beb2 vents of1 #bgd2 basic sills of1 #cjd2 gabbro intrusions of1 #cbg2 acid intrusions of1 #cfd1 #cgi1 #dcj1 #dcg2 dislocations of1 #deb2 denudation of1 #dee ,heddle1 ,dr41 i4 #bgd1 #cjb2 ii4 #gh1 #gi1 #bdf1 #cjg1 #djf ,helland1 ,prof4 ,a41 ii4 #aia1 #bfa1 #bfc1 #bfd ,henderson1 ,mr4 ,j41 i4 #ddi ,henry ,mountains1 laccolites of1 i4 #hf2 ii4 #cfb ,henslow1 ,j4 ,s41 ii4 #bb1 #bbd ,heterogeneity in volcanic magmas1 i4 #he1 #ij2 ii1 #aij1 #ccd ,hett ,dyke1 ii4 #a1 #g1 #adg ,hibbert1 ,s41 i4 #df ,hicks1 ,dr4 ,h41 i4 #abf1 #ade1 #aed1 #aeh1 #aei1 #aff1 #bjf ,hill1 ,mr4 ,j4 ,b41 ii4 #adj ,hill1 ,rev4 ,e41 i4 #ace ,hinde1 ,dr4 ,g4 ,j41 i4 #aih2 ii4 #ce ,hinxman1 ,mr4 ,l41 i4 #aad1 #cdd2 ii4 #aba ,hobson1 ,mr4 ,b41 i4 #bfj2 ii4 #bc1 #bg1 #if1 #ii ,holden1 ,mr4 ,j4 ,s41 ii4 #bjd ,holl1 ,h4 ,b41 i4 #acc1 #acd1 #agj ,holland1 ,mr4 ,p41 i4 #agg1 #agh1 #agi2 ii4 #bc ,hollybush ,sandstone1 i4 #acc1 #agj ,holocrystalline structure1 i4 #gh2 ii4 #acf1 #ahd ,hopkins1 ,w41 ii4 #agg1 #agi1 #bfh ,hornblende1 ejected crystals of1 i4 #agh1 #aha2 ii4 #di1 #ea1 #eh1 #gi ,hornblende-schists formed from basic igneous rocks1 i4 #ge1 #aad1 #aah1 #aai1 #abd1 #abi ,horne1 ,mr4 ,john1 i4 #aad1 #aif1 #aii1 #bjj1 #cdd1 #cde1 #cge2 ii4 #bc1 #add1 #bib ,hornito of a lava-stream1 i4 #ee2 ii4 #bfd ,hornstone1 i4 #aca1 #acf1 #bgg1 #bgh 7analyses71 #cbd ,houston ,marls1 i4 #dbc1 #dcf1 #ddj1 #ddd1 #dff ,howard1 ,mr4 ,h4 ,t41 i4 #bjg ,howell1 ,mr4 ,h4 ,h41 i4 #bid1 #cjg1 #cfd ,hoy1 ,island of1 i4 #cej ,hughes1 ,prof4 ,t4 ,m',k41 i4 #abf1 #add1 #afj1 #afa1 #aff1 #afh1 #bbj1 #bbb1 #bbc1 #bbg ,hull1 ,mr4 ,e41 ii4 #db1 #ie1 #ajc1 #bgb1 #dba1 #dbf1 #ddi ,hurlet ,limestone1 i4 #cfj1 #cff1 #cid1 #daj1 #dae1 #ddd1 #deb1 #def1 #dfg1 #dgj1 #dgd ,huronian rocks1 i4 #aaa ,hutchings1 ,mr4 ,w4 ,m41 i4 #bbg1 #bcj1 #bcc ,hutton1 ,james1 i4 #cfc2 ii4 #i1 #aaj ,hutton1 ,w41 ii4 #c ,hyperite1 i4 #bgi ,hysgeir1 pitchstone of1 ii4 #bdf ,iceland1 ,tertiary basalts of1 ii4 #ahb1 #bfj2 ,tertiary gabbros and liparites or granophyres of1 #bfa2 continuity of volcanic phenomenon of1 #bfa2 lava-fields of1 i4 #bd1 #db1 #ec1 #ajj2 ii4 #bfj2 lava-domes of1 i4 #aj2 ii4 #bfe2 fissures of1 i4 #gj2 ii4 #bfb1 #bga1 #ded2 dykes of1 #abb1 #bfa2 cinder cones of1 #bfd1 #bga2 subsidence of1 #ddg ,idaho1 lava-fields of1 ii4 #bfg ,iddings1 ,prof41 i4 #bh1 #bi1 #cj1 #gh1 #gi1 #hb1 #hd1 #ij2 ii4 #abh1 #agh ,idiomorphic crystals1 i4 #ba1 #dag1 #dbj2 ii4 #dj ,index ,limestone of the ,scottish coal-fields1 i4 #cfj1 #ddd1 #deb ,india1 fissure-eruptions of1 i4 #aj2 volcanic plateau of1 ii4 #ahj ,intermediate volcanic rocks1 silica-percentage of1 i4 #ad ,intersertal structure1 i4 #dag2 ii4 #acf ,intrusive rooks1 defined1 i4 #ad2 occasional cellular character of1 #af2 flow-structure in1 #bb1 #afa2 varieties of1 #gg2 textures of1 #gh1 #ddi2 ii4 #bgd1 #cfj1 #cib2 in sheets1 sills1 and laccolites1 i4 #hc2 melting of rocks by1 #hb1 #hd2 ii4 #abi1 #afc1 #cib2 consolidation of1 i4 #hd2 banding of1 #hd1 #dej2 ii4 #cbi1 #cdb2 heterogeneity of1 i4 #he2 ii4 #cdd2 metamorphism by1 i4 #id1 #dea2 influence of surrounding rocks on1 #ie2 conditions of their intrusion1 #ig2 columnar structure in1 ii4 #ahg1 #bia1 #cja ---- ,pre-,cambrian1 i4 #aaf2 ,cambrian1 #aef2 ,silurian1 #ahg1 #aie1 #bjf1 #baf1 #bce1 #bcg1 #bdh1 #bdi2 ,devonian1 #bfa2 ,old ,red ,sandstone1 #bgg1 #bia1 #cba1 #cce1 #cch1 #cdc1 #cde2 ,carboniferous1 #djf1 #djh1 #dbj1 #ddf2 ii4 #a1 #ba1 #cj1 #cc1 #dj1 #dh2 ,permian1 #eh1 #fd2 ,tertiary1 #bgj1 #bih ,ireland1 submarine eruptions of1 i4 #dh2 ,dalradian rocks of1 #abb1 #abc1 #abf2 ,arenig rocks in1 #abc2 ,silurian volcanic rocks of1 #bci1 #bea2 granites of1 #bij2 ,old ,red ,sandstone volcanic rocks of1 #cdf1 #cdh2 ,carboniferous volcanic rocks of1 #cei2 ii4 #cg2 early observers among the ,tertiary volcanic rooks of1 #aji2 ,tertiary basalt plateau of1 #cfd1 #cgj1 #cga2 gabbros of1 #cei2 acid rocks of1 #dbj ,iron-ore1 pisolitic 7,arenig71 i4 #aha1 #bjh2 ,tertiary1 of ,antrim1 ii4 #bjd ,irvine1 ,mr4 ,d4 ,r41 i4 #bid1 #bii ,irving1 ,rev4 ,a41 ii4 #ie ,isogeotherms1 shifting of1 i4 #ih ,italy1 old volcanoes of1 i4 #d2 ii4 #dgd1 #dgg ,jack1 ,mr4 ,r4 ,l41 i4 #bid1 #cjh1 #cfi1 #cge1 #cif1 #djd2 ii4 #eg1 #ade ,jameson1 ,robert1 i4 #bfh1 #bfi1 #cag1 #cfc2 ii4 #aji1 #afa1 #bdd1 #ccc1 #cee1 #cfd ,jan ,mayen1 ii4 #ahb ,jedburgh type of dolerite and basalt1 i4 #dah ,jennings1 ,mr4 ,c4 ,v41 i4 #agf1 #agg1 #agi1 #ahj1 #aha1 #ahd1 #ahe1 #ahf1 #ahg1 #ahh ,johnston-,lavis1 ,dr41 ii4 #bfa ,joints in dykes1 ii4 #acb ,judd1 ,prof4 ,j4 ,w41 i4 #aeg1 #bge2 ii4 #aae1 #aaf1 #acd1 #acg1 #afb1 #ahe1 #bji1 #baa1 #bde1 #bdg1 #bfg1 #bgd1 #bgh1 #bhj1 #cjc1 #cjg1 #cji1 #cae1 #caf1 #cai1 #cbb1 #cbh1 #cbi1 #ccb1 #ccc1 #cdi1 #cef1 #cfj1 #cgb1 #chh1 #daj1 #dci ,jukes1 ,j4 ,b41 i4 #adc1 #aga1 #age1 #bjh1 #bah1 #bai1 #bde1 #bdf1 #bej1 #bed1 #caf2 ii4 #aj1 #bj1 #db1 #dg1 #di1 #aja1 #ajc1 #aje ,jurassic period1 physical conditions of the1 ii4 #ajh1 #ahb ,kelly1 ,j41 i4 #cad ,kenmare1 ,old ,red ,sandstone volcanic rocks of1 i4 #cej ,keratophyre1 i4 #bdg ,kerrera1 ,isle of1 i4 #cdb ,kersantite1 i4 #bfa ,keswick1 i4 #bbi ,kildare1 ,chair of1 ,bala volcanic rocks at1 i4 #bde1 #bef ,killarney1 nodular lavas of1 i4 #bj1 #bgb1 #cdf ,kilpatrick ,hills1 i4 #che1 #chh1 #djc1 #daj ,kilroe1 ,mr4 ,j4 ,r41 i4 #bea1 #bec1 #cae ,kilsyth type of dolerite and basalt1 i4 #dah ,kinahan1 ,mr4 ,g4 ,h41 i4 #cdi2 ii4 #de1 #di1 #dbf ,kincardineshire1 volcanic necks of1 i4 #bha1 #bhf1 #bic1 #bii2 ,old ,red ,sandstone of1 #cja ,king1 ,mr4 ,clarence1 i4 #bg ,king's ,county1 volcanic necks of1 ii4 #cg ,kippie ,law type of basalt1 i4 #dah ,kirkby1 ,mr4 ,j41 ii4 #ajf ,kirwan1 ,r41 ii4 #aaj ,knockfeerina1 ,old ,red ,sandstone volcanic rocks of1 i4 #cdi ,knocklayd1 ii4 #bjj ,kynaston1 ,mr4 ,h41 i4 #cdc ,labyrinthodonts1 i4 #cef ,laccolites1 i4 #gg1 #hc1 #hf1 #hh1 #ih1 #ii1 #aij2 ii4 #cfc ,lacroix1 ,prof41 i4 #if ,lacustrine volcanic eruptions1 i4 #di ,lagorio1 ,dr4 ,a41 ii4 #acg ,lake1 ,mr4 ,p41 i4 #agg1 #agi ,lake-,district1 i4 #bbg1 #bij2 ,vesuvian cone of1 i4 #db1 #de ,lake ,caledonia1 i4 #bgb1 #bid1 #bif ,lake of ,lorne1 i4 #cda ,lake ,orcadie1 i4 #bff1 #bga1 #cdc1 #cej ,lakes1 eruptions in1 i4 #di2 crater1 #eh2 of ,old ,red ,sandstone1 #bfd ,lambay ,island1 conglomerates of1 i4 #bdd ,lammermuir1 granites of1 i4 #bij1 #cdj ,lamplugh1 ,mr4 ,g4 ,w41 i4 #cb1 #bbj2 ii4 #bc1 #bh ,lamprophyre1 i4 #bia1 #bic ,lanarkshire1 i4 #bia1 #cfh1 #daf ,land1 sculpture of the1 i4 #aja1 #ajb ,landslips1 ii4 #bjj1 #bhg ,lankester1 ,prof4 ,e4 ,ray1 i4 #caj ,lapilli1 volcanic1 i4 #cc1 #cd1 #fa1 #aea ,lapworth1 ,prof4 ,c41 i4 #acj1 #acb1 #acg1 #aga1 #agb1 #ahi1 #aij1 #aif ,largs1 volcanic vent near1 i4 #ef1 #cif1 #cig1 #dja ,lasaulx1 ,prof4 von1 ii4 #cfe1 #cga1 #dbf ,laurentian gneiss1 i4 #aaj ,lavas1 classification of1 i4 #ad2 flow-structure of1 #af1 #ba2 vesicular structure of1 #ag2 glass in1 #ah2 devitrification of1 #ai2 bedding of1 #bd2 effect of water on molten1 #be1 #ccd2 sack-like or pillow-structure of1 #bf1 #ahd1 #aic1 #bja1 #bdj1 #bdd1 #beb2 seldom occur in solitary sheets1 #bf2 variations in structure in1 #bg2 sequence of1 in eruptions1 #bh1 #ib1 #cgg1 #chf2 crusts of1 disrupted in volcanic explosions1 #eh1 #ei1 #fj2 ii4 #ahi2 alternations of acid and basic1 i4 #bh1 #fa1 #aeb1 #aeg1 #afe1 #bjg1 #bac1 #bhd1 #cah2 ii4 #bcf1 #bff2 contrasted with intrusive rocks1 i4 #gh2 sandstone veins in1 #bhc1 #cjj1 #cjc1 #cbj1 #cbg1 #ccc1 #ccg2 ii4 #ei1 #ih2 shattered or agglomerate structure of1 #ii2 metamorphism of1 i4 #bca1 #bdj1 #cch2 ii4 #bgb1 #bgf1 #ccg1 #cci1 #cdj1 #cdg1 #cee1 #cgi1 #chf1 #cig1 #cii1 #djj1 #djd1 #dac ---- ,cambrian1 i4 #aeb1 #afh ---- ,silurian of ,merionethshire1 i4 #ahc2 ,scotland1 #aia2 ,builth1 #bjc2 ,pembrokeshire1 #bje2 ,caernarvonshire1 #bjg2 ,berwyn ,hills1 #bah2 ,anglesey1 #bai2 ,lake ,district1 #bbg2 ,gloucestershire1 #bch2 ,ireland1 #bci1 #bed ---- ,lower ,old ,red ,sandstone1 i4 #bgc1 #bha1 #bid1 #cag ---- ,carboniferous1 i4 #cgg1 #chd1 #dag1 #dcf1 #ddj1 #ddc2 ii4 #h1 #ah1 #cd1 #de ---- ,permian1 ii4 #fh1 #if ---- ,tertiary1 ii4 #ahc2 types of1 #ahf2 banding of1 #ahi2 thickness of1 #aib2 lenticular character of1 #aic2 of ,antrim1 #aii2 irregular bedding of the vitreous1 #bdc ---- modern ,icelandic eruptions of1 ii4 #bfa ,lava-domes1 ii4 #bfe ,lava-plug of volcanic funnels1 permanence of1 i4 #dj1 #da1 #ee1 #gc1 #gf1 #dcj ,lawson1 ,prof4 ,a4 ,c41 i4 #hb ,leaf-beds1 ii4 #aih ,lebour1 ,prof41 i4 #ccf2 ii4 #b1 #c1 #e1 #g ,leckstone1 i4 #dai1 #ddb1 #ddc ,lecoq1 ,h41 i4 #de2 ii4 #cgc ,leinster granite1 ii4 #bde1 #bdi1 #bij ,lewisian ,gneiss1 i4 #ha1 #aaj1 #aaa1 #aac1 #aah ,liddesdale1 ,carboniferous volcanic vents of1 i4 #ee1 #daf1 #dbe1 #ddj1 #dge ,life1 earliest traces of1 i4 #adj ,lignite in ,tertiary volcanic series1 ii4 #aih ,limburgite1 i4 #cgg1 #djh1 #dag1 #dbj1 #ddh2 ii4 #dj1 #df ---- type1 i4 #dah ,limerick1 ,carboniferous volcanic rocks of1 i4 #dba1 #dcj2 ii4 #da2 ,old ,red ,sandstone volcanic rocks of1 i4 #cdh ,limestone1 metamorphism of1 i4 #gb1 #dea2 ii4 #ad1 #bb1 #afd1 #bhj1 #chc ,lindley and ,hutton on ,eigg conifer1 ii4 #bch ,lingula ,flags1 i4 #add1 #agg ,linlithgowshire 7see ,west ,lothian7 ,lintrathen1 porphyry of1 i4 #bgg1 #bib1 #caa ,lion's haunch type of dolerite and basalt1 i4 #dah ,lithomarge1 ii4 #aig1 #bjd ,lizard1 rocks at the1 i4 #aid ,llanberis1 ,pass of1 i4 #aei1 #afc ---- group1 i4 #add ,llandeilo group1 i4 #age1 #aif1 #bdb2 volcanic rocks of1 #ahf1 #bjb1 #bba1 #bbg1 #bda ,llandeiniolen1 i4 #afj ,llandovery group1 i4 #age1 #aif2 possible volcanic rocks of1 #bch ,llangadock1 i4 #bje ,llangefni1 i4 #bbj ,llanllyfni1 i4 #afa ,llanwrtyd1 i4 #bjd ,lleyn ,peninsula1 i4 #bjh1 #bji1 #bac1 #bae ,lloyd ,morgan1 ,prof41 i4 #ade1 #adg1 #aed ,llyn ,padarn1 i4 #aeg1 #aei1 #afj ,loch ,carron1 pre-,cambrian rocks of1 i4 #aae1 #aag ,loch ,lomond1 dykes at1 ii4 #ahj ,loch ,tay ,limestone1 i4 #abb1 #abd1 #abe ,lomas1 ,mr4 ,j41 ii4 #aia1 #cbb ,longulites1 ii4 #ace ,lonsdale1 ,w41 i4 #beg ,longmyndian rocks1 i4 #aaa1 #abi1 #acb ,lorne1 volcanic rocks of1 i4 #ajb1 #bga1 #bha1 #cda ,lough ,mask1 ,silurian volcanic rocks of1 i4 #bea ---- ,nafooey1 ,silurian volcanic rocks of1 i4 #bea ---- ,neagh1 subsidence of site of1 ii4 #bja1 #bje2 history of1 #ddh ,ludlow group1 i4 #age ,lycopods1 fossil1 i4 #agd ,maare1 volcanic1 i4 #eh2 ii4 #bhg1 #bhh1 #bif ,macconochie1 ,mr4 ,a41 ii4 #eh ,macculloch1 ,john1 i4 #ie1 #bfi1 #bgj2 ii4 #bb1 #aaa1 #aac1 #abc1 #adj1 #aed1 #aef1 #aei1 #agb1 #agd1 #age1 #bac1 #bag1 #bca1 #bcg1 #bdd1 #bea1 #bhj1 #bic1 #cjd-cjg1 #caj1 #cad1 #cae1 #cbg1 #cdi1 #cfd1 #cga1 #djc1 #djf1 #djh1 #dji1 #dah ,macknight1 ,dr41 i4 #bfh1 #cag ,maclaren1 ,charles1 i4 #bfi1 #cag1 #cbe1 #cfc1 #cgb1 #cgc1 #dea1 #dfb2 ii4 #fg ,m',henry1 ,mr4 ,a41 i4 #cb1 #bdj1 #bdb1 #bdd1 #cad1 #cdg2 ii4 #bja1 #bjd1 #bgb1 #bic1 #dbf1 #dbg1 #dbh1 #dbi ,m',mahon1 ,general1 i4 #bfj2 ii4 #ce1 #cf ,magma1 volcanic1 explosive energy of1 i4 #ac2 gases and vapours dissolved in1 #ac1 #gb1 #ig1 #ii2 differentiation of1 #bb1 #hd1 #ia2 solvent action of1 #hb1 #hd1 #he1 #ii2 ii4 #cib1 #dae1 #dbb1 #dcc2 heterogeneity of1 i4 #he1 #ij1 #ia2 ii4 #cdd1 #cfj1 #dgf2 metamorphic action of1 i4 #id2 alteration of1 by incorporation of foreign material1 #if2 ii4 #chf1 #cij1 #cib2 conditions for the injection of1 i4 #ig1 #ii ,magnesian ,limestone1 ii4 #ed ,malvern1 pre-,cambrian volcanic rocks of1 i4 #acc2 ,cambrian volcanic rocks of1 #afi ,man1 ,isle of1 ,carboniferous volcanic rocks of1 ii4 #bb ,manod1 i4 #ahd ,marl ,slate1 ii4 #ed ,marl1 volcanic1 i4 #dbc1 #dcf1 #ddj1 #ddd1 #dff ,marr1 ,mr4 ,j4 ,e41 i4 #bbg1 #bbh1 #bcj1 #bca1 #bcb1 #bcf1 #bcg1 #bch1 #bij2 ii4 #ahi ,matlock ,bath1 ii4 #ac1 #bb ,mediterranean1 earthquakes and volcanoes of1 i4 #a ,melaphyre1 i4 #aca ,mello1 ,mr4 ,j4 ,m41 ii4 #bb ,melrose1 rocks near1 i4 #cig1 #cih1 #djj1 #dbe ,melting of rocks by igneous intrusions1 i4 #hb1 #hd1 #he1 #if1 #ii2 ii4 #afc1 #cib1 #dae1 #dbb1 #dcc ,menai ,strait1 i4 #aei ,menevian group1 i4 #aad ,merse1 volcanic plateau of the1 i4 #cge ,metamorphism of tuffs1 i4 #aeg2 of lavas 7see under ,lavas7 ---- by lavas1 i4 #bg2 by sills and bosses1 #hg1 #id1 #ie1 #baf1 #bcf1 #cca1 #cch1 #dca2 ii4 #g1 #bb1 #cf1 #aca1 #adh1 #afc1 #bii1 #cjj1 #caj1 #ccg1 #cci1 #cdj1 #cdg1 #cee1 #cef1 #ceg1 #ceh1 #cfb1 #cgh1 #chc1 #chf1 #cig1 #cii1 #djj1 #djd1 #dac ---- in vents1 i4 #fg1 #ga1 #hb1 #ic1 #cii1 #djd2 ii4 #ci1 #gh1 #bib ---- around vents1 i4 #gb1 #cdi1 #cej1 #ceb1 #cii1 #djd1 #dcb2 ii4 #gf1 #bgb1 #bgc1 #bgf1 #bhj1 #bib ---- regional1 i4 #aba1 #abc2 ii4 #ce ,mica1 ejected crystals of1 in volcanic breccias1 ii4 #di1 #eh1 #gi1 #hj ,mica-porphyrite1 i4 #bgg1 #cch ,michel ,lvy1 ,m41 i4 #ah1 #ba1 #df1 #hh2 ii4 #cgc1 #cgd ,microgranite1 i4 #aca1 #bae1 #bce1 #bdi2 ii4 #cfg1 #dcg ,microlites of igneous rocks1 i4 #ah1 #ba1 #cc2 ii4 #ace1 #bge ,micropegmatitic structure1 i4 #bj1 #dah1 #ddi2 ii4 #e1 #cfh1 #dcg ,microscopic examination of rocks1 i4 #ah1 #ba ,midlands1 eruptive rocks of ,english1 ii4 #ajj ,midlothian1 ,carboniferous volcanic plateau of1 i4 #cgc1 #che1 #chg2 sills of1 #ddf ,miller1 ,hugh1 ii4 #bcg ---- ,mr4 ,h41 i4 #ccf ,mills1 ,abraham1 ii4 #aji ,millstone grit1 i4 #ceh1 #cfj1 #cff ,minette1 i4 #bgg1 #bgh1 #bia1 #bic ,minto ,crags1 i4 #cge1 #cig ,mitchell1 ,rev4 ,hugh1 i4 #cja ,moel ,siabod1 i4 #age ---- ,wyn1 i4 #age1 #agf1 #ahd1 #ahe ,monckton1 ,mr4 ,h4 ,w41 i4 #ddi2 ii4 #bbd ,montana1 lava-fields of1 ii4 #aae1 #bfg ,montrose1 volcanoes of1 i4 #bii ,moray ,firth1 basin of1 i4 #bga1 #cdc ,morton1 ,mr4 ,g4 ,h41 i4 #ahi ,morven1 basalt-plateau of1 ii4 #bjh ,mountain-chains1 origin of1 i4 #aa1 #ab1 #ih ,mourne mountains1 granite of1 i4 #ic2 ii4 #abd1 #cff1 #cfg1 #dbj ,muck1 ,isle of1 ii4 #bae ,mud-lava1 ii4 #he ,mudstone1 volcanic1 i4 #dbc1 #dcf1 #ddj1 #ddd1 #dff2 ii4 #hf1 #bbb1 #beh ,mull1 branching amygdales of1 i4 #ag2 perlitic glass from1 #ai2 pale lavas of1 ii4 #ahd1 #bac2 basalt of1 #ahh1 #aib1 #aic2 breccias of1 #aif2 plateau of1 i4 #bd1 ii4 #bjh2 non-volcanic breccias in1 #aif1 #baa2 flint gravel in1 #baa2 leaf-beds of1 #bab2 vents in1 #bgd1 #bgh2 gabbro of1 #cee2 acid bosses of1 i4 #bj2 ii4 #cie2 acid sills of1 #dcj2 acid dykes and veins of1 #ddc2 enormous denudation of1 #deg1 #dfa ,murchison1 ,r4 ,i41 i4 #aac1 #aba1 #abi1 #adb1 #agc1 #age1 #ahi1 #bjd1 #bje1 #bjg1 #bch1 #beg2 ii4 #ef1 #ie ,mynydd-mawr1 i4 #bji1 #baa1 #baf ,mythology1 influence of earthquakes and volcanoes on1 i4 #b ,nant ,francon1 i4 #afa ,naples1 puys of1 i4 #ajj1 #dbi ,napoleonite1 i4 #bb ,necker1 ,l4 ,a41 ii4 #aab1 #abc1 #aci1 #adj1 #adf ,necks1 volcanic1 i4 #ef2 of fragmentary materials1 #ef2 of non-volcanic detritus1 #eg1 #bhi1 #cdc1 #dbf2 of agglomerate1 #eh2 ii4 #bgf2 internal stratification in1 i4 #fc2 ii4 #hj1 #bid2 with central lava-plug1 i4 #fd1 #dcj2 with dykes and veins1 #ff1 #dcj2 ii1 #bia2 of lava-form material1 i4 #fg1 #dcj2 ii4 #bga2 parasitic1 i4 #fi2 connection of1 with cones1 #gj1 #dce2 ii1 #gj1 #hi1 #bgg1 #bha1 #bij2 metamorphism of1 i4 #fg1 #ga1 #hb1 #ic1 #cii2 ii4 #ci1 #gh2 metamorphism of rocks around1 i4 #gb1 #cdi1 #cej1 #ceb1 #cii1 #djd1 #dcb2 ii4 #gf1 #bgb1 #bgc1 #bgd1 #bgf1 #bhj1 #bib2 inward dip of rocks towards1 i4 #gc1 #ceb2 ii4 #hj2 connection of1 with bosses1 i4 #ic2 ii4 #bgf1 #bhd2 entombment and exposure of1 i4 #dcd2 connection of with valleys1 #bgb1 #cff1 #cge2 ii4 #fa1 #fe2 relation between their size and the character of the agglomerate1 #gf2 connection of1 with sheets of tuff or lava1 #gj1 #hi1 #bgg1 #bhd ---- ,silurian1 i4 #bae1 #bce2 ,old ,red ,sandstone1 #bgg1 #bhh1 #bic1 #caa1 #cah1 #cbc1 #cbh2 ,carboniferous1 #cid1 #cii1 #djj1 #djd1 #djf1 #dbd1 #dfe2 ii4 #ac1 #bh1 #dg2 ,permian1 ii4 #fb1 #fg2 ,tertiary1 ii4 #bjb1 #bgj1 #bgf ,neptunist and ,plutonist controversy1 i4 #cfc2 ii4 #fg1 #ie1 #aaj1 #aab ,new ,mexico1 necks in1 i4 #fh ,newry granite1 i4 #bij ,nicholson1 ,prof4 ,alleyne1 i4 #bbh1 #bbi ,nicol1 ,james1 i4 #caa1 #cfi ---- ,w41 ii4 #bch ,nigrine1 ii4 #gi ,nithsdale1 ,permian volcanic rocks of1 ii4 #eh1 #fj1 #fb1 #fe ,nodular structure of lavas1 i4 #bj1 #afb1 #bjd1 #bjf1 #bjg1 #baa1 #bcb1 #bdg1 #bee1 #bgd1 #cdf ,nolan1 ,mr4 ,j41 i4 #bdj1 #bea1 #cae2 ii4 #dbc1 #dbd1 #dbe ,non-volcanic debris1 among volcanic rocks1 i4 #ca1 #eg1 #bhi1 #cac1 #cde1 #cha1 #cii1 #djb1 #dbb1 #dbf1 #dcg2 ii4 #ah1 #bg1 #bh1 #eh1 #fd1 #gf1 #gh1 #ii1 #aie1 #aif1 #bha1 #dbc2 indicates comparatively feeble eruptions1 i4 #eg1 #bhi1 #cde1 #dbf1 #dch2 ii4 #bic2 points to earliest eruptions of a vent1 ii4 #gf ,nordenskjld1 ,mr4 ,o41 i4 #abj ,north ,berwick ,law1 i4 #cga1 #cgc1 #djc ,north1 ,mr4 ,barker1 ii4 #bdd ,norway1 eruptive rocks of1 i4 #bh ,nuneaton1 i4 #aga ,obsidian1 i4 #ah1 #ai2 ii4 #cgj ,ochil ,hills1 i4 #bgd1 #bgf1 #bgg1 #bgi1 #bha1 #bhf1 #bhg1 #bhh1 #bic1 #cjc1 #cjh1 #caa ,oil-shales of the ,lothians1 i4 #cfa1 #cfb1 #dfb ,o',kelly1 ,j41 i4 #cdi2 ii4 #di ,oldham1 ,t41 ii4 #bii ,old ,red ,sandstone1 lines of vents in1 i4 #fi2 granite protrusions of1 #bcf1 #bgb1 #bgg1 #bij1 #ccg2 of ,county ,waterford1 #bea2 distribution in ,britain1 #beg2 an exceptional stratigraphical type1 #beh2 conditions of its deposit1 #bei1 #bfc1 #big2 original scenery of1 #bfe2 vegetation of1 #bfe2 isolation of the water-basins of1 shown by fossil evidence1 #bfe2 classification of1 #bfj2 history of the investigation of1 #bfh2 volcanic centres in1 #bga2 nature of volcanic products in1 #bgc2 structure of lavas and tuffs of1 #bha2 volcanoes of1 #bei1 #bfc1 #bid1 #cjc1 #cbc1 #cbe1 #ccg1 #cda1 #cdc1 #cdf1 #cdh1 #ceb2 subdivisions of1 #big2 thickest conglomerates of1 #cja2 composition of conglomerates of1 #cjb1 #cae1 #cda2 unconformabilities in1 #bfg1 #cbh1 #ccc2 ,upper division of1 #cdh1 #cge1 #chc2 ii4 #db ,olenellus-zone1 i4 #aab1 #acj1 #acb1 #adj1 #add ,olenus-zone1 i4 #add ,olivine1 i4 #aed1 #dah1 #dbj2 ii4 #eh1 #ace ,omagh1 i4 #cae ,ophitic structure1 i4 #ba1 #dag2 ii4 #acf1 #ahd1 #bgd ---- type of dolerite1 i4 #dah1 #dba ,oregon1 crater lake in1 i4 #eh ,orkney ,isles1 i4 #bga1 #cdd1 #cej2 ii4 #aba ,orthoclase1 ejected crystals of1 ii4 #gi ,orthophyre1 i4 #bgc1 #bgf1 #bgg1 #cjh ,osann1 ,a41 ii4 #aia ,oyenhausen1 ,c4 von1 ii4 #aab1 #bhj1 #ccc1 #cdj1 #cfg1 #cgb1 #cha ,palopicrite1 i4 #bfa ,palozoic systems1 i4 #aci2 volcanic rocks resemble modern1 i4 #cj ,palagonite1 i4 #cc1 #fa1 #aea1 #ahj1 #bdf1 #dbb1 #dbc2 ii4 #dd1 #df1 #eg1 #bbc ,paradoxides-zone1 i4 #add ,paramorphism1 i4 #bdi ,peach1 ,mr4 ,b4 ,n41 i4 #aad1 #abh1 #adg1 #afh1 #aia1 #aib1 #aie1 #aif1 #aig1 #aih1 #aii1 #bjj1 #baf1 #bdj1 #bgg-bid1 #cjg1 #cjh1 #cbi1 #cca1 #cdd1 #cde1 #cfi1 #cge1 #dbe1 #dbf1 #dgf2 ii4 #acc1 #ade ,pebidian1 i4 #ade ,pegmatite1 i4 #bj1 #aai1 #abg1 #dah1 #ddi2 ii4 #e1 #cfh1 #dcg ,pembrokeshire1 volcanic rocks of1 i4 #ade1 #aei1 #bje ,penmaen-mawr1 i4 #bji1 #bae ,pennant1 ,t41 ii4 #aji ,pennine chain1 ii4 #h ,pentland ,hills1 volcanic series of the1 i4 #ajb1 #bfi1 #bgc1 #bgf1 #bgi1 #bha1 #bhe1 #bhg1 #bhi1 #bia1 #caa1 #cag ,perlite1 i4 #acj ,perlitic structure1 i4 #ai1 #aif1 #aii1 #bjf1 #baa1 #baf1 #bcb1 #bgd ,permian system1 geographical conditions accompanying the deposition of1 ii4 #ec1 #ig2 subdivisions of1 in ,s4 ,w4 ,england1 #id2 volcanic phenomena of1 i4 #df2 ii4 #ee2 lavas and tuffs of1 #eg1 #eh2 vents of1 #fb1 #fg1 #gj1 #if2 sills of1 #ia1 #ajj ,permo-carboniferous strata1 ii4 #ed ,petersen1 ,dr41 i4 #bge1 #ccf ,phillips1 ,j41 i4 #acc1 #agj1 #bje1 #bch2 ii4 #c ---- ,j4 ,a41 i4 #bfj1 #bfa ---- ,w41 i4 #aga2 ii4 #ie ,phonolite1 i4 #chj 7analysis72 ii4 #cge ,phyllite1 i4 #afb1 #bbb ,picrite1 i4 #cgg1 #dag1 #dbj1 #ddh1 #dej2 ii4 #eg ---- type1 i4 #dah ,pillow-structure in lavas1 i4 #bf1 #ahd1 #aic1 #bja1 #bdj1 #bdd1 #beb2 ii4 #ahi1 #bei ,pitchstone1 i4 #ah1 #ai1 #acj2 ii4 #acd1 #agd1 #bjd1 #bch1 #bdb1 #bdf1 #cgj1 #dcg1 #ddd ,plagioclase1 ejected crystals of1 ii4 #gi ,plants fossil1 in tuffs1 i4 #cib2 ii4 #aac1 #aih1 #bab1 #bbb ,platania1 ,g41 i4 #bf ,plateau-type of volcanoes1 i4 #db1 #ajj1 #cjh1 #cda1 #cfd ,plateaux1 ,carboniferous1 of ,scotland1 i4 #cfd2 distribution of1 #cfg2 composition of1 #cgg2 structure of1 #chc2 lavas and tuffs of1 #chc2 vents of1 #cid2 dykes and sills of1 #djf2 close of eruption of1 #daj2 ,tertiary1 ii4 #aha2 formation of modern ,icelandic1 #bfe ,player1 ,mr4 ,j4 ,h41 analyses by1 i4 #cgg1 #cha2 ii4 #ach1 #ccj ,playfair1 ,john1 i4 #cfc2 ii4 #aaj ,plinthite1 ii4 #aig ,pliocene 7supposed7 of ,lough ,neagh1 ii4 #ddi ,plutonic operations of volcanoes1 i4 #gg2 granite1 #hh ,plutonists and ,neptunists1 i4 #cfc2 ii4 #fg1 #ie1 #aaj1 #aab ,pomeroy1 volcanic series near1 i4 #cae ,porphyrite1 i4 #aij1 #aic1 #bjg1 #bbi1 #bdj1 #beb1 #bgc1 #bgd1 #cgg1 #cgi ,porphyritic structure1 i4 #ai1 #bgd2 ii4 #abh ,portlock1 ,j4 ,e41 ii4 #aaj1 #aaa1 #aac1 #aii1 #bja1 #bii1 #cfd ,portraine1 conglomerates at1 i4 #bdd ,portrush1 shells in supposed basalt at1 ii4 #aaj1 #bii ,potstone1 i4 #abe ,pre-,cambrian rocks1 i4 #aaa1 #aba1 #abf ,pressure1 experimental proof of effects of1 i4 #bd ,prestwich1 ,sir ,j41 ii4 #ajc ,propylites1 ii4 #ahe1 #chh ,proterobase1 i4 #bdg ,pterygotus1 i4 #bfe ,pumice in tuffs1 i4 #bdd1 #dbb2 ii4 #ag1 #bg1 #bh1 #cb1 #dd1 #df1 #bhf1 #bhh2 in volcanic necks1 i4 #fj1 #ahj2 ii4 #ag1 #ci1 #aie ,pumiceous structure1 i4 #ae1 #cc1 #cd1 #fj ,puys1 as a type of volcano1 i4 #aj1 #dd1 #ajj1 #dad2 probable subrial nature of some1 #dcb2 ,carboniferous1 #cjh1 #cfd1 #dad1 #dbd1 #dfc2 ii4 #ac1 #bh1 #cd1 #dg2 ,permian1 ii4 #fb2 ,tertiary1 #bga1 #bgf ,puy de ,chopine1 i4 #cb2 ii4 #cgd ---- ,dme1 ii4 #cgc ---- ,montchar1 i4 #cb ---- ,pariou1 i4 #ff1 #gj2 ii4 #ca1 #bha ,pyroclastic detritus1 i4 #ca1 #eh1 #fa ,pyromeride1 i4 #baa ,pyrope1 ii4 #eh1 #gi ,pyroxene1 ii4 #ace ,quartz-porphyry1 i4 #ai1 #aef1 #aei1 #afj1 #afa1 #afe1 #bgg1 #bia1 #cad2 ii4 #if1 #cfi1 #dbj1 #dbc1 #dcj1 #dca ,quartz-trachyte1 ii4 #cga ,quartzite1 i4 #aab1 #agj ,raasay1 basalt of1 ii4 #aib2 neck-like breccias in1 #bic2 acid sill of1 #dcj ,raddling or red-staining of rocks1 i4 #bej1 #bfa ,radiolarian cherts1 i4 #abc1 #afg1 #afi1 #agc1 #agd1 #ahd1 #aif1 #bja1 #bdd ,rain-pittings in strata1 i4 #cdb ,raisin1 ,miss1 i4 #afa1 #afc1 #afd1 #afe1 #baj ,ramsay1 ,a4 ,c41 i4 #abf1 #adb1 #adc1 #add1 #ade1 #aeh1 #aei1 #afh1 #age1 #agf1 #agg1 #agh1 #agi1 #ahj1 #ahb1 #ahc1 #bjd1 #bje1 #bjh1 #baj1 #bab1 #bad1 #bbc1 #bcg1 #cfd ,ratho type of dolerite1 i4 #dah1 #dba ,red ,head1 section at1 i4 #cjj ,red ,hills1 ,skye1 scenery of1 i4 #aje2 ii4 #cgi ,reed1 ,mr4 ,cowper1 i4 #bje ,reid1 ,mr4 ,clement1 ii4 #ddi1 #dej ,renard1 ,prof4 ,a41 i4 #adh1 #adi ,renfrewshire1 ,carboniferous volcanic rocks of1 i4 #cfh1 #che1 #cig1 #djj1 #djd1 #djh1 #daf1 #dcj1 #ddg ,reyer1 ,prof4 ,e41 ii4 #dgd ,reynolds1 ,mr4 ,s4 ,h41 i4 #agg1 #agi1 #bef ,rhobel ,fawr1 i4 #agg1 #agh1 #ahf ,rhyolite1 i4 #ai1 #bb1 #bd1 #aca1 #afa1 #afe1 #afg1 #afh1 #agh1 #bjd1 #baj1 #bca1 #bcb1 #bee1 #bgf1 #bgh2 ii4 #ahe1 #bje1 #cga1 #dbd1 #dcg ,rhyolitic conglomerate1 ii4 #aie1 #bjf1 #dbi ,richardson1 ,rev4 ,w41 ii4 #aaj ,richthofen1 ,f4 von1 i4 #bh2 ii4 #aae1 #aaf ,rivers made to shift their channels by volcanic eruptions1 i4 #db1 #di2 of the ,tertiary volcanic period1 ii4 #bag1 #bbh1 #bca1 #bcd1 #def ,rocks1 oldest known1 i4 #aaj ,roscommon1 volcanic rocks of1 i4 #caf ,rosenbusch1 ,prof4 ,h41 ii4 #acf1 #acg ,ross1 ,mr4 ,alexander1 ii4 #djf1 #dji ,rothliegende1 ii4 #ie ,roxburghshire1 ,carboniferous vents of1 i4 #ee1 #djc1 #djd ,rubers ,law1 i4 #cge1 #chj1 #djd ,rum1 i4 #aab2 basalt-plateau of1 ii4 #bae2 gabbros of1 #ccb1 #cdi2 acid bosses of1 #djc2 acid sills of1 #dca2 pitchstone of1 #dde ,rutley1 ,mr4 ,f41 i4 #aca1 #acc1 #agj1 #bjg1 #baj1 #bbg1 #bca1 #bcb1 #bch1 #bfj2 ii4 #bc1 #ce ,sahlite found in volcanic vents1 i4 #fb ,saline ,hill1 volcanic vents of1 i4 #dcc1 #dce1 #ddj ,sanday1 basalts of1 ii4 #bae2 conglomerates of1 #bbf ,sandstone altered into quartzite1 i4 #gb1 #cdi1 #cej1 #djd1 #dcb1 #dea2 ii4 #gf1 #afd2 veinings of1 in lava1 i4 #bhc1 #cjj1 #cjc1 #cbj1 #cbg1 #ccc1 #ccg2 ii4 #ei1 #ih ,sandwich ,islands1 lava-cones of1 i4 #aj ,sanidine ejected from volcanic vents1 ii4 #eh1 #gi ,sanquhar1 ,silurian volcanic rocks at1 i4 #aib1 #aie1 #aii2 ,permian volcanic rocks at1 ii4 #fb ,santorin1 ii4 #acd ,saponite1 ii4 #gi ,scenery1 origin of1 i4 #h1 #ajj ,schalstein1 i4 #bfb2 ii4 #cf ,schists1 primeval1 i4 #aaj1 #aad1 #aah1 #aai2 produced by deformation of igneous rocks1 #ge1 #aad1 #aah1 #aai1 #aba1 #afb1 #bdj1 #bdi1 #beb1 #bfa ,schmidt1 ,dr4 ,c4 ,w41 ii4 #bff ,scoriaceous structure1 i4 #ae1 #af1 #bhb1 #cbg1 #cci ,scorpions1 fossil1 i4 #agd1 #cef1 #dff ,scotland1 lines of fault in1 i4 #aa2 ,vesuvian cones of1 #db2 plateaux of1 #dc2 puys of1 #df2 submarine lavas of1 #dh2 ,carboniferous vents of1 #ee2 volcanic scenery of1 #ajd2 pre-,cambrian rocks of1 #aaa2 ,cambrian rocks of1 #aab2 ,lewisian gneiss of1 #aad2 ,dalradian rocks of1 #aba2 ,arenig rocks of1 #abc1 #aia2 ,old ,red ,sandstone of1 #bff1 #bgc1 #bha1 #bia2 ,carboniferous geography of1 #cef2 ,carboniferous volcanoes of1 #cei2 ,carboniferous plateaux of1 #cfg2 ,carboniferous puys of1 #dad2 ,permian volcanoes of1 ii4 #ee2 ,tertiary dykes of1 #abb2 ,tertiary basalt-plateaux of1 #bjh1 #bgd1 #cjd2 ,tertiary gabbros of1 #cbg2 ,tertiary acid rocks of1 #cgi ,scrope1 ,g4 ,p41 i4 #bg1 #hb1 #de1 #aaf2 ii4 #cgc1 #cgd1 #cha ,sedgwick1 ,a41 i4 #adb1 #aff1 #age1 #bah1 #bbg1 #beg2 ii4 #a1 #b1 #c1 #e1 #aac1 #aci1 #aec1 #aeg ,segregation 7see ,differentiation7 ,segregation-veins1 i4 #hd1 #ib2 ii4 #ff1 #acj1 #cjj1 #cjc ,selwyn1 ,mr4 ,a4 ,c4 ,r41 i4 #adc1 #age1 #bjh1 #bba ,semi-opal1 ii4 #gi ,sepulchre ,mountain1 i4 #gi ,serpentine1 i4 #aie1 #bic ,shale1 alteration of1 i4 #gb1 #dea2 ii4 #afd ,shap1 granite of1 i4 #bcf1 #bch1 #bga1 #bij ,sheets1 intrusive 7see ,sills7 ,shelve1 i4 #agf1 #aij ,shetland1 i4 #bga1 #bhi1 #bib1 #bic1 #cde ,shiant ,isles1 ii4 #cjg ,shineton ,shales1 i4 #add ,shore-lines1 traces of ancient1 i4 #bie1 #cje ,shropshire1 ancient volcanic rocks of1 i4 #abi1 #ahi2 latest eruptive rocks of1 ii4 #aja ,sicily1 i4 #bf ,sidlaw ,hills1 i4 #bhf1 #bid1 #cjc ,sills1 vitreous margins of1 i4 #ah2 tectonic relations of1 #gg1 #hc1 #dea2 origin of name1 #hc2 differentiation 7segregation7 in1 #ha1 #dej2 ii4 #dgf2 ordinary stratigraphical position of1 i4 #he2 considered as parts of incompleted volcanoes1 #hf2 metamorphism by1 #hg1 #id1 #dea2 ii4 #bii1 #cjc1 #caj2 conditions for injection of1 i4 #ig1 #ih1 #ii1 #deh2 columnar structure of1 ii4 #ahg1 #bia1 #cja1 #cjf1 #cjh1 #cai2 amygdaloidal structure in1 #bii1 #cab2 banding of1 #cji2 split by later sills1 #caj1 #caf2 extreme subdivision of1 #caa2 slaggy surface in some1 #cab2 give off veins1 #cac2 double and multiple1 #cah1 #dcd2 connection with vents1 #cbb ---- pre-,cambrian1 i4 #aah1 #abd2 ,cambrian1 #aee1 #agj1 #aga2 ,silurian1 #ahg1 #aie1 #bjf1 #baf1 #bcg1 #bdi2 ,devonian1 #bfa2 ,old ,red ,sandstone1 #bgg1 #bia1 #cba1 #cce1 #cdc1 #cde2 ,carboniferous1 #djh1 #ddf1 #dgb2 ii4 #b1 #ba1 #cj1 #dh2 ,permian1 #fd1 #ff2 of ,midlands1 #ajb1 #ajc2 ,tertiary1 7#a7 ,basic1 #bih2 7#b7 ,acid1 #cff1 #dcj ,silurian system1 i4 #agc2 vegetation of1 #agd2 geography of1 #bfc2 volcanoes of1 #age2 classification of1 #age2 two volcanic series of1 #agg ---- volcanoes in ,shropshire1 i4 #ahi2 in ,scotland1 #aia2 at ,builth1 #bjc2 in ,pembrokeshire1 #bje2 in ,caernarvonshire1 #bjg2 in the ,berwyn ,hills1 #bah2 in ,anglesey1 #bai2 in the ,lake ,district1 #bbg2 in ,gloucestershire1 #bch2 in ,ireland1 #bci1 #bed ,skae1 ,h4 ,m41 i4 #bid1 #bii1 #cjf1 #cge2 ii4 #eg ,skiddaw1 i4 #bbh2 granite of1 #bcf ---- ,slate1 i4 #bbi ,skomer ,island1 i4 #bjg ,skye1 spherulitic dykes and sills of1 i4 #bj2 ophitic structure from1 #bj2 basalt-terraces of1 #bd2 metamorphism by granophyre of1 #ie1 #if2 volcanic scenery of1 #ajc1 #aje2 dykes of1 ii4 #abc1 #abd1 #abi1 #aci1 #adj1 #adf1 #aej1 #aeb1 #aed1 #afj1 #afb1 #afd1 #afe1 #agc1 #bfi2 bedded basalts of1 #aib1 #bdi1 #bfi2 tuffs of1 #bea2 connection of dykes and superficial lavas in1 #bfi2 vents in1 #bhj2 sills of1 #cjd2 gabbro bosses of1 i4 #aaf2 ii4 #ccd2 acid bosses of1 #cgi2 acid sills of1 #dca2 acid dykes of1 #dcg2 pitchstone veins of1 #dde2 subsidence of1 #ddg ,slaggy structure1 i4 #af1 #cc1 #ei1 #bhb1 #cbg1 #cci2 ii4 #ih1 #ahg ,slane1 volcanic rocks near1 i4 #bdd ,slate-tuffs1 i4 #ahj1 #bac1 #bcd ,slemish a volcanic neck1 ii4 #bga ,slieve ,foye1 ii4 #dba ---- ,gallion1 ii4 #bjj ---- ,gullion1 ii4 #dbb ,small ,isles1 basalt-plateau of1 ii4 #bae2 vents of1 #bhh2 sills of1 #cah2 acid bosses of1 #djc2 acid sills of1 #dca ,small1 ,mr4 ,e4 ,w41 i4 #bjg ,smaragdite found in volcanic vents1 i4 #fb ,snowdon1 volcanic rocks of1 i4 #bj1 #db1 #dg1 #ajb1 #age1 #bjh1 #bji1 #baj1 #baa1 #bab1 #bac1 #bah1 #bbf ,soda-felsites1 i4 #ahc1 #aif1 #bdg ,solfataric action1 i4 #ga2 ii4 #ahe1 #bje1 #chh ,sollas1 ,prof41 i4 #if2 ii4 #age1 #bic1 #dae1 #dba1 #dbb ,solway1 ,carboniferous volcanic plateau of1 i4 #cge1 #che1 #dac ,somerset1 volcanic rocks of1 ii4 #cb ,somma1 denudation of1 i4 #c1 #ajj ,spheroidal structure of dolerite1 i4 #def ,spherulitic structure1 i4 #ai1 #bj1 #ie1 #abj1 #acj1 #aee1 #afb1 #ahd1 #baa1 #bcb1 #bce1 #cdf2 ii4 #cfi1 #cha1 #cib1 #dcb1 #dce1 #dcg1 #dda1 #ddf ,spilosite1 i4 #bfb ,springs1 mineral1 connected with volcanic action1 i4 #cij1 #dde ,st4 ,abb's ,head1 i4 #cch ,staffa1 i4 #be2 first notice of1 ii4 #aji2 columnar basalts of1 #ahf1 #ahh1 #baj2 basalt conglomerate of1 #aie ,staffordshire1 latest eruptive rocks of1 ii4 #aja1 #ajc ,st4 ,andrews1 old volcanoes near1 ii4 #ga1 #gc1 #hg ,st4 ,david's1 ,cambrian volcanic rocks of1 i4 #ade ---- ,head1 i4 #bje ,steam in volcanic action1 i4 #ac1 #ae1 #af1 #ga ,stecher1 ,dr41 i4 #dba1 #dea2 ii4 #afe ,st4 ,kilda1 dykes of1 ii4 #agc1 #daf2 gabbro of1 #ceh2 general account of geology of1 #dje2 granophyre of1 #djh ,stocks1 or bosses1 i4 #gh1 #hh ,strahan1 ,mr4 ,a41 i4 #aga2 ii4 #aj1 #ab1 #bc1 #bh1 #cb ,strathaird1 ii4 #abc1 #adj1 #afd1 #bfi ,strathbogie1 i4 #cdd2 ii4 #aba ,strathmore1 i4 #cjd ,stromboli1 i4 #d ,sublimations1 traces of ancient1 i4 #dde ,submarine eruptions1 i4 #dh ,sub-ophitic structure1 i4 #dag ,subsidence and volcanic action1 i4 #bie1 #big1 #ddd1 #dfc2 ii4 #db1 #bje1 #ddg1 #dgj ,subterranean igneous injections1 i4 #gg 7see ,bosses1 ,dykes1 ,sills7 ,suess1 ,prof4 ,e41 ii4 #dgd ,sun-cracks1 i4 #cdb ,sweden1 ,archan volcanic rocks of1 i4 #abj ,syenite1 ii4 #cff ,symes1 ,mr4 ,r4 ,g41 i4 #caa1 #cdc1 #cfi2 ii4 #bja1 #dbh ,tate1 ,g41 ii4 #c1 #aac ---- ,r41 ii4 #bjd ,tatlock1 ,mr4 ,r4 ,r41 i4 #bgc1 #bgh ,tawney1 ,e4 ,b41 i4 #aeg ,teall1 ,mr4 ,j4 ,j4 ,h41 i4 #ij1 #aad1 #aaf1 #aag1 #aah1 #aai1 #abj1 #aef1 #aib1 #aid1 #bjj1 #bjg1 #baj1 #bge1 #bgg1 #bij1 #caa1 #ccf1 #cch1 #cdf1 #djg1 #ddi2 ii4 #b1 #c1 #e1 #g1 #aa1 #cb1 #dd1 #aac1 #aca1 #acd1 #ace1 #acg1 #ach1 #adj1 #add1 #adi1 #bib1 #bic1 #cbi1 #cfg1 #cfh1 #cfi ,teesdale1 i4 #bbh ,termier1 ,m4 ,p41 ii4 #cge ,terrestrial volcanic eruptions1 i4 #ej ,tertiary ,volcanic ,series1 ii4 #aha ,subaerial character of eruptions1 ii4 #ajc1 #aih ,scenery of1 ii4 #bee1 #bef1 #cdi1 #cia1 #dje1 #djh ,the ,plateaux1 ii4 #ahc1 #bdi2 lavas of1 #ahc1 #bah1 #bcf1 #bef2 thickness of individual sheets1 #aib1 #bjf1 #bed1 #beg2 lenticular character of lavas of1 #aic1 #beg2 greatest depth of1 #baj1 #baa1 #bac1 #bfj2 tuffs and clays of1 #aid1 #bjb1 #bjd1 #baa1 #bbb1 #bbe1 #bea1 #beh1 #bgg1 #bhd1 #bhg2 non-volcanic fragments in1 #aif1 #baa1 #bac1 #bai2 lignites of1 #aih1 #bjc1 #bjh1 #bac1 #bea2 gravels and conglomerates of1 #aih1 #bab1 #bch1 #bef2 coal of1 #bac1 #bea1 #bef1 #bhg2 leaf-beds of1 #bjd1 #bab1 #bbb1 #bbe1 #bhh2 carbonaceous nature of the upper parts of intercalated sediments in1 #bbc1 #bbf1 #bbg1 #bbi1 #bcb1 #bea1 #bhh2 evidence for intervals between the eruptions in1 #bjc1 #bje1 #bjh1 #bba1 #bbh1 #bdj1 #bde1 #bea1 #bed1 #bhh2 no evidence of great central vents in1 #bjh1 #bad1 #bee1 #beh1 #bfj1 #bfg2 faulted condition of1 #bjj1 #bjh1 #bji1 #deb2 subsidences of1 #bje1 #bjh1 #bji1 #bad1 #ddg2 ancient river channels of1 #bag1 #bbh1 #bca1 #bcd1 #def2 volcanic cones of1 #bjb1 #bah1 #bcj1 #bgg1 #bha1 #bhe2 paralleled by the modern ,icelandic eruptions1 #bfj2 vents of1 #bjb1 #bah1 #bcj1 #bgj1 #bgf ,the ,basic sills1 ii4 #bih1 #cjd ,the ,gabbro bosses1 ii4 #cbg1 #cdi1 #cee1 #ceh2 history of the gabbro intrusions1 #cei ,the ,acid rocks1 ii4 #cfd2 petrography of1 #cff2 history of their investigation1 #cga2 analogies with trachytes of ,central ,france1 #cgc2 intruded at base of the gabbros or of the bedded basalts1 #ccg1 #cec1 #ceg1 #dca1 #dcb1 #ddd2 bosses of ,skye1 #cgh2 of ,mull1 #cie2 of ,small ,isles1 #dje2 of ,st4 ,kilda1 #dje2 of ,arran1 #dah2 of ,carlingford1 #dbj2 of ,slieve ,foye1 and ,barnavave1 #dba2 of ,slieve ,gullion1 #dbb2 of ,antrim1 #dbf2 acid sills1 #dcj2 acid dykes and veins1 #dcg ,metamorphism of the basalts1 ii4 #bgb1 #bgf1 #ccg1 #cci1 #cdj1 #cdg1 #cee1 #cef1 #ceg1 #ceh1 #cfb1 #cgh1 #chc1 #chf1 #cig1 #cii1 #djj1 #djd1 #dac ,texture1 varieties of1 in igneous rocks1 i4 #gh1 #ddi2 ii4 #e1 #bii1 #cfj ,tholeiites1 ii4 #acg1 #aeh ,tholeiite type of basalt1 i4 #dai1 #dba ,thornhill1 volcanic rocks of1 ii4 #fj ,thoroddsen1 ,th41 ii4 #bfa1 #bfb1 #bfc1 #bfd1 #bfe1 #bff1 #bgh ,thrust-planes1 i4 #bbi ,time in geological history1 ii4 #ajg1 #dfa1 #dfe ,timmins1 ,j4 ,h41 i4 #acc ,tinto1 i4 #bgh1 #bhh1 #cbi ,titterstone ,clee ,hill1 ii4 #aja ,toadstones of ,derbyshire1 i4 #cei2 ii4 #h ,topley1 ,w41 i4 #adg2 ii4 #c1 #e1 #g ,torridonian rocks1 i4 #aaa1 #aab1 #aac1 #abj2 ii4 #cej ,tortworth1 volcanic rocks at1 i4 #bch ,tourmakeady1 volcanic rocks of ,bala age at1 i4 #bea ,townson1 ,r41 i4 #cfc ,trachyte1 i4 #ahc1 #bcj1 #bdf1 #bgc1 #bgf 7analysis71 #cgi 7analysis71 #chf1 #djc1 #djg1 #dba2 ii4 #cf1 #dg1 #if1 #ach1 #aeb1 #ahd1 #bcf ,traill1 ,mr4 ,w41 ii4 #age1 #dba1 #dbb ,traprain ,law1 i4 #cgb1 #chj1 #djc1 #dje ,traquair1 ,dr4 ,r4 ,h41 i4 #bff ,tremadoc group1 i4 #add1 #agg ,trevelyan1 ,w4 ,c41 ii4 #c ,triassic eruptive rocks1 i4 #bi2 geography1 ii4 #ajh ,trichites1 i4 #ai2 ii4 #acf ,troctolite1 ii4 #ccb ,tuffs1 i4 #ca2 association of1 #cc2 composition of1 #cd2 alternations of1 #cd1 #fa2 blending of1 with non-volcanic sediment1 #ce1 #dcg2 fossiliferous1 #cf2 without lava1 #cf2 necks of1 #eh2 relation of1 to lavas1 #fa2 metamorphism of1 ii4 #bbd ---- pre-,cambrian1 i1 #abe1 #ace2 ,cambrian1 #adg1 #aea1 #aee1 #afc1 #afe1 #afg2 ,silurian1 #agh1 #ahi1 #aij1 #aie1 #bje1 #bji1 #bab1 #bac1 #bbb1 #bbd1 #bbi1 #bcb1 #bda1 #bde1 #bdf1 #bed1 #bee2 ,devonian1 #bfb2 ,old ,red ,sandstone1 #bgi1 #bha1 #bhi1 #ccg1 #cci1 #cea2 ,carboniferous1 #cha1 #chd1 #chg1 #cii1 #dbb1 #dbg1 #dbi1 #dcb1 #dcf1 #dff2 ii4 #aa1 #ah1 #bd1 #cf2 ,permian1 #eg1 #eh2 ,tertiary1 #aid1 #aig1 #bjb1 #bjd1 #baa1 #bbb ,tyrol1 ,triassic eruptive rocks of1 i4 #bi ,tyrone1 ,old ,red ,sandstone of1 i4 #cad ,ulster1 ,old ,red ,sandstone volcanic rocks of1 i4 #cad ,ultra-basic rocks1 i4 #ad1 #aah1 #cgg1 #dag ,unconformability1 deceptive case of1 i4 #afc ,urgneiss1 i4 #aaj ,uriconian volcanic rocks1 i4 #abi ,ussher1 ,mr4 ,w4 ,a4 ,e41 i4 #bfj1 #bfb2 ii4 #ce1 #ie ,utah1 laccolites of1 i4 #hf2 volcanic regions of1 ii4 #aae1 #bfg ,valleys1 tendency of vents to appear in1 i4 #bgb1 #cfh1 #cgf2 ii4 #fa1 #fe1 #if ,vapours1 action of volcanic1 i4 #ac1 #ae1 #af1 #ag1 #ca1 #eg1 #gb1 #gh1 #ig1 #ii1 #ahj1 #bhi1 #dbf ,variolitic structure1 i4 #ba1 #bjf1 #bce ,veins1 intrusive1 i4 #ff1 #gg1 #gi1 #ih1 #dbf1 #dbi2 ii4 #caa1 #cac1 #djj1 #daj1 #dcb1 #dcg ,velay1 volcanic rocks of1 i4 #bf1 #bg1 #bi1 #fj2 ii4 #bga1 #cgc1 #cge ,vents1 volcanic1 i4 #ec2 ground-plans of1 #ed2 size of1 #ee2 filled with non-volcanic detritus1 #eg2 ejected crystals found in1 #fb2 agglomerates of1 #fb2 ii4 #ac1 #bh1 #dg1 #fa1 #fi1 #bgf1 #bhj1 #bhd1 #bhh1 #bhi2 stratification in1 i4 #fc2 ii4 #hj2 metamorphism in1 i4 #fg1 #ga2 metamorphism of rocks around1 #gb1 #cdi1 #cej1 #ceb1 #cii1 #djd1 #dcb2 ii4 #gf1 #bgb1 #bhj2 connection of1 with geological structure-lines1 i4 #fh2 occurrence of1 in lines and in groups1 #fi2 double and multiple1 #fi2 possible indications of length of activity of1 #gb2 inward dip of strata around1 #gc1 #ceb2 ii4 #gf1 #bie2 stages in history of1 i4 #gd2 tendency of1 to rise in lines of valley1 #bgb1 #cfh1 #cgf2 ii4 #fa1 #fe1 #if1 #dfh2 criteria for the relative ages of1 #bgj2 connection of1 with later eruptive bosses1 #bhj1 #chd1 #cii1 #djj ---- ,silurian1 i4 #bji1 #bad1 #bcd2 ,old ,red ,sandstone1 #bgb1 #bhg1 #bih1 #cje1 #cbc1 #cbh1 #ccg2 ,carboniferous1 #cid1 #cii1 #djj1 #djd1 #djf1 #dbd1 #dfe2 ii4 #ac1 #bh1 #dg2 ,permian1 #fa1 #fi2 ,tertiary1 #bjb1 #bgj1 #bid1 #djj ,vesicular structure of lavas1 i4 #ae2 ii4 #ahg ,vesuvius1 denudation of1 i4 #c2 as an active volcano1 #d2 as a type of volcano1 #aj1 #ci1 #ec1 #ajj2 ii4 #ajh1 #aae1 #bfa1 #bff ,vicary1 ,mr4 ,w41 ii4 #ie ,vogesite1 i4 #bgg1 #bic ,volcanello ,island1 i4 #gj ,volcanic action1 permanent traces of1 i4 #d2 of present time elucidates that of the past1 #e2 submarine1 #e2 transient effects of1 #h2 chief factors in1 #aj2 explosive energy of1 #ac1 #ii2 uniformity of1 in geological time1 #ac1 ii4 #dgj2 metamorphism by1 i4 #fg1 #ga2 underground phases of1 #gg2 proofs of gradual quiescence of1 #aee1 #aeg1 #aff2 connected with subsidence1 #bie1 #big2 ii4 #bje1 #ddd1 #dfc1 #dgj2 repetition of1 in the same region1 i4 #cfh1 #cge1 #cgg2 ii4 #db1 #fi1 #id1 #dfg2 developed along continental borders1 #dff2 persistence of1 in ,britain1 i4 #g2 ii4 #dff2 connection of1 with lines of geological structure1 #dfh2 connection of1 with terrestrial disturbance1 #dfi2 gradual decline of1 during ,palozoic time1 #dga2 quiescence of1 during ,mesozoic time1 #dgb ,volcanic cycles1 i4 #bg1 #ib ---- products1 general characters of1 i4 #ad2 persistent uniformity of1 #cj1 #df2 thickest mass of1 in ,britain1 #bbi 7see ,agglomerate1 ,lava1 ,tuff7 ,volcano1 ,island of1 i4 #d1 #bd ,volcanoes1 their influence on mythology1 i4 #a1 #b2 denudation of1 #c2 number of extinct1 #d2 ancient1 of ,britain1 #f2 influence on scenery1 #h1 #ajj1 #ajb2 defined1 #aj2 types of1 #aj1 #ci2 ii4 #dga2 determination of relative dates of1 i4 #df2 their geographical condition in old times1 how ascertained1 #dh2 parasitic1 #fi2 contemporaneous denudation of1 #gc1 #ajj2 connected with granite1 #hi2 incompleted1 #hf1 #ic1 #ii ,vom ,rath1 ,g41 ii4 #dgd ,wacke1 i4 #aeg ,walcott1 ,mr41 i4 #cj ,wales1 pre-,cambrian rocks of1 i4 #abf1 #adb2 early geological work in1 #adb2 ,cambrian volcanoes of1 #ade1 #aei2 volcanic scenery of1 #agf2 ,silurian volcanoes of1 #agf1 #bjb1 #bje1 #bjg1 #bah1 #bai2 ,old ,red ,sandstone of1 #beg1 #bei ,waller1 ,mr4 ,t4 ,h41 i4 #aga1 #bgh ,ward1 ,j4 ,c41 i4 #bbg1 #bbh1 #bbi1 #bcj1 #bca1 #bcc1 #bcd1 #bce1 #bcf1 #bcg2 ii4 #bc ,warwickshire1 ,cambrian rocks of1 i4 #acg1 #aga ,waterford1 volcanic region of1 i4 #bdg ,watts1 ,mr4 ,w4 ,w41 i4 #aca1 #acb1 #ace1 #acg1 #ahi1 #aij1 #aia1 #bdc1 #bgf1 #bgh1 #ccf1 #cdg1 #dag1 #dba1 #dbc2 ii4 #dj1 #db1 #dc1 #de1 #eg1 #if1 #ahd1 #bjd1 #bbd1 #bgb1 #dbd1 #dbe ,weaver1 ,t41 i4 #bch ,wenlock group1 i4 #age2 volcanic rocks of1 #eeb ,wernerian ,school1 ii4 #aji ,west ,lothian1 volcanic rocks of1 i4 #dg1 #ee1 #dae1 #dcc1 #dcg ,whin ,sill of ,england1 i4 #hc1 #he1 #ig1 #ddi2 ii4 #b ,whitehurst1 ,j41 ii4 #i1 #aji ,white trap1 i4 #if1 #dbf1 #ddi1 #def2 ii4 #fe1 #hg1 #ajc1 #afe1 #beb ,williams1 ,mr4 ,g4 ,j41 i4 #agi1 #ahe1 #ahf1 #ahh ,williamson1 ,w4 ,c41 i4 #cib ,wilson1 ,mr4 ,j4 ,s4 ,grant1 i4 #adh1 #adi1 #aec1 #bgf1 #cdd1 #cge1 #cgi1 #chj2 ii4 #acg1 #afd ,wilson1 ,mr4 ,a41 ii4 #di ,winch1 ,n4 ,t41 ii4 #aac1 #adg ,witham1 ,h4 ,t4 ,m41 ii4 #aac1 #bch ,wood1 ,n41 ii4 #aac ,woods1 ,mr4 ,h41 i4 #bjd ,woodward1 ,dr4 ,henry1 ii4 #ddi ,woodward1 ,mr4 ,h4 ,b41 ii4 #cb1 #dce1 #dec ,worcestershire1 latest eruptive rocks of1 ii4 #aja ,worth1 ,mr4 ,r4 ,n41 ii4 #ii ,wrekin1 i4 #acj ,wright1 ,j4 ,r41 ii4 #ajb ,wunsch1 ,e41 i4 #cfi1 #cib ,wrtemberg1 puys of1 i4 #df ,wyoming1 lava-fields of1 ii4 #aae ,yates1 ,j41 i4 #aga ,yellowstone ,park1 volcanic phenomena of1 i4 #bi1 #ca ,y-foel-frs1 i4 #bji1 #bad ,y ,glyder-,fach1 i4 #bji ,yoredale group1 ii4 #i1 #ac1 #ag ,young1 ,mr4 ,john1 i4 #cfi1 #cib ,young1 ,prof4 ,john1 i4 #bid1 #cjh ,zircon1 found in volcanic vents1 i4 #fb ,zirkel1 ,prof41 ii4 #cbg1 #cbi1 #ccd1 #cef1 #cfd1 #cgj1 #cgb1 #cgi1 #dcj ,,the ,,end ,printed by ,r4 ,r4 ,clark1 ,limited1 ,edinburgh4 ,,the ,,cambridge ,,natural ,,history4 ,edited by ,s4 ,f4 ,harmer1 ,m4,a1 ,fellow of ,king's ,college1 ,cambridge1 ,superintendent of the ,university ,museum of ,zoology2 and ,a4 ,e4 ,shipley1 ,m4,a41 ,fellow of ,christ's ,college1 ,cambridge1 ,university ,lecturer on the ,morphology of ,invertebrates4 ,to be completed in ,ten ,volumes4 #hvo4 price #ags4 net each4 ,intended in all respects to be a ,standard ,natural ,history accurate enough to be of use to the ,student1 and at the same time popular enough for the general reader who desires trustworthy information as to the structure and habits of all members of the ,animal ,kingdom1 from the ,protozoa to the ,mammals4 ,the ,volumes are fully illustrated by original figures drawn where possible from nature4 ,when complete1 the ,series is one which should be indispensable in all libraries1 whether public or private4 ,,now ,,ready4 ,,worms1 ,,leeches1 etc41 ,,volume ,,ii4 ,,flatworms4 ,by ,f4 ,w4 ,gamble1 ,m4,sc41 ,vict41 ,owens ,college4 ,,nemertines4 ,by ,miss ,l4 ,sheldon1 ,newnham ,college1 ,cambridge4 ,,thread-,,worms1 ,etc4 ,by ,a4 ,e4 ,shipley1 ,m4,a41 ,fellow of ,christ's ,college1 ,cambridge4 ,,rotifers4 ,by ,marcus ,hartog1 ,m4,a41 ,trinity ,college1 ,cambridge1 ,d4,sc4 ,lond41 ,professor of ,natural ,history in the ,queen's ,college1 ,cork4 ,,polychaet ,,worms4 ,by ,w4 ,blaxland ,benham1 ,d4,sc4 ,lond41 ,hon4 ,m4,a4 ,oxon41 ,aldrichian ,demonstrator of ,comparative ,anatomy in the ,university of ,oxford4 ,,earthworms ,,and ,,leeches4 ,by ,f4 ,e4 ,beddard1 ,m4,a4 ,oxon41 ,f4,r4,s41 ,prosector to the ,zoological ,society1 ,london4 ,,gephyrea1 ,etc4 ,by ,a4 ,e4 ,shipley1 ,m4,a41 ,fellow of ,christ's ,college1 ,cambridge4 ,,polyzoa4 ,by ,s4 ,f4 ,harmer1 ,m4,a41 ,fellow of ,king's ,college1 ,cambridge4 ,,natural ,,science4--,this second volume of the ,cambridge ,natural ,history is certain to prove a most welcome addition to ,english ,zoological literature4 ,it deals with a series of animal groups1 all deeply interesting to the specialist in ,morphology2 some important from their economic relations to other living things1 others in their life-histories rivalling the marvels of fairy-tales4 ,and the style in which they are here treated is also interesting3 history and the early observations of the older writers lend their charm2 accounts of habits1 and mode of occurrence1 of life1 in a word1 from the cradle to the grave1 are given in ample detail1 accompanied by full references to modern and current literature4 ,the whole is admirably illustrated4 ,,shells4 ,,volume ,,iii4 ,,molluscs ,,and ,,brachiopods4 ,by the ,rev4 ,a4 ,h4 ,cooke1 ,m4,a41 ,a4 ,e4 ,shipley1 ,m4,a41 and ,f4 ,r4 ,c4 ,reed1 ,m4,a4 ,,field4--,we know of no book available to the general reader which affords such a vast fund of information on the structure and habits of molluscs4 ,,daily ,,news4--,promises to be1 in its own department of science1 the most important work of the day4444 ,this large and exhaustive work is written especially for general readers4 ,no special training in science is needed for the easy comprehension of this first volume at any rate4 ,but while the writers treat their respective subjects in what is called a popular way1 they are careful to give due importance to the latest results of scientific research4444 ,the volume is liberally and admirably illustrated4 ,,knowledge4--,if succeeding volumes are like this one1 the ,cambridge ,natural ,history will rank as one of the finest works on natural history ever published4 ,,westminster ,,review4--,should find a place in the library of every naturalist4 ,,insects and ,,centipedes4 ,,volume ,v4 ,,peripatus4 ,by ,adam ,sedgwick1 ,m4,a41 ,f4,r4,s4 ,,myriapods4 ,by ,f4 ,g4 ,sinclair1 ,m4,a4 ,,insects4 ,part ,i4 ,by ,david ,sharp1 ,m4,a4 ,cantab41 ,m4,b4 ,edin41 ,f4,r4,s4 ,,field4--,although written for the student and the specialist1 the book is not the less adapted to all intelligent readers who wish to make themselves thoroughly acquainted with the habits1 structure1 and the modern classification of the animals of which it treats4 ,to such it cannot be recommended too strongly4 ,,athen,,um4--,this book will find a place in the library of most entomologists1 prove a welcome boon to weak brethren who are compelled to lecture--docenda discimus--and occupy by the side of ,westwood's ',modern ,classification' and ,burmester's ',manual' a niche that has long-been empty4 ,,nature ,,notes4--,when the following volume is completed1 ,mr4 ,sharp's work will1 we think1 last for many years as the standard text-book on ,entomology in ,england4 ,,science ,,progress4--,the work is excellently illustrated1 and besides being full of interest for the general reader will prove extremely useful to the student4 ,the next ,volumes to appear will be ,vol4 ,,ix4 on ,,birds1 by ,mr4 ,a4 ,h4 ,evans1 and ,vol4 ,,vii4 on ,,fishes and ,kindred ,forms1 by ,professor ,bridge1 and other contributors4 ,these will be published in the course of #ahig1 and the remaining ,volumes are all in active progress4 ,,macmillan ,,and ,,co41 ,ltd41 ,,london4 ,,nature ,,series4 ,crown #hvo4 ,,popular ,,lectures ,,and ,,addresses4 ,by ,lord ,kelvin1 ,f4,r4,s4 ,in ,three ,vols4 ,vol4 ,i4 ,the ,constitution of ,matter4 ,second ,edition4 #gs4 #f;d4 ,vol4 ,,ii4 ,geology and ,general ,physics4 #gs4 #f;d4 ,vol4 ,,iii4 ,papers on ,navigation4 #gs4 #f;d4 ,,how ,,to ,,draw ,a ,,straight ,,line3 a ,lecture on ,linkages4 ,by ,a4 ,b4 ,kempe1 ,f4,r4,s4 ,illustrated4 #as4 #f;d4 ,,on ,,light4 ,the ,burnett ,lectures4 ,by ,sir ,george ,gabriel ,stokes1 ,f4,r4,s4 ,three ,courses4 ,i4 ,on the ,nature of ,light4 ,,ii4 ,on ,light as a means of ,investigation4 ,,iii4 ,on ,beneficial ,effects of ,light4 #gs4 #f;d4 ,,light3 a ,series of ,simple1 ,entertaining1 and ,useful ,experiments4 ,by ,a4 ,m4 ,mayer and ,c4 ,barnard4 ,illustrated4 #bs4 #f;d4 ,,polarisation ,,of ,,light4 ,by ,w4 ,spottiswoode1 ,f4,r4,s4 ,illustrated4 #cs4 #f;d4 ,,sound3 a ,series of ,simple1 ,entertaining1 and ,inexpensive ,experiments4 ,by ,a4 ,m4 ,mayer4 #cs4 #f;d4 ,,modern ,,views ,,of ,,electricity4 ,by ,prof4 ,o4 ,j4 ,lodge1 ,f4,r4,s4 ,illustrated4 #fs4 #f;d4 ,,the ,,chemistry ,,of ,,the ,,secondary ,,batteries ,,of ,,plante ,,and ,,faure4 ,by ,j4 ,h4 ,gladstone1 ,f4,r4,s41 and ,a4 ,tribe4 #bs4 #f;d4 ,,the ,,chemistry ,,of ,,photography4 ,by ,prof4 ,r4 ,meldola1 ,f4,r4,s4 ,illustrated4 #fs4 ,,charles ,,darwin4 ,memorial ,notices reprinted from ,nature4 ,by ,thomas ,h4 ,huxley1 ,f4,r4,s42 ,g4 ,j4 ,romanes1 ,f4,r4,s42 ,sir ,archibald ,geikie1 ,f4,r4,s42 and ,w4 ,t4 ,dyer1 ,f4,r4,s4 #bs4 #f;d4 ,,the ,,scientific ,,evidences ,,of ,,organic ,,evolution4 ,by ,george ,j4 ,romanes1 ,f4,r4,s4 #bs4 #f;d4 ,,timber ,,and ,,some ,,of ,,its ,,diseases4 ,by ,prof4 ,h4 ,m4 ,ward1 ,f4,r4,s4 ,illustrated4 #fs4 ,,on ,,the ,,colours ,,of ,,flowers4 ,by ,grant ,allen4 ,illustrated4 #cs4 #f;d4 ,,flowers1 ,,fruits1 ,,and ,,leaves4 ,by ,sir ,john ,lubbock1 ,m4,p41 ,f4,r4,s4 ,illustrated4 #ds4 #f;d4 ,,on ,,british ,,wild ,,flowers ,,considered ,,in ,,relation ,,to ,,insects4 ,by ,sir ,john ,lubbock1 ,m4,p41 ,f4,r4,s4 ,illustrated4 #ds4 #f;d4 ,,the ,,origin ,,and ,,metamorphoses ,,of ,,insects4 ,by ,sir ,john ,lubbock1 ,m4,p41 ,f4,r4,s4 ,with ,illustrations4 #cs4 #f;d4 ,,seeing ,,and ,,thinking4 ,by ,prof4 ,w4 ,k4 ,clifford1 ,f4,r4,s4 ,diagrams4 #cs4 #f;d4 ,,macmillan ,,and ,,co41 ,ltd41 ,,london4 ,,by ,,the ,,same ,,author4 ,,text-,,book ,,of ,,geology4 ,with ,illustrations4 ,third ,edition4 ,revised and ,enlarged4 ,medium #hvo4 #bhs4 ,,class-,,book ,,of ,,geology4 ,illustrated with woodcuts4 ,second ,edition4 ,crown #hvo4 #ds4 #f;d4 ,,elementary ,,lessons ,,in ,,physical ,,geography4 ,illustrated with woodcuts and ten plates4 ,fcap4 #hvo4 #ds4 #f;d4 ,questions for the use of ,schools4 ,fcap4 #hvo4 #as4 #f;d4 ,,physical ,,geography4 ,illustrated4 ,pott #hvo4 #as4 ,science ,primers4 ,,the ,,scenery ,,of ,,scotland ,,viewed ,,in ,,connexion ,,with ,,its ,,physical ,,geology4 ,with a ,geological ,map and ,illustrations4 ,second ,edition4 ,crown #hvo4 #abs4 #f;d4 ,,geology4 ,with ,illustrations4 ,pott #hvo4 #as4 ,science ,primers4 ,,outlines ,,of ,,field ,,geology4 ,new and revised ,edition4 ,extra ,fcap4 #hvo4 #cs4 #f;d4 ,,geological ,,sketches ,,at ,,home ,,and ,,abroad4 ,with ,illustrations4 #hvo4 #ajs4 #f;d4 ,,the ,,teaching ,,of ,,geography4 ,suggestions regarding principles and methods for the use of teachers4 ,globe #hvo4 #bs4 ,geographical ,series4 ,,geography ,,of ,,the ,,british ,,isles4 ,pott #hvo4 #as4 ,geographical ,series4 99 99 99 99 99 ,box of ,geological ,specimens to illustrate ,geikie's ,primer of ,geology4 #ajs4 #f;d4 ,,macmillan ,,and ,,co41 ,ltd41 ,,london4 ,,macmillan ,,and ,,co4',s ,,books ,,for ,,students ,,of ,,physical ,,geography1 ,,geology1 ,,and ,,mineralogy4 ,,physics ,,of ,,the ,,earth',s ,,crust4 ,by ,rev4 ,osmond ,fisher1 ,m4,a41 ,f4,g4,s41 ,hon4 ,fellow of ,king's ,college1 ,london4 #bnd ,ed41 enlarged4 #hvo4 #abs4 ,,tables ,,for ,,the ,,determination ,,of ,,the ,,rock-,,forming ,,minerals4 ,compiled by ,f4 ,l4 ,loewinson-,lessing4 ,trans4 by ,j4 ,w4 ,gregory1 ,b4,sc41 ,f4,g4,s4 ,glossary by ,prof4 ,g4 ,a4 ,j4 ,cole1 ,f4,g4,s4 #hvo4 #ds4 #f;d4 net4 ,,microscopical ,,physiography ,,of ,,the ,,rock-,,making ,,minerals4 ,by ,h4 ,rosenbusch4 ,trans4 by ,j4 ,p4 ,iddings4 #hvo4 #bds4 ,,meteorology4 ,by ,t4 ,russell4 #hvo4 #afs4 net4 ,,physiography ,,for ,,beginners4 ,by ,a4 ,t4 ,simmons1 ,b4,sc4 ,globe #hvo4 #bs4 #f;d4 ,a ,,popular ,,treatise ,,on ,,the ,,winds4 ,by ,w4 ,ferrel1 ,m4,a41 ,member of the ,american ,national ,academy of ,sciences4 #hvo4 #ags4 net4 ,,economic ,,geology ,,of ,,the ,u4,s4 ,by ,r4 ,s4 ,tarr1 ,b4,s4 #hvo4 #afs4 net4 ,,elementary ,,physical ,,geography4 ,by ,r4 ,s4 ,tarr1 ,b4,s4 ,crown #hvo4 #gs4 #f;d4 net4 ,,elements ,,of ,,crystallography1 for ,students of ,chemistry1 ,physics1 and ,mineralogy4 ,by ,g4 ,h4 ,williams1 ,ph4,d4 ,crown #hvo4 #fs4 ,,popular ,,lectures ,,and ,,addresses4 ,by ,lord ,kelvin1 ,p4,r4,s4 ,vol4 ,,ii4 ,geology and ,general ,physics4 ,crown #hvo4 #gs4 #f;d4 ,,the ,,planet ,,earth4 ,by ,r4 ,a4 ,gregory1 ,f4,r4,a4,s4 ,crown #hvo4 #bs4 ,,physiography4 ,an ,introduction to the ,study of ,nature4 ,by the ,eight ,hon4 ,t4 ,h4 ,huxley1 ,f4,r4,s4 ,illustrated4 ,crown #hvo4 #fs4 ,,short ,,studies ,,in ,,earth ,,knowledge4 ,introduction to ,physiography4 ,by ,william ,gee4 ,illustrated4 ,globe #hvo4 #cs4 #f;d4 ,,outlines ,,of ,,physiography--,,the ,,movements ,,of ,,the ,,earth4 ,by ,j4 ,norman ,lockyer1 ,f4,r4,s4 ,illustrated4 ,crown #hvo4 ,sewed1 #as4 #f;d4 ,,macmillan ,,and ,,co41 ,ltd41 ,,london4 99 99 99 99 99 ,transcriber ,note ,minor typos were corrected4 ,a paragraph break was inserted in the last paragraph on pages #bga and #ccd to accommodate placement of illustrations4 ,to prevent images from splitting paragraphs1 some text was moved4