,transcriber ,note ,text emphasis denoted by ,italics and ,bold4 ,,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 ,and41 ,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 ,,naturalists1 ,,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 ,i ,london ,,macmillan ,,and ,,co41 ,limited4 ,,new ,,york3 ,,the ,,macmillan ,,company4 #ahig ,all rights reserved ,,to ,m4 ,ferdinand ,fouqu ,,member ,,of ,,the ,,institute ,,professor ,,of ,,the ,,natural ,,history ,,of ,,inorganic ,,bodies ,,in ,,the ,,coll,,ge ,,de ,,france ,,and ,m4 ,auguste ,michel-,lvy ,,member ,,of ,,the ,,institute ,,director ,,of ,,the ,,geological ,,survey ,,of ,,france ,,distinguished ,,representatives ,,of ,,that ,,french ,,school ,,of ,,geology ,,which ,,by ,,the ,,hands ,,of ,,desmarest ,,founded ,,the ,,study ,,of ,,ancient ,,volcanoes ,,and ,,has ,,since ,,done ,,so ,,much ,,to ,,promote ,,its ,,progress ,,these ,,volumes ,,are ,,inscribed ,,with ,,the ,,highest ,,admiration ,,and ,,esteem ,,preface ,in no department of science is the slow and chequered progress of investigation more conspicuous than in that branch of ,geology which treats of volcanoes4 ,although from the earliest dawn of history1 men had been familiar with the stupendous events of volcanic eruptions1 they were singularly slow in recognizing these phenomena as definite and important parts of the natural history of the earth4 ,even within the present century1 the dominant geological school in ,europe taught that volcanoes were mere accidents1 due to the combustion of subterranean beds of coal casually set on fire by lightning1 or by the decomposition of pyrites4 ,burning mountains1 as they were called1 were believed to be only local and fortuitous appearances1 depending on the position of the coal-fields1 and having no essential connection with the internal structure and past condition of our planet4 ,so long as such fantastic conceptions prevailed1 it was impossible that any solid progress could be made in this branch of science4 ,a juster appreciation of the nature of the earth's interior was needed before men could recognize that volcanic action had once been vigorous and prolonged in many countries1 where no remains of volcanoes can now be seen4 ,to ,france1 which has led the way in so many departments of human inquiry1 belongs the merit of having laid the foundations of the systematic study of ancient volcanoes4 ,her groups of ,puys furnished the earliest inspiration in this subject1 and have ever since been classic ground to which the geological pilgrim has made his way from all parts of the world4 ,as far back as the year #ageb1 ,guettard recognised that these marvellous hills were volcanic cones that had poured forth streams of lava4 ,but it was reserved for ,desmarest twelve years later to examine the question in detail1 and to establish the investigation of former volcanic action upon a broad and firm basis of careful observation and sagacious inference4 ,his method of research was as well conceived as the region of ,auvergne was admirably fitted to be the field of exploration4 ,he soon discovered that the volcanoes of ,central ,france were not all of one age1 but had made their appearance in a long series1 whereof the individual members became less perfect and distinct in proportion to their antiquity4 ,beginning with the cones1 craters1 and lava-streams which stand out so fresh that they might almost be supposed to have been erupted only a few generations ago1 ,desmarest traced the volcanic series backward in time1 through successive stages of the decay and degradation wrought upon them by the influence of the atmosphere1 rain and running water4 ,he was thus able1 as it were1 to watch the gradual obliteration of the cones1 the removal of the ashes and scori1 and the erosion of the lava-streams1 until he could point to mere isolated remnants of lava1 perched upon the hills1 and overlooking the valleys which had been excavated through them4 ,he showed how every step in this process of denudation could be illustrated by examples of its occurrence in ,auvergne1 and how1 in this way1 the various eruptions could be grouped according to their place in the chronological sequence4 ,to this illustrious ,frenchman geology is thus indebted1 not only for the foundation of the scientific study of former volcanic action1 but for the first carefully worked out example of the potency of subrial erosion in the excavation of valleys and the transformation of the scenery of the land4 ,while these fruitful researches were in progress in ,france1 others of hardly less moment were advancing in ,scotland4 ,there likewise ,nature had provided ample material to arrest the attention of all who cared to make themselves acquainted with the past history of our globe4 ,hutton1 as a part of his immortal ,theory of the ,earth1 had conceived the idea that much molten material had been injected from below into the terrestrial crust1 and he had found many proofs of such intrusion among the rocks alike of the ,lowlands and ,highlands of his native country4 ,his observations1 confirmed and extended by ,playfair and ,hall1 and subsequently by ,macculloch1 opened up the investigation of the subterranean phases of ancient volcanic action4 ,under the influence of these great pioneers1 volcanic geology would have made steady and perhaps rapid progress in the later decades of last century1 and the earlier years of the present1 but for the theoretical views unfortunately adopted by ,werner4 ,that illustrious teacher1 to whom volcanoes seemed to be a blot on the system of nature which he had devised1 did all in his power to depreciate their importance4 ,adopting the old and absurd notion that they were caused by the combustion of coal under ground1 he laboured to show that they were mere modern accidents1 and had no connection with his universal formations4 ,he proclaimed1 as an obvious axiom in science1 that the basalts1 so widely spread over ,central and ,western ,europe1 and which the observations of ,desmarest had shown to mark the sites of old volcanoes1 were really chemical precipitates from a primeval universal ocean4 ,yet he had actually before him in ,saxony examples of basalt hills which entirely disprove his assertions4 ,fortunately for the progress of natural knowledge1 ,werner disliked the manual labour of penmanship4 ,consequently he wrote little4 ,but his wide range of acquirement1 not in mineralogy only1 his precision of statement1 his absolute certainty about the truth of his own opinions1 and his hardly disguised contempt for opinions that differed from them1 combined with his enthusiasm1 eloquence and personal charm1 fired his pupils with emulation of his zeal and turned them into veritable propagandists4 ,misled as to the structure of the country in which their master taught1 and undisciplined to investigate nature with an impartial mind1 they travelled into other lands for the purpose of applying there the artificial system which they had learnt at ,freiberg4 ,the methodical but cumbrous terminology in which ,werner had trained them was translated by them into their own languages1 where it looked still more uncouth than in its native ,german4 ,besides imbibing their teacher's system1 they acquired and even improved upon his somewhat disdainful manner towards all conclusions different from those of the ,saxon ,mining ,school4 ,such was the spirit in which the pupils of ,werner proceeded to set the geognosy of ,europe to rights4 ,the views1 announced by ,desmarest1 that various rocks1 far removed from any active volcano1 were yet of volcanic origin1 had been slowly gaining ground when the militant students from ,saxony spread themselves over the ,continent4 ,these views1 however1 being irreconcilable with the tenets enunciated from the ,freiberg ,chair1 were now either ignored or contemptuously rejected4 ,werner's disciples loved to call themselves by their teacher's term geognosts1 and claimed that they confined themselves to the strict investigation of fact with regard to the structure of the earth1 in apparent unconsciousness that their terminology and methods were founded on baseless assumptions and almost puerile hypotheses4 ,with such elements ready for controversy1 it was no wonder that before long a battle arose over the origin of basalt and the part played by volcanoes in the past history of the globe4 ,the disciples of ,werner1 champions of a universal ocean and the deposition of everything from water1 were dubbed ,neptunists1 while their opponents1 equally stubborn in defence of the potency of volcanic fire1 were known as ,vulcanists or ,plutonists4 ,for more than a generation this futile warfare was waged with extraordinary bitterness--dogmatism and authority doing their best to stop the progress of impartial observation and honest opinion4 ,one of the most notable incidents in the campaign is to be found in the way in which the tide of battle was at last turned against the ,wernerians4 ,cuvier tells us that when some of the ardent upholders of the ,freiberg faith came to consult ,desmarest1 the old man1 who took no part in the fray1 would only answer1 ,go and see4 ,he felt that in his memoir and maps he had demonstrated the truth of his conclusions1 and that an unprejudiced observer had only to visit ,auvergne to be convinced4 ,by a curious irony of fate it was from that very ,auvergne that the light broke which finally chased away the ,wernerian darkness1 and it was by two of ,werner's most distinguished disciples that the reaction was begun4 ,daubuisson1 a favourite pupil of the ,freiberg professor1 had written and published at ,paris in #ahjc a volume on the ,basalts of ,saxony1 conceived in the true ,wernerian spirit1 and treating these rocks1 as he had been taught to regard then1 as chemical precipitates from a former universal ocean4 ,in the following year the young and accomplished ,frenchman went to ,auvergne and the ,vivarais that he might see with his own eyes the alleged proofs of the volcanic origin of basalt4 ,greatly no doubt to his own surprise1 he found these proofs to be irrefragable4 ,with praiseworthy frankness he lost no time in publicly announcing his recantation of the ,wernerian doctrine on the subject1 and ever afterwards he did good service in making the cause of truth and progress prevail4 ,still more sensational was the conversion of a yet more illustrious prophet of the ,freiberg school--the great ,leopold von ,buch4 ,he too had been educated in the strictest ,wernerian faith4 ,but eventually1 after a journey to ,italy1 he made his way to ,auvergne in #ahjb1 and there1 in presence of the astonishing volcanic records of that region1 the scales seem to have fallen from his eyes also4 ,with evident reluctance he began to doubt his master's teaching in regard to basalt and volcanoes4 ,he went into raptures over the clear presentation of volcanic phenomena to be found in ,central ,france1 traced each detail among the puys1 as in the examination of a series of vast models1 and remarked that while we may infer what takes place at ,vesuvius1 we can actually see what has transpired at the ,puy de ,pariou4 ,with the enthusiasm of a convert he rushed into the discussion of the phenomena1 but somehow omitted to make any mention of ,desmarest1 who had taught the truth so many years before4 ,impressed by the example of such men as ,daubuisson and ,von ,buch1 the ,wernerian disciples gradually slackened in zeal for their master's tenets4 ,they clung to their errors longer perhaps in ,scotland than anywhere else out of ,germany--a singular paradox only explicable by another personal influence4 ,jameson1 trained at ,freiberg1 carried thence to the ,university of ,edinburgh the most implicit acceptance of the tenets of the ,saxon school1 and continued to maintain the aqueous origin of basalt for many years after the notion had been abandoned by some of his most distinguished contemporaries4 ,but the error1 though it died hard1 was confessed at last even by ,jameson4 ,after the close of this protracted and animated controversy the study of former volcanic action resumed its place among the accepted subjects of geological research4 ,from the peculiarly favourable structure of the country1 ,britain has been enabled to make many important contributions to the investigation of the subject4 ,de la ,beche1 ,murchison and ,sedgwick led the way in recognizing1 even among the most ancient stratified formations of ,england and ,wales1 the records of contemporaneous volcanoes and of their subterranean intrusions4 ,scrope threw himself with ardour into the study of the volcanoes of ,italy and of ,central ,france4 ,maclaren made known the structure of some of the volcanic groups of the lowlands of ,scotland4 ,ramsay1 ,selwyn1 and ,jukes1 following these pioneers1 were the first to map out a ,palozoic volcanic region in ample detail4 ,sorby1 applying to the study of rocks the method of microscopic examination by thin slices1 devised by ,william ,nicol of ,edinburgh for the study of fossil plants1 opened up a new and vast field in the domain of observational geology1 and furnished the geologist with a key to solve many of the problems of volcanism4 ,thus1 alike from the stratigraphical and petrographical sides1 the igneous rocks of this country have received constantly increasing attention4 ,the present work is intended to offer a summary of what has now been ascertained regarding the former volcanoes of the ,british ,isles4 ,the subject has occupied much of my time and thought all through life4 ,born among the crags that mark the sites of some of these volcanoes1 ,i was led in my boyhood to interest myself in their structure and history4 ,the fascination which they then exercised has lasted till now1 impelling me to make myself acquainted with the volcanic records all over our islands1 and to travel into the volcanic regions of ,europe and ,western ,america for the purpose of gaining clearer conceptions of the phenomena4 ,from time to time during a period of almost forty years ,i have communicated chiefly to the ,geological ,society of ,london and the ,royal ,society of ,edinburgh the results of my researches4 ,as materials accumulated1 the desire arose to combine them into a general narrative of the whole progress of volcanic action from the remotest geological periods down to the time when the latest eruptions ceased4 ,an opportunity of partially putting this design into execution occurred when1 as ,president of the ,geological ,society1 the duty devolved upon me of giving the ,annual ,addresses in #ahia and #ahib4 ,within the limits permissible to such essays1 it was not possible to present more than a full summary of the subject4 ,since that time ,i have continued my researches in the field1 especially among the ,tertiary volcanic areas1 and have now expanded the two ,addresses by the incorporation of a large amount of new matter and of portions of my published papers4 ,in the onward march of science a book which is abreast of our knowledge to-day begins to be left behind to-morrow4 ,nevertheless it may serve a useful purpose if it does no more than make a definite presentation of the condition of that knowledge at a particular time4 ,such a statement becomes a kind of landmark by which subsequent progress may be measured4 ,it may also be of service in indicating the gaps that have to be filled up1 and the fields where fresh research may most hopefully be undertaken4 ,i have to thank the ,councils of the ,royal ,society of ,edinburgh and the ,geological ,society for their permission to use a number of the illustrations which have accompanied my papers published in their ,transactions and ,journal4 ,to ,colonel ,evans and ,miss ,thom of ,canna ,i am indebted for the photographs which they have kindly taken for me4 ,to those of my colleagues in the ,geological ,survey who have furnished me with information my best thanks are due4 ,their contributions are acknowledged where they have been made use of in the text4 ,the illustrations of these volumes are chiefly from my own note-books and sketch-books4 ,but besides the photographs just referred to1 ,i have availed myself of a series taken by ,mr4 ,robert ,lunn for the ,geological ,survey among the volcanic districts of ,central ,scotland4 ,geological ,survey ,office1 #bh ,jermyn ,street1 ,london1 #ast ,january #ahig4 ,,contents ,,book ,i ,,general ,,principles ,,and ,,methods ,,of ,,investigation ,,chapter ,i ,,page ,earliest ,knowledge of ,volcanoes--,their ,influence on ,mythology and ,superstition--,part taken by ,volcanic ,rocks in ,scenery--,progress of the ,denudation of ,volcanoes--,value of the ,records of former ,volcanoes as illustrating ,modern ,volcanic ,action--,favourable ,position of ,britain for the ,study of this ,subject #a ,,chapter ,,ii ,the ,nature and ,causes of ,volcanic ,action--,modern ,volcanoes #aj ,,chapter ,,iii ,ancient ,volcanoes3 ,proofs of their existence derived from the ,nature of the ,rocks erupted from the ,earth's ,interior4 ,a4 ,materials erupted at the ,surface--,extrusive ,series4 i4 ,lavas1 their ,general ,characters4 ,volcanic ,cycles4 ii4 ,agglomerates1 ,breccias and ,tuffs #ad ,,chapter ,,iv ,materials erupted at the ,surface--,extrusive ,series--continued4 iii4 ,types of old ,volcanoes--#a4 ,the ,vesuvian ,type2 #b4 ,the ,plateau or ,fissure ,type2 #c4 ,the ,puy ,type4 iv4 ,determination of the relative ,geological ,dates of ,ancient ,volcanoes4 v4 ,how the ,physical ,geography associated with ,ancient ,volcanoes is ascertained #ci ,,chapter ,v ,underground ,phases of ,volcanic ,action4 ,b4 ,materials injected or consolidated beneath the ,surface--,intrusive ,series3 ,i4 ,vents of ,eruption--i4 ,necks of ,fragmentary ,materials2 ii4 ,necks of ,lava-form ,materials2 iii4 ,distribution of ,vents in relation to ,geological ,structure-,lines2 iv4 ,metamorphism in and around ,volcanic ,cones1 ,solfataric ,action2 v4 ,inward ,dip of ,rocks towards ,necks2 vi4 ,influence of contemporaneous ,denudation upon ,volcanic ,cones2 vii4 ,stages in the ,history of old ,volcanic ,vents #eb ,,chapter ,,vi ,underground ,phases of ,volcanic ,action--continued4 ,,ii4 ,subterranean ,movements of the ,magma3 i4 ,dykes and ,veins2 ii4 ,sills and ,laccolites2 iii4 ,bosses 7,stocks1 ,culots71 ,conditions that govern the ,intrusion of ,molten ,rock within the ,terrestrial ,crust #gg ,,chapter ,,vii ,influence of ,volcanic ,rocks on the ,scenery of the ,land--,effects of ,denudation #ajj ,,book ,,ii ,,volcanic ,,action ,,in ,,pre-,,cambrian ,,time ,,chapter ,,viii ,pre-,cambrian ,volcanoes ,the ,beginnings of ,geological ,history--,difficulties in fixing on a generally applicable ,terminology--i4 ,the ,lewisian 7,archan7 ,gneiss2 ii4 ,the ,dalradian or ,younger ,schists of ,scotland2 iii4 ,the ,gneisses and ,schists of ,anglesey2 iv4 ,the ,uriconian ,volcanoes2 v4 ,the ,malvern ,volcano2 vi4 ,the ,charnwood ,forest ,volcano #aji ,,book ,,iii ,,the ,,cambrian ,,volcanoes ,,chapter ,,ix ,characteristics of the ,cambrian ,system in ,britain ,the ,physical ,geography of the ,cambrian ,period--,the ,pioneers of ,palozoic ,geology in ,britain--,work of the ,geological ,survey in ,wales--,subdivisions of the ,cambrian ,system in ,britain #aci ,,chapter ,x ,the ,cambrian ,volcanoes of ,south ,wales #ade ,,chapter ,,xi ,the ,cambrian ,volcanoes of ,north ,wales1 the ,malvern ,hills and ,warwickshire #aei ,,book ,,iv ,,the ,,silurian ,,volcanoes ,,chapter ,,xii ,characters of the ,silurian ,system in ,britain4 ,the ,arenig ,volcanoes ,the ,land and ,sea of ,silurian time--,classification of the ,silurian ,system--,general ,petrography of the ,silurian ,volcanic ,rocks--,i4 ,the ,eruptions of ,arenig ,age #agc ,,chapter ,,xiii ,the ,eruptions of ,llandeilo and ,bala ,age i4 ,the ,builth ,volcano--ii4 ,the ,volcanoes of ,pembrokeshire--iii4 ,the ,caernarvonshire ,volcanoes of the ,bala ,period--iv4 ,the ,volcanic ,district of the ,berwyn ,hills--v4 ,the ,volcanoes of ,anglesey--vi4 ,the ,volcanoes of the ,lake ,district2 ,arenig to close of ,bala ,period--vii4 ,upper ,silurian 787 ,volcanoes of ,gloucestershire #bjb ,,chapter ,,xiv ,the ,silurian ,volcanoes of ,ireland #bci ,,book ,v ,,the ,,volcanoes ,,of ,,devonian ,,and ,,old ,,red ,,sandstone ,,time ,,chapter ,,xv ,the ,devonian ,volcanoes #beg ,,chapter ,,xvi ,the ,volcanoes of the ,old ,red ,sandstone ,geological ,revolutions at the close of the ,silurian ,period ,physical ,geography of the ,old ,red ,sandstone--,old ,lake-basins1 their ,flora and ,fauna--,abundance of ,volcanoes--,history of ,investigation in the ,subject #bfc ,,chapter ,,xvii ,distribution of the ,volcanic ,centres in the ,lower ,old ,red ,sandstone--,characters of the ,materials ,erupted by the ,volcanoes #bga ,,chapter ,,xviii ,structure and ,arrangement of the ,lower ,old ,red ,sandstone ,volcanic ,rocks in the ,field #bha ,,chapter ,,xix ,volcanoes of the ,lower ,old ,red ,sandstone of ,lake ,caledonia ,description of the several ,volcanic ,districts3 ,lake ,caledonia1 its ,chains of ,volcanoes--,the ,northern ,chain3 ,montrose ,group--,ochil and ,sidlaw ,hills--the ,arran and ,cantyre ,centre--the ,ulster ,centre #bid ,,chapter ,,xx ,volcanoes of the ,lower ,old ,red ,sandstone of ,lake ,caledonia--continued ,the ,southern ,chain--,the ,pentland ,volcano--,the ,biggar ,centre--,the ,duneaton ,centre--,the ,ayrshire ,volcanoes #cag ,,chapter ,,xxi ,volcanoes of the ,lower ,old ,red ,sandstone of the ,cheviot ,hills1 ,lorne1 ,lake ,orcadie and ,killarney #ccf ,,chapter ,,xxii ,volcanoes of the ,upper ,old ,red ,sandstone--,the ,south-,west of ,ireland1 the ,north of ,scotland #cdh ,,book ,,vi ,,the ,,carboniferous ,,volcanoes ,,chapter ,,xxiii ,the ,carboniferous ,system of ,britain and its ,volcanic ,records ,geography and ,scenery of the ,carboniferous ,period--,range of ,volcanic ,eruptions during that time--,i4 ,the ,carboniferous ,volcanoes of ,scotland--,distribution1 ,arrangement and ,local ,characters of the ,carboniferous ,system in ,scotland--,sketch of the ,work of previous ,observers in this ,subject #cee ,,chapter ,,xxiv ,carboniferous ,volcanic ,plateaux of ,scotland ,i4 ,the ,plateau-type restricted to ,scotland--i4 ,distribution in the ,different ,areas of ,eruption--ii4 ,nature of the ,materials ,erupted #cfg ,,chapter ,,xxv ,geological ,structure of the ,carboniferous ,volcanic ,plateaux of ,scotland #a4 ,bedded ,lavas and ,tuffs2 ,upper ,limits and ,original ,areas and ,slopes of the ,plateaux2 #b4 ,vents2 ,necks of ,agglomerate and ,tuff2 ,necks of ,massive ,rock2 ,composite ,necks2 #c4 ,dykes and ,sills2 #d4 ,close of the ,plateau-eruptions #chc ,,chapter ,,xxvi ,the ,carboniferous ,puys of ,scotland i4 ,general ,character and ,distribution of the ,puys2 ii4 ,nature of the ,materials ,erupted--,lavas ,ejected at the ,surface--,intrusive ,sheets--,necks and ,dykes--,tuffs #dad ,,chapter ,,xxvii ,geological ,structure of the ,carboniferous ,puys of ,scotland #a4 ,vents3 ,relation of the ,necks to the ,rocks through which they rise--,evidence of the probable ,subrial ,character of some of the ,cones or ,puys of ,tuff--,entombment of the ,volcanic ,cones and their ,relation to the ,superficial ,ejections4 #b4 ,bedded ,tuffs and ,lavas--,effects of ,subsequent ,dislocations4 #c4 ,sills1 ,bosses1 and ,dykes #dbd ,,chapter ,,xxviii ,illustrative ,examples of the ,carboniferous ,puys of ,scotland ,the ,basin of the ,firth of ,forth--,north ,ayrshire--,liddesdale #dfb ,,list ,,of ,,illustrations ,,fig4 ,,page #a4 ,vesicular structure1 ,lava from ,ascension ,island1 slightly less than natural size #ae #b4 ,elongation and branching of steam-vesicles in a lava1 ,kilninian1 ,isle of ,mull1 a little less than natural size #ag #c4 ,microlites of the ,pitchstone of ,arran 7magnified #gj diameters7 #ai #d4 ,perlitic structure in ,felsitic ,glass1 ,isle of ,mull 7magnified7 #ai #e4 ,spherulitic structure 7magnified7 #ai #f4 ,micropegmatitic or ,granophyric structure in ,granophyre1 ,mull 7magnified7 #bj #g4 ,ophitic structure in ,dolerite1 ,gortacloghan1 ,co4 ,derry 7magnified7 #bj #h4 ,variolitic or orbicular structure1 ,napoleonite1 ,corsica 7nat4 size7 #bb #i4 ,flow-structure in ,rhyolite1 ,antrim1 slightly reduced #bc #aj4 ,lumpy1 irregular trachytic lava-streams 7,carboniferous71 ,east ,linton1 ,haddingtonshire #bd #aa4 ,view at the entrance of the ,svinofjord1 ,faroe ,islands1 illustrating the terraced forms assumed by basic lavas #be #ab4 ,sack-like or pillow-form structure of basic lavas 7,lower ,silurian71 ,bennan ,head1 ,ballantrae1 ,ayrshire #bf #ac4 ,alternations of coarser and finer ,tuff #cd #ad4 ,alternations of ,tuff with non-volcanic sediment #ce #ae4 ,ejected block of basalt which has fallen among ,carboniferous shales and limestones1 shore1 ,pettycur1 ,fife #cg #af4 ,effects of denudation on a ,vesuvian cone #dj #ag4 ,section to illustrate the structure of the ,plateau type #dc #ah4 ,diagram illustrating the structure and denudation of ,puys #de #ai4 ,section illustrating submarine eruptions2 alternations of lavas and tuffs with limestones and shales full of marine organisms #dh #bj4 ,diagram illustrating volcanic eruptions on a river-plain #di #ba4 ,diagram illustrating volcanic eruptions on a land-surface #ej #bb4 ,ground-plans of some volcanic vents from the ,carboniferous districts of ,scotland #ee #bc4 ,view of an old volcanic ,neck 7,the ,knock1 ,largs1 ,ayrshire1 a vent of ,lower ,carboniferous age7 #ef #bd4 ,section of neck of agglomerate1 rising through sandstones and shales #eh #be4 ,neck filled with stratified tuff #fd #bf4 ,section of neck of agglomerate with plug of lava #fe #bg4 ,section of agglomerate neck with dykes and veins #ff #bh4 ,section of neck filled with massive rock #fh #bi4 ,successive shiftings of vents giving rise to double or triple cones #gj #cj4 ,section to show the connection of a neck with a cone and surrounding bedded tuffs #ga #ca4 ,diagram illustrating the gradual emergence of buried volcanic cones through the influence of prolonged denudation #ge #cb4 ,dyke1 ,vein1 and ,sill #hj #cc4 ,section of ,sill or ,intrusive ,sheet #hc #cd4 ,ideal section of three ,laccolites4 7,after ,mr4 ,gilbert7 #hf #ce4 ,diagram illustrating the stratigraphical relations of the pre-,cambrian and ,cambrian rocks of the ,north-west ,highlands of ,scotland #aab #cf4 ,map of a portion of the ,lewisian gneiss of ,ross-shire #aah #cg4 ,section showing the position of sills in the mica-schist series between ,loch ,tay and ,amulree #abd #ch4 ,sketch of crushed basic igneous rock among the schists1 ,e4 side of ,porth-tywyn-mawr1 ,e4 side of ,holyhead ,straits #abh #ci4 ,section across the ,uriconian series of ,caer ,caradoc #acb #dj4 ,map of the volcanic district of ,st4 ,david's #adf #da4 ,section showing the interstratification of tuff and conglomerate above ,lower ,mill1 ,st4 ,david's #aed #db4 ,basic dyke traversing quartz-porphyry and converted into a kind of slate by cleavage4 ,west side of ,llyn ,padarn #afb #dc4 ,section of well-cleaved tuff1 grit and breccia passing up into rudely-cleaved conglomerate and well-bedded cleaved fine conglomerate and grit4 ,east side of ,llyn ,padarn #afc #dd4 ,section of ,clegyr on the north-east side of ,llyn ,padarn1 near the lower end #afd #de4 ,section across the ,cambrian formations of the ,malvern ,hills1 showing the position of the intercalated igneous rocks4 ,after ,phillips #agj #df4 ,section across ,rhobell ,fawr #agh #dg4 ,section at the ,slate ,quarry1 ,penrhyn ,gwyn1 north slopes of ,cader ,idris #ahj #dh4 ,sketch-section across ,cader ,idris #ahb #di4 ,section across the ,moelwyn ,range #ahe #ej4 ,section across the anticline of ,corndon #aij #ea4 ,structure in finely-amygdaloidal diabase lava1 south of mouth of ,stinchar ,river1 ,ayrshire #aic #eb4 ,view of ,knockdolian ,hill from the east #aid #ec4 ,section across the ,lower ,silurian volcanic series in the south of ,ayrshire 7,b4 ,n4 ,peach7 #aig #ed4 ,section of part of the ,arenig volcanic group1 stream south of ,bennane ,head1 ,ayrshire #aih #ee4 ,flow-structure in the lowest felsite on the track from ,llanberis to the top of ,snowdon #baa #ef4 ,section of ,snowdon #bab #eg4 ,section across the ,berwyn ,hills4 7,reduced from ,horizontal ,section1 ,geol4 ,surv4 ,sheet #ce7 #bai #eh4 ,section of the strata on the shore at ,porth ,wen1 west of ,amlwch #bbc #ei4 ,section of intercalated black shale in the volcanic series at ,porth yr hwch1 south of ,carmel ,point1 ,anglesey #bbd #fj4 ,green slates overlain with volcanic breccia1 ,carmel ,point #bbd #fa4 ,blue shale or slate passing into volcanic breccia east of ,porth ,padrig1 near ,carmel ,point #bbe #fb4 ,section of felsites in the ,coniston ,limestone group1 west of ,stockdale #bcb #fc4 ,fine tuff with coarser bands near ,quayfoot ,quarries1 ,borrowdale #bcd #fd4 ,diagram of the general relations of the different groups of rock in the ,lower ,silurian volcanic district along the western shore of ,lough ,mask #bec #fe4 ,veins and nests of sandstone due to the washing of sand into fissures and cavities of an ,old ,red ,sandstone lava4 ,turnberry ,point1 ,ayrshire #bhc #ff4 ,ground-plan of reticulated cracks in the upper surface of an ,old ,red ,sandstone lava filled in with sandstone4 ,red ,head1 ,forfarshire #bhd #fg4 ,section across the volcanic series of ,forfarshire #bhf #fh4 ,section across two necks above ,tillicoultry1 ,ochil ,hills #bhh #fi4 ,section of the granite core between ,merrick and ,corscrine #bij #gj4 ,section across the three ,dirrington ,laws1 ,berwickshire #bia #ga4 ,section of ,papa ,stour1 ,shetlands1 showing sill of spherulitic felsite traversing ,old ,red ,sandstone and bedded porphyrites 7,messrs4 ,peach and ,horne7 #bib #gb4 ,section across ,northmavine1 from ,okrea ,head to ,skea ,ness1 ,shetland1 showing dykes and connected sill of granite and felsite 7,messrs4 ,peach and ,horne7 #bib #gc4 ,section at the edge of one of the bays of ,lower ,old ,red ,sandstone along the northern margin of ,lake ,caledonia1 near ,ochtertyre #bie #gd4 ,craig ,beinn-nan-,eun 7#bjfg feet71 east of ,uam ,var1 ,braes of ,doune4 ,old ,red ,conglomerate1 with the truncated ends of the strata looking across into the ,highlands2 moraines of ,corry ,beach in the foreground #bif #ge4 ,section showing the top of the volcanic series at the foot of the precipice of the ,red ,head1 ,forfarshire #cjj #gf4 ,andesite with sandstone veinings and overlying conglomerate4 ,todhead1 south of ,caterline1 coast of ,kincardineshire #cjc #gg4 ,section across the ,boundary-fault of the ,highlands at ,glen ,turrit1 ,perthshire #cje #gh4 ,section across the chain of the ,sidlaw ,hills near ,kilspindie #cjf #gi4 ,section across the ,eastern ,ochil ,hills from near ,newburgh to near ,auchtermuchty #cjg #hj4 ,generalized section across the heart of the ,ochil ,hills from ,dunning on the north to the ,fife coal-field near ,saline on the south #cjh #ha4 ,diagram of the volcanic series of the ,western ,ochil ,hills #cji #hb4 ,view of ,cnoc ,garbh1 ,southend1 ,campbeltown4 ,a volcanic neck of ,lower ,old ,red ,sandstone age1 about #djj yards wide in its longer diameter #cab #hc4 ,section of volcanic series on beach1 ,southend1 ,campbeltown #cac #hd4 ,section of the base of the volcanic series1 ,reclain1 five miles south of ,pomeroy #caf #he4 ,section of shales and breccias at ,crossna ,chapel1 north-east of ,boyle #caf #hf4 ,section across the north end of the ,pentland ,hills1 from ,warklaw ,hill to ,pentland ,mains4 ,length about five miles #cah #hg4 ,view of the lava-escarpments of ,warklaw ,hill1 ,pentland chain1 from the north-west #cai #hh4 ,section across the ,pentland ,hills through ,north ,black ,hill and ,scald ,law4 ,length about three miles #cbb #hi4 ,section from the valley of the ,gutterford ,burn through ,green ,law and ,braid ,law to ,eight-,mile ,burn #cbb #ij4 ,section across the north end of the ,pentland ,hills1 and the southern edge of the ,braid ,hill vent4 ,length about two miles #cbd #ia4 ,section across the northern end of the ,biggar volcanic group1 from ,fadden ,hill to beyond ,mendick ,hill #cbf #ib4 ,section across the southern part of the ,biggar volcanic group from ,covington to ,culter #cbh #ic4 ,section from ,thankerton ,moor across ,tinto to ,lamington #cbh #id4 ,section across the ,duneaton volcanic district from the head of the ,duneaton ,water to ,kirklea ,hill #ccj #ie4 ,cavernous spaces in andesite1 filled in with sandstone1 ,john o' ,groats ,port1 ,turnberry1 ,ayrshire #ccc #if4 ,section of andesites1 ,turnberry ,castle1 ,ayrshire #ccd #ig4 ,lenticular form of a brecciated andesite 7shown in ,fig4 #if71 ,turnberry1 ,ayrshire #ccd #ih4 ,section across the volcanic area of ,st4 ,abb's ,head 7after ,prof4 ,j4 ,geikie7 #cci #ii4 ,view of terraced andesite hills resting on massive conglomerate1 south of ,oban #cda #ajj4 ,section of lava-escarpment at ,beinn ,lora1 north side of mouth of ,loch ,etive1 ,argyllshire #cdb #aja4 ,section across ,strathbogie1 below ,rhyme1 showing the position of the volcanic band #cdd #ajb4 ,view of ,knockfeerina1 ,limerick1 from the north-east--a volcanic neck of ,upper ,old ,red ,sandstone age #cdi #ajc4 ,section of the volcanic zone in the ,upper ,old ,red ,sandstone1 ,cam of ,hoy1 ,orkney #cea #ajd4 ,section of the volcanic zone in the ,upper ,old ,red ,sandstone at ,black ,ness1 ,rackwick1 ,hoy #cea #aje4 ,section across the volcanic band and its associated necks1 ,hoy1 ,orkney #ceb #ajf4 ,ground-plan of volcanic neck piercing the ,caithness ,flagstone series on the beach near ,john o' ,groat's ,house #cec #ajg4 ,view of the escarpment of the ,clyde ,plateau in the ,little ,cumbrae1 from the south-west #cfh #ajh4 ,view of the edge of the ,volcanic ,plateau south of ,campbeltown1 ,argyllshire #cgj #aji4 ,view of ,north ,berwick ,law from the east1 a phonolite neck marking one of the chief vents of the ,garleton ,plateau4 7,from a photograph7 #cga #aaj4 ,the ,bass ,rock1 a trachytic neck belonging to the ,garleton plateau1 from the shore at ,canty ,bay #cgb #aaa4 ,corston ,hill--a fragment of the ,midlothian ,plateau1 seen from the north #cgc #aab4 ,view of ,arthur ,seat from ,calton ,hill to the north #cgd #aac4 ,view of ,arkleton ,fell1 part of the ,solway ,plateau1 from the south-west #cgf #aad4 ,vertical sections of the escarpment of the ,clyde plateau from north-east to south-west #chd #aae4 ,section of ,craiglockhart ,hill1 ,edinburgh #chg #aaf4 ,section of the bottom of the ,midlothian ,plateau1 ,linnhouse ,water above ,mid-,calder ,oilworks #chg #aag4 ,section of the top of the ,midlothian ,plateau in the ,murieston ,water #chh #aah4 ,section of ,calton ,hill1 ,edinburgh #chi #aai4 ,cliff of tuff and agglomerate1 east side of ,oxroad ,bay1 a little east from ,tantallon ,castle1 ,east ,lothian #cia #abj4 ,section across part of the ,clyde ,plateau to the west of ,bowling 7reduced from ,sheet #f of the ,horizontal ,sections of the ,geological ,survey of ,scotland7 #cib #aba4 ,diagram illustrating the thinning away southwards of the lavas of the ,clyde ,plateau between ,largs and ,ardrossan4 ,length about #aj miles #cic #abb4 ,diagram illustrating the thinning away eastwards of the lavas of the ,clyde ,plateau in the ,fintry ,hills4 ,length about #ab miles #cid #abc4 ,view of the two necks ,dumgoyn and ,dumfoyn1 ,stirlingshire1 taken from the south #cie #abd4 ,ground-plan of ,plateau-vents near ,strathblane1 ,stirlingshire1 on the scale of #f inches to a mile #cie #abe4 ,ground-plans of double and triple necks in the ,plateau series1 on the scale of #f inches to a mile #cif #abf4 ,ground-plan of tuff-neck1 shore east of ,dunbar #cih #abg4 ,section across the vents ,dumgoyn and ,dumfoyn1 and the edge of the ,clyde plateau above ,strathblane1 ,stirlingshire #djj #abh4 ,section through the large vent of the ,campsie ,hills #djj #abi4 ,diagrammatic section across the central vent of the ,clyde plateau in ,renfrewshire #djj #acj4 ,section across ,southern ,berwickshire1 to show the relation of the volcanic plateau to the vents lying south from it #dja #aca4 ,section of south end of ,dumbuck ,hill4 ,east of ,dumbarton #djc #acb4 ,section across the ,east ,lothian plateau1 to show the relative position of one of the necks #djc #acc4 ,view of ,traprain ,law from the south1 a phonolite neck of the ,garleton ,plateau #dje #acd4 ,veins and dykes traversing the agglomerate and tuff of the great ,renfrewshire vent #djh #ace4 ,the ,yellow ,man1 a dyke in volcanic tuff and conglomerate on the shore a little east of ,north ,berwick #dji #acf4 ,trachytic sills1 ,knockvadie1 ,kilpatrick ,hills #daj #acg4 ,section across the edge of the ,clyde plateau1 south-east of ,beith #daa #ach4 ,section across the upper part of the ,clyde plateau at ,kilbirnie1 ,ayrshire #daa #aci4 ,section across the upper surface of the ,clyde volcanic plateau1 ,burnhead1 north-west of ,kilsyth #dab #adj4 ,section across the upper surface of the ,clyde volcanic plateau at ,campsie #dab #ada4 ,section across western edge of the ,garlton plateau #dab #adb4 ,section across the ,solway plateau #dac #adc4 ,section of volcanic vent at ,east ,grange1 ,perthshire coal-field1 constructed by ,mr4 ,b4 ,n4 ,peach from the rocks exposed in a railway-cutting1 and from plans of ironstone- and coal-pits #dbf #add4 ,view of the ,binn of ,burntisland--a volcanic neck of agglomerate #dbh #ade4 ,view of part of the cliffs of vertical agglomerate1 ,binn of ,burntisland #dca #adf4 ,diagram of buried volcanic cone near ,dalry1 ,ayrshire4 ,constructed from information obtained in mining operations #dcd #adg4 ,diagram to illustrate how ,volcanic ,necks may be concealed and exposed #dcd #adh4 ,section across the ,saline ,hills1 ,fife #dce #adi4 ,section across the ,binn of ,burntisland1 in an east and west direction #dcf #aej4 ,section in old quarry1 west of ,wester ,ochiltree1 ,linlithgowshire4 ,calciferous ,sandstone series #dcg #aea4 ,ejected volcanic block in ,carboniferous strata1 ,burntisland #dch #aeb4 ,view of volcanic agglomerate becoming finer above east end of ,kingswood ,craig1 two miles east from ,burntisland #dci #aec4 ,alternations of basalt and tuff1 with shale1 etc41 of ,kingswood ,craig1 ,burntisland #dda #aed4 ,section of the upper surface of a diabase 7leckstone7 sheet1 ,skolie ,burn1 south-east of ,bathgate #ddc #aee4 ,section across the volcanic ridge of the ,linlithgow and ,bathgate ,hills1 showing the intercalation of limestones that mark important stratigraphical horizons #ddd #aef4 ,section in ,wardlaw ,quarry1 ,linlithgowshire #dde #aeg4 ,section from ,linlithgow ,loch to the ,firth of ,forth #ddf #aeh4 ,section across the ,campsie ,fells illustrating the contrast between the sills below and above the plateau-lavas #ddg #aei4 ,section showing the position of the basic sills in relation to the volcanic series at ,burntisland1 ,fife #ddh #afj4 ,sills between shales and sandstones1 ,hound ,point1 ,linlithgowshire #ddi #afa4 ,section of ,sill1 ,cramond ,railway1 ,barnton1 near ,edinburgh #dej #afb4 ,intrusive dolerite sheet enclosing and sending threads into portions of shale1 ,salisbury ,crags1 ,edinburgh #deb #afc4 ,intrusive sheet invading limestone and shale1 ,dodhead ,quarry1 near ,burntisland #deb #afd4 ,spheroidal weathering of dolerite sill1 quarry east of ,north ,queensferry1 ,fife4 #dee #afe4 ,two thin sills of white trap injected into black carbonaceous shale overlying the ,hurlet ,limestone1 ,hillhouse ,quarry1 ,linlithgow #def #aff4 ,dyke cutting the agglomerate of a neck4 ,binn of ,burntisland #deg #afg4 ,boss of diabase cutting the ,burdiehouse ,limestone and sending sills and veins into the overlying shales4 ,railway cutting1 ,west ,quarry1 ,east ,calder1 ,midlothian #deh #afh4 ,side of columnar basalt-dyke in the same agglomerate as in ,fig4 #aff #dei #afi4 ,dyke rising through the ,hurlet ,limestone and its overlying shales4 ,silvermine ,quarry1 ,linlithgowshire #dfj #agj4 ,junction of amygdaloidal basalt with shales and limestone1 shore1 half a mile east from ,kinghorn1 ,fife #dfd #aga4 ,columnar basalt1 ,pettycur1 ,kinghorn1 ,fife #dfi #agb4 ,section across the ,fife band of ,sills #dgc #agc4 ,section across the upper volcanic band of north ,ayrshire4 ,length about four miles #dgd #agd4 ,section showing the connection of the two volcanic bands in ,liddesdale #dgf #age4 ,diagram to show the position of a mass of ,upper ,old ,red ,sandstone which has fallen into the great vent near ,tudhope ,hill1 east of ,mosspaul #dgf ,,maps ,i4 ,general map of the ,volcanic districts of the ,british ,isles--,at the end of the volume ,,ii4 ,map of the ,cambrian and ,silurian volcanic region of ,north ,wales ,to face p4 #bef ,,iii4 ,map of the ,old ,red ,sandstone volcanic region of ,lake ,caledonia in ,central ,scotland and ,north ,ireland ,to face p4 #ccd ,,iv4 ,map of the ,carboniferous volcanic districts of ,scotland ,to face p4 #dgf ,,book ,i ,,general ,,principles ,,and ,,methods ,,of ,,investigation ,,chapter ,i ,earliest ,knowledge of ,volcanoes--,their ,influence on ,mythology and ,superstition--,part taken by ,volcanic ,rocks in ,scenery--,progress of the ,denudation of ,volcanoes--,value of the ,records of former ,volcanoes as illustrating ,modern ,volcanic ,action--,favourable ,position of ,britain for the ,study of this ,subject4 ,among the influences which affected the infancy of mankind1 the most potent were those of environment4 ,whatever in outer nature stimulated or repressed courage1 inventiveness1 endurance1 whatever tended to harden or to weaken the bodily faculties1 whatever appealed to the imagination or excited the fancy1 became a powerful factor in human development4 ,thus1 in the dawn of civilization1 the frequent recurrence of earthquakes and volcanic eruptions throughout the basin of the ,mediterranean could not but have a marked effect on the peoples that dwelt by the borders of that sea4 ,while every part of the region was from time to time shaken by underground commotion1 there were certain places that became specially noteworthy for the wonder and terror of their catastrophes4 ,when1 after successive convulsions1 vast clouds of black smoke rose from a mountain and overspread the sky1 when the brightness of noon was rapidly replaced by the darkness of midnight1 when the air grew thick with stifling dust and a rain of stones and ashes fell from it on all the surrounding country1 when streams of what looked like liquid fire poured forth and desolated gardens1 vineyards1 fields and villages--then did men feel sure that the gods were angry4 ,the contrast between the peacefulness and beauty of the ordinary landscape and the hideous warfare of the elements at these times of volcanic fury could not but powerfully impress the imagination and give a colour to early human conceptions of nature and religion4 ,it was not only in one limited district that these manifestations of underground convulsion showed themselves4 ,the islands of the gean had their volcanoes1 and the ,greeks who dwelt among them watched their glowing fires by night and their clouds of steam by day1 culminating now and then in a stupendous explosion1 like that which1 in prehistoric time1 destroyed the island of ,santorin4 ,as the islanders voyaged eastward they would see1 on the coast of ,asia ,minor1 the black bristling lavas of the ,burnt ,country1 perhaps even then flowing from their rugged heaps of cinders4 ,or when1 more adventurously still1 they sailed westward into the ,tyrrhenian waters1 they beheld the snowy cone of ,etna1 with its dark canopy of smoke and the lurid nocturnal gleam of its fires2 while from time to time they witnessed there on a still more stupendous scale the horrors of a great volcanic eruption4 ,from all sides1 therefore1 the early ,greek voyagers would carry back to the mother-country marvellous tales of convulsion and disaster4 ,they would tell how the sky rapidly darkened even in the blaze of mid-day1 how the land was smothered with dust and stones1 how over the sea there spread such a covering of ashes that the oarsmen could hardly drive their vessels onward1 how red-hot stones1 whirling high overhead1 rained down on sails and deck1 and crushed or burnt whatever they fell upon1 and how1 as the earth shook and the sea rose in sudden waves and the mountain gave forth an appalling din of constant explosion1 it verily seemed that the end of the world had come4 ,to the actual horrors of such scenes there could hardly fail to be added the usual embellishments of travellers' tales4 ,thus1 in the end1 the volcanoes of the ,mediterranean basin came to play a not unimportant part in ,hellenic mythology4 ,they seemed to stand up as everlasting memorials of the victory of ,zeus over the giants and monsters of an earlier time4 ,and as the lively ,greek beheld ,mount ,etna in eruption1 his imagination readily pictured the imprisoned ,titan buried under the burning roots of the mountain1 breathing forth fire and smoke1 and convulsing the country far and near1 as he turned himself on his uneasy pallet4 ,when in later centuries the scientific spirit began to displace the popular and mythological interpretation of natural phenomena1 the existence of volcanoes and their extraordinary phenomena offered a fruitful field for speculation and conjecture4 ,as men journeyed outward from the ,mediterranean cradle of civilization1 they met with volcanic manifestations in many other parts of the world4 ,when they eventually penetrated into the ,far ,east1 they encountered volcanoes on a colossal scale and in astonishing abundance4 ,when they had discovered the ,new ,world they learnt that1 in that hemisphere also1 burning mountains were numerous and of gigantic dimensions4 ,gradually it was ascertained that vast lines of volcanic activity encircle the globe4 ,by slow degrees the volcano was recognized to be as normal a part of the mechanism of our planet as the rivers that flow on the terrestrial surface4 ,and now at last men devote themselves to the task of critically watching the operations of volcanoes with as much enthusiasm as they display in the investigation of any other department of nature4 ,they feel that their knowledge of the earth extends to little beyond its mere outer skin1 and that the mystery which still hangs over the vast interior of the planet can only1 if ever1 be dispelled by the patient study of these vents of communication between the interior and the surface4 ,if1 however1 we desire to form some adequate idea of the part which volcanic action has played in the past history of the earth1 we should be misled were we to confine our attention to the phenomena of the eruptions of the present day4 ,an attentive examination of any modern volcano will convince us that of some of the most startling features of an eruption no enduring memorial remains4 ,the convulsive earthquakes that accompany a great volcanic paroxysm1 unless where they actually fissure the ground1 leave little or no trace behind them4 ,lamentably destructive as they are to human life and property1 the havoc which they work is mostly superficial4 ,in a year or two the ruins have been cleared away1 the earth-falls have been healed over1 the prostrated trees have been removed1 and1 save in the memories and chronicles of the inhabitants1 no record of the catastrophe may survive4 ,the clouds of dust and showers of ashes which destroyed the crops and crushed in the roofs of houses soon disappear from the air1 and the covering which they leave over the surface of a district gradually mingles with the soil4 ,vegetation eventually regains its place1 and the landscape becomes again as smiling as before4 ,even where the materials thrown out from the crater accumulate in much greater mass1 where thick deposits of ashes or solid sheets of lava bury the old land-surface1 the look of barren desolation1 though in some cases it may endure for long centuries1 may in others vanish in a few years4 ,the surface-features of the district are altered indeed1 but the new topography soon ceases to look new4 ,another generation of inhabitants loses recollection of the old landmarks1 and can hardly realize that what has become so familiar to itself differs so much from what was familiar to its fathers4 ,but even when the volcanic covering1 thus thrown athwart a wide tract of country1 has been concealed under a new growth of soil and vegetation1 it still remains a prey to the ceaseless processes of decay and degradation which everywhere affect the surface of the land4 ,no feature of a modern volcano is more impressive than the lesson which it conveys of the reality and potency of this continual waste4 ,the northern slopes of ,vesuvius1 for example1 are trenched with deep ravines1 which in the course of centuries have been dug out of the lavas and tuffs of ,monte ,somma by rain and melted snow4 ,year by year these chasms are growing deeper and wider1 while the ridges between them are becoming narrower4 ,in some cases1 indeed1 the intervening ridges have been reduced to sharp crests which are split up and lowered by the unceasing influence of the weather4 ,the slopes of such a volcanic cone have been aptly compared to a half-opened umbrella4 ,it requires little effort of imagination to picture a time1 by no means remote in a geological sense1 when1 unless renovated by the effects of fresh eruptions1 the cone will have been so levelled with the surrounding country that the peasants of the future will trail their vines and build their cots over the site of the old volcano1 in happy ignorance of what has been the history of the ground beneath their feet4 ,what is here predicted as probable or certain in the future has undoubtedly happened again and again in the past4 ,over many districts of ,europe and ,western ,america extinct volcanoes may be seen in every stage of decay4 ,the youngest may still show1 perfect and bare of vegetation1 their cones and their craters1 with the streams of lava that escaped from them4 ,those of older date have been worn down into mere low rounded hills1 or the whole cone has been cleared away1 and there is only left the hard core of material that solidified in the funnel below the surface4 ,the lava-sheets have been cut through by streams1 and now remain in mere scattered patches capping detached hills1 which only a trained eye can recognize as relics of a once continuous level sheet of solid rock4 ,by this resistless degradation1 a volcanic district is step by step stripped of every trace of its original surface4 ,all that the eruptions did to change the face of the landscape may be entirely obliterated4 ,cones and craters1 ashes and lavas1 may be gradually effaced4 ,and yet enough may be left to enable a geologist to make sure that volcanic action was once rife there4 ,as the volcano marks a channel of direct communication between the interior of the earth and the atmosphere outside1 there are subterranean as well as superficial manifestations of its activity1 and while the latter are removed by denudation1 the former are one by one brought into light4 ,the progress of denudation is a process of dissection1 whereby every detail in the structure of a volcano is successively cut down and laid bare4 ,but for this process1 our knowledge of the mechanism and history of volcanic action would be much less full and definite than happily it is4 ,in active volcanoes the internal and subterranean structure can only be conjectured2 in those of ancient date1 which have been deeply eroded1 this underground structure is open to the closest examination4 ,by gathering together evidence of this nature over the surface of the globe1 we learn that abundantly as still active volcanoes are distributed on that surface1 they form but a small fraction of the total number of vents which have at various times been in eruption4 ,in ,italy1 for example1 while ,vesuvius is active on the mainland1 and ,etna1 ,stromboli and ,volcano display their vigour among the islands1 there are scores of old volcanoes that have been silent and cold ever since the beginning of history1 yet show by their cones of cinders and streams of bristling lava that they were energetic enough in their day4 ,but the ,italian volcanic region is only one of many to be found on the ,european ,continent4 ,if we travel eastward into ,hungary1 or northward into the ,eifel1 or into the heart of ,france1 we encounter abundant cones and craters1 many of them so fresh that1 though there is no historical record of their activity1 they look as if they had been in eruption only a few generations ago4 ,but when the geologist begins to search among rocks of still older date than these comparatively recent volcanic memorials1 he meets with abundant relics of far earlier eruptions4 ,and as he arranges the chronicles of the earth's history1 he discovers that each section of the long cycle of geological ages has preserved its records of former volcanoes4 ,in a research of this kind he can best realize how much he owes to the process of denudation4 ,the volcanic remains of former geological periods have in most cases been buried under younger deposits1 and have sunk sometimes thousands of feet below the level of the sea4 ,they have been dislocated and upheaved again during successive commotions of the terrestrial crust1 and have at last been revealed by the gradual removal of the pile of material under which they had lain4 ,hence we learn that the active volcanoes of the present time1 which really embrace but a small part of the volcanic history of our planet1 are the descendants of a long line of ancestors4 ,their distribution and activity should be considered not merely from the evidence they themselves supply1 but in the light derived from a study of that ancestry4 ,it is only when we take this broad view of the subject that we can be in a position to form some adequate conception of the nature and history of volcanoes in the geological evolution of the globe4 ,in this research it is obvious that the presently active volcano must be the basis and starting-point of inquiry4 ,at that channel of communication between the unknown inside and the familiar outside of our globe1 we can watch what takes place in times of quiescence or of activity4 ,we can there study each successive phase of an eruption1 measure temperatures1 photograph passing phenomena1 collect gases and vapours1 register the fall of ashes or the flow of lavas1 and gather a vast body of facts regarding the materials that are ejected from the interior1 and the manner of their emission4 ,indispensable as this information is for the comprehension of volcanic action1 it obviously affords after all but a superficial glimpse of that action4 ,we cannot see beyond the bottom of the crater4 ,we cannot tell anything about the subterranean ducts1 or how the molten and fragmental materials behave in them4 ,all the underground mechanism of volcanoes is necessarily hidden from our eyes4 ,but much of this concealed structure has been revealed in the case of ancient volcanic masses1 which have been buried and afterwards upraised and laid bare by denudation4 ,in yet another important aspect modern volcanoes do not permit us to obtain full knowledge of the subject4 ,the terrestrial vents1 from which we derive our information1 by no means represent all the existing points of direct connection between the interior and the exterior of the planet4 ,we know that some volcanic eruptions occur under the sea1 and doubtless vast numbers more take place there of which we know nothing4 ,but the conditions under which these submarine discharges are effected1 the behaviour of the outflowing lava under a body of oceanic water1 and the part played by fragmentary materials in the explosions1 can only be surmised4 ,now and then a submarine volcano pushes its summit above the sea-level1 and allows its operations to be seen1 but in so doing it becomes practically a terrestrial volcano1 and the peculiar submarine phenomena are still effectually concealed from observation4 ,the volcanic records of former geological periods1 however1 are in large measure those of eruptions under the sea4 ,in studying them we are permitted1 as it were1 to explore the sea-bottom4 ,we can trace how sheets of coral and groves of crinoids were buried under showers of ashes and stones1 and how the ooze and silt of the sea-floor were overspread with streams of lava4 ,we are thus1 in some degree1 enabled to realize what must now happen over many parts of the bed of the existing ocean4 ,the geologist who undertakes an investigation into the history of volcanic action within the area of the ,british ,isles during past time1 with a view to the better comprehension of this department of terrestrial physics1 finds himself in a situation of peculiar advantage4 ,probably no region on the face of the globe is better fitted than these islands to furnish a large and varied body of evidence regarding the progress of volcanic energy in former ages4 ,this special fitness may be traced to four causes--#ast1 ,the remarkable completeness of the geological record in ,britain2 #bnd1 ,the geographical position of the region on the oceanic border of a continent2 #crd1 ,the singularly ample development to be found there of volcanic rocks belonging to a long succession of geological ages2 and #dth1 ,the extent to which this full chronicle of volcanic activity has been laid bare by denudation4 #a4 ,in the first place1 the geological record of ,britain is singularly complete4 ,it has often been remarked how largely all the great periods of geological time are represented within the narrow confines of these islands4 ,the gaps in the chronicle are comparatively few1 and for the most part are not of great moment4 ,thanks to the restricted area of the country and to the large number of observers1 this remarkably full record of geological history has been studied with a minute care which has hardly been equalled in any other country4 ,the detailed succession of all the formations has been so fully determined in ,britain that the very names first applied here to them and to their subdivisions have in large measure passed into the familiar language of geology all over the globe4 ,every definite platform in the stratigraphical series has been more or less fully worked out4 ,a basis has thus been laid for referring each incident in the geological history of the region to its proper relative date4 #b4 ,in the second place1 the geographical position of ,britain gives it a notable advantage in regard to the manifestations of volcanic energy4 ,rising from the margin of a great ocean-basin and extending along the edge of a continent1 these islands have lain on that critical border-zone of the terrestrial surface1 where volcanic action is apt to be most vigorous and continuous4 ,it has long been remarked that volcanoes are generally placed not far from the sea4 ,from the earliest geological periods the site of ,britain1 even when submerged below the sea1 has never lain far from the land which supplied the vast accumulations of sediment that went to form the ,palozoic and later formations1 while1 on the other hand1 it frequently formed part of the land of former geological periods4 ,it was thus most favourably situated as a theatre for both terrestrial and submarine volcanic activity4 #c4 ,in the third place1 this advantageous geographical position is found to have been attended with an altogether remarkable abundance and persistence of volcanic eruptions4 ,no tract of equal size yet known on the face of the globe furnishes so ample a record of volcanic activity from the earliest geological periods down into ,tertiary time4 ,every degree of energy may be signalized in that record1 from colossal eruptions which piled up thousands of feet of rock down to the feeblest discharge of dust and stones4 ,every known type of volcano is represented--great central cones like ,etna or ,vesuvius1 scattered groups of small cones like the puys of ,france1 and fissure- or dyke-eruptions like those of recent times in ,iceland4 ,moreover1 the accurate manner in which the stratigraphy of the country has been established permits each successive era in the long volcanic history to be precisely determined1 and allows us to follow the whole progress of that history stage by stage1 from the beginning to the end4 ,these characteristics may be instructively represented on a map1 such as that which accompanies the present volume 7,map ,i474 ,the reader will there observe how repeatedly volcanic eruptions have taken place1 not merely within the general area of the ,british ,isles1 but even within the same limited region of that area4 ,the broad midland valley of ,scotland has been especially the theatre for their display4 ,from the early part of the ,lower ,silurian period1 through the ages of the ,old ,red ,sandstone1 ,carboniferous and ,permian systems1 hundreds of volcanic vents were active in that region1 while in long subsequent time there came the fissure-eruptions of the ,tertiary series4 #d4 ,in the fourth place1 the geological revolutions of successive ages have made this long volcanic chronicle fully accessible to observation4 ,had the lavas and ashes of one period remained buried under the sedimentary accumulations of the next1 their story would have been lost to us4 ,we should only have been able to decipher the latest records which might happen to lie on the surface4 ,fortunately for the progress of geology1 the endless vicissitudes of a continental border have brought up the very oldest rocks once more to the surface4 ,all the later formations of the earth's crust have likewise been upraised and exposed to denudation during long cycles of time4 ,in this manner1 the rocky framework of the country has been laid bare1 and each successive chapter of its geological history may be satisfactorily deciphered4 ,the singularly complete volcanic chronicle1 after being entombed under younger deposits1 has been broken up and raised once more into view4 ,the active vents of former periods have been dissected1 submarine streams of lava have been uncovered1 sheets of ashes that fell over the sea-bottom have been laid bare4 ,the progress of denudation is specially favoured in such a variable and moist climate as that of ,britain1 and thus by the co-operation of underground and meteoric causes the marvellous volcanic records of this country have been laid open in minutest detail4 ,there is yet another respect in which the volcanic geology of ,britain possesses a special value4 ,popular imagination has long been prone to see signs of volcanic action in the more prominent rocky features of landscape4 ,a bold crag1 a deep and precipitous ravine1 a chasm in the side of a mountain1 have been unhesitatingly set down as proof of volcanic disturbance4 ,many a cauldron-shaped recess1 like the corries of ,scotland or the cwms of ,wales1 has been cited as an actual crater1 with its encircling walls still standing almost complete4 ,the relics of former volcanoes in this country furnish ample proofs to dispel these common misconceptions4 ,they show that not a single crater anywhere remains1 save where it has been buried under lava2 that no trace of the original cones has survived1 except in a few doubtful cases where they may have been preserved under subsequent accumulations of material2 that in the rugged tracts1 where volcanic action has been thought to have been most rife1 there may be not a vestige of it1 while1 on the other hand1 where the uneducated eye would never suspect the presence of any remnant of volcanic energy1 lavas and ashes may abound4 ,we are thus presented with some of the most impressive contrasts in geological history1 while1 at the same time1 this momentous lesson is borne in upon the mind1 that the existing inequalities in the configuration of a landscape are generally due far less to the influence of subterranean force than to the action of the superficial agents which are ceaselessly carving the face of the land4 ,those rocks which from their hardness or structure are best able to withstand that destruction rise into prominence1 while the softer material around them is worn away4 ,volcanic rocks are no exception to this rule1 as the geological structure of ,britain amply proves4 ,in the following chapters1 forming ,book ,i4 of this work1 ,i propose to begin by offering some general remarks regarding the nature and causes of volcanic action1 so far as these are known to us4 ,i shall then proceed to consider the character of the evidence that may be expected to be met with respecting the former prevalence of that action at any particular locality where volcanic disturbances have long since ceased4 ,the most telling evidence of old volcanoes is naturally to be found in the materials which they have left behind them1 and the reader's attention will be asked to the special characteristics of these materials1 in so far as they give evidence of former volcanic activity4 ,as has been already remarked1 many of the most prominent phenomena of a modern volcano are only of transient importance4 ,the earthquakes and tremors1 and the constant disengagement of steam and gases1 that play so conspicuous a part in an eruption1 may leave no sensible record behind them4 ,but even the cones of ashes and lava1 which are piled up into mountainous masses1 have no true permanence3 they are liable to ceaseless erosion by the meteoric agencies of waste1 and every stage in their degradation may be traced4 ,in successive examples we can follow them as they are cut down to the very core1 until in the end they are entirely effaced4 ,we may well1 therefore1 ask at the outset by what more enduring records we may hope to detect the traces of former volcanic action4 ,the following introductory chapters will be devoted to an attempt to answer this question4 ,i shall try to show the nature and relative importance of the records of ancient volcanoes2 how these records1 generally so fragmentary1 may be pieced together so as to be made to furnish the history which they contain2 how their relative chronology may be established2 how their testimony may be supplemented in such wise that the position of long vanished seas1 lands1 rivers1 and lakes may be ascertained2 and how1 after ages of geological revolution1 volcanic rocks that have lain long buried under the surface now influence the scenery of the regions where they have once more been exposed to view4 ,from this groundwork of ascertained fact and reasonable inference1 we shall enter in ,book ,,ii4 upon the story of the old volcanoes of the ,british ,isles4 ,it is usual to treat geological history in chronological order1 beginning with the earliest ages4 ,and this method1 as on the whole the most convenient1 will be adopted in the present work4 ,at the same time1 the plan so persistently followed by ,lyell1 of working backward from the present into the past1 has some distinct advantages4 ,the volcanic records of the later ages are much simpler and clearer than those of older times1 and the student may1 in some respects1 profitably study the history of the ,tertiary eruptions before he proceeds to make himself acquainted with the scantier chronicles of the eruptions of the ,palozoic periods4 ,but as ,i wish to follow the gradual evolution of volcanic phenomena1 and to show how volcanic energy has varied1 waxing and waning through successive vast intervals of time1 ,i will adhere to the chronological sequence4 ,,chapter ,,ii ,the ,nature and ,causes of ,volcanic ,action--,modern ,volcanoes4 ,a volcano is a conical or dome-shaped hill or mountain1 consisting of materials which have been erupted from an orifice leading down from the surface into the heated interior of the earth4 ,among modern and recent volcanoes three types may be recognized4 ,in the first and most familiar of these1 the lavas and ashes ejected from the central vent have gathered around it by successive eruptions1 until they have built up a central cone like those of ,etna and ,vesuvius4 ,as this cone grows in height and diameter1 lateral or parasitic cones are formed on its flanks1 and may become themselves the chief actively erupting vents4 ,this type of volcano1 which has been so long well known from its ,mediterranean examples1 was until recently believed by geologists to be the normal1 or indeed the only1 phase of volcanic energy on the face of the earth4 ,a modification of this type is to be found in a few regions where fragmentary discharges are small in amount and where the eruptions are almost wholly confined to the emission of tolerably liquid lava4 ,a vast dome with gently sloping declivities may in this way be formed1 as in the ,sandwich ,islands and in certain parts of ,iceland4 ,the second type of volcano is at the present day extensively developed only in ,iceland1 but in ,tertiary time it appears to have had a wide range over the globe1 for stupendous memorials of it are preserved in ,north-,western ,europe1 in ,western ,america1 and in ,india4 ,it is distinguished by the formation of numerous parallel fissures from which the lava gushes forth1 either with or without the formation of small cinder-cones along the lines of the chasms4 ,the third type is distinguished by the formation of groups of cinder-cones or lava-domes1 which from their admirable development in ,central ,france have received the name of ,puys4 ,from these vents considerable streams of lava have sometimes been discharged4 ,without entering here into a detailed inquiry regarding the nature and causes of ,volcanic ,action1 we may with advantage consider briefly the two main factors on which this action appears to depend4 #a4 ,much uncertainty still exists as to the condition and composition of the earth's interior4 ,the wide distribution of volcanoes over the globe1 together with the general similarity of materials brought by them up to the surface1 formerly led to the belief that our planet consists of a central mass of molten rock enclosed within a comparatively thin solid crust4 ,physical arguments1 however1 have since demonstrated that the earth1 with such a structure1 would have undergone great tidal deformation1 but that in actual fact it has a greater rigidity than if it were made of solid glass or steel4 ,from all the evidence obtainable it is certain that the temperature of the earth's interior must be high4 ,the rate of increase of this temperature downward from the surface differs from place to place2 but an increase is always observed4 ,at a depth of a few miles1 every known substance must be much hotter than its melting point at the surface4 ,but at the great pressures within the earth1 actual liquefaction is no doubt prevented1 and the nucleus remains solid1 though at a temperature at which1 but for the pressure1 it would be like so much molten iron4 ,any cause which will diminish the pressure may allow the intensely hot material within the globe to pass into the liquid state4 ,there is one known cause which will bring about this result4 ,the downward increment of temperature proves that our planet is continually losing heat4 ,as the outer crust is comparatively cool1 and does not become sensibly hotter by the uprise of heat from within1 the hot nucleus must cool faster than the crust is doing4 ,now cooling involves contraction4 ,the hot interior is contracting faster than the cooler shell which encloses it1 and that shell is thus forced to subside4 ,in its descent it has to adjust itself to a constantly diminishing diameter4 ,it can do so only by plication or by rupture4 ,when the terrestrial crust1 under the strain of contraction1 is compressed into folds1 the relief thus obtained is not distributed uniformly over the whole surface of the planet4 ,from an early geological period it appears to have followed certain lines4 ,how these came to be at first determined we cannot tell4 ,but it is certain that they have served again and again1 during successive periods of terrestrial readjustment4 ,these lines of relief coincide1 on the whole1 with the axes of our continents4 ,the land-areas of the globe may be regarded as owing their existence above sea-level to this result of terrestrial contraction4 ,the crust underneath them has been repeatedly wrinkled1 fractured and thrust upward by the vast oceanic subsidence around them4 ,the long mountain-chains are thus1 so to speak1 the crests of the waves into which the crust has from time to time been thrown4 ,again1 the great lines of fracture in the crust of the earth probably lie in large measure within the land-areas1 or at least parallel with their axes and close to their borders4 ,where the disposition of the chief ruptures and of the predominant plications can be examined1 these leading structural features are found to be1 on the whole1 coincident4 ,in the ,british ,islands1 for instance1 the prevalent trend of the axes of folding from early ,palozoic to ,tertiary time has been from south-west to north-east4 ,how profoundly this direction of earth-movement has affected the structure of the region is shown by any ordinary map1 in the long hill-ranges of the land and in the long inlets of the sea4 ,a geological map makes the dependence of the scenery upon the building of the rocks still more striking4 ,not only have these rocks been plicated into endless foldings1 the axes of which traverse the ,british ,islands with a north-easterly trend3 they have likewise been dislocated by many gigantic ruptures1 which tend on the whole to follow the same direction4 ,the line of the ,great ,glen1 the southern front of the ,highlands1 and the northern boundary of the ,southern ,uplands of ,scotland1 are conspicuous examples of the position and effect of some of the greater fractures in the structure of this country4 ,the ridging up of any part of the terrestrial crust will afford some relief from pressure to the parts of the interior immediately underneath4 ,if1 as is probable1 the material of the earth's interior is at the melting point proper for the pressure at each depth1 then any diminution of the pressure may allow the intensely heated substance to pass into the liquid state4 ,it would be along the lines of terrestrial uplift that this relief would be given4 ,it is there that active volcanoes are found4 ,the molten material is forced upward under these upraised ridges by the subsidence of the surrounding regions4 ,and where by rupture of the crust this material can make its way to the surface1 we may conceive that it will be ejected as lava or as stones and ashes4 ,viewed in a broad way1 such appears to be the mechanism involved in the formation and distribution of volcanoes over the surface of the earth4 ,but obviously this explanation only carries us so far in the elucidation of volcanic action4 ,if the molten magma flowed out merely in virtue of the influence of terrestrial contraction1 it might do so for the most part tranquilly1 though it would probably be affected by occasional sudden snaps1 as the crust yielded to accumulations of pressure4 ,human experience has no record of the actual elevation of a mountain-chain4 ,we may believe that if such an event were to happen suddenly or rapidly1 it would be attended with gigantic catastrophes over the surface of the globe4 ,we can hardly conceive what would be the scale of a volcanic eruption attending upon so colossal a disturbance of the terrestrial crust4 ,but the eruptions which have taken place within the memory of man have been the accompaniments of no such disturbance4 ,although they have been many in number and sometimes powerful in effect1 they have seldom been attended with any marked displacement of the surrounding parts of the terrestrial crust4 ,contraction is1 of course1 continuously and regularly in progress1 and we may suppose that the consequent subsidence1 though it results in intermittent wrinkling and uplifting of the terrestrial ridges1 may also be more or less persistent in the regions lying outside these ridges4 ,there will thus be a constant pressure of the molten magma into the roots of volcanoes1 and a persistent tendency for the magma to issue at the surface at every available rent or orifice4 ,the energy and duration of outflow1 if they depended wholly upon the effects of contraction1 would thus vary with the rate of subsidence of the sinking areas1 probably assuming generally a feeble development1 but sometimes bursting into fountains of molten rock hundreds of feet in height1 like those observed from time to time in ,hawaii4 #b4 ,the actual phenomena of volcanic eruptions1 however1 show that a source of explosive energy is almost always associated with them1 and that while the transference of the subterranean molten magma towards the volcanic vents may be referred to the results of terrestrial contraction1 the violent discharge of materials from those vents must be assigned to some kind of energy stored up in the substance of the earth's interior4 ,the deep-seated magma from which lavas ascend contains various vapours and gases which1 under the enormous pressure within and beneath the terrestrial crust1 are absorbed or dissolved in it4 ,so great is the tension of these gaseous constituents1 that when from any cause the pressure on the magma is suddenly relieved1 they are liberated with explosive violence4 ,a volcanic paroxysm is thus immediately the effect of the rapid escape of these imprisoned gases and vapours4 ,with such energy does the explosion sometimes take place1 that the ascending column of molten lava is blown into the finest impalpable dust1 which may load the air around a volcano for many days before it falls to the ground1 or may be borne in the upper regions of the atmosphere round the globe4 ,the proportion of dissolved gases varies in different lavas1 while the lavas themselves differ in the degree of their liquidity4 ,some flow out tranquilly like molten iron1 others issue in a pasty condition and rapidly congeal into scori and clinkers4 ,thus within the magma itself the amount of explosive energy is far from being always the same4 ,it is to the co-operation of these two causes--terrestrial contraction and its effects on the one hand1 and the tension of absorbed gases and vapours the other--that the phenomena of volcanoes appear to be mainly due4 ,there is no reason to believe that modern volcanoes differ in any essential respect from those of past ages in the earth's history4 ,it might1 indeed1 have been anticipated that the general energy of the planet would manifest itself in far more stupendous volcanic eruptions in early times than those of the modern period4 ,but there is certainly no geological evidence in favour of such a difference4 ,one of the objects of the present work is to trace the continuity of volcanic phenomena back to the very earliest epochs1 and to show that1 so far as the geological records go1 the interior of the planet has reacted on its exterior in the same way and with the same results4 ,we may now proceed to inquire how far volcanoes leave behind them evidence of their existence4 ,i shall devote the next two or three chapters to a consideration of the proofs of volcanic action furnished by the very nature of the materials brought up from the interior of the earth1 by the arrangement of these materials at the surface1 by the existence of the actual funnels or ducts from which they were discharged above ground1 and by the disposition of the masses of rock which1 at various depths below the surface1 have been injected into and have solidified within the terrestrial crust4 ,,chapter ,,iii ,ancient ,volcanoes3 ,proofs of their existence derived from the ,nature of the ,rocks erupted from the ,earth's ,interior4 ,a4 ,materials erupted at the ,surface--,extrusive ,series4 i4 ,lavas1 their ,general ,characters4 ,volcanic ,cycles4 ii4 ,volcanic ,agglomerates1 ,breccias and ,tuffs4 ,the materials brought by volcanic action from the earth's interior have certain common characters which distinguish them from other constituents of the terrestrial crust4 ,hence the occurrence of these materials on any part of the earth's surface affords convincing proofs of former volcanic eruptions1 even where all outward trace of actual volcanoes may have been effaced from the topographical features of the ground4 ,volcanic products may be classed in two divisions--#ast1 ,those which have been ejected at the surface of the earth1 or the ,extrusive series2 and #bnd1 ,those which have been injected into the terrestrial crust at a greater or less distance below the surface1 and which are known as the ,intrusive series4 ,extrusive rocks may be further classified in two great groups--7i47 ,the ,lavas1 or those which have been poured out in a molten condition at the surface2 and 7ii47 ,the ,fragmental ,materials1 including all kinds of pyroclastic detritus discharged from volcanic vents4 ,taking first the ,extrusive volcanic rocks1 we may in the present chapter consider those characters in them which are of most practical value in the investigation of the volcanic phenomena of former geological periods4 i4 ,,lavas ,the term ,lava is a convenient and comprehensive designation for all those volcanic products which have flowed out in a molten condition4 ,they differ from each other in composition and structure1 but their variations are comprised within tolerably definite limits4 ,as regards their composition they are commonly classed in three divisions--#ast1 ,the ,acid lavas1 in which the proportion of silicic acid ranges from a little below #gj per cent upwards2 #bnd1 ,the ,intermediate lavas1 wherein the percentage of silica may vary from #ee to near #gj2 and #crd1 ,the ,basic lavas1 where the acid constituent ranges from #ee per cent downwards4 ,sometimes the most basic kinds are distinguished as a fourth group under the name of ,ultrabasic1 in which the percentage of silica may fall below #dj4 ,the structures of lavas1 however1 furnish their most easily appreciated characteristics4 ,four of these structures deserve more particular attention3 #ast1 ,cellular1 vesicular or pumiceous structure2 #bnd1 ,the presence of glass1 or some result of the devitrification of an original glass2 #crd1 ,flow-structure2 and #dth1 ,the arrangement of the rocks in sheets or beds1 with columnar and other structures4 ,illustration3 ,fig4 #a4--,vesicular structure1 ,lava from ,ascension ,island1 slightly less than natural size4 #a4 ,the ,,cellular1 ,,vesicular1 ,,scoriaceous or ,,pumiceous ,,structure of volcanic rocks 7,fig4 #a7 could only have arisen in molten masses from the expansion of imprisoned vapours or gases1 and is thus of crucial importance in deciding the once liquid condition of the rocks which display it4 ,the vesicles may be of microscopic minuteness1 but are generally quite visible to the naked eye1 and are often large and conspicuous4 ,sometimes these cavities have been subsequently filled up with calcite1 quartz1 agate1 zeolites or other mineral deposition4 ,as the kernels thus produced are frequently flattened or almond-shaped 7amygdales71 owing to elongation of the steam-holes by movement of the lava before its consolidation1 the rocks containing them are said to be amygdaloidal4 ,this structure1 though eminently characteristic of superficial lavas1 is not always by itself sufficient to distinguish them from the intrusive rocks4 ,examples will be given in later chapters where dykes1 sills and other masses of injected igneous material are conspicuously cellular in some parts4 ,but1 in such cases1 the cavities are generally comparatively small1 usually spherical or approximately so1 tolerably uniform in size and distribution1 and1 especially when they occur in dykes1 distributed more particularly along certain lines or bands1 sometimes with considerable regularity 7see ,figs4 #ij1 #ia1 and #bcf74 ,among the superficial lavas1 however1 such regularity is rarely to be seen4 ,now and then1 indeed1 a lava1 which is not on the whole cellular1 may be found to have rows of vesicles arranged parallel to its under or upper surface1 or it may have acquired a peculiar banded structure from the arrangement of its vesicles in parallel layers along the direction of flow4 ,the last-named peculiarity is widely distributed among the ,tertiary lavas of ,north-,western ,europe1 and gives to their weathered surfaces a deceptive resemblance to tuffs or other stratified rocks 7see ,figs4 #bfj1 #caj and #caa74 ,it will be more particularly referred to a few pages further on4 ,in general1 however1 we may say that the steam-cavities of lavas are quite irregular in size1 shape and distribution1 sometimes increasing to such relative proportions as to occupy most of the bulk of the rock1 and in other places disappearing1 so as to leave the lava tolerably compact4 ,when a lava presents an irregularly vesicular character1 like that of the slags of an iron-furnace1 it is said to be slaggy4 ,when its upper surface is rugged and full of steam-vesicles of all sizes up to large cavernous spaces1 it is said to be scoriaceous1 and fragments of such a rock ejected from a volcanic vent are spoken of as scori4 ,attention to the flattening of the steam-vesicles in cellular lavas1 which has just been alluded to as the result of the onward movement of the still molten mass1 may show1 by the trend and grouping of these elongated cavities1 the probable direction of the flow of the lava before it came to rest4 ,sometimes the vesicles have been drawn out and flattened to such a degree that the rock has acquired in consequence a fissile structure4 ,in other instances1 the vesicles have been originally formed as long parallel and even branching tubes1 like the burrows of ,annelids or the borings of ,teredo4 ,some remarkable examples of this exceptional structure have been obtained from the ,tertiary plateau-basalts of the ,western ,isles1 of which an example is represented in ,fig4 #b4 ,in many cases the vesicles extend through the whole thickness of a lava4 ,frequently they may be found most developed towards the top and bottom2 the central portion of the sheet being compact1 while the top and bottom are rugged1 cavernous or scoriaceous4 ,though originally the vesicles and cavernous spaces1 blown open by the expansion of the vapours dissolved in molten lava1 remained empty on the consolidation of the rock1 they have generally been subsequently filled up by the deposit within them of mineral substances carried in aqueous solution4 ,the minerals thus introduced are such as might have been derived from the removal of their constituent ingredients by the solvent action of water on the surrounding rock4 ,and as amygdaloids are generally more decayed than the non-vesicular lavas1 it has been generally believed that the abstraction of mineral material and its re-deposit within the steam-vesicles have been due to the influence of meteoric water1 which at atmospheric temperatures and pressures has slowly percolated from the surface through the cellular lava1 long after the latter had consolidated and cooled1 and even after volcanic energy at the locality had entirely ceased4 ,illustration3 ,fig4 #b4--;,elongation and branching of steam-vesicles in a lava1 ,kilninian1 ,isle of ,mull1 a little less than natural size4 ,examples1 however1 are now accumulating which certainly prove that1 in some cases1 the vesicles were filled up during the volcanic period4 ,among the ,tertiary basalt-plateaux of the ,inner ,hebrides1 for instance1 it can be shown that the lavas were already amygdaloidal before the protrusion of the gabbros and granophyres which mark later stages of the same continuous volcanic history1 and even before the outpouring of much of the basalt of these plateaux4 ,not improbably the mineral secretions were largely due to the influence of hot volcanic vapours during the eruption of the basalts4 ,this subject will be again referred to in the description of the ,tertiary volcanic series4 ,vesicular structure is more commonly and perfectly developed among the lavas which are basic and intermediate in composition than among those which are acid4 ,while the existence of a highly vesicular or scoriaceous structure may generally be taken as proof that the rock displaying it flowed out at the surface as a lava1 other evidence pointing to the same conclusion may often be gathered from the rocks with which the supposed lava is associated4 ,where1 for example1 a scoriaceous lava is covered with stratified deposits which contain pieces of that lava1 we may be confident that the rock is an interstratified or contemporaneous sheet4 ,it has been erupted after the deposition of the strata on which it rests1 and before that of the strata which cover it and contain pieces of it4 ,in such a case1 the geological date of the eruption could be precisely defined4 ,illustrations of this reasoning will be given in ,chapter iv41 and in the account of the volcanic series of ,carboniferous age in ,central ,scotland1 where a basic lava can sometimes be proved to be a true flow and not an intrusive sill by the fact that portions of its upper slaggy surface are enclosed in overlying sandstone1 shale or limestone4 #b4 ,the presence of ,,glass1 or of some result of the devitrification of an original glass1 is an indication that the rock which exhibits it has once been in a state of fusion4 ,even where no trace of the original vitreous condition may remain1 stages in its devitrification1 that is1 in its conversion into a stony or lithoid condition1 may be traceable4 ,thus what are called spherulitic and perlitic structures 7which will be immediately described71 either visible to the naked eye or only observable with the aid of the microscope1 afford evidence of the consolidation and conversion of a glassy into a lithoid substance4 ,striking evidence of the former glassy1 and therefore molten1 condition of many rocks now lithoid is to be gained by the examination of thin slices of them under the microscope4 ,not only are vestiges of the original glass recognizable1 but the whole progress of devitrification may be followed into a crystalline structure4 ,the primitive crystallites or microlites of different minerals may be seen to have grouped themselves together into more or less perfect crystals1 while scattered crystals of earlier consolidation have been partially dissolved in and corroded by the molten glass4 ,these and other characteristics of once fused rocks have to a considerable extent been imitated artificially by ,,mm4 ,fouqu and ,michel ,lvy1 who have fused the constituent minerals in the proper proportions4 ,since traces of glass or of its representative devitrified structures are so abundantly discoverable in lavas1 we may infer the original condition of most lavas to have been vitreous4 ,where1 for instance1 the outer selvages of a basic dyke or sill are coated with a layer of black glass which rapidly passes into a fine-grained crystalline basalt1 and then again into a more largely crystalline or doleritic texture in the centre1 there can be no hesitation in believing that glassy coating to be due to the sudden chilling and consolidation of the lava injected between the cool rocks that enclose it4 ,the part that solidified first may be regarded as probably representing the condition of the whole body of lava at the time of intrusion4 ,the lithoid or crystalline portion between the two vitreous outer layers shows the condition which the molten rock finally assumed as it cooled more slowly4 ,some lavas1 such as obsidians and pitchstones1 have consolidated in the glassy form4 ,more usually1 however1 a lithoid structure has been developed1 the original glass being only discoverable by the microscope1 and often not even by its aid4 ,two varieties of devitrification may be observed among lavas1 which1 though not marked off from each other by any sharp lines1 are on the whole distinctive of the two great groups of acid and basic rocks4 7#a7 ,among the acid rocks1 what is called the ,felsitic type of devitrification is characteristic4 ,thus1 obsidians pass by intermediate stages from a clear transparent or translucent glass into a dull flinty or horny mass4 ,when thin slices of these transitional forms are examined under the microscope1 minute hairs and fibres or trichites1 which may be observed even in the most perfectly glassy rocks1 are seen to increase in number until they entirely take the place of the glass4 ,microlites of definite minerals may likewise be observed1 together with indefinite granules1 and the rock finally becomes a rhyolite1 felsite or allied variety 7,fig4 #c74 ,illustration3 ,fig4 #c4--,microlites of the ,pitchstone of ,arran 7magnified #gj diameters74 ,at the same time it should be observed that1 even in the vitreous condition of a lava1 definite crystals of an early consolidation were generally already present4 ,felspars and quartz1 usually in large porphyritic forms1 may be seen in the glass1 often so corroded as to indicate that they were in course of being dissolved in the magma at the time of the cooling and solidification of the mass4 ,in obsidians and pitchstones such relics of an earlier or derived series of crystallized minerals may often be recognized1 while in felsites and quartz-porphyries they are equally prominent4 ,where large dispersed crystals form a prominent characteristic in a rock they give rise to what is termed the ,porphyritic structure4 ,illustration3 ,fig4 #d4--,perlitic structure in ,felsitic ,glass1 ,isle of ,mull 7magnified74 ,illustration3 ,fig4 #e4--,spherulitic structure 7magnified74 ,accompanying the passage of glass into stone1 various structures make their appearance1 sometimes distinctly visible to the naked eye1 at other times only perceptible with the aid of the microscope4 ,one of these structures1 known as ,perlitic 7,fig4 #d71 consists in the formation of minute curved or straight cracks between which the vitreous or felsitic substance1 during its contraction in cooling1 assumed a finely globular form4 ,another structure1 termed ,spherulitic 7,fig4 #e71 shows the development of globules or spherules which may range from grains of microscopic minuteness up to balls two inches or more in diameter4 ,these not infrequently present a well-formed internal fibrous radiation1 which gives a black cross between crossed ,nicol prisms4 ,spherulites are more especially developed along the margins of intrusive rocks1 and may be found in dykes1 sills and bosses 7see ,figs4 #cge and #cgg74 ,where the injected mass is not thick it may be spherulitic to the very centre1 as can be seen among the felsitic and granophyric dykes of ,skye4 ,some felsitic lavas possess a peculiar nodular structure1 which was developed during the process of consolidation4 ,so marked does this arrangement sometimes become that the rocks which display it have actually been mistaken for conglomerates4 ,it is well exhibited among the ,lower ,silurian lavas of ,snowdon1 the ,upper ,silurian lavas of ,dingle1 and the ,lower ,old ,red ,sandstone lavas near ,killarney4 ,illustration3 ,fig4 #f4--,micropegmatitic or ,granophyric structure in ,granophyre1 ,mull 7magnified74 ,illustration3 ,fig4 #g4--,ophitic structure in ,dolerite1 ,gortacloghan1 ,co4 ,derry 7magnified74 ,a marked structure among some intrusive rocks1 especially of an acid composition1 is that called ,micropegmatitic or ,granophyric4 ,it consists in a minute intergrowth of two component minerals1 especially quartz and felspar1 and is more especially characteristic of certain granitic or granitoid rocks which have consolidated at some distance from the surface and occur as bosses1 sills and dykes4 ,it is also met with1 however1 in some basic sills4 ,examples of all these and other structures will occur in the course of the following description of ,british volcanic rocks4 7#b7 ,the second type of devitrification1 conspicuous in rocks of more basic composition1 is marked by a more complete development of crystallization4 ,among basic1 as among acid rocks1 there are proofs of the consolidation of definite minerals at more than one period4 ,where the molten material has suddenly cooled into a black glass1 porphyritic felspars or other minerals are often to be seen which were already floating in the magma in its molten condition4 ,during devitrification1 however1 other felspars of a later period of generation made their appearance1 but they are generally distinguishable from their predecessors4 ,probably most basic and intermediate rocks1 when poured out at the surface as lavas1 were no longer mere vitreous material1 but had already advanced to various stages of progress towards a stony condition4 ,these stages are still to some extent traceable by the aid of the microscope4 ,microlites of the component minerals are first developed1 which1 if the process of aggregation is not arrested1 build up more or less perfect crystals or crystalline grains of the minerals4 ,eventually the glass may be so completely devitrified by the development of its constituent minerals as to be wholly used up1 the rock then becoming entirely crystalline1 or to survive only in scanty interstitial spaces4 ,in the family of the basalts and dolerites the gradual transition from a true glass into a holocrystalline compound may be followed with admirable clearness4 ,the component minerals have sometimes crystallized in their own distinct crystallographic forms 7idiomorphic72 in other cases1 though thoroughly crystalline1 they have assumed externally different irregular shapes1 fitting into each other without their ,proper geometric boundaries 7allotriomorphic74 ,a specially characteristic feature of many basic rocks is the presence of what is termed an ,ophitic structure 7,fig4 #g74 ,thus the component crystals of pyroxene occur as large plates separated and penetrated by small needles and crystals of felspar4 ,the portions of pyroxene1 divided by the enclosed felspar1 are seen under the microscope to be in optical continuity1 and to have crystallized round the already formed felspar4 ,this structure is never found in metamorphic crystalline rocks4 ,it has been reproduced artificially from fusion by ,messrs4 ,fouqu and ,michel ,lvy4 ,the name ,variolitic is applied to another structure of basic rocks 7,fig4 #h71 in which1 especially towards the margin of eruptive masses1 abundant spheroidal aggregates have been developed from the size of a millet-seed to that of a walnut1 imbedded in a fine-grained or compact greenish matrix into which the kernels seem to shade off4 ,these kernels consist of silicates arranged either radially or in concentric zones4 #c4 ,flow-structure is an arrangement of the crystals1 vesicles1 spherulites1 or devitrification-streaks in bands or lines1 which sweep round any enclosed object1 such as a porphyritic crystal or detached spherulite1 and represent the curving flow of a mobile or viscous mass4 ,admirable examples of this structure may often be observed in old lavas1 as well as in dykes and sills1 the streaky lines of flow being marked as distinctly as the lines of foam that curve round the boulders projecting from the surface of a mountain-brook4 ,flow-structure is most perfectly developed among the obsidians1 rhyolites1 felsites and other acid rocks1 of which it may be said to be a frequently conspicuous character 7,fig4 #i74 ,in these rocks it is revealed by the parallel arrangement of the minute hair-like bodies and crystals1 or by alternate layers of glassy and lithoid material4 ,the streaky lines thus developed are sometimes almost as thin and parallel as the leaves of a book4 ,but they generally show interruptions and curvatures1 and may be seen to bend round larger enclosed crystals1 or to gather into eddy-like curves1 in such a manner as vividly to portray the flow of a viscous substance4 ,these lines represent on a minute scale the same flow-structure which may be traced in large sheets among the lavas4 ,the porphyritic crystals and the spherulites are also drawn out in rows in the same general direction4 ,sometimes1 indeed1 the spherulites have been so symmetrically grouped in parallel rows that they appear as rod-like aggregates which extend along the margin of a dyke4 ,illustration3 ,fig4 #h4--,variolitic or ,orbicular structure1 ,napoleonite1 ,corsica 7nat4 size74 ,among lavas of more basic composition flow-structure is not so often well displayed4 ,it most frequently shows itself by the orientation of porphyritic felspars or of lines of steam-vesicles4 ,occasionally1 however1 sheets of basalt may be found in which a distinct streakiness has been developed owing to variations in the differentiation of the original molten magma4 ,a remarkable and widespread occurrence of such a structure is met with among the ,tertiary basalt-plateaux of the ,inner ,hebrides and the ,faroe ,islands4 ,in the lower parts of these thick accumulations of successive lava-sheets1 a banded character is so marked as to give the rocks the aspect of truly stratified deposits4 ,the observer1 indeed1 can hardly undeceive himself as to their real nature until he examines them closely4 ,as a full description of this structure will be given in a later chapter1 it may suffice to state here that the banding arises from two causes4 ,in some cellular lavas1 the vesicles are arranged in layers which lie parallel with the upper and under surfaces of the sheets4 ,these layers either project as ribs or recede into depressions along the outcrop1 and thus impart a distinctly stratified aspect to the rock4 ,more frequently1 however1 the banded structure is produced by the alternation of different varieties of texture1 and even of composition1 in the same sheet of basalt4 ,lenticular seams of olivine-basalt may be found intercalated in a more largely crystalline dolerite4 ,these differences appear to point to considerable variations in the constitution of the magma from which the lavas issued--variations which already existed before the discharge of these lavas1 and which showed themselves in the successive outflow of basaltic and doleritic material during the eruption of what was really1 as regards its appearance at the surface1 one continuous stream of molten rock4 ,it is impossible to account for such variations in the same sheet of lava by any process of differentiation in the melted material during its outflow and cooling4 ,analogous variations occur among the basic sills and bosses of the ,tertiary volcanic series of ,britain4 ,these1 as will be more fully discussed in later chapters1 indicate a considerable amount of heterogeneity in the deep-seated magma from which the intrusive sheets and bosses were supplied 7see vol4 ii4 pp4 #cbi1 #cdb74 ,illustration3 ,fig4 #i4--;,flow-structure in ,rhyolite1 ,antrim1 slightly reduced4 ,it is a common error to assume that flow-structure is a distinctive character of lavas that have flowed out at the surface4 ,in reality some of the most perfect examples of the structure occur in dykes and sills1 both among acid and basic rocks4 ,innumerable instances might be quoted from the ,british ,isles in support of this statement4 ,although1 in the vast majority of cases1 the presence of flow-structure may be confidently assumed to indicate a former molten condition of the rock in which it occurs1 it is not an absolutely reliable test for an igneous rock4 ,experiment has shown that under enormous pressure even solid metals may be made to flow into cavities prepared for their reception4 ,under the vast compression to which the earth's crust is subjected during terrestrial contraction1 the most obdurate rocks are crushed into fragments varying from large blocks to the finest powder4 ,this comminuted material is driven along in the direction of thrust1 and when it comes to rest presents a streakiness1 with curving lines of flow round the larger fragments1 closely simulating the structure of many rhyolites and obsidians4 ,it is only by attention to the local surroundings that such deceptive resemblances can be assigned to their true cause4 ,illustration3 ,fig4 #aj4--,lumpy1 irregular trachytic ,lava-streams 7,carboniferous71 ,east ,linton1 ,haddingtonshire4 #d4 ,the ,,disposition ,,of ,,lavas ,,in ,,sheets ,,or ,,beds is the result of successive outflows of molten rock4 ,such sheets may range from only a yard or two to several hundred feet in thickness4 ,as a rule1 though with many exceptions1 the basic lavas1 such as the basalts1 appear in thinner beds than the acid forms4 ,this difference is well brought out if we compare1 for instance1 the massive rhyolites or felsites of ,north ,wales with the thin sheets of basalt in ,antrim and the ,inner ,hebrides4 ,the regularity of the bedded character is likewise more definite among the basic than among the acid rocks1 and this contrast also is strikingly illustrated by the two series of rocks just referred to4 ,the rhyolites and felsites1 sometimes also the trachytes and andesites1 assume lumpy1 irregular forms1 and some little care may be required to trace their upper and under surfaces1 and to ascertain that they really do form continuous sheets1 though varying much in thickness from place to place 7,fig4 #aj74 ,like modern acid lavas1 they seem to have flowed out in a pasty condition1 and to have been heaped up round the vents in the form of domes1 or with an irregular hummocky or mounded surface4 ,the basalts1 and dolerites1 and sometimes the andesites1 have issued in a more fluid condition1 and have spread out in sheets of more uniform thickness1 as may be instructively seen in the sea-cliffs of ,antrim1 ,mull1 ,skye1 and the ,faroe ,islands1 where the horizontal or gently-inclined flows of basalt lie upon each other in even parallel beds traceable for considerable distances along the face of the precipices 7,figs4 #aa1 #bfe1 and #bhf74 ,the andesites of the ,old ,red ,sandstone 7,figs4 #ii1 #ajj7 and ,carboniferous series 7,figs4 #ajg1 #ajh1 #aaa1 #aab1 #aac1 #abc7 in ,scotland likewise form terraced hills4 ,the length of a lava-stream may vary within wide limits4 ,sometimes an outflow of lava has not reached the base of the cone from the side of which it issued1 like the obsidian stream on the flanks of the little cone of the island of ,volcano4 ,in other cases1 the molten rock has flowed for forty or fifty miles1 like the copious ,icelandic lava-floods of #aghc4 ,in the basalt-plateaux of the ,inner ,hebrides a single sheet may sometimes be traced for several miles4 ,illustration3 ,fig4 #aa4--,view at the entrance of the ,svinofjord1 ,faroe ,islands1 illustrating the terraced forms assumed by basic lavas4 ,the island on the left is ,bor1 that in the centre ,vider1 and that on the right ,svin4 ,some lavas1 more especially among the basic series1 assume in cooling a ,columnar structure1 of which two types may be noticed4 ,in one of these the columns pass with regularity and parallelism from the top to the bottom of a bed 7,figs4 #aga1 #bbe74 ,the basalt in which ,fingal's ,cave1 in the isle of ,staffa1 has been hollowed out may be taken as a characteristic example 7,fig4 #bff;a74 ,not infrequently the columns are curved1 as at the well-known ,clam-shell ,cave of ,staffa4 ,in the other type1 the columns or prisms are not persistent1 but die out into each other and have a wavy1 irregular shape1 somewhat like prisms of starch4 ,these two types may occur in successive sheets of basalt1 or may even pass into each other4 ,at ,staffa the regularly columnar bed is immediately overlain with one of the starch-like character4 ,the columnar structure in either case is a contraction phenomenon1 produced during the cooling and shrinking of the lava4 ,but it is difficult to say what special conditions in the lava were required for its production1 or why it should sometimes have assumed the regular1 at others the irregular form4 ,it may be found not only in superficial lavas but in equal perfection in some dykes and intrusive sills or injections1 as among the ,tertiary volcanic rocks of the island of ,canna 7,figs4 #cjg and #cjh74 ,the precipitation of a lava-stream into a lake or the sea may cause the outer crust of the rock to break up with violence1 so that the still molten material inside may rush into the water4 ,some basic lavas on flowing into water or into a watery silt have assumed a remarkable spheroidal sack-like or pillow-like structure1 the spheroids being sometimes pressed into shapes like piles of sacks4 ,a good instance of this structure occurs in a basalt at ,acicastello in ,sicily4#a ,a similar appearance will be described in a later chapter as peculiarly characteristic of certain ,lower ,silurian lavas associated with radiolarian cherts in ,britain and in other countries 7,fig4 #ab74 ,footnote #a3 ,see ,prof4 ,g4 ,platania in ,dr4 ,johnston-,lavis' ,south ,italian ,volcanoes1 ,naples 7#ahia71 p4 #da and plate xii4 ,illustration3 ,fig4 #ab4--,sack-like or pillow-form structure of basic lavas 7,lower ,silurian71 ,bennan ,head1 ,ballantrae1 ,ayrshire4 ,it probably seldom happens that a solitary sheet of lava occurs among non-volcanic sedimentary strata1 with no other indication around it of former volcanic activity4 ,such an isolated record does not seem to have been met with in the remarkably ample volcanic register of the ,british ,isles4 ,the outpouring of molten rock has generally been accompanied with the ejection of fragmentary materials4 ,hence among the memorials of volcanic eruptions1 while intercalated lavas are generally associated with sheets of tuff1 bands of tuff may not infrequently be encountered in a sedimentary series without any lava4 ,instances in illustration of these statements may be culled from the ,british ,palozoic formations back even into the ,cambrian system4 ,a characteristic feature of some interest in connection with the flow of lava is the effect produced by it on the underlying rocks4 ,if these are not firmly compacted they may be ploughed up or even dislocated4 ,thus the tuffs of the ,velay have sometimes been plicated1 inverted1 and fractured by the movement of a flowing current of basalt4#b ,the great heat of the lava has frequently induced considerable alteration upon the underlying rocks4 ,induration is the most common result1 often accompanied with a reddening of the altered substance4 ,occasionally a beautifully prismatic structure has been developed in the soft material immediately beneath a basalt1 as in ferruginous clay near the village of ,esplot in the ,velay1 in which the close-set columns are #ej centimetres long and #d to #e centimetres in diameter4#c ,changes of this nature1 however1 are more frequent among sills than among superficial lavas4 ,many examples of them may be gathered from the ,scottish ,carboniferous districts4 ,footnote #b3 ,m4 ,boule1 ,bull4 ,cart4 ,gol4 ,france1 ,no4 #bh1 tom4 iv4 7#ahib71 p4 #bce4 ,footnote #c3 ,m4 ,boule4 ,op4 cit4 p4 #bcd4 ,variations of structure in single lava-sheets4--,from what has been said above in regard to certain kinds of flow-structure among basic rocks1 it will be evident that some considerable range of chemical1 but more particularly of mineralogical1 composition may be sometimes observed even within the same sheet of lava4 ,such differences1 it is true1 are more frequent among intrusive rocks1 especially thick sills and large bosses4 ,but they have been met with in so many instances among superficial lavas as to show that they are the results of some general law1 which probably has a wide application among the surface-products of volcanic action4 ,scrope expressed the opinion that in the focus of a volcano there may be a kind of filtration of the constituents of a molten mass1 the heavier minerals sinking through the lighter1 so that the upper portions of the mass will become more felspathic and the lower parts more augitic and ferruginous4#d ,footnote #d3 ,volcanoes1 p4 #abe4 ,leopold von ,buch found that in some of the highly glassy lavas of the ,canary ,islands the felspar increases towards the bottom of the mass1 becoming so abundant as almost to exclude the matrix1 and giving rise to a compound that might be mistaken for a primitive rock4#e ,footnote #e3 ,description ,physique des ,isles ,canaries 7#ahcf71 p4 #aij4 ,darwin observed that in a grey basalt filling up the hollow of an old crater in ,james ,island1 one of the ,galapagos group1 the felspar crystals became much more abundant in the lower scoriaceous part1 and he discussed the question of the descent of crystals by virtue of their specific gravity through a still molten lava4#f ,footnote #f3 ,geological ,observations on ,volcanic ,islands 7#ahdd71 p4 #aag4 ,mr4 ,clarence ,king during a visit to ,hawaii found that in every case where he broke newly-congealed streamlets of lava1 the bottom of the flow was thickly crowded with triclinic felspars and augites1 while the whole upper part of the stream was of nearly pure isotropic and acid glass4#g ,this subject will be again referred to when we come to discuss the characters of intrusive sills and bosses1 for it is among them that the most marked petrographical variations may be observed4 ,examples will be cited both from the intrusive and extrusive volcanic groups of ,britain4 ,footnote #g3 ,u4,s4 ,geol4 ,exploration of the ,fortieth ,parallel1 vol4 i4 7#ahgh71 p4 #gaf4 ,volcanic cycles4--,closely related to the problem of the range of structure and composition in a single mass of lava is another problem presented by the remarkable sequence of different types of lava which are erupted within a given district during a single volcanic period4 ,nearly thirty years ago ,baron von ,richthofen drew attention to the sequence of volcanic materials erupted within the same geographical area4 ,he showed1 more especially from observations in ,western ,america1 that a definite order of appearance in the successive species of lava could be established1 the earliest eruptions consisting of materials of an intermediate or average composition1 and those of subsequent outflows becoming on the whole progressively more acid1 but finishing by an abrupt transition to a basic type4 ,his sequence was as follows3 #a4 ,propylite2 #b4 ,andesite2 #c4 ,trachyte2 #d4 ,rhyolite2 #e4 ,basalt4#h ,this generalisation has been found to hold good over wide regions of the ,old ,world as well as the ,new4 ,it is not1 however1 of universal application4#i ,examples are not uncommon of an actual alternation of acid and basic lavas from the same1 or at least from adjacent vents4 ,such an alternation occurs among the ,tertiary eruptions of ,central ,france and among those of ,old ,red ,sandstone age in ,scotland4 ,footnote #h3 ,trans4 ,acad4 ,california1 #ahfh4 ,prof4 ,iddings' ,journ4 ,geol41 vol4 i4 7#ahic71 p4 #fjf4 ,footnote #i3 ,see ,prof4 ,brgger1 ,die ,eruptivgesteine des ,kristianiagebietes1 part ii4 7#ahie71 p4 #age2 ,zeitsch4 ,kryst4 und ,mineral1 vol4 xvi4 7#ahij7 p4 #hc4 ,this author would1 from this point of view1 draw a distinction between rocks which have consolidated deep within the earth and those which have flowed out at the surface1 since he thinks that we are not justified in applying our experience of the order of sequence in the one series to the other4 ,yet there can be no doubt that in many old volcanic districts the masses that may be presumed to have consolidated at a great depth have been in unbroken connection with masses that reached the surface4 ,these latter1 as ,prof4 ,iddings has urged1 furnish a much larger body of evidence than the intrusive sheets and bosses4 ,the range of variation in the nature of the eruptive rocks during the whole of a volcanic period in any district may be termed a volcanic cycle4 ,in ,britain1 where the records of many volcanic periods have been preserved1 a number of such cycles may be studied4 ,in this way the evolution of the subterranean magma during one geological age may be compared with that of another4 ,it will be one of the objects of the following chapters to trace out this evolution in each period where the requisite materials for the purpose are available4 ,we shall find that back to ,archan time a number of distinct cycles may be observed1 differing in many respects from each other1 but agreeing in the general order of development of the successive eruptions4 ,leaving these ,british examples for future consideration1 it may be useful to cite here a few from the large series now collected from the ,european continent and ,north ,america4#aj ,footnote #aj3 ,prof4 ,m4 ,bertrand in a suggestive paper published in #ahhh dealt with the general order of appearance of eruptive rocks in different provinces of ,europe4 ,but the materials then at his command probably did not warrant him in offering more than a sketch of the subject1 ,bull4 ,soc4 ,geol41 ,france1 xvi4 p4 #egc4 ,in the same volume there is a paper by ,m4 ,le ,verrier1 who announces his opinion that the eruption of the basic rocks takes place in times of terrestrial calm1 while that of the acid rocks occurs in periods of great disturbance1 op4 cit4 p4 #dih4 ,compare also ,prof4 ,brgger1 ,die ,eruptivgesteine des ,kristianiagebietes1 ii4 p4 #afi4 ,among the older rocks of the ,european continent1 ,prof4 ,brgger has shown that in the ,christiania district the eruptive rocks which traverse the ,cambrian and ,silurian formations began with the outburst of basic material such as melaphyre1 augite-porphyrite1 and gabbro-diabase1 having from about #dd to about #eb per cent of silica4 ,these were followed by rocks with a silica-percentage ranging from about #ej to #fa1 including some characteristic ,norwegian rocks1 like the rhomben-porphyry4 ,the acidity continued to increase1 for in the next series of eruptions the silica-percentage rose to between #fj and #fg1 the characteristic rock being a quartz-syenite4 ,then came deep-seated protrusions of highly acid rocks1 varieties of granite1 containing from #fh to #ge per cent of silica4 ,the youngest eruptive masses in the district show a complete change of character4 ,they are basic dykes 7proterobase1 diabase1 etc474#aa ,footnote #aa3 ,eruptivgest4 ,kristianiageb41 #ahie4 ,the same author institutes a comparison between the post-,silurian eruptive series of ,christiania and that of the ,triassic system in the ,tyrol1 and believes that the two cycles closely agree4#ab ,footnote #ab3 ,op4 cit4 ,he supposes in each case the pre-existence of a parent magma from which the eruptive series started and which had a silica-percentage of about #fd or #fe4 ,in this difficult subject it is of the utmost importance to accumulate fact before proceeding to speculation4 ,during ,tertiary time in ,central ,france more than one cycle may be made out in distinct districts4 ,thus in the ,velay1 during the ,miocene ,period1 volcanic activity began with the outpouring of basalts1 followed successively by trachytes1 labradorites and augitic andesites1 phonolites and basalts4 ,the ,pliocene eruptions showed a reversion to the intermediate types of augitic andesites and trachytes1 followed by abundant basalts1 which continued to be poured forth in ,pleistocene time4#ac ,footnote #ac3 ,m4 ,boule1 ,description ,gologique du ,velay1 ,bull4 ,carte4 ,gol4 ,france1 #ahib4 ,this author draws special attention to the evidence for the alternation of basic and more acid material in the ,tertiary eruptions of ,central ,france4 ,further north1 in ,auvergne1 where the eruptions come down to a later period1 the volcanic sequence appears to have been first a somewhat acid group of lavas 7trachytes or domites71 followed by a series of basalts1 then by andesites and labradorites1 the latest outflows again consisting of basalts4#ad ,footnote #ad3 ,m4 ,michel ,lvy1 ,bull4 ,soc4 ,gol4 ,france1 #ahij1 p4 #gjd4 ,not less striking is the succession of lavas in the ,yellowstone region1 as described by ,mr4 ,iddings4 ,the first eruptions consisted of andesites4 ,these were followed by abundant discharges of basalt1 succeeded by later outflows of andesite1 and of basalt like that previously erupted4 ,after a period of extensive erosion1 occupying a prolonged interval of time1 volcanic energy was renewed by the eruption of a vast flood of rhyolite1 after which came a feebler outflow of basalt that brought the cycle to a close1 though geysers and fumaroles show that the volcanic fires are not yet entirely extinguished below4#ae ,footnote #ae3 ,journal of ,geology1 ,chicago1 i4 7#ahic7 p4 #fjf4 ,see also this author's excellent monograph on ,electric ,peak and ,sepulchre ,mountain1 #abth ,ann4 ,rep4 ,u4,s4 ,geol4 ,survey 7#ahij-ia71 and ,mr4 ,h4 ,w4 ,turner on ,the ,succession of ,tertiary ,volcanic ,rocks in the ,sierra ,nevada of ,north ,america1 #adth ,ann4 ,rep4 ,u4,s4 ,geol4 ,survey 7#ahib-ic71 p4 #dic4 ,but not only is there evidence of a remarkable evolution or succession or erupted material within the volcanic cycle of a single geological period4 ,one of the objects of the present work is to bring forward proofs that such cycles have succeeded each other again and again1 at widely separated intervals1 within the same region4 ,after the completion of a cycle and the relapse of volcanic energy into repose1 there has been a renewal of the previous condition of the subterranean magma1 giving rise ultimately to a similar succession of erupted materials4 ,if we are at a loss to account for the changes in the sequence of lavas during a single volcanic cycle1 our difficulties are increased when we find that in some way the magma is restored each time to somewhat the same initial condition4 ,analogies may be traced between the differentiation which has taken place within a plutonic intrusive boss or sill and the sequence of lavas in volcanic cycles4 ,it can be shown that though the original magma that supplied the intrusive mass may be supposed to have had a fairly uniform composition deep down in its reservoir1 differentiation set in long before the intrusive mass consolidated1 the more basic constituents travelling outwards to the margin and leaving the central parts more acid4 ,if some such process takes place within a lava-reservoir1 it may account for a sequence of variations in composition4 ,but this would not meet all the difficulties of the case1 nor explain the determining cause of the separation of the constituents within the reservoir of molten rock1 whether arising from temperature1 specific gravity1 or other influence4 ,this subject will be further considered in connection with intrusive ,bosses4 ,another fact which may be regarded as now well established is the persistence of composition and structure in the lavas of all ages4 ,notwithstanding the oft-repeated cycles in the character of the magma1 the materials erupted to the surface1 whether acid or basic1 have retained with wonderful uniformity the same fundamental characteristics4 ,no part of the contribution of ,british geology to the elucidation of the history of volcanic action is of more importance than the evidence which it furnishes for this persistent sameness of the subterranean magma4 ,an artificial line has sometimes been drawn between the volcanic products of ,tertiary time and those of earlier ages4 ,but a careful study of the eruptive rocks of ,britain shows that no such line of division is based upon any fundamental differences4 ,the lavas of ,palozoic time have of course been far longer exposed to alterations of every kind than those of the ,tertiary periods1 and certain superficial distinctions may be made between them4 ,but when these accidental differences are eliminated1 we find that the oldest known lavas exhibit the same types of structure and composition that are familiar in those of ,tertiary and recent volcanoes4 ,many illustrations of this statement will be furnished in later chapters4 ,as a particularly striking instance1 ,i may cite here the most ancient and most modern lavas of the ,grand ,caon of the ,colorado4 ,mr4 ,walcott and ,mr4 ,iddings have shown that in the ,lower ,cambrian1 or possibly pre-,cambrian1 formations of that great gorge1 certain basic lavas were contemporaneously interstratified1 which1 in spite of their vast antiquity1 are only slightly different from the modern basalts that have been poured over the surrounding plateau4#af ,footnote #af3 #adth ,annual ,report ,u4,s4 ,geol4 ,survey 7#ahib-ic74 ,the chief lavas found in ,britain4--,of the lavas which have been poured out at the surface within the region of the ,british ,isles1 the following varieties are of most frequent occurrence4 ,in the acid series are ,rhyolites and ,felsites1 but the vitreous forms are probably all intrusive4 ,the intermediate series is represented by ,trachytes and ,andesites 7,porphyrites71 which range from a glassy to a holocrystalline structure4 ,the basic series includes ,dolerites1 ,diabases1 ,basalts1 ,limburgites 7or ,magma-basalts1 containing little or no felspar71 and ,picrites or other varieties of ,peridotites4 ,the intrusive rocks display a greater variety of composition and structure4 ii4 ,,volcanic ,,agglomerates1 ,,breccias ,,and ,,tuffs ,the coarser fragmentary materials thrown from volcanic vents are known as ,agglomerates where they show no definite arrangement1 and especially where they actually fill up the old funnels of discharge4 ,when they have accumulated in sheets or strata of angular detritus outside an active vent they are termed ,breccias1 or if their component stones have been water-worn1 ,conglomerates4 ,the finer ejected materials may be comprehended under the general name of ,tuffs4 ,although these various forms of pyroclastic detritus consist as a rule of thoroughly volcanic material1 they may include fragments of non-volcanic rocks4 ,this is especially the case among those which represent the earliest explosions of a volcano4 ,the first efforts to establish an eruptive vent lead to the shattering of the terrestrial crust1 and the consequent discharge of abundant debris of that crust4 ,the breccias or agglomerates thus produced may contain1 indeed1 little or no truly volcanic material1 but may be made up of fragments of granite1 gneiss1 sandstone1 limestone1 shale1 or whatever may happen to be the rocks through which the eruptive orifice has been drilled4 ,if the first explosions exhausted the energy of the vent1 it may happen that the only discharges from it consisted merely of non-volcanic debris4 ,examples of this kind have been cited from various old volcanic districts4 ,a striking case occurs at ,sepulchre ,mountain in the ,yellowstone ,park1 where the lower breccias1 the product of the earliest explosions of the ,electric ,peak volcano1 and attaining a thickness of #ejj feet1 are full of pieces of the ,archan rocks which underlie the younger formations of that district4#ag ,these non-volcanic stones do not occur among the breccias higher up4 ,obviously1 however1 though most abundant at first1 pieces of the underlying rocks may reappear in subsequent discharges1 wherever by the energy of explosion1 fragments are broken from the walls of a volcanic chimney and hurled out of the crater4 ,illustrations of these features will be given in the account of the ,british ,carboniferous1 ,permian and ,tertiary volcanic rocks4 ,footnote #ag3 ,prof4 ,j4 ,p4 ,iddings1 #abth ,ann4 ,rep4 ,u4,s4 ,geol4 ,survey 7#ahij-ia71 p4 #fcd4 ,it will be obvious that where the component materials of such fragmentary accumulations consist entirely of non-volcanic rocks1 great caution must be exercised in attributing them to volcanic agency4 ,two sources of error in such cases may here be pointed out4 ,in the first place1 where angular detritus has been precipitated into still water1 as1 for instance1 from a crag or rocky declivity into a lake1 a very coarse and tumultuous kind of breccia may be formed4 ,it is conceivable that1 in course of time1 such a breccia may be buried under ordinary sediments1 and may thereby be preserved1 while all trace of its parent precipice may have disappeared4 ,the breccia might resemble some true volcanic agglomerates1 but the resemblance would be entirely deceptive4 ,in the second place1 notice must be taken of the frequent results of movements within the terrestrial crust1 whereby rocks have not only been ruptured but1 as already pointed out1 have been crushed into fragments4 ,in this way1 important masses of breccia or conglomerate have been formed1 sometimes running for a number of miles and attaining a breadth of several hundred feet4 ,the stones1 often in huge blocks1 have been derived from the surrounding rocks1 and while sometimes angular1 are sometimes well-rounded4 ,they are imbedded in a finer matrix of the same material1 and may be scattered promiscuously through the mass1 in such a way as to present the closest resemblance to true volcanic breccia4 ,where the crushed material has included ancient igneous rocks it might deceive even an experienced geologist4 ,indeed1 some rocks which have been mapped and described as volcanic tuffs or agglomerates are now known to be only examples of crush-conglomerates4#ah ,footnote #ah3 ,for an account of crush-conglomerates1 see ,mr4 ,lamplugh's paper on those of the ,isle of ,man1 ,quart4 ,journ4 ,geol4 ,soc41 li4 7#ahie71 p4 #efc4 ,mr4 ,m',henry has pointed to probable cases of mistake of this kind in ,ireland1 ,nature1 #eth ,march #ahif4 ,a4 ,geikie1 ,geol4 ,mag4 ,november #ahif4 ,not only have vast quantities of detritus of non-volcanic rocks been shot forth from volcanic vents1 but sometimes enormous solid masses of rock have been brought up by ascending lavas or have been ejected by explosive vapours4 ,every visitor to the puys of ,auvergne will remember the great cliff-like prominence of granite and mica-schist which1 as described long ago by ,scrope1 has been carried up by the trachyte of the ,puy ,chopine1 and forms one of the summits of the dome 7,fig4 #cdd74 ,the same phenomenon is observable at the ,puy de ,montchar1 where large blocks of granite have been transported from the underlying platform4 ,abich has described some remarkable examples in the region of ,erzeroum4 ,the huge crater of ,palandokn1 #ifhg feet above the sea contains1 in cliff-like projections from its walls as well as scattered over its uneven bottom1 great masses of marmorised limestone and alabaster1 associated with pieces of the green chloritic schists1 serpentines and gabbros of the underlying non-volcanic platform4 ,these rocks1 which form an integral part of the structure of the crater1 have been carried up by masses of trachydoleritic1 andesitic and quartz-trachytic lavas4#ai ,examples will be given in a later chapter showing how gigantic blocks of mica-schist and other rocks have been carried many hundred feet upwards and buried among sheets of lava or masses of agglomerates during the ,tertiary volcanic period in ,britain 7,fig4 #bfb74 ,footnote #ai3 ,abich1 ,geologie des ,armenischen ,hochlandes 7,part ii41 western half71 #ahhb1 p4 #gf4 ,in the vast majority of cases1 the fragmentary substances ejected by ancient volcanic explosions1 like those of the present day1 have consisted wholly or mainly of material which existed in a molten condition within the earth1 and which has been violently expelled to the surface4 ,such ejected detritus varies from the finest impalpable dust or powder up to huge masses of solid rock4 ,these various materials may come from more than one source4 ,where a volcanic orifice is blown out through already solidified lavas belonging to previous eruptions1 the fragments of these lavas may accumulate within or around the vent1 and be gradually consolidated into agglomerate or breccia4 ,again1 explosions within the funnel may break up lava-crusts that have there formed over the cooling upper surface of the column of molten rock4 ,or the uprising lava in the chimney may be spurted out in lumps of slag and bombs1 or may be violently blown out in the form of minute lapilli1 or of extremely fine dust and ashes4 ,although in theory these several varieties of origin may be discriminated1 it is hardly possible always to distinguish them among the products of ancient volcanic action4 ,in the great majority of cases old tuffs1 having been originally deposited in water1 have undergone a good deal of decomposition1 and such early alteration has been aggravated by the subsequent influence of percolating meteoric water4 ,where disintegration has not proceeded too far1 the finer particles of tuffs may be seen to consist of minute angular pieces of altered glass1 or of microlites or crystals1 or of some vitreous or semi-vitreous substance1 in which such microlites and crystals are enclosed4 ,it has already been stated that the occurrence of glass1 or of any substance which has resulted from the devitrification of glass1 may be taken as good evidence of former volcanic activity4 ,most commonly1 especially in the case of tuffs of high antiquity1 like those associated with the ,palozoic formations1 the fresh glassy and microlitic constituents1 so conspicuous in modern volcanic ashes1 are hardly to be recognised4 ,the finer dust which no doubt contained these characteristic substances has generally passed into dull1 earthy1 granular1 or structureless material1 though here and there1 among basic tuffs1 remaining as palagonite4 ,in the midst of this decayed matrix1 the lapilli of disrupted lavas may endure1 but it may be difficult or impossible to decide whether they were derived from the breaking up of older lavas by explosion1 or from the blowing out of the lava-crusts within the funnel4 ,the cellular structure1 which we have seen to be a markedly volcanic peculiarity among the lavas1 is not less so in their fragments among the agglomerates1 breccias and tuffs2 indeed it may be said to be eminently characteristic of them4 ,the vesicles in the lapilli1 bombs1 and blocks are sometimes of large size1 as in masses of ejected slag1 but they range down to microscopic minuteness4 ,the lapilli of many old tuffs are sometimes so crowded with such minute pores1 as to show that they were originally true pumice4 ,the composition of tuffs must obviously depend upon that of the lavas from which they were derived4 ,but their frequently decayed condition makes it less easy1 in their case1 to draw definite boundary-lines between varieties4 ,in a group of acid lavas1 the tuffs may be expected to be also acid1 while among intermediate or basic lavas1 the tuffs will generally be found to correspond4 ,there are1 however1 exceptions to this general rule4 ,as will be afterwards described in detail1 abundant felsitic tuffs may be seen among the andesitic lavas of ,lower ,old ,red ,sandstone age in ,scotland1 and rhyolitic tuffs occur also among the ,tertiary basalts of ,antrim4 ,as a rule1 basic and intermediate tuffs1 like the lavas from which they have been derived1 are rather more prone to decomposition than the acid varieties4 ,one of their most characteristic features is the presence in them of lapilli of a minutely vesicular pumice1 which will be more particularly described in connection with volcanic necks1 of which it is a characteristic constituent4 ,occasional detached crystals of volcanic minerals1 either entire or broken1 may be detected in them1 though perhaps less frequently than in the agglomerates4 ,the earthy matrix is generally greenish in colour1 varying into shades of brick-red1 purple and brown4 ,the acid tuffs are1 on the whole1 paler in colour than the others1 sometimes indeed they are white or pale grey1 but graduate into tones of hmatitic red or brown1 the varying ferruginous tints being indicative of stages in the oxidation of the iron-bearing constituent minerals4 ,small rounded lapilli or angular fragments of felsite or rhyolite may be noticed among them1 sometimes exhibiting the most perfect flow-structure4 ,as typical examples of such tuffs1 ,i may refer to those of the ,pentland ,hills1 near ,edinburgh1 and those that lie between the two groups of basalt in ,antrim4 ,illustration3 ,fig4 #ac4--;,alternations of coarser and finer ,tuff4 ,thrown out promiscuously from active vents1 the materials that form tuffs arrange themselves1 on the whole1 according to relative size over the surface on which they come to rest1 the largest being generally grouped nearest to the focus of discharge1 and the finest extending farthest from it4 ,as the volcanoes of which records have been preserved among the geological formations were chiefly subaqueous1 the fragmentary substances1 as they fell into water1 would naturally be to some extent spread out by the action of currents or waves4 ,they would thus tend to take a more or less distinctly stratified arrangement4 ,moreover1 as during an eruption there might be successive paroxysms of violence in the discharges1 coarser and finer detritus would successively fall over the same spot4 ,in this way1 rapid alternations of texture would arise 7,fig4 #ac74 ,a little experience will enable the observer to distinguish between such truly volcanic variations and those of ordinary sedimentation1 where1 for instance1 layers of gravel and sand repeatedly alternate4 ,besides the volcanic nature of the fragments and their non-water-worn forms1 he will notice that here and there the larger blocks may be placed on end--a position the reverse of that usual in the disposal of aqueous sediments1 but one which is not infrequently assumed by ejected stones1 even when they fall through some little depth of water4 ,further1 the occurrence of large pieces of lava1 scattered at random through deposits of fine tuff1 would lead him to recognize the tumultuous discharges of a volcanic focus1 rather than the sorting and sifting action of moving water4 ,admirable illustrations of these various characteristics may be gathered in endless number from the ,palozoic volcanic chronicles of ,britain4 ,i may especially cite the basin of the ,firth of ,forth as a classical region for the study of ,carboniferous examples4 ,illustration3 ,fig4 #ad4--;,alternations of ,tuff 7t1 t17 with non-volcanic sediment 7l1 l74 ,when the conditions of modern volcanic eruptions are considered1 it will be seen that where ejected ashes and stones fall into water1 they will there mingle with any ordinary sediment that may be in course of deposition at the time4 ,there will thus be a blending of volcanic and non-volcanic detritus1 and the transition between the two may be so insensible that no hard line of demarcation can be drawn4 ,such intermingling has continually taken place during past ages4 ,one of the first lessons learnt by the geologist1 who begins the study of ancient volcanic records1 is the necessity of recognizing this gradation of material1 and likewise the frequently recurring alternations of true tuff with shale1 sandstone1 limestone or other entirely non-volcanic detritus 7,fig4 #ad74 ,he soon perceives that such facts as these furnish him with some of the most striking proofs of the reality and progress of former eruptions4 ,the intermingling of much ordinary detritus with the volcanic material may be regarded as indicative either of comparatively feeble activity1 or at least of considerable distance from the focus of discharge4 ,it is sometimes possible to trace such intermixtures through gradually augmenting proportions of volcanic dust and stones1 until the deposit becomes wholly volcanic in composition1 and so coarse in texture as to indicate the proximity of the eruptive vent4 ,on the other hand1 the gradual decrease of the volcanic ejections can be followed in the upward sequence of a series of stratified deposits1 until the whole material becomes entirely non-volcanic4 ,the occurrence of thin partings of tuff between ordinary sedimentary strata points to occasional intermittent eruptions of ashes or stones1 the vigour and duration of each eruptive interval being roughly indicated by the thickness and coarseness of the volcanic detritus4 ,the pauses in the volcanic activity allowed the deposit of ordinary sediment to proceed unchecked4 ,the nature of such non-volcanic intercalations gives a clue to the physical conditions of sedimentation at the time1 while their thickness affords some indication of the relative duration of the periods of volcanic repose4 ,a little reflection will convince the observer that in such a section as that represented in ,fig4 #ad the volcanic intercalations must be regarded as a mere local accident4 ,evidently the normal conditions of sedimentation at the time these strata were accumulated are indicated by the limestone bands 7l1 l74 ,had there been no volcanic eruptions1 a continuous mass of limestone would have been deposited1 but this continuity was from time to time interrupted by the explosions that gave rise to the intercalated bands of tuff 7t1 t74 ,the application of these rules of geological evidence will be best understood from actual examples of their use4 ,many illustrations of them will be subsequently given1 more especially from the volcanic records of the ,carboniferous period4 ,one of the most interesting peculiarities of interstratified tuffs is the not infrequent occurrence of the remains of plants and animals imbedded in them4 ,such remains would have been entombed in the ordinary sediment had there been no volcanic eruptions1 and their presence in the tuffs is another convincing proof of contemporaneous volcanic action during the deposition of a sedimentary series4 ,but they may be made to furnish much more information as to the chronology of the eruptions and the physical geography of the localities where the volcanoes were active1 as will be set forth farther on4 ,tuffs1 as already remarked1 frequently occur without any accompaniment of lava1 although lavas seldom appear without some tuff4 ,we thus learn that in the past1 as at present1 discharges of fragmentary materials alone were more common than the outflow of lava by itself4 ,the relative proportions of the lavas and tuffs in a volcanic series vary indefinitely4 ,in the ,tertiary basalt-plateaux of ,britain1 the lavas succeed each other1 sheet above sheet1 for hundreds of feet1 with few and trifling fragmental intercalations4 ,among the ,carboniferous volcanic ejections1 on the other hand1 many solitary or successive bands of tuff may be observed without any visible sheets of lava4 ,viewed broadly1 however1 in their general distribution during geological time1 the two great groups of volcanic material may be regarded as having generally appeared together4 ,in all the great volcanic series1 from the base of the ,palozoic systems up to the ,tertiary plateaux1 lavas and tuffs are found associated1 much as they are among the ejections of modern volcanoes4 ,they often alternate1 and thus furnish evidence as to oscillations of energy at the eruptive vents4 ,now and then1 by the explosions from a volcano at the present day1 a single stone may be ejected at such an angle and with such force as to fall to the ground at a long distance from the vent4 ,in like manner1 among the volcanic records of former periods1 we may occasionally come upon a single block of lava imbedded among tuffs or even in non-volcanic strata4 ,where such a stone has fallen upon soft sediment1 it can be seen to have sunk into it1 pressing down the layers beneath it1 and having the subsequently deposited layers heaped over it4 ,an ejected block of this nature is represented among the tuffs shown in ,fig4 #ac4 ,another instance from a group of non-volcanic sediments is given in ,fig4 #ae1 and is selected from a number of illustrations of this interesting feature which have been observed among the ,lower ,carboniferous formations of the basin of the ,firth of ,forth4 ,a solitary block1 imbedded in a series of strata1 would not1 of course1 by itself afford a demonstration of volcanic activity4 ,there are various ways in which such stones may be transported and dropped over a muddy water-bottom4 ,they may1 for example1 be floated off attached to sea-weeds1 or wrapped round by the roots of trees4 ,but where a block of basalt or other volcanic rock has obviously descended with such force as to crush down the deposits on which it fell1 and when lavas and tuffs are known to exist in the vicinity1 there can be little hesitation in regarding such a block as having been ejected from a neighbouring vent1 either during an explosion of exceptional violence or with an unusually low angle of projection4 ,illustration3 ,fig4 #ae4--;,ejected block of ,basalt which has fallen among ,carboniferous shales and limestones1 shore1 ,pettycur1 ,fife4 ,in conclusion1 reference may conveniently be made here to another variety of fragmental volcanic materials which cannot always be satisfactorily distinguished from true tuffs1 although arising from a thoroughly non-volcanic agency4 ,where a mass of lava has been exposed to denudation1 as1 for instance1 when a volcanic island has been formed in a lake or in the sea1 the detritus worn away from it may be spread out like any other kind of sediment4 ,though derived from the degradation of lava1 such a mechanical deposit is not properly a tuff1 nor can it even be included among strictly volcanic formations4 ,it may be called a volcanic conglomerate1 rhyolitic conglomerate1 diabase sandstone1 felsitic shale1 or by any other name that will adequately denote its composition and texture4 ,but it may not afford proof of strictly contemporaneous volcanic activity4 ,all that we are entitled to infer from such a deposit is that1 at the time when it was laid down1 volcanic rocks of a certain kind were exposed at the surface and were undergoing degradation4 ,but the date of their original eruption may have been long previous to that of the formation of the detrital deposit from their waste4 ,nevertheless1 it is sometimes possible to make sure that the conglomerate or sandstone1 though wholly due to the mechanical destruction of already erupted lavas1 was in a general sense contemporaneous with the volcanoes that gave forth these lavas4 ,the detrital formation may be traced perhaps up to the lavas from which it was derived1 and may be found to be intercalated in the same sedimentary series with which they are associated4 ,or it may contain large bombs and slags1 such as most probably came either directly from explosions or from the washing down of cinder-cones or other contemporaneously existing volcanic heaps4 ,examples of such intercalated conglomerates will be given from the ,lower ,old ,red ,sandstone of ,central ,scotland and from the ,tertiary volcanic plateaux of the ,inner ,hebrides4 ,,chapter ,,iv ,materials erupted at the ,surface--,extrusive ,series--continued4 iii4 ,types of old ,volcanoes--#a4 ,the ,vesuvian ,type2 #b4 ,the ,plateau or ,fissure ,type2 #c4 ,the ,puy ,type4 iv4 ,determination of the relative ,geological ,dates of ancient ,volcanoes4 v4 ,how the ,physical ,geography associated with ancient ,volcanoes is ascertained4 ,having now taken note of the various materials ejected to the surface from volcanic orifices1 we may pass to the consideration of these orifices themselves1 with the view of ascertaining under what various conditions volcanic action has taken place in the geological past4 ,we have seen that modern and not long extinct volcanoes may be grouped into three types1 and a study of the records of ancient volcanoes shows that the same types may be recognized in the eruptions of former ages4 ,the following chapters will supply many illustrations of each type from the geological history of the ,british ,isles4 ,in dealing with these illustrations1 however1 we must ever bear in mind the all-powerful influence of denudation4 ,we ought not to expect to meet with the original forms of the volcanoes4 ,some little reflection and experience may be required before we can realize under what aspect we may hope to recognize ancient and much-denuded volcanoes4 ,it may therefore be of advantage to consider here1 in a broad way1 which of the original characters are most permanent1 and should be looked for as mementoes of ancient volcanoes after long ages of denudation4 iii4 ,,types ,,of ,,old ,,volcanoes ,the three forms of ancient volcanoes now to be discussed are--#ast1 the ,vesuvian type2 #bnd1 the ,plateau or ,fissure type2 and #crd1 the ,puy type4 #a4 ,the ,vesuvian ,type4--,in this kind of volcano1 lavas and fragmental ejections are discharged from a central vent1 which is gradually built up by successive eruptions of these materials4 ,as the cone increases in size1 parasitic cones appear on its sides1 and in the energy and completeness of their phenomena become true volcanoes1 almost rivalling their parent mountain4 ,streams of lava descend upon the lower grounds1 while showers of dust and ashes are spread far and wide over the surrounding country4 ,if a transverse section could be made of a modern ,vesuvian cone1 the volcanic pile would be found to consist of alternations of lavas and tuffs1 thickest at the centre1 and thinning away in all directions4 ,at some distance from the crater1 these volcanic materials might be seen to include layers of soil and remains of land-vegetation1 marking pauses between the eruptions1 during which soil accumulated and plants sprang up upon it4 ,where the lavas and ashes had made their way into sheets of fresh water or into the sea1 they would probably be found interstratified with layers of ordinary sediment containing remains of the animal or vegetable life of the time4 ,illustration3 ,fig4 #af4--;,effects of denudation on a ,vesuvian cone4 ,conceive now the effects of prolonged denudation upon such a pile of volcanic rocks4 ,the cone will eventually be worn down1 the crater will disappear1 and the only relics of the eruptive orifice may be the top of the central lava-column and of any fragmental materials that solidified within the vent 7,fig4 #af74 ,the waste will1 on the whole1 be greater at the cone than on the more level areas beyond4 ,it might1 in course of time1 reach the original surface of the ground on which the volcano built up its heap of ejected material4 ,the central lava-plug might thus be left as an isolated eminence rising from a platform of older non-volcanic rocks1 and the distance between it and the dwindling sheets of lava and tuff which came out of it would then be continually increased as their outcrop receded under constant degradation4 ,this piece of volcanic history is diagrammatically illustrated in ,fig4 #af4 ,the original forms of the central volcano and of its parasitic cones are suggested by the dotted lines in the upper half of the ,figure4 ,all this upper portion has been removed by denudation1 and the present surface of the ground is shown by the uppermost continuous line4 ,the general structure of the volcanic pile is indicated underneath that line--the lenticular sheets of lava and tuff 7l1 l71 the dykes 7d1 d71 and the lavas 7p1 p7 and agglomerates 7a1 a7 of the central vent and of the subordinate cones4 ,the waste1 though greatest on the higher ground of the great cone1 would not stop there4 ,it would extend over the flatter area around the volcano4 ,streams flowing over the plain would cut their way down through the lavas and tuffs1 eroding ravines in them1 and leaving them in detached and ever diminishing outliers on the crests of the intervening ridges4 ,we can easily picture a time when the last of these relics would have been worn away1 and when every vestige of the volcanic ejections would have been removed1 save the lava-column marking the site of the former vent4 ,every stage in this process of effacement may be recognized in actual progress among the extinct volcanoes of the earth's surface4 ,probably nowhere may the phenomena be more conveniently and impressively studied than among the volcanic districts of ,central ,france4 ,on the one hand1 we meet there with cinder-cones so perfect that it is hard to believe them to have been silent ever since the beginnings of history4 ,on the other hand1 we see solitary cones of agglomerate or of lava1 which have been left isolated1 while their once overlying and encircling sheets of ejected material have been so extensively worn away as to remain merely in scattered patches capping the neighbouring hills4 ,valleys many hundreds of feet in depth have been cut by the rivers through the volcanic sheets and the underlying ,tertiary strata and granite since the older eruptions ceased4 ,and yet these eruptions belong to a period which1 in a geological sense1 is quite recent4 ,it is not difficult to contemplate a future time1 geologically not very remote1 when in the valley of the ,loire not a trace will remain of the wonderfully varied and interesting volcanic chronicle of that district1 save the plugs that will mark the positions of the former active vents4 ,in the ,british ,islands1 ancient volcanoes of the ,vesuvian type are well represented among the ,palozoic systems of strata4 ,their preservation has been largely due to the fact that they made their appearance in areas that were undergoing slow subsidence4 ,their piles of erupted lava and ashes were chiefly heaped up over the sea-floor1 and were buried under the sand1 silt and ooze that gathered there4 ,thus covered up1 they were protected from denudation4 ,it is only in much later geological ages that1 owing to upheaval1 gradual degradation of the surface1 and removal of their overlying cover of stratified formations1 they have at last been exposed to waste4 ,the process of disinterment may be observed in many different stages of progress4 ,in some localities1 only the tops of the sheets of lava and tuff have begun to show themselves2 in others1 everything is gone except the indestructible lava-plug4 ,these inequalities of denudation arise not only from variations in the durability of volcanic rocks1 but still more from the relative position of these rocks in the terrestrial crust1 and the geological period at which1 in the course of the general lowering of the surface1 they have been laid bare4 ,not only are volcanic rocks of many different ages1 and lie1 therefore1 on many successive platforms within the crust of the earth3 their places have been still further dependent upon changes in the arrangement of that crust4 ,fracture1 upheaval1 depression1 curvature1 unconformable deposition of strata1 have contributed to protect some portions1 while leaving others exposed to attack4 ,hence it happens that the volcanic record varies greatly in its fulness of detail from one geological system or one district to another4 ,some chapters have been recorded with the most surprising minuteness1 so that the events which they reveal can be realized as vividly as those of a modern volcano4 ,others1 again1 are meagre and fragmentary1 because the chronicle is still for the most part buried underground1 or because it has been so long exposed at the surface that only fragments of it now remain there4 ,in the descriptions which will subsequently be given of ancient ,british volcanoes of the ,vesuvian type1 it will be shown that at many successive periods during ,palozoic time1 and at many distinct centres1 lavas and tuffs have been piled up to a depth of frequently more than #ejjj feet--that is to say1 higher than the height of ,vesuvius4 ,sometimes the vent from which these materials were ejected can be recognized4 ,in other places1 it is still buried under later formations1 or has been so denuded as to be represented now merely by the column of molten or fragmental rock that finally solidified in it4 ,examples will be quoted of such ancient vents1 measuring not less than two miles in diameter1 with subsidiary necks on their flanks1 like the parasitic cones on ,etna4 ,i shall show that while the ejected volcanic products have accumulated in greatest depth close to the vent that discharged them1 they die away as they recede from it1 sometimes so rapidly that a volcanic pile which is #gjjj feet thick around its source may entirely thin out and disappear in a distance of not more than ten or twelve miles4 ,i shall point out how1 as the lavas and tuffs are followed outwards from their centre1 they not only get thinner1 but are increasingly interstratified among the sedimentary deposits with which they were coeval1 and that in this way their limits1 their age1 and the geographical conditions under which they were accumulated can be satisfactorily fixed4 ,as illustrations of the ,vesuvian type in the volcanic history of ,britain1 ,i may refer to the great ,lower ,silurian volcanoes of ,cader ,idris1 ,arenig1 ,snowdon and the ,lake ,district1 and to the ,old ,red ,sandstone volcanoes of ,central ,scotland4 #b4 ,the ,plateau or ,fissure type is1 among modern volcanoes1 best developed in ,iceland1 as will be more fully detailed in ,chapter xl4 ,in that island1 during a volcanic eruption1 the ground is rent open into long parallel fissures1 only a few feet or yards in width1 but traceable sometimes for many miles1 and descending to an unknown depth into the interior4 ,from these fissures lava issues--in some cases flowing out tranquilly in broad streams from either side1 in other cases issuing with the discharge of slags and blocks of lava which are piled up into small cones set closely together along the line of the rent4 ,it was from a fissure of this kind that the great eruption of #aghc took place--the most stupendous outpouring of lava within historic time4 ,by successive discharges of lava from fissures1 or from vents opening on lines of fissure1 wide plains may be covered with a floor of rock hundreds or thousands of feet in thickness1 made up of horizontal beds4 ,the original topography1 which might have been undulating and varied1 is completely buried under a vast level lava-desert4 ,the rivers which drained the country before the beginning of the volcanic history will have their channels filled up1 and will be driven to seek new courses across the lava-fields4 ,again and again1 as fresh eruptions take place1 these streams will be compelled to shift their line of flow1 each river-bed being in turn sealed up in lava1 with all its gravels1 silts and drift-wood4 ,but the rain will continue to fall1 and the drainage to seek its way seaward4 ,when the last eruption ceases1 and the rivers are at length left undisturbed at their task of erosion1 they will carve that lava-floor into deep gorges or open valleys4 ,where they flow between the lavas and the slopes against which these ended1 they will cut back the volcanic pile1 until in course of time the lavas will present a bold mural escarpment to the land that once formed their limit4 ,the volcanic plain will become a plateau1 ending off in this vertical wall and deeply trenched by the streams that wind across it4 ,and if the denudation is continued long enough1 the plateau will be reduced to detached hills1 separated by deep and wide valleys4 ,illustration3 ,fig4 #ag4--,section to illustrate the structure of the ,plateau type4 ,this geological history is illustrated by the diagram in ,fig4 #ag4 ,the stippled ground underneath 7x1 x7 represents the original undulating surface of the country on which the plateau eruptions were poured out4 ,the lavas of these eruptions are shown by the horizontal lines to have entirely buried the heights and hollows of the old land1 and to have risen up to the upper dotted line1 which may be taken to mark the limit reached by the accumulation of volcanic material4 ,the dark lines 7d1 d7 which come up through the bedded lavas indicate the dykes with their connected vents4 ,denudation has since stripped off the upper part of the volcanic series down to the uppermost continuous black line which represents the existing surface of the ground4 ,the level sheets of lava have been deeply trenched1 and in one instance the valley has not only been cut through the volcanic pile1 but has been partly eroded out of the older rocks below4 ,to the right and left1 the lavas end off abruptly in great escarpments4 ,the succession of events here depicted has occurred more than once in ,britain4 ,the ,plateau type is chiefly developed in this country among the great ,tertiary basalt districts of ,antrim and the ,inner ,hebrides1 which reappear in the ,faroe ,islands1 and again still farther north in ,iceland4 ,but it also occurs among the volcanic rocks of the ,old ,red ,sandstone and ,carboniferous periods4 ,as compared with the other volcanic types1 that of the ,plateaux is distinguished by the wide extent of surface which its rocks cover1 by the great preponderance of lavas over tuffs1 and by the regularity and persistence of the individual sheets of rock4 ,in ,britain1 the plateau-lavas are even still often approximately horizontal1 and lie piled on each other in tolerably regular beds to a thickness of #ajjj1 and in one place to more than #cjjj feet4 ,they form wide level or gently undulating tablelands1 which rise in bold escarpments from the surrounding country and have been deeply carved into valleys4 ,the sides of their cliffs and slopes are marked by parallel lines of terrace1 arising from the outcrop of successive sheets of lava 7,figs4 #aa1 #bfe74 ,with the ,tertiary basalt-plateaux are connected thousands of dykes1 that follow each other along nearly parallel lines in a general north-westerly direction1 and mark the position of fissures up which the molten lava ascended4 ,occasional necks of agglomerate or basalt indicate the sites of some of the eruptive vents4 ,the ,carboniferous volcanic plateaux have been more extensively denuded than those of ,tertiary age1 so that a large number of their vents have been laid bare4 ,in general these vents are of comparatively small size1 though larger than those of the ,carboniferous ,puys4 ,in some districts1 abundant dykes traverse the rocks on which the plateaux rest1 though the fissures seem to have been less numerous than in ,tertiary time4 #c4 ,the ,puy type1 as before remarked1 takes its name from the well-known puys1 or volcanic cones1 of ,central ,france4 ,volcanoes of this type form conical hills1 generally of small size1 consisting usually of fragmental materials1 sometimes of lava4 ,where a cone is partially effaced by a second1 and even by a third1 successive slight shiftings of the vent are to be inferred 7see ,figs4 #bi and #bad74 ,in many cases1 no lava has issued from such cones1 nor were the ashes and cinders dispersed far from the vent4 ,hence1 in the progress of denudation1 cones of this kind are easily effaced4 ,from the uniformity of composition of their materials1 the simplicity and regularity of their forms1 and their small size1 it may be inferred that many of these cones were the products of single eruptions4 ,they may conceivably have been thrown up in a few days1 or even in a single day4 ,the history of ,monte ,nuovo1 in the ,bay of ,naples1 which was formed within twenty-four hours in the year #aech1 is a memorable example of the rapidity with which a cone more than #djj feet high may be thrown up at some distance from a central vent4 ,the smallest independent volcanoes are included in the ,puy type4 ,in many instances the diameter of the funnel has not exceeded a few yards2 the largest examples of the type seldom exceed #ajjj feet in width4 ,where lavas have been discharged1 as well as ashes and stones1 a more vigorous activity is indicated than where merely cones of tuff were formed4 ,the lavas may come from more than one side of a cone1 and may flow in opposite directions for a distance of several miles4 ,it is observable that considerable streams of lava have issued from the base of a cinder-cone without disturbing it4 ,the molten rock has found a passage between the loose materials and the surface on which they rest1#bj though1 in some cases1 the cone may have been thrown up after the emission of the lava4 ,footnote #bj3 ,m4 ,boule1 ,bull4 ,carte ,gol4 ,france1 ,no4 #bh1 tome iv4 p4 #bcb4 ,in the history of a puy there is commonly a first discharge of fragmentary material2 then lava may flow out1 followed by a final discharge of loose stones and ashes4 ,hence the products of such a vent group themselves into three layers--two of breccia separated by an intervening sheet of lava4#ba ,footnote #ba3 ,m4 ,boule1 ,bull4 ,carte ,gol4 ,france1 ,no4 #bh1 tome iv4 ,great changes are wrought on puys and their connected lavas and tuffs during the progress of denudation4 ,the cones are eventually destroyed1 and only a stump of agglomerate or lava is left to mark its place4 ,the connection of a lava-stream with its parent vent may likewise be effaced1 and the lava itself may be reduced to merely a few separate patches1 perhaps capping a ridge1 while the surrounding ground has been hollowed into valleys4 ,if the waste continues long enough1 even these outliers will disappear1 and nothing but the neck or stump of the little volcano will remain4 ,illustration3 ,fig4 #ah4--;,diagram illustrating the structure and denudation of ,puys4 ,the accompanying diagram 7,fig4 #ah7 may help to make these changes more intelligible4 ,the upper dotted lines show the original forms of three puys with the covering of loose materials discharged by them over the surrounding ground4 ,the lower shaded portion represents the surface as left by denudation1 and a section of the three vents beneath that surface4 ,the whole of the cones and craters has here been swept away1 and only the volcanic neck is in each case left4 ,in the vent to the right1 the material that fills it up is a coarse agglomerate1 which projects as a rounded dome above the surrounding country4 ,the central pipe is filled with fragmentary materials1 through which molten rock has risen1 giving off dykes and veins4 ,in the vent to the left hand1 only lava is seen to occupy the orifice1 representing the column of molten rock which solidified there and brought the activity of this little volcano to an end4 ,it will be observed that in each of these volcanic hills the present outlines are very far from being those of the original volcano1 and that the eminence projects because of its greater resistance to the forces of denudation that have not only removed the superficial volcanic material1 but have made some progress in lowering the level of the ground on which that material was accumulated4 ,the typical area for the study of ,puys is the extraordinarily interesting volcanic region of ,central ,france4 ,there the volcanic cones are clustered in irregular groups1 sometimes so close as to be touching each other2 elsewhere separated by intervals of several miles4 ,they may be traced in all stages of decay1 from the most perfect cones and craters to the isolated eminence that marks the site of a once active chimney4 ,their lavas1 too1 may be seen as detached fragments of plateaux1 many hundred feet above the valleys that have been excavated since they flowed4#bb ,footnote #bb3 ,see ,desmarest's classic map and his papers in ,mem4 ,acad4 ,roy4 ,sciences1 ,paris1 #aggd1 #aggi2 ,journ4 de ,physique1 #aggi2 ,scrope's ,geology of ,central ,france1 #ahbg1 and ,extinct ,volcanoes of ,central ,france1 #aheh2 ,lecoq's poques ,gologiques de l',auvergne1 #ahfg2 ,m4 ,michel ,lvy1 ,bull4 ,soc4 ,gol4 ,france1 #ahij1 p4 #fhh2 ,m4 ,boule1 ,bull4 ,carte ,gol4 ,france1 ,no4 #bh1 tome iv4 #ahib4 ,another well-known region of modern ,puys is that of the ,eifel1 where the cones and craters are often so fresh that it is difficult to believe them to be prehistoric4#bc ,one of the most remarkable denuded puy-regions in ,europe covers a wide territory in the ,swabian ,alps of ,wrtemberg4 ,no fewer than #abe denuded necks filled with tuff1 agglomerate and basalt have there been mapped and described4 ,they are of higher antiquity than the ,upper ,miocene strata1 and have thus probably been exposed to prolonged denudation4 ,in external aspect and internal structure they present the closest parallel to the ,carboniferous and ,permian necks of ,britain described in ,books ,,vi4 and ,,vii4 of the present work4#bd ,among the ,palozoic volcanoes of ,britain many admirable illustrations of the ,puy type are to be found4 ,their cones are almost always entirely gone1 though traces of them occasionally appear4 ,the necks that show the position of the vents are in some districts crowded together as thickly as those of ,auvergne4 ,during the ,carboniferous and ,permian periods in ,central ,scotland1 clusters of such little volcanoes must have risen among shallow lagoons and inland sheets of water1 casting out their ashes and pouring forth their little streams of lava over the water-bottom around them and then dying out4 ,as these eruptions took place in a region that was gradually subsiding1 the cones and their ejected ashes and lavas were one by one submerged1 the looser materials being washed down and spread out among the silt1 sand or mud1 and enveloping the remains of any plants or animals that might be strewn over the floor of the lake or sea4 ,hence the ,puys of ,palozoic time in ,britain have been preserved with extraordinary fulness of detail4 ,they have been dissected by denudation1 both among the hills of the interior and along the margin of the sea4 ,their structure can thus be in some respects made out even more satisfactorily than that of the much younger and more perfect cones of ,central ,france4 ,footnote #bc3 ,the ,eifel district has been fully described by ,hibbert1 ,von ,dechen1 and other writers4 ,von ,dechen's little handbooks to the ,eifel and ,siebengebirge are useful guides4 ,footnote #bd3 ,these ,wrtemberg vents have been elaborately described and discussed by ,professor ,w4 ,branco of ,tbingen in his ,schwabens #abe ,vulkan-,embryonen und deren tufferflte ,ausbruchsrhren1 das grsste ,gebiet chemaliger ,maare auf der ,erde1 ,stuttgart1 #ahid4 iv4 ,,determination ,,of ,,the ,,relative ,,geological ,,dates ,,of ,,ancient ,,volcanoes ,in themselves1 accumulations of volcanic materials do not furnish any exact or reliable evidence of the geological period in which they were erupted4 ,the lavas of the early ,palozoic ages may1 indeed1 on careful examination1 be distinguished from those of ,tertiary date1 but1 as we have seen1 the difference is rather due to the effects of age and gradual alteration than to any inherent fundamental distinction between them4 ,in all essential particulars of composition and internal structure1 the lavas of the ,cambrian or ,silurian period resemble those of ,tertiary and modern volcanoes4 ,the igneous magmas which supply volcanic vents thus appear to have been very much what they are now from early geological epochs4 ,at least no important difference1 according to relative age1 has yet been satisfactorily established among them4 ,but although the rocks themselves afford no precise or trustworthy clue to their date1 yet where they have been intercalated contemporaneously among fossiliferous stratified formations1 of which the geological horizon can be determined from included organic remains1 it is easy to assign them to their exact place in geological chronology4 ,a determination of this kind is only an application of the general principle on which the sequence of the geological record is defined4 ,a few illustrations will suffice to make this point quite obvious4 ,among the volcanic tuffs in the upper part of ,snowdon various fossils occur1 which are identical with those found in the well-known ,bala ,limestone4 ,as the accepted reading of such evidence1 we conclude that these tuffs must therefore be of the same geological age as that limestone4 ,now the position of this seam of rock has been well established as a definite horizon in the series of ,lower ,silurian formations4 ,and we consequently without hesitation place the eruptions of the ,snowdon volcano on that same platform1 and speak of them as belonging to the ,bala division of the ,lower ,silurian period4 ,again1 in ,west ,lothian the tuffs and lavas ejected from many scattered puys were interstratified among shales and limestones in which the characteristic fossils of the ,carboniferous ,limestone are abundant4 ,there cannot1 therefore1 be any doubt that these eruptions were much younger than those of ,snowdon1 and that they took place at the time when the ,carboniferous ,limestone was being deposited4 ,we thus speak of them as belonging to volcanoes which were active in that early part of the ,carboniferous period to which the thick ,mountain ,limestone of ,ireland and ,derbyshire belongs4 ,as yet another illustration of the determination of geological age1 an example from the plateau-type of eruption may be given4 ,the great basalt-plateaux of ,antrim and the ,inner ,hebrides are built up of lavas that lie unconformably on the ,chalk4 ,they are thus proved to be later than the ,cretaceous system1 and this deduction would hold true even if no organic remains were found associated with the volcanic rocks4 ,but here and there1 intercalated between the basalts1 lie layers of shale1 limestone and tuff containing well-preserved remains of plants which are recognizable as older ,tertiary forms of vegetation4 ,this fossil evidence definitely places the date of the eruptions in older ,tertiary time4 ,it is clear that1 proceeding on this basis of reasoning1 we may arrange the successive volcanic eruptions of any given district1 make out their order of sequence in time1 and thus obtain materials for a consecutive history of them4 ,or1 proceeding from that district into other regions1 we may compare its volcanic phenomena with theirs1 determine the relative dates of their respective eruptions1 and in this way compile a wider history of volcanic action in past time4 ,it is on these principles that the general and detailed chronology of the volcanic rocks of the ,british ,isles has been worked out1 and that the following chapters have been arranged4 v4 ,,how ,,the ,,physical ,,geography ,,associated ,,with ,,ancient ,,volcanoes ,,is ,,ascertained ,while the materials erupted from old volcanic vents tell plainly enough their subterranean origin1 they may leave us quite in the dark as to the conditions under which they were thrown out at the surface4 ,yet a careful examination of the strata associated with them may throw much light on the circumstances in which the eruptions took place4 ,many of the results of such examination will be given in subsequent chapters4 ,i will here submit illustrations of how four different phases of physical geography during former volcanic eruptions may be satisfactorily determined4 ,illustration3 ,fig4 #ai4--,section illustrating submarine eruptions2 alternations of lavas and tuffs with limestones and shales full of marine organisms4 #a4 ,submarine ,eruptions4--,as by far the largest accessible part of the crust of the earth consists of old marine sediments1 it is natural that the volcanic records preserved in that crust should be mainly those of submarine eruptions4 ,that many lavas during the geological past were poured out upon the sea-bottom is plainly shown by the thick beds of marine organisms which they have overspread and which lie above them 7,fig4 #ai74 ,in ,central ,scotland1 for example1 sheets of basalt have flowed over a sea-bottom on which thick groves of crinoids1 bunches of coral and crowds of sea-shells were living4 ,not less striking is the evidence supplied by bands of tuff4 ,around ,limerick1 for instance1 the thick ,carboniferous ,limestone is interrupted by many thin layers of tuff marking intervals when showers of volcanic dust fell over the sea-bottom1 killing off the organisms that lived there4 ,but the limestone that overlies these volcanic intercalations is again crowded with fossils1 proving that the crinoids1 corals and shells once more spread over the place and flourished as abundantly as ever above the tuff4 ,the accompanying diagram 7,fig4 #ai7 illustrates these statements4 ,at the bottom a thick mass of limestone 7l7 full of crinoids1 corals1 brachiopods and other marine organisms bears witness to a long time of repose1 when the clear sea-water teemed with life4 ,at last a volcanic explosion took place1 which threw out the first seam of tuff 7t74 ,but this was only a transient interruption1 for the accumulation of calcareous sediment was immediately resumed1 and the next band of limestone was laid down4 ,thereafter a more prolonged or vigorous eruption ejected a larger mass of dust and stones1 which fell over the bottom and prevented the continuation of the limestone4 ,but that the sea still abounded in life is shown by the numerous organisms imbedded in the second stratified band of tuff4 ,at last an access of volcanic vigour gave vent to a stream of slaggy lava1 which rolled over the sea-bottom and solidified in the thick sheet of amydaloidal basalt marked ,b4 ,this outflow was followed by a further discharge of ashes and stones1 which1 from their absence of stratification1 may be supposed to have been the result of a single explosion1 or at least to have fallen too rapidly for the marine currents to rearrange them in layers4 ,when the water cleared1 the abundant sea-creatures returned1 and from their crowded remains limestone once more gathered over the bottom4 ,yet the volcanic history had not then reached its close1 for again there came a discharge of ashes1 followed by the outpouring of a second lava1 which consolidated as a sheet of columnar basalt 7,b'74 ,it is not necessary1 in order to prove the eruptions to have been submarine1 that organic remains should be found in the tuffs or between them4 ,if the volcanic ejections are intercalated among strata which elsewhere can be proved to be marine1 their discharge must obviously have taken place under the sea4 ,the vent that discharged them may have raised its head above the sea-level1 but its lavas and tuffs were spread out over the adjoining sea-floor4 #b4 ,lacustrine ,eruptions4--,the same line of evidence furnishes proof that some volcanoes arose in inland sheets of water4 ,if their products are interstratified among sandstones1 gravels and shell-marls1 wherein the remains of land-plants1 insects and lacustrine shells1 are preserved1 we may be confident that the eruptions took place in or near to some lake-basin4 ,the older lavas and tuffs of ,central ,france supply an instructive example of such an association4 ,in ,britain1 the abundant and extensive outpouring of lavas and tuffs during the time of the ,lower ,old ,red ,sandstone probably occurred in large lakes4 ,among the sediments of these bodies of water1 interstratified between the volcanic sheets1 remains of land-plants are abundant1 together with1 here and there1 those of myriapods washed down from the woodlands1 and of many forms of ganoid fishes4 ,illustration3 ,fig4 #bj4--;,diagram illustrating volcanic eruptions on a river-plain4 #c4 ,fluviatile ,eruptions4--,volcanoes have sometimes arisen on river-plains or on the edges of valleys and gorges1 and have poured out their lavas and discharged their ashes over the channels or alluvial lands of the streams4 ,volcanic materials1 usurping the water-channels1 bury or are interstratified with fluviatile sand or shingle1 containing perhaps remains of the vegetation or animal life of the surrounding land4 ,there may thus be a constant shifting of the river-courses1 and a consequent deposit of fluviatile sediment at many successive levels among the lavas and tuffs4 ,in ,fig4 #bj some of these changes are indicated in a series of bedded lavas 7l74 ,the lower part of the diagram shows the dying out of a bed of river gravel 7g7 against the sloping end of a lava-stream1 and the sealing up of this intercalation by a fresh outpouring of lava4 ,higher up in the diagram a section is shown of a gully or ravine which has been cut out of the lavas by a stream1 and has become choked up with water-worn detritus4 ,subsequent outflows of lava have rolled over this channel and sealed it up4 ,examples of such intercalations of lava with old river deposits1 and of the burying of water-courses1 will be cited in the account of the ,tertiary volcanic plateaux of ,britain in ,chapter xxxviii4 #d4 ,terrestrial ,eruptions4--,that volcanoes in former times broke out on land as well as in water may readily be expected4 ,but it is obvious that the proofs of a terrestrial origin may not be always easy to obtain1 for every land-surface is exposed to denudation2 and thus the relics of the eruptions of one age may be effaced by the winds1 rains1 frosts and rivers of the next4 ,in assigning any volcanic group to a terrestrial origin1 we may be guided partly by negative evidence1 such as the absence of all trace of marine organisms in any of the sedimentary layers associated with the group4 ,but such evidence standing by itself would not be satisfactory or sufficient4 ,if1 however1 between the sheets of lava there occur occasional depressions1 filled with hardened sediment full of land-plants1 with possibly traces of insects and other terrestrial organisms1 we may with some confidence infer that these silted-up hollows represent pools or lakes that gathered on the surface of the lava-sheets1 and into which the vegetation of the surrounding ground was blown or washed4 ,rain falling on the rugged surface of a lava-field would naturally gather into pools and lakes1 as the bottoms of the hollows became puddled by the gradual decay of the rock and the washing of fine silt into the crevices of the lava4 ,illustration3 ,fig4 #ba4--;,diagram illustrating volcanic eruptions on a land-surface4 ,again1 it may be expected that prolonged exposure to the air would give rise to disintegration of the lava and to the consequent formation of soil4 ,terrestrial vegetation would naturally spring up on such soil2 trees might take root upon it4 ,hence1 if another lava-flood deluged the surface1 the soil and its vegetable mantle would be entombed under the molten rock4 ,these geological changes are represented diagrammatically in ,fig4 #ba4 ,two hollows among the lavas are there shown to have been filled with silt1 including successive layers of vegetation now converted into coal4 ,one of the soils 7s7 is marked between the lavas1 and the charred stump of a tree with its roots still in another layer of soil higher up is seen to have been engulphed in the overlying sheet of melted rock4 ,admirable illustrations of this succession of events are to be encountered among the great ,tertiary basaltic plateaux which cover so large an area in the north-west of ,europe4 ,not only has no trace of any marine organism been found among their interstratified sedimentary layers1 but they have yielded a terrestrial flora which is preserved in hollows between the successive sheets of basalt4 ,a full account of these rocks will be given in ,book ,,viii4 ,,chapter ,v ,underground ,phases of ,volcanic ,action4 ,b4 ,materials injected or consolidated beneath the ,surface--,intrusive ,series3 ,i4 ,vents of ,eruption--i4 ,necks of ,fragmentary ,materials2 ii4 ,necks of ,lava-form ,materials2 iii4 ,distribution of ,vents in relation to ,geological ,structure-,lines2 iv4 ,metamorphism in and around ,volcanic ,cones1 ,solfataric ,action2 v4 ,inward ,dip of ,rocks towards ,necks2 vi4 ,influence of contemporaneous ,denudation upon ,volcanic ,cones2 vii4 ,stages in the ,history of old ,volcanic ,vents4 ,in our profound ignorance of the nature of the earth's interior1 we know as yet nothing certain regarding the condition and distribution there of those molten materials which form the prime visible source of volcanic energy4 ,by the study of volcanoes and their products we learn that the fused substances are not everywhere precisely the same and do not remain absolutely uniform1 even in the same volcanic region4 ,but in what manner and from what causes these variations arise is still unknown4 ,we are further aware that the molten magma1 under a centre of volcanic disturbance1 manifests from time to time energetic movements which culminate in eruptions at the surface4 ,but what may be the exciting cause of these movements1 to what depth they descend1 and over what extent of superficies they spread1 are matters regarding which nothing better than conjecture can as yet be offered4 ,it is true that1 in some cases1 a magma of fairly uniform composition has been erupted over a vast tract of the earth's surface1 and must have had a correspondingly wide extent within the terrestrial crust4 ,thus in the case of the older ,tertiary volcanic eruptions of ,north-,western ,europe1 basalt of practically the same composition was discharged from thousands of fissures and vents distributed from the south of ,antrim northward beyond the ,inner ,hebrides1 through the chain of the ,faroe ,islands and over the whole breadth of ,iceland4 ,under the ,british ,isles alone1 the subterranean reservoirs of molten lavas must have been at least #dj1jjj square miles in united area4 ,if they stretched continuously northwards below the ,faroe ,islands and ,iceland1 as is highly probable1 that is1 for #fjj miles further1 their total extent may have been comparable to such a region as ,scandinavia4 ,was this vast underground body of lava part of a universal liquid mass within the globe1 or was it rather of the nature of one or more lakes or large vesicles within the crust8 ,we can only offer speculation for answer4 ,on the other hand1 there seems to be good proof that in some districts1 both now and in former geological periods1 such differences exist between the materials ejected from vents not far distant from each other as to show the existence of more limited distinct reservoirs of liquid rock underneath4 ,some of the questions here asked will be further dealt with in later pages in connection with such geological evidence as can be produced regarding them4 ,but it will be found that at every step in the endeavour to ascertain the origin of volcanic phenomena difficulties present themselves which are now and may long remain insoluble4 ,i4 ,vents of ,eruption ,it is a general belief that the first stage in the formation of a volcano of the ,vesuvian type by the efforts of subterranean energy is the rending of the terrestrial crust in a line of fissure4 ,some of the most remarkable groups of active volcanoes on the face of the globe are certainly placed in rows1 as if they had risen along some such great rents4 ,the actual fissure1 however1 is not there seen1 and its existence is only a matter of probable inference4 ,undoubtedly the effect of successive eruptions must be to conceal the fissure1 even if it ever revealed itself at the surface4 ,what is supposed to have marked the initial step in the formation of a great volcano is occasionally repeated in the subsequent history of the mountain4 ,during the convulsive shocks that precede and accompany an eruption1 the sides of the cone1 and even sometimes part of the ground beyond1 are rent open1 occasionally for a distance of several miles1 and on the fissures thus formed minor volcanoes are built up4 ,it is in ,iceland1 as already stated1 that the phenomena of fissures are best displayed4 ,there the great deserts of lava are from time to time dislocated by new lines of rent1 which ascend up to the surface and stretch for horizontal distances of many miles4 ,from these long narrow chasms lava flows out to either side2 while cones of slag and scori usually form upon them4 ,this interesting eruptive phase will be more fully described in the chapters dealing with the ,tertiary volcanic rocks of ,britain4 ,there can be no doubt1 however1 that in a vast number of volcanic vents of all geological periods no trace can be discovered of their connection with any fissure in the earth's crust4 ,such fissures may indeed exist underneath1 and may have served as passages for the ascent of lava to within a greater or less distance from the surface4 ,but it is certain that volcanic energy has the power of blowing out an opening for itself through the upper part of the crust without the existence of any visible fissure there4 ,what may be the limits of depth at which this mode of communication with the outer air is possible we do not yet know4 ,they must obviously vary greatly according to the structure of the terrestrial crust on the one hand1 and the amount and persistence of volcanic energy on the other4 ,we may suppose that where a fissure terminates upward under a great depth of overlying rock1 the internal magma may rise up to the end of the rent1 and even be injected laterally into the surrounding parts of the crust1 but may be unable to complete the formation of a volcano by opening a passage to the surface4 ,but where the thickness of rock above the end of the fissure is not too great1 the expansive energy of the vapours absorbed in the magma may overcome the resistance of that cover1 and blow out an orifice by which the volcanic materials can reach the surface4 ,in the formation of new cones within the historic period at a distance from any central volcano1 the existence of an open fissure at the surface has not been generally observed4 ,when1 for example1 ,monte ,nuovo was formed1 it rose close to the shore among fields and gardens1 but without the appearance of any rent from which its materials were discharged4 ,that in innumerable instances during the geological past1 similar vents have been opened without the aid of fissures that reached the surface1 will be made clear from the evidence to be drawn from the volcanic history of the ,british ,isles4 ,so abundant1 indeed1 are these instances that they may be taken as proving that1 at least in the ,puy type of volcanoes1 the actual vents have generally been blown out by explosions rather than by the ascent of fissures to the open air4 ,in cases where1 as in ,iceland1 fissures open at the surface and discharge lava there1 the channel of ascent is the open space between the severed walls of the rent4 ,within this space the lava will eventually cool and solidify as a dyke4 ,it is obvious that a comparatively small amount of denudation will suffice to remove all trace of the connection of such a dyke with the stream of lava that issued from it4 ,among the thousands of dykes belonging to the ,tertiary period in the ,british ,islands1 it is probable that many may have served as lines of escape for the basalt at the surface4 ,but it is now apparently impossible to distinguish between those which had such a communication with the outer air and those that ended upward within the crust of the earth4 ,the structure of dykes will be subsequently discussed among the subterranean intrusions of volcanic material4 ,in an ordinary volcanic orifice the ground-plan is usually irregularly circular or elliptical4 ,if that portion of the crust of the earth through which the vent is drilled should be of uniform structure1 and would thus yield equally to the effects of the volcanic energy1 we might anticipate that the ascent and explosion of successive globular masses of highly heated vapours would give rise to a cylindrical pipe4 ,but in truth the rocks of the terrestrial crust vary greatly in structure2 while the direction and force of volcanic explosions are liable to change4 ,hence considerable irregularities of ground-plan are to be looked for among vents4 ,some of these irregularities are depicted in ,fig4 #bb1 which represents the ground plan of some vents from the ,carboniferous volcanic districts of ,scotland4 ,they are all drawn on the same scale4 ,other examples will be cited in later chapters from the same and other parts of the ,british ,isles4 ,some of the most marked departures from the normal and simple type of vent occur where two orifices have been opened close to each other1 or where the same vent has shifted its position 7,figs4 #bi1 #abe1 #bje1 and #bad74 ,curiously irregular or elongated forms may thus arise in the resultant necks now visible at the surface4 ,many striking examples of these features may be seen among the ,carboniferous and ,permian volcanoes to be afterwards described4 ,occasionally where an open fissure has served as a vent it has given rise to a long dyke-like mass 7,no4 #a in ,fig4 #bb74 ,illustration3 ,fig4 #bb4--;,ground-plans of some ,volcanic vents from the ,carboniferous districts of ,scotland4 #a4 ,linhope ,burn1 near ,mosspaul1 ,roxburghshire2 the shaded parts are intrusions of trachytic material4 #b4 ,hazelside ,hill1 two miles ,w4 from ,newcastleton1 ,roxburghshire4 #c4 ,st4 ,magdalen's1 ,linlithgow4 #d4 ,south-west side of ,coom's ,fell 7see ,fig4 #agd74 #e4 ,neck on ,greatmoor1 ,roxburghshire4 #f4 ,pester ,hill1 ,tarras ,water4 #g4 ,head of ,routing ,burn1 ,s4,e4 side of ,hartsgarth ,fell1 ,liddesdale4 #h4 ,hartsgarth ,flow1 ,liddesdale4 ,the size of a volcanic vent may vary indefinitely from a diameter of not more than a yard or two up to one or two or more miles4 ,as a rule1 the smaller the vents the more numerously are they crowded together4 ,in the case of large central volcanoes like ,etna1 where many subsidiary vents1 some of them forming not inconsiderable hills1 may spring up along the sides of the parent cone1 denudation will ultimately remove all the material that was heaped up on the surface1 and leave the stumps or necks of the parasitic vents in groups around the central funnel4 ,each volcanic chimney1 by which vapours1 ashes or lava are discharged at the surface1 may be conceived to descend in a more or less nearly vertical direction until it reaches the surface of the lava whence the eruptions proceed4 ,after the cessation of volcanic activity1 this pipe will be left filled up with the last material discharged1 which will usually take the form of a rudely cylindrical column reaching from the bottom of the crater down to the lava-reservoir4 ,it will be obvious that no matter how great may be the denudation of the volcano1 or how extensive may be the removal of the various materials discharged over the surrounding ground1 the pipe or funnel with its column of solid rock must still remain4 ,no amount of waste of the surface of the land can efface that column4 ,successively lower and yet lower levels may be laid bare in it1 but the column itself goes still further down4 ,it will continue to make its appearance at the surface until its roots are laid bare in the lava of the subterranean magma4 ,hence1 of all the relics of volcanic action1 the filled-up chimney of the eruptive vent is the most enduring4 ,save where it may have been of the less deep-seated nature of a hornito upon a lava-stream1 we may regard it as practically permanent4 ,the full meaning of these statements will be best understood from a consideration of the numerous illustrations to be afterwards given4 ,the stumps of volcanic columns of this nature1 after prolonged denudation1 generally project above the surrounding ground as rounded or conical eminences known as ,necks 7,fig4 #bc4 ,see also ,figs4 #eb1 #hb1 #ajb1 #aji1 #abc1 #acc1 #add1 #agh1 #aib1 #aie1 #bjc1 #bjd1 #bji1 #bid1 #bih1 #cjf and #caj74 ,their outlines1 however1 vary with the nature of their component materials4 ,the softer rocks1 such as tuffs and agglomerates1 are apt to assume the form of smooth domes or cones1 while the harder and especially the crystalline rocks rise into irregular1 craggy hills4 ,occasionally1 indeed1 it may happen that a neck makes no prominence on the surface of the ground1 and its existence may only be discoverable by a careful examination of the geological structure of the locality4 ,now and then an old vent will be found not to form a hill1 but to sink into a hollow4 ,such variations1 however1 have little or no reference to original volcanic contours in the history of the localities which display them4 ,they arise mainly from the differing hardness and structure of the materials that have filled the vents1 and the consequent diversity in the amount of resistance which they have offered to the progress of denudation4 ,illustration3 ,fig4 #bc4--,view of an old volcanic ,neck 7,the ,knock1 ,largs1 ,ayrshire1 a vent of ,lower ,carboniferous age74 ,the materials now found in volcanic funnels are of two kinds3 #ast1 ,fragmentary1 derived from volcanic explosions2 and #bnd1 ,lava-form1 arising from the ascent and consolidation of molten rock within the funnel4 i4 ,necks of ,fragmentary ,materials ,by far the most satisfactory evidence of a former volcanic orifice is furnished by a neck of fragmentary materials4 ,where bosses of crystalline rock rise to the surface and assume the outward form of necks1 we cannot always be certain that they may not have been produced by subterranean intrusions that never effected any connection with the surface4 ,in other words1 such bosses may not mark volcanic orifices at all1 though they may have been part of the underground protrusions of volcanoes in their neighbourhood4 ,but where the chimney has been filled with debris1 there can be no doubt that it truly marks the site of a once active volcano4 ,the fragmentary material is an eloquent memorial of the volcanic explosions that drilled the vent1 kept it open1 and finally filled it up4 ,these explosions could not have taken place unless the elastic vapours which caused them had found an escape from the pressure under which they lay within the crust of the earth4 ,now and then1 indeed1 where the outpouring of lava or some other cause has left cavernous spaces within the crust1 there may conceivably be some feeble explosion there1 and some trifling accumulation of fragmentary materials4 ,but we may regard it as practically certain that the mass of tumultuous detritus now found in volcanic necks could not have been formed unless where a free passage had been opened from the molten magma underneath to the outer surface of the planet4 ,considerable diversity may be observed in the nature and arrangement of the fragmentary materials in volcanic necks4 ,the chief varieties may be arranged in four groups3 7#a7 ,necks of non-volcanic detritus2 7#b7 ,necks of volcanic agglomerate or tuff2 7#c7 ,necks of agglomerate or tuff with a central plug of lava2 and 7#d7 ,necks of agglomerate or tuff with veins1 dykes or some lateral irregular mass of lava4 7#a7 ,necks of non-volcanic ,detritus4--,during the first convulsive efforts of a volcanic focus to find a vent at the surface1 the explosions that eventually form the orifice do so by blowing out in fragments the solid rocks of the exterior of the terrestrial crust4 ,of the detritus thus produced1 shot up the funnel and discharged into the air1 part may gather round the mouth of the opening and build up there a cone with an enclosed crater1 while part will fall back into the chimney1 either to accumulate there1 should the explosions cease1 or to be thrown out again1 should they continue4 ,in the feeblest or most transient kinds of volcanic energy1 the explosive vapours may escape without any accompanying ascent of the molten magma to the surface1 and even without any sensible discharge of volcanic ashes from that magma4 ,in such cases1 as ,i have already pointed out1 the detritus of the non-volcanic rocks1 whatever they may be1 through which volcanic energy has made an opening1 accumulate in the pipe and eventually consolidate there4 ,examples of this nature will be adduced in later chapters from the volcanic districts of ,britain4 ,where only non-volcanic materials fill up a vent we may reasonably infer that the eruptions were comparatively feeble1 never advancing beyond the initial stage when elastic vapours made their escape with explosive violence1 but did not lead to the outflow of lava or the discharge of ashes4 ,in the great majority of necks1 however1 traces of the earliest eruptions have been destroyed by subsequent explosions1 and the uprise of thoroughly volcanic fragments4 ,yet even among these fragments1 occasional blocks may be detected which have been detached from the rocks forming the walls of the funnel4 ,the general name of ,agglomerate1 as already stated1 is given to all accumulations of coarse1 usually unstratified1 detritus in volcanic funnels1 irrespective of the lithological nature of the materials4 ,for further and more precise designation1 when an agglomerate is mainly made up of fragments of one particular rock1 the name of that rock may be prefixed as sandstone-agglomerate1 granite-agglomerate1 basalt-agglomerate1 trachyte-agglomerate4 ,volcanic agglomerate is a useful general term that may include all the coarser detritus ejected by volcanic action4 ,where volcanic explosions have been of sufficient violence or long continuance1 the upper part of the funnel may be left empty1 and on the cessation of volcanic activity1 may be filled with water and become a lake4 ,the ejected detritus left round the edge of the orifice sometimes hardly forms any wall1 the crater-bottom being but little below the level of the surrounding ground4 ,explosion-lakes are not infrequent in ,central ,france and the ,eifel 7,maare74 ,a more gigantic illustration is afforded by the perfectly circular crater of ,coon ,butte in ,arizona1 about #djjj feet in diameter and #fjj feet deep4 ,it has been blown out in limestone1 the debris of which forms a rampart #bjj feet high around it4 ,examples will afterwards be cited from the ,tertiary volcanic plateaux of ,north-,western ,europe4 ,vents may also be formed by an engulphment or subsidence of the material1 like that which has taken place at the great lava cauldron of ,hawaii1 still an active volcano4 ,the picturesque ,crater ,lake of ,oregon is an admirable instance of this structure4 7#b7 ,necks of ,agglomerate or ,tuff4--,in the vast majority of cases1 the explosions that clear out a funnel through the rocks of the upper part of the crust do not end by merely blowing out these rocks in fragments4 ,the elastic vapours that escape from the molten lava underneath are usually followed by an uprise of the lava within the pipe4 ,relieved from the enormous pressure under which it had before lain1 the lava as it ascends is kept in ebullition1 or may be torn into bombs which are sent whirling up into the air1 or may even be blown into the finest dust by the sudden expansion of the imprisoned steam4 ,if its ascent is arrested within the vent1 and a crust is formed on the upper surface of the lava-column1 this congealed crust may be disrupted and thrown out in scattered pieces by successive explosions1 but may re-form again and again4 ,illustration3 ,fig4 #bd4--,section of neck of agglomerate1 rising through sandstones and shales4 ,in many vents1 both in recent and in ancient times1 volcanic progress has never advanced beyond this early stage of the ejection of stones and dust4 ,the column of lava1 though rising near enough to the surface to supply by its ebullition abundant pyroclastic detritus1 coarse and fine1 has not flowed out above ground1 nor even ascended to the top of the funnel4 ,it may have formed1 at the surface1 cones of stones and cinders with enclosed craters4 ,but thereafter the eruptions have ceased4 ,the vents1 filled up with the fragmentary ejected material1 have given passage only to hot vapours and gases4 ,as these gradually ceased1 the volcanoes have become finally extinct4 ,denudation has attacked their sides and crests4 ,if submerged in the sea or a lake1 the cones have been washed down1 and their materials have been strewn over the bottom of the water4 ,if standing on the land1 they have been gradually levelled1 until perhaps only the projecting knob or neck of solidified rubbish in each funnel has remained to mark its site4 ,the buried column of compacted fragmentary material will survive as the only memorial of the eruptions 7,fig4 #bd4 ,for views of necks formed of agglomerate or tuff see ,figs4 #bc1 #hb1 #ajb1 #abc1 #add1 #agh1 #aib1 #bjc1 #bjd1 #bji1 #baj1 #bab1 #baf74 ,the volcanic agglomerates of such vents sometimes include1 among their non-volcanic materials1 pieces of rock which bear evidence of having been subjected to considerable heat 7see vol4 ii4 p4 #gh74 ,carbonaceous shales1 for instance1 have had their volatile constituents driven off1 limestones have been converted into marble1 and a general induration or baking may be perceptible4 ,in other cases1 however1 the fragments exhibit no sensible alteration4 ,fossiliferous limestones and shales often retain their organic remains so unchanged that specimens taken out of the agglomerate cannot be distinguished from those gathered from the strata lying in situ outside4 ,some stones have evidently been derived from a deeper part of the chimney1 where they have been exposed to a higher temperature than others1 or they may have been lain longer within the influence of hot ascending vapours4 ,the volcanic materials in agglomerate range in size from the finest dust to blocks several yards in length1 with occasionally even much larger masses4 ,the proportions of dust to stones vary indefinitely1 the finer material sometimes merely filling in the interstices between the stones1 at other times forming a considerable part of the whole mass4 ,the stones of an agglomerate may be angular or subangular1 but are more usually somewhat rounded4 ,many of them are obviously pieces that have been broken from already solid rock and have had their edges rounded by attrition1 probably by knocking against each other and the walls of the chimney as they were hurled up and fell back again4 ,their frequently angular shapes negative the supposition that they could have been produced by the discharge of spurts of still liquid lava4 ,as already stated1 they have probably been in large measure derived from the violent disruption of the solidified cake or crust on the top of the column of lava in the pipe4 ,many of them may have been broken off from the layer of congealed lava that partially coated the rough walls of the funnel after successive uprises of the molten material4 ,among them may be observed many large and small blocks that appear to have been derived from the disruption of true lava-streams1 as if beds of lava had been pierced in the formation of the vent1 or as if those that congealed on the slopes of the cone had been broken up by subsequent explosions4 ,these fragments of lava are sometimes strongly amygdaloidal4 ,a characteristic feature1 indeed1 of the blocks of volcanic material in the agglomerates is their frequent cellular structure4 ,many of them may be described as rough slags or scori4 ,these have generally come from the spongy crust or upper part of the lava where the imprisoned steam1 relieved from pressure1 is able to expand and gather into vesicles4 ,less frequently evidence is obtainable that the blocks were partially or wholly molten at the time of expulsion4 ,sometimes1 for example1 a mass which presents on one side such a broken face as to indicate that it came from already solidified material1 will show on the other that its steam-vesicles have been pulled out in such a way as to conform to the rounded surface of the block4 ,this elongation could only take place in lava that was not yet wholly consolidated4 ,it seems to indicate that such blocks were derived from a thin hardened crust lying upon still molten material1 and that they carried up parts of that material with them4 ,as each stone went whirling up the funnel into the open air1 its melted part would be drawn round the gyrating mass1 and would rapidly cool there4 ,in other cases1 we encounter true volcanic bombs1 that is1 rounded or bomb-shaped blocks of lava1 with their vesicles elongated all round them and conforming to their spherical shape4 ,sometimes such blocks are singularly vesicular in the centre1 with a more close-grained crust on the outside4 ,their rapid centrifugal motion during flight would allow of the greater expansion of the dissolved steam in the central part of each mass1 while the outer parts would be quickly chilled1 and would assume a more compact texture4 ,bombs of this kind are met with among ancient volcanic products1 and1 like those of modern volcanoes1 have obviously been produced by the ejection of spurts or gobbets of lava from the surface of a mass in a state of violent ebullition4 ,occasionally they are hollow inside1 the rotation in these cases having probably been exceptionally rapid4 ,passing from the larger blocks to the smaller fragments1 we notice the great abundance of nut-like subangular or rounded pieces of lava in the agglomerates4 ,these include lumps of fine grain not specially vesicular1 and probably derived from the disruption of solidified rock4 ,but in many agglomerates1 especially those associated with the outpouring of basalts or other basic lavas 7as those of ,carboniferous and ,tertiary age described in later chapters71 they comprise also vast numbers of very finely cellular material or pumice4 ,these pumiceous lapilli have been already alluded to as ingredients of the stratified tuffs4 ,but they are still more characteristic of the necks1 and reach there a larger size1 ranging from the finest grains up to lumps as large as a hen's egg1 or even larger4 ,the peculiar distinctions of this ejected pumice are the extreme minuteness of its vesicles1 their remarkable abundance1 their prevalent spherical forms1 and the thinness of the walls which separate them4 ,in these respects they present a marked contrast to the large irregularly-shaped steam-cavities of the outflowing lavas1 or even of the scori in the agglomerates4 ,this characteristic minutely vesicular pumice is basic in composition4 ,where not too much decayed1 it may be recognized as a basic glass4 ,thus among the remarkable agglomerates which fill up the ,pliocene or ,pleistocene vents of the ,velay1 the fragments consist of a dark very basic glass1 which encloses such a multitude of minute steam-cavities that1 when seen under the microscope1 they are found to be separated from each other by walls so thin that the slice looks like a pattern of delicate lace4#be ,in necks of earlier date1 such as those of older ,tertiary1 and still more of ,palozoic1 time1 the glass has generally been altered into some palagonitic material4 ,footnote #be3 ,m4 ,boule1 ,bull4 ,cart4 ,gol4 ,france1 ,no4 #bh1 tome iv4 7#ahib7 p4 #aic4 ,this finely pumiceous substance appears to be peculiar to the vents and to the deposits of tuff immediately derived from them4 ,it is not found1 so far as ,i know1 among any of the superficial lavas1 and1 of course1 would not be looked for among intrusive rocks4 ,it was evidently a special product of the volcanic chimney1 as distinguished from the mass of the magma below4 ,we may perhaps regards it as in some way due to a process of quiet simmering within the vent1 when the continual passage of ascending vapours kept the molten lava there in ebullition1 and gave it its special frothy or finely pumiceous character4 ,the compacted dust1 sand or gravelly detritus found in necks1 and comprised under the general name of ,tuff1 consists partly of the finer particles produced during the violent disruption of already solidified rocks1 partly of the detritus arising from the friction and impact of stones ascending and descending above an active vent during times of eruption1 and partly of the extremely light dust or ash into which molten lava may be blown by violent volcanic explosions4 ,in old volcanic necks1 where the rocks have long been subjected to the influence of percolating meteoric water1 it is not perhaps possible to discriminate1 except in a rough way1 the products from these three sources4 ,the more minutely comminuted material has generally undergone considerable alteration1 so that under the microscope it seldom reveals any distinctive structures4 ,here and there in a slide1 traces may occasionally be detected of loose volcanic microlites1 though more usually these can only be found in lapilli of altered glass or finely pumiceous lava4 ,the composition of the detritus in a neck of agglomerate or tuff has almost always a close relation to that of any lavas which may have been emitted from that vent4 ,if the lavas have been of an acid character1 such as rhyolites1 felsites or obsidians1 the pyroclastic materials will almost always be found to be also acid4 ,where1 on the other hand1 the lavas have been intermediate or basic1 so also will be the tuffs and agglomerates4 ,occasionally1 however1 as has already been pointed out1 from the same or closely adjoining vents lavas of very different chemical composition have been successively erupted4 ,felsites or rhyolites have alternated with diabases1 basalts or andesites4 ,in such cases1 a commingling of acid and basic detritus may be observed1 as1 for example1 among the volcanoes of the ,old ,red ,sandstone4 ,it has even happened sometimes that such a mixture of material has taken place when only one class of lavas has been poured out at the surface1 as in the agglomerates that fill vents among the basalts of the ,inner ,hebrides4 ,but we may be sure that1 though not discharged at the surface1 the lavas of which pieces are found in the tuffs must have risen high enough in the vents to be actually blown out in a fragmentary form4 ,the occurrence of felsitic fragments among the otherwise basic agglomerates of ,mull and ,skye will be described in subsequent pages1 likewise the intercalation of rhyolitic detritus between the basalts of ,antrim4 ,a similar association occurs among the modern vents of ,iceland4 ,among the contents of the tuffs and agglomerates that occupy old volcanic vents1 some are occasionally to be observed of which the source is not easily conjectured4 ,detached crystals of various minerals sometimes occur abundantly which were certainly not formed in situ1 but must have been ejected as loose lapilli with the other volcanic detritus4 ,where these crystals belong to minerals that enter into the composition of the lavas of the district in which they are found1 they may be regarded as having probably been derived from the explosion of such lavas in the vents1 the molten magma being blown into dust1 and its already formed crystals being liberated and expelled as separate grains4 ,but it seems to be extremely rare to find any neighbouring lava in which the minerals in question are so largely and so perfectly crystallized as they are in these loose crystals of the neck4 ,the beautifully complete crystals of augite found in the old tuffs of ,vesuvius and on the flanks of ,stromboli may be paralleled among ,palozoic tuffs and agglomerates in ,britain4 ,thus the necks belonging to the ,arenig and ,llandeilo volcanoes of southern ,scotland are sometimes crowded with augite1 varying from minute seed-like grains up to perfectly formed crystals as large as hazel nuts4 ,the conditions under which such well-shaped idiomorphic minerals were formed were probably different from those that governed the cooling and consolidation of the ordinary lavas4 ,but besides the minerals that may be claimed as belonging to the volcanic series of a district1 others occur not infrequently in some tuff-necks1 the origin of which is extremely puzzling4 ,such are the large felspars1 micas1 garnets and the various gems that have been obtained from necks4 ,the large size of some of these crystals and their frequently perfect crystallographic forms negative the idea that they can1 as a rule1 be derived from the destruction of any known rocks1 though they may sometimes be conceivably the residue left after the solution of the other constituents of a rock by the underground magma1 like the large residual felspars enclosed in some dykes4 ,the crystals in question1 however1 seem rather to point to some chemical processes still unknown1 which1 in the depths of a volcanic focus1 under conditions of pressure and temperature which we may speculate about but can perhaps hardly ever imitate in our laboratories1 lead to the elaboration of the diamond1 garnet1 sahlite1 smaragdite1 zircon and other minerals4#bf ,examples of such foreign or deep-seated crystals will be described from the probably ,permian necks of ,central ,scotland4 ,footnote #bf3 ,for lists of the minerals found in the diamond-bearing necks of ,kimberley1 see ,m4 ,boutan in ,frmy's ,encyclopdie ,chimique 7#ahhf71 vol4 ii4 p4 #afh2 ,dr4 ,m4 ,bauer's ,edelsteinkunde 7#ahie71 p4 #bbc4 ,whatsoever may be the source and nature of the fragmentary materials that fill old volcanic vents1 they present1 as a general rule1 no definite arrangement in the necks4 ,blocks of all sizes are scattered promiscuously through the agglomerate1 just as they fell back into the chimney and came to rest there4 ,the larger masses are placed at all angles1 or stand on end1 and are sometimes especially conspicuous in the centre of a neck1 though more usually dispersed through the whole4 ,such a thoroughly tumultuous accumulation is precisely what might be expected where explosions have taken place in still liquid and in already consolidated lavas1 and where the materials1 violently discharged to the surface1 have fallen back and come finally to rest in the chimney of the volcano4 ,nevertheless1 this absence of arrangement sometimes gives place to a stratification which becomes more distinct in proportion as the material of the vent passes from coarse agglomerate into fine tuff4 ,it is possible that the existence and development of this structure depend on the depth at which the materials accumulate in the funnel4 ,we may conceive1 for instance1 that in the lower parts of the chimney1 the stones and dust1 tumultuously falling and rebounding from projections of the rugged walls1 will hardly be likely to show much trace of arrangement1 though even there1 if the explosions continue to keep an open though diminishing passage in the vent1 alternations of coarser and finer layers1 marking varying phases of eruptivity1 may be formed in the gradually heightening pile of agglomerate4 ,rude indications of some such alternations may sometimes be detected in what are otherwise quite unstratified necks4 ,in the upper part of a volcanic funnel1 however1 close to and even within the crater1 the conditions are not so unfavourable to the production of a stratified arrangement4 ,as the pipe is filled up1 and the activity of eruption lessens1 explosions may occur only from the very middle of the orifice4 ,the debris that falls back into the vent will gather most thickly round the walls1 whence it will slide down to the central1 still eruptive hole4 ,it will thus assume a stratified arrangement1 the successive layers lying at the steepest angles of repose1 or from #cj to #ce1 and dipping down in an inverted conical disposition towards the centre4 ,if the process should continue long enough1 the crater itself may be partially or completely filled up with detritus 7,fig4 #be74 ,of this gradual infilling of a volcanic chimney with stratified agglomerate and tuff1 examples belonging to different geological periods will be cited in subsequent chapters4 ,i may here especially allude to one of the most recently observed and best marked illustrations1 which occurs on the west side of ,strom1 in the ,faroe ,islands 7see ,figs4 #caj1 #caa1 #cab74 ,a neck has there been filled up with coarse agglomerate1 which is rudely stratified1 the layers dipping steeply into the centre1 where the tumultuous assemblage of large blocks no doubt points to the final choking up of the diminished orifice of explosion4 ,the walls of the neck are nearly vertical1 and consist of the bedded basaltic lavas through which the vent has been opened4 ,they terminate upward in a conical expansion1 evidently the old crater1 which has subsequently been filled up by the inroads of several lava-streams from adjacent vents4 ,it is here manifest that the bedded agglomerate belongs to the uppermost part of the volcanic funnel4 ,illustration3 ,fig4 #be4--,neck filled with stratified tuff4 ,a4 ground plan2 ,b4 transverse section4 ,where vents have been filled up with tuff rather than with agglomerate1 the stratified structure is best developed4 ,alternations of coarser and finer detritus give rise to more or less definite layers1 which1 though inconstant and irregular1 serve to impart a distinctly stratified character to the mass4 ,where there has been no subsequent disturbance within a vent1 these layers show the same inward dip towards the centre just referred to1 at the ordinary angles of repose4 ,now and then1 where a neck with this structure has been laid bare on a beach1 its denuded cross-section presents a series of concentric rings of strata from the walls towards the centre4 ,good illustrations of these features are supplied by the probably ,permian necks of eastern ,fife 7,figs4 #be ,a and #bag74bg ,footnote #bg3 ,see also the sections of vents on the west coast of ,strom ,faroes1 above referred to4 ,it has frequently happened1 however1 that1 owing to subsidence of the materials filling up the vents or to later volcanic disturbances1 the compacted tuffs have been broken up and thrown into various positions1 large masses being even placed on end4 ,among the ,carboniferous and ,permian necks of ,central ,scotland such dislocated and vertical tuffs are of common occurrence 7see ,figs4 #ade1 #bah74 ,if1 as is probable1 we are justified in regarding the stratified parts of necks as indicative of the uppermost parts of volcanic funnels1 not far from the surface1 the importance of this inference will be best understood when the ,carboniferous and ,permian volcanoes are described4 7#c7 ,necks with a central ,lava-plug4--,some vents of agglomerate or tuff are pierced by a plug of lava1 as may be instructively seen in many of the ,carboniferous and ,permian necks of the centre and south of ,scotland 7,fig4 #bf2 compare also ,figs4 #adh1 #agd1 #bjg1 and #bbf74 ,where this structure shows itself1 the contrast in hardness and durability between the more destructible fragmentary material and the solid resisting lava leads to a topographical distinction in the outer forms of necks4 ,the smooth declivities of the friable tuffs are crowned or interrupted by more craggy features1 which mark the position of the harder intrusive rock4 ,illustration3 ,fig4 #bf4--,section of neck of agglomerate 7a a7 with plug of lava 7b74 ,the plug1 like the pipe up which it has risen1 is in general irregularly circular in ground-plan4 ,it may be conceived to be a column of rock1 descending to an unknown depth into the interior1 with a casing of pyroclastic debris surrounding it4 ,it may vary considerably in the proportion which its cross-section bears to that of the surrounding fragmental material4 ,sometimes it does not occupy more than a small part of the whole1 often appearing in the centre4 ,in other cases1 it more than equals all the rest of the material in the vent1 while instances may be noted where only occasional patches of tuff or agglomerate are visible between the lava-plug and the wall of the pipe4 ,from these we naturally pass to the second type of vent1 where no fragmentary material is to be seen1 but where the chimney is now entirely filled with some massive once-molten rock4 ,a neck with a lava-plug probably contains the records of two stages in volcanic progress1 the first of which1 indicated by the tuff or agglomerate1 was confined to the discharge of fragmentary materials2 while the second1 shown by the lava-plug1 belonged to the time when1 after the earlier explosions1 lava ascended in the vent and solidified there1 thus bringing the eruptions from that particular orifice to an end4 ,where a small central column of lava rises through the tuff1 we may suppose that the funnel had been mainly choked up by the accumulation in it of ejected detritus1 which was compacted to a solid mass adhering to the wall of the funnel1 but leaving a central orifice to be kept open by the gradually waning energy of the volcano4 ,by a final effort that impelled molten rock up that duct and allowed it to consolidate there1 the operations of the vent were brought to a close4 ,where1 on the other hand1 only occasional strips of tuff or agglomerate are to be found between the lava-plug and the wall of the pipe1 the last uprise of lava may be supposed to have been preceded by more vigorous explosions which cleared the throat of the volcano1 driving out the accumulated detritus and leaving only scattered patches adhering to the sides of the funnel4 ,there is1 no doubt1 some downward limit to the production of fragmentary material1 and if we could lay bare successive levels in the chimney of a volcano we should find the agglomerate eventually replaced entirely by lava4 ,the materials of the lava-plugs vary widely in composition4 ,sometimes they are remarkably basic1 and present rocks of the picrite or limburgite type2 in other cases they are thoroughly acid rocks such as felsite and granophyre4 ,many intermediate varieties may be found between these extremes4 ,it is noteworthy that1 in districts where the lavas erupted to the surface have been andesitic or basaltic1 the material which has finally solidified in the vents is often more acid in composition1 trachytic rocks being specially frequent4 ,illustration3 ,fig4 #bg4--,section of agglomerate neck 7a a7 with dykes and veins 7b b74 7#d7 ,necks with ,dykes1 ,veins1 or irregular intrusions of ,lava4--,while the presence of a central plug of lava in a neck of fragmental material may indicate that the vent was still to some extent open1 there is another structure which seems to point to the ascent of lava after the funnel has been choked up4 ,numerous instances have been observed where lava has been forced upward through rents in a mass of tuff or agglomerate1 and has solidified there in the form of dykes or veins 7,fig4 #bg74 ,illustrations of this structure abound among the ,carboniferous and ,permian necks of ,britain4 ,here1 again1 though on a less marked scale1 the contrast in the amount and character of the weathering of the two groups of rock gives rise to corresponding topographical features1 which are especially observable in cliffs and coast-sections1 where the dykes and veins project out of the tuffs as dark prominent walls 7,figs4 #ace1 #adi1 #aff1 #afh1 #bai1 #bba1 #bbb74 ,these intrusive injections are generally irregular in their forms1 the lava having evidently been driven through a mass of material which1 not having yet consolidated sufficiently to acquire a jointed structure1 afforded few dominant lines of division along which it could ascend4 ,now and then1 however1 sharply defined dykes or veins1 which at a distance look like dark ribbons1 may be seen running vertically or at a high angle1 and with a straight or wavy course1 through the fine compacted tuff of a vent4 ,frequently the injected material has found its readiest line of ascent along the walls of the funnel1 between the tuff and the surrounding rocks4 ,occasionally it has made its way into rents in these rocks1 as well as into the body of the neck4 ,it is worthy of remark in passing that complete consolidation of the fragmentary material does not appear to be always requisite in order to allow of the formation of such fissures as are needed for the production of dykes4 ,a singularly interesting illustration of this fact may be seen on the northern crest of the outer crater of the ,puy ,pariou in ,auvergne4 ,a dyke of andesite #h or #aj feet broad may there be traced running for a distance of about #cjj yards through the loose material of the cone4 ,the rock is highly vesicular1 and the vesicles have been elongated in the direction of the course of the dyke so as to impart a somewhat fissile structure to the mass4 ,there can be little doubt that the dykes and veins which traverse necks of agglomerate belong to one of the closing phases in the history of the vents in which they occur4 ,they could only have been injected after the pipes had been so choked up that explosions had almost or entirely ceased1 and eruptions had consequently become nearly or quite impossible4 ,they show1 however1 that volcanic energy still continued to manifest itself by impelling the molten magma into these extinct funnels1 while at the same time it may have been actively discharging materials from other still open vents in the same neighbourhood4 ,with regard to the composition of these dykes and veins1 it may be remarked that in a district of acid lavas they may be expected to be felsitic or rhyolitic1 sometimes granophyric4 ,where1 on the other hand1 the lavas poured out at the surface have been intermediate or basic1 the veins in the necks may be andesites1 basalts or other still more basic compounds4 ,but it is observable1 as in the case of the lava-plugs1 that the injections into the necks may be much more acid than any of the superficial lavas4 ,the advent of acid material in the later part of a volcano's history has been already alluded to1 and many examples of it will be given in this work4 ,after all explosions and eruptions have ceased1 heated vapours may still for a long period continue to make their way upward through the loose spongy detritus filling up the vent4 ,the ascent of such vapours1 and more particularly of steam1 may induce considerable metamorphism of the agglomerate1 as is more particularly noticed at p4 #ga4 ii4 ,necks of ,lava-form ,material ,the second type of neck is that in which the volcanic pipe has been entirely filled up with some massive or crystalline rock4 ,as already remarked1 it is not always possible to be certain that bosses of rock1 having the external form of necks of this kind1 mark the sites of actual volcanic orifices4 ,eruptive material that has never reached the surface1 but has been injected into the crust of the earth1 has sometimes solidified there in forms which1 when subsequently exposed by denudation1 present a deceptive resemblance to true volcanic necks4 ,each example must be examined by itself1 and its probable origin must be determined by a consideration of all the circumstances connected with it4 ,where other evidence exists of volcanic activity1 such1 for instance1 as the presence of bedded tuffs or intercalated sheets of lava1 the occurrence of neck-like eminences or bosses of felsite1 andesite1 dolerite1 basalt or other eruptive rock1 would furnish a presumption that these marked the sites of some of the active vents of the period to which the tuffs and lavas belonged4 ,if a neck-like eminence of this kind were found to possess a circular or elliptical ground-plan1 and to descend vertically like a huge pillar into the crust of the earth2 if the surrounding rocks were bent down towards it and altered in the manner which ,i shall afterwards describe in detail2 if1 moreover1 the material composing the eminence were ascertained to be closely related petrographically to some parts of the surrounding volcanic series1 it might with some confidence be set down as marking the place of one of the active vents from which that series was ejected4 ,the chief contrast in external form between this type of neck and that formed of fragmentary material arises from differences in the relative durability of their component substance4 ,the various kinds of lava-form rock found in necks are1 as a whole1 much harder and more indestructible than agglomerates and tuffs4 ,consequently bosses of them are apt to stand out more prominently4 ,they mount into higher points1 present steeper declivities1 and are scarped into more rugged crags4 ,but essentially they are characterized by similar conical outlines1 and by rising in the same solitary and abrupt way from lower ground around them 7see ,figs4 #aji1 #acc1 and #aie1 #bid74 ,illustration3 ,fig4 #bh4--,section of neck filled with massive rock4 ,various joint-structures may be observed in these necks4 ,in some cases there is a tendency to separate into joints parallel to the bounding walls1 and occasionally this arrangement goes so far that the rock has acquired a fissile structure as if it were composed of vertical strata4 ,in other instances1 the rock shows a columnar structure1 the columns diverging from the outer margin1 or curving inwards1 or displaying various irregular groupings4 ,more usually1 however1 this jointing is so indefinite that no satisfactory connection can be traced between it and the walls of the orifice in which the rock has solidified4 ,some of the most remarkable examples of necks ever figured and described are those to which attention was called by ,captain ,dutton as displayed in the ,zuni plateau of ,new ,mexico1 where1 amid wide denuded sheets of basalt1 numerous prominent crags mark the sites of eruptive vents4 ,the basalt of these eminences is columnar1 the columns standing or lying in all sorts of attitudes1 and in most cases curved4#bh ,in the ,upper ,velay1 in ,central ,france1 numerous conspicuous domes and cones of phonolite rise amidst the much-worn basalt-plateau of that region 7,fig4 #cde74 ,many instances will be cited in later chapters from the ,british ,isles4 ,footnote #bh3 ,u4,s4 ,geol4 ,survey1 #fth ,annual ,report1 #ahhd-he1 p4 #agb4 iii4 ,distribution of ,vents in ,relation to ,geological ,structure-lines ,where the positions of true volcanic necks can be accurately determined1 it is interesting to study their distribution and their relation to the main lines of geological structure around them4 ,sometimes a distinct linear arrangement can be detected in their grouping4 ,those of the ,lower ,old ,red ,sandstone of ,central ,scotland1 for instance1 can be followed in lines for distances of many miles 7,map ,no4 ,,iii74 ,yet when we try to trace the connection of such an arrangement with any known great lines of dislocation in the terrestrial crust1 we can seldom establish it satisfactorily4 ,in the case of the ,scottish ,old ,red ,sandstone just cited1 it is obvious that the vents were opened along a broad belt of subsidence between the mountains of crystalline schist on the north1 and those of convoluted ,silurian strata on the south1 either margin of that belt being subsequently1 if not then1 defined by lines of powerful fault4 ,no vents have risen along these faults1 nor has any relation been detected between the sites of the volcanic foci and dislocations in the area of ancient depression4 ,indeed1 it may be asserted of the vents of ,britain that they are usually entirely independent of any faults that traverse at least the upper visible part of the earth's crust4 ,they sometimes rise close to such lines of fracture without touching them1 but they are equally well developed where no fractures are to be found4 ,now and then one of them may be observed rising along a line of fault1 but such a coincidence could hardly fail occasionally to happen4 ,from the evidence in the ,british ,isles1 it is quite certain that if volcanic vents have1 as is possible1 risen preferably along lines of fissure in the terrestrial crust1 these lines are seldom those of the visible superficial faults1 but must lie much deeper1 and are not generally prolonged upward to the surface4 ,the frequent recurrence of volcanic outbursts at successive geological periods from the same or adjacent vents seems to point to the existence of lines or points of weakness deep down in the crust1 within reach of the internal molten magma1 but far beneath the horizon of the stratified formations at the surface1 with their more superficial displacements4 ,while sometimes running in lines1 old volcanic vents of the ,vesuvian and ,puy types often occur also in scattered groups4 ,two or three may be found together within an area of a few hundred yards4 ,then may come an interval where none1 or possibly only a solitary individual1 may appear4 ,and beyond that space may rise another sporadic group4 ,these features are well exhibited by the ,carboniferous and ,permian series of ,scotland1 to the account of which the reader is referred4 ,a large neck may have a number of smaller ones placed around it1 just as a modern ,vesuvian cone has smaller parasitic cones upon its flanks4 ,an instructive example of this arrangement is to be seen at the great vent of the ,braid ,hills belonging to the ,lower ,old ,red ,sandstone and described in ,chapter xx4 ,other instances may be cited from the ,carboniferous and ,permian volcanic series 7see ,figs4 #ij1 #adh1 #bac74 ,not infrequently the irregularities in the ground-plan of a neck1 as already remarked1 may be accounted for on the supposition that they mark the site of more than one vent4 ,sometimes1 indeed1 it is possible to demonstrate the existence of two or even more vents which have been successively opened nearly on the same spot4 ,the first orifice having become choked up1 another has broken out a little to one side1 which in turn ceasing to be effective from the same or some other cause1 has been succeeded by a third 7,fig4 #bi74 ,the three cones and craters of the little island of ,volcanello supply a singularly perfect recent instance of this structure 7,fig4 #bad74 ,here the funnel has twice shifted its position1 each cone becoming successively smaller and partially effacing that which preceded it4 ,in ,auvergne1 the ,puy de ,pariou has long been celebrated as an example of a fresh cinder-cone partially effacing an earlier one4 ,in the much denuded ,palozoic volcanic tracts of ,britain1 where the cones have long since disappeared and only the stumps of the volcanic cylinders are left1 many illustrations occur of a similar displacement of the funnel1 especially among the volcanoes of the ,carboniferous system4 ,among the irregularities of necks that may indicate a connection with lines of fissure1 reference may be made here to dykes or dyke-like masses of agglomerate which are sometimes to be seen among the volcanic districts of ,britain4 ,in these cases the fragmentary materials1 instead of lying in a more or less cylindrical pipe1 appear to fill up a long fissure4 ,we may suppose that the explosions which produced them did actually occur in fissures instead of in ordinary vents4 ,the remarkable ,icelandic fissures with their long rows of cinder cones are doubtless1 at least in their upper parts1 largely filled up with slag and scori4 ,some illustrations of this structure will be given in the account of the ,carboniferous volcanic rocks of ,scotland 7see ,no4 #a in ,fig4 #bb74 ,illustration3 ,fig4 #bi4--,successive shiftings of vents giving rise to double or triple cones4 ,a1 ground-plan2 ,b1 vertical section4 ,there is yet another consideration in regard to the form and size of necks which deserves attention4 ,where the actual margin of a neck and its line of vertical junction with the rocks through which it has been drilled can be seen1 there is no room for dispute as to the diameter of the original funnel1 which must have been that of the actual neck4 ,but in many cases it is impossible to observe the boundary2 not merely because of superficial soil or drift1 but occasionally because the volcanic detritus extends beyond the actual limits of the funnel4 ,in such cases the necks have retained some portion of the original volcanic cone which accumulated on the surface around the eruptive vent4 ,it may even chance that what appears to be a large neck would be considerably reduced in diameter1 and might be shown to include more than one pipe if all this outer casing could be removed from it4 ,in ,fig4 #cj1 for example1 a section is given of a neck 7n7 from which on the right-hand side all the cone and surrounding tuffs 7t7 have been removed by denudation1 the original form of the volcano being suggested by the dotted lines4 ,on the left side1 however1 the tuffs which were interstratified with the contemporaneous sediments are still connected with the neck1 denudation not having yet severed them from it4 ,the overlying strata 7l1 l7 which originally overspread the extinct volcano have been bent into an anticline1 and the neck of the vent has thus been laid bare by the removal of the crest of the arch4 ,illustration3 ,fig4 #cj4--,section to show the connection of a neck with a cone and surrounding bedded tuffs4 ,the instances where a structure of this kind is concealed are probably fewer in number in proportion to their antiquity4 ,but among ,tertiary cones they may perhaps not be so rare4 ,the possibility of their occurrence should be kept in view during the investigation of extinct volcanoes4 ,the term ,neck ought not properly to be applied to such degraded volcanic cones4 ,the true neck still remains preserved in the inside of them4 ,as illustrative of the structure here referred to1 ,i may cite the example of the ,saline ,hill 7,fig4 #adh7 and of ,largo ,law 7,fig4 #bbf71 both in ,fife4 iv4 ,metamorphism in and around ,volcanic ,vents--,solfataric ,action ,the prolonged ascent of hot vapours1 stones1 dust and lava1 in the funnel of a volcano must necessarily affect the rocks through which the funnel has been driven4 ,we may therefore expect some signs of alteration in the material forming the walls of a volcanic neck4 ,the nature of the metamorphism will no doubt depend1 in the first place1 on the character and duration of the agents producing it1 and in the second1 on the susceptibility of the rocks to undergo change4 ,mere heat will indurate rocks1 baking sandstone1 for instance1 into quartzite1 and shales into porcellanite4 ,but there will almost invariably be causes of alteration other than mere high temperature4 ,water-vapour1 for instance1 has probably always been one of the most abundant and most powerful of them4 ,the copious evolution of steam from volcanoes is one of their most characteristic features at the present day1 and that it was equally so in past time seems to be put beyond question by the constantly recurring vesicular structure in ancient lavas and in the lapilli and ejected blocks of old agglomerates and tuffs4 ,direct experiment has demonstrated1 in the hands of various skilful observers1 from the time of ,sir ,james ,hall to that of ,professor ,daubre1 how powerfully rocks are acted upon when exposed to superheated vapour of water under great pressure4 ,but the steam of volcanoes often contains other vapours or mineralizing agents dissolved in it1 which increase its metamorphic influence4 ,the mineral acids1 for instance1 must exert a powerful effect in corroding most minerals and rocks4 ,at the ,solfatara of ,naples and at other volcanic orifices in different parts of ,italy1 considerable alteration is seen to be due to this cause4 ,bearing these well-known facts in mind1 we may be prepared to find various proofs of metamorphism around and within old volcanic vents4 ,the surrounding rocks are generally much hardened immediately contiguous to a neck1 whether its materials be fragmental or massive4 ,sandstones1 for example1 are often markedly bleached1 acquire the vitreous lustre and texture of quartzite1 lose their usual fissility1 break irregularly into angular blocks1 and on an exposed surface project above the level of the unaltered parts beyond4 ,shales are baked into a kind of porcelain-like substance4 ,coal-seams are entirely destroyed for economic purposes1 having been burnt into a kind of cinder or fused into a blistered slag-like mass4 ,limestones likewise lose their usual bluish-grey tint1 become white and hard1 and assume the saccaroid texture of marble4 ,the distance to which this metamorphism extends from the wall is1 among the exposed necks in ,britain1 smaller than might be anticipated4 ,thus ,i have seldom been able to trace it among those of ,carboniferous or ,permian age for more than #ae or #bj yards in ordinary arenaceous and argillaceous strata1 even where every detail of a neck and its surroundings has been laid bare in plan upon a beach4 ,the alteration seems to reach furthest in carbonaceous seams1 such as coals4 ,it is evident that the element of time must enter into the question of the amount of metamorphism produced in the terrestrial crust immediately surrounding a volcanic pipe4 ,a volcano1 of which the eruptions begin and end within an interval of a few days or hours1 cannot be expected to have had much metamorphic influence on the rocks through which its vent was opened4 ,on the other hand1 around a funnel which served for many centuries as a channel for the escape of hot vapours1 ashes or lava to the surface1 there could hardly fail to be a considerable amount of alteration4 ,the absence or comparatively slight development of metamorphism at the ,carboniferous and ,permian necks of ,scotland may perhaps be regarded as some indication that these volcanoes were generally short-lived4 ,on the other hand1 more extensive alteration may be taken as pointing to a longer continuance of eruptive vigour4 ,the same causes which have induced metamorphism in the rocks surrounding a volcanic vent might obviously effect it also among the fragmentary materials by which the vent may have been filled up4 ,when the eruptions ceased and the funnel was left choked with volcanic debris1 hot vapours and gases would no doubt still continue for a time to find their way upward through the loose or partially compacted mass4 ,in their ascent they would permeate this material1 and in the end produce in it a series of changes similar to1 and possibly even more pronounced than1 those traceable in the walls of the vent4 ,instances of this kind of metamorphism will be cited in the following chapters 7see in particular p4 #djd74 v4 ,inward ,dip of ,docks towards ,necks ,one concluding observation requires to be made regarding the relation of old volcanic necks to the rocks which immediately surround them4 ,where a vent has been opened through massive rocks1 such as granite1 felsite1 andesite or basalt1 it is generally difficult or impossible to determine whether there has been any displacement of these rocks1 beyond the disruption of them caused by the explosions that blew out the orifice4 ,but where the pipe has been drilled through stratified rocks1 especially when these still lie nearly flat1 the planes of stratification usually supply a ready test and measure of any such movement4 ,investigation of the volcanic rocks of ,britain has shown me that where any displacement can be detected at a neck1 it is almost invariably in a downward direction4 ,the strata immediately around the vent tend to dip towards it1 whatever may be their prevalent inclination in the ground beyond 7,fig4 #bd74 ,this is the reverse of the position which might have been expected4 ,it is so frequent1 however1 that it appears to indicate a general tendency to subsidence at the sites of volcanic vents4 ,after copious eruptions1 large cavernous spaces may conceivably be left at the roots of volcanoes1 and the materials that have filled the vents1 losing support underneath1 will tend to gravitate downwards1 and if firmly welded to their surrounding walls may drag these irregularly down with them4 ,examples of such sagging structures are abundantly to be seen among the dissected vents of the ,carboniferous and ,permian volcanic series of ,scotland4 vi4 ,influence of ,contemporaneous ,denudation upon ,volcanic ,cones ,it must be remembered that former vents1 except those of the later geological periods1 are revealed at the surface now only after extensive denudation4 ,as a rule1 the volcanoes that formed them appeared and continued in eruption during periods of general subsidence1 and were one by one submerged and buried beneath subaqueous deposits4 ,we can conceive that1 while a volcanic cone was sinking under water1 it might be seriously altered in form and height by waves and currents4 ,if it consisted of loose ashes and stones1 it might be entirely levelled1 and its material might be strewn over the floor of the sea or lake in which it stood4 ,but1 as has been already pointed out1 the destruction of the cone would still leave the choked-up pipe or funnel from which the materials of that cone had been ejected4 ,though1 during the subsidence1 every outward vestige of the actual volcano might disappear1 yet the agglomerate or lava that solidified in the funnel underneath would remain4 ,and if these materials had risen some way within the cone or crater1 or if they reached at least a higher level in the funnel than the surrounding water-bottom or land-surface1 the destruction of the cone might leave a projecting knob or neck to be surrounded and covered by the accumulating sediments of the time4 ,it is thus evident that the levelling of a cone of loose ashes during gradual subsidence1 and the deposition of a contemporary series of sedimentary deposits1 might give rise to a true neck1 which would be coeval with the geological period of the volcano itself4 ,in practice it is extremely difficult to decide how far any now visible neck may have been reduced to the condition of a mere stump or core of a volcano before being buried under the stratified accumulations of its time4 ,in every case the existence of the neck is a proof of denudation1 and perhaps1 in most cases1 the chief amount of that denudation is to be ascribed not to the era of the original volcano1 but to the comparatively recent interval that has elapsed since1 in the progress of degradation1 the volcanic rocks1 after being long buried within the crust1 were once more laid bare by the continuous waste and lowering of the level of the land4 vii4 ,stages in the ,history of old ,volcanic ,vents ,let us now try to follow the successive stages in the history of a volcano after its fires had quite burnt out1 and when1 slowly sinking in the waters of the sea or lake wherein it had burst forth1 it was buried under an ever-growing accumulation of sedimentary material4 ,the sand1 mud1 calcareous ooze1 shell-banks1 or whatever may have been the sediment that was gathering there1 gradually crept over the submerged cone or neck1 and would no doubt be more or less mixed with any volcanic detritus which waves or currents could stir up4 ,if the cone escaped being levelled1 or if it left a projecting neck1 this subaqueous feature would be entombed and preserved beneath these detrital deposits4 ,hundreds or thousands of feet of strata might be laid down over the site of the volcano1 which would then remain hidden and preserved for an indefinite period1 until in the course of geological revolutions it might once again be brought to the surface4 ,these successive changes involve no theory or supposition4 ,they must obviously have taken place again and again in past time4 ,that they actually did occur is demonstrated by many examples in the ,british ,isles4 ,i need only refer here to the interesting cases brought to light by mining operations in the ,dairy coal-fields of ,ayrshire1 which are more fully described in ,chapter xxvii4 7p4 #dcc74 ,in that district a number of cones of tuff1 one of which is #gjj feet in height1 have been met with in the course of boring and mining for ironstone and coal4 ,the well-known mineral seams of the coal-field can be followed up to and over these hidden hills of volcanic tuff which in the progress of denudation have not yet been laid bare 7,fig4 #adf74 ,the subsidence which carried down the water-bottom and allowed the volcanic vents to be entombed in sedimentary deposits may have been in most cases tolerably equable1 so that at any given point these deposits would be sensibly horizontal4 ,but subsequent terrestrial disturbances might seriously affect this regularity4 ,the sedimentary formations1 piled above each other to a great depth1 and acquiring solidity by compression1 might be thrown into folds1 dislocated1 upheaved or depressed4 ,the buried volcanic funnels would1 of course1 share in the effects of these disturbances1 and eventually might be so squeezed and broken as to be with difficulty recognizable4 ,it is possible that some of the extreme stages of such subterranean commotions are revealed among the ,dalradian rocks of ,scotland4 ,certain green schists which were evidently originally sediments1 and probably tuffs1 are associated with numerous sills and bosses of eruptive material4 ,the way in which these various rocks are grouped together strikingly suggests a series of volcanic products1 some of the crushed bosses recalling the forms of true necks in younger formations4 ,but they have been so enormously compressed and sheared that the very lavas which originally were massive amorphous crystalline rocks have passed into fissile hornblende-schists4 ,illustration3 ,fig4 #ca4--;,diagram illustrating the gradual emergence of buried volcanic cones through the influence of prolonged denudation4 ,among the ,palozoic systems of ,britain1 however1 where considerable fracture and displacement have taken place1 examples of successive stages in the reappearance of buried volcanic cones and necks may be gathered in abundance4 ,as an illustrative diagram of the process of revelation by the gradual denudation of an upheaved tract of country1 ,fig4 #ca may be referred to 7compare also ,fig4 #adg74 ,here three volcanic vents are represented in different stages of re-emergence4 ,in the first 7,a7 we see a cone and funnel which1 after having been buried under sedimentary deposits 7s1 s17 have been tilted up by subterranean movements4 ,the overlying strata have been brought within the influence of denudation1 and their exposed basset edges along the present surface of the land 7g1 g7 bear witness to the loss which they have suffered4 ,already1 in the progress of degradation1 a portion of the volcanic materials which1 ejected from that vent1 were interstratified with the contemporaneous sediments of the surrounding sea-floor1 has been exposed at t4 ,a geologist coming to that volcanic intercalation would be sure that it pointed to the existence of some volcanic vent in the neighbourhood1 but without further evidence he would be unable to tell whether it lay to right or left1 whether it was now at the surface or lay still buried under cover of the stratified deposits which were laid down upon it4 ,in the second or central example 7,b7 we have a pipe and cone which have been similarly disturbed4 ,but in this case denudation has proceeded so far as to reveal the cone and even to cut away a portion of it1 as shown by the dotted lines to the right hand4 ,owing1 however1 to the general inclination of the rocks towards the left1 that side of the cone1 together with the tuffs or lavas connected with it1 still lies buried and protected under cover of the sedimentary formations 7s1 s74 ,the third example 7,c7 shows a much more advanced stage of destruction4 ,here the whole of the cone has been worn away4 ,all the lavas and tuffs which were ejected from it towards the right have likewise disappeared1 and strata older than the eruptions of this vent now come to the surface there4 ,to the left1 however1 a little portion of its lavas still remains at l1 though all the intervening volcanic material has been removed4 ,that solitary fragment of the outpourings of this volcano once extended further to the left hand1 but the occurrence of the large dislocation 7f7 has carried this extension for down below the surface4 ,the vent in this instance1 owing to its position1 has suffered more from denudation than the other two4 ,yet1 judged by the size of its neck1 it was probably larger than either of them1 and threw out a more extensive pile of volcanic material4 ,its funnel has been filled with agglomerate 7a71 through which a central plug of lava 7p7 has ascended1 and into which dykes or veins 7d1 d71 the last efforts of eruption1 have been injected4 ,this diagram will serve to illustrate the fact already so often insisted on1 that although denudation may entirely remove a volcanic cone1 and also all the lavas and tuffs which issued from it1 the actual filled-up pipe cannot be so effaced1 but is practically permanent4 ,,chapter ,,vi ,underground ,phases of ,volcanic ,action--continued4 ,,ii4 ,subterranean ,movements of the ,magma3 i4 ,dykes and ,veins2 ii4 ,sills and ,laccolites2 iii4 ,bosses 7,stocks1 ,culots7--,conditions that govern the ,intrusion of ,molten ,rock within the ,terrestrial ,crust4 ,,ii4 ,subterranean ,movements of the ,magma ,in the foregoing pages attention has been more specially directed to those aspects of volcanic energy which reveal themselves above ground and in eruptive vents4 ,we have now to consider the various ways in which the molten magma is injected into the crust of the earth4 ,such injection must obviously take place during the expulsion of volcanic materials to the surface4 ,if the explosive violence of an eruption1 or the concomitant movements of the earth's crust1 should lead to ruptures among the subterranean rocks1 the molten magma will be forced into these rents4 ,it is evident that this may happen either with or without any discharge of lava at the surface4 ,it may be either entirely a plutonic1 that is1 a deep-seated phenomenon1 or it may be part of a truly volcanic series of events4 ,it is clear that1 by the study of old volcanoes that have had their structure laid bare by denudation1 we may hope to obtain fresh light in regard to some of the more deeply-seated features of volcanic energy1 which in a modern volcano are entirely concealed from view4 ,a little reflection will convince us that the conditions for consolidation within the crust are so different from those at the surface that we may expect them to make themselves visible in the internal characters of the rocks4 ,an essential distinction between underground propulsions of molten rock and superficial outflows of the same material lies in the fact that while the latter are free to take any shape which the form and slope of the ground may permit1 the subterranean injections1 like metal poured into a mould1 are always bounded by the walls of the aperture into which they are thrust4 ,according1 therefore1 to the shape of this aperture a convenient classification of such intrusions may be made4 ,where the molten material has risen up vertical fissures or irregular cracks1 it has solidified as ,dykes and ,veins4 ,where it has been thrust between the divisional planes either of stratified or unstratified rocks1 so as to form beds1 these are conveniently known as ,sills1 ,laccolites or ,intrusive ,sheets4 ,where it has taken the form of large cylindrical masses1 which1 ascending through the crust1 appear at the surface in rounded1 elliptical or irregularly-shaped eminences1 these are called ,bosses 7,stocks1 ,culots74 ,further contrasts between the superficial and subterranean consolidation of molten material are to be found in the respective textures and minute structures of the rocks4 ,the deep-seated intrusions are commonly characterized by a general and markedly greater coarseness of crystallization than is possessed by lavas poured out at the surface4 ,this difference of texture1 obviously in great measure the result of slower cooling1 shows itself in acid1 intermediate1 and basic magmas4 ,a lava which at the surface has cooled as a fine-grained1 compact black basalt1 in which neither with the naked eye nor with the lens can the constituent minerals be distinctly determined1 may conceivably be represented at the roots of its parent volcano by a coarse-textured gabbro1 in which the felspars and pyroxenes may have grown into crystals or crystalline aggregates an inch or more in length4 ,mr4 ,iddings has pointed out that the various porphyrites which form the dykes and sills of ,electric ,peak are connected with a central boss of coarsely crystalline diorite4#bi ,examples of the same relation from different volcanic centres in ,britain will be cited in later chapters4 ,footnote #bi3 #abth ,ann4 ,rep4 ,u4,s4 ,geol4 ,survey 7#ahij-ia71 p4 #eie4 ,this greater coarseness of texture is shown by microscopic examination to be accompanied by other notable differences4 ,in particular1 the glassy residuum1 or its devitrified representatives1 which may be so frequently detected among the crystals of outflowing lavas1 is less often traceable in the body of subterranean intrusive rocks1 though it may sometimes be noticed at their outer margins where they have been rapidly chilled by contact with the cool upper part of the crust into which they have been impelled4 ,various minerals1 the constituents of which exist in the original magma1 but which may be hardly or not all recognisable in the superficial lavas1 have had leisure to crystallize out in the deep-seated intrusions and appear sometimes among the components of the general body of the rock1 or as well-terminated crystals in its drusy cavities4 ,considerable though the variations may be between the petrographical characters of the intrusive and extrusive rocks of a given district and of the same eruptive period1 they appear generally to lie within such limits as to suggest a genetic relation between the whole series4 ,conditions of temperature and pressure1 and the retention or escape of the absorbed vapours which play so large a part in volcanic activity1 must exercise great influence on the crystallization of constituent minerals1 and on the consolidation and ultimate texture of the rocks4 ,slow cooling under great pressure and with the mineralizing vapours still largely retained seems to be pre-eminently favourable for the production of a holocrystalline texture in deep-seated portions of the magma1 while rapid cooling under merely atmospheric pressure and with a continuous disengagement of vapours1 appears to be required for the finer grain1 more glassy structure1 and more vesicular character of lavas poured out at the surface4 ,besides these differences1 however1 there is evidence of a migration of the constituent minerals in the body of large intrusive masses before consolidation4 ,in particular1 the heavier and more basic constituents travel towards the cooling margin1 leaving the central portions more acid4 ,this subject will be more fully considered in connection with the internal constitution of ,bosses1 and some ,british examples will then be cited4 ,reference1 however1 may here be made to one of the most exhaustive and instructive studies of the relations of the subterranean and superficial erupted rocks of an old volcano1 which will be found in the monograph by ,mr4 ,iddings on ,electric ,peak and ,sepulchre ,mountain in the ,yellowstone ,park of ,western ,america4 ,from the data there obtainable he draws the deduction that one parent magma1 retaining the same chemical composition1 may result in the ultimate production of rocks strikingly different from each other in structure and mineralogical constitution1 yet chemically identical4 ,electric ,peak includes the central funnel filled up with coarsely crystalline diorite1 and having a connected series of sills and dykes of various porphyrites4 ,sepulchre ,mountain1 separated from its neighbouring eminence by a fault of #djjj feet1 displays some of the superficial discharges from the vent--coarse breccias with andesite-lavas4 ,these rocks are not chemically distinguishable from the intrusive series1 but the lavas are1 on the whole1 more glassy1 while the materials of the bosses1 sills and dykes are more crystalline4 ,the latter display much more visible quartz and biotite4#cj ,footnote #cj3 #abth ,ann4 ,rep4 ,u4,s4 ,geol4 ,survey1 #ahij-ia4 ,as already stated1 the eruptions of this volcanic centre became progressively more acid1 and this change appears to be exhibited by the extrusive lavas as well as by the intrusive rocks4 ,by practice in the field1 supplemented by investigation with the aid of the microscope1 a geologist acquires a power of discriminating with fair accuracy1 even in hand specimens1 the superficial from the subterranean igneous rocks of an old volcanic district4 ,denudation1 while laying bare the underground mechanism of an ancient volcano1 has not always revealed the evidence of the actual structural relations of the rocks1 or has first exposed and then destroyed it4 ,sometimes a mass of eruptive rock has been worn down and left in such an isolated condition that its connection with the rest of the volcanic network cannot be determined4 ,so far as its position goes1 it might perhaps be either a remnant of a lava-stream or the projecting part of some deeper-seated protrusion4 ,but its texture and internal structure will often enable a confident opinion to be expressed regarding the true relations of such a solitary mass4 i4 ,dykes and ,veins ,for the study of these manifestations of volcanic energy1 the ,british ,isles may be regarded as a typical region4 ,it was thence that the word dyke passed into geological literature4 ,thousands of examples of both dykes and veins may be seen from the ,outer ,hebrides southwards across the length and breadth of the southern half of ,scotland1 far into the north of ,england and towards the centre of ,ireland4 ,they may be found cutting the crests of the mountains and extending as reefs below the level of the sea4 ,they are thus exposed in every conceivable divergence of position and in endless varieties of enclosing rock4 ,moreover1 they can be shown to represent a vast range of geological time4 ,one system of them belongs to some remote part of the ,archan periods1 another is as young as the older ,tertiary ages4 ,illustration3 ,fig4 #cb4--;,dyke1 ,vein and ,sill4 ,the dyke 7d7 rises along a small fault among sandstones1 shales1 and ironstones 7sh71 and gives off a vein 7v7 and an intrusive sheet or sill 7b74 ,full details regarding these interesting relics of volcanic activity will be given in later chapters1 especially in ,chapters xxxiv4 and xxxv4 ,it may suffice here to note that each of the three types of old volcanoes above described has1 in ,britain1 its accompaniment of dykes and veins4 ,the plateaux1 however1 present by far the most abundant and varied development of them4 ,the dykes of this series are characterized not only by their prodigious numbers in and around some of the plateaux1 but by the long distances to which they may be traced beyond these limits4 ,they are chiefly found in connection with the ,tertiary basalt-plateaux1 though the ,carboniferous andesite-plateaux present a feebler display of them4 ,the ,tertiary dykes are pre-eminently distinguished by their persistent rectilinear lines1 sometimes for distances of many miles1 and their general north-westerly direction4 ,they form a vast system extending over an area of some #dj1jjj square miles4 ,throughout that wide region their persistence of direction and of petrographical characters point to the former existence of one or more reservoirs of an andesitic and basaltic magma underneath the northern half of ,britain1 and to the rupture of the crust overlying this subterranean reservoir by thousands of parallel fissures4 ,they thus constitute perhaps the most astonishing feature in the volcanic history of ,tertiary time4 ,the dykes and veins connected with the puys are mainly to be found at or close to the vents4 ,not infrequently they traverse the agglomerates of the necks1 and are sometimes to be traced to a central pipe or core of basalt4 ,the larger cones are likewise intersected with similar vertical1 inclined or tortuously irregular walls of intruded lava4 ,occasionally a radiate arrangement may be observed in such cases1 like that noticeable at some modern volcanoes1 the dykes diverging from the eruptive centre4 ,many dykes exist regarding which there is no evidence to connect them with any actual volcanic rocks4 ,they have been injected into fissures1 but whether this took place during volcanic paroxysms1 or owing to some subterranean movements which never culminated in any eruption1 cannot be decided4 ,the question of the age of dykes1 like that of intrusive masses of all kinds1 is often difficult or impossible to decide4 ,a dyke must of course be younger than the rocks which it traverses1 and a limit to its antiquity is thus easily fixed4 ,but we cannot always affirm that because a dyke stops short of a particular rock1 or series of rocks1 it is older than these4 ,the ,hett ,dyke1 in the north of ,england1 rises through the ,coal-measures1 but stops at the ,magnesian ,limestone2 yet this cessation does not necessarily imply that the dyke was in place before the deposition of that limestone4 ,the structure may have arisen from the dyke-fissure having ended at the bottom of the limestone4 ,where dykes rise up to the base of an unconformable formation without in any single case entering it1 and where fragments of them are enclosed in that formation1 they must be of higher antiquity1 and must have been laid bare by extensive denudation before the unconformable strata were deposited upon them4 ,the great system of dykes in the ,lewisian ,gneiss of the north-west of ,scotland is in this way proved to be much more ancient than the ,torridon ,sandstones under which it passes 7,figs4 #ce1 #cf74 ,where two dykes cross each other1 it is sometimes not difficult to decide upon their relative antiquity4 ,in intrusive rocks1 the finest-grained parts are those which lie nearest the outer margin1 where the molten material was rapidly chilled by coming in contact with cool surfaces of rock4 ,such chilled margins of closer grain are common characteristics of dykes4 ,wherever a dyke carries its chilled margin across another dyke1 it must be the younger of the two1 and wherever such a margin is interrupted by another dyke1 it must belong to the older4 ,as a rule1 the uprise of molten material in a fissure has so effectually sealed it up that in the subsequent disturbances of the terrestrial crust the fissure has not been reopened1 though others may have been produced near it1 or across it4 ,sometimes1 however1 the enormous tension to which the crust was exposed opened the fissure once more1 sometimes even splitting a dyke along its centre1 and a new ascent of molten rock took place within the rent4 ,hence double or treble or compound dykes have been produced4 ,the second or later infillings are generally somewhat different from the original dyke4 ,occasionally1 indeed1 they present a strong contrast to it4 ,thus1 among the dykes of ,skye examples occur where the centre is occupied by an acid granophyre1 while the sides are occupied by dykes of basalt4 ,instances of this compound type of dyke will be given in the account of the ,tertiary volcanic rocks of ,britain4 ,it is obvious that in a wide fissure the central portion may remain molten for some time after the sides have consolidated4 ,if the fissure served as a channel for the ascent of lava to the surface1 it is conceivable that the central still fluid part might be driven out and be replaced by other material from below1 and that this later material might differ considerably in composition from that which first filled the opening4 ,such1 according to ,mr4 ,iddings1 has been the probable history of some of the dykes at the old volcano of ,electric ,peak4#ca ,but we can hardly suppose that this explanation of compound dykes can have any wide application4 ,it could only hold good of broad fissures having an outlet1 and is probably inadmissible in the case of the numerous compound dykes not more than #aj or #ae feet in diameter1 where the several bands of rock are sharply marked off from each other4 ,the abrupt demarcation of the materials in these dykes1 their closer texture along their mutual boundaries1 the indications of solution of the older parts of the group by the younger1 and of injection of the latter into the former1 show that they belong to separate and unconnected intrusions4 ,these questions will be again referred to in the account of the ,british ,tertiary dykes 7,chapter xxxv4 vol4 ii4 p4 #aei74 ,footnote #ca3 #abth ,ann4 ,rep4 ,u4,s4 ,geol4 ,survey 7#ahij-ia71 p4 #ehg4 ,another kind of compound dyke has arisen from the manner in which the original fissure has been produced4 ,while1 in general1 the dislocation has taken the form of a single rectilinear rent1 which on opening has left two clean-cut walls1 cases occur where the rupture has followed several parallel lines1 and the magma on rising into the rents appears as two or more vertical sheets or dykes1 separated by intervening partitions of the surrounding rock4 ,examples of this structure are not infrequent among the ,tertiary dykes of ,scotland4 ,one of these may be noticed rising through the cliffs of ,lewisian gneiss on the east coast of the island of ,lewis1 south of ,stornoway4 ,one of the most extraordinary instances of the same structure yet observed is that described by ,professor ,a4 ,c4 ,lawson from the ,laurentian rocks at the mouth of ,white ,gravel ,river1 on the ,n4,e4 coast of ,lake ,superior4 ,in a breadth of only about #ad feet no less than #bh vertically intrusive sheets or dykes of diabase1 from #a inch to #f inches broad1 rise through the granite1 which is thus split into #bg thin sheets4 ,the diabase undoubtedly cuts the granite1 some of the sheets actually anastomosing and sending veins into the older rock4#cb ,footnote #cb3 ,american ,geologist 7#ahid71 p4 #bic4 ,from the evidence supplied by the modern eruptions of ,iceland1 it is evident that gaping fissures1 which are filled by ascending lava and thereby converted into dykes1 in many instances serve as channels by which molten rock escapes to the surface4 ,it would be interesting if any test could be discovered whereby those dykes could be distinguished which had ever established a connection with the outer air4 ,if the lava continued to ascend in the fissures1 and to pour out in superficial streams for a long time1 the rocks on either side would be likely to undergo considerably more metamorphism than where there was only one rapid injection of the magma1 which would soon cool4 ,possibly in the much greater alteration of the same rocks by some dykes than by others1 a sign of such a connection with the surface may survive4 ,this subject will be again referred to in the account of the ,tertiary dykes of ,britain in ,book ,,viii41 where the whole of the phenomena of this phase of volcanic action will be fully discussed 7see vol4 ii4 p4 #afc74 ii4 ,sills and ,laccolites ,the word sill1 derived from a remarkable sheet of eruptive rock in the north of ,england1 known as the ,great ,whin ,sill 7,chapter xxix471 is now applied as a convenient general term to masses of intrusive material1 which have been injected between such divisional planes as those of stratification1 and which now appear as sheets or beds 7,fig4 #cc74 ,these masses are likewise called ,intrusive ,sheets1 and where the injected material has accumulated in large blister-like expansions1 these are known as ,laccolites 7,fig4 #cd74 ,illustration3 ,fig4 #cc4--,section of ,sill or ,intrusive ,sheet4 ,sills vary from only an inch or two up to #ejj feet or more in thickness4 ,lying1 as they frequently do1 parallel with strata above and below them1 they resemble in some respects true lava-sheets erupted contemporaneously with the series of sediments among which they are intercalated4 ,and1 indeed1 cases occur in which it is hardly possible to decide whether to regard a given mass as a sill or as a superficial lava4 ,in general1 however1 sills exhibit the coarser texture above referred to as specially characteristic of subterranean eruptive masses4 ,moreover they are usually1 though not always1 free from the vesicular and amygdaloidal structures of true surface-lavas4 ,their under and upper surfaces1 unlike the more scoriaceous parts of lavas1 are commonly much closer in grain than the general body of the mass2 in other words1 they possess chilled borders1 the result of more rapid consolidation by contact with cooler rock4 ,again1 instead of conforming to the stratification of the formations among which they lie1 as truly interstratified lavas do1 they may be seen to break across the bedding and pursue their course on a higher or lower platform4 ,the strata that overlie them1 instead of enclosing pieces of them and wrapping round irregularities on their surface1 as in the case of contemporaneously erupted lava-sheets1 are usually indurated1 sometimes even considerably altered1 while in many cases they are invaded by veins from the eruptive sheet1 or portions of them are involved in it1 and are then much hardened or metamorphosed4 ,the petrographical character of the sills in a volcanic district depends primarily on the constitution of the parent magma1 whence both they and the outflowing lavas have issued4 ,where the lavas are rhyolites or felsites the sills are acid1 where basalts have been erupted the sills are basic1 though there has often been a tendency towards the appearance of more acid material1 such as trachyte4 ,as we have seen1 considerable differences in petrographical characters may arise between the intrusive and extrusive offshoots from the same parent magma during the course of a volcanic cycle4 ,this question will be more appropriately discussed together with the leading characters of ,bosses4 ,between the upper and under surface of a thick sill considerable petrographical variation may sometimes be observed1 especially where the rock is of basic constitution4 ,differences both of texture and even to some extent of composition can be detected4 ,sometimes what have been called segregation veins traverse the mass1 consisting of the same minerals as the general body of the rock1 but in larger crystals and in somewhat different proportions4 ,that these veins belong to the period of original consolidation appears to be shown by the absence of fine-grained1 chilled margins1 and by the way in which the component crystals of the veins are interlocked with those of the body of the rock4 ,other veins of finer grain and more acid composition probably belong to a later phase of consolidation1 when1 after the separation and crystallization of the more basic minerals1 the more acid mother liquor that remained was1 in consequence of terrestrial movements1 injected into cracks in the now solidified1 though still highly heated1 rock4 ,examples of these features will be cited from various geological formations in the following chapters4 ,reference has already been made to the difference occasionally perceptible between the constitution of the upper and that of the under portions of superficial lavas4 ,a similar variation is sometimes strongly marked among sills1 especially those of a basic character1 the felspars remaining most abundant above1 while the olivines and augites preponderate below4 ,mr4 ,iddings has observed some excellent illustrations of this character in the great series of sills connected with the volcanic pipe of ,electric ,peak in the ,yellowstone country4#cc ,some examples of the same structure will subsequently be cited from the ,carboniferous volcanic series of ,central ,scotland4 ,footnote #cc3 ,electric ,peak and ,sepulchre ,mountain1 #abth ,ann4 ,rep4 ,u4,s4 ,geol4 ,survey 7#ahij-ia71 p4 #ehd4 ,the greatest extreme of difference which ,i have observed in the petrographical characters of any group of sills is that displayed by the ,tertiary gabbros of ,skye4 ,these rocks occur as sheets interposed among the bedded basalts1 and injected between each other in such a manner as to form thick piles of rudely stratified sills4 ,they possess a remarkable banded structure1 due to the aggregation of their component minerals in distinct layers1 some of which are dark in colour1 from the abundance of their iron-ore1 pyroxene and olivine2 while others are light-coloured1 from the predominance of their felspar4 ,from the manner in which the component minerals of one band interlace with those of the contiguous bands1 it is quite certain that the structure is not due to successive injections of material among already consolidated rocks1 but belongs to the original conditions of expulsion of the gabbro as a whole4 ,it seems to indicate that the magma which supplied the sills was at the time of its extrusion heterogeneous in composition1 and that the banding arises from the simultaneous or rapidly successive protrusion of different portions of this variously-constituted magma4 ,the details of the structure will be described in the general account to be given of the ,tertiary volcanic rocks 7,chapters xliii4 and xliv474 ,besides such visible differences in the composition of sills1 others much less obtrusive may occasionally be detected with the aid of microscopic or chemical research4 ,the outer parts of some sills are thus discovered to be more basic or more acid than the inner portions4 ,or evidence may be obtained pointing to the probable melting down of surrounding rocks by the erupted magma1 with a consequent local change in the chemical and mineralogical constitution of the mass4 ,in regard to their position in the geological structure of an old volcanic district ,i may here remark that sills1 seldom entirely absent1 are more especially developed either among the rocks through which the volcano has driven its vent1 or about the base of the erupted lavas and tuffs4 ,many illustrations of this distribution will be described from the various volcanic areas of ,britain belonging to ,palozoic and ,tertiary time4 ,at the base of the great ,cambrian and ,lower ,silurian volcanic series of ,merionethshire1 sills are admirably developed1 while among the basaltic eruptions which closed the long volcanic record in the north of ,ireland and the ,inner ,hebrides1 they play a notable part4 ,from the frequent place which sills take at the base of a volcanic series1 it may be inferred that they generally belong to a late phase in the history of an eruptive episode or cycle1 when the orifices of discharge had become choked up1 and when the volcanic energy found an easier passage laterally between the strata underneath the volcanic pile or between the sheets of that pile itself1 than upward through the ever-increasing thickness of ejected material4 ,while there is an obvious relation between most sills and some eruptive centre in their neighbourhood1 cases occur in which no trace of any contemporaneous volcano can be found1 but where the intrusive sheet remains as the sole evidence of the movements of the subterranean magma4 ,the ,great ,whin ,sill1 one of the most extensive intrusive sheets in the ,british ,isles1 is an instance of this kind4 ,though this large mass of injected material can be traced for a distance of about #hj miles1 and though the strata beneath and above it are well exposed in innumerable sections1 no evidence has yet been detected to show that it was connected with any vent that formed a volcano at the surface 7see vol4 ii4 p4 #b74 ,the absence of this evidence may1 of course1 arise from the failure of denudation to uncover the site of the vent1 which may possibly still remain buried under the ,carboniferous strata that overlie the sill towards the south-east4 ,but it may be due to the non-existence of any such vent4 ,we can quite conceive that volcanic energy should sometimes have failed to complete the formation of an actual volcano4 ,aided by subterranean movements1 it might have been potent enough to disrupt the lower parts of the terrestrial crust1 to propel the molten magma into fissures1 even to inject it for many miles between the planes of stratification1 which would be lines of least resistance1 and yet in default of available rents1 might have been unable to force its way through the upper layers and so reach the surface4 ,examples of such incompleted volcanoes are perhaps to be recognized among solitary sills1 which not infrequently present themselves in the geological structure of ,britain4 ,but the positive decision of this question is almost always frustrated by the imperfection of the evidence1 and the consequent possibility that a connected vent may still lie concealed under overlying strata4 ,besides the more usual intrusions of molten material in the form of sheets of which the vertical thickness bears but a small proportion to the horizontal extent1 there occur also large and thick cakes of intruded material in which the vertical thickness may approach1 or perhaps even surpass1 the horizontal diameter4 ,these dome-shaped or irregular expansions form a connecting link between ordinary sills and the bosses to be subsequently described4 ,they have received the name of ,laccolites from ,mr4 ,g4 ,k4 ,gilbert1 who worked out this peculiar type of structure in the case of the ,henry ,mountains in southern ,utah#cd 7,fig4 #cd74 ,the same type has since been found distributed over ,arizona and ,colorado1 and it has been recognized as essentially that of many eruptive masses or bosses in all parts of the world4 ,footnote #cd3 ,geology of the ,henry ,mountains1 ,u4,s4 ,geog4 and ,geol4 ,survey of the ,rocky ,mountain ,region1 #ahgg4 ,for a review of the whole subject of laccolites in ,western ,america see a paper by ,mr4 ,whitman ,cross1 in the #adth ,annual ,report of the ,director of the ,u4,s4 ,geological ,survey1 #ahib-ic 7pub4 #ahie71 p4 #aeg4 ,illustration3 ,fig4 #cd4--;,ideal section of three ,laccolites4 7,after ,mr4 ,gilbert47 ,in ,western ,america1 owing in large measure to the previously undisturbed condition of the sedimentary formations1 the relations of the injected igneous material to these formations can be satisfactorily ascertained4 ,the geological structure of the various isolated laccolites thus clearly presented1 helps to explain the structure of other intrusive bodies which1 having been injected among plicated and dislocated rocks1 do not so readily admit of interpretation4 ,in ,colorado1 ,utah and ,arizona the eruptive magma1 usually a porphyrite1 diorite or quartz-porphyry1 has risen in one or more pipes1 and has then intruded itself laterally between the planes of the sedimentary formations which1 over the centre of intrusion1 have been pushed upward into a vast dome-shaped or blister-like elevation4 ,the horizon on which this lateral and vertical expansion of the intruded material took place would seem to have lain several thousand feet below the surface4 ,it ranges from the ,cambrian to the ,tertiary formations4 ,subsequent denudation has cut down the upraised mantle of sedimentary layers1 and has revealed more or less of the igneous rock underneath1 which is thus allowed to protrude and to be affected by atmospheric erosion4 ,in this way1 wide plains of horizontal or gently undulating ,secondary and ,tertiary strata have been diversified by the appearance of cones1 detached or in groups1 which have become more peaked and varied in outline in proportion as their original sedimentary covering has been removed from them4 ,the largest of the laccolitic masses in the ,henry ,mountains is about #gjjj feet deep and about #d miles in diameter4 ,less than one-half of the cover of overarching strata has been removed1 and denudation has cut deeply into the remaining part4 ,that the type of structure1 so well exhibited among the ,henry ,mountains1 has not been more abundantly recognized elsewhere probably arises from the fact not that it is rare1 but that the conditions for its development are seldom so favourable as in ,western ,america4 ,obviously where stratified rocks have been much disturbed1 they cease to furnish definite or regular platforms for the reception of eruptive material1 and to afford convenient datum-lines for estimating what was probably the shape of the intruded magma4 ,we may believe that the effect of the propulsion of eruptive material is usually to upheave the overlying crust1 and thus to give rise to a laccolitic form of intrusion4 ,the upheaval relatively to the surrounding country will be apt to be practically permanent1 the intruded body of rock being welded to the surrounding formations1 and forming in this way a solid and resisting core directly united by pipes or funnels with the great magma-reservoir underneath4 ,on the other hand1 where the molten rock1 instead of consolidating underground1 has been copiously discharged at the surface1 its emission must tend towards the production of cavernous spaces within the crust4 ,the falling in of the roofs of such caverns will give rise to shocks of earthquakes4 ,subsequent uprisings of the magma may fill these spaces up1 and when the rock has solidified in the form of laccolites or bosses1 it may effectually put an end there to further eruptions4 ,some contact metamorphism may be observed along the upper and under surfaces of large sills4 ,the rocks over the ,american laccolites have sometimes been highly altered4 ,but as the change is the same in kind as that attendant upon ,bosses1 though generally less in degree1 it will be considered with these intrusive masses4 ,the problems in terrestrial physics suggested by the intrusion of such thick and persistent masses of eruptive material as those which form sills and laccolites will likewise be discussed in connection with the mechanism of the remaining intrusive masses which have now to be described4 iii4 ,bosses 7,stocks1 ,culots7 ,the term ,boss has been applied to masses of intrusive rock which form at the surface rounded1 craggy or variously-shaped eminences1 having a circular1 elliptical or irregular ground-plan1 and descending into the terrestrial crust with vertical or steeply-inclined sides 7,fig4 #bh74 ,sometimes they can be seen to have pushed the surrounding rocks aside4 ,in other places they seem to occupy the place of these rocks through which1 as it were1 an opening has been punched for the reception of the intrusive material4 ,occasionally1 more especially in the case of large bosses1 like those in which granite so frequently appears1 the eruptive mass may be observed to rise here and there in detached knobs through the surrounding rocks1 or to enclose patches of these1 in such a manner as to indicate that the large body of eruptive material terminates upward in a very irregular surface1 of which only the more prominent parts project through the cake of overlying rocks4 ,in true bosses1 unlike sills or laccolites1 we do not get to any bottom on which the eruptive material rests4 ,laccolites1 indeed1 may be regarded as intermediate between the typical sill and the typical boss4 ,the difference between a laccolite and a boss lies in the fact that the body of the laccolite does not descend into an unknown depth in the crust1 but lies upon a platform on which it has accumulated1 the magma having ascended by one or more ducts1 which generally bear but a small proportion in area to the mass of the laccolite4 ,the boss1 on the other hand1 is not known to lie on any horizon1 nor to proceed from smaller ducts underneath1 but plunges as a great pillar or irregular mass1 which may frequently be noticed to widen downwards into the crust4 ,there can be no doubt1 however1 that many masses of eruptive rock1 which1 according to the definition here given1 should be called bosses1 would be found to be truly laccolites if their structure below ground could be ascertained4 ,it is obvious that our failure to find any platform on which the body of a boss lies1 may arise merely from denudation having been as yet insufficient to lay such a platform bare4 ,it is hardly probable that a boss several miles in diameter should descend as a column of that magnitude to the magma-reservoir from which its material came4 ,more probably it has been supplied through one or more smaller ducts4 ,the large boss now visible at the surface may thus be really a laccolitic expansion on one or more horizons4 ,m4 ,michel ,lvy lays stress on the general widening of granitic bosses as they descend into the crust4#ce ,while his observations are supported by many illustrations from all parts of the globe1 and are probably true of the deeper-seated masses of granite1 it is no less true that numerous examples have been met with where a granite boss is sharply marked off from the rocks which it has invaded and on which it may be seen to lie4 ,apart from the cases where granite seems to form part of a vast internal1 once molten mass1 into which its encircling gneisses seem to graduate1 there are others in which this rock1 as now visible1 has been injected into the crust as a boss or as a laccolite4 ,instances will be described in later chapters where such bosses have risen through ,cambrian1 ,silurian1 ,devonian and ,carboniferous formations4 ,it may be said that between such granitic intrusions and volcanic operations no connection can be traced4 ,but reasons will be brought forward in later chapters to regard some of the granitic bosses as parts of the mechanism of ,palozoic volcanoes4 ,it will also be shown that among the intrusive rocks of the ,tertiary volcanic series of ,britain there occur bosses of truly granophyric and granitic material4 ,hence1 though mainly what is called a plutonic rock1 granite has made its appearance among the subterranean protrusions of volcanoes4 ,footnote #ce3 ,m4 ,michel ,lvy1 ,bull4 ,carte ,gol4 ,france1 ,no4 #ce1 tome v4 7#ahic71 p4 #cb4 ,the view stated in the text is also that adopted by ,prof4 ,brgger with reference to the granite of the ,christiania district4 ,die ,eruptivgesteine des ,kristianiagebietes4 ,it is no doubt true that many intrusive masses1 which must be included under the general name of bosses1 have probably had no connection whatever with volcanic action properly so called4 ,they are plutonic injections1 that is1 portions of the subterranean magma which have been intruded into the terrestrial crust during its periods of disturbance1 and have not been accompanied with any superficial discharges1 which are essential in truly volcanic energy4 ,it has been proposed to draw a distinction between such deep-seated intrusions and those which represent volcanic funnels4#cf ,if this were always practicable it would certainly be desirable4 ,but the distinction is not one that can in every case be satisfactorily drawn4 ,even in regard to granitic bosses1 which may generally be assumed to be plutonic in origin1 the ,british examples just referred to have in all likelihood been connected with undoubted volcanic outbursts4 ,without1 therefore1 attempting here to separate the obviously volcanic necks of eruptive material from the probably plutonic bosses1 ,i propose to describe briefly the general characters of bosses considered as a group of intrusive rocks1 together with the phenomena which accompany them1 and the conditions under which they may have been injected4 ,footnote #cf3 ,m4 ,michel ,lvy1 ,bull4 ,carte ,gol4 ,france1 ,no4 #ce1 tome v4 7#ahic74 ,bosses1 whether of plutonic or volcanic origin1 are frequently not merely single masses of eruptive rock1 but are accompanied with a system of dykes and veins1 some of which can be traced directly into the parent-mass1 while others traverse it as well as the surrounding rocks4 ,hence the history of a boss may be considerably more complex than the external form of the mass might suggest4 ,the petrographical characters of bosses link them with the other underground injections of igneous material1 more especially with sills and laccolites4 ,indeed1 on mere lithological grounds no satisfactory line could be drawn between these various forms of intrusive rocks4 ,the larger the mass the more coarsely crystalline it may be expected to be4 ,but the whole range of structure1 texture and composition1 from those of the narrowest vein to those of the widest boss1 constitutes one connected series of gradations4 ,acid1 intermediate and basic rocks are abundantly displayed among the bosses4 ,huge masses of granite1 granophyre1 quartz-porphyry1 felsite or rhyolite1 represent the acid series4 ,intermediate varieties consist of trachyte1 phonolite1 diorite1 andesite or other rock4 ,the basic bosses include varieties of gabbro1 dolerite1 basalt1 picrite1 and other compounds4 ,in a boss of large size1 a considerable range of texture1 composition and structure may often be observed4 ,the rock is generally much coarser in grain than that of thin sills or dykes4 ,sometimes it exhibits a finer texture along the margin than in the centre1 though this variation is not usually so marked as in sills and dykes4 ,the rapidly-chilled and therefore more close-textured selvage seems to have been developed much more fully in small than in large masses of eruptive material4 ,the latter1 cooling more slowly1 allowed even their marginal parts to retain their heat1 and sometimes perhaps even their molten condition1 longer than small injections4 ,some influence must also have been exercised by the temperature of the rocks into which the eruptive material was intruded4 ,where this temperature was high1 as in deep-seated parts of the crust1 it would allow the intrusive magma to cool more slowly1 and thus to assume a more coarsely crystalline condition4 ,the absence of a close grain round the margins of granitic bosses may be due to this cause4 ,but a much more important distinction may be traced between the central and marginal parts of some large bosses and thick sills4 ,i have already alluded to the fact that while the middle of a large intrusive mass may be decidedly acid1 taking even the form of granite1 the outer borders are sometimes found to be much more basic1 passing into such a rock as gabbro1 or even into some ultra-basic compound4 ,between these extremes of composition no sharp division is sometimes discoverable1 such as might have been expected had the one rock been intruded into the other4 ,the differences graduate so insensibly into each other as to suggest that originally the whole mass of the rock formed one continuous body of eruptive material4 ,it is possible that in some cases the magma itself was heterogeneous at the time of intrusion4#cg ,but the frequency of the distribution of the basic ingredients towards the outer margin1 and the acid towards the centre1 points rather to a process of differentiation among the constituents of the boss before consolidation4 ,in some instances the differentiation would appear to have taken place before crystallization to any great extent had set in1 because the minerals ultimately developed in the central parts differ from those at the sides4 ,in other cases1 the transference of material would seem to have been in progress after the component minerals had crystallized out of the magma1 for they are the same throughout the whole intrusive mass1 but differ in relative proportions from centre to circumference4#ch ,footnote #cg3 ,the ,tertiary gabbros of the ,inner ,hebrides have already been cited1 and will be more fully described in a later chapter as exhibiting the heterogeneousness of an eruptive magma4 ,footnote #ch3 ,see ,messrs4 ,dakyns and ,teall1 ,quart4 ,journ4 ,geol4 ,soc4 xlviii4 7#ahib71 p4 #ajd2 ,prof4 ,brgger1 op4 cit4 #a4 7#ahid71 p4 #ae2 ,mr4 ,a4 ,harker1 op4 cit4 p4 #cbj2 ,prof4 ,iddings1 ,journ4 ,geol4 ,chicago1 i4 7#ahic71 p4 #hcc2 ,bull4 ,phil4 ,soc4 ,washington1 ii4 7#ahij71 p4 #aia2 #ahib1 p4 #hi4 ,as illustrations of these features ,i may cite two good examples1 one from ,scotland and one from ,england4 ,the mass of ,garabol ,hill1 in the ,loch ,lomond district1 consists mainly of granite1 occupying an area of about #ab square miles4 ,messrs4 ,dakyns and ,teall have shown that while the central portions consist of granite1 the south-eastern margin affords a remarkable series of intermediate rocks1 such as hornblende-biotite-granite1 tonalite 7quartz-mica-diorite71 diorite and augite-diorite1 which lead us outwards into highly basic compounds1 including wehrlites 7olivine-diallage rocks71 picrites 7olivine-augite rocks71 serpentine 7possibly representing dunites1 saxonites1 and lherzolites71 and a peculiar rock consisting essentially of enstatite1 diallage1 brown hornblende and biotite4 ,the authors regard the whole of these widely different rocks as the products of one original magma1 the more basic marginal area having consolidated first as peridotites1 followed by diorites1 tonalites and granites in the order of increasing acidity4 ,the most acid rock in the whole series consists of felspar and quartz1 is almost devoid of ferro-magnesian minerals1 and occurs in narrow veins in the granite and tonalite4 ,it indicates that after the segregation and consolidation of the whole boss1 ruptures occurred which were filled in by the ascent of the very latest and most acid remaining portion of still fluid magma4#ci ,footnote #ci3 ,messrs4 ,dakyns and ,teall1 ,quart4 ,journ4 ,geol4 ,soc4 xlviii4 7#ahib71 p4 #ajd4 ,the case of ,carrock ,fell in ,cumberland has been described by ,mr4 ,a4 ,harker1 who has ascertained that the gabbro of this boss has in its central portions a specific gravity of less than #bhe and a silica-percentage sometimes as high as #eidf1 whilst its marginal zone gives a specific gravity above #bie and a silica-percentage as low as #cbej4 ,the migration of the heavy iron ores towards the margin is readily apparent to the naked eye1 and is well established by chemical analysis1 the oxides of iron amounting in the centre to #fbd 7,fe#b,o#c #cfj1 ,fe,o #bfd71 and at the margin to #beed 7,fe#b,o#c #hdd1 ,fe,o #agaj74dj ,neither in this instance nor in that of ,garabol ,hill has any evidence been noticed which would suggest that the basic and acid rocks belong to different periods of intrusion4 ,they pass so insensibly into each other as to form in each case one graduated mass4 ,footnote #dj3 ,mr4 ,a4 ,harker1 op4 cit4 p4 #cbj4 ,from these and other examples which have been observed1 it is difficult to escape the conclusion that the differences between the basic margin and the acid centre are due to some process of segregation or differentiation while the mass was still in a liquid condition1 and its constituents could pass from one part of the boss to another4 ,according to ,professor ,brgger1 it may be stated as a general law that differentiation sets in during consolidation1 and is determined by1 and dependent on1 the laws of crystallization in a magma1 in so far as the compounds which1 on given conditions1 would first crystallize out1 diffuse themselves towards the cooling margin so as to produce in the contact-stratum a peculiar chemical composition in the still liquid material before crystallization takes place4#da ,footnote #da3 ,this general conclusion is stated by ,professor ,brgger from his investigation of the rocks of ,gran1 ,quart4 ,journ4 ,geol4 ,soc4 l4 7#ahid71 p4 #cf4 ,if during the process of differentiation1 and before consolidation1 injections of the magma occur1 they may be expected to differ in character according to the portion of the magma from which they are derived4 ,professor ,brgger believes that among the basic eruptive rocks of ,gran in the ,christiania district1 one and the same magma has in the bosses solidified as olivine-gabbro-diabases1 and in the dykes as camptonites1 bostonites1 pyroxenites1 hornblendites1 and more acid augite-diorites4#db ,footnote #db3 ,quart4 ,journ4 ,geol4 ,soc4 l4 7#ahid71 p4 #ce4 ,various opinions have been propounded as to the cause or causes of this so-called differentiation1 but none of them are entirely satisfactory4 ,we must await the results of further exploration in the field and of continued research in the laboratory4 ,what appears to have taken place within a subterranean molten magma which has been propelled into the earth's crust as a boss or laccolite1 with or without a connected system of dykes1 may possibly be made to throw some light on the remarkable changes in the characters of lavas successively erupted from the same vent during the continuance of a volcanic cycle4 ,whether or not any such process of differentiation can be proved to take place within a subterranean volcanic reservoir1 the sequence of erupted lavas bears a curious resemblance to the order in which the constituents of some large bosses succeed each other from margin to centre4 ,the earliest lavas may be of an intermediate or even basic character1 but they generally tend to become more acid4 ,nevertheless alternations of basic and acid lavas which have been noted in various districts would seem to show that if there be a process of differentiation in the magma-basins1 it is not regular and continuous1 but liable to interruption and renewal4 ,the return to basic eruptions1 which so often marks the close of a volcanic cycle1 is likewise not easily explicable on the supposition of continuous differentiation4 ,where no sensible evidence of differentiation is traceable in the general body of a large intrusive mass1 indications that some such process has there been in progress are perhaps supplied by the more acid dykes or veins1 and the so-called segregation veins1 which have been already alluded to as traversing large intrusive masses4 ,though these portions differ to a greater or less extent in texture and composition from the main substance of the boss1 the differences are not such as to prevent us from regarding them as really parts of the same parent magma4 ,the veins1 which are more acid than the rock that they traverse1 may be regarded as having emanated from some central or deeper-seated part of a boss1 which still remained fluid after the marginal or upper portion had consolidated sufficiently far to be capable of being rent open during subterranean disturbance4 ,but that the mass1 though coherent enough to be fissured1 still remained at a high temperature1 may be inferred from the general absence of chilled edges to these veins4 ,the evidence of differentiation supplied by segregation veins has been referred to in the case of ,sills4 ,the study of the petrographical variations in the constitution of large eruptive bosses has a twofold interest for the geologist4 ,in the first place1 it affords him material for an investigation of the changes which a volcanic magma undergoes during its eruption and consolidation1 and thereby provides him with some data for an elucidation of the cause of the sequence of erupted products during a volcanic cycle4 ,in the second place1 it yields to him some interesting analogies with the structures of ancient gneisses1 and thus helps towards the comprehension of the origin and history of these profoundly difficult but deeply fascinating rocks4 ,bosses1 like sills1 occur in the midst of volcanic sheets1 and also as solitary protrusions4 ,where they rise amidst interstratified lavas and tuffs they may often be recognized as occupying the position of volcanic vents4 ,they are then necks1 and their characters in this connection have already been given4 ,where1 however1 as so frequently happens1 they appear among rocks in which no trace of any contemporaneous volcanic material is to be detected1 their relation to former volcanic activity remains uncertain4 ,of this doubtful nature some of the most notable examples are supplied by the great granitic bosses which occur so frequently among the older ,palozoic rocks of ,britain4 ,the age of these can sometimes be approximately fixed1 and is then found to correspond more or less closely with some volcanic episode4 ,thus the granite-bosses of ,galloway1 in the south of ,scotland1 disrupt ,upper ,silurian strata1 but are older than the ,upper ,old ,sandstone4 ,hence they probably belong to the period of the ,lower ,old ,red ,sandstone1 which was eminently characterized by the vigour and long continuance of its volcanoes4 ,the granite of ,arran and of the ,mourne ,mountains can be shown by one line of reasoning to be younger than surrounding ,carboniferous formations1 by other arguments to be probably later than the ,permian period1 and by a review of the whole evidence to form almost certainly part of the volcanic history of ,tertiary time4 ,but even where it can be shown that the uprise of a huge boss of eruptive material was geologically contemporaneous with energetic volcanic action1 this coincidence may not warrant the conclusion that the boss therefore marks one of the volcanic centres of activity4 ,each example must be judged by itself4 ,there have1 doubtless1 been many cases of the intrusion of molten material in bosses1 as well as in sills1 without the establishment of any connection with the surface4 ,such incompleted volcanoes have been revealed by denudation after the removal of a great thickness of superincumbent rock4 ,the evidence which would have decided the question to what extent any of them became true volcanic vents has thus been destroyed4 ,we can only reason tentatively from a careful collation of all the facts that are now recoverable4 ,illustrations of this kind of reasoning will be fully given in subsequent chapters4 ,it has been supposed that a test for the discrimination of a subterranean protrusion from a true volcanic chimney may be found in the condition of the surrounding rocks1 which in the case of the prolonged flow of molten matter up a vent would be likely to undergo far more metamorphism than would be the case in the injection of a single eruptive mass4#dc ,but1 as has been already pointed out1 no special or excessive metamorphism of the encircling rocks is noticeable around many vents4 ,there is certainly no more alteration contiguous to numerous true necks than around bosses1 which there is no reason to suppose ever communicated directly with the surface1 and which were probably the result of a single intrusion4 ,we must always remember that the denudation which has revealed these bosses has generally removed the evidence of their upward termination and of their possible connection with any volcanic ejections4 ,many of them may mark the sites of true vents from which only single eruptions took place4 ,the opening of a volcanic vent does not necessarily imply a prolonged ascent of volcanic material4 ,in a vast number of cases the original eruption was the first and last effort of the volcano1 so that in such circumstances there seems no more reason for much alteration of the walls of the chimney than for the metamorphism of the rocks round a boss1 laccolite1 sill or dyke4 ,footnote #dc3 ,see1 for example1 ,mr4 ,harker1 ,quart4 ,journ4 ,geol4 ,soc4 l4 7#ahid71 p4 #cbi4 ,the metamorphism produced by intrusions of molten material upon the rocks with which they have come in contact has long been studied4 ,its amount varies so greatly in different cases that the conditions on which it has specially depended are not easily determined4 ,three factors have obviously been of great importance--first1 the bulk of the intruded material2 secondly1 the chemical composition and lithological texture and structure of the rocks affected2 and thirdly1 the constitution and temperature of the invading magma4 #a4 ,it is clear that a huge boss of eruptive material will be likely to effect much more alteration of the surrounding rocks than a small boss1 sill or dyke4 ,its initial temperature will probably be higher at the time of its assuming its final place than that of the same material after it has found its way into the narrower space of a thin sill or dyke4 ,it will likewise take much longer to cool4 ,hence the influence of its heat and its vapours will continue to act long after those of the dyke or sill have ceased to manifest themselves4 #b4 ,it is equally evident that much of the resultant metamorphism will depend on the susceptibility of the rocks to change4 ,an obdurate material such as pure quartz-sand1 for example1 will resist further alteration than mere hardening into quartzite4 ,shales and mudstones may be indurated into cherty substances of various textures4 ,limestones and dolomites1 on the other hand1 may become entirely crystalline1 and may even have new minerals1 such as garnet1 tremolite1 pyroxene1 etc41 developed in them4 ,hence in comparing the amount of metamorphism attendant on two separate bosses we must always take into account the nature of the rocks in which it has been induced4 #c4 ,but perhaps the most effective cause of variation in the nature and amount of contact metamorphism has been the constitution of the eruptive magma4 ,a broad distinction may be drawn between the alteration produced by basic and by acid rocks4 ,the intrusion of basic material has often produced singularly little change1 even when the eruptive mass has been of considerable size4 ,the greatest amount of alteration is to be found where the basic boss has caught up and enveloped portions of the surrounding rocks4 ,thus where the gabbro of ,carrock ,fell has invaded the basic ,lower ,silurian lavas of the ,lake ,district1 the enveloped portions of the latter show considerable modification4 ,their groundmass becomes darker and more lustrous1 the felspars assume a clearer appearance and lose some of their conspicuous inclusions1 the pyroxenic constituents are converted into pale amphibole1 and the glassy base disappears4 ,at the actual line of contact the felspars of the lavas have become disengaged from their original matrix1 which seems to have been dissolved and absorbed in the gabbro-magma4 ,brown mica has been exceptionally developed in the altered lava4 ,at the same time1 a change is noticeable in the character of the gabbro itself near the contact4 ,brown mica is there to be seen1 though not a constituent of the rock elsewhere4 ,the eruptive material has incorporated the basic groundmass of the lavas1 leaving the felspars undissolved4#dd ,footnote #dd3 ,mr4 ,harker1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 l4 7#ahid71 p4 #cca4 ,much more serious are the changes produced by intrusions of acid material1 though here again the metamorphism varies within wide limits1 being sometimes hardly perceptible1 and in other cases advancing so far as to convert mere sedimentary material into thoroughly crystalline rocks4 ,small sills and dykes of felsite and granophyre may produce very slight change even upon shales and limestones1 as may be seen among the eruptive rocks of ,skye and ,raasay4 ,large bosses of granophyre1 and still more of granite1 have been accompanied with the most extensive metamorphism4 ,round these eruptive masses every gradation may be traced among sandy and argillaceous sediments1 until they pass into crystalline mica-schists1 which do not appear to be distinguishable from rocks of ,archan age4 ,admirable examples of this extreme alteration may be observed around the great granite bosses of ,galloway4#de ,again1 among calcareous rocks a transition may be traced from dull grey ordinary fossiliferous limestones and dolomites into pure white crystalline marbles1 full of crystals of tremolite1 zoisite1 garnet and other minerals4 ,the alteration of the fossiliferous ,cambrian limestones of ,strath in ,skye by the intrusive bosses of ,tertiary granite well illustrates this change4#df ,footnote #de3 ,see ,explanation to ,sheet #i of the ,geological ,survey of ,scotland1 p4 #bb2 ,prof4 ,bonney and ,mr4 ,allport1 ,proc4 ,roy4 ,soc4 xvi4 7#ahhi72 ,miss ,gardiner1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xlvi4 7#ahij71 p4 #efi4 ,footnote #df3 ,macculloch1 ,trans4 ,geol4 ,soc4 vol4 iii4 7#ahaf71 p4 #a2 ,description of the ,western ,isles1 vol4 i4 p4 #cbb4 ,see also ,quart4 ,journ4 ,geol4 ,soc4 vol4 xiv4 7#aheg71 p4 #a2 and vol4 xliv4 7#ahhh71 p4 #fb4 ,without entering further here into the wide subject of contact metamorphism1 to which a large literature has now been devoted1 we may note the effects which have been produced in the eruptive material itself by its contact with the surrounding rocks4 ,not only have these rocks been altered1 but very considerable modifications have likewise taken place in the active agent of the change4 ,sometimes the alteration of the invading material has been effected without any sensible absorption of the mineral constituents of the rocks invaded4 ,this appears to be the case in those instances where sheets of basalt1 intruded among coals or highly carbonaceous shales1 have lost their compact crystalline character and have become mere clays4 ,in the coal-fields of ,britain1 where many examples of this change have been noted1 the igneous material is known as white trap4 ,the iron oxides have been in great part removed1 or1 together with the lime of the component minerals1 have been converted into carbonates4 ,traces of the original felspar crystals may still be detected1 but the groundmass has been changed into a dull1 earthy1 friable and decomposed substance4 ,nearly always1 however1 the alteration of the intrusive magma has resulted from the incorporation of portions of the surrounding rocks4 ,reference has been made above to the alteration of the ,carrock ,fell gabbro by the absorption of some of the basic lavas around it4 ,but still more remarkable is the change produced in some acid rocks by the incorporation of basic material into their substance4 ,professor ,sollas has described in great detail a remarkable instance of this effect in the probably ,tertiary eruptive rocks of the ,carlingford district in the north-east of ,ireland4 ,he has ascertained that the eruptive gabbro of that district is older than the granite1 for it is traversed by granophyre dykes which enclose pieces of it4 ,the granophyre dykes1 on the other hand1 often show a lithoidal or chilled margin1 which is not visible in the gabbro4 ,he believes that the gabbro is not only older than the acid protrusions1 but was already completely solid1 traversed by contraction-joints1 and probably fractured by earth-movements1 before the injection of the granophyric material1 which at the time of its intrusion was in a state of extreme fluidity1 for it has found its way into the minutest cracks and crevices4 ,he has especially studied the alteration produced by the granophyre upon the enclosed pieces of basic rock4 ,the diallage1 isolated from the other constituents of the gabbro1 may commonly be seen to have broken up into numerous granules1 like the augite grains of basalt1 while in some cases biotite and hornblende have been developed with the concomitant excretion of magnetite4 ,the acid rock itself has undergone considerable modification owing to the incorporation of basic material into its substance4 ,professor ,sollas distinguishes the following varieties of the rock3--,biotite-granophyre1 biotite-amphibole-granophyre1 augite-granophyre1 diallage-amphibole-augite-granophyre4#dg ,footnote #dg3 ,trans4 ,roy4 ,irish ,acad4 xxx4 7#ahid71 part xii4 p4 #dgg4 ,similar phenomena have been described by ,mr4 ,harker as occurring where granophyre has invaded the gabbro of ,carrock ,fell4#dh ,the same observer has more recently detected some interesting examples furnished by injections of ,tertiary granophyre in the agglomerates of ,skye4 ,the acid rock is roughly estimated by him to have taken up about one-fourth of its bulk of gabbro fragments4 ,he has investigated the minute structure of the rock thus constituted1 and has been able to recognize the augite of the original gabbro1 in various stages of alteration and completely isolated1 the other minerals having been dissolved in the acid magma4#di ,footnote #dh3 ,quart4 ,journ4 ,geol4 ,soc4 li4 7#ahie71 p4 #ahc4 ,footnote #di3 ,op4 cit4 lii4 ,the metamorphism produced upon fragments of different kinds of foreign material enclosed within various igneous rocks has in recent years been studied in great detail by ,professor ,lacroix--,les ,enclaves des ,roches ,volcaniques1 ,macon1 #ahic4 ,it is not easy to comprehend the conditions under which large masses of molten material have been injected into the crust of the earth4 ,the two main factors in volcanic action--terrestrial contraction and the energy of the vapours in the magma--have no doubt played the chief part in the process4 ,but the relative share of each and the way in which the enormous load of overlying rock has been overcome are not readily intelligible4 ,let us first consider for a moment the pressure of the superincumbent crust under which the injection in many cases took place4 ,the ,whin ,sill of ,england may serve as a good illustration of the difficulties of the problem4 ,this notable mass of intrusive rock has been forced between the stratification planes of the ,carboniferous ,limestone series in one1 or sometimes more than one1 sheet4 ,it stretches for a horizontal distance of not less than #hj miles with an average thickness of between #hj and #ajj feet4 ,from the area over which it can be traced its total extent underground must be at least #djj square miles 7see ,chapter xxix474 ,in any single section the ,whin ,sill might be supposed to be a truly interstratified sheet1 so evenly does it seem to be intercalated between the sedimentary strata4 ,but here and there it diverges upward or downward in such a way as to prove it to be really a vast injected sheet4 ,the age of the injection cannot be precisely fixed4 ,it must be later than the ,carboniferous ,limestone4 ,there is no trace of any stratigraphical break in the ,carboniferous system of the region traversed by the sill4 ,if the injection took place during the ,carboniferous period1 it does not appear to have been attended with any local disturbance1 such as we might suppose would have been likely to accompany the extravasation of so enormous a mass of igneous material4 ,if the date of injection be assigned to the next volcanic episode in the geological history of ,britain--that of the ,permian period--it will follow that the ,whin ,sill was intruded into its present position under the superincumbent weight of the whole of the ,carboniferous system higher than the platform followed by the injected rock4 ,the overlying body of strata would thus exceed #ejjj feet in thickness1 or in round numbers would amount at least to an ,english mile4 ,the pressure of this mass of superincumbent material1 at the depth at which the injected magma was forced between the strata1 must have been so gigantic that it is difficult to believe that the energy of the magma would have been able to achieve of itself so stupendous a task as the formation of the ,great ,whin ,sill4 ,the volume of injected material is likewise deserving of special attention4 ,many sills exceed #cjj or #djj feet in thickness2 and some laccolites must enormously surpass these limits4 ,the intrusion of so vast a body of new material into the terrestrial crust will necessitate either a corresponding elevation of that part of the crust overlying the injected magma or a subsidence of that part underlying it1 or some combination of both movements4 ,it is conceivable that1 where the body of protruded magma was large and the thickness of overlying crust was small1 the expansive force of the vapours under high tension in the molten rock may have sufficed for the uplift4 ,this result will be most likely to be effected around a volcanic chimney where the magma has the least amount of overlying load1 and encounters that relief from pressure which enables it to become a powerful agent in terrestrial physics4 ,but in the case of the larger bodies of injected rock1 especially where they do not seem to have been accompanied by the opening of any volcanic vents1 the propulsion of the igneous material into the crust has probably been effected as a consequence of disturbance of the terrestrial crust4 ,when the strain of contraction leads to the pushing upward of the terrestrial areas intervening between wide regions of subsidence1 even though the differential movement may be slight1 the isogeotherms undergo deformation4 ,the intensely hot nucleus is squeezed upward1 and if in the process of compression ruptures take place in the crust1 and cavities in it are consequently opened1 the magma will at once be forced into them4 ,such ruptures may be expected to take place along lines of weakness4 ,rocks will split along their stratification-planes1 and the tendency to separation along these lines may be aided by the readiness of the energetic magma to find its way into and to enlarge every available opening4 ,hence we may expect that1 besides vertical fractures1 leading to the production of dykes and bosses1 there will often be horizontal thrusts and ruptures1 which will give rise to the formation of sills4 ,there is still another feature of terrestrial contraction which may help us to follow the behaviour of the magma within the crust4 ,plication of the crust is one of the most characteristic results of the contracting strain4 ,where a great series of sedimentary formations has been violently compressed so that its component strata have been thrown into rapid folds and squeezed into a vertical position1 the portion of the crust thus treated may possibly be on the whole strengthened against the uprise of molten material through it4 ,but the folding is often accompanied with dislocation4 ,not only are the rocks thrown into endless plications1 but portions of them are ruptured and even driven horizontally over other parts4 ,such greatly disturbed areas of the crust are not infrequently found to have been plentifully injected with igneous rocks in the form of dykes1 veins1 sills1 laccolites and bosses4 ,the elevation of a mountain-chain is known to be accompanied with a diminution of density in the crust underneath4 ,mr4 ,o4 ,fisher has suggested that along such lines of terrestrial uplift there may be a double bulge in the crust1 one portion rising to form the upheaved land and the other sinking down into the hot nucleus4 ,if the lighter descending crust were there melted it might form a magma ready to be poured out as lava on the opening of any vent4 ,the lava thus ejected would be of the lighter kinds4 ,it has been remarked as certainly a curious fact that the lavas which issue from high mountain ranges are generally much more acid than the heavy basic lavas which are so characteristic of volcanoes close to the level of the sea4 ,but even where no actual mountain-chain is formed1 there are gentle undulations of the crust which no doubt also affect the isogeotherms4 ,if any series of disturbances should give rise to a double system of such undulations1 one crossing the other1 there would be limited dome-shaped elevations at the intersections of these waves1 and if at the same time actual rupture of the crust should take place1 the magma might find its way upward under such domes and give rise to the formation of laccolitic intrusions4 ,cessation of the earth-movements might allow the intruded material slowly to solidify without ever making an opening to the surface and forming a volcano4 ,doubtless many sills1 laccolites and bosses represent such early or arrested stages in volcanic history4 ,propelled into the crust at a high temperature1 and endowed with great energy from the tension of its absorbed vapours and gases1 the magma will avail itself of every rent which may be opened in the surrounding crust1 and where it has succeeded in reaching the surface1 its own explosive violence may enable it to rupture the crust still further1 and open for itself many new passages4 ,thus an eruptive laccolite or boss is often fringed with veins1 dykes and sills which proceed from its mass into the rocks around4 ,the question how far an ascending mass of magma can melt down its walls is one to which no definite answer can yet be given4 ,recent observations show that where the difference in the silica percentage between the magma and the rock attacked is great1 there may be considerable dissolution of material from this cause4 ,allusion has already been made to ,mr4 ,harker's computation that some of the acid granophyres of ,skye have melted down about a fourth of their bulk of the basic gabbros4 ,if such a reaction should take place between the magma of a boss1 sill or laccolite and the rocks among which it has been intruded1 great changes might result in the composition of the intruded rock4 ,we are not yet1 however1 in possession of evidence to indicate that absorption of this kind really takes place on an extensive scale within the earth's crust4 ,if it did occur to a large extent1 we should expect much greater varieties in the composition of eruptive rocks than usually occur1 and also some observable relation between the composition of the igneous material and that of the rocks into which it has been injected4 ,but enough is not yet known of this subject to warrant any decided opinion regarding it4 ,,chapter ,,vii ,influence of ,volcanic ,rocks on the ,scenery of the ,land--,effects of ,denudation4 ,as considerable popular misapprehension exists respecting the part which volcanism has played in the evolution of the existing topography of the earth's surface1 and as the ,british ,isles1 from their varied geological structure1 offer special facilities for the discussion of this subject1 it may not be out of place to devote a final section of the present ,introduction to a consideration of the real topographical influence of volcanic action4 ,with modern1 and especially with active1 volcanoes we need not here concern ourselves4 ,their topographical forms are well known1 and give rise to no difficulty4 ,the lofty cones of the ,vesuvian type1 with their widespread lavas and ashes1 their vast craters and their abundant parasitic volcanoes2 the crowded1 but generally diminutive1 cones and domes of the puy type1 so well displayed in ,auvergne1 the ,eifel and the ,bay of ,naples2 and the vast lava deserts of the plateaux1 so characteristically developed in ,iceland and ,western ,america1 illustrate the various ways in which volcanic energy directly changes the contours of a terrestrial surface4 ,but the circumstances are altered when we deal with the topographical influence of long extinct volcanoes4 ,other agencies then come into play1 and some caution may be needed in the effort to disentangle the elements of the complicated problem1 and to assign to each contributing cause its own proper effect4 ,reference has already been made to the continuous denudation of volcanic hills from the time that they are first erupted4 ,but the comparative rapidity of the waste and the remarkable topographical changes which it involves can hardly be adequately realized without the inspection of an actual example4 ,a visit to the back of ,monte ,somma1 already alluded to1 will teach the observer1 far more vividly than books can do1 how a volcanic cone is affected by daily meteoric changes4 ,the sides of such a cone may remain tolerably uniform slopes so long as they are always being renewed by deposits from fresh eruptions4 ,but when the volcanic activity ceases1 and the declivities undergo no such reparation1 they are rapidly channelled by the descent of rain-water1 until the furrows grow by degrees wide and deep ravines1 with only narrow and continually-diminishing crests between them4 ,if unchecked by any fresh discharge of volcanic material1 the degradation will at last have removed the whole cone4 ,it is thus obvious that purely volcanic topography1 that is1 the terrestrial scenery due directly to the eruption of materials from within the earth1 can never become in a geological sense very old4 ,it can only endure so long as it is continually renewed by fresh eruptions1 or where it is carried down by subsidence under water and is there buried under a cover of protecting sediments4 ,when1 therefore1 we meet with volcanic rocks of ancient date exposed at the surface1 we may be quite certain that their present contours are not those of the original volcano1 but have been brought about by the processes of denudation4 ,it is true that1 in the general erosion of the surface of the land1 volcanic rocks of ancient date sometimes rise into wonderfully craggy heights1 including1 perhaps1 cones and deep crater-like hollows1 which to popular imagination betoken contours left by now extinguished volcanic fires4 ,examples of such scenery are familiar in various parts of ,britain2 but the resemblance to recent volcanic topography is deceptive4 ,there are1 indeed1 a few hills wherein the progress of denudation seems not as yet to have entirely removed the lavas and tuffs that gathered round the original vents4 ,some of the tuff-cones of eastern ,fife1 for example1 present cases of this kind4 ,again1 the great granophyre domes and cones of the ,tertiary volcanic series of the ,inner ,hebrides1 though they have undoubtedly been extensively denuded1 may possibly retain contours that do not greatly differ from those which these protruded bosses originally assumed under the mass of rock which has been removed from them4 ,nevertheless1 putting such doubtful exceptions aside1 we may confidently affirm that hills composed of ancient volcanic material give no clue to the forms of the original volcanoes4 ,it can hardly be too often repeated that the fundamental law in the universal decay and sculpture of the land is that the waste is proportioned to the resistance offered to it3 the softer rocks are worn down with comparative rapidity1 while the harder varieties are left projecting above them4 ,as a general rule1 volcanic rocks are more durable than those among which they are interstratified1 and hence project above them1 but this is not always the case4 ,no universal rule can1 indeed1 be laid down with regard to the relative durability of any rocks4 ,while1 therefore1 topographic contours afford a valuable indication of the nature and disposition of the rocks below the surface1 they cannot be relied upon as in all circumstances an infallible guide in this respect4 ,no better proof can be offered of the caution that is needed in tracing such contours back to their origin than is furnished by the old volcanic rocks of ,britain4 ,these eruptive masses1 consisting usually of durable materials and ranging through a vast cycle of geological time1 usually rise into prominent features and thus support the general law4 ,but they include also many easily eroded members1 which1 instead of forming eminences1 are worn into hollows4 ,they include1 in short1 every type of scenery1 from featureless plains and rolling lowlands to craggy and spiry mountains4 ,the first point1 then1 which is established in an investigation of the topographical influence of old volcanic rocks is that their prevailing prominence arises from relative durability amidst universal degradation4 ,when we proceed further to inquire why they vary so much from each other in different places1 and how their complicated details of feature have been elaborated1 we soon learn that such local peculiarities have arisen mainly from variations in the internal structure and grouping of the rocks themselves4 ,here again the general law of sculpture comes into play4 ,the local features have depended upon the comparative resistance offered to the sculpturing agents by the different portions of a volcanic series4 ,each distinct variety of rock possesses its own characteristic internal structure4 ,the lines along which atmospheric disintegration will most effectually carry on its carving work are thus already traced in the very substance and architecture of the rock itself4 ,each rock consequently yields in its own way to the processes of disintegration1 and thus contributes its own distinctive share to topographical feature4 ,among the massive rocks abundant examples of such special types of weathering may be cited1 from the acid and basic series1 and from superficial lavas as well as from intrusive bosses and sills4 ,acid bosses1 such as those of granite1 granophyre and quartz-porphyry1 tend to weather into blocks and finally into sand1 and as this tendency is somewhat uniformly distributed through the rocks1 they are apt to assume rounded1 dome-shaped or conical forms which1 at a distance1 may seem to have smooth declivities1 but on examination are generally found to be covered with a slowly-descending sheet of disintegrated blocks and debris 7,fig4 #cdf74 ,when less prone to decay1 and especially where traversed by a strongly-defined system of vertical joints1 they may shoot up into tower-like heights1 with prominent spires and obelisks4 ,basic bosses1 when their materials decay somewhat rapidly1 give rise to analogous topographical forms1 though the more fertile soils which they produce generally lead to their being clothed with vegetation4 ,where they consist of an obdurate rock1 much jointed and fissured1 like the gabbro of the ,inner ,hebrides1 they form exceedingly rugged mountains1 terminating upward in serrated crests and groups of aiguilles 7,figs4 #cca1 #ccc74 ,acid lavas that have been superficially erupted weather into irregularly craggy hills1 like the flanks of ,snowdon4 ,those of intermediate composition1 where they have accumulated in thick masses1 are apt to weather into conical forms1 as may be seen among the ,cheviot1 ,pentland and ,garleton ,hills 7,figs4 #aji1 #aaj1 #acc72 but where they have been poured out in successive thin sheets they have built up undulating plateaux with terraced sides1 as among the ,ayrshire and ,campsie ,fells and the hills of ,lorne 7,figs4 #ii1 #ajg74 ,basic lavas have issued in comparatively thin sheets1 frequently columnar or slaggy1 forming flat-topped hills and terraced escarpments1 such as are typically developed among the ,tertiary basalt-plateaux of the ,inner ,hebrides and the ,faroe ,islands 7,figs4 #aa1 #bfe1 #bhc1 #bhd1 #bhf74 ,one of the most frequent causes of local peculiarities of topography among old volcanic rocks is the intercalation of very distinct varieties of material in the same volcanic series4 ,where1 for instance1 lavas and tuffs alternate1 great inequalities of surface may be produced4 ,the tuffs1 being generally more friable1 decay faster and give rise to hollows1 while the lavas1 being more durable1 project in bold ridges or rise into mural escarpments 7,fig4 #bfe74 ,again1 where dykes weather more readily than the rocks which they traverse1 they originate deep narrow clefts1 while where they weather more slowly than the rocks around them1 they project as dark ribs4 ,thus in ,skye some dykes which rise through the obdurate gabbro are marked by chasms which reach up even to the highest crests of the mountains 7,fig4 #ccc71 while of those which run in the pale crumbling granophyre1 some stand up as black walls that can be followed with the eye across the ridges even from a long distance4 ,many further illustrations of these principles might be cited here from the old volcanic districts of ,britain4 ,but they will present themselves successively in later chapters4 ,for my present purpose it is enough to show that the scenery of these districts is not directly due to volcanic action1 but is the immediate result of denudation acting upon volcanic rocks1 modified and directed by their geological structure4 ,it may1 however1 be useful1 in concluding the discussion of this subject1 to cite some typical volcanic regions in the ,british ,isles as illustrations of the relations between geology and topography1 which1 besides impressing the main lesson here enforced1 may serve also to show some of the striking contrasts which geology reveals between the present and former conditions of the surface of the globe4 ,among these contrasts none are more singular than those offered by tracts where volcanic action has once been rife1 and where the picture of ancient geography presented in the rocks differs so widely from the scenery of the same places to-day as to appeal vividly to the imagination4 ,the first district to which ,i may refer where ancient volcanic rocks are well developed is that of ,devonshire4 ,the story of the ,devonian volcanoes will be told in some detail in later chapters1 when it will be shown that the eruptions were again and again renewed during a long course of ages4 ,yet1 abundant as the intercalated lavas and tuffs are1 they can hardly be said to have had any marked effect on the scenery1 though here and there a harder or larger mass of diabase rises into a prominent knoll or isolated hill4 ,when the amount of volcanic material in this region is considered1 we may feel some surprise at the trifling influence which it has exerted in the general denudation of the surface4 ,to one who wanders over the rich champaign of southern ,devonshire1 and surveys from some higher prominence the undulating tree-crowned ridges that slope down into orchard-filled hollows1 and the green uplands that sweep in successive waves of verdure to the distant blue tors of ,dartmoor1 the scene appears as a type of all that is most peaceful1 varied and fertile in ,english landscape4 ,in the trim luxuriance that meets the eye on every side1 the hand of man is apparent1 though from many a point of vantage no sound may be heard for a time to show that he himself is anywhere near us4 ,yet ever and anon from the deep lanes1 hidden out of sight under their canopy of foliage1 there will come the creak of the groaning waggon and the crack of the waggoner's whip1 as evidence that there are roads and human traffic through this bosky silent country4 ,amid so much quiet beauty1 where every feature seems to be eloquent of long generations of undisturbed repose1 it must surely stir the imagination to be told that underneath these orchards1 meadows and woodlands lie the mouldering remnants of once active and long-lived volcanoes4 ,yet we have only to descend into one of the deep lanes to find the crumbling lavas and ashes of the old eruptions4 ,the landscape has1 in truth1 been carved out of these volcanic rocks1 and their decomposition has furnished the rich loam that nourishes so luxuriant a vegetation4 ,not less impressive is the contrast presented between the present and former condition of the broad pastoral uplands of the south of ,scotland4 ,nowhere in the ,british ,islands can the feeling of mere loneliness be more perfectly experienced than among these elevated tracts of bare moorland4 ,they have nothing of the grandeur of outline peculiar to mountain tracts4 ,sometimes1 for miles around one of their conspicuous summits1 we may see no projecting knob or pinnacle4 ,the rocks have been gently rounded off into broad featureless hills1 which sink into winding valleys1 each with its thread of streamlet and its farms along the bottom1 and its scattered remnants of birch-wood or alder-copse along its slopes and dingles4 ,across miles of heathy pasture and moorland1 on the summits of this great tableland1 we may perchance see no sign of man or his handiwork1 though the bleating of the sheep and the far-off barking of the collie tell that we are here within the quiet domain of the south-country shepherd4 ,in this pastoral territory1 also1 though they hardly affect the scenery1 volcanic rocks come to the surface where the foldings of the earth's crust have brought up the oldest formations4 ,their appearance extends over so wide an area as to show that a large part of these uplands lies on a deeply-buried volcanic floor4 ,a whole series of submarine volcanoes1 extending over an area of many hundreds of square miles1 and still in great part overlain with the accumulated sands and silts of the sea-bottom1 now hardened into stone1 underlies these quiet hills and lonely valleys4 ,a contrast of another type meets us in the broad midland valley of ,scotland4 ,around the city of ,edinburgh1 for instance1 the landscape is diversified by many hills and crags which show where harder rocks project from amidst the sediments of the ,carboniferous system4 ,on some of these crags the forts of the early races1 the towers of ,celt and ,saxon1 and the feudal castles of the middle ages were successively planted1 and round their base clustered for protection the cots of the peasants and the earliest homesteads of the future city4 ,beneath these crags many of the most notable events in the stormy annals of the country were transacted4 ,under their shadow1 and not without inspiration from their local form and colour1 literature1 art and science have arisen and flourished4 ,nowhere1 in short1 within the compass of the ,british ,isles has the political and intellectual progress of the people been more plainly affected by the environment than in this central district of ,scotland4 ,when now we inquire into the origin and history of the topography which has so influenced the population around it1 we find that its prominences are relics of ancient volcanoes4 ,the feudal towers are based on sills and dykes and necks4 ,the fields and gardens1 monuments and roadways1 overlie sheets of lava or beds of volcanic ashes4 ,not only is every conspicuous eminence immediately around of volcanic origin1 but even the ranges of blue hills that close in the distant view to south and north and east and west are mainly built up of lavas and tuffs4 ,the eruptions of which these heights are memorials belong to a vast range of geological ages1 the latest of them having passed away long before the advent of man4 ,but they have left their traces deeply engraven in the rocky framework of the landscape4 ,while human history1 stormy or peaceful1 has been slowly evolving itself during the progress of the centuries in these fertile lowlands1 the crags and heights have remained as memorials of an earlier history when ,central ,scotland continued for many ages to be the theatre of vigorous volcanic activity4 ,as a final illustration of the influence of volcanic rocks in scenery1 and of the contrast between their origin and their present condition1 ,i may cite the more prominent groups of hills in the ,inner ,hebrides4 ,in the singularly varied landscapes of that region three distinct types of topography attract the eye of the traveller4 ,these are best combined and most fully developed in the island of ,skye4 ,throughout the northern half of that picturesque island1 the ground rises into a rolling tableland1 deeply penetrated by arms of the sea1 into which it slopes in green declivities1 while along its outer borders it plunges in ranges of precipice into the ,atlantic4 ,everywhere1 alike on the cliffs and the inland slopes1 long parallel lines of rock-terrace meet the eye4 ,these mount one above another from the shores up to the flat tops of the highest hills1 presenting level or gently-inclined bars of dark crag that rise above slopes of debris1 green sward and bracken4 ,it is these parallel1 sharply-defined bars of rock1 with their intervening strips of verdure1 that give its distinctive character to the scenery of northern ,skye4 ,on hillside after hillside and in valley after valley1 they reappear with the same almost artificial monotony4 ,and far beyond the limits of ,skye they are repeated in one island after another1 all down the chain of the ,inner ,hebrides4 ,in striking contrast to this scenery1 and abruptly bounding it on the south1 rise the ,red ,hills of ,skye--a singular group of connected cones4 ,alike in form and in colour1 these hills stand apart from everything around them4 ,the verdure of the northern terraced tableland here entirely disappears4 ,the slopes are sheets of angular debris1--huge blocks of naked stone and trails of sand1 amidst which hardly any vegetation finds a footing4 ,the decay of the rock gives it a pale yellowish-grey hue1 which after rain deepens into russet1 so that in favourable lights these strange cones gleam with a warm glow as if they1 in some special way1 could catch and reflect the radiance of the sky4 ,immediately to the west of these pale smooth-sloped cones1 the dark mass of the ,cuillin ,hills completes the interruption of the northern tableland4 ,in almost every topographical feature these hills present a contrast to the other two kinds of scenery4 ,their forms are more rugged than those of any other hill-group in ,britain 7,fig4 #cca74 ,every declivity among them is an irregular pile of crags1 every crest is notched like a saw1 every peak is sharpened into a pinnacle4 ,instead of being buried under vast sheets of their own debris1 these hills show everywhere their naked rock1 which seems to brave the elements as few other rocks can do4 ,unlike the pale ,red ,hills1 they are dark1 almost black in tone1 though when canopied with cloud they assume a hue of deepest violet4 ,each of these three distinct types of topography owes its existence to the way in which a special kind of volcanic rock yields to the influences of denudation4 ,the terraced tableland of the north is built up of hundreds of sheets of basaltic lava1 each of the long level ledges of brown rock marking the outcrop of one or more of these once molten streams4 ,the black rugged mass of the ,cuillin ,hills consists of a vast protruded body of eruptive material1 which1 in the form of endless sills and bosses of gabbro and dolerite1 has invaded the basalt-plateau1 and has now been revealed by the gradual removal of the portion of that plateau which it upraised4 ,the pale cones and domes of the ,red ,hills mark the place of one of the last protrusions in the volcanic history of ,britain--that of large masses of an acid magma1 which broke through the basalt-plateau and also disrupted the earlier gabbro4 ,in no part of ,north-,western ,europe has volcanic activity left more varied and abundant records of its operations than in these three contiguous tracts of ,skye4 ,it is interesting therefore to note the striking contrast between the former and the present landscapes of the region4 ,the lavas of the basaltic tableland crumble into a rich loam1 that in the mild moist climate of the ,hebrides supports a greener verdure than any of the other rocks around will yield4 ,the uplands have accordingly become pasture-grounds for herds of sheep and cattle4 ,the strips of lowland along the valleys and in the recesses of the coast-line furnish the chief tracts of arable land in the island1 and are thus the main centres of the crofter population4 ,the bays and creeks of the much-indented shores form natural harbours1 which in former days attracted the ,norse sea-rovers1 and supplied them with sites for their settlements4 ,norse names still linger on headland and inlet1 but the spirit of adventure has passed away1 and a few poor fishing-boats1 here and there drawn up on the beach1 are usually the only token that the islanders make any attempt to gather the harvest of the sea4 ,the mountain groups which so abruptly bound the basalt-plateau on the south1 and present in their topographical features such distinctive scenery1 comprise a region too lofty1 too rugged and too barren for human occupation4 ,the black ,cuillins and the pale ,red ,hills are solitudes left to the few wild creatures that have not yet been exterminated4 ,the corries are the home of the red deer4 ,the gabbro cliffs are haunts of the eagle and the raven4 ,where patches of soil have gathered in the crannies of the gabbro1 alpine plants find their home4 ,in the chasms left by the decay of the dykes between the vertical walls of their fissures1 the winter snows linger into summer1 and conceal with their thick drifts the mouldering surface of the once molten rock beneath them4 ,on every side and at every turn a mute appeal is made to the imagination by the strange contrasts between the quiet restfulness of to-day1 when the sculpture-tools of nature are each busily carving the features of the landscape1 and the tumult of the time when the rocks1 now so silent1 were erupted4 99 99 99 99 99 ,the general discussion of the subject of ,volcanism in this ,introduction will1 ,i hope1 have prepared the reader who has no special geological training for entering upon the more detailed descriptions in the rest of this treatise4 ,as already stated1 the chronological order of arrangement will be followed4 ,beginning with the records of the earliest ages1 we shall follow the story of volcanic action down to the end of the latest eruptions4 ,each great geological system will be taken as a whole1 representing a long period of time1 and its volcanic evolution will be traced from the beginning of the period to the close4 ,some variety of treatment is necessarily entailed by the wide range in the nature and amount of the evidence for the volcanic history of different ages4 ,but where practicable1 an outline will first be given of what can be gathered respecting the physical geography of each geological period in ,britain4 ,in the description which will then follow of the volcanic phenomena1 an account of the general characters of the erupted rocks will precede the more detailed narrative of the history of the volcanic eruptions in the several regions where they took place4 ,references to the published literature of each formation will be given in the first part of each section1 or will be introduced in subsequent pages1 as may be found most convenient4 ,,book ,,ii ,,volcanic ,,action ,,in ,,pre-,,cambrian ,,time ,,chapter ,,viii ,,pre-,,cambrian ,,volcanoes ,the ,beginnings of ,geological ,history--,difficulties in fixing on a generally-applicable ,terminology--i4 ,the ,lewisian 7,archan7 ,gneiss2 ii4 ,the ,dalradian or ,younger ,schists of ,scotland2 iii4 ,the ,gneisses and ,schists of ,anglesey2 iv4 ,the ,uriconian ,volcanoes2 v4 ,the ,malvern ,volcano2 vi4 ,the ,charnwood ,forest ,volcano4 ,the early geological history of this globe1 like the early history of mankind1 must be drawn from records at once scanty and hardly decipherable4 ,exposed to the long series of revolutions which the surface of the planet has undergone1 these records1 never perhaps complete at the first1 have been in large measure obliterated4 ,even where they still exist1 their meaning is often so doubtful that1 in trying to interpret it1 we find little solid footing1 and feel ourselves to be groping1 as it were1 in the dimness of mythological legend1 rather than working in the light of trustworthy and intelligible chronicles4 ,these primeval records have been more particularly the objects of sedulous study during the last twenty years all over ,europe and in ,north ,america4 ,a certain amount of progress in their decipherment has been made4 ,but the problems they still present for solution are numerous and obscure4 ,fortunately1 with many of these problems the subject of the present treatise is not immediately connected4 ,we need only concern ourselves with those which are related to the history of primeval volcanic activity4 ,to the earliest and least definite division of the geological annals various names have been applied4 ,some writers1 believing that this period preceded the first appearance of plants or animals upon the globe1 have named it ,azoic--the lifeless age of geological history4 ,but the absence of any hitherto detected trace of organic existence among the oldest known rocks cannot be held to prove that these rocks were formed before the advent of living things on the surface of the earth4 ,the chance discovery of a single fossil1 which might at any moment be made1 would show the name ,azoic to be a misnomer4 ,other geologists1 believing that1 as a matter of fact1 organic structures of low types do actually occur in them1 have called these old rocks ,eozoic1 to denote that they were deposited during the dawn of life upon our planet4 ,but the supposed organisms have not been everywhere accepted as evidence of former life4 ,by many able observers they are regarded as mere mineral aggregates4 ,another term1 ,archan1 has been proposed for the primeval ages of geological history1 which are recorded in rocks that carry us as far as may ever be possible towards the beginnings of that history4 ,in choosing some general term to include the oldest known parts of the earth's crust1 geologists are apt unconsciously to assume that the rocks thus classed together represent a definite section of geological time1 comparable1 for instance1 to that denoted by one of the ,palozoic systems4 ,yet it is obvious that1 under one of these general terms of convenient classification1 a most multifarious series of rocks may be included1 representing not one but possibly many1 and widely separated1 periods of geological history4 ,in many countries the oldest sedimentary accumulations1 whether fossiliferous or not1 are underlain by a series of crystalline rocks1 which consist in great part of coarse massive gneisses and other schists4 ,all over the world these rocks present a singular sameness of structure and composition4 ,what might be found below them no man can say4 ,they are in each country the oldest rocks of which anything is yet known1 and whatsoever may be our theory of their origin1 we must1 at least for the present1 start from them as the fundamental platform of the terrestrial crust4 ,but though crystalline rocks of this persistent character are widely distributed1 both in the ,old ,world and in the ,new1 they in themselves furnish no means of determining their precise geological age4 ,no method has yet been devised whereby the oldest gneiss of one country can be shown to be the true stratigraphical equivalent of the oldest gneiss of another4 ,palontology is here of no avail1 and ,petrology has not yet provided us with such a genetic scheme as will enable us to make use of minerals and rock-structures1 as we do of fossils1 in the determination of geological horizons4 ,all that can be positively affirmed regarding the stratigraphical relations of the rocks in question is that they are vastly more ancient than the oldest sedimentary and fossiliferous formations in each country where they are found4 ,the ,lewisian gneiss of the north-west of ,scotland1 the ,urgneiss of ,central ,europe1 and the ,laurentian gneiss of ,canada occupy similar stratigraphical positions1 and present a close resemblance in lithological characters4 ,we may conveniently class them under one common name to denote this general relationship4 ,but we have1 as yet1 no means of determining how far they belong to one continuous period of geological history4 ,they may really be of vastly different degrees of antiquity4 ,from the very nature of the case1 any name by which we may choose to designate such ancient rocks cannot possess the precise stratigraphical value of the terms applied to the fossiliferous formations4 ,yet the convenience of possessing such a general descriptive epithet is obvious4 ,until much more knowledge of the subject has been gained1 any terminology which may be proposed must be regarded as more or less provisional4 ,the comprehensive term pre-,cambrian may be usefully adopted as a general designation for all rocks older than the base of the ,cambrian system1 irrespective of their nature and origin4 ,already it is well known that under this term a vast series of rocks1 igneous and sedimentary1 is included4 ,in some regions several successive formations1 or systems of formations1 may be recognized in this series4 ,but until some method has been devised for determining the stratigraphical relations of these formations in different regions1 it would seem safest not to attempt to introduce general names for universal adoption1 but to let the sequence of rocks in each distinct geological province be expressed by a local terminology4 ,this caution is more especially desirable in the case of sedimentary deposits4 ,we may surmise as to the equivalence of the rocks called ,huronian1 ,torridonian and ,longmyndian1 but whilst so much is mere conjecture1 it is certainly injudicious to transfer the local names of one province to the rocks of another4 ,the only relaxation of this general precaution which ,i think may at present be made is the adoption of a common name for the oldest type of gneisses4 ,the term ,archan has been applied to these rocks1 and if it is used simply to express a common petrographical type1 occupying the lowest horizon in the stratigraphical series of a country1 it has obvious advantages4 ,but ,i would still retain the local names as subordinate terms to mark the local characteristics of the ,archan rocks of each province4 ,thus the ,laurentian rocks of ,canada and the ,lewisian rocks of ,scotland are widely-separated representatives of the peculiar stratigraphical series which is known as ,archan4 ,the pre-,cambrian rocks of ,britain include several distinct systems or groups4 ,how far those of even one part of this comparatively limited region are the proper equivalents of those of another and distant part is a problem still unsolved4 ,hence each distinct area1 with its own type of rocks1 will here be treated by itself4 ,the following rock-types will be described3 ,i4 ,the ,lewisian 7,archan7 ,gneiss2 ,,ii4 ,the ,younger 7,dalradian7 ,schists of ,scotland2 ,,iii4 ,the ,gneisses and ,schists of ,anglesey2 ,,iv4 ,the ,uriconian ,group2 ,v4 ,the ,malvern ,group2 ,,vi4 ,the ,charnwood ,forest ,group 7see ,map ,i474 i4 ,,the ,,lewisian 7,,archa,n7 ,,gneiss ,the ,british ,isles are singularly fortunate in possessing an admirable development of pre-,cambrian rocks4 ,these ancient masses rise up in various parts of the islands1 but the region where they are most extensively displayed1 and where their stratigraphical position and sequence are most clearly shown1 lies in the north-west of ,scotland4#ej ,in that territory they form the whole chain of the ,outer ,hebrides1 and likewise extend as an irregular selvage along the western margin of the counties of ,sutherland and ,ross4 ,the lowest known platform of the fossiliferous formations has there been discovered and has been traced for a distance of more than #ajj miles4 ,from this definite horizon1 the high antiquity of all that lies below it is impressively demonstrated4 ,the accompanying diagram 7,fig4 #ce7 will explain the general relations of the various geological formations of the region4 ,footnote #ej3 ,these rocks have been the subject of much discussion1 but geologists are now agreed as to their succession and structure4 ,a full summary of the literature of the controversy regarding them will be found in the ,quarterly ,journal of the ,geological ,society1 vol4 xliv4 7#ahhh71 p4 #cgh4 ,in certain dark shales 7b7 which occupy a well-defined and readily-traceable position among the rocks of ,sutherland and ,ross1 numerous specimens of the trilobite genus ,olenellus1 together with other fossils1 have been found4 ,by common consent among geologists1 the zone of rock in which this genus appears is taken as the lowest stage of the ,cambrian system4 ,in ,britain it marks the oldest known group of fossiliferous strata--the platform on which the whole of the ,palozoic systems rest4 ,illustration3 ,fig4 #ce4--;,diagram illustrating the stratigraphical relations of the pre-,cambrian and ,cambrian rocks of the ,north-west ,highlands of ,scotland4 c1 ,durness ,limestones1 with ,upper ,cambrian and perhaps ,lower ,silurian fossils1 #aejj feet1 top nowhere seen4 b1 ,serpulite grit and fucoid shales1 #gj to #hj feet1 containing the ,olenellus-zone4 a1 ,quartzite1 with abundant annelid tubes1 about #fjj feet4 ,,ii4 ,red ,sandstones and ,conglomerates1 sometimes #hjjj feet or more 7,torridonian74 ,i4 ,gneiss with dykes1 etc4 7,lewisian74 ,from the definite geological epoch indicated by this platform1 we can go backward into pre-,cambrian time1 and realize in some measure how prodigious must be the antiquity of the successive groups of rock which emerge from beneath the base of the ,palozoic systems4 ,nowhere is this antiquity more impressively proclaimed than in the north-west of ,scotland4 ,from below the ,olenellus-zone with its underlying sheets of quartzite 7a71 a thick group of dull red sandstones and conglomerates 7,,ii47 rises into a series of detached conical or pyramidal mountains1 which form one of the most characteristic features in the scenery of that region4 ,as this detrital formation is well developed around ,loch ,torridon1 it has been termed ,torridonian4 ,it attains a thickness of at least #hjjj or #aj1jjj feet1 and is traceable all the way from the extreme northern headlands of ,sutherland to the southern cliffs of the island of ,rum4 ,in judging of the chronological significance of the geological structure of the north-west of ,scotland1 we are first impressed by the stratigraphical break between the base of the ,cambrian system and the ,torridonian deposits below4 ,this break is so complete that here and there the thick intervening mass of sandstones and conglomerates has been nearly or wholly removed by denudation before the lowest ,cambrian strata were laid down4 ,such a discordance marks the passage of a protracted interval of time4 ,again1 when the composition of the ,torridonian rocks is considered1 further striking evidence is obtained of the lapse of long periods4 ,the sandstones1 conglomerates and shales of this pre-,cambrian system present no evidence of cataclysmal action4 ,on the contrary1 they bear testimony that they were accumulated much in the same way and at the same rate as the subsequent ,palozoic systems4 ,in that primeval period1 as now1 sand and silt were spread out under lakes and seas1 were ripple-marked by the agitation of the water1 and were gradually buried under other layers of similar sediment4 ,the accumulation of #aj1jjj feet of such gradually-assorted detritus must have demanded a long series of ages4 ,here1 then1 in the internal structure of the ,torridonian rocks1 there is proof that in passing across them1 from their summit to their base1 we make another vast stride backward into the early past of geological history4 ,but when attention is directed to the relations of the ,torridonian strata to the rocks beneath them1 a still more striking proof of an enormously protracted period of time is obtained4 ,between the two series of formations lies one of the most marked stratigraphical breaks in the geological structure of the ,british ,isles4 ,there is absolutely nothing in common between them1 save that the conglomerates and sandstones have been largely made out of the waste of the underlying gneiss4 ,the denudation of the crystalline rocks before the deposition of any of the ,torridonian sediments must have been prolonged and gigantic4 ,the more1 indeed1 we study the gneiss1 the more do we feel impressed by the evidence for the lapse of a vast interval of time1 here unrecorded in rock1 between the last terrestrial movements indicated by the gneiss and the earliest of the ,torridonian sediments4 ,in this manner1 reasoning backward from the horizon of the ,olenellus-zone1 we are enabled to form some conception of the vastness of the antiquity of the fundamental rocks of the ,north-west ,highlands4 ,the nature and origin of these rocks acquire a special interest from a consideration of their age4 ,they contain the chronicles of the very beginnings of geological history1 in so far as this history is contained in the crust of the earth4 ,no part of the geological record is so obscure as this earliest chapter1 but we need not here enter further into its difficulties than may be necessary for the purpose of understanding what light it can be made to throw on the earliest manifestations of volcanic action4 ,under the term ,lewisian ,gneiss 7,i4 in ,fig4 #ce7 a series of rocks is comprised which differ from each other in composition1 structure and age1 though most of them possess such crystalline and generally foliated characters as may be conveniently included under the designation of gneiss4 ,the complexity of these ancient crystalline masses was not recognized at the time when ,murchison called them the ,fundamental or ,lewisian gneiss4 ,it is only since the ,geological ,survey began to study and map them in full detail that their true nature and history have begun to be understood4#ea ,footnote #ea3 ,see the ,report of this ,survey work by ,messrs4 ,peach1 ,horne1 ,gunn1 ,clough1 ,cadell and ,hinxman1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xliv4 7#ahhh71 pp4 #cgh-dda2 and ,annual ,reports of ,director-,general of the ,geological ,survey in the ,report of ,the ,science and ,art ,department for #ahid1 p4 #bgi1 and #ahie1 p4 #ag of reprint4 ,the general area of the gneiss is shown in ,map ,i4 ,the researches of the ,survey have shown the so-called ,lewisian gneiss to comprise the following five groups of rock3 #a4 ,a group of various more or less banded and foliated rocks which form together the oldest and chief part of the gneiss 7,fundamental complex72 #b4 ,highly basic dykes cutting the first group2 #c4 ,dykes and sills of dolerite1 epidiorite and hornblende-schist2 #d4 ,a few dykes of peculiar composition2 #e4 ,gneissose granite and pegmatite4 ,the first of these groups1 forming the main body of the gneiss1 has been critically studied on the mainland from ,cape ,wrath to ,skye4 ,but its development in the ,outer ,hebrides has not yet been worked out1 although the name ,lewisian was actually taken from that chain of islands4 ,so far as at present known1 however1 the gneiss of the ,hebrides repeats the essential characters of that of the mainland4 ,mr4 ,teall1 as the result of a careful investigation in the field and with the microscope1 has ascertained that on the mainland between ,skye and ,cape ,wrath the rocks of the fundamental complex are essentially composed of olivine1 hypersthene1 augite 7including diallage71 hornblende1 biotite1 plagioclase1 orthoclase1 microcline and quartz4 ,he has further observed that these minerals are associated together in the same manner as in peridotites1 gabbros1 diorites and granites4 ,treating the rocks in accordance with their composition and partly with their structure1 but excluding theoretical considerations1 he has arranged them in the following five subdivisions3-- #a4 ,rocks composed of ferro-magnesian minerals1 without felspar or quartz--,pyroxenites1 ,hornblendites4 #b4 ,rocks in which pyroxenes are the dominating ferro-magnesian constituents1 felspar always being present1 sometimes quartz3 ,a1 ,without quartz1 ,hypersthene-augite-rocks 7pyroxene granulites2 rocks of the ,baltimore-gabbro type7 and augite-rocks 7gabbros72 ,b1 ,with quartz1 ,augite-gneiss4 #c4 ,rocks in which hornblende is the prevalent ferro-magnesian constituent3 ,a1 ,without quartz1 or containing it only in small quantity2 rocks basic in composition3 7a7 massive or only slightly foliated 7,amphibolites1 as epidote-amphibolite1 zoisite-amphibolite1 garnet-amphibolite72 7b7 foliated 7,hornblende-schist74 ,b1 ,with quartz2 rocks intermediate or acid in composition3 7a7 with compact hornblende and a granular structure 7,hornblende-gneiss proper72 7b7 with hornblende occurring in fibrous or other aggregates2 7c7 with compact hornblende and a more or less granulitic structure 7,granulitic hornblende-gneiss74 #d4 ,rocks in which biotite is the predominant ferro-magnesian constituent2 felspar and quartz both present3 7a7 ,biotite occurring as independent plates or in aggregates of two or three large individuals 7,biotite-gneiss72 7b7 ,biotite occurring in aggregates of numerous small individuals 7rare type72 7c7 ,biotite occurring as independent plates in a granulitic structure4 #e4 ,rocks in which muscovite and biotite are present1 together with felspar and quartz--,muscovite-biotite-gneiss4 ,these1 though not forming a well-defined natural group1 are placed together for purposes of description4 ,they are all foliated1 some having the aspect of mica-schists1 others being typical augen-gneisses1 or light grey gneisses with abundant oligoclase and inclusions of microlitic epidote4 ,the rocks of each of these types are usually restricted to relatively small areas1 and they succeed each other with much irregularity all the way from ,skye to ,cape ,wrath4 ,their chemical and mineralogical composition proves them to have decided affinities with the plutonic igneous masses of the earth's crust4 ,the only exceptions to this prevalent igneous type occur in the districts of ,gairloch and ,loch ,carron1 where the gneiss appears to be associated with a group of mica-schists1 graphitic-schists1 quartzites and siliceous granulites1 limestones1 dolomites1 chlorite-schists and other schists4 ,that these are altered sedimentary formations can hardly be doubted4 ,what their precise relations to the fundamental complex of the gneiss may be has not yet been satisfactorily determined4 ,they are certainly far older than the ,torridon sandstone which covers them unconformably4 ,possibly they may represent a sedimentary formation still more ancient than the gneiss4 ,save these obscure relics of a pre-,torridonian system of strata1 the gneiss never presents any structure which suggests the alteration of clastic constituents4 ,everywhere its mineral composition points to a connection with the subterranean intrusions of different igneous magmas1 while the manner in which its different rock-groups are associated together1 and the internal structure of some of them1 still further link it with phenomena which will be described in succeeding chapters as parts of the records of volcanic action4 ,an interesting feature of the fundamental complex1 as bearing on the origin of the gneiss1 is to be found in the occurrence of bosses and bands which are either non-foliated or foliated only in a slight degree4 ,these comparatively structureless portions present much of the character of bosses or sills of true eruptive rocks4 ,they occur in various parts of ,sutherland and ,ross4 ,their external margins are not well defined1 and they pass insensibly into the ordinary gneiss1 the dark basic massive rocks shading off into coarse basic gneisses1 and the pegmatites of quartz and felspar which traverse them merging into bands of grey quartzose gneiss4 ,so far1 therefore1 as present knowledge goes1 the main body or fundamental complex of the ,lewisian gneiss in the ,north-west ,highlands of ,scotland consists of what may have been originally a mass of various eruptive rocks4 ,it has subsequently undergone a succession of deformations from enormous stresses within the terrestrial crust1 which have been investigated with great care by the ,geological ,survey4 ,but it presents structures which1 in spite of the abundant proofs of great mechanical deformation1 are yet1 ,i venture to think1 original1 or at least belong to the time of igneous protrusion before deformation took place4 ,the alternation of rocks of different petrographical constitution suggests a succession of extravasations of eruptive materials1 though it may not be always possible now to determine the order in which these followed each other4 ,in the feebly foliated or massive bands and bosses there is a parallel arrangement of their constituent minerals or of fine and coarse crystalline layers which recalls sometimes very strikingly the flow-structure of rhyolites and other lavas4 ,this resemblance was strongly insisted on by ,poulett ,scrope1 who believed that the laminar structure of such rocks as gneiss and mica-schist was best explained by the supposition of the flow of a granitic magma under great pressure within the earth's crust4#eb ,footnote #eb3 ,volcanoes1 pp4 #adj1 #bhc1 #bii4 ,the conviction that these parallel structures do1 in some cases1 really represent traces of movements in the original unconsolidated igneous masses1 not yet wholly effaced by later mechanical stresses1 has been greatly strengthened in my mind by a recent study of the structures of various eruptive bosses1 especially those of gabbro in the ,tertiary volcanic series of the ,inner ,hebrides4 ,the banded structure1 the separation of the constituent minerals into distinct layers or zones1 the alternation of markedly basic with more acid layers1 and the puckering and plication of those bands1 can be seen as perfectly among the ,tertiary gabbro bosses of ,skye as in the ,lewisian gneiss 7see ,figs4 #ccf and #ccg74 ,it cannot be contended that such structures in the gabbro are due to any subsequent terrestrial disturbance and consequent deformation4 ,they must be accepted as part of the original structure of the molten magma4#ec ,it seems to me1 therefore1 highly probable that the parallel banding in the uncrushed cores of the ,lewisian gneiss reveals to us some of the movements of the original magma at the time of its extrusion and before it underwent those great mechanical stresses which have so largely contributed to the production of many of its most characteristic structures4 ,footnote #ec3 ,see ,a4 ,geikie and ,j4 ,j4 ,h4 ,teall1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 #a4 7#ahid71 p4 #fde4 ,while the material of the oldest gneiss presents many affinities to plutonic rocks of much younger date1 a wide region of mere speculation opens out when we try to picture the conditions under which this material was accumulated4 ,some geologists have boldly advanced the doctrine that the ,archan gneisses represent the earliest crust that consolidated upon the surface of the globe4 ,but these rocks offer no points of resemblance to the ordinary aspect of superficial volcanic ejections4 ,on the contrary1 the coarsely-crystalline condition even of those portions of the gneiss which seem most nearly to represent original structure1 the absence of anything like scori or fragmental bands of any kind1 and the resemblances which may be traced between parts of the gneiss and intrusive bosses of igneous rock compel us to seek the nearest analogies to the original gneiss in deep-seated masses of eruptive material4 ,it is difficult to conceive that any rocks approaching in character to the gabbros1 picrites1 granulites and other coarsely-crystalline portions of the old gneiss could have consolidated at or near the surface4 ,when the larger area of gneiss forming the chain of the ,outer ,hebrides is studied1 we may obtain additional information regarding the probable origin and the earliest structures of the fundamental complex of the ,lewisian gneiss4 ,in particular1 we may look for some unfoliated cores of a more acid character1 and perhaps for evidence which will show that both acid and basic materials were successively protruded4 ,we may even entertain a faint hope that some trace may be discovered of superficial or truly volcanic products connected with the bosses which recall those of later date and obviously eruptive nature4 ,but up to the present time no indication of any such superficial accompaniments has been detected4 ,if any portions of the old gneiss represent the deeper parts of columns of molten rock that flowed out at the surface as lava1 with discharges of fragmentary materials1 all this superincumbent material1 at least in the regions which have been studied in detail1 had disappeared entirely before the deposition of the very oldest part of the ,torridonian rocks1 unless some trace of it may remain among the pebbles of the ,torridonian conglomerates1 to which reference will be immediately made4 ,so far1 then1 as the evidence now available allows a conclusion to be drawn1 the ,lewisian gneiss reveals to us a primeval group of eruptive rocks presenting a strong resemblance to some which in later formations are connected1 as underground continuations1 with bedded lavas and tuffs that were erupted at the surface2 and although no proof has yet been obtained of true volcanic ejections associated with the fundamental complex1 the rocks seem to be most readily understood if we regard them as having consolidated from igneous fusion at some depth1 and we may plausibly infer that they may have been actually connected with the discharge of volcanic materials at the surface4 ,the graphite-schists1 mica-schists1 and limestones of the ,gairloch and ,loch ,carron may thus be surviving fragments of the stratified crust into which these deep-seated masses were intruded1 and through which any volcanic eruptions that were connected with them had to make their way4 ,the limited areas occupied by the several varieties of rock in the fundamental complex suggests the successive protrusion of different magmas1 or of different portions from one gradually changing magma4 ,mr4 ,teall has ascertained that whenever in this series of rocks the relative ages of two petrographical types can be clearly ascertained1 the more basic is older than the more acid4 ,but besides all the complexity arising from original diversity of area1 structure and composition among the successive intrusions1 a further intricacy has been produced by the subsequent terrestrial disturbances1 which on a gigantic scale affected the north-west of ,europe after the formation of the fundamental complex of the old gneiss1 but long before the ,torridonian period4 ,by a series of terrestrial stresses that came as precursors of those which in later geological times worked such great changes among the rocks of the ,scottish ,highlands1 the original bosses and sheets of the gneiss were compressed1 plicated1 fractured and rolled out1 acquiring in this process a crumpled1 foliated structure4 ,whether or not these disturbances were accompanied by any manifestations of superficial volcanic action has not yet been determined4 ,but we know that they were followed by a succession of dyke-eruptions1 to which1 for extent and variety1 there is no parallel in the geological structure of ,britain1 save in the remarkable assemblage of dykes belonging to the ,tertiary volcanic period#ed 7,fig4 #cf74 ,footnote #ed3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xliv4 7#ahhh71 p4 #chi et seq4 ,illustration3 ,walker ,boutall sc4 ,fig4 #cf4--,map of a portion of the ,lewisian gneiss of ,ross-shire4 ,taken from ,sheet #ajg of the ,geological ,survey of ,scotland on the scale of one inch to a mile4 ,the white ground 7,a7 marks the general body of the ,lewisian gneiss4 ,this is traversed by dykes of dolerite 7,b71 which are cut by later dykes of highly basic material 7peridotite1 picrite1 etc41 ,p74 ,the gneiss and its system of dykes is overlain unconformably by the nearly horizontal ,torridon ,sandstone 7t71 which is injected by sheets of oligoclase-porphyry 7,f74 ,for the production of these dykes a series of fissures was first opened through the fundamental complex of the gneiss1 having a general trend from ,e4,s4,e4 to ,w4,n4,w41 running in parallel lines for many miles1 and so close together in some places that fifteen or twenty of them occurred within a horizontal space of one mile4 ,the fissures were probably not all formed at the same time2 at all events1 the molten materials that rose in them exhibit distinct evidence of a succession of upwellings from the igneous magma below4 ,considered simply from the petrographical point of view1 the materials that have filled the fissures have been arranged by ,mr4 ,teall in the following groups3 #a4 ,ultrabasic dykes1 sometimes massive 7peridotites71 sometimes foliated 7talcose schists containing carbonates and sometimes gedrite72 #b4 ,basic dykes which where massive take the forms of dolerite and epidiorite1 and where foliated appear as hornblende-schist1 the same dyke often presenting the three conditions of dolerite1 epidiorite and hornblende-schist2 #c4 ,dykes of peculiar composition1 comprising microcline-mica rocks and biotite-diorite with macro-poikilitic plagioclase2 #d4 ,granites and gneissose granites 7biotite-granite with microcline72 #e4 ,pegmatites 7microcline-quartz rocks with a variable amount of oligoclase or albite74#ee ,footnote #ee3 ,annual ,report of ,geological ,survey for #ahie1 p4 #ah of reprint4 ,distinct evidence of a succession of eruptions can be made out among these rocks4 ,by far the largest proportion of the dykes consists of basic materials4 ,the oldest and most abundant of them are of plagioclase-augite rocks1 which1 where uncrushed1 differ in no essential feature of structure or composition from the dolerites and basalts of more modern periods1 though they have been plentifully changed into epidiorite and hornblende-schist4#ef ,they present1 too1 most of the broad features that characterize the dykes of later times--the central more coarsely-crystalline portion1 the marginal band of finer grain1 passing occasionally into what was probably a basic glass1 and the transverse jointing4 ,they belong to more than one period of emission1 for they cross each other4 ,they vary in width up to nearly #bjj feet1 and sometimes run with singular persistence completely across the whole breadth of the strip of gneiss in the west of ,sutherland and ,ross4 ,dozens of dykes have been followed by the ,geological ,survey for distances of ten or twelve miles4 ,footnote #ef3 ,see ,mr4 ,teall1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xli4 7#ahhe71 p4 #acc4 ,later in time1 and much less abundant1 are certain highly basic dykes--peridotites with schistose modifications--which cut across the dolerites in a more nearly east-and-west direction4 ,there are likewise occasional dykes of peculiar composition1 which1 as above stated1 have been distinguished by ,mr4 ,teall as microcline-mica rocks and biotite-diorite4 ,last of all comes a group of thoroughly acid rocks--varieties of granite and pegmatite--which form intrusive sheets and dykes4 ,the granites contain biotite with microcline1 and are sometimes gneissose4 ,the pegmatites are microcline-quartz rocks with a variable amount of oligoclase or albite4 ,these dykes coincide in direction with the basalts and dolerites1 but they are apt to run together into belts of granite and pegmatite1 sometimes #aejj feet broad4 ,up to the present time no evidence has been found of any superficial outpouring of material in connection with this remarkable series of dykes in the ,lewisian gneiss4 ,that they may have been concomitant with true volcanic eruptions may be plausibly inferred from the close analogy which1 in spite of their antiquity and the metamorphism they have undergone1 they still present to the system of dykes that forms a part of the great ,tertiary volcanic series of ,antrim and the ,inner ,hebrides4 ,the close-set fissures running in a ,w4,n4,w direction1 the abundant uprise into these fissures of basic igneous rocks1 followed by a later and more feeble extravasation of acid material1 are features which in a singular manner anticipate the volcanic phenomena of ,tertiary time4 ,there can be no question as to the high antiquity of these dykes4 ,they were already in place before the advent of those extraordinary vertical lines of shearing which have so greatly affected both the gneiss and the dykes2 and these movements1 in turn1 had long been accomplished before the ,torridon ,sandstone was laid down1 for the dykes1 with their abundant deformation1 run up to and pass beneath the sandstone which buries them and all the rocks with which they are associated4 ,though later than the original fundamental complex1 the dykes have become so integral and essential a part of the gneiss as it now exists that they must be unhesitatingly grouped with it4 ,with so wide an extension of the subterranean relics of volcanic energy1 it is surely not too much to hope that somewhere there may have been preserved1 and may still be discovered1 proofs that these eruptive rocks opened a connection with the surface1 and that we may thus recognize vestiges of the superficial products of actual ,archan volcanoes4 ,among the pebbles in the conglomerates of the ,torridon ,sandstone there occur1 indeed1 fragments of felsites which possess great interest from the perfection with which they retain some of the characteristic features of younger lavas4 ,mr4 ,teall has described their minute structure4 ,they are dark1 purplish1 compact rocks1 consisting of a spherulitic micropegmatitic1 micropoikilitic or microcrystalline groundmass1 in which are imbedded porphyritic crystals or crystal-groups of felspar1 often oligoclase4 ,these spherulitic rocks occasionally show traces of perlitic structure4 ,they bear a striking resemblance to some of the ,uriconian felsites of ,shropshire1 pebbles from which occur in the ,longmynd rocks4#eg ,these fragments suggest the existence of volcanic materials at the surface when the ,torridon ,sandstone was deposited4 ,possibly they may represent some vanished ,lewisian lavas4 ,but the time between the uprise of the dykes and the formation of the ,torridonian series was vast enough for the advent of many successive volcanic episodes4 ,the pebbles may therefore be the relics of eruptions that took place long after the period of the dykes4 ,footnote #eg3 ,annual ,report of ,geological ,survey for #ahie1 p4 #ba of reprint4 ,among the ,torridonian strata no undoubted trace of any contemporaneous volcanic eruptions has been met with4#eh ,the only relics of volcanic rocks in this enormous accumulation of sediments are the pebbles just referred to1 which may be referable to a time long anterior to the very oldest parts of the ,torridonian series4 ,footnote #eh3 ,the supposed tuff referred to in ,quart4 ,journ4 ,geol4 ,soc4 vol4 xlviii4 7#ahib71 p4 #afh1 is probably not of truly volcanic origin4 ,that ,archan time witnessed volcanic eruptions on a considerable scale1 and with great variety of petrographical material1 has recently been shown in detail by ,mr4 ,otto ,nordenskjld from a study of the rocks of ,smland in ,sweden4 ,he has described a series of acid outbursts1 including masses of rhyolite and dacite1 together with agglomerates and tuffs1 likewise basic eruptions1 with dioritic rocks1 augite-porphyrite and breccia4 ,he refers these rocks to the same age as most of the ,scandinavian gneisses1 and remarks that though they have undergone much mechanical deformation and metamorphism1 they have yet here and there retained some of their distinctive volcanic structures1 such as the spherulitic4#ei ,when the large area of ,lewisian gneiss forming the chain of the ,outer ,hebrides is investigated it may possibly supply examples of a similar series of ancient volcanic masses4 ,footnote #ei3 ber ,archische ,ergussgesteine aus ,smland1 ,sveriges ,geol4 ,underskn1 ,no4 #ace 7#ahid74 ii4 ,,the ,,dalradian ,,or ,,younger ,,schists ,,of ,,scotland ,we now come to one of the great gaps in the geological record4 ,the ,lewisian gneiss affords us glimpses of probable volcanic activity at the very beginning of geological history4 ,an enormous lapse of time1 apparently unrepresented in ,britain by any geological record1 must be marked by the unconformability between the gneiss and the ,torridon ,sandstone4 ,another prodigious interval is undoubtedly shown by the ,torridonian series4 ,neither this thick accumulation of sediment nor the ,cambrian formations1 which to a depth of some #bjjj feet overlie the ,torridon ,sandstone1 have yielded any evidence of true superficial eruptions1 though they are traversed by numerous dykes1 sills and bosses4 ,the age of these intrusive masses cannot be precisely fixed2 a large proportion of them is certainly older than the great terrestrial displacements and concurrent metamorphism of the ,north-,west ,highlands4 ,while from the ,lewisian gneiss upward to the highest visible ,cambrian platform in ,sutherland1 no vestige of contemporaneous volcanic rocks is to be seen1 the continuity of the geological record is abruptly broken at the top of the ,durness ,limestone4 ,by a series of the most stupendous dislocations1 the rocks of the terrestrial crust have there been displaced to such a degree that portions have been thrust westward for a horizontal distance of sometimes as much as ten miles1 while they have been so crushed and sheared as to have often lost entirely their original structures1 and to have passed into the crystalline and foliated condition of schists4 ,portions of the floor of ,lewisian gneiss1 and large masses of the ,torridon ,sandstone1 which had been buried under the ,cambrian sediments1 have been torn up and driven over the ,durness ,limestone and quartzite4 ,though much care has been bestowed by the officers of the ,geological ,survey on the investigation of the complicated mass of material which1 pushed over the ,cambrian strata1 forms the mountainous ground that lies to the east of a line drawn from ,loch ,eribol1 in the north of ,sutherland1 to the south-east of ,skye1 some uncertainty still exists as to the age and history of the rocks of that region4 ,for the purposes of this work1 therefore1 the rest of the country eastwards to the line of the ,great ,glen--that remarkable valley which cuts ,scotland in two--may be left out of account4 ,to the east of the ,great ,glen the ,scottish ,highlands display a vast succession of crystalline schists1 the true stratigraphical relations of which to the ,lewisian gneiss have still to be determined1 but which1 taken as a whole1 no one now seriously doubts must be greatly younger than that ancient rock4 ,murchison first suggested that the quartzites and limestones found in this newer series are the equivalents of those of the ,north-,west4 ,this identification may yet be shown to be correct1 but must be regarded as still unproved4 ,traces of fossils 7annelid-pipes7 have been found in some of the quartzites1 but they afford little or no help in determining the horizons of the rocks4 ,in ,donegal1 where similar quartzites1 limestones and schists are well developed1 obscure indications of organic remains 7corals and graptolites7 have likewise been detected1 but they also fail to supply any satisfactory basis for stratigraphical comparison4 ,essentially the schists of the ,scottish ,highlands east of the ,great ,glen consist of altered sedimentary rocks4 ,besides quartzites and limestones1 there occur thick masses of clay-slate and other slates and schists1 with bands of graphitic schist1 greywacke1 pebbly grit1 quartzite1 boulder-beds and conglomerates4 ,among rocks that have been so disturbed and foliated it is necessarily difficult to determine the true order of succession4 ,in the ,central ,highlands1 however1 a certain definite sequence has been found to continue as far as the ground has yet been mapped4 ,were the rocks always severely contorted1 broken and placed at high angles1 this sequence might be deceptive1 and leave still uncertain the original order of deposition of the whole series4 ,but over many square miles the angles of inclination are low1 and the successive bands may be traced from hill to hill1 across strath and glen1 forming escarpments along the slopes and outliers on the summits1 precisely as gently-undulating beds of sandstone and limestone may be seen to do in the dales of ,yorkshire4 ,it is difficult to resist the belief1 though it may1 perhaps1 be premature to conclude1 that this obvious and persistent order of succession really marks the original sequence of deposition4 ,in ,donegal also a definite arrangement of the rock-groups has been ascertained which1 when followed across the country1 gives the key to its geological structure4#fj ,footnote #fj3 ,geol4 ,survey ,memoirs3 ,geology of ,n4,w4 ,donegal1 #ahia4 ,in the order of succession which has been recognized during the progress of the ,geological ,survey through the ,central and ,southern ,highlands1 it is hard in many places to determine whether the sequence that can be recognized is in an upward or downward direction4 ,two bands of limestone1 which appear to retain their relative positions across ,scotland for a distance of some #bcj miles1 may afford a solution of this difficulty1 and if1 as is probable1 they are to be identified with the similar limestones of ,donegal1 ,mayo and ,galway1 their assistance will thus be available across a tract of more than #djj miles4 ,what is regarded as the lower zone of limestone is particularly well seen about ,loch ,tay2 what is believed to be the upper is typically displayed in the heart of ,perthshire1 about ,blair-,athol4 ,from under the ,loch ,tay ,limestone a great thickness of mica-schists1 green schists1 schistose grits and conglomerates1 slates and greywackes1 emerges up to the border of the ,highlands4 ,above that calcareous band thick masses of mica-schist and sericite-schist are succeeded by a well-marked zone of quartzite1 which forms the mountains of ,ben-y-,glo and ,schihallion1 and stretches south-westward across ,argyllshire into ,islay and ,jura4 ,the second or ,blair-,athol ,limestone lies next to this quartzite4 ,if the limestones are identical with those of ,donegal1 ,mayo and ,galway1 the quartzites may doubtless be also regarded as continued in those of the same ,irish counties1 where they form some of the most conspicuous features in the scenery1 since they rise into such conspicuous mountains as ,erigal1 ,slieve ,league1 ,nephin1 and the twelve ,bins of ,connemara4 ,the age of this vast system of altered rocks has still to be determined4 ,it is possible that they may include some parts of the ,torridonian series1 or even here and there a wedge of the ,lewisian gneiss driven into position by gigantic disruptions1 like those of the ,north-,west ,highlands4 ,but there can be no doubt that the schists1 quartzites and limestones form an assemblage of metamorphosed sedimentary strata which differs much in variety of petrographical character1 as well as in thickness1 from the ,torridonian sandstone1 and which has not been identified as the equivalent of any known ,palozoic system or group of formations in ,britain4 ,it may conceivably embrace the ,cambrian series of the ,north-,west ,highlands1 and also the sedimentary deposits that succeeded the ,durness ,limestone1 of which no recognizable vestige remains in ,sutherland or ,ross4 ,that the metamorphic rocks east of the line of the ,great ,glen are at least older than the ,arenig formation of the ,lower ,silurian system may be inferred from an interesting discovery recently made by the officers of the ,geological ,survey4 ,a narrow strip of rocks has been found which1 from their remarkable petrographical characters1 their order of sequence and their scanty fossil contents 7,radiolaria71 are with some confidence identified with a peculiar assemblage of rocks on the ,arenig horizon of the ,silurian system in the ,southern ,uplands of ,scotland1 to which fuller reference will be made in ,chapter xii4 ,this strip or wedge of probably ,lower ,silurian strata intervenes between the ,highland schists and the ,old ,red ,sandstone in ,kincardineshire1 ,forfarshire and ,dumbartonshire4 ,it has been recognized also1 occupying a similar position1 in ,tyrone in ,ireland4 ,the schists in some places retain their foliated character up to the abrupt line of junction with the presumably ,lower ,silurian strata1 while in other districts1 as at ,aberfoyle1 they have been so little affected that it is hardly possible to draw a line between the ,highland rocks and those of this border-zone1 which indeed are there perhaps more metamorphosed than the ,highland grits to the north of them4 ,the metamorphism of the schists may have been mainly effected before the final disturbances that wedged in this strip of ,silurian strata along the ,highland border1 though some amount of crushing and schist-making seems to have accompanied these disturbances4 ,no trace of any similar strip of ,palozoic rocks has ever been detected among the folds of the schists further into the ,highlands4 ,but some of the ,highland rocks in the region of ,loch ,awe lose their metamorphosed character1 and pass into sedimentary strata which1 so far as petrographical characters are concerned1 might well be ,palozoic4 ,until some clue is found to the age of the ,younger or ,eastern schists1 quartzites and limestones of the ,highlands1 it is desirable to have some short convenient adjective to distinguish them4 ,as a provisional term for them ,i have proposed the term ,dalradian1 from ,dalriada1 the name of the old ,celtic kingdom of the north of ,ireland and south-west of ,scotland4#fa ,footnote #fa3 ,presidential ,address to ,geological ,society1 #ahia1 p4 #ci4 ,the special feature for which this ,dalradian series is cited in the present volume is the evidence it furnishes of powerful and extensive volcanic action4 ,in a series of rocks so greatly dislocated1 crumpled and metamorphosed1 we cannot look for the usual clear proofs of contemporaneous eruptions4 ,nevertheless all over the ,scottish ,highlands1 from the far coast of ,aberdeenshire to the ,mull of ,cantyre1 and across the west of ,ireland from the headlands of ,donegal into ,galway1 there occurs abundant evidence of the existence of rocks which1 though now forming an integral part of the schists1 can be paralleled with masses of undoubtedly volcanic origin4 ,illustration3 ,fig4 #cg4--,section showing the position of ,sills in the mica-schist series between ,loch ,tay and ,amulree4 a1 ,mica-schist2 b1 b1 ,sills4 ,intercalated in the vast pile of altered sediments lie numerous sheets of epidiorite and hornblende-schist1 which were erupted as molten materials1 not improbably as varieties of diabase-lava4 ,most of these sheets are doubtless intrusive sills1 for they can be observed to break across from one horizon to another4 ,but some of them may possibly be contemporaneous lava-streams4 ,a sheet may sometimes be followed for many miles1 occupying the same stratigraphical platform4 ,thus a band of sills may be traced from the coast of ,banffshire to near ,ben ,ledi1 a distance of more than #ajj miles4 ,among the hornblendic sills of this band some occur on a number of horizons between the group of ,ben ,voirlich grits and the ,ben-y-,glo quartzite4 ,one of the most marked of these is a sheet1 sometimes #bjj feet thick1 which underlies the ,loch ,tay ,limestone4 ,another interesting group in the same great band has been mapped by the ,geological ,survey on the hills between ,loch ,tay and ,amulree1 some of them being traceable for several miles among the mica-schists with which they alternate 7,fig4 #cg74 ,in ,argyllshire also1 between ,loch ,tarbert and ,loch ,awe1 and along the eastern coasts of the islands of ,islay and ,jura1 an abundant series of sheets of epidiorite1 amphibolite and hornblende-schist runs with the prevalent strike of the schists1 grits and limestones of that region4 ,similar rocks reappear in a like position in ,donegal1 where1 as in ,scotland1 the frequency of the occurrence of these eruptive rocks on the horizons of the limestones is worthy of remark4 ,the persistence1 number and aggregate thickness of the sills in this great band mark it out as the most extensive series of intrusive sheets in the ,british ,isles4 ,in addition to the sills there occur also bosses of similar material1 which in their form and their obvious relation to the sheets recall the structure of volcanic necks4 ,they consist of hornblendic rocks1 like the sills1 but are usually tolerably massive1 and show much less trace of superinduced foliation4 ,besides the obviously eruptive masses there is another abundant group of rocks which1 ,i believe1 furnishes important evidence as to contemporaneous volcanic action during the accumulation of the ,dalradian series4 ,throughout the ,central and ,south-,western ,highlands certain zones of green schist have long occupied the attention of the officers of the ,geological ,survey4 ,they occur more especially on two horizons between the ,loch ,tay ,limestone and a much lower series of grits and fine conglomerates1 which run through the ,trossachs and form the craggy ridges of ,ben ,ledi1 ,ben ,voirlich and other mountains near the ,highland border4 ,in the lower group of green schists1 thick hornblendic sills begin to make their appearance1 increasing in number upwards4 ,the upper group of green schists lies between two bands of garnetiferous mica-schist1 above the higher of which comes the ,loch ,tay ,limestone4 ,the peculiar greenish tint and corresponding mineral constituents of these schists1 however1 are likewise found diffused through higher parts of the series4 ,so much do the green schists vary in structure and composition that no single definition of them is always applicable4 ,at one extreme are dull green chlorite-schists1 passing into a potstone1 which1 like that of ,trondhjem1 can be cut into blocks for architectural purposes4#fb ,at the other extreme lie grits and quartzites1 with a slight admixture of the same greenish-coloured constituent4 ,between these limits almost every stage may be met with1 the proportion of chlorite or hornblende and of granular or pebbly quartz varying continually1 not only vertically1 but even in the extension of the same bed4 ,the quartz-pebbles are sometimes opalescent1 and occasionally larger than peas4 ,an average specimen from one of the zones of green schists is found1 on closer examination1 to be a thoroughly schistose rock1 composed of a matrix of granular quartz1 through which acicular hornblende and biotite crystals1 or actinolite and chlorite1 are ranged along the planes of foliation4 ,footnote #fb3 ,from such a rock1 which crosses the upper part of ,loch ,fyne1 the ,duke of ,argyll's residence at ,inveraray has been built4 ,that these rocks are essentially of detrital origin admits of no doubt4 ,they differ1 however1 from the other sedimentary members of the ,dalradian series in the persistence and abundance of the magnesian silicates diffused through them4 ,the idea which they suggested to my mind some years ago was that the green colouring-matter represents fine basic volcanic dust1 which was showered out during the accumulation of ordinary quartzose1 argillaceous and calcareous sediments1 and that1 under the influence of the metamorphism which has so greatly affected all the rocks of the region1 the original pyroxenes and felspars suffered the usual conversion into hornblendes1 chlorites and micas4 ,this view has occurred also to my colleagues on the ,survey1 and is now generally adopted by them4 ,not only are these green schists traceable all through the ,central and ,south-,western ,highlands1 rocks of similar character1 and not improbably on the same horizons1 reappear in the north-west of ,ireland1 and run thence south-westward as far as the ,dalradian rocks extend4 ,if we are justified in regarding them as metamorphosed tuffs and ashy sediments1 they mark a widespread and long-continued volcanic period during the time when the later half of the ,dalradian series was deposited4 ,besides the extensive development of basic sills which1 though probably in great part later than the green schists1 may belong to the same prolonged period of subterranean activity1 numerous acid protrusions are to be observed in the ,dalradian series of ,scotland and ,ireland4 ,that these masses were erupted at several widely-separated intervals is well shown by their relation to the schists among which they occur4 ,some of the great bosses and sills of granite were undoubtedly injected before the metamorphism of the schists was completed1 for they have shared in the foliation of the region4 ,others have certainly appeared after the metamorphism was complete1 for they show no trace of having suffered from its effects4 ,thus some of the vast tracts of newer granite in the ,grampian chain1 which cover many square miles of ground1 must be among the newest rocks of that area4 ,they have recently been found by ,mr4 ,g4 ,barrow1 of the ,geological ,survey1 to send veins into the belt of probably ,lower ,silurian strata which flanks the ,highland schists4 ,they are thus later than the ,arenig period4 ,not impossibly they may be referable to the great granite intrusions which formed so striking a feature in the history of the ,lower ,old ,red ,sandstone4 iii4 ,,the ,,gneisses ,,and ,,schists ,,of ,,anglesey ,in the island of ,anglesey an interesting series of schists and quartzites presents many points of resemblance to the ,dalradian or younger schists of the ,highlands4 ,at present the geologist possesses no means of determining whether these ,welsh rocks are the equivalents of the ,scottish in stratigraphical position1 but their remarkable similarity justifies a brief allusion to them in this place4 ,much controversy has arisen regarding the geology of ,anglesey1 but into this dispute it is not necessary for my present purpose to enter4#fc ,i will content myself with expressing what seems to me1 after several traverses1 to be the geological structure of the ground4 ,footnote #fc3 ,the literature of ,anglesey geology is now somewhat voluminous1 but ,i may refer to the following as the chief authorities4 ,the island is mapped in ,sheet #gh of the ,geological ,survey of ,england and ,wales1 and its structure is illustrated in ,horizontal ,sections1 ,sheet #dj4 ,a full account of its various formations and of their relations to each other is given in vol4 iii4 of the ,memoirs of the ,geological ,survey1 ,the ,geology of ,north ,wales1 by ,sir ,a4 ,c4 ,ramsay1 #bnd edit4 #ahha4 ,the subject has been discussed by ,professor ,hughes1 ,quart4 ,journ4 ,geol4 ,soc4 vols4 xxxiv4 7#ahgh7 p4 #acg1 xxxv4 7#ahgi7 p4 #fhb1 xxxvi4 7#ahhj7 p4 #bcg1 xxxviii4 7#ahhb7 p4 #af2 ,brit4 ,assoc4 ,rep4 7#ahha7 pp4 #fdc1 #fdd2 ,proc4 ,camb4 ,phil4 ,soc4 vol4 iii4 pp4 #fg1 #hi1 #cda2 by ,professor ,bonney1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xxxv4 7#ahgi7 pp4 #cjj1 #cba2 ,geol4 ,mag4 7#ahhj7 p4 #abe2 by ,dr4 ,h4 ,hicks1 ,quart4 ,journ4 ,geol4 ,soc4 vols4 xxxiv4 7#ahgh7 p4 #adg1 xxxv4 7#ahgi7 p4 #bie2 ,geol4 ,mag4 7#ahgi7 pp4 #dcc1 #ebh 7#ahic7 p4 #edh2 by ,dr4 ,c4 ,callaway1 ,quart4 ,journ4 ,geol4 ,soc4 vols4 xxxvii4 7#ahha7 p4 #baj1 xl4 7#ahhd7 p4 #efg2 and by the ,rev4 ,j4 ,f4 ,blake1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xliv4 7#ahhh7 p4 #dfc4 ,further references to the work of these observers in ,anglesey are given in ,chapter xiii4 p4 #bbj et seq4 ,the ,pre-,cambrian areas of ,anglesey are shown in ,map ,,ii4 ,there are two groups of rocks in ,anglesey to which a pre-,cambrian age may with probability be assigned4 ,in the heart of the island lies a core of gneiss which1 if petrographical characters may be taken as a guide1 must certainly be looked upon as ,archan4 ,in visiting that district with my colleague ,mr4 ,teall ,i was much astonished to find there so striking a counterpart to portions of the ,lewisian gneiss of the north-west of ,sutherland and ,ross4 ,the very external features of the ground recall the peculiar hummocky surface which so persistently characterizes the areas of this rock throughout the north-west of ,scotland4 ,if the geologist could be suddenly transported from the rounded rocky knolls of ,sutherland1 ,ross-shire or the ,hebrides to those in the middle of ,anglesey1 south of ,llanerchymedd1 he would hardly be aware of the change1 save in the greater verdure of the hollows1 which has resulted from a more advanced state of decomposition of the rocks at the surface1 as well as from a better climate and agriculture4 ,when we examine these rocky hummocks in detail we find them to consist of coarse gneisses1 the foliation of which has a prevalent dip to ,n4,n4,w4 ,some portions abound in dark hornblende and garnets1 others are rich in brown mica1 the folia being coarsely crystalline and rudely banded1 as in the more massive gneisses of ,sutherland4 ,abundant veins of coarse pegmatite may here and there be seen1 with pinkish and white felspars and milky quartz4 ,occasionally the gneiss is traversed by bands of a dark greenish-grey rock1 which remind one of the dykes of the north-west of ,scotland4 ,there are other rocks1 some of them probably intrusive and of later date1 to be seen in the same area2 but they require more detailed study than they have yet received4 ,the relation of this core of gneiss and its associated rocks to the second group of pre-,cambrian rocks has not hitherto been satisfactorily ascertained4 ,the core may conceivably be an eruptive boss in that group1 and may have acquired its foliation during the movements that produced the foliation of the surrounding schists4 ,but it seems more probable that the gneiss is much older than these schists1 though it would undoubtedly participate in the effects of the mechanical movements which gave rise to their deformation1 cleavage and foliation4 ,the second group of rocks occupies a large area in the west and in the centre and south of ,anglesey4 ,the schists of which it consists are obviously in the main a clastic series4 ,one of their most conspicuous members is quartzite1 which1 besides occurring sporadically all over the island1 forms the prominent mass of ,holyhead ,mountain4 ,there are likewise flaggy chloritic schists1 green and purple phyllites or slates1 and bands of grit1 while parts of the so-called grey gneiss consist of pebbly sandstones that have acquired a crystalline structure4 ,that some order of sequence among these various strata may yet be worked out is not impossible1 but the task will be one of no ordinary difficulty1 for the plications and fractures are numerous1 and much of the surface of the ground is obscured by the spread of ,palozoic formations and superficial deposits4 ,these ,anglesey schists are so obviously an altered sedimentary series that it is not surprising that they should have been regarded as metamorphosed ,cambrian strata4 ,all that can be positively affirmed regarding their age is that they are not only older than the lowest fossiliferous rocks around them--that is1 than ,arenig or even ,tremadoc strata--but that they had already acquired their present metamorphic character before these strata were laid down unconformably upon them4 ,there is no actual proof that they include no altered ,cambrian rocks4 ,but when we consider their distinctly crystalline structure1 and the absence of such a structure from any portion of the ,cambrian areas of the mainland2 when1 moreover1 we reflect that the metamorphism which has affected them is of the regional type1 and can hardly have been restricted to merely the limited area of ,anglesey2 we must agree with those observers who1 in spite of the absence of positive proof of their true geological horizon1 have regarded these rocks as of much higher antiquity than the ,cambrian strata of the neighbourhood4 ,no one familiar with the ,dalradian rocks of ,scotland and ,ireland can fail to be struck with the close resemblance which these younger ,anglesey schists bear to them1 down even into the minutest details4 ,petrographically they are precisely the counterparts of the quartzites and schists of ,perthshire and ,donegal1 and a further connection may be established of a palontological kind4 ,the upper part of the ,holyhead quartzite was found by ,mr4 ,b4 ,n4 ,peach and myself in the autumn of the year #ahij to be at one place crowded with annelid-pipes1 and ,i subsequently found the same to be the case with some of the flaggy quartzites near the ,south ,stack4 ,illustration3 ,fig4 #ch4--,sketch of crushed basic igneous rock among the schists1 ,e4 side of ,porth-tywyn-mawr1 ,e4 side of ,holyhead ,straits4 ,for the purpose of the inquiry which forms the theme of this work1 the feature of greatest interest about these younger schists of ,anglesey is the association of igneous rocks with them4 ,they include bands of dark basic material1 the less crushed parts of which resemble the diabases of later formations1 while the sheared portions pass into epidiorites and true hornblende-schists4 ,as in other regions where eruptive rocks have been crushed down and changed into the schistose modification1 it is frequently possible to see groups of uncrushed cores round which1 under severe mechanical stresses1 the rock has undergone this conversion4 ,lines of movement through the body of the rock may be detected by bands of schist1 the gradation from the solid core to the hornblende-schist being quite gradual4 ,the accompanying figure 7,fig4 #ch7 represents a portion of one of these crushed basic igneous rocks on the east side of ,holyhead ,straits4 ,as in the ,dalradian series of the ,highlands1 many1 perhaps most1 of these igneous bands are probably intrusive sills1 but others may be intercalated contemporaneous sheets4 ,they occur across the whole breadth of the island from the ,menai ,strait to the shores of ,holyhead4 ,besides these undoubtedly igneous rocks1 the green chloritic slates of ,anglesey deserve notice4 ,they are well-bedded strata1 consisting of alternations of foliated fine grit or sandstone1 with layers more largely made up of schistose chlorite4 ,the gritty bands sometimes contain pebbles of blue quartz1 and evidently represent original layers of sandy sediment1 but with an admixture of chloritic material4 ,the manner in which this green chloritic constituent is diffused through the whole succession of strata1 and likewise aggregated into bands with comparatively little quartzose sediment1 reminds one of the green schists of the ,central ,highlands and ,donegal1 and suggests a similar explanation4 ,taken in connection with the associated basic igneous rocks1 these chloritic schists seem to me to represent a thick group of volcanic tuffs and interstratified sandy and clayey layers4 ,if this inference is well founded1 and if we are justified in grouping these ,anglesey rocks with the ,dalradian schists of ,scotland and ,ireland1 a striking picture is presented to the mind of the wide extent and persistent activity of the volcanoes of that primeval period in ,britain4#fd ,footnote #fd3 ,mr4 ,e4 ,greenly1 late of the ,geological ,survey of ,scotland1 has recently established himself on the ,menai ,strait for the purpose of working out in detail the geological structure of this interesting and complicated region4 ,we may therefore hope that some of the still unsolved problems presented by the rocks of ,anglesey will before long be satisfactorily explained4 iv4 ,,the ,,uriconian ,,volcanoes ,along the eastern borders of ,wales a ridge of ancient rocks1 much broken by faults and presenting several striking unconformabilities1 has long been classic ground in geology from the descriptions and illustrations of ,murchison's ,silurian ,system4#fe ,the main outlines of the structure of that district1 first admirably worked out by this great pioneer1 were delineated on the maps and sections of the ,geological ,survey1 wherein it was shown that in the ,longmynd an enormously thick group of stratified rocks1 which1 though unfossiliferous1 were referred to the ,cambrian system1 rose in the very heart of the country2 that to the east of these rocks lay strata of ,caradoc or ,bala age2 that by a great hiatus in the stratigraphy the ,upper ,silurian series transgressively wrapped round everything below it2 that yet again the ,coal-measures crept over all these various ,palozoic formations1 followed once more unconformably by ,permian and ,triassic deposits4#ff ,besides all this evidence of extraordinary and repeated terrestrial movement1 it was found that the region was traversed by some of the most powerful dislocations in this country1 while to complete the picture of disturbance1 many protrusions of igneous rocks were recognized4 ,footnote #fe3 ,see especially chap4 xix4 vol4 i4 p4 #bbe4 ,footnote #ff3 ,the area is embraced in ,sheet #fa of the ,geological ,survey1 and is illustrated by ,nos4 #cc and #cf of the sheets of ,horizontal ,sections4 ,in the early editions of the ,survey maps the felspathic traps and the greenstones of the ,wrekin district were distinguished by separate colours1 but unfortunately this useful and so far correct discrimination was given up in subsequent editions1 where all the acid and basic rocks are merged into one4 ,in a territory so complicated1 though it had been sedulously and skilfully explored1 there could hardly fail to remain features of structure which had escaped the notice of the first observers4 ,in particular1 the igneous rocks had been dealt with only in a general way1 and they consequently offered a favourable field for more detailed study2 while by a more searching examination of some of the rocks for fossils1 important corrections of the earlier work might yet be made4 ,a notable step towards a revision of the received opinions regarding the igneous rocks of this region was taken by ,mr4 ,allport1 who showed that the so-called greenstone included masses of devitrified spherulitic pitchstones and perlites1 together with indurated volcanic breccias1 agglomerates and ashes4#fg ,subsequently ,professor ,bonney described more fully the petrographical characters of the ,wrekin igneous rocks1 confirming and extending the observations of ,mr4 ,allport4#fh ,footnote #fg3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xxxiii4 7#ahgg7 p4 #ddi4 ,footnote #fh3 ,op4 cit4 vol4 xxxv4 7#ahgi7 p4 #ffb2 vol4 xxxviii4 7#ahhb7 p4 #abd4 ,but the correction of the prevalent error as to the geological age of these rocks was due to ,dr4 ,callaway1 who1 after spending much time and labour in ascertaining1 by a careful search for fossils1 the position of the superincumbent rocks 7wherein he discovered ,cambrian organisms71 and in a detailed investigation of the structure and relationships of the igneous masses themselves1 was led to regard them as part of an ancient pre-,cambrian ridge2 and he proposed for the volcanic group the name of ,uriconian1 from the name of the former ,roman town which stood not far to the west of them4#fi ,he has shown how essentially volcanic this ancient series of rocks is1 how seldom they present any clearly-marked evidence of stratification1 and how small is the proportion of sedimentary material associated with them4#gj ,footnote #fi3 ,quart4 ,journ4 ,geol4 ,soc4 vols4 xxx4 7#ahgd7 p4 #aif1 xxxiv4 7#ahgh7 p4 #ged1 xxxv4 7#ahgi7 p4 #fdc1 xlii4 7#ahhf7 p4 #dha4 ,for a criticism of ,dr4 ,callaway's views as to the order of succession among the rocks of this district1 see ,prof4 ,blake1 op4 cit4 vol4 xlvi4 7#ahij7 p4 #chf1 and ,dr4 ,callaway's reply1 vol4 xlvii4 7#ahia7 p4 #aji4 ,footnote #gj3 ,op4 cit4 vol4 xlvii4 7#ahia7 p4 #abc4 ,subsequently ,professor ,lapworth1 by his discovery of the ,olenellus-fauna1 marking the lowest known fossiliferous ,cambrian zone in the ,wrekin district1 and his recognition of ,cambrian fossils under the ,coal-measures of ,warwickshire1 supplied valuable evidence for the discussion of the geological position of the older rocks of the ,midlands4 ,he has mapped in minute detail the rocks of the ,wrekin1 and has exhausted all the evidence that is at present obtainable on the subject4 ,but unfortunately the publication of his researches is still delayed4#ga ,footnote #ga3 ,geol4 ,mag4 7#ahhb7 p4 #efc1 7#ahhf7 p4 #cai1 7#ahhg7 p4 #gh1 7#ahhh7 p4 #dhd2 and a joint paper with ,mr4 ,w4 ,w4 ,watts on the ,geology of ,south ,shropshire1 ,proc4 ,geol4 ,assoc4 vol4 xiii4 7#ahid7 pp4 #cjb1 #cce4 ,it is now recognized that the core of the ancient ridge1 extending from near ,wellington through the ,wrekin1 ,caer ,caradoc and other hills1 until it sinks beneath the ,upper ,silurian formations1 is formed of igneous rocks that consist partly of lavas1 partly of volcanic breccias and fine tuffs4 ,the lavas are thoroughly acid rocks of the felsitic or rhyolitic type4 ,one of them1 about #ajj feet thick1 which forms a prominent feature on the flanks and crest of ,caer ,caradoc1 shows abundant finely-banded flow-structure1 often curved or on end1 while its bottom and upper parts are strongly amygdaloidal1 the cavities being occasionally pulled out in the direction of flow and lined with quartz or chalcedony4 ,some of the detached areas of eruptive rocks show the beautiful spherulitic and perlitic structures first noticed in this region by ,mr4 ,allport4 ,more recently the structures of these acid rocks have been described by ,mr4 ,f4 ,rutley4#gb ,footnote #gb3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xlvii4 7#ahia7 p4 #edj4 ,mr4 ,rutley more particularly describes those of ,caradoc ,hill4 ,the breccias and tuffs appear to consist mainly of felsitic material4 ,in the coarser varieties1 fragments of finely-banded felsite may be noticed1 while the finer kinds pass into a kind of hornstone 7hlleflinta71 which in hand-specimens could hardly be distinguished from close-grained felsite4 ,in some places1 these pyroclastic rocks are well stratified1 but elsewhere no satisfactory bedding can be recognized in them4 ,various other rocks1 which are probably intrusive1 occur in the ridge4 ,at either end of the ,wrekin there is a mass of pink microgranite1 while at ,caer ,caradoc numerous sheets of greenstone1 intercalated in the fine tuffs1 sweep across the hill4 ,mr4 ,rutley has published an account of these basic rocks1 which he classes as melaphyres1 or altered forms of basalt or andesite4#gc ,that at least some of them are intrusive is manifest by the way in which they ramify through the surrounding strata4 ,but others are so strongly amygdaloidal and slaggy that they may possibly be true interbedded lavas1 though there may be some hesitation in admitting that such basic outflows could be erupted in the midst of thoroughly acid ejections4#gd ,leaving these doubtful flows out of account1 we have here a group of undoubted volcanic rocks represented by acid lavas and pyroclastic materials1 by intrusive bosses of acid rocks1 and by younger basic sills4 ,the general lithological characters of these masses and the sequence of their appearance thus strongly resemble those of subsequent ,palozoic volcanic episodes4 ,footnote #gc3 ,op4 cit4 p4 #ecd4 ,footnote #gd3 ,this difficulty1 however1 need not be in itself insuperable1 as is evident from the remarkable alternation of basic and acid lavas and tuffs in the ,cambrian volcanic group of ,st4 ,david's and in the ,old ,red ,sandstone series of the ,pentland ,hills4 ,the geological age of this volcanic group is a question of much interest and importance in regard to the history of volcanism in this country4 ,an inferior limit to the antiquity of the group can at once be fixed by the fact that1 as originally pointed out by ,dr4 ,callaway1 the quartzite which overlies the volcanic rocks passes under a limestone containing ,cambrian fossils in which ,professor ,lapworth has since recognized ,olenellus1 ,paradoxides and other ,lower ,cambrian forms4 ,the eruptions1 therefore1 must be at least as old as the earlier part of the ,cambrian period4 ,but it is affirmed that the quartzite rests with a complete unconformability on the volcanic rocks4 ,if this be so1 then the epoch of eruption must be shifted much farther back4 ,illustration3 ,fig4 #ci4--,section across the ,uriconian series of ,caer ,caradoc4 ,s#c1 ,upper ,silurian2 ,s#b1 ,bala group2 ,s#a1 ,arenig group2 ,c1 ,cambrian2 ,l1 ,longmyndian2 u1 ,uriconian2 f f1 faults4 ,the evidence adduced in favour of this great break appears to me to be threefold4 ,in the first place1 the quartzite contains fragments of the volcanic rocks4 ,i do not think much stress can be laid on this fact4 ,when ,i visited the ground1 what struck me most in the composition of the quartzite was its singularly pure quartzose character1 and the comparative scarcity of felsite-pebbles in it4 ,any deposit laid down conformably upon the top of the breccias and tuffs might obviously contain some of these materials1 while1 if laid down unconformably1 it might reasonably be expected to be full of them4 ,in the second place1 this quartzite is alleged to pass transgressively across the edges of successive sheets of the volcanic group1 and thus to have a quite discordant dip and strike4 ,i failed to find satisfactory evidence of this unconformability in the northern part of the district4 ,but in the ,caer ,caradoc area the quartzite does appear to steal across the outcrops of the older rocks1 which plunge at nearly right angles in an opposite direction4 ,in the third place1 the felsitic volcanic group is believed by ,professor ,lapworth to pass upwards into the ,longmynd rocks4 ,obviously1 if this group lies at the very bottom of the vast ,longmynd series1 the discordance between it and the quartzite must be enormous1 and the date of the volcanic eruptions must be placed vastly farther back in geological antiquity4 ,though the evidence does not seem to me to amount to clear proof1 ,i am disposed1 in the meantime1 to accept it as affording the most probable solution of the difficulties presented by the structure of the ground4 ,the sequence of the rocks around ,caer ,caradoc is partly concealed by surface accumulations1 but if these could be cleared away the structure of the ground would be1 according to ,messrs4 ,lapworth and ,watts1 as shown in ,fig4 #ci4ge ,footnote #ge3 ,proc4 ,geol4 ,assoc4 vol4 xiii4 7#ahid71 pp4 #cad1 #cae4 ,if1 then1 this volcanic group underlies the whole of the ,longmynd series1 and if1 as it now appears1 that series is older than the ,olenellus-zone of the ,lower ,cambrian rocks1 we can hardly include the volcanic rocks of the ,wrekin and ,caer ,caradoc in the ,cambrian system4 ,they must belong to a still older geological formation1 and ,i think we cannot do better than adopt for them ,dr4 ,callaway's name1 ,uriconian4 ,there are still1 however1 many problems to be solved before the geological history of that region is completely understood4 ,the rocks of the ,longmynd must be more fully worked out4 ,it is improbable that strata which look so likely to yield fossils should for ever prove barren4 ,the lower half at least may be hopefully searched1 although the upper massive reddish sandstones and conglomerates offer less prospect of success4 ,on the west side of the ,longmynd1 above ,pontesbury1 there occurs a small area of volcanic rocks like those of the ,wrekin district1 including a well-marked nodular felsite and fine tuffs4 ,these rocks have been regarded by ,dr4 ,callaway as another axis of the ,uriconian series4 ,it is very difficult1 however1 by any combination of geological structures1 to bring up a portion of the very bottom of the ,longmynd series and place it apparently at the top4 ,this is a feat which a detailed study of the region1 and the detection of unconformabilities in the ,longmynd1 may possibly accomplish4 ,in the meantime1 however1 ,i would venture to suggest whether it is not more probable that we have here a detached area of much younger volcanic rocks1 like those which1 in various districts1 may be included in the ,cambrian system1 and which will be referred to in some detail in subsequent pages4 ,v4 ,,the ,,malvern ,,volcano ,regarding the age and origin of the oldest rocks of the ,malvern ,hills some controversy has arisen1 and no general agreement has yet been reached4#gf ,it is clear that the core of crystalline rocks which is overlain unconformably by the ,hollybush ,sandstone must be older than the ,upper ,cambrian rocks4 ,there is no good evidence of any stratigraphical break in the ,cambrian system of ,england or ,wales1 and it may be reasonably inferred that the break seen at the base of the ,hollybush ,sandstones indicates that the rocks underneath that horizon are pre-,cambrian4 ,some portions of these certainly very ancient rocks are gneisses or schists2 others have been described as felsites1 and have been regarded as passing into schists1 and as the original material from which portions of the foliated series of the range have been produced by mechanical deformation4 ,not improbably the whole series of rocks is of igneous origin1 but has been subsequently rendered more or less schistose4 ,footnote #gf3 ,there is no room here for a full bibliography of the geological literature devoted to this locality4 ,in the monograph by ,j4 ,phillips in vol4 ii4 part i4 of the ,memoirs of the ,geological ,survey1 a list of writings is given up to the time of its publication in #ahdh4 ,since that year many additional papers have appeared4 ,i may especially refer to ,h4 ,b4 ,holl1 ,quart4 ,journ4 ,geol4 ,soc4 xxi4 7#ahfe7 p4 #gb2 ,j4 ,h4 ,timins1 op4 cit4 xxii4 7#ahfg72 ,mr4 ,f4 ,rutley1 op4 cit4 xliii4 7#ahhg7 p4 #dha2 ,dr4 ,callaway1 op4 cit4 xliii4 7#ahhg7 p4 #ebe1 xlv4 7#ahhi7 p4 #dge1 xlix4 7#ahic7 p4 #cih2 ,prof4 ,green1 op4 cit4 li4 7#ahie7 p4 #a2 ,mr4 ,h4 ,d4 ,acland1 ,geol4 ,mag4 #ahid1 p4 #dh4 ,there is one area where the rocks have escaped metamorphism1 and where they present some of the well-known features of ancient volcanic materials4 ,this tract was first indicated by ,dr4 ,h4 ,b4 ,holl as one occupied by altered primordial rocks and post-primordial trap4 ,its evidently igneous materials have been examined and described by different observers1 among whom ,dr4 ,callaway has contributed some detailed papers on the subject4 ,more recently ,professor ,green1 who had the advantage of sections exposed in the excavations for the construction of a reservoir for supplying water to ,great ,malvern1 came to the conclusion that the rocks consist mainly of felsites1 having many of the characters of rhyolites4 ,with these are associated felsitic tuffs1 while bands of dolerite1 probably intrusive1 form likewise part of the series4 ,so far as the somewhat meagre evidence allows an opinion to be formed1 there appears to be an alternation of felsites1 lavas and tuffs placed in a more or less vertical position1 striking in a northerly direction1 and traversed by several sheets of intrusive dolerite4 ,no junction has been found between these unfoliated volcanic rocks and the schists that form the core of the range4 ,judging merely from their present relative condition1 one would naturally infer that the volcanic rocks must be the younger of the two groups4 ,but1 as ,professor ,green has pointed out1 it is conceivable that the latter may have locally escaped crushing1 and yet be of the same age as the felsites and epidiorites of the neighbouring ,raggedstone ,hill1 which have been in part considerably affected by mechanical movements4#gg ,footnote #gg3 ,op4 cit4 p4 #g4 ,the metamorphism of the igneous rocks of the ,malvern ,hills into schists has been especially investigated by ,dr4 ,callaway4 ,for our present inquiry it is perhaps sufficient to take note that in the heart of the ,malvern ,hills there lies a remnant of a volcanic district1 probably of pre-,cambrian age1 the rocks of which had been raised up into a vertical position so as to form islets or reefs in the sea in which the ,upper ,cambrian strata 7,hollybush ,sandstone and ,upper ,lingula shales7 were deposited4 ,until some more precise evidence is obtained as to the geological age of these rocks it may be convenient to place them provisionally with the volcanic ,uriconian series4 vi4 ,,the ,,charnwood ,,forest ,,volcano ,in the heart of ,england the great ,triassic plain is diversified by the uprise through it of the peaks and crests of an old ,triassic land-surface1 which are embraced in the district known as ,charnwood ,forest4 ,these scattered eminences consist of materials not only immensely older than the ,trias1 but once doubtless buried under thousands of feet of ,palozoic strata4 ,they had been laid bare by denudation and carved into picturesque crags and pinnacles before the ,new ,red ,sandstone was deposited around and above them4 ,to these vestiges of an early ,mesozoic land1 still half buried among ,triassic strata1 a peculiar interest attaches from the obviously high antiquity of their rocks and their uprise in the very centre of the island4 ,various opinions have been expressed as to the age of their component rocks4 ,when they were mapped by the ,geological ,survey they were recognized to be as old as any group of rocks then known1 and they were accordingly placed in the ,cambrian system4 ,more recent research has suggested that they may be still more ancient1 and may be regarded as pre-,cambrian4 ,the rocks of ,charnwood ,forest have been the subject of an exhaustive research by the ,rev4 ,e4 ,hill and ,professor ,bonney1 to whom most of our knowledge regarding them is due4 ,these observers first pointed out the truly volcanic nature of the coarse clastic rocks of the district4 ,they have traced their relations in the field1 and have likewise described their structure and composition as shown by the microscope4 ,subsequently the district has been re-mapped on the scale of six inches to a mile by ,mr4 ,fox ,strangways for the ,geological ,survey1 while ,mr4 ,w4 ,w4 ,watts1 another member of the ,survey1 has studied the petrography of the ground1 and has traced the boundaries of the several rock-groups so far as these can be determined4 ,confirming generally the stratigraphical arrangement sketched by ,messrs4 ,hill and ,bonney1 ,mr4 ,watts has proposed the following classification of the rocks3--#gh #g4 ,groby and ,swithland slates4 #f4 ,hanging ,rocks conglomerate and ,the ,brand series4 ,bradgate quartzite4 #e4 ,woodhouse beds 7ashy grits74 #d4 ,slate-agglomerate of ,roecliffe4 ,the ,maplewell series 7volcanic tuffs and agglomerates74 #c4 ,hornstone beds of ,beacon ,hill4 #b4 ,felsitic agglomerate of ,benscliffe4 #a4 ,quartzose1 felspathic and felsitic grits4 ,the ,blackbrook series4 ,footnote #gh3 ,annual ,report of ,director-,general of the ,geological ,survey1 in the ,report of ,science and ,art ,department for #ahie4 ,under any computation or measurement1 the total thickness of detrital material in this series of formations must amount to several thousand feet4 ,the chief interest centres in the middle series1 which consists largely of fragmental volcanic rocks1 with intercalations of slate and grit4 ,as was first shown by ,mr4 ,hill and ,professor ,bonney1 these volcanic materials vary from exceedingly coarse agglomerates to fine1 ashy or felspathic slates4 ,in most cases distinct bedding can be recognized in them1 but more particularly in the fine-grained material4 ,yet even among the massive agglomerates a tendency may be seen towards an orientation of the blocks with their long axes parallel4 ,that this arrangement is not entirely due to the effects of cleavage may be inferred from the many exceptions to it1 which would hardly have occurred had such powerful cleavage affected the whole district1 as would be needed to rearrange the large blocks in the agglomerates4 ,besides1 the coarser parts often intercalate with fine felspathic grits1 which distinctly mark the stratification of the whole4 ,the remarkably coarse breccia of ,benscliffe is mainly made up of blocks of quartz-porphyry1 felsite or rhyolite1 with slate fragments4 ,the ,roecliffe agglomerate1 another extraordinarily coarse rock1 consists of slate fragments imbedded in an andesitic matrix1 some of the blocks of slate being six feet long4 ,the finer tuffs have been ascertained to consist of felsitic or andesitic detritus1 sometimes forming exceedingly compact flinty rocks or hornstones4 ,in this thick accumulation of detrital rocks we are presented with a series of alternations of coarser and finer pyroclastic material1 interstratified among green1 grey and purple slates and grits1 which probably represent the non-volcanic sediments of the time of eruption4 ,the succession of strata bears witness to a long series of eruptions of varying intensity1 but culminating at two distinct periods in the discharge of huge blocks of rock 7,benscliffe and ,roecliffe agglomerates74 ,after some search ,i have been unable to detect a single vesicular fragment among the stones in the breccias and tuffs1 and ,messrs4 ,hill and ,bonney were not more successful4 ,not a trace of anything in the least degree scoriaceous is anywhere to be found4 ,the paste in which the blocks lie consists of such fine material as would result from the trituration of felsite and slate4 ,it contains many broken crystals of felspar1 with grains of clear quartz4 ,a gradation can be traced from the coarser into the finer bands of volcanic and non-volcanic material1 fine slates being also interleaved with highly-felspathic partings of grit4 ,having looked with some care for a trace of a true volcanic neck in the district1 ,i have not seen anything that could be unhesitatingly so designated4 ,even in the north-western part of the district1 where the breccias are coarsest1 and there is least trace of ordinary sediments1 some signs of bedding can usually be detected in the position of the imbedded stones and the partings of finer tuff4 ,both the coarser and finer detritus suggest the kind of material discharged from vents before the uprise of any lava4 ,the entire absence of scoriaceous fragments is noteworthy1 and the abundance of slate blocks rather points to the early eruptions of a volcanic focus4 ,possibly1 while the chief centre of eruption lay towards the north-west1 numerous vents may have been opened all over the district1 discharging abundant showers of dust and stones1 but seldom or never culminating in the actual outpouring of lava4 ,no indubitable lava-sheet has1 in my judgment1 been yet recognized in ,charnwood ,forest4 ,various opinions have been expressed as to some of the more compact close-grained rocks1 and even the verdicts of the same observers have varied from time to time1 the rocks once considered as felsites being afterwards regarded as tuffs1 and subsequently placed with the felsites or andesites after all4 ,it is not necessary for my present purpose to enter into these questions1 which are rather of local interest4 ,i will only say that1 in my opinion1 the rocks of ,sharpley1 ,peldar1 and ,bardon ,hill are massive rocks1 as they have finally been classed by ,messrs4 ,hill and ,bonney4 ,but ,i cannot look upon them as lavas1 at least ,i have seen no evidence to lead me to believe that they were ever erupted at the surface4 ,i have fully considered the arguments of ,mr4 ,hill and ,professor ,bonney on this point4#gi ,there can1 ,i think1 be no doubt of the close association of these felsitic rocks and the breccias1 but the structure of the rocks in the field seems to me to be decidedly in favour of the view expressed above4 ,the microscope affords no assistance in the question4#hj ,the doubtful rocks seem to me rather to be intrusive masses which have been protruded into the volcanic sedimentary series among which they rise4 ,they are acid1 fine-grained1 porphyritic rocks1 which would formerly have been included under the general name of felsites or quartz-porphyries4 ,their coarse porphyritic parts rapidly pass into close-grained felsitic material4 ,many of the blocks in the breccias are precisely like parts of these rocks4 ,it might hence be asserted that these fragmental deposits are later than the eruptive bosses4 ,at least it is obvious that rocks of the same type as those of ,sharpley1 ,peldar1 and ,bardon ,hill must have been disrupted to produce the coarse breccias4 ,footnote #gi3 ,quart4 ,journ4 ,geol4 ,soc4 xlvii4 7#ahia71 pp4 #hj-hh4 ,footnote #hj3 ,see ,messrs4 ,hill and ,bonney1 op4 cit4 xxxiv4 7#ahgh71 p4 #baa4 ,later eruptive rocks1 consisting of masses of syenite and granite1 with still younger dykes of dolerite1 andesite1 diorite and felsite1 have successively made their appearance1 and add to the diversity of the igneous phenomena of this district4 ,the question of the age of this isolated volcanic series is one of much interest1 but of great perplexity4 ,though a resemblance may be admitted to exist between some of the slates and parts of the ,cambrian system of ,north ,wales1 the difference between the ,charnwood rocks and the undoubted ,cambrian series of ,warwickshire1 only thirteen miles to the south-west1 is such as to indicate that the former are probably older than the latter4 ,while the ,charnwood rocks have been intensely cleaved and crushed1 those of ,warwickshire have undergone no such change4 ,the argillaceous strata in the one region have been converted into slates1 in the other they remain mere shales4 ,though cleavage is sometimes irregularly developed1 its rapid disappearance in so short a distance as the interval between ,charnwood ,forest and ,nuneaton seems most explicable if we suppose that the rocks at the more easterly locality were cleaved before those towards the west were deposited4 ,if this inference be well grounded the pre-,cambrian age of the ,charnwood volcanoes would be established4 ,but the argument is not conclusive4 ,no fossils of any kind have yet been found in any of the old rocks of ,charnwood4#ha ,merely lithological resemblances or differences are all that can be used as a guide to the geological age of these masses4 ,mr4 ,watts has suggested that possibly the quartzite of ,bradgate 7,no4 #f of the ,charnwood groups7 may be the equivalent of the quartzite which in ,shropshire and ,warwickshire forms the base of the sedimentary ,cambrian formations4 ,if that correlation could be established1 the volcanic series below the quartzite in ,charnwood might be regarded as representing the ,uriconian volcanic series of ,shropshire4 ,footnote #ha3 ,since this page was in type1 ,professor ,lapworth has found a worm-burrow low down in the ,brand ,series1 and one or two additional examples have since been obtained by ,mr4 ,j4 ,rhodes of the ,geological ,survey4 ,these are the first undoubted organisms from the ,charnwood ,forest rocks4 ,mr4 ,watts1 ,geol4 ,mag4 #ahif1 p4 #dhg4 ,,book ,,iii ,,the ,,cambrian ,,volcanoes ,,chapter ,,ix ,,characteristics ,,of ,,the ,,cambrian ,,system ,,in ,,britain ,the ,physical ,geography of the ,cambrian ,period--,the ,pioneers of ,palozoic ,geology in ,britain--,work of the ,geological ,survey in ,wales--,subdivisions of the ,cambrian ,system in ,britain4 ,in leaving the investigation of the pre-,cambrian rocks and entering upon that of the ,palozoic systems1 that is1 the great series of sedimentary formations which include the earliest records of organized life upon the surface of the globe1 the geologist feels much as the historian when1 quitting the domain of legend and tradition1 he can tread firmly in the region of documentary evidence4 ,from the bottom of the ,cambrian system upward through the long series of geological formations1 the chronicle1 though often sadly incomplete1 is usually clear and legible4 ,as we follow the lowest fossiliferous strata across a territory1 we recognize that they bear witness to the same processes of denudation and deposition which have been going on uninterruptedly on the face of the globe ever since4 ,the beds of conglomerate represent the gravels and shingles of old coast-lines and river-beds4 ,the sandstones recall the familiar features of sandy sea-bottoms not far from land4 ,the shales bear witness to the fall of fine sediment in stiller water1 such as now takes place in the deeper parts of seas and lakes4 ,notwithstanding their vast antiquity1 the strata themselves exhibit no exceptional peculiarities of origin4 ,they seem to be just such familiar deposits as are gathering under fitting conditions at the present time4 ,some writers have speculated on the far greater intensity of all geological activities in the early times of the planet's history4 ,but if we may interpret the record of the stratified formations by the phenomena of to-day1 there is for these speculations no confirmation in the sedimentation of the oldest stratified deposits4 ,it is of course quite intelligible1 if not probable1 that many geological forces may have been more vigorous in primeval times than they afterwards became4 ,but of the gigantic tides1 prodigious denudation and violent huddling together of the waste of the earth's surface1 which have been postulated for the early ,palozoic ages1 there is assuredly nowhere any indication among the stratified formations4 ,in those vast orderly repositories1 layer succeeds layer among thinly-laminated shales1 as gently and equably as the fine silt of each tide sinks to-day over the floor of a sheltered estuary4 ,at the primeval period of which these sediments are the memorial1 the waters receded from flat shores and left tracts of mud bare to the sky1 precisely as they do still4 ,then as now1 the sun shone and dried such mud-flats1 covering their surfaces with a network of cracks2 the rain fell in heavy drops1 that left their imprints on the drying mud2 and the next tide rose so gently as to overflow these records of sunshine and shower without effacing them1 but spreading over them a fresh film of sediment1 to be succeeded by other slowly-accumulating layers1 under which they have lain preserved during the long cycles of geological history4 ,that organized creatures had already appeared upon the earth's surface before the beginning of the ,cambrian period cannot be doubted4 ,the animal remains in the lowest ,cambrian strata are far from being the simple forms which might be expected to indicate the first start of animal life upon the surface of the earth4 ,on the contrary1 though they are comparatively scanty in types1 and often rare or absent throughout a thick mass of sedimentary deposits1 they show beyond dispute that1 when they flourished1 invertebrate life had already reached such a stage of advancement and differentiation that various leading types had appeared which have descended1 in some cases with generic identity1 down to our own day4 ,there must have been a long pedigree to these organisms of the oldest known fossiliferous rocks4 ,and somewhere on the earth's surface we may yet hope to find the remains of their progenitors in pre-,cambrian deposits4 ,the researches of many explorers in ,europe and ,north ,america have brought to light an interesting series of organic remains from the ,cambrian system4 ,of the plants of the time hardly any traces have survived1 save some markings which have been referred to sea-weeds4 ,the earliest known sponges and corals occur in this system1 likewise the ancestors of the graptolites1 which played so prominent a part in the life of the next or ,silurian period4 ,there were already representatives of crinoids and star-fishes1 besides examples of the extinct group of cystideans4 ,sea-worms crawled over the muddy and sandy sea-bottom1 for they have left their trails and burrows in the hardened sediments4 ,molluscs had by this time appeared in their four great divisions of ,brachiopods1 ,lamellibranchs1 ,gasteropods and ,cephalopods1 though the forms yet discovered among ,cambrian rocks are comparatively few4 ,the most abundant and characteristic inhabitants of the ,cambrian seas were the trilobites1 of which many genera have been disinterred from the strata4 ,in the lowest fossiliferous ,cambrian group the trilobitic genus ,olenellus1 already referred to1 is the characteristic form4 ,higher up ,paradoxides is predominant1 while towards the top of the system the most characteristic genus is ,olenus4 ,from the organic remains which have been preserved1 we may legitimately infer the existence of others which have entirely disappeared4 ,there seems no reason to doubt that the leading grades of invertebrate life which are wanting in the known ,cambrian fauna were really represented in the ,cambrian seas4 ,the chance discovery of a band of limestone may any day entirely alter our knowledge as to the relative proportions of the several divisions of the animal kingdom in the earliest ,palozoic rocks4 ,sand is rather adverse to the preservation of a varied representation of the organisms of the overlying sea-water4 ,mud is generally favourable1 but calcareous accumulations are greatly more so1 and they usually consist almost entirely of organic remains4 ,thus in the ,cambrian series of the north-west of ,scotland the quartzites that form the lower group1 though sometimes crowded with worm-burrows1 contain hardly any other sign of organisms4 ,the overlying shales1 besides their abundant worm-castings1 have yielded perfect specimens of ,olenellus and other fossils4 ,but in the uppermost group1 consisting of limestones1 every particle of the sediment appears to have passed through the intestines of worms1 and as it gathered on the sea-bottom it enclosed and has preserved a varied and abundant assemblage of organisms1 including trilobites1 gasteropods and a number of cephalopods4 ,while in the ,cambrian rocks of ,europe calcareous bands are comparatively rare1 in those of ,north ,america they are not infrequent4 ,hence it is largely from ,american deposits that our knowledge of the ,cambrian fauna has been derived4 ,not a vestige of any vertebrate organism has yet been detected among the earlier ,palozoic sediments4 ,so far as we know1 there were no fishes in the ,cambrian seas4 ,the highest organisms then existing were chambered shells1 a once abundant and singularly varied class1 of which the living ,nautilus is now the sole representative4 ,in trying to realize the general geographical conditions of ,cambrian time1 the geologist finds himself entirely without any evidence as to the character of the terrestrial vegetation4 ,we can hardly doubt that the land was clothed with plants1 probably including lycopods and ferns1 possibly even cycads and conifers4 ,but no remains of this flora have yet been recovered4 ,nor have any traces of land-animals been detected4 ,all that we yet know of the life of the period has been gleaned from marine sediments1 which show that the invertebrate population by which the sea was then tenanted embraced some of the leading types of structure that have survived through all the long vista of geological time down to our own day4 ,some of the shore-lines of the ,cambrian waters may still be traced1 and it is possible to say where the land of the time stood and where lay the sea4 ,in the ,british area the largest relic of ,cambrian land is found in the far north-west of ,scotland4 ,formed partly of the ,lewisian ,gneiss and partly of the ,torridon ,sandstone1 it takes in the whole chain of the ,outer ,hebrides and likewise part of the present western seaboard of ,sutherland and ,ross4 ,along the margin of that northern land the white sand was laid down which now gleams in sheets of snow-like quartzite on most of the higher mountains from ,cape ,wrath to ,skye4 ,the sea lay to the east and1 so far as we know1 may have stretched across the rest of ,scotland1 and the north and centre of ,england4 ,another vestige of the land of this ancient era occurs in ,anglesey4 ,there1 and likewise over scattered tracts in the ,midlands1 and in the south-west of ,england1 the geologist seems to descry the last relics of islets that rose out of the ,cambrian sea1 and are now surrounded with its hardened sediments4 ,while such was the general aspect of the region of the ,british ,isles during ,cambrian time1 volcanic action manifested itself at various localities over the area1 breaking out on the sea-bottom1 and pouring forth sheets of lava and showers of ashes1 which mingled with the sand and silt that were settling there at the time4 ,in the northern or ,scottish tract no trace of this subterranean activity has been found2 but in the ,english ,midlands and over much of ,wales abundant evidence has been obtained to show that in those districts the ,cambrian period was marked by frequent and prolonged eruptions4 ,as its name denotes1 the ,cambrian system is typically developed in ,wales4 ,it was there that ,sedgwick first worked out the stratigraphical relations of its ancient sediments1 and that ,murchison demonstrated the succession of organic remains contained in them1 applying to them the principles of classification laid down by ,william ,smith in regard to the ,secondary formations4 ,it was there too that some of the earliest and most memorable achievements were made in the investigation of ancient volcanic rocks4 ,sedgwick and ,murchison1 besides the admirable work which they accomplished in establishing the stratigraphy of the older ,palozoic formations1 clearly recognized that among these formations there were preserved the records of contemporaneous submarine eruptions4 ,sedgwick showed that the mountainous masses of eruptive rock in ,north ,wales were really lavas and ashes1 which had been discharged over the sea-floor at the time when the ancient sediments of that region were deposited1 while ,murchison established the same fact by numerous observations in the east and south of ,wales1 and in the bordering ,english counties4 ,de la ,beche had found similar evidence among the grauwacke rocks of ,devonshire4#hb ,footnote #hb3 ,for early researches on the older ,palozoic volcanic rocks of ,britain1 see ,sedgwick1 ,proc4 ,geol4 ,soc4 vols4 ii4 7#ahch7 pp4 #fgh1 #fgi1 iii4 7#ahda7 p4 #edh1 iv4 7#ahdc7 p4 #bae2 ,quart4 ,journ4 ,geol4 ,soc4 vols4 i4 7#ahde7 pp4 #h-ag1 iii4 7#ahdg7 p4 #acd4 ,murchison1 ,proc4 ,geol4 ,soc4 vol4 ii4 7#ahcc-cd7 p4 #he2 ,silurian ,system 7#ahci7 pp4 #bbe1 #beh1 #bfh1 #bhg1 #cag1 #cbd1 #dja2 ,siluria1 #dth edit4 7#ahfg7 p4 #gf et seq4 ,de la ,beche1 ,mem4 ,geol4 ,survey1 vol4 i4 7#ahdf7 pp4 #bi-cf4 ,a4 ,c4 ,ramsay in the ,maps and ,horizontal ,sections of ,wales published by the ,geological ,survey2 also ,descriptive ,catalogue of the ,rock-,specimens in the ,museum of ,practical ,geology1 #ast edit4 7#aheh71 #bnd edit4 7#ahei71 #crd edit4 7#ahfb72 ,the ,geology of ,north ,wales1 forming vol4 iii4 of ,memoirs of the ,geological ,survey1 #ast edit4 7#ahff71 #bnd edit4 7#ahha74 ,following in the track thus opened up by these great masters1 the officers of the ,geological ,survey were enabled to unravel1 as had never before been attempted1 the complicated structure of the old volcanic regions of ,wales4 ,at the outset of the following discussion ,i wish to express my admiration of the labours of the early pioneers who thus laid for us the foundation of our knowledge of volcanic action in the ,palozoic periods4 ,to ,de la ,beche and his associates in the ,survey a special measure of gratitude is due from all who have followed in their steps and profited by their work4 ,when we consider the condition of geological science1 and especially of the department of petrography1 in this country at the time when these early and detailed investigations were carried on1 when we remember the imperfection of much of the topography on the old one-inch ,ordnance maps 7which were the only maps then available71 when we call to mind the rugged and lofty nature of the ground where some of the most complicated geological structures are displayed1 we must admit that at the period when these maps and sections were produced they could not have been better done2 nay1 that as in some important respects they were distinctly in advance of their time1 their publication marked an era in the progress of structural1 and especially of volcanic1 geology4 ,the separation of lavas and tuffs over hundreds of square miles in a mountainous region1 the discrimination of intrusive sheets and eruptive bosses1 the determination of successive stratigraphical zones of volcanic activity among some of the oldest fossiliferous formations1 were achievements which will ever place the names of ,ramsay1 ,selwyn1 ,jukes and their associates high in the bede-roll of geological science4 ,no one ever thinks now of making a geological excursion into ,wales without carrying with him the sheets of the ,geological ,survey map4 ,these form his guide and handbook1 and furnish him with the basis of information from which he starts in his own researches4 ,but science does not stand still4 ,the most perfect geological map that can be made to-day will be capable of improvement thirty or forty years hence4 ,the maps of the ,geological ,survey are no exception to this rule4 ,in criticizing and correcting them1 however1 let us judge them not by the standard of knowledge which we have now reached1 but by that of the time when they were prepared4 ,it is easy to criticize2 it is not so easy to recognize how much we owe to the very work which we pronounce to be imperfect4 ,the ancient volcanoes of ,wales1 thanks mainly to the admirable labours of my former friend and chief1 ,sir ,andrew ,c4 ,ramsay1 have taken a familiar place in geological literature4 ,but a good deal has been learnt regarding them since he mapped and wrote4 ,the volcanic history1 as he viewed it1 began in the ,arenig period4 ,the progress of subsequent inquiry1 however1 has shown that there are volcanic rocks in ,wales of much older date4 ,i shall show that the ,cambrian period1 both in ,south and ,north ,wales1 was eminently volcanic4 ,much controversy having arisen as to the respective limits and nomenclature of the older ,palozoic rocks1 let me state1 at the outset of the inquiry into the volcanic eruptions of ,cambrian time1 that under the term ,cambrian ,i class all the known ,palozoic rocks which lie below the bottom of what is termed the ,arenig group4 ,it was maintained by ,sir ,andrew ,ramsay and his colleagues on the ,geological ,survey that on the mainland of ,wales no base is ever found to the ,cambrian system4 ,more recently certain conglomerates have been fixed upon as the true ,cambrian base1 both in ,south and ,north ,wales1 and endeavours have been made to trace an unconformability at that line1 all rocks below it being treated as pre-,cambrian4 ,but conglomerates do not necessarily mark a stratigraphical discordance1 and in ,south ,wales there is no trace of any unconformability between the strata above and below the supposed line of break4#hc ,professor ,bonney has shown that in ,north ,wales several zones of conglomerate have been erroneously identified as the supposed basal platform of the ,cambrian series1 and more recently ,mr4 ,blake has pointed out that some of these conglomerates are unquestionably ,lower ,silurian4 ,footnote #hc3 ,see a discussion of this subject in ,quart4 ,journ4 ,geol4 ,soc4 vol4 xxxix4 7#ahhc71 p4 #cje4 ,my own examination so far confirms the conclusions arrived at by these observers4 ,like my predecessors in the ,geological ,survey1 however1 ,i have been unable to detect anywhere in ,caernarvonshire or ,merionethshire a base to the ,cambrian system1 and ,i am compelled to agree with them in regarding as ,cambrian 7partly even as ,lower ,silurian7 all the rocks from ,bangor to ,llanllyfni1 which have more recently been classed as pre-,cambrian4 ,but though thus supporting their general stratigraphy1 ,i am bound to acknowledge that they failed to recognize the existence of a great volcanic series below the ,arenig horizon4 ,the existence of this series1 noticed by ,sedgwick1 was first definitely stated by ,professor ,hughes1#hd and his statements have been confirmed and extended by subsequent observers1 notably by ,professor ,bonney and ,mr4 ,blake4 ,the ,cambrian period is thus proved to have been perhaps even more continuously volcanic than the ,lower ,silurian period was in ,wales4 ,footnote #hd3 ,proc4 ,camb4 ,phil4 ,soc4 vol4 iii4 7#ahgg71 p4 #hi4 ,the ,cambrian volcanic areas of ,north ,wales are represented in ,map ,,ii4 ,the following table shows the subdivisions of the ,cambrian system now recognized in ,britain3-- ---------------------------------------- ----------------------------- ,wales4 7,ranging up to ,western ,england4 ,n4,w4 ,scotland4 #ab1jjj feet 7,about #cjjj feet47 7,about #bjjj feet47 or more47 ---------------------------------------- ----------------------------- ,upper or ,olenus ,zones4 ,tremadoc ,slates ,shineton ,shales ,limestones1 about #aejj ,lingula ,flags 7,dictyograptus feet thick1 divisible 7,lingulella1 or ,dictyonema1 into seven groups ,olenus1 etc474 ,olenus1 etc474 7,archocyathus1 ,maclurea1 ,ophileta1 ,middle or ,paradoxides ,zones4 ,murchisonia1 ,menevian group ,conglomerates and ,orthoceras1 7,paradoxides74 limestones 7,comley71 and vast quantities of 7,comley71 with annelid castings74 ,paradoxides1 etc4 ,lower or ,olenellus ,zones4 ,harlech and ,thin quartzite ,shales 7fucoid beds71 ,llanberis passing up into with ,olenellus1 group with green flags1 grits1 ,salterella1 etc4 basement volcanic shales ,quartzites with annelid rocks2 bottom not and sandstone burrows4 ,the base of seen4 7,comley ,sandstone71 the series lies containing unconformably on ,olenellus4 pre-,cambrian rocks4 ---------------------------------------- ----------------------------- ,,chapter ,x ,,the ,,cambrian ,,volcanoes ,,of ,,south ,,wales ,in the southern part of the ,principality of ,wales a remarkably varied display of ,british ,cambrian volcanic rocks has been preserved4 ,the district around ,st4 ,david's has the distinction of being the first in which volcanic rocks of such high antiquity were recognized4 ,as far back as the year #ahdb1 ,ramsay found that felspathic volcanic ash was associated with other proofs of igneous action1 and this fact was recorded by him on the published ,horizontal ,sections of the ,geological ,survey4 ,unfortunately he afterwards came to regard the rocks as altered ,cambrian1 thus following certain hypothetical views which1 as will be further alluded to in the sequel1 he had adopted in explanation of the phenomena in ,caernarvonshire and in ,anglesey4 ,the volcanic nature of these ancient materials was subsequently rediscovered by ,dr4 ,hicks1 who has devoted much time and labour to their study4 ,distinguishing the volcanic series of ,st4 ,david's by the name ,pebidian1 he has contended that it forms a pre-,cambrian system separated by an unconformability from the base of the ,cambrian formations4 ,he likewise endeavoured to show that an older system of rhyolitic lavas1 felsitic breccias and hlleflintas could be distinguished1 which he termed ,arvonian2 and more ancient still1 a core of granitoid or gneissic rocks1 which he separated under the name of ,dimetian4 ,my own investigation of the ground thoroughly convinced me that there are no pre-,cambrian rocks at ,st4 ,david's2 that the ,arvonian and ,dimetian series are merely intrusive rocks 7quartz-porphyry1 granite1 etc47 which have invaded the volcanic series2 and that the ,pebidian1 instead of being a pre-,cambrian formation on which the ,cambrian base rests unconformably1 is a group of volcanic rocks into which the ,cambrian strata pass down conformably1 and which in the ,st4 ,david's district constitutes the lowest group of the ,cambrian system4#he ,footnote #he3 ,for ,dr4 ,hicks' views1 see especially his papers in the ,quart4 ,journ4 ,geol4 ,soc4 vols4 xxxi4 xxxiii4 xxxiv4 xl4 ,my criticism of them will be found in op4 cit4 vol4 xxxix4 7#ahhc71 subsequently in the main confirmed by ,prof4 ,lloyd ,morgan1 op4 cit4 xlvi4 p4 #bda4 ,see also ,prof4 ,blake1 op4 cit4 xl4 7#ahhd74 ,dr4 ,hicks in his more recent papers has merely reiterated his previously published opinions4 ,illustration3 ,walker ,boutall sc4 ,fig4 #dj4--,map of the volcanic district of ,st4 ,david's4 ,the volcanic geology of ,st4 ,david's possesses a special interest inasmuch as it embraces a tolerably full development of various features which characterize the volcanic groups of later ,palozoic systems4 ,though the rocks are chiefly tuffs1 they include also sheets of lava1 as well as sills1 dykes and bosses4 ,they show a remarkable range in chemical composition from quite basic to highly acid materials4 ,they present the amplest proofs of having been erupted and spread out over the sea-bottom1 and they likewise afford clear evidence of alternation with the ordinary non-volcanic sediment of the time to which they belong4 ,in these respects they are particularly noteworthy1 for they prove that in the earliest ,palozoic ages the essential features of volcanic action were already as well developed as in any subsequent epoch of geological history4 ,the volcanic group of ,st4 ,david's attains a visible thickness of about #ahjj feet4 ,its upper part graduates upward into purple and green ,lower ,cambrian sandstones4 ,the base of the group is not seen owing to the plicated structure of the district4 ,hence the total thickness of volcanic material cannot be determined1 neither can we tell on what it rests1 whether on a still lower sedimentary series or on some platform of pre-,cambrian rocks4 ,the structure of the group1 notwithstanding all that has been written about it1 has never yet been adequately worked out4 ,the unfortunate and barren controversy about supposed pre-,cambrian rocks at ,st4 ,david's has tended to obscure the real importance of these rocks as the oldest well-preserved record of volcanic action in ,britain4 ,they deserve to be carefully surveyed on maps of a large scale1 in the same detailed manner as has been so successfully applied to the elucidation of younger volcanic tracts4 ,until such detailed investigation is made1 any account of them which is given can be little more than a general outline of the subject4 ,the following description is the result of my examination of the ground in company with my colleague ,mr4 ,b4 ,n4 ,peach1 and afterwards with the late ,mr4 ,w4 ,topley4#hf ,a few additional observations1 from the subsequent exploration of ,professor ,lloyd ,morgan1#hg are incorporated in the narrative4 ,footnote #hf3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xxxix4 7#ahhc71 p4 #bid et seq4 ,while the essential parts of the investigation are given in the following pages1 ,i would refer the reader to this paper for details not transferred to the present volume4 ,footnote #hg3 ,op4 cit4 vol4 xlvi4 7#ahij71 p4 #bda4 ,the geologist who traces these ,st4 ,david's rocks in the field cannot fail to be struck with their general resemblance to volcanic masses of later ,palozoic date4 ,many of the lavas and tuffs are in outward characters quite indistinguishable from those of the ,lower ,old ,red ,sandstone and ,carboniferous systems of ,britain4 ,so many points of detail may be observed to be common to the ,palozoic eruptive rocks all over the country from the ,cambrian to the ,permian periods as to indicate that volcanic phenomena must have recurred under much the same conditions throughout ,palozoic time4 ,by far the larger part of the ,cambrian volcanic group of ,st4 ,david's consists of bedded tuffs1 though a few lavas are interstratified in it1 particularly towards the top4 ,the whole has subsequently been invaded by acid protrusions1 and lastly by basic dykes4 #a4 ,bedded ,tuffs and ,lavas4--,the tuffs1 which are the predominant members of the volcanic group1 present many varieties of colour1 from dark purple1 through tints of brick-red and lilac1 to pale pink1 yellow and creamy white1 but not unfrequently assume various shades of dull green4 ,they vary likewise in texture from somewhat coarse breccias or agglomerates1 through many gradations1 into fine silky schists in which the tuffaceous character is almost lost4 ,generally they are distinctly granular1 presenting to the naked eye abundant angular and subangular lapilli1 among which broken crystals of a white1 somewhat kaolinized1 felspar and fragments of fine-grained felsite are often conspicuous4 ,the greater part of the tuffs1 particularly the purple1 red and dark-green varieties1 which constitute so large a proportion of the whole1 has been derived from the explosion of basic rocks similar in character to the diabases now found associated with them4 ,on the other hand1 the paler varieties1 both in the form of fine tuffs and of breccias1 have probably resulted mainly from the destruction of more siliceous lavas1 probably felsites 7rhyolites7 or other acid rocks4 ,that many of the tuffs are due to the destruction of diabase-lavas may be surmised from their close general external resemblance to these rocks1 and from the way in which they are associated with the contemporaneous sheets of diabase4 ,some of the dull dark-purple tuffs might almost at first sight be mistaken for truly eruptive rocks4 ,the analyses of two typical examples of these basic tuffs 7,nos4 ,i4 and ,,ii471 and one 7,no4 ,,iii47 of an intermediate variety containing an admixture of acid fragments1 are given in the subjoined table4 ---------------------------------------- ------------- ,i4 ,,ii4 ,,iii4 ---------------------------------------- ------------- ,si,o#b #eabe #dhaa #faed ,al#b,o#c #bjda #accj #afcj ,fe#b,o#c #cjb #cgj #ddj ,fe,o #cia #haj #cff ,mn,o #jba #adc #jcb ,ca,o #dec #hdh #cjh ,mg,o #gbb #iea #bii ,k#b,o #bic #aeg #afb ,na#b,o #ahb #aif #bha ,h#b,o and ,loss on ,ignition4 #ejb #dba #bii ,total4 #ajjcb #ajjcg #iiga ,specific ,gravity4 #bhd #bib 444 ---------------------------------------- ------------- ,i4 ,purplish-red shaly tuff from below olivine-diabase1 ,crag ,rhosson4 ,analysis by ,mr4 ,j4 ,s4 ,grant ,wilson4 ,,ii4 ,dull purple and green tuff from the lowest group of tuffs between ,pen-maen-melyn and ,pen-y-foel4 ,analysis by ,mr4 ,wilson4 ,,iii4 ,greenish shaly finely granular tuff1 from road-side1 north of ,board ,schools1 ,st4 ,david's4 ,analysis by ,prof4 ,a4 ,renard of ,ghent4 ,although the majority of the tuffs are more or less basic1 they frequently contain evidence in the form of small felsitic lapilli that acid lavas were present in the eruptive vents1 while the pale tuffs show that at the time of their discharge it was these acid lavas and not the diabases that were blown out by the explosions4 ,appended are three analyses of the acid tuffs 7,nos4 ,,iv4 ,v4 and ,,vi474 ---------------------------------------- ------------- ,,iv4 ,v4 ,,vi4 ---------------------------------------- ------------- ,si,o#b #hjei #gcdb #gbfc ,al#b,o#c #aabi #abji #afbc ,fe#b,o#c #jbh #jia #bgj ,fe,o #ada #cac #jdh ,mn,o trace #jbe 444 ,ca,o #jeb #bid #jah ,mg,o #jie #aab #acf ,k#b,o #bih #afg #cce ,na#b,o #jgb #chh #jae ,h#b,o and ,loss on ,ignition #aif #abh #cjj ,total4 #ajjgj #ajjfi #ajjab ,specific ,gravity4 #bee #bgd 444 ---------------------------------------- ------------- ,,iv4 ,greenish felsitic breccia1 ,clegyr ,hill2 angular fragments of various felsites in a greenish base4 ,analysis by ,mr4 ,j4 ,s4 ,grant ,wilson4 ,v4 ,grey granular felsitic tuff1 ,bridge over ,allan ,river north from ,st4 ,david's ,board ,schools4 ,analysis by ,mr4 ,wilson4 ,,vi4 ,pale pinkish-white1 finely schistose tuff--a characteristic sample of the ,porth-lisky schists4 ,analysis by ,prof4 ,renard4 ,many varieties of texture can be traced among the tuffs1 from coarse breccias or agglomerates1 with blocks a yard or more in length1 to fine schistose mudstones or sericitic schists4 ,one of the most remarkable of the finer kinds1 found near ,pen-y-foel1 is externally dirty-green1 compact and tolerably homogeneous1 but with distinct evidence of its clastic character4 ,under the microscope it is found to be composed mainly of lapilli of a peculiar rock1 which is characterized by the abundance and freshness of its plagioclase 7an unusual feature in the volcanic group of ,st4 ,david's72 by the large1 well-defined crystals 7one of which measured #jjbb inch by #jjabe inch7 of augite2 by large crystals replaced by green decomposition-products1 but having the external form of olivine2 by the absence or scantiness of any base or groundmass2 and1 in many of the lapilli1 by the abundance of spherical cells1 either empty or filled up as amygdales with decomposition-products4 ,these spherical vapour-vesicles1 so characteristic of the basic or palagonitic lapilli in many ,palozoic volcanic vents1 were found in one fragment1 where they were particularly abundant1 to range from a minimum of #jjjjh inch to a maximum of #jjjcc inch1 with a mean of about #jjjah4 ,the rock from which these lapilli have been derived comes nearest to one of the diabases from the same part of the district 7which will afterwards be referred to71 but shows a closer approach to basalt rocks4 ,another interesting tuff is that of which the analysis 7,no4 ,,ii47 has been given4 ,it occurs not far from the horizon of the rock just described4 ,under a low power1 it is seen to be composed mainly of fragments of diabase like the rocks of ,rhosson and ,clegyr ,foig4 ,these fragments are subangular1 or irregular in shape1 and vary considerably in size4 ,they are sometimes finely cellular--the cavities1 as in the case just referred to1 being spherical4 ,the plagioclase crystals in the diabase-lapilli are everywhere conspicuous2 so also is the augite1 which occurs in larger forms than in the rock of ,rhosson or ,clegyr ,foig4 ,next in abundance to these basic fragments are rounded or subangular pieces of felsite4 ,these weather out in conspicuous grey rough projections on the exposed face of the rock2 under the microscope they are seen to consist of fine granular felsite1 which shows a groundmass remaining dark between crossed nicols1 but with luminous points and filaments1 and an occasional spherulite giving the usual cross in polarized light4 ,lapilli of an older tuff may here and there be detected4 ,a few angular and subangular grains of quartz are scattered through the rock4 ,the lapilli are bound together by a finely-granular dirty-green substance4 ,as a typical illustration of the minute structure of the felsitic tuffs1 ,i may refer to the rock ,no4 ,v4 of the foregoing analyses4 ,it is composed mainly of fragments of various felsites1 many of which show good flow-structure4 ,large1 and usually broken1 crystals of orthoclase are dispersed among the other ingredients4 ,here and there a fragment of diabase may be detected2 but ,i could find no trace of pieces of the peculiar microcrystalline spherulitic quartz-porphyries of ,st4 ,david's4 ,there is but little that could be called matrix cementing the lapilli together4 ,the presence of fragments of diabase may possibly reduce the proportion of silica and increase that of magnesia1 as compared with what would otherwise have been present in the rock4 ,some of the tuffs appear to have been a kind of volcanic mud4 ,a specimen of this nature collected from the road-side section1 north of the ,board ,school1 presents a finely-granular paste enclosing abundant angular and subangular lapilli of diabase1 a smaller proportion of felsite 7sometimes displaying perfect flow-structure71 broken plagioclase crystals1 and a greenish micaceous mineral which has been subsequently developed out of the matrix between the lapilli4 ,though they lie in the sedimentary series above the main volcanic group1 ,i may refer here to certain thin bands of tuff at ,castell1 on account of their interest in relation to the true ,cambrian age of the volcanic group4 ,they are not quite so fresh as the tuff that occurs in thicker masses1 but their volcanic origin is readily observable4 ,one band appears to be made up of the debris of some basic rock1 like the diabase of the district1 through which detached plagioclase crystals are scattered4 ,the lapilli are subangular2 and around their border a granular deposit of hmatite has taken place1 giving a red colour to the rock4 ,another band presents small angular lapilli1 almost entirely composed of a substance which to the naked eye1 or with a lens1 is dull1 white and clay-like1 easily scratched1 and slightly unctuous to the touch4 ,under the microscope1 with a low power1 it becomes pale greyish-green and transparent1 and is seen to consist in large part of altered felspar crystals1 partially kaolinized and partially changed into white mica and calcite4 ,these scattered crystals are true volcanic lapilli1 and have not been derived from the mechanical waste of any pre-existing volcanic rock4 ,in the tuffs interstratified with the conglomerate1 at the quarry above ,porth-clais1 though much decomposed1 crystals of plagioclase can likewise still be traced4 ,these strata are also true tuffs1 and not mere detritus due to mechanical degradation 7see ,fig4 #da74 ,the general result of the study of the microscopic structure of the ,cambrian tuffs of ,st4 ,david's may be briefly summed up as follows3-- #a4 ,these pyroclastic deposits are almost wholly composed of fragments of eruptive rocks1 sometimes rounded1 but usually angular or subangular4 ,in the more granular varieties very little matrix is present2 it consists of fine debris of the same materials4 ,no detached microlites have been noted1 such as are common among modern volcanic ashes2 but there are abundant ejected crystals4 ,in these respects the ,cambrian tuffs resemble those of the other ,palozoic systems4 ,a mingling of grains of quartz-sand may indicate the intermixture of ordinary with volcanic sediment4 #b4 ,they may be divided into two groups--one composed mainly of fragments of diabase or other similar basic rocks1 the other of felsite4 ,the former group has doubtless been derived from the explosion of such rocks as the diabase-sheets of the district4 ,the felsitic tuffs have not been observed to contain any fragments of the microcrystalline quartz-porphyries of ,st4 ,david's4 ,they have been derived from true fine-grained felsites or rhyolites4 ,there are various intermediate varieties of tuff1 due to the mingling in various proportions of the two kinds of debris4 #c4 ,they are marked by the presence of some characteristic features of the volcanic vents of later ,palozoic time1 and in particular by presenting the following peculiarities3 7a7 lapilli of a minutely-cellular pumice with spherical cells2 7b7 lapilli with well-developed flow-structure2 7c7 lapilli consisting of a pale green serpentinous substance resembling altered palagonite and probably originally glass2 7d7 lapilli derived from the destruction of older tuffs2 and 7e7 lapilli consisting of ejected crystals1 especially of felspars1 sometimes entire1 often broken4 #d4 ,they frequently show that they have undergone metamorphism1 by the development of a pale greenish micaceous mineral between the lapilli1 the change advancing until the fine tuffs occasionally pass into fine silky schists4 ,in my study of the ,st4 ,david's district1 ,i was unable to observe any evidence that the basic and siliceous tuffs characterize two distinct periods of volcanicity4 ,from the foregoing analyses it appears that some of the oldest visible tuffs which are seen between ,pen-maen-melyn and ,pen-y-foel contain only #dhaa per cent of silica2 while a specimen from ,porth-lisky yielded #gbfc per cent of that ingredient4 ,specimens taken even from adjacent beds show great differences in the percentage of silica1 as may be seen in the analyses ,nos4 ,,iii4 and ,v4 ,this alternation of basic and siliceous fragmental materials has its parallel in the neighbouring eruptive rocks1 some of which are olivine-diabases containing only #de per cent of silica1 while others are highly siliceous quartz-porphyries4 ,but all the siliceous eruptive rocks1 so far as ,i have been able to discover1 are intrusive1 and belong1 ,i believe1 to a later period than that of the volcanic group2 in no single instance do they appear to me to be true superficial lava-flows4 ,on the other hand1 the basic eruptive rocks occur both as contemporaneous sheets and as intrusive masses4 ,the presence of both siliceous and basic lavas in the ,cambrian volcanic reservoirs1 however1 is proved by the character of the tuffs4 ,it would appear from the evidence at present known1 that while the basic lavas were most abundant in the vents during the volcanic period recorded by the rocks of ,st4 ,david's1 furnishing the material for most of the fragmental eruptions1 and issuing in occasional superficial streams of molten rock1 the siliceous lavas did not flow forth at the surface1 though their debris was copiously discharged in the form of dust and lapilli4 ,the rise of both basic and acid lavas at different periods in the same or adjoining vents1 so familiar in recent volcanic phenomena1 thus appears to have also characterized some of the oldest examples of volcanic action4 ,an interesting parallel may be traced between the succession of events at ,st4 ,david's and that which occurred in the volcanic group of the ,lower ,old ,red ,sandstone of the ,pentland ,hills1 near ,edinburgh1 of which a detailed account will be given in ,chapter xx4 of this volume4 ,it is also worthy of remark that in the latest of the volcanic episodes in ,british geology a remarkable similarity to the ,st4 ,david's volcanic group may be observed4 ,some of the older ,tertiary agglomerates are full of pieces of acid rocks 7felsites1 rhyolites or granophyres71 while the lavas poured out at the surface were mainly basalts4 ,in the volcanic group of ,st4 ,david's the tuffs contain evidence that ordinary sedimentation was not entirely interrupted by the volcanic discharges4 ,thus1 in the ,allan valley1 west from the ,cathedral1 one of the schistose tuffs is full of well-rounded pebbles of white quartz4 ,occasional shaly bands indicate the deposit of mud with the tuffs4 ,excluding the granites and porphyries 7which are described at p4 #aee71 two kinds of eruptive rocks are associated with the volcanic group4 ,one of these is certainly intrusive and of late date1 viz4 dykes and veins of diabase1 to be afterwards referred to4 ,the other kind occurs in long parallel sheets1 some of which1 if not all1 are true contemporaneous lava-streams1 erupted at intervals during the accumulation of the volcanic group4 ,they form prominent crags to the west of ,st4 ,david's1 such as ,clegyr ,foig1 ,rhosson1 and the rocky ground rising from the eastern shores of ,ramsey ,sound4 ,their dip and strike coincide with those of the tuffs above and below them4 ,it is possible that some of these sheets may be intrusive sills intercalated along the bedding of the tuffs2 and in one or two cases ,i have observed indications of what1 on further and more careful exploration1 may prove to be disruption across the bedding4 ,but it is the interbedded sheets that possess the chief interest as superficial lava-streams of such venerable antiquity4 ,they present many of the ordinary features of true lava-flows4 ,in particular a slaggy structure may be detected at the bottom of a sheet1 the vesicles being here and there lengthened in the direction of flow4 ,some of the sheets are in part amygdaloidal4 ,the alternation of these sheets with tuffs1 evidently derived from lavas of similar character1 is another argument in favour of their contemporaneous date4 ,one of the best localities for studying these features lies between ,clegyr ,foig and the coast1 west of ,rhosson4 ,the eruptive rocks thicken towards the south-west1 as if the main vents had lain in that direction4 ,there are doubtless intrusive as well as contemporaneously interbedded masses in the rough ground between ,pen-maen-melyn and ,treginnis4 ,to separate these out would be a most interesting and beautiful piece of mapping for any competent geologist in possession of a good map on a sufficiently large scale4 ,the interbedded lavas1 so far as ,i have had an opportunity of studying them1 appear to present remarkable uniformity of petrographical characters4 ,megascopically they are dull1 fine-grained to compact1 sparingly porphyritic1 ranging in colour from an epidote-green to dull blackish-green and dark chocolate-brown4 ,some of them are finely porphyritic from the presence of small glistening surfaces which present the colour and metallic lustre of hmatite and yield its characteristic streak4 ,obviously basic rocks1 they present1 as ,i have said1 a close external resemblance to many of the lavas of the ,lower ,old ,red ,sandstone and ,carboniferous districts of ,scotland4 ,from their chemical composition and microscopic structure they may be most appropriately ranged among the diabases4 ,the analyses of two of the most conspicuous diabases of this class in the district1 those of ,rhosson 7,,vii47 and ,clegyr ,foig 7,,viii471 by ,mr4 ,j4 ,s4 ,grant ,wilson1 are shown in the following table3-- ---------------------------------------- ------ ,,vii4 ,,viii4 ---------------------------------------- ------ ,si,o#b #deib #dech ,al#b ,o#c #ahaf #affb ,fe#b ,o#c #aah #djf ,fe,o #ibg #hfc ,mn,o #jai #jad ,ca,o #gai #hai ,mg,o #ajjg #ida ,k#b,o #agh #jga ,na#b,o #bab #bbj ,h#b,o and ,loss on ,ignition4 #dbb #dcd ,insoluble ,residue4 #jjd #jjh ,total4 #ajjad #iigf ,specific ,gravity4 #bif #b4ii ---------------------------------------- ------ ,the two rocks here analyzed1 likewise that from the crag south of ,castell and that from the cliffs at the southern end of the promontory between ,ramsey ,sound and ,pen-y-foel1 show under the microscope a general similarity of composition and structure4 ,they present a variable quantity of a base1 which under a objective is resolved into ill-defined coalescent globulites and fibre-like bodies1 that remain dark when rotated between crossed nicols4 ,in some varieties1 as in part of ,rhosson ,crag1 the base is nearly lost in the crowd of crystalline constituents2 in others1 as in the crag south of ,castell1 it forms a large part of the whole mass1 and may be seen in distinct spaces free from any crystalline particles4 ,through this base are diffused1 in vast numbers1 irregularly-shaped grains of augite1 seldom showing idiomorphic forms4 ,these grains1 or granules1 may perhaps average about #jjjc inch in diameter4 ,plagioclase is generally hardly to be recognized1 though here and there a crystal with characteristic twinning may be detected in the base4 ,magnetite occurs abundantly--its minute octahedra1 with their peculiar colour and lustre1 being apparent with reflected light on the fresher specimens1 though apt to be lost as diffused ferruginous blotches in the more decomposed varieties4 ,but perhaps the most remarkable ingredient is olivine4 ,red hmatitic crystals are visible1 even to the naked eye1 dispersed through the groundmass of the rocks4 ,with a lens these may be observed to be orthorhombic in form1 and to be evidently pseudomorphs after some mineral which has been converted chiefly into hmatite4 ,such red pseudomorphs are common in ,carboniferous and ,old ,red ,sandstone lavas1 where in some cases they appear to be after hornblende1 and in others after augite1 but occasionally are suggestive of olivine1 though with no trace of the original substance of that mineral4 ,in the lava associated with the tuffs at the south end of the promontory between ,ramsey ,sound and ,pen-y-foel1 however1 certain large1 well-developed pseudomorphs are undoubtedly after olivine4 ,they have the characteristic contour of that mineral and its peculiar transverse curved and irregular fractures4 ,the average length of these pseudomorphs was found1 from the measurement of six examples1 to be #jjbc inch1 the largest being #jjcd1 and the smallest #jjad4 ,seen by transmitted light they present a structureless pale-green material nearly inert in polarized light1 round the borders and across fissures in which an opaque substance has been developed1 as serpentine and magnetite have been grouped in the familiar alteration of olivine4 ,the opaque material is bright brick-red in reflected light1 and is evidently now chiefly oxidized into hmatite4 ,every stage may be traced1 from orthorhombic forms with the incipient development of transverse fissures filled with iron-oxide1 to others of distorted shapes in which the ferruginous matter occupies the whole1 or nearly the whole1 of the mould of the original crystal4 ,the rocks now described differ from the ,palozoic andesites or porphyrites1 with which ,i am acquainted1 in their more basic composition1 in the less abundance of their microscopic base1 in the comparatively inconspicuous development of their felspars of later consolidation1 in the absence of large porphyritic felspars of earlier consolidation1 in the extraordinary prominence of the granular augite1 and in the presence of olivine4 ,in composition and structure they are essentially forms of olivine-diabase4 ,illustration3 ,fig4 #da4--,section showing the interstratification of tuff and conglomerate above ,lower ,mill1 ,st4 ,david's4 ,above the volcanic group of ,st4 ,david's lies a band of quartz-conglomerate which has been taken by ,dr4 ,hicks to mark the base of the ,cambrian system4 ,this rock1 though mainly composed of quartz and quartzite1 contains fragments of the underlying volcanic rocks4 ,but that it does not mark any decided break in the sedimentation1 much less the violent unconformability and vast interval of time which it has been erroneously supposed to do1 is well illustrated by the occurrence of bands of tuff1 as well as diffused volcanic dust1 in the conglomerate and also in the green and red shales and sandstones which conformably overlie it4 ,an example of this intercalation of volcanic material is represented in ,fig4 #da4 ,on the left side vertical layers of fine reddish tuff 7a7 are succeeded by a band of quartz conglomerate 7b7 of the usual character4 ,parallel to this conglomerate comes a band1 about six inches thick1 of fine tuff 7c71 followed by ashy sandstone 7d71 which graduates into conglomerate 7e74 ,no more complete evidence could be desired of the perfect inosculation of the conglomerate with the volcanic group4 ,on the coast at ,nun's ,chapel similar evidence presents itself4 ,the conglomerate there contains some thin seams of tuff1 and is intercalated between a series of dull green agglomerates and tuffs and some greenish shales and sandstones with layers of tuff4 ,not less striking is the evidence of the contemporaneous eruption of fine volcanic dust in the overlying shales and sandstones4#hh ,some of the red shales are full of this material1 which here and there is gathered into the thin seams or ribs of which the microscopic characters have already been described4 ,this diffused volcanic detritus marks1 no doubt1 the enfeebled discharges of fine dust towards the close of the volcanic episode in the ,lower ,cambrian period at ,st4 ,david's4 ,it would be difficult to find an instance of a more perfect transition from a series of thoroughly volcanic masses into a series of ordinary mechanical sediments4 ,footnote #hh3 ,these are a portion of ,dr4 ,hicks' ,caerfai group in the ,lower ,cambrian series4 ,they have yielded ,lower ,cambrian fossils4 #b4 ,intrusive ,granite and ,quartz-,porphyry4--,the heart of the volcanic group is pierced by a mass of granite which also cuts the conglomerate and overlying shales and sandstones on the east side4 ,the age of this intrusive boss cannot be more definitely fixed than by saying that it must be later than the volcanic group4 ,this rock has been the subject of a remarkable amount of description1 and has been dignified by being actually elevated into a distinct ,archan formation composed of highly crystalline gneissic rocks1 with bands of limestone1 hornblende1 chlorite1 and micaceous schists4#hi ,into this somewhat dreary chapter of ,english geological literature it is fortunately not necessary to enter here4 ,i will only say that the rock is unquestionably a granite1 with no essential differences from many other bosses regarding which there has been no controversy4 ,it is a holocrystalline rock with a thoroughly granitic texture1 and composed of the ordinary minerals of granite--quartz1 orthoclase and plagioclase1 among which a green chloritic mineral1 more or less plentiful1 probably represents original hornblende1 biotite or augite4 ,sometimes the quartz and felspar in the body of the rock show a micropegmatitic arrangement1 and the same structure occurs in veins that traverse it4 ,this structure gives the rock some resemblance to the ,tertiary granites and granophyres of ,ireland and ,scotland4 ,footnote #hi3 ,see the papers cited on p4 #ade and my discussion of the relations of this granite in ,quart4 ,journ4 ,geol4 ,soc4 vol4 xxxix42 also ,prof4 ,lloyd ,morgan1 op4 cit4 vol4 xlvi4 7#ahij74 ,this granite has undergone a good deal of decomposition1 for its felspars are turbid1 and its original ferro-magnesian constituents are always replaced by green chloritic aggregates1 while epidote is also present4 ,the rock tends to become finer in grain towards the margin1 and then sometimes assumes a more decidedly pegmatitic structure1 like graphic granite4 ,at the northern end of the granite ridge1 a gradation can be traced from the ordinary texture through increasingly fine-grained varieties until we pass into a microcrystalline spherulitic porphyry4 ,after a careful examination of the ground ,i satisfied myself that the spherulitic quartz-porphyries1 which form a conspicuous feature in the geology of ,st4 ,david's1 are really offshoots from this granitic core4#ij ,footnote #ij3 ,these apophyses from the granite constitute the ,arvonian formation of ,dr4 ,hicks' pre-,cambrian series of ,st4 ,david's4 ,these spherulitic rocks have been fully described4#ia ,they consist of a base composed of a microcrystalline aggregate of quartz and orthoclase1 which is distributed between the spherulites4 ,these have been developed in remarkable beauty and perfection4 ,while the microcrystalline structure is everywhere recognizable1 the spherulites occasionally disappear4 ,but their absence is merely local1 and they may be found both in large dykes and narrow veins4 ,a further porphyritic structure is given to the rocks by the presence in them of abundant quartz1 which takes the form of conspicuous rounded blebs or worn crystals sometimes distinctly dihexihedral1 but with somewhat blunted angles4 ,porphyritic plagioclase is also common4 ,flow-structure is occasionally traceable4 ,some parts of these rocks where the porphyritic elements are locally absent might be cursorily classed as felsites2 but they all possess a microcrystalline and not a felsitic base4 ,they cannot be confounded with the true felsites of which fragments occur in the tuffs4 ,footnote #ia3 ,see1 for example1 ,j4 ,davies1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xxiv4 p4 #afd1 xxxv4 p4 #bjc2 also the paper already referred to1 op4 cit4 xxxix4 p4 #cae2 and ,mr4 ,teall's ,british ,petrography1 p4 #ccd4 ,in addition to the parallelism that may be traced between the earliest ,palozoic agglomerates and those of the youngest volcanic series of ,britain1 a close analogy may also be noticed between the acid intrusive rocks of the two widely-separated periods4 ,in both cases we have a granitic core sending out apophyses which assume a spherulitic structure and traverse earlier volcanic products of the district4 ,these spherulitic quartz-porphyries of ,st4 ,david's occur as bosses1 dykes 7elvans7 or veins1 cutting through all horizons of the volcanic group1 and in one case apparently1 if not actually1 reaching the quartz conglomerate4 ,one of the best exposures of this intrusive character may be seen in the cliff below ,nun's ,chapel1 where the elvan runs along the face of the cliff through the uppermost zone of the volcanic group1 cutting the strata somewhat irregularly4 ,apparently in connection with this dyke1 a network of intrusions of decomposed quartz-porphyry may be observed in the shales along the face of the cliff immediately below ,nun's ,chapel4 ,on the whole1 the intruded material has forced its way along the bedding-planes of the shales1 but has also broken across them1 sending out finger-like branches4 #c4 ,diabase ,dykes and ,sills4--,the latest rocks of the ,st4 ,david's district are dykes and intrusive sheets of diabase1 which traverse all the other formations4 ,the dykes are specially abundant in the granite4 ,one or two may be detected in almost every artificial opening which has been made in that rock2 while on the coast-section they are here and there profusely abundant4 ,they are likewise frequent in the quartz-porphyries1 and occur also in the volcanic group as well as in the sandstones and shales above the conglomerate1 but become fewer in number as they recede from the granite centre4#ib ,footnote #ib3 ,the occurrence of these dykes is paralleled by that of the similar intrusions in the quartz-felsite of ,llyn ,padarn to be afterwards described4 ,in external characters1 the rock composing these dykes and sheets may be described as usually a dull dirty-green or yellowish-brown mass1 to which the old name of wacke might appropriately be given4 ,it exhibits the texture and mode of weathering of the more distinctly crystalline members of the basalt family4 ,it is occasionally amygdaloidal or cellular1 the kernels or cavities being arranged parallel with the sides of the dyke4 ,here and there a rudely prismatic structure extends between the walls4 ,the microscopic structure of this rock has been described by ,professor ,judd1 ,mr4 ,davies and ,mr4 ,tawney4 ,it is a diabase1 but more allied in structure to true basalt than the olivine-diabase of the volcanic group4 ,it especially differs from the older rock in the abundance and freshness of its felspars1 in the comparative scarcity of its augite1 and in the absence of olivine4 ,the magnesian silicates are very generally replaced by green decomposition-products diffused through the mass4 ,an occasional crystal of hornblende1 recognizable by its cleavages and dichroism1 may be detected4 ,some of the diabase dykes present excellent examples of flow-structure4 ,a beautiful instance occurs in a dyke that cuts the shales1 in a small cove to the east of ,nun's ,chapel4 ,the shale and eruptive rock are in contact2 and the small acicular prisms of felspar1 besides ranging themselves in line parallel to the side of the dyke1 stream round the larger felspar crystals4 ,some of the dykes or veins are only three inches broad4 ,they send out fingers1 and sometimes break abruptly across from one line to another4 ,they appear generally to have followed the lines of joint in the granite1 as ,mr4 ,tawney has observed2#ic consequently they must be posterior to the development of the system of jointing in that rock4 ,footnote #ic3 ,proc4 ,nat4 ,hist4 ,soc4 ,bristol1 vol4 ii4 part ii4 7#ahgi71 p4 #aae4 ,besides the abundant dykes1 there has been a more limited extrusion of the same material in sheets parallel 7or approximately so7 to the bedding of the sandstones and shales4 ,these sheets are well displayed at ,st4 ,john's ,point1 where evidence of their being intrusive1 and not truly bedded1 may be seen along the fine cliffs which have been cut by the waves on this part of the coast-line4 ,the sedimentary series which overlies the volcanic group of ,st4 ,david's1 and contains the fossils of the lower part of the ,cambrian system1 gradually loses all trace of volcanic material1 as its members are followed upward in stratigraphical order4#id ,we thus learn that the eruptions of this district came to an end in an early part of the ,cambrian period4 ,but as we shall see in the following pages1 volcanic activity was subsequently renewed at no great distance in the next or ,silurian period4 ,footnote #id3 ,dr4 ,hicks has noted the occurrence of volcanic tuff in the ,lower ,lingula ,flags of ,porth-y-,rhaw1 a little to the east of ,st4 ,david's 7,quart4 ,journ4 ,geol4 ,soc4 vol4 xx4 #ahfd1 p4 #bdj74 ,this intercalation is marked as a dyke in the ,,ms4 notes of ,sir ,a4 ,c4 ,ramsay on a copy of the ,geological ,survey map of the district4 ,,chapter ,,xi ,,the ,,cambrian ,,volcanoes ,,of ,,north ,,wales1 ,,the ,,malvern ,,hills ,,and ,,warwickshire ,,north ,,wales ,the ,cambrian volcanic rocks in the northern part of the ,welsh ,principality have their main development in ,caernarvonshire4 ,southwards from that tract1 though the ,lower ,cambrian strata form a vast pile of sedimentary material in the ,harlech anticline1 which is estimated by the ,geological ,survey to be from #fjjj to #gjjj feet thick1 they have yielded no trace of any contemporaneous volcanic rocks4#ie ,the purple slates that rise along the centre of the anticline dip below the grits and conglomerates on either side without disclosing a glimpse of the base of the system4 ,this enormous accumulation of sedimentary deposits seems to diminish in thickness as it is traced northwards1 for towards the ,menai ,strait it does not reach more than a fourth part of the depth which it is said to display in the ,harlech anticline4#if ,in the ,pass of ,llanberis the series of grits that overlies the purple slates is estimated to be about #acjj feet thick4#ig ,this gradual thinning away of the ,cambrian series towards the north was1 in the opinion of ,sir ,andrew ,ramsay1 accompanied by an increasing metamorphism of the lower portions of the system4 ,in his view1 the long ridge of quartz-porphyry which crosses the lower end of ,llyn ,padarn represents the result of the extreme alteration of the stratified rocks4 ,he believed that he could trace an insensible passage from the slates1 grits and conglomerates into the porphyry1 and he was led to the conviction that the solid porphyry itself is nothing but the result of the alteration of the stratified masses carried a stage further than the stage of porcellanite1 into the condition of that kind of absolute fusion that in many other regions seems to have resulted in the formation of granites1 syenites and other rocks1 commonly called intrusive4#ih ,certain structural lines in the quartz-porphyry he looked upon as indicating traces of stratification in a rock1 the original felspathic and quartzose material of which has been metamorphosed into true porphyry4#ii ,in conformity with these ideas1 the remarkable felspathic strata which lie nearest the porphyry were regarded as metamorphosed ,cambrian rocks1 and where similar rocks reappear over a large area near ,bangor they were coloured on the map with the same tint and lettering as were used for the so-called altered ,cambrian of ,anglesey4 ,footnote #ie3 ,mem4 ,geol4 ,surv4 vol4 iii4 #bnd edit4 ,geology of ,north ,wales1 p4 #ba4 ,it is possible that this thickness has been somewhat overestimated4 ,dr4 ,hicks 7,geol4 ,mag4 #ahhj1 p4 #eai7 has referred to certain highly felsitic rocks1 for the most part a metamorphic series of schists1 alternating with harder felsitic bands1 probably originally felsitic ashes1 lying at the bottom of the whole pile1 and he has claimed them as pre-,cambrian4 ,but ,i have not found any evidence of such rocks1 nor any trace of igneous materials save dykes and sills1 acid and basic1 such as are indicated on the ,survey map4 ,footnote #if3 ,ibid4 p4 #bd4 ,footnote #ig3 ,ibid4 p4 #agc4 ,footnote #ih3 ,mem4 ,geol4 ,surv4 vol4 iii4 #bnd edit4 p4 #agc4 ,footnote #ii3 ,ibid4 p4 #agd4 ,no one who has examined this ,caernarvonshire ground can have failed to find the sections which doubtless led my predecessor to form the convictions to which he gave expression in the passages ,i have just quoted4 ,it is easy to see how these sections1 wherein it is certainly difficult to draw a sharp line between the igneous rock and the clastic materials derived from it1 would be welcomed as appearing to offer confirmation of the ideas concerning metamorphism which were then in vogue4 ,there cannot1 however1 be any doubt that my friend was mistaken in his interpretation of the structure of that part of the country4 ,it is to me a subject of keen regret that in his later years1 when the subject was revived1 he was no longer able to re-examine this ground himself1 for no one would have confessed more frankly his error1 and done more ample justice to those who1 coming after him1 have been able in some parts to correct his work4 ,the quartz-porphyry1 felsite or rhyolite of ,llyn ,padarn1 as well as that of ,llandeiniolen1 is not a metamorphic but an eruptive rock1 as has been demonstrated by ,professors ,hughes and ,bonney4 ,there is no true passage of the sedimentary rocks into it2 on the contrary1 the conglomerates which abut against it are in great part made out of its fragments1 so that it was already in existence before these ,cambrian strata were deposited upon it4 ,these conclusions must be regarded as wholly indisputable4 ,but most of the critics of the work of the ,geological ,survey have proceeded to certain further deductions4 ,they have maintained that the presence of fragments of the porphyry in the overlying conglomerate marks an unconformability between the two rocks1 that the conglomerate shows the base of the ,cambrian system1 and that the porphyry is therefore pre-,cambrian4 ,these assertions and inferences do not seem to me to be warranted4 ,they have1 in my judgment1 been disproved by ,mr4 ,blake1#ajj who shows that there is no break in the ,cambrian series1 that the various porphyries and their accompaniments are parts of that series1 and that there is no certain proof of the existence of any pre-,cambrian rocks in the whole district4#aja ,footnote #ajj3 ,in an excellent memoir read before the ,geological ,society in #ahhh1 with the main conclusions of which ,i agree4 ,footnote #aja3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xliv4 p4 #bga4 ,for subsequent papers by ,mr4 ,blake1 see op4 cit4 vols4 xlviii4 7#ahib7 p4 #bdc1 xlix4 7#ahic7 p4 #dda4 ,that the igneous rocks of the ,llyn ,padarn area mark a volcanic period has been recognized by most writers since ,professor ,bonney pointed out the flow-structure of the quartz-porphyry1 and other proofs of active volcanic eruptions have been traced by him1 as well as by ,professor ,hughes and ,mr4 ,blake1 in the stratified rocks which stretch north-eastwards to ,bangor4 ,the extent and persistence of these ancient volcanic phenomena1 and their probable connection with the remarkable northward attenuation of the ,cambrian sedimentary rocks1 deserve special attention4 ,it is generally agreed that the rocks variously termed quartz-porphyries1 felsites or rhyolites form the oldest members of this volcanic series4#ajb ,they come to the surface in two long ridges1 one running from ,caernarvon to near ,bangor1 the other from ,llanllyfni to ,ann's ,chapel1 at the mouth of ,nant ,francon 7,map4 ,,ii474 ,whether the materials of these two ridges are parts of one originally continuous sheet or group of sheets1 or1 if different protrusions1 whether they belong to the same geological horizon1 or whether1 as ,mr4 ,blake believes1 they are distinct masses1 separated by a considerable thickness of detrital material1 cannot in the present state of our knowledge be positively decided4 ,it seems to me probable that they are connected underground1 as a continuous platform beneath the overlying pyroclastic materials4 ,footnote #ajb3 ,whether the granitic rock of ,twt ,hill1 ,caernarvon1 is connected with the porphyry or belongs to an older eruption is immaterial for my present purpose4 ,these acid rocks have been regarded by some observers as intrusive sheets1 by others as lava-streams that were poured out at the surface4 ,if account be taken simply of their petrographical characters1 they find their nearest analogies among the intrusive quartz-porphyries of older geological periods4 ,the presence of flow-structure in them has been thought to indicate that they were superficial streams4 ,but this structure may be found in dykes and intrusive sheets as perfectly as in lava-flows1 so that it cannot by itself be taken as proof of a surface-discharge of lava4 ,it must be confessed that1 both in the main mass of quartz-porphyry and in the abundant fragments of it in the overlying conglomerates and breccias1 there is an absence of such scoriform portions as one would naturally look for in a superficial lava-stream2#ajc while1 on the other hand1 the rock generally presents the tolerably uniform flinty texture so familiar in intrusive sheets of similar material4#ajd ,my own impression is that these igneous masses were probably erupted to the surface as long banks which rose above the waves2 that they were thus exposed to prolonged subrial and marine denudation2 that by this means any upper more cellular portions of the lava which may have existed were broken up and pounded down into detritus1 and thus that what is now visible is a part of the eruptive rock which originally lay at some depth within its body4 ,this view is confirmed by a study of other lavas which are found on different platforms in the detrital deposits that overlie the ,llyn ,padarn quartz-porphyry4 ,footnote #ajc3 ,but the ,llyn ,padarn rock1 like many acid lavas1 may never have possessed a vesicular structure in any portion of its mass4 ,the sheets of felsite which occur among the overlying tuffs are not cellular1 but present the closest resemblance to the main mass below4 ,footnote #ajd3 ,mr4 ,blake brought forward the evidence of a section on the north or under side of the ,llyn ,padarn ridge to show that the rock has there been intruded into the ,cambrian strata 7,quart4 ,journ4 ,geol4 ,soc4 vol4 xliv4 7#ahhh71 p4 #bhc74 ,but the rock supposed by him to be altered slate has been shown to be intrusive greenstone 7,miss ,raisin1 op4 cit4 vol4 xlvii4 7#ahia71 p4 #ccf74 ,that the material of each of the two main ridges is the result of more than one eruption has been inferred from the supposed intercalation of bands of slate and of breccia in the rock4#aje ,considerable lithological differences may be detected in each mass1 but they are not greater than may be observed in single sills and bosses4 ,in some parts of the ,llyn ,padarn porphyry a distinct nodular structure appears which shades off into bands and lenticular streaks1 reminding one of the characters of some of the ,bala rhyolites4 ,other portions are markedly brecciated1 the separated fragments being surrounded in a matrix of the rock1 which shows flow-structure sweeping past them4 ,on ,moel ,gronw angular fragments of a dark pinkish tint are scattered through the general mass4 ,again1 some parts are crowded with quartz-grains1 while others are comparatively free of these1 and occasionally a spherulitic structure has been observed4#ajf ,footnote #aje3 ,see for example1 ,prof4 ,bonney1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xxxv4 7#ahgi71 p4 #cab2 ,mr4 ,blake1 op4 cit4 vol4 xliv4 7#ahhh71 pp4 #bgg1 #bhg4 ,but some at least of the supposed slates1 as stated in a previous footnote1 have been since shown to be dykes4 ,footnote #ajf3 ,mr4 ,blake1 ibid4 p4 #bgg4 ,the microscopic structure of this ancient eruptive rock has been studied by ,professor ,bonney1 who found that the general type was a compact dull grey felsite1 with porphyritic crystals of felspar and grains of quartz1 closely resembling some modern rhyolites4 ,though unable to detect any actual glass in the base1 he had no doubt that the rock was originally vitreous1 and he found abundant and fresh examples of the most perfect flow-structure4#ajg ,footnote #ajg3 ,op4 cit4 vol4 xxxv4 p4 #cab4 ,illustration3 ,fig4 #db4--;,basic dyke traversing quartz-porphyry and converted into a kind of slate by cleavage4 ,west side of ,llyn ,padarn4 p p1 quartz-porphyry2 d d1 dyke and connected veins4 ,reference may be made here to the remarkable influence of the intense cleavage of the district upon this rock4#ajh ,along its southern margin1 where it has been exposed to pressure from the south-east1 the quartz-porphyry has been so crushed that it passes here and there into a fine unctuous slate or almost a schist4 ,nowhere can this change be more clearly seen than on the slopes of ,mynydd y ,cilgwyn4 ,the cleavage planes strike about ,n4 #dj ,e41 with an inclination to dip towards the ,n4,w4 ,within a space of a few yards a series of specimens may be collected showing at one end an ordinary or only slightly-sheared quartz-porphyry with abundant quartz-blebs1 and at the other a fine greenish sericitic slate or phyllite1 wherein the quartz has been almost entirely crushed down4 ,lines of shearing may be detected across the breadth of the porphyry ridge1 each of them coinciding with the prevalent trend of the cleavage4 ,sometimes also certain basic dykes1 which traverse the porphyry in some numbers1 have undergone considerable deformation from the same cause4 ,their thinner portions are so well cleaved that they have been mistaken for included bands of green slate 7,fig4 #db74 ,but these cleaved branches may sometimes be traced into a thicker and more solid dyke1 whose uncrushed cores still preserve the original character of the rock and prove it to be eruptive4 ,footnote #ajh3 ,the secondary planes due to cleavage must not be confounded with the original flow-structure4 ,illustration3 ,fig4 #dc4--,section of well-cleaved tuff1 grit and breccia passing up into rudely-cleaved conglomerate and well-bedded cleaved fine conglomerate and grit4 ,east side of ,llyn ,padarn4 ,the rocks which succeed the porphyry in the ,valley of ,llanberis are of great interest1 for they contain abundant proof of contemporaneous volcanic activity1 and they show that1 so far from there being any marked hiatus here1 there is evidence of the persistence of eruptions even into the time of the ,llanberis ,slates4#aji ,considerable misapprehension has arisen from the attempt to make one of the conglomerates the base of the ,cambrian series1 and the real significance of the volcanic detrital strata in association with it was consequently missed4 ,the conglomerate does not lie on one definite horizon4 ,in truth1 there are several zones of conglomerate1 each with some difference of composition1 thickness or extent4#aaj ,these may be well studied both on the south and the north side of the porphyry ridge at the lower end of ,llyn ,padarn4 ,they are intercalated among fine tuffs1 grits1 volcanic breccias and purple slates1 sometimes full of fine ashy material4 ,on the south-east side of the ridge1 where the rocks have suffered intense cleavage1 they assume a fissile unctuous character1 and then resemble parts of the cleaved ,cambrian tuffs at ,st4 ,david's4 ,but on the north-west side1 where they have in large measure escaped the effects of the cleavage-movements1 their original structures are well preserved4 ,footnote #aji3 ,the sections in the ,vale of ,llanberis on either side of ,llyn ,padarn have been again and again described and fought over4 ,some of the papers are cited in the following pages1 but it would be impossible in this volume to find room for a full discussion of the differences of opinion4 ,what is stated in the text is the result of my own study of the rocks on the ground1 coupled with a careful consideration of the work of other observers4 ,footnote #aaj3 ,i can find no evidence of unconformability beneath any of the conglomerates4 ,the section described by ,professor ,green1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xli4 7#ahhe71 p4 #gd1 merely shows the difference between the effects of cleavage on the fine tuffs and the more massive resisting conglomerate which overlies them4 ,this section is represented in ,fig4 #dc4 ,at first sight the conglomerate appears to be lying on the vertical edges of an older group of slates1 but any one acquainted with cleavage can trace this structure from the tuffs into the conglomerate and resuming its course again in the finer sediments above4 ,the whole series of deposits in the section is continuous and conformable4 ,the section on the slate railway has deceived ,mr4 ,blake as well as ,professor ,green 7,quart4 ,journ4 ,geol4 ,soc4 vol4 xlix4 7#ahic71 p4 #dde74 ,the correct interpretation is given by ,professor ,bonney and ,miss ,raisin 7op4 cit4 vol4 l4 p4 #eib74 ,one of the first features of these detrital deposits to arrest attention is the amount and variety of the fragments of igneous rocks in them4 ,some of the conglomerates1 though enclosing pebbles of quartz1 quartzite1 granite and other rocks not found in situ in the immediate district1 are mainly composed of the debris of the quartz-porphyry of the ridge4 ,indeed1 this latter material appears to have contributed a large proportion of the detritus of which the general body of strata here is made up4 ,but there are to be noticed among the contents of the conglomerates and breccias pieces of many volcanic rocks not to be found on the porphyry ridge4 ,among these1 besides felsites showing sometimes beautiful flow-structure 7rhyolites7 and various quartz-porphyries1 there occur abundant fragments of less acid lavas 7andesites7 and pieces of older tuffs4 ,some of the fragmental rocks are green in colour1 probably from the abundance of fine basic volcanic dust in them4 ,certain bands are full of large angular pieces of shale1 similar in character to the ,cambrian slates1 and doubtless due to the disruption of pre-existing ,cambrian strata by volcanic explosions4 ,it is clear that from vents in this neighbourhood there continued to be an abundant discharge of dust and various andesitic and other lapilli1 which1 falling on the sea-floor1 mingled there with the ordinary mechanical sediment that was being deposited at the time4#aaa ,footnote #aaa3 ,on the composition of the conglomerates or breccias1 see ,professor ,bonney and ,miss ,raisin1 ,quart4 ,jour4 ,geol4 ,soc4 vol4 l4 7#ahid71 p4 #eih4 ,illustration3 ,fig4 #dd4--,section of ,clegyr on the north-east side of ,llyn ,padarn1 near the lower end4 ,but we have evidence that1 during the period when these showers of volcanic detritus were thrown out1 streams of lava1 though on a greatly diminished scale1 continued to be poured forth4 ,the hill of ,clegyr 7,fig4 #dd71 near the lower end of ,llyn ,padarn1 on the north-east side1 consists mainly of cleaved tuffs 7t7 and slates with conglomerates 7c71 overlying the quartz-porphyry 7p74 ,near the summit a band of felsite is intercalated in these rocks4 ,still more striking are the sections on the south-west side of the lake4#aab ,starting from the porphyry of the ridge1 we cross a zone of conglomerate and grit largely composed of the debris of the porphyry1 until we reach a band of felsite or quartz-porphyry1 which at its eastern end is about ten feet thick1 while it seems to increase in dimensions westwards4#aac ,in the centre the rock is dark purplish-red1 exceedingly compact or flinty1 sprinkled with a variable proportion of quartz-blebs and felspar crystals4 ,towards its southern or upper edge 7for the rocks1 though nearly vertical1 dip southwards7 it has been cleaved into a variety of purple slate1 and would there at once be classed among the ordinary slates of the neighbourhood4 ,but the fissile character is merely a marginal structure which the rock shares with the highly-cleaved tuffs that follow it4 ,traced westwards1 this bed is found to enclose a core of quartziferous porphyry1 which1 though it has escaped from the most complete results of crushing1 is nevertheless cleaved along its margin as well as partially in its interior4 ,it would not be possible to distinguish parts of this intercalated less crushed core from portions of the porphyry of the main ridge4 ,the difference of colour does not count for much1 for even in this core the purple tint gives place to greenish grey1 and what in the centre at the east end is a solid dark purplish-red felsite passes westward into a greenish slate1 like that already noticed on ,mynydd y ,cilgwyn4 ,footnote #aab3 ,for various readings of these sections1 compare ,mr4 ,blake 7,quart4 ,jour4 ,geol4 ,soc4 vol4 xlix4 7#ahic71 p4 #dej7 with ,professor ,bonney and ,miss ,raisin 7op4 cit4 vol4 l4 7#ahid71 p4 #eha74 ,footnote #aac3 ,see ,professor ,bonney and ,miss ,raisin1 op4 cit4 p4 #eic et seq4 ,the microscopical examination of this rock shows it to be a true felsite of the rhyolitic type1 which in the central uncleaved part exhibits a wavy flow-structure like that found in the quartz-porphyry of the ridge4 ,so intense has been the cleavage in its upper part that the original structure of the rock is there effaced4 ,the immediately overlying tuffs1 which are likewise so thoroughly cleaved that it is not easy to draw a sharp and continuous line between them and the intercalated lava1 precisely resemble those found below the conglomerate on the opposite side of the lake4 ,they include bands of coarse volcanic breccia as well as fine compact material1 showing the varying intensity of the volcanic discharges4 ,their included stones consist of various felsites1 andesites and slates4#aad ,footnote #aad3 ,i see no reason to doubt that the less acid igneous fragments were ejected during the closing phases of volcanic action1 even though no such rocks have been found at the surface in situ4 ,we must remember how frequently mixtures of acid and basic materials are to be found in the same continuous series of volcanic ejections and even in the same vent1 of which illustration will be given in subsequent pages4 ,nor should we forget what a mere fragment of a volcanic group is exposed at the surface in the ,llanberis district4 ,see ,professor ,bonney and ,miss ,raisin1 op4 cit4 p4 #eif1 footnote4 ,the thin sheet of interstratified quartz-porphyry here described is not the only one to be found in the section4 ,others thinner and more intensely cleaved lie among the tuffs higher up4 ,they have been sheared into mere pale unctous slates1 but the remains of their quartz-blebs may still be detected in them4 ,the tuffs1 with their interstratified bands of porphyry1 become more and more mingled with ordinary argillaceous and sandy sediment as they are followed in upward succession4 ,among them occur bands of grit and fine conglomerate containing pebbles of porphyry and pieces of slate4 ,some of these grits are mainly composed of white felspar1 felsite and clear grains of quartz1 evidently derived from the disintegration of a rock like the porphyry of the main ridge4 ,as the ordinary sediment of the ,llanberis group sets in1 the tuffs are restricted to thinner and more widely-separated bands4 ,some thin layers of felspathic breccia1 seen among the slates close to the ,glyn ,peris ,hotel1 probably mark the last discharges of the slowly-expiring vents of this region4 ,here1 as at ,st4 ,david's1 from the most ancient of our volcanic records1 striking evidence is furnished of the gradual extinction of volcanic action4 ,through many hundreds of feet of strata which now supervene1 representing the closing ages of the ,cambrian and the earlier ages of the ,silurian period1 no trace of volcanic material has been found in this district until we reach the ,bala lavas and agglomerates of ,snowdon and the ,pass of ,llanberis4 ,in the neighbourhood of ,bangor another area of similar rocks wraps round the northern end of the western porphyry ridge4 ,the ,geological ,survey map1 in conformity with the ideas that governed its representation of the older rocks of ,anglesey and ,caernarvon1 colours these as altered ,cambrian4 ,that this error should have been made1 or1 when made1 should not have been speedily corrected1 is all the more surprising when we consider the thorough mastery which the surveyors had acquired of the aspects and the interpretation of ancient volcanic rocks in ,wales1 and when1 moreover1 we remember that as far back as #ahdc1 long before the ,survey of ,caernarvonshire was published1 ,sedgwick had pointed out the true volcanic nature of the rocks4 ,that great pioneer recognized the presence of trappean conglomerates and trappean shales 7,schaalstein7 among these deposits at ,bangor2 but he could not separate them from the ,cambrian series of the rest of ,wales4#aae ,and in his section he represents them as undulating towards the east and passing under the great mass of the ,caernarvonshire slates and porphyries4 ,footnote #aae3 ,proc4 ,geol4 ,soc4 vol4 iv4 p4 #bab2 ,quart4 ,journ4 ,geol4 ,soc4 vol4 iii4 7#ahdg71 p4 #acf4 ,this interpretation1 which ,i believe to be essentially accurate1 was modified by ,professor ,hughes1 who1 fixing on a conglomerate as the base of the ,cambrian system1 regarded all the rocks below it1 or what he termed his ,bangor group1 as pre-,cambrian4#aaf ,he has been followed in this view by subsequent writers2#aag but ,mr4 ,blake has argued that here1 as in the ,llanberis district1 there is no evidence to separate the volcanic detrital deposits above the porphyry from the ,cambrian system4#aah ,footnote #aaf3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xxxiv4 7#ahgh71 p4 #acg4 ,footnote #aag3 ,prof4 ,bonney1 op4 cit4 vol4 xxxv4 7#ahgi71 p4 #caf2 ,dr4 ,hicks1 ibid4 p4 #bif4 ,footnote #aah3 ,op4 cit4 vol4 xliv4 7#ahhh71 p4 #bgh4 ,a little southward from ,bangor the quartz-porphyry is overlain by a most interesting group of fragmental rocks1 the ,bangor group of ,professor ,hughes4 ,largely of volcanic origin1 they must be some hundreds of feet thick1 and pass under the dark shales and grits of the ,lower ,silurian 7,arenig7 series4 ,some of the most persistent bands among them are conglomerates1 which differ from each other in composition1 but most of which consist largely of fragments of various igneous rocks4 ,some of the coarser masses might be termed agglomerates1 for they show little or no trace of bedding1 and are essentially made up of blocks of volcanic material4 ,there are abundant beds of grit1 sometimes pebbly or finely conglomeratic1 alternating with tuffs and with bands of more ordinary sediment4 ,courses of purple shale and sandstone1 green shale and dark grey sandy shale occasionally occur to mark pauses in the volcanic explosions4 ,perhaps the most striking feature in the pyroclastic materials is the great abundance of very fine compact pale tuffs 7hlleflintas of some writers71 sometimes thinly laminated1 sometimes occurring in ribbon-like bands1 each of which presents internally a close-grained1 almost felsitic or flinty texture4#aai ,footnote #aai3 ,the occurrence of flinty or cherty deposits1 in association with volcanic rocks of ,lower ,silurian age1 is well established in ,britain1 and will be more particularly referred to in the sequel4 ,a cursory examination of the contents of the conglomerates1 breccias and grits shows them to consist largely of different felsites1 with fragments of more basic lavas4 ,some of these might obviously have been derived from the rock of the porphyry ridge1 but1 as at ,llyn ,padarn1 there is a far greater variety of material than can be found in that ridge4 ,some of the fragments show perfect flow-structure4 ,professor ,bonney has described the microscopic characters of some of these fragments1 and has especially remarked upon their glassy character4 ,among the slides prepared from specimens collected by myself1 besides the abundant fragments of felsite 7rhyolite71 there are also numerous pieces of different andesitic lavas and fine tuffs1 as well as grains of quartz and felspar1 and sometimes a good deal of granular iron-ore4 ,that a large proportion of the material of the so-called ,bangor beds was directly derived from volcanic explosions can hardly be doubted4 ,there appears to have been a prolonged succession of eruptions1 varying in intensity1 and somewhat also in the nature as well as in the relative fineness of the material discharged4 ,on the one hand1 coarse massive agglomerates were probably accumulated not far from the active vents1 as the result of more violent or transient explosions2 on the other hand1 exceedingly fine and well-stratified tuffs1 which attain a great thickness1 serve to indicate a phase of eruptivity marked by the long-continued discharge of fine volcanic dust4 ,ordinary sediment was doubtless drifted over the sea-bottom in this district during the volcanic episode1 but the comparative infrequence of distinct interstratifications of shale or sandstone may be taken to imply that as a rule the pauses between the eruptions were not long enough to allow any considerable accumulation of sand or mud to take place4 ,no satisfactory proof has yet been obtained of any interstratified lavas among the tuffs of the ,bangor district4 ,some rocks1 indeed1 can be seen on the road between the ,george ,hotel and ,hendrewen1 which1 if there were better exposures1 might possibly furnish the required proof2 but at present little can be made of them1 for their relations to the surrounding rocks are everywhere concealed4 ,from what ,i have now adduced1 it is obvious that while both felsitic and andesitic lavas existed within the volcanic foci1 and were ejected in fragments to form the tuffs and breccias1 the lavas poured out at the surface during the ,cambrian period in ,caernarvonshire were mainly1 if not entirely1 felsites 7rhyolites7 in which the chief porphyritic constituent was quartz4 ,these lavas thus stand entirely by themselves in the volcanic history of ,wales4 ,though felsites of various types were afterwards poured out1 nothing of the same quartziferous kind1 so far as we yet know1 ever again appeared4 ,further south1 in ,merionethshire1 as will be shown in ,chapter xii41 the ,cambrian volcanic eruptions appear to have been on the whole less acid1 and to have begun with the outpouring of andesitic lavas4 ,i have now to consider the relation of the volcanic group of ,bangor to the strata which overlie it4 ,the geological horizon of these strata is not1 perhaps1 very definitely fixed4 ,it may be ,arenig1 possibly even older4 ,but for my present purpose it will be sufficient to consider the strata in question as lying at the bottom of the ,lower ,silurian series4 ,professors ,hughes and ,bonney have taken as their base a marked but impersistent band of conglomerate4 ,mr4 ,blake1 however1 has more recently shown that1 as this band is succeeded by tuffs like those below it1 it cannot be claimed as marking the upper limit of the volcanic group4 ,he therefore classes it in that group and traces what he thinks is an overlap or unconformability at the bottom of the ,lower ,silurian strata to the east4 ,mr4 ,b4 ,n4 ,peach1 who accompanied me in an examination of this ground1 agreed with me in confirming ,mr4 ,blake's observation as to the position of the conglomerate1 which is undoubtedly overlain by the same flinty felsitic tuffs as are found below it4 ,but we were unable to trace any unconformability4 ,according to the numerous observations which we made1 there does not seem to be any discordance in strike or dip between the flinty tuffs and the overlying shales and grits4 ,the two groups of rock appeared to us to be conformable and to pass into each other1 as at ,llyn ,padarn4#abj ,footnote #abj3 ,see ,mr4 ,blake on this point1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xlviii4 7#ahib71 p4 #beb1 note4 ,i retain the opinion expressed above4 ,an unconformable junction here would1 in some respects1 have been welcome1 for it would at once have accounted for the superposition of ,lower ,silurian strata directly upon the ,cambrian volcanic series1 and for the disappearance of the ,llanberis slates and grits which form so conspicuous a feature above the tuffs and conglomerates at ,llyn ,padarn4 ,in the absence of such a structure we must accept the order of succession as apparently unbroken1 and rely on some such explanation as was proposed by ,sir ,andrew ,ramsay to account for the overlap of the ,arenig rocks on everything older than themselves as they are traced northwards4#aba ,but this explanation will not entirely remove the difficulties of the case4 ,the inosculation of the volcanic group of ,bangor with the base of the ,lower ,silurian series cannot be accounted for by any such overlap2 it seems only explicable on the supposition that the volcanic activity1 which ceased in the ,llyn ,padarn district about the time that the ,llanberis ,slates were deposited1 was continued in the ,bangor area until ,arenig time1 or was then renewed4 ,the thick volcanic group of ,bangor would thus be the stratigraphical equivalent not only of the thin volcanic group of ,llyn ,padarn1 but of the overlying mass of strata up to the ,arenig rocks4 ,in confirmation of this view1 ,i shall show in a later chapter that volcanic action seems to have been prolonged in ,anglesey to a still later geological period1 that it appeared during the deposition of the ,arenig strata1 and that it attained a great development throughout the time of the ,bala group4 ,that a series of volcanic rocks1 with associated cherty strata1 may be the stratigraphical equivalent of a great thickness of ordinary sediments in other districts will be dwelt upon in the description of the ,lower ,silurian volcanic geology of the ,southern ,uplands of ,scotland4#abb ,footnote #aba3 ,mem4 ,geol4 ,surv4 vol4 iii4 #bnd edit4 p4 #beb4 ,footnote #abb3 ,a group of cherts and mudstones not more than #fj or #gj feet thick appear in that region to be stratigraphically equivalent to the great depth of sedimentary material which elsewhere constitutes the ,upper ,arenig and ,lower and ,middle ,llandeilo formations4 ,see ,annual ,report of the ,geological ,survey for #ahie1 p4 #bg of reprint4 ,in the areas of ,north ,wales which have now been described1 volcanic action appears to have begun and ended within the limits of the ,cambrian period4 ,southwards1 in the district of ,dolgelly1 another distinct and1 in some respects1 very different development of ,cambrian volcanic activity may be recognized4 ,in that district there is evidence that the volcanoes which distinguished the earlier part of the ,silurian period had already begun their eruptions during ,cambrian time4 ,as their records1 however1 are intimately linked with those of ,silurian age1 an account of them is deferred to the next chapter4 ,,the ,,malvern ,,hills ,although the chief surviving records of ,cambrian volcanic action in ,britain are found in ,wales1 there is no evidence that the volcanoes of the period lay chiefly in that region4 ,it is certainly a suggestive fact that1 in the few districts where ,cambrian strata appear from under younger formations in ,england1 they are generally accompanied with igneous rocks1 though the age of the latter may be older or later than the ,cambrian period4 ,if the oldest ,palozoic rocks could be uncovered over the ,english counties1 a more abundant development of volcanic materials might be laid bare than is now to be seen in ,wales4 ,taking1 however1 the extremely limited exposures of ,cambrian strata1 we find two tracts that specially deserve attention4 ,reference has already been made to the ancient eruptive rocks of the ,malvern ,hills1 the antiquity of which is proved by the position of the ,cambrian fossiliferous strata that overlie them4 ,but these strata themselves include certain igneous rocks which point to a recrudescence of eruptive energy in a far later geological period4 ,nearly half a century has passed away since ,john ,phillips mentioned the intercalation of igneous rocks in the series of strata which is now classed as ,upper ,cambrian in the ,malvern ,hills4 ,since that date hardly anything has been added to the information which he collected4 ,the existence of a group of rocks of such high antiquity1 asserted to be of truly volcanic origin1 and the precise horizon of which could be fixed by the stratigraphical aid of organic remains1 seems to have almost dropped out of sight4 ,phillips noted the occurrence of what he regarded as truly volcanic materials in the ,hollybush ,sandstone and the overlying dark 7,lingula7 shales1 and he clearly recognized that a wide difference of age separated them from the far more ancient igneous rocks of the central core of the chain4 ,the ,hollybush ,sandstones were observed by him to have often a trappean aspect and to be traversed with felspathic dykes4 ,he found the overlying black shales to include layers of trappean ashy sandstone4 ,but it was at the top of these shales that he obtained what he regarded as the most conspicuous evidence of contemporaneous volcanic action4 ,he there encountered a zone of interposed trap rocks varying up to #ej feet in thickness1 consisting of porphyritic and greenstone masses1 which1 erupted from below1 have flowed in limited streams over the surface of the black shales4 ,he recognized amygdaloidal and prismatic structures among them4#abc ,the position of these eruptive rocks is shown in ,fig4 #de4 ,footnote #abc3 ,mem4 ,geol4 ,survey1 vol4 ii4 part i4 pp4 #eb1 #ee2 also ,horizontal ,sections of the ,geol4 ,survey1 ,sheet #ac1 ,no4 #h1 and ,sheet #ae4 ,reference to the igneous rocks of this area will be found in the remarkable essay by ,de la ,beche in vol4 i4 of the ,mem4 ,geol4 ,surv4 pp4 #cd1 #ch4 ,illustration3 ,fig4 #de4--,section across the ,cambrian formations of the ,malvern ,hills1 showing the position of the intercalated igneous rocks 7p p74 ,after ,phillips4 ,these rocks were afterwards observed and described by ,dr4 ,holl1 who found what he considered to be four true lava sheets interstratified in the ,hollybush ,sandstones4 ,he noted the intercalation of numerous beds of volcanic ash1 grit and lava in the black shales4#abd ,footnote #abd3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xxi4 7#ahfe71 pp4 #hg-ia4 ,so far as ,i am aware1 no more recent account of these rocks has been published4 ,their true stratigraphical and petrographical relations require to be more precisely determined4 ,if they are really contemporaneous lavas1 they point to volcanic eruptions at the time when the middle division of the ,cambrian system was being deposited4 ,if1 on the other hand1 they should prove to be intrusive1 they would indicate probable volcanic activity in this part of ,england at some time later than the middle of the ,cambrian period4 ,,warwickshire ,some fifty miles to the north-east of the ,malvern ,hills1 in the heart of the rich ,midlands1 and among the coal-fields and the ,new ,red ,sandstone to which these ,midlands owe so much of their manufacturing industry and their agricultural fertility1 another little tract of ,cambrian rocks rises to the surface on the east side of the ,warwickshire coal-field between ,nuneaton and ,atherstone4 ,so unobtrusively do these ancient strata take their place among their younger peers1 that their venerable antiquity was for a long time undetected4#abe ,they were actually regarded as parts of the ,carboniferous series1 which at first sight they seem to underlie conformably4 ,it was not until #ahhb that the mistake was corrected by ,professor ,lapworth1 who proved the rocks to be ,cambrian by finding undoubted ,upper ,cambrian fossils in them4#abf ,subsequent investigation enabled him to work out the detailed sequence of these strata4 ,he found that the supposed ,millstone ,grit is a thick-bedded quartzite perhaps #ajjj feet in thickness1 and resembling the well-known quartzites of the ,lickey and ,caer ,caradoc4 ,the ,coal-shales proved to be a series 7possibly #bjjj feet thick7 of purple1 green1 grey and black shales1 which from their fossils could be paralleled with the dark shales of the ,upper ,cambrian series of the ,malvern ,hills4#abg ,these shales are immediately overlain by the ,coal-measures4 ,footnote #abe3 ,their antiquity was recognized by ,yates as far back as #ahbe 7,trans4 ,geol4 ,soc4 #bnd series1 vol4 ii4 p4 #bfa74 ,they had been confounded with ,millstone ,grit and ,coal-shale by ,conybeare and ,phillips1 and this mistake was adopted on the maps and memoirs of the ,geological ,survey4 ,footnote #abf3 ,geol4 ,mag4 7#ahhb71 p4 #efc4 ,footnote #abg3 ,op4 cit4 7#ahhf71 p4 #cai4 ,for our present inquiry1 however1 the chief feature of interest in these discoveries is the recognition of a group of volcanic rocks underneath the quartzite4 ,this group was named the ,caldecote ,volcanic ,rocks by ,professor ,lapworth1 who first recognized its nature and relations4 ,its rocks have been studied by ,mr4 ,t4 ,h4 ,waller#abh and ,mr4 ,f4 ,rutley1#abi and have been traced upon a revised edition of the ,geological ,survey map by ,mr4 ,a4 ,strahan4#acj ,they consist of a thin series of well-stratified tuffs apparently derived from andesitic lavas4 ,their base is not seen owing to the fault which brings down the ,new ,red ,sandstone against them4 ,they are surmounted by the quartzite1 which at its base is conglomeratic and contains blocks of the tuff4 ,a mass of quartz-felsite is possibly intrusive in these strata1 and is associated with a diabase-porphyrite4 ,in these rocks1 but still more in the shales which overlie them1 numerous sills of diorite and diabase occur4 ,the total thickness of rocks from the lowest visible part of the ,caldecote volcanic series to the base of the ,coal-measures is probably between #bjjj and #cjjj feet4 ,footnote #abh3 ,op4 cit4 p4 #cbc4 ,footnote #abi3 ,op4 cit4 p4 #eeg4 ,footnote #acj3 ,geol4 ,mag4 7#ahhf71 p4 #edj4 ,in this paper full references will be found to the previous papers on the geology of the district4 ,jukes had recognized that the rocks below the coal-bearing strata were older than the ,upper ,silurian1 perhaps older than any ,silurian1 ,mem4 ,geol4 ,survey1 ,south ,staffordshire ,coal-field 7#ahei71 p4 #acd4 ,there can be no doubt as to the geological position of the dark fossiliferous shales and their underlying quartzite4 ,the fact that the basement conglomerate of the quartzite is partly made up of the underlying volcanic series may possibly mark a wide difference of age between them1 and may indicate that the eruption of the tuffs took place long before ,upper ,cambrian time4 ,on the other hand1 the tuffs have the same strike and angle of dip with the quartzite1 and as ,professor ,lapworth admits1 the break between them may not be of great moment4 ,it is at least certain that the intrusive sills of the district are later than the tuffs1 and later also than the sedimentary ,cambrian groups4 ,,book ,,iv ,,the ,,silurian ,,volcanoes ,,chapter ,,xii ,,characters ,,of ,,the ,,silurian ,,system ,,in ,,britain4 ,,the ,,arenig ,,volcanoes ,the ,land and ,sea of ,silurian time--,classification of the ,silurian ,system--,general ,petrography of the ,silurian ,volcanic ,rocks--,i4 ,the ,eruptions of ,arenig ,age4 ,the next great geological period1 to which ,murchison gave the name of ,silurian1 has in ,britain a fuller record than the period which preceded it4 ,the rocks that tell its history are more varied in origin and structure4 ,they are displayed at the surface over a far wider area1 and1 what gives them special interest and value1 they contain a much larger assemblage of organic remains4 ,for the immediate subject of the present volume1 they have likewise the additional attraction that they include a singularly complete and widespread volcanic chronicle4 ,they display in many admirable sections the piled-up lavas and tuffs of scores of volcanoes1 scattered all over the three kingdoms1 from the headlands of ,kerry to the hills of ,lammermuir4 ,they thus enable us to form a truer conception of what the early ,palozoic volcanoes were than is possible from the more limited evidence furnished by the ,cambrian system4 ,at the beginning of the ,silurian period most of the area of the ,british ,isles lay under the sea4 ,but if we may judge from the sedimentary strata which represent the floor of that sea1 the water1 during most of the time1 was of no great depth4 ,there is evidence1 indeed1 that during a part of the period the sea was deep enough to admit of the accumulation of wide tracts of radiolarian ooze1 with but little admixture of mechanical sediment4 ,but1 for the most part1 sand and mud were drifted from neighbouring lands1 the more important of which probably lay to the north1 over what are now the ,highlands of ,scotland and the north and north-west districts of ,ireland4 ,no general change in topography or in physical conditions took place at the close of ,cambrian time4 ,the older era glided insensibly into the newer1 unmarked by any such catastrophe as was once supposed to have intervened at the end of each great geological period4 ,there are traces1 indeed1 of slight local disturbances1 but these only make the general gradual transition more marked4 ,of the vegetation which covered the ,silurian lands hardly anything is known4 ,traces of lycopods and ferns have been detected1 and these probably formed the chief constituents in what must have been rather a sombre and monotonous flora4 ,the character of the terrestrial fauna is still hidden from us1 though we do know that insects winged their way through those green flowerless forests1 and that scorpions likewise harboured there4 ,that these primeval arachnoids were air-breathers is shown by their breathing stigmata2 and from the fact that they possessed a well-developed poison-gland and sting1 we may believe that there were already living at the same time other land-animals1 possibly of higher grade1 on which they preyed4 ,but of these ancestral types no actual relics have yet been discovered4 ,it is the life of the sea-floor that has mainly been chronicled among the sedimentary formations4 ,taking the ,silurian system as a whole1 we find it to be the repository of a remarkably varied assemblage of organisms4 ,among the simpler forms1 ,radiolaria deserve especial notice1 from their wide range in space and time1 and the comparative indestructibility of the highly-siliceous1 fine-grained1 flinty strata1 which have preserved them in abundance and have a wide distribution over the ,british ,isles4 ,the ,graptolites1 so specially characteristic of the system1 range entirely through it1 and by their successive differences of specific and generic forms1 furnish a basis for the division of the whole series of rocks into more or less definite stratigraphical zones4 ,hardly less important for purposes of correlation are the ,trilobites which in the ,silurian period reached the culmination of their development in regard to number of species and genera4 ,these interesting extinct types of crustacean life must have swarmed over some parts of the sea-bottom1 for their remains abound in its hardened silts4 ,the ,brachiopods are likewise numerously represented among ,silurian strata2 and since the vertical range of the species is generally not great1 they serve as useful guides in fixing stratigraphical horizons4 ,lamellibranchs1 ,gasteropods1 and ,cephalopods become increasingly numerous and varied as we follow the succession of strata from the base to the summit of the ,silurian system4 ,that there were fishes also in the ,silurian seas is proved by the occurrence of their remains1 more particularly in the higher formations4 ,from the organic remains which have been preserved in the rocks1 it may be inferred that the animal life of the globe became more varied in ,silurian time2 higher types made their appearance1 until vertebrates took the place of pre-eminence which they have ever since maintained4 ,the volcanic activity that had marked the passage of ,cambrian time in ,britain was prolonged into the ,silurian period4 ,in ,north ,wales1 indeed1 it is clear that though the eruptions began in the earlier era of geological history they continued to be comparatively feeble until they broke out into full activity in the succeeding epoch4 ,there is no hiatus or essential difference between the volcanic phenomena1 any more than there is between the sedimentary deposits1 of the two periods4 ,although it may be only owing to the fact that the ,silurian formations come much more extensively to the surface of the land than the underlying ,cambrian are permitted to do1 yet it is at least noteworthy that the relics of ,silurian volcanoes are spread over a far wider area of the ,british ,isles than those of the earlier period4 ,throughout a large part of ,wales they form some of the most prominent mountains1 such as ,cader ,idris1 the ,arans1 ,arenig ,fawr1 ,moel ,wyn1 ,moel ,siabod1 and ,snowdon4 ,they rise into the picturesque hill-groups of the ,lake ,district1 they appear at many detached places throughout the south of ,scotland1 and form conspicuous eminences in ,carrick4 ,in ,ireland they abound all down the east side of the island1 and even reappear on the far western headlands of the ,dingle coast-line4 ,to the same pioneers1 by whom the foundations of our knowledge of the ,cambrian volcanoes were laid1 we are indebted for the first broad outlines of the history of volcanic action in ,silurian time4 ,the writings of ,sedgwick and ,murchison1 but still more the detailed mapping of ,de la ,beche1 ,ramsay1 ,selwyn1 ,jukes1 and the other members of the ,geological ,survey1 have given to the ,silurian volcanic rocks of ,wales a classic interest in the history of geology4 ,to these labours further reference will be made in subsequent pages4#aca ,footnote #aca3 ,for references to the older literature see ante1 p4 #adb4 ,the amount of material being so ample for the compilation of a record of volcanic action in ,britain during ,silurian time1 it will be desirable to arrange it in stratigraphical order4 ,for this purpose invaluable assistance is afforded by the evidence of organic remains1 whereby the whole ,silurian system has been subdivided into sections1 each characterized throughout the whole region by certain distinctive fossils4 ,the following tabular statement exhibits the chief stratigraphical divisions of the system1 and the short black lines in it mark the positions of separate volcanic platforms in each of the three kingdoms3-- ---------------------------------------- ---------------------------- ,england ,wales ,scotland,ireland ---------------------------------------- ---------------------------- ,ludlow ,group 444 444 444 444 ,upper ,silurian ,wenlock ,group 444 444 444 --- ,llandovery ,group 8 444 444 444 ,bala and ,caradoc ,group --- --- --- --- ,lower ,silurian ,llandeilo ,group --- --- --- --- ,arenig ,group --- --- --- --- ---------------------------------------- ---------------------------- ,it will be most convenient1 following the combined stratigraphical and geographical arrangement of this table1 to discuss first the volcanic history of the ,lower ,silurian period as recorded in each of the three kingdoms1 and then that of the ,upper ,silurian4 ,i4 ,,the ,,eruptions ,,of ,,arenig ,,age i4 ,,merionethshire ,placing the upper limit of the ,cambrian system at the top of the ,tremadoc group1 we pass into the records of another series of volcanic eruptions which marked various epochs during the ,silurian period over the area of the ,british ,isles4 ,the earliest of these volcanic episodes has left its memorials in some of the most impressive scenery of ,north ,wales4 ,to the picturesque forms sculptured out of the lavas and ashes of that early time1 we owe the noble range of cliffs and peaks that sweeps in a vast semicircle through the heights of ,cader ,idris1 ,aran ,mawddwy1 ,arenig1 and ,moel ,wyn4 ,to the east other volcanic masses1 perhaps in part coeval with these1 rise from amidst younger formations in the groups of the ,berwyn and ,breidden ,hills1 and the long ridges of the ,shelve and ,corndon country4 ,far to the south1 traces of ,silurian volcanoes are met with near ,builth1 while still more remote are the sheets of lava and tuff interstratified among the ,lower ,silurian rocks of ,pembrokeshire1 and those which extend into ,skomer ,island4 ,the most important of these districts is unquestionably that of ,merionethshire4 ,in this area1 as was pointed out in the last chapter1 the eruptions certainly began before the close of the ,cambrian period1 for traces of them occur in the ,tremadoc and ,lingula ,flag groups4 ,but below these strata1 in the vast pile of grits and conglomerates of the ,harlech anticline1 there does not appear to be any trace of contemporaneous volcanic action4 ,at the time when the ,geological ,survey maps of this region were prepared1 the ,cambrian and ,lower ,silurian rocks had not been subdivided into the various palontological groups which are now recognized4 ,nor had any attempt been made to separate the various kinds of contemporaneous igneous masses from each other and from the tuffs in so extensive and complicated a mountain-region4 ,the task undertaken by the ,survey was beset with difficulties1 some of which geologists1 furnished with the advantages of a later time1 can hardly perhaps realize4 ,the imperfections of the mapping were long ago recognized by the original surveyors1 and various corrections of them were made from time to time4 ,first of all1 the volcanic rocks1 which originally had been all massed under one colour1 were traced out separately on the ground1 according to their structure and mode of origin1 and were distinguished from each other on the maps4#acb ,subsequently divisional lines were followed out between some of the larger stratigraphical groups1 the maps and sections were still further modified1 and the results were summed up in the volume on the ,geology of ,north ,wales4#acc ,but short of actually resurveying the whole of that rugged tract1 it was impossible to bring the maps abreast of the onward march of science4 ,they consequently remain1 as a whole1 very much as they were some thirty or forty years ago4 ,footnote #acb3 ,mem4 ,geol4 ,surv4 vol4 iii4 #bnd edit4 p4 #ie1 note4 ,footnote #acc3 ,some of the modifications introduced are1 ,i think1 to be regretted1 for the earlier editions of the maps and sections are in certain respects more accurate than the later4 ,on this point ,i concur with the criticism made by ,messrs4 ,cole and ,jennings1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xlv4 7#ahhi71 p4 #dcf4 ,sir ,andrew ,ramsay1 in his great ,monograph on the geology of ,north ,wales1 has described the ,merionethshire volcanic district in considerable detail4 ,he seems finally to have come to the conclusion that the eruptions of that area were included within the ,arenig period4#acd ,he shows1 indeed1 that on ,rhobell ,fawr the ejected materials lie directly on disturbed ,lingula ,flags without the intervention of the ,tremadoc group1 which is nevertheless present in full development in the near neighbourhood4#ace ,and in trying to account for this remarkable fact he evidently had in his mind the possibility that volcanic eruptions had taken place long before as well as after the beginning of the deposition of the ,arenig grit and slates4#acf ,he seems eventually1 however1 to have looked on the ,rhobell ,fawr sections as exceptional and possibly to be accounted for by some local disturbance and intrusion of eruptive rock4#acg ,he clearly recognized that there were two great epochs of volcanic activity during the ,silurian period in ,wales1 one belonging to the time of the ,arenig1 the other to that of the ,bala rocks1 and he pointed out that the records of these two periods are separated by a thick accumulation of sedimentary strata which1 being free from interstratifications of contemporaneous igneous rocks1 may be taken to indicate a long interval of quiescence among the subterranean forces4#ach ,footnote #acd3 ,mem4 ,geol4 ,survey1 vol4 iii4 #bnd ed41 p4 #if4 ,footnote #ace3 ,the ashes and agglomerates of ,rhobell ,fawr can be seen in various places to rest on the highest members of the ,lingula ,flags4 ,see ,messrs4 ,cole and ,holland1 ,geol4 ,mag4 7#ahij71 p4 #dea4 ,footnote #acf3 ,op4 cit4 p4 #gb4 ,footnote #acg3 ,he was disposed to regard ,rhobell ,fawr as one of the great centres of eruption of the district4 ,see ,memoir of ,a4 ,c4 ,ramsay1 p4 #ha1 and ,geology of ,north ,wales1 #bnd edit4 p4 #ih4 ,footnote #ach3 ,op4 cit4 pp4 #ga1 #if1 #aje4 ,the lower limit of the ,arenig rocks has been fixed at a band or bands of grit or conglomerate 7,garth grit7 which can be followed with some slight interruptions all round the great dome of ,cambrian strata from ,llanegrin on the south to the shore at ,criccieth on the north4 ,the volcanic group doubtless lies1 generally speaking1 above that basement platform4 ,but1 besides the sections at ,rhobell ,fawr just referred to1 where the volcanic materials lie on the ,lingula ,flags1 the same relation may1 ,i think1 be observed on the north flank of ,cader ,idris4 ,messrs4 ,cole1 ,jennings1 and ,holland have come to the conclusion that the eruptions began at a rather earlier date than that assigned to them in the ,survey ,memoirs1 and my own examination of the ground led me to accept their conclusion4#aci ,i inferred that the earliest discharges in the southern part of the region took place in ,cambrian time1 at or possibly before the close of the deposition of the ,lingula ,flags1 and that intermittent outbursts occurred at many intervals during the time when the ,tremadoc and ,arenig rocks were deposited4 ,footnote #aci3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xlv4 7#ahhi71 p4 #dcf2 ,geol4 ,mag4 7#ahij71 p4 #ddg4 ,pres4 ,address ,geol4 ,soc4 #ahij1 p4 #ajg4 ,important confirmation of this view of the ,cambrian age of the earlier volcanic eruptions of the ,cader ,idris region has recently been obtained by ,messrs4 ,p4 ,lake and ,s4 ,h4 ,reynolds who1 in the ground intervening between the lower slopes of ,cader ,idris and ,dolgelly1 have ascertained the existence of a marked band of andesitic lava traceable for some distance in the upper ,lingula ,flags4 ,they have also observed a higher volcanic group reposing upon the ,tremadoc strata at the top of the ,cambrian system1 and consisting of rhyolite with rhyolite-tuffs4#adj ,footnote #adj3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 lii4 7#ahif71 p4 #eaa4 ,some of the most stupendous memorials of the earlier eruptions are to be seen in the huge mountain mass of ,rhobell ,fawr 7#bdjc feet74 ,they consist mainly of agglomerates and tuffs1 one of the most remarkable varieties of which is distinguished by its abundant scattered crystals of hornblende and of augite4 ,the fragments of rock included in these rocks are scori and lumps of various lavas1 especially basaltic and trachytic andesites4 ,the tuffs become finer towards the top of the mountain where they are interleaved with grits4 ,among the pyroclastic materials occasional lavas 7basaltic andesites7 occur which may be contemporaneous streams1 but most of the lava-form rocks appear to be intrusive4 ,they include dolerites 7augite-aphanites71 basaltic andesites1 and trachytic andesites4#ada ,footnote #ada3 ,prof4 ,cole1 ,geol4 ,mag4 7#ahic71 p4 #ccg4 ,illustration3 ,fig4 #df4--,section across ,rhobell ,fawr4#adb ,l ,l1 ,lingula flags2 t1 tuffs and ashy slates2 s1 slates and grits2 ,f ,f1 ,arenig volcanic series2 ,d1 dolerite4 ,footnote #adb3 ,after ,messrs4 ,cole and ,holland1 ,geol4 ,mag4 7#ahij71 p4 #dej4 ,the materials from the ,rhobell ,fawr volcano are clearly distinguishable from those of the ,arenig volcanoes in the neighbourhood4 ,the latter begin to make their appearance among the black slates at the base of the northern declivities of ,cader ,idris1 and extend upward through that mountain into the country beyond4 ,an upper limit to this volcanic group is not easily traceable2 partly1 no doubt1 from the gradual cessation of the eruptions and partly from the want of any marked and persistent stratigraphical horizon near the top of the group4 ,sir ,andrew ,ramsay1 indeed1 refers to the well-known band of pisolitic iron-ore as lying at or near to the top of the ,arenig rocks4#adc ,there can be no doubt1 however1 that the volcanic intercalations continue far above that horizon in the southern part of the district4 ,footnote #adc3 ,mem4 ,geol4 ,survey1 vol4 iii4 #bnd edit4 pp4 #bdi1 #bej4 ,in spite of the extent to which the volcanic masses of the ,arenig period have been covered by later ,palozoic formations1 it is still possible to fix approximately the northern1 western1 and southern limits of the district over which the ashes and lavas were distributed4 ,these materials die out as they are traced southwards from ,cader ,idris and north-westwards from ,tremadoc4#add ,the greatest diameter of ground across which they are now continuously traceable is about twenty-eight miles4 ,they attain their greatest thickness1 upwards of #ejjj feet1 in ,aran ,mawddwy1 which rises from their most easterly escarpment4 ,we may therefore infer that the main vent or vents lay somewhere in that direction4 ,the noble range of precipices facing westwards shows how greatly the limits of the volcanic rocks have been reduced by denudation4 ,there can be little doubt that at least the finer tuffs extended westwards as far as a line drawn from ,tremadoc to ,llanegrin--that is1 some fifteen miles or more beyond the cliffs of ,aran ,mawddwy1 thus stretching across much of the site of what is now the great ,harlech anticline4 ,footnote #add3 ,op4 cit4 p4 #if4 ,this compact1 well-defined volcanic area1 in spite of the faults which traverse it and the disturbed positions into which its rocks have been thrown1 is1 in many respects1 one of the simplest and most easily studied among the ,palozoic formations of this country4 ,its main features have been delineated on the maps of the ,geological ,survey and have been described in ,sir ,andrew ,ramsay's monograph4 ,but these publications cannot be regarded as more than a first broad1 though masterly1 outline of the whole subject4 ,there is an ample field for further and more minute research wherein1 with the larger and better ,ordnance maps now available1 and with the advantage of the numerous modern petrographical aids1 a more exhaustive account may be given of the district4 ,the whole volcanic succession from base to summit is laid bare in innumerable magnificent natural sections along ranges of hills for a distance of some forty miles1 and a careful study and re-mapping of it could not fail to add greatly to our knowledge of the early history of volcanic action4#ade ,footnote #ade3 ,the excellent papers of ,professor ,cole1 ,mr4 ,jennings1 ,mr4 ,holland1 ,mr4 ,g4 ,j4 ,williams1 ,mr4 ,p4 ,lake and ,mr4 ,s4 ,h4 ,reynolds are illustrations of how the published work of the ,geological ,survey may be modified and elaborated4 ,according to the observations of the ,geological ,survey1 the ,arenig volcanic rocks of ,merionethshire naturally arrange themselves in three great bands1 each of which is described as tolerably persistent throughout the whole district3--#ast1 a lower series of ashes and conglomerates1 sometimes #ccjj feet thick 7,aran ,mawddwy72 #bnd1 a middle group of felstones and porphyries1 consisting partly of true contemporaneous lava-streams and partly of intrusive sheets1 and reaching a thickness of #aejj feet2 #crd1 an upper series of fragmental deposits like that beneath1 the extreme thickness of which is #hjj feet 7,arenig mountain74 ,a re-mapping of the ground on the six-inch maps would1 no doubt1 show many local departures from this general scheme4 ,the pyroclastic members of this volcanic series present many features of interest both to the field-geologist and the petrographer2 but they have as yet been only partially studied4 ,at the southern end of the district it is remarkable to what a large extent the earliest eruptions must have been mere gaseous explosions1 with the discharge of comparatively little volcanic material4 ,many of the tuffs that are interstratified with black slates 78 ,lingula ,flags7 at the foot of the long northern slope of ,cader ,idris1 consist mainly of black-slate fragments like the slate underneath1 with a variable proportion of grey volcanic dust4 ,illustration3 ,fig4 #dg4--,section at the ,slate ,quarry1 ,penrhyn ,gwyn1 north slopes of ,cader ,idris4 ,the accompanying section 7,fig4 #dg7 represents the arrangement of the rocks exposed at the ,slate ,quarry of ,penrhyn ,gwyn4 ,about #ej feet of black slate 7a7 are there seen1 the bedding in which dips ,s4 at #bj1 while the cleavage is inclined towards ,s4,w4 at a slightly higher angle4 ,the next #bj feet of slate 7b7 are distinguished by many intercalations of slate-tuff or breccia1 varying from less than an inch to three feet in thickness4 ,an intrusive sheet of andesite 7c71 which varies from two or three to ten feet in thickness1 and is strongly cellular in the centre1 interrupts the slates and hardens them4 ,above this sill the indurated slate and tuff 7d71 containing abundant felspar crystals1 pass under a flinty porphyritic felsite 7e7 or exceedingly fine tuff1 enclosing a band of granular tuff4 ,beyond this band the black slates with their seams of tuff continue up the hill and include a sheet of slaggy felsitic lava #h or #aj feet thick4 ,this section1 affording as it does the first glimpse of the volcanic history of ,cader ,idris1 indicates a continued series of feeble gaseous discharges1 probably from one or more small vents1 whereby the black clay on the sea-floor was blown out1 the fragments falling back again to be covered up under a gradual accumulation of similar dark mud4 ,by degrees1 as the vigour of eruption increased1 lava-dust and detached felspar crystals were ejected1 and eventually lava rose to the surface and flowed over the sea-bottom in thin sheets4 ,but elsewhere1 and likewise at a later period in this same southern part of the district1 the fragmental discharges consisted mainly of volcanic material4 ,sir ,andrew ,ramsay has described the coarse conglomerates composed of subangular and rounded blocks of different porphyries1 sometimes #bj inches in diameter1 embedded in a fine matrix of similar materials4 ,the true nature of the component fragments in these rocks has still to be worked out4 ,messrs4 ,cole and ,jennings have noticed that the grey volcanic dust of the older slate-tuff of ,cader ,idris is seen under the microscope to abound in particles of scoriaceous andesite-glass1 now converted into a green palagonite4#adf ,their investigations show that while the same kinds of volcanic rocks continue to be met with from the bottom to the top1 nevertheless there is an increase in the acid character of the lapilli as the section is traced upwards4 ,some of the fragments consist of colourless devitrified glass1 with pieces of pumice1 as if derived from the breaking up of previously-formed tuffs4 ,others resemble quartz-andesites1 rhyolites1 or trachytes1 while in at least one instance1 somewhat low down in the section1 quartz-grains with intruded material point to the existence of some fairly acid and vitreous lava4#adg ,on the south side of ,llyn ,cau1 that is towards the top of the volcanic group1 ,i found a coarse agglomerate with blocks of felsitic lavas1 sometimes three feet across 7see ,fig4 #dh74 ,this gradual increase of acidity in the lapilli of the tuffs finds an interesting confirmation in the contemporaneous lava-sheets to which ,i shall afterwards allude4 ,footnote #adf3 ,quart4 ,journ4 ,geol4 ,soc41 vol4 xlv4 7#ahhi71 p4 #dbd2 ,geol4 ,mag4 7#ahij71 p4 #ddg4 ,footnote #adg3 ,op4 cit4 p4 #dbi4 ,a tuff lying below the ironstone near ,cross ,foxes1 east of ,dolgelly1 likewise contains fragments of trachytic lavas4 ,one of the most noticeable features in the tuffs of this volcanic group is the great abundance of entire and broken crystals dispersed through them4 ,these crystals have certainly not been formed in situ1 but were discharged from the vents as part of the volcanic dust4 ,they usually consist of felspar which1 at least in the southern portion of the district1 appears generally to be plagioclase4 ,frequent reference to these crystals as evidence of volcanic explosions may be found in the publications of the ,survey4 ,nowhere can they be better seen than in the black slate-tuffs of ,cader ,idris4 ,they are there white1 more or less kaolinized1 and as they lie dispersed through the black base1 they give the rock a deceptive resemblance to some dark porphyry4 ,the large crystals of hornblende and augite abundantly scattered through much of the tuff of ,rhobell ,fawr have been already referred to4 ,in the central parts of the district thick bands of ashes were mapped by the ,survey1 and described as consisting almost wholly of volcanic materials1 but containing occasional thin bands of slate which suffice to mark pauses in the eruptions1 when ordinary sediment was strewn over the sea-bottom4 ,in the ,cader ,idris ground1 on the other hand1 interstratifications of non-volcanic material are of such frequent recurrence as to show that there1 instead of constant and vigorous discharges accumulating a vast pile of ashes1 the eruptions followed each other after intervals of sufficient duration to allow of the usual dark sediment spreading for a depth of many feet over the sea-bottom4 ,one of the most interesting deposits of these interludes of quiescence is that of the pisolitic ironstone and its accompanying strata on the north front of ,cader ,idris 7i in ,fig4 #dh74 ,a coarse pumiceous conglomerate with large slag-like blocks of andesite and other rocks1 seen near ,llyn-y-,gadr1 passes upward into a fine bluish grit and shale1 among which lies the bed of pisolitic 7or rather oolitic7 ironstone which is so widely diffused over ,north ,wales4 ,the finely-oolitic structure of this band is obviously original1 but the substance was probably deposited as carbonate of lime under quiet conditions of precipitation4 ,the presence of numerous small ,lingul in the rock shows that molluscan life flourished on the spot at the time4 ,the iron exists in the ore mainly as magnetite1 the original calcite or aragonite having been first replaced by carbonate of iron1 which was subsequently broken up so as to leave a residue of minute cubes of magnetite4#adh ,footnote #adh3 ,messrs4 ,cole and ,jennings1 op4 cit4 p4 #dbf4 ,above the ironstone some more blue and black shale and grit pass under a coarse volcanic conglomerate like that below1 lying at the base of the high precipice of ,cader ,idris4 ,hence this intercalated group of sedimentary strata marks a pause in the discharge of ashes and lavas1 during which the peculiar conditions of sedimentation indicated by the ironstone spread over at least the southern part of the volcanic area4 ,some few miles to the east1 where the ironstone has been excavated near ,cross ,foxes1 the band is again found lying among tuffs and grits full of volcanic lapilli4 ,between a lower and an upper band of tuff in the ,arenig volcanic group the ,maps and ,memoirs of the ,geological ,survey distinguish a central zone of felspathic porphyry1 which attains a maximum thickness of #aejj feet 7see ,fig4 #dh74 ,from ,sir ,andrew ,ramsay's descriptions1 it is clear that he recognized in this zone both intrusive and extrusive sheets1 and that the latter1 where thickest1 were not to be regarded as one mighty lava-flow1 but rather as the result of successive outpourings1 with occasional intervals marked by the intercalation of bands of slate or of tuff4 ,to a certain extent the intruded sheets are separated on the map from the contemporaneous lavas2 but this has been done only in a broad and sketchy way4 ,one of the most important1 and at the same time most difficult1 tasks yet to be accomplished in this district is the separation of the rocks which were probably poured out at the surface from those that were injected underneath it4 ,my own traverses of the ground have convinced me that good evidence of superficial outflows may be found in tracts which have been mapped as entirely intrusive2 while1 on the other hand1 some of the so-called lavas may more probably be of the nature of sills4 ,illustration3 ,fig4 #dh4--,sketch-section across ,cader ,idris4 st1 slates and tuffs and ashy slates2 s1 slates and grits2 i1 ironstone2 b1 volcanic breccias2 a1 slaggy andesitic and more basic lavas2 e1 microgranite or eurite2 f1 felsites2 d1 greenstone 7dolerites1 diabases1 etc474 ,the petrography of the rocks1 moreover1 still requires much study4 ,among the so-called felspathic porphyries of the ,survey maps a considerable variety of texture1 structure and composition will doubtless be detected4 ,in the ,descriptive ,catalogue of ,rock-,specimens in the ,museum of ,practical ,geology 7#crd edit41 #ahfb7 the rocks that form the lava-streams of ,llandeilo age1 in ,merionethshire1 are named felstone1 felspar-porphyry1 felstone-porphyry1 felspathic-porphyry1 and calcareous amygdaloid4 ,the most interesting feature which my own slight personal acquaintance with the region has brought before me is the clear evidence of a succession from comparatively basic lavas in the lower part of the group to much more acid masses in the higher part4 ,in the ,survey map numerous sheets of intrusive greenstone are shown traversing the ,lingula ,flags1 ,tremadoc slates1 and lower part of the volcanic group along the northern slopes of ,cader ,idris4 ,the true intrusive nature of much of this material is clearly established by transgressive lines of junction and by contact-metamorphism1 as well as by the distinctive crystalline texture of the rocks themselves4 ,but the surveyors were evidently puzzled by some parts of the ground4 ,sir ,andrew ,ramsay speaks of the great mass of problematical vesicular and sometimes calcareous rock which is in places almost ashy-looking4 ,after several oscillations of opinion1 he seems to have come finally to the conclusion that this vesicular material1 which occurs also in the upper part of the mountain1 passes into1 and cannot be separated from1 the undoubted intrusive greenstones4#adi ,footnote #adi3 ,mem4 ,geol4 ,surv4 vol4 iii4 #bnd edit4 p4 #cf2 see also pp4 #ca1 #cb4 ,the true solution of the difficulty will be found1 ,i believe1 in the recognition of a group of scoriaceous lavas among these greenstones4 ,the presence of a cellular structure might not be sufficient to demonstrate that the rocks in which it appears are true lava-beds1 for such a structure is far from unknown both among dykes and sills4 ,but in the present case there is other corroborative testimony that some of these ,cader ,idris amygdaloids were really poured out at the surface4 ,below ,llyn-y-,gadr--the dark tarn at the foot of the vast wall of ,cader ,idris--the beds of coarse volcanic conglomerate 7b in ,fig4 #dh71 to which ,i have already alluded1 are largely composed of blocks of the vesicular greenstones on which they lie4 ,these greenstones1 moreover1 have many of the most striking characteristics of true lavas 7a in ,fig4 #dh74 ,they are extraordinarily cellular2 their upper surfaces sometimes present a mass of bomb-like slags with flow-structure1 and the vesicles are not infrequently arranged in rows and bands along the dip-planes4 ,a microscopic examination of two slides cut from these rocks shows them to be of a trachytic or andesitic type1 with porphyritic crystals of a kaolinized felspar embedded in a microlitic groundmass4 ,the rocks are much impregnated with calcite1 which fills their vesicles and ramifies through their mass4 ,a few miles to the east some remarkable felsitic rocks take the place of these vesicular lavas immediately below the pisolitic iron ore4 ,i have not determined satisfactorily their relations to the surrounding rocks1 and in particular am uncertain whether they are interbedded lavas or intrusive sheets4 ,dr4 ,f4 ,h4 ,hatch found that their microscopic characters show a close resemblance to the soda-felsites described by him from the ,bala series of the south-east of ,ireland4 ,the slopes of ,cader ,idris are partly obscured with debris1 from above which rises the great precipitous face formed by the escarpment of porphyry1 here intrusively interposed among the ,arenig volcanic rocks4 ,this enormous sill will be referred to a little further on in connection with the other intrusive sheets of the region4 ,the remarkably cellular rock which forms the peak of ,cader ,idris is coloured on the ,survey map as an intrusive sill of greenstone1 which in the ,memoir is said to alter the contiguous slates and to appear to cut across them diagonally4 ,i am disposed1 however1 to think that these appearances of intrusion are deceptive4 ,on the southern declivity of the mountain this rock presents one of the most curious structures to be seen in the whole district4 ,its surface displays a mass of spheroidal or pillow-shaped blocks aggregated together1 each having a tendency to divide internally into prisms which diverge from the outside towards the centre4#aej ,some portions are extremely slaggy1 and round these more solid portions finely crystalline parts are drawn1 suggestive rather of free motion at the surface than of the conditions under which a subterranean sill must be formed4 ,the idea occurred to me on the ground that while the band of rock marked as greenstone on the map is probably1 in the main1 an interstratified lava1 there may nevertheless be basic intrusions along its course1 as in the lower part of the mountain4 ,the minute structure of this amygdaloid1 as revealed by the microscope1 shows it to be an epidiorite wherein the hornblende1 paramorphic after augite1 has been again partially altered along the margins into chlorite4 ,footnote #aej3 ,this peculiar structure of the more basic ,arenig lavas1 where the rock looks as if built up of irregularly-spheroidal1 sack-like or pillow-shaped blocks1 will be again referred to in connection with the ,arenig 7and ,llandeilo7 lavas of ,scotland and ,ireland4 ,it appears to be widely distributed1 and especially in connection with the occurrence of radiolarian cherts4 ,the black slate above the ,cader ,idris amygdaloid would1 in a similar position in ,scotland1 be associated with such cherts1 but these have not yet been noticed at this locality4 ,with the spheroidal internally-radiating prismatic structure of the ,cader ,idris rock1 compare that of the lava at ,acicastello already noticed on p4 #bf4 ,the highest lavas of ,cader ,idris1 forming the ridge to the south of ,llyn ,cau1 are separated from the amygdaloid just described by a thick zone of black slate with thin ashy intercalations1 beyond which comes the coarse volcanic agglomerate already referred to as containing blocks of felsite a yard or more in diameter4 ,these lavas are true felsites1 sometimes beautifully spherulitic and exhibiting abundant flow-structure1 like some of the felsites of the next or ,bala volcanic period4#aea ,the petrography of these rocks still remains to be worked out4 ,footnote #aea3 ,messrs4 ,cole and ,jennings1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xlv4 7#ahhi71 p4 #dcj4 ,from the examination of slices prepared from a few of the felsites of the ,dolgelly district1 ,dr4 ,hatch observed a striking difference between their characters and those of the ,cambrian felsites of ,caernarvonshire4 ,the porphyritic constituent is now no longer quartz1 but felspar 7plagioclase71 and the rocks belong1 not to the rhyolitic1 but rather to the less acid trachytes1 perhaps even to the andesites4 ,the volcanic series of ,cader ,idris sweeps northward through the chain of ,aran and ,arenig1 and then curves westward through the group of ,manod and ,moelwyn1 beyond which it rapidly dies out4 ,in its course of about #de miles it undergoes considerable variation1 as may be seen by comparing a section through ,moelwyn with that through ,cader ,idris already given4 ,according to the researches of ,mr4 ,jennings and ,mr4 ,williams1#aeb the main mass of volcanic material in the northern part of the region consists of fragmentary rocks varying in texture from agglomerates into fine tuffs1 but showing some differences in the succession of beds in different localities4 ,footnote #aeb3 ,quart4 ,journ4 ,geol4 ,soc4 xlvii4 7#ahia71 p4 #cfh4 ,the ,tremadoc group of strata clearly underlies the volcanic series of these more northerly tracts4 ,but it contains1 so far as appears1 no intercalation of volcanic material4 ,the inference may thus be drawn that the eruptions began in the ,cader ,idris district1 and did not extend into that of ,manod and ,moelwyn until after the beginning of the ,arenig period4 ,above the ,tremadoc group lies the well-marked and persistent band1 about #ac feet thick1 known as the ,garth grit1 which has been already referred to as a convenient base-line to the ,arenig group4 ,illustration3 ,fig4 #di4--,section across the ,moelwyn ,range4#aec #a1 ,tremadoc ,group2 #b1 ,garth or ,arenig grit 7base of ,arenig group72 #c1 ,arenig slates1 etc42 #ca1 ,lower slate band2 #cb1 ,middle slate band2 #cc1 ,upper slate band2 #da1 ,lower agglomerate2 #db1 ,middle agglomerate2 #dc1 ,upper agglomerate2 #e1 ,llandeilo group2 ,g1 ,granite boss of ,moel tan y ,grisiau4 ,footnote #aec3 ,after ,messrs4 ,jennings and ,williams1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xlvii4 7#ahia71 p4 #cga1 and ,horizont4 ,sect4 ,geol4 ,surv4 ,sheet #bh4 ,in this northern district1 among the sediments which overlie the ,garth grit1 layers of fine tuff begin to make their appearance1 which north of ,cwm ,orthin thicken out into a considerable mass between the grit and the lowest of the great agglomerates4 ,these tuffs1 which mark the beginning of the volcanic eruptions of the district1 are followed by a band of slate which in some places has yielded a ,lingula1 ,orthis ,carausii1 and a ,tetragraptus1 and points to an interval of quiescence in the volcanic history4 ,we now enter upon an enormous thickness of agglomerates and tuffs separated by several bands of slate4 ,taking advantage of the slaty intercalations1 ,messrs4 ,jennings and ,williams have divided this great accumulation of fragmentary volcanic material into three beds 7,fig4 #di74 ,the matrix of the agglomerates is compact and pale1 so as to resemble and to have been called felstone1 but showing its fragmentary nature on weathered surfaces4 ,the blocks imbedded in this paste range up to sometimes as much as #aa feet in length by #d feet in width4 ,their minute petrographical characters have not been studied1 but the blocks are stated to consist for the most part of slaty and schistose fragments mixed with rounded pebbles of fine-grained 'felstone4' ,they are heaped together as in true agglomerates4 ,in the upper agglomerate1 fragments of cleaved slate containing ,lingula have been observed4 ,the name of felstone is restricted by ,messrs4 ,jennings and ,williams to certain fine-grained varieties of rock1 of which a thin band lies at the base of the lower agglomerate1 while another of considerably greater importance occurs in the middle of the upper agglomerate4 ,these bands consist of a fine compact greenish base1 and weather with a dull white crust2 sometimes1 as in the thicker sheet1 a columnar structure shows itself4 ,whether these rocks are to be regarded as lavas or sills1 or even as finer varieties of tuff1 is a question that awaits further inquiry4 ,but it is clear1 from the investigation of the two observers just cited1 that the pyroclastic constituents must vastly preponderate in the volcanic series over the northern part of the region4 ,all these rocks1 whether coarse or fine-grained1 appear to be rather acid in composition1 and no evidence has yet been obtained of a sequence among them from a more basic to a more acid series1 as in ,cader ,idris4 ,the highest agglomerate bed of the ,manod and ,moelwyn area is covered by slates which contain ,llandeilo graptolites4 ,in this way1 by means of palontological evidence1 the upward and downward limits of the ,arenig volcanic series in this part of ,wales are definitely fixed4 ,hardly any information has yet been obtained as to the situation and character of the vents from which the lavas and ashes of ,merionethshire were discharged4 ,in the course of the mapping of the ground1 the ,geological ,survey recognized that1 as the greatest bulk of erupted material lies in the eastern and south-eastern parts of the region1 the chief centres of emission were to be looked for in that quarter1 and that possibly some of the intrusive masses which break through the rocks west of the great escarpment may mark the site of vents1 such as ,tyddyn-rhiw1 ,gelli-llwyd-fawr1 ,y-,foel-ddu1 ,rhobell ,fawr1 and certain bosses near ,arenig4#aed ,the distribution of the volcanic materials indicates that there were certainly more than one active crater4 ,while the southward thickening of the whole volcanic group points to some specially vigorous volcano in that quarter1 the notable thinning away of the upper tuffs southward and their great depth about ,arenig suggest their having come from some vent in this neighbourhood4 ,on the other hand1 the lower tuffs are absent at ,arenig1 while on ,aran ,mawddwy1 only nine miles to the south1 they reach a depth of #cjjj feet4 ,still farther to the south these volcanic ejections become more and more divided by intercalated bands of ordinary sediment4 ,one of the most important volcanoes of the region evidently rose somewhere in the neighbourhood of what is now ,aran ,mawddwy4 ,there seems reason to surmise that the sites of the chief vents now lie to the east and south of the great escarpment1 buried under the thick sedimentary formations which cover all that region4 ,footnote #aed3 ,mem4 ,geol4 ,surv4 vol4 iii4 #bnd edit4 p4 #ih2 see also pp4 #dd1 #ed1 #eh1 #ga4 ,if we are justified1 on stratigraphical and petrographical grounds1 in connecting the lowest volcanic rocks of the ,berwyn range with those of ,merionethshire1 we may speculate on the existence of a group of submarine vents1 coming into eruption at successive intervals1 from some epoch during the period of the ,lingula ,flags up to that of the ,bala rocks1 and covering with lavas and ashes a space of sea-bottom at least forty miles from east to west by more than twenty miles from north to south1 or roughly1 an area of some #hjj square miles4#aee ,footnote #aee3 ,the ,berwyn ,hills1 however1 will be described in later pages as a distinct volcanic district4 ,besides the materials ejected to the surface1 the ancient volcanic region of ,merionethshire was marked by the intrusion of a vast amount of igneous rock between and across the bedding-planes of the strata deep underground4 ,one of the most prominent features of the ,geological ,survey map is the great number of sills represented as running with the general strike of the strata1 especially between the top of the ,harlech grits and the base of the volcanic series4 ,on the north side of the valley of the ,mawddach1 between ,barmouth and ,rhaiadr ,mawddach1 in a distance of twelve miles the ,survey mapped more than #aej intrusions varying from a few yards to nearly a mile in length4#aef ,this zone of sills is equally marked on the south side of the valley4 ,it may be traced all round the ,harlech anticline until it dies out1 as the bedded masses also do1 towards ,towyn on the south and about ,tremadoc on the north4 ,footnote #aef3 ,mem4 ,geol4 ,surv4 vol4 iii4 p4 #bf4 ,the presence of such a zone of intrusive sheets at the base of an ancient volcanic series is a characteristic feature in the geology of ,britain4 ,it is met with again and again among the ,palozoic systems1 and appears on a striking scale in association with the ,tertiary basaltic plateaux of ,antrim and the ,inner ,hebrides4 ,but nowhere1 perhaps1 is it more strongly developed than beneath the ,arenig group of lavas and tuffs in ,north ,wales4 ,abundant as are the protrusions marked on the ,geological ,survey map1 they fall short of the actual number to be met with on the ground4 ,indeed1 to represent them as they really are would require laborious surveying and the use of maps on a far larger scale than one inch to a mile4 ,the vast majority of these sills are basic rocks1 or1 in the old and convenient terminology1 greenstones4 ,those of the ,cader ,idris district have been examined by ,messrs4 ,cole and ,jennings1 who found that1 notwithstanding the considerable alteration everywhere shown by the abundant epidote and calcite1 the coarser varieties may be recognized as having originally been dolerites approaching gabbro1 with a well-developed ophitic character1 the general range of structure being from dolerites without olivine and aphanites to andesitic rocks with an originally glassy matrix4#aeg ,dr4 ,hatch confirmed this diagnosis from slides prepared from my specimens4 ,the ophitic structure is usually characteristic and well preserved1 in spite of the alteration indicated by epidote1 chlorite1 uralite1 and leucoxene4 ,footnote #aeg3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xlv4 7#ahhi71 p4 #dcb4 ,that this zone of greenstone sills belongs to the period of the ,merionethshire volcanoes may be reasonably concluded4 ,the way in which they follow the line of the great escarpment1 their almost entire absence from the ,cambrian dome to the west1 their cessation as the overlying lavas and tuffs die out laterally1 and their scarcity above the lower part of the volcanic group1 seem to indicate their close relationship to that group4 ,moreover1 that they must have been as a whole later than the main part of the lavas and tuffs may be inferred from their position4 ,the molten material of which they were formed could hardly have forced its way between and across the strata unless egress to the surface had been impeded by some thick overlying mass4 ,the greenstones may therefore be regarded as lateral emanations from funnels of more basic lava towards the close of the volcanic period4 ,possibly some at least of the highly slaggy and vesicular bands to which ,i have referred may represent portions of this material1 which actually flowed out as streams of lava at the surface4 ,but there is likewise evidence of extensive intrusion of more siliceous rocks4 ,on the ,geological ,survey map1 besides the numerous greenstones1 various sheets of felspathic porphyry are represented as running with the general strike of the region1 but here and there breaking across it4 ,one of the most remarkable of these acid sills is that which1 in the noble precipice of ,cader ,idris1 has a thickness of about #aejj feet and a length of three or four miles4 ,it is shown on the map to be transgressive across other rocks1 and1 as seen on the ground1 it maintains the uniformity of texture which is characteristic rather of sheets that have solidified underneath than of those which have congealed with comparative rapidity at the surface4 ,on a fresh fracture the rock presents a pale bluish-grey tint1 becoming yellowish or brownish as the result of weathering4 ,its texture is finely granular1 with occasional disseminated felspars4 ,under the microscope a section of it was found by ,dr4 ,hatch to exhibit the characteristic structure of a microgranite1 a confused holocrystalline aggregate of quartz and felspar1 with a few porphyritic felspars4 ,messrs4 ,cole and ,jennings have proposed to revive for this rock ,daubuisson's name ,eurite4#aeh ,footnote #aeh3 ,mr4 ,harker speaks of the rock as a granophyre4 ,a similar rock occurs at a lower horizon among the ,lingula ,flags at ,gelli-llwyd-fawr1 two miles south-west of ,dolgelly1#aei and much microgranite has been injected along the slopes above ,tyddyn-mawr4 ,footnote #aei3 ,messrs4 ,cole and ,jennings1 op4 cit4 p4 #dce4 ,the chronological relation of these acid sheets and bosses to the more basic intrusions has not yet been definitely determined4 ,that some of them may have solidified in vents and may have been directly connected with the protrusion of the later or more highly siliceous lavas is not at all improbable4 ,others again would seem to belong to a much later geological period than the ,arenig volcanoes4 ,in this late series the well-known boss of ,tan-y-grisiau near ,festiniog should probably be included4 ,this mass of eruptive material was mapped by the ,geological ,survey as intrusive syenite4 ,it has been more recently examined and described by ,messrs4 ,jennings and ,williams as a granitite4#afj ,these observers have noticed not only that it intrusively traverses and alters the ,tremadoc group1 but that its intrusion appears to have taken place subsequent to the cleavage which affects the ,llandeilo as well as older formations4 ,this granitic boss has thus probably no connection with the ,arenig volcanoes1 but belongs to a later period in the volcanic history of the ,principality4 ,footnote #afj3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xlvii4 7#ahia71 p4 #cgi4 ,the remarkable scarcity of dykes in the volcanic districts of ,wales has been noticed by more than one observer4 ,among the intrusive greenstones of ,merionethshire some occasionally assume the dyke form1 and through the agglomerates and tuffs of ,rhobell ,fawr dykes of olivine-diabase have worked their way4 ,in the ,festiniog district various altered andesitic dykes have been noted4 ,but there has been no widespread fissuring of the ground and uprise of lava in the rents1 such as may be seen in the ,archan gneiss1 and in the later ,palozoic1 but still more in the ,tertiary volcanic regions4 ,this feature becomes all the more notable when it is viewed in connection with the great development of sills1 and the evidence thereby afforded of widespread and extremely vigorous subterranean volcanic action4 ,in the ,merionethshire region there certainly was a long period of quiescence between the close of the ,arenig and the beginning of the ,bala eruptions4 ,moreover1 no evidence has yet been found that active vents ever again appeared in that district1 the subterranean energy at its next outburst having broken out farther to the east and north4 ,in ,anglesey1 however1 where1 as ,i shall point out1 there is proof of contemporaneous tuffs among the ,arenig rocks1 it is possible that a continuous record of volcanic action may yet be traced from ,arenig well onward into ,bala time4 ii4 ,,shropshire ,about #ce miles to the south-east of the great volcanic range of ,merionethshire a small tract of ,arenig rocks rises from amidst younger formations1 and forms the picturesque country between ,church ,stoke and ,pontesbury4 ,murchison in his excellent account of this district clearly recognized the presence of both intrusive and interstratified igneous rocks4#afa ,the ground has in recent years been more carefully worked over by ,mr4 ,g4 ,h4 ,morton#afb and ,professor ,lapworth4#afc ,footnote #afa3 ,silurian ,system 7#ahci71 chap4 xix42 ,siluria1 #dth edit4 7#ahfg71 pp4 #bf1 #di4 ,footnote #afb3 ,proc4 ,liverpool ,geol4 ,soc4 x4 7#ahed71 p4 #fb4 ,footnote #afc3 ,geol4 ,mag4 7#ahhg71 p4 #gh4 ,at the top of the ,arenig group of this district lies a zone of well-stratified andesitic tuff and breccia 7,stapeley ,ash71 with frequent intercalations of shales1 and occasionally fossiliferous4#afd ,there is thus satisfactory proof of contemporaneous eruptions at intervals during the accumulation of the later ,arenig sediments4 ,that there were also outflows of lava is shown by the presence of sheets of augite- and hypersthene-andesite4 ,these volcanic intercalations form marked ridges1 having a general northerly trend4 ,they are folded over the broad laccolitic ridge of ,corndon1 on the east side of which they are thrown into a synclinal trough1 so that successive parallel outcrops of them are exposed4 ,according to the mapping of the ,geological ,survey they are thickest towards the west1 and become more split up with intercalated sediments as they range eastward4 ,footnote #afd3 ,prof4 ,lapworth and ,mr4 ,w4 ,w4 ,watts1 ,proc4 ,geol4 ,assoc4 xiii4 7#ahid71 pp4 #cag1 #ccg4 ,volcanic eruptions in this ,shropshire region continued from the ,arenig into the ,bala period4 ,they are marked among the ,llandeilo strata by occasional tuffs and by two massive beds of volcanic grit1 described by ,murchison1#afe but they appear to have been rather less vigorous in the interval represented by this subdivision of the ,silurian system4 ,those of ,bala time gave forth abundant discharges of ash1 of which the lowest accumulation1 locally known as the ,hagley ,ash1 consists of andesitic detritus4 ,occasional layers of tuff are intercalated in the overlying ,hagley ,shales1 above which comes an important band called the ,whittery ,ash1 consisting of andesitic and rhyolitic breccias and conglomerates1 fine ashes with curious spherulitic or pisolitic structures1 and bands of shale often fossiliferous4#aff ,it is evident that the eruptions of the ,shelve district came from independent vents in that neighbourhood1 and never reached the importance of the great volcanoes of ,arenig age in ,montgomeryshire or of ,bala age in ,caernarvonshire4 ,footnote #afe3 ,silurian ,system1 p4 #bbi4 ,footnote #aff3 ,messrs4 ,lapworth and ,watts1 op4 cit4 p4 #cah4 ,illustration3 ,fig4 #ej4--,section across the anticline of ,corndon4#afg ,a1 ,arenig flags and shales2 ,b1 andesites and tuffs2 ,c1 intrusive dolerite4 ,footnote #afg3 ,after ,prof4 ,lapworth and ,mr4 ,watts1 op4 cit4 p4 #cdb4 ,numerous dykes and sills traverse the rocks of this district4 ,they consist chiefly of hypersthene-dolerite4 ,they appear to belong to a much later period than the interstratified volcanic series2 at least some of them are found altering the ,pentamerus limestones1 and these must be later than the ,llandovery rocks4#afh ,the most important sill is that which forms ,corndon1 the central igneous mass of the district4 ,this body of dolerite was ascertained by ,mr4 ,watts not to be a boss but a laccolite1 which wedges out both towards the north-west and south-east1 as shown in ,fig4 #ej4 ,footnote #afh3 ,op4 cit4 p4 #cci4 ,six miles to the north of the ,shelve and ,corndon district the ,breidden ,hills rise on the border of ,shropshire and ,montgomeryshire1 and include a mass of volcanic material belonging to a distinct area of eruption4 ,in the ridge that extends for about three and a half miles through ,moel-y-golfa and ,middletown ,hill1 a synclinal trough of volcanic rocks lies upon shales1 which from their fossils have been placed in the ,bala group4 ,the volcanic series appears to exceed #ajjj feet in thickness4 ,the lowest part of it on ,moel-y-golfa consists of andesitic lavas about #djj feet thick1 followed by tuffs and volcanic conglomerates4 ,the lavas resemble some of the porphyrites of the ,old ,red ,sandstone1 and contain two forms of pyroxene--one rhombic1 probably enstatite1 and the other monoclinic augite4 ,there are likewise considerable masses of intrusive rock1 which are varieties of diabase or dolerite4#afi ,footnote #afi3 ,see ,mr4 ,w4 ,w4 ,watts on the ,igneous and ,associated ,rocks of the ,breidden ,hills1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xli4 7#ahhe71 p4 #ecb4 iii4 ,,scotland ,from the centre of ,england we must in imagination transport ourselves into the ,southern ,uplands of ,scotland1 where a widely distributed series of ,silurian volcanic rocks has been preserved4 ,it was1 until recently1 supposed that the ,silurian system north of the ,tweed contains no contemporaneously erupted volcanic rocks4 ,yet1 as far back as the year #ahfj1 ,i pointed to the abundant existence of volcanic detritus in these strata throughout the southern counties as a probable indication of volcanic activity at the time and in the area within which the strata were deposited4#agj ,some years later1 when the microscope had been introduced as an aid to field-geology1 ,i sliced some of the ,silurian sediments of that region and found them1 particularly certain shales and grits of ,moffatdale1 to contain a large admixture of perfectly fresh unworn felspar crystals1 which ,i felt tolerably certain had been supplied by volcanic explosions4 ,as no trace1 however1 had then been detected of an intercalated volcanic group in any part of the ,silurian series of the south of ,scotland1 ,i used at that time to speculate on the possibility of the volcanic detritus having been wind-borne from the volcanoes of the ,lake ,district4 ,i had at that time no suspicion that its source was rather to be sought under my feet4 ,the presence of volcanic rocks underneath the uplands of the south of ,scotland would have been a welcome explanation of the frequent felspathic composition of many of the ,silurian greywackes and shales of that region1 and particularly the abundance of andesitic and felsitic fragments in them4 ,footnote #agj3 ,trans4 ,roy4 ,soc4 ,edin4 xxii4 7#ahfj71 p4 #fcf4 ,it had been long known that the ,scottish ,silurian formations1 besides having undergone extensive plication1 have also been injected by protrusions of igneous material of various kinds4 ,the intrusive character of many of these is so obvious that a similar origin was attributed even to those bosses which could not be proved to be intrusive4 ,recent work of the ,geological ,survey1 however1 and more especially the numerous and careful traverses of my friend and colleague ,mr4 ,peach1 have revealed the unlooked-for and important fact that a large number of these supposed intrusions are really portions of a volcanic group brought up on the crests of anticlinal folds1 and laid bare by denudation4 ,this group can be traced for at least #ajj miles from north-east to south-west over a belt of country sometimes #cj miles broad4 ,its original limits cannot be ascertained1 but they obviously exceeded those within which the rocks can now be seen4 ,nevertheless the present boundaries embrace an area of nearly #bjjj square miles4 ,this ,palozoic volcanic region is thus one of the most extensive in the ,british ,isles4 ,owing1 however1 to the constant plication of the strata1 and the wide space which the overlying sedimentary deposits are thus made to cover1 the volcanic group only comes occasionally into view1 and thus occupies but a mere fraction of the superficial extent of the region over which its scattered outcrops appear4 ,these exposures1 sometimes only a few square yards in extent1 may always be looked for where the anticlinal folds bring up a sufficiently low portion of the ,silurian system2 they prove that a vast volcanic floor underlies the visible ,lower ,silurian grits and shales over the length and breadth of the ,southern ,uplands of ,scotland4 ,without anticipating details which will properly appear in the official ,memoirs of the ,geological ,survey1 ,i may briefly indicate the visible boundaries of the volcanic group1 and refer to some of the localities where it may best be seen4 ,the most easterly points where it has been recognized by ,mr4 ,peach stand on the crests of some sharp anticlinal folds near ,st4 ,mary's ,loch and near ,leadburn and ,winkstone in ,peeblesshire4 ,farther westwards it appears at many places along the northern border of the ,silurian territory1 as at ,romanno ,bridge1 ,wrae1 ,kilbucho1 ,culter ,water and ,abington1 the length and breadth of each exposure depending partly on the breadth of the anticline and partly on the depth to which it has been cut down by denudation4 ,near ,sanquhar the volcanic series opens out for a breadth of more than a mile1 and is seen at intervals across the wild moorlands of ,carrick1 until from the ,stinchar valley it widens out seaward and occupies much of the coast-line of ,ayrshire between ,girvan and the mouth of ,loch ,ryan4 ,it probably rises again along a fold near ,portpatrick1 and it is seen at various points along the southern borders of the ,silurian uplands1 as near ,castle-,douglas1 at ,glenkiln1 ,bell ,craig near ,moffat1 and the head of ,ettrickdale4 ,the best sections are those exposed along the coast to the north and south of ,ballantrae4 ,when that ground was first examined by the ,geological ,survey1 the hypothetical views in regard to metamorphism already referred to were in full ascendant1 and the rocks were mapped on the same general principles as those which had been followed in ,wales4 ,professor ,bonney1 however1 a few years later recognized the true igneous nature of many of the rocks4 ,he found among them porphyrite lavas and agglomerates which he regarded as of ,old ,red ,sandstone age1 likewise intrusive serpentines and gabbros4#aga ,footnote #aga3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xxxiv4 7#ahgh71 p4 #gfi4 ,the volcanic rocks of this wide district include both lavas and their pyroclastic accompaniments1 as well as intrusive sills and bosses of various materials4 ,they have recently been studied by ,mr4 ,j4 ,j4 ,h4 ,teall1 and full descriptions of them by him will appear in a forthcoming volume of the ,memoirs of the ,geological ,survey4 ,he has ascertained that though generally more or less decomposed1 the lavas would be classed by ,german petrographers as diabases and diabase-porphyrites4 ,the former are compact dark-green non-porphyritic rocks1 often containing numerous small spherical amygdales2 while the latter are markedly porphyritic1 enclosing large phenocrysts of more or less altered plagioclase1 often measuring half an inch across4 ,these two groups of rock are connected by transitional varieties4 ,they were probably1 in the first instance1 composed of plagioclase1 augite1 iron-ores1 and a variable quantity of imperfectly crystallized interstitial matter4 ,some of these rocks closely resemble in outward appearance the andesites 7porphyrites7 of the ,old ,red ,sandstone of the district not many miles to the north1 that is1 fine purplish-red rocks with a compact base through which porphyritic felspars are abundantly scattered4 ,occasionally they are markedly slaggy1 and show even a ropy surface1 while the breccias associated with them contain blocks of similar slag4 ,illustration3 ,fig4 #ea4--,structure in finely-amygdaloidal diabase lava1 south of mouth of ,stinchar ,river1 ,ayrshire4 ,the fine dots and circles mark the lines of amygdales4 ,but the most characteristic external feature of these lavas is their tendency to assume irregularly-elliptical1 sack-like or pillow-shaped forms4 ,on a weathered face they sometimes look like a pile of partially-filled sacks heaped on each other1 the prominences of one projecting into corresponding hollows in the next4 ,the general aspect of this structure is shown in ,fig4 #ab1 which represents a face of rock about eight feet high and six feet broad4 ,the rocks exhibiting this peculiarity are usually finely amygdaloidal1 and it may be observed that the vesicles are grouped in lines parallel to the outer surface of the pillow-like block in which they occur4 ,the diagram in ,fig4 #ea represents in ground-plan a surface about twelve feet square on the shore immediately to the south of the mouth of the ,river ,stinchar4 ,in the heart of the spheroids enclosed fragments of other lavas are sometimes observable4 ,this singular structure has already 7p4 #ahd7 been referred to as strikingly displayed in a rock at the top of ,cader ,idris4 ,it is found in dark basic lavas probably of ,arenig age1 which will be afterwards referred to as occurring along the southern flanks of the ,scottish ,highlands and also in the north of ,ireland4 ,it has been observed by ,mr4 ,teall among the rocks of the ,lizard1 and has been described as occurring in ,saxony and ,california4#agb ,in these different localities it is associated with jaspers and cherts1 some of which contain abundant ,radiolaria4 ,the same structure has been found among the variolitic diabases of ,mont ,genvre1#agc and likewise in some modern lavas1 as in that of ,acicastello already referred to 7ante1 p4 #bf74 ,footnote #agb3 ,mr4 ,j4 ,j4 ,h4 ,teall1 ,roy4 ,geol4 ,soc4 ,cornwall1 #ahid1 p4 #c4 ,mr4 ,l4 ,ransome1 ,bull4 ,depart4 ,geol4 ,university of ,california1 vol4 i4 p4 #ajf4 ,footnote #agc3 ,messrs4 ,cole and ,gregory1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xlvi4 7#ahij71 p4 #caa4 ,illustration3 ,fig4 #eb4--,view of ,knockdolian ,hill from the east4 ,the volcanic agglomerates and breccias1 in the south-west of ,ayrshire1 attain a great development in several centres probably at or near the original volcanic vents4 ,they present several distinct petrographical types4 ,the remarkable neck-like hill of ,knockdolian in the ,stinchar ,valley is made of a coarse breccia composed mainly of angular pieces of dull greyish-green fine-grained diabase4 ,the breccias and agglomerates of ,bennane ,head in some parts consist largely of broken-up shales1 flinty mudstone1 black radiolarian flint or chert1 and abundant fragments of andesites and felsites4 ,in other parts the volcanic material predominates1 including angular and subangular fragments of various somewhat basic lavas1 lumps of vesicular slag and pieces of pumice4 ,here and there much calcite is diffused through the matrix in strings1 veins and patches1 which enclose the lapilli4 ,the agglomerate north of ,lendalfoot possesses a greenish1 somewhat serpentinous matrix1 through which immense numbers of tabular felspar crystals are scattered4 ,similar crystals also occur abundantly in embedded blocks of one of the purplish diabase-porphyrites1 which occurs in mass on the shore and inland1 and closely resembles the rock of ,carnethy in the ,old ,red ,sandstone volcanic series of the ,pentland ,hills4 ,yet another and very distinct type of agglomerate is to be seen on the ,mains ,hill south-east of ,ballantrae4 ,it is a coarse rock1 enclosing blocks up to a yard or more in diameter1 of a fine compact purplish porphyrite1 with large crystals of plagioclase and smaller ones of augite4 ,in some places immense numbers of the small lapilli in the matrix consist of an extremely fine vesicular pumice4 ,small perfect and larger broken crystals of augite are likewise abundant in some of the greenish1 more basic parts of the mass4 ,these greenish serpentinous parts and the numerous augite crystals point to the explosion of some tolerably basic pyroxenic lava4 ,a similar dark green1 almost black1 rock1 with augite crystals1 which sometimes measure a quarter of an inch in diameter1 occurs near ,sanquhar in ,nithsdale4 ,it presents a close resemblance to the agglomerate of ,rhobell ,fawr1 already alluded to4 ,so far as these ,scottish agglomerates have yet been microscopically examined1 they have been found to be composed of crystals1 crystal-fragments1 and lapilli derived partly from lavas similar to those above described1 and partly from felsitic and other rocks which have not yet been observed here in the form of lavas4 ,the finer tuffs show likewise a considerable range of composition4 ,according to ,mr4 ,peach's observations along the south-eastern parts of the volcanic area1 the ejected materials have consisted largely of fine dust 7probably in great measure felsitic71 which towards the north-east is gradually interleaved with ordinary sediment till the ashy character disappears4 ,as ,i have already remarked1 there is reason to believe that the overlying greywackes and shales derived part of their material either directly from volcanic explosions or from the attrition of banks of lavas and tuffs exposed to denudation4 ,but besides the interstratified lavas and fragmental rocks there occur numerous intrusive masses which are so intimately associated with the volcanic series that they may with little hesitation be regarded as forming part of it4 ,they consist of various gabbros and serpentines1 which are especially developed where the volcanic series comes out in greatest force in the south-west of ,ayrshire4 ,they also include more acid intrusions which1 as in the case of the rock of ,byne ,hill1 near ,girvan1 even assume the characters of granite4 ,the dying out of the volcanic material towards the north-east probably indicates that the vents of the period lay rather in the central or south-western parts of the district4 ,unfortunately1 the limited extent of the exposures of the rocks makes it a hopeless task to search for traces of these vents over by far the largest part of the area4 ,there are two localities1 however1 where the search may be made with better prospect of success4 ,one of these is a tract to the north of ,sanquhar in ,nithsdale1 which still requires to be studied in detail with reference to the sequence and structure of its volcanic rocks4 ,the other area is that south-western part of ,ayrshire which has been already cited as displaying so large a development of the volcanic series4 ,here the coast-sections reveal the intercalation of fossiliferous bands which show the true stratigraphical horizons of the lavas and tuffs4 ,under ,bennane ,head1 ,professor ,lapworth some years ago found1 in certain hardened black shales1 a group of graptolites which mark an undoubted ,arenig platform4#agd ,recently the ground has been carefully re-examined by ,messrs ,peach and ,horne1 who have detected a number of other fossiliferous zones which confirm and extend previous observations4 ,they have also been able to unravel the complicated structure of the volcanic series1 and to represent it on the #f-;inch maps of the ,geological ,survey1 of which a reduction on the scale of #a inch to a mile is now in course of preparation4 ,the following tabular summary1 taken partly from notes made by myself during a series of traverses of the ground with ,mr4 ,peach when the revision was begun1 and partly from memoranda supplied by that geologist himself1 may suffice as a general outline of the volcanic history of this exceedingly interesting and important region4 ,footnote #agd3 ,geol4 ,mag4 #ahhi1 p4 #bb4 ,llandovery4 ,pentamerus grit4 ,conglomerate 7,mulloch ,hill74 ,caradoc4 ,shales1 sandstones1 grits1 etc4 7,ardmillan1 ,balcletchie74 ,thick conglomerate 7,byne ,hill1 ,bennane1 etc474 ,thick fossiliferous limestone 7,stinchar1 ,girvan74 7,on this horizon come the perlitic felsites and soda-felsites of ,winkstone and ,wrae47 ,sandstone 7,orthis confinis7 passing down into thick conglomerate4 ,unconformability4 ,upper ,llandeilo4 ,green mudstones1 grits and greywackes4 ,thin band of dark mudstone with ,upper ,llandeilo graptolites4 ,arenig and ,lower and ,middle ,llandeilo4 ,group of ,radiolarian cherts 7about #gj feet7 with alternating tuffs4 ,tuff or volcanic conglomerate1 with occasional lava-flows4 ,black shale 7#aj feet7 with ,arenig graptolites4 ,volcanic breccias around local centres 7,knockdolian1 etc474 ,thick group of porphyrite and diabase lavas4 ,red flinty mudstones with ,arenig graptolites4 ,porphyrites1 etc4 ,fine tuffs1 etc41 with ,lower ,arenig fossils4 ,diabase lavas1 etc4 7base not seen74 ,it will be noticed from this table that the bottom of the volcanic series is not reached1 so that no estimate can be formed of its full thickness1 nor on what geological platform it begins4 ,possibly its visible portions represent merely the closing scenes of a long volcanic history1 which1 over the area of the south of ,scotland1 extended into ,cambrian time1 like the contemporary series of ,cader ,idris4 ,among the lowest lavas there are interstratified courses of fine tuffs1 flinty shales and thin limestones1 which sometimes fill in the hollows between the pillow-like blocks above referred to4 ,among the characteristic ,lower ,arenig graptolites of these intercalated layers are ,tetragraptus bryonoides1 ,t4 fruticosus1 ,t4 quadribrachiatus1 and ,t4 ,headi together with ,caryocaris ,wrightii4 ,considerable variation is to be seen in the development of the upper part of the volcanic series4 ,in some places the lavas ascend almost to the top2 in others1 thick masses of breccia or agglomerate take their place4 ,these fragmentary materials are locally developed round particular centres1 which probably lie near the sites of active vents whence large quantities of pyroclastic material were discharged4 ,one of the volcanic centres must have been situated close to the position of ,knockdolian ,hill already referred to4 ,the exceedingly coarse breccia of that eminence is rudely stratified in alternations of coarser and finer material1 which was probably to some extent assorted under water around the cinder-cone that discharged it4 ,the date of the explosions of this hill has been ascertained by ,mr4 ,peach from the intercalation of black shales containing ,arenig graptolites among the breccias4 ,another vent lay somewhere in the immediate neighbourhood of the ,mains ,hill agglomerate1 if not actually on part of the site of that rock4 ,though probably not more than a mile from the ,knockdolian volcano1 and belonging to the same epoch of eruption1 this vent1 to judge from the peculiarities of its ejected material1 must have been quite distinct in its source4 ,a third vent lay somewhere in the immediate vicinity of ,bennane ,head1 and threw out the extraordinary masses of agglomerate and the sheets of lava seen on the coast at that locality4 ,a fourth may be traced by its separate group of fine tuffs on the coast three miles south of ,ballantrae4 ,illustration3 ,fig4 #ec4--,section across the ,lower ,silurian volcanic series in the south of ,ayrshire 7,b4 ,n4 ,peach74 ,b1 ,interstratified lavas in ,arenig group2 t1 tuffs2 r1 radolarian cherts2 b#b1 ,llandeilo group2 b#c ,caradoc group4 1 ,serpentine4 ,g1 ,gabbro4 ,a feature of singular interest in the material erupted from these various centres of activity consists in the evidence that the explosions occurred at intervals during the deposition of the ,lower ,silurian formations1 and that these formations were successively disrupted by submarine explosions4 ,mr4 ,peach has found1 for example1 abundant pieces of the peculiar and easily recognized radiolarian cherts imbedded in the volcanic series4 ,that these cherts were deposited contemporaneously with the volcanic eruptions is proved by their intercalation among the breccias4 ,yet among these very breccias lie abundant fragments of chert which must have already solidified before disruption4 ,it is thus evident that this siliceous ooze not only accumulated but set into solid stone on the sea-floor1 between periods of volcanic outburst1 and that such an occurrence took place several times in succession over the same area4 ,these facts derive further interest from the organic origin of the chert4 ,it is now some years since ,mr4 ,peach and his colleagues observed that between the ,glenkiln ,shale with its ,upper ,llandeilo graptolites and the top of the volcanic group in the central part of the ,silurian uplands1 alternations of green1 grey or red shaly mudstones and flinty greywackes are interleaved with fine tuffs1 and are specially marked by the occurrence in them of nodules and bands of black1 grey and reddish chert4 ,this latter substance1 on being submitted to ,dr4 ,hinde1 was found by him to yield twenty-three new species of ,radiolaria belonging to twelve genera1 of which half are new4 ,it thus appears that during the volcanic activity there must have been intervals of such quiescence1 and such slow1 tranquil sedimentation in clear1 perhaps moderately deep water1 that a true radiolarian ooze gathered over the sea-bottom4#age ,footnote #age3 ,ann4 ,mag4 ,nat4 ,hist4 7#ahij71 #fth ser4 vi4 p4 #dj4 ,that the deposition of this ooze probably occupied a prolonged lapse of time seems clearly indicated by the evidence of the fossils that occur below and above the cherts4 ,the graptolites underneath indicate a horizon in the ,middle ,arenig group1 those overlying the cherts are unmistakably ,upper ,llandeilo4 ,thus the great depth of strata which elsewhere constitute the ,upper ,arenig and ,lower and ,middle ,llandeilo subdivisions is here represented by only some #fj or #gj feet of radiolarian cherts4 ,these fine siliceous1 organic sediments probably accumulated with extreme slowness in a sea of some depth and over a part of the sea-floor which lay outside the area of the transport and deposit of the land-derived sediment of the time4#agf ,footnote #agf3 ,annual ,report of the ,geol4 ,surv4 for #ahie1 p4 #bg of reprint4 ,illustration3 ,fig4 #ed4--,section of part of the ,arenig volcanic group1 stream south of ,bennane ,head1 ,ayrshire4 ,as an illustration of some of the characteristic features in the succession of deposits in the volcanic series of the south-west of ,ayrshire1 the accompanying section 7,fig4 #ed7 is inserted4 ,in descending order we come first upon a group of greywackes and grey shattery mudstones 7a71 followed by grey-green and dark banded cherts1 containing ,radiolaria and much plicated4 ,next comes a group of dark-grey1 black and red cherts1 with numerous partings and thin bands of tuff and volcanic conglomerate 7c74 ,the siliceous bands were certainly deposited during the volcanic eruptions1 and they are moulded round the rugose1 slaggy upper surface of the band of lavas 7d7 on which they directly lie4 ,these lavas have the sack-like or pillow structure already described1 and they enclose lumps of chert containing ,radiolaria4 ,a few yards to the west of the line of section bands of nodular tuff are interposed between the top of the lavas and the overlying cherts1 with which also they are interstratified4 ,these tuffs contain blocks of lava six inches or more in diameter4 ,below the belt of lavas come black cherts and shales 7e7 succeeded below by volcanic breccias and tuffs 7f7 alternating with shales in thin inconstant courses4 ,these coarse detrital rocks are thoroughly volcanic in origin1 and they contain fragments of the black cherts which lie still lower in the series4 ,the whole depth of strata represented in this section does not amount to much more than #ajj feet4 ,while in some parts of the ,ayrshire district the coarse breccias that accumulated around their parent vents form most of the upper part of the volcanic series1 in others the lavas are succeeded by fine tuffs which are intercalated among the ordinary sediments1 and show a gradual decline and cessation of volcanic energy4 ,south of ,ballantrae1 for example1 the lavas occupy more than two miles of coast1 in which space they display hardly any intercalations of sedimentary material1 though they show more or less distinctly that they consist of many separate flows4 ,where they at last end1 bands of nodular and fine tuff make their appearance1 together with bands of ashy shale and the characteristic zone of the red radiolarian cherts or flints4 ,above these1 in conformable sequence1 come bands of black shale1 containing abundant ,upper ,llandeilo graptolites1 overlain by greenish or olive-coloured shaly mudstones1 which pass upward into a thick overlying group of greywackes4 ,in this section the alternation of fine pyroclastic with ordinary sediment shows that the volcanic eruptions in the southern part of the ,ballantrae district came to an end by a slowly-lessening series of explosions4 ,the ashy material gradually dies out1 and does not reappear all through the thick group of sandy and muddy sediments which here overlies the volcanic series4 ,we thus learn from the evidence of the ,ayrshire sections that volcanic action was in full vigour in the south-west of ,scotland during the ,arenig period1 but gradually died out before the end of the ,llandeilo period4 ,the rocks in which this volcanic history is chronicled have been very greatly disturbed and plicated1 so that though from their frequently vertical position they might be thought to attain a vast depth1 they very possibly do not exceed #ejj feet in thickness4 ,as the volcanic series is followed north-eastwards it exhibits a gradual diminution in extent and variety1 but this may be at least partly due to the much less depth of it exposed on the crests of the narrow anticlines that bring it to the surface4 ,there is evidence in that region that the eruptions did not everywhere terminate in the ,llandeilo period1 but were in some districts prolonged into the age of the ,bala rocks4 ,thus in the neighbourhood of ,sanquhar volcanic breccias1 tuffs and lavas have been found by ,messrs4 ,peach and ,horne intercalated in strata apparently belonging to the ,bala group4 ,again1 in the district of ,hartfell1 a moderately coarse volcanic agglomerate occurs in the heart of the so-called barren mudstones of the ,hartfell black-shale group1 which1 from its graptolites1 is placed on the horizon of the ,bala rocks4 ,at ,winkstone1 ,hamilton ,hill1 and ,wrae in ,peeblesshire1 perlitic felsites and soda-felsites have been detected by ,messrs4 ,peach and ,horne and determined by ,mr4 ,teall4 ,they are associated with the ,bala limestone1 which in some of its conglomeratic bands contains pebbles of felsite4 ,the intrusive rocks which accompany the ,lower ,silurian volcanic series of the south of ,scotland are best displayed in the south-west of ,ayrshire1 between ,girvan and ,ballantrae1 where they appear to be on the whole later than at least the great mass of the interstratified lavas and tuffs4 ,the most abundant rocks and the earliest to be injected are complex basic masses which include serpentine1 olivine-enstatite rock1 troctolite1 gabbro and other compounds1 all which may be different modifications of the same original basic magma4 ,they do not show a finer texture where they respectively meet1 nor any other symptom of having been subsequently intruded into each other1 though they do exhibit such structures along their lines of contact with the surrounding rocks1 into which they are intrusive4 ,these more basic masses have subsequently been invaded by irregular bosses and dyke-like protrusions1 which1 when small1 are fine-grained dolerites1 but when in larger bodies take the form of gabbro1 sometimes exhibiting a mineral banding and foliated structure4 ,these banded varieties much resemble the banded ,tertiary gabbros of ,skye and some parts of the ,lewisian gneiss4 ,at the ,byne ,hill1 near ,girvan1 a large intrusive boss or ridge displays on its outer margin a fine-grained texture1 where it comes in contact with the serpentine4 ,further inwards it becomes a fine dolerite1 passing into gabbro and increasing in coarseness of grain as well as in acidity of composition1 through stages of what in the field would be called diorite and quartz-diorite1 into a central granitic rock1 whereof milky or blue quartz forms the prominent constituent4 ,the intrusive rocks of this district have generally been injected parallel to the stratification-planes1 and take on the whole the form of sills4 ,some time after the close of the volcanic episode in the ,silurian period of the south of ,scotland1 the rocks were locally subjected to considerable disturbance and elevation1 whereby parts of the volcanic series were exposed to extensive denudation4 ,hence the overlying unconformable ,caradoc conglomerates are in some places largely made up of the detritus of the volcanic rocks4 ,it is interesting to find this evidence of waste during the very next stage of the ,silurian period1 for it affords good evidence that the extensive sheets of intrusive material could not have had any large amount of overlying strata resting upon them at the time of their injection4 ,pieces of these intrusive rocks1 such as the serpentine1 occur abundantly in the ,caradoc conglomerates1 some of which indeed are almost wholly composed of their detritus4 ,probably the total thickness of the overlying cover of rock under which the sills were injected did not amount to as much as #bjj or #cjj feet4 ,yet we see that among the sills were coarse gabbros and granitoid rocks4 ,we may therefore infer that for the injection of such intrusive masses1 great depth and enormous superincumbent pressure are possibly not always necessary4 ,during the progress of the ,geological ,survey along the southern borders of the ,highlands1 a remarkable group of rocks has been observed1 intervening as a narrow interrupted strip between the schistose masses to the north and the great boundary-fault which brings the ,old ,red ,sandstone in vertical strata against them4 ,between ,cortachy in ,forfarshire and ,stonehaven on the east coast1 these rocks have been mapped by ,mr4 ,g4 ,barrow1 who has carefully worked out their relations4 ,they appear again between ,callander and ,loch ,lomond1 where their extent and structure have been mapped by ,mr4 ,c4 ,t4 ,clough4 ,for the purpose of our present inquiry two chief features of interest are presented by these rocks4 ,they include a group of sedimentary strata among which occur bands of jasper or chert containing radiolaria1 and one of their most conspicuous members is a series of volcanic rocks consisting chiefly of dolerites and basalts1 some of which have been much crushed and cleaved1 but in which vesicular structures can still occasionally be recognized4 ,the striking resemblance of both the aqueous and igneous members of this marginal strip of rocks along the ,highland border to the ,arenig cherts and their accompanying lavas in the south of ,scotland1 the remarkable association of the same kinds of material in the same order of sequence1 the occurrence of radiolaria in the siliceous bands in both regions1 furnish strong presumptive evidence that a strip of ,arenig rocks has been wedged in against the ,highland schists4 ,in many respects1 these dull green diabasic lavas of the ,highland border resemble those of the ,ayrshire coast4 ,in particular1 the same peculiar sack-like or pillow-shaped masses are conspicuous in the ,forfarshire ravines4 ,as in ,ayrshire1 igneous materials underlie the cherts which are doubled over and repeated by many successive folds4 ,unfortunately1 it is only a narrow strip of these probably ,arenig lavas that has been preserved1 and no trace has been detected of tuffs1 agglomerates or necks4 ,if1 however1 we may regard the rocks as truly of ,arenig age1 they furnish interesting additional proof of the wide extent of the earliest ,silurian volcanoes4 ,the distance between the last ,arenig volcanic outcrop in the ,southern uplands and the band of similar lavas along the margin of the ,highlands is about #ej miles4 ,if the volcanic ejections were continuous across the intervening tract1 the total area over which the lavas and tuffs of the ,arenig volcanoes were distributed must be increased by at least #fjjj square miles in ,scotland4 ,but it is in the north of ,ireland that this northern extension of what may probably be regarded as an ,arenig series of volcanic rocks attains its greatest development4 ,of this ,irish prolongation a brief account is given in ,chapter xiv41 where the whole of the ,silurian volcanic rocks of the island are discussed4 ,,chapter ,,xiii ,,the ,,eruptions ,,of ,,llandeilo ,,and ,,bala ,,age i4 ,the ,builth ,volcano--ii4 ,the ,volcanoes of ,pembrokeshire--iii4 ,the ,caernarvonshire ,volcanoes of the ,bala ,period--iv4 ,the ,volcanic ,district of the ,berwyn ,hills--v4 ,the ,volcanoes of ,anglesey--vi4 ,the ,volcanoes of the ,lake ,district2 ,arenig to close of ,bala ,period--vii4 ,upper ,silurian 787 volcanoes of ,gloucestershire4 ,the stratigraphical subdivisions of geology are necessarily more or less arbitrary4 ,the sequence in the sedimentary deposits of one region always differs in some degree from that of adjoining regions4 ,in drawing up a table of stratigraphical equivalents for separate countries1 we must be content to accept a general parallelism1 without insisting on too close an identity in either the character of the strata or the grouping of their organic remains4 ,we need especially to guard against the assumption that the limit assigned to a geological formation in any country marks a chronological epoch which will practically agree with that denoted by the limit fixed for the same formation in another country4 ,the desirability of caution in this respect is well shown by the vagueness of the horizons between the several subdivisions of the ,lower ,silurian system4 ,so long as the areas of comparison are near each other1 no great error may perhaps be committed if their stratigraphical equivalents are taken to have been in a broad geological sense contemporary4 ,but in proportion as the element of distance comes in1 there enters with it the element of uncertainty4 ,even within so limited a region as the ,british ,isles1 this difficulty makes itself strongly felt4 ,thus1 in the typical regions of ,wales1 the several subdivisions of the ,lower ,silurian strata are tolerably well marked1 both by lithological nature and by fossils4 ,but as they are followed into other parts of the country1 they assume new features1 sometimes increasing sometimes diminishing in thickness1 changing their sedimentary character1 and altering the association or range of their organisms4 ,the subdivisions into which the geologist groups them may thus be vaguely defined by limits which1 in different parts of the region1 may be far from representing the same periods of time4 ,hence1 in trying to ascertain how far the volcanic eruptions of one area during the ,silurian period may have been contemporary with those of another area1 we must be content to allow a wide margin for error4 ,it is hardly possible to adhere strictly to the stratigraphical arrangement1 for the geological record shows that in the volcanic districts the sedimentary formations by which the chronology might have been worked out are not infrequently absent or obscure4 ,it will be more convenient to treat the rest of the ,lower ,silurian formations as the records of one long and tolerable definite section of geological time1 without attempting in each case to distinguish between the eruptions of the successive included periods1 so long as the actual volcanic sequence is distinctly kept in view4 ,i will therefore take the history of each district in turn and follow its changes from the close of the ,arenig period to the end of ,upper ,silurian time4 ,the stages in the volcanic evolution of each tract will thus be clearly seen4 ,above the ,arenig group with its voluminous volcanic records comes the great group of sediments known as the ,llandeilo formation1 in which also there are proofs of contemporaneous volcanic activity over various parts of the sea-floor within the site of ,britain4 ,we have seen that in the south of ,scotland the eruptions of ,arenig time were probably continued into the period of the ,llandeilo rocks1 or even still later into that of the ,bala group4 ,but it is in ,wales that the history of the ,llandeilo volcanoes is most fully preserved4 ,a series of detached areas of volcanic rocks1 intercalated among the ,llandeilo sediments1 may be followed for nearly #ajj miles1 from the northern end of the ,breidden ,hills in ,montgomeryshire1 by ,shelve1 ,builth1 ,llanwrtyd and ,llangadock1 to the mouth of the ,taf river4 ,but some #ce miles further west another group of lavas and tuffs appears on the coast of ,pembrokeshire1 from ,abereiddy ,bay to beyond ,fishguard4 ,the want of continuity in these scattered outcrops is no doubt partly due to concealment by geological structure4 ,but from the comparative thinness of the volcanic accumulations and their apparent thinning out along the strike it may be inferred that no large ,llandeilo volcano existed in ,wales4 ,there would rather seem to have been a long line of minor vents which in the south-east part of the area appear to have only discharged ashes4 ,certainly1 if we may judge from their visible relics1 these eruptions never rivalled the magnitude of the discharges from the ,arenig volcanoes that preceded1 or the ,bala volcanoes that followed them4 i4 ,,the ,,volcano ,,of ,,builth ,,and ,,its ,,neighbours ,so far as the available evidence goes1 the most important volcanic centre down the eastern side of ,wales during the ,llandeilo period was one which lay not far from the centre of the long line of vents just referred to4 ,its visible remains form an isolated tract of hilly ground1 some seven miles long1 and four or five miles broad1 immediately north from the town of ,builth4 ,this area is almost entirely surrounded by unconformable ,upper ,silurian strata1 so that its total extent is not seen1 and may be much more considerable than the area now laid bare by denudation4 ,the volcanic rocks of ,builth were first described in the ,silurian ,system4 ,murchison clearly recognized that they included some which were evolved from volcanic apertures during the submarine accumulation of the ,lower ,silurian rocks1 and also unbedded volcanic masses which had been intruded subsequently1 dismembering and altering all the strata with which they came in contact4#agg ,these igneous rocks were mapped in some detail by the ,geological ,survey1 and their general relations were expressed in lines of horizontal section4#agh ,they were likewise described by ,ramsay in the ,catalogue of the ,rock-specimens in the ,jermyn ,street ,museum1 specimens of them being displayed in that collection4#agi ,the tuffs and lavas were distinguished1 and likewise the intrusive greenstones4 ,but no attempt was made towards petrographical detail4 ,footnote #agg3 ,silurian ,system1 #ahci1 p4 #ccj4 ,the occurrence of trappean ash with fossils in the ,builth district was noticed by ,de la ,beche1 ,mem4 ,geol4 ,surv4 vol4 i4 7#ahdf71 p4 #ca4 ,footnote #agh3 ,see ,sheet #ef of the one-inch map and ,sheets #e and #f of the ,horizontal ,sections4 ,footnote #agi3 ,catalogue of ,rock ,specimens1 #crd edit4 #ahfb1 p4 #cf et seq4 ,this interesting district has recently been studied by ,mr4 ,henry ,woods1#ahj who has grouped the igneous rocks in probable order of appearance1 as follows3--#ast1 ,andesites2 #bnd1 ,andesitic ash2 #crd1 ,rhyolites2 #dth1 ,diabase-porphyrite2 and #eth1 ,diabase4 ,footnote #ahj3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 l4 7#ahid71 p4 #eff4 ,some of the andesites are described as intrusive in the ,llandeilo strata4 ,the ash in its lower part contains numerous well-rounded pebbles of andesite1 usually five or six inches in diameter1 but sometimes having a length of two feet4 ,it contains fossils 7,orthis calligramma1 ,leptna sericea1 ,serpulites dispar1 etc471 and as it is overlain with shales containing ,ogygia ,buchii1 it may be regarded as probably of ,lower ,llandeilo age4 ,the rhyolites are feebly represented1 and some of them may possibly be intrusive4 ,among them a nodular variety has been noticed1 the nodules being solid throughout1 varying up to two inches in diameter1 and formed of microcrystalline quartz and felspar1 with no trace of any radial or concentric internal arrangement4 ,the diabase-porphyrite1 the most conspicuous rock of the district1 is intrusive in the andesites and ashes1 and occurs in four separate masses or sills4 ,the diabases are all intrusive and of later date than any of the other igneous rocks1 and as they traverse also the ,llandeilo shales1 they are probably considerably later than the previous eruptions4 ,but as they do not enter the surrounding ,llandovery and ,wenlock strata1 they are regarded by ,mr4 ,woods as of intermediate age between the time of the ,llandeilo and that of the ,upper ,silurian formations4 ,about nine miles in a west-south-westerly direction from the southern extremity of the ,builth volcanic area1 another much smaller exposure of igneous rocks has been mapped by the ,geological ,survey at the village of ,llanwrtyd4 ,this tract is only about three miles long and half a mile broad4 ,the volcanic rocks are represented as consisting of three or more bands of felspathic trap interstratified in the ,lower ,silurian strata1 and folded into an anticline along the ridge of ,caer ,cwm4 ,no published line of section runs across this ground1 and the band of rock does not appear to have been described4#aha ,footnote #aha3 ,the locality is referred to by ,de la ,beche1 ,mem4 ,geol4 ,surv4 vol4 i4 p4 #ca1 and by ,ramsay in the ,descriptive ,catalogue of ,rock-specimens in the ,museum of ,practical ,geology1 #crd edit4 p4 #ch1 but no specimens from it are in the collection4 ,seventeen miles to the south-west a still feebler display of intercalated volcanic material occurs in the ,llandeilo formation near the village of ,llangadock4 ,the ,geological ,survey map represents one or more bands of ash associated with limestone1 and thrown into a succession of folds4 ,in the ,horizontal ,section 7,sheet ,,iii4 ,section #c7 a band1 #ajj to #bjj feet thick1 of trappean ash with fossils is shown among the shales1 limestones and grits1 and in the ,catalogue of ,rock-specimens the same rock is referred to as brecciated ash in connection with specimens of it in the ,museum1 which are described as not purely ashy1 but containing many slate-fragments and broken felspar-crystals together with organic remains4#ahb ,footnote #ahb3 ,op4 cit4 p4 #ch4 ,about twenty-four miles still further in the same south-westerly direction1 two patches of ash are shown upon the ,survey map1 near the mouth of the river ,taf4 ,no description of these rocks is given4#ahc ,footnote #ahc3 ,one of the patches was shown by ,j4 ,phillips in ,horizontal ,section1 ,sheet ,,iii4 ,section #f1 as a felspathic trap1 near which the shales are bleached4 ,the map1 however1 was subsequently altered1 so as to make the igneous rocks pyroclastic4 ii4 ,,the ,,volcanoes ,,of ,,pembrokeshire ,in north-western ,pembrokeshire1 the observations of ,murchison1 ,de la ,beche and ,ramsay showed the existence of an important volcanic district1 where numerous igneous bands are interstratified among the ,lower ,silurian rocks1 over an area extending from ,st4 ,david's ,head for thirty miles to the eastward4#ahd ,on the maps of the ,geological ,survey1 lavas1 tuffs1 sills and bosses were discriminated1 but no description of these rocks was published4 ,since the publication of the ,survey map very little has yet been added to our information on the subject4 ,footnote #ahd3 ,see ,silurian ,system1 p4 #dja2 ,sheet #dj of the ,geological ,survey ,map2 ,memoir of ,a4 ,c4 ,ramsay1 p4 #bcb et seq42 ,de la ,beche1 ,trans4 ,geol4 ,soc4 #bnd series1 vol4 ii4 part i4 7#ahbf71 p4 #c4 ,there appear to have been at least three principal groups of vents4 ,one may be indicated by the bands of felspathic trap which have been mapped as extending from near ,st4 ,lawrence for fourteen miles to the east4 ,another must have existed in the neighbourhood of ,fishguard4 ,a third is shown to have lain between ,abereiddy ,bay and ,mathry1 by the abundant bands of lava and tuff and intrusive sills there to be seen4 ,of these areas the only one which has yet been examined and described in some detail is that of ,fishguard1 of which an account has recently been published by ,mr4 ,cowper ,reed4#ahe ,this observer has shown that the eruptions began there during the deposition of the ,lower ,llandeilo rocks1 and continued intermittently into the ,bala period4 ,the earliest consisted of felsites and tuffs intercalated between ,lower ,llandeilo black slates containing ,didymograptus ,murchisoni1 the tuffs themselves being sometimes fossiliferous4 ,a second great volcanic belt1 composed of felsitic lavas1 breccias and tuffs1 lies at the base of the ,upper ,llandeilo strata and shows the maximum of volcanic energy4 ,the breccias are partly coarse agglomerates1 which probably represent1 or lie not far from1 some of the eruptive vents of the time4 ,a higher band of lavas and breccias appears to be referable to the ,bala formation4 ,the whole volcanic series is stated to thin out towards the south-west1 so that the chief focus of eruption probably lay somewhere in the neighbourhood of ,fishguard4 ,footnote #ahe3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 li4 7#ahie71 p4 #adi4 ,the lavas may all be included under the general term felsite4 ,their specific gravity ranges from #bfj to #bgf1 and their silica percentage from #fh to #gb4 ,mr4 ,cowper ,reed observed among them three conspicuous types of structure4 ,some are characterized by a distinct arrangement in fine light and dark bands which rapidly alternate1 and are sometimes thrown into folds and convolutions4 ,a second structure1 observed only at one locality1 consists in the development of pale grey or whitish ovate nodules1 about half an inch in length1 with a clear quartz-grain in their centre1 or else hollow4 ,the third type is shown by the appearance of perlitic structure on the weathered surface4#ahf ,footnote #ahf3 ,mr4 ,cowper ,reed enters into a detailed account of the microscopic structures and chemical composition of these rocks4 ,they have rather a high percentage of alumina1 potash and soda1 and are obviously akin to the keratophyres of other districts4 ,the tuffs and breccias are chiefly developed at the base and top of each volcanic group4 ,some of them contain highly vesicular fragments1 as well as pieces of slate and broken crystals of quartz and felspar4 ,a characteristic feature of this volcanic district is the occurrence in it of sills and irregularly-intruded masses of greenstone4 ,under that name are comprised basalts1 dolerites1 andesitic dolerites with tachylitic modifications1 as well as diabases and gabbros4#ahg ,some of these rocks exhibit a variolitic structure4 ,as regards age1 some of the intrusions appear to have taken place before the tilting1 cleavage and faulting of the strata4 ,they have not been noticed in the surrounding ,upper ,silurian strata1 and we may perhaps infer that here1 as at ,builth1 they are of ,lower ,silurian date4 ,mr4 ,cowper ,reed1 however1 is inclined to regard the large ,strumble ,head masses as later than the tilting and folding of the rocks4#ahh ,footnote #ahg3 ,mr4 ,cowper ,reed1 op4 cit4 p4 #ahj4 ,footnote #ahh3 ,op4 cit4 p4 #aic4 ,a few miles to the south-west of the ,fishguard district1 on the coast of ,abereiddy ,bay1 good sections have been laid bare of the volcanic rocks of this region4 ,dr4 ,hicks has shown that the bands of tuff there displayed are intercalated among the black slates of the ,lower ,llandeilo group1 and that there was probably a renewal of volcanic activity during the deposition of the upper group4#ahi ,but the volcanic history of this area still remains to be properly investigated4 ,footnote #ahi3 ,quart4 ,journ4 ,geol4 ,soc4 xxxi4 7#ahge71 p4 #agg4 ,in southern ,pembrokeshire two conspicuous bands of eruptive rocks have long been known and described4 ,their general characters and distribution were sketched by ,de la ,beche1#aij and further details were afterwards added by ,murchison4#aia ,as traced by the officers of the ,geological ,survey1 they were represented as consisting of greenstone1 syenite and granite4 ,the more northerly band was shown to run in a nearly east and west line from ,lawrenny to the ,stack ,rock1 west of ,talbenny1 a distance of about fourteen miles4 ,the second band1 placed a short way farther south1 stretches in the same general line1 from ,milford ,haven at ,dall ,road into ,skomer ,island1 a distance of about seven miles4 ,footnote #aij3 ,trans4 ,geol4 ,soc41 #bnd ser4 vol4 ii4 7#ahbc71 p4 #f et seq4 ,footnote #aia3 ,silurian ,system1 p4 #dja et seq4 ,the relations of these rocks to the surrounding formations and their geological age have been variously interpreted4 ,de la ,beche regarded the different masses as intrusive1 and probably later than even the adjoining ,coal-measures4#aib ,murchison1 on the other hand1 considered the bedded eruptive rocks of ,skomer ,island to be undoubtedly lavas contemporaneous with the strata among which they are intercalated4#aic ,footnote #aib3 ,mem4 ,geol4 ,survey1 vol4 i4 p4 #bca4 ,footnote #aic3 ,silurian ,system1 p4 #djd4 ,the rocks have been studied petrographically by various observers4 ,mr4 ,rutley gave a full description of the remarkable nodular and banded felsites of ,skomer ,island4#aid ,mr4 ,teall has also noticed these rocks1 likewise a magnificent series of basic lava-flows in the same island1 and a number of porphyrites4 ,the basic lavas seemed to him to contain too much felspar and too little olivine to be regarded as perfectly typical olivine-basalts1 and he found them to lie sometimes in very thin and highly vesicular sheets4 ,the porphyrites he placed on the border-line between basic and intermediate rocks4#aie ,footnote #aid3 ,the ,felsitic ,lavas of ,england and ,wales1 ,mem4 ,geol4 ,survey 7#ahhe71 pp4 #af1 #ah4 ,footnote #aie3 ,british ,petrography1 pp4 #bbd1 #bhd1 #ccf4 ,more recently this southern district of ,pembrokeshire has been examined by ,messrs4 ,f4 ,t4 ,howard and ,e4 ,w4 ,small1 who have obtained further evidence of the interbedded character of the igneous series4 ,below an upper basalt they have noted the occurrence of bands of felsitic conglomerate1 sandstone1 shale and breccia lying upon and obviously derived from a banded spherulitic felsite1 below which comes a lower group of basalts4 ,the age of this interesting alteration of basic and acid eruptions has not been precisely determined1 but is conjectured to be that of the ,bala or ,llandovery rocks4#aif ,footnote #aif3 ,rep4 ,brit4 ,assoc4 #ahic1 p4 #gff2 ,geol4 ,mag4 #ahif1 p4 #dha4 iii4 ,,the ,,caernarvonshire ,,volcanoes ,,of ,,the ,,bala ,,period ,owing to the effects partly of plication and partly of denudation1 the rocks of the next volcanic episode in ,wales1 that of the ,bala period1 occupy a less compact and defined area than those of the ,arenig group in ,merionethshire4 ,from the latter they are separated1 as we have seen1 by a considerable depth of strata1#aig whence we may infer1 with the ,geological ,survey1 that the eruptions of ,arenig1 the ,arans and ,cader ,idris were succeeded by a long period of repose1 the ,llandeilo outbreaks described in the foregoing pages not having extended apparently into ,north ,wales4 ,when the next outbreaks took place1 the vents are found to have shifted northwards into ,caernarvonshire1 where they fixed themselves along a line not much to the east of where the ,cambrian porphyries and tuffs now appear at the surface4 ,the lavas and ashes that were thrown out from these vents form the highest and most picturesque mountains of ,north ,wales1 culminating in the noble cone of ,snowdon4 ,they stretch northwards to ,diganwy1 beyond ,conway1 and southwards1 at least as far as the neighbourhood of ,criccieth4 ,they die out north-eastwards beyond ,bala ,lake1 and there can be but little doubt that they thin out also eastwards under the ,upper ,bala rocks4 ,the lavas and tuffs that rise up on a similar horizon among the ,bala rocks of the ,berwyn ,hills evidently came not from the ,snowdonian vents1 but from another minor volcanic centre some miles to the east1 while still more remote lay the vents of the ,breidden ,hills and the sheets of andesitic tuff that probably spread from them over the ground east of ,chirbury 7,map ,,ii474 ,footnote #aig3 ,estimated at from #fjjj to #gjjj feet1 ,mem4 ,geol4 ,surv4 vol4 iii4 #bnd edit4 p4 #aca4 ,the ,caernarvonshire volcanic group extends from north to south for fully thirty miles1 with an extreme breadth of about fifteen miles2 while1 if we include the rocks of the ,lleyn peninsula1 the area will be prolonged some twenty miles farther to the south-west4 ,the general stratigraphical horizon of this volcanic group has been well determined by the careful mapping of ,ramsay1 ,selwyn and ,jukes on the maps of the ,geological ,survey4 ,these observers brought forward ample evidence to show that the lavas and tuffs were erupted during the deposition of the ,bala strata of the ,lower ,silurian series1 that the ,bala ,limestone is in places full of ashy material1 and that this well-marked fossiliferous band passes laterally into stratified volcanic tuffs containing the same species of fossils4#aih ,but the progress of stratigraphical geology1 and the increasing value found to attach to organic remains as marking even minor stratigraphical horizons1 give us reason to believe that a renewed and still more detailed study of the ,bala rocks of ,north ,wales would probably furnish data for more precisely defining the platforms of successive eruptions1 and would thus fill in the details of the broad sketch which ,sir ,andrew ,ramsay and his associates so admirably traced4 ,besides the ,bala ,limestone there may be other lithological horizons which1 like the ,garth grit and the pisolitic iron-ore of the ,arenig group1 might be capable of being followed among the cwms and crests as well as the opener valleys of ,caernarvonshire4 ,until some such detailed mapping is accomplished1 we cannot safely advance much beyond the point where the stratigraphy was left by the ,survey4 ,footnote #aih3 ,mem4 ,geol4 ,surv4 vol4 iii4 #bnd edit4 pp4 #abf1 #abh1 #aca1 #aci1 etc4 ,from the ,survey maps and sections it is not difficult to follow the general volcanic succession1 and to perceive that the erupted materials must altogether be several thousand feet in thickness from the lowest lavas in the north to the highest on the crest of ,snowdon4 ,in that mountain the total mass of volcanic material is set down as #cajj feet4 ,but this includes only the higher part of the whole volcanic group4 ,below it come the lavas of ,y ,glyder-,fach1 which1 according to the ,survey measurements1 are about #aejj feet thick1 while still lower lie the ancient coules of ,carnedd ,dafydd and those that run north from the vent of ,y-foel-frs1 which must reach a united thickness of many hundred feet4 ,we can thus hardly put the total depth of volcanic material at a maximum of less than #fjjj to #hjjj feet4 ,the pile is1 of course1 thickest round the vents of discharge1 so that no measurement1 however carefully made at one locality1 would be found to hold good for more than a short distance4 ,though little is said in the ,survey ,memoir of the vents from which this vast amount of volcanic material was erupted1 the probable positions of a number of these orifices may be inferred from the maps4 ,from the shore west of ,conway a series of remarkable eminences may be traced south-westwards for a distance of nearly forty miles into the peninsula of ,lleyn4 ,at the northern extremity of this line stands the prominent boss of ,penmaen-mawr1 while southward beyond the large mass of ,y-foel-frs1 with the smaller knobs west of ,nant ,francon1 and the great dome of ,mynydd-mawr1 the eye ranges as far as the striking group of puy-like cones that rise from the sea around ,yr ,eifl and ,nevin4 ,some of these hills1 particularly ,y-foel-frs1 were recognized by the ,survey as vents4#aii ,but the first connected account of them and of their probable relation to the volcanic district in which they occur has been given by ,mr4 ,harker in his exceedingly able essay on ,the ,bala ,volcanic ,series of ,caernarvonshire1#bjj--the most important contribution to the volcanic history of ,wales which has been made since the publications of the ,geological ,survey appeared4 ,i shall refer to these vents more specially in the sequel4 ,i allude to them here for the purpose of showing at the outset the marvellous completeness of the volcanic records of ,caernarvonshire4 ,so great has been the denudation of the region that the pile of lavas and tuffs which accumulated immediately around and above these orifices has been swept away4 ,no trace of any portion of that pile has survived to the west of the line of bosses2 while to the east1 owing to curvature and subsequent denudation1 the rocks have been dissected from top to bottom1 until almost every phase of the volcanic activity is revealed4 ,footnote #aii3 ,op4 cit4 pp4 #acg1 #bbj4 ,footnote #bjj3 ,this was the ,sedgwick ,prize ,essay for #ahhh1 and was published in #ahhi4 ,the volcanic products discharged from these vents consist of a succession of lava-streams separated by bands of slate1 tuff1 conglomerate and breccia4 ,these fragmental intercalations1 which vary from a few yards to many hundred feet in thickness1 are important not only as marking pauses in the emission of lava or in the activity of the volcanoes1 but as affording a means of tracing the several lavas to their respective vents4 ,essentially1 however1 the volcanic materials consist of lava-flows1 the intercalations of fragmentary materials1 though numerous1 being comparatively thin4 ,the thickest accumulation of tuffs is that forming much of the upper part of ,snowdon4 ,it is set down by my predecessor at #abjj feet in thickness1 but ,i should be inclined to reduce this estimate4 ,i shall have occasion to show that the summit and upper shoulders of ,snowdon are capped with andesites interstratified among the tuffs4 ,sir ,andrew ,ramsay has referred with justice to the difficulty of always discriminating in the field between the fine tuffs and some of the lavas4#bja ,yet ,i am compelled to admit that1 if the ground were to be re-mapped now1 the area represented as covered by fragmental rocks would be considerably restricted4 ,mr4 ,harker is undoubtedly correct when he remarks that1 taken as a whole1 the ,bala volcanic series of ,caernarvonshire is rather remarkable for the paucity of genuine ashes and agglomerates4#bjb ,footnote #bja3 ,op4 cit4 p4 #adh4 ,footnote #bjb3 ,bala ,volcanic ,rocks1 p4 #be4 ,the lavas of the ,bala volcanic group1 like those of the ,arenig series1 were mapped by the ,survey as porphyries1 felstones1 or felspathic traps4 ,they were shown to be acid-lavas1 having often a well-developed flow-structure comparable with that of obsidian and pitchstone1 and to consist of successive sheets that were poured out over the sea-floor4 ,their petrography has subsequently been studied more in detail by many observers1 among whom ,i need only cite ,professor ,bonney1 ,professor ,cole1 ,mr4 ,rutley1 ,mr4 ,teall1 and ,miss ,raisin2 the most important recent additions to our knowledge of this subject having been made by ,mr4 ,harker in the ,essay to which ,i have just referred4 ,the great majority of these lavas are thoroughly acid rocks1 and present close analogies of composition and structure to modern rhyolites1 though ,i prefer to retain for them the old name of felsites4 ,their silica-percentage ranges from #ge to more than #hj4 ,to the naked eye they are externally pale greyish1 or even white1 but when broken into below the thick decomposed and decoloured crust1 they are bluish-grey to dark iron-grey1 or even black4 ,they break with a splintery or almost conchoidal fracture1 and show on a fresh surface an exceedingly fine-grained1 tolerably uniform texture1 with minute scattered felspars4 ,one of their most striking features is the frequency and remarkable development of their flow-structure4 ,not merely as a microscopic character1 but on such a scale as to be visible at a little distance on the face of a cliff or crag1 this structure may be followed for some way along the crops of particular flows4 ,the darker and lighter bands of devitrification1 with their lenticular forms1 rude parallelism and twisted curvature1 have been compared to the structure of mica-schist and gneiss4 ,one aspect of this structure1 however1 appears to have escaped observation1 or1 at least1 has attracted less notice than it seems to me to deserve4 ,in many cases it is not difficult to detect1 from the manner in which the lenticles and strips of the flow-structure have been curled over and pushed onward1 what was the direction in which the lava was moving while still a viscous mass4 ,by making a sufficient number of observations of this direction1 it might in some places be possible to ascertain the quarter from which the several flows proceeded4 ,as an illustration1 ,i would refer to one of the basement-felsites of ,snowdon1 which forms a line of picturesque crags on the slope facing ,llanberis4 ,the layers of variously-devitrified matter curl and fold over each other1 and have been rolled into balls1 or have been broken up and enclosed one within the other 7,fig4 #ee74 ,the general push indicated by them points to a movement from the westward4 ,turning round from the crags1 and looking towards the west1 we see before us on the other side of the deep vale of ,llyn ,cwellyn1 at a distance of little more than three miles1 the great dome-shaped ,mynydd-mawr1 which1 there is every reason to believe1 marks one of the orifices of eruption4 ,it might in this way be practicable to obtain information regarding even some of the vents that still lie deeply buried under volcanic or sedimentary rocks4 ,that these felsites were poured forth in a glassy condition may be inferred from the occurrence of the minute perlitic and spherulitic forms so characteristic of the devitrification of once vitreous rocks4 ,mr4 ,rutley was the first who called attention to this interesting proof of the close resemblance between ,palozoic felsites and modern obsidians1 and other observers have since confirmed and extended his observations4#bjc ,footnote #bjc3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xxxv4 7#ahgi71 p4 #ejh4 ,illustration3 ,fig4 #ee4--;,flow-structure in the lowest felsite on the track from ,llanberis to the top of ,snowdon4 ,length about #d feet1 height #b feet4 ,another remarkable aspect of the felsites is that nodular structure so often to be seen among them1 and regarding the origin of which so much has already been written4 ,i agree with ,professor ,cole and ,mr4 ,harker in looking upon the nodules as derived from original spherulites by a process of alteration1 of which almost every successive stage may be traced until the original substance of the rock has been converted into a flinty or agate-like material4 ,if this be the true explanation of the structure1 some of the original lavas must have exhibited perlitic and spherulitic forms on a gigantic scale4 ,there can1 ,i think1 be little doubt that this peculiar structure was very generally misunderstood by the earlier observers1 who naturally looked upon it as of clastic origin1 and who therefore believed that large beds of rock consisted of volcanic conglomerate1 which we should now map as nodular felsite 7pyromeride74#bjd ,footnote #bjd3 ,another source of error may probably be traced in the occasional brecciated structure of the felsites1 which has been mistaken for true volcanic breccia1 but which can be traced disappearing into the solid rock4 ,sometimes this structure has resulted from the breaking up of the lenticles of flow1 sometimes from later crushing4 ,while by far the larger proportion of the ,caernarvonshire lavas consists of thoroughly acid rocks1 the oldest outflows are much less acid than those erupted at the height of the volcanic activity1 when the rocks of ,snowdon were poured forth4#bje ,but towards the close of the period there was apparently a falling off in the acidity of the magma1 for at the top of the group the andesitic lavas to which ,i have already alluded are encountered4 ,sir ,andrew ,ramsay has shown the existence of an upper felstone or felspathic porphyry1 almost entirely removed by denudation1 but of which outliers occur on ,crib-goch1 ,lliwedd1 and other crests around ,snowdon1 and likewise on ,moel ,hebog4#bjf ,mr4 ,harker alludes to these remnants1 and speaks of them as less acid than the older lavas1 but he gives no details as to their structure and composition4#bjg ,in an examination of ,snowdon ,i was surprised to find that the summit of the mountain1 instead of consisting of bedded ashes as hitherto represented1 is formed of a group of lava-sheets having a total thickness of perhaps from #ajj to #aej feet 7#f in ,fig4 #ef74 ,the apex of ,yr ,wyddfa1 the peak of ,snowdon1 consists of fossiliferous shale lying on a dull grey rock that weathers with elongated vesicles1 somewhat like a cleaved amygdaloid1 but a good deal decomposed4 ,a thin slice of this latter rock shows under the microscope irregular grains and microlites of felspar1 with a few grams of quartz1 the whole much sheared and calcified4 ,below this bed comes a felsite1 or devitrified obsidian1 showing in places good spherulitic structure1 and followed by a grey amygdaloid4 ,the latter is a markedly cellular rock1 and1 though rather decayed1 shows under the microscope a microlitic felspathic groundmass1 through which granules of magnetite are dispersed4 ,footnote #bje3 ,mr4 ,harker1 op4 cit4 p4 #abg4 ,footnote #bjf3 ,mem4 ,geol4 ,surv4 vol4 iii4 #bnd edit4 pp4 #ada1 #add1 #ade1 #adg1 #afa4 ,footnote #bjg3 ,bala ,volcanic ,series1 pp4 #aj1 #bc1 #abe4 ,he refers also to lavas occupying a similar position at ,nant ,gwynant and ,moel ,hebog2 but he adds that he had not had an opportunity of studying them4 ,illustration3 ,fig4 #ef4--,section of ,snowdon4#bjh #a4 ,grits and slates2 #b4 ,felsite with good flow-structure2 #c4 ,volcanic tuffs2 #d4 ,felsite2 #e4 ,tuffs with sheets of felsite and andesite2 #f4 ,group of andesitic lavas on summit of ,snowdon2 #g4 ,intrusive greenstones4 ,footnote #bjh3 ,after the ,geological ,survey ,section 7,horizont4 ,sect4 ,sheet #bh71 slightly modified4 ,underneath this upper group of lavas lie the tuffs for which ,snowdon has been so long celebrated4 ,but1 as ,i have already stated1 there does not appear to me to be such a continuous thickness of fragmental material as has been supposed4 ,there cannot1 ,i think1 be any doubt that not only at the top1 but at many horizons throughout this supposed thick accumulation of tuff1 some of the beds of rock are really lava-flows4 ,some of these lavas have suffered considerably from the cleavage which has affected the whole of the rocks of the mountain1 while the results of centuries of atmospheric disintegration1 so active in that high exposed locality1 have still further contributed to alter them4 ,they consequently present on their weathered faces a resemblance to the pyroclastic rocks among which they lie4 ,where1 however1 the lavas are thicker and more massive1 and have resisted cleavage better1 some of them appear as cellular dull grey andesites or trachytes1 while a few are felsites4 ,many instructive sections of such bands among the true tuffs may be seen on the eastern precipices of ,snowdon above ,glas-lyn4 ,it thus appears that the latest lavas which flowed from the ,snowdonian vent were1 on the whole1 decidedly more basic than the main body of felsites that immediately preceded them4 ,they occur also in thinner sheets1 and are far more abundantly accompanied with ashes4 ,at the same time it is deserving of special notice that among these less acid outflows there are intercalated sheets of felsite1 and that some of these still retain the spherulitic structure formed by the devitrification of an original volcanic glass4 ,far to the south-west1 in the promontory of ,lleyn1 another group of volcanic rocks exists which may have been in a general sense contemporaneous with those of the ,snowdon region1 but which were certainly erupted from independent vents4 ,mr4 ,harker has described them as quartzless pyroxene-andesites1 sometimes markedly cellular1 and though their geological relations are rather obscure1 he regards them as lava-flows interbedded among strata of ,bala age and occurring below the chief rhyolites of the district4 ,if this be their true position1 they indicate the outflow of much less highly siliceous lavas before the eruption of the acid felsites4 ,in the ,snowdon area any such intermediate rocks which may have been poured out before the time of the felsitic outflows have been buried under these4 ,the tuffs of the ,bala series in ,caernarvonshire have not received the same attention as the lavas4 ,one of the first results of a more careful study of them will probably be a modification of the published maps by a reduction of the area over which these rocks have been represented4 ,they range from coarse volcanic breccias to exceedingly fine compacted volcanic dust1 which cannot easily be distinguished1 either in the field or under the microscope1 from the finer crushed forms of felsite4 ,among the oldest tuffs pieces of dark blue shale as well as of felsite may be recognized1 pointing to the explosions by which the vents were drilled through the older ,silurian sediments already deposited and consolidated4 ,sometimes1 indeed1 they recall the dark slate-tuffs of ,cader ,idris1 like which they are plentifully sprinkled with kaolinized felspar crystals4 ,the beds of volcanic breccia intercalated between the lower felsites of ,snowdon include magnificent examples of the accumulation of coarse volcanic detritus4 ,the blocks of various felsites in them are often a yard or more in diameter4 ,among the felsite fragments smaller scattered pieces of andesitic rocks may be found4 ,this mixture of more basic materials appears to increase upwards1 the highest ashes containing detritus of andesitic lavas like those which occur among them as flows4 ,the tuffs in the upper part of ,snowdon are well-bedded deposits made up partly of volcanic detritus and partly of ordinary muddy sediment4#bji ,layers of blue shale or slate interstratified among them indicate that the enfeebled volcanic activity marked by the fine tuffs passed occasionally into a state of quiescence4 ,as is well known1 numerous fossils characteristic of the ,bala rocks occur in these tuffs4 ,the volcanic discharges are thus proved to have been submarine and to have occurred during ,bala time4 ,footnote #bji3 ,see the interesting account of these tuffs given by ,sir ,a4 ,ramsay1 ,mem4 ,geol4 ,survey1 vol4 iii4 #bnd edit4 p4 #adb4 ,i have already alluded to some of the probable vents from which the lavas and tuffs were discharged1 and to their position along a line drawn from ,penmaen-mawr into the peninsula of ,lleyn4 ,it will be observed that they lie outside the area of the bedded volcanic rocks and rise through parts of the ,silurian system older than these rocks4 ,the largest and most important of them is unquestionably that formed by ,y-foel-frs and its neighbouring heights4 ,as mapped by the ,geological ,survey1 this mass of igneous rock is irregularly elliptical1 measures about six square miles in area1 and consists mainly of intrusive felstone-porphyry passing into hornblendic greenstone4#baj ,mr4 ,harker1 however1 has made an important correction of this petrography1 by showing that a large part of the area consists of augitic granophyre1 while the so-called greenstone is partly diabase and partly andesitic ashes and agglomerates4 ,he suggests that an older vent has here been destroyed by a later and larger protrusion of igneous matter4#baa ,this high and somewhat inaccessible tract of ground is still in need of detailed mapping and closer study1 for undoubtedly it is the most important volcanic vent now visible in ,north ,wales4 ,my former colleague in the ,geological ,survey1 ,mr4 ,e4 ,greenly1 spent a week upon it some years ago1 and kindly supplied me with the following notes of his observations3--,the central and largest area of the neck is mainly occupied with diabases and andesites1 while the ashes and agglomerates1 which are intimately connected with them1 seem to run as a belt or ring round them1 and to occur in one or more patches in the midst of them4 ,portions of green amygdaloid run through the pyroclastic masses4 ,outside the ring of agglomerate and ashes an interrupted border of felsite can be traced1 which may be presumed to be older than they1 for a block of it was observed in them4 ,the granophyre1 on the other hand1 which is interposed between the fragmental masses and the surrounding rocks on the western wall of the vent1 seems to be of later date4 ,dykes or small bosses of diabase1 like the material of the sills1 pierce both the agglomerates and the rocks of the centre4#bab ,footnote #baj3 ,mem4 ,geol4 ,survey1 vol4 iii4 #bnd edit4 pp4 #acg1 #aci4 ,footnote #baa3 ,bala ,volcanic ,series1 pp4 #da1 #ga1 #gb1 #abc4 ,footnote #bab3 ,mr4 ,greenly has made a sketch map of this interesting locality4 ,as he has now established his home in ,north ,wales1 ,i trust he may find an opportunity of returning to ,y-foel-frs and completing his investigations4 ,no agglomerate appears to have been noticed by any observer among the other supposed vents along the line that runs south-westwards from ,penmaen-mawr1 to the promontory of ,lleyn4 ,these bosses are rudely circular in ground-plan and rise vertically out of the ,lower ,silurian or ,cambrian strata1 or partake more of the nature of lenticular sheets or laccolites which have been thrust between the planes of bedding4 ,there is usually an observable alteration of the surrounding rocks along the line of contact4 ,the material of these bosses is sometimes thoroughly acid1 as is the granophyre of ,y-foel-frs1 the microgranite of ,mynydd-mawr with its riebeckite crystals1 the augite-granite-porphyry of ,clynog-fawr1 and the granophyric and rhyolitic quartz-porphyries of the ,rivals4 ,in other cases the rock is of an intermediate grade1 as in the enstatite-diorite of ,penmaen-mawr1 the pyroxene-andesite of ,carn ,boduan1 and the quartz-augite-syenite of ,llanfoglen4#bac ,a few bosses of still more basic material occur in the ,sarn district1 including hornblende-diabase and hornblende-picrite4 ,sometimes both the acid and the more basic rocks are found in the same boss1 as in the large mass of ,y-foel-frs4 ,footnote #bac3 ,the geological relations and petrographical characters of these various rocks are given by ,mr4 ,harker in the fourth and fifth sections of his ,essay4 ,it must be confessed that there is no absolute proof that any of these masses mark the actual sites of eruptive vents1 except probably the boss of ,y-foel-frs4 ,some of them may have been intruded without establishing any outlet to the surface4#bad ,but that a few of them really represent orifices from which the ,bala volcanic group was erupted may be plausibly inferred from their neck-like form1 from their positions with reference to the volcanic district1 from the obvious thickening of the lavas and tuffs in the direction of these bosses1 and from the petrographical relation that exists between their component materials and rocks that were discharged at the surface4 ,this last-named feature has been well pointed out by ,mr4 ,harker1 who has established1 by a study of microscopic slides1 a gradation from the granophyric material of the bosses into structures greatly resembling those of the bedded felsites1 and likewise a close similarity between the intermediate rocks of the other bosses and the andesites which have elsewhere been poured out at the surface4#bae ,but perhaps the most impressive evidence as to the sites of the chief centres of eruption is supplied by the lavas and tuffs themselves as they thicken in certain directions and thin away in others4 ,this feature of their distribution has been well expressed in the maps and sections of the ,survey1 and has been clearly summarized by ,mr4 ,harker4#baf ,the oldest lavas now visible lie at the northern end of the district1 and the vents from which they proceeded may1 with considerable probability1 be placed somewhere in the tract which includes the chain of bosses of ,penmaen-mawr1 ,y-foel-frs1 and ,y ,drosgl4 ,the chief centre of eruption no doubt lay somewhere in the ,snowdon tract1 where the lavas and tuffs attain their greatest thickness1 and whence they thin away in all directions4 ,the ,mynydd-mawr boss may be presumed to have been one of the main vents4 ,but there were not improbably others1 now concealed under the deep cover of their own ejections4 ,footnote #bad3 ,mr4 ,harker speaks of some of them as laccolites4 ,footnote #bae3 ,op4 cit4 pp4 #eg1 #gb4 ,footnote #baf3 ,see especially pp4 #i1 #abj et seq41 and fig4 #f of his ,essay4 ,more diligent search1 with a special eye to the discovery of such vents1 might indeed be rewarded1 even in the midst of the volcanic district itself4 ,to the north-east of ,capel ,curig1 for example1 there is a prominent knob of agglomerate1#bag which ,i visited with ,mr4 ,b4 ,n4 ,peach1 and which we regarded as probably marking one of the minor vents4 ,the material of this eminence has a base which by itself would probably be regarded by the field-geologist as a felsite4 ,but through this compact matrix are dispersed abundant stones of all sizes up to six inches or more in diameter4 ,they are mostly subangular or somewhat rounded-off at the edges1 and generally markedly cellular4 ,among them may be observed pieces of trachyte1 felsite1 and a rock that is probably a devitrified pitchstone or obsidian4 ,the vesicles in these stones are sometimes lined with an acicular zeolite4 ,traces of rude bedding can be detected1 dipping at high angles4 ,on the north-east side of the hill finer agglomerate is seen to alternate with ashy grits and grey shales1 which1 dipping ,e4,n4,e4 at #bj-cj1 pass under a group of felsites1 one at least of which retains a very fine perlitic structure and evidently flowed as a true glass4 ,some of these lavas are full of enclosed pieces of various flinty cellular and porphyritic felsites and andesites or trachytes1 like the stones which occur abundantly in the agglomerate4 ,the connection of these bedded lavas and tuffs with the agglomerate-neck seems obvious4 ,footnote #bag3 ,this rock is referred to in the ,geological ,survey ,memoir as a short thick band of conglomeratic ash1 which strikes northwards about half a mile and then disappears 7p4 #acd74 ,the ,caernarvonshire volcanic area furnishes another admirable example of the intrusion of basic sills as a final phase of eruptivity4 ,these masses have been carefully separated out on the maps of the ,geological ,survey1 which present a striking picture of their distribution and their relation to the other igneous rocks4 ,an examination of the maps shows at once that the basic sheets tend to lie parallel with the bedding along certain horizons4 ,in the southern and western portions of the area they have forced themselves among the ,lower ,silurian sedimentary strata that underlie the ,bala volcanic group--a position analogous to that taken by the corresponding sills of the ,arenig series4 ,but they likewise invade the volcanic group itself4 ,along the eastern borders of the district they abound1 especially in the higher parts of the volcanic pile1 where they have been injected between the flows1 and have subsequently participated in the abundant plication of the rocks between the mountains and the line of the ,river ,conway4 ,the curvatures into which the rocks of the region have been thrown1 and the consequent breadth of country over which the volcanic sheets can now be examined1 furnish a much better field than ,merionethshire for the attempt to trace the probable centre or centres from which the basic magma of the sills was protruded4 ,a study of the ,survey maps soon leads to a conviction that the intrusions were not connected1 except perhaps to a trifling extent1 with the great line of western vents4 ,it is remarkable that the older strata which emerge from under the volcanic group on its western outcrop are1 on the whole1 singularly free from sills1 though some conspicuous examples are shown opposite to ,mynydd-mawr1 while a few more occur further north along the same line4 ,their lenticular forms1 their short outcrops1 and their appearance on different horizons at widely separated points seem to indicate that the sills probably proceeded from many distinct subterranean pipes4 ,their greater abundance along the eastern part of the district may be taken to indicate that the ducts lay for the most part considerably to the eastward of the line of western vents4 ,they may have risen in minor funnels1 like that of ,capel ,curig4 ,it is noteworthy that so abundant an extravasation of basic material should have taken place without the formation of numerous dykes4 ,we have here a repetition of the phenomena that distinguished the preceding ,arenig volcanic period in ,merionethshire1 and it will be remembered that the ,llandeilo eruptions of ,builth were likewise followed by the injection of large bodies of basic rock4 ,as an enormous amount of igneous magma may thus be impelled into the ,earth's crust without the formation of dykes1 it is evident that the conditions for the production of sills must be in some important respects different from those required for dykes4 ,no evidence has yet been obtained that any one of these sills established a connection with the surface4 ,not a trace can be found of the outpouring of any such basic lava-streams1 nor have fragments of such materials been met with in any of the tuffs4 ,on the other hand1 there is abundant proof of the usual contact-metamorphism4 ,though the sills conform on the whole to the bedding1 they frequently break across it4 ,they swell into thick irregular masses1 and thin out rapidly4 ,in short1 they behave as true intrusive sheets1 and not as bedded lavas4 ,in regard to their internal character1 they show the customary uniformity of texture throughout each mass4 ,they are mapped under the general name of greenstones by the ,geological ,survey1 and are described in the ,memoir as hornblendic4#bah ,the more precise modern methods of examination1 however1 prove them to be true diabases1 in which the felspar has1 as a rule1 consolidated before the augite1 giving as a result the various types of diabasic structure4#bai ,footnote #bah3 ,op4 cit4 p4 #aef4 ,footnote #bai3 ,mr4 ,harker1 ,bala ,volcanic ,series1 p4 #hc4 ,the date of the intrusion of these basic sills can be fixed by the same process of reasoning as was applied to those of the ,arenig volcanic group4 ,their connection with the other igneous rocks of ,caernarvonshire is so obvious that they must be included as part of the volcanic history of the ,bala period4 ,but they clearly belong to a late stage1 perhaps the very latest stage1 of that history4 ,they probably could not have been injected into their present positions1 unless a considerable mass of rocky material had overlain them4 ,some of them are certainly younger than the tuffs of ,snowdon and ,moel ,hebog1 which belong to a late part of the volcanic period4 ,on the other hand1 they had been intruded before the curvature and compression of the region1 for they share in the foldings and cleavage of the rocks among which they lie4 ,the terrestrial movements that produced this disturbance have been proved to have occurred after the time when the uppermost ,bala rocks were deposited1 and before that of the accumulation of the ,upper ,silurian formations4#bbj ,the epoch of intrusion is thus narrowed down to some part of the ,upper ,bala period4 ,with this subterranean manifestation1 volcanic action in this part of the country finally died out4 ,footnote #bbj3 ,mem4 ,geol4 ,sur4 vol4 iii4 #bnd edit4 p4 #cbf4 ,see also ,mr4 ,harker's ,bala ,volcanic ,series1 p4 #gf4 iv4 ,,the ,,volcanic ,,centre ,,of ,,the ,,berwyn ,,hills ,among the thick group of sedimentary formations which overlies the great volcanic ridge of the ,arans and ,arenig1 and undulates eastwards across the ,bala ,valley1 occasional thin intercalations of tuff point to the existence of another centre of volcanic activity which lay somewhere in the region of the ,berwyn ,hills4 ,the structure of this ground1 first indicated by ,sedgwick1 was investigated in detail by ,j4 ,b4 ,jukes and his colleagues1 whose work was embodied in the ,maps1 ,sections and ,memoirs of the ,geological ,survey4#bba ,the distinguishing characteristics of the volcanic rocks of this district are the occurrence of both lavas and tuffs as comparatively thin solitary bands in the midst of the ordinary sediments1 and the persistence of these bands for a distance of sometimes more than #bd miles4 ,the position of the vent or vents from which this extensive outpouring of volcanic material took place has not been revealed4 ,as the bands tend to thin away eastwards1 it may be surmised that the chief focus of eruption lay rather towards the west1 perhaps under the trough of ,upper ,silurian strata somewhere in the neighbourhood of ,llandderfel4 ,there was probably another in the ,hirnant district4 ,footnote #bba3 ,see ,sheet #gd of the one-inch map2 ,sheets #cb1 #ce1 #cg and #ch of the ,horizontal ,sections2 and chapter xxxi4 of the ,memoir on the ,geology of ,north ,wales4 ,the mapping of the officers of the ,survey showed that in the ,berwyn ,hills there are representatives of both the great volcanic periods of ,north ,wales4 ,a lower series of felstones and greenstones probably belongs to the older period1 which began towards the end of ,cambrian time and lasted in some districts even into the time of the ,llandeilo formation4 ,an upper group of tuffs1 lying among the ,bala rocks1 is evidently equivalent1 on the whole1 to the much thicker volcanic series of the ,snowdon region4 ,the lowest visible volcanic rocks occur among the hills to the north-west of ,llanrhaiadr yn ,mochnant4 ,they are described as consisting of felstone of a pale greenish-grey colour and compact texture1 like those of ,arenig1 and ashes distinctly interstratified with the slates4 ,no exact petrographical examination of these rocks has yet been made4 ,from the account given in the ,survey ,memoir there appears to be here a group of lavas and tuffs intercalated in ,llandeilo perhaps partly in ,upper ,arenig1 strata which form the broken dome of the ,berwyn anticline4 ,the lavas are represented as lying on four or five platforms1 a single band reaching a thickness of #cjj feet and separated from the next band by sometimes #ajjj or #aejj feet of non-volcanic sediment4 ,these lower lavas1 according to the measurements of ,jukes1 are overlain by more than #djjj feet of sedimentary strata before the upper or ,bala volcanic series is reached4 ,three successive ash-beds constitute this upper series4 ,of these the lowest band1 about #ej or #fj feet thick1 was named a greenstone ash in contradistinction to a felstone ash1 and was not traceable for more than a short distance4 ,above it1 after an intervening thickness of several hundred feet of sedimentary strata1 comes a second and much more continuous band of tuff1 known as the ,lower ash-bed1 about #ajj feet thick on the west front of the ,berwyn range4 ,still higher1 after an interval of about #aejj feet of slates1 lies the ,upper ash-bed1 which on the same line of section has a thickness of about #bjj feet4 ,this is the most persistent of all the volcanic horizons1 for it can be followed continuously round the whole range of the ,berwyns until it is overlain by the ,carboniferous ,limestone near ,selattyn1 a distance of not less than twenty-four miles4 ,the same band1 but much more feebly developed1 has been traced through the faulted country on both sides of ,bala ,lake1 where it formed a useful platform in the investigation of the complicated geological structure of that area4 ,along the north side of the ,berwyn ,hills another thin band of tuff lies from #aej to #bjj feet still higher up in the series1 and has been traced for a distance of about twelve miles4 ,the ,bala limestone comes in about #hjj or #ajjj feet above the ,upper ash-bed4 ,illustration3 ,fig4 #eg4--,section across the ,berwyn ,hills4 7,reduced from ,horizontal ,section1 ,geol4 ,surv41 ,sheet #ce74 ,l1 ,llandeilo ,flags2 ,b1 ,bala group2 ,b ,l1 ,bala ,limestone2 t t1 volcanic tuffs2 ,d1 intrusive greenstones4 ,besides the rocks now enumerated1 the ,survey maps show the intercalation of four or five sheets of greenstone1 which are represented as following with marked regularity the strike of the strata4 ,until these sheets have been more precisely examined it is impossible to decide regarding their true petrographical character1 or to determine whether they are sills1 or interstratified lavas1 or include rocks of both these types4 ,v4 ,,the ,,volcanoes ,,of ,,anglesey ,we now turn to another part of the country1 about which much has been written and keen controversy has arisen4 ,in the centre of ,anglesey1 among the rocks grouped together by the ,geological ,survey as altered ,cambrian1 there occur masses of breccia1 the probable volcanic origin of which was1 so far as ,i know1 first suggested by ,professor ,hughes4#bbb ,dr4 ,callaway regards them as pre-,cambrian1#bbc while ,professor ,blake places them in his ,monian system4#bbd ,when ,i went over them some years ago1 ,i accepted the view that they are volcanic agglomerates4#bbe ,subsequent examination1 however1 has convinced me that notwithstanding their remarkable resemblance to true agglomerates they are not really of volcanic origin1 but are essentially crush-conglomerates1 like those in the ,isle of ,man1 so well described by ,mr4 ,lamplugh4#bbf ,footnote #bbb3 ,proc4 ,camb4 ,phil4 ,soc4 vol4 iii4 7#ahhj71 p4 #cdg4 ,footnote #bbc3 ,quart4 ,journ4 ,geol4 ,soc4 ,footnote #bbd3 ,op4 cit4 ,footnote #bbe3 ,presidential ,address ,geol4 ,soc4 vol4 xlvii4 7#ahia71 p4 #acj4 ,footnote #bbf3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 li4 7#ahie71 p4 #efc4 ,see ,geol4 ,mag4 #ahif1 p4 #dha4 ,but though their present coarse1 agglomerate-like structure is1 ,i think1 entirely due to the mechanical crushing of the rocks in situ and not to volcanic explosions1 it does not follow that the rocks which have been broken up do not contain evidence of volcanic action contemporaneous with their original formation4 ,obviously1 pyroclastic materials may be subjected to deformation and disruption as well as any other components of the earth's crust1 and may be equally converted into crush-conglomerates4 ,and in ,anglesey it can1 ,i think1 be shown that some of the rocks which have been broken up were originally tuffs and volcanic breccias4 ,throughout ,anglesey the stratified rocks present evidence of having undergone very great compression1 deformation and rupture4 ,thus at ,llanerchymedd thick-bedded ,lower ,silurian grits1 with their intercalations of shale1 have been broken up by numerous small faults1 and have been pushed over each other in large irregular blocks1 the shales being now pinched out1 and now pressed up into the interstices between the dislocated harder and more resisting grits4 ,this condition of rupture may be regarded as one of the stages towards the formation of a conglomerate by the crushing together of rocks in situ4 ,a few miles further south at the beginning of the railway cuttings of ,llangefni1 green1 red and purple slates and grits appear in a rather more crushed state1 and immediately beyond these strata come the coarse breccias4 ,neither in their composition nor in their structural condition do these ,llangefni strata appear to be marked off from the undoubted ,lower ,silurian rocks as parts of a different system4 ,the railway cuttings at ,llangefni reveal a series of rocks which appear to have been originally shales1 with thin bands of siliceous grit4 ,the argillaceous portions of this series are now green and phyllitic1 and remind one of the finer parts of some basic tuffs among the older ,palozoic systems4 ,they include1 however1 pale flinty bands1 such as might have been derived from fine felsitic dust4 ,the grits are for the most part fine-grained and highly siliceous1 but they include also coarser varieties with clear quartz-grains4 ,the enormous deformation which these strata have undergone is at once apparent4 ,they seem to have been plicated1 ruptured and thrust over each other1 the harder parts surviving longest1 but being eventually broken into small fragments4 ,every stage may be traced from a recognizable band of grit down to the rounded or elliptical pebbles of the same material entirely isolated in this phyllitic matrix of crushed shale4 ,but while the volcanic origin of these coarsely-fragmental masses cannot be maintained1 there is elsewhere evidence that the older ,palozoic rocks of ,anglesey include relics of contemporaneous volcanic eruptions4 ,seven miles to the south-east of ,holyhead1 in the basal ,lower ,silurian conglomerates which1 as before referred to1 ,mr4 ,selwyn found lying unconformably on the green schists1 there occur abundant fragments of volcanic rocks1 besides the prevalent detritus of the schists of the neighbourhood4 ,some of the bands have somewhat the character of volcanic breccias or tuffs1 and they show an evident resemblance to portions of the ,bangor group and the rocks of ,llyn ,padarn1 though they are doubtless of much later age4 ,that these volcanic fragments were not derived from the waste of rocks of a much earlier period is made tolerably certain by the intercalation of true tuffs among the black shales higher up in the order of succession4 ,here1 then1 we have evidence of contemporaneous volcanic action in the very basement ,lower ,silurian strata of ,anglesey1 which by their fossil contents are shown to be on the horizon of the lowest ,arenig or even ,tremadoc group4 ,but still further and fuller evidence of ,silurian volcanism is to be obtained by an examination of the northern coast-line4 ,i have already referred to the elliptical fault which is marked on the ,geological ,survey map as running from the north-western headland to the eastern coast beyond ,amlwch4 ,the necessity for inserting this fault1 apart from any actual visible trace of its occurrence1 arose when the conclusion was arrived at that the rocks of the extreme north of ,anglesey were essentially altered ,cambrian strata4#bbg ,for immediately to the south of these rocks black shales1 obviously ,silurian1 were seen to dip to the north--a structure which could only be accounted for by a dislocation letting them down into that position4 ,the same necessity for a fault has of course been felt by all writers who have subsequently treated the northern area as pre-,cambrian4 ,but it is deserving of notice that in the original mapping of the ,survey no continuous abrupt hiatus is shown by the line which was afterwards marked as a continuous line of fault4 ,on the contrary1 on one of the field-maps in1 ,i believe1 ,mr4 ,selwyn's handwriting the remark occurs3--,the gradual passage from the black shale to the upper green gritty slates of ,llanfechell is best seen at ,bothedd1 on road from ,llanfaethlu to ,llyn-llygeirian4#bbh ,footnote #bbg3 ,i have fully considered the evidence adduced by ,dr4 ,callaway and ,professor ,blake1 and have examined the ground1 and can come to no other conclusion than that stated in the text4 ,but see ,mr4 ,blake's remarks1 ,geol4 ,mag4 #ahia1 p4 #dhc4 ,footnote #bbh3 ,there is no continuous section now visible at this place1 but the two groups of rock can be traced to within a few feet of each other1 both inclined as usual in the same direction1 and the black shales appearing to pass under the others4 ,it is no part of my aim to disprove the existence of faults along the line referred to4 ,these may quite well exist2 but there is assuredly no one gigantic displacement1 such as the theory ,i am combating would require2 while any faults which do occur cannot be greatly different from the others of the district1 and do not prevent the true relations of the rocks from being discoverable4 ,where the supposed elliptical fault reaches the shore at ,carmel ,point1 the two groups of rock seem to me to follow each other in unbroken sequence4#bbi ,the black slates1 which are admittedly ,lower ,silurian1 dip underneath a breccia and greenish 7,amlwch7 slates4 ,not only so1 but bands of similar black slates occur higher up1 interstratified with and shading-off into tuffs and greenish slates4 ,further1 bands of coarse volcanic breccia occur among the black slates south of the supposed break4 ,these1 in accordance with the exigencies of theory1 are represented as separated by a network of faults from the black slates amid which they lie4 ,but good evidence may be found that they are truly interbedded in these slates4 ,in short1 the whole of the rocks in that part of ,anglesey form one great series1 consisting partly of black slates1 partly of greenish slates1 with abundant intercalations of volcanic detritus4 ,the age of the base of this series is moreover determined by the occurrence of ,bala fossils in a band of limestone near ,carmel ,point4 ,footnote #bbi3 ,i cannot admit that there is any evidence of a thrust-plane here4 ,to insert one is merely to modify field-evidence to suit theory4 ,see ,geol4 ,mag4 #ahia1 p4 #dhc4 ,the rocks which extend eastward along the coast from the north-western headland of ,anglesey are marked on the ,survey map as green1 grey and purple slates with conglomeratic and siliceous beds4 ,the truly volcanic nature of a considerable proportion of these strata has been clearly stated by ,mr4 ,blake4#bcj ,as they dip in a general northerly direction1 higher portions of the series present themselves as far as the most northern projection of the island near ,porth ,wen 7,fig4 #eh74 ,they have been greatly crumpled and crushed1 so that the slates pass into phyllites4 ,they include some thick seams of blue limestone and white quartzite1 also courses of black shale containing ,lower ,silurian graptolites4 ,among their uppermost strata several 7probably ,bala7 fossils1 including ,orthis ,bailyana1 have been obtained by ,professor ,hughes4 ,it has been supposed that the higher bands of black shale may also have been brought into their present positions by faults1 and that they do not really belong to the series of strata among which they lie4 ,but this suggestion is completely disproved by the coast-sections1 which exhibit many thin interstratified leaves of black shale1 sometimes less than an inch thick4 ,these and the ashy layers containing the ,orthis and other fossils form an integral part of the so-called ,amlwch slates4#bca ,footnote #bcj3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xliv4 7#ahhh71 p4 #eag4 ,see his further remarks in ,geol4 ,mag4 #ahia1 p4 #dhc4 ,footnote #bca3 ,the ,amlwch slates exhibit on a great scale the puckering that points to intense compression4 ,this gnarled structure1 as ,prof4 ,hughes called it1 has been illustrated by ,mr4 ,harker1 ,british ,assoc4 ,report 7#ahhe71 pp4 #hci1 #hdj4 ,as evidence of the regular intercalation of the black shales and tuffs in this sedimentary series1 a portion of the coast section at ,porth ,wen is here given 7,fig4 #eh74 ,the lowest member 7#a7 of the series is a white quartzite much jumbled in its bedding1 but yet distinctly interstratified with the other sediments1 and containing intercalated courses of green tuff and highly carbonaceous shale4 ,markings like worm-pipes are here and there to be seen4 ,the next group of strata 7#b7 consists of black shale followed by yellow conglomeratic sandstone and pebbly tuffs4 ,the shales enclose rounded and angular fragments of quartzite4 ,the sandstone passes upward into pinkish and yellowish conglomerate 7#c71 with an abundant lustrous phyllitic matrix1 which when free from pebbles closely resembles some of the tuffs of ,llyn ,padarn4 ,the next band 7#d7 is one of yellow1 sandy1 felspathic grit1 quartz-conglomerate and fine tuffs1 with leaves of dark shale towards the base4 ,it was in the lower part of this band that the ,orthis above mentioned was found4 ,the black shales contain markings which are probably graptolites4 ,reddish quartzite and quartz-conglomerate 7#e7 next succeed4 ,these strata have the same phyllitic base just noticed4 ,the highest group here shown is one of black1 yellow and green shales mixed with patches and bands of volcanic breccia and tuff1 the whole being greatly disturbed1 cleavage and bedding seeming as it were to be struggling for the mastery4 ,these last strata look as if they were about to pass up vertically into the ordinary dark ,lower ,silurian shales or slates4 ,illustration3 ,fig4 #eh4--,section of the strata on the shore at ,porth ,wen1 west of ,amlwch4 ,there can be no doubt regarding the serious amount of crushing which the rocks of this coast-line have undergone4 ,some of the bands might even be described as crush-conglomerates4 ,yet the intercalation of seams of black shale and limestone1 and the occurrence of the exactly similar but thicker group of black shales at ,porth ,prydd1 which are admitted to be ,lower ,silurian1 unite the whole series of strata as parts of one formation4 ,it thus appears that the area coloured altered ,cambrian on the ,survey map1 and regarded as pre-,cambrian by some later observers1 is proved by the evidence of fossils at its base1 towards its centre and at its top1 to belong to the ,lower ,silurian series1 probably to the ,bala division4 ,that this was the geological horizon of part at least of the area was recognized by ,sir ,a4 ,ramsay1 though he confessed himself unable precisely to determine on the north coast of ,anglesey how much of the strata are of ,silurian and how much of ,cambrian age4#bcb ,professor ,hughes was the first to suggest that the whole of these rocks should be referred to the ,bala group4#bcc ,footnote #bcb3 ,mem4 ,geol4 ,surv4 vol4 iii4 #bnd edit4 p4 #bdb4 ,footnote #bcc3 ,proc4 ,camb4 ,phil4 ,soc4 vol4 iii4 7#ahhj71 pp4 #cda-cdh4 ,illustration3 ,fig4 #ei4--,section of intercalated black shale in the volcanic series at ,porth yr hwch1 south of ,carmel ,point4 ,illustration3 ,fig4 #fj4--;,green slates overlain with volcanic breccia1 ,carmel ,point1 ,anglesey4 ,i have dwelt on the determination of the true geological age of the rocks of the north of ,anglesey because of the diversity of opinion respecting them1 and because of their great interest in regard to the history of volcanic action in ,wales4 ,these rocks contain a record of volcanic eruptions1 probably contemporaneous on the whole with those of the ,bala period in ,caernarvonshire1 yet independent of them and belonging to a different type of volcanic energy4 ,some of the vents probably lay in the north-western part of ,anglesey4 ,the materials ejected from them were1 so far as we know1 entirely of a fragmentary kind4 ,vast quantities of detritus1 largely in the form of fine dust1 were thrown out2 but no trace has yet been found of the outflow of any lava4 ,the lower part of this volcanic series consists of bedded breccias which are sometimes remarkably coarse4 ,their included stones1 ranging up to six inches or more in diameter1 are usually more or less angular1 and consist mainly of various felsites4 ,layers of more rounded pebbles occasionally occur1 while the bedding is still further indicated by finer and coarser bands1 and even by intercalations of fine tuffs and ashy shales4 ,towards their upper limits some of these volcanic bands shade off into pale grey or greenish ashy shale1 followed by black sandy shale of the usual kind4 ,the relation of the peculiar greenish shale of the ,amlwch type to these tuffs and breccias is well shown east of ,carmel ,point4 ,this shale is interleaved with tuff and contains frequent repetitions of finer or coarser volcanic breccia1 as well as occasional seams of black shale4 ,an illustration of this structure is given in ,fig4 #ei1 where some yellow decomposing breccias 7#a71 cut by a fault 7f71 are overlain by about #dj or #ej feet of black shale 7#b71 above which lies a flinty felsitic rock 7#c7 that appears to run in bands or dykes through the agglomerate4 ,at ,carmel ,point 7,fig4 #fj7 a similar structure may be observed to that at ,llyn ,padarn already referred to 7p4 #afc74 ,the cleavage1 which is well developed in the green slates 7a71 is much more faintly marked in the overlying breccia 7b71 but the bedding can still be detected in both rocks running parallel to their mutual boundary-line4 ,beyond ,porth ,padrig1 which lies east from ,carmel ,point1 the section may be seen which is shown in ,fig4 #fa4 ,here the blue or lead-coloured shale or slate 7a7 marked as ,silurian on the ,geological ,survey map passes up into a mass of fine yellowish felsitic tuff and breccia 7b74 ,the shale at the junction intercalates in thin leaves with the tuff4 ,illustration3 ,fig4 #fa4--;,blue shale or slate passing into volcanic breccia east of ,porth ,padrig1 near ,carmel ,point1 ,anglesey4 ,the breccias south of ,carmel ,point1 though they are chiefly made up of felsitic detritus1 sometimes show a preponderance of fragments of shale4 ,they vary also rapidly in texture and composition4 ,these variations may indicate that the vent or vents from which their materials were derived stood somewhere in the near neighbourhood1 if indeed they are not to be recognized in some of the boss-like eminences that rise above the shore4 ,at the same time1 the enormous amount of crushing and shearing which the rocks of this region have undergone has doubtless introduced crush-conglomerates into the structure of the ground4 ,and some patient labour may be required before the nature and origin of the different fragmental masses are determined4 ,certain remarkably coarse1 tumultuous breccias1 exposed on the coast at ,mynyddwylfa and ,cemmaes1 were formerly regarded by me as volcanic agglomerates4 ,but more recent examination has satisfied me that these1 like the breccias at ,llangefni1 are not of volcanic origin but are crush-conglomerates4#bcd ,footnote #bcd3 ,presidential ,address1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xlvii4 p4 #acd2 ,rep4 ,brit4 ,assoc4 #ahif1 ,section ,c2 ,geol4 ,mag4 #ahif1 p4 #dha4 ,while the lower breccias are sometimes tolerably coarse1 the volcanic detritus becomes much finer in the higher parts of the ,amlwch slates4 ,above the limestones and black shales of ,cemmaes volcanic breccias and ashes1 with limestone1 quartzite1 conglomerate and thin seams of black shale1 continue to the extreme northern headlands4 ,the amount of fine volcanic detritus distributed through these strata is very great4 ,we can clearly make out that while ordinary sedimentation was in progress1 an almost constant but variable discharge of fragmental materials took place from the vents in the neighbourhood4 ,sometimes a special paroxysm of explosion would give rise to a distinct band of breccia or of tuff1 but even where1 during a time of comparative quiescence1 the ordinary sand or mud predominated1 it was generally mingled with more or less volcanic dust4 ,some bands of conglomerate in this group of strata deserve particular notice4 ,the most conspicuous of these1 already referred to as seen at ,porth ,wen1 is made up of quartz and quartzite blocks1 embedded in a reddish matrix largely composed of ashy material1 and recalling the red spotted tuffs of ,llyn ,padarn4 ,the occurrence of strong conglomerates near the top of a volcanic series has been noted at ,st4 ,david's1 ,llyn ,padarn and ,bangor4 ,in none of these localities1 as ,i have tried to show1 do the conglomerates mark an unconformability or serious break between two widely-separated groups of rock4 ,the ,anglesey section entirely supports this view1 for the conglomerates are there merely intercalations in a continuous sequence of deposits2 they are succeeded by tuffs and shales like those which underlie them4 ,the interposition of such coarse materials1 however1 may undoubtedly indicate local disturbance1 connected1 perhaps1 in this and the other localities1 with terrestrial readjustments consequent upon the waning of volcanic energy4 ,the detailed geological structure of ,anglesey is still far from being completely understood4 ,besides the serious crushing here referred to1 there is reason to suspect that considerable plication1 perhaps even inversion1 of the strata has taken place1 and that1 by denudation1 detached portions of some of the higher groups have been left in different parts of the island4 ,the occurrence of ,upper ,silurian fossils in several localities adds to the perplexity of the problem by indicating that1 among the folds and hardly distinguishable from the older slates1 portions of ,upper ,silurian formations may have been caught and preserved4 ,these difficulties1 moreover1 involve in some obscurity the closing phases of volcanic activity in ,wales2 for until they are1 to some extent at least1 removed1 we shall be left in doubt whether the vents in the north of ,anglesey1 which were in eruption probably during ,bala time1 were the last of the long succession of ,welsh volcanoes4 ,if the black shales of ,parys ,mountain are really referable to the horizon of the ,mayhill ,sandstone1 the two great igneous bands between which they lie would seem to mark an outbreak of volcanic energy during ,upper ,silurian time4 ,no other indications1 however1 of eruptions of that age having been met with in ,great ,britain 7though they occur in the south-west of ,ireland and possibly in ,gloucestershire71 more careful investigation is required before such a position can be safely assigned to any rocks in ,anglesey4 ,putting these doubtful rocks aside for the present1 we may1 in conclusion1 contrast the type of eruption in ,anglesey with that of the great ,snowdonian region4 ,while the ,caernarvonshire volcanoes were pouring forth their volumes of felsitic lava1 and piling them up for thousands of feet on the sea-floor1 the northern ,anglesey vents1 not more than some five-and-twenty miles away1 threw out only stones and dust1 but continued their intermittent explosions until they had strewn the sea-bottom with detritus to a depth of many hundred feet4 ,there is yet another feature of interest in this independent group of submarine vents in ,anglesey4 ,their operations appear to have begun before the earliest eruptions of the ,bala volcanoes in ,caernarvonshire4 ,their first beginnings may1 indeed1 have been coeval with the explosions that produced the older ,arenig tuffs of ,merionethshire2 their latest discharges were possibly the last manifestations of volcanic energy in ,wales4 ,they seem thus to bridge over the vast interval from ,tremadoc to ,upper ,bala1 possibly even to ,upper ,silurian time4 ,but we may1 perhaps1 connect them with the still earlier period of ,cambrian volcanism4 ,i have referred to the evidence which appears to show that the vents whence the lavas and tuffs of ,moel ,trefan and ,llyn ,padarn were erupted gradually moved northwards1 and continued in eruption until after the beginning of the deposition of the black slates that are generally regarded as ,arenig4 ,the ,anglesey tuffs and breccias may thus be looked upon as evidence of a still further shifting of the active orifices northward4 ,in this view1 while the ,aran and ,cader ,idris volcanoes broke out in ,upper ,cambrian and continued through ,arenig time1 and the ,snowdonian group was confined to ,bala time1 a line of vents opened to the north-west in the ,cambrian period before the epoch of the ,llanberis slates1 and1 dying out in the south1 continued to manifest a minor degree of energy1 frequently discharging fragmental materials1 but no lava1 over the sea-bottom1 until1 towards the close of the ,bala period1 possibly even in ,upper ,silurian time1 they finally became extinct4 vi4 ,,the ,,volcanoes ,,of ,,the ,,lake ,,district 7,,arenig ,,to ,,close ,,of ,,bala ,,period7 ,from the time of the appearance of ,sedgwick's classic letters to ,wordsworth1 no volcanic area of ,britain has probably been so well known in a general sense to the ordinary travelling public as the district of the ,english ,lakes4 ,many geologists have since then visited the ground1 and not a few of them have published additions to our knowledge respecting what is now known as the ,borrowdale ,volcanic ,series4 ,the most elaborate and detailed account of any part of it is that given by the late ,mr4 ,j4 ,c4 ,ward in the ,geological ,survey ,memoirs1 wherein he embodied the results of his minute investigation and mapping of the northern portion of the district4#bce ,notices of the petrography of some of the more interesting rocks have subsequently been published by ,mr4 ,rutley1 ,professor ,bonney1 ,mr4 ,harker1 ,mr4 ,marr1 ,mr4 ,hutchings and others4 ,but up to the present time no complete memoir on the volcanic geology of the ,lake ,district as a whole has appeared4 ,the sheets of the ,geological ,survey map present a graphic view of the general distribution of the rocks1 but so rapid has the progress of certain branches of geology been since these sheets were published1 that the map is even now susceptible of considerable improvement4 ,footnote #bce3 ,sheet #aja ,s4 ,e4 of the ,geological ,survey of ,england and ,wales and ,explanation illustrating the same2 and papers by him in ,quart4 ,journ4 ,geol4 ,soc4 vols4 xxxi4 xxxii4 7#ahge-gf74 ,see also ,messrs4 ,aveline and ,hughes1 ,mem4 ,geol4 ,survey1 ,sheet #ih ,n4,e4 7,kendal1 ,sedbergh1 etc474 ,in estimating the area over which the volcanic rocks of the ,lake ,district are spread1 geologists are apt to consider only the tract which lies to the south of ,keswick and stretches southward to a line drawn from the ,duddon ,sands to ,shap4 ,but it can easily be shown that this area falls far short of the extent of that wherein the rocks can still be traced1 and yet further short of that over which the lavas and ashes originally spread4 ,for1 in the first place1 the volcanic group can be followed round the eastern end of the mountain-group which culminates in ,skiddaw1 and along the northern base of these heights to ,cockermouth1 though only a narrow fringe of it emerges from underneath the ,carboniferous series4 ,it is thus manifest that the volcanic rocks once stretched completely across ,skiddaw and its neighbours1 and that they extend northwards below the ,whitehaven ,coal-field4 ,but1 in the next place1 far beyond these limits1 volcanic rocks1 which there can be little doubt were originally continuous with those of the ,lakes1 emerge from beneath the base of the ,cross ,fell escarpment1#bcf and still further to the east a prolongation of the same group rises for a brief space to the surface from under the great limestone sheets of ,upper ,teesdale4 ,between the north-western and south-eastern limits within which the rocks can now be seen there intervenes a distance of some #aa miles1 while the extreme length of the tract from south-west to north-east is about #ej miles4 ,even if we take these figures as marking the approximate boundaries of the region covered by the volcanic ejections1 it cannot be less than #eej square miles4 ,but this is probably much less than the original area4 ,footnote #bcf3 ,for an account of the ,cross ,fell inlier of ,silurian rocks see the paper by ,professor ,nicholson and ,mr4 ,marr1 with the petrographical appendix by ,mr4 ,harker4 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xlvii4 7#ahia71 pp4 #ejj1 #eab4 ,the thickness of the accumulated volcanic materials is proportionate to the large tract of country over which they have been spread4 ,from various causes1 it is difficult to arrive satisfactorily at any precise statement on this question4 ,in a volcanic series bedding is apt to be obscure where1 as in the present case1 there are no interstratified bands of ordinary sedimentary strata to mark it off4 ,it tends1 moreover1 to vary considerably and rapidly within short distances1 not only from subsequent unequal movements of subsidence or elevation1 but from the very conditions of original accumulation4 ,mr4 ,ward considered that the maximum thickness of the volcanic group of the ,lake ,district might be taken to range from #ab1jjj to #ae1jjj feet4#bcg ,professors ,harkness and ,nicholson1 on the other hand1 gave the average thickness as not more than #ejjj feet4#bch ,my own impression is that the truth is to be found somewhere between these two estimates1 and that the maximum thickness probably does not exceed #hjjj or #ijjj feet4 ,in any case there cannot1 ,i think1 be much doubt that we have here the thickest accumulation of volcanic material1 belonging to a single geological period1 anywhere known to exist in ,britain4 ,footnote #bcg3 ,ward1 op4 cit4 p4 #df4 ,footnote #bch3 ,brit4 ,assoc4 7#ahgj7 ,sectional ,reports1 p4 #gd4 ,the geological age of this remarkable volcanic episode is fortunately fixed by definite palontological horizons both below and above4 ,the base of the volcanic group rests upon and is interstratified with the upper part of the ,skiddaw ,slate1#bci which from the evidence of its fossils is paralleled with the ,arenig rocks of ,wales4 ,the highest members of the group are interstratified with the ,coniston ,limestone1 which1 from its abundant fauna1 can without hesitation be placed on the same platform as the ,bala ,limestone of ,wales1 and is immediately followed by the ,upper ,silurian series4 ,thus the volcanic history comprises the geological interval that elapsed between the later part of the ,arenig period and the close of the ,bala period4 ,it begins probably not so far back as that of the ,arenig group of ,merionethshire1 and its termination was perhaps coincident with the dying out of the ,snowdonian volcanoes4 ,but it contains no record of a great break or interval of quiescence like that which separated the ,arenig from the ,bala eruptions in ,wales4 ,footnote #bci3 ,mr4 ,dakyns has expressed his belief that the volcanic group lies unconformably on the ,skiddaw ,slate 7,geol4 ,mag4 #ahfi1 pp4 #ef1 #aaf71 and ,professor ,nicholson has formed the same opinion 7op4 cit4 pp4 #aje1 #afg2 ,proc4 ,geol4 ,assoc4 vol4 iii4 p4 #ajf74 ,mr4 ,goodchild1 however1 has shown that in the ,cross ,fell inlier the oldest tuffs are interstratified with the ,skiddaw ,slates 7,proc4 ,geol4 ,assoc4 vol4 xi4 7#ahhi71 p4 #bfa74 ,mr4 ,ward in mapping the district inserted a complex series of faults along the junction-line between the volcanic series and the ,skiddaw ,slates4 ,when ,i went over the ground with him some years before his death ,i discussed this boundary-line with him and could not adopt his view that it was so dislocated4 ,more recent re-examination has confirmed me in my dissent4 ,a large number of the faults inserted on the ,geological ,survey map to separate the ,skiddaw ,slates from the ,borrowdale volcanic series cannot be proved1 and probably do not exist4 ,others may be of the nature of thrust-planes4 ,but see ,mr4 ,ward's explanation of his views1 op4 cit4 p4 #dh4 ,the materials that form this enormous volcanic pile consist entirely of lavas and ashes4 ,no intercalations of ordinary sedimentary material have been met with in it1 save at the bottom and at the top4 ,the lower lavas1 well seen among the hills to the south of ,keswick1 were shown by ,mr4 ,ward to be intermediate between felsites and dolerites in regard to their silica percentage1 and he proposed for them the name of felsi-dolerites4 ,they are comprised in the group of the andesites or porphyrites4 ,from the analyses published by ,mr4 ,ward1 the amount of silica appears to range up to about #fj per cent4#bdj ,they are usually close-grained1 dull dark-grey to black rocks1 breaking1 where fresh1 with a splintery or conchoidal fracture1 showing a few minute striated felspars1 apt to weather with a pale-brown or yellowish-grey crust1 and sometimes strongly vesicular or amygdaloidal4 ,they present many external resemblances to some of the porphyrites or altered andesites of the ,lower ,old ,red ,sandstone of ,scotland4 ,a microscopic examination of specimens collected by ,dr4 ,hatch and myself from the hills to the south of ,keswick showed the rocks to be true andesites1 composed of a multitude of slender laths 7sometimes large porphyritic crystals7 of felspar with a brownish glassy groundmass1 and with some chloritic material probably representing augite1 but with no trace of quartz4#bda ,footnote #bdj3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xxxi4 7#ahge7 p4 #djh1 vol4 xxxii4 7#ahgf7 p4 #bd4 ,geology of ,northern ,part of ,lake ,district 7,mem4 ,geol4 ,survey71 p4 #bb4 ,in a subsequent paper the more basic lavas of ,eycott ,hill are compared with dolerites 7,monthly ,microscopical ,journ4 #ahgg1 p4 #bdf74 ,footnote #bda3 ,these rocks were mapped as tuffs by ,mr4 ,ward4 ,their microscopic characters have been described by ,messrs4 ,harker and ,marr1 ,quart4 ,journ4 ,geol4 ,soc4 xlvii4 7#ahia71 p4 #bib2 by ,mr4 ,harker1 op4 cit4 p4 #eag2 and by ,mr4 ,w4 ,m4 ,hutchings1 ,geol4 ,mag4 #ahia1 p4 #ecg2 #ahib1 pp4 #bbg1 #edj4 ,another type of andesite has been found by ,mr4 ,hutchings to occur abundantly at ,harter ,fell1 ,mardale1 between the ,nan ,bield ,pass and ,high ,street1 and in the cliffs on the right side of the ,kentmere ,valley4 ,it consists of rocks mostly of a grey-green or grey-blue colour with resinous lustre and extremely splintery fracture4 ,they are augite-andesites of a much more vitreous nature than the dominant type of lavas of the ,lake ,district4 ,their groundmass under the microscope is seen to have originally varied from a wholly glassy base to an intimate mixture of glass and exceedingly minute felspar-microlites4 ,this groundmass is permeated with chlorite in minute flakelets1 and encloses numerous porphyritic sharply-defined felspar-crystals1 together with chlorite-pseudomorphs after augite4#bdb ,gradations from these rocks to the ordinary more coarse-grained andesites may be observed4 ,footnote #bdb3 ,mr4 ,hutchings1 ,geol4 ,mag4 #ahia1 p4 #eci4 ,this observer describes a quartz-andesite or dacite from near ,dunmail ,raise4 ,some of the andesites appear to have a trachytic facies1 where the felspars of the groundmass consist largely of untwinned laths and appear to be mainly orthoclase4#bdc ,footnote #bdc3 ,op4 cit4 p4 #edc4 ,among the lavas of the ,lake ,district there occur many which are decidedly more basic than the andesites1 and which should rather be classed among the dolerites and basalts1 though they do not appear to contain olivine4 ,these rocks occur at ,eycott ,hill1 above ,easedale ,tarn1 ,scarf ,gap ,pass1 ,dale ,head1 ,high ,scawdell1 ,seatoller ,fell and other places4 ,analyses of those from ,eycott ,hill were published by ,mr4 ,ward1 and their silica percentage was shown to range from #ea to #ecc4bdd ,the microscopic characters of the group have been more recently determined by ,mr4 ,hutchings#bde and ,messrs4 ,harker and ,marr4#bdf ,footnote #bdd3 ,monthly ,microscopical ,journal1 #ahgg1 p4 #bdf4 ,footnote #bde3 ,geol4 ,mag4 #ahia1 p4 #ech4 ,footnote #bdf3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xlix4 7#ahic71 p4 #chi4 ,mr4 ,harker1 op4 cit4 vol4 xlvii4 7#ahia74 ,the andesitic and more basic lavas are particularly developed in the lower and central part of the volcanic group4 ,they rise into ranges of craggy hills above the ,skiddaw ,slates1 and form1 with their accompanying tuffs1 the most rugged and lofty ground in the ,lake ,district4 ,they extend even to the southern margin of the volcanic area at one locality to the south-west of ,coniston1 where they may be seen with their characteristic vesicular structure forming a succession of distinct flows or beds1 striking at the ,coniston ,limestone which lies upon them with a decided1 though probably very local1 unconformability4#bdg ,one of the flows from this locality was found by ,dr4 ,hatch1 under the microscope1 to belong to the more basic series4 ,it approaches a basalt1 containing porphyritic crystals of fresh augite instead of the usual felspars1 and showing a groundmass of felspar microlites with some granules of augite and dispersed magnetite4 ,this local increase of basic composition is interesting as occurring towards the top of the volcanic group4 ,a porphyritic and somewhat vesicular andesite1 with large crystals of striated felspar in a dark almost isotropic groundmass1 occurs under the ,coniston ,limestone near ,stockdale4 ,footnote #bdg3 ,this unconformability has been described and discussed by various observers4 ,the general impression has been1 ,i think1 that the break is only of local importance4 ,mr4 ,aveline1 however1 believed it to be much more serious1 and he regarded the volcanic rocks which were ejected during the deposition of the ,coniston ,limestone series as much later in date than those of the ,borrowdale group4 ,see ,mem4 ,geol4 ,survey1 ,explanation to ,sheet #ih ,n4,e4 #bnd edit4 p4 #h 7#ahhh74 ,mr4 ,ward was much impressed with the widespread metamorphism which he believed all the volcanic rocks of this region had undergone1 and as a consequence of which arose the difficulty he found in discriminating between close-grained lavas and fine tuffs4 ,there is1 of course1 a general induration of the rocks1 while cleavage has widely1 and sometimes very seriously1 affected them4 ,there is also local metamorphism round such bosses as the ,shap granite1 but the evidence of any general and serious metamorphism of the whole area does not seem to me to be convincing4#bdh ,footnote #bdh3 ,the metamorphism of all the rocks1 aqueous and igneous1 around the ,shap granite has been well worked out by ,messrs4 ,harker and ,marr1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xlvii4 7#ahia7 p4 #bff1 xlix4 7#ahic7 p4 #cei4 ,with regard to the original structure and subsequent alteration of some of the andesitic lavas1 an interesting section has recently been cut along the road up ,borrowdale a little south of the ,bowder ,stone4 ,several bands of coarse amygdaloidal lava may there be seen interstratified among tuffs4 ,the calcite amygdales in these rocks are arranged parallel to the bedding and therefore in the planes of flow1 while those lined with chlorite are more usually deformed parallel to the direction of the cleavage4 ,this difference suggests that before the cleavage took place1 not improbably during the volcanic period1 the rocks had been traversed by heated water producing internal alteration and rearrangements1 in virtue of which the vesicles along certain paths of permeation were filled up with calcite1 so as then to offer some resistance to the cleavage1 while those which remained empty1 or which had been merely lined with infiltrated substance1 were flattened and pulled out of shape4 ,messrs4 ,harker and ,marr have shown that the amygdaloidal kernels had already been introduced into the cellular lavas before the intrusion of the ,shap granite4 ,in the account to be given of the ,tertiary plateau-basalts 7,chapter xxxvi47 evidence will be adduced that this filling up of the steam-cavities of lava may take place during a volcanic period1 and that it is probably connected with the passage of heated vapours or water through the rocks4 ,though acid lavas are not wholly absent from the central and lower parts of the volcanic group1 it is at the top that their chief development appears to occur4 ,these rocks may be grouped together as felsites or rhyolites4 ,they probably play a much larger part in the structure of the southern part of the volcanic area than the published maps would suggest1 and a detailed survey and petrographical study of them would well reward the needful labour4#bdi ,a fine series of felsites is interbedded in the lower part of the ,coniston ,limestone1 and spreads out underneath it along the southern margin of the volcanic district from the ,shap granite south-westward for some miles#bej 7,fig4 #fb74 ,between the valleys of the ,sprint and ,kent these felsites 7which farther east are said to be #gjj feet thick7 may be seen interposed between the limestone and the fossiliferous calcareous shales below it1 while from underneath the latter other sheets rise up into the range of hills behind4 ,footnote #bdi3 ,see ,mr4 ,f4 ,rutley1 ,the ,felsitic ,lavas of ,england and ,wales1 ,mem4 ,geol4 ,surv4 #ahhe1 pp4 #ab-ae2 also the description of ,messrs4 ,harker and ,marr1 ,quart4 ,journ4 ,geol4 ,soc4 xlvii4 7#ahia71 p4 #cja4 ,footnote #bej3 ,unfortunately these acid lavas are not distinguished from the others in the ,geological ,survey maps4 ,illustration3 ,fig4 #fb4--,section of felsites on the ,coniston ,limestone group1 west of ,stockdale4 a1 ,felsites more or less cleaved2 b1 ,calcareous shales with fossils1 much cleaved2 c1 ,cleaved felsite2 d1 ,coniston ,limestone2 e1 ,stockdale ,shales 7with graptolites74 ,these acid lavas are generally grey1 cream-coloured1 or pink1 with a white weathered crust4 ,their texture when fresh is flinty or horny1 or at least extremely fine-grained and compact4 ,they are seldom markedly porphyritic4 ,they frequently display good flow-structure1 and sometimes split up readily along the planes of flow4 ,occasionally the flow-lines on the outer crust have broken up in the movement of the rock1 giving rise to irregular fragments which have been carried forward4 ,short1 extremely irregular1 branching veins of a fine cherty felsitic substance1 which occasionally shows a well-marked flow-structure parallel to the walls1 traverse certain parts of a dark-grey felsite1 near ,brockstones1 between the valleys of the ,kent and ,sprint4#bea ,occasionally a distinct nodular structure may be observed in these acid lavas1 sometimes minute1 like an oolite1 in other parts1 as on ,great ,yarlside1 presenting large rounded balls4 ,this nodular structure is not confined to the lava-flows1 but has been detected by ,messrs4 ,harker and ,marr in what appears to be an intrusive rock near ,shap ,wells4 ,the microscopic characters of some of the ,lake ,district rhyolites were described by ,mr4 ,rutley1 who found them to exhibit beautiful perlitic and spherulitic structures4#beb ,that such rocks as these were poured out in a vitreous condition1 like obsidian or pitchstone1 cannot be doubted4 ,chemical analysis shows that the ,lake ,district rhyolites agree exactly with those of ,north ,wales in their composition4 ,they contain about #gf per cent of silica4#bec ,footnote #bea3 ,compare the structure described by ,mr4 ,harker from the ,cross ,fell inlier1 ,quart4 ,journ4 ,geol4 ,soc4 xlvii4 7#ahia71 p4 #eah4 ,footnote #beb3 ,geology of ,kendal1 etc41 ,mem4 ,geol4 ,survey1 ,sheet #ih ,n4,e4 #bnd edit4 p4 #i4 ,footnote #bec3 ,messrs4 ,harker and ,marr1 op4 cit4 p4 #cjb4 ,the rhyolitic lavas have been seriously affected by the general cleavage of the region4 ,in some places they have been so intensely cleaved as to become a kind of fissile slate1 and there seems good reason to believe that in this altered condition they have often been mistaken for tuffs4 ,where they assume a nodular structure1 the nodules have sometimes been flattened and elongated in the direction of the prevalent cleavage4 ,the abundance and persistence of thoroughly acid lavas along the southern edge of the volcanic area where the youngest outflows are found1 is a fact of much interest and importance in the history of the eruptions of this region4 ,it harmonizes with the observations made in ,wales1 where in the ,arenig1 and less distinctly in the ,bala group1 a marked increase in acidity is noticeable in the later volcanic products4 ,at the same time1 as above mentioned1 there is evidence also of the discharge of more basic materials towards the close of the eruptions1 and even of the outflow of a lava approaching in character to basalt4 ,according to the ,geological ,survey maps1 by far the largest part of the volcanic district consists of pyroclastic materials4 ,when my lamented friend1 the late ,mr4 ,ward1 was engaged in mapping the northern part of the district1 which he did with so much enthusiasm1 ,i had an opportunity of going over some of the ground with him1 and of learning from him his ideas as to the nature and distribution of the rocks and the general structure of the region4 ,i remember the difficulty ,i had in recognizing as tuff much of what he had mapped as such1 and ,i felt that had ,i been myself required1 without his experience of the ground1 to map the rocks1 ,i should probably have greatly enlarged the area coloured as lava1 with a corresponding reduction of that coloured as tuff4 ,a recent visit to the district has revived these doubts4 ,it is quite true1 as ,mr4 ,ward maintains1 that where the finer-grained tuffs have undergone some degree of induration or metamorphism1 they can hardly1 by any test in the field1 be distinguished from compact lavas4 ,he was himself quite aware of the objections that might be made to his mapping1#bed but the conclusions he reached had been deduced only after years of unremitting study in the field and with the microscope1 and in the light of experience gained in other volcanic regions4 ,nevertheless ,i think that he has somewhat exaggerated the amount of fragmental material in the northern part of the ,lake ,district1 and that the mapping1 so consistently and ably carried out by him1 and followed by those members of the ,survey who mapped the rest of the ground1 led to similar over-representation there4 ,some portions of the so-called tuffs of the ,keswick region are undoubtedly andesites2 other parts in the southern tracts include intercalated bands of felsite as well as andesite4 ,footnote #bed3 ,he says3 ,i shall be very much surprised if my mapping of many parts of the district be not severely criticized and found fault with by those who examine only one small area and do not take into consideration all the facts gathered together1 during the course of several years1 from every mountain flank and summit 7op4 cit4 p4 #be74 ,mr4 ,hutchings has expressed his agreement with the opinions stated in the text4 ,he likewise coincides in the belief that there are many of these ,lake ,district volcanic rocks1 regarding which it is impossible to decide whether they are lavas or ashes 7,geol4 ,mag4 #ahia1 p4 #edd74 ,but even with this limitation1 the pyroclastic material in the ,lake ,district is undoubtedly very great in amount4 ,it varies in texture from coarse breccia or agglomerate1 with blocks measuring several yards across1 to the most impalpable compacted volcanic dust4 ,in the lower parts of the group some of the tuffs abound in blocks and chips of ,skiddaw ,slate4 ,some good examples of this kind may be seen in ,borrowdale1 below ,falcon ,crag and at the ,quayfoot quarries4 ,where the tuff is largely made up of fragments of dark blue slate1 it much resembles the slate-tuffs of ,cader ,idris4 ,some of the pieces of slate are six or eight inches long and are now placed parallel to the cleavage of the rock4 ,among the slate debris1 however1 felspar crystals and felsitic fragments may be observed4 ,bands of coarser and finer green tuff show very clearly the bedding in spite of the marked cleavage 7,fig4 #fc74 ,illustration3 ,fig4 #fc4--;,fine tuff with coarser bands near ,quayfoot quarries1 ,borrowdale4 ,the highly-inclined fine lines show the cleavage4 ,the more gently dipping bands and lines mark the bedding4 ,but throughout the whole volcanic group the material of the tuff is chiefly of thoroughly volcanic origin1 and its distribution appears to agree on the whole with that of the bedded lavas4 ,in the older portions of the group it is probably mainly derived from andesitic rocks1 though with an occasional intermingling of felsitic or rhyolitic detritus1 while in the higher parts many of the tuffs are markedly rhyolitic4 ,among the lapilli minute crystals of felspar1 broken or entire1 may be detected with the microscope4 ,some of the ejected ash must have been an exceedingly fine dust4 ,compacted layers of such material form bands of green slates1 which may occasionally be seen to consist of alternations of coarser and finer detritus1 now and then false-bedded4 ,such tuffs bring vividly before the mind the intermittent explosions1 varying a little in intensity1 by which so much of the fabric of the ,lake mountains was built up4 ,breccias of varying coarseness are likewise abundant1 composed of fragments of andesite and older tuffs in the central and lower parts of the volcanic group1 and mainly of felsitic or rhyolitic detritus in the upper parts4 ,some of these rocks1 wherein the blocks measure several yards across1 are probably not far from the eruptive vents1 as at ,sourmilk ,gill and below ,honister ,pass4 ,generally the stones are angular1 but occasionally more or less rounded4 ,stratification can generally be detected among these fragmental rocks1 but it is apt to be concealed or effaced by the cleavage1 while it is further obscured by that widespread induration on which ,mr4 ,ward has laid so much stress4 ,the extreme state of comminution of the volcanic dust that went to form the tuffs has probably caused them to be more liable to metamorphism than the lavas4#bee ,footnote #bee3 ,the microscopic and chemical characters of the ,ash-,slates of the ,lake ,district have been investigated by ,mr4 ,hutchings1 ,geol4 ,mag4 #ahib1 pp4 #aee1 #bah4 ,little has yet been done in identifying any of the vents from which the vast mass of volcanic material in the ,lake ,district was ejected4 ,mr4 ,ward believed that the diabase boss forming the ,castle ,head of ,keswick marks the site of one of the main volcanic centres of this particular district1#bef whence the great lava sheets to the southward flowed out4 ,there are obviously two groups of bosses on the northern side of the district1 some of which may possibly mark the position of vents4 ,a few of them are occupied by more basic1 others by more acid rocks4 ,it is not necessary to suppose that the andesitic lavas ascended only from the former and the felsites from the latter4 ,while the felsites on the whole are younger than the more basic lavas1 they may have been erupted from vents which had previously emitted andesites1 so that the present plug may represent only the later and more acid protrusions4 ,footnote #bef3 ,op4 cit4 p4 #gj4 ,besides the boss of ,castle ,head there are numerous smaller basic intrusions farther down the ,derwent ,valley on either side of ,bassenthwaite ,lake4 ,among these are the highly basic rocks forming the picrite on the east side of the ,dash ,beck and the dykes on ,bassenthwaite ,common4 ,all these bosses1 sills1 and dykes rise through the ,skiddaw ,slates1 but there is no positive proof that they belong to the ,lower ,silurian volcanic series2 they may possibly be much later4 ,the most important and most interesting of all the intrusive masses of basic material is that which constitutes a large part of the eminence that culminates in ,carrock ,fell4 ,the remarkable variations in the composition of this mass have been already referred to4 ,mr4 ,harker has shown that while the centre of the mass is a quartz-gabbro1 it becomes progressively more basic towards the margin4 ,through the gabbro a mass of granophyre has subsequently made its way1 and along the line of junction has incorporated into its own substance so much of the basic rock as to undergo a marked modification in its structure and composition4 ,whether these intruded bodies of basic and acid material have ascended in one of the old volcanic funnels and have been injected laterally in laccolitic fashion has not been ascertained4 ,mr4 ,harker1 indeed1 is rather inclined to refer the intrusions to a time not only later than the ,borrowdale volcanoes1 but later even than the terrestrial movements that subsequently affected the district and gave the rocks their present cleaved and faulted structures4 ,besides the gabbro and granophyre of this locality1 igneous activity has manifested itself in the uprise of numerous later dykes and veins1 intermediate to basic in composition4 ,some of these are glassy 7tachylyte7 and spherulitic or variolitic4#beg ,footnote #beg3 ,mr4 ,harker1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 l4 7#ahid7 p4 #cab1 li4 7#ahie7 p4 #abe4 ,geol4 ,mag4 #ahid1 p4 #eea4 ,throughout the ,lake ,district a considerable number of bosses of more acid rocks rise through the ,skiddaw ,slates1 and likewise through the volcanic group even up to its highest members4 ,some of these bosses may possibly indicate the site of volcanic vents4 ,two of them1 which form conspicuous features on either side of the ,vale of ,st4 ,john1 consist of microgranite1 and rise like great plugs through the ,skiddaw ,slates1 as well as through the base of the volcanic group4 ,the view of the more eastern hill1 as seen from the west1 is at once suggestive of a neck4 ,these masses measure roughly about a square mile each4 ,with the acid intrusions may possibly be associated some of the other masses of granophyre1 microgranite and granite 7felsite1 felstone1 quartz-felsite1 syenitic granite1 quartz-syenite1 elvanite71 which have long attracted attention in this region4 ,the largest of these intrusions is the tract of granite which stretches from ,eskdale down to near the sea-coast as a belt about eleven miles long and from one to three miles broad4 ,another large mass is the granophyre or syenite of ,ennerdale4 ,numerous other intrusions of smaller dimensions have been mapped4 ,to what extent any of these eruptive masses were associated with the volcanic phenomena remains still to be worked out4 ,there seems to be little doubt that a number of them must belong to a much later period4 ,mr4 ,harker has expressed his belief that the intrusion of some of these igneous rocks was intimately associated with the post-,silurian terrestrial movements of which cleavage is one of the memorials4#beh ,the ,skiddaw granite1 though it does not touch any part of the volcanic group1 but is confined to the underlying ,skiddaw ,slates1 was erupted after the cleavage of the district1 which affects the volcanic as well as the sedimentary series4 ,in other instances also1 as in that of ,carrock ,fell1 the intrusion seems to have been later than the disturbances of the crust4#bei ,the amount of metamorphism around some of the bosses of granite is considerable4 ,that of the ,skiddaw region has been well described by ,j4 ,c4 ,ward1#bfj while that of the volcanic group by the ,shap granite has been carefully worked out by ,mr4 ,harker and ,mr4 ,marr4#bfa ,footnote #beh3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 li4 7#ahie71 p4 #add4 ,footnote #bei3 ,op4 cit4 p4 #abf4 ,footnote #bfj3 ,geology of ,northern ,part of the ,english ,lake ,district1 ,mem4 ,geol4 ,surv4 #ahgf1 chap4 iii4 ,the metamorphism around the diorites and dolerites1 and the granophyres and felsites1 is described in the same chapter4 ,footnote #bfa3 ,quart4 ,journ4 ,geol4 ,soc4 xlvii4 7#ahia7 p4 #bff1 xlix4 7#ahic7 p4 #cei4 ,the ,shap granite comes through the very highest member of the volcanic series1 and even alters the ,upper ,silurian strata4 ,it must thus be of much younger date than the volcanic history of the ,lake ,district4 ,it presents some features in common with the granite bosses of the south of ,scotland4 ,like these1 it is later than ,upper ,silurian and older than ,lower ,carboniferous or ,upper ,old ,red ,sandstone time4 ,its protrusion may thus have been coeval with the great volcanic eruptions of the period of the ,lower ,old ,red ,sandstone4 ,it will accordingly be again referred to in a later chapter4 ,it must be confessed that none of the large bosses of massive rocks1 whether diabases1 gabbros1 felsites1 granophyres1 or granites1 appear to afford any satisfactory proof of the position of the vents which supplied the lavas and tuffs of the ,lake ,district4 ,nor can such a decided accumulation of the volcanic materials in certain directions be established as to indicate the quarters where the centres of eruption should be sought4 ,on the contrary1 the confused commingling of materials1 and the comparative shortness of the outcrop of the several sheets which have been traced1 rather suggest that if any one great central volcano existed1 its site must lie outside of the present volcanic district1 or more probably1 that many scattered vents threw out their lavas and ashes over no very wide area1 but near enough to each other to allow their ejected materials to meet and mingle4 ,the scene may have been rather of the type of the ,phlegran fields than of ,etna and ,vesuvius4 ,if this surmise be true1 we may expect yet to recognize little necks scattered over the volcanic district and marking the positions of some of these vanished cones4 ,what appears to have been one of these small vents stands near ,grange at the mouth of ,borrowdale1 where ,i came upon it in #ahij4 ,in the little ,comb ,beck1 the ,skiddaw ,slates are pierced by a mass of extremely coarse agglomerate1 forming a rudely-circular boss4 ,the slates are greatly disturbed along the edges of the boss1 so much so1 indeed1 that it is in some places difficult to draw a line between them and the material of the agglomerate4 ,that material is made up of angular blocks1 varying in size up to three feet long1 stuck in every position and angle in an intensely-indurated matrix formed apparently of comminuted debris like the stones4 ,the blocks consist of a finely-stratified shale1 which is now hardened into a kind of hornstone1 with some felsitic fragments4 ,i could see no slags or bombs of any kind4 ,there is no trace of cleavage among the blocks1 nor is the matrix itself sensibly cleaved4 ,i believe this to be a small volcanic neck and not a crush-conglomerate4 ,it has been blown through the ,skiddaw ,slates1 and is now filled up with the debris of these slates4 ,its formation seems to have taken place before the cleavage of the strata1 and its firm position and great induration enabled it to resist the cleavage which has so powerfully affected the slates and many members of the volcanic group4 ,it was the opinion of my predecessor1 ,sir ,andrew ,ramsay1 and likewise of ,mr4 ,ward1 that the ,cumbrian volcanic action was mainly subrial4 ,this opinion was founded chiefly on the fact that1 save at the bottom and top of the series1 there is no evidence of any interstratified sediment of non-volcanic kind4 ,the absence of such interstratification may undoubtedly furnish a presumption in favour of this view1 but1 of course1 it is by no means a proof4 ,better evidence is furnished by the unconformability already mentioned between the ,coniston ,limestone and the lavas on which it lies4 ,besides angular pieces of lava1 probably derived from direct volcanic explosion1 this limestone contains fragments of amygdaloidal andesite1 and also rolled crystals of striated felspar4#bfb ,these ingredients seem to indicate that some part of the volcanic group was above water when the ,coniston ,limestone was deposited4 ,footnote #bfb3 ,messrs4 ,harker and ,marr1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xlvii4 7#ahia71 p4 #caj4 ,the absence of interstratifications of ordinary non-volcanic sediment in the ,borrowdale group might conceivably arise from the eruptions following each other so continuously on the sea-floor1 and at so great a distance from land that no deposition of sand or mud from the outside could sensibly affect the accumulation of volcanic material4 ,certainly some miles to the east at the ,cross ,fell inlier1 as already mentioned1 there is evidence of the alternation of tuffs with the sandy and muddy sediment of the sea-bottom4 ,here1 at the outer confines of the volcanic district1 the ejected materials evidently fell on the sea-floor1 mingled there with ordinary sediment1 and enclosed the same organic remains4 ,the well-defined stratification of many of the fine tuffs is rather suggestive to my mind of subaqueous than of subrial accumulation4 ,at the same time1 there seems no reason why1 here and there at least1 the volcanic cones should not have risen above the water1 though their materials would be washed down and spread out by the waves4 ,one of the most marked points of contrast between the ,cumbrian and the ,welsh volcanic districts is to be found in the great paucity of sills in the former region4 ,a few sheets of diorite and diabase have been mapped1 especially in the lower parts of the volcanic group and in the underlying ,skiddaw ,slates4 ,on the other hand1 dykes are in some parts of the district not unfrequent1 and certainly play a much more prominent part here than they do in the ,welsh volcanic districts4 ,the majority of them consist of felsites1 quartz-porphyries1 diorites1 and mica-traps4 ,but there is reason to suspect that where they are crowded together near the granite1 as around ,shap ,fells1 they ought to be connected with the uprise of the post-,silurian granitic magma rather than with the history of the volcanic group4#bfc ,if this series of dykes be eliminated1 there remain comparatively few that can with any confidence be associated with the eruption of the ,borrowdale rocks4 ,footnote #bfc3 ,for a description of the dykes around the ,shap granite see the paper by ,messrs4 ,harker and ,marr1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xlvii4 7#ahia71 p4 #bhe4 vii4 ,,upper ,,silurian 787 ,,volcanoes ,,of ,,gloucestershire ,a remarkable group of igneous materials has long been known to rise among the ,silurian rocks of the ,tortworth district at the north end of the ,bristol coal-field4 ,they were believed to be aqueous deposits in the ,wernerian sense by ,weaver4#bfd ,murchison regarded them as intrusive sheets2#bfe ,phillips looked on them as partly intrusive and partly interstratified4#bff ,they consist largely of coarsely-amygdaloidal basalts1 some of which have been microscopically examined4#bfg ,but their field-relations as well as their petrography have not yet been adequately determined4 ,they are represented on the ,geological ,survey ,map as forming a number of parallel bands in strata classed as ,upper ,llandovery4 ,if1 as seems probable1 some of them are really interstratified1 they form the youngest group of ,silurian volcanic rocks in ,england1 ,scotland1 or ,wales4 ,footnote #bfd3 ,trans4 ,geol4 ,soc4 #bnd ser4 vol4 i4 7#ahai71 pp4 #cbd-ccd4 ,footnote #bfe3 ,silurian ,system 7#ahci71 p4 #deg4 ,footnote #bff3 ,mem4 ,geol4 ,surv4 vol4 ii4 part i4 7#ahdh71 p4 #aid4 ,footnote #bfg3 ,geology of ,east ,somerset1 etc41 in ,mem4 ,geol4 ,surv4 7#ahgf71 p4 #baj2 descriptions by ,mr4 ,f4 ,rutley4 ,,chapter ,,xiv ,,the ,,silurian ,,volcanoes ,,of ,,ireland ,abundant as are the volcanic records of the ,silurian period in ,england1 ,wales and ,scotland1 the description of them would be incomplete without an account of those of ,ireland4 ,the eruptions of ,arenig1 ,llandeilo and ,bala time1 which we have followed from the south of ,caermarthenshire to the borders of the ,scottish ,highlands1 had their counterparts all down the east of ,ireland4 ,the ,irish register of them1 however1 supplies some details which are less clearly preserved in the sister island4 ,but the most distinctive feature of the ,silurian volcanic history in ,ireland is the preservation of memorials of eruptions during the ,upper ,silurian period4 ,in no part of ,great ,britain has any unquestionable trace been found of volcanic activity during that part of the geological record1 the last eruptions of which the age is known being those of the ,bala rocks4 ,but in the south-west of ,ireland there is evidence that for a time active vents appeared over the sea-floor on which the earlier deposits of ,upper ,silurian time were laid down4 ,i4 ,the ,lower ,silurian ,series i4 ,eruptions probably of ,arenig ,age ,it is in that part of ,ireland which lies east of a line drawn from ,strabane to ,dungarvan ,harbour that the records of ,lower ,silurian volcanic activity are to be found4 ,in the north the development of volcanic rocks resembles that in ,scotland1 in the south it corresponds rather with the volcanic districts of ,wales4 ,the ,irish ,silurian volcanic rocks have been traced with more or less detail on the maps of the ,geological ,survey4 ,since these maps were published1 however1 great advances have been made in the study of the petrography of volcanic rocks1 as well as in the art of tracing their structure upon maps4 ,much1 therefore1 now remains to be done to bring our knowledge of the older volcanic history of ,ireland abreast of that of the rest of the ,british ,isles4 ,in the following summary ,i have had to rely mainly on my own traverses of the ground1 guided by the maps and memoirs of the ,survey1 and with the personal assistance of some of my colleagues4 ,the remarkable zone of crushed cherts1 igneous rocks and sandstones1 probably of ,lower ,silurian age1 which ,i have referred to 7p4 #bja7 as wedged in between the schists and the ,old ,red ,sandstone along the southern margin of the ,highlands of ,scotland1 reappears in ,ireland4 ,it occupies an area in the ,county ,tyrone1 about #bd miles long and about #i miles broad at the broadest part1 but disappearing towards the north-east and south-west4#bfh ,lying between the ,palozoic formations on the south and the schists on the north1 it occupies a similar position to the ,scottish belt1 but presents a much broader area1 and thus affords greater facilities for examining the rocks4 ,it presents the same indefinite or faulted boundaries as in ,scotland1 so that its relations to the rocks along its flanks have not been satisfactorily determined4 ,that the rocks of this area are older than the ,silurian strata to the south of them seems to be established by the occurrence of fragments of them in these strata1 and that they are younger than the schists may be inferred from their non-foliated character4 ,but they have undoubtedly undergone considerable crushing by powerful terrestrial movements which have placed them in their present position4 ,footnote #bfh3 ,this area was mapped by ,mr4 ,j4 ,nolan for the ,geological ,survey1 and was described by him in the ,geol4 ,mag4 for #ahgi4 ,i visited it in company with my colleagues1 ,mr4 ,b4 ,n4 ,peach and ,mr4 ,a4 ,m',henry1 in #ahij and again in #ahid4 ,my first conclusion was that the volcanic rocks should be regarded as part of the schistose series lying to the north of them 7,pres4 ,address ,geol4 ,soc4 #ahia1 p4 #gg74 ,but on the second visit1 after having studied the rocks of the border of the ,scottish ,highlands1 ,i formed the opinion stated in the text4 ,the special feature of interest in this ,irish area is the remarkable development of volcanic materials which is there to be seen1 spreading over a far wider area than in ,scotland4 ,the rocks include lavas associated with tuffs and agglomerates1 likewise a varied series of intrusive masses4 ,the lavas are chiefly dull greenish1 fine-grained rocks1 having the general character of diabases and porphyrites4 ,they are sometimes quite slaggy1 and where the amygdaloidal kernels remain1 these are usually of calcite4 ,under the microscope1 the diabases show in some parts that their lath-shaped felspars1 and the augite which these penetrate1 are tolerably fresh1 while in other parts fibrous chlorite1 granular epidote and veins of calcite bear witness to the metamorphism which they have undergone4 ,one of the most conspicuous features in some of these lavas is the occurrence of the same sack-like or pillow-shaped structure which has been already referred to as so marked among the ,arenig lavas of ,scotland4 ,though the vesicles of these rocks are often quite uncrushed1 showing that there has been no general subsequent deformation of the whole mass1 there occur local tracts where evidence of considerable movement may be noticed4 ,thus close to a mass of gneiss1 and elsewhere along their margin1 the lavas are apt to be much jointed and broken with numerous lines of shear1 along which the crushed material assumes more or less of a schistose structure4 ,yet in the solid cores between these bands of crushing the original forms of the vesicles are retained4 ,these greenish lavas are occasionally interleaved with grey flinty mudstones1 cherts and red jaspers1 which are more particularly developed immediately above4 ,in lithological character1 and in their relation to the diabases1 these siliceous bands bear the closest resemblance to those of ,arenig age in ,scotland4 ,but no recognizable ,radiolaria have yet been detected in them4 ,besides the more basic lavas1 there occur also1 but less abundantly1 platy felsitic rocks which have suffered much from shearing1 and consequently have acquired a fissile slaty structure4 ,the agglomerates are made up of angular1 subangular and rounded fragments imbedded in a matrix of similar composition4 ,this matrix has in places become quite schistose1 and then closely resembles some parts of the green schists of the ,scottish ,highlands4 ,of the inclosed stones the great majority consist of various felsites1 which1 weathering with a thick white opaque crust1 are internally close-grained1 dull-grey or even black1 sometimes showing flow-structure1 and of all sizes up to eight inches in diameter or more4 ,there are also fragments of the basic lavas1 and likewise pieces of chert and jasper4 ,on many of the rocky hummocks no distinct bedding can be made out in the agglomerate1 but in others the rock is tolerably well stratified4 ,the tuffs are fine silky schistose rocks1 and seem to have been largely derived from basic lavas4 ,they have suffered more than any of the other rocks from mechanical deformation1 for they pass into green chloritic schists4 ,some portions of them are not unlike the slaty tuffs of ,llyn ,padarn in ,caernarvonshire4 ,accompanying the fragmental volcanic rocks1 some ordinary sedimentary intercalations are to be found--red shales and pebbly quartzites1 that seem to have escaped much crushing4 ,the true order of succession in the volcanic series has not yet been determined4 ,but apparently above this series come some dark shales1 such as might yield graptolites1 pale grits and occasional limestones4 ,later than the lavas and the pyroclastic material are various intrusive masses1 which in bands and bosses form numerous craggy hills throughout the area4 ,so far as ,i have been able to observe1 these rocks include two groups4 ,of these the older consists of basic injections1 such as gabbros and allied rocks1 some of which remind me of the so-called hypersthene-rock of ,lendalfoot1 in ,ayrshire4 ,the coarser varieties1 as at ,carrickmore or ,termon rock1 are sometimes traversed by fine-grained veins from an inch to several feet in breadth4 ,portions of the slaggy diabases may be observed inclosed in these intrusive masses4 ,the younger group is of more acid composition 7granite1 quartz-porphyry1 etc471 and sends veins into the older4 ii4 ,eruptions of ,llandeilo and ,bala ,age ,into the east of ,ireland the ,lower ,silurian rocks are prolonged from ,scotland1 from the ,lake ,district and from ,wales4 ,though greatly concealed under younger formations across the breadth of the island1 and occasionally interrupted by what are regarded as older strata of ,cambrian age1 they nevertheless occupy by much the larger part of the maritime counties from ,belfast ,lough to the southern coast-line of ,waterford1 even as far as ,dungarvan ,harbour4 ,with the same lithological types of sedimentary deposits as in other parts of the ,united ,kingdom1 they carry with them here also their characteristic records of contemporaneous volcanic action4 ,though nowhere piled into such magnificent mountain-masses as in ,westmoreland and ,north ,wales1 these records become increasingly abundant and interesting as they are traced southwards1 until they are abruptly terminated by the coast-line along the south of the counties of ,wexford and ,waterford4 ,while much remains to be done1 both in the field and in the laboratory and microscope-room1 before our acquaintance with the ,irish ,silurian volcanic rocks is as complete as our knowledge of their equivalents in other portions of the ,united ,kingdom1 a serious preliminary difficulty must be recognized in the fact that the several geological horizons of these rocks have only been approximately fixed4 ,great difficulty was experienced by the ,geological ,survey in drawing any satisfactory line between the ,llandeilo and ,bala formations4 ,this arose not so much from deficiency of fossil evidence as from the way in which the fossils of each group seemed to occur in alternating bands in what were regarded as a continuous series of strata4 ,indeed1 in some localities it almost appeared as if the occurrence of one or other facies of fossils depended mainly on lithological characters indicative of original conditions of deposit1 for the ,llandeilo forms recurred where black shales set in1 while ,bala forms made their reappearance where calcareous and gritty strata predominated4#bfi ,more recent work among the ,silurian formations in ,england and ,scotland1 however1 indicates that the parallel repetition of the two types of fossils is due to rapid and constant plication of the rocks1 whereby the two formations1 neither of them1 perhaps1 of great thickness1 have been folded with each other in such a way that without the evidence of an established sequence of fossils1 or the aid of continuous sections1 it becomes extremely difficult to make out the stratigraphical order in any district4 ,when the ground is attacked anew in detail1 with the assistance of such palontological and lithological horizons as have permitted the complicated structure of the southern uplands of ,scotland to be unravelled1 we may be enabled to tabulate the successive phases of the volcanic history of the region in a way which is for the present impossible4 ,we have as yet no palontological evidence that in the ,silurian region of the east of ,ireland1 which extends from ,belfast ,lough to the south coast of ,county ,waterford1 any of the anticlinal folds bring up to the surface a portion of the ,lower ,arenig formation1 though possibly some of the lowest visible strata may be of ,upper ,arenig age4 ,a considerable part of the region must be referred to the ,llandovery and other ,upper ,silurian formations1 but the precise limits of the two divisions of the ,silurian system have not yet been determined1 except for the region north of ,dublin1 which has recently been re-examined for the ,geological ,survey by ,mr4 ,f4 ,w4 ,egan and ,mr4 ,a4 ,m',henry4 ,footnote #bfi3 ,jukes was disposed to regard the two faunas as essentially coeval1 but inhabiting different kinds of sea-bottom4 ,see his note1 ,explanation of ,sheets #afg1 #afh1 #agh1 #agi1 p4 #cj4 ,these observers have ascertained that1 as in ,southern ,scotland1 by far the larger part of the ,silurian region of the north-east of ,ireland is occupied by strata belonging to the upper division of the system4 ,the ,lower ,silurian formations1 including the ,llandeilo and ,bala groups1 form a belt varying up to six miles in breadth1 which stretches from the coast of ,down1 between the mouth of ,belfast ,lough and ,copeland ,island1 in a south-westerly direction to near the valley of the ,shannon in ,county ,longford4 ,south of this belt the ,lower ,silurian rocks rise to the surface only here and there on the crests of anticlinal folds1 and it is in these scattered inliers that the volcanic and intrusive rocks are found4 ,so far as the available evidence goes1 the volcanic history of this part of ,ireland is entirely to be assigned to ,lower ,silurian time1 and more especially to the interval between the beginning of the ,llandeilo and the close of the ,bala period4 ,i must for the present content myself with this general limit of geological chronology1 and make no attempt to trace the relative antiquity of the igneous rocks in the several districts in which they are distributed4#bgj ,footnote #bgj3 ,the task of revising the ,irish maps and tracing out the respective areas of ,upper and ,lower ,silurian rocks over the whole island is now in progress by the ,geological ,survey1 ,mr4 ,egan and ,mr4 ,m',henry being entirely engaged on it4 ,viewing the volcanic region of ,eastern ,ireland as a whole1 we are first struck by the feebleness of the manifestations of eruptivity in the north1 and their increasing development as we advance southwards4 ,at the northern end of the ,silurian area in ,county ,down1 thin bands of felstone and ash have been mapped by the ,geological ,survey as interstratified with the highly inclined and plicated ,silurian rocks4#bga ,as the latter are plainly a continuation of the strata which have been mapped out zone by zone in the south of ,scotland1 their igneous intercalations may be looked upon as probably equivalents of some of those in the ,silurian districts of ,wigtonshire and ,kirkcudbrightshire4 ,but in ,county ,down no representative has yet been detected of the ,arenig and ,llandeilo volcanic series of the southern uplands of ,scotland4 ,nor has more precise petrographical examination confirmed the reference of any of the igneous rocks in the ,silurian area of that district to truly contemporaneously intercalated volcanic rocks4 ,all the eruptive material appears to be of an intrusive character4 ,it occurs in the form of dykes of lamprophyre or mica-trap belonging to the groups of minettes and kersantites4 ,nothing definite is known of the age of these intrusions3 they are possibly referable to the time of the ,lower ,old ,red ,sandstone4#bgb ,footnote #bga3 ,see ,sheet #di ,geol4 ,survey1 ,ireland1 and ,explanation thereto 7#ahga71 pp4 #af1 #cg1 #ci4 ,the so-called ashes of the ,explanation are probably parts of dykes which have been more or less crushed4 ,footnote #bgb3 ,guide to the ,collection of ,rocks and ,fossils belonging to the ,geological ,survey of ,ireland1 by ,messrs4 ,m',henry and ,watts1 ,dublin1 #ahie1 p4 #gd4 ,far in the interior several bands of felspathic ash and massive agglomerate are shown on the ,survey map as running through the counties of ,monaghan and ,cavan4#bgc ,in one locality south of the ,drumcalpin ,loughs a large exposure of this ash is visible3 brown crumbly beds1 with small rounded pebbles1 give place to a massive bed of agglomerate1 the enclosed blocks of which are always of one species of felstone1 sometimes measuring #aj #ab #ah inches1 and not always rounded4 ,south of ,carrickatee ,lough1 and a few miles farther to the south-west1 near ,lackan ,bridge1 considerable exposures of these rocks occur4 ,one crag in particular displays a thickness of more than #gj feet of tough flaky breccias1 thick agglomerates with small and large blocks of felstone1 and thin beds of fine pale green compact grit without pebbles1 and a few flags4 ,one of the flaky beds contains numerous white worn crystals of felspar2 the imbedded blocks of felstone are of the usual kind--pale compact matrix showing dark oblong patches1 vesicular and amygdaloidal1 the cavities being filled with chlorite4 ,footnote #bgc3 ,sheet #fi ,geol4 ,survey1 ,ireland1 and ,explanation of ,sheets #fh and #fi1 pp4 #i1 #ac1 #ae4 ,further south a more extensive area of igneous rocks has been mapped on the borders of ,louth and ,meath1 where1 according to the ,geological ,survey map1 a group of lavas and tuffs extends for about twelve miles near ,slane4#bgd ,other bands of ash and felstone have been mapped in the ,silurian area south of ,drogheda4 ,thus at ,hilltown1 west from the racecourse1 a bluish crystalline felstone1 showing in places lines of viscous flow1 is stated to be overlain by indurated felspathic ash and tuff1 felstone1 and indurated shale in alternating beds4#bge ,on a recent visit to this locality ,i found that the porcellanite or indurated shale is a greenish-grey chert1 full of ,radiolaria and finely-diffused volcanic dust4 ,this association of radiolarian chert with contemporaneous volcanic activity is of much interest1 as showing the extension of the same physical conditions of the ,lower ,silurian sea from ,scotland into ,ireland4 ,the ,lower ,llandeilo age of the volcanic intercalations in ,county ,meath is further indicated by the occurrence of ,didymograptus ,murchisoni in grey shales in the same neighbourhood with the radiolarian cherts4 ,in the ,lower ,silurian district of ,balbriggan numerous intrusive bosses and sills have been mapped by the ,geological ,survey4 ,i have found1 however1 that among these rocks there occur bands of volcanic breccia1 containing abundant angular fragments of a minutely-vesicular pumice1 and also that some of the diabase-masses display the pillow-structure and amygdaloidal texture4 ,hence1 though most of the igneous rocks are no doubt intrusive1 they appear to include lavas and tuffs of ,bala age4 ,footnote #bgd3 ,ibid4 ,sheets #ha and #ia4 ,these rocks are chiefly augitic andesites1 a few are basalts1 and some seem related to felstones4 ,probably many of them are intrusive sills of uncertain age4 ,the ashes contain fragments of felsite and porphyrite often of considerable size 7,guide to ,irish ,rock-,collection1 p4 #cf74 ,footnote #bge3 ,ibid4 ,sheets #ia and #ib and ,explanation to these ,sheets 7#ahga71 p4 #aj2 ,guide to ,irish ,rock-,collection1 p4 #cf4 ,some of these lavas are andesites1 others are felsites4 ,mr4 ,m',henry has contended that certain ashes and agglomerates1 particularly those exposed on the coast at ,portraine1 opposite ,lambay ,island1 are crush-conglomerates due to terrestrial disturbances1 which have affected both intrusive igneous rocks and the sedimentary series into which these have been injected4 ,when the numerous ,silurian cores of the mountain-groups in the interior of ,ireland shall have been searched for traces of contemporaneous volcanic action1 it is not improbable that these will be found4 ,one of the smaller ,silurian inliers which diversify the great ,carboniferous plain1 that of the ,chair of ,kildare1 has long been known to have igneous rocks associated with its abundantly fossiliferous ,bala limestone4#bgf ,on recently visiting this locality ,i found that1 besides the amygdaloidal and porphyritic andesites and basalts described by ,jukes and ,du ,noyer1 the fossiliferous conglomerates contain pebbles of rocks like those of the ,chair1 together with worn crystals of felspar1 while intercalated with them are thin courses of volcanic tuff4 ,there is thus evidence here of contemporaneous volcanic activity during the accumulation of the ,bala group of strata4 ,the limited area over which the rocks are exposed1 however1 affords merely a glimpse of this volcanic centre4 ,footnote #bgf3 ,see ,explanation to ,quarter ,sheet #ce ,n4,e4 7,sheet #aai of newer numeration7 of ,geol4 ,survey ,ireland 7#aheh71 p4 #af4 7,see note1 p4 #bef47 ,crossing over the broad belt of ,carboniferous ,limestone through which the ,liffey flows into ,dublin ,bay1 we come to the great continuous tract of older ,palozoic rocks which stretches southward to the cliffs of ,waterford4 ,through this tract runs the huge ridge of the ,wicklow and ,carlow granite4 ,on the west side of this intrusive mass1 bands of greenstone-ash1 as well as felspathic ashes1 have been traced among the ,silurian rocks by the ,geological ,survey4 ,but it is on the south-east side of the granite that the volcanic intercalations are best displayed4 ,indeed1 from ,wicklow ,head to ,dungarvan ,harbour there is an almost continuous development of igneous rocks1 rising into rocky eminences1 trenched into ravines by the numerous streams1 and laid bare by the waves in fine coast-cliffs4 ,it is in this south-eastern region1 comprising the counties of ,wicklow1 ,wexford and ,waterford1 that the ,irish ,lower ,silurian igneous rocks can best be studied4 ,there are obviously various distinct centres of eruption in this long belt of country4 ,the ,rathdrum and ,castletimon tract forms one of these4 ,another of less size culminates in ,kilpatrick ,hill1 a few miles to the southward4 ,arklow ,head marks the position of a third4 ,the lavas and tuffs which set in a few miles to the south of that promontory1 and may be said to extend without interruption to the south coast1 were probably thrown out by a series of vents which1 placed along a north-east and south-west line1 united their ejections into one long submarine volcanic bank4 ,there can be no doubt that the most active vents lay at the southern end of the belt1 for there the volcanic materials are piled up in thickest mass1 and succeed each other with comparatively trifling intercalations of ordinary sedimentary material4 ,some of these vents1 as ,i shall relate in the sequel1 have been cut open by the sea along a range of precipitous cliffs4 ,the comparatively feeble character of the volcanic energy during ,lower ,silurian time over the greater part of the south-east of ,ireland is shown by the great contrast between the thickness of the volcanic intercalations there and in ,wales and the ,lake country1 but still more strikingly by innumerable sections where thin interstratifications of fine tuff or volcanic breccia occur among the ordinary sedimentary strata1 and are sometimes crowded with ,bala fossils4 ,some interesting illustrations of this feature are to be seen in the ,enniscorthy district1 where layers of fine felsitic tuff1 sometimes less than an inch in thickness1 lie among the shales4 ,in some of the tuffs the lapilli are fragments of trachytic or andesitic rocks4 ,a striking example of rapid alternations of pyroclastic material with ordinary sediment lies far to the south in ,county ,waterford1 close to ,dunhill ,bridge1 where a group of fine volcanic breccias and grits has been laid bare by quarrying4#bgg ,these strata consist of coarser and finer detritus1 enclosing angular fragments of felsites and grey and black shale4 ,the felsite-lapilli vary in texture1 some of them presenting beautiful flow-structure4 ,the stones are stuck at random through each bed1 the largest being often at the bottom4 ,the beds of breccia vary from a few inches to a foot or more in thickness4 ,there can1 ,i think1 be little doubt that each of these breccia-bands points to a single volcanic explosion1 whereby felsitic fragments were thrown out1 mingled with pieces of the ,silurian strata through which the vents were drilled4 ,in a vertical thickness of some fifty feet of rock there must thus be a record of ten or twelve such explosions4 ,footnote #bgg3 ,see ,explanation of ,sheets #afg1 #afh1 #agh and #agi1 ,geol4 ,surv4 ,ireland1 p4 #ef4 ,nearer the active vents the fragmental deposits become1 as usual1 coarser and thicker4 ,but ,i have not observed any thick masses of tuff like those of ,north ,wales4 ,so far as my examination has gone1 the tuffs are mainly felsitic4 ,the so-called greenstone-ash of the ,survey maps is certainly in many cases not a true tuff4 ,this term was proposed by ,jukes for certain apple-green to olive-brown flaky fissile rocks only found in association with masses of greenstone4#bgh ,some years ago ,i had occasion to make a series of traverses in ,wicklow and ,wexford1 and then convinced myself that in that part of the country the greenstone-ashes were probably crushed bands of basic sills4 ,dr4 ,hatch has proved this to be their origin from a series of microscopic slides prepared from specimens collected by himself on the ground4#bgi ,in other cases the greenstone-ashes seem to be excessively-cleaved or sheared felsites1 which have acquired a soapy feel and a dull green colour2 but they also do include true tuffs4 ,thus1 in one instance1 at ,ballyvoyle cross-roads1 in the south of ,county ,waterford1 a greenstone-ash is a dull green tuff full of fragments of felspar 7chiefly plagioclase7 and pieces of dark andesitic lavas4 ,another example may be found to the west of the ,metal ,man1 near ,tramore1 where the tuff is full of fragments of felspar and shale cemented in a greenish-yellow material which may be palagonite4 ,footnote #bgh3 ,explanation of ,sheets #abi1 #acj1 p4 #ac 7#ahfi74 ,footnote #bgi3 ,explanation of ,sheets #ach1 #aci4 ,the felsites of the south-east of ,ireland form by much the largest proportion of the whole volcanic series4 ,they occur as lenticular sheets from a few feet to several hundred feet in thickness1 and occasionally traceable for some miles4 ,on the whole1 they are compact dull grey rocks1 weathering with a white crust4 ,a geologist familiar with the contemporary lavas of ,north ,wales cannot fail to be struck with the absence of the coarse flow-structure so often characteristic of the felsites in that region4 ,this structure1 indeed1 is not entirely absent from the ,irish rocks1 but it occurs1 so far1 at least1 as ,i have seen1 rather as a fine streakiness than in the bold lenticular bands so common in ,caernarvonshire4 ,in like manner the nodular structure1 though not entirely absent1 is rare4#bhj ,footnote #bhj3 ,in ,waterford nodular felsites occur with concretions varying from the size of a pea to several inches in diameter4 ,explanation to ,sheets #afg1 #afh1 #agh and #agi1 p4 #aa4 ,until these felsites have been subjected to more detailed investigation1 little can be said as to their petrography1 and as to the points of resemblance or difference between them and those of other ,lower ,silurian districts in the ,united ,kingdom4 ,an important step1 however1 in this direction was taken by ,dr4 ,hatch1 who studied them on the ground1 in the laboratory1 and with the microscope4 ,he found that some of them were soda-felsites or keratophyres 7with albite as their felspar71 that others were potash-felsites 7with orthoclase as their felspar71 while a third group contained both soda and potash1 the last-named greatly preponderating4#bha ,the existence of soda-felsites had not been previously detected among ,british volcanic rocks1 and it remains to be seen how far they may occur in the large and somewhat varied group of rocks combined under the general term felsites4 ,dr4 ,hatch believed that these rocks probably graduate into the normal or orthoclase felsites2 but it has not yet been possible to test this view on the ground1 nor to ascertain whether there is any essential difference between the mode of occurrence of the two types4 ,footnote #bha3 ,explanation of ,sheets #ach1 #aci1 p4 #di2 and ,geol4 ,mag4 #ahhi1 p4 #ede4 ,besides the more abundant felsites1 occasional bands of andesite have been detected4 ,various other eruptive rocks occur1 probably in most or all cases intrusive4 ,such are quartz-mica-diorites1 quartz-diorites1 augite-diorites or proterobases1 dolerites1 gabbros1 diabases and epidiorites4#bhb ,footnote #bhb3 ,guide to ,irish ,rock-,collections1 pp4 #cd1 #ce4 ,i have said that the chief theatre of eruption lay towards the south-west end of the volcanic belt of the south-east of ,ireland4 ,the coast-line of ,county ,waterford1 from ,tramore westward to ,ballyvoyle ,head--a distance of nearly fifteen miles--presents1 perhaps1 the most wonderful series of sections of volcanic vents within the ,british ,islands4 ,no one coming from the inland is prepared for either the striking character of the cliff scenery or the extraordinary geological structure there presented1 for the country is1 on the whole1 rather featureless1 and much of it is smoothed over and obscured by a covering of drift1 through which occasional knobs of the harder felsites protrude4 ,the cliffs for mile after mile range from #ajj to #aej or #bjj feet in height1 and present naked vertical walls of rock1 trenched by occasional gullies1 through which a descent may be made to the beach4 ,throughout the whole distance agglomerates and felsites succeed each other in bewildering confusion1 varied here and there by the intercalation of ,lower ,silurian shales and limestones involved and pierced by the igneous rocks4 ,hardly any bedded volcanic material is to be recognized from one end to the other4 ,the sea has laid bare a succession of volcanic vents placed so close to each other that it will be difficult or impossible to separate them out4 ,a careful study and detailed mapping of this marvellous coast-section1 however1 is a task well worthy of the labour of any one desirous of making himself acquainted with some of the conditions of volcanism during older ,palozoic time4 ,at the east end of the section1 black shales containing ,llandeilo graptolites1 and calcareous bands full of ,bala fossils1 dip westward below a group of soda-felsites and felsitic tuffs1 which seem to lie quite conformably on these strata4 ,here1 then1 we start with proof that the volcanic eruptions of this locality began during some part of the ,bala period4 ,but immediately to the west1 these bedded igneous rocks are broken through by a neck of coarse agglomerate stuck full of chips and blocks of shale1 some of them a foot long1 with abundant fragments of scoriform and flinty felsites4 ,some columnar dykes of dolerite cut through the neck1 and a larger intrusion seems to have risen up the same funnel4 ,the bedded tuffs appear again for a short distance1 but they are soon replaced by a tumultuous mass of agglomerate4 ,and from this part of the coast onwards for some distance all is disorder4 ,the agglomerates are crowded with blocks of various felsites and micro-granites sometimes #ah inches in diameter1 many of them presenting the most exquisite streaky flow-structure4 ,the angularity of these stones and the abrupt truncation of their lines of flow prove that they were derived from the shattering of already consolidated rocks4 ,in other places the ejected materials consist almost wholly of black shale fragments1 but with an intermixture of felsite-lapilli4 ,it is difficult to convey an adequate idea of the way in which the agglomerates are traversed by dykes1 veins and bosses of various felsites1 and of how these break in endless confusion through each other4 ,some of the intrusive rocks are compact and amorphous1 others are vesicular1 others close-grained and columnar4 ,again and again they present the most perfect flow-structure1 and it is noticeable that the lines of flow follow the inequalities of the walls of the fissure up which the rock has ascended1 and not only so1 but even of the surfaces of detached blocks of shale or felsite which have been caught up and enclosed in the still moving mass4 ,a few of these intrusive rocks were examined in thin slices by ,dr4 ,hatch4 ,most of them appear to be soda-felsites1 but they include also rather decomposed rocks1 some of which are probably diorites and quartz-diorites4 ,occasionally1 thoroughly basic dykes 7dolerite7 may be observed4 ,in the midst of this tumultuous assemblage of volcanic masses1 representing the roots of a group of ancient vents1 there occur occasional interspaces occupied by ordinary stratified rocks4 ,in the eastern part of the section these consist mainly of black shale1 sometimes with calcareous bands1 from which a series of ,bala fossils has been obtained4#bhc ,a very cursory examination suffices to show that these intercalations do not mark pauses in the volcanic eruptions4 ,they are1 in fact1 portions of the marine accumulations under the sea-floor through which the vents were blown2 they have been tossed about1 crushed and invaded by dykes and veins of felsite4 ,footnote #bhc3 ,but see the ,geol4 ,survey ,memoir on ,sheets #afg1 #afh1 #agh and #agi1 ,ireland 7#ahfe71 p4 #bh1 for a description of the association of ,bala and ,llandeilo fossils on that coast-line4 ,but certain other intercalated strips of stratified rocks present a special interest1 for they bring before us examples of volcanic ashes that gathered on the sea-floor1 but which were disrupted by later explosions4 ,thus1 at the ,knockmahon headland1 well-bedded felspathic grits and ashy shales occur1 thrown in among the general mass of eruptive material4 ,as ,i have already remarked1 it is difficult or impossible to fix the horizons of the stratified patches that are involved among the igneous ejections of this coast-section1 save where they contain recognizable fossils1 but the intercalation of true bedded tuffs among them is a proof that volcanic action had been in operation there long before the outbreak of the vents which are now laid bare along the cliffs4 ,in the south-east of ,ireland there is the usual association of acid and basic sills with the evidence of a superficial outpouring of lavas and ashes4 ,but these intrusive masses play a much less imposing part than in ,wales4 ,they may be regarded1 indeed1 as bearing somewhat the same proportion to the comparatively feeble display of extrusive rocks in this region that the abundant and massive sheets of ,merionethshire and ,caernarvonshire do to the enormous piles of lavas and tuffs which overlie them4 ,among the acid intrusive sheets the most conspicuous are those mapped by the ,survey as elvans4 ,these rocks1 as they occur in ,wicklow and ,wexford1 have been examined by ,dr4 ,hatch1 who finds them to be micro-granitic in structure1 occasionally exhibiting micropegmatitic or granophyric modifications4#bhd ,the true stratigraphical relations of these rocks have not yet been adequately investigated4 ,those of them which occur on the flanks of the great granite ridge are not improbably connected with that mass1 and if so are much younger than the ,lower ,silurian volcanoes4#bhe ,footnote #bhd3 ,explanation of ,sheets #ach and #aci1 p4 #ec4 ,footnote #bhe3 ,the ,leinster granite is certainly later than the ,lower ,silurian rocks and older than the ,carboniferous rocks of the south-east of ,ireland4 ,it may belong to the great epoch of granite protrusion during the ,old ,red ,sandstone period4 ,the basic sills1 or greenstones1 consist largely of diabase1 frequently altered into epidiorite2 they include also varieties of diorite4#bhf ,that they were intruded before the plication and cleavage of the rocks among which they lie is well shown by their crushed and sheared margins where they are in thick mass1 and by their cleaved and almost schistose condition where they are thinner4 ,the intense compression and crushing to which they have been subjected are well shown by the state of their component minerals1 and notably by the paramorphism of the original augite into hornblende4 ,footnote #bhf3 ,dr4 ,hatch1 op4 cit4 p4 #di4 ,the scarcity of dykes associated with ,silurian volcanic action is as noticeable in the south-east of ,ireland as it is in ,wales4 ,i have observed a considerable number1 indeed1 but they are confined to the line of old vents on the ,waterford coast1 and1 but for the clear cliff-sections cut by the sea1 they would certainly have escaped observation1 for they make no feature on the ground in the interior4 ,they are sometimes distinctly columnar1 and vary from less than a foot to many yards in width4 ,they traverse both the agglomerates and the intrusive felsites4 ,most of them are of felsite1 sometimes cellular2 but in some cases they are dolerites4 ,there is obviously no clue to the dates of these dykes4 ,that some at least of the vents along the south coast of ,county ,waterford may be vastly younger than the ,lower ,silurian rocks through which they have forced their way is suggested1 if not proved1 by a section which is in some respects the most extraordinary of the whole of this remarkable series4 ,the occurrence of a group of red strata was carefully noted by the late ,mr4 ,du ,noyer at ,ballydouane ,bay1 when he was engaged in carrying on the ,geological ,survey of that part of the country4 ,at first he regarded them as belonging to the ,old ,red ,sandstone1 which comes on in great force only a few miles to the west2 but he subsequently arrived at the belief that they are really an integral part of the ,lower ,silurian rocks of the district4 ,professor ,jukes had previously expressed himself in favour of this latter idea1 which was thought to receive support from the occurrence of some reddish strata in the ,lower ,silurian rocks of ,tagoat1 ,county ,wexford4#bhg ,footnote #bhg3 ,explanation of ,sheets #afg1 #afh1 #agh and #agi of the ,geological ,survey of ,ireland 7#ahfe71 pp4 #aj1 #ei4 ,the occurrence of red rocks among ,silurian strata1 which are not usually red1 might quite reasonably be looked for in the neighbourhood of ,old ,red ,sandstone1 ,permian or ,triassic deposits4 ,if these deposits once spread over the ,silurian formations1 a more or less decided raddling of the latter may have taken place4 ,but in the present instance1 though the ,old ,red ,sandstone begins not many miles to the west1 no such explanation of the colour of the strata is possible4 ,the cliffs of ,ballydouane ,bay consist of red sandstone1 red sandy shale and conglomerate4 ,the red tint is of that dull chocolate tone so characteristic of the ,lower ,old ,red ,sandstone4 ,the conglomerates are immense accumulations of ancient shingle1 consisting largely of pieces of white vein-quartz and quartzite1 sometimes a foot long and often well water-worn4 ,some of the sandy beds are full of large scales of white mica1 as if derived from some granitic or schistose region at no great distance4 ,taken as a whole1 the strata are much less indurated and broken than the ,silurian grits and shales of the district2 some of them1 indeed1 weather into mere incoherent sand that crumbles under the fingers4 ,there does not appear to be any positive proof that the red rocks are truly bedded with the ordinary ,silurian strata1 the junctions being faulted or obscured by intrusive igneous masses4 ,nowhere in the ,british ,islands1 so far as ,i am aware1 is there a similar group of strata among the ,lower ,silurian rocks4 ,if they belong to so ancient a series1 they show that in the south of ,ireland1 during ,lower ,silurian time1 there arose a set of peculiar physical conditions precisely like those that determined the accumulation of the ,old ,red ,sandstone in the same region at a later geological period4 ,and in that case it is hardly possible to conceive that these conditions could have been confined to the extreme south of ,ireland4 ,we should certainly expect to meet with evidence of them elsewhere1 at least in the same ,silurian region4#bhh ,footnote #bhh3 ,the nearest approach of any ,silurian group of strata to the character of these conglomerates is furnished by the remarkably coarse conglomerates1 boulder-beds and pebbly grits of the ,bala and ,llandovery series in the region between ,killary ,harbour and ,lough ,mask1 to which further reference is made in a later part of this chapter4 ,while ,i hesitate to express a decided opinion in opposition to the conclusions of such experienced observers as ,jukes and ,du ,noyer1 ,i incline to believe that the rocks in question really belong to the ,old ,red ,sandstone4 ,if such shall finally be determined to be their geological position1 they will supply evidence that some at least of the volcanic vents of the coast-line cannot be older than the ,old ,red ,sandstone4 ,they are pierced by masses of soda-felsite and by a coarse red agglomerate containing abundant pieces of felsite4 ,these volcanic rocks belong to the same type as those which break through the undoubted ,silurian rocks on either side4 ,they may thus come to prove a recrudescence of volcanic energy in this same district at a much later geological period2 and a new problem will arise to task the skill of the most accomplished field-geologist and petrographer--to unravel the structure and history of this chain of volcanic vents1 and1 in so doing1 to detect and separate the eruptions of ,lower ,silurian time from those of the ,lower ,old ,red ,sandstone4 ,in the far west of ,ireland1 another group of ,lower ,silurian volcanoes has left its remains in the mountainous tract of country between the western shores of ,lough ,mask and ,killary ,harbour4#bhi ,there appear to have been at least three separate centres of eruption along a line stretching in a north-easterly direction for about #af miles from the western end of ,lough ,nafooey to the hamlet of ,derrindaffdery beyond ,tourmakeady1 where the older rocks are unconformably overlain by the lower ,carboniferous strata4 ,as shown by the mapping of the ,geological ,survey1 the most northerly area1 which may be called the ,tourmakeady centre1 has a breadth of about a mile1 and dies out southward after a course of nearly six miles4 ,about a mile to the south-west of the last visible prolongation of its rocks1 we encounter a second volcanic centre which occupies an area of about a square mile in the valley of ,glensaul4 ,the third centre stretches from the western shores of ,lough ,mask across ,lough ,nafooey1 where it forms a mass of high rugged ground1 and reaches a length of some six or seven miles before it finally dies out4#bij ,footnote #bhi3 ,this group was placed in the ,upper ,silurian series by the officers of the ,geological ,survey who mapped the region 7see ,sheets #hd1 #he1 #id and #ie of the ,geological ,map of ,ireland and accompanying ,explanation71 and on their testimony ,i formerly referred to the volcanic rocks as of ,upper ,silurian age4 ,mr4 ,baily1 however1 had pointed out that the limestone associated with the lavas and agglomerates contains ,bala fossils4 ,yet1 in spite of this palontological testimony1 the fossils were considered to be derivative1 and the rocks were removed from the series of formations to which they would naturally be assigned4 ,a recent examination of the ground1 in company with ,mr4 ,j4 ,r4 ,kilroe of the ,geological ,survey1 has satisfied me that the volcanic rocks are interstratified with sedimentary deposits of ,bala age1 and must consequently be grouped with the rest of the ,lower ,silurian series of ,ireland4 ,the results of this examination are given in the text4 ,footnote #bij3 ,these areas were carefully mapped for the ,survey by ,mr4 ,nolan1 and the lines of division marked by him fairly represent the general distribution of the rocks4 ,the rocks in each of these three areas are similar4 ,one of their distinguishing features is the intercalation among them of a fossiliferous limestone and calcareous fossiliferous tuffs1 which contain well-preserved species of organisms characteristic of the ,bala division of the ,lower ,silurian rocks4#bia ,there cannot be any question that these organisms were living at the time the strata in which their remains occur are found4 ,the most delicate parts of the sculpture on ,illnus ,bowmanni and ,orthis elegantula are well preserved4 ,nor have the limestones been pushed into their present places by volcanic agency1 or by faults in the terrestrial crust4 ,they are not only regularly intercalated among the volcanic rocks1 but the limestone in some places abounds in volcanic dust1 while above it come calcareous tuffs1 also containing the same fossils4 ,it is thus clearly established that the volcanic series now to be described has its geological age definitely fixed as that of the ,bala period4 ,footnote #bia3 ,see the list of fossils as determined by ,mr4 ,baily in ,explanatory ,memoir to accompany ,sheets #gc1 #gd1 #hc and #hd of the ,geological ,survey of ,ireland1 p4 #fh 7#ahgf74 ,the lavas of the ,lough ,mask region consist of felsites and andesites with rocks of probably more basic composition4 ,the felsites are generally quartziferous porphyries1 which occupy a considerable space in each of the three districts4 ,to what extent they are intrusive rather than interstratified remains for investigation4 ,some of them have undoubtedly invaded other members of the volcanic series4 ,but1 on the other hand1 fragments of similar quartz-porphyries and felsites abound in the intercalated bands of volcanic breccia4 ,the andesites and more basic lavas are finely-crystalline or compact1 dull-green to chocolate-purple rocks1 often resembling the porphyrites of the ,old ,red ,sandstone4 ,some of them are strongly vesicular1 the cavities being filled with calcite on fresh fracture1 though empty on weathered surfaces4 ,the sack-like or pillow structure1 already referred to as characteristic of many ,lower ,silurian lavas1 appears conspicuously among some of these rocks4 ,at ,bohaun1 nine miles south from ,westport1 where a prolongation of the volcanic series rises to the surface from under the overlying coarse conglomerates1 ,i observed that1 owing to the compression which the rocks have there undergone1 the pillow-shaped blocks have been squeezed together into rudely polygonal forms1 while their vesicles have been greatly drawn out in the direction of tension4 ,where the rocks have been still more sheared1 the distinct pillow-shaped blocks with their vesicular structure disappear1 while the more fine-grained crusts that surround them have been broken up and appear as fragments involved in a matrix of green schist4 ,intercalated with the lavas are numerous bands of volcanic breccia and fine tuff4 ,the stones in these breccias consist chiefly of various felsites with andesites and more basic lavas4 ,but pieces of jasper1 chert1 shale and grit are not infrequent4 ,in some places abundant blocks of black shale are to be noticed1 probably derived from the ,llandeilo group which exists below1 and which has here and there been ridged up to the surface in the midst of the volcanic rocks4#bib ,near ,shangort ,i noticed in one of these breccias one block measuring #ab feet1 another #bj feet in length and #c or #d feet thick1 composed of alternating bands of grit and slate4 ,it is interesting to note that these strata had already undergone cleavage before disruption1 the bands of slate being strongly cleaved obliquely to the bedding4 ,none of the ,llandeilo or other rocks in the neighbourhood display this structure4 ,the blocks seem to have been derived from some deeper group of strata4 ,they are laid down parallel with the rude bedding of the breccia in which they lie4 ,footnote #bib3 ,in re-examining this region1 ,mr4 ,kilroe has found in the stream west of the monastery1 ,tourmakeady1 an uprise of graptolitic black shale containing forms belonging to the very lowest ,llandeilo or ,upper ,arenig strata1 and a similar band above ,leenane1 ,killary ,harbour4 ,the fine tuffs and thin ashy limestones associated with the thicker band of limestone show the renewal of volcanic explosions after the interval marked by the calcareous deposit which is sometimes #bj or #dj feet thick4 ,in many places this limestone is brecciated and much mingled with volcanic dust and lapilli4 ,at ,shangort1 for example1 the thick tolerably pure limestone is truncated on the west and north sides by a coarse agglomerate probably filling a volcanic vent4 ,a few hundred yards further north1 beyond the interrupting agglomerate1 the limestone reappears on the same line of strike1 but is then found to be nodular and brecciated and much mingled with volcanic detritus4 ,it lies among ashy grits and tuffs4 ,illustration3 ,fig4 #fd4--;,diagram of the general relations of the different groups of rock in the ,lower ,silurian volcanic district along the western shore of ,lough ,mask4 a1 ,llandeilo shales1 cherts and grits2 b1 ,volcanic breccias2 c1 ,felsites and andesites2 d1 ,tuffs and ashy grits and shales2 e1 ,limestone with ,bala fossils2 f1 ,calcareous tuffs and thin bands of ashy limestone with fossils2 g1 ,coarse conglomerate and grits2 h1 ,wenlock strata resting unconformably on the ,bala rocks and passing southwards from these to overlie an older series of schists2 991 ,fault4 ,the general structure of the ground occupied by the ,lough ,mask volcanic rocks is diagrammatically represented in ,fig4 #fd4 ,the thickness of the volcanic series must amount to many hundred feet1 but it has not been precisely determined4 ,the uppermost parts of the series pass under a great thickness of coarse conglomerates and pebbly grits which form the ridge of ,formnamore1 and stretch thence westwards along ,killary ,harbour and through the ,mweelrea mountains4 ,these strata are classed as the ,upper ,silurian on the ,geological ,survey map4 ,since1 however1 they conformably overlie rocks containing ,bala fossils1 and in the ,killary district include green shales which have yielded fossils of the same age1 they doubtless belong in large part to the ,lower ,silurian division4 ,the remarkable coarseness of these conglomerates towards the south1 and their rapid passage into much finer grits and shales towards the north1 probably indicate that they were formed close to the shores of a land composed of schistose rocks1 quartzite and granite1 of which the mountainous tracts of ,connemara are the last relics4 ,a base to the volcanic series is found in the occasional uprise of a short axis of ,llandeilo1 or perhaps even upper ,arenig strata1 containing bands of dark chert and black graptolitic shales4 ,unfortunately the relations of these underlying rocks to the volcanic masses are not very clear1 being obscured by superficial accumulations and also by faulting4 ,it is thus hardly possible to be certain whether they pass up conformably into the base of the volcanic series1 or are covered by it unconformably4 ,the position of this isolated volcanic district in the far west of ,ireland1 the abundance1 variety and thickness of the erupted materials1 and the definite intercalation of these materials in the ,bala or highest division of the ,lower ,silurian series1 acquire a special interest from the history of the nearest ,silurian volcanic area which has now to be described--that of the western shores of the ,dingle promontory4 ,,ii4 ,the ,upper ,silurian ,series ,the latest volcanic eruptions of ,silurian time yet definitely known took place during the accumulation of the ,wenlock and ,ludlow rocks in the far west of ,ireland4 ,no satisfactory record of any contemporaneous phenomena of a like kind has yet been met with in any other ,upper ,silurian district in the ,british ,isles1 unless at ,tortworth in ,gloucestershire1 as above described4 ,so far as at present known1 only one centre of activity has been preserved4 ,it lies among the headlands of ,kerry1 where the land projects furthest west into the stormy ,atlantic4 ,the occurrence of volcanic rocks in this remote area and their geological horizon have been clearly indicated on the maps of the ,geological ,survey4 ,more than thirty years1 however1 have elapsed since some of the mapping was done1 and we must therefore be prepared to find it1 more especially in its petrography1 capable of modification and improvement now4 ,in the country known as the ,dingle promontory1 these traces of contemporaneous volcanic rocks are to be observed at various localities and on several horizons4 ,to the east1 near ,anascaul1 on the northern shore of ,dingle ,bay1 some tuffs occur in what are believed to be ,llandovery strata4 ,but it is on the western coast1 among the headlands and coves that lie to the north and south of ,clogher ,head1 that the best sections are to be seen4 ,the succession of the rocks in this locality was well worked out by ,du ,noyer1 and the ,memoir prepared by him1 with the general introduction by ,jukes1 is an invaluable guide to the geologist who would explore this somewhat inaccessible region4#bic ,the most important correction that will require to be made in the work arises from a mistake as to the true nature of certain rocks which were described as pisolitic tuffs1 but which are nodular felsites4 ,footnote #bic3 ,sheets #afj and #aga of the one-inch map1 and ,memoir on ,sheets #afj1 #afa1 #aga and #agb4 ,by far the most striking geological feature of this singularly interesting and impressive coast-line is to be found in the interstratification of lavas with bands of tuff among abundantly fossiliferous strata which1 from their organic contents1 are unmistakably of the age of the ,wenlock group4 ,these lavas occur in a number of sheets1 separated from each other by tuffs and other fragmental deposits4 ,they thus point to a series of eruptions over a sea-bottom that teemed with ,upper ,silurian life4 ,they consist for the most part of remarkably fine typical nodular felsites4 ,the nodules vary in dimensions from less than a pea to the size of a hen's egg4 ,they are sometimes hollow and lined with quartz-crystals4 ,they vary greatly in number1 some parts being almost free from them and others entirely made up of them4 ,the matrix1 where a fresh fracture can be obtained1 is horny in texture1 and often exhibits an exceedingly beautiful and fine flow-structure4 ,on weathered faces there may be seen thick parallel strips and lenticles of flow-structure like those of the ,snowdon lavas4 ,the upper portions of some of the sheets enclose fragments of foreign rocks4 ,the microscopic examination of a few slices cut from these lavas shows them to be true felsites 7rhyolites7 composed of a microcrystalline aggregate of quartz and felspar1 with layers and patches of cryptocrystalline matter1 and only occasional porphyritic crystals of orthoclase and plagioclase4 ,the pyroclastic rocks associated with these lavas vary from exceedingly fine tuff to coarse agglomerate4 ,some of the finer tuffs contain pumiceous fragments and pieces of grey and red shale2 they pass into fine ashy sandstones and shales1 crowded with fossils1 and into gravelly breccias made up of fragments of different volcanic rocks4 ,but the most extraordinary of these intercalated fragmental strata is a breccia or agglomerate1 about #ae feet thick1 which lies in a thick group of fossiliferous dull-yellow1 ashy and ochreous sandstones4 ,the stones of this bed consist chiefly of blocks of different felsites1 varying up to three feet in length4 ,some of them show most perfect flow-structure2 others are spongy and cellular1 like lumps of pumice4 ,the calcareous sandstone on the top of the breccia is crowded with fossils chiefly in the form of empty casts1 and the same material1 still full of brachiopods1 crinoids1 corals1 etc41 fills up the interstices among the blocks down to the bottom of the breccia1 where similar fossiliferous strata underlie it4 ,nowhere has the volcanic history of a portion of ,palozoic time been more clearly and eloquently recorded than in this remote line of cliffs swept by the gales of the ,atlantic4 ,we see that the ordinary sedimentation of ,upper ,silurian time was quietly proceeding1 fine mud and sand being deposited1 and enclosing the remains of the marine organisms that swarmed over the sea-bottom when volcanic eruptions began4 ,first came discharges of fine dust and small stones1 which sometimes fell so lightly as not seriously to disturb the fauna on the sea-floor1 but at other times followed so rapidly and continuously as to mask the usual sediment and form sheets of tuff and volcanic gravel4 ,occasionally there would come more paroxysmal explosions1 whereby large blocks of lava were hurled forth until they gathered in a thick layer over the bottom4 ,but the life that teemed in the sea1 though temporarily destroyed or driven out1 soon returned4 ,corals1 crinoids and shells found their way back again1 and fine sediment carried their remains with it and filled up the crevices4 ,the ejected volcanic blocks are thus enclosed in a highly fossiliferous matrix4 ,a succession of lava-streams1 of which the strongly-nodular sheet of ,clogher ,head is the thickest and most conspicuous1 mark the culmination of the volcanic energy1 and show how at this late part of the ,silurian period felsites that reproduce some of the most striking peculiarities of earlier time were once more poured out at the surface4 ,a few more discharges of tuff and the outflow of a greenish flinty felsite brought this series of eruptions to an end1 and closed in ,britain the long and varied record of older ,palozoic volcanic activity4#bid ,footnote #bid3 ,as this sheet is passing through the press1 the interesting paper by ,messrs4 ,s4 ,h4 ,reynolds and ,c4 ,j4 ,gardiner1 ,on the ,kildare ,inlier has appeared 7,quart4 ,journ4 ,geol4 ,soc4 vol4 lii4 p4 #ehg74 ,these authors give petrographical details regarding the lavas1 which they show to be andesites and basalts of ,bala age1 associated with highly fossiliferous tuffs4 ,illustration ,,to ,,accompany ,,sir ,,archibald ,,geikie',s ,,ancient ,,volcanoes ,,of ,,britain ,map ,,ii ,,map ,,of ,,the ,,silurian ,,volcanic ,,districts ,,of ,,north ,,wales ,reduced from the ,maps of the ,geological ,survey4 ,the ,edinburgh ,geographical ,institute -- ,copyright -- ,j4 ,g4 ,bartholomew4 ,,book ,v ,,the ,,volcanoes ,,of ,,devonian ,,and ,,old ,,red ,,sandstone ,,time ,,chapter ,,xv ,,the ,,devonian ,,volcanoes ,throughout the whole region of the ,british ,isles1 wherever the uppermost strata of the ,silurian system can be seen to graduate into any later series of sedimentary deposits1 they are found to pass up conformably into an enormous accumulation of red sandstones1 marls1 cornstones1 and conglomerates1 which have long been grouped together under the name of ,old ,red ,sandstone4 ,in ,england and ,wales1 in ,scotland and in ,ireland1 this upward succession is so well shown that at first ,british geologists were naturally disposed to believe it to represent the normal order of the geological record4 ,when1 however1 ,sedgwick and ,murchison demonstrated that in the counties of ,devon and ,cornwall a very different group of strata contained an abundant assemblage of organic remains1 including types which ,lonsdale showed to be intermediate between those of the ,silurian and the ,carboniferous systems2 when1 moreover1 this palontological facies of the south-west of ,england1 termed by its discoverers ,devonian1 was found to be abundantly developed on the ,continent1 and to be there indeed the prevalent stratigraphical type of the formations intervening between ,silurian and ,carboniferous1 geologists began to perceive that the ,old ,red ,sandstone must be regarded as the record of peculiar local conditions of sedimentation1 while the ,devonian type was evidently the more usual development of the same geological period4 ,from the remote ,shetland ,isles1 across the whole of ,scotland and ,england1 down to the northern shores of the ,bristol ,channel1 the ,old ,red ,sandstone maintains its general characters4 ,nowhere1 indeed1 are these characters more typically developed than in ,south ,wales1 where many thousands of feet of red sediments1 almost entirely devoid of organic remains1 emerge from under the escarpments of ,carboniferous ,limestone1 and stretch into broad uplands until they are lost at the top of the ,silurian system4 ,but when the geologist crosses the ,bristol ,channel to the opposite shores of ,north ,devon1 he encounters a remarkably different assemblage of rocks4 ,it is true that he has not yet been able to detect there any equivalents of the uppermost ,silurian strata of ,glamorganshire1 nor does he find any conspicuous band of ,carboniferous ,limestone1 such as that which encircles the ,welsh ,coal-field4 ,he is thus unable to start from a known definite horizon in the attempt to work out the order of succession1 either in an upward or downward direction4 ,lithological characters likewise afford him no means of establishing any satisfactory parallelism4 ,as he follows the ,devonian strata1 however1 he finds them to disappear conformably under the ,culm-measures1 which1 though strangely unlike the ,carboniferous strata on the opposite coast1 are yet proved by their fossils to belong to the ,carboniferous system4 ,hence the ,devonian type1 like the ,old ,red ,sandstone1 is proved to be immediately anterior to1 and to graduate into1 the ,carboniferous rocks4 ,there is no stratigraphical change in ,britain so rapid and complete as that from the ,old ,red ,sandstone on the one side of the ,bristol ,channel to the ,devonian series on the other4 ,no satisfactory explanation has yet been found for this sudden transformation1 which still remains one of the unsolved problems in ,british geology4 ,as the observer follows the ,devonian assemblage across the land to the southern coast-line1 he is conscious that its general characters1 both lithological and palontological1 depart more and more from the type of the ,old ,red ,sandstone1 and approach more closely to the common ,devonian facies of the ,continent4 ,he is forced to admit that the ,old ,red ,sandstone1 notwithstanding its extensive development in ,britain1 must be regarded as an exceptional type of sedimentation1 while the ,devonian facies represents that which is most widely prevalent1 not only in ,europe1 but generally over the world4 ,the broad estuary of the ,bristol ,channel unfortunately conceals from view the tract which lies between the typical ,old ,red ,sandstone of ,glamorganshire and the typical ,devonian formations of ,devonshire4 ,whether this intervening space of some fifteen miles was occupied by a physical barrier1 which separated the respective areas of deposit of these two types1 or the circumstances of sedimentation in the one region merged insensibly into those of the other1 must remain matter for speculation4 ,the geographical conditions betokened by the ,old ,red ,sandstone will be considered in the next chapter4 ,there can be no doubt that those indicated by the ,devonian system were marine4 ,the organic remains so plentifully distributed through the argillaceous and arenaceous sediments of that system1 and so crowded together in its limestones1 were obviously denizens of the open sea4 ,yet the only tract of ,britain over which this sea can be shown to have spread was the south of ,england4 ,to the north of that belt1 the site of ,britain during ,devonian time appears to have been partly land and partly wide water-basins in which the ,old ,red ,sandstone was deposited4 ,in that half terrestrial half lacustrine territory that stretched northwards to beyond the ,shetland ,isles1 many volcanoes were active1 of which the chronicles will be described in later pages4 ,the most southerly of these centres of eruption yet known was the district of the ,cheviot ,hills4 ,throughout the rest of ,england and ,wales no trace of any contemporary volcanic action has been detected in the ,old ,red ,sandstone4 ,it is true that over most of that region rocks of this age have been concealed under younger formations4 ,yet throughout ,wales1 where the ,old ,red ,sandstone attains so vast a thickness1 and covers so wide an area1 it has not yet yielded a vestige of any contemporaneous volcanic eruptions4 ,but over the sea-floor that covered the south of ,england1 and stretched thence into the heart of ,europe1 abundant volcanoes have left behind them proofs of their activity4 ,the first geologist who recognized these proofs and traced their extent on the ground appears to have been ,de la ,beche1 who1 by his detailed maps and careful description of the igneous rocks of ,devonshire1 did so much to advance the study of ancient volcanic action4 ,this great pioneer not only determined the former existence of ,devonian volcanoes1 but he was likewise the first to detect and map the volcanic rocks associated with the ,carboniferous and ,new ,red ,sandstone formations of the same region4 ,the broad outlines traced by him among the volcanic products of these three geological periods in the south-west of ,england still remain but little changed4 ,nor are they likely to be much improved until the ground is resurveyed on a larger and more accurate map1 and with better petrographical equipment than were available in his day4 ,not long after the observations of ,de la ,beche came those of ,a4 ,c4 ,godwin-,austen1 who devoted much time to a sedulous exploration of the rocks of ,south ,devon1 and satisfied himself that contemporaneous volcanic sheets were intercalated among the limestones of that district4 ,the coral limestones1 he says1 are in many places superincumbent on great sheets of volcanic materials1 with which1 in some instances1 as at ,north ,whilborough1 they alternate4 ,he pointed out that the interstratified volcanic rocks are of two periods1 one ,devonian and the other ,carboniferous4#bie ,footnote #bie3 ,trans4 ,geol4 ,soc4 #bnd ser4 vol4 vi4 7#ahdb71 pp4 #dfe1 #dgj1 #dgc4 ,in his ,geological ,maps of ,devon and ,cornwall1 which are to the present time those issued by the ,geological ,survey1 ,de la ,beche made no attempt to discriminate between the varieties of igneous rocks1 save that the basic greenstones were distinguished from the acid bosses of granite and the elvans4 ,but in his classic ,report much more detail was inserted1 showing that he clearly recognized the existence both of volcanic ashes and of lavas1 as well as of intrusive sheets4 ,at the outset of his account of the ,grauwacke1 he remarks that the sedimentary deposits are accompanied with igneous products1 a portion of which may also be termed sedimentary1 inasmuch as it would seem to have been deposited in beds among contemporaneous rocks of the former description by the agency of water1 after having been ejected from fissures or craters in the shape of ashes and cinders1 precisely as we may now expect would happen with the ashes and cinders ejected from volcanoes1 particularly insular and littoral volcanoes1 into the sea4#bif ,again he speaks of two kinds of trappean rocks having probably been erupted1 one in the state of igneous fusion1 and the other in that of ash1 during the time that the mud1 now forming slates1 was deposited1 the mixtures of volcanic and sedimentary materials being irregular from the irregular action of the respective causes which produced them2 so that though the one may have been derived from igneous action1 and the other from the ordinary abrasion of pre-existing solid rocks1 they were geologically contemporaneous4#big ,he recognized the origin of the amygdaloidal varieties of rock1 and by dissolving out the calcite from their cells showed how close was their resemblance to modern pumice4#bih ,footnote #bif3 ,report on the ,geology of ,cornwall1 ,devon and ,west ,somerset1 ,mem4 ,geol4 ,survey1 #ahci1 p4 #cg4 ,footnote #big3 ,op4 cit4 p4 #eg4 ,footnote #bih3 ,op4 cit4 pp4 #eg1 #fa4 ,since these early researches many geologists have studied the igneous rocks of ,devonshire4 ,i would especially refer to the labours of ,mr4 ,allport1#bii the late ,j4 ,a4 ,phillips1#cjj ,mr4 ,rutley1#cja the late ,mr4 ,champernowne1#cjb ,mr4 ,w4 ,a4 ,e4 ,ussher1#cjc ,mr4 ,hobson1#cjd and ,general ,m',mahon4#cje ,mr4 ,champernowne in particular has shown the abundance of volcanic material among the rocks of ,devonshire1 and the resemblance which in this respect they offer to the ,devonian system of ,north ,germany4 ,footnote #bii3 ,quart4 ,journ4 ,geol4 ,soc4 xxxii4 7#ahgf71 p4 #dah4 ,footnote #cjj3 ,op4 cit4 xxxi4 7#ahge7 p4 #cbe1 xxxii4 7#ahgf7 p4 #aee1 xxxiv4 7#ahgh7 p4 #dga4 ,footnote #cja3 ,brent ,tor1 ,mem4 ,geol4 ,surv4 p4 #ah2 ,quart4 ,journ4 ,geol4 ,soc4 lii4 7#ahif71 p4 #ff4 ,footnote #cjb3 ,see in particular his last paper ,on the ,ashprington ,volcanic ,series of ,south ,devon1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xlv4 7#ahhi71 p4 #cfi4 ,footnote #cjc3 ,this geologist has spent many laborious years in the investigation of the geology of ,devonshire1 and has published numerous papers on the subject1 in the ,transactions of the ,devonshire ,association and of the ,royal ,cornwall ,geological ,society1 in the ,proceedings of the ,somersetshire ,archological and ,natural ,history ,society1 and of the ,geologists' ,association1 in the ,geological ,magazine1 and the ,quarterly ,journal of the ,geological ,society4 ,reference may especially be made to his ,memoir in the last named journal1 vol4 xlvi4 7#ahij71 p4 #dhg4 ,footnote #cjd3 ,quart4 ,journ4 ,geol4 ,soc4 xlviii4 7#ahib71 p4 #dif4 ,footnote #cje3 ,op4 cit4 xlix4 7#ahic71 p4 #che4 ,unfortunately the geological structure of the ,palozoic rocks of the ,south-west of ,england has been complicated to an amazing extent by plication and fracture1 with concomitant cleavage and metamorphism4 ,hence it is a task of extreme difficulty to trace out with any certainty definite stratigraphical horizons1 and to determine the range of contemporaneous volcanic action4 ,mr4 ,ussher has shown with what success this task may be accomplished when it is pursued on a basis of minute mapping1 combined with a sedulous collection and determination of fossils4#cjf ,but years must necessarily elapse before such detailed work is carried over the whole ,devonian region1 and probably not till then will the story of the volcanic history of the rocks be adequately made out4 ,footnote #cjf3 ,see ,memoir cited in a previous note4 ,in the meantime1 it has been established that while there is a singular absence of igneous rocks in ,north ,devon1 a strip of country extending from ,newton ,abbot and ,torquay westwards by ,plymouth across ,cornwall to ,penzance contains abundant records of volcanic action4 ,it has not yet been possible to map out1 among what were formerly all grouped together as greenstones1 the respective limits of the bedded lavas and the tuffs1 to distinguish the true sills1 and to fix on the position of the chief vents of eruption4 ,so intense have been the compression and shearing of the rocks that solid sheets of diabase have been crushed into fissile schists1 which can hardly be distinguished from tuffs4 ,moreover1 owing perhaps in large measure to the mantle of red ,permian 7or ,triassic7 strata1 which has been stripped off by denudation from large tracts of this region once overspread by it1 the ,devonian rocks have been deeply raddled1 or stained red4 ,but probably one of the main sources of difficulty in studying the petrography of the area is to be found in the results of atmospheric weathering4 ,devonshire lies in that southern non-glaciated strip of ,england1 where the rocks have been undergoing continuous decay since long before the ,ice ,age4 ,no ploughshare of ice has there swept off the deep weathered crust1 so as to leave hard surfaces of rock1 fresh and bare1 under a protecting sheet of boulder-clay4 ,it is seldom that a really fresh piece of any igneous rock can be procured among the lanes and shallow pits of ,devon1 where alone1 for the most part1 the materials are exposed4 ,much1 therefore1 remains to be done1 both in the stratigraphy and petrography of the ,devonian volcanic rocks of this country4 ,to the late ,j4 ,a4 ,phillips geology is indebted for the first detailed chemical and microscopical investigation of these rocks4 ,he clearly showed the truly volcanic origin of many of the so-called greenstones4 ,he believed that certain slaty blue elvans1 which he found to have a composition identical with that of altered dolerites1 might be highly metamorphosed tuffs1 and that others might have been originally sheets of volcanic mud4 ,after studying the chemical composition and minute structure of a large collection of greenstones1 he demonstrated that in all essential particulars they were dolerites1 though somewhat altered from their original character4#cjg ,subsequently they were studied by ,dr4 ,hatch1 who found the fresher specimens generally to possess an ophitic structure1 while some are granular1 others porphyritic4#cjh ,footnote #cjg3 ,see especially ,quart4 ,journ4 ,geol4 ,soc4 vols4 xxxii4 and xxxiv4 ,footnote #cjh3 ,a few of the eruptive rocks of ,devonshire have recently been studied by ,k4 ,busz4 ,he finds most of his specimens 7chiefly from the ,torquay district7 to be varieties of diabase1 but describes a palopicrite from ,highweek near ,newton ,bushel1 and a kersantite from ,south ,brent on the south-east edge of ,dartmoor 7,neues ,jahrb4 #ahif1 p4 #eg74 ,although the rocks have undergone so much crushing1 solid cores of them1 showing the original structure1 may be obtained1 also examples of the amygdaloidal1 vesicular or slaggy character4 ,they occur in sheets either singly or in groups1 and appear generally to be regularly interstratified in the slates and grits4 ,while some of these intercalations1 especially the amygdaloidal sheets1 may be true superficial lavas1 it can hardly be doubted that others are sills1 especially those which assume the crystalline structure and composition of gabbros1 and show an entire absence of the vesicular structure4 ,but no one has yet attempted to separate the two types from each other4 ,with these rocks are associated abundant diabase-tuffs 7schalstein71 frequently mingled with ordinary non-volcanic detrital matter1 and shading off into the surrounding grits and slates4 ,there is thus clear evidence of the outpouring of basic lavas and showers of ashes during the ,devonian period in the south-west of ,england1 under conditions analogous to those which characterized the deposition of the ,devonian system in ,nassau and the ,harz4 ,the exact range of these eruptions in geological time has still to be ascertained4 ,so far as at present determined1 volcanic activity was not awakened during the accumulation of the ,lower ,devonian formations4 ,it was not until the sporadic coral-reefs and shell-banks had grown up1 which form the basement limestones of the ,middle ,devonian group1 that the first eruptions took place4 ,as ,godwin-,austen1 ,champernowne and ,mr4 ,ussher have shown1 some of these reefs were overwhelmed with streams of lava or buried under showers of ashes4 ,the volcanic discharges1 however1 were peculiarly local1 probably from many scattered vents1 and never on any great scale4 ,some districts remained little or not at all affected by them1 so that the growth of limestone went on without interruption1 or at least with no serious break4 ,in other areas again the place of the limestone is taken by volcanic materials4 ,the chief epoch of this volcanic action1 marked by the ,ashprington ,volcanic ,series1 appears to have occurred about midway in the ,middle ,devonian period4 ,but in certain districts it extended into ,upper ,devonian time4 ,intrusive sills of diabase may mark the later phases of the volcanic history4 ,but the occurrence of such sills even in the ,upper ,devonian rocks1 and the alteration of the strata in contact with them 7spilosite71 point to the continuance or renewal of subterranean disturbance even in the later ,devonian ages1 if not in subsequent geological time4 ,that volcanic activity accompanied the deposition of the ,carboniferous rocks of ,devonshire has long been well known 7see ,chapter xxix474 ,,chapter ,,xvi ,,the ,,volcanoes ,,of ,,the ,,old ,,red ,,sandstone ,geological ,revolutions at the close of the ,silurian ,period--,physical ,geography of the ,old ,red ,sandstone--,old ,lake-basins1 their ,flora and ,fauna--,abundance of ,volcanoes--,history of ,investigation in the ,subject4 ,we now enter upon the consideration of the records of a notable era in the geological evolution of north-western ,europe4 ,up to the close of the ,silurian period the long history embodied in the rocks presents a constant succession of slowly sinking sea-floors4 ,wide tracts of ocean stretched over most of ,europe1 and across the shifting bottom1 sand and mud1 washed from lands that have long vanished1 spread in an ever-accumulating pile4 ,now and then1 some terrestrial movement of more than usual potency upraised this monotonous sea-bed1 but the old conditions of ceaseless waste continued1 and fresh sheets of detritus were thrown down upon the broken-up heaps of older sediment4 ,all through the vast cycles of time denoted by these accumulations of strata1 generations of sea-creatures came and went in long procession1 leaving their relics amidst the ooze of the bottom4 ,genera and families1 once abundant1 gradually died out1 and gave place to others1 the onward march of life being slow but uninterrupted4 ,of the land of the time or of the plants and animals that lived on its surface1 hardly anything is known4 ,the chronicles that have come down to us are almost wholly records of the vicissitudes of the ocean-bed4 ,over the centre and south of ,europe1 the marine conditions of ,silurian time were prolonged1 as we have seen1 into the next period1 when the ,devonian formations were deposited4 ,in that wide region1 no marked break has been traced between either the sedimentation or the animal life of the ,silurian and ,devonian periods4 ,but in the north-west of ,europe a striking departure took place from the protracted monotony of marine conditions4 ,by a series of terrestrial movements that affected the area lying to the north of the line of the ,bristol ,channel1 and extended not only to the furthest limit of the ,british ,isles1 but probably as far as ,norway1 and perhaps even into northern ,russia1 the previous widespread conditions of marine sedimentation were entirely altered4 ,instead of the fine oceanic silts and sands with their abundant organic remains1 and the thick limestones with their masses of coral and crowds of crinoids1 there were now laid down1 over these northern regions1 vast piles of deep red sediment1 from which traces of animal life are almost wholly absent4 ,the shelving land against which these ferruginous sands and gravels gathered can still in part be recognized4 ,as the observer follows its margin1 notes the varying local peculiarities of its sediment1 and detects1 sometimes in great abundance1 remains of the vegetation which clothed it1 the conviction grows in his mind that the remarkable contrast between these deposits1 known as the ,old ,red ,sandstone1 and those of the ,silurian and ,devonian systems is not to be accounted for by any mere rearrangement of the sea-bottom1 or redistribution of the land that supplied that sea-bottom with sediment4 ,it has long been the general belief among geologists that the subterranean movements which1 over the greater part of ,britain1 brought the deposition of the ,upper ,silurian formations to a close1 led to a total alteration of the geography of the region affected1 that the sea-floor was elevated1 and that1 over the upraised tract1 large lakes or inland seas were eventually formed1 in which the peculiar sediments of the ,old ,red ,sandstone were accumulated4 ,the records of this series of geographical changes are too fragmentary to enable us to follow1 except in a very general way1 the sequence of events in the transformation of the ,silurian sea into the peculiar topographical conditions in which the ,old ,red ,sandstone was laid down4 ,while there was a widespread elevation of the sea-floor1 and of such ridges of insular land as may have risen above sea-level1 the upheaval appears to have been of a somewhat complicated kind1 and to have been combined with many local subsidences4 ,the area of disturbance was probably thrown into a series of parallel ridges and troughs1 the former continuing to be pushed upward1 while the latter tended to subside4 ,the ridges thus became land surfaces1 and their prolonged elevation may have more or less compensated for the denudation to which1 on their emergence1 they were necessarily exposed4 ,the troughs1 on the other hand1 which sank down1 may in many cases have subsided below the sea4 ,but where the general upheaval of the crust was most pronounced1 some of the depressions would be isolated above sea-level and become lake-basins in the terrestrial areas4 ,of some of these water-basins the outlines can still in some measure be defined4 ,the rocks that rose into hills around them1 and from which their enormous accumulations of detritus were derived1 still partially survive4 ,we can explore these piles of sediment1 and from them can form some idea of the condition of the water in the lakes1 and the nature of the vegetation on the surrounding land4 ,the frequent occurrence and exceeding coarseness of the conglomerates1 which appear on many successive horizons throughout the deposits of these basins1 probably indicate contemporaneous terrestrial disturbances4 ,the same causes that led to the wrinkling of the crust into parallel ridges and troughs no doubt still continued in operation4 ,from time to time the ridges1 much worn down by prolonged denudation1 were pushed upward1 either by gradual uprise or by more rapid jerks4 ,the troughs may in like manner have been still affected by their old tendency to subsidence4 ,hence1 in spite of the effects of degradation and deposition1 it is possible that the ridges may not1 on the whole1 have varied much in height1 nor the basins in depth1 during the time when thousands of feet were stripped off the land and strewn in detritus over the bottoms of the lakes4 ,let us try to realize a little more definitely the general aspect of the region in which the ,old ,red ,sandstone water-basins lay4 ,as the axes of the folds into which the crust of the earth was thrown ran in a north-east and south-west direction1 they gave this trend to the lakes and to the tracts of land that separated them4 ,these intervening ridges must in some instances have been hilly or even mountainous4 ,thus1 the ,scottish ,highlands rose between two of the lakes1 and poured into them an abundant tribute of gravel1 sand and silt4 ,the terrestrial vegetation of the time has been partially preserved4 ,the hills seem to have been clothed with conifers1 while the lower slopes and swamps were green with sigillari1 lepidodendra and calamites4 ,one of the most characteristic plants was ,psilophyton1 of which large matted sheets were drifted across the lakes and entombed in the silt of the bottom4 ,a grass-like vegetation1 with long linear leaves1 seems to have grown thickly in some of the shallows of the lakes4 ,of the land animals we have still less knowledge than of the vegetation4 ,doubtless various forms of insect life flitted through the woodlands1 though no relics of their forms have yet been recovered4 ,but the remains of myriapods have been found in ,forfarshire4#cji ,these early relics of the animal life of the land inhabited the woodlands1 like our modern gally-worms1 and were swept down into the lakes1 together with large quantities of vegetation4 ,footnote #cji3 ,mr4 ,b4 ,n4 ,peach1 ,proceedings of ,royal ,physical ,society of ,edinburgh1 vol4 vii4 7#ahhb74 ,some of the lakes1 especially in the earlier part of their history1 abounded in eurypterid crustacea4 ,these animals inhabited the seas in ,upper ,silurian time1 and appear to have been isolated in the water-basins of the ,old ,red ,sandstone4 ,certain species of ,pterygotus1 a ,silurian genus found also in the ,lower ,old ,red ,sandstone1 reached a length of six feet4 ,but the most abundant forms of animal life were fishes4 ,these furnish additional evidence in favour of the lacustrine nature of the waters in which they lived4 ,such characteristically marine forms as the sharks and rays of the ,silurian seas were replaced by genera of ,acanthodians1 ,ostracoderms1 ,dipnoids1 ,teleostomes1 ,placoderms1 and ,paloniscids1 which abounded in the more northerly waters4 ,the distinctive outward characters of many of these early vertebrates were their bony scales and plates4 ,some of them had their heads encased in an armature of bone1 of large size and massive thickness4 ,in several genera the bone was coated with a layer of glittering enamel4 ,even now1 after the vast lapse of time since their day1 the cuirasses and scale-armour of these fishes keep their bright sheen in the hardened sand and mud from which they are disinterred4 ,a difference is observable between the faunas of the different water-basins4 ,even where the same genus occurs in two adjacent areas1 the species are often distinct4 ,two large lakes1 separated by the tract of the ,scottish ,highlands1 had each its own assemblage of fishes1 not a single genus being common to the two basins4 ,such contrasts1 whether the two lakes were geologically contemporaneous1 or the northern arose later than the southern1 undoubtedly indicate long-continued isolation and the gradual evolution of new forms under different conditions of environment4#caj ,footnote #caj3 ,in my memoir ,on the ,old ,red ,sandstone of ,western ,europe 7,trans4 ,roy4 ,soc4 ,edin4 vol4 xxviii4 #ahgh71 ,i argued for the probable geological contemporaneity of the conglomerates1 sandstones and flagstones on either side of the ,grampian chain1 even although their organic contents were so unlike4 ,the stratigraphical evidence favours this view4 ,in each case a thick series of strata is covered unconformably by ,upper ,old ,red ,sandstone1 containing ,holoptychius nobilissimus and other fishes4 ,the question cannot perhaps be definitely settled by the data available in ,scotland4 ,it is quite possible that the basin on the northern side of the ,grampians1 which ,i have termed ,lake ,orcadie1 came into existence after that on the southern side4 ,but ,i do not think the differences in their respective faunas are to be accounted for simply by lapse of time and the gradual organic evolution in progress over one continuous region4 ,the more the ,old ,red ,sandstone is studied1 the more local do its various fish-faunas appear to have been4 ,these strongly-marked diversities appear to me rather to point to prolonged isolation of the basins from each other1 as stated above4 ,dr4 ,traquair has drawn attention to the remarkable fact that1 even in what appears to be one continuous series of strata of no great thickness forming the ,upper ,old ,red ,sandstone of the ,moray ,firth basin1 the fishes found about ,nairn are entirely different from those met with in the rest of the region4 ,such1 in brief1 were the aspects of the physical geography of the time on the further consideration of which we are now to enter4 ,the subterranean disturbances1 so characteristic of the period1 were accompanied by a display of volcanic activity more widespread1 perhaps1 than any which had yet taken place in the geological history of ,britain4 ,nevertheless1 it is worthy of remark that this manifestation of underground energy did not begin with the commencement of these displacements of the crust4 ,the earliest eruptions only took place after the geography of the region had been completely changed2 at least no trace of them is to be found in the earliest portions of the ,old ,red ,sandstone4 ,after the last lingering ,silurian volcanoes in the west of ,ireland had died out1 a protracted quiescence of the subterranean fires ensued4 ,in the latest ages of ,silurian time there was not in ,britain1 so far as at present known1 a single volcanic eruption4 ,not until after the inauguration of the ,old ,red ,sandstone topography1 when the lakes had taken shape and had begun to be filled with sediment from the surrounding hills1 did a series of new volcanoes burst into activity over the northern half of ,britain4 ,rising in the midst of the lakes in groups of separate cones1 these vents poured out floods of lava1 together with clouds of ashes and stones4 ,their sites1 the history of their eruptions1 and the piles of material ejected by them1 can still be ascertained1 and ,i shall now proceed to give some account of them4 ,the thick mass of sedimentary material known as the ,old ,red ,sandstone1 lying between the top of the ,silurian and the base of the ,carboniferous system1 has been divided into two sections1 which1 however1 are of unequal dimensions1 and doubtless represent very unequal periods of time4 ,the older series1 or ,lower ,old ,red ,sandstone1 is by far the more important and interesting in its extent1 thickness1 palontological riches1 and1 what specially concerns us in the present inquiry1 in its volcanic records4 ,wherever its true base can be seen1 this series passes down conformably into ,upper ,silurian strata4 ,it sometimes reaches a thickness of #ae1jjj and even #bj1jjj feet4 ,there is generally a marked break between its highest visible strata and all younger formations4 ,even the upper division of the ,old ,red ,sandstone rests unconformably upon the lower4#caa ,such a hiatus undoubtedly points to a considerable lapse of geological time1 and to the advent of important geographical changes that considerably modified the remarkable topography of the older part of the period4 ,footnote #caa3 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xvi4 7#ahfj71 p4 #cab4 ,in ,wales no break has actually been discovered between the two divisions of the ,old ,red ,sandstone1 though it is suspected to exist there also4 ,the younger division or ,upper ,old ,red ,sandstone passes upward conformably into the base of the ,carboniferous system4 ,its red and yellow sandstones1 conglomerates and breccias1 covering much more restricted areas1 and attaining a much less thickness than those of the lower division1 indicate the diminution and gradual effacement of the lakes of the older time1 and the eventual return of the sea to the tracts from which it had been so long excluded4 ,so vast an interval elapsed between the time recorded in the deposits respectively of the two sections of the ,old ,red ,sandstone that the characteristic forms of animal life in the earlier ages had entirely passed away1 and their places had been taken by other types when the diminished lake-basins of the second period began to be filled up4 ,volcanic action also dwindled to such a degree that in contrast to the abundant vents of the older period1 only one or two widely scattered groups of vents are known to have existed in the area of the ,british ,isles during the later period1 and these1 after a feeble activity1 gave way to a prolonged volcanic quiescence1 which lasted until the earlier ages of the succeeding or ,carboniferous period4 ,although geologists are in the habit of grouping the ,old ,red ,sandstone and the ,devonian rocks as equivalent or homotaxial formations1 deposited in distinct areas under considerably different conditions of sedimentation1 the attempt to follow out the sequence of strata in ,devonshire1 and to trace some analogy between the ,devonian succession and that of the ,old ,red ,sandstone1 presents many difficulties for which no obvious solution suggests itself4 ,into these problems it is not needful to enter further than was done in the last chapter4 ,we may assume that not improbably some of the eruptions now to be described were coeval with those of ,devonian time in the south-west of ,england1 though we may hesitate to decide which of them should be brought into parallelism4 ,as we trace the shore-lines of the ancient basins of the ,lower ,old ,red ,sandstone1 and walk over the shingle of their beaches1 or as we examine the silt of their deeper gulfs1 and exhume the remains of the plants that shaded their borders1 and of the fishes that swarmed in their waters1 we gradually learn that although the sediments which accumulated in some of these basins amount to many thousand feet in thickness2 yet from bottom to top they abound in evidence of shallow-water conditions of deposit4 ,the terrestrial disturbances above referred to continued for a vast interval1 and while1 as already suggested1 the floors of the basins sank1 and the intervening tracts were ridged up1 as the results of one great movement of the earth's crust1 the denudation of the surface of the land contributed to the basins such a constant influx of sediment as1 on the whole1 compensated for the gradual depression of their bottoms4 ,we need not suppose that these movements of subsidence and upheaval were uninterrupted and uniform4 ,indeed1 the abundant coarse conglomerates1 which play so prominent a part in the materials thrown into the basins1 suggest that at various intervals during the prolonged sedimentation subterranean disturbances were specially vigorous4 ,but the occurrence of strong unconformabilities among the deposits of the basins sets this question at rest1 by proving that the terrestrial movements were so great as sometimes to break up the floor of a lake1 and to place its older sediments on end1 in which position they were covered up and deeply buried by the succeeding deposits4#cab ,footnote #cab3 ,an unconformability of this kind occurs between the south end of the ,pentland ,hills and ,tinto in ,lanarkshire1 and another in ,ayrshire4 ,it is not surprising to discover1 among these evidences of great terrestrial disturbance1 that eventually groups of volcanoes rose in long lines from the waters of most of the lakes1 and threw out enormous quantities of lava and ashes over tracts hundreds of square miles in extent4 ,so vast1 indeed1 were these discharges1 across what is now the ,midland ,valley of ,scotland1 that the portions of sheets of lava and tuff visible at the surface form some of the most conspicuous ranges of hills in that district1 stretching continuously for #dj or #ej miles and reaching heights of more than #bjjj feet above the sea4 ,exposed in hundreds of ravines and escarpments1 and dissected by the waves along both the eastern and western coasts of the country1 these volcanic records may be studied with a fulness of detail which cannot be found among earlier ,palozoic formations4 ,it might have been supposed that a series of rocks so well displayed and so full of interest1 would long ere this have been fully examined and described4 ,but they can hardly be said to have yet received1 as a whole1 the attention they deserve4 ,without enumerating all the writers who1 each in his own measure1 have added to the sum of our knowledge of the subject1 ,i may refer to the labours of ,jameson1#cac ,macknight#cad and ,fleming1#cae among the observers who began the investigation4 ,but of the early pioneers1 by far the most important in regard to the igneous rocks of the ,old ,red ,sandstone was ,ami ,bou4 ,while attending the ,university of ,edinburgh1 where he took the degree of ,m4,d4 in the year #ahaf1 he imbibed from ,jameson a love of mineralogy and geognosy1 and for several years spent his leisure time in personally visiting many parts of ,scotland1 in order to study the geological structure of the country4 ,probably in #ahbj he published in ,french his now classic ,essai4#caf ,the value of this work as an original contribution to the geology of the ,british ,isles has probably never been adequately acknowledged4 ,for this want of due recognition the author himself was no doubt in some measure to blame4 ,he refers distinctly enough to various previous writers1 notably to ,jameson and ,macculloch1 but he mingles the results of his own personal examinations with theirs in such a way that it is hardly possible to ascertain what portions are the outcome of his own original observations4 ,less credit has accordingly been given to him than he could fairly have claimed for solid additions to the subjects of which he treated4 ,in the later years of his life ,i had opportunities of learning personally from him how extensive had been his early peregrinations in ,scotland1 and how vivid were the recollections which1 after the lapse of half a century1 he still retained of them4 ,judged simply as a well-ordered summary of all the known facts regarding the geology of ,scotland1 his ,essai must be regarded as a work of very great value4 ,especially important is his arrangement of the volcanic phenomena of the country1 which stands far in advance of anything of the kind previously attempted4 ,under the head of the ,terrain ,volcanique1 he treats of the basaltic formations1 distinguishing them as sheets 7nappes1 coules7 and dykes2 and of the felspathic or trachytic formations1 which he subdivides into phonolites1 trachytes1 porphyries 7forming mountains and also sheets7 and felspathic or trachytic dykes4 ,in the details supplied under each of these sections he gives facts and deductions which were obviously the result of his own independent examination of the ground1 and he likewise marshals the data accumulated by ,jameson1 ,macculloch and others1 in such a way as to present a more comprehensive and definite picture of the volcanic phenomena of ,scotland than any previous writer had ventured to give4 ,footnote #cac3 ,memoirs of the ,wernerian ,society1 vol4 ii4 7#ahaa71 pp4 #agh1 #bag1 #fah2 vol4 iii4 7#ahbj71 p4 #bbj1 #bbe4 ,footnote #cad3 ,op4 cit4 vol4 ii4 pp4 #abc1 #dfa4 ,footnote #cae3 ,op4 cit4 vol4 i4 7#ahjh71 p4 #afb2 vol4 ii4 7#ahaa71 pp4 #ach1 #cci4 ,footnote #caf3 ,essai gologique sur l'cosse 7,paris2 no date1 but probably about #ahbj74 ,he acknowledges his indebtedness to ,jameson1 whose demonstrations of the geology of the ,edinburgh district he partly reproduced in his book4 ,jameson's early writings in the ,wernerian ,memoirs and in separate works were mere mineralogical or geognostical descriptions4 ,his later lectures became more valuable but were never published1 save indirectly in so far as they influenced the opinions of his pupils who published writings on the same subjects4 ,see1 for instance1 ,hay ,cunningham's ,geology of the ,lothians1 p4 #ei1 footnote4 ,compare an article on ,bou1 ,edinburgh ,review for ,may #ahbc 7vol4 xxxviii4 p4 #dac74 ,the account which ,bou wrote of the ,old ,red ,sandstone and its associated igneous rocks marked the first great forward step in the investigation of this section of the geological record4 ,he was the earliest observer to divide what he calls the roches feldspathiques et trappennes into groups according to their geological position and mineralogical character1 and to regard them as of igneous origin and of the age1 or nearly of the age1 of the red sandstone of ,central ,scotland4 ,of later writers who have treated of the volcanic rocks of the ,old ,red ,sandstone1 my old friend ,charles ,maclaren deserves special recognition4 ,his survey and description of the ,pentland ,hills embodied the first detailed and accurate investigation of any portion of these rocks1 and his ,geology of ,fife and the ,lothians may still be read with pleasure and instruction4#cag ,bou had indicated roughly on the little sketch-map accompanying his ,essai the chief bands of his felspathic and trappean rocks of the ,old ,red ,sandstone1 but their position and limits were more precisely defined in ,macculloch's ,geological ,map of ,scotland1 which was published in #ahdj1 five years after the sudden and tragic death of its author4 ,the observers who have more recently studied these rocks have been chiefly members of the ,geological ,survey1 and to some of the more important results obtained by them ,i shall refer in the sequel4 ,footnote #cag3 ,geology of ,fife and the ,lothians1 #ahci4 ,more detailed reference will be made in later pages to this classic4 ,for many years ,i have devoted much time to the investigation of the ,old ,red ,sandstone and its volcanic rocks4 ,in the year #ahei ,i ascertained the existence of the great hiatus between the ,lower and ,upper divisions of the system4#cah ,a first sketch of the volcanic history of the ,old ,red ,sandstone was given by me in #ahfa1cai which was subsequently enlarged and filled in with more detail in #ahgi4cbj ,but it was not until #ahib that ,i published a somewhat detailed outline of the whole subject1 tracing the history of volcanic action during the period of the ,old ,red ,sandstone1 the distribution of the volcanoes1 and the character of the materials erupted by them4#cba ,this outline ,i now proceed to amplify1 filling in details that were necessarily omitted before1 though there are still several districts regarding which information is scanty4 ,footnote #cah3 ,on the ,old ,red ,sandstone of the ,south of ,scotland1 ,quart4 ,journ4 ,geol4 ,soc4 xvi4 7#ahfj71 p4 #cab4 ,footnote #cai3 ,on the ,chronology of the ,trap-,rocks of ,scotland1 ,trans4 ,roy4 ,soc4 ,edin4 vol4 xxii4 7#ahfa71 p4 #fc4 ,footnote #cbj3 ,article ,geology1 in ,ninth ,edition of the ,encyclopdia ,britannica1 vol4 x4 7#ahgi71 p4 #cdc4 ,reprinted in my ,text-,book of ,geology1 of which the first edition appeared in #ahhb4 ,footnote #cba3 ,presidential ,address to the ,geological ,society1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xlviii4 7#ahib74 ,in arranging the treatment of the subject ,i shall divide the record into two main sections1 the first and much the more important being devoted to the ,lower and the second to the ,upper ,old ,red ,sandstone4 ,in the first of these divisions it will be convenient to begin by taking note of the distribution of the various districts over which the geological evidence is spread4 ,we may then proceed to consider the general character of the volcanic rocks and the manner in which they are arranged in the stratigraphy of the country1 taking in consecutive order 7#a7 the superficial lavas and tuffs2 7#b7 the vents2 7#c7 the dykes and sills4 ,from these general considerations we may pass to the detailed history of events in each of the separate volcanic areas1 and thus obtain1 as far as the evidence at present permits1 a broad view of the progress of volcanic action during the time of the ,lower ,old ,red ,sandstone in ,britain4 ,,chapter ,,xvii ,,distribution ,,of ,,the ,,volcanic ,,centres ,,in ,,the ,,lower ,,old ,,red ,,sandstone--,,characters ,,of ,,the ,,materials ,,erupted ,,by ,,the ,,volcanoes i4 ,,distribution ,,of ,,volcanic ,,centres ,the area within which volcanic rocks belonging to the ,lower ,old ,red ,sandstone appear is one of the most extensive regions over which the volcanic eruptions of any geological period can be traced in the ,british ,isles 7,map ,i474 ,its northern limit reaches as far as the islet of ,uya in ,shetland1 and its southern appears in ,england in the ,cheviot ,hills--a distance of about #bej miles4 ,but volcanic rocks of probably corresponding age occur even as far to the south as the hills near ,killarney4 ,the most easterly margin of this area is defined by the ,north ,sea on the coast of ,berwickshire1 and its extreme western boundary extends to near ,lough ,erne in the north of ,ireland--a distance of some #bcj miles4 ,if we include the post-,silurian bosses and dykes1 like those of ,shap1 and likewise the ,devonian volcanic rocks of ,devon and ,cornwall1 as contemporaneous with those of the ,old ,red ,sandstone1 the area of eruption will be greatly enlarged4 ,but leaving these out of account for the present1 and confining our attention to the ,lower ,old ,red ,sandstone series1 we find that1 within the wide limits over which the volcanic rocks are distributed1 a number of distinct and often widely separated centres of eruption may be traced4 ,taking these as they lie from north to south1 we may specially enumerate the following3-- #a4 ,the ,shetland and ,orkney ,islands1 together with the basin of the ,moray ,firth4 ,this region includes several distinct volcanic groups1 of which the most northerly extends through the centre to the north-western headlands of the mainland of ,shetland1 another lies in the island of ,shapinshay1 one of the ,orkneys1 while at least two can be recognized on the south side of the ,moray ,firth4 ,to this wide region of ,old ,red ,sandstone ,i have given the general designation of ,lake ,orcadie4#cbb ,footnote #cbb3 ,trans4 ,roy4 ,soc4 ,edin4 vol4 xxviii4 7#ahgh71 p4 #ced4 #b4 ,the basin of ,lorne1 on the west of the mainland of ,argyllshire1 extending from ,loch ,creran to ,loch ,melfort and the hills on the west side of ,loch ,awe4 #c4 ,the great central basin of ,scotland1 which1 for the sake of distinctness1 ,i have called ,lake ,caledonia1#cbc stretching between the ,highlands and the ,southern ,uplands1 from the east coast south-westwards across ,arran and the south end of ,cantire into ,ireland as far as ,lough ,erne4 ,numerous distinct volcanic groups occur in this great basin1 and their volcanic history will be discussed in detail in later chapters 7see ,map ,,iii474 ,footnote #cbc3 ,op4 cit4 #d4 ,the basin of the ,cheviot ,hills and ,berwickshire1 with these hills as the chief area1 but including also other tracts1 probably independent1 which are cut off by the sea along the eastern coast of ,berwickshire between ,st4 ,abb's ,head and ,eyemouth4 #e4 ,the ,killarney tract1 including the hills lying around ,lough ,guitane in the east of ,county ,kerry4 ,at the outset we may take note of a feature in the volcanic history of ,britain1 first prominently noticeable in the records of the ,old ,red ,sandstone1 and becoming increasingly distinct during the rest of the long sequence of ,palozoic eruptions1 namely1 the persistence with which the vents have been opened in the valleys and have avoided the high grounds4 ,i formerly dwelt on this relation1 with reference to the ,carboniferous volcanic phenomena1#cbd but the observation may be greatly extended4 ,with regard to the ,old ,red ,sandstone of ,central ,scotland1 though the lavas and tuffs that were discharged over the floor of the sheet of water which occupied that region gradually rose along the flanks of the northern and southern hills1 yet it was on the lake-bottom and not among the hills that the orifices of eruption broke forth4 ,footnote #cbd3 ,trans4 ,roy4 ,soc4 ,edin4 vol4 xxix4 7#ahgi71 p4 #ded4 ,so far as ,i am aware1 no undoubted vents of the age of the ,lower ,old ,red ,sandstone have been detected among the high grounds of the ,highlands on the one hand1 or among the ,silurian uplands on the other1 although a fringe of the lavas may be traced here and there along the base of the hills4#cbe ,in some cases1 doubtless1 the position of the valleys may have been determined by lines of fault that might well serve as lines of relief along which volcanic vents would be opened4 ,but in many instances it can be proved that1 though the vents have risen in valleys and low grounds1 they have not selected lines of fault visible at the surface1 even when these existed in their neighbourhood4 ,any fissures up which the volcanic ejections made their way must have lain at great depths beneath the formations that now form the surface rocks4 ,footnote #cbe3 ,certain remarkable necks of breccia have been detected by ,mr4 ,j4 ,r4 ,dakyns rising through the schists at the upper end of ,loch ,lomond2 but there is not sufficient evidence to connect them with the volcanic series of the ,lower ,old ,red ,sandstone4 ,some of the younger granite bosses are not improbably to be referred to this volcanic series4 ,the latest granites of the eastern ,grampians1 as already stated1 have lately been found by ,mr4 ,barrow cutting the band of probably ,lower ,silurian strata along the southern border of the ,highlands4 ,those of ,galloway are younger than the ,upper ,silurian formations1 which they invade1 and older than the conglomerates of the ,upper ,old ,red ,sandstone1 which contain pebbles of them4 ,these eruptive bosses will be further discussed in the sequel4 ii4 ,,characters ,,of ,,the ,,materials ,,erupted ,,by ,,the ,,volcanoes ,a general summary of the petrographical characters of the igneous rocks of the ,lower ,old ,red ,sandstone may here find a place4 ,further details will be given in the account of ,lake ,caledonia1 which is the typical area for them2 but1 on the whole1 the prevailing types in one region are found to be repeated in the others4 #a4 ,bedded ,lavas4--,beginning with the lavas which were poured out at the surface1 we have to notice a considerable range of chemical composition among them1 although1 as a rule1 they are characterized by general similarity of external appearance4 ,at the one end1 come diabases and other ancient forms of basalt or dolerite1 wherein the silica percentage is below or little above #ej4 ,by far the largest proportion of the lavas1 however1 are porphyrites or altered andesites1 having about #fj per cent of silica4 ,with these are associated lavas containing more or less unstriped felspar and a somewhat higher proportion of silica1 which may be grouped as trachytes1 though no very sharp line can be drawn between them and the andesites4 ,in the ,pentland ,hills1 and some other areas1 orthophyres flowed out alternately with the more basic lavas1 and were associated with felsitic tuffs and breccias4 ,it is noteworthy that the lava-sheets of the ,lower ,old ,red ,sandstone1 if we consider the character of the prevalent type1 hold an intermediate grade between the average chemical composition of those of ,silurian and of those of later ,carboniferous time4 ,on the one hand1 they rarely assume the character of thoroughly acid rocks1 like the nodular rhyolites of the ,bala and ,upper ,silurian series2#cbf on the other hand1 they seldom include such basic lavas as the basalts1 so common among the puy-eruptions of the ,carboniferous system1 and never1 so far as ,i know1 contain varieties comparable to the ultra-basic compounds which ,i shall have occasion to allude to as characteristic of a particular volcanic zone in that system4 ,footnote #cbf3 ,the only examples known to me are those of ,benaun ,more and other hills in ,county ,kerry4 7a7 ,the ,diabase-lavas are typically developed in the chain of the ,pentland ,hills1 where they form long bands intercalated between felsitic tuffs--a remarkable association1 to which ,i shall make more detailed reference in a later chapter4 ,they range in texture from a compact dark greenish base to a dull earthy amygdaloid4 ,one of their most remarkable varieties is a fine-grained green porphyry1 with large flat tabular crystals of plagioclase arranged parallel to the direction of flow 7,carnethy ,hill74 ,most of them1 however1 are more or less amygdaloidal1 and some of them 7,warklaw ,hill7 strongly so4 ,the following analyses1 made in the laboratory of the ,royal ,school of ,mines under the direction of ,prof4 ,e4 ,frankland1 show the chemical composition of some of the diabases of the ,pentland ,hills3#cbg-- ,footnote #cbg3 ,for analyses of some ,shetland diabases of ,old ,red ,sandstone age1 see ,mr4 ,r4 ,r4 ,tatlock1 ,trans4 ,roy4 ,soc4 ,edin4 vol4 xxxii4 7#ahhg71 p4 #chg4 ---------------------------------------- ----------------------- ,carnethy ,buiselaw4 ,warklaw ,hill4 ,hill#cbh ,sp4 grav4 #bhj4 ,sp4 grav4 #bgg4 ---------------------------------------- ----------------------- ,soluble ,insoluble ,soluble ,insoluble in ,h,cl in ,h,cl in ,h,cl in ,h,cl ---------------------------------------- ----------------------- ,si,o#b #eaaf 444 #ebjj 444 #dggg ,al#b,o#c #bbbg #acj #agdf #ebc #acjh ,fe#b,o#c #bid #aec #ghe #gcb #jhd ,fe,o #djb #aad 444 444 444 ,ca,o #efa #bdc #fhj #ghh #djg ,mg,o #cdf #jih #ajf #cfe #jcj ,k#b,o #bdb 444 #aff 444 #aag ,na#b,o #beh 444 #dag 444 #bcj ,h#b,o #cdb 444 #bfh 444 #bdh ,p#b,o#e #jdh #jcb 444 #jab 444 ,,co#b #abh 444 444 #eja 444 ---------------------------------------- ----------------------- ,footnote #cbh3 ,there was a trace of manganous oxide in this specimen4 7b7 ,the ,andesites1 or1 as they were formerly called1 ,porphyrites1 which constitute by far the largest proportion of the lavas1 have a characteristic but limited range of lithological varieties4 ,the prevailing type presents a close-grained1 rather dull texture1 and a colour varying from pinkish grey1 through many shades of green and brown1 to purplish red1 which last is1 on the whole1 the predominant hue4 ,minute lath-shaped felspars may frequently be detected with the naked eye on fresh surfaces1 while scattered crystals1 which are generally hmatitic pseudomorphs after some pyroxene1 occasionally after hornblende or mica1 may often be observed4 ,the usual porphyritic constituents are plagioclase felspars1 occasionally in abundant tabular crystals measuring half an inch or more across1 also one or more pyroxenes 7augite1 enstatite71 and sometimes brown or black mica4 ,where large felspar-crystals occur in a compact green matrix1 the rock assumes a resemblance to the verde antique of the ancients4#cbi ,one of the ,cheviot andesites lying at the bottom of the series is distinguished by its large and abundant plates of black mica4#ccj ,footnote #cbi3 ,an instance of this rock occurs in ,kincardineshire1 from which the large flat twins of labradorite have been analyzed by ,dr4 ,heddle 7,trans4 ,roy4 ,soc4 ,edin4 vol4 xxviii4 7#ahgi71 p4 #beg74 ,footnote #ccj3 ,c4 ,t4 ,clough1 ,the ,cheviot ,hills1 ,mem4 ,geol4 ,survey 7#ahhh71 p4 #ab4 ,the texture of the andesites occasionally becomes faintly resinous1 where a considerable proportion of glass still remains undevitrified1 as in the well-known varieties from the ,cheviot ,hills1 and in another pitchstone-like rock from above ,airthrey ,castle in the ,ochil ,hills1 near ,bridge of ,allan4 ,it sometimes presents a nodular or coarsely perlitic character1 weathering out in nut-like balls1 like the rock of ,buckham's ,wall ,burn in the ,cheviot ,hills4#cca ,much more frequent is a well-developed amygdaloidal structure1 which indeed may be said to be the most obvious characteristic of these rocks as a whole4 ,the steam-vesicles1 now filled with agate1 quartz1 calcite or zeolite1 vary in size from mere granules up to large irregular cavities a foot or more in diameter4 ,where the kernels are coated with pale-green earth and lie in a dark brown matrix1 they give rise to some of the most beautiful varieties of rock in any volcanic series in this country1 as may be seen on the ,ayrshire coast at ,culzean and ,turnberry4 ,some rocks contain the vesicles only as rare individuals1 others have them so crowded together as to form the greater part of the cubic contents of the mass4 ,when the infiltration-products have weathered out1 some of the amygdaloids present a striking resemblance to recent slaggy brown lavas2 lumps of them must have been originally light enough to float in water4 ,footnote #cca3 ,ibid4 p4 #aa4 ,my colleague in the ,geological ,survey1 ,mr4 ,j4 ,s4 ,grant ,wilson1 some years ago made for me a large series of determinations of the specific gravity of the volcanic rocks of the ,lower ,old ,red ,sandstone of ,scotland4 ,he found that the andesites collected from various districts to illustrate the more typical varieties of rock averaged about #bff4 ,he also made a series of chemical analyses of a number of the same rocks from the ,cheviot ,hills1 where they are well preserved4 ,the results are shown in the following table3-- ---------------------------------------- ------------------------------- ,scawd,rennie-,cunrie-,duncan's,whitton, cuddies' ,cocklaw- ,more- ,law ston ston ,dubs ,hill ,tops foot battle ---------------------------------------- ------------------------------- ,si,o#b #eibi #fbha #fcch #eidd #fjgj #fjeh #fbbi #eihb ,al#b,o#cafcj #afdj #aegg #afae #agih #abbe #agjc #afif ,fe#b,o#c #agg #ee #gc #aje #ff #aja #ic #bj ,fe,o #cgj #cbg #bfe #bhc #beh #dac #bdd #feg ,mn,o #da #ha #jh #cg #bj #ae #ba #ae ,ca,o #dha #ddf #ddd #fgj #gjg #ddj #cib #dgc ,mg,o #cae #afd #ahh #bdf #bbj #bhf #bga #bhd ,k#b,o #dai #cfj #ahh #cah #ceg #bai #aad #bfc ,na#b,o #cdd #cjb #ded #cgj #bie #cfa #cbj #cjd ,h#b,o #chd #djd #dfi #cce #cde 444 #biccb 444 ,h#b,,so#d 444 444 444 444 444 #ee #cg trace ,loss4 444 444 444 444 444 #bae #dha #aih ---------------------------------------- ------------------------------- ,footnote #ccb3 ,this is ,,co#b4 ,the microscopic structure of the andesites of the ,lower ,old ,red ,sandstone has been partially investigated1 especially those of the ,cheviot ,hills1 by ,mr4 ,teall#ccc and by ,dr4 ,petersen1#ccd who both give chemical analyses of the rocks4 ,much1 however1 still remains to be done before our knowledge of this branch of ,british petrography can be regarded as adequate4 ,the groundmass in some of the rocks consists mainly of a brown glass with a streaky structure 7as in the well-known variety of ,kirk ,yetholm1 and in the rock1 still more like pitchstone1 from near ,airthrey ,castle in the ,ochil chain72 more usually it has been devitrified more or less completely by the appearance of felspathic microlites1 until it presents a confused felspar aggregate4 ,the porphyritic felspars are often large1 generally striped1 but sometimes including crystals that show no striping4 ,they are frequently found to be full of inclusions of the base1 and these sometimes consist of glass4 ,the ferro-magnesian constituents are usually rather decomposed1 being now represented by chloritic pseudomorphs2 but augite1 and perhaps still more frequently enstatite1 may be recognized1 or its presence may be inferred among them4 ,the beautiful resinous or pitchstone-like rock from near ,airthrey ,castle has been found by ,mr4 ,watts to be a glassy hypersthene-augite-andesite1 since among its phenocrysts of plagioclase1 augite and hypersthene both occur4 ,magnetite is commonly traceable1 and apatite may be occasionally detected4 ,as the result of decomposition1 calcite1 chlorite and limonite are very generally diffused through the rocks4#cce ,footnote #ccc3 ,geol4 ,mag4 for #ahhc1 pp4 #ajj1 #ade1 #beb4 ,footnote #ccd3 ,mikroskopische und chemische ,untersuchungen am ,enstatit-porphyrit aus den ,cheviot ,hills1 ,inaug4 ,dissert4 ,kiel1 #ahhd4 ,descriptions have also been published of detached rocks from other districts1 such as those by ,prof4 ,judd and ,mr4 ,durham of specimens from the ,eastern ,ochils1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xlii4 7#ahhf71 p4 #dah4 ,footnote #cce3 ,dr4 ,f4 ,h4 ,hatch supplied notes on microscopic structure which are incorporated in the text1 together with particulars afterwards furnished by ,mr4 ,watts4 7c7 ,the lavas which may be separated as ,trachytes offer no distinctive features externally by which they may be distinguished from the andesites4 ,indeed1 both groups of rocks appear to be connected by intermediate varieties4 ,in the ,cheviot ,hills some of the lavas are found1 on microscopic examination1 to contain a large admixture of unstriped porphyritic felspars1 which can occasionally be recognized as sanidine in ,carlsbad twins4 ,the groundmass is sometimes a brown glass1 but is usually more or less completely devitrified1 portions of it being inclosed in the large felspars4 ,chlorite1 pseudomorphic after augite or enstatite1 may be detected1 and sometimes a brown mica4 ,a specimen of one of these rocks1 from a locality to the north-west of ,whitton1 near ,jedburgh1 was found by ,mr4 ,j4 ,s4 ,grant ,wilson to have the following composition3-- ----------------------- ,n4,w4 of ,whitton ,hill1 ,jedburgh 7,no4 #aich7 ,sp4 gr4 #bee4 ---------------------- ,si,o#b #fbdd ,al#b,o#c #ahii ,fe#b,o#c #cce ,fe,o #ah ,mn,o #be ,ca,o #ahd ,mg,o #acg ,k#b,o #ejb ,na#b,o #bfe ,h#b,o #bdh ---------------------- ,total4 #ajjai ---------------------- 7d7 ,acid rocks such as ,felsites and ,rhyolites are rare among the lavas poured out at the surface during the time of the ,lower ,old ,red ,sandstone4 ,they occur in the ,pentland ,hills1 also near ,dolphinton in the ,biggar district1 and in the ,ochil ,hills near ,auchterarder1 associated with extensive accumulations of felsitic tuffs and breccias4 ,they are usually so much decomposed that it is hardly possible to procure fresh specimens of them4 ,some of them display beautiful flow-structure4 ,they appear to be generally orthoclase-felsites or orthophyres4 ,dull1 fine-grained to flinty in texture1 they hardly ever display free quartz1 so that they can seldom be placed among the typical rhyolites1 though in their banded flow-structure they often strongly resemble some lithoid varieties of these rocks1 especially such varieties as that represented in ,fig4 #i4 ,mr4 ,watts1 to whom ,i submitted1 for microscopic examination1 a number of specimens from the ,pentland and ,ochil ,hills1 has found them to consist of a brown felsitic groundmass in which are embedded a generation of small stumpy prisms of orthoclase and a set of larger phenocrysts1 generally consisting of orthoclase and plagioclase in equal proportions4 ,brown mica is usually present and zircons are not uncommon4 ,the rocks1 when they undergo weathering1 pass into the varieties formerly comprised under the name claystone4 ,the only nodular felsite of this age which ,i have met with is that of ,lough ,guitane among the ,dingle ,beds1 near ,killarney1 to which reference will be made in later pages4 #b4 ,intrusive ,bosses1 ,sills and ,dykes4--,while the interbedded lava-sheets are mainly andesites1 the intrusive rocks are generally more acid1 and most of them may be grouped under the convenient head of felsites4 ,some intrusive andesites1 and even more basic rocks1 do indeed occur in dykes and sills1 as well as also filling vents4 ,but the rule remains of general application over the whole country that the materials which have consolidated in the volcanic orifices of the ,old ,red ,sandstone1 or have been thrust among the rocks in dykes1 bosses or sills1 are decidedly acid4 ,in this series of rocks a greater range of types may be traced than among the extrusive lavas4 ,at the one end we find true granites or granitites1 as in the intrusive bosses of ,spango ,water and of ,galloway1 which1 for reasons which ,i will afterwards adduce1 may with some probability be assigned to the volcanic history of the ,lower ,old ,red ,sandstone period4 ,among the bosses1 many of which probably mark the positions of eruptive vents1 orthophyres are especially prominent4 ,these rocks frequently contain no mica1 but1 on the other hand1 they sometimes show abundant quartz in their groundmass4 ,the augite-granitite of the ,cheviot ,hills1 so well described by ,mr4 ,teall1 has invaded the bedded andesites of that region4#ccf ,a similar rock has been noticed by my brother1 ,prof4 ,james ,geikie1 associated with the ,lower ,old ,red ,sandstone volcanic rocks of the east of ,ayrshire4 ,a remarkable petrographical variety has been mapped by ,mr4 ,b4 ,n4 ,peach1 rising as a small boss through the lower part of the great lava-sheets of the ,ochil ,hills1 above ,tillicoultry4 ,it is a granophyric quartz-diorite1 which1 under the microscope1 is seen to be composed of short1 thick-set prisms of plagioclase1 with abundant granophyric quartz1 a pleochroic hypersthene1 and needles of apatite4 ,sometimes the pyroxene is replaced by green chloritic pseudomorphs4#ccg ,footnote #ccf3 ,geol4 ,mag4 for #ahhc1 pp4 #ajj1 #ade1 #beb2 and ,british ,petrography1 pp4 #bgb1 #bgh4 ,footnote #ccg3 ,notes by ,dr4 ,hatch4 ,at the other end of the series come the felsites1 quartz-porphyries1 mica-porphyrites1 minettes1 vogesites1 hornstones and claystones 7or decayed felsites71 which have a close-grained texture1 often with porphyritic felspars1 quartz or black mica1 generally a whitish1 pale buff1 orange1 pink or purplish-grey colour1 and a specific gravity of about #bee4cch ,footnote #cch3 ,the intrusive porphyry of ,lintrathen in ,forfarshire 7which may be younger than the ,old ,red ,sandstone7 is a bright red rock with porphyritic felspar1 quartz1 white mica and a very singular black mica 7,mr4 ,teall's ,british ,petrography1 p4 #bhf74 ,though ,i class these rocks as intrusive1 ,i am not prepared to assert that in none of the instances where they occur as sheets may they possibly have been erupted at the surface as lavas4 ,in one or two cases the evidence either way is doubtful1 but as the great majority of the acid rocks can be shown to be intrusive in their behaviour1 ,i have preferred to keep them all in the same category4 ,i am prepared to find1 however1 that1 as so vast an amount of felsitic debris was ejected to form the tuffs1 more of this material may have flowed out in streams of lava than is at present recognized4 ,the following table shows the chemical composition of some acid sills and dykes from the ,lower ,old ,red ,sandstone1 as determined in the laboratory of ,prof4 ,e4 ,frankland3#cci-- ,footnote #cci3 ,two analyses of rhyolites from ,shetland by ,mr4 ,tatlock will be found in ,trans4 ,roy4 ,soc4 ,edin4 vol4 xxxii4 7#ahhg71 p4 #chg4 ,their silica percentage is #gbcb and #gcgj4 ,an analysis of a quartz-felsite from the ,cheviot ,hills by ,mr4 ,t4 ,waller is given in the ,geological ,survey ,memoir on the ,cheviot ,hills1 p4 #be4 ,the proportion of silica in this rock is #fgi4 ---------------------------------------- ------------------- ,tinto1 ,hornstone4 ,lanarkshire3 ,torgeith ,hornstone4 ,soluble in ,knowe1 ,braid hydrochloric,insoluble ,pentlands ,hills#cdj acid in ditto4 ---------------------------------------- ------------------- ,si,o#b #gcia #fdgc #jd #gjbh ,al#b,o#c #adda #agja #aja #abed ,fe#b,o#c #gf #bce #abd #dc ,mn,o #jg #bd 444 444 ,ca,o #aba #dai #ib #ia ,mg,o #dij #ff #eb 444 ,k#b,o #ccf #cbg 444 #cib ,na#b,o #aeg #cge 444 #ehd ,p#b,o#e 444 #bf #af 444 ,h#b,o #ij #bgh 444 #aii ---------------------------------------- ------------------- ,footnote #cdj3 ,this specimen also yielded #jac of ferrous oxide1 and #bdb of carbon dioxide4 ,the rock of ,tinto1 which may be considered typical of the prevailing acid intrusive rocks of the series1 presents several slightly different varieties4 ,dr4 ,hatch1 as the result of his examination of a number of microscopic slides prepared from specimens taken by me from various parts of the hill1 found some to be minettes1 showing small isolated crystals of orthoclase and rare flakes of biotite1 sometimes granules of quartz1 imbedded in a brown1 finely microlitic groundmass of felspar powdered over with calcite2 while other specimens had a granular instead of a microlitic groundmass1 and contained a considerable amount of quartz in addition to the constituents just mentioned4 ,a conspicuous knob on the south side of ,tinto1 called the ,pap ,craig1 is a mass of augite-diorite1 which has risen through the other rocks#cda 7see ,fig4 #ic74 ,the sills in the same region show still further differences4 ,some are true felspar-porphyries1 and quartz-porphyries varying in the relative abundance and size of their porphyritic orthoclase and quartz1 while others1 by the introduction of hornblende or pseudomorphs after that mineral1 pass into vogesites4 ,footnote #cda3 ,this rock differs considerably from the other intrusive masses in its neighbourhood4 ,dr4 ,hatch found it to be composed chiefly of lath-shaped striped felspar1 with some granular augite1 magnetite and interstitial quartz4 ,basic sills and bosses are chiefly developed among the ,ochil and ,sidlaw ,hills4 ,they may generally be classed as diabases4 ,but sometimes their pyroxenic constituent is partly hypersthene1 as in a coarsely crystalline boss about a mile south of ,dunning1 which has been determined by ,mr4 ,watts to consist of augite and hypersthene imbedded in and occurring amongst large plagioclase prisms4 ,some iron-ore is also present2 the rock is a hyperite4 #c4 ,tuffs and ,agglomerates4--,the fragmental materials1 ejected from or filling up the vents1 vary from the finest compacted dust up to some of the coarsest agglomerates in this country4 ,in general they consist mainly of detritus of andesite1 and have been derived from the blowing up of already consolidated masses of that rock4 ,the fragments are usually angular1 and range from minute grains up to blocks as large as a cottage4 ,the tuffs are often more or less mixed with ordinary non-volcanic sediment1 and as they are traced away from the centres of eruption they pass insensibly into sandstones and conglomerates4 ,but while1 as might be expected1 the tuffs are most commonly made up of debris of the same kind of lavas as those that usually form the sheets which were poured out at the surface1 they include also bands of material derived from the destruction of much more acid rocks4 ,throughout the chain of the ,ochil ,hills1 for example1 in the midst of the bedded andesite-lavas1 many of the thin courses of fine tuff consist largely of felsitic fragments1 with scattered felspar crystals4 ,the most remarkable examples of this nature1 however1 are to be met with at the great vent of the ,braid ,hills1 in the chain of the ,pentland ,hills which runs south-westward from it1 and in the ,biggar volcanic district still further south4 ,these acid tuffs are generally pale flesh-coloured or lilac in tint1 and compact in texture1 but1 like the felsitic lavas from which they were derived1 they are apt to weather into yellow or buff claystones4 ,the finer varieties are so compact as to present to the naked eye no distinguishable grains2 they might be mistaken for felsites1 and indeed1 except where they contain recognizable fragments of rock or broken crystals of felspar1 can hardly be discriminated from them4 ,they consist of an exceedingly fine compacted felsitic dust4 ,here and there1 however1 the scattered crystals of felspar and small angular fragments of felsite1 which may be detected in them1 increase in number until they form the whole of the rock1 which is then a brecciated tuff or fine volcanic breccia1 made up of different felsites1 among which1 even with the naked eye1 delicate flow-structures may be detected4 ,in these pale acid tuffs1 fragments of different andesites may often be observed1 which increase in number as the rocks are traced away from the main vents of eruption4 ,at my request my colleague1 ,mr4 ,george ,barrow1 determined the silica percentages in a few specimens which ,i selected as showing some of the more characteristic varieties of these tuffs from the ,braid and ,pentland ,hills4 ,his results are exhibited in the following table3-- ,silica percentage4 #a4 ,quarry above ,woodhouselee #fcc #b4 ,south-west side of ,castlelaw ,hill #gcae #c4 ,quarry on road1 mile ,n4,e4 of ,swanston 7,braid ,hill vent7 #gda #d4 ,south-west side of ,castlelaw ,hill #gej #e4 ,castlelaw ,hill #gfjj #f4 ,south side of ,white ,hill ,plantation #ijjj ,from these analyses it may be inferred that the average amount of silica in the more typical varieties is between #gj and #ge per cent4 ,the last specimen in the table1 with its abnormally high percentage of acid1 must be regarded as an exceptional variety1 where there has either been an excessive removal of some of the bases1 or where silica has been added by infiltration4 ,the microscopic examination of these rocks has not added much to the information derivable from a study of them in the field4 ,in their most close-grained varieties1 as above remarked1 they are hardly to be distinguished from felsites4 ,but they generally show traces of the minute detrital particles of felsite of which they are essentially composed4 ,the brecciated varieties exhibit finely-streaked flow-structure in some of the fragments4 ,pieces of andesite1 grains of quartz1 and other extraneous ingredients appear in these rocks towards the southern limits of the volcanic area of the ,pentland ,hills1 where the acid tuffs are associated with and pass laterally and vertically into ordinary non-volcanic sedimentary strata4 ,further details as to the part which these tuffs play in the volcanic history of the regions wherein they occur will be given in later pages4 ,,chapter ,,xviii ,,structure ,,and ,,arrangement ,,of ,,the ,,lower ,,old ,,red ,,sandstone ,,volcanic ,,rocks ,,in ,,the ,,field ,we have now to consider the manner in which the various volcanic products have been grouped around and within the orifices of discharge4 ,the first feature to arrest the eye of a trained geologist who approaches them as they are displayed in one of the ranges of hills in ,central ,scotland is the bedded aspect of the rocks4 ,if1 for example1 he looks eastward from the head of the ,firth of ,tay1 he marks on the right hand1 running for many miles through the county of ,fife1 a succession of parallel escarpments1 of which the steep fronts face northwards1 while their long dip-slopes descend towards the south4 ,on his left hand a similar but higher series of escarpments1 stretching far eastwards into ,forfarshire1 through the chain of the ,sidlaw ,hills1 repeats the same features1 but in opposite directions4 ,if he stands on the alluvial plain of the ,forth1 near ,stirling1 and looks towards the north1 he can trace bar after bar of brown rock and grassy slope rising from base to summit of the western end of the ,ochil ,hills4 ,if1 again1 from any height on the southern outskirts of the city of ,edinburgh1 he lets his eye range along the north-western front of the chain of the ,pentland ,hills1 especially towards evening1 he can follow the same parallel banding as a conspicuous feature on each successive hill that mounts above the plain4 ,or if1 as he traverses the west of ,argyllshire1 he comes in sight of the uplands of ,lorne1 he at once recognizes the terraced contours of the hills between ,loch ,awe and the western sea1 presenting so strange a contrast to the rugged and irregular outlines of the more ancient schist and granite mountains all around 7see ,fig4 #ii74 i4 ,,bedded ,,lavas ,,and ,,tuffs ,on a nearer inspection1 the dominant topographical features are found to correspond with a well-marked stratification of the whole volcanic series4 ,where two sheets of andesite are separated by layers of tuff1 sandstone or conglomerate1 a well-marked hollow will often be found to indicate the junction-line2 but even where the lavas follow each other without such interstratifications1 their differences of texture and consequent variations in mode and amount of weathering usually suffice to mark them off from each other1 and to indicate their trend along the surface in successive terraces4 ,even where the angles of inclination are high1 the bedded arrangement can generally be detected4 ,it is in the picturesque and instructive coast-sections1 however1 that the details of this bedded structure are most clearly displayed4 ,on both sides of the country1 along the shores of ,ayrshire on the west1 and those of ,kincardineshire and ,forfarshire on the east1 the volcanic group has been admirably dissected by the waves4 ,the lava-beds have been cut in vertical section1 so that their structure and their mode of superposition1 one over another1 can be conveniently studied1 while at the same time1 the upper surfaces of many of the flows have been once more laid bare as they existed before they were buried under the sedimentary accumulations of the waters in which they were erupted4 ,though distinctly bedded1 the ,lavas show little of the regularity and persistence so characteristic of those of ,carboniferous and of ,tertiary time4 ,some of them are not more than from four to ten feet thick1 and generally1 on the coast-cliffs1 they appear to be less than fifty feet4 ,a continuous group of sheets can sometimes be traced for ten miles or more from the probable vent of discharge4 ,that many of these lavas were erupted in a markedly pasty condition may be inferred from certain of their more prominent characteristics4 ,sometimes1 indeed1 they appear as tolerably dense homogeneous masses1 breaking with a kind of prismatic jointing2 but more frequently they are strongly amygdaloidal1 and sometimes so much so that1 as already stated1 the amygdales form the larger proportion of their bulk4 ,where the secondary infiltration-products have weathered out1 the rough scoriform rock looks as if it might only recently have been erupted4 ,in a few instances ,i have observed an undulating rope-like surface1 which reminded me of well-known ,vesuvian lavas4 ,usually the top and bottom of each sheet assume a strikingly slaggy aspect1 which here and there is exaggerated to such an extent that between the more solid and homogeneous parts of two consecutive flows an intermediate band occurs1 ten or twelve feet thick1 made up of clinker-like lumps of slag1 the interspaces being filled in with hardened sand4 ,in some cases these agglomeratic layers may actually consist in part of ejected blocks2 but the way in which many of the lavas have cooled in rugged scoriaceous surfaces is as conspicuous as on any modern coule4 ,the loosened slags1 or the broken-up cakes and blocks of lava1 have sometimes been caught up in the still moving1 pasty current1 which has congealed with its vesicles drawn out round the enclosed fragments1 giving rise to a mass that might be taken for a breccia or agglomerate4 ,now and then we may observe that the upper slaggy portion of a sheet has assumed a bright red colour from the oxidation of its ferruginous minerals2 and from the contrast it thus presents to the rest of the rock we may perhaps legitimately infer that the disintegration took place before the outflow of the next succeeding lava4 ,if this inference be well founded1 and it is confirmed by other evidence which will be subsequently adduced1 it points to the probable lapse of considerable intervals of time between some of the outflows of lava4 ,illustration3 ,fig4 #fe4--,veins and nests of sandstone due to the washing of sand into fissures and cavities of an ,old ,red ,sandstone lava4 ,turnberry ,point1 ,ayrshire4 ,but perhaps the most singular structure displayed by these lavas is to be seen in the manner in which they are traversed by and enclose portions of sandstone4 ,since ,i originally observed this feature on the ,ayrshire coast1 near ,turnberry ,point1 many years ago1#cdb ,i have repeatedly met with it in the various volcanic districts of the ,lower ,old ,red ,sandstone across the whole of the ,midland ,valley of ,scotland4 ,the first and natural inference which a cursory examination of it suggests is that the molten rock has caught up and carried along pieces of already consolidated sandstone4 ,but a little further observation will show that the lines of stratification in the sandstone1 even in what appear to be detached fragments1 are marked by a general parallelism1 and lie in the same general plane with the surface of the bed of lava in which the sandy material is enclosed4 ,in a vertical section the sandstone is seen to occur sometimes in narrow dykes with even1 parallel walls1 but more usually in irregular twisting and branching veins1 or even in lumps which1 though probably once connected with some of these veins1 now appear as if entirely detached from them 7,fig4 #fe74 ,frequently1 indeed1 the nodular slaggy andesite and the sandstone are so mixed up that the observer may hesitate whether to describe the mass as a sandstone enclosing balls and blocks of lava1 or as a scoriaceous lava permeated with hardened sand4 ,a close connection may be traced between these sandstone-inclosures and the beds of sandstone interstratified between the successive lavas4 ,we can follow the sandy material downwards from these intercalated beds into the andesites below them4 ,on exposed upper surfaces of the lava1 an intricate reticulation of sandstone veins may be noticed1 in each of which the stratification of the material runs across the veins1 showing sometimes distinct current-bedding1 but maintaining a general parallelism with the bedding of the volcanic sheets and their fragmentary accompaniments 7,fig4 #ff74 ,if we could remove the sandstone-veinings and aggregates1 we should find the upper surfaces of these igneous masses to present a singularly fissured and slaggy appearance1 reminding us of the rugged1 rent and clinker-loaded slopes of a modern viscous lava1 like some of those in the ,atrio del ,cavallo on ,vesuvius4 ,there cannot1 therefore1 be any doubt that the sandstone1 so irregularly dispersed through these lavas1 was introduced originally as loose sand washed in from above so as to fill the numerous rents and cavernous interspaces of the volcanic rock4 ,a more striking proof of the subaqueous character of the eruptions could hardly be conceived4 ,this interesting feature in lavas erupted under water is not confined to the volcanic series of the ,old ,red ,sandstone4 ,we shall find that it is hardly less distinct among the basic lavas of the ,permian series both in ,scotland and in ,devonshire4 ,footnote #cdb3 ,see ,jukes' ,manual of ,geology1 #crd edit4 7#ahgb71 ,fig4 #aaa1 p4 #bgf4 ,illustration3 ,fig4 #ff4--;,ground-plan of reticulated cracks in the upper surface of an ,old ,red ,sandstone lava filled in with sandstone4 ,red ,head1 ,forfarshire4 ,a remarkable exception to the general type of dark basic and intermediate lavas is furnished by the pale1 decomposing felsites of the ,pentland and ,dolphinton ,hills4 ,those which issue from the great eruptive centre of the ,braid ,hills1 alternate with the andesites and the diabases1 gradually diminishing like these in a southward direction and dying out in some six or seven miles4 ,beyond the limits of these lavas1 another similar thick group was erupted from a separate vent at the northern end of the ,biggar district near ,dolphinton4 ,the same occurrence has been ascertained also in the area of the ,ochil chain4 ,fuller reference will be made to these interesting rocks in the descriptions to be afterwards given of the structure and history of the volcanic areas of the ,pentland ,hills1 the ,biggar centre and the ,ochil ,hills4 ,it is certainly a notable feature in the volcanism of ,old ,red ,sandstone time that from the same1 or from closely adjoining vents1 lavas should be alternately poured forth1 differing so much from each other1 alike in chemical composition and petrographical characters1 as andesites and diabases on the one hand1 and felsites on the other4 ,additional examples1 from widely different geological systems1 will be cited in subsequent pages4 ,it will be shown that even in the very latest volcanic period in ,britain1 that of older ,tertiary time1 highly basic and markedly acid materials were ejected from the same centres of eruption4 ,the part taken by the ,tuffs in the structure of the ground agrees with what might have been expected in the accompaniments of extremely slaggy and viscid lavas4 ,these pyroclastic intercalations are1 in most of the volcanic districts1 comparatively insignificant in amount1 by far the largest proportion of solid material ejected from the various vents having consisted of streams of lava4 ,round or within some of the vents the fragmentary materials attain a remarkable coarseness1 as may be seen in the great agglomerates of ,dumyat1 near ,stirling1 the largest of which is more than #gjj feet thick4 ,these massive accumulations doubtless represent a long series of explosive discharges from the summit of the lava column in one or more adjacent vents4 ,traced away from the orifices of emission1 the tuffs rapidly grow finer in grain1 less in thickness1 and more mixed with ordinary detritus1 until they pass into ordinary non-volcanic sediment or die out between the lava-sheets4 ,good sections1 showing the nature and arrangement of the thin intercalations of andesite-tuff between the successive outpourings of lava1 may be examined on the coast4 ,thus1 near ,turnberry ,point1 in ,ayrshire1 upwards of a dozen successive flows of lava1 with their sandy and ashy intervening layers1 are exposed in plan upon the beach1 and partly also in section along the cliffs on which the ruins of the historic castle of ,turnberry stand4 7,figs4 #ie1 #if1 #ig74 ,again1 along the coast of ,forfarshire1 from the ,red ,head to ,montrose1 the numerous sheets of andesite are separated by layers of dull purplish tuff passing into conglomerate1 with blocks of porphyrite a yard or more in diameter4 ,the most remarkable interstratified tuffs in the ,lower ,old ,red ,sandstone are the felsitic varieties4 ,those which proceed from the great vent of the ,braid ,hills1 extend south-westwards for eight or nine miles1 and their peculiar materials1 mixed with ordinary sediment1 may be traced several miles further4 ,they occur in successive sheets1 which1 from a maximum thickness and number at the north end1 gradually thin away southwards1 like the felsitic lavas which they accompany1 and from the explosion of which they no doubt were derived4 ,they consist to a large extent of extremely fine volcanic dust1 and since they are generally much decomposed1 it is often1 as already remarked1 hardly possible to distinguish between them and the equally decayed felsites4 ,in some parts of the hills they present a distinct fissile bedding2 but still more satisfactory is the occasional fine brecciated structure which they assume1 when they are seen to consist of angular lapilli of different felsites4 ,the amount of volcanic material ejected from the more important vents was much greater than the height of the present hills would lead us to suppose4 ,the rocks have generally been tilted into positions much more inclined than those which they originally occupied1 so that to measure their actual thickness we must take a line approximately perpendicular to the dip4 ,in this way we ascertain that the accumulated mass of lavas and tuffs immediately outside the vent at the north end of the ,pentland ,hills must be at least #gjjj feet thick1 for the base of the series is concealed under the unconformable overlap of the ,lower ,carboniferous ,sandstones1 while the top is cut off by a fault which brings down the ,carboniferous formations against the eastern flank of the hills4 ,probably not less voluminous is the pile of ejected material in the ,ochil ,hills1 where1 though the base of the whole is concealed by the fault which throws down the coal-field1 some #fejj feet of lavas1 tuffs and conglomerates can be seen4 ,there were thus1 during the time of the ,lower ,old ,red ,sandstone1 more than one volcano in ,central ,scotland which might be compared in bulk of ejected material to ,vesuvius4 ,illustration3 ,fig4 #fg4--,section across the volcanic series of ,forfarshire4 a1 conglomerates1 sandstones and flagstones2 b1 sheets of andesitic lava4 ,that the eruptions were mainly subaqueous is indicated1 as ,i have shown1 by the intercalated bands of sandstone and conglomerate between the successive lavas1 as these are traced away from the centres of discharge1 and likewise1 even more impressively1 by the hardened sand which has been washed into former fissures and crevices in the lava4 ,but that1 in some cases1 the volcanic cones were built up above the surface of the lake may be legitimately inferred from the remarkable volcanic conglomerates which occur1 more particularly in the great chain of the ,ochil and ,sidlaw ,hills4 ,these thick accumulations of well-rounded and water-worn blocks are interspersed between sheets of andesite1 and are mainly made up of andesite fragments4 ,impressive sections of them may be seen along the ,kincardineshire coast4 ,the conglomerates are sometimes so remarkably coarse1 many of their blocks exceeding two feet in diameter1 and so rudely bedded1 that it is only by noting the position of oblong boulders that one can make out the general direction of the stratification4 ,in their smooth rounded forms1 these blocks resemble the materials of storm-beaches on an exposed coast4 ,the trituration of the andesite fragments has given rise to a certain amount of green paste1 which firmly wraps round the stones1 and retains casts of them after they have dropped out4 ,it is further deserving of remark that while in some districts1 as in the central ,ochils1 the materials were entirely derived from the destruction of volcanic rocks1 in others a large proportion of non-volcanic materials is mingled with the debris of the lavas4 ,south of ,stonehaven1 for example1 large boulders of quartzite form a conspicuous feature in the conglomerates1 of which in places they make up quite half of the total constituents4 ,there can be little doubt1 ,i think1 that the materials of these coarse detrital accumulations were gathered together as shingle-beaches1 and were derived in part from volcanic cones which had risen above the level of the lake4 ,they seem to suggest considerable degradation of these cones by breaker-action1 whereby blocks of rock a yard or more in diameter could be rounded and smoothed4 ,another inference deducible from such conglomerates1 and to which ,i have already alluded1 is that considerable intervals of time took place between some of the eruptions4 ,round the vents1 indeed1 where the successive sheets of volcanic material follow each other continuously1 it is perhaps impossible to form any definite opinion as to the relative chronological value of the lines of separation between different ejections4 ,but where some hundreds of feet of coarse conglomerate1 chiefly composed of well-rounded andesite blocks1 intervene between two streams of lava1 we may conclude that the interval between the outpouring of these streams must have been of considerable duration4 ,other evidence of a similar tendency may be recognized in the intercalation of groups of varied sedimentary accumulations1 such as those which were deposited over the site of ,eastern ,forfarshire and ,kincardineshire during the time that elapsed between two successive floods of lava4 ,in the ,den of ,canterland1 for example1 in the midst of the volcanic sheets we find interesting evidence of one of these intervals of quiescence1 during which layers of fine olive shales were laid quietly down1 while macerated vegetation1 drifting over the lake-bottom1 was buried with remains of fishes1 and abundant gally-worms 7,kampecaris1 ,archidesmus71 washed from the neighbouring land4#cdc ,so undisturbed were the conditions of deposition that calcareous sediment gathered round some of the organisms and encased them in limestone nodules4 ,footnote #cdc3 ,an abundant organism in some of these deposits1 named ,parka1 was first regarded as a plant1 was afterwards believed to be the egg-packets of crustacea1 and is now pronounced by competent authorities to belong to an aquatic plant with creeping stems1 linear leaves and sessile sporocarps4 ,in some of the districts the discharges of volcanic material were so abundant or so continuous that no recognizable deposition of ordinary sediment has taken place between them4 ,thus1 at the north end of the ,pentland ,hills the rocks are entirely of volcanic origin1 and though1 as we trace them southwards away from the centre of eruption1 they diminish in thickness1 they include hardly any interstratified sandstones and conglomerates until they finally begin to die out4 ,the distances to which the lavas and tuffs have been erupted from the chief vents of a district vary up to #ae or #bj miles4 ,those of the ,pentland ,hills extend from the ,braid ,hill vent for #aj miles to the south-west4 ,those of the ,biggar centre stretch for about #af miles to the north-east4 ,those of the ,ochil ,hills1 which probably came from a number of distinct vents1 can be traced for nearly #ej miles4 ii4 ,,vents ,on the whole the actual vents of the volcanoes of ,lower ,old ,red ,sandstone time are less clearly distinguishable than those of subsequent volcanic periods4 ,this deficiency doubtless arises from the geological structure of the districts in which the formation is chiefly developed4 ,thus1 in the great ,midland ,valley of ,scotland1 where the ,old ,red ,sandstone covers a large part of the surface1 the vents seem to have been placed along the central parts of the long trough rather than among the older rocks on either margin4 ,hence they are in large measure buried either under the volcanic and sedimentary accumulations of their own period or under ,carboniferous strata4 ,illustration3 ,fig4 #fh4--,section across two necks above ,tillicoultry1 ,ochil ,hills4 #a #a1 ,andesite lavas2 #b #b1 ,tuffs and volcanic conglomerates2 #c #c1 ,the two necks2 #d #d1 ,dykes of felsite1 etc42 #e1 ,coal-measures2 f1 ,fault4 ,certain bosses of massive rocks lying well within the volcanic area may with some confidence be regarded as the sites of eruptive centres4 ,they occur either singly or in groups1 and may be specially noticed along the chain of the ,ochil and ,sidlaw ,hills4 ,yet it seems to me probable that these visible bosses1 even if we are correct in regarding them as marking the positions of true vents1 do not indicate the chief orifices of discharge4 ,if we consider their size and their distribution with reference to the areas of lava and tuff discharged at the surface1 we are rather led to look upon them as subsidiary vents1 the more important orifices1 from which the main bulk of the eruptions took place1 being still concealed under the ,carboniferous rocks of the ,midland ,valley4 ,the bosses which rise through different portions of the volcanic series are obviously not the oldest or original vents4 ,the great felsitic mass of ,tinto in ,lanarkshire 7,fig4 #ic71 indeed1 pierces strata which lie near the base of the ,lower ,old ,red ,sandstone1 but the smaller cone of ,quothquan in its neighbourhood appears in the midst of the lavas 7,fig4 #ib74 ,in the south-western part of the ,ochil chain the bosses or necks are chiefly small in size1 seldom exceeding half a mile in diameter4 ,they have been filled sometimes with crystalline1 sometimes with fragmental materials4 ,two of them1 containing the remarkable granophyric quartz-diorite already referred to1 emerge from among the tuffs in a low part of the series1 immediately above the village of ,tillicoultry in ,clackmannan 7,fig4 #fh74 ,two or three more1 which are occupied by orthophyres and quartz-felsites1 pierce the volcanic group a few miles to the west of ,loch ,leven4 ,the whole of the visible bosses of the ,ochil ,hills may be regarded as one connected group1 subsidiary to the main orifices which lay rather further to the south and west4 ,more particular reference to this district will be made in the following chapter 7p4 #cjc74 ,vents which have been filled up with agglomerate1 and which thus furnish the most obvious proofs of their connection with the eruptions of the volcanic series1 though not frequent1 may be observed in a number of the volcanic districts4 ,their fragmentary materials generally consist mainly of the detritus of andesites or diabases like those which form the bedded lavas4 ,but where more acid lavas have risen to the surface1 fragments of felsite may occur more or less abundantly4 ,in the great vent of the ,braid ,hills the tuffs and breccias are almost wholly acid4 ,non-volcanic materials may often be found in the agglomerates1 and occasionally even to the exclusion of volcanic detritus4 ,thus1 in the far north of ,scotland several examples occur among the ,shetland ,isles of necks filled entirely with blocks of the surrounding flagstones and sandstones4 ,such cases1 as has been already pointed out1 probably represent incompleted volcanoes1 when the explosive vapours were powerful enough to drill orifices in the crust of the earth and eject the shattered debris from them1 but were not sufficiently vigorous or lasting to bring up any solid or liquid volcanic material to the surface4 ,these ,shetland examples are further noticed on p4 #cde4 ,necks of agglomerate in the ,lower ,old ,red ,sandstone vary in size from a great orifice measuring two miles across to little plugs only a few yards in diameter4 ,they may be found in limited numbers in most of the volcanic districts4 ,no examples have been observed rising through older rocks than the ,old ,red ,sandstone1 all the known instances being eruptive through some part of the volcanic series or of the sandstones1 and therefore not belonging to the earliest eruptions4 ,the largest1 and in some respects the most interesting1 vent in the ,lower ,old ,red ,sandstone1 that of the ,braid ,hills near ,edinburgh1 described in ,chapter xx41 covers an area of more than two square miles1 and is filled with felsitic breccias and tuffs1 through which bosses and veins of acid and basic rocks have been injected4 ,it completely truncates the bedded lavas and tuffs of the ,pentland ,hills1 and not improbably marks the chief centre from which these rocks were erupted4 ,several smaller necks rise a little beyond its southern margin1 marking1 perhaps1 lateral cones on the main volcano4 ,in the small area of ,lower ,old ,red ,sandstone lying between ,campbeltown and the ,mull of ,cantyre1 several necks of agglomerate occur1 which have been partly dissected by the waves along the shore1 thus revealing their internal structure and their relation to the surrounding conglomerates4 ,an account of them will be found at p4 #caa4 ,one of the series1 which lies back from the coast-line1 forms a prominent rounded hill measuring about #djj yards in its longest diameter4 ,its general contour is represented in ,fig4 #hb4 ,of the eruptive bosses of massive rock outside the limits of the ,old ,red ,sandstone which may be plausibly referred to the volcanic phenomena of the period1 though they cannot be proved to be actually part of them1 the most notable are the bosses of granite and other acid material which rise through the ,silurian strata of the ,southern ,uplands of ,scotland4#cdd ,the largest are the well-known masses of ,galloway 7,fig4 #fi71 with which must be grouped the bosses near ,new ,cumnock1 that of the ,spango ,water 7,fig4 #id71 and those of ,cockburn ,law and ,priestlaw in ,lammermuir1 together with a number of masses of felsitic material scattered over the same region1 such as the ,dirrington ,laws of ,berwickshire 7,fig4 #gj74 ,these bosses present some points of structure in common with true vents4 ,they come like great vertical columns through highly-folded and puckered strata1 and1 as they truncate the ,llandovery and ,wenlock formations1 they are certainly younger than the greater part of the ,upper ,silurian series4 ,they must be later1 too1 than the chief plication and cleavage of these strata2 but they are older than the ,upper ,old ,red ,sandstone or basement ,carboniferous rocks which contain pebbles of them4 ,their date of eruption is thus narrowed to the interval between the later part of the ,upper ,silurian period and the beginning of the ,upper ,old ,red ,sandstone4 ,i have myself little doubt that they are to be associated with the volcanic epoch we are now considering1 as it was the only known great episode of igneous activity in this region during the interval within which the protrusion of these granites must have taken place4 ,in the ,cheviot ,hills1 indeed1 we have evidence of the eruption of a large mass of augite-granitite through the porphyrite-lavas of the ,lower ,old ,red ,sandstone1 with abundant veins projecting from it into them1 as will be narrated in later pages4#cde ,footnote #cdd3 ,i suggested this possible connection many years ago in ,trans4 ,geol4 ,soc4 ,edin4 vol4 ii4 7#ahgd7 p4 #ba4 ,footnote #cde3 ,the volcanic geology of the ,cheviot ,hills is described by ,mr4 ,teall1 ,geol4 ,mag4 for #ahhc1 p4 #ajf2 and by ,mr4 ,clough1 ,mem4 ,geol4 ,survey1 ,geology of the ,cheviot ,hills1 ,sheet #ajh ,n4,e41 #ahhh1 p4 #bd4 ,illustration3 ,fig4 #fi4--,section of the granite core between ,merrick and ,corscrine4 a1 ,silurian greywackes1 grits and shales2 b1 granite4 ,not improbably many other granite protrusions throughout the ,british ,isles are to be referred to the volcanic operations of the ,lower ,old ,red ,sandstone4 ,such are those of the ,lake ,district1 notably that of ,shap1#cdf the granites of ,newry and ,leinster in the east of ,ireland1 which are later than the ,silurian rocks and older than the ,carboniferous ,limestone1 and the younger ,grampian granites1 which pierce the presumably ,arenig belt along the ,highland border4 ,whether or not these granitic protrusions were connected with superficial volcanic discharges of which no remains have survived1 they seem to indicate the wide extent and remarkable vigour of the subterranean igneous action of this geological period4 ,footnote #cdf3 ,see the descriptions of the ,shap granite by ,messrs4 ,marr and ,harker1 ,quart4 ,journ4 ,geol4 ,soc4 xlvii4 7#ahia7 p4 #bff1 and xlix4 7#ahic7 p4 #cei4 ,illustration3 ,fig4 #gj4--,section across the three ,dirrington ,laws1 ,berwickshire4 a1 ,upper ,silurian strata2 b1 ,necks probably of ,lower ,old ,red ,sandstone age2 c1 ,upper ,old ,red ,sandstone lying unconformably both on a and b4 ,viewed as a whole1 the materials which now occupy the vents of the volcanic chains in the ,lower ,old ,red ,sandstone of the ,british ,isles are more acid than the lavas erupted at the surface4 ,in the ,pentland district1 indeed1 and in some other areas this acid material was ejected at intervals in abundant discharges of dust and lapilli and in outflows of felsitic lavas1 while between these successive discharges copious streams of diabasic and andesitic lavas1 either from the same or from some closely-adjoining vent1 were poured out4 ,throughout the whole region1 however1 as a closing phase of the volcanic history1 the acid magma rose after the outpouring of the more basic lavas and filled such chimneys of the volcanoes as were not already blocked with agglomerate4 ,it was probably after these pipes were plugged that the final efforts of volcanic energy were expended in the protrusion of the acid material as sills between the bedding-planes of the surrounding rocks1 and as dykes and veins in and around the vents4 iii4 ,,sills ,,and ,,dykes ,nowhere throughout the volcanic tracts of the ,lower ,old ,red ,sandstone is there any such development of sills as may be seen beneath the ,silurian volcanic sheets of ,north ,wales4 ,those which occur are most abundant in the ,lanarkshire district1 to the north-west and south-west of ,tinto1 and in the south of ,ayrshire4 ,from the village of ,muirkirk to the gorge of the ,clyde1 below the ,falls1 the ,upper ,silurian and ,lower ,old ,red ,sandstone strata are traversed by numerous intrusive sheets of pink and yellow felsite1 quartz-porphyry1 minette1 lamprophyre and allied rocks1 which are no doubt to be regarded as part of the volcanic phenomena with which we are here concerned4 ,in the south of ,ayrshire1 between the villages of ,dalmellington and ,barr1 there is a copious development of similar sills1 especially along one or more horizons near the base of the ,old ,red ,sandstone4 ,garleffin ,fell1 ,glenalla ,fell1 ,turgeny and other heights are conspicuous prominences formed of these rocks2 above the sills lie thick conglomerates and sandstones on which the great andesite-sheets rest4 ,in the ,pentland ,hills1 as will be described in ,chapter xx41 a massive felsitic sill forms a conspicuous feature along the north side of the chain1 and there are probably others which have not yet been separated from the felsitic tuffs and orthophyres which they so much resemble4 ,perhaps the most remarkable acid sills in the ,old ,red ,sandstone of ,britain are those which occur at the extreme northern end of the region among the volcanic phenomena of the ,shetland ,isles 7,figs4 #ga1 #gb74 ,the largest of them1 consisting mainly of granite and felsite1 is believed to reach a length of #bj and a breadth of from three to four miles4#cdg ,footnote #cdg3 ,messrs4 ,b4 ,n4 ,peach and ,j4 ,horne1 ,trans4 ,roy4 ,soc4 ,edin4 xxxii4 7#ahhd71 p4 #cei4 ,illustration3 ,fig4 #ga4--,section of ,papa ,stour1 ,shetlands1 showing sill of spherulitic felsite traversing ,old ,red ,sandstone and bedded porphyrites 7,messrs4 ,peach and ,horne74 #a4 ,red sandstones and flagstones2 #b4 ,purple diabase-porphyrites2 #c4 ,great sheet of pink spherulitic felsite4 ,illustration3 ,fig4 #gb4--,section across ,northmavine1 from ,okrea ,head to ,skea ,ness1 ,shetland1 showing dykes and connected sill of granite and felsite 7,messrs4 ,peach and ,horne74 #a4 ,schists1 etc42 #b4 ,serpentine2 #c4 ,granite and quartz-felsite2 #d4 ,breccia of serpentine fragments2 #e4 ,bedded andesites and tuffs4 f1 ,fault4 ,a group of sills composed of a bright red quartz-porphyry has been traced along the southern flanks of the ,highlands for upwards of #ah miles4#cdh ,this rock1 already referred to as the ,lintrathen porphyry1 lies chiefly among the conglomerates and sandstones1 but also intersects the lavas1 and may be later than the ,old ,red ,sandstone 7p4 #bgg74 ,an extension of it is found even on the north side of the boundary fault1 cutting the andesites which there lie unconformably on the schists4 ,footnote #cdh3 ,see ,sheet #ef of the ,geological ,survey of ,scotland4 ,examples1 however1 occur of sills much less acid in composition4 ,in the ,dundee district1 for instance1 the intrusive sheets are andesites and diabases4 ,they send veins into and bake the sandstones among which they have been intruded1 and are sometimes full of fragments of such indurated sandstone1 as may be well seen on the northern shore of the ,firth of ,tay1 west of ,dundee4 ,a conspicuous characteristic of most of the volcanic tracts of the ,lower ,old ,red ,sandstone is the comparative scarcity of contemporaneous dykes4 ,in the band of acid sills between ,muirkirk and the ,clyde1 a considerable number of dykes have been mapped1 which must be regarded as due to the same series of movements and protrusions of the magma that produced the adjacent sills4 ,throughout the length of the ,southern ,uplands dykes of felsite1 minette1 lamprophyre1 vogesite and other varieties1 which may also be connected with the volcanic phenomena of the ,lower ,old ,red ,sandstone1 not infrequently occur among the ,silurian rocks4 ,on the ,kincardineshire coast1 south of ,bervie1 a number of dykes of pink quartz-porphyry traverse the conglomerates and sandstones4 ,the coast south of ,montrose displays some singularly picturesque sections1 where a porphyry dyke running through andesitic lavas and agglomerates stands up in wall-like and tower-like projections4 ,on the shore at ,gourdon1 as well as inland1 intrusive dykes of serpentine occur4 ,a line of these1 possibly along the same fissure1 has been traced for more than a dozen of miles from above ,cortachy ,castle to near ,bamff4 ,but there is no evidence to connect them with the volcanic phenomena of the ,old ,red ,sandstone4 ,not improbably they belong to a later geological period4 ,one would expect to meet with a network of dykes in and around the volcanic vents2 but even there they are usually not conspicuous either for number or size4 ,in the great vent of the ,braid ,hills only a few have been noticed4 ,in the ,ochil ,hills groups of dykes of felsite and andesite may be observed1 especially near the necks4 ,they are fairly numerous in the neighbourhood of ,dollar 7see ,fig4 #fh74 ,one of the most abundant series yet observed traverses the tract around the granite boss of the ,cheviot ,hills1 from which many dykes of granite1 felsite1 quartz-porphyry and andesite radiate4 ,this district will be more fully referred to in ,chapter xxi4 ,another remarkable development of dykes occurs in ,shetland 7,fig4 #gb71 where they consist of granite1 felsite and rhyolite1 and are associated with the acid sills above referred to4 ,,chapter ,,xix ,,volcanoes ,,of ,,the ,,lower ,,old ,,red ,,sandstone ,,of ,,lake ,,caledonia ,description of the several ,volcanic ,districts3 ,lake ,caledonia1 its ,chains of ,volcanoes--,the ,northern ,chain3 ,montrose ,group1 ,ochil and ,sidlaw ,hills1 the ,arran and ,cantyre ,centre1 the ,ulster ,centre4 ,i now propose to give some account of each of the districts which have been separate areas of volcanic action during the time of the ,lower ,old ,red ,sandstone1 tracing its general structure1 the arrangement and sequence of its volcanic rocks and the history of its eruptions4 ,as by far the most varied development of the ,old ,red ,sandstone is to be found in the great ,midland ,valley of ,scotland1 and as it is there that the remarkable volcanic phenomena of the system have been most abundantly displayed and are most clearly recorded1 ,i shall begin my description of the volcanic eruptions of the ,lower ,old ,red ,sandstone with a detailed account of the different centres of volcanic activity in that region4 ,the phenomena are so fully displayed there that a more summary treatment of the subject will suffice for the other regions4 ,under the designation of ,lake ,caledonia1 as already remarked1 ,i include the whole of the ,midland ,valley of ,scotland between the ,highlands and the ,southern ,uplands1 likewise the continuation of the same ancient hollow by ,arran and the south of ,cantyre across the north of ,ireland to ,lough ,erne4#cdi ,throughout most of the area thus defined1 the present limits of the ,lower ,old ,red ,sandstone are sharply marked off by large parallel faults4 ,on the north-west side one1 or rather a parallel series1 of such dislocations runs from ,stonehaven along the flank of the ,highland mountains to the ,clyde1 thus traversing the whole breadth of the island4 ,on the south-east side another similar series of faults1 which there skirts the edge of the ,silurian tableland1 has nearly the same effect in precisely defining the margin of the ,old ,red ,sandstone4 ,as thus limited1 the tract has a breadth of about #ej miles in ,scotland1 while the portion of it now visible in the ,british ,isles has an extreme length of about #bhj miles 7,map ,,iii474 ,footnote #cdi3 ,my own investigations of this region have been continued over an interval of forty years4 ,besides personally traversing every portion of it1 ,i have mapped in detail1 for the ,geological ,survey1 many hundreds of square miles of its area from the outskirts of ,edinburgh south-westwards into ,lanarkshire1 in ,ayrshire1 and in the counties of ,fife1 ,perth and ,kinross4 ,the ,geological ,survey maps of the volcanic tracts of the ,sidlaw ,hills have been prepared by my brother1 ,prof4 ,james ,geikie1 and ,messrs4 ,h4 ,m4 ,skae and ,d4 ,r4 ,irvine4 ,the ,western ,ochils were mapped chiefly by ,mr4 ,b4 ,n4 ,peach1 partly by ,prof4 ,j4 ,young1 ,mr4 ,r4 ,l4 ,jack and myself2 the ,eastern ,ochils were surveyed mainly by ,mr4 ,h4 ,h4 ,howell2 while the volcanic belt between the tracts mapped by me in ,lanarkshire and in ,ayrshire was chiefly traced out by ,mr4 ,peach4 ,as a rule1 each of these geologists has described in the ,survey ,memoirs the portions of country surveyed by him4 ,but though the boundary-faults determine1 on the whole1 the present limits of the tract of ,old ,red ,sandstone1 they do not necessarily indicate the shore-lines of the sheet of water in which that great series of deposits was laid down4 ,they point to an enormous subsidence of the tract between them--a prolonged and extensive sagging of the strip of country that stretches across the ,midland ,valley of ,scotland into the north of ,ireland4#cej ,this downward movement began as far back as the close of the ,silurian period1 but the marginal fractures and the disruption and plication of the thick masses of sandstone and conglomerate which were accumulated in the lake chiefly took place after the close of the period of the ,lower ,old ,red ,sandstone4 ,i think we may reasonably connect these movements with the general sinking of the area consequent upon the enormous outpouring of volcanic materials during that period4 ,footnote #cej3 ,in some of the dislocations along the ,highland border1 the ,old ,red ,sandstone is bent back upon itself1 and the older schists are thus made to recline upon it1 as if there had been a push over from the ,highland area4 ,along both the northern and southern margins of the basin there occur1 on the farther side of the boundary faults1 outlying patches of ,lower ,old ,red ,sandstone that rest unconformably on the rocks forming the flanks of the hills4 ,these areas possess a peculiar interest1 inasmuch as they reveal some parts of the shore-line of the lake1 and show the relation between the earlier rocks and the sediments of the ,old ,red ,sandstone4 ,we learn from them that the shore-line was indented with wide bays1 but nevertheless ran in a general north-easterly direction4 ,it thus corresponded in trend with the present ,midland ,valley1 with the axes of plication among the schists of the ,highlands as well as among the ,silurian rocks of the ,southern ,uplands1 and with the subsequent faulting and folding of the ,old ,red ,sandstone4 ,illustration3 ,fig4 #gc4--,section at the edge of one of the bays of ,lower ,old ,red ,sandstone along the northern margin of ,lake ,caledonia1 near ,ochtertyre4 a1 slates and phyllites2 b1 volcanic conglomerates2 c1 andesite-lava4 ,i may remark in passing that the conglomerates and other associated materials which have been preserved in these bays and hollows beyond the lines of the great faults1 though they lie unconformably on the rocks beneath1 are not the basement portions of the ,old ,red ,sandstone4 ,on the contrary1 where their probable stratigraphical horizons can be recognized or inferred1 they are found to belong to parts of the series considerably above the base of the whole4 ,they point to the gradual sinking of the basin and the creeping of the waters with their littoral shingles further and further up the slopes of the hills on either side 7,fig4 #gc74 ,but this is not all the evidence that can be adduced to show that the limits of the lake extended considerably beyond the lines of dislocation between which the present area of ,old ,red ,sandstone mainly lies4 ,no one can look at the noble escarpments of the ,braes of ,doune on the one side 7,fig4 #gd71 or walk over the upturned conglomerates and andesites which flank the ,lanarkshire uplands on the other1 without being convinced that if the effects of the boundary faults could be undone1 so as to restore the original structure of the ground1 the prolongations of the rocks1 now removed by denudation1 would be found sweeping far into the ,highlands on the north and into the ,silurian ,uplands on the south4 ,illustration3 ,fig4 #gd4--;,craig ,beinn nan-,eun 7#bjfg feet71 east of ,uam ,var1 ,braes of ,doune4 ,old ,red ,conglomerate1 with the truncated ends of the strata looking across into the ,highlands2 moraines of ,corry ,beach in the foreground4 ,if the area of ,lake ,caledonia were taken to be defined by the boundary faults1 it covered a space of about #aj1jjj square miles4 ,but1 as we know that it certainly stretched beyond the limits marked by these faults1 it must have been of still greater extent4 ,we shall probably not exaggerate if we regard it as somewhat larger than the present ,lake ,erie1 the superficies of which is about #iijj square miles4 ,in this long narrow basin the remarkable volcanic history was enacted of which ,i now proceed to give some account4 ,the ,lower ,old ,red ,sandstone of ,central ,scotland may be conveniently divided into three great groups1 each of which marks a distinct epoch in the history of the basin wherein they were successively accumulated4 ,the lowest of these groups indicates a time of quiet sedimentation during which the basin was defined by plication of the terrestrial crust1 and when1 by the same subterranean movements1 some parts of the floor of the lake were pushed upward above water1 and were then denuded and buried4 ,the middle group consists largely of volcanic rocks4 ,it points to the existence of lines of active volcanic cones situated along the length of the lake4 ,the uppermost group records the extinction of volcanic action and the gradual obliteration of the lake1 partly by the pouring of sediment into it1 and partly no doubt by the continued terrestrial movements which had originally produced the basin4 ,it is evident from these records that though volcanic activity continued vigorous for a vast period of time1 it had entirely ceased in ,lake ,caledonia long before the last sediments of the ,lower ,old ,red ,sandstone were laid down4 ,the great cones of the ,ochil ,hills1 for example1 sank below the waters of the lake in which they had long been a conspicuous feature1 and so protracted was the subsidence of the lake-bottom that the site of these volcanoes was buried under #hjjj or #ijjj feet of sandstones and conglomerates1 among which no trace of any volcanic eruptions has yet been found4 ,the sagging of the terrestrial crust over an area from which such an enormous amount of volcanic products had been discharged would doubtless be a protracted process4 ,long after the subsidence of the lake-bottom and the accumulation of its thick mass of sediments1 after even the entire effacement of the topography and the deposition of the thick ,carboniferous formations over its site1 the downward movement showed itself in the production of gigantic north-east faults1 and the sinking of the ,carboniferous rocks for several thousand feet4 ,these dislocations1 as was natural1 have run through the heart of some of the volcanic groups1 carrying much of the evidence of the ancient volcanoes out of sight1 and leaving us only fragments from which to piece together the records of a volcanic period which is by no means the least interesting in the geological history of this country4 ,confining our attention for the present to the records of the middle or volcanic group1 we find evidence of a number of distinct clusters of volcanoes ranged along the whole length of the basin4 ,the independence of these volcanic districts may be inferred from the following facts3--#ast1 ,the actual vents of discharge may in some cases be recognized2 #bnd1 ,even where these vents have been buried1 we may often observe1 as we approach their probable sites1 a marked increase in the thickness of the volcanic accumulations1 as well as a great development of agglomerates and tuffs2 #crd1 ,traced in opposite directions1 the volcanic materials are found to thin away or even to disappear4 ,those from one centre of discharge may be observed now and then to overlap those from another1 but the two series remain distinct4 ,reasoning from these data and studying the distribution of the various volcanic areas1 we are led to recognize the former existence of two parallel chains of vents1 running along the length of the lake at a distance from each other of somewhere about twenty miles4 ,they may be conveniently distinguished as the northern and the southern chain4 ,the northern band runs from the coast-line near ,stonehaven south-westward through the ,sidlaw and ,ochil ,hills4 ,it is then abruptly truncated by a large fault and by the unconformable superposition of the ,carboniferous formations4 ,but #fj miles further to the south-west1 where the ,old ,red ,sandstone comes out on the west side of the ,firth of ,clyde1 a continuation of the volcanic band has recently been detected by ,mr4 ,w4 ,gunn of the ,geological ,survey in the ,island of ,arran4 ,twenty-five miles still further in the same direction a much ampler development of the volcanic rocks occurs to the south of ,campbeltown in ,cantyre4 ,if we cross the #bb miles of sea that separate the ,argyllshire coast-sections from those of ,red ,bay in ,ireland1 we find near ,cushendall a repetition of the ,scottish volcanic conglomerates1 while still further along the same persistent line1 some #ej miles into the interior1 the hills of ,tyrone include sheets of lava precisely like those of ,central ,scotland4 ,the total length of this northern chain of volcanoes is thus not much less than #bej miles1 and as its north-eastern end is now cut off by the ,north ,sea it must have been still longer4 ,it ran parallel to the north-western coast-line of the lake1 at a distance which1 over the site of the ,midland ,valley of ,scotland1 seems to have varied from #aj to #bj miles1 but which greatly lessened further to the south-west4 ,at a distance of some twenty miles to the south of the northern belt1 the second parallel chain of volcanoes ran in a nearly straight line1 which is now traceable from the southern suburbs of ,edinburgh to the coast of ,ayrshire1 a distance of about #ge miles1 but as its north-eastern end is concealed by ,carboniferous formations1 and its south-western passes under the sea1 its true length is probably considerably more4 ,if the areas which present evidence of distinct and independent vents are grouped according to their positions on these two lines1 they naturally arrange themselves as in the following list3-- ,i4 ,northern ,chain of ,volcanoes #a4 ,the ,montrose ,centre4 #b4 ,the ,sidlaw and ,ochil ,group4 #c4 ,the ,arran and ,cantyre ,centre4 #d4 ,the ,ulster ,centres4 ,,ii4 ,southern ,chain of ,volcanoes #e4 ,the ,pentland ,volcano4 #f4 ,the ,biggar ,centre4 #g4 ,the ,duneaton ,centre4 #h4 ,the ,ayrshire ,group4 ,the distribution of these various volcanic areas will be most easily understood from an examination of ,map ,,iii4 accompanying this volume4 ,i4 ,,the ,,northern ,,chain ,,of ,,volcanoes ,,in ,,lake ,,caledonia #a4 ,the ,montrose ,centre ,beginning at the north-eastern end of the area1 we first encounter a series of volcanic rocks which attain their maximum thickness in ,forfarshire around the town of ,montrose4 ,the main vents probably lay somewhere to the east of the present coast1 under the floor of the ,north ,sea2 at least no clear indication of their existence either on the coast or inland has been detected4 ,from ,montrose1 both to the north-east and south-west1 the lavas thin away1 becoming intercalated among the sandstones1 flagstones and conglomerates1 and gradually dying out4 ,the total length of the volcanic belt is about #ah miles1 that is nine miles from the central thick mass in a north-easterly and the same distance in a south-westerly direction4#cea ,the volcanic pile must be several thousand feet thick1 but owing to the prolongation of the great ,ochil anticline1 the lavas roll over and do not allow their base to be seen4 ,the axis of the fold must pass out to sea1 through the hollow on which the town of ,montrose stands4 ,the volcanic series consists of andesite-sheets with volcanic conglomerates4 ,it contains little ordinary tuff1 but the conglomerates no doubt partly represent ejected fragmental material1 as well as the waste of exposed lavas4 ,a section across the anticlinal fold from ,forfar to ,panbride1 a little to the south-west of ,montrose1 would reveal the structure shown in ,fig4 #fg4 ,footnote #cea3 ,the south-western part of this area from ,arbroath to ,johnshaven was mapped for the ,geological ,survey by the late ,mr4 ,h4 ,m4 ,skae1 the north-eastern part by ,mr4 ,d4 ,r4 ,irvine4 ,my account of it is mainly taken from notes made by myself on the ground preliminary to the commencement of the mapping of the ,survey4 ,in the north-eastern prolongation of the volcanic series from the ,montrose centre1 successively lower members are exposed along the coast-line4 ,but the lavas are dying out in that direction1 and sometimes many hundreds of feet of ordinary sediment intervene between two successive flows4 ,it was in one of these long pauses near the top of the whole pile of lavas that the strata of ,canterland were deposited1 to which reference has already been made4 ,south-west from ,montrose the thick volcanic mass rapidly diminishes1 and is prolonged to the end only by three or four bands separated by sandstones and flagstones4 ,it is in these intercalated groups of sedimentary material that the ,forfarshire flags occur4 ,nowhere can the details of the ,old ,red ,sandstone volcanic rocks be more conveniently studied than along the coast-section in this district from the ,red ,head to ,stonehaven4 ,the rocks have not only been cut into vertical cliffs1 but along many parts of the shore they have been also laid bare in ground-plan1 so that a complete dissection of them is presented to the geologist4 ,at the south end1 the top of the volcanic series appears at the bold promontory of the ,red ,head4 ,there1 at the base of the cliffs of red sandstone1 the accompanying section may be seen4 ,beneath the red false-bedded and sometimes pebbly sandstones 7e71 which form nearly the whole precipice1 lies a band of dull purplish ashy conglomerate 7d71 composed almost wholly of fragments of different andesites1 imbedded in a paste of the same comminuted material4 ,towards the south1 this rock rapidly becomes coarser1 until it passes into a kind of agglomerate1 in which the andesite blocks are sometimes a yard or more in diameter4 ,it includes bands of sandstone1 which increase in number and thickness towards the north1 and sometimes intervene underneath the conglomerate4 ,the lowest rocks here visible are sheets of andesite or porphyrite 7a71 separated from each other by irregular bright red layers of tufaceous sand and agglomerate4 ,these lavas are dull purplish-grey to green1 some of them being tolerably compact1 others highly amygdaloidal1 with large steam-cavities often drawn out in the direction of flow4 ,illustration3 ,fig4 #ge4--,section showing the top of the volcanic series at the foot of the precipice of the ,red ,head1 ,forfarshire4 a1 ,top of slaggy andesite2 b1 coarse volcanic conglomerate2 c1 ,red sandstone2 d1 ,tuff and volcanic conglomerate2 e1 ,red sandstones4 ,one of the most striking features in the andesites of this coast is the remarkable manner in which they include the veinings of pale green and red sandstone already described 7see ,figs4 #fe1 #ff74 ,some of the sheets have in cooling cracked into rude polygons4 ,they are likewise traversed by large cavernous spaces and intricate fissures or steam-cavities4 ,into all these openings the sand has been washed1 filling them up and solidifying into well-stratified sandstone1 the bedding of which is generally parallel with that of the rocks that enclose it1 the dip of the whole series of strata being gently seawards4 ,but a still more intimate mixture of the sand with the lava-sheets is to be remarked where these rocks assume their most slaggy character4 ,in some of them the upper part1 to a depth of ten or twelve feet1 consists of mere rugged lumps of slag which1 while the mass was in motion1 were probably in large measure loose1 and rolled over each other as they were borne onward4 ,the sand has found its way into all the interstices of these clinker-beds1 and now binds the whole mass firmly together4 ,at first sight1 these bands might be taken for agglomerates of ejected blocks1 and as already suggested1 some of the slags may have been thrown out as loose pieces1 but a little examination will show that in the main the rough scoriaceous lumps are pieces of the lava underneath4 ,in these instances1 also1 it is clear that the blocks were in position before the fine sand was sifted into their interspaces1 for the pale green sandstone is horizontally stratified through its intricate ramifications among the pile of dark clinkers4 ,the seaward inclination of the rocks allows the succession of lavas to be seen as the coast is followed westward into ,lunan ,bay4 ,on the further side of that inlet1 after passing over a group of sandstones that underlie the volcanic series of the ,red ,head1 the observer meets with a second and lower succession of lavas which in the five miles northward to ,montrose ,harbour are admirably exposed both along coast-cliffs and on the beach4 ,they resemble those of the ,red ,head1 being made up of alternations of highly vesicular andesite with more compact varieties1 and showing similar sandstone veinings4 ,here and there1 as at ,fishtown of ,usar1 the sea has cut them down into a platform from which the harder parts rise as fantastic half-tide stacks4 ,in some cases1 the more durable rock consists of the slaggy upper portions of the flows1 and in one case this material stands up as a rude pillar twelve feet high1 composed of clinkers firmly cemented with veinings of sandstone4 ,the geologist who wanders over this coast-line is arrested at every turn by the marvellously fresh volcanic aspect of many of the lavas4 ,their upper parts are so cellular that if the calcite1 chalcedony and other infiltrated minerals were removed from their vesicles1 they would be transformed into surfaces of mere slag4 ,in one respect would their antiquity still be evident4 ,these slaggy bands are generally a good deal reddened1 as if they had been long exposed to oxidation before being covered by the overlying sheets of lava--a feature already cited1 as probably indicating the lapse of some considerable interval of time between successive outflows4 ,along this coast-section the absence of intercalated tuffs is soon remarked4 ,the volcanic ejections seem to have consisted almost entirely of andesitic lavas1 though it is possible that here and there the very slaggy bands between the more solid parts of the sheets may include a little pyroclastic material4 ,the lowest portion of the volcanic group here visible is reached at ,montrose ,harbour1 where1 in the flagstones and shales of ,ferryden1 the late ,rev4 ,hugh ,mitchell obtained some of the fossil-fishes of the formation4 ,a space of more than three miles now intervenes where the rocks are concealed by blown sand and other superficial accumulations4 ,it is through this hollow1 as already stated1 that the great ,ochil anticline runs out to sea4 ,on the north side of the ,north ,esk ,river1 we again come upon the same band of lavas as to the south of ,montrose1 but with a dip to the north-west4 ,this inclination1 however1 soon bends round more westerly1 and the result of the change is to expose a slowly descending section all the way to the ,highland fault at ,stonehaven4 ,a picturesque line of high inland cliff1 running northwards beyond ,st4 ,cyrus1 reveals with great clearness the bedded structure of the andesites4 ,but as one moves northward1 owing to the change in the direction of dip1 one finally passes out of this volcanic belt and begins gradually to descend into the thick ,kincardineshire ,old ,red ,sandstone4 ,the amount of conglomerate exposed along this part of the coast-line probably considerably surpasses in thickness any other conglomerate series in the ,lower ,old ,red ,sandstone of ,britain4 ,throughout the enormous depth of sedimentary material1 the conglomerates are well-bedded1 consisting of a dull green paste1 composed in large degree of comminuted andesitic debris1 and interstratified with green felspathic sandstones4 ,they are often remarkably coarse1 the pebbles sometimes measuring three feet in length4 ,interposed among them are some ten or twelve bands1 probably often single outflows of andesite1 sometimes compact and porphyritic1 at other times highly amygdaloidal4 ,such is the succession of rocks for many miles along the shore2 and as the inclination varies from a little north of west to west1 or even west by south1 the observer gradually passes over a thickness of rather more than #bjjj feet from the base of the ,st4 ,cyrus andesites to ,gourdon4 ,in this accumulation of coarse1 well water-worn material1 with abundant intercalations of finer sandstone and occasional sheets of lava1 there is the record of prolonged and powerful denudation with intermittent volcanic activity4 ,dykes of a quartziferous porphyry cut the conglomerates1 and at ,gourdon they are pierced by the intrusion of serpentine above referred to4 ,the proportion of andesite fragments in the conglomerates of this part of the coast varies1 but is generally much lower than that of the rocks from the ,highlands4 ,thus at ,johnshaven1 out of #ajj blocks1 broken promiscuously from the conglomerate1 ,i found that only #h per cent were of andesite1 while #dd per cent were of quartzite1 and the remainder consisted of various quartz-porphyries1 granites and schists4 ,it is evident1 therefore1 that some area of crystalline rocks was subjected to enormous waste1 and that its detritus was strewn over the floor of ,lake ,caledonia1 at the same time that from the ,montrose volcanic vents many streams of andesitic lava were poured forth4 ,a vast mass of coarse conglomerate intervenes between ,gourdon and ,dunnottar1 and forms a nearly continuous line of precipices which in some places rise #bjj feet above the waves4 ,the bedding is everywhere distinctly marked1 so that there is no difficulty in following the succession of the strata1 and estimating their thickness4 ,from the last of the lavas at ,gourdon to the base of the conglomerates near ,stonehaven1 there lies an accumulation of conglomerate at least #hjjj feet thick4 ,the boulders and pebbles in these deposits are generally well-rounded1 and vary up to four feet or more in length4 ,i observed one of quartz-porphyry at ,kinneff which measured seven feet long and six feet broad4 ,the proportion of andesite fragments in these conglomerates continues to be small4 ,i ascertained that in the coarsest mass at ,kinneff they numbered only #ad per cent2 at ,todhead ,point1 a mile and a half to the north1 #bj per cent1 and at ,caterline1 three quarters of a mile further in the same direction1 #ba per cent4 ,in the midst of this gigantic accumulation of the very coarsest water-worn detritus1 there are still records of contemporaneous volcanic action4 ,near ,kinneff the beautiful andesite1 with large tabular crystals of plagioclase1 alluded to on p4 #bgd1 occurs in the conglomerate4#ceb ,south of ,caterline two flows1 lying still lower in the system1 project into the sea4 ,one of these presents a section of much interest4 ,it shows a central solid portion1 jointed into rudely prismatic blocks1 with an indefinite platy structure1 which gives it a roughly-bedded aspect4 ,its upper ten or twelve feet are sharply marked off by their slaggy structure1 ending upwards in a wavy surface like that of the ,vesuvian lava of #aheh4 ,into its fissures1 steam-cavities and irregular hollows1 fine sand has been washed from above1 as at ,red ,head1 while immediately above it comes a coarse conglomerate of the usual character 7,fig4 #gf74 ,still lower down1 beneath some #ijj feet of remarkably coarse conglomerate1 another group of sheets of andesite abuts at ,crawton upon the coast1 with which1 at a short distance inland1 it runs parallel for more than two miles1 coming back to the sea at ,thornyhive ,bay and at ,maidenkaim4 ,we have then to pass over about #ejjj feet of similar conglomerates1 until1 after having crossed several intercalated sheets of andesite1 we meet with the last and lowest of the whole volcanic series of this region in the form of some bands of porphyrite at the ,bellman's ,head1 ,stonehaven4 ,the peculiar geographical conditions that led to the formation of the coarse conglomerates appear to have been established at the same time that the volcanic eruptions began1 for as we descend in the long coast section1 we find that the coarse sediment and the intercalated lavas cease on the same general horizon4 ,below that platform lie some #ejjj feet of red sandstones and red shales1 yet the base of the series is not seen1 for the lowest visible strata have been faulted against the schists of the ,highlands4 ,it is thus obvious that more than #ejjj feet of sediment had been laid down over this part of the floor of ,lake ,caledonia before the first lavas were here erupted4 ,footnote #ceb3 ,for an analysis of the felspar in this rock1 see ,prof4 ,heddle's paper1 ,trans4 ,roy4 ,soc4 ,edin4 xxviii4 7#ahgi71 p4 #beg4 ,illustration3 ,fig4 #gf4--;,andesite with sandstone veinings and overlying conglomerate4 ,todhead1 south of ,caterline1 coast of ,kincardineshire4 #b4 ,the ,sidlaw and ,ochil ,group ,the volcanoes which poured out the masses of material that now form the chain of the ,ochil and ,sidlaw ,hills appear to have been among the most vigorous in the whole region of ,lake ,caledonia4 ,their chief vents probably lay towards the south-west in the neighbourhood of ,stirling1 where the lavas1 agglomerates and tuffs discharged from them reach a thickness of not less than #fejj feet1 without revealing their bottom4 ,from that centre the lavas range continuously for nearly fifty miles to the north-east1 until they reach the sea at ,tayport2 but they are prolonged on the north side of the ,firth of ,tay from ,broughty ,ferry to near ,arbroath1 so as to overlap those of the ,montrose group4 ,they thus attain a total length of nearly sixty miles in a north-easterly line4 ,how far they stretched south-west cannot now be ascertained1 for they have been dislocated and buried in that direction under the ,carboniferous formations of the ,midland ,valley4 ,it will be observed from the map 7,no4 ,,iii47 that the great volcanic ridge of the ,ochil ,hills continues unbroken for twenty-two miles1 from ,stirling to ,bridge of ,earn4 ,thereafter it branches into two divergent portions1 one of which runs on through the north of ,fife to the southern promontory of the estuary of the ,tay1 while the other1 after sinking below the alluvial plains of the ,earn and the ,tay1 mounts once more into a high ridge near ,perth1 and thence stretches eastward into ,forfarshire as the chain of the ,sidlaw ,hills4 ,this bifurcation is due to the opening out and denudation of the great anticlinal fold above mentioned4 ,the rocks in the northern limb dip north-westward1 those in the southern limb dip south-eastward4 ,the lower members of the ,old ,red ,sandstone1 underlying the volcanic series1 ought to be seen beneath them along the crest of the anticline4 ,unfortunately1 however1 partly by the action of faults along the boundaries of the volcanic bands1 but chiefly from the unconformable overspread of ,upper ,old ,red ,sandstone and ,lower ,carboniferous rocks across the plains of the ,carse of ,gowrie and of the ,earn1 the lower parts of the system are there concealed 7see ,fig4 #gh74 ,as already remarked1 this important anticlinal fold runs to the north-east across ,forfarshire1 and passes out to sea north of ,montrose4 ,through the ,ochil chain the fold runs obliquely in a south-westerly direction1 until it is truncated by the great fault which lets down the ,clackmannan coal-field4 ,the total traceable length of this anticline is thus about sixty miles4 ,it flattens down towards the south-west2 consequently the rocks in the western part of the ,ochil ,hills are so gently inclined that the same bands may be followed winding round the sides of the valleys1 and giving to the steep declivities the terraced contours to which allusion has already been made 7see ,fig4 #fh74 ,another result of this structure is that the base of the volcanic series is entirely concealed by its higher portions4 ,from an examination of the map it will be further obvious that the whole wide plain of ,strathmore--that is the great hollow1 more than #hj miles long and about ten or twelve miles broad1 which stretches between the base of the ,highland mountains and the north-western slopes of the ,ochil and ,sidlaw chain--is underlain with volcanic rocks of ,lower ,old ,red ,sandstone age4 ,this plain lies on a broad synclinal fold1 along the south-east side of which the lavas1 tuffs and conglomerates of the ,ochil and ,sidlaw ,hills dip under a thick accumulation of red sandstone and flagstone4 ,on the north-west side similar lavas and tuffs rise again to the surface1 both on the southern side of the great boundary faults1 and also in the little bays which here and there survive on the northern side of the dislocations 7,fig4 #gg74 ,i have already alluded to these interesting relics of the shore-line of ,lake ,caledonia1 and to the fact that though they lie unconformably on the ,highland schists1 they do not belong to the actual basement members of the ,old ,red ,sandstone 7ante1 p4 #bie1 and ,fig4 #gc74 ,we have seen that below the bottom of the volcanic series a thickness of #ejjj feet of sandstones and shales emerges on the ,stonehaven coast1 and yet that even there the base of the whole system is not visible1 owing to the effect of the ,highland boundary fault4 ,it is thus evident that over the bottom of ,lake ,caledonia a very thick deposit of tolerably fine sedimentary material was spread before the commencement of the ,ochil and ,sidlaw eruptions1--that when the lavas were poured out and the coarse conglomerates began to be formed1 these materials overlapped the older deposits and gradually encroached upon the subsiding area of the ,highlands4 ,the lavas rolled across the floor of the lake and entered the successive bays of the northern coast-line1 where their outlying patches may still be seen4 ,illustration3 ,fig4 #gg4--,section across the ,boundary-fault of the ,highlands at ,glen ,turrit1 ,perthshire4 s1 ,crystalline schists of the ,highlands2 c c1 conglomerates and sandstones 7,lower ,old ,red ,sandstone7 with interstratified volcanic rocks 7v v72 f1 fault4 ,from these facts it is clear that to the actually visible area of volcanic material in the ,ochil and ,sidlaw region1 and to the anticlinal tract whence the andesites have been removed by denudation1 we have to add the area that lies under the plain of ,strathmore1 which may be computed to be at least #hjj square miles1 making a total of probably not less than #acjj square miles4 ,but it will be remembered that practically only one side of the anticlinal fold is accessible to observation4 ,we cannot tell how far in a southerly direction the lavas of the ,ochil ,hills may extend4 ,it is quite possible that not a half of the total area covered by the eruptions of this volcanic group is now within reach1 either of observation or of well-founded inference4 ,one further general characteristic of this volcanic district will be obvious from an inspection of the map4 ,while the thickest mass of lavas and tuffs1 lying towards the south-west1 points to the existence of the most active vents in that part of the area1 the actual positions of these vents have not been detected4 ,probably they lie somewhere to the south of the edge of the ,ochil chain1 under the tract which is overspread with the coal-field4 ,but other and possibly minor orifices of eruption appear to have risen at irregular intervals towards the north-east along the length of the lake4 ,thus there are numerous bosses of felsitic and andesitic rocks among the central ,ochils1 some of which may mark the positions of active vents4 ,for some miles to the east of that area an interval occurs1 marked by the presence of only a few small intrusive masses4 ,but as the broad anticline of the ,firth of ,tay opens out and allows the lower or pre-volcanic members of the ,old ,red ,sandstone to approach the surface1 another group of bosses emerges from the lower sandstones and flagstones4 ,some of these cover a considerable space at the surface1 though a portion of their visible area may be due to lateral extravasation from adjacent pipes1 the true dimensions of which are thereby obscured4 ,some of the masses are undoubtedly sills4 ,in the case of ,dundee ,law we probably see both the pipe and the sill which proceeded from it2 the prominent1 well-defined hill marking the former1 while the band of rock which stretches from it south-westwards to the shore belongs to the latter4 ,the material that forms the bosses and sills in this neighbourhood is generally a dark compact andesite4 ,the rock of ,dundee ,law was found by ,dr4 ,hatch to show under the microscope striped lath-shaped felspars abundantly imbedded in a finely granular groundmass1 speckled with granules of magnetite1 but showing no unaltered ferro-magnesian constituents4 ,here and there in the same district a solitary neck may be observed filled with agglomerate 7,fig4 #gh74 ,illustration3 ,fig4 #gh4--,section across the chain of the ,sidlaw ,hills1 near ,kilspindie4 #a4 ,lower ,old ,red ,flagstones and ,sandstones2 #b4 ,andesite lavas2 #c4 ,volcanic tuff2 #d4 ,volcanic conglomerates and sandstones2 ,n1 ,volcanic neck2 #e4 ,upper ,old ,red ,sandstone under ,carse of ,gowrie1 lying unconformably on the lower division2 f1 ,fault2 d1 ,basic dyke4 ,the variations in the structure of the ,ochil and ,sidlaw volcanic group will be most easily understood from a series of parallel sections4 ,beginning on the north-eastern or ,sidlaw branch of the volcanic band1 we find the arrangement of the rocks to be as is shown in the accompanying figure#cec 7,fig4 #gh74 ,as is usually the case in this region1 the base of the volcanic series is here concealed by the fault which brings down the ,upper ,old ,red ,sandstone under the alluvial deposits of the ,carse of ,gowrie4 ,the total thickness of the series in this section is about #bejj feet4 ,the rocks consist of successive sheets of andesite of the familiar types1 varying in colour through shades of blue1 purple and red1 and in texture from a dull compact almost felsitic character to more coarsely crystalline varieties4 ,they are often amygdaloidal1 especially in the upper and lower portions of the individual flows4 ,they are not infrequently separated from each other by courses of conglomerate or ashy sandstone and grit4 ,of these intercalations four are of sufficient thickness and persistence to be mapped1 and are shown on the ,geological ,survey ,sheet #dh4 ,the stones in the conglomerates vary up to blocks two feet in diameter1 and consist chiefly of andesites1 but include also some pink felsites and pieces of greenish hardened sandstone4 ,generally they are more or less well-rounded2 but occasionally they become angular like those of volcanic agglomerates4 ,footnote #cec3 ,this section and the notes accompanying it have been supplied by ,prof4 ,james ,geikie1 who mapped the western half of the ,sidlaw range for the ,geological ,survey4 ,the eastern half was mapped by the late ,mr4 ,h4 ,m4 ,skae4 ,one of the most interesting features in this section is the neck which at ,over ,durdie rises through the volcanic series4 ,oval in form1 it measures #fcj yards in one diameter and #cej in another1 and is filled with pinkish granular tuff1 full of andesitic lapilli and blocks4 ,a much smaller neck of similar material lies about #ajj yards further to the south-west4 ,there seems no reason to doubt that these necks mark two of the volcanic vents belonging to a late part of the volcanic history of the district4 ,the structure of the ,sidlaw range is repeated among the hills of east ,fife on the southern side of the great anticlinal fold4#ced ,thus a section from near ,newburgh on the ,firth of ,tay southward to near ,auchtermuchty in ,stratheden gives the arrangement of rocks shown in ,fig4 #gi4 ,in this traverse a thick mass of fragmental material occurs in the higher part of the series of volcanic rocks4 ,though on the whole stratified and forming a group of conglomerate-beds between the lavas1 the material is in places an amorphous agglomerate of volcanic blocks varying in size up to two feet in diameter4 ,these portions show abundant angular and subangular blocks1 many of which1 after having undergone some attrition1 have been finally broken across before reaching their present resting-places4 ,sharply fractured surfaces can be picked out of the felspathic ashy matrix4 ,the stones are chiefly varieties of andesite1 but they include also pink felsites and pieces of some older fine-grained tuff4 ,footnote #ced3 ,the eastern part of the ,ochils was mapped for the ,geological ,survey by ,mr4 ,h4 ,h4 ,howell and ,mr4 ,b4 ,n4 ,peach4 ,illustration3 ,fig4 #gi4--,section across the ,eastern ,ochil ,hills from near ,newburgh to near ,auchtermuchty4 #a4 ,lower ,old ,red ,sandstones and conglomerates2 #b4 ,andesite lavas2 #c4 ,volcanic conglomerates2 #d4 ,upper ,old ,red ,sandstone4 ,these fragmental materials form a local deposit about nine miles long1 and probably not less than #agjj feet thick4 ,they are partly interstratified with flows of andesite4 ,though1 from the rounded forms of some of the pebbles1 wave-action may be inferred to have been concerned in their accumulation1 they seem to be mainly due to volcanic explosions4 ,no trace1 however1 has been found of the vent from which the eruptions took place4 ,not improbably its site lies somewhere to the south in the area now concealed under the ,upper ,old ,red ,sandstone and ,carboniferous formations4 ,the large size of many of the blocks suggests that they do not lie far from their parent focus of discharge4 ,it is impossible to tell how much of the volcanic series is here concealed by the unconformable overlap of the younger formations4 ,illustration3 ,fig4 #hj4--;,generalized section across the heart of the ,ochil ,hills1 from ,dunning on the north to the ,fife ,coal-field near ,saline on the south4 #a4 ,volcanic tuffs and agglomerates2 #b4 ,andesite lavas2 #c4 ,lower ,old ,red ,sandstone and conglomerate2 #d4 ,necks of felsitic rocks2 #e4 ,upper ,old ,red ,sandstone and ,calciferous ,sandstones2 #f4 ,representative of the ,plateau lavas and tuffs of the ,lower ,carboniferous series2 #g4 ,hurlet 7,carboniferous7 ,limestone2 #h4 ,dolerite sill2 #i4 ,sandstones1 shales and coals of the ,carboniferous ,limestone series2 #aj4 ,neck of the ,puy series 7,carboniferous72 f1 ,fault4 ,a section across the centre of the ,ochil chain1#cee from ,dunning in ,strathearn to the ,crook of ,devon and the ,fife ,coal-field1 gives the structure which is generalized in ,fig4 #hj4 ,at the north end the volcanic series is found to be gradually split up into separate lava-sheets until it dips under the red sandstones of ,strathearn4 ,traced southwards the rocks become entirely volcanic4 ,some of their most conspicuous and interesting members are pale felsitic tuffs1 which occupy a considerable tract of ground about ,craig ,rossie1 south-east of ,auchterarder4 ,as the dip gradually lessens the harder lavas are able to spread over wider tracts of ground1 capping the hills and ridges1 while underneath them thick masses of tuff and conglomerate are laid bare in the valleys4 ,a number of bosses of orthophyre rise through these rocks and are accompanied by many veins and dykes of similar material4 ,it is not improbable that some of these bosses1 as already suggested1 may represent vents4 ,they are especially prominent among the hills due south of ,auchterarder4 ,one of these eminences1 known as the ,black ,maller1 is composed of a typical orthoclase-felsite without mica4 ,another1 about four and a half miles further south1 forms the conspicuous summit of ,ben ,shee overlooking ,glen ,devon1 and consists of a similar rock with a characteristic platy structure4 ,footnote #cee3 ,the central portion of the ,ochils was mapped for the ,geological ,survey by ,mr4 ,b4 ,n4 ,peach1 ,prof4 ,james ,geikie1 ,prof4 ,j4 ,young1 ,mr4 ,r4 ,l4 ,jack and myself4 ,no necks of agglomerate have been observed in this part of the chain4 ,it will be seen from the section that the lowest visible parts of the ,ochil volcanic series are here truncated by a fault which brings in the lower part of the ,carboniferous system4 ,by a curious conjuncture1 immediately on the south side of this fault1 a band of tuff appears1 lying on the platform of the ,carboniferous plateau-lavas1 to be hereafter considered1 and passing below the well-known ,hurlet seam of the ,carboniferous ,limestone1 while through these strata rises one of the puys belonging to the second phase of volcanic activity in ,carboniferous time in ,scotland4 ,the best sections to show the nature and sequence of the volcanic series of the ,ochil ,hills are to be observed at the west end of the chain4 ,but as the whole succession of rocks cannot conveniently be obtained along one line1 it is better to make several traverses1 starting in each case from a known horizon4 ,in this way1 by means of three parallel sections1 we may obtain the whole series of lavas and tuffs in continuous order4 ,the first line of section starts in the lowest part of the tuffs represented at the bottom of the group in ,fig4 #hj1 and runs up to the first thick ashy intercalation among the lavas4 ,following this bed south-westward to the ,burn of ,sorrow1 we make from that horizon a second traverse across the strike to the summit of ,king's ,seat ,hill 7#baaa feet above the sea71 where we meet with a well-marked lava which can be traced south-westwards1 gradually descending the southern escarpment of the hills until it reaches the boundary fault near the village of ,menstrie4 ,starting again from this definite horizon1 we take a third line across the top of ,dumyat 7#acgc feet7 to the plain of ,sheriffmuir1 and there pass beyond the volcanic series into the overlying red sandstones4 ,arranged thus in continuous vertical sequence the succession is found to be as represented in ,fig4 #ha4 ,the total thickness of volcanic material amounts to more than #fejj feet4 ,illustration3 ,fig4 #ha4--;,diagram of the volcanic series of the ,western ,ochil ,hills4 ,the bands with vertical lines are various lavas 7a72 the tuffs and volcanic breccias are shown by the dotted bands 7b72 the uppermost portion of the section above the last thick group of lavas consists of conglomerates and sandstones 7c7 with a sheet of lava4 ,in this vast pile of volcanic ejections the lavas are almost entirely andesites of the usual characters4 ,they include many slaggy and amygdaloidal varieties1 some beautiful porphyries with large tabular felspars1 likewise the resinous or glassy variety already referred to as occurring above ,airthrey ,castle4 ,their upper and under surfaces show the same structure as already described in those of the coast-sections in the ,montrose tract4 ,they include also more acid lavas1 like the pale pink decomposing felsites of the ,pentland ,hills4 ,the tuffs and conglomerates occur on many platforms throughout the succession of lava-sheets4 ,they form the lowest visible part of the whole volcanic series1 but they are most abundant towards the top1 and are best displayed at the western end of the hills4 ,in ,dumyat they form a conspicuous feature4 ,the whole of that hill consists of a constant alternation of lavas 7chiefly slaggy andesites1 but including also one felsitic flow7 with bands of coarse and finer tuff and volcanic conglomerate4 ,the greatest continuous mass of this fragmental material is #fjj or #gjj feet thick4 ,from the extraordinary size of its included blocks it obviously must have been formed of ashes1 stones and huge pieces of lava ejected from some vent in the near neighbourhood4 ,some of the individual blocks in this mass are as large as a ,highland crofter's cottage4 ,the uppermost lavas of ,dumyat dip under a still higher series of coarse volcanic conglomerates entirely made up of andesitic debris and reaching a thickness of about #ajjj feet4 ,this enormous accumulation was probably due partly to the abrasion of exposed cones and lava-ridges1 and partly to volcanic discharges of fragmentary materials4 ,yet it is worthy of note that even amidst these evidences of the most vigorous volcanic activity we have also proofs of quiet sedimentation and traces of the fishes that lived in the waters of the lake4 ,this particular zone of coarse conglomerate as it extends in a south-westerly direction becomes finer1 and its upper part passes into a chocolate-coloured sandstone which has been quarried at ,wolfe's ,hole1 ,westerton1 ,bridge of ,allan1 at a distance of about three miles from where the line of section runs1 which is embodied in the diagram1 ,fig4 #ha4 ,it was from this locality that the specimens of ,eucephalaspis1 ,pteraspis and ,scaphaspis were obtained which were described by ,professor ,ray ,lankester4#cef ,footnote #cef3 ,palontographical ,society1 vols4 xxi4 7#ahfg7 and xxiii4 7#ahfi74 ,above the last-named thick group of coarse volcanic conglomerates a solitary sheet of dark slaggy andesite may be observed4 ,this lava is then overlain by the great depth of chocolate-coloured and red sandstones and marls of the plain of ,strathmore 7c in ,fig4 #ha74 ,nevertheless a few hundred feet up in these sedimentary deposits we meet with yet one further thin sheet of lava--the last known eruption of the long volcanic history of this district4 ,before quitting the ,ochil range ,i may refer to the evidence there obtainable as to the horizontal extent of separate sheets of lava4 ,the western end of this range affords great facilities for following out individual beds of andesite along the bare terraced front of the great escarpment4 ,thus1 the easily recognizable porphyrite which caps ,king's ,seat ,hill1 above ,tillicoultry 7see ,fig4 #fh71 can be traced winding along the hill-slopes until it descends to the plain1 and is then lost under the great fault1 at the foot of ,dumyat--a distance of more than six miles4 ,there is1 therefore1 no difficulty in supposing that from the ,ochil line of vents streams of lava should have rolled along the floor of the lake across to the base of the ,highland slopes1 #aj or #ab miles distant4 ,we cannot tell1 of course1 whether any buried vents lie below the plain of ,strathmore1 but certainly no unquestionable trace of vents has yet been found among the crystalline rocks along the borders of the ,highlands4#ceg ,footnote #ceg3 ,allusion has already been made to the possible connection of the younger ,highland granites with the volcanic series of the north-eastern part of ,lake ,caledonia2 also to the occurrence of isolated masses of breccia piercing the crystalline schists near ,loch ,lomond 7ante1 p4 #bgb74 ,reference has already been made to the comparative scarcity of sills in this region1 and to the occurrence of the acid group of ,lintrathen porphyry and the more basic sheets between the ,firth of ,tay and ,forfar4 ,this scarcity no doubt arises in part from the extent to which the rocks that underlie the volcanic series are concealed4 ,yet it is noteworthy that along the coast-section of these rocks near ,stonehaven hardly any intrusive sheets are to be seen4 #c4 ,the ,arran and ,cantyre ,centre ,it is unfortunate that the ,ochil chain should be broken across and buried under younger formations at the very place where some of the most interesting vents in the whole area of the ,old ,red ,sandstone might have been looked for4#ceh ,we have to pass westwards across the ,firth of ,clyde to the ,isle of ,arran before we again meet with rocks of the same age and character4 ,footnote #ceh3 ,the ,ochil area is not the only example of the abrupt termination of a volcanic band near its centre owing to faults or overlaps4 ,the sudden disappearance of the ,pentland lavas and tuffs on the northern side of the ,braid ,hills is another striking illustration4 ,in the course of the recent work of the ,geological ,survey in that island1 ,mr4 ,w4 ,gunn has discovered that the ,lower ,old ,red ,sandstone includes some interstratified volcanic rocks on the north side of ,north ,glen ,sannox1 and he has supplied me with the following notes regarding them4 ,the area in which the volcanic intercalations occur is much faulted and only a part of it has been mapped in detail1 but the position of the interbedded igneous rocks is quite clear4 ,the ,old ,red ,sandstone here consists of three distinct members1 the lowest of which is made up of coarse1 well-rounded conglomerates1 alternating with sandstones and purple mudstones4 ,above this1 and apparently unconformable to it1 is a middle series of light coloured conglomerates and sandstones1 the pebbles in which are mainly of quartz4 ,finally comes an upper series of red sandstones and conglomerates1 which occupy nearly the whole of the coast section1 and it is this series which has generally been taken as the typical ,old ,red ,sandstone of the island4 ,the volcanic series is intercalated between the middle and upper divisions given above1 and may be seen in several places on the hillside between the shepherd's house at ,north ,sannox and ,laggan4 ,it consists mainly of old lava-beds of a dull reddish or purplish colour1 often soft1 and in places much decomposed4 ,it seems basic in character4 ,a specimen from near the ,fallen ,rocks1 examined by ,mr4 ,teall1 was found to be too much altered for precise determination1 but was probably a basalt originally4 ,these rocks do not occur on the coast4 ,in the southern extremity of ,cantyre some important relics of the volcanic rocks of the ,lower ,old ,red ,sandstone have been recently detected and mapped for the ,geological ,survey by ,mr4 ,r4 ,g4 ,symes4#cei ,this division of the system has been ascertained by him to be extensively developed to the south of ,campbeltown1 and to include some small but interesting remains of the volcanic action which was so marked a feature in the areas of ,lake ,caledonia1 lying further to the east4 ,to the student of volcanic geology1 indeed1 this small tract at the extreme southern end of ,argyllshire has a peculiar interest1 for in no other part of the ,british ,isles have the phenomena of the eruptive vents of the ,lower ,old ,red ,sandstone been more admirably laid bare4 ,not only are there necks in the interior like that represented in ,fig4 #hb2 but others have been dissected by the waves along the southern shore1 and their relations to the deposits of fragmentary material showered over the bottom of the lake have been more or less clearly exposed4 ,footnote #cei3 ,the late ,prof4 ,james ,nicol published in #aheb an account of the geology of the southern portion of ,cantyre4 ,he grouped all the igneous rocks of the district as one series1 which he regarded as later than the ,coal-formation and possibly of the same age as those of the north-east of ,ireland4 ,he made no distinction between the ,lower ,old ,red ,sandstone and the younger unconformable conglomerates 7,quart4 ,journ4 ,geol4 ,soc4 vol4 viii4 7#aheb71 p4 #djf74 ,illustration3 ,fig4 #hb4--,view of ,cnoc ,garbh1 ,southend1 ,campbeltown4 ,a volcanic neck of ,lower ,old ,red ,sandstone age1 about #djj yards wide in its longer diameter4 ,at ,keil ,point1 a little to the east of the most southerly headland of the ,mull of ,cantyre1 some reddish and purplish highly felspathic sandstones 7a in ,fig4 #hc7 dipping towards the east are found to pass upward into coarse volcanic breccias 7b71 which1 followed eastwards1 lose almost all trace of stratification1 and are then abruptly succeeded by a neck of coarse agglomerate 7c7 measuring #be yards from north to south1 where its limits can be seen1 and at least #ab yards from west to east4 ,it is hardly possible to distinguish between the breccias to the west and the agglomerate of the neck1 except by the rude bedding of the former which pass down into the well-bedded sandstones4 ,the agglomerate is a thoroughly volcanic rock4 ,the materials consist chiefly of angular blocks of a pale purplish or lilac highly porphyritic mica-porphyrite1 with large white felspars and hexagonal tables of black mica4 ,these blocks might sometimes be mistaken for slags from their cavernous1 weathered surfaces1 but this rough aspect is found on examination to be due to the decay of their felspars4 ,illustration3 ,fig4 #hc4--,section of volcanic series on beach1 ,southend1 ,campbeltown4 a1 ,fine reddish and purplish highly felspathic sandstones1 largely composed of porphyry-debris and passing up into coarse breccias2 b1 volcanic breccias1 coarse and only rudely stratified1 formed of blocks of porphyry1 sandstone fine tuff and andesite1 together with water-worn quartzite pebbles derived from some conglomerate2 c1 coarse unstratified agglomerate forming a neck4 ,perhaps the most singular feature among the contents of this neck is the number of well-rounded and smoothed pebbles and boulders of quartzite4 ,these are dispersed at random through the mass1 and are often placed on end4 ,there can be no doubt that they are water-worn stones1 but the contrast of their smooth surfaces and rounded forms with the rough angular blocks of igneous material is so striking as to lead at once to the conclusion that they cannot have acquired their water-worn character in the deposit where they now lie4 ,their positions and their occurrence with ejected volcanic blocks suggest that they too were discharged by volcanic explosions4 ,they so exactly resemble the quartzite boulders and pebbles in the neighbouring ,old ,red ,conglomerates that there can be little hesitation in regarding them as derived from these conglomerates4 ,they seem to me to have come from a lower part of the ,old ,red ,sandstone1 which was shattered by volcanic energy either before the conglomerates were firmly consolidated or afterwards by such violent explosions as served to separate the pebbles from the matrix of the rock4 ,there occur also in the agglomerate blocks of fine tuff and ashy sandstone sometimes four feet long1 and often stuck on end1 showing that the deposits of earlier eruptions were broken up during the drilling of this little vent4 ,a few hundred yards further east a larger neck rises on the beach1 immediately to the south of the old ,celtic chapel of ,st4 ,columba4 ,it consists also of exceedingly coarse agglomerate1 with andesite blocks three and four yards in diameter4 ,it is about #abe yards broad from east to west1 on which sides it is seen to be flanked by coarse volcanic breccias and conglomerates1 resembling in composition the materials of the neck1 but showing an increasingly definite stratification as they are traced eastward in the ascending succession of deposits4 ,following the section in still the same easterly direction along the coast1 we find that bands of fine felspathic sandstone1 marking probably intervals of quiescence1 are again and again succeeded by coarse brecciated conglomerates of igneous materials1 which may be inferred to have been due to a renewal of violent eruptions4 ,by degrees the evidence of stratification and of attrition among the volcanic materials becomes more pronounced as the ascending section is followed2 blocks of andesite1 even #ah inches or two feet in diameter1 assume well-rolled1 rounded1 water-worn forms1 like the pebbles of quartzite associated with them1 and eventually the strata return to the usual aspect of the conglomerates of the district4 ,i have never seen anywhere better proofs of volcanic explosions1 contemporaneous with a group of strata1 and of the distribution of volcanic fragmentary material round the vents4 ,a further point of much interest is the additional evidence furnished by this shore-section of considerable wave-action during the accumulation of the coarse conglomerates4 ,to give to blocks of porphyrite two feet in diameter a smoothed and rounded form must have required the action of water in considerable agitation4 #d4 ,the ,ulster ,centres ,from the volcanic breccias and conglomerates of the ,mull of ,cantyre to the coast of ,antrim in a straight line is a distance of little more than twenty miles4 ,on a clear day the ,old ,red ,sandstone of ,cross ,slieve1 and the range of cliffs in which it abruptly descends to the sea between ,cushendall and ,cushendun1 can be distinctly seen from the ,argyllshire shore4 ,the geologist who passes from the ,scottish to the ,irish sections cannot fail to be impressed with the resemblance of the rocks in the two countries1 and with the persistence of the types of conglomerate in ,lake ,caledonia4 ,a picturesque section has been laid bare between the ,coastguard ,station south of ,cushendall and ,cushendun ,bay4#cfj ,at the south side of the little inlet of ,cushendall1 a compact dull quartz-porphyry is exposed in crags along the shore4 ,this rock ranges in colour from dark brown and purple to pale-green and buff4 ,its texture also varies1 as well as the proportion of its felspar-crystals and quartz-blebs4 ,some parts have a cavernous structure1 like that of an amygdaloid1 the small globular cavities being filled with green decomposition products4 ,footnote #cfj3 ,for descriptions of this district see ,j4 ,bryce1 ,proc4 ,geol4 ,soc4 i4 7#ahcd7 p4 #cif1 v4 7#ahcg7 p4 #fi2 ,j4 ,kelly1 ,proc4 ,roy4 ,irish ,acad4 x4 7#ahfh71 p4 #bci4 ,the area is contained in ,sheet #ad of the ,geological ,survey of ,ireland1 and was mapped by ,mr4 ,a4 ,m',henry and described by him in the accompanying ,explanatory ,memoir 7#ahhf71 pp4 #ab1 #be4 ,the stratigraphical relations of this rock are not quite clear1 but it is certainly older than the ,old ,red conglomerates which lie to the north of it1 for these are largely made up of its fragments4 ,the matrix of these detrital masses consists mainly of the comminuted debris of the porphyry4 ,the pebbles include all the varieties of that rock1 and are tolerably well-rounded4 ,there is no distinct evidence of volcanic action among these conglomerates4 ,they resemble1 however1 many of the conglomerates in the ,midland ,valley of ,scotland1 which1 as in the case of those on the ,forfarshire and ,kincardineshire coast1 are in great part made of the detritus of andesitic lavas4 ,the ,cushendall rocks become coarser as they are traced northwards into lower members of the series1 while at the same time the proportion of porphyry-debris in their constitution diminishes1 and materials from the metamorphic series take its place4 ,thus at ,cushendun the percentage of quartz-pebbles rises to #gj or #hj4 ,these blocks1 of all sizes up to two feet or more in diameter1 are admirably rounded and smoothed1 like those in the ,stonehaven section and those among the conglomerates at the south end of ,cantyre4 ,fragments of the porphyry1 however1 still continue to appear1 and the matrix shows an admixture of the finer detritus of that rock4 ,i may remark in passing that no conglomerates of the ,old ,red ,sandstone show more strikingly than these at ,cushendun the effects of mechanical crushing subsequent to deposition and consolidation4 ,in many parts of the rock it is hardly possible to find a rounded block that has not been fractured4 ,some of them1 indeed1 may be seen cut into half a dozen slices1 which have been pushed over each other under the strain of strong lateral or vertical pressure4 ,in the interior of the country1 after passing over the broad ,tertiary basaltic plateau of ,antrim1 we come upon a large area of ,lower ,old ,red ,sandstone in ,tyrone4 ,it stretches from ,pomeroy to ,loch ,erne1 a distance of about #cj miles1 and is about #ab miles broad4 ,in lithological character the strata of this tract exactly resemble parts of the deposits of ,lake ,caledonia in ,central ,scotland4 ,they include also a volcanic series which1 down to the smallest points of detail1 may be paralleled in the sister island4#cfa ,this interesting westward prolongation of the volcanic record consists of a number of outlying patches confined to the eastern part of the district4 ,footnote #cfa3 ,this area of ,old ,red ,sandstone is represented on ,sheets #cc1 #cd1 #de and #df of the ,geological ,survey of ,ireland1 and the igneous rocks are described in the ,memoirs on ,sheets #cc 7#ahhf1 p4 #ag7 by ,mr4 ,j4 ,r4 ,kilroe1 and #cd 7#ahgh1 p4 #af7 by ,mr4 ,j4 ,nolan4 ,the largest of these patches lies to the south of ,pomeroy1 where it forms a line of hills about four miles long1 and covers an area of some five square miles4 ,the rocks consist of successive sheets of andesite-lavas4 ,these1 as a rule1 are not markedly cellular1 though they include some characteristic amygdaloids4 ,a distinguishing feature of some of the sheets is their remarkably well-developed flow-structure4 ,thus on ,sentry ,box1 at the north-western end of the ridge1 the fissility resulting from this structure so perfectly divides the rock into parallel flags that the material might easily be mistaken for a bedded rock4 ,where this structure has been produced in a cellular lava1 the cavities have been drawn out and flattened in the direction of flow4 ,i have not observed true tuffs in any of the sections traversed by me in this district4 ,but the conglomerates furnish abundant evidence of the contemporaneous outpouring of the lavas4 ,thus1 in a brook a little west of ,reclain1 five miles south of ,pomeroy1 the section shown in ,fig4 #hd may be seen4 ,at the base lies a coarse conglomerate 7a7 largely composed of andesite-debris1 the stones being here1 as elsewhere in the district1 well rounded4 ,then comes a series of green and reddish highly-felspathic sandstones 7b71 followed by an exceedingly coarse conglomerate 7c71 formed mainly of the debris of andesites1 especially lumps of slag4 ,some of the stones measure #ah inches in diameter1 and all are well water-worn4 ,immediately over this mass of detritus lies the lowest sheet of andesite-lava 7d74 ,illustration3 ,fig4 #hd4--,section of the base of the volcanic series1 ,reclain1 five miles south of ,pomeroy4 ,some sections visible in the neighbourhood of ,omagh afford further evidence of volcanic action at the time of the deposition of the ,old ,red ,sandstone of this region4 ,at ,farm ,hill1 a little to the east of the town1 felspathic sandstones and breccias enclose angular and subangular pieces of various andesites1 and occasionally even pieces of tuff4 ,near these strata a decayed andesite occurs in the bed of a stream1 and a fresher variety is quarried at ,farm ,hill4 ,a little further south another variety of andesite is exposed in two quarries at ,recarson ,meeting-,house--a fine granular purplish-grey rock1 with abundantly-diffused hmatite pseudomorphs1 probably after a pyroxene1 and sometimes strongly amygdaloidal4 ,illustration3 ,fig4 #he4 ,section of shales and breccias at ,crossna ,chapel1 north-east of ,boyle4 a a1 ,green and grey shales2 b b1 green and grey hard sandstones and grits1 some bands strongly felspathic2 c1 fine compact felspathic breccia1 with angular chips of different felsites and andesites1 etc4 ,there can thus be no doubt that this region of ,ulster included several centres of volcanic activity during the deposition of the red sandstones and conglomerates1 and that the lavas and volcanic conglomerates belonged to precisely the same types as those of the same geological age which occur so abundantly in ,scotland4 ,further south-west1 near ,boyle1 in the county of ,roscommon1 certain curious felspathic breccias in the ,old ,red ,sandstone have been mapped as felstone4#cfb ,so far as ,i have been able to examine them1 however1 they are entirely of fragmental origin4 ,they contain pieces of andesitic and felsitic rocks1 with fragments of devitrified glass1 which undoubtedly point to the occurrence of volcanic eruptions during their deposition1 though no tuffs and lavas appear to crop out in the narrow strip of the formation there exposed4 ,footnote #cfb3 ,see ,sheet #ff ,geological ,survey of ,ireland1 and ,explanation to that sheet 7#ahgh71 p4 #ae4 ,the rocks were previously described by ,jukes and ,foot1 ,journ4 ,roy4 ,geol4 ,soc4 ,ireland1 vol4 i4 7#ahff71 p4 #bdi4 ,the accompanying section 7,fig4 #he7 may be seen on the hills to the north-east of ,boyle4 ,where quarried on the road-side to the north of ,boyle1 the series of deposits here represented contains a bed of coarse and exceedingly compact breccia1 similar to that just referred to1 but containing angular and subangular fragments six or eight inches long4 ,the joints of these compact strata are remarkably sharp and clean cut1 so that where the fragmentary character is not very distinct the rocks might easily be mistaken on casual inspection for felsites4 ,,chapter ,,xx ,,volcanoes ,,of ,,the ,,lower ,,old ,,red ,,sandstone ,,of ,,lake ,,caledonia--continued ,the ,southern ,chain--,the ,pentland ,volcano--,the ,biggar ,centre--,the ,duneaton ,centre--,the ,ayrshire ,volcanoes4 ,we have now to note the leading features of the groups of volcanic rocks distributed along the southern line of vents already described4 ,at least four different centres of eruption may be observed on that line4 ,their mutual limits are1 on the whole1 better seen than those of the northern line1 for from the north-eastern to the south-western end of the volcanic belt the ,old ,red ,sandstone and rocks of older date are almost continuously exposed at the surface4 ,the encroaching areas of ,carboniferous formations in ,lanarkshire and ,ayrshire interrupt but do not entirely conceal the volcanic tracts4 ,,ii4 ,,the ,,southern ,,chain ,,of ,,volcanoes ,,in ,,lake ,,caledonia #e4 ,the ,pentland ,volcano ,beginning at the north-east end of the line we first come upon the classic area of the ,pentland ,hills1 for the study of which the geologist is prepared by the admirable description of ,charles ,maclaren1#cfc and the earlier geognostical papers of ,jameson4#cfd ,the area mapped in detail is represented in ,sheet #cb of the ,geological ,survey of ,scotland1 published in #ahei1 and described in the ,memoir accompanying that sheet4 ,footnote #cfc3 ,a ,sketch of the ,geology of ,fife and the ,lothians1 #ahci4 ,the detailed descriptions in this work are accompanied with a map and two plates of sections4 ,in the map all the volcanic rocks are represented by one colour4 ,in the sections the bedding of the rocks is shown1 and an indication is given of the succession of their chief varieties4 ,footnote #cfd3 ,see specially ,mem4 ,wernerian ,soc4 vol4 ii42 also ,mac,knight in vol4 i4 ,the account of the ,pentland ,hills by ,hay ,cunningham in vol4 vii4 7#ahch7 is clear but brief4 ,when in these early days ,i surveyed this ground ,i found it extremely difficult to understand4 ,being then myself but a beginner in geology1 and the study of old volcanic rocks not having yet advanced much beyond its elementary stage1 ,i failed to disentangle the puzzle4 ,not until after more than twenty years1 largely spent in the investigation of volcanic rocks elsewhere1 had ,i an opportunity of resurveying the ground and bringing to its renewed study a wider knowledge of the subject4 ,a new edition of the map was issued in #ahib1 and ,i shall here embody in my summary the chief results obtained in the course of this revision4 ,the most obvious features in the ,pentland area are the marked development of the volcanic rocks at the north end of the chain1 their rapid diminution and disappearance towards the south-west1 the abrupt truncation of the bedded masses by the line of craggy declivity which forms the northern termination of the hills1 and lastly1 the continuation of the volcanic series northward in a totally different form in the lower eminences of the ,braid ,hills4 ,the length of the whole volcanic tract is about eleven miles2 its breadth at the widest northern part is four miles1 but from that maximum it dwindles southwards and dies out in seven miles4 ,its western side is in large measure flanked by the unconformable overlap of the ,upper ,old ,red ,sandstone and ,lower ,carboniferous formations1 though in some places the base of the volcanic series is seen4 ,the eastern boundary is chiefly formed by a large fault which brings down the ,carboniferous rocks against the volcanic ridge4 ,at the northern end1 this ridge plunges unconformably under the ,upper ,old ,red ,sandstone of the southern outskirts of ,edinburgh4 ,the bedded aspect of the truncated end of the ,pentland chain1 as seen from the north1 has been already alluded to 7p4 #bha74 ,the rocks dip to the south-east1 hence the lower members of the series are to be found along the north-west side of the hills4 ,illustration3 ,fig4 #hf4--,section across the north end of the ,pentland ,hills1 from ,warklaw ,hill to ,pentland ,mains4 ,length about five miles4 #a4 ,upper ,silurian grits and shales1 not seen where the line of section crosses2 #b #b4 ,andesites and diabases in numerous interstratified sheets2 #b s4 ,intercalated sandstones and conglomerates2 #c4 ,felsitic tuffs and breccias and orthophyre sheets2 n1 ,volcanic neck2 #d4 ,lower ,carboniferous strata lying unconformably on and overlapping the volcanic series2 #e4 ,calciferous ,sandstones and ,carboniferous ,limestone series brought down against the volcanic series by a fault 7f74 ,it will be noticed from the ,geological ,survey map that the volcanic rocks of the main body of the ,pentland ,hills are arranged in alternations of somewhat basic and more acid bands4 ,the most basic sheets are some amygdaloidal diabases at the bottom of the whole series which make their appearance in ,warklaw ,hill 7,fig4 #hf74 ,the greater number of the dark lavas are varieties of andesite1 sometimes tolerably compact1 sometimes highly cellular and amygdaloidal4 ,but interstratified with these are thick sheets of what used to be called claystone1 a term which here comprised decayed felsites 7orthophyres71 and also felsitic tuffs and breccias4 ,the remarkably acid nature of some of these rocks has been already pointed out4 ,the total thickness of the volcanic series at the north end of the hills is about #gjjj feet1 but as neither the top nor the bottom is there visible1 it may be considerably greater4 ,at these maximum dimensions the rocks form the high scarped front of the ,pentland ,hills1 which rises into so prominent a feature in the southern landscape of ,edinburgh4 ,a series of transverse sections across the chain from north to south will illustrate its structure and history4 ,these ,i shall here describe1 reserving for subsequent consideration the great vent of the ,braid ,hills4 ,a section taken through the north end of the chain1 where the maximum depth of volcanic material is exposed1 presents the arrangement represented in ,fig4 #hf4 ,it will be seen that the base of the series is here concealed by the unconformable overlap of the ,lower ,carboniferous rocks on the west side1 while the top is cut off by the great fault which on the east side brings down the ,midlothian ,coal-field4 ,illustration3 ,fig4 #hg4--,view of the lava-escarpments of ,warklaw ,hill1 ,pentland chain1 from the north-west4 ,the ,lower ,carboniferous conglomerates 7#d7 creep over the edge and up the slopes of the volcanic series of the ,pentland ,hills4 ,they contain abundant pebbles of the lavas1 and were evidently laid down along a shore from which the ,pentland rocks rose steeply into land4 ,though the actual base of the lavas is not seen here1 two miles further to the south highly-inclined ,upper ,silurian shales and mudstones are found emerging unconformably from under the volcanic pile1 and similar strata probably underlie ,warklaw ,hill as indicated in the figure4 ,the ,upper ,silurian strata pass up into a lower group of the ,lower ,old ,red ,sandstone1 which has also been covered unconformably by the volcanic series4 ,in these underlying deposits we have evidence of the pre-volcanic accumulations of the lake1 which were broken up and tilted at the beginning of the volcanic eruptions4 ,the lowest lavas1 consisting of well-marked beds of diabase 7#b71 present their escarpments to the north-west and dip into the rising ground1 as sketched in ,fig4 #hg4 ,their characters have been already noticed in the general petrography of the ,old ,red ,sandstone volcanic rocks4 ,dark solid compact portions of them pass rapidly into coarsely cellular slag1 especially along the upper and under parts of the several sheets4 ,no tuff has been noticed between these basic flows1 but here and there thin lenticular layers of sandstone1 lying in hollows of the lava-sheets1 are connected with vertical or highly-inclined ramifying veins of similar material1 with the plains of stratification passing across the breadth of the veins4 ,these features are an exact reproduction of those above described in ,forfarshire and ,kincardineshire4 ,the amygdales consist of chalcedony1 crystallized quartz and calcite4 ,torduff ,hill1 which rises to the east of ,warklaw1 consists of a mass of coarse volcanic breccia or agglomerate 7n71 markedly felsitic in its materials4 ,it probably forms a neck marking a small volcanic vent1 like some others at the north end of the chain to be afterwards referred to4 ,in the lower part of ,capelaw ,hill1 the next eminence in an easterly direction1 bedded andesites1 with an intercalated band of sandstone and conglomerate 7#bs71 appear and pass under rocks of so decomposing a kind that no good sections of them are to be found4 ,the hill is covered with grass1 but among the rubbish of the screes pieces of felsite-like rocks and breccias may be observed4 ,some of these blocks show an alternation of layers of felsitic breccia with a fine felsite-like material which may be a tuff4 ,these rocks1 conspicuous by the light colours of their screes1 alternate further up with other dark andesitic lavas1 and run south-westward for about five miles4 ,beyond ,capelaw ,hill1 upon a band of these pale rocks1 comes a thick group of sheets of dark andesite1 which form the main mass of ,allermuir ,hill4 ,they are well seen from the south side and likewise from the north1 dipping towards the south-east at angles of from #ce to #dj1 and weathering along the crest of the hills into a succession of scars and slopes which show the bedded character of the lavas4 ,at ,caerketton ,hill another band of pale material forms the conspicuous craggy face so familiar in the aspect of the ,pentland ,hills as seen from ,edinburgh4 ,this band consists of pale felsitic breccia1 and amorphous1 compact1 much-decayed rock1 regarding which it is difficult to decide whether it should be considered as a fine felsitic tuff1 or as a decomposed felsite4 ,the band is better seen when traced southwards4 ,the light colour of its screes makes it easily followed by the eye even from a distance along the hill-tops and declivities4 ,on the next hill to the south-west1 known as ,castlelaw ,hill1 this pale band of rock is exposed in a few crags and quarries1 and its debris1 protruding through the scanty herbage1 slips down the slopes4 ,on its north side the screes display the same felsitic breccias and compact1 decayed felsitic rocks1 occasionally showing a structure like the flow-structure of rhyolite4 ,the breccia which projects in blocks from the summit of the hill has been quarried immediately below the crest on the south side1 where it overlies a thin intercalated band of a dull1 much-decomposed porphyry4 ,the breccias are composed almost entirely of thoroughly acid rock-fragments1 as may be judged from the percentage of silica shown to occur in them4 ,these fragments vary from the finest lapilli up to angular pieces several inches long4 ,they not infrequently display a fine and extremely beautiful flow-structure4 ,it is thus quite certain that there are acid breccias intercalated among the more basic lavas of the northern ,pentlands1 and that among the constituents of these breccias are fragments of felsite or perhaps even lithoid rhyolite4 ,we may therefore be prepared to find that actual outflows of felsitic lava accompanied the discharge of these highly-siliceous tuffs4 ,unfortunately the manner in which the rocks decay and conceal themselves under their own debris makes it difficult to separate the undoubtedly fragmental bands from those which may be true lavas4 ,but an occasional opening1 and here and there a scattered loose block1 serve to indicate that the two groups of rock certainly do coexist in this pale band1 which can be followed through the chain for upwards of six miles until it is cut off by the eastern boundary fault4 ,at the south-west end of ,castlelaw ,hill1 where a quarry has been opened above the ,kirk ,burn1 blocks of felsite may be observed showing flow-structure on a large scale4 ,the bands of varied devitrification are sometimes a quarter of an inch broad1 and weather out in lighter and darker tints4 ,some of them have retained their felsitic texture better than others1 which have become more thoroughly kaolinized4 ,that these are not deceptive layers of different texture in fine tuffs is made quite clear by some characteristic rhyolitic structures4 ,the bands are not quite parallel1 but1 on the contrary1 are developed lenticularly1 and may be observed to be occasionally puckered1 and to be even bent back and folded over as in ordinary rhyolites4 ,there is no contortion to be observed among the stratified tuffs of the hills4 ,this irregularity in the layers is obviously original1 and can only be due to the flow of a moving lava4 ,on the east side of ,castlelaw ,hill1 as shown in ,fig4 #hf1 dull reddish andesites overlie the pale belt of felsitic rocks4 ,their lower bands are marked by the presence of well-formed crystals of a dark green mica4 ,their central and higher portions consist of porphyrites of the prevalent type1 both compact and vesicular4 ,these lavas continue as far as any rock can be seen4 ,beyond the boundary fault1 the ,burdiehouse ,limestone and oil-shales of the ,lower ,carboniferous series are met with1 inclined at high angles against the hills4 ,it is impossible to say how much of the volcanic series has here been removed from sight by the dislocation4 ,if now we move three miles further to the south-west and take a second section across the ,pentland ,hills1 it will be found to expose the arrangement of rocks represented in ,fig4 #hh4 ,at the western end the ,upper ,old ,red ,sandstones 7#d7 and ,lower ,carboniferous series 7#e7 are seen lying unconformably on the upturned edges of the ,upper ,silurian shales 7#a74 ,north ,black ,hill consists of a large intrusive sheet of pale felsite 7,f7 that has broken through the ,silurian strata and has in places thrust itself between them and the conglomerates of the ,lower ,old ,red ,sandstone which lie unconformably upon them4 ,in the neighbouring ,logan ,burn1 at the bottom of the ,habbie's ,howe ,waterfall1 the felsite can be seen injected into the conglomerate4 ,the felsitic sill of ,north ,black ,hill runs for a mile and a half along the western base of the volcanic series1 and has a breadth of about half a mile4 ,it is the only important intrusive mass in the ,pentland ,hills4 ,illustration3 ,fig4 #hh4--,section across the ,pentland ,hills through ,north ,black ,hill and ,scald ,law 7length about three miles74 ,to the south of the ,silurian shales that lie against the southern flank of ,north ,black ,hill1 pale felsitic tuffs 7#c7 occur1 which are a continuation of those already referred to as running southwards from ,capelaw ,hill4 ,above them a series of andesites 7#b71 with intercalated bands of tuff1 sandstone and conglomerate 7#bs71 occupy the bottom of the ,logan valley and part of the slopes on both sides4 ,in the thickest band of tuffs1 which is well-exposed along the road by the side of the ,loganlee ,reservoir1 a group of well-bedded strata occurs from less than an inch to a foot or more in thickness4 ,generally they are pale in colour1 and are made up of white felsitic detritus1 but with a sprinkling of dull purplish-red fragments1 and occasional larger rounded pieces of different andesites4 ,some of the rocks might be called felspathic sandstones4 ,other bands in the group are dark purplish-red in tint1 and consist mainly of andesitic debris1 with a dusting of white felsitic grains and fragments4 ,there would thus seem to have been showers both of felsitic and of andesitic ashes and lapilli4 ,the dark lavas that overlie the tuffs are likewise well displayed along the same road-section4 ,they vary rapidly from extremely compact homogeneous dark blue rocks1 that weather with a greenish crust1 to coarse1 slaggy masses and amygdaloids4 ,illustration3 ,fig4 #hi4--,section from the valley of the ,gutterford ,burn through ,green ,law and ,braid ,law to ,eight-,mile ,burn4 ,these more basic lavas are a continuation of those of ,allermuir ,hill1 and1 as at that locality1 they plunge here also under the same band of white tuffs1 breccias and felsites 7#c71 which has been referred to as stretching southward from ,caerketton ,crags4 ,this band must here be at least #ejj feet thick4 ,it forms ,scald ,law 7#ahih feet7 and the surrounding summits1 and thus occupies the highest elevations in the ,pentland chain4 ,it dips beneath the uppermost group of andesites1 which1 as before1 are here truncated by the eastern fault 7f71 the ,calciferous ,sandstones and ,carboniferous ,limestone series 7#f7 being thrown against them4 ,a third section 7,fig4 #hi71 taken two miles still further south1 shows a remarkable attenuation of the volcanic series1 and the appearance of a thick group of conglomerates 7#b7 lying conformably below that series1 but resting on the upturned edges of the upper ,silurian shales 7#a74 ,the thick ,allermuir porphyrites are here reduced to a few thin beds 7#c7 intercalated among the conglomerates and sandstones1 amidst which the whole volcanic series dies out southward4 ,a detailed section of the rocks exposed on the western front of ,braid ,law shows the following succession3-- ,white felsitic rocks of ,braid ,law 7#d in ,fig4 #hi74 ,coarse conglomerate passing down into sandstone4 ,about #bj feet visible4 ,dark andesite1 #d feet4 ,parting of yellow felspathic grit1 #h or #aj inches4 ,andesite1 #aj feet4 ,hard felspathic grit1 #f feet4 ,dark green amygdaloidal andesite1 #b feet4 ,yellow felspathic sandstone and grit1 #b feet4 ,dark green amygdaloidal andesite1 #f feet4 ,felspathic grit and red and brown sandstone1 #d feet4 ,dark andesite1 perhaps #f or #h feet4 ,great conglomerate with alternating courses of sandstone1 rapidly increasing in thickness southwards4 ,above these dwindling representatives of the northern andesitic lavas comes the continuation of the white band of tuffs and breccias of ,caerketton and ,scald ,law 7#d71 which in turn dips under the highest group of andesites4 ,the ,carboniferous strata 7#e7 are brought in by the fault 7f74 ,in little more than two miles beyond this line of section the volcanic series disappears1 and the ,old ,red ,sandstone for a brief space consists only of sedimentary deposits4 ,besides the remarkable alternation of basic and acid ejections1 there is a further notable feature in the geology of the ,pentland ,hills4 ,this volcanic centre presents us with one of the most remarkable vents anywhere to be seen among the volcanic rocks of ,britain4 ,the full significance of this feature may best be perceived if we advance along the hills from their south-western end4 ,as has now been made clear1 the volcanic materials which begin about the line of the ,north ,esk near ,carlops rapidly augment in thickness until1 in a distance of not more than seven miles1 they attain a thickness of about #gjjj feet1 and then form the great scarped front of the hills that look over ,edinburgh4 ,but at the base of that wall their continuity abruptly ceases4 ,the lower ground1 which extends thence to the southern suburbs of ,edinburgh1 and includes the group of the ,braid ,hills1 is occupied by another and more complex group of rocks in which the parallelism and persistence so marked in the ,pentland chain entirely disappear4 ,this abrupt truncation of the bedded lavas and tuffs marks approximately the southern margin of a large vent from which at least some1 if not most1 of these rocks were probably ejected4 ,the size of this vent cannot be precisely ascertained on account of the unconformable overspread of ,lower ,carboniferous strata4 ,but that it must have been a large and important volcanic orifice may be inferred from the fact that the visible area of the materials that fill it up measures two miles from north-east to south-west1 and a mile and a half from south-east to north-west1 thus including a space of rather more than two square miles4 ,its original limits towards the north and south can be traced by help of the bedded lavas that partially surround it1 but on the two other sides they are concealed by the younger formations4 ,we shall probably not over-estimate the original area of the vent if we state it at about four square miles4 ,illustration3 ,fig4 #ij4--,section across the north end of the ,pentland ,hills1 and the southern edge of the ,braid ,hill vent4 ,length about two miles4 #a #a4 ,andesites2 #b4 ,fine tuffs1 etc41 of the ,braid ,hill vent2 #c #c #c4 ,agglomerate in lateral necks with felsitic intrusions 7#d74 ,the materials that now fill this important orifice consist mainly of claystones1 like those of the ,pentland series--dull rocks1 meagre to the touch1 varying in texture from the rough porous aspect of a sinter through stages of increasing firmness till they become almost felsitic1 and ranging in colour from a dark purple-red1 through shades of lilac and yellow1 to nearly white1 but often strikingly mottled4 ,a more or less laminar structure is often to be observed among them1 indicating a dip in various directions 7but especially towards the north7 and at considerable angles4 ,throughout this exceedingly fine-grained material1 lines of small lapilli may occasionally be detected1 also bands of breccia1 consisting of broken-up tuff of the same character1 and of fine hornstone and felsite1 with delicate flow-structure4 ,exhibiting on the whole so little structure1 this tract may be regarded as consisting largely of fine volcanic dust derived from the explosion of felsitic or orthophyric lavas4 ,some portions indeed are not improbably composed of decayed felsites1 like those which present so many difficulties to the geologist who would try to trace their course among the other lavas and tuffs of the ,pentland chain4 ,various veins1 dykes and small bosses of felsite1 andesite and even more basic material1 such as fine dolerite1 have been intruded into the general body of the mass4 ,on the outskirts of the main vent some subordinate necks may be observed 7#c1 #c in ,fig4 #ij71 perhaps1 like ,torduff ,hill1 already noticed 7,fig4 #hf71 marking lateral eruptions from the flanks of the great cone4 ,three of these occur in a line more than half a mile long1 possibly indicating a fissure on the side of the old volcano1 running in a south-westerly direction from the southern edge of the vent4 ,the smallest of them measures about #ejj feet in diameter2 the largest is oblong in shape1 its shorter diameter being about #ejj feet1 and its longer about #ajjj feet4 ,the materials that fill these lateral vents are coarse agglomerates1 traversed by veins and irregular intrusions of a fine horny or flinty felsite4 ,from the acid character of most of the rocks that now fill the wide vent of the ,braid ,hills it may be inferred that at least the last eruptions from it consisted chiefly of acid tuffs and lavas4 ,the upper portion of the volcanic series being everywhere concealed1 there are no means left to verify this inference from an examination of the ejected material4 ,it may be remarked1 however1 that the pale yellow sandstones which lie on the east side of the fault and are exposed in the ,lyne ,water above ,west ,linton are in great measure composed of fine felsitic material4#cfe ,they certainly belong to a higher horizon than the most southerly lavas of the ,pentland ,hills1 and if they have not derived their volcanic detritus from the ,biggar volcanic area1 it may be assumed that they obtained it from the vent of the ,braid ,hills4 ,in any case they show that after the lavas of the southern end of the ,pentland ,hills were buried1 acid volcanic detritus continued to be abundantly distributed over this part of the floor of ,lake ,caledonia4 ,footnote #cfe3 ,explanation to ,sheet #bd of the ,geological ,survey of ,scotland1 pp4 #aj1 #ab4 #f4 ,the ,biggar ,centre#cff ,footnote #cff3 ,this area is included in ,sheets #bc and #bd of the ,geological ,survey of ,scotland4 ,it was mapped and described by myself4 7,explanations of ,sheets #bc and #bd47 ,various parts of it have been referred to by earlier writers1 particularly ,maclaren1 ,geology of ,fife1 etc41 p4 #agf4 ,another distinct group of volcanoes had its centre about #be miles south-westward from the ,braid vent1 and on the same line as those of the ,pentland ,hills4 ,in no part of the basin can the isolation of the different volcanic clusters be so impressively observed as in the area to the south-west of these hills4 ,on the one hand1 the lavas and tuffs from the ,braid vent die out1 and on the other1 as we follow the conglomerates south-westwards1 a new volcanic series immediately makes its appearance4 ,the space between the last extremity of the ,pentland lavas and the beginning of the ,biggar series does not exceed some #ejj yards4 ,it will be remembered that the lower half of the ,pentland volcanic series dies out long before it reaches the southern end of the hills1 and that it is by lavas on the horizon of some of the dark andesites of ,allermuir ,hill that the volcanic band is finally prolonged to its extreme southern limit4 ,the most northerly extension of the ,biggar lavas lies somewhere on the same general platform4 ,but whereas1 at the north end of the ,pentland chain1 the volcanic sheets rest on the edges of the ,upper ,silurian shales1 at the south end1 several hundred feet of coarse conglomerate and sandstone intervene between the ,silurian shales and the porphyrites4 ,so rapidly does the bulk of these sedimentary formations increase that in the course of two miles they must be #cjjj feet in thickness below the most northerly of the ,biggar lavas just referred to4 ,but after that point1 when they cross the ,lyne ,water1 they begin to be more and more interstratified with thin sheets of andesite4 ,these lavas1 the beginning of the ,biggar series1 soon number nine or ten distinct bands1 and so quickly do they usurp the place of the sedimentary materials that in a distance of not more than twelve miles they form1 where traversed by the river ,clyde1 the whole breadth of the visible tract of ,old ,red ,sandstone1 to the exclusion of the conglomerates4 ,unfortunately1 soon after the lavas make their appearance at the north end they are in great measure overlapped unconformably by the red sandstones at the base of the ,carboniferous system1 but where the ,medwin ,water has cut through this covering1 they can be seen here and there underneath on their southerly course4 ,a section through the northern end of the ,biggar series1 where the successive lavas are dying out northwards among the conglomerates1 shows the structure given in ,fig4 #ia4 ,the sedimentary strata consist largely of debris of andesite1 and the lavas include dark red or purple andesites and also pale felsites1 both having the same characters as those of the ,pentland ,hills4 ,illustration3 ,fig4 #ia4--,section across the northern end of the ,biggar volcanic group1 from ,fadden ,hill to beyond ,mendick ,hill4 #a4 ,conglomerates and sandstones2 #b4 ,lavas1 the lowest being an olivine-diabase or basalt1 the main mass being andesites2 #c4 ,felsites and tuffs2 #d4 ,upper ,old ,red ,sandstone4 f1 ,fault4 ,in one important respect the volcanic series in the northern part of the ,biggar area differs from that of the ,pentland ,hills1 for whereas the uppermost parts of the latter are concealed by faults which bring down the ,carboniferous strata against the base of the hills1 the lavas at the north end of the ,biggar district pass conformably under a thick group of ,lower ,old ,red conglomerates and sandstones4 ,we thus learn that here the volcanic eruptions ceased long before the close of the deposition of the ,lower ,old ,red ,sandstone4 ,the overlying sedimentary series is disposed in a long synclinal trough1 corresponding in direction with the general north-easterly strike of the volcanic rocks which reappear from under the sandstones and conglomerates along its south-eastern border1 where they are abruptly truncated by the fault 7f1 ,fig4 #ib71 which brings them against the flanks of the ,silurian ,uplands4 ,it is interesting to note that by this dislocation the lavas of the ,lower ,old ,red ,sandstone are placed almost in immediate contact with those of the ,lower ,silurian series1 which appear here on the crests of numerous anticlinal folds that are obliquely cut off by the fault4 ,there is yet another feature of interest in the northern part of the ,biggar volcanic centre4 ,while the lowest visible lava is an olivine-diabase not unlike parts of the ,warklaw group of the ,pentland ,hills1 those which occur above it are partly andesites and partly orthoclase-felsites4 ,the latter form1 among the hills near ,dolphinton1 an important group which reaches its greatest development in the ,black ,mount 7#afhi feet74 ,these rocks cover a breadth of more than a mile of ground1 and probably attain a thickness of not less than #bjjj feet4 ,they so closely resemble in their general characters the corresponding rocks of the ,pentland ,hills that a brief description of them may suffice4 ,as in that chain of hills1 they are so prone to decomposition that they are in large part concealed under a covering of their own debris and of herbage1 though their fragments form abundant screes1 and numerous projecting knobs of rock suffice to show the main features of the lavas and their accompaniments4 ,the felsites weather into pale yellow and greyish claystones1 but where fresher sections can be procured they often show darker tints of lilac and purple4 ,they are close-grained1 sometimes flinty1 generally porphyritic with scattered highly-kaolinized white felspars1 but without quartz1 often presenting beautiful flow-structure1 and not infrequently showing a brecciated appearance1 which in the usual weathered blocks is hardly to be distinguished from the breccia of interstratified tuffs4 ,a locality where some of these features may be satisfactorily examined is a dry ravine in the farm of ,bank1 on the south-east side of the ,black ,mount4 ,here the felsite possesses such a perfectly developed flow-structure as to split into slabs which1 dipping ,s4,e4 at about #be1 might deceive the observer into the belief that it is a sedimentary rock4 ,a fresh fracture shows the lamin of flow1 many of which are as thin as sheets of paper1 to be lilac in colour1 some of the more decomposed layers assuming tints of grey4 ,the felspars and micas are arranged with their long axes parallel to the lines of flow4 ,the rock is not vesicular1 but it breaks up here and there into the brecciated condition just referred to4 ,below the sheet which displays the most perfect flow-structure1 what is probably a true volcanic breccia makes its appearance4 ,it consists of angular fragments of a similar lilac felsite1 of all sizes up to pieces two or three inches in length1 cemented in a matrix of the same material stained reddish-brown4 ,in this breccia the stones show little or no flow-structure4 ,above the group of felsites and felsitic breccias1 grey andesites make their appearance1 like some of those in the ,pentland ,hills4 ,they are sometimes extraordinarily vesicular1 the vesicles in the body of the rock being filled with calcite1 agate1 etc4 ,such lavas must have been originally sheets of rough slag4 ,the elongated steam-vesicles have been partly filled up with micaceous sand and fine red mud that were washed into crannies of the lava in direct communication with the overlying water4 ,it is evident that in the northern part of the ,biggar centre the succession of volcanic events followed closely the order observable in the ,pentland ,hills1 but on a feebler scale4 ,we may suppose that the lower diabases and andesites are the equivalents of those of ,warklaw and ,allermuir1 that the felsites and breccias were contemporaneous with those of ,capelaw1 ,caerketton and ,castlelaw1 and that the last andesites made their appearance together with those which form the highest lavas of the ,pentland chain4 ,illustration3 ,fig4 #ib4--,section across the southern part of the ,biggar volcanic group from ,covington to ,culter4 #a4 ,lower ,silurian strata2 #b4 ,lower ,old ,red ,sandstone 7pre-volcanic group72 #c4 ,andesite lavas with intercalated sandstones and conglomerates2 #d4 ,felsite neck4 f1 ,the boundary-fault on northern edge of ,southern ,uplands4 ,a section across the southern end of the ,biggar volcanic belt shows less diversity of structure 7,fig4 #ib74 ,the lavas 7#c7 are there found to flatten out and to spread unconformably over the older part of the ,lower ,old ,red ,sandstone 7#b71 which1 as already stated1 passes down into the ,upper ,silurian shales4 ,a few intercalations of conglomerate1 mainly made up of volcanic detritus1 are here and there to be detected among these lavas4 ,but the chocolate sandstones and conglomerates that lie unconformably below them contain no such detritus1 for they belong to the pre-volcanic part of the history of ,lake ,caledonia1 and were here locally upraised1 perhaps as an accompaniment of the terrestrial disturbances that preceded or attended the first outburst of volcanic energy4 ,followed south-westwards1 the stratigraphical break in the ,lower ,old ,red ,sandstone disappears1 and1 as will be shown in the account of the ,duneaton centre1 a continuous succession can there be traced from the ,upper ,silurian shales up into the volcanic series4 ,an interesting feature in this district is the felsitic boss of ,quothquan already alluded to 7p4 #bhh7 as rising up through the andesites1 and possibly marking one of the vents of the district4 ,it is one of a number of felsitic intrusions in this neighbourhood1 of which the most important is ,tinto4 ,illustration3 ,fig4 #ic4--,section from ,thankerton ,moor across ,tinto to ,lamington4 #a;a4 ,lower ,silurian2 #a4 ,upper ,silurian strata2 #b4 ,lower ,old ,red ,sandstone with two marked bands of conglomerate2 #c4 ,lower ,old ,sandstone 7pre-volcanic chocolate sandstones72 #d4 ,andesite lavas with sandstones1 conglomerates and tuffs lying unconformably on ,no4 #c2 #e4 ,felsite sill of ,tinto with the smaller sill of the ,pap ,craig 7#f74 f1 ,fault bounding the ,silurian uplands on the north4 ,a small patch of the unconformable ,lower ,old ,red conglomerate is seen on the south side of the fault4 ,a third section taken across ,tinto1 from ,thankerton ,moor on the north side to ,lamington on the south1 will serve further to illustrate the great unconformability in the ,lower ,old ,red ,sandstone of this district1 and to show the relation of the largest felsitic intrusion to the surrounding rocks 7,fig4 #ic74 ,the conglomerates and sandstones that appear on the south slopes of ,tinto lie near the base of the ,old ,red ,sandstone1 and if we could bore among the overlying andesites we should probably meet with the ,upper ,silurian shales among or conformably beneath the red passage-beds1 as in the ,lesmahagow district4 ,the andesitic lavas creep over the upturned denuded edges of these strata and sweep round the flanks of ,tinto4 ,this conspicuous hill reaches a height of #bcce feet above the sea1 and consists of the felsitic rocks already described 7p4 #bgh74 ,seen from many points of view it rises as a graceful cone1 distinguished from all the other eminences around it by the pinkish colour of its screes4 ,in reality it forms a continuous ridge which runs in an east and west direction for about five and a half miles1 with a breadth of about a mile4 ,some part at least1 and possibly the whole of this oblong mass1 is in the form of a sill or laccolite which dips towards the north4 ,conglomerates and sandstones plunge under it on the southern side1 and similar sandstones overlie it on the north4 ,if there be a neck in this mass1 as one might infer from the shape of the hill1 its precise limits are concealed4 ,the rock does not break through the andesites1 and may belong to an earlier period of eruptivity than the lavas immediately around it4 ,there were other1 though smaller1 vents in the immediate neighbourhood4 ,besides the cone of ,quothquan just referred to1 another may be marked by the felsite boss which overlooks the village of ,douglas1 four miles to the south-west of the ,tinto ridge1 while a third rises into a low rounded hill close to the village of ,symington4 ,the lavas spread out again to the south-west of ,tinto in a group of hills1 until they are interrupted by a fault which brings in the ,douglas coal-field4#cfg ,this dislocation abruptly terminates the ,biggar volcanic band in a south-westerly direction1 after extending for a length of #bf miles1 with a breadth of sometimes as much as five miles4 ,footnote #cfg3 ,see ,explanation to ,sheet #bc of the ,geological ,survey of ,scotland 7#ahgc71 p4 #ae4 ,this ground was mapped and described by ,mr4 ,b4 ,n4 ,peach4 #g4 ,the ,duneaton ,centre ,among the high bleak muirlands on the confines of the three counties of ,lanark1 ,ayr and ,dumfries1 traversed by the ,duneaton ,water1 a distinct volcanic area may be traced4#cfh ,its boundaries1 however1 cannot be satisfactorily fixed4 ,it is overspread with ,carboniferous rocks both to the north-east and south-west1 so that its rocks are only visible along a strip about seven miles long and two miles broad4 ,on the north-western side its lower members are seen lying interstratified among the sandstones and conglomerates which thence pass down conformably into the ,upper ,silurian series 7,fig4 #id74 ,but although we thus get below the volcanic series we meet with no vents or sills among the lower rocks4 ,on the south-east side the highest lavas and tuffs are overlain by some #ejjj feet of red sandstones and conglomerates 7#b1 #c71 which completely bury all traces of the volcanic history4 ,footnote #cfh3 ,this area was mapped by ,mr4 ,b4 ,n4 ,peach in ,sheet #ae of the ,geological ,survey of ,scotland1 and is described by him in the accompanying ,memoir4 ,the volcanic series in this limited district reaches an estimated thickness of #djjj feet1 built up of purple and green slaggy andesites1 dark heavy diabases 7melaphyres7 and tuffs1 with abundant interstratification of sandstone1 especially towards the base4 ,one of its chief features of interest is the manner in which it exhibits1 better1 perhaps1 than can be found in any of the other volcanic areas1 the frequent and rapid alternations of lavas and tuffs with sandstone and conglomerate4 ,in this part of the region the volcanic discharges were obviously frequent and intermittent1 while at the same time the transport and deposition of sediment were continuous4 ,this sediment consisted largely1 indeed1 of volcanic detritus mixed with ordinary sand and silt4 ,that these conditions of sedimentation were not wholly inimical to animal life is shown by the occasional occurrence of worm-burrows in the ashy sandstones4#cfi ,footnote #cfi3 ,memoir on ,sheet #ae ,geol4 ,surv4 ,scotland 7#ahga71 p4 #bb4 ,illustration3 ,fig4 #id4--,section across the ,duneaton volcanic district from the head of the ,duneaton ,water to ,kirklea ,hill4 #a4 ,silurian strata2 #b4 ,lower ,old ,red ,sandstone and conglomerate2 #c4 ,coarse conglomerate2 #d4 ,andesite-lavas2 #e4 ,stratified tuffs2 #f4 ,spango granite2 #g4 ,upper ,old ,red ,sandstone4 ,the thick accumulation of sandstones and conglomerates above the main mass of lavas has been derived almost wholly from the waste of the volcanic rocks 7#c74 ,blocks of andesite1 well rounded and often from six to twelve inches in diameter1 may be seen in the remarkable band of coarse conglomerate which runs as a nearly continuous ridge from the ,nith to the ,clyde--a distance of more than twenty miles4 ,nothing impresses the geologist more1 as he wanders over this district1 than the evidence of the prodigious waste which the volcanic series underwent before it was finally buried4 ,some part of the detritus may have been supplied1 indeed1 by occasional discharges of fragmental matter1 as has already been suggested in the case of the ,ochil and ,montrose conglomerates4 ,but the nature of the pebbles in these masses of ancient shingle shows them to be not bombs1 but pieces worn away from sheets of lava4 ,that the lavas underlie these piles of detritus and extend southwards even up to the very edge of the ,silurian ,uplands is shown by the rise of a number of successive beds from under the trough into which the conglomerate has been thrown4 ,these lavas1 however1 are almost immediately cut off by the great boundary fault 7f7 which flanks the ,silurian territory4 ,that they are not met with now to the south-east of the dislocation1 where they must once have lain1 is an evidence of the great denudation which the district has undergone4 ,fig4 #id1 which gives a section across the broadest part of the area1 from the edge of the ,muirkirk coal-field to the ,silurian uplands1 shows the general structure of the ground4 ,no satisfactory evidence regarding the position of any of the vents of the period has been met with in this district4 ,the rocks to the south of the boundary-fault are older than the ,old ,red ,sandstone1 and as they must have been for some distance overspread by the conglomerates1 sandstones and volcanic series1 we might hope to find somewhere among them traces of necks or bosses4 ,the only mass of eruptive rock in that part of the district is the tract of ,spango granite which has been already referred to in connection with the subject of the vents and granite protrusions of ,old ,red ,sandstone time4 ,this mass1 about four miles long and two miles broad1 rises through ,silurian strata1 and by means of the boundary fault is brought against the higher group of conglomerates and sandstones4 ,the ,silurian shales and sandstones around the granite have undergone contact-metamorphism1 becoming highly micaceous and schistose4 ,the ascent of this granite must have taken place between the upheaval and contortion of the ,upper ,silurian strata1 and the deposition of the higher parts of the ,lower ,old ,red ,sandstone of this region4 ,its date might thus come within the limits of the volcanic period4 ,but one must frankly own that there is no positive evidence to connect its production with the volcanic history4 #h4 ,the ,ayrshire ,group of ,vents ,the original limits of the volcanic districts in the remaining portion of the ,old ,red ,sandstone area on the mainland of ,scotland1 from the valley of the ,nith to the ,firth of ,clyde1 can only be vaguely indicated4#cgj ,there is a difficulty in ascertaining the south-western termination of the ,duneaton area1 and in deciding whether the lavas and tuffs of ,corsincone in ,nithsdale should be assigned to that district or be placed with those further to the south-west4 ,between ,corsincone and the next visible volcanic rocks of the ,lower ,old ,red ,sandstone there intervenes a space of six miles1 along which1 owing to the effect of the great fault that flanks the north-western margin of the ,southern ,uplands1 the ,carboniferous ,limestone and even the ,coal-measures are brought against the ,silurian formations1 every intermediate series of rocks being there cut out4 ,it may therefore be1 on the whole1 better to include all the volcanic rocks on the left side of the ,nith as part of the ,duneaton series4 ,there will still remain a tract of five miles of blank intermediate ground before we enter upon the volcanic rocks of ,ayrshire4 ,footnote #cgj3 ,the mapping of the ,old ,red ,sandstone volcanic areas of ,ayrshire for the ,geological ,survey was thus distributed3--,the district east of ,dalmellington was surveyed by ,mr4 ,b4 ,n4 ,peach1 that between ,dalmellington and ,straiton by ,prof4 ,james ,geikie1 and all from the line of the ,girvan ,valley south of ,straiton westward to the sea by myself4 ,the ground is embraced in ,sheets #h1 #ac and #ad of the ,map of ,scotland1 and is described in accompanying ,explanations4 ,owing to complicated faults1 extensive unconformable overlaps of the ,carboniferous formations1 and enormous denudation1 the volcanic tracts of ,old ,red ,sandstone age in ,ayrshire have been reduced to mere scattered patches1 the true relations of which are not always easily discoverable4 ,one of these isolated areas flanks the ,silurian ,uplands as a belt from a mile to a mile and a half in breadth and about six miles long1 but with its limits everywhere defined by faults4 ,a second much more diversified district extends for about ten miles to the south-west of ,dalmellington4 ,it too forms a belt1 averaging about four miles in breadth1 but presenting a singularly complicated geological structure4 ,owing to faults1 curvatures and denudation1 the volcanic rocks have there been isolated into a number of detached portions1 between some of which the older parts of the ,old ,red ,sandstone1 and even the ,silurian rocks1 have been laid bare1 while between others the ground is overspread with ,carboniferous strata4 ,a third unbroken area forms the ,brown ,carrick ,hills1 south of the town of ,ayr1 and is of special interest from the fact that its rocks have been exposed along a range of sea-cliffs and of beach-sections for a distance of nearly four miles4 ,other detached tracts of volcanic rocks are displayed on the shore at ,turnberry and ,port ,garrick1 on the hills between ,mochrum and the vale of the ,girvan1 and on the low ground between ,dalrymple and ,kirkmichael4 ,the isolation of these various outliers and separated districts is probably not entirely due to the effects of subsequent geological revolutions4 ,more probably some of the areas were always independent of each other1 and their igneous rocks were discharged from distinct volcanic centres4 ,we may conjecture that one of these centres lay somewhere in the neighbourhood of ,new ,cumnock1 for the lavas between that town and ,dalmellington appear to diminish in thickness and number as they are traced south-westward4 ,another vent1 or more probably a group of vents1 may have stood on the site of the present hills to the right and left of the ,girvan ,valley1 south of the village of ,straiton4 ,a third probably rose somewhere between ,dailly and ,crosshill1 and poured out the lavas of the ridges between ,maybole and the ,dailly coal-field4 ,the important centre of eruption that produced the thick and extensive lavas of the ,brown ,carrick ,hills may be concealed under these hills1 or may have stood somewhere to the west of ,maybole4 ,still another vent1 perhaps now under the sea1 appears to be indicated by the porphyrites of the coast-section between ,turnberry and ,culzean ,bay4 ,owing to the complicated structure of the ground1 several important points in the history of the ,old ,red ,sandstone of this region have not been established beyond dispute4 ,in particular1 the unconformability which undoubtedly exists in that system in the south-west of ,ayrshire has not been traced far enough eastwards to determine whether it affects the volcanic belt east of ,dalmellington1 or whether the break took place before or after the eruption of that belt4 ,west of ,dalmellington it clearly separates a higher group of sandstones1 conglomerates and volcanic rocks from everything older than themselves4 ,the structure is similar to that in the ,pentland ,hills1 a marked disturbance having taken place here as well as there after a considerable portion of the ,lower ,old ,red ,sandstone had been deposited4 ,these earlier strata were upraised1 and on their denuded ends another group of sandstones and conglomerates was laid down1 followed by an extensive eruption of volcanic materials4 ,it is the upper unconformable series that requires to be considered here1 as it includes all the volcanic rocks of the ,old ,red ,sandstone lying to the west of the meridian of ,dalmellington4 ,the position of these rocks on their underlying conglomerates is admirably exposed among the hills between the valleys of the ,doon and the ,girvan1 as well as on ,bennan ,hill to the south of ,straiton4 ,the andesites rise in a craggy escarpment crowning long green slopes that more or less conceal the conglomerates and sandstones below4 ,along the coast-sections the structure of the volcanic rocks may be most advantageously studied4 ,the shore from the ,heads of ,ayr to ,culzean ,castle affords a fine series of exposures1 where every feature in the succession of the lavas may be observed4 ,still more instructive1 perhaps1 is the mile and a half of beach between ,turnberry ,bay and ,douglaston1 of which ,i shall here give a condensed account1 for comparison with the coast-sections of ,kincardineshire and ,forfarshire already described4 ,the special feature of this part of the ,ayrshire coast-line is the number of distinct andesite sheets which can be discriminated by means of the thin layers of sandstone and sandy tuff that intervene between them4 ,in the short space of a mile and a half somewhere about thirty sheets can be recognized1 each marking a separate outflow of lava4 ,it was in this section that ,i first observed the sandstone-veinings which have been described in previous pages1 and nowhere are they more clearly developed4 ,almost every successive stream of andesite has been more or less fissured in cooling1 and its rents and irregular cavernous hollows have been filled with fine sand silted in from above4 ,the connection may often be observed between these sandstone partitions or patches and the bed of the same material1 which overspread the surface of the lava at the time that the fissures were being filled up4 ,illustration3 ,fig4 #ie4--;,cavernous spaces in andesite1 filled in with sandstone1 ,john o' ,groat's ,port1 ,turnberry1 ,ayrshire4 ,the andesites of the ,turnberry shore are of the usual dark purplish-red to green colours1 more or less compact in the centre and vesicular towards the top and bottom4 ,they display with great clearness the large empty spaces that were apt to be formed in such viscous slaggy lavas as they moved along the lake-bottom4 ,these spaces1 afterwards filled with fine sand1 now appear as irregular enclosures of hard green sandstone embedded in the andesite4 ,the example shown in ,fig4 #ie may be seen in one of the lavas at ,john o' ,groat's ,port4 ,illustration3 ,fig4 #if4--,section of andesites1 ,turnberry ,castle1 ,ayrshire4 ,from the arrangement of the veins of sandstone it is evident that irregularly divergent1 but often more or less stellate1 fissures opened in the lavas as they cooled4 ,sometimes1 indeed1 the molten rock appears to have broken up into a shattered mass of fragments1 as must often have happened when lavas were poured over the lake-floor4 ,what may be an instance of this effect is to be seen on the cliff under the walls of ,turnberry ,castle1 whence the annexed sketch 7,fig4 #if7 was taken4 ,the lower andesite 7a7 is highly amygdaloidal towards the top1 and is traversed in all directions with irregular veins and nests of sandstone which can be traced upward to the bed 7b71 consisting of sandstone1 but so full of lumps or slags of amygdaloidal andesite that one is here and there puzzled whether to regard it as a sedimentary deposit1 or as the upper layer of clinkers of a lava-stream strewn with sand4 ,above this fragmentary layer lies another bed of andesite 7c7 of a coarsely amygdaloidal structure1 which encloses patches of the underlying sandstone4 ,it passes upward1 in a space of from four to six feet1 into a mass of angular scoriaceous fragments 7d7 of all sizes up to blocks #ah inches in length cemented in a vein-stuff of calcite1 chalcedony and quartz4 ,this brecciated structure ascends for about #ac or #ad feet1 and is then succeeded by a greenish compact andesite 7e71 which further north becomes amygdaloidal and much veined with sandstone1 passing into a breccia of lava fragments and sandstone4 ,illustration3 ,fig4 #ig4--,lenticular form of a brecciated andesite 7shown in ,fig4 #if71 ,turnberry1 ,ayrshire4 ,illustration3 ,map ,,iii4 ,,map ,,of ,,the ,,volcanic ,,districts ,,of ,,the ,,lower ,,old ,,red ,,sandstone ,,of ,,lake ,,caledonia ,,in ,,central ,,scotland ,,north ,,east ,,ireland ,the remarkable brecciated band 7d7 in this cliff1 though #ac or #ad feet in the centre1 immediately thins out on either side1 until in the course of a few yards it completely disappears and allows the lavas c and e to come together1 as shown in ,fig4 #ig4 ,we may suppose that this section reveals the structure of the terminal portion of a highly viscous lava which was shattered into fragments as it moved along under water4 ,no clear evidence of the sites of any of the volcanic vents has yet been detected in the ,old ,red ,sandstone of ,ayrshire4 ,possibly some of the numerous felsitic bosses to the south-west of ,dalmellington may partly mark their positions4 ,but the sills connected with the volcanic series are well exposed in the #ab miles of hilly ground between ,dalmellington and ,barr4 ,two groups of intrusive sheets may there be seen4 ,the most numerous consist of pale or dark-pink felsite1 often full of crystals of mica4 ,they form prominent hills1 such as ,turgeny1 ,knockskae and ,garleffin ,fell4 ,the second group comprises various diabase-sheets which have been intruded near the base of the red sandstones and conglomerates1 over a distance of seven miles on the north side of the ,stinchar ,valley above ,barr4 ,they attain their greatest development on ,jedburgh ,hill1 where they form a series of successive sills1 the largest of which unite northwards into one thick mass and die out southward among the sandstones and conglomerates4 ,,chapter ,,xxi ,,volcanoes ,,of ,,the ,,lower ,,old ,,red ,,sandstone ,,of ,,the ,,cheviot ,,hills1 ,,lorne1 ,,lake ,,orcadie ,,and ,,killarney ,,the ,,cheviot ,,and ,,berwickshire ,,district ,in the south-east of ,scotland1 and extending thence into the north of ,england1 the remains of several distinct volcanic centres of the ,lower ,old ,red ,sandstone may still be recognized4 ,of these the largest and most interesting forms the mass of the ,cheviot ,hills2 a second has been partially dissected by the sea along the coast south from ,st4 ,abb's ,head2 while possibly relics of others may survive in detached bosses of eruptive rock which rise through the ,silurian formations of ,berwickshire4 ,the water-basin in which these volcanic groups were active was named by me ,lake ,cheviot1#cga to distinguish it from the other basins of the same geological period 7,map ,i474 ,footnote #cga3 ,trans4 ,roy4 ,soc4 ,edin4 xxviii4 7#ahgh71 p4 #ced4 ,the volcanic rocks of the ,cheviot ,hills1 though their limits have been reduced by faults1 unconformable overlap of younger formations and severe denudation1 still cover about #bcj square miles of ground1 and rise to a height of #bfgf feet above the sea4 ,as they have been mapped in detail by the ,geological ,survey1 both on the ,english and the ,scottish sides of the ,border1 their structure is now known4#cgb ,no good horizontal section1 however1 has yet been constructed to show this structure--a deficiency which1 it is hoped1 may before long be supplied4 ,footnote #cgb3 ,the ,geology of the ,cheviot ,hills is comprised in ,sheets #ajh ,n4,e41 #aji ,n4,w41 and #aaj ,s4,w4 of the ,geological ,survey of ,england and ,wales1 and in ,sheets #ag1 #ah and #bf of the ,geological ,survey of ,scotland4 ,for descriptive accounts the ,memoirs to some of these ,sheets may be consulted1 particularly ,geology of the ,cheviot ,hills 7,english side71 by ,c4 ,t4 ,clough 7,mem4 ,geol4 ,surv4 #ahhh72 ,geology of ,otterburn and ,elsdon1 by ,h4 ,miller and ,c4 ,t4 ,clough 7,mem4 ,geol4 ,surv4 #ahhg72 ,geology of ,part of ,northumberland between ,wooler and ,coldstream1 by ,w4 ,gunn and ,c4 ,t4 ,clough1 with ,petrographical ,notes by ,w4 ,w4 ,watts 7,mem4 ,geol4 ,surv4 #ahie74 ,other descriptions have been published by ,professor ,james ,geikie1 ,good ,words1 vol4 xvii4 7#ahgf71 reprinted in ,fragments of ,earth-lore 7#ahic71 and by ,prof4 ,lebour1 ,outlines of the ,geology of ,northumberland1 #bnd edit4 #ahhf4 ,for the petrography of the rocks consult ,mr4 ,j4 ,j4 ,h4 ,teall1 ,geol4 ,mag4 #ahhc1 pp4 #ajj1 #ade1 #beb1 #cdd2 #ahhd1 p4 #bbf2 #ahhe1 p4 #ajf2 ,proc4 ,geol4 ,assoc4 ix4 7#ahhf7 p4 #ege2 and his ,british ,petrography1 #ahhh2 ,dr4 ,j4 ,petersen1 ,mikroskopische und chemische ,untersuchungen am ,enstatit-porphyrit aus den ,cheviot ,hills1 ,inaugural ,dissertation1 ,kiel1 #ahhd4 ,this volcanic pile1 consisting mainly of bedded andesites which rest unconformably on the upturned edges of ,wenlock shales and grits1 presents a most typical display of the lavas of the ,lower ,old ,red ,sandstone4 ,these rocks range from vitreous or resinous pitchstone-like varieties to coarsely porphyritic forms1 on the one hand1 and to highly vesicular and amygdaloidal kinds1 on the other4 ,analyses of some of these rocks1 and an account of their petrography1 have already been given4 ,the lavas are often separated by thin partings of tuff1 and their upper surfaces show the fissured character with sandstone infillings1 so characteristic among the lavas of ,lake ,caledonia4#cgc ,tuffs form a very subordinate part of the whole volcanic series4 ,one of the most important bands is a thick mass at the base of the series1 lying immediately on the highly inclined ,silurian shales4 ,the fragments are generally of a fine-grained purple mica-andesite1 often two or three feet and sometimes at least five feet long4 ,for a few feet near the bottom of this mass of tuff1 pieces of ,silurian shale an inch in length may be noticed4 ,mr4 ,clough remarks that distinct bedding is not usual among the tuffs4 ,though no doubt most of the fragmental materials really lie intercalated between successive lava-streams1 yet some of the isolated patches of coarse volcanic breccia may mark the sites of eruptive vents4 ,one such probable neck has been mapped on the ,scottish side between ,cocklawfoot1 at the head of the ,bowmont ,water1 and ,king's ,seat1 while others may perhaps occur among the detached patches that have been observed on the ,northumbrian side4 ,no thick conglomerates or sandstones have been noticed in the ,cheviot ,district4 ,the volcanic eruptions appear to have usually succeeded each other without the spread of any notable amount of ordinary detritus over the floor of the water-basin4 ,it is difficult to estimate the total thickness of volcanic material here piled up1 but it probably amounts to several thousand feet4 ,the top of the series is not visible1 having been partly removed by denudation and partly buried under the ,carboniferous formations4 ,footnote #cgc3 ,clough1 ,geology of the ,cheviot ,hills1 p4 #ae4 ,it will thus be seen that the ,cheviot area stands apart from the other volcanic districts of the ,lower ,old ,red ,sandstone in the great relative thickness of its accumulated lavas1 the comparative thinness of its tuffs1 and the absence of the thick intercalations of coarse conglomerate so abundantly developed among the volcanic series all over ,central ,scotland4 ,but there is yet another characteristic in which this area is pre-eminently conspicuous4 ,in the heart of the andesites lies a core of augite-granitite1 around which these rocks are traversed with dykes4 ,this interesting granitic boss rises into the highest summit of the whole ,cheviot range1 and covers an area of rather more than #bj square miles4 ,while its petrographical characters have been described by ,mr4 ,teall1 its boundary has been mapped by ,mr4 ,clough1 who found the line difficult to trace1 owing partly to the prevalent covering of peat1 and partly to the jagged and irregular junction caused by the protrusion of dykes from and into the boss4 ,he obtained evidence that the granite has broken through the bedded andesites1 and that it is in turn traversed by dykes composed of a material indistinguishable from that of some of the flows4 ,he therefore considered that it is essentially of the same age as the rest of the volcanic series1 and not improbably the deep-seated source of it4#cgd ,mr4 ,teall also1 from a chemical and microscopical examination of the rocks1 drew a similar conclusion4#cge ,footnote #cgd3 ,op4 cit4 p4 #bd4 ,footnote #cge3 ,geol4 ,mag4 #ahhe1 p4 #ajf4 ,the andesites around the granite have undergone contact-metamorphism1 but the nature and extent of the change have not yet been studied4 ,there occur around the granite many dykes of felsite and quartz-felsite1 to the petrographical character of which reference has already been made4 ,but the most abundant and remarkable dykes of the district are those of a reddish mica-porphyrite1 of which ,mr4 ,clough has mapped no fewer than forty1 besides those in the granitic area4 ,he has called attention to the significant manner in which all the dykes of the district tend to point in a general way to the great core of granite1 as if that were the nucleus from which they had radiated4#cgf ,footnote #cgf3 ,op4 cit4 pp4 #bf-bh4 ,the central granite of the ,cheviot ,hills1 with its peripheral dykes1 has no accompanying agglomerates nor any decided proof that it ever communicated with the surface4 ,when1 however1 we consider its petrographical and chemical constitution1 its position as a core among the bedded lavas1 and the intimate way in which it is linked with these rocks by the network of dykes1 we are1 ,i think1 justified in accepting the inference that it belongs to the volcanic series4 ,it possesses some curious and interesting features in common with the great granophyre bosses of ,tertiary age in the ,inner ,hebrides4 ,like these it has no visible accompaniment of superficial discharges4 ,yet it may have ascended by means of some central vent or group of vents which1 offering to it a weak part of the crust1 allowed it to communicate with the surface and give rise to the outflow of lavas and fragmental ejections4 ,in any case1 it affords us a most interesting and instructive insight into one of the deeper-seated ducts of a volcanic region1 and the relation of a volcanic focus to the ascent of the granitic magma4 99 99 99 99 99 ,about twenty miles to the north of the ,cheviot ,hills1 and separated from them by the ,carboniferous and ,upper ,old ,red ,sandstones which spread across the broad plain of the ,merse1 a group of volcanic rocks has been laid open in a singularly instructive manner along the coast of ,berwickshire1 between the village of ,eyemouth and the promontory of ,st4 ,abb's ,head4 ,not only the actual vents1 but the lavas and tuffs connected with them1 have there been admirably dissected by the forces of denudation4 ,that this volcanic area was quite distinct from that of the ,cheviot ,hills may be inferred from its coarse agglomerates1 and from the fact that when the rocks are followed inland in a south-westerly direction1 that is1 towards the ,cheviot area1 they are found to diminish in thickness and to disappear among the ordinary sediments4 ,for the same reason we may regard the area as independent of any vents which may have risen further west about ,cockburn ,law and the ,dirrington ,laws4 ,unfortunately1 however1 only a small part of the area comes into view1 the rest of it lying beneath the waters of the ,north ,sea4#cgg ,footnote #cgg3 ,this area lies in ,sheet #cd ,geological ,survey of ,scotland1 and was described by myself in the ,memoir to accompany that ,sheet 7,geology of ,eastern ,berwickshire1 #ahfd1 p4 #bj74 ,more recently the shore between ,st4 ,abb's ,head and ,coldingham has been re-mapped by ,professor ,james ,geikie who has also studied the microscopic character of the rocks1 ,proc4 ,roy4 ,soc4 ,edin4 xiv4 7#ahhg74 ,of the several vents dissected along this coast-line1 one may be seen at ,eyemouth1 filled with a very coarse tumultuous agglomerate of andesite fragments embedded in a compact felspathic matrix1 through which are scattered broken crystals of felspar1 and imperfect tabular crystals of black mica4 ,another of similar character is exposed for more than a mile and a half along the shore at ,coldingham4 ,it contains blocks1 sometimes more than a yard in diameter1 of different varieties of andesite1 and1 as at ,eyemouth1 is much invaded by veins and bosses of intrusive andesite4 ,illustration3 ,fig4 #ih4--,section across the volcanic area of ,st4 ,abb's ,head 7after ,prof4 ,j4 ,geikie74 #aa4 ,silurian formations2 #b4 ,lower ,old ,red ,conglomerate and ,sandstone2 #c #c4 ,sheets of andesitic lava2 #d4 ,volcanic tuffs1 largely composed of scori in the higher parts2 #e4 ,volcanic agglomerate of neck on shore2 #f4 ,intrusive andesites4 f1 ,fault4 ,to the north of ,coldingham1 a series of bedded volcanic rocks which form the picturesque headland of ,st4 ,abb's ,head1 are1 according to the estimate of ,professor ,james ,geikie1 about #ajjj to #abjj feet thick1 but neither their bottom nor their top is seen4 ,the same observer found them to consist of three groups of andesite sheets separated and overlain by bedded tuffs4 ,the lowest lavas have their base concealed under the sea1 and are covered by a thick band of coarse agglomeratic tuff1 above which lies the second group of andesites1 about #bej feet thick4 ,an intercalation of various tuffs from #dj to #ej feet thick then succeeds1 followed by the third lava-group1 #bej or #cjj feet in depth4 ,the highest member of the series is a mass of bedded tuffs some #djj feet thick4 ,the andesites lie in beds varying from about #ae to about #ej feet or more in thickness4 ,they are fine-grained1 purplish-blue1 or greyish-blue1 often reddish rocks1 of the usual type4 ,generally rather close-grained1 they are not as a rule very porphyritic1 but often highly scoriaceous and amygdaloidal1 especially towards the top and bottom of each bed4 ,the more slaggy portions are sometimes so filled in with fine tuff that the rock might be mistaken for one of fragmental origin4 ,the bedded tuffs are usually well stratified deposits4 ,the most important band of them is that which forms the highest member of the volcanic series4 ,it consists of successive beds that vary from fine red mudstones up to volcanic breccias with blocks one foot or more in diameter4 ,the materials have been derived from the explosion of andesitic lavas4 ,most of the lapilli are vesicular or amygdaloidal1 and many of them have evidently come from vitreous scoriaceous lavas4 ,professor ,geikie remarks that from their highly vesicular character1 they might well have floated in water at the time of their ejection--they are in short mere cinders4 ,he could detect no trace of ordinary sediment in the matrix1 the whole material being thoroughly volcanic in origin4 ,the lavas1 tuffs and agglomerates have been abundantly invaded by intrusive rocks1 chiefly andesites4#cgh ,footnote #cgh3 ,see ,prof4 ,j4 ,geikie1 op4 cit4 ,the agglomerates of this ,berwickshire coast extend for a short way inland from the ,coldingham and ,eyemouth vents1 but the fragmental material soon becomes finer and more water-rolled1 and assumes a distinctly stratified structure1 as it is gradually and increasingly interleaved with layers of ordinary sediment4 ,hence in passing towards the south-west1 away from the coast-line1 we are obviously receding from the vents of eruption and entering into the usual non-volcanic deposits of the time4 ,that these deposits belong to the ,lower ,old ,red ,sandstone was first ascertained during the progress of the ,geological ,survey in this district by the discovery of abundant plant-remains in the form of linear grass-like strips1 and also pieces of ,pterygotus in some of the green shales interstratified among fine tuffs and ashy sandstones4#cgi ,before the volcanic detritus disappears from the strata as they are followed in a south-westerly direction1 the whole series is unconformably overlain by the ,upper ,old ,red ,sandstone4 ,the lower division of the formation is not again seen until it rises from under the southern margin of the plain of the ,merse into the ,cheviot ,hills4 ,footnote #cgi3 ,geology of ,eastern ,berwickshire1 ,mem4 ,geol4 ,surv4 ,scotland 7#ahfd71 pp4 #bf1 #bg1 #eg4 ,about ten miles to the south-west of the large ,coldingham neck the great boss of ,cockburn ,law and ,stoneshiel ,hill rises out of the ,silurian rocks4#chj ,five miles still further in the same direction the group of the beautiful cones of ,dirrington 7,fig4 #gj7 overlooks the wide ,merse of ,berwickshire1#cha and six miles to the north of these hills1 in the very heart of ,lammermuir1 lies the solitary boss of the ,priestlaw granite4#chb ,to these protrusions of igneous material reference has already been made as possible volcanic vents connected with the eruptions of the ,lower ,old ,red ,sandstone4 ,as regards their age they must certainly be younger than the ,llandovery rocks which they disrupt1 and older than the ,upper ,old ,red ,sandstone1 of which the conglomerates1 largely made from their debris1 lie on them unconformably4 ,it seems therefore probable that these great bosses may form a part of the volcanic history of the ,lower ,old ,red ,sandstone period4 ,but no positive proof has yet been obtained that any one of them was the site of an eruptive vent1 and no trace has been detected around them of any lavas or tuffs which might have proceeded from them4 ,footnote #chj3 ,see ,the ,geology of ,eastern ,berwickshire 7,sheet #cd71 ,mem4 ,geol4 ,surv4 ,scotland 7#ahfd71 p4 #bi4 ,footnote #cha3 ,these hills are chiefly represented in ,sheet #be4 ,but see ,the ,geology of ,east ,lothian1 ,mem4 ,geol4 ,surv4 ,scotland 7#ahff71 p4 #bf4 ,footnote #chb3 ,geology of ,east ,lothian1 ,mem4 ,geol4 ,surv4 ,scotland1 p4 #ae1 and authorities there cited4 ,,the ,,lake ,,of ,,lorne ,illustration3 ,fig4 #ii4--,view of terraced andesite hills resting on massive conglomerate1 south of ,oban4 ,the basin of ,lorne has not yet been carefully examined and described1 though various writers have referred to different parts of it 7,map ,i474 ,my own observations have been too few to enable me to give an adequate account of it4 ,the volcanic sheets of this area form a notable feature in the scenery of the ,west ,highlands1 for they are the materials out of which the remarkable terraced hills have been carved1 which stretch from ,loch ,melfort to ,loch ,creran 7,fig4 #ii71 and which reappear in picturesque outliers among the mountains traversed by ,glen ,coe4 ,between the ancient schists that form the foundation-rocks of this district and the base of the volcanic series1 lies a group of sedimentary strata which in the western part of the district must be #ejj feet thick4 ,this group consists of exceedingly coarse breccias at the bottom1 above which come massive conglomerates1 volcanic grits or tuffs1 fine sandstones and courses of shale4 ,while the basement-breccias are composed mainly of detritus of the underlying schists1 including blocks six feet long1 they pass up into coarse conglomerates made up almost entirely of fragments of different lavas 7andesites1 diabases1 etc471 and more than #ajj feet thick1 which often show little or no trace of stratification1 but break up into large quadrangular blocks by means of joints which cut across the imbedded boulders4 ,these volcanic conglomerates form some of the more conspicuous features of the coast to the south and north of ,oban1 and are well exposed in the ,isle of ,kerrera4 ,they offer many points of resemblance to those of ,lake ,caledonia1 but no certain proof has been noted that they belong to the ,lower ,old ,red ,sandstone4 ,they have obviously been derived from lava-sheets that were exposed to strong breaker-action1 which at the same time transported and rounded blocks of granite1 schist and other crystalline rocks derived from the adjacent hills4 ,during the intervals of quieter sedimentation indicated by the fine sandstones and shales1 volcanic explosions continued1 as may be seen by the occurrence of occasional large bombs which have fallen upon and pressed down the fine ashy silt that was gathering on the bottom4 ,it would seem from the characters of some of the strata in this sedimentary series that over the area of deposit portions of the shallower waters were occasionally laid bare to the sun and air4 ,among the conglomerates there lie certain bands of reddish sandy1 ripple-marked1 sun-cracked and rain-pitted shales and fine sandstones4 ,such accumulations1 indicative of the ultimate exposure of fine sediment that silted up hollows in the great banks of coarse shingle1 may be noticed at the south end of the ,island of ,kerrera1 on at least two horizons which are separated from each other by thick masses of conglomerate and fine felspathic grit4 ,we may infer1 therefore1 that the fine littoral mud1 which gathered during pauses in the heaping up of the coarse gravel and shingle1 was occasionally laid dry4 ,similar strata may be observed behind ,oban1 where the alternation of exceedingly fine sediment and granular ashy bands is well exhibited4 ,illustration3 ,fig4 #ajj4--,section of lava-escarpment at ,beinn ,lora1 north side of mouth of ,loch ,etive1 ,argyllshire4 #a4 ,phyllites2 #b4 ,thick conglomerate2 #c4 ,successive sheets of andesite4 ,but the explosions that gave rise to the volcanic materials so largely represented in these lower conglomerates1 were merely preliminary to those which led to the outflow of the great sheets of lava that now constitute so large a part of the hills of ,lorne4 ,in the few traverses which ,i have made across different parts of this district ,i have noted the general resemblance of the lavas to those of the ,lower ,old ,red ,sandstone of the ,midland ,valley of ,scotland1 their bedded character1 and the occurrence of occasional layers of stratified material between them4 ,the prominent features of these rocks1 and their relation to the volcanic conglomerates below them1 and to the underlying slates and schists are well displayed on ,beinn ,lora at the mouth of ,loch ,etive 7,fig4 #ajj74 ,there the black slates of the district are unconformably covered by the coarse volcanic conglomerate1 formed chiefly of blocks of andesite1 cemented in a hard matrix of similar composition4 ,about #aej or #bjj feet of this material underlie the great escarpment of the lavas1 which here rise in successive beds to the top of the hill1 #ajjj feet above its base4 ,on the south side of ,loch ,etive the base of the volcanic series1 with its underlying conglomerate1 may be followed westward to ,oban and thence southward to ,loch ,feochan4 ,the lavas cover most of the ground from the western shore eastwards to near ,loch ,awe4 ,but this area is still very imperfectly known4 ,the ,geological ,survey1 however1 has now advanced to this part of the country1 so that we shall before long be in possession of more detailed information regarding the character and sequence of its volcanic history and the geological age of the eruptions4 ,mr4 ,h4 ,kynaston1 who has begun the mapping of the eastern portion of the district1 finds that there1 as further west1 the bottom of the volcanic series is generally a breccia or conglomerate4 ,he has met with two leading types among the lavas1 the more abundant being strongly vesicular1 the other more compact4 ,he has observed also numerous dykes and sills of intrusive porphyrite1 trending in a general ,n4,n4,e4 and ,s4,s4,w4 direction1 and pointing towards the great granite mass of ,ben ,cruachan4#chc ,footnote #chc3 ,ann4 ,report ,geol4 ,surv4 7#ahie71 p4 #bi of reprint4 ,mr4 ,r4 ,g4 ,symes has traced the volcanic series to the north and south of ,oban4 ,while visiting with him part of his ground1 ,i was much struck with the evidence of an intrusive mass at the base of the volcanic series in the ,sound of ,kerrera4 ,a prominent feature on the east side of the channel1 known as ,dun ,uabairtich and #bgj feet high1 consists of andesite which appears to combine both a central boss and a sill4 ,the rock breaks through the black slates and the overlying conglomerates and sandstones1 and has wedged itself into the unconformable junction between the two formations4 ,it is beautifully columnar on its sea-covered face1 some of the columns being #abj feet or more in length1 and gently curved4 ,,lake ,,orcadie ,we now cross the whole breadth of the ,scottish ,highlands in order to peruse the records of another of the great detached water-basins of the ,old ,red ,sandstone1 which for the sake of brevity of reference ,i have named and described as ,lake ,orcadie 7,map ,i474 ,this area has its southern limits along the base of the hills that enclose the wide ,moray ,firth4 ,it spreads northward over the ,orkney and much of the ,shetland ,islands1 but its boundaries in that direction are lost under the sea4 ,in the extensive sheet of water which spread over all that northern region the peculiar ,caithness ,flags1 with their associated sandstones and conglomerates1 were deposited to a total depth of #af1jjj feet4 ,a sigillaroid and lycopodiaceous vegetation flourished on the surrounding land1 together with ferns1 ,psilophyton and conifers4 ,the waters teemed with fishes of which many genera and species have now been described4 ,the remains of these creatures lie crowded upon each other in the flagstones in such a manner as to indicate that from time to time vast quantities of fish were suddenly killed4 ,not impossibly1 these destructions may have been connected with the volcanic activity which has now to be described4 ,in the year #ahgh ,i called attention to the evidence for the existence of contemporaneous volcanic rocks in the ,old ,red ,sandstone north of the range of the ,grampians1 and specially noted three localities where this evidence could be seen--,strathbogie1 ,buckie and ,shetland4#chd ,since that time ,messrs4 ,b4 ,n4 ,peach and ,j4 ,horne have added a fourth centre in the ,orkney ,islands4 ,at present1 therefore1 we are acquainted with the records of four separate groups of volcanic vents in ,lake ,orcadie4 ,footnote #chd3 ,trans4 ,roy4 ,soc4 ,edin4 xxviii4 7#ahgh74 ,the southern margin of this water-basin appears to have indented the land with long fjord-like inlets4 ,one of these now forms the vale of ,strathbogie1 which runs into the hills for a distance of fully #bj miles beyond what seems to have been the general trend of the coast-line4 ,in this valley ,i found a bed of dark vesicular diabase intercalated among the red sandstones and high above the base of the formation1 as exposed on the west side of the valley near ,burn of ,craig4 ,on the east side a similar band has since been mapped for the ,geological ,survey by ,mr4 ,l4 ,hinxman who has traced it for some miles down the ,strath4#che ,this latter band1 as shown in ,fig4 #aja1 lies not far above the bottom of the ,old ,red ,sandstone of this district1 and is thus probably distinct from the ,craig outcrop4 ,there would thus appear to be evidence of two separate outflows of basic lava in this fjord of the ,old ,red ,sandstone period4 ,footnote #che3 ,see ,sheet #gf of the ,geological ,survey of ,scotland4 ,illustration3 ,fig4 #aja4--,section across ,strathbogie1 below ,rhynie1 showing the position of the volcanic band4 #a4 ,knotted schists2 #b4 ,diorite intrusive in schists2 #c4 ,old ,red ,conglomerate2 #d4 ,volcanic band2 #e4 ,shales with calcareous nodules2 #f4 ,sandstones of ,rhynie2 #g4 ,shales and sandstones4 f1 ,fault4 ,no vestige has here been found of any vent1 nor is the lava accompanied with tuff4 ,the eruptions took place some time after the earlier sediments of the basin were accumulated1 but ceased before the thick mass of upper sandstones and shales was deposited4 ,a section across the valley gives the structure represented in the accompanying diagram 7,fig4 #aja74 ,twenty-five miles further north a still smaller andesite band has been detected by ,mr4 ,j4 ,grant ,wilson among the sandstones and conglomerates near ,buckie4#chf ,it is a truly contemporaneous flow1 for pebbles of it occur in the overlying strata4 ,but again it forms only a solitary bed1 and no trace of any accompanying tuff has been met with1 nor of the vent from which it came4 ,both this vent and that of ,strathbogie must have been situated near the southern coast-line of the lake4 ,footnote #chf3 ,see ,sheet #ie of the ,geological ,survey of ,scotland and ,trans4 ,roy4 ,soc4 ,edin4 vol4 xxviii4 7#ahgh71 p4 #dce4 ,at a distance of some #ij miles northward from these ,moray ,firth vents another volcanic district lies in the very heart of the ,orkney ,islands4#chg ,the lavas which were there ejected occur at the south-eastern corner of the island of ,shapinshay1 where they are regularly bedded with the flagstones4 ,they consist of dark green olivine-diabases with highly amygdaloidal and vesicular upper surfaces4 ,their thickness cannot be ascertained1 as their base is not seen1 but they have been cut by the sea into trenches which show them to exceed #cj feet in depth4 ,the position of the vent from which they came has not been ascertained4 ,neither here nor in the ,moray ,firth area do any sills accompany the interbedded sheets1 and in both cases the volcanic action would seem to have been of a feeble and short-lived character4 ,footnote #chg3 ,messrs4 ,b4 ,n4 ,peach and ,j4 ,horne1 ,proc4 ,roy4 ,phys4 ,soc4 ,edin4 vol4 v4 7#ahgi71 p4 #hj4 ,much more important were the volcanoes that broke out nearly #ajj miles still further north1 where the ,mainland of ,shetland now lies4 ,i shall never forget the pleasure with which ,i first recognized the traces of these eruptions1 and found near the most northerly limits of the ,british ,isles proofs of volcanic activity in the ,lower ,old ,red ,sandstone4 ,since my observations were published1#chh ,mr4 ,peach1 who accompanied me in ,shetland1 has returned to the district1 and1 in concert with his colleague ,mr4 ,horne1 has extended our knowledge of the subject4#chi ,the chief vent or vents lay towards the west and north-west of the ,mainland and ,north ,mavine2 others of a less active and persistent type were blown out some #be miles to the east1 where the islands of ,bressay and ,noss now stand4 ,in the western district streams of slaggy andesite and diabase with showers of fine tuff and coarse agglomerate were ejected1 until the total accumulation reached a thickness of not less than #ejj feet4 ,the volcanic eruptions took place contemporaneously with the deposition of the red sandstones1 for the lavas and tuffs are intercalated in these strata4 ,the lavas and volcanic conglomerates are traceable from the southern coast of ,papa ,stour across ,st4 ,magnus' ,bay to the western headlands of ,esha ,ness1 a distance of more than #ad miles4 ,they have been cut by the ,atlantic into a picturesque range of cliffs1 which exhibit in some places1 as at the singular sea-stalk of ,doreholm1 rough banks of andesitic lava with the conglomerate deposited against and over them1 and in other places1 as along the cliffs of ,esha ,ness1 sheets of lava overlying the conglomerates4 ,footnote #chh3 ,trans4 ,roy4 ,soc4 ,edin4 vol4 xxviii4 7#ahgh71 p4 #dah4 ,footnote #chi3 ,ibid4 vol4 xxxii4 7#ahhd71 p4 #cei4 ,no trace of any vents has been found in the western and chief volcanic district1 but in ,noss ,sound a group of small necks occurs1 filled with a coarse agglomerate composed of pieces of sandstone1 flagstone and shale4 ,messrs4 ,peach and ,horne infer that these little orifices never discharged any streams of lava4 ,more probably they were opened by explosions which only gave forth vapours and fragmentary discharges1 such as a band of tuff which is intercalated among the flagstones in their neighbourhood4 ,but one of the most striking features of the volcanic phenomena of this remote region is the relative size and number of the sills and dykes which here as elsewhere mark the latest phases of subterranean activity4 ,messrs4 ,peach and ,horne have shown us that three great sheets of acid rocks 7granites and spherulitic felsites1 to which reference has already been made1 p4 #bib7 have been injected among the sandstones and basic lavas1 that abundant veins of granite1 quartz-felsite and rhyolite radiate from these acid sills1 and that the latest phase of igneous action in this region was the intrusion of a series of bosses and dykes of basic rocks 7diabases7 which traverse the sills4 ,the ,killarney ,district ,in the south of ,ireland the ,upper ,silurian strata are followed upwards conformably by the great series of red sandstones and conglomerates known as the ,dingle ,beds4 ,lithologically these rocks present the closest resemblance to the ,lower ,old ,red ,sandstone of ,central ,scotland4 ,they occupy a similar stratigraphical position1 and though they have not yielded any palontological data for comparison1 there can1 ,i think1 be no hesitation in classing them with the ,scottish ,lower ,old ,red ,sandstone1 and regarding them as having been deposited under similar geographical conditions4 ,they offer one feature of special interest for the purpose of the present inquiry1 since they contain a well-marked group of contemporaneous volcanic rocks1 including nodular felsites1 like those so characteristic of the ,silurian period4 ,the area where this remote and isolated volcanic group is best developed forms a range of high rugged ground along the northern front of the hills that stretch eastward from the ,lakes of ,killarney4 ,their general distribution is shown on ,sheets #ahd and #ahe of the ,geological ,survey of ,ireland2#cij though ,i may again remark that petrography has made great strides during the thirty years and more that have passed since these maps and their accompanying ,memoirs were published1 and that1 were the district to be surveyed now1 probably a considerable tract of ground coloured as ash would be marked as felsite4 ,at the same time the existence of both these rocks here cannot be gainsaid4 ,footnote #cij3 ,see the ,memoir 7by ,j4 ,b4 ,jukes and ,g4 ,v4 ,du ,noyer7 on ,sheet #ahd1 p4 #ae4 ,other volcanic rocks have been mapped at ,valentia ,harbour in the ,dingle ,beds1 but these ,i have not had an opportunity of personally examining4 ,the felsite was long ago brought into notice by ,dr4 ,haughton1 who published an analysis of it4#cia ,it is also referred to by ,mr4 ,teall for its spherulitic structure4#cib ,seen on the ground it appears as a pale greenish-grey close-grained rock1 sometimes exhibiting flow-structure in a remarkably clear manner1 the lamin of devitrification following each other in wavy lines1 sometimes twisted and delicately puckered or frilled1 as in some schists4 ,portions of the rock are strongly nodular1 the nodules varying in size from less than a pea to that of a hen's egg4 ,footnote #cia3 ,trans4 ,roy4 ,irish ,acad4 vol4 xxiii4 7#ahei71 p4 #fae4 ,footnote #cib3 ,british ,petrography1 p4 #cdi4 ,the close resemblance of this rock to many of the ,lower ,silurian nodular felsites of ,wales cannot but strike the geologist4 ,it presents analogies also to the ,upper ,silurian felsites of ,dingle4 ,but its chief interest arises from the geological horizon on which it occurs4 ,lying in the so-called ,dingle-,beds1 which may be regarded as the equivalents of the ,lower ,old ,red ,sandstone of ,england and ,scotland1 it is1 so far as my observations go1 the only example of such a nodular felsite of later date than the ,silurian period4 ,we recognize in it a survival1 as it were1 of the peculiar ,silurian type of acid lava1 the last preceding eruption of which took place not many miles to the west1 in the ,dingle promontory4 ,but elsewhere this type does not appear to have survived the end of the ,silurian period4 ,the detrital rocks accompanying the felsite1 in the district east of ,killarney1 vary from such closed-grained felsitic material as cannot readily be distinguished from the felsite itself to unmistakable felsitic breccias4 ,even in the finest parts of them1 occasional rounded quartz-pebbles may be detected1 while here and there a reddish shaly band1 or a layer of fine pebbly conglomerate with quartz-pebbles an inch in length1 shows at once the bedding and the dip4 ,mr4 ,w4 ,w4 ,watts1 who1 with ,mr4 ,a4 ,m',henry of the ,irish ,staff of the ,geological ,survey1 accompanied me over this ground1 found that a microscopic examination of the slides which were prepared from the specimens we collected completely confirmed the conclusions reached from inspection of the rocks in the field4#cic ,he detected among the angular grains slightly damaged crystals of felspar1 chiefly orthoclase4 ,many portions of these felspathic grits much resemble the detrital ,cambrian rocks which in the ,vale of ,llanberis have been made out of the pale felsite of that locality4 ,footnote #cic3 ,mr4 ,watts also examined the microscopic structure of the felsite of ,benaun ,more4 ,he found that the spherulites appear to have a micropegmatitic structure1 owing to the intergrowth of quartz and felspar4 ,in some parts of the rock the spherulites1 from #jb to #ja inch in diameter1 are surrounded by exceedingly minute green needles1 possibly of hornblende1 while inside some of them are small quartz-grains4 ,larger porphyritic felspars occur outside the spherulites1 some being of plagioclase1 but most of orthoclase4 ,the spherulitic structure is not so well developed near the felspars4 ,a few of the large nodules are hollow and lined with crystals1 while some of them show a finely concentric lamination like the successive layers of an agate4 ,,chapter ,,xxii ,,volcanoes ,,of ,,the ,,upper ,,old ,,red ,,sandstone--,,the ,,south-,,west ,,of ,,ireland1 ,,the ,,north ,,of ,,scotland ,in the northern half of ,britain1 where the ,old ,red ,sandstone is so well displayed1 the two great divisions into which this series of sedimentary deposits is there divisible are separated from each other by a strongly marked unconformability4 ,the interval of time represented by this break must have been of long duration1 for it witnessed the effacement of the old water-basins1 the folding1 fracture1 and elevation of their thick sedimentary and volcanic accumulations1 and the removal by denudation of1 in some places1 several thousand feet of these rocks4 ,the ,upper ,old ,red ,sandstone1 consisting so largely as it does of red sandstones and conglomerates1 indicates the return or persistence of geographical conditions not unlike those that marked the deposition of the lower subdivision4 ,but in one important respect its history differs greatly from that which ,i have sketched for the older part of the system4 ,though the ,upper ,old ,red ,sandstone is well developed across the southern districts of ,scotland from the ,ochil to the ,cheviot ,hills1 and appears in scattered areas over so much of ,england and ,wales1 no trace has ever been there detected in it of any contemporaneously erupted volcanic rocks4 ,the topographical changes which preceded its deposition must have involved no inconsiderable amount of subterranean disturbance1 yet the volcanic energy1 which had died out so completely long before the close of the time of the ,lower ,old ,red ,sandstone1 does not appear to have been rekindled until the beginning of the ,carboniferous period4 ,two widely separated tracts in the ,british ,isles have yielded traces of contemporaneous volcanic rocks in the ,upper ,old ,red ,sandstone4 ,one of these lies in the south-west of ,ireland1 the other in the far north of ,scotland4 ,,the ,,south-,,west ,,of ,,ireland ,the ,irish locality is situated a few miles to the south of the town of ,limerick1 where the ,carboniferous ,limestone has been thrown into long folds1 and where1 along the anticlines1 strips of the underlying red sandstones have been brought up to the surface4 ,two such ridges of ,upper ,old ,red ,sandstone bear1 each on its crest1 a small but interesting relic of volcanic activity#cid 7,map ,i474 ,footnote #cid3 ,see ,sheet #aec of the ,geological ,survey of ,ireland1 and ,explanation to that ,sheet 7#ahfa71 by ,messrs4 ,g4 ,h4 ,kinahan and ,j4 ,o',kelly4 ,the account of the ground above given is from notes which ,i made during a personal visit4 ,the more northerly ridge rises in the conical eminence of ,knockfeerina to a height of #idi feet above the sea4 ,even from a distance the resemblance of this hill 7,fig4 #ajb7 to many of the ,carboniferous necks of ,scotland at once attracts the eye of the geologist4 ,the resemblance is found to hold still more closely when the internal structure of the ground is examined4 ,the cone consists mainly of a coarse agglomerate1 with blocks generally somewhat rounded and varying in size up to two feet in length4 ,the most prominent of these1 on the lower eastern slopes1 are pieces of a fine flinty felsite weathering white1 but there also occur fragments of grit and baked shale4 ,the matrix is dull-green in colour1 and among its ingredients are abundant small lapilli of a finely vesicular andesite or diabase4 ,these more basic ingredients increase in number towards the top of the eminence1 where much of the agglomerate is almost wholly made up of them4 ,no marked dip is observable over most of the hill1 the rock appearing as a tumultuous agglomerate1 though here and there1 particularly near the top and on the south side1 a rude bedding may be detected dipping outwards4 ,on the west side the agglomerate is flanked with yellow sandstone baked into quartzite1 so that the line of junction there between the two rocks not improbably gives us the actual wall of the vent4 ,the induration of the surrounding sandstones is a familiar feature among the ,carboniferous vents4 ,some intrusive dark flinty rock traverses the agglomerate near the top on the north side4 ,illustration3 ,fig4 #ajb4--,view of ,knockfeerina1 ,limerick1 from the north-east--a volcanic neck of ,upper ,old ,red ,sandstone age4 ,retiring eastwards from the cone1 the observer finds evidence of the intercalation of tuff among the surrounding ,upper ,old ,red ,sandstone4 ,at the east end of the village of ,knockfeerina a red nodular tuff1 with rounded pieces of andesite1 grit and sandstone1 sometimes #ab inches long1 is seen to dip under yellow1 grey and red sandstones and shales1 while other shales and sandstones underlie this tuff and crop out between it and the agglomerate4 ,there is thus evidence of the intercalation of volcanic tuff in the ,upper ,old ,red ,sandstone of this district4 ,and there seems no reason to doubt that the tuff was ejected from the adjoining vent of ,knockfeerina4 ,on the next ridge of ,old ,red ,sandstone1 which runs parallel to that of ,knockfeerina at a distance of little more than a mile to the south1 another mass of volcanic material rises into a prominence at ,ballinleeny4 ,on the north side it consists of agglomerate like that just described1 and is flanked by sandstone baked into quartzite4 ,here again we probably see the edge of a volcanic funnel4 ,there may possibly be more than one vent in this area4 ,but well-bedded tuffs can be observed to dip away from the centre and to pass under sandstones and shales which are full of fine ashy material4 ,gradations can be traced from the tuff into ordinary sediment4 ,in this instance1 therefore1 there is additional proof of contemporaneous volcanic action in the ,upper ,old ,red ,sandstone4 ,there can be no uncertainty as to the horizon of the strata in which these records have been preserved1 for they dip conformably under the shales and limestones at the base of the ,carboniferous ,limestone series4 ,they are said to have yielded the characteristic fern ,palopteris of ,kiltorcan4#cie ,footnote #cie3 ,there may be some other examples of ,upper ,old ,red ,sandstone volcanic rocks in ,ireland which ,i have not yet been able personally to examine4 ,on the maps of the ,geological ,survey 7,sheet #aih1 and ,explanation1 pp4 #h1 #ag7 contemporaneous rocks of this age are marked as occurring at ,cod's ,head and ,dursey ,island1 on the south side of the mouth of the ,kenmare estuary4 ,,the ,,north ,,of ,,scotland ,the only district in ,england or ,scotland wherein traces of volcanic action during the time of the ,upper ,old ,red ,sandstone have been observed lies far to the north among the ,orkney ,islands1 near the centre of the scattered outliers which ,i have united as parts of the deposits of ,lake ,orcadie#cif 7,map ,i474 ,the thick group of yellow and red sandstones which form most of the high island of ,hoy1 and which1 there can be little doubt1 are correctly referred to the ,upper ,old ,red ,sandstone1 rest with a marked unconformability on the edges of the ,caithness flagstones 7,fig4 #ajc74 ,at the base of these pale sandstones1 and regularly interstratified with them1 lies a band of lavas and tuffs which can be traced from the base of the rounded hills to the edge of the cliffs at the ,cam1 along the face of which it runs as a conspicuous feature1 gradually sloping to a lower level1 till it reaches the sea4 ,at the ,cam of ,hoy it is about #bjj feet thick1 and consists of three or more sheets of andesite4 ,the upper #ej or #fj feet show a strongly slaggy structure1 the central portion is rudely columnar1 and the lower part presents a kind of horizontal jointing or bedding4 ,there can be no question that this rock is not a sill but a group of contemporaneous lava-flows4 ,beneath it1 and lying across the edges of the flagstones below1 there is a zone of dull-red1 fine-grained tuff1 banded with seams of hard red and yellow sandstone4 ,this tuff zone dies out to the eastward of the ,cam4 ,footnote #cif3 ,first noticed in ,geol4 ,mag4 ,february #ahgh2 and ,trans4 ,roy4 ,soc4 ,edin4 xxviii4 7#ahgh71 p4 #daa4 ,illustration3 ,fig4 #ajc4--,section of the volcanic zone in the ,upper ,old ,red ,sandstone1 ,cam of ,hoy1 ,orkney4 #a4 ,caithness flagstones2 #b4 ,dull-red tuff and bands of sandstone2 #c4 ,lava zone in three bands2 #d4 ,yellow and red sandstone4 ,illustration3 ,fig4 #ajd4--,section of the volcanic zone in the ,upper ,old ,red ,sandstone at ,black ,ness1 ,rackwick1 ,hoy4 ,a few miles south of the ,cam the volcanic zone appears again as the platform on which the picturesque natural obelisk of the ,old ,man of ,hoy stands4 ,here the lava runs out as a promontory from the base of the cliff1 and on this projection the ,old ,man has been left isolated from the main precipice4 ,the cliffs of ,hoy are traversed by numerous small faults which have shifted the volcanic zone4 ,but on the great face of rock behind the ,old ,man there appears to be a second volcanic zone lying several hundred feet above that just described4 ,it is probably this upper zone which emerges from under the hills a mile and a half to the south at ,black ,ness in the bay of ,rackwick4 ,a good section is there visible1 which is represented in ,fig4 #ajd4 ,the ordinary red and yellow sandstones 7a7 appear from under the volcanic rocks at this locality1 and stretch southwards to the most southerly headland of ,hoy4 ,the lowest volcanic band in the section is one of red sandy well-bedded tuffs 7b74 ,some of the layers are coarse and almost agglomeratic1 while others are fine marly and sandy1 with dispersed bombs1 blocks and lapilli of diabase and andesite4 ,hard ribs of a sandy tufaceous material also occur4 ,these fragmental deposits are immediately overlain by a dark-blue rudely prismatic diabase with slaggy top 7c74 ,it is about #aej feet thick at its thickest part1 but rapidly thins away in a westerly direction4 ,it passes under a zone of red tuff 7d7 like that below1 and above this highest member of the volcanic group comes the great overlying pile of yellow and reddish sandstone of ,hoy4 ,followed westward for a short distance1 the whole volcanic zone is found to die out and the sandstones below and above it then come together4 ,the interest of this little volcanic centre in ,hoy is heightened by the fact that the progress of denudation has revealed some of the vents belonging to it4 ,on the low ground to the east of the ,cam1 and immediately to the north of the volcanic escarpment1 the flagstones which there emerge from under the base of the unconformable upper sandstones are pierced by three volcanic necks which we may with little hesitation recognize as marking the sites of vents from which this series of lavas and tuffs was discharged 7,fig4 #aje74 ,the largest of them forms a conspicuous hill about #dej feet high1 the smallest is only a few yards in diameter1 and rises from the surface of a flagstone ridge4 ,they are filled with a coarse1 dull-green1 volcanic agglomerate1 made up of fragments of the lavas with pieces of hardened yellow sandstone and flagstone4 ,around the chief vent the flagstones through which it has been opened have been greatly hardened and blistered4 ,the most easterly vent1 which has been laid bare on the beach at ,bring1 due east of ,hoy ,hill1 can be seen to pierce the flagstones1 which1 although their general dip is westerly at from #aj to #ae1 yet at their junction with the agglomerate are bent in towards the neck1 and are otherwise much jumbled and disturbed4 ,illustration3 ,fig4 #aje4--,section across the volcanic band and its associated necks1 ,hoy1 ,orkney4 #a4 ,caithness flagstones2 #b4 ,volcanic band lying on red sandstones and conglomerates and dying out eastwards2 #c #c4 ,two vents between the base of the hills and the sea2 their connection with the volcanic band is shown by dotted lines2 #d4 ,overlying mass of ,upper ,old ,red ,sandstone forming the hills of ,hoy4 ,on the northern coast of ,caithness ,i have described a remarkable volcanic vent about #cjj feet in diameter1 which rises through the uppermost group of the ,caithness flagstones4 ,it is filled with a coarse agglomerate consisting of a dull-greenish diabase paste crowded with blocks of diabase1 sometimes three feet in diameter1 and others of red sandstone1 flagstone1 limestone1 gneiss and lumps of black cleavable augite 7,fig4 #ajf74cig ,the sandstones around it present the usual disrupted1 indurated and jointed character1 and are traversed by a small diabase dyke close to the western margin of the neck4 ,another similar neck has since been found by the officers of the ,geological ,survey on the same coast4 ,that these volcanic orifices were active about the same time with those in the opposite island of ,hoy may be legitimately inferred4 ,footnote #cig3 ,see ,trans4 ,roy4 ,soc4 ,edin4 xxviii4 7#ahgh71 p4 #dje2 also p4 #dhb of the same volume for an account of the cleavable augite4 ,illustration3 ,fig4 #ajf4--;,ground-plan of volcanic neck piercing the ,caithness ,flagstone series on the beach near ,john o' ,groat's ,house4 ,these northern volcanoes made their appearance in a district where during the preceding ,lower ,old ,red ,sandstone period there had been several widely separated groups of active volcanic vents4 ,so far as the fragmentary nature of the geological evidence permits an opinion to be formed1 they seem to have broken out at the beginning1 or at least at an early stage1 of the deposition of the ,upper ,old ,red ,sandstone1 and to have become entirely extinct after the lavas of ,hoy were poured forth4 ,no higher platform of volcanic materials has been met with in that region4 ,with these brief and limited ,orcadian explosions the long record of ,old ,red ,sandstone volcanic activity in the area of the ,british ,isles comes to an end4#cih ,footnote #cih3 ,there appear to be traces of volcanic eruptions contemporaneous with the ,upper ,old ,red ,sandstone of ,berwickshire1 but as they merely formed a prelude to the great volcanic activity of ,carboniferous time1 they are included in the account of the ,carboniferous plateau of ,berwickshire in ,chapters xxiv4 and xxv4 ,,book ,,vi ,,the ,,carboniferous ,,volcanoes ,,chapter ,,xxiii ,,the ,,carboniferous ,,system ,,of ,,britain ,,and ,,its ,,volcanic ,,records ,geography and ,scenery of the ,carboniferous ,period--,range of ,volcanic ,eruptions during that time--,i4 ,the ,carboniferous ,volcanoes of ,scotland--,distribution1 ,arrangement and ,local ,characters of the ,carboniferous ,system in ,scotland--,sketch of the ,work of previous ,observers in this ,subject4 ,within the area of the ,british ,isles1 the geological record is comparatively full and continuous from the base of the ,upper ,old ,red ,sandstone to the top of the ,coal-measures4 ,we learn from it that the local basins of deposit in which the later portion of the ,old ,red ,sandstone was accumulated sank steadily in a wide general subsidence1 that allowed the clear sea of the ,carboniferous ,limestone ultimately to spread for some #gjj miles from the west coast of ,ireland into ,westphalia4 ,over the centre of ,england this ,carboniferous ,mediterranean had a breadth of at least #aej miles1 gradually shallowing northwards in the direction of land in ,scotland and ,northern ,ireland4 ,the gentle sinking of the floor of the basin continued until more than #fjjj feet of sediment1 chiefly composed of the remains of crinoids1 corals and other marine organisms1 had been piled up in the deeper parts4 ,traces of the southern margin of this sea1 or at least of a long insular ridge that rose out of its waters1 are to be seen in the protuberances of older rocks which appear at intervals from under the ,coal-measures and later formations between the borders of ,wales and the heart of ,leicestershire1 and of which the crags of ,charnwood ,forest are among the few peaks that still remain visible4 ,to the south of this ridge1 open sea extended far southward and westward over the site of the ,mendip ,hills and the uplands of ,south ,wales4 ,the ,carboniferous period1 as chronicled by its sedimentary deposits1 was a time of slow submergence and quiet sedimentation1 terrestrial and marine conditions alternating along the margins of the sinking land1 according as the rate of depression surpassed or fell short of that of the deposition of sediment4 ,there is no trace of any general disturbance among the strata1 such as would be marked by an important and widely extended unconformability4 ,but many indications may be observed that the rate of subsidence did not continue uniform1 if1 indeed1 the downward movement was not locally arrested1 and even exchanged for a movement in the opposite direction4 ,it is difficult1 for instance1 to believe the ancient ridge of the ,midlands to have been so lofty that even the prolonged subsidence required for the accumulation of the whole ,carboniferous system was insufficient to carry its highest crests below the level of the coal-jungles4 ,more probably the depression reached its maximum along certain lines or bands running in a general north-easterly direction1 the intervals between these lines sinking less1 or possibly even undergoing some measure of uplift4 ,one of the subsiding tracts1 that of the wide lowlands of ,central ,scotland1 was flanked on the south by a ridge which1 while its north-eastern portion was buried under the ,upper ,old ,red ,sandstone and ,lower ,carboniferous rocks1 remained above water towards the south-west1 and does not appear to have been wholly submerged there even at the close of the ,carboniferous period4 ,so abundant and varied are the sedimentary formations of ,carboniferous time1 and so fully have they preserved remains of the contemporary plants and animals1 that it is not difficult to realise in some measure the general aspect of the scenery of the time1 and the succession of changes which it underwent from the beginning to the end of the period4 ,the land was green with a luxuriant if somewhat monotonous vegetation4 ,large pine trees flourished on the drier uplands4 ,the lower grounds nourished dense groves of cycads or plants allied to them1 which rose as slim trees twenty or thirty feet high1 with long hard green leaves and catkins that grew into berries4 ,the swamps and wetter lands bore a rank growth of various gigantic kinds of club-moss1 equisetaceous reeds and ferns4 ,nor was the hum of insect-life absent from these forests4 ,ancestral types of cockroaches1 mayflies and beetles lived there4 ,scorpions swarmed along the margins of the shallow waters1 for their remains1 washed away with the decayed vegetation among which they harboured1 are now found in abundance throughout many of the dark shales4 ,the waters were haunted by numerous kinds of fish quite distinct from those of the ,old ,red ,sandstone4 ,in the lagoons1 shoals of small ganoids lived on the cyprids that peopled the bottom1 and they were in turn preyed on by larger ganoids with massive armature of bone4 ,now and then a shark from the opener sea would find its way into these more inland waters4 ,the highest types of animal life yet known to have existed at this time were various amphibians of the extinct order of ,labyrinthodonts4 ,the open sea1 too1 teemed with life4 ,wide tracts of its floor supported a thick growth of crinoids whose jointed stems1 piled over each other generation after generation1 grew into masses of limestone many hundreds of feet in thickness4 ,corals of various kinds lived singly and in colonies1 here and there even growing into reefs4 ,foraminifera1 sponges1 sea-urchins1 brachiopods1 gasteropods1 lamellibranchs and cephalopods1 in many genera and species1 mingled their remains with the dead crinoids and corals to furnish materials for the wide and thick accumulation of ,carboniferous ,limestone4 ,looking broadly at the history of the ,carboniferous period1 and bearing in mind the evidence of prolonged depression already referred to1 we can recognize in it three great eras4 ,during the first1 the wide clear sea of the ,carboniferous ,limestone spread over the centre and south of ,britain1 interrupted here and there by islands that rose from long ridges whereby the sea-floor was divided into separate basins4 ,next came a time of lessened depression1 when the sea-bottom was overspread with sand1 mud and gravel1 and was even in part silted up1 as has been chronicled in the ,millstone ,grit4 ,the third stage brings before us the jungles of the ,coal-measures1 when the former sea-floor became a series of shallow lagoons where1 as in the mangrove-swamps of our own time1 a terrestrial vegetation sprang up and mingled its remains with those of marine shells and fishes4 ,such a state of balance among the geological forces as is indicated by the stratigraphy of the ,carboniferous system would not prepare us for the discovery of the relics of any serious display of contemporary volcanic activity4 ,and1 indeed1 throughout the ,carboniferous rocks of ,western ,europe there is for the most part little trace of contemporaneous volcanic eruptions4 ,yet striking evidence exists that1 along the western borders of the continental area1 in ,france as well as over much of ,britain1 which had for so many previous geological ages been the theatre of subterranean activity1 the older half of ,carboniferous time witnessed an abundant1 though less stupendous and prolonged1 renewal of volcanic energy4 ,from the very commencement of the ,carboniferous period to the epoch when the ,coal-measures began to be accumulated1 the area of the ,british ,isles continued to be a scene of active volcanism4 ,in the course of that prolonged interval of geological time the vents shifted their positions1 and gradually grew less energetic1 but there does not appear to have been any protracted section of the interval when the subterranean activity became everywhere entirely quiescent4 ,the geologist who traces1 from older to younger formations1 the progress of some persistent operation of nature1 observes the evidence gradually to increase in amount and clearness as it is furnished by successively later parts of the record4 ,he finds that the older rocks have generally been so dislocated and folded1 and are often so widely covered by younger formations1 that the evidence which they no doubt actually contain may be difficult to decipher1 or may be altogether concealed from view4 ,in following1 for instance1 the progress of volcanic action1 he is impressed1 as he passes from the older to the younger ,palozoic chronicles1 by the striking contrast between the fulness and legibility of the ,carboniferous records and the comparative meagreness and obscurity of those of the earlier periods4 ,the ,carboniferous rocks have undergone far less disturbance than the ,cambrian and ,silurian formations2 while over wide tracts1 where their volcanic chapters are fullest and most interesting1 they lie at the surface1 and can thus be subjected to the closest scrutiny4 ,hence the remains of the volcanic phenomena of the later ,palozoic periods present a curiously modern aspect1 when contrasted with the fragmentary and antique look of those of older date4 ,the history of volcanic action during the ,carboniferous period in ,britain is almost wholly comprised in the records of the earlier half of that period1 that is1 during the long interval represented by the ,carboniferous ,limestone series and the ,millstone ,grit4 ,it was chiefly in the northern part of the region that volcanic activity manifested itself4 ,in ,scotland there is the chronicle of a long succession of eruptions across the district of the central and southern counties1 from the very beginning of ,carboniferous time down to the epoch when the ,coal-measures began to be accumulated4 ,in ,england1 on the other hand1 the traces of ,carboniferous volcanoes are confined within a limited range in the ,carboniferous ,limestone1 while in ,ireland they appear to be likewise restricted to the same lower division of the system4 ,during the whole of the vast interval represented by the ,coal-measures volcanic energy1 so far as at present known1 was entirely dormant over the region of the ,british ,isles4 ,these general statements will be more clearly grasped from the accompanying table1 which shows the various sections into which the ,carboniferous system of ,britain has been divided1 and also1 by black vertical lines1 the range of volcanic intercalations in each of the three kingdoms4 ---------------------------------------- -------------------------------- ,england4 ,scotland4 ,ireland4 ---------------------------------------- -------------------------------- ,coal-measures4 ,upper ,red ,sandstones with ,spirorbis-limestone4 ,middle or chief coal-bearing measures4 ,gannister group4 ,millstone ,grit4 ,grits1 flagstones and shales with thin coals4 ,carboniferous ,limestone4 ,yoredale group of shales and grits with limestones4 ,thick 7,scaur or ,main7 ,limestone of ,england1 with sandstones and coals in ,scotland4 ,lower ,limestone ,shale 7,calciferous ,sandstones of ,scotland74 ---------------------------------------- -------------------------------- ,such being the general range in time of the ,carboniferous volcanic phenomena1 it may be convenient1 in this preliminary survey1 to take note of the general distribution of the volcanic districts over the ,british ,isles1 as in this way we may best realise the extent and grouping of the eruptions1 which will then be considered in further detail 7see ,map ,i474 ,not only were the ,carboniferous volcanoes most abundant and persistent in ,scotland1 but they attained there a variety and development which give their remains an altogether exceptional interest in the study of volcanic geology4 ,they were distributed over the wide central valley1 from the south of ,cantyre to beyond the mouth of the estuary of the ,forth4 ,on the southern side of the ,silurian ,uplands1 they were likewise numerous and active4 ,there is thus no considerable tract of ,lower ,carboniferous rocks in ,scotland which does not furnish its evidence of contemporaneous volcanic action4 ,although some portions of the ,scottish ,carboniferous igneous rocks run for a short distance into ,england1 it is remarkable that1 when these at last die out southwards1 no other relics of contemporaneous volcanic energy take their place4 ,along the ,pennine chain1 from the ,border into the heart of ,england1 though natural sections are abundant1 no trace of included volcanic rocks appears until we reach ,derbyshire4 ,the whole of that wide interval of #aej miles1 so far as the present evidence goes1 remained during ,carboniferous time entirely free from any volcanic eruption4 ,but from the picturesque country of the ,peak southwards1 the sea-floor of the ,carboniferous ,limestone1 in what is now the heart of ,england1 was dotted with vents whence the sheets of toadstone were ejected1 which have so long been a familiar feature in ,english geology4 ,beyond this limited volcanic district the ,carboniferous formations of the south-west of ,england remain1 on the whole1 devoid of contemporaneous volcanic intercalations1 traces of ,carboniferous volcanic action having been recognized only in ,west ,somerset and ,devonshire4 ,in the ,mendip district and in the ridges of limestone near ,weston-super-,mare bands of cellular lava and tuff have been observed4 ,to the west of ,dartmoor1 ,brent ,tor and some of the surrounding igneous masses may mark the positions of eruptive vents during an early part of the ,carboniferous period4 ,at the south end of the ,isle of ,man relics remain of a group of vents among the ,carboniferous limestones4 ,passing across to ,ireland1 where these limestones attain so great a thickness and cover so large a proportion of the surface of the island1 we search in vain for any continuation of the abundant and varied volcanic phenomena of ,central ,scotland4 ,so far as observation has yet gone1 only two widely separated areas of ,carboniferous volcanic rocks are known to occur in ,ireland4#cii ,one of these shows that a little group of vents probably rose from the floor of the ,carboniferous ,limestone sea1 near ,philipstown1 in ,king's ,county4 ,the other lies far to the west in the ,golden ,vale of ,limerick1 where a more important series of vents poured out successive streams of lava with showers of ashes1 from an early part of the ,carboniferous period up to about the beginning of the time of the ,coal-measures4 ,footnote #cii3 ,the supposed ,carboniferous volcanic rocks of ,bearhaven on the coast of ,cork are noticed on p4 #di1 vol4 ii4 ,the total area within which the volcanic eruptions of ,carboniferous time took place was thus less than that over which the volcanoes of the ,lower ,old ,red ,sandstone were distributed1 yet they were scattered across the larger part of the site of the ,british ,isles4 ,from the vents of ,fife to those of ,limerick is a distance of above #cjj miles2 from the latter eastward to those of ,devonshire is an interval of #bej miles2 while the space between the ,devonshire volcanoes and those of ,fife is about #djj miles4 ,in this triangular space volcanic action manifested itself at each of the apices1 to a slight extent along the centre of the eastern side1 but with much the greatest vigour throughout the northern part of the area4 ,since the volcanic phenomena of ,carboniferous time are exhibited on a much more extensive scale in ,scotland than in any other region of the world yet studied1 it will be desirable to describe that area in considerable detail4 ,the other tracts in ,britain where volcanic rocks of the same age occur need not be so fully treated1 except where they help to a better comprehension of the general volcanic history4 99 99 99 99 99 ,it is in the southern half of ,scotland that the ,carboniferous system is developed 7,map ,,iv474 ,a line drawn from ,machrihanish ,bay1 near the ,mull of ,cantyre1 north-eastward across ,arran and ,bute to the south end of ,loch ,lomond1 and thence eastward by ,bridge of ,allan1 ,kinross and ,cupar to ,st4 ,andrews ,bay1 forms the northern limit of this system4 ,south of that line ,carboniferous volcanic intercalations are to be met with in nearly every county across into the borders of ,northumberland4 ,that we may follow intelligently the remarkably varied volcanic history of this region1 it is desirable to begin by taking note of the nature and sequence of the sedimentary formations among which the volcanic rocks are intercalated1 for these serve to bring before us the general conditions of the geography of the period4 ,the subjoined table exhibits the subdivisions into which the ,carboniferous system in ,scotland has been grouped3-- ,coal-measures4 ,upper ,red ,sandstone group1 nearly devoid of coal-seams4 ,coal-bearing1 white1 yellow and grey sandstones1 dark shales and ironstones 7,upper ,coal series74 ,millstone ,grit4 ,thick white and reddish sandstones and grits4 ,carboniferous ,limestone series4 ,sandstones1 shales1 fireclays1 coal-seams1 ironstones and three seams of marine limestone1 of which the uppermost is known as the ,castlecary seam1 the second as the ,calmy or ,arden1 and the lowest as the ,index 7,lower ,coal series74 ,bands of marine limestone intercalated among sandstones1 shales and some coal-seams4 ,a thick band of limestone lying at or near the bottom of the group1 traceable all over ,central ,scotland1 is known as the ,hurlet or ,main ,limestone4 ,some higher and thinner seams are called ,hosie's 7see ,fig4 #aee74 ,calciferous ,sandstones4#djj ,in the basin of the ,firth of ,forth1 below the ,hurlet ,limestone1 comes a varied series of white and yellow sandstones1 black shales 7oil-shales71 cyprid shales and limestones 7,burdiehouse71 and occasional coal-seams 7,houston71 having a total depth of about #cjjj feet4 ,this local group abounds in fossil plants1 entomostraca and ganoid fishes4 ,it passes down into the ,cement-stone group1 which1 however1 is feebly developed in this district1 unless it is partly represented by the sandstones1 shales1 limestones and coals just mentioned4 ,cement-stone group consisting of red1 blue and green marls and shales1 red and grey sandstones1 and thin bands of cement-stone3 fossils scarce4 ,reddish and grey sandstones and shales1 with occasional plant- remains1 passing down into the deep red 7sometimes yellow7 sandstones1 red marls and cornstones of the ,upper ,old ,red ,sandstone4 ,footnote #djj3 ,the ,calciferous ,sandstones are the stratigraphical equivalents of the ,limestone ,shale and lower portion of the ,carboniferous ,limestone of ,england4 ,from this table the gradual geographical evolution of the ,carboniferous period in ,scotland may be gleaned4 ,we observe that at the beginning1 the conditions under which the ,old ,red ,sandstone had been accumulated still in part continued4 ,the great lacustrine basins of the ,lower ,old ,red ,sandstone had indeed been effaced1 and their sites were occupied by comparatively shallow areas of fresh or brackish water in which the ,upper ,old ,red ,sandstone was laid down4 ,their conglomerates and sandstones had been uplifted and fractured4 ,their vast ranges of volcanic material1 after being deeply buried under sediment1 had been once more laid bare1 and extended as ridges of land1 separating the pools and lagoons which they supplied with sand and silt4 ,this singular topography had not been entirely effaced at the beginning of the ,carboniferous period1 for we find that many of the ridges which bounded the basins of the ,upper ,old ,red ,sandstone remained as land until they sank beneath the waters in which the earliest ,carboniferous strata accumulated4 ,thus1 while no trace of an unconformability has yet been detected at the top of the ,upper ,old ,red ,sandstone1 there is often a strong overlap of the succeeding deposits4 ,at the south end of the ,pentland ,hills1 for example1 the ,upper ,old ,red ,sandstone attains a thickness of #ajjj feet1 but only three miles further south it entirely disappears1 together with all the overlying mass of ,calciferous ,sandstones1 and the ,carboniferous ,limestone then rests directly on the ,lower ,old ,red ,sandstone4 ,again1 at the north end of the same chain the upper division of the ,old ,red ,sandstone dies out against the lower1 which is eventually overlapped by the ,calciferous ,sandstones4 ,the change from the physical conditions of the ,scottish ,old ,red ,sandstone to those of the ,carboniferous system was no doubt gradual and slow4 ,the peculiar red sandy sediment continued to be laid down in basins that were apparently being gradually widened by access of water from the open sea4 ,yet it would seem that in ,scotland these basins still for a long time continued saline or1 from some other cause1 unfavourable to life2 for the red1 blue and green shales or marls1 and occasional impure limestones or cement-stones and gypseous layers1 which were deposited in them1 are in general unfossiliferous1 though drifted plants from the neighbouring land are here and there common enough4 ,the sediments of these early ,carboniferous waters are met with all over the southern half of ,scotland1 but in very unequal development1 and constitute what is known as the ,cement-stone ,group4 ,it was while these strata were in course of deposition that the earliest ,carboniferous volcanoes broke into eruption4 ,in some localities a thickness of several hundred feet of the ,cement-stone group underlies the lowest lavas4 ,in other places the lavas occur in and rest on the ,upper ,old ,red ,sandstone and have the ,cement-stone group wholly above them2 while in yet other districts the volcanic rocks seem entirely to take the place of that group4 ,so vigorous was the earliest display of volcanic action in ,carboniferous times that from the borders of ,northumberland to the uplands of ,galloway1 and from the slopes of the ,lammermuirs to ,stirlingshire and thence across the estuary of the ,clyde to ,cantyre1 innumerable vents were opened and large bodies of lava and ashes were ejected4 ,the ,cement-stone group1 save where succeeded by volcanic intercalations1 passes up conformably into the lowest crinoidal limestones of the ,carboniferous ,limestone series4 ,in the basin of the ,firth of ,forth1 however1 the cement-stones1 feebly represented there1 are overlain by a remarkable assemblage of white sandstones1 black carbonaceous shales1 or oil-shales1 cyprid limestones1 occasional marine limestones and thin seams of coal1 the whole having a thickness of more than #cjjj feet4 ,these strata1 unlike the typical ,cement-stone group1 abound in fossils both vegetable and animal4 ,they prove that1 over the area of the ,forth1 the insalubrious basins wherein the red and green sediments of the ,cement-stone group were laid down1 gave place to opener and clearer water with occasional access of the sea4 ,the peculiar lagoon-conditions which favoured the formation of coal were thus developed in ,central ,scotland earlier than elsewhere in ,britain4 ,we shall see in later pages that these conditions were accompanied by a fresh outbreak of volcanic activity1 in a phase less vigorous but more enduring and extensive than that of the first ,carboniferous eruptions4 ,the ,carboniferous ,limestone sea over the site of the southern half of ,scotland appears never to have reached the depth which it attained in ,england and ,ireland4 ,to the north of it lay the land from which large quantities of sand and mud were carried into it1 as shown by the deep accumulations of sandstone and shale1 which far surpass in thickness the few comparatively thin marine limestones intercalated in them4 ,there is thus a striking contrast between the thick masses of limestone in central and south-western ,england and their dwindled representatives in the north4 ,another marked difference between the ,scottish and ,english developments of this formation is to be noticed in the abundant proof that the comparatively shallow waters of the northern basin were plentifully dotted over with active volcanoes4 ,the eruptions were especially vigorous and prolonged in the basin of the ,firth of ,forth4 ,they continued at intervals1 even after the peculiar geographical conditions of the ,carboniferous ,limestone had ceased4 ,but they had died out by the time of the beginning of the ,coal-measures4 ,owing to the number and variety of the natural sections1 the ,carboniferous volcanic rocks of ,scotland have been the subject of numerous observations and descriptions1 from the early days of geology down to the present time4 ,the mere enumeration of the titles of the various publications regarding them would make a long list4 ,these rocks formed the subject of some of ,hutton's early observations1 and furnished him with facts from which he established the igneous origin of whinstone4#dja ,they supplied ,playfair with numerous apt illustrations in support of ,hutton's views1 and he seems to have made himself thoroughly familiar with them4#djb ,in the hands of ,sir ,james ,hall they became the groundwork of those remarkable experiments on the fusion of whinstone which may be said to have laid the foundation of experimental geology4#djc ,in the controversies of the ,neptunian and ,plutonian schools these rocks were frequently appealed to by each side in confirmation of its dogmas4 ,the appointment in #ahjd of ,jameson to the ,chair of ,natural ,history in the ,edinburgh ,university gave increased impetus to the study of the igneous rocks of ,scotland4 ,though he did not himself publish much regarding them1 we know that he was constantly in the habit of conducting his class to the hills1 ravines and quarries around ,edinburgh1 and that the views which he taught were imbibed and extended by his pupils4#djd ,among the early writers the names of ,allan1#dje ,townson1#djf ,lord ,greenock1#djg and ,ami ,bou1#djh deserve especial mention4 ,footnote #dja3 ,hutton's ,theory of the ,earth1 vol4 i4 p4 #aee et seq4 ,footnote #djb3 ,playfair's ,illustrations of the ,huttonian ,theory1 #bee et seq4 ,footnote #djc3 ,trans4 ,roy4 ,soc4 ,edin4 7#ahje71 vol4 v4 p4 #dc4 ,footnote #djd3 ,mem4 ,wern4 ,soc4 ii4 #agh1 #fah2 iii4 #be2 ,edin4 ,phil4 ,journ4 i4 #ach1 #ceb2 xv4 #chf4 ,footnote #dje3 ,trans4 ,roy4 ,soc4 ,edin4 7#ahaa71 vi4 p4 #dje4 ,footnote #djf3 ,tracts and ,observations in ,natural ,history and ,physiology1 #hvo1 ,lond4 #agii4 ,footnote #djg3 ,trans4 ,roy4 ,soc4 ,edin4 7#ahcc71 xiii4 pp4 #ci1 #ajg4 ,footnote #djh3 ,essai gologique sur l'cosse4 ,paris2 no date1 probably #ahbj4 ,the first broad general sketch of the ,carboniferous igneous rocks of a large district of the country was that given by ,hay ,cunningham in his valuable essay on the geology of the ,lothians4#dji ,he separated them into two series1 the ,felspathic1 including porphyry and clinkstone1 and the ,augitic or ,trap rocks4 ,to these he added ,trap-tufa1 which he considered to be identical in origin with modern volcanic tuff4 ,it was the eruptive character of the igneous rocks on which he specially dwelt1 showing by numerous sections the effects which the protrusion of the molten masses have had upon the surrounding rocks4 ,he did not attempt to separate the intrusive from the interstratified sheets1 nor to form a chronological arrangement of the whole4 ,footnote #dji3 ,mem4 ,wern4 ,soc4 vii4 p4 #a4 ,published separately1 #ahch4 ,still more important was the sketch given by ,maclaren1 in his classic ,geology of ,fife and the ,lothians4#daj ,this author clearly recognized that many of the igneous rocks were thrown out contemporaneously with the strata among which they now lie4 ,he constantly sought for analogies among modern volcanic phenomena1 and presented the ,carboniferous igneous rocks of the ,lothians not as so many petrographical varieties1 but as monuments of different phases of volcanic action previous to the formation of the ,coal-measures4 ,his detailed descriptions of ,arthur ,seat and the rocks immediately around ,edinburgh1 which alone the work was originally intended to embrace1 may be cited as models of exact and luminous research4 ,the portions referring to the rest of the basin of the ,forth did not profess to be more than a mere sketch of the subject4 ,footnote #daj3 ,small #hvo1 ,edin4 #ahch1 first partly published as articles in the ,scotsman newspaper4 ,a second edition1 which was little more than a reprint of the first1 appeared in #ahff4 ,various papers of more local interest1 to some of which allusion will be made in the sequel1 appeared during the next quarter of a century4 ,but no systematic study of the volcanic phenomena of any part of ,scotland was resumed until the extension in #ahed of the ,geological ,survey to the north of the ,tweed by ,a4 ,c4 ,ramsay4 ,the volcanic rocks of the ,lothians and ,fife were mapped by ,mr4 ,h4 ,h4 ,howell and myself4 ,the maps of that district began to be published in the year #ahei1 and the ,memoirs two years later4 ,in #ahfa1 in a chronological grouping of the whole of the volcanic phenomena of ,scotland1 ,i gave an outline of the ,carboniferous eruptions4#daa ,by degrees the detailed mapping of the ,geological ,survey was pushed across the whole of the rest of the south of ,scotland1 and the ,carboniferous volcanic rocks of each area were then for the first time carefully traced and assigned to their various stratigraphical horizons4 ,in the following pages reference will be given to the more important features of the ,survey maps and ,memoirs4 ,in the year #ahgi1 availing myself of the large amount of information which my own traverses and the work of the ,survey had enabled me to acquire1 ,i published a ,memoir on the geology and petrography of the volcanic rocks of the basin of the ,firth of ,forth2#dab and lastly1 in my ,presidential ,address to the ,geological ,society in #ahib1 ,i gave a summary of all that had then been ascertained on the subject of the volcanic rocks of ,carboniferous time in the ,british ,isles4#dac ,footnote #daa3 ,trans4 ,roy4 ,soc4 ,edin4 vol4 xxii4 ,footnote #dab3 ,ibid4 vol4 xxix4 7#ahgi71 p4 #dcg4 ,footnote #dac3 ,quart4 ,journ4 ,geol4 ,soc4 xlviii4 7#ahib71 p4 #ajd4 ,this summary1 with additional details and illustrations1 is embodied in the text4 ,two well-marked types of volcanic accumulations are recognizable in the ,british ,isles1 which may be conveniently termed ,plateaux and ,puys4 #a4 ,plateaux4--,in this type1 the volcanic materials were discharged over wide tracts of country1 so that they now form broad tablelands or ranges of hills1 reaching sometimes an extent of many hundreds of square miles and a thickness of more than #ajjj feet4 ,plateaux of this character occur within the ,british area only in ,scotland1 where they are the predominant phase of volcanic intercalations in the ,carboniferous system4 ,it is noteworthy that the ,carboniferous plateaux appeared during a well-marked interval of geological time4 ,the earliest examples of them date from the close of the ,upper ,old ,red ,sandstone4 ,they were all in vigorous eruption during the time of the ,calciferous ,sandstones1 but in no case did they survive into that of the ,hurlet and later limestones4 ,they are thus eminently characteristic of the earliest portion of the ,carboniferous period4 #b4 ,puys4--,in this type1 the ejections were often confined to the discharge of a small amount of fragmentary materials from a single solitary vent1 and even where the vents were more numerous and the outpourings of lava and showers of ash more copious1 the ejected material usually covered only a small area round the centres of eruption4 ,occasionally streams of basic lava and accumulations of tuff were piled up into long ridges4 ,volcanoes of this character were specially abundant in the basin of the ,firth of ,forth1 and more sparingly in ,ayrshire and ,roxburghshire4 ,they form the persistent type throughout the rest of the ,british ,isles4 ,the ,puys also occupy a well-defined stratigraphical position4 ,they did not begin until some of the volcanic plateaux had become extinct4 ,from the top of the ,cement-stone group up into the ,carboniferous ,limestone series1 their lavas and tuffs are met with on many platforms1 but none occur above that series save in ,ayrshire1 where some of the eruptions appear to have been as late as about the beginning of the ,coal-measures4 ,arranged in tabular form the stratigraphical and geographical distribution of the two great volcanic types of the ,carboniferous system in ,scotland will be more easily followed4 ,i have therefore drawn up the accompanying scheme3-- ,location ,key3 ,a4 ,ayrshire and ,renfrewshire4 ,b4 ,stirlingshire4 ,c4 ,west ,lothian4 ,d4 ,midlothian4 ,e4 ,east ,lothian4 ,f4 ,fife4 ,g4 ,berwick ,roxburghshire4 ---------------------------------------- ------------------------------ ,plateau-type4 ,puy-type4 ---------------------------------------- ------------------- ,a4,b4,d4,e4,g4,a4,b4,c4,d4,e4,f4,g4 ,coal ,measures 444444444444444444444444 ,millstone ,grit 4444444444 444444444444 ,carboniferous ,limestone ,series4 ,castlecary ,limestone 4444444444 444444444444 ,calmy 4444444444 444444444444 ,index 4444444444 44 4444 44 ,hurlet 44444444444444 4444 ,calciferous ,sandstone ,series4 ,burdiehouse ,limestone and ,oil-shale ,group 44 444444 4444 ,cement-stone ,group 44444444 4444 44 ,red ,sandstones passing down 44444444 44444444444444 into ,upper ,old ,red ,sandstone44444444 44444444444444 ---------------------------------------- ------------------- ,,chapter ,,xxiv ,,carboniferous ,,volcanic ,,plateaux ,,of ,,scotland ,i4 ,the ,plateau-type restricted to ,scotland--i4 ,distribution in the ,different ,areas of ,eruption--ii4 ,nature of the ,materials erupted4 ,in the division of the ,plateaux ,i group all the more copious eruptions during the ,carboniferous period1 when the fragmentary materials generally formed but a small part of the discharges1 but when the lavas were poured out so abundantly and frequently as to form lava-fields sometimes more than #bjjj square miles in area1 and to build up piles of volcanic material sometimes upwards of #cjjj feet in thickness4 ,as already remarked1 this phase of volcanic action1 especially characteristic of the earlier part of the ,carboniferous period across the south of ,scotland1 but not found elsewhere in the same system in ,britain1 preceded the type of the ,puys4 ,its eruptions extended from about the close of the ,old ,red ,sandstone period through that section of ,carboniferous time which was marked by the deposition of the ,calciferous ,sandstones1 but they entirely ceased before the accumulation of the ,main or ,hurlet ,limestone1 at the base of the ,carboniferous ,limestone ,series of ,scotland4 ,its stratigraphical limits1 however1 are not everywhere the same4 ,in the eastern part of the region1 the lavas appear to be intercalated with1 and certainly lie directly upon1 the ,upper ,old ,red ,sandstone containing scales of ,bothriolepis and other characteristic fishes1 and they are covered by the ,cement-stone group of the ,calciferous ,sandstones4 ,in the western district a considerable thickness of ,carboniferous strata sometimes underlies the volcanic sheets4 ,on the other hand1 the type of the ,puys1 although it appeared in ,fife1 ,linlithgowshire and ,midlothian during the time of the ,calciferous ,sandstones1 attained its chief development during that of the ,carboniferous ,limestone1 and did not finally die out in ,ayrshire until the beginning of the deposition of the ,coal-measures4 i4 ,,distribution ,,of ,,the ,,plateaux ,notwithstanding the effects of many powerful faults and extensive denudation1 the general position of the ,plateaux and their independence of each other can still be traced4 ,they are entirely confined1 as ,i have said1 to the southern half of ,scotland 7see ,map ,,iv474 ,in noting their situations we are once more brought face to face with the remarkable fact1 so strikingly manifested in the geological history of ,britain1 that volcanic action has been apt to recur again and again in or near to the same areas4 ,the ,carboniferous volcanic plateaux were poured out from vents1 some of which not impossibly rose among the extinct vents of the ,old ,red ,sandstone4 ,another fact1 to which also ,i have already alluded as partially recognizable in the records of ,old ,red ,sandstone volcanism1 now becomes increasingly evident--the tendency of volcanic vents to be opened along lines of valley rather than over tracts of hill4 ,the vents that supplied the materials of the largest of the ,carboniferous volcanic plateaux broke forth1 like the ,old ,red ,sandstone volcanoes1 along the broad ,midland ,valley of ,scotland1 between the ridge of the ,highlands on the north and that of the ,southern ,uplands on the south4 ,others appeared in the long hollow between the southern side of these uplands1 and the ,cheviot ,hills and hills of the ,lake ,district4 ,it is not a question of the rise of volcanic vents merely along lines of fault1 but over broad tracts of low ground rather than on the surrounding or neighbouring heights4 ,it can easily be shown that this distribution is not the result of better preservation in the valleys and greater denudation from the higher grounds1 for1 as has been already remarked in regard to the volcanoes of the ,old ,red ,sandstone1 these higher grounds are singularly free from traces of necks which1 had any vents ever existed there1 would certainly have remained as memorials of them4 ,the following summary of the position and extent of the ,plateaux will afford some idea of their general characters3-- ,illustration3 ,fig4 #ajg4--,view of the escarpment of the ,clyde ,plateau in the ,little ,cumbrae1 from the south-west4 #a4 ,the ,clyde ,plateau4--,the chief plateau rises into one of the most conspicuous features in the scenery of ,central ,scotland4 ,beginning at ,stirling1 it forms the tableland of the ,fintry1 ,kilsyth1 ,campsie and ,kilpatrick ,hills1 stretching westwards to the ,clyde near ,dumbarton4 ,it rises again on the south side of that river1 sweeping southwards into the hilly moorlands which range from ,greenock to ,ardrossan1 and spreading eastwards along the high watershed between ,renfrewshire1 ,ayrshire1 and ,lanarkshire to ,galston and ,strathavon4 ,but it is not confined to the mainland1 for its prolongation can be traced down the broad expanse of the ,firth of ,clyde by the islands of ,cumbrae to the southern end of ,bute1 and thence by the east of ,arran to ,campbeltown in ,cantyre4 ,its visible remnants thus extend for more than #ajj miles from north-east to south-west1 with a width of some thirty-five miles in the broadest part4 ,we shall probably not exaggerate if we estimate the original extent of this great volcanic area as not less than between #bjjj and #cjjj square miles4 ,it is in this tract that the phenomena of the plateaux are most admirably displayed4 ,ranges of lofty escarpments reveal the succession of the several eruptions1 and the lower ground in front of these escarpments presents to us1 as the result of stupendous denudation1 many of the vents from which the materials of the plateau were ejected1 while in the western portion of the area admirable coast-sections lay bare to view the minutest details of structure4#dad ,footnote #dad3 ,this plateau is represented in ,sheets #ab1 #ba1 #bi1 #cj1 #ca and #ci of the ,geological ,survey1 and is described in the accompanying ,memoirs as far as published4 ,the eastern part of the ,campsie ,hills was surveyed by ,mr4 ,b4 ,n4 ,peach1 the western part by ,mr4 ,r4 ,l4 ,jack1 who also mapped the rest of the plateau to the ,clyde1 and a portion of the high ground of ,renfrewshire and ,ayrshire2 the rest of the area1 south to ,ardrossan1 was surveyed by myself4 ,the tract from ,stewarton to ,strathavon was surveyed by ,mr4 ,james ,geikie1 the ,cumbraes and ,bute by ,mr4 ,w4 ,gunn1 and southern ,cantyre by ,mr4 ,r4 ,g4 ,symes4 ,the ,campsie ,hills have been partly described by ,mr4 ,john ,young in the first volume of the ,transactions of the ,glasgow ,geological ,society4 ,the occurrence of plants in the tuffs of the east coast of ,arran was discovered by ,mr4 ,e4 ,wunsch4 ,the ,campbeltown igneous rocks were described by ,j4 ,nicol1 ,quart4 ,journ4 ,geol4 ,soc4 viii4 7#aheb71 p4 #djf4 ,see also ,j4 ,bryce's ,arran and ,clydesdale4 ,it will be seen from the map 7,no4 ,,iv471 that the ,clyde plateau extends in a general north-east and south-west direction4 ,it is inclined on the whole towards the east1 where1 when not interrupted by faults1 its highest lavas and tuffs may be seen to pass under the ,carboniferous ,limestone series4 ,its greatest elevations are thus towards its escarpment1 which1 commencing above the plains of the ,forth a little to the west of ,stirling1 extends as a striking feature to the ,clyde above ,dumbarton4 ,on the south side of the great estuary the escarpment again stretches in a noble range of terraced slopes for many miles into ,ayrshire4 ,it is well developed in the ,little ,cumbrae ,island 7,fig4 #ajg71 and in the south of ,bute1 where its successive platforms of lava mount in terraces and green slopes above the ,firth4 ,even as far as the southern coast of ,cantyre the characteristic plateau scenery reappears in the outliers which there cap the hills and descend the slopes 7,fig4 #ajh74 ,while the escarpment side of this plateau is comparatively unfaulted1 so that the order of succession of the lavas and their superposition in the sedimentary rocks can be distinctly seen1 the eastern or dip side is almost everywhere dislocated4 ,innumerable local ruptures have taken place1 allowing the limestone series to subside1 and giving to the margin of the volcanic area a remarkably notched appearance4 ,to the effects of this faulting may be attributed the way in which the plateau has been separated into detached blocks with intervening younger strata4 ,thus a complex series of dislocations brings in a long strip of ,carboniferous ,limestone which extends from ,johnston to ,ardrossan1 while another series lets in the limestone that runs from ,barrhead to near ,dalry4 ,in each of these instances1 the continuity of the volcanic plateau is interrupted4 ,to the same cause we owe the occasional reappearance of a portion of the plateau beyond the limits of the main mass1 as for instance in the detached area which occurs in the valley of the ,garnock above ,kilwinning4 ,illustration3 ,fig4 #ajh4--,view of the edge of the ,volcanic ,plateau south of ,campbeltown1 ,argyllshire4 ,the uppermost of the three zones is the volcanic series with its lava-ridges4 ,the central band is the ,upper ,old ,red ,sandstone1 lying conformably beneath the lavas1 with its cornstone which has been quarried4 ,the lowest band1 tinted dark1 is the ,lower ,old ,red ,sandstone1 on which the other rocks rest unconformably4 ,denudation has likewise come into play1 not only in reducing the area of the plateau1 but in isolating portions of it into outliers1 with or without the assistance of faults4 ,the site of the ,cumbraes and ,bute was no doubt at one time covered with a continuous sheet of volcanic material1 and there appears to be no reason for refusing to believe that this sheet formed part of that which caps the opposite uplands of ,ayrshire4 ,from the southern end of ,bute it is only about seven miles across to the shore of ,arran near ,corrie1 where the lavas and tuffs reappear4 ,they are so poorly represented there1 however1 that we are evidently not far from the limit of the plateau in that direction4 ,so vast has been the denudation of the region that it is now impossible to determine whether the volcanic ejections of ,campbeltown1 which occupy the same geological platform as those of ,arran1 ,bute and ,ayrshire1 were also actually continuous with them4 ,but as the distance between the denuded fragments of the volcanic series in ,arran and in ,cantyre is only about #bj miles it is not improbable that this continuity existed1 and thus that the volcanic accumulations reached at least as far as the southern end of ,argyllshire1 where they now slip under the sea4 ,illustration3 ,fig4 #aji4--,view of ,north ,berwick ,law from the east1 a trachyte neck marking one of the chief vents of the ,garleton ,plateau4 7,from a photograph47 ,this illustration and ,figs4 #aai1 #acc and #ace are from photographs taken by ,mr4 ,robert ,lunn for the ,geological ,survey4 #b4 ,the ,east ,lothian or ,garleton ,plateau4--,some #ej miles to the east of the ,clyde volcanic district1 and entirely independent of it1 lies the plateau of the ,garleton ,hills in ,east ,lothian1 which1 as its limits towards the east and north have been reduced by denudation1 and towards the west are hidden under the ,carboniferous ,limestone series of ,haddington1 covers now an area of not more than about #fj square miles4#dae ,that the eruptions from this area did not extend far to the north is shown by the absence of all trace of them among the ,lower ,carboniferous rocks of ,fife4 ,a relic of them occurs above ,borthwick1 in ,midlothian1 about twelve miles to the south-west of the nearest margin of the plateau4 ,the area over which the lavas and tuffs were discharged may not have exceeded #aej square miles4 ,small though this plateau is1 it possesses much interest from the remarkable variety of petrographical character in its lavas1 from the size and composition of its necks1 and from the picturesque coast-line where its details have been admirably dissected by the waves4 ,in many respects it stands by itself as an exception to the general type of the other plateaux4 ,footnote #dae3 ,this plateau is represented in ,sheets #cc and #da of the ,geological ,survey of ,scotland1 and is described in the ,explanation to accompany ,sheet #cc4 ,illustration3 ,fig4 #aaj4--,the ,bass ,rock1 a trachytic neck belonging to the ,garleton plateau1 from the shore at ,canty ,bay4 ,from its proximity to ,edinburgh this volcanic area has been often studied and described4 ,the memoirs of ,hay ,cunningham and ,maclaren gave the fullest account of it until its structure was mapped by the ,geological ,survey4 ,its scenery differs from that of the other plateaux chiefly in the absence of the terraced contour which in them is so characteristic4 ,the peculiar lavas of the ,garleton ,hills form irregularly-uneven ground1 rising to not more than #fjj feet above the sea4 ,they slope gradually down to the coast1 where a succession of fine sections of the volcanic series has been laid bare for a distance of altogether about ten miles4 ,nowhere1 indeed1 can the phenomena of the plateau-tuffs and their association with the ,carboniferous strata be so well studied as along the coast-line from ,north ,berwick to ,dunbar4 ,among the necks of this plateau distinguished for their size1 conspicuous prominence and component materials1 the most important are those that form the conical eminences of ,north ,berwick ,law 7,fig4 #aji71 ,traprain ,law 7,fig4 #acc71 and the ,bass ,rock 7,fig4 #aaj74 #c4 ,the ,midlothian ,plateau4--,on the same general stratigraphical horizon as the other volcanic plateaux1 a narrow band of lavas and tuffs can be followed from the eastern outskirts of the city of ,edinburgh into ,lanarkshire1 a distance of about #bc miles4 ,it is not continuously visible1 often disappearing altogether1 and varying much in thickness and composition4 ,this volcanic tract1 which may be conveniently termed the ,midlothian ,plateau1 is the smallest and most fragmentary of all the series4 ,its most easterly outliers form ,arthur ,seat and ,calton ,hill at ,edinburgh4#daf ,three miles to the south-west a third detached portion is known as ,craiglockhart ,hill4 ,after another interval of ten miles1 the largest remaining fragment forms the prominent ridge of ,corston ,hill 7,fig4 #aaa71 whence a discontinuous narrow strip may be traced nearly as far as the ,river ,clyde4 ,footnote #daf3 ,i formerly classed these eminences with the ,puys1 but ,i am now of opinion that they ought rather to be regarded as fragments of a long and somewhat narrow plateau4 ,their basic lavas and overlying sheets of porphyrite repeat the usual sequence of the plateaux1 which is not met with among the ,puys4 ,but1 as will be pointed out in the sequel1 ,arthur ,seat in long subsequent time became again the site of a volcanic vent4 ,illustration3 ,fig4 #aaa4--;,corston ,hill--a fragment of the ,midlothian ,plateau1 seen from the north4 ,the volcanic rocks form a cake on the top1 the slopes lying across the edges of the ,calciferous ,sandstones4 ,the well-known ,arthur ,seat and ,calton ,hill have been fully described by ,maclaren1 and have been the subject of numerous observations by other geologists4#dag ,they have been likewise mapped in detail on a large scale by the ,geological ,survey1 and have been described in the ,survey ,memoirs4 ,the rest of the plateau to the south-west is much less familiar4 ,footnote #dag3 ,maclaren's ,geology of ,fife and the ,lothians1 #ahci1 pp4 #a-fg2 and ,hay ,cunningham1 ,mem4 ,wer4 ,soc4 vii4 pp4 #ea-fb4 ,the plateau is represented in ,sheets #bd and #cb of the ,geological ,survey1 and ,arthur ,seat and ,calton ,hill will be found on ,sheet #b of the ,geological ,survey map of ,edinburghshire on the scale of #f inches to a mile4 ,in ,fig4 #aab the great escarpment which descends from the right towards the centre is the sill of ,salisbury ,crags4 ,the long dark crag 7,long ,row7 rising between the two valleys is the lowest of the interstratified lavas4 ,the slope that rises above it has been cut out of well-bedded tuffs1 on which lie the basalts and andesites in successive sheets that form all the eastern or left side of the hill4 ,the rocks around the summit belong to a much later period of volcanic eruption1 and are referred to in ,chapter xxxi4 ,illustration3 ,fig4 #aab4--,view of ,arthur ,seat from ,calton ,hill to the north4 ,the rocks of this plateau are comparatively limited in thickness1 and have a much more restricted vertical range than those of other districts4 ,at ,arthur ,seat and ,corston ,hill they begin above the cement-stones and cease in a low part of the great group of white sandstones and dark shales which form the upper half of the ,calciferous ,sandstones of ,midlothian4 ,they do not ascend as high as the ,burdiehouse ,limestone1 which to the west of ,corston ,hill is seen to come on above them4 ,one of their most remarkable features is the manner in which they diminish to a single thin bed and then die out altogether1 reappearing again in a similar attenuated form on the same horizon4 ,this impersistence is well seen in the south-western part of the area1 between ,buteland1 in the parish of ,currie1 and ,crosswood1 in the parish of ,mid-,calder4 ,the lowest more basic band may there be traced at intervals for many miles without the overlying andesitic group4 ,yet that andesites followed the basalts1 as in other plateaux1 is well shown by large remnants of these less basic lavas left in ,arthur ,seat and ,calton ,hill4 ,on the extreme southern margin of the area also a thin band of porphyrite with a group of overlying tuffs is seen above the red sandstones near ,dunsyre4#dah ,the eruptions over the site of this plateau seem to have been much more local and limited than in the other plateaux4 ,they appear to have gathered chiefly around two centres of activity1 one of which lay about the position of ,edinburgh1 the other in the neighbourhood of ,corston ,hill4 ,it is worthy of remark that this tract of volcanic material flanks the much older range of lavas and tuffs of the ,pentland ,hills and wraps round the south-western end of this range1 thus furnishing another illustration of the renewal of volcanic activity in the same region during successive geological periods4 ,footnote #dah3 ,explanation1 ,geol4 ,surv4 ,scotland1 ,sheet #bd1 p4 #ac 7#ahfi74 #d4 ,the ,berwickshire ,plateau4--,another and entirely disconnected area occurs in the broad plain or ,merse of the lower portion of the valley of the ,tweed4#dai ,the northern limit of its volcanic tuff occurs in the ,river ,whitadder above ,duns1 whence the erupted materials rapidly widen and thicken towards the south-west by ,stitchell and ,kelso1 until they die out against the flanks of the ,cheviot ,hills4 ,the eastern extension of the area is lost beneath the ,cement-stone group which covers the ,merse down to the sea4 ,its western boundary must once have reached far beyond its present limits1 for the low ,silurian ground in that direction is dotted over with scattered vents to a distance of ten miles or more from the present outcrop of the bedded lavas1 extensive denudation having cleared away the erupted materials and exposed the volcanic pipes over many square miles of country4 ,among the more prominent of these old vents are the ,eildon ,hills1 ,minto ,crags and ,rubers ,law1 as well as many other eminences familiar in ,border story4 ,footnote #dai3 ,this plateau is shown on ,sheets #ag1 #be1 #bf and #cc of the ,geological ,survey ,map of ,scotland4 ,it was chiefly mapped by ,prof4 ,james ,geikie and ,mr4 ,b4 ,n4 ,peach4 ,the bedded volcanic rocks of this area form a marked feature in the topography and geology of the district4 ,they rise above the plain of the ,merse as a band of undulating hills1 of which the eminence crowned by ,hume ,castle1 about #fjj feet above the sea1 is the most conspicuous height4 ,in the geological structure of this part of ,scotland they are mainly interposed between the ,upper ,old ,red ,sandstone and the base of the ,carboniferous system1 which they thus serve to divide from each other4 ,but their lowest sheets appear to be in some places intercalated in the ,old ,red ,sandstone1 so that their eruption probably began before the beginning of the ,carboniferous period4 ,they form a band that curves round the end of the great ,carboniferous trough at ,kelso and skirts the northern edge of the andesites of the ,lower ,old ,red ,sandstone in the ,cheviot ,hills4 #e4 ,the ,solway ,plateau4--,the last plateau1 that of the ,solway basin1 though its present visible eastern limits approach those reached by the lavas from the ,berwickshire area1 was quite distinct1 and had its chief vents at some distance towards the south-west4#dbj ,on the north-western flanks of the ,cheviot ,hills1 the ,upper ,old ,red ,sandstone is overlain by the lowest ,carboniferous strata1 without the intercalation of any volcanic zone1 so that there must have been some intermediate ground that escaped being flooded with lava from the vents of the ,merse on the one hand1 and of the ,solway on the other4 ,the ,solway lavas form a much thinner group than those of ,berwickshire4 ,from the wild moorland between the sources of the ,liddell and the ,rule ,water1 they run in a narrow and much-faulted band south-westward across ,eskdale and the foot of ,annandale1 and are traceable in occasional patches on the farther side of the ,nith along the southern flanks of ,criffel1 even as far as ,torrorie on the coast of ,kirkcudbright--a total distance of about #de miles4 ,it is probable that this long outcrop presents merely the northern edge of a volcanic platform which is mainly buried under the ,carboniferous rocks of the ,solway basin4 ,yet it exhibits many of the chief characters of the other plateaux1 and even occasionally rivals them in the dignity of the escarpments which mark its progress through the lonely uplands between the head of ,liddesdale and the ,ewes ,water 7,figs4 #aac1 #adb74 ,footnote #dbj3 ,for a delineation of the distribution and structure of this plateau see ,sheets #e1 #f1 #aj1 #aa and #ag of the ,geological ,survey of ,scotland4 ,in the upper part of ,liddesdale1 ,ewesdale and ,tarras it was mapped by ,mr4 ,b4 ,n4 ,peach2 in lower ,liddesdale and ,eskdale by ,mr4 ,r4 ,l4 ,jack and ,mr4 ,j4 ,s4 ,grant ,wilson2 from ,langholm to the ,annan by ,mr4 ,h4 ,skae2 and in ,kirkcudbright by ,mr4 ,john ,horne4 ,illustration3 ,fig4 #aac4--,view of ,arkleton ,fell1 part of the ,solway ,plateau1 from the south-west4 ,the lower slopes below the single bird1 round to the left side of the sketch1 are on the ,upper ,old ,red ,sandstone2 the line of crag below the two birds marks the volcanic group above which lies an outlier of the ,calciferous ,sandstone series1 forming the upper part of the hill 7three birds74 ,the knobs under the four birds are bosses of andesite4 ,the plateaux of the ,merse and the ,solway illustrate in a striking manner the distribution of the volcanic eruptions along valleys and low plains4 ,the vents from which the lavas and tuffs proceeded are chiefly to be found on the lower grounds1 though these bedded volcanic rocks rise to a height of #agab feet 7the ,pikes7 to the west of the ,cheviot ,hills4 ,between the ,silurian uplands of ,selkirkshire and ,berwickshire on the north and the ridge of the ,cheviot ,hills on the south1 the broad plain was dotted with volcanic vents and flooded with lava1 while to the south-west the corresponding hollow between the uplands of ,dumfries and ,galloway on the one side1 and those of ,cumberland on the other1 was similarly overspread4 ,the significance of these facts will be more apparent when the grouping of the vents has been described4 ,we shall then also be better able to realize the validity of the inference that the present plateaux are mere fragments of what they originally were1 wide areas having been removed from the one side of them by denudation1 and having been concealed on the other under later portions of the ,carboniferous system4 ,the same two plateaux likewise supply further illustrations of the outflow of similar volcanic materials in the same locality at widely separated intervals of time4 ,they may be traced up to and round the margin of the great pile of andesites of ,lower ,old ,red ,sandstone age forming the ,cheviot ,hills4 ii4 ,,nature ,,of ,,the ,,materials ,,erupted ,the volcanic materials characteristic of the plateau-type of eruptions consist mainly of lavas in successive sheets1 but include also various tuffs in frequent thin courses1 and less commonly in thick local accumulations4 ,the lavas are chiefly andesites in the altered condition of porphyrites4 ,they vary a good deal in the relative proportions of silica4 ,some of them are decidedly basic and take the form of dolerites and olivine-basalts4 ,with these rocks are occasionally associated ultra-basic varieties1 where the felspar almost disappears and the material consists mainly of ferro-magnesian minerals4 ,the more basic rocks are generally found towards the bottom of the volcanic series1 where they appear as the oldest flows4 ,in the ,garleton ,hills lavas of a much more acid nature are met with--true sanidine-trachytes1 which overlie the porphyrites and basalts of the earlier eruptions4 ,no adequate investigation has yet been made of the chemical and microscopic characters of these various rocks1 regarded as a great volcanic series belonging to a definite geological age1 though many of the individual rocks and the petrography of different districts have been more or less fully described4 ,i cannot here enter into much detail on the subject1 but must content myself with such a summary as will convey some idea of the general composition and structure of this very interesting volcanic series4 7a7 ,augite-olivine ,rocks 7,picrites and ,limburgites74--,towards the bottom of the plateaux there are found here and there sheets of ultra-basic material1 some of which appear to be bedded with the other rocks and to have flowed out as surface-lavas1 though it may be impossible to prove that they are not sills4 ,thus at ,whitelaw ,hill1 on the south side of the ,garleton ,hills1 a dark heavy rock is found to contain hardly any felspar1 but to be made up mainly of olivine and augite4 ,dr4 ,hatch has published a description and drawing of this rock1 together with the following analysis by ,mr4 ,player3#dba-- ,silica #djb ,titanic oxide #bi ,alumina #abh ,ferric oxide #dj ,ferrous oxide #ajd ,lime #ajd ,magnesia #aai ,potash #jh ,soda #bg ,loss by ignition #cd ---- ,spec4 grav4 #cjc4 #iie ,footnote #dba3 ,trans4 ,roy4 ,soc4 ,edin4 vol4 xxxvii4 7#ahic71 p4 #aaf4 7b7 ,dolerites and ,basalts4#dbb--,these rocks are found both as interstratified lavas and as intrusive masses4 ,in the former condition they take a conspicuous place among the sheets of the plateaux1 but especially in the lower parts of the series4 ,they are dark1 often black1 usually more or less porphyritic1 with large felspars1 frequently also large crystals of augite or olivine1 and may be described as porphyritic olivine-dolerites and olivine-basalts1 more rarely as olivine-free dolerites and basalts4 ,their groundmass consists of short laths or microlites of felspar 7probably labradorite7 and granules or small crystals of augite and magnetite1 with sometimes a little fibrous brown mica4 ,the large porphyritic felspars are striped 7probably labradorite71 the augites are frequently chloritized1 and the olivines are generally more or less serpentinized4 ,but in some cases all these minerals are as fresh as in a recent basalt4 ,the rocks are sometimes beautifully columnar1 as at ,arthur ,seat4 ,footnote #dbb3 ,a general classification of the whole series of ,scottish ,carboniferous dolerites and basalts1 including both the plateau and puy examples1 will be given in the account of the rocks of the puys in ,chapter ,,xxvi4 7p4 #dah74 ,of these basic lavas conspicuous examples may be seen at ,arthur ,seat1 ,calton ,hill and ,craiglockhart ,hill4 ,the eastern part of ,arthur ,seat1 known as ,whinny ,hill1 furnishes examples of olivine-dolerites of the ,jedburgh type 7p4 #dah74 ,the beautiful basalt of ,craiglockhart with its large porphyritic olivines and augites has afforded a distinct type of ,carboniferous basalt 7p4 #dah74 ,the same type occurs on the ,calton ,hill in the cliff below the gaol4 ,similar basic lavas are especially abundant and remarkable in the ,clyde plateau near ,campbeltown in ,argyllshire1 and at the south end of ,bute and in the ,cumbraes1 where they are associated with an interesting series of dykes and sills4 ,but even where1 as in the ,garleton ,hills1 the lavas are for the most part somewhat acid in composition1 those first poured out1 which form the lowest band1 include some typical olivine-basalts1 of which a characteristic example occurs at ,kippie ,law at the base of the ,garleton plateau 7p4 #dah74 ,it has been described by ,dr4 ,hatch as exhibiting under the microscope porphyritic crystals of felspar and olivine lying in a groundmass composed of lath-shaped felspars1 granular olivine and magnetite1 and microlitic augite4 ,the olivine1 originally the most abundant constituent1 has been converted into a fibrous aggregate of serpentine4 ,all the minerals are more or less idiomorphic1 but especially the augite1 which crowds the groundmass in delicately-shaped prisms1 most of which are terminated at both ends by faces of the hemi-pyramid4 ,the analysis of this rock is given in the accompanying table of analyses of ,garleton basalts4 ,the ,kippie ,law type of basalt was recognized by ,dr4 ,hatch among the ,geological ,survey collections from other districts1 as in the intrusive bosses of ,neides ,law and ,bonchester near ,jedburgh1 and from the ,campsie plateau a mile and a half north of ,lennoxtown4#dbc ,footnote #dbc3 ,trans4 ,roy4 ,soc4 ,edin4 vol4 xxxvii4 7#ahic71 pp4 #aag-aai4 ,at ,hailes ,castle1 in the ,garleton plateau1 the lower basic lavas include another olivine-basalt somewhat more felspathic than that just described1 and at ,markle quarry the rock is still more felspathic and contains the olivine only in small sporadic grains4 ,the composition of these basic rocks of the ,garleton plateau is shown in the subjoined table of analyses by ,mr4 ,j4 ,s4 ,grant ,wilson3-- ---------------------------------------- ------------------ ,kippie ,law1 ,hailes ,castle1 ,markle ,quarry1 specific specific specific gravity #bh gravity #bgf gravity #bg ---------------------------------------- ------------------ ,si,o#b #dfja #dijg #died ,al#b,o#c #aiai #aidc #bbbc ,fe#b,o#c #eia #ajeh #iee ,fe,o #fge #bce #aab ,mn,o #jai #jcb #jjh ,ca,o #hfh #ghg #gai ,mg,o #fha #dcf #bhj ,k#b,o #abj #jih #aha ,na#b,o #cbg #cca #def ,h#b,o #cjg #bbf #bdb ,total #ajajh #ajjec #ajacj ---------------------------------------- ------------------ ,olivine-dolerites are more especially developed in the district around ,jedburgh1 where they form some of the most prominent bosses1 such as ,dunian and ,black ,law4 ,they show a sub-ophitic groundmass1 with inconspicuous porphyritic crystals1 among which those of olivine are more prominent than the felspars 7p4 #dah74 7c7 ,andesites 7,porphyrites74--,these are the most abundant lavas of the plateaux4 ,they occur in every district1 and usually form the main constituents of the pile of volcanic material4 ,they vary in colour from a pale pinkish grey1 through many shades of red1 purple1 brown and yellow1 to sometimes a dark green or nearly black rock4 ,their texture ranges from almost semi-vitreous1 through different degrees of compactness1 to open1 cellular1 slaggy masses4 ,generally through their base porphyritic felspars are abundantly disseminated1 sometimes in large1 flat1 tabular forms1 like those of the ,lower ,old ,red ,sandstone already referred to4 ,the amygdaloidal kernels consist of calcite1 zeolites1 chalcedony or quartz4 ,it is from the amygdaloids on either side of the ,clyde that the fine examples of zeolites have been chiefly obtained for which the south of ,scotland has long been famed4 ,occasionally1 as at the south end of ,bute1 the andesitic lavas display a marked columnar structure4 ,under the microscope these rocks present the usual fine felted aggregate of felspar microlites1 with granules or crystals of magnetite and sometimes pyroxene4 ,the porphyritic felspars1 often large and well defined1 generally contain inclusions of the groundmass4 ,occasionally some of the large porphyritic constituents are augite1 or pseudomorphs after that mineral4 ,the alteration of the rocks has oxidized some of the iron-ore and given rise to the prevalent purplish and reddish tints4 7d7 ,trachytes4--,some of the most remarkable lavas to be found in any of the plateaux are those which constitute a large part of the ,garleton ,hills4 ,they overlie the lower andesite and basalt platform1 which surrounds them as a narrow belt1 while they occupy the central and much the largest part of the area4 ,they have been included among the porphyrites1 but are pale rocks1 generally with a yellowish crust1 presenting when quite fresh a grey1 compact1 felsitic base with large porphyritic crystals of unstriped felspar4 ,a number of specimens selected as illustrative of the different varieties have been analyzed and the results are stated in the subjoined table4#dbd ,the specific gravity of the rocks is about #bf4 ,footnote #dbd3 ,the first two analyses are by ,mr4 ,j4 ,s4 ,grant ,wilson1 the last two by ,mr4 ,a4 ,dick jun41 and that from ,hopetoun ,monument by ,mr4 ,g4 ,barrow4 ,trans4 ,roy4 ,soc4 ,edin4 vol4 xxxvii4 p4 #abb4 ---------------------------------------- --------------------- ,pepper ,kae ,hopetoun ,phantassie ,bangley ,craig ,heughs ,monument ,quarry ---------------------------------------- --------------------- ,si,o#b #fbfa #face #fbej #eiej #ehej ,al#b,o#c #ahag #afhh #ahea #ahbe #baab ,fe#b,o#c #jcb #jda #dci #dha #dfh ,fe,o #dbe #eja #bcd 444 ,mn,o #jba #jbf 444 444 444 ,ca,o #beh #bci #bjj #baj #cgj ,mg,o #jgd #jdd #jfa #jgj #jic ,k#b,o #djb #fab #fca #fcj #ehd ,na#b,o #fdi #ebf #cdd #ejc #cij ,h#b,o #jhj #agj #baj #afj #bjj ,total #ajjai #iihb #iihf #ajjfc #ajjfg ---------------------------------------- --------------------- ,the microscopic characters of these rocks1 as worked out by ,dr4 ,hatch1 show them to be well-marked and wonderfully fresh sanidine-trachytes4 ,some of them are porphyritic1 with large crystals of perfectly unaltered sanidine1 sometimes also oligoclase4 ,small but well-formed crystals of yellowish-green augite1 in addition to the porphyritic felspars1 are imbedded in a fine groundmass composed chiefly of microlites of sanidine1 but with granules of augite and magnetite plentifully interspersed1 and occasionally prisms of apatite4 ,there is a group in which the porphyritic felspars are scarce or absent4 ,in these there is little or no ferro-magnesian constituent4 ,other trachytes1 rather less basic than the augite-bearing varieties here referred to1 occur as bosses in the ,garleton ,hills district1 and are referred to in the following section 7e74#dbe ,footnote #dbe3 ,for fuller petrographical details consult ,dr4 ,hatch's paper above cited4 7e7 ,rocks of the ,necks4--,in the necks connected with the plateaux other types of massive rock are to be found4 ,among these perhaps the most frequent are trachytes1 grey to pink in colour1 but apt to weather yellow1 exceedingly compact1 sparingly porphyritic1 and with a peculiar platy structure and waxy lustre4 ,rocks of this character also appear as sills and dykes4 ,other varieties that occur in similar positions are more basic in composition1 including dark1 coarse1 granular diabases4 ,in the ,jedburgh district the most frequent rocks are beautiful varieties of olivine-dolerite and olivine-basalt1 which form most of the prominent hills of the neighbourhood4 ,these bosses are sometimes associated with agglomerates as at ,rubers ,law4 ,in the ,garleton ,hills district1 some of the necks present another petrographical type which directly connects them with the remarkable lavas of the higher part of that plateau4 ,thus the rock of ,traprain ,law was ascertained by ,dr4 ,hatch to be a true phonolite4 ,in its general platy structure and sonorous ring under the hammer it reminds one of typical phonolites4 ,under the microscope the rock is found to consist mainly of small lath-shaped crystals of sanidine arranged in a marked minute flow-structure1 but with few porphyritic crystals4 ,it contains small crystals and ophitic patches of a light green soda-augite1 with practically no magnesia in it4 ,a small quantity of iron-ore and isolated granules of apatite are also present1 together with patches of nepheline which1 though generally decomposed and replaced with zeolitic products1 occasionally display six- and four-sided crystal-contours4 ,an analysis of the ,traprain phonolite by ,mr4 ,player is subjoined3--#dbf ,silica #efh ,titanic acid #je ,alumina #aig ,ferric oxide #bb ,ferrous oxide #ce ,manganous oxide #jb ,lime #bb ,magnesia #jd ,soda #dc ,potash #ga ,loss by ignition #be ---- ,spec4 grav4 #behh #iid ,footnote #dbf3 ,trans4 ,roy4 ,soc4 ,edin4 vol4 xxxvii4 p4 #abe4 ,the neck of ,north ,berwick ,law was found by ,dr4 ,hatch to be a trachyte1 showing a plexus of lath-shaped sanidines that diminish in size to minute microlites1 but with no porphyritic or ferro-magnesian constituent4 ,the ,bass ,rock1 though its geological relations are concealed by the sea1 is in all probability another neck of this district4 ,it is likewise a mass of trachyte1 composed almost entirely of lath-shaped crystals of sanidine1 with no ferro-magnesian constituent1 but a good deal of iron ore4 ,it shows none of the large porphyritic felspars so characteristic of the ,garleton ,hills lavas1 but it closely resembles the non-porphyritic varieties1 particularly the lavas of ,score ,hill1 ,pencraig1 ,lock ,pit ,hill1 and ,craigie ,hill4#dbg ,footnote #dbg3 ,the composition of the rocks of ,north ,berwick ,law and the ,bass closely resembles that of the trachytic lavas of the plateau4 ,for analyses1 see ,dr4 ,hatch's ,paper1 ibid4 pp4 #abc1 #abd4 7f7 ,tuffs4--,the fragmentary ejections of the plateaux vary in texture from the finest-grained tuffs to coarse agglomerates4#dbh ,as they have been derived from the explosion of andesite-lavas1 they consist mainly of the debris of these rocks4 ,they are often deep red in colour1 as for example those of ,dunbar1 but are most frequently greenish4 ,they have a granular texture1 due to the small lapilli of various porphyrites imbedded in a fine dust of the same material4 ,grains of quartz1 frequently to be detected even in the finer tuffs1 may either have been ejected from the volcanic vents1 or may have been grains of sand in the ordinary sediment of the sea-bottom4 ,both at the base and at the top of the plateau-series1 the tuffs are interstratified with and blend into sandstones and shales1 so that specimens may be collected showing a gradual passage from volcanic into non-volcanic detritus4 ,in many of the tuffs of the necks fragments of sandstone and other stratified rocks occur1 representing the strata through which the vents were drilled4 ,in the tuffs of the ,eaglesham district pieces of grey and pink granite have been met with which1 if they are portions of an old granite mass below1 must have come from a great depth4#dbi ,in the coarser tuffs and agglomerates a larger variety of lava-form rocks is to be found than can be seen among the bedded lavas of the ,plateaux4 ,they include felsites and quartz-porphyries1 and more rarely basic lavas 7diabases1 etc474 ,footnote #dbh3 ,for accounts of these rocks1 see ,explanation of ,sheet #cc ,geol4 ,surv4 ,scot4 p4 #cb2 ,sheet #bb1 pp4 #aa-ad2 ,sheet #ca1 pp4 #ad-ag4 ,footnote #dbi3 ,explanation of ,sheet #bb ,geol4 ,surv4 ,scot4 p4 #ab4 ,,chapter ,,xxv ,,geological ,,structure ,,of ,,the ,,carboniferous ,,volcanic ,,plateaux ,,of ,,scotland #a4 ,bedded ,lavas and ,tuffs2 ,upper ,limits and ,original ,areas and ,slopes of the ,plateaux2 #b4 ,vents2 ,necks of ,agglomerate and ,tuff2 ,necks of ,massive ,rock2 ,composite ,necks2 #c4 ,dykes and ,sills2 #d4 ,close of the ,plateau-eruptions4 ,the structure of the various plateaux presents a general similarity1 with many local variations4 ,each plateau is built up entirely1 or almost entirely1 of sheets of volcanic material1 the intercalations of ordinary sedimentary layers being1 for the most part1 few and unimportant1 and usually occurring either towards the base or the top of the volcanic series1 though at a few localities interstratifications of shale and sandstone1 marking pauses in the eruptions1 occur throughout that series4 ,the vents of eruption are in some instances still to be recognized on the plateaux themselves4 ,more usually they occur on the lower ground flanking the volcanic escarpments1 where they have been laid bare by denudation4 ,dykes1 though seldom abundant1 are associated with the plateaux1 while the sills which may mark the latest manifestations of volcanic energy1 though not developed on so large a scale as among the ,cambrian and ,silurian volcanoes1 can nevertheless be distinctly recognized4 ,it is a question of some interest to determine the geological date of the commencement of the plateau-eruptions by fixing the precise stratigraphical horizon on which the base of the volcanic series rests4 ,i have already referred to the fact that this base does not always lie on the same platform among the ,lower ,carboniferous formations4 ,in ,berwickshire1 as above mentioned1 the earliest eruptions appear to have taken place before the close of the ,upper ,old ,red ,sandstone period4 ,these are the earliest of the whole series4 ,in ,cantyre1 the lowest lavas and tuffs come directly upon the sandstones1 marls and cornstones of the ,upper ,old ,red ,sandstone4 ,in ,stirlingshire1 ,renfrewshire and ,ayrshire several hundred feet of the ,cement-stone group are sometimes interposed between the bottom of the volcanic rocks and the top of the ,old ,red ,sandstone4 ,this divergence doubtless indicates that the eruptions began earlier in some districts than in others4 ,but there were also probably unequal terrestrial movements preceding1 and perhaps accompanying1 the volcanic outbursts4 ,in the case of the ,clyde plateau1 for example1 if we examine its base in the neighbourhood of ,fintry1 we find that it lies upon some #ejj feet of ,carboniferous white sandstone1 red and green marls and cement-stones1 which rest on the ,upper ,old ,red ,sandstone4 ,yet only eight miles to the eastward1 this considerable mass of strata disappears1 and the bottom of the lavas comes down upon the red sandstones4 ,five miles still further in the same direction the volcanic masses likewise die out1 and then the ,carboniferous ,limestone series is found at ,abbey ,craig to lie1 with scarcely any representative of the ,cement-stone group1 on the ,upper ,old ,red ,sandstone 7,fig4 #aad74 ,again1 to the south-west of ,fintry1 the zone of cement-stones below the volcanic series continues to vary considerably in thickness and sometimes almost to disappear1 while in ,ayrshire the lavas lie immediately on the red sandstones4 ,illustration3 ,fig4 #aad4--,vertical sections of the escarpment of the ,clyde plateau from north-east to south-west4 ,i4 ,section at the east end of the ,campsie ,hills1 four miles west from ,stirling4 ,,ii4 ,section above ,glins1 six miles west from ,no4 ,i4 ,,iii4 ,section at ,strathblane ,hill1 eight miles further south-west4 ,,iv4 ,section at ,lang ,craig1 east from ,dumbarton1 eight miles south-west from ,no4 ,,iii4 ,v4 ,section above ,fort ,matilda1 ,greenock1 eleven miles from the previous section and on the south side of the ,clyde4 #a4 ,lower ,old ,red ,sandstone2 #b4 ,upper ,old ,red ,sandstone2 #c4 ,carboniferous shales1 sandstones and cement-stones 7the ,ballagan beds72 #d4 ,thick white sandstone which comes in above the ,ballagan beds2 #e4 ,andesite lava-sheets2 #f4 ,interstratified tuffs4 ,the dotted lines connect the base of the volcanic series4 ,these irregularities1 not improbably indicative of inequalities of subsidence and of deposition1 may have been connected with the subterranean disturbances which culminated in the abundant outbreak of volcanic action4 ,but though the volcanic rocks of the plateaux may be traced overlapping the underlying strata1 no evidence has anywhere been detected of an unconformability between them and the ,lower ,carboniferous or ,upper ,old ,red ,sandstone series4 #a4 ,,bedded ,,lavas ,,and ,,tuffs ,the successive sheets of lava in a plateau usually form thin and widespread beds which are only occasionally separated by intercalations of tuff or of red marl4 ,in this1 as well as in other respects1 they present much resemblance to the lavas of the ,tertiary plateaux of ,antrim and the ,inner ,hebrides4 ,they are generally marked off from each other by the slaggy upper and under portions of the successive flows1 and this structure gives a distinctly bedded aspect to the escarpments1 as in the ,campsie and ,largs ,hills1 or still more conspicuously in ,little ,cumbrae 7,fig4 #ajg7 and the southern end of ,bute4 ,considerable diversity of structure may be noticed among these sheets4 ,some present a compact jointed centre passing up and down into the slaggy material just referred to2 others have assumed a vesicular character throughout1 the vesicles being often elongated in the direction of flow4 ,where1 as usually occurs1 the vesicular is replaced by the amygdaloidal structure1 some of the rocks have long been famous for the minerals found in their cavities4 ,the beautiful zeolites of the ,kilpatrick and ,renfrewshire ,hills1 for example1 may be found in every large mineralogical collection in the country4 ,well-developed columnar structure occasionally appears among the lavas of the plateaux1 but chiefly1 so far as ,i have observed1 in the lower or more basic group1 as in the basalts along the east side of the ,dry ,dam at ,arthur ,seat4 ,in each plateau the lavas may be observed to thicken in one direction1 or more usually towards more than one1 and this increase no doubt indicates in which quarters the chief centres of discharge lay4 ,thus in the ,clyde plateau1 several areas of maximum development may be detected4 ,in the ,kilpatrick ,hills the total thickness of lavas and tuffs exceeds #cjjj feet 7,fig4 #abj74 ,above ,largs it is more than #aejj feet1 rapidly thinning away towards the south4 ,the continuation of the plateau far to the north-east in the ,campsie ,fells reveals a thickness of about #ajjj feet of lavas at ,kilsyth1 which become thicker further west1 but eastward rapidly diminish in collective bulk1 until in about twelve or thirteen miles they disappear altogether1 and then1 as already remarked1 the ,calciferous ,sandstone series closes up without any volcanic intercalation4 ,in the ,solway plateau1 the lavas attain a maximum development about ,birrenswark1 whence they diminish in bulk towards the north-east and south-west4 ,the ,berwickshire plateau reaches its thickest mass about ,stitchill1 whence it rapidly thins away towards the north-east1 until at a distance of some twelve miles it disappears altogether1 the last trace of it in that direction being a band of tuff which dies out in the ,calciferous ,sandstones to the north of ,duns4 ,in the ,midlothian ,plateau1 the development of the volcanic series is more irregular than in any of the others4 ,as already remarked1 there appear to have been at least two chief centres of discharge in this region1 one at ,edinburgh and one some fourteen miles to the south-west4 ,at the former1 the volcanic materials attain in ,arthur ,seat and ,calton ,hill a thickness of about #aajj feet4 ,in ,craiglockhart ,hill1 three miles distant1 they are still about #fjj feet thick4 ,but beyond that eminence they cease to be traceable for about eight miles1 either because they entirely die out1 or because their dwindling outcrops are concealed under superficial deposits4 ,as we approach the south-western centre of eruption around ,corston ,hill a new volcanic group begins and soon increases in bulk4 ,a distinguishing feature of the plateaux is found in the difference between the lavas that were first erupted and those which followed them4 ,the earlier eruptions1 as above remarked1 were generally basic1 sometimes highly so4 ,thus at ,arthur ,seat the thick series of lavas which form the eastern part of the hill have at their base several sheets of columnar basalt1 over which come the andesites that make up the main mass of the erupted material4 ,in the ,calton ,hill the same sequence may be observed4 ,underneath the andesites of ,campbeltown comes a well-marked and persistent band of olivine-dolerite4 ,still more basic are some portions of the earliest lavas of the ,garleton plateau where1 as already stated1 rocks present themselves composed mainly of olivine and augite4 ,it is worthy of notice that where the lavas of a plateau diminish greatly in thickness or become impersistent1 the lowest basic group may continue while the overlying andesites disappear4 ,this feature has been already mentioned as well seen in the ,midlothian plateau4 ,the thick group of andesites in ,arthur ,seat and ,calton ,hill is not to be found in the next volcanic eminence1 ,craiglockhart ,hill2 but the basalts with their underlying tuffs continue4 ,in the south-western tract from ,harper ,rig to ,hare ,law in ,lanarkshire1 the thin lava-band1 which can be found only at intervals along the line of outcrop of the volcanic series for about nine miles1 is a dolerite often highly slaggy in structure4 ,again1 at ,corrie in ,arran1 the lavas which appear upon the shore1 apparently at the extreme western limits of the ,clyde plateau1 are basic rocks4 ,but whether or not the lowest and more basic lavas appear in any plateau1 the main mass of the molten material erupted has usually consisted of varieties of andesite4 ,the successive discharges of these intermediate lavas have flowed out in sheets1 some of which must have been little more than heaps of clinkers and scori1 while others were more fluid and rolled along with a ropy or slaggy surface4 ,occasionally the upper part of an andesite shows the reddened and decomposed character that suggests some degree of disintegration or weathering before the next lava-stream buried it4 ,the intervals between successive outflows of these lavas are not1 as a rule1 defined by any marked breaks or by the intercalation of other material4 ,in general1 the plateaux are mainly built up of successive sheets of lava which have followed each other at intervals sufficiently short to prevent the accumulation of much detritus between them4 ,thus the ,campsie ,hills have the upper #fjj feet of their mass formed of admirably-well-defined sheets of andesite1 separated sometimes by thin partings of tuff1 but more usually only by the slaggy vesicular surfaces between successive flows4 ,where the lavas consisted of trachytes they were apt to assume more irregular forms4 ,of this tendency the rocks of the ,garleton ,hills supply an excellent example4 ,as already stated1 their lumpy character gives to these hills an outline which offers strong contrast to the ordinary symmetrical terraced contours of the andesitic plateaux4 ,illustration3 ,fig4 #aae4--,section of ,craiglockhart ,hill1 ,edinburgh4 #a4 ,red sandstones and clays2 #b4 ,green stratified tuffs2 #c4 ,columnar basalt2 #d4 ,dark shales1 ironstones and sandstones1 with plants4 ,illustration3 ,fig4 #aaf4--,section of the bottom of the ,midlothian ,plateau1 ,linnhouse ,water above ,mid-,calder ,oilworks4 #a4 ,shales and cement-stones2 #b4 ,sandstones2 #c4 ,highly vesicular lava2 #d4 ,tuffs and sandstone bands4 f1 ,fault4 ,although tuffs play1 on the whole1 a comparatively unimportant part among the constituents of the plateaux1 they attain in a few localities an exceptionally great development1 and even where they occur only as thin partings between the successive lava-flows1 they are always interesting memorials of the volcanic activity of a district4 ,in many portions of the plateaux1 the lowest members of the volcanic series are tuffs and agglomerates1 showing that the eruptions often began with the discharge of fragmentary materials4 ,thus in the ,midlothian plateau at ,arthur ,seat1 though the lowest interbedded volcanic sheet is a dolerite1 it is immediately followed by a series of bedded tuffs1 before the main mass of the lavas of that hill make their appearance4 ,at ,craiglockhart ,hill1 three miles distant 7,fig4 #aae71 this lowest lava is absent1 and a group of tuffs about #cjj feet thick rests immediately on the red ,carboniferous sandstones and shales1 and is overlain by sheets of columnar basalt4 ,the scoriaceous bottom of the latter rock may here and there be seen to have cut out parts of the tuff as it rolled over the still unconsolidated material4 ,in the same district1 a few miles further to the south-west1 some interesting sections of the ,midlothian plateau are laid bare in the streams which descend from the western slopes of the ,pentland ,hills4 ,i may cite1 in particular1 those exposed in the course of the ,linnhouse ,water4 ,at the railway viaduct near the foot of ,corston ,hill1 a good section is displayed of the ,cement-stone group--thick reddish1 purplish1 and greenish-blue marly shales or clays1 with thin ribs and bands of cement-stone and grey compact cyprid-limestone1 as well as lenticular seams and thicker beds of grey shaly sandstone1 sometimes full of ripple-marks and sun-cracks4 ,these strata1 which exactly reproduce the typical lithological characters of the ,cement-stone group of ,stirlingshire 7,ballagan ,beds71 ,ayrshire and ,berwickshire1 are surmounted by a group of reddish1 yellow and brown sandstones1 sometimes pebbly and containing a band of conglomerate4 ,among the stones in this band1 pieces of the radiolarian cherts of the ,lower ,silurian series of the ,southern ,uplands are conspicuous1 likewise pieces of andesite which may have come from the neighbouring ,pentland ,hills4 ,above these strata lie the lavas of ,corston ,hill4 ,these are highly vesicular in some parts1 and include bands of tuff which are well exposed further down the same stream1 immediately above the railway bridge near the ,mid-,calder oilworks 7,fig4 #aaf74 ,there the lavas1 though much decomposed1 show a highly vesicular structure with a rugged upper surface1 in the hollows and over the prominences of which fine flaky and sandy tuffs have been deposited1 while thin seams of vesicular lava are intercalated among these strata4 ,illustration3 ,fig4 #aag4--,section of the top of the ,midlothian ,plateau in the ,murieston ,water4 ,the upper part of the same plateau1 as exposed in the course of the ,murieston ,water1 contains evidence that the last eruptions consisted of tuff4 ,the highly slaggy lava 7#a in ,fig4 #aag7 is there surmounted by a thick mass of grey and greenish-white well-bedded granular tuff 7#b7 including occasional lumps of the basic lava1 and passing up into black shale 7#c74 ,but that the volcanic eruptions continued during the accumulation of the shale is proved by the intercalation of thin partings and thicker layers of tuff in the black sediment4 ,a short way higher up the ,burdiehouse ,limestone comes in4 ,the great lava-escarpment of the ,kilpatrick ,hills rests on a continuous band of tuff which is thickest towards the west1 near the group of vents above ,dumbarton1 while it thins away eastward and disappears in ,strathblane1 the lavas then forming the base of the volcanic series4 ,but perhaps the most remarkable group of basal tuffs is that which underlies the lavas of the ,garleton plateau1 to which further reference will be immediately made4 ,extensive accumulations of tuff form in one or two localities a large proportion of the thickness of the whole volcanic series of a plateau4 ,thus in the north-eastern part of ,ayrshire1 between ,eaglesham and the valley of the ,irvine1 the lavas die out for a space and give place to tuffs4 ,during the discharge of the fragmentary materials over that ground no lava seems to have flowed out for a long period4 ,ordinary sediment1 however1 mingled with the volcanic detritus1 and there were even pauses in the eruptions when layers of ironstone were deposited1 together with thin impure limestone that inclosed shells of ,productus giganteus4#dcj ,footnote #dcj3 ,explanation of ,sheet #bb ,geol4 ,surv4 ,scotland1 p4 #ab4 ,in some of the plateaux1 particularly within the older part of the volcanic series1 intercalations of ordinary sediment among the tuffs and lavas show that eruptions occurred only occasionally1 and that during the long intervals between them the deposition of sand and mud went on as before4 ,thus the lower #djj feet of the ,campsie ,fells are built up of slaggy andesites and thick beds of fine-grained stratified tuff1 with bands of red1 green and grey clays and cement-stone and a zone of white sandstone4 ,the ,calton ,hill at ,edinburgh 7,fig4 #aah7 affords an excellent illustration of the interstratification both of tuffs and ordinary sediments among the successive outflows of lava4 ,in the total thickness of about #aajj feet of volcanic material in this hill1 at least eight intervals in the discharge of the lavas are marked by the intercalation of as many bands of nodular tuff1 together with seams of shale and sandstone more or less charged with volcanic detritus4 ,the highest lava is immediately covered by the white sandstones and black shales of the ,calciferous ,sandstone series4 ,illustration3 ,fig4 #aah4--,section of ,calton ,hill1 ,edinburgh4 #a4 ,lower ,carboniferous sandstones2 #b4 ,basic lava at the bottom of the volcanic series2 #c4 ,tuff often interstratified with sandstones and shales2 #d4 ,sheets of andesite-lava frequently separated by layers of tuff2 #e4 ,shale passing into tuff2 #f4 ,white sandstone and black carbonaceous shales overlying the volcanic series4 ,the tuffs1 as might be expected1 are coarsest in texture and thickest in mass where they approach most nearly to some of the vents of eruption1 and1 on the other hand1 become finer as they recede from these4 ,as a rule1 they are distinctly stratified1 and consist of layers varying in the size of their component lapilli4 ,here and there1 near the centres of discharge1 the bedding becomes hardly traceable or disappears1 and the fragmentary materials take the form of agglomerate4 ,in the admirable range of coast-cliffs which extend from ,north ,berwick to ,dunbar1 we learn that above the red sandstones at the base of the ,carboniferous system1 a thick pile of volcanic ashes was accumulated by numerous discharges from vents in the immediate neighbourhood4 ,some of the explosions were so vigorous that blocks of different lavas1 sometimes a yard or more in length1 were thrown out and heaped up in irregular mounds and hollows4 ,others discharged exceedingly fine dust1 and between these two extremes every degree of coarseness of material may be recognized4 ,as an illustration of the remarkable alternation of coarse and fine materials1 according to the varying intensity of the volcanic paroxysm1 ,fig4 #aai is here introduced4 ,it represents a portion of the tuff-cliffs east of ,tantallon ,castle1 and shows at the bottom fine well-stratified tuff1 over which a shower of large blocks of lava has fallen4 ,fine detritus is seen to cover the deposits of this shower1 and successive discharges of large stones may be noticed higher up on more or less well-defined horizons4 ,the space over which this pyroclastic material can now be traced1 large though it is1 does not represent the whole of the original area included within the range of the discharges of ash and stones1 for much has been removed by denudation4 ,during pauses of various length between the eruptions1 waves and currents washed down the heaps of volcanic material and distributed ordinary sediment over the bottom of the water4 ,hence1 abundantly interstratified in some parts of the tuff1 seams of sandstone1 blue and green shale1 cement-stone and limestone occur4 ,one thick band of limestone may be traced from near ,tynningham ,house to ,whittinghame1 a distance of about four miles2 another patch appears near ,rockville ,house2 and a third at ,rhodes1 near ,north ,berwick4 ,no fossils have been noticed in these limestones4 ,the calcareous matter1 together sometimes with silica1 appears to have been supplied1 at least in part1 by springs1 which may have been connected with the volcanic phenomena of the district4 ,the ,north ,berwick limestone1 in particular1 has the peculiar carious wavy structure with minute mamillated interstices so common among sinters4 ,it contains grains of pyrites1 flakes of white kaolin1 which probably represent decayed prisms or tufts of natrolite1 and cavities lined with dog-tooth spar4 ,some portions give out a strongly ftid odour when freshly broken4 ,after the tuffs of the ,garleton plateau had accumulated to a depth of perhaps #bjj feet or more1 lavas began to be poured out4 ,first came basic outflows 7olivine-basalts with picrites7 and andesites 7porphyrites71 which form a thin but continuous sheet all over the area4 ,these were succeeded by the series of trachytes which distinguish this area4 ,although the observer remarks the absence there of the usual terraced arrangement1 yet from some points of view1 particularly from the westward1 a succession of low escarpments and longer dip-slopes can be detected among the trachytes of the ,garleton ,hills1 while there can be no doubt that1 in spite of their irregular lumpy contours1 these lavas lie as a great cake above the lower platform of more basic flows 7,fig4 #aj74 ,there is evidence that during the emission of the trachytes occasional eruptions of andesite took place4 ,not the least striking and interesting feature of this plateau is the size and distribution of its necks1 to which reference will be made in the sequel4 ,the latest eruption in the ,garleton area had ceased and the cones and lava sheets had probably been buried under sediment before the commencement of the deposition of the ,hurlet or thick ,main ,limestone of the ,carboniferous ,limestone series which lies immediately to the west of the plateau4 ,the tuffs of the plateaux are seldom fossiliferous1 probably for the same reason that fossils are scarce in the ,cement-stone group which the plateau volcanic rocks overspread and with which they are interstratified4 ,occasional stems and other fragments of vegetation occur in the plateau-tuffs1 as in those of ,north ,berwick1 where ,i have found a decayed coniferous trunk three feet in length4 ,the green tuff at the base of the volcanic group of ,arthur ,seat contains abundant macerated plant-remains1 together with scales of ,rhizodus and other fishes4 ,in some places the plants are represented by trunks or roots1 which appear to remain in their positions of growth4 ,a remarkable instance of this nature occurs in some bands of tuff in the volcanic group of the east coast of the ,isle of ,arran1 first brought to notice by ,mr4 ,e4 ,wunsch1#dca and of which the plants have been so fully investigated by ,professor ,williamson4#dcb ,footnote #dca3 ,trans4 ,geol4 ,soc4 ,glasgow1 vol4 ii4 7#ahfg7 p4 #ig4 ,footnote #dcb3 ,phil4 ,trans4 #ahga-ahhc4 ,illustration3 ,fig4 #aai4--;,cliff of tuff and agglomerate1 east side of ,oxroad ,bay1 a little east from ,tantallon ,castle1 ,east ,lothian4 ,plant-remains also occasionally occur in the stratified layers intercalated among the lavas and tuffs of the plateaux4 ,some of the best examples of their occurrence are to be found in the shales and tuffs interstratified among the enormous pile of volcanic material near ,bowling4 ,not only does abundant vegetable debris occur distributed through the detrital strata in the volcanic series at that locality1 but it is even aggregated into thin seams of coal which have been examined and described by various observers4#dcc ,it may be remarked that the plant remains thus found intercalated in the volcanic series1 especially when they have been entombed in tuff1 have often had their internal structure admirably preserved1 the organic tissues having been delicately replaced by calcite or other petrifying medium4 ,the remarkably perfect structure of some of these plants has been demonstrated by ,professor ,williamson1 especially in the case of the ,arran deposit just referred to4 ,mr4 ,john ,young has also found the structure well preserved among the ,sigillari and ,stigmari that occur in the stratified intercalations between the lavas near ,bowling4 ,footnote #dcc3 ,see in particular ,j4 ,young1 ,trans4 ,geol4 ,soc4 ,glasgow1 vol4 iv4 7#ahgd7 p4 #abc4 ,illustration3 ,fig4 #abj4--,section across part of the ,clyde ,plateau to the west of ,bowling 7reduced from ,sheet #f of the ,horizontal ,sections of the ,geological ,survey of ,scotland74 #a4 ,ballagan ,beds2 #b4 ,white sandstone2 #c4 ,tuffs1 #fjj feet thick1 with a thin sheet of andesite2 #d4 ,andesite sheets1 #ejj feet2 #e4 ,stratified tuffs with thin coals1 shales1 fireclays and plant-remains1 #ejj or #fjj feet2 #f #f4 ,a series of andesite-lavas1 about #aejj feet thick1 enclosing a thin coal-seam at 992 #g4 ,stratified tuffs1 #bjj feet2 #h4 ,shales with plants and coaly seams1 #aej feet2 #i4 ,base of another andesite series1 which must be some hundreds of feet thick2 #aj and #aa4 ,necks of agglomerate4 ,upper ,limits and ,original ,areas and ,slopes of the ,plateaux4--,where the highest members of the volcanic series can be seen passing conformably under the overlying ,carboniferous strata they are frequently found to be mainly composed of fine tuffs1 the last feeble efforts of the plateau-volcanoes having consisted in the discharge of showers of ashes4 ,these materials were mingled with a gradually increasing proportion of ordinary mechanical sediment1 which finally overspread and buried the volcanic tracts of ground1 as these slowly sank in the general subsidence of the region4 ,the characteristic corals1 crinoids and shells of the ,carboniferous ,limestone begin to appear in these ashy sediments4 ,there is thus an insensible passage from volcanic detritus into fossiliferous shales and limestones4 ,examples of this gradation may be seen in many natural sections along the flanks of the ,ayrshire plateau from above ,kilbirnie to ,strathavon4 ,it is still possible to fix in some quarters the limits beyond which neither the lavas nor the tuffs extended1 and thus partially to map out the original areas of the plateaux4 ,for example1 in certain directions the ,carboniferous formations can be followed continuously downward below the ,main ,limestone1 without the intervention of any volcanic material1 or with only a slight intermixture of fine volcanic lapilli1 such as might have been carried by a strong wind from some neighbouring active vents4 ,by this kind of evidence and by the proved thinning-out of the materials of the plateau1 we can demonstrate that in the north of ,ayrshire the southern limits of the great volcanic bank did not pass beyond a line drawn from near ,ardrossan to ,galston4 ,we can show1 too1 that the lavas of the ,campsie ,fells ended off about a mile beyond ,stirling before they reached the line of the ,ochil heights1 and that the coules which flowed from the ,solway vents did not quite join with those from the ,berwickshire volcanoes4 ,illustration3 ,fig4 #aba4--;,diagram illustrating the thinning away southwards of the lavas of the ,clyde ,plateau between ,largs and ,ardrossan4 ,length about #aj miles4 #a4 ,upper ,old ,red ,sandstone2 #b4 ,sandstones1 shales1 etc41 with ,ballagan ,beds2 #c4 ,tuffs2 #d4 ,andesite lavas2 #e4 ,carboniferous ,limestone series4 ,moreover1 evidence enough remains to enable us to form a tolerably clear conception of the original average slopes of the surface of some of the plateaux4 ,thus in the great escarpment above ,largs and the high ground eastward to ,kilbirnie the volcanic series1 as already stated1 must be at least #aejj feet thick4 ,this thick mass of lavas and tuffs thins away southwards and probably disappears a short distance south from ,ardrossan in a space of about ten miles 7,fig4 #aba74 ,the original southward slope of the plateau would thus appear to have been about #a in #ce4 ,again1 the northward slope of the same plateau may be estimated from observations in the ,campsie ,fells4 ,we have seen that above ,kilsyth the total depth of the volcanic sheets is about #ajjj feet1 while to the westward it is much thicker4 ,from the top of the ,meikle ,bin 7#ahgj feet7 above ,kilsyth north-eastwards to ,causewayhead1 where the whole volcanic series has died out1 is a distance of #ab miles1 so that the slope of the surface of erupted materials on this side was about #a in #fc 7,fig4 #abb74 ,judging from the sections exposed along the faces of the escarpments1 we may infer that the volcanic sheets had a tolerably uniform surface which sloped gently away from the chief vents1 but with local inequalities according to the irregularities of the lava-streams that were heaped up round the vents and flowed outward in different directions and to various distances from them4 ,at the beginning1 these flat volcanic domes were certainly subaqueous4 ,while they were being formed1 continuous subsidence appears to have been in progress4 ,but the great thickness of the volcanic accumulations1 as in the ,kilpatrick and ,renfrewshire areas1 and the paucity of ordinary sedimentary strata among them1 make it not improbable that at least their higher parts rose above the water4 ,where this was the case there may have been considerable degradation of the lava-banks before these were reduced or were by subsidence submerged beneath the water-level4 ,evidence of this waste is probably to be recognized in the bands of conglomerate1 occasionally of considerable thickness1 which1 particularly in some parts of ,ayrshire1 intervene between the top of the volcanic group and the ,hurlet ,limestone4 ,as ,i shall have occasion to point out further on1 there seems to be some amount of evidence in favour of the view that a considerable interval of time elapsed between the close of the plateau-eruptions and the date of that widespread depression which led to the deposition of the ,hurlet ,limestone over the whole of ,central ,scotland4 ,if such an interval did occur it would include a prolonged abrasion of any projecting parts of the plateaux1 and the production and deposition of volcanic conglomerate4 ,illustration3 ,fig4 #abb4--;,diagram illustrating the thinning away eastwards of the lavas of the ,clyde ,plateau in the ,fintry ,hills4 ,length about #ab miles4 #a4 ,upper ,old ,red ,sandstone2 #b4 ,white sandstone1 blue shales and cement-stones 7,ballagan ,beds72 #c4 ,andesite sheet1 about #ajj feet thick2 #d4 ,tuffs 7#bej feet71 with an included band of ashy sandstone containing plant-remains2 #e4 ,andesite lavas2 #f4 ,carboniferous ,limestone series1 which to the east lies immediately on the ,upper ,old ,red ,sandstone4 #b4 ,,vents ,we have now to consider the external forms1 internal contents and distribution of the vents from which the material of the plateaux was discharged4 ,in the ,carboniferous system these interesting relics of former volcanoes are far more distinctly defined and better preserved than in older geological formations4 ,moreover1 in ,scotland1 they are laid bare to greater advantage1 both inland and along the sea-coast1 and may indeed be studied there as typical illustrations of this kind of geological structure4 ,illustration3 ,fig4 #abc4--,view of the two necks ,dumgoyn and ,dumfoyn1 ,stirlingshire1 taken from the south4 ,these two necks form a conspicuous feature in front of and below the lava plateau1 a portion of which is shown on the right hand4 ,the ground-plan of the same necks is shown in ,fig4 #abd4 ,illustration3 ,fig4 #abd4--;,ground-plan of ,plateau-vents near ,strathblane1 ,stirlingshire1 on the scale of #f inches to a mile4 ,in external form the necks connected both with the plateaux and the puys generally rise from the surrounding ground as isolated1 rounded1 conical or dome-shaped prominences1 their details of contour depending mainly upon the materials of which they consist4 ,when these materials are of agglomerate1 tuff or other readily disintegrated rock1 the surface of the domes is generally smooth and grass-covered4 ,where1 on the other hand1 they consist wholly or in part of dolerite1 basalt1 diabase1 andesite1 trachyte or other crystalline rock1 they present more irregular rocky outlines4 ,illustrations of some of those varying forms are given in ,figs4 #bc and #abc4 ,in rare instances the vent is marked at the surface not by a hill but by a hollow1 as in the great neck in the heart of the ,campsie ,fells 7,fig4 #abh74 ,illustration3 ,fig4 #abe4--;,ground-plans of double and triple necks in the ,plateau series1 on the scale of #f inches to a mile4 ,a4 ,barwood ,hill and ,ravenscraig1 east of ,dumbarton1 double vent4 ,b4 ,the ,knock ,hill1 ,largs1 ,ayrshire1 double vent 7see ,fig4 #bc74 ,c4 ,dumbowie and ,dumbuck1 east of ,dumbarton1 triple vent4 ,as regards their ground-plan1 which affords a cross-section of the original volcanic funnel1 the plateau-vents present considerable variety4 ,the simplest cases are those in which the form is approximately circular or somewhat elliptical4 ,here the outline corresponds to the cross-section of a single and normal orifice4 ,some examples of this simple type are given in ,fig4 #abd1 which represents a group of vents on the edge of the ,clyde plateau near ,strathblane4 ,the two larger necks here shown are the same which appear in the view in ,fig4 #abc4dcd ,where two vents have been successively opened close to each other1 or where the same vent has shifted its position1 the ground-plan may be greatly modified4 ,in some instances the double funnel can be distinctly traced4 ,thus in the conspicuous ,knock ,hill above ,largs in ,ayrshire 7,fig4 #abe1 ,b7 there are two conjoined necks1 and such appears to be also the structure shown by the ground-plan of the neck of ,barwood ,hill and ,raven's ,craig1 east of ,dumbarton 7,fig4 #abe1 ,a74#dce ,but more complex forms occur which point to a still larger number of coalescing necks4 ,a group of hills to the east of ,dumbarton gives the ground-plan shown in ,c1 ,fig4 #abe1 where traces may be detected of three separate vents4 ,still more irregular are long narrow dyke-like masses of tuff or agglomerate which have probably risen along lines of fissure 7,fig4 #bb1 ,no4 #a74 ,the most striking example of these1 however1 occur in association with the puys and will be described in later pages4 ,footnote #dcd3 ,the illustrations in ,figs4 #abd and #abe are taken from the field-maps of the ,geological ,survey on the scale of #f inches to a mile4 ,the ground represented in ,fig4 #abd was mapped by ,mr4 ,r4 ,l4 ,jack4 ,footnote #dce3 ,these ground-plans are likewise taken from the field-maps of the ,geological ,survey4 ,a and ,c were mapped by ,mr4 ,jack1 ,b by myself4 ,the shaded parts are intrusive andesites and dolerites2 the dark bars in ,a and ,c being dolerite dykes of much later date than the necks4 ,the dotted portions mark tuff and agglomerate4 ,connected with their ground-plan is the relative size of the plateau-vents4 ,on the whole they are larger than those of the puy series4 ,the simple circular or elliptical type presents the smallest necks1 some of them not exceeding #ajj feet in diameter4 ,the more complex forms are generally also of larger dimensions4 ,by much the largest vent or connected group of vents is that which lies among the uplands of ,misty ,law in the heart of the ,renfrewshire part of the ,clyde plateau1 where a connected mass of tuff and agglomerate now occupies a space of about #d miles in length by #b miles in breadth 7,fig4 #abi74 ,it has not been found possible1 however1 to trace the boundaries of the separate vents of this tract1 nor to distinguish the material of the necks from that which surrounds them4 ,another large mass which from its shape may be conjectured to represent more than one vent is the great tract north of ,melrose1 which measures #hhjj by #dbjj feet4#dcf ,footnote #dcf3 ,the following measurements are1 like those in the text1 taken from the field-maps of the ,geological ,survey4 ,carewood ,rig1 on the borders of ,roxburghshire and ,dumfriesshire1 #gjjj #bdjj feet2 the great vent in the middle of the ,campsie ,fells1 #ebjj #bfjj2 ,black ,law1 between ,bedrule and ,jedburgh1 #cdjj #afjj2 ,dumgoyn1 ,strathblane1 #bcjj #acjj2 ,rubers ,law1 #aejj #ajjj2 ,minto ,hill 7south71 #bcjj #afej2 ,minto ,hill 7north71 #aejj #aajj2 ,doughnot ,hill1 ,kilpatrick range1 #ajjj #gjj2 four of the smallest agglomerate vents along the northern escarpment of the ,clyde plateau between ,strathblane and ,fintry1 #ejj #dej1 #dej #djj1 #bej #ajj1 #bjj #bjj2 ,pike ,law1 ,arkleton1 ,tarras ,water1 #ejj #ejj2 ,harwood1 ,stonedge1 #e miles ,s4,e4 from ,hawick1 #ejj #cjj2 ,arkleton ,burn1 ,dumfriesshire1 #djj #ajj2 ,dalbate ,burn1 #bej #abj4 ,the distribution of the necks can best be understood from the maps of the ,geological ,survey1 where they have been carefully indicated4 ,as might have been expected1 they are not found outside the original limits within which it may be reasonably inferred that the lavas and tuffs were erupted4 ,they occur most abundantly and attain their largest size in and around the districts where the plateaux are most extensively developed4 ,no doubt a large number of them are concealed under these plateaux4 ,a few appear at the surface among the lavas and tuffs1 but by far the largest number now visible have been revealed by denudation1 the escarpments having been cut back so as to lay bare the underlying rocks through which the necks rise4 ,thus1 along the flanks of the great escarpment that extends from near ,stirling by ,fintry and ,strathblane to ,dumbarton1 more than two dozen of agglomerate necks may be counted in a distance of about sixteen miles1 while if the necks of lava-form material are included1 the number of vents must be about fifty4 ,nowhere in ,scotland do such necks form a more conspicuous feature in the scenery as well as the geology than they do between ,fintry and ,strathblane1 where1 standing out as bold isolated hills in front of the escarpments1 their conical and rounded outlines present a striking contrast to the terraced escarpments behind them4 ,i would especially refer again to the two remarkable cones of ,dumfoyn and ,dumgoyn above ,strathblane 7,figs4 #abc1 #abd1 #abg74 ,along the west front of the hills between ,gourock and ,ardrossan seventeen agglomerate-vents occur in a distance of sixteen miles4 ,in ,roxburghshire a group of large agglomerate-necks is dotted over the ,silurian country around ,melrose and ,selkirk#dcg 7see ,fig4 #acj74 ,footnote #dcg3 ,in this region and farther southward1 besides the plateau-eruptions1 a later group of puys is to be seen1 and it is difficult to discriminate between the necks belonging to the two groups4 ,those which lie to the east are probably connected with the plateaux1 those to the west with the puys4 ,the latter are referred to on p4 #dge4 ,illustration3 ,fig4 #abf4--;,ground-plan of tuff-neck1 shore east of ,dunbar4 ,the surrounding rocks are sandstones1 which are much hardened round the vent in the zone marked by the short divergent lines4 ,the arrows mark the direction of dip4 ,see ,geology of ,east ,lothian1 ,mem4 ,geol4 ,survey1 p4 #dd4 ,from the evidence of these necks it is plain that the volcanic materials of the plateaux must in each case have been supplied not from great central orifices1 but from abundant vents standing sometimes singly1 with intervening spaces of several miles1 often in groups of four or five within a single square mile4 ,in the interior of the country1 it is seldom possible to examine the actual junction of necks with the rocks through which they rise1 the boundary-line being usually obscured by debris or herbage4 ,on the coast1 the vents of the plateaux have not been bared by the sea so fully as in the case of the much younger series of the east of ,fife to be described in later pages4 ,but where the ,east ,lothian plateau touches the shore1 the waves have laid bare a number of its minor vents1 which have thus been dissected in ground-plan on the beach4 ,as an illustration of these vents an example is given in ,fig4 #abf1 from the shore east of ,dunbar4 ,here the sandstones1 which are inclined in an easterly direction at #bj to #be1 are pierced by an irregular mass of tuff4 ,it is observable that in this instance long tongue-like projections of the sandstones protrude into the neck2 more frequently the material of a neck sends veins or dykes into the surrounding walls4 ,a volcanic chimney would seem to have been often much shattered and fissured in the course of the volcanic explosions1 and the fragmentary material has fallen or been injected into the rents thus caused4 ,as a rule1 the rocks immediately around the ,carboniferous necks are more or less indurated1 as in this instance from the ,dunbar shore4 ,the materials which have filled up the vents connected with the plateau-eruptions generally consist of 7a7 agglomerates or tuffs1 but occasionally of 7b7 some kind of lava1 and frequently 7c7 of both these kinds of rock combined4 7a7 ,necks of ,agglomerate or ,tuff4--,these materials vary greatly in the nature and relative proportions of their constituents4 ,usually the included blocks and lapilli are pieces of andesite1 diabase1 basalt or other lava1 like the rocks of the plateaux4 ,but with these occur also fragments probably detached from the sides of the funnels through which the explosions took place1 such as pieces of greywacke1 sandstone1 limestone and shale4 ,considerable induration may be observed among these non-volcanic ingredients4 ,in some cases1 as in that of the occurrence of pieces of granite referred to on p4 #chb1 the stones have probably been brought up from some considerable depth4 ,in others it is easy to see that the blocks have slipped down from some higher group of strata now removed from the surrounding surface by denudation4 ,some striking illustrations of this feature will be cited from necks of the puy-series in the south of ,roxburghshire 7p4 #dgf74 ,the lava blocks in the tuffs and agglomerates are usually rounded or subangular4 ,pear-shaped blocks1 or flattened discs1 or hollow spherical balls are hardly ever to be observed1 though ,i have noticed a few examples in the tuffs of ,dunbar4 ,a frequent character of the blocks is that of roughly rounded1 highly amygdaloidal pieces of lava1 the cellular structure being specially developed in the interior1 and the cells on the outside being often much drawn out round the circumference of the mass4 ,such blocks were probably torn from the cavernous1 partially consolidated1 or at least rather viscous1 top of a lava column4 ,most of the stones1 however1 suggest that they were produced by the explosion of already solidified lava1 and were somewhat rounded by attrition in their ascent and descent4 ,the vents filled with such materials must have been the scene of prolonged and intermittent activity2 successive paroxysms resulting in the clearing out of the hardened lava column in the throat of the volcano1 and in the rise of fresh lava1 with abundant ejection of dust and lapilli4 ,illustration3 ,fig4 #abg4--,section across the vents ,dumgoyn and ,dumfoyn1 and the edge of the ,clyde plateau above ,strathblane1 ,stirlingshire4 #a4 ,upper ,old ,red ,sandstone2 #b4 ,shales1 cement-stones and sandstones 7,ballagan beds72 #c4 ,white sandstone2 #d4 ,andesite lavas2 #e4 ,agglomerate 7shown by the dotted portions71 traversed by intrusive diabase4 f1 ,fault4 ,d4 ,late dolerite dyke4 ,necks formed entirely of agglomerate are abundant among the vents connected with the plateaux4 ,as examples of them ,i may refer to the series already mentioned as fronting the escarpment of the ,clyde plateau from ,fintry to ,largs4 ,another interesting group rises through the ,silurian and ,old ,red ,sandstone rocks to the west of the escarpment of the ,berwickshire plateau1 that near ,melrose forming one of the largest in ,scotland4 ,illustration3 ,fig4 #abh4--,section through the large vent of the ,campsie ,hills4 #a4 ,andesite lavas2 #b4 ,agglomerate and tuff2 #c4 ,trachytic and andesitic intrusive rocks4 ,illustration3 ,fig4 #abi4--;,diagrammatic section across the central vent of the ,clyde plateau in ,renfrewshire4 #a4 ,andesite lavas2 #b4 ,agglomerates and fine tuffs often much altered2 #c4 ,dykes of trachytic and andesitic rocks2 #d4 ,later dykes of dolerite and basalt4 ,illustrations of the varying structure of these vents are given in the accompanying figures4 ,in ,fig4 #abg1 a section is drawn through the two necks ,dumgoyn and ,dumfoyn1 which have already been shown in outline and in ground-plan4 ,the relation of these two vents to the neighbouring plateau to the right can here be seen4 ,fig4 #abh gives a section taken through the great vent of the ,campsie ,hills1 with the minor adjacent necks of ,dungoil1 ,bin ,bairn1 and the ,meikle ,bin4 ,the diagram in ,fig4 #abi is meant to convey in a general way what appears to be the structure of the central vent of the ,renfrewshire plateaux1 to be afterwards referred to4 ,but1 as already mentioned1 the limits of the various rocks are too much obscured to allow an accurate delineation to be given of their areas and relations to each other4 ,the ,berwickshire plateau supplies abundant interesting examples of tuff necks which rise through the ,old ,red ,sandstone many miles distant from the edge of the lavas4 ,this structure is shown in ,fig4 #acj4 ,illustration3 ,fig4 #acj4--,section across ,southern ,berwickshire to show the relation of the volcanic plateau to the vents lying south from it4 #a4 ,upper ,silurian strata2 #b4 ,upper ,old ,red ,sandstone2 #c4 ,the volcanic plateau2 #d4 ,agglomerate and tuff of the vents2 #e4 ,basalt and dolerite2 #f4 ,lower ,carboniferous strata4 ,indications may occasionally be observed of an agglomerate vent having been first occupied by one kind of material and then1 after being in great measure cleared out by explosions1 having been subsequently filled up with another4 ,as an example of this structure ,i may cite again the double neck of the ,knock ,hill a little to the north of ,largs1 of which the outline is shown in ,fig4 #bc1 and the ground-plan in ,fig4 #abe1 ,b4 ,this hill rises from the red sandstone slopes that front the great ,ayrshire plateau and forms a conspicuous cone the top of which is rather more than #gjj feet above the sea4 ,its summit commands a remarkably extensive and interesting panorama of the scenery of the ,clyde1 but to the geologist perhaps the most striking feature in the landscape is the range of terraced hills behind1 mounting up into the great vents of the ,renfrewshire uplands4 ,on these declivities the successive lava-streams that have built up the plateau can be seen piled over each other for a thickness of more than #ajjj feet1 and presenting their escarpments as parallel lines of brown crag with green slopes between4 ,the ,knock has had its upper part artificially dressed1 for lines of trench have been cut out of its rocks by some early race that converted the summit of the hill into a strongly intrenched camp4 ,from the apex of the cone the ground falls rapidly westward into a hollow1 beyond which rises a lower rounded ridge of similar materials4 ,it is possible that this western ridge may really form part of the main hill1 but the grass-covered ground does not afford sufficient exposures of the rocks to settle this point4 ,from the contours of the surface1 it may be inferred that there are two closely adjacent vents1 and that the western and lower eminence is the older of the two4 ,this hill or ridge consists partly of a coarse agglomerate1 and partly of veins and irregular protrusions of a dark1 compact1 slightly cellular lava4 ,the stones in the fragmental rock are different olivine-basalts1 or other basic lavas1 and sandstones4 ,the paste is rough1 loose and granular4 ,the sandstone fragments are much indurated and sometimes bleached4 ,the ,knock itself is formed mainly of a remarkably coarse and strikingly volcanic agglomerate4 ,round the outside1 and particularly on the south-east1 the rock is finer in texture1 compact1 and gravelly1 or like a mudstone1 with few or no imbedded blocks1 dull-green to red in colour1 and breaking with a clean fracture which shows angular lapilli of various basalts or diabases4 ,at the southern end of the neck1 where the surrounding red sandstone can be seen within a few feet of the tuff1 the latter is bright red in colour1 and contains much debris of red sandstone and marl4 ,possibly this finer tuff1 which is traceable as an irregular band round the outside of the neck1 may mark an older infilling of the vent than the agglomerate of the centre2 but there is no sharp line to be drawn between the two1 though a hollow can sometimes be traced on the surface where they join4 ,the agglomerate of this locality is one of the most characteristic among the plateau-necks of the ,clyde region4 ,its blocks sometimes measure from two to three feet in diameter4 ,they consist almost wholly of a dark crystalline porphyritic olivine-basalt4 ,these blocks are subangular in form1 often with clean-fractured surfaces4 ,though occasionally slightly cellular1 they are never slaggy so far as ,i could see1 nor are any true scori to be noticed among them4 ,the blocks suggest that they were derived from the disruption of an already solidified mass of lava4 ,the agglomerate is entirely without any trace of stratification4 ,through this tumultuous accumulation of volcanic debris some irregular veins of olivine-basalt1 sometimes glassy in structure1 have been injected1 and reach nearly to the summit of the hill4 ,this intrusive material resembles generally some of the dark intrusive masses in the ,dumbartonshire necks4 ,like these1 it exhibits a tendency to assume a more or less distinctly columnar structure1 its columns having the same characteristic wavy sides and irregular curvature4 ,the intrusive rocks in the two eminences of the ,knock may be paralleled among the stones in the agglomerate4 ,the neck on its north-eastern side rises steeply from the red sandstones which it pierces1 but which1 although they are much jointed and broken1 are not sensibly indurated4 ,unfortunately the actual junction of the igneous and sedimentary rocks is concealed under herbage4 ,as a rule1 the fragmental materials of the plateau-necks are quite unstratified4 ,their included blocks1 distributed irregularly through the mass1 have evidently undergone little or no assortment after they fell back into the vents4 ,occasionally1 however1 a more or less distinct bedding of the agglomerate or tuff may be observed1 the layers having a tendency to dip inward into the centre4 ,one of the most conspicuous examples of this structure is to be found in the hill of ,dumbuck1 to the east of ,dumbarton4 ,this neck1 which forms so prominent a feature in the landscape1 presents a precipitous face towards the south1 and allows the disposition of its component materials to be there seen4 ,the agglomerate consists of a succession of rudely stratified beds of coarser and finer detritus1 which on both sides are inclined towards the centre1 where a plug of fine-grained olivine-basalt has risen and spread out into a columnar sheet above 7,fig4 #aca74 ,in general form this basalt resembles such intrusions as that of ,largo ,law1 to be afterwards described 7,fig4 #bbf71 where what may have been the hollow or bottom of the crater is filled with basalt4 ,illustration3 ,fig4 #aca4--,section of south end of ,dumbuck ,hill4 ,east of ,dumbarton4 ,illustration3 ,fig4 #acb4--,section across the ,east ,lothian plateau to show the relative position of one of the necks4 #a4 ,lower ,carboniferous sandstones and shales2 #b4 ,red and green tuffs with a seam of limestone 7l72 #c4 ,band of basic sheets at the base of the lavas2 #d4 ,trachytes2 #e4 ,phonolite neck4 7b7 ,necks of ,andesite1 ,trachyte1 ,dolerite1 ,diabase1 or other massive ,rock4--,when the vents have been filled by the uprise of some molten rock1 it is generally1 as we have seen1 of a more acid character than the ordinary lavas of the plateaux4 ,frequently it consists of some variety of trachyte or andesite1 commonly of a dull yellow or grey tint and waxy lustre4 ,good examples may be seen among the remarkable group of necks on either side of the valley north of the village of ,strathblane and in those above ,bowling4 ,the three great necks in ,east ,lothian1 already alluded to1--,traprain ,law 7,figs4 #acb1 #acc71 ,north ,berwick ,law 7,fig4 #aji71 and the ,bass ,rock 7,fig4 #aaj7--;are masses of phonolite and trachyte1 obviously related to the trachytes of the adjacent plateau4 ,a smaller but very perfect instance of a vent similarly filled is to be seen in the same neighbourhood on the shore to the east of ,north ,berwick ,law4#dch ,footnote #dch3 ,see ,geology of ,east ,lothian1 ,geological ,survey ,memoir1 p4 #dj4 ,examples occur where the funnels of eruption have been finally sealed up by the rise of more basic material1 and this has happened even in a district where most of the lava-form necks consist of trachyte or some other intermediate lava4 ,thus1 in the ,campsie ,fells1 several such bosses appear1 of which the most conspicuous forms the hill of ,dungoil 7#acif feet1 ,fig4 #abh74 ,further west1 among the ,kilpatrick ,hills1 bosses of this kind are still more numerous4 ,the group of bosses near ,ancrum and ,jedburgh is mainly made up of olivine-dolerites and olivine-basalts 7,fig4 #acj74 ,this more basic composition of itself suggests that these bosses may be connected rather with the puy- than with the plateau-eruptions4 7c7 ,necks of ,composite ,character4--,in not a few examples1 the vents have been filled with agglomerate which has been pierced by a plug or veins of lava-form material4 ,many illustrations of this composite structure may be observed along the west front of the great escarpments from ,fintry to ,ardrossan 7see ,figs4 #abd1 #abe1 #abg and #abh74 ,in that region the intruded rock is often a dull yellowish or grey trachytic or andesitic material4 ,olivine-basalt is the chief rock intruded in the vents in the ,dumbarton district4 ,among the ,roxburghshire vents1 where the injected material is commonly olivine-basalt or dolerite1 it occasionally happens1 as in ,rubers ,law1 that the uprise of the lava has almost entirely cleared out or concealed the agglomerate1 and in some of the bosses1 where no agglomerate is now to be seen1 the basalt may have taken its place 7,fig4 #acj74 ,the largest and most interesting vents connected with this type of ,carboniferous volcano1 are those which occur within the limits of the plateaux1 where they are still surrounded with lavas and tuffs that probably came out of them4 ,of these by far the most extensive and remarkable lies among the high moorlands of ,renfrewshire between ,largs and ,lochwinnoch1 where the ground rises to more than #agjj feet above the sea 7see ,fig4 #abi74 ,this area1 as already remarked1 is unfortunately much obscured with drift and peat1 so that the limits of its rocks cannot be so satisfactorily traced as might be desired4 ,i think it probable that several successive vents have here been opened close to each other1 but their erupted ashes probably cannot be distinguished4 ,over a space measuring about four miles in length by two and a half in breadth1 the rocks exposed at the surface are fine tuffs1 breccias and coarse agglomerates1 largely made up of trachytic1 andesitic or felsitic material1 and pierced by innumerable protrusions of various andesitic1 trachytic or felsitic rocks in bosses and veins1 as well as also by dykes of a more basic kind1 such as dolerites and basalts4 ,some of the tuffs present a curiously indurated condition2 and they are frequently much decayed at the surface4#dci ,another large mass of tuff and agglomerate lies a little to the south-west of the main area4 ,footnote #dci3 ,this tract of ground was mapped for the ,geological ,survey by ,mr4 ,r4 ,l4 ,jack1 now in charge of the ,geological ,survey of ,queensland4 ,see ,sheet #ca1 ,geological ,survey of ,scotland4 ,after the explosions ceased1 by which the vents were opened and the cones of debris were heaped up1 heated vapours would in many cases1 as in modern volcanoes1 continue for a long while to ascend in the funnels4 ,the experiments of ,daubre on the effects of water and vapour upon silicates under great pressure and at a low red heat1 have shown how great may be the lithological changes thereby superinduced4 ,it is improbable that where a mass of tuff and lava1 lying deep within a volcanic vent1 was thoroughly permeated with constantly ascending heated vapours1 it should escape some kind of change4 ,i am inclined to attribute to this cause the frequent conversion of the sandstones round the walls of the vents into quartzite4 ,the most remarkable example of metamorphism within a vent which ,i have observed among the plateaux1 occurs in the heart of the ,campsie ,fells1 where1 instead of forming a prominence1 the neck is marked by a great hollow1 measuring about a mile in length and half a mile in breadth 7,fig4 #abh74ddj ,it is occupied mainly by a coarse tumultuous agglomerate1 like that of other necks in the same district1 but with a matrix rather more indurated1 and assuming in certain parts a crystalline texture1 so as to be at first sight hardly distinguishable from some of the surrounding andesites4 ,even in this altered condition1 however1 its included fragments may be recognized1 particularly blocks of sandstone which have been hardened into quartzite4 ,numerous small veins of pink and yellow trachyte traverse the agglomerate1 and are found also cutting the bedded andesites that encircle it4 ,footnote #ddj3 ,see ,explanation to ,sheet #ca1 ,geological ,survey of ,scotland1 par4 #ba 7#ahgh74 ,illustration3 ,fig4 #acc4--,view of ,traprain ,law from the south1 a phonolite neck of the ,garleton ,plateau4 #c4 ,,dykes ,,and ,,sills ,intrusive masses both in the form of dykes and of sills are of frequent occurrence in connection with the ,carboniferous volcanic plateaux4 ,from the variety of their component materials it may be inferred that these rocks belong to different ages of intrusion4 ,dykes4--,the great majority of the ,dykes consist of trachyte or of andesite1 resembling in lithological characters the material of the necks and doubtless connected with its uprise4 ,there occur also dykes of diabase1 basalt or dolerite4 ,some of the latter1 especially those which run for many miles1 cutting every rock in the districts in which they occur1 and crossing large faults without deviation1 are certainly long posterior to the plateau volcanic period4 ,whether the small inconstant dykes of more basic composition1 found in the same districts with the trachytes1 are to be looked upon as part of the volcanic phenomena of the plateaux1 is a question to which at present no definite answer can be given4 ,i shall have occasion to show that in the next volcanic period the lavas that flowed from the puys are more basic than most of those of the plateaux1 and that they are associated with more basic dykes and sills4 ,in ,roxburghshire1 where it is so difficult to distinguish between the denuded vents of the two periods1 the dark heavy olivine-basalts and dolerites of the bosses may possibly belong rather to the later than to the earlier volcanic episode4 ,and if that be their true age1 the dykes of similar material may be connected with them4 ,at the same time it must be remembered that the earliest eruptions of the plateaux were markedly basic1 that many vents in the plateaux are pierced by basic intrusions1 and that basic dykes may have been associated with the uprise of the same magma4 ,the dykes occur in considerable numbers and in two distinct positions1 though these may be closely related to each other3 #ast1 among the rocks outside and beneath the plateau-lavas1 or cutting these lavas2 and #bnd1 in and around the vents4 #a4 ,among the rocks which emerge from under the ,carboniferous volcanic plateaux1 dykes are sometimes to be observed in considerable numbers4 ,they may be compared to the far more extensive series connected with the ,tertiary basalt-plateaux1 like which they may have had a close relation to the actual building up of the successive sheets of andesite1 trachyte and basalt that were erupted at the surface4 ,they are particularly well developed in the ,clyde plateau1 where by extensive denudation they have been admirably exposed4 ,i would especially refer to those that traverse the tract of red sandstones which underlie the volcanic series along the flanks of the great escarpments from ,fintry to ,strathblane and ,dumbarton1 and between ,gourock and ,ardrossan4 ,these dykes have been dissected by the sea along both sides of the estuary of the ,clyde and in the islands of ,cumbrae4 ,in these islands and in ,bute they have recently been mapped in great detail for the ,geological ,survey by my colleague1 ,mr4 ,w4 ,gunn1 who has supplied me with notes of his observations on the subject1 from which the following summary is compiled4 ,there are at least four distinct groups of intrusive rocks in the ,greater ,cumbrae4 ,the oldest of these is trachytic in character1 and occurs both as dykes and sheets1 which run generally in the same ,e4,n4,e4 direction4 ,the rock is usually pinkish in colour1 sometimes grey or purplish4 ,a specimen from the dyke of the ,hawk's ,nest1 north of ,farland ,point1 analyzed by ,mr4 ,teall1 was found to contain #aa per cent of alkalies1 principally potash1 while the percentages of lime and iron were very low4 ,sometimes these rocks are fine in grain with only a few porphyritic orthoclase crystals1 though numerous small crystals of this mineral are found with the aid of the microscope4 ,these red trachyte dykes are almost confined to the ,upper ,old ,red ,sandstone1 rarely entering the overlying white ,calciferous ,sandstones1 and never invading the plateau-lavas4 ,they are therefore probably of early ,carboniferous age4 ,the next group follows the same general direction1 but clearly traverses the trachytes1 and must therefore be of later date4 ,the dykes of this group are the most numerous of the whole1 the greater part of the island being intersected by them4 ,in the north-east corner about #dj of them may be counted in half a mile of coast-line1 some being of large size4 ,all of them which can be clearly made out are porphyritic olivine-basalts of the type of the ,lion's ,haunch at ,arthur's ,seat4 ,they are generally grey in colour and finer at the edges than in the centre1 which is often coarsely porphyritic and amygdaloidal4 ,olivine seems always characteristic1 but has often been replaced by hmatite or calcite4 ,in ,bute a good many dykes have been mapped to the north of ,kilchattan ,bay resembling this basalt series of ,cumbrae1 and running in the same direction4 ,but they appear to be all porphyritic andesites4 ,the second group of dykes1 though it cuts the first and is thus proved to be later in date1 is nevertheless confined within the same stratigraphical limits4 ,it may thus belong nearly to the same period of intrusion4 ,the dykes of the third group are dolerites without olivine1 and follow on the whole an east and west direction4 ,they cut both of the two foregoing sets of dykes1 and likewise the lavas of the plateau4 ,they must thus belong to a far later period of intrusion4 ,they may be connected with other dykes and sills on the mainland1 which traverse the ,coal-measures1 and would thus be not older than late ,carboniferous or ,permian time4 ,the fourth group of dykes intersects all the others1 and is probably of ,tertiary age4 ,the prevalent direction of these dykes in the ,cumbraes is ,n4,n4,w4 ,the ,tertiary dykes are more fully described in ,chapters xxxiv4 and xxxv4 ,the great group of tuffs which underlies the lavas of the ,east ,lothian plateau is traversed by numerous dykes and sills1 of which many good examples may be seen in the coast-cliffs of ,north ,berwick4 ,among these rocks are beautiful olivine-basalts with singularly fresh olivine1 as on the shore at ,north ,berwick4 ,some of them are still more basic1 as in the case of a limburgite intrusion at the ,gin ,head1 ,tantallon ,castle4 ,illustration3 ,fig4 #acd4--,veins and dykes traversing the agglomerate and tuff of the great ,renfrewshire vent4 #b4 ,in the necks1 dykes are sometimes abundant1 and they may be observed occasionally to traverse the surrounding lavas4 ,they consist of similar materials to those found outside the plateaux4 ,some of the larger necks are intersected by a network of dykes and veins4 ,the great vent or group of vents among the uplands of ,renfrewshire1 already described 7,fig4 #abi71 furnishes some admirable examples of this characteristic volcanic feature4 ,an illustration from that locality forms the subject of ,fig4 #acd4 ,the agglomerate which fills the large hollow among the ,campsie ,hills may be quoted as another illustration 7,fig4 #abh74 ,further instances will be found in some of the sections given in preceding pages 7see ,figs4 #abd1 #abe1 #abg74 ,the general aspect of a dyke in the volcanic series is shown in ,fig4 #ace4 ,illustration3 ,fig4 #ace4--,the ,yellow ,man1 a dyke in volcanic tuff and conglomerate on the shore a little east of ,north ,berwick4 ,the ,sills associated with the plateau-type of ,carboniferous volcanic action form a less prominent feature than they do among the earlier ,palozoic formations or in the puy-type which succeeded them4 ,they consist in general of short lenticular sheets of andesite or trachyte1 like the necks and dykes in proximity to which they commonly appear4 ,the best area for the study of them is the ground which stretches out from the base of the great escarpments of the ,campsie1 ,kilpatrick and ,ayrshire ,hills 7,fig4 #acf71 where1 among the agglomerate-vents and abundant dykes1 intrusive sheets have likewise been injected between the bedding-planes of the red sandstones4 ,but these sheets are of comparatively trifling dimensions4 ,very few of them reach a mile in length1 the great majority falling far short of that size4 ,in the ,cumbraes and in ,bute1 ,mr4 ,gunn has observed that the trachytic1 olivine-basalt and dolerite dykes are apt to pass into intrusive sheets4 ,that the sills1 as well as the dykes and bosses of the same material1 are not of older date than the lavas of the plateaux is proved by the manner in which they pierce these lavas1 especially towards the bottom of the series4 ,the general absence of basic sills1 when we consider how thick a mass of these rocks has sometimes been poured out in the plateaux1 is not a little remarkable4 ,only in the basin of the ,firth of ,forth do we encounter thick basic sills near the plateaux1 such1 for instance1 as ,salisbury ,crags at ,edinburgh4 ,but it is doubtful whether they ought not rather to be classed with the sills of the puys1 to be afterwards described4 ,illustration3 ,fig4 #acf4--,trachytic sills1 ,knockvadie1 ,kilpatrick ,hills4 #a4 ,upper ,old ,red ,sandstone2 #b4 ,ballagan ,beds2 #c4 ,tuffs2 #d4 ,lavas of the ,plateau2 #e4 ,agglomerate of necks2 #f4 ,trachyte sills2 #g4 ,dolerite dyke 78 ,tertiary74 #d4 ,close of the ,plateau-eruptions ,the relative geological date when the eruptions of each plateau ceased can fortunately be determined with much more precision than the time of their beginning4 ,the ,hurlet ,limestone1 so well known as the lowest thick calcareous seam in the ,carboniferous ,limestone series1 of which it is generally taken as the base1 can be identified over the whole of ,central ,scotland1 and thus forms an excellent stratigraphical horizon1 from which the upward termination of the volcanic sheets underneath it can be measured4 ,when the volcanic episode of the plateau-eruptions came to an end1 such banks or cones as rose above the level of the shallow sea which then overspread ,central ,scotland were brought beneath the water1 as ,i have already remarked1 either by prolonged denudation or more probably in large part by the continued subsidence of the region4 ,the downward movement may possibly for a time have been accelerated1 especially in some districts4 ,thus the ,hurlet ,limestone1 though usually not more than five or six feet thick1 increases locally to a much greater thickness4 ,at ,petersfield1 near ,bathgate1 for example1 it is between #gj and #hj feet in depth1 while at ,beith1 in ,north ,ayrshire1 it increases to #ajj feet 7,fig4 #acg71 which is the thickest mass of ,carboniferous ,limestone known to exist in ,scotland4 ,at both of these localities the limestone lies upon a series of volcanic rocks1 and we may perhaps infer that the subsidence advanced there somewhat more rapidly or to a greater extent1 so as to form hollows in which the limestone could gather to an abnormal depth4 ,the water would appear to have become for a time tolerably free from mechanical sediment4 ,the limestone is hence comparatively pure1 and is extensively quarried all over the country for industrial purposes4 ,it is a crinoidal rock1 abounding in many species of corals1 brachiopods1 lamellibranchs1 and gasteropods1 with trilobites1 cephalopods1 and fishes4 ,illustration3 ,fig4 #acg4--,section across the edge of the ,clyde plateau1 south-east of ,beith4 #a4 ,plateau-lavas2 #b4 ,tuffs and volcanic conglomerates2 #c4 ,hurlet ,limestone2 #d4 ,coal-bearing strata above the limestone2 #e4 ,dolerite dyke4 ,a variable thickness of strata intervenes between the top of the volcanic series and the ,main ,limestone4 ,sometimes these deposits consist in large measure of a mixture of ordinary sandy and muddy material with the washed-down tuff of the cones1 and probably with volcanic dust and lapilli thrown out by the latest eruptions4 ,thus along the flank of the hills from ,barrhead to ,strathavon1 yellow and green ashy sandstones1 grits and conglomerates are succeeded by ordinary sandstones1 black shales and ironstones1 while here and there true volcanic tuff and conglomerate make their appearance4#dda ,further west1 in the ,kilbirnie district1 the limestone lies directly on the tuffs that rest upon the andesites 7,figs4 #acg1 #ach74 ,footnote #dda3 ,explanation of ,sheet #bb1 ,geol4 ,surv4 ,scotland1 p4 #ab4 ,illustration3 ,fig4 #ach4--,section across the upper part of the ,clyde plateau at ,kilbirnie1 ,ayrshire4 #a #a4 ,plateau-lavas2 #b #b4 ,tuffs2 #c #c4 ,hurlet ,limestone2 #d4 ,black-band ,ironstone4 f f4 ,faults4 ,but perhaps the most striking contrast between adjacent localities in regard to the distance between the limestone and the top of the volcanic series is to be observed along the southern front of the ,campsie ,fells4 ,in spite of the abundant faults which have there so broken up the regular sequence of the rocks1 we can see that at ,banton and ,burnhead the limestone lies almost immediately on the volcanic series 7,fig4 #aci74 ,but a little to the westward1 sandstones1 conglomerates1 shales and thin limestones begin to intervene between the volcanic series and the ,hurlet ,limestone and swell out so rapidly that on ,craigmaddie ,muir and ,south ,hill of ,campsie1 only some five miles off1 they must form a total thickness of not less than from #fjj to #hjj feet of ordinary non-volcanic deposits1 chiefly thick pebbly sandstones 7,fig4 #adj74 ,such local variations not improbably serve to indicate hollows on the flanks of the plateaux that were filled up with detritus before the depression and clearing of the water that led to the deposition of the ,hurlet ,limestone4 ,illustration3 ,fig4 #aci4--,section across the upper surface of the ,clyde volcanic plateau1 ,burnhead1 north-west of ,kilsyth4 #a4 ,lavas of the plateau2 #b4 ,tuffs2 #c4 ,hurlet ,limestone2 #d4 ,hosie's ,limestone2 f1 ,fault4 ,illustration3 ,fig4 #adj4--,section across the upper surface of the ,clyde volcanic plateau at ,campsie4 #a4 ,shales1 sandstones1 cement-stones1 etc4 7,ballagan ,beds72 #b4 ,lavas of the plateau2 #c4 ,thick white sandstone and conglomerate2 #d4 ,hurlet ,limestone2 #e4 ,hosie's ,limestone2 f4 ,fault4 ,illustration3 ,fig4 #ada4--,section across western edge of the ,garleton plateau4 #a4 ,trachyte lavas of the plateau2 #b4 ,calciferous ,sandstones2 #c4 ,hurlet ,limestone4 ,i have already remarked that the eruptions of the plateau period lasted longer in the western than in the eastern parts of the region4 ,in the ,garleton district1 where the peculiar viscous trachytic lavas probably gave rise to a more uneven surface or more prominent cones than was usual among the andesitic plateaux1 the eruptions ceased some time before the deposition of the ,hurlet ,limestone4 ,as the area sank1 the successive zones of the ,calciferous ,sandstones crept over the flanks of the trachytes1 until at last they had completely buried these rocks before the limestone spread over the area 7,fig4 #ada74 ,in consequence1 probably1 of the uneven surface of this plateau1 there is here a strong overlap of the higher part of the ,calciferous ,sandstones4 ,on the west side of the volcanic area there can hardly be more than some #bjj feet of strata between the top of the trachytic series and the limestone1 while on the south side there must be greatly more than that thickness4 ,this structure probably indicates that the ,garleton volcanoes became extinct after having piled up a mass of tuffs and lavas to such a height that its summits were not submerged until the area had subsided #hjj or #ajjj feet in the waters1 over the floor of which the ,calciferous ,sandstones were laid down4 ,hence1 in spite of the proximity of the lavas to the limestone1 there may have been a vast interval of time between their respective epochs1 as has been already suggested with regard to other plateaux4 ,this subject will be again referred to in discussing the relative chronology of the plateaux and puys4 ,in the ,berwickshire and ,solway districts1 the extinction of the plateau-vents appears to have taken place at a still earlier part of the ,carboniferous period1 for there the andesites1 while they rest on the ,upper ,old ,red ,sandstone1 are covered with at least the higher group of the ,calciferous ,sandstones 7,fig4 #adb74 ,the equivalent of the ,hurlet ,limestone of ,central ,scotland must lie many hundred feet above them4 ,the submergence of the plateaux1 and their entombment under the thick ,carboniferous ,limestone series1 did not mark the close of volcanic activity in ,central ,scotland during ,carboniferous time4 ,the plateau-type of eruption ceased and was not repeated1 but a new type arose1 to which ,i would now call the reader's attention4 ,illustration3 ,fig4 #adb4--,section across the ,solway plateau from ,birrenswark to ,kirtlebridge4 #a4 ,upper ,silurian strata2 #b4 ,upper ,old ,red ,sandstone2 #c4 ,plateau-lavas2 #d4 ,calciferous ,sandstones and ,carboniferous ,limestone series2 #e4 ,trias4 ,,chapter ,,xxvi ,,the ,,carboniferous ,,puys ,,of ,,scotland i4 ,general ,character and ,distribution of the ,puys2 ii4 ,nature of the ,materials ,erupted--,lavas ,ejected at the ,surface--,intrusive ,sheets--,necks and ,dykes--,tuffs4 i4 ,,general ,,character ,,and ,,distribution ,after the beginning of the ,carboniferous ,limestone period1 when eruptions of the plateau-type had generally ceased1 volcanic activity showed itself over the area of the ,british ,isles in a different guise both as regards the nature of its products and the manner and scale of their discharge4 ,instead of widely extended lava-sheets and tuffs1 piled above each other sometimes to a thickness of many hundred feet1 and stretching over hundreds of square miles1 we have now to study the records of another phase of volcanism1 where scattered groups and rows of ,puys1 or small volcanic cones1 threw out in most instances merely tuffs1 and these often only in trifling quantity1 though here and there their vents also poured forth lavas and gradually piled up volcanic ridges which1 in a few cases1 almost rivalled some of the plateaux4 ,the evidence for these less vigorous manifestations of volcanic activity is furnished 7#a7 by layers of tuff and sheets of basaltic-lavas intercalated among the strata that were being deposited at the time of the eruptions1 7#b7 by necks of tuff1 agglomerate1 or different lava-form rocks that mark the positions of the orifices of discharge1 and 7#c7 by sills1 bosses1 and dykes that indicate the subterranean efforts of the volcanoes4 ,the comparatively small thickness of the accumulations usually formed by these vents1 their extremely local character1 the numerous distinct horizons on which they appear1 and the intimate way in which they mingle and alternate with the ordinary ,carboniferous strata are features which at once arrest the attention of the geologist1 presenting1 as they do1 so striking a contrast to those of the plateaux4 ,from the clear intercalation of these volcanic materials on successive platforms of the ,carboniferous system1 the limits of geological time within which they were erupted can be fixed with considerable precision4 ,it may be said that1 in a broad sense1 they coincided with the period of the ,carboniferous ,limestone1 and certainly it was during the deposition of that formation that the eruptions which produced them reached their greatest vigour and widest extent4 ,here and there in ,scotland evidence may be found that the phase of the ,puys began during that earlier section of ,carboniferous time recorded by the ,calciferous ,sandstones4 ,this is markedly the case in ,liddesdale and the neighbouring territory4 ,over the western part of ,midlothian also1 the eastern portion of ,west ,lothian1 and the southern margin of ,fife1 abundant traces occur of puy-eruptions during the deposition of the ,calciferous ,sandstones4 ,elsewhere in ,central ,scotland there is no evidence of the vents having been opened until after the deposition of the ,hurlet ,limestone1 which1 as we have seen1 may conveniently be taken as the base of the ,scottish ,carboniferous ,limestone series4 ,the volcanoes remained active in ,west ,lothian until near the close of the time represented by that series2 but in ,ayrshire they continued in eruption until the beginning of the accumulation of the ,coal-measures4 ,these western examples of the puy-type are1 so far as ,i am aware1 the latest known in ,britain4 ,whether or not the earliest puy-eruptions began before the latest plateau-lavas and tuffs were accumulated is a question that cannot be readily answered4 ,it will be remembered that in the basin of the ,firth of ,forth a thickness of more than #cjjj feet of sedimentary strata1 including the ,burdiehouse ,limestone and numerous oil-shales as well as thin coal-seams1 lies above the red and green marls1 shales1 sandstones and cement-stones of the ,calciferous ,sandstone series4 ,this remarkable assemblage of strata is absent in the western parts of the country1 where the top of the ,clyde volcanic plateau is almost immediately overlain by the ,hurlet ,limestone4 ,if we were to judge of the sequence of events merely from the stratigraphy1 as expressed in such sections as ,figs4 #acg1 #ach1 #aci and #adj1 we might naturally infer that as no trace of any break occurs at the top of the ,clyde plateau1 the tuffs shading upward there into the limestone series1 no important pause in sedimentation took place1 but that the last volcanic eruptions were soon succeeded by the conditions that led to the deposition of the widespread encrinite-limestones4 ,if this inference were well founded it would follow that while the plateau-eruptions in the west lasted till the time of the ,hurlet ,limestone1 those in the east ceased long before that time and were succeeded by the puys of ,fife and the ,lothians4 ,there would thus be an overlap of the two phases of volcanic action4 ,i am inclined to believe1 however1 that in spite of the superposition of the ,hurlet ,limestone almost immediately upon the volcanic rocks of the ,clyde plateau1 and the absence of any trace of a break in the process of sedimentation1 a long interval nevertheless elapsed between the last eruptions and the deposit of that limestone4 ,the ,campsie section 7,fig4 #adj7 shows us how rapidly a thick mass of strata can come in along that horizon4 ,the volcanic ridges may have remained partly unsubmerged for such time as was required for the subsidence of the ,forth basin and the deposit of the thick ,calciferous ,sandstone series there1 and their summits may only have finally sunk under the sea not long before the ,hurlet ,limestone grew as a continuous floor of calcareous material over the whole area of central ,scotland4 ,in these circumstances1 the puy-eruptions of that basin would be long subsequent to the eruptions of the ,clyde plateau1 as they certainly were to those of the plateaux of ,midlothian and the ,garleton ,hills4 ,in tracing the geographical distribution of the puy-eruptions we are first impressed with the force of the evidence for their extremely local and restricted character 7,map ,,iv474 ,thus in the area of the basin of the ,firth of ,forth1 which may be regarded as the typical region in ,britain for the study of this form of ,carboniferous volcano1 traces of them are abundant to the west of the line of the ,pentland ,hills4 ,to the east of that line1 however1 not a vestige of puy-eruptions1 save a few sills of uncertain relationship1 can be detected1 though the same series of stratigraphical horizons is well developed on both sides of the ,lothian coal-field4 ,again1 to the westward of the ,forth basin over the area of ,stirlingshire1 ,lanarkshire and ,renfrewshire lying to the north of the great volcanic plateau1 no record of puy-eruptions has been noticed4 ,immediately to the south of that plateau1 however1 these eruptions were numerous in the north of ,ayrshire4 ,yet the rest of the ,carboniferous area in that large county has supplied no relics of these eruptions save at one locality--the ,heads of ,ayr4 ,lastly1 while no trace of any younger display of volcanic activity occurs in the ,merse of ,berwickshire1 east of the plateau series of that district1 the ground immediately to the west abounds in puys1 and contains likewise extensive sheets of tuff and beds of basic lavas connected with these vents4 ,another fact which at once attracts notice in ,scotland is the way in which the puy-vents have generally avoided the areas of the plateaux1 though they sometimes approach them closely4 ,as a rule1 it is possible to distinguish the tuffs and agglomerates which have filled up these vents from those that mark the sites of the eruptive orifices of the plateaux4 ,there are1 no doubt1 some instances1 as in ,liddesdale1 where puys have appeared on the sites of the older lavas1 but these are exceptional collocations4#ddb ,on the other hand1 many examples may be found where puys have risen in the interspace between the limits of the eruptions of two plateau-areas4 ,thus the tract between the ,clyde plateau-eruptions on the west and those of the ,garleton ,hills on the east was dotted over with puys4 ,again1 the southern margin of the ,clyde plateau in ,ayrshire1 from ,dalry to ,galston is flanked with puys and long sheets of their lavas and tuffs4#ddc ,footnote #ddb3 ,a means of definitely placing some of these vents in the series of puy-eruptions is stated further on1 at p4 #dgf4 ,footnote #ddc3 ,reference may again be made here to the remarkable similarity between the ,scottish ,carboniferous puy-vents and those of older ,tertiary time in the ,swabian ,alps so fully described by ,professor ,branco in the work already cited p4 #df4 ,denudation in that region has bared the cones and exposed the structure of the necks which1 down to even minute details1 repeat the phenomena of ,carboniferous and ,permian time in ,scotland4 ii4 ,,nature ,,of ,,the ,,materials ,,erupted ,a4 ,the ,lava-form ,rocks ,we have now to consider the nature of the materials erupted by the volcanic activity of the puys4 ,the geologist who passes from the study of the plateau lavas to those of the puys at once remarks the prevalent more basic character of the latter4 ,the great majority of them are basalts1 generally olivine-bearing1 in the various types embraced in the table on the following page4 ,the olivine-free dolerites are generally found as intrusive bosses1 sills and dykes4 ,such more acid rocks as andesites occur only rarely1 and still more seldom are quartziferous masses met with in some of the bosses4 ,dolerites and ,basalts4--,the great majority of the lava-form rocks connected with the puys are basic in composition1 and belong to the family of the ,dolerites and ,basalts4 ,they graduate1 on the one hand1 into ultra-basic rocks such as limburgite and picrite1 and on the other1 into compounds that approach andesites or trachytes in composition4 ,a large series of specimens from ,central ,scotland was studied a few years ago by ,dr4 ,hatch1 who proposed a petrographical classification of the rocks1 and arranged them in a number of types which he named after localities where they are well developed4#ddd ,more recently the rocks have again been subjected to microscopic investigation by my colleague ,mr4 ,watts1 who1 confirming generally ,dr4 ,hatch's discriminations1 has made some modifications of them4 ,he has furnished me with a revised classification 7p4 #dah71 based on purely petrographical considerations4 ,the doleritic and basaltic series may be grouped into two divisions1 one with1 and the other without1 olivine1 and each division may be further separated into a dolerite group1 which presents an ophitic or sub-ophitic structure1 and a basalt-group in which the groundmass is made up of felspar and granular augite1 and possesses the intersertal structure of ,rosenbusch1 or consists of idiomorphic augite embedded in felspar substance4 ,the term sub-ophitic is employed by ,mr4 ,watts to imply that the augite grains are neither very large nor very continuous1 optically1 and that they rarely contain entire felspar-crystals imbedded in them1 merely the ends of a group of these crystals as a rule penetrating into them4 ,footnote #ddd3 ,this classification was given in my ,presidential ,address to the ,geological ,society1 #ahib1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xlviii4 p4 #abi4 ,see ,report of ,geological ,survey for #ahif4 ,transitional forms occur between many of the following types by the increase or diminution in the relative proportions of the constituents4 ,thus it is not easy to draw a line between #b;b and #b;c2 the latter again shades into #b;d and #b;e by the decrease of the felspar4 ,mr4 ,watts has further observed that the rocks containing no olivine offer greater difficulties in classification than those in which that mineral is present4 ,the very distinction1 he remarks1 between dolerites and basalts is less marked1 the types are much less sharply distinguished1 and decomposition and masking of the structure are more common4 ,while using the term ,dolerite for such rocks as have a sub-ophitic structure1 ,i have extended it to those rocks in which evidence exists that a great part of the crystallization took place under intratelluric conditions4 ,although not quite holocrystalline1 the crystals of felspar1 augite and magnetite are large and the structure coarse-grained1 while the groundmass is confined to comparatively small interstitial patches4 ,in these rocks there is usually no one dominant porphyritic ingredient4 ,i4 ,the ,olivine-bearing ,series #a4 ,olivine-,dolerites #a;a4 ,porphyritic elements inconspicuous1 olivine being the principal1 and felspar of secondary importance2 ,jedburgh ,type4 groundmass sub-ophitic4 #a;b4 ,strongly porphyritic2 felspar-phenocrysts large2 ,kilsyth ,type4 olivine smaller2 groundmass sub-ophitic4 #a;c4 ,porphyritic olivine1 but not felspar2 groundmass ,gallaston ,type4 sub-ophitic4 #b4 ,olivine-,basalts #b;a4 ,porphyritic olivine1 augite and felspar2 groundmass ,lion's ,haunch of felspar-laths imbedded in granules of augite4 ,type4 7,see ,fig4 #bjg47 #b;b4 ,porphyritic olivine and augite2 groundmass of felspar-laths imbedded in granules of augite4 ,more ,craiglockhart rarely the groundmass is made of idiomorphic augite ,type4 imbedded in felspar-substance4 #b;c4 ,porphyritic olivine abundant1 augite much less common1 and felspar very rare or absent2 groundmass ,dalmeny ,type4 with granular or idiomorphic augite 7one of the most common types74 #b;d4 ,porphyritic olivine more common than augite2 groundmass of granules of augite set amongst lath-like ,picrite ,type4 felspars which are much fewer in number than in #b;c4 #b;e4 ,porphyritic olivine more common than augite2 groundmass of idiomorphic augite imbedded in ,limburgite felspathic material which is not abundant4 ,type4 #b;f4 ,porphyritic olivine and felspar1 without augite2 ,kippie ,law ,type4 groundmass of granular or idiomorphic augite1 with 7,for analysis lath-shaped felspars4 see p4 #cgi74 ,,ii4 ,the ,non-olivine-bearing ,series #c4 ,olivine-free ,dolerites ,felspar1 augite1 magnetite in coarse-grained aggregate usually ophitic or sub-ophitic2 groundmass not plentiful4 #c;a4 ,groundmass absent ,ophitic ,type4 #c;b4 ,groundmass micropegmatitic ,ratho ,type4 #c;c4 ,groundmass an unstriated felspar 7not orthoclase7 and occasionally some interstitial altered glass or a ,burntisland little quartz4 ,sill ,type4 #d4 ,doleritic ,basalts ,felspar1 augite and magnetite in coarse-grained aggregate2 groundmass rather more plentiful and often in large patches4 #d;a4 ,felspar and augite1 related sub-ophitically where together1 but augite showing crystalline contours in ,bowden contact with the groundmass2 some interstitial quartz ,hill ,type4 and unstriated felspar4 #e4 ,basalts ,finer-grained rocks1 generally with a porphyritic ingredient and much scattered interstitial matter in the groundmass4 #e;a4 ,porphyritic felspar1 and occasionally a little augite2 groundmass of granular augite1 felspar needles ,binny ,craig and magnetite with some interstitial matter4 ,type4 #e;b4 ,porphyritic felspars not conspicuous and small2 the rock mainly made up of a mesh of fine felspar-laths set ,tholeiite amongst granular augite1 magnetite and base4 ,type4 #e;c4 ,similar to the last but even finer-grained1 and ,crypto- with the base in a cryptocrystalline condition4 crystalline ,type4 ,taking first the superficial lavas1 ,i know of only one locality where picrite occurs in such a position that it may be included among the surface outflows4 ,this is the quarry near ,blackburn1 to the east of ,bathgate1 where ,i originally observed it4#dde ,the rock occurs there on the line of the basalt-flows from the ,bathgate ,hills1 and ,i mapped it as one of them before the microscope revealed the remarkable composition of the mass4 ,i still believe it to be a lava like the leckstone described on p4 #ddc1 though the other known examples of this rock in the basin of the ,firth of ,forth are intrusive sheets4 ,the rock locally known as leckstone or lakestone has long been quarried for the purpose of constructing the soles of bakers' ovens1 as it stands a considerable temperature without cracking4 ,its microscopic structure is now well known4 ,as exposed in ,blackburn quarry1 an interesting difference is observable between the lower and upper parts of the sheet4 ,the lower portion is a picrite1 with abundant serpentinized olivine1 large crystals of augite1 and a considerable amount of ores4 ,the upper portion1 on the other hand1 has plagioclase as its most abundant definite mineral1 with a minor quantity of minute prisms of augite and of iron-ores1 and scattered crystals of olivine4 ,here1 within the compass of a few yards and in one continuous mass of rock1 we have a transition from a variety of olivine-basalt into a picrite4 ,footnote #dde3 ,trans4 ,roy4 ,soc4 ,edin4 vol4 xxix4 7#ahgi7 p4 #ejf4 ,the great majority of the puy lavas belong to the olivine-bearing series4 ,a few of them are dolerites1 but most are true basalts of the ,dalmeny type1 of which typical examples may be seen at the ,kirkton quarries1 ,bathgate1 and in the coast section between ,pettycur and ,kinghorn4 ,occasionally they present transitions towards picrite1 as in the sheet overlying the lowest limestone at ,kirkton1 and in the lowest lava of ,king ,alexander's ,crag1 ,burntisland4 ,these puy lavas exhibit considerable variety of structure as seen in the field4 ,some are solid1 compact1 black rocks1 not infrequently columnar and weathering into spheroidal exfoliating forms4 ,others are somewhat granular in texture1 acquiring green and brown tints by weathering1 often showing amygdaloidal kernels1 and even passing into well-marked amygdaloids4 ,many of them exhibit a slaggy structure at their upper and under surfaces 7,figs4 #aec1 #agj1 #aga74 ,these external differences are an index to the corresponding variations in composition and microscopic structure enumerated in the foregoing tabular arrangement4 ,as a rule1 the basic rocks which occur intrusively in connection with the puys1 especially where they form a considerable mass1 have assumed a much more coarsely crystalline texture than those of similar composition which have been poured out at the surface4 ,they are generally dolerites rather than basalts4 ,but with this obvious distinction1 the two groups have so much in common1 that the geologist who passes from the study of the subterranean phenomena of the ,plateaux to that of the corresponding phenomena of the ,puys is at once impressed with the close relationship between the material which1 in the case of the puys1 has consolidated above ground1 and that which has been injected below4 ,there is no such contrast between them1 for example1 as that between the basic and intermediate lavas of the plateaux and the more acid intrusions associated with them4 ,by far the largest number of the basic sills1 bosses and dykes associated with the puys are somewhat coarsely crystalline dolerites without olivine4 ,they include1 however1 olivine-dolerites and basalts1 and even some extremely basic compounds4 ,of these last1 a typical example is supplied by the now well-known picrite of ,inchcolm1 in the ,firth of ,forth1 which occurs as an intrusive sheet among the ,lower ,carboniferous ,sandstones4#ddf ,in recent years one or two other picrite-sills have been observed in the same district4 ,an interesting example has been described from a railway cutting between ,edinburgh and ,cramond where the rock invades and alters shales4 ,more detailed reference to it will be made in the account of the sills connected with the puys4 ,another instance of the occurrence of this rock is in a railway cutting immediately to the west of ,burntisland where it has been intruded among the ,calciferous ,sandstones below the ,burdiehouse ,limestone4 ,footnote #ddf3 ,trans4 ,roy4 ,soc4 ,edin4 vol4 xxix4 7#ahgi7 p4 #ejf4 ,teall1 ,british ,petrography1 p4 #id4 ,rocks approaching limburgite occur among the sills and bosses which pierce the ,carboniferous ,limestone series of ,fife between ,cowdenbeath and ,inverkeithing4 ,one of these is found at ,pitandrew1 near ,fordel ,castle4 ,dr4 ,hatch observed that it consists of numerous porphyritic crystals of olivine1 with a few grains of augite and an occasional small lath-shaped crystal of felspar imbedded in a groundmass which is composed principally of idiomorphic augite microlites1 small crystals of a brown mica1 granules of magnetite and prisms of apatite4 ,in addition1 there is a considerable amount of interstitial matter1 which is partly colourless glass1 and partly shows a slight reaction between crossed nicols4 ,another example of the same type of rock occurs as a plug or boss in the tuff-vent of the ,hill of ,beath1 and a further display of the limburgite type is to be seen in ,dunearn ,hill near ,burntisland4 ,although olivine-basalts of the ,dalmeny type are most frequently met with as interstratified lavas1 they also occur as bosses and sills4 ,the typical example from ,dalmeny is itself intrusive4 ,other illustrations are to be found in the ,castle ,rock of ,edinburgh and in the sheets near ,crossgates and ,blairadam in ,fife4 ,the presence or absence of olivine1 however1 may sometimes be a mere accident of cooling or otherwise4 ,i have shown that in the same mass of rock at ,blackburn a gradation can be traced from a rock largely composed of altered olivine into one consisting mainly of felspar with but little olivine1 and another example occurs in the picrite-sill between ,edinburgh and ,cramond4 ,dr4 ,stecher has ascertained that the marginal portions of the sills in the basin of the ,firth of ,forth1 which cooled first and rapidly1 and may be taken1 therefore1 to indicate the mineral composition of the rock at the time of extrusion1 are often rich in olivine1 while that mineral may be hardly or not at all discernible in the main body of the rock4#ddg ,footnote #ddg3 ,dr4 ,stecher1 ,tschermak's ,mineralog4 ,mittheil4 vol4 ix4 7#ahhg7 p4 #aic4 ,proc4 ,roy4 ,soc4 ,edin4 vol4 xv4 7#ahhh7 p4 #afb4 ,of the ordinary and characteristic dolerites without olivine which constitute most of the intrusive masses1 the various types enumerated in the tabular arrangement are abundantly developed in ,central ,scotland4 ,thus the normal ophitic type is displayed by the uppermost sill of the ,burntisland series1 and by the rock which forms the plug of the ,binns ,hill neck in ,linlithgowshire4 ,the ,ratho type is well seen in the large sill at ,ratho1 likewise in the extensive intrusive sheets in the west of ,linlithgowshire as at ,muckraw and ,carribber4 ,the ,burntisland sill type is shown by the lower sills of ,burntisland and by some others in the same region1 especially by that of ,colinswell1 and by another on the shore east from the ,poorhouse1 near ,kinghorn4 ,the great boss among the ,bathgate ,hills likewise displays it4 ,the ,bowden ,hill type occurs in well-marked development at ,bowden ,hill1 three miles south-west of ,linlithgow1 and in the massive sill at ,st4 ,margaret's1 west from ,north ,queensferry4 ,the non-olivine-bearing basalts are found in various bosses and sheets in the basin of the ,firth of ,forth4 ,thus the ,binny ,craig type occurs in the prominent and picturesque sill from which it is named1 likewise among the intrusive sheets near ,kirkcaldy1 in ,fife4 ,sometimes the same mass of rock displays more than one type of structure1 as in the case of the great ,galabraes neck among the ,bathgate ,hills wherein both the ,tholeiite and ,burntisland sill types may be recognized4 ,some of the sills in ,west ,lothian1 as ,i pointed out many years ago1 contain bitumen and give off a bituminous odour when freshly broken4 ,they have been injected into bituminous shales or coal-seams4#ddh ,footnote #ddh3 ,geol4 ,survey ,memoir on ,geology of ,edinburgh 7,sheet #cb1 ,scotland71 p4 #df4 #b4 ,andesites4--,rocks referable to this series appear to have been of rare occurrence among the puy-eruptions4 ,examples of them containing as much as #fj per cent of silica occur among the lavas of the ,limerick basin4 ,some of the necks and what may be sills in the same district likewise consist of them4 #c4 ,trachytes and ,quartz-bearing ,rocks4--,acid rocks1 as ,i have already said1 are extremely rare among the puy-eruptions4 ,the only important examples known to me are those around the ,limerick basin1 where they rise apparently in old vents and form conspicuous rounded or conical hills4 ,these rocks have been examined microscopically by ,mr4 ,w4 ,w4 ,watts4 ,one of the most interesting varieties1 which occurs at the ,standing ,stone near ,oola1 was found by him to show quartz enclosing ophitically the felspars which1 with well-terminated prisms1 project into it4 ,further west1 near ,knockaunavoher1 another boss occurs with conspicuous quartz4 ,these rocks have much in common with trachytes but have a wholly crystalline structure4 ,they will be described in the account of the ,limerick basin4 ,b4 ,tuffs ,the fragmental rocks connected with the puy-eruptions form a well-marked group1 easily distinguishable1 for the most part1 from the tuffs of the plateaux4 ,they vary from exceedingly fine compacted dust or volcanic mud1 through various stages of increasing coarseness of texture1 to basalt-conglomerates and tumultuous agglomerates4 ,the fragmentary material found in the necks of the puys is generally an agglomerate of a dull dirty-green colour4 ,the matrix ranges from a fine compact volcanic mud to a thoroughly granular detritus1 and sometimes shows a spheroidal concentric structure in weathering4 ,in this matrix the lapilli are distributed with great irregularity and in constantly varying proportions4 ,they consist in large measure of a pale yellowish-green1 sometimes pale grey1 very basic1 finely vesicular1 devitrified glass1 which is generally much decomposed and cuts easily with the knife4 ,this highly basic substance is a kind of palagonite4 ,so minute are its vesicles that under the microscope a thin slice may present a delicate lace-like network of connected walls1 the palagonite occupying much less space than the vesicles4 ,the material has been a finely frothed-up pumice4 ,besides this generally distributed basic pumice1 the stones in the agglomerate of the necks likewise include fragments of older volcanic grits or tuffs1 blocks of basalt or diabase1 as well as pieces of the ,carboniferous strata of the district1 especially shale1 sandstone and limestone4 ,not infrequently also1 they comprise angular blocks of fossil wood4 ,the materials which fill the necks are generally much coarser than those that form intercalated beds4 ,but while in numerous cases huge blocks of basalt and large masses of sandstone1 shale1 limestone1 ironstone or other strata may be seen wrapped up in a matrix of coarse basalt-tuff1 in not a few instances the material in the necks may be observed to consist of a tuff quite as fine as that of the interstratified bands4 ,such necks appear to mark the sites of tuff-cones where only fine ashes and lapilli were ejected1 and where1 after sometimes a brief and feeble period of activity1 the orifice became extinct4 ,the bedded tuffs interstratified with the ordinary ,carboniferous strata do not essentially differ in composition from the material of the necks4 ,they are basalt- 7diabase-7 tuffs and basalt- 7diabase-7 conglomerates1 usually dull green in colour and granular in texture1 the lapilli consisting in great measure of various more or less decayed basalts1 but containing the same highly vesicular basic glass or pumice above referred to4 ,they are mainly to be distinguished by their conspicuous stratification1 and especially by their rapid alternations of coarser and finer material1 by the intercalation of shales1 limestones1 sandstones or ironstones in them1 and by the insensible gradations by which they pass both vertically and laterally into ordinary sediments4 ,occasional large blocks or bombs1 indicating some paroxysm of explosion1 may be observed even among the finer tuffs1 shales and other strata1 which round the sides of these masses have had their layers bent down by the fall of heavy blocks4#ddi ,many of the bedded tuffs contain fossils1 such as crinoids1 corals1 brachiopods1 fish-teeth or macerated fragments of land-plants4 ,coal-seams also are occasionally interstratified among them4 ,footnote #ddi3 ,ante1 p4 #cf1 and ,figs4 #ae and #aea4 ,see also ,geol4 ,mag4 i4 7#ahfd71 p4 #bb2 ,trans4 ,roy4 ,soc4 ,edin4 vol4 xxix4 7#ahgi7 p4 #eae4 ,of the finer kinds1 the best example is furnished by a remarkable group of green and red marls which lie above a seam of coal 7,houston ,coal7 in the ,calciferous ,sandstones of ,west ,lothian4#dej ,these strata1 which differ much from any of the rocks with which they are associated1 are exceedingly fine in grain1 dull sage-green and brownish or chocolate-red in colour1 not well laminated like the shales1 but breaking under the influence of weathering into angular fragments1 sometimes with a conchoidal fracture4 ,they look like indurated mud4 ,mr4 ,h4 ,m4 ,cadell1 who has recently re-examined them in connection with a revision of the ,geological ,survey ,map 7,sheet #cb7 has found them passing into ordinary granular tuff4 ,footnote #dej3 ,memoir on ,sheet #cb ,geol4 ,surv4 ,scotland 7#ahfa71 p4 #db4 ,the stratigraphical position of these ,houston ,marls1 as they are locally called1 is indicated in ,fig4 #aee4 ,palagonitic-tuff is of frequent occurrence4 ,it is met with in the ,firth of ,forth district1#dea and ,mr4 ,watts has detected fragments of palagonite among the tuffs of the ,limerick basin4 ,footnote #dea3 ,trans4 ,roy4 ,soc4 ,edin4 vol4 xxix4 7#ahai7 p4 #eae4 ,,chapter ,,xxvii ,,geological ,,structure ,,of ,,the ,,carboniferous ,,puys ,,of ,,scotland #a4 ,vents3 ,relation of the ,necks to the ,rocks through which they rise--,evidence of the probable ,subrial ,character of some of the ,cones or ,puys of ,tuff--,entombment of the ,volcanic ,cones and their ,relation to the ,superficial ,ejections4 #b4 ,bedded ,tuffs and ,lavas--,effects of ,subsequent ,dislocations4 #c4 ,sills1 ,bosses and ,dykes4 ,the puy-type of volcanic hill differs widely in one respect from those which we have hitherto been considering4 ,in the earlier epochs of volcanism within the ,british area1 it is the masses of material discharged from the vent1 rather than the vents themselves which arrest attention4 ,indeed1 so copiously have these masses been erupted that the vents are often buried1 or their positions have been rendered doubtful1 by the uprise in and around them of sills and bosses of molten rock4 ,but among the ,carboniferous puys the vent is often the only record that remains of the volcanic activity4 ,in some cases we know that it never ejected any igneous material to the surface4 ,in others1 though it may be filled with volcanic agglomerate or tuff1 there is no record of any shower of such detritus having been discharged from it4 ,in yet a third class of examples1 we see that lava rose in the vent1 but no evidence remains as to whether or not it ever flowed out above ground4 ,other cases occur where beds of lava or of tuff1 or of both together1 have been intercalated in a group of strata1 but with no trace now visible of the vent from which they came4 ,the most complete chronicle1 preserving at once a record of the outflow of lava1 of the showering forth of ashes and bombs1 and of the necks that mark the vents of eruption1 is only to be found in some of the districts4 ,i shall therefore1 in the present instance1 reverse the order of arrangement followed in the previous chapters1 and treat first of the vents1 then of the materials emitted from them1 and lastly of the sills and dykes4 i4 ,,vents ,a large number of vents rise through the ,carboniferous rocks of ,scotland4 ,some of these are not associated with any interbedded volcanic material1 so that their geological age cannot be more precisely defined than by saying that they must be later than the particular formations which they pierce4 ,some of them1 as ,i shall endeavour to show1 are in all probability of ,permian age4 ,but many1 from their position with reference to the nearest intercalated lavas and tuffs1 are to be regarded as almost certainly belonging to the ,carboniferous period4 ,those which are immediately surrounded by sheets of lava and tuff1 similar in character to the materials in the vents themselves1 may without hesitation be connected with these sheets as marking the orifices of discharge4 ,the vents of the puys are in general much less than those of the plateaux4 ,their smallest examples measure only a few yards in diameter1 their largest seldom much exceed half a mile4#deb ,footnote #deb3 ,the following measurements of necks belonging to the puy-eruptions in different parts of ,scotland are taken from the #f-;inch field-maps of the ,geological ,survey3--,saline ,hill1 ,fife1 #fjjj #djjj feet2 ,binn of ,burntisland1 #cejj #aejj2 ,hill of ,beath1 ,fife1 #bijj #aeej2 ,binns ,hill1 ,linlithgowshire1 #dhjj #bbjj2 ,tor ,hill1 ,ecclesmachan1 ,linlithgowshire1 #aijj #ajjj 7,fig4 #aee72 ,great ,moor1 near ,maiden ,pap1 ,roxburghshire1 #bfjj #bdjj2 ,tinnis ,hill1 ,liddesdale1 #aejj #ajjj2 ,roan ,fell1 ,liddesdale1 #cjj #bjj2 ,hadsgarth ,burn1 ,liddesdale1 #bej #bjj2 ,dalbate ,burn1 #bej #abj4 ,in some cases1 especially in those of the larger necks1 it is probable that the tuff belongs to more than one funnel4 ,thus the ,binn of ,burntisland almost certainly includes two necks1 a smaller one to the west and a much larger one to the east4 ,saline ,hill may also conceal more than one vent4 ,but in the continuous mass of tuff at the surface it is at present impossible to determine precisely the number and boundaries of the several orifices4 ,the dislocations of the ,carboniferous system are probably on the whole later than its volcanic phenomena4 ,it is at least certain that the lavas and tuffs of the puys have been extensively faulted1 like the surrounding sedimentary strata1 and the vents seldom show any apparent relation to faults4 ,it may sometimes be observed1 however1 that the vents are arranged in lines suggestive of fissures underneath4 ,a remarkable instance of the linear distribution is furnished by the chain of necks which extends from the ,vale of the ,tweed at ,melrose south-westwards across the watershed and down ,liddesdale4 ,the most notable part of this line lies among the uplands to the east of the ,old ,mosspaul ,inn at the head of the ,ewes ,water4 ,a string of masses of agglomerate has there solidified in a fissure among the ,silurian greywackes and shales1 running in a north-easterly direction for several miles4 ,the largest connected mass of agglomerate is #dgjj feet long1 and from #cej to #fjj feet broad 7see ,no4 #a in ,fig4 #bb74 ,that this curious vent1 or connected line of vents along a great fissure1 belongs to the puy-eruptions of ,liddesdale is shown by the abundant fragments of yellow sandstone and cement-stone which occur in the agglomerate4#dec ,footnote #dec3 ,these facts were ascertained by ,mr4 ,peach in mapping the ground for the ,geological ,survey4 ,see ,sheet #ag1 ,scotland4 ,most frequently the vents are distributed irregularly in groups4 ,as examples of this arrangement ,i may cite those of the west of ,fife1 of ,west ,lothian and of the north of ,ayrshire4 ,a convenient classification of the vents may be made by dividing them into four groups according to the nature of the material that now fills them3 #ast1 ,necks of non-volcanic debris2 #bnd1 ,necks of tuff and agglomerate2 #crd1 ,necks of similar materials1 but with a central plug of basalt2 #dth1 ,bosses of basalt or other lava1 without agglomerate or tuff4 #a4 ,necks of ,non-volcanic ,debris4--,in a few instances the orifices of eruption have been filled up entirely with non-volcanic debris4 ,they have served as funnels for the discharge of explosive vapours only1 without the expulsion of any solid volcanic materials4 ,at least no trace of fragmentary lavas is met with in them1 nor are any beds of tuff or lava intercalated among the surrounding strata4 ,some interesting examples of this kind were laid bare in the open ironstone-workings near ,carluke in ,lanarkshire4 ,they were circular in ground-plan1 descended vertically into the strata1 and were somewhat wider at the top of the quarry than at the bottom4 ,they were filled with angular pieces of ,carboniferous sandstone1 shale1 limestone1 ironstone and other rocks1 these materials being rudely arranged with a dip towards the centre of the neck1 where the blocks were largest in size4 ,though no fragments of igneous rocks were observed among the debris1 a few string-like veins of white trap1 or altered basalt1 were seen to traverse the agglomerate here and there4 ,the necks and the strata surrounding them were highly impregnated with pyrites and sulphate of lime4#ded ,footnote #ded3 ,prof4 ,jas4 ,geikie1 ,mem4 ,geol4 ,surv4 ,scotland1 ,explanation of ,sheet #bc1 p4 #ci4 ,illustration3 ,fig4 #adc4--,section of volcanic vent at ,east ,grange1 ,perthshire coal-field1 constructed by ,mr4 ,b4 ,n4 ,peach from the rocks exposed in a railway-cutting1 and from plans of ironstone- and coal-pits4 #a4 ,three feet coal2 #b4 ,ontake coal2 #c4 ,upper and ,lower ,black-band ,ironstones2 #d4 ,index ,limestone2 #e4 ,gas ,coal and ,janet ,peat ,coal2 #f4 ,calmy ,limestone2 #g4 ,neck4 ,a vent of the same nature1 but on a much larger scale1 has been mapped by ,mr4 ,peach in the south of ,perthshire1 near ,east ,grange1 where it rises through the higher coal-bearing part of the ,carboniferous ,limestone series 7,fig4 #adc74 ,it has been encountered in the mining of coal and ironstone1 and its cross-section has been ascertained in the underground workings which have been carried up to its margin4 ,it measures #aejj feet in diameter from east to west and #bjjj feet from north to south4 ,it does not appear ever to have emitted any ashes or lava4 ,mr4 ,peach found it filled with dark sandy crumbling clays1 full of fragments of sandstone1 shale and coal4 ,these sediments are arranged in layers that dip in the same general direction as the strata surrounding the vent4 ,they contain abundant calcareous nodules of all sizes from that of a hazel-nut up to concretions #ah feet in diameter4 ,the clays likewise include many of the common shells and crinoids of the ,carboniferous ,limestone sea1 and the same fossils are enclosed in the nodules4 ,a remarkable feature in this vent is the occurrence of abundant vertical rents1 which have been filled partly with the same material that forms the nodules1 and partly with sandstone4 ,the formation of the neck took place after the deposition of the ,index ,limestone1 and probably about the time of the accumulation of the next limestone1 which lies immediately to the west somewhat higher in the series4 ,it would appear that the eruption which produced this funnel gave forth only gaseous explosions1 and occurred on the sea-floor2 that the low crater-walls were washed down to such an extent that the sea entered and carried some of its characteristic organisms into the lagoon or maar within2 further1 that as the silt gathered inside1 successive subsidences occurred1 whereby the sediment was rent by cracks into which sand and calcareous mud were washed from above4#dee ,footnote #dee3 ,the vent is shown in ,sheet #ci1 ,geol4 ,surv4 ,scotland4 ,many necks occur wherein non-volcanic materials1 though not forming the whole of the agglomerate1 make up by far the larger part1 with only a slight admixture of volcanic tuff between them4 ,among the ,burntisland necks of ,fife1 for instance1 abundant fragments of the well-marked cyprid limestone and shale may be observed1 while at ,niddry in ,linlithgowshire blocks several yards in length1 and consisting of different layers of shale and cement-stone still adhering to each other1 may be seen imbedded at all angles in the tuff4 ,where only the debris of non-volcanic rocks occupies a vent1 we may infer that the volcanic action was limited to the explosion of steam1 whereby the rocks were dislocated1 and an orifice communicating with the surface was drilled through them1 and that while no true volcanic rock in such a case appeared1 the pipe was filled up to perhaps not far from the surface by the falling back of the shattered detritus4 ,a little greater intensity or farther prolongation of the volcanic action would bring the column of lava up the funnel1 and allow its upper part to be blown out as dust and lapilli2 while still more vigorous activity would be marked by the rise of the lava into rents of the cone or its actual outflow at the surface4 ,every gradation in this scale of progress may be detected among the ,carboniferous volcanoes of the basin of the ,firth of ,forth4 #b4 ,necks of ,tuff and ,agglomerate4--,the majority of the necks connected with the puys consist of tuff or agglomerate4 ,externally they generally appear as smooth rounded grassy hills that rise disconnected from other eminences4 ,in some districts their materials consist of a greenish granular often stratified tuff1 enclosing rounded balls of various basic lavas and pieces of sandstone1 shale1 limestone or other strata through which they have been drilled4 ,this is their usual character in the ,forth region4 ,but in some cases1 the tuff becomes a coarse agglomerate1 made up partly of large blocks of basalt and other volcanic rocks and partly of the sedimentary strata around them1 of which large masses1 many cubic yards in bulk1 may be seen4 ,among the enclosed fragments it is not unusual to find pieces of older stratified tuff4 ,these resemble in general petrographical character parts of the tuff among which they are imbedded4 ,sometimes they have been ,illustration3 ,fig4 #add4--,view of the ,binn of ,burntisland--a volcanic neck of agglomerate4 7,this illustration and ,figs4 #ade1 #aeb1 #aec1 #afd1 #aff and #afh are from photographs taken by ,mr4 ,robert ,lunn for the ,geological ,survey47 derived from previous tuffs which1 interstratified among the sedimentary strata1 had been broken up by the opening of a new vent4 ,but probably in most cases they should be regarded as portions of the volcanic debris which1 having solidified inside the crater1 was blown out in fragments by subsequent explosions4 ,in a modern volcano a considerable amount of stratified tuff may be formed inside the crater4 ,the ashes and stones thrown out during a period of activity fall not only on the outer slopes of the cone1 but on the steep inner declivities of the crater1 where they arrange themselves in beds that dip at high angles towards the crater bottom4 ,this feature is well seen in some of the extinct cones in the ,neapolitan district4 ,in some of the ,scottish puys the tuff is stratified and has tumbled down into a highly inclined or vertical position 7,fig4 #ade74 ,as a good illustration of the variety and relative proportions of the ejected blocks in the green tuff of the puy-vents1 ,i may cite the following table of percentages which ,i took many years ago in the tuff which rises through the ,cement-stone group on the beach at the ,heads of ,ayr4 ,diabase and basalt #eg per cent4 ,older tuff #c ,andesite 7probably from ,old ,red ,sandstone volcanic series of the neighbourhood7 #ad ,limestone 7cement-stone1 etc47 #bj ,shale #c ,sandstone #b ,fossil wood #a ---- #ajj ,while many examples might be cited where no molten rock of any kind has risen in the vents1 or where at least all the visible materials are of a fragmentary character1 yet small veins and dykes of basalt have not infrequently been injected into the tuff or agglomerate4 ,these seldom run far1 and usually present a more or less tortuous course4 ,thus1 on the south front of the ,binn of ,burntisland 7,figs4 #aff1 #afh7 a number of basalt-dykes1 which vary in breadth from five or six feet to scarcely so many inches1 bifurcate and rapidly disappear in the tuff1 one of them ascending tortuously to near the top of the cliff4 ,they at once recall the appearance of the well-known dykes in the great crater wall of ,somma4 ,though not by any means the largest or most perfect of the vents in the basin of the ,firth of ,forth1 the ,binn of ,burntisland1 of which a view is given in ,fig4 #add1 may be cited in illustration of their general characters4 ,it presents in detail some of the most strikingly volcanic aspects of scenery anywhere to be seen in that region4 ,consisting of a dull green granular volcanic tuff1 it rises abruptly out of the ,lower ,carboniferous formations to a height of #fca feet above the sea4 ,the southern edge of this neck has been so extensively denuded1 that it presents steep craggy slopes and rugged precipices1 which descend from the very summit of the cone to the plain below--a vertical distance of nearly #ejj feet4 ,here and there the action of atmospheric waste has hollowed out huge crater-like chasms in the crumbling tuff4 ,standing in one of these1 the geologist can realize what must have been the aspect of the interior of these ancient ,carboniferous volcanic cones4 ,the scene at once reminds him of the crater-walls of a modern or not long extinct volcano4 ,the dull-green rudely stratified tuff rises around in verdureless crumbling sheets of naked rock1 roughened by the innumerable blocks of lava1 which form so conspicuous an element in the composition of the mass4 ,the ribs or veins of columnar basalt run up the declivities as black shattered walls4 ,the frosts and rains of many centuries have restored to the tuff its original loose gravelly character4 ,it disintegrates rapidly1 and rolls down the slopes in long grey lines of volcanic sand1 precisely as it no doubt did at the time of its ejection1 when it fell on the outer and inner declivities of the original cone4 ,some of these features may be partly realized from ,fig4 #ade1 which represents a portion of the south front of the hill4 ,sections of this neck are given in ,figs4 #adi and #aei4 7#c7 ,necks of ,tuff or ,agglomerate with a ,central ,plug of ,basalt or other ,lava4--,it has often happened that1 after the explosions in a vent have begun to decrease in vigour1 or have at last ceased1 lava has risen in the chimney and finally sealed it up4 ,in such cases the main mass of the rock may consist of tuff or agglomerate1 which the enfeebled volcanic activity has been unable to expel from the orifice1 while a plug of basalt1 dolerite1 or even more basic material1 of much smaller dimensions1 may have risen up the pipe in the centre or towards one side4 ,binns ,hill1 ,west ,lothian1 the ,beath and ,saline ,hills of ,fife1 and ,tinnis ,hill in ,liddesdale are good examples of this structure4 7,see ,figs4 #bf1 #adh1 #adi and #agd74 7#d7 ,necks of ,basalt1 ,dolerite1 etc4--,in other cases no fragmental material is present in the vent1 or possibly traces of it may be seen here and there adhering to the walls of the funnel1 the prevailing rock being some form of lava4 ,necks of this kind are much less frequent in the puy- than in the plateau-type4 ,but examples may be found in several districts4 ,the most striking with which ,i am acquainted are those which form so picturesque a group of isolated cones around the volcanic basin of ,limerick1 to be afterwards described 7,figs4 #aie1 #aif74 ,the vents there have been filled by the uprise of much more acid rocks than the lavas of the basin1 for1 as ,i have already stated1 they include even quartziferous trachytes4 ,in the basin of the ,firth of ,forth some prominent bosses of basalt probably mark the sites of former vents1 such as ,dunearn ,hill in ,fife1 the ,castle ,rock of ,edinburgh1 and ,galabraes ,hill near ,bathgate4 ,some striking vents which occur in the ,jedburgh district1 in the debateable land between the plateau series on the east and the puy-series on the west1 show the nearly complete usurpation of the funnel by basalt1 but with portions of the tuff still remaining visible4 ,relation of the ,necks to the ,rocks through which they rise4--,a remarkable feature among the ,carboniferous and ,permian vents of central ,scotland is presented by the effect which has been produced on the strata immediately surrounding them4 ,in the interior of the country this effect is often ,illustration3 ,fig4 #ade4--,view of part of the cliffs of vertical agglomerate1 ,binn of ,burntisland4 concealed by herbage1 but where the rocks have been laid bare by the sea it may be most instructively studied4 ,in such shore-sections1 a singular change of dip is often observable among the strata round the edge of a vent4 ,no matter what may be the normal inclination at the locality1 the beds are bent sharply down towards the wall of the neck1 and are frequently placed on end4 ,this structure 7shown in ,figs4 #bd1 #adc1 #adg1 #adh and #adi7 is precisely the reverse of what might have been anticipated1 and can hardly be due to upward volcanic explosions4 ,it is frequently associated with considerable metamorphism in the disturbed strata4 ,shales are converted into porcellanite or various jaspery rocks1 according to their composition4 ,sandstones pass into quartzite1 with its characteristic lustrous fracture4 ,it is common to find vents surrounded with a ring of this contact-metamorphism1 which1 from the hardness and frequently vertical or highly inclined bedding of its strata1 stands up prominently on the beach 7as in ,figs4 #abf and #baj71 and serves to mark the position of the necks from a distance4 ,i have not been able to find an altogether satisfactory explanation of this inward dip of the strata around vents4 ,taking it in connection with the metamorphism1 ,i am inclined to believe that it arose after the close of the long-continued volcanic action which had hardened the rocks around the volcanic pipe1 and as the result of some kind of subsidence within the vent4 ,the outpouring of so much tuff and lava as escaped from many of the volcanoes would doubtless often be apt to produce cavities underneath them1 and on the decay of volcanic energy there might be a tendency in the solid or cavernous column filling up the funnel1 to settle down by mere gravitation4 ,so firmly1 however1 did much of it cohere to the sides of the pipe1 that if it sank at all1 it could hardly fail to drag down a portion of these sides4 ,so general is this evidence of downward movement in all the volcanic districts of ,scotland where the necks have been adequately exposed1 that the structure may be regarded as normal to these volcanic vents4 ,it has been observed among the shore-sections of the volcanoes of the ,auckland district1 ,new ,zealand4 ,mr4 ,c4 ,heaphy1 in an interesting paper upon that district1 gives a drawing of a crater and lava-stream abutting on the edge of a cliff where the strata bend down towards the point of eruption1 as in the numerous cases in ,scotland4#def ,footnote #def3 ,quart4 ,journ4 ,geol4 ,soc4 #ahfj1 vol4 xvi4 p4 #bde4 ,evidence for the probable subrial ,character of some of the ,cones or ,puys of ,tuff4--,from the stratigraphical data furnished by the basin of the ,firth of ,forth1 it is certain that this region1 during a great part of the ,carboniferous period1 existed as a wide shallow lagoon1 sometimes overspread with sea-water deep enough to allow of the growth of corals1 crinoids1 and brachiopods2 at other times1 shoaled to such an extent with sand and mud as to be covered with wide jungles of a lepidodendroid and calamitoid vegetation4 ,as volcanic action went on interruptedly during a vast section of that period1 the vents1 though generally submarine1 may occasionally have been subrial4 ,indeed1 we may suppose that the same vent might begin as a subaqueous orifice and continue to eject volcanic materials1 until1 as these rose above the level of the water1 the vent became subrial4 ,an instance of a submarine vent has been cited from the ,perthshire coal-field 7p4 #dbf74 ,among the evidence which may be collected to show that some ,carboniferous volcanoes probably rose as insular cones of tuff above the surrounding waters1 the structure of the tuff in many necks may be cited1 for it suggests subrial rather than subaqueous stratification4 ,the way in which the stones1 large and small1 are grouped together in lenticular seams may be paralleled on the slopes of many a modern volcano4 ,another indication of this mode of origin is supplied by the traces of wood to be met with in some of the tuff-necks4 ,the vents of ,fife and ,linlithgowshire contain these traces sometimes in great abundance4 ,the specimens are always angular fragments1 and are frequently encrusted with calcite4#deg ,sometimes they present the glossy fracture and clear ligneous structure shown by sticks of well-made wood charcoal4 ,in a neck at ,st4 ,magdalen's1 near ,linlithgow1 the wood fragments occur as numerous black chips4 ,so far as can be ascertained from the slices already prepared for the microscope1 the wood is always coniferous4 ,these woody fragments seldom occur in the interstratified tuffs or in the associated strata where ,stigmaria1 ,lepidodendron1 etc41 are common4 ,they are specially characteristic of the necks and adjacent tuffs4 ,the parent trees may have grown on the volcanic cones1 which as dry insular spots would support a different vegetation from the club-mosses and reeds of the surrounding swamps4 ,as the fragments occur in the tuffs which1 on the grounds already stated1 may be held to have been deposited within the crater1 they seem to point to intervals of volcanic quiescence1 when the dormant or extinct craters were filled with a terrestrial flora1 as ,vesuvius was between the years #aejj and #afca1 when no eruptions took place4 ,some of the cones1 such as ,saline ,hill and the ,binn of ,burntisland1 may have risen several hundred feet above the water4 ,clothed with dark pine woods1 they would form a notable feature in the otherwise monotonous scenery of central ,scotland during the ,carboniferous period4 ,footnote #deg3 ,the largest ,i have observed is a portion of a stem about two feet long and six inches broad1 in the 7,permian87 neck below ,st4 ,monan's church4 ,entombment of the ,volcanic ,cones and their relation to the bedded ,lavas and ,tuffs4--,from the facts above detailed1 it is evident that in most cases the necks represent1 as it were1 the mere denuded stumps of the volcanoes4 ,as the puys took their rise in areas which1 on the whole1 were undergoing a movement of subsidence1 they were eventually submerged and buried under sedimentary accumulations4 ,their loose ashes would be apt to be washed down and strewn over the sea-bottom1 so that only the lower and inner part of a cone might remain4 ,we can hardly hope to discover any of the actual craters among these volcanic relics4 ,the cones having been submerged and buried under many hundred feet of sediment1 their present position at the surface is due to subsequent elevation and prolonged denudation4 ,it is obvious that there must still be many buried cones which the progress of denudation has not yet reached4 ,some of these have been revealed in the course of mining operations4 ,valuable seams of coal1 ironstone and oil-shale in the ,scottish ,carboniferous ,limestone and ,calciferous ,sandstone series are extensively worked1 and in the underground operations many illustrations of former volcanic action have been met with4 ,the most remarkable instances of the discovery of buried volcanoes have occurred in the ,dalry coal-field in the north of ,ayrshire4 ,in one pit-shaft about a mile and a half to the south-west of the village of ,dalry1 a thickness of #aae fathoms of tuff was passed through1 and in another pit #ij fathoms of similar tuff were sunk into before the position of the black-band ironstone of that mineral field was reached by driving levels through the tuff into the sedimentary strata outside of it4 ,only a short distance from these thick piles of tuff1 their place is entirely taken up by the ordinary sedimentary strata of the district4 ,the working-plans of the mines show the tuff to occur in irregular patches and strips1 between which the ironstone is workable4 ,from these data we perceive that the shafts have in some cases been sunk directly upon the tops of puys of tuff1 which were1 in one case1 nearly #gjj feet1 and in another instance1 #edj high#deh 7,fig4 #adf74 ,footnote #deh3 ,explanation of ,sheet #bb1 ,geol4 ,surv4 of ,scotland1 p4 #af4 ,illustration3 ,fig4 #adf4--;,diagram of buried volcanic cone near ,dalry1 ,ayrshire4 ,constructed from information obtained in mining operations4 #a4 ,hurlet ,limestone4 #b4 ,clayband ,ironstone4 #c4 ,black-band ,ironstone4 #d4 ,borestone ,coal4 #e4 ,wee ,coal4 #f4 ,highfield ,limestone4 #g and #h4 ,thin ,limestones4 #i4 ,linn ,limestone4 #aj4 ,volcanic neck and cone of tuff4 ,it is obvious that from the condition of a completely buried and concealed cone every stage may be expected to occur up to the deeply worn-down neck representing merely the stump of the volcanic column4 ,the subjoined diagram 7,fig4 #adg7 may serve to illustrate this process of gradual re-emergence4 ,illustration3 ,fig4 #adg4--;,diagram to illustrate how ,volcanic ,necks may be concealed and exposed4 #a1 ,neck1 still buried under the succeeding sedimentary accumulations2 #b1 ,neck uncovered and denuded4 ,when1 in the progress of denudation1 a volcanic cone began to show itself from under the cover of removed strata1 it would still for a time maintain its connection with the sheets of tuff or of lava which1 when active1 it had erupted4 ,a number of examples of this structure may be observed in the basin of the ,firth of ,forth1 where the degradation of the surface has not yet proceeded so far as to isolate the column of agglomerate or tuff from the sheets of tuff that were strewn around the old volcano4 ,in such cases1 the actual limits of the vent are still more or less concealed1 or at least no sharp line can be drawn between the vent and its ejections4 ,as an illustration of this connection of a volcanic pipe with the materials ejected from it over the surrounding country ,i would cite ,saline ,hill in the west of ,fife4 ,that eminence rises to a height of #aagh feet above the sea1 out of a band of tuff which can be traced across the country for fully three miles4 ,numerous sections in the water-courses show that this tuff is regularly interbedded in the ,carboniferous ,limestone series1 so that the relative geological date of its eruption can be precisely fixed4 ,on the south of ,saline ,hill1 coal and ironstone1 worked under the tuff1 prove that this portion of the mass belongs to the general sheet of loose ashes and dust1 extending outwards from the original cone over the floor of the sheet of water in which the ,carboniferous ,limestone series of strata was being deposited4 ,but the central portion of the hill is occupied by one or more volcanic pipes4 ,a section across the eminence from north-west to south-east would probably show the structure represented in ,fig4 #adh4 ,immediately to the east of the ,saline ,hill lies another eminence1 known as the ,knock ,hill1 which marks the site of another eruptive vent4 ,a coal-seam 7the ,little ,parrot or ,gas ,coal7 is worked along its southern base1 and is found to plunge down steeply towards the volcanic rocks4 ,this seam1 however1 is not the same as that worked under the ,saline ,hill1 but lies some #fjj feet below it4 ,probably the whole of the ,knock ,hill occupies the place of a former vent4 ,illustration3 ,fig4 #adh4--,section across the ,saline ,hills1 ,fife4 ,the thick parallel black lines mark the position of seams of coal and ironstone1 some of which are worked under ,saline ,hill4 ,t1 ,tuff of the necks2 t1 ,tuff at a little distance from the cone1 interstratified with the ordinary sedimentary beds2 ,b1 ,basalt4 ,the larger eminence is ,saline ,hill1 the lower is ,knock ,hill4 ,a further stage of decay and denudation brings before us the entire severance of the volcanic column from the materials that were ejected from it4 ,an excellent example of this isolation of the neck in the midst of surrounding masses of tuff and lava which proceeded from it is presented by the ,binn of ,burntisland1 to which ,i have already alluded4 ,a section across that eminence gives the geological structure represented in ,fig4 #adi4 ,the dip of the rocks away from the volcanic pipe at this locality has been produced long after the volcanic phenomena had ceased4 ,the arch here shown is really the prolongation and final disappearance of the great anticlinal fold of which the ,pentland ,hills form the axis on the opposite side of the ,firth4 ,but if we restore the rocks to a horizontal1 or approximately horizontal position1 we find the ,binn of ,burntisland rising among them in one or more necks1 which doubtless mark centres of volcanic activity in that district4 ,a series of smaller neck-like eminences runs for two miles westward4 ,striking as the forms of many of the necks are1 and much as their present conical forms resemble those of active and extinct volcanoes1 the evidence of extensive denudation proves that these contours are not the original outlines of the ,carboniferous vents1 but are in every case the result of prolonged waste4 ,what we now see is a section of the volcanic chimney1 and the conical form is due to the way in which the materials filling the chimney have yielded to the forces of denudation4 ,illustration3 ,fig4 #adi4--,section across the ,binn of ,burntisland1 in an ,east and ,west direction4 #a1 ,sandstones2 #b1 ,limestone 7,burdiehouse72 #c1 ,shales1 etc42 b1 b1 ,interstratified basalts2 t t1 ,bedded tuff1 etc42 ,t1 ,tuff of the great neck of ,burntisland2 ,b1 ,basalt veins4 ii4 ,,bedded ,,tuffs ,,and ,,lavas ,during at least the earlier part of the period of the puys1 in some districts or from certain vents1 such as those of ,east ,fife1 ,western ,midlothian1 ,eastern ,linlithgowshire1 ,northern ,ayrshire1 ,heads of ,ayr and ,lower ,eskdale1 only fine tuff seems to have been thrown out1 which we now find intercalated among the surrounding strata4 ,these eruptions1 neither so vigorous nor so long-continued as those of the plateaux1 never gave forth such thick and widespread sheets of fragmentary materials as those associated with the plateaux in ,east ,lothian and the north-east of ,ayrshire4 ,a single discharge of ashes seems in many cases to have been the sole achievement of one of those little volcanoes2 at least only one thin band of tuff may be discoverable to mark its activity4 ,the tuff of these solitary bands is seldom coarse in texture4 ,it usually consists of the ordinary dull green paste1 with dust and lapilli of basic pumice4 ,the local variations in the tuffs of the puys generally arise mainly from differences in the composition1 size and numbers of the included ejected blocks4 ,generally the most abundant stones are pieces of different diabases1 or basalts2 then come fragments from the surrounding ,carboniferous strata1 from older tuffs and rarely from rocks of much deeper-seated origin4 ,now and then the eruptions of tuff have consisted of extremely fine volcanic dust1 which1 mingling with water1 took the form of a compact mudstone1 as in the case of the ,houston ,marls 7p4 #dbc71 which remind one of a volcanic mud4 ,but in most localities the discharge of tuff1 though for a time it may have completely obscured the ordinary contemporaneous sedimentation1 was intermittent1 so that in the intervals between successive showers of detritus1 the deposition of non-volcanic sediment went on as usual4 ,hence it is that bands of tuff1 whether they lie among lavas or among sedimentary formations1 are apt to contain interstratifications of sandstone1 shale1 limestone or other detrital deposit1 and to pass insensibly into these4 ,the extremely gentle gradation from volcanic into non-volcanic sediment1 and the occasional reappearance of thin partings of tuff bring vividly before the mind the slow dying out of volcanic energy among the ,carboniferous lagoons4 ,illustration3 ,fig4 #aej4--,section in old quarry1 west of ,wester ,ochiltree1 ,linlithgowshire4 ,calciferous ,sandstone series4 ,the comparatively quiet character of the volcanic explosions1 and the contemporaneous undisturbed deposition of sediment during the earlier part of the puy period1 are exemplified in many sections throughout the areas above enumerated1 as will be more fully illustrated in subsequent pages4 ,two typical examples may suffice for this general statement of the characters of the discharges of tuff in the puy-eruptions4 ,in the ,linlithgowshire quarry represented in ,fig4 #aej1 where about ten feet of strata have been exposed1 a black shale 7#a7 of the usual carbonaceous character1 so common in the ,oil-shale series of this region1 may be seen at the bottom of the section4 ,it is covered by a bed of nodular bluish-grey tuff 7#b7 containing black shale fragments4 ,a second black shale 7#c7 is succeeded by a second thin band of fine pale yellowish tuff 7#d74 ,black shale 7#e7 again supervenes1 containing rounded fragments of tuff1 perhaps ejected lapilli1 and passing up into a layer of tuff 7#f74 ,it is evident that we have here a continuous deposit of black shale which was three times interrupted by showers of volcanic dust and stones4 ,at the close of the third interruption1 the deposition of the shale was renewed and continued1 with sufficient slowness to permit of the segregation of thin seams and nodules of clay ironstone round the decomposing organic remains of the muddy bottom 7#g74 ,a fourth volcanic interlude now took place1 and the floor of the water was once more covered with tuff 7#h74 ,but the old conditions of deposit were immediately afterwards resumed 7#i72 the muddy bottom was abundantly peopled with ostracod crustaceans1 while many fishes1 whose coprolites have been left in the mud1 haunted the locality4 ,at last1 however1 a much more serious volcanic explosion took place4 ,a coarse agglomeratic tuff 7#aj71 with blocks sometimes nearly three feet in diameter1 was then thrown out1 and overspread the lagoon4#dei ,footnote #dei3 ,see ,geol4 ,surv4 ,memoir of ,edinburgh1 p4 #de4 ,these tuffs are further described on pp4 #dfe et seq4 ,the second illustration may be taken from the admirable coast-section between ,burntisland and ,kinghorn1 where the number of intercalations of tuff is very great4 ,besides thicker well-marked bands1 successive innumerable thin layers occur there among the associated zones of sedimentary strata which separate the sheets of basalt4 ,the character of these tuff-seams may be inferred from the following details of less than two feet of rock at ,pettycur ,point3-- ,tuff #ae inch4 ,limestone #jb ,tuff #je ,shale #jb ,tuff #ja ,shale and tuff #aj ,shale #jb ,limestone #je ,shale full of volcanic dust #ce ,shaly limestone #ae ,laminated tufaceous limestone #bj ,limestone in thin bands1 with thin lamin of tuff #jh ,granular tuff #jf ,argillaceous limestone1 with diffused tuff #ji ,fine granular tuff #jg ,argillaceous limestone1 with diffused tuff #ae ,laminated limestone #ja ,limestone1 with parting of granular tuff in middle #ji ,tufaceous shale #bj ,limestone #jd ,shaly tuff #abe ,laminated limestone #ja ,tuff #ab ----- #bafe inches4 ,illustration3 ,fig4 #aea4--;,ejected volcanic block in ,carboniferous strata1 ,burntisland4 #a4 ,brown shaly fire-clay with rootlets1 about five inches2 #b4 ,impure coal1 five or six inches1 pressed down in its upper layers by the impact and weight of the stone2 #c4 ,green crumbling ashy fire-clay1 one foot1 with its lower layers pressed down by the stone while the upper layers rise over it1 showing that the stone fell at the time when half this seam was deposited4 ,the fire-clay passes up into dark greenish and black ashy shale 7#d7 about six inches thick and containing plant-remains4 ,the stone is a pale diabase weighing about six or eight pounds4 ,such a section as this brings vividly before the mind a long-continued intermittent feeble volcanic action during pauses between successive outbursts of lava4 ,in such intervals of quiescence1 the ordinary sediment of the lagoons accumulated1 and was mixed up with the debris supplied by occasional showers of volcanic dust4 ,in this ,fife volcanic series1 thin layers of sandstone1 streaked with remains of the ,carboniferous vegetation2 beds of shale full of cyprid-cases1 ganoid scales1 and fragmentary ferns2 thin beds of limestone1 and bands of fire-clay supporting seams of coal1 are interleaved with strata of tuff and sheets of basalt4 ,now and then a sharp discharge of larger stones is seen to have taken place1 as in the case of the block many years ago described by me as having fallen and crushed down a still soft bed of coal 7,fig4 #aea74dfj ,footnote #dfj3 ,geol4 ,mag4 vol4 i4 p4 #bb4 ,this ,fife coast-section is given in full at p4 #dgj4 ,illustration3 ,fig4 #aeb4--,view of volcanic agglomerate becoming finer above4 ,east end of ,kingswood ,craig1 two miles east from ,burntisland4 ,the ,fife coast-section from which these details are taken supplies almost endless instances of the varying characters of the pyroclastic materials of the puy-eruptions4 ,the very same cliff1 bank or reef will show at one point an accumulation of excessively coarse volcanic debris and at another thin lamin of the finest dust and lapilli4 ,these rapid gradations are illustrated in ,fig4 #aeb1 which is taken from the east end of the ,kingswood ,craig4 ,the lower part of the declivity is a coarse agglomerate which passes upward into finer tuff4 ,besides the thin partings and thicker layers of tuff which1 intercalated among the sedimentary strata of the ,carboniferous system1 mark a comparatively feeble and intermittent volcanic activity1 we meet in some localities with examples where the puys have piled up much thicker accumulations of fragmentary material without any intercalated streams of lava1 or interstratified sandstone1 shale or limestone4 ,thus the widespread ,houston marls above described reach a thickness of some #bjj feet4 ,the vents of the ,saline ,hills in ,fife covered the sea-floor with volcanic ashes to a depth of several hundred feet4 ,in the north of ,ayrshire the first eruptions of the puys have formed a continuous band of fine tuff traceable for some #ae miles1 and in places at least #bjj feet thick4 ,where volcanic energy reached its highest intensity during the time of the puys1 not only tuffs but sheets of lava were emitted1 which1 gathering round the vents1 formed cones or long1 connected banks and ridges4 ,of these there are four conspicuous examples in ,scotland--the hills of the ,burntisland district1 the ,bathgate ,hills1 the ground between ,dalry and ,galston in north ,ayrshire1 and a broken tract in ,liddesdale4 ,nowhere in the volcanic history of this country have even the minutest details of that history been more admirably preserved than among the materials erupted from puys in these respective districts4 ,lava-cones1 answering to solitary tuff-cones among the fragmental eruptions1 do not appear to have existed1 or1 like some of those in the great lava-fields of ,northern ,iceland and ,western ,america1 must have been mere small heaps of slag and cinders at the top of the lava-column1 which were washed down and effaced during the subsidence and entombment of the volcanic materials4 ,the lavas never occur without traces of fragmentary discharges4 ,two successive streams of basalt may indeed be found at a given locality without any visible intercalation of tuff1 but proofs of the eruption of fragmental material will generally be observed to occur somewhere in the neighbourhood1 associated with one or both of them1 or with other lavas above or below them4 ,where the phenomena of the puys have been most typically developed1 lavas and tuffs succeed each other in rapid succession1 with numerous or occasional interstratifications of ordinary sediment4 ,perhaps the most complete and interesting example of this association is to be found on the coast between ,burntisland and ,kirkcaldy1 where1 out of a total thickness of rock which may be computed to be between #aejj and #bjjj feet1 it will probably be a fair estimate to say that the igneous materials constitute four-fifths1 or from ,illustration3 ,fig4 #aec4--;,alternations of basalt and tuff with shale1 etc41 ,kingswood ,craig1 ,burntisland4 #abjj to #afjj feet4 ,the lavas are varieties of basalt ranging in character from a black compact columnar to a dirty green earthy cellular or slaggy rock4 ,each separate flow may be on the average about #bj or #cj feet in thickness4 ,columnar and amorphous sheets succeed each other without any interposition of fragmentary material 7,fig4 #aga74 ,but along the junctions of the separate flows layers of red clay1 like the bole between the basalts of the ,giant's ,causeway1 may frequently be noticed4 ,the characteristic slaggy aspect of the upper parts of these ancient coules is sometimes remarkably striking4 ,the full details of this most interesting section will be given in later pages 7p4 #dgj74 ,but some of its more characteristic external features may be understood from the views which are presented in ,figs4 #aeb1 #aec1 #agj1 #aga4 ,the general bedded character of the volcanic series is well shown in ,fig4 #aec1 which represents the alternations of lavas and tuffs in the ,kingswood ,craig two miles to the east of ,burntisland4 ,the harder basalts will be seen to project as bold crags while the tuffs and other stratified deposits between them give rise to grassy slopes and hollows4 ,a nearer view of the alternation of lavas and tuffs with non-volcanic sedimentary deposits is supplied in ,fig4 #agj1 which is taken from a part of the ,fife coast a little further to the east than the last illustration4 ,here one of the limestones of the ,carboniferous ,limestone series is overlain with shale and tuff1 which1 being easily disintegrated1 have been cut away by the waves1 leaving the lava above to overhang and fall off in blocks4 ,the columnar structure of some of the basalts of this coast is well brought out in ,fig4 #aga1 which shows further how the columns sometimes merge into an amorphous part of the same sheet4 ,these ,fife basalts illustrate admirably the peculiarities of the sheets of lava which are intercalated among the ,carboniferous strata4 ,they show how easy it generally is to discriminate between such sheets and intrusive sills4 ,the true lavas are never so largely crystalline1 nor spread out in such thick sheets as the sills2 they are frequently slaggy and amygdaloidal1 especially towards the top and bottom1 the central portion being generally more fine-grained and sometimes porphyritic4 ,where most highly cellular they often decompose into a dull1 earthy1 dirty-green rock4 ,where they form a thick mass they are usually composed of different beds of varying texture4 ,except the differences between the more compact centre and the slaggy layer above and below1 the bedded lavas do not present any marked variation in composition or structure within the same sheet4 ,a striking exception to this rule1 however1 is furnished by the ,bathgate leckstone already described4#dfa ,this mass forms a continuation of the great basaltic ridge of the ,bathgate ,hills1 and though its exact relations to the surrounding strata are concealed1 it appears to be an interbedded and not an intrusive sheet4 ,the remarkable separation of its constituent minerals into an upper1 lighter felspathic layer1 and a lower1 heavier layer1 rich in olivine1 augite and iron-ores1 is a structure which might be more naturally expected to occur in a sill4 ,an instance of its development in an undoubted sill will be described further on4 ,nevertheless1 if we follow the trend of the volcanic band of the ,bathgate ,hills southward for only two miles beyond the picrite quarry1 we find in the ,skolie ,burn a rock in many respects similar1 and quarried for the same purpose of building oven-soles4 ,this leckstone is there seen to be surmounted by a group of calcareous shales and thin limestones4 ,the section laid bare in the stream is represented in ,fig4 #aed4 ,immediately above the diabase1 which is highly cellular1 lies a green felspathic sandstone or shale containing detached fragments of the amygdaloid together with ,lingul and other shells4 ,there seems no reason to doubt that this is a true interstratified lava4#dfb ,footnote #dfa3 ,trans4 ,roy4 ,soc4 ,edin4 xxix4 7#ahgi7 p4 #ejd4 ,footnote #dfb3 ,trans4 ,roy4 ,soc4 ,edin4 xxix4 7#ahgi71 pp4 #eje-ejg4 ,illustration3 ,fig4 #aed4--,section of the upper surface of a diabase 7leckstone7 sheet1 ,skolie ,burn1 south-east of ,bathgate4 #a4 ,slaggy diabase2 #b4 ,green sandy shale and shaly sandstone containing ,lingul1 also pieces of slag from the underlying lava1 which are completely wrapped round in the sediment2 #c4 ,yellow calcareous shelly sandstone2 #d4 ,dark shale with ,spirifer1 etc42 #e4 ,bed of blue crinoidal limestone2 #f4 ,clays and thin coal2 #g4 ,black and blue calcareous shales and thin limestones4 ,where the puys attained their greatest development in ,scotland1 they rose in the shallow lagoons1 and here and there from deeper parts of the sea-bottom1 until by their successive discharges of lavas and tuffs they gradually built up piles of material1 which1 in the ,linlithgow and ,bathgate district1 may have been nearly #bjjj feet in thickness4 ,it must be remembered1 however1 that the eruptions took place in a subsiding area1 and that even the thickest volcanic ejections1 if the downward movement kept pace with the volcanic activity1 need not have grown into a lofty volcanic hill4 ,indeed1 largely as the lavas and tuffs bulk in the geology of some parts of ,central ,scotland1 their eruption does not seem to have seriously interfered with the broader physical changes that were in progress over the whole region4 ,thus the subsidence which led to the spread of a marine and limestone-making fauna over much of ,central ,scotland included also the volcanic districts4 ,the limestones1 formed of crinoids1 corals and other marine organisms1 extended over the submerged lavas and tuffs1 and were even interstratified with them4 ,while the volcanic materials are found to replace locally the ordinary ,carboniferous sedimentary strata1 it is interesting in this regard to note that1 during pauses in the volcanic activity1 while the subsidence doubtless was still going on1 some groups of sandstones1 shales or limestones extended themselves across the volcanic ridges so as to interpose1 on more than one platform1 a mass of ordinary sediment between the lavas or tuffs already erupted and those of succeeding discharges1 and thus to furnish valuable geological chronometers by which to define the stratigraphical horizons of the successive phases of volcanic energy4 ,illustration3 ,fig4 #aee4--,section across the volcanic ridge of the ,linlithgow and ,bathgate ,hills1 showing the intercalation of limestones that mark important stratigraphical horizons4 #a4 ,houston ,coal2 #b4 ,houston ,marls and tuffs2 #c4 ,interstratified sheets of basic lavas with occasional tuffs and intercalations of shale1 sandstone1 etc42 #d4 ,tartraven ,limestone2 #e4 ,hurlet ,limestone with tuffs1 shales and sandstones above and below2 #f4 ,wardlaw ,limestone2 #g4 ,index ,limestone2 #h4 ,highest band of tuff--upward limit of the volcanic series2 #i #i4 ,volcanic necks2 #aj4 ,sill of basalt2 #aa4 ,levenseat or ,castlecary ,limestone2 #ab4 ,millstone ,grit2 #ac4 ,base of ,coal-measures2 #ad4 ,thick doleritic sill2 #ae4 ,dolerite dyke 78 ,tertiary74 ,the volcanic banks or ridges not improbably emerged as islets out of the water1 and were sometimes ten miles or more in length4 ,their materials were supplied from many separate vents along their surface1 but probably never attained to anything approaching the elevation which they would have reached had they been poured out upon a stable platform4 ,this feature in the history of the volcanic ridges is admirably shown by the fact just referred to1 that recognizable stratigraphical horizons can sometimes be traced right through the heart of the thickest volcanic accumulations4 ,one of the largest areas of basalts and tuffs connected with the puys is that of the ,linlithgow and ,bathgate ,hills1 where1 as already remarked1 a depth of some #bjjj feet of igneous rocks has been piled up4 ,yet several well-known seams of stone can be traced through it1 such as the ,hurlet ,limestone and the ,index ,limestone 7,fig4 #aee74 ,only at the north end1 where the volcanic mass is thickest and the surface-exposures of rock are not continuous1 has it been impossible to subdivide the mass by mapping intercalations of sedimentary strata across it4 ,it would thus seem that1 even where the amplest accumulations gathered round the puys1 they formed low flat domes1 rather than prominent hills1 which1 as subsidence went on and the tuff-cones were washed down1 gradually sank under water1 and were buried under the accumulating silt of the sea-floor4 ,as a detailed illustration of the manner in which the growth of organically-formed limestones and the deposit of ordinary sediment took place concurrently with the occasional outflow of lava-streams over the sea-bottom1 ,i may cite the section presented in another ,linlithgowshire quarry 7,fig4 #aef74 ,at the bottom of the group of strata there exposed1 a pale amygdaloidal1 somewhat altered basalt 7,a7 marks the upper surface of one of the submarine lavas of the period4 ,directly over it comes a bed of limestone 7,b7 #ae feet thick1 the lower layers of which are made up of a dense growth of the thin-stemmed coral ,lithostrotion irregulare4 ,the next stratum is a band of dark shale 7,c7 about two feet thick1 followed by about the same thickness of an impure limestone with shale seams 7,d74 ,the conditions for coral and crinoid growth were evidently not favourable1 for this argillaceous limestone was eventually arrested first by the deposit of a dark mud1 now to be seen in the form of three or four inches of a black pyritous shale 7,e71 and next by the inroad of a large quantity of dark sandy mud and drift vegetation1 which has been preserved as a sandy shale 7,f71 containing ,calamites1 ,producti1 ganoid scales and other traces of the life of the time4 ,finally1 a great sheet of lava1 represented by the uppermost amygdaloid 7,g71 overspread the area1 and sealed up these records of ,palozoic history4#dfc ,footnote #dfc3 ,geol4 ,surv4 ,mem4 ,geology of ,edinburgh1 p4 #eh4 ,illustration3 ,fig4 #aef4--,section in ,wardlaw ,quarry1 ,linlithgowshire4 ,among the phenomena associated with the ,carboniferous volcanoes mention may1 in conclusion1 be made of the evidence for the former existence of thermal springs and saline sublimations or incrustations4 ,among the plateau-tuffs of ,north ,berwick1 as has been already pointed out 7p4 #cij71 a ftid limestone has been quarried1 which bears indications of having been deposited by springs1 probably in connection with the volcanic action of the district4 ,the lower limestones of ,bathgate furnish abundant lamin of silica interleaved with calcareous matter1 the whole probably due to the action of siliceous and calcareous springs connected with the active puys of that district4 ,some portions of the limestone are full of cellular spaces1 lined with chalcedony4#dfd ,a saline water has been met with among the volcanic rocks to the west of ,linlithgow1 in a bore which was sunk to a depth of #cdh feet in these rocks without reaching their bottom4 ,the water that rose from the bore-hole was found to contain as much as #ace grains of chloride of sodium in the gallon4 ,it is not improbable that this salt was originally produced by incrustations on the ,carboniferous lavas immediately after their eruption1 as has happened so often in recent times at ,vesuvius1 and that it was then buried under succeeding showers of tuff and streams of lava4#dfe ,footnote #dfd3 ,ibid4 p4 #di1 et seq4 ,footnote #dfe3 ,proc4 ,roy4 ,soc4 ,edin4 vol4 ix4 p4 #cfg4 ,besides chloride of sodium the water contained also chlorides of calcium1 magnesium and potassium1 carbonates of lime and magnesia1 sulphate of lime1 and other ingredients in minute proportions4 ,subsequent ,dislocation of ,bedded ,lavas and ,tuffs4--,as the interstratified volcanic materials were laid down in sheets at the surface1 they necessarily behave like the ordinary sedimentary strata1 and have undergone with them the various curvatures and fractures which have occurred since ,carboniferous times4 ,notwithstanding their volcanic nature1 they can be traced and mapped precisely as if they had been limestones or sandstones4 ,this perfect conformability with the associated stratified rocks is strikingly seen in the case of the sheets of lava which lie imbedded in the heart of the great volcanic ridge of ,linlithgowshire4 ,the overlying strata having been removed from their surface for some distance1 and the ground having been broken by faults1 these volcanic rocks might at first be taken for irregular intrusive bosses1 but their true character is that shown in ,fig4 #aeg1 where by a succession of faults1 with a throw in the same direction1 the upper basalts of ,bonnytoun ,hill are gradually brought down to the level of the ,firth of ,forth4 ,illustration3 ,fig4 #aeg4--,section from ,linlithgow ,loch to the ,firth of ,forth4 iii4 ,,sills1 ,,bosses ,,and ,,dykes ,one of the characteristic features of ,central ,scotland is the great number1 and often the large size and extraordinary persistence1 of the masses of eruptive1 more or less basic material1 which have been injected among the ,carboniferous strata4 ,the precise geological age of these intrusions cannot1 of course1 be more exactly defined than by stating that they are younger than the rocks which they traverse1 though in many cases their association with the necks1 lavas and tuffs is such as to show that they must be regarded as part of the ,carboniferous volcanic phenomena4 ,sills4--,with regard to the sills ,i have been led1 for the following reasons1 to connect the great majority of them with the puys1 though some are certainly of far later date1 while others should possibly be assigned to the plateaux4 ,in the first place1 the sills obviously connected with the plateaux are in great measure intermediate1 or even somewhat acid rocks1 while those of the puy series are much more basic4 ,it is hardly possible1 however1 in all cases to decide to which series a particular sill should be assigned4 ,this difficulty is particularly manifest in the western part of ,midlothian1 where the plateau of that district exhibits such frequent interruption1 and where it often consists only of a single basaltic sheet4 ,to the west of it lie the abundant puys with their lavas and tuffs1 and between the two volcanic areas numerous sills of dolerite and diabase make their appearance4 ,in the difficulty of deciding to which series these sills should be referred1 it will be convenient to consider them with those of the puys4 ,illustration3 ,fig4 #aeh4--,section across the ,campsie ,fells illustrating the contrast between the sills below and above the plateau-lavas4 #a4 ,upper ,old ,red ,sandstone2 #b4 ,ballagan ,beds2 #c4 ,tuffs2 #d4 ,lavas of the ,campsie district of the ,clyde plateau2 #e #e4 ,necks belonging to the plateau volcanic series2 #f4 ,trachytic sills belonging to the plateau2 #g4 ,carboniferous ,limestone series2 #h4 ,dolerite sills cutting the ,carboniferous ,limestone series4 f1 ,fault4 ,a remarkable illustration of the contrast in petrographical character between the typical sills of the plateaux and those of the puys is furnished by the chain of the ,campsie ,fells1 where1 on the north side1 among the ,calciferous ,sandstones which emerge from under the andesitic lavas of the ,clyde plateau1 many intrusive sheets and bosses of trachytic material may be seen1 while on the southern side come the great basic sills which1 from ,milngavie by ,kilsyth to ,stirling1 run in the ,carboniferous ,limestone series 7,fig4 #aeh74 ,a similar contrast may be observed in ,renfrewshire between the trachytic sills below the plateau-lavas south of ,greenock and the basic sills above these lavas in the ,carboniferous ,limestone series around ,johnstone and ,paisley4 ,in the second place1 the more basic sills1 as a rule1 appear on platforms higher in stratigraphical position than the plateaux1 and wherever this is their position there cannot be any hesitation in deciding against their association with the older phase of volcanic activity4 ,in the third place1 the basic sills often occur in obvious connection with the vents or bedded lavas and tuffs of the puy series4 ,a conspicuous example of this dependence is supplied by the intrusive sheets of ,burntisland1 underlying the basalts and tuffs of that district in the immediate neighbourhood of some of the vents from which these bedded rocks were erupted 7,fig4 #aei74 ,in the fourth place1 even where no visible vents appear now at the surface near the sills1 the latter generally occupy horizons within the stratigraphical range indicated by the interbedded volcanic rocks4 ,it must be remembered that all the ,carboniferous vents were deeply buried under sedimentary deposits1 and that large as is the number of them which has been exposed by denudation1 it is probably much smaller than the number still concealed from our view4 ,the sills are to be regarded as deep-seated parts of the volcanic protrusions1 and they more especially appear at the surface where the strata between which they were injected crop out from under some of the higher members of the ,carboniferous system4 ,thus the remarkable group of sills between ,kilsyth and ,stirling 7,fig4 #aeh7 may quite possibly be connected with a group of vents lying not far to the eastward1 but now buried under the higher parts of the ,carboniferous ,limestone1 ,millstone ,grit and ,coal-measures4 ,again1 the great series of sills that gives rise to such a conspicuous range of hills in the north and middle of ,fife may have depended for its origin upon the efforts of a line of vents running east and west through the centre of the county1 but now buried under the ,coal-measures4 ,some vents1 indeed1 have been laid bare in that district1 such as the conspicuous groups of the ,saline ,hills and the ,hill of ,beath1 but many more may be concealed under higher ,carboniferous strata further east4 ,illustration3 ,fig4 #aei4--,section showing the position of the basic sills in relation to the volcanic series at ,burntisland1 ,fife4 #a4 ,calciferous ,sandstone series2 #b4 ,burdiehouse ,limestone2 #c4 ,sandstones1 shales and tuffs2 #d4 ,basalts and tuffs1 with intercalations of sandstone1 shale and limestone2 #e4 ,agglomerate of the ,binn of ,burntisland neck2 #f4 ,basalt dyke2 #g4 ,dyke and sill2 #h #h #h4 ,three sills4 ,in the fifth place1 the materials of which the sills consist link them in petrographical character with those that proceeded from the puys4 ,the rocks of the intrusive sheets in ,west ,lothian1 ,midlothian and ,fife are very much what an examination of the bedded lavas of the puys in the same region would lead us to expect4 ,there is1 of course1 the marked textural difference between masses of molten rock which have cooled very slowly within the crust of the earth and those which have solidified with rapidity at the surface1 the sills being for the most part much more coarsely crystalline than the lavas1 and more uniform in texture throughout1 though generally finer at the margins than at the centre4 ,there is likewise the further contrast arising from differences in the composition of the volcanic magma at widely-separated periods of its extravasation4 ,at the time when the streams of basalt flowed out from the puys its constitution was comparatively basic1 in some localities even extremely basic4 ,any sills dating from that time may be expected to show an equal proportion of bases4 ,but those which were injected at a long subsequent stage in the volcanic period may well have been considerably more acid4 ,in actual fact the petrographical range of the sills reasonably referable to the puy-eruptions varies from picrite or limburgite to dolerite without olivine4 ,the great majority of these sheets in the basin of the ,firth of ,forth1 where they are chiefly displayed1 are dolerites 7diabases71 sometimes with1 but more frequently without1 olivine4 ,they include all the more coarsely crystalline rocks of the region1 though occasionally they are ordinary close-grained basalts4 ,their texture may be observed to bear some relation to their mass1 so far at least as that1 where they occur in beds only two or three feet or yards in thickness1 they are almost invariably closer-grained4 ,a cellular or amygdaloidal texture is seldom to be observed among them1 and never where they are largely crystalline4 ,this texture is most often to be found in thin sills which have been injected among carbonaceous shales or coals4 ,these intrusive sheets are generally finely cellular1 and more or less decayed 7white trap74 ,illustration3 ,fig4 #afj4--,sills between shales and sandstones1 ,hound ,point1 ,linlithgowshire4 ,differences of texture may often be observed within short distances in the same sill1 and likewise considerable varieties in colour and composition4 ,the most finely crystalline portions are1 as usual1 those along the junction with the stratified rocks1 the most crystalline occurring in the central parts of the mass4 ,a diminution in the size of the crystalline constituents may be traced not only at the base1 but also at the top of a sheet1 or at any intermediate portion which has come in contact with a large mass of the surrounding rock4 ,a good illustration is supplied by the intrusive sheet at ,hound ,point 7,fig4 #afj71 to the east of ,south ,queensferry1 where some layers of shale have been involved in the igneous rock1 which becomes remarkably close-grained along the junction4#dff ,this change in texture and absence of cellular structure form a well-marked distinction between these sheets and those which have flowed out at the surface as true lava-streams4 ,footnote #dff3 ,see ,hay ,cunningham's ,essay1 p4 #ff1 and plate ix42 and ,geol4 ,survey ,memoir on ,geology of ,edinburgh1 p4 #aad4 ,some of the larger doleritic sills display a somewhat coarsely crystalline texture in their central portions1 and occasionally present a notable micropegmatitic aggregate1 which plays the part of interstitial substance enclosing the other minerals4 ,mr4 ,teall has referred to the frequent occurrence of this structure in the coarser parts of the ,whin ,sill of the north of ,england4#dfg ,it occurs also in a marked degree in the ,ratho sill and in some portions of the great doleritic sill of which the crags of ,stirling form a part4#dfh ,footnote #dfg3 ,british ,petrography1 p4 #bjh4 ,footnote #dfh3 ,mr4 ,h4 ,w4 ,monckton4 ,quart4 ,journal ,geol4 ,soc4 vol4 li4 7#ahie71 p4 #dhb4 ,but beside the differences in texture1 mainly due to varying rates of cooling1 the sills sometimes exhibit striking varieties of composition in the same mass of rock4 ,these variations are more especially noticeable among the larger sills1 and particularly where the material is most markedly basic4 ,the special type of differentiation1 so noticeable in the ,bathgate diabase and picrite mass already alluded to1 is likewise well exhibited in an intrusive sheet or group of sheets1 recently exposed at ,barnton1 in the cutting of a railway from ,edinburgh to ,cramond#dfi 7,fig4 #afa74 ,the intrusive nature of the several bands of igneous rock which occur here is made quite evident by the alteration they have produced upon the shales with which they have come in contact4 ,it is the uppermost and most extensive of these sills which specially deserves notice1 for the differentiation of its constituents4 ,it stretches along the cutting for several hundred yards at an angle of dip of about #ae4 ,at the western or upper part of the mass its actual contact with the superincumbent sedimentary strata is not visible1 but as the igneous rock is there a good deal finer in grain than elsewhere1 its upper surface cannot be many feet distant4 ,the upper visible portion is a light well-crystallized dolerite with a rudely bedded structure1 the planes dipping westwards at #ae4 ,about #bj or #cj feet below the upper visible termination of the mass1 the dark ferro-magnesian minerals begin rapidly to increase in relative proportion to the pale felspar1 and the rock consequently becomes dark-greenish brown4 ,the change is particularly noticeable in certain bands which run parallel with the general dip4 ,there is no definite line between the pale and dark body of the rock1 the two graduating into each other and the darker part becoming deeper in colour1 heavier and more decomposing1 until it becomes a true typical picrite4 ,even in this ultra-basic portion the same rude bedding or banding may be observed4 ,footnote #dfi3 ,this rock has been described by ,mr4 ,j4 ,henderson and ,mr4 ,goodchild1 ,trans4 ,geol4 ,soc4 ,edin4 vi4 7#ahic7 pp4 #big1 #cja1 and by ,mr4 ,h4 ,w4 ,monckton1 ,quart4 ,journ4 ,geol4 ,soc4 l4 7#ahid7 p4 #ci4 ,mr4 ,goodchild recognized the occurrence of picrite1 and ,mr4 ,monckton has described the succession of rocks1 and given a diagram of them4 ,illustration3 ,fig4 #afa4--,section of ,sill1 ,cramomd ,railway1 ,barnton1 near ,edinburgh4 #a4 ,baked shale2 #b4 ,sill of very felspathic dolerite about1 nine feet thick2 #c4 ,baked shale1 eight inches2 #d4 ,dolerite showing chilled fine-grained edge and adhering firmly to the shale below2 it rapidly passes up into 7#e7 ,picrite with white felspathic veins 7#f72 #g4 ,junction of picrite and dolerite with a similar vein along the line of contact2 #h4 ,large globular body of dolerite enclosing a mass of picrite4 ,veins in which felspar predominates over the darker minerals traverse the rock1 sometimes parallel with the bedding1 sometimes across it4 ,they vary from less than an inch to a foot in width1 sometimes dividing and enclosing parts of the surrounding mass4 ,but that they are on the whole contemporaneous with the sill itself1 and not long subsequent injections1 is shown by the way in which the dark ferro-magnesian minerals project from the picrite into the veins and lock the two together4 ,but besides these injections1 which doubtless represent the last and more acid portions of the magma injected into the basic parts before the final consolidation of the whole1 there are to be observed irregular concretionary patches1 of similar character to the veins1 distributed through the picrite4 ,on the other hand1 towards its base the sill becomes a coarse dolerite round which the picrite is wrapped1 and which encloses a detached portion of that rock4 ,it is deserving of note that while the ultra-basic portion descends almost to the very bottom of the sill1 the lowest five feet show the same change as occurs at the top of the mass4 ,there the felspar rapidly begins to predominate over the darker minerals1 and the dolerite into which the rock passes shows a fine-grained margin adhering firmly to the shales on which it rests4 ,this lower doleritic band1 showing as it does the effect of chilling upon its under surface1 may be due to more rapid cooling and crystallization1 while in the overlying parts the mass remained sufficiently mobile to allow of a separation of the heavier minerals from the felspars1 which appear in predominant quantity towards the top4 ,it must be frankly admitted1 however1 that we are still very ignorant of the causes which led to this separation of ingredients in a few sills1 while they were entirely absent or non-efficient in most of them4 ,the intrusive character of the ,carboniferous sills of ,central ,scotland and their contact-metamorphism have been fully described1 and some of them have become1 as it were1 household words in geology4#dgj ,exposed in so many fine natural sections in the vicinity of ,edinburgh1 they early attracted the notice of geologists1 and furnished a battle-ground on which many a conflict took place between the ,plutonist and ,neptunist champions at the beginning of the present century4 ,footnote #dgj3 ,see1 for instance1 ,maclaren's ,geology of ,fife and the ,lothians1 #ahci2 ,hay ,cunningham's ,essay1 previously cited2 ,geological ,survey ,memoir on the ,geology of ,edinburgh 7,sheet #cb71 #ahfa2 ,mr4 ,allport1 ,quart4 ,journ4 ,geol4 ,soc4 vol4 xxx4 7#ahgd7 p4 #eec2 ,teall1 ,british ,petrography1 p4 #ahg2 ,e4 ,stecher1 ,contacterscheinungen an schottischen ,olivindiabasen1 ,tschermak's ,mineralog4 ,mittheil4 vol4 ix4 7#ahhg7 p4 #ade2 ,proc4 ,roy4 ,soc4 ,edin4 vol4 xv4 7#ahhh7 p4 #afj4 ,as the sills frequently lie in even sheets perfectly parallel with the bedding of the strata between which they have been injected1 care is required in some cases to establish that they are of intrusive origin4 ,one of the most obvious tests for this purpose is furnished by the alteration they produce among the strata through which they have made their way1 whether these lie above or below them4 ,the strata are sometimes crumpled up in such a manner as to indicate considerable pressure4 ,they are occasionally broken into fragments1 though this may have been due rather to the effects of gaseous explosions than to the actual protrusion of melted rock4 ,but the most frequent change superinduced upon them is an induration which varies greatly in amount even along the edge of the same intrusive sheet4 ,sandstones are hardened into quartzite1 breaking with a smooth clear glistening fracture4 ,coals are converted into a soft sooty substance1 sometimes into anthracite4 ,limestones acquire a crystalline saccharoid structure4 ,shales pass generally into a kind of porcellanite1 but occasionally exhibit other types of contact-metamorphism4 ,thus below the thick picrite sill at ,barnton1 near ,edinburgh1 the shales have assumed a finely concretionary structure by the appearance in them of spherical pea-like aggregates4 ,another proof of intrusion is to be found in the manner in which sills catch up and completely enclose portions of the overlying strata4 ,the well-known examples on ,salisbury ,crags 7,fig4 #afb7 are paralleled by scores of other instances in different parts of the same region4 ,moreover1 sills do not always remain on the same horizon2 that is1 between the same strata4 ,they may be observed to steal across or break through the beds1 so as to lie successively between different layers4 ,no more instructive example of this relation on a small scale could be cited than that of the intrusive sheet which has been laid open in the ,dodhead ,limestone ,quarry1 near ,burntisland4 ,as shown in the accompanying figure 7,fig4 #afc71 this rock breaks through the limestone and then spreads out among the overlying shales1 across which it passes obliquely4 ,illustration3 ,fig4 #afb4--;,intrusive dolerite sheet enclosing and sending threads into portions of shale1 ,salisbury ,crags1 ,edinburgh4 ,among the larger sills this transgressive character is seen to be sometimes manifested on a great scale4 ,thus1 along the important belt of intrusive rocks that runs from ,kilsyth to ,stirling1 the ,hurlet ,limestone lies in one place below1 in another above1 the invading mass1 but in the intervening ground has been engulphed in it4 ,similar evidence of the widely separate horizons occupied by different parts of the same sill is supplied at ,kilsyth1 where the intrusive sheet lies about #gj or #hj fathoms below the ,index ,limestone1 while at ,croy1 in the same neighbourhood1 it actually passes above that seam4#dga ,footnote #dga3 ,explanation of ,sheet #ca1 ,geological ,survey of ,scotland1 #dc and #hc4 ,illustration3 ,fig4 #afc4--;,intrusive sheet invading limestone and shale1 ,dodhead ,quarry1 near ,burntisland4 ,other interesting evidence of the intrusive nature of the ,carboniferous dolerite sills of ,central ,scotland is supplied by the internal modifications which the eruptive rock has undergone by contact with the strata between which it has been thrust4 ,these alterations1 though partly visible to the naked eye1 are best studied in thin slices with the aid of the microscope4 ,tracing the variations of an intrusive dolerite outwards in the direction of the rocks which it has invaded1 we perceive change first in the augite4 ,the large crystals and kernels of that mineral grow smaller until they pass into a granulated form like that characteristic of basalts4 ,the large plates and amorphous patches of titaniferous iron or magnetite give place to minute particles1 which tend to group themselves into long club-shaped bodies4 ,the labradorite continues but little affected1 except that its prisms1 though as defined1 may not be quite so large4 ,the interstitial glassy groundmass remains in much the same condition and relative amount as in the centre of the rock4 ,along the line of contact1 while the dolerite becomes exceedingly close-grained1 its felspar crystals are still quite distinct even up to the very edge4 ,but they become fewer in relative number1 and still smaller in size1 though an occasional prism two or three millimetres in length may occur4 ,they retain also their sharpness of outline1 and their comparative freedom from enclosures of any kind4 ,they tend to range themselves parallel with the surface of the contact-rock4 ,the augite exists as a finely granular pale green substance1 which might at first be taken for a glass1 but it gives the characteristic action of augite with polarized light4 ,it is intimately mixed through the clear glass of the groundmass1 which it far exceeds in quantity4 ,the iron oxides now appear as a fine granular dust1 which is frequently aggregated into elongated club-shaped objects1 as if round some inner pellucid or translucent microlite4 ,in patches throughout the field1 however1 the oxides take the form of a geometrically perfect network of interlacing rods4 ,this beautiful structure1 described and figured by ,zirkel and others1#dgb is never to be seen in any of the dolerites1 except close to the line of contact with the surrounding rocks4 ,it occurs also in some of the dykes4 ,i have not succeeded in detecting any microlites in the sandstones at the edge of a dolerite sheet1 though ,i have had many slices prepared for the purpose4 ,footnote #dgb3 ,mikroskopische ,beschaffenheit der ,mineralien und ,gesteine1 p4 #bgc2 ,vogelsang's ,krystalliten4 ,where one of the dolerite sills has invaded sandstone1 there is usually a tolerably sharp line of demarcation between the two rocks1 though it is seldom easy to procure a hand-specimen showing the actual contact1 for the stone is apt to break along the junction-line4 ,where1 however1 the rock traversed by the igneous mass is argillaceous shale1 we may find a thorough welding of the two substances into each other4 ,in such cases the dolerite at the actual contact becomes a dark opaque rock1 which in thin slices under the microscope is found to be formed of a mottled or curdled segregation of exceedingly minute black grains and hairs in a clear glassy matrix1 in which the augite and felspar are not individualized4 ,but even in this tachylyte-like rock perfectly formed and very sharply defined crystals of triclinic felspar may be observed ranging themselves as usual parallel to the bounding surfaces of the rock4 ,these characters are well seen in the contact of the intrusive sheet of dolerite with shale and sandstone at ,hound ,point 7,fig4 #afj74 ,another instructive example is furnished by the small threads which proceed from the dolerite of ,salisbury ,crags1 and traverse enclosed fragments of shale 7,fig4 #afb74 ,some of these miniature dykes are not more than one-eighth of an inch in diameter1 and may therefore easily be included1 together with part of the surrounding rock1 in the field of the microscope4 ,the dolerite in these ramifications assumes an exceedingly fine texture4 ,the felspar is the only mineral distinctly formed into definite crystals4 ,it occurs in prisms of an early consolidation1 sometimes one-fifth of an inch long1 and therefore readily recognizable by the naked eye4 ,these prisms are perfectly shaped1 contain abundant twin lamell1 and show enclosures of the iron of the base4 ,they had been already completely formed at the time of injection2 for occasionally they may be observed projecting beyond the wall of the vein into the adjacent shale or sandstone1 and they have ranged themselves parallel to the sides of the vein4#dgc ,the black ground1 from which these large well-defined crystals stand out prominently1 consists of a devitrified glass1 rendered dark by the multitude of its enclosed black opaque microlites4 ,these are very minute grains and rudely feathered rods1 with a tendency to group themselves here and there into forms like portions of the rhombohedral skeletons of titaniferous iron4 ,so thoroughly fused and liquid has the dolerite been at the time of its injection1 that little threads of it1 less than #a_/ajj of an inch in diameter1 consisting of the same dark base1 with well-defined felspars1 may be seen isolated within the surrounding sedimentary rock4 ,minute grains and rounded portions of the latter may also be noticed in the marginal parts of the dolerite4 ,footnote #dgc3 ,the infusibility of the felspar was well shown in some experiments on the rocks of the neighbourhood of ,edinburgh1 made at my request by ,dr4 ,r4 ,s4 ,marsden1 who subjected some of these rocks to fusion at the laboratory of the ,university of ,edinburgh4 ,microscopic sections were prepared of the products obtained4 ,the basalt of ,lion's ,haunch is peculiarly instructive4 ,its large labradorite crystals have resisted the intense white heat which1 continued for four hours1 has reduced the rest of the minerals to a perfect glass4 ,we can thus well understand how large definite crystals of felspar should have survived or appeared in dykes and veins while the rock was still thoroughly liquid4 ,the glass obtained from the ,lion's ,haunch rock is of a honey-yellow1 and contains translucent tufted microlites4 ,the iron forms beautiful dendritic films in the cracks4 ,altogether1 the glass presents a strong resemblance to the palagonitic substance so abundant among the lapilli in the tuffs of the vents4 ,it is thus evident that specimens taken from the edge of an intrusive sheet1 where the rock has rapidly chilled and solidified1 represent to us an earlier stage in the history of the whole mass than specimens taken from its central portions4 ,in fact1 a series of samples collected at short intervals from the outer contact to the inner mass shows1 as it were1 the successive stages in the consolidation of the molten rock4 ,from the observations just described1 it appears that the triclinic felspars began to assume the shape of large definite crystals before any of the other minerals4 ,these felspars already existed when the molten mass forced its way among the shales1 for they can be seen lying with their long axes parallel to the surface of shale1 precisely as1 in the well-known flow-structures1 they behave round a large crystal embedded in the heart of a rock4 ,a few feet from where the consolidation was not so rapid1 the iron oxides have grouped themselves into incipient crystalline forms and skeleton crystals2 the felspar crystals abundantly occur1 and the augite has been left in the finely granular condition4 ,still further towards the interior of the mass1 the normal character of the dolerite is gradually assumed4#dgd ,footnote #dgd3 ,for a further and more detailed investigation of the contact phenomena of the ,carboniferous doleritic sills of the basin of the ,firth of ,forth1 see the papers by ,dr4 ,stecher1 quoted on p4 #dea4 ,illustration3 ,fig4 #afd4--,spheroidal weathering of dolerite sill1 quarry east of ,north ,queensferry1 ,fife4 ,where a sill has been injected among carbonaceous shales and coals it has undergone great alteration along the contact1 and if the sheet is only a few inches or feet thick1 the change extends throughout its whole mass4 ,black basalts and dolerites1 in such circumstances1 pass into a substance like a white or pale yellow clay1 which at first might be mistaken for some band of fire-clay intercalated among the other sediments4 ,but evidence of actual intrusion may usually be found1 as where the igneous rock has caught up or broken through the adjacent strata1 besides altering them4 ,such white traps1 as they have been called1 have long been familiar in the coal-fields of ,scotland and ,central ,england4 ,illustration3 ,fig4 #afe4--,two thin sills of ,white ,trap injected into black carbonaceous shale overlying the ,hurlet ,limestone1 ,hillhouse ,quarry1 ,linlithgow4 #a4 ,hurlet ,limestone2 #b4 ,black shales2 #c #c4 ,two sills of ,white ,trap2 #d4 ,columnar ,basalts4 ,as a good illustration of the behaviour of such thin sills among carbonaceous shales ,i give here a section 7,fig4 #afe7 exposed in the old limestone quarry of ,hillhouse1 south of ,linlithgow4 ,at the bottom lies the ,hurlet ,limestone which has once been extensively mined at this locality4 ,above it comes a group of black shales which in turn are surmounted by a sheet of beautifully columnar basalt4 ,the shales seem at first sight to include two layers of pale fire-clay1 each only a few inches in thickness4 ,closer inspection1 however1 will show that these two thin intercalations are really sills which1 though on the whole parallel with the bedding of the shale1 may be seen to cut across it1 and even at one place to send a finger into it4 ,the upper example may also be observed to diminish rapidly in thickness in one direction4 ,the dimensions of the sills vary within tolerably wide limits4 ,although here and there the injected material dwindles down to an inch or less in thickness1 running away even into threads1 it more usually forms sheets of considerable depth4 ,the rock of ,salisbury ,crags1 for example1 is fully #aej feet thick at its maximum4 ,that of ,corstorphine ,hill is probably about #cej feet4 ,the great sheet which runs among the lower limestones from ,kilsyth by ,denny to ,stirling has been bored through to a depth of #bgf feet1 but as the bore started on the rock1 and not in overlying strata1 some addition may need to be made to that thickness4 ,the spheroidal weathering so characteristic of basic eruptive rocks is nowhere more characteristically displayed than among the great doleritic sills of the basin of the ,firth of ,forth4 ,as an illustration of this structure an example is taken here from the large sheet at ,north ,queensferry 7,fig4 #afd74 ,while one is struck with the great size and extent of some of the sills connected with the puys1 as compared with the small and local sheets underneath the plateaux1 there is a further fact regarding them that ,illustration3 ,fig4 #aff4--;,dyke cutting the agglomerate of a neck4 ,binn of ,burntisland4 deserves remark--their capricious distribution4 ,their occurrence seems to have little or no relation to the measure of volcanic energy as manifested in superficial eruptions4 ,thus in the north of ,ayrshire1 where a long band of lavas and tuffs1 pointing to vigorous activity1 lies at the top of the ,carboniferous ,limestone series1 and where the strata underneath it are abundantly exposed at the surface1 the sills occur as thin and inconstant bands in the central and eastern parts of the district only4 ,the bedded lavas and tuffs at the head of the ,slitrig ,water have no visible accompaniment of sills4 ,on the other hand1 in the ,edinburgh and ,burntisland districts1 the sills bear a large proportion to the amount of bedded lavas and tuffs1 while in the ,bathgate and ,linlithgow district1 where the superficial eruptions were especially vigorous and prolonged1 the sills are of trifling extent4 ,it would seem from these facts that the extent to which the crust of the earth round a volcanic orifice is injected with molten rock1 in the form of intrusive sheets between the strata1 does not depend upon the energy of the volcano as gauged by its superficial outpourings1 but on other considerations not quite apparent4 ,possibly1 the more effectively volcanic energy succeeded in expelling materials from the vent1 the less opportunity was afforded for subterranean injections4 ,and if the protrusion of the sills took place after the vents were solidly sealed up with agglomerate or lava1 it would doubtless often be easier for the impelled magma to open a way for itself laterally between the bedding-planes of the strata than vertically through the thick solid crust4 ,the size and extent of the sills may thus be a record of the intensity of this latest phase of the volcanic eruptions4 ,illustration3 ,fig4 #afg4--;,boss of diabase cutting the ,burdiehouse ,limestone and sending sills and veins into the overlying shales4 ,railway cutting1 ,west ,quarry1 ,east ,calder1 ,midlothian4 #a4 ,burdiehouse ,limestone2 #b4 ,shales2 #c4 ,diabase4 99 99 99 99 99 ,bosses4--,the rounded1 oval or irregularly shaped masses of igneous rock included under this head are found in some cases to be only denuded domes of sills1 as1 for example1 in the apparently isolated patches in the oil-shale district of ,linlithgowshire1 which have been found to unite under ground4 7,compare ,fig4 #aeg74 ,in other instances1 bosses possibly1 or almost certainly1 mark the position of volcanic funnels1 as at the ,castle ,rock of ,edinburgh1 ,dunearn ,hill1 ,burntisland1 and ,galabraes1 near ,bathgate4 ,but many examples occur which can only be regarded as the exposed ends of irregular bodies of molten material which has been protruded upwards into the ,illustration3 ,fig4 #afh4--,side of columnar basalt-dyke in the same agglomerate as in ,fig4 #aff4 ,carboniferous formations4 ,the area between ,edinburgh and ,linlithgow and the hills of the north of ,fife furnish many examples4 ,the connection between bosses and intrusive sheets is instructively exhibited in a railway cutting to the west of ,edinburgh1 where the section shown in ,fig4 #afg may be seen4 ,in the space of a few yards no fewer than four distinct bands of diabase traverse the shale1 thickening rapidly in one direction and uniting with a large boss of more coarsely crystalline material4 ,such connections must exist in all sills1 for the material injected as a sheet between stratified formations cannot but be united to some column1 dyke or irregular protrusion which descends to the parent magma in the interior4 ,but it is very rarely that the geologist is permitted to see them4 99 99 99 99 99 ,illustration3 ,fig4 #afi4--;,dyke rising through the ,hurlet ,limestone and its overlying shales4 ,silvermine ,quarry1 ,linlithgowshire4 ,dykes take a comparatively unimportant place in the eruptive phenomena of the puys4 ,they occur in some numbers1 but on a small scale1 among the tuff vents1 and there they can without much hesitation be set down as part of the phenomena of eruption through these pipes4 ,the ,binn of ,burntisland1 which has been so often referred to in this ,chapter1 may again be cited as a typical vent for the display of this series of dykes 7,figs4 #adi and #aei74 ,two additional illustrations from this locality are here given4 ,in ,fig4 #aff a dyke of compact black basalt is seen on the right hand running up the steep slopes of the agglomerate4 ,some of these dykes are distinctly columnar1 the columns diverging from the walls on each side4 ,where the encasing agglomerate has been removed by the weather1 the side of the dyke presents a reticulated network of prism-ends4 ,a narrow basalt-dyke of this character near the top of the ,binn vent is represented in ,fig4 #afh4 ,but dykes also occur apart from vents and without any apparent relation to these4 ,they are sometimes associated with sills and bosses in such a manner as to suggest that the whole of these forms of injected material belong to one connected series of intrusions4 ,among the ,bathgate ,hills1 for example1 from which ,i have already cited instances of sills and a boss1 the section represented in ,fig4 #afi occurs4 ,yet in this same district there is a group of large east and west dykes which cut all the other rocks including the bedded lavas and tuffs1 and must be of later date than the highest part of the ,coal-measures 7,fig4 #aee74 ,it is difficult to ascertain the age of the dykes which rise through the ,carboniferous formations at a distance from any interbedded sheets of lava and tuff1 or from any recognizable vent4 ,the south-east and north-west dykes1 increasing in number as they go westward1 and attaining a prodigious development in the great volcanic area of ,antrim and the ,inner ,hebrides1 are probably of ,tertiary date4#dge ,others may possibly be ,permian1 while a certain number may reasonably be looked upon as ,carboniferous4 ,in petrographical characters the latter resemble the dolerites and basalts 7diabases7 of the finer-grained sills4 ,footnote #dge3 ,these are fully described in ,chapters xxxiv4 and xxxv4 ,,chapter ,,xxviii ,,illustrative ,,examples ,,of ,,the ,,carboniferous ,,puys ,,of ,,scotland ,the ,basin of the ,firth of ,forth--,north ,ayrshire--,liddesdale4 ,though many of the geological details of each of the ,scottish districts of ,puys have been given in the foregoing pages1 it will be of advantage to describe in connected sequence the structure and geological history of a few typical areas4 ,by far the fullest and most varied record of this phase of volcanic activity has been preserved in the basin of the ,firth of ,forth2 but the north of ,ayrshire and the district of ,liddesdale furnish also many interesting characteristics4 #a4 ,,basin ,,of ,,the ,,firth ,,of ,,forth ,reference has already been made to the remarkable peculiarity in the development of the lower part of the ,carboniferous system in this district4#dgf ,elsewhere throughout ,scotland the ,cement-stone group and the plateau lavas are immediately overlain by the ,carboniferous ,limestone series4 ,but in the basin of the ,firth of ,forth a varied succession of strata1 more than #cjjj feet in thickness1 intervenes between the ,cement-stones and the ,hurlet ,limestone4 ,the lower portion of this thick mass of sediment may represent a part of the ,cement-stone group of other districts1 but even if some deduction is made on this account there remain many hundred feet of stratified deposits1 for which there does not appear to be any stratigraphical equivalent elsewhere in ,scotland4 ,the distinguishing features of this series of strata are the thick zones of white sandstone1 with occasional bands of fine conglomerate1 the abundant seams of dark shale1 often highly carbonaceous 7oil-shales71 the cyprid limestones and the seams of coal4 ,such an association of deposits may indicate a more humid climate and more varied conditions of denudation and deposition than are presented by the typical ,cement-stones4 ,the muddy floor of the shallow water must1 in many places1 have supported a luxuriant growth of vegetation1 which is preserved in occasional seams and streaks of coal4 ,numerous epiphytic ferns grew on the subrial stems and branches of the lycopodiaceous trees4 ,large coniferae clothed the higher grounds1 from which the streams brought down copious supplies of sediment1 and whence a flood now and then transported huge prostrate trunks of pine4 ,in the lagoons animal life abounded4 ,cyprids swarmed to such a degree as to form by their accumulated remains bands of limestone1 which in the well-known ,burdiehouse seam sometimes attain a thickness of #gj feet4 ,fishes of many genera haunted the waters1 for their scales1 bones and coprolites are found in profusion among the shales and limestones4 ,footnote #dgf3 ,see ,maclaren's ,geology of ,fife and the ,lothians1 the ,memoirs of the ,geological ,survey of ,scotland1 on ,sheets #ca and #cb1 and my ,memoir1 already cited1 ,trans4 ,roy4 ,soc4 ,edin4 vol4 xxix4 7#ahgi7 p4 #dcg4 ,when the puys began their activity1 this district was gradually dotted over with little volcanic cones4 ,at the same time it was affected by the general movement of slow subsidence which embraced all ,central ,scotland4 ,cone after cone1 more or less effaced by the waters which closed over it1 was carried down and buried under the growing accumulation of sediment4 ,new vents1 however1 continued to be opened elsewhere1 throwing out for a time their showers of dust and stones1 and then lapsing into quiescence as they sank into the lagoon4 ,two groups of volcanoes emitted streams of lava and built up two long volcanic ridges--those of ,fife and ,west ,lothian4 ,the occasional presence of the sea over the area is well shown by the occurrence of thin bands of limestone or shale1 containing such fossils as species of ,orthoceras1 ,bellerophon and ,discina1 which suffice to prove the strata to be stratigraphical equivalents of the ,lower ,limestone shale1 and part of the ,carboniferous ,limestone of ,england 7,fig4 #agj74 ,yet the general estuarine or freshwater character of the accumulations seems satisfactorily established1 not only by the absence of undoubtedly marine forms from most of the strata1 but by the abundance of cyprids and small ganoids1 the profusion of vegetable remains1 and the occasional seams of coal4 ,the portion of the ,forth basin within which the puys are displayed extends from near ,leven in ,fife1 on the north1 to ,crosswood ,burn near the borders of ,lanarkshire1 on the south1 a distance of about #cf miles1 and from near ,culross in ,fife and the line of the ,almond ,river between ,stirlingshire and ,linlithgowshire1 on the west1 to the island of ,inchkeith on the east1 a distance of about #af miles 7,map ,,iv474 ,but these limits do not precisely mark the original boundaries of the eruptions4 ,to the north and south1 indeed1 we can trace the gradual dying out of the volcanic intercalations1 until we reach ground over which no trace of either lavas or tuffs can be detected4 ,to the east1 the waters of the ,firth conceal the geology of a considerable area1 the island of ,inchkeith with its bedded lavas and tuffs showing that these rocks extend some way farther eastwards than the position of that island4 ,but in ,midlothian there is no evidence that any of the puy-eruptions took place to the east of the line of the ,pentland ,hills4 ,on the west side1 the volcanic rocks dip under the ,millstone ,grit and ,coal-measures1 so that we do not know how far they extend in that direction4 ,but as the ,carboniferous ,limestone series1 when it rises again to the surface on the west side of the ,stirlingshire coal-field1 is destitute of included lavas and tuffs1 the westward limit of the eruptions cannot lie much beyond the line of the ,river ,illustration3 ,fig4 #agj4--;,junction of amygdaloidal basalt with shales and limestone1 ,shore1 half a mile east from ,kinghorn1 ,fife4 7,from a photograph by ,mr4 ,r4 ,lunn47 ,almond4 ,we shall probably be within the mark if we set down the original area over which puys broke out and spread abroad their lavas and tuffs as covering about #cjj square miles of the lagoons and jungles of ,central ,scotland4 ,i have already shown that the range in geological time of the puy-eruptions in this district extends from a low horizon among the ,calciferous ,sandstones through the ,carboniferous ,limestone series1 up to nearly the level of the ,calmy ,limestone1 which lies not far from the top of that series4 ,the vertical thickness of strata between these two stratigraphical limits1 when there are no intercalated volcanic rocks1 is probably more than #djjj feet4 ,the vents from which the volcanic materials were ejected1 so far as they are now to be observed at the surface1 may be divided into two groups1 one lying to the north1 the other to the south of the ,firth of ,forth4 ,the northern or ,fife group may be followed over an area #ae miles long1 and about three miles broad4 ,some fifteen separate vents may be recognized in it1 distributed chiefly at the two ends of the belt1 a cluster of about six rising around ,burntisland1 while another of nearly as many appears at ,saline4 ,the characters of some of these necks have been already given in the foregoing pages4 ,the southern or ,west ,lothian group includes about a dozen vents which are scattered over an area of some #fj square miles1 extending from the coast-line between ,borrowstounness and ,queensferry southwards to ,bathgate and ,uphall4 ,in this group ,binns ,hill1 a mile long by almost half a mile broad1 and rising to a height of nearly #cjj feet above the sea1 forms the most prominent individual4 ,but the vents are generally smaller in the southern than in the northern group4 ,the manner in which the vents have been left filled with volcanic material has been described in previous pages4 ,most of them are occupied by tuff or agglomerate4 ,in many cases the neck consists entirely of pyroclastic detritus1 as in most of the vents of eastern ,linlithgowshire and many of those in ,fife4 ,not infrequently a column of basalt has risen in the funnel and solidified there1 as exemplified by ,binns ,hill and ,saline ,hill1 or the basalt has filled rents in the tuff and now appears in dykes like those on the ,binn of ,burntisland 7,figs4 #adh1 #adi1 #aei1 #aff1 #afh74 ,but it is possible that in some cases vents may be represented by bosses of basalt or dolerite1 unaccompanied by any agglomerate or tuff4 ,perhaps the best example of this suggested origin is supplied by ,galabraes ,hill1 which rises through the ,hurlet limestone and the volcanic series of the ,bathgate ,hills1 about a mile north-east from the town of ,bathgate4 ,this eminence rises to a height of #idj feet above the sea1 and consists of a rudely elliptical boss of basalt1 measuring #cejj feet in its greater and #cjjj feet in its minor axis4 ,it disrupts the sedimentary and volcanic series1 which can be traced up to it on all sides4 ,some of the smaller circular or elliptical bosses in eastern ,linlithgowshire and western ,fife may perhaps belong to the same category4 ,but undoubtedly most of the intrusive basalts and dolerites of this volcanic region are sills4 ,over the greater part of the district1 only fine tuffs were ejected4 ,these occur as interstratifications among the ordinary sediments1 and vary from mere thin partings1 marking the feeblest and briefest explosions1 up to continuous accumulations several hundred feet thick4 ,as an example of the least vigorous emission of tuff ,i may refer to the sections already given on pp4 #dcg1 #dch4 ,the thicker bands are well illustrated by that which lies some way above the ,houston ,coal1 between ,drumcross and ,west ,broadlaw in ,linlithgowshire1 and by the great mass of tuff which occurs immediately below the ,calmy ,limestone on the ,river ,avon near ,linlithgow ,bridge1 and which may be #cjj feet thick4 ,it is a striking characteristic of the tuffs that they may be met with in their solitary beds intercalated in the midst of ordinary sediments at a distance from any other trace of volcanic activity1 their parent vents not being visible4 ,i may cite in illustration an interesting case in the ,swear ,burn1 near the southern end of the volcanic district4 ,a band of tuff about ten feet thick lies there intercalated in a group of dark shales and thin coals4 ,below it there is a seam of coal four inches thick1 and among the blue shales overlying it is another coal ten inches thick4 ,the tuff is pale green1 almost white in colour1 fine in texture1 like a volcanic mud1 while some of its component beds1 one foot in thickness1 are made up of fine lamin and are even false-bedded4 ,we might infer that the volcanic vent lay at some distance1 so that only the finest dust fell over the swamps in which the coal-vegetation was accumulating1 but for the presence of occasional blocks of basalt one foot in diameter scattered through the tuff4 ,when the eruptions ceased1 the deposition of ordinary mud and the accumulation of plant-remains went on as before1 and animal life crowded in again over the spot1 for between the partings of the coal above the tuff1 abundant fragments of eurypterids and scorpions may be found4 ,one of the most extensive volcanic discharges of fragmentary material was that which produced the ,houston marls already referred to4 ,these strata appear to mark a peculiar phase in the volcanic history of the ,lower ,carboniferous rocks of the ,firth of ,forth1 when exceedingly fine ash1 or perhaps even volcanic mud1 was erupted in considerable quantity4 ,the marls attain in some places a thickness of nearly #bjj feet1 and can be traced through most of the eastern part of ,linlithgowshire1 over an area of perhaps more than #ej square miles4 ,this volcanic platform1 which has been followed in mining for oil-shale1 is one of the most extensive among the puy-eruptions4 ,the material1 which probably came from one or more vents among the ,bathgate ,hills1 is not always of equal fineness1 but passes into and even alternates with ordinary granular tuff4 ,thus in the ,niddry ,burn1 above ,ecclesmachan1 the dull sage-green and brownish red ,houston marls contain a few inconstant layers of green tuff1 in which may be noticed pieces of black shale and lapilli of the usual basic pumice4 ,not far to the west1 beyond ,wester ,ochiltree1 and thus probably nearer to the active vents that ejected the dust and ashes1 the ,houston marls are replaced by or include a bedded granular tuff or basalt-agglomerate1 which lies above the #b-;feet coal of the district4 ,the matrix of this rock is in part a dull green granular mudstone1 wrapping round the lapilli and ejected stones1 which1 when they fall out under the action of the weather1 leave casts of their forms behind them4 ,the enclosed fragments vary in size up to blocks three feet in diameter1 and consist in great measure of a compact volcanic grit1 composed of a fine mud mixed with minute fragments of black shale1 grains of sand and flakes of mica4 ,there are likewise blocks of cement-stone and shale4 ,thin courses of black shale interlaminated with the tuff show its bedding4 ,the thickest and most continuous accumulations of tuff occur round some of the larger tuff cones1 particularly round the ,saline ,hills1 and in the ,burntisland district4 ,in the first-named area the copious eruptions of fragmentary material brought the volcanic history there to an end2 but around ,burntisland they were only the prelude to a prolonged and varied series of discharges4 ,i have already remarked that in the area of the puys of the ,forth-basin1 while the majority of the vents were tuff-cones1 and emitted only fragmentary discharges1 there were two well-marked tracts where lavas were poured out extensively and during a long geological interval4 ,one of those lies in the southern1 the other in the northern area4 ,the southern or ,linlithgowshire lava-ridge forms now what are known as the ,bathgate and ,linlithgow ,hills4 ,the lavas extend for about #ad miles from north to south1 dying out in both directions1 while their present visible breadth is about three miles at its widest part4 ,the highest summit reaches a height of about #ajjj feet above the sea4 ,the structure of this long ridge reveals an interesting record of volcanic eruptions4 ,it consists mainly of sheets of basalt1 sometimes separated by layers of tuff 7,fig4 #aee74 ,but on one or two horizons the volcanic rocks cease1 and ordinary sedimentary deposits take their place4 ,as has been already stated1 the ,main or ,hurlet ,limestone can be traced through the heart of the volcanic masses4 ,this seam attains there an exceptional thickness of as much as #gj to #hj feet1 and is nowhere more abundantly fossiliferous4 ,during its deposition there seems to have been a subsidence of the area1 together with a cessation of volcanic activity for a time4 ,the crinoids1 corals1 brachiopods1 bryozoa1 lamellibranchs1 gasteropods1 cephalopods and fishes1 which swarmed in the clear water1 built up a thick calcareous layer above the lavas and tuffs of the sea-bottom4 ,among the sandstones and shales that cover the limestone1 bands of tuff make their appearance1 indicating the renewal of volcanic activity4 ,these are immediately surmounted by another thick pile of basalt-sheets4 ,subsequently1 during pauses in the eruptions1 while the general subsidence continued1 renewed deposits of sediment spread over the submerged volcanic bank4 ,one of the longest periods of quiescence was that during which the coals and even the ,index ,limestone of ,bathgate crept northwards over the sunken lavas and tuffs4 ,but the whole of the ridge does not seem to have disappeared at that time under water1 at least these intercalated strata have not been traced across the thick pile of volcanic material near ,linlithgow4 ,during the final period of eruption1 the outpouring of lava and discharge of ashes1 neither in united thickness nor in horizontal extent1 equalled those which had preceded them4 ,when the volcanoes ceased their activity1 the area continued to sink1 and over the submerged lavas marine organisms crowded the sea-floor1 so as to build up the ,calmy ,limestone4 ,after that time volcanic action seems to have become extinct in this region1 for no trace of any intercalated lava or tuff has been detected either in the overlying ,millstone ,grit or in the ,coal-measures4 ,the total thickness of rock in the ,linlithgowshire volcanic ridge is about #bbjj feet4 ,it will probably not be an exaggeration to place the proportion of lava and tuff in that depth of material at nearly #bjjj feet4 ,the northern or ,fifeshire district over which lavas were abundantly erupted stretches along the coast from ,aberdour to ,kirkcaldy and inland to near ,lochgelly1 as well as seawards to ,inchkeith4 ,it may comprise an area of about #cj square miles4 ,in many respects this is the most important locality in ,britain for the study of ,carboniferous volcanic history4 ,the sea has cut an admirable coast-section in which the structures of the rocks are laid bare4 ,the bottom and top of the whole volcanic series can be seen4 ,the vents and their relation to the lavas and tuffs that were emitted from them may easily be made out2 while the interstratification of well-known seams of rock in the ,scottish ,carboniferous system permits the sequence and chronology of the whole volcanic series to be traced with great clearness4 ,most of these features have already been described in foregoing pages1 for the district is a typical one for the study of ,carboniferous volcanic phenomena4 ,thus the group of vents about ,burntisland has been illustrated by the ,binn of ,burntisland rising among the bedded lavas and tuffs4 ,the characters of the ,carboniferous basalt-sheets have been enumerated1 together with their intercalated layers of tuff and bole1 and their fine partings of ashy material that was thrown out over the lagoons during the intervals between two outbursts of lava4 ,but it may be of service if ,i insert here a detailed section of the whole volcanic series as it is displayed along the coast-section between ,burntisland and ,kinghorn4 ,the lowest intercalated lavas of that section lie a little above the horizon of the ,burdiehouse ,limestone1 and are thus probably rather earlier than those of ,linlithgowshire4 ,the highest reach up to the base of the ,hurlet ,limestone4 ,the volcanic energy manifestly died out here long before it ceased on the south side of the ,firth4 ,yet so vigorous was its activity while it continued1 that it piled up one of the thickest masses of volcanic material anywhere to be seen among the puy-eruptions of the ,british ,isles4 ,the following tabular statement of the alternations of material in this great mass in descending order1 was drawn up by me on the ground many years ago1 before the construction of fortifications and other changes partly concealed the rocks4 ,illustration3 ,fig4 #aga4--;,columnar basalt1 ,pettycur1 ,kinghorn1 ,fife4 7,from a photograph taken for the ,geological ,survey by ,mr4 ,r4 ,lunn47 ,section of the ,volcanic ,series below the ,hurlet or ,main ,limestone on the ,coast of ,fife1 west of ,kinghorn1 in descending order#dgg ,footnote #dgg3 ,the succession of rocks in this interesting coast-section was briefly given by ,dr4 ,p4 ,neill in his translation of ,daubuisson's ,basalts of ,saxony1 ,edinburgh1 #ahad1 note f1 p4 #bae4 ,he was secretary of the ,wernerian ,society1 and in his enumeration the ,wernerian terminology is used without a hint that any single band in the whole series is of volcanic origin4 #ge4 ,reddish and white sandstones4 #gd4 ,shale with hard ribs of limestone and ironstone nodules4 ,fossils abundant4 #gc4 ,limestone1 crinoidal1 #h or #i feet4 #gb4 ,blue shale1 becoming calcareous towards the top1 where shells are plentiful4 #ga4 ,reddish false-bedded sandstones1 with bands of reddish and blue shale4 #gj4 ,basalt in two sills separated by #b or #c feet of sandstone and shale4 #fi4 ,dark fissile sandy shale1 passing up into white shaly sandstone1 and including a thin parting of impure coal4 #fh4 ,limestone 7,hurlet or ,main ,seam7 in a number of bands having a united thickness of #be feet4 ,abundant fossils4 #fg4 ,black shale becoming calcareous at top1 and then enclosing abundant ,productus1 etc41 #h or #aj feet4 #ff4 ,red and green tufaceous marl and tuff4 ,about #cj feet4 #fe4 ,basalt1 the lower part strongly amygdaloidal4 #fd4 ,tufaceous red marl and tuff2 comparatively coarse below1 becoming finer above1 #c or #d feet4 #fc4 ,basalt1 earthy and amygdaloidal1 with an irregular bottom involving masses of the shales below4 #fb4 ,dark calcareous shale and dull green tufaceous marly shale1 #b or #c feet4 #fa4 ,crinoidal limestone in several bands with a united thickness of #aj feet4 #fj4 ,shale1 #a foot4 #ei4 ,fine green sandy tuffs in a number of bands of varying coarseness1 about #f feet4 #eh4 ,dark shale with abundance of ,aviculopecten immediately under the tuffs above1 #a feet4 #eg4 ,soft1 light1 marly shale with fragmentary plants1 #a feet4 #ef4 ,dark fissile shale1 full of fish-scales1 plants1 etc41 #c feet4 #ee4 ,basalt1 rudely columnar1 dark fine-grained in centre1 becoming highly amygdaloidal and scoriaceous at bottom and top4 #ed4 ,basalt1 like the sheet above1 vesicular at top and bottom1 with a parting of red clay on top4 #ec4 ,fissile rippled sandy shale1 with plants1 having a red and green marly parting at the top1 #ab or #ad feet4 #eb4 ,basalts2 a group of beds1 probably in part sills1 involving three bands of sandstone or quartzite4 #ea4 ,quartzite--a hard white altered sandstone1 #b to #c feet4 #ej4 ,basalt1 light green1 earthy1 amygdaloidal4 #di4 ,sandstones and shales with plants1 #be feet4 #dh4 ,basalt1 with a highly amygdaloidal central band4 ,there may be several sheets here4 #dg4 ,green tufaceous shale and marl1 #a foot4 #df4 ,basalt1 dark1 firm and amygdaloidal4 #de4 ,sandstones and shales with plants4 #dd4 ,basalt forming west side of ,kinghorn ,bay1 and including more than one sheet4 ,the rock is very black1 compact1 irregularly columnar1 with the usual amygdaloidal earthy band at the base1 and forms the crag called the ,carlinehead ,rocks4 ,an irregular and inconstant band of dull green tufaceous shale1 sometimes #b feet thick1 serves to separate two of the basalt-sheets4 ,below it lies a remarkable scoriaceous almost brecciated basalt1 which has been broken up on cooling in such a manner that at first it might be mistaken for a volcanic conglomerate4 #dc4 ,basalt1 a compact black solid rock1 with a vesicular and amygdaloidal bottom1 about #dj feet4 ,this sheet runs out into the promontory of ,kinghorn ,ness4 #db4 ,basalt1 firm1 compact and highly amygdaloidal throughout1 #ae feet4 #da4 ,basalt1 earthy1 amygdaloidal and scoriaceous in the upper part1 compact below4 #dj4 ,red tufaceous marl1 clay or bole1 a few inches thick4 #ci4 ,basalt3 one of the most compact sheets of the whole series1 about #dj feet4 ,the top is formed of a thick zone of scoriaceous and brecciated material1 the bottom is singularly uneven owing to the very irregular surface of the underlying bed4 #ch4 ,basalt more or less scoriaceous throughout1 especially at the bottom1 the vesicles being drawn out round the slag-like blocks4 #cg4 ,green tufaceous shales with bands of fine green tuff1 #g to #h feet4 ,the lower bands consist of a gravelly tuff passing up into a fine volcanic mudstone1 with scattered lapilli of basalt an inch or more in diameter4 #cf4 ,basalt1 with an upper1 earthy and highly amygdaloidal portion1 #cj feet4 #ce4 ,tufaceous sandstone and shale1 #f to #h feet4 #cd4 ,basalt1 in a thick bed1 having an earthy1 slaggy top and a scoriaceous bottom4 #cc4 ,basalt1 very slaggy below with a compact centre4 #cb4 ,basalt1 like that below it4 #ca4 ,basalt1 firm1 compact1 black rock1 with a rough1 green earthy band1 from #f inches to a foot1 at the bottom1 and becoming again very slaggy at the top4 #cj4 ,green shale like that below the underlying limestone1 a few inches in thickness4 #bi4 ,coarse1 green1 sandy tufaceous limestone1 averaging #a foot in thickness4 #bh4 ,black shale with plants1 #ab or #ad feet1 becoming green and tufaceous at the top4 #bg4 ,basalt--the most striking of the whole section--a fine compact black olivine-bearing rock1 beautifully columnar1 #cj feet4 ,the columns reach to within a foot of the bottom of the bed and cease about #aj feet from the top1 the upper portion of the bed being massive1 with vesicles which are drawn out parallel to the bedding of the series4 ,the lowest part of the bed is a broken brecciated band1 #c or #d inches thick4 7,see ,fig4 #aga47 #bf4 ,black shale with fragmentary plants1 #c feet4 #be4 ,basalt1 with plentiful olivine1 #ab to #af feet4 ,the base is not highly scoriaceous1 but finely vesicular4 ,towards the top it becomes green1 earthy and roughly brecciated4 ,it partly cuts out the tuff underneath4 #bd4 ,tuff1 green1 fine-grained and well-stratified1 consisting chiefly of fine volcanic dust1 but becoming coarser towards the top1 where it contains lapilli and occasional bombs of highly vesicular lavas4 #bc4 ,black carbonaceous shale1 #c feet2 approaching to the character of an impure coal in the lower part1 and becoming more argillaceous above with some thin nodular calcareous bands4 #bb4 ,green tuff1 #ab feet1 well stratified and fine-grained1 with minute lapilli of highly vesicular basic lavas2 becomes shaly at the bottom4 #ba4 ,basalt1 compact1 amygdaloidal1 with highly vesicular upper surface1 #bj feet4 #bj4 ,basalt1 hard1 black and full of olivine2 an irregular bed #c to #f feet thick4 #ai4 ,basalt1 dull brownish-green to black1 full of kernels and strings of calcite1 and showing harder and softer bands parallel with upper and under surfaces1 which give it a stratified appearance4 #ah4 ,basalt1 some parts irregularly compact1 others earthy and scoriaceous4 ,the distinguishing feature of this bed is the abundance of its enclosed fragments of shale1 ironstone and limestone1 which here and there form half of its bulk4 ,the roughly scoriaceous upper portion is especially full of these fragments4 ,in the ironstone balls coprolites may be detected1 and occasional pieces of plant-stems are embedded in the basalt4 ,this lava has evidently broken up and involved some of the underlying strata over which it flowed4 ,this rock overhangs ,pettycur ,harbour4 #ag4 ,shales and limestone bands more or less tufaceous1 #h to #aj feet1 with plants1 cyprids1 etc4 ,the intercalation of fine partings of tuff in this band has been already cited on p4 #dch1 as an illustration of the feeble intermittent character of many of the volcanic explosions between successive outflowings of lava4 ,owing to a change in the direction of strike the rocks now wheel round and for a time run nearly parallel with the coast-line1 while they are partly concealed by blown sand and herbage4 ,the shales and limestones just mentioned are not constant1 and are soon lost1 but about a quarter of a mile westward a band of tuff begins on the same horizon or near it1 and increases in thickness towards the west1 where it is associated with other sediments4 ,the shore ceases to furnish a continuous section1 so that recourse must be had to the craggy slopes immediately to the north1 where the rocks can be examined on a cliff face 7,fig4 #aec74 ,there the tuff just referred to1 together with some overlying bands of sandstone1 is seen to pass under the group of basalts last enumerated4 ,it is a green1 stratified rock1 perhaps #fj feet thick at its maximum1 but dying out rapidly to north-west and south-east4 ,it encloses balls of basalt and subangular and rounded fragments of sandstone1 limestone and shale4 ,a mass of coarse volcanic agglomerate which is connected with it and cuts across the ends of some of the basalts below1 probably marks the position of the vent from which the tuff was ejected 7,fig4 #aeb74 #af4 ,black and grey shales forming a thin band at the summit of ,king ,alexander's ,crag4 #ae4 ,basalt1 dark compact rock1 with an upper and lower highly scoriaceous and amygdaloidal band1 #ae feet4 #ad4 ,black shales1 tufaceous green shales1 sandstone1 and #f inches of coal1 forming a group of strata about #ab feet thick between two basalts2 plants and cyprids abundant4 7,the coal seam is shown in ,fig4 #aea47 #ac4 ,basalt1 dull1 earthy and highly amygdaloidal1 with abundant calcite in kernels and veins2 about #ae feet1 but varying in thickness4 #ab4 ,basalt1 forming a well-marked bed from #ab to #be feet thick4 ,it is a compact black olivine-bearing rock1 sparingly amygdaloidal1 but showing the usual dull green1 earthy scoriform base4 ,the upper surface is singularly irregular1 having1 in flowing1 broken up into large clinker-like blocks1 which are involved in the immediately overlying basalt4 ,the bottom also is very uneven1 for the basalt has in some places cut out the underlying shales1 so as to rest directly upon the basalt below4 #aa4 ,black shale1 varying up to #f inches1 but sometimes entirely removed by the overlying lava-stream4 #aj4 ,basalt1 containing large irregularly spheroidal masses of hard black finely vesicular material enclosed in more earthy and coarsely vesicular rock4 ,the vesicles are sometimes elongated parallel to the bedding1 but have often been drawn out round a spheroid2 some of them measure nearly a foot in length by #b or #c inches in breadth4 ,the upper surface is uneven and coarsely amygdaloidal4 #i4 ,basalt1 hard black1 with abundant olivine1 and a columnar structure4 #h4 ,green shale1 #f inches to #a foot1 much baked and involved in the overlying basalt4 #g4 ,basalt1 dull-green1 earthy1 amygdaloidal1 varying from #aj to #dj feet in thickness4 #f4 ,blue shale1 disappearing where the basalt above it unites with that below4 #e4 ,basalt with olivine1 forming a thick irregular bed1 which in some places is black and compact1 in others green1 earthy and amygdaloidal4 ,the upper part is particularly cellular4 #d4 ,sandstones forming a thick group of beds which have long been quarried for building-stone at the ,grange and elsewhere4 #c4 ,black shales4 #b4 ,limestone 7,burdiehouse74 #a4 ,sandstones1 shales and thin limestones forming the strata at ,burntisland through which the sills of that district have been injected 7,fig4 #aei74 ,the phenomena of sills are abundantly developed among the ,carboniferous rocks of the basin of the ,firth of ,forth1 and some of the more remarkable examples in this district have been already cited4 ,taking now a general survey of this part of the volcanic history1 ,i may observe that if the sills are for a moment considered simply as they appear at the surface1 and apart from the geological horizons on which they lie1 they form a wide ring surrounding the ,falkirk and ,stirlingshire coal-field4 ,beginning at the ,abbey ,craig1 near ,stirling1 we may trace this ring as a continuous belt of high ground from ,stirling to the ,river ,carron4 ,thence it splits up into minor masses in different portions of the ,carboniferous system1 and doubtless belonging to different periods of volcanic disturbance1 but yet sweeping as a whole across the north-eastern part of the ,clyde coal-field1 and then circling round into ,stirlingshire and ,linlithgowshire4 ,there are no visible masses to fill up the portion of the ring back to ,abbey ,craig4 ,but through ,linlithgowshire and the west of ,edinburghshire a number of intrusive sheets form an eastward prolongation of the ring4 ,large as some of these sheets are at the surface1 for they sometimes exceed two or three square miles in area1 a much larger portion of their mass is generally concealed below ground4 ,mining operations1 for example1 have proved that in the south-east of ,linlithgowshire areas of intrusive rock which appear as detached bosses or bands at the surface are connected underneath as portions of one continuous sill1 which must be several square miles in extent4 ,illustration3 ,fig4 #agb4--,section across the ,fife band of ,sills4 #a4 ,upper ,old ,red ,sandstone2 #b4 ,calciferous ,sandstones2 #c4 ,carboniferous ,limestone series2 #d4 ,millstone ,grit2 #e4 ,coal-measures2 #f4 ,dolerite ,sills4 f1 ,fault4 ,but it is in ,fife that the sills reach their greatest development among the ,carboniferous rocks of ,scotland 7,fig4 #agb74 ,a nearly continuous belt of them runs from the ,cult ,hill near ,saline on the west1 to near ,st4 ,andrews on the east1 a distance of about #ce miles4 ,this remarkable band is connected with a less extensive one1 which extends from ,torryburn on the west1 to near ,kirkcaldy on the east4 ,in two districts of the ,fife region of sills1 a connection seems to be traceable between the intrusive sheets and volcanic vents1 at least groups of necks are found in the midst of the sills4 ,one of these districts is that of the ,saline ,hills already described1 the other is that of ,burntisland4 ,in the latter case the evidence is especially striking1 for the vents are connected above with bedded lavas and tuffs1 while below lie three well-marked sills 7,fig4 #aei74 ,it is certainly worthy of remark that sills are generally absent from those areas where no traces of contemporaneous volcanic activity are to be found4 ,no contrast in this respect can be stronger than that between the ground to the east and west of the old axis of the ,pentland ,hills4 ,in the western district1 where the puys are so well displayed1 sills abound1 but in the eastern tract both disappear4 ,another question of importance in dealing with the history of these sills is their stratigraphical position4 ,by far the larger proportion of them lies in the ,carboniferous ,limestone series4 ,thus the great sill between ,stirling and ,kilsyth keeps among the lower parts of that series4 ,on the same general horizon are the vast sheets of dolerite which stretch through ,fife in the chain of the ,cult1 ,cleish1 and ,lomond ,hills on the one side1 and in the eminences from ,torryburn to ,kinghorn on the other1 though the intrusive material sometimes descends almost to the ,old ,red ,sandstone4 ,in ,linlithgowshire and ,edinburghshire1 as well as in the south of ,fife1 the sills traverse the ,calciferous ,sandstone groups4 ,if the horizons of the sills furnished any reliable clue to their age1 it might be inferred that the rocks were all intruded during the ,carboniferous period1 and as most of them lie beneath the upper stratigraphical limit of the puy-eruptions1 the further deduction might be drawn that they are connected with these eruptions4 ,i have little doubt that in a general sense both conclusions are well-founded4 ,but that there are exceptions to the generalization must be frankly conceded4 ,on close examination it will be observed that the same intrusive mass sometimes extends from the lower into the upper parts of the ,carboniferous groups4 ,thus1 in the west of ,linlithgowshire1 a large protrusion which lies upon the ,upper ,limestones1 crosses most of the ,millstone ,grit1 and reaches up almost as high as the ,coal-measures4 ,again1 in ,fife1 to the east of ,loch ,leven1 a spur of the great ,lomond sill1 crossing the ,carboniferous limestone1 advances southward into the coal-field of ,kinglassie1 ,in ,stirlingshire and ,lanarkshire numerous large dolerite sheets have invaded the ,millstone ,grit and ,coal-measures1 including even the upper red sandstones1 which form the top of the ,carboniferous system in this region4 ,it is thus obvious that if the puy-eruptions in the basin of the ,forth ceased towards the close of the deposition of the ,carboniferous ,limestone series1 there must have been a subsequent injection of basic lava on a gigantic scale in central ,scotland4 ,i shall recur to this subject in ,chapter xxxi4 ,illustration3 ,fig4 #agc4--,section across the ,upper ,volcanic ,band of north ,ayrshire4 ,length about four miles4 #a4 ,andesite lavas of the ,clyde ,plateau2 #b4 ,tuffs closing the ,plateau volcanic series2 #c4 ,hurlet ,limestone2 #d4 ,carboniferous ,limestone series with coal-seams2 #e4 ,lower tuff zone of the ,upper volcanic band2 #f4 ,basic lavas2 #g4 ,upper tuff zone2 #h4 ,basic sill2 #i4 ,coal-measures4 #b4 ,,north ,,of ,,ayrshire ,in this part of the country another group of puys and their associated tuffs and lavas may be traced from near ,dairy on the west1 to near ,galston on the east 7,map ,,iv474 ,the length of the tract is about sixteen miles1 while its breadth varies from about a furlong to nearly a mile and a half4 ,i have had occasion to allude to this marked band of volcanic materials which here intervenes between the ,carboniferous ,limestone and the ,coal-measures1 and from its position appears to mark the latest ,carboniferous volcanoes4 ,its component rocks reach a thickness of sometimes #fjj feet1 and as they dip southwards under the ,coal-measures1 they may extend for some distance in that direction4 ,they have been met with in borings sunk through the northern part of the ,irvine coal-field4 ,even what of them can be seen at the surface1 in spite of the effects of faults and denudation1 shows that they form one of the most persistent platforms of volcanic rock among the puy-eruptions of ,scotland4 ,where best developed this volcanic band has a zone of tuff at the bottom1 a central and much thicker zone of bedded basalts1 and an upper group of tuffs1 on which the ,coal-measures rest conformably4 ,a few vents1 probably connected with it1 are to be seen at the surface between ,fenwick and ,ardrossan4 ,but others have been buried under the ,carboniferous sedimentary rocks1 and1 as already described1 have been discovered in the underground workings for coal and ironstone 7p4 #dcd74 ,these mining operations have1 indeed1 revealed the presence of far more volcanic material below ground than would be surmized from what can be seen at the surface4 ,here and there1 thin layers of tuff appear in brook-sections1 indicating what might be conjectured to have been trifling discharges of volcanic material4 ,but the prosecution of the ironstone-mining has proved that1 at the time when the seam of ,black-band ,ironstone of that district was accumulated1 the floor of the shallow sea or lagoon where this deposition took place was dotted over with cones of tuff1 in the hollows between which the ferruginous and other sediments gathered into layers4 ,that seam is in one place thick and of good quality2 yet only a short distance off it is found to be so mixed with fine tuff as to be worthless1 and even to die out altogether4#dgh ,footnote #dgh3 ,see ,explanation of ,sheet #bb1 ,geol4 ,surv4 of ,scotland1 pars4 #bi1 #cc1 #de4 #c4 ,,liddesdale ,a remarkable development of puys lies in that little-visited tract of country which stretches from the valleys of the ,teviot and ,rule ,water south-westwards across the high moorland watershed1 and down ,liddesdale4 ,through this district a zone of bedded olivine-basalts and associated tuffs runs in a broken band which1 owing to numerous faults and extensive denudation1 covers now only a few scattered patches of the site over which it once spread4 ,the geological horizon of this zone lies in the ,calciferous ,sandstones1 many hundred feet above the position of the top of the plateau-lavas 7,map ,,iv474 ,so great an amount of material has been here removed by denudation that not only has the volcanic zone been bared away1 but the vents which supplied its materials have been revealed in the most remarkable manner over an area some twenty miles long and eight miles broad4 ,upwards of forty necks of agglomerate may be seen in this district1 rising through the ,silurian1 ,old ,red ,sandstone1 and lowest ,carboniferous rocks4 ,it fills the geologist with wonder to meet with those stumps of old volcanoes far to the west among the ,silurian lowlands1 sometimes fully ten miles away from the nearest relic of the bedded lavas connected with them4#dgi ,that these vents1 though they rose through ground which at the time of their activity was covered with the volcanic series of the plateaux1 do not belong to that series1 but are of younger date1 has been proved in several cases by ,mr4 ,peach4 ,he has found that among the blocks composing their agglomerates1 pieces of the sandstones1 fossiliferous limestones and shales of the ,cement-stone group1 overlying the plateau-lavas1 are to be recognized4 ,these vents were therefore drilled through some part at least of the ,calciferous ,sandstones1 which are thus shown to have extended across the tract dotted with vents4 ,after the volcanic activity ceased1 fragments of these strata tumbled down from the sides into the funnels4 ,denudation has since stripped off the ,calciferous ,sandstones1 but the pieces detached from them1 and sealed up at a lower level in the agglomerates1 still remain4 ,mr4 ,peach's observations indicate to how considerable an extent sagging of the walls of these orifices took place1 with the precipitation not merely of blocks1 but of enormous masses of rock1 into the volcanic chimneys4 ,in one instance1 between ,tudhope ,hill and ,anton ,heights1 a long neck1 or perhaps group of necks1 which crosses the watershed1 shows a mass of the red sandstone many acres in extent1 and large enough to be inserted on the one-inch map1 which has fallen into the vent 7,fig4 #age74 ,footnote #dgi3 ,they have been recognized and mapped by ,mr4 ,b4 ,n4 ,peach for the ,geological ,survey4 ,see ,sheets #aa and #ag1 ,geol4 ,surv4 ,scotland4 ,illustration3 ,fig4 #agd4--,section showing the connection of the two volcanic bands in ,liddesdale4 #a4 ,upper ,silurian strata2 #b4 ,upper ,old ,red ,sandstone2 #c4 ,the lavas of the ,solway plateau2 #d4 ,agglomerate neck with lava plug1 belonging to the plateau system2 #e4 ,calciferous ,sandstone series2 #f4 ,thick ,carboniferous ,limestones2 #g4 ,tuff1 and #h4 ,lavas1 of the upper volcanic band1 connected with the puys2 #i4 ,agglomerate neck with lava plug belonging to the puy-system2 #aj4 ,basic sill4 ,illustration3 ,fig4 #age4--;,diagram to show the position of a mass of ,upper ,old ,red ,sandstone which has fallen into the great vent near ,tudhope ,hill1 east of ,mosspaul4 #a4 ,upper ,silurian strata2 #b4 ,outlier of ,upper ,old ,red ,sandstone2 #b4 ,large mass of this formation in the vent2 #c4 ,agglomerate of the neck with andesite intrusion 7#d74 ,the materials ejected from the ,liddesdale vents include both basaltic lavas and tuffs4 ,the former are sometimes highly vesicular1 especially along the upper parts of the flows4 ,they are thickest towards the north1 and in ,windburgh ,hill attain a depth of at least #cjj or #djj feet4 ,in that part of the district they form the lower and main part of the volcanic series1 being there covered by a group of tuffs4 ,but a few miles southwards1 not far to the west of ,kershopefoot1 they die out4 ,the tuffs then form the whole of the volcanic band which1 intercalated in a well-marked group of limestones1 can be followed across the moors for some six miles into the valley of the ,esk1 where an interesting section of them and of the associated limestone and shales is exposed 7,fig4 #agd74 ,at ,kershopefoot1 a higher band of basic lava overlies the ,kershopefoot limestone1 and can be traced in scattered patches both on the ,scottish and ,english side of the ,border4 ,,end ,,of ,,vol4 ,i4 ,printed by ,r4 ,r4 ,clark1 ,limited1 ,edinburgh4 ,illustration3 ,,to ,,accompany ,,sir ,,archibald ,,geikie',s ,,ancient ,,volcanoes ,,of ,,britain ,map ,,iv4 ,,map ,,of ,,the ,,carboniferous ,,volcanoes ,,of ,,scotland ,english ,miles ,,explanation ,,of ,,colouring ,basalts and thin ,tuffs ,puy ,series ,thicker ,sheets of ,tuff ,lavas and thin ,tuffs ,plateau ,series ,thicker ,sheets of ,tuff ,vents filled with ,agglomerate or ,tuff ,basic ,sills and ,bosses ,intermediate and acid ,sills and ,bosses ,the ,edinburgh ,geographical ,institute ,copyright ,j4 ,g4 ,bartholomew4 ,illustration3 ,,map #a ,,map ,,of ,,the ,,volcanic ,,districts ,,of ,,the ,,british ,,isles ,,by ,sir ,,archibald ,,geikie1 ,d4,c4,l41 ,f4,r4,s4 ,,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 ,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,s41 ,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 ,right ,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 #hs4 #f;d4 ,,outlines ,,of ,,physiography--,,the ,,movements ,,of ,,the ,,earth4 ,by ,j4 ,norman ,lockyer1 ,f4,r4,s4 ,illustrated4 ,crown #hvo4 ,sewed1 #as4 #f;d4 ,,physiography ,,for ,,beginners4 ,by ,a4 ,t4 ,simmons1 ,b4,sc4 ,globe #hvo4 #bs4 #f;d4 ,,macmillan ,,and ,,co4 ,ltd41 ,,london4 99 99 99 99 99 ,transcriber ,note ,minor typos corrected4 ,several tables were rotated so they will fit in the limits of the text format4 ,text rearranged to avoid split paragraphs at full page images4