,geological ,observations on the ,volcanic ,islands by ,charles ,darwin ,contents ,,editorial ,,note ,,detailed ,,table ,,of ,,contents ,,critical ,,introduction ,,chapter ,i4--,,st4 ,,jago1 ,,in ,,the ,,cape ,,de ,,verde ,,archipelago ,,chapter ,,ii4--,,fernando ,,noronha2 ,,terceira2 ,,tahiti1 ,,etc ,,chapter ,,iii4--,,ascension ,,chapter ,,iv4--,,st4 ,,helena ,,chapter ,v4--,,galapagos ,,archipelago ,,chapter ,,vi4--,,trachyte ,,and ,,basalt4--,,distribution ,,of ,,volcanic ,,isles ,,chapter ,,vii4--,,australia2 ,,new ,,zealand2 ,,cape ,,of ,,good ,,hope ,,index ,,to ,,volcanic ,,islands ,,editorial ,,note ,although in some respects more technical in their subjects and style than ,darwin's 8,journal10 the books here reprinted will never lose their value and interest for the originality of the observations they contain4 ,many parts of them are admirably adapted for giving an insight into problems regarding the structure and changes of the earth's surface1 and in fact they form a charming introduction to physical geology and physiography in their application to special domains4 ,the books themselves cannot be obtained for many times the price of the present volume1 and both the general reader1 who desires to know more of ,darwin's work1 and the student of geology1 who naturally wishes to know how a master mind reasoned on most important geological subjects1 will be glad of the opportunity of possessing them in a convenient and cheap form4 ,the three introductions1 which my friend ,professor ,judd has kindly furnished1 give critical and historical information which makes this edition of special value4 ,g4,t4,b4 ,,volcanic ,,islands4 ,,detailed ,,table ,,of ,,contents ,i4--,,st4 ,,jago1 ,,in ,,the ,,cape ,,de ,,verde ,,archipelago4 ,rocks of the lowest series4--,a calcareous sedimentary deposit1 with recent shells1 altered by the contact of superincumbent lava1 its horizontality and extent4--,subsequent volcanic eruptions1 associated with calcareous matter in an earthy and fibrous form1 and often enclosed within the separate cells of the scoriae4--,ancient and obliterated orifices of eruption of small size4--,difficulty of tracing over a bare plain recent streams of lava4--,inland hills of more ancient volcanic rock4--,decomposed olivine in large masses4--,feldspathic rocks beneath the upper crystalline basaltic strata4--,uniform structure and form of the more ancient volcanic hills4--,form of the valleys near the coast4--,conglomerate now forming on the sea beach4 ,,ii4--,,fernando ,,noronha2 ,,terceira2 ,,tahiti1 ,,etc4 ,,fernando ,,noronha4--,precipitous hill of phonolite4 ,,terceira4--,trachytic rocks3 their singular decomposition by steam of high temperature4 ,,tahiti4--,passage from wacke into trap2 singular volcanic rock with the vesicles half-filled with mesotype4 ,,mauritius4--,proofs of its recent elevation4--,structure of its more ancient mountains2 similarity with ,st4 ,jago4 ,,st4 ,,paul',s ,,rocks4--,not of volcanic origin4--,their singular mineralogical composition4 ,,iii4--,,ascension4 ,basaltic lavas4--,numerous craters truncated on the same side4--,singular structure of volcanic bombs4--,aeriform explosions4--,ejected granite fragments4--,trachytic rocks4--,singular veins4--,jasper1 its manner of formation4--,concretions in pumiceous tuff4--,calcareous deposits and frondescent incrustations on the coast4--,remarkable laminated beds1 alternating with1 and passing into obsidian4--,origin of obsidian4-- ,lamination of volcanic rocks4 ,,iv4--,,st4 ,,helena4 ,lavas of the feldspathic1 basaltic1 and submarine series4--,section of ,flagstaff ,hill and of the ,barn4--,dikes4--,turk's ,cap and ,prosperous ,bays4-- ,basaltic ring4--,central crateriform ridge1 with an internal ledge and a parapet4--,cones of phonolite4--,superficial beds of calcareous sandstone4-- ,extinct land-shells4--,beds of detritus4--,elevation of the land4-- ,denudation4--,craters of elevation4 ,v4--,,galapagos ,,archipelago4 ,chatham ,island4--,craters composed of a peculiar kind of tuff4--,small basaltic craters1 with hollows at their bases4--,albemarle ,island2 fluid lavas1 their composition4--,craters of tuff2 inclination of their exterior diverging strata1 and structure of their interior converging strata4--,james ,island1 segment of a small basaltic crater2 fluidity and composition of its lava-streams1 and of its ejected fragments4--,concluding remarks on the craters of tuff1 and on the breached condition of their southern sides4-- ,mineralogical composition of the rocks of the archipelago4--,elevation of the land4--,direction of the fissures of eruption4 ,,vi4--,,trachyte ,,and ,,basalt4--,,distribution ,,of ,,volcanic ,,isles4 ,the sinking of crystals in fluid lava4--,specific gravity of the constituent parts of trachyte and of basalt1 and their consequent separation4-- ,obsidian4--,apparent non-separation of the elements of plutonic rocks4-- ,origin of trap-dikes in the plutonic series4--,distribution of volcanic islands2 their prevalence in the great oceans4--,they are generally arranged in lines4--,the central volcanoes of ,von ,buch doubtful4--,volcanic islands bordering continents4--,antiquity of volcanic islands1 and their elevation in mass4--,eruptions on parallel lines of fissure within the same geological period4 ,,vii4--,,australia2 ,,new ,,zealand2 ,,cape ,,of ,,good ,,hope4 ,new ,south ,wales4--,sandstone formation4--,embedded pseudo-fragments of shale4--,stratification4--,current-cleav age4--,great valleys4--,van ,diemen's ,land4--,palaeozoic formation4--,newer formation with volcanic rocks4-- ,travertin with leaves of extinct plants4--,elevation of the land4--,new ,zealand4--,king ,george's ,sound4--,superficial ferruginous beds4--,superficial calcareous deposits1 with casts of branches2 its origin from drifted particles of shells and corals4--,their extent4--,cape of ,good ,hope4-- ,junction of the granite and clay-slate4--,sandstone formation4 ,,index4 ,,geological ,,observations ,,on ,,volcanic ,,islands4 ,,critical ,,introduction4 ,the preparation of the series of works published under the general title 8,geology of the ,voyage of the #8,beagle'0 occupied a great part of ,darwin's time during the ten years that followed his return to ,england4 ,the second volume of the series1 entitled 8,geological ,observations on ,volcanic ,islands1 with ,brief ,notices on the ,geology of ,australia and the ,cape of ,good ,hope10 made its appearance in #ahdd4 ,the materials for this volume were collected in part during the outward voyage1 when the 8,beagle0 called at ,st4 ,jago in the ,cape de ,verde ,islands1 and ,st4 ,paul's ,rocks1 and at ,fernando ,noronha1 but mainly during the homeward cruise2 then it was that the ,galapagos ,islands were surveyed1 the ,low ,archipelago passed through1 and ,tahiti visited2 after making calls at the ,bay of ,islands1 in ,new ,zealand1 and also at ,sydney1 ,hobart ,town and ,king ,george's ,sound in ,australia1 the 8,beagle0 sailed across the ,indian ,ocean to the little group of the ,keeling or ,cocos ,islands1 which ,darwin has rendered famous by his observations1 and thence to ,mauritius2 calling at the ,cape of ,good ,hope on her way1 the ship then proceeded successively to ,st4 ,helena and ,ascension1 and revisited the ,cape de ,verde ,islands before finally reaching ,england4 ,although ,darwin was thus able to gratify his curiosity by visits to a great number of very interesting volcanic districts1 the voyage opened for him with a bitter disappointment4 ,he had been reading ,humboldt's 8,personal ,narrative0 during his last year's residence in ,cambridge1 and had copied out from it long passages about ,teneriffe4 ,he was actually making inquiries as to the best means of visiting that island1 when the offer was made to him to accompany ,captain ,fitzroy in the 8,beagle4 8 ,his friend ,henslow too1 on parting with him1 had given him the advice to procure and read the recently published first volume of the 8,principles of ,geology10 though he warned him against accepting the views advocated by its author4 ,during the time the 8,beagle0 was beating backwards and forwards when the voyage commenced1 ,darwin1 although hardly ever able to leave his berth1 was employing all the opportunities which the terrible sea-sickness left him1 in studying ,humboldt and ,lyell4 ,we may therefore form an idea of his feelings when1 on the ship reaching ,santa ,cruz1 and the ,peak of ,teneriffe making its appearance among the clouds1 they were suddenly informed that an outbreak of cholera would prevent any landing6 ,ample compensation for this disappointment was found1 however1 when the ship reached ,porta ,praya in ,st4 ,jago1 the largest of the ,cape de ,verde ,islands4 ,here he spent three most delightful weeks1 and really commenced his work as a geologist and naturalist4 ,writing to his father he says1 8,geologising in a volcanic country is most delightful2 besides the interest attached to itself1 it leads you into most beautiful and retired spots4 ,nobody but a person fond of ,natural ,history can imagine the pleasure of strolling under cocoa-nuts in a thicket of bananas and coffee-plants1 and an endless number of wild flowers4 ,and this island1 that has given me so much instruction and delight1 is reckoned the most uninteresting place that we perhaps shall touch at during our voyage4 ,it certainly is generally very barren1 but the valleys are more exquisitely beautiful1 from the very contrast4 ,it is utterly useless to say anything about the scenery2 it would be as profitable to explain to a blind man colours1 as to a person who has not been out of ,europe1 the total dissimilarity of a tropical view4 ,whenever ,i enjoy anything1 ,i always look forward to writing it down1 either in my log-book 7which increases in bulk71 or in a letter2 so you must excuse raptures1 and those raptures badly expressed4 ,i find my collections are increasing wonderfully1 and from ,rio ,i think ,i shall be obliged to send a cargo home40 ,the indelible impression made on ,darwin's mind by this first visit to a volcanic island1 is borne witness to by a remarkable passage in the 8,autobiography0 written by him in #ahgf4 8,the geology of ,st4 ,jago is very striking1 yet simple2 a stream of lava formerly flowed over the bed of the sea1 formed of triturated recent shells and corals1 which it has baked into a hard white rock4 ,since then the whole island has been upheaved4 ,but the line of white rock revealed to me a new and important fact1 namely that there had been afterwards subsidence round the craters which had since been in action1 and had poured forth lava4 ,it then first dawned on me that ,i might perhaps write a book on the geology of the various countries visited1 and this made me thrill with delight4 ,that was a memorable hour to me1 and how distinctly ,i can call to mind the low cliff of lava beneath which ,i rested1 with the sun glaring hot1 a few strange desert plants growing near and with living corals in the tidal pools at my feet40 ,only five years before1 when listening to poor ,professor ,jameson's lectures on the effete ,wernerianism1 which at that time did duty for geological teaching1 ,darwin had found them 8incredibly dull10 and he declared that 8the sole effect they produced on me was a determination never so long as ,i lived to read a book on ,geology1 or in any way to study the science40 ,what a contrast we find in the expressions which he makes use of in referring to ,geological ,science1 in his letters written home from the 8,beagle60 ,after alluding to the delight of collecting and studying marine animals1 he exclaims1 8,but ,geology carries the day60 ,writing to ,henslow he says1 8,i am quite charmed with ,geology1 but1 like the wise animal between two bundles of hay1 ,i do not know which to like best2 the old crystalline group of rocks1 or the softer and more fossiliferous beds40 ,and just as the long voyage is about to come to a close he again writes1 8,i find in ,geology a never-failing interest2 as it has been remarked1 it creates the same grand ideas respecting this world which ,astronomy does for the ,universe40 ,in this passage ,darwin doubtless refers to a remark of ,sir ,john ,herschel's in his admirable 8,preliminary ,discourse on the ,study of ,natural ,philosophy10--a book which exercised a most remarkable and beneficial influence on the mind of the young naturalist4 ,if there cannot be any doubt as to the strong predilection in ,darwin's mind for geological studies1 both during and after the memorable voyage1 there is equally little difficulty in perceiving the school of geological thought which1 in spite of the warnings of ,sedgwick and ,henslow1 had obtained complete ascendancy over his mind4 ,he writes in #ahgf3 8,the very first place which ,i examined1 namely ,st4 ,jago in the ,cape de ,verde ,islands1 showed me clearly the wonderful superiority of ,lyell's manner of treating ,geology1 compared with that of any other author1 whose works ,i had with me1 or ever afterwards read40 ,and again1 8,the science of ,geology is enormously indebted to ,lyell--more so1 as ,i believe1 than to any other man who ever lived444,i am proud to remember that the first place1 namely1 ,st4 ,jago1 in the ,cape de ,verde ,archipelago1 in which ,i geologised1 convinced me of the infinite superiority of ,lyell's views over those advocated in any other work known to me40 ,the passages ,i have cited will serve to show the spirit in which ,darwin entered upon his geological studies1 and the perusal of the following pages will furnish abundant proofs of the enthusiasm1 acumen1 and caution with which his researches were pursued4 ,large collections of rocks and minerals were made by ,darwin during his researches1 and sent home to ,cambridge1 to be kept under the care of his faithful friend ,henslow4 ,after visiting his relations and friends1 ,darwin's first care on his return to ,england was to unpack and examine these collections4 ,he accordingly1 at the end of #ahcf1 took lodgings for three months in ,fitzwilliam ,street1 ,cambridge1 so as to be near ,henslow2 and in studying and determining his geological specimens received much valuable aid from the eminent crystallographer and mineralogist1 ,professor ,william ,hallows ,miller4 ,the actual writing of the volume upon volcanic islands was not commenced till #ahdc1 when ,darwin had settled in the spot which became his home for the rest of his life--the famous house at ,down1 in ,kent4 ,writing to his friend ,mr4 ,fox1 on ,march #bhth1 #ahdc1 he says1 8,i am very slowly progressing with a volume1 or rather pamphlet1 on the volcanic islands which we visited3 ,i manage only a couple of hours per day1 and that not very regularly4 ,it is uphill work writing books1 which cost money in publishing1 and which are not read even by geologists40 ,the work occupied ,darwin during the whole of the year #ahdc1 and was issued in the spring of the following year1 the actual time engaged in preparing it being recorded in his diary as 8from the summer of #ahdb to ,january #ahdd20 but the author does not appear to have been by any means satisfied with the result when the book was finished4 ,he wrote to ,lyell1 8,you have pleased me much by saying that you intend looking through my #8,volcanic ,islands2' it cost me eighteen months666 and ,i have heard of very few who have read it4 ,now ,i shall feel1 whatever little 7and little it is7 there is confirmatory of old work1 or new1 will work its effect and not be lost40 ,to ,sir ,joseph ,hooker he wrote1 8,i have just finished a little volume on the volcanic islands which we visited4 ,i do not know how far you care for dry simple geology1 but ,i hope you will let me send you a copy40 ,every geologist knows how full of interest and suggestiveness is this book of ,darwin's on volcanic islands4 ,probably the scant satisfaction which its author seemed to find in it may be traced to the effect of a contrast which he felt between the memory of glowing delights he had experienced when1 hammer in hand1 he roamed over new and interesting scenes1 and the slow1 laborious1 and less congenial task of re-writing and arranging his notes in book-form4 ,in #ahgd1 in writing an account of the ancient volcanoes of the ,hebrides1 ,i had frequent occasion to quote ,mr4 ,darwin's observations on the ,atlantic volcanoes1 in illustration of the phenomena exhibited by the relics of still older volcanoes in our own islands4 ,darwin1 in writing to his old friend ,sir ,charles ,lyell upon the subject1 says1 8,i was not a little pleased to see my volcanic book quoted1 for ,i thought it was completely dead and forgotten40 ,two years later the original publishers of this book and of that on ,south ,america proposed to re-issue them4 ,darwin at first hesitated1 for he seemed to think there could be little of abiding interest in them2 he consulted me upon the subject in one of the conversations which ,i used to have with him at that time1 and ,i strongly urged upon him the reprint of the works4 ,i was much gratified when he gave way upon the point1 and consented to their appearing just as originally issued4 ,in his preface he says1 8,owing to the great progress which ,geology has made in recent times1 my views on some few points may be somewhat antiquated1 but ,i have thought it best to leave them as they originally appeared40 ,it may be interesting to indicate1 as briefly as possible1 the chief geological problem upon which the publication of ,darwin's 8,volcanic ,islands0 threw new and important light4 ,the merit of the work consisted in supplying interesting observations1 which in some cases have proved of crucial value in exploding prevalent fallacies2 in calling attention to phenomena and considerations that had been quite overlooked by geologists1 but have since exercised an important influence in moulding geological speculation2 and lastly in showing the importance which attaches to small and seemingly insignificant causes1 some of which afford a key to the explanation of very curious geological problems4 ,visiting as he did the districts in which ,von ,buch and others had found what they thought to be evidence of the truth of 8,elevation-craters10 ,darwin was able to show that the facts were capable of a totally different interpretation4 ,the views originally put forward by the old ,german geologist and traveller1 and almost universally accepted by his countrymen1 had met with much support from ,elie de ,beaumont and ,dufrenoy1 the leaders of geological thought in ,france4 ,they were1 however1 stoutly opposed by ,scrope and ,lyell in this country1 and by ,constant ,prevost and ,virlet on the other side of the channel4 ,darwin1 in the work before us1 shows how little ground there is for the assumption that the great ring-craters of the ,atlantic islands have originated in gigantic blisters of the earth's surface which1 opening at the top1 have given origin to the craters4 ,admitting the influence of the injection of lava into the structure of the volcanic cones1 in increasing their bulk and elevation1 he shows that1 in the main1 the volcanoes are built up by repeated ejections causing an accumulation of materials around the vent4 ,while1 however1 agreeing on the whole with ,scrope and ,lyell1 as to the explosive origin of ordinary volcanic craters1 ,darwin clearly saw that1 in some cases1 great craters might be formed or enlarged1 by the subsidence of the floors after eruptions4 ,the importance of this agency1 to which too little attention has been directed by geologists1 has recently been shown by ,professor ,dana1 in his admirable work on ,kilauea and the other great volcanoes of the ,hawaiian ,archipelago4 ,the effects of subsidence at a volcanic centre in producing a downward dip of the strata around it1 was first pointed out by ,darwin1 as the result of his earliest work in the ,cape de ,verde ,islands4 ,striking illustrations of the same principle have since been pointed out by ,m4 ,robert and others in ,iceland1 by ,mr4 ,heaphy in ,new ,zealand1 and by myself in the ,western ,isles of ,scotland4 ,darwin again and again called attention to the evidence that volcanic vents exhibit relations to one another which can only be explained by assuming the existence of lines of fissure in the earth's crust1 along which the lavas have made their way to the surface4 ,but he1 at the same time1 clearly saw that there was no evidence of the occurrence of great deluges of lava along such fissures2 he showed how the most remarkable plateaux1 composed of successive lava sheets1 might be built up by repeated and moderate ejections from numerous isolated vents2 and he expressly insists upon the rapidity with which the cinder-cones around the orifices of ejection and the evidences of successive outflows of lava would be obliterated by denudation4 ,one of the most striking parts of the book is that in which he deals with the effects of denudation in producing 8basal wrecks0 or worn down stumps of volcanoes4 ,he was enabled to examine a series of cases in which could be traced every gradation1 from perfect volcanic cones down to the solidified plugs which had consolidated in the vents from which ejections had taken place4 ,darwin's observations on these points have been of the greatest value and assistance to all who have essayed to study the effects of volcanic action during earlier periods of the earth's history4 ,like ,lyell1 he was firmly persuaded of the continuity of geological history1 and ever delighted in finding indications1 in the present order of nature1 that the phenomena of the past could be accounted for by means of causes which are still in operation4 ,lyell's last work in the field was carried on about his home in ,forfarshire1 and only a few months before his death he wrote to ,darwin3 8,all the work which ,i have done has confirmed me in the belief that the only difference between ,palaeozoic and recent volcanic rocks is no more than we must allow for1 by the enormous time to which the products of the oldest volcanoes have been subjected to chemical changes40 ,darwin was greatly impressed1 as the result of his studies of volcanic phenomena1 followed by an examination of the great granite-masses of the ,andes1 with the relations between the so-called ,plutonic rocks and those of undoubtedly volcanic origin4 ,it was indeed a fortunate circumstance1 that after studying some excellent examples of recent volcanic rocks1 he proceeded to examine in ,south ,america many fine illustrations of the older igneous rock-masses1 and especially of the most highly crystalline types of the same1 and then on his way home had opportunities of reviving the impression made upon him by the fresh and unaltered volcanic rocks4 ,some of the general considerations suggested by these observations were discussed in a paper read by him before the ,geological ,society1 on ,march #gth1 #ahch1 under the title 8,on the ,connection of ,certain ,volcanic ,phenomena1 and ,on the ,formation of ,mountain-chains1 and the ,effect of ,continental ,elevations40 ,the exact bearing of these two classes of facts upon one another are more fully discussed in his book on ,south ,american geology4 ,the proofs of recent elevation around many of the volcanic islands led ,darwin to conclude that volcanic areas were1 as a rule1 regions in which upward movements were taking place1 and he was naturally led to contrast them with the areas in which1 as he showed1 the occurrence of atolls1 encircling reefs1 and barrier-reefs afford indication of subsidence4 ,in this way he was able to map out the oceanic areas in different zones1 along which opposite kinds of movement were taking place4 ,his conclusions on this subject were full of novelty and suggestiveness4 ,very clearly did ,darwin recognise the importance of the fact that most of the oceanic islands appear to be of volcanic origin1 though he was careful to point out the remarkable exceptions which somewhat invalidate the generalisation4 ,in his 8,origin of ,species0 he has elaborated the idea and suggested the theory of the permanence of ocean-basins1 a suggestion which has been adopted and pushed farther by subsequent authors1 than we think its originator would have approved4 ,his caution and fairness of mind on this and similar speculative questions was well-known to all who were in the habit of discussing them with him4 ,some years before the voyage of the 8,beagle10 ,mr4 ,poulett ,scrope had pointed out the remarkable analogies that exist between certain igneous rocks of banded structure1 as seen in the ,ponza ,islands1 and the foliated crystalline schists4 ,it does not appear that ,darwin was acquainted with this remarkable memoir1 but quite independently he called attention to the same phenomena when he came to study some very similar rocks which occur in the island of ,ascension4 ,coming fresh from the study of the great masses of crystalline schist in the ,south ,american continent1 he was struck by the circumstance that in the undoubtedly igneous rocks of ,ascension we find a similar separation of the constituent minerals along parallel 8folia40 ,these observations led ,darwin to the same conclusion as that arrived at some time before by ,scrope--namely that when crystallisation takes place in rock masses under the influence of great deforming stresses1 a separation and parallel arrangement of the constituent minerals will result4 ,this is a process which is now fully recognised as having been a potent factor in the production of the metamorphic rock1 and has been called by more recent writers 8dynamo-metamorphism40 ,in this1 and in many similar discussions1 in which exact mineralogical knowledge was required1 it is remarkable how successful ,darwin was in making out the true facts with regard to the rocks he studied by the simple aid of a penknife and pocket-lens1 supplemented by a few chemical tests and the constant use of the blowpipe4 ,since his day1 the method of study of rocks by thin sections under the microscope has been devised1 and has become a most efficient aid in all petrographical inquiries4 ,during the voyage of ,h4,m4,s4 8,challenger10 many of the islands studied by ,darwin have been revisited and their rocks collected4 ,the results of their study by one of the greatest masters of the science of micropetrography--,professor ,renard of ,brussels--have been recently published in one of the volumes of 8,reports on the #8,challenger' ,expedition40 ,while much that is new and valuable has been contributed to geological science by these more recent investigations1 and many changes have been made in nomenclature and other points of detail1 it is interesting to find that all the chief facts described by ,darwin and his friend ,professor ,miller have stood the test of time and further study1 and remain as a monument of the acumen and accuracy in minute observation of these pioneers in geological research4 ,,john ,w4 ,,judd4 ,,chapter ,i4 ,,st4 ,,jago1 ,,in ,,the ,,cape ,,de ,,verde ,,archipelago4 ,rocks of the lowest series4 ,a calcareous sedimentary deposit1 with recent shells1 altered by the contact of superincumbent lava1 its horizontality and extent4 ,subsequent volcanic eruptions1 associated with calcareous matter in an earthy and fibrous form1 and often enclosed within the separate cells of the scoriae4 ,ancient and obliterated orifices of eruption of small size4 ,difficulty of tracing over a bare plain recent streams of lava4 ,inland hills of more ancient volcanic rock4 ,decomposed olivine in large masses4 ,feldspathic rocks beneath the upper crystalline basaltic strata4 ,uniform structure and form of the more ancient volcanic hills4 ,form of the valleys near the coast4 ,conglomerate now forming on the sea beach4 7,,figure #a3 ,,map #a3 ,,part ,,of ,,st4 ,,jago1 ,,one ,,of ,,the ,,cape ,,de ,,verde ,,islands47 ,the island of ,st4 ,jago extends in a ,n4,n4,w4 and ,s4,s4,e4 direction1 thirty miles in length by about twelve in breadth4 ,my observations1 made during two visits1 were confined to the southern portion within the distance of a few leagues from ,porto ,praya4 ,the country1 viewed from the sea1 presents a varied outline3 smooth conical hills of a reddish colour 7like ,red ,hill in ,figure #a 7,map #a74 7,the outline of the coast1 the position of the villages1 streamlets1 and of most of the hills in this woodcut1 are copied from the chart made on board ,h4,m4,s4 8,leven40 ,the square-topped hills 7,a1 ,b1 ,c1 etc47 are put in merely by eye1 to illustrate my description4771 and others less regular1 flat-topped1 and of a blackish colour 7like ,a1 ,b1 ,c17 rise from successive1 step-formed plains of lava4 ,at a distance1 a chain of mountains1 many thousand feet in height1 traverses the interior of the island4 ,there is no active volcano in ,st4 ,jago1 and only one in the group1 namely at ,fogo4 ,the island since being inhabited has not suffered from destructive earthquakes4 ,the lowest rocks exposed on the coast near ,porto ,praya1 are highly crystalline and compact2 they appear to be of ancient1 submarine1 volcanic origin2 they are unconformably covered by a thin1 irregular1 calcareous deposit1 abounding with shells of a late tertiary period2 and this again is capped by a wide sheet of basaltic lava1 which has flowed in successive streams from the interior of the island1 between the square-topped hills marked ,a1 ,b1 ,c1 etc4 ,still more recent streams of lava have been erupted from the scattered cones1 such as ,red and ,signal ,post ,hills4 ,the upper strata of the square-topped hills are intimately related in mineralogical composition1 and in other respects1 with the lowest series of the coast- rocks1 with which they seem to be continuous4 ,,mineralogical ,,description ,,of ,,the ,,rocks ,,of ,,the ,,lowest ,,series4 ,these rocks possess an extremely varying character2 they consist of black1 brown1 and grey1 compact1 basaltic bases1 with numerous crystals of augite1 hornblende1 olivine1 mica1 and sometimes glassy feldspar4 ,a common variety is almost entirely composed of crystals of augite with olivine4 ,mica1 it is known1 seldom occurs where augite abounds2 nor probably does the present case offer a real exception1 for the mica 7at least in my best characterised specimen1 in which one nodule of this mineral is nearly half an inch in length7 is as perfectly rounded as a pebble in a conglomerate1 and evidently has not been crystallised in the base1 in which it is now enclosed1 but has proceeded from the fusion of some pre-existing rock4 ,these compact lavas alternate with tuffs1 amygdaloids1 and wacke1 and in some places with coarse conglomerate4 ,some of the argillaceous wackes are of a dark green colour1 others1 pale yellowish-green1 and others nearly white2 ,i was surprised to find that some of the latter varieties1 even where whitest1 fused into a jet black enamel1 whilst some of the green varieties afforded only a pale gray bead4 ,numerous dikes1 consisting chiefly of highly compact augitic rocks1 and of gray amygdaloidal varieties1 intersect the strata1 which have in several places been dislocated with considerable violence1 and thrown into highly inclined positions4 ,one line of disturbance crosses the northern end of ,quail ,island 7an islet in the ,bay of ,porto ,praya71 and can be followed to the mainland4 ,these disturbances took place before the deposition of the recent sedimentary bed2 and the surface1 also1 had previously been denuded to a great extent1 as is shown by many truncated dikes4 ,,description ,,of ,,the ,,calcareous ,,deposit ,,overlying ,,the ,,foregoing ,,volcanic ,,rocks4 ,this stratum is very conspicuous from its white colour1 and from the extreme regularity with which it ranges in a horizontal line for some miles along the coast4 ,its average height above the sea1 measured from the upper line of junction with the superincumbent basaltic lava1 is about sixty feet2 and its thickness1 although varying much from the inequalities of the underlying formation1 may be estimated at about twenty feet4 ,it consists of quite white calcareous matter1 partly composed of organic debris1 and partly of a substance which may be aptly compared in appearance with mortar4 ,fragments of rock and pebbles are scattered throughout this bed1 often forming1 especially in the lower part1 a conglomerate4 ,many of the fragments of rock are whitewashed with a thin coating of calcareous matter4 ,at ,quail ,island1 the calcareous deposit is replaced in its lowest part by a soft1 brown1 earthy tuff1 full of ,turritellae2 this is covered by a bed of pebbles1 passing into sandstone1 and mixed with fragments of echini1 claws of crabs1 and shells2 the oyster-shells still adhering to the rock on which they grew4 ,numerous white balls appearing like pisolitic concretions1 from the size of a walnut to that of an apple1 are embedded in this deposit2 they usually have a small pebble in their centres4 ,although so like concretions1 a close examination convinced me that they were ,nulliporae1 retaining their proper forms1 but with their surfaces slightly abraded3 these bodies 7plants as they are now generally considered to be7 exhibit under a microscope of ordinary power1 no traces of organisation in their internal structure4 ,mr4 ,george ,r4 ,sowerby has been so good as to examine the shells which ,i collected3 there are fourteen species in a sufficiently perfect condition for their characters to be made out with some degree of certainty1 and four which can be referred only to their genera4 ,of the fourteen shells1 of which a list is given in the ,appendix1 eleven are recent species2 one1 though undescribed1 is perhaps identical with a species which ,i found living in the harbour of ,porto ,praya2 the two remaining species are unknown1 and have been described by ,mr4 ,sowerby4 ,until the shells of this ,archipelago and of the neighbouring coasts are better known1 it would be rash to assert that even these two latter shells are extinct4 ,the number of species which certainly belong to existing kinds1 although few in number1 are sufficient to show that the deposit belongs to a late tertiary period4 ,from its mineralogical character1 from the number and size of the embedded fragments1 and from the abundance of ,patellae1 and other littoral shells1 it is evident that the whole was accumulated in a shallow sea1 near an ancient coast-line4 ,,effects ,,produced ,,by ,,the ,,flowing ,,of ,,the ,,superincumbent ,,basaltic ,,lava ,,over ,,the ,,calcareous ,,deposit4 ,these effects are very curious4 ,the calcareous matter is altered to the depth of about a foot beneath the line of junction2 and a most perfect gradation can be traced1 from loosely aggregated1 small1 particles of shells1 corallines1 and ,nulliporae1 into a rock1 in which not a trace of mechanical origin can be discovered1 even with a microscope4 ,where the metamorphic change has been greatest1 two varieties occur4 ,the first is a hard1 compact1 white1 fine-grained rock1 striped with a few parallel lines of black volcanic particles1 and resembling a sandstone1 but which1 upon close examination1 is seen to be crystallised throughout1 with the cleavages so perfect that they can be readily measured by the reflecting goniometer4 ,in specimens1 where the change has been less complete1 when moistened and examined under a strong lens1 the most interesting gradation can be traced1 some of the rounded particles retaining their proper forms1 and others insensibly melting into the granulo-crystalline paste4 ,the weathered surface of this stone1 as is so frequently the case with ordinary limestones1 assumes a brick-red colour4 ,the second metamorphosed variety is likewise a hard rock1 but without any crystalline structure4 ,it consists of a white1 opaque1 compact1 calcareous stone1 thickly mottled with rounded1 though regular1 spots of a soft1 earthy1 ochraceous substance4 ,this earthy matter is of a pale yellowish- brown colour1 and appears to be a mixture of carbonate of lime with iron2 it effervesces with acids1 is infusible1 but blackens under the blowpipe1 and becomes magnetic4 ,the rounded form of the minute patches of earthy substance1 and the steps in the progress of their perfect formation1 which can be followed in a suit of specimens1 clearly show that they are due either to some power of aggregation in the earthy particles amongst themselves1 or more probably to a strong attraction between the atoms of the carbonate of line1 and consequently to the segregation of the earthy extraneous matter4 ,i was much interested by this fact1 because ,i have often seen quartz rocks 7for instance1 in the ,falkland ,islands1 and in the lower ,silurian strata of the ,stiper-stones in ,shropshire71 mottled in a precisely analogous manner1 with little spots of a white1 earthy substance 7earthy feldspar872 and these rocks1 there was good reason to suppose1 had undergone the action of heat1--a view which thus receives confirmation4 ,this spotted structure may possibly afford some indication in distinguishing those formations of quartz1 which owe their present structure to igneous action1 from those produced by the agency of water alone2 a source of doubt1 which ,i should think from my own experience1 that most geologists1 when examining arenaceo-quartzose districts must have experienced4 ,the lowest and most scoriaceous part of the lava1 in rolling over the sedimentary deposit at the bottom of the sea1 has caught up large quantities of calcareous matter1 which now forms a snow-white1 highly crystalline basis to a breccia1 including small pieces of black1 glossy scoriae4 ,a little above this1 where the lime is less abundant1 and the lava more compact1 numerous little balls1 composed of spicula of calcareous spar1 radiating from common centres1 occupy the interstices4 ,in one part of ,quail ,island1 the lime has thus been crystallised by the heat of the superincumbent lava1 where it is only thirteen feet in thickness2 nor had the lava been originally thicker1 and since reduced by degradation1 as could be told from the degree of cellularity of its surface4 ,i have already observed that the sea must have been shallow in which the calcareous deposit was accumulated4 ,in this case1 therefore1 the carbonic acid gas has been retained under a pressure1 insignificant compared with that 7a column of water1 #a1gjh feet in height7 originally supposed by ,sir ,james ,hall to be requisite for this end3 but since his experiments1 it has been discovered that pressure has less to do with the retention of carbonic acid gas1 than the nature of the circumjacent atmosphere2 and hence1 as is stated to be the case by ,mr4 ,faraday1 masses of limestone are sometimes fused and crystallised even in common limekilns4 7,i am much indebted to ,mr4 ,e4,w4 ,brayley in having given me the following references to papers on this subject3 ,faraday in the 8,edinburgh ,new ,philosophical ,journal0 volume #ae page #cih2 ,gay-,lussac in 8,annales de ,chem4 et ,phys40 tome #fc page #bai translated in the 8,london and ,edinburgh ,philosophical ,magazine0 volume #aj page #dif47 ,carbonate of lime can be heated to almost any degree1 according to ,faraday1 in an atmosphere of carbonic acid gas1 without being decomposed2 and ,gay-,lussac found that fragments of limestone1 placed in a tube and heated to a degree1 not sufficient by itself to cause their decomposition1 yet immediately evolved their carbonic acid1 when a stream of common air or steam was passed over them3 ,gay-,lussac attributes this to the mechanical displacement of the nascent carbonic acid gas4 ,the calcareous matter beneath the lava1 and especially that forming the crystalline spicula between the interstices of the scoriae1 although heated in an atmosphere probably composed chiefly of steam1 could not have been subjected to the effects of a passing stream2 and hence it is1 perhaps1 that they have retained their carbonic acid1 under a small amount of pressure4 ,the fragments of scoriae1 embedded in the crystalline calcareous basis1 are of a jet black colour1 with a glossy fracture like pitchstone4 ,their surfaces1 however1 are coated with a layer of a reddish-orange1 translucent substance1 which can easily be scratched with a knife2 hence they appear as if overlaid by a thin layer of rosin4 ,some of the smaller fragments are partially changed throughout into this substance3 a change which appears quite different from ordinary decomposition4 ,at the ,galapagos ,archipelago 7as will be described in a future chapter71 great beds are formed of volcanic ashes and particles of scoriae1 which have undergone a closely similar change4 ,,the ,,extent ,,and ,,horizontality ,,of ,,the ,,calcareous ,,stratum4 7,,figure #b3 ,,signal ,,post ,,hill4 7,section with ,a low and ,c high47 ,a4--,ancient volcanic rocks4 ,b4--,calcareous stratum4 ,c4--,upper basaltic lava47 ,the upper line of surface of the calcareous stratum1 which is so conspicuous from being quite white and so nearly horizontal1 ranges for miles along the coast1 at the height of about sixty feet above the sea4 ,the sheet of basalt1 by which it is capped1 is on an average eighty feet in thickness4 ,westward of ,porto ,praya beyond ,red ,hill1 the white stratum with the superincumbent basalt is covered up by more recent streams4 ,northward of ,signal ,post ,hill1 ,i could follow it with my eye1 trending away for several miles along the sea cliffs4 ,the distance thus observed is about seven miles2 but ,i cannot doubt from its regularity that it extends much farther4 ,in some ravines at right angles to the coast1 it is seen gently dipping towards the sea1 probably with the same inclination as when deposited round the ancient shores of the island4 ,i found only one inland section1 namely1 at the base of the hill marked ,a1 where1 at the height of some hundred feet1 this bed was exposed2 it here rested on the usual compact augitic rock associated with wacke1 and was covered by the widespread sheet of modern basaltic lava4 ,some exceptions occur to the horizontality of the white stratum3 at ,quail ,island1 its upper surface is only forty feet above the level of the sea2 here also the capping of lava is only between twelve and fifteen feet in thickness2 on the other hand1 at the north-east side of ,porto ,praya harbour1 the calcareous stratum1 as well as the rock on which it rests1 attain a height above the average level3 the inequality of level in these two cases is not1 as ,i believe1 owing to unequal elevation1 but to original irregularities at the bottom of the sea4 ,of this fact1 at ,quail ,island1 there was clear evidence in the calcareous deposit being in one part of much greater than the average thickness1 and in another part being entirely absent2 in this latter case1 the modern basaltic lavas rested directly on those of more ancient origin4 ,under ,signal ,post ,hill1 the white stratum dips into the sea in a remarkable manner4 ,this hill is conical1 #dej feet in height1 and retains some traces of having had a crateriform structure2 it is composed chiefly of matter erupted posteriorly to the elevation of the great basaltic plain1 but partly of lava of apparently submarine origin and of considerable antiquity4 ,the surrounding plain1 as well as the eastern flank of this hill1 has been worn into steep precipices1 overhanging the sea4 ,in these precipices1 the white calcareous stratum may be seen1 at the height of about seventy feet above the beach1 running for some miles both northward and southward of the hill1 in a line appearing to be perfectly horizontal2 but for a space of a quarter of a mile directly under the hill1 it dips into the sea and disappears4 ,on the south side the dip is gradual1 on the north side it is more abrupt1 as is shown in ,figure #b4 ,as neither the calcareous stratum1 nor the superincumbent basaltic lava 7as far as the latter can be distinguished from the more modern ejections71 appears to thicken as it dips1 ,i infer that these strata were not originally accumulated in a trough1 the centre of which afterwards became a point of eruption2 but that they have subsequently been disturbed and bent4 ,we may suppose either that ,signal ,post ,hill subsided after its elevation with the surrounding country1 or that it never was uplifted to the same height with it4 ,this latter seems to me the most probable alternative1 for during the slow and equable elevation of this portion of the island1 the subterranean motive power1 from expending part of its force in repeatedly erupting volcanic matter from beneath this point1 would1 it is likely1 have less force to uplift it4 ,something of the same kind seems to have occurred near ,red ,hill1 for when tracing upwards the naked streams of lava from near ,porto ,praya towards the interior of the island1 ,i was strongly induced to suspect1 that since the lava had flowed1 the slope of the land had been slightly modified1 either by a small subsidence near ,red ,hill1 or by that portion of the plain having been uplifted to a less height during the elevation of the whole area4 ,,the ,,basaltic ,,lava1 ,,superincumbent ,,on ,,the ,,calcareous ,,deposit4 ,this lava is of a pale grey colour1 fusing into a black enamel2 its fracture is rather earthy and concretionary2 it contains olivine in small grains4 ,the central parts of the mass are compact1 or at most crenulated with a few minute cavities1 and are often columnar4 ,at ,quail ,island this structure was assumed in a striking manner2 the lava in one part being divided into horizontal laminae1 which became in another part split by vertical fissures into five-sided plates2 and these again1 being piled on each other1 insensibly became soldered together1 forming fine symmetrical columns4 ,the lower surface of the lava is vesicular1 but sometimes only to the thickness of a few inches2 the upper surface1 which is likewise vesicular1 is divided into balls1 frequently as much as three feet in diameter1 made up of concentric layers4 ,the mass is composed of more than one stream2 its total thickness being1 on an average1 about eighty feet3 the lower portion has certainly flowed beneath the sea1 and probably likewise the upper portion4 ,the chief part of this lava has flowed from the central districts1 between the hills marked ,a1 ,b1 ,c1 etc41 in the woodcut- map4 ,the surface of the country1 near the coast1 is level and barren2 towards the interior1 the land rises by successive terraces1 of which four1 when viewed from a distance1 could be distinctly counted4 ,,volcanic ,,eruptions ,,subsequent ,,to ,,the ,,elevation ,,of ,,the ,,coastland2 ,,the ,,ejected ,,matter ,,associated ,,with ,,earthy ,,lime4 ,these recent lavas have proceeded from those scattered1 conical1 reddish- coloured hills1 which rise abruptly from the plain-country near the coast4 ,i ascended some of them1 but will describe only one1 namely1 ,,red ,,hill1 which may serve as a type of its class1 and is remarkable in some especial respects4 ,its height is about six hundred feet2 it is composed of bright red1 highly scoriaceous rock of a basaltic nature2 on one side of its summit there is a hollow1 probably the last remnant of a crater4 ,several of the other hills of this class1 judging from their external forms1 are surmounted by much more perfect craters4 ,when sailing along the coast1 it was evident that a considerable body of lava had flowed from ,red ,hill1 over a line of cliff about one hundred and twenty feet in height1 into the sea3 this line of cliff is continuous with that forming the coast1 and bounding the plain on both sides of this hill2 these streams1 therefore1 were erupted1 after the formation of the coast-cliffs1 from ,red ,hill1 when it must have stood1 as it now does1 above the level of the sea4 ,this conclusion accords with the highly scoriaceous condition of all the rock on it1 appearing to be of subaerial formation3 and this is important1 as there are some beds of calcareous matter near its summit1 which might1 at a hasty glance1 have been mistaken for a submarine deposit4 ,these beds consist of white1 earthy1 carbonate of lime1 extremely friable so as to be crushed with the least pressure2 the most compact specimens not resisting the strength of the fingers4 ,some of the masses are as white as quicklime1 and appear absolutely pure2 but on examining them with a lens1 minute particles of scoriae can always be seen1 and ,i could find none which1 when dissolved in acids1 did not leave a residue of this nature4 ,it is1 moreover1 difficult to find a particle of the lime which does not change colour under the blowpipe1 most of them even becoming glazed4 ,the scoriaceous fragments and the calcareous matter are associated in the most irregular manner1 sometimes in obscure beds1 but more generally as a confused breccia1 the lime in some parts and the scoriae in others being most abundant4 ,sir ,h4 ,de la ,beche has been so kind as to have some of the purest specimens analysed1 with a view to discover1 considering their volcanic origin1 whether they contained much magnesia2 but only a small portion was found1 such as is present in most limestones4 ,fragments of the scoriae embedded in the calcareous mass1 when broken1 exhibit many of their cells lined and partly filled with a white1 delicate1 excessively fragile1 moss-like1 or rather conferva-like1 reticulation of carbonate of lime4 ,these fibres1 examined under a lens of one-tenth of an inch focal distance1 appear cylindrical2 they are rather above one- thousandth of an inch in diameter2 they are either simply branched1 or more commonly united into an irregular mass of network1 with the meshes of very unequal sizes and of unequal numbers of sides4 ,some of the fibres are thickly covered with extremely minute spicula1 occasionally aggregated into little tuffs2 and hence they have a hairy appearance4 ,these spicula are of the same diameter throughout their length2 they are easily detached1 so that the object-glass of the microscope soon becomes scattered over with them4 ,within the cells of many fragments of the scoria1 the lime exhibits this fibrous structure1 but generally in a less perfect degree4 ,these cells do not appear to be connected with one another4 ,there can be no doubt1 as will presently be shown1 that the lime was erupted1 mingled with the lava in its fluid state1 and therefore ,i have thought it worth while to describe minutely this curious fibrous structure1 of which ,i know nothing analogous4 ,from the earthy condition of the fibres1 this structure does not appear to be related to crystallisation4 ,other fragments of the scoriaceous rock from this hill1 when broken1 are often seen marked with short and irregular white streaks1 which are owing to a row of separate cells being partly1 or quite1 filled with white calcareous powder4 ,this structure immediately reminded me of the appearance in badly kneaded dough1 of balls and drawn-out streaks of flour1 which have remained unmixed with the paste2 and ,i cannot doubt that small masses of the lime1 in the same manner remaining unmixed with the fluid lava1 have been drawn out when the whole was in motion4 ,i carefully examined1 by trituration and solution in acids1 pieces of the scoriae1 taken from within half-an-inch of those cells which were filled with the calcareous powder1 and they did not contain an atom of free lime4 ,it is obvious that the lava and lime have on a large scale been very imperfectly mingled2 and where small portions of the lime have been entangled within a piece of the viscid lava1 the cause of their now occupying1 in the form of a powder or of a fibrous reticulation1 the vesicular cavities1 is1 ,i think1 evidently due to the confined gases having most readily expanded at the points where the incoherent lime rendered the lava less adhesive4 ,a mile eastward of the town of ,praya1 there is a steep-sided gorge1 about one hundred and fifty yards in width1 cutting through the basaltic plain and underlying beds1 but since filled up by a stream of more modern lava4 ,this lava is dark grey1 and in most parts compact and rudely columnar2 but at a little distance from the coast1 it includes in an irregular manner a brecciated mass of red scoriae mingled with a considerable quantity of white1 friable1 and in some parts1 nearly pure earthy lime1 like that on the summit of ,red ,hill4 ,this lava1 with its entangled lime1 has certainly flowed in the form of a regular stream2 and1 judging from the shape of the gorge1 towards which the drainage of the country 7feeble though it now be7 still is directed1 and from the appearance of the bed of loose water-worn blocks with their interstices unfilled1 like those in the bed of a torrent1 on which the lava rests1 we may conclude that the stream was of subaerial origin4 ,i was unable to trace it to its source1 but1 from its direction1 it seemed to have come from ,signal ,post ,hill1 distant one mile and a quarter1 which1 like ,red ,hill1 has been a point of eruption subsequent to the elevation of the great basaltic plain4 ,it accords with this view1 that ,i found on ,signal ,post ,hill1 a mass of earthy1 calcareous matter of the same nature1 mingled with scoriae4 ,i may here observe that part of the calcareous matter forming the horizontal sedimentary bed1 especially the finer matter with which the embedded fragments of rock are whitewashed1 has probably been derived from similar volcanic eruptions1 as well as from triturated organic remains3 the underlying1 ancient1 crystalline rocks1 also1 are associated with much carbonate of lime1 filling amygdaloidal cavities1 and forming irregular masses1 the nature of which latter ,i was unable to understand4 ,considering the abundance of earthy lime near the summit of ,red ,hill1 a volcanic cone six hundred feet in height1 of subaerial growth1--considering the intimate manner in which minute particles and large masses of scoriae are embedded in the masses of nearly pure lime1 and on the other hand1 the manner in which small kernels and streaks of the calcareous powder are included in solid pieces of the scoriae1--considering1 also1 the similar occurrence of lime and scoriae within a stream of lava1 also supposed1 with good reason1 to have been of modern subaerial origin1 and to have flowed from a hill1 where earthy lime also occurs3 ,i think1 considering these facts1 there can be no doubt that the lime has been erupted1 mingled with the molten lava4 ,i am not aware that any similar case has been described3 it appears to me an interesting one1 inasmuch as most geologists must have speculated on the probable effects of a volcanic focus1 bursting through deep-seated beds of different mineralogical composition4 ,the great abundance of free silex in the trachytes of some countries 7as described by ,beudant in ,hungary1 and by ,p4 ,scrope in the ,panza ,islands71 perhaps solves the inquiry with respect to deep-seated beds of quartz2 and we probably here see it answered1 where the volcanic action has invaded subjacent masses of limestone4 ,one is naturally led to conjecture in what state the now earthy carbonate of lime existed1 when ejected with the intensely heated lava3 from the extreme cellularity of the scoriae on ,red ,hill1 the pressure cannot have been great1 and as most volcanic eruptions are accompanied by the emission of large quantities of steam and other gases1 we here have the most favourable conditions1 according to the views at present entertained by chemists1 for the expulsion of the carbonic acid4 7,whilst deep beneath the surface1 the carbonate of lime was1 ,i presume1 in a fluid state4 ,hutton1 it is known1 thought that all amygdaloids were produced by drops of molten limestone floating in the trap1 like oil in water3 this no doubt is erroneous1 but if the matter forming the summit of ,red ,hill had been cooled under the pressure of a moderately deep sea1 or within the walls of a dike1 we should1 in all probability1 have had a trap rock associated with large masses of compact1 crystalline1 calcareous spar1 which1 according to the views entertained by many geologists1 would have been wrongly attributed to subsequent infiltration47 ,has the slow re- absorption of this gas1 it may be asked1 given to the lime in the cells of the lava1 that peculiar fibrous structure1 like that of an efflorescing salt8 ,finally1 ,i may remark on the great contrast in appearance between this earthy lime1 which must have been heated in a free atmosphere of steam and other gases1 while the white1 crystalline1 calcareous spar1 produced by a single thin sheet of lava 7as at ,quail ,island7 rolling over similar earthy lime and the debris of organic remains1 at the bottom of a shallow sea4 ,,signal ,,post ,,hill4 ,this hill has already been several times mentioned1 especially with reference to the remarkable manner in which the white calcareous stratum1 in other parts so horizontal 7,figure #b71 dips under it into the sea4 ,it has a broad summit1 with obscure traces of a crateriform structure1 and is composed of basaltic rocks 7,of these1 one common variety is remarkable for being full of small fragments of a dark jasper-red earthy mineral1 which1 when examined carefully1 shows an indistinct cleavage2 the little fragments are elongated in form1 are soft1 are magnetic before and after being heated1 and fuse with difficulty into a dull enamel4 ,this mineral is evidently closely related to the oxides of iron1 but ,i cannot ascertain what it exactly is4 ,the rock containing this mineral is crenulated with small angular cavities1 which are lined and filled with yellowish crystals of carbonate of lime471 some compact1 others highly cellular with inclined beds of loose scoriae1 of which some are associated with earthy lime4 ,like ,red ,hill1 it has been the source of eruptions1 subsequently to the elevation of the surrounding basaltic plain2 but unlike that hill1 it has undergone considerable denudation1 and has been the seat of volcanic action at a remote period1 when beneath the sea4 ,i judge of this latter circumstance from finding on its inland flank the last remains of three small points of eruption4 ,these points are composed of glossy scoriae1 cemented by crystalline calcareous spar1 exactly like the great submarine calcareous deposit1 where the heated lava has rolled over it3 their demolished state can1 ,i think1 be explained only by the denuding action of the waves of the sea4 ,i was guided to the first orifice by observing a sheet of lava1 about two hundred yards square1 with steepish sides1 superimposed on the basaltic plain with no adjoining hillock1 whence it could have been erupted2 and the only trace of a crater which ,i was able to discover1 consisted of some inclined beds of scoriae at one of its corners4 ,at the distance of fifty yards from a second level-topped patch of lava1 but of much smaller size1 ,i found an irregular circular group of masses of cemented1 scoriaceous breccia1 about six feet in height1 which doubtless had once formed the point of eruption4 ,the third orifice is now marked only by an irregular circle of cemented scoriae1 about four yards in diameter1 and rising in its highest point scarcely three feet above the level of the plain1 the surface of which1 close all round1 exhibits its usual appearance3 here we have a horizontal basal section of a volcanic spiracle1 which1 together with all its ejected matter1 has been almost totally obliterated4 ,the stream of lava1 which fills the narrow gorge eastward of the town of ,praya1 judging from its course1 seems1 as before remarked1 to have come from ,signal ,post ,hill1 and to have flowed over the plain1 after its elevation 7,the sides of this gorge1 where the upper basaltic stratum is intersected1 are almost perpendicular4 ,the lava1 which has since filled it up1 is attached to these sides1 almost as firmly as a dike is to its walls4 ,in most cases1 where a stream of lava has flowed down a valley1 it is bounded on each side by loose scoriaceous masses473 the same observation applies to a stream 7possibly part of the same one7 capping the sea cliffs1 a little eastward of the gorge4 ,when ,i endeavoured to follow these streams over the stony level plain1 which is almost destitute of soil and vegetation1 ,i was much surprised to find1 that although composed of hard basaltic matter1 and not having been exposed to marine denudation1 all distant traces of them soon became utterly lost4 ,but ,i have since observed at the ,galapagos ,archipelago1 that it is often impossible to follow even great deluges of quite recent lava across older streams1 except by the size of the bushes growing on them1 or by the comparative states of glossiness of their surfaces1--characters which a short lapse of time would be sufficient quite to obscure4 ,i may remark1 that in a level country1 with a dry climate1 and with the wind blowing always in one direction 7as at the ,cape de ,verde ,archipelago71 the effects of atmospheric degradation are probably much greater than would at first be expected2 for soil in this case accumulates only in a few protected hollows1 and being blown in one direction1 it is always travelling towards the sea in the form of the finest dust1 leaving the surface of the rocks bare1 and exposed to the full effects of renewed meteoric action4 ,,inland ,,hills ,,of ,,more ,,ancient ,,volcanic ,,rocks4 ,these hills are laid down by eye1 and marked as ,a1 ,b1 ,c1 etc41 in ,map #a4 ,they are related in mineralogical composition1 and are probably directly continuous with the lowest rocks exposed on the coast4 ,these hills1 viewed from a distance1 appear as if they had once formed part of an irregular tableland1 and from their corresponding structure and composition this probably has been the case4 ,they have flat1 slightly inclined summits1 and are1 on an average1 about six hundred feet in height2 they present their steepest slope towards the interior of the island1 from which point they radiate outwards1 and are separated from each other by broad and deep valleys1 through which the great streams of lava1 forming the coast-plains1 have descended4 ,their inner and steeper escarpments are ranged in an irregular curve1 which rudely follows the line of the shore1 two or three miles inland from it4 ,i ascended a few of these hills1 and from others1 which ,i was able to examine with a telescope1 ,i obtained specimens1 through the kindness of ,mr4 ,kent1 the assistant-surgeon of the 8,beagle02 although by these means ,i am acquainted with only a part of the range1 five or six miles in length1 yet ,i scarcely hesitate1 from their uniform structure1 to affirm that they are parts of one great formation1 stretching round much of the circumference of the island4 ,the upper and lower strata of these hills differ greatly in composition4 ,the upper are basaltic1 generally compact1 but sometimes scoriaceous and amygdaloidal1 with associated masses of wacke3 where the basalt is compact1 it is either fine-grained or very coarsely crystallised2 in the latter case it passes into an augitic rock1 containing much olivine2 the olivine is either colourless1 or of the usual yellow and dull reddish shades4 ,on some of the hills1 beds of calcareous matter1 both in an earthy and in a crystalline form1 including fragments of glossy scoriae1 are associated with the basaltic strata4 ,these strata differ from the streams of basaltic lava forming the coast-plains1 only in being more compact1 and in the crystals of augite1 and in the grains of olivine being of much greater size2--characters which1 together with the appearance of the associated calcareous beds1 induce me to believe that they are of submarine formation4 ,some considerable masses of wacke1 which are associated with these basaltic strata1 and which likewise occur in the basal series on the coast1 especially at ,quail ,island1 are curious4 ,they consist of a pale yellowish- green argillaceous substance1 of a crumbling texture when dry1 but unctuous when moist3 in its purest form1 it is of a beautiful green tint1 with translucent edges1 and occasionally with obscure traces of an original cleavage4 ,under the blowpipe it fuses very readily into a dark grey1 and sometimes even black bead1 which is slightly magnetic4 ,from these characters1 ,i naturally thought that it was one of the pale species1 decomposed1 of the genus augite2--a conclusion supported by the unaltered rock being full of large separate crystals of black augite1 and of balls and irregular streaks of dark grey augitic rock4 ,as the basalt ordinarily consists of augite1 and of olivine often tarnished and of a dull red colour1 ,i was led to examine the stages of decomposition of this latter mineral1 and ,i found1 to my surprise1 that ,i could trace a nearly perfect gradation from unaltered olivine to the green wacke4 ,part of the same grain under the blowpipe would in some instances behave like olivine1 its colour being only slightly changed1 and part would give a black magnetic bead4 ,hence ,i can have no doubt that the greenish wacke originally existed as olivine2 but great chemical changes must have been effected during the act of decomposition thus to have altered a very hard1 transparent1 infusible mineral1 into a soft1 unctuous1 easily melted1 argillaceous substance4 7,d',aubuisson 8,traite de ,geognosie0 tome #b page #efi mentions1 on the authority of ,m4 ,marcel de ,serres1 masses of green earth near ,montpellier1 which are supposed to be due to the decomposition of olivine4 ,i do not1 however1 find1 that the action of this mineral under the blowpipe being entirely altered1 as it becomes decomposed1 has been noticed2 and the knowledge of this fact is important1 as at first it appears highly improbable that a hard1 transparent1 refractory mineral should be changed into a soft1 easily fused clay1 like this of ,st4 ,jago4 ,i shall hereafter describe a green substance1 forming threads within the cells of some vesicular basaltic rocks in ,van ,diemen's ,land1 which behave under the blowpipe like the green wacke of ,st4 ,jago2 but its occurrence in cylindrical threads1 shows it cannot have resulted from the decomposition of olivine1 a mineral always existing in the form of grains or crystals47 ,the basal strata of these hills1 as well as some neighbouring1 separate1 bare1 rounded hillocks1 consist of compact1 fine-grained1 non-crystalline 7or so slightly as scarcely to be perceptible71 ferruginous1 feldspathic rocks1 and generally in a state of semi-decomposition4 ,their fracture is exceedingly irregular1 and splintery2 yet small fragments are often very tough4 ,they contain much ferruginous matter1 either in the form of minute grains with a metallic lustre1 or of brown hair-like threads3 the rock in this latter case assuming a pseudo-brecciated structure4 ,these rocks sometimes contain mica and veins of agate4 ,their rusty brown or yellowish colour is partly due to the oxides of iron1 but chiefly to innumerable1 microscopically minute1 black specks1 which1 when a fragment is heated1 are easily fused1 and evidently are either hornblende or augite4 ,these rocks1 therefore1 although at first appearing like baked clay or some altered sedimentary deposit1 contain all the essential ingredients of trachyte2 from which they differ only in not being harsh1 and in not containing crystals of glassy feldspar4 ,as is so often the case with trachytic formation1 no stratification is here apparent4 ,a person would not readily believe that these rocks could have flowed as lava2 yet at ,st4 ,helena there are well-characterised streams 7as will be described in an ensuing chapter7 of nearly similar composition4 ,amidst the hillocks composed of these rocks1 ,i found in three places1 smooth conical hills of phonolite1 abounding with fine crystals of glassy feldspar1 and with needles of hornblende4 ,these cones of phonolite1 ,i believe1 bear the same relation to the surrounding feldspathic strata which some masses of coarsely crystallised augitic rock1 in another part of the island1 bear to the surrounding basalt1 namely1 that both have been injected4 ,the rocks of a feldspathic nature being anterior in origin to the basaltic strata1 which cap them1 as well as to the basaltic streams of the coast-plains1 accords with the usual order of succession of these two grand divisions of the volcanic series4 ,the strata of most of these hills in the upper part1 where alone the planes of division are distinguishable1 are inclined at a small angle from the interior of the island towards the sea-coast4 ,the inclination is not the same in each hill2 in that marked ,a it is less than in ,b1 ,d1 or ,e2 in ,c the strata are scarcely deflected from a horizontal plane1 and in ,f 7as far as ,i could judge without ascending it7 they are slightly inclined in a reverse direction1 that is1 inwards and towards the centre of the island4 ,notwithstanding these differences of inclination1 their correspondence in external form1 and in the composition both of their upper and lower parts1- -their relative position in one curved line1 with their steepest sides turned inwards1--all seem to show that they originally formed parts of one platform2 which platform1 as before remarked1 probably extended round a considerable portion of the circumference of the island4 ,the upper strata certainly flowed as lava1 and probably beneath the sea1 as perhaps did the lower feldspathic masses3 how then come these strata to hold their present position1 and whence were they erupted8 ,in the centre of the island there are lofty mountains1 but they are separated from the steep inland flanks of these hills by a wide space of lower country3 the interior mountains1 moreover1 seem to have been the source of those great streams of basaltic lava which1 contracting as they pass between the bases of the hills in question1 expand into the coast- plains4 7,i saw very little of the inland parts of the island4 ,near the village of ,st4 ,domingo1 there are magnificent cliffs of rather coarsely crystallised basaltic lava4 ,following the little stream in this valley1 about a mile above the village1 the base of the great cliff was formed of a compact fine-grained basalt1 conformably covered by a bed of pebbles4 ,near ,fuentes1 ,i met with pap-formed hills of the compact feldspathic series of rocks47 ,round the shores of ,st4 ,helena there is a rudely formed ring of basaltic rocks1 and at ,mauritius there are remnants of another such a ring round part1 if not round the whole1 of the island2 here again the same question immediately occurs1 how came these masses to hold their present position1 and whence were they erupted8 ,the same answer1 whatever it may be1 probably applies in these three cases2 and in a future chapter we shall recur to this subject4 ,,valleys ,,near ,,the ,,coast4 ,these are broad1 very flat1 and generally bounded by low cliff-formed sides4 ,portions of the basaltic plain are sometimes nearly or quite isolated by them2 of which fact1 the space on which the town of ,praya stands offers an instance4 ,the great valley west of the town has its bottom filled up to a depth of more than twenty feet by well-rounded pebbles1 which in some parts are firmly cemented together by white calcareous matter4 ,there can be no doubt1 from the form of these valleys1 that they were scooped out by the waves of the sea1 during that equable elevation of the land1 of which the horizontal calcareous deposit1 with its existing species of marine remains1 gives evidence4 ,considering how well shells have been preserved in this stratum1 it is singular that ,i could not find even a single small fragment of shell in the conglomerate at the bottom of the valleys4 ,the bed of pebbles in the valley west of the town is intersected by a second valley joining it as a tributary1 but even this valley appears much too wide and flat-bottomed to have been formed by the small quantity of water1 which falls only during one short wet season2 for at other times of the year these valleys are absolutely dry4 ,,recent ,,conglomerate4 ,on the shores of ,quail ,island1 ,i found fragments of brick1 bolts of iron1 pebbles1 and large fragments of basalt1 united by a scanty base of impure calcareous matter into a firm conglomerate4 ,to show how exceedingly firm this recent conglomerate is1 ,i may mention1 that ,i endeavoured with a heavy geological hammer to knock out a thick bolt of iron1 which was embedded a little above low-water mark1 but was quite unable to succeed4 ,,chapter ,,ii4 ,,fernando ,,noronha2 ,,terceira2 ,,tahiti1 ,,etc4 ,,fernando ,,noronha4 ,precipitous hill of phonolite4 ,,terceira4 ,trachytic rocks3 their singular decomposition by steam of high temperature4 ,,tahiti4 ,passage from wacke into trap2 singular volcanic rock with the vesicles half-filled with mesotype4 ,,mauritius4 ,proofs of its recent elevation4 ,structure of its more ancient mountains2 similarity with ,st4 ,jago4 ,,st4 ,,paul',s ,,rocks4 ,not of volcanic origin4 ,their singular mineralogical composition4 ,,fernando ,,noronha4 ,during our short visit at this and the four following islands1 ,i observed very little worthy of description4 ,fernando ,noronha is situated in the ,atlantic ,ocean1 in latitude #c degrees #ej minutes ,s41 and #bcj miles distant from the coast of ,south ,america4 ,it consists of several islets1 together nine miles in length by three in breadth4 ,the whole seems to be of volcanic origin2 although there is no appearance of any crater1 or of any one central eminence4 ,the most remarkable feature is a hill #a1jjj feet high1 of which the upper #djj feet consist of a precipitous1 singularly shaped pinnacle1 formed of columnar phonolite1 containing numerous crystals of glassy feldspar1 and a few needles of hornblende4 ,from the highest accessible point of this hill1 ,i could distinguish in different parts of the group several other conical hills1 apparently of the same nature4 ,at ,st4 ,helena there are similar1 great1 conical1 protuberant masses of phonolite1 nearly one thousand feet in height1 which have been formed by the injection of fluid feldspathic lava into yielding strata4 ,if this hill has had1 as is probable1 a similar origin1 denudation has been here effected on an enormous scale4 ,near the base of this hill1 ,i observed beds of white tuff1 intersected by numerous dikes1 some of amygdaloidal basalt and others of trachyte2 and beds of slaty phonolite with the planes of cleavage directed ,n4,w4 and ,s4,e4 ,parts of this rock1 where the crystals were scanty1 closely resembled common clay-slate1 altered by the contact of a trap-dike4 ,the lamination of rocks1 which undoubtedly have once been fluid1 appears to me a subject well deserving attention4 ,on the beach there were numerous fragments of compact basalt1 of which rock a distant facade of columns seemed to be formed4 ,,terceira ,,in ,,the ,,azores4 ,the central parts of this island consist of irregularly rounded mountains of no great elevation1 composed of trachyte1 which closely resembles in general character the trachyte of ,ascension1 presently to be described4 ,this formation is in many parts overlaid1 in the usual order of superposition1 by streams of basaltic lava1 which near the coast compose nearly the whole surface4 ,the course which these streams have followed from their craters1 can often be followed by the eye4 ,the town of ,angra is overlooked by a crateriform hill 7,mount ,brazil71 entirely built of thin strata of fine-grained1 harsh1 brown-coloured tuff4 ,the upper beds are seen to overlap the basaltic streams on which the town stands4 ,this hill is almost identical in structure and composition with numerous crateriformed hills in the ,galapagos ,archipelago4 ,,effects ,,of ,,steam ,,on ,,the ,,trachytic ,,rocks4 ,in the central part of the island there is a spot1 where steam is constantly issuing in jets from the bottom of a small ravine-like hollow1 which has no exit1 and which abuts against a range of trachytic mountains4 ,the steam is emitted from several irregular fissures3 it is scentless1 soon blackens iron1 and is of much too high temperature to be endured by the hand4 ,the manner in which the solid trachyte is changed on the borders of these orifices is curious3 first1 the base becomes earthy1 with red freckles evidently due to the oxidation of particles of iron2 then it becomes soft2 and lastly1 even the crystals of glassy feldspar yield to the dissolving agent4 ,after the mass is converted into clay1 the oxide of iron seems to be entirely removed from some parts1 which are left perfectly white1 whilst in other neighbouring parts1 which are of the brightest red colour1 it seems to be deposited in greater quantity2 some other masses are marbled with two distinct colours4 ,portions of the white clay1 now that they are dry1 cannot be distinguished by the eye from the finest prepared chalk2 and when placed between the teeth they feel equally soft-grained2 the inhabitants use this substance for white-washing their houses4 ,the cause of the iron being dissolved in one part1 and close by being again deposited1 is obscure2 but the fact has been observed in several other places4 7,spallanzani1 ,dolomieu1 and ,hoffman have described similar cases in the ,italian volcanic islands4 ,dolomieu says the iron at the ,panza ,islands is redeposited in the form of veins 7page #hf 8,memoire sur les ,isles ,ponces074 ,these authors likewise believe that the steam deposits silica3 it is now experimentally known that vapour of a high temperature is able to dissolve silica47 ,in some half-decayed specimens1 ,i found small1 globular aggregations of yellow hyalite1 resembling gum-arabic1 which no doubt had been deposited by the steam4 ,as there is no escape for the rain-water1 which trickles down the sides of the ravine-like hollow1 whence the steam issues1 it must all percolate downwards through the fissures at its bottom4 ,some of the inhabitants informed me that it was on record that flames 7some luminous appearance87 had originally proceeded from these cracks1 and that the flames had been succeeded by the steam2 but ,i was not able to ascertain how long this was ago1 or anything certain on the subject4 ,when viewing the spot1 ,i imagined that the injection of a large mass of rock4 like the cone of phonolite at ,fernando ,noronha1 in a semi-fluid state1 by arching the surface might have caused a wedge-shaped hollow with cracks at the bottom1 and that the rain- water percolating to the neighbourhood of the heated mass1 would during many succeeding years be driven back in the form of steam4 ,,tahiti 7,,otaheite74 ,i visited only a part of the north-western side of this island1 and this part is entirely composed of volcanic rocks4 ,near the coast there are several varieties of basalt1 some abounding with large crystals of augite and tarnished olivine1 others compact and earthy1--some slightly vesicular1 and others occasionally amygdaloidal4 ,these rocks are generally much decomposed1 and to my surprise1 ,i found in several sections that it was impossible to distinguish1 even approximately1 the line of separation between the decayed lava and the alternating beds of tuff4 ,since the specimens have become dry1 it is rather more easy to distinguish the decomposed igneous rocks from the sedimentary tuffs4 ,this gradation in character between rocks having such widely different origins1 may ,i think be explained by the yielding under pressure of the softened sides of the vesicular cavities1 which in many volcanic rocks occupy a large proportion of their bulk4 ,as the vesicles generally increase in size and number in the upper parts of a stream of lava1 so would the effects of their compression increase2 the yielding1 moreover1 of each lower vesicle must tend to disturb all the softened matter above it4 ,hence we might expect to trace a perfect gradation from an unaltered crystalline rock to one in which all the particles 7although originally forming part of the same solid mass7 had undergone mechanical displacement2 and such particles could hardly be distinguished from others of similar composition1 which had been deposited as sediment4 ,as lavas are sometimes laminated in their upper parts even horizontal lines1 appearing like those of aqueous deposition1 could not in all cases be relied on as a criterion of sedimentary origin4 ,from these considerations it is not surprising that formerly many geologists believed in real transitions from aqueous deposits1 through wacke1 into igneous traps4 ,in the valley of ,tia-auru1 the commonest rocks are basalts with much olivine1 and in some cases almost composed of large crystals of augite4 ,i picked up some specimens1 with much glassy feldspar1 approaching in character to trachyte4 ,there were also many large blocks of vesicular basalt1 with the cavities beautifully lined with chabasie 7871 and radiating bundles of mesotype4 ,some of these specimens presented a curious appearance1 owing to a number of the vesicles being half filled up with a white1 soft1 earthy mesotypic mineral1 which intumesced under the blowpipe in a remarkable manner4 ,as the upper surfaces in all the half-filled cells are exactly parallel1 it is evident that this substance has sunk to the bottom of each cell from its weight4 ,sometimes1 however1 it entirely fills the cells4 ,other cells are either quite filled1 or lined1 with small crystals1 apparently of chabasie2 these crystals1 also1 frequently line the upper half of the cells partly filled with the earthy mineral1 as well as the upper surface of this substance itself1 in which case the two minerals appear to blend into each other4 ,i have never seen any other amygdaloid with the cells half filled in the manner here described2 and it is difficult to imagine the causes which determined the earthy mineral to sink from its gravity to the bottom of the cells1 and the crystalline mineral to adhere in a coating of equal thickness round the sides of the cells4 7,mac,culloch1 however1 has described and given a plate of 78,geolog4 ,trans40 #ast series volume #d page #bbe7 a trap rock1 with cavities filled up horizontally with quartz and chalcedony4 ,the upper halves of these cavities are often filled by layers1 which follow each irregularity of the surface1 and by little depending stalactites of the same siliceous substances47 ,the basic strata on the sides of the valley are gently inclined seaward1 and ,i nowhere observed any sign of disturbance2 the strata are separated from each other by thick1 compact beds of conglomerate1 in which the fragments are large1 some being rounded1 but most angular4 ,from the character of these beds1 from the compact and crystalline condition of most of the lavas1 and from the nature of the infiltrated minerals1 ,i was led to conjecture that they had originally flowed beneath the sea4 ,this conclusion agrees with the fact that the ,rev4 ,w4 ,ellis found marine remains at a considerable height1 which he believes were interstratified with volcanic matter2 as is likewise described to be the case by ,messrs4 ,tyerman and ,bennett at ,huaheine1 an island in this same archipelago4 ,mr4 ,stutchbury also discovered near the summit of one of the loftiest mountains of ,tahiti1 at the height of several thousand feet1 a stratum of semi-fossil coral4 ,none of these remains have been specifically examined4 ,on the coast1 where masses of coral-rock would have afforded the clearest evidence1 ,i looked in vain for any signs of recent elevation4 ,for references to the above authorities1 and for more detailed reasons for not believing that ,tahiti has been recently elevated1 ,i must refer to the 8,structure and ,distribution of ,coral-,reefs40 ,,mauritius4 ,approaching this island on the northern or north-western side1 a curved chain of bold mountains1 surmounted by rugged pinnacles1 is seen to rise from a smooth border of cultivated land1 which gently slopes down to the coast4 ,at the first glance1 one is tempted to believe that the sea lately reached the base of these mountains1 and upon examination1 this view1 at least with respect to the inferior parts of the border1 is found to be perfectly correct4 ,several authors have described masses of upraised coral- rock round the greater part of the circumference of the island4 7,captain ,carmichael1 in ,hooker's 8,bot4 ,misc40 volume #b page #cja4 ,captain ,lloyd has lately1 in the 8,proceedings of the ,geological ,society0 7volume #c page #cag71 described carefully some of these masses4 ,in the 8,voyage a l',isle de ,france1 par un ,officier du ,roi10 many interesting facts are given on this subject4 ,consult also 8,voyage aux ,quatre ,isles d',afrique1 par ,m4 ,bory ,st4 ,vincent407 ,between ,tamarin ,bay and the ,great ,black ,river ,i observed1 in company with ,captain ,lloyd1 two hillocks of coral-rock1 formed in their lower part of hard calcareous sandstone1 and in their upper of great blocks1 slightly aggregated1 of ,astraea and ,madrepora1 and of fragments of basalt2 they were divided into beds dipping seaward1 in one case at an angle of #h degrees1 and in the other at #ah degrees2 they had a water-worn appearance1 and they rose abruptly from a smooth surface1 strewed with rolled debris of organic remains1 to a height of about twenty feet4 ,the ,officier du ,roi1 in his most interesting tour in #agfh round the island1 has described masses of upraised coral-rocks1 still retaining that moat-like structure 7see my 8,coral ,reefs07 which is characteristic of the living reefs4 ,on the coast northward of ,port ,louis1 ,i found the lava concealed for a considerable space inland by a conglomerate of corals and shells1 like those on the beach1 but in parts consolidated by red ferruginous matter4 ,m4 ,bory ,st4 ,vincent has described similar calcareous beds over nearly the whole of the plain of ,pamplemousses4 ,near ,port ,louis1 when turning over some large stones1 which lay in the bed of a stream at the head of a protected creek1 and at the height of some yards above the level of spring tides1 ,i found several shells of serpula still adhering to their under sides4 ,the jagged mountains near ,port ,louis rise to a height of between two and three thousand feet2 they consist of strata of basalt1 obscurely separated from each other by firmly aggregated beds of fragmentary matter2 and they are intersected by a few vertical dikes4 ,the basalt in some parts abounds with large crystals of augite and olivine1 and is generally compact4 ,the interior of the island forms a plain1 raised probably about a thousand feet above the level of the sea1 and composed of streams of lava which have flowed round and between the rugged basaltic mountains4 ,these more recent lavas are also basaltic1 but less compact1 and some of them abound with feldspar1 so that they even fuse into a pale coloured glass4 ,on the banks of the ,great ,river1 a section is exposed nearly five hundred feet deep1 worn through numerous thin sheets of the lava of this series1 which are separated from each other by beds of scoriae4 ,they seem to have been of subaerial formation1 and to have flowed from several points of eruption on the central platform1 of which the ,piton du ,milieu is said to be the principal one4 ,there are also several volcanic cones1 apparently of this modern period1 round the circumference of the island1 especially at the northern end1 where they form separate islets4 ,the mountains composed of the more compact and crystalline basalt1 form the main skeleton of the island4 ,m4 ,bailly 78,voyage aux ,terres ,australes0 tome #a page #ed47 states that they all 8se developpent autour d'elle comme une ceinture d'immenses remparts1 toutes affectant une pente plus ou moins enclinee vers le rivage de la mer2 tandis1 au contraire1 que vers le centre de l'ile elles presentent une coupe abrupte1 et souvent taillee a pic4 ,toutes ces montagnes sont formees de couches paralleles inclinees du centre de l'ile vers la mer40 ,these statements have been disputed1 though not in detail1 by ,m4 ,quoy1 in the voyage of ,freycinet4 ,as far as my limited means of observation went1 ,i found them perfectly correct4 7,m4 ,lesson1 in his account of this island1 in the 8,voyage of the #8,coquille'10 seems to follow ,m4 ,bailly's views47 ,the mountains on the ,n4,w4 side of the island1 which ,i examined1 namely1 ,la ,pouce1 ,peter ,botts1 ,corps de ,garde1 ,les ,mamelles1 and apparently another farther southward1 have precisely the external shape and stratification described by ,m4 ,bailly4 ,they form about a quarter of his girdle of ramparts4 ,although these mountains now stand quite detached1 being separated from each other by breaches1 even several miles in width1 through which deluges of lava have flowed from the interior of the island2 nevertheless1 seeing their close general similarity1 one must feel convinced that they originally formed parts of one continuous mass4 ,judging from the beautiful map of the ,mauritius1 published by the ,admiralty from a ,french ,,ms41 there is a range of mountains 7,m4 ,bamboo7 on the opposite side of the island1 which correspond in height1 relative position1 and external form1 with those just described4 ,whether the girdle was ever complete may well be doubted2 but from ,m4 ,bailly's statements1 and my own observations1 it may be safely concluded that mountains with precipitous inland flanks1 and composed of strata dipping outwards1 once extended round a considerable portion of the circumference of the island4 ,the ring appears to have been oval and of vast size2 its shorter axis1 measured across from the inner sides of the mountains near ,port ,louis and those near ,grand ,port1 being no less than thirteen geographical miles in length4 ,m4 ,bailly boldly supposes that this enormous gulf1 which has since been filled up to a great extent by streams of modern lava1 was formed by the sinking in of the whole upper part of one great volcano4 ,it is singular in how many respects those portions of ,st4 ,jago and of ,mauritius which ,i visited agree in their geological history4 ,at both islands1 mountains of similar external form1 stratification1 and 7at least in their upper beds7 composition1 follow in a curved chain the coast-line4 ,these mountains in each case appear originally to have formed parts of one continuous mass4 ,the basaltic strata of which they are composed1 from their compact and crystalline structure1 seem1 when contrasted with the neighbouring basaltic streams of subaerial formation1 to have flowed beneath the pressure of the sea1 and to have been subsequently elevated4 ,we may suppose that the wide breaches between the mountains were in both cases worn by the waves1 during their gradual elevation--of which process1 within recent times1 there is abundant evidence on the coast-land of both islands4 ,at both1 vast streams of more recent basaltic lavas have flowed from the interior of the island1 round and between the ancient basaltic hills2 at both1 moreover1 recent cones of eruption are scattered around the circumference of the island2 but at neither have eruptions taken place within the period of history4 ,as remarked in the last chapter1 it is probable that these ancient basaltic mountains1 which resemble 7at least in many respects7 the basal and disturbed remnants of two gigantic volcanoes1 owe their present form1 structure1 and position1 to the action of similar causes4 ,,st4 ,,paul',s ,,rocks4 ,this small island is situated in the ,atlantic ,ocean1 nearly one degree north of the equator1 and #edj miles distant from ,south ,america1 in #bi degrees #ae minutes west longitude4 ,its highest point is scarcely fifty feet above the level of the sea2 its outline is irregular1 and its entire circumference barely three-quarters of a mile4 ,this little point of rock rises abruptly out of the ocean2 and1 except on its western side1 soundings were not obtained1 even at the short distance of a quarter of a mile from its shore4 ,it is not of volcanic origin2 and this circumstance1 which is the most remarkable point in its history 7as will hereafter be referred to71 properly ought to exclude it from the present volume4 ,it is composed of rocks1 unlike any which ,i have met with1 and which ,i cannot characterise by any name1 and must therefore describe4 ,the simplest1 and one of the most abundant kinds1 is a very compact1 heavy1 greenish-black rock1 having an angular1 irregular fracture1 with some points just hard enough to scratch glass1 and infusible4 ,this variety passes into others of paler green tints1 less hard1 but with a more crystalline fracture1 and translucent on their edges2 and these are fusible into a green enamel4 ,several other varieties are chiefly characterised by containing innumerable threads of dark-green serpentine1 and by having calcareous matter in their interstices4 ,these rocks have an obscure1 concretionary structure1 and are full of variously coloured angular pseudo fragments4 ,these angular pseudo fragments consist of the first-described dark green rock1 of a brown softer kind1 of serpentine1 and of a yellowish harsh stone1 which1 perhaps1 is related to serpentine rock4 ,there are other vesicular1 calcareo-ferruginous1 soft stones4 ,there is no distinct stratification1 but parts are imperfectly laminated2 and the whole abounds with innumerable veins1 and vein-like masses1 both small and large4 ,of these vein-like masses1 some calcareous ones1 which contain minute fragments of shells1 are clearly of subsequent origin to the others4 ,a ,,glossy ,,incrustation4 ,extensive portions of these rocks are coated by a layer of a glossy polished substance1 with a pearly lustre and of a greyish white colour2 it follows all the inequalities of the surface1 to which it is firmly attached4 ,when examined with a lens1 it is found to consist of numerous exceedingly thin layers1 their aggregate thickness being about the tenth of an inch4 ,it is considerably harder than calcareous spar1 but can be scratched with a knife2 under the blowpipe it scales off1 decrepitates1 slightly blackens1 emits a fetid odour1 and becomes strongly alkaline3 it does not effervesce in acids4 7,in my 8,journal0 ,i have described this substance2 ,i then believed that it was an impure phosphate of lime47 ,i presume this substance has been deposited by water draining from the birds' dung1 with which the rocks are covered4 ,at ,ascension1 near a cavity in the rocks which was filled with a laminated mass of infiltrated birds' dung1 ,i found some irregularly formed1 stalactitical masses of apparently the same nature4 ,these masses1 when broken1 had an earthy texture2 but on their outsides1 and especially at their extremities1 they were formed of a pearly substance1 generally in little globules1 like the enamel of teeth1 but more translucent1 and so hard as just to scratch plate-glass4 ,this substance slightly blackens under the blowpipe1 emits a bad smell1 then becomes quite white1 swelling a little1 and fuses into a dull white enamel2 it does not become alkaline2 nor does it effervesce in acids4 ,the whole mass had a collapsed appearance1 as if in the formation of the hard glossy crust the whole had shrunk much4 ,at the ,abrolhos ,islands on the coast of ,brazil1 where also there is much birds' dung1 ,i found a great quantity of a brown1 arborescent substance adhering to some trap-rock4 ,in its arborescent form1 this substance singularly resembles some of the branched species of ,nullipora4 ,under the blowpipe1 it behaves like the specimens from ,ascension2 but it is less hard and glossy1 and the surface has not the shrunk appearance4 ,,chapter ,,iii4 ,,ascension4 ,basaltic lavas4 ,numerous craters truncated on the same side4 ,singular structure of volcanic bombs4 ,aeriform explosions4 ,ejected granitic fragments4 ,trachytic rocks4 ,singular veins4 ,jasper1 its manner of formation4 ,concretions in pumiceous tuff4 ,calcareous deposits and frondescent incrustations on the coast4 ,remarkable laminated beds1 alternating with1 and passing into1 obsidian4 ,origin of obsidian4 ,lamination of volcanic rocks4 7,,map #b3 ,,the ,,island ,,of ,,ascension47 ,this island is situated in the ,atlantic ,ocean1 in latitude #h degrees ,s41 longitude #ad degrees ,w4 ,it has the form of an irregular triangle 7see ,map #b71 each side being about six miles in length4 ,its highest point is #b1hgj feet 78,geographical ,journal0 volume #e page #bdc47 above the level of the sea4 ,the whole is volcanic1 and1 from the absence of proofs to the contrary1 ,i believe of subaerial origin4 ,the fundamental rock is everywhere of a pale colour1 generally compact1 and of a feldspathic nature4 ,in the ,s4,e4 portion of the island1 where the highest land is situated1 well characterised trachyte1 and other congenerous rocks of that varying family1 occur4 ,nearly the entire circumference is covered up by black and rugged streams of basaltic lava1 with here and there a hill or single point of rock 7one of which near the sea-coast1 north of the ,fort1 is only two or three yards across7 of the trachyte still remaining exposed4 ,,basaltic ,,rocks4 ,the overlying basaltic lava is in some parts extremely vesicular1 in others little so2 it is of a black colour1 but sometimes contains crystals of glassy feldspar1 and seldom much olivine4 ,these streams appear to have possessed singularly little fluidity2 their side walls and lower ends being very steep1 and even as much as between twenty and thirty feet in height4 ,their surface is extraordinarily rugged1 and from a short distance appears as if studded with small craters4 ,these projections consist of broad1 irregularly conical1 hillocks1 traversed by fissures1 and composed of the same unequally scoriaceous basalt with the surrounding streams1 but having an obscure tendency to a columnar structure2 they rise to a height between ten and thirty feet above the general surface1 and have been formed1 as ,i presume1 by the heaping up of the viscid lava at points of greater resistance4 ,at the base of several of these hillocks1 and occasionally likewise on more level parts1 solid ribs1 composed of angulo-globular masses of basalt1 resembling in size and outline arched sewers or gutters of brickwork1 but not being hollow1 project between two or three feet above the surface of the streams2 what their origin may have been1 ,i do not know4 ,many of the superficial fragments from these basaltic streams present singularly convoluted forms2 and some specimens could hardly be distinguished from logs of dark-coloured wood without their bark4 ,many of the basaltic streams can be traced1 either to points of eruption at the base of the great central mass of trachyte1 or to separate1 conical1 red-coloured hills1 which are scattered over the northern and western borders of the island4 ,standing on the central eminence1 ,i counted between twenty and thirty of these cones of eruption4 ,the greater number of them had their truncated summits cut off obliquely1 and they all sloped towards the ,s4,e41 whence the trade-wind blows4 7,m4 ,lesson in the 8,zoology of the ,voyage of the #8,coquille'0 page #dij has observed this fact4 ,mr4 ,hennah 78,geolog4 ,proceedings0 #ahce page #ahi7 further remarks that the most extensive beds of ashes at ,ascension invariably occur on the leeward side of the island47 ,this structure no doubt has been caused by the ejected fragments and ashes being always blown1 during eruptions1 in greater quantity towards one side than towards the other4 ,m4 ,moreau de ,jonnes has made a similar observation with respect to the volcanic orifices in the ,west ,indian ,islands4 ,,volcanic ,,bombs4 7,,figure #c3 ,,fragment ,,of ,a ,,spherical ,,volcanic ,,bomb1 with the interior parts coarsely cellular1 coated by a concentric layer of compact lava1 and this again by a crust of finely cellular rock4 ,,figure #d3 ,,volcanic ,,bomb ,,of ,,obsidian ,,from ,,australia4 ,the upper figure gives a front view2 the lower a side view of the same object47 ,these occur in great numbers strewed on the ground1 and some of them lie at considerable distances from any points of eruption4 ,they vary in size from that of an apple to that of a man's body2 they are either spherical or pear-shaped1 or with the hinder part 7corresponding to the tail of a comet7 irregular1 studded with projecting points1 and even concave4 ,their surfaces are rough1 and fissured with branching cracks2 their internal structure is either irregularly scoriaceous and compact1 or it presents a symmetrical and very curious appearance4 ,an irregular segment of a bomb of this latter kind1 of which ,i found several1 is accurately represented in ,figure #c4 ,its size was about that of a man's head4 ,the whole interior is coarsely cellular2 the cells averaging in diameter about the tenth of an inch2 but nearer the outside they gradually decrease in size4 ,this part is succeeded by a well-defined shell of compact lava1 having a nearly uniform thickness of about the third of an inch2 and the shell is overlaid by a somewhat thicker coating of finely cellular lava 7the cells varying from the fiftieth to the hundredth of an inch in diameter71 which forms the external surface3 the line separating the shell of compact lava from the outer scoriaceous crust is distinctly defined4 ,this structure is very simply explained1 if we suppose a mass of viscid1 scoriaceous matter1 to be projected with a rapid1 rotatory motion through the air2 for whilst the external crust1 from cooling1 became solidified 7in the state we now see it71 the centrifugal force1 by relieving the pressure in the interior parts of the bomb1 would allow the heated vapours to expand their cells2 but these being driven by the same force against the already-hardened crust1 would become1 the nearer they were to this part1 smaller and smaller or less expanded1 until they became packed into a solid1 concentric shell4 ,as we know that chips from a grindstone 7,nichol 8,architecture of the ,heavens407 can be flirted off1 when made to revolve with sufficient velocity1 we need not doubt that the centrifugal force would have power to modify the structure of a softened bomb1 in the manner here supposed4 ,geologists have remarked1 that the external form of a bomb at once bespeaks the history of its aerial course1 and few now see that the internal structure can speak1 with almost equal plainness1 of its rotatory movement4 ,m4 ,bory ,st4 ,vincent 78,voyage aux ,quatre ,isles d',afrique0 tome #a page #bbb47 has described some balls of lava from the ,isle of ,bourbon1 which have a closely similar structure4 ,his explanation1 however 7if ,i understand it rightly71 is very different from that which ,i have given2 for he supposes that they have rolled1 like snowballs1 down the sides of the crater4 ,m4 ,beudant 78,voyage en ,hongrie0 tome #b page #bad471 also1 has described some singular little balls of obsidian1 never more than six or eight inches in diameter1 which he found strewed on the surface of the ground3 their form is always oval2 sometimes they are much swollen in the middle1 and even spindle-shaped3 their surface is regularly marked with concentric ridges and furrows1 all of which on the same ball are at right angles to one axis3 their interior is compact and glassy4 ,m4 ,beudant supposes that masses of lava1 when soft1 were shot into the air1 with a rotatory movement round the same axis1 and that the form and superficial ridges of the bombs were thus produced4 ,sir ,thomas ,mitchell has given me what at first appears to be the half of a much flattened oval ball of obsidian2 it has a singular artificial-like appearance1 which is well represented 7of the natural size7 in ,figure #d4 ,it was found in its present state1 on a great sandy plain between the rivers ,darling and ,murray1 in ,australia1 and at the distance of several hundred miles from any known volcanic region4 ,it seems to have been embedded in some reddish tufaceous matter2 and may have been transported either by the aborigines or by natural means4 ,the external saucer consists of compact obsidian1 of a bottle-green colour1 and is filled with finely cellular black lava1 much less transparent and glassy than the obsidian4 ,the external surface is marked with four or five not quite perfect ridges1 which are represented rather too distinctly in ,figure #d4 ,here1 then1 we have the external structure described by ,m4 ,beudant1 and the internal cellular condition of the bombs from ,ascension4 ,the lip of the saucer is slightly concave1 exactly like the margin of a soup-plate1 and its inner edge overlaps a little the central cellular lava4 ,this structure is so symmetrical round the entire circumference1 that one is forced to suppose that the bomb burst during its rotatory course1 before being quite solidified1 and that the lip and edges were thus slightly modified and turned inwards4 ,it may be remarked that the superficial ridges are in planes1 at right angles to an axis1 transverse to the longer axis of the flattened oval3 to explain this circumstance1 we may suppose that when the bomb burst1 the axis of rotation changed4 ,,aeriform ,,explosions4 ,the flanks of ,green ,mountain and the surrounding country are covered by a great mass1 some hundred feet in thickness1 of loose fragments4 ,the lower beds generally consist of fine-grained1 slightly consolidated tuffs 7,some of this peperino1 or tuff1 is sufficiently hard not to be broken by the greatest force of the fingers471 and the upper beds of great loose fragments1 with alternating finer beds4 7,on the northern side of the ,green ,mountain a thin seam1 about an inch in thickness1 of compact oxide of iron1 extends over a considerable area2 it lies conformably in the lower part of the stratified mass of ashes and fragments4 ,this substance is of a reddish- brown colour1 with an almost metallic lustre2 it is not magnetic1 but becomes so after having been heated under the blowpipe1 by which it is blackened and partly fused4 ,this seam of compact stone1 by intercepting the little rain-water which falls on the island1 gives rise to a small dripping spring1 first discovered by ,dampier4 ,it is the only fresh water on the island1 so that the possibility of its being inhabited has entirely depended on the occurrence of this ferruginous layer47 ,one white ribbon- like layer of decomposed1 pumiceous breccia1 was curiously bent into deep unbroken curves1 beneath each of the large fragments in the superincumbent stratum4 ,from the relative position of these beds1 ,i presume that a narrow- mouthed crater1 standing nearly in the position of ,green ,mountain1 like a great air-gun1 shot forth1 before its final extinction1 this vast accumulation of loose matter4 ,subsequently to this event1 considerable dislocations have taken place1 and an oval circus has been formed by subsidence4 ,this sunken space lies at the north-eastern foot of ,green ,mountain1 and is well represented in ,map #b4 ,its longer axis1 which is connected with a ,n4,e4 and ,s4,w4 line of fissure1 is three-fifths of a nautical mile in length2 its sides are nearly perpendicular1 except in one spot1 and about four hundred feet in height2 they consist1 in the lower part1 of a pale basalt with feldspar1 and in the upper part1 of the tuff and loose ejected fragments2 the bottom is smooth and level1 and under almost any other climate a deep lake would have been formed here4 ,from the thickness of the bed of loose fragments1 with which the surrounding country is covered1 the amount of aeriform matter necessary for their projection must have been enormous2 hence we may suppose it probable that after the explosions vast subterranean caverns were left1 and that the falling in of the roof of one of these produced the hollow here described4 ,at the ,galapagos ,archipelago1 pits of a similar character1 but of a much smaller size1 frequently occur at the bases of small cones of eruption4 ,,ejected ,,granitic ,,fragments4 ,in the neighbourhood of ,green ,mountain1 fragments of extraneous rock are not unfrequently found embedded in the midst of masses of scoriae4 ,lieutenant ,evans1 to whose kindness ,i am indebted for much information1 gave me several specimens1 and ,i found others myself4 ,they nearly all have a granitic structure1 are brittle1 harsh to the touch1 and apparently of altered colours4 ,,first1 a white syenite1 streaked and mottled with red2 it consists of well- crystallised feldspar1 numerous grains of quartz1 and brilliant1 though small1 crystals of hornblende4 ,the feldspar and hornblende in this and the succeeding cases have been determined by the reflecting goniometer1 and the quartz by its action under the blowpipe4 ,the feldspar in these ejected fragments1 like the glassy kind in the trachyte1 is from its cleavage a potash-feldspar4 ,,secondly1 a brick-red mass of feldspar1 quartz1 and small dark patches of a decayed mineral2 one minute particle of which ,i was able to ascertain1 by its cleavage1 to be hornblende4 ,,thirdly1 a mass of confusedly crystallised white feldspar1 with little nests of a dark-coloured mineral1 often carious1 externally rounded1 having a glossy fracture1 but no distinct cleavage3 from comparison with the second specimen1 ,i have no doubt that it is fused hornblende4 ,,fourthly1 a rock1 which at first appears a simple aggregation of distinct and large-sized crystals of dusty-coloured ,labrador feldspar 7,professor ,miller has been so kind as to examine this mineral4 ,he obtained two good cleavages of #hf degrees #cj minutes and #hf degrees #ej minutes4 ,the mean of several1 which ,i made1 was #hf degrees #cj minutes4 ,professor ,miller states that these crystals1 when reduced to a fine powder1 are soluble in hydrochloric acid1 leaving some undissolved silex behind2 the addition of oxalate of ammonia gives a copious precipitate of lime4 ,he further remarks1 that according to ,von ,kobell1 anorthite 7a mineral occurring in the ejected fragments at ,mount ,somma7 is always white and transparent1 so that if this be the case1 these crystals from ,ascension must be considered as ,labrador feldspar4 ,professor ,miller adds1 that he has seen an account1 in ,erdmann's 8,journal fur tecnische ,chemie10 of a mineral ejected from a volcano which had the external characters of ,labrador feldspar1 but differed in the analysis from that given by mineralogists of this mineral3 the author attributed this difference to an error in the analysis of ,labrador feldspar1 which is very old472 but in their interstices there is some white granular feldspar1 abundant scales of mica1 a little altered hornblende1 and1 as ,i believe1 no quartz4 ,i have described these fragments in detail1 because it is rare to find granitic rocks ejected from volcanoes with their ,,minerals ,,unchanged1 as is the case with the first specimen1 and partially with the second4 7,daubeny1 in his work on ,volcanoes page #chf1 remarks that this is the case2 and ,humboldt1 in his 8,personal ,narrative0 volume #a page #bcf1 says 8,in general1 the masses of known primitive rocks1 ,i mean those which perfectly resemble our granites1 gneiss1 and mica-slate1 are very rare in lavas3 the substances we generally denote by the name of granite1 thrown out by ,vesuvius1 are mixtures of nepheline1 mica1 and pyroxene407 ,one other large fragment1 found in another spot1 is deserving of notice2 it is a conglomerate1 containing small fragments of granitic1 cellular1 and jaspery rocks1 and of hornstone porphyries1 embedded in a base of wacke1 threaded by numerous thin layers of a concretionary pitchstone passing into obsidian4 ,these layers are parallel1 slightly tortuous1 and short2 they thin out at their ends1 and resemble in form the layers of quartz in gneiss4 ,it is probable that these small embedded fragments were not separately ejected1 but were entangled in a fluid volcanic rock1 allied to obsidian2 and we shall presently see that several varieties of this latter series of rock assume a laminated structure4 ,,trachytic ,,series ,,of ,,rocks4 ,those occupy the more elevated and central1 and likewise the south-eastern1 parts of the island4 ,the trachyte is generally of a pale brown colour1 stained with small darker patches2 it contains broken and bent crystals of glassy feldspar1 grains of specular iron1 and black microscopical points1 which latter1 from being easily fused1 and then becoming magnetic1 ,i presume are hornblende4 ,the greater number of the hills1 however1 are composed of a quite white1 friable stone1 appearing like a trachytic tuff4 ,obsidian1 hornstone1 and several kinds of laminated feldspathic rocks1 are associated with the trachyte4 ,there is no distinct stratification2 nor could ,i distinguish a crateriform structure in any of the hills of this series4 ,considerable dislocations have taken place2 and many fissures in these rocks are yet left open1 or are only partially filled with loose fragments4 ,within the space 7,this space is nearly included by a line sweeping round ,green ,mountain1 and joining the hills1 called the ,weather ,port ,signal1 ,holyhead1 and that denominated 7improperly in a geological sense7 8the ,crater of an old volcano4071 mainly formed of trachyte1 some basaltic streams have burst forth2 and not far from the summit of ,green ,mountain1 there is one stream of quite black1 vesicular basalt1 containing minute crystals of glassy feldspar1 which have a rounded appearance4 ,the soft white stone above mentioned is remarkable from its singular resemblance1 when viewed in mass1 to a sedimentary tuff3 it was long before ,i could persuade myself that such was not its origin2 and other geologists have been perplexed by closely similar formations in trachytic regions4 ,in two cases1 this white earthy stone formed isolated hills2 in a third1 it was associated with columnar and laminated trachyte2 but ,i was unable to trace an actual junction4 ,it contains numerous crystals of glassy feldspar and black microscopical specks1 and is marked with small darker patches1 exactly as in the surrounding trachyte4 ,its basis1 however1 when viewed under the microscope1 is generally quite earthy2 but sometimes it exhibits a decidedly crystalline structure4 ,on the hill marked 8,crater of an old volcano10 it passes into a pale greenish-grey variety1 differing only in its colour1 and in not being so earthy2 the passage was in one case effected insensibly2 in another1 it was formed by numerous1 rounded and angular1 masses of the greenish variety1 being embedded in the white variety2--in this latter case1 the appearance was very much like that of a sedimentary deposit1 torn up and abraded during the deposition of a subsequent stratum4 ,both these varieties are traversed by innumerable tortuous veins 7presently to be described71 which are totally unlike injected dikes1 or indeed any other veins which ,i have ever seen4 ,both varieties include a few scattered fragments1 large and small1 of dark- coloured scoriaceous rocks1 the cells of some of which are partially filled with the white earthy stone2 they likewise include some huge blocks of a cellular porphyry4 7,the porphyry is dark coloured2 it contains numerous1 often fractured1 crystals of white opaque feldspar1 also decomposing crystals of oxide of iron2 its vesicles include masses of delicate1 hair- like1 crystals1 apparently of analcime47 ,these fragments project from the weathered surface1 and perfectly resemble fragments embedded in a true sedimentary tuff4 ,but as it is known that extraneous fragments of cellular rock are sometimes included in columnar trachyte1 in phonolite 7,d',aubuisson 8,traite de ,geognosie0 tome #b page #edh471 and in other compact lavas1 this circumstance is not any real argument for the sedimentary origin of the white earthy stone4 7,dr4 ,daubeny on ,volcanoes1 page #ahj seems to have been led to believe that certain trachytic formations of ,ischia and of the ,puy de ,dome1 which closely resemble these of ,ascension1 were of sedimentary origin1 chiefly from the frequent presence in them 8of scoriform portions1 different in colour from the matrix40 ,dr4 ,daubeny adds1 that on the other hand1 ,brocchi1 and other eminent geologists1 have considered these beds as earthy varieties of trachyte2 he considers the subject deserving of further attention47 ,the insensible passage of the greenish variety into the white one1 and likewise the more abrupt passage by fragments of the former being embedded in the latter1 might result from slight differences in the composition of the same mass of molten stone1 and from the abrading action of one such part still fluid on another part already solidified4 ,the curiously formed veins have1 ,i believe1 been formed by siliceous matter being subsequently segregated4 ,but my chief reason for believing that these soft earthy stones1 with their extraneous fragments1 are not of sedimentary origin1 is the extreme improbability of crystals of feldspar1 black microscopical specks1 and small stains of a darker colour occurring in the same proportional numbers in an aqueous deposit1 and in masses of solid trachyte4 ,moreover1 as ,i have remarked1 the microscope occasionally reveals a crystalline structure in the apparently earthy basis4 ,on the other hand1 the partial decomposition of such great masses of trachyte1 forming whole mountains1 is undoubtedly a circumstance of not easy explanation4 ,,veins ,,in ,,the ,,earthy ,,trachytic ,,masses4 ,these veins are extraordinarily numerous1 intersecting in the most complicated manner both coloured varieties of the earthy trachyte3 they are best seen on the flanks of the 8,crater of the old volcano40 ,they contain crystals of glassy feldspar1 black microscopical specks and little dark stains1 precisely as in the surrounding rock2 but the basis is very different1 being exceedingly hard1 compact1 somewhat brittle1 and of rather less easy fusibility4 ,the veins vary much1 and suddenly1 from the tenth of an inch to one inch in thickness2 they often thin out1 not only on their edges1 but in their central parts1 thus leaving round1 irregular apertures2 their surfaces are rugged4 ,they are inclined at every possible angle with the horizon1 or are horizontal2 they are generally curvilinear1 and often interbranch one with another4 ,from their hardness they withstand weathering1 and projecting two or three feet above the ground1 they occasionally extend some yards in length2 these plate-like veins1 when struck1 emit a sound1 almost like that of a drum1 and they may be distinctly seen to vibrate2 their fragments1 which are strewed on the ground1 clatter like pieces of iron when knocked against each other4 ,they often assume the most singular forms2 ,i saw a pedestal of the earthy trachyte1 covered by a hemispherical portion of a vein1 like a great umbrella1 sufficiently large to shelter two persons4 ,i have never met with1 or seen described1 any veins like these2 but in form they resemble the ferruginous seams1 due to some process of segregation1 occurring not uncommonly in sandstones1--for instance1 in the ,new ,red sandstone of ,england4 ,numerous veins of jasper and of siliceous sinter1 occurring on the summit of this same hill1 show that there has been some abundant source of silica1 and as these plate-like veins differ from the trachyte only in their greater hardness1 brittleness1 and less easy fusibility1 it appears probable that their origin is due to the segregation or infiltration of siliceous matter1 in the same manner as happens with the oxides of iron in many sedimentary rocks4 ,,siliceous ,,sinter ,,and ,,jasper4 ,the siliceous sinter is either quite white1 of little specific gravity1 and with a somewhat pearly fracture1 passing into pinkish pearl quartz2 or it is yellowish white1 with a harsh fracture1 and it then contains an earthy powder in small cavities4 ,both varieties occur1 either in large irregular masses in the altered trachyte1 or in seams included in broad1 vertical1 tortuous1 irregular veins of a compact1 harsh stone of a dull red colour1 appearing like a sandstone4 ,this stone1 however1 is only altered trachyte2 and a nearly similar variety1 but often honeycombed1 sometimes adheres to the projecting plate-like veins1 described in the last paragraph4 ,the jasper is of an ochre yellow or red colour2 it occurs in large irregular masses1 and sometimes in veins1 both in the altered trachyte and in an associated mass of scoriaceous basalt4 ,the cells of the scoriaceous basalt are lined or filled with fine1 concentric layers of chalcedony1 coated and studded with bright-red oxide of iron4 ,in this rock1 especially in the rather more compact parts1 irregular angular patches of the red jasper are included1 the edges of which insensibly blend into the surrounding mass2 other patches occur having an intermediate character between perfect jasper and the ferruginous1 decomposed1 basaltic base4 ,in these patches1 and likewise in the large vein-like masses of jasper1 there occur little rounded cavities1 of exactly the same size and form with the air-cells1 which in the scoriaceous basalt are filled and lined with layers of chalcedony4 ,small fragments of the jasper1 examined under the microscope1 seem to resemble the chalcedony with its colouring matter not separated into layers1 but mingled in the siliceous paste1 together with some impurities4 ,i can understand these facts1--namely1 the blending of the jasper into the semi-decomposed basalt1--its occurrence in angular patches1 which clearly do not occupy pre-existing hollows in the rock1--and its containing little vesicles filled with chalcedony1 like those in the scoriaceous lava1--only on the supposition that a fluid1 probably the same fluid which deposited the chalcedony in the air-cells1 removed in those parts where there were no cavities1 the ingredients of the basaltic rock1 and left in their place silica and iron1 and thus produced the jasper4 ,in some specimens of silicified wood1 ,i have observed1 that in the same manner as in the basalt1 the solid parts were converted into a dark-coloured homogeneous stone1 whereas the cavities formed by the larger sap-vessels 7which may be compared with the air-vesicles in the basaltic lava7 and other irregular hollows1 apparently produced by decay1 were filled with concentric layers of chalcedony2 in this case1 there can be little doubt that the same fluid deposited the homogeneous base and the chalcedonic layers4 ,after these considerations1 ,i cannot doubt but that the jasper of ,ascension may be viewed as a volcanic rock silicified1 in precisely the same sense as this term is applied to wood1 when silicified2 we are equally ignorant of the means by which every atom of wood1 whilst in a perfect state1 is removed and replaced by atoms of silica1 as we are of the means by which the constituent parts of a volcanic rock could be thus acted on4 7,beudant 8,voyage en ,hongrie0 tome #c pages #ejb1 #ejd describes kidney-shaped masses of jasper-opal1 which either blend into the surrounding trachytic conglomerate1 or are embedded in it like chalk-flints2 and he compares them with the fragments of opalised wood1 which are abundant in this same formation4 ,beudant1 however1 appears to have viewed the process of their formation rather as one of simple infiltration than of molecular exchange2 but the presence of a concretion1 wholly different from the surrounding matter1 if not formed in a pre-existing hollow1 clearly seems to me to require1 either a molecular or mechanical displacement of the atoms1 which occupied the space afterwards filled by it4 ,the jasper-opal of ,hungary passes into chalcedony1 and therefore in this case1 as in that of ,ascension1 jasper seems to be intimately related in origin with chalcedony47 ,i was led to the careful examination of these rocks1 and to the conclusion here given1 from having heard the ,rev4 ,professor ,henslow express a similar opinion1 regarding the origin in trap-rocks of many chalcedonies and agates4 ,siliceous deposits seem to be very general1 if not of universal occurrence1 in partially decomposed trachytic tuffs 7,beudant 8,voyage ,min40 tome #c page #ejg enumerates cases in ,hungary1 ,germany1 ,central ,france1 ,italy1 ,greece1 and ,mexico472 and as these hills1 according to the view above given1 consist of trachyte softened and altered in situ1 the presence of free silica in this case may be added as one more instance to the list4 ,,concretions ,,in ,,pumiceous ,,tuff4 ,the hill1 marked in ,map #b 8,crater of an old volcano10 has no claims to this appellation1 which ,i could discover1 except in being surmounted by a circular1 very shallow1 saucer-like summit1 nearly half a mile in diameter4 ,this hollow has been nearly filled up with many successive sheets of ashes and scoriae1 of different colours1 and slightly consolidated4 ,each successive saucer-shaped layer crops out all round the margin1 forming so many rings of various colours1 and giving to the hill a fantastic appearance4 ,the outer ring is broad1 and of a white colour2 hence it resembles a course round which horses have been exercised1 and has received the name of the ,devil's ,riding ,school1 by which it is most generally known4 ,these successive layers of ashes must have fallen over the whole surrounding country1 but they have all been blown away except in this one hollow1 in which probably moisture accumulated1 either during an extraordinary year when rain fell1 or during the storms often accompanying volcanic eruptions4 ,one of the layers of a pinkish colour1 and chiefly derived from small1 decomposed fragments of pumice1 is remarkable1 from containing numerous concretions4 ,these are generally spherical1 from half an inch to three inches in diameter2 but they are occasionally cylindrical1 like those of iron-pyrites in the chalk of ,europe4 ,they consist of a very tough1 compact1 pale-brown stone1 with a smooth and even fracture4 ,they are divided into concentric layers by thin white partitions1 resembling the external superficies2 six or eight of such layers are distinctly defined near the outside2 but those towards the inside generally become indistinct1 and blend into a homogeneous mass4 ,i presume that these concentric layers were formed by the shrinking of the concretion1 as it became compact4 ,the interior part is generally fissured by minute cracks or septaria1 which are lined1 both by black1 metallic1 and by other white and crystalline specks1 the nature of which ,i was unable to ascertain4 ,some of the larger concretions consist of a mere spherical shell1 filled with slightly consolidated ashes4 ,the concretions contain a small proportion of carbonate of lime3 a fragment placed under the blowpipe decrepitates1 then whitens and fuses into a blebby enamel1 but does not become caustic4 ,the surrounding ashes do not contain any carbonate of lime2 hence the concretions have probably been formed1 as is so often the case1 by the aggregation of this substance4 ,i have not met with any account of similar concretions2 and considering their great toughness and compactness1 their occurrence in a bed1 which probably has been subjected only to atmospheric moisture1 is remarkable4 ,,formation ,,of ,,calcareous ,,rocks ,,on ,,the ,,sea-,,coast4 ,on several of the sea-beaches1 there are immense accumulations of small1 well-rounded particles of shells and corals1 of white1 yellowish1 and pink colours1 interspersed with a few volcanic particles4 ,at the depth of a few feet1 these are found cemented together into stone1 of which the softer varieties are used for building2 there are other varieties1 both coarse and fine-grained1 too hard for this purpose3 and ,i saw one mass divided into even layers half an inch in thickness1 which were so compact that when struck with a hammer they rang like flint4 ,it is believed by the inhabitants1 that the particles become united in the course of a single year4 ,the union is effected by calcareous matter2 and in the most compact varieties1 each rounded particle of shell and volcanic rock can be distinctly seen to be enveloped in a husk of pellucid carbonate of lime4 ,extremely few perfect shells are embedded in these agglutinated masses2 and ,i have examined even a large fragment under a microscope1 without being able to discover the least vestige of striae or other marks of external form3 this shows how long each particle must have been rolled about1 before its turn came to be embedded and cemented4 7,the eggs of the turtle being buried by the parent1 sometimes become enclosed in the solid rock4 ,mr4 ,lyell has given a figure 78,principles of ,geology0 book #c chapter #ag7 of some eggs1 containing the bones of young turtles1 found thus entombed47 ,one of the most compact varieties1 when placed in acid1 was entirely dissolved1 with the exception of some flocculent animal matter2 its specific gravity was #b4fc4 ,the specific gravity of ordinary limestone varies from #b4f to #b4ge2 pure ,carrara marble was found by ,sir ,h4 ,de la ,beche to be #b4g4 78,researches in ,theoretical ,geology0 page #ab47 ,it is remarkable that these rocks of ,ascension1 formed close to the surface1 should be nearly as compact as marble1 which has undergone the action of heat and pressure in the plutonic regions4 ,the great accumulation of loose calcareous particles1 lying on the beach near the ,settlement1 commences in the month of ,october1 moving towards the ,s4,w41 which1 as ,i was informed by ,lieutenant ,evans1 is caused by a change in the prevailing direction of the currents4 ,at this period the tidal rocks1 at the ,s4,w4 end of the beach1 where the calcareous sand is accumulating1 and round which the currents sweep1 become gradually coated with a calcareous incrustation1 half an inch in thickness4 ,it is quite white1 compact1 with some parts slightly spathose1 and is firmly attached to the rock4 ,after a short time it gradually disappears1 being either redissolved1 when the water is less charged with lime1 or more probably is mechanically abraded4 ,lieutenant ,evans has observed these facts1 during the six years he has resided at ,ascension4 ,the incrustation varies in thickness in different years3 in #ahca it was unusually thick4 ,when ,i was there in ,july1 there was no remnant of the incrustation2 but on a point of basalt1 from which the quarrymen had lately removed a mass of the calcareous freestone1 the incrustation was perfectly preserved4 ,considering the position of the tidal-rocks1 and the period at which they become coated1 there can be no doubt that the movement and disturbance of the vast accumulation of calcareous particles1 many of them being partially agglutinated together1 cause the waves of the sea to be so highly charged with carbonate of lime1 that they deposit it on the first objects against which they impinge4 ,i have been informed by ,lieutenant ,holland1 ,r4,n41 that this incrustation is formed on many parts of the coast1 on most of which1 ,i believe1 there are likewise great masses of comminuted shells4 ,a ,,frondescent ,,calcareous ,,incrustation4 7,,figure #e4 ,,an ,,incrustation ,,of ,,calcareous ,,and ,,animal ,,matter1 coating the tidal-rocks at ,ascension47 ,in many respects this is a singular deposit2 it coats throughout the year the tidal volcanic rocks1 that project from the beaches composed of broken shells4 ,its general appearance is well represented in ,figure #e2 but the fronds or discs1 of which it is composed1 are generally so closely crowded together as to touch4 ,these fronds have their sinuous edges finely crenulated1 and they project over their pedestals or supports2 their upper surfaces are either slightly concave1 or slightly convex2 they are highly polished1 and of a dark grey or jet black colour2 their form is irregular1 generally circular1 and from the tenth of an inch to one inch and a half in diameter2 their thickness1 or amount of their projection from the rock on which they stand1 varies much1 about a quarter of an inch being perhaps most usual4 ,the fronds occasionally become more and more convex1 until they pass into botryoidal masses with their summits fissured2 when in this state1 they are glossy and of an intense black1 so as to resemble some fused metallic substance4 ,i have shown the incrustation1 both in this latter and in its ordinary state to several geologists1 but not one could conjecture its origin1 except that perhaps it was of volcanic nature6 ,the substance forming the fronds has a very compact and often almost crystalline fracture2 the edges being translucent1 and hard enough easily to scratch calcareous spar4 ,under the blowpipe it immediately becomes white1 and emits a strong animal odour1 like that from fresh shells4 ,it is chiefly composed of carbonate of lime2 when placed in muriatic acid it froths much1 leaving a residue of sulphate of lime1 and of an oxide of iron1 together with a black powder1 which is not soluble in heated acids4 ,this latter substance seems to be carbonaceous1 and is evidently the colouring matter4 ,the sulphate of lime is extraneous1 and occurs in distinct1 excessively minute1 lamellar plates1 studded on the surface of the fronds1 and embedded between the fine layers of which they are composed2 when a fragment is heated in the blowpipe1 these lamellae are immediately rendered visible4 ,the original outline of the fronds may often be traced1 either to a minute particle of shell fixed in a crevice of the rock1 or to several cemented together2 these first become deeply corroded1 by the dissolving power of the waves1 into sharp ridges1 and then are coated with successive layers of the glossy1 grey1 calcareous incrustation4 ,the inequalities of the primary support affect the outline of every successive layer1 in the same manner as may often be seen in bezoar-stones1 when an object like a nail forms the centre of aggregation4 ,the crenulated edges1 however1 of the frond appear to be due to the corroding power of the surf on its own deposit1 alternating with fresh depositions4 ,on some smooth basaltic rocks on the coast of ,st4 ,jago1 ,i found an exceedingly thin layer of brown calcareous matter1 which under a lens presented a miniature likeness of the crenulated and polished fronds of ,ascension2 in this case a basis was not afforded by any projecting extraneous particles4 ,although the incrustation at ,ascension is persistent throughout the year2 yet from the abraded appearance of some parts1 and from the fresh appearance of other parts1 the whole seems to undergo a round of decay and renovation1 due probably to changes in the form of the shifting beach1 and consequently in the action of the breakers3 hence probably it is1 that the incrustation never acquires a great thickness4 ,considering the position of the encrusted rocks in the midst of the calcareous beach1 together with its composition1 ,i think there can be no doubt that its origin is due to the dissolution and subsequent deposition of the matter composing the rounded particles of shells and corals4 7,the selenite1 as ,i have remarked is extraneous1 and must have been derived from the sea-water4 ,it is an interesting circumstance thus to find the waves of the ocean1 sufficiently charged with sulphate of lime1 to deposit it on the rocks1 against which they dash every tide4 ,dr4 ,webster has described 78,voyage of the #8,chanticleer'0 volume #b page #cai7 beds of gypsum and salt1 as much as two feet in thickness1 left by the evaporation of the spray on the rocks on the windward coast4 ,beautiful stalactites of selenite1 resembling in form those of carbonate of lime1 are formed near these beds4 ,amorphous masses of gypsum1 also1 occur in caverns in the interior of the island2 and at ,cross ,hill 7an old crater7 ,i saw a considerable quantity of salt oozing from a pile of scoriae4 ,in these latter cases1 the salt and gypsum appear to be volcanic products47 ,from this source it derives its animal matter1 which is evidently the colouring principle4 ,the nature of the deposit1 in its incipient stage1 can often be well seen upon a fragment of white shell1 when jammed between two of the fronds2 it then appears exactly like the thinnest wash of a pale grey varnish4 ,its darkness varies a little1 but the jet blackness of some of the fronds and of the botryoidal masses seems due to the translucency of the successive grey layers4 ,there is1 however1 this singular circumstance1 that when deposited on the under side of ledges of rock or in fissures1 it appears always to be of a pale1 pearly grey colour1 even when of considerable thickness3 hence one is led to suppose1 that an abundance of light is necessary to the development of the dark colour1 in the same manner as seems to be the case with the upper and exposed surfaces of the shells of living mollusca1 which are always dark1 compared with their under surfaces and with the parts habitually covered by the mantle of the animal4 ,in this circumstance1--in the immediate loss of colour and in the odour emitted under the blowpipe1--in the degree of hardness and translucency of the edges1--and in the beautiful polish of the surface 7,from the fact described in my 8,journal of ,researches0 of a coating of oxide of iron1 deposited by a streamlet on the rocks in its bed 7like a nearly similar coating at the great cataracts of the ,orinoco and ,nile71 becoming finely polished where the surf acts1 ,i presume that the surf in this instance1 also1 is the polishing agent471 rivalling when in a fresh state that of the finest ,oliva1 there is a striking analogy between this inorganic incrustation and the shells of living molluscous animals4 7,in the section descriptive of ,st4 ,paul's ,rocks1 ,i have described a glossy1 pearly substance1 which coats the rocks1 and an allied stalactitical incrustation from ,ascension1 the crust of which resembles the enamel of teeth1 but is hard enough to scratch plate-glass4 ,both these substances contain animal matter1 and seem to have been derived from water in filtering through birds' dung47 ,this appears to me to be an interesting physiological fact4 7,mr4 ,horner and ,sir ,david ,brewster have described 8,philosophical ,transactions0 #ahcf page #fe a singular 8artificial substance1 resembling shell40 ,it is deposited in fine1 transparent1 highly polished1 brown- coloured laminae1 possessing peculiar optical properties1 on the inside of a vessel1 in which cloth1 first prepared with glue and then with lime1 is made to revolve rapidly in water4 ,it is much softer1 more transparent1 and contains more animal matter1 than the natural incrustation at ,ascension2 but we here again see the strong tendency which carbonate of lime and animal matter evince to form a solid substance allied to shell47 ,,singular ,,laminated ,,beds ,,alternating ,,with ,,and ,,passing ,,into ,,obsidian4 ,these beds occur within the trachytic district1 at the western base of ,green ,mountain1 under which they dip at a high inclination4 ,they are only partially exposed1 being covered up by modern ejections2 from this cause1 ,i was unable to trace their junction with the trachyte1 or to discover whether they had flowed as a stream of lava1 or had been injected amidst the overlying strata4 ,there are three principal beds of obsidian1 of which the thickest forms the base of the section4 ,the alternating stony layers appear to me eminently curious1 and shall be first described1 and afterwards their passage into the obsidian4 ,they have an extremely diversified appearance2 five principal varieties may be noticed1 but these insensibly blend into each other by endless gradations4 ,,first4 ,a pale grey1 irregularly and coarsely laminated 7,this term is open to some misinterpretation1 as it may be applied both to rocks divided into laminae of exactly the same composition1 and to layers firmly attached to each other1 with no fissile tendency1 but composed of different minerals1 or of different shades of colour4 ,the term 8laminated10 in this chapter1 is applied in these latter senses2 where a homogeneous rock splits1 as in the former sense1 in a given direction1 like clay-slate1 ,i have used the term 8fissile4071 harsh-feeling rock1 resembling clay-slate which has been in contact with a trap-dike1 and with a fracture of about the same degree of crystalline structure4 ,this rock1 as well as the following varieties1 easily fuses into a pale glass4 ,the greater part is honeycombed with irregular1 angular1 cavities1 so that the whole has a curious appearance1 and some fragments resemble in a remarkable manner silicified logs of decayed wood4 ,this variety1 especially where more compact1 is often marked with thin whitish streaks1 which are either straight or wrap round1 one behind the other1 the elongated carious hollows4 ,,secondly4 ,a bluish grey or pale brown1 compact1 heavy1 homogeneous stone1 with an angular1 uneven1 earthy fracture2 viewed1 however1 under a lens of high power1 the fracture is seen to be distinctly crystalline1 and even separate minerals can be distinguished4 ,,thirdly4 ,a stone of the same kind with the last1 but streaked with numerous1 parallel1 slightly tortuous1 white lines of the thickness of hairs4 ,these white lines are more crystalline than the parts between them2 and the stone splits along them3 they frequently expand into exceedingly thin cavities1 which are often only just perceptible with a lens4 ,the matter forming the white lines becomes better crystallised in these cavities1 and ,professor ,miller was fortunate enough1 after several trials1 to ascertain that the white crystals1 which are the largest1 were of quartz 7,professor ,miller informs me that the crystals which he measured had the faces ,p1 z1 m of the figure 7#adg7 given by ,haidinger in his ,translation of ,mohs2 and he adds1 that it is remarkable1 that none of them had the slightest trace of faces r of the regular six-sided prism471 and that the minute green transparent needles were augite1 or1 as they would more generally be called1 diopside3 besides these crystals1 there are some minute1 dark specks without a trace of crystalline1 and some fine1 white1 granular1 crystalline matter which is probably feldspar4 ,minute fragments of this rock are easily fusible4 ,,fourthly4 ,a compact crystalline rock1 banded in straight lines with innumerable layers of white and grey shades of colour1 varying in width from the thirtieth to the two-hundredth of an inch2 these layers seem to be composed chiefly of feldspar1 and they contain numerous perfect crystals of glassy feldspar1 which are placed lengthways2 they are also thickly studded with microscopically minute1 amorphous1 black specks1 which are placed in rows1 either standing separately1 or more frequently united1 two or three or several together1 into black lines1 thinner than a hair4 ,when a small fragment is heated in the blowpipe1 the black specks are easily fused into black brilliant beads1 which become magnetic1--characters that apply to no common mineral except hornblende or augite4 ,with the black specks there are mingled some others of a red colour1 which are magnetic before being heated1 and no doubt are oxide of iron4 ,round two little cavities1 in a specimen of this variety1 ,i found the black specks aggregated into minute crystals1 appearing like those of augite or hornblende1 but too dull and small to be measured by the goniometer2 in the specimen1 also1 ,i could distinguish amidst the crystalline feldspar1 grains1 which had the aspect of quartz4 ,by trying with a parallel ruler1 ,i found that the thin grey layers and the black hair-like lines were absolutely straight and parallel to each other4 ,it is impossible to trace the gradation from the homogeneous grey rocks to these striped varieties1 or indeed the character of the different layers in the same specimen1 without feeling convinced that the more or less perfect whiteness of the crystalline feldspathic matter depends on the more or less perfect aggregation of diffused matter1 into the black and red specks of hornblende and oxide of iron4 ,,fifthly4 ,a compact heavy rock1 not laminated1 with an irregular1 angular1 highly crystalline1 fracture2 it abounds with distinct crystals of glassy feldspar1 and the crystalline feldspathic base is mottled with a black mineral1 which on the weathered surface is seen to be aggregated into small crystals1 some perfect1 but the greater number imperfect4 ,i showed this specimen to an experienced geologist1 and asked him what it was2 he answered1 as ,i think every one else would have done1 that it was a primitive greenstone4 ,the weathered surface1 also1 of the banded variety in ,figure #d1 strikingly resembles a worn fragment of finely laminated gneiss4 ,these five varieties1 with many intermediate ones1 pass and repass into each other4 ,as the compact varieties are quite subordinate to the others1 the whole may be considered as laminated or striped4 ,the laminae1 to sum up their characteristics1 are either quite straight1 or slightly tortuous1 or convoluted2 they are all parallel to each other1 and to the intercalating strata of obsidian2 they are generally of extreme thinness2 they consist either of an apparently homogeneous1 compact rock1 striped with different shades of grey and brown colours1 or of crystalline feldspathic layers in a more or less perfect state of purity1 and of different thicknesses1 with distinct crystals of glassy feldspar placed lengthways1 or of very thin layers chiefly composed of minute crystals of quartz and augite1 or composed of black and red specks of an augitic mineral and of an oxide of iron1 either not crystallised or imperfectly so4 ,after having fully described the obsidian1 ,i shall return to the subject of the lamination of rocks of the trachytic series4 ,the passage of the foregoing beds into the strata of glassy obsidian is effected in several ways3 first1 angulo-modular masses of obsidian1 both large and small1 abruptly appear disseminated in a slaty1 or in an amorphous1 pale-coloured1 feldspathic rock1 with a somewhat pearly fracture4 ,secondly1 small irregular nodules of the obsidian1 either standing separately1 or united into thin layers1 seldom more than the tenth of an inch in thickness1 alternate repeatedly with very thin layers of a feldspathic rock1 which is striped with the finest parallel zones of colour1 like an agate1 and which sometimes passes into the nature of pitchstone2 the interstices between the nodules of obsidian are generally filled by soft white matter1 resembling pumiceous ashes4 ,thirdly1 the whole substance of the bounding rock suddenly passes into an angulo-concretionary mass of obsidian4 ,such masses 7as well as the small nodules7 of obsidian are of a pale green colour1 and are generally streaked with different shades of colour1 parallel to the laminae of the surrounding rock2 they likewise generally contain minute white sphaerulites1 of which half is sometimes embedded in a zone of one shade of colour1 and half in a zone of another shade4 ,the obsidian assumes its jet black colour and perfectly conchoidal fracture1 only when in large masses2 but even in these1 on careful examination and on holding the specimens in different lights1 ,i could generally distinguish parallel streaks of different shades of darkness4 7,,figure #f4 ,,opaque ,,brown ,,sphaerulites1 drawn on an enlarged scale4 ,the upper ones are externally marked with parallel ridges4 ,the internal radiating structure of the lower ones1 is much too plainly represented4 ,,figure #g4 ,a ,,layer ,,formed ,,by ,,the ,,union ,,of ,,minute ,,brown ,,sphaerulites1 ,,intersecting ,,two ,,other ,,similar ,,layers3 the whole represented of nearly the natural size47 ,one of the commonest transitional rocks deserves in several respects a further description4 ,it is of a very complicated nature1 and consists of numerous thin1 slightly tortuous layers of a pale-coloured feldspathic stone1 often passing into an imperfect pitchstone1 alternating with layers formed of numberless little globules of two varieties of obsidian1 and of two kinds of sphaerulites1 embedded in a soft or in a hard pearly base4 ,the sphaerulites are either white and translucent1 or dark brown and opaque2 the former are quite spherical1 of small size1 and distinctly radiated from their centre4 ,the dark brown sphaerulites are less perfectly round1 and vary in diameter from the twentieth to the thirtieth of an inch2 when broken they exhibit towards their centres1 which are whitish1 an obscure radiating structure2 two of them when united sometimes have only one central point of radiation2 there is occasionally a trace of or a hollow crevice in their centres4 ,they stand either separately1 or are united two or three or many together into irregular groups1 or more commonly into layers1 parallel to the stratification of the mass4 ,this union in many cases is so perfect1 that the two sides of the layer thus formed1 are quite even2 and these layers1 as they become less brown and opaque1 cannot be distinguished from the alternating layers of the pale-coloured feldspathic stone4 ,the sphaerulites1 when not united1 are generally compressed in the plane of the lamination of the mass2 and in this same plane1 they are often marked internally1 by zones of different shades of colour1 and externally by small ridges and furrows4 ,in the upper part of ,figure #f1 the sphaerulites with the parallel ridges and furrows are represented on an enlarged scale1 but they are not well executed2 and in the lower part1 their usual manner of grouping is shown4 ,in another specimen1 a thin layer formed of the brown sphaerulites closely united together1 intersects1 as represented in ,figure #g1 a layer of similar composition2 and after running for a short space in a slightly curved line1 again intersects it1 and likewise a second layer lying a little way beneath that first intersected4 ,the small nodules also of obsidian are sometimes externally marked with ridges and furrows1 parallel to the lamination of the mass1 but always less plainly than the sphaerulites4 ,these obsidian nodules are generally angular1 with their edges blunted3 they are often impressed with the form of the adjoining sphaerulites1 than which they are always larger2 the separate nodules seldom appear to have drawn each other out by exerting a mutually attractive force4 ,had ,i not found in some cases1 a distinct centre of attraction in these nodules of obsidian1 ,i should have been led to have considered them as residuary matter1 left during the formation of the pearlstone1 in which they are embedded1 and of the sphaerulitic globules4 ,the sphaerulites and the little nodules of obsidian in these rocks so closely resemble1 in general form and structure1 concretions in sedimentary deposits1 that one is at once tempted to attribute to them an analogous origin4 ,they resemble ordinary concretions in the following respects3 in their external form1--in the union of two or three1 or of several1 into an irregular mass1 or into an even-sided layer1--in the occasional intersection of one such layer by another1 as in the case of chalk-flints1- -in the presence of two or three kinds of nodules1 often close together1 in the same basis1--in their fibrous1 radiating structure1 with occasional hollows in their centres1--in the co-existence of a laminary1 concretionary1 and radiating structure1 as is so well developed in the concretions of magnesian limestone1 described by ,professor ,sedgwick4 78,geological ,transactions0 volume #c part #a page #cg47 ,concretions in sedimentary deposits1 it is known1 are due to the separation from the surrounding mass of the whole or part of some mineral substance1 and its aggregation round certain points of attraction4 ,guided by this fact1 ,i have endeavoured to discover whether obsidian and the sphaerulites 7to which may be added marekanite and pearlstone1 both of them occurring in nodular concretions in the trachytic series7 differ in their constituent parts1 from the minerals generally composing trachytic rocks4 ,it appears from three analyses1 that obsidian contains on an average #gf per cent of silica2 from one analysis1 that sphaerulites contain #gi4ab2 from two1 that marekanite contains #gi4be2 and from two other analyses1 that pearlstone contains #ge4fb of silica4 7,the foregoing analyses are taken from ,beudant 8,traite de ,mineralogie0 tome #b page #aac2 and one analysis of obsidian from ,phillips 8,mineralogy407 ,now1 the constituent parts of trachyte1 as far as they can be distinguished consist of feldspar1 containing #fe4ba of silica2 or of albite1 containing #fi4ji2 of hornblende1 containing #ee4bg 7,these analyses are taken from ,von ,kobell 8,grundzuge der ,mineralogie0 #ahch471 and of oxide of iron3 so that the foregoing glassy concretionary substances all contain a larger proportion of silica than that occurring in ordinary feldspathic or trachytic rocks4 ,d',aubuisson 78,traite de ,geogn40 tome #b page #ece471 also1 has remarked on the large proportion of silica compared with alumina1 in six analyses of obsidian and pearlstone given in ,brongniart's 8,mineralogy40 ,hence ,i conclude1 that the foregoing concretions have been formed by a process of aggregation1 strictly analogous to that which takes place in aqueous deposits1 acting chiefly on the silica1 but likewise on some of the other elements of the surrounding mass1 and thus producing the different concretionary varieties4 ,from the well-known effects of rapid cooling 7,this is seen in the manufacture of common glass1 and in ,gregory ,watts's experiments on molten trap2 also on the natural surfaces of lava- streams1 and on the side-walls of dikes47 in giving glassiness of texture1 it is probably necessary that the entire mass1 in cases like that of ,ascension1 should have cooled at a certain rate2 but considering the repeated and complicated alterations of nodules and thin layers of a glassy texture with other layers quite stony or crystalline1 all within the space of a few feet or even inches1 it is hardly possible that they could have cooled at different rates1 and thus have acquired their different textures4 ,the natural sphaerulites in these rocks very closely resemble those produced in glass1 when slowly cooled4 7,i do not know whether it is generally known1 that bodies having exactly the same appearance as sphaerulites1 sometimes occur in agates4 ,mr4 ,robert ,brown showed me in an agate1 formed within a cavity in a piece of silicified wood1 some little specks1 which were only just visible to the naked eye3 these specks1 when placed by him under a lens of high power1 presented a beautiful appearance3 they were perfectly circular1 and consisted of the finest fibres of a brown colour1 radiating with great exactness from a common centre4 ,these little radiating stars are occasionally intersected1 and portions are quite cut off by the fine1 ribbon-like zones of colour in the agate4 ,in the obsidian of ,ascension1 the halves of a sphaerulite often lie in different zones of colour1 but they are not cut off by them1 as in the agate47 ,in some fine specimens of partially devitrified glass1 in the possession of ,mr4 ,stokes1 the sphaerulites are united into straight layers with even sides1 parallel to each other1 and to one of the outer surfaces1 exactly as in the obsidian4 ,these layers sometimes interbranch and form loops2 but ,i did not see any case of actual intersection4 ,they form the passage from the perfectly glassy portions1 to those nearly homogeneous and stony1 with only an obscure concretionary structure4 ,in the same specimen1 also1 sphaerulites differing slightly in colour and in structure1 occur embedded close together4 ,considering these facts1 it is some confirmation of the view above given of the concretionary origin of the obsidian and natural sphaerulites1 to find that ,m4 ,dartigues 78,journal de ,physique0 tome #ei #ahjd pages #aj1 #ab471 in his curious paper on this subject1 attributes the production of sphaerulites in glass1 to the different ingredients obeying their own laws of attraction and becoming aggregated4 ,he is led to believe that this takes place1 from the difficulty in remelting sphaerulitic glass1 without the whole be first thoroughly pounded and mixed together2 and likewise from the fact1 that the change takes place most readily in glass composed of many ingredients4 ,in confirmation of ,m4 ,dartigues' view1 ,i may remark1 that ,m4 ,fleuriau de ,bellevue 7,idem tome #fj #ahje page #dah47 found that the sphaerulitic portions of devitrified glass were acted on both by nitric acid and under the blowpipe1 in a different manner from the compact paste in which they were embedded4 ,,comparison ,,of ,,the ,,obsidian ,,beds ,,and ,,alternating ,,strata ,,of ,,ascension1 ,,with ,,those ,,of ,,other ,,countries4 ,i have been struck with much surprise1 how closely the excellent description of the obsidian rocks of ,hungary1 given by ,beudant 78,voyage en ,hongrie0 tome #a page #ccj2 tome #b pages #bba and #cae2 tome #c pages #cfi1 #cga1 #cgg1 #cha471 and that by ,humboldt1 of the same formation in ,mexico and ,peru 78,essai ,geognostique0 pages #agf1 #cbf1 #cbh471 and likewise the descriptions given by several authors 7,p4 ,scrope 8,geological ,transactions0 volume #b second series page #aie4 ,consult also ,dolomieu 8,voyage aux ,isles ,lipari0 and ,d',aubuisson 8,traite de ,geogn40 tome #b page #ecd47 of the trachytic regions in the ,italian islands1 agree with my observations at ,ascension4 ,many passages might have been transferred without alteration from the works of the above authors1 and would have been applicable to this island4 ,they all agree in the laminated and stratified character of the whole series2 and ,humboldt speaks of some of the beds of obsidian being ribboned like jasper4 7,in ,mr4 ,stokes' fine collection of obsidians from ,mexico1 ,i observe that the sphaerulites are generally much larger than those of ,ascension2 they are generally white1 opaque1 and are united into distinct layers3 there are many singular varieties1 different from any at ,ascension4 ,the obsidians are finely zoned1 in quite straight or curved lines1 with exceedingly slight differences of tint1 of cellularity1 and of more or less perfect degrees of glassiness4 ,tracing some of the less perfectly glassy zones1 they are seen to become studded with minute white sphaerulites1 which become more and more numerous1 until at last they unite and form a distinct layer3 on the other hand1 at ,ascension1 only the brown sphaerulites unite and form layers2 the white ones always being irregularly disseminated4 ,some specimens at the ,geological ,society1 said to belong to an obsidian formation from ,mexico1 have an earthy fracture1 and are divided in the finest parallel laminae1 by specks of a black mineral1 like the augitic or hornblendic specks in the rocks at ,ascension47 ,they all agree in the nodular or concretionary character of the obsidian1 and of the passage of these nodules into layers4 ,they all refer to the repeated alterations1 often in undulatory planes1 of glassy1 pearly1 stony1 and crystalline layers3 the crystalline layers1 however1 seem to be much more perfectly developed at ,ascension1 than in the above-named countries4 ,humboldt compares some of the stony beds1 when viewed from a distance1 to strata of a schistose sandstone4 ,sphaerulites are described as occurring abundantly in all cases2 and they everywhere seem to mark the passage1 from the perfectly glassy to the stony and crystalline beds4 ,beudant's account 7,beudant 8,voyage0 tome #c page #cgc47 of his 8perlite lithoide globulaire0 in every1 even the most trifling particular1 might have been written for the little brown sphaerulitic globules of the rocks of ,ascension4 ,from the close similarity in so many respects1 between the obsidian formations of ,hungary1 ,mexico1 ,peru1 and of some of the ,italian islands1 with that of ,ascension1 ,i can hardly doubt that in all these cases1 the obsidian and the sphaerulites owe their origin to a concretionary aggregation of the silica1 and of some of the other constituent elements1 taking place whilst the liquified mass cooled at a certain required rate4 ,it is1 however1 well-known1 that in several places1 obsidian has flowed in streams like lava2 for instance1 at ,teneriffe1 at the ,lipari ,islands1 and at ,iceland4 7,for ,teneriffe see von ,buch 8,descript4 des ,isles ,canaries0 pages #ahd and #aij2 for the ,lipari ,islands see ,dolomieu 8,voyage0 page #cd2 for ,iceland see ,mackenzie 8,travels0 page #cfi47 ,in these cases1 the superficial parts are the most perfectly glassy1 the obsidian passing at the depth of a few feet into an opaque stone4 ,in an analysis by ,vauquelin of a specimen of obsidian from ,hecla1 which probably flowed as lava1 the proportion of silica is nearly the same as in the nodular or concretionary obsidian from ,mexico4 ,it would be interesting to ascertain1 whether the opaque interior portions and the superficial glassy coating contained the same proportional constituent parts3 we know from ,m4 ,dufrenoy 78,memoires pour servir a une ,descript4 ,geolog4 de la ,france0 tome #d page #cga47 that the exterior and interior parts of the same stream of lava sometimes differ considerably in their composition4 ,even should the whole body of the stream of obsidian turn out to be similarly composed with nodular obsidian1 it would only be necessary1 in accordance with the foregoing facts1 to suppose that lava in these instances had been erupted with its ingredients mixed in the same proportion1 as in the concretionary obsidian4 ,,lamination ,,of ,,volcanic ,,rocks ,,of ,,the ,,trachytic ,,series4 ,we have seen that1 in several and widely distant countries1 the strata alternating with beds of obsidian1 are highly laminated4 ,the nodules1 also1 both large and small1 of the obsidian1 are zoned with different shades of colour2 and ,i have seen a specimen from ,mexico in ,mr4 ,stokes' collection1 with its external surface weathered 7,mac,culloch states 8,classification of ,rocks0 page #eca that the exposed surfaces of the pitchstone dikes in ,arran are furrowed 8with undulating lines1 resembling certain varieties of marbled paper1 and which evidently result from some corresponding difference of laminar structure407 into ridges and furrows1 corresponding with the zones of different degrees of glassiness3 ,humboldt 78,personal ,narrative0 volume #a page #bbb471 moreover1 found on the ,peak of ,teneriffe1 a stream of obsidian divided by very thin1 alternating1 layers of pumice4 ,many other lavas of the feldspathic series are laminated2 thus1 masses of common trachyte at ,ascension are divided by fine earthy lines1 along which the rock splits1 separating thin layers of slightly different shades of colour2 the greater number1 also1 of the embedded crystals of glassy feldspar are placed lengthways in the same direction4 ,mr4 ,p4 ,scrope 78,geological ,transactions0 volume #b second series page #aie47 has described a remarkable columnar trachyte in the ,panza ,islands1 which seems to have been injected into an overlying mass of trachytic conglomerate3 it is striped with zones1 often of extreme tenuity1 of different textures and colours2 the harder and darker zones appearing to contain a larger proportion of silica4 ,in another part of the island1 there are layers of pearlstone and pitchstone1 which in many respects resemble those of ,ascension4 ,the zones in the columnar trachyte are generally contorted2 they extend uninterruptedly for a great length in a vertical direction1 and apparently parallel to the walls of the dike-like mass4 ,von ,buch 78,description des ,iles ,canaries0 page #ahd47 has described at ,teneriffe1 a stream of lava containing innumerable thin1 plate-like crystals of feldspar1 which are arranged like white threads1 one behind the other1 and which mostly follow the same direction4 ,dolomieu 78,voyage aux ,isles de ,lipari0 pages #ce and #he47 also states1 that the grey lavas of the modern cone of ,vulcano1 which have a vitreous texture1 are streaked with parallel white lines3 he further describes a solid pumice-stone which possesses a fissile structure1 like that of certain micaceous schists4 ,phonolite1 which ,i may observe is often1 if not always1 an injected rock1 also1 often has a fissile structure2 this is generally due to the parallel position of the embedded crystals of feldspar1 but sometimes1 as at ,fernando ,noronha1 seems to be nearly independent of their presence4 7,in this case1 and in that of the fissile pumice-stone1 the structure is very different from that in the foregoing cases1 where the laminae consist of alternate layers of different composition or texture4 ,in some sedimentary formations1 however1 which apparently are homogeneous and fissile1 as in glossy clay-slate1 there is reason to believe1 according to ,d',aubuisson1 that the laminae are really due to excessively thin1 alternating1 layers of mica47 ,from these facts we see1 that various rocks of the feldspathic series have either a laminated or fissile structure1 and that it occurs both in masses which have injected into overlying strata1 and in others which have flowed as streams of lava4 ,the laminae of the beds1 alternating with the obsidian at ,ascension1 dip at a high angle under the mountain1 at the base of which they are situated2 and they do not appear as if they had been inclined by violence4 ,a high inclination is common to these beds in ,mexico1 ,peru1 and in some of the ,italian islands 7,see ,phillips 8,mineralogy0 for the ,italian ,islands page #acf4 ,for ,mexico and ,peru see ,humboldt 8,essai ,geognostique40 ,mr4 ,edwards also describes the high inclination of the obsidian rocks of the ,cerro del ,navaja in ,mexico in the 8,proc4 of the ,geolog4 ,soc40 ,june #ahch473 on the other hand1 in ,hungary1 the layers are horizontal2 the laminae1 also1 of some of the lava-streams above referred to1 as far as ,i can understand the descriptions given of them1 appear to be highly inclined or vertical4 ,i doubt whether in any of these cases1 the laminae have been tilted into their present position2 and in some instances1 as in that of the trachyte described by ,mr4 ,scrope1 it is almost certain that they have been originally formed with a high inclination4 ,in many of these cases1 there is evidence that the mass of liquified rock has moved in the direction of the laminae4 ,at ,ascension1 many of the air-cells have a drawn out appearance1 and are crossed by coarse semi-glassy fibres1 in the direction of the laminae2 and some of the layers1 separating the sphaerulitic globules1 have a scored appearance1 as if produced by the grating of the globules4 ,i have seen a specimen of zoned obsidian from ,mexico1 in ,mr4 ,stokes' collection1 with the surfaces of the best-defined layers streaked or furrowed with parallel lines2 and these lines or streaks precisely resembled those1 produced on the surface of a mass of artificial glass by its having been poured out of a vessel4 ,humboldt1 also1 has described little cavities1 which he compares to the tails of comets1 behind sphaerulites in laminated obsidian rocks from ,mexico1 and ,mr4 ,scrope has described other cavities behind fragments embedded in his laminated trachyte1 and which he supposes to have been produced during the movement of the mass4 78,geological ,transactions0 volume #b second series page #bjj etc4 ,these embedded fragments1 in some instances1 consist of the laminated trachyte broken off and 8enveloped in those parts1 which still remained liquid40 ,beudant1 also1 frequently refers in his great work on 8,hungary0 tome #c page #chf1 to trachytic rocks1 irregularly spotted with fragments of the same varieties1 which in other parts form the parallel ribbons4 ,in these cases1 we must suppose1 that after part of the molten mass had assumed a laminated structure1 a fresh irruption of lava broke up the mass1 and involved fragments1 and that subsequently the whole became relaminated47 ,from such facts1 most authors have attributed the lamination of these volcanic rocks to their movement whilst liquified4 ,although it is easy to perceive1 why each separate air-cell1 or each fibre in pumice-stone 7,dolomieu 8,voyage0 page #fd471 should be drawn out in the direction of the moving mass2 it is by no means at first obvious why such air-cells and fibres should be arranged by the movement1 in the same planes1 in laminae absolutely straight and parallel to each other1 and often of extreme tenuity2 and still less obvious is it1 why such layers should come to be of slightly different composition and of different textures4 ,in endeavouring to make out the cause of the lamination of these igneous feldspathic rocks1 let us return to the facts so minutely described at ,ascension4 ,we there see1 that some of the thinnest layers are chiefly formed by numerous1 exceedingly minute1 though perfect1 crystals of different minerals2 that other layers are formed by the union of different kinds of concretionary globules1 and that the layers thus formed1 often cannot be distinguished from the ordinary feldspathic and pitchstone layers1 composing a large portion of the entire mass4 ,the fibrous radiating structure of the sphaerulites seems1 judging from many analogous cases1 to connect the concretionary and crystalline forces3 the separate crystals1 also1 of feldspar all lie in the same parallel planes4 7,the formation1 indeed1 of a large crystal of any mineral in a rock of mixed composition implies an aggregation of the requisite atoms1 allied to concretionary action4 ,the cause of the crystals of feldspar in these rocks of ,ascension1 being all placed lengthways1 is probably the same with that which elongates and flattens all the brown sphaerulitic globules 7which behave like feldspar under the blowpipe7 in this same direction47 ,these allied forces1 therefore1 have played an important part in the lamination of the mass1 but they cannot be considered the primary force2 for the several kinds of nodules1 both the smallest and largest1 are internally zoned with excessively fine shades of colour1 parallel to the lamination of the whole2 and many of them are1 also1 externally marked in the same direction with parallel ridges and furrows1 which have not been produced by weathering4 ,some of the finest streaks of colour in the stony layers1 alternating with the obsidian1 can be distinctly seen to be due to an incipient crystallisation of the constituent minerals4 ,the extent to which the minerals have crystallised can1 also1 be distinctly seen to be connected with the greater or less size1 and with the number1 of the minute1 flattened1 crenulated air-cavities or fissures4 ,numerous facts1 as in the case of geodes1 and of cavities in silicified wood1 in primary rocks1 and in veins1 show that crystallisation is much favoured by space4 ,hence1 ,i conclude1 that1 if in a mass of cooling volcanic rock1 any cause produced in parallel planes a number of minute fissures or zones of less tension 7which from the pent-up vapours would often be expanded into crenulated air-cavities71 the crystallisation of the constituent parts1 and probably the formation of concretions1 would be superinduced or much favoured in such planes2 and thus1 a laminated structure of the kind we are here considering would be generated4 ,that some cause does produce parallel zones of less tension in volcanic rocks1 during their consolidation1 we must admit in the case of the thin alternate layers of obsidian and pumice described by ,humboldt1 and of the small1 flattened1 crenulated air-cells in the laminated rocks of ,ascension2 for on no other principle can we conceive why the confined vapours should through their expansion form air-cells or fibres in separate1 parallel planes1 instead of irregularly throughout the mass4 ,in ,mr4 ,stokes' collection1 ,i have seen a beautiful example of this structure1 in a specimen of obsidian from ,mexico1 which is shaded and zoned1 like the finest agate1 with numerous1 straight1 parallel layers1 more or less opaque and white1 or almost perfectly glassy2 the degree of opacity and glassiness depending on the number of microscopically minute1 flattened air-cells2 in this case1 it is scarcely possible to doubt but that the mass1 to which the fragment belonged1 must have been subjected to some1 probably prolonged1 action1 causing the tension slightly to vary in the successive planes4 ,several causes appear capable of producing zones of different tension1 in masses semi-liquified by heat4 ,in a fragment of devitrified glass1 ,i have observed layers of sphaerulites which appeared1 from the manner in which they were abruptly bent1 to have been produced by the simple contraction of the mass in the vessel1 in which it cooled4 ,in certain dikes on ,mount ,etna1 described by ,m4 ,elie de ,beaumont 78,mem4 pour servir0 etc4 tome #d page #aca471 as bordered by alternating bands of scoriaceous and compact rock1 one is led to suppose that the stretching movement of the surrounding strata1 which originally produced the fissures1 continued whilst the injected rock remained fluid4 ,guided1 however1 by ,professor ,forbes' 78,edinburgh ,new ,phil4 ,journal0 #ahdb page #cej47 clear description of the zoned structure of glacier-ice1 far the most probable explanation of the laminated structure of these feldspathic rocks appears to be1 that they have been stretched whilst slowly flowing onwards in a pasty condition 7,i presume that this is nearly the same explanation which ,mr4 ,scrope had in his mind1 when he speaks 78,geolog4 ,transact40 volume #b second series page #bbh7 of the ribboned structure of his trachytic rocks1 having arisen1 from 8a linear extension of the mass1 while in a state of imperfect liquidity1 coupled with a concretionary process4071 in precisely the same manner as ,professor ,forbes believes1 that the ice of moving glaciers is stretched and fissured4 ,in both cases1 the zones may be compared to those in the finest agates2 in both1 they extend in the direction in which the mass has flowed1 and those exposed on the surface are generally vertical3 in the ice1 the porous laminae are rendered distinct by the subsequent congelation of infiltrated water1 in the stony feldspathic lavas1 by subsequent crystalline and concretionary action4 ,the fragment of glassy obsidian in ,mr4 ,stokes' collection1 which is zoned with minute air-cells must strikingly resemble1 judging from ,professor ,forbes' descriptions1 a fragment of the zoned ice2 and if the rate of cooling and nature of the mass had been favourable to its crystallisation or to concretionary action1 we should here have had the finest parallel zones of different composition and texture4 ,in glaciers1 the lines of porous ice and of minute crevices seem to be due to an incipient stretching1 caused by the central parts of the frozen stream moving faster than the sides and bottom1 which are retarded by friction3 hence in glaciers of certain forms and towards the lower end of most glaciers1 the zones become horizontal4 ,may we venture to suppose that in the feldspathic lavas with horizontal laminae1 we see an analogous case8 ,all geologists1 who have examined trachytic regions1 have come to the conclusion1 that the lavas of this series have possessed an exceedingly imperfect fluidity2 and as it is evident that only matter thus characterised would be subject to become fissured and to be formed into zones of different tensions1 in the manner here supposed1 we probably see the reason why augitic lavas1 which appear generally to have possessed a high degree of fluidity1 are not1 like the feldspathic lavas1 divided into laminae of different composition and texture4 7,basaltic lavas1 and many other rocks1 are not unfrequently divided into thick laminae or plates1 of the same composition1 which are either straight or curved2 these being crossed by vertical lines of fissure1 sometimes become united into columns4 ,this structure seems related1 in its origin1 to that by which many rocks1 both igneous and sedimentary1 become traversed by parallel systems of fissures47 ,moreover1 in the augitic series1 there never appears to be any tendency to concretionary action1 which we have seen plays an important part in the lamination of rocks1 of the trachytic series1 or at least in rendering that structure apparent4 ,whatever may be thought of the explanation here advanced of the laminated structure of the rocks of the trachytic series1 ,i venture to call the attention of geologists to the simple fact1 that in a body of rock at ,ascension1 undoubtedly of volcanic origin1 layers often of extreme tenuity1 quite straight1 and parallel to each other1 have been produced2--some composed of distinct crystals of quartz and diopside1 mingled with amorphous augitic specks and granular feldspar1--others entirely composed of these black augitic specks1 with granules of oxide of iron1--and lastly1 others formed of crystalline feldspar1 in a more or less perfect state of purity1 together with numerous crystals of feldspar1 placed lengthways4 ,at this island1 there is reason to believe1 and in some analogous cases1 it is certainly known1 that the laminae have originally been formed with their present high inclination4 ,facts of this nature are manifestly of importance1 with relation to the structural origin of that grand series of plutonic rocks1 which like the volcanic have undergone the action of heat1 and which consist of alternate layers of quartz1 feldspar1 mica and other minerals4 ,,chapter ,,iv4 ,,st4 ,,helena4 ,lavas of the feldspathic1 basaltic1 and submarine series4 ,section of ,flagstaff ,hill and of the ,barn4 ,dikes4 ,turk's ,cap and ,prosperous ,bays4 ,basaltic ring4 ,central crateriform ridge1 with an internal ledge and a parapet4 ,cones of phonolite4 ,superficial beds of calcareous sandstone4 ,extinct land-shells4 ,beds of detritus4 ,elevation of the land4 ,denudation4 ,craters of elevation4 ,the whole island is of volcanic origin2 its circumference1 according to ,beatson1 is about twenty-eight miles4 7,governor ,beatson 8,account of ,st4 ,helena407 ,the central and largest part consists of rocks of a feldspathic nature1 generally decomposed to an extraordinary degree2 and when in this state1 presenting a singular assemblage of alternating1 red1 purple1 brown1 yellow1 and white1 soft1 argillaceous beds4 ,from the shortness of our visit1 ,i did not examine these beds with care2 some of them1 especially those of the white1 yellow1 and brown shades1 originally existed as streams of lava1 but the greater number were probably ejected in the form of scoriae and ashes3 other beds of a purple tint1 porphyritic with crystal- shaped patches of a white1 soft substance1 which are now unctuous1 and yield1 like wax1 a polished streak to the nail1 seem once to have existed as solid claystone-porphyries3 the red argillaceous beds generally have a brecciated structure1 and no doubt have been formed by the decomposition of scoriae4 ,several extensive streams1 however1 belonging to this series1 retain their stony character2 these are either of a blackish-green colour1 with minute acicular crystals of feldspar1 or of a very pale tint1 and almost composed of minute1 often scaly1 crystals of feldspar1 abounding with microscopical black specks2 they are generally compact and laminated2 others1 however1 of similar composition1 are cellular and somewhat decomposed4 ,none of these rocks contain large crystals of feldspar1 or have the harsh fracture peculiar to trachyte4 ,these feldspathic lavas and tuffs are the uppermost or those last erupted2 innumerable dikes1 however1 and great masses of molten rock1 have subsequently been injected into them4 ,they converge1 as they rise1 towards the central curved ridge1 of which one point attains the elevation of #b1gjj feet4 ,this ridge is the highest land in the island2 and it once formed the northern rim of a great crater1 whence the lavas of this series flowed3 from its ruined condition1 from the southern half having been removed1 and from the violent dislocation which the whole island has undergone1 its structure is rendered very obscure4 ,,basaltic ,,series4 ,the margin of the island is formed by a rude circle of great1 black1 stratified1 ramparts of basalt1 dipping seaward1 and worn into cliffs1 which are often nearly perpendicular1 and vary in height from a few hundred feet to two thousand4 ,this circle1 or rather horse-shoe shaped ring1 is open to the south1 and is breached by several other wide spaces4 ,its rim or summit generally projects little above the level of the adjoining inland country2 and the more recent feldspathic lavas1 sloping down from the central heights1 generally abut against and overlap its inner margin2 on the north-western side of the island1 however1 they appear 7judging from a distance7 to have flowed over and concealed portions of it4 ,in some parts1 where the basaltic ring has been breached1 and the black ramparts stand detached1 the feldspathic lavas have passed between them1 and now overhang the sea-coast in lofty cliffs4 ,the basaltic rocks are of a black colour and thinly stratified2 they are generally highly vesicular1 but occasionally compact2 some of them contain numerous crystals of glassy feldspar and octahedrons of titaniferous iron2 others abound with crystals of augite and grains of olivine4 ,the vesicles are frequently lined with minute crystals 7of chabasie87 and even become amygdaloidal with them4 ,the streams are separated from each other by cindery matter1 or by a bright red1 friable1 saliferous tuff1 which is marked by successive lines like those of aqueous deposition2 and sometimes it has an obscure1 concretionary structure4 ,the rocks of this basaltic series occur nowhere except near the coast4 ,in most volcanic districts the trachytic lavas are of anterior origin to the basaltic2 but here we see1 that a great pile of rock1 closely related in composition to the trachytic family1 has been erupted subsequently to the basaltic strata3 the number1 however1 of dikes1 abounding with large crystals of augite1 with which the feldspathic lavas have been injected1 shows perhaps some tendency to a return to the more usual order of superposition4 ,,basal ,,submarine ,,lavas4 ,the lavas of this basal series lie immediately beneath both the basaltic and feldspathic rocks4 ,according to ,mr4 ,seale1 they may be seen at intervals on the sea-beach round the entire island4 78,geognosy of the ,island of ,st4 ,helena40 ,mr4 ,seale has constructed a gigantic model of ,st4 ,helena1 well worth visiting1 which is now deposited at ,addiscombe ,college1 in ,surrey47 ,in the sections which ,i examined1 their nature varied much2 some of the strata abound with crystals of augite2 others are of a brown colour1 either laminated or in a rubbly condition2 and many parts are highly amygdaloidal with calcareous matter4 ,the successive sheets are either closely united together1 or are separated from each other by beds of scoriaceous rock and of laminated tuff1 frequently containing well-rounded fragments4 ,the interstices of these beds are filled with gypsum and salt2 the gypsum also sometimes occurring in thin layers4 ,from the large quantity of these two substances1 from the presence of rounded pebbles in the tuffs1 and from the abundant amygdaloids1 ,i cannot doubt that these basal volcanic strata flowed beneath the sea4 ,this remark ought perhaps to be extended to a part of the superincumbent basaltic rocks2 but on this point1 ,i was not able to obtain clear evidence4 ,the strata of the basal series1 whenever ,i examined them1 were intersected by an extraordinary number of dikes4 ,,flagstaff ,,hill ,,and ,,the ,,barn4 7,,figure #h4 ,,flagstaff ,,hill ,,and ,,the ,,barn4 7,section ,west 7left7 to ,east 7right77 ,flagstaff ,hill1 #b1bgb feet high to ,the ,barn1 #b1jae feet high4 ,the double lines represent the basaltic strata2 the single1 the basal submarine strata2 the dotted1 the upper feldspathic strata2 the dikes are shaded transversely47 ,i will now describe some of the more remarkable sections1 and will commence with these two hills1 which form the principal external feature on the north-eastern side of the island4 ,the square1 angular outline1 and black colour of the ,barn1 at once show that it belongs to the basaltic series2 whilst the smooth1 conical figure1 and the varied bright tints of ,flagstaff ,hill1 render it equally clear1 that it is composed of the softened1 feldspathic rocks4 ,these two lofty hills are connected 7as is shown in ,figure #h7 by a sharp ridge1 which is composed of the rubbly lavas of the basal series4 ,the strata of this ridge dip westward1 the inclination becoming less and less towards the ,flagstaff2 and the upper feldspathic strata of this hill can be seen1 though with some difficulty1 to dip conformably to the ,w4,s4,w4 ,close to the ,barn1 the strata of the ridge are nearly vertical1 but are much obscured by innumerable dikes2 under this hill1 they probably change from being vertical into being inclined into an opposite direction2 for the upper or basaltic strata1 which are about eight hundred or one thousand feet in thickness1 are inclined north-eastward1 at an angle between thirty and forty degrees4 ,this ridge1 and likewise the ,barn and ,flagstaff ,hills1 are interlaced by dikes1 many of which preserve a remarkable parallelism in a ,n4,n4,w4 and ,s4,s4,e4 direction4 ,the dikes chiefly consist of a rock1 porphyritic with large crystals of augite2 others are formed of a fine-grained and brown- coloured trap4 ,most of these dikes are coated by a glossy layer1 from one to two-tenths of an inch in thickness1 which1 unlike true pitchstone1 fuses into a black enamel2 this layer is evidently analogous to the glossy superficial coating of many lava streams4 7,this circumstance has been observed 7,lyell 8,principles of ,geology0 volume #d chapter #aj page #i7 in the dikes of the ,atrio del ,cavallo1 but apparently it is not of very common occurrence4 ,sir ,g4 ,mackenzie1 however1 states 7page #cgb 8,travels in ,iceland07 that all the veins in ,iceland have a 8black vitreous coating on their sides40 ,captain ,carmichael1 speaking of the dikes in ,tristan d',acunha1 a volcanic island in the ,southern ,atlantic1 says 78,linnaean ,transactions0 volume #ab page #dhe7 that their sides1 8where they come in contact with the rocks1 are invariably in a semi-vitrified state407 ,the dikes can often be followed for great lengths both horizontally and vertically1 and they seem to preserve a nearly uniform thickness 78,geognosy of the ,island of ,st4 ,helena0 plate #e473 ,mr4 ,seale states1 that one near the ,barn1 in a height of #a1bfj feet1 decreases in width only four inches1--from nine feet at the bottom1 to eight feet and eight inches at the top4 ,on the ridge1 the dikes appear to have been guided in their course1 to a considerable degree1 by the alternating soft and hard strata3 they are often firmly united to the harder strata1 and they preserve their parallelism for such great lengths1 that in very many instances it was impossible to conjecture1 which of the beds were dikes1 and which streams of lava4 ,the dikes1 though so numerous on this ridge1 are even more numerous in the valleys a little south of it1 and to a degree ,i never saw equalled anywhere else3 in these valleys they extend in less regular lines1 covering the ground with a network1 like a spider's web1 and with some parts of the surface even appearing to consist wholly of dikes1 interlaced by other dikes4 ,from the complexity produced by the dikes1 from the high inclination and anticlinal dip of the strata of the basal series1 which are overlaid1 at the opposite ends of the short ridge1 by two great masses of different ages and of different composition1 ,i am not surprised that this singular section has been misunderstood4 ,it has even been supposed to form part of a crater2 but so far is this from having been the case1 that the summit of ,flagstaff ,hill once formed the lower extremity of a sheet of lava and ashes1 which were erupted from the central1 crateriform ridge4 ,judging from the slope of the contemporaneous streams in an adjoining and undisturbed part of the island1 the strata of the ,flagstaff ,hill must have been upturned at least twelve hundred feet1 and probably much more1 for the great truncated dikes on its summit show that it has been largely denuded4 ,the summit of this hill now nearly equals in height the crateriform ridge2 and before having been denuded1 it was probably higher than this ridge1 from which it is separated by a broad and much lower tract of country2 we here1 therefore1 see that the lower extremities of a set of lava-streams have been tilted up to as great a height as1 or perhaps greater height than1 the crater1 down the flanks of which they originally flowed4 ,i believe that dislocations on so grand a scale are extremely rare in volcanic districts4 7,m4 ,constant ,prevost 8,mem4 de la ,soc4 ,geolog40 tome #b observes that 8les produits volcaniques n'ont que localement et rarement meme derange le sol1 a travers lequel ils se sont fait jour407 ,the formation of such numbers of dikes in this part of the island shows that the surface must here have been stretched to a quite extraordinary degree3 this stretching1 on the ridge between ,flagstaff and ,barn ,hills1 probably took place subsequently 7though perhaps immediately so7 to the strata being tilted2 for had the strata at that time extended horizontally1 they would in all probability have been fissured and injected transversely1 instead of in the planes of their stratification4 ,although the space between the ,barn and ,flagstaff ,hill presents a distinct anticlinal line extending north and south1 and though most of the dikes range with much regularity in the same line1 nevertheless1 at only a mile due south of the ridge the strata lie undisturbed4 ,hence the disturbing force seems to have acted under a point1 rather than along a line4 ,the manner in which it has acted1 is probably explained by the structure of ,little ,stony-top1 a mountain #b1jjj feet high1 situated a few miles southward of the ,barn2 we there see1 even from a distance1 a dark-coloured1 sharp1 wedge of compact columnar rock1 with the bright-coloured feldspathic strata1 sloping away on each side from its uncovered apex4 ,this wedge1 from which it derives its name of ,stony-top1 consists of a body of rock1 which has been injected whilst liquified into the overlying strata2 and if we may suppose that a similar body of rock lies injected1 beneath the ridge connecting the ,barn and ,flagstaff1 the structure there exhibited would be explained4 ,,turk',s ,,cap ,,and ,,prosperous ,,bays4 7,,figure #i4 ,,prosperous ,,hill ,,and ,,the ,,barn4 7,section ,s4,s4,e4 7left7 to ,n4,n4,w4 7right7 ,prosperous ,hill through ,hold-fast-,tom and ,flagstaff ,hill to ,the ,barn4 ,the double lines represent the basaltic strata2 the single1 the basal submarine strata2 the dotted1 the upper feldspathic strata47 ,prosperous ,hill is a great1 black1 precipitous mountain1 situated two miles and a half south of the ,barn1 and composed1 like it1 of basaltic strata4 ,these rest1 in one part1 on the brown-coloured1 porphyritic beds of the basal series1 and in another part1 on a fissured mass of highly scoriaceous and amygdaloidal rock1 which seems to have formed a small point of eruption beneath the sea1 contemporaneously with the basal series4 ,prosperous ,hill1 like the ,barn1 is traversed by many dikes1 of which the greater number range north and south1 and its strata dip1 at an angle of about #bj degrees1 rather obliquely from the island towards the sea4 ,the space between ,prosperous ,hill and the ,barn1 as represented in ,figure #i1 consists of lofty cliffs1 composed of the lavas of the upper or feldspathic series1 which rest1 though unconformably1 on the basal submarine strata1 as we have seen that they do at ,flagstaff ,hill4 ,differently1 however1 from in that hill1 these upper strata are nearly horizontal1 gently rising towards the interior of the island2 and they are composed of greenish-black1 or more commonly1 pale brown1 compact lavas1 instead of softened and highly coloured matter4 ,these brown-coloured1 compact lavas1 consist almost entirely of small glimmering scales1 or of minute acicular crystals1 of feldspar1 placed close by the side of each other1 and abounding with minute black specks1 apparently of hornblende4 ,the basaltic strata of ,prosperous ,hill project only a little above the level of the gently-sloping1 feldspathic streams1 which wind round and abut against their upturned edges4 ,the inclination of the basaltic strata seems to be too great to have been caused by their having flowed down a slope1 and they must have been tilted into their present position before the eruption of the feldspathic streams4 ,,basaltic ,,ring4 ,proceeding round the ,island1 the lavas of the upper series1 southward of ,prosperous ,hill1 overhang the sea in lofty precipices4 ,further on1 the headland1 called ,great ,stony-top1 is composed1 as ,i believe1 of basalt2 as is ,long ,range ,point1 on the inland side of which the coloured beds abut4 ,on the southern side of the island1 we see the basaltic strata of the ,south ,barn1 dipping obliquely seaward at a considerable angle2 this headland1 also1 stands a little above the level of the more modern1 feldspathic lavas4 ,further on1 a large space of coast1 on each side of ,sandy ,bay1 has been much denuded1 and there seems to be left only the basal wreck of the great1 central crater4 ,the basaltic strata reappear1 with their seaward dip1 at the foot of the hill1 called ,man-and-,horse2 and thence they are continued along the whole north-western coast to ,sugar-,loaf ,hill1 situated near to the ,flagstaff2 and they everywhere have the same seaward inclination1 and rest1 in some parts at least1 on the lavas of the basal series4 ,we thus see that the circumference of the island is formed by a much-broken ring1 or rather1 a horse-shoe1 of basalt1 open to the south1 and interrupted on the eastern side by many wide breaches4 ,the breadth of this marginal fringe on the north-western side1 where alone it is at all perfect1 appears to vary from a mile to a mile and a half4 ,the basaltic strata1 as well as those of the subjacent basal series1 dip1 with a moderate inclination1 where they have not been subsequently disturbed1 towards the sea4 ,the more broken state of the basaltic ring round the eastern half1 compared with the western half of the island1 is evidently due to the much greater denuding power of the waves on the eastern or windward side1 as is shown by the greater height of the cliffs on that side1 than to leeward4 ,whether the margin of basalt was breached1 before or after the eruption of the lavas of the upper series1 is doubtful2 but as separate portions of the basaltic ring appear to have been tilted before that event1 and from other reasons1 it is more probable1 that some at least of the breaches were first formed4 ,reconstructing in imagination1 as far as is possible1 the ring of basalt1 the internal space or hollow1 which has since been filled up with the matter erupted from the great central crater1 appears to have been of an oval figure1 eight or nine miles in length by about four miles in breadth1 and with its axis directed in a ,n4,e4 and ,s ,w4 line1 coincident with the present longest axis of the island4 ,,the ,,central ,,curved ,,ridge4 ,this ridge consists1 as before remarked1 of grey feldspathic lavas1 and of red1 brecciated1 argillaceous tuffs1 like the beds of the upper coloured series4 ,the grey lavas contain numerous1 minute1 black1 easily fusible specks2 and but very few large crystals of feldspar4 ,they are generally much softened2 with the exception of this character1 and of being in many parts highly cellular1 they are quite similar to those great sheets of lava which overhang the coast at ,prosperous ,bay4 ,considerable intervals of time appear to have elapsed1 judging from the marks of denudation1 between the formation of the successive beds1 of which this ridge is composed4 ,on the steep northern slope1 ,i observed in several sections a much worn undulating surface of red tuff1 covered by grey1 decomposed1 feldspathic lavas1 with only a thin earthy layer interposed between them4 ,in an adjoining part1 ,i noticed a trap-dike1 four feet wide1 cut off and covered up by the feldspathic lava1 as is represented in ,figure #i4 ,the ridge ends on the eastern side in a hook1 which is not represented clearly enough in any map which ,i have seen2 towards the western end1 it gradually slopes down and divides into several subordinate ridges4 ,the best defined portion between ,diana's ,peak and ,nest ,lodge1 which supports the highest pinnacles in the island varying from #b1jjj to #b1gjj feet1 is rather less than three miles long in a straight line4 ,throughout this space the ridge has a uniform appearance and structure2 its curvature resembles that of the coast-line of a great bay1 being made up of many smaller curves1 all open to the south4 ,the northern and outer side is supported by narrow ridges or buttresses1 which slope down to the adjoining country4 ,the inside is much steeper1 and is almost precipitous2 it is formed of the basset edges of the strata1 which gently decline outwards4 ,along some parts of the inner side1 a little way beneath the summit1 a flat ledge extends1 which imitates in outline the smaller curvatures of the crest4 ,ledges of this kind occur not unfrequently within volcanic craters1 and their formation seems to be due to the sinking down of a level sheet of hardened lava1 the edges of which remain 7like the ice round a pool1 from which the water has been drained7 adhering to the sides4 7,a most remarkable instance of this structure is described in ,ellis 8,polynesian ,researches0 second edition where an admirable drawing is given of the successive ledges or terraces1 on the borders of the immense crater at ,hawaii1 in the ,sandwich ,islands47 7,,figure #aj4 ,,dike4 7,section showing layers #a1 #b and #c from top to bottom47 #a4 ,grey feldspathic lava4 #b4 ,a layer1 one inch in thickness1 of a reddish earthy matter4 #c4 ,brecciated1 red1 argillaceous tuff47 ,in some parts1 the ridge is surmounted by a wall or parapet1 perpendicular on both sides4 ,near ,diana's ,peak this wall is extremely narrow4 ,at the ,galapagos ,archipelago ,i observed parapets1 having a quite similar structure and appearance1 surmounting several of the craters2 one1 which ,i more particularly examined1 was composed of glossy1 red scoriae firmly cemented together2 being externally perpendicular1 and extending round nearly the whole circumference of the crater1 it rendered it almost inaccessible4 ,the ,peak of ,teneriffe and ,cotopaxi1 according to ,humboldt1 are similarly constructed2 he states that 8at their summits a circular wall surrounds the crater1 which wall1 at a distance1 has the appearance of a small cylinder placed on a truncated cone4 78,personal ,narrative0 volume #a page #aga47 ,on ,cotopaxi this peculiar structure is visible to the naked eye at more than two thousand toises' distance2 and no person has ever reached its crater4 7,humboldt 8,picturesque ,atlas0 folio plate #aj47 ,on the ,peak of ,teneriffe1 the parapet is so high1 that it would be impossible to reach the caldera1 if on the eastern side there did not exist a breach40 ,the origin of these circular parapets is probably due to the heat or vapours from the crater1 penetrating and hardening the sides to a nearly equal depth1 and afterwards to the mountain being slowly acted on by the weather1 which would leave the hardened part1 projecting in the form of a cylinder or circular parapet4 ,from the points of structure in the central ridge1 now enumerated1--namely1 from the convergence towards it of the beds of the upper series1--from the lavas there becoming highly cellular1--from the flat ledge1 extending along its inner and precipitous side1 like that within some still active craters1--from the parapet-like wall on its summit1--and lastly1 from its peculiar curvature1 unlike that of any common line of elevation1 ,i cannot doubt that this curved ridge forms the last remnant of a great crater4 ,in endeavouring1 however1 to trace its former outline1 one is soon baffled2 its western extremity gradually slopes down1 and1 branching into other ridges1 extends to the sea-coast2 the eastern end is more curved1 but it is only a little better defined4 ,some appearances lead me to suppose that the southern wall of the crater joined the present ridge near ,nest ,lodge2 in this case the crater must have been nearly three miles long1 and about a mile and a half in breadth4 ,had the denudation of the ridge and the decomposition of its constituent rocks proceeded a few steps further1 and had this ridge1 like several other parts of the island1 been broken up by great dikes and masses of injected matter1 we should in vain have endeavoured to discover its true nature4 ,even now we have seen that at ,flagstaff ,hill the lower extremity and most distant portion of one sheet of the erupted matter has been upheaved to as great a height as the crater down which it flowed1 and probably even to a greater height4 ,it is interesting thus to trace the steps by which the structure of a volcanic district becomes obscured1 and finally obliterated3 so near to this last stage is ,st4 ,helena1 that ,i believe no one has hitherto suspected that the central ridge or axis of the island is the last wreck of the crater1 whence the most modern volcanic streams were poured forth4 ,the great hollow space or valley southward of the central curved ridge1 across which the half of the crater must once have extended1 is formed of bare1 water-worn hillocks and ridges of red1 yellow1 and brown rocks1 mingled together in chaos-like confusion1 interlaced by dikes1 and without any regular stratification4 ,the chief part consists of red decomposing scoriae1 associated with various kinds of tuff and yellow argillaceous beds1 full of broken crystals1 those of augite being particularly large4 ,here and there masses of highly cellular and amygdaloidal lavas protrude4 ,from one of the ridges in the midst of the valley1 a conical precipitous hill1 called ,lot1 boldly stands up1 and forms a most singular and conspicuous object4 ,it is composed of phonolite1 divided in one part into great curved laminae1 in another1 into angular concretionary balls1 and in a third part into outwardly radiating columns4 ,at its base the strata of lava1 tuff1 and scoriae1 dip away on all sides 7,abich in his 8,views of ,vesuvius0 plate #f has shown the manner in which beds1 under nearly similar circumstances1 are tilted up4 ,the upper beds are more turned up than the lower2 and he accounts for this1 by showing that the lava insinuates itself horizontally between the lower beds472 the uncovered portion is #aig feet in height 7,this height is given by ,mr4 ,seale in his ,geognosy of the island4 ,the height of the summit above the level of the sea is said to be #a1ddd feet471 and its horizontal section gives an oval figure4 ,the phonolite is of a greenish-grey colour1 and is full of minute acicular crystals of feldspar2 in most parts it has a conchoidal fracture1 and is sonorous1 yet it is crenulated with minute air-cavities4 ,in a ,s4,w4 direction from ,lot1 there are some other remarkable columnar pinnacles1 but of a less regular shape1 namely1 ,lot's ,wife1 and the ,asses' ,ears1 composed of allied kinds of rock4 ,from their flattened shape1 and their relative position to each other1 they are evidently connected on the same line of fissure4 ,it is1 moreover1 remarkable that this same ,n4,e4 and ,s4,w4 line1 joining ,lot and ,lot's ,wife1 if prolonged would intersect ,flagstaff ,hill1 which1 as before stated1 is crossed by numerous dikes running in this direction1 and which has a disturbed structure1 rendering it probable that a great body of once fluid rock lies injected beneath it4 ,in this same great valley there are several other conical masses of injected rock 7one1 ,i observed1 was composed of compact greenstone71 some of which are not connected1 as far as is apparent1 with any line of dike2 whilst others are obviously thus connected4 ,of these dikes1 three or four great lines stretch across the valley in a ,n4,e4 and ,s4,w4 direction1 parallel to that one connecting the ,asses' ,ears1 ,lot's ,wife1 and probably ,lot4 ,the number of these masses of injected rock is a remarkable feature in the geology of ,st4 ,helena4 ,besides those just mentioned1 and the hypothetical one beneath ,flagstaff ,hill1 there is ,little ,stony-top and others1 as ,i have reason to believe1 at the ,man-and-,horse1 and at ,high ,hill4 ,most of these masses1 if not all of them1 have been injected subsequently to the last volcanic eruptions from the central crater4 ,the formation of conical bosses of rock on lines of fissure1 the walls of which are in most cases parallel1 may probably be attributed to inequalities in the tension1 causing small transverse fissures1 and at these points of intersection the edges of the strata would naturally yield1 and be easily turned upwards4 ,finally1 ,i may remark1 that hills of phonolite everywhere are apt to assume singular and even grotesque shapes1 like that of ,lot 7,d',aubuisson in his 8,traite de ,geognosie0 tome #b page #edj particularly remarks that this is the case473 the peak at ,fernando ,noronha offers an instance2 at ,st4 ,jago1 however1 the cones of phonolite1 though tapering1 have a regular form4 ,supposing1 as seems probable1 that all such hillocks or obelisks have originally been injected1 whilst liquified1 into a mould formed by yielding strata1 as certainly has been the case with ,lot1 how are we to account for the frequent abruptness and singularity of their outlines1 compared with similarly injected masses of greenstone and basalt8 ,can it be due to a less perfect degree of fluidity1 which is generally supposed to be characteristic of the allied trachytic lavas8 ,,superficial ,,deposits4 ,soft calcareous sandstone occurs in extensive1 though thin1 superficial beds1 both on the northern and southern shores of the island4 ,it consists of very minute1 equal-sized1 rounded particles of shells1 and other organic bodies1 which partially retain their yellow1 brown1 and pink colours1 and occasionally1 though very rarely1 present an obscure trace of their original external forms4 ,i in vain endeavoured to find a single unrolled fragment of a shell4 ,the colour of the particles is the most obvious character by which their origin can be recognised1 the tints being affected 7and an odour produced7 by a moderate heat1 in the same manner as in fresh shells4 ,the particles are cemented together1 and are mingled with some earthy matter3 the purest masses1 according to ,beatson1 contain #gj per cent of carbonate of lime4 ,the beds1 varying in thickness from two or three feet to fifteen feet1 coat the surface of the ground2 they generally lie on that side of the valley which is protected from the wind1 and they occur at the height of several hundred feet above the level of the sea4 ,their position is the same which sand1 if now drifted by the trade-wind1 would occupy2 and no doubt they thus originated1 which explains the equal size and minuteness of the particles1 and likewise the entire absence of whole shells1 or even of moderately-sized fragments4 ,it is remarkable that at the present day there are no shelly beaches on any part of the coast1 whence calcareous dust could be drifted and winnowed2 we must1 therefore1 look back to a former period when before the land was worn into the present great precipices1 a shelving coast1 like that of ,ascension1 was favourable to the accumulation of shelly detritus4 ,some of the beds of this limestone are between six hundred and seven hundred feet above the sea2 but part of this height may possibly be due to an elevation of the land1 subsequent to the accumulation of the calcareous sand4 ,the percolation of rain-water has consolidated parts of these beds into a solid rock1 and has formed masses of dark brown1 stalagmitic limestone4 ,at the ,sugar-,loaf quarry1 fragments of rock on the adjoining slopes have been thickly coated by successive fine layers of calcareous matter4 7,in the earthy detritus on several parts of this hill1 irregular masses of very impure1 crystallised sulphate of lime occur4 ,as this substance is now being abundantly deposited by the surf at ,ascension1 it is possible that these masses may thus have originated2 but if so1 it must have been at a period when the land stood at a much lower level4 ,this earthy selenite is now found at a height of between six hundred and seven hundred feet47 ,it is singular1 that many of these pebbles have their entire surfaces coated1 without any point of contact having been left uncovered2 hence1 these pebbles must have been lifted up by the slow deposition between them of the successive films of carbonate of lime4 ,masses of white1 finely oolitic rock are attached to the outside of some of these coated pebbles4 ,von ,buch has described a compact limestone at ,lanzarote1 which seems perfectly to resemble the stalagmitic deposition just mentioned3 it coats pebbles1 and in parts is finely oolitic3 it forms a far-extended layer1 from one inch to two or three feet in thickness1 and it occurs at the height of #hjj feet above the sea1 but only on that side of the island exposed to the violent north-western winds4 ,von ,buch remarks1 that it is not found in hollows1 but only on the unbroken and inclined surfaces of the mountain4 78,description des ,isles ,canaries0 page #bic47 ,he believes1 that it has been deposited by the spray which is borne over the whole island by these violent winds4 ,it appears1 however1 to me much more probable that it has been formed1 as at ,st4 ,helena1 by the percolation of water through finely comminuted shells3 for when sand is blown on a much-exposed coast1 it always tends to accumulate on broad1 even surfaces1 which offer a uniform resistance to the winds4 ,at the neighbouring island1 moreover1 of ,feurteventura1 there is an earthy limestone1 which1 according to ,von ,buch1 is quite similar to specimens which he has seen from ,st4 ,helena1 and which he believes to have been formed by the drifting of shelly detritus4 7,idem pages #cad and #cgd47 ,the upper beds of the limestone1 at the above-mentioned quarry on the ,sugar-,loaf ,hill1 are softer1 finer-grained and less pure1 than the lower beds4 ,they abound with fragments of land-shells1 and with some perfect ones2 they contain1 also1 the bones of birds1 and the large eggs1 apparently of water-fowl4 7,colonel ,wilkes1 in a catalogue presented with some specimens to the ,geological ,society1 states that as many as ten eggs were found by one person4 ,dr4 ,buckland has remarked 78,geolog4 ,trans40 volume #e page #dgd7 on these eggs47 ,it is probable that these upper beds remained long in an unconsolidated form1 during which time1 these terrestrial productions were embedded4 ,mr4 ,g4,r4 ,sowerby has kindly examined three species of land-shells1 which ,i procured from this bed1 and has described them in detail4 ,one of them is a ,succinea1 identical with a species now living abundantly on the island2 the two others1 namely1 ,cochlogena fossilis and ,helix biplicata1 are not known in a recent state3 the latter species was also found in another and different locality1 associated with a species of ,cochlogena which is undoubtedly extinct4 ,,beds ,,of ,,extinct ,,land-,,shells4 ,land-shells1 all of which appear to be species now extinct1 occur embedded in earth1 in several parts of the island4 ,the greater number have been found at a considerable height on ,flagstaff ,hill4 ,on the ,n4,w4 side of this hill1 a rain-channel exposes a section of about twenty feet in thickness1 of which the upper part consists of black vegetable mould1 evidently washed down from the heights above1 and the lower part of less black earth1 abounding with young and old shells1 and with their fragments3 part of this earth is slightly consolidated by calcareous matter1 apparently due to the partial decomposition of some of the shells4 ,mr4 ,seale1 an intelligent resident1 who first called attention to these shells1 gave me a large collection from another locality1 where the shells appear to have been embedded in very black earth4 ,mr4 ,g4,r4 ,sowerby has examined these shells1 and has described them4 ,there are seven species1 namely1 one ,cochlogena1 two species of the genus ,cochlicopa1 and four of ,helix2 none of these are known in a recent state1 or have been found in any other country4 ,the smaller species were picked out of the inside of the large shells of the ,cochlogena aurisvulpina4 ,this last-mentioned species is in many respects a very singular one2 it was classed1 even by ,lamarck1 in a marine genus1 and having thus been mistaken for a sea-shell1 and the smaller accompanying species having been overlooked1 the exact localities where it was found have been measured1 and the elevation of this island thus deduced6 ,it is very remarkable that all the shells of this species found by me in one spot1 form a distinct variety1 as described by ,mr4 ,sowerby1 from those procured from another locality by ,mr4 ,seale4 ,as this ,cochlogena is a large and conspicuous shell1 ,i particularly inquired from several intelligent countrymen whether they had ever seen it alive2 they all assured me that they had not1 and they would not even believe that it was a land animal3 ,mr4 ,seale1 moreover1 who was a collector of shells all his life at ,st4 ,helena1 never met with it alive4 ,possibly some of the smaller species may turn out to be yet living kinds2 but1 on the other hand1 the two land- shells which are now living on the island in great numbers1 do not occur embedded1 as far as is yet known1 with the extinct species4 ,i have shown in my 8,journal0 78,journal of ,researches0 page #ehb471 that the extinction of these land-shells possibly may not be an ancient event2 as a great change took place in the state of the island about one hundred and twenty years ago1 when the old trees died1 and were not replaced by young ones1 these being destroyed by the goats and hogs1 which had run wild in numbers1 from the year #aejb4 ,mr4 ,seale states1 that on ,flagstaff ,hill1 where we have seen that the embedded land-shells are especially numerous1 traces are everywhere discoverable1 which plainly indicate that it was once thickly clothed with trees2 at present not even a bush grows there4 ,the thick bed of black vegetable mould which covers the shell-bed1 on the flanks of this hill1 was probably washed down from the upper part1 as soon as the trees perished1 and the shelter afforded by them was lost4 ,,elevation ,,of ,,the ,,land4 ,seeing that the lavas of the basal series1 which are of submarine origin1 are raised above the level of the sea1 and at some places to the height of many hundred feet1 ,i looked out for superficial signs of the elevation of the land4 ,the bottoms of some of the gorges1 which descend to the coast1 are filled up to the depth of about a hundred feet1 by rudely divided layers of sand1 muddy clay1 and fragmentary masses2 in these beds1 ,mr4 ,seale has found the bones of the tropic-bird and of the albatross2 the former now rarely1 and the latter never visiting the island4 ,from the difference between these layers1 and the sloping piles of detritus which rest on them1 ,i suspect that they were deposited1 when the gorges stood beneath the sea4 ,mr4 ,seale1 moreover1 has shown that some of the fissure- like gorges become1 with a concave outline1 gradually rather wider at the bottom than at the top2 and this peculiar structure was probably caused by the wearing action of the sea1 when it entered the lower part of these gorges4 7,a fissure-like gorge1 near ,stony-top1 is said by ,mr4 ,seale to be #hdj feet deep1 and only #aae feet in width47 ,at greater heights1 the evidence of the rise of the land is even less clear3 nevertheless1 in a bay-like depression on the table-land behind ,prosperous ,bay1 at the height of about a thousand feet1 there are flat-topped masses of rock1 which it is scarcely conceivable1 could have been insulated from the surrounding and similar strata1 by any other agency than the denuding action of a sea- beach4 ,much denudation1 indeed1 has been effected at great elevations1 which it would not be easy to explain by any other means3 thus1 the flat summit of the ,barn1 which is #b1jjj feet high1 presents1 according to ,mr4 ,seale1 a perfect network of truncated dikes2 on hills like the ,flagstaff1 formed of soft rock1 we might suppose that the dikes had been worn down and cut off by meteoric agency1 but we can hardly suppose this possible with the hard1 basaltic strata of the ,barn4 ,,coast ,,denudation4 ,the enormous cliffs1 in many parts between one and two thousand feet in height1 with which this prison-like island is surrounded1 with the exception of only a few places1 where narrow valleys descend to the coast1 is the most striking feature in its scenery4 ,we have seen that portions of the basaltic ring1 two or three miles in length by one or two miles in breadth1 and from one to two thousand feet in height1 have been wholly removed4 ,there are1 also1 ledges and banks of rock1 rising out of profoundly deep water1 and distant from the present coast between three and four miles1 which1 according to ,mr4 ,seale1 can be traced to the shore1 and are found to be the continuations of certain well-known great dikes4 ,the swell of the ,atlantic ,ocean has obviously been the active power in forming these cliffs2 and it is interesting to observe that the lesser1 though still great1 height of the cliffs on the leeward and partially protected side of the island 7extending from the ,sugar-,loaf ,hill to ,south ,west ,point71 corresponds with the lesser degree of exposure4 ,when reflecting on the comparatively low coasts of many volcanic islands1 which also stand exposed in the open ocean1 and are apparently of considerable antiquity1 the mind recoils from an attempt to grasp the number of centuries of exposure1 necessary to have ground into mud and to have dispersed the enormous cubic mass of hard rock which has been pared off the circumference of this island4 ,the contrast in the superficial state of ,st4 ,helena1 compared with the nearest island1 namely1 ,ascension1 is very striking4 ,at ,ascension1 the surfaces of the lava-streams are glossy1 as if just poured forth1 their boundaries are well defined1 and they can often be traced to perfect craters1 whence they were erupted2 in the course of many long walks1 ,i did not observe a single dike2 and the coast round nearly the entire circumference is low1 and has been eaten back 7though too much stress must not be placed on this fact1 as the island may have been subsiding7 into a little wall only from ten to thirty feet high4 ,yet during the #cdj years1 since ,ascension has been known1 not even the feeblest signs of volcanic action have been recorded4 7,in the 8,nautical ,magazine0 for #ahce page #fdb1 and for #ahch page #cfa1 and in the 8,comptes ,rendus0 ,april #ahch1 accounts are given of a series of volcanic phenomena--earthquakes--troubled water--floating scoriae and columns of smoke--which have been observed at intervals since the middle of the last century1 in a space of open sea between longitudes #bj degrees and #bb degrees west1 about half a degree south of the equator4 ,these facts seem to show1 that an island or an archipelago is in process of formation in the middle of the ,atlantic3 a line joining ,st4 ,helena and ,ascension1 prolonged1 intersects this slowly nascent focus of volcanic action47 ,on the other hand1 at ,st4 ,helena1 the course of no one stream of lava can be traced1 either by the state of its boundaries or of its superficies2 the mere wreck of one great crater is left2 not the valleys only1 but the surfaces of some of the highest hills1 are interlaced by worn-down dikes1 and1 in many places1 the denuded summits of great cones of injected rock stand exposed and naked2 lastly1 as we have seen1 the entire circuit of the island has been deeply worn back into the grandest precipices4 ,,craters ,,of ,,elevation4 ,there is much resemblance in structure and in geological history between ,st4 ,helena1 ,st4 ,jago1 and ,mauritius4 ,all three islands are bounded 7at least in the parts which ,i was able to examine7 by a ring of basaltic mountains1 now much broken1 but evidently once continuous4 ,these mountains have1 or apparently once had1 their escarpments steep towards the interior of the island1 and their strata dip outwards4 ,i was able to ascertain1 only in a few cases1 the inclination of the beds2 nor was this easy1 for the stratification was generally obscure1 except when viewed from a distance4 ,i feel1 however1 little doubt that1 according to the researches of ,m4 ,elie de ,beaumont1 their average inclination is greater than that which they could have acquired1 considering their thickness and compactness1 by flowing down a sloping surface4 ,at ,st4 ,helena1 and at ,st4 ,jago1 the basaltic strata rest on older and probably submarine beds of different composition4 ,at all three islands1 deluges of more recent lavas have flowed from the centre of the island1 towards and between the basaltic mountains2 and at ,st4 ,helena the central platform has been filled up by them4 ,all three islands have been raised in mass4 ,at ,mauritius the sea1 within a late geological period1 must have reached to the foot of the basaltic mountains1 as it now does at ,st4 ,helena2 and at ,st4 ,jago it is cutting back the intermediate plain towards them4 ,in these three islands1 but especially at ,st4 ,jago and at ,mauritius1 when1 standing on the summit of one of the old basaltic mountains1 one looks in vain towards the centre of the island1--the point towards which the strata beneath one's feet1 and of the mountains on each side1 rudely converge1--for a source whence these strata could have been erupted2 but one sees only a vast hollow platform stretched beneath1 or piles of matter of more recent origin4 ,these basaltic mountains come1 ,i presume1 into the class of ,craters of elevation3 it is immaterial whether the rings were ever completely formed1 for the portions which now exist have so uniform a structure1 that1 if they do not form fragments of true craters1 they cannot be classed with ordinary lines of elevation4 ,with respect to their origin1 after having read the works of ,mr4 ,lyell 78,principles of ,geology0 fifth edition volume #b page #aga471 and of ,,mm4 ,c4 ,prevost and ,virlet1 ,i cannot believe that the great central hollows have been formed by a simple dome-shaped elevation1 and the consequent arching of the strata4 ,on the other hand1 ,i have very great difficulty in admitting that these basaltic mountains are merely the basal fragments of great volcanoes1 of which the summits have either been blown off1 or more probably swallowed up by subsidence4 ,these rings are1 in some instances1 so immense1 as at ,st4 ,jago and at ,mauritius1 and their occurrence is so frequent1 that ,i can hardly persuade myself to adopt this explanation4 ,moreover1 ,i suspect that the following circumstances1 from their frequent concurrence1 are someway connected together1--a connection not implied in either of the above views3 namely1 first1 the broken state of the ring2 showing that the now detached portions have been exposed to great denudation1 and in some cases1 perhaps1 rendering it probable that the ring never was entire2 secondly1 the great amount of matter erupted from the central area after or during the formation of the ring2 and thirdly1 the elevation of the district in mass4 ,as far as relates to the inclination of the strata being greater than that which the basal fragments of ordinary volcanoes would naturally possess1 ,i can readily believe that this inclination might have been slowly acquired by that amount of elevation1 of which1 according to ,m4 ,elie de ,beaumont1 the numerous upfilled fissures or dikes are the evidence and the measure1--a view equally novel and important1 which we owe to the researches of that geologist on ,mount ,etna4 ,a conjecture1 including the above circumstances1 occurred to me1 when1-- with my mind fully convinced1 from the phenomena of #ahce in ,south ,america1 that the forces which eject matter from volcanic orifices and raise continents in mass are identical1--,i viewed that part of the coast of ,st4 ,jago1 where the horizontally upraised1 calcareous stratum dips into the sea1 directly beneath a cone of subsequently erupted lava4 7,i have given a detailed account of these phenomena1 in a paper read before the ,geological ,society in ,march #ahch4 ,at the instant of time1 when an immense area was convulsed and a large tract elevated1 the districts immediately surrounding several of the great vents in the ,cordillera remained quiescent2 the subterranean forces being apparently relieved by the eruptions1 which then recommenced with great violence4 ,an event of somewhat the same kind1 but on an infinitely smaller scale1 appears to have taken place1 according to ,abich 78,views of ,vesuvius0 plates #a and #i71 within the great crater of ,vesuvius1 where a platform on one side of a fissure was raised in mass twenty feet1 whilst on the other side1 a train of small volcanoes burst forth in eruption47 ,the conjecture is that1 during the slow elevation of a volcanic district or island1 in the centre of which one or more orifices continue open1 and thus relieve the subterranean forces1 the borders are elevated more than the central area2 and that the portions thus upraised do not slope gently into the central1 less elevated area1 as does the calcareous stratum under the cone at ,st4 ,jago1 and as does a large part of the circumference of ,iceland1 but that they are separated from it by curved faults4 7,it appears1 from information communicated to me in the most obliging manner by ,m4 ,e4 ,robert1 that the circumferential parts of ,iceland1 which are composed of ancient basaltic strata alternating with tuff1 dip inland1 thus forming a gigantic saucer4 ,m4 ,robert found that this was the case1 with a few and quite local exceptions1 for a space of coast several hundred miles in length4 ,i find this statement corroborated1 as far as regards one place1 by ,mackenzie in his 8,travels0 page #cgg1 and in another place by some ,,ms4 notes kindly lent me by ,dr4 ,holland4 ,the coast is deeply indented by creeks1 at the head of which the land is generally low4 ,m4 ,robert informs me1 that the inwardly dipping strata appear to extend as far as this line1 and that their inclination usually corresponds with the slope of the surface1 from the high coast-mountains to the low land at the head of these creeks4 ,in the section described by ,sir ,g4 ,mackenzie1 the dip is #abj4 ,the interior parts of the island chiefly consist1 as far as is known1 of recently erupted matter4 ,the great size1 however1 of ,iceland1 equalling the bulkiest part of ,england1 ought perhaps to exclude it from the class of islands we have been considering2 but ,i cannot avoid suspecting that if the coast-mountains1 instead of gently sloping into the less elevated central area1 had been separated from it by irregularly curved faults1 the strata would have been tilted seaward1 and a 8,crater of elevation10 like that of ,st4 ,jago or that of ,mauritius1 but of much vaster dimensions1 would have been formed4 ,i will only further remark1 that the frequent occurrence of extensive lakes at the foot of large volcanoes1 and the frequent association of volcanic and fresh-water strata1 seem to indicate that the areas around volcanoes are apt to be depressed beneath the level of the adjoining country1 either from having been less elevated1 or from the effects of subsidence47 ,we might expect1 from what we see along ordinary faults1 that the strata on the upraised side1 already dipping outwards from their original formation as lava-streams1 would be tilted from the line of fault1 and thus have their inclination increased4 ,according to this hypothesis1 which ,i am tempted to extend only to some few cases1 it is not probable that the ring would ever be formed quite perfect2 and from the elevation being slow1 the upraised portions would generally be exposed to much denudation1 and hence the ring become broken2 we might also expect to find occasional inequalities in the dip of the upraised masses1 as is the case at ,st4 ,jago4 ,by this hypothesis the elevation of the districts in mass1 and the flowing of deluges of lava from the central platforms1 are likewise connected together4 ,on this view the marginal basaltic mountains of the three foregoing islands might still be considered as forming 8,craters of elevation20 the kind of elevation implied having been slow1 and the central hollow or platform having been formed1 not by the arching of the surface1 but simply by that part having been upraised to a less height4 ,,chapter ,v4 ,,galapagos ,,archipelago4 ,chatham ,island4 ,craters composed of a peculiar kind of tuff4 ,small basaltic craters1 with hollows at their bases4 ,albemarle ,island2 fluid lavas1 their composition4 ,craters of tuff2 inclination of their exterior diverging strata1 and structure of their interior converging strata4 ,james ,island1 segment of a small basaltic crater2 fluidity and composition of its lava-streams1 and of its ejected fragments4 ,concluding remarks on the craters of tuff1 and on the breached condition of their southern sides4 ,mineralogical composition of the rocks of the archipelago4 ,elevation of the land4 ,direction of the fissures of eruption4 7,,figure #aa4 ,,map #c4 ,,galapagos ,,archipelago4 ,showing ,wenman1 ,abingdon1 ,bindloes1 ,tower1 ,narborough1 ,albemarle1 ,james1 ,indefatigable1 ,barrington1 ,chatham1 ,charles and ,hood's ,islands47 ,this archipelago is situated under the equator1 at a distance of between five and six hundred miles from the west coast of ,south ,america4 ,it consists of five principal islands1 and of several small ones1 which together are equal in area1 but not in extent of land1 to ,sicily1 conjointly with the ,ionian ,islands4 7,i exclude from this measurement1 the small volcanic islands of ,culpepper and ,wenman1 lying seventy miles northward of the group4 ,craters were visible on all the islands of the group1 except on ,towers ,island1 which is one of the lowest2 this island is1 however1 formed of volcanic rocks47 ,they are all volcanic3 on two1 craters have been seen in eruption1 and on several of the other islands1 streams of lava have a recent appearance4 ,the larger islands are chiefly composed of solid rock1 and they rise with a tame outline to a height of between one and four thousand feet4 ,they are sometimes1 but not generally1 surmounted by one principal orifice4 ,the craters vary in size from mere spiracles to huge caldrons several miles in circumference2 they are extraordinarily numerous1 so that ,i should think1 if enumerated1 they would be found to exceed two thousand2 they are formed either of scoriae and lava1 or of a brown-coloured tuff2 and these latter craters are in several respects remarkable4 ,the whole group was surveyed by the officers of the 8,beagle40 ,i visited myself four of the principal islands1 and received specimens from all the others4 ,under the head of the different islands ,i will describe only that which appears to me deserving of attention4 ,,chatham ,,island4 ,,craters ,,composed ,,of ,a ,,singular ,,kind ,,of ,,tuff4 ,towards the eastern end of this island there occur two craters composed of two kinds of tuff2 one kind being friable1 like slightly consolidated ashes2 and the other compact1 and of a different nature from anything which ,i have met with described4 ,this latter substance1 where it is best characterised1 is of a yellowish-brown colour1 translucent1 and with a lustre somewhat resembling resin2 it is brittle1 with an angular1 rough1 and very irregular fracture1 sometimes1 however1 being slightly granular1 and even obscurely crystalline3 it can readily be scratched with a knife1 yet some points are hard enough just to mark common glass2 it fuses with ease into a blackish-green glass4 ,the mass contains numerous broken crystals of olivine and augite1 and small particles of black and brown scoriae2 it is often traversed by thin seams of calcareous matter4 ,it generally affects a nodular or concretionary structure4 ,in a hand specimen1 this substance would certainly be mistaken for a pale and peculiar variety of pitchstone2 but when seen in mass its stratification1 and the numerous layers of fragments of basalt1 both angular and rounded1 at once render its subaqueous origin evident4 ,an examination of a series of specimens shows that this resin-like substance results from a chemical change on small particles of pale and dark-coloured scoriaceous rocks2 and this change could be distinctly traced in different stages round the edges of even the same particle4 ,the position near the coast of all the craters composed of this kind of tuff or peperino1 and their breached condition1 renders it probable that they were all formed when standing immersed in the sea2 considering this circumstance1 together with the remarkable absence of large beds of ashes in the whole archipelago1 ,i think it highly probable that much the greater part of the tuff has originated from the trituration of fragments of the grey1 basaltic lavas in the mouths of craters standing in the sea4 ,it may be asked whether the heated water within these craters has produced this singular change in the small scoriaceous particles and given to them their translucent1 resin-like fracture4 ,or has the associated lime played any part in this change8 ,i ask these questions from having found at ,st4 ,jago1 in the ,cape de ,verde ,islands1 that where a great stream of molten lava has flowed over a calcareous bottom into the sea1 the outermost film1 which in other parts resembles pitchstone1 is changed1 apparently by its contact with the carbonate of lime1 into a resin-like substance1 precisely like the best characterised specimens of the tuff from this archipelago4 7,the concretions containing lime1 which ,i have described at ,ascension1 as formed in a bed of ashes1 present some degree of resemblance to this substance1 but they have not a resinous fracture4 ,at ,st4 ,helena1 also1 ,i found veins of a somewhat similar1 compact1 but non- resinous substance1 occurring in a bed of pumiceous ashes1 apparently free from calcareous matter3 in neither of these cases could heat have acted47 ,to return to the two craters3 one of them stands at the distance of a league from the coast1 the intervening tract consisting of a calcareous tuff1 apparently of submarine origin4 ,this crater consists of a circle of hills some of which stand quite detached1 but all have a very regular1 qua- qua versal dip1 at an inclination of between thirty and forty degrees4 ,the lower beds1 to the thickness of several hundred feet1 consist of the resin- like stone1 with embedded fragments of lava4 ,the upper beds1 which are between thirty and forty feet in thickness1 are composed of a thinly stratified1 fine-grained1 harsh1 friable1 brown-coloured tuff1 or peperino4 7,those geologists who restrict the term of 8tuff0 to ashes of a white colour1 resulting from the attrition of feldspathic lavas1 would call these brown-coloured strata 8peperino407 ,a central mass without any stratification1 which must formerly have occupied the hollow of the crater1 but is now attached only to a few of the circumferential hills1 consists of a tuff1 intermediate in character between that with a resin-like1 and that with an earthy fracture4 ,this mass contains white calcareous matter in small patches4 ,the second crater 7#ebj feet in height7 must have existed until the eruption of a recent1 great stream of lava1 as a separate islet2 a fine section1 worn by the sea1 shows a grand funnel-shaped mass of basalt1 surrounded by steep1 sloping flanks of tuff1 having in parts an earthy1 and in others a semi-resinous fracture4 ,the tuff is traversed by several broad1 vertical dikes1 with smooth and parallel sides1 which ,i did not doubt were formed of basalt1 until ,i actually broke off fragments4 ,these dikes1 however1 consist of tuff like that of the surrounding strata1 but more compact1 and with a smoother fracture2 hence we must conclude1 that fissures were formed and filled up with the finer mud or tuff from the crater1 before its interior was occupied1 as it now is1 by a solidified pool of basalt4 ,other fissures have been subsequently formed1 parallel to these singular dikes1 and are merely filled with loose rubbish4 ,the change from ordinary scoriaceous particles to the substance with a semi-resinous fracture1 could be clearly followed in portions of the compact tuff of these dikes4 7,,figure #ab4 ,,the ,,kicker ,,rock1 #djj ,,feet ,,high47 ,at the distance of a few miles from these two craters1 stands the ,kicker ,rock1 or islet1 remarkable from its singular form4 ,it is unstratified1 and is composed of compact tuff1 in parts having the resin-like fracture4 ,it is probable that this amorphous mass1 like that similar mass in the case first described1 once filled up the central hollow of a crater1 and that its flanks1 or sloping walls1 have since been worn quite away by the sea1 in which it stands exposed4 ,,small ,,basaltic ,,craters4 ,a bare1 undulating tract1 at the eastern end of ,chatham ,island1 is remarkable from the number1 proximity1 and form of the small basaltic craters with which it is studded4 ,they consist1 either of a mere conical pile1 or1 but less commonly1 of a circle1 of black and red1 glossy scoriae1 partially cemented together4 ,they vary in diameter from thirty to one hundred and fifty yards1 and rise from about fifty to one hundred feet above the level of the surrounding plain4 ,from one small eminence1 ,i counted sixty of these craters1 all of which were within a third of a mile from each other1 and many were much closer4 ,i measured the distance between two very small craters1 and found that it was only thirty yards from the summit-rim of one to the rim of the other4 ,small streams of black1 basaltic lava1 containing olivine and much glassy feldspar1 have flowed from many1 but not from all of these craters4 ,the surfaces of the more recent streams were exceedingly rugged1 and were crossed by great fissures2 the older streams were only a little less rugged2 and they were all blended and mingled together in complete confusion4 ,the different growth1 however1 of the trees on the streams1 often plainly marked their different ages4 ,had it not been for this latter character1 the streams could in few cases have been distinguished2 and1 consequently1 this wide undulatory tract might have 7as probably many tracts have7 been erroneously considered as formed by one great deluge of lava1 instead of by a multitude of small streams1 erupted from many small orifices4 ,in several parts of this tract1 and especially at the base of the small craters1 there are circular pits1 with perpendicular sides1 from twenty to forty feet deep4 ,at the foot of one small crater1 there were three of these pits4 ,they have probably been formed1 by the falling in of the roofs of small caverns4 7,m4 ,elie de ,beaumont has described 78,mem4 pour servir0 etc4 tome #d page #aac7 many 8petits cirques d'eboulement0 on ,etna1 of some of which the origin is historically known47 ,in other parts1 there are mammiform hillocks1 which resemble great bubbles of lava1 with their summits fissured by irregular cracks1 which appeared1 upon entering them1 to be very deep2 lava has not flowed from these hillocks4 ,there are1 also1 other very regular1 mammiform hillocks1 composed of stratified lava1 and surmounted by circular1 steep-sided hollows1 which1 ,i suppose have been formed by a body of gas1 first1 arching the strata into one of the bubble- like hillocks1 and then1 blowing off its summit4 ,these several kinds of hillocks and pits1 as well as the numerous1 small1 scoriaceous craters1 all show that this tract has been penetrated1 almost like a sieve1 by the passage of heated vapours4 ,the more regular hillocks could only have been heaved up1 whilst the lava was in a softened state4 7,sir ,g4 ,mackenzie 8,travels in ,iceland0 pages #chi to #cib1 has described a plain of lava at the foot of ,hecla1 everywhere heaved up into great bubbles or blisters4 ,sir ,george states that this cavernous lava composes the uppermost stratum2 and the same fact is affirmed by ,von ,buch 8,descript4 des ,isles ,canaries0 page #aei1 with respect to the basaltic stream near ,rialejo1 in ,teneriffe4 ,it appears singular that it should be the upper streams that are chiefly cavernous1 for one sees no reason why the upper and lower should not have been equally affected at different times2--have the inferior streams flowed beneath the pressure of the sea1 and thus been flattened1 after the passage through them1 of bodies of gas87 ,,albemarle ,,island4 ,this island consists of five1 great1 flat-topped craters1 which1 together with the one on the adjoining island of ,narborough1 singularly resemble each other1 in form and height4 ,the southern one is #d1gjj feet high1 two others are #c1gbj feet1 a third only #ej feet higher1 and the remaining ones apparently of nearly the same height4 ,three of these are situated on one line1 and their craters appear elongated in nearly the same direction4 ,the northern crater1 which is not the largest1 was found by the triangulation to measure1 externally1 no less than three miles and one-eighth of a mile in diameter4 ,over the lips of these great1 broad caldrons1 and from little orifices near their summits1 deluges of black lava have flowed down their naked sides4 ,,fluidity ,,of ,,different ,,lavas4 ,near ,tagus or ,banks' ,cove1 ,i examined one of these great streams of lava1 which is remarkable from the evidence of its former high degree of fluidity1 especially when its composition is considered4 ,near the sea-coast this stream is several miles in width4 ,it consists of a black1 compact base1 easily fusible into a black bead1 with angular and not very numerous air-cells1 and thickly studded with large1 fractured crystals of glassy albite1 varying from the tenth of an inch to half an inch in diameter4 7,in the ,cordillera of ,chile1 ,i have seen lava very closely resembling this variety at the ,galapagos ,archipelago4 ,it contained1 however1 besides the albite1 well-formed crystals of augite1 and the base 7perhaps in consequence of the aggregation of the augitic particles7 was a shade lighter in colour4 ,i may here remark1 that in all these cases1 ,i call the feldspathic crystals1 8albite10 from their cleavage-planes 7as measured by the reflecting goniometer7 corresponding with those of that mineral4 ,as1 however1 other species of this genus have lately been discovered to cleave in nearly the same planes with albite1 this determination must be considered as only provisional4 ,i examined the crystals in the lavas of many different parts of the ,galapagos group1 and ,i found that none of them1 with the exception of some crystals from one part of ,james ,island1 cleaved in the direction of orthite or potash-feldspar47 ,this lava1 although at first sight appearing eminently porphyritic1 cannot properly be considered so1 for the crystals have evidently been enveloped1 rounded1 and penetrated by the lava1 like fragments of foreign rock in a trap-dike4 ,this was very clear in some specimens of a similar lava1 from ,abingdon ,island1 in which the only difference was1 that the vesicles were spherical and more numerous4 ,the albite in these lavas is in a similar condition with the leucite of ,vesuvius1 and with the olivine1 described by ,von ,buch1 as projecting in great balls from the basalt of ,lanzarote4 78,description des ,isles ,canaries0 page #bie47 ,besides the albite1 this lava contains scattered grains of a green mineral1 with no distinct cleavage1 and closely resembling olivine 7,humboldt mentions that he mistook a green augitic mineral1 occurring in the volcanic rocks of the ,cordillera of ,quito1 for olivine472 but as it fuses easily into a green glass1 it belongs probably to the augitic family3 at ,james ,island1 however1 a similar lava contained true olivine4 ,i obtained specimens from the actual surface1 and from a depth of four feet1 but they differed in no respect4 ,the high degree of fluidity of this lava-stream was at once evident1 from its smooth and gently sloping surface1 from the manner in which the main stream was divided by small inequalities into little rills1 and especially from the manner in which its edges1 far below its source1 and where it must have been in some degree cooled1 thinned out to almost nothing2 the actual margin consisting of loose fragments1 few of which were larger than a man's head4 ,the contrast between this margin1 and the steep walls1 above twenty feet high1 bounding many of the basaltic streams at ,ascension1 is very remarkable4 ,it has generally been supposed that lavas abounding with large crystals1 and including angular vesicles1 have possessed little fluidity2 but we see that the case has been very different at ,albemarle ,island4 7,the irregular and angular form of the vesicles is probably caused by the unequal yielding of a mass composed1 in almost equal proportion1 of solid crystals and of a viscid base4 ,it certainly seems a general circumstance1 as might have been expected1 that in lava1 which has possessed a high degree of fluidity1 ,,as ,,well ,,as ,,an ,,even-,,sized ,,grain1 the vesicles are internally smooth and spherical47 ,the degree of fluidity in different lavas1 does not seem to correspond with any ,,apparent corresponding amount of difference in their composition3 at ,chatham ,island1 some streams1 containing much glassy albite and some olivine1 are so rugged1 that they may be compared to a sea frozen during a storm2 whilst the great stream at ,albemarle ,island is almost as smooth as a lake when ruffled by a breeze4 ,at ,james ,island1 black basaltic lava1 abounding with small grains of olivine1 presents an intermediate degree of roughness2 its surface being glossy1 and the detached fragments resembling1 in a very singular manner1 folds of drapery1 cables1 and pieces of the bark of trees4 7,a specimen of basaltic lava1 with a few small broken crystals of albite1 given me by one of the officers1 is perhaps worthy of description4 ,it consists of cylindrical ramifications1 some of which are only the twentieth of an inch in diameter1 and are drawn out into the sharpest points4 ,the mass has not been formed like a stalactite1 for the points terminate both upwards and downwards4 ,globules1 only the fortieth of an inch in diameter1 have dropped from some of the points1 and adhere to the adjoining branches4 ,the lava is vesicular1 but the vesicles never reach the surface of the branches1 which are smooth and glossy4 ,as it is generally supposed that vesicles are always elongated in the direction of the movement of the fluid mass1 ,i may observe1 that in these cylindrical branches1 which vary from a quarter to only the twentieth of an inch in diameter1 every air-cell is spherical47 ,,craters ,,of ,,tuff4 ,about a mile southward of ,banks' ,cove1 there is a fine elliptic crater1 about five hundred feet in depth1 and three-quarters of a mile in diameter4 ,its bottom is occupied by a lake of brine1 out of which some little crateriform hills of tuff rise4 ,the lower beds are formed of compact tuff1 appearing like a subaqueous deposit2 whilst the upper beds1 round the entire circumference1 consist of a harsh1 friable tuff1 of little specific gravity1 but often containing fragments of rock in layers4 ,this upper tuff contains numerous pisolitic balls1 about the size of small bullets1 which differ from the surrounding matter1 only in being slightly harder and finer grained4 ,the beds dip away very regularly on all sides1 at angles varying1 as ,i found by measurement1 from twenty-five to thirty degrees4 ,the external surface of the crater slopes at a nearly similar inclination1 and is formed by slightly convex ribs1 like those on the shell of a pecten or scallop1 which become broader as they extend from the mouth of the crater to its base4 ,these ribs are generally from eight to twenty feet in breadth1 but sometimes they are as much as forty feet broad2 and they resemble old1 plastered1 much flattened vaults1 with the plaster scaling off in plates3 they are separated from each other by gullies1 deepened by alluvial action4 ,at their upper and narrow ends1 near the mouth of the crater1 these ribs often consist of real hollow passages1 like1 but rather smaller than1 those often formed by the cooling of the crust of a lava-stream1 whilst the inner parts have flowed onward2--of which structure ,i saw many examples at ,chatham ,island4 ,there can be no doubt but that these hollow ribs or vaults have been formed in a similar manner1 namely1 by the setting or hardening of a superficial crust on streams of mud1 which have flowed down from the upper part of the crater4 ,in another part of this same crater1 ,i saw open concave gutters between one and two feet wide1 which appear to have been formed by the hardening of the lower surface of a mud stream1 instead of1 as in the former case1 of the upper surface4 ,from these facts ,i think it is certain that the tuff must have flowed as mud4 7,this conclusion is of some interest1 because ,m4 ,dufrenoy 8,mem4 pour servir0 tome #d page #bgd1 has argued from strata of tuff1 apparently of similar composition with that here described1 being inclined at angles between #ah degrees and #bj degrees1 that ,monte ,nuevo and some other craters of ,southern ,italy have been formed by upheaval4 ,from the facts given above1 of the vaulted character of the separate rills1 and from the tuff not extending in horizontal sheets round these crateriform hills1 no one will suppose that the strata have here been produced by elevation2 and yet we see that their inclination is above #bj degrees1 and often as much as #cj degrees4 ,the consolidated strata also1 of the internal talus1 as will be immediately seen1 dips at an angle of above #cj degrees47 ,this mud may have been formed either within the crater1 or from ashes deposited on its upper parts1 and afterwards washed down by torrents of rain4 ,the former method1 in most of the cases1 appears the more probable one2 at ,james ,island1 however1 some beds of the friable kind of tuff extend so continuously over an uneven surface1 that probably they were formed by the falling of showers of ashes4 ,within this same crater1 strata of coarse tuff1 chiefly composed of fragments of lava1 abut1 like a consolidated talus1 against the inside walls4 ,they rise to a height of between one hundred and one hundred and fifty feet above the surface of the internal brine-lake2 they dip inwards1 and are inclined at an angle varying from thirty to thirty-six degrees4 ,they appear to have been formed beneath water1 probably at a period when the sea occupied the hollow of the crater4 ,i was surprised to observe that beds having this great inclination did not1 as far as they could be followed1 thicken towards their lower extremities4 ,,banks' ,,cove4 7,,figure #ac4 ,a ,,sectional ,,sketch ,,of ,,the ,,headlands ,,forming ,,banks' ,,cove1 showing the diverging crateriform strata1 and the converging stratified talus4 ,the highest point of these hills is #hag feet above the sea47 ,this harbour occupies part of the interior of a shattered crater of tuff larger than that last described4 ,all the tuff is compact1 and includes numerous fragments of lava2 it appears like a subaqueous deposit4 ,the most remarkable feature in this crater is the great development of strata converging inwards1 as in the last case1 at a considerable inclination1 and often deposited in irregular curved layers4 ,these interior converging beds1 as well as the proper1 diverging crateriform strata1 are represented in ,figure #ac1 a rude1 sectional sketch of the headlands1 forming this ,cove4 ,the internal and external strata differ little in composition1 and the former have evidently resulted from the wear and tear1 and redeposition of the matter forming the external crateriform strata4 ,from the great development of these inner beds1 a person walking round the rim of this crater might fancy himself on a circular anticlinal ridge of stratified sandstone and conglomerate4 ,the sea is wearing away the inner and outer strata1 and especially the latter2 so that the inwardly converging strata will1 perhaps1 in some future age1 be left standing alone--a case which might at first perplex a geologist4 7,i believe that this case actually occurs in the ,azores1 where ,dr4 ,webster 8,description0 page #ahe1 has described a basin-formed1 little island1 composed of ,,strata ,,of ,,tuff1 dipping inwards and bounded externally by steep sea-worn cliffs4 ,dr4 ,daubeny supposes 8,volcanoes0 page #bff1 that this cavity must have been formed by a circular subsidence4 ,it appears to me far more probable1 that we here have strata which were originally deposited within the hollow of a crater1 of which the exterior walls have since been removed by the sea47 ,,james ,,island4 ,two craters of tuff on this island are the only remaining ones which require any notice4 ,one of them lies a mile and a half inland from ,puerto ,grande3 it is circular1 about the third of a mile in diameter1 and #djj feet in depth4 ,it differs from all the other tuff-craters which ,i examined1 in having the lower part of its cavity1 to the height of between one hundred and one hundred and fifty feet1 formed by a precipitous wall of basalt1 giving to the crater the appearance of having burst through a solid sheet of rock4 ,the upper part of this crater consists of strata of the altered tuff1 with a semi-resinous fracture4 ,its bottom is occupied by a shallow lake of brine1 covering layers of salt1 which rest on deep black mud4 ,the other crater lies at the distance of a few miles1 and is only remarkable from its size and perfect condition4 ,its summit is #a1bjj feet above the level of the sea1 and the interior hollow is #fjj feet deep4 ,its external sloping surface presented a curious appearance from the smoothness of the wide layers of tuff1 which resembled a vast plastered floor4 ,brattle ,island is1 ,i believe1 the largest crater in the ,archipelago composed of tuff2 its interior diameter is nearly a nautical mile4 ,at present it is in a ruined condition1 consisting of little more than half a circle open to the south2 its great size is probably due1 in part1 to internal degradation1 from the action of the sea4 ,,segment ,,of ,a ,,basaltic ,,crater4 7,,figure #ad4 ,,segment ,,of ,a ,,very ,,small ,,orifice ,,of ,,eruption1 on the beach of ,fresh-water ,bay47 ,one side of ,fresh-water ,bay1 in ,james ,island1 is bounded by a promontory1 which forms the last wreck of a great crater4 ,on the beach of this promontory1 a quadrant-shaped segment of a small subordinate point of eruption stands exposed4 ,it consists of nine separate little streams of lava piled upon each other2 and of an irregular pinnacle1 about fifteen feet high1 of reddish-brown1 vesicular basalt1 abounding with large crystals of glassy albite1 and with fused augite4 ,this pinnacle1 and some adjoining paps of rock on the beach1 represent the axis of the crater4 ,the streams of lava can be followed up a little ravine1 at right angles to the coast1 for between ten and fifteen yards1 where they are hidden by detritus3 along the beach they are visible for nearly eighty yards1 and ,i do not believe that they extend much further4 ,the three lower streams are united to the pinnacle2 and at the point of junction 7as shown in ,figure #ad1 a rude sketch made on the spot71 they are slightly arched1 as if in the act of flowing over the lip of the crater4 ,the six upper streams no doubt were originally united to this same column before it was worn down by the sea4 ,the lava of these streams is of similar composition with that of the pinnacle1 excepting that the crystals of albite appear to be more comminuted1 and the grains of fused augite are absent4 ,each stream is separated from the one above it by a few inches1 or at most by one or two feet in thickness1 of loose fragmentary scoriae1 apparently derived from the abrasion of the streams in passing over each other4 ,all these streams are very remarkable from their thinness4 ,i carefully measured several of them2 one was eight inches thick1 but was firmly coated with three inches above1 and three inches below1 of red scoriaceous rock 7which is the case with all the streams71 making altogether a thickness of fourteen inches3 this thickness was preserved quite uniformly along the entire length of the section4 ,a second stream was only eight inches thick1 including both the upper and lower scoriaceous surfaces4 ,until examining this section1 ,i had not thought it possible that lava could have flowed in such uniformly thin sheets over a surface far from smooth4 ,these little streams closely resemble in composition that great deluge of lava at ,albemarle ,island1 which likewise must have possessed a high degree of fluidity4 ,,pseudo-,,extraneous1 ,,ejected ,,fragments4 ,in the lava and in the scoriae of this little crater1 ,i found several fragments1 which1 from their angular form1 their granular structure1 their freedom from air-cells1 their brittle and burnt condition1 closely resembled those fragments of primary rocks which are occasionally ejected1 as at ,ascension1 from volcanoes4 ,these fragments consist of glassy albite1 much mackled1 and with very imperfect cleavages1 mingled with semi-rounded grains1 having tarnished1 glossy surfaces1 of a steel-blue mineral4 ,the crystals of albite are coated by a red oxide of iron1 appearing like a residual substance2 and their cleavage-planes also are sometimes separated by excessively fine layers of this oxide1 giving to the crystals the appearance of being ruled like a glass micrometer4 ,there was no quartz4 ,the steel-blue mineral1 which is abundant in the pinnacle1 but which disappears in the streams derived from the pinnacle1 has a fused appearance1 and rarely presents even a trace of cleavage2 ,i obtained1 however1 one measurement1 which proved that it was augite2 and in one other fragment1 which differed from the others1 in being slightly cellular1 and in gradually blending into the surrounding matrix the small grains of this mineral were tolerably well crystallised4 ,although there is so wide a difference in appearance between the lava of the little streams1 and especially of their red scoriaceous crusts1 and one of these angular ejected fragments1 which at first sight might readily be mistaken for syenite1 yet ,i believe that the lava has originated from the melting and movement of a mass of rock of absolutely similar composition with the fragments4 ,besides the specimen above alluded to1 in which we see a fragment becoming slightly cellular1 and blending into the surrounding matrix1 some of the grains of the steel-blue augite also have their surfaces becoming very finely vesicular1 and passing into the nature of the surrounding paste2 other grains are throughout1 in an intermediate condition4 ,the paste seems to consist of the augite more perfectly fused1 or1 more probably1 merely disturbed in its softened state by the movement of the mass1 and mingled with the oxide of iron and with finely comminuted1 glassy albite4 ,hence probably it is that the fused albite1 which is abundant in the pinnacle1 disappears in the streams4 ,the albite is in exactly the same state1 with the exception of most of the crystals being smaller in the lava and in the embedded fragments2 but in the fragments they appear to be less abundant3 this1 however1 would naturally happen from the intumescence of the augitic base1 and its consequent apparent increase in bulk4 ,it is interesting thus to trace the steps by which a compact granular rock becomes converted into a vesicular1 pseudo-porphyritic lava1 and finally into red scoriae4 ,the structure and composition of the embedded fragments show that they are parts either of a mass of primary rock which has undergone considerable change from volcanic action1 or more probably of the crust of a body of cooled and crystallised lava1 which has afterwards been broken up and re-liquified2 the crust being less acted on by the renewed heat and movement4 ,,concluding ,,remarks ,,on ,,the ,,tuff-,,craters4 ,these craters1 from the peculiarity of the resin-like substance which enters largely into their composition1 from their structure1 their size and number1 present the most striking feature in the geology of this ,archipelago4 ,the majority of them form either separate islets1 or promontories attached to the larger islands2 and those which now stand at some little distance from the coast are worn and breached1 as if by the action of the sea4 ,from this general circumstance of their position1 and from the small quantity of ejected ashes in any part of the ,archipelago1 ,i am led to conclude1 that the tuff has been chiefly produced1 by the grinding together of fragments of lava within active craters1 communicating with the sea4 ,in the origin and composition of the tuff1 and in the frequent presence of a central lake of brine and of layers of salt1 these craters resemble1 though on a gigantic scale1 the 8salses10 or hillocks of mud1 which are common in some parts of ,italy and in other countries4 7,d',aubuisson 8,traite de ,geognosie0 tome #a page #ahi4 ,i may remark1 that ,i saw at ,terceira1 in the ,azores1 a crater of tuff or peperino1 very similar to these of the ,galapagos ,archipelago4 ,from the description given in ,freycinet 8,voyage10 similar ones occur at the ,sandwich ,islands2 and probably they are present in many other places47 ,their closer connection1 however1 in this ,archipelago1 with ordinary volcanic action1 is shown by the pools of solidified basalt1 with which they are sometimes filled up4 ,it at first appears very singular1 that all the craters formed of tuff have their southern sides1 either quite broken down and wholly removed1 or much lower than the other sides4 ,i saw and received accounts of twenty-eight of these craters2 of these1 twelve form separate islets 7,these consist of the three ,crossman ,islets1 the largest of which is #fjj feet in height2 ,enchanted ,island2 ,gardner ,island 7#gfj feet high72 ,champion ,island 7#cca feet high72 ,enderby ,island2 ,brattle ,island2 two islets near ,indefatigable ,island2 and one near ,james ,island4 ,a second crater near ,james ,island 7with a salt lake in its centre7 has its southern side only about twenty feet high1 whilst the other parts of the circumference are about three hundred feet in height471 and now exist as mere crescents quite open to the south1 with occasionally a few points of rock marking their former circumference3 of the remaining sixteen1 some form promontories1 and others stand at a little distance inland from the shore2 but all have their southern sides either the lowest1 or quite broken down4 ,two1 however1 of the sixteen had their northern sides also low1 whilst their eastern and western sides were perfect4 ,i did not see1 or hear of1 a single exception to the rule1 of these craters being broken down or low on the side1 which faces a point of the horizon between ,s4,e4 and ,s4,w4 ,this rule does not apply to craters composed of lava and scoriae4 ,the explanation is simple3 at this ,archipelago1 the waves from the trade-wind1 and the swell propagated from the distant parts of the open ocean1 coincide in direction 7which is not the case in many parts of the ,pacific71 and with their united forces attack the southern sides of all the islands2 and consequently the southern slope1 even when entirely formed of hard basaltic rock1 is invariably steeper than the northern slope4 ,as the tuff-craters are composed of a soft material1 and as probably all1 or nearly all1 have at some period stood immersed in the sea1 we need not wonder that they should invariably exhibit on their exposed sides the effects of this great denuding power4 ,judging from the worn condition of many of these craters1 it is probable that some have been entirely washed away4 ,as there is no reason to suppose1 that the craters formed of scoriae and lava were erupted whilst standing in the sea1 we can see why the rule does not apply to them4 ,at ,ascension1 it was shown that the mouths of the craters1 which are there all of terrestrial origin1 have been affected by the trade-wind2 and this same power might here1 also1 aid in making the windward and exposed sides of some of the craters originally the lowest4 ,,mineralogical ,,composition ,,of ,,the ,,rocks4 ,in the northern islands1 the basaltic lavas seem generally to contain more albite than they do in the southern half of the ,archipelago2 but almost all the streams contain some4 ,the albite is not unfrequently associated with olivine4 ,i did not observe in any specimen distinguishable crystals of hornblende or augite2 ,i except the fused grains in the ejected fragments1 and in the pinnacle of the little crater1 above described4 ,i did not meet with a single specimen of true trachyte2 though some of the paler lavas1 when abounding with large crystals of the harsh and glassy albite1 resemble in some degree this rock2 but in every case the basis fuses into a black enamel4 ,beds of ashes and far-ejected scoriae1 as previously stated1 are almost absent2 nor did ,i see a fragment of obsidian or of pumice4 ,von ,buch believes that the absence of pumice on ,mount ,etna is consequent on the feldspar being of the ,labrador variety 78,description des ,isles ,canaries0 page #cbh472 if the presence of pumice depends on the constitution of the feldspar1 it is remarkable1 that it should be absent in this archipelago1 and abundant in the ,cordillera of ,south ,america1 in both of which regions the feldspar is of the albitic variety4 ,owing to the absence of ashes1 and the general indecomposable character of the lava in this ,archipelago1 the islands are slowly clothed with a poor vegetation1 and the scenery has a desolate and frightful aspect4 ,,elevation ,,of ,,the ,,land4 ,proofs of the rising of the land are scanty and imperfect4 ,at ,chatham ,island1 ,i noticed some great blocks of lava1 cemented by calcareous matter1 containing recent shells2 but they occurred at the height of only a few feet above high-water mark4 ,one of the officers gave me some fragments of shells1 which he found embedded several hundred feet above the sea1 in the tuff of two craters1 distant from each other4 ,it is possible1 that these fragments may have been carried up to their present height in an eruption of mud2 but as1 in one instance1 they were associated with broken oyster- shells1 almost forming a layer1 it is more probable that the tuff was uplifted with the shells in mass4 ,the specimens are so imperfect that they can be recognised only as belonging to recent marine genera4 ,on ,charles ,island1 ,i observed a line of great rounded blocks1 piled on the summit of a vertical cliff1 at the height of fifteen feet above the line1 where the sea now acts during the heaviest gales4 ,this appeared1 at first1 good evidence in favour of the elevation of the land2 but it was quite deceptive1 for ,i afterwards saw on an adjoining part of this same coast1 and heard from eye- witnesses1 that wherever a recent stream of lava forms a smooth inclined plane1 entering the sea1 the waves during gales have the power of ,,rolling ,,up ,,rounded blocks to a great height1 above the line of their ordinary action4 ,as the little cliff in the foregoing case is formed by a stream of lava1 which1 before being worn back1 must have entered the sea with a gently sloping surface1 it is possible or rather it is probable1 that the rounded boulders1 now lying on its summit1 are merely the remnants of those which had been ,,rolled ,,up during storms to their present height4 ,,direction ,,of ,,the ,,fissures ,,of ,,eruption4 ,the volcanic orifices in this group cannot be considered as indiscriminately scattered4 ,three great craters on ,albermarle ,island form a well-marked line1 extending ,n4,w4 by ,n4 and ,s4,e4 by ,s4 ,narborough ,island1 and the great crater on the rectangular projection of ,albemarle ,island1 form a second parallel line4 ,to the east1 ,hood's ,island1 and the islands and rocks between it and ,james ,island1 form another nearly parallel line1 which1 when prolonged1 includes ,culpepper and ,wenman ,islands1 lying seventy miles to the north4 ,the other islands lying further eastward1 form a less regular fourth line4 ,several of these islands1 and the vents on ,albemarle ,island1 are so placed1 that they likewise fall on a set of rudely parallel lines1 intersecting the former lines at right angles2 so that the principal craters appear to lie on the points where two sets of fissures cross each other4 ,the islands themselves1 with the exception of ,albemarle ,island1 are not elongated in the same direction with the lines on which they stand4 ,the direction of these islands is nearly the same with that which prevails in so remarkable a manner in the numerous archipelagoes of the great ,pacific ,ocean4 ,finally1 ,i may remark1 that amongst the ,galapagos ,islands there is no one dominant vent much higher than all the others1 as may be observed in many volcanic archipelagoes3 the highest is the great mound on the south- western extremity of ,albemarle ,island1 which exceeds by barely a thousand feet several other neighbouring craters4 ,,chapter ,,vi4 ,,trachyte ,,and ,,basalt4--,,distribution ,,of ,,volcanic ,,isles4 ,the sinking of crystals in fluid lava4 ,specific gravity of the constituent parts of trachyte and of basalt1 and their consequent separation4 ,obsidian4 ,apparent non-separation of the elements of plutonic rocks4 ,origin of trap-dikes in the plutonic series4 ,distribution of volcanic islands2 their prevalence in the great oceans4 ,they are generally arranged in lines4 ,the central volcanoes of ,von ,buch doubtful4 ,volcanic islands bordering continents4 ,antiquity of volcanic islands1 and their elevation in mass4 ,eruptions on parallel lines of fissure within the same geological period4 ,,on ,,the ,,separation ,,of ,,the ,,constituent ,,minerals ,,of ,,lava1 ,,according ,,to ,,their ,,specific ,,gravities4 ,one side of ,fresh-water ,bay1 in ,james ,island1 is formed by the wreck of a large crater1 mentioned in the last chapter1 of which the interior has been filled up by a pool of basalt1 about two hundred feet in thickness4 ,this basalt is of a grey colour1 and contains many crystals of glassy albite1 which become much more numerous in the lower1 scoriaceous part4 ,this is contrary to what might have been expected1 for if the crystals had been originally disseminated in equal numbers1 the greater intumescence of this lower scoriaceous part would have made them appear fewer in number4 ,von ,buch has described a stream of obsidian on the ,peak of ,teneriffe1 in which the crystals of feldspar become more and more numerous1 as the depth or thickness increases1 so that near the lower surface of the stream the lava even resembles a primary rock4 78,description des ,isles ,canaries0 pages #aij and #aia47 ,von ,buch further states1 that ,m4 ,dree1 in his experiments in melting lava1 found that the crystals of feldspar always tended to precipitate themselves to the bottom of the crucible4 ,in these cases1 ,i presume there can be no doubt that the crystals sink from their weight4 7,in a mass of molten iron1 it is found 78,edinburgh ,new ,philosophical ,journal0 volume #bd page #ff7 that the substances1 which have a closer affinity for oxygen than iron has1 rise from the interior of the mass to the surface4 ,but a similar cause can hardly apply to the separation of the crystals of these lava-streams4 ,the cooling of the surface of lava seems1 in some cases1 to have affected its composition2 for ,dufrenoy 78,mem4 pour servir0 tome #d page #bga7 found that the interior parts of a stream near ,naples contained two-thirds of a mineral which was acted on by acids1 whilst the surface consisted chiefly of a mineral unattackable by acids47 ,the specific gravity of feldspar varies from #b4d to #b4eh1 whilst obsidian seems commonly to be from #b4c to #b4d2 and in a fluidified state its specific gravity would probably be less1 which would facilitate the sinking of the crystals of feldspar4 7,i have taken the specific gravities of the simple minerals from ,von ,kobell1 one of the latest and best authorities1 and of the rocks from various authorities4 ,obsidian1 according to ,phillips1 is #b4ce2 and ,jameson says it never exceeds #b4d2 but a specimen from ,ascension1 weighed by myself1 was #b4db47 ,at ,james ,island1 the crystals of albite1 though no doubt of less weight than the grey basalt1 in the parts where compact1 might easily be of greater specific gravity than the scoriaceous mass1 formed of melted lava and bubbles of heated gas4 ,the sinking of crystals through a viscid substance like molten rock1 as is unequivocally shown to have been the case in the experiments of ,m4 ,dree1 is worthy of further consideration1 as throwing light on the separation of the trachytic and basaltic series of lavas4 ,mr4 ,p4 ,scrope has speculated on this subject2 but he does not seem to have been aware of any positive facts1 such as those above given2 and he has overlooked one very necessary element1 as it appears to me1 in the phenomenon--namely1 the existence of either the lighter or heavier mineral in globules or in crystals4 ,in a substance of imperfect fluidity1 like molten rock1 it is hardly credible1 that the separate1 infinitely small atoms1 whether of feldspar1 augite1 or of any other mineral1 would have power from their slightly different gravities to overcome the friction caused by their movement2 but if the atoms of any one of these minerals became1 whilst the others remained fluid1 united into crystals or granules1 it is easy to perceive that from the lessened friction1 their sinking or floating power would be greatly increased4 ,on the other hand1 if all the minerals became granulated at the same time1 it is scarcely possible1 from their mutual resistance1 that any separation could take place4 ,a valuable1 practical discovery1 illustrating the effect of the granulation of one element in a fluid mass1 in aiding its separation1 has lately been made3 when lead containing a small proportion of silver1 is constantly stirred whilst cooling1 it becomes granulated1 and the grains of imperfect crystals of nearly pure lead sink to the bottom1 leaving a residue of melted metal much richer in silver2 whereas if the mixture be left undisturbed1 although kept fluid for a length of time1 the two metals show no signs of separating4 7,a full and interesting account of this discovery1 by ,mr4 ,pattinson1 was read before the ,british ,association in ,september #ahch4 ,in some alloys1 according to ,turner 8,chemistry0 page #baj1 the heaviest metal sinks1 and it appears that this takes place whilst both metals are fluid4 ,where there is a considerable difference in gravity1 as between iron and the slag formed during the fusion of the ore1 we need not be surprised at the atoms separating1 without either substance being granulated47 ,the sole use of the stirring seems to be1 the formation of detached granules4 ,the specific gravity of silver is #aj4d1 and of lead #aa4ce3 the granulated lead1 which sinks1 is never absolutely pure1 and the residual fluid metal contains1 when richest1 only #a_/aai part of silver4 ,as the difference in specific gravity1 caused by the different proportions of the two metals1 is so exceedingly small1 the separation is probably aided in a great degree by the difference in gravity between the lead1 when granular though still hot1 and when fluid4 ,in a body of liquified volcanic rock1 left for some time without any violent disturbance1 we might expect1 in accordance with the above facts1 that if one of the constituent minerals became aggregated into crystals or granules1 or had been enveloped in this state from some previously existing mass1 such crystals or granules would rise or sink1 according to their specific gravity4 ,now we have plain evidence of crystals being embedded in many lavas1 whilst the paste or basis has continued fluid4 ,i need only refer1 as instances1 to the several1 great1 pseudo-porphyritic streams at the ,galapagos ,islands1 and to the trachytic streams in many parts of the world1 in which we find crystals of feldspar bent and broken by the movement of the surrounding1 semi-fluid matter4 ,lavas are chiefly composed of three varieties of feldspar1 varying in specific gravity from #b4d to #b4gd2 of hornblende and augite1 varying from #c4j to #c4d2 of olivine1 varying from #c4c to #c4d2 and lastly1 of oxides of iron1 with specific gravities from #d4h to #e4b4 ,hence crystals of feldspar1 enveloped in a mass of liquified1 but not highly vesicular lava1 would tend to rise to the upper parts2 and crystals or granules of the other minerals1 thus enveloped1 would tend to sink4 ,we ought not1 however1 to expect any perfect degree of separation in such viscid materials4 ,trachyte1 which consists chiefly of feldspar1 with some hornblende and oxide of iron1 has a specific gravity of about #b4de2 whilst basalt1 composed chiefly of augite and feldspar1 often with much iron and olivine1 has a gravity of about #c4j4 7,trachyte from ,java was found by ,von ,buch to be #b4dg2 from ,auvergne1 by ,de la ,beche1 it was #b4db2 from ,ascension1 by myself1 it was #b4db4 ,jameson and other authors give to basalt a specific gravity of #c4j2 but specimens from ,auvergne were found1 by ,de la ,beche1 to be only #b4gh2 and from the ,giant's ,causeway1 to be #b4ia47 ,accordingly we find1 that where both trachytic and basaltic streams have proceeded from the same orifice1 the trachytic streams have generally been first erupted owing1 as we must suppose1 to the molten lava of this series having accumulated in the upper parts of the volcanic focus4 ,this order of eruption has been observed by ,beudant1 ,scrope1 and by other authors2 three instances1 also1 have been given in this volume4 ,as the later eruptions1 however1 from most volcanic mountains1 burst through their basal parts1 owing to the increased height and weight of the internal column of molten rock1 we see why1 in most cases1 only the lower flanks of the central1 trachytic masses1 are enveloped by basaltic streams4 ,the separation of the ingredients of a mass of lava1 would1 perhaps1 sometimes take place within the body of a volcanic mountain1 if lofty and of great dimensions1 instead of within the underground focus2 in which case1 trachytic streams might be poured forth1 almost contemporaneously1 or at short recurrent intervals1 from its summit1 and basaltic streams from its base3 this seems to have taken place at ,teneriffe4 7,consult ,von ,buch's well-known and admirable 8,description ,physique0 of this island1 which might serve as a model of descriptive geology47 ,i need only further remark1 that from violent disturbances the separation of the two series1 even under otherwise favourable conditions1 would naturally often be prevented1 and likewise their usual order of eruption be inverted4 ,from the high degree of fluidity of most basaltic lavas1 these perhaps1 alone1 would in many cases reach the surface4 ,as we have seen that crystals of feldspar1 in the instance described by ,von ,buch1 sink in obsidian1 in accordance with their known greater specific gravity1 we might expect to find in every trachytic district1 where obsidian has flowed as lava1 that it had proceeded from the upper or highest orifices4 ,this1 according to ,von ,buch1 holds good in a remarkable manner both at the ,lipari ,islands and on the ,peak of ,teneriffe2 at this latter place obsidian has never flowed from a less height than #i1bjj feet4 ,obsidian1 also1 appears to have been erupted from the loftiest peaks of the ,peruvian ,cordillera4 ,i will only further observe1 that the specific gravity of quartz varies from #b4f to #b4h2 and therefore1 that when present in a volcanic focus1 it would not tend to sink with the basaltic bases2 and this1 perhaps1 explains the frequent presence1 and the abundance of this mineral1 in the lavas of the trachytic series1 as observed in previous parts of this volume4 ,an objection to the foregoing theory will1 perhaps1 be drawn from the plutonic rocks not being separated into two evidently distinct series1 of different specific gravities2 although1 like the volcanic1 they have been liquified4 ,in answer1 it may first be remarked1 that we have no evidence of the atoms of any one of the constituent minerals in the plutonic series having been aggregated1 whilst the others remained fluid1 which we have endeavoured to show is an almost necessary condition of their separation2 on the contrary1 the crystals have generally impressed each other with their forms4 7,the crystalline paste of phonolite is frequently penetrated by long needles of hornblende2 from which it appears that the hornblende1 though the more fusible mineral1 has crystallised before1 or at the same time with a more refractory substance4 ,phonolite1 as far as my observations serve1 in every instance appears to be an injected rock1 like those of the plutonic series2 hence probably1 like these latter1 it has generally been cooled without repeated and violent disturbances4 ,those geologists who have doubted whether granite could have been formed by igneous liquefaction1 because minerals of different degrees of fusibility impress each other with their forms1 could not have been aware of the fact of crystallised hornblende penetrating phonolite1 a rock undoubtedly of igneous origin4 ,the viscidity1 which it is now known1 that both feldspar and quartz retain at a temperature much below their points of fusion1 easily explains their mutual impressment4 ,consult on this subject ,mr4 ,horner's paper on ,bonn 8,geolog4 ,transact40 volume #d page #dci2 and 8,l',institut0 with respect to quartz #ahci page #afa47 ,in the second place1 the perfect tranquillity1 under which it is probable that the plutonic masses1 buried at profound depths1 have cooled1 would1 most likely1 be highly unfavourable to the separation of their constituent minerals2 for1 if the attractive force1 which during the progressive cooling draws together the molecules of the different minerals1 has power sufficient to keep them together1 the friction between such half-formed crystals or pasty globules would effectually prevent the heavier ones from sinking1 or the lighter ones from rising4 ,on the other hand1 a small amount of disturbance1 which would probably occur in most volcanic foci1 and which we have seen does not prevent the separation of granules of lead from a mixture of molten lead and silver1 or crystals of feldspar from streams of lava1 by breaking and dissolving the less perfectly formed globules1 would permit the more perfect and therefore unbroken crystals1 to sink or rise1 according to their specific gravity4 ,although in plutonic rocks two distinct species1 corresponding to the trachytic and basaltic series1 do not exist1 ,i much suspect that a certain amount of separation of their constituent parts has often taken place4 ,i suspect this from having observed how frequently dikes of greenstone and basalt intersect widely extended formations of granite and the allied metamorphic rocks4 ,i have never examined a district in an extensive granitic region without discovering dikes2 ,i may instance the numerous trap-dikes1 in several districts of ,brazil1 ,chile1 and ,australia1 and at the ,cape of ,good ,hope3 many dikes likewise occur in the great granitic tracts of ,india1 in the north of ,europe1 and in other countries4 ,whence1 then1 has the greenstone and basalt1 forming these dikes1 come8 ,are we to suppose1 like some of the elder geologists1 that a zone of trap is uniformly spread out beneath the granitic series1 which composes1 as far as we know1 the foundations of the earth's crust8 ,is it not more probable1 that these dikes have been formed by fissures penetrating into partially cooled rocks of the granitic and metamorphic series1 and by their more fluid parts1 consisting chiefly of hornblende1 oozing out1 and being sucked into such fissures8 ,at ,bahia1 in ,brazil1 in a district composed of gneiss and primitive greenstone1 ,i saw many dikes1 of a dark augitic 7for one crystal certainly was of this mineral7 or hornblendic rock1 which1 as several appearances clearly proved1 either had been formed before the surrounding mass had become solid1 or had together with it been afterwards thoroughly softened4 7,portions of these dikes have been broken off1 and are now surrounded by the primary rocks1 with their laminae conformably winding round them4 ,dr4 ,hubbard also 78,silliman's ,journal0 volume #cd page #aai71 has described an interlacement of trap-veins in the granite of the ,white ,mountains1 which he thinks must have been formed when both rocks were soft47 ,on both sides of one of these dikes1 the gneiss was penetrated1 to the distance of several yards1 by numerous1 curvilinear threads or streaks of dark matter1 which resembled in form clouds of the class called cirrhi- comae2 some few of these threads could be traced to their junction with the dike4 ,when examining them1 ,i doubted whether such hair-like and curvilinear veins could have been injected1 and ,i now suspect1 that instead of having been injected from the dike1 they were its feeders4 ,if the foregoing views of the origin of trap-dikes in widely extended granitic regions far from rocks of any other formation1 be admitted as probable1 we may further admit1 in the case of a great body of plutonic rock1 being impelled by repeated movements into the axis of a mountain-chain1 that its more liquid constituent parts might drain into deep and unseen abysses2 afterwards1 perhaps1 to be brought to the surface under the form1 either of injected masses of greenstone and augitic porphyry1 or of basaltic eruptions4 7,mr4 ,phillips 8,lardner's ,encyclop40 volume #b page #aae quotes ,von ,buch's statement1 that augitic porphyry ranges parallel to1 and is found constantly at the base of1 great chains of mountains4 ,humboldt1 also1 has remarked the frequent occurrence of trap-rock1 in a similar position2 of which fact ,i have observed many examples at the foot of the ,chilian ,cordillera4 ,the existence of granite in the axes of great mountain chains is always probable1 and ,i am tempted to suppose1 that the laterally injected masses of augitic porphyry and of trap1 bear nearly the same relation to the granitic axes which basaltic lavas bear to the central trachytic masses1 round the flanks of which they have so frequently been erupted47 ,much of the difficulty which geologists have experienced when they have compared the composition of volcanic with plutonic formations1 will1 ,i think1 be removed1 if we may believe that most plutonic masses have been1 to a certain extent1 drained of those comparatively weighty and easily liquified elements1 which compose the trappean and basaltic series of rocks4 ,,on ,,the ,,distribution ,,of ,,volcanic ,,islands4 ,during my investigations on coral-reefs1 ,i had occasion to consult the works of many voyagers1 and ,i was invariably struck with the fact1 that with rare exceptions1 the innumerable islands scattered throughout the ,pacific1 ,indian1 and ,atlantic ,oceans1 were composed either of volcanic1 or of modern coral-rocks4 ,it would be tedious to give a long catalogue of all the volcanic islands2 but the exceptions which ,i have found are easily enumerated3 in the ,atlantic1 we have ,st4 ,paul's ,rock1 described in this volume1 and the ,falkland ,islands1 composed of quartz and clay-slate2 but these latter islands are of considerable size1 and lie not very far from the ,south ,american coast 7,judging from ,forster's imperfect observation1 perhaps ,georgia is not volcanic4 ,dr4 ,allan is my informant with regard to the ,seychelles4 ,i do not know of what formation ,rodriguez1 in the ,indian ,ocean1 is composed473 in the ,indian ,ocean1 the ,seychelles 7situated in a line prolonged from ,madagascar7 consist of granite and quartz3 in the ,pacific ,ocean1 ,new ,caledonia1 an island of large size1 belongs 7as far as is known7 to the primary class4 ,new ,zealand1 which contains much volcanic rock and some active volcanoes1 from its size cannot be classed with the small islands1 which we are now considering4 ,the presence of a small quantity of non-volcanic rock1 as of clay-slate on three of the ,azores 7,this is stated on the authority of ,count ,v4 de ,bedemar1 with respect to ,flores and ,graciosa 7,charlsworth 8,magazine of ,nat4 ,hist40 volume #a page #eeg74 ,st4 ,maria has no volcanic rock1 according to ,captain ,boyd 7,von ,buch 8,descript40 page #cfe74 ,chatham ,island has been described by ,dr4 ,dieffenbach in the 8,geographical ,journal0 #ahda page #bja4 ,as yet we have received only imperfect notices on ,kerguelen ,land1 from the ,antarctic ,expedition471 or of tertiary limestone at ,madeira1 or of clay-slate at ,chatham ,island in the ,pacific1 or of lignite at ,kerguelen ,land1 ought not to exclude such islands or archipelagoes1 if formed chiefly of erupted matter1 from the volcanic class4 ,the composition of the numerous islands scattered through the great oceans being with such rare exceptions volcanic1 is evidently an extension of that law1 and the effect of those same causes1 whether chemical or mechanical1 from which it results1 that a vast majority of the volcanoes now in action stand either as islands in the sea1 or near its shores4 ,this fact of the ocean-islands being so generally volcanic is also interesting in relation to the nature of the mountain-chains on our continents1 which are comparatively seldom volcanic2 and yet we are led to suppose that where our continents now stand an ocean once extended4 ,do volcanic eruptions1 we may ask1 reach the surface more readily through fissures formed during the first stages of the conversion of the bed of the ocean into a tract of land8 ,looking at the charts of the numerous volcanic archipelagoes1 we see that the islands are generally arranged either in single1 double1 or triple rows1 in lines which are frequently curved in a slight degree4 7,professors ,william and ,henry ,darwin ,rogers have lately insisted much1 in a memoir read before the ,american ,association1 on the regularly curved lines of elevation in parts of the ,appalachian range47 ,each separate island is either rounded1 or more generally elongated in the same direction with the group in which it stands1 but sometimes transversely to it4 ,some of the groups which are not much elongated present little symmetry in their forms2 ,m4 ,virlet 78,bulletin de la ,soc4 ,geolog40 tome #c page #aaj47 states that this is the case with the ,grecian ,archipelago3 in such groups ,i suspect 7for ,i am aware how easy it is to deceive oneself on these points71 that the vents are generally arranged on one line1 or on a set of short parallel lines1 intersecting at nearly right angles another line1 or set of lines4 ,the ,galapagos ,archipelago offers an example of this structure1 for most of the islands and the chief orifices on the largest island are so grouped as to fall on a set of lines ranging about ,n4,w4 by ,n41 and on another set ranging about ,w4,s4,w43 in the ,canary ,archipelago we have a simpler structure of the same kind3 in the ,cape de ,verde group1 which appears to be the least symmetrical of any oceanic volcanic archipelago1 a ,n4,w4 and ,s4,e4 line formed by several islands1 if prolonged1 would intersect at right angles a curved line1 on which the remaining islands are placed4 ,von ,buch 78,description des ,isles ,canaries0 page #cbd47 has classed all volcanoes under two heads1 namely1 ,,central ,,volcanoes1 round which numerous eruptions have taken place on all sides1 in a manner almost regular1 and ,,volcanic ,,chains4 ,in the examples given of the first class1 as far as position is concerned1 ,i can see no grounds for their being called 8central20 and the evidence of any difference in mineralogical nature between ,,central ,,volcanoes and ,,volcanic ,,chains appears slight4 ,no doubt some one island in most small volcanic archipelagoes is apt to be considerably higher than the others2 and in a similar manner1 whatever the cause may be1 that on the same island one vent is generally higher than all the others4 ,von ,buch does not include in his class of volcanic chains small archipelagoes1 in which the islands are admitted by him1 as at the ,azores1 to be arranged in lines2 but when viewing on a map of the world how perfect a series exists from a few volcanic islands placed in a row to a train of linear archipelagoes following each other in a straight line1 and so on to a great wall like the ,cordillera of ,america1 it is difficult to believe that there exists any essential difference between short and long volcanic chains4 ,von ,buch 7,idem page #cic47 states that his volcanic chains surmount1 or are closely connected with1 mountain-ranges of primary formation3 but if trains of linear archipelagoes are1 in the course of time1 by the long- continued action of the elevatory and volcanic forces1 converted into mountain-ranges1 it would naturally result that the inferior primary rocks would often be uplifted and brought into view4 ,some authors have remarked that volcanic islands occur scattered1 though at very unequal distances1 along the shores of the great continents1 as if in some measure connected with them4 ,in the case of ,juan ,fernandez1 situated #ccj miles from the coast of ,chile1 there was undoubtedly a connection between the volcanic forces acting under this island and under the continent1 as was shown during the earthquake of #ahce4 ,the islands1 moreover1 of some of the small volcanic groups which thus border continents1 are placed in lines1 related to those along which the adjoining shores of the continents trend2 ,i may instance the lines of intersection at the ,galapagos1 and at the ,cape de ,verde ,archipelagoes1 and the best marked line of the ,canary ,islands4 ,if these facts be not merely accidental1 we see that many scattered volcanic islands and small groups are related not only by proximity1 but in the direction of the fissures of eruption to the neighbouring continents--a relation1 which ,von ,buch considers1 characteristic of his great volcanic chains4 ,in volcanic archipelagoes1 the orifices are seldom in activity on more than one island at a time2 and the greater eruptions usually recur only after long intervals4 ,observing the number of craters1 that are usually found on each island of a group1 and the vast amount of matter which has been erupted from them1 one is led to attribute a high antiquity even to those groups1 which appear1 like the ,galapagos1 to be of comparatively recent origin4 ,this conclusion accords with the prodigious amount of degradation1 by the slow action of the sea1 which their originally sloping coasts must have suffered1 when they are worn back1 as is so often the case1 into grand precipices4 ,we ought not1 however1 to suppose1 in hardly any instance1 that the whole body of matter1 forming a volcanic island1 has been erupted at the level1 on which it now stands3 the number of dikes1 which seem invariably to intersect the interior parts of every volcano1 show1 on the principles explained by ,m4 ,elie de ,beaumont1 that the whole mass has been uplifted and fissured4 ,a connection1 moreover1 between volcanic eruptions and contemporaneous elevations in mass has1 ,i think1 been shown to exist in my work on ,coral-,reefs1 both from the frequent presence of upraised organic remains1 and from the structure of the accompanying coral-reefs4 7,a similar conclusion is forced on us1 by the phenomena1 which accompanied the earthquake of #ahce1 at ,concepcion1 and which are detailed in my paper 7volume #e page #fja7 in the 8,geological ,transactions407 ,finally1 ,i may remark1 that in the same ,archipelago1 eruptions have taken place within the historical period on more than one of the parallel lines of fissure3 thus1 at the ,galapagos ,archipelago1 eruptions have taken place from a vent on ,narborough ,island1 and from one on ,albemarle ,island1 which vents do not fall on the same line2 at the ,canary ,islands1 eruptions have taken place in ,teneriffe and ,lanzarote2 and at the ,azores1 on the three parallel lines of ,pico1 ,st4 ,jorge1 and ,terceira4 ,believing that a mountain-axis differs essentially from a volcano1 only in plutonic rocks having been injected1 instead of volcanic matter having been ejected1 this appears to me an interesting circumstance2 for we may infer from it as probable1 that in the elevation of a mountain-chain1 two or more of the parallel lines forming it may be upraised and injected within the same geological period4 ,,chapter ,,vii4 ,,australia2 ,,new ,,zealand2 ,,cape ,,of ,,good ,,hope4 ,new ,south ,wales4 ,sandstone formation4 ,embedded pseudo-fragments of shale4 ,stratification4 ,current-cleavage4 ,great valleys4 ,van ,diemen's ,land4 ,palaeozoic formation4 ,newer formation with volcanic rocks4 ,travertin with leaves of extinct plants4 ,elevation of the land4 ,new ,zealand4 ,king ,george's ,sound4 ,superficial ferruginous beds4 ,superficial calcareous deposits1 with casts of branches4 ,their origin from drifted particles of shells and corals4 ,their extent4 ,cape of ,good ,hope4 ,junction of the granite and clay-slate4 ,sandstone formation4 ,the 8,beagle10 in her homeward voyage1 touched at ,new ,zealand1 ,australia1 ,van ,diemen's ,land1 and the ,cape of ,good ,hope4 ,in order to confine the ,third ,part of these ,geological ,observations to ,south ,america1 ,i will here briefly describe all that ,i observed at these places worthy of the attention of geologists4 ,,new ,,south ,,wales4 ,my opportunities of observation consisted of a ride of ninety geographical miles to ,bathurst1 in a ,w4,n4,w4 direction from ,sydney4 ,the first thirty miles from the coast passes over a sandstone country1 broken up in many places by trap-rocks1 and separated by a bold escarpment overhanging the river ,nepean1 from the great sandstone platform of the ,blue ,mountains4 ,this upper platform is #a1jjj feet high at the edge of the escarpment1 and rises in a distance of twenty-five miles to between three and four thousand feet above the level of the sea4 ,at this distance the road descends to a country rather less elevated1 and composed in chief part of primary rocks4 ,there is much granite1 in one part passing into a red porphyry with octagonal crystals of quartz1 and intersected in some places by trap-dikes4 ,near the ,downs of ,bathurst ,i passed over much pale-brown1 glossy clay-slate1 with the shattered laminae running north and south2 ,i mention this fact1 because ,captain ,king informs me that1 in the country a hundred miles southward1 near ,lake ,george1 the mica-slate ranges so invariably north and south that the inhabitants take advantage of it in finding their way through the forests4 ,the sandstone of the ,blue ,mountains is at least #a1bjj feet thick1 and in some parts is apparently of greater thickness2 it consists of small grains of quartz1 cemented by white earthy matter1 and it abounds with ferruginous veins4 ,the lower beds sometimes alternate with shales and coal3 at ,wolgan ,i found in carbonaceous shale leaves of the ,glossopteris ,brownii1 a fern which so frequently accompanies the coal of ,australia4 ,the sandstone contains pebbles of quartz2 and these generally increase in number and size 7seldom1 however1 exceeding an inch or two in diameter7 in the upper beds3 ,i observed a similar circumstance in the grand sandstone formation at the ,cape of ,good ,hope4 ,on the ,south ,american coast1 where tertiary and supra- tertiary beds have been extensively elevated1 ,i repeatedly noticed that the uppermost beds were formed of coarser materials than the lower3 this appears to indicate that1 as the sea became shallower1 the force of the waves or currents increased4 ,on the lower platform1 however1 between the ,blue ,mountains and the coast1 ,i observed that the upper beds of the sandstone frequently passed into argillaceous shale1--the effect1 probably1 of this lower space having been protected from strong currents during its elevation4 ,the sandstone of the ,blue ,mountains evidently having been of mechanical origin1 and not having suffered any metamorphic action1 ,i was surprised at observing that1 in some specimens1 nearly all the grains of quartz were so perfectly crystallised with brilliant facets that they evidently had not in their ,,present form been aggregated in any previously existing rock4 7,i have lately seen1 in a paper by ,smith 7the father of ,english geologists71 in the 8,magazine of ,natural ,history10 that the grains of quartz in the millstone grit of ,england are often crystallised4 ,sir ,david ,brewster1 in a paper read before the ,british ,association1 #ahdj1 states1 that in old decomposed glass1 the silex and metals separate into concentric rings1 and that the silex regains its crystalline structure1 as is shown by its action on light47 ,it is difficult to imagine how these crystals could have been formed2 one can hardly believe that they were separately precipitated in their present crystallised state4 ,is it possible that rounded grains of quartz may have been acted on by a fluid corroding their surfaces1 and depositing on them fresh silica8 ,i may remark that1 in the sandstone formation of the ,cape of ,good ,hope1 it is evident that silica has been profusely deposited from aqueous solution4 ,in several parts of the sandstone ,i noticed patches of shale which might at the first glance have been mistaken for extraneous fragments2 their horizontal laminae1 however1 being parallel with those of the sandstone1 showed that they were the remnants of thin1 continuous beds4 ,one such fragment 7probably the section of a long narrow strip7 seen in the face of a cliff1 was of greater vertical thickness than breadth1 which proves that this bed of shale must have been in some slight degree consolidated1 after having been deposited1 and before being worn away by the currents4 ,each patch of the shale shows1 also1 how slowly many of the successive layers of sandstone were deposited4 ,these pseudo-fragments of shale will perhaps explain1 in some cases1 the origin of apparently extraneous fragments in crystalline metamorphic rocks4 ,i mention this1 because ,i found near ,rio de ,janeiro a well-defined angular fragment1 seven yards long by two yards in breadth1 of gneiss containing garnets and mica in layers1 enclosed in the ordinary1 stratified1 porphyritic gneiss of the country4 ,the laminae of the fragment and of the surrounding matrix ran in exactly the same direction1 but they dipped at different angles4 ,i do not wish to affirm that this singular fragment 7a solitary case1 as far as ,i know7 was originally deposited in a layer1 like the shale in the ,blue ,mountains1 between the strata of the porphyritic gneiss1 before they were metamorphosed2 but there is sufficient analogy between the two cases to render such an explanation possible4 ,,stratification ,,of ,,the ,,escarpment4 ,the strata of the ,blue ,mountains appear to the eye horizontal2 but they probably have a similar inclination with the surface of the platform1 which slopes from the west towards the escarpment over the ,nepean1 at an angle of one degree1 or of one hundred feet in a mile4 7,this is stated on the authority of ,sir ,t4 ,mitchell in 8,travels0 volume #b page #ceg47 ,the strata of the escarpment dip almost conformably with its steeply inclined face1 and with so much regularity1 that they appear as if thrown into their present position2 but on a more careful examination1 they are seen to thicken and to thin out1 and in the upper part to be succeeded and almost capped by horizontal beds4 ,these appearances render it probable1 that we here see an original escarpment1 not formed by the sea having eaten back into the strata1 but by the strata having originally extended only thus far4 ,those who have been in the habit of examining accurate charts of sea-coasts1 where sediment is accumulating1 will be aware1 that the surfaces of the banks thus formed1 generally slope from the coast very gently towards a certain line in the offing1 beyond which the depth in most cases suddenly becomes great4 ,i may instance the great banks of sediment within the ,west ,indian ,archipelago 7,i have described these very curious banks in the ,appendix to my volume on the structure of ,coral-,reefs4 ,i have ascertained the inclination of the edges of the banks1 from information given me by ,captain ,b4 ,allen1 one of the surveyors1 and by carefully measuring the horizontal distances between the last sounding on the bank and the first in the deep water4 ,widely extended banks in all parts of the ,west ,indies have the same general form of surface471 which terminate in submarine slopes1 inclined at angles of between thirty and forty degrees1 and sometimes even at more than forty degrees3 every one knows how steep such a slope would appear on the land4 ,banks of this nature1 if uplifted1 would probably have nearly the same external form as the platform of the ,blue ,mountains1 where it abruptly terminates over the ,nepean4 ,,current-,,cleavage4 ,the strata of sandstone in the low coast country1 and likewise on the ,blue ,mountains1 are often divided by cross or current laminae1 which dip in different directions1 and frequently at an angle of forty-five degrees4 ,most authors have attributed these cross layers to successive small accumulations on an inclined surface2 but from a careful examination in some parts of the ,new ,red ,sandstone of ,england1 ,i believe that such layers generally form parts of a series of curves1 like gigantic tidal ripples1 the tops of which have since been cut off1 either by nearly horizontal layers1 or by another set of great ripples1 the folds of which do not exactly coincide with those below them4 ,it is well-known to surveyors that mud and sand are disturbed during storms at considerable depths1 at least from three hundred to four hundred and fifty feet 7,see ,martin ,white on 8,soundings in the ,british ,channel0 pages #d and #aff471 so that the nature of the bottom even becomes temporarily changed2 the bottom1 also1 at a depth between sixty and seventy feet1 has been observed to be broadly rippled4 7,m4 ,siau on the 8,action of ,waves0 8,edin4 ,new ,phil4 ,journ40 volume #ca page #bde47 ,one may1 therefore1 be allowed to suspect1 from the appearance just mentioned in the ,new ,red ,sandstone1 that at greater depths1 the bed of the ocean is heaped up during gales into great ripple-like furrows and depressions1 which are afterwards cut off by the currents during more tranquil weather1 and again furrowed during gales4 ,,valleys ,,in ,,the ,,sandstone ,,platforms4 ,the grand valleys1 by which the ,blue ,mountains and the other sandstone platforms of this part of ,australia are penetrated1 and which long offered an insuperable obstacle to the attempts of the most enterprising colonist to reach the interior country1 form the most striking feature in the geology of ,new ,south ,wales4 ,they are of grand dimensions1 and are bordered by continuous links of lofty cliffs4 ,it is not easy to conceive a more magnificent spectacle1 than is presented to a person walking on the summit- plains1 when without any notice he arrives at the brink of one of these cliffs1 which are so perpendicular1 that he can strike with a stone 7as ,i have tried7 the trees growing1 at the depth of between one thousand and one thousand five hundred feet below him2 on both hands he sees headland beyond headland of the receding line of cliff1 and on the opposite side of the valley1 often at the distance of several miles1 he beholds another line rising up to the same height with that on which he stands1 and formed of the same horizontal strata of pale sandstone4 ,the bottoms of these valleys are moderately level1 and the fall of the rivers flowing in them1 according to ,sir ,t4 ,mitchell1 is gentle4 ,the main valleys often send into the platform great baylike arms1 which expand at their upper ends2 and on the other hand1 the platform often sends promontories into the valley1 and even leaves in them great1 almost insulated1 masses4 ,so continuous are the bounding lines of cliff1 that to descend into some of these valleys1 it is necessary to go round twenty miles2 and into others1 the surveyors have only lately penetrated1 and the colonists have not yet been able to drive in their cattle4 ,but the most remarkable point of structure in these valleys1 is1 that although several miles wide in their upper parts1 they generally contract towards their mouths to such a degree as to become impassable4 ,the ,surveyor-,general1 ,sir ,t4 ,mitchell1 in vain endeavoured1 first on foot and then by crawling between the great fallen fragments of sandstone1 to ascend through the gorge by which the river ,grose joins the ,nepean 78,travels in ,australia0 volume #a page #aed4--;,i must express my obligation to ,sir ,t4 ,mitchell for several interesting personal communications on the subject of these great valleys of ,new ,south ,wales472 yet the valley of the ,grose in its upper part1 as ,i saw1 forms a magnificent basin some miles in width1 and is on all sides surrounded by cliffs1 the summits of which are believed to be nowhere less than #c1jjj feet above the level of the sea4 ,when cattle are driven into the valley of the ,wolgan by a path 7which ,i descended7 partly cut by the colonists1 they cannot escape2 for this valley is in every other part surrounded by perpendicular cliffs1 and eight miles lower down1 it contracts1 from an average width of half a mile1 to a mere chasm impassable to man or beast4 ,sir ,t4 ,mitchell states1 that the great valley of the ,cox river with all its branches contracts1 where it unites with the ,nepean1 into a gorge #b1bjj yards wide1 and about one thousand feet in depth4 7,idem volume #b page #ceh47 ,other similar cases might have been added4 ,the first impression1 from seeing the correspondence of the horizontal strata1 on each side of these valleys and great amphitheatre-like depressions1 is that they have been in chief part hollowed out1 like other valleys1 by aqueous erosion2 but when one reflects on the enormous amount of stone1 which on this view must have been removed1 in most of the above cases through mere gorges or chasms1 one is led to ask whether these spaces may not have subsided4 ,but considering the form of the irregularly branching valleys1 and of the narrow promontories1 projecting into them from the platforms1 we are compelled to abandon this notion4 ,to attribute these hollows to alluvial action1 would be preposterous2 nor does the drainage from the summit-level always fall1 as ,i remarked near the ,weatherboard1 into the head of these valleys1 but into one side of their bay-like recesses4 ,some of the inhabitants remarked to me1 that they never viewed one of these baylike recesses1 with the headlands receding on both hands1 without being struck with their resemblance to a bold sea-coast4 ,this is certainly the case2 moreover1 the numerous fine harbours1 with their widely branching arms1 on the present coast of ,new ,south ,wales1 which are generally connected with the sea by a narrow mouth1 from one mile to a quarter of a mile in width1 passing through the sandstone coast-cliffs1 present a likeness1 though on a miniature scale1 to the great valleys of the interior4 ,but then immediately occurs the startling difficulty1 why has the sea worn out these great1 though circumscribed1 depressions on a wide platform1 and left mere gorges1 through which the whole vast amount of triturated matter must have been carried away8 ,the only light ,i can throw on this enigma1 is by showing that banks appear to be forming in some seas of the most irregular forms1 and that the sides of such banks are so steep 7as before stated7 that a comparatively small amount of subsequent erosion would form them into cliffs3 that the waves have power to form high and precipitous cliffs1 even in landlocked harbours1 ,i have observed in many parts of ,south ,america4 ,in the ,red ,sea1 banks with an extremely irregular outline and composed of sediment1 are penetrated by the most singularly shaped creeks with narrow mouths3 this is likewise the case1 though on a larger scale1 with the ,bahama ,banks4 ,such banks1 ,i have been led to suppose1 have been formed by currents heaping sediment on an irregular bottom4 7,see the 8,appendix0 to the ,part on ,coral-,reefs4 ,the fact of the sea heaping up mud round a submarine nucleus1 is worthy of the notice of geologists3 for outlyers of the same composition with the coast banks are thus formed2 and these1 if upheaved and worn into cliffs1 would naturally be thought to have been once connected together47 ,that in some cases1 the sea1 instead of spreading out sediment in a uniform sheet1 heaps it round submarine rocks and islands1 it is hardly possible to doubt1 after having examined the charts of the ,west ,indies4 ,to apply these ideas to the sandstone platforms of ,new ,south ,wales1 ,i imagine that the strata might have been heaped on an irregular bottom by the action of strong currents1 and of the undulations of an open sea2 and that the valley-like spaces thus left unfilled might1 during a slow elevation of the land1 have had their steeply sloping flanks worn into cliffs2 the worn-down sandstone being removed1 either at the time when the narrow gorges were cut by the retreating sea1 or subsequently by alluvial action4 ,,van ,,diemen',s ,,land4 ,the southern part of this island is mainly formed of mountains of greenstone1 which often assumes a syenitic character1 and contains much hypersthene4 ,these mountains1 in their lower half1 are generally encased by strata containing numerous small corals and some shells4 ,these shells have been examined by ,mr4 ,g4,b4 ,sowerby1 and have been described by him3 they consist of two species of ,producta1 and of six of ,spirifera2 two of these1 namely1 ,p4 rugata and ,s4 rotundata1 resemble1 as far as their imperfect condition allows of comparison1 ,british mountain-limestone shells4 ,mr4 ,lonsdale has had the kindness to examine the corals2 they consist of six undescribed species1 belonging to three genera4 ,species of these genera occur in the ,silurian1 ,devonian1 and ,carboniferous strata of ,europe4 ,mr4 ,lonsdale remarks1 that all these fossils have undoubtedly a ,palaeozoic character1 and that probably they correspond in age to a division of the system above the ,silurian formations4 ,the strata containing these remains are singular from the extreme variability of their mineralogical composition4 ,every intermediate form is present1 between flinty-slate1 clay-slate passing into grey wacke1 pure limestone1 sandstone1 and porcellanic rock2 and some of the beds can only be described as composed of a siliceo-calcareo-clay-slate4 ,the formation1 as far as ,i could judge1 is at least a thousand feet in thickness3 the upper few hundred feet usually consist of a siliceous sandstone1 containing pebbles and no organic remains2 the inferior strata1 of which a pale flinty slate is perhaps the most abundant1 are the most variable2 and these chiefly abound with the remains4 ,between two beds of hard crystalline limestone1 near ,newtown1 a layer of white soft calcareous matter is quarried1 and is used for whitewashing houses4 ,from information given to me by ,mr4 ,frankland1 the ,surveyor-,general1 it appears that this ,palaeozoic formation is found in different parts of the whole island2 from the same authority1 ,i may add1 that on the north-eastern coast and in ,bass' ,straits primary rocks extensively occur4 ,the shores of ,storm ,bay are skirted1 to the height of a few hundred feet1 by strata of sandstone1 containing pebbles of the formation just described1 with its characteristic fossils1 and therefore belonging to a subsequent age4 ,these strata of sandstone often pass into shale1 and alternate with layers of impure coal2 they have in many places been violently disturbed4 ,near ,hobart ,town1 ,i observed one dike1 nearly a hundred yards in width1 on one side of which the strata were tilted at an angle of #fj degrees1 and on the other they were in some parts vertical1 and had been altered by the effects of the heat4 ,on the west side of ,storm ,bay1 ,i found these strata capped by streams of basaltic lava with olivine2 and close by there was a mass of brecciated scoriae1 containing pebbles of lava1 which probably marks the place of an ancient submarine crater4 ,two of these streams of basalt were separated from each other by a layer of argillaceous wacke1 which could be traced passing into partially altered scoriae4 ,the wacke contained numerous rounded grains of a soft1 grass-green mineral1 with a waxy lustre1 and translucent on its edges3 under the blowpipe it instantly blackened1 and the points fused into a strongly magnetic1 black enamel4 ,in these characters1 it resembles those masses of decomposed olivine1 described at ,st4 ,jago in the ,cape de ,verde group2 and ,i should have thought that it had thus originated1 had ,i not found a similar substance1 in cylindrical threads1 within the cells of the vesicular basalt1--a state under which olivine never appears2 this substance1 ,i believe1 would be classed as bole by mineralogists4 7,chlorophaeite1 described by ,dr4 ,mac,culloch 78,western ,islands0 volume #a page #ejd7 as occurring in a basaltic amygdaloid1 differs from this substance1 in remaining unchanged before the blowpipe1 and in blackening from exposure to the air4 ,may we suppose that olivine1 in undergoing the remarkable change described at ,st4 ,jago1 passes through several states87 ,,travertin ,,with ,,extinct ,,plants4 ,behind ,hobart ,town there is a small quarry of a hard travertin1 the lower strata of which abound with distinct impressions of leaves4 ,mr4 ,robert ,brown has had the kindness to look at my specimens1 and he informed me that there are four or five kinds1 none of which he recognises as belonging to existing species4 ,the most remarkable leaf is palmate1 like that of a fan- palm1 and no plant having leaves of this structure has hitherto been discovered in ,van ,diemen's ,land4 ,the other leaves do not resemble the most usual form of the ,eucalyptus 7of which tribe the existing forests are chiefly composed71 nor do they resemble that class of exceptions to the common form of the leaves of the ,eucalyptus1 which occur in this island4 ,the travertin containing this remnant of a lost vegetation1 is of a pale yellow colour1 hard1 and in parts even crystalline2 but not compact1 and is everywhere penetrated by minute1 tortuous1 cylindrical pores4 ,it contains a very few pebbles of quartz1 and occasionally layers of chalcedonic nodules1 like those of chert in our ,greensand4 ,from the pureness of this calcareous rock1 it has been searched for in other places1 but has never been found4 ,from this circumstance1 and from the character of the deposit1 it was probably formed by a calcareous spring entering a small pool or narrow creek4 ,the strata have subsequently been tilted and fissured2 and the surface has been covered by a singular mass1 with which1 also1 a large fissure has been filled up1 formed of balls of trap embedded in a mixture of wacke and a white1 earthy1 alumino-calcareous substance4 ,hence it would appear1 as if a volcanic eruption had taken place on the borders of the pool1 in which the calcareous matter was depositing1 and had broken it up and drained it4 ,,elevation ,,of ,,the ,,land4 ,both the eastern and western shores of the bay1 in the neighbourhood of ,hobart ,town1 are in most parts covered to the height of thirty feet above the level of high-water mark1 with broken shells1 mingled with pebbles4 ,the colonists attribute these shells to the aborigines having carried them up for food3 undoubtedly1 there are many large mounds1 as was pointed out to me by ,mr4 ,frankland1 which have been thus formed2 but ,i think from the numbers of the shells1 from their frequent small size1 from the manner in which they are thinly scattered1 and from some appearances in the form of the land1 that we must attribute the presence of the greater number to a small elevation of the land4 ,on the shore of ,ralph ,bay 7opening into ,storm ,bay7 ,i observed a continuous beach about fifteen feet above high-water mark1 clothed with vegetation1 and by digging into it1 pebbles encrusted with ,serpulae were found3 along the banks1 also1 of the river ,derwent1 ,i found a bed of broken sea-shells above the surface of the river1 and at a point where the water is now much too fresh for sea-shells to live2 but in both these cases1 it is just possible1 that before certain spits of sand and banks of mud in ,storm ,bay were accumulated1 the tides might have risen to the height where we now find the shells4 7 ,it would appear that some changes are now in progress in ,ralph ,bay1 for ,i was assured by an intelligent farmer1 that oysters were formerly abundant in it1 but that about the year #ahcd they had1 without any apparent cause1 disappeared4 ,in the 8,transactions of the ,maryland ,academy0 volume #a part #a page #bh there is an account by ,mr4 ,ducatel of vast beds of oysters and clams having been destroyed by the gradual filling up of the shallow lagoons and channels1 on the shores of the southern ,united ,states4 ,at ,chiloe1 in ,south ,america1 ,i heard of a similar loss1 sustained by the inhabitants1 in the disappearance from one part of the coast of an edible species of ,ascidia47 ,evidence more or less distinct of a change of level between the land and water1 has been detected on almost all the land on this side of the globe4 ,captain ,grey1 and other travellers1 have found in ,southern ,australia upraised shells1 belonging either to the recent1 or to a late tertiary period4 ,the ,french naturalists in ,baudin's expedition1 found shells similarly circumstanced on the ,s4,w4 coast of ,australia4 ,the ,rev4 ,w4,b4 ,clarke finds proofs of the elevation of the land1 to the amount of #djj feet1 at the ,cape of ,good ,hope4 78,proceedings of the ,geological ,society0 volume #c page #dbj47 ,in the neighbourhood of the ,bay of ,islands in ,new ,zealand1 ,i observed that the shores were scattered to some height1 as at ,van ,diemen's ,land1 with sea-shells1 which the colonists attribute to the natives4 7,i will here give a catalogue of the rocks which ,i met with near the ,bay of ,islands1 in ,new ,zealand3--#ast1 ,much basaltic lava1 and scoriform rocks1 forming distinct craters2--#bnd1 ,a castellated hill of horizontal strata of flesh-coloured limestone1 showing when fractured distinct crystalline facets3 the rain has acted on this rock in a remarkable manner1 corroding its surface into a miniature model of an ,alpine country3 ,i observed here layers of chert and clay ironstone2 and in the bed of a stream1 pebbles of clay-slate2--#crd1 ,the shores of the ,bay of ,islands are formed of a feldspathic rock1 of a bluish-grey colour1 often much decomposed1 with an angular fracture1 and crossed by numerous ferruginous seams1 but without any distinct stratification or cleavage4 ,some varieties are highly crystalline1 and would at once be pronounced to be trap2 others strikingly resembled clay-slate1 slightly altered by heat3 ,i was unable to form any decided opinion on this formation47 ,whatever may have been the origin of these shells1 ,i cannot doubt1 after having seen a section of the valley of the ,thames ,river 7#cg degrees ,s471 drawn by the ,rev4 ,w4 ,williams1 that the land has been there elevated3 on the opposite sides of this great valley1 three step-like terraces1 composed of an enormous accumulation of rounded pebbles1 exactly correspond with each other3 the escarpment of each terrace is about fifty feet in height4 ,no one after having examined the terraces in the valleys on the western shores of ,south ,america1 which are strewed with sea-shells1 and have been formed during intervals of rest in the slow elevation of the land1 could doubt that the ,new ,zealand terraces have been similarly formed4 ,i may add1 that ,dr4 ,dieffenbach1 in his description of the ,chatham ,islands 78,geographical ,journal0 volume #aa pages #bjb1 #bje47 7,s4,w4 of ,new ,zealand71 states that it is manifest 8that the sea has left many places bare which were once covered by its waters40 ,,king ,,george',s ,,sound4 ,this settlement is situated at the south-western angle of the ,australian continent3 the whole country is granitic1 with the constituent minerals sometimes obscurely arranged in straight or curved laminae4 ,in these cases1 the rock would be called by ,humboldt1 gneiss-granite1 and it is remarkable that the form of the bare conical hills1 appearing to be composed of great folding layers1 strikingly resembles1 on a small scale1 those composed of gneiss-granite at ,rio de ,janeiro1 and those described by ,humboldt at ,venezuela4 ,these plutonic rocks are1 in many places1 intersected by trappean-dikes2 in one place1 ,i found ten parallel dikes ranging in an ,e4 and ,w4 line2 and not far off another set of eight dikes1 composed of a different variety of trap1 ranging at right angles to the former ones4 ,i have observed in several primary districts1 the occurrence of systems of dikes parallel and close to each other4 ,,superficial ,,ferruginous ,,beds4 ,the lower parts of the country are everywhere covered by a bed1 following the inequalities of the surface1 of a honeycombed sandstone1 abounding with oxides of iron4 ,beds of nearly similar composition are common1 ,i believe1 along the whole western coast of ,australia1 and on many of the ,east ,indian islands4 ,at the ,cape of ,good ,hope1 at the base of the mountains formed of granite and capped with sandstone1 the ground is everywhere coated either by a fine-grained1 rubbly1 ochraceous mass1 like that at ,king ,george's ,sound1 or by a coarser sandstone with fragments of quartz1 and rendered hard and heavy by an abundance of the hydrate of iron1 which presents1 when freshly broken1 a metallic lustre4 ,both these varieties have a very irregular texture1 including spaces either rounded or angular1 full of loose sand3 from this cause the surface is always honeycombed4 ,the oxide of iron is most abundant on the edges of the cavities1 where alone it affords a metallic fracture4 ,in these formations1 as well as in many true sedimentary deposits1 it is evident that iron tends to become aggregated1 either in the form of a shell1 or of a network4 ,the origin of these superficial beds1 though sufficiently obscure1 seems to be due to alluvial action on detritus abounding with iron4 ,,superficial ,,calcareous ,,deposit4 ,a calcareous deposit on the summit of ,bald ,head1 containing branched bodies1 supposed by some authors to have been corals1 has been celebrated by the descriptions of many distinguished voyagers4 7,i visited this hill1 in company with ,captain ,fitzroy1 and we came to a similar conclusion regarding these branching bodies47 ,it folds round and conceals irregular hummocks of granite1 at the height of #fjj feet above the level of the sea4 ,it varies much in thickness2 where stratified1 the beds are often inclined at high angles1 even as much as at thirty degrees1 and they dip in all directions4 ,these beds are sometimes crossed by oblique and even-sided laminae4 ,the deposit consists either of a fine1 white calcareous powder1 in which not a trace of structure can be discovered1 or of exceedingly minute1 rounded grains1 of brown1 yellowish1 and purplish colours2 both varieties being generally1 but not always1 mixed with small particles of quartz1 and being cemented into a more or less perfect stone4 ,the rounded calcareous grains1 when heated in a slight degree1 instantly lose their colours2 in this and in every other respect1 closely resembling those minute1 equal- sized particles of shells and corals1 which at ,st4 ,helena have been drifted up the side of the mountains1 and have thus been winnowed of all coarser fragments4 ,i cannot doubt that the coloured calcareous particles here have had a similar origin4 ,the impalpable powder has probably been derived from the decay of the rounded particles2 this certainly is possible1 for on the coast of ,peru1 ,i have traced ,,large ,,unbroken shells gradually falling into a substance as fine as powdered chalk4 ,both of the above-mentioned varieties of calcareous sandstone frequently alternate with1 and blend into1 thin layers of a hard substalagmitic rock1 which1 even when the stone on each side contains particles of quartz1 is entirely free from them 7,i adopt this term from ,lieutenant ,nelson's excellent paper on the ,bermuda ,islands 8,geolog4 ,trans40 volume #e page #ajf1 for the hard1 compact1 cream- or brown- coloured stone1 without any crystalline structure1 which so often accompanies superficial calcareous accumulations4 ,i have observed such superficial beds1 coated with substalagmitic rock1 at the ,cape of ,good ,hope1 in several parts of ,chile1 and over wide spaces in ,la ,plata and ,patagonia4 ,some of these beds have been formed from decayed shells1 but the origin of the greater number is sufficiently obscure4 ,the causes which determine water to dissolve lime1 and then soon to redeposit it1 are not1 ,i think1 known4 ,the surface of the substalagmitic layers appears always to be corroded by the rain-water4 ,as all the above-mentioned countries have a long dry season1 compared with the rainy one1 ,i should have thought that the presence of the substalagmitic was connected with the climate1 had not ,lieutenant ,nelson found this substance forming under sea-water4 ,disintegrated shell seems to be extremely soluble2 of which ,i found good evidence1 in a curious rock at ,coquimbo in ,chile1 which consisted of small1 pellucid1 empty husks1 cemented together4 ,a series of specimens clearly showed that these husks had originally contained small rounded particles of shells1 which had been enveloped and cemented together by calcareous matter 7as often happens on sea-beaches71 and which subsequently had decayed1 and been dissolved by water1 that must have penetrated through the calcareous husks1 without corroding them1--of which processes every stage could be seen473 hence we must suppose that these layers1 as well as certain vein- like masses1 have been formed by rain dissolving the calcareous matter and re-precipitating it1 as has happened at ,st4 ,helena4 ,each layer probably marks a fresh surface1 when the1 now firmly cemented1 particles existed as loose sand4 ,these layers are sometimes brecciated and re-cemented1 as if they had been broken by the slipping of the sand when soft4 ,i did not find a single fragment of a sea-shell2 but bleached shells of the ,helix melo1 an existing land species1 abound in all the strata2 and ,i likewise found another ,helix1 and the case of an ,oniscus4 ,the branches are absolutely undistinguishable in shape from the broken and upright stumps of a thicket2 their roots are often uncovered1 and are seen to diverge on all sides2 here and there a branch lies prostrate4 ,the branches generally consist of the sandstone1 rather firmer than the surrounding matter1 with the central parts filled1 either with friable1 calcareous matter1 or with a substalagmitic variety2 this central part is also frequently penetrated by linear crevices1 sometimes1 though rarely1 containing a trace of woody matter4 ,these calcareous1 branching bodies1 appear to have been formed by fine calcareous matter being washed into the casts or cavities1 left by the decay of branches and roots of thickets1 buried under drifted sand4 ,the whole surface of the hill is now undergoing disintegration1 and hence the casts1 which are compact and hard1 are left projecting4 ,in calcareous sand at the ,cape of ,good ,hope1 ,i found the casts1 described by ,abel1 quite similar to these at ,bald ,head2 but their centres are often filled with black carbonaceous matter not yet removed4 ,it is not surprising1 that the woody matter should have been almost entirely removed from the casts on ,bald ,head2 for it is certain1 that many centuries must have elapsed since the thickets were buried2 at present1 owing to the form and height of the narrow promontory1 no sand is drifted up1 and the whole surface1 as ,i have remarked1 is wearing away4 ,we must1 therefore1 look back to a period when the land stood lower1 of which the ,french naturalists 7,see ,m4 ,peron 8,voyage0 tome #a page #bjd47 found evidence in upraised shells of recent species1 for the drifting on ,bald ,head of the calcareous and quartzose sand1 and the consequent embedment of the vegetable remains4 ,there was only one appearance which at first made me doubt concerning the origin of the cast1--namely1 that the finer roots from different stems sometimes became united together into upright plates or veins2 but when the manner is borne in mind in which fine roots often fill up cracks in hard earth1 and that these roots would decay and leave hollows1 as well as the stems1 there is no real difficulty in this case4 ,besides the calcareous branches from the ,cape of ,good ,hope1 ,i have seen casts1 of exactly the same forms1 from ,madeira99 and from ,bermuda2 at this latter place1 the surrounding calcareous rocks1 judging from the specimens collected by ,lieutenant ,nelson1 are likewise similar1 as is their subaerial formation4 ,reflecting on the stratification of the deposit on ,bald ,head1--on the irregularly alternating layers of substalagmitic rock1--on the uniformly sized1 and rounded particles1 apparently of sea-shells and corals1--on the abundance of land-shells throughout the mass1--and finally1 on the absolute resemblance of the calcareous casts1 to the stumps1 roots1 and branches of that kind of vegetation1 which would grow on sand-hillocks1 ,i think there can be no reasonable doubt1 notwithstanding the different opinion of some authors1 that a true view of their origin has been here given4 997,dr4 ,j4 ,macaulay has fully described 78,edinb4 ,new ,phil4 ,journ40 volume #bi page #cej7 the casts from ,madeira4 ,he considers 7differently from ,mr4 ,smith of ,jordan ,hill7 these bodies to be corals1 and the calcareous deposit to be of subaqueous origin4 ,his arguments chiefly rest 7for his remarks on their structure are vague7 on the great quantity of the calcareous matter1 and on the casts containing animal matter1 as shown by their evolving ammonia4 ,had ,dr4 ,macaulay seen the enormous masses of rolled particles of shells and corals on the beach of ,ascension1 and especially on coral-reefs2 and had he reflected on the effects of long-continued1 gentle winds1 in drifting up the finer particles1 he would hardly have advanced the argument of quantity1 which is seldom trustworthy in geology4 ,if the calcareous matter has originated from disintegrated shells and corals1 the presence of animal matter is what might have been expected4 ,mr4 ,anderson analysed for ,dr4 ,macaulay part of a cast1 and he found it composed of3-- ,carbonate of lime444444#gc4ae ,silica44444444444444444#aa4ij ,phosphate of lime4444444#h4ha ,animal matter44444444444#d4be ,sulphate of lime444444a trace #ih4aa7 ,calcareous deposits1 like these of ,king ,george's ,sound1 are of vast extent on the ,australian shores4 ,dr4 ,fitton remarks1 that 8recent calcareous breccia 7by which term all these deposits are included7 was found during ,baudin's voyage1 over a space of no less than twenty-five degrees of latitude and an equal extent of longitude1 on the southern1 western1 and north-western coasts40 7,for ample details on this formation consult ,dr4 ,fitton 8,appendix to ,captain ,king's ,voyage40 ,dr4 ,fitton is inclined to attribute a concretionary origin to the branching bodies3 ,i may remark1 that ,i have seen in beds of sand in ,la ,plata cylindrical stems which no doubt thus originated2 but they differed much in appearance from these at ,bald ,head1 and the other places above specified47 ,it appears also from ,m4 ,peron1 with whose observations and opinions on the origin of the calcareous matter and branching casts mine entirely accord1 that the deposit is generally much more continuous than near ,king ,george's ,sound4 ,at ,swan ,river1 ,archdeacon ,scott states that in one part it extends ten miles inland4 78,proceedings of the ,geolog4 ,soc40 volume #a page #cbj47 ,captain ,wickham1 moreover1 informs me that during his late survey of the western coast1 the bottom of the sea1 wherever the vessel anchored1 was ascertained1 by crowbars being let down1 to consist of white calcareous matter4 ,hence it seems that along this coast1 as at ,bermuda and at ,keeling ,atoll1 submarine and subaerial deposits are contemporaneously in process of formation1 from the disintegration of marine organic bodies4 ,the extent of these deposits1 considering their origin1 is very striking2 and they can be compared in this respect only with the great coral-reefs of the ,indian and ,pacific ,oceans4 ,in other parts of the world1 for instance in ,south ,america1 there are ,,superficial calcareous deposits of great extent1 in which not a trace of organic structure is discoverable2 these observations would lead to the inquiry1 whether such deposits may not1 also1 have been formed from disintegrated shells and corals4 ,,cape ,,of ,,good ,,hope4 ,after the accounts given by ,barrow1 ,carmichael1 ,basil ,hall1 and ,w4,b4 ,clarke of the geology of this district1 ,i shall confine myself to a few observations on the junction of the three principal formations4 ,the fundamental rock is granite 7,in several places ,i observed in the granite1 small dark-coloured balls1 composed of minute scales of black mica in a tough basis4 ,in another place1 ,i found crystals of black schorl radiating from a common centre4 ,dr4 ,andrew ,smith found1 in the interior parts of the country1 some beautiful specimens of granite1 with silvery mica radiating or rather branching1 like moss1 from central points4 ,at the ,geological ,society1 there are specimens of granite with crystallised feldspar branching and radiating in like manner471 overlaid by clay-slate3 the latter is generally hard1 and glossy from containing minute scales of mica2 it alternates with1 and passes into1 beds of slightly crystalline1 feldspathic1 slaty rock4 ,this clay-slate is remarkable from being in some places 7as on the ,lion's ,rump7 decomposed1 even to the depth of twenty feet1 into a pale-coloured1 sandstone-like rock1 which has been mistaken1 ,i believe1 by some observers1 for a separate formation4 ,i was guided by ,dr4 ,andrew ,smith to a fine junction at ,green ,point between the granite and clay-slate3 the latter at the distance of a quarter of a mile from the spot1 where the granite appears on the beach 7though1 probably1 the granite is much nearer underground71 becomes slightly more compact and crystalline4 ,at a less distance1 some of the beds of clay-slate are of a homogeneous texture1 and obscurely striped with different zones of colour1 whilst others are obscurely spotted4 ,within a hundred yards of the first vein of granite1 the clay-slate consists of several varieties2 some compact with a tinge of purple1 others glistening with numerous minute scales of mica and imperfectly crystallised feldspar2 some obscurely granular1 others porphyritic with small1 elongated spots of a soft white mineral1 which being easily corroded1 gives to this variety a vesicular appearance4 ,close to the granite1 the clay-slate is changed into a dark-coloured1 laminated rock1 having a granular fracture1 which is due to imperfect crystals of feldspar1 coated by minute1 brilliant scales of mica4 ,the actual junction between the granitic and clay-slate districts extends over a width of about two hundred yards1 and consists of irregular masses and of numerous dikes of granite1 entangled and surrounded by the clay- slate3 most of the dikes range in a ,n4,w4 and ,s4,e4 line1 parallel to the cleavage of the slate4 ,as we leave the junction1 thin beds1 and lastly1 mere films of the altered clay-slate are seen1 quite isolated1 as if floating1 in the coarsely crystallised granite2 but although completely detached1 they all retain traces of the uniform ,n4,w4 and ,s4,e4 cleavage4 ,this fact has been observed in other similar cases1 and has been advanced by some eminent geologists 7,see ,m4 ,keilhau 8,theory on ,granite0 translated in the 8,edinburgh ,new ,philosophical ,journal0 volume #bd page #djb471 as a great difficulty on the ordinary theory1 of granite having been injected whilst liquified2 but if we reflect on the probable state of the lower surface of a laminated mass1 like clay-slate1 after having been violently arched by a body of molten granite1 we may conclude that it would be full of fissures parallel to the planes of cleavage2 and that these would be filled with granite1 so that wherever the fissures were close to each other1 mere parting layers or wedges of the slate would depend into the granite4 ,should1 therefore1 the whole body of rock afterwards become worn down and denuded1 the lower ends of these dependent masses or wedges of slate would be left quite isolated in the granite2 yet they would retain their proper lines of cleavage1 from having been united1 whilst the granite was fluid1 with a continuous covering of clay-slate4 ,following1 in company with ,dr4 ,a4 ,smith1 the line of junction between the granite and the slate1 as it stretched inland1 in a ,s4,e4 direction1 we came to a place1 where the slate was converted into a fine-grained1 perfectly characterised gneiss1 composed of yellow-brown granular feldspar1 of abundant black brilliant mica1 and of few and thin laminae of quartz4 ,from the abundance of the mica in this gneiss1 compared with the small quantity and excessively minute scales1 in which it exists in the glossy clay-slate1 we must conclude1 that it has been here formed by the metamorphic action--a circumstance doubted1 under nearly similar circumstances1 by some authors4 ,the laminae of the clay-slate are straight2 and it was interesting to observe1 that as they assumed the character of gneiss1 they became undulatory with some of the smaller flexures angular1 like the laminae of many true metamorphic schists4 ,,sandstone ,,formation4 ,this formation makes the most imposing feature in the geology of ,southern ,africa4 ,the strata are in many parts horizontal1 and attain a thickness of about two thousand feet4 ,the sandstone varies in character2 it contains little earthy matter1 but is often stained with iron2 some of the beds are very fine-grained and quite white2 others are as compact and homogeneous as quartz rock4 ,in some places ,i observed a breccia of quartz1 with the fragments almost dissolved in a siliceous paste4 ,broad veins of quartz1 often including large and perfect crystals1 are very numerous2 and it is evident in nearly all the strata1 that silica has been deposited from solution in remarkable quantity4 ,many of the varieties of quartzite appeared quite like metamorphic rocks2 but from the upper strata being as siliceous as the lower1 and from the undisturbed junctions with the granite1 which in many places can be examined1 ,i can hardly believe that these sandstone-strata have been exposed to heat4 7,the ,rev4 ,w4,b4 ,clarke1 however1 states1 to my surprise 78,geolog4 ,proceedings0 volume #c page #dbb71 that the sandstone in some parts is penetrated by granitic dikes3 such dikes must belong to an epoch altogether subsequent to that when the molten granite acted on the clay-slate47 ,on the lines of junction between these two great formations1 ,i found in several places the granite decayed to the depth of a few inches1 and succeeded1 either by a thin layer of ferruginous shale1 or by four or five inches in thickness of the re-cemented crystals of the granite1 on which the great pile of sandstone immediately rested4 ,mr4 ,schomburgk has described 78,geographical ,journal0 volume #aj page #bdf47 a great sandstone formation in ,northern ,brazil1 resting on granite1 and resembling to a remarkable degree1 in composition and in the external form of the land1 this formation of the ,cape of ,good ,hope4 ,the sandstones of the great platforms of ,eastern ,australia1 which also rest on granite1 differ in containing more earthy and less siliceous matter4 ,no fossil remains have been discovered in these three vast deposits4 ,finally1 ,i may add that ,i did not see any boulders of far-transported rocks at the ,cape of ,good ,hope1 or on the eastern and western shores of ,australia1 or at ,van ,diemen's ,land4 ,in the northern island of ,new ,zealand1 ,i noticed some large blocks of greenstone1 but whether their parent rock was far distant1 ,i had no opportunity of determining4 ,,index ,,to ,,volcanic ,,islands4 ,abel1 ,m41 on calcareous casts at the ,cape of ,good ,hope4 ,abingdon island4 ,abrolhos islands1 incrustation on4 ,aeriform explosions at ,ascension4 ,albatross1 driven from ,st4 ,helena4 ,albemarle island4 ,albite1 at the ,galapagos archipelago4 ,amygdaloidal cells1 half filled4 ,amygdaloids1 calcareous origin of4 ,ascension1 arborescent incrustation on rocks of4 -absence of dikes1 freedom from volcanic action1 and state of lava-streams4 ,ascidia1 extinction of4 ,atlantic ,ocean1 new volcanic focus in4 ,augite1 fused4 ,australia4 ,azores4 ,bahia in ,brazil1 dikes at4 ,bailly1 ,m41 on the mountains of ,mauritius4 ,bald ,head4 ,banks' ,cove4 ,barn1 ,the1 ,st4 ,helena4 ,basalt1 specific gravity of4 ,basaltic coast-mountains at ,mauritius4 -at ,st4 ,helena4 -at ,st4 ,jago4 ,beaumont1 ,m4 ,elie de1 on circular subsidences in lava4 -on dikes indicating elevation4 -on inclination of lava-streams4 -on laminated dikes4 ,bermuda1 calcareous rocks of4 ,beudant1 ,m41 on bombs4 -on jasper4 -on laminated trachyte4 -on obsidian of ,hungary4 -on silex in trachyte4 ,bole4 ,bombs1 volcanic4 ,bory ,st4 ,vincent1 on bombs4 ,boulders1 absence in ,australia and ,cape of ,good ,hope4 ,brattle island4 ,brewster1 ,sir ,d41 on a calcareo-animal substance4 -on decomposed glass4 ,brown1 ,mr4 ,r41 on extinct plants from ,van ,diemen's land4 -on sphaerulitic bodies in silicified wood4 ,buch1 ,von1 on cavernous lava4 -on central volcanoes4 -on crystals sinking in obsidian4 -on laminated lava4 -on obsidian streams4 -on olivine in basalt4 -on superficial calcareous beds in the ,canary islands4 ,calcareous deposit at ,st4 ,jago affected by heat4 -fibrous matter1 entangled in streaks in scoriae4 -freestone at ,ascension4 -incrustations at ,ascension4 -sandstone at ,st4 ,helena4 -superficial beds at ,king ,george's sound4 ,cape of ,good ,hope4 ,carbonic acid1 expulsion of1 by heat4 ,carmichael1 ,capt41 on glassy coatings to dikes4 ,casts1 calcareous1 of branches4 ,chalcedonic nodules4 ,chalcedony in basalt and in silicified wood4 ,chatham island4 ,chlorophaeite4 ,clarke1 ,rev4 ,w41 on the ,cape of ,good ,hope4 ,clay-slate1 its decomposition and junction with granite at the ,cape of ,good ,hope4 ,cleavage of clay-slate in ,australia4 ,cleavage1 cross1 in sandstone4 ,coast denudation at ,st4 ,helena4 ,columnar basalt4 8,comptes ,rendus10 account of volcanic phenomena in the ,atlantic4 ,concepcion1 earthquake of4 ,concretions in aqueous and igneous rocks compared4 -in tuff4 -of obsidian4 ,conglomerate1 recent1 at ,st4 ,jago4 ,coquimbo1 curious rock of4 ,corals1 fossil1 from ,van ,diemen's ,land4 ,crater1 segment of1 at the ,galapagos4 -great central one at ,st4 ,helena4 -internal ledges round1 and parapet on4 ,craters1 basaltic1 at ,ascension4 -form of1 affected by the trade wind4 -of elevation4 -of tuff at ,terceira4 -of tuff at the ,galapagos archipelago4 -their breached state4 -small basaltic at ,st4 ,jago4 --at the ,galapagos archipelago4 ,crystallisation favoured by space4 ,dartigues1 ,m41 on sphaerulites4 ,daubeny1 ,dr41 on a basin-formed island4 -on fragments in trachyte4 ,d',aubuisson on hills of phonolite4 -on the composition of obsidian4 -on the lamination of clay-slate4 ,de la ,beche1 ,sir ,h41 on magnesia in erupted lime4 -on specific gravity of limestones4 ,denudation of coast at ,st4 ,helena4 ,diana's ,peak1 ,st4 ,helena4 ,dieffenbach1 ,dr41 on the ,chatham ,islands4 ,dikes1 truncated1 on central crateriform ridge of ,st4 ,helena4 -at ,st4 ,helena2 number of2 coated by a glossy layer2 uniform thickness of4 -great parallel ones at ,st4 ,helena4 -not observed at ,ascension4 -of tuff4 -of trap in the plutonic series4 -remnants of1 extending far into the sea round ,st4 ,helena4 ,dislocations at ,ascension4 -at ,st4 ,helena4 ,distribution of volcanic islands4 ,dolomieu1 on decomposed trachyte4 -on laminated lava4 -on obsidian4 ,dree1 ,m41 on crystals sinking in lava4 ,dufrenoy1 ,m41 on the composition of the surface of certain lava-streams4 -on the inclination of tuff-strata4 ,eggs of birds embedded at ,st4 ,helena4 -of turtle at ,ascension4 ,ejected fragments at ,ascension4 -at the ,galapagos archipelago4 ,elevation of ,st4 ,helena4 -the ,galapagos archipelago4 -,van ,diemen's ,land1 ,cape of ,good ,hope1 ,new ,zealand1 ,australia1 and ,chatham island4 -of volcanic islands4 ,ellis1 ,rev4 ,w41 on ledges within the great crater at ,hawaii4 -on marine remains at ,otaheite4 ,eruption1 fissures of4 ,extinction of land-shells at ,st4 ,helena4 ,faraday1 ,mr41 on the expulsion of carbonic acid gas4 ,feldspar1 fusibility of4 -in radiating crystals4 -,labrador1 ejected4 ,feldspathic lavas4 -at ,st4 ,helena4 -rock1 alternating with obsidian4 -lamination1 and origin of4 ,fernando ,noronha4 ,ferruginous superficial beds4 ,fibrous calcareous matter at ,st4 ,jago4 ,fissures of eruption4 ,fitton1 ,dr41 on calcareous breccia4 ,flagstaff ,hill1 ,st4 ,helena4 ,fleurian de ,bellevue on sphaerulites4 ,fluidity of lavas4 ,forbes1 ,professor1 on the structure of glaciers4 ,fragments ejected at ,ascension4 -at the ,galapagos archipelago4 ,freshwater ,bay4 ,fuerteventura 7,feurteventura71 calcareous beds of4 ,galapagos archipelago4 -parapets round craters4 ,gay ,lussac1 on the expulsion of carbonic acid gas4 ,glaciers1 their structure4 ,glossiness of texture1 origin of4 ,gneiss1 derived from clay-slate4 -with a great embedded fragment4 ,gneiss-granite1 form of hills of4 ,good ,hope1 ,cape of4 ,gorges1 narrow1 at ,st4 ,helena4 ,granite1 junction with clay-slate1 at the ,cape of ,good ,hope4 ,granitic ejected fragments4 ,gravity1 specific1 of lavas4 ,gypsum1 at ,ascension4 -in volcanic strata at ,st4 ,helena4 -on surface of the ground at ditto4 ,hall1 ,sir ,j41 on the expulsion of carbonic acid gas4 ,heat1 action of1 on calcareous matter4 ,hennah1 ,mr41 on ashes at ,ascension4 ,henslow1 ,prof41 on chalcedony4 ,hoffmann1 on decomposed trachyte4 ,holland1 ,dr41 on ,iceland4 ,horner1 ,mr41 on a calcareo-animal substance4 -on fusibility of feldspar4 ,hubbard1 ,dr41 on dikes4 ,humboldt on ejected fragments4 -on obsidian formations4 -on parapets round craters4 -on sphaerulites4 ,hutton on amygdaloids4 ,hyalite in decomposed trachyte4 ,iceland1 stratification of the circumferential hills4 ,islands1 volcanic1 distribution of4 -their elevation4 ,incrustation1 on ,st4 ,paul's rocks4 ,incrustations1 calcareous1 at ,ascension4 ,jago1 ,st4 ,james island4 ,jasper1 origin of4 ,jonnes1 ,m4 ,moreau de1 on craters affected by wind4 ,juan ,fernandez4 ,keilhau1 ,m41 on granite4 ,kicker ,rock4 ,king ,george's sound4 ,labrador feldspar1 ejected4 ,lakes at bases of volcanoes4 ,lamination of volcanic rocks4 ,land-shells1 extinct1 at ,st4 ,helena4 ,lanzarote1 calcareous beds of4 ,lava1 adhesion to sides of a gorge4 -feldspathic4 -with cells semi-amygdaloidal4 ,lavas1 specific gravity of4 ,lava-streams blending together at ,st4 ,jago4 -composition of surface of4 -differences in the state of their surfaces4 -extreme thinness of4 -heaved up into hillocks at the ,galapagos archipelago4 -their fluidity4 -with irregular hummocks at ,ascension4 ,lead1 separation from silver4 ,lesson1 ,m41 on craters at ,ascension4 ,leucite4 ,lime1 sulphate of1 at ,ascension4 ,lonsdale1 ,mr41 on fossil-corals from ,van ,diemen's land4 ,lot1 ,st4 ,helena4 ,lyell1 ,mr41 on craters of elevation4 -on embedded turtles' eggs4 -on glossy coating to dikes4 ,macaulay1 ,dr41 on calcareous casts at ,madeira4 ,mac,culloch1 ,dr41 on an amygdaloid4 -on chlorophaeite4 -on laminated pitchstone4 ,mackenzie1 ,sir ,g41 on cavernous lava-streams4 -on glossy coatings to dikes4 -on obsidian streams4 -on stratification in ,iceland4 ,madeira1 calcareous casts at4 8,magazine1 ,nautical10 account of volcanic phenomena in the ,atlantic4 ,marekanite4 ,mauritius1 crater of elevation of4 ,mica1 in rounded nodules4 -origin in metamorphic slate4 -radiating form of4 ,miller1 ,prof41 on ejected ,labrador feldspar4 -on quartz crystals in obsidian beds4 ,mitchell1 ,sir ,t41 on bombs4 -on the ,australian valleys4 ,mud streams at the ,galapagos archipelago4 ,narborough island4 ,nelson1 ,lieut41 on the ,bermuda islands4 ,new ,caledonia4 ,new ,red sandstone1 cross cleavage of4 ,new ,south ,wales4 ,new ,zealand4 ,nulliporae 7fossil71 resembling concretions4 ,obsidian1 absent at the ,galapagos archipelago4 -bombs of4 -composition and origin of4 -crystals of feldspar sink in4 -its irruption from lofty craters4 -passage of beds into4 -specific gravity of4 -streams of4 ,olivine decomposed at ,st4 ,jago4 -at ,van ,diemen's land4 -in the lavas at the ,galapagos archipelago4 ,oolitic structure of recent calcareous beds at ,st4 ,helena4 ,otaheite4 ,oysters1 extinction of4 ,panza islands1 laminated trachyte of4 ,pattinson1 ,mr41 on the separation of lead and silver4 ,paul's1 ,st41 rocks of4 ,pearlstone4 ,peperino4 ,peron1 ,m41 on calcareous rocks of ,australia4 ,phonolite1 hills of4 -laminated4 -with more fusible hornblende4 ,pitchstone4 -dikes of4 ,plants1 extinct4 ,plutonic rocks1 separation of constituent parts of1 by gravity4 ,porto ,praya4 ,prevost1 ,m4 ,c41 on rarity of great dislocations in volcanic islands4 ,prosperous hill1 ,st4 ,helena4 ,pumice1 absent at the ,galapagos archipelago4 -laminated4 ,puy de ,dome1 trachyte of4 ,quail island1 ,st4 ,jago4 ,quartz1 crystals of1 in beds alternating with obsidian4 -crystallised in sandstone4 -fusibility of4 -rock1 mottled from metamorphic action with earthy matter4 ,red hill4 ,resin-like altered scoriae4 ,rio de ,janeiro1 gneiss of4 ,robert1 ,m41 on strata of ,iceland4 ,rogers1 ,professor1 on curved lines of elevation4 ,salses1 compared with tuff craters4 ,salt deposited by the sea4 -in volcanic strata4 -lakes of1 in craters4 ,sandstone of ,brazil4 -of the ,cape of ,good ,hope4 -platforms of1 in ,new ,south ,wales4 ,schorl1 radiating4 ,scrope1 ,mr4 ,p41 on laminated trachyte4 -on obsidian4 -on separation of trachyte and basalt4 -on silex in trachyte4 -on sphaerulites4 ,seale1 ,mr41 geognosy of ,st4 ,helena4 -on dikes4 -on embedded birds' bones4 ,seale1 on extinct shells of ,st4 ,helena4 ,sedgwick1 ,professor1 on concretions4 ,septaria1 in concretions in tuff4 ,serpulae on upraised rocks4 ,seychelles4 ,shells1 colour of1 affected by light4 -from ,van ,diemen's land4 -land1 extinct1 at ,st4 ,helena4 -particles of1 drifted by the wind at ,st4 ,helena4 ,shelly matter deposited by the waves4 ,siau1 ,m41 on ripples4 ,signal ,post ,hill4 ,silica1 deposited by steam4 -large proportion of1 in obsidian4 -specific gravity of4 ,siliceous sinter4 ,smith1 ,dr4 ,a41 on junction of granite and clay-slate4 ,spallanzani on decomposed trachyte4 ,specific gravity of recent calcareous rocks and of limestone4 -of lavas4 ,sphaerulites in glass and in silicified wood4 -in obsidian4 ,sowerby1 ,mr4 ,g4,b41 on fossil-shells from ,van ,diemen's land4 -from ,st4 ,jago4 -land-shells from ,st4 ,helena4 ,st4 ,helena4 -crater of elevation of4 ,st4 ,jago1 crater of elevation of4 -effects of calcareous matter on lava4 ,st4 ,paul's rocks4 ,stokes1 ,mr41 collections of sphaerulites and of obsidians4 ,stony-top1 ,little4 -,great4 ,stratification of sandstone in ,new ,south ,wales4 ,streams of obsidian4 ,stutchbury1 ,mr41 on marine remains at ,otaheite4 ,subsided space at ,ascension4 ,tahiti4 ,talus1 stratified1 within tuff craters4 ,terceira4 ,tertiary deposit of ,st4 ,jago4 ,trachyte1 absent at the ,galapagos archipelago4 -at ,ascension4 -at ,terceira4 -decomposition of1 by steam4 -its lamination4 -its separation from basalt4 -softened at ,ascension4 -specific gravity of4 -with singular veins4 ,trap-dikes in the plutonic series4 -at ,king ,george's sound4 ,travertin at ,van ,diemen's land4 ,tropic-bird1 now rare1 at ,st4 ,helena4 ,tuff1 craters of4 -their breached state4 -peculiar kind of4 ,turner1 ,mr41 on the separation of molten metals4 ,tyerman and ,bennett on marine remains at ,huaheine4 ,valleys1 gorge-like1 at ,st4 ,helena4 -in ,new ,south ,wales4 -in ,st4 ,jago4 ,van ,diemen's land4 ,veins in trachyte4 -of jasper4 ,vincent1 ,bory ,st41 on bombs4 ,volcanic bombs4 -island in process of formation in the ,atlantic4 -islands1 their distribution4 ,wacke1 its passage into lava4 ,wackes1 argillaceous4 ,webster1 ,dr41 on a basin-formed island4 -on gypsum at ,ascension4 ,white1 ,martin1 on soundings4 ,wind1 effects of1 on the form of craters4