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diff --git a/.gitattributes b/.gitattributes new file mode 100644 index 0000000..6833f05 --- /dev/null +++ b/.gitattributes @@ -0,0 +1,3 @@ +* text=auto +*.txt text +*.md text diff --git a/3054-0.txt b/3054-0.txt new file mode 100644 index 0000000..6b950d6 --- /dev/null +++ b/3054-0.txt @@ -0,0 +1,6416 @@ +The Project Gutenberg eBook of Geological Observations on the Volcanic Islands, by Charles Darwin + +This eBook is for the use of anyone anywhere in the United States and +most other parts of the world at no cost and with almost no restrictions +whatsoever. You may copy it, give it away or re-use it under the terms +of the Project Gutenberg License included with this eBook or online at +www.gutenberg.org. If you are not located in the United States, you +will have to check the laws of the country where you are located before +using this eBook. + +Title: Geological Observations on the Volcanic Islands + +Author: Charles Darwin + +Release Date: February, 2002 [eBook #3054] +[Most recently updated: March 29, 2021] + +Language: English + +Character set encoding: UTF-8 + +Produced by: Sue Asscher + +*** START OF THE PROJECT GUTENBERG EBOOK VOLCANIC ISLANDS *** + + + + +Geological Observations on the Volcanic Islands + +by Charles Darwin + + +Contents + + EDITORIAL NOTE + DETAILED TABLE OF CONTENTS + CRITICAL INTRODUCTION + CHAPTER I.—ST. JAGO, IN THE CAPE DE VERDE ARCHIPELAGO + CHAPTER II.—FERNANDO NORONHA; TERCEIRA; TAHITI, ETC + CHAPTER III.—ASCENSION + CHAPTER IV.—ST. HELENA + CHAPTER V.—GALAPAGOS ARCHIPELAGO + CHAPTER VI.—TRACHYTE AND BASALT.—DISTRIBUTION OF VOLCANIC ISLES + CHAPTER VII.—AUSTRALIA; NEW ZEALAND; CAPE OF GOOD HOPE + INDEX TO VOLCANIC ISLANDS + + + + +EDITORIAL NOTE + + +Although in some respects more technical in their subjects and style +than Darwin’s “Journal,” the books here reprinted will never lose their +value and interest for the originality of the observations they +contain. Many parts of them are admirably adapted for giving an insight +into problems regarding the structure and changes of the earth’s +surface, and in fact they form a charming introduction to physical +geology and physiography in their application to special domains. The +books themselves cannot be obtained for many times the price of the +present volume, and both the general reader, who desires to know more +of Darwin’s work, and the student of geology, who naturally wishes to +know how a master mind reasoned on most important geological subjects, +will be glad of the opportunity of possessing them in a convenient and +cheap form. + +The three introductions, which my friend Professor Judd has kindly +furnished, give critical and historical information which makes this +edition of special value. + +G.T.B. + + + + +VOLCANIC ISLANDS. + + + + +DETAILED TABLE OF CONTENTS + + +I.—ST. JAGO, IN THE CAPE DE VERDE ARCHIPELAGO. + +Rocks of the lowest series.—A calcareous sedimentary deposit, with +recent shells, altered by the contact of superincumbent lava, its +horizontality and extent.—Subsequent volcanic eruptions, associated +with calcareous matter in an earthy and fibrous form, and often +enclosed within the separate cells of the scoriae.—Ancient and +obliterated orifices of eruption of small size.—Difficulty of tracing +over a bare plain recent streams of lava.—Inland hills of more ancient +volcanic rock.—Decomposed olivine in large masses.—Feldspathic rocks +beneath the upper crystalline basaltic strata.—Uniform structure and +form of the more ancient volcanic hills.—Form of the valleys near the +coast.—Conglomerate now forming on the sea beach. + +II.—FERNANDO NORONHA; TERCEIRA; TAHITI, ETC. + +FERNANDO NORONHA.—Precipitous hill of phonolite. + +TERCEIRA.—Trachytic rocks: their singular decomposition by steam of +high temperature. + +TAHITI.—Passage from wacke into trap; singular volcanic rock with the +vesicles half-filled with mesotype. + +MAURITIUS.—Proofs of its recent elevation.—Structure of its more +ancient mountains; similarity with St. Jago. + +ST. PAUL’S ROCKS.—Not of volcanic origin.—Their singular mineralogical +composition. + +III.—ASCENSION. + +Basaltic lavas.—Numerous craters truncated on the same side.—Singular +structure of volcanic bombs.—Aeriform explosions.—Ejected granite +fragments.—Trachytic rocks.—Singular veins.—Jasper, its manner of +formation.—Concretions in pumiceous tuff.—Calcareous deposits and +frondescent incrustations on the coast.—Remarkable laminated beds, +alternating with, and passing into obsidian.—Origin of obsidian.— +Lamination of volcanic rocks. + +IV.—ST. HELENA. + +Lavas of the feldspathic, basaltic, and submarine series.—Section of +Flagstaff Hill and of the Barn.—Dikes.—Turk’s Cap and Prosperous Bays.— +Basaltic ring.—Central crateriform ridge, with an internal ledge and a +parapet.—Cones of phonolite.—Superficial beds of calcareous sandstone.— +Extinct land-shells.—Beds of detritus.—Elevation of the land.— +Denudation.—Craters of elevation. + +V.—GALAPAGOS ARCHIPELAGO. + +Chatham Island.—Craters composed of a peculiar kind of tuff.—Small +basaltic craters, with hollows at their bases.—Albemarle Island; fluid +lavas, their composition.—Craters of tuff; inclination of their +exterior diverging strata, and structure of their interior converging +strata.—James Island, segment of a small basaltic crater; fluidity and +composition of its lava-streams, and of its ejected +fragments.—Concluding remarks on the craters of tuff, and on the +breached condition of their southern sides.— Mineralogical composition +of the rocks of the archipelago.—Elevation of the land.—Direction of +the fissures of eruption. + +VI.—TRACHYTE AND BASALT.—DISTRIBUTION OF VOLCANIC ISLES. + +The sinking of crystals in fluid lava.—Specific gravity of the +constituent parts of trachyte and of basalt, and their consequent +separation.— Obsidian.—Apparent non-separation of the elements of +plutonic rocks.— Origin of trap-dikes in the plutonic +series.—Distribution of volcanic islands; their prevalence in the great +oceans.—They are generally arranged in lines.—The central volcanoes of +Von Buch doubtful.—Volcanic islands bordering continents.—Antiquity of +volcanic islands, and their elevation in mass.—Eruptions on parallel +lines of fissure within the same geological period. + +VII.—AUSTRALIA; NEW ZEALAND; CAPE OF GOOD HOPE. + +New South Wales.—Sandstone formation.—Embedded pseudo-fragments of +shale.—Stratification.—Current-cleavage.—Great valleys.—Van Diemen’s +Land.—Palaeozoic formation.—Newer formation with volcanic rocks.— +Travertin with leaves of extinct plants.—Elevation of the land.—New +Zealand.—King George’s Sound.—Superficial ferruginous beds.—Superficial +calcareous deposits, with casts of branches; its origin from drifted +particles of shells and corals.—Their extent.—Cape of Good Hope.— +Junction of the granite and clay-slate.—Sandstone formation. + +INDEX. + + + + +GEOLOGICAL OBSERVATIONS ON VOLCANIC ISLANDS. + + + + +CRITICAL INTRODUCTION. + + +The preparation of the series of works published under the general +title “Geology of the Voyage of the ‘Beagle’” occupied a great part of +Darwin’s time during the ten years that followed his return to England. +The second volume of the series, entitled “Geological Observations on +Volcanic Islands, with Brief Notices on the Geology of Australia and +the Cape of Good Hope,” made its appearance in 1844. The materials for +this volume were collected in part during the outward voyage, when the +“Beagle” called at St. Jago in the Cape de Verde Islands, and St. +Paul’s Rocks, and at Fernando Noronha, but mainly during the homeward +cruise; then it was that the Galapagos Islands were surveyed, the Low +Archipelago passed through, and Tahiti visited; after making calls at +the Bay of Islands, in New Zealand, and also at Sydney, Hobart Town and +King George’s Sound in Australia, the “Beagle” sailed across the Indian +Ocean to the little group of the Keeling or Cocos Islands, which Darwin +has rendered famous by his observations, and thence to Mauritius; +calling at the Cape of Good Hope on her way, the ship then proceeded +successively to St. Helena and Ascension, and revisited the Cape de +Verde Islands before finally reaching England. + +Although Darwin was thus able to gratify his curiosity by visits to a +great number of very interesting volcanic districts, the voyage opened +for him with a bitter disappointment. He had been reading Humboldt’s +“Personal Narrative” during his last year’s residence in Cambridge, and +had copied out from it long passages about Teneriffe. He was actually +making inquiries as to the best means of visiting that island, when the +offer was made to him to accompany Captain Fitzroy in the “Beagle. “ +His friend Henslow too, on parting with him, had given him the advice +to procure and read the recently published first volume of the +“Principles of Geology,” though he warned him against accepting the +views advocated by its author. During the time the “Beagle” was beating +backwards and forwards when the voyage commenced, Darwin, although +hardly ever able to leave his berth, was employing all the +opportunities which the terrible sea-sickness left him, in studying +Humboldt and Lyell. We may therefore form an idea of his feelings when, +on the ship reaching Santa Cruz, and the Peak of Teneriffe making its +appearance among the clouds, they were suddenly informed that an +outbreak of cholera would prevent any landing! + +Ample compensation for this disappointment was found, however, when the +ship reached Porta Praya in St. Jago, the largest of the Cape de Verde +Islands. Here he spent three most delightful weeks, and really +commenced his work as a geologist and naturalist. Writing to his father +he says, “Geologising in a volcanic country is most delightful; besides +the interest attached to itself, it leads you into most beautiful and +retired spots. Nobody but a person fond of Natural History can imagine +the pleasure of strolling under cocoa-nuts in a thicket of bananas and +coffee-plants, and an endless number of wild flowers. And this island, +that has given me so much instruction and delight, is reckoned the most +uninteresting place that we perhaps shall touch at during our voyage. +It certainly is generally very barren, but the valleys are more +exquisitely beautiful, from the very contrast. It is utterly useless to +say anything about the scenery; it would be as profitable to explain to +a blind man colours, as to a person who has not been out of Europe, the +total dissimilarity of a tropical view. Whenever I enjoy anything, I +always look forward to writing it down, either in my log-book (which +increases in bulk), or in a letter; so you must excuse raptures, and +those raptures badly expressed. I find my collections are increasing +wonderfully, and from Rio I think I shall be obliged to send a cargo +home.” + +The indelible impression made on Darwin’s mind by this first visit to a +volcanic island, is borne witness to by a remarkable passage in the +“Autobiography” written by him in 1876. “The geology of St. Jago is +very striking, yet simple; a stream of lava formerly flowed over the +bed of the sea, formed of triturated recent shells and corals, which it +has baked into a hard white rock. Since then the whole island has been +upheaved. But the line of white rock revealed to me a new and important +fact, namely that there had been afterwards subsidence round the +craters which had since been in action, and had poured forth lava. It +then first dawned on me that I might perhaps write a book on the +geology of the various countries visited, and this made me thrill with +delight. That was a memorable hour to me, and how distinctly I can call +to mind the low cliff of lava beneath which I rested, with the sun +glaring hot, a few strange desert plants growing near and with living +corals in the tidal pools at my feet.” + +Only five years before, when listening to poor Professor Jameson’s +lectures on the effete Wernerianism, which at that time did duty for +geological teaching, Darwin had found them “incredibly dull,” and he +declared that “the sole effect they produced on me was a determination +never so long as I lived to read a book on Geology, or in any way to +study the science.” + +What a contrast we find in the expressions which he makes use of in +referring to Geological Science, in his letters written home from the +“Beagle!” After alluding to the delight of collecting and studying +marine animals, he exclaims, “But Geology carries the day!” Writing to +Henslow he says, “I am quite charmed with Geology, but, like the wise +animal between two bundles of hay, I do not know which to like best; +the old crystalline group of rocks, or the softer and more +fossiliferous beds.” And just as the long voyage is about to come to a +close he again writes, “I find in Geology a never-failing interest; as +it has been remarked, it creates the same grand ideas respecting this +world which Astronomy does for the Universe.” In this passage Darwin +doubtless refers to a remark of Sir John Herschel’s in his admirable +“Preliminary Discourse on the Study of Natural Philosophy,”—a book +which exercised a most remarkable and beneficial influence on the mind +of the young naturalist. + +If there cannot be any doubt as to the strong predilection in Darwin’s +mind for geological studies, both during and after the memorable +voyage, there is equally little difficulty in perceiving the school of +geological thought which, in spite of the warnings of Sedgwick and +Henslow, had obtained complete ascendancy over his mind. He writes in +1876: “The very first place which I examined, namely St. Jago in the +Cape de Verde Islands, showed me clearly the wonderful superiority of +Lyell’s manner of treating Geology, compared with that of any other +author, whose works I had with me, or ever afterwards read.” And again, +“The science of Geology is enormously indebted to Lyell—more so, as I +believe, than to any other man who ever lived...I am proud to remember +that the first place, namely, St. Jago, in the Cape de Verde +Archipelago, in which I geologised, convinced me of the infinite +superiority of Lyell’s views over those advocated in any other work +known to me.” + +The passages I have cited will serve to show the spirit in which Darwin +entered upon his geological studies, and the perusal of the following +pages will furnish abundant proofs of the enthusiasm, acumen, and +caution with which his researches were pursued. + +Large collections of rocks and minerals were made by Darwin during his +researches, and sent home to Cambridge, to be kept under the care of +his faithful friend Henslow. After visiting his relations and friends, +Darwin’s first care on his return to England was to unpack and examine +these collections. He accordingly, at the end of 1836, took lodgings +for three months in Fitzwilliam Street, Cambridge, so as to be near +Henslow; and in studying and determining his geological specimens +received much valuable aid from the eminent crystallographer and +mineralogist, Professor William Hallows Miller. + +The actual writing of the volume upon volcanic islands was not +commenced till 1843, when Darwin had settled in the spot which became +his home for the rest of his life—the famous house at Down, in Kent. +Writing to his friend Mr. Fox, on March 28th, 1843, he says, “I am very +slowly progressing with a volume, or rather pamphlet, on the volcanic +islands which we visited: I manage only a couple of hours per day, and +that not very regularly. It is uphill work writing books, which cost +money in publishing, and which are not read even by geologists.” + +The work occupied Darwin during the whole of the year 1843, and was +issued in the spring of the following year, the actual time engaged in +preparing it being recorded in his diary as “from the summer of 1842 to +January 1844;” but the author does not appear to have been by any means +satisfied with the result when the book was finished. He wrote to +Lyell, “You have pleased me much by saying that you intend looking +through my ‘Volcanic Islands;’ it cost me eighteen months!!! and I have +heard of very few who have read it. Now I shall feel, whatever little +(and little it is) there is confirmatory of old work, or new, will work +its effect and not be lost.” To Sir Joseph Hooker he wrote, “I have +just finished a little volume on the volcanic islands which we visited. +I do not know how far you care for dry simple geology, but I hope you +will let me send you a copy.” + +Every geologist knows how full of interest and suggestiveness is this +book of Darwin’s on volcanic islands. 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 when, hammer in hand, he roamed over new and interesting +scenes, and the slow, laborious, and less congenial task of re-writing +and arranging his notes in book-form. + +In 1874, in writing an account of the ancient volcanoes of the +Hebrides, I had frequent occasion to quote Mr. Darwin’s observations on +the Atlantic volcanoes, in illustration of the phenomena exhibited by +the relics of still older volcanoes in our own islands. Darwin, in +writing to his old friend Sir Charles Lyell upon the subject, says, “I +was not a little pleased to see my volcanic book quoted, for I thought +it was completely dead and forgotten.” + +Two years later the original publishers of this book and of that on +South America proposed to re-issue them. Darwin at first hesitated, for +he seemed to think there could be little of abiding interest in them; +he consulted me upon the subject in one of the conversations which I +used to have with him at that time, and I strongly urged upon him the +reprint of the works. I was much gratified when he gave way upon the +point, and consented to their appearing just as originally issued. In +his preface he says, “Owing to the great progress which Geology has +made in recent times, my views on some few points may be somewhat +antiquated, but I have thought it best to leave them as they originally +appeared.” + +It may be interesting to indicate, as briefly as possible, the chief +geological problem upon which the publication of Darwin’s “Volcanic +Islands” threw new and important light. The merit of the work consisted +in supplying interesting observations, which in some cases have proved +of crucial value in exploding prevalent fallacies; in calling attention +to phenomena and considerations that had been quite overlooked by +geologists, but have since exercised an important influence in moulding +geological speculation; and lastly in showing the importance which +attaches to small and seemingly insignificant causes, some of which +afford a key to the explanation of very curious geological problems. + +Visiting as he did the districts in which Von Buch and others had found +what they thought to be evidence of the truth of “Elevation-craters,” +Darwin was able to show that the facts were capable of a totally +different interpretation. The views originally put forward by the old +German geologist and traveller, and almost universally accepted by his +countrymen, had met with much support from Elie de Beaumont and +Dufrenoy, the leaders of geological thought in France. They were, +however, stoutly opposed by Scrope and Lyell in this country, and by +Constant Prevost and Virlet on the other side of the channel. Darwin, +in the work before us, 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 which, opening +at the top, have given origin to the craters. Admitting the influence +of the injection of lava into the structure of the volcanic cones, in +increasing their bulk and elevation, he shows that, in the main, the +volcanoes are built up by repeated ejections causing an accumulation of +materials around the vent. + +While, however, agreeing on the whole with Scrope and Lyell, as to the +explosive origin of ordinary volcanic craters, Darwin clearly saw that, +in some cases, great craters might be formed or enlarged, by the +subsidence of the floors after eruptions. The importance of this +agency, to which too little attention has been directed by geologists, +has recently been shown by Professor Dana, in his admirable work on +Kilauea and the other great volcanoes of the Hawaiian Archipelago. + +The effects of subsidence at a volcanic centre in producing a downward +dip of the strata around it, was first pointed out by Darwin, as the +result of his earliest work in the Cape de Verde Islands. Striking +illustrations of the same principle have since been pointed out by M. +Robert and others in Iceland, by Mr. Heaphy in New Zealand, and by +myself in the Western Isles of Scotland. + +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 crust, along +which the lavas have made their way to the surface. But he, at the same +time, clearly saw that there was no evidence of the occurrence of great +deluges of lava along such fissures; he showed how the most remarkable +plateaux, composed of successive lava sheets, might be built up by +repeated and moderate ejections from numerous isolated vents; 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 denudation. + +One of the most striking parts of the book is that in which he deals +with the effects of denudation in producing “basal wrecks” or worn down +stumps of volcanoes. He was enabled to examine a series of cases in +which could be traced every gradation, from perfect volcanic cones down +to the solidified plugs which had consolidated in the vents from which +ejections had taken place. 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 history. Like Lyell, he was firmly persuaded of the continuity +of geological history, and ever delighted in finding indications, in +the present order of nature, that the phenomena of the past could be +accounted for by means of causes which are still in operation. Lyell’s +last work in the field was carried on about his home in Forfarshire, +and only a few months before his death he wrote to Darwin: “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 for, by the enormous time to which the products of the +oldest volcanoes have been subjected to chemical changes.” + +Darwin was greatly impressed, as the result of his studies of volcanic +phenomena, followed by an examination of the great granite-masses of +the Andes, with the relations between the so-called Plutonic rocks and +those of undoubtedly volcanic origin. It was indeed a fortunate +circumstance, that after studying some excellent examples of recent +volcanic rocks, he proceeded to examine in South America many fine +illustrations of the older igneous rock-masses, and especially of the +most highly crystalline types of the same, and then on his way home had +opportunities of reviving the impression made upon him by the fresh and +unaltered volcanic rocks. Some of the general considerations suggested +by these observations were discussed in a paper read by him before the +Geological Society, on March 7th, 1838, under the title “On the +Connection of Certain Volcanic Phenomena, and On the Formation of +Mountain-chains, and the Effect of Continental Elevations.” The exact +bearing of these two classes of facts upon one another are more fully +discussed in his book on South American geology. + +The proofs of recent elevation around many of the volcanic islands led +Darwin to conclude that volcanic areas were, as a rule, regions in +which upward movements were taking place, and he was naturally led to +contrast them with the areas in which, as he showed, the occurrence of +atolls, encircling reefs, and barrier-reefs afford indication of +subsidence. In this way he was able to map out the oceanic areas in +different zones, along which opposite kinds of movement were taking +place. His conclusions on this subject were full of novelty and +suggestiveness. + +Very clearly did Darwin recognise the importance of the fact that most +of the oceanic islands appear to be of volcanic origin, though he was +careful to point out the remarkable exceptions which somewhat +invalidate the generalisation. In his “Origin of Species” he has +elaborated the idea and suggested the theory of the permanence of +ocean-basins, a suggestion which has been adopted and pushed farther by +subsequent authors, than we think its originator would have approved. +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 him. + +Some years before the voyage of the “Beagle,” Mr. Poulett Scrope had +pointed out the remarkable analogies that exist between certain igneous +rocks of banded structure, as seen in the Ponza Islands, and the +foliated crystalline schists. It does not appear that Darwin was +acquainted with this remarkable memoir, 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 Ascension. Coming fresh from +the study of the great masses of crystalline schist in the South +American continent, 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 “folia.” 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 stresses, a separation and +parallel arrangement of the constituent minerals will result. This is a +process which is now fully recognised as having been a potent factor in +the production of the metamorphic rock, and has been called by more +recent writers “dynamo-metamorphism.” + +In this, and in many similar discussions, in which exact mineralogical +knowledge was required, 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-lens, supplemented by a few +chemical tests and the constant use of the blowpipe. Since his day, the +method of study of rocks by thin sections under the microscope has been +devised, and has become a most efficient aid in all petrographical +inquiries. During the voyage of H.M.S. “Challenger,” many of the +islands studied by Darwin have been revisited and their rocks +collected. 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 “Reports on the +‘Challenger’ Expedition.” While much that is new and valuable has been +contributed to geological science by these more recent investigations, +and many changes have been made in nomenclature and other points of +detail, 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 study, and remain as a monument of the acumen and accuracy in +minute observation of these pioneers in geological research. + +JOHN W. JUDD. + + + + +CHAPTER I. +ST. JAGO, IN THE CAPE DE VERDE ARCHIPELAGO. + + +Rocks of the lowest series. A calcareous sedimentary deposit, with +recent shells, altered by the contact of superincumbent lava, its +horizontality and extent. Subsequent volcanic eruptions, associated +with calcareous matter in an earthy and fibrous form, and often +enclosed within the separate cells of the scoriae. Ancient and +obliterated orifices of eruption of small size. Difficulty of tracing +over a bare plain recent streams of lava. Inland hills of more ancient +volcanic rock. Decomposed olivine in large masses. Feldspathic rocks +beneath the upper crystalline basaltic strata. Uniform structure and +form of the more ancient volcanic hills. Form of the valleys near the +coast. Conglomerate now forming on the sea beach. + +(FIGURE 1: MAP 1: PART OF ST. JAGO, ONE OF THE CAPE DE VERDE ISLANDS.) + +The island of St. Jago extends in a N.N.W. and S.S.E. direction, thirty +miles in length by about twelve in breadth. My observations, made +during two visits, were confined to the southern portion within the +distance of a few leagues from Porto Praya. The country, viewed from +the sea, presents a varied outline: smooth conical hills of a reddish +colour (like Red Hill in Figure 1 (Map 1). (The outline of the coast, +the position of the villages, streamlets, and of most of the hills in +this woodcut, are copied from the chart made on board H.M.S. “Leven.” +The square-topped hills (A, B, C, etc.) are put in merely by eye, to +illustrate my description.)), and others less regular, flat-topped, and +of a blackish colour (like A, B, C,) rise from successive, step-formed +plains of lava. At a distance, a chain of mountains, many thousand feet +in height, traverses the interior of the island. There is no active +volcano in St. Jago, and only one in the group, namely at Fogo. The +island since being inhabited has not suffered from destructive +earthquakes. + +The lowest rocks exposed on the coast near Porto Praya, are highly +crystalline and compact; they appear to be of ancient, submarine, +volcanic origin; they are unconformably covered by a thin, irregular, +calcareous deposit, abounding with shells of a late tertiary period; +and this again is capped by a wide sheet of basaltic lava, which has +flowed in successive streams from the interior of the island, between +the square-topped hills marked A, B, C, etc. Still more recent streams +of lava have been erupted from the scattered cones, such as Red and +Signal Post Hills. The upper strata of the square-topped hills are +intimately related in mineralogical composition, and in other respects, +with the lowest series of the coast- rocks, with which they seem to be +continuous. + +MINERALOGICAL DESCRIPTION OF THE ROCKS OF THE LOWEST SERIES. + +These rocks possess an extremely varying character; they consist of +black, brown, and grey, compact, basaltic bases, with numerous crystals +of augite, hornblende, olivine, mica, and sometimes glassy feldspar. A +common variety is almost entirely composed of crystals of augite with +olivine. Mica, it is known, seldom occurs where augite abounds; nor +probably does the present case offer a real exception, for the mica (at +least in my best characterised specimen, in which one nodule of this +mineral is nearly half an inch in length) is as perfectly rounded as a +pebble in a conglomerate, and evidently has not been crystallised in +the base, in which it is now enclosed, but has proceeded from the +fusion of some pre-existing rock. These compact lavas alternate with +tuffs, amygdaloids, and wacke, and in some places with coarse +conglomerate. Some of the argillaceous wackes are of a dark green +colour, others, pale yellowish-green, and others nearly white; I was +surprised to find that some of the latter varieties, even where +whitest, fused into a jet black enamel, whilst some of the green +varieties afforded only a pale gray bead. Numerous dikes, consisting +chiefly of highly compact augitic rocks, and of gray amygdaloidal +varieties, intersect the strata, which have in several places been +dislocated with considerable violence, and thrown into highly inclined +positions. One line of disturbance crosses the northern end of Quail +Island (an islet in the Bay of Porto Praya), and can be followed to the +mainland. These disturbances took place before the deposition of the +recent sedimentary bed; and the surface, also, had previously been +denuded to a great extent, as is shown by many truncated dikes. + +DESCRIPTION OF THE CALCAREOUS DEPOSIT OVERLYING THE FOREGOING VOLCANIC +ROCKS. + +This stratum is very conspicuous from its white colour, and from the +extreme regularity with which it ranges in a horizontal line for some +miles along the coast. Its average height above the sea, measured from +the upper line of junction with the superincumbent basaltic lava, is +about sixty feet; and its thickness, although varying much from the +inequalities of the underlying formation, may be estimated at about +twenty feet. It consists of quite white calcareous matter, partly +composed of organic debris, and partly of a substance which may be +aptly compared in appearance with mortar. Fragments of rock and pebbles +are scattered throughout this bed, often forming, especially in the +lower part, a conglomerate. Many of the fragments of rock are +whitewashed with a thin coating of calcareous matter. At Quail Island, +the calcareous deposit is replaced in its lowest part by a soft, brown, +earthy tuff, full of Turritellae; this is covered by a bed of pebbles, +passing into sandstone, and mixed with fragments of echini, claws of +crabs, and shells; the oyster-shells still adhering to the rock on +which they grew. Numerous white balls appearing like pisolitic +concretions, from the size of a walnut to that of an apple, are +embedded in this deposit; they usually have a small pebble in their +centres. Although so like concretions, a close examination convinced me +that they were Nulliporae, retaining their proper forms, but with their +surfaces slightly abraded: these bodies (plants as they are now +generally considered to be) exhibit under a microscope of ordinary +power, no traces of organisation in their internal structure. Mr. +George R. Sowerby has been so good as to examine the shells which I +collected: there are fourteen species in a sufficiently perfect +condition for their characters to be made out with some degree of +certainty, and four which can be referred only to their genera. Of the +fourteen shells, of which a list is given in the Appendix, eleven are +recent species; one, though undescribed, is perhaps identical with a +species which I found living in the harbour of Porto Praya; the two +remaining species are unknown, and have been described by Mr. Sowerby. +Until the shells of this Archipelago and of the neighbouring coasts are +better known, it would be rash to assert that even these two latter +shells are extinct. The number of species which certainly belong to +existing kinds, although few in number, are sufficient to show that the +deposit belongs to a late tertiary period. From its mineralogical +character, from the number and size of the embedded fragments, and from +the abundance of Patellae, and other littoral shells, it is evident +that the whole was accumulated in a shallow sea, near an ancient +coast-line. + +EFFECTS PRODUCED BY THE FLOWING OF THE SUPERINCUMBENT BASALTIC LAVA +OVER THE CALCAREOUS DEPOSIT. + +These effects are very curious. The calcareous matter is altered to the +depth of about a foot beneath the line of junction; and a most perfect +gradation can be traced, from loosely aggregated, small, particles of +shells, corallines, and Nulliporae, into a rock, in which not a trace +of mechanical origin can be discovered, even with a microscope. Where +the metamorphic change has been greatest, two varieties occur. The +first is a hard, compact, white, fine-grained rock, striped with a few +parallel lines of black volcanic particles, and resembling a sandstone, +but which, upon close examination, is seen to be crystallised +throughout, with the cleavages so perfect that they can be readily +measured by the reflecting goniometer. In specimens, where the change +has been less complete, when moistened and examined under a strong +lens, the most interesting gradation can be traced, some of the rounded +particles retaining their proper forms, and others insensibly melting +into the granulo-crystalline paste. The weathered surface of this +stone, as is so frequently the case with ordinary limestones, assumes a +brick-red colour. + +The second metamorphosed variety is likewise a hard rock, but without +any crystalline structure. It consists of a white, opaque, compact, +calcareous stone, thickly mottled with rounded, though regular, spots +of a soft, earthy, ochraceous substance. This earthy matter is of a +pale yellowish- brown colour, and appears to be a mixture of carbonate +of lime with iron; it effervesces with acids, is infusible, but +blackens under the blowpipe, and becomes magnetic. The rounded form of +the minute patches of earthy substance, and the steps in the progress +of their perfect formation, which can be followed in a suit of +specimens, clearly show that they are due either to some power of +aggregation in the earthy particles amongst themselves, or more +probably to a strong attraction between the atoms of the carbonate of +line, and consequently to the segregation of the earthy extraneous +matter. I was much interested by this fact, because I have often seen +quartz rocks (for instance, in the Falkland Islands, and in the lower +Silurian strata of the Stiper-stones in Shropshire), mottled in a +precisely analogous manner, with little spots of a white, earthy +substance (earthy feldspar?); and these rocks, there was good reason to +suppose, had undergone the action of heat,—a view which thus receives +confirmation. This spotted structure may possibly afford some +indication in distinguishing those formations of quartz, which owe +their present structure to igneous action, from those produced by the +agency of water alone; a source of doubt, which I should think from my +own experience, that most geologists, when examining arenaceo-quartzose +districts must have experienced. + +The lowest and most scoriaceous part of the lava, in rolling over the +sedimentary deposit at the bottom of the sea, has caught up large +quantities of calcareous matter, which now forms a snow-white, highly +crystalline basis to a breccia, including small pieces of black, glossy +scoriae. A little above this, where the lime is less abundant, and the +lava more compact, numerous little balls, composed of spicula of +calcareous spar, radiating from common centres, occupy the interstices. +In one part of Quail Island, the lime has thus been crystallised by the +heat of the superincumbent lava, where it is only thirteen feet in +thickness; nor had the lava been originally thicker, and since reduced +by degradation, as could be told from the degree of cellularity of its +surface. I have already observed that the sea must have been shallow in +which the calcareous deposit was accumulated. In this case, therefore, +the carbonic acid gas has been retained under a pressure, insignificant +compared with that (a column of water, 1,708 feet in height) originally +supposed by Sir James Hall to be requisite for this end: but since his +experiments, it has been discovered that pressure has less to do with +the retention of carbonic acid gas, than the nature of the circumjacent +atmosphere; and hence, as is stated to be the case by Mr. Faraday, +masses of limestone are sometimes fused and crystallised even in common +limekilns. (I am much indebted to Mr. E.W. Brayley in having given me +the following references to papers on this subject: Faraday in the +“Edinburgh New Philosophical Journal” volume 15 page 398; Gay-Lussac in +“Annales de Chem. et Phys.” tome 63 page 219 translated in the “London +and Edinburgh Philosophical Magazine” volume 10 page 496.) Carbonate of +lime can be heated to almost any degree, according to Faraday, in an +atmosphere of carbonic acid gas, without being decomposed; and +Gay-Lussac found that fragments of limestone, placed in a tube and +heated to a degree, not sufficient by itself to cause their +decomposition, yet immediately evolved their carbonic acid, when a +stream of common air or steam was passed over them: Gay-Lussac +attributes this to the mechanical displacement of the nascent carbonic +acid gas. The calcareous matter beneath the lava, and especially that +forming the crystalline spicula between the interstices of the scoriae, +although heated in an atmosphere probably composed chiefly of steam, +could not have been subjected to the effects of a passing stream; and +hence it is, perhaps, that they have retained their carbonic acid, +under a small amount of pressure. + +The fragments of scoriae, embedded in the crystalline calcareous basis, +are of a jet black colour, with a glossy fracture like pitchstone. +Their surfaces, however, are coated with a layer of a reddish-orange, +translucent substance, which can easily be scratched with a knife; +hence they appear as if overlaid by a thin layer of rosin. Some of the +smaller fragments are partially changed throughout into this substance: +a change which appears quite different from ordinary decomposition. At +the Galapagos Archipelago (as will be described in a future chapter), +great beds are formed of volcanic ashes and particles of scoriae, which +have undergone a closely similar change. + +THE EXTENT AND HORIZONTALITY OF THE CALCAREOUS STRATUM. + +(FIGURE 2: SIGNAL POST HILL. (Section with A low and C high.) + +A.—Ancient volcanic rocks. + +B.—Calcareous stratum. + +C.—Upper basaltic lava.) + +The upper line of surface of the calcareous stratum, which is so +conspicuous from being quite white and so nearly horizontal, ranges for +miles along the coast, at the height of about sixty feet above the sea. +The sheet of basalt, by which it is capped, is on an average eighty +feet in thickness. Westward of Porto Praya beyond Red Hill, the white +stratum with the superincumbent basalt is covered up by more recent +streams. Northward of Signal Post Hill, I could follow it with my eye, +trending away for several miles along the sea cliffs. The distance thus +observed is about seven miles; but I cannot doubt from its regularity +that it extends much farther. In some ravines at right angles to the +coast, it is seen gently dipping towards the sea, probably with the +same inclination as when deposited round the ancient shores of the +island. I found only one inland section, namely, at the base of the +hill marked A, where, at the height of some hundred feet, this bed was +exposed; it here rested on the usual compact augitic rock associated +with wacke, and was covered by the widespread sheet of modern basaltic +lava. Some exceptions occur to the horizontality of the white stratum: +at Quail Island, its upper surface is only forty feet above the level +of the sea; here also the capping of lava is only between twelve and +fifteen feet in thickness; on the other hand, at the north-east side of +Porto Praya harbour, the calcareous stratum, as well as the rock on +which it rests, attain a height above the average level: the inequality +of level in these two cases is not, as I believe, owing to unequal +elevation, but to original irregularities at the bottom of the sea. Of +this fact, at Quail Island, there was clear evidence in the calcareous +deposit being in one part of much greater than the average thickness, +and in another part being entirely absent; in this latter case, the +modern basaltic lavas rested directly on those of more ancient origin. + +Under Signal Post Hill, the white stratum dips into the sea in a +remarkable manner. This hill is conical, 450 feet in height, and +retains some traces of having had a crateriform structure; it is +composed chiefly of matter erupted posteriorly to the elevation of the +great basaltic plain, but partly of lava of apparently submarine origin +and of considerable antiquity. The surrounding plain, as well as the +eastern flank of this hill, has been worn into steep precipices, +overhanging the sea. In these precipices, the white calcareous stratum +may be seen, at the height of about seventy feet above the beach, +running for some miles both northward and southward of the hill, in a +line appearing to be perfectly horizontal; but for a space of a quarter +of a mile directly under the hill, it dips into the sea and disappears. +On the south side the dip is gradual, on the north side it is more +abrupt, as is shown in Figure 2. As neither the calcareous stratum, nor +the superincumbent basaltic lava (as far as the latter can be +distinguished from the more modern ejections), appears to thicken as it +dips, I infer that these strata were not originally accumulated in a +trough, the centre of which afterwards became a point of eruption; but +that they have subsequently been disturbed and bent. We may suppose +either that Signal Post Hill subsided after its elevation with the +surrounding country, or that it never was uplifted to the same height +with it. This latter seems to me the most probable alternative, for +during the slow and equable elevation of this portion of the island, +the subterranean motive power, from expending part of its force in +repeatedly erupting volcanic matter from beneath this point, would, it +is likely, have less force to uplift it. Something of the same kind +seems to have occurred near Red Hill, for when tracing upwards the +naked streams of lava from near Porto Praya towards the interior of the +island, I was strongly induced to suspect, that since the lava had +flowed, the slope of the land had been slightly modified, either by a +small subsidence near Red Hill, or by that portion of the plain having +been uplifted to a less height during the elevation of the whole area. + +THE BASALTIC LAVA, SUPERINCUMBENT ON THE CALCAREOUS DEPOSIT. + +This lava is of a pale grey colour, fusing into a black enamel; its +fracture is rather earthy and concretionary; it contains olivine in +small grains. The central parts of the mass are compact, or at most +crenulated with a few minute cavities, and are often columnar. At Quail +Island this structure was assumed in a striking manner; the lava in one +part being divided into horizontal laminae, which became in another +part split by vertical fissures into five-sided plates; and these +again, being piled on each other, insensibly became soldered together, +forming fine symmetrical columns. The lower surface of the lava is +vesicular, but sometimes only to the thickness of a few inches; the +upper surface, which is likewise vesicular, is divided into balls, +frequently as much as three feet in diameter, made up of concentric +layers. The mass is composed of more than one stream; its total +thickness being, on an average, about eighty feet: the lower portion +has certainly flowed beneath the sea, and probably likewise the upper +portion. The chief part of this lava has flowed from the central +districts, between the hills marked A, B, C, etc., in the woodcut- map. +The surface of the country, near the coast, is level and barren; +towards the interior, the land rises by successive terraces, of which +four, when viewed from a distance, could be distinctly counted. + +VOLCANIC ERUPTIONS SUBSEQUENT TO THE ELEVATION OF THE COASTLAND; THE +EJECTED MATTER ASSOCIATED WITH EARTHY LIME. + +These recent lavas have proceeded from those scattered, conical, +reddish- coloured hills, which rise abruptly from the plain-country +near the coast. I ascended some of them, but will describe only one, +namely, RED HILL, which may serve as a type of its class, and is +remarkable in some especial respects. Its height is about six hundred +feet; it is composed of bright red, highly scoriaceous rock of a +basaltic nature; on one side of its summit there is a hollow, probably +the last remnant of a crater. Several of the other hills of this class, +judging from their external forms, are surmounted by much more perfect +craters. When sailing along the coast, it was evident that a +considerable body of lava had flowed from Red Hill, over a line of +cliff about one hundred and twenty feet in height, into the sea: this +line of cliff is continuous with that forming the coast, and bounding +the plain on both sides of this hill; these streams, therefore, were +erupted, after the formation of the coast-cliffs, from Red Hill, when +it must have stood, as it now does, above the level of the sea. This +conclusion accords with the highly scoriaceous condition of all the +rock on it, appearing to be of subaerial formation: and this is +important, as there are some beds of calcareous matter near its summit, +which might, at a hasty glance, have been mistaken for a submarine +deposit. These beds consist of white, earthy, carbonate of lime, +extremely friable so as to be crushed with the least pressure; the most +compact specimens not resisting the strength of the fingers. Some of +the masses are as white as quicklime, and appear absolutely pure; but +on examining them with a lens, minute particles of scoriae can always +be seen, and I could find none which, when dissolved in acids, did not +leave a residue of this nature. It is, moreover, difficult to find a +particle of the lime which does not change colour under the blowpipe, +most of them even becoming glazed. The scoriaceous fragments and the +calcareous matter are associated in the most irregular manner, +sometimes in obscure beds, but more generally as a confused breccia, +the lime in some parts and the scoriae in others being most abundant. +Sir H. De la Beche has been so kind as to have some of the purest +specimens analysed, with a view to discover, considering their volcanic +origin, whether they contained much magnesia; but only a small portion +was found, such as is present in most limestones. + +Fragments of the scoriae embedded in the calcareous mass, when broken, +exhibit many of their cells lined and partly filled with a white, +delicate, excessively fragile, moss-like, or rather conferva-like, +reticulation of carbonate of lime. These fibres, examined under a lens +of one-tenth of an inch focal distance, appear cylindrical; they are +rather above one- thousandth of an inch in diameter; they are either +simply branched, or more commonly united into an irregular mass of +network, with the meshes of very unequal sizes and of unequal numbers +of sides. Some of the fibres are thickly covered with extremely minute +spicula, occasionally aggregated into little tuffs; and hence they have +a hairy appearance. These spicula are of the same diameter throughout +their length; they are easily detached, so that the object-glass of the +microscope soon becomes scattered over with them. Within the cells of +many fragments of the scoria, the lime exhibits this fibrous structure, +but generally in a less perfect degree. These cells do not appear to be +connected with one another. There can be no doubt, as will presently be +shown, that the lime was erupted, mingled with the lava in its fluid +state, and therefore I have thought it worth while to describe minutely +this curious fibrous structure, of which I know nothing analogous. From +the earthy condition of the fibres, this structure does not appear to +be related to crystallisation. + +Other fragments of the scoriaceous rock from this hill, when broken, +are often seen marked with short and irregular white streaks, which are +owing to a row of separate cells being partly, or quite, filled with +white calcareous powder. This structure immediately reminded me of the +appearance in badly kneaded dough, of balls and drawn-out streaks of +flour, which have remained unmixed with the paste; and I cannot doubt +that small masses of the lime, in the same manner remaining unmixed +with the fluid lava, have been drawn out when the whole was in motion. +I carefully examined, by trituration and solution in acids, pieces of +the scoriae, taken from within half-an-inch of those cells which were +filled with the calcareous powder, and they did not contain an atom of +free lime. It is obvious that the lava and lime have on a large scale +been very imperfectly mingled; and where small portions of the lime +have been entangled within a piece of the viscid lava, the cause of +their now occupying, in the form of a powder or of a fibrous +reticulation, the vesicular cavities, is, I think, evidently due to the +confined gases having most readily expanded at the points where the +incoherent lime rendered the lava less adhesive. + +A mile eastward of the town of Praya, there is a steep-sided gorge, +about one hundred and fifty yards in width, cutting through the +basaltic plain and underlying beds, but since filled up by a stream of +more modern lava. This lava is dark grey, and in most parts compact and +rudely columnar; but at a little distance from the coast, it includes +in an irregular manner a brecciated mass of red scoriae mingled with a +considerable quantity of white, friable, and in some parts, nearly pure +earthy lime, like that on the summit of Red Hill. This lava, with its +entangled lime, has certainly flowed in the form of a regular stream; +and, judging from the shape of the gorge, towards which the drainage of +the country (feeble though it now be) still is directed, and from the +appearance of the bed of loose water-worn blocks with their interstices +unfilled, like those in the bed of a torrent, on which the lava rests, +we may conclude that the stream was of subaerial origin. I was unable +to trace it to its source, but, from its direction, it seemed to have +come from Signal Post Hill, distant one mile and a quarter, which, like +Red Hill, has been a point of eruption subsequent to the elevation of +the great basaltic plain. It accords with this view, that I found on +Signal Post Hill, a mass of earthy, calcareous matter of the same +nature, mingled with scoriae. I may here observe that part of the +calcareous matter forming the horizontal sedimentary bed, especially +the finer matter with which the embedded fragments of rock are +whitewashed, has probably been derived from similar volcanic eruptions, +as well as from triturated organic remains: the underlying, ancient, +crystalline rocks, also, are associated with much carbonate of lime, +filling amygdaloidal cavities, and forming irregular masses, the nature +of which latter I was unable to understand. + +Considering the abundance of earthy lime near the summit of Red Hill, a +volcanic cone six hundred feet in height, of subaerial +growth,—considering the intimate manner in which minute particles and +large masses of scoriae are embedded in the masses of nearly pure lime, +and on the other hand, the manner in which small kernels and streaks of +the calcareous powder are included in solid pieces of the +scoriae,—considering, also, the similar occurrence of lime and scoriae +within a stream of lava, also supposed, with good reason, to have been +of modern subaerial origin, and to have flowed from a hill, where +earthy lime also occurs: I think, considering these facts, there can be +no doubt that the lime has been erupted, mingled with the molten lava. +I am not aware that any similar case has been described: it appears to +me an interesting one, inasmuch as most geologists must have speculated +on the probable effects of a volcanic focus, bursting through +deep-seated beds of different mineralogical composition. The great +abundance of free silex in the trachytes of some countries (as +described by Beudant in Hungary, and by P. Scrope in the Panza +Islands), perhaps solves the inquiry with respect to deep-seated beds +of quartz; and we probably here see it answered, where the volcanic +action has invaded subjacent masses of limestone. One is naturally led +to conjecture in what state the now earthy carbonate of lime existed, +when ejected with the intensely heated lava: from the extreme +cellularity of the scoriae on Red Hill, the pressure cannot have been +great, and as most volcanic eruptions are accompanied by the emission +of large quantities of steam and other gases, we here have the most +favourable conditions, according to the views at present entertained by +chemists, for the expulsion of the carbonic acid. (Whilst deep beneath +the surface, the carbonate of lime was, I presume, in a fluid state. +Hutton, it is known, thought that all amygdaloids were produced by +drops of molten limestone floating in the trap, like oil in water: this +no doubt is erroneous, but if the matter forming the summit of Red Hill +had been cooled under the pressure of a moderately deep sea, or within +the walls of a dike, we should, in all probability, have had a trap +rock associated with large masses of compact, crystalline, calcareous +spar, which, according to the views entertained by many geologists, +would have been wrongly attributed to subsequent infiltration.) Has the +slow re- absorption of this gas, it may be asked, given to the lime in +the cells of the lava, that peculiar fibrous structure, like that of an +efflorescing salt? Finally, I may remark on the great contrast in +appearance between this earthy lime, which must have been heated in a +free atmosphere of steam and other gases, while the white, crystalline, +calcareous spar, produced by a single thin sheet of lava (as at Quail +Island) rolling over similar earthy lime and the debris of organic +remains, at the bottom of a shallow sea. + +SIGNAL POST HILL. + +This hill has already been several times mentioned, especially with +reference to the remarkable manner in which the white calcareous +stratum, in other parts so horizontal (Figure 2), dips under it into +the sea. It has a broad summit, with obscure traces of a crateriform +structure, and is composed of basaltic rocks (Of these, one common +variety is remarkable for being full of small fragments of a dark +jasper-red earthy mineral, which, when examined carefully, shows an +indistinct cleavage; the little fragments are elongated in form, are +soft, are magnetic before and after being heated, and fuse with +difficulty into a dull enamel. This mineral is evidently closely +related to the oxides of iron, but I cannot ascertain what it exactly +is. The rock containing this mineral is crenulated with small angular +cavities, which are lined and filled with yellowish crystals of +carbonate of lime.), some compact, others highly cellular with inclined +beds of loose scoriae, of which some are associated with earthy lime. +Like Red Hill, it has been the source of eruptions, subsequently to the +elevation of the surrounding basaltic plain; but unlike that hill, it +has undergone considerable denudation, and has been the seat of +volcanic action at a remote period, when beneath the sea. I judge of +this latter circumstance from finding on its inland flank the last +remains of three small points of eruption. These points are composed of +glossy scoriae, cemented by crystalline calcareous spar, exactly like +the great submarine calcareous deposit, where the heated lava has +rolled over it: their demolished state can, I think, be explained only +by the denuding action of the waves of the sea. I was guided to the +first orifice by observing a sheet of lava, about two hundred yards +square, with steepish sides, superimposed on the basaltic plain with no +adjoining hillock, whence it could have been erupted; and the only +trace of a crater which I was able to discover, consisted of some +inclined beds of scoriae at one of its corners. At the distance of +fifty yards from a second level-topped patch of lava, but of much +smaller size, I found an irregular circular group of masses of +cemented, scoriaceous breccia, about six feet in height, which +doubtless had once formed the point of eruption. The third orifice is +now marked only by an irregular circle of cemented scoriae, about four +yards in diameter, and rising in its highest point scarcely three feet +above the level of the plain, the surface of which, close all round, +exhibits its usual appearance: here we have a horizontal basal section +of a volcanic spiracle, which, together with all its ejected matter, +has been almost totally obliterated. + +The stream of lava, which fills the narrow gorge eastward of the town +of Praya, judging from its course, seems, as before remarked, to have +come from Signal Post Hill, and to have flowed over the plain, after +its elevation (The sides of this gorge, where the upper basaltic +stratum is intersected, are almost perpendicular. The lava, which has +since filled it up, is attached to these sides, almost as firmly as a +dike is to its walls. In most cases, where a stream of lava has flowed +down a valley, it is bounded on each side by loose scoriaceous +masses.): the same observation applies to a stream (possibly part of +the same one) capping the sea cliffs, a little eastward of the gorge. +When I endeavoured to follow these streams over the stony level plain, +which is almost destitute of soil and vegetation, I was much surprised +to find, that although composed of hard basaltic matter, and not having +been exposed to marine denudation, all distant traces of them soon +became utterly lost. But I have since observed at the Galapagos +Archipelago, that it is often impossible to follow even great deluges +of quite recent lava across older streams, except by the size of the +bushes growing on them, or by the comparative states of glossiness of +their surfaces,—characters which a short lapse of time would be +sufficient quite to obscure. I may remark, that in a level country, +with a dry climate, and with the wind blowing always in one direction +(as at the Cape de Verde Archipelago), the effects of atmospheric +degradation are probably much greater than would at first be expected; +for soil in this case accumulates only in a few protected hollows, and +being blown in one direction, it is always travelling towards the sea +in the form of the finest dust, leaving the surface of the rocks bare, +and exposed to the full effects of renewed meteoric action. + +INLAND HILLS OF MORE ANCIENT VOLCANIC ROCKS. + +These hills are laid down by eye, and marked as A, B, C, etc., in Map +1. They are related in mineralogical composition, and are probably +directly continuous with the lowest rocks exposed on the coast. These +hills, viewed from a distance, appear as if they had once formed part +of an irregular tableland, and from their corresponding structure and +composition this probably has been the case. They have flat, slightly +inclined summits, and are, on an average, about six hundred feet in +height; they present their steepest slope towards the interior of the +island, from which point they radiate outwards, and are separated from +each other by broad and deep valleys, through which the great streams +of lava, forming the coast-plains, have descended. Their inner and +steeper escarpments are ranged in an irregular curve, which rudely +follows the line of the shore, two or three miles inland from it. I +ascended a few of these hills, and from others, which I was able to +examine with a telescope, I obtained specimens, through the kindness of +Mr. Kent, the assistant-surgeon of the “Beagle”; although by these +means I am acquainted with only a part of the range, five or six miles +in length, yet I scarcely hesitate, from their uniform structure, to +affirm that they are parts of one great formation, stretching round +much of the circumference of the island. + +The upper and lower strata of these hills differ greatly in +composition. The upper are basaltic, generally compact, but sometimes +scoriaceous and amygdaloidal, with associated masses of wacke: where +the basalt is compact, it is either fine-grained or very coarsely +crystallised; in the latter case it passes into an augitic rock, +containing much olivine; the olivine is either colourless, or of the +usual yellow and dull reddish shades. On some of the hills, beds of +calcareous matter, both in an earthy and in a crystalline form, +including fragments of glossy scoriae, are associated with the basaltic +strata. These strata differ from the streams of basaltic lava forming +the coast-plains, only in being more compact, and in the crystals of +augite, and in the grains of olivine being of much greater +size;—characters which, together with the appearance of the associated +calcareous beds, induce me to believe that they are of submarine +formation. + +Some considerable masses of wacke, which are associated with these +basaltic strata, and which likewise occur in the basal series on the +coast, especially at Quail Island, are curious. They consist of a pale +yellowish- green argillaceous substance, of a crumbling texture when +dry, but unctuous when moist: in its purest form, it is of a beautiful +green tint, with translucent edges, and occasionally with obscure +traces of an original cleavage. Under the blowpipe it fuses very +readily into a dark grey, and sometimes even black bead, which is +slightly magnetic. From these characters, I naturally thought that it +was one of the pale species, decomposed, of the genus augite;—a +conclusion supported by the unaltered rock being full of large separate +crystals of black augite, and of balls and irregular streaks of dark +grey augitic rock. As the basalt ordinarily consists of augite, and of +olivine often tarnished and of a dull red colour, I was led to examine +the stages of decomposition of this latter mineral, and I found, to my +surprise, that I could trace a nearly perfect gradation from unaltered +olivine to the green wacke. Part of the same grain under the blowpipe +would in some instances behave like olivine, its colour being only +slightly changed, and part would give a black magnetic bead. Hence I +can have no doubt that the greenish wacke originally existed as +olivine; but great chemical changes must have been effected during the +act of decomposition thus to have altered a very hard, transparent, +infusible mineral, into a soft, unctuous, easily melted, argillaceous +substance. (D’Aubuisson “Traite de Geognosie” tome 2 page 569 mentions, +on the authority of M. Marcel de Serres, masses of green earth near +Montpellier, which are supposed to be due to the decomposition of +olivine. I do not, however, find, that the action of this mineral under +the blowpipe being entirely altered, as it becomes decomposed, has been +noticed; and the knowledge of this fact is important, as at first it +appears highly improbable that a hard, transparent, refractory mineral +should be changed into a soft, easily fused clay, like this of St. +Jago. I shall hereafter describe a green substance, forming threads +within the cells of some vesicular basaltic rocks in Van Diemen’s Land, +which behave under the blowpipe like the green wacke of St. Jago; but +its occurrence in cylindrical threads, shows it cannot have resulted +from the decomposition of olivine, a mineral always existing in the +form of grains or crystals.) + +The basal strata of these hills, as well as some neighbouring, +separate, bare, rounded hillocks, consist of compact, fine-grained, +non-crystalline (or so slightly as scarcely to be perceptible), +ferruginous, feldspathic rocks, and generally in a state of +semi-decomposition. Their fracture is exceedingly irregular, and +splintery; yet small fragments are often very tough. They contain much +ferruginous matter, either in the form of minute grains with a metallic +lustre, or of brown hair-like threads: the rock in this latter case +assuming a pseudo-brecciated structure. These rocks sometimes contain +mica and veins of agate. Their rusty brown or yellowish colour is +partly due to the oxides of iron, but chiefly to innumerable, +microscopically minute, black specks, which, when a fragment is heated, +are easily fused, and evidently are either hornblende or augite. These +rocks, therefore, although at first appearing like baked clay or some +altered sedimentary deposit, contain all the essential ingredients of +trachyte; from which they differ only in not being harsh, and in not +containing crystals of glassy feldspar. As is so often the case with +trachytic formation, no stratification is here apparent. A person would +not readily believe that these rocks could have flowed as lava; yet at +St. Helena there are well-characterised streams (as will be described +in an ensuing chapter) of nearly similar composition. Amidst the +hillocks composed of these rocks, I found in three places, smooth +conical hills of phonolite, abounding with fine crystals of glassy +feldspar, and with needles of hornblende. These cones of phonolite, I +believe, bear the same relation to the surrounding feldspathic strata +which some masses of coarsely crystallised augitic rock, in another +part of the island, bear to the surrounding basalt, namely, that both +have been injected. The rocks of a feldspathic nature being anterior in +origin to the basaltic strata, which cap them, as well as to the +basaltic streams of the coast-plains, accords with the usual order of +succession of these two grand divisions of the volcanic series. + +The strata of most of these hills in the upper part, where alone the +planes of division are distinguishable, are inclined at a small angle +from the interior of the island towards the sea-coast. The inclination +is not the same in each hill; in that marked A it is less than in B, D, +or E; in C the strata are scarcely deflected from a horizontal plane, +and in F (as far as I could judge without ascending it) they are +slightly inclined in a reverse direction, that is, inwards and towards +the centre of the island. Notwithstanding these differences of +inclination, their correspondence in external form, and in the +composition both of their upper and lower parts,- -their relative +position in one curved line, with their steepest sides turned +inwards,—all seem to show that they originally formed parts of one +platform; which platform, as before remarked, probably extended round a +considerable portion of the circumference of the island. The upper +strata certainly flowed as lava, and probably beneath the sea, as +perhaps did the lower feldspathic masses: how then come these strata to +hold their present position, and whence were they erupted? + +In the centre of the island there are lofty mountains, but they are +separated from the steep inland flanks of these hills by a wide space +of lower country: the interior mountains, moreover, seem to have been +the source of those great streams of basaltic lava which, contracting +as they pass between the bases of the hills in question, expand into +the coast- plains. (I saw very little of the inland parts of the +island. Near the village of St. Domingo, there are magnificent cliffs +of rather coarsely crystallised basaltic lava. Following the little +stream in this valley, about a mile above the village, the base of the +great cliff was formed of a compact fine-grained basalt, conformably +covered by a bed of pebbles. Near Fuentes, I met with pap-formed hills +of the compact feldspathic series of rocks.) Round the shores of St. +Helena there is a rudely formed ring of basaltic rocks, and at +Mauritius there are remnants of another such a ring round part, if not +round the whole, of the island; here again the same question +immediately occurs, how came these masses to hold their present +position, and whence were they erupted? The same answer, whatever it +may be, probably applies in these three cases; and in a future chapter +we shall recur to this subject. + +VALLEYS NEAR THE COAST. + +These are broad, very flat, and generally bounded by low cliff-formed +sides. Portions of the basaltic plain are sometimes nearly or quite +isolated by them; of which fact, the space on which the town of Praya +stands offers an instance. The great valley west of the town has its +bottom filled up to a depth of more than twenty feet by well-rounded +pebbles, which in some parts are firmly cemented together by white +calcareous matter. There can be no doubt, from the form of these +valleys, that they were scooped out by the waves of the sea, during +that equable elevation of the land, of which the horizontal calcareous +deposit, with its existing species of marine remains, gives evidence. +Considering how well shells have been preserved in this stratum, it is +singular that I could not find even a single small fragment of shell in +the conglomerate at the bottom of the valleys. The bed of pebbles in +the valley west of the town is intersected by a second valley joining +it as a tributary, but even this valley appears much too wide and +flat-bottomed to have been formed by the small quantity of water, which +falls only during one short wet season; for at other times of the year +these valleys are absolutely dry. + +RECENT CONGLOMERATE. + +On the shores of Quail Island, I found fragments of brick, bolts of +iron, pebbles, and large fragments of basalt, united by a scanty base +of impure calcareous matter into a firm conglomerate. To show how +exceedingly firm this recent conglomerate is, I may mention, that I +endeavoured with a heavy geological hammer to knock out a thick bolt of +iron, which was embedded a little above low-water mark, but was quite +unable to succeed. + + + + +CHAPTER II. +FERNANDO NORONHA; TERCEIRA; TAHITI, ETC. + + +FERNANDO NORONHA. Precipitous hill of phonolite. + +TERCEIRA. Trachytic rocks: their singular decomposition by steam of +high temperature. + +TAHITI. Passage from wacke into trap; singular volcanic rock with the +vesicles half-filled with mesotype. + +MAURITIUS. Proofs of its recent elevation. Structure of its more +ancient mountains; similarity with St. Jago. + +ST. PAUL’S ROCKS. Not of volcanic origin. Their singular mineralogical +composition. + +FERNANDO NORONHA. + +During our short visit at this and the four following islands, I +observed very little worthy of description. Fernando Noronha is +situated in the Atlantic Ocean, in latitude 3 degrees 50 minutes S., +and 230 miles distant from the coast of South America. It consists of +several islets, together nine miles in length by three in breadth. The +whole seems to be of volcanic origin; although there is no appearance +of any crater, or of any one central eminence. The most remarkable +feature is a hill 1,000 feet high, of which the upper 400 feet consist +of a precipitous, singularly shaped pinnacle, formed of columnar +phonolite, containing numerous crystals of glassy feldspar, and a few +needles of hornblende. From the highest accessible point of this hill, +I could distinguish in different parts of the group several other +conical hills, apparently of the same nature. At St. Helena there are +similar, great, conical, protuberant masses of phonolite, nearly one +thousand feet in height, which have been formed by the injection of +fluid feldspathic lava into yielding strata. If this hill has had, as +is probable, a similar origin, denudation has been here effected on an +enormous scale. Near the base of this hill, I observed beds of white +tuff, intersected by numerous dikes, some of amygdaloidal basalt and +others of trachyte; and beds of slaty phonolite with the planes of +cleavage directed N.W. and S.E. Parts of this rock, where the crystals +were scanty, closely resembled common clay-slate, altered by the +contact of a trap-dike. The lamination of rocks, which undoubtedly have +once been fluid, appears to me a subject well deserving attention. On +the beach there were numerous fragments of compact basalt, of which +rock a distant facade of columns seemed to be formed. + +TERCEIRA IN THE AZORES. + +The central parts of this island consist of irregularly rounded +mountains of no great elevation, composed of trachyte, which closely +resembles in general character the trachyte of Ascension, presently to +be described. This formation is in many parts overlaid, in the usual +order of superposition, by streams of basaltic lava, which near the +coast compose nearly the whole surface. The course which these streams +have followed from their craters, can often be followed by the eye. The +town of Angra is overlooked by a crateriform hill (Mount Brazil), +entirely built of thin strata of fine-grained, harsh, brown-coloured +tuff. The upper beds are seen to overlap the basaltic streams on which +the town stands. This hill is almost identical in structure and +composition with numerous crateriformed hills in the Galapagos +Archipelago. + +EFFECTS OF STEAM ON THE TRACHYTIC ROCKS. + +In the central part of the island there is a spot, where steam is +constantly issuing in jets from the bottom of a small ravine-like +hollow, which has no exit, and which abuts against a range of trachytic +mountains. The steam is emitted from several irregular fissures: it is +scentless, soon blackens iron, and is of much too high temperature to +be endured by the hand. The manner in which the solid trachyte is +changed on the borders of these orifices is curious: first, the base +becomes earthy, with red freckles evidently due to the oxidation of +particles of iron; then it becomes soft; and lastly, even the crystals +of glassy feldspar yield to the dissolving agent. After the mass is +converted into clay, the oxide of iron seems to be entirely removed +from some parts, which are left perfectly white, whilst in other +neighbouring parts, which are of the brightest red colour, it seems to +be deposited in greater quantity; some other masses are marbled with +two distinct colours. Portions of the white clay, now that they are +dry, cannot be distinguished by the eye from the finest prepared chalk; +and when placed between the teeth they feel equally soft-grained; the +inhabitants use this substance for white-washing their houses. The +cause of the iron being dissolved in one part, and close by being again +deposited, is obscure; but the fact has been observed in several other +places. (Spallanzani, Dolomieu, and Hoffman have described similar +cases in the Italian volcanic islands. Dolomieu says the iron at the +Panza Islands is redeposited in the form of veins (page 86 “Memoire sur +les Isles Ponces”). These authors likewise believe that the steam +deposits silica: it is now experimentally known that vapour of a high +temperature is able to dissolve silica.) In some half-decayed +specimens, I found small, globular aggregations of yellow hyalite, +resembling gum-arabic, which no doubt had been deposited by the steam. + +As there is no escape for the rain-water, which trickles down the sides +of the ravine-like hollow, whence the steam issues, it must all +percolate downwards through the fissures at its bottom. Some of the +inhabitants informed me that it was on record that flames (some +luminous appearance?) had originally proceeded from these cracks, and +that the flames had been succeeded by the steam; but I was not able to +ascertain how long this was ago, or anything certain on the subject. +When viewing the spot, I imagined that the injection of a large mass of +rock. like the cone of phonolite at Fernando Noronha, in a semi-fluid +state, by arching the surface might have caused a wedge-shaped hollow +with cracks at the bottom, and that the rain- water percolating to the +neighbourhood of the heated mass, would during many succeeding years be +driven back in the form of steam. + +TAHITI (OTAHEITE). + +I visited only a part of the north-western side of this island, and +this part is entirely composed of volcanic rocks. Near the coast there +are several varieties of basalt, some abounding with large crystals of +augite and tarnished olivine, others compact and earthy,—some slightly +vesicular, and others occasionally amygdaloidal. These rocks are +generally much decomposed, and to my surprise, I found in several +sections that it was impossible to distinguish, even approximately, the +line of separation between the decayed lava and the alternating beds of +tuff. Since the specimens have become dry, it is rather more easy to +distinguish the decomposed igneous rocks from the sedimentary tuffs. +This gradation in character between rocks having such widely different +origins, may I think be explained by the yielding under pressure of the +softened sides of the vesicular cavities, which in many volcanic rocks +occupy a large proportion of their bulk. As the vesicles generally +increase in size and number in the upper parts of a stream of lava, so +would the effects of their compression increase; the yielding, +moreover, of each lower vesicle must tend to disturb all the softened +matter above it. Hence we might expect to trace a perfect gradation +from an unaltered crystalline rock to one in which all the particles +(although originally forming part of the same solid mass) had undergone +mechanical displacement; and such particles could hardly be +distinguished from others of similar composition, which had been +deposited as sediment. As lavas are sometimes laminated in their upper +parts even horizontal lines, appearing like those of aqueous +deposition, could not in all cases be relied on as a criterion of +sedimentary origin. From these considerations it is not surprising that +formerly many geologists believed in real transitions from aqueous +deposits, through wacke, into igneous traps. + +In the valley of Tia-auru, the commonest rocks are basalts with much +olivine, and in some cases almost composed of large crystals of augite. +I picked up some specimens, with much glassy feldspar, approaching in +character to trachyte. There were also many large blocks of vesicular +basalt, with the cavities beautifully lined with chabasie (?), and +radiating bundles of mesotype. Some of these specimens presented a +curious appearance, owing to a number of the vesicles being half filled +up with a white, soft, earthy mesotypic mineral, which intumesced under +the blowpipe in a remarkable manner. As the upper surfaces in all the +half-filled cells are exactly parallel, it is evident that this +substance has sunk to the bottom of each cell from its weight. +Sometimes, however, it entirely fills the cells. Other cells are either +quite filled, or lined, with small crystals, apparently of chabasie; +these crystals, also, frequently line the upper half of the cells +partly filled with the earthy mineral, as well as the upper surface of +this substance itself, in which case the two minerals appear to blend +into each other. I have never seen any other amygdaloid with the cells +half filled in the manner here described; and it is difficult to +imagine the causes which determined the earthy mineral to sink from its +gravity to the bottom of the cells, and the crystalline mineral to +adhere in a coating of equal thickness round the sides of the cells. +(MacCulloch, however, has described and given a plate of (“Geolog. +Trans.” 1st series volume 4 page 225) a trap rock, with cavities filled +up horizontally with quartz and chalcedony. The upper halves of these +cavities are often filled by layers, which follow each irregularity of +the surface, and by little depending stalactites of the same siliceous +substances.) + +The basic strata on the sides of the valley are gently inclined +seaward, and I nowhere observed any sign of disturbance; the strata are +separated from each other by thick, compact beds of conglomerate, in +which the fragments are large, some being rounded, but most angular. +From the character of these beds, from the compact and crystalline +condition of most of the lavas, and from the nature of the infiltrated +minerals, I was led to conjecture that they had originally flowed +beneath the sea. This conclusion agrees with the fact that the Rev. W. +Ellis found marine remains at a considerable height, which he believes +were interstratified with volcanic matter; as is likewise described to +be the case by Messrs. Tyerman and Bennett at Huaheine, an island in +this same archipelago. Mr. Stutchbury also discovered near the summit +of one of the loftiest mountains of Tahiti, at the height of several +thousand feet, a stratum of semi-fossil coral. None of these remains +have been specifically examined. On the coast, where masses of +coral-rock would have afforded the clearest evidence, I looked in vain +for any signs of recent elevation. For references to the above +authorities, and for more detailed reasons for not believing that +Tahiti has been recently elevated, I must refer to the “Structure and +Distribution of Coral-Reefs.” + +MAURITIUS. + +Approaching this island on the northern or north-western side, a curved +chain of bold mountains, surmounted by rugged pinnacles, is seen to +rise from a smooth border of cultivated land, which gently slopes down +to the coast. At the first glance, one is tempted to believe that the +sea lately reached the base of these mountains, and upon examination, +this view, at least with respect to the inferior parts of the border, +is found to be perfectly correct. Several authors have described masses +of upraised coral- rock round the greater part of the circumference of +the island. (Captain Carmichael, in Hooker’s “Bot. Misc.” volume 2 page +301. Captain Lloyd has lately, in the “Proceedings of the Geological +Society” (volume 3 page 317), described carefully some of these masses. +In the “Voyage a l’Isle de France, par un Officier du Roi,” many +interesting facts are given on this subject. Consult also “Voyage aux +Quatre Isles d’Afrique, par M. Bory St. Vincent.”) Between Tamarin Bay +and the Great Black River I observed, in company with Captain Lloyd, +two hillocks of coral-rock, formed in their lower part of hard +calcareous sandstone, and in their upper of great blocks, slightly +aggregated, of Astraea and Madrepora, and of fragments of basalt; they +were divided into beds dipping seaward, in one case at an angle of 8 +degrees, and in the other at 18 degrees; they had a water-worn +appearance, and they rose abruptly from a smooth surface, strewed with +rolled debris of organic remains, to a height of about twenty feet. The +Officier du Roi, in his most interesting tour in 1768 round the island, +has described masses of upraised coral-rocks, still retaining that +moat-like structure (see my “Coral Reefs”) which is characteristic of +the living reefs. On the coast northward of Port Louis, I found the +lava concealed for a considerable space inland by a conglomerate of +corals and shells, like those on the beach, but in parts consolidated +by red ferruginous matter. M. Bory St. Vincent has described similar +calcareous beds over nearly the whole of the plain of Pamplemousses. +Near Port Louis, when turning over some large stones, which lay in the +bed of a stream at the head of a protected creek, and at the height of +some yards above the level of spring tides, I found several shells of +serpula still adhering to their under sides. + +The jagged mountains near Port Louis rise to a height of between two +and three thousand feet; they consist of strata of basalt, obscurely +separated from each other by firmly aggregated beds of fragmentary +matter; and they are intersected by a few vertical dikes. The basalt in +some parts abounds with large crystals of augite and olivine, and is +generally compact. The interior of the island forms a plain, raised +probably about a thousand feet above the level of the sea, and composed +of streams of lava which have flowed round and between the rugged +basaltic mountains. These more recent lavas are also basaltic, but less +compact, and some of them abound with feldspar, so that they even fuse +into a pale coloured glass. On the banks of the Great River, a section +is exposed nearly five hundred feet deep, worn through numerous thin +sheets of the lava of this series, which are separated from each other +by beds of scoriae. They seem to have been of subaerial formation, and +to have flowed from several points of eruption on the central platform, +of which the Piton du Milieu is said to be the principal one. There are +also several volcanic cones, apparently of this modern period, round +the circumference of the island, especially at the northern end, where +they form separate islets. + +The mountains composed of the more compact and crystalline basalt, form +the main skeleton of the island. M. Bailly (“Voyage aux Terres +Australes” tome 1 page 54.) states that they all “se developpent autour +d’elle comme une ceinture d’immenses remparts, toutes affectant une +pente plus ou moins enclinee vers le rivage de la mer; tandis, au +contraire, que vers le centre de l’ile elles presentent une coupe +abrupte, et souvent taillee a pic. Toutes ces montagnes sont formees de +couches paralleles inclinees du centre de l’ile vers la mer.” These +statements have been disputed, though not in detail, by M. Quoy, in the +voyage of Freycinet. As far as my limited means of observation went, I +found them perfectly correct. (M. Lesson, in his account of this +island, in the “Voyage of the ‘Coquille’,” seems to follow M. Bailly’s +views.) The mountains on the N.W. side of the island, which I examined, +namely, La Pouce, Peter Botts, Corps de Garde, Les Mamelles, and +apparently another farther southward, have precisely the external shape +and stratification described by M. Bailly. They form about a quarter of +his girdle of ramparts. Although these mountains now stand quite +detached, being separated from each other by breaches, even several +miles in width, through which deluges of lava have flowed from the +interior of the island; nevertheless, seeing their close general +similarity, one must feel convinced that they originally formed parts +of one continuous mass. Judging from the beautiful map of the +Mauritius, published by the Admiralty from a French MS., there is a +range of mountains (M. Bamboo) on the opposite side of the island, +which correspond in height, relative position, and external form, with +those just described. Whether the girdle was ever complete may well be +doubted; but from M. Bailly’s statements, and my own observations, it +may be safely concluded that mountains with precipitous inland flanks, +and composed of strata dipping outwards, once extended round a +considerable portion of the circumference of the island. The ring +appears to have been oval and of vast size; its shorter axis, measured +across from the inner sides of the mountains near Port Louis and those +near Grand Port, being no less than thirteen geographical miles in +length. M. Bailly boldly supposes that this enormous gulf, which has +since been filled up to a great extent by streams of modern lava, was +formed by the sinking in of the whole upper part of one great volcano. + +It is singular in how many respects those portions of St. Jago and of +Mauritius which I visited agree in their geological history. At both +islands, mountains of similar external form, stratification, and (at +least in their upper beds) composition, follow in a curved chain the +coast-line. These mountains in each case appear originally to have +formed parts of one continuous mass. The basaltic strata of which they +are composed, from their compact and crystalline structure, seem, when +contrasted with the neighbouring basaltic streams of subaerial +formation, to have flowed beneath the pressure of the sea, and to have +been subsequently elevated. We may suppose that the wide breaches +between the mountains were in both cases worn by the waves, during +their gradual elevation—of which process, within recent times, there is +abundant evidence on the coast-land of both islands. At both, vast +streams of more recent basaltic lavas have flowed from the interior of +the island, round and between the ancient basaltic hills; at both, +moreover, recent cones of eruption are scattered around the +circumference of the island; but at neither have eruptions taken place +within the period of history. As remarked in the last chapter, it is +probable that these ancient basaltic mountains, which resemble (at +least in many respects) the basal and disturbed remnants of two +gigantic volcanoes, owe their present form, structure, and position, to +the action of similar causes. + +ST. PAUL’S ROCKS. + +This small island is situated in the Atlantic Ocean, nearly one degree +north of the equator, and 540 miles distant from South America, in 29 +degrees 15 minutes west longitude. Its highest point is scarcely fifty +feet above the level of the sea; its outline is irregular, and its +entire circumference barely three-quarters of a mile. This little point +of rock rises abruptly out of the ocean; and, except on its western +side, soundings were not obtained, even at the short distance of a +quarter of a mile from its shore. It is not of volcanic origin; and +this circumstance, which is the most remarkable point in its history +(as will hereafter be referred to), properly ought to exclude it from +the present volume. It is composed of rocks, unlike any which I have +met with, and which I cannot characterise by any name, and must +therefore describe. + +The simplest, and one of the most abundant kinds, is a very compact, +heavy, greenish-black rock, having an angular, irregular fracture, with +some points just hard enough to scratch glass, and infusible. This +variety passes into others of paler green tints, less hard, but with a +more crystalline fracture, and translucent on their edges; and these +are fusible into a green enamel. Several other varieties are chiefly +characterised by containing innumerable threads of dark-green +serpentine, and by having calcareous matter in their interstices. These +rocks have an obscure, concretionary structure, and are full of +variously coloured angular pseudo fragments. These angular pseudo +fragments consist of the first-described dark green rock, of a brown +softer kind, of serpentine, and of a yellowish harsh stone, which, +perhaps, is related to serpentine rock. There are other vesicular, +calcareo-ferruginous, soft stones. There is no distinct stratification, +but parts are imperfectly laminated; and the whole abounds with +innumerable veins, and vein-like masses, both small and large. Of these +vein-like masses, some calcareous ones, which contain minute fragments +of shells, are clearly of subsequent origin to the others. + +A GLOSSY INCRUSTATION. + +Extensive portions of these rocks are coated by a layer of a glossy +polished substance, with a pearly lustre and of a greyish white colour; +it follows all the inequalities of the surface, to which it is firmly +attached. When examined with a lens, it is found to consist of numerous +exceedingly thin layers, their aggregate thickness being about the +tenth of an inch. It is considerably harder than calcareous spar, but +can be scratched with a knife; under the blowpipe it scales off, +decrepitates, slightly blackens, emits a fetid odour, and becomes +strongly alkaline: it does not effervesce in acids. (In my “Journal” I +have described this substance; I then believed that it was an impure +phosphate of lime.) I presume this substance has been deposited by +water draining from the birds’ dung, with which the rocks are covered. +At Ascension, near a cavity in the rocks which was filled with a +laminated mass of infiltrated birds’ dung, I found some irregularly +formed, stalactitical masses of apparently the same nature. These +masses, when broken, had an earthy texture; but on their outsides, and +especially at their extremities, they were formed of a pearly +substance, generally in little globules, like the enamel of teeth, but +more translucent, and so hard as just to scratch plate-glass. This +substance slightly blackens under the blowpipe, emits a bad smell, then +becomes quite white, swelling a little, and fuses into a dull white +enamel; it does not become alkaline; nor does it effervesce in acids. +The whole mass had a collapsed appearance, as if in the formation of +the hard glossy crust the whole had shrunk much. At the Abrolhos +Islands on the coast of Brazil, where also there is much birds’ dung, I +found a great quantity of a brown, arborescent substance adhering to +some trap-rock. In its arborescent form, this substance singularly +resembles some of the branched species of Nullipora. Under the +blowpipe, it behaves like the specimens from Ascension; but it is less +hard and glossy, and the surface has not the shrunk appearance. + + + + +CHAPTER III. +ASCENSION. + + +Basaltic lavas. Numerous craters truncated on the same side. Singular +structure of volcanic bombs. Aeriform explosions. Ejected granitic +fragments. Trachytic rocks. Singular veins. Jasper, its manner of +formation. Concretions in pumiceous tuff. Calcareous deposits and +frondescent incrustations on the coast. Remarkable laminated beds, +alternating with, and passing into, obsidian. Origin of obsidian. +Lamination of volcanic rocks. + +(MAP 2: THE ISLAND OF ASCENSION.) + +This island is situated in the Atlantic Ocean, in latitude 8 degrees +S., longitude 14 degrees W. It has the form of an irregular triangle +(see Map 2), each side being about six miles in length. Its highest +point is 2,870 feet (“Geographical Journal” volume 5 page 243.) above +the level of the sea. The whole is volcanic, and, from the absence of +proofs to the contrary, I believe of subaerial origin. The fundamental +rock is everywhere of a pale colour, generally compact, and of a +feldspathic nature. In the S.E. portion of the island, where the +highest land is situated, well characterised trachyte, and other +congenerous rocks of that varying family, occur. Nearly the entire +circumference is covered up by black and rugged streams of basaltic +lava, with here and there a hill or single point of rock (one of which +near the sea-coast, north of the Fort, is only two or three yards +across) of the trachyte still remaining exposed. + +BASALTIC ROCKS. + +The overlying basaltic lava is in some parts extremely vesicular, in +others little so; it is of a black colour, but sometimes contains +crystals of glassy feldspar, and seldom much olivine. These streams +appear to have possessed singularly little fluidity; their side walls +and lower ends being very steep, and even as much as between twenty and +thirty feet in height. Their surface is extraordinarily rugged, and +from a short distance appears as if studded with small craters. These +projections consist of broad, irregularly conical, hillocks, traversed +by fissures, and composed of the same unequally scoriaceous basalt with +the surrounding streams, but having an obscure tendency to a columnar +structure; they rise to a height between ten and thirty feet above the +general surface, and have been formed, as I presume, by the heaping up +of the viscid lava at points of greater resistance. At the base of +several of these hillocks, and occasionally likewise on more level +parts, solid ribs, composed of angulo-globular masses of basalt, +resembling in size and outline arched sewers or gutters of brickwork, +but not being hollow, project between two or three feet above the +surface of the streams; what their origin may have been, I do not know. +Many of the superficial fragments from these basaltic streams present +singularly convoluted forms; and some specimens could hardly be +distinguished from logs of dark-coloured wood without their bark. + +Many of the basaltic streams can be traced, either to points of +eruption at the base of the great central mass of trachyte, or to +separate, conical, red-coloured hills, which are scattered over the +northern and western borders of the island. Standing on the central +eminence, I counted between twenty and thirty of these cones of +eruption. The greater number of them had their truncated summits cut +off obliquely, and they all sloped towards the S.E., whence the +trade-wind blows. (M. Lesson in the “Zoology of the Voyage of the +‘Coquille’” page 490 has observed this fact. Mr. Hennah (“Geolog. +Proceedings” 1835 page 189) further remarks that the most extensive +beds of ashes at Ascension invariably occur on the leeward side of the +island.) This structure no doubt has been caused by the ejected +fragments and ashes being always blown, during eruptions, in greater +quantity towards one side than towards the other. M. Moreau de Jonnes +has made a similar observation with respect to the volcanic orifices in +the West Indian Islands. + +VOLCANIC BOMBS. + +(FIGURE 3: FRAGMENT OF A SPHERICAL VOLCANIC BOMB, with the interior +parts coarsely cellular, coated by a concentric layer of compact lava, +and this again by a crust of finely cellular rock. + +FIGURE 4: VOLCANIC BOMB OF OBSIDIAN FROM AUSTRALIA. The upper figure +gives a front view; the lower a side view of the same object.) + +These occur in great numbers strewed on the ground, and some of them +lie at considerable distances from any points of eruption. They vary in +size from that of an apple to that of a man’s body; they are either +spherical or pear-shaped, or with the hinder part (corresponding to the +tail of a comet) irregular, studded with projecting points, and even +concave. Their surfaces are rough, and fissured with branching cracks; +their internal structure is either irregularly scoriaceous and compact, +or it presents a symmetrical and very curious appearance. An irregular +segment of a bomb of this latter kind, of which I found several, is +accurately represented in Figure 3. Its size was about that of a man’s +head. The whole interior is coarsely cellular; the cells averaging in +diameter about the tenth of an inch; but nearer the outside they +gradually decrease in size. This part is succeeded by a well-defined +shell of compact lava, having a nearly uniform thickness of about the +third of an inch; and the shell is overlaid by a somewhat thicker +coating of finely cellular lava (the cells varying from the fiftieth to +the hundredth of an inch in diameter), which forms the external +surface: the line separating the shell of compact lava from the outer +scoriaceous crust is distinctly defined. This structure is very simply +explained, if we suppose a mass of viscid, scoriaceous matter, to be +projected with a rapid, rotatory motion through the air; for whilst the +external crust, from cooling, became solidified (in the state we now +see it), the centrifugal force, by relieving the pressure in the +interior parts of the bomb, would allow the heated vapours to expand +their cells; but these being driven by the same force against the +already-hardened crust, would become, the nearer they were to this +part, smaller and smaller or less expanded, until they became packed +into a solid, concentric shell. As we know that chips from a grindstone +(Nichol “Architecture of the Heavens.”) can be flirted off, when made +to revolve with sufficient velocity, we need not doubt that the +centrifugal force would have power to modify the structure of a +softened bomb, in the manner here supposed. Geologists have remarked, +that the external form of a bomb at once bespeaks the history of its +aerial course, and few now see that the internal structure can speak, +with almost equal plainness, of its rotatory movement. + +M. Bory St. Vincent (“Voyage aux Quatre Isles d’Afrique” tome 1 page +222.) has described some balls of lava from the Isle of Bourbon, which +have a closely similar structure. His explanation, however (if I +understand it rightly), is very different from that which I have given; +for he supposes that they have rolled, like snowballs, down the sides +of the crater. M. Beudant (“Voyage en Hongrie” tome 2 page 214.), also, +has described some singular little balls of obsidian, never more than +six or eight inches in diameter, which he found strewed on the surface +of the ground: their form is always oval; sometimes they are much +swollen in the middle, and even spindle-shaped: their surface is +regularly marked with concentric ridges and furrows, all of which on +the same ball are at right angles to one axis: their interior is +compact and glassy. M. Beudant supposes that masses of lava, when soft, +were shot into the air, with a rotatory movement round the same axis, +and that the form and superficial ridges of the bombs were thus +produced. Sir Thomas Mitchell has given me what at first appears to be +the half of a much flattened oval ball of obsidian; it has a singular +artificial-like appearance, which is well represented (of the natural +size) in Figure 4. It was found in its present state, on a great sandy +plain between the rivers Darling and Murray, in Australia, and at the +distance of several hundred miles from any known volcanic region. It +seems to have been embedded in some reddish tufaceous matter; and may +have been transported either by the aborigines or by natural means. The +external saucer consists of compact obsidian, of a bottle-green colour, +and is filled with finely cellular black lava, much less transparent +and glassy than the obsidian. The external surface is marked with four +or five not quite perfect ridges, which are represented rather too +distinctly in Figure 4. Here, then, we have the external structure +described by M. Beudant, and the internal cellular condition of the +bombs from Ascension. The lip of the saucer is slightly concave, +exactly like the margin of a soup-plate, and its inner edge overlaps a +little the central cellular lava. This structure is so symmetrical +round the entire circumference, that one is forced to suppose that the +bomb burst during its rotatory course, before being quite solidified, +and that the lip and edges were thus slightly modified and turned +inwards. It may be remarked that the superficial ridges are in planes, +at right angles to an axis, transverse to the longer axis of the +flattened oval: to explain this circumstance, we may suppose that when +the bomb burst, the axis of rotation changed. + +AERIFORM EXPLOSIONS. + +The flanks of Green Mountain and the surrounding country are covered by +a great mass, some hundred feet in thickness, of loose fragments. The +lower beds generally consist of fine-grained, slightly consolidated +tuffs (Some of this peperino, or tuff, is sufficiently hard not to be +broken by the greatest force of the fingers.), and the upper beds of +great loose fragments, with alternating finer beds. (On the northern +side of the Green Mountain a thin seam, about an inch in thickness, of +compact oxide of iron, extends over a considerable area; it lies +conformably in the lower part of the stratified mass of ashes and +fragments. This substance is of a reddish- brown colour, with an almost +metallic lustre; it is not magnetic, but becomes so after having been +heated under the blowpipe, by which it is blackened and partly fused. +This seam of compact stone, by intercepting the little rain-water which +falls on the island, gives rise to a small dripping spring, first +discovered by Dampier. It is the only fresh water on the island, so +that the possibility of its being inhabited has entirely depended on +the occurrence of this ferruginous layer.) One white ribbon- like layer +of decomposed, pumiceous breccia, was curiously bent into deep unbroken +curves, beneath each of the large fragments in the superincumbent +stratum. From the relative position of these beds, I presume that a +narrow- mouthed crater, standing nearly in the position of Green +Mountain, like a great air-gun, shot forth, before its final +extinction, this vast accumulation of loose matter. Subsequently to +this event, considerable dislocations have taken place, and an oval +circus has been formed by subsidence. This sunken space lies at the +north-eastern foot of Green Mountain, and is well represented in Map 2. +Its longer axis, which is connected with a N.E. and S.W. line of +fissure, is three-fifths of a nautical mile in length; its sides are +nearly perpendicular, except in one spot, and about four hundred feet +in height; they consist, in the lower part, of a pale basalt with +feldspar, and in the upper part, of the tuff and loose ejected +fragments; the bottom is smooth and level, and under almost any other +climate a deep lake would have been formed here. From the thickness of +the bed of loose fragments, with which the surrounding country is +covered, the amount of aeriform matter necessary for their projection +must have been enormous; hence we may suppose it probable that after +the explosions vast subterranean caverns were left, and that the +falling in of the roof of one of these produced the hollow here +described. At the Galapagos Archipelago, pits of a similar character, +but of a much smaller size, frequently occur at the bases of small +cones of eruption. + +EJECTED GRANITIC FRAGMENTS. + +In the neighbourhood of Green Mountain, fragments of extraneous rock +are not unfrequently found embedded in the midst of masses of scoriae. +Lieutenant Evans, to whose kindness I am indebted for much information, +gave me several specimens, and I found others myself. They nearly all +have a granitic structure, are brittle, harsh to the touch, and +apparently of altered colours. + +FIRST, a white syenite, streaked and mottled with red; it consists of +well- crystallised feldspar, numerous grains of quartz, and brilliant, +though small, crystals of hornblende. The feldspar and hornblende in +this and the succeeding cases have been determined by the reflecting +goniometer, and the quartz by its action under the blowpipe. The +feldspar in these ejected fragments, like the glassy kind in the +trachyte, is from its cleavage a potash-feldspar. + +SECONDLY, a brick-red mass of feldspar, quartz, and small dark patches +of a decayed mineral; one minute particle of which I was able to +ascertain, by its cleavage, to be hornblende. + +THIRDLY, a mass of confusedly crystallised white feldspar, with little +nests of a dark-coloured mineral, often carious, externally rounded, +having a glossy fracture, but no distinct cleavage: from comparison +with the second specimen, I have no doubt that it is fused hornblende. + +FOURTHLY, a rock, which at first appears a simple aggregation of +distinct and large-sized crystals of dusty-coloured Labrador feldspar +(Professor Miller has been so kind as to examine this mineral. He +obtained two good cleavages of 86 degrees 30 minutes and 86 degrees 50 +minutes. The mean of several, which I made, was 86 degrees 30 minutes. +Professor Miller states that these crystals, when reduced to a fine +powder, are soluble in hydrochloric acid, leaving some undissolved +silex behind; the addition of oxalate of ammonia gives a copious +precipitate of lime. He further remarks, that according to Von Kobell, +anorthite (a mineral occurring in the ejected fragments at Mount Somma) +is always white and transparent, so that if this be the case, these +crystals from Ascension must be considered as Labrador feldspar. +Professor Miller adds, that he has seen an account, in Erdmann’s +“Journal fur tecnische Chemie,” of a mineral ejected from a volcano +which had the external characters of Labrador feldspar, but differed in +the analysis from that given by mineralogists of this mineral: the +author attributed this difference to an error in the analysis of +Labrador feldspar, which is very old.); but in their interstices there +is some white granular feldspar, abundant scales of mica, a little +altered hornblende, and, as I believe, no quartz. I have described +these fragments in detail, because it is rare to find granitic rocks +ejected from volcanoes with their MINERALS UNCHANGED, as is the case +with the first specimen, and partially with the second. (Daubeny, in +his work on Volcanoes page 386, remarks that this is the case; and +Humboldt, in his “Personal Narrative” volume 1 page 236, says “In +general, the masses of known primitive rocks, I mean those which +perfectly resemble our granites, gneiss, and mica-slate, are very rare +in lavas: the substances we generally denote by the name of granite, +thrown out by Vesuvius, are mixtures of nepheline, mica, and +pyroxene.”) One other large fragment, found in another spot, is +deserving of notice; it is a conglomerate, containing small fragments +of granitic, cellular, and jaspery rocks, and of hornstone porphyries, +embedded in a base of wacke, threaded by numerous thin layers of a +concretionary pitchstone passing into obsidian. These layers are +parallel, slightly tortuous, and short; they thin out at their ends, +and resemble in form the layers of quartz in gneiss. It is probable +that these small embedded fragments were not separately ejected, but +were entangled in a fluid volcanic rock, allied to obsidian; and we +shall presently see that several varieties of this latter series of +rock assume a laminated structure. + +TRACHYTIC SERIES OF ROCKS. + +Those occupy the more elevated and central, and likewise the +south-eastern, parts of the island. The trachyte is generally of a pale +brown colour, stained with small darker patches; it contains broken and +bent crystals of glassy feldspar, grains of specular iron, and black +microscopical points, which latter, from being easily fused, and then +becoming magnetic, I presume are hornblende. The greater number of the +hills, however, are composed of a quite white, friable stone, appearing +like a trachytic tuff. Obsidian, hornstone, and several kinds of +laminated feldspathic rocks, are associated with the trachyte. There is +no distinct stratification; nor could I distinguish a crateriform +structure in any of the hills of this series. Considerable dislocations +have taken place; and many fissures in these rocks are yet left open, +or are only partially filled with loose fragments. Within the space +(This space is nearly included by a line sweeping round Green Mountain, +and joining the hills, called the Weather Port Signal, Holyhead, and +that denominated (improperly in a geological sense) “the Crater of an +old volcano.”), mainly formed of trachyte, some basaltic streams have +burst forth; and not far from the summit of Green Mountain, there is +one stream of quite black, vesicular basalt, containing minute crystals +of glassy feldspar, which have a rounded appearance. + +The soft white stone above mentioned is remarkable from its singular +resemblance, when viewed in mass, to a sedimentary tuff: it was long +before I could persuade myself that such was not its origin; and other +geologists have been perplexed by closely similar formations in +trachytic regions. In two cases, this white earthy stone formed +isolated hills; in a third, it was associated with columnar and +laminated trachyte; but I was unable to trace an actual junction. It +contains numerous crystals of glassy feldspar and black microscopical +specks, and is marked with small darker patches, exactly as in the +surrounding trachyte. Its basis, however, when viewed under the +microscope, is generally quite earthy; but sometimes it exhibits a +decidedly crystalline structure. On the hill marked “Crater of an old +volcano,” it passes into a pale greenish-grey variety, differing only +in its colour, and in not being so earthy; the passage was in one case +effected insensibly; in another, it was formed by numerous, rounded and +angular, masses of the greenish variety, being embedded in the white +variety;—in this latter case, the appearance was very much like that of +a sedimentary deposit, torn up and abraded during the deposition of a +subsequent stratum. Both these varieties are traversed by innumerable +tortuous veins (presently to be described), which are totally unlike +injected dikes, or indeed any other veins which I have ever seen. Both +varieties include a few scattered fragments, large and small, of dark- +coloured scoriaceous rocks, the cells of some of which are partially +filled with the white earthy stone; they likewise include some huge +blocks of a cellular porphyry. (The porphyry is dark coloured; it +contains numerous, often fractured, crystals of white opaque feldspar, +also decomposing crystals of oxide of iron; its vesicles include masses +of delicate, hair- like, crystals, apparently of analcime.) These +fragments project from the weathered surface, and perfectly resemble +fragments embedded in a true sedimentary tuff. But as it is known that +extraneous fragments of cellular rock are sometimes included in +columnar trachyte, in phonolite (D’Aubuisson “Traite de Geognosie” tome +2 page 548.), and in other compact lavas, this circumstance is not any +real argument for the sedimentary origin of the white earthy stone. +(Dr. Daubeny on Volcanoes, page 180 seems to have been led to believe +that certain trachytic formations of Ischia and of the Puy de Dome, +which closely resemble these of Ascension, were of sedimentary origin, +chiefly from the frequent presence in them “of scoriform portions, +different in colour from the matrix.” Dr. Daubeny adds, that on the +other hand, Brocchi, and other eminent geologists, have considered +these beds as earthy varieties of trachyte; he considers the subject +deserving of further attention.) The insensible passage of the greenish +variety into the white one, and likewise the more abrupt passage by +fragments of the former being embedded in the latter, might result from +slight differences in the composition of the same mass of molten stone, +and from the abrading action of one such part still fluid on another +part already solidified. The curiously formed veins have, I believe, +been formed by siliceous matter being subsequently segregated. But my +chief reason for believing that these soft earthy stones, with their +extraneous fragments, are not of sedimentary origin, is the extreme +improbability of crystals of feldspar, black microscopical specks, and +small stains of a darker colour occurring in the same proportional +numbers in an aqueous deposit, and in masses of solid trachyte. +Moreover, as I have remarked, the microscope occasionally reveals a +crystalline structure in the apparently earthy basis. On the other +hand, the partial decomposition of such great masses of trachyte, +forming whole mountains, is undoubtedly a circumstance of not easy +explanation. + +VEINS IN THE EARTHY TRACHYTIC MASSES. + +These veins are extraordinarily numerous, intersecting in the most +complicated manner both coloured varieties of the earthy trachyte: they +are best seen on the flanks of the “Crater of the old volcano.” They +contain crystals of glassy feldspar, black microscopical specks and +little dark stains, precisely as in the surrounding rock; but the basis +is very different, being exceedingly hard, compact, somewhat brittle, +and of rather less easy fusibility. The veins vary much, and suddenly, +from the tenth of an inch to one inch in thickness; they often thin +out, not only on their edges, but in their central parts, thus leaving +round, irregular apertures; their surfaces are rugged. They are +inclined at every possible angle with the horizon, or are horizontal; +they are generally curvilinear, and often interbranch one with another. +From their hardness they withstand weathering, and projecting two or +three feet above the ground, they occasionally extend some yards in +length; these plate-like veins, when struck, emit a sound, almost like +that of a drum, and they may be distinctly seen to vibrate; their +fragments, which are strewed on the ground, clatter like pieces of iron +when knocked against each other. They often assume the most singular +forms; I saw a pedestal of the earthy trachyte, covered by a +hemispherical portion of a vein, like a great umbrella, sufficiently +large to shelter two persons. I have never met with, or seen described, +any veins like these; but in form they resemble the ferruginous seams, +due to some process of segregation, occurring not uncommonly in +sandstones,—for instance, in the New Red sandstone of England. Numerous +veins of jasper and of siliceous sinter, occurring on the summit of +this same hill, show that there has been some abundant source of +silica, and as these plate-like veins differ from the trachyte only in +their greater hardness, brittleness, and less easy fusibility, it +appears probable that their origin is due to the segregation or +infiltration of siliceous matter, in the same manner as happens with +the oxides of iron in many sedimentary rocks. + +SILICEOUS SINTER AND JASPER. + +The siliceous sinter is either quite white, of little specific gravity, +and with a somewhat pearly fracture, passing into pinkish pearl quartz; +or it is yellowish white, with a harsh fracture, and it then contains +an earthy powder in small cavities. Both varieties occur, either in +large irregular masses in the altered trachyte, or in seams included in +broad, vertical, tortuous, irregular veins of a compact, harsh stone of +a dull red colour, appearing like a sandstone. This stone, however, is +only altered trachyte; and a nearly similar variety, but often +honeycombed, sometimes adheres to the projecting plate-like veins, +described in the last paragraph. The jasper is of an ochre yellow or +red colour; it occurs in large irregular masses, and sometimes in +veins, both in the altered trachyte and in an associated mass of +scoriaceous basalt. The cells of the scoriaceous basalt are lined or +filled with fine, concentric layers of chalcedony, coated and studded +with bright-red oxide of iron. In this rock, especially in the rather +more compact parts, irregular angular patches of the red jasper are +included, the edges of which insensibly blend into the surrounding +mass; other patches occur having an intermediate character between +perfect jasper and the ferruginous, decomposed, basaltic base. In these +patches, and likewise in the large vein-like masses of jasper, there +occur little rounded cavities, of exactly the same size and form with +the air-cells, which in the scoriaceous basalt are filled and lined +with layers of chalcedony. Small fragments of the jasper, examined +under the microscope, seem to resemble the chalcedony with its +colouring matter not separated into layers, but mingled in the +siliceous paste, together with some impurities. I can understand these +facts,—namely, the blending of the jasper into the semi-decomposed +basalt,—its occurrence in angular patches, which clearly do not occupy +pre-existing hollows in the rock,—and its containing little vesicles +filled with chalcedony, like those in the scoriaceous lava,—only on the +supposition that a fluid, probably the same fluid which deposited the +chalcedony in the air-cells, removed in those parts where there were no +cavities, the ingredients of the basaltic rock, and left in their place +silica and iron, and thus produced the jasper. In some specimens of +silicified wood, I have observed, that in the same manner as in the +basalt, the solid parts were converted into a dark-coloured homogeneous +stone, whereas the cavities formed by the larger sap-vessels (which may +be compared with the air-vesicles in the basaltic lava) and other +irregular hollows, apparently produced by decay, were filled with +concentric layers of chalcedony; in this case, there can be little +doubt that the same fluid deposited the homogeneous base and the +chalcedonic layers. After these considerations, I cannot doubt but that +the jasper of Ascension may be viewed as a volcanic rock silicified, in +precisely the same sense as this term is applied to wood, when +silicified; we are equally ignorant of the means by which every atom of +wood, whilst in a perfect state, is removed and replaced by atoms of +silica, as we are of the means by which the constituent parts of a +volcanic rock could be thus acted on. (Beudant “Voyage en Hongrie” tome +3 pages 502, 504 describes kidney-shaped masses of jasper-opal, which +either blend into the surrounding trachytic conglomerate, or are +embedded in it like chalk-flints; and he compares them with the +fragments of opalised wood, which are abundant in this same formation. +Beudant, however, appears to have viewed the process of their formation +rather as one of simple infiltration than of molecular exchange; but +the presence of a concretion, wholly different from the surrounding +matter, if not formed in a pre-existing hollow, clearly seems to me to +require, either a molecular or mechanical displacement of the atoms, +which occupied the space afterwards filled by it. The jasper-opal of +Hungary passes into chalcedony, and therefore in this case, as in that +of Ascension, jasper seems to be intimately related in origin with +chalcedony.) I was led to the careful examination of these rocks, and +to the conclusion here given, from having heard the Rev. Professor +Henslow express a similar opinion, regarding the origin in trap-rocks +of many chalcedonies and agates. Siliceous deposits seem to be very +general, if not of universal occurrence, in partially decomposed +trachytic tuffs (Beudant “Voyage Min.” tome 3 page 507 enumerates cases +in Hungary, Germany, Central France, Italy, Greece, and Mexico.); and +as these hills, according to the view above given, consist of trachyte +softened and altered in situ, the presence of free silica in this case +may be added as one more instance to the list. + +CONCRETIONS IN PUMICEOUS TUFF. + +The hill, marked in Map 2 “Crater of an old volcano,” has no claims to +this appellation, which I could discover, except in being surmounted by +a circular, very shallow, saucer-like summit, nearly half a mile in +diameter. This hollow has been nearly filled up with many successive +sheets of ashes and scoriae, of different colours, and slightly +consolidated. Each successive saucer-shaped layer crops out all round +the margin, forming so many rings of various colours, and giving to the +hill a fantastic appearance. The outer ring is broad, and of a white +colour; hence it resembles a course round which horses have been +exercised, and has received the name of the Devil’s Riding School, by +which it is most generally known. These successive layers of ashes must +have fallen over the whole surrounding country, but they have all been +blown away except in this one hollow, in which probably moisture +accumulated, either during an extraordinary year when rain fell, or +during the storms often accompanying volcanic eruptions. One of the +layers of a pinkish colour, and chiefly derived from small, decomposed +fragments of pumice, is remarkable, from containing numerous +concretions. These are generally spherical, from half an inch to three +inches in diameter; but they are occasionally cylindrical, like those +of iron-pyrites in the chalk of Europe. They consist of a very tough, +compact, pale-brown stone, with a smooth and even fracture. They are +divided into concentric layers by thin white partitions, resembling the +external superficies; six or eight of such layers are distinctly +defined near the outside; but those towards the inside generally become +indistinct, and blend into a homogeneous mass. I presume that these +concentric layers were formed by the shrinking of the concretion, as it +became compact. The interior part is generally fissured by minute +cracks or septaria, which are lined, both by black, metallic, and by +other white and crystalline specks, the nature of which I was unable to +ascertain. Some of the larger concretions consist of a mere spherical +shell, filled with slightly consolidated ashes. The concretions contain +a small proportion of carbonate of lime: a fragment placed under the +blowpipe decrepitates, then whitens and fuses into a blebby enamel, but +does not become caustic. The surrounding ashes do not contain any +carbonate of lime; hence the concretions have probably been formed, as +is so often the case, by the aggregation of this substance. I have not +met with any account of similar concretions; and considering their +great toughness and compactness, their occurrence in a bed, which +probably has been subjected only to atmospheric moisture, is +remarkable. + +FORMATION OF CALCAREOUS ROCKS ON THE SEA-COAST. + +On several of the sea-beaches, there are immense accumulations of +small, well-rounded particles of shells and corals, of white, +yellowish, and pink colours, interspersed with a few volcanic +particles. At the depth of a few feet, these are found cemented +together into stone, of which the softer varieties are used for +building; there are other varieties, both coarse and fine-grained, too +hard for this purpose: and I saw one mass divided into even layers half +an inch in thickness, which were so compact that when struck with a +hammer they rang like flint. It is believed by the inhabitants, that +the particles become united in the course of a single year. The union +is effected by calcareous matter; and in the most compact varieties, +each rounded particle of shell and volcanic rock can be distinctly seen +to be enveloped in a husk of pellucid carbonate of lime. Extremely few +perfect shells are embedded in these agglutinated masses; and I have +examined even a large fragment under a microscope, without being able +to discover the least vestige of striae or other marks of external +form: this shows how long each particle must have been rolled about, +before its turn came to be embedded and cemented. (The eggs of the +turtle being buried by the parent, sometimes become enclosed in the +solid rock. Mr. Lyell has given a figure (“Principles of Geology” book +3 chapter 17) of some eggs, containing the bones of young turtles, +found thus entombed.) One of the most compact varieties, when placed in +acid, was entirely dissolved, with the exception of some flocculent +animal matter; its specific gravity was 2.63. The specific gravity of +ordinary limestone varies from 2.6 to 2.75; pure Carrara marble was +found by Sir H. De la Beche to be 2.7. (“Researches in Theoretical +Geology” page 12.) It is remarkable that these rocks of Ascension, +formed close to the surface, should be nearly as compact as marble, +which has undergone the action of heat and pressure in the plutonic +regions. + +The great accumulation of loose calcareous particles, lying on the +beach near the Settlement, commences in the month of October, moving +towards the S.W., which, as I was informed by Lieutenant Evans, is +caused by a change in the prevailing direction of the currents. At this +period the tidal rocks, at the S.W. end of the beach, where the +calcareous sand is accumulating, and round which the currents sweep, +become gradually coated with a calcareous incrustation, half an inch in +thickness. It is quite white, compact, with some parts slightly +spathose, and is firmly attached to the rock. After a short time it +gradually disappears, being either redissolved, when the water is less +charged with lime, or more probably is mechanically abraded. Lieutenant +Evans has observed these facts, during the six years he has resided at +Ascension. The incrustation varies in thickness in different years: in +1831 it was unusually thick. When I was there in July, there was no +remnant of the incrustation; but on a point of basalt, from which the +quarrymen had lately removed a mass of the calcareous freestone, the +incrustation was perfectly preserved. Considering the position of the +tidal-rocks, and the period at which they become coated, there can be +no doubt that the movement and disturbance of the vast accumulation of +calcareous particles, many of them being partially agglutinated +together, cause the waves of the sea to be so highly charged with +carbonate of lime, that they deposit it on the first objects against +which they impinge. I have been informed by Lieutenant Holland, R.N., +that this incrustation is formed on many parts of the coast, on most of +which, I believe, there are likewise great masses of comminuted shells. + +A FRONDESCENT CALCAREOUS INCRUSTATION. + +(FIGURE 5. AN INCRUSTATION OF CALCAREOUS AND ANIMAL MATTER, coating the +tidal-rocks at Ascension.) + +In many respects this is a singular deposit; it coats throughout the +year the tidal volcanic rocks, that project from the beaches composed +of broken shells. Its general appearance is well represented in Figure +5; but the fronds or discs, of which it is composed, are generally so +closely crowded together as to touch. These fronds have their sinuous +edges finely crenulated, and they project over their pedestals or +supports; their upper surfaces are either slightly concave, or slightly +convex; they are highly polished, and of a dark grey or jet black +colour; their form is irregular, generally circular, and from the tenth +of an inch to one inch and a half in diameter; their thickness, or +amount of their projection from the rock on which they stand, varies +much, about a quarter of an inch being perhaps most usual. The fronds +occasionally become more and more convex, until they pass into +botryoidal masses with their summits fissured; when in this state, they +are glossy and of an intense black, so as to resemble some fused +metallic substance. I have shown the incrustation, both in this latter +and in its ordinary state to several geologists, but not one could +conjecture its origin, except that perhaps it was of volcanic nature! + +The substance forming the fronds has a very compact and often almost +crystalline fracture; the edges being translucent, and hard enough +easily to scratch calcareous spar. Under the blowpipe it immediately +becomes white, and emits a strong animal odour, like that from fresh +shells. It is chiefly composed of carbonate of lime; when placed in +muriatic acid it froths much, leaving a residue of sulphate of lime, +and of an oxide of iron, together with a black powder, which is not +soluble in heated acids. This latter substance seems to be +carbonaceous, and is evidently the colouring matter. The sulphate of +lime is extraneous, and occurs in distinct, excessively minute, +lamellar plates, studded on the surface of the fronds, and embedded +between the fine layers of which they are composed; when a fragment is +heated in the blowpipe, these lamellae are immediately rendered +visible. The original outline of the fronds may often be traced, either +to a minute particle of shell fixed in a crevice of the rock, or to +several cemented together; these first become deeply corroded, by the +dissolving power of the waves, into sharp ridges, and then are coated +with successive layers of the glossy, grey, calcareous incrustation. +The inequalities of the primary support affect the outline of every +successive layer, in the same manner as may often be seen in +bezoar-stones, when an object like a nail forms the centre of +aggregation. The crenulated edges, however, of the frond appear to be +due to the corroding power of the surf on its own deposit, alternating +with fresh depositions. On some smooth basaltic rocks on the coast of +St. Jago, I found an exceedingly thin layer of brown calcareous matter, +which under a lens presented a miniature likeness of the crenulated and +polished fronds of Ascension; in this case a basis was not afforded by +any projecting extraneous particles. Although the incrustation at +Ascension is persistent throughout the year; yet from the abraded +appearance of some parts, and from the fresh appearance of other parts, +the whole seems to undergo a round of decay and renovation, due +probably to changes in the form of the shifting beach, and consequently +in the action of the breakers: hence probably it is, that the +incrustation never acquires a great thickness. Considering the position +of the encrusted rocks in the midst of the calcareous beach, together +with its composition, 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 corals. (The selenite, as +I have remarked is extraneous, and must have been derived from the +sea-water. It is an interesting circumstance thus to find the waves of +the ocean, sufficiently charged with sulphate of lime, to deposit it on +the rocks, against which they dash every tide. Dr. Webster has +described (“Voyage of the ‘Chanticleer’” volume 2 page 319) beds of +gypsum and salt, as much as two feet in thickness, left by the +evaporation of the spray on the rocks on the windward coast. Beautiful +stalactites of selenite, resembling in form those of carbonate of lime, +are formed near these beds. Amorphous masses of gypsum, also, occur in +caverns in the interior of the island; and at Cross Hill (an old +crater) I saw a considerable quantity of salt oozing from a pile of +scoriae. In these latter cases, the salt and gypsum appear to be +volcanic products.) From this source it derives its animal matter, +which is evidently the colouring principle. The nature of the deposit, +in its incipient stage, can often be well seen upon a fragment of white +shell, when jammed between two of the fronds; it then appears exactly +like the thinnest wash of a pale grey varnish. Its darkness varies a +little, but the jet blackness of some of the fronds and of the +botryoidal masses seems due to the translucency of the successive grey +layers. There is, however, this singular circumstance, that when +deposited on the under side of ledges of rock or in fissures, it +appears always to be of a pale, pearly grey colour, even when of +considerable thickness: hence one is led to suppose, that an abundance +of light is necessary to the development of the dark colour, in the +same manner as seems to be the case with the upper and exposed surfaces +of the shells of living mollusca, which are always dark, compared with +their under surfaces and with the parts habitually covered by the +mantle of the animal. In this circumstance,—in the immediate loss of +colour and in the odour emitted under the blowpipe,—in the degree of +hardness and translucency of the edges,—and in the beautiful polish of +the surface (From the fact described in my “Journal of Researches” of a +coating of oxide of iron, deposited by a streamlet on the rocks in its +bed (like a nearly similar coating at the great cataracts of the +Orinoco and Nile), becoming finely polished where the surf acts, I +presume that the surf in this instance, also, is the polishing agent.), +rivalling when in a fresh state that of the finest Oliva, there is a +striking analogy between this inorganic incrustation and the shells of +living molluscous animals. (In the section descriptive of St. Paul’s +Rocks, I have described a glossy, pearly substance, which coats the +rocks, and an allied stalactitical incrustation from Ascension, the +crust of which resembles the enamel of teeth, but is hard enough to +scratch plate-glass. Both these substances contain animal matter, and +seem to have been derived from water in filtering through birds’ dung.) +This appears to me to be an interesting physiological fact. (Mr. Horner +and Sir David Brewster have described “Philosophical Transactions” 1836 +page 65 a singular “artificial substance, resembling shell.” It is +deposited in fine, transparent, highly polished, brown- coloured +laminae, possessing peculiar optical properties, on the inside of a +vessel, in which cloth, first prepared with glue and then with lime, is +made to revolve rapidly in water. It is much softer, more transparent, +and contains more animal matter, than the natural incrustation at +Ascension; but we here again see the strong tendency which carbonate of +lime and animal matter evince to form a solid substance allied to +shell.) + +SINGULAR LAMINATED BEDS ALTERNATING WITH AND PASSING INTO OBSIDIAN. + +These beds occur within the trachytic district, at the western base of +Green Mountain, under which they dip at a high inclination. They are +only partially exposed, being covered up by modern ejections; from this +cause, I was unable to trace their junction with the trachyte, or to +discover whether they had flowed as a stream of lava, or had been +injected amidst the overlying strata. There are three principal beds of +obsidian, of which the thickest forms the base of the section. The +alternating stony layers appear to me eminently curious, and shall be +first described, and afterwards their passage into the obsidian. They +have an extremely diversified appearance; five principal varieties may +be noticed, but these insensibly blend into each other by endless +gradations. + +FIRST. + +A pale grey, irregularly and coarsely laminated (This term is open to +some misinterpretation, as it may be applied both to rocks divided into +laminae of exactly the same composition, and to layers firmly attached +to each other, with no fissile tendency, but composed of different +minerals, or of different shades of colour. The term “laminated,” in +this chapter, is applied in these latter senses; where a homogeneous +rock splits, as in the former sense, in a given direction, like +clay-slate, I have used the term “fissile.”), harsh-feeling rock, +resembling clay-slate which has been in contact with a trap-dike, and +with a fracture of about the same degree of crystalline structure. This +rock, as well as the following varieties, easily fuses into a pale +glass. The greater part is honeycombed with irregular, angular, +cavities, so that the whole has a curious appearance, and some +fragments resemble in a remarkable manner silicified logs of decayed +wood. This variety, especially where more compact, is often marked with +thin whitish streaks, which are either straight or wrap round, one +behind the other, the elongated carious hollows. + +SECONDLY. + +A bluish grey or pale brown, compact, heavy, homogeneous stone, with an +angular, uneven, earthy fracture; viewed, however, under a lens of high +power, the fracture is seen to be distinctly crystalline, and even +separate minerals can be distinguished. + +THIRDLY. + +A stone of the same kind with the last, but streaked with numerous, +parallel, slightly tortuous, white lines of the thickness of hairs. +These white lines are more crystalline than the parts between them; and +the stone splits along them: they frequently expand into exceedingly +thin cavities, which are often only just perceptible with a lens. The +matter forming the white lines becomes better crystallised in these +cavities, and Professor Miller was fortunate enough, after several +trials, to ascertain that the white crystals, which are the largest, +were of quartz (Professor Miller informs me that the crystals which he +measured had the faces P, z, m of the figure (147) given by Haidinger +in his Translation of Mohs; and he adds, that it is remarkable, that +none of them had the slightest trace of faces r of the regular +six-sided prism.), and that the minute green transparent needles were +augite, or, as they would more generally be called, diopside: besides +these crystals, there are some minute, dark specks without a trace of +crystalline, and some fine, white, granular, crystalline matter which +is probably feldspar. Minute fragments of this rock are easily fusible. + +FOURTHLY. + +A compact crystalline rock, banded in straight lines with innumerable +layers of white and grey shades of colour, varying in width from the +thirtieth to the two-hundredth of an inch; these layers seem to be +composed chiefly of feldspar, and they contain numerous perfect +crystals of glassy feldspar, which are placed lengthways; they are also +thickly studded with microscopically minute, amorphous, black specks, +which are placed in rows, either standing separately, or more +frequently united, two or three or several together, into black lines, +thinner than a hair. When a small fragment is heated in the blowpipe, +the black specks are easily fused into black brilliant beads, which +become magnetic,—characters that apply to no common mineral except +hornblende or augite. With the black specks there are mingled some +others of a red colour, which are magnetic before being heated, and no +doubt are oxide of iron. Round two little cavities, in a specimen of +this variety, I found the black specks aggregated into minute crystals, +appearing like those of augite or hornblende, but too dull and small to +be measured by the goniometer; in the specimen, also, I could +distinguish amidst the crystalline feldspar, grains, which had the +aspect of quartz. By trying with a parallel ruler, I found that the +thin grey layers and the black hair-like lines were absolutely straight +and parallel to each other. It is impossible to trace the gradation +from the homogeneous grey rocks to these striped varieties, or indeed +the character of the different layers in the same specimen, 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 matter, into the black and red specks of +hornblende and oxide of iron. + +FIFTHLY. + +A compact heavy rock, not laminated, with an irregular, angular, highly +crystalline, fracture; it abounds with distinct crystals of glassy +feldspar, and the crystalline feldspathic base is mottled with a black +mineral, which on the weathered surface is seen to be aggregated into +small crystals, some perfect, but the greater number imperfect. I +showed this specimen to an experienced geologist, and asked him what it +was; he answered, as I think every one else would have done, that it +was a primitive greenstone. The weathered surface, also, of the banded +variety in Figure 4, strikingly resembles a worn fragment of finely +laminated gneiss. + +These five varieties, with many intermediate ones, pass and repass into +each other. As the compact varieties are quite subordinate to the +others, the whole may be considered as laminated or striped. The +laminae, to sum up their characteristics, are either quite straight, or +slightly tortuous, or convoluted; they are all parallel to each other, +and to the intercalating strata of obsidian; they are generally of +extreme thinness; they consist either of an apparently homogeneous, +compact rock, striped with different shades of grey and brown colours, +or of crystalline feldspathic layers in a more or less perfect state of +purity, and of different thicknesses, with distinct crystals of glassy +feldspar placed lengthways, or of very thin layers chiefly composed of +minute crystals of quartz and augite, or composed of black and red +specks of an augitic mineral and of an oxide of iron, either not +crystallised or imperfectly so. After having fully described the +obsidian, I shall return to the subject of the lamination of rocks of +the trachytic series. + +The passage of the foregoing beds into the strata of glassy obsidian is +effected in several ways: first, angulo-modular masses of obsidian, +both large and small, abruptly appear disseminated in a slaty, or in an +amorphous, pale-coloured, feldspathic rock, with a somewhat pearly +fracture. Secondly, small irregular nodules of the obsidian, either +standing separately, or united into thin layers, seldom more than the +tenth of an inch in thickness, alternate repeatedly with very thin +layers of a feldspathic rock, which is striped with the finest parallel +zones of colour, like an agate, and which sometimes passes into the +nature of pitchstone; the interstices between the nodules of obsidian +are generally filled by soft white matter, resembling pumiceous ashes. +Thirdly, the whole substance of the bounding rock suddenly passes into +an angulo-concretionary mass of obsidian. Such masses (as well as the +small nodules) of obsidian are of a pale green colour, and are +generally streaked with different shades of colour, parallel to the +laminae of the surrounding rock; they likewise generally contain minute +white sphaerulites, of which half is sometimes embedded in a zone of +one shade of colour, and half in a zone of another shade. The obsidian +assumes its jet black colour and perfectly conchoidal fracture, only +when in large masses; but even in these, on careful examination and on +holding the specimens in different lights, I could generally +distinguish parallel streaks of different shades of darkness. + +(FIGURE 6. OPAQUE BROWN SPHAERULITES, drawn on an enlarged scale. The +upper ones are externally marked with parallel ridges. The internal +radiating structure of the lower ones, is much too plainly represented. + +FIGURE 7. A LAYER FORMED BY THE UNION OF MINUTE BROWN SPHAERULITES, +INTERSECTING TWO OTHER SIMILAR LAYERS: the whole represented of nearly +the natural size.) + +One of the commonest transitional rocks deserves in several respects a +further description. It is of a very complicated nature, and consists +of numerous thin, slightly tortuous layers of a pale-coloured +feldspathic stone, often passing into an imperfect pitchstone, +alternating with layers formed of numberless little globules of two +varieties of obsidian, and of two kinds of sphaerulites, embedded in a +soft or in a hard pearly base. The sphaerulites are either white and +translucent, or dark brown and opaque; the former are quite spherical, +of small size, and distinctly radiated from their centre. The dark +brown sphaerulites are less perfectly round, and vary in diameter from +the twentieth to the thirtieth of an inch; when broken they exhibit +towards their centres, which are whitish, an obscure radiating +structure; two of them when united sometimes have only one central +point of radiation; there is occasionally a trace of or a hollow +crevice in their centres. They stand either separately, or are united +two or three or many together into irregular groups, or more commonly +into layers, parallel to the stratification of the mass. This union in +many cases is so perfect, that the two sides of the layer thus formed, +are quite even; and these layers, as they become less brown and opaque, +cannot be distinguished from the alternating layers of the +pale-coloured feldspathic stone. The sphaerulites, when not united, are +generally compressed in the plane of the lamination of the mass; and in +this same plane, they are often marked internally, by zones of +different shades of colour, and externally by small ridges and furrows. +In the upper part of Figure 6, the sphaerulites with the parallel +ridges and furrows are represented on an enlarged scale, but they are +not well executed; and in the lower part, their usual manner of +grouping is shown. In another specimen, a thin layer formed of the +brown sphaerulites closely united together, intersects, as represented +in Figure 7, a layer of similar composition; and after running for a +short space in a slightly curved line, again intersects it, and +likewise a second layer lying a little way beneath that first +intersected. The small nodules also of obsidian are sometimes +externally marked with ridges and furrows, parallel to the lamination +of the mass, but always less plainly than the sphaerulites. These +obsidian nodules are generally angular, with their edges blunted: they +are often impressed with the form of the adjoining sphaerulites, than +which they are always larger; the separate nodules seldom appear to +have drawn each other out by exerting a mutually attractive force. Had +I not found in some cases, a distinct centre of attraction in these +nodules of obsidian, I should have been led to have considered them as +residuary matter, left during the formation of the pearlstone, in which +they are embedded, and of the sphaerulitic globules. + +The sphaerulites and the little nodules of obsidian in these rocks so +closely resemble, in general form and structure, concretions in +sedimentary deposits, that one is at once tempted to attribute to them +an analogous origin. They resemble ordinary concretions in the +following respects: in their external form,—in the union of two or +three, or of several, into an irregular mass, or into an even-sided +layer,—in the occasional intersection of one such layer by another, as +in the case of chalk-flints,- -in the presence of two or three kinds of +nodules, often close together, in the same basis,—in their fibrous, +radiating structure, with occasional hollows in their centres,—in the +co-existence of a laminary, concretionary, and radiating structure, as +is so well developed in the concretions of magnesian limestone, +described by Professor Sedgwick. (“Geological Transactions” volume 3 +part 1 page 37.) Concretions in sedimentary deposits, it is known, are +due to the separation from the surrounding mass of the whole or part of +some mineral substance, and its aggregation round certain points of +attraction. Guided by this fact, I have endeavoured to discover whether +obsidian and the sphaerulites (to which may be added marekanite and +pearlstone, both of them occurring in nodular concretions in the +trachytic series) differ in their constituent parts, from the minerals +generally composing trachytic rocks. It appears from three analyses, +that obsidian contains on an average 76 per cent of silica; from one +analysis, that sphaerulites contain 79.12; from two, that marekanite +contains 79.25; and from two other analyses, that pearlstone contains +75.62 of silica. (The foregoing analyses are taken from Beudant “Traite +de Mineralogie” tome 2 page 113; and one analysis of obsidian from +Phillips “Mineralogy.”) Now, the constituent parts of trachyte, as far +as they can be distinguished consist of feldspar, containing 65.21 of +silica; or of albite, containing 69.09; of hornblende, containing 55.27 +(These analyses are taken from Von Kobell “Grundzuge der Mineralogie” +1838.), and of oxide of iron: so that the foregoing glassy +concretionary substances all contain a larger proportion of silica than +that occurring in ordinary feldspathic or trachytic rocks. D’Aubuisson +(“Traite de Geogn.” tome 2 page 535.), also, has remarked on the large +proportion of silica compared with alumina, in six analyses of obsidian +and pearlstone given in Brongniart’s “Mineralogy.” Hence I conclude, +that the foregoing concretions have been formed by a process of +aggregation, strictly analogous to that which takes place in aqueous +deposits, acting chiefly on the silica, but likewise on some of the +other elements of the surrounding mass, and thus producing the +different concretionary varieties. From the well-known effects of rapid +cooling (This is seen in the manufacture of common glass, and in +Gregory Watts’s experiments on molten trap; also on the natural +surfaces of lava- streams, and on the side-walls of dikes.) in giving +glassiness of texture, it is probably necessary that the entire mass, +in cases like that of Ascension, should have cooled at a certain rate; +but considering the repeated and complicated alterations of nodules and +thin layers of a glassy texture with other layers quite stony or +crystalline, all within the space of a few feet or even inches, it is +hardly possible that they could have cooled at different rates, and +thus have acquired their different textures. + +The natural sphaerulites in these rocks very closely resemble those +produced in glass, when slowly cooled. (I do not know whether it is +generally known, that bodies having exactly the same appearance as +sphaerulites, sometimes occur in agates. Mr. Robert Brown showed me in +an agate, formed within a cavity in a piece of silicified wood, some +little specks, which were only just visible to the naked eye: these +specks, when placed by him under a lens of high power, presented a +beautiful appearance: they were perfectly circular, and consisted of +the finest fibres of a brown colour, radiating with great exactness +from a common centre. These little radiating stars are occasionally +intersected, and portions are quite cut off by the fine, ribbon-like +zones of colour in the agate. In the obsidian of Ascension, the halves +of a sphaerulite often lie in different zones of colour, but they are +not cut off by them, as in the agate.) In some fine specimens of +partially devitrified glass, in the possession of Mr. Stokes, the +sphaerulites are united into straight layers with even sides, parallel +to each other, and to one of the outer surfaces, exactly as in the +obsidian. These layers sometimes interbranch and form loops; but I did +not see any case of actual intersection. They form the passage from the +perfectly glassy portions, to those nearly homogeneous and stony, with +only an obscure concretionary structure. In the same specimen, also, +sphaerulites differing slightly in colour and in structure, occur +embedded close together. Considering these facts, it is some +confirmation of the view above given of the concretionary origin of the +obsidian and natural sphaerulites, to find that M. Dartigues (“Journal +de Physique” tome 59 1804 pages 10, 12.), in his curious paper on this +subject, attributes the production of sphaerulites in glass, to the +different ingredients obeying their own laws of attraction and becoming +aggregated. He is led to believe that this takes place, from the +difficulty in remelting sphaerulitic glass, without the whole be first +thoroughly pounded and mixed together; and likewise from the fact, that +the change takes place most readily in glass composed of many +ingredients. In confirmation of M. Dartigues’ view, I may remark, that +M. Fleuriau de Bellevue (Idem tome 60 1805 page 418.) found that the +sphaerulitic portions of devitrified glass were acted on both by nitric +acid and under the blowpipe, in a different manner from the compact +paste in which they were embedded. + +COMPARISON OF THE OBSIDIAN BEDS AND ALTERNATING STRATA OF ASCENSION, +WITH THOSE OF OTHER COUNTRIES. + +I have been struck with much surprise, how closely the excellent +description of the obsidian rocks of Hungary, given by Beudant (“Voyage +en Hongrie” tome 1 page 330; tome 2 pages 221 and 315; tome 3 pages +369, 371, 377, 381.), and that by Humboldt, of the same formation in +Mexico and Peru (“Essai Geognostique” pages 176, 326, 328.), and +likewise the descriptions given by several authors (P. Scrope +“Geological Transactions” volume 2 second series page 195. Consult also +Dolomieu “Voyage aux Isles Lipari” and D’Aubuisson “Traite de Geogn.” +tome 2 page 534.) of the trachytic regions in the Italian islands, +agree with my observations at Ascension. Many passages might have been +transferred without alteration from the works of the above authors, and +would have been applicable to this island. They all agree in the +laminated and stratified character of the whole series; and Humboldt +speaks of some of the beds of obsidian being ribboned like jasper. (In +Mr. Stokes’ fine collection of obsidians from Mexico, I observe that +the sphaerulites are generally much larger than those of Ascension; +they are generally white, opaque, and are united into distinct layers: +there are many singular varieties, different from any at Ascension. The +obsidians are finely zoned, in quite straight or curved lines, with +exceedingly slight differences of tint, of cellularity, and of more or +less perfect degrees of glassiness. Tracing some of the less perfectly +glassy zones, they are seen to become studded with minute white +sphaerulites, which become more and more numerous, until at last they +unite and form a distinct layer: on the other hand, at Ascension, only +the brown sphaerulites unite and form layers; the white ones always +being irregularly disseminated. Some specimens at the Geological +Society, said to belong to an obsidian formation from Mexico, have an +earthy fracture, and are divided in the finest parallel laminae, by +specks of a black mineral, like the augitic or hornblendic specks in +the rocks at Ascension.) They all agree in the nodular or concretionary +character of the obsidian, and of the passage of these nodules into +layers. They all refer to the repeated alterations, often in undulatory +planes, of glassy, pearly, stony, and crystalline layers: the +crystalline layers, however, seem to be much more perfectly developed +at Ascension, than in the above-named countries. Humboldt compares some +of the stony beds, when viewed from a distance, to strata of a +schistose sandstone. Sphaerulites are described as occurring abundantly +in all cases; and they everywhere seem to mark the passage, from the +perfectly glassy to the stony and crystalline beds. Beudant’s account +(Beudant “Voyage” tome 3 page 373.) of his “perlite lithoide +globulaire” in every, even the most trifling particular, might have +been written for the little brown sphaerulitic globules of the rocks of +Ascension. + +From the close similarity in so many respects, between the obsidian +formations of Hungary, Mexico, Peru, and of some of the Italian +islands, with that of Ascension, I can hardly doubt that in all these +cases, the obsidian and the sphaerulites owe their origin to a +concretionary aggregation of the silica, and of some of the other +constituent elements, taking place whilst the liquified mass cooled at +a certain required rate. It is, however, well-known, that in several +places, obsidian has flowed in streams like lava; for instance, at +Teneriffe, at the Lipari Islands, and at Iceland. (For Teneriffe see +von Buch “Descript. des Isles Canaries” pages 184 and 190; for the +Lipari Islands see Dolomieu “Voyage” page 34; for Iceland see Mackenzie +“Travels” page 369.) In these cases, the superficial parts are the most +perfectly glassy, the obsidian passing at the depth of a few feet into +an opaque stone. In an analysis by Vauquelin of a specimen of obsidian +from Hecla, which probably flowed as lava, the proportion of silica is +nearly the same as in the nodular or concretionary obsidian from +Mexico. It would be interesting to ascertain, whether the opaque +interior portions and the superficial glassy coating contained the same +proportional constituent parts: we know from M. Dufrenoy (“Memoires +pour servir a une Descript. Geolog. de la France” tome 4 page 371.) +that the exterior and interior parts of the same stream of lava +sometimes differ considerably in their composition. Even should the +whole body of the stream of obsidian turn out to be similarly composed +with nodular obsidian, it would only be necessary, in accordance with +the foregoing facts, to suppose that lava in these instances had been +erupted with its ingredients mixed in the same proportion, as in the +concretionary obsidian. + +LAMINATION OF VOLCANIC ROCKS OF THE TRACHYTIC SERIES. + +We have seen that, in several and widely distant countries, the strata +alternating with beds of obsidian, are highly laminated. The nodules, +also, both large and small, of the obsidian, are zoned with different +shades of colour; and I have seen a specimen from Mexico in Mr. Stokes’ +collection, with its external surface weathered (MacCulloch states +“Classification of Rocks” page 531 that the exposed surfaces of the +pitchstone dikes in Arran are furrowed “with undulating lines, +resembling certain varieties of marbled paper, and which evidently +result from some corresponding difference of laminar structure.”) into +ridges and furrows, corresponding with the zones of different degrees +of glassiness: Humboldt (“Personal Narrative” volume 1 page 222.), +moreover, found on the Peak of Teneriffe, a stream of obsidian divided +by very thin, alternating, layers of pumice. Many other lavas of the +feldspathic series are laminated; thus, masses of common trachyte at +Ascension are divided by fine earthy lines, along which the rock +splits, separating thin layers of slightly different shades of colour; +the greater number, also, of the embedded crystals of glassy feldspar +are placed lengthways in the same direction. Mr. P. Scrope (“Geological +Transactions” volume 2 second series page 195.) has described a +remarkable columnar trachyte in the Panza Islands, which seems to have +been injected into an overlying mass of trachytic conglomerate: it is +striped with zones, often of extreme tenuity, of different textures and +colours; the harder and darker zones appearing to contain a larger +proportion of silica. In another part of the island, there are layers +of pearlstone and pitchstone, which in many respects resemble those of +Ascension. The zones in the columnar trachyte are generally contorted; +they extend uninterruptedly for a great length in a vertical direction, +and apparently parallel to the walls of the dike-like mass. Von Buch +(“Description des Iles Canaries” page 184.) has described at Teneriffe, +a stream of lava containing innumerable thin, plate-like crystals of +feldspar, which are arranged like white threads, one behind the other, +and which mostly follow the same direction. Dolomieu (“Voyage aux Isles +de Lipari” pages 35 and 85.) also states, that the grey lavas of the +modern cone of Vulcano, which have a vitreous texture, are streaked +with parallel white lines: he further describes a solid pumice-stone +which possesses a fissile structure, like that of certain micaceous +schists. Phonolite, which I may observe is often, if not always, an +injected rock, also, often has a fissile structure; this is generally +due to the parallel position of the embedded crystals of feldspar, but +sometimes, as at Fernando Noronha, seems to be nearly independent of +their presence. (In this case, and in that of the fissile pumice-stone, +the structure is very different from that in the foregoing cases, where +the laminae consist of alternate layers of different composition or +texture. In some sedimentary formations, however, which apparently are +homogeneous and fissile, as in glossy clay-slate, there is reason to +believe, according to D’Aubuisson, that the laminae are really due to +excessively thin, alternating, layers of mica.) From these facts we +see, that various rocks of the feldspathic series have either a +laminated or fissile structure, and that it occurs both in masses which +have injected into overlying strata, and in others which have flowed as +streams of lava. + +The laminae of the beds, alternating with the obsidian at Ascension, +dip at a high angle under the mountain, at the base of which they are +situated; and they do not appear as if they had been inclined by +violence. A high inclination is common to these beds in Mexico, Peru, +and in some of the Italian islands (See Phillips “Mineralogy” for the +Italian Islands page 136. For Mexico and Peru see Humboldt “Essai +Geognostique.” Mr. Edwards also describes the high inclination of the +obsidian rocks of the Cerro del Navaja in Mexico in the “Proc. of the +Geolog. Soc.” June 1838.): on the other hand, in Hungary, the layers +are horizontal; the laminae, also, of some of the lava-streams above +referred to, as far as I can understand the descriptions given of them, +appear to be highly inclined or vertical. I doubt whether in any of +these cases, the laminae have been tilted into their present position; +and in some instances, as in that of the trachyte described by Mr. +Scrope, it is almost certain that they have been originally formed with +a high inclination. In many of these cases, there is evidence that the +mass of liquified rock has moved in the direction of the laminae. At +Ascension, many of the air-cells have a drawn out appearance, and are +crossed by coarse semi-glassy fibres, in the direction of the laminae; +and some of the layers, separating the sphaerulitic globules, have a +scored appearance, as if produced by the grating of the globules. I +have seen a specimen of zoned obsidian from Mexico, in Mr. Stokes’ +collection, with the surfaces of the best-defined layers streaked or +furrowed with parallel lines; and these lines or streaks precisely +resembled those, produced on the surface of a mass of artificial glass +by its having been poured out of a vessel. Humboldt, also, has +described little cavities, which he compares to the tails of comets, +behind sphaerulites in laminated obsidian rocks from Mexico, and Mr. +Scrope has described other cavities behind fragments embedded in his +laminated trachyte, and which he supposes to have been produced during +the movement of the mass. (“Geological Transactions” volume 2 second +series page 200 etc. These embedded fragments, in some instances, +consist of the laminated trachyte broken off and “enveloped in those +parts, which still remained liquid.” Beudant, also, frequently refers +in his great work on “Hungary” tome 3 page 386, to trachytic rocks, +irregularly spotted with fragments of the same varieties, which in +other parts form the parallel ribbons. In these cases, we must suppose, +that after part of the molten mass had assumed a laminated structure, a +fresh irruption of lava broke up the mass, and involved fragments, and +that subsequently the whole became relaminated.) From such facts, most +authors have attributed the lamination of these volcanic rocks to their +movement whilst liquified. Although it is easy to perceive, why each +separate air-cell, or each fibre in pumice-stone (Dolomieu “Voyage” +page 64.), should be drawn out in the direction of the moving mass; it +is by no means at first obvious why such air-cells and fibres should be +arranged by the movement, in the same planes, in laminae absolutely +straight and parallel to each other, and often of extreme tenuity; and +still less obvious is it, why such layers should come to be of slightly +different composition and of different textures. + +In endeavouring to make out the cause of the lamination of these +igneous feldspathic rocks, let us return to the facts so minutely +described at Ascension. We there see, that some of the thinnest layers +are chiefly formed by numerous, exceedingly minute, though perfect, +crystals of different minerals; that other layers are formed by the +union of different kinds of concretionary globules, and that the layers +thus formed, often cannot be distinguished from the ordinary +feldspathic and pitchstone layers, composing a large portion of the +entire mass. The fibrous radiating structure of the sphaerulites seems, +judging from many analogous cases, to connect the concretionary and +crystalline forces: the separate crystals, also, of feldspar all lie in +the same parallel planes. (The formation, indeed, of a large crystal of +any mineral in a rock of mixed composition implies an aggregation of +the requisite atoms, allied to concretionary action. The cause of the +crystals of feldspar in these rocks of Ascension, being all placed +lengthways, is probably the same with that which elongates and flattens +all the brown sphaerulitic globules (which behave like feldspar under +the blowpipe) in this same direction.) These allied forces, therefore, +have played an important part in the lamination of the mass, but they +cannot be considered the primary force; for the several kinds of +nodules, both the smallest and largest, are internally zoned with +excessively fine shades of colour, parallel to the lamination of the +whole; and many of them are, also, externally marked in the same +direction with parallel ridges and furrows, which have not been +produced by weathering. + +Some of the finest streaks of colour in the stony layers, alternating +with the obsidian, can be distinctly seen to be due to an incipient +crystallisation of the constituent minerals. The extent to which the +minerals have crystallised can, also, be distinctly seen to be +connected with the greater or less size, and with the number, of the +minute, flattened, crenulated air-cavities or fissures. Numerous facts, +as in the case of geodes, and of cavities in silicified wood, in +primary rocks, and in veins, show that crystallisation is much favoured +by space. Hence, I conclude, that, if in a mass of cooling volcanic +rock, any cause produced in parallel planes a number of minute fissures +or zones of less tension (which from the pent-up vapours would often be +expanded into crenulated air-cavities), the crystallisation of the +constituent parts, and probably the formation of concretions, would be +superinduced or much favoured in such planes; and thus, a laminated +structure of the kind we are here considering would be generated. + +That some cause does produce parallel zones of less tension in volcanic +rocks, during their consolidation, we must admit in the case of the +thin alternate layers of obsidian and pumice described by Humboldt, and +of the small, flattened, crenulated air-cells in the laminated rocks of +Ascension; for on no other principle can we conceive why the confined +vapours should through their expansion form air-cells or fibres in +separate, parallel planes, instead of irregularly throughout the mass. +In Mr. Stokes’ collection, I have seen a beautiful example of this +structure, in a specimen of obsidian from Mexico, which is shaded and +zoned, like the finest agate, with numerous, straight, parallel layers, +more or less opaque and white, or almost perfectly glassy; the degree +of opacity and glassiness depending on the number of microscopically +minute, flattened air-cells; in this case, it is scarcely possible to +doubt but that the mass, to which the fragment belonged, must have been +subjected to some, probably prolonged, action, causing the tension +slightly to vary in the successive planes. + +Several causes appear capable of producing zones of different tension, +in masses semi-liquified by heat. In a fragment of devitrified glass, I +have observed layers of sphaerulites which appeared, from the manner in +which they were abruptly bent, to have been produced by the simple +contraction of the mass in the vessel, in which it cooled. In certain +dikes on Mount Etna, described by M. Elie de Beaumont (“Mem. pour +servir” etc. tome 4 page 131.), as bordered by alternating bands of +scoriaceous and compact rock, one is led to suppose that the stretching +movement of the surrounding strata, which originally produced the +fissures, continued whilst the injected rock remained fluid. Guided, +however, by Professor Forbes’ (“Edinburgh New Phil. Journal” 1842 page +350.) clear description of the zoned structure of glacier-ice, far the +most probable explanation of the laminated structure of these +feldspathic rocks appears to be, that they have been stretched whilst +slowly flowing onwards in a pasty condition (I presume that this is +nearly the same explanation which Mr. Scrope had in his mind, when he +speaks (“Geolog. Transact.” volume 2 second series page 228) of the +ribboned structure of his trachytic rocks, having arisen, from “a +linear extension of the mass, while in a state of imperfect liquidity, +coupled with a concretionary process.”), in precisely the same manner +as Professor Forbes believes, that the ice of moving glaciers is +stretched and fissured. In both cases, the zones may be compared to +those in the finest agates; in both, they extend in the direction in +which the mass has flowed, and those exposed on the surface are +generally vertical: in the ice, the porous laminae are rendered +distinct by the subsequent congelation of infiltrated water, in the +stony feldspathic lavas, by subsequent crystalline and concretionary +action. The fragment of glassy obsidian in Mr. Stokes’ collection, +which is zoned with minute air-cells must strikingly resemble, judging +from Professor Forbes’ descriptions, a fragment of the zoned ice; and +if the rate of cooling and nature of the mass had been favourable to +its crystallisation or to concretionary action, we should here have had +the finest parallel zones of different composition and texture. In +glaciers, the lines of porous ice and of minute crevices seem to be due +to an incipient stretching, caused by the central parts of the frozen +stream moving faster than the sides and bottom, which are retarded by +friction: hence in glaciers of certain forms and towards the lower end +of most glaciers, the zones become horizontal. May we venture to +suppose that in the feldspathic lavas with horizontal laminae, we see +an analogous case? All geologists, who have examined trachytic regions, +have come to the conclusion, that the lavas of this series have +possessed an exceedingly imperfect fluidity; and as it is evident that +only matter thus characterised would be subject to become fissured and +to be formed into zones of different tensions, in the manner here +supposed, we probably see the reason why augitic lavas, which appear +generally to have possessed a high degree of fluidity, are not, like +the feldspathic lavas, divided into laminae of different composition +and texture. (Basaltic lavas, and many other rocks, are not +unfrequently divided into thick laminae or plates, of the same +composition, which are either straight or curved; these being crossed +by vertical lines of fissure, sometimes become united into columns. +This structure seems related, in its origin, to that by which many +rocks, both igneous and sedimentary, become traversed by parallel +systems of fissures.) Moreover, in the augitic series, there never +appears to be any tendency to concretionary action, which we have seen +plays an important part in the lamination of rocks, of the trachytic +series, or at least in rendering that structure apparent. + +Whatever may be thought of the explanation here advanced of the +laminated structure of the rocks of the trachytic series, I venture to +call the attention of geologists to the simple fact, that in a body of +rock at Ascension, undoubtedly of volcanic origin, layers often of +extreme tenuity, quite straight, and parallel to each other, have been +produced;—some composed of distinct crystals of quartz and diopside, +mingled with amorphous augitic specks and granular feldspar,—others +entirely composed of these black augitic specks, with granules of oxide +of iron,—and lastly, others formed of crystalline feldspar, in a more +or less perfect state of purity, together with numerous crystals of +feldspar, placed lengthways. At this island, there is reason to +believe, and in some analogous cases, it is certainly known, that the +laminae have originally been formed with their present high +inclination. Facts of this nature are manifestly of importance, with +relation to the structural origin of that grand series of plutonic +rocks, which like the volcanic have undergone the action of heat, and +which consist of alternate layers of quartz, feldspar, mica and other +minerals. + + + + +CHAPTER IV. +ST. HELENA. + + +Lavas of the feldspathic, basaltic, and submarine series. Section of +Flagstaff Hill and of the Barn. Dikes. Turk’s Cap and Prosperous Bays. +Basaltic ring. Central crateriform ridge, with an internal ledge and a +parapet. Cones of phonolite. Superficial beds of calcareous sandstone. +Extinct land-shells. Beds of detritus. Elevation of the land. +Denudation. Craters of elevation. + +The whole island is of volcanic origin; its circumference, according to +Beatson, is about twenty-eight miles. (Governor Beatson “Account of St. +Helena.”) The central and largest part consists of rocks of a +feldspathic nature, generally decomposed to an extraordinary degree; +and when in this state, presenting a singular assemblage of +alternating, red, purple, brown, yellow, and white, soft, argillaceous +beds. From the shortness of our visit, I did not examine these beds +with care; some of them, especially those of the white, yellow, and +brown shades, originally existed as streams of lava, but the greater +number were probably ejected in the form of scoriae and ashes: other +beds of a purple tint, porphyritic with crystal- shaped patches of a +white, soft substance, which are now unctuous, and yield, like wax, a +polished streak to the nail, seem once to have existed as solid +claystone-porphyries: the red argillaceous beds generally have a +brecciated structure, and no doubt have been formed by the +decomposition of scoriae. Several extensive streams, however, belonging +to this series, retain their stony character; these are either of a +blackish-green colour, with minute acicular crystals of feldspar, or of +a very pale tint, and almost composed of minute, often scaly, crystals +of feldspar, abounding with microscopical black specks; they are +generally compact and laminated; others, however, of similar +composition, are cellular and somewhat decomposed. None of these rocks +contain large crystals of feldspar, or have the harsh fracture peculiar +to trachyte. These feldspathic lavas and tuffs are the uppermost or +those last erupted; innumerable dikes, however, and great masses of +molten rock, have subsequently been injected into them. They converge, +as they rise, towards the central curved ridge, of which one point +attains the elevation of 2,700 feet. This ridge is the highest land in +the island; and it once formed the northern rim of a great crater, +whence the lavas of this series flowed: from its ruined condition, from +the southern half having been removed, and from the violent dislocation +which the whole island has undergone, its structure is rendered very +obscure. + +BASALTIC SERIES. + +The margin of the island is formed by a rude circle of great, black, +stratified, ramparts of basalt, dipping seaward, and worn into cliffs, +which are often nearly perpendicular, and vary in height from a few +hundred feet to two thousand. This circle, or rather horse-shoe shaped +ring, is open to the south, and is breached by several other wide +spaces. Its rim or summit generally projects little above the level of +the adjoining inland country; and the more recent feldspathic lavas, +sloping down from the central heights, generally abut against and +overlap its inner margin; on the north-western side of the island, +however, they appear (judging from a distance) to have flowed over and +concealed portions of it. In some parts, where the basaltic ring has +been breached, and the black ramparts stand detached, the feldspathic +lavas have passed between them, and now overhang the sea-coast in lofty +cliffs. The basaltic rocks are of a black colour and thinly stratified; +they are generally highly vesicular, but occasionally compact; some of +them contain numerous crystals of glassy feldspar and octahedrons of +titaniferous iron; others abound with crystals of augite and grains of +olivine. The vesicles are frequently lined with minute crystals (of +chabasie?) and even become amygdaloidal with them. The streams are +separated from each other by cindery matter, or by a bright red, +friable, saliferous tuff, which is marked by successive lines like +those of aqueous deposition; and sometimes it has an obscure, +concretionary structure. The rocks of this basaltic series occur +nowhere except near the coast. In most volcanic districts the trachytic +lavas are of anterior origin to the basaltic; but here we see, that a +great pile of rock, closely related in composition to the trachytic +family, has been erupted subsequently to the basaltic strata: the +number, however, of dikes, abounding with large crystals of augite, +with which the feldspathic lavas have been injected, shows perhaps some +tendency to a return to the more usual order of superposition. + +BASAL SUBMARINE LAVAS. + +The lavas of this basal series lie immediately beneath both the +basaltic and feldspathic rocks. According to Mr. Seale, they may be +seen at intervals on the sea-beach round the entire island. (“Geognosy +of the Island of St. Helena.” Mr. Seale has constructed a gigantic +model of St. Helena, well worth visiting, which is now deposited at +Addiscombe College, in Surrey.) In the sections which I examined, their +nature varied much; some of the strata abound with crystals of augite; +others are of a brown colour, either laminated or in a rubbly +condition; and many parts are highly amygdaloidal with calcareous +matter. The successive sheets are either closely united together, or +are separated from each other by beds of scoriaceous rock and of +laminated tuff, frequently containing well-rounded fragments. The +interstices of these beds are filled with gypsum and salt; the gypsum +also sometimes occurring in thin layers. From the large quantity of +these two substances, from the presence of rounded pebbles in the +tuffs, and from the abundant amygdaloids, I cannot doubt that these +basal volcanic strata flowed beneath the sea. This remark ought perhaps +to be extended to a part of the superincumbent basaltic rocks; but on +this point, I was not able to obtain clear evidence. The strata of the +basal series, whenever I examined them, were intersected by an +extraordinary number of dikes. + +FLAGSTAFF HILL AND THE BARN. + +(FIGURE 8. FLAGSTAFF HILL AND THE BARN. (Section West (left) to East +(right)) Flagstaff Hill, 2,272 feet high to The Barn, 2,015 feet high. + +The double lines represent the basaltic strata; the single, the basal +submarine strata; the dotted, the upper feldspathic strata; the dikes +are shaded transversely.) + +I will now describe some of the more remarkable sections, and will +commence with these two hills, which form the principal external +feature on the north-eastern side of the island. The square, angular +outline, and black colour of the Barn, at once show that it belongs to +the basaltic series; whilst the smooth, conical figure, and the varied +bright tints of Flagstaff Hill, render it equally clear, that it is +composed of the softened, feldspathic rocks. These two lofty hills are +connected (as is shown in Figure 8) by a sharp ridge, which is composed +of the rubbly lavas of the basal series. The strata of this ridge dip +westward, the inclination becoming less and less towards the Flagstaff; +and the upper feldspathic strata of this hill can be seen, though with +some difficulty, to dip conformably to the W.S.W. Close to the Barn, +the strata of the ridge are nearly vertical, but are much obscured by +innumerable dikes; under this hill, they probably change from being +vertical into being inclined into an opposite direction; for the upper +or basaltic strata, which are about eight hundred or one thousand feet +in thickness, are inclined north-eastward, at an angle between thirty +and forty degrees. + +This ridge, and likewise the Barn and Flagstaff Hills, are interlaced +by dikes, many of which preserve a remarkable parallelism in a N.N.W. +and S.S.E. direction. The dikes chiefly consist of a rock, porphyritic +with large crystals of augite; others are formed of a fine-grained and +brown- coloured trap. Most of these dikes are coated by a glossy layer, +from one to two-tenths of an inch in thickness, which, unlike true +pitchstone, fuses into a black enamel; this layer is evidently +analogous to the glossy superficial coating of many lava streams. (This +circumstance has been observed (Lyell “Principles of Geology” volume 4 +chapter 10 page 9) in the dikes of the Atrio del Cavallo, but +apparently it is not of very common occurrence. Sir G. Mackenzie, +however, states (page 372 “Travels in Iceland”) that all the veins in +Iceland have a “black vitreous coating on their sides.” Captain +Carmichael, speaking of the dikes in Tristan d’Acunha, a volcanic +island in the Southern Atlantic, says (“Linnaean Transactions” volume +12 page 485) that their sides, “where they come in contact with the +rocks, are invariably in a semi-vitrified state.”) The dikes can often +be followed for great lengths both horizontally and vertically, and +they seem to preserve a nearly uniform thickness (“Geognosy of the +Island of St. Helena” plate 5.): Mr. Seale states, that one near the +Barn, in a height of 1,260 feet, decreases in width only four +inches,—from nine feet at the bottom, to eight feet and eight inches at +the top. On the ridge, the dikes appear to have been guided in their +course, to a considerable degree, by the alternating soft and hard +strata: they are often firmly united to the harder strata, and they +preserve their parallelism for such great lengths, that in very many +instances it was impossible to conjecture, which of the beds were +dikes, and which streams of lava. The dikes, though so numerous on this +ridge, are even more numerous in the valleys a little south of it, and +to a degree I never saw equalled anywhere else: in these valleys they +extend in less regular lines, covering the ground with a network, like +a spider’s web, and with some parts of the surface even appearing to +consist wholly of dikes, interlaced by other dikes. + +From the complexity produced by the dikes, from the high inclination +and anticlinal dip of the strata of the basal series, which are +overlaid, at the opposite ends of the short ridge, by two great masses +of different ages and of different composition, I am not surprised that +this singular section has been misunderstood. It has even been supposed +to form part of a crater; but so far is this from having been the case, +that the summit of Flagstaff Hill once formed the lower extremity of a +sheet of lava and ashes, which were erupted from the central, +crateriform ridge. Judging from the slope of the contemporaneous +streams in an adjoining and undisturbed part of the island, the strata +of the Flagstaff Hill must have been upturned at least twelve hundred +feet, and probably much more, for the great truncated dikes on its +summit show that it has been largely denuded. The summit of this hill +now nearly equals in height the crateriform ridge; and before having +been denuded, it was probably higher than this ridge, from which it is +separated by a broad and much lower tract of country; we here, +therefore, see that the lower extremities of a set of lava-streams have +been tilted up to as great a height as, or perhaps greater height than, +the crater, down the flanks of which they originally flowed. I believe +that dislocations on so grand a scale are extremely rare in volcanic +districts. (M. Constant Prevost “Mem. de la Soc. Geolog.” tome 2 +observes that “les produits volcaniques n’ont que localement et +rarement meme derange le sol, a travers lequel ils se sont fait jour.”) +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 +degree: this stretching, on the ridge between Flagstaff and Barn Hills, +probably took place subsequently (though perhaps immediately so) to the +strata being tilted; for had the strata at that time extended +horizontally, they would in all probability have been fissured and +injected transversely, instead of in the planes of their +stratification. Although the space between the Barn and Flagstaff Hill +presents a distinct anticlinal line extending north and south, and +though most of the dikes range with much regularity in the same line, +nevertheless, at only a mile due south of the ridge the strata lie +undisturbed. Hence the disturbing force seems to have acted under a +point, rather than along a line. The manner in which it has acted, is +probably explained by the structure of Little Stony-top, a mountain +2,000 feet high, situated a few miles southward of the Barn; we there +see, even from a distance, a dark-coloured, sharp, wedge of compact +columnar rock, with the bright-coloured feldspathic strata, sloping +away on each side from its uncovered apex. This wedge, from which it +derives its name of Stony-top, consists of a body of rock, which has +been injected whilst liquified into the overlying strata; and if we may +suppose that a similar body of rock lies injected, beneath the ridge +connecting the Barn and Flagstaff, the structure there exhibited would +be explained. + +TURK’S CAP AND PROSPEROUS BAYS. + +(FIGURE 9. PROSPEROUS HILL AND THE BARN. (Section S.S.E. (left) to +N.N.W. (right) Prosperous Hill through Hold-fast-Tom and Flagstaff Hill +to The Barn. + +The double lines represent the basaltic strata; the single, the basal +submarine strata; the dotted, the upper feldspathic strata.) + +Prosperous Hill is a great, black, precipitous mountain, situated two +miles and a half south of the Barn, and composed, like it, of basaltic +strata. These rest, in one part, on the brown-coloured, porphyritic +beds of the basal series, and in another part, on a fissured mass of +highly scoriaceous and amygdaloidal rock, which seems to have formed a +small point of eruption beneath the sea, contemporaneously with the +basal series. Prosperous Hill, like the Barn, is traversed by many +dikes, of which the greater number range north and south, and its +strata dip, at an angle of about 20 degrees, rather obliquely from the +island towards the sea. The space between Prosperous Hill and the Barn, +as represented in Figure 9, consists of lofty cliffs, composed of the +lavas of the upper or feldspathic series, which rest, though +unconformably, on the basal submarine strata, as we have seen that they +do at Flagstaff Hill. Differently, however, from in that hill, these +upper strata are nearly horizontal, gently rising towards the interior +of the island; and they are composed of greenish-black, or more +commonly, pale brown, compact lavas, instead of softened and highly +coloured matter. These brown-coloured, compact lavas, consist almost +entirely of small glimmering scales, or of minute acicular crystals, of +feldspar, placed close by the side of each other, and abounding with +minute black specks, apparently of hornblende. The basaltic strata of +Prosperous Hill project only a little above the level of the +gently-sloping, feldspathic streams, which wind round and abut against +their upturned edges. The inclination of the basaltic strata seems to +be too great to have been caused by their having flowed down a slope, +and they must have been tilted into their present position before the +eruption of the feldspathic streams. + +BASALTIC RING. + +Proceeding round the Island, the lavas of the upper series, southward +of Prosperous Hill, overhang the sea in lofty precipices. Further on, +the headland, called Great Stony-top, is composed, as I believe, of +basalt; as is Long Range Point, on the inland side of which the +coloured beds abut. On the southern side of the island, we see the +basaltic strata of the South Barn, dipping obliquely seaward at a +considerable angle; this headland, also, stands a little above the +level of the more modern, feldspathic lavas. Further on, a large space +of coast, on each side of Sandy Bay, has been much denuded, and there +seems to be left only the basal wreck of the great, central crater. The +basaltic strata reappear, with their seaward dip, at the foot of the +hill, called Man-and-Horse; and thence they are continued along the +whole north-western coast to Sugar-Loaf Hill, situated near to the +Flagstaff; and they everywhere have the same seaward inclination, and +rest, in some parts at least, on the lavas of the basal series. We thus +see that the circumference of the island is formed by a much-broken +ring, or rather, a horse-shoe, of basalt, open to the south, and +interrupted on the eastern side by many wide breaches. The breadth of +this marginal fringe on the north-western side, where alone it is at +all perfect, appears to vary from a mile to a mile and a half. The +basaltic strata, as well as those of the subjacent basal series, dip, +with a moderate inclination, where they have not been subsequently +disturbed, towards the sea. The more broken state of the basaltic ring +round the eastern half, compared with the western half of the island, +is evidently due to the much greater denuding power of the waves on the +eastern or windward side, as is shown by the greater height of the +cliffs on that side, than to leeward. Whether the margin of basalt was +breached, before or after the eruption of the lavas of the upper +series, is doubtful; but as separate portions of the basaltic ring +appear to have been tilted before that event, and from other reasons, +it is more probable, that some at least of the breaches were first +formed. Reconstructing in imagination, as far as is possible, the ring +of basalt, the internal space or hollow, which has since been filled up +with the matter erupted from the great central crater, appears to have +been of an oval figure, eight or nine miles in length by about four +miles in breadth, and with its axis directed in a N.E. and S W. line, +coincident with the present longest axis of the island. + +THE CENTRAL CURVED RIDGE. + +This ridge consists, as before remarked, of grey feldspathic lavas, and +of red, brecciated, argillaceous tuffs, like the beds of the upper +coloured series. The grey lavas contain numerous, minute, black, easily +fusible specks; and but very few large crystals of feldspar. They are +generally much softened; with the exception of this character, and of +being in many parts highly cellular, they are quite similar to those +great sheets of lava which overhang the coast at Prosperous Bay. +Considerable intervals of time appear to have elapsed, judging from the +marks of denudation, between the formation of the successive beds, of +which this ridge is composed. On the steep northern slope, I observed +in several sections a much worn undulating surface of red tuff, covered +by grey, decomposed, feldspathic lavas, with only a thin earthy layer +interposed between them. In an adjoining part, I noticed a trap-dike, +four feet wide, cut off and covered up by the feldspathic lava, as is +represented in Figure 9. The ridge ends on the eastern side in a hook, +which is not represented clearly enough in any map which I have seen; +towards the western end, it gradually slopes down and divides into +several subordinate ridges. The best defined portion between Diana’s +Peak and Nest Lodge, which supports the highest pinnacles in the island +varying from 2,000 to 2,700 feet, is rather less than three miles long +in a straight line. Throughout this space the ridge has a uniform +appearance and structure; its curvature resembles that of the +coast-line of a great bay, being made up of many smaller curves, all +open to the south. The northern and outer side is supported by narrow +ridges or buttresses, which slope down to the adjoining country. The +inside is much steeper, and is almost precipitous; it is formed of the +basset edges of the strata, which gently decline outwards. Along some +parts of the inner side, a little way beneath the summit, a flat ledge +extends, which imitates in outline the smaller curvatures of the crest. +Ledges of this kind occur not unfrequently within volcanic craters, and +their formation seems to be due to the sinking down of a level sheet of +hardened lava, the edges of which remain (like the ice round a pool, +from which the water has been drained) adhering to the sides. (A most +remarkable instance of this structure is described in Ellis “Polynesian +Researches” second edition where an admirable drawing is given of the +successive ledges or terraces, on the borders of the immense crater at +Hawaii, in the Sandwich Islands.) + +(FIGURE 10. DIKE. (Section showing layers 1, 2 and 3 from top to +bottom.) + +1. Grey feldspathic lava. + +2. A layer, one inch in thickness, of a reddish earthy matter. + +3. Brecciated, red, argillaceous tuff.) + +In some parts, the ridge is surmounted by a wall or parapet, +perpendicular on both sides. Near Diana’s Peak this wall is extremely +narrow. At the Galapagos Archipelago I observed parapets, having a +quite similar structure and appearance, surmounting several of the +craters; one, which I more particularly examined, was composed of +glossy, red scoriae firmly cemented together; being externally +perpendicular, and extending round nearly the whole circumference of +the crater, it rendered it almost inaccessible. The Peak of Teneriffe +and Cotopaxi, according to Humboldt, are similarly constructed; he +states that “at their summits a circular wall surrounds the crater, +which wall, at a distance, has the appearance of a small cylinder +placed on a truncated cone. (“Personal Narrative” volume 1 page 171.) +On Cotopaxi this peculiar structure is visible to the naked eye at more +than two thousand toises’ distance; and no person has ever reached its +crater. (Humboldt “Picturesque Atlas” folio plate 10.) On the Peak of +Teneriffe, the parapet is so high, that it would be impossible to reach +the caldera, if on the eastern side there did not exist a breach.” The +origin of these circular parapets is probably due to the heat or +vapours from the crater, penetrating and hardening the sides to a +nearly equal depth, and afterwards to the mountain being slowly acted +on by the weather, which would leave the hardened part, projecting in +the form of a cylinder or circular parapet. + +From the points of structure in the central ridge, now +enumerated,—namely, from the convergence towards it of the beds of the +upper series,—from the lavas there becoming highly cellular,—from the +flat ledge, extending along its inner and precipitous side, like that +within some still active craters,—from the parapet-like wall on its +summit,—and lastly, from its peculiar curvature, unlike that of any +common line of elevation, I cannot doubt that this curved ridge forms +the last remnant of a great crater. In endeavouring, however, to trace +its former outline, one is soon baffled; its western extremity +gradually slopes down, and, branching into other ridges, extends to the +sea-coast; the eastern end is more curved, but it is only a little +better defined. Some appearances lead me to suppose that the southern +wall of the crater joined the present ridge near Nest Lodge; in this +case the crater must have been nearly three miles long, and about a +mile and a half in breadth. Had the denudation of the ridge and the +decomposition of its constituent rocks proceeded a few steps further, +and had this ridge, like several other parts of the island, been broken +up by great dikes and masses of injected matter, we should in vain have +endeavoured to discover its true nature. 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 flowed, and probably even to a greater height. +It is interesting thus to trace the steps by which the structure of a +volcanic district becomes obscured, and finally obliterated: so near to +this last stage is St. Helena, that I believe no one has hitherto +suspected that the central ridge or axis of the island is the last +wreck of the crater, whence the most modern volcanic streams were +poured forth. + +The great hollow space or valley southward of the central curved ridge, +across which the half of the crater must once have extended, is formed +of bare, water-worn hillocks and ridges of red, yellow, and brown +rocks, mingled together in chaos-like confusion, interlaced by dikes, +and without any regular stratification. The chief part consists of red +decomposing scoriae, associated with various kinds of tuff and yellow +argillaceous beds, full of broken crystals, those of augite being +particularly large. Here and there masses of highly cellular and +amygdaloidal lavas protrude. From one of the ridges in the midst of the +valley, a conical precipitous hill, called Lot, boldly stands up, and +forms a most singular and conspicuous object. It is composed of +phonolite, divided in one part into great curved laminae, in another, +into angular concretionary balls, and in a third part into outwardly +radiating columns. At its base the strata of lava, tuff, and scoriae, +dip away on all sides (Abich in his “Views of Vesuvius” plate 6 has +shown the manner in which beds, under nearly similar circumstances, are +tilted up. The upper beds are more turned up than the lower; and he +accounts for this, by showing that the lava insinuates itself +horizontally between the lower beds.); the uncovered portion is 197 +feet in height (This height is given by Mr. Seale in his Geognosy of +the island. The height of the summit above the level of the sea is said +to be 1,444 feet.), and its horizontal section gives an oval figure. +The phonolite is of a greenish-grey colour, and is full of minute +acicular crystals of feldspar; in most parts it has a conchoidal +fracture, and is sonorous, yet it is crenulated with minute +air-cavities. In a S.W. direction from Lot, there are some other +remarkable columnar pinnacles, but of a less regular shape, namely, +Lot’s Wife, and the Asses’ Ears, composed of allied kinds of rock. From +their flattened shape, and their relative position to each other, they +are evidently connected on the same line of fissure. It is, moreover, +remarkable that this same N.E. and S.W. line, joining Lot and Lot’s +Wife, if prolonged would intersect Flagstaff Hill, which, as before +stated, is crossed by numerous dikes running in this direction, and +which has a disturbed structure, rendering it probable that a great +body of once fluid rock lies injected beneath it. + +In this same great valley there are several other conical masses of +injected rock (one, I observed, was composed of compact greenstone), +some of which are not connected, as far as is apparent, with any line +of dike; whilst others are obviously thus connected. Of these dikes, +three or four great lines stretch across the valley in a N.E. and S.W. +direction, parallel to that one connecting the Asses’ Ears, Lot’s Wife, +and probably Lot. The number of these masses of injected rock is a +remarkable feature in the geology of St. Helena. Besides those just +mentioned, and the hypothetical one beneath Flagstaff Hill, there is +Little Stony-top and others, as I have reason to believe, at the +Man-and-Horse, and at High Hill. Most of these masses, if not all of +them, have been injected subsequently to the last volcanic eruptions +from the central crater. The formation of conical bosses of rock on +lines of fissure, the walls of which are in most cases parallel, may +probably be attributed to inequalities in the tension, causing small +transverse fissures, and at these points of intersection the edges of +the strata would naturally yield, and be easily turned upwards. +Finally, I may remark, that hills of phonolite everywhere are apt to +assume singular and even grotesque shapes, like that of Lot +(D’Aubuisson in his “Traite de Geognosie” tome 2 page 540 particularly +remarks that this is the case.): the peak at Fernando Noronha offers an +instance; at St. Jago, however, the cones of phonolite, though +tapering, have a regular form. Supposing, as seems probable, that all +such hillocks or obelisks have originally been injected, whilst +liquified, into a mould formed by yielding strata, as certainly has +been the case with Lot, how are we to account for the frequent +abruptness and singularity of their outlines, compared with similarly +injected masses of greenstone and basalt? Can it be due to a less +perfect degree of fluidity, which is generally supposed to be +characteristic of the allied trachytic lavas? + +SUPERFICIAL DEPOSITS. + +Soft calcareous sandstone occurs in extensive, though thin, superficial +beds, both on the northern and southern shores of the island. It +consists of very minute, equal-sized, rounded particles of shells, and +other organic bodies, which partially retain their yellow, brown, and +pink colours, and occasionally, though very rarely, present an obscure +trace of their original external forms. I in vain endeavoured to find a +single unrolled fragment of a shell. The colour of the particles is the +most obvious character by which their origin can be recognised, the +tints being affected (and an odour produced) by a moderate heat, in the +same manner as in fresh shells. The particles are cemented together, +and are mingled with some earthy matter: the purest masses, according +to Beatson, contain 70 per cent of carbonate of lime. The beds, varying +in thickness from two or three feet to fifteen feet, coat the surface +of the ground; they generally lie on that side of the valley which is +protected from the wind, and they occur at the height of several +hundred feet above the level of the sea. Their position is the same +which sand, if now drifted by the trade-wind, would occupy; and no +doubt they thus originated, which explains the equal size and +minuteness of the particles, and likewise the entire absence of whole +shells, or even of moderately-sized fragments. It is remarkable that at +the present day there are no shelly beaches on any part of the coast, +whence calcareous dust could be drifted and winnowed; we must, +therefore, look back to a former period when before the land was worn +into the present great precipices, a shelving coast, like that of +Ascension, was favourable to the accumulation of shelly detritus. Some +of the beds of this limestone are between six hundred and seven hundred +feet above the sea; but part of this height may possibly be due to an +elevation of the land, subsequent to the accumulation of the calcareous +sand. + +The percolation of rain-water has consolidated parts of these beds into +a solid rock, and has formed masses of dark brown, stalagmitic +limestone. At the Sugar-Loaf quarry, fragments of rock on the adjoining +slopes have been thickly coated by successive fine layers of calcareous +matter. (In the earthy detritus on several parts of this hill, +irregular masses of very impure, crystallised sulphate of lime occur. +As this substance is now being abundantly deposited by the surf at +Ascension, it is possible that these masses may thus have originated; +but if so, it must have been at a period when the land stood at a much +lower level. This earthy selenite is now found at a height of between +six hundred and seven hundred feet.) It is singular, that many of these +pebbles have their entire surfaces coated, without any point of contact +having been left uncovered; hence, these pebbles must have been lifted +up by the slow deposition between them of the successive films of +carbonate of lime. Masses of white, finely oolitic rock are attached to +the outside of some of these coated pebbles. Von Buch has described a +compact limestone at Lanzarote, which seems perfectly to resemble the +stalagmitic deposition just mentioned: it coats pebbles, and in parts +is finely oolitic: it forms a far-extended layer, from one inch to two +or three feet in thickness, and it occurs at the height of 800 feet +above the sea, but only on that side of the island exposed to the +violent north-western winds. Von Buch remarks, that it is not found in +hollows, but only on the unbroken and inclined surfaces of the +mountain. (“Description des Isles Canaries” page 293.) He believes, +that it has been deposited by the spray which is borne over the whole +island by these violent winds. It appears, however, to me much more +probable that it has been formed, as at St. Helena, by the percolation +of water through finely comminuted shells: for when sand is blown on a +much-exposed coast, it always tends to accumulate on broad, even +surfaces, which offer a uniform resistance to the winds. At the +neighbouring island, moreover, of Feurteventura, there is an earthy +limestone, which, according to Von Buch, is quite similar to specimens +which he has seen from St. Helena, and which he believes to have been +formed by the drifting of shelly detritus. (Idem pages 314 and 374.) + +The upper beds of the limestone, at the above-mentioned quarry on the +Sugar-Loaf Hill, are softer, finer-grained and less pure, than the +lower beds. They abound with fragments of land-shells, and with some +perfect ones; they contain, also, the bones of birds, and the large +eggs, apparently of water-fowl. (Colonel Wilkes, in a catalogue +presented with some specimens to the Geological Society, states that as +many as ten eggs were found by one person. Dr. Buckland has remarked +(“Geolog. Trans.” volume 5 page 474) on these eggs.) It is probable +that these upper beds remained long in an unconsolidated form, during +which time, these terrestrial productions were embedded. Mr. G.R. +Sowerby has kindly examined three species of land-shells, which I +procured from this bed, and has described them in detail. One of them +is a Succinea, identical with a species now living abundantly on the +island; the two others, namely, Cochlogena fossilis and Helix +biplicata, are not known in a recent state: the latter species was also +found in another and different locality, associated with a species of +Cochlogena which is undoubtedly extinct. + +BEDS OF EXTINCT LAND-SHELLS. + +Land-shells, all of which appear to be species now extinct, occur +embedded in earth, in several parts of the island. The greater number +have been found at a considerable height on Flagstaff Hill. On the N.W. +side of this hill, a rain-channel exposes a section of about twenty +feet in thickness, of which the upper part consists of black vegetable +mould, evidently washed down from the heights above, and the lower part +of less black earth, abounding with young and old shells, and with +their fragments: part of this earth is slightly consolidated by +calcareous matter, apparently due to the partial decomposition of some +of the shells. Mr. Seale, an intelligent resident, who first called +attention to these shells, gave me a large collection from another +locality, where the shells appear to have been embedded in very black +earth. Mr. G.R. Sowerby has examined these shells, and has described +them. There are seven species, namely, one Cochlogena, two species of +the genus Cochlicopa, and four of Helix; none of these are known in a +recent state, or have been found in any other country. The smaller +species were picked out of the inside of the large shells of the +Cochlogena aurisvulpina. This last-mentioned species is in many +respects a very singular one; it was classed, even by Lamarck, in a +marine genus, and having thus been mistaken for a sea-shell, and the +smaller accompanying species having been overlooked, the exact +localities where it was found have been measured, and the elevation of +this island thus deduced! It is very remarkable that all the shells of +this species found by me in one spot, form a distinct variety, as +described by Mr. Sowerby, from those procured from another locality by +Mr. Seale. As this Cochlogena is a large and conspicuous shell, I +particularly inquired from several intelligent countrymen whether they +had ever seen it alive; they all assured me that they had not, and they +would not even believe that it was a land animal: Mr. Seale, moreover, +who was a collector of shells all his life at St. Helena, never met +with it alive. Possibly some of the smaller species may turn out to be +yet living kinds; but, on the other hand, the two land- shells which +are now living on the island in great numbers, do not occur embedded, +as far as is yet known, with the extinct species. I have shown in my +“Journal” (“Journal of Researches” page 582.), that the extinction of +these land-shells possibly may not be an ancient event; as a great +change took place in the state of the island about one hundred and +twenty years ago, when the old trees died, and were not replaced by +young ones, these being destroyed by the goats and hogs, which had run +wild in numbers, from the year 1502. Mr. Seale states, that on +Flagstaff Hill, where we have seen that the embedded land-shells are +especially numerous, traces are everywhere discoverable, which plainly +indicate that it was once thickly clothed with trees; at present not +even a bush grows there. The thick bed of black vegetable mould which +covers the shell-bed, on the flanks of this hill, was probably washed +down from the upper part, as soon as the trees perished, and the +shelter afforded by them was lost. + +ELEVATION OF THE LAND. + +Seeing that the lavas of the basal series, which are of submarine +origin, are raised above the level of the sea, and at some places to +the height of many hundred feet, I looked out for superficial signs of +the elevation of the land. The bottoms of some of the gorges, which +descend to the coast, are filled up to the depth of about a hundred +feet, by rudely divided layers of sand, muddy clay, and fragmentary +masses; in these beds, Mr. Seale has found the bones of the tropic-bird +and of the albatross; the former now rarely, and the latter never +visiting the island. From the difference between these layers, and the +sloping piles of detritus which rest on them, I suspect that they were +deposited, when the gorges stood beneath the sea. Mr. Seale, moreover, +has shown that some of the fissure- like gorges become, with a concave +outline, gradually rather wider at the bottom than at the top; and this +peculiar structure was probably caused by the wearing action of the +sea, when it entered the lower part of these gorges. (A fissure-like +gorge, near Stony-top, is said by Mr. Seale to be 840 feet deep, and +only 115 feet in width.) At greater heights, the evidence of the rise +of the land is even less clear: nevertheless, in a bay-like depression +on the table-land behind Prosperous Bay, at the height of about a +thousand feet, there are flat-topped masses of rock, which it is +scarcely conceivable, could have been insulated from the surrounding +and similar strata, by any other agency than the denuding action of a +sea- beach. Much denudation, indeed, has been effected at great +elevations, which it would not be easy to explain by any other means: +thus, the flat summit of the Barn, which is 2,000 feet high, presents, +according to Mr. Seale, a perfect network of truncated dikes; on hills +like the Flagstaff, formed of soft rock, we might suppose that the +dikes had been worn down and cut off by meteoric agency, but we can +hardly suppose this possible with the hard, basaltic strata of the +Barn. + +COAST DENUDATION. + +The enormous cliffs, in many parts between one and two thousand feet in +height, with which this prison-like island is surrounded, with the +exception of only a few places, where narrow valleys descend to the +coast, is the most striking feature in its scenery. We have seen that +portions of the basaltic ring, two or three miles in length by one or +two miles in breadth, and from one to two thousand feet in height, have +been wholly removed. There are, also, ledges and banks of rock, rising +out of profoundly deep water, and distant from the present coast +between three and four miles, which, according to Mr. Seale, can be +traced to the shore, and are found to be the continuations of certain +well-known great dikes. The swell of the Atlantic Ocean has obviously +been the active power in forming these cliffs; and it is interesting to +observe that the lesser, though still great, height of the cliffs on +the leeward and partially protected side of the island (extending from +the Sugar-Loaf Hill to South West Point), corresponds with the lesser +degree of exposure. When reflecting on the comparatively low coasts of +many volcanic islands, which also stand exposed in the open ocean, and +are apparently of considerable antiquity, the mind recoils from an +attempt to grasp the number of centuries of exposure, 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 island. The +contrast in the superficial state of St. Helena, compared with the +nearest island, namely, Ascension, is very striking. At Ascension, the +surfaces of the lava-streams are glossy, as if just poured forth, their +boundaries are well defined, and they can often be traced to perfect +craters, whence they were erupted; in the course of many long walks, I +did not observe a single dike; and the coast round nearly the entire +circumference is low, and has been eaten back (though too much stress +must not be placed on this fact, as the island may have been subsiding) +into a little wall only from ten to thirty feet high. Yet during the +340 years, since Ascension has been known, not even the feeblest signs +of volcanic action have been recorded. (In the “Nautical Magazine” for +1835 page 642, and for 1838 page 361, and in the “Comptes Rendus” April +1838, 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 century, in a space of open sea between longitudes 20 degrees and +22 degrees west, about half a degree south of the equator. These facts +seem to show, that an island or an archipelago is in process of +formation in the middle of the Atlantic: a line joining St. Helena and +Ascension, prolonged, intersects this slowly nascent focus of volcanic +action.) On the other hand, at St. Helena, the course of no one stream +of lava can be traced, either by the state of its boundaries or of its +superficies; the mere wreck of one great crater is left; not the +valleys only, but the surfaces of some of the highest hills, are +interlaced by worn-down dikes, and, in many places, the denuded summits +of great cones of injected rock stand exposed and naked; lastly, as we +have seen, the entire circuit of the island has been deeply worn back +into the grandest precipices. + +CRATERS OF ELEVATION. + +There is much resemblance in structure and in geological history +between St. Helena, St. Jago, and Mauritius. All three islands are +bounded (at least in the parts which I was able to examine) by a ring +of basaltic mountains, now much broken, but evidently once continuous. +These mountains have, or apparently once had, their escarpments steep +towards the interior of the island, and their strata dip outwards. I +was able to ascertain, only in a few cases, the inclination of the +beds; nor was this easy, for the stratification was generally obscure, +except when viewed from a distance. I feel, however, little doubt that, +according to the researches of M. Elie de Beaumont, their average +inclination is greater than that which they could have acquired, +considering their thickness and compactness, by flowing down a sloping +surface. At St. Helena, and at St. Jago, the basaltic strata rest on +older and probably submarine beds of different composition. At all +three islands, deluges of more recent lavas have flowed from the centre +of the island, towards and between the basaltic mountains; and at St. +Helena the central platform has been filled up by them. All three +islands have been raised in mass. At Mauritius the sea, within a late +geological period, must have reached to the foot of the basaltic +mountains, as it now does at St. Helena; and at St. Jago it is cutting +back the intermediate plain towards them. In these three islands, but +especially at St. Jago and at Mauritius, when, standing on the summit +of one of the old basaltic mountains, one looks in vain towards the +centre of the island,—the point towards which the strata beneath one’s +feet, and of the mountains on each side, rudely converge,—for a source +whence these strata could have been erupted; but one sees only a vast +hollow platform stretched beneath, or piles of matter of more recent +origin. + +These basaltic mountains come, I presume, into the class of Craters of +elevation: it is immaterial whether the rings were ever completely +formed, for the portions which now exist have so uniform a structure, +that, if they do not form fragments of true craters, they cannot be +classed with ordinary lines of elevation. With respect to their origin, +after having read the works of Mr. Lyell (“Principles of Geology” fifth +edition volume 2 page 171.), and of MM. C. Prevost and Virlet, I cannot +believe that the great central hollows have been formed by a simple +dome-shaped elevation, and the consequent arching of the strata. On the +other hand, I have very great difficulty in admitting that these +basaltic mountains are merely the basal fragments of great volcanoes, +of which the summits have either been blown off, or more probably +swallowed up by subsidence. These rings are, in some instances, so +immense, as at St. Jago and at Mauritius, and their occurrence is so +frequent, that I can hardly persuade myself to adopt this explanation. +Moreover, I suspect that the following circumstances, from their +frequent concurrence, are someway connected together,—a connection not +implied in either of the above views: namely, first, the broken state +of the ring; showing that the now detached portions have been exposed +to great denudation, and in some cases, perhaps, rendering it probable +that the ring never was entire; secondly, the great amount of matter +erupted from the central area after or during the formation of the +ring; and thirdly, the elevation of the district in mass. As far as +relates to the inclination of the strata being greater than that which +the basal fragments of ordinary volcanoes would naturally possess, I +can readily believe that this inclination might have been slowly +acquired by that amount of elevation, of which, according to M. Elie de +Beaumont, the numerous upfilled fissures or dikes are the evidence and +the measure,—a view equally novel and important, which we owe to the +researches of that geologist on Mount Etna. + +A conjecture, including the above circumstances, occurred to me, when,— +with my mind fully convinced, from the phenomena of 1835 in South +America, that the forces which eject matter from volcanic orifices and +raise continents in mass are identical,—I viewed that part of the coast +of St. Jago, where the horizontally upraised, calcareous stratum dips +into the sea, directly beneath a cone of subsequently erupted lava. (I +have given a detailed account of these phenomena, in a paper read +before the Geological Society in March 1838. At the instant of time, +when an immense area was convulsed and a large tract elevated, the +districts immediately surrounding several of the great vents in the +Cordillera remained quiescent; the subterranean forces being apparently +relieved by the eruptions, which then recommenced with great violence. +An event of somewhat the same kind, but on an infinitely smaller scale, +appears to have taken place, according to Abich (“Views of Vesuvius” +plates 1 and 9), within the great crater of Vesuvius, where a platform +on one side of a fissure was raised in mass twenty feet, whilst on the +other side, a train of small volcanoes burst forth in eruption.) The +conjecture is that, during the slow elevation of a volcanic district or +island, in the centre of which one or more orifices continue open, and +thus relieve the subterranean forces, the borders are elevated more +than the central area; and that the portions thus upraised do not slope +gently into the central, less elevated area, as does the calcareous +stratum under the cone at St. Jago, and as does a large part of the +circumference of Iceland, but that they are separated from it by curved +faults. (It appears, from information communicated to me in the most +obliging manner by M. E. Robert, that the circumferential parts of +Iceland, which are composed of ancient basaltic strata alternating with +tuff, dip inland, thus forming a gigantic saucer. M. Robert found that +this was the case, with a few and quite local exceptions, for a space +of coast several hundred miles in length. I find this statement +corroborated, as far as regards one place, by Mackenzie in his +“Travels” page 377, and in another place by some MS. notes kindly lent +me by Dr. Holland. The coast is deeply indented by creeks, at the head +of which the land is generally low. M. Robert informs me, that the +inwardly dipping strata appear to extend as far as this line, and that +their inclination usually corresponds with the slope of the surface, +from the high coast-mountains to the low land at the head of these +creeks. In the section described by Sir G. Mackenzie, the dip is 120. +The interior parts of the island chiefly consist, as far as is known, +of recently erupted matter. The great size, however, of Iceland, +equalling the bulkiest part of England, ought perhaps to exclude it +from the class of islands we have been considering; but I cannot avoid +suspecting that if the coast-mountains, instead of gently sloping into +the less elevated central area, had been separated from it by +irregularly curved faults, the strata would have been tilted seaward, +and a “Crater of elevation,” like that of St. Jago or that of +Mauritius, but of much vaster dimensions, would have been formed. I +will only further remark, that the frequent occurrence of extensive +lakes at the foot of large volcanoes, and the frequent association of +volcanic and fresh-water strata, seem to indicate that the areas around +volcanoes are apt to be depressed beneath the level of the adjoining +country, either from having been less elevated, or from the effects of +subsidence.) We might expect, from what we see along ordinary faults, +that the strata on the upraised side, already dipping outwards from +their original formation as lava-streams, would be tilted from the line +of fault, and thus have their inclination increased. According to this +hypothesis, which I am tempted to extend only to some few cases, it is +not probable that the ring would ever be formed quite perfect; and from +the elevation being slow, the upraised portions would generally be +exposed to much denudation, and hence the ring become broken; we might +also expect to find occasional inequalities in the dip of the upraised +masses, as is the case at St. Jago. By this hypothesis the elevation of +the districts in mass, and the flowing of deluges of lava from the +central platforms, are likewise connected together. On this view the +marginal basaltic mountains of the three foregoing islands might still +be considered as forming “Craters of elevation;” the kind of elevation +implied having been slow, and the central hollow or platform having +been formed, not by the arching of the surface, but simply by that part +having been upraised to a less height. + + + + +CHAPTER V. +GALAPAGOS ARCHIPELAGO. + + +Chatham Island. Craters composed of a peculiar kind of tuff. Small +basaltic craters, with hollows at their bases. Albemarle Island; fluid +lavas, their composition. Craters of tuff; inclination of their +exterior diverging strata, and structure of their interior converging +strata. James Island, segment of a small basaltic crater; fluidity and +composition of its lava-streams, and of its ejected fragments. +Concluding remarks on the craters of tuff, and on the breached +condition of their southern sides. Mineralogical composition of the +rocks of the archipelago. Elevation of the land. Direction of the +fissures of eruption. + +(FIGURE 11. MAP 3. GALAPAGOS ARCHIPELAGO. + +Showing Wenman, Abingdon, Bindloes, Tower, Narborough, Albemarle, +James, Indefatigable, Barrington, Chatham, Charles and Hood’s Islands.) + +This archipelago is situated under the equator, at a distance of +between five and six hundred miles from the west coast of South +America. It consists of five principal islands, and of several small +ones, which together are equal in area, but not in extent of land, to +Sicily, conjointly with the Ionian Islands. (I exclude from this +measurement, the small volcanic islands of Culpepper and Wenman, lying +seventy miles northward of the group. Craters were visible on all the +islands of the group, except on Towers Island, which is one of the +lowest; this island is, however, formed of volcanic rocks.) They are +all volcanic: on two, craters have been seen in eruption, and on +several of the other islands, streams of lava have a recent appearance. +The larger islands are chiefly composed of solid rock, and they rise +with a tame outline to a height of between one and four thousand feet. +They are sometimes, but not generally, surmounted by one principal +orifice. The craters vary in size from mere spiracles to huge caldrons +several miles in circumference; they are extraordinarily numerous, so +that I should think, if enumerated, they would be found to exceed two +thousand; they are formed either of scoriae and lava, or of a +brown-coloured tuff; and these latter craters are in several respects +remarkable. The whole group was surveyed by the officers of the +“Beagle.” I visited myself four of the principal islands, and received +specimens from all the others. Under the head of the different islands +I will describe only that which appears to me deserving of attention. + +CHATHAM ISLAND. CRATERS COMPOSED OF A SINGULAR KIND OF TUFF. + +Towards the eastern end of this island there occur two craters composed +of two kinds of tuff; one kind being friable, like slightly +consolidated ashes; and the other compact, and of a different nature +from anything which I have met with described. This latter substance, +where it is best characterised, is of a yellowish-brown colour, +translucent, and with a lustre somewhat resembling resin; it is +brittle, with an angular, rough, and very irregular fracture, +sometimes, however, being slightly granular, and even obscurely +crystalline: it can readily be scratched with a knife, yet some points +are hard enough just to mark common glass; it fuses with ease into a +blackish-green glass. The mass contains numerous broken crystals of +olivine and augite, and small particles of black and brown scoriae; it +is often traversed by thin seams of calcareous matter. It generally +affects a nodular or concretionary structure. In a hand specimen, this +substance would certainly be mistaken for a pale and peculiar variety +of pitchstone; but when seen in mass its stratification, and the +numerous layers of fragments of basalt, both angular and rounded, at +once render its subaqueous origin evident. 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 rocks; and this change could be distinctly traced in +different stages round the edges of even the same particle. The +position near the coast of all the craters composed of this kind of +tuff or peperino, and their breached condition, renders it probable +that they were all formed when standing immersed in the sea; +considering this circumstance, together with the remarkable absence of +large beds of ashes in the whole archipelago, I think it highly +probable that much the greater part of the tuff has originated from the +trituration of fragments of the grey, basaltic lavas in the mouths of +craters standing in the sea. 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 translucent, resin-like +fracture. Or has the associated lime played any part in this change? I +ask these questions from having found at St. Jago, in the Cape de Verde +Islands, that where a great stream of molten lava has flowed over a +calcareous bottom into the sea, the outermost film, which in other +parts resembles pitchstone, is changed, apparently by its contact with +the carbonate of lime, into a resin-like substance, precisely like the +best characterised specimens of the tuff from this archipelago. (The +concretions containing lime, which I have described at Ascension, as +formed in a bed of ashes, present some degree of resemblance to this +substance, but they have not a resinous fracture. At St. Helena, also, +I found veins of a somewhat similar, compact, but non- resinous +substance, occurring in a bed of pumiceous ashes, apparently free from +calcareous matter: in neither of these cases could heat have acted.) + +To return to the two craters: one of them stands at the distance of a +league from the coast, the intervening tract consisting of a calcareous +tuff, apparently of submarine origin. This crater consists of a circle +of hills some of which stand quite detached, but all have a very +regular, qua- qua versal dip, at an inclination of between thirty and +forty degrees. The lower beds, to the thickness of several hundred +feet, consist of the resin- like stone, with embedded fragments of +lava. The upper beds, which are between thirty and forty feet in +thickness, are composed of a thinly stratified, fine-grained, harsh, +friable, brown-coloured tuff, or peperino. (Those geologists who +restrict the term of “tuff” to ashes of a white colour, resulting from +the attrition of feldspathic lavas, would call these brown-coloured +strata “peperino.”) A central mass without any stratification, which +must formerly have occupied the hollow of the crater, but is now +attached only to a few of the circumferential hills, consists of a +tuff, intermediate in character between that with a resin-like, and +that with an earthy fracture. This mass contains white calcareous +matter in small patches. The second crater (520 feet in height) must +have existed until the eruption of a recent, great stream of lava, as a +separate islet; a fine section, worn by the sea, shows a grand +funnel-shaped mass of basalt, surrounded by steep, sloping flanks of +tuff, having in parts an earthy, and in others a semi-resinous +fracture. The tuff is traversed by several broad, vertical dikes, with +smooth and parallel sides, which I did not doubt were formed of basalt, +until I actually broke off fragments. These dikes, however, consist of +tuff like that of the surrounding strata, but more compact, and with a +smoother fracture; hence we must conclude, that fissures were formed +and filled up with the finer mud or tuff from the crater, before its +interior was occupied, as it now is, by a solidified pool of basalt. +Other fissures have been subsequently formed, parallel to these +singular dikes, and are merely filled with loose rubbish. The change +from ordinary scoriaceous particles to the substance with a +semi-resinous fracture, could be clearly followed in portions of the +compact tuff of these dikes. + +(FIGURE 12. THE KICKER ROCK, 400 FEET HIGH.) + +At the distance of a few miles from these two craters, stands the +Kicker Rock, or islet, remarkable from its singular form. It is +unstratified, and is composed of compact tuff, in parts having the +resin-like fracture. It is probable that this amorphous mass, like that +similar mass in the case first described, once filled up the central +hollow of a crater, and that its flanks, or sloping walls, have since +been worn quite away by the sea, in which it stands exposed. + +SMALL BASALTIC CRATERS. + +A bare, undulating tract, at the eastern end of Chatham Island, is +remarkable from the number, proximity, and form of the small basaltic +craters with which it is studded. They consist, either of a mere +conical pile, or, but less commonly, of a circle, of black and red, +glossy scoriae, partially cemented together. They vary in diameter from +thirty to one hundred and fifty yards, and rise from about fifty to one +hundred feet above the level of the surrounding plain. From one small +eminence, I counted sixty of these craters, all of which were within a +third of a mile from each other, and many were much closer. I measured +the distance between two very small craters, and found that it was only +thirty yards from the summit-rim of one to the rim of the other. Small +streams of black, basaltic lava, containing olivine and much glassy +feldspar, have flowed from many, but not from all of these craters. The +surfaces of the more recent streams were exceedingly rugged, and were +crossed by great fissures; the older streams were only a little less +rugged; and they were all blended and mingled together in complete +confusion. The different growth, however, of the trees on the streams, +often plainly marked their different ages. Had it not been for this +latter character, the streams could in few cases have been +distinguished; and, consequently, this wide undulatory tract might have +(as probably many tracts have) been erroneously considered as formed by +one great deluge of lava, instead of by a multitude of small streams, +erupted from many small orifices. + +In several parts of this tract, and especially at the base of the small +craters, there are circular pits, with perpendicular sides, from twenty +to forty feet deep. At the foot of one small crater, there were three +of these pits. They have probably been formed, by the falling in of the +roofs of small caverns. (M. Elie de Beaumont has described (“Mem. pour +servir” etc. tome 4 page 113) many “petits cirques d’eboulement” on +Etna, of some of which the origin is historically known.) In other +parts, there are mammiform hillocks, which resemble great bubbles of +lava, with their summits fissured by irregular cracks, which appeared, +upon entering them, to be very deep; lava has not flowed from these +hillocks. There are, also, other very regular, mammiform hillocks, +composed of stratified lava, and surmounted by circular, steep-sided +hollows, which, I suppose have been formed by a body of gas, first, +arching the strata into one of the bubble- like hillocks, and then, +blowing off its summit. These several kinds of hillocks and pits, as +well as the numerous, small, scoriaceous craters, all show that this +tract has been penetrated, almost like a sieve, by the passage of +heated vapours. The more regular hillocks could only have been heaved +up, whilst the lava was in a softened state. (Sir G. Mackenzie “Travels +in Iceland” pages 389 to 392, has described a plain of lava at the foot +of Hecla, everywhere heaved up into great bubbles or blisters. Sir +George states that this cavernous lava composes the uppermost stratum; +and the same fact is affirmed by Von Buch “Descript. des Isles +Canaries” page 159, with respect to the basaltic stream near Rialejo, +in Teneriffe. It appears singular that it should be the upper streams +that are chiefly cavernous, for one sees no reason why the upper and +lower should not have been equally affected at different times;—have +the inferior streams flowed beneath the pressure of the sea, and thus +been flattened, after the passage through them, of bodies of gas?) + +ALBEMARLE ISLAND. + +This island consists of five, great, flat-topped craters, which, +together with the one on the adjoining island of Narborough, singularly +resemble each other, in form and height. The southern one is 4,700 feet +high, two others are 3,720 feet, a third only 50 feet higher, and the +remaining ones apparently of nearly the same height. Three of these are +situated on one line, and their craters appear elongated in nearly the +same direction. The northern crater, which is not the largest, was +found by the triangulation to measure, externally, no less than three +miles and one-eighth of a mile in diameter. Over the lips of these +great, broad caldrons, and from little orifices near their summits, +deluges of black lava have flowed down their naked sides. + +FLUIDITY OF DIFFERENT LAVAS. + +Near Tagus or Banks’ Cove, I examined one of these great streams of +lava, which is remarkable from the evidence of its former high degree +of fluidity, especially when its composition is considered. Near the +sea-coast this stream is several miles in width. It consists of a +black, compact base, easily fusible into a black bead, with angular and +not very numerous air-cells, and thickly studded with large, fractured +crystals of glassy albite, varying from the tenth of an inch to half an +inch in diameter. (In the Cordillera of Chile, I have seen lava very +closely resembling this variety at the Galapagos Archipelago. It +contained, however, besides the albite, well-formed crystals of augite, +and the base (perhaps in consequence of the aggregation of the augitic +particles) was a shade lighter in colour. I may here remark, that in +all these cases, I call the feldspathic crystals, “albite,” from their +cleavage-planes (as measured by the reflecting goniometer) +corresponding with those of that mineral. As, however, other species of +this genus have lately been discovered to cleave in nearly the same +planes with albite, this determination must be considered as only +provisional. I examined the crystals in the lavas of many different +parts of the Galapagos group, and I found that none of them, with the +exception of some crystals from one part of James Island, cleaved in +the direction of orthite or potash-feldspar.) This lava, although at +first sight appearing eminently porphyritic, cannot properly be +considered so, for the crystals have evidently been enveloped, rounded, +and penetrated by the lava, like fragments of foreign rock in a +trap-dike. This was very clear in some specimens of a similar lava, +from Abingdon Island, in which the only difference was, that the +vesicles were spherical and more numerous. The albite in these lavas is +in a similar condition with the leucite of Vesuvius, and with the +olivine, described by Von Buch, as projecting in great balls from the +basalt of Lanzarote. (“Description des Isles Canaries” page 295.) +Besides the albite, this lava contains scattered grains of a green +mineral, with no distinct cleavage, and closely resembling olivine +(Humboldt mentions that he mistook a green augitic mineral, occurring +in the volcanic rocks of the Cordillera of Quito, for olivine.); but as +it fuses easily into a green glass, it belongs probably to the augitic +family: at James Island, however, a similar lava contained true +olivine. I obtained specimens from the actual surface, and from a depth +of four feet, but they differed in no respect. The high degree of +fluidity of this lava-stream was at once evident, from its smooth and +gently sloping surface, from the manner in which the main stream was +divided by small inequalities into little rills, and especially from +the manner in which its edges, far below its source, and where it must +have been in some degree cooled, thinned out to almost nothing; the +actual margin consisting of loose fragments, few of which were larger +than a man’s head. The contrast between this margin, and the steep +walls, above twenty feet high, bounding many of the basaltic streams at +Ascension, is very remarkable. It has generally been supposed that +lavas abounding with large crystals, and including angular vesicles, +have possessed little fluidity; but we see that the case has been very +different at Albemarle Island. (The irregular and angular form of the +vesicles is probably caused by the unequal yielding of a mass composed, +in almost equal proportion, of solid crystals and of a viscid base. It +certainly seems a general circumstance, as might have been expected, +that in lava, which has possessed a high degree of fluidity, AS WELL AS +AN EVEN-SIZED GRAIN, the vesicles are internally smooth and spherical.) +The degree of fluidity in different lavas, does not seem to correspond +with any APPARENT corresponding amount of difference in their +composition: at Chatham Island, some streams, containing much glassy +albite and some olivine, are so rugged, that they may be compared to a +sea frozen during a storm; whilst the great stream at Albemarle Island +is almost as smooth as a lake when ruffled by a breeze. At James +Island, black basaltic lava, abounding with small grains of olivine, +presents an intermediate degree of roughness; its surface being glossy, +and the detached fragments resembling, in a very singular manner, folds +of drapery, cables, and pieces of the bark of trees. (A specimen of +basaltic lava, with a few small broken crystals of albite, given me by +one of the officers, is perhaps worthy of description. It consists of +cylindrical ramifications, some of which are only the twentieth of an +inch in diameter, and are drawn out into the sharpest points. The mass +has not been formed like a stalactite, for the points terminate both +upwards and downwards. Globules, only the fortieth of an inch in +diameter, have dropped from some of the points, and adhere to the +adjoining branches. The lava is vesicular, but the vesicles never reach +the surface of the branches, which are smooth and glossy. As it is +generally supposed that vesicles are always elongated in the direction +of the movement of the fluid mass, I may observe, that in these +cylindrical branches, which vary from a quarter to only the twentieth +of an inch in diameter, every air-cell is spherical.) + +CRATERS OF TUFF. + +About a mile southward of Banks’ Cove, there is a fine elliptic crater, +about five hundred feet in depth, and three-quarters of a mile in +diameter. Its bottom is occupied by a lake of brine, out of which some +little crateriform hills of tuff rise. The lower beds are formed of +compact tuff, appearing like a subaqueous deposit; whilst the upper +beds, round the entire circumference, consist of a harsh, friable tuff, +of little specific gravity, but often containing fragments of rock in +layers. This upper tuff contains numerous pisolitic balls, about the +size of small bullets, which differ from the surrounding matter, only +in being slightly harder and finer grained. The beds dip away very +regularly on all sides, at angles varying, as I found by measurement, +from twenty-five to thirty degrees. The external surface of the crater +slopes at a nearly similar inclination, and is formed by slightly +convex ribs, like those on the shell of a pecten or scallop, which +become broader as they extend from the mouth of the crater to its base. +These ribs are generally from eight to twenty feet in breadth, but +sometimes they are as much as forty feet broad; and they resemble old, +plastered, much flattened vaults, with the plaster scaling off in +plates: they are separated from each other by gullies, deepened by +alluvial action. At their upper and narrow ends, near the mouth of the +crater, these ribs often consist of real hollow passages, like, but +rather smaller than, those often formed by the cooling of the crust of +a lava-stream, whilst the inner parts have flowed onward;—of which +structure I saw many examples at Chatham Island. There can be no doubt +but that these hollow ribs or vaults have been formed in a similar +manner, namely, by the setting or hardening of a superficial crust on +streams of mud, which have flowed down from the upper part of the +crater. In another part of this same crater, I saw open concave gutters +between one and two feet wide, which appear to have been formed by the +hardening of the lower surface of a mud stream, instead of, as in the +former case, of the upper surface. From these facts I think it is +certain that the tuff must have flowed as mud. (This conclusion is of +some interest, because M. Dufrenoy “Mem. pour servir” tome 4 page 274, +has argued from strata of tuff, apparently of similar composition with +that here described, being inclined at angles between 18 degrees and 20 +degrees, that Monte Nuevo and some other craters of Southern Italy have +been formed by upheaval. From the facts given above, of the vaulted +character of the separate rills, and from the tuff not extending in +horizontal sheets round these crateriform hills, no one will suppose +that the strata have here been produced by elevation; and yet we see +that their inclination is above 20 degrees, and often as much as 30 +degrees. The consolidated strata also, of the internal talus, as will +be immediately seen, dips at an angle of above 30 degrees.) This mud +may have been formed either within the crater, or from ashes deposited +on its upper parts, and afterwards washed down by torrents of rain. The +former method, in most of the cases, appears the more probable one; at +James Island, however, some beds of the friable kind of tuff extend so +continuously over an uneven surface, that probably they were formed by +the falling of showers of ashes. + +Within this same crater, strata of coarse tuff, chiefly composed of +fragments of lava, abut, like a consolidated talus, against the inside +walls. They rise to a height of between one hundred and one hundred and +fifty feet above the surface of the internal brine-lake; they dip +inwards, and are inclined at an angle varying from thirty to thirty-six +degrees. They appear to have been formed beneath water, probably at a +period when the sea occupied the hollow of the crater. I was surprised +to observe that beds having this great inclination did not, as far as +they could be followed, thicken towards their lower extremities. + +BANKS’ COVE. + +(FIGURE 13. A SECTIONAL SKETCH OF THE HEADLANDS FORMING BANKS’ COVE, +showing the diverging crateriform strata, and the converging stratified +talus. The highest point of these hills is 817 feet above the sea.) + +This harbour occupies part of the interior of a shattered crater of +tuff larger than that last described. All the tuff is compact, and +includes numerous fragments of lava; it appears like a subaqueous +deposit. The most remarkable feature in this crater is the great +development of strata converging inwards, as in the last case, at a +considerable inclination, and often deposited in irregular curved +layers. These interior converging beds, as well as the proper, +diverging crateriform strata, are represented in Figure 13, a rude, +sectional sketch of the headlands, forming this Cove. The internal and +external strata differ little in composition, and the former have +evidently resulted from the wear and tear, and redeposition of the +matter forming the external crateriform strata. From the great +development of these inner beds, a person walking round the rim of this +crater might fancy himself on a circular anticlinal ridge of stratified +sandstone and conglomerate. The sea is wearing away the inner and outer +strata, and especially the latter; so that the inwardly converging +strata will, perhaps, in some future age, be left standing alone—a case +which might at first perplex a geologist. (I believe that this case +actually occurs in the Azores, where Dr. Webster “Description” page +185, has described a basin-formed, little island, composed of STRATA OF +TUFF, dipping inwards and bounded externally by steep sea-worn cliffs. +Dr. Daubeny supposes “Volcanoes” page 266, that this cavity must have +been formed by a circular subsidence. It appears to me far more +probable, that we here have strata which were originally deposited +within the hollow of a crater, of which the exterior walls have since +been removed by the sea.) + +JAMES ISLAND. + +Two craters of tuff on this island are the only remaining ones which +require any notice. One of them lies a mile and a half inland from +Puerto Grande: it is circular, about the third of a mile in diameter, +and 400 feet in depth. It differs from all the other tuff-craters which +I examined, in having the lower part of its cavity, to the height of +between one hundred and one hundred and fifty feet, formed by a +precipitous wall of basalt, giving to the crater the appearance of +having burst through a solid sheet of rock. The upper part of this +crater consists of strata of the altered tuff, with a semi-resinous +fracture. Its bottom is occupied by a shallow lake of brine, covering +layers of salt, which rest on deep black mud. The other crater lies at +the distance of a few miles, and is only remarkable from its size and +perfect condition. Its summit is 1,200 feet above the level of the sea, +and the interior hollow is 600 feet deep. Its external sloping surface +presented a curious appearance from the smoothness of the wide layers +of tuff, which resembled a vast plastered floor. Brattle Island is, I +believe, the largest crater in the Archipelago composed of tuff; its +interior diameter is nearly a nautical mile. At present it is in a +ruined condition, consisting of little more than half a circle open to +the south; its great size is probably due, in part, to internal +degradation, from the action of the sea. + +SEGMENT OF A BASALTIC CRATER. + +(FIGURE 14. SEGMENT OF A VERY SMALL ORIFICE OF ERUPTION, on the beach +of Fresh-water Bay.) + +One side of Fresh-water Bay, in James Island, is bounded by a +promontory, which forms the last wreck of a great crater. On the beach +of this promontory, a quadrant-shaped segment of a small subordinate +point of eruption stands exposed. It consists of nine separate little +streams of lava piled upon each other; and of an irregular pinnacle, +about fifteen feet high, of reddish-brown, vesicular basalt, abounding +with large crystals of glassy albite, and with fused augite. This +pinnacle, and some adjoining paps of rock on the beach, represent the +axis of the crater. The streams of lava can be followed up a little +ravine, at right angles to the coast, for between ten and fifteen +yards, where they are hidden by detritus: along the beach they are +visible for nearly eighty yards, and I do not believe that they extend +much further. The three lower streams are united to the pinnacle; and +at the point of junction (as shown in Figure 14, a rude sketch made on +the spot), they are slightly arched, as if in the act of flowing over +the lip of the crater. The six upper streams no doubt were originally +united to this same column before it was worn down by the sea. The lava +of these streams is of similar composition with that of the pinnacle, +excepting that the crystals of albite appear to be more comminuted, and +the grains of fused augite are absent. Each stream is separated from +the one above it by a few inches, or at most by one or two feet in +thickness, of loose fragmentary scoriae, apparently derived from the +abrasion of the streams in passing over each other. All these streams +are very remarkable from their thinness. I carefully measured several +of them; one was eight inches thick, but was firmly coated with three +inches above, and three inches below, of red scoriaceous rock (which is +the case with all the streams), making altogether a thickness of +fourteen inches: this thickness was preserved quite uniformly along the +entire length of the section. A second stream was only eight inches +thick, including both the upper and lower scoriaceous surfaces. Until +examining this section, I had not thought it possible that lava could +have flowed in such uniformly thin sheets over a surface far from +smooth. These little streams closely resemble in composition that great +deluge of lava at Albemarle Island, which likewise must have possessed +a high degree of fluidity. + +PSEUDO-EXTRANEOUS, EJECTED FRAGMENTS. + +In the lava and in the scoriae of this little crater, I found several +fragments, which, from their angular form, their granular structure, +their freedom from air-cells, their brittle and burnt condition, +closely resembled those fragments of primary rocks which are +occasionally ejected, as at Ascension, from volcanoes. These fragments +consist of glassy albite, much mackled, and with very imperfect +cleavages, mingled with semi-rounded grains, having tarnished, glossy +surfaces, of a steel-blue mineral. The crystals of albite are coated by +a red oxide of iron, appearing like a residual substance; and their +cleavage-planes also are sometimes separated by excessively fine layers +of this oxide, giving to the crystals the appearance of being ruled +like a glass micrometer. There was no quartz. The steel-blue mineral, +which is abundant in the pinnacle, but which disappears in the streams +derived from the pinnacle, has a fused appearance, and rarely presents +even a trace of cleavage; I obtained, however, one measurement, which +proved that it was augite; and in one other fragment, which differed +from the others, in being slightly cellular, and in gradually blending +into the surrounding matrix the small grains of this mineral were +tolerably well crystallised. Although there is so wide a difference in +appearance between the lava of the little streams, and especially of +their red scoriaceous crusts, and one of these angular ejected +fragments, which at first sight might readily be mistaken for syenite, +yet I believe that the lava has originated from the melting and +movement of a mass of rock of absolutely similar composition with the +fragments. Besides the specimen above alluded to, in which we see a +fragment becoming slightly cellular, and blending into the surrounding +matrix, some of the grains of the steel-blue augite also have their +surfaces becoming very finely vesicular, and passing into the nature of +the surrounding paste; other grains are throughout, in an intermediate +condition. The paste seems to consist of the augite more perfectly +fused, or, more probably, merely disturbed in its softened state by the +movement of the mass, and mingled with the oxide of iron and with +finely comminuted, glassy albite. Hence probably it is that the fused +albite, which is abundant in the pinnacle, disappears in the streams. +The albite is in exactly the same state, with the exception of most of +the crystals being smaller in the lava and in the embedded fragments; +but in the fragments they appear to be less abundant: this, however, +would naturally happen from the intumescence of the augitic base, and +its consequent apparent increase in bulk. It is interesting thus to +trace the steps by which a compact granular rock becomes converted into +a vesicular, pseudo-porphyritic lava, and finally into red scoriae. 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 action, or more probably of the crust of a body of +cooled and crystallised lava, which has afterwards been broken up and +re-liquified; the crust being less acted on by the renewed heat and +movement. + +CONCLUDING REMARKS ON THE TUFF-CRATERS. + +These craters, from the peculiarity of the resin-like substance which +enters largely into their composition, from their structure, their size +and number, present the most striking feature in the geology of this +Archipelago. The majority of them form either separate islets, or +promontories attached to the larger islands; and those which now stand +at some little distance from the coast are worn and breached, as if by +the action of the sea. From this general circumstance of their +position, and from the small quantity of ejected ashes in any part of +the Archipelago, I am led to conclude, that the tuff has been chiefly +produced, by the grinding together of fragments of lava within active +craters, communicating with the sea. In the origin and composition of +the tuff, and in the frequent presence of a central lake of brine and +of layers of salt, these craters resemble, though on a gigantic scale, +the “salses,” or hillocks of mud, which are common in some parts of +Italy and in other countries. (D’Aubuisson “Traite de Geognosie” tome 1 +page 189. I may remark, that I saw at Terceira, in the Azores, a crater +of tuff or peperino, very similar to these of the Galapagos +Archipelago. From the description given in Freycinet “Voyage,” similar +ones occur at the Sandwich Islands; and probably they are present in +many other places.) Their closer connection, however, in this +Archipelago, with ordinary volcanic action, is shown by the pools of +solidified basalt, with which they are sometimes filled up. + +It at first appears very singular, that all the craters formed of tuff +have their southern sides, either quite broken down and wholly removed, +or much lower than the other sides. I saw and received accounts of +twenty-eight of these craters; of these, twelve form separate islets +(These consist of the three Crossman Islets, the largest of which is +600 feet in height; Enchanted Island; Gardner Island (760 feet high); +Champion Island (331 feet high); Enderby Island; Brattle Island; two +islets near Indefatigable Island; and one near James Island. A second +crater near James Island (with a salt lake in its centre) has its +southern side only about twenty feet high, whilst the other parts of +the circumference are about three hundred feet in height.), and now +exist as mere crescents quite open to the south, with occasionally a +few points of rock marking their former circumference: of the remaining +sixteen, some form promontories, and others stand at a little distance +inland from the shore; but all have their southern sides either the +lowest, or quite broken down. Two, however, of the sixteen had their +northern sides also low, whilst their eastern and western sides were +perfect. I did not see, or hear of, a single exception to the rule, of +these craters being broken down or low on the side, which faces a point +of the horizon between S.E. and S.W. This rule does not apply to +craters composed of lava and scoriae. The explanation is simple: at +this Archipelago, the waves from the trade-wind, and the swell +propagated from the distant parts of the open ocean, coincide in +direction (which is not the case in many parts of the Pacific), and +with their united forces attack the southern sides of all the islands; +and consequently the southern slope, even when entirely formed of hard +basaltic rock, is invariably steeper than the northern slope. As the +tuff-craters are composed of a soft material, and as probably all, or +nearly all, have at some period stood immersed in the sea, we need not +wonder that they should invariably exhibit on their exposed sides the +effects of this great denuding power. Judging from the worn condition +of many of these craters, it is probable that some have been entirely +washed away. As there is no reason to suppose, that the craters formed +of scoriae and lava were erupted whilst standing in the sea, we can see +why the rule does not apply to them. At Ascension, it was shown that +the mouths of the craters, which are there all of terrestrial origin, +have been affected by the trade-wind; and this same power might here, +also, aid in making the windward and exposed sides of some of the +craters originally the lowest. + +MINERALOGICAL COMPOSITION OF THE ROCKS. + +In the northern islands, the basaltic lavas seem generally to contain +more albite than they do in the southern half of the Archipelago; but +almost all the streams contain some. The albite is not unfrequently +associated with olivine. I did not observe in any specimen +distinguishable crystals of hornblende or augite; I except the fused +grains in the ejected fragments, and in the pinnacle of the little +crater, above described. I did not meet with a single specimen of true +trachyte; though some of the paler lavas, when abounding with large +crystals of the harsh and glassy albite, resemble in some degree this +rock; but in every case the basis fuses into a black enamel. Beds of +ashes and far-ejected scoriae, as previously stated, are almost absent; +nor did I see a fragment of obsidian or of pumice. Von Buch believes +that the absence of pumice on Mount Etna is consequent on the feldspar +being of the Labrador variety (“Description des Isles Canaries” page +328.); if the presence of pumice depends on the constitution of the +feldspar, it is remarkable, that it should be absent in this +archipelago, and abundant in the Cordillera of South America, in both +of which regions the feldspar is of the albitic variety. Owing to the +absence of ashes, and the general indecomposable character of the lava +in this Archipelago, the islands are slowly clothed with a poor +vegetation, and the scenery has a desolate and frightful aspect. + +ELEVATION OF THE LAND. + +Proofs of the rising of the land are scanty and imperfect. At Chatham +Island, I noticed some great blocks of lava, cemented by calcareous +matter, containing recent shells; but they occurred at the height of +only a few feet above high-water mark. One of the officers gave me some +fragments of shells, which he found embedded several hundred feet above +the sea, in the tuff of two craters, distant from each other. It is +possible, that these fragments may have been carried up to their +present height in an eruption of mud; but as, in one instance, they +were associated with broken oyster- shells, almost forming a layer, it +is more probable that the tuff was uplifted with the shells in mass. +The specimens are so imperfect that they can be recognised only as +belonging to recent marine genera. On Charles Island, I observed a line +of great rounded blocks, piled on the summit of a vertical cliff, at +the height of fifteen feet above the line, where the sea now acts +during the heaviest gales. This appeared, at first, good evidence in +favour of the elevation of the land; but it was quite deceptive, for I +afterwards saw on an adjoining part of this same coast, and heard from +eye- witnesses, that wherever a recent stream of lava forms a smooth +inclined plane, entering the sea, the waves during gales have the power +of ROLLING UP ROUNDED blocks to a great height, above the line of their +ordinary action. As the little cliff in the foregoing case is formed by +a stream of lava, which, before being worn back, must have entered the +sea with a gently sloping surface, it is possible or rather it is +probable, that the rounded boulders, now lying on its summit, are +merely the remnants of those which had been ROLLED UP during storms to +their present height. + +DIRECTION OF THE FISSURES OF ERUPTION. + +The volcanic orifices in this group cannot be considered as +indiscriminately scattered. Three great craters on Albermarle Island +form a well-marked line, extending N.W. by N. and S.E. by S. Narborough +Island, and the great crater on the rectangular projection of Albemarle +Island, form a second parallel line. To the east, Hood’s Island, and +the islands and rocks between it and James Island, form another nearly +parallel line, which, when prolonged, includes Culpepper and Wenman +Islands, lying seventy miles to the north. The other islands lying +further eastward, form a less regular fourth line. Several of these +islands, and the vents on Albemarle Island, are so placed, that they +likewise fall on a set of rudely parallel lines, intersecting the +former lines at right angles; so that the principal craters appear to +lie on the points where two sets of fissures cross each other. The +islands themselves, with the exception of Albemarle Island, are not +elongated in the same direction with the lines on which they stand. 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 Ocean. Finally, I may remark, that amongst the Galapagos +Islands there is no one dominant vent much higher than all the others, +as may be observed in many volcanic archipelagoes: the highest is the +great mound on the south- western extremity of Albemarle Island, which +exceeds by barely a thousand feet several other neighbouring craters. + + + + +CHAPTER VI. +TRACHYTE AND BASALT.—DISTRIBUTION OF VOLCANIC ISLES. + + +The sinking of crystals in fluid lava. Specific gravity of the +constituent parts of trachyte and of basalt, and their consequent +separation. Obsidian. Apparent non-separation of the elements of +plutonic rocks. Origin of trap-dikes in the plutonic series. +Distribution of volcanic islands; their prevalence in the great oceans. +They are generally arranged in lines. The central volcanoes of Von Buch +doubtful. Volcanic islands bordering continents. Antiquity of volcanic +islands, and their elevation in mass. Eruptions on parallel lines of +fissure within the same geological period. + +ON THE SEPARATION OF THE CONSTITUENT MINERALS OF LAVA, ACCORDING TO +THEIR SPECIFIC GRAVITIES. + +One side of Fresh-water Bay, in James Island, is formed by the wreck of +a large crater, mentioned in the last chapter, of which the interior +has been filled up by a pool of basalt, about two hundred feet in +thickness. This basalt is of a grey colour, and contains many crystals +of glassy albite, which become much more numerous in the lower, +scoriaceous part. This is contrary to what might have been expected, +for if the crystals had been originally disseminated in equal numbers, +the greater intumescence of this lower scoriaceous part would have made +them appear fewer in number. Von Buch has described a stream of +obsidian on the Peak of Teneriffe, in which the crystals of feldspar +become more and more numerous, as the depth or thickness increases, so +that near the lower surface of the stream the lava even resembles a +primary rock. (“Description des Isles Canaries” pages 190 and 191.) Von +Buch further states, that M. Dree, in his experiments in melting lava, +found that the crystals of feldspar always tended to precipitate +themselves to the bottom of the crucible. In these cases, I presume +there can be no doubt that the crystals sink from their weight. (In a +mass of molten iron, it is found (“Edinburgh New Philosophical Journal” +volume 24 page 66) that the substances, which have a closer affinity +for oxygen than iron has, rise from the interior of the mass to the +surface. But a similar cause can hardly apply to the separation of the +crystals of these lava-streams. The cooling of the surface of lava +seems, in some cases, to have affected its composition; for Dufrenoy +(“Mem. pour servir” tome 4 page 271) found that the interior parts of a +stream near Naples contained two-thirds of a mineral which was acted on +by acids, whilst the surface consisted chiefly of a mineral +unattackable by acids.) The specific gravity of feldspar varies from +2.4 to 2.58, whilst obsidian seems commonly to be from 2.3 to 2.4; and +in a fluidified state its specific gravity would probably be less, +which would facilitate the sinking of the crystals of feldspar. (I have +taken the specific gravities of the simple minerals from Von Kobell, +one of the latest and best authorities, and of the rocks from various +authorities. Obsidian, according to Phillips, is 2.35; and Jameson says +it never exceeds 2.4; but a specimen from Ascension, weighed by myself, +was 2.42.) At James Island, the crystals of albite, though no doubt of +less weight than the grey basalt, in the parts where compact, might +easily be of greater specific gravity than the scoriaceous mass, formed +of melted lava and bubbles of heated gas. + +The sinking of crystals through a viscid substance like molten rock, as +is unequivocally shown to have been the case in the experiments of M. +Dree, is worthy of further consideration, as throwing light on the +separation of the trachytic and basaltic series of lavas. Mr. P. Scrope +has speculated on this subject; but he does not seem to have been aware +of any positive facts, such as those above given; and he has overlooked +one very necessary element, as it appears to me, in the +phenomenon—namely, the existence of either the lighter or heavier +mineral in globules or in crystals. In a substance of imperfect +fluidity, like molten rock, it is hardly credible, that the separate, +infinitely small atoms, whether of feldspar, augite, or of any other +mineral, would have power from their slightly different gravities to +overcome the friction caused by their movement; but if the atoms of any +one of these minerals became, whilst the others remained fluid, united +into crystals or granules, it is easy to perceive that from the +lessened friction, their sinking or floating power would be greatly +increased. On the other hand, if all the minerals became granulated at +the same time, it is scarcely possible, from their mutual resistance, +that any separation could take place. A valuable, practical discovery, +illustrating the effect of the granulation of one element in a fluid +mass, in aiding its separation, has lately been made: when lead +containing a small proportion of silver, is constantly stirred whilst +cooling, it becomes granulated, and the grains of imperfect crystals of +nearly pure lead sink to the bottom, leaving a residue of melted metal +much richer in silver; whereas if the mixture be left undisturbed, +although kept fluid for a length of time, the two metals show no signs +of separating. (A full and interesting account of this discovery, by +Mr. Pattinson, was read before the British Association in September +1838. In some alloys, according to Turner “Chemistry” page 210, the +heaviest metal sinks, and it appears that this takes place whilst both +metals are fluid. Where there is a considerable difference in gravity, +as between iron and the slag formed during the fusion of the ore, we +need not be surprised at the atoms separating, without either substance +being granulated.) The sole use of the stirring seems to be, the +formation of detached granules. The specific gravity of silver is 10.4, +and of lead 11.35: the granulated lead, which sinks, is never +absolutely pure, and the residual fluid metal contains, when richest, +only 1/119 part of silver. As the difference in specific gravity, +caused by the different proportions of the two metals, is so +exceedingly small, the separation is probably aided in a great degree +by the difference in gravity between the lead, when granular though +still hot, and when fluid. + +In a body of liquified volcanic rock, left for some time without any +violent disturbance, we might expect, in accordance with the above +facts, that if one of the constituent minerals became aggregated into +crystals or granules, or had been enveloped in this state from some +previously existing mass, such crystals or granules would rise or sink, +according to their specific gravity. Now we have plain evidence of +crystals being embedded in many lavas, whilst the paste or basis has +continued fluid. I need only refer, as instances, to the several, +great, pseudo-porphyritic streams at the Galapagos Islands, and to the +trachytic streams in many parts of the world, in which we find crystals +of feldspar bent and broken by the movement of the surrounding, +semi-fluid matter. Lavas are chiefly composed of three varieties of +feldspar, varying in specific gravity from 2.4 to 2.74; of hornblende +and augite, varying from 3.0 to 3.4; of olivine, varying from 3.3 to +3.4; and lastly, of oxides of iron, with specific gravities from 4.8 to +5.2. Hence crystals of feldspar, enveloped in a mass of liquified, but +not highly vesicular lava, would tend to rise to the upper parts; and +crystals or granules of the other minerals, thus enveloped, would tend +to sink. We ought not, however, to expect any perfect degree of +separation in such viscid materials. Trachyte, which consists chiefly +of feldspar, with some hornblende and oxide of iron, has a specific +gravity of about 2.45; whilst basalt, composed chiefly of augite and +feldspar, often with much iron and olivine, has a gravity of about 3.0. +(Trachyte from Java was found by Von Buch to be 2.47; from Auvergne, by +De la Beche, it was 2.42; from Ascension, by myself, it was 2.42. +Jameson and other authors give to basalt a specific gravity of 3.0; but +specimens from Auvergne were found, by De la Beche, to be only 2.78; +and from the Giant’s Causeway, to be 2.91.) Accordingly we find, that +where both trachytic and basaltic streams have proceeded from the same +orifice, the trachytic streams have generally been first erupted owing, +as we must suppose, to the molten lava of this series having +accumulated in the upper parts of the volcanic focus. This order of +eruption has been observed by Beudant, Scrope, and by other authors; +three instances, also, have been given in this volume. As the later +eruptions, however, from most volcanic mountains, burst through their +basal parts, owing to the increased height and weight of the internal +column of molten rock, we see why, in most cases, only the lower flanks +of the central, trachytic masses, are enveloped by basaltic streams. +The separation of the ingredients of a mass of lava, would, perhaps, +sometimes take place within the body of a volcanic mountain, if lofty +and of great dimensions, instead of within the underground focus; in +which case, trachytic streams might be poured forth, almost +contemporaneously, or at short recurrent intervals, from its summit, +and basaltic streams from its base: this seems to have taken place at +Teneriffe. (Consult Von Buch’s well-known and admirable “Description +Physique” of this island, which might serve as a model of descriptive +geology.) I need only further remark, that from violent disturbances +the separation of the two series, even under otherwise favourable +conditions, would naturally often be prevented, and likewise their +usual order of eruption be inverted. From the high degree of fluidity +of most basaltic lavas, these perhaps, alone, would in many cases reach +the surface. + +As we have seen that crystals of feldspar, in the instance described by +Von Buch, sink in obsidian, in accordance with their known greater +specific gravity, we might expect to find in every trachytic district, +where obsidian has flowed as lava, that it had proceeded from the upper +or highest orifices. This, according to Von Buch, holds good in a +remarkable manner both at the Lipari Islands and on the Peak of +Teneriffe; at this latter place obsidian has never flowed from a less +height than 9,200 feet. Obsidian, also, appears to have been erupted +from the loftiest peaks of the Peruvian Cordillera. I will only further +observe, that the specific gravity of quartz varies from 2.6 to 2.8; +and therefore, that when present in a volcanic focus, it would not tend +to sink with the basaltic bases; and this, perhaps, explains the +frequent presence, and the abundance of this mineral, in the lavas of +the trachytic series, as observed in previous parts of this volume. + +An objection to the foregoing theory will, perhaps, be drawn from the +plutonic rocks not being separated into two evidently distinct series, +of different specific gravities; although, like the volcanic, they have +been liquified. In answer, it may first be remarked, that we have no +evidence of the atoms of any one of the constituent minerals in the +plutonic series having been aggregated, whilst the others remained +fluid, which we have endeavoured to show is an almost necessary +condition of their separation; on the contrary, the crystals have +generally impressed each other with their forms. (The crystalline paste +of phonolite is frequently penetrated by long needles of hornblende; +from which it appears that the hornblende, though the more fusible +mineral, has crystallised before, or at the same time with a more +refractory substance. Phonolite, as far as my observations serve, in +every instance appears to be an injected rock, like those of the +plutonic series; hence probably, like these latter, it has generally +been cooled without repeated and violent disturbances. Those geologists +who have doubted whether granite could have been formed by igneous +liquefaction, because minerals of different degrees of fusibility +impress each other with their forms, could not have been aware of the +fact of crystallised hornblende penetrating phonolite, a rock +undoubtedly of igneous origin. The viscidity, which it is now known, +that both feldspar and quartz retain at a temperature much below their +points of fusion, easily explains their mutual impressment. Consult on +this subject Mr. Horner’s paper on Bonn “Geolog. Transact.” volume 4 +page 439; and “L’Institut” with respect to quartz 1839 page 161.) + +In the second place, the perfect tranquillity, under which it is +probable that the plutonic masses, buried at profound depths, have +cooled, would, most likely, be highly unfavourable to the separation of +their constituent minerals; for, if the attractive force, which during +the progressive cooling draws together the molecules of the different +minerals, has power sufficient to keep them together, the friction +between such half-formed crystals or pasty globules would effectually +prevent the heavier ones from sinking, or the lighter ones from rising. +On the other hand, a small amount of disturbance, which would probably +occur in most volcanic foci, and which we have seen does not prevent +the separation of granules of lead from a mixture of molten lead and +silver, or crystals of feldspar from streams of lava, by breaking and +dissolving the less perfectly formed globules, would permit the more +perfect and therefore unbroken crystals, to sink or rise, according to +their specific gravity. + +Although in plutonic rocks two distinct species, corresponding to the +trachytic and basaltic series, do not exist, I much suspect that a +certain amount of separation of their constituent parts has often taken +place. I suspect this from having observed how frequently dikes of +greenstone and basalt intersect widely extended formations of granite +and the allied metamorphic rocks. I have never examined a district in +an extensive granitic region without discovering dikes; I may instance +the numerous trap-dikes, in several districts of Brazil, Chile, and +Australia, and at the Cape of Good Hope: many dikes likewise occur in +the great granitic tracts of India, in the north of Europe, and in +other countries. Whence, then, has the greenstone and basalt, forming +these dikes, come? Are we to suppose, like some of the elder +geologists, that a zone of trap is uniformly spread out beneath the +granitic series, which composes, as far as we know, the foundations of +the earth’s crust? Is it not more probable, that these dikes have been +formed by fissures penetrating into partially cooled rocks of the +granitic and metamorphic series, and by their more fluid parts, +consisting chiefly of hornblende, oozing out, and being sucked into +such fissures? At Bahia, in Brazil, in a district composed of gneiss +and primitive greenstone, I saw many dikes, of a dark augitic (for one +crystal certainly was of this mineral) or hornblendic rock, which, as +several appearances clearly proved, either had been formed before the +surrounding mass had become solid, or had together with it been +afterwards thoroughly softened. (Portions of these dikes have been +broken off, and are now surrounded by the primary rocks, with their +laminae conformably winding round them. Dr. Hubbard also (“Silliman’s +Journal” volume 34 page 119), has described an interlacement of +trap-veins in the granite of the White Mountains, which he thinks must +have been formed when both rocks were soft.) On both sides of one of +these dikes, the gneiss was penetrated, to the distance of several +yards, by numerous, curvilinear threads or streaks of dark matter, +which resembled in form clouds of the class called cirrhi- comae; some +few of these threads could be traced to their junction with the dike. +When examining them, I doubted whether such hair-like and curvilinear +veins could have been injected, and I now suspect, that instead of +having been injected from the dike, they were its feeders. If the +foregoing views of the origin of trap-dikes in widely extended granitic +regions far from rocks of any other formation, be admitted as probable, +we may further admit, in the case of a great body of plutonic rock, +being impelled by repeated movements into the axis of a mountain-chain, +that its more liquid constituent parts might drain into deep and unseen +abysses; afterwards, perhaps, to be brought to the surface under the +form, either of injected masses of greenstone and augitic porphyry, or +of basaltic eruptions. (Mr. Phillips “Lardner’s Encyclop.” volume 2 +page 115 quotes Von Buch’s statement, that augitic porphyry ranges +parallel to, and is found constantly at the base of, great chains of +mountains. Humboldt, also, has remarked the frequent occurrence of +trap-rock, in a similar position; of which fact I have observed many +examples at the foot of the Chilian Cordillera. The existence of +granite in the axes of great mountain chains is always probable, and I +am tempted to suppose, that the laterally injected masses of augitic +porphyry and of trap, bear nearly the same relation to the granitic +axes which basaltic lavas bear to the central trachytic masses, round +the flanks of which they have so frequently been erupted.) Much of the +difficulty which geologists have experienced when they have compared +the composition of volcanic with plutonic formations, will, I think, be +removed, if we may believe that most plutonic masses have been, to a +certain extent, drained of those comparatively weighty and easily +liquified elements, which compose the trappean and basaltic series of +rocks. + +ON THE DISTRIBUTION OF VOLCANIC ISLANDS. + +During my investigations on coral-reefs, I had occasion to consult the +works of many voyagers, and I was invariably struck with the fact, that +with rare exceptions, the innumerable islands scattered throughout the +Pacific, Indian, and Atlantic Oceans, were composed either of volcanic, +or of modern coral-rocks. It would be tedious to give a long catalogue +of all the volcanic islands; but the exceptions which I have found are +easily enumerated: in the Atlantic, we have St. Paul’s Rock, described +in this volume, and the Falkland Islands, composed of quartz and +clay-slate; but these latter islands are of considerable size, and lie +not very far from the South American coast (Judging from Forster’s +imperfect observation, perhaps Georgia is not volcanic. Dr. Allan is my +informant with regard to the Seychelles. I do not know of what +formation Rodriguez, in the Indian Ocean, is composed.): in the Indian +Ocean, the Seychelles (situated in a line prolonged from Madagascar) +consist of granite and quartz: in the Pacific Ocean, New Caledonia, an +island of large size, belongs (as far as is known) to the primary +class. New Zealand, which contains much volcanic rock and some active +volcanoes, from its size cannot be classed with the small islands, +which we are now considering. The presence of a small quantity of +non-volcanic rock, as of clay-slate on three of the Azores (This is +stated on the authority of Count V. de Bedemar, with respect to Flores +and Graciosa (Charlsworth “Magazine of Nat. Hist.” volume 1 page 557). +St. Maria has no volcanic rock, according to Captain Boyd (Von Buch +“Descript.” page 365). Chatham Island has been described by Dr. +Dieffenbach in the “Geographical Journal” 1841 page 201. As yet we have +received only imperfect notices on Kerguelen Land, from the Antarctic +Expedition.), or of tertiary limestone at Madeira, or of clay-slate at +Chatham Island in the Pacific, or of lignite at Kerguelen Land, ought +not to exclude such islands or archipelagoes, if formed chiefly of +erupted matter, from the volcanic class. + +The composition of the numerous islands scattered through the great +oceans being with such rare exceptions volcanic, is evidently an +extension of that law, and the effect of those same causes, whether +chemical or mechanical, from which it results, that a vast majority of +the volcanoes now in action stand either as islands in the sea, or near +its shores. This fact of the ocean-islands being so generally volcanic +is also interesting in relation to the nature of the mountain-chains on +our continents, which are comparatively seldom volcanic; and yet we are +led to suppose that where our continents now stand an ocean once +extended. Do volcanic eruptions, we may ask, 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 land? + +Looking at the charts of the numerous volcanic archipelagoes, we see +that the islands are generally arranged either in single, double, or +triple rows, in lines which are frequently curved in a slight degree. +(Professors William and Henry Darwin Rogers have lately insisted much, +in a memoir read before the American Association, on the regularly +curved lines of elevation in parts of the Appalachian range.) Each +separate island is either rounded, or more generally elongated in the +same direction with the group in which it stands, but sometimes +transversely to it. Some of the groups which are not much elongated +present little symmetry in their forms; M. Virlet (“Bulletin de la Soc. +Geolog.” tome 3 page 110.) states that this is the case with the +Grecian Archipelago: in such groups I suspect (for I am aware how easy +it is to deceive oneself on these points), that the vents are generally +arranged on one line, or on a set of short parallel lines, intersecting +at nearly right angles another line, or set of lines. The Galapagos +Archipelago offers an example of this structure, 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 N.W. by N., and on another set +ranging about W.S.W.: in the Canary Archipelago we have a simpler +structure of the same kind: in the Cape de Verde group, which appears +to be the least symmetrical of any oceanic volcanic archipelago, a N.W. +and S.E. line formed by several islands, if prolonged, would intersect +at right angles a curved line, on which the remaining islands are +placed. + +Von Buch (“Description des Isles Canaries” page 324.) has classed all +volcanoes under two heads, namely, CENTRAL VOLCANOES, round which +numerous eruptions have taken place on all sides, in a manner almost +regular, and VOLCANIC CHAINS. In the examples given of the first class, +as far as position is concerned, I can see no grounds for their being +called “central;” and the evidence of any difference in mineralogical +nature between CENTRAL VOLCANOES and VOLCANIC CHAINS appears slight. No +doubt some one island in most small volcanic archipelagoes is apt to be +considerably higher than the others; and in a similar manner, whatever +the cause may be, that on the same island one vent is generally higher +than all the others. Von Buch does not include in his class of volcanic +chains small archipelagoes, in which the islands are admitted by him, +as at the Azores, to be arranged in lines; 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 line, and so on to a great wall like the Cordillera of +America, it is difficult to believe that there exists any essential +difference between short and long volcanic chains. Von Buch (Idem page +393.) states that his volcanic chains surmount, or are closely +connected with, mountain-ranges of primary formation: but if trains of +linear archipelagoes are, in the course of time, by the long- continued +action of the elevatory and volcanic forces, converted into +mountain-ranges, it would naturally result that the inferior primary +rocks would often be uplifted and brought into view. + +Some authors have remarked that volcanic islands occur scattered, +though at very unequal distances, along the shores of the great +continents, as if in some measure connected with them. In the case of +Juan Fernandez, situated 330 miles from the coast of Chile, there was +undoubtedly a connection between the volcanic forces acting under this +island and under the continent, as was shown during the earthquake of +1835. The islands, moreover, of some of the small volcanic groups which +thus border continents, are placed in lines, related to those along +which the adjoining shores of the continents trend; I may instance the +lines of intersection at the Galapagos, and at the Cape de Verde +Archipelagoes, and the best marked line of the Canary Islands. If these +facts be not merely accidental, we see that many scattered volcanic +islands and small groups are related not only by proximity, but in the +direction of the fissures of eruption to the neighbouring continents—a +relation, which Von Buch considers, characteristic of his great +volcanic chains. + +In volcanic archipelagoes, the orifices are seldom in activity on more +than one island at a time; and the greater eruptions usually recur only +after long intervals. Observing the number of craters, that are usually +found on each island of a group, and the vast amount of matter which +has been erupted from them, one is led to attribute a high antiquity +even to those groups, which appear, like the Galapagos, to be of +comparatively recent origin. This conclusion accords with the +prodigious amount of degradation, by the slow action of the sea, which +their originally sloping coasts must have suffered, when they are worn +back, as is so often the case, into grand precipices. We ought not, +however, to suppose, in hardly any instance, that the whole body of +matter, forming a volcanic island, has been erupted at the level, on +which it now stands: the number of dikes, which seem invariably to +intersect the interior parts of every volcano, show, on the principles +explained by M. Elie de Beaumont, that the whole mass has been uplifted +and fissured. A connection, moreover, between volcanic eruptions and +contemporaneous elevations in mass has, I think, been shown to exist in +my work on Coral-Reefs, both from the frequent presence of upraised +organic remains, and from the structure of the accompanying +coral-reefs. (A similar conclusion is forced on us, by the phenomena, +which accompanied the earthquake of 1835, at Concepcion, and which are +detailed in my paper (volume 5 page 601) in the “Geological +Transactions.”) Finally, I may remark, that in the same Archipelago, +eruptions have taken place within the historical period on more than +one of the parallel lines of fissure: thus, at the Galapagos +Archipelago, eruptions have taken place from a vent on Narborough +Island, and from one on Albemarle Island, which vents do not fall on +the same line; at the Canary Islands, eruptions have taken place in +Teneriffe and Lanzarote; and at the Azores, on the three parallel lines +of Pico, St. Jorge, and Terceira. Believing that a mountain-axis +differs essentially from a volcano, only in plutonic rocks having been +injected, instead of volcanic matter having been ejected, this appears +to me an interesting circumstance; for we may infer from it as +probable, that in the elevation of a mountain-chain, two or more of the +parallel lines forming it may be upraised and injected within the same +geological period. + + + + +CHAPTER VII. +AUSTRALIA; NEW ZEALAND; CAPE OF GOOD HOPE. + + +New South Wales. Sandstone formation. Embedded pseudo-fragments of +shale. Stratification. Current-cleavage. Great valleys. Van Diemen’s +Land. Palaeozoic formation. Newer formation with volcanic rocks. +Travertin with leaves of extinct plants. Elevation of the land. New +Zealand. King George’s Sound. Superficial ferruginous beds. Superficial +calcareous deposits, with casts of branches. Their origin from drifted +particles of shells and corals. Their extent. Cape of Good Hope. +Junction of the granite and clay-slate. Sandstone formation. + +The “Beagle,” in her homeward voyage, touched at New Zealand, +Australia, Van Diemen’s Land, and the Cape of Good Hope. In order to +confine the Third Part of these Geological Observations to South +America, I will here briefly describe all that I observed at these +places worthy of the attention of geologists. + +NEW SOUTH WALES. + +My opportunities of observation consisted of a ride of ninety +geographical miles to Bathurst, in a W.N.W. direction from Sydney. The +first thirty miles from the coast passes over a sandstone country, +broken up in many places by trap-rocks, and separated by a bold +escarpment overhanging the river Nepean, from the great sandstone +platform of the Blue Mountains. This upper platform is 1,000 feet high +at the edge of the escarpment, and rises in a distance of twenty-five +miles to between three and four thousand feet above the level of the +sea. At this distance the road descends to a country rather less +elevated, and composed in chief part of primary rocks. There is much +granite, in one part passing into a red porphyry with octagonal +crystals of quartz, and intersected in some places by trap-dikes. Near +the Downs of Bathurst I passed over much pale-brown, glossy clay-slate, +with the shattered laminae running north and south; I mention this +fact, because Captain King informs me that, in the country a hundred +miles southward, near Lake George, the mica-slate ranges so invariably +north and south that the inhabitants take advantage of it in finding +their way through the forests. + +The sandstone of the Blue Mountains is at least 1,200 feet thick, and +in some parts is apparently of greater thickness; it consists of small +grains of quartz, cemented by white earthy matter, and it abounds with +ferruginous veins. The lower beds sometimes alternate with shales and +coal: at Wolgan I found in carbonaceous shale leaves of the +Glossopteris Brownii, a fern which so frequently accompanies the coal +of Australia. The sandstone contains pebbles of quartz; and these +generally increase in number and size (seldom, however, exceeding an +inch or two in diameter) in the upper beds: I observed a similar +circumstance in the grand sandstone formation at the Cape of Good Hope. +On the South American coast, where tertiary and supra- tertiary beds +have been extensively elevated, I repeatedly noticed that the uppermost +beds were formed of coarser materials than the lower: this appears to +indicate that, as the sea became shallower, the force of the waves or +currents increased. On the lower platform, however, between the Blue +Mountains and the coast, I observed that the upper beds of the +sandstone frequently passed into argillaceous shale,—the effect, +probably, of this lower space having been protected from strong +currents during its elevation. The sandstone of the Blue Mountains +evidently having been of mechanical origin, and not having suffered any +metamorphic action, I was surprised at observing that, in some +specimens, 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 rock. (I have +lately seen, in a paper by Smith (the father of English geologists), in +the “Magazine of Natural History,” that the grains of quartz in the +millstone grit of England are often crystallised. Sir David Brewster, +in a paper read before the British Association, 1840, states, that in +old decomposed glass, the silex and metals separate into concentric +rings, and that the silex regains its crystalline structure, as is +shown by its action on light.) It is difficult to imagine how these +crystals could have been formed; one can hardly believe that they were +separately precipitated in their present crystallised state. Is it +possible that rounded grains of quartz may have been acted on by a +fluid corroding their surfaces, and depositing on them fresh silica? I +may remark that, in the sandstone formation of the Cape of Good Hope, +it is evident that silica has been profusely deposited from aqueous +solution. + +In several parts of the sandstone I noticed patches of shale which +might at the first glance have been mistaken for extraneous fragments; +their horizontal laminae, however, being parallel with those of the +sandstone, showed that they were the remnants of thin, continuous beds. +One such fragment (probably the section of a long narrow strip) seen in +the face of a cliff, was of greater vertical thickness than breadth, +which proves that this bed of shale must have been in some slight +degree consolidated, after having been deposited, and before being worn +away by the currents. Each patch of the shale shows, also, how slowly +many of the successive layers of sandstone were deposited. These +pseudo-fragments of shale will perhaps explain, in some cases, the +origin of apparently extraneous fragments in crystalline metamorphic +rocks. I mention this, because I found near Rio de Janeiro a +well-defined angular fragment, seven yards long by two yards in +breadth, of gneiss containing garnets and mica in layers, enclosed in +the ordinary, stratified, porphyritic gneiss of the country. The +laminae of the fragment and of the surrounding matrix ran in exactly +the same direction, but they dipped at different angles. I do not wish +to affirm that this singular fragment (a solitary case, as far as I +know) was originally deposited in a layer, like the shale in the Blue +Mountains, between the strata of the porphyritic gneiss, before they +were metamorphosed; but there is sufficient analogy between the two +cases to render such an explanation possible. + +STRATIFICATION OF THE ESCARPMENT. + +The strata of the Blue Mountains appear to the eye horizontal; but they +probably have a similar inclination with the surface of the platform, +which slopes from the west towards the escarpment over the Nepean, at +an angle of one degree, or of one hundred feet in a mile. (This is +stated on the authority of Sir T. Mitchell in “Travels” volume 2 page +357.) The strata of the escarpment dip almost conformably with its +steeply inclined face, and with so much regularity, that they appear as +if thrown into their present position; but on a more careful +examination, they are seen to thicken and to thin out, and in the upper +part to be succeeded and almost capped by horizontal beds. These +appearances render it probable, that we here see an original +escarpment, not formed by the sea having eaten back into the strata, +but by the strata having originally extended only thus far. Those who +have been in the habit of examining accurate charts of sea-coasts, +where sediment is accumulating, will be aware, that the surfaces of the +banks thus formed, generally slope from the coast very gently towards a +certain line in the offing, beyond which the depth in most cases +suddenly becomes great. I may instance the great banks of sediment +within the West Indian Archipelago (I have described these very curious +banks in the Appendix to my volume on the structure of Coral-Reefs. I +have ascertained the inclination of the edges of the banks, from +information given me by Captain B. Allen, one of the surveyors, and by +carefully measuring the horizontal distances between the last sounding +on the bank and the first in the deep water. Widely extended banks in +all parts of the West Indies have the same general form of surface.), +which terminate in submarine slopes, inclined at angles of between +thirty and forty degrees, and sometimes even at more than forty +degrees: every one knows how steep such a slope would appear on the +land. Banks of this nature, if uplifted, would probably have nearly the +same external form as the platform of the Blue Mountains, where it +abruptly terminates over the Nepean. + +CURRENT-CLEAVAGE. + +The strata of sandstone in the low coast country, and likewise on the +Blue Mountains, are often divided by cross or current laminae, which +dip in different directions, and frequently at an angle of forty-five +degrees. Most authors have attributed these cross layers to successive +small accumulations on an inclined surface; but from a careful +examination in some parts of the New Red Sandstone of England, I +believe that such layers generally form parts of a series of curves, +like gigantic tidal ripples, the tops of which have since been cut off, +either by nearly horizontal layers, or by another set of great ripples, +the folds of which do not exactly coincide with those below them. It is +well-known to surveyors that mud and sand are disturbed during storms +at considerable depths, at least from three hundred to four hundred and +fifty feet (See Martin White on “Soundings in the British Channel” +pages 4 and 166.), so that the nature of the bottom even becomes +temporarily changed; the bottom, also, at a depth between sixty and +seventy feet, has been observed to be broadly rippled. (M. Siau on the +“Action of Waves” “Edin. New Phil. Journ.” volume 31 page 245.) One +may, therefore, be allowed to suspect, from the appearance just +mentioned in the New Red Sandstone, that at greater depths, the bed of +the ocean is heaped up during gales into great ripple-like furrows and +depressions, which are afterwards cut off by the currents during more +tranquil weather, and again furrowed during gales. + +VALLEYS IN THE SANDSTONE PLATFORMS. + +The grand valleys, by which the Blue Mountains and the other sandstone +platforms of this part of Australia are penetrated, and which long +offered an insuperable obstacle to the attempts of the most +enterprising colonist to reach the interior country, form the most +striking feature in the geology of New South Wales. They are of grand +dimensions, and are bordered by continuous links of lofty cliffs. It is +not easy to conceive a more magnificent spectacle, than is presented to +a person walking on the summit- plains, when without any notice he +arrives at the brink of one of these cliffs, which are so +perpendicular, that he can strike with a stone (as I have tried) the +trees growing, at the depth of between one thousand and one thousand +five hundred feet below him; on both hands he sees headland beyond +headland of the receding line of cliff, and on the opposite side of the +valley, often at the distance of several miles, he beholds another line +rising up to the same height with that on which he stands, and formed +of the same horizontal strata of pale sandstone. The bottoms of these +valleys are moderately level, and the fall of the rivers flowing in +them, according to Sir T. Mitchell, is gentle. The main valleys often +send into the platform great baylike arms, which expand at their upper +ends; and on the other hand, the platform often sends promontories into +the valley, and even leaves in them great, almost insulated, masses. So +continuous are the bounding lines of cliff, that to descend into some +of these valleys, it is necessary to go round twenty miles; and into +others, the surveyors have only lately penetrated, and the colonists +have not yet been able to drive in their cattle. But the most +remarkable point of structure in these valleys, is, that although +several miles wide in their upper parts, they generally contract +towards their mouths to such a degree as to become impassable. The +Surveyor-General, Sir T. Mitchell, in vain endeavoured, first on foot +and then by crawling between the great fallen fragments of sandstone, +to ascend through the gorge by which the river Grose joins the Nepean +(“Travels in Australia” volume 1 page 154.—I must express my obligation +to Sir T. Mitchell for several interesting personal communications on +the subject of these great valleys of New South Wales.); yet the valley +of the Grose in its upper part, as I saw, forms a magnificent basin +some miles in width, and is on all sides surrounded by cliffs, the +summits of which are believed to be nowhere less than 3,000 feet above +the level of the sea. When cattle are driven into the valley of the +Wolgan by a path (which I descended) partly cut by the colonists, they +cannot escape; for this valley is in every other part surrounded by +perpendicular cliffs, and eight miles lower down, it contracts, from an +average width of half a mile, to a mere chasm impassable to man or +beast. Sir T. Mitchell states, that the great valley of the Cox river +with all its branches contracts, where it unites with the Nepean, into +a gorge 2,200 yards wide, and about one thousand feet in depth. (Idem +volume 2 page 358.) Other similar cases might have been added. + +The first impression, from seeing the correspondence of the horizontal +strata, on each side of these valleys and great amphitheatre-like +depressions, is that they have been in chief part hollowed out, like +other valleys, by aqueous erosion; but when one reflects on the +enormous amount of stone, which on this view must have been removed, in +most of the above cases through mere gorges or chasms, one is led to +ask whether these spaces may not have subsided. But considering the +form of the irregularly branching valleys, and of the narrow +promontories, projecting into them from the platforms, we are compelled +to abandon this notion. To attribute these hollows to alluvial action, +would be preposterous; nor does the drainage from the summit-level +always fall, as I remarked near the Weatherboard, into the head of +these valleys, but into one side of their bay-like recesses. Some of +the inhabitants remarked to me, that they never viewed one of these +baylike recesses, with the headlands receding on both hands, without +being struck with their resemblance to a bold sea-coast. This is +certainly the case; moreover, the numerous fine harbours, with their +widely branching arms, on the present coast of New South Wales, which +are generally connected with the sea by a narrow mouth, from one mile +to a quarter of a mile in width, passing through the sandstone +coast-cliffs, present a likeness, though on a miniature scale, to the +great valleys of the interior. But then immediately occurs the +startling difficulty, why has the sea worn out these great, though +circumscribed, depressions on a wide platform, and left mere gorges, +through which the whole vast amount of triturated matter must have been +carried away? The only light I can throw on this enigma, is by showing +that banks appear to be forming in some seas of the most irregular +forms, and that the sides of such banks are so steep (as before stated) +that a comparatively small amount of subsequent erosion would form them +into cliffs: that the waves have power to form high and precipitous +cliffs, even in landlocked harbours, I have observed in many parts of +South America. In the Red Sea, banks with an extremely irregular +outline and composed of sediment, are penetrated by the most singularly +shaped creeks with narrow mouths: this is likewise the case, though on +a larger scale, with the Bahama Banks. Such banks, I have been led to +suppose, have been formed by currents heaping sediment on an irregular +bottom. (See the “Appendix” to the Part on Coral-Reefs. The fact of the +sea heaping up mud round a submarine nucleus, is worthy of the notice +of geologists: for outlyers of the same composition with the coast +banks are thus formed; and these, if upheaved and worn into cliffs, +would naturally be thought to have been once connected together.) That +in some cases, the sea, instead of spreading out sediment in a uniform +sheet, heaps it round submarine rocks and islands, it is hardly +possible to doubt, after having examined the charts of the West Indies. +To apply these ideas to the sandstone platforms of New South Wales, I +imagine that the strata might have been heaped on an irregular bottom +by the action of strong currents, and of the undulations of an open +sea; and that the valley-like spaces thus left unfilled might, during a +slow elevation of the land, have had their steeply sloping flanks worn +into cliffs; the worn-down sandstone being removed, either at the time +when the narrow gorges were cut by the retreating sea, or subsequently +by alluvial action. + +VAN DIEMEN’S LAND. + +The southern part of this island is mainly formed of mountains of +greenstone, which often assumes a syenitic character, and contains much +hypersthene. These mountains, in their lower half, are generally +encased by strata containing numerous small corals and some shells. +These shells have been examined by Mr. G.B. Sowerby, and have been +described by him: they consist of two species of Producta, and of six +of Spirifera; two of these, namely, P. rugata and S. rotundata, +resemble, as far as their imperfect condition allows of comparison, +British mountain-limestone shells. Mr. Lonsdale has had the kindness to +examine the corals; they consist of six undescribed species, belonging +to three genera. Species of these genera occur in the Silurian, +Devonian, and Carboniferous strata of Europe. Mr. Lonsdale remarks, +that all these fossils have undoubtedly a Palaeozoic character, and +that probably they correspond in age to a division of the system above +the Silurian formations. + +The strata containing these remains are singular from the extreme +variability of their mineralogical composition. Every intermediate form +is present, between flinty-slate, clay-slate passing into grey wacke, +pure limestone, sandstone, and porcellanic rock; and some of the beds +can only be described as composed of a siliceo-calcareo-clay-slate. The +formation, as far as I could judge, is at least a thousand feet in +thickness: the upper few hundred feet usually consist of a siliceous +sandstone, containing pebbles and no organic remains; the inferior +strata, of which a pale flinty slate is perhaps the most abundant, are +the most variable; and these chiefly abound with the remains. Between +two beds of hard crystalline limestone, near Newtown, a layer of white +soft calcareous matter is quarried, and is used for whitewashing +houses. From information given to me by Mr. Frankland, the +Surveyor-General, it appears that this Palaeozoic formation is found in +different parts of the whole island; from the same authority, I may +add, that on the north-eastern coast and in Bass’ Straits primary rocks +extensively occur. + +The shores of Storm Bay are skirted, to the height of a few hundred +feet, by strata of sandstone, containing pebbles of the formation just +described, with its characteristic fossils, and therefore belonging to +a subsequent age. These strata of sandstone often pass into shale, and +alternate with layers of impure coal; they have in many places been +violently disturbed. Near Hobart Town, I observed one dike, nearly a +hundred yards in width, on one side of which the strata were tilted at +an angle of 60 degrees, and on the other they were in some parts +vertical, and had been altered by the effects of the heat. On the west +side of Storm Bay, I found these strata capped by streams of basaltic +lava with olivine; and close by there was a mass of brecciated scoriae, +containing pebbles of lava, which probably marks the place of an +ancient submarine crater. Two of these streams of basalt were separated +from each other by a layer of argillaceous wacke, which could be traced +passing into partially altered scoriae. The wacke contained numerous +rounded grains of a soft, grass-green mineral, with a waxy lustre, and +translucent on its edges: under the blowpipe it instantly blackened, +and the points fused into a strongly magnetic, black enamel. In these +characters, it resembles those masses of decomposed olivine, described +at St. Jago in the Cape de Verde group; and I should have thought that +it had thus originated, had I not found a similar substance, in +cylindrical threads, within the cells of the vesicular basalt,—a state +under which olivine never appears; this substance, I believe, would be +classed as bole by mineralogists. (Chlorophaeite, described by Dr. +MacCulloch (“Western Islands” volume 1 page 504) as occurring in a +basaltic amygdaloid, differs from this substance, in remaining +unchanged before the blowpipe, and in blackening from exposure to the +air. May we suppose that olivine, in undergoing the remarkable change +described at St. Jago, passes through several states?) + +TRAVERTIN WITH EXTINCT PLANTS. + +Behind Hobart Town there is a small quarry of a hard travertin, the +lower strata of which abound with distinct impressions of leaves. Mr. +Robert Brown has had the kindness to look at my specimens, and he +informed me that there are four or five kinds, none of which he +recognises as belonging to existing species. The most remarkable leaf +is palmate, like that of a fan- palm, and no plant having leaves of +this structure has hitherto been discovered in Van Diemen’s Land. The +other leaves do not resemble the most usual form of the Eucalyptus (of +which tribe the existing forests are chiefly composed), nor do they +resemble that class of exceptions to the common form of the leaves of +the Eucalyptus, which occur in this island. The travertin containing +this remnant of a lost vegetation, is of a pale yellow colour, hard, +and in parts even crystalline; but not compact, and is everywhere +penetrated by minute, tortuous, cylindrical pores. It contains a very +few pebbles of quartz, and occasionally layers of chalcedonic nodules, +like those of chert in our Greensand. From the pureness of this +calcareous rock, it has been searched for in other places, but has +never been found. From this circumstance, and from the character of the +deposit, it was probably formed by a calcareous spring entering a small +pool or narrow creek. The strata have subsequently been tilted and +fissured; and the surface has been covered by a singular mass, with +which, also, a large fissure has been filled up, formed of balls of +trap embedded in a mixture of wacke and a white, earthy, +alumino-calcareous substance. Hence it would appear, as if a volcanic +eruption had taken place on the borders of the pool, in which the +calcareous matter was depositing, and had broken it up and drained it. + +ELEVATION OF THE LAND. + +Both the eastern and western shores of the bay, in the neighbourhood of +Hobart Town, are in most parts covered to the height of thirty feet +above the level of high-water mark, with broken shells, mingled with +pebbles. The colonists attribute these shells to the aborigines having +carried them up for food: undoubtedly, there are many large mounds, as +was pointed out to me by Mr. Frankland, which have been thus formed; +but I think from the numbers of the shells, from their frequent small +size, from the manner in which they are thinly scattered, and from some +appearances in the form of the land, that we must attribute the +presence of the greater number to a small elevation of the land. On the +shore of Ralph Bay (opening into Storm Bay) I observed a continuous +beach about fifteen feet above high-water mark, clothed with +vegetation, and by digging into it, pebbles encrusted with Serpulae +were found: along the banks, also, of the river Derwent, I found a bed +of broken sea-shells above the surface of the river, and at a point +where the water is now much too fresh for sea-shells to live; but in +both these cases, it is just possible, that before certain spits of +sand and banks of mud in Storm Bay were accumulated, the tides might +have risen to the height where we now find the shells. ( It would +appear that some changes are now in progress in Ralph Bay, for I was +assured by an intelligent farmer, that oysters were formerly abundant +in it, but that about the year 1834 they had, without any apparent +cause, disappeared. In the “Transactions of the Maryland Academy” +volume 1 part 1 page 28 there is an account by Mr. Ducatel of vast beds +of oysters and clams having been destroyed by the gradual filling up of +the shallow lagoons and channels, on the shores of the southern United +States. At Chiloe, in South America, I heard of a similar loss, +sustained by the inhabitants, in the disappearance from one part of the +coast of an edible species of Ascidia.) + +Evidence more or less distinct of a change of level between the land +and water, has been detected on almost all the land on this side of the +globe. Captain Grey, and other travellers, have found in Southern +Australia upraised shells, belonging either to the recent, or to a late +tertiary period. The French naturalists in Baudin’s expedition, found +shells similarly circumstanced on the S.W. coast of Australia. The Rev. +W.B. Clarke finds proofs of the elevation of the land, to the amount of +400 feet, at the Cape of Good Hope. (“Proceedings of the Geological +Society” volume 3 page 420.) In the neighbourhood of the Bay of Islands +in New Zealand, I observed that the shores were scattered to some +height, as at Van Diemen’s Land, with sea-shells, which the colonists +attribute to the natives. (I will here give a catalogue of the rocks +which I met with near the Bay of Islands, in New Zealand:—1st, Much +basaltic lava, and scoriform rocks, forming distinct craters;—2nd, A +castellated hill of horizontal strata of flesh-coloured limestone, +showing when fractured distinct crystalline facets: the rain has acted +on this rock in a remarkable manner, corroding its surface into a +miniature model of an Alpine country: I observed here layers of chert +and clay ironstone; and in the bed of a stream, pebbles of +clay-slate;—3rd, The shores of the Bay of Islands are formed of a +feldspathic rock, of a bluish-grey colour, often much decomposed, with +an angular fracture, and crossed by numerous ferruginous seams, but +without any distinct stratification or cleavage. Some varieties are +highly crystalline, and would at once be pronounced to be trap; others +strikingly resembled clay-slate, slightly altered by heat: I was unable +to form any decided opinion on this formation.) Whatever may have been +the origin of these shells, I cannot doubt, after having seen a section +of the valley of the Thames River (37 degrees S.), drawn by the Rev. W. +Williams, that the land has been there elevated: on the opposite sides +of this great valley, three step-like terraces, composed of an enormous +accumulation of rounded pebbles, exactly correspond with each other: +the escarpment of each terrace is about fifty feet in height. No one +after having examined the terraces in the valleys on the western shores +of South America, which are strewed with sea-shells, and have been +formed during intervals of rest in the slow elevation of the land, +could doubt that the New Zealand terraces have been similarly formed. I +may add, that Dr. Dieffenbach, in his description of the Chatham +Islands (“Geographical Journal” volume 11 pages 202, 205.) (S.W. of New +Zealand), states that it is manifest “that the sea has left many places +bare which were once covered by its waters.” + +KING GEORGE’S SOUND. + +This settlement is situated at the south-western angle of the +Australian continent: the whole country is granitic, with the +constituent minerals sometimes obscurely arranged in straight or curved +laminae. In these cases, the rock would be called by Humboldt, +gneiss-granite, and it is remarkable that the form of the bare conical +hills, appearing to be composed of great folding layers, strikingly +resembles, on a small scale, those composed of gneiss-granite at Rio de +Janeiro, and those described by Humboldt at Venezuela. These plutonic +rocks are, in many places, intersected by trappean-dikes; in one place, +I found ten parallel dikes ranging in an E. and W. line; and not far +off another set of eight dikes, composed of a different variety of +trap, ranging at right angles to the former ones. I have observed in +several primary districts, the occurrence of systems of dikes parallel +and close to each other. + +SUPERFICIAL FERRUGINOUS BEDS. + +The lower parts of the country are everywhere covered by a bed, +following the inequalities of the surface, of a honeycombed sandstone, +abounding with oxides of iron. Beds of nearly similar composition are +common, I believe, along the whole western coast of Australia, and on +many of the East Indian islands. At the Cape of Good Hope, at the base +of the mountains formed of granite and capped with sandstone, the +ground is everywhere coated either by a fine-grained, rubbly, +ochraceous mass, like that at King George’s Sound, or by a coarser +sandstone with fragments of quartz, and rendered hard and heavy by an +abundance of the hydrate of iron, which presents, when freshly broken, +a metallic lustre. Both these varieties have a very irregular texture, +including spaces either rounded or angular, full of loose sand: from +this cause the surface is always honeycombed. The oxide of iron is most +abundant on the edges of the cavities, where alone it affords a +metallic fracture. In these formations, as well as in many true +sedimentary deposits, it is evident that iron tends to become +aggregated, either in the form of a shell, or of a network. The origin +of these superficial beds, though sufficiently obscure, seems to be due +to alluvial action on detritus abounding with iron. + +SUPERFICIAL CALCAREOUS DEPOSIT. + +A calcareous deposit on the summit of Bald Head, containing branched +bodies, supposed by some authors to have been corals, has been +celebrated by the descriptions of many distinguished voyagers. (I +visited this hill, in company with Captain Fitzroy, and we came to a +similar conclusion regarding these branching bodies.) It folds round +and conceals irregular hummocks of granite, at the height of 600 feet +above the level of the sea. It varies much in thickness; where +stratified, the beds are often inclined at high angles, even as much as +at thirty degrees, and they dip in all directions. These beds are +sometimes crossed by oblique and even-sided laminae. The deposit +consists either of a fine, white calcareous powder, in which not a +trace of structure can be discovered, or of exceedingly minute, rounded +grains, of brown, yellowish, and purplish colours; both varieties being +generally, but not always, mixed with small particles of quartz, and +being cemented into a more or less perfect stone. The rounded +calcareous grains, when heated in a slight degree, instantly lose their +colours; in this and in every other respect, closely resembling those +minute, equal- sized particles of shells and corals, which at St. +Helena have been drifted up the side of the mountains, and have thus +been winnowed of all coarser fragments. I cannot doubt that the +coloured calcareous particles here have had a similar origin. The +impalpable powder has probably been derived from the decay of the +rounded particles; this certainly is possible, for on the coast of +Peru, I have traced LARGE UNBROKEN shells gradually falling into a +substance as fine as powdered chalk. Both of the above-mentioned +varieties of calcareous sandstone frequently alternate with, and blend +into, thin layers of a hard substalagmitic rock, which, even when the +stone on each side contains particles of quartz, is entirely free from +them (I adopt this term from Lieutenant Nelson’s excellent paper on the +Bermuda Islands “Geolog. Trans.” volume 5 page 106, for the hard, +compact, cream- or brown- coloured stone, without any crystalline +structure, which so often accompanies superficial calcareous +accumulations. I have observed such superficial beds, coated with +substalagmitic rock, at the Cape of Good Hope, in several parts of +Chile, and over wide spaces in La Plata and Patagonia. Some of these +beds have been formed from decayed shells, but the origin of the +greater number is sufficiently obscure. The causes which determine +water to dissolve lime, and then soon to redeposit it, are not, I +think, known. The surface of the substalagmitic layers appears always +to be corroded by the rain-water. As all the above-mentioned countries +have a long dry season, compared with the rainy one, I should have +thought that the presence of the substalagmitic was connected with the +climate, had not Lieutenant Nelson found this substance forming under +sea-water. Disintegrated shell seems to be extremely soluble; of which +I found good evidence, in a curious rock at Coquimbo in Chile, which +consisted of small, pellucid, empty husks, cemented together. A series +of specimens clearly showed that these husks had originally contained +small rounded particles of shells, which had been enveloped and +cemented together by calcareous matter (as often happens on +sea-beaches), and which subsequently had decayed, and been dissolved by +water, that must have penetrated through the calcareous husks, without +corroding them,—of which processes every stage could be seen.): hence +we must suppose that these layers, as well as certain vein- like +masses, have been formed by rain dissolving the calcareous matter and +re-precipitating it, as has happened at St. Helena. Each layer probably +marks a fresh surface, when the, now firmly cemented, particles existed +as loose sand. These layers are sometimes brecciated and re-cemented, +as if they had been broken by the slipping of the sand when soft. I did +not find a single fragment of a sea-shell; but bleached shells of the +Helix melo, an existing land species, abound in all the strata; and I +likewise found another Helix, and the case of an Oniscus. + +The branches are absolutely undistinguishable in shape from the broken +and upright stumps of a thicket; their roots are often uncovered, and +are seen to diverge on all sides; here and there a branch lies +prostrate. The branches generally consist of the sandstone, rather +firmer than the surrounding matter, with the central parts filled, +either with friable, calcareous matter, or with a substalagmitic +variety; this central part is also frequently penetrated by linear +crevices, sometimes, though rarely, containing a trace of woody matter. +These calcareous, branching bodies, appear to have been formed by fine +calcareous matter being washed into the casts or cavities, left by the +decay of branches and roots of thickets, buried under drifted sand. The +whole surface of the hill is now undergoing disintegration, and hence +the casts, which are compact and hard, are left projecting. In +calcareous sand at the Cape of Good Hope, I found the casts, described +by Abel, quite similar to these at Bald Head; but their centres are +often filled with black carbonaceous matter not yet removed. It is not +surprising, that the woody matter should have been almost entirely +removed from the casts on Bald Head; for it is certain, that many +centuries must have elapsed since the thickets were buried; at present, +owing to the form and height of the narrow promontory, no sand is +drifted up, and the whole surface, as I have remarked, is wearing away. +We must, therefore, look back to a period when the land stood lower, of +which the French naturalists (See M. Peron “Voyage” tome 1 page 204.) +found evidence in upraised shells of recent species, for the drifting +on Bald Head of the calcareous and quartzose sand, and the consequent +embedment of the vegetable remains. There was only one appearance which +at first made me doubt concerning the origin of the cast,—namely, that +the finer roots from different stems sometimes became united together +into upright plates or veins; but when the manner is borne in mind in +which fine roots often fill up cracks in hard earth, and that these +roots would decay and leave hollows, as well as the stems, there is no +real difficulty in this case. Besides the calcareous branches from the +Cape of Good Hope, I have seen casts, of exactly the same forms, from +Madeira* and from Bermuda; at this latter place, the surrounding +calcareous rocks, judging from the specimens collected by Lieutenant +Nelson, are likewise similar, as is their subaerial formation. +Reflecting on the stratification of the deposit on Bald Head,—on the +irregularly alternating layers of substalagmitic rock,—on the uniformly +sized, and rounded particles, apparently of sea-shells and corals,—on +the abundance of land-shells throughout the mass,—and finally, on the +absolute resemblance of the calcareous casts, to the stumps, roots, and +branches of that kind of vegetation, which would grow on sand-hillocks, +I think there can be no reasonable doubt, notwithstanding the different +opinion of some authors, that a true view of their origin has been here +given. + +*(Dr. J. Macaulay has fully described (“Edinb. New Phil. Journ.” volume +29 page 350) the casts from Madeira. He considers (differently from Mr. +Smith of Jordan Hill) these bodies to be corals, and the calcareous +deposit to be of subaqueous origin. His arguments chiefly rest (for his +remarks on their structure are vague) on the great quantity of the +calcareous matter, and on the casts containing animal matter, as shown +by their evolving ammonia. Had Dr. Macaulay seen the enormous masses of +rolled particles of shells and corals on the beach of Ascension, and +especially on coral-reefs; and had he reflected on the effects of +long-continued, gentle winds, in drifting up the finer particles, he +would hardly have advanced the argument of quantity, which is seldom +trustworthy in geology. If the calcareous matter has originated from +disintegrated shells and corals, the presence of animal matter is what +might have been expected. Mr. Anderson analysed for Dr. Macaulay part +of a cast, and he found it composed of:— + +Carbonate of lime......73.15 Silica.................11.90 Phosphate of +lime.......8.81 Animal matter...........4.25 Sulphate of lime......a +trace 98.11) + +Calcareous deposits, like these of King George’s Sound, are of vast +extent on the Australian shores. Dr. Fitton remarks, that “recent +calcareous breccia (by which term all these deposits are included) was +found during Baudin’s voyage, over a space of no less than twenty-five +degrees of latitude and an equal extent of longitude, on the southern, +western, and north-western coasts.” (For ample details on this +formation consult Dr. Fitton “Appendix to Captain King’s Voyage.” Dr. +Fitton is inclined to attribute a concretionary origin to the branching +bodies: I may remark, that I have seen in beds of sand in La Plata +cylindrical stems which no doubt thus originated; but they differed +much in appearance from these at Bald Head, and the other places above +specified.) It appears also from M. Peron, with whose observations and +opinions on the origin of the calcareous matter and branching casts +mine entirely accord, that the deposit is generally much more +continuous than near King George’s Sound. At Swan River, Archdeacon +Scott states that in one part it extends ten miles inland. +(“Proceedings of the Geolog. Soc.” volume 1 page 320.) Captain Wickham, +moreover, informs me that during his late survey of the western coast, +the bottom of the sea, wherever the vessel anchored, was ascertained, +by crowbars being let down, to consist of white calcareous matter. +Hence it seems that along this coast, as at Bermuda and at Keeling +Atoll, submarine and subaerial deposits are contemporaneously in +process of formation, from the disintegration of marine organic bodies. +The extent of these deposits, considering their origin, is very +striking; and they can be compared in this respect only with the great +coral-reefs of the Indian and Pacific Oceans. In other parts of the +world, for instance in South America, there are SUPERFICIAL calcareous +deposits of great extent, in which not a trace of organic structure is +discoverable; these observations would lead to the inquiry, whether +such deposits may not, also, have been formed from disintegrated shells +and corals. + +CAPE OF GOOD HOPE. + +After the accounts given by Barrow, Carmichael, Basil Hall, and W.B. +Clarke of the geology of this district, I shall confine myself to a few +observations on the junction of the three principal formations. The +fundamental rock is granite (In several places I observed in the +granite, small dark-coloured balls, composed of minute scales of black +mica in a tough basis. In another place, I found crystals of black +schorl radiating from a common centre. Dr. Andrew Smith found, in the +interior parts of the country, some beautiful specimens of granite, +with silvery mica radiating or rather branching, like moss, from +central points. At the Geological Society, there are specimens of +granite with crystallised feldspar branching and radiating in like +manner.), overlaid by clay-slate: the latter is generally hard, and +glossy from containing minute scales of mica; it alternates with, and +passes into, beds of slightly crystalline, feldspathic, slaty rock. +This clay-slate is remarkable from being in some places (as on the +Lion’s Rump) decomposed, even to the depth of twenty feet, into a +pale-coloured, sandstone-like rock, which has been mistaken, I believe, +by some observers, for a separate formation. I was guided by Dr. Andrew +Smith to a fine junction at Green Point between the granite and +clay-slate: the latter at the distance of a quarter of a mile from the +spot, where the granite appears on the beach (though, probably, the +granite is much nearer underground), becomes slightly more compact and +crystalline. At a less distance, some of the beds of clay-slate are of +a homogeneous texture, and obscurely striped with different zones of +colour, whilst others are obscurely spotted. Within a hundred yards of +the first vein of granite, the clay-slate consists of several +varieties; some compact with a tinge of purple, others glistening with +numerous minute scales of mica and imperfectly crystallised feldspar; +some obscurely granular, others porphyritic with small, elongated spots +of a soft white mineral, which being easily corroded, gives to this +variety a vesicular appearance. Close to the granite, the clay-slate is +changed into a dark-coloured, laminated rock, having a granular +fracture, which is due to imperfect crystals of feldspar, coated by +minute, brilliant scales of mica. + +The actual junction between the granitic and clay-slate districts +extends over a width of about two hundred yards, and consists of +irregular masses and of numerous dikes of granite, entangled and +surrounded by the clay- slate: most of the dikes range in a N.W. and +S.E. line, parallel to the cleavage of the slate. As we leave the +junction, thin beds, and lastly, mere films of the altered clay-slate +are seen, quite isolated, as if floating, in the coarsely crystallised +granite; but although completely detached, they all retain traces of +the uniform N.W. and S.E. cleavage. This fact has been observed in +other similar cases, and has been advanced by some eminent geologists +(See M. Keilhau “Theory on Granite” translated in the “Edinburgh New +Philosophical Journal” volume 24 page 402.), as a great difficulty on +the ordinary theory, of granite having been injected whilst liquified; +but if we reflect on the probable state of the lower surface of a +laminated mass, like clay-slate, after having been violently arched by +a body of molten granite, we may conclude that it would be full of +fissures parallel to the planes of cleavage; and that these would be +filled with granite, so that wherever the fissures were close to each +other, mere parting layers or wedges of the slate would depend into the +granite. Should, therefore, the whole body of rock afterwards become +worn down and denuded, the lower ends of these dependent masses or +wedges of slate would be left quite isolated in the granite; yet they +would retain their proper lines of cleavage, from having been united, +whilst the granite was fluid, with a continuous covering of clay-slate. + +Following, in company with Dr. A. Smith, the line of junction between +the granite and the slate, as it stretched inland, in a S.E. direction, +we came to a place, where the slate was converted into a fine-grained, +perfectly characterised gneiss, composed of yellow-brown granular +feldspar, of abundant black brilliant mica, and of few and thin laminae +of quartz. From the abundance of the mica in this gneiss, compared with +the small quantity and excessively minute scales, in which it exists in +the glossy clay-slate, we must conclude, that it has been here formed +by the metamorphic action—a circumstance doubted, under nearly similar +circumstances, by some authors. The laminae of the clay-slate are +straight; and it was interesting to observe, that as they assumed the +character of gneiss, they became undulatory with some of the smaller +flexures angular, like the laminae of many true metamorphic schists. + +SANDSTONE FORMATION. + +This formation makes the most imposing feature in the geology of +Southern Africa. The strata are in many parts horizontal, and attain a +thickness of about two thousand feet. The sandstone varies in +character; it contains little earthy matter, but is often stained with +iron; some of the beds are very fine-grained and quite white; others +are as compact and homogeneous as quartz rock. In some places I +observed a breccia of quartz, with the fragments almost dissolved in a +siliceous paste. Broad veins of quartz, often including large and +perfect crystals, are very numerous; and it is evident in nearly all +the strata, that silica has been deposited from solution in remarkable +quantity. Many of the varieties of quartzite appeared quite like +metamorphic rocks; but from the upper strata being as siliceous as the +lower, and from the undisturbed junctions with the granite, which in +many places can be examined, I can hardly believe that these +sandstone-strata have been exposed to heat. (The Rev. W.B. Clarke, +however, states, to my surprise (“Geolog. Proceedings” volume 3 page +422), that the sandstone in some parts is penetrated by granitic dikes: +such dikes must belong to an epoch altogether subsequent to that when +the molten granite acted on the clay-slate.) On the lines of junction +between these two great formations, I found in several places the +granite decayed to the depth of a few inches, and succeeded, either by +a thin layer of ferruginous shale, or by four or five inches in +thickness of the re-cemented crystals of the granite, on which the +great pile of sandstone immediately rested. + +Mr. Schomburgk has described (“Geographical Journal” volume 10 page +246.) a great sandstone formation in Northern Brazil, resting on +granite, and resembling to a remarkable degree, in composition and in +the external form of the land, this formation of the Cape of Good Hope. +The sandstones of the great platforms of Eastern Australia, which also +rest on granite, differ in containing more earthy and less siliceous +matter. No fossil remains have been discovered in these three vast +deposits. Finally, I may add that I did not see any boulders of +far-transported rocks at the Cape of Good Hope, or on the eastern and +western shores of Australia, or at Van Diemen’s Land. In the northern +island of New Zealand, I noticed some large blocks of greenstone, but +whether their parent rock was far distant, I had no opportunity of +determining. + + + +INDEX TO VOLCANIC ISLANDS. + + +Abel, M., on calcareous casts at the Cape of Good Hope. + +Abingdon island. + +Abrolhos islands, incrustation on. + +Aeriform explosions at Ascension. + +Albatross, driven from St. Helena. + +Albemarle island. + +Albite, at the Galapagos archipelago. + +Amygdaloidal cells, half filled. + +Amygdaloids, calcareous origin of. + +Ascension, arborescent incrustation on rocks of. -absence of dikes, +freedom from volcanic action, and state of lava-streams. + +Ascidia, extinction of. + +Atlantic Ocean, new volcanic focus in. + +Augite, fused. + +Australia. + +Azores. + +Bahia in Brazil, dikes at. + +Bailly, M., on the mountains of Mauritius. + +Bald Head. + +Banks’ Cove. + +Barn, The, St. Helena. + +Basalt, specific gravity of. + +Basaltic coast-mountains at Mauritius. -at St. Helena. -at St. Jago. + +Beaumont, M. Elie de, on circular subsidences in lava. -on dikes +indicating elevation. -on inclination of lava-streams. -on laminated +dikes. + +Bermuda, calcareous rocks of. + +Beudant, M., on bombs. -on jasper. -on laminated trachyte. -on obsidian +of Hungary. -on silex in trachyte. + +Bole. + +Bombs, volcanic. + +Bory St. Vincent, on bombs. + +Boulders, absence in Australia and Cape of Good Hope. + +Brattle island. + +Brewster, Sir D., on a calcareo-animal substance. -on decomposed glass. + +Brown, Mr. R., on extinct plants from Van Diemen’s land. -on +sphaerulitic bodies in silicified wood. + +Buch, Von, on cavernous lava. -on central volcanoes. -on crystals +sinking in obsidian. -on laminated lava. -on obsidian streams. -on +olivine in basalt. -on superficial calcareous beds in the Canary +islands. + +Calcareous deposit at St. Jago affected by heat. -fibrous matter, +entangled in streaks in scoriae. -freestone at Ascension. +-incrustations at Ascension. -sandstone at St. Helena. -superficial +beds at King George’s sound. + +Cape of Good Hope. + +Carbonic acid, expulsion of, by heat. + +Carmichael, Capt., on glassy coatings to dikes. + +Casts, calcareous, of branches. + +Chalcedonic nodules. + +Chalcedony in basalt and in silicified wood. + +Chatham island. + +Chlorophaeite. + +Clarke, Rev. W., on the Cape of Good Hope. + +Clay-slate, its decomposition and junction with granite at the Cape of +Good Hope. + +Cleavage of clay-slate in Australia. + +Cleavage, cross, in sandstone. + +Coast denudation at St. Helena. + +Columnar basalt. + +“Comptes Rendus,” account of volcanic phenomena in the Atlantic. + +Concepcion, earthquake of. + +Concretions in aqueous and igneous rocks compared. -in tuff. -of +obsidian. + +Conglomerate, recent, at St. Jago. + +Coquimbo, curious rock of. + +Corals, fossil, from Van Diemen’s Land. + +Crater, segment of, at the Galapagos. -great central one at St. Helena. +-internal ledges round, and parapet on. + +Craters, basaltic, at Ascension. -form of, affected by the trade wind. +-of elevation. -of tuff at Terceira. -of tuff at the Galapagos +archipelago. -their breached state. -small basaltic at St. Jago. —at +the Galapagos archipelago. + +Crystallisation favoured by space. + +Dartigues, M., on sphaerulites. + +Daubeny, Dr., on a basin-formed island. -on fragments in trachyte. + +D’Aubuisson on hills of phonolite. -on the composition of obsidian. -on +the lamination of clay-slate. + +De la Beche, Sir H., on magnesia in erupted lime. -on specific gravity +of limestones. + +Denudation of coast at St. Helena. + +Diana’s Peak, St. Helena. + +Dieffenbach, Dr., on the Chatham Islands. + +Dikes, truncated, on central crateriform ridge of St. Helena. -at St. +Helena; number of; coated by a glossy layer; uniform thickness of. +-great parallel ones at St. Helena. -not observed at Ascension. -of +tuff. -of trap in the plutonic series. -remnants of, extending far into +the sea round St. Helena. + +Dislocations at Ascension. -at St. Helena. + +Distribution of volcanic islands. + +Dolomieu, on decomposed trachyte. -on laminated lava. -on obsidian. + +Dree, M., on crystals sinking in lava. + +Dufrenoy, M., on the composition of the surface of certain +lava-streams. -on the inclination of tuff-strata. + +Eggs of birds embedded at St. Helena. -of turtle at Ascension. + +Ejected fragments at Ascension. -at the Galapagos archipelago. + +Elevation of St. Helena. -the Galapagos archipelago. -Van Diemen’s +Land, Cape of Good Hope, New Zealand, Australia, and Chatham island. +-of volcanic islands. + +Ellis, Rev. W., on ledges within the great crater at Hawaii. -on marine +remains at Otaheite. + +Eruption, fissures of. + +Extinction of land-shells at St. Helena. + +Faraday, Mr., on the expulsion of carbonic acid gas. + +Feldspar, fusibility of. -in radiating crystals. -Labrador, ejected. + +Feldspathic lavas. -at St. Helena. -rock, alternating with obsidian. +-lamination, and origin of. + +Fernando Noronha. + +Ferruginous superficial beds. + +Fibrous calcareous matter at St. Jago. + +Fissures of eruption. + +Fitton, Dr., on calcareous breccia. + +Flagstaff Hill, St. Helena. + +Fleurian de Bellevue on sphaerulites. + +Fluidity of lavas. + +Forbes, Professor, on the structure of glaciers. + +Fragments ejected at Ascension. -at the Galapagos archipelago. + +Freshwater Bay. + +Fuerteventura (Feurteventura), calcareous beds of. + +Galapagos archipelago. -parapets round craters. + +Gay Lussac, on the expulsion of carbonic acid gas. + +Glaciers, their structure. + +Glossiness of texture, origin of. + +Gneiss, derived from clay-slate. -with a great embedded fragment. + +Gneiss-granite, form of hills of. + +Good Hope, Cape of. + +Gorges, narrow, at St. Helena. + +Granite, junction with clay-slate, at the Cape of Good Hope. + +Granitic ejected fragments. + +Gravity, specific, of lavas. + +Gypsum, at Ascension. -in volcanic strata at St. Helena. -on surface of +the ground at ditto. + +Hall, Sir J., on the expulsion of carbonic acid gas. + +Heat, action of, on calcareous matter. + +Hennah, Mr., on ashes at Ascension. + +Henslow, Prof., on chalcedony. + +Hoffmann, on decomposed trachyte. + +Holland, Dr., on Iceland. + +Horner, Mr., on a calcareo-animal substance. -on fusibility of +feldspar. + +Hubbard, Dr., on dikes. + +Humboldt on ejected fragments. -on obsidian formations. -on parapets +round craters. -on sphaerulites. + +Hutton on amygdaloids. + +Hyalite in decomposed trachyte. + +Iceland, stratification of the circumferential hills. + +Islands, volcanic, distribution of. -their elevation. + +Incrustation, on St. Paul’s rocks. + +Incrustations, calcareous, at Ascension. + +Jago, St. + +James island. + +Jasper, origin of. + +Jonnes, M. Moreau de, on craters affected by wind. + +Juan Fernandez. + +Keilhau, M., on granite. + +Kicker Rock. + +King George’s sound. + +Labrador feldspar, ejected. + +Lakes at bases of volcanoes. + +Lamination of volcanic rocks. + +Land-shells, extinct, at St. Helena. + +Lanzarote, calcareous beds of. + +Lava, adhesion to sides of a gorge. -feldspathic. -with cells +semi-amygdaloidal. + +Lavas, specific gravity of. + +Lava-streams blending together at St. Jago. -composition of surface of. +-differences in the state of their surfaces. -extreme thinness of. +-heaved up into hillocks at the Galapagos archipelago. -their fluidity. +-with irregular hummocks at Ascension. + +Lead, separation from silver. + +Lesson, M., on craters at Ascension. + +Leucite. + +Lime, sulphate of, at Ascension. + +Lonsdale, Mr., on fossil-corals from Van Diemen’s land. + +Lot, St. Helena. + +Lyell, Mr., on craters of elevation. -on embedded turtles’ eggs. -on +glossy coating to dikes. + +Macaulay, Dr., on calcareous casts at Madeira. + +MacCulloch, Dr., on an amygdaloid. -on chlorophaeite. -on laminated +pitchstone. + +Mackenzie, Sir G., on cavernous lava-streams. -on glossy coatings to +dikes. -on obsidian streams. -on stratification in Iceland. + +Madeira, calcareous casts at. + +“Magazine, Nautical,” account of volcanic phenomena in the Atlantic. + +Marekanite. + +Mauritius, crater of elevation of. + +Mica, in rounded nodules. -origin in metamorphic slate. -radiating form +of. + +Miller, Prof., on ejected Labrador feldspar. -on quartz crystals in +obsidian beds. + +Mitchell, Sir T., on bombs. -on the Australian valleys. + +Mud streams at the Galapagos archipelago. + +Narborough island. + +Nelson, Lieut., on the Bermuda islands. + +New Caledonia. + +New Red sandstone, cross cleavage of. + +New South Wales. + +New Zealand. + +Nulliporae (fossil), resembling concretions. + +Obsidian, absent at the Galapagos archipelago. -bombs of. -composition +and origin of. -crystals of feldspar sink in. -its irruption from lofty +craters. -passage of beds into. -specific gravity of. -streams of. + +Olivine decomposed at St. Jago. -at Van Diemen’s land. -in the lavas at +the Galapagos archipelago. + +Oolitic structure of recent calcareous beds at St. Helena. + +Otaheite. + +Oysters, extinction of. + +Panza islands, laminated trachyte of. + +Pattinson, Mr., on the separation of lead and silver. + +Paul’s, St., rocks of. + +Pearlstone. + +Peperino. + +Peron, M., on calcareous rocks of Australia. + +Phonolite, hills of. -laminated. -with more fusible hornblende. + +Pitchstone. -dikes of. + +Plants, extinct. + +Plutonic rocks, separation of constituent parts of, by gravity. + +Porto Praya. + +Prevost, M. C., on rarity of great dislocations in volcanic islands. + +Prosperous hill, St. Helena. + +Pumice, absent at the Galapagos archipelago. -laminated. + +Puy de Dome, trachyte of. + +Quail island, St. Jago. + +Quartz, crystals of, in beds alternating with obsidian. -crystallised +in sandstone. -fusibility of. -rock, mottled from metamorphic action +with earthy matter. + +Red hill. + +Resin-like altered scoriae. + +Rio de Janeiro, gneiss of. + +Robert, M., on strata of Iceland. + +Rogers, Professor, on curved lines of elevation. + +Salses, compared with tuff craters. + +Salt deposited by the sea. -in volcanic strata. -lakes of, in craters. + +Sandstone of Brazil. -of the Cape of Good Hope. -platforms of, in New +South Wales. + +Schorl, radiating. + +Scrope, Mr. P., on laminated trachyte. -on obsidian. -on separation of +trachyte and basalt. -on silex in trachyte. -on sphaerulites. + +Seale, Mr., geognosy of St. Helena. -on dikes. -on embedded birds’ +bones. + +Seale, on extinct shells of St. Helena. + +Sedgwick, Professor, on concretions. + +Septaria, in concretions in tuff. + +Serpulae on upraised rocks. + +Seychelles. + +Shells, colour of, affected by light. -from Van Diemen’s land. -land, +extinct, at St. Helena. -particles of, drifted by the wind at St. +Helena. + +Shelly matter deposited by the waves. + +Siau, M., on ripples. + +Signal Post Hill. + +Silica, deposited by steam. -large proportion of, in obsidian. +-specific gravity of. + +Siliceous sinter. + +Smith, Dr. A., on junction of granite and clay-slate. + +Spallanzani on decomposed trachyte. + +Specific gravity of recent calcareous rocks and of limestone. -of +lavas. + +Sphaerulites in glass and in silicified wood. -in obsidian. + +Sowerby, Mr. G.B., on fossil-shells from Van Diemen’s land. -from St. +Jago. -land-shells from St. Helena. + +St. Helena. -crater of elevation of. + +St. Jago, crater of elevation of. -effects of calcareous matter on +lava. + +St. Paul’s rocks. + +Stokes, Mr., collections of sphaerulites and of obsidians. + +Stony-top, Little. -Great. + +Stratification of sandstone in New South Wales. + +Streams of obsidian. + +Stutchbury, Mr., on marine remains at Otaheite. + +Subsided space at Ascension. + +Tahiti. + +Talus, stratified, within tuff craters. + +Terceira. + +Tertiary deposit of St. Jago. + +Trachyte, absent at the Galapagos archipelago. -at Ascension. -at +Terceira. -decomposition of, by steam. -its lamination. -its separation +from basalt. -softened at Ascension. -specific gravity of. -with +singular veins. + +Trap-dikes in the plutonic series. -at King George’s sound. + +Travertin at Van Diemen’s land. + +Tropic-bird, now rare, at St. Helena. + +Tuff, craters of. -their breached state. -peculiar kind of. + +Turner, Mr., on the separation of molten metals. + +Tyerman and Bennett on marine remains at Huaheine. + +Valleys, gorge-like, at St. Helena. -in New South Wales. -in St. Jago. + +Van Diemen’s land. + +Veins in trachyte. -of jasper. + +Vincent, Bory St., on bombs. + +Volcanic bombs. -island in process of formation in the Atlantic. +-islands, their distribution. + +Wacke, its passage into lava. + +Wackes, argillaceous. + +Webster, Dr., on a basin-formed island. -on gypsum at Ascension. + +White, Martin, on soundings. + +Wind, effects of, on the form of craters. + + + + +*** END OF THE PROJECT GUTENBERG EBOOK VOLCANIC ISLANDS *** + +***** This file should be named 3054-0.txt or 3054-0.zip ***** +This and all associated files of various formats will be found in: + https://www.gutenberg.org/3/0/5/3054/ + +Updated editions will replace the previous one--the old editions will +be renamed. + +Creating the works from print editions not protected by U.S. copyright +law means that no one owns a United States copyright in these works, +so the Foundation (and you!) can copy and distribute it in the +United States without permission and without paying copyright +royalties. 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You may copy it, give it away or re-use it under the terms +of the Project Gutenberg License included with this eBook or online +at <a href="https://www.gutenberg.org">www.gutenberg.org</a>. If you +are not located in the United States, you will have to check the laws of the +country where you are located before using this eBook. +</div> +<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Title: Geological Observations on the Volcanic Islands</div> +<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Author: Charles Darwin</div> +<div style='display:block;margin:1em 0'>Release Date: February, 2002 [eBook #3054]<br /> +[Most recently updated: March 29, 2021]</div> +<div style='display:block;margin:1em 0'>Language: English</div> +<div style='display:block;margin:1em 0'>Character set encoding: UTF-8</div> +<div style='display:block; margin-left:2em; text-indent:-2em'>Produced by: Sue Asscher </div> +<div style='margin-top:2em;margin-bottom:4em'>*** START OF THE PROJECT GUTENBERG EBOOK VOLCANIC ISLANDS ***</div> + +<h1>Geological Observations on the Volcanic Islands</h1> + +<h2 class="no-break">by Charles Darwin</h2> + +<hr /> + +<h2>Contents</h2> + +<table summary="" style=""> + +<tr> +<td> <a href="#pref01">EDITORIAL NOTE</a></td> +</tr> + +<tr> +<td> <a href="#toc01">DETAILED TABLE OF CONTENTS</a></td> +</tr> + +<tr> +<td> <a href="#pref02">CRITICAL INTRODUCTION</a></td> +</tr> + +<tr> +<td> <a href="#chap01">CHAPTER I.—ST. JAGO, IN THE CAPE DE VERDE ARCHIPELAGO</a></td> +</tr> + +<tr> +<td> <a href="#chap02">CHAPTER II.—FERNANDO NORONHA; TERCEIRA; TAHITI, ETC</a></td> +</tr> + +<tr> +<td> <a href="#chap03">CHAPTER III.—ASCENSION</a></td> +</tr> + +<tr> +<td> <a href="#chap04">CHAPTER IV.—ST. HELENA</a></td> +</tr> + +<tr> +<td> <a href="#chap05">CHAPTER V.—GALAPAGOS ARCHIPELAGO</a></td> +</tr> + +<tr> +<td> <a href="#chap06">CHAPTER VI.—TRACHYTE AND BASALT.—DISTRIBUTION OF VOLCANIC ISLES</a></td> +</tr> + +<tr> +<td> <a href="#chap07">CHAPTER VII.—AUSTRALIA; NEW ZEALAND; CAPE OF GOOD HOPE</a></td> +</tr> + +<tr> +<td> <a href="#chap08">INDEX TO VOLCANIC ISLANDS</a></td> +</tr> + +</table> + +<div class="chapter"> + +<h2><a name="pref01"></a>EDITORIAL NOTE</h2> + +<p> +Although in some respects more technical in their subjects and style than +Darwin’s “Journal,” the books here reprinted will never lose +their value and interest for the originality of the observations they contain. +Many parts of them are admirably adapted for giving an insight into problems +regarding the structure and changes of the earth’s surface, and in fact +they form a charming introduction to physical geology and physiography in their +application to special domains. The books themselves cannot be obtained for +many times the price of the present volume, and both the general reader, who +desires to know more of Darwin’s work, and the student of geology, who +naturally wishes to know how a master mind reasoned on most important +geological subjects, will be glad of the opportunity of possessing them in a +convenient and cheap form. +</p> + +<p> +The three introductions, which my friend Professor Judd has kindly furnished, +give critical and historical information which makes this edition of special +value. +</p> + +<h3>G.T.B.</h3> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2>VOLCANIC ISLANDS.</h2> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="toc01"></a>DETAILED TABLE OF CONTENTS</h2> + +<h3><a href="#chap01">I.—ST. JAGO, IN THE CAPE DE VERDE ARCHIPELAGO.</a></h3> + +<p> +Rocks of the lowest series.—A calcareous sedimentary deposit, with recent +shells, altered by the contact of superincumbent lava, its horizontality and +extent.—Subsequent volcanic eruptions, associated with calcareous matter +in an earthy and fibrous form, and often enclosed within the separate cells of +the scoriae.—Ancient and obliterated orifices of eruption of small +size.—Difficulty of tracing over a bare plain recent streams of +lava.—Inland hills of more ancient volcanic rock.—Decomposed +olivine in large masses.—Feldspathic rocks beneath the upper crystalline +basaltic strata.—Uniform structure and form of the more ancient volcanic +hills.—Form of the valleys near the coast.—Conglomerate now forming +on the sea beach. +</p> + +<h3><a href="#chap02">II.—FERNANDO NORONHA; TERCEIRA; TAHITI, ETC.</a></h3> + +<p> +FERNANDO NORONHA.—Precipitous hill of phonolite. +</p> + +<p> +TERCEIRA.—Trachytic rocks: their singular decomposition by steam of high +temperature. +</p> + +<p> +TAHITI.—Passage from wacke into trap; singular volcanic rock with the +vesicles half-filled with mesotype. +</p> + +<p> +MAURITIUS.—Proofs of its recent elevation.—Structure of its more +ancient mountains; similarity with St. Jago. +</p> + +<p> +ST. PAUL’S ROCKS.—Not of volcanic origin.—Their singular +mineralogical composition. +</p> + +<h3><a href="#chap03">III.—ASCENSION.</a></h3> + +<p> +Basaltic lavas.—Numerous craters truncated on the same +side.—Singular structure of volcanic bombs.—Aeriform +explosions.—Ejected granite fragments.—Trachytic +rocks.—Singular veins.—Jasper, its manner of +formation.—Concretions in pumiceous tuff.—Calcareous deposits and +frondescent incrustations on the coast.—Remarkable laminated beds, +alternating with, and passing into obsidian.—Origin of obsidian.— +Lamination of volcanic rocks. +</p> + +<h3><a href="#chap04">IV.—ST. HELENA.</a></h3> + +<p> +Lavas of the feldspathic, basaltic, and submarine series.—Section of +Flagstaff Hill and of the Barn.—Dikes.—Turk’s Cap and +Prosperous Bays.— Basaltic ring.—Central crateriform ridge, with an +internal ledge and a parapet.—Cones of phonolite.—Superficial beds +of calcareous sandstone.— Extinct land-shells.—Beds of +detritus.—Elevation of the land.— Denudation.—Craters of +elevation. +</p> + +<h3><a href="#chap05">V.—GALAPAGOS ARCHIPELAGO.</a></h3> + +<p> +Chatham Island.—Craters composed of a peculiar kind of tuff.—Small +basaltic craters, with hollows at their bases.—Albemarle Island; fluid +lavas, their composition.—Craters of tuff; inclination of their exterior +diverging strata, and structure of their interior converging +strata.—James Island, segment of a small basaltic crater; fluidity and +composition of its lava-streams, and of its ejected fragments.—Concluding +remarks on the craters of tuff, and on the breached condition of their southern +sides.— Mineralogical composition of the rocks of the +archipelago.—Elevation of the land.—Direction of the fissures of +eruption. +</p> + +<h3><a href="#chap06">VI.—TRACHYTE AND BASALT.—DISTRIBUTION OF VOLCANIC ISLES.</a></h3> + +<p> +The sinking of crystals in fluid lava.—Specific gravity of the +constituent parts of trachyte and of basalt, and their consequent +separation.— Obsidian.—Apparent non-separation of the elements of +plutonic rocks.— Origin of trap-dikes in the plutonic +series.—Distribution of volcanic islands; their prevalence in the great +oceans.—They are generally arranged in lines.—The central volcanoes +of Von Buch doubtful.—Volcanic islands bordering +continents.—Antiquity of volcanic islands, and their elevation in +mass.—Eruptions on parallel lines of fissure within the same geological +period. +</p> + +<h3><a href="#chap07">VII.—AUSTRALIA; NEW ZEALAND; CAPE OF GOOD HOPE.</a></h3> + +<p> +New South Wales.—Sandstone formation.—Embedded pseudo-fragments of +shale.—Stratification.—Current-cleavage.—Great +valleys.—Van Diemen’s Land.—Palaeozoic formation.—Newer +formation with volcanic rocks.— Travertin with leaves of extinct +plants.—Elevation of the land.—New Zealand.—King +George’s Sound.—Superficial ferruginous beds.—Superficial +calcareous deposits, with casts of branches; its origin from drifted particles +of shells and corals.—Their extent.—Cape of Good Hope.— +Junction of the granite and clay-slate.—Sandstone formation. +</p> + +<h3><a href="#chap08">INDEX.</a></h3> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2>GEOLOGICAL OBSERVATIONS ON VOLCANIC ISLANDS.</h2> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="pref02"></a>CRITICAL INTRODUCTION.</h2> + +<p> +The preparation of the series of works published under the general title +“Geology of the Voyage of the ‘Beagle’” occupied a +great part of Darwin’s time during the ten years that followed his return +to England. The second volume of the series, entitled “Geological +Observations on Volcanic Islands, with Brief Notices on the Geology of +Australia and the Cape of Good Hope,” made its appearance in 1844. The +materials for this volume were collected in part during the outward voyage, +when the “Beagle” called at St. Jago in the Cape de Verde Islands, +and St. Paul’s Rocks, and at Fernando Noronha, but mainly during the +homeward cruise; then it was that the Galapagos Islands were surveyed, the Low +Archipelago passed through, and Tahiti visited; after making calls at the Bay +of Islands, in New Zealand, and also at Sydney, Hobart Town and King +George’s Sound in Australia, the “Beagle” sailed across the +Indian Ocean to the little group of the Keeling or Cocos Islands, which Darwin +has rendered famous by his observations, and thence to Mauritius; calling at +the Cape of Good Hope on her way, the ship then proceeded successively to St. +Helena and Ascension, and revisited the Cape de Verde Islands before finally +reaching England. +</p> + +<p> +Although Darwin was thus able to gratify his curiosity by visits to a great +number of very interesting volcanic districts, the voyage opened for him with a +bitter disappointment. He had been reading Humboldt’s “Personal +Narrative” during his last year’s residence in Cambridge, and had +copied out from it long passages about Teneriffe. He was actually making +inquiries as to the best means of visiting that island, when the offer was made +to him to accompany Captain Fitzroy in the “Beagle. “ His friend +Henslow too, on parting with him, had given him the advice to procure and read +the recently published first volume of the “Principles of Geology,” +though he warned him against accepting the views advocated by its author. +During the time the “Beagle” was beating backwards and forwards +when the voyage commenced, Darwin, although hardly ever able to leave his +berth, was employing all the opportunities which the terrible sea-sickness left +him, in studying Humboldt and Lyell. We may therefore form an idea of his +feelings when, on the ship reaching Santa Cruz, and the Peak of Teneriffe +making its appearance among the clouds, they were suddenly informed that an +outbreak of cholera would prevent any landing! +</p> + +<p> +Ample compensation for this disappointment was found, however, when the ship +reached Porta Praya in St. Jago, the largest of the Cape de Verde Islands. Here +he spent three most delightful weeks, and really commenced his work as a +geologist and naturalist. Writing to his father he says, “Geologising in +a volcanic country is most delightful; besides the interest attached to itself, +it leads you into most beautiful and retired spots. Nobody but a person fond of +Natural History can imagine the pleasure of strolling under cocoa-nuts in a +thicket of bananas and coffee-plants, and an endless number of wild flowers. +And this island, that has given me so much instruction and delight, is reckoned +the most uninteresting place that we perhaps shall touch at during our voyage. +It certainly is generally very barren, but the valleys are more exquisitely +beautiful, from the very contrast. It is utterly useless to say anything about +the scenery; it would be as profitable to explain to a blind man colours, as to +a person who has not been out of Europe, the total dissimilarity of a tropical +view. Whenever I enjoy anything, I always look forward to writing it down, +either in my log-book (which increases in bulk), or in a letter; so you must +excuse raptures, and those raptures badly expressed. I find my collections are +increasing wonderfully, and from Rio I think I shall be obliged to send a cargo +home.” +</p> + +<p> +The indelible impression made on Darwin’s mind by this first visit to a +volcanic island, is borne witness to by a remarkable passage in the +“Autobiography” written by him in 1876. “The geology of St. +Jago is very striking, yet simple; a stream of lava formerly flowed over the +bed of the sea, formed of triturated recent shells and corals, which it has +baked into a hard white rock. Since then the whole island has been upheaved. +But the line of white rock revealed to me a new and important fact, namely that +there had been afterwards subsidence round the craters which had since been in +action, and had poured forth lava. It then first dawned on me that I might +perhaps write a book on the geology of the various countries visited, and this +made me thrill with delight. That was a memorable hour to me, and how +distinctly I can call to mind the low cliff of lava beneath which I rested, +with the sun glaring hot, a few strange desert plants growing near and with +living corals in the tidal pools at my feet.” +</p> + +<p> +Only five years before, when listening to poor Professor Jameson’s +lectures on the effete Wernerianism, which at that time did duty for geological +teaching, Darwin had found them “incredibly dull,” and he declared +that “the sole effect they produced on me was a determination never so +long as I lived to read a book on Geology, or in any way to study the +science.” +</p> + +<p> +What a contrast we find in the expressions which he makes use of in referring +to Geological Science, in his letters written home from the +“Beagle!” After alluding to the delight of collecting and studying +marine animals, he exclaims, “But Geology carries the day!” Writing +to Henslow he says, “I am quite charmed with Geology, but, like the wise +animal between two bundles of hay, I do not know which to like best; the old +crystalline group of rocks, or the softer and more fossiliferous beds.” +And just as the long voyage is about to come to a close he again writes, +“I find in Geology a never-failing interest; as it has been remarked, it +creates the same grand ideas respecting this world which Astronomy does for the +Universe.” In this passage Darwin doubtless refers to a remark of Sir +John Herschel’s in his admirable “Preliminary Discourse on the +Study of Natural Philosophy,”—a book which exercised a most +remarkable and beneficial influence on the mind of the young naturalist. +</p> + +<p> +If there cannot be any doubt as to the strong predilection in Darwin’s +mind for geological studies, both during and after the memorable voyage, there +is equally little difficulty in perceiving the school of geological thought +which, in spite of the warnings of Sedgwick and Henslow, had obtained complete +ascendancy over his mind. He writes in 1876: “The very first place which +I examined, namely St. Jago in the Cape de Verde Islands, showed me clearly the +wonderful superiority of Lyell’s manner of treating Geology, compared +with that of any other author, whose works I had with me, or ever afterwards +read.” And again, “The science of Geology is enormously indebted to +Lyell—more so, as I believe, than to any other man who ever lived...I am +proud to remember that the first place, namely, St. Jago, in the Cape de Verde +Archipelago, in which I geologised, convinced me of the infinite superiority of +Lyell’s views over those advocated in any other work known to me.” +</p> + +<p> +The passages I have cited will serve to show the spirit in which Darwin entered +upon his geological studies, and the perusal of the following pages will +furnish abundant proofs of the enthusiasm, acumen, and caution with which his +researches were pursued. +</p> + +<p> +Large collections of rocks and minerals were made by Darwin during his +researches, and sent home to Cambridge, to be kept under the care of his +faithful friend Henslow. After visiting his relations and friends, +Darwin’s first care on his return to England was to unpack and examine +these collections. He accordingly, at the end of 1836, took lodgings for three +months in Fitzwilliam Street, Cambridge, so as to be near Henslow; and in +studying and determining his geological specimens received much valuable aid +from the eminent crystallographer and mineralogist, Professor William Hallows +Miller. +</p> + +<p> +The actual writing of the volume upon volcanic islands was not commenced till +1843, when Darwin had settled in the spot which became his home for the rest of +his life—the famous house at Down, in Kent. Writing to his friend Mr. +Fox, on March 28th, 1843, he says, “I am very slowly progressing with a +volume, or rather pamphlet, on the volcanic islands which we visited: I manage +only a couple of hours per day, and that not very regularly. It is uphill work +writing books, which cost money in publishing, and which are not read even by +geologists.” +</p> + +<p> +The work occupied Darwin during the whole of the year 1843, and was issued in +the spring of the following year, the actual time engaged in preparing it being +recorded in his diary as “from the summer of 1842 to January 1844;” +but the author does not appear to have been by any means satisfied with the +result when the book was finished. He wrote to Lyell, “You have pleased +me much by saying that you intend looking through my ‘Volcanic +Islands;’ it cost me eighteen months!!! and I have heard of very few who +have read it. Now I shall feel, whatever little (and little it is) there is +confirmatory of old work, or new, will work its effect and not be lost.” +To Sir Joseph Hooker he wrote, “I have just finished a little volume on +the volcanic islands which we visited. I do not know how far you care for dry +simple geology, but I hope you will let me send you a copy.” +</p> + +<p> +Every geologist knows how full of interest and suggestiveness is this book of +Darwin’s on volcanic islands. 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 when, hammer in +hand, he roamed over new and interesting scenes, and the slow, laborious, and +less congenial task of re-writing and arranging his notes in book-form. +</p> + +<p> +In 1874, in writing an account of the ancient volcanoes of the Hebrides, I had +frequent occasion to quote Mr. Darwin’s observations on the Atlantic +volcanoes, in illustration of the phenomena exhibited by the relics of still +older volcanoes in our own islands. Darwin, in writing to his old friend Sir +Charles Lyell upon the subject, says, “I was not a little pleased to see +my volcanic book quoted, for I thought it was completely dead and +forgotten.” +</p> + +<p> +Two years later the original publishers of this book and of that on South +America proposed to re-issue them. Darwin at first hesitated, for he seemed to +think there could be little of abiding interest in them; he consulted me upon +the subject in one of the conversations which I used to have with him at that +time, and I strongly urged upon him the reprint of the works. I was much +gratified when he gave way upon the point, and consented to their appearing +just as originally issued. In his preface he says, “Owing to the great +progress which Geology has made in recent times, my views on some few points +may be somewhat antiquated, but I have thought it best to leave them as they +originally appeared.” +</p> + +<p> +It may be interesting to indicate, as briefly as possible, the chief geological +problem upon which the publication of Darwin’s “Volcanic +Islands” threw new and important light. The merit of the work consisted +in supplying interesting observations, which in some cases have proved of +crucial value in exploding prevalent fallacies; in calling attention to +phenomena and considerations that had been quite overlooked by geologists, but +have since exercised an important influence in moulding geological speculation; +and lastly in showing the importance which attaches to small and seemingly +insignificant causes, some of which afford a key to the explanation of very +curious geological problems. +</p> + +<p> +Visiting as he did the districts in which Von Buch and others had found what +they thought to be evidence of the truth of “Elevation-craters,” +Darwin was able to show that the facts were capable of a totally different +interpretation. The views originally put forward by the old German geologist +and traveller, and almost universally accepted by his countrymen, had met with +much support from Elie de Beaumont and Dufrenoy, the leaders of geological +thought in France. They were, however, stoutly opposed by Scrope and Lyell in +this country, and by Constant Prevost and Virlet on the other side of the +channel. Darwin, in the work before us, 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 which, opening at +the top, have given origin to the craters. Admitting the influence of the +injection of lava into the structure of the volcanic cones, in increasing their +bulk and elevation, he shows that, in the main, the volcanoes are built up by +repeated ejections causing an accumulation of materials around the vent. +</p> + +<p> +While, however, agreeing on the whole with Scrope and Lyell, as to the +explosive origin of ordinary volcanic craters, Darwin clearly saw that, in some +cases, great craters might be formed or enlarged, by the subsidence of the +floors after eruptions. The importance of this agency, to which too little +attention has been directed by geologists, has recently been shown by Professor +Dana, in his admirable work on Kilauea and the other great volcanoes of the +Hawaiian Archipelago. +</p> + +<p> +The effects of subsidence at a volcanic centre in producing a downward dip of +the strata around it, was first pointed out by Darwin, as the result of his +earliest work in the Cape de Verde Islands. Striking illustrations of the same +principle have since been pointed out by M. Robert and others in Iceland, by +Mr. Heaphy in New Zealand, and by myself in the Western Isles of Scotland. +</p> + +<p> +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 crust, along which the lavas +have made their way to the surface. But he, at the same time, clearly saw that +there was no evidence of the occurrence of great deluges of lava along such +fissures; he showed how the most remarkable plateaux, composed of successive +lava sheets, might be built up by repeated and moderate ejections from numerous +isolated vents; 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 denudation. +</p> + +<p> +One of the most striking parts of the book is that in which he deals with the +effects of denudation in producing “basal wrecks” or worn down +stumps of volcanoes. He was enabled to examine a series of cases in which could +be traced every gradation, from perfect volcanic cones down to the solidified +plugs which had consolidated in the vents from which ejections had taken place. +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 history. Like Lyell, he was firmly +persuaded of the continuity of geological history, and ever delighted in +finding indications, in the present order of nature, that the phenomena of the +past could be accounted for by means of causes which are still in operation. +Lyell’s last work in the field was carried on about his home in +Forfarshire, and only a few months before his death he wrote to Darwin: +“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 for, by the enormous time to which the products of the oldest +volcanoes have been subjected to chemical changes.” +</p> + +<p> +Darwin was greatly impressed, as the result of his studies of volcanic +phenomena, followed by an examination of the great granite-masses of the Andes, +with the relations between the so-called Plutonic rocks and those of +undoubtedly volcanic origin. It was indeed a fortunate circumstance, that after +studying some excellent examples of recent volcanic rocks, he proceeded to +examine in South America many fine illustrations of the older igneous +rock-masses, and especially of the most highly crystalline types of the same, +and then on his way home had opportunities of reviving the impression made upon +him by the fresh and unaltered volcanic rocks. Some of the general +considerations suggested by these observations were discussed in a paper read +by him before the Geological Society, on March 7th, 1838, under the title +“On the Connection of Certain Volcanic Phenomena, and On the Formation of +Mountain-chains, and the Effect of Continental Elevations.” The exact +bearing of these two classes of facts upon one another are more fully discussed +in his book on South American geology. +</p> + +<p> +The proofs of recent elevation around many of the volcanic islands led Darwin +to conclude that volcanic areas were, as a rule, regions in which upward +movements were taking place, and he was naturally led to contrast them with the +areas in which, as he showed, the occurrence of atolls, encircling reefs, and +barrier-reefs afford indication of subsidence. In this way he was able to map +out the oceanic areas in different zones, along which opposite kinds of +movement were taking place. His conclusions on this subject were full of +novelty and suggestiveness. +</p> + +<p> +Very clearly did Darwin recognise the importance of the fact that most of the +oceanic islands appear to be of volcanic origin, though he was careful to point +out the remarkable exceptions which somewhat invalidate the generalisation. In +his “Origin of Species” he has elaborated the idea and suggested +the theory of the permanence of ocean-basins, a suggestion which has been +adopted and pushed farther by subsequent authors, than we think its originator +would have approved. 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 him. +</p> + +<p> +Some years before the voyage of the “Beagle,” Mr. Poulett Scrope +had pointed out the remarkable analogies that exist between certain igneous +rocks of banded structure, as seen in the Ponza Islands, and the foliated +crystalline schists. It does not appear that Darwin was acquainted with this +remarkable memoir, 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 Ascension. Coming fresh from the study of the great masses of +crystalline schist in the South American continent, 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 +“folia.” 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 stresses, a separation and parallel arrangement of the constituent +minerals will result. This is a process which is now fully recognised as having +been a potent factor in the production of the metamorphic rock, and has been +called by more recent writers “dynamo-metamorphism.” +</p> + +<p> +In this, and in many similar discussions, in which exact mineralogical +knowledge was required, 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-lens, supplemented by a few chemical tests and the constant +use of the blowpipe. Since his day, the method of study of rocks by thin +sections under the microscope has been devised, and has become a most efficient +aid in all petrographical inquiries. During the voyage of H.M.S. +“Challenger,” many of the islands studied by Darwin have been +revisited and their rocks collected. 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 +“Reports on the ‘Challenger’ Expedition.” While much +that is new and valuable has been contributed to geological science by these +more recent investigations, and many changes have been made in nomenclature and +other points of detail, 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 study, and remain as a monument of the acumen and accuracy in +minute observation of these pioneers in geological research. +</p> + +<h3>JOHN W. JUDD.</h3> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap01"></a>CHAPTER I.<br /> +ST. JAGO, IN THE CAPE DE VERDE ARCHIPELAGO.</h2> + +<p> +Rocks of the lowest series. A calcareous sedimentary deposit, with recent +shells, altered by the contact of superincumbent lava, its horizontality and +extent. Subsequent volcanic eruptions, associated with calcareous matter in an +earthy and fibrous form, and often enclosed within the separate cells of the +scoriae. Ancient and obliterated orifices of eruption of small size. Difficulty +of tracing over a bare plain recent streams of lava. Inland hills of more +ancient volcanic rock. Decomposed olivine in large masses. Feldspathic rocks +beneath the upper crystalline basaltic strata. Uniform structure and form of +the more ancient volcanic hills. Form of the valleys near the coast. +Conglomerate now forming on the sea beach. +</p> + +<h3>(FIGURE 1: MAP 1: PART OF ST. JAGO, ONE OF THE CAPE DE VERDE ISLANDS.) +</h3> + +<p> +The island of St. Jago extends in a N.N.W. and S.S.E. direction, thirty miles +in length by about twelve in breadth. My observations, made during two visits, +were confined to the southern portion within the distance of a few leagues from +Porto Praya. The country, viewed from the sea, presents a varied outline: +smooth conical hills of a reddish colour (like Red Hill in Figure 1 (Map 1). +(The outline of the coast, the position of the villages, streamlets, and of +most of the hills in this woodcut, are copied from the chart made on board +H.M.S. “Leven.” The square-topped hills (A, B, C, etc.) are put in +merely by eye, to illustrate my description.)), and others less regular, +flat-topped, and of a blackish colour (like A, B, C,) rise from successive, +step-formed plains of lava. At a distance, a chain of mountains, many thousand +feet in height, traverses the interior of the island. There is no active +volcano in St. Jago, and only one in the group, namely at Fogo. The island +since being inhabited has not suffered from destructive earthquakes. +</p> + +<p> +The lowest rocks exposed on the coast near Porto Praya, are highly crystalline +and compact; they appear to be of ancient, submarine, volcanic origin; they are +unconformably covered by a thin, irregular, calcareous deposit, abounding with +shells of a late tertiary period; and this again is capped by a wide sheet of +basaltic lava, which has flowed in successive streams from the interior of the +island, between the square-topped hills marked A, B, C, etc. Still more recent +streams of lava have been erupted from the scattered cones, such as Red and +Signal Post Hills. The upper strata of the square-topped hills are intimately +related in mineralogical composition, and in other respects, with the lowest +series of the coast- rocks, with which they seem to be continuous. +</p> + +<h3>MINERALOGICAL DESCRIPTION OF THE ROCKS OF THE LOWEST SERIES.</h3> + +<p> +These rocks possess an extremely varying character; they consist of black, +brown, and grey, compact, basaltic bases, with numerous crystals of augite, +hornblende, olivine, mica, and sometimes glassy feldspar. A common variety is +almost entirely composed of crystals of augite with olivine. Mica, it is known, +seldom occurs where augite abounds; nor probably does the present case offer a +real exception, for the mica (at least in my best characterised specimen, in +which one nodule of this mineral is nearly half an inch in length) is as +perfectly rounded as a pebble in a conglomerate, and evidently has not been +crystallised in the base, in which it is now enclosed, but has proceeded from +the fusion of some pre-existing rock. These compact lavas alternate with tuffs, +amygdaloids, and wacke, and in some places with coarse conglomerate. Some of +the argillaceous wackes are of a dark green colour, others, pale +yellowish-green, and others nearly white; I was surprised to find that some of +the latter varieties, even where whitest, fused into a jet black enamel, whilst +some of the green varieties afforded only a pale gray bead. Numerous dikes, +consisting chiefly of highly compact augitic rocks, and of gray amygdaloidal +varieties, intersect the strata, which have in several places been dislocated +with considerable violence, and thrown into highly inclined positions. One line +of disturbance crosses the northern end of Quail Island (an islet in the Bay of +Porto Praya), and can be followed to the mainland. These disturbances took +place before the deposition of the recent sedimentary bed; and the surface, +also, had previously been denuded to a great extent, as is shown by many +truncated dikes. +</p> + +<p> +DESCRIPTION OF THE CALCAREOUS DEPOSIT OVERLYING THE FOREGOING VOLCANIC ROCKS. +</p> + +<p> +This stratum is very conspicuous from its white colour, and from the extreme +regularity with which it ranges in a horizontal line for some miles along the +coast. Its average height above the sea, measured from the upper line of +junction with the superincumbent basaltic lava, is about sixty feet; and its +thickness, although varying much from the inequalities of the underlying +formation, may be estimated at about twenty feet. It consists of quite white +calcareous matter, partly composed of organic debris, and partly of a substance +which may be aptly compared in appearance with mortar. Fragments of rock and +pebbles are scattered throughout this bed, often forming, especially in the +lower part, a conglomerate. Many of the fragments of rock are whitewashed with +a thin coating of calcareous matter. At Quail Island, the calcareous deposit is +replaced in its lowest part by a soft, brown, earthy tuff, full of Turritellae; +this is covered by a bed of pebbles, passing into sandstone, and mixed with +fragments of echini, claws of crabs, and shells; the oyster-shells still +adhering to the rock on which they grew. Numerous white balls appearing like +pisolitic concretions, from the size of a walnut to that of an apple, are +embedded in this deposit; they usually have a small pebble in their centres. +Although so like concretions, a close examination convinced me that they were +Nulliporae, retaining their proper forms, but with their surfaces slightly +abraded: these bodies (plants as they are now generally considered to be) +exhibit under a microscope of ordinary power, no traces of organisation in +their internal structure. Mr. George R. Sowerby has been so good as to examine +the shells which I collected: there are fourteen species in a sufficiently +perfect condition for their characters to be made out with some degree of +certainty, and four which can be referred only to their genera. Of the fourteen +shells, of which a list is given in the Appendix, eleven are recent species; +one, though undescribed, is perhaps identical with a species which I found +living in the harbour of Porto Praya; the two remaining species are unknown, +and have been described by Mr. Sowerby. Until the shells of this Archipelago +and of the neighbouring coasts are better known, it would be rash to assert +that even these two latter shells are extinct. The number of species which +certainly belong to existing kinds, although few in number, are sufficient to +show that the deposit belongs to a late tertiary period. From its mineralogical +character, from the number and size of the embedded fragments, and from the +abundance of Patellae, and other littoral shells, it is evident that the whole +was accumulated in a shallow sea, near an ancient coast-line. +</p> + +<p> +EFFECTS PRODUCED BY THE FLOWING OF THE SUPERINCUMBENT BASALTIC LAVA OVER THE +CALCAREOUS DEPOSIT. +</p> + +<p> +These effects are very curious. The calcareous matter is altered to the depth +of about a foot beneath the line of junction; and a most perfect gradation can +be traced, from loosely aggregated, small, particles of shells, corallines, and +Nulliporae, into a rock, in which not a trace of mechanical origin can be +discovered, even with a microscope. Where the metamorphic change has been +greatest, two varieties occur. The first is a hard, compact, white, +fine-grained rock, striped with a few parallel lines of black volcanic +particles, and resembling a sandstone, but which, upon close examination, is +seen to be crystallised throughout, with the cleavages so perfect that they can +be readily measured by the reflecting goniometer. In specimens, where the +change has been less complete, when moistened and examined under a strong lens, +the most interesting gradation can be traced, some of the rounded particles +retaining their proper forms, and others insensibly melting into the +granulo-crystalline paste. The weathered surface of this stone, as is so +frequently the case with ordinary limestones, assumes a brick-red colour. +</p> + +<p> +The second metamorphosed variety is likewise a hard rock, but without any +crystalline structure. It consists of a white, opaque, compact, calcareous +stone, thickly mottled with rounded, though regular, spots of a soft, earthy, +ochraceous substance. This earthy matter is of a pale yellowish- brown colour, +and appears to be a mixture of carbonate of lime with iron; it effervesces with +acids, is infusible, but blackens under the blowpipe, and becomes magnetic. The +rounded form of the minute patches of earthy substance, and the steps in the +progress of their perfect formation, which can be followed in a suit of +specimens, clearly show that they are due either to some power of aggregation +in the earthy particles amongst themselves, or more probably to a strong +attraction between the atoms of the carbonate of line, and consequently to the +segregation of the earthy extraneous matter. I was much interested by this +fact, because I have often seen quartz rocks (for instance, in the Falkland +Islands, and in the lower Silurian strata of the Stiper-stones in Shropshire), +mottled in a precisely analogous manner, with little spots of a white, earthy +substance (earthy feldspar?); and these rocks, there was good reason to +suppose, had undergone the action of heat,—a view which thus receives +confirmation. This spotted structure may possibly afford some indication in +distinguishing those formations of quartz, which owe their present structure to +igneous action, from those produced by the agency of water alone; a source of +doubt, which I should think from my own experience, that most geologists, when +examining arenaceo-quartzose districts must have experienced. +</p> + +<p> +The lowest and most scoriaceous part of the lava, in rolling over the +sedimentary deposit at the bottom of the sea, has caught up large quantities of +calcareous matter, which now forms a snow-white, highly crystalline basis to a +breccia, including small pieces of black, glossy scoriae. A little above this, +where the lime is less abundant, and the lava more compact, numerous little +balls, composed of spicula of calcareous spar, radiating from common centres, +occupy the interstices. In one part of Quail Island, the lime has thus been +crystallised by the heat of the superincumbent lava, where it is only thirteen +feet in thickness; nor had the lava been originally thicker, and since reduced +by degradation, as could be told from the degree of cellularity of its surface. +I have already observed that the sea must have been shallow in which the +calcareous deposit was accumulated. In this case, therefore, the carbonic acid +gas has been retained under a pressure, insignificant compared with that (a +column of water, 1,708 feet in height) originally supposed by Sir James Hall to +be requisite for this end: but since his experiments, it has been discovered +that pressure has less to do with the retention of carbonic acid gas, than the +nature of the circumjacent atmosphere; and hence, as is stated to be the case +by Mr. Faraday, masses of limestone are sometimes fused and crystallised even +in common limekilns. (I am much indebted to Mr. E.W. Brayley in having given me +the following references to papers on this subject: Faraday in the +“Edinburgh New Philosophical Journal” volume 15 page 398; +Gay-Lussac in “Annales de Chem. et Phys.” tome 63 page 219 +translated in the “London and Edinburgh Philosophical Magazine” +volume 10 page 496.) Carbonate of lime can be heated to almost any degree, +according to Faraday, in an atmosphere of carbonic acid gas, without being +decomposed; and Gay-Lussac found that fragments of limestone, placed in a tube +and heated to a degree, not sufficient by itself to cause their decomposition, +yet immediately evolved their carbonic acid, when a stream of common air or +steam was passed over them: Gay-Lussac attributes this to the mechanical +displacement of the nascent carbonic acid gas. The calcareous matter beneath +the lava, and especially that forming the crystalline spicula between the +interstices of the scoriae, although heated in an atmosphere probably composed +chiefly of steam, could not have been subjected to the effects of a passing +stream; and hence it is, perhaps, that they have retained their carbonic acid, +under a small amount of pressure. +</p> + +<p> +The fragments of scoriae, embedded in the crystalline calcareous basis, are of +a jet black colour, with a glossy fracture like pitchstone. Their surfaces, +however, are coated with a layer of a reddish-orange, translucent substance, +which can easily be scratched with a knife; hence they appear as if overlaid by +a thin layer of rosin. Some of the smaller fragments are partially changed +throughout into this substance: a change which appears quite different from +ordinary decomposition. At the Galapagos Archipelago (as will be described in a +future chapter), great beds are formed of volcanic ashes and particles of +scoriae, which have undergone a closely similar change. +</p> + +<h3>THE EXTENT AND HORIZONTALITY OF THE CALCAREOUS STRATUM.</h3> + +<p> +(FIGURE 2: SIGNAL POST HILL. (Section with A low and C high.) +</p> + +<p> +A.—Ancient volcanic rocks. +</p> + +<p> +B.—Calcareous stratum. +</p> + +<p> +C.—Upper basaltic lava.) +</p> + +<p> +The upper line of surface of the calcareous stratum, which is so conspicuous +from being quite white and so nearly horizontal, ranges for miles along the +coast, at the height of about sixty feet above the sea. The sheet of basalt, by +which it is capped, is on an average eighty feet in thickness. Westward of +Porto Praya beyond Red Hill, the white stratum with the superincumbent basalt +is covered up by more recent streams. Northward of Signal Post Hill, I could +follow it with my eye, trending away for several miles along the sea cliffs. +The distance thus observed is about seven miles; but I cannot doubt from its +regularity that it extends much farther. In some ravines at right angles to the +coast, it is seen gently dipping towards the sea, probably with the same +inclination as when deposited round the ancient shores of the island. I found +only one inland section, namely, at the base of the hill marked A, where, at +the height of some hundred feet, this bed was exposed; it here rested on the +usual compact augitic rock associated with wacke, and was covered by the +widespread sheet of modern basaltic lava. Some exceptions occur to the +horizontality of the white stratum: at Quail Island, its upper surface is only +forty feet above the level of the sea; here also the capping of lava is only +between twelve and fifteen feet in thickness; on the other hand, at the +north-east side of Porto Praya harbour, the calcareous stratum, as well as the +rock on which it rests, attain a height above the average level: the inequality +of level in these two cases is not, as I believe, owing to unequal elevation, +but to original irregularities at the bottom of the sea. Of this fact, at Quail +Island, there was clear evidence in the calcareous deposit being in one part of +much greater than the average thickness, and in another part being entirely +absent; in this latter case, the modern basaltic lavas rested directly on those +of more ancient origin. +</p> + +<p> +Under Signal Post Hill, the white stratum dips into the sea in a remarkable +manner. This hill is conical, 450 feet in height, and retains some traces of +having had a crateriform structure; it is composed chiefly of matter erupted +posteriorly to the elevation of the great basaltic plain, but partly of lava of +apparently submarine origin and of considerable antiquity. The surrounding +plain, as well as the eastern flank of this hill, has been worn into steep +precipices, overhanging the sea. In these precipices, the white calcareous +stratum may be seen, at the height of about seventy feet above the beach, +running for some miles both northward and southward of the hill, in a line +appearing to be perfectly horizontal; but for a space of a quarter of a mile +directly under the hill, it dips into the sea and disappears. On the south side +the dip is gradual, on the north side it is more abrupt, as is shown in Figure +2. As neither the calcareous stratum, nor the superincumbent basaltic lava (as +far as the latter can be distinguished from the more modern ejections), appears +to thicken as it dips, I infer that these strata were not originally +accumulated in a trough, the centre of which afterwards became a point of +eruption; but that they have subsequently been disturbed and bent. We may +suppose either that Signal Post Hill subsided after its elevation with the +surrounding country, or that it never was uplifted to the same height with it. +This latter seems to me the most probable alternative, for during the slow and +equable elevation of this portion of the island, the subterranean motive power, +from expending part of its force in repeatedly erupting volcanic matter from +beneath this point, would, it is likely, have less force to uplift it. +Something of the same kind seems to have occurred near Red Hill, for when +tracing upwards the naked streams of lava from near Porto Praya towards the +interior of the island, I was strongly induced to suspect, that since the lava +had flowed, the slope of the land had been slightly modified, either by a small +subsidence near Red Hill, or by that portion of the plain having been uplifted +to a less height during the elevation of the whole area. +</p> + +<h3>THE BASALTIC LAVA, SUPERINCUMBENT ON THE CALCAREOUS DEPOSIT.</h3> + +<p> +This lava is of a pale grey colour, fusing into a black enamel; its fracture is +rather earthy and concretionary; it contains olivine in small grains. The +central parts of the mass are compact, or at most crenulated with a few minute +cavities, and are often columnar. At Quail Island this structure was assumed in +a striking manner; the lava in one part being divided into horizontal laminae, +which became in another part split by vertical fissures into five-sided plates; +and these again, being piled on each other, insensibly became soldered +together, forming fine symmetrical columns. The lower surface of the lava is +vesicular, but sometimes only to the thickness of a few inches; the upper +surface, which is likewise vesicular, is divided into balls, frequently as much +as three feet in diameter, made up of concentric layers. The mass is composed +of more than one stream; its total thickness being, on an average, about eighty +feet: the lower portion has certainly flowed beneath the sea, and probably +likewise the upper portion. The chief part of this lava has flowed from the +central districts, between the hills marked A, B, C, etc., in the woodcut- map. +The surface of the country, near the coast, is level and barren; towards the +interior, the land rises by successive terraces, of which four, when viewed +from a distance, could be distinctly counted. +</p> + +<p> +VOLCANIC ERUPTIONS SUBSEQUENT TO THE ELEVATION OF THE COASTLAND; THE EJECTED +MATTER ASSOCIATED WITH EARTHY LIME. +</p> + +<p> +These recent lavas have proceeded from those scattered, conical, reddish- +coloured hills, which rise abruptly from the plain-country near the coast. I +ascended some of them, but will describe only one, namely, RED HILL, which may +serve as a type of its class, and is remarkable in some especial respects. Its +height is about six hundred feet; it is composed of bright red, highly +scoriaceous rock of a basaltic nature; on one side of its summit there is a +hollow, probably the last remnant of a crater. Several of the other hills of +this class, judging from their external forms, are surmounted by much more +perfect craters. When sailing along the coast, it was evident that a +considerable body of lava had flowed from Red Hill, over a line of cliff about +one hundred and twenty feet in height, into the sea: this line of cliff is +continuous with that forming the coast, and bounding the plain on both sides of +this hill; these streams, therefore, were erupted, after the formation of the +coast-cliffs, from Red Hill, when it must have stood, as it now does, above the +level of the sea. This conclusion accords with the highly scoriaceous condition +of all the rock on it, appearing to be of subaerial formation: and this is +important, as there are some beds of calcareous matter near its summit, which +might, at a hasty glance, have been mistaken for a submarine deposit. These +beds consist of white, earthy, carbonate of lime, extremely friable so as to be +crushed with the least pressure; the most compact specimens not resisting the +strength of the fingers. Some of the masses are as white as quicklime, and +appear absolutely pure; but on examining them with a lens, minute particles of +scoriae can always be seen, and I could find none which, when dissolved in +acids, did not leave a residue of this nature. It is, moreover, difficult to +find a particle of the lime which does not change colour under the blowpipe, +most of them even becoming glazed. The scoriaceous fragments and the calcareous +matter are associated in the most irregular manner, sometimes in obscure beds, +but more generally as a confused breccia, the lime in some parts and the +scoriae in others being most abundant. Sir H. De la Beche has been so kind as +to have some of the purest specimens analysed, with a view to discover, +considering their volcanic origin, whether they contained much magnesia; but +only a small portion was found, such as is present in most limestones. +</p> + +<p> +Fragments of the scoriae embedded in the calcareous mass, when broken, exhibit +many of their cells lined and partly filled with a white, delicate, excessively +fragile, moss-like, or rather conferva-like, reticulation of carbonate of lime. +These fibres, examined under a lens of one-tenth of an inch focal distance, +appear cylindrical; they are rather above one- thousandth of an inch in +diameter; they are either simply branched, or more commonly united into an +irregular mass of network, with the meshes of very unequal sizes and of unequal +numbers of sides. Some of the fibres are thickly covered with extremely minute +spicula, occasionally aggregated into little tuffs; and hence they have a hairy +appearance. These spicula are of the same diameter throughout their length; +they are easily detached, so that the object-glass of the microscope soon +becomes scattered over with them. Within the cells of many fragments of the +scoria, the lime exhibits this fibrous structure, but generally in a less +perfect degree. These cells do not appear to be connected with one another. +There can be no doubt, as will presently be shown, that the lime was erupted, +mingled with the lava in its fluid state, and therefore I have thought it worth +while to describe minutely this curious fibrous structure, of which I know +nothing analogous. From the earthy condition of the fibres, this structure does +not appear to be related to crystallisation. +</p> + +<p> +Other fragments of the scoriaceous rock from this hill, when broken, are often +seen marked with short and irregular white streaks, which are owing to a row of +separate cells being partly, or quite, filled with white calcareous powder. +This structure immediately reminded me of the appearance in badly kneaded +dough, of balls and drawn-out streaks of flour, which have remained unmixed +with the paste; and I cannot doubt that small masses of the lime, in the same +manner remaining unmixed with the fluid lava, have been drawn out when the +whole was in motion. I carefully examined, by trituration and solution in +acids, pieces of the scoriae, taken from within half-an-inch of those cells +which were filled with the calcareous powder, and they did not contain an atom +of free lime. It is obvious that the lava and lime have on a large scale been +very imperfectly mingled; and where small portions of the lime have been +entangled within a piece of the viscid lava, the cause of their now occupying, +in the form of a powder or of a fibrous reticulation, the vesicular cavities, +is, I think, evidently due to the confined gases having most readily expanded +at the points where the incoherent lime rendered the lava less adhesive. +</p> + +<p> +A mile eastward of the town of Praya, there is a steep-sided gorge, about one +hundred and fifty yards in width, cutting through the basaltic plain and +underlying beds, but since filled up by a stream of more modern lava. This lava +is dark grey, and in most parts compact and rudely columnar; but at a little +distance from the coast, it includes in an irregular manner a brecciated mass +of red scoriae mingled with a considerable quantity of white, friable, and in +some parts, nearly pure earthy lime, like that on the summit of Red Hill. This +lava, with its entangled lime, has certainly flowed in the form of a regular +stream; and, judging from the shape of the gorge, towards which the drainage of +the country (feeble though it now be) still is directed, and from the +appearance of the bed of loose water-worn blocks with their interstices +unfilled, like those in the bed of a torrent, on which the lava rests, we may +conclude that the stream was of subaerial origin. I was unable to trace it to +its source, but, from its direction, it seemed to have come from Signal Post +Hill, distant one mile and a quarter, which, like Red Hill, has been a point of +eruption subsequent to the elevation of the great basaltic plain. It accords +with this view, that I found on Signal Post Hill, a mass of earthy, calcareous +matter of the same nature, mingled with scoriae. I may here observe that part +of the calcareous matter forming the horizontal sedimentary bed, especially the +finer matter with which the embedded fragments of rock are whitewashed, has +probably been derived from similar volcanic eruptions, as well as from +triturated organic remains: the underlying, ancient, crystalline rocks, also, +are associated with much carbonate of lime, filling amygdaloidal cavities, and +forming irregular masses, the nature of which latter I was unable to +understand. +</p> + +<p> +Considering the abundance of earthy lime near the summit of Red Hill, a +volcanic cone six hundred feet in height, of subaerial +growth,—considering the intimate manner in which minute particles and +large masses of scoriae are embedded in the masses of nearly pure lime, and on +the other hand, the manner in which small kernels and streaks of the calcareous +powder are included in solid pieces of the scoriae,—considering, also, +the similar occurrence of lime and scoriae within a stream of lava, also +supposed, with good reason, to have been of modern subaerial origin, and to +have flowed from a hill, where earthy lime also occurs: I think, considering +these facts, there can be no doubt that the lime has been erupted, mingled with +the molten lava. I am not aware that any similar case has been described: it +appears to me an interesting one, inasmuch as most geologists must have +speculated on the probable effects of a volcanic focus, bursting through +deep-seated beds of different mineralogical composition. The great abundance of +free silex in the trachytes of some countries (as described by Beudant in +Hungary, and by P. Scrope in the Panza Islands), perhaps solves the inquiry +with respect to deep-seated beds of quartz; and we probably here see it +answered, where the volcanic action has invaded subjacent masses of limestone. +One is naturally led to conjecture in what state the now earthy carbonate of +lime existed, when ejected with the intensely heated lava: from the extreme +cellularity of the scoriae on Red Hill, the pressure cannot have been great, +and as most volcanic eruptions are accompanied by the emission of large +quantities of steam and other gases, we here have the most favourable +conditions, according to the views at present entertained by chemists, for the +expulsion of the carbonic acid. (Whilst deep beneath the surface, the carbonate +of lime was, I presume, in a fluid state. Hutton, it is known, thought that all +amygdaloids were produced by drops of molten limestone floating in the trap, +like oil in water: this no doubt is erroneous, but if the matter forming the +summit of Red Hill had been cooled under the pressure of a moderately deep sea, +or within the walls of a dike, we should, in all probability, have had a trap +rock associated with large masses of compact, crystalline, calcareous spar, +which, according to the views entertained by many geologists, would have been +wrongly attributed to subsequent infiltration.) Has the slow re- absorption of +this gas, it may be asked, given to the lime in the cells of the lava, that +peculiar fibrous structure, like that of an efflorescing salt? Finally, I may +remark on the great contrast in appearance between this earthy lime, which must +have been heated in a free atmosphere of steam and other gases, while the +white, crystalline, calcareous spar, produced by a single thin sheet of lava +(as at Quail Island) rolling over similar earthy lime and the debris of organic +remains, at the bottom of a shallow sea. +</p> + +<h3>SIGNAL POST HILL.</h3> + +<p> +This hill has already been several times mentioned, especially with reference +to the remarkable manner in which the white calcareous stratum, in other parts +so horizontal (Figure 2), dips under it into the sea. It has a broad summit, +with obscure traces of a crateriform structure, and is composed of basaltic +rocks (Of these, one common variety is remarkable for being full of small +fragments of a dark jasper-red earthy mineral, which, when examined carefully, +shows an indistinct cleavage; the little fragments are elongated in form, are +soft, are magnetic before and after being heated, and fuse with difficulty into +a dull enamel. This mineral is evidently closely related to the oxides of iron, +but I cannot ascertain what it exactly is. The rock containing this mineral is +crenulated with small angular cavities, which are lined and filled with +yellowish crystals of carbonate of lime.), some compact, others highly cellular +with inclined beds of loose scoriae, of which some are associated with earthy +lime. Like Red Hill, it has been the source of eruptions, subsequently to the +elevation of the surrounding basaltic plain; but unlike that hill, it has +undergone considerable denudation, and has been the seat of volcanic action at +a remote period, when beneath the sea. I judge of this latter circumstance from +finding on its inland flank the last remains of three small points of eruption. +These points are composed of glossy scoriae, cemented by crystalline calcareous +spar, exactly like the great submarine calcareous deposit, where the heated +lava has rolled over it: their demolished state can, I think, be explained only +by the denuding action of the waves of the sea. I was guided to the first +orifice by observing a sheet of lava, about two hundred yards square, with +steepish sides, superimposed on the basaltic plain with no adjoining hillock, +whence it could have been erupted; and the only trace of a crater which I was +able to discover, consisted of some inclined beds of scoriae at one of its +corners. At the distance of fifty yards from a second level-topped patch of +lava, but of much smaller size, I found an irregular circular group of masses +of cemented, scoriaceous breccia, about six feet in height, which doubtless had +once formed the point of eruption. The third orifice is now marked only by an +irregular circle of cemented scoriae, about four yards in diameter, and rising +in its highest point scarcely three feet above the level of the plain, the +surface of which, close all round, exhibits its usual appearance: here we have +a horizontal basal section of a volcanic spiracle, which, together with all its +ejected matter, has been almost totally obliterated. +</p> + +<p> +The stream of lava, which fills the narrow gorge eastward of the town of Praya, +judging from its course, seems, as before remarked, to have come from Signal +Post Hill, and to have flowed over the plain, after its elevation (The sides of +this gorge, where the upper basaltic stratum is intersected, are almost +perpendicular. The lava, which has since filled it up, is attached to these +sides, almost as firmly as a dike is to its walls. In most cases, where a +stream of lava has flowed down a valley, it is bounded on each side by loose +scoriaceous masses.): the same observation applies to a stream (possibly part +of the same one) capping the sea cliffs, a little eastward of the gorge. When I +endeavoured to follow these streams over the stony level plain, which is almost +destitute of soil and vegetation, I was much surprised to find, that although +composed of hard basaltic matter, and not having been exposed to marine +denudation, all distant traces of them soon became utterly lost. But I have +since observed at the Galapagos Archipelago, that it is often impossible to +follow even great deluges of quite recent lava across older streams, except by +the size of the bushes growing on them, or by the comparative states of +glossiness of their surfaces,—characters which a short lapse of time +would be sufficient quite to obscure. I may remark, that in a level country, +with a dry climate, and with the wind blowing always in one direction (as at +the Cape de Verde Archipelago), the effects of atmospheric degradation are +probably much greater than would at first be expected; for soil in this case +accumulates only in a few protected hollows, and being blown in one direction, +it is always travelling towards the sea in the form of the finest dust, leaving +the surface of the rocks bare, and exposed to the full effects of renewed +meteoric action. +</p> + +<h3>INLAND HILLS OF MORE ANCIENT VOLCANIC ROCKS.</h3> + +<p> +These hills are laid down by eye, and marked as A, B, C, etc., in Map 1. They +are related in mineralogical composition, and are probably directly continuous +with the lowest rocks exposed on the coast. These hills, viewed from a +distance, appear as if they had once formed part of an irregular tableland, and +from their corresponding structure and composition this probably has been the +case. They have flat, slightly inclined summits, and are, on an average, about +six hundred feet in height; they present their steepest slope towards the +interior of the island, from which point they radiate outwards, and are +separated from each other by broad and deep valleys, through which the great +streams of lava, forming the coast-plains, have descended. Their inner and +steeper escarpments are ranged in an irregular curve, which rudely follows the +line of the shore, two or three miles inland from it. I ascended a few of these +hills, and from others, which I was able to examine with a telescope, I +obtained specimens, through the kindness of Mr. Kent, the assistant-surgeon of +the “Beagle”; although by these means I am acquainted with only a +part of the range, five or six miles in length, yet I scarcely hesitate, from +their uniform structure, to affirm that they are parts of one great formation, +stretching round much of the circumference of the island. +</p> + +<p> +The upper and lower strata of these hills differ greatly in composition. The +upper are basaltic, generally compact, but sometimes scoriaceous and +amygdaloidal, with associated masses of wacke: where the basalt is compact, it +is either fine-grained or very coarsely crystallised; in the latter case it +passes into an augitic rock, containing much olivine; the olivine is either +colourless, or of the usual yellow and dull reddish shades. On some of the +hills, beds of calcareous matter, both in an earthy and in a crystalline form, +including fragments of glossy scoriae, are associated with the basaltic strata. +These strata differ from the streams of basaltic lava forming the coast-plains, +only in being more compact, and in the crystals of augite, and in the grains of +olivine being of much greater size;—characters which, together with the +appearance of the associated calcareous beds, induce me to believe that they +are of submarine formation. +</p> + +<p> +Some considerable masses of wacke, which are associated with these basaltic +strata, and which likewise occur in the basal series on the coast, especially +at Quail Island, are curious. They consist of a pale yellowish- green +argillaceous substance, of a crumbling texture when dry, but unctuous when +moist: in its purest form, it is of a beautiful green tint, with translucent +edges, and occasionally with obscure traces of an original cleavage. Under the +blowpipe it fuses very readily into a dark grey, and sometimes even black bead, +which is slightly magnetic. From these characters, I naturally thought that it +was one of the pale species, decomposed, of the genus augite;—a +conclusion supported by the unaltered rock being full of large separate +crystals of black augite, and of balls and irregular streaks of dark grey +augitic rock. As the basalt ordinarily consists of augite, and of olivine often +tarnished and of a dull red colour, I was led to examine the stages of +decomposition of this latter mineral, and I found, to my surprise, that I could +trace a nearly perfect gradation from unaltered olivine to the green wacke. +Part of the same grain under the blowpipe would in some instances behave like +olivine, its colour being only slightly changed, and part would give a black +magnetic bead. Hence I can have no doubt that the greenish wacke originally +existed as olivine; but great chemical changes must have been effected during +the act of decomposition thus to have altered a very hard, transparent, +infusible mineral, into a soft, unctuous, easily melted, argillaceous +substance. (D’Aubuisson “Traite de Geognosie” tome 2 page 569 +mentions, on the authority of M. Marcel de Serres, masses of green earth near +Montpellier, which are supposed to be due to the decomposition of olivine. I do +not, however, find, that the action of this mineral under the blowpipe being +entirely altered, as it becomes decomposed, has been noticed; and the knowledge +of this fact is important, as at first it appears highly improbable that a +hard, transparent, refractory mineral should be changed into a soft, easily +fused clay, like this of St. Jago. I shall hereafter describe a green +substance, forming threads within the cells of some vesicular basaltic rocks in +Van Diemen’s Land, which behave under the blowpipe like the green wacke +of St. Jago; but its occurrence in cylindrical threads, shows it cannot have +resulted from the decomposition of olivine, a mineral always existing in the +form of grains or crystals.) +</p> + +<p> +The basal strata of these hills, as well as some neighbouring, separate, bare, +rounded hillocks, consist of compact, fine-grained, non-crystalline (or so +slightly as scarcely to be perceptible), ferruginous, feldspathic rocks, and +generally in a state of semi-decomposition. Their fracture is exceedingly +irregular, and splintery; yet small fragments are often very tough. They +contain much ferruginous matter, either in the form of minute grains with a +metallic lustre, or of brown hair-like threads: the rock in this latter case +assuming a pseudo-brecciated structure. These rocks sometimes contain mica and +veins of agate. Their rusty brown or yellowish colour is partly due to the +oxides of iron, but chiefly to innumerable, microscopically minute, black +specks, which, when a fragment is heated, are easily fused, and evidently are +either hornblende or augite. These rocks, therefore, although at first +appearing like baked clay or some altered sedimentary deposit, contain all the +essential ingredients of trachyte; from which they differ only in not being +harsh, and in not containing crystals of glassy feldspar. As is so often the +case with trachytic formation, no stratification is here apparent. A person +would not readily believe that these rocks could have flowed as lava; yet at +St. Helena there are well-characterised streams (as will be described in an +ensuing chapter) of nearly similar composition. Amidst the hillocks composed of +these rocks, I found in three places, smooth conical hills of phonolite, +abounding with fine crystals of glassy feldspar, and with needles of +hornblende. These cones of phonolite, I believe, bear the same relation to the +surrounding feldspathic strata which some masses of coarsely crystallised +augitic rock, in another part of the island, bear to the surrounding basalt, +namely, that both have been injected. The rocks of a feldspathic nature being +anterior in origin to the basaltic strata, which cap them, as well as to the +basaltic streams of the coast-plains, accords with the usual order of +succession of these two grand divisions of the volcanic series. +</p> + +<p> +The strata of most of these hills in the upper part, where alone the planes of +division are distinguishable, are inclined at a small angle from the interior +of the island towards the sea-coast. The inclination is not the same in each +hill; in that marked A it is less than in B, D, or E; in C the strata are +scarcely deflected from a horizontal plane, and in F (as far as I could judge +without ascending it) they are slightly inclined in a reverse direction, that +is, inwards and towards the centre of the island. Notwithstanding these +differences of inclination, their correspondence in external form, and in the +composition both of their upper and lower parts,- -their relative position in +one curved line, with their steepest sides turned inwards,—all seem to +show that they originally formed parts of one platform; which platform, as +before remarked, probably extended round a considerable portion of the +circumference of the island. The upper strata certainly flowed as lava, and +probably beneath the sea, as perhaps did the lower feldspathic masses: how then +come these strata to hold their present position, and whence were they erupted? +</p> + +<p> +In the centre of the island there are lofty mountains, but they are separated +from the steep inland flanks of these hills by a wide space of lower country: +the interior mountains, moreover, seem to have been the source of those great +streams of basaltic lava which, contracting as they pass between the bases of +the hills in question, expand into the coast- plains. (I saw very little of the +inland parts of the island. Near the village of St. Domingo, there are +magnificent cliffs of rather coarsely crystallised basaltic lava. Following the +little stream in this valley, about a mile above the village, the base of the +great cliff was formed of a compact fine-grained basalt, conformably covered by +a bed of pebbles. Near Fuentes, I met with pap-formed hills of the compact +feldspathic series of rocks.) Round the shores of St. Helena there is a rudely +formed ring of basaltic rocks, and at Mauritius there are remnants of another +such a ring round part, if not round the whole, of the island; here again the +same question immediately occurs, how came these masses to hold their present +position, and whence were they erupted? The same answer, whatever it may be, +probably applies in these three cases; and in a future chapter we shall recur +to this subject. +</p> + +<h3>VALLEYS NEAR THE COAST.</h3> + +<p> +These are broad, very flat, and generally bounded by low cliff-formed sides. +Portions of the basaltic plain are sometimes nearly or quite isolated by them; +of which fact, the space on which the town of Praya stands offers an instance. +The great valley west of the town has its bottom filled up to a depth of more +than twenty feet by well-rounded pebbles, which in some parts are firmly +cemented together by white calcareous matter. There can be no doubt, from the +form of these valleys, that they were scooped out by the waves of the sea, +during that equable elevation of the land, of which the horizontal calcareous +deposit, with its existing species of marine remains, gives evidence. +Considering how well shells have been preserved in this stratum, it is singular +that I could not find even a single small fragment of shell in the conglomerate +at the bottom of the valleys. The bed of pebbles in the valley west of the town +is intersected by a second valley joining it as a tributary, but even this +valley appears much too wide and flat-bottomed to have been formed by the small +quantity of water, which falls only during one short wet season; for at other +times of the year these valleys are absolutely dry. +</p> + +<h3>RECENT CONGLOMERATE.</h3> + +<p> +On the shores of Quail Island, I found fragments of brick, bolts of iron, +pebbles, and large fragments of basalt, united by a scanty base of impure +calcareous matter into a firm conglomerate. To show how exceedingly firm this +recent conglomerate is, I may mention, that I endeavoured with a heavy +geological hammer to knock out a thick bolt of iron, which was embedded a +little above low-water mark, but was quite unable to succeed. +</p> + + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap02"></a>CHAPTER II.<br /> +FERNANDO NORONHA; TERCEIRA; TAHITI, ETC.</h2> + +<p> +FERNANDO NORONHA. Precipitous hill of phonolite. +</p> + +<p> +TERCEIRA. Trachytic rocks: their singular decomposition by steam of high +temperature. +</p> + +<p> +TAHITI. Passage from wacke into trap; singular volcanic rock with the vesicles +half-filled with mesotype. +</p> + +<p> +MAURITIUS. Proofs of its recent elevation. Structure of its more ancient +mountains; similarity with St. Jago. +</p> + +<p> +ST. PAUL’S ROCKS. Not of volcanic origin. Their singular mineralogical +composition. +</p> + +<h3>FERNANDO NORONHA.</h3> + +<p> +During our short visit at this and the four following islands, I observed very +little worthy of description. Fernando Noronha is situated in the Atlantic +Ocean, in latitude 3 degrees 50 minutes S., and 230 miles distant from the +coast of South America. It consists of several islets, together nine miles in +length by three in breadth. The whole seems to be of volcanic origin; although +there is no appearance of any crater, or of any one central eminence. The most +remarkable feature is a hill 1,000 feet high, of which the upper 400 feet +consist of a precipitous, singularly shaped pinnacle, formed of columnar +phonolite, containing numerous crystals of glassy feldspar, and a few needles +of hornblende. From the highest accessible point of this hill, I could +distinguish in different parts of the group several other conical hills, +apparently of the same nature. At St. Helena there are similar, great, conical, +protuberant masses of phonolite, nearly one thousand feet in height, which have +been formed by the injection of fluid feldspathic lava into yielding strata. If +this hill has had, as is probable, a similar origin, denudation has been here +effected on an enormous scale. Near the base of this hill, I observed beds of +white tuff, intersected by numerous dikes, some of amygdaloidal basalt and +others of trachyte; and beds of slaty phonolite with the planes of cleavage +directed N.W. and S.E. Parts of this rock, where the crystals were scanty, +closely resembled common clay-slate, altered by the contact of a trap-dike. The +lamination of rocks, which undoubtedly have once been fluid, appears to me a +subject well deserving attention. On the beach there were numerous fragments of +compact basalt, of which rock a distant facade of columns seemed to be formed. +</p> + +<h3>TERCEIRA IN THE AZORES.</h3> + +<p> +The central parts of this island consist of irregularly rounded mountains of no +great elevation, composed of trachyte, which closely resembles in general +character the trachyte of Ascension, presently to be described. This formation +is in many parts overlaid, in the usual order of superposition, by streams of +basaltic lava, which near the coast compose nearly the whole surface. The +course which these streams have followed from their craters, can often be +followed by the eye. The town of Angra is overlooked by a crateriform hill +(Mount Brazil), entirely built of thin strata of fine-grained, harsh, +brown-coloured tuff. The upper beds are seen to overlap the basaltic streams on +which the town stands. This hill is almost identical in structure and +composition with numerous crateriformed hills in the Galapagos Archipelago. +</p> + +<h3>EFFECTS OF STEAM ON THE TRACHYTIC ROCKS.</h3> + +<p> +In the central part of the island there is a spot, where steam is constantly +issuing in jets from the bottom of a small ravine-like hollow, which has no +exit, and which abuts against a range of trachytic mountains. The steam is +emitted from several irregular fissures: it is scentless, soon blackens iron, +and is of much too high temperature to be endured by the hand. The manner in +which the solid trachyte is changed on the borders of these orifices is +curious: first, the base becomes earthy, with red freckles evidently due to the +oxidation of particles of iron; then it becomes soft; and lastly, even the +crystals of glassy feldspar yield to the dissolving agent. After the mass is +converted into clay, the oxide of iron seems to be entirely removed from some +parts, which are left perfectly white, whilst in other neighbouring parts, +which are of the brightest red colour, it seems to be deposited in greater +quantity; some other masses are marbled with two distinct colours. Portions of +the white clay, now that they are dry, cannot be distinguished by the eye from +the finest prepared chalk; and when placed between the teeth they feel equally +soft-grained; the inhabitants use this substance for white-washing their +houses. The cause of the iron being dissolved in one part, and close by being +again deposited, is obscure; but the fact has been observed in several other +places. (Spallanzani, Dolomieu, and Hoffman have described similar cases in the +Italian volcanic islands. Dolomieu says the iron at the Panza Islands is +redeposited in the form of veins (page 86 “Memoire sur les Isles +Ponces”). These authors likewise believe that the steam deposits silica: +it is now experimentally known that vapour of a high temperature is able to +dissolve silica.) In some half-decayed specimens, I found small, globular +aggregations of yellow hyalite, resembling gum-arabic, which no doubt had been +deposited by the steam. +</p> + +<p> +As there is no escape for the rain-water, which trickles down the sides of the +ravine-like hollow, whence the steam issues, it must all percolate downwards +through the fissures at its bottom. Some of the inhabitants informed me that it +was on record that flames (some luminous appearance?) had originally proceeded +from these cracks, and that the flames had been succeeded by the steam; but I +was not able to ascertain how long this was ago, or anything certain on the +subject. When viewing the spot, I imagined that the injection of a large mass +of rock. like the cone of phonolite at Fernando Noronha, in a semi-fluid state, +by arching the surface might have caused a wedge-shaped hollow with cracks at +the bottom, and that the rain- water percolating to the neighbourhood of the +heated mass, would during many succeeding years be driven back in the form of +steam. +</p> + +<h3>TAHITI (OTAHEITE).</h3> + +<p> +I visited only a part of the north-western side of this island, and this part +is entirely composed of volcanic rocks. Near the coast there are several +varieties of basalt, some abounding with large crystals of augite and tarnished +olivine, others compact and earthy,—some slightly vesicular, and others +occasionally amygdaloidal. These rocks are generally much decomposed, and to my +surprise, I found in several sections that it was impossible to distinguish, +even approximately, the line of separation between the decayed lava and the +alternating beds of tuff. Since the specimens have become dry, it is rather +more easy to distinguish the decomposed igneous rocks from the sedimentary +tuffs. This gradation in character between rocks having such widely different +origins, may I think be explained by the yielding under pressure of the +softened sides of the vesicular cavities, which in many volcanic rocks occupy a +large proportion of their bulk. As the vesicles generally increase in size and +number in the upper parts of a stream of lava, so would the effects of their +compression increase; the yielding, moreover, of each lower vesicle must tend +to disturb all the softened matter above it. Hence we might expect to trace a +perfect gradation from an unaltered crystalline rock to one in which all the +particles (although originally forming part of the same solid mass) had +undergone mechanical displacement; and such particles could hardly be +distinguished from others of similar composition, which had been deposited as +sediment. As lavas are sometimes laminated in their upper parts even horizontal +lines, appearing like those of aqueous deposition, could not in all cases be +relied on as a criterion of sedimentary origin. From these considerations it is +not surprising that formerly many geologists believed in real transitions from +aqueous deposits, through wacke, into igneous traps. +</p> + +<p> +In the valley of Tia-auru, the commonest rocks are basalts with much olivine, +and in some cases almost composed of large crystals of augite. I picked up some +specimens, with much glassy feldspar, approaching in character to trachyte. +There were also many large blocks of vesicular basalt, with the cavities +beautifully lined with chabasie (?), and radiating bundles of mesotype. Some of +these specimens presented a curious appearance, owing to a number of the +vesicles being half filled up with a white, soft, earthy mesotypic mineral, +which intumesced under the blowpipe in a remarkable manner. As the upper +surfaces in all the half-filled cells are exactly parallel, it is evident that +this substance has sunk to the bottom of each cell from its weight. Sometimes, +however, it entirely fills the cells. Other cells are either quite filled, or +lined, with small crystals, apparently of chabasie; these crystals, also, +frequently line the upper half of the cells partly filled with the earthy +mineral, as well as the upper surface of this substance itself, in which case +the two minerals appear to blend into each other. I have never seen any other +amygdaloid with the cells half filled in the manner here described; and it is +difficult to imagine the causes which determined the earthy mineral to sink +from its gravity to the bottom of the cells, and the crystalline mineral to +adhere in a coating of equal thickness round the sides of the cells. +(MacCulloch, however, has described and given a plate of (“Geolog. +Trans.” 1st series volume 4 page 225) a trap rock, with cavities filled +up horizontally with quartz and chalcedony. The upper halves of these cavities +are often filled by layers, which follow each irregularity of the surface, and +by little depending stalactites of the same siliceous substances.) +</p> + +<p> +The basic strata on the sides of the valley are gently inclined seaward, and I +nowhere observed any sign of disturbance; the strata are separated from each +other by thick, compact beds of conglomerate, in which the fragments are large, +some being rounded, but most angular. From the character of these beds, from +the compact and crystalline condition of most of the lavas, and from the nature +of the infiltrated minerals, I was led to conjecture that they had originally +flowed beneath the sea. This conclusion agrees with the fact that the Rev. W. +Ellis found marine remains at a considerable height, which he believes were +interstratified with volcanic matter; as is likewise described to be the case +by Messrs. Tyerman and Bennett at Huaheine, an island in this same archipelago. +Mr. Stutchbury also discovered near the summit of one of the loftiest mountains +of Tahiti, at the height of several thousand feet, a stratum of semi-fossil +coral. None of these remains have been specifically examined. On the coast, +where masses of coral-rock would have afforded the clearest evidence, I looked +in vain for any signs of recent elevation. For references to the above +authorities, and for more detailed reasons for not believing that Tahiti has +been recently elevated, I must refer to the “Structure and Distribution +of Coral-Reefs.” +</p> + +<h3>MAURITIUS.</h3> + +<p> +Approaching this island on the northern or north-western side, a curved chain +of bold mountains, surmounted by rugged pinnacles, is seen to rise from a +smooth border of cultivated land, which gently slopes down to the coast. At the +first glance, one is tempted to believe that the sea lately reached the base of +these mountains, and upon examination, this view, at least with respect to the +inferior parts of the border, is found to be perfectly correct. Several authors +have described masses of upraised coral- rock round the greater part of the +circumference of the island. (Captain Carmichael, in Hooker’s “Bot. +Misc.” volume 2 page 301. Captain Lloyd has lately, in the +“Proceedings of the Geological Society” (volume 3 page 317), +described carefully some of these masses. In the “Voyage a l’Isle +de France, par un Officier du Roi,” many interesting facts are given on +this subject. Consult also “Voyage aux Quatre Isles d’Afrique, par +M. Bory St. Vincent.”) Between Tamarin Bay and the Great Black River I +observed, in company with Captain Lloyd, two hillocks of coral-rock, formed in +their lower part of hard calcareous sandstone, and in their upper of great +blocks, slightly aggregated, of Astraea and Madrepora, and of fragments of +basalt; they were divided into beds dipping seaward, in one case at an angle of +8 degrees, and in the other at 18 degrees; they had a water-worn appearance, +and they rose abruptly from a smooth surface, strewed with rolled debris of +organic remains, to a height of about twenty feet. The Officier du Roi, in his +most interesting tour in 1768 round the island, has described masses of +upraised coral-rocks, still retaining that moat-like structure (see my +“Coral Reefs”) which is characteristic of the living reefs. On the +coast northward of Port Louis, I found the lava concealed for a considerable +space inland by a conglomerate of corals and shells, like those on the beach, +but in parts consolidated by red ferruginous matter. M. Bory St. Vincent has +described similar calcareous beds over nearly the whole of the plain of +Pamplemousses. Near Port Louis, when turning over some large stones, which lay +in the bed of a stream at the head of a protected creek, and at the height of +some yards above the level of spring tides, I found several shells of serpula +still adhering to their under sides. +</p> + +<p> +The jagged mountains near Port Louis rise to a height of between two and three +thousand feet; they consist of strata of basalt, obscurely separated from each +other by firmly aggregated beds of fragmentary matter; and they are intersected +by a few vertical dikes. The basalt in some parts abounds with large crystals +of augite and olivine, and is generally compact. The interior of the island +forms a plain, raised probably about a thousand feet above the level of the +sea, and composed of streams of lava which have flowed round and between the +rugged basaltic mountains. These more recent lavas are also basaltic, but less +compact, and some of them abound with feldspar, so that they even fuse into a +pale coloured glass. On the banks of the Great River, a section is exposed +nearly five hundred feet deep, worn through numerous thin sheets of the lava of +this series, which are separated from each other by beds of scoriae. They seem +to have been of subaerial formation, and to have flowed from several points of +eruption on the central platform, of which the Piton du Milieu is said to be +the principal one. There are also several volcanic cones, apparently of this +modern period, round the circumference of the island, especially at the +northern end, where they form separate islets. +</p> + +<p> +The mountains composed of the more compact and crystalline basalt, form the +main skeleton of the island. M. Bailly (“Voyage aux Terres +Australes” tome 1 page 54.) states that they all “se developpent +autour d’elle comme une ceinture d’immenses remparts, toutes +affectant une pente plus ou moins enclinee vers le rivage de la mer; tandis, au +contraire, que vers le centre de l’ile elles presentent une coupe +abrupte, et souvent taillee a pic. Toutes ces montagnes sont formees de couches +paralleles inclinees du centre de l’ile vers la mer.” These +statements have been disputed, though not in detail, by M. Quoy, in the voyage +of Freycinet. As far as my limited means of observation went, I found them +perfectly correct. (M. Lesson, in his account of this island, in the +“Voyage of the ‘Coquille’,” seems to follow M. +Bailly’s views.) The mountains on the N.W. side of the island, which I +examined, namely, La Pouce, Peter Botts, Corps de Garde, Les Mamelles, and +apparently another farther southward, have precisely the external shape and +stratification described by M. Bailly. They form about a quarter of his girdle +of ramparts. Although these mountains now stand quite detached, being separated +from each other by breaches, even several miles in width, through which deluges +of lava have flowed from the interior of the island; nevertheless, seeing their +close general similarity, one must feel convinced that they originally formed +parts of one continuous mass. Judging from the beautiful map of the Mauritius, +published by the Admiralty from a French MS., there is a range of mountains (M. +Bamboo) on the opposite side of the island, which correspond in height, +relative position, and external form, with those just described. Whether the +girdle was ever complete may well be doubted; but from M. Bailly’s +statements, and my own observations, it may be safely concluded that mountains +with precipitous inland flanks, and composed of strata dipping outwards, once +extended round a considerable portion of the circumference of the island. The +ring appears to have been oval and of vast size; its shorter axis, measured +across from the inner sides of the mountains near Port Louis and those near +Grand Port, being no less than thirteen geographical miles in length. M. Bailly +boldly supposes that this enormous gulf, which has since been filled up to a +great extent by streams of modern lava, was formed by the sinking in of the +whole upper part of one great volcano. +</p> + +<p> +It is singular in how many respects those portions of St. Jago and of Mauritius +which I visited agree in their geological history. At both islands, mountains +of similar external form, stratification, and (at least in their upper beds) +composition, follow in a curved chain the coast-line. These mountains in each +case appear originally to have formed parts of one continuous mass. The +basaltic strata of which they are composed, from their compact and crystalline +structure, seem, when contrasted with the neighbouring basaltic streams of +subaerial formation, to have flowed beneath the pressure of the sea, and to +have been subsequently elevated. We may suppose that the wide breaches between +the mountains were in both cases worn by the waves, during their gradual +elevation—of which process, within recent times, there is abundant +evidence on the coast-land of both islands. At both, vast streams of more +recent basaltic lavas have flowed from the interior of the island, round and +between the ancient basaltic hills; at both, moreover, recent cones of eruption +are scattered around the circumference of the island; but at neither have +eruptions taken place within the period of history. As remarked in the last +chapter, it is probable that these ancient basaltic mountains, which resemble +(at least in many respects) the basal and disturbed remnants of two gigantic +volcanoes, owe their present form, structure, and position, to the action of +similar causes. +</p> + +<h3>ST. PAUL’S ROCKS.</h3> + +<p> +This small island is situated in the Atlantic Ocean, nearly one degree north of +the equator, and 540 miles distant from South America, in 29 degrees 15 minutes +west longitude. Its highest point is scarcely fifty feet above the level of the +sea; its outline is irregular, and its entire circumference barely +three-quarters of a mile. This little point of rock rises abruptly out of the +ocean; and, except on its western side, soundings were not obtained, even at +the short distance of a quarter of a mile from its shore. It is not of volcanic +origin; and this circumstance, which is the most remarkable point in its +history (as will hereafter be referred to), properly ought to exclude it from +the present volume. It is composed of rocks, unlike any which I have met with, +and which I cannot characterise by any name, and must therefore describe. +</p> + +<p> +The simplest, and one of the most abundant kinds, is a very compact, heavy, +greenish-black rock, having an angular, irregular fracture, with some points +just hard enough to scratch glass, and infusible. This variety passes into +others of paler green tints, less hard, but with a more crystalline fracture, +and translucent on their edges; and these are fusible into a green enamel. +Several other varieties are chiefly characterised by containing innumerable +threads of dark-green serpentine, and by having calcareous matter in their +interstices. These rocks have an obscure, concretionary structure, and are full +of variously coloured angular pseudo fragments. These angular pseudo fragments +consist of the first-described dark green rock, of a brown softer kind, of +serpentine, and of a yellowish harsh stone, which, perhaps, is related to +serpentine rock. There are other vesicular, calcareo-ferruginous, soft stones. +There is no distinct stratification, but parts are imperfectly laminated; and +the whole abounds with innumerable veins, and vein-like masses, both small and +large. Of these vein-like masses, some calcareous ones, which contain minute +fragments of shells, are clearly of subsequent origin to the others. +</p> + +<h3>A GLOSSY INCRUSTATION.</h3> + +<p> +Extensive portions of these rocks are coated by a layer of a glossy polished +substance, with a pearly lustre and of a greyish white colour; it follows all +the inequalities of the surface, to which it is firmly attached. When examined +with a lens, it is found to consist of numerous exceedingly thin layers, their +aggregate thickness being about the tenth of an inch. It is considerably harder +than calcareous spar, but can be scratched with a knife; under the blowpipe it +scales off, decrepitates, slightly blackens, emits a fetid odour, and becomes +strongly alkaline: it does not effervesce in acids. (In my +“Journal” I have described this substance; I then believed that it +was an impure phosphate of lime.) I presume this substance has been deposited +by water draining from the birds’ dung, with which the rocks are covered. +At Ascension, near a cavity in the rocks which was filled with a laminated mass +of infiltrated birds’ dung, I found some irregularly formed, +stalactitical masses of apparently the same nature. These masses, when broken, +had an earthy texture; but on their outsides, and especially at their +extremities, they were formed of a pearly substance, generally in little +globules, like the enamel of teeth, but more translucent, and so hard as just +to scratch plate-glass. This substance slightly blackens under the blowpipe, +emits a bad smell, then becomes quite white, swelling a little, and fuses into +a dull white enamel; it does not become alkaline; nor does it effervesce in +acids. The whole mass had a collapsed appearance, as if in the formation of the +hard glossy crust the whole had shrunk much. At the Abrolhos Islands on the +coast of Brazil, where also there is much birds’ dung, I found a great +quantity of a brown, arborescent substance adhering to some trap-rock. In its +arborescent form, this substance singularly resembles some of the branched +species of Nullipora. Under the blowpipe, it behaves like the specimens from +Ascension; but it is less hard and glossy, and the surface has not the shrunk +appearance. +</p> + + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap03"></a>CHAPTER III.<br /> +ASCENSION.</h2> + +<p> +Basaltic lavas. Numerous craters truncated on the same side. Singular structure +of volcanic bombs. Aeriform explosions. Ejected granitic fragments. Trachytic +rocks. Singular veins. Jasper, its manner of formation. Concretions in +pumiceous tuff. Calcareous deposits and frondescent incrustations on the coast. +Remarkable laminated beds, alternating with, and passing into, obsidian. Origin +of obsidian. Lamination of volcanic rocks. +</p> + +<h3>(MAP 2: THE ISLAND OF ASCENSION.)</h3> + +<p> +This island is situated in the Atlantic Ocean, in latitude 8 degrees S., +longitude 14 degrees W. It has the form of an irregular triangle (see Map 2), +each side being about six miles in length. Its highest point is 2,870 feet +(“Geographical Journal” volume 5 page 243.) above the level of the +sea. The whole is volcanic, and, from the absence of proofs to the contrary, I +believe of subaerial origin. The fundamental rock is everywhere of a pale +colour, generally compact, and of a feldspathic nature. In the S.E. portion of +the island, where the highest land is situated, well characterised trachyte, +and other congenerous rocks of that varying family, occur. Nearly the entire +circumference is covered up by black and rugged streams of basaltic lava, with +here and there a hill or single point of rock (one of which near the sea-coast, +north of the Fort, is only two or three yards across) of the trachyte still +remaining exposed. +</p> + +<h3>BASALTIC ROCKS.</h3> + +<p> +The overlying basaltic lava is in some parts extremely vesicular, in others +little so; it is of a black colour, but sometimes contains crystals of glassy +feldspar, and seldom much olivine. These streams appear to have possessed +singularly little fluidity; their side walls and lower ends being very steep, +and even as much as between twenty and thirty feet in height. Their surface is +extraordinarily rugged, and from a short distance appears as if studded with +small craters. These projections consist of broad, irregularly conical, +hillocks, traversed by fissures, and composed of the same unequally scoriaceous +basalt with the surrounding streams, but having an obscure tendency to a +columnar structure; they rise to a height between ten and thirty feet above the +general surface, and have been formed, as I presume, by the heaping up of the +viscid lava at points of greater resistance. At the base of several of these +hillocks, and occasionally likewise on more level parts, solid ribs, composed +of angulo-globular masses of basalt, resembling in size and outline arched +sewers or gutters of brickwork, but not being hollow, project between two or +three feet above the surface of the streams; what their origin may have been, I +do not know. Many of the superficial fragments from these basaltic streams +present singularly convoluted forms; and some specimens could hardly be +distinguished from logs of dark-coloured wood without their bark. +</p> + +<p> +Many of the basaltic streams can be traced, either to points of eruption at the +base of the great central mass of trachyte, or to separate, conical, +red-coloured hills, which are scattered over the northern and western borders +of the island. Standing on the central eminence, I counted between twenty and +thirty of these cones of eruption. The greater number of them had their +truncated summits cut off obliquely, and they all sloped towards the S.E., +whence the trade-wind blows. (M. Lesson in the “Zoology of the Voyage of +the ‘Coquille’” page 490 has observed this fact. Mr. Hennah +(“Geolog. Proceedings” 1835 page 189) further remarks that the most +extensive beds of ashes at Ascension invariably occur on the leeward side of +the island.) This structure no doubt has been caused by the ejected fragments +and ashes being always blown, during eruptions, in greater quantity towards one +side than towards the other. M. Moreau de Jonnes has made a similar observation +with respect to the volcanic orifices in the West Indian Islands. +</p> + +<h3>VOLCANIC BOMBS.</h3> + +<p> +(FIGURE 3: FRAGMENT OF A SPHERICAL VOLCANIC BOMB, with the interior parts +coarsely cellular, coated by a concentric layer of compact lava, and this again +by a crust of finely cellular rock. +</p> + +<p> +FIGURE 4: VOLCANIC BOMB OF OBSIDIAN FROM AUSTRALIA. The upper figure gives a +front view; the lower a side view of the same object.) +</p> + +<p> +These occur in great numbers strewed on the ground, and some of them lie at +considerable distances from any points of eruption. They vary in size from that +of an apple to that of a man’s body; they are either spherical or +pear-shaped, or with the hinder part (corresponding to the tail of a comet) +irregular, studded with projecting points, and even concave. Their surfaces are +rough, and fissured with branching cracks; their internal structure is either +irregularly scoriaceous and compact, or it presents a symmetrical and very +curious appearance. An irregular segment of a bomb of this latter kind, of +which I found several, is accurately represented in Figure 3. Its size was +about that of a man’s head. The whole interior is coarsely cellular; the +cells averaging in diameter about the tenth of an inch; but nearer the outside +they gradually decrease in size. This part is succeeded by a well-defined shell +of compact lava, having a nearly uniform thickness of about the third of an +inch; and the shell is overlaid by a somewhat thicker coating of finely +cellular lava (the cells varying from the fiftieth to the hundredth of an inch +in diameter), which forms the external surface: the line separating the shell +of compact lava from the outer scoriaceous crust is distinctly defined. This +structure is very simply explained, if we suppose a mass of viscid, scoriaceous +matter, to be projected with a rapid, rotatory motion through the air; for +whilst the external crust, from cooling, became solidified (in the state we now +see it), the centrifugal force, by relieving the pressure in the interior parts +of the bomb, would allow the heated vapours to expand their cells; but these +being driven by the same force against the already-hardened crust, would +become, the nearer they were to this part, smaller and smaller or less +expanded, until they became packed into a solid, concentric shell. As we know +that chips from a grindstone (Nichol “Architecture of the +Heavens.”) can be flirted off, when made to revolve with sufficient +velocity, we need not doubt that the centrifugal force would have power to +modify the structure of a softened bomb, in the manner here supposed. +Geologists have remarked, that the external form of a bomb at once bespeaks the +history of its aerial course, and few now see that the internal structure can +speak, with almost equal plainness, of its rotatory movement. +</p> + +<p> +M. Bory St. Vincent (“Voyage aux Quatre Isles d’Afrique” tome +1 page 222.) has described some balls of lava from the Isle of Bourbon, which +have a closely similar structure. His explanation, however (if I understand it +rightly), is very different from that which I have given; for he supposes that +they have rolled, like snowballs, down the sides of the crater. M. Beudant +(“Voyage en Hongrie” tome 2 page 214.), also, has described some +singular little balls of obsidian, never more than six or eight inches in +diameter, which he found strewed on the surface of the ground: their form is +always oval; sometimes they are much swollen in the middle, and even +spindle-shaped: their surface is regularly marked with concentric ridges and +furrows, all of which on the same ball are at right angles to one axis: their +interior is compact and glassy. M. Beudant supposes that masses of lava, when +soft, were shot into the air, with a rotatory movement round the same axis, and +that the form and superficial ridges of the bombs were thus produced. Sir +Thomas Mitchell has given me what at first appears to be the half of a much +flattened oval ball of obsidian; it has a singular artificial-like appearance, +which is well represented (of the natural size) in Figure 4. It was found in +its present state, on a great sandy plain between the rivers Darling and +Murray, in Australia, and at the distance of several hundred miles from any +known volcanic region. It seems to have been embedded in some reddish tufaceous +matter; and may have been transported either by the aborigines or by natural +means. The external saucer consists of compact obsidian, of a bottle-green +colour, and is filled with finely cellular black lava, much less transparent +and glassy than the obsidian. The external surface is marked with four or five +not quite perfect ridges, which are represented rather too distinctly in Figure +4. Here, then, we have the external structure described by M. Beudant, and the +internal cellular condition of the bombs from Ascension. The lip of the saucer +is slightly concave, exactly like the margin of a soup-plate, and its inner +edge overlaps a little the central cellular lava. This structure is so +symmetrical round the entire circumference, that one is forced to suppose that +the bomb burst during its rotatory course, before being quite solidified, and +that the lip and edges were thus slightly modified and turned inwards. It may +be remarked that the superficial ridges are in planes, at right angles to an +axis, transverse to the longer axis of the flattened oval: to explain this +circumstance, we may suppose that when the bomb burst, the axis of rotation +changed. +</p> + +<h3>AERIFORM EXPLOSIONS.</h3> + +<p> +The flanks of Green Mountain and the surrounding country are covered by a great +mass, some hundred feet in thickness, of loose fragments. The lower beds +generally consist of fine-grained, slightly consolidated tuffs (Some of this +peperino, or tuff, is sufficiently hard not to be broken by the greatest force +of the fingers.), and the upper beds of great loose fragments, with alternating +finer beds. (On the northern side of the Green Mountain a thin seam, about an +inch in thickness, of compact oxide of iron, extends over a considerable area; +it lies conformably in the lower part of the stratified mass of ashes and +fragments. This substance is of a reddish- brown colour, with an almost +metallic lustre; it is not magnetic, but becomes so after having been heated +under the blowpipe, by which it is blackened and partly fused. This seam of +compact stone, by intercepting the little rain-water which falls on the island, +gives rise to a small dripping spring, first discovered by Dampier. It is the +only fresh water on the island, so that the possibility of its being inhabited +has entirely depended on the occurrence of this ferruginous layer.) One white +ribbon- like layer of decomposed, pumiceous breccia, was curiously bent into +deep unbroken curves, beneath each of the large fragments in the superincumbent +stratum. From the relative position of these beds, I presume that a narrow- +mouthed crater, standing nearly in the position of Green Mountain, like a great +air-gun, shot forth, before its final extinction, this vast accumulation of +loose matter. Subsequently to this event, considerable dislocations have taken +place, and an oval circus has been formed by subsidence. This sunken space lies +at the north-eastern foot of Green Mountain, and is well represented in Map 2. +Its longer axis, which is connected with a N.E. and S.W. line of fissure, is +three-fifths of a nautical mile in length; its sides are nearly perpendicular, +except in one spot, and about four hundred feet in height; they consist, in the +lower part, of a pale basalt with feldspar, and in the upper part, of the tuff +and loose ejected fragments; the bottom is smooth and level, and under almost +any other climate a deep lake would have been formed here. From the thickness +of the bed of loose fragments, with which the surrounding country is covered, +the amount of aeriform matter necessary for their projection must have been +enormous; hence we may suppose it probable that after the explosions vast +subterranean caverns were left, and that the falling in of the roof of one of +these produced the hollow here described. At the Galapagos Archipelago, pits of +a similar character, but of a much smaller size, frequently occur at the bases +of small cones of eruption. +</p> + +<h3>EJECTED GRANITIC FRAGMENTS.</h3> + +<p> +In the neighbourhood of Green Mountain, fragments of extraneous rock are not +unfrequently found embedded in the midst of masses of scoriae. Lieutenant +Evans, to whose kindness I am indebted for much information, gave me several +specimens, and I found others myself. They nearly all have a granitic +structure, are brittle, harsh to the touch, and apparently of altered colours. +</p> + +<p> +FIRST, a white syenite, streaked and mottled with red; it consists of well- +crystallised feldspar, numerous grains of quartz, and brilliant, though small, +crystals of hornblende. The feldspar and hornblende in this and the succeeding +cases have been determined by the reflecting goniometer, and the quartz by its +action under the blowpipe. The feldspar in these ejected fragments, like the +glassy kind in the trachyte, is from its cleavage a potash-feldspar. +</p> + +<p> +SECONDLY, a brick-red mass of feldspar, quartz, and small dark patches of a +decayed mineral; one minute particle of which I was able to ascertain, by its +cleavage, to be hornblende. +</p> + +<p> +THIRDLY, a mass of confusedly crystallised white feldspar, with little nests of +a dark-coloured mineral, often carious, externally rounded, having a glossy +fracture, but no distinct cleavage: from comparison with the second specimen, I +have no doubt that it is fused hornblende. +</p> + +<p> +FOURTHLY, a rock, which at first appears a simple aggregation of distinct and +large-sized crystals of dusty-coloured Labrador feldspar (Professor Miller has +been so kind as to examine this mineral. He obtained two good cleavages of 86 +degrees 30 minutes and 86 degrees 50 minutes. The mean of several, which I +made, was 86 degrees 30 minutes. Professor Miller states that these crystals, +when reduced to a fine powder, are soluble in hydrochloric acid, leaving some +undissolved silex behind; the addition of oxalate of ammonia gives a copious +precipitate of lime. He further remarks, that according to Von Kobell, +anorthite (a mineral occurring in the ejected fragments at Mount Somma) is +always white and transparent, so that if this be the case, these crystals from +Ascension must be considered as Labrador feldspar. Professor Miller adds, that +he has seen an account, in Erdmann’s “Journal fur tecnische +Chemie,” of a mineral ejected from a volcano which had the external +characters of Labrador feldspar, but differed in the analysis from that given +by mineralogists of this mineral: the author attributed this difference to an +error in the analysis of Labrador feldspar, which is very old.); but in their +interstices there is some white granular feldspar, abundant scales of mica, a +little altered hornblende, and, as I believe, no quartz. I have described these +fragments in detail, because it is rare to find granitic rocks ejected from +volcanoes with their MINERALS UNCHANGED, as is the case with the first +specimen, and partially with the second. (Daubeny, in his work on Volcanoes +page 386, remarks that this is the case; and Humboldt, in his “Personal +Narrative” volume 1 page 236, says “In general, the masses of known +primitive rocks, I mean those which perfectly resemble our granites, gneiss, +and mica-slate, are very rare in lavas: the substances we generally denote by +the name of granite, thrown out by Vesuvius, are mixtures of nepheline, mica, +and pyroxene.”) One other large fragment, found in another spot, is +deserving of notice; it is a conglomerate, containing small fragments of +granitic, cellular, and jaspery rocks, and of hornstone porphyries, embedded in +a base of wacke, threaded by numerous thin layers of a concretionary pitchstone +passing into obsidian. These layers are parallel, slightly tortuous, and short; +they thin out at their ends, and resemble in form the layers of quartz in +gneiss. It is probable that these small embedded fragments were not separately +ejected, but were entangled in a fluid volcanic rock, allied to obsidian; and +we shall presently see that several varieties of this latter series of rock +assume a laminated structure. +</p> + +<h3>TRACHYTIC SERIES OF ROCKS.</h3> + +<p> +Those occupy the more elevated and central, and likewise the south-eastern, +parts of the island. The trachyte is generally of a pale brown colour, stained +with small darker patches; it contains broken and bent crystals of glassy +feldspar, grains of specular iron, and black microscopical points, which +latter, from being easily fused, and then becoming magnetic, I presume are +hornblende. The greater number of the hills, however, are composed of a quite +white, friable stone, appearing like a trachytic tuff. Obsidian, hornstone, and +several kinds of laminated feldspathic rocks, are associated with the trachyte. +There is no distinct stratification; nor could I distinguish a crateriform +structure in any of the hills of this series. Considerable dislocations have +taken place; and many fissures in these rocks are yet left open, or are only +partially filled with loose fragments. Within the space (This space is nearly +included by a line sweeping round Green Mountain, and joining the hills, called +the Weather Port Signal, Holyhead, and that denominated (improperly in a +geological sense) “the Crater of an old volcano.”), mainly formed +of trachyte, some basaltic streams have burst forth; and not far from the +summit of Green Mountain, there is one stream of quite black, vesicular basalt, +containing minute crystals of glassy feldspar, which have a rounded appearance. +</p> + +<p> +The soft white stone above mentioned is remarkable from its singular +resemblance, when viewed in mass, to a sedimentary tuff: it was long before I +could persuade myself that such was not its origin; and other geologists have +been perplexed by closely similar formations in trachytic regions. In two +cases, this white earthy stone formed isolated hills; in a third, it was +associated with columnar and laminated trachyte; but I was unable to trace an +actual junction. It contains numerous crystals of glassy feldspar and black +microscopical specks, and is marked with small darker patches, exactly as in +the surrounding trachyte. Its basis, however, when viewed under the microscope, +is generally quite earthy; but sometimes it exhibits a decidedly crystalline +structure. On the hill marked “Crater of an old volcano,” it passes +into a pale greenish-grey variety, differing only in its colour, and in not +being so earthy; the passage was in one case effected insensibly; in another, +it was formed by numerous, rounded and angular, masses of the greenish variety, +being embedded in the white variety;—in this latter case, the appearance +was very much like that of a sedimentary deposit, torn up and abraded during +the deposition of a subsequent stratum. Both these varieties are traversed by +innumerable tortuous veins (presently to be described), which are totally +unlike injected dikes, or indeed any other veins which I have ever seen. Both +varieties include a few scattered fragments, large and small, of dark- coloured +scoriaceous rocks, the cells of some of which are partially filled with the +white earthy stone; they likewise include some huge blocks of a cellular +porphyry. (The porphyry is dark coloured; it contains numerous, often +fractured, crystals of white opaque feldspar, also decomposing crystals of +oxide of iron; its vesicles include masses of delicate, hair- like, crystals, +apparently of analcime.) These fragments project from the weathered surface, +and perfectly resemble fragments embedded in a true sedimentary tuff. But as it +is known that extraneous fragments of cellular rock are sometimes included in +columnar trachyte, in phonolite (D’Aubuisson “Traite de +Geognosie” tome 2 page 548.), and in other compact lavas, this +circumstance is not any real argument for the sedimentary origin of the white +earthy stone. (Dr. Daubeny on Volcanoes, page 180 seems to have been led to +believe that certain trachytic formations of Ischia and of the Puy de Dome, +which closely resemble these of Ascension, were of sedimentary origin, chiefly +from the frequent presence in them “of scoriform portions, different in +colour from the matrix.” Dr. Daubeny adds, that on the other hand, +Brocchi, and other eminent geologists, have considered these beds as earthy +varieties of trachyte; he considers the subject deserving of further +attention.) The insensible passage of the greenish variety into the white one, +and likewise the more abrupt passage by fragments of the former being embedded +in the latter, might result from slight differences in the composition of the +same mass of molten stone, and from the abrading action of one such part still +fluid on another part already solidified. The curiously formed veins have, I +believe, been formed by siliceous matter being subsequently segregated. But my +chief reason for believing that these soft earthy stones, with their extraneous +fragments, are not of sedimentary origin, is the extreme improbability of +crystals of feldspar, black microscopical specks, and small stains of a darker +colour occurring in the same proportional numbers in an aqueous deposit, and in +masses of solid trachyte. Moreover, as I have remarked, the microscope +occasionally reveals a crystalline structure in the apparently earthy basis. On +the other hand, the partial decomposition of such great masses of trachyte, +forming whole mountains, is undoubtedly a circumstance of not easy explanation. +</p> + +<h3>VEINS IN THE EARTHY TRACHYTIC MASSES.</h3> + +<p> +These veins are extraordinarily numerous, intersecting in the most complicated +manner both coloured varieties of the earthy trachyte: they are best seen on +the flanks of the “Crater of the old volcano.” They contain +crystals of glassy feldspar, black microscopical specks and little dark stains, +precisely as in the surrounding rock; but the basis is very different, being +exceedingly hard, compact, somewhat brittle, and of rather less easy +fusibility. The veins vary much, and suddenly, from the tenth of an inch to one +inch in thickness; they often thin out, not only on their edges, but in their +central parts, thus leaving round, irregular apertures; their surfaces are +rugged. They are inclined at every possible angle with the horizon, or are +horizontal; they are generally curvilinear, and often interbranch one with +another. From their hardness they withstand weathering, and projecting two or +three feet above the ground, they occasionally extend some yards in length; +these plate-like veins, when struck, emit a sound, almost like that of a drum, +and they may be distinctly seen to vibrate; their fragments, which are strewed +on the ground, clatter like pieces of iron when knocked against each other. +They often assume the most singular forms; I saw a pedestal of the earthy +trachyte, covered by a hemispherical portion of a vein, like a great umbrella, +sufficiently large to shelter two persons. I have never met with, or seen +described, any veins like these; but in form they resemble the ferruginous +seams, due to some process of segregation, occurring not uncommonly in +sandstones,—for instance, in the New Red sandstone of England. Numerous +veins of jasper and of siliceous sinter, occurring on the summit of this same +hill, show that there has been some abundant source of silica, and as these +plate-like veins differ from the trachyte only in their greater hardness, +brittleness, and less easy fusibility, it appears probable that their origin is +due to the segregation or infiltration of siliceous matter, in the same manner +as happens with the oxides of iron in many sedimentary rocks. +</p> + +<h3>SILICEOUS SINTER AND JASPER.</h3> + +<p> +The siliceous sinter is either quite white, of little specific gravity, and +with a somewhat pearly fracture, passing into pinkish pearl quartz; or it is +yellowish white, with a harsh fracture, and it then contains an earthy powder +in small cavities. Both varieties occur, either in large irregular masses in +the altered trachyte, or in seams included in broad, vertical, tortuous, +irregular veins of a compact, harsh stone of a dull red colour, appearing like +a sandstone. This stone, however, is only altered trachyte; and a nearly +similar variety, but often honeycombed, sometimes adheres to the projecting +plate-like veins, described in the last paragraph. The jasper is of an ochre +yellow or red colour; it occurs in large irregular masses, and sometimes in +veins, both in the altered trachyte and in an associated mass of scoriaceous +basalt. The cells of the scoriaceous basalt are lined or filled with fine, +concentric layers of chalcedony, coated and studded with bright-red oxide of +iron. In this rock, especially in the rather more compact parts, irregular +angular patches of the red jasper are included, the edges of which insensibly +blend into the surrounding mass; other patches occur having an intermediate +character between perfect jasper and the ferruginous, decomposed, basaltic +base. In these patches, and likewise in the large vein-like masses of jasper, +there occur little rounded cavities, of exactly the same size and form with the +air-cells, which in the scoriaceous basalt are filled and lined with layers of +chalcedony. Small fragments of the jasper, examined under the microscope, seem +to resemble the chalcedony with its colouring matter not separated into layers, +but mingled in the siliceous paste, together with some impurities. I can +understand these facts,—namely, the blending of the jasper into the +semi-decomposed basalt,—its occurrence in angular patches, which clearly +do not occupy pre-existing hollows in the rock,—and its containing little +vesicles filled with chalcedony, like those in the scoriaceous lava,—only +on the supposition that a fluid, probably the same fluid which deposited the +chalcedony in the air-cells, removed in those parts where there were no +cavities, the ingredients of the basaltic rock, and left in their place silica +and iron, and thus produced the jasper. In some specimens of silicified wood, I +have observed, that in the same manner as in the basalt, the solid parts were +converted into a dark-coloured homogeneous stone, whereas the cavities formed +by the larger sap-vessels (which may be compared with the air-vesicles in the +basaltic lava) and other irregular hollows, apparently produced by decay, were +filled with concentric layers of chalcedony; in this case, there can be little +doubt that the same fluid deposited the homogeneous base and the chalcedonic +layers. After these considerations, I cannot doubt but that the jasper of +Ascension may be viewed as a volcanic rock silicified, in precisely the same +sense as this term is applied to wood, when silicified; we are equally ignorant +of the means by which every atom of wood, whilst in a perfect state, is removed +and replaced by atoms of silica, as we are of the means by which the +constituent parts of a volcanic rock could be thus acted on. (Beudant +“Voyage en Hongrie” tome 3 pages 502, 504 describes kidney-shaped +masses of jasper-opal, which either blend into the surrounding trachytic +conglomerate, or are embedded in it like chalk-flints; and he compares them +with the fragments of opalised wood, which are abundant in this same formation. +Beudant, however, appears to have viewed the process of their formation rather +as one of simple infiltration than of molecular exchange; but the presence of a +concretion, wholly different from the surrounding matter, if not formed in a +pre-existing hollow, clearly seems to me to require, either a molecular or +mechanical displacement of the atoms, which occupied the space afterwards +filled by it. The jasper-opal of Hungary passes into chalcedony, and therefore +in this case, as in that of Ascension, jasper seems to be intimately related in +origin with chalcedony.) I was led to the careful examination of these rocks, +and to the conclusion here given, from having heard the Rev. Professor Henslow +express a similar opinion, regarding the origin in trap-rocks of many +chalcedonies and agates. Siliceous deposits seem to be very general, if not of +universal occurrence, in partially decomposed trachytic tuffs (Beudant +“Voyage Min.” tome 3 page 507 enumerates cases in Hungary, Germany, +Central France, Italy, Greece, and Mexico.); and as these hills, according to +the view above given, consist of trachyte softened and altered in situ, the +presence of free silica in this case may be added as one more instance to the +list. +</p> + +<h3>CONCRETIONS IN PUMICEOUS TUFF.</h3> + +<p> +The hill, marked in Map 2 “Crater of an old volcano,” has no claims +to this appellation, which I could discover, except in being surmounted by a +circular, very shallow, saucer-like summit, nearly half a mile in diameter. +This hollow has been nearly filled up with many successive sheets of ashes and +scoriae, of different colours, and slightly consolidated. Each successive +saucer-shaped layer crops out all round the margin, forming so many rings of +various colours, and giving to the hill a fantastic appearance. The outer ring +is broad, and of a white colour; hence it resembles a course round which horses +have been exercised, and has received the name of the Devil’s Riding +School, by which it is most generally known. These successive layers of ashes +must have fallen over the whole surrounding country, but they have all been +blown away except in this one hollow, in which probably moisture accumulated, +either during an extraordinary year when rain fell, or during the storms often +accompanying volcanic eruptions. One of the layers of a pinkish colour, and +chiefly derived from small, decomposed fragments of pumice, is remarkable, from +containing numerous concretions. These are generally spherical, from half an +inch to three inches in diameter; but they are occasionally cylindrical, like +those of iron-pyrites in the chalk of Europe. They consist of a very tough, +compact, pale-brown stone, with a smooth and even fracture. They are divided +into concentric layers by thin white partitions, resembling the external +superficies; six or eight of such layers are distinctly defined near the +outside; but those towards the inside generally become indistinct, and blend +into a homogeneous mass. I presume that these concentric layers were formed by +the shrinking of the concretion, as it became compact. The interior part is +generally fissured by minute cracks or septaria, which are lined, both by +black, metallic, and by other white and crystalline specks, the nature of which +I was unable to ascertain. Some of the larger concretions consist of a mere +spherical shell, filled with slightly consolidated ashes. The concretions +contain a small proportion of carbonate of lime: a fragment placed under the +blowpipe decrepitates, then whitens and fuses into a blebby enamel, but does +not become caustic. The surrounding ashes do not contain any carbonate of lime; +hence the concretions have probably been formed, as is so often the case, by +the aggregation of this substance. I have not met with any account of similar +concretions; and considering their great toughness and compactness, their +occurrence in a bed, which probably has been subjected only to atmospheric +moisture, is remarkable. +</p> + +<h3>FORMATION OF CALCAREOUS ROCKS ON THE SEA-COAST.</h3> + +<p> +On several of the sea-beaches, there are immense accumulations of small, +well-rounded particles of shells and corals, of white, yellowish, and pink +colours, interspersed with a few volcanic particles. At the depth of a few +feet, these are found cemented together into stone, of which the softer +varieties are used for building; there are other varieties, both coarse and +fine-grained, too hard for this purpose: and I saw one mass divided into even +layers half an inch in thickness, which were so compact that when struck with a +hammer they rang like flint. It is believed by the inhabitants, that the +particles become united in the course of a single year. The union is effected +by calcareous matter; and in the most compact varieties, each rounded particle +of shell and volcanic rock can be distinctly seen to be enveloped in a husk of +pellucid carbonate of lime. Extremely few perfect shells are embedded in these +agglutinated masses; and I have examined even a large fragment under a +microscope, without being able to discover the least vestige of striae or other +marks of external form: this shows how long each particle must have been rolled +about, before its turn came to be embedded and cemented. (The eggs of the +turtle being buried by the parent, sometimes become enclosed in the solid rock. +Mr. Lyell has given a figure (“Principles of Geology” book 3 +chapter 17) of some eggs, containing the bones of young turtles, found thus +entombed.) One of the most compact varieties, when placed in acid, was entirely +dissolved, with the exception of some flocculent animal matter; its specific +gravity was 2.63. The specific gravity of ordinary limestone varies from 2.6 to +2.75; pure Carrara marble was found by Sir H. De la Beche to be 2.7. +(“Researches in Theoretical Geology” page 12.) It is remarkable +that these rocks of Ascension, formed close to the surface, should be nearly as +compact as marble, which has undergone the action of heat and pressure in the +plutonic regions. +</p> + +<p> +The great accumulation of loose calcareous particles, lying on the beach near +the Settlement, commences in the month of October, moving towards the S.W., +which, as I was informed by Lieutenant Evans, is caused by a change in the +prevailing direction of the currents. At this period the tidal rocks, at the +S.W. end of the beach, where the calcareous sand is accumulating, and round +which the currents sweep, become gradually coated with a calcareous +incrustation, half an inch in thickness. It is quite white, compact, with some +parts slightly spathose, and is firmly attached to the rock. After a short time +it gradually disappears, being either redissolved, when the water is less +charged with lime, or more probably is mechanically abraded. Lieutenant Evans +has observed these facts, during the six years he has resided at Ascension. The +incrustation varies in thickness in different years: in 1831 it was unusually +thick. When I was there in July, there was no remnant of the incrustation; but +on a point of basalt, from which the quarrymen had lately removed a mass of the +calcareous freestone, the incrustation was perfectly preserved. Considering the +position of the tidal-rocks, and the period at which they become coated, there +can be no doubt that the movement and disturbance of the vast accumulation of +calcareous particles, many of them being partially agglutinated together, cause +the waves of the sea to be so highly charged with carbonate of lime, that they +deposit it on the first objects against which they impinge. I have been +informed by Lieutenant Holland, R.N., that this incrustation is formed on many +parts of the coast, on most of which, I believe, there are likewise great +masses of comminuted shells. +</p> + +<h3>A FRONDESCENT CALCAREOUS INCRUSTATION.</h3> + +<p> +(FIGURE 5. AN INCRUSTATION OF CALCAREOUS AND ANIMAL MATTER, coating the +tidal-rocks at Ascension.) +</p> + +<p> +In many respects this is a singular deposit; it coats throughout the year the +tidal volcanic rocks, that project from the beaches composed of broken shells. +Its general appearance is well represented in Figure 5; but the fronds or +discs, of which it is composed, are generally so closely crowded together as to +touch. These fronds have their sinuous edges finely crenulated, and they +project over their pedestals or supports; their upper surfaces are either +slightly concave, or slightly convex; they are highly polished, and of a dark +grey or jet black colour; their form is irregular, generally circular, and from +the tenth of an inch to one inch and a half in diameter; their thickness, or +amount of their projection from the rock on which they stand, varies much, +about a quarter of an inch being perhaps most usual. The fronds occasionally +become more and more convex, until they pass into botryoidal masses with their +summits fissured; when in this state, they are glossy and of an intense black, +so as to resemble some fused metallic substance. I have shown the incrustation, +both in this latter and in its ordinary state to several geologists, but not +one could conjecture its origin, except that perhaps it was of volcanic nature! +</p> + +<p> +The substance forming the fronds has a very compact and often almost +crystalline fracture; the edges being translucent, and hard enough easily to +scratch calcareous spar. Under the blowpipe it immediately becomes white, and +emits a strong animal odour, like that from fresh shells. It is chiefly +composed of carbonate of lime; when placed in muriatic acid it froths much, +leaving a residue of sulphate of lime, and of an oxide of iron, together with a +black powder, which is not soluble in heated acids. This latter substance seems +to be carbonaceous, and is evidently the colouring matter. The sulphate of lime +is extraneous, and occurs in distinct, excessively minute, lamellar plates, +studded on the surface of the fronds, and embedded between the fine layers of +which they are composed; when a fragment is heated in the blowpipe, these +lamellae are immediately rendered visible. The original outline of the fronds +may often be traced, either to a minute particle of shell fixed in a crevice of +the rock, or to several cemented together; these first become deeply corroded, +by the dissolving power of the waves, into sharp ridges, and then are coated +with successive layers of the glossy, grey, calcareous incrustation. The +inequalities of the primary support affect the outline of every successive +layer, in the same manner as may often be seen in bezoar-stones, when an object +like a nail forms the centre of aggregation. The crenulated edges, however, of +the frond appear to be due to the corroding power of the surf on its own +deposit, alternating with fresh depositions. On some smooth basaltic rocks on +the coast of St. Jago, I found an exceedingly thin layer of brown calcareous +matter, which under a lens presented a miniature likeness of the crenulated and +polished fronds of Ascension; in this case a basis was not afforded by any +projecting extraneous particles. Although the incrustation at Ascension is +persistent throughout the year; yet from the abraded appearance of some parts, +and from the fresh appearance of other parts, the whole seems to undergo a +round of decay and renovation, due probably to changes in the form of the +shifting beach, and consequently in the action of the breakers: hence probably +it is, that the incrustation never acquires a great thickness. Considering the +position of the encrusted rocks in the midst of the calcareous beach, together +with its composition, 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 corals. (The selenite, as I have remarked is +extraneous, and must have been derived from the sea-water. It is an interesting +circumstance thus to find the waves of the ocean, sufficiently charged with +sulphate of lime, to deposit it on the rocks, against which they dash every +tide. Dr. Webster has described (“Voyage of the +‘Chanticleer’” volume 2 page 319) beds of gypsum and salt, as +much as two feet in thickness, left by the evaporation of the spray on the +rocks on the windward coast. Beautiful stalactites of selenite, resembling in +form those of carbonate of lime, are formed near these beds. Amorphous masses +of gypsum, also, occur in caverns in the interior of the island; and at Cross +Hill (an old crater) I saw a considerable quantity of salt oozing from a pile +of scoriae. In these latter cases, the salt and gypsum appear to be volcanic +products.) From this source it derives its animal matter, which is evidently +the colouring principle. The nature of the deposit, in its incipient stage, can +often be well seen upon a fragment of white shell, when jammed between two of +the fronds; it then appears exactly like the thinnest wash of a pale grey +varnish. Its darkness varies a little, but the jet blackness of some of the +fronds and of the botryoidal masses seems due to the translucency of the +successive grey layers. There is, however, this singular circumstance, that +when deposited on the under side of ledges of rock or in fissures, it appears +always to be of a pale, pearly grey colour, even when of considerable +thickness: hence one is led to suppose, that an abundance of light is necessary +to the development of the dark colour, in the same manner as seems to be the +case with the upper and exposed surfaces of the shells of living mollusca, +which are always dark, compared with their under surfaces and with the parts +habitually covered by the mantle of the animal. In this circumstance,—in +the immediate loss of colour and in the odour emitted under the +blowpipe,—in the degree of hardness and translucency of the +edges,—and in the beautiful polish of the surface (From the fact +described in my “Journal of Researches” of a coating of oxide of +iron, deposited by a streamlet on the rocks in its bed (like a nearly similar +coating at the great cataracts of the Orinoco and Nile), becoming finely +polished where the surf acts, I presume that the surf in this instance, also, +is the polishing agent.), rivalling when in a fresh state that of the finest +Oliva, there is a striking analogy between this inorganic incrustation and the +shells of living molluscous animals. (In the section descriptive of St. +Paul’s Rocks, I have described a glossy, pearly substance, which coats +the rocks, and an allied stalactitical incrustation from Ascension, the crust +of which resembles the enamel of teeth, but is hard enough to scratch +plate-glass. Both these substances contain animal matter, and seem to have been +derived from water in filtering through birds’ dung.) This appears to me +to be an interesting physiological fact. (Mr. Horner and Sir David Brewster +have described “Philosophical Transactions” 1836 page 65 a singular +“artificial substance, resembling shell.” It is deposited in fine, +transparent, highly polished, brown- coloured laminae, possessing peculiar +optical properties, on the inside of a vessel, in which cloth, first prepared +with glue and then with lime, is made to revolve rapidly in water. It is much +softer, more transparent, and contains more animal matter, than the natural +incrustation at Ascension; but we here again see the strong tendency which +carbonate of lime and animal matter evince to form a solid substance allied to +shell.) +</p> + +<h3>SINGULAR LAMINATED BEDS ALTERNATING WITH AND PASSING INTO OBSIDIAN.</h3> + +<p> +These beds occur within the trachytic district, at the western base of Green +Mountain, under which they dip at a high inclination. They are only partially +exposed, being covered up by modern ejections; from this cause, I was unable to +trace their junction with the trachyte, or to discover whether they had flowed +as a stream of lava, or had been injected amidst the overlying strata. There +are three principal beds of obsidian, of which the thickest forms the base of +the section. The alternating stony layers appear to me eminently curious, and +shall be first described, and afterwards their passage into the obsidian. They +have an extremely diversified appearance; five principal varieties may be +noticed, but these insensibly blend into each other by endless gradations. +</p> + +<h3>FIRST.</h3> + +<p> +A pale grey, irregularly and coarsely laminated (This term is open to some +misinterpretation, as it may be applied both to rocks divided into laminae of +exactly the same composition, and to layers firmly attached to each other, with +no fissile tendency, but composed of different minerals, or of different shades +of colour. The term “laminated,” in this chapter, is applied in +these latter senses; where a homogeneous rock splits, as in the former sense, +in a given direction, like clay-slate, I have used the term +“fissile.”), harsh-feeling rock, resembling clay-slate which has +been in contact with a trap-dike, and with a fracture of about the same degree +of crystalline structure. This rock, as well as the following varieties, easily +fuses into a pale glass. The greater part is honeycombed with irregular, +angular, cavities, so that the whole has a curious appearance, and some +fragments resemble in a remarkable manner silicified logs of decayed wood. This +variety, especially where more compact, is often marked with thin whitish +streaks, which are either straight or wrap round, one behind the other, the +elongated carious hollows. +</p> + +<h3>SECONDLY.</h3> + +<p> +A bluish grey or pale brown, compact, heavy, homogeneous stone, with an +angular, uneven, earthy fracture; viewed, however, under a lens of high power, +the fracture is seen to be distinctly crystalline, and even separate minerals +can be distinguished. +</p> + +<h3>THIRDLY.</h3> + +<p> +A stone of the same kind with the last, but streaked with numerous, parallel, +slightly tortuous, white lines of the thickness of hairs. These white lines are +more crystalline than the parts between them; and the stone splits along them: +they frequently expand into exceedingly thin cavities, which are often only +just perceptible with a lens. The matter forming the white lines becomes better +crystallised in these cavities, and Professor Miller was fortunate enough, +after several trials, to ascertain that the white crystals, which are the +largest, were of quartz (Professor Miller informs me that the crystals which he +measured had the faces P, z, m of the figure (147) given by Haidinger in his +Translation of Mohs; and he adds, that it is remarkable, that none of them had +the slightest trace of faces r of the regular six-sided prism.), and that the +minute green transparent needles were augite, or, as they would more generally +be called, diopside: besides these crystals, there are some minute, dark specks +without a trace of crystalline, and some fine, white, granular, crystalline +matter which is probably feldspar. Minute fragments of this rock are easily +fusible. +</p> + +<h3>FOURTHLY.</h3> + +<p> +A compact crystalline rock, banded in straight lines with innumerable layers of +white and grey shades of colour, varying in width from the thirtieth to the +two-hundredth of an inch; these layers seem to be composed chiefly of feldspar, +and they contain numerous perfect crystals of glassy feldspar, which are placed +lengthways; they are also thickly studded with microscopically minute, +amorphous, black specks, which are placed in rows, either standing separately, +or more frequently united, two or three or several together, into black lines, +thinner than a hair. When a small fragment is heated in the blowpipe, the black +specks are easily fused into black brilliant beads, which become +magnetic,—characters that apply to no common mineral except hornblende or +augite. With the black specks there are mingled some others of a red colour, +which are magnetic before being heated, and no doubt are oxide of iron. Round +two little cavities, in a specimen of this variety, I found the black specks +aggregated into minute crystals, appearing like those of augite or hornblende, +but too dull and small to be measured by the goniometer; in the specimen, also, +I could distinguish amidst the crystalline feldspar, grains, which had the +aspect of quartz. By trying with a parallel ruler, I found that the thin grey +layers and the black hair-like lines were absolutely straight and parallel to +each other. It is impossible to trace the gradation from the homogeneous grey +rocks to these striped varieties, or indeed the character of the different +layers in the same specimen, 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 matter, into the black and red specks of +hornblende and oxide of iron. +</p> + +<h3>FIFTHLY.</h3> + +<p> +A compact heavy rock, not laminated, with an irregular, angular, highly +crystalline, fracture; it abounds with distinct crystals of glassy feldspar, +and the crystalline feldspathic base is mottled with a black mineral, which on +the weathered surface is seen to be aggregated into small crystals, some +perfect, but the greater number imperfect. I showed this specimen to an +experienced geologist, and asked him what it was; he answered, as I think every +one else would have done, that it was a primitive greenstone. The weathered +surface, also, of the banded variety in Figure 4, strikingly resembles a worn +fragment of finely laminated gneiss. +</p> + +<p> +These five varieties, with many intermediate ones, pass and repass into each +other. As the compact varieties are quite subordinate to the others, the whole +may be considered as laminated or striped. The laminae, to sum up their +characteristics, are either quite straight, or slightly tortuous, or +convoluted; they are all parallel to each other, and to the intercalating +strata of obsidian; they are generally of extreme thinness; they consist either +of an apparently homogeneous, compact rock, striped with different shades of +grey and brown colours, or of crystalline feldspathic layers in a more or less +perfect state of purity, and of different thicknesses, with distinct crystals +of glassy feldspar placed lengthways, or of very thin layers chiefly composed +of minute crystals of quartz and augite, or composed of black and red specks of +an augitic mineral and of an oxide of iron, either not crystallised or +imperfectly so. After having fully described the obsidian, I shall return to +the subject of the lamination of rocks of the trachytic series. +</p> + +<p> +The passage of the foregoing beds into the strata of glassy obsidian is +effected in several ways: first, angulo-modular masses of obsidian, both large +and small, abruptly appear disseminated in a slaty, or in an amorphous, +pale-coloured, feldspathic rock, with a somewhat pearly fracture. Secondly, +small irregular nodules of the obsidian, either standing separately, or united +into thin layers, seldom more than the tenth of an inch in thickness, alternate +repeatedly with very thin layers of a feldspathic rock, which is striped with +the finest parallel zones of colour, like an agate, and which sometimes passes +into the nature of pitchstone; the interstices between the nodules of obsidian +are generally filled by soft white matter, resembling pumiceous ashes. Thirdly, +the whole substance of the bounding rock suddenly passes into an +angulo-concretionary mass of obsidian. Such masses (as well as the small +nodules) of obsidian are of a pale green colour, and are generally streaked +with different shades of colour, parallel to the laminae of the surrounding +rock; they likewise generally contain minute white sphaerulites, of which half +is sometimes embedded in a zone of one shade of colour, and half in a zone of +another shade. The obsidian assumes its jet black colour and perfectly +conchoidal fracture, only when in large masses; but even in these, on careful +examination and on holding the specimens in different lights, I could generally +distinguish parallel streaks of different shades of darkness. +</p> + +<p> +(FIGURE 6. OPAQUE BROWN SPHAERULITES, drawn on an enlarged scale. The upper +ones are externally marked with parallel ridges. The internal radiating +structure of the lower ones, is much too plainly represented. +</p> + +<p> +FIGURE 7. A LAYER FORMED BY THE UNION OF MINUTE BROWN SPHAERULITES, +INTERSECTING TWO OTHER SIMILAR LAYERS: the whole represented of nearly the +natural size.) +</p> + +<p> +One of the commonest transitional rocks deserves in several respects a further +description. It is of a very complicated nature, and consists of numerous thin, +slightly tortuous layers of a pale-coloured feldspathic stone, often passing +into an imperfect pitchstone, alternating with layers formed of numberless +little globules of two varieties of obsidian, and of two kinds of sphaerulites, +embedded in a soft or in a hard pearly base. The sphaerulites are either white +and translucent, or dark brown and opaque; the former are quite spherical, of +small size, and distinctly radiated from their centre. The dark brown +sphaerulites are less perfectly round, and vary in diameter from the twentieth +to the thirtieth of an inch; when broken they exhibit towards their centres, +which are whitish, an obscure radiating structure; two of them when united +sometimes have only one central point of radiation; there is occasionally a +trace of or a hollow crevice in their centres. They stand either separately, or +are united two or three or many together into irregular groups, or more +commonly into layers, parallel to the stratification of the mass. This union in +many cases is so perfect, that the two sides of the layer thus formed, are +quite even; and these layers, as they become less brown and opaque, cannot be +distinguished from the alternating layers of the pale-coloured feldspathic +stone. The sphaerulites, when not united, are generally compressed in the plane +of the lamination of the mass; and in this same plane, they are often marked +internally, by zones of different shades of colour, and externally by small +ridges and furrows. In the upper part of Figure 6, the sphaerulites with the +parallel ridges and furrows are represented on an enlarged scale, but they are +not well executed; and in the lower part, their usual manner of grouping is +shown. In another specimen, a thin layer formed of the brown sphaerulites +closely united together, intersects, as represented in Figure 7, a layer of +similar composition; and after running for a short space in a slightly curved +line, again intersects it, and likewise a second layer lying a little way +beneath that first intersected. The small nodules also of obsidian are +sometimes externally marked with ridges and furrows, parallel to the lamination +of the mass, but always less plainly than the sphaerulites. These obsidian +nodules are generally angular, with their edges blunted: they are often +impressed with the form of the adjoining sphaerulites, than which they are +always larger; the separate nodules seldom appear to have drawn each other out +by exerting a mutually attractive force. Had I not found in some cases, a +distinct centre of attraction in these nodules of obsidian, I should have been +led to have considered them as residuary matter, left during the formation of +the pearlstone, in which they are embedded, and of the sphaerulitic globules. +</p> + +<p> +The sphaerulites and the little nodules of obsidian in these rocks so closely +resemble, in general form and structure, concretions in sedimentary deposits, +that one is at once tempted to attribute to them an analogous origin. They +resemble ordinary concretions in the following respects: in their external +form,—in the union of two or three, or of several, into an irregular +mass, or into an even-sided layer,—in the occasional intersection of one +such layer by another, as in the case of chalk-flints,- -in the presence of two +or three kinds of nodules, often close together, in the same basis,—in +their fibrous, radiating structure, with occasional hollows in their +centres,—in the co-existence of a laminary, concretionary, and radiating +structure, as is so well developed in the concretions of magnesian limestone, +described by Professor Sedgwick. (“Geological Transactions” volume +3 part 1 page 37.) Concretions in sedimentary deposits, it is known, are due to +the separation from the surrounding mass of the whole or part of some mineral +substance, and its aggregation round certain points of attraction. Guided by +this fact, I have endeavoured to discover whether obsidian and the sphaerulites +(to which may be added marekanite and pearlstone, both of them occurring in +nodular concretions in the trachytic series) differ in their constituent parts, +from the minerals generally composing trachytic rocks. It appears from three +analyses, that obsidian contains on an average 76 per cent of silica; from one +analysis, that sphaerulites contain 79.12; from two, that marekanite contains +79.25; and from two other analyses, that pearlstone contains 75.62 of silica. +(The foregoing analyses are taken from Beudant “Traite de +Mineralogie” tome 2 page 113; and one analysis of obsidian from Phillips +“Mineralogy.”) Now, the constituent parts of trachyte, as far as +they can be distinguished consist of feldspar, containing 65.21 of silica; or +of albite, containing 69.09; of hornblende, containing 55.27 (These analyses +are taken from Von Kobell “Grundzuge der Mineralogie” 1838.), and +of oxide of iron: so that the foregoing glassy concretionary substances all +contain a larger proportion of silica than that occurring in ordinary +feldspathic or trachytic rocks. D’Aubuisson (“Traite de +Geogn.” tome 2 page 535.), also, has remarked on the large proportion of +silica compared with alumina, in six analyses of obsidian and pearlstone given +in Brongniart’s “Mineralogy.” Hence I conclude, that the +foregoing concretions have been formed by a process of aggregation, strictly +analogous to that which takes place in aqueous deposits, acting chiefly on the +silica, but likewise on some of the other elements of the surrounding mass, and +thus producing the different concretionary varieties. From the well-known +effects of rapid cooling (This is seen in the manufacture of common glass, and +in Gregory Watts’s experiments on molten trap; also on the natural +surfaces of lava- streams, and on the side-walls of dikes.) in giving +glassiness of texture, it is probably necessary that the entire mass, in cases +like that of Ascension, should have cooled at a certain rate; but considering +the repeated and complicated alterations of nodules and thin layers of a glassy +texture with other layers quite stony or crystalline, all within the space of a +few feet or even inches, it is hardly possible that they could have cooled at +different rates, and thus have acquired their different textures. +</p> + +<p> +The natural sphaerulites in these rocks very closely resemble those produced in +glass, when slowly cooled. (I do not know whether it is generally known, that +bodies having exactly the same appearance as sphaerulites, sometimes occur in +agates. Mr. Robert Brown showed me in an agate, formed within a cavity in a +piece of silicified wood, some little specks, which were only just visible to +the naked eye: these specks, when placed by him under a lens of high power, +presented a beautiful appearance: they were perfectly circular, and consisted +of the finest fibres of a brown colour, radiating with great exactness from a +common centre. These little radiating stars are occasionally intersected, and +portions are quite cut off by the fine, ribbon-like zones of colour in the +agate. In the obsidian of Ascension, the halves of a sphaerulite often lie in +different zones of colour, but they are not cut off by them, as in the agate.) +In some fine specimens of partially devitrified glass, in the possession of Mr. +Stokes, the sphaerulites are united into straight layers with even sides, +parallel to each other, and to one of the outer surfaces, exactly as in the +obsidian. These layers sometimes interbranch and form loops; but I did not see +any case of actual intersection. They form the passage from the perfectly +glassy portions, to those nearly homogeneous and stony, with only an obscure +concretionary structure. In the same specimen, also, sphaerulites differing +slightly in colour and in structure, occur embedded close together. Considering +these facts, it is some confirmation of the view above given of the +concretionary origin of the obsidian and natural sphaerulites, to find that M. +Dartigues (“Journal de Physique” tome 59 1804 pages 10, 12.), in +his curious paper on this subject, attributes the production of sphaerulites in +glass, to the different ingredients obeying their own laws of attraction and +becoming aggregated. He is led to believe that this takes place, from the +difficulty in remelting sphaerulitic glass, without the whole be first +thoroughly pounded and mixed together; and likewise from the fact, that the +change takes place most readily in glass composed of many ingredients. In +confirmation of M. Dartigues’ view, I may remark, that M. Fleuriau de +Bellevue (Idem tome 60 1805 page 418.) found that the sphaerulitic portions of +devitrified glass were acted on both by nitric acid and under the blowpipe, in +a different manner from the compact paste in which they were embedded. +</p> + +<p> +COMPARISON OF THE OBSIDIAN BEDS AND ALTERNATING STRATA OF ASCENSION, WITH THOSE +OF OTHER COUNTRIES. +</p> + +<p> +I have been struck with much surprise, how closely the excellent description of +the obsidian rocks of Hungary, given by Beudant (“Voyage en +Hongrie” tome 1 page 330; tome 2 pages 221 and 315; tome 3 pages 369, +371, 377, 381.), and that by Humboldt, of the same formation in Mexico and Peru +(“Essai Geognostique” pages 176, 326, 328.), and likewise the +descriptions given by several authors (P. Scrope “Geological +Transactions” volume 2 second series page 195. Consult also Dolomieu +“Voyage aux Isles Lipari” and D’Aubuisson “Traite de +Geogn.” tome 2 page 534.) of the trachytic regions in the Italian +islands, agree with my observations at Ascension. Many passages might have been +transferred without alteration from the works of the above authors, and would +have been applicable to this island. They all agree in the laminated and +stratified character of the whole series; and Humboldt speaks of some of the +beds of obsidian being ribboned like jasper. (In Mr. Stokes’ fine +collection of obsidians from Mexico, I observe that the sphaerulites are +generally much larger than those of Ascension; they are generally white, +opaque, and are united into distinct layers: there are many singular varieties, +different from any at Ascension. The obsidians are finely zoned, in quite +straight or curved lines, with exceedingly slight differences of tint, of +cellularity, and of more or less perfect degrees of glassiness. Tracing some of +the less perfectly glassy zones, they are seen to become studded with minute +white sphaerulites, which become more and more numerous, until at last they +unite and form a distinct layer: on the other hand, at Ascension, only the +brown sphaerulites unite and form layers; the white ones always being +irregularly disseminated. Some specimens at the Geological Society, said to +belong to an obsidian formation from Mexico, have an earthy fracture, and are +divided in the finest parallel laminae, by specks of a black mineral, like the +augitic or hornblendic specks in the rocks at Ascension.) They all agree in the +nodular or concretionary character of the obsidian, and of the passage of these +nodules into layers. They all refer to the repeated alterations, often in +undulatory planes, of glassy, pearly, stony, and crystalline layers: the +crystalline layers, however, seem to be much more perfectly developed at +Ascension, than in the above-named countries. Humboldt compares some of the +stony beds, when viewed from a distance, to strata of a schistose sandstone. +Sphaerulites are described as occurring abundantly in all cases; and they +everywhere seem to mark the passage, from the perfectly glassy to the stony and +crystalline beds. Beudant’s account (Beudant “Voyage” tome 3 +page 373.) of his “perlite lithoide globulaire” in every, even the +most trifling particular, might have been written for the little brown +sphaerulitic globules of the rocks of Ascension. +</p> + +<p> +From the close similarity in so many respects, between the obsidian formations +of Hungary, Mexico, Peru, and of some of the Italian islands, with that of +Ascension, I can hardly doubt that in all these cases, the obsidian and the +sphaerulites owe their origin to a concretionary aggregation of the silica, and +of some of the other constituent elements, taking place whilst the liquified +mass cooled at a certain required rate. It is, however, well-known, that in +several places, obsidian has flowed in streams like lava; for instance, at +Teneriffe, at the Lipari Islands, and at Iceland. (For Teneriffe see von Buch +“Descript. des Isles Canaries” pages 184 and 190; for the Lipari +Islands see Dolomieu “Voyage” page 34; for Iceland see Mackenzie +“Travels” page 369.) In these cases, the superficial parts are the +most perfectly glassy, the obsidian passing at the depth of a few feet into an +opaque stone. In an analysis by Vauquelin of a specimen of obsidian from Hecla, +which probably flowed as lava, the proportion of silica is nearly the same as +in the nodular or concretionary obsidian from Mexico. It would be interesting +to ascertain, whether the opaque interior portions and the superficial glassy +coating contained the same proportional constituent parts: we know from M. +Dufrenoy (“Memoires pour servir a une Descript. Geolog. de la +France” tome 4 page 371.) that the exterior and interior parts of the +same stream of lava sometimes differ considerably in their composition. Even +should the whole body of the stream of obsidian turn out to be similarly +composed with nodular obsidian, it would only be necessary, in accordance with +the foregoing facts, to suppose that lava in these instances had been erupted +with its ingredients mixed in the same proportion, as in the concretionary +obsidian. +</p> + +<h3>LAMINATION OF VOLCANIC ROCKS OF THE TRACHYTIC SERIES.</h3> + +<p> +We have seen that, in several and widely distant countries, the strata +alternating with beds of obsidian, are highly laminated. The nodules, also, +both large and small, of the obsidian, are zoned with different shades of +colour; and I have seen a specimen from Mexico in Mr. Stokes’ collection, +with its external surface weathered (MacCulloch states “Classification of +Rocks” page 531 that the exposed surfaces of the pitchstone dikes in +Arran are furrowed “with undulating lines, resembling certain varieties +of marbled paper, and which evidently result from some corresponding difference +of laminar structure.”) into ridges and furrows, corresponding with the +zones of different degrees of glassiness: Humboldt (“Personal +Narrative” volume 1 page 222.), moreover, found on the Peak of Teneriffe, +a stream of obsidian divided by very thin, alternating, layers of pumice. Many +other lavas of the feldspathic series are laminated; thus, masses of common +trachyte at Ascension are divided by fine earthy lines, along which the rock +splits, separating thin layers of slightly different shades of colour; the +greater number, also, of the embedded crystals of glassy feldspar are placed +lengthways in the same direction. Mr. P. Scrope (“Geological +Transactions” volume 2 second series page 195.) has described a +remarkable columnar trachyte in the Panza Islands, which seems to have been +injected into an overlying mass of trachytic conglomerate: it is striped with +zones, often of extreme tenuity, of different textures and colours; the harder +and darker zones appearing to contain a larger proportion of silica. In another +part of the island, there are layers of pearlstone and pitchstone, which in +many respects resemble those of Ascension. The zones in the columnar trachyte +are generally contorted; they extend uninterruptedly for a great length in a +vertical direction, and apparently parallel to the walls of the dike-like mass. +Von Buch (“Description des Iles Canaries” page 184.) has described +at Teneriffe, a stream of lava containing innumerable thin, plate-like crystals +of feldspar, which are arranged like white threads, one behind the other, and +which mostly follow the same direction. Dolomieu (“Voyage aux Isles de +Lipari” pages 35 and 85.) also states, that the grey lavas of the modern +cone of Vulcano, which have a vitreous texture, are streaked with parallel +white lines: he further describes a solid pumice-stone which possesses a +fissile structure, like that of certain micaceous schists. Phonolite, which I +may observe is often, if not always, an injected rock, also, often has a +fissile structure; this is generally due to the parallel position of the +embedded crystals of feldspar, but sometimes, as at Fernando Noronha, seems to +be nearly independent of their presence. (In this case, and in that of the +fissile pumice-stone, the structure is very different from that in the +foregoing cases, where the laminae consist of alternate layers of different +composition or texture. In some sedimentary formations, however, which +apparently are homogeneous and fissile, as in glossy clay-slate, there is +reason to believe, according to D’Aubuisson, that the laminae are really +due to excessively thin, alternating, layers of mica.) From these facts we see, +that various rocks of the feldspathic series have either a laminated or fissile +structure, and that it occurs both in masses which have injected into overlying +strata, and in others which have flowed as streams of lava. +</p> + +<p> +The laminae of the beds, alternating with the obsidian at Ascension, dip at a +high angle under the mountain, at the base of which they are situated; and they +do not appear as if they had been inclined by violence. A high inclination is +common to these beds in Mexico, Peru, and in some of the Italian islands (See +Phillips “Mineralogy” for the Italian Islands page 136. For Mexico +and Peru see Humboldt “Essai Geognostique.” Mr. Edwards also +describes the high inclination of the obsidian rocks of the Cerro del Navaja in +Mexico in the “Proc. of the Geolog. Soc.” June 1838.): on the other +hand, in Hungary, the layers are horizontal; the laminae, also, of some of the +lava-streams above referred to, as far as I can understand the descriptions +given of them, appear to be highly inclined or vertical. I doubt whether in any +of these cases, the laminae have been tilted into their present position; and +in some instances, as in that of the trachyte described by Mr. Scrope, it is +almost certain that they have been originally formed with a high inclination. +In many of these cases, there is evidence that the mass of liquified rock has +moved in the direction of the laminae. At Ascension, many of the air-cells have +a drawn out appearance, and are crossed by coarse semi-glassy fibres, in the +direction of the laminae; and some of the layers, separating the sphaerulitic +globules, have a scored appearance, as if produced by the grating of the +globules. I have seen a specimen of zoned obsidian from Mexico, in Mr. +Stokes’ collection, with the surfaces of the best-defined layers streaked +or furrowed with parallel lines; and these lines or streaks precisely resembled +those, produced on the surface of a mass of artificial glass by its having been +poured out of a vessel. Humboldt, also, has described little cavities, which he +compares to the tails of comets, behind sphaerulites in laminated obsidian +rocks from Mexico, and Mr. Scrope has described other cavities behind fragments +embedded in his laminated trachyte, and which he supposes to have been produced +during the movement of the mass. (“Geological Transactions” volume +2 second series page 200 etc. These embedded fragments, in some instances, +consist of the laminated trachyte broken off and “enveloped in those +parts, which still remained liquid.” Beudant, also, frequently refers in +his great work on “Hungary” tome 3 page 386, to trachytic rocks, +irregularly spotted with fragments of the same varieties, which in other parts +form the parallel ribbons. In these cases, we must suppose, that after part of +the molten mass had assumed a laminated structure, a fresh irruption of lava +broke up the mass, and involved fragments, and that subsequently the whole +became relaminated.) From such facts, most authors have attributed the +lamination of these volcanic rocks to their movement whilst liquified. Although +it is easy to perceive, why each separate air-cell, or each fibre in +pumice-stone (Dolomieu “Voyage” page 64.), should be drawn out in +the direction of the moving mass; it is by no means at first obvious why such +air-cells and fibres should be arranged by the movement, in the same planes, in +laminae absolutely straight and parallel to each other, and often of extreme +tenuity; and still less obvious is it, why such layers should come to be of +slightly different composition and of different textures. +</p> + +<p> +In endeavouring to make out the cause of the lamination of these igneous +feldspathic rocks, let us return to the facts so minutely described at +Ascension. We there see, that some of the thinnest layers are chiefly formed by +numerous, exceedingly minute, though perfect, crystals of different minerals; +that other layers are formed by the union of different kinds of concretionary +globules, and that the layers thus formed, often cannot be distinguished from +the ordinary feldspathic and pitchstone layers, composing a large portion of +the entire mass. The fibrous radiating structure of the sphaerulites seems, +judging from many analogous cases, to connect the concretionary and crystalline +forces: the separate crystals, also, of feldspar all lie in the same parallel +planes. (The formation, indeed, of a large crystal of any mineral in a rock of +mixed composition implies an aggregation of the requisite atoms, allied to +concretionary action. The cause of the crystals of feldspar in these rocks of +Ascension, being all placed lengthways, is probably the same with that which +elongates and flattens all the brown sphaerulitic globules (which behave like +feldspar under the blowpipe) in this same direction.) These allied forces, +therefore, have played an important part in the lamination of the mass, but +they cannot be considered the primary force; for the several kinds of nodules, +both the smallest and largest, are internally zoned with excessively fine +shades of colour, parallel to the lamination of the whole; and many of them +are, also, externally marked in the same direction with parallel ridges and +furrows, which have not been produced by weathering. +</p> + +<p> +Some of the finest streaks of colour in the stony layers, alternating with the +obsidian, can be distinctly seen to be due to an incipient crystallisation of +the constituent minerals. The extent to which the minerals have crystallised +can, also, be distinctly seen to be connected with the greater or less size, +and with the number, of the minute, flattened, crenulated air-cavities or +fissures. Numerous facts, as in the case of geodes, and of cavities in +silicified wood, in primary rocks, and in veins, show that crystallisation is +much favoured by space. Hence, I conclude, that, if in a mass of cooling +volcanic rock, any cause produced in parallel planes a number of minute +fissures or zones of less tension (which from the pent-up vapours would often +be expanded into crenulated air-cavities), the crystallisation of the +constituent parts, and probably the formation of concretions, would be +superinduced or much favoured in such planes; and thus, a laminated structure +of the kind we are here considering would be generated. +</p> + +<p> +That some cause does produce parallel zones of less tension in volcanic rocks, +during their consolidation, we must admit in the case of the thin alternate +layers of obsidian and pumice described by Humboldt, and of the small, +flattened, crenulated air-cells in the laminated rocks of Ascension; for on no +other principle can we conceive why the confined vapours should through their +expansion form air-cells or fibres in separate, parallel planes, instead of +irregularly throughout the mass. In Mr. Stokes’ collection, I have seen a +beautiful example of this structure, in a specimen of obsidian from Mexico, +which is shaded and zoned, like the finest agate, with numerous, straight, +parallel layers, more or less opaque and white, or almost perfectly glassy; the +degree of opacity and glassiness depending on the number of microscopically +minute, flattened air-cells; in this case, it is scarcely possible to doubt but +that the mass, to which the fragment belonged, must have been subjected to +some, probably prolonged, action, causing the tension slightly to vary in the +successive planes. +</p> + +<p> +Several causes appear capable of producing zones of different tension, in +masses semi-liquified by heat. In a fragment of devitrified glass, I have +observed layers of sphaerulites which appeared, from the manner in which they +were abruptly bent, to have been produced by the simple contraction of the mass +in the vessel, in which it cooled. In certain dikes on Mount Etna, described by +M. Elie de Beaumont (“Mem. pour servir” etc. tome 4 page 131.), as +bordered by alternating bands of scoriaceous and compact rock, one is led to +suppose that the stretching movement of the surrounding strata, which +originally produced the fissures, continued whilst the injected rock remained +fluid. Guided, however, by Professor Forbes’ (“Edinburgh New Phil. +Journal” 1842 page 350.) clear description of the zoned structure of +glacier-ice, far the most probable explanation of the laminated structure of +these feldspathic rocks appears to be, that they have been stretched whilst +slowly flowing onwards in a pasty condition (I presume that this is nearly the +same explanation which Mr. Scrope had in his mind, when he speaks +(“Geolog. Transact.” volume 2 second series page 228) of the +ribboned structure of his trachytic rocks, having arisen, from “a linear +extension of the mass, while in a state of imperfect liquidity, coupled with a +concretionary process.”), in precisely the same manner as Professor +Forbes believes, that the ice of moving glaciers is stretched and fissured. In +both cases, the zones may be compared to those in the finest agates; in both, +they extend in the direction in which the mass has flowed, and those exposed on +the surface are generally vertical: in the ice, the porous laminae are rendered +distinct by the subsequent congelation of infiltrated water, in the stony +feldspathic lavas, by subsequent crystalline and concretionary action. The +fragment of glassy obsidian in Mr. Stokes’ collection, which is zoned +with minute air-cells must strikingly resemble, judging from Professor +Forbes’ descriptions, a fragment of the zoned ice; and if the rate of +cooling and nature of the mass had been favourable to its crystallisation or to +concretionary action, we should here have had the finest parallel zones of +different composition and texture. In glaciers, the lines of porous ice and of +minute crevices seem to be due to an incipient stretching, caused by the +central parts of the frozen stream moving faster than the sides and bottom, +which are retarded by friction: hence in glaciers of certain forms and towards +the lower end of most glaciers, the zones become horizontal. May we venture to +suppose that in the feldspathic lavas with horizontal laminae, we see an +analogous case? All geologists, who have examined trachytic regions, have come +to the conclusion, that the lavas of this series have possessed an exceedingly +imperfect fluidity; and as it is evident that only matter thus characterised +would be subject to become fissured and to be formed into zones of different +tensions, in the manner here supposed, we probably see the reason why augitic +lavas, which appear generally to have possessed a high degree of fluidity, are +not, like the feldspathic lavas, divided into laminae of different composition +and texture. (Basaltic lavas, and many other rocks, are not unfrequently +divided into thick laminae or plates, of the same composition, which are either +straight or curved; these being crossed by vertical lines of fissure, sometimes +become united into columns. This structure seems related, in its origin, to +that by which many rocks, both igneous and sedimentary, become traversed by +parallel systems of fissures.) Moreover, in the augitic series, there never +appears to be any tendency to concretionary action, which we have seen plays an +important part in the lamination of rocks, of the trachytic series, or at least +in rendering that structure apparent. +</p> + +<p> +Whatever may be thought of the explanation here advanced of the laminated +structure of the rocks of the trachytic series, I venture to call the attention +of geologists to the simple fact, that in a body of rock at Ascension, +undoubtedly of volcanic origin, layers often of extreme tenuity, quite +straight, and parallel to each other, have been produced;—some composed +of distinct crystals of quartz and diopside, mingled with amorphous augitic +specks and granular feldspar,—others entirely composed of these black +augitic specks, with granules of oxide of iron,—and lastly, others formed +of crystalline feldspar, in a more or less perfect state of purity, together +with numerous crystals of feldspar, placed lengthways. At this island, there is +reason to believe, and in some analogous cases, it is certainly known, that the +laminae have originally been formed with their present high inclination. Facts +of this nature are manifestly of importance, with relation to the structural +origin of that grand series of plutonic rocks, which like the volcanic have +undergone the action of heat, and which consist of alternate layers of quartz, +feldspar, mica and other minerals. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap04"></a>CHAPTER IV.<br /> +ST. HELENA.</h2> + +<p> +Lavas of the feldspathic, basaltic, and submarine series. Section of Flagstaff +Hill and of the Barn. Dikes. Turk’s Cap and Prosperous Bays. Basaltic +ring. Central crateriform ridge, with an internal ledge and a parapet. Cones of +phonolite. Superficial beds of calcareous sandstone. Extinct land-shells. Beds +of detritus. Elevation of the land. Denudation. Craters of elevation. +</p> + +<p> +The whole island is of volcanic origin; its circumference, according to +Beatson, is about twenty-eight miles. (Governor Beatson “Account of St. +Helena.”) The central and largest part consists of rocks of a feldspathic +nature, generally decomposed to an extraordinary degree; and when in this +state, presenting a singular assemblage of alternating, red, purple, brown, +yellow, and white, soft, argillaceous beds. From the shortness of our visit, I +did not examine these beds with care; some of them, especially those of the +white, yellow, and brown shades, originally existed as streams of lava, but the +greater number were probably ejected in the form of scoriae and ashes: other +beds of a purple tint, porphyritic with crystal- shaped patches of a white, +soft substance, which are now unctuous, and yield, like wax, a polished streak +to the nail, seem once to have existed as solid claystone-porphyries: the red +argillaceous beds generally have a brecciated structure, and no doubt have been +formed by the decomposition of scoriae. Several extensive streams, however, +belonging to this series, retain their stony character; these are either of a +blackish-green colour, with minute acicular crystals of feldspar, or of a very +pale tint, and almost composed of minute, often scaly, crystals of feldspar, +abounding with microscopical black specks; they are generally compact and +laminated; others, however, of similar composition, are cellular and somewhat +decomposed. None of these rocks contain large crystals of feldspar, or have the +harsh fracture peculiar to trachyte. These feldspathic lavas and tuffs are the +uppermost or those last erupted; innumerable dikes, however, and great masses +of molten rock, have subsequently been injected into them. They converge, as +they rise, towards the central curved ridge, of which one point attains the +elevation of 2,700 feet. This ridge is the highest land in the island; and it +once formed the northern rim of a great crater, whence the lavas of this series +flowed: from its ruined condition, from the southern half having been removed, +and from the violent dislocation which the whole island has undergone, its +structure is rendered very obscure. +</p> + +<h3>BASALTIC SERIES.</h3> + +<p> +The margin of the island is formed by a rude circle of great, black, +stratified, ramparts of basalt, dipping seaward, and worn into cliffs, which +are often nearly perpendicular, and vary in height from a few hundred feet to +two thousand. This circle, or rather horse-shoe shaped ring, is open to the +south, and is breached by several other wide spaces. Its rim or summit +generally projects little above the level of the adjoining inland country; and +the more recent feldspathic lavas, sloping down from the central heights, +generally abut against and overlap its inner margin; on the north-western side +of the island, however, they appear (judging from a distance) to have flowed +over and concealed portions of it. In some parts, where the basaltic ring has +been breached, and the black ramparts stand detached, the feldspathic lavas +have passed between them, and now overhang the sea-coast in lofty cliffs. The +basaltic rocks are of a black colour and thinly stratified; they are generally +highly vesicular, but occasionally compact; some of them contain numerous +crystals of glassy feldspar and octahedrons of titaniferous iron; others abound +with crystals of augite and grains of olivine. The vesicles are frequently +lined with minute crystals (of chabasie?) and even become amygdaloidal with +them. The streams are separated from each other by cindery matter, or by a +bright red, friable, saliferous tuff, which is marked by successive lines like +those of aqueous deposition; and sometimes it has an obscure, concretionary +structure. The rocks of this basaltic series occur nowhere except near the +coast. In most volcanic districts the trachytic lavas are of anterior origin to +the basaltic; but here we see, that a great pile of rock, closely related in +composition to the trachytic family, has been erupted subsequently to the +basaltic strata: the number, however, of dikes, abounding with large crystals +of augite, with which the feldspathic lavas have been injected, shows perhaps +some tendency to a return to the more usual order of superposition. +</p> + +<h3>BASAL SUBMARINE LAVAS.</h3> + +<p> +The lavas of this basal series lie immediately beneath both the basaltic and +feldspathic rocks. According to Mr. Seale, they may be seen at intervals on the +sea-beach round the entire island. (“Geognosy of the Island of St. +Helena.” Mr. Seale has constructed a gigantic model of St. Helena, well +worth visiting, which is now deposited at Addiscombe College, in Surrey.) In +the sections which I examined, their nature varied much; some of the strata +abound with crystals of augite; others are of a brown colour, either laminated +or in a rubbly condition; and many parts are highly amygdaloidal with +calcareous matter. The successive sheets are either closely united together, or +are separated from each other by beds of scoriaceous rock and of laminated +tuff, frequently containing well-rounded fragments. The interstices of these +beds are filled with gypsum and salt; the gypsum also sometimes occurring in +thin layers. From the large quantity of these two substances, from the presence +of rounded pebbles in the tuffs, and from the abundant amygdaloids, I cannot +doubt that these basal volcanic strata flowed beneath the sea. This remark +ought perhaps to be extended to a part of the superincumbent basaltic rocks; +but on this point, I was not able to obtain clear evidence. The strata of the +basal series, whenever I examined them, were intersected by an extraordinary +number of dikes. +</p> + +<h3>FLAGSTAFF HILL AND THE BARN.</h3> + +<p> +(FIGURE 8. FLAGSTAFF HILL AND THE BARN. (Section West (left) to East (right)) +Flagstaff Hill, 2,272 feet high to The Barn, 2,015 feet high. +</p> + +<p> +The double lines represent the basaltic strata; the single, the basal submarine +strata; the dotted, the upper feldspathic strata; the dikes are shaded +transversely.) +</p> + +<p> +I will now describe some of the more remarkable sections, and will commence +with these two hills, which form the principal external feature on the +north-eastern side of the island. The square, angular outline, and black colour +of the Barn, at once show that it belongs to the basaltic series; whilst the +smooth, conical figure, and the varied bright tints of Flagstaff Hill, render +it equally clear, that it is composed of the softened, feldspathic rocks. These +two lofty hills are connected (as is shown in Figure 8) by a sharp ridge, which +is composed of the rubbly lavas of the basal series. The strata of this ridge +dip westward, the inclination becoming less and less towards the Flagstaff; and +the upper feldspathic strata of this hill can be seen, though with some +difficulty, to dip conformably to the W.S.W. Close to the Barn, the strata of +the ridge are nearly vertical, but are much obscured by innumerable dikes; +under this hill, they probably change from being vertical into being inclined +into an opposite direction; for the upper or basaltic strata, which are about +eight hundred or one thousand feet in thickness, are inclined north-eastward, +at an angle between thirty and forty degrees. +</p> + +<p> +This ridge, and likewise the Barn and Flagstaff Hills, are interlaced by dikes, +many of which preserve a remarkable parallelism in a N.N.W. and S.S.E. +direction. The dikes chiefly consist of a rock, porphyritic with large crystals +of augite; others are formed of a fine-grained and brown- coloured trap. Most +of these dikes are coated by a glossy layer, from one to two-tenths of an inch +in thickness, which, unlike true pitchstone, fuses into a black enamel; this +layer is evidently analogous to the glossy superficial coating of many lava +streams. (This circumstance has been observed (Lyell “Principles of +Geology” volume 4 chapter 10 page 9) in the dikes of the Atrio del +Cavallo, but apparently it is not of very common occurrence. Sir G. Mackenzie, +however, states (page 372 “Travels in Iceland”) that all the veins +in Iceland have a “black vitreous coating on their sides.” Captain +Carmichael, speaking of the dikes in Tristan d’Acunha, a volcanic island +in the Southern Atlantic, says (“Linnaean Transactions” volume 12 +page 485) that their sides, “where they come in contact with the rocks, +are invariably in a semi-vitrified state.”) The dikes can often be +followed for great lengths both horizontally and vertically, and they seem to +preserve a nearly uniform thickness (“Geognosy of the Island of St. +Helena” plate 5.): Mr. Seale states, that one near the Barn, in a height +of 1,260 feet, decreases in width only four inches,—from nine feet at the +bottom, to eight feet and eight inches at the top. On the ridge, the dikes +appear to have been guided in their course, to a considerable degree, by the +alternating soft and hard strata: they are often firmly united to the harder +strata, and they preserve their parallelism for such great lengths, that in +very many instances it was impossible to conjecture, which of the beds were +dikes, and which streams of lava. The dikes, though so numerous on this ridge, +are even more numerous in the valleys a little south of it, and to a degree I +never saw equalled anywhere else: in these valleys they extend in less regular +lines, covering the ground with a network, like a spider’s web, and with +some parts of the surface even appearing to consist wholly of dikes, interlaced +by other dikes. +</p> + +<p> +From the complexity produced by the dikes, from the high inclination and +anticlinal dip of the strata of the basal series, which are overlaid, at the +opposite ends of the short ridge, by two great masses of different ages and of +different composition, I am not surprised that this singular section has been +misunderstood. It has even been supposed to form part of a crater; but so far +is this from having been the case, that the summit of Flagstaff Hill once +formed the lower extremity of a sheet of lava and ashes, which were erupted +from the central, crateriform ridge. Judging from the slope of the +contemporaneous streams in an adjoining and undisturbed part of the island, the +strata of the Flagstaff Hill must have been upturned at least twelve hundred +feet, and probably much more, for the great truncated dikes on its summit show +that it has been largely denuded. The summit of this hill now nearly equals in +height the crateriform ridge; and before having been denuded, it was probably +higher than this ridge, from which it is separated by a broad and much lower +tract of country; we here, therefore, see that the lower extremities of a set +of lava-streams have been tilted up to as great a height as, or perhaps greater +height than, the crater, down the flanks of which they originally flowed. I +believe that dislocations on so grand a scale are extremely rare in volcanic +districts. (M. Constant Prevost “Mem. de la Soc. Geolog.” tome 2 +observes that “les produits volcaniques n’ont que localement et +rarement meme derange le sol, a travers lequel ils se sont fait jour.”) +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 degree: this +stretching, on the ridge between Flagstaff and Barn Hills, probably took place +subsequently (though perhaps immediately so) to the strata being tilted; for +had the strata at that time extended horizontally, they would in all +probability have been fissured and injected transversely, instead of in the +planes of their stratification. Although the space between the Barn and +Flagstaff Hill presents a distinct anticlinal line extending north and south, +and though most of the dikes range with much regularity in the same line, +nevertheless, at only a mile due south of the ridge the strata lie undisturbed. +Hence the disturbing force seems to have acted under a point, rather than along +a line. The manner in which it has acted, is probably explained by the +structure of Little Stony-top, a mountain 2,000 feet high, situated a few miles +southward of the Barn; we there see, even from a distance, a dark-coloured, +sharp, wedge of compact columnar rock, with the bright-coloured feldspathic +strata, sloping away on each side from its uncovered apex. This wedge, from +which it derives its name of Stony-top, consists of a body of rock, which has +been injected whilst liquified into the overlying strata; and if we may suppose +that a similar body of rock lies injected, beneath the ridge connecting the +Barn and Flagstaff, the structure there exhibited would be explained. +</p> + +<h3>TURK’S CAP AND PROSPEROUS BAYS.</h3> + +<p> +(FIGURE 9. PROSPEROUS HILL AND THE BARN. (Section S.S.E. (left) to N.N.W. +(right) Prosperous Hill through Hold-fast-Tom and Flagstaff Hill to The Barn. +</p> + +<p> +The double lines represent the basaltic strata; the single, the basal submarine +strata; the dotted, the upper feldspathic strata.) +</p> + +<p> +Prosperous Hill is a great, black, precipitous mountain, situated two miles and +a half south of the Barn, and composed, like it, of basaltic strata. These +rest, in one part, on the brown-coloured, porphyritic beds of the basal series, +and in another part, on a fissured mass of highly scoriaceous and amygdaloidal +rock, which seems to have formed a small point of eruption beneath the sea, +contemporaneously with the basal series. Prosperous Hill, like the Barn, is +traversed by many dikes, of which the greater number range north and south, and +its strata dip, at an angle of about 20 degrees, rather obliquely from the +island towards the sea. The space between Prosperous Hill and the Barn, as +represented in Figure 9, consists of lofty cliffs, composed of the lavas of the +upper or feldspathic series, which rest, though unconformably, on the basal +submarine strata, as we have seen that they do at Flagstaff Hill. Differently, +however, from in that hill, these upper strata are nearly horizontal, gently +rising towards the interior of the island; and they are composed of +greenish-black, or more commonly, pale brown, compact lavas, instead of +softened and highly coloured matter. These brown-coloured, compact lavas, +consist almost entirely of small glimmering scales, or of minute acicular +crystals, of feldspar, placed close by the side of each other, and abounding +with minute black specks, apparently of hornblende. The basaltic strata of +Prosperous Hill project only a little above the level of the gently-sloping, +feldspathic streams, which wind round and abut against their upturned edges. +The inclination of the basaltic strata seems to be too great to have been +caused by their having flowed down a slope, and they must have been tilted into +their present position before the eruption of the feldspathic streams. +</p> + +<h3>BASALTIC RING.</h3> + +<p> +Proceeding round the Island, the lavas of the upper series, southward of +Prosperous Hill, overhang the sea in lofty precipices. Further on, the +headland, called Great Stony-top, is composed, as I believe, of basalt; as is +Long Range Point, on the inland side of which the coloured beds abut. On the +southern side of the island, we see the basaltic strata of the South Barn, +dipping obliquely seaward at a considerable angle; this headland, also, stands +a little above the level of the more modern, feldspathic lavas. Further on, a +large space of coast, on each side of Sandy Bay, has been much denuded, and +there seems to be left only the basal wreck of the great, central crater. The +basaltic strata reappear, with their seaward dip, at the foot of the hill, +called Man-and-Horse; and thence they are continued along the whole +north-western coast to Sugar-Loaf Hill, situated near to the Flagstaff; and +they everywhere have the same seaward inclination, and rest, in some parts at +least, on the lavas of the basal series. We thus see that the circumference of +the island is formed by a much-broken ring, or rather, a horse-shoe, of basalt, +open to the south, and interrupted on the eastern side by many wide breaches. +The breadth of this marginal fringe on the north-western side, where alone it +is at all perfect, appears to vary from a mile to a mile and a half. The +basaltic strata, as well as those of the subjacent basal series, dip, with a +moderate inclination, where they have not been subsequently disturbed, towards +the sea. The more broken state of the basaltic ring round the eastern half, +compared with the western half of the island, is evidently due to the much +greater denuding power of the waves on the eastern or windward side, as is +shown by the greater height of the cliffs on that side, than to leeward. +Whether the margin of basalt was breached, before or after the eruption of the +lavas of the upper series, is doubtful; but as separate portions of the +basaltic ring appear to have been tilted before that event, and from other +reasons, it is more probable, that some at least of the breaches were first +formed. Reconstructing in imagination, as far as is possible, the ring of +basalt, the internal space or hollow, which has since been filled up with the +matter erupted from the great central crater, appears to have been of an oval +figure, eight or nine miles in length by about four miles in breadth, and with +its axis directed in a N.E. and S W. line, coincident with the present longest +axis of the island. +</p> + +<h3>THE CENTRAL CURVED RIDGE.</h3> + +<p> +This ridge consists, as before remarked, of grey feldspathic lavas, and of red, +brecciated, argillaceous tuffs, like the beds of the upper coloured series. The +grey lavas contain numerous, minute, black, easily fusible specks; and but very +few large crystals of feldspar. They are generally much softened; with the +exception of this character, and of being in many parts highly cellular, they +are quite similar to those great sheets of lava which overhang the coast at +Prosperous Bay. Considerable intervals of time appear to have elapsed, judging +from the marks of denudation, between the formation of the successive beds, of +which this ridge is composed. On the steep northern slope, I observed in +several sections a much worn undulating surface of red tuff, covered by grey, +decomposed, feldspathic lavas, with only a thin earthy layer interposed between +them. In an adjoining part, I noticed a trap-dike, four feet wide, cut off and +covered up by the feldspathic lava, as is represented in Figure 9. The ridge +ends on the eastern side in a hook, which is not represented clearly enough in +any map which I have seen; towards the western end, it gradually slopes down +and divides into several subordinate ridges. The best defined portion between +Diana’s Peak and Nest Lodge, which supports the highest pinnacles in the +island varying from 2,000 to 2,700 feet, is rather less than three miles long +in a straight line. Throughout this space the ridge has a uniform appearance +and structure; its curvature resembles that of the coast-line of a great bay, +being made up of many smaller curves, all open to the south. The northern and +outer side is supported by narrow ridges or buttresses, which slope down to the +adjoining country. The inside is much steeper, and is almost precipitous; it is +formed of the basset edges of the strata, which gently decline outwards. Along +some parts of the inner side, a little way beneath the summit, a flat ledge +extends, which imitates in outline the smaller curvatures of the crest. Ledges +of this kind occur not unfrequently within volcanic craters, and their +formation seems to be due to the sinking down of a level sheet of hardened +lava, the edges of which remain (like the ice round a pool, from which the +water has been drained) adhering to the sides. (A most remarkable instance of +this structure is described in Ellis “Polynesian Researches” second +edition where an admirable drawing is given of the successive ledges or +terraces, on the borders of the immense crater at Hawaii, in the Sandwich +Islands.) +</p> + +<p> +(FIGURE 10. DIKE. (Section showing layers 1, 2 and 3 from top to bottom.) +</p> + +<p> +1. Grey feldspathic lava. +</p> + +<p> +2. A layer, one inch in thickness, of a reddish earthy matter. +</p> + +<p> +3. Brecciated, red, argillaceous tuff.) +</p> + +<p> +In some parts, the ridge is surmounted by a wall or parapet, perpendicular on +both sides. Near Diana’s Peak this wall is extremely narrow. At the +Galapagos Archipelago I observed parapets, having a quite similar structure and +appearance, surmounting several of the craters; one, which I more particularly +examined, was composed of glossy, red scoriae firmly cemented together; being +externally perpendicular, and extending round nearly the whole circumference of +the crater, it rendered it almost inaccessible. The Peak of Teneriffe and +Cotopaxi, according to Humboldt, are similarly constructed; he states that +“at their summits a circular wall surrounds the crater, which wall, at a +distance, has the appearance of a small cylinder placed on a truncated cone. +(“Personal Narrative” volume 1 page 171.) On Cotopaxi this peculiar +structure is visible to the naked eye at more than two thousand toises’ +distance; and no person has ever reached its crater. (Humboldt +“Picturesque Atlas” folio plate 10.) On the Peak of Teneriffe, the +parapet is so high, that it would be impossible to reach the caldera, if on the +eastern side there did not exist a breach.” The origin of these circular +parapets is probably due to the heat or vapours from the crater, penetrating +and hardening the sides to a nearly equal depth, and afterwards to the mountain +being slowly acted on by the weather, which would leave the hardened part, +projecting in the form of a cylinder or circular parapet. +</p> + +<p> +From the points of structure in the central ridge, now +enumerated,—namely, from the convergence towards it of the beds of the +upper series,—from the lavas there becoming highly cellular,—from +the flat ledge, extending along its inner and precipitous side, like that +within some still active craters,—from the parapet-like wall on its +summit,—and lastly, from its peculiar curvature, unlike that of any +common line of elevation, I cannot doubt that this curved ridge forms the last +remnant of a great crater. In endeavouring, however, to trace its former +outline, one is soon baffled; its western extremity gradually slopes down, and, +branching into other ridges, extends to the sea-coast; the eastern end is more +curved, but it is only a little better defined. Some appearances lead me to +suppose that the southern wall of the crater joined the present ridge near Nest +Lodge; in this case the crater must have been nearly three miles long, and +about a mile and a half in breadth. Had the denudation of the ridge and the +decomposition of its constituent rocks proceeded a few steps further, and had +this ridge, like several other parts of the island, been broken up by great +dikes and masses of injected matter, we should in vain have endeavoured to +discover its true nature. 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 flowed, and +probably even to a greater height. It is interesting thus to trace the steps by +which the structure of a volcanic district becomes obscured, and finally +obliterated: so near to this last stage is St. Helena, that I believe no one +has hitherto suspected that the central ridge or axis of the island is the last +wreck of the crater, whence the most modern volcanic streams were poured forth. +</p> + +<p> +The great hollow space or valley southward of the central curved ridge, across +which the half of the crater must once have extended, is formed of bare, +water-worn hillocks and ridges of red, yellow, and brown rocks, mingled +together in chaos-like confusion, interlaced by dikes, and without any regular +stratification. The chief part consists of red decomposing scoriae, associated +with various kinds of tuff and yellow argillaceous beds, full of broken +crystals, those of augite being particularly large. Here and there masses of +highly cellular and amygdaloidal lavas protrude. From one of the ridges in the +midst of the valley, a conical precipitous hill, called Lot, boldly stands up, +and forms a most singular and conspicuous object. It is composed of phonolite, +divided in one part into great curved laminae, in another, into angular +concretionary balls, and in a third part into outwardly radiating columns. At +its base the strata of lava, tuff, and scoriae, dip away on all sides (Abich in +his “Views of Vesuvius” plate 6 has shown the manner in which beds, +under nearly similar circumstances, are tilted up. The upper beds are more +turned up than the lower; and he accounts for this, by showing that the lava +insinuates itself horizontally between the lower beds.); the uncovered portion +is 197 feet in height (This height is given by Mr. Seale in his Geognosy of the +island. The height of the summit above the level of the sea is said to be 1,444 +feet.), and its horizontal section gives an oval figure. The phonolite is of a +greenish-grey colour, and is full of minute acicular crystals of feldspar; in +most parts it has a conchoidal fracture, and is sonorous, yet it is crenulated +with minute air-cavities. In a S.W. direction from Lot, there are some other +remarkable columnar pinnacles, but of a less regular shape, namely, Lot’s +Wife, and the Asses’ Ears, composed of allied kinds of rock. From their +flattened shape, and their relative position to each other, they are evidently +connected on the same line of fissure. It is, moreover, remarkable that this +same N.E. and S.W. line, joining Lot and Lot’s Wife, if prolonged would +intersect Flagstaff Hill, which, as before stated, is crossed by numerous dikes +running in this direction, and which has a disturbed structure, rendering it +probable that a great body of once fluid rock lies injected beneath it. +</p> + +<p> +In this same great valley there are several other conical masses of injected +rock (one, I observed, was composed of compact greenstone), some of which are +not connected, as far as is apparent, with any line of dike; whilst others are +obviously thus connected. Of these dikes, three or four great lines stretch +across the valley in a N.E. and S.W. direction, parallel to that one connecting +the Asses’ Ears, Lot’s Wife, and probably Lot. The number of these +masses of injected rock is a remarkable feature in the geology of St. Helena. +Besides those just mentioned, and the hypothetical one beneath Flagstaff Hill, +there is Little Stony-top and others, as I have reason to believe, at the +Man-and-Horse, and at High Hill. Most of these masses, if not all of them, have +been injected subsequently to the last volcanic eruptions from the central +crater. The formation of conical bosses of rock on lines of fissure, the walls +of which are in most cases parallel, may probably be attributed to inequalities +in the tension, causing small transverse fissures, and at these points of +intersection the edges of the strata would naturally yield, and be easily +turned upwards. Finally, I may remark, that hills of phonolite everywhere are +apt to assume singular and even grotesque shapes, like that of Lot +(D’Aubuisson in his “Traite de Geognosie” tome 2 page 540 +particularly remarks that this is the case.): the peak at Fernando Noronha +offers an instance; at St. Jago, however, the cones of phonolite, though +tapering, have a regular form. Supposing, as seems probable, that all such +hillocks or obelisks have originally been injected, whilst liquified, into a +mould formed by yielding strata, as certainly has been the case with Lot, how +are we to account for the frequent abruptness and singularity of their +outlines, compared with similarly injected masses of greenstone and basalt? Can +it be due to a less perfect degree of fluidity, which is generally supposed to +be characteristic of the allied trachytic lavas? +</p> + +<h3>SUPERFICIAL DEPOSITS.</h3> + +<p> +Soft calcareous sandstone occurs in extensive, though thin, superficial beds, +both on the northern and southern shores of the island. It consists of very +minute, equal-sized, rounded particles of shells, and other organic bodies, +which partially retain their yellow, brown, and pink colours, and occasionally, +though very rarely, present an obscure trace of their original external forms. +I in vain endeavoured to find a single unrolled fragment of a shell. The colour +of the particles is the most obvious character by which their origin can be +recognised, the tints being affected (and an odour produced) by a moderate +heat, in the same manner as in fresh shells. The particles are cemented +together, and are mingled with some earthy matter: the purest masses, according +to Beatson, contain 70 per cent of carbonate of lime. The beds, varying in +thickness from two or three feet to fifteen feet, coat the surface of the +ground; they generally lie on that side of the valley which is protected from +the wind, and they occur at the height of several hundred feet above the level +of the sea. Their position is the same which sand, if now drifted by the +trade-wind, would occupy; and no doubt they thus originated, which explains the +equal size and minuteness of the particles, and likewise the entire absence of +whole shells, or even of moderately-sized fragments. It is remarkable that at +the present day there are no shelly beaches on any part of the coast, whence +calcareous dust could be drifted and winnowed; we must, therefore, look back to +a former period when before the land was worn into the present great +precipices, a shelving coast, like that of Ascension, was favourable to the +accumulation of shelly detritus. Some of the beds of this limestone are between +six hundred and seven hundred feet above the sea; but part of this height may +possibly be due to an elevation of the land, subsequent to the accumulation of +the calcareous sand. +</p> + +<p> +The percolation of rain-water has consolidated parts of these beds into a solid +rock, and has formed masses of dark brown, stalagmitic limestone. At the +Sugar-Loaf quarry, fragments of rock on the adjoining slopes have been thickly +coated by successive fine layers of calcareous matter. (In the earthy detritus +on several parts of this hill, irregular masses of very impure, crystallised +sulphate of lime occur. As this substance is now being abundantly deposited by +the surf at Ascension, it is possible that these masses may thus have +originated; but if so, it must have been at a period when the land stood at a +much lower level. This earthy selenite is now found at a height of between six +hundred and seven hundred feet.) It is singular, that many of these pebbles +have their entire surfaces coated, without any point of contact having been +left uncovered; hence, these pebbles must have been lifted up by the slow +deposition between them of the successive films of carbonate of lime. Masses of +white, finely oolitic rock are attached to the outside of some of these coated +pebbles. Von Buch has described a compact limestone at Lanzarote, which seems +perfectly to resemble the stalagmitic deposition just mentioned: it coats +pebbles, and in parts is finely oolitic: it forms a far-extended layer, from +one inch to two or three feet in thickness, and it occurs at the height of 800 +feet above the sea, but only on that side of the island exposed to the violent +north-western winds. Von Buch remarks, that it is not found in hollows, but +only on the unbroken and inclined surfaces of the mountain. (“Description +des Isles Canaries” page 293.) He believes, that it has been deposited by +the spray which is borne over the whole island by these violent winds. It +appears, however, to me much more probable that it has been formed, as at St. +Helena, by the percolation of water through finely comminuted shells: for when +sand is blown on a much-exposed coast, it always tends to accumulate on broad, +even surfaces, which offer a uniform resistance to the winds. At the +neighbouring island, moreover, of Feurteventura, there is an earthy limestone, +which, according to Von Buch, is quite similar to specimens which he has seen +from St. Helena, and which he believes to have been formed by the drifting of +shelly detritus. (Idem pages 314 and 374.) +</p> + +<p> +The upper beds of the limestone, at the above-mentioned quarry on the +Sugar-Loaf Hill, are softer, finer-grained and less pure, than the lower beds. +They abound with fragments of land-shells, and with some perfect ones; they +contain, also, the bones of birds, and the large eggs, apparently of +water-fowl. (Colonel Wilkes, in a catalogue presented with some specimens to +the Geological Society, states that as many as ten eggs were found by one +person. Dr. Buckland has remarked (“Geolog. Trans.” volume 5 page +474) on these eggs.) It is probable that these upper beds remained long in an +unconsolidated form, during which time, these terrestrial productions were +embedded. Mr. G.R. Sowerby has kindly examined three species of land-shells, +which I procured from this bed, and has described them in detail. One of them +is a Succinea, identical with a species now living abundantly on the island; +the two others, namely, Cochlogena fossilis and Helix biplicata, are not known +in a recent state: the latter species was also found in another and different +locality, associated with a species of Cochlogena which is undoubtedly extinct. +</p> + +<h3>BEDS OF EXTINCT LAND-SHELLS.</h3> + +<p> +Land-shells, all of which appear to be species now extinct, occur embedded in +earth, in several parts of the island. The greater number have been found at a +considerable height on Flagstaff Hill. On the N.W. side of this hill, a +rain-channel exposes a section of about twenty feet in thickness, of which the +upper part consists of black vegetable mould, evidently washed down from the +heights above, and the lower part of less black earth, abounding with young and +old shells, and with their fragments: part of this earth is slightly +consolidated by calcareous matter, apparently due to the partial decomposition +of some of the shells. Mr. Seale, an intelligent resident, who first called +attention to these shells, gave me a large collection from another locality, +where the shells appear to have been embedded in very black earth. Mr. G.R. +Sowerby has examined these shells, and has described them. There are seven +species, namely, one Cochlogena, two species of the genus Cochlicopa, and four +of Helix; none of these are known in a recent state, or have been found in any +other country. The smaller species were picked out of the inside of the large +shells of the Cochlogena aurisvulpina. This last-mentioned species is in many +respects a very singular one; it was classed, even by Lamarck, in a marine +genus, and having thus been mistaken for a sea-shell, and the smaller +accompanying species having been overlooked, the exact localities where it was +found have been measured, and the elevation of this island thus deduced! It is +very remarkable that all the shells of this species found by me in one spot, +form a distinct variety, as described by Mr. Sowerby, from those procured from +another locality by Mr. Seale. As this Cochlogena is a large and conspicuous +shell, I particularly inquired from several intelligent countrymen whether they +had ever seen it alive; they all assured me that they had not, and they would +not even believe that it was a land animal: Mr. Seale, moreover, who was a +collector of shells all his life at St. Helena, never met with it alive. +Possibly some of the smaller species may turn out to be yet living kinds; but, +on the other hand, the two land- shells which are now living on the island in +great numbers, do not occur embedded, as far as is yet known, with the extinct +species. I have shown in my “Journal” (“Journal of +Researches” page 582.), that the extinction of these land-shells possibly +may not be an ancient event; as a great change took place in the state of the +island about one hundred and twenty years ago, when the old trees died, and +were not replaced by young ones, these being destroyed by the goats and hogs, +which had run wild in numbers, from the year 1502. Mr. Seale states, that on +Flagstaff Hill, where we have seen that the embedded land-shells are especially +numerous, traces are everywhere discoverable, which plainly indicate that it +was once thickly clothed with trees; at present not even a bush grows there. +The thick bed of black vegetable mould which covers the shell-bed, on the +flanks of this hill, was probably washed down from the upper part, as soon as +the trees perished, and the shelter afforded by them was lost. +</p> + +<h3>ELEVATION OF THE LAND.</h3> + +<p> +Seeing that the lavas of the basal series, which are of submarine origin, are +raised above the level of the sea, and at some places to the height of many +hundred feet, I looked out for superficial signs of the elevation of the land. +The bottoms of some of the gorges, which descend to the coast, are filled up to +the depth of about a hundred feet, by rudely divided layers of sand, muddy +clay, and fragmentary masses; in these beds, Mr. Seale has found the bones of +the tropic-bird and of the albatross; the former now rarely, and the latter +never visiting the island. From the difference between these layers, and the +sloping piles of detritus which rest on them, I suspect that they were +deposited, when the gorges stood beneath the sea. Mr. Seale, moreover, has +shown that some of the fissure- like gorges become, with a concave outline, +gradually rather wider at the bottom than at the top; and this peculiar +structure was probably caused by the wearing action of the sea, when it entered +the lower part of these gorges. (A fissure-like gorge, near Stony-top, is said +by Mr. Seale to be 840 feet deep, and only 115 feet in width.) At greater +heights, the evidence of the rise of the land is even less clear: nevertheless, +in a bay-like depression on the table-land behind Prosperous Bay, at the height +of about a thousand feet, there are flat-topped masses of rock, which it is +scarcely conceivable, could have been insulated from the surrounding and +similar strata, by any other agency than the denuding action of a sea- beach. +Much denudation, indeed, has been effected at great elevations, which it would +not be easy to explain by any other means: thus, the flat summit of the Barn, +which is 2,000 feet high, presents, according to Mr. Seale, a perfect network +of truncated dikes; on hills like the Flagstaff, formed of soft rock, we might +suppose that the dikes had been worn down and cut off by meteoric agency, but +we can hardly suppose this possible with the hard, basaltic strata of the Barn. +</p> + +<h3>COAST DENUDATION.</h3> + +<p> +The enormous cliffs, in many parts between one and two thousand feet in height, +with which this prison-like island is surrounded, with the exception of only a +few places, where narrow valleys descend to the coast, is the most striking +feature in its scenery. We have seen that portions of the basaltic ring, two or +three miles in length by one or two miles in breadth, and from one to two +thousand feet in height, have been wholly removed. There are, also, ledges and +banks of rock, rising out of profoundly deep water, and distant from the +present coast between three and four miles, which, according to Mr. Seale, can +be traced to the shore, and are found to be the continuations of certain +well-known great dikes. The swell of the Atlantic Ocean has obviously been the +active power in forming these cliffs; and it is interesting to observe that the +lesser, though still great, height of the cliffs on the leeward and partially +protected side of the island (extending from the Sugar-Loaf Hill to South West +Point), corresponds with the lesser degree of exposure. When reflecting on the +comparatively low coasts of many volcanic islands, which also stand exposed in +the open ocean, and are apparently of considerable antiquity, the mind recoils +from an attempt to grasp the number of centuries of exposure, 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 island. The contrast in the +superficial state of St. Helena, compared with the nearest island, namely, +Ascension, is very striking. At Ascension, the surfaces of the lava-streams are +glossy, as if just poured forth, their boundaries are well defined, and they +can often be traced to perfect craters, whence they were erupted; in the course +of many long walks, I did not observe a single dike; and the coast round nearly +the entire circumference is low, and has been eaten back (though too much +stress must not be placed on this fact, as the island may have been subsiding) +into a little wall only from ten to thirty feet high. Yet during the 340 years, +since Ascension has been known, not even the feeblest signs of volcanic action +have been recorded. (In the “Nautical Magazine” for 1835 page 642, +and for 1838 page 361, and in the “Comptes Rendus” April 1838, +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 century, in a space of open sea between longitudes 20 degrees and +22 degrees west, about half a degree south of the equator. These facts seem to +show, that an island or an archipelago is in process of formation in the middle +of the Atlantic: a line joining St. Helena and Ascension, prolonged, intersects +this slowly nascent focus of volcanic action.) On the other hand, at St. +Helena, the course of no one stream of lava can be traced, either by the state +of its boundaries or of its superficies; the mere wreck of one great crater is +left; not the valleys only, but the surfaces of some of the highest hills, are +interlaced by worn-down dikes, and, in many places, the denuded summits of +great cones of injected rock stand exposed and naked; lastly, as we have seen, +the entire circuit of the island has been deeply worn back into the grandest +precipices. +</p> + +<h3>CRATERS OF ELEVATION.</h3> + +<p> +There is much resemblance in structure and in geological history between St. +Helena, St. Jago, and Mauritius. All three islands are bounded (at least in the +parts which I was able to examine) by a ring of basaltic mountains, now much +broken, but evidently once continuous. These mountains have, or apparently once +had, their escarpments steep towards the interior of the island, and their +strata dip outwards. I was able to ascertain, only in a few cases, the +inclination of the beds; nor was this easy, for the stratification was +generally obscure, except when viewed from a distance. I feel, however, little +doubt that, according to the researches of M. Elie de Beaumont, their average +inclination is greater than that which they could have acquired, considering +their thickness and compactness, by flowing down a sloping surface. At St. +Helena, and at St. Jago, the basaltic strata rest on older and probably +submarine beds of different composition. At all three islands, deluges of more +recent lavas have flowed from the centre of the island, towards and between the +basaltic mountains; and at St. Helena the central platform has been filled up +by them. All three islands have been raised in mass. At Mauritius the sea, +within a late geological period, must have reached to the foot of the basaltic +mountains, as it now does at St. Helena; and at St. Jago it is cutting back the +intermediate plain towards them. In these three islands, but especially at St. +Jago and at Mauritius, when, standing on the summit of one of the old basaltic +mountains, one looks in vain towards the centre of the island,—the point +towards which the strata beneath one’s feet, and of the mountains on each +side, rudely converge,—for a source whence these strata could have been +erupted; but one sees only a vast hollow platform stretched beneath, or piles +of matter of more recent origin. +</p> + +<p> +These basaltic mountains come, I presume, into the class of Craters of +elevation: it is immaterial whether the rings were ever completely formed, for +the portions which now exist have so uniform a structure, that, if they do not +form fragments of true craters, they cannot be classed with ordinary lines of +elevation. With respect to their origin, after having read the works of Mr. +Lyell (“Principles of Geology” fifth edition volume 2 page 171.), +and of MM. C. Prevost and Virlet, I cannot believe that the great central +hollows have been formed by a simple dome-shaped elevation, and the consequent +arching of the strata. On the other hand, I have very great difficulty in +admitting that these basaltic mountains are merely the basal fragments of great +volcanoes, of which the summits have either been blown off, or more probably +swallowed up by subsidence. These rings are, in some instances, so immense, as +at St. Jago and at Mauritius, and their occurrence is so frequent, that I can +hardly persuade myself to adopt this explanation. Moreover, I suspect that the +following circumstances, from their frequent concurrence, are someway connected +together,—a connection not implied in either of the above views: namely, +first, the broken state of the ring; showing that the now detached portions +have been exposed to great denudation, and in some cases, perhaps, rendering it +probable that the ring never was entire; secondly, the great amount of matter +erupted from the central area after or during the formation of the ring; and +thirdly, the elevation of the district in mass. As far as relates to the +inclination of the strata being greater than that which the basal fragments of +ordinary volcanoes would naturally possess, I can readily believe that this +inclination might have been slowly acquired by that amount of elevation, of +which, according to M. Elie de Beaumont, the numerous upfilled fissures or +dikes are the evidence and the measure,—a view equally novel and +important, which we owe to the researches of that geologist on Mount Etna. +</p> + +<p> +A conjecture, including the above circumstances, occurred to me, when,— +with my mind fully convinced, from the phenomena of 1835 in South America, that +the forces which eject matter from volcanic orifices and raise continents in +mass are identical,—I viewed that part of the coast of St. Jago, where +the horizontally upraised, calcareous stratum dips into the sea, directly +beneath a cone of subsequently erupted lava. (I have given a detailed account +of these phenomena, in a paper read before the Geological Society in March +1838. At the instant of time, when an immense area was convulsed and a large +tract elevated, the districts immediately surrounding several of the great +vents in the Cordillera remained quiescent; the subterranean forces being +apparently relieved by the eruptions, which then recommenced with great +violence. An event of somewhat the same kind, but on an infinitely smaller +scale, appears to have taken place, according to Abich (“Views of +Vesuvius” plates 1 and 9), within the great crater of Vesuvius, where a +platform on one side of a fissure was raised in mass twenty feet, whilst on the +other side, a train of small volcanoes burst forth in eruption.) The conjecture +is that, during the slow elevation of a volcanic district or island, in the +centre of which one or more orifices continue open, and thus relieve the +subterranean forces, the borders are elevated more than the central area; and +that the portions thus upraised do not slope gently into the central, less +elevated area, as does the calcareous stratum under the cone at St. Jago, and +as does a large part of the circumference of Iceland, but that they are +separated from it by curved faults. (It appears, from information communicated +to me in the most obliging manner by M. E. Robert, that the circumferential +parts of Iceland, which are composed of ancient basaltic strata alternating +with tuff, dip inland, thus forming a gigantic saucer. M. Robert found that +this was the case, with a few and quite local exceptions, for a space of coast +several hundred miles in length. I find this statement corroborated, as far as +regards one place, by Mackenzie in his “Travels” page 377, and in +another place by some MS. notes kindly lent me by Dr. Holland. The coast is +deeply indented by creeks, at the head of which the land is generally low. M. +Robert informs me, that the inwardly dipping strata appear to extend as far as +this line, and that their inclination usually corresponds with the slope of the +surface, from the high coast-mountains to the low land at the head of these +creeks. In the section described by Sir G. Mackenzie, the dip is 120. The +interior parts of the island chiefly consist, as far as is known, of recently +erupted matter. The great size, however, of Iceland, equalling the bulkiest +part of England, ought perhaps to exclude it from the class of islands we have +been considering; but I cannot avoid suspecting that if the coast-mountains, +instead of gently sloping into the less elevated central area, had been +separated from it by irregularly curved faults, the strata would have been +tilted seaward, and a “Crater of elevation,” like that of St. Jago +or that of Mauritius, but of much vaster dimensions, would have been formed. I +will only further remark, that the frequent occurrence of extensive lakes at +the foot of large volcanoes, and the frequent association of volcanic and +fresh-water strata, seem to indicate that the areas around volcanoes are apt to +be depressed beneath the level of the adjoining country, either from having +been less elevated, or from the effects of subsidence.) We might expect, from +what we see along ordinary faults, that the strata on the upraised side, +already dipping outwards from their original formation as lava-streams, would +be tilted from the line of fault, and thus have their inclination increased. +According to this hypothesis, which I am tempted to extend only to some few +cases, it is not probable that the ring would ever be formed quite perfect; and +from the elevation being slow, the upraised portions would generally be exposed +to much denudation, and hence the ring become broken; we might also expect to +find occasional inequalities in the dip of the upraised masses, as is the case +at St. Jago. By this hypothesis the elevation of the districts in mass, and the +flowing of deluges of lava from the central platforms, are likewise connected +together. On this view the marginal basaltic mountains of the three foregoing +islands might still be considered as forming “Craters of +elevation;” the kind of elevation implied having been slow, and the +central hollow or platform having been formed, not by the arching of the +surface, but simply by that part having been upraised to a less height. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap05"></a>CHAPTER V.<br /> +GALAPAGOS ARCHIPELAGO.</h2> + +<p> +Chatham Island. Craters composed of a peculiar kind of tuff. Small basaltic +craters, with hollows at their bases. Albemarle Island; fluid lavas, their +composition. Craters of tuff; inclination of their exterior diverging strata, +and structure of their interior converging strata. James Island, segment of a +small basaltic crater; fluidity and composition of its lava-streams, and of its +ejected fragments. Concluding remarks on the craters of tuff, and on the +breached condition of their southern sides. Mineralogical composition of the +rocks of the archipelago. Elevation of the land. Direction of the fissures of +eruption. +</p> + +<h3>(FIGURE 11. MAP 3. GALAPAGOS ARCHIPELAGO.</h3> + +<p> +Showing Wenman, Abingdon, Bindloes, Tower, Narborough, Albemarle, James, +Indefatigable, Barrington, Chatham, Charles and Hood’s Islands.) +</p> + +<p> +This archipelago is situated under the equator, at a distance of between five +and six hundred miles from the west coast of South America. It consists of five +principal islands, and of several small ones, which together are equal in area, +but not in extent of land, to Sicily, conjointly with the Ionian Islands. (I +exclude from this measurement, the small volcanic islands of Culpepper and +Wenman, lying seventy miles northward of the group. Craters were visible on all +the islands of the group, except on Towers Island, which is one of the lowest; +this island is, however, formed of volcanic rocks.) They are all volcanic: on +two, craters have been seen in eruption, and on several of the other islands, +streams of lava have a recent appearance. The larger islands are chiefly +composed of solid rock, and they rise with a tame outline to a height of +between one and four thousand feet. They are sometimes, but not generally, +surmounted by one principal orifice. The craters vary in size from mere +spiracles to huge caldrons several miles in circumference; they are +extraordinarily numerous, so that I should think, if enumerated, they would be +found to exceed two thousand; they are formed either of scoriae and lava, or of +a brown-coloured tuff; and these latter craters are in several respects +remarkable. The whole group was surveyed by the officers of the +“Beagle.” I visited myself four of the principal islands, and +received specimens from all the others. Under the head of the different islands +I will describe only that which appears to me deserving of attention. +</p> + +<h3>CHATHAM ISLAND. CRATERS COMPOSED OF A SINGULAR KIND OF TUFF.</h3> + +<p> +Towards the eastern end of this island there occur two craters composed of two +kinds of tuff; one kind being friable, like slightly consolidated ashes; and +the other compact, and of a different nature from anything which I have met +with described. This latter substance, where it is best characterised, is of a +yellowish-brown colour, translucent, and with a lustre somewhat resembling +resin; it is brittle, with an angular, rough, and very irregular fracture, +sometimes, however, being slightly granular, and even obscurely crystalline: it +can readily be scratched with a knife, yet some points are hard enough just to +mark common glass; it fuses with ease into a blackish-green glass. The mass +contains numerous broken crystals of olivine and augite, and small particles of +black and brown scoriae; it is often traversed by thin seams of calcareous +matter. It generally affects a nodular or concretionary structure. In a hand +specimen, this substance would certainly be mistaken for a pale and peculiar +variety of pitchstone; but when seen in mass its stratification, and the +numerous layers of fragments of basalt, both angular and rounded, at once +render its subaqueous origin evident. 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 rocks; and this change could be +distinctly traced in different stages round the edges of even the same +particle. The position near the coast of all the craters composed of this kind +of tuff or peperino, and their breached condition, renders it probable that +they were all formed when standing immersed in the sea; considering this +circumstance, together with the remarkable absence of large beds of ashes in +the whole archipelago, I think it highly probable that much the greater part of +the tuff has originated from the trituration of fragments of the grey, basaltic +lavas in the mouths of craters standing in the sea. 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 translucent, resin-like +fracture. Or has the associated lime played any part in this change? I ask +these questions from having found at St. Jago, in the Cape de Verde Islands, +that where a great stream of molten lava has flowed over a calcareous bottom +into the sea, the outermost film, which in other parts resembles pitchstone, is +changed, apparently by its contact with the carbonate of lime, into a +resin-like substance, precisely like the best characterised specimens of the +tuff from this archipelago. (The concretions containing lime, which I have +described at Ascension, as formed in a bed of ashes, present some degree of +resemblance to this substance, but they have not a resinous fracture. At St. +Helena, also, I found veins of a somewhat similar, compact, but non- resinous +substance, occurring in a bed of pumiceous ashes, apparently free from +calcareous matter: in neither of these cases could heat have acted.) +</p> + +<p> +To return to the two craters: one of them stands at the distance of a league +from the coast, the intervening tract consisting of a calcareous tuff, +apparently of submarine origin. This crater consists of a circle of hills some +of which stand quite detached, but all have a very regular, qua- qua versal +dip, at an inclination of between thirty and forty degrees. The lower beds, to +the thickness of several hundred feet, consist of the resin- like stone, with +embedded fragments of lava. The upper beds, which are between thirty and forty +feet in thickness, are composed of a thinly stratified, fine-grained, harsh, +friable, brown-coloured tuff, or peperino. (Those geologists who restrict the +term of “tuff” to ashes of a white colour, resulting from the +attrition of feldspathic lavas, would call these brown-coloured strata +“peperino.”) A central mass without any stratification, which must +formerly have occupied the hollow of the crater, but is now attached only to a +few of the circumferential hills, consists of a tuff, intermediate in character +between that with a resin-like, and that with an earthy fracture. This mass +contains white calcareous matter in small patches. The second crater (520 feet +in height) must have existed until the eruption of a recent, great stream of +lava, as a separate islet; a fine section, worn by the sea, shows a grand +funnel-shaped mass of basalt, surrounded by steep, sloping flanks of tuff, +having in parts an earthy, and in others a semi-resinous fracture. The tuff is +traversed by several broad, vertical dikes, with smooth and parallel sides, +which I did not doubt were formed of basalt, until I actually broke off +fragments. These dikes, however, consist of tuff like that of the surrounding +strata, but more compact, and with a smoother fracture; hence we must conclude, +that fissures were formed and filled up with the finer mud or tuff from the +crater, before its interior was occupied, as it now is, by a solidified pool of +basalt. Other fissures have been subsequently formed, parallel to these +singular dikes, and are merely filled with loose rubbish. The change from +ordinary scoriaceous particles to the substance with a semi-resinous fracture, +could be clearly followed in portions of the compact tuff of these dikes. +</p> + +<h3>(FIGURE 12. THE KICKER ROCK, 400 FEET HIGH.)</h3> + +<p> +At the distance of a few miles from these two craters, stands the Kicker Rock, +or islet, remarkable from its singular form. It is unstratified, and is +composed of compact tuff, in parts having the resin-like fracture. It is +probable that this amorphous mass, like that similar mass in the case first +described, once filled up the central hollow of a crater, and that its flanks, +or sloping walls, have since been worn quite away by the sea, in which it +stands exposed. +</p> + +<h3>SMALL BASALTIC CRATERS.</h3> + +<p> +A bare, undulating tract, at the eastern end of Chatham Island, is remarkable +from the number, proximity, and form of the small basaltic craters with which +it is studded. They consist, either of a mere conical pile, or, but less +commonly, of a circle, of black and red, glossy scoriae, partially cemented +together. They vary in diameter from thirty to one hundred and fifty yards, and +rise from about fifty to one hundred feet above the level of the surrounding +plain. From one small eminence, I counted sixty of these craters, all of which +were within a third of a mile from each other, and many were much closer. I +measured the distance between two very small craters, and found that it was +only thirty yards from the summit-rim of one to the rim of the other. Small +streams of black, basaltic lava, containing olivine and much glassy feldspar, +have flowed from many, but not from all of these craters. The surfaces of the +more recent streams were exceedingly rugged, and were crossed by great +fissures; the older streams were only a little less rugged; and they were all +blended and mingled together in complete confusion. The different growth, +however, of the trees on the streams, often plainly marked their different +ages. Had it not been for this latter character, the streams could in few cases +have been distinguished; and, consequently, this wide undulatory tract might +have (as probably many tracts have) been erroneously considered as formed by +one great deluge of lava, instead of by a multitude of small streams, erupted +from many small orifices. +</p> + +<p> +In several parts of this tract, and especially at the base of the small +craters, there are circular pits, with perpendicular sides, from twenty to +forty feet deep. At the foot of one small crater, there were three of these +pits. They have probably been formed, by the falling in of the roofs of small +caverns. (M. Elie de Beaumont has described (“Mem. pour servir” +etc. tome 4 page 113) many “petits cirques d’eboulement” on +Etna, of some of which the origin is historically known.) In other parts, there +are mammiform hillocks, which resemble great bubbles of lava, with their +summits fissured by irregular cracks, which appeared, upon entering them, to be +very deep; lava has not flowed from these hillocks. There are, also, other very +regular, mammiform hillocks, composed of stratified lava, and surmounted by +circular, steep-sided hollows, which, I suppose have been formed by a body of +gas, first, arching the strata into one of the bubble- like hillocks, and then, +blowing off its summit. These several kinds of hillocks and pits, as well as +the numerous, small, scoriaceous craters, all show that this tract has been +penetrated, almost like a sieve, by the passage of heated vapours. The more +regular hillocks could only have been heaved up, whilst the lava was in a +softened state. (Sir G. Mackenzie “Travels in Iceland” pages 389 to +392, has described a plain of lava at the foot of Hecla, everywhere heaved up +into great bubbles or blisters. Sir George states that this cavernous lava +composes the uppermost stratum; and the same fact is affirmed by Von Buch +“Descript. des Isles Canaries” page 159, with respect to the +basaltic stream near Rialejo, in Teneriffe. It appears singular that it should +be the upper streams that are chiefly cavernous, for one sees no reason why the +upper and lower should not have been equally affected at different +times;—have the inferior streams flowed beneath the pressure of the sea, +and thus been flattened, after the passage through them, of bodies of gas?) +</p> + +<h3>ALBEMARLE ISLAND.</h3> + +<p> +This island consists of five, great, flat-topped craters, which, together with +the one on the adjoining island of Narborough, singularly resemble each other, +in form and height. The southern one is 4,700 feet high, two others are 3,720 +feet, a third only 50 feet higher, and the remaining ones apparently of nearly +the same height. Three of these are situated on one line, and their craters +appear elongated in nearly the same direction. The northern crater, which is +not the largest, was found by the triangulation to measure, externally, no less +than three miles and one-eighth of a mile in diameter. Over the lips of these +great, broad caldrons, and from little orifices near their summits, deluges of +black lava have flowed down their naked sides. +</p> + +<h3>FLUIDITY OF DIFFERENT LAVAS.</h3> + +<p> +Near Tagus or Banks’ Cove, I examined one of these great streams of lava, +which is remarkable from the evidence of its former high degree of fluidity, +especially when its composition is considered. Near the sea-coast this stream +is several miles in width. It consists of a black, compact base, easily fusible +into a black bead, with angular and not very numerous air-cells, and thickly +studded with large, fractured crystals of glassy albite, varying from the tenth +of an inch to half an inch in diameter. (In the Cordillera of Chile, I have +seen lava very closely resembling this variety at the Galapagos Archipelago. It +contained, however, besides the albite, well-formed crystals of augite, and the +base (perhaps in consequence of the aggregation of the augitic particles) was a +shade lighter in colour. I may here remark, that in all these cases, I call the +feldspathic crystals, “albite,” from their cleavage-planes (as +measured by the reflecting goniometer) corresponding with those of that +mineral. As, however, other species of this genus have lately been discovered +to cleave in nearly the same planes with albite, this determination must be +considered as only provisional. I examined the crystals in the lavas of many +different parts of the Galapagos group, and I found that none of them, with the +exception of some crystals from one part of James Island, cleaved in the +direction of orthite or potash-feldspar.) This lava, although at first sight +appearing eminently porphyritic, cannot properly be considered so, for the +crystals have evidently been enveloped, rounded, and penetrated by the lava, +like fragments of foreign rock in a trap-dike. This was very clear in some +specimens of a similar lava, from Abingdon Island, in which the only difference +was, that the vesicles were spherical and more numerous. The albite in these +lavas is in a similar condition with the leucite of Vesuvius, and with the +olivine, described by Von Buch, as projecting in great balls from the basalt of +Lanzarote. (“Description des Isles Canaries” page 295.) Besides the +albite, this lava contains scattered grains of a green mineral, with no +distinct cleavage, and closely resembling olivine (Humboldt mentions that he +mistook a green augitic mineral, occurring in the volcanic rocks of the +Cordillera of Quito, for olivine.); but as it fuses easily into a green glass, +it belongs probably to the augitic family: at James Island, however, a similar +lava contained true olivine. I obtained specimens from the actual surface, and +from a depth of four feet, but they differed in no respect. The high degree of +fluidity of this lava-stream was at once evident, from its smooth and gently +sloping surface, from the manner in which the main stream was divided by small +inequalities into little rills, and especially from the manner in which its +edges, far below its source, and where it must have been in some degree cooled, +thinned out to almost nothing; the actual margin consisting of loose fragments, +few of which were larger than a man’s head. The contrast between this +margin, and the steep walls, above twenty feet high, bounding many of the +basaltic streams at Ascension, is very remarkable. It has generally been +supposed that lavas abounding with large crystals, and including angular +vesicles, have possessed little fluidity; but we see that the case has been +very different at Albemarle Island. (The irregular and angular form of the +vesicles is probably caused by the unequal yielding of a mass composed, in +almost equal proportion, of solid crystals and of a viscid base. It certainly +seems a general circumstance, as might have been expected, that in lava, which +has possessed a high degree of fluidity, AS WELL AS AN EVEN-SIZED GRAIN, the +vesicles are internally smooth and spherical.) The degree of fluidity in +different lavas, does not seem to correspond with any APPARENT corresponding +amount of difference in their composition: at Chatham Island, some streams, +containing much glassy albite and some olivine, are so rugged, that they may be +compared to a sea frozen during a storm; whilst the great stream at Albemarle +Island is almost as smooth as a lake when ruffled by a breeze. At James Island, +black basaltic lava, abounding with small grains of olivine, presents an +intermediate degree of roughness; its surface being glossy, and the detached +fragments resembling, in a very singular manner, folds of drapery, cables, and +pieces of the bark of trees. (A specimen of basaltic lava, with a few small +broken crystals of albite, given me by one of the officers, is perhaps worthy +of description. It consists of cylindrical ramifications, some of which are +only the twentieth of an inch in diameter, and are drawn out into the sharpest +points. The mass has not been formed like a stalactite, for the points +terminate both upwards and downwards. Globules, only the fortieth of an inch in +diameter, have dropped from some of the points, and adhere to the adjoining +branches. The lava is vesicular, but the vesicles never reach the surface of +the branches, which are smooth and glossy. As it is generally supposed that +vesicles are always elongated in the direction of the movement of the fluid +mass, I may observe, that in these cylindrical branches, which vary from a +quarter to only the twentieth of an inch in diameter, every air-cell is +spherical.) +</p> + +<h3>CRATERS OF TUFF.</h3> + +<p> +About a mile southward of Banks’ Cove, there is a fine elliptic crater, +about five hundred feet in depth, and three-quarters of a mile in diameter. Its +bottom is occupied by a lake of brine, out of which some little crateriform +hills of tuff rise. The lower beds are formed of compact tuff, appearing like a +subaqueous deposit; whilst the upper beds, round the entire circumference, +consist of a harsh, friable tuff, of little specific gravity, but often +containing fragments of rock in layers. This upper tuff contains numerous +pisolitic balls, about the size of small bullets, which differ from the +surrounding matter, only in being slightly harder and finer grained. The beds +dip away very regularly on all sides, at angles varying, as I found by +measurement, from twenty-five to thirty degrees. The external surface of the +crater slopes at a nearly similar inclination, and is formed by slightly convex +ribs, like those on the shell of a pecten or scallop, which become broader as +they extend from the mouth of the crater to its base. These ribs are generally +from eight to twenty feet in breadth, but sometimes they are as much as forty +feet broad; and they resemble old, plastered, much flattened vaults, with the +plaster scaling off in plates: they are separated from each other by gullies, +deepened by alluvial action. At their upper and narrow ends, near the mouth of +the crater, these ribs often consist of real hollow passages, like, but rather +smaller than, those often formed by the cooling of the crust of a lava-stream, +whilst the inner parts have flowed onward;—of which structure I saw many +examples at Chatham Island. There can be no doubt but that these hollow ribs or +vaults have been formed in a similar manner, namely, by the setting or +hardening of a superficial crust on streams of mud, which have flowed down from +the upper part of the crater. In another part of this same crater, I saw open +concave gutters between one and two feet wide, which appear to have been formed +by the hardening of the lower surface of a mud stream, instead of, as in the +former case, of the upper surface. From these facts I think it is certain that +the tuff must have flowed as mud. (This conclusion is of some interest, because +M. Dufrenoy “Mem. pour servir” tome 4 page 274, has argued from +strata of tuff, apparently of similar composition with that here described, +being inclined at angles between 18 degrees and 20 degrees, that Monte Nuevo +and some other craters of Southern Italy have been formed by upheaval. From the +facts given above, of the vaulted character of the separate rills, and from the +tuff not extending in horizontal sheets round these crateriform hills, no one +will suppose that the strata have here been produced by elevation; and yet we +see that their inclination is above 20 degrees, and often as much as 30 +degrees. The consolidated strata also, of the internal talus, as will be +immediately seen, dips at an angle of above 30 degrees.) This mud may have been +formed either within the crater, or from ashes deposited on its upper parts, +and afterwards washed down by torrents of rain. The former method, in most of +the cases, appears the more probable one; at James Island, however, some beds +of the friable kind of tuff extend so continuously over an uneven surface, that +probably they were formed by the falling of showers of ashes. +</p> + +<p> +Within this same crater, strata of coarse tuff, chiefly composed of fragments +of lava, abut, like a consolidated talus, against the inside walls. They rise +to a height of between one hundred and one hundred and fifty feet above the +surface of the internal brine-lake; they dip inwards, and are inclined at an +angle varying from thirty to thirty-six degrees. They appear to have been +formed beneath water, probably at a period when the sea occupied the hollow of +the crater. I was surprised to observe that beds having this great inclination +did not, as far as they could be followed, thicken towards their lower +extremities. +</p> + +<h3>BANKS’ COVE.</h3> + +<p> +(FIGURE 13. A SECTIONAL SKETCH OF THE HEADLANDS FORMING BANKS’ COVE, +showing the diverging crateriform strata, and the converging stratified talus. +The highest point of these hills is 817 feet above the sea.) +</p> + +<p> +This harbour occupies part of the interior of a shattered crater of tuff larger +than that last described. All the tuff is compact, and includes numerous +fragments of lava; it appears like a subaqueous deposit. The most remarkable +feature in this crater is the great development of strata converging inwards, +as in the last case, at a considerable inclination, and often deposited in +irregular curved layers. These interior converging beds, as well as the proper, +diverging crateriform strata, are represented in Figure 13, a rude, sectional +sketch of the headlands, forming this Cove. The internal and external strata +differ little in composition, and the former have evidently resulted from the +wear and tear, and redeposition of the matter forming the external crateriform +strata. From the great development of these inner beds, a person walking round +the rim of this crater might fancy himself on a circular anticlinal ridge of +stratified sandstone and conglomerate. The sea is wearing away the inner and +outer strata, and especially the latter; so that the inwardly converging strata +will, perhaps, in some future age, be left standing alone—a case which +might at first perplex a geologist. (I believe that this case actually occurs +in the Azores, where Dr. Webster “Description” page 185, has +described a basin-formed, little island, composed of STRATA OF TUFF, dipping +inwards and bounded externally by steep sea-worn cliffs. Dr. Daubeny supposes +“Volcanoes” page 266, that this cavity must have been formed by a +circular subsidence. It appears to me far more probable, that we here have +strata which were originally deposited within the hollow of a crater, of which +the exterior walls have since been removed by the sea.) +</p> + +<h3>JAMES ISLAND.</h3> + +<p> +Two craters of tuff on this island are the only remaining ones which require +any notice. One of them lies a mile and a half inland from Puerto Grande: it is +circular, about the third of a mile in diameter, and 400 feet in depth. It +differs from all the other tuff-craters which I examined, in having the lower +part of its cavity, to the height of between one hundred and one hundred and +fifty feet, formed by a precipitous wall of basalt, giving to the crater the +appearance of having burst through a solid sheet of rock. The upper part of +this crater consists of strata of the altered tuff, with a semi-resinous +fracture. Its bottom is occupied by a shallow lake of brine, covering layers of +salt, which rest on deep black mud. The other crater lies at the distance of a +few miles, and is only remarkable from its size and perfect condition. Its +summit is 1,200 feet above the level of the sea, and the interior hollow is 600 +feet deep. Its external sloping surface presented a curious appearance from the +smoothness of the wide layers of tuff, which resembled a vast plastered floor. +Brattle Island is, I believe, the largest crater in the Archipelago composed of +tuff; its interior diameter is nearly a nautical mile. At present it is in a +ruined condition, consisting of little more than half a circle open to the +south; its great size is probably due, in part, to internal degradation, from +the action of the sea. +</p> + +<h3>SEGMENT OF A BASALTIC CRATER.</h3> + +<p> +(FIGURE 14. SEGMENT OF A VERY SMALL ORIFICE OF ERUPTION, on the beach of +Fresh-water Bay.) +</p> + +<p> +One side of Fresh-water Bay, in James Island, is bounded by a promontory, which +forms the last wreck of a great crater. On the beach of this promontory, a +quadrant-shaped segment of a small subordinate point of eruption stands +exposed. It consists of nine separate little streams of lava piled upon each +other; and of an irregular pinnacle, about fifteen feet high, of reddish-brown, +vesicular basalt, abounding with large crystals of glassy albite, and with +fused augite. This pinnacle, and some adjoining paps of rock on the beach, +represent the axis of the crater. The streams of lava can be followed up a +little ravine, at right angles to the coast, for between ten and fifteen yards, +where they are hidden by detritus: along the beach they are visible for nearly +eighty yards, and I do not believe that they extend much further. The three +lower streams are united to the pinnacle; and at the point of junction (as +shown in Figure 14, a rude sketch made on the spot), they are slightly arched, +as if in the act of flowing over the lip of the crater. The six upper streams +no doubt were originally united to this same column before it was worn down by +the sea. The lava of these streams is of similar composition with that of the +pinnacle, excepting that the crystals of albite appear to be more comminuted, +and the grains of fused augite are absent. Each stream is separated from the +one above it by a few inches, or at most by one or two feet in thickness, of +loose fragmentary scoriae, apparently derived from the abrasion of the streams +in passing over each other. All these streams are very remarkable from their +thinness. I carefully measured several of them; one was eight inches thick, but +was firmly coated with three inches above, and three inches below, of red +scoriaceous rock (which is the case with all the streams), making altogether a +thickness of fourteen inches: this thickness was preserved quite uniformly +along the entire length of the section. A second stream was only eight inches +thick, including both the upper and lower scoriaceous surfaces. Until examining +this section, I had not thought it possible that lava could have flowed in such +uniformly thin sheets over a surface far from smooth. These little streams +closely resemble in composition that great deluge of lava at Albemarle Island, +which likewise must have possessed a high degree of fluidity. +</p> + +<h3>PSEUDO-EXTRANEOUS, EJECTED FRAGMENTS.</h3> + +<p> +In the lava and in the scoriae of this little crater, I found several +fragments, which, from their angular form, their granular structure, their +freedom from air-cells, their brittle and burnt condition, closely resembled +those fragments of primary rocks which are occasionally ejected, as at +Ascension, from volcanoes. These fragments consist of glassy albite, much +mackled, and with very imperfect cleavages, mingled with semi-rounded grains, +having tarnished, glossy surfaces, of a steel-blue mineral. The crystals of +albite are coated by a red oxide of iron, appearing like a residual substance; +and their cleavage-planes also are sometimes separated by excessively fine +layers of this oxide, giving to the crystals the appearance of being ruled like +a glass micrometer. There was no quartz. The steel-blue mineral, which is +abundant in the pinnacle, but which disappears in the streams derived from the +pinnacle, has a fused appearance, and rarely presents even a trace of cleavage; +I obtained, however, one measurement, which proved that it was augite; and in +one other fragment, which differed from the others, in being slightly cellular, +and in gradually blending into the surrounding matrix the small grains of this +mineral were tolerably well crystallised. Although there is so wide a +difference in appearance between the lava of the little streams, and especially +of their red scoriaceous crusts, and one of these angular ejected fragments, +which at first sight might readily be mistaken for syenite, yet I believe that +the lava has originated from the melting and movement of a mass of rock of +absolutely similar composition with the fragments. Besides the specimen above +alluded to, in which we see a fragment becoming slightly cellular, and blending +into the surrounding matrix, some of the grains of the steel-blue augite also +have their surfaces becoming very finely vesicular, and passing into the nature +of the surrounding paste; other grains are throughout, in an intermediate +condition. The paste seems to consist of the augite more perfectly fused, or, +more probably, merely disturbed in its softened state by the movement of the +mass, and mingled with the oxide of iron and with finely comminuted, glassy +albite. Hence probably it is that the fused albite, which is abundant in the +pinnacle, disappears in the streams. The albite is in exactly the same state, +with the exception of most of the crystals being smaller in the lava and in the +embedded fragments; but in the fragments they appear to be less abundant: this, +however, would naturally happen from the intumescence of the augitic base, and +its consequent apparent increase in bulk. It is interesting thus to trace the +steps by which a compact granular rock becomes converted into a vesicular, +pseudo-porphyritic lava, and finally into red scoriae. 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 action, +or more probably of the crust of a body of cooled and crystallised lava, which +has afterwards been broken up and re-liquified; the crust being less acted on +by the renewed heat and movement. +</p> + +<h3>CONCLUDING REMARKS ON THE TUFF-CRATERS.</h3> + +<p> +These craters, from the peculiarity of the resin-like substance which enters +largely into their composition, from their structure, their size and number, +present the most striking feature in the geology of this Archipelago. The +majority of them form either separate islets, or promontories attached to the +larger islands; and those which now stand at some little distance from the +coast are worn and breached, as if by the action of the sea. From this general +circumstance of their position, and from the small quantity of ejected ashes in +any part of the Archipelago, I am led to conclude, that the tuff has been +chiefly produced, by the grinding together of fragments of lava within active +craters, communicating with the sea. In the origin and composition of the tuff, +and in the frequent presence of a central lake of brine and of layers of salt, +these craters resemble, though on a gigantic scale, the “salses,” +or hillocks of mud, which are common in some parts of Italy and in other +countries. (D’Aubuisson “Traite de Geognosie” tome 1 page +189. I may remark, that I saw at Terceira, in the Azores, a crater of tuff or +peperino, very similar to these of the Galapagos Archipelago. From the +description given in Freycinet “Voyage,” similar ones occur at the +Sandwich Islands; and probably they are present in many other places.) Their +closer connection, however, in this Archipelago, with ordinary volcanic action, +is shown by the pools of solidified basalt, with which they are sometimes +filled up. +</p> + +<p> +It at first appears very singular, that all the craters formed of tuff have +their southern sides, either quite broken down and wholly removed, or much +lower than the other sides. I saw and received accounts of twenty-eight of +these craters; of these, twelve form separate islets (These consist of the +three Crossman Islets, the largest of which is 600 feet in height; Enchanted +Island; Gardner Island (760 feet high); Champion Island (331 feet high); +Enderby Island; Brattle Island; two islets near Indefatigable Island; and one +near James Island. A second crater near James Island (with a salt lake in its +centre) has its southern side only about twenty feet high, whilst the other +parts of the circumference are about three hundred feet in height.), and now +exist as mere crescents quite open to the south, with occasionally a few points +of rock marking their former circumference: of the remaining sixteen, some form +promontories, and others stand at a little distance inland from the shore; but +all have their southern sides either the lowest, or quite broken down. Two, +however, of the sixteen had their northern sides also low, whilst their eastern +and western sides were perfect. I did not see, or hear of, a single exception +to the rule, of these craters being broken down or low on the side, which faces +a point of the horizon between S.E. and S.W. This rule does not apply to +craters composed of lava and scoriae. The explanation is simple: at this +Archipelago, the waves from the trade-wind, and the swell propagated from the +distant parts of the open ocean, coincide in direction (which is not the case +in many parts of the Pacific), and with their united forces attack the southern +sides of all the islands; and consequently the southern slope, even when +entirely formed of hard basaltic rock, is invariably steeper than the northern +slope. As the tuff-craters are composed of a soft material, and as probably +all, or nearly all, have at some period stood immersed in the sea, we need not +wonder that they should invariably exhibit on their exposed sides the effects +of this great denuding power. Judging from the worn condition of many of these +craters, it is probable that some have been entirely washed away. As there is +no reason to suppose, that the craters formed of scoriae and lava were erupted +whilst standing in the sea, we can see why the rule does not apply to them. At +Ascension, it was shown that the mouths of the craters, which are there all of +terrestrial origin, have been affected by the trade-wind; and this same power +might here, also, aid in making the windward and exposed sides of some of the +craters originally the lowest. +</p> + +<h3>MINERALOGICAL COMPOSITION OF THE ROCKS.</h3> + +<p> +In the northern islands, the basaltic lavas seem generally to contain more +albite than they do in the southern half of the Archipelago; but almost all the +streams contain some. The albite is not unfrequently associated with olivine. I +did not observe in any specimen distinguishable crystals of hornblende or +augite; I except the fused grains in the ejected fragments, and in the pinnacle +of the little crater, above described. I did not meet with a single specimen of +true trachyte; though some of the paler lavas, when abounding with large +crystals of the harsh and glassy albite, resemble in some degree this rock; but +in every case the basis fuses into a black enamel. Beds of ashes and +far-ejected scoriae, as previously stated, are almost absent; nor did I see a +fragment of obsidian or of pumice. Von Buch believes that the absence of pumice +on Mount Etna is consequent on the feldspar being of the Labrador variety +(“Description des Isles Canaries” page 328.); if the presence of +pumice depends on the constitution of the feldspar, it is remarkable, that it +should be absent in this archipelago, and abundant in the Cordillera of South +America, in both of which regions the feldspar is of the albitic variety. Owing +to the absence of ashes, and the general indecomposable character of the lava +in this Archipelago, the islands are slowly clothed with a poor vegetation, and +the scenery has a desolate and frightful aspect. +</p> + +<h3>ELEVATION OF THE LAND.</h3> + +<p> +Proofs of the rising of the land are scanty and imperfect. At Chatham Island, I +noticed some great blocks of lava, cemented by calcareous matter, containing +recent shells; but they occurred at the height of only a few feet above +high-water mark. One of the officers gave me some fragments of shells, which he +found embedded several hundred feet above the sea, in the tuff of two craters, +distant from each other. It is possible, that these fragments may have been +carried up to their present height in an eruption of mud; but as, in one +instance, they were associated with broken oyster- shells, almost forming a +layer, it is more probable that the tuff was uplifted with the shells in mass. +The specimens are so imperfect that they can be recognised only as belonging to +recent marine genera. On Charles Island, I observed a line of great rounded +blocks, piled on the summit of a vertical cliff, at the height of fifteen feet +above the line, where the sea now acts during the heaviest gales. This +appeared, at first, good evidence in favour of the elevation of the land; but +it was quite deceptive, for I afterwards saw on an adjoining part of this same +coast, and heard from eye- witnesses, that wherever a recent stream of lava +forms a smooth inclined plane, entering the sea, the waves during gales have +the power of ROLLING UP ROUNDED blocks to a great height, above the line of +their ordinary action. As the little cliff in the foregoing case is formed by a +stream of lava, which, before being worn back, must have entered the sea with a +gently sloping surface, it is possible or rather it is probable, that the +rounded boulders, now lying on its summit, are merely the remnants of those +which had been ROLLED UP during storms to their present height. +</p> + +<h3>DIRECTION OF THE FISSURES OF ERUPTION.</h3> + +<p> +The volcanic orifices in this group cannot be considered as indiscriminately +scattered. Three great craters on Albermarle Island form a well-marked line, +extending N.W. by N. and S.E. by S. Narborough Island, and the great crater on +the rectangular projection of Albemarle Island, form a second parallel line. To +the east, Hood’s Island, and the islands and rocks between it and James +Island, form another nearly parallel line, which, when prolonged, includes +Culpepper and Wenman Islands, lying seventy miles to the north. The other +islands lying further eastward, form a less regular fourth line. Several of +these islands, and the vents on Albemarle Island, are so placed, that they +likewise fall on a set of rudely parallel lines, intersecting the former lines +at right angles; so that the principal craters appear to lie on the points +where two sets of fissures cross each other. The islands themselves, with the +exception of Albemarle Island, are not elongated in the same direction with the +lines on which they stand. 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 Ocean. Finally, I may remark, that amongst +the Galapagos Islands there is no one dominant vent much higher than all the +others, as may be observed in many volcanic archipelagoes: the highest is the +great mound on the south- western extremity of Albemarle Island, which exceeds +by barely a thousand feet several other neighbouring craters. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap06"></a>CHAPTER VI.<br /> +TRACHYTE AND BASALT.—DISTRIBUTION OF VOLCANIC +ISLES.</h2> + +<p> +The sinking of crystals in fluid lava. Specific gravity of the constituent +parts of trachyte and of basalt, and their consequent separation. Obsidian. +Apparent non-separation of the elements of plutonic rocks. Origin of trap-dikes +in the plutonic series. Distribution of volcanic islands; their prevalence in +the great oceans. They are generally arranged in lines. The central volcanoes +of Von Buch doubtful. Volcanic islands bordering continents. Antiquity of +volcanic islands, and their elevation in mass. Eruptions on parallel lines of +fissure within the same geological period. +</p> + +<p> +ON THE SEPARATION OF THE CONSTITUENT MINERALS OF LAVA, ACCORDING TO THEIR +SPECIFIC GRAVITIES. +</p> + +<p> +One side of Fresh-water Bay, in James Island, is formed by the wreck of a large +crater, mentioned in the last chapter, of which the interior has been filled up +by a pool of basalt, about two hundred feet in thickness. This basalt is of a +grey colour, and contains many crystals of glassy albite, which become much +more numerous in the lower, scoriaceous part. This is contrary to what might +have been expected, for if the crystals had been originally disseminated in +equal numbers, the greater intumescence of this lower scoriaceous part would +have made them appear fewer in number. Von Buch has described a stream of +obsidian on the Peak of Teneriffe, in which the crystals of feldspar become +more and more numerous, as the depth or thickness increases, so that near the +lower surface of the stream the lava even resembles a primary rock. +(“Description des Isles Canaries” pages 190 and 191.) Von Buch +further states, that M. Dree, in his experiments in melting lava, found that +the crystals of feldspar always tended to precipitate themselves to the bottom +of the crucible. In these cases, I presume there can be no doubt that the +crystals sink from their weight. (In a mass of molten iron, it is found +(“Edinburgh New Philosophical Journal” volume 24 page 66) that the +substances, which have a closer affinity for oxygen than iron has, rise from +the interior of the mass to the surface. But a similar cause can hardly apply +to the separation of the crystals of these lava-streams. The cooling of the +surface of lava seems, in some cases, to have affected its composition; for +Dufrenoy (“Mem. pour servir” tome 4 page 271) found that the +interior parts of a stream near Naples contained two-thirds of a mineral which +was acted on by acids, whilst the surface consisted chiefly of a mineral +unattackable by acids.) The specific gravity of feldspar varies from 2.4 to +2.58, whilst obsidian seems commonly to be from 2.3 to 2.4; and in a fluidified +state its specific gravity would probably be less, which would facilitate the +sinking of the crystals of feldspar. (I have taken the specific gravities of +the simple minerals from Von Kobell, one of the latest and best authorities, +and of the rocks from various authorities. Obsidian, according to Phillips, is +2.35; and Jameson says it never exceeds 2.4; but a specimen from Ascension, +weighed by myself, was 2.42.) At James Island, the crystals of albite, though +no doubt of less weight than the grey basalt, in the parts where compact, might +easily be of greater specific gravity than the scoriaceous mass, formed of +melted lava and bubbles of heated gas. +</p> + +<p> +The sinking of crystals through a viscid substance like molten rock, as is +unequivocally shown to have been the case in the experiments of M. Dree, is +worthy of further consideration, as throwing light on the separation of the +trachytic and basaltic series of lavas. Mr. P. Scrope has speculated on this +subject; but he does not seem to have been aware of any positive facts, such as +those above given; and he has overlooked one very necessary element, as it +appears to me, in the phenomenon—namely, the existence of either the +lighter or heavier mineral in globules or in crystals. In a substance of +imperfect fluidity, like molten rock, it is hardly credible, that the separate, +infinitely small atoms, whether of feldspar, augite, or of any other mineral, +would have power from their slightly different gravities to overcome the +friction caused by their movement; but if the atoms of any one of these +minerals became, whilst the others remained fluid, united into crystals or +granules, it is easy to perceive that from the lessened friction, their sinking +or floating power would be greatly increased. On the other hand, if all the +minerals became granulated at the same time, it is scarcely possible, from +their mutual resistance, that any separation could take place. A valuable, +practical discovery, illustrating the effect of the granulation of one element +in a fluid mass, in aiding its separation, has lately been made: when lead +containing a small proportion of silver, is constantly stirred whilst cooling, +it becomes granulated, and the grains of imperfect crystals of nearly pure lead +sink to the bottom, leaving a residue of melted metal much richer in silver; +whereas if the mixture be left undisturbed, although kept fluid for a length of +time, the two metals show no signs of separating. (A full and interesting +account of this discovery, by Mr. Pattinson, was read before the British +Association in September 1838. In some alloys, according to Turner +“Chemistry” page 210, the heaviest metal sinks, and it appears that +this takes place whilst both metals are fluid. Where there is a considerable +difference in gravity, as between iron and the slag formed during the fusion of +the ore, we need not be surprised at the atoms separating, without either +substance being granulated.) The sole use of the stirring seems to be, the +formation of detached granules. The specific gravity of silver is 10.4, and of +lead 11.35: the granulated lead, which sinks, is never absolutely pure, and the +residual fluid metal contains, when richest, only 1/119 part of silver. As the +difference in specific gravity, caused by the different proportions of the two +metals, is so exceedingly small, the separation is probably aided in a great +degree by the difference in gravity between the lead, when granular though +still hot, and when fluid. +</p> + +<p> +In a body of liquified volcanic rock, left for some time without any violent +disturbance, we might expect, in accordance with the above facts, that if one +of the constituent minerals became aggregated into crystals or granules, or had +been enveloped in this state from some previously existing mass, such crystals +or granules would rise or sink, according to their specific gravity. Now we +have plain evidence of crystals being embedded in many lavas, whilst the paste +or basis has continued fluid. I need only refer, as instances, to the several, +great, pseudo-porphyritic streams at the Galapagos Islands, and to the +trachytic streams in many parts of the world, in which we find crystals of +feldspar bent and broken by the movement of the surrounding, semi-fluid matter. +Lavas are chiefly composed of three varieties of feldspar, varying in specific +gravity from 2.4 to 2.74; of hornblende and augite, varying from 3.0 to 3.4; of +olivine, varying from 3.3 to 3.4; and lastly, of oxides of iron, with specific +gravities from 4.8 to 5.2. Hence crystals of feldspar, enveloped in a mass of +liquified, but not highly vesicular lava, would tend to rise to the upper +parts; and crystals or granules of the other minerals, thus enveloped, would +tend to sink. We ought not, however, to expect any perfect degree of separation +in such viscid materials. Trachyte, which consists chiefly of feldspar, with +some hornblende and oxide of iron, has a specific gravity of about 2.45; whilst +basalt, composed chiefly of augite and feldspar, often with much iron and +olivine, has a gravity of about 3.0. (Trachyte from Java was found by Von Buch +to be 2.47; from Auvergne, by De la Beche, it was 2.42; from Ascension, by +myself, it was 2.42. Jameson and other authors give to basalt a specific +gravity of 3.0; but specimens from Auvergne were found, by De la Beche, to be +only 2.78; and from the Giant’s Causeway, to be 2.91.) Accordingly we +find, that where both trachytic and basaltic streams have proceeded from the +same orifice, the trachytic streams have generally been first erupted owing, as +we must suppose, to the molten lava of this series having accumulated in the +upper parts of the volcanic focus. This order of eruption has been observed by +Beudant, Scrope, and by other authors; three instances, also, have been given +in this volume. As the later eruptions, however, from most volcanic mountains, +burst through their basal parts, owing to the increased height and weight of +the internal column of molten rock, we see why, in most cases, only the lower +flanks of the central, trachytic masses, are enveloped by basaltic streams. The +separation of the ingredients of a mass of lava, would, perhaps, sometimes take +place within the body of a volcanic mountain, if lofty and of great dimensions, +instead of within the underground focus; in which case, trachytic streams might +be poured forth, almost contemporaneously, or at short recurrent intervals, +from its summit, and basaltic streams from its base: this seems to have taken +place at Teneriffe. (Consult Von Buch’s well-known and admirable +“Description Physique” of this island, which might serve as a model +of descriptive geology.) I need only further remark, that from violent +disturbances the separation of the two series, even under otherwise favourable +conditions, would naturally often be prevented, and likewise their usual order +of eruption be inverted. From the high degree of fluidity of most basaltic +lavas, these perhaps, alone, would in many cases reach the surface. +</p> + +<p> +As we have seen that crystals of feldspar, in the instance described by Von +Buch, sink in obsidian, in accordance with their known greater specific +gravity, we might expect to find in every trachytic district, where obsidian +has flowed as lava, that it had proceeded from the upper or highest orifices. +This, according to Von Buch, holds good in a remarkable manner both at the +Lipari Islands and on the Peak of Teneriffe; at this latter place obsidian has +never flowed from a less height than 9,200 feet. Obsidian, also, appears to +have been erupted from the loftiest peaks of the Peruvian Cordillera. I will +only further observe, that the specific gravity of quartz varies from 2.6 to +2.8; and therefore, that when present in a volcanic focus, it would not tend to +sink with the basaltic bases; and this, perhaps, explains the frequent +presence, and the abundance of this mineral, in the lavas of the trachytic +series, as observed in previous parts of this volume. +</p> + +<p> +An objection to the foregoing theory will, perhaps, be drawn from the plutonic +rocks not being separated into two evidently distinct series, of different +specific gravities; although, like the volcanic, they have been liquified. In +answer, it may first be remarked, that we have no evidence of the atoms of any +one of the constituent minerals in the plutonic series having been aggregated, +whilst the others remained fluid, which we have endeavoured to show is an +almost necessary condition of their separation; on the contrary, the crystals +have generally impressed each other with their forms. (The crystalline paste of +phonolite is frequently penetrated by long needles of hornblende; from which it +appears that the hornblende, though the more fusible mineral, has crystallised +before, or at the same time with a more refractory substance. Phonolite, as far +as my observations serve, in every instance appears to be an injected rock, +like those of the plutonic series; hence probably, like these latter, it has +generally been cooled without repeated and violent disturbances. Those +geologists who have doubted whether granite could have been formed by igneous +liquefaction, because minerals of different degrees of fusibility impress each +other with their forms, could not have been aware of the fact of crystallised +hornblende penetrating phonolite, a rock undoubtedly of igneous origin. The +viscidity, which it is now known, that both feldspar and quartz retain at a +temperature much below their points of fusion, easily explains their mutual +impressment. Consult on this subject Mr. Horner’s paper on Bonn +“Geolog. Transact.” volume 4 page 439; and +“L’Institut” with respect to quartz 1839 page 161.) +</p> + +<p> +In the second place, the perfect tranquillity, under which it is probable that +the plutonic masses, buried at profound depths, have cooled, would, most +likely, be highly unfavourable to the separation of their constituent minerals; +for, if the attractive force, which during the progressive cooling draws +together the molecules of the different minerals, has power sufficient to keep +them together, the friction between such half-formed crystals or pasty globules +would effectually prevent the heavier ones from sinking, or the lighter ones +from rising. On the other hand, a small amount of disturbance, which would +probably occur in most volcanic foci, and which we have seen does not prevent +the separation of granules of lead from a mixture of molten lead and silver, or +crystals of feldspar from streams of lava, by breaking and dissolving the less +perfectly formed globules, would permit the more perfect and therefore unbroken +crystals, to sink or rise, according to their specific gravity. +</p> + +<p> +Although in plutonic rocks two distinct species, corresponding to the trachytic +and basaltic series, do not exist, I much suspect that a certain amount of +separation of their constituent parts has often taken place. I suspect this +from having observed how frequently dikes of greenstone and basalt intersect +widely extended formations of granite and the allied metamorphic rocks. I have +never examined a district in an extensive granitic region without discovering +dikes; I may instance the numerous trap-dikes, in several districts of Brazil, +Chile, and Australia, and at the Cape of Good Hope: many dikes likewise occur +in the great granitic tracts of India, in the north of Europe, and in other +countries. Whence, then, has the greenstone and basalt, forming these dikes, +come? Are we to suppose, like some of the elder geologists, that a zone of trap +is uniformly spread out beneath the granitic series, which composes, as far as +we know, the foundations of the earth’s crust? Is it not more probable, +that these dikes have been formed by fissures penetrating into partially cooled +rocks of the granitic and metamorphic series, and by their more fluid parts, +consisting chiefly of hornblende, oozing out, and being sucked into such +fissures? At Bahia, in Brazil, in a district composed of gneiss and primitive +greenstone, I saw many dikes, of a dark augitic (for one crystal certainly was +of this mineral) or hornblendic rock, which, as several appearances clearly +proved, either had been formed before the surrounding mass had become solid, or +had together with it been afterwards thoroughly softened. (Portions of these +dikes have been broken off, and are now surrounded by the primary rocks, with +their laminae conformably winding round them. Dr. Hubbard also +(“Silliman’s Journal” volume 34 page 119), has described an +interlacement of trap-veins in the granite of the White Mountains, which he +thinks must have been formed when both rocks were soft.) On both sides of one +of these dikes, the gneiss was penetrated, to the distance of several yards, by +numerous, curvilinear threads or streaks of dark matter, which resembled in +form clouds of the class called cirrhi- comae; some few of these threads could +be traced to their junction with the dike. When examining them, I doubted +whether such hair-like and curvilinear veins could have been injected, and I +now suspect, that instead of having been injected from the dike, they were its +feeders. If the foregoing views of the origin of trap-dikes in widely extended +granitic regions far from rocks of any other formation, be admitted as +probable, we may further admit, in the case of a great body of plutonic rock, +being impelled by repeated movements into the axis of a mountain-chain, that +its more liquid constituent parts might drain into deep and unseen abysses; +afterwards, perhaps, to be brought to the surface under the form, either of +injected masses of greenstone and augitic porphyry, or of basaltic eruptions. +(Mr. Phillips “Lardner’s Encyclop.” volume 2 page 115 quotes +Von Buch’s statement, that augitic porphyry ranges parallel to, and is +found constantly at the base of, great chains of mountains. Humboldt, also, has +remarked the frequent occurrence of trap-rock, in a similar position; of which +fact I have observed many examples at the foot of the Chilian Cordillera. The +existence of granite in the axes of great mountain chains is always probable, +and I am tempted to suppose, that the laterally injected masses of augitic +porphyry and of trap, bear nearly the same relation to the granitic axes which +basaltic lavas bear to the central trachytic masses, round the flanks of which +they have so frequently been erupted.) Much of the difficulty which geologists +have experienced when they have compared the composition of volcanic with +plutonic formations, will, I think, be removed, if we may believe that most +plutonic masses have been, to a certain extent, drained of those comparatively +weighty and easily liquified elements, which compose the trappean and basaltic +series of rocks. +</p> + +<h3>ON THE DISTRIBUTION OF VOLCANIC ISLANDS.</h3> + +<p> +During my investigations on coral-reefs, I had occasion to consult the works of +many voyagers, and I was invariably struck with the fact, that with rare +exceptions, the innumerable islands scattered throughout the Pacific, Indian, +and Atlantic Oceans, were composed either of volcanic, or of modern +coral-rocks. It would be tedious to give a long catalogue of all the volcanic +islands; but the exceptions which I have found are easily enumerated: in the +Atlantic, we have St. Paul’s Rock, described in this volume, and the +Falkland Islands, composed of quartz and clay-slate; but these latter islands +are of considerable size, and lie not very far from the South American coast +(Judging from Forster’s imperfect observation, perhaps Georgia is not +volcanic. Dr. Allan is my informant with regard to the Seychelles. I do not +know of what formation Rodriguez, in the Indian Ocean, is composed.): in the +Indian Ocean, the Seychelles (situated in a line prolonged from Madagascar) +consist of granite and quartz: in the Pacific Ocean, New Caledonia, an island +of large size, belongs (as far as is known) to the primary class. New Zealand, +which contains much volcanic rock and some active volcanoes, from its size +cannot be classed with the small islands, which we are now considering. The +presence of a small quantity of non-volcanic rock, as of clay-slate on three of +the Azores (This is stated on the authority of Count V. de Bedemar, with +respect to Flores and Graciosa (Charlsworth “Magazine of Nat. +Hist.” volume 1 page 557). St. Maria has no volcanic rock, according to +Captain Boyd (Von Buch “Descript.” page 365). Chatham Island has +been described by Dr. Dieffenbach in the “Geographical Journal” +1841 page 201. As yet we have received only imperfect notices on Kerguelen +Land, from the Antarctic Expedition.), or of tertiary limestone at Madeira, or +of clay-slate at Chatham Island in the Pacific, or of lignite at Kerguelen +Land, ought not to exclude such islands or archipelagoes, if formed chiefly of +erupted matter, from the volcanic class. +</p> + +<p> +The composition of the numerous islands scattered through the great oceans +being with such rare exceptions volcanic, is evidently an extension of that +law, and the effect of those same causes, whether chemical or mechanical, from +which it results, that a vast majority of the volcanoes now in action stand +either as islands in the sea, or near its shores. This fact of the +ocean-islands being so generally volcanic is also interesting in relation to +the nature of the mountain-chains on our continents, which are comparatively +seldom volcanic; and yet we are led to suppose that where our continents now +stand an ocean once extended. Do volcanic eruptions, we may ask, 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 land? +</p> + +<p> +Looking at the charts of the numerous volcanic archipelagoes, we see that the +islands are generally arranged either in single, double, or triple rows, in +lines which are frequently curved in a slight degree. (Professors William and +Henry Darwin Rogers have lately insisted much, in a memoir read before the +American Association, on the regularly curved lines of elevation in parts of +the Appalachian range.) Each separate island is either rounded, or more +generally elongated in the same direction with the group in which it stands, +but sometimes transversely to it. Some of the groups which are not much +elongated present little symmetry in their forms; M. Virlet (“Bulletin de +la Soc. Geolog.” tome 3 page 110.) states that this is the case with the +Grecian Archipelago: in such groups I suspect (for I am aware how easy it is to +deceive oneself on these points), that the vents are generally arranged on one +line, or on a set of short parallel lines, intersecting at nearly right angles +another line, or set of lines. The Galapagos Archipelago offers an example of +this structure, 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 N.W. by N., +and on another set ranging about W.S.W.: in the Canary Archipelago we have a +simpler structure of the same kind: in the Cape de Verde group, which appears +to be the least symmetrical of any oceanic volcanic archipelago, a N.W. and +S.E. line formed by several islands, if prolonged, would intersect at right +angles a curved line, on which the remaining islands are placed. +</p> + +<p> +Von Buch (“Description des Isles Canaries” page 324.) has classed +all volcanoes under two heads, namely, CENTRAL VOLCANOES, round which numerous +eruptions have taken place on all sides, in a manner almost regular, and +VOLCANIC CHAINS. In the examples given of the first class, as far as position +is concerned, I can see no grounds for their being called +“central;” and the evidence of any difference in mineralogical +nature between CENTRAL VOLCANOES and VOLCANIC CHAINS appears slight. No doubt +some one island in most small volcanic archipelagoes is apt to be considerably +higher than the others; and in a similar manner, whatever the cause may be, +that on the same island one vent is generally higher than all the others. Von +Buch does not include in his class of volcanic chains small archipelagoes, in +which the islands are admitted by him, as at the Azores, to be arranged in +lines; 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 line, and so on to a great wall like the +Cordillera of America, it is difficult to believe that there exists any +essential difference between short and long volcanic chains. Von Buch (Idem +page 393.) states that his volcanic chains surmount, or are closely connected +with, mountain-ranges of primary formation: but if trains of linear +archipelagoes are, in the course of time, by the long- continued action of the +elevatory and volcanic forces, converted into mountain-ranges, it would +naturally result that the inferior primary rocks would often be uplifted and +brought into view. +</p> + +<p> +Some authors have remarked that volcanic islands occur scattered, though at +very unequal distances, along the shores of the great continents, as if in some +measure connected with them. In the case of Juan Fernandez, situated 330 miles +from the coast of Chile, there was undoubtedly a connection between the +volcanic forces acting under this island and under the continent, as was shown +during the earthquake of 1835. The islands, moreover, of some of the small +volcanic groups which thus border continents, are placed in lines, related to +those along which the adjoining shores of the continents trend; I may instance +the lines of intersection at the Galapagos, and at the Cape de Verde +Archipelagoes, and the best marked line of the Canary Islands. If these facts +be not merely accidental, we see that many scattered volcanic islands and small +groups are related not only by proximity, but in the direction of the fissures +of eruption to the neighbouring continents—a relation, which Von Buch +considers, characteristic of his great volcanic chains. +</p> + +<p> +In volcanic archipelagoes, the orifices are seldom in activity on more than one +island at a time; and the greater eruptions usually recur only after long +intervals. Observing the number of craters, that are usually found on each +island of a group, and the vast amount of matter which has been erupted from +them, one is led to attribute a high antiquity even to those groups, which +appear, like the Galapagos, to be of comparatively recent origin. This +conclusion accords with the prodigious amount of degradation, by the slow +action of the sea, which their originally sloping coasts must have suffered, +when they are worn back, as is so often the case, into grand precipices. We +ought not, however, to suppose, in hardly any instance, that the whole body of +matter, forming a volcanic island, has been erupted at the level, on which it +now stands: the number of dikes, which seem invariably to intersect the +interior parts of every volcano, show, on the principles explained by M. Elie +de Beaumont, that the whole mass has been uplifted and fissured. A connection, +moreover, between volcanic eruptions and contemporaneous elevations in mass +has, I think, been shown to exist in my work on Coral-Reefs, both from the +frequent presence of upraised organic remains, and from the structure of the +accompanying coral-reefs. (A similar conclusion is forced on us, by the +phenomena, which accompanied the earthquake of 1835, at Concepcion, and which +are detailed in my paper (volume 5 page 601) in the “Geological +Transactions.”) Finally, I may remark, that in the same Archipelago, +eruptions have taken place within the historical period on more than one of the +parallel lines of fissure: thus, at the Galapagos Archipelago, eruptions have +taken place from a vent on Narborough Island, and from one on Albemarle Island, +which vents do not fall on the same line; at the Canary Islands, eruptions have +taken place in Teneriffe and Lanzarote; and at the Azores, on the three +parallel lines of Pico, St. Jorge, and Terceira. Believing that a mountain-axis +differs essentially from a volcano, only in plutonic rocks having been +injected, instead of volcanic matter having been ejected, this appears to me an +interesting circumstance; for we may infer from it as probable, that in the +elevation of a mountain-chain, two or more of the parallel lines forming it may +be upraised and injected within the same geological period. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap07"></a>CHAPTER VII.<br /> +AUSTRALIA; NEW ZEALAND; CAPE OF GOOD HOPE.</h2> + +<p> +New South Wales. Sandstone formation. Embedded pseudo-fragments of shale. +Stratification. Current-cleavage. Great valleys. Van Diemen’s Land. +Palaeozoic formation. Newer formation with volcanic rocks. Travertin with +leaves of extinct plants. Elevation of the land. New Zealand. King +George’s Sound. Superficial ferruginous beds. Superficial calcareous +deposits, with casts of branches. Their origin from drifted particles of shells +and corals. Their extent. Cape of Good Hope. Junction of the granite and +clay-slate. Sandstone formation. +</p> + +<p> +The “Beagle,” in her homeward voyage, touched at New Zealand, +Australia, Van Diemen’s Land, and the Cape of Good Hope. In order to +confine the Third Part of these Geological Observations to South America, I +will here briefly describe all that I observed at these places worthy of the +attention of geologists. +</p> + +<h3>NEW SOUTH WALES.</h3> + +<p> +My opportunities of observation consisted of a ride of ninety geographical +miles to Bathurst, in a W.N.W. direction from Sydney. The first thirty miles +from the coast passes over a sandstone country, broken up in many places by +trap-rocks, and separated by a bold escarpment overhanging the river Nepean, +from the great sandstone platform of the Blue Mountains. This upper platform is +1,000 feet high at the edge of the escarpment, and rises in a distance of +twenty-five miles to between three and four thousand feet above the level of +the sea. At this distance the road descends to a country rather less elevated, +and composed in chief part of primary rocks. There is much granite, in one part +passing into a red porphyry with octagonal crystals of quartz, and intersected +in some places by trap-dikes. Near the Downs of Bathurst I passed over much +pale-brown, glossy clay-slate, with the shattered laminae running north and +south; I mention this fact, because Captain King informs me that, in the +country a hundred miles southward, near Lake George, the mica-slate ranges so +invariably north and south that the inhabitants take advantage of it in finding +their way through the forests. +</p> + +<p> +The sandstone of the Blue Mountains is at least 1,200 feet thick, and in some +parts is apparently of greater thickness; it consists of small grains of +quartz, cemented by white earthy matter, and it abounds with ferruginous veins. +The lower beds sometimes alternate with shales and coal: at Wolgan I found in +carbonaceous shale leaves of the Glossopteris Brownii, a fern which so +frequently accompanies the coal of Australia. The sandstone contains pebbles of +quartz; and these generally increase in number and size (seldom, however, +exceeding an inch or two in diameter) in the upper beds: I observed a similar +circumstance in the grand sandstone formation at the Cape of Good Hope. On the +South American coast, where tertiary and supra- tertiary beds have been +extensively elevated, I repeatedly noticed that the uppermost beds were formed +of coarser materials than the lower: this appears to indicate that, as the sea +became shallower, the force of the waves or currents increased. On the lower +platform, however, between the Blue Mountains and the coast, I observed that +the upper beds of the sandstone frequently passed into argillaceous +shale,—the effect, probably, of this lower space having been protected +from strong currents during its elevation. The sandstone of the Blue Mountains +evidently having been of mechanical origin, and not having suffered any +metamorphic action, I was surprised at observing that, in some specimens, +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 rock. (I have lately seen, in a paper by Smith (the father +of English geologists), in the “Magazine of Natural History,” that +the grains of quartz in the millstone grit of England are often crystallised. +Sir David Brewster, in a paper read before the British Association, 1840, +states, that in old decomposed glass, the silex and metals separate into +concentric rings, and that the silex regains its crystalline structure, as is +shown by its action on light.) It is difficult to imagine how these crystals +could have been formed; one can hardly believe that they were separately +precipitated in their present crystallised state. Is it possible that rounded +grains of quartz may have been acted on by a fluid corroding their surfaces, +and depositing on them fresh silica? I may remark that, in the sandstone +formation of the Cape of Good Hope, it is evident that silica has been +profusely deposited from aqueous solution. +</p> + +<p> +In several parts of the sandstone I noticed patches of shale which might at the +first glance have been mistaken for extraneous fragments; their horizontal +laminae, however, being parallel with those of the sandstone, showed that they +were the remnants of thin, continuous beds. One such fragment (probably the +section of a long narrow strip) seen in the face of a cliff, was of greater +vertical thickness than breadth, which proves that this bed of shale must have +been in some slight degree consolidated, after having been deposited, and +before being worn away by the currents. Each patch of the shale shows, also, +how slowly many of the successive layers of sandstone were deposited. These +pseudo-fragments of shale will perhaps explain, in some cases, the origin of +apparently extraneous fragments in crystalline metamorphic rocks. I mention +this, because I found near Rio de Janeiro a well-defined angular fragment, +seven yards long by two yards in breadth, of gneiss containing garnets and mica +in layers, enclosed in the ordinary, stratified, porphyritic gneiss of the +country. The laminae of the fragment and of the surrounding matrix ran in +exactly the same direction, but they dipped at different angles. I do not wish +to affirm that this singular fragment (a solitary case, as far as I know) was +originally deposited in a layer, like the shale in the Blue Mountains, between +the strata of the porphyritic gneiss, before they were metamorphosed; but there +is sufficient analogy between the two cases to render such an explanation +possible. +</p> + +<h3>STRATIFICATION OF THE ESCARPMENT.</h3> + +<p> +The strata of the Blue Mountains appear to the eye horizontal; but they +probably have a similar inclination with the surface of the platform, which +slopes from the west towards the escarpment over the Nepean, at an angle of one +degree, or of one hundred feet in a mile. (This is stated on the authority of +Sir T. Mitchell in “Travels” volume 2 page 357.) The strata of the +escarpment dip almost conformably with its steeply inclined face, and with so +much regularity, that they appear as if thrown into their present position; but +on a more careful examination, they are seen to thicken and to thin out, and in +the upper part to be succeeded and almost capped by horizontal beds. These +appearances render it probable, that we here see an original escarpment, not +formed by the sea having eaten back into the strata, but by the strata having +originally extended only thus far. Those who have been in the habit of +examining accurate charts of sea-coasts, where sediment is accumulating, will +be aware, that the surfaces of the banks thus formed, generally slope from the +coast very gently towards a certain line in the offing, beyond which the depth +in most cases suddenly becomes great. I may instance the great banks of +sediment within the West Indian Archipelago (I have described these very +curious banks in the Appendix to my volume on the structure of Coral-Reefs. I +have ascertained the inclination of the edges of the banks, from information +given me by Captain B. Allen, one of the surveyors, and by carefully measuring +the horizontal distances between the last sounding on the bank and the first in +the deep water. Widely extended banks in all parts of the West Indies have the +same general form of surface.), which terminate in submarine slopes, inclined +at angles of between thirty and forty degrees, and sometimes even at more than +forty degrees: every one knows how steep such a slope would appear on the land. +Banks of this nature, if uplifted, would probably have nearly the same external +form as the platform of the Blue Mountains, where it abruptly terminates over +the Nepean. +</p> + +<h3>CURRENT-CLEAVAGE.</h3> + +<p> +The strata of sandstone in the low coast country, and likewise on the Blue +Mountains, are often divided by cross or current laminae, which dip in +different directions, and frequently at an angle of forty-five degrees. Most +authors have attributed these cross layers to successive small accumulations on +an inclined surface; but from a careful examination in some parts of the New +Red Sandstone of England, I believe that such layers generally form parts of a +series of curves, like gigantic tidal ripples, the tops of which have since +been cut off, either by nearly horizontal layers, or by another set of great +ripples, the folds of which do not exactly coincide with those below them. It +is well-known to surveyors that mud and sand are disturbed during storms at +considerable depths, at least from three hundred to four hundred and fifty feet +(See Martin White on “Soundings in the British Channel” pages 4 and +166.), so that the nature of the bottom even becomes temporarily changed; the +bottom, also, at a depth between sixty and seventy feet, has been observed to +be broadly rippled. (M. Siau on the “Action of Waves” “Edin. +New Phil. Journ.” volume 31 page 245.) One may, therefore, be allowed to +suspect, from the appearance just mentioned in the New Red Sandstone, that at +greater depths, the bed of the ocean is heaped up during gales into great +ripple-like furrows and depressions, which are afterwards cut off by the +currents during more tranquil weather, and again furrowed during gales. +</p> + +<h3>VALLEYS IN THE SANDSTONE PLATFORMS.</h3> + +<p> +The grand valleys, by which the Blue Mountains and the other sandstone +platforms of this part of Australia are penetrated, and which long offered an +insuperable obstacle to the attempts of the most enterprising colonist to reach +the interior country, form the most striking feature in the geology of New +South Wales. They are of grand dimensions, and are bordered by continuous links +of lofty cliffs. It is not easy to conceive a more magnificent spectacle, than +is presented to a person walking on the summit- plains, when without any notice +he arrives at the brink of one of these cliffs, which are so perpendicular, +that he can strike with a stone (as I have tried) the trees growing, at the +depth of between one thousand and one thousand five hundred feet below him; on +both hands he sees headland beyond headland of the receding line of cliff, and +on the opposite side of the valley, often at the distance of several miles, he +beholds another line rising up to the same height with that on which he stands, +and formed of the same horizontal strata of pale sandstone. The bottoms of +these valleys are moderately level, and the fall of the rivers flowing in them, +according to Sir T. Mitchell, is gentle. The main valleys often send into the +platform great baylike arms, which expand at their upper ends; and on the other +hand, the platform often sends promontories into the valley, and even leaves in +them great, almost insulated, masses. So continuous are the bounding lines of +cliff, that to descend into some of these valleys, it is necessary to go round +twenty miles; and into others, the surveyors have only lately penetrated, and +the colonists have not yet been able to drive in their cattle. But the most +remarkable point of structure in these valleys, is, that although several miles +wide in their upper parts, they generally contract towards their mouths to such +a degree as to become impassable. The Surveyor-General, Sir T. Mitchell, in +vain endeavoured, first on foot and then by crawling between the great fallen +fragments of sandstone, to ascend through the gorge by which the river Grose +joins the Nepean (“Travels in Australia” volume 1 page 154.—I +must express my obligation to Sir T. Mitchell for several interesting personal +communications on the subject of these great valleys of New South Wales.); yet +the valley of the Grose in its upper part, as I saw, forms a magnificent basin +some miles in width, and is on all sides surrounded by cliffs, the summits of +which are believed to be nowhere less than 3,000 feet above the level of the +sea. When cattle are driven into the valley of the Wolgan by a path (which I +descended) partly cut by the colonists, they cannot escape; for this valley is +in every other part surrounded by perpendicular cliffs, and eight miles lower +down, it contracts, from an average width of half a mile, to a mere chasm +impassable to man or beast. Sir T. Mitchell states, that the great valley of +the Cox river with all its branches contracts, where it unites with the Nepean, +into a gorge 2,200 yards wide, and about one thousand feet in depth. (Idem +volume 2 page 358.) Other similar cases might have been added. +</p> + +<p> +The first impression, from seeing the correspondence of the horizontal strata, +on each side of these valleys and great amphitheatre-like depressions, is that +they have been in chief part hollowed out, like other valleys, by aqueous +erosion; but when one reflects on the enormous amount of stone, which on this +view must have been removed, in most of the above cases through mere gorges or +chasms, one is led to ask whether these spaces may not have subsided. But +considering the form of the irregularly branching valleys, and of the narrow +promontories, projecting into them from the platforms, we are compelled to +abandon this notion. To attribute these hollows to alluvial action, would be +preposterous; nor does the drainage from the summit-level always fall, as I +remarked near the Weatherboard, into the head of these valleys, but into one +side of their bay-like recesses. Some of the inhabitants remarked to me, that +they never viewed one of these baylike recesses, with the headlands receding on +both hands, without being struck with their resemblance to a bold sea-coast. +This is certainly the case; moreover, the numerous fine harbours, with their +widely branching arms, on the present coast of New South Wales, which are +generally connected with the sea by a narrow mouth, from one mile to a quarter +of a mile in width, passing through the sandstone coast-cliffs, present a +likeness, though on a miniature scale, to the great valleys of the interior. +But then immediately occurs the startling difficulty, why has the sea worn out +these great, though circumscribed, depressions on a wide platform, and left +mere gorges, through which the whole vast amount of triturated matter must have +been carried away? The only light I can throw on this enigma, is by showing +that banks appear to be forming in some seas of the most irregular forms, and +that the sides of such banks are so steep (as before stated) that a +comparatively small amount of subsequent erosion would form them into cliffs: +that the waves have power to form high and precipitous cliffs, even in +landlocked harbours, I have observed in many parts of South America. In the Red +Sea, banks with an extremely irregular outline and composed of sediment, are +penetrated by the most singularly shaped creeks with narrow mouths: this is +likewise the case, though on a larger scale, with the Bahama Banks. Such banks, +I have been led to suppose, have been formed by currents heaping sediment on an +irregular bottom. (See the “Appendix” to the Part on Coral-Reefs. +The fact of the sea heaping up mud round a submarine nucleus, is worthy of the +notice of geologists: for outlyers of the same composition with the coast banks +are thus formed; and these, if upheaved and worn into cliffs, would naturally +be thought to have been once connected together.) That in some cases, the sea, +instead of spreading out sediment in a uniform sheet, heaps it round submarine +rocks and islands, it is hardly possible to doubt, after having examined the +charts of the West Indies. To apply these ideas to the sandstone platforms of +New South Wales, I imagine that the strata might have been heaped on an +irregular bottom by the action of strong currents, and of the undulations of an +open sea; and that the valley-like spaces thus left unfilled might, during a +slow elevation of the land, have had their steeply sloping flanks worn into +cliffs; the worn-down sandstone being removed, either at the time when the +narrow gorges were cut by the retreating sea, or subsequently by alluvial +action. +</p> + +<h3>VAN DIEMEN’S LAND.</h3> + +<p> +The southern part of this island is mainly formed of mountains of greenstone, +which often assumes a syenitic character, and contains much hypersthene. These +mountains, in their lower half, are generally encased by strata containing +numerous small corals and some shells. These shells have been examined by Mr. +G.B. Sowerby, and have been described by him: they consist of two species of +Producta, and of six of Spirifera; two of these, namely, P. rugata and S. +rotundata, resemble, as far as their imperfect condition allows of comparison, +British mountain-limestone shells. Mr. Lonsdale has had the kindness to examine +the corals; they consist of six undescribed species, belonging to three genera. +Species of these genera occur in the Silurian, Devonian, and Carboniferous +strata of Europe. Mr. Lonsdale remarks, that all these fossils have undoubtedly +a Palaeozoic character, and that probably they correspond in age to a division +of the system above the Silurian formations. +</p> + +<p> +The strata containing these remains are singular from the extreme variability +of their mineralogical composition. Every intermediate form is present, between +flinty-slate, clay-slate passing into grey wacke, pure limestone, sandstone, +and porcellanic rock; and some of the beds can only be described as composed of +a siliceo-calcareo-clay-slate. The formation, as far as I could judge, is at +least a thousand feet in thickness: the upper few hundred feet usually consist +of a siliceous sandstone, containing pebbles and no organic remains; the +inferior strata, of which a pale flinty slate is perhaps the most abundant, are +the most variable; and these chiefly abound with the remains. Between two beds +of hard crystalline limestone, near Newtown, a layer of white soft calcareous +matter is quarried, and is used for whitewashing houses. From information given +to me by Mr. Frankland, the Surveyor-General, it appears that this Palaeozoic +formation is found in different parts of the whole island; from the same +authority, I may add, that on the north-eastern coast and in Bass’ +Straits primary rocks extensively occur. +</p> + +<p> +The shores of Storm Bay are skirted, to the height of a few hundred feet, by +strata of sandstone, containing pebbles of the formation just described, with +its characteristic fossils, and therefore belonging to a subsequent age. These +strata of sandstone often pass into shale, and alternate with layers of impure +coal; they have in many places been violently disturbed. Near Hobart Town, I +observed one dike, nearly a hundred yards in width, on one side of which the +strata were tilted at an angle of 60 degrees, and on the other they were in +some parts vertical, and had been altered by the effects of the heat. On the +west side of Storm Bay, I found these strata capped by streams of basaltic lava +with olivine; and close by there was a mass of brecciated scoriae, containing +pebbles of lava, which probably marks the place of an ancient submarine crater. +Two of these streams of basalt were separated from each other by a layer of +argillaceous wacke, which could be traced passing into partially altered +scoriae. The wacke contained numerous rounded grains of a soft, grass-green +mineral, with a waxy lustre, and translucent on its edges: under the blowpipe +it instantly blackened, and the points fused into a strongly magnetic, black +enamel. In these characters, it resembles those masses of decomposed olivine, +described at St. Jago in the Cape de Verde group; and I should have thought +that it had thus originated, had I not found a similar substance, in +cylindrical threads, within the cells of the vesicular basalt,—a state +under which olivine never appears; this substance, I believe, would be classed +as bole by mineralogists. (Chlorophaeite, described by Dr. MacCulloch +(“Western Islands” volume 1 page 504) as occurring in a basaltic +amygdaloid, differs from this substance, in remaining unchanged before the +blowpipe, and in blackening from exposure to the air. May we suppose that +olivine, in undergoing the remarkable change described at St. Jago, passes +through several states?) +</p> + +<h3>TRAVERTIN WITH EXTINCT PLANTS.</h3> + +<p> +Behind Hobart Town there is a small quarry of a hard travertin, the lower +strata of which abound with distinct impressions of leaves. Mr. Robert Brown +has had the kindness to look at my specimens, and he informed me that there are +four or five kinds, none of which he recognises as belonging to existing +species. The most remarkable leaf is palmate, like that of a fan- palm, and no +plant having leaves of this structure has hitherto been discovered in Van +Diemen’s Land. The other leaves do not resemble the most usual form of +the Eucalyptus (of which tribe the existing forests are chiefly composed), nor +do they resemble that class of exceptions to the common form of the leaves of +the Eucalyptus, which occur in this island. The travertin containing this +remnant of a lost vegetation, is of a pale yellow colour, hard, and in parts +even crystalline; but not compact, and is everywhere penetrated by minute, +tortuous, cylindrical pores. It contains a very few pebbles of quartz, and +occasionally layers of chalcedonic nodules, like those of chert in our +Greensand. From the pureness of this calcareous rock, it has been searched for +in other places, but has never been found. From this circumstance, and from the +character of the deposit, it was probably formed by a calcareous spring +entering a small pool or narrow creek. The strata have subsequently been tilted +and fissured; and the surface has been covered by a singular mass, with which, +also, a large fissure has been filled up, formed of balls of trap embedded in a +mixture of wacke and a white, earthy, alumino-calcareous substance. Hence it +would appear, as if a volcanic eruption had taken place on the borders of the +pool, in which the calcareous matter was depositing, and had broken it up and +drained it. +</p> + +<h3>ELEVATION OF THE LAND.</h3> + +<p> +Both the eastern and western shores of the bay, in the neighbourhood of Hobart +Town, are in most parts covered to the height of thirty feet above the level of +high-water mark, with broken shells, mingled with pebbles. The colonists +attribute these shells to the aborigines having carried them up for food: +undoubtedly, there are many large mounds, as was pointed out to me by Mr. +Frankland, which have been thus formed; but I think from the numbers of the +shells, from their frequent small size, from the manner in which they are +thinly scattered, and from some appearances in the form of the land, that we +must attribute the presence of the greater number to a small elevation of the +land. On the shore of Ralph Bay (opening into Storm Bay) I observed a +continuous beach about fifteen feet above high-water mark, clothed with +vegetation, and by digging into it, pebbles encrusted with Serpulae were found: +along the banks, also, of the river Derwent, I found a bed of broken sea-shells +above the surface of the river, and at a point where the water is now much too +fresh for sea-shells to live; but in both these cases, it is just possible, +that before certain spits of sand and banks of mud in Storm Bay were +accumulated, the tides might have risen to the height where we now find the +shells. ( It would appear that some changes are now in progress in Ralph Bay, +for I was assured by an intelligent farmer, that oysters were formerly abundant +in it, but that about the year 1834 they had, without any apparent cause, +disappeared. In the “Transactions of the Maryland Academy” volume 1 +part 1 page 28 there is an account by Mr. Ducatel of vast beds of oysters and +clams having been destroyed by the gradual filling up of the shallow lagoons +and channels, on the shores of the southern United States. At Chiloe, in South +America, I heard of a similar loss, sustained by the inhabitants, in the +disappearance from one part of the coast of an edible species of Ascidia.) +</p> + +<p> +Evidence more or less distinct of a change of level between the land and water, +has been detected on almost all the land on this side of the globe. Captain +Grey, and other travellers, have found in Southern Australia upraised shells, +belonging either to the recent, or to a late tertiary period. The French +naturalists in Baudin’s expedition, found shells similarly circumstanced +on the S.W. coast of Australia. The Rev. W.B. Clarke finds proofs of the +elevation of the land, to the amount of 400 feet, at the Cape of Good Hope. +(“Proceedings of the Geological Society” volume 3 page 420.) In the +neighbourhood of the Bay of Islands in New Zealand, I observed that the shores +were scattered to some height, as at Van Diemen’s Land, with sea-shells, +which the colonists attribute to the natives. (I will here give a catalogue of +the rocks which I met with near the Bay of Islands, in New Zealand:—1st, +Much basaltic lava, and scoriform rocks, forming distinct craters;—2nd, A +castellated hill of horizontal strata of flesh-coloured limestone, showing when +fractured distinct crystalline facets: the rain has acted on this rock in a +remarkable manner, corroding its surface into a miniature model of an Alpine +country: I observed here layers of chert and clay ironstone; and in the bed of +a stream, pebbles of clay-slate;—3rd, The shores of the Bay of Islands +are formed of a feldspathic rock, of a bluish-grey colour, often much +decomposed, with an angular fracture, and crossed by numerous ferruginous +seams, but without any distinct stratification or cleavage. Some varieties are +highly crystalline, and would at once be pronounced to be trap; others +strikingly resembled clay-slate, slightly altered by heat: I was unable to form +any decided opinion on this formation.) Whatever may have been the origin of +these shells, I cannot doubt, after having seen a section of the valley of the +Thames River (37 degrees S.), drawn by the Rev. W. Williams, that the land has +been there elevated: on the opposite sides of this great valley, three +step-like terraces, composed of an enormous accumulation of rounded pebbles, +exactly correspond with each other: the escarpment of each terrace is about +fifty feet in height. No one after having examined the terraces in the valleys +on the western shores of South America, which are strewed with sea-shells, and +have been formed during intervals of rest in the slow elevation of the land, +could doubt that the New Zealand terraces have been similarly formed. I may +add, that Dr. Dieffenbach, in his description of the Chatham Islands +(“Geographical Journal” volume 11 pages 202, 205.) (S.W. of New +Zealand), states that it is manifest “that the sea has left many places +bare which were once covered by its waters.” +</p> + +<h3>KING GEORGE’S SOUND.</h3> + +<p> +This settlement is situated at the south-western angle of the Australian +continent: the whole country is granitic, with the constituent minerals +sometimes obscurely arranged in straight or curved laminae. In these cases, the +rock would be called by Humboldt, gneiss-granite, and it is remarkable that the +form of the bare conical hills, appearing to be composed of great folding +layers, strikingly resembles, on a small scale, those composed of +gneiss-granite at Rio de Janeiro, and those described by Humboldt at Venezuela. +These plutonic rocks are, in many places, intersected by trappean-dikes; in one +place, I found ten parallel dikes ranging in an E. and W. line; and not far off +another set of eight dikes, composed of a different variety of trap, ranging at +right angles to the former ones. I have observed in several primary districts, +the occurrence of systems of dikes parallel and close to each other. +</p> + +<h3>SUPERFICIAL FERRUGINOUS BEDS.</h3> + +<p> +The lower parts of the country are everywhere covered by a bed, following the +inequalities of the surface, of a honeycombed sandstone, abounding with oxides +of iron. Beds of nearly similar composition are common, I believe, along the +whole western coast of Australia, and on many of the East Indian islands. At +the Cape of Good Hope, at the base of the mountains formed of granite and +capped with sandstone, the ground is everywhere coated either by a +fine-grained, rubbly, ochraceous mass, like that at King George’s Sound, +or by a coarser sandstone with fragments of quartz, and rendered hard and heavy +by an abundance of the hydrate of iron, which presents, when freshly broken, a +metallic lustre. Both these varieties have a very irregular texture, including +spaces either rounded or angular, full of loose sand: from this cause the +surface is always honeycombed. The oxide of iron is most abundant on the edges +of the cavities, where alone it affords a metallic fracture. In these +formations, as well as in many true sedimentary deposits, it is evident that +iron tends to become aggregated, either in the form of a shell, or of a +network. The origin of these superficial beds, though sufficiently obscure, +seems to be due to alluvial action on detritus abounding with iron. +</p> + +<h3>SUPERFICIAL CALCAREOUS DEPOSIT.</h3> + +<p> +A calcareous deposit on the summit of Bald Head, containing branched bodies, +supposed by some authors to have been corals, has been celebrated by the +descriptions of many distinguished voyagers. (I visited this hill, in company +with Captain Fitzroy, and we came to a similar conclusion regarding these +branching bodies.) It folds round and conceals irregular hummocks of granite, +at the height of 600 feet above the level of the sea. It varies much in +thickness; where stratified, the beds are often inclined at high angles, even +as much as at thirty degrees, and they dip in all directions. These beds are +sometimes crossed by oblique and even-sided laminae. The deposit consists +either of a fine, white calcareous powder, in which not a trace of structure +can be discovered, or of exceedingly minute, rounded grains, of brown, +yellowish, and purplish colours; both varieties being generally, but not +always, mixed with small particles of quartz, and being cemented into a more or +less perfect stone. The rounded calcareous grains, when heated in a slight +degree, instantly lose their colours; in this and in every other respect, +closely resembling those minute, equal- sized particles of shells and corals, +which at St. Helena have been drifted up the side of the mountains, and have +thus been winnowed of all coarser fragments. I cannot doubt that the coloured +calcareous particles here have had a similar origin. The impalpable powder has +probably been derived from the decay of the rounded particles; this certainly +is possible, for on the coast of Peru, I have traced LARGE UNBROKEN shells +gradually falling into a substance as fine as powdered chalk. Both of the +above-mentioned varieties of calcareous sandstone frequently alternate with, +and blend into, thin layers of a hard substalagmitic rock, which, even when the +stone on each side contains particles of quartz, is entirely free from them (I +adopt this term from Lieutenant Nelson’s excellent paper on the Bermuda +Islands “Geolog. Trans.” volume 5 page 106, for the hard, compact, +cream- or brown- coloured stone, without any crystalline structure, which so +often accompanies superficial calcareous accumulations. I have observed such +superficial beds, coated with substalagmitic rock, at the Cape of Good Hope, in +several parts of Chile, and over wide spaces in La Plata and Patagonia. Some of +these beds have been formed from decayed shells, but the origin of the greater +number is sufficiently obscure. The causes which determine water to dissolve +lime, and then soon to redeposit it, are not, I think, known. The surface of +the substalagmitic layers appears always to be corroded by the rain-water. As +all the above-mentioned countries have a long dry season, compared with the +rainy one, I should have thought that the presence of the substalagmitic was +connected with the climate, had not Lieutenant Nelson found this substance +forming under sea-water. Disintegrated shell seems to be extremely soluble; of +which I found good evidence, in a curious rock at Coquimbo in Chile, which +consisted of small, pellucid, empty husks, cemented together. A series of +specimens clearly showed that these husks had originally contained small +rounded particles of shells, which had been enveloped and cemented together by +calcareous matter (as often happens on sea-beaches), and which subsequently had +decayed, and been dissolved by water, that must have penetrated through the +calcareous husks, without corroding them,—of which processes every stage +could be seen.): hence we must suppose that these layers, as well as certain +vein- like masses, have been formed by rain dissolving the calcareous matter +and re-precipitating it, as has happened at St. Helena. Each layer probably +marks a fresh surface, when the, now firmly cemented, particles existed as +loose sand. These layers are sometimes brecciated and re-cemented, as if they +had been broken by the slipping of the sand when soft. I did not find a single +fragment of a sea-shell; but bleached shells of the Helix melo, an existing +land species, abound in all the strata; and I likewise found another Helix, and +the case of an Oniscus. +</p> + +<p> +The branches are absolutely undistinguishable in shape from the broken and +upright stumps of a thicket; their roots are often uncovered, and are seen to +diverge on all sides; here and there a branch lies prostrate. The branches +generally consist of the sandstone, rather firmer than the surrounding matter, +with the central parts filled, either with friable, calcareous matter, or with +a substalagmitic variety; this central part is also frequently penetrated by +linear crevices, sometimes, though rarely, containing a trace of woody matter. +These calcareous, branching bodies, appear to have been formed by fine +calcareous matter being washed into the casts or cavities, left by the decay of +branches and roots of thickets, buried under drifted sand. The whole surface of +the hill is now undergoing disintegration, and hence the casts, which are +compact and hard, are left projecting. In calcareous sand at the Cape of Good +Hope, I found the casts, described by Abel, quite similar to these at Bald +Head; but their centres are often filled with black carbonaceous matter not yet +removed. It is not surprising, that the woody matter should have been almost +entirely removed from the casts on Bald Head; for it is certain, that many +centuries must have elapsed since the thickets were buried; at present, owing +to the form and height of the narrow promontory, no sand is drifted up, and the +whole surface, as I have remarked, is wearing away. We must, therefore, look +back to a period when the land stood lower, of which the French naturalists +(See M. Peron “Voyage” tome 1 page 204.) found evidence in upraised +shells of recent species, for the drifting on Bald Head of the calcareous and +quartzose sand, and the consequent embedment of the vegetable remains. There +was only one appearance which at first made me doubt concerning the origin of +the cast,—namely, that the finer roots from different stems sometimes +became united together into upright plates or veins; but when the manner is +borne in mind in which fine roots often fill up cracks in hard earth, and that +these roots would decay and leave hollows, as well as the stems, there is no +real difficulty in this case. Besides the calcareous branches from the Cape of +Good Hope, I have seen casts, of exactly the same forms, from Madeira* and from +Bermuda; at this latter place, the surrounding calcareous rocks, judging from +the specimens collected by Lieutenant Nelson, are likewise similar, as is their +subaerial formation. Reflecting on the stratification of the deposit on Bald +Head,—on the irregularly alternating layers of substalagmitic +rock,—on the uniformly sized, and rounded particles, apparently of +sea-shells and corals,—on the abundance of land-shells throughout the +mass,—and finally, on the absolute resemblance of the calcareous casts, +to the stumps, roots, and branches of that kind of vegetation, which would grow +on sand-hillocks, I think there can be no reasonable doubt, notwithstanding the +different opinion of some authors, that a true view of their origin has been +here given. +</p> + +<p class="footnote"> +*(Dr. J. Macaulay has fully described (“Edinb. New Phil. Journ.” +volume 29 page 350) the casts from Madeira. He considers (differently from Mr. +Smith of Jordan Hill) these bodies to be corals, and the calcareous deposit to +be of subaqueous origin. His arguments chiefly rest (for his remarks on their +structure are vague) on the great quantity of the calcareous matter, and on the +casts containing animal matter, as shown by their evolving ammonia. Had Dr. +Macaulay seen the enormous masses of rolled particles of shells and corals on +the beach of Ascension, and especially on coral-reefs; and had he reflected on +the effects of long-continued, gentle winds, in drifting up the finer +particles, he would hardly have advanced the argument of quantity, which is +seldom trustworthy in geology. If the calcareous matter has originated from +disintegrated shells and corals, the presence of animal matter is what might +have been expected. Mr. Anderson analysed for Dr. Macaulay part of a cast, and +he found it composed of:— +</p> + +<pre xml:space="preserve"> +Carbonate of lime......73.15 +Silica.................11.90 +Phosphate of lime.......8.81 +Animal matter...........4.25 +Sulphate of lime......a trace + 98.11) +</pre> + +<p> +Calcareous deposits, like these of King George’s Sound, are of vast +extent on the Australian shores. Dr. Fitton remarks, that “recent +calcareous breccia (by which term all these deposits are included) was found +during Baudin’s voyage, over a space of no less than twenty-five degrees +of latitude and an equal extent of longitude, on the southern, western, and +north-western coasts.” (For ample details on this formation consult Dr. +Fitton “Appendix to Captain King’s Voyage.” Dr. Fitton is +inclined to attribute a concretionary origin to the branching bodies: I may +remark, that I have seen in beds of sand in La Plata cylindrical stems which no +doubt thus originated; but they differed much in appearance from these at Bald +Head, and the other places above specified.) It appears also from M. Peron, +with whose observations and opinions on the origin of the calcareous matter and +branching casts mine entirely accord, that the deposit is generally much more +continuous than near King George’s Sound. At Swan River, Archdeacon Scott +states that in one part it extends ten miles inland. (“Proceedings of the +Geolog. Soc.” volume 1 page 320.) Captain Wickham, moreover, informs me +that during his late survey of the western coast, the bottom of the sea, +wherever the vessel anchored, was ascertained, by crowbars being let down, to +consist of white calcareous matter. Hence it seems that along this coast, as at +Bermuda and at Keeling Atoll, submarine and subaerial deposits are +contemporaneously in process of formation, from the disintegration of marine +organic bodies. The extent of these deposits, considering their origin, is very +striking; and they can be compared in this respect only with the great +coral-reefs of the Indian and Pacific Oceans. In other parts of the world, for +instance in South America, there are SUPERFICIAL calcareous deposits of great +extent, in which not a trace of organic structure is discoverable; these +observations would lead to the inquiry, whether such deposits may not, also, +have been formed from disintegrated shells and corals. +</p> + +<h3>CAPE OF GOOD HOPE.</h3> + +<p> +After the accounts given by Barrow, Carmichael, Basil Hall, and W.B. Clarke of +the geology of this district, I shall confine myself to a few observations on +the junction of the three principal formations. The fundamental rock is granite +(In several places I observed in the granite, small dark-coloured balls, +composed of minute scales of black mica in a tough basis. In another place, I +found crystals of black schorl radiating from a common centre. Dr. Andrew Smith +found, in the interior parts of the country, some beautiful specimens of +granite, with silvery mica radiating or rather branching, like moss, from +central points. At the Geological Society, there are specimens of granite with +crystallised feldspar branching and radiating in like manner.), overlaid by +clay-slate: the latter is generally hard, and glossy from containing minute +scales of mica; it alternates with, and passes into, beds of slightly +crystalline, feldspathic, slaty rock. This clay-slate is remarkable from being +in some places (as on the Lion’s Rump) decomposed, even to the depth of +twenty feet, into a pale-coloured, sandstone-like rock, which has been +mistaken, I believe, by some observers, for a separate formation. I was guided +by Dr. Andrew Smith to a fine junction at Green Point between the granite and +clay-slate: the latter at the distance of a quarter of a mile from the spot, +where the granite appears on the beach (though, probably, the granite is much +nearer underground), becomes slightly more compact and crystalline. At a less +distance, some of the beds of clay-slate are of a homogeneous texture, and +obscurely striped with different zones of colour, whilst others are obscurely +spotted. Within a hundred yards of the first vein of granite, the clay-slate +consists of several varieties; some compact with a tinge of purple, others +glistening with numerous minute scales of mica and imperfectly crystallised +feldspar; some obscurely granular, others porphyritic with small, elongated +spots of a soft white mineral, which being easily corroded, gives to this +variety a vesicular appearance. Close to the granite, the clay-slate is changed +into a dark-coloured, laminated rock, having a granular fracture, which is due +to imperfect crystals of feldspar, coated by minute, brilliant scales of mica. +</p> + +<p> +The actual junction between the granitic and clay-slate districts extends over +a width of about two hundred yards, and consists of irregular masses and of +numerous dikes of granite, entangled and surrounded by the clay- slate: most of +the dikes range in a N.W. and S.E. line, parallel to the cleavage of the slate. +As we leave the junction, thin beds, and lastly, mere films of the altered +clay-slate are seen, quite isolated, as if floating, in the coarsely +crystallised granite; but although completely detached, they all retain traces +of the uniform N.W. and S.E. cleavage. This fact has been observed in other +similar cases, and has been advanced by some eminent geologists (See M. Keilhau +“Theory on Granite” translated in the “Edinburgh New +Philosophical Journal” volume 24 page 402.), as a great difficulty on the +ordinary theory, of granite having been injected whilst liquified; but if we +reflect on the probable state of the lower surface of a laminated mass, like +clay-slate, after having been violently arched by a body of molten granite, we +may conclude that it would be full of fissures parallel to the planes of +cleavage; and that these would be filled with granite, so that wherever the +fissures were close to each other, mere parting layers or wedges of the slate +would depend into the granite. Should, therefore, the whole body of rock +afterwards become worn down and denuded, the lower ends of these dependent +masses or wedges of slate would be left quite isolated in the granite; yet they +would retain their proper lines of cleavage, from having been united, whilst +the granite was fluid, with a continuous covering of clay-slate. +</p> + +<p> +Following, in company with Dr. A. Smith, the line of junction between the +granite and the slate, as it stretched inland, in a S.E. direction, we came to +a place, where the slate was converted into a fine-grained, perfectly +characterised gneiss, composed of yellow-brown granular feldspar, of abundant +black brilliant mica, and of few and thin laminae of quartz. From the abundance +of the mica in this gneiss, compared with the small quantity and excessively +minute scales, in which it exists in the glossy clay-slate, we must conclude, +that it has been here formed by the metamorphic action—a circumstance +doubted, under nearly similar circumstances, by some authors. The laminae of +the clay-slate are straight; and it was interesting to observe, that as they +assumed the character of gneiss, they became undulatory with some of the +smaller flexures angular, like the laminae of many true metamorphic schists. +</p> + +<h3>SANDSTONE FORMATION.</h3> + +<p> +This formation makes the most imposing feature in the geology of Southern +Africa. The strata are in many parts horizontal, and attain a thickness of +about two thousand feet. The sandstone varies in character; it contains little +earthy matter, but is often stained with iron; some of the beds are very +fine-grained and quite white; others are as compact and homogeneous as quartz +rock. In some places I observed a breccia of quartz, with the fragments almost +dissolved in a siliceous paste. Broad veins of quartz, often including large +and perfect crystals, are very numerous; and it is evident in nearly all the +strata, that silica has been deposited from solution in remarkable quantity. +Many of the varieties of quartzite appeared quite like metamorphic rocks; but +from the upper strata being as siliceous as the lower, and from the undisturbed +junctions with the granite, which in many places can be examined, I can hardly +believe that these sandstone-strata have been exposed to heat. (The Rev. W.B. +Clarke, however, states, to my surprise (“Geolog. Proceedings” +volume 3 page 422), that the sandstone in some parts is penetrated by granitic +dikes: such dikes must belong to an epoch altogether subsequent to that when +the molten granite acted on the clay-slate.) On the lines of junction between +these two great formations, I found in several places the granite decayed to +the depth of a few inches, and succeeded, either by a thin layer of ferruginous +shale, or by four or five inches in thickness of the re-cemented crystals of +the granite, on which the great pile of sandstone immediately rested. +</p> + +<p> +Mr. Schomburgk has described (“Geographical Journal” volume 10 page +246.) a great sandstone formation in Northern Brazil, resting on granite, and +resembling to a remarkable degree, in composition and in the external form of +the land, this formation of the Cape of Good Hope. The sandstones of the great +platforms of Eastern Australia, which also rest on granite, differ in +containing more earthy and less siliceous matter. No fossil remains have been +discovered in these three vast deposits. Finally, I may add that I did not see +any boulders of far-transported rocks at the Cape of Good Hope, or on the +eastern and western shores of Australia, or at Van Diemen’s Land. In the +northern island of New Zealand, I noticed some large blocks of greenstone, but +whether their parent rock was far distant, I had no opportunity of determining. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap08"></a>INDEX TO VOLCANIC ISLANDS.</h2> + +<p> +Abel, M., on calcareous casts at the Cape of Good Hope. +</p> + +<p> +Abingdon island. +</p> + +<p> +Abrolhos islands, incrustation on. +</p> + +<p> +Aeriform explosions at Ascension. +</p> + +<p> +Albatross, driven from St. Helena. +</p> + +<p> +Albemarle island. +</p> + +<p> +Albite, at the Galapagos archipelago. +</p> + +<p> +Amygdaloidal cells, half filled. +</p> + +<p> +Amygdaloids, calcareous origin of. +</p> + +<p> +Ascension, arborescent incrustation on rocks of. -absence of dikes, freedom +from volcanic action, and state of lava-streams. +</p> + +<p> +Ascidia, extinction of. +</p> + +<p> +Atlantic Ocean, new volcanic focus in. +</p> + +<p> +Augite, fused. +</p> + +<p> +Australia. +</p> + +<p> +Azores. +</p> + +<p> +Bahia in Brazil, dikes at. +</p> + +<p> +Bailly, M., on the mountains of Mauritius. +</p> + +<p> +Bald Head. +</p> + +<p> +Banks’ Cove. +</p> + +<p> +Barn, The, St. Helena. +</p> + +<p> +Basalt, specific gravity of. +</p> + +<p> +Basaltic coast-mountains at Mauritius. -at St. Helena. -at St. Jago. +</p> + +<p> +Beaumont, M. Elie de, on circular subsidences in lava. -on dikes indicating +elevation. -on inclination of lava-streams. -on laminated dikes. +</p> + +<p> +Bermuda, calcareous rocks of. +</p> + +<p> +Beudant, M., on bombs. -on jasper. -on laminated trachyte. -on obsidian of +Hungary. -on silex in trachyte. +</p> + +<p> +Bole. +</p> + +<p> +Bombs, volcanic. +</p> + +<p> +Bory St. Vincent, on bombs. +</p> + +<p> +Boulders, absence in Australia and Cape of Good Hope. +</p> + +<p> +Brattle island. +</p> + +<p> +Brewster, Sir D., on a calcareo-animal substance. -on decomposed glass. +</p> + +<p> +Brown, Mr. R., on extinct plants from Van Diemen’s land. -on sphaerulitic +bodies in silicified wood. +</p> + +<p> +Buch, Von, on cavernous lava. -on central volcanoes. -on crystals sinking in +obsidian. -on laminated lava. -on obsidian streams. -on olivine in basalt. -on +superficial calcareous beds in the Canary islands. +</p> + +<p> +Calcareous deposit at St. Jago affected by heat. -fibrous matter, entangled in +streaks in scoriae. -freestone at Ascension. -incrustations at Ascension. +-sandstone at St. Helena. -superficial beds at King George’s sound. +</p> + +<p> +Cape of Good Hope. +</p> + +<p> +Carbonic acid, expulsion of, by heat. +</p> + +<p> +Carmichael, Capt., on glassy coatings to dikes. +</p> + +<p> +Casts, calcareous, of branches. +</p> + +<p> +Chalcedonic nodules. +</p> + +<p> +Chalcedony in basalt and in silicified wood. +</p> + +<p> +Chatham island. +</p> + +<p> +Chlorophaeite. +</p> + +<p> +Clarke, Rev. W., on the Cape of Good Hope. +</p> + +<p> +Clay-slate, its decomposition and junction with granite at the Cape of Good +Hope. +</p> + +<p> +Cleavage of clay-slate in Australia. +</p> + +<p> +Cleavage, cross, in sandstone. +</p> + +<p> +Coast denudation at St. Helena. +</p> + +<p> +Columnar basalt. +</p> + +<p> +“Comptes Rendus,” account of volcanic phenomena in the Atlantic. +</p> + +<p> +Concepcion, earthquake of. +</p> + +<p> +Concretions in aqueous and igneous rocks compared. -in tuff. -of obsidian. +</p> + +<p> +Conglomerate, recent, at St. Jago. +</p> + +<p> +Coquimbo, curious rock of. +</p> + +<p> +Corals, fossil, from Van Diemen’s Land. +</p> + +<p> +Crater, segment of, at the Galapagos. -great central one at St. Helena. +-internal ledges round, and parapet on. +</p> + +<p> +Craters, basaltic, at Ascension. -form of, affected by the trade wind. -of +elevation. -of tuff at Terceira. -of tuff at the Galapagos archipelago. -their +breached state. -small basaltic at St. Jago. —at the Galapagos +archipelago. +</p> + +<p> +Crystallisation favoured by space. +</p> + +<p> +Dartigues, M., on sphaerulites. +</p> + +<p> +Daubeny, Dr., on a basin-formed island. -on fragments in trachyte. +</p> + +<p> +D’Aubuisson on hills of phonolite. -on the composition of obsidian. -on +the lamination of clay-slate. +</p> + +<p> +De la Beche, Sir H., on magnesia in erupted lime. -on specific gravity of +limestones. +</p> + +<p> +Denudation of coast at St. Helena. +</p> + +<p> +Diana’s Peak, St. Helena. +</p> + +<p> +Dieffenbach, Dr., on the Chatham Islands. +</p> + +<p> +Dikes, truncated, on central crateriform ridge of St. Helena. -at St. Helena; +number of; coated by a glossy layer; uniform thickness of. -great parallel ones +at St. Helena. -not observed at Ascension. -of tuff. -of trap in the plutonic +series. -remnants of, extending far into the sea round St. Helena. +</p> + +<p> +Dislocations at Ascension. -at St. Helena. +</p> + +<p> +Distribution of volcanic islands. +</p> + +<p> +Dolomieu, on decomposed trachyte. -on laminated lava. -on obsidian. +</p> + +<p> +Dree, M., on crystals sinking in lava. +</p> + +<p> +Dufrenoy, M., on the composition of the surface of certain lava-streams. -on +the inclination of tuff-strata. +</p> + +<p> +Eggs of birds embedded at St. Helena. -of turtle at Ascension. +</p> + +<p> +Ejected fragments at Ascension. -at the Galapagos archipelago. +</p> + +<p> +Elevation of St. Helena. -the Galapagos archipelago. -Van Diemen’s Land, +Cape of Good Hope, New Zealand, Australia, and Chatham island. -of volcanic +islands. +</p> + +<p> +Ellis, Rev. W., on ledges within the great crater at Hawaii. -on marine remains +at Otaheite. +</p> + +<p> +Eruption, fissures of. +</p> + +<p> +Extinction of land-shells at St. Helena. +</p> + +<p> +Faraday, Mr., on the expulsion of carbonic acid gas. +</p> + +<p> +Feldspar, fusibility of. -in radiating crystals. -Labrador, ejected. +</p> + +<p> +Feldspathic lavas. -at St. Helena. -rock, alternating with obsidian. +-lamination, and origin of. +</p> + +<p> +Fernando Noronha. +</p> + +<p> +Ferruginous superficial beds. +</p> + +<p> +Fibrous calcareous matter at St. Jago. +</p> + +<p> +Fissures of eruption. +</p> + +<p> +Fitton, Dr., on calcareous breccia. +</p> + +<p> +Flagstaff Hill, St. Helena. +</p> + +<p> +Fleurian de Bellevue on sphaerulites. +</p> + +<p> +Fluidity of lavas. +</p> + +<p> +Forbes, Professor, on the structure of glaciers. +</p> + +<p> +Fragments ejected at Ascension. -at the Galapagos archipelago. +</p> + +<p> +Freshwater Bay. +</p> + +<p> +Fuerteventura (Feurteventura), calcareous beds of. +</p> + +<p> +Galapagos archipelago. -parapets round craters. +</p> + +<p> +Gay Lussac, on the expulsion of carbonic acid gas. +</p> + +<p> +Glaciers, their structure. +</p> + +<p> +Glossiness of texture, origin of. +</p> + +<p> +Gneiss, derived from clay-slate. -with a great embedded fragment. +</p> + +<p> +Gneiss-granite, form of hills of. +</p> + +<p> +Good Hope, Cape of. +</p> + +<p> +Gorges, narrow, at St. Helena. +</p> + +<p> +Granite, junction with clay-slate, at the Cape of Good Hope. +</p> + +<p> +Granitic ejected fragments. +</p> + +<p> +Gravity, specific, of lavas. +</p> + +<p> +Gypsum, at Ascension. -in volcanic strata at St. Helena. -on surface of the +ground at ditto. +</p> + +<p> +Hall, Sir J., on the expulsion of carbonic acid gas. +</p> + +<p> +Heat, action of, on calcareous matter. +</p> + +<p> +Hennah, Mr., on ashes at Ascension. +</p> + +<p> +Henslow, Prof., on chalcedony. +</p> + +<p> +Hoffmann, on decomposed trachyte. +</p> + +<p> +Holland, Dr., on Iceland. +</p> + +<p> +Horner, Mr., on a calcareo-animal substance. -on fusibility of feldspar. +</p> + +<p> +Hubbard, Dr., on dikes. +</p> + +<p> +Humboldt on ejected fragments. -on obsidian formations. -on parapets round +craters. -on sphaerulites. +</p> + +<p> +Hutton on amygdaloids. +</p> + +<p> +Hyalite in decomposed trachyte. +</p> + +<p> +Iceland, stratification of the circumferential hills. +</p> + +<p> +Islands, volcanic, distribution of. -their elevation. +</p> + +<p> +Incrustation, on St. Paul’s rocks. +</p> + +<p> +Incrustations, calcareous, at Ascension. +</p> + +<p> +Jago, St. +</p> + +<p> +James island. +</p> + +<p> +Jasper, origin of. +</p> + +<p> +Jonnes, M. Moreau de, on craters affected by wind. +</p> + +<p> +Juan Fernandez. +</p> + +<p> +Keilhau, M., on granite. +</p> + +<p> +Kicker Rock. +</p> + +<p> +King George’s sound. +</p> + +<p> +Labrador feldspar, ejected. +</p> + +<p> +Lakes at bases of volcanoes. +</p> + +<p> +Lamination of volcanic rocks. +</p> + +<p> +Land-shells, extinct, at St. Helena. +</p> + +<p> +Lanzarote, calcareous beds of. +</p> + +<p> +Lava, adhesion to sides of a gorge. -feldspathic. -with cells +semi-amygdaloidal. +</p> + +<p> +Lavas, specific gravity of. +</p> + +<p> +Lava-streams blending together at St. Jago. -composition of surface of. +-differences in the state of their surfaces. -extreme thinness of. -heaved up +into hillocks at the Galapagos archipelago. -their fluidity. -with irregular +hummocks at Ascension. +</p> + +<p> +Lead, separation from silver. +</p> + +<p> +Lesson, M., on craters at Ascension. +</p> + +<p> +Leucite. +</p> + +<p> +Lime, sulphate of, at Ascension. +</p> + +<p> +Lonsdale, Mr., on fossil-corals from Van Diemen’s land. +</p> + +<p> +Lot, St. Helena. +</p> + +<p> +Lyell, Mr., on craters of elevation. -on embedded turtles’ eggs. -on +glossy coating to dikes. +</p> + +<p> +Macaulay, Dr., on calcareous casts at Madeira. +</p> + +<p> +MacCulloch, Dr., on an amygdaloid. -on chlorophaeite. -on laminated pitchstone. +</p> + +<p> +Mackenzie, Sir G., on cavernous lava-streams. -on glossy coatings to dikes. -on +obsidian streams. -on stratification in Iceland. +</p> + +<p> +Madeira, calcareous casts at. +</p> + +<p> +“Magazine, Nautical,” account of volcanic phenomena in the +Atlantic. +</p> + +<p> +Marekanite. +</p> + +<p> +Mauritius, crater of elevation of. +</p> + +<p> +Mica, in rounded nodules. -origin in metamorphic slate. -radiating form of. +</p> + +<p> +Miller, Prof., on ejected Labrador feldspar. -on quartz crystals in obsidian +beds. +</p> + +<p> +Mitchell, Sir T., on bombs. -on the Australian valleys. +</p> + +<p> +Mud streams at the Galapagos archipelago. +</p> + +<p> +Narborough island. +</p> + +<p> +Nelson, Lieut., on the Bermuda islands. +</p> + +<p> +New Caledonia. +</p> + +<p> +New Red sandstone, cross cleavage of. +</p> + +<p> +New South Wales. +</p> + +<p> +New Zealand. +</p> + +<p> +Nulliporae (fossil), resembling concretions. +</p> + +<p> +Obsidian, absent at the Galapagos archipelago. -bombs of. -composition and +origin of. -crystals of feldspar sink in. -its irruption from lofty craters. +-passage of beds into. -specific gravity of. -streams of. +</p> + +<p> +Olivine decomposed at St. Jago. -at Van Diemen’s land. -in the lavas at +the Galapagos archipelago. +</p> + +<p> +Oolitic structure of recent calcareous beds at St. Helena. +</p> + +<p> +Otaheite. +</p> + +<p> +Oysters, extinction of. +</p> + +<p> +Panza islands, laminated trachyte of. +</p> + +<p> +Pattinson, Mr., on the separation of lead and silver. +</p> + +<p> +Paul’s, St., rocks of. +</p> + +<p> +Pearlstone. +</p> + +<p> +Peperino. +</p> + +<p> +Peron, M., on calcareous rocks of Australia. +</p> + +<p> +Phonolite, hills of. -laminated. -with more fusible hornblende. +</p> + +<p> +Pitchstone. -dikes of. +</p> + +<p> +Plants, extinct. +</p> + +<p> +Plutonic rocks, separation of constituent parts of, by gravity. +</p> + +<p> +Porto Praya. +</p> + +<p> +Prevost, M. C., on rarity of great dislocations in volcanic islands. +</p> + +<p> +Prosperous hill, St. Helena. +</p> + +<p> +Pumice, absent at the Galapagos archipelago. -laminated. +</p> + +<p> +Puy de Dome, trachyte of. +</p> + +<p> +Quail island, St. Jago. +</p> + +<p> +Quartz, crystals of, in beds alternating with obsidian. -crystallised in +sandstone. -fusibility of. -rock, mottled from metamorphic action with earthy +matter. +</p> + +<p> +Red hill. +</p> + +<p> +Resin-like altered scoriae. +</p> + +<p> +Rio de Janeiro, gneiss of. +</p> + +<p> +Robert, M., on strata of Iceland. +</p> + +<p> +Rogers, Professor, on curved lines of elevation. +</p> + +<p> +Salses, compared with tuff craters. +</p> + +<p> +Salt deposited by the sea. -in volcanic strata. -lakes of, in craters. +</p> + +<p> +Sandstone of Brazil. -of the Cape of Good Hope. -platforms of, in New South +Wales. +</p> + +<p> +Schorl, radiating. +</p> + +<p> +Scrope, Mr. P., on laminated trachyte. -on obsidian. -on separation of trachyte +and basalt. -on silex in trachyte. -on sphaerulites. +</p> + +<p> +Seale, Mr., geognosy of St. Helena. -on dikes. -on embedded birds’ bones. +</p> + +<p> +Seale, on extinct shells of St. Helena. +</p> + +<p> +Sedgwick, Professor, on concretions. +</p> + +<p> +Septaria, in concretions in tuff. +</p> + +<p> +Serpulae on upraised rocks. +</p> + +<p> +Seychelles. +</p> + +<p> +Shells, colour of, affected by light. -from Van Diemen’s land. -land, +extinct, at St. Helena. -particles of, drifted by the wind at St. Helena. +</p> + +<p> +Shelly matter deposited by the waves. +</p> + +<p> +Siau, M., on ripples. +</p> + +<p> +Signal Post Hill. +</p> + +<p> +Silica, deposited by steam. -large proportion of, in obsidian. -specific +gravity of. +</p> + +<p> +Siliceous sinter. +</p> + +<p> +Smith, Dr. A., on junction of granite and clay-slate. +</p> + +<p> +Spallanzani on decomposed trachyte. +</p> + +<p> +Specific gravity of recent calcareous rocks and of limestone. -of lavas. +</p> + +<p> +Sphaerulites in glass and in silicified wood. -in obsidian. +</p> + +<p> +Sowerby, Mr. G.B., on fossil-shells from Van Diemen’s land. -from St. +Jago. -land-shells from St. Helena. +</p> + +<p> +St. Helena. -crater of elevation of. +</p> + +<p> +St. Jago, crater of elevation of. -effects of calcareous matter on lava. +</p> + +<p> +St. Paul’s rocks. +</p> + +<p> +Stokes, Mr., collections of sphaerulites and of obsidians. +</p> + +<p> +Stony-top, Little. -Great. +</p> + +<p> +Stratification of sandstone in New South Wales. +</p> + +<p> +Streams of obsidian. +</p> + +<p> +Stutchbury, Mr., on marine remains at Otaheite. +</p> + +<p> +Subsided space at Ascension. +</p> + +<p> +Tahiti. +</p> + +<p> +Talus, stratified, within tuff craters. +</p> + +<p> +Terceira. +</p> + +<p> +Tertiary deposit of St. Jago. +</p> + +<p> +Trachyte, absent at the Galapagos archipelago. -at Ascension. -at Terceira. +-decomposition of, by steam. -its lamination. -its separation from basalt. +-softened at Ascension. -specific gravity of. -with singular veins. +</p> + +<p> +Trap-dikes in the plutonic series. -at King George’s sound. +</p> + +<p> +Travertin at Van Diemen’s land. +</p> + +<p> +Tropic-bird, now rare, at St. Helena. +</p> + +<p> +Tuff, craters of. -their breached state. -peculiar kind of. +</p> + +<p> +Turner, Mr., on the separation of molten metals. +</p> + +<p> +Tyerman and Bennett on marine remains at Huaheine. +</p> + +<p> +Valleys, gorge-like, at St. Helena. -in New South Wales. -in St. Jago. +</p> + +<p> +Van Diemen’s land. +</p> + +<p> +Veins in trachyte. -of jasper. +</p> + +<p> +Vincent, Bory St., on bombs. +</p> + +<p> +Volcanic bombs. -island in process of formation in the Atlantic. -islands, +their distribution. +</p> + +<p> +Wacke, its passage into lava. +</p> + +<p> +Wackes, argillaceous. +</p> + +<p> +Webster, Dr., on a basin-formed island. -on gypsum at Ascension. +</p> + +<p> +White, Martin, on soundings. +</p> + +<p> +Wind, effects of, on the form of craters. +</p> + +</div><!--end chapter--> + +<div style='display:block;margin-top:4em'>*** END OF THE PROJECT GUTENBERG EBOOK VOLCANIC ISLANDS ***</div> +<div style='display:block;margin:1em 0;'>This file should be named 3054-h.htm or 3054-h.zip</div> +<div style='display:block;margin:1em 0;'>This and all associated files of various formats will be found in https://www.gutenberg.org/3/0/5/3054/</div> +<div style='display:block; margin:1em 0'> +Updated editions will replace the previous one—the old editions will +be renamed. +</div> + +<div style='display:block; margin:1em 0'> +Creating the works from print editions not protected by U.S. copyright +law means that no one owns a United States copyright in these works, +so the Foundation (and you!) can copy and distribute it in the United +States without permission and without paying copyright +royalties. 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FOR PUBLIC DOMAIN ETEXTS*Ver.04.07.00*END* + + + + + +This etext was prepared by Sue Asscher <asschers@dingoblue.net.au> + + + + + +VOLCANIC ISLANDS + +BY + +CHARLES DARWIN + + +EDITORIAL NOTE. + +Although in some respects more technical in their subjects and style than +Darwin's "Journal," the books here reprinted will never lose their value +and interest for the originality of the observations they contain. Many +parts of them are admirably adapted for giving an insight into problems +regarding the structure and changes of the earth's surface, and in fact +they form a charming introduction to physical geology and physiography in +their application to special domains. The books themselves cannot be +obtained for many times the price of the present volume, and both the +general reader, who desires to know more of Darwin's work, and the student +of geology, who naturally wishes to know how a master mind reasoned on most +important geological subjects, will be glad of the opportunity of +possessing them in a convenient and cheap form. + +The three introductions, which my friend Professor Judd has kindly +furnished, give critical and historical information which makes this +edition of special value. + +G.T.B. + + + +VOLCANIC ISLANDS. + + +TABLE OF CONTENTS. + + +CRITICAL INTRODUCTION. + + +CHAPTER I.--ST. JAGO, IN THE CAPE DE VERDE ARCHIPELAGO. + +Rocks of the lowest series.--A calcareous sedimentary deposit, with recent +shells, altered by the contact of superincumbent lava, its horizontality +and extent.--Subsequent volcanic eruptions, associated with calcareous +matter in an earthy and fibrous form, and often enclosed within the +separate cells of the scoriae.--Ancient and obliterated orifices of +eruption of small size.--Difficulty of tracing over a bare plain recent +streams of lava.--Inland hills of more ancient volcanic rock.--Decomposed +olivine in large masses.--Feldspathic rocks beneath the upper crystalline +basaltic strata.--Uniform structure and form of the more ancient volcanic +hills.--Form of the valleys near the coast.--Conglomerate now forming on +the sea beach. + + +CHAPTER II.--FERNANDO NORONHA; TERCEIRA; TAHITI, ETC. + +FERNANDO NORONHA.--Precipitous hill of phonolite. + +TERCEIRA.--Trachytic rocks: their singular decomposition by steam of high +temperature. + +TAHITI.--Passage from wacke into trap; singular volcanic rock with the +vesicles half-filled with mesotype. + +MAURITIUS.--Proofs of its recent elevation.--Structure of its more ancient +mountains; similarity with St. Jago. + +ST. PAUL'S ROCKS.--Not of volcanic origin.--Their singular mineralogical +composition. + + +CHAPTER III.--ASCENSION. + +Basaltic lavas.--Numerous craters truncated on the same side.--Singular +structure of volcanic bombs.--Aeriform explosions.--Ejected granite +fragments.--Trachytic rocks.--Singular veins.--Jasper, its manner of +formation.--Concretions in pumiceous tuff.--Calcareous deposits and +frondescent incrustations on the coast.--Remarkable laminated beds, +alternating with, and passing into obsidian.--Origin of obsidian.-- +Lamination of volcanic rocks. + + +CHAPTER IV.--ST. HELENA. + +Lavas of the feldspathic, basaltic, and submarine series.--Section of +Flagstaff Hill and of the Barn.--Dikes.--Turk's Cap and Prosperous Bays.-- +Basaltic ring.--Central crateriform ridge, with an internal ledge and a +parapet.--Cones of phonolite.--Superficial beds of calcareous sandstone.-- +Extinct land-shells.--Beds of detritus.--Elevation of the land.-- +Denudation.--Craters of elevation. + + +CHAPTER V.--GALAPAGOS ARCHIPELAGO. + +Chatham Island.--Craters composed of a peculiar kind of tuff.--Small +basaltic craters, with hollows at their bases.--Albemarle Island; fluid +lavas, their composition.--Craters of tuff; inclination of their exterior +diverging strata, and structure of their interior converging strata.--James +Island, segment of a small basaltic crater; fluidity and composition of its +lava-streams, and of its ejected fragments.--Concluding remarks on the +craters of tuff, and on the breached condition of their southern sides.-- +Mineralogical composition of the rocks of the archipelago.--Elevation of +the land.--Direction of the fissures of eruption. + + +CHAPTER VI.--TRACHYTE AND BASALT.--DISTRIBUTION OF VOLCANIC ISLES. + +The sinking of crystals in fluid lava.--Specific gravity of the constituent +parts of trachyte and of basalt, and their consequent separation.-- +Obsidian.--Apparent non-separation of the elements of plutonic rocks.-- +Origin of trap-dikes in the plutonic series.--Distribution of volcanic +islands; their prevalence in the great oceans.--They are generally arranged +in lines.--The central volcanoes of Von Buch doubtful.--Volcanic islands +bordering continents.--Antiquity of volcanic islands, and their elevation +in mass.--Eruptions on parallel lines of fissure within the same geological +period. + + +CHAPTER VII.--AUSTRALIA; NEW ZEALAND; CAPE OF GOOD HOPE. + +New South Wales.--Sandstone formation.--Embedded pseudo-fragments of +shale.--Stratification.--Current-cleavage.--Great valleys.--Van Diemen's +Land.--Palaeozoic formation.--Newer formation with volcanic rocks.-- +Travertin with leaves of extinct plants.--Elevation of the land.--New +Zealand.--King George's Sound.--Superficial ferruginous beds.--Superficial +calcareous deposits, with casts of branches; its origin from drifted +particles of shells and corals.--Their extent.--Cape of Good Hope.-- +Junction of the granite and clay-slate.--Sandstone formation. + + +INDEX. + + + + +GEOLOGICAL OBSERVATIONS ON VOLCANIC ISLANDS. + + +CRITICAL INTRODUCTION. + +The preparation of the series of works published under the general title +"Geology of the Voyage of the 'Beagle'" occupied a great part of Darwin's +time during the ten years that followed his return to England. The second +volume of the series, entitled "Geological Observations on Volcanic +Islands, with Brief Notices on the Geology of Australia and the Cape of +Good Hope," made its appearance in 1844. The materials for this volume were +collected in part during the outward voyage, when the "Beagle" called at +St. Jago in the Cape de Verde Islands, and St. Paul's Rocks, and at +Fernando Noronha, but mainly during the homeward cruise; then it was that +the Galapagos Islands were surveyed, the Low Archipelago passed through, +and Tahiti visited; after making calls at the Bay of Islands, in New +Zealand, and also at Sydney, Hobart Town and King George's Sound in +Australia, the "Beagle" sailed across the Indian Ocean to the little group +of the Keeling or Cocos Islands, which Darwin has rendered famous by his +observations, and thence to Mauritius; calling at the Cape of Good Hope on +her way, the ship then proceeded successively to St. Helena and Ascension, +and revisited the Cape de Verde Islands before finally reaching England. + +Although Darwin was thus able to gratify his curiosity by visits to a great +number of very interesting volcanic districts, the voyage opened for him +with a bitter disappointment. He had been reading Humboldt's "Personal +Narrative" during his last year's residence in Cambridge, and had copied +out from it long passages about Teneriffe. He was actually making inquiries +as to the best means of visiting that island, when the offer was made to +him to accompany Captain Fitzroy in the "Beagle. " His friend Henslow too, +on parting with him, had given him the advice to procure and read the +recently published first volume of the "Principles of Geology," though he +warned him against accepting the views advocated by its author. During the +time the "Beagle" was beating backwards and forwards when the voyage +commenced, Darwin, although hardly ever able to leave his berth, was +employing all the opportunities which the terrible sea-sickness left him, +in studying Humboldt and Lyell. We may therefore form an idea of his +feelings when, on the ship reaching Santa Cruz, and the Peak of Teneriffe +making its appearance among the clouds, they were suddenly informed that an +outbreak of cholera would prevent any landing! + +Ample compensation for this disappointment was found, however, when the +ship reached Porta Praya in St. Jago, the largest of the Cape de Verde +Islands. Here he spent three most delightful weeks, and really commenced +his work as a geologist and naturalist. Writing to his father he says, +"Geologising in a volcanic country is most delightful; besides the interest +attached to itself, it leads you into most beautiful and retired spots. +Nobody but a person fond of Natural History can imagine the pleasure of +strolling under cocoa-nuts in a thicket of bananas and coffee-plants, and +an endless number of wild flowers. And this island, that has given me so +much instruction and delight, is reckoned the most uninteresting place that +we perhaps shall touch at during our voyage. It certainly is generally very +barren, but the valleys are more exquisitely beautiful, from the very +contrast. It is utterly useless to say anything about the scenery; it would +be as profitable to explain to a blind man colours, as to a person who has +not been out of Europe, the total dissimilarity of a tropical view. +Whenever I enjoy anything, I always look forward to writing it down, either +in my log-book (which increases in bulk), or in a letter; so you must +excuse raptures, and those raptures badly expressed. I find my collections +are increasing wonderfully, and from Rio I think I shall be obliged to send +a cargo home." + +The indelible impression made on Darwin's mind by this first visit to a +volcanic island, is borne witness to by a remarkable passage in the +"Autobiography" written by him in 1876. "The geology of St. Jago is very +striking, yet simple; a stream of lava formerly flowed over the bed of the +sea, formed of triturated recent shells and corals, which it has baked into +a hard white rock. Since then the whole island has been upheaved. But the +line of white rock revealed to me a new and important fact, namely that +there had been afterwards subsidence round the craters which had since been +in action, and had poured forth lava. It then first dawned on me that I +might perhaps write a book on the geology of the various countries visited, +and this made me thrill with delight. That was a memorable hour to me, and +how distinctly I can call to mind the low cliff of lava beneath which I +rested, with the sun glaring hot, a few strange desert plants growing near +and with living corals in the tidal pools at my feet." + +Only five years before, when listening to poor Professor Jameson's lectures +on the effete Wernerianism, which at that time did duty for geological +teaching, Darwin had found them "incredibly dull," and he declared that +"the sole effect they produced on me was a determination never so long as I +lived to read a book on Geology, or in any way to study the science." + +What a contrast we find in the expressions which he makes use of in +referring to Geological Science, in his letters written home from the +"Beagle!" After alluding to the delight of collecting and studying marine +animals, he exclaims, "But Geology carries the day!" Writing to Henslow he +says, "I am quite charmed with Geology, but, like the wise animal between +two bundles of hay, I do not know which to like best; the old crystalline +group of rocks, or the softer and more fossiliferous beds." And just as the +long voyage is about to come to a close he again writes, "I find in Geology +a never-failing interest; as it has been remarked, it creates the same +grand ideas respecting this world which Astronomy does for the Universe." +In this passage Darwin doubtless refers to a remark of Sir John Herschel's +in his admirable "Preliminary Discourse on the Study of Natural +Philosophy,"--a book which exercised a most remarkable and beneficial +influence on the mind of the young naturalist. + +If there cannot be any doubt as to the strong predilection in Darwin's mind +for geological studies, both during and after the memorable voyage, there +is equally little difficulty in perceiving the school of geological thought +which, in spite of the warnings of Sedgwick and Henslow, had obtained +complete ascendancy over his mind. He writes in 1876: "The very first place +which I examined, namely St. Jago in the Cape de Verde Islands, showed me +clearly the wonderful superiority of Lyell's manner of treating Geology, +compared with that of any other author, whose works I had with me, or ever +afterwards read." And again, "The science of Geology is enormously indebted +to Lyell--more so, as I believe, than to any other man who ever lived...I +am proud to remember that the first place, namely, St. Jago, in the Cape de +Verde Archipelago, in which I geologised, convinced me of the infinite +superiority of Lyell's views over those advocated in any other work known +to me." + +The passages I have cited will serve to show the spirit in which Darwin +entered upon his geological studies, and the perusal of the following pages +will furnish abundant proofs of the enthusiasm, acumen, and caution with +which his researches were pursued. + +Large collections of rocks and minerals were made by Darwin during his +researches, and sent home to Cambridge, to be kept under the care of his +faithful friend Henslow. After visiting his relations and friends, Darwin's +first care on his return to England was to unpack and examine these +collections. He accordingly, at the end of 1836, took lodgings for three +months in Fitzwilliam Street, Cambridge, so as to be near Henslow; and in +studying and determining his geological specimens received much valuable +aid from the eminent crystallographer and mineralogist, Professor William +Hallows Miller. + +The actual writing of the volume upon volcanic islands was not commenced +till 1843, when Darwin had settled in the spot which became his home for +the rest of his life--the famous house at Down, in Kent. Writing to his +friend Mr. Fox, on March 28th, 1843, he says, "I am very slowly progressing +with a volume, or rather pamphlet, on the volcanic islands which we +visited: I manage only a couple of hours per day, and that not very +regularly. It is uphill work writing books, which cost money in publishing, +and which are not read even by geologists." + +The work occupied Darwin during the whole of the year 1843, and was issued +in the spring of the following year, the actual time engaged in preparing +it being recorded in his diary as "from the summer of 1842 to January +1844;" but the author does not appear to have been by any means satisfied +with the result when the book was finished. He wrote to Lyell, "You have +pleased me much by saying that you intend looking through my 'Volcanic +Islands;' it cost me eighteen months!!! and I have heard of very few who +have read it. Now I shall feel, whatever little (and little it is) there is +confirmatory of old work, or new, will work its effect and not be lost." To +Sir Joseph Hooker he wrote, "I have just finished a little volume on the +volcanic islands which we visited. I do not know how far you care for dry +simple geology, but I hope you will let me send you a copy." + +Every geologist knows how full of interest and suggestiveness is this book +of Darwin's on volcanic islands. 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 when, +hammer in hand, he roamed over new and interesting scenes, and the slow, +laborious, and less congenial task of re-writing and arranging his notes in +book-form. + +In 1874, in writing an account of the ancient volcanoes of the Hebrides, I +had frequent occasion to quote Mr. Darwin's observations on the Atlantic +volcanoes, in illustration of the phenomena exhibited by the relics of +still older volcanoes in our own islands. Darwin, in writing to his old +friend Sir Charles Lyell upon the subject, says, "I was not a little +pleased to see my volcanic book quoted, for I thought it was completely +dead and forgotten." + +Two years later the original publishers of this book and of that on South +America proposed to re-issue them. Darwin at first hesitated, for he seemed +to think there could be little of abiding interest in them; he consulted me +upon the subject in one of the conversations which I used to have with him +at that time, and I strongly urged upon him the reprint of the works. I was +much gratified when he gave way upon the point, and consented to their +appearing just as originally issued. In his preface he says, "Owing to the +great progress which Geology has made in recent times, my views on some few +points may be somewhat antiquated, but I have thought it best to leave them +as they originally appeared." + +It may be interesting to indicate, as briefly as possible, the chief +geological problem upon which the publication of Darwin's "Volcanic +Islands" threw new and important light. The merit of the work consisted in +supplying interesting observations, which in some cases have proved of +crucial value in exploding prevalent fallacies; in calling attention to +phenomena and considerations that had been quite overlooked by geologists, +but have since exercised an important influence in moulding geological +speculation; and lastly in showing the importance which attaches to small +and seemingly insignificant causes, some of which afford a key to the +explanation of very curious geological problems. + +Visiting as he did the districts in which Von Buch and others had found +what they thought to be evidence of the truth of "Elevation-craters," +Darwin was able to show that the facts were capable of a totally different +interpretation. The views originally put forward by the old German +geologist and traveller, and almost universally accepted by his countrymen, +had met with much support from Elie de Beaumont and Dufrenoy, the leaders +of geological thought in France. They were, however, stoutly opposed by +Scrope and Lyell in this country, and by Constant Prevost and Virlet on the +other side of the channel. Darwin, in the work before us, 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 which, opening at the top, have given origin to the craters. +Admitting the influence of the injection of lava into the structure of the +volcanic cones, in increasing their bulk and elevation, he shows that, in +the main, the volcanoes are built up by repeated ejections causing an +accumulation of materials around the vent. + +While, however, agreeing on the whole with Scrope and Lyell, as to the +explosive origin of ordinary volcanic craters, Darwin clearly saw that, in +some cases, great craters might be formed or enlarged, by the subsidence of +the floors after eruptions. The importance of this agency, to which too +little attention has been directed by geologists, has recently been shown +by Professor Dana, in his admirable work on Kilauea and the other great +volcanoes of the Hawaiian Archipelago. + +The effects of subsidence at a volcanic centre in producing a downward dip +of the strata around it, was first pointed out by Darwin, as the result of +his earliest work in the Cape de Verde Islands. Striking illustrations of +the same principle have since been pointed out by M. Robert and others in +Iceland, by Mr. Heaphy in New Zealand, and by myself in the Western Isles +of Scotland. + +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 crust, along which the +lavas have made their way to the surface. But he, at the same time, clearly +saw that there was no evidence of the occurrence of great deluges of lava +along such fissures; he showed how the most remarkable plateaux, composed +of successive lava sheets, might be built up by repeated and moderate +ejections from numerous isolated vents; 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 +denudation. + +One of the most striking parts of the book is that in which he deals with +the effects of denudation in producing "basal wrecks" or worn down stumps +of volcanoes. He was enabled to examine a series of cases in which could be +traced every gradation, from perfect volcanic cones down to the solidified +plugs which had consolidated in the vents from which ejections had taken +place. 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 history. Like Lyell, +he was firmly persuaded of the continuity of geological history, and ever +delighted in finding indications, in the present order of nature, that the +phenomena of the past could be accounted for by means of causes which are +still in operation. Lyell's last work in the field was carried on about his +home in Forfarshire, and only a few months before his death he wrote to +Darwin: "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 for, by the enormous time to which the products of the +oldest volcanoes have been subjected to chemical changes." + +Darwin was greatly impressed, as the result of his studies of volcanic +phenomena, followed by an examination of the great granite-masses of the +Andes, with the relations between the so-called Plutonic rocks and those of +undoubtedly volcanic origin. It was indeed a fortunate circumstance, that +after studying some excellent examples of recent volcanic rocks, he +proceeded to examine in South America many fine illustrations of the older +igneous rock-masses, and especially of the most highly crystalline types of +the same, and then on his way home had opportunities of reviving the +impression made upon him by the fresh and unaltered volcanic rocks. Some of +the general considerations suggested by these observations were discussed +in a paper read by him before the Geological Society, on March 7th, 1838, +under the title "On the Connection of Certain Volcanic Phenomena, and On +the Formation of Mountain-chains, and the Effect of Continental +Elevations." The exact bearing of these two classes of facts upon one +another are more fully discussed in his book on South American geology. + +The proofs of recent elevation around many of the volcanic islands led +Darwin to conclude that volcanic areas were, as a rule, regions in which +upward movements were taking place, and he was naturally led to contrast +them with the areas in which, as he showed, the occurrence of atolls, +encircling reefs, and barrier-reefs afford indication of subsidence. In +this way he was able to map out the oceanic areas in different zones, along +which opposite kinds of movement were taking place. His conclusions on this +subject were full of novelty and suggestiveness. + +Very clearly did Darwin recognise the importance of the fact that most of +the oceanic islands appear to be of volcanic origin, though he was careful +to point out the remarkable exceptions which somewhat invalidate the +generalisation. In his "Origin of Species" he has elaborated the idea and +suggested the theory of the permanence of ocean-basins, a suggestion which +has been adopted and pushed farther by subsequent authors, than we think +its originator would have approved. 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 him. + +Some years before the voyage of the "Beagle," Mr. Poulett Scrope had +pointed out the remarkable analogies that exist between certain igneous +rocks of banded structure, as seen in the Ponza Islands, and the foliated +crystalline schists. It does not appear that Darwin was acquainted with +this remarkable memoir, 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 Ascension. Coming fresh from the study of the great masses of +crystalline schist in the South American continent, 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 "folia." +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 stresses, a separation +and parallel arrangement of the constituent minerals will result. This is a +process which is now fully recognised as having been a potent factor in the +production of the metamorphic rock, and has been called by more recent +writers "dynamo-metamorphism." + +In this, and in many similar discussions, in which exact mineralogical +knowledge was required, 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-lens, supplemented by a few chemical tests and +the constant use of the blowpipe. Since his day, the method of study of +rocks by thin sections under the microscope has been devised, and has +become a most efficient aid in all petrographical inquiries. During the +voyage of H.M.S. "Challenger," many of the islands studied by Darwin have +been revisited and their rocks collected. 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 +"Reports on the 'Challenger' Expedition." While much that is new and +valuable has been contributed to geological science by these more recent +investigations, and many changes have been made in nomenclature and other +points of detail, 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 study, and remain as a monument of the acumen and accuracy +in minute observation of these pioneers in geological research. + +JOHN W. JUDD. + + + +CHAPTER I.--ST. JAGO, IN THE CAPE DE VERDE ARCHIPELAGO. + +Rocks of the lowest series. +A calcareous sedimentary deposit, with recent shells, altered by the +contact of superincumbent lava, its horizontality and extent. +Subsequent volcanic eruptions, associated with calcareous matter in an +earthy and fibrous form, and often enclosed within the separate cells of +the scoriae. +Ancient and obliterated orifices of eruption of small size. +Difficulty of tracing over a bare plain recent streams of lava. +Inland hills of more ancient volcanic rock. +Decomposed olivine in large masses. +Feldspathic rocks beneath the upper crystalline basaltic strata. +Uniform structure and form of the more ancient volcanic hills. +Form of the valleys near the coast. +Conglomerate now forming on the sea beach. + +(FIGURE 1: MAP 1: PART OF ST. JAGO, ONE OF THE CAPE DE VERDE ISLANDS.) + +The island of St. Jago extends in a N.N.W. and S.S.E. direction, thirty +miles in length by about twelve in breadth. My observations, made during +two visits, were confined to the southern portion within the distance of a +few leagues from Porto Praya. The country, viewed from the sea, presents a +varied outline: smooth conical hills of a reddish colour (like Red Hill in +Figure 1 (Map 1). (The outline of the coast, the position of the villages, +streamlets, and of most of the hills in this woodcut, are copied from the +chart made on board H.M.S. "Leven." The square-topped hills (A, B, C, etc.) +are put in merely by eye, to illustrate my description.)), and others less +regular, flat-topped, and of a blackish colour (like A, B, C,) rise from +successive, step-formed plains of lava. At a distance, a chain of +mountains, many thousand feet in height, traverses the interior of the +island. There is no active volcano in St. Jago, and only one in the group, +namely at Fogo. The island since being inhabited has not suffered from +destructive earthquakes. + +The lowest rocks exposed on the coast near Porto Praya, are highly +crystalline and compact; they appear to be of ancient, submarine, volcanic +origin; they are unconformably covered by a thin, irregular, calcareous +deposit, abounding with shells of a late tertiary period; and this again is +capped by a wide sheet of basaltic lava, which has flowed in successive +streams from the interior of the island, between the square-topped hills +marked A, B, C, etc. Still more recent streams of lava have been erupted +from the scattered cones, such as Red and Signal Post Hills. The upper +strata of the square-topped hills are intimately related in mineralogical +composition, and in other respects, with the lowest series of the coast- +rocks, with which they seem to be continuous. + +MINERALOGICAL DESCRIPTION OF THE ROCKS OF THE LOWEST SERIES. + +These rocks possess an extremely varying character; they consist of black, +brown, and grey, compact, basaltic bases, with numerous crystals of augite, +hornblende, olivine, mica, and sometimes glassy feldspar. A common variety +is almost entirely composed of crystals of augite with olivine. Mica, it is +known, seldom occurs where augite abounds; nor probably does the present +case offer a real exception, for the mica (at least in my best +characterised specimen, in which one nodule of this mineral is nearly half +an inch in length) is as perfectly rounded as a pebble in a conglomerate, +and evidently has not been crystallised in the base, in which it is now +enclosed, but has proceeded from the fusion of some pre-existing rock. +These compact lavas alternate with tuffs, amygdaloids, and wacke, and in +some places with coarse conglomerate. Some of the argillaceous wackes are +of a dark green colour, others, pale yellowish-green, and others nearly +white; I was surprised to find that some of the latter varieties, even +where whitest, fused into a jet black enamel, whilst some of the green +varieties afforded only a pale gray bead. Numerous dikes, consisting +chiefly of highly compact augitic rocks, and of gray amygdaloidal +varieties, intersect the strata, which have in several places been +dislocated with considerable violence, and thrown into highly inclined +positions. One line of disturbance crosses the northern end of Quail Island +(an islet in the Bay of Porto Praya), and can be followed to the mainland. +These disturbances took place before the deposition of the recent +sedimentary bed; and the surface, also, had previously been denuded to a +great extent, as is shown by many truncated dikes. + +DESCRIPTION OF THE CALCAREOUS DEPOSIT OVERLYING THE FOREGOING VOLCANIC +ROCKS. + +This stratum is very conspicuous from its white colour, and from the +extreme regularity with which it ranges in a horizontal line for some miles +along the coast. Its average height above the sea, measured from the upper +line of junction with the superincumbent basaltic lava, is about sixty +feet; and its thickness, although varying much from the inequalities of the +underlying formation, may be estimated at about twenty feet. It consists of +quite white calcareous matter, partly composed of organic debris, and +partly of a substance which may be aptly compared in appearance with +mortar. Fragments of rock and pebbles are scattered throughout this bed, +often forming, especially in the lower part, a conglomerate. Many of the +fragments of rock are whitewashed with a thin coating of calcareous matter. +At Quail Island, the calcareous deposit is replaced in its lowest part by a +soft, brown, earthy tuff, full of Turritellae; this is covered by a bed of +pebbles, passing into sandstone, and mixed with fragments of echini, claws +of crabs, and shells; the oyster-shells still adhering to the rock on which +they grew. Numerous white balls appearing like pisolitic concretions, from +the size of a walnut to that of an apple, are embedded in this deposit; +they usually have a small pebble in their centres. Although so like +concretions, a close examination convinced me that they were Nulliporae, +retaining their proper forms, but with their surfaces slightly abraded: +these bodies (plants as they are now generally considered to be) exhibit +under a microscope of ordinary power, no traces of organisation in their +internal structure. Mr. George R. Sowerby has been so good as to examine +the shells which I collected: there are fourteen species in a sufficiently +perfect condition for their characters to be made out with some degree of +certainty, and four which can be referred only to their genera. Of the +fourteen shells, of which a list is given in the Appendix, eleven are +recent species; one, though undescribed, is perhaps identical with a +species which I found living in the harbour of Porto Praya; the two +remaining species are unknown, and have been described by Mr. Sowerby. +Until the shells of this Archipelago and of the neighbouring coasts are +better known, it would be rash to assert that even these two latter shells +are extinct. The number of species which certainly belong to existing +kinds, although few in number, are sufficient to show that the deposit +belongs to a late tertiary period. From its mineralogical character, from +the number and size of the embedded fragments, and from the abundance of +Patellae, and other littoral shells, it is evident that the whole was +accumulated in a shallow sea, near an ancient coast-line. + +EFFECTS PRODUCED BY THE FLOWING OF THE SUPERINCUMBENT BASALTIC LAVA OVER +THE CALCAREOUS DEPOSIT. + +These effects are very curious. The calcareous matter is altered to the +depth of about a foot beneath the line of junction; and a most perfect +gradation can be traced, from loosely aggregated, small, particles of +shells, corallines, and Nulliporae, into a rock, in which not a trace of +mechanical origin can be discovered, even with a microscope. Where the +metamorphic change has been greatest, two varieties occur. The first is a +hard, compact, white, fine-grained rock, striped with a few parallel lines +of black volcanic particles, and resembling a sandstone, but which, upon +close examination, is seen to be crystallised throughout, with the +cleavages so perfect that they can be readily measured by the reflecting +goniometer. In specimens, where the change has been less complete, when +moistened and examined under a strong lens, the most interesting gradation +can be traced, some of the rounded particles retaining their proper forms, +and others insensibly melting into the granulo-crystalline paste. The +weathered surface of this stone, as is so frequently the case with ordinary +limestones, assumes a brick-red colour. + +The second metamorphosed variety is likewise a hard rock, but without any +crystalline structure. It consists of a white, opaque, compact, calcareous +stone, thickly mottled with rounded, though regular, spots of a soft, +earthy, ochraceous substance. This earthy matter is of a pale yellowish- +brown colour, and appears to be a mixture of carbonate of lime with iron; +it effervesces with acids, is infusible, but blackens under the blowpipe, +and becomes magnetic. The rounded form of the minute patches of earthy +substance, and the steps in the progress of their perfect formation, which +can be followed in a suit of specimens, clearly show that they are due +either to some power of aggregation in the earthy particles amongst +themselves, or more probably to a strong attraction between the atoms of +the carbonate of line, and consequently to the segregation of the earthy +extraneous matter. I was much interested by this fact, because I have often +seen quartz rocks (for instance, in the Falkland Islands, and in the lower +Silurian strata of the Stiper-stones in Shropshire), mottled in a precisely +analogous manner, with little spots of a white, earthy substance (earthy +feldspar?); and these rocks, there was good reason to suppose, had +undergone the action of heat,--a view which thus receives confirmation. +This spotted structure may possibly afford some indication in +distinguishing those formations of quartz, which owe their present +structure to igneous action, from those produced by the agency of water +alone; a source of doubt, which I should think from my own experience, that +most geologists, when examining arenaceo-quartzose districts must have +experienced. + +The lowest and most scoriaceous part of the lava, in rolling over the +sedimentary deposit at the bottom of the sea, has caught up large +quantities of calcareous matter, which now forms a snow-white, highly +crystalline basis to a breccia, including small pieces of black, glossy +scoriae. A little above this, where the lime is less abundant, and the lava +more compact, numerous little balls, composed of spicula of calcareous +spar, radiating from common centres, occupy the interstices. In one part of +Quail Island, the lime has thus been crystallised by the heat of the +superincumbent lava, where it is only thirteen feet in thickness; nor had +the lava been originally thicker, and since reduced by degradation, as +could be told from the degree of cellularity of its surface. I have already +observed that the sea must have been shallow in which the calcareous +deposit was accumulated. In this case, therefore, the carbonic acid gas has +been retained under a pressure, insignificant compared with that (a column +of water, 1,708 feet in height) originally supposed by Sir James Hall to be +requisite for this end: but since his experiments, it has been discovered +that pressure has less to do with the retention of carbonic acid gas, than +the nature of the circumjacent atmosphere; and hence, as is stated to be +the case by Mr. Faraday, masses of limestone are sometimes fused and +crystallised even in common limekilns. (I am much indebted to Mr. E.W. +Brayley in having given me the following references to papers on this +subject: Faraday in the "Edinburgh New Philosophical Journal" volume 15 +page 398; Gay-Lussac in "Annales de Chem. et Phys." tome 63 page 219 +translated in the "London and Edinburgh Philosophical Magazine" volume 10 +page 496.) Carbonate of lime can be heated to almost any degree, according +to Faraday, in an atmosphere of carbonic acid gas, without being +decomposed; and Gay-Lussac found that fragments of limestone, placed in a +tube and heated to a degree, not sufficient by itself to cause their +decomposition, yet immediately evolved their carbonic acid, when a stream +of common air or steam was passed over them: Gay-Lussac attributes this to +the mechanical displacement of the nascent carbonic acid gas. The +calcareous matter beneath the lava, and especially that forming the +crystalline spicula between the interstices of the scoriae, although heated +in an atmosphere probably composed chiefly of steam, could not have been +subjected to the effects of a passing stream; and hence it is, perhaps, +that they have retained their carbonic acid, under a small amount of +pressure. + +The fragments of scoriae, embedded in the crystalline calcareous basis, are +of a jet black colour, with a glossy fracture like pitchstone. Their +surfaces, however, are coated with a layer of a reddish-orange, translucent +substance, which can easily be scratched with a knife; hence they appear as +if overlaid by a thin layer of rosin. Some of the smaller fragments are +partially changed throughout into this substance: a change which appears +quite different from ordinary decomposition. At the Galapagos Archipelago +(as will be described in a future chapter), great beds are formed of +volcanic ashes and particles of scoriae, which have undergone a closely +similar change. + +THE EXTENT AND HORIZONTALITY OF THE CALCAREOUS STRATUM. + +(FIGURE 2: SIGNAL POST HILL. (Section with A low and C high.) + +A.--Ancient volcanic rocks. + +B.--Calcareous stratum. + +C.--Upper basaltic lava.) + +The upper line of surface of the calcareous stratum, which is so +conspicuous from being quite white and so nearly horizontal, ranges for +miles along the coast, at the height of about sixty feet above the sea. The +sheet of basalt, by which it is capped, is on an average eighty feet in +thickness. Westward of Porto Praya beyond Red Hill, the white stratum with +the superincumbent basalt is covered up by more recent streams. Northward +of Signal Post Hill, I could follow it with my eye, trending away for +several miles along the sea cliffs. The distance thus observed is about +seven miles; but I cannot doubt from its regularity that it extends much +farther. In some ravines at right angles to the coast, it is seen gently +dipping towards the sea, probably with the same inclination as when +deposited round the ancient shores of the island. I found only one inland +section, namely, at the base of the hill marked A, where, at the height of +some hundred feet, this bed was exposed; it here rested on the usual +compact augitic rock associated with wacke, and was covered by the +widespread sheet of modern basaltic lava. Some exceptions occur to the +horizontality of the white stratum: at Quail Island, its upper surface is +only forty feet above the level of the sea; here also the capping of lava +is only between twelve and fifteen feet in thickness; on the other hand, at +the north-east side of Porto Praya harbour, the calcareous stratum, as well +as the rock on which it rests, attain a height above the average level: the +inequality of level in these two cases is not, as I believe, owing to +unequal elevation, but to original irregularities at the bottom of the sea. +Of this fact, at Quail Island, there was clear evidence in the calcareous +deposit being in one part of much greater than the average thickness, and +in another part being entirely absent; in this latter case, the modern +basaltic lavas rested directly on those of more ancient origin. + +Under Signal Post Hill, the white stratum dips into the sea in a remarkable +manner. This hill is conical, 450 feet in height, and retains some traces +of having had a crateriform structure; it is composed chiefly of matter +erupted posteriorly to the elevation of the great basaltic plain, but +partly of lava of apparently submarine origin and of considerable +antiquity. The surrounding plain, as well as the eastern flank of this +hill, has been worn into steep precipices, overhanging the sea. In these +precipices, the white calcareous stratum may be seen, at the height of +about seventy feet above the beach, running for some miles both northward +and southward of the hill, in a line appearing to be perfectly horizontal; +but for a space of a quarter of a mile directly under the hill, it dips +into the sea and disappears. On the south side the dip is gradual, on the +north side it is more abrupt, as is shown in Figure 2. As neither the +calcareous stratum, nor the superincumbent basaltic lava (as far as the +latter can be distinguished from the more modern ejections), appears to +thicken as it dips, I infer that these strata were not originally +accumulated in a trough, the centre of which afterwards became a point of +eruption; but that they have subsequently been disturbed and bent. We may +suppose either that Signal Post Hill subsided after its elevation with the +surrounding country, or that it never was uplifted to the same height with +it. This latter seems to me the most probable alternative, for during the +slow and equable elevation of this portion of the island, the subterranean +motive power, from expending part of its force in repeatedly erupting +volcanic matter from beneath this point, would, it is likely, have less +force to uplift it. Something of the same kind seems to have occurred near +Red Hill, for when tracing upwards the naked streams of lava from near +Porto Praya towards the interior of the island, I was strongly induced to +suspect, that since the lava had flowed, the slope of the land had been +slightly modified, either by a small subsidence near Red Hill, or by that +portion of the plain having been uplifted to a less height during the +elevation of the whole area. + +THE BASALTIC LAVA, SUPERINCUMBENT ON THE CALCAREOUS DEPOSIT. + +This lava is of a pale grey colour, fusing into a black enamel; its +fracture is rather earthy and concretionary; it contains olivine in small +grains. The central parts of the mass are compact, or at most crenulated +with a few minute cavities, and are often columnar. At Quail Island this +structure was assumed in a striking manner; the lava in one part being +divided into horizontal laminae, which became in another part split by +vertical fissures into five-sided plates; and these again, being piled on +each other, insensibly became soldered together, forming fine symmetrical +columns. The lower surface of the lava is vesicular, but sometimes only to +the thickness of a few inches; the upper surface, which is likewise +vesicular, is divided into balls, frequently as much as three feet in +diameter, made up of concentric layers. The mass is composed of more than +one stream; its total thickness being, on an average, about eighty feet: +the lower portion has certainly flowed beneath the sea, and probably +likewise the upper portion. The chief part of this lava has flowed from the +central districts, between the hills marked A, B, C, etc., in the woodcut- +map. The surface of the country, near the coast, is level and barren; +towards the interior, the land rises by successive terraces, of which four, +when viewed from a distance, could be distinctly counted. + +VOLCANIC ERUPTIONS SUBSEQUENT TO THE ELEVATION OF THE COASTLAND; THE +EJECTED MATTER ASSOCIATED WITH EARTHY LIME. + +These recent lavas have proceeded from those scattered, conical, reddish- +coloured hills, which rise abruptly from the plain-country near the coast. +I ascended some of them, but will describe only one, namely, RED HILL, +which may serve as a type of its class, and is remarkable in some especial +respects. Its height is about six hundred feet; it is composed of bright +red, highly scoriaceous rock of a basaltic nature; on one side of its +summit there is a hollow, probably the last remnant of a crater. Several of +the other hills of this class, judging from their external forms, are +surmounted by much more perfect craters. When sailing along the coast, it +was evident that a considerable body of lava had flowed from Red Hill, over +a line of cliff about one hundred and twenty feet in height, into the sea: +this line of cliff is continuous with that forming the coast, and bounding +the plain on both sides of this hill; these streams, therefore, were +erupted, after the formation of the coast-cliffs, from Red Hill, when it +must have stood, as it now does, above the level of the sea. This +conclusion accords with the highly scoriaceous condition of all the rock on +it, appearing to be of subaerial formation: and this is important, as there +are some beds of calcareous matter near its summit, which might, at a hasty +glance, have been mistaken for a submarine deposit. These beds consist of +white, earthy, carbonate of lime, extremely friable so as to be crushed +with the least pressure; the most compact specimens not resisting the +strength of the fingers. Some of the masses are as white as quicklime, and +appear absolutely pure; but on examining them with a lens, minute particles +of scoriae can always be seen, and I could find none which, when dissolved +in acids, did not leave a residue of this nature. It is, moreover, +difficult to find a particle of the lime which does not change colour under +the blowpipe, most of them even becoming glazed. The scoriaceous fragments +and the calcareous matter are associated in the most irregular manner, +sometimes in obscure beds, but more generally as a confused breccia, the +lime in some parts and the scoriae in others being most abundant. Sir H. De +la Beche has been so kind as to have some of the purest specimens analysed, +with a view to discover, considering their volcanic origin, whether they +contained much magnesia; but only a small portion was found, such as is +present in most limestones. + +Fragments of the scoriae embedded in the calcareous mass, when broken, +exhibit many of their cells lined and partly filled with a white, delicate, +excessively fragile, moss-like, or rather conferva-like, reticulation of +carbonate of lime. These fibres, examined under a lens of one-tenth of an +inch focal distance, appear cylindrical; they are rather above one- +thousandth of an inch in diameter; they are either simply branched, or more +commonly united into an irregular mass of network, with the meshes of very +unequal sizes and of unequal numbers of sides. Some of the fibres are +thickly covered with extremely minute spicula, occasionally aggregated into +little tuffs; and hence they have a hairy appearance. These spicula are of +the same diameter throughout their length; they are easily detached, so +that the object-glass of the microscope soon becomes scattered over with +them. Within the cells of many fragments of the scoria, the lime exhibits +this fibrous structure, but generally in a less perfect degree. These cells +do not appear to be connected with one another. There can be no doubt, as +will presently be shown, that the lime was erupted, mingled with the lava +in its fluid state, and therefore I have thought it worth while to describe +minutely this curious fibrous structure, of which I know nothing analogous. +From the earthy condition of the fibres, this structure does not appear to +be related to crystallisation. + +Other fragments of the scoriaceous rock from this hill, when broken, are +often seen marked with short and irregular white streaks, which are owing +to a row of separate cells being partly, or quite, filled with white +calcareous powder. This structure immediately reminded me of the appearance +in badly kneaded dough, of balls and drawn-out streaks of flour, which have +remained unmixed with the paste; and I cannot doubt that small masses of +the lime, in the same manner remaining unmixed with the fluid lava, have +been drawn out when the whole was in motion. I carefully examined, by +trituration and solution in acids, pieces of the scoriae, taken from within +half-an-inch of those cells which were filled with the calcareous powder, +and they did not contain an atom of free lime. It is obvious that the lava +and lime have on a large scale been very imperfectly mingled; and where +small portions of the lime have been entangled within a piece of the viscid +lava, the cause of their now occupying, in the form of a powder or of a +fibrous reticulation, the vesicular cavities, is, I think, evidently due to +the confined gases having most readily expanded at the points where the +incoherent lime rendered the lava less adhesive. + +A mile eastward of the town of Praya, there is a steep-sided gorge, about +one hundred and fifty yards in width, cutting through the basaltic plain +and underlying beds, but since filled up by a stream of more modern lava. +This lava is dark grey, and in most parts compact and rudely columnar; but +at a little distance from the coast, it includes in an irregular manner a +brecciated mass of red scoriae mingled with a considerable quantity of +white, friable, and in some parts, nearly pure earthy lime, like that on +the summit of Red Hill. This lava, with its entangled lime, has certainly +flowed in the form of a regular stream; and, judging from the shape of the +gorge, towards which the drainage of the country (feeble though it now be) +still is directed, and from the appearance of the bed of loose water-worn +blocks with their interstices unfilled, like those in the bed of a torrent, +on which the lava rests, we may conclude that the stream was of subaerial +origin. I was unable to trace it to its source, but, from its direction, it +seemed to have come from Signal Post Hill, distant one mile and a quarter, +which, like Red Hill, has been a point of eruption subsequent to the +elevation of the great basaltic plain. It accords with this view, that I +found on Signal Post Hill, a mass of earthy, calcareous matter of the same +nature, mingled with scoriae. I may here observe that part of the +calcareous matter forming the horizontal sedimentary bed, especially the +finer matter with which the embedded fragments of rock are whitewashed, has +probably been derived from similar volcanic eruptions, as well as from +triturated organic remains: the underlying, ancient, crystalline rocks, +also, are associated with much carbonate of lime, filling amygdaloidal +cavities, and forming irregular masses, the nature of which latter I was +unable to understand. + +Considering the abundance of earthy lime near the summit of Red Hill, a +volcanic cone six hundred feet in height, of subaerial growth,--considering +the intimate manner in which minute particles and large masses of scoriae +are embedded in the masses of nearly pure lime, and on the other hand, the +manner in which small kernels and streaks of the calcareous powder are +included in solid pieces of the scoriae,--considering, also, the similar +occurrence of lime and scoriae within a stream of lava, also supposed, with +good reason, to have been of modern subaerial origin, and to have flowed +from a hill, where earthy lime also occurs: I think, considering these +facts, there can be no doubt that the lime has been erupted, mingled with +the molten lava. I am not aware that any similar case has been described: +it appears to me an interesting one, inasmuch as most geologists must have +speculated on the probable effects of a volcanic focus, bursting through +deep-seated beds of different mineralogical composition. The great +abundance of free silex in the trachytes of some countries (as described by +Beudant in Hungary, and by P. Scrope in the Panza Islands), perhaps solves +the inquiry with respect to deep-seated beds of quartz; and we probably +here see it answered, where the volcanic action has invaded subjacent +masses of limestone. One is naturally led to conjecture in what state the +now earthy carbonate of lime existed, when ejected with the intensely +heated lava: from the extreme cellularity of the scoriae on Red Hill, the +pressure cannot have been great, and as most volcanic eruptions are +accompanied by the emission of large quantities of steam and other gases, +we here have the most favourable conditions, according to the views at +present entertained by chemists, for the expulsion of the carbonic acid. +(Whilst deep beneath the surface, the carbonate of lime was, I presume, in +a fluid state. Hutton, it is known, thought that all amygdaloids were +produced by drops of molten limestone floating in the trap, like oil in +water: this no doubt is erroneous, but if the matter forming the summit of +Red Hill had been cooled under the pressure of a moderately deep sea, or +within the walls of a dike, we should, in all probability, have had a trap +rock associated with large masses of compact, crystalline, calcareous spar, +which, according to the views entertained by many geologists, would have +been wrongly attributed to subsequent infiltration.) Has the slow re- +absorption of this gas, it may be asked, given to the lime in the cells of +the lava, that peculiar fibrous structure, like that of an efflorescing +salt? Finally, I may remark on the great contrast in appearance between +this earthy lime, which must have been heated in a free atmosphere of steam +and other gases, while the white, crystalline, calcareous spar, produced by +a single thin sheet of lava (as at Quail Island) rolling over similar +earthy lime and the debris of organic remains, at the bottom of a shallow +sea. + +SIGNAL POST HILL. + +This hill has already been several times mentioned, especially with +reference to the remarkable manner in which the white calcareous stratum, +in other parts so horizontal (Figure 2), dips under it into the sea. It has +a broad summit, with obscure traces of a crateriform structure, and is +composed of basaltic rocks (Of these, one common variety is remarkable for +being full of small fragments of a dark jasper-red earthy mineral, which, +when examined carefully, shows an indistinct cleavage; the little fragments +are elongated in form, are soft, are magnetic before and after being +heated, and fuse with difficulty into a dull enamel. This mineral is +evidently closely related to the oxides of iron, but I cannot ascertain +what it exactly is. The rock containing this mineral is crenulated with +small angular cavities, which are lined and filled with yellowish crystals +of carbonate of lime.), some compact, others highly cellular with inclined +beds of loose scoriae, of which some are associated with earthy lime. Like +Red Hill, it has been the source of eruptions, subsequently to the +elevation of the surrounding basaltic plain; but unlike that hill, it has +undergone considerable denudation, and has been the seat of volcanic action +at a remote period, when beneath the sea. I judge of this latter +circumstance from finding on its inland flank the last remains of three +small points of eruption. These points are composed of glossy scoriae, +cemented by crystalline calcareous spar, exactly like the great submarine +calcareous deposit, where the heated lava has rolled over it: their +demolished state can, I think, be explained only by the denuding action of +the waves of the sea. I was guided to the first orifice by observing a +sheet of lava, about two hundred yards square, with steepish sides, +superimposed on the basaltic plain with no adjoining hillock, whence it +could have been erupted; and the only trace of a crater which I was able to +discover, consisted of some inclined beds of scoriae at one of its corners. +At the distance of fifty yards from a second level-topped patch of lava, +but of much smaller size, I found an irregular circular group of masses of +cemented, scoriaceous breccia, about six feet in height, which doubtless +had once formed the point of eruption. The third orifice is now marked only +by an irregular circle of cemented scoriae, about four yards in diameter, +and rising in its highest point scarcely three feet above the level of the +plain, the surface of which, close all round, exhibits its usual +appearance: here we have a horizontal basal section of a volcanic spiracle, +which, together with all its ejected matter, has been almost totally +obliterated. + +The stream of lava, which fills the narrow gorge eastward of the town of +Praya, judging from its course, seems, as before remarked, to have come +from Signal Post Hill, and to have flowed over the plain, after its +elevation (The sides of this gorge, where the upper basaltic stratum is +intersected, are almost perpendicular. The lava, which has since filled it +up, is attached to these sides, almost as firmly as a dike is to its walls. +In most cases, where a stream of lava has flowed down a valley, it is +bounded on each side by loose scoriaceous masses.): the same observation +applies to a stream (possibly part of the same one) capping the sea cliffs, +a little eastward of the gorge. When I endeavoured to follow these streams +over the stony level plain, which is almost destitute of soil and +vegetation, I was much surprised to find, that although composed of hard +basaltic matter, and not having been exposed to marine denudation, all +distant traces of them soon became utterly lost. But I have since observed +at the Galapagos Archipelago, that it is often impossible to follow even +great deluges of quite recent lava across older streams, except by the size +of the bushes growing on them, or by the comparative states of glossiness +of their surfaces,--characters which a short lapse of time would be +sufficient quite to obscure. I may remark, that in a level country, with a +dry climate, and with the wind blowing always in one direction (as at the +Cape de Verde Archipelago), the effects of atmospheric degradation are +probably much greater than would at first be expected; for soil in this +case accumulates only in a few protected hollows, and being blown in one +direction, it is always travelling towards the sea in the form of the +finest dust, leaving the surface of the rocks bare, and exposed to the full +effects of renewed meteoric action. + +INLAND HILLS OF MORE ANCIENT VOLCANIC ROCKS. + +These hills are laid down by eye, and marked as A, B, C, etc., in Map 1. +They are related in mineralogical composition, and are probably directly +continuous with the lowest rocks exposed on the coast. These hills, viewed +from a distance, appear as if they had once formed part of an irregular +tableland, and from their corresponding structure and composition this +probably has been the case. They have flat, slightly inclined summits, and +are, on an average, about six hundred feet in height; they present their +steepest slope towards the interior of the island, from which point they +radiate outwards, and are separated from each other by broad and deep +valleys, through which the great streams of lava, forming the coast-plains, +have descended. Their inner and steeper escarpments are ranged in an +irregular curve, which rudely follows the line of the shore, two or three +miles inland from it. I ascended a few of these hills, and from others, +which I was able to examine with a telescope, I obtained specimens, through +the kindness of Mr. Kent, the assistant-surgeon of the "Beagle"; although +by these means I am acquainted with only a part of the range, five or six +miles in length, yet I scarcely hesitate, from their uniform structure, to +affirm that they are parts of one great formation, stretching round much of +the circumference of the island. + +The upper and lower strata of these hills differ greatly in composition. +The upper are basaltic, generally compact, but sometimes scoriaceous and +amygdaloidal, with associated masses of wacke: where the basalt is compact, +it is either fine-grained or very coarsely crystallised; in the latter case +it passes into an augitic rock, containing much olivine; the olivine is +either colourless, or of the usual yellow and dull reddish shades. On some +of the hills, beds of calcareous matter, both in an earthy and in a +crystalline form, including fragments of glossy scoriae, are associated +with the basaltic strata. These strata differ from the streams of basaltic +lava forming the coast-plains, only in being more compact, and in the +crystals of augite, and in the grains of olivine being of much greater +size;--characters which, together with the appearance of the associated +calcareous beds, induce me to believe that they are of submarine formation. + +Some considerable masses of wacke, which are associated with these basaltic +strata, and which likewise occur in the basal series on the coast, +especially at Quail Island, are curious. They consist of a pale yellowish- +green argillaceous substance, of a crumbling texture when dry, but unctuous +when moist: in its purest form, it is of a beautiful green tint, with +translucent edges, and occasionally with obscure traces of an original +cleavage. Under the blowpipe it fuses very readily into a dark grey, and +sometimes even black bead, which is slightly magnetic. From these +characters, I naturally thought that it was one of the pale species, +decomposed, of the genus augite;--a conclusion supported by the unaltered +rock being full of large separate crystals of black augite, and of balls +and irregular streaks of dark grey augitic rock. As the basalt ordinarily +consists of augite, and of olivine often tarnished and of a dull red +colour, I was led to examine the stages of decomposition of this latter +mineral, and I found, to my surprise, that I could trace a nearly perfect +gradation from unaltered olivine to the green wacke. Part of the same grain +under the blowpipe would in some instances behave like olivine, its colour +being only slightly changed, and part would give a black magnetic bead. +Hence I can have no doubt that the greenish wacke originally existed as +olivine; but great chemical changes must have been effected during the act +of decomposition thus to have altered a very hard, transparent, infusible +mineral, into a soft, unctuous, easily melted, argillaceous substance. +(D'Aubuisson "Traite de Geognosie" tome 2 page 569 mentions, on the +authority of M. Marcel de Serres, masses of green earth near Montpellier, +which are supposed to be due to the decomposition of olivine. I do not, +however, find, that the action of this mineral under the blowpipe being +entirely altered, as it becomes decomposed, has been noticed; and the +knowledge of this fact is important, as at first it appears highly +improbable that a hard, transparent, refractory mineral should be changed +into a soft, easily fused clay, like this of St. Jago. I shall hereafter +describe a green substance, forming threads within the cells of some +vesicular basaltic rocks in Van Diemen's Land, which behave under the +blowpipe like the green wacke of St. Jago; but its occurrence in +cylindrical threads, shows it cannot have resulted from the decomposition +of olivine, a mineral always existing in the form of grains or crystals.) + +The basal strata of these hills, as well as some neighbouring, separate, +bare, rounded hillocks, consist of compact, fine-grained, non-crystalline +(or so slightly as scarcely to be perceptible), ferruginous, feldspathic +rocks, and generally in a state of semi-decomposition. Their fracture is +exceedingly irregular, and splintery; yet small fragments are often very +tough. They contain much ferruginous matter, either in the form of minute +grains with a metallic lustre, or of brown hair-like threads: the rock in +this latter case assuming a pseudo-brecciated structure. These rocks +sometimes contain mica and veins of agate. Their rusty brown or yellowish +colour is partly due to the oxides of iron, but chiefly to innumerable, +microscopically minute, black specks, which, when a fragment is heated, are +easily fused, and evidently are either hornblende or augite. These rocks, +therefore, although at first appearing like baked clay or some altered +sedimentary deposit, contain all the essential ingredients of trachyte; +from which they differ only in not being harsh, and in not containing +crystals of glassy feldspar. As is so often the case with trachytic +formation, no stratification is here apparent. A person would not readily +believe that these rocks could have flowed as lava; yet at St. Helena there +are well-characterised streams (as will be described in an ensuing chapter) +of nearly similar composition. Amidst the hillocks composed of these rocks, +I found in three places, smooth conical hills of phonolite, abounding with +fine crystals of glassy feldspar, and with needles of hornblende. These +cones of phonolite, I believe, bear the same relation to the surrounding +feldspathic strata which some masses of coarsely crystallised augitic rock, +in another part of the island, bear to the surrounding basalt, namely, that +both have been injected. The rocks of a feldspathic nature being anterior +in origin to the basaltic strata, which cap them, as well as to the +basaltic streams of the coast-plains, accords with the usual order of +succession of these two grand divisions of the volcanic series. + +The strata of most of these hills in the upper part, where alone the planes +of division are distinguishable, are inclined at a small angle from the +interior of the island towards the sea-coast. The inclination is not the +same in each hill; in that marked A it is less than in B, D, or E; in C the +strata are scarcely deflected from a horizontal plane, and in F (as far as +I could judge without ascending it) they are slightly inclined in a reverse +direction, that is, inwards and towards the centre of the island. +Notwithstanding these differences of inclination, their correspondence in +external form, and in the composition both of their upper and lower parts,- +-their relative position in one curved line, with their steepest sides +turned inwards,--all seem to show that they originally formed parts of one +platform; which platform, as before remarked, probably extended round a +considerable portion of the circumference of the island. The upper strata +certainly flowed as lava, and probably beneath the sea, as perhaps did the +lower feldspathic masses: how then come these strata to hold their present +position, and whence were they erupted? + +In the centre of the island there are lofty mountains, but they are +separated from the steep inland flanks of these hills by a wide space of +lower country: the interior mountains, moreover, seem to have been the +source of those great streams of basaltic lava which, contracting as they +pass between the bases of the hills in question, expand into the coast- +plains. (I saw very little of the inland parts of the island. Near the +village of St. Domingo, there are magnificent cliffs of rather coarsely +crystallised basaltic lava. Following the little stream in this valley, +about a mile above the village, the base of the great cliff was formed of a +compact fine-grained basalt, conformably covered by a bed of pebbles. Near +Fuentes, I met with pap-formed hills of the compact feldspathic series of +rocks.) Round the shores of St. Helena there is a rudely formed ring of +basaltic rocks, and at Mauritius there are remnants of another such a ring +round part, if not round the whole, of the island; here again the same +question immediately occurs, how came these masses to hold their present +position, and whence were they erupted? The same answer, whatever it may +be, probably applies in these three cases; and in a future chapter we shall +recur to this subject. + +VALLEYS NEAR THE COAST. + +These are broad, very flat, and generally bounded by low cliff-formed +sides. Portions of the basaltic plain are sometimes nearly or quite +isolated by them; of which fact, the space on which the town of Praya +stands offers an instance. The great valley west of the town has its bottom +filled up to a depth of more than twenty feet by well-rounded pebbles, +which in some parts are firmly cemented together by white calcareous +matter. There can be no doubt, from the form of these valleys, that they +were scooped out by the waves of the sea, during that equable elevation of +the land, of which the horizontal calcareous deposit, with its existing +species of marine remains, gives evidence. Considering how well shells have +been preserved in this stratum, it is singular that I could not find even a +single small fragment of shell in the conglomerate at the bottom of the +valleys. The bed of pebbles in the valley west of the town is intersected +by a second valley joining it as a tributary, but even this valley appears +much too wide and flat-bottomed to have been formed by the small quantity +of water, which falls only during one short wet season; for at other times +of the year these valleys are absolutely dry. + +RECENT CONGLOMERATE. + +On the shores of Quail Island, I found fragments of brick, bolts of iron, +pebbles, and large fragments of basalt, united by a scanty base of impure +calcareous matter into a firm conglomerate. To show how exceedingly firm +this recent conglomerate is, I may mention, that I endeavoured with a heavy +geological hammer to knock out a thick bolt of iron, which was embedded a +little above low-water mark, but was quite unable to succeed. + + +CHAPTER II.--FERNANDO NORONHA; TERCEIRA; TAHITI, ETC. + +FERNANDO NORONHA. +Precipitous hill of phonolite. + +TERCEIRA. +Trachytic rocks: their singular decomposition by steam of high temperature. + +TAHITI. +Passage from wacke into trap; singular volcanic rock with the vesicles +half-filled with mesotype. + +MAURITIUS. +Proofs of its recent elevation. +Structure of its more ancient mountains; similarity with St. Jago. + +ST. PAUL'S ROCKS. +Not of volcanic origin. +Their singular mineralogical composition. + + +FERNANDO NORONHA. + +During our short visit at this and the four following islands, I observed +very little worthy of description. Fernando Noronha is situated in the +Atlantic Ocean, in latitude 3 degrees 50 minutes S., and 230 miles distant +from the coast of South America. It consists of several islets, together +nine miles in length by three in breadth. The whole seems to be of volcanic +origin; although there is no appearance of any crater, or of any one +central eminence. The most remarkable feature is a hill 1,000 feet high, of +which the upper 400 feet consist of a precipitous, singularly shaped +pinnacle, formed of columnar phonolite, containing numerous crystals of +glassy feldspar, and a few needles of hornblende. From the highest +accessible point of this hill, I could distinguish in different parts of +the group several other conical hills, apparently of the same nature. At +St. Helena there are similar, great, conical, protuberant masses of +phonolite, nearly one thousand feet in height, which have been formed by +the injection of fluid feldspathic lava into yielding strata. If this hill +has had, as is probable, a similar origin, denudation has been here +effected on an enormous scale. Near the base of this hill, I observed beds +of white tuff, intersected by numerous dikes, some of amygdaloidal basalt +and others of trachyte; and beds of slaty phonolite with the planes of +cleavage directed N.W. and S.E. Parts of this rock, where the crystals were +scanty, closely resembled common clay-slate, altered by the contact of a +trap-dike. The lamination of rocks, which undoubtedly have once been fluid, +appears to me a subject well deserving attention. On the beach there were +numerous fragments of compact basalt, of which rock a distant facade of +columns seemed to be formed. + +TERCEIRA IN THE AZORES. + +The central parts of this island consist of irregularly rounded mountains +of no great elevation, composed of trachyte, which closely resembles in +general character the trachyte of Ascension, presently to be described. +This formation is in many parts overlaid, in the usual order of +superposition, by streams of basaltic lava, which near the coast compose +nearly the whole surface. The course which these streams have followed from +their craters, can often be followed by the eye. The town of Angra is +overlooked by a crateriform hill (Mount Brazil), entirely built of thin +strata of fine-grained, harsh, brown-coloured tuff. The upper beds are seen +to overlap the basaltic streams on which the town stands. This hill is +almost identical in structure and composition with numerous crateriformed +hills in the Galapagos Archipelago. + +EFFECTS OF STEAM ON THE TRACHYTIC ROCKS. + +In the central part of the island there is a spot, where steam is +constantly issuing in jets from the bottom of a small ravine-like hollow, +which has no exit, and which abuts against a range of trachytic mountains. +The steam is emitted from several irregular fissures: it is scentless, soon +blackens iron, and is of much too high temperature to be endured by the +hand. The manner in which the solid trachyte is changed on the borders of +these orifices is curious: first, the base becomes earthy, with red +freckles evidently due to the oxidation of particles of iron; then it +becomes soft; and lastly, even the crystals of glassy feldspar yield to the +dissolving agent. After the mass is converted into clay, the oxide of iron +seems to be entirely removed from some parts, which are left perfectly +white, whilst in other neighbouring parts, which are of the brightest red +colour, it seems to be deposited in greater quantity; some other masses are +marbled with two distinct colours. Portions of the white clay, now that +they are dry, cannot be distinguished by the eye from the finest prepared +chalk; and when placed between the teeth they feel equally soft-grained; +the inhabitants use this substance for white-washing their houses. The +cause of the iron being dissolved in one part, and close by being again +deposited, is obscure; but the fact has been observed in several other +places. (Spallanzani, Dolomieu, and Hoffman have described similar cases in +the Italian volcanic islands. Dolomieu says the iron at the Panza Islands +is redeposited in the form of veins (page 86 "Memoire sur les Isles +Ponces"). These authors likewise believe that the steam deposits silica: it +is now experimentally known that vapour of a high temperature is able to +dissolve silica.) In some half-decayed specimens, I found small, globular +aggregations of yellow hyalite, resembling gum-arabic, which no doubt had +been deposited by the steam. + +As there is no escape for the rain-water, which trickles down the sides of +the ravine-like hollow, whence the steam issues, it must all percolate +downwards through the fissures at its bottom. Some of the inhabitants +informed me that it was on record that flames (some luminous appearance?) +had originally proceeded from these cracks, and that the flames had been +succeeded by the steam; but I was not able to ascertain how long this was +ago, or anything certain on the subject. When viewing the spot, I imagined +that the injection of a large mass of rock. like the cone of phonolite at +Fernando Noronha, in a semi-fluid state, by arching the surface might have +caused a wedge-shaped hollow with cracks at the bottom, and that the rain- +water percolating to the neighbourhood of the heated mass, would during +many succeeding years be driven back in the form of steam. + +TAHITI (OTAHEITE). + +I visited only a part of the north-western side of this island, and this +part is entirely composed of volcanic rocks. Near the coast there are +several varieties of basalt, some abounding with large crystals of augite +and tarnished olivine, others compact and earthy,--some slightly vesicular, +and others occasionally amygdaloidal. These rocks are generally much +decomposed, and to my surprise, I found in several sections that it was +impossible to distinguish, even approximately, the line of separation +between the decayed lava and the alternating beds of tuff. Since the +specimens have become dry, it is rather more easy to distinguish the +decomposed igneous rocks from the sedimentary tuffs. This gradation in +character between rocks having such widely different origins, may I think +be explained by the yielding under pressure of the softened sides of the +vesicular cavities, which in many volcanic rocks occupy a large proportion +of their bulk. As the vesicles generally increase in size and number in the +upper parts of a stream of lava, so would the effects of their compression +increase; the yielding, moreover, of each lower vesicle must tend to +disturb all the softened matter above it. Hence we might expect to trace a +perfect gradation from an unaltered crystalline rock to one in which all +the particles (although originally forming part of the same solid mass) had +undergone mechanical displacement; and such particles could hardly be +distinguished from others of similar composition, which had been deposited +as sediment. As lavas are sometimes laminated in their upper parts even +horizontal lines, appearing like those of aqueous deposition, could not in +all cases be relied on as a criterion of sedimentary origin. From these +considerations it is not surprising that formerly many geologists believed +in real transitions from aqueous deposits, through wacke, into igneous +traps. + +In the valley of Tia-auru, the commonest rocks are basalts with much +olivine, and in some cases almost composed of large crystals of augite. I +picked up some specimens, with much glassy feldspar, approaching in +character to trachyte. There were also many large blocks of vesicular +basalt, with the cavities beautifully lined with chabasie (?), and +radiating bundles of mesotype. Some of these specimens presented a curious +appearance, owing to a number of the vesicles being half filled up with a +white, soft, earthy mesotypic mineral, which intumesced under the blowpipe +in a remarkable manner. As the upper surfaces in all the half-filled cells +are exactly parallel, it is evident that this substance has sunk to the +bottom of each cell from its weight. Sometimes, however, it entirely fills +the cells. Other cells are either quite filled, or lined, with small +crystals, apparently of chabasie; these crystals, also, frequently line the +upper half of the cells partly filled with the earthy mineral, as well as +the upper surface of this substance itself, in which case the two minerals +appear to blend into each other. I have never seen any other amygdaloid +with the cells half filled in the manner here described; and it is +difficult to imagine the causes which determined the earthy mineral to sink +from its gravity to the bottom of the cells, and the crystalline mineral to +adhere in a coating of equal thickness round the sides of the cells. +(MacCulloch, however, has described and given a plate of ("Geolog. Trans." +1st series volume 4 page 225) a trap rock, with cavities filled up +horizontally with quartz and chalcedony. The upper halves of these cavities +are often filled by layers, which follow each irregularity of the surface, +and by little depending stalactites of the same siliceous substances.) + +The basic strata on the sides of the valley are gently inclined seaward, +and I nowhere observed any sign of disturbance; the strata are separated +from each other by thick, compact beds of conglomerate, in which the +fragments are large, some being rounded, but most angular. From the +character of these beds, from the compact and crystalline condition of most +of the lavas, and from the nature of the infiltrated minerals, I was led to +conjecture that they had originally flowed beneath the sea. This conclusion +agrees with the fact that the Rev. W. Ellis found marine remains at a +considerable height, which he believes were interstratified with volcanic +matter; as is likewise described to be the case by Messrs. Tyerman and +Bennett at Huaheine, an island in this same archipelago. Mr. Stutchbury +also discovered near the summit of one of the loftiest mountains of Tahiti, +at the height of several thousand feet, a stratum of semi-fossil coral. +None of these remains have been specifically examined. On the coast, where +masses of coral-rock would have afforded the clearest evidence, I looked in +vain for any signs of recent elevation. For references to the above +authorities, and for more detailed reasons for not believing that Tahiti +has been recently elevated, I must refer to the "Structure and Distribution +of Coral-Reefs." + +MAURITIUS. + +Approaching this island on the northern or north-western side, a curved +chain of bold mountains, surmounted by rugged pinnacles, is seen to rise +from a smooth border of cultivated land, which gently slopes down to the +coast. At the first glance, one is tempted to believe that the sea lately +reached the base of these mountains, and upon examination, this view, at +least with respect to the inferior parts of the border, is found to be +perfectly correct. Several authors have described masses of upraised coral- +rock round the greater part of the circumference of the island. (Captain +Carmichael, in Hooker's "Bot. Misc." volume 2 page 301. Captain Lloyd has +lately, in the "Proceedings of the Geological Society" (volume 3 page 317), +described carefully some of these masses. In the "Voyage a l'Isle de +France, par un Officier du Roi," many interesting facts are given on this +subject. Consult also "Voyage aux Quatre Isles d'Afrique, par M. Bory St. +Vincent.") Between Tamarin Bay and the Great Black River I observed, in +company with Captain Lloyd, two hillocks of coral-rock, formed in their +lower part of hard calcareous sandstone, and in their upper of great +blocks, slightly aggregated, of Astraea and Madrepora, and of fragments of +basalt; they were divided into beds dipping seaward, in one case at an +angle of 8 degrees, and in the other at 18 degrees; they had a water-worn +appearance, and they rose abruptly from a smooth surface, strewed with +rolled debris of organic remains, to a height of about twenty feet. The +Officier du Roi, in his most interesting tour in 1768 round the island, has +described masses of upraised coral-rocks, still retaining that moat-like +structure (see my "Coral Reefs") which is characteristic of the living +reefs. On the coast northward of Port Louis, I found the lava concealed for +a considerable space inland by a conglomerate of corals and shells, like +those on the beach, but in parts consolidated by red ferruginous matter. M. +Bory St. Vincent has described similar calcareous beds over nearly the +whole of the plain of Pamplemousses. Near Port Louis, when turning over +some large stones, which lay in the bed of a stream at the head of a +protected creek, and at the height of some yards above the level of spring +tides, I found several shells of serpula still adhering to their under +sides. + +The jagged mountains near Port Louis rise to a height of between two and +three thousand feet; they consist of strata of basalt, obscurely separated +from each other by firmly aggregated beds of fragmentary matter; and they +are intersected by a few vertical dikes. The basalt in some parts abounds +with large crystals of augite and olivine, and is generally compact. The +interior of the island forms a plain, raised probably about a thousand feet +above the level of the sea, and composed of streams of lava which have +flowed round and between the rugged basaltic mountains. These more recent +lavas are also basaltic, but less compact, and some of them abound with +feldspar, so that they even fuse into a pale coloured glass. On the banks +of the Great River, a section is exposed nearly five hundred feet deep, +worn through numerous thin sheets of the lava of this series, which are +separated from each other by beds of scoriae. They seem to have been of +subaerial formation, and to have flowed from several points of eruption on +the central platform, of which the Piton du Milieu is said to be the +principal one. There are also several volcanic cones, apparently of this +modern period, round the circumference of the island, especially at the +northern end, where they form separate islets. + +The mountains composed of the more compact and crystalline basalt, form the +main skeleton of the island. M. Bailly ("Voyage aux Terres Australes" tome +1 page 54.) states that they all "se developpent autour d'elle comme une +ceinture d'immenses remparts, toutes affectant une pente plus ou moins +enclinee vers le rivage de la mer; tandis, au contraire, que vers le centre +de l'ile elles presentent une coupe abrupte, et souvent taillee a pic. +Toutes ces montagnes sont formees de couches paralleles inclinees du centre +de l'ile vers la mer." These statements have been disputed, though not in +detail, by M. Quoy, in the voyage of Freycinet. As far as my limited means +of observation went, I found them perfectly correct. (M. Lesson, in his +account of this island, in the "Voyage of the 'Coquille'," seems to follow +M. Bailly's views.) The mountains on the N.W. side of the island, which I +examined, namely, La Pouce, Peter Botts, Corps de Garde, Les Mamelles, and +apparently another farther southward, have precisely the external shape and +stratification described by M. Bailly. They form about a quarter of his +girdle of ramparts. Although these mountains now stand quite detached, +being separated from each other by breaches, even several miles in width, +through which deluges of lava have flowed from the interior of the island; +nevertheless, seeing their close general similarity, one must feel +convinced that they originally formed parts of one continuous mass. Judging +from the beautiful map of the Mauritius, published by the Admiralty from a +French MS., there is a range of mountains (M. Bamboo) on the opposite side +of the island, which correspond in height, relative position, and external +form, with those just described. Whether the girdle was ever complete may +well be doubted; but from M. Bailly's statements, and my own observations, +it may be safely concluded that mountains with precipitous inland flanks, +and composed of strata dipping outwards, once extended round a considerable +portion of the circumference of the island. The ring appears to have been +oval and of vast size; its shorter axis, measured across from the inner +sides of the mountains near Port Louis and those near Grand Port, being no +less than thirteen geographical miles in length. M. Bailly boldly supposes +that this enormous gulf, which has since been filled up to a great extent +by streams of modern lava, was formed by the sinking in of the whole upper +part of one great volcano. + +It is singular in how many respects those portions of St. Jago and of +Mauritius which I visited agree in their geological history. At both +islands, mountains of similar external form, stratification, and (at least +in their upper beds) composition, follow in a curved chain the coast-line. +These mountains in each case appear originally to have formed parts of one +continuous mass. The basaltic strata of which they are composed, from their +compact and crystalline structure, seem, when contrasted with the +neighbouring basaltic streams of subaerial formation, to have flowed +beneath the pressure of the sea, and to have been subsequently elevated. We +may suppose that the wide breaches between the mountains were in both cases +worn by the waves, during their gradual elevation--of which process, within +recent times, there is abundant evidence on the coast-land of both islands. +At both, vast streams of more recent basaltic lavas have flowed from the +interior of the island, round and between the ancient basaltic hills; at +both, moreover, recent cones of eruption are scattered around the +circumference of the island; but at neither have eruptions taken place +within the period of history. As remarked in the last chapter, it is +probable that these ancient basaltic mountains, which resemble (at least in +many respects) the basal and disturbed remnants of two gigantic volcanoes, +owe their present form, structure, and position, to the action of similar +causes. + +ST. PAUL'S ROCKS. + +This small island is situated in the Atlantic Ocean, nearly one degree +north of the equator, and 540 miles distant from South America, in 29 +degrees 15 minutes west longitude. Its highest point is scarcely fifty feet +above the level of the sea; its outline is irregular, and its entire +circumference barely three-quarters of a mile. This little point of rock +rises abruptly out of the ocean; and, except on its western side, soundings +were not obtained, even at the short distance of a quarter of a mile from +its shore. It is not of volcanic origin; and this circumstance, which is +the most remarkable point in its history (as will hereafter be referred +to), properly ought to exclude it from the present volume. It is composed +of rocks, unlike any which I have met with, and which I cannot characterise +by any name, and must therefore describe. + +The simplest, and one of the most abundant kinds, is a very compact, heavy, +greenish-black rock, having an angular, irregular fracture, with some +points just hard enough to scratch glass, and infusible. This variety +passes into others of paler green tints, less hard, but with a more +crystalline fracture, and translucent on their edges; and these are fusible +into a green enamel. Several other varieties are chiefly characterised by +containing innumerable threads of dark-green serpentine, and by having +calcareous matter in their interstices. These rocks have an obscure, +concretionary structure, and are full of variously coloured angular pseudo +fragments. These angular pseudo fragments consist of the first-described +dark green rock, of a brown softer kind, of serpentine, and of a yellowish +harsh stone, which, perhaps, is related to serpentine rock. There are other +vesicular, calcareo-ferruginous, soft stones. There is no distinct +stratification, but parts are imperfectly laminated; and the whole abounds +with innumerable veins, and vein-like masses, both small and large. Of +these vein-like masses, some calcareous ones, which contain minute +fragments of shells, are clearly of subsequent origin to the others. + +A GLOSSY INCRUSTATION. + +Extensive portions of these rocks are coated by a layer of a glossy +polished substance, with a pearly lustre and of a greyish white colour; it +follows all the inequalities of the surface, to which it is firmly +attached. When examined with a lens, it is found to consist of numerous +exceedingly thin layers, their aggregate thickness being about the tenth of +an inch. It is considerably harder than calcareous spar, but can be +scratched with a knife; under the blowpipe it scales off, decrepitates, +slightly blackens, emits a fetid odour, and becomes strongly alkaline: it +does not effervesce in acids. (In my "Journal" I have described this +substance; I then believed that it was an impure phosphate of lime.) I +presume this substance has been deposited by water draining from the birds' +dung, with which the rocks are covered. At Ascension, near a cavity in the +rocks which was filled with a laminated mass of infiltrated birds' dung, I +found some irregularly formed, stalactitical masses of apparently the same +nature. These masses, when broken, had an earthy texture; but on their +outsides, and especially at their extremities, they were formed of a pearly +substance, generally in little globules, like the enamel of teeth, but more +translucent, and so hard as just to scratch plate-glass. This substance +slightly blackens under the blowpipe, emits a bad smell, then becomes quite +white, swelling a little, and fuses into a dull white enamel; it does not +become alkaline; nor does it effervesce in acids. The whole mass had a +collapsed appearance, as if in the formation of the hard glossy crust the +whole had shrunk much. At the Abrolhos Islands on the coast of Brazil, +where also there is much birds' dung, I found a great quantity of a brown, +arborescent substance adhering to some trap-rock. In its arborescent form, +this substance singularly resembles some of the branched species of +Nullipora. Under the blowpipe, it behaves like the specimens from +Ascension; but it is less hard and glossy, and the surface has not the +shrunk appearance. + + +CHAPTER III.--ASCENSION. + +Basaltic lavas. +Numerous craters truncated on the same side. +Singular structure of volcanic bombs. +Aeriform explosions. +Ejected granitic fragments. +Trachytic rocks. +Singular veins. +Jasper, its manner of formation. +Concretions in pumiceous tuff. +Calcareous deposits and frondescent incrustations on the coast. +Remarkable laminated beds, alternating with, and passing into, obsidian. +Origin of obsidian. +Lamination of volcanic rocks. + +(MAP 2: THE ISLAND OF ASCENSION.) + +This island is situated in the Atlantic Ocean, in latitude 8 degrees S., +longitude 14 degrees W. It has the form of an irregular triangle (see Map +2), each side being about six miles in length. Its highest point is 2,870 +feet ("Geographical Journal" volume 5 page 243.) above the level of the +sea. The whole is volcanic, and, from the absence of proofs to the +contrary, I believe of subaerial origin. The fundamental rock is everywhere +of a pale colour, generally compact, and of a feldspathic nature. In the +S.E. portion of the island, where the highest land is situated, well +characterised trachyte, and other congenerous rocks of that varying family, +occur. Nearly the entire circumference is covered up by black and rugged +streams of basaltic lava, with here and there a hill or single point of +rock (one of which near the sea-coast, north of the Fort, is only two or +three yards across) of the trachyte still remaining exposed. + +BASALTIC ROCKS. + +The overlying basaltic lava is in some parts extremely vesicular, in others +little so; it is of a black colour, but sometimes contains crystals of +glassy feldspar, and seldom much olivine. These streams appear to have +possessed singularly little fluidity; their side walls and lower ends being +very steep, and even as much as between twenty and thirty feet in height. +Their surface is extraordinarily rugged, and from a short distance appears +as if studded with small craters. These projections consist of broad, +irregularly conical, hillocks, traversed by fissures, and composed of the +same unequally scoriaceous basalt with the surrounding streams, but having +an obscure tendency to a columnar structure; they rise to a height between +ten and thirty feet above the general surface, and have been formed, as I +presume, by the heaping up of the viscid lava at points of greater +resistance. At the base of several of these hillocks, and occasionally +likewise on more level parts, solid ribs, composed of angulo-globular +masses of basalt, resembling in size and outline arched sewers or gutters +of brickwork, but not being hollow, project between two or three feet above +the surface of the streams; what their origin may have been, I do not know. +Many of the superficial fragments from these basaltic streams present +singularly convoluted forms; and some specimens could hardly be +distinguished from logs of dark-coloured wood without their bark. + +Many of the basaltic streams can be traced, either to points of eruption at +the base of the great central mass of trachyte, or to separate, conical, +red-coloured hills, which are scattered over the northern and western +borders of the island. Standing on the central eminence, I counted between +twenty and thirty of these cones of eruption. The greater number of them +had their truncated summits cut off obliquely, and they all sloped towards +the S.E., whence the trade-wind blows. (M. Lesson in the "Zoology of the +Voyage of the 'Coquille'" page 490 has observed this fact. Mr. Hennah +("Geolog. Proceedings" 1835 page 189) further remarks that the most +extensive beds of ashes at Ascension invariably occur on the leeward side +of the island.) This structure no doubt has been caused by the ejected +fragments and ashes being always blown, during eruptions, in greater +quantity towards one side than towards the other. M. Moreau de Jonnes has +made a similar observation with respect to the volcanic orifices in the +West Indian Islands. + +VOLCANIC BOMBS. + +(FIGURE 3: FRAGMENT OF A SPHERICAL VOLCANIC BOMB, with the interior parts +coarsely cellular, coated by a concentric layer of compact lava, and this +again by a crust of finely cellular rock. + +FIGURE 4: VOLCANIC BOMB OF OBSIDIAN FROM AUSTRALIA. The upper figure gives +a front view; the lower a side view of the same object.) + +These occur in great numbers strewed on the ground, and some of them lie at +considerable distances from any points of eruption. They vary in size from +that of an apple to that of a man's body; they are either spherical or +pear-shaped, or with the hinder part (corresponding to the tail of a comet) +irregular, studded with projecting points, and even concave. Their surfaces +are rough, and fissured with branching cracks; their internal structure is +either irregularly scoriaceous and compact, or it presents a symmetrical +and very curious appearance. An irregular segment of a bomb of this latter +kind, of which I found several, is accurately represented in Figure 3. Its +size was about that of a man's head. The whole interior is coarsely +cellular; the cells averaging in diameter about the tenth of an inch; but +nearer the outside they gradually decrease in size. This part is succeeded +by a well-defined shell of compact lava, having a nearly uniform thickness +of about the third of an inch; and the shell is overlaid by a somewhat +thicker coating of finely cellular lava (the cells varying from the +fiftieth to the hundredth of an inch in diameter), which forms the external +surface: the line separating the shell of compact lava from the outer +scoriaceous crust is distinctly defined. This structure is very simply +explained, if we suppose a mass of viscid, scoriaceous matter, to be +projected with a rapid, rotatory motion through the air; for whilst the +external crust, from cooling, became solidified (in the state we now see +it), the centrifugal force, by relieving the pressure in the interior parts +of the bomb, would allow the heated vapours to expand their cells; but +these being driven by the same force against the already-hardened crust, +would become, the nearer they were to this part, smaller and smaller or +less expanded, until they became packed into a solid, concentric shell. As +we know that chips from a grindstone (Nichol "Architecture of the +Heavens.") can be flirted off, when made to revolve with sufficient +velocity, we need not doubt that the centrifugal force would have power to +modify the structure of a softened bomb, in the manner here supposed. +Geologists have remarked, that the external form of a bomb at once bespeaks +the history of its aerial course, and few now see that the internal +structure can speak, with almost equal plainness, of its rotatory movement. + +M. Bory St. Vincent ("Voyage aux Quatre Isles d'Afrique" tome 1 page 222.) +has described some balls of lava from the Isle of Bourbon, which have a +closely similar structure. His explanation, however (if I understand it +rightly), is very different from that which I have given; for he supposes +that they have rolled, like snowballs, down the sides of the crater. M. +Beudant ("Voyage en Hongrie" tome 2 page 214.), also, has described some +singular little balls of obsidian, never more than six or eight inches in +diameter, which he found strewed on the surface of the ground: their form +is always oval; sometimes they are much swollen in the middle, and even +spindle-shaped: their surface is regularly marked with concentric ridges +and furrows, all of which on the same ball are at right angles to one axis: +their interior is compact and glassy. M. Beudant supposes that masses of +lava, when soft, were shot into the air, with a rotatory movement round the +same axis, and that the form and superficial ridges of the bombs were thus +produced. Sir Thomas Mitchell has given me what at first appears to be the +half of a much flattened oval ball of obsidian; it has a singular +artificial-like appearance, which is well represented (of the natural size) +in Figure 4. It was found in its present state, on a great sandy plain +between the rivers Darling and Murray, in Australia, and at the distance of +several hundred miles from any known volcanic region. It seems to have been +embedded in some reddish tufaceous matter; and may have been transported +either by the aborigines or by natural means. The external saucer consists +of compact obsidian, of a bottle-green colour, and is filled with finely +cellular black lava, much less transparent and glassy than the obsidian. +The external surface is marked with four or five not quite perfect ridges, +which are represented rather too distinctly in Figure 4. Here, then, we +have the external structure described by M. Beudant, and the internal +cellular condition of the bombs from Ascension. The lip of the saucer is +slightly concave, exactly like the margin of a soup-plate, and its inner +edge overlaps a little the central cellular lava. This structure is so +symmetrical round the entire circumference, that one is forced to suppose +that the bomb burst during its rotatory course, before being quite +solidified, and that the lip and edges were thus slightly modified and +turned inwards. It may be remarked that the superficial ridges are in +planes, at right angles to an axis, transverse to the longer axis of the +flattened oval: to explain this circumstance, we may suppose that when the +bomb burst, the axis of rotation changed. + +AERIFORM EXPLOSIONS. + +The flanks of Green Mountain and the surrounding country are covered by a +great mass, some hundred feet in thickness, of loose fragments. The lower +beds generally consist of fine-grained, slightly consolidated tuffs (Some +of this peperino, or tuff, is sufficiently hard not to be broken by the +greatest force of the fingers.), and the upper beds of great loose +fragments, with alternating finer beds. (On the northern side of the Green +Mountain a thin seam, about an inch in thickness, of compact oxide of iron, +extends over a considerable area; it lies conformably in the lower part of +the stratified mass of ashes and fragments. This substance is of a reddish- +brown colour, with an almost metallic lustre; it is not magnetic, but +becomes so after having been heated under the blowpipe, by which it is +blackened and partly fused. This seam of compact stone, by intercepting the +little rain-water which falls on the island, gives rise to a small dripping +spring, first discovered by Dampier. It is the only fresh water on the +island, so that the possibility of its being inhabited has entirely +depended on the occurrence of this ferruginous layer.) One white ribbon- +like layer of decomposed, pumiceous breccia, was curiously bent into deep +unbroken curves, beneath each of the large fragments in the superincumbent +stratum. From the relative position of these beds, I presume that a narrow- +mouthed crater, standing nearly in the position of Green Mountain, like a +great air-gun, shot forth, before its final extinction, this vast +accumulation of loose matter. Subsequently to this event, considerable +dislocations have taken place, and an oval circus has been formed by +subsidence. This sunken space lies at the north-eastern foot of Green +Mountain, and is well represented in Map 2. Its longer axis, which is +connected with a N.E. and S.W. line of fissure, is three-fifths of a +nautical mile in length; its sides are nearly perpendicular, except in one +spot, and about four hundred feet in height; they consist, in the lower +part, of a pale basalt with feldspar, and in the upper part, of the tuff +and loose ejected fragments; the bottom is smooth and level, and under +almost any other climate a deep lake would have been formed here. From the +thickness of the bed of loose fragments, with which the surrounding country +is covered, the amount of aeriform matter necessary for their projection +must have been enormous; hence we may suppose it probable that after the +explosions vast subterranean caverns were left, and that the falling in of +the roof of one of these produced the hollow here described. At the +Galapagos Archipelago, pits of a similar character, but of a much smaller +size, frequently occur at the bases of small cones of eruption. + +EJECTED GRANITIC FRAGMENTS. + +In the neighbourhood of Green Mountain, fragments of extraneous rock are +not unfrequently found embedded in the midst of masses of scoriae. +Lieutenant Evans, to whose kindness I am indebted for much information, +gave me several specimens, and I found others myself. They nearly all have +a granitic structure, are brittle, harsh to the touch, and apparently of +altered colours. + +FIRST, a white syenite, streaked and mottled with red; it consists of well- +crystallised feldspar, numerous grains of quartz, and brilliant, though +small, crystals of hornblende. The feldspar and hornblende in this and the +succeeding cases have been determined by the reflecting goniometer, and the +quartz by its action under the blowpipe. The feldspar in these ejected +fragments, like the glassy kind in the trachyte, is from its cleavage a +potash-feldspar. + +SECONDLY, a brick-red mass of feldspar, quartz, and small dark patches of a +decayed mineral; one minute particle of which I was able to ascertain, by +its cleavage, to be hornblende. + +THIRDLY, a mass of confusedly crystallised white feldspar, with little +nests of a dark-coloured mineral, often carious, externally rounded, having +a glossy fracture, but no distinct cleavage: from comparison with the +second specimen, I have no doubt that it is fused hornblende. + +FOURTHLY, a rock, which at first appears a simple aggregation of distinct +and large-sized crystals of dusty-coloured Labrador feldspar (Professor +Miller has been so kind as to examine this mineral. He obtained two good +cleavages of 86 degrees 30 minutes and 86 degrees 50 minutes. The mean of +several, which I made, was 86 degrees 30 minutes. Professor Miller states +that these crystals, when reduced to a fine powder, are soluble in +hydrochloric acid, leaving some undissolved silex behind; the addition of +oxalate of ammonia gives a copious precipitate of lime. He further remarks, +that according to Von Kobell, anorthite (a mineral occurring in the ejected +fragments at Mount Somma) is always white and transparent, so that if this +be the case, these crystals from Ascension must be considered as Labrador +feldspar. Professor Miller adds, that he has seen an account, in Erdmann's +"Journal fur tecnische Chemie," of a mineral ejected from a volcano which +had the external characters of Labrador feldspar, but differed in the +analysis from that given by mineralogists of this mineral: the author +attributed this difference to an error in the analysis of Labrador +feldspar, which is very old.); but in their interstices there is some white +granular feldspar, abundant scales of mica, a little altered hornblende, +and, as I believe, no quartz. I have described these fragments in detail, +because it is rare to find granitic rocks ejected from volcanoes with their +MINERALS UNCHANGED, as is the case with the first specimen, and partially +with the second. (Daubeny, in his work on Volcanoes page 386, remarks that +this is the case; and Humboldt, in his "Personal Narrative" volume 1 page +236, says "In general, the masses of known primitive rocks, I mean those +which perfectly resemble our granites, gneiss, and mica-slate, are very +rare in lavas: the substances we generally denote by the name of granite, +thrown out by Vesuvius, are mixtures of nepheline, mica, and pyroxene.") +One other large fragment, found in another spot, is deserving of notice; it +is a conglomerate, containing small fragments of granitic, cellular, and +jaspery rocks, and of hornstone porphyries, embedded in a base of wacke, +threaded by numerous thin layers of a concretionary pitchstone passing into +obsidian. These layers are parallel, slightly tortuous, and short; they +thin out at their ends, and resemble in form the layers of quartz in +gneiss. It is probable that these small embedded fragments were not +separately ejected, but were entangled in a fluid volcanic rock, allied to +obsidian; and we shall presently see that several varieties of this latter +series of rock assume a laminated structure. + +TRACHYTIC SERIES OF ROCKS. + +Those occupy the more elevated and central, and likewise the south-eastern, +parts of the island. The trachyte is generally of a pale brown colour, +stained with small darker patches; it contains broken and bent crystals of +glassy feldspar, grains of specular iron, and black microscopical points, +which latter, from being easily fused, and then becoming magnetic, I +presume are hornblende. The greater number of the hills, however, are +composed of a quite white, friable stone, appearing like a trachytic tuff. +Obsidian, hornstone, and several kinds of laminated feldspathic rocks, are +associated with the trachyte. There is no distinct stratification; nor +could I distinguish a crateriform structure in any of the hills of this +series. Considerable dislocations have taken place; and many fissures in +these rocks are yet left open, or are only partially filled with loose +fragments. Within the space (This space is nearly included by a line +sweeping round Green Mountain, and joining the hills, called the Weather +Port Signal, Holyhead, and that denominated (improperly in a geological +sense) "the Crater of an old volcano."), mainly formed of trachyte, some +basaltic streams have burst forth; and not far from the summit of Green +Mountain, there is one stream of quite black, vesicular basalt, containing +minute crystals of glassy feldspar, which have a rounded appearance. + +The soft white stone above mentioned is remarkable from its singular +resemblance, when viewed in mass, to a sedimentary tuff: it was long before +I could persuade myself that such was not its origin; and other geologists +have been perplexed by closely similar formations in trachytic regions. In +two cases, this white earthy stone formed isolated hills; in a third, it +was associated with columnar and laminated trachyte; but I was unable to +trace an actual junction. It contains numerous crystals of glassy feldspar +and black microscopical specks, and is marked with small darker patches, +exactly as in the surrounding trachyte. Its basis, however, when viewed +under the microscope, is generally quite earthy; but sometimes it exhibits +a decidedly crystalline structure. On the hill marked "Crater of an old +volcano," it passes into a pale greenish-grey variety, differing only in +its colour, and in not being so earthy; the passage was in one case +effected insensibly; in another, it was formed by numerous, rounded and +angular, masses of the greenish variety, being embedded in the white +variety;--in this latter case, the appearance was very much like that of a +sedimentary deposit, torn up and abraded during the deposition of a +subsequent stratum. Both these varieties are traversed by innumerable +tortuous veins (presently to be described), which are totally unlike +injected dikes, or indeed any other veins which I have ever seen. Both +varieties include a few scattered fragments, large and small, of dark- +coloured scoriaceous rocks, the cells of some of which are partially filled +with the white earthy stone; they likewise include some huge blocks of a +cellular porphyry. (The porphyry is dark coloured; it contains numerous, +often fractured, crystals of white opaque feldspar, also decomposing +crystals of oxide of iron; its vesicles include masses of delicate, hair- +like, crystals, apparently of analcime.) These fragments project from the +weathered surface, and perfectly resemble fragments embedded in a true +sedimentary tuff. But as it is known that extraneous fragments of cellular +rock are sometimes included in columnar trachyte, in phonolite (D'Aubuisson +"Traite de Geognosie" tome 2 page 548.), and in other compact lavas, this +circumstance is not any real argument for the sedimentary origin of the +white earthy stone. (Dr. Daubeny on Volcanoes, page 180 seems to have been +led to believe that certain trachytic formations of Ischia and of the Puy +de Dome, which closely resemble these of Ascension, were of sedimentary +origin, chiefly from the frequent presence in them "of scoriform portions, +different in colour from the matrix." Dr. Daubeny adds, that on the other +hand, Brocchi, and other eminent geologists, have considered these beds as +earthy varieties of trachyte; he considers the subject deserving of further +attention.) The insensible passage of the greenish variety into the white +one, and likewise the more abrupt passage by fragments of the former being +embedded in the latter, might result from slight differences in the +composition of the same mass of molten stone, and from the abrading action +of one such part still fluid on another part already solidified. The +curiously formed veins have, I believe, been formed by siliceous matter +being subsequently segregated. But my chief reason for believing that these +soft earthy stones, with their extraneous fragments, are not of sedimentary +origin, is the extreme improbability of crystals of feldspar, black +microscopical specks, and small stains of a darker colour occurring in the +same proportional numbers in an aqueous deposit, and in masses of solid +trachyte. Moreover, as I have remarked, the microscope occasionally reveals +a crystalline structure in the apparently earthy basis. On the other hand, +the partial decomposition of such great masses of trachyte, forming whole +mountains, is undoubtedly a circumstance of not easy explanation. + +VEINS IN THE EARTHY TRACHYTIC MASSES. + +These veins are extraordinarily numerous, intersecting in the most +complicated manner both coloured varieties of the earthy trachyte: they are +best seen on the flanks of the "Crater of the old volcano." They contain +crystals of glassy feldspar, black microscopical specks and little dark +stains, precisely as in the surrounding rock; but the basis is very +different, being exceedingly hard, compact, somewhat brittle, and of rather +less easy fusibility. The veins vary much, and suddenly, from the tenth of +an inch to one inch in thickness; they often thin out, not only on their +edges, but in their central parts, thus leaving round, irregular apertures; +their surfaces are rugged. They are inclined at every possible angle with +the horizon, or are horizontal; they are generally curvilinear, and often +interbranch one with another. From their hardness they withstand +weathering, and projecting two or three feet above the ground, they +occasionally extend some yards in length; these plate-like veins, when +struck, emit a sound, almost like that of a drum, and they may be +distinctly seen to vibrate; their fragments, which are strewed on the +ground, clatter like pieces of iron when knocked against each other. They +often assume the most singular forms; I saw a pedestal of the earthy +trachyte, covered by a hemispherical portion of a vein, like a great +umbrella, sufficiently large to shelter two persons. I have never met with, +or seen described, any veins like these; but in form they resemble the +ferruginous seams, due to some process of segregation, occurring not +uncommonly in sandstones,--for instance, in the New Red sandstone of +England. Numerous veins of jasper and of siliceous sinter, occurring on the +summit of this same hill, show that there has been some abundant source of +silica, and as these plate-like veins differ from the trachyte only in +their greater hardness, brittleness, and less easy fusibility, it appears +probable that their origin is due to the segregation or infiltration of +siliceous matter, in the same manner as happens with the oxides of iron in +many sedimentary rocks. + +SILICEOUS SINTER AND JASPER. + +The siliceous sinter is either quite white, of little specific gravity, and +with a somewhat pearly fracture, passing into pinkish pearl quartz; or it +is yellowish white, with a harsh fracture, and it then contains an earthy +powder in small cavities. Both varieties occur, either in large irregular +masses in the altered trachyte, or in seams included in broad, vertical, +tortuous, irregular veins of a compact, harsh stone of a dull red colour, +appearing like a sandstone. This stone, however, is only altered trachyte; +and a nearly similar variety, but often honeycombed, sometimes adheres to +the projecting plate-like veins, described in the last paragraph. The +jasper is of an ochre yellow or red colour; it occurs in large irregular +masses, and sometimes in veins, both in the altered trachyte and in an +associated mass of scoriaceous basalt. The cells of the scoriaceous basalt +are lined or filled with fine, concentric layers of chalcedony, coated and +studded with bright-red oxide of iron. In this rock, especially in the +rather more compact parts, irregular angular patches of the red jasper are +included, the edges of which insensibly blend into the surrounding mass; +other patches occur having an intermediate character between perfect jasper +and the ferruginous, decomposed, basaltic base. In these patches, and +likewise in the large vein-like masses of jasper, there occur little +rounded cavities, of exactly the same size and form with the air-cells, +which in the scoriaceous basalt are filled and lined with layers of +chalcedony. Small fragments of the jasper, examined under the microscope, +seem to resemble the chalcedony with its colouring matter not separated +into layers, but mingled in the siliceous paste, together with some +impurities. I can understand these facts,--namely, the blending of the +jasper into the semi-decomposed basalt,--its occurrence in angular patches, +which clearly do not occupy pre-existing hollows in the rock,--and its +containing little vesicles filled with chalcedony, like those in the +scoriaceous lava,--only on the supposition that a fluid, probably the same +fluid which deposited the chalcedony in the air-cells, removed in those +parts where there were no cavities, the ingredients of the basaltic rock, +and left in their place silica and iron, and thus produced the jasper. In +some specimens of silicified wood, I have observed, that in the same manner +as in the basalt, the solid parts were converted into a dark-coloured +homogeneous stone, whereas the cavities formed by the larger sap-vessels +(which may be compared with the air-vesicles in the basaltic lava) and +other irregular hollows, apparently produced by decay, were filled with +concentric layers of chalcedony; in this case, there can be little doubt +that the same fluid deposited the homogeneous base and the chalcedonic +layers. After these considerations, I cannot doubt but that the jasper of +Ascension may be viewed as a volcanic rock silicified, in precisely the +same sense as this term is applied to wood, when silicified; we are equally +ignorant of the means by which every atom of wood, whilst in a perfect +state, is removed and replaced by atoms of silica, as we are of the means +by which the constituent parts of a volcanic rock could be thus acted on. +(Beudant "Voyage en Hongrie" tome 3 pages 502, 504 describes kidney-shaped +masses of jasper-opal, which either blend into the surrounding trachytic +conglomerate, or are embedded in it like chalk-flints; and he compares them +with the fragments of opalised wood, which are abundant in this same +formation. Beudant, however, appears to have viewed the process of their +formation rather as one of simple infiltration than of molecular exchange; +but the presence of a concretion, wholly different from the surrounding +matter, if not formed in a pre-existing hollow, clearly seems to me to +require, either a molecular or mechanical displacement of the atoms, which +occupied the space afterwards filled by it. The jasper-opal of Hungary +passes into chalcedony, and therefore in this case, as in that of +Ascension, jasper seems to be intimately related in origin with +chalcedony.) I was led to the careful examination of these rocks, and to +the conclusion here given, from having heard the Rev. Professor Henslow +express a similar opinion, regarding the origin in trap-rocks of many +chalcedonies and agates. Siliceous deposits seem to be very general, if not +of universal occurrence, in partially decomposed trachytic tuffs (Beudant +"Voyage Min." tome 3 page 507 enumerates cases in Hungary, Germany, Central +France, Italy, Greece, and Mexico.); and as these hills, according to the +view above given, consist of trachyte softened and altered in situ, the +presence of free silica in this case may be added as one more instance to +the list. + +CONCRETIONS IN PUMICEOUS TUFF. + +The hill, marked in Map 2 "Crater of an old volcano," has no claims to this +appellation, which I could discover, except in being surmounted by a +circular, very shallow, saucer-like summit, nearly half a mile in diameter. +This hollow has been nearly filled up with many successive sheets of ashes +and scoriae, of different colours, and slightly consolidated. Each +successive saucer-shaped layer crops out all round the margin, forming so +many rings of various colours, and giving to the hill a fantastic +appearance. The outer ring is broad, and of a white colour; hence it +resembles a course round which horses have been exercised, and has received +the name of the Devil's Riding School, by which it is most generally known. +These successive layers of ashes must have fallen over the whole +surrounding country, but they have all been blown away except in this one +hollow, in which probably moisture accumulated, either during an +extraordinary year when rain fell, or during the storms often accompanying +volcanic eruptions. One of the layers of a pinkish colour, and chiefly +derived from small, decomposed fragments of pumice, is remarkable, from +containing numerous concretions. These are generally spherical, from half +an inch to three inches in diameter; but they are occasionally cylindrical, +like those of iron-pyrites in the chalk of Europe. They consist of a very +tough, compact, pale-brown stone, with a smooth and even fracture. They are +divided into concentric layers by thin white partitions, resembling the +external superficies; six or eight of such layers are distinctly defined +near the outside; but those towards the inside generally become indistinct, +and blend into a homogeneous mass. I presume that these concentric layers +were formed by the shrinking of the concretion, as it became compact. The +interior part is generally fissured by minute cracks or septaria, which are +lined, both by black, metallic, and by other white and crystalline specks, +the nature of which I was unable to ascertain. Some of the larger +concretions consist of a mere spherical shell, filled with slightly +consolidated ashes. The concretions contain a small proportion of carbonate +of lime: a fragment placed under the blowpipe decrepitates, then whitens +and fuses into a blebby enamel, but does not become caustic. The +surrounding ashes do not contain any carbonate of lime; hence the +concretions have probably been formed, as is so often the case, by the +aggregation of this substance. I have not met with any account of similar +concretions; and considering their great toughness and compactness, their +occurrence in a bed, which probably has been subjected only to atmospheric +moisture, is remarkable. + +FORMATION OF CALCAREOUS ROCKS ON THE SEA-COAST. + +On several of the sea-beaches, there are immense accumulations of small, +well-rounded particles of shells and corals, of white, yellowish, and pink +colours, interspersed with a few volcanic particles. At the depth of a few +feet, these are found cemented together into stone, of which the softer +varieties are used for building; there are other varieties, both coarse and +fine-grained, too hard for this purpose: and I saw one mass divided into +even layers half an inch in thickness, which were so compact that when +struck with a hammer they rang like flint. It is believed by the +inhabitants, that the particles become united in the course of a single +year. The union is effected by calcareous matter; and in the most compact +varieties, each rounded particle of shell and volcanic rock can be +distinctly seen to be enveloped in a husk of pellucid carbonate of lime. +Extremely few perfect shells are embedded in these agglutinated masses; and +I have examined even a large fragment under a microscope, without being +able to discover the least vestige of striae or other marks of external +form: this shows how long each particle must have been rolled about, before +its turn came to be embedded and cemented. (The eggs of the turtle being +buried by the parent, sometimes become enclosed in the solid rock. Mr. +Lyell has given a figure ("Principles of Geology" book 3 chapter 17) of +some eggs, containing the bones of young turtles, found thus entombed.) One +of the most compact varieties, when placed in acid, was entirely dissolved, +with the exception of some flocculent animal matter; its specific gravity +was 2.63. The specific gravity of ordinary limestone varies from 2.6 to +2.75; pure Carrara marble was found by Sir H. De la Beche to be 2.7. +("Researches in Theoretical Geology" page 12.) It is remarkable that these +rocks of Ascension, formed close to the surface, should be nearly as +compact as marble, which has undergone the action of heat and pressure in +the plutonic regions. + +The great accumulation of loose calcareous particles, lying on the beach +near the Settlement, commences in the month of October, moving towards the +S.W., which, as I was informed by Lieutenant Evans, is caused by a change +in the prevailing direction of the currents. At this period the tidal +rocks, at the S.W. end of the beach, where the calcareous sand is +accumulating, and round which the currents sweep, become gradually coated +with a calcareous incrustation, half an inch in thickness. It is quite +white, compact, with some parts slightly spathose, and is firmly attached +to the rock. After a short time it gradually disappears, being either +redissolved, when the water is less charged with lime, or more probably is +mechanically abraded. Lieutenant Evans has observed these facts, during the +six years he has resided at Ascension. The incrustation varies in thickness +in different years: in 1831 it was unusually thick. When I was there in +July, there was no remnant of the incrustation; but on a point of basalt, +from which the quarrymen had lately removed a mass of the calcareous +freestone, the incrustation was perfectly preserved. Considering the +position of the tidal-rocks, and the period at which they become coated, +there can be no doubt that the movement and disturbance of the vast +accumulation of calcareous particles, many of them being partially +agglutinated together, cause the waves of the sea to be so highly charged +with carbonate of lime, that they deposit it on the first objects against +which they impinge. I have been informed by Lieutenant Holland, R.N., that +this incrustation is formed on many parts of the coast, on most of which, I +believe, there are likewise great masses of comminuted shells. + +A FRONDESCENT CALCAREOUS INCRUSTATION. + +(FIGURE 5. AN INCRUSTATION OF CALCAREOUS AND ANIMAL MATTER, coating the +tidal-rocks at Ascension.) + +In many respects this is a singular deposit; it coats throughout the year +the tidal volcanic rocks, that project from the beaches composed of broken +shells. Its general appearance is well represented in Figure 5; but the +fronds or discs, of which it is composed, are generally so closely crowded +together as to touch. These fronds have their sinuous edges finely +crenulated, and they project over their pedestals or supports; their upper +surfaces are either slightly concave, or slightly convex; they are highly +polished, and of a dark grey or jet black colour; their form is irregular, +generally circular, and from the tenth of an inch to one inch and a half in +diameter; their thickness, or amount of their projection from the rock on +which they stand, varies much, about a quarter of an inch being perhaps +most usual. The fronds occasionally become more and more convex, until they +pass into botryoidal masses with their summits fissured; when in this +state, they are glossy and of an intense black, so as to resemble some +fused metallic substance. I have shown the incrustation, both in this +latter and in its ordinary state to several geologists, but not one could +conjecture its origin, except that perhaps it was of volcanic nature! + +The substance forming the fronds has a very compact and often almost +crystalline fracture; the edges being translucent, and hard enough easily +to scratch calcareous spar. Under the blowpipe it immediately becomes +white, and emits a strong animal odour, like that from fresh shells. It is +chiefly composed of carbonate of lime; when placed in muriatic acid it +froths much, leaving a residue of sulphate of lime, and of an oxide of +iron, together with a black powder, which is not soluble in heated acids. +This latter substance seems to be carbonaceous, and is evidently the +colouring matter. The sulphate of lime is extraneous, and occurs in +distinct, excessively minute, lamellar plates, studded on the surface of +the fronds, and embedded between the fine layers of which they are +composed; when a fragment is heated in the blowpipe, these lamellae are +immediately rendered visible. The original outline of the fronds may often +be traced, either to a minute particle of shell fixed in a crevice of the +rock, or to several cemented together; these first become deeply corroded, +by the dissolving power of the waves, into sharp ridges, and then are +coated with successive layers of the glossy, grey, calcareous incrustation. +The inequalities of the primary support affect the outline of every +successive layer, in the same manner as may often be seen in bezoar-stones, +when an object like a nail forms the centre of aggregation. The crenulated +edges, however, of the frond appear to be due to the corroding power of the +surf on its own deposit, alternating with fresh depositions. On some smooth +basaltic rocks on the coast of St. Jago, I found an exceedingly thin layer +of brown calcareous matter, which under a lens presented a miniature +likeness of the crenulated and polished fronds of Ascension; in this case a +basis was not afforded by any projecting extraneous particles. Although the +incrustation at Ascension is persistent throughout the year; yet from the +abraded appearance of some parts, and from the fresh appearance of other +parts, the whole seems to undergo a round of decay and renovation, due +probably to changes in the form of the shifting beach, and consequently in +the action of the breakers: hence probably it is, that the incrustation +never acquires a great thickness. Considering the position of the encrusted +rocks in the midst of the calcareous beach, together with its composition, +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 corals. (The selenite, as I have remarked is extraneous, and +must have been derived from the sea-water. It is an interesting +circumstance thus to find the waves of the ocean, sufficiently charged with +sulphate of lime, to deposit it on the rocks, against which they dash every +tide. Dr. Webster has described ("Voyage of the 'Chanticleer'" volume 2 +page 319) beds of gypsum and salt, as much as two feet in thickness, left +by the evaporation of the spray on the rocks on the windward coast. +Beautiful stalactites of selenite, resembling in form those of carbonate of +lime, are formed near these beds. Amorphous masses of gypsum, also, occur +in caverns in the interior of the island; and at Cross Hill (an old crater) +I saw a considerable quantity of salt oozing from a pile of scoriae. In +these latter cases, the salt and gypsum appear to be volcanic products.) +From this source it derives its animal matter, which is evidently the +colouring principle. The nature of the deposit, in its incipient stage, can +often be well seen upon a fragment of white shell, when jammed between two +of the fronds; it then appears exactly like the thinnest wash of a pale +grey varnish. Its darkness varies a little, but the jet blackness of some +of the fronds and of the botryoidal masses seems due to the translucency of +the successive grey layers. There is, however, this singular circumstance, +that when deposited on the under side of ledges of rock or in fissures, it +appears always to be of a pale, pearly grey colour, even when of +considerable thickness: hence one is led to suppose, that an abundance of +light is necessary to the development of the dark colour, in the same +manner as seems to be the case with the upper and exposed surfaces of the +shells of living mollusca, which are always dark, compared with their under +surfaces and with the parts habitually covered by the mantle of the animal. +In this circumstance,--in the immediate loss of colour and in the odour +emitted under the blowpipe,--in the degree of hardness and translucency of +the edges,--and in the beautiful polish of the surface (From the fact +described in my "Journal of Researches" of a coating of oxide of iron, +deposited by a streamlet on the rocks in its bed (like a nearly similar +coating at the great cataracts of the Orinoco and Nile), becoming finely +polished where the surf acts, I presume that the surf in this instance, +also, is the polishing agent.), rivalling when in a fresh state that of the +finest Oliva, there is a striking analogy between this inorganic +incrustation and the shells of living molluscous animals. (In the section +descriptive of St. Paul's Rocks, I have described a glossy, pearly +substance, which coats the rocks, and an allied stalactitical incrustation +from Ascension, the crust of which resembles the enamel of teeth, but is +hard enough to scratch plate-glass. Both these substances contain animal +matter, and seem to have been derived from water in filtering through +birds' dung.) This appears to me to be an interesting physiological fact. +(Mr. Horner and Sir David Brewster have described "Philosophical +Transactions" 1836 page 65 a singular "artificial substance, resembling +shell." It is deposited in fine, transparent, highly polished, brown- +coloured laminae, possessing peculiar optical properties, on the inside of +a vessel, in which cloth, first prepared with glue and then with lime, is +made to revolve rapidly in water. It is much softer, more transparent, and +contains more animal matter, than the natural incrustation at Ascension; +but we here again see the strong tendency which carbonate of lime and +animal matter evince to form a solid substance allied to shell.) + +SINGULAR LAMINATED BEDS ALTERNATING WITH AND PASSING INTO OBSIDIAN. + +These beds occur within the trachytic district, at the western base of +Green Mountain, under which they dip at a high inclination. They are only +partially exposed, being covered up by modern ejections; from this cause, I +was unable to trace their junction with the trachyte, or to discover +whether they had flowed as a stream of lava, or had been injected amidst +the overlying strata. There are three principal beds of obsidian, of which +the thickest forms the base of the section. The alternating stony layers +appear to me eminently curious, and shall be first described, and +afterwards their passage into the obsidian. They have an extremely +diversified appearance; five principal varieties may be noticed, but these +insensibly blend into each other by endless gradations. + +FIRST. + +A pale grey, irregularly and coarsely laminated (This term is open to some +misinterpretation, as it may be applied both to rocks divided into laminae +of exactly the same composition, and to layers firmly attached to each +other, with no fissile tendency, but composed of different minerals, or of +different shades of colour. The term "laminated," in this chapter, is +applied in these latter senses; where a homogeneous rock splits, as in the +former sense, in a given direction, like clay-slate, I have used the term +"fissile."), harsh-feeling rock, resembling clay-slate which has been in +contact with a trap-dike, and with a fracture of about the same degree of +crystalline structure. This rock, as well as the following varieties, +easily fuses into a pale glass. The greater part is honeycombed with +irregular, angular, cavities, so that the whole has a curious appearance, +and some fragments resemble in a remarkable manner silicified logs of +decayed wood. This variety, especially where more compact, is often marked +with thin whitish streaks, which are either straight or wrap round, one +behind the other, the elongated carious hollows. + +SECONDLY. + +A bluish grey or pale brown, compact, heavy, homogeneous stone, with an +angular, uneven, earthy fracture; viewed, however, under a lens of high +power, the fracture is seen to be distinctly crystalline, and even separate +minerals can be distinguished. + +THIRDLY. + +A stone of the same kind with the last, but streaked with numerous, +parallel, slightly tortuous, white lines of the thickness of hairs. These +white lines are more crystalline than the parts between them; and the stone +splits along them: they frequently expand into exceedingly thin cavities, +which are often only just perceptible with a lens. The matter forming the +white lines becomes better crystallised in these cavities, and Professor +Miller was fortunate enough, after several trials, to ascertain that the +white crystals, which are the largest, were of quartz (Professor Miller +informs me that the crystals which he measured had the faces P, z, m of the +figure (147) given by Haidinger in his Translation of Mohs; and he adds, +that it is remarkable, that none of them had the slightest trace of faces r +of the regular six-sided prism.), and that the minute green transparent +needles were augite, or, as they would more generally be called, diopside: +besides these crystals, there are some minute, dark specks without a trace +of crystalline, and some fine, white, granular, crystalline matter which is +probably feldspar. Minute fragments of this rock are easily fusible. + +FOURTHLY. + +A compact crystalline rock, banded in straight lines with innumerable +layers of white and grey shades of colour, varying in width from the +thirtieth to the two-hundredth of an inch; these layers seem to be composed +chiefly of feldspar, and they contain numerous perfect crystals of glassy +feldspar, which are placed lengthways; they are also thickly studded with +microscopically minute, amorphous, black specks, which are placed in rows, +either standing separately, or more frequently united, two or three or +several together, into black lines, thinner than a hair. When a small +fragment is heated in the blowpipe, the black specks are easily fused into +black brilliant beads, which become magnetic,--characters that apply to no +common mineral except hornblende or augite. With the black specks there are +mingled some others of a red colour, which are magnetic before being +heated, and no doubt are oxide of iron. Round two little cavities, in a +specimen of this variety, I found the black specks aggregated into minute +crystals, appearing like those of augite or hornblende, but too dull and +small to be measured by the goniometer; in the specimen, also, I could +distinguish amidst the crystalline feldspar, grains, which had the aspect +of quartz. By trying with a parallel ruler, I found that the thin grey +layers and the black hair-like lines were absolutely straight and parallel +to each other. It is impossible to trace the gradation from the homogeneous +grey rocks to these striped varieties, or indeed the character of the +different layers in the same specimen, 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 matter, into +the black and red specks of hornblende and oxide of iron. + +FIFTHLY. + +A compact heavy rock, not laminated, with an irregular, angular, highly +crystalline, fracture; it abounds with distinct crystals of glassy +feldspar, and the crystalline feldspathic base is mottled with a black +mineral, which on the weathered surface is seen to be aggregated into small +crystals, some perfect, but the greater number imperfect. I showed this +specimen to an experienced geologist, and asked him what it was; he +answered, as I think every one else would have done, that it was a +primitive greenstone. The weathered surface, also, of the banded variety in +Figure 4, strikingly resembles a worn fragment of finely laminated gneiss. + +These five varieties, with many intermediate ones, pass and repass into +each other. As the compact varieties are quite subordinate to the others, +the whole may be considered as laminated or striped. The laminae, to sum up +their characteristics, are either quite straight, or slightly tortuous, or +convoluted; they are all parallel to each other, and to the intercalating +strata of obsidian; they are generally of extreme thinness; they consist +either of an apparently homogeneous, compact rock, striped with different +shades of grey and brown colours, or of crystalline feldspathic layers in a +more or less perfect state of purity, and of different thicknesses, with +distinct crystals of glassy feldspar placed lengthways, or of very thin +layers chiefly composed of minute crystals of quartz and augite, or +composed of black and red specks of an augitic mineral and of an oxide of +iron, either not crystallised or imperfectly so. After having fully +described the obsidian, I shall return to the subject of the lamination of +rocks of the trachytic series. + +The passage of the foregoing beds into the strata of glassy obsidian is +effected in several ways: first, angulo-modular masses of obsidian, both +large and small, abruptly appear disseminated in a slaty, or in an +amorphous, pale-coloured, feldspathic rock, with a somewhat pearly +fracture. Secondly, small irregular nodules of the obsidian, either +standing separately, or united into thin layers, seldom more than the tenth +of an inch in thickness, alternate repeatedly with very thin layers of a +feldspathic rock, which is striped with the finest parallel zones of +colour, like an agate, and which sometimes passes into the nature of +pitchstone; the interstices between the nodules of obsidian are generally +filled by soft white matter, resembling pumiceous ashes. Thirdly, the whole +substance of the bounding rock suddenly passes into an angulo-concretionary +mass of obsidian. Such masses (as well as the small nodules) of obsidian +are of a pale green colour, and are generally streaked with different +shades of colour, parallel to the laminae of the surrounding rock; they +likewise generally contain minute white sphaerulites, of which half is +sometimes embedded in a zone of one shade of colour, and half in a zone of +another shade. The obsidian assumes its jet black colour and perfectly +conchoidal fracture, only when in large masses; but even in these, on +careful examination and on holding the specimens in different lights, I +could generally distinguish parallel streaks of different shades of +darkness. + +(FIGURE 6. OPAQUE BROWN SPHAERULITES, drawn on an enlarged scale. The upper +ones are externally marked with parallel ridges. The internal radiating +structure of the lower ones, is much too plainly represented. + +FIGURE 7. A LAYER FORMED BY THE UNION OF MINUTE BROWN SPHAERULITES, +INTERSECTING TWO OTHER SIMILAR LAYERS: the whole represented of nearly the +natural size.) + +One of the commonest transitional rocks deserves in several respects a +further description. It is of a very complicated nature, and consists of +numerous thin, slightly tortuous layers of a pale-coloured feldspathic +stone, often passing into an imperfect pitchstone, alternating with layers +formed of numberless little globules of two varieties of obsidian, and of +two kinds of sphaerulites, embedded in a soft or in a hard pearly base. The +sphaerulites are either white and translucent, or dark brown and opaque; +the former are quite spherical, of small size, and distinctly radiated from +their centre. The dark brown sphaerulites are less perfectly round, and +vary in diameter from the twentieth to the thirtieth of an inch; when +broken they exhibit towards their centres, which are whitish, an obscure +radiating structure; two of them when united sometimes have only one +central point of radiation; there is occasionally a trace of or a hollow +crevice in their centres. They stand either separately, or are united two +or three or many together into irregular groups, or more commonly into +layers, parallel to the stratification of the mass. This union in many +cases is so perfect, that the two sides of the layer thus formed, are quite +even; and these layers, as they become less brown and opaque, cannot be +distinguished from the alternating layers of the pale-coloured feldspathic +stone. The sphaerulites, when not united, are generally compressed in the +plane of the lamination of the mass; and in this same plane, they are often +marked internally, by zones of different shades of colour, and externally +by small ridges and furrows. In the upper part of Figure 6, the +sphaerulites with the parallel ridges and furrows are represented on an +enlarged scale, but they are not well executed; and in the lower part, +their usual manner of grouping is shown. In another specimen, a thin layer +formed of the brown sphaerulites closely united together, intersects, as +represented in Figure 7, a layer of similar composition; and after running +for a short space in a slightly curved line, again intersects it, and +likewise a second layer lying a little way beneath that first intersected. +The small nodules also of obsidian are sometimes externally marked with +ridges and furrows, parallel to the lamination of the mass, but always less +plainly than the sphaerulites. These obsidian nodules are generally +angular, with their edges blunted: they are often impressed with the form +of the adjoining sphaerulites, than which they are always larger; the +separate nodules seldom appear to have drawn each other out by exerting a +mutually attractive force. Had I not found in some cases, a distinct centre +of attraction in these nodules of obsidian, I should have been led to have +considered them as residuary matter, left during the formation of the +pearlstone, in which they are embedded, and of the sphaerulitic globules. + +The sphaerulites and the little nodules of obsidian in these rocks so +closely resemble, in general form and structure, concretions in sedimentary +deposits, that one is at once tempted to attribute to them an analogous +origin. They resemble ordinary concretions in the following respects: in +their external form,--in the union of two or three, or of several, into an +irregular mass, or into an even-sided layer,--in the occasional +intersection of one such layer by another, as in the case of chalk-flints,- +-in the presence of two or three kinds of nodules, often close together, in +the same basis,--in their fibrous, radiating structure, with occasional +hollows in their centres,--in the co-existence of a laminary, +concretionary, and radiating structure, as is so well developed in the +concretions of magnesian limestone, described by Professor Sedgwick. +("Geological Transactions" volume 3 part 1 page 37.) Concretions in +sedimentary deposits, it is known, are due to the separation from the +surrounding mass of the whole or part of some mineral substance, and its +aggregation round certain points of attraction. Guided by this fact, I have +endeavoured to discover whether obsidian and the sphaerulites (to which may +be added marekanite and pearlstone, both of them occurring in nodular +concretions in the trachytic series) differ in their constituent parts, +from the minerals generally composing trachytic rocks. It appears from +three analyses, that obsidian contains on an average 76 per cent of silica; +from one analysis, that sphaerulites contain 79.12; from two, that +marekanite contains 79.25; and from two other analyses, that pearlstone +contains 75.62 of silica. (The foregoing analyses are taken from Beudant +"Traite de Mineralogie" tome 2 page 113; and one analysis of obsidian from +Phillips "Mineralogy.") Now, the constituent parts of trachyte, as far as +they can be distinguished consist of feldspar, containing 65.21 of silica; +or of albite, containing 69.09; of hornblende, containing 55.27 (These +analyses are taken from Von Kobell "Grundzuge der Mineralogie" 1838.), and +of oxide of iron: so that the foregoing glassy concretionary substances all +contain a larger proportion of silica than that occurring in ordinary +feldspathic or trachytic rocks. D'Aubuisson ("Traite de Geogn." tome 2 page +535.), also, has remarked on the large proportion of silica compared with +alumina, in six analyses of obsidian and pearlstone given in Brongniart's +"Mineralogy." Hence I conclude, that the foregoing concretions have been +formed by a process of aggregation, strictly analogous to that which takes +place in aqueous deposits, acting chiefly on the silica, but likewise on +some of the other elements of the surrounding mass, and thus producing the +different concretionary varieties. From the well-known effects of rapid +cooling (This is seen in the manufacture of common glass, and in Gregory +Watts's experiments on molten trap; also on the natural surfaces of lava- +streams, and on the side-walls of dikes.) in giving glassiness of texture, +it is probably necessary that the entire mass, in cases like that of +Ascension, should have cooled at a certain rate; but considering the +repeated and complicated alterations of nodules and thin layers of a glassy +texture with other layers quite stony or crystalline, all within the space +of a few feet or even inches, it is hardly possible that they could have +cooled at different rates, and thus have acquired their different textures. + +The natural sphaerulites in these rocks very closely resemble those +produced in glass, when slowly cooled. (I do not know whether it is +generally known, that bodies having exactly the same appearance as +sphaerulites, sometimes occur in agates. Mr. Robert Brown showed me in an +agate, formed within a cavity in a piece of silicified wood, some little +specks, which were only just visible to the naked eye: these specks, when +placed by him under a lens of high power, presented a beautiful appearance: +they were perfectly circular, and consisted of the finest fibres of a brown +colour, radiating with great exactness from a common centre. These little +radiating stars are occasionally intersected, and portions are quite cut +off by the fine, ribbon-like zones of colour in the agate. In the obsidian +of Ascension, the halves of a sphaerulite often lie in different zones of +colour, but they are not cut off by them, as in the agate.) In some fine +specimens of partially devitrified glass, in the possession of Mr. Stokes, +the sphaerulites are united into straight layers with even sides, parallel +to each other, and to one of the outer surfaces, exactly as in the +obsidian. These layers sometimes interbranch and form loops; but I did not +see any case of actual intersection. They form the passage from the +perfectly glassy portions, to those nearly homogeneous and stony, with only +an obscure concretionary structure. In the same specimen, also, +sphaerulites differing slightly in colour and in structure, occur embedded +close together. Considering these facts, it is some confirmation of the +view above given of the concretionary origin of the obsidian and natural +sphaerulites, to find that M. Dartigues ("Journal de Physique" tome 59 1804 +pages 10, 12.), in his curious paper on this subject, attributes the +production of sphaerulites in glass, to the different ingredients obeying +their own laws of attraction and becoming aggregated. He is led to believe +that this takes place, from the difficulty in remelting sphaerulitic glass, +without the whole be first thoroughly pounded and mixed together; and +likewise from the fact, that the change takes place most readily in glass +composed of many ingredients. In confirmation of M. Dartigues' view, I may +remark, that M. Fleuriau de Bellevue (Idem tome 60 1805 page 418.) found +that the sphaerulitic portions of devitrified glass were acted on both by +nitric acid and under the blowpipe, in a different manner from the compact +paste in which they were embedded. + +COMPARISON OF THE OBSIDIAN BEDS AND ALTERNATING STRATA OF ASCENSION, WITH +THOSE OF OTHER COUNTRIES. + +I have been struck with much surprise, how closely the excellent +description of the obsidian rocks of Hungary, given by Beudant ("Voyage en +Hongrie" tome 1 page 330; tome 2 pages 221 and 315; tome 3 pages 369, 371, +377, 381.), and that by Humboldt, of the same formation in Mexico and Peru +("Essai Geognostique" pages 176, 326, 328.), and likewise the descriptions +given by several authors (P. Scrope "Geological Transactions" volume 2 +second series page 195. Consult also Dolomieu "Voyage aux Isles Lipari" and +D'Aubuisson "Traite de Geogn." tome 2 page 534.) of the trachytic regions +in the Italian islands, agree with my observations at Ascension. Many +passages might have been transferred without alteration from the works of +the above authors, and would have been applicable to this island. They all +agree in the laminated and stratified character of the whole series; and +Humboldt speaks of some of the beds of obsidian being ribboned like jasper. +(In Mr. Stokes' fine collection of obsidians from Mexico, I observe that +the sphaerulites are generally much larger than those of Ascension; they +are generally white, opaque, and are united into distinct layers: there are +many singular varieties, different from any at Ascension. The obsidians are +finely zoned, in quite straight or curved lines, with exceedingly slight +differences of tint, of cellularity, and of more or less perfect degrees of +glassiness. Tracing some of the less perfectly glassy zones, they are seen +to become studded with minute white sphaerulites, which become more and +more numerous, until at last they unite and form a distinct layer: on the +other hand, at Ascension, only the brown sphaerulites unite and form +layers; the white ones always being irregularly disseminated. Some +specimens at the Geological Society, said to belong to an obsidian +formation from Mexico, have an earthy fracture, and are divided in the +finest parallel laminae, by specks of a black mineral, like the augitic or +hornblendic specks in the rocks at Ascension.) They all agree in the +nodular or concretionary character of the obsidian, and of the passage of +these nodules into layers. They all refer to the repeated alterations, +often in undulatory planes, of glassy, pearly, stony, and crystalline +layers: the crystalline layers, however, seem to be much more perfectly +developed at Ascension, than in the above-named countries. Humboldt +compares some of the stony beds, when viewed from a distance, to strata of +a schistose sandstone. Sphaerulites are described as occurring abundantly +in all cases; and they everywhere seem to mark the passage, from the +perfectly glassy to the stony and crystalline beds. Beudant's account +(Beudant "Voyage" tome 3 page 373.) of his "perlite lithoide globulaire" in +every, even the most trifling particular, might have been written for the +little brown sphaerulitic globules of the rocks of Ascension. + +From the close similarity in so many respects, between the obsidian +formations of Hungary, Mexico, Peru, and of some of the Italian islands, +with that of Ascension, I can hardly doubt that in all these cases, the +obsidian and the sphaerulites owe their origin to a concretionary +aggregation of the silica, and of some of the other constituent elements, +taking place whilst the liquified mass cooled at a certain required rate. +It is, however, well-known, that in several places, obsidian has flowed in +streams like lava; for instance, at Teneriffe, at the Lipari Islands, and +at Iceland. (For Teneriffe see von Buch "Descript. des Isles Canaries" +pages 184 and 190; for the Lipari Islands see Dolomieu "Voyage" page 34; +for Iceland see Mackenzie "Travels" page 369.) In these cases, the +superficial parts are the most perfectly glassy, the obsidian passing at +the depth of a few feet into an opaque stone. In an analysis by Vauquelin +of a specimen of obsidian from Hecla, which probably flowed as lava, the +proportion of silica is nearly the same as in the nodular or concretionary +obsidian from Mexico. It would be interesting to ascertain, whether the +opaque interior portions and the superficial glassy coating contained the +same proportional constituent parts: we know from M. Dufrenoy ("Memoires +pour servir a une Descript. Geolog. de la France" tome 4 page 371.) that +the exterior and interior parts of the same stream of lava sometimes differ +considerably in their composition. Even should the whole body of the stream +of obsidian turn out to be similarly composed with nodular obsidian, it +would only be necessary, in accordance with the foregoing facts, to suppose +that lava in these instances had been erupted with its ingredients mixed in +the same proportion, as in the concretionary obsidian. + +LAMINATION OF VOLCANIC ROCKS OF THE TRACHYTIC SERIES. + +We have seen that, in several and widely distant countries, the strata +alternating with beds of obsidian, are highly laminated. The nodules, also, +both large and small, of the obsidian, are zoned with different shades of +colour; and I have seen a specimen from Mexico in Mr. Stokes' collection, +with its external surface weathered (MacCulloch states "Classification of +Rocks" page 531 that the exposed surfaces of the pitchstone dikes in Arran +are furrowed "with undulating lines, resembling certain varieties of +marbled paper, and which evidently result from some corresponding +difference of laminar structure.") into ridges and furrows, corresponding +with the zones of different degrees of glassiness: Humboldt ("Personal +Narrative" volume 1 page 222.), moreover, found on the Peak of Teneriffe, a +stream of obsidian divided by very thin, alternating, layers of pumice. +Many other lavas of the feldspathic series are laminated; thus, masses of +common trachyte at Ascension are divided by fine earthy lines, along which +the rock splits, separating thin layers of slightly different shades of +colour; the greater number, also, of the embedded crystals of glassy +feldspar are placed lengthways in the same direction. Mr. P. Scrope +("Geological Transactions" volume 2 second series page 195.) has described +a remarkable columnar trachyte in the Panza Islands, which seems to have +been injected into an overlying mass of trachytic conglomerate: it is +striped with zones, often of extreme tenuity, of different textures and +colours; the harder and darker zones appearing to contain a larger +proportion of silica. In another part of the island, there are layers of +pearlstone and pitchstone, which in many respects resemble those of +Ascension. The zones in the columnar trachyte are generally contorted; they +extend uninterruptedly for a great length in a vertical direction, and +apparently parallel to the walls of the dike-like mass. Von Buch +("Description des Iles Canaries" page 184.) has described at Teneriffe, a +stream of lava containing innumerable thin, plate-like crystals of +feldspar, which are arranged like white threads, one behind the other, and +which mostly follow the same direction. Dolomieu ("Voyage aux Isles de +Lipari" pages 35 and 85.) also states, that the grey lavas of the modern +cone of Vulcano, which have a vitreous texture, are streaked with parallel +white lines: he further describes a solid pumice-stone which possesses a +fissile structure, like that of certain micaceous schists. Phonolite, which +I may observe is often, if not always, an injected rock, also, often has a +fissile structure; this is generally due to the parallel position of the +embedded crystals of feldspar, but sometimes, as at Fernando Noronha, seems +to be nearly independent of their presence. (In this case, and in that of +the fissile pumice-stone, the structure is very different from that in the +foregoing cases, where the laminae consist of alternate layers of different +composition or texture. In some sedimentary formations, however, which +apparently are homogeneous and fissile, as in glossy clay-slate, there is +reason to believe, according to D'Aubuisson, that the laminae are really +due to excessively thin, alternating, layers of mica.) From these facts we +see, that various rocks of the feldspathic series have either a laminated +or fissile structure, and that it occurs both in masses which have injected +into overlying strata, and in others which have flowed as streams of lava. + +The laminae of the beds, alternating with the obsidian at Ascension, dip at +a high angle under the mountain, at the base of which they are situated; +and they do not appear as if they had been inclined by violence. A high +inclination is common to these beds in Mexico, Peru, and in some of the +Italian islands (See Phillips "Mineralogy" for the Italian Islands page +136. For Mexico and Peru see Humboldt "Essai Geognostique." Mr. Edwards +also describes the high inclination of the obsidian rocks of the Cerro del +Navaja in Mexico in the "Proc. of the Geolog. Soc." June 1838.): on the +other hand, in Hungary, the layers are horizontal; the laminae, also, of +some of the lava-streams above referred to, as far as I can understand the +descriptions given of them, appear to be highly inclined or vertical. I +doubt whether in any of these cases, the laminae have been tilted into +their present position; and in some instances, as in that of the trachyte +described by Mr. Scrope, it is almost certain that they have been +originally formed with a high inclination. In many of these cases, there is +evidence that the mass of liquified rock has moved in the direction of the +laminae. At Ascension, many of the air-cells have a drawn out appearance, +and are crossed by coarse semi-glassy fibres, in the direction of the +laminae; and some of the layers, separating the sphaerulitic globules, have +a scored appearance, as if produced by the grating of the globules. I have +seen a specimen of zoned obsidian from Mexico, in Mr. Stokes' collection, +with the surfaces of the best-defined layers streaked or furrowed with +parallel lines; and these lines or streaks precisely resembled those, +produced on the surface of a mass of artificial glass by its having been +poured out of a vessel. Humboldt, also, has described little cavities, +which he compares to the tails of comets, behind sphaerulites in laminated +obsidian rocks from Mexico, and Mr. Scrope has described other cavities +behind fragments embedded in his laminated trachyte, and which he supposes +to have been produced during the movement of the mass. ("Geological +Transactions" volume 2 second series page 200 etc. These embedded +fragments, in some instances, consist of the laminated trachyte broken off +and "enveloped in those parts, which still remained liquid." Beudant, also, +frequently refers in his great work on "Hungary" tome 3 page 386, to +trachytic rocks, irregularly spotted with fragments of the same varieties, +which in other parts form the parallel ribbons. In these cases, we must +suppose, that after part of the molten mass had assumed a laminated +structure, a fresh irruption of lava broke up the mass, and involved +fragments, and that subsequently the whole became relaminated.) From such +facts, most authors have attributed the lamination of these volcanic rocks +to their movement whilst liquified. Although it is easy to perceive, why +each separate air-cell, or each fibre in pumice-stone (Dolomieu "Voyage" +page 64.), should be drawn out in the direction of the moving mass; it is +by no means at first obvious why such air-cells and fibres should be +arranged by the movement, in the same planes, in laminae absolutely +straight and parallel to each other, and often of extreme tenuity; and +still less obvious is it, why such layers should come to be of slightly +different composition and of different textures. + +In endeavouring to make out the cause of the lamination of these igneous +feldspathic rocks, let us return to the facts so minutely described at +Ascension. We there see, that some of the thinnest layers are chiefly +formed by numerous, exceedingly minute, though perfect, crystals of +different minerals; that other layers are formed by the union of different +kinds of concretionary globules, and that the layers thus formed, often +cannot be distinguished from the ordinary feldspathic and pitchstone +layers, composing a large portion of the entire mass. The fibrous radiating +structure of the sphaerulites seems, judging from many analogous cases, to +connect the concretionary and crystalline forces: the separate crystals, +also, of feldspar all lie in the same parallel planes. (The formation, +indeed, of a large crystal of any mineral in a rock of mixed composition +implies an aggregation of the requisite atoms, allied to concretionary +action. The cause of the crystals of feldspar in these rocks of Ascension, +being all placed lengthways, is probably the same with that which elongates +and flattens all the brown sphaerulitic globules (which behave like +feldspar under the blowpipe) in this same direction.) These allied forces, +therefore, have played an important part in the lamination of the mass, but +they cannot be considered the primary force; for the several kinds of +nodules, both the smallest and largest, are internally zoned with +excessively fine shades of colour, parallel to the lamination of the whole; +and many of them are, also, externally marked in the same direction with +parallel ridges and furrows, which have not been produced by weathering. + +Some of the finest streaks of colour in the stony layers, alternating with +the obsidian, can be distinctly seen to be due to an incipient +crystallisation of the constituent minerals. The extent to which the +minerals have crystallised can, also, be distinctly seen to be connected +with the greater or less size, and with the number, of the minute, +flattened, crenulated air-cavities or fissures. Numerous facts, as in the +case of geodes, and of cavities in silicified wood, in primary rocks, and +in veins, show that crystallisation is much favoured by space. Hence, I +conclude, that, if in a mass of cooling volcanic rock, any cause produced +in parallel planes a number of minute fissures or zones of less tension +(which from the pent-up vapours would often be expanded into crenulated +air-cavities), the crystallisation of the constituent parts, and probably +the formation of concretions, would be superinduced or much favoured in +such planes; and thus, a laminated structure of the kind we are here +considering would be generated. + +That some cause does produce parallel zones of less tension in volcanic +rocks, during their consolidation, we must admit in the case of the thin +alternate layers of obsidian and pumice described by Humboldt, and of the +small, flattened, crenulated air-cells in the laminated rocks of Ascension; +for on no other principle can we conceive why the confined vapours should +through their expansion form air-cells or fibres in separate, parallel +planes, instead of irregularly throughout the mass. In Mr. Stokes' +collection, I have seen a beautiful example of this structure, in a +specimen of obsidian from Mexico, which is shaded and zoned, like the +finest agate, with numerous, straight, parallel layers, more or less opaque +and white, or almost perfectly glassy; the degree of opacity and glassiness +depending on the number of microscopically minute, flattened air-cells; in +this case, it is scarcely possible to doubt but that the mass, to which the +fragment belonged, must have been subjected to some, probably prolonged, +action, causing the tension slightly to vary in the successive planes. + +Several causes appear capable of producing zones of different tension, in +masses semi-liquified by heat. In a fragment of devitrified glass, I have +observed layers of sphaerulites which appeared, from the manner in which +they were abruptly bent, to have been produced by the simple contraction of +the mass in the vessel, in which it cooled. In certain dikes on Mount Etna, +described by M. Elie de Beaumont ("Mem. pour servir" etc. tome 4 page +131.), as bordered by alternating bands of scoriaceous and compact rock, +one is led to suppose that the stretching movement of the surrounding +strata, which originally produced the fissures, continued whilst the +injected rock remained fluid. Guided, however, by Professor Forbes' +("Edinburgh New Phil. Journal" 1842 page 350.) clear description of the +zoned structure of glacier-ice, far the most probable explanation of the +laminated structure of these feldspathic rocks appears to be, that they +have been stretched whilst slowly flowing onwards in a pasty condition (I +presume that this is nearly the same explanation which Mr. Scrope had in +his mind, when he speaks ("Geolog. Transact." volume 2 second series page +228) of the ribboned structure of his trachytic rocks, having arisen, from +"a linear extension of the mass, while in a state of imperfect liquidity, +coupled with a concretionary process."), in precisely the same manner as +Professor Forbes believes, that the ice of moving glaciers is stretched and +fissured. In both cases, the zones may be compared to those in the finest +agates; in both, they extend in the direction in which the mass has flowed, +and those exposed on the surface are generally vertical: in the ice, the +porous laminae are rendered distinct by the subsequent congelation of +infiltrated water, in the stony feldspathic lavas, by subsequent +crystalline and concretionary action. The fragment of glassy obsidian in +Mr. Stokes' collection, which is zoned with minute air-cells must +strikingly resemble, judging from Professor Forbes' descriptions, a +fragment of the zoned ice; and if the rate of cooling and nature of the +mass had been favourable to its crystallisation or to concretionary action, +we should here have had the finest parallel zones of different composition +and texture. In glaciers, the lines of porous ice and of minute crevices +seem to be due to an incipient stretching, caused by the central parts of +the frozen stream moving faster than the sides and bottom, which are +retarded by friction: hence in glaciers of certain forms and towards the +lower end of most glaciers, the zones become horizontal. May we venture to +suppose that in the feldspathic lavas with horizontal laminae, we see an +analogous case? All geologists, who have examined trachytic regions, have +come to the conclusion, that the lavas of this series have possessed an +exceedingly imperfect fluidity; and as it is evident that only matter thus +characterised would be subject to become fissured and to be formed into +zones of different tensions, in the manner here supposed, we probably see +the reason why augitic lavas, which appear generally to have possessed a +high degree of fluidity, are not, like the feldspathic lavas, divided into +laminae of different composition and texture. (Basaltic lavas, and many +other rocks, are not unfrequently divided into thick laminae or plates, of +the same composition, which are either straight or curved; these being +crossed by vertical lines of fissure, sometimes become united into columns. +This structure seems related, in its origin, to that by which many rocks, +both igneous and sedimentary, become traversed by parallel systems of +fissures.) Moreover, in the augitic series, there never appears to be any +tendency to concretionary action, which we have seen plays an important +part in the lamination of rocks, of the trachytic series, or at least in +rendering that structure apparent. + +Whatever may be thought of the explanation here advanced of the laminated +structure of the rocks of the trachytic series, I venture to call the +attention of geologists to the simple fact, that in a body of rock at +Ascension, undoubtedly of volcanic origin, layers often of extreme tenuity, +quite straight, and parallel to each other, have been produced;--some +composed of distinct crystals of quartz and diopside, mingled with +amorphous augitic specks and granular feldspar,--others entirely composed +of these black augitic specks, with granules of oxide of iron,--and lastly, +others formed of crystalline feldspar, in a more or less perfect state of +purity, together with numerous crystals of feldspar, placed lengthways. At +this island, there is reason to believe, and in some analogous cases, it is +certainly known, that the laminae have originally been formed with their +present high inclination. Facts of this nature are manifestly of +importance, with relation to the structural origin of that grand series of +plutonic rocks, which like the volcanic have undergone the action of heat, +and which consist of alternate layers of quartz, feldspar, mica and other +minerals. + + +CHAPTER IV.--ST. HELENA. + +Lavas of the feldspathic, basaltic, and submarine series. +Section of Flagstaff Hill and of the Barn. +Dikes. +Turk's Cap and Prosperous Bays. +Basaltic ring. +Central crateriform ridge, with an internal ledge and a parapet. +Cones of phonolite. +Superficial beds of calcareous sandstone. +Extinct land-shells. +Beds of detritus. +Elevation of the land. +Denudation. +Craters of elevation. + +The whole island is of volcanic origin; its circumference, according to +Beatson, is about twenty-eight miles. (Governor Beatson "Account of St. +Helena.") The central and largest part consists of rocks of a feldspathic +nature, generally decomposed to an extraordinary degree; and when in this +state, presenting a singular assemblage of alternating, red, purple, brown, +yellow, and white, soft, argillaceous beds. From the shortness of our +visit, I did not examine these beds with care; some of them, especially +those of the white, yellow, and brown shades, originally existed as streams +of lava, but the greater number were probably ejected in the form of +scoriae and ashes: other beds of a purple tint, porphyritic with crystal- +shaped patches of a white, soft substance, which are now unctuous, and +yield, like wax, a polished streak to the nail, seem once to have existed +as solid claystone-porphyries: the red argillaceous beds generally have a +brecciated structure, and no doubt have been formed by the decomposition of +scoriae. Several extensive streams, however, belonging to this series, +retain their stony character; these are either of a blackish-green colour, +with minute acicular crystals of feldspar, or of a very pale tint, and +almost composed of minute, often scaly, crystals of feldspar, abounding +with microscopical black specks; they are generally compact and laminated; +others, however, of similar composition, are cellular and somewhat +decomposed. None of these rocks contain large crystals of feldspar, or have +the harsh fracture peculiar to trachyte. These feldspathic lavas and tuffs +are the uppermost or those last erupted; innumerable dikes, however, and +great masses of molten rock, have subsequently been injected into them. +They converge, as they rise, towards the central curved ridge, of which one +point attains the elevation of 2,700 feet. This ridge is the highest land +in the island; and it once formed the northern rim of a great crater, +whence the lavas of this series flowed: from its ruined condition, from the +southern half having been removed, and from the violent dislocation which +the whole island has undergone, its structure is rendered very obscure. + +BASALTIC SERIES. + +The margin of the island is formed by a rude circle of great, black, +stratified, ramparts of basalt, dipping seaward, and worn into cliffs, +which are often nearly perpendicular, and vary in height from a few hundred +feet to two thousand. This circle, or rather horse-shoe shaped ring, is +open to the south, and is breached by several other wide spaces. Its rim or +summit generally projects little above the level of the adjoining inland +country; and the more recent feldspathic lavas, sloping down from the +central heights, generally abut against and overlap its inner margin; on +the north-western side of the island, however, they appear (judging from a +distance) to have flowed over and concealed portions of it. In some parts, +where the basaltic ring has been breached, and the black ramparts stand +detached, the feldspathic lavas have passed between them, and now overhang +the sea-coast in lofty cliffs. The basaltic rocks are of a black colour and +thinly stratified; they are generally highly vesicular, but occasionally +compact; some of them contain numerous crystals of glassy feldspar and +octahedrons of titaniferous iron; others abound with crystals of augite and +grains of olivine. The vesicles are frequently lined with minute crystals +(of chabasie?) and even become amygdaloidal with them. The streams are +separated from each other by cindery matter, or by a bright red, friable, +saliferous tuff, which is marked by successive lines like those of aqueous +deposition; and sometimes it has an obscure, concretionary structure. The +rocks of this basaltic series occur nowhere except near the coast. In most +volcanic districts the trachytic lavas are of anterior origin to the +basaltic; but here we see, that a great pile of rock, closely related in +composition to the trachytic family, has been erupted subsequently to the +basaltic strata: the number, however, of dikes, abounding with large +crystals of augite, with which the feldspathic lavas have been injected, +shows perhaps some tendency to a return to the more usual order of +superposition. + +BASAL SUBMARINE LAVAS. + +The lavas of this basal series lie immediately beneath both the basaltic +and feldspathic rocks. According to Mr. Seale, they may be seen at +intervals on the sea-beach round the entire island. ("Geognosy of the +Island of St. Helena." Mr. Seale has constructed a gigantic model of St. +Helena, well worth visiting, which is now deposited at Addiscombe College, +in Surrey.) In the sections which I examined, their nature varied much; +some of the strata abound with crystals of augite; others are of a brown +colour, either laminated or in a rubbly condition; and many parts are +highly amygdaloidal with calcareous matter. The successive sheets are +either closely united together, or are separated from each other by beds of +scoriaceous rock and of laminated tuff, frequently containing well-rounded +fragments. The interstices of these beds are filled with gypsum and salt; +the gypsum also sometimes occurring in thin layers. From the large quantity +of these two substances, from the presence of rounded pebbles in the tuffs, +and from the abundant amygdaloids, I cannot doubt that these basal volcanic +strata flowed beneath the sea. This remark ought perhaps to be extended to +a part of the superincumbent basaltic rocks; but on this point, I was not +able to obtain clear evidence. The strata of the basal series, whenever I +examined them, were intersected by an extraordinary number of dikes. + +FLAGSTAFF HILL AND THE BARN. + +(FIGURE 8. FLAGSTAFF HILL AND THE BARN. (Section West (left) to East +(right)) Flagstaff Hill, 2,272 feet high to The Barn, 2,015 feet high. + +The double lines represent the basaltic strata; the single, the basal +submarine strata; the dotted, the upper feldspathic strata; the dikes are +shaded transversely.) + +I will now describe some of the more remarkable sections, and will commence +with these two hills, which form the principal external feature on the +north-eastern side of the island. The square, angular outline, and black +colour of the Barn, at once show that it belongs to the basaltic series; +whilst the smooth, conical figure, and the varied bright tints of Flagstaff +Hill, render it equally clear, that it is composed of the softened, +feldspathic rocks. These two lofty hills are connected (as is shown in +Figure 8) by a sharp ridge, which is composed of the rubbly lavas of the +basal series. The strata of this ridge dip westward, the inclination +becoming less and less towards the Flagstaff; and the upper feldspathic +strata of this hill can be seen, though with some difficulty, to dip +conformably to the W.S.W. Close to the Barn, the strata of the ridge are +nearly vertical, but are much obscured by innumerable dikes; under this +hill, they probably change from being vertical into being inclined into an +opposite direction; for the upper or basaltic strata, which are about eight +hundred or one thousand feet in thickness, are inclined north-eastward, at +an angle between thirty and forty degrees. + +This ridge, and likewise the Barn and Flagstaff Hills, are interlaced by +dikes, many of which preserve a remarkable parallelism in a N.N.W. and +S.S.E. direction. The dikes chiefly consist of a rock, porphyritic with +large crystals of augite; others are formed of a fine-grained and brown- +coloured trap. Most of these dikes are coated by a glossy layer, from one +to two-tenths of an inch in thickness, which, unlike true pitchstone, fuses +into a black enamel; this layer is evidently analogous to the glossy +superficial coating of many lava streams. (This circumstance has been +observed (Lyell "Principles of Geology" volume 4 chapter 10 page 9) in the +dikes of the Atrio del Cavallo, but apparently it is not of very common +occurrence. Sir G. Mackenzie, however, states (page 372 "Travels in +Iceland") that all the veins in Iceland have a "black vitreous coating on +their sides." Captain Carmichael, speaking of the dikes in Tristan +d'Acunha, a volcanic island in the Southern Atlantic, says ("Linnaean +Transactions" volume 12 page 485) that their sides, "where they come in +contact with the rocks, are invariably in a semi-vitrified state.") The +dikes can often be followed for great lengths both horizontally and +vertically, and they seem to preserve a nearly uniform thickness ("Geognosy +of the Island of St. Helena" plate 5.): Mr. Seale states, that one near the +Barn, in a height of 1,260 feet, decreases in width only four inches,--from +nine feet at the bottom, to eight feet and eight inches at the top. On the +ridge, the dikes appear to have been guided in their course, to a +considerable degree, by the alternating soft and hard strata: they are +often firmly united to the harder strata, and they preserve their +parallelism for such great lengths, that in very many instances it was +impossible to conjecture, which of the beds were dikes, and which streams +of lava. The dikes, though so numerous on this ridge, are even more +numerous in the valleys a little south of it, and to a degree I never saw +equalled anywhere else: in these valleys they extend in less regular lines, +covering the ground with a network, like a spider's web, and with some +parts of the surface even appearing to consist wholly of dikes, interlaced +by other dikes. + +From the complexity produced by the dikes, from the high inclination and +anticlinal dip of the strata of the basal series, which are overlaid, at +the opposite ends of the short ridge, by two great masses of different ages +and of different composition, I am not surprised that this singular section +has been misunderstood. It has even been supposed to form part of a crater; +but so far is this from having been the case, that the summit of Flagstaff +Hill once formed the lower extremity of a sheet of lava and ashes, which +were erupted from the central, crateriform ridge. Judging from the slope of +the contemporaneous streams in an adjoining and undisturbed part of the +island, the strata of the Flagstaff Hill must have been upturned at least +twelve hundred feet, and probably much more, for the great truncated dikes +on its summit show that it has been largely denuded. The summit of this +hill now nearly equals in height the crateriform ridge; and before having +been denuded, it was probably higher than this ridge, from which it is +separated by a broad and much lower tract of country; we here, therefore, +see that the lower extremities of a set of lava-streams have been tilted up +to as great a height as, or perhaps greater height than, the crater, down +the flanks of which they originally flowed. I believe that dislocations on +so grand a scale are extremely rare in volcanic districts. (M. Constant +Prevost "Mem. de la Soc. Geolog." tome 2 observes that "les produits +volcaniques n'ont que localement et rarement meme derange le sol, a travers +lequel ils se sont fait jour.") 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 degree: this stretching, on the ridge +between Flagstaff and Barn Hills, probably took place subsequently (though +perhaps immediately so) to the strata being tilted; for had the strata at +that time extended horizontally, they would in all probability have been +fissured and injected transversely, instead of in the planes of their +stratification. Although the space between the Barn and Flagstaff Hill +presents a distinct anticlinal line extending north and south, and though +most of the dikes range with much regularity in the same line, +nevertheless, at only a mile due south of the ridge the strata lie +undisturbed. Hence the disturbing force seems to have acted under a point, +rather than along a line. The manner in which it has acted, is probably +explained by the structure of Little Stony-top, a mountain 2,000 feet high, +situated a few miles southward of the Barn; we there see, even from a +distance, a dark-coloured, sharp, wedge of compact columnar rock, with the +bright-coloured feldspathic strata, sloping away on each side from its +uncovered apex. This wedge, from which it derives its name of Stony-top, +consists of a body of rock, which has been injected whilst liquified into +the overlying strata; and if we may suppose that a similar body of rock +lies injected, beneath the ridge connecting the Barn and Flagstaff, the +structure there exhibited would be explained. + +TURK'S CAP AND PROSPEROUS BAYS. + +(FIGURE 9. PROSPEROUS HILL AND THE BARN. (Section S.S.E. (left) to N.N.W. +(right) Prosperous Hill through Hold-fast-Tom and Flagstaff Hill to The +Barn. + +The double lines represent the basaltic strata; the single, the basal +submarine strata; the dotted, the upper feldspathic strata.) + +Prosperous Hill is a great, black, precipitous mountain, situated two miles +and a half south of the Barn, and composed, like it, of basaltic strata. +These rest, in one part, on the brown-coloured, porphyritic beds of the +basal series, and in another part, on a fissured mass of highly scoriaceous +and amygdaloidal rock, which seems to have formed a small point of eruption +beneath the sea, contemporaneously with the basal series. Prosperous Hill, +like the Barn, is traversed by many dikes, of which the greater number +range north and south, and its strata dip, at an angle of about 20 degrees, +rather obliquely from the island towards the sea. The space between +Prosperous Hill and the Barn, as represented in Figure 9, consists of lofty +cliffs, composed of the lavas of the upper or feldspathic series, which +rest, though unconformably, on the basal submarine strata, as we have seen +that they do at Flagstaff Hill. Differently, however, from in that hill, +these upper strata are nearly horizontal, gently rising towards the +interior of the island; and they are composed of greenish-black, or more +commonly, pale brown, compact lavas, instead of softened and highly +coloured matter. These brown-coloured, compact lavas, consist almost +entirely of small glimmering scales, or of minute acicular crystals, of +feldspar, placed close by the side of each other, and abounding with minute +black specks, apparently of hornblende. The basaltic strata of Prosperous +Hill project only a little above the level of the gently-sloping, +feldspathic streams, which wind round and abut against their upturned +edges. The inclination of the basaltic strata seems to be too great to have +been caused by their having flowed down a slope, and they must have been +tilted into their present position before the eruption of the feldspathic +streams. + +BASALTIC RING. + +Proceeding round the Island, the lavas of the upper series, southward of +Prosperous Hill, overhang the sea in lofty precipices. Further on, the +headland, called Great Stony-top, is composed, as I believe, of basalt; as +is Long Range Point, on the inland side of which the coloured beds abut. On +the southern side of the island, we see the basaltic strata of the South +Barn, dipping obliquely seaward at a considerable angle; this headland, +also, stands a little above the level of the more modern, feldspathic +lavas. Further on, a large space of coast, on each side of Sandy Bay, has +been much denuded, and there seems to be left only the basal wreck of the +great, central crater. The basaltic strata reappear, with their seaward +dip, at the foot of the hill, called Man-and-Horse; and thence they are +continued along the whole north-western coast to Sugar-Loaf Hill, situated +near to the Flagstaff; and they everywhere have the same seaward +inclination, and rest, in some parts at least, on the lavas of the basal +series. We thus see that the circumference of the island is formed by a +much-broken ring, or rather, a horse-shoe, of basalt, open to the south, +and interrupted on the eastern side by many wide breaches. The breadth of +this marginal fringe on the north-western side, where alone it is at all +perfect, appears to vary from a mile to a mile and a half. The basaltic +strata, as well as those of the subjacent basal series, dip, with a +moderate inclination, where they have not been subsequently disturbed, +towards the sea. The more broken state of the basaltic ring round the +eastern half, compared with the western half of the island, is evidently +due to the much greater denuding power of the waves on the eastern or +windward side, as is shown by the greater height of the cliffs on that +side, than to leeward. Whether the margin of basalt was breached, before or +after the eruption of the lavas of the upper series, is doubtful; but as +separate portions of the basaltic ring appear to have been tilted before +that event, and from other reasons, it is more probable, that some at least +of the breaches were first formed. Reconstructing in imagination, as far as +is possible, the ring of basalt, the internal space or hollow, which has +since been filled up with the matter erupted from the great central crater, +appears to have been of an oval figure, eight or nine miles in length by +about four miles in breadth, and with its axis directed in a N.E. and S W. +line, coincident with the present longest axis of the island. + +THE CENTRAL CURVED RIDGE. + +This ridge consists, as before remarked, of grey feldspathic lavas, and of +red, brecciated, argillaceous tuffs, like the beds of the upper coloured +series. The grey lavas contain numerous, minute, black, easily fusible +specks; and but very few large crystals of feldspar. They are generally +much softened; with the exception of this character, and of being in many +parts highly cellular, they are quite similar to those great sheets of lava +which overhang the coast at Prosperous Bay. Considerable intervals of time +appear to have elapsed, judging from the marks of denudation, between the +formation of the successive beds, of which this ridge is composed. On the +steep northern slope, I observed in several sections a much worn undulating +surface of red tuff, covered by grey, decomposed, feldspathic lavas, with +only a thin earthy layer interposed between them. In an adjoining part, I +noticed a trap-dike, four feet wide, cut off and covered up by the +feldspathic lava, as is represented in Figure 9. The ridge ends on the +eastern side in a hook, which is not represented clearly enough in any map +which I have seen; towards the western end, it gradually slopes down and +divides into several subordinate ridges. The best defined portion between +Diana's Peak and Nest Lodge, which supports the highest pinnacles in the +island varying from 2,000 to 2,700 feet, is rather less than three miles +long in a straight line. Throughout this space the ridge has a uniform +appearance and structure; its curvature resembles that of the coast-line of +a great bay, being made up of many smaller curves, all open to the south. +The northern and outer side is supported by narrow ridges or buttresses, +which slope down to the adjoining country. The inside is much steeper, and +is almost precipitous; it is formed of the basset edges of the strata, +which gently decline outwards. Along some parts of the inner side, a little +way beneath the summit, a flat ledge extends, which imitates in outline the +smaller curvatures of the crest. Ledges of this kind occur not unfrequently +within volcanic craters, and their formation seems to be due to the sinking +down of a level sheet of hardened lava, the edges of which remain (like the +ice round a pool, from which the water has been drained) adhering to the +sides. (A most remarkable instance of this structure is described in Ellis +"Polynesian Researches" second edition where an admirable drawing is given +of the successive ledges or terraces, on the borders of the immense crater +at Hawaii, in the Sandwich Islands.) + +(FIGURE 10. DIKE. (Section showing layers 1, 2 and 3 from top to bottom.) + +1. Grey feldspathic lava. + +2. A layer, one inch in thickness, of a reddish earthy matter. + +3. Brecciated, red, argillaceous tuff.) + +In some parts, the ridge is surmounted by a wall or parapet, perpendicular +on both sides. Near Diana's Peak this wall is extremely narrow. At the +Galapagos Archipelago I observed parapets, having a quite similar structure +and appearance, surmounting several of the craters; one, which I more +particularly examined, was composed of glossy, red scoriae firmly cemented +together; being externally perpendicular, and extending round nearly the +whole circumference of the crater, it rendered it almost inaccessible. The +Peak of Teneriffe and Cotopaxi, according to Humboldt, are similarly +constructed; he states that "at their summits a circular wall surrounds the +crater, which wall, at a distance, has the appearance of a small cylinder +placed on a truncated cone. ("Personal Narrative" volume 1 page 171.) On +Cotopaxi this peculiar structure is visible to the naked eye at more than +two thousand toises' distance; and no person has ever reached its crater. +(Humboldt "Picturesque Atlas" folio plate 10.) On the Peak of Teneriffe, +the parapet is so high, that it would be impossible to reach the caldera, +if on the eastern side there did not exist a breach." The origin of these +circular parapets is probably due to the heat or vapours from the crater, +penetrating and hardening the sides to a nearly equal depth, and afterwards +to the mountain being slowly acted on by the weather, which would leave the +hardened part, projecting in the form of a cylinder or circular parapet. + +From the points of structure in the central ridge, now enumerated,--namely, +from the convergence towards it of the beds of the upper series,--from the +lavas there becoming highly cellular,--from the flat ledge, extending along +its inner and precipitous side, like that within some still active +craters,--from the parapet-like wall on its summit,--and lastly, from its +peculiar curvature, unlike that of any common line of elevation, I cannot +doubt that this curved ridge forms the last remnant of a great crater. In +endeavouring, however, to trace its former outline, one is soon baffled; +its western extremity gradually slopes down, and, branching into other +ridges, extends to the sea-coast; the eastern end is more curved, but it is +only a little better defined. Some appearances lead me to suppose that the +southern wall of the crater joined the present ridge near Nest Lodge; in +this case the crater must have been nearly three miles long, and about a +mile and a half in breadth. Had the denudation of the ridge and the +decomposition of its constituent rocks proceeded a few steps further, and +had this ridge, like several other parts of the island, been broken up by +great dikes and masses of injected matter, we should in vain have +endeavoured to discover its true nature. 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 flowed, and probably even to a greater height. It is +interesting thus to trace the steps by which the structure of a volcanic +district becomes obscured, and finally obliterated: so near to this last +stage is St. Helena, that I believe no one has hitherto suspected that the +central ridge or axis of the island is the last wreck of the crater, whence +the most modern volcanic streams were poured forth. + +The great hollow space or valley southward of the central curved ridge, +across which the half of the crater must once have extended, is formed of +bare, water-worn hillocks and ridges of red, yellow, and brown rocks, +mingled together in chaos-like confusion, interlaced by dikes, and without +any regular stratification. The chief part consists of red decomposing +scoriae, associated with various kinds of tuff and yellow argillaceous +beds, full of broken crystals, those of augite being particularly large. +Here and there masses of highly cellular and amygdaloidal lavas protrude. +From one of the ridges in the midst of the valley, a conical precipitous +hill, called Lot, boldly stands up, and forms a most singular and +conspicuous object. It is composed of phonolite, divided in one part into +great curved laminae, in another, into angular concretionary balls, and in +a third part into outwardly radiating columns. At its base the strata of +lava, tuff, and scoriae, dip away on all sides (Abich in his "Views of +Vesuvius" plate 6 has shown the manner in which beds, under nearly similar +circumstances, are tilted up. The upper beds are more turned up than the +lower; and he accounts for this, by showing that the lava insinuates itself +horizontally between the lower beds.); the uncovered portion is 197 feet in +height (This height is given by Mr. Seale in his Geognosy of the island. +The height of the summit above the level of the sea is said to be 1,444 +feet.), and its horizontal section gives an oval figure. The phonolite is +of a greenish-grey colour, and is full of minute acicular crystals of +feldspar; in most parts it has a conchoidal fracture, and is sonorous, yet +it is crenulated with minute air-cavities. In a S.W. direction from Lot, +there are some other remarkable columnar pinnacles, but of a less regular +shape, namely, Lot's Wife, and the Asses' Ears, composed of allied kinds of +rock. From their flattened shape, and their relative position to each +other, they are evidently connected on the same line of fissure. It is, +moreover, remarkable that this same N.E. and S.W. line, joining Lot and +Lot's Wife, if prolonged would intersect Flagstaff Hill, which, as before +stated, is crossed by numerous dikes running in this direction, and which +has a disturbed structure, rendering it probable that a great body of once +fluid rock lies injected beneath it. + +In this same great valley there are several other conical masses of +injected rock (one, I observed, was composed of compact greenstone), some +of which are not connected, as far as is apparent, with any line of dike; +whilst others are obviously thus connected. Of these dikes, three or four +great lines stretch across the valley in a N.E. and S.W. direction, +parallel to that one connecting the Asses' Ears, Lot's Wife, and probably +Lot. The number of these masses of injected rock is a remarkable feature in +the geology of St. Helena. Besides those just mentioned, and the +hypothetical one beneath Flagstaff Hill, there is Little Stony-top and +others, as I have reason to believe, at the Man-and-Horse, and at High +Hill. Most of these masses, if not all of them, have been injected +subsequently to the last volcanic eruptions from the central crater. The +formation of conical bosses of rock on lines of fissure, the walls of which +are in most cases parallel, may probably be attributed to inequalities in +the tension, causing small transverse fissures, and at these points of +intersection the edges of the strata would naturally yield, and be easily +turned upwards. Finally, I may remark, that hills of phonolite everywhere +are apt to assume singular and even grotesque shapes, like that of Lot +(D'Aubuisson in his "Traite de Geognosie" tome 2 page 540 particularly +remarks that this is the case.): the peak at Fernando Noronha offers an +instance; at St. Jago, however, the cones of phonolite, though tapering, +have a regular form. Supposing, as seems probable, that all such hillocks +or obelisks have originally been injected, whilst liquified, into a mould +formed by yielding strata, as certainly has been the case with Lot, how are +we to account for the frequent abruptness and singularity of their +outlines, compared with similarly injected masses of greenstone and basalt? +Can it be due to a less perfect degree of fluidity, which is generally +supposed to be characteristic of the allied trachytic lavas? + +SUPERFICIAL DEPOSITS. + +Soft calcareous sandstone occurs in extensive, though thin, superficial +beds, both on the northern and southern shores of the island. It consists +of very minute, equal-sized, rounded particles of shells, and other organic +bodies, which partially retain their yellow, brown, and pink colours, and +occasionally, though very rarely, present an obscure trace of their +original external forms. I in vain endeavoured to find a single unrolled +fragment of a shell. The colour of the particles is the most obvious +character by which their origin can be recognised, the tints being affected +(and an odour produced) by a moderate heat, in the same manner as in fresh +shells. The particles are cemented together, and are mingled with some +earthy matter: the purest masses, according to Beatson, contain 70 per cent +of carbonate of lime. The beds, varying in thickness from two or three feet +to fifteen feet, coat the surface of the ground; they generally lie on that +side of the valley which is protected from the wind, and they occur at the +height of several hundred feet above the level of the sea. Their position +is the same which sand, if now drifted by the trade-wind, would occupy; and +no doubt they thus originated, which explains the equal size and minuteness +of the particles, and likewise the entire absence of whole shells, or even +of moderately-sized fragments. It is remarkable that at the present day +there are no shelly beaches on any part of the coast, whence calcareous +dust could be drifted and winnowed; we must, therefore, look back to a +former period when before the land was worn into the present great +precipices, a shelving coast, like that of Ascension, was favourable to the +accumulation of shelly detritus. Some of the beds of this limestone are +between six hundred and seven hundred feet above the sea; but part of this +height may possibly be due to an elevation of the land, subsequent to the +accumulation of the calcareous sand. + +The percolation of rain-water has consolidated parts of these beds into a +solid rock, and has formed masses of dark brown, stalagmitic limestone. At +the Sugar-Loaf quarry, fragments of rock on the adjoining slopes have been +thickly coated by successive fine layers of calcareous matter. (In the +earthy detritus on several parts of this hill, irregular masses of very +impure, crystallised sulphate of lime occur. As this substance is now being +abundantly deposited by the surf at Ascension, it is possible that these +masses may thus have originated; but if so, it must have been at a period +when the land stood at a much lower level. This earthy selenite is now +found at a height of between six hundred and seven hundred feet.) It is +singular, that many of these pebbles have their entire surfaces coated, +without any point of contact having been left uncovered; hence, these +pebbles must have been lifted up by the slow deposition between them of the +successive films of carbonate of lime. Masses of white, finely oolitic rock +are attached to the outside of some of these coated pebbles. Von Buch has +described a compact limestone at Lanzarote, which seems perfectly to +resemble the stalagmitic deposition just mentioned: it coats pebbles, and +in parts is finely oolitic: it forms a far-extended layer, from one inch to +two or three feet in thickness, and it occurs at the height of 800 feet +above the sea, but only on that side of the island exposed to the violent +north-western winds. Von Buch remarks, that it is not found in hollows, but +only on the unbroken and inclined surfaces of the mountain. ("Description +des Isles Canaries" page 293.) He believes, that it has been deposited by +the spray which is borne over the whole island by these violent winds. It +appears, however, to me much more probable that it has been formed, as at +St. Helena, by the percolation of water through finely comminuted shells: +for when sand is blown on a much-exposed coast, it always tends to +accumulate on broad, even surfaces, which offer a uniform resistance to the +winds. At the neighbouring island, moreover, of Feurteventura, there is an +earthy limestone, which, according to Von Buch, is quite similar to +specimens which he has seen from St. Helena, and which he believes to have +been formed by the drifting of shelly detritus. (Idem pages 314 and 374.) + +The upper beds of the limestone, at the above-mentioned quarry on the +Sugar-Loaf Hill, are softer, finer-grained and less pure, than the lower +beds. They abound with fragments of land-shells, and with some perfect +ones; they contain, also, the bones of birds, and the large eggs, +apparently of water-fowl. (Colonel Wilkes, in a catalogue presented with +some specimens to the Geological Society, states that as many as ten eggs +were found by one person. Dr. Buckland has remarked ("Geolog. Trans." +volume 5 page 474) on these eggs.) It is probable that these upper beds +remained long in an unconsolidated form, during which time, these +terrestrial productions were embedded. Mr. G.R. Sowerby has kindly examined +three species of land-shells, which I procured from this bed, and has +described them in detail. One of them is a Succinea, identical with a +species now living abundantly on the island; the two others, namely, +Cochlogena fossilis and Helix biplicata, are not known in a recent state: +the latter species was also found in another and different locality, +associated with a species of Cochlogena which is undoubtedly extinct. + +BEDS OF EXTINCT LAND-SHELLS. + +Land-shells, all of which appear to be species now extinct, occur embedded +in earth, in several parts of the island. The greater number have been +found at a considerable height on Flagstaff Hill. On the N.W. side of this +hill, a rain-channel exposes a section of about twenty feet in thickness, +of which the upper part consists of black vegetable mould, evidently washed +down from the heights above, and the lower part of less black earth, +abounding with young and old shells, and with their fragments: part of this +earth is slightly consolidated by calcareous matter, apparently due to the +partial decomposition of some of the shells. Mr. Seale, an intelligent +resident, who first called attention to these shells, gave me a large +collection from another locality, where the shells appear to have been +embedded in very black earth. Mr. G.R. Sowerby has examined these shells, +and has described them. There are seven species, namely, one Cochlogena, +two species of the genus Cochlicopa, and four of Helix; none of these are +known in a recent state, or have been found in any other country. The +smaller species were picked out of the inside of the large shells of the +Cochlogena aurisvulpina. This last-mentioned species is in many respects a +very singular one; it was classed, even by Lamarck, in a marine genus, and +having thus been mistaken for a sea-shell, and the smaller accompanying +species having been overlooked, the exact localities where it was found +have been measured, and the elevation of this island thus deduced! It is +very remarkable that all the shells of this species found by me in one +spot, form a distinct variety, as described by Mr. Sowerby, from those +procured from another locality by Mr. Seale. As this Cochlogena is a large +and conspicuous shell, I particularly inquired from several intelligent +countrymen whether they had ever seen it alive; they all assured me that +they had not, and they would not even believe that it was a land animal: +Mr. Seale, moreover, who was a collector of shells all his life at St. +Helena, never met with it alive. Possibly some of the smaller species may +turn out to be yet living kinds; but, on the other hand, the two land- +shells which are now living on the island in great numbers, do not occur +embedded, as far as is yet known, with the extinct species. I have shown in +my "Journal" ("Journal of Researches" page 582.), that the extinction of +these land-shells possibly may not be an ancient event; as a great change +took place in the state of the island about one hundred and twenty years +ago, when the old trees died, and were not replaced by young ones, these +being destroyed by the goats and hogs, which had run wild in numbers, from +the year 1502. Mr. Seale states, that on Flagstaff Hill, where we have seen +that the embedded land-shells are especially numerous, traces are +everywhere discoverable, which plainly indicate that it was once thickly +clothed with trees; at present not even a bush grows there. The thick bed +of black vegetable mould which covers the shell-bed, on the flanks of this +hill, was probably washed down from the upper part, as soon as the trees +perished, and the shelter afforded by them was lost. + +ELEVATION OF THE LAND. + +Seeing that the lavas of the basal series, which are of submarine origin, +are raised above the level of the sea, and at some places to the height of +many hundred feet, I looked out for superficial signs of the elevation of +the land. The bottoms of some of the gorges, which descend to the coast, +are filled up to the depth of about a hundred feet, by rudely divided +layers of sand, muddy clay, and fragmentary masses; in these beds, Mr. +Seale has found the bones of the tropic-bird and of the albatross; the +former now rarely, and the latter never visiting the island. From the +difference between these layers, and the sloping piles of detritus which +rest on them, I suspect that they were deposited, when the gorges stood +beneath the sea. Mr. Seale, moreover, has shown that some of the fissure- +like gorges become, with a concave outline, gradually rather wider at the +bottom than at the top; and this peculiar structure was probably caused by +the wearing action of the sea, when it entered the lower part of these +gorges. (A fissure-like gorge, near Stony-top, is said by Mr. Seale to be +840 feet deep, and only 115 feet in width.) At greater heights, the +evidence of the rise of the land is even less clear: nevertheless, in a +bay-like depression on the table-land behind Prosperous Bay, at the height +of about a thousand feet, there are flat-topped masses of rock, which it is +scarcely conceivable, could have been insulated from the surrounding and +similar strata, by any other agency than the denuding action of a sea- +beach. Much denudation, indeed, has been effected at great elevations, +which it would not be easy to explain by any other means: thus, the flat +summit of the Barn, which is 2,000 feet high, presents, according to Mr. +Seale, a perfect network of truncated dikes; on hills like the Flagstaff, +formed of soft rock, we might suppose that the dikes had been worn down and +cut off by meteoric agency, but we can hardly suppose this possible with +the hard, basaltic strata of the Barn. + +COAST DENUDATION. + +The enormous cliffs, in many parts between one and two thousand feet in +height, with which this prison-like island is surrounded, with the +exception of only a few places, where narrow valleys descend to the coast, +is the most striking feature in its scenery. We have seen that portions of +the basaltic ring, two or three miles in length by one or two miles in +breadth, and from one to two thousand feet in height, have been wholly +removed. There are, also, ledges and banks of rock, rising out of +profoundly deep water, and distant from the present coast between three and +four miles, which, according to Mr. Seale, can be traced to the shore, and +are found to be the continuations of certain well-known great dikes. The +swell of the Atlantic Ocean has obviously been the active power in forming +these cliffs; and it is interesting to observe that the lesser, though +still great, height of the cliffs on the leeward and partially protected +side of the island (extending from the Sugar-Loaf Hill to South West +Point), corresponds with the lesser degree of exposure. When reflecting on +the comparatively low coasts of many volcanic islands, which also stand +exposed in the open ocean, and are apparently of considerable antiquity, +the mind recoils from an attempt to grasp the number of centuries of +exposure, 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 island. The contrast in the superficial state of St. Helena, +compared with the nearest island, namely, Ascension, is very striking. At +Ascension, the surfaces of the lava-streams are glossy, as if just poured +forth, their boundaries are well defined, and they can often be traced to +perfect craters, whence they were erupted; in the course of many long +walks, I did not observe a single dike; and the coast round nearly the +entire circumference is low, and has been eaten back (though too much +stress must not be placed on this fact, as the island may have been +subsiding) into a little wall only from ten to thirty feet high. Yet during +the 340 years, since Ascension has been known, not even the feeblest signs +of volcanic action have been recorded. (In the "Nautical Magazine" for 1835 +page 642, and for 1838 page 361, and in the "Comptes Rendus" April 1838, +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 century, in a space of open sea +between longitudes 20 degrees and 22 degrees west, about half a degree +south of the equator. These facts seem to show, that an island or an +archipelago is in process of formation in the middle of the Atlantic: a +line joining St. Helena and Ascension, prolonged, intersects this slowly +nascent focus of volcanic action.) On the other hand, at St. Helena, the +course of no one stream of lava can be traced, either by the state of its +boundaries or of its superficies; the mere wreck of one great crater is +left; not the valleys only, but the surfaces of some of the highest hills, +are interlaced by worn-down dikes, and, in many places, the denuded summits +of great cones of injected rock stand exposed and naked; lastly, as we have +seen, the entire circuit of the island has been deeply worn back into the +grandest precipices. + +CRATERS OF ELEVATION. + +There is much resemblance in structure and in geological history between +St. Helena, St. Jago, and Mauritius. All three islands are bounded (at +least in the parts which I was able to examine) by a ring of basaltic +mountains, now much broken, but evidently once continuous. These mountains +have, or apparently once had, their escarpments steep towards the interior +of the island, and their strata dip outwards. I was able to ascertain, only +in a few cases, the inclination of the beds; nor was this easy, for the +stratification was generally obscure, except when viewed from a distance. I +feel, however, little doubt that, according to the researches of M. Elie de +Beaumont, their average inclination is greater than that which they could +have acquired, considering their thickness and compactness, by flowing down +a sloping surface. At St. Helena, and at St. Jago, the basaltic strata rest +on older and probably submarine beds of different composition. At all three +islands, deluges of more recent lavas have flowed from the centre of the +island, towards and between the basaltic mountains; and at St. Helena the +central platform has been filled up by them. All three islands have been +raised in mass. At Mauritius the sea, within a late geological period, must +have reached to the foot of the basaltic mountains, as it now does at St. +Helena; and at St. Jago it is cutting back the intermediate plain towards +them. In these three islands, but especially at St. Jago and at Mauritius, +when, standing on the summit of one of the old basaltic mountains, one +looks in vain towards the centre of the island,--the point towards which +the strata beneath one's feet, and of the mountains on each side, rudely +converge,--for a source whence these strata could have been erupted; but +one sees only a vast hollow platform stretched beneath, or piles of matter +of more recent origin. + +These basaltic mountains come, I presume, into the class of Craters of +elevation: it is immaterial whether the rings were ever completely formed, +for the portions which now exist have so uniform a structure, that, if they +do not form fragments of true craters, they cannot be classed with ordinary +lines of elevation. With respect to their origin, after having read the +works of Mr. Lyell ("Principles of Geology" fifth edition volume 2 page +171.), and of MM. C. Prevost and Virlet, I cannot believe that the great +central hollows have been formed by a simple dome-shaped elevation, and the +consequent arching of the strata. On the other hand, I have very great +difficulty in admitting that these basaltic mountains are merely the basal +fragments of great volcanoes, of which the summits have either been blown +off, or more probably swallowed up by subsidence. These rings are, in some +instances, so immense, as at St. Jago and at Mauritius, and their +occurrence is so frequent, that I can hardly persuade myself to adopt this +explanation. Moreover, I suspect that the following circumstances, from +their frequent concurrence, are someway connected together,--a connection +not implied in either of the above views: namely, first, the broken state +of the ring; showing that the now detached portions have been exposed to +great denudation, and in some cases, perhaps, rendering it probable that +the ring never was entire; secondly, the great amount of matter erupted +from the central area after or during the formation of the ring; and +thirdly, the elevation of the district in mass. As far as relates to the +inclination of the strata being greater than that which the basal fragments +of ordinary volcanoes would naturally possess, I can readily believe that +this inclination might have been slowly acquired by that amount of +elevation, of which, according to M. Elie de Beaumont, the numerous +upfilled fissures or dikes are the evidence and the measure,--a view +equally novel and important, which we owe to the researches of that +geologist on Mount Etna. + +A conjecture, including the above circumstances, occurred to me, when,-- +with my mind fully convinced, from the phenomena of 1835 in South America, +that the forces which eject matter from volcanic orifices and raise +continents in mass are identical,--I viewed that part of the coast of St. +Jago, where the horizontally upraised, calcareous stratum dips into the +sea, directly beneath a cone of subsequently erupted lava. (I have given a +detailed account of these phenomena, in a paper read before the Geological +Society in March 1838. At the instant of time, when an immense area was +convulsed and a large tract elevated, the districts immediately surrounding +several of the great vents in the Cordillera remained quiescent; the +subterranean forces being apparently relieved by the eruptions, which then +recommenced with great violence. An event of somewhat the same kind, but on +an infinitely smaller scale, appears to have taken place, according to +Abich ("Views of Vesuvius" plates 1 and 9), within the great crater of +Vesuvius, where a platform on one side of a fissure was raised in mass +twenty feet, whilst on the other side, a train of small volcanoes burst +forth in eruption.) The conjecture is that, during the slow elevation of a +volcanic district or island, in the centre of which one or more orifices +continue open, and thus relieve the subterranean forces, the borders are +elevated more than the central area; and that the portions thus upraised do +not slope gently into the central, less elevated area, as does the +calcareous stratum under the cone at St. Jago, and as does a large part of +the circumference of Iceland, but that they are separated from it by curved +faults. (It appears, from information communicated to me in the most +obliging manner by M. E. Robert, that the circumferential parts of Iceland, +which are composed of ancient basaltic strata alternating with tuff, dip +inland, thus forming a gigantic saucer. M. Robert found that this was the +case, with a few and quite local exceptions, for a space of coast several +hundred miles in length. I find this statement corroborated, as far as +regards one place, by Mackenzie in his "Travels" page 377, and in another +place by some MS. notes kindly lent me by Dr. Holland. The coast is deeply +indented by creeks, at the head of which the land is generally low. M. +Robert informs me, that the inwardly dipping strata appear to extend as far +as this line, and that their inclination usually corresponds with the slope +of the surface, from the high coast-mountains to the low land at the head +of these creeks. In the section described by Sir G. Mackenzie, the dip is +120. The interior parts of the island chiefly consist, as far as is known, +of recently erupted matter. The great size, however, of Iceland, equalling +the bulkiest part of England, ought perhaps to exclude it from the class of +islands we have been considering; but I cannot avoid suspecting that if the +coast-mountains, instead of gently sloping into the less elevated central +area, had been separated from it by irregularly curved faults, the strata +would have been tilted seaward, and a "Crater of elevation," like that of +St. Jago or that of Mauritius, but of much vaster dimensions, would have +been formed. I will only further remark, that the frequent occurrence of +extensive lakes at the foot of large volcanoes, and the frequent +association of volcanic and fresh-water strata, seem to indicate that the +areas around volcanoes are apt to be depressed beneath the level of the +adjoining country, either from having been less elevated, or from the +effects of subsidence.) We might expect, from what we see along ordinary +faults, that the strata on the upraised side, already dipping outwards from +their original formation as lava-streams, would be tilted from the line of +fault, and thus have their inclination increased. According to this +hypothesis, which I am tempted to extend only to some few cases, it is not +probable that the ring would ever be formed quite perfect; and from the +elevation being slow, the upraised portions would generally be exposed to +much denudation, and hence the ring become broken; we might also expect to +find occasional inequalities in the dip of the upraised masses, as is the +case at St. Jago. By this hypothesis the elevation of the districts in +mass, and the flowing of deluges of lava from the central platforms, are +likewise connected together. On this view the marginal basaltic mountains +of the three foregoing islands might still be considered as forming +"Craters of elevation;" the kind of elevation implied having been slow, and +the central hollow or platform having been formed, not by the arching of +the surface, but simply by that part having been upraised to a less height. + + +CHAPTER V.--GALAPAGOS ARCHIPELAGO. + +Chatham Island. +Craters composed of a peculiar kind of tuff. +Small basaltic craters, with hollows at their bases. +Albemarle Island; fluid lavas, their composition. +Craters of tuff; inclination of their exterior diverging strata, and +structure of their interior converging strata. +James Island, segment of a small basaltic crater; fluidity and composition +of its lava-streams, and of its ejected fragments. +Concluding remarks on the craters of tuff, and on the breached condition of +their southern sides. +Mineralogical composition of the rocks of the archipelago. +Elevation of the land. +Direction of the fissures of eruption. + +(FIGURE 11. MAP 3. GALAPAGOS ARCHIPELAGO. + +Showing Wenman, Abingdon, Bindloes, Tower, Narborough, Albemarle, James, +Indefatigable, Barrington, Chatham, Charles and Hood's Islands.) + +This archipelago is situated under the equator, at a distance of between +five and six hundred miles from the west coast of South America. It +consists of five principal islands, and of several small ones, which +together are equal in area, but not in extent of land, to Sicily, +conjointly with the Ionian Islands. (I exclude from this measurement, the +small volcanic islands of Culpepper and Wenman, lying seventy miles +northward of the group. Craters were visible on all the islands of the +group, except on Towers Island, which is one of the lowest; this island is, +however, formed of volcanic rocks.) They are all volcanic: on two, craters +have been seen in eruption, and on several of the other islands, streams of +lava have a recent appearance. The larger islands are chiefly composed of +solid rock, and they rise with a tame outline to a height of between one +and four thousand feet. They are sometimes, but not generally, surmounted +by one principal orifice. The craters vary in size from mere spiracles to +huge caldrons several miles in circumference; they are extraordinarily +numerous, so that I should think, if enumerated, they would be found to +exceed two thousand; they are formed either of scoriae and lava, or of a +brown-coloured tuff; and these latter craters are in several respects +remarkable. The whole group was surveyed by the officers of the "Beagle." I +visited myself four of the principal islands, and received specimens from +all the others. Under the head of the different islands I will describe +only that which appears to me deserving of attention. + +CHATHAM ISLAND. CRATERS COMPOSED OF A SINGULAR KIND OF TUFF. + +Towards the eastern end of this island there occur two craters composed of +two kinds of tuff; one kind being friable, like slightly consolidated +ashes; and the other compact, and of a different nature from anything which +I have met with described. This latter substance, where it is best +characterised, is of a yellowish-brown colour, translucent, and with a +lustre somewhat resembling resin; it is brittle, with an angular, rough, +and very irregular fracture, sometimes, however, being slightly granular, +and even obscurely crystalline: it can readily be scratched with a knife, +yet some points are hard enough just to mark common glass; it fuses with +ease into a blackish-green glass. The mass contains numerous broken +crystals of olivine and augite, and small particles of black and brown +scoriae; it is often traversed by thin seams of calcareous matter. It +generally affects a nodular or concretionary structure. In a hand specimen, +this substance would certainly be mistaken for a pale and peculiar variety +of pitchstone; but when seen in mass its stratification, and the numerous +layers of fragments of basalt, both angular and rounded, at once render its +subaqueous origin evident. 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 rocks; and this change +could be distinctly traced in different stages round the edges of even the +same particle. The position near the coast of all the craters composed of +this kind of tuff or peperino, and their breached condition, renders it +probable that they were all formed when standing immersed in the sea; +considering this circumstance, together with the remarkable absence of +large beds of ashes in the whole archipelago, I think it highly probable +that much the greater part of the tuff has originated from the trituration +of fragments of the grey, basaltic lavas in the mouths of craters standing +in the sea. 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 translucent, resin-like fracture. Or has the associated +lime played any part in this change? I ask these questions from having +found at St. Jago, in the Cape de Verde Islands, that where a great stream +of molten lava has flowed over a calcareous bottom into the sea, the +outermost film, which in other parts resembles pitchstone, is changed, +apparently by its contact with the carbonate of lime, into a resin-like +substance, precisely like the best characterised specimens of the tuff from +this archipelago. (The concretions containing lime, which I have described +at Ascension, as formed in a bed of ashes, present some degree of +resemblance to this substance, but they have not a resinous fracture. At +St. Helena, also, I found veins of a somewhat similar, compact, but non- +resinous substance, occurring in a bed of pumiceous ashes, apparently free +from calcareous matter: in neither of these cases could heat have acted.) + +To return to the two craters: one of them stands at the distance of a +league from the coast, the intervening tract consisting of a calcareous +tuff, apparently of submarine origin. This crater consists of a circle of +hills some of which stand quite detached, but all have a very regular, qua- +qua versal dip, at an inclination of between thirty and forty degrees. The +lower beds, to the thickness of several hundred feet, consist of the resin- +like stone, with embedded fragments of lava. The upper beds, which are +between thirty and forty feet in thickness, are composed of a thinly +stratified, fine-grained, harsh, friable, brown-coloured tuff, or peperino. +(Those geologists who restrict the term of "tuff" to ashes of a white +colour, resulting from the attrition of feldspathic lavas, would call these +brown-coloured strata "peperino.") A central mass without any +stratification, which must formerly have occupied the hollow of the crater, +but is now attached only to a few of the circumferential hills, consists of +a tuff, intermediate in character between that with a resin-like, and that +with an earthy fracture. This mass contains white calcareous matter in +small patches. The second crater (520 feet in height) must have existed +until the eruption of a recent, great stream of lava, as a separate islet; +a fine section, worn by the sea, shows a grand funnel-shaped mass of +basalt, surrounded by steep, sloping flanks of tuff, having in parts an +earthy, and in others a semi-resinous fracture. The tuff is traversed by +several broad, vertical dikes, with smooth and parallel sides, which I did +not doubt were formed of basalt, until I actually broke off fragments. +These dikes, however, consist of tuff like that of the surrounding strata, +but more compact, and with a smoother fracture; hence we must conclude, +that fissures were formed and filled up with the finer mud or tuff from the +crater, before its interior was occupied, as it now is, by a solidified +pool of basalt. Other fissures have been subsequently formed, parallel to +these singular dikes, and are merely filled with loose rubbish. The change +from ordinary scoriaceous particles to the substance with a semi-resinous +fracture, could be clearly followed in portions of the compact tuff of +these dikes. + +(FIGURE 12. THE KICKER ROCK, 400 FEET HIGH.) + +At the distance of a few miles from these two craters, stands the Kicker +Rock, or islet, remarkable from its singular form. It is unstratified, and +is composed of compact tuff, in parts having the resin-like fracture. It is +probable that this amorphous mass, like that similar mass in the case first +described, once filled up the central hollow of a crater, and that its +flanks, or sloping walls, have since been worn quite away by the sea, in +which it stands exposed. + +SMALL BASALTIC CRATERS. + +A bare, undulating tract, at the eastern end of Chatham Island, is +remarkable from the number, proximity, and form of the small basaltic +craters with which it is studded. They consist, either of a mere conical +pile, or, but less commonly, of a circle, of black and red, glossy scoriae, +partially cemented together. They vary in diameter from thirty to one +hundred and fifty yards, and rise from about fifty to one hundred feet +above the level of the surrounding plain. From one small eminence, I +counted sixty of these craters, all of which were within a third of a mile +from each other, and many were much closer. I measured the distance between +two very small craters, and found that it was only thirty yards from the +summit-rim of one to the rim of the other. Small streams of black, basaltic +lava, containing olivine and much glassy feldspar, have flowed from many, +but not from all of these craters. The surfaces of the more recent streams +were exceedingly rugged, and were crossed by great fissures; the older +streams were only a little less rugged; and they were all blended and +mingled together in complete confusion. The different growth, however, of +the trees on the streams, often plainly marked their different ages. Had it +not been for this latter character, the streams could in few cases have +been distinguished; and, consequently, this wide undulatory tract might +have (as probably many tracts have) been erroneously considered as formed +by one great deluge of lava, instead of by a multitude of small streams, +erupted from many small orifices. + +In several parts of this tract, and especially at the base of the small +craters, there are circular pits, with perpendicular sides, from twenty to +forty feet deep. At the foot of one small crater, there were three of these +pits. They have probably been formed, by the falling in of the roofs of +small caverns. (M. Elie de Beaumont has described ("Mem. pour servir" etc. +tome 4 page 113) many "petits cirques d'eboulement" on Etna, of some of +which the origin is historically known.) In other parts, there are +mammiform hillocks, which resemble great bubbles of lava, with their +summits fissured by irregular cracks, which appeared, upon entering them, +to be very deep; lava has not flowed from these hillocks. There are, also, +other very regular, mammiform hillocks, composed of stratified lava, and +surmounted by circular, steep-sided hollows, which, I suppose have been +formed by a body of gas, first, arching the strata into one of the bubble- +like hillocks, and then, blowing off its summit. These several kinds of +hillocks and pits, as well as the numerous, small, scoriaceous craters, all +show that this tract has been penetrated, almost like a sieve, by the +passage of heated vapours. The more regular hillocks could only have been +heaved up, whilst the lava was in a softened state. (Sir G. Mackenzie +"Travels in Iceland" pages 389 to 392, has described a plain of lava at the +foot of Hecla, everywhere heaved up into great bubbles or blisters. Sir +George states that this cavernous lava composes the uppermost stratum; and +the same fact is affirmed by Von Buch "Descript. des Isles Canaries" page +159, with respect to the basaltic stream near Rialejo, in Teneriffe. It +appears singular that it should be the upper streams that are chiefly +cavernous, for one sees no reason why the upper and lower should not have +been equally affected at different times;--have the inferior streams flowed +beneath the pressure of the sea, and thus been flattened, after the passage +through them, of bodies of gas?) + +ALBEMARLE ISLAND. + +This island consists of five, great, flat-topped craters, which, together +with the one on the adjoining island of Narborough, singularly resemble +each other, in form and height. The southern one is 4,700 feet high, two +others are 3,720 feet, a third only 50 feet higher, and the remaining ones +apparently of nearly the same height. Three of these are situated on one +line, and their craters appear elongated in nearly the same direction. The +northern crater, which is not the largest, was found by the triangulation +to measure, externally, no less than three miles and one-eighth of a mile +in diameter. Over the lips of these great, broad caldrons, and from little +orifices near their summits, deluges of black lava have flowed down their +naked sides. + +FLUIDITY OF DIFFERENT LAVAS. + +Near Tagus or Banks' Cove, I examined one of these great streams of lava, +which is remarkable from the evidence of its former high degree of +fluidity, especially when its composition is considered. Near the sea-coast +this stream is several miles in width. It consists of a black, compact +base, easily fusible into a black bead, with angular and not very numerous +air-cells, and thickly studded with large, fractured crystals of glassy +albite, varying from the tenth of an inch to half an inch in diameter. (In +the Cordillera of Chile, I have seen lava very closely resembling this +variety at the Galapagos Archipelago. It contained, however, besides the +albite, well-formed crystals of augite, and the base (perhaps in +consequence of the aggregation of the augitic particles) was a shade +lighter in colour. I may here remark, that in all these cases, I call the +feldspathic crystals, "albite," from their cleavage-planes (as measured by +the reflecting goniometer) corresponding with those of that mineral. As, +however, other species of this genus have lately been discovered to cleave +in nearly the same planes with albite, this determination must be +considered as only provisional. I examined the crystals in the lavas of +many different parts of the Galapagos group, and I found that none of them, +with the exception of some crystals from one part of James Island, cleaved +in the direction of orthite or potash-feldspar.) This lava, although at +first sight appearing eminently porphyritic, cannot properly be considered +so, for the crystals have evidently been enveloped, rounded, and penetrated +by the lava, like fragments of foreign rock in a trap-dike. This was very +clear in some specimens of a similar lava, from Abingdon Island, in which +the only difference was, that the vesicles were spherical and more +numerous. The albite in these lavas is in a similar condition with the +leucite of Vesuvius, and with the olivine, described by Von Buch, as +projecting in great balls from the basalt of Lanzarote. ("Description des +Isles Canaries" page 295.) Besides the albite, this lava contains scattered +grains of a green mineral, with no distinct cleavage, and closely +resembling olivine (Humboldt mentions that he mistook a green augitic +mineral, occurring in the volcanic rocks of the Cordillera of Quito, for +olivine.); but as it fuses easily into a green glass, it belongs probably +to the augitic family: at James Island, however, a similar lava contained +true olivine. I obtained specimens from the actual surface, and from a +depth of four feet, but they differed in no respect. The high degree of +fluidity of this lava-stream was at once evident, from its smooth and +gently sloping surface, from the manner in which the main stream was +divided by small inequalities into little rills, and especially from the +manner in which its edges, far below its source, and where it must have +been in some degree cooled, thinned out to almost nothing; the actual +margin consisting of loose fragments, few of which were larger than a man's +head. The contrast between this margin, and the steep walls, above twenty +feet high, bounding many of the basaltic streams at Ascension, is very +remarkable. It has generally been supposed that lavas abounding with large +crystals, and including angular vesicles, have possessed little fluidity; +but we see that the case has been very different at Albemarle Island. (The +irregular and angular form of the vesicles is probably caused by the +unequal yielding of a mass composed, in almost equal proportion, of solid +crystals and of a viscid base. It certainly seems a general circumstance, +as might have been expected, that in lava, which has possessed a high +degree of fluidity, AS WELL AS AN EVEN-SIZED GRAIN, the vesicles are +internally smooth and spherical.) The degree of fluidity in different +lavas, does not seem to correspond with any APPARENT corresponding amount +of difference in their composition: at Chatham Island, some streams, +containing much glassy albite and some olivine, are so rugged, that they +may be compared to a sea frozen during a storm; whilst the great stream at +Albemarle Island is almost as smooth as a lake when ruffled by a breeze. At +James Island, black basaltic lava, abounding with small grains of olivine, +presents an intermediate degree of roughness; its surface being glossy, and +the detached fragments resembling, in a very singular manner, folds of +drapery, cables, and pieces of the bark of trees. (A specimen of basaltic +lava, with a few small broken crystals of albite, given me by one of the +officers, is perhaps worthy of description. It consists of cylindrical +ramifications, some of which are only the twentieth of an inch in diameter, +and are drawn out into the sharpest points. The mass has not been formed +like a stalactite, for the points terminate both upwards and downwards. +Globules, only the fortieth of an inch in diameter, have dropped from some +of the points, and adhere to the adjoining branches. The lava is vesicular, +but the vesicles never reach the surface of the branches, which are smooth +and glossy. As it is generally supposed that vesicles are always elongated +in the direction of the movement of the fluid mass, I may observe, that in +these cylindrical branches, which vary from a quarter to only the twentieth +of an inch in diameter, every air-cell is spherical.) + +CRATERS OF TUFF. + +About a mile southward of Banks' Cove, there is a fine elliptic crater, +about five hundred feet in depth, and three-quarters of a mile in diameter. +Its bottom is occupied by a lake of brine, out of which some little +crateriform hills of tuff rise. The lower beds are formed of compact tuff, +appearing like a subaqueous deposit; whilst the upper beds, round the +entire circumference, consist of a harsh, friable tuff, of little specific +gravity, but often containing fragments of rock in layers. This upper tuff +contains numerous pisolitic balls, about the size of small bullets, which +differ from the surrounding matter, only in being slightly harder and finer +grained. The beds dip away very regularly on all sides, at angles varying, +as I found by measurement, from twenty-five to thirty degrees. The external +surface of the crater slopes at a nearly similar inclination, and is formed +by slightly convex ribs, like those on the shell of a pecten or scallop, +which become broader as they extend from the mouth of the crater to its +base. These ribs are generally from eight to twenty feet in breadth, but +sometimes they are as much as forty feet broad; and they resemble old, +plastered, much flattened vaults, with the plaster scaling off in plates: +they are separated from each other by gullies, deepened by alluvial action. +At their upper and narrow ends, near the mouth of the crater, these ribs +often consist of real hollow passages, like, but rather smaller than, those +often formed by the cooling of the crust of a lava-stream, whilst the inner +parts have flowed onward;--of which structure I saw many examples at +Chatham Island. There can be no doubt but that these hollow ribs or vaults +have been formed in a similar manner, namely, by the setting or hardening +of a superficial crust on streams of mud, which have flowed down from the +upper part of the crater. In another part of this same crater, I saw open +concave gutters between one and two feet wide, which appear to have been +formed by the hardening of the lower surface of a mud stream, instead of, +as in the former case, of the upper surface. From these facts I think it is +certain that the tuff must have flowed as mud. (This conclusion is of some +interest, because M. Dufrenoy "Mem. pour servir" tome 4 page 274, has +argued from strata of tuff, apparently of similar composition with that +here described, being inclined at angles between 18 degrees and 20 degrees, +that Monte Nuevo and some other craters of Southern Italy have been formed +by upheaval. From the facts given above, of the vaulted character of the +separate rills, and from the tuff not extending in horizontal sheets round +these crateriform hills, no one will suppose that the strata have here been +produced by elevation; and yet we see that their inclination is above 20 +degrees, and often as much as 30 degrees. The consolidated strata also, of +the internal talus, as will be immediately seen, dips at an angle of above +30 degrees.) This mud may have been formed either within the crater, or +from ashes deposited on its upper parts, and afterwards washed down by +torrents of rain. The former method, in most of the cases, appears the more +probable one; at James Island, however, some beds of the friable kind of +tuff extend so continuously over an uneven surface, that probably they were +formed by the falling of showers of ashes. + +Within this same crater, strata of coarse tuff, chiefly composed of +fragments of lava, abut, like a consolidated talus, against the inside +walls. They rise to a height of between one hundred and one hundred and +fifty feet above the surface of the internal brine-lake; they dip inwards, +and are inclined at an angle varying from thirty to thirty-six degrees. +They appear to have been formed beneath water, probably at a period when +the sea occupied the hollow of the crater. I was surprised to observe that +beds having this great inclination did not, as far as they could be +followed, thicken towards their lower extremities. + +BANKS' COVE. + +(FIGURE 13. A SECTIONAL SKETCH OF THE HEADLANDS FORMING BANKS' COVE, +showing the diverging crateriform strata, and the converging stratified +talus. The highest point of these hills is 817 feet above the sea.) + +This harbour occupies part of the interior of a shattered crater of tuff +larger than that last described. All the tuff is compact, and includes +numerous fragments of lava; it appears like a subaqueous deposit. The most +remarkable feature in this crater is the great development of strata +converging inwards, as in the last case, at a considerable inclination, and +often deposited in irregular curved layers. These interior converging beds, +as well as the proper, diverging crateriform strata, are represented in +Figure 13, a rude, sectional sketch of the headlands, forming this Cove. +The internal and external strata differ little in composition, and the +former have evidently resulted from the wear and tear, and redeposition of +the matter forming the external crateriform strata. From the great +development of these inner beds, a person walking round the rim of this +crater might fancy himself on a circular anticlinal ridge of stratified +sandstone and conglomerate. The sea is wearing away the inner and outer +strata, and especially the latter; so that the inwardly converging strata +will, perhaps, in some future age, be left standing alone--a case which +might at first perplex a geologist. (I believe that this case actually +occurs in the Azores, where Dr. Webster "Description" page 185, has +described a basin-formed, little island, composed of STRATA OF TUFF, +dipping inwards and bounded externally by steep sea-worn cliffs. Dr. +Daubeny supposes "Volcanoes" page 266, that this cavity must have been +formed by a circular subsidence. It appears to me far more probable, that +we here have strata which were originally deposited within the hollow of a +crater, of which the exterior walls have since been removed by the sea.) + +JAMES ISLAND. + +Two craters of tuff on this island are the only remaining ones which +require any notice. One of them lies a mile and a half inland from Puerto +Grande: it is circular, about the third of a mile in diameter, and 400 feet +in depth. It differs from all the other tuff-craters which I examined, in +having the lower part of its cavity, to the height of between one hundred +and one hundred and fifty feet, formed by a precipitous wall of basalt, +giving to the crater the appearance of having burst through a solid sheet +of rock. The upper part of this crater consists of strata of the altered +tuff, with a semi-resinous fracture. Its bottom is occupied by a shallow +lake of brine, covering layers of salt, which rest on deep black mud. The +other crater lies at the distance of a few miles, and is only remarkable +from its size and perfect condition. Its summit is 1,200 feet above the +level of the sea, and the interior hollow is 600 feet deep. Its external +sloping surface presented a curious appearance from the smoothness of the +wide layers of tuff, which resembled a vast plastered floor. Brattle Island +is, I believe, the largest crater in the Archipelago composed of tuff; its +interior diameter is nearly a nautical mile. At present it is in a ruined +condition, consisting of little more than half a circle open to the south; +its great size is probably due, in part, to internal degradation, from the +action of the sea. + +SEGMENT OF A BASALTIC CRATER. + +(FIGURE 14. SEGMENT OF A VERY SMALL ORIFICE OF ERUPTION, on the beach of +Fresh-water Bay.) + +One side of Fresh-water Bay, in James Island, is bounded by a promontory, +which forms the last wreck of a great crater. On the beach of this +promontory, a quadrant-shaped segment of a small subordinate point of +eruption stands exposed. It consists of nine separate little streams of +lava piled upon each other; and of an irregular pinnacle, about fifteen +feet high, of reddish-brown, vesicular basalt, abounding with large +crystals of glassy albite, and with fused augite. This pinnacle, and some +adjoining paps of rock on the beach, represent the axis of the crater. The +streams of lava can be followed up a little ravine, at right angles to the +coast, for between ten and fifteen yards, where they are hidden by +detritus: along the beach they are visible for nearly eighty yards, and I +do not believe that they extend much further. The three lower streams are +united to the pinnacle; and at the point of junction (as shown in Figure +14, a rude sketch made on the spot), they are slightly arched, as if in the +act of flowing over the lip of the crater. The six upper streams no doubt +were originally united to this same column before it was worn down by the +sea. The lava of these streams is of similar composition with that of the +pinnacle, excepting that the crystals of albite appear to be more +comminuted, and the grains of fused augite are absent. Each stream is +separated from the one above it by a few inches, or at most by one or two +feet in thickness, of loose fragmentary scoriae, apparently derived from +the abrasion of the streams in passing over each other. All these streams +are very remarkable from their thinness. I carefully measured several of +them; one was eight inches thick, but was firmly coated with three inches +above, and three inches below, of red scoriaceous rock (which is the case +with all the streams), making altogether a thickness of fourteen inches: +this thickness was preserved quite uniformly along the entire length of the +section. A second stream was only eight inches thick, including both the +upper and lower scoriaceous surfaces. Until examining this section, I had +not thought it possible that lava could have flowed in such uniformly thin +sheets over a surface far from smooth. These little streams closely +resemble in composition that great deluge of lava at Albemarle Island, +which likewise must have possessed a high degree of fluidity. + +PSEUDO-EXTRANEOUS, EJECTED FRAGMENTS. + +In the lava and in the scoriae of this little crater, I found several +fragments, which, from their angular form, their granular structure, their +freedom from air-cells, their brittle and burnt condition, closely +resembled those fragments of primary rocks which are occasionally ejected, +as at Ascension, from volcanoes. These fragments consist of glassy albite, +much mackled, and with very imperfect cleavages, mingled with semi-rounded +grains, having tarnished, glossy surfaces, of a steel-blue mineral. The +crystals of albite are coated by a red oxide of iron, appearing like a +residual substance; and their cleavage-planes also are sometimes separated +by excessively fine layers of this oxide, giving to the crystals the +appearance of being ruled like a glass micrometer. There was no quartz. The +steel-blue mineral, which is abundant in the pinnacle, but which disappears +in the streams derived from the pinnacle, has a fused appearance, and +rarely presents even a trace of cleavage; I obtained, however, one +measurement, which proved that it was augite; and in one other fragment, +which differed from the others, in being slightly cellular, and in +gradually blending into the surrounding matrix the small grains of this +mineral were tolerably well crystallised. Although there is so wide a +difference in appearance between the lava of the little streams, and +especially of their red scoriaceous crusts, and one of these angular +ejected fragments, which at first sight might readily be mistaken for +syenite, yet I believe that the lava has originated from the melting and +movement of a mass of rock of absolutely similar composition with the +fragments. Besides the specimen above alluded to, in which we see a +fragment becoming slightly cellular, and blending into the surrounding +matrix, some of the grains of the steel-blue augite also have their +surfaces becoming very finely vesicular, and passing into the nature of the +surrounding paste; other grains are throughout, in an intermediate +condition. The paste seems to consist of the augite more perfectly fused, +or, more probably, merely disturbed in its softened state by the movement +of the mass, and mingled with the oxide of iron and with finely comminuted, +glassy albite. Hence probably it is that the fused albite, which is +abundant in the pinnacle, disappears in the streams. The albite is in +exactly the same state, with the exception of most of the crystals being +smaller in the lava and in the embedded fragments; but in the fragments +they appear to be less abundant: this, however, would naturally happen from +the intumescence of the augitic base, and its consequent apparent increase +in bulk. It is interesting thus to trace the steps by which a compact +granular rock becomes converted into a vesicular, pseudo-porphyritic lava, +and finally into red scoriae. 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 action, or more probably of +the crust of a body of cooled and crystallised lava, which has afterwards +been broken up and re-liquified; the crust being less acted on by the +renewed heat and movement. + +CONCLUDING REMARKS ON THE TUFF-CRATERS. + +These craters, from the peculiarity of the resin-like substance which +enters largely into their composition, from their structure, their size and +number, present the most striking feature in the geology of this +Archipelago. The majority of them form either separate islets, or +promontories attached to the larger islands; and those which now stand at +some little distance from the coast are worn and breached, as if by the +action of the sea. From this general circumstance of their position, and +from the small quantity of ejected ashes in any part of the Archipelago, I +am led to conclude, that the tuff has been chiefly produced, by the +grinding together of fragments of lava within active craters, communicating +with the sea. In the origin and composition of the tuff, and in the +frequent presence of a central lake of brine and of layers of salt, these +craters resemble, though on a gigantic scale, the "salses," or hillocks of +mud, which are common in some parts of Italy and in other countries. +(D'Aubuisson "Traite de Geognosie" tome 1 page 189. I may remark, that I +saw at Terceira, in the Azores, a crater of tuff or peperino, very similar +to these of the Galapagos Archipelago. From the description given in +Freycinet "Voyage," similar ones occur at the Sandwich Islands; and +probably they are present in many other places.) Their closer connection, +however, in this Archipelago, with ordinary volcanic action, is shown by +the pools of solidified basalt, with which they are sometimes filled up. + +It at first appears very singular, that all the craters formed of tuff have +their southern sides, either quite broken down and wholly removed, or much +lower than the other sides. I saw and received accounts of twenty-eight of +these craters; of these, twelve form separate islets (These consist of the +three Crossman Islets, the largest of which is 600 feet in height; +Enchanted Island; Gardner Island (760 feet high); Champion Island (331 feet +high); Enderby Island; Brattle Island; two islets near Indefatigable +Island; and one near James Island. A second crater near James Island (with +a salt lake in its centre) has its southern side only about twenty feet +high, whilst the other parts of the circumference are about three hundred +feet in height.), and now exist as mere crescents quite open to the south, +with occasionally a few points of rock marking their former circumference: +of the remaining sixteen, some form promontories, and others stand at a +little distance inland from the shore; but all have their southern sides +either the lowest, or quite broken down. Two, however, of the sixteen had +their northern sides also low, whilst their eastern and western sides were +perfect. I did not see, or hear of, a single exception to the rule, of +these craters being broken down or low on the side, which faces a point of +the horizon between S.E. and S.W. This rule does not apply to craters +composed of lava and scoriae. The explanation is simple: at this +Archipelago, the waves from the trade-wind, and the swell propagated from +the distant parts of the open ocean, coincide in direction (which is not +the case in many parts of the Pacific), and with their united forces attack +the southern sides of all the islands; and consequently the southern slope, +even when entirely formed of hard basaltic rock, is invariably steeper than +the northern slope. As the tuff-craters are composed of a soft material, +and as probably all, or nearly all, have at some period stood immersed in +the sea, we need not wonder that they should invariably exhibit on their +exposed sides the effects of this great denuding power. Judging from the +worn condition of many of these craters, it is probable that some have been +entirely washed away. As there is no reason to suppose, that the craters +formed of scoriae and lava were erupted whilst standing in the sea, we can +see why the rule does not apply to them. At Ascension, it was shown that +the mouths of the craters, which are there all of terrestrial origin, have +been affected by the trade-wind; and this same power might here, also, aid +in making the windward and exposed sides of some of the craters originally +the lowest. + +MINERALOGICAL COMPOSITION OF THE ROCKS. + +In the northern islands, the basaltic lavas seem generally to contain more +albite than they do in the southern half of the Archipelago; but almost all +the streams contain some. The albite is not unfrequently associated with +olivine. I did not observe in any specimen distinguishable crystals of +hornblende or augite; I except the fused grains in the ejected fragments, +and in the pinnacle of the little crater, above described. I did not meet +with a single specimen of true trachyte; though some of the paler lavas, +when abounding with large crystals of the harsh and glassy albite, resemble +in some degree this rock; but in every case the basis fuses into a black +enamel. Beds of ashes and far-ejected scoriae, as previously stated, are +almost absent; nor did I see a fragment of obsidian or of pumice. Von Buch +believes that the absence of pumice on Mount Etna is consequent on the +feldspar being of the Labrador variety ("Description des Isles Canaries" +page 328.); if the presence of pumice depends on the constitution of the +feldspar, it is remarkable, that it should be absent in this archipelago, +and abundant in the Cordillera of South America, in both of which regions +the feldspar is of the albitic variety. Owing to the absence of ashes, and +the general indecomposable character of the lava in this Archipelago, the +islands are slowly clothed with a poor vegetation, and the scenery has a +desolate and frightful aspect. + +ELEVATION OF THE LAND. + +Proofs of the rising of the land are scanty and imperfect. At Chatham +Island, I noticed some great blocks of lava, cemented by calcareous matter, +containing recent shells; but they occurred at the height of only a few +feet above high-water mark. One of the officers gave me some fragments of +shells, which he found embedded several hundred feet above the sea, in the +tuff of two craters, distant from each other. It is possible, that these +fragments may have been carried up to their present height in an eruption +of mud; but as, in one instance, they were associated with broken oyster- +shells, almost forming a layer, it is more probable that the tuff was +uplifted with the shells in mass. The specimens are so imperfect that they +can be recognised only as belonging to recent marine genera. On Charles +Island, I observed a line of great rounded blocks, piled on the summit of a +vertical cliff, at the height of fifteen feet above the line, where the sea +now acts during the heaviest gales. This appeared, at first, good evidence +in favour of the elevation of the land; but it was quite deceptive, for I +afterwards saw on an adjoining part of this same coast, and heard from eye- +witnesses, that wherever a recent stream of lava forms a smooth inclined +plane, entering the sea, the waves during gales have the power of ROLLING +UP ROUNDED blocks to a great height, above the line of their ordinary +action. As the little cliff in the foregoing case is formed by a stream of +lava, which, before being worn back, must have entered the sea with a +gently sloping surface, it is possible or rather it is probable, that the +rounded boulders, now lying on its summit, are merely the remnants of those +which had been ROLLED UP during storms to their present height. + +DIRECTION OF THE FISSURES OF ERUPTION. + +The volcanic orifices in this group cannot be considered as +indiscriminately scattered. Three great craters on Albermarle Island form a +well-marked line, extending N.W. by N. and S.E. by S. Narborough Island, +and the great crater on the rectangular projection of Albemarle Island, +form a second parallel line. To the east, Hood's Island, and the islands +and rocks between it and James Island, form another nearly parallel line, +which, when prolonged, includes Culpepper and Wenman Islands, lying seventy +miles to the north. The other islands lying further eastward, form a less +regular fourth line. Several of these islands, and the vents on Albemarle +Island, are so placed, that they likewise fall on a set of rudely parallel +lines, intersecting the former lines at right angles; so that the principal +craters appear to lie on the points where two sets of fissures cross each +other. The islands themselves, with the exception of Albemarle Island, are +not elongated in the same direction with the lines on which they stand. 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 +Ocean. Finally, I may remark, that amongst the Galapagos Islands there is +no one dominant vent much higher than all the others, as may be observed in +many volcanic archipelagoes: the highest is the great mound on the south- +western extremity of Albemarle Island, which exceeds by barely a thousand +feet several other neighbouring craters. + + +CHAPTER VI.--TRACHYTE AND BASALT.--DISTRIBUTION OF VOLCANIC ISLES. + +The sinking of crystals in fluid lava. +Specific gravity of the constituent parts of trachyte and of basalt, and +their consequent separation. +Obsidian. +Apparent non-separation of the elements of plutonic rocks. +Origin of trap-dikes in the plutonic series. +Distribution of volcanic islands; their prevalence in the great oceans. +They are generally arranged in lines. +The central volcanoes of Von Buch doubtful. +Volcanic islands bordering continents. +Antiquity of volcanic islands, and their elevation in mass. +Eruptions on parallel lines of fissure within the same geological period. + +ON THE SEPARATION OF THE CONSTITUENT MINERALS OF LAVA, ACCORDING TO THEIR +SPECIFIC GRAVITIES. + +One side of Fresh-water Bay, in James Island, is formed by the wreck of a +large crater, mentioned in the last chapter, of which the interior has been +filled up by a pool of basalt, about two hundred feet in thickness. This +basalt is of a grey colour, and contains many crystals of glassy albite, +which become much more numerous in the lower, scoriaceous part. This is +contrary to what might have been expected, for if the crystals had been +originally disseminated in equal numbers, the greater intumescence of this +lower scoriaceous part would have made them appear fewer in number. Von +Buch has described a stream of obsidian on the Peak of Teneriffe, in which +the crystals of feldspar become more and more numerous, as the depth or +thickness increases, so that near the lower surface of the stream the lava +even resembles a primary rock. ("Description des Isles Canaries" pages 190 +and 191.) Von Buch further states, that M. Dree, in his experiments in +melting lava, found that the crystals of feldspar always tended to +precipitate themselves to the bottom of the crucible. In these cases, I +presume there can be no doubt that the crystals sink from their weight. (In +a mass of molten iron, it is found ("Edinburgh New Philosophical Journal" +volume 24 page 66) that the substances, which have a closer affinity for +oxygen than iron has, rise from the interior of the mass to the surface. +But a similar cause can hardly apply to the separation of the crystals of +these lava-streams. The cooling of the surface of lava seems, in some +cases, to have affected its composition; for Dufrenoy ("Mem. pour servir" +tome 4 page 271) found that the interior parts of a stream near Naples +contained two-thirds of a mineral which was acted on by acids, whilst the +surface consisted chiefly of a mineral unattackable by acids.) The specific +gravity of feldspar varies from 2.4 to 2.58, whilst obsidian seems commonly +to be from 2.3 to 2.4; and in a fluidified state its specific gravity would +probably be less, which would facilitate the sinking of the crystals of +feldspar. (I have taken the specific gravities of the simple minerals from +Von Kobell, one of the latest and best authorities, and of the rocks from +various authorities. Obsidian, according to Phillips, is 2.35; and Jameson +says it never exceeds 2.4; but a specimen from Ascension, weighed by +myself, was 2.42.) At James Island, the crystals of albite, though no doubt +of less weight than the grey basalt, in the parts where compact, might +easily be of greater specific gravity than the scoriaceous mass, formed of +melted lava and bubbles of heated gas. + +The sinking of crystals through a viscid substance like molten rock, as is +unequivocally shown to have been the case in the experiments of M. Dree, is +worthy of further consideration, as throwing light on the separation of the +trachytic and basaltic series of lavas. Mr. P. Scrope has speculated on +this subject; but he does not seem to have been aware of any positive +facts, such as those above given; and he has overlooked one very necessary +element, as it appears to me, in the phenomenon--namely, the existence of +either the lighter or heavier mineral in globules or in crystals. In a +substance of imperfect fluidity, like molten rock, it is hardly credible, +that the separate, infinitely small atoms, whether of feldspar, augite, or +of any other mineral, would have power from their slightly different +gravities to overcome the friction caused by their movement; but if the +atoms of any one of these minerals became, whilst the others remained +fluid, united into crystals or granules, it is easy to perceive that from +the lessened friction, their sinking or floating power would be greatly +increased. On the other hand, if all the minerals became granulated at the +same time, it is scarcely possible, from their mutual resistance, that any +separation could take place. A valuable, practical discovery, illustrating +the effect of the granulation of one element in a fluid mass, in aiding its +separation, has lately been made: when lead containing a small proportion +of silver, is constantly stirred whilst cooling, it becomes granulated, and +the grains of imperfect crystals of nearly pure lead sink to the bottom, +leaving a residue of melted metal much richer in silver; whereas if the +mixture be left undisturbed, although kept fluid for a length of time, the +two metals show no signs of separating. (A full and interesting account of +this discovery, by Mr. Pattinson, was read before the British Association +in September 1838. In some alloys, according to Turner "Chemistry" page +210, the heaviest metal sinks, and it appears that this takes place whilst +both metals are fluid. Where there is a considerable difference in gravity, +as between iron and the slag formed during the fusion of the ore, we need +not be surprised at the atoms separating, without either substance being +granulated.) The sole use of the stirring seems to be, the formation of +detached granules. The specific gravity of silver is 10.4, and of lead +11.35: the granulated lead, which sinks, is never absolutely pure, and the +residual fluid metal contains, when richest, only 1/119 part of silver. As +the difference in specific gravity, caused by the different proportions of +the two metals, is so exceedingly small, the separation is probably aided +in a great degree by the difference in gravity between the lead, when +granular though still hot, and when fluid. + +In a body of liquified volcanic rock, left for some time without any +violent disturbance, we might expect, in accordance with the above facts, +that if one of the constituent minerals became aggregated into crystals or +granules, or had been enveloped in this state from some previously existing +mass, such crystals or granules would rise or sink, according to their +specific gravity. Now we have plain evidence of crystals being embedded in +many lavas, whilst the paste or basis has continued fluid. I need only +refer, as instances, to the several, great, pseudo-porphyritic streams at +the Galapagos Islands, and to the trachytic streams in many parts of the +world, in which we find crystals of feldspar bent and broken by the +movement of the surrounding, semi-fluid matter. Lavas are chiefly composed +of three varieties of feldspar, varying in specific gravity from 2.4 to +2.74; of hornblende and augite, varying from 3.0 to 3.4; of olivine, +varying from 3.3 to 3.4; and lastly, of oxides of iron, with specific +gravities from 4.8 to 5.2. Hence crystals of feldspar, enveloped in a mass +of liquified, but not highly vesicular lava, would tend to rise to the +upper parts; and crystals or granules of the other minerals, thus +enveloped, would tend to sink. We ought not, however, to expect any perfect +degree of separation in such viscid materials. Trachyte, which consists +chiefly of feldspar, with some hornblende and oxide of iron, has a specific +gravity of about 2.45; whilst basalt, composed chiefly of augite and +feldspar, often with much iron and olivine, has a gravity of about 3.0. +(Trachyte from Java was found by Von Buch to be 2.47; from Auvergne, by De +la Beche, it was 2.42; from Ascension, by myself, it was 2.42. Jameson and +other authors give to basalt a specific gravity of 3.0; but specimens from +Auvergne were found, by De la Beche, to be only 2.78; and from the Giant's +Causeway, to be 2.91.) Accordingly we find, that where both trachytic and +basaltic streams have proceeded from the same orifice, the trachytic +streams have generally been first erupted owing, as we must suppose, to the +molten lava of this series having accumulated in the upper parts of the +volcanic focus. This order of eruption has been observed by Beudant, +Scrope, and by other authors; three instances, also, have been given in +this volume. As the later eruptions, however, from most volcanic mountains, +burst through their basal parts, owing to the increased height and weight +of the internal column of molten rock, we see why, in most cases, only the +lower flanks of the central, trachytic masses, are enveloped by basaltic +streams. The separation of the ingredients of a mass of lava, would, +perhaps, sometimes take place within the body of a volcanic mountain, if +lofty and of great dimensions, instead of within the underground focus; in +which case, trachytic streams might be poured forth, almost +contemporaneously, or at short recurrent intervals, from its summit, and +basaltic streams from its base: this seems to have taken place at +Teneriffe. (Consult Von Buch's well-known and admirable "Description +Physique" of this island, which might serve as a model of descriptive +geology.) I need only further remark, that from violent disturbances the +separation of the two series, even under otherwise favourable conditions, +would naturally often be prevented, and likewise their usual order of +eruption be inverted. From the high degree of fluidity of most basaltic +lavas, these perhaps, alone, would in many cases reach the surface. + +As we have seen that crystals of feldspar, in the instance described by Von +Buch, sink in obsidian, in accordance with their known greater specific +gravity, we might expect to find in every trachytic district, where +obsidian has flowed as lava, that it had proceeded from the upper or +highest orifices. This, according to Von Buch, holds good in a remarkable +manner both at the Lipari Islands and on the Peak of Teneriffe; at this +latter place obsidian has never flowed from a less height than 9,200 feet. +Obsidian, also, appears to have been erupted from the loftiest peaks of the +Peruvian Cordillera. I will only further observe, that the specific gravity +of quartz varies from 2.6 to 2.8; and therefore, that when present in a +volcanic focus, it would not tend to sink with the basaltic bases; and +this, perhaps, explains the frequent presence, and the abundance of this +mineral, in the lavas of the trachytic series, as observed in previous +parts of this volume. + +An objection to the foregoing theory will, perhaps, be drawn from the +plutonic rocks not being separated into two evidently distinct series, of +different specific gravities; although, like the volcanic, they have been +liquified. In answer, it may first be remarked, that we have no evidence of +the atoms of any one of the constituent minerals in the plutonic series +having been aggregated, whilst the others remained fluid, which we have +endeavoured to show is an almost necessary condition of their separation; +on the contrary, the crystals have generally impressed each other with +their forms. (The crystalline paste of phonolite is frequently penetrated +by long needles of hornblende; from which it appears that the hornblende, +though the more fusible mineral, has crystallised before, or at the same +time with a more refractory substance. Phonolite, as far as my observations +serve, in every instance appears to be an injected rock, like those of the +plutonic series; hence probably, like these latter, it has generally been +cooled without repeated and violent disturbances. Those geologists who have +doubted whether granite could have been formed by igneous liquefaction, +because minerals of different degrees of fusibility impress each other with +their forms, could not have been aware of the fact of crystallised +hornblende penetrating phonolite, a rock undoubtedly of igneous origin. The +viscidity, which it is now known, that both feldspar and quartz retain at a +temperature much below their points of fusion, easily explains their mutual +impressment. Consult on this subject Mr. Horner's paper on Bonn "Geolog. +Transact." volume 4 page 439; and "L'Institut" with respect to quartz 1839 +page 161.) + +In the second place, the perfect tranquillity, under which it is probable +that the plutonic masses, buried at profound depths, have cooled, would, +most likely, be highly unfavourable to the separation of their constituent +minerals; for, if the attractive force, which during the progressive +cooling draws together the molecules of the different minerals, has power +sufficient to keep them together, the friction between such half-formed +crystals or pasty globules would effectually prevent the heavier ones from +sinking, or the lighter ones from rising. On the other hand, a small amount +of disturbance, which would probably occur in most volcanic foci, and which +we have seen does not prevent the separation of granules of lead from a +mixture of molten lead and silver, or crystals of feldspar from streams of +lava, by breaking and dissolving the less perfectly formed globules, would +permit the more perfect and therefore unbroken crystals, to sink or rise, +according to their specific gravity. + +Although in plutonic rocks two distinct species, corresponding to the +trachytic and basaltic series, do not exist, I much suspect that a certain +amount of separation of their constituent parts has often taken place. I +suspect this from having observed how frequently dikes of greenstone and +basalt intersect widely extended formations of granite and the allied +metamorphic rocks. I have never examined a district in an extensive +granitic region without discovering dikes; I may instance the numerous +trap-dikes, in several districts of Brazil, Chile, and Australia, and at +the Cape of Good Hope: many dikes likewise occur in the great granitic +tracts of India, in the north of Europe, and in other countries. Whence, +then, has the greenstone and basalt, forming these dikes, come? Are we to +suppose, like some of the elder geologists, that a zone of trap is +uniformly spread out beneath the granitic series, which composes, as far as +we know, the foundations of the earth's crust? Is it not more probable, +that these dikes have been formed by fissures penetrating into partially +cooled rocks of the granitic and metamorphic series, and by their more +fluid parts, consisting chiefly of hornblende, oozing out, and being sucked +into such fissures? At Bahia, in Brazil, in a district composed of gneiss +and primitive greenstone, I saw many dikes, of a dark augitic (for one +crystal certainly was of this mineral) or hornblendic rock, which, as +several appearances clearly proved, either had been formed before the +surrounding mass had become solid, or had together with it been afterwards +thoroughly softened. (Portions of these dikes have been broken off, and are +now surrounded by the primary rocks, with their laminae conformably winding +round them. Dr. Hubbard also ("Silliman's Journal" volume 34 page 119), has +described an interlacement of trap-veins in the granite of the White +Mountains, which he thinks must have been formed when both rocks were +soft.) On both sides of one of these dikes, the gneiss was penetrated, to +the distance of several yards, by numerous, curvilinear threads or streaks +of dark matter, which resembled in form clouds of the class called cirrhi- +comae; some few of these threads could be traced to their junction with the +dike. When examining them, I doubted whether such hair-like and curvilinear +veins could have been injected, and I now suspect, that instead of having +been injected from the dike, they were its feeders. If the foregoing views +of the origin of trap-dikes in widely extended granitic regions far from +rocks of any other formation, be admitted as probable, we may further +admit, in the case of a great body of plutonic rock, being impelled by +repeated movements into the axis of a mountain-chain, that its more liquid +constituent parts might drain into deep and unseen abysses; afterwards, +perhaps, to be brought to the surface under the form, either of injected +masses of greenstone and augitic porphyry, or of basaltic eruptions. (Mr. +Phillips "Lardner's Encyclop." volume 2 page 115 quotes Von Buch's +statement, that augitic porphyry ranges parallel to, and is found +constantly at the base of, great chains of mountains. Humboldt, also, has +remarked the frequent occurrence of trap-rock, in a similar position; of +which fact I have observed many examples at the foot of the Chilian +Cordillera. The existence of granite in the axes of great mountain chains +is always probable, and I am tempted to suppose, that the laterally +injected masses of augitic porphyry and of trap, bear nearly the same +relation to the granitic axes which basaltic lavas bear to the central +trachytic masses, round the flanks of which they have so frequently been +erupted.) Much of the difficulty which geologists have experienced when +they have compared the composition of volcanic with plutonic formations, +will, I think, be removed, if we may believe that most plutonic masses have +been, to a certain extent, drained of those comparatively weighty and +easily liquified elements, which compose the trappean and basaltic series +of rocks. + +ON THE DISTRIBUTION OF VOLCANIC ISLANDS. + +During my investigations on coral-reefs, I had occasion to consult the +works of many voyagers, and I was invariably struck with the fact, that +with rare exceptions, the innumerable islands scattered throughout the +Pacific, Indian, and Atlantic Oceans, were composed either of volcanic, or +of modern coral-rocks. It would be tedious to give a long catalogue of all +the volcanic islands; but the exceptions which I have found are easily +enumerated: in the Atlantic, we have St. Paul's Rock, described in this +volume, and the Falkland Islands, composed of quartz and clay-slate; but +these latter islands are of considerable size, and lie not very far from +the South American coast (Judging from Forster's imperfect observation, +perhaps Georgia is not volcanic. Dr. Allan is my informant with regard to +the Seychelles. I do not know of what formation Rodriguez, in the Indian +Ocean, is composed.): in the Indian Ocean, the Seychelles (situated in a +line prolonged from Madagascar) consist of granite and quartz: in the +Pacific Ocean, New Caledonia, an island of large size, belongs (as far as +is known) to the primary class. New Zealand, which contains much volcanic +rock and some active volcanoes, from its size cannot be classed with the +small islands, which we are now considering. The presence of a small +quantity of non-volcanic rock, as of clay-slate on three of the Azores +(This is stated on the authority of Count V. de Bedemar, with respect to +Flores and Graciosa (Charlsworth "Magazine of Nat. Hist." volume 1 page +557). St. Maria has no volcanic rock, according to Captain Boyd (Von Buch +"Descript." page 365). Chatham Island has been described by Dr. Dieffenbach +in the "Geographical Journal" 1841 page 201. As yet we have received only +imperfect notices on Kerguelen Land, from the Antarctic Expedition.), or of +tertiary limestone at Madeira, or of clay-slate at Chatham Island in the +Pacific, or of lignite at Kerguelen Land, ought not to exclude such islands +or archipelagoes, if formed chiefly of erupted matter, from the volcanic +class. + +The composition of the numerous islands scattered through the great oceans +being with such rare exceptions volcanic, is evidently an extension of that +law, and the effect of those same causes, whether chemical or mechanical, +from which it results, that a vast majority of the volcanoes now in action +stand either as islands in the sea, or near its shores. This fact of the +ocean-islands being so generally volcanic is also interesting in relation +to the nature of the mountain-chains on our continents, which are +comparatively seldom volcanic; and yet we are led to suppose that where our +continents now stand an ocean once extended. Do volcanic eruptions, we may +ask, 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 +land? + +Looking at the charts of the numerous volcanic archipelagoes, we see that +the islands are generally arranged either in single, double, or triple +rows, in lines which are frequently curved in a slight degree. (Professors +William and Henry Darwin Rogers have lately insisted much, in a memoir read +before the American Association, on the regularly curved lines of elevation +in parts of the Appalachian range.) Each separate island is either rounded, +or more generally elongated in the same direction with the group in which +it stands, but sometimes transversely to it. Some of the groups which are +not much elongated present little symmetry in their forms; M. Virlet +("Bulletin de la Soc. Geolog." tome 3 page 110.) states that this is the +case with the Grecian Archipelago: in such groups I suspect (for I am aware +how easy it is to deceive oneself on these points), that the vents are +generally arranged on one line, or on a set of short parallel lines, +intersecting at nearly right angles another line, or set of lines. The +Galapagos Archipelago offers an example of this structure, 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 N.W. by N., and on another set ranging +about W.S.W.: in the Canary Archipelago we have a simpler structure of the +same kind: in the Cape de Verde group, which appears to be the least +symmetrical of any oceanic volcanic archipelago, a N.W. and S.E. line +formed by several islands, if prolonged, would intersect at right angles a +curved line, on which the remaining islands are placed. + +Von Buch ("Description des Isles Canaries" page 324.) has classed all +volcanoes under two heads, namely, CENTRAL VOLCANOES, round which numerous +eruptions have taken place on all sides, in a manner almost regular, and +VOLCANIC CHAINS. In the examples given of the first class, as far as +position is concerned, I can see no grounds for their being called +"central;" and the evidence of any difference in mineralogical nature +between CENTRAL VOLCANOES and VOLCANIC CHAINS appears slight. No doubt some +one island in most small volcanic archipelagoes is apt to be considerably +higher than the others; and in a similar manner, whatever the cause may be, +that on the same island one vent is generally higher than all the others. +Von Buch does not include in his class of volcanic chains small +archipelagoes, in which the islands are admitted by him, as at the Azores, +to be arranged in lines; 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 line, and so on to +a great wall like the Cordillera of America, it is difficult to believe +that there exists any essential difference between short and long volcanic +chains. Von Buch (Idem page 393.) states that his volcanic chains surmount, +or are closely connected with, mountain-ranges of primary formation: but if +trains of linear archipelagoes are, in the course of time, by the long- +continued action of the elevatory and volcanic forces, converted into +mountain-ranges, it would naturally result that the inferior primary rocks +would often be uplifted and brought into view. + +Some authors have remarked that volcanic islands occur scattered, though at +very unequal distances, along the shores of the great continents, as if in +some measure connected with them. In the case of Juan Fernandez, situated +330 miles from the coast of Chile, there was undoubtedly a connection +between the volcanic forces acting under this island and under the +continent, as was shown during the earthquake of 1835. The islands, +moreover, of some of the small volcanic groups which thus border +continents, are placed in lines, related to those along which the adjoining +shores of the continents trend; I may instance the lines of intersection at +the Galapagos, and at the Cape de Verde Archipelagoes, and the best marked +line of the Canary Islands. If these facts be not merely accidental, we see +that many scattered volcanic islands and small groups are related not only +by proximity, but in the direction of the fissures of eruption to the +neighbouring continents--a relation, which Von Buch considers, +characteristic of his great volcanic chains. + +In volcanic archipelagoes, the orifices are seldom in activity on more than +one island at a time; and the greater eruptions usually recur only after +long intervals. Observing the number of craters, that are usually found on +each island of a group, and the vast amount of matter which has been +erupted from them, one is led to attribute a high antiquity even to those +groups, which appear, like the Galapagos, to be of comparatively recent +origin. This conclusion accords with the prodigious amount of degradation, +by the slow action of the sea, which their originally sloping coasts must +have suffered, when they are worn back, as is so often the case, into grand +precipices. We ought not, however, to suppose, in hardly any instance, that +the whole body of matter, forming a volcanic island, has been erupted at +the level, on which it now stands: the number of dikes, which seem +invariably to intersect the interior parts of every volcano, show, on the +principles explained by M. Elie de Beaumont, that the whole mass has been +uplifted and fissured. A connection, moreover, between volcanic eruptions +and contemporaneous elevations in mass has, I think, been shown to exist in +my work on Coral-Reefs, both from the frequent presence of upraised organic +remains, and from the structure of the accompanying coral-reefs. (A similar +conclusion is forced on us, by the phenomena, which accompanied the +earthquake of 1835, at Concepcion, and which are detailed in my paper +(volume 5 page 601) in the "Geological Transactions.") Finally, I may +remark, that in the same Archipelago, eruptions have taken place within the +historical period on more than one of the parallel lines of fissure: thus, +at the Galapagos Archipelago, eruptions have taken place from a vent on +Narborough Island, and from one on Albemarle Island, which vents do not +fall on the same line; at the Canary Islands, eruptions have taken place in +Teneriffe and Lanzarote; and at the Azores, on the three parallel lines of +Pico, St. Jorge, and Terceira. Believing that a mountain-axis differs +essentially from a volcano, only in plutonic rocks having been injected, +instead of volcanic matter having been ejected, this appears to me an +interesting circumstance; for we may infer from it as probable, that in the +elevation of a mountain-chain, two or more of the parallel lines forming it +may be upraised and injected within the same geological period. + + +CHAPTER VII.--AUSTRALIA; NEW ZEALAND; CAPE OF GOOD HOPE. + +New South Wales. +Sandstone formation. +Embedded pseudo-fragments of shale. +Stratification. +Current-cleavage. +Great valleys. +Van Diemen's Land. +Palaeozoic formation. +Newer formation with volcanic rocks. +Travertin with leaves of extinct plants. +Elevation of the land. +New Zealand. +King George's Sound. +Superficial ferruginous beds. +Superficial calcareous deposits, with casts of branches. +Their origin from drifted particles of shells and corals. +Their extent. +Cape of Good Hope. +Junction of the granite and clay-slate. +Sandstone formation. + +The "Beagle," in her homeward voyage, touched at New Zealand, Australia, +Van Diemen's Land, and the Cape of Good Hope. In order to confine the Third +Part of these Geological Observations to South America, I will here briefly +describe all that I observed at these places worthy of the attention of +geologists. + +NEW SOUTH WALES. + +My opportunities of observation consisted of a ride of ninety geographical +miles to Bathurst, in a W.N.W. direction from Sydney. The first thirty +miles from the coast passes over a sandstone country, broken up in many +places by trap-rocks, and separated by a bold escarpment overhanging the +river Nepean, from the great sandstone platform of the Blue Mountains. This +upper platform is 1,000 feet high at the edge of the escarpment, and rises +in a distance of twenty-five miles to between three and four thousand feet +above the level of the sea. At this distance the road descends to a country +rather less elevated, and composed in chief part of primary rocks. There is +much granite, in one part passing into a red porphyry with octagonal +crystals of quartz, and intersected in some places by trap-dikes. Near the +Downs of Bathurst I passed over much pale-brown, glossy clay-slate, with +the shattered laminae running north and south; I mention this fact, because +Captain King informs me that, in the country a hundred miles southward, +near Lake George, the mica-slate ranges so invariably north and south that +the inhabitants take advantage of it in finding their way through the +forests. + +The sandstone of the Blue Mountains is at least 1,200 feet thick, and in +some parts is apparently of greater thickness; it consists of small grains +of quartz, cemented by white earthy matter, and it abounds with ferruginous +veins. The lower beds sometimes alternate with shales and coal: at Wolgan I +found in carbonaceous shale leaves of the Glossopteris Brownii, a fern +which so frequently accompanies the coal of Australia. The sandstone +contains pebbles of quartz; and these generally increase in number and size +(seldom, however, exceeding an inch or two in diameter) in the upper beds: +I observed a similar circumstance in the grand sandstone formation at the +Cape of Good Hope. On the South American coast, where tertiary and supra- +tertiary beds have been extensively elevated, I repeatedly noticed that the +uppermost beds were formed of coarser materials than the lower: this +appears to indicate that, as the sea became shallower, the force of the +waves or currents increased. On the lower platform, however, between the +Blue Mountains and the coast, I observed that the upper beds of the +sandstone frequently passed into argillaceous shale,--the effect, probably, +of this lower space having been protected from strong currents during its +elevation. The sandstone of the Blue Mountains evidently having been of +mechanical origin, and not having suffered any metamorphic action, I was +surprised at observing that, in some specimens, 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 rock. (I have lately seen, in a paper by Smith (the father of +English geologists), in the "Magazine of Natural History," that the grains +of quartz in the millstone grit of England are often crystallised. Sir +David Brewster, in a paper read before the British Association, 1840, +states, that in old decomposed glass, the silex and metals separate into +concentric rings, and that the silex regains its crystalline structure, as +is shown by its action on light.) It is difficult to imagine how these +crystals could have been formed; one can hardly believe that they were +separately precipitated in their present crystallised state. Is it possible +that rounded grains of quartz may have been acted on by a fluid corroding +their surfaces, and depositing on them fresh silica? I may remark that, in +the sandstone formation of the Cape of Good Hope, it is evident that silica +has been profusely deposited from aqueous solution. + +In several parts of the sandstone I noticed patches of shale which might at +the first glance have been mistaken for extraneous fragments; their +horizontal laminae, however, being parallel with those of the sandstone, +showed that they were the remnants of thin, continuous beds. One such +fragment (probably the section of a long narrow strip) seen in the face of +a cliff, was of greater vertical thickness than breadth, which proves that +this bed of shale must have been in some slight degree consolidated, after +having been deposited, and before being worn away by the currents. Each +patch of the shale shows, also, how slowly many of the successive layers of +sandstone were deposited. These pseudo-fragments of shale will perhaps +explain, in some cases, the origin of apparently extraneous fragments in +crystalline metamorphic rocks. I mention this, because I found near Rio de +Janeiro a well-defined angular fragment, seven yards long by two yards in +breadth, of gneiss containing garnets and mica in layers, enclosed in the +ordinary, stratified, porphyritic gneiss of the country. The laminae of the +fragment and of the surrounding matrix ran in exactly the same direction, +but they dipped at different angles. I do not wish to affirm that this +singular fragment (a solitary case, as far as I know) was originally +deposited in a layer, like the shale in the Blue Mountains, between the +strata of the porphyritic gneiss, before they were metamorphosed; but there +is sufficient analogy between the two cases to render such an explanation +possible. + +STRATIFICATION OF THE ESCARPMENT. + +The strata of the Blue Mountains appear to the eye horizontal; but they +probably have a similar inclination with the surface of the platform, which +slopes from the west towards the escarpment over the Nepean, at an angle of +one degree, or of one hundred feet in a mile. (This is stated on the +authority of Sir T. Mitchell in "Travels" volume 2 page 357.) The strata of +the escarpment dip almost conformably with its steeply inclined face, and +with so much regularity, that they appear as if thrown into their present +position; but on a more careful examination, they are seen to thicken and +to thin out, and in the upper part to be succeeded and almost capped by +horizontal beds. These appearances render it probable, that we here see an +original escarpment, not formed by the sea having eaten back into the +strata, but by the strata having originally extended only thus far. Those +who have been in the habit of examining accurate charts of sea-coasts, +where sediment is accumulating, will be aware, that the surfaces of the +banks thus formed, generally slope from the coast very gently towards a +certain line in the offing, beyond which the depth in most cases suddenly +becomes great. I may instance the great banks of sediment within the West +Indian Archipelago (I have described these very curious banks in the +Appendix to my volume on the structure of Coral-Reefs. I have ascertained +the inclination of the edges of the banks, from information given me by +Captain B. Allen, one of the surveyors, and by carefully measuring the +horizontal distances between the last sounding on the bank and the first in +the deep water. Widely extended banks in all parts of the West Indies have +the same general form of surface.), which terminate in submarine slopes, +inclined at angles of between thirty and forty degrees, and sometimes even +at more than forty degrees: every one knows how steep such a slope would +appear on the land. Banks of this nature, if uplifted, would probably have +nearly the same external form as the platform of the Blue Mountains, where +it abruptly terminates over the Nepean. + +CURRENT-CLEAVAGE. + +The strata of sandstone in the low coast country, and likewise on the Blue +Mountains, are often divided by cross or current laminae, which dip in +different directions, and frequently at an angle of forty-five degrees. +Most authors have attributed these cross layers to successive small +accumulations on an inclined surface; but from a careful examination in +some parts of the New Red Sandstone of England, I believe that such layers +generally form parts of a series of curves, like gigantic tidal ripples, +the tops of which have since been cut off, either by nearly horizontal +layers, or by another set of great ripples, the folds of which do not +exactly coincide with those below them. It is well-known to surveyors that +mud and sand are disturbed during storms at considerable depths, at least +from three hundred to four hundred and fifty feet (See Martin White on +"Soundings in the British Channel" pages 4 and 166.), so that the nature of +the bottom even becomes temporarily changed; the bottom, also, at a depth +between sixty and seventy feet, has been observed to be broadly rippled. +(M. Siau on the "Action of Waves" "Edin. New Phil. Journ." volume 31 page +245.) One may, therefore, be allowed to suspect, from the appearance just +mentioned in the New Red Sandstone, that at greater depths, the bed of the +ocean is heaped up during gales into great ripple-like furrows and +depressions, which are afterwards cut off by the currents during more +tranquil weather, and again furrowed during gales. + +VALLEYS IN THE SANDSTONE PLATFORMS. + +The grand valleys, by which the Blue Mountains and the other sandstone +platforms of this part of Australia are penetrated, and which long offered +an insuperable obstacle to the attempts of the most enterprising colonist +to reach the interior country, form the most striking feature in the +geology of New South Wales. They are of grand dimensions, and are bordered +by continuous links of lofty cliffs. It is not easy to conceive a more +magnificent spectacle, than is presented to a person walking on the summit- +plains, when without any notice he arrives at the brink of one of these +cliffs, which are so perpendicular, that he can strike with a stone (as I +have tried) the trees growing, at the depth of between one thousand and one +thousand five hundred feet below him; on both hands he sees headland beyond +headland of the receding line of cliff, and on the opposite side of the +valley, often at the distance of several miles, he beholds another line +rising up to the same height with that on which he stands, and formed of +the same horizontal strata of pale sandstone. The bottoms of these valleys +are moderately level, and the fall of the rivers flowing in them, according +to Sir T. Mitchell, is gentle. The main valleys often send into the +platform great baylike arms, which expand at their upper ends; and on the +other hand, the platform often sends promontories into the valley, and even +leaves in them great, almost insulated, masses. So continuous are the +bounding lines of cliff, that to descend into some of these valleys, it is +necessary to go round twenty miles; and into others, the surveyors have +only lately penetrated, and the colonists have not yet been able to drive +in their cattle. But the most remarkable point of structure in these +valleys, is, that although several miles wide in their upper parts, they +generally contract towards their mouths to such a degree as to become +impassable. The Surveyor-General, Sir T. Mitchell, in vain endeavoured, +first on foot and then by crawling between the great fallen fragments of +sandstone, to ascend through the gorge by which the river Grose joins the +Nepean ("Travels in Australia" volume 1 page 154.--I must express my +obligation to Sir T. Mitchell for several interesting personal +communications on the subject of these great valleys of New South Wales.); +yet the valley of the Grose in its upper part, as I saw, forms a +magnificent basin some miles in width, and is on all sides surrounded by +cliffs, the summits of which are believed to be nowhere less than 3,000 +feet above the level of the sea. When cattle are driven into the valley of +the Wolgan by a path (which I descended) partly cut by the colonists, they +cannot escape; for this valley is in every other part surrounded by +perpendicular cliffs, and eight miles lower down, it contracts, from an +average width of half a mile, to a mere chasm impassable to man or beast. +Sir T. Mitchell states, that the great valley of the Cox river with all its +branches contracts, where it unites with the Nepean, into a gorge 2,200 +yards wide, and about one thousand feet in depth. (Idem volume 2 page 358.) +Other similar cases might have been added. + +The first impression, from seeing the correspondence of the horizontal +strata, on each side of these valleys and great amphitheatre-like +depressions, is that they have been in chief part hollowed out, like other +valleys, by aqueous erosion; but when one reflects on the enormous amount +of stone, which on this view must have been removed, in most of the above +cases through mere gorges or chasms, one is led to ask whether these spaces +may not have subsided. But considering the form of the irregularly +branching valleys, and of the narrow promontories, projecting into them +from the platforms, we are compelled to abandon this notion. To attribute +these hollows to alluvial action, would be preposterous; nor does the +drainage from the summit-level always fall, as I remarked near the +Weatherboard, into the head of these valleys, but into one side of their +bay-like recesses. Some of the inhabitants remarked to me, that they never +viewed one of these baylike recesses, with the headlands receding on both +hands, without being struck with their resemblance to a bold sea-coast. +This is certainly the case; moreover, the numerous fine harbours, with +their widely branching arms, on the present coast of New South Wales, which +are generally connected with the sea by a narrow mouth, from one mile to a +quarter of a mile in width, passing through the sandstone coast-cliffs, +present a likeness, though on a miniature scale, to the great valleys of +the interior. But then immediately occurs the startling difficulty, why has +the sea worn out these great, though circumscribed, depressions on a wide +platform, and left mere gorges, through which the whole vast amount of +triturated matter must have been carried away? The only light I can throw +on this enigma, is by showing that banks appear to be forming in some seas +of the most irregular forms, and that the sides of such banks are so steep +(as before stated) that a comparatively small amount of subsequent erosion +would form them into cliffs: that the waves have power to form high and +precipitous cliffs, even in landlocked harbours, I have observed in many +parts of South America. In the Red Sea, banks with an extremely irregular +outline and composed of sediment, are penetrated by the most singularly +shaped creeks with narrow mouths: this is likewise the case, though on a +larger scale, with the Bahama Banks. Such banks, I have been led to +suppose, have been formed by currents heaping sediment on an irregular +bottom. (See the "Appendix" to the Part on Coral-Reefs. The fact of the sea +heaping up mud round a submarine nucleus, is worthy of the notice of +geologists: for outlyers of the same composition with the coast banks are +thus formed; and these, if upheaved and worn into cliffs, would naturally +be thought to have been once connected together.) That in some cases, the +sea, instead of spreading out sediment in a uniform sheet, heaps it round +submarine rocks and islands, it is hardly possible to doubt, after having +examined the charts of the West Indies. To apply these ideas to the +sandstone platforms of New South Wales, I imagine that the strata might +have been heaped on an irregular bottom by the action of strong currents, +and of the undulations of an open sea; and that the valley-like spaces thus +left unfilled might, during a slow elevation of the land, have had their +steeply sloping flanks worn into cliffs; the worn-down sandstone being +removed, either at the time when the narrow gorges were cut by the +retreating sea, or subsequently by alluvial action. + +VAN DIEMEN'S LAND. + +The southern part of this island is mainly formed of mountains of +greenstone, which often assumes a syenitic character, and contains much +hypersthene. These mountains, in their lower half, are generally encased by +strata containing numerous small corals and some shells. These shells have +been examined by Mr. G.B. Sowerby, and have been described by him: they +consist of two species of Producta, and of six of Spirifera; two of these, +namely, P. rugata and S. rotundata, resemble, as far as their imperfect +condition allows of comparison, British mountain-limestone shells. Mr. +Lonsdale has had the kindness to examine the corals; they consist of six +undescribed species, belonging to three genera. Species of these genera +occur in the Silurian, Devonian, and Carboniferous strata of Europe. Mr. +Lonsdale remarks, that all these fossils have undoubtedly a Palaeozoic +character, and that probably they correspond in age to a division of the +system above the Silurian formations. + +The strata containing these remains are singular from the extreme +variability of their mineralogical composition. Every intermediate form is +present, between flinty-slate, clay-slate passing into grey wacke, pure +limestone, sandstone, and porcellanic rock; and some of the beds can only +be described as composed of a siliceo-calcareo-clay-slate. The formation, +as far as I could judge, is at least a thousand feet in thickness: the +upper few hundred feet usually consist of a siliceous sandstone, containing +pebbles and no organic remains; the inferior strata, of which a pale flinty +slate is perhaps the most abundant, are the most variable; and these +chiefly abound with the remains. Between two beds of hard crystalline +limestone, near Newtown, a layer of white soft calcareous matter is +quarried, and is used for whitewashing houses. From information given to me +by Mr. Frankland, the Surveyor-General, it appears that this Palaeozoic +formation is found in different parts of the whole island; from the same +authority, I may add, that on the north-eastern coast and in Bass' Straits +primary rocks extensively occur. + +The shores of Storm Bay are skirted, to the height of a few hundred feet, +by strata of sandstone, containing pebbles of the formation just described, +with its characteristic fossils, and therefore belonging to a subsequent +age. These strata of sandstone often pass into shale, and alternate with +layers of impure coal; they have in many places been violently disturbed. +Near Hobart Town, I observed one dike, nearly a hundred yards in width, on +one side of which the strata were tilted at an angle of 60 degrees, and on +the other they were in some parts vertical, and had been altered by the +effects of the heat. On the west side of Storm Bay, I found these strata +capped by streams of basaltic lava with olivine; and close by there was a +mass of brecciated scoriae, containing pebbles of lava, which probably +marks the place of an ancient submarine crater. Two of these streams of +basalt were separated from each other by a layer of argillaceous wacke, +which could be traced passing into partially altered scoriae. The wacke +contained numerous rounded grains of a soft, grass-green mineral, with a +waxy lustre, and translucent on its edges: under the blowpipe it instantly +blackened, and the points fused into a strongly magnetic, black enamel. In +these characters, it resembles those masses of decomposed olivine, +described at St. Jago in the Cape de Verde group; and I should have thought +that it had thus originated, had I not found a similar substance, in +cylindrical threads, within the cells of the vesicular basalt,--a state +under which olivine never appears; this substance, I believe, would be +classed as bole by mineralogists. (Chlorophaeite, described by Dr. +MacCulloch ("Western Islands" volume 1 page 504) as occurring in a basaltic +amygdaloid, differs from this substance, in remaining unchanged before the +blowpipe, and in blackening from exposure to the air. May we suppose that +olivine, in undergoing the remarkable change described at St. Jago, passes +through several states?) + +TRAVERTIN WITH EXTINCT PLANTS. + +Behind Hobart Town there is a small quarry of a hard travertin, the lower +strata of which abound with distinct impressions of leaves. Mr. Robert +Brown has had the kindness to look at my specimens, and he informed me that +there are four or five kinds, none of which he recognises as belonging to +existing species. The most remarkable leaf is palmate, like that of a fan- +palm, and no plant having leaves of this structure has hitherto been +discovered in Van Diemen's Land. The other leaves do not resemble the most +usual form of the Eucalyptus (of which tribe the existing forests are +chiefly composed), nor do they resemble that class of exceptions to the +common form of the leaves of the Eucalyptus, which occur in this island. +The travertin containing this remnant of a lost vegetation, is of a pale +yellow colour, hard, and in parts even crystalline; but not compact, and is +everywhere penetrated by minute, tortuous, cylindrical pores. It contains a +very few pebbles of quartz, and occasionally layers of chalcedonic nodules, +like those of chert in our Greensand. From the pureness of this calcareous +rock, it has been searched for in other places, but has never been found. +From this circumstance, and from the character of the deposit, it was +probably formed by a calcareous spring entering a small pool or narrow +creek. The strata have subsequently been tilted and fissured; and the +surface has been covered by a singular mass, with which, also, a large +fissure has been filled up, formed of balls of trap embedded in a mixture +of wacke and a white, earthy, alumino-calcareous substance. Hence it would +appear, as if a volcanic eruption had taken place on the borders of the +pool, in which the calcareous matter was depositing, and had broken it up +and drained it. + +ELEVATION OF THE LAND. + +Both the eastern and western shores of the bay, in the neighbourhood of +Hobart Town, are in most parts covered to the height of thirty feet above +the level of high-water mark, with broken shells, mingled with pebbles. The +colonists attribute these shells to the aborigines having carried them up +for food: undoubtedly, there are many large mounds, as was pointed out to +me by Mr. Frankland, which have been thus formed; but I think from the +numbers of the shells, from their frequent small size, from the manner in +which they are thinly scattered, and from some appearances in the form of +the land, that we must attribute the presence of the greater number to a +small elevation of the land. On the shore of Ralph Bay (opening into Storm +Bay) I observed a continuous beach about fifteen feet above high-water +mark, clothed with vegetation, and by digging into it, pebbles encrusted +with Serpulae were found: along the banks, also, of the river Derwent, I +found a bed of broken sea-shells above the surface of the river, and at a +point where the water is now much too fresh for sea-shells to live; but in +both these cases, it is just possible, that before certain spits of sand +and banks of mud in Storm Bay were accumulated, the tides might have risen +to the height where we now find the shells. ( It would appear that some +changes are now in progress in Ralph Bay, for I was assured by an +intelligent farmer, that oysters were formerly abundant in it, but that +about the year 1834 they had, without any apparent cause, disappeared. In +the "Transactions of the Maryland Academy" volume 1 part 1 page 28 there is +an account by Mr. Ducatel of vast beds of oysters and clams having been +destroyed by the gradual filling up of the shallow lagoons and channels, on +the shores of the southern United States. At Chiloe, in South America, I +heard of a similar loss, sustained by the inhabitants, in the disappearance +from one part of the coast of an edible species of Ascidia.) + +Evidence more or less distinct of a change of level between the land and +water, has been detected on almost all the land on this side of the globe. +Captain Grey, and other travellers, have found in Southern Australia +upraised shells, belonging either to the recent, or to a late tertiary +period. The French naturalists in Baudin's expedition, found shells +similarly circumstanced on the S.W. coast of Australia. The Rev. W.B. +Clarke finds proofs of the elevation of the land, to the amount of 400 +feet, at the Cape of Good Hope. ("Proceedings of the Geological Society" +volume 3 page 420.) In the neighbourhood of the Bay of Islands in New +Zealand, I observed that the shores were scattered to some height, as at +Van Diemen's Land, with sea-shells, which the colonists attribute to the +natives. (I will here give a catalogue of the rocks which I met with near +the Bay of Islands, in New Zealand:--1st, Much basaltic lava, and scoriform +rocks, forming distinct craters;--2nd, A castellated hill of horizontal +strata of flesh-coloured limestone, showing when fractured distinct +crystalline facets: the rain has acted on this rock in a remarkable manner, +corroding its surface into a miniature model of an Alpine country: I +observed here layers of chert and clay ironstone; and in the bed of a +stream, pebbles of clay-slate;--3rd, The shores of the Bay of Islands are +formed of a feldspathic rock, of a bluish-grey colour, often much +decomposed, with an angular fracture, and crossed by numerous ferruginous +seams, but without any distinct stratification or cleavage. Some varieties +are highly crystalline, and would at once be pronounced to be trap; others +strikingly resembled clay-slate, slightly altered by heat: I was unable to +form any decided opinion on this formation.) Whatever may have been the +origin of these shells, I cannot doubt, after having seen a section of the +valley of the Thames River (37 degrees S.), drawn by the Rev. W. Williams, +that the land has been there elevated: on the opposite sides of this great +valley, three step-like terraces, composed of an enormous accumulation of +rounded pebbles, exactly correspond with each other: the escarpment of each +terrace is about fifty feet in height. No one after having examined the +terraces in the valleys on the western shores of South America, which are +strewed with sea-shells, and have been formed during intervals of rest in +the slow elevation of the land, could doubt that the New Zealand terraces +have been similarly formed. I may add, that Dr. Dieffenbach, in his +description of the Chatham Islands ("Geographical Journal" volume 11 pages +202, 205.) (S.W. of New Zealand), states that it is manifest "that the sea +has left many places bare which were once covered by its waters." + +KING GEORGE'S SOUND. + +This settlement is situated at the south-western angle of the Australian +continent: the whole country is granitic, with the constituent minerals +sometimes obscurely arranged in straight or curved laminae. In these cases, +the rock would be called by Humboldt, gneiss-granite, and it is remarkable +that the form of the bare conical hills, appearing to be composed of great +folding layers, strikingly resembles, on a small scale, those composed of +gneiss-granite at Rio de Janeiro, and those described by Humboldt at +Venezuela. These plutonic rocks are, in many places, intersected by +trappean-dikes; in one place, I found ten parallel dikes ranging in an E. +and W. line; and not far off another set of eight dikes, composed of a +different variety of trap, ranging at right angles to the former ones. I +have observed in several primary districts, the occurrence of systems of +dikes parallel and close to each other. + +SUPERFICIAL FERRUGINOUS BEDS. + +The lower parts of the country are everywhere covered by a bed, following +the inequalities of the surface, of a honeycombed sandstone, abounding with +oxides of iron. Beds of nearly similar composition are common, I believe, +along the whole western coast of Australia, and on many of the East Indian +islands. At the Cape of Good Hope, at the base of the mountains formed of +granite and capped with sandstone, the ground is everywhere coated either +by a fine-grained, rubbly, ochraceous mass, like that at King George's +Sound, or by a coarser sandstone with fragments of quartz, and rendered +hard and heavy by an abundance of the hydrate of iron, which presents, when +freshly broken, a metallic lustre. Both these varieties have a very +irregular texture, including spaces either rounded or angular, full of +loose sand: from this cause the surface is always honeycombed. The oxide of +iron is most abundant on the edges of the cavities, where alone it affords +a metallic fracture. In these formations, as well as in many true +sedimentary deposits, it is evident that iron tends to become aggregated, +either in the form of a shell, or of a network. The origin of these +superficial beds, though sufficiently obscure, seems to be due to alluvial +action on detritus abounding with iron. + +SUPERFICIAL CALCAREOUS DEPOSIT. + +A calcareous deposit on the summit of Bald Head, containing branched +bodies, supposed by some authors to have been corals, has been celebrated +by the descriptions of many distinguished voyagers. (I visited this hill, +in company with Captain Fitzroy, and we came to a similar conclusion +regarding these branching bodies.) It folds round and conceals irregular +hummocks of granite, at the height of 600 feet above the level of the sea. +It varies much in thickness; where stratified, the beds are often inclined +at high angles, even as much as at thirty degrees, and they dip in all +directions. These beds are sometimes crossed by oblique and even-sided +laminae. The deposit consists either of a fine, white calcareous powder, in +which not a trace of structure can be discovered, or of exceedingly minute, +rounded grains, of brown, yellowish, and purplish colours; both varieties +being generally, but not always, mixed with small particles of quartz, and +being cemented into a more or less perfect stone. The rounded calcareous +grains, when heated in a slight degree, instantly lose their colours; in +this and in every other respect, closely resembling those minute, equal- +sized particles of shells and corals, which at St. Helena have been drifted +up the side of the mountains, and have thus been winnowed of all coarser +fragments. I cannot doubt that the coloured calcareous particles here have +had a similar origin. The impalpable powder has probably been derived from +the decay of the rounded particles; this certainly is possible, for on the +coast of Peru, I have traced LARGE UNBROKEN shells gradually falling into a +substance as fine as powdered chalk. Both of the above-mentioned varieties +of calcareous sandstone frequently alternate with, and blend into, thin +layers of a hard substalagmitic rock, which, even when the stone on each +side contains particles of quartz, is entirely free from them (I adopt this +term from Lieutenant Nelson's excellent paper on the Bermuda Islands +"Geolog. Trans." volume 5 page 106, for the hard, compact, cream- or brown- +coloured stone, without any crystalline structure, which so often +accompanies superficial calcareous accumulations. I have observed such +superficial beds, coated with substalagmitic rock, at the Cape of Good +Hope, in several parts of Chile, and over wide spaces in La Plata and +Patagonia. Some of these beds have been formed from decayed shells, but the +origin of the greater number is sufficiently obscure. The causes which +determine water to dissolve lime, and then soon to redeposit it, are not, I +think, known. The surface of the substalagmitic layers appears always to be +corroded by the rain-water. As all the above-mentioned countries have a +long dry season, compared with the rainy one, I should have thought that +the presence of the substalagmitic was connected with the climate, had not +Lieutenant Nelson found this substance forming under sea-water. +Disintegrated shell seems to be extremely soluble; of which I found good +evidence, in a curious rock at Coquimbo in Chile, which consisted of small, +pellucid, empty husks, cemented together. A series of specimens clearly +showed that these husks had originally contained small rounded particles of +shells, which had been enveloped and cemented together by calcareous matter +(as often happens on sea-beaches), and which subsequently had decayed, and +been dissolved by water, that must have penetrated through the calcareous +husks, without corroding them,--of which processes every stage could be +seen.): hence we must suppose that these layers, as well as certain vein- +like masses, have been formed by rain dissolving the calcareous matter and +re-precipitating it, as has happened at St. Helena. Each layer probably +marks a fresh surface, when the, now firmly cemented, particles existed as +loose sand. These layers are sometimes brecciated and re-cemented, as if +they had been broken by the slipping of the sand when soft. I did not find +a single fragment of a sea-shell; but bleached shells of the Helix melo, an +existing land species, abound in all the strata; and I likewise found +another Helix, and the case of an Oniscus. + +The branches are absolutely undistinguishable in shape from the broken and +upright stumps of a thicket; their roots are often uncovered, and are seen +to diverge on all sides; here and there a branch lies prostrate. The +branches generally consist of the sandstone, rather firmer than the +surrounding matter, with the central parts filled, either with friable, +calcareous matter, or with a substalagmitic variety; this central part is +also frequently penetrated by linear crevices, sometimes, though rarely, +containing a trace of woody matter. These calcareous, branching bodies, +appear to have been formed by fine calcareous matter being washed into the +casts or cavities, left by the decay of branches and roots of thickets, +buried under drifted sand. The whole surface of the hill is now undergoing +disintegration, and hence the casts, which are compact and hard, are left +projecting. In calcareous sand at the Cape of Good Hope, I found the casts, +described by Abel, quite similar to these at Bald Head; but their centres +are often filled with black carbonaceous matter not yet removed. It is not +surprising, that the woody matter should have been almost entirely removed +from the casts on Bald Head; for it is certain, that many centuries must +have elapsed since the thickets were buried; at present, owing to the form +and height of the narrow promontory, no sand is drifted up, and the whole +surface, as I have remarked, is wearing away. We must, therefore, look back +to a period when the land stood lower, of which the French naturalists (See +M. Peron "Voyage" tome 1 page 204.) found evidence in upraised shells of +recent species, for the drifting on Bald Head of the calcareous and +quartzose sand, and the consequent embedment of the vegetable remains. +There was only one appearance which at first made me doubt concerning the +origin of the cast,--namely, that the finer roots from different stems +sometimes became united together into upright plates or veins; but when the +manner is borne in mind in which fine roots often fill up cracks in hard +earth, and that these roots would decay and leave hollows, as well as the +stems, there is no real difficulty in this case. Besides the calcareous +branches from the Cape of Good Hope, I have seen casts, of exactly the same +forms, from Madeira* and from Bermuda; at this latter place, the +surrounding calcareous rocks, judging from the specimens collected by +Lieutenant Nelson, are likewise similar, as is their subaerial formation. +Reflecting on the stratification of the deposit on Bald Head,--on the +irregularly alternating layers of substalagmitic rock,--on the uniformly +sized, and rounded particles, apparently of sea-shells and corals,--on the +abundance of land-shells throughout the mass,--and finally, on the absolute +resemblance of the calcareous casts, to the stumps, roots, and branches of +that kind of vegetation, which would grow on sand-hillocks, I think there +can be no reasonable doubt, notwithstanding the different opinion of some +authors, that a true view of their origin has been here given. + +*(Dr. J. Macaulay has fully described ("Edinb. New Phil. Journ." volume 29 +page 350) the casts from Madeira. He considers (differently from Mr. Smith +of Jordan Hill) these bodies to be corals, and the calcareous deposit to be +of subaqueous origin. His arguments chiefly rest (for his remarks on their +structure are vague) on the great quantity of the calcareous matter, and on +the casts containing animal matter, as shown by their evolving ammonia. Had +Dr. Macaulay seen the enormous masses of rolled particles of shells and +corals on the beach of Ascension, and especially on coral-reefs; and had he +reflected on the effects of long-continued, gentle winds, in drifting up +the finer particles, he would hardly have advanced the argument of +quantity, which is seldom trustworthy in geology. If the calcareous matter +has originated from disintegrated shells and corals, the presence of animal +matter is what might have been expected. Mr. Anderson analysed for Dr. +Macaulay part of a cast, and he found it composed of:-- +Carbonate of lime......73.15 +Silica.................11.90 +Phosphate of lime.......8.81 +Animal matter...........4.25 +Sulphate of lime......a trace + 98.11) + +Calcareous deposits, like these of King George's Sound, are of vast extent +on the Australian shores. Dr. Fitton remarks, that "recent calcareous +breccia (by which term all these deposits are included) was found during +Baudin's voyage, over a space of no less than twenty-five degrees of +latitude and an equal extent of longitude, on the southern, western, and +north-western coasts." (For ample details on this formation consult Dr. +Fitton "Appendix to Captain King's Voyage." Dr. Fitton is inclined to +attribute a concretionary origin to the branching bodies: I may remark, +that I have seen in beds of sand in La Plata cylindrical stems which no +doubt thus originated; but they differed much in appearance from these at +Bald Head, and the other places above specified.) It appears also from M. +Peron, with whose observations and opinions on the origin of the calcareous +matter and branching casts mine entirely accord, that the deposit is +generally much more continuous than near King George's Sound. At Swan +River, Archdeacon Scott states that in one part it extends ten miles +inland. ("Proceedings of the Geolog. Soc." volume 1 page 320.) Captain +Wickham, moreover, informs me that during his late survey of the western +coast, the bottom of the sea, wherever the vessel anchored, was +ascertained, by crowbars being let down, to consist of white calcareous +matter. Hence it seems that along this coast, as at Bermuda and at Keeling +Atoll, submarine and subaerial deposits are contemporaneously in process of +formation, from the disintegration of marine organic bodies. The extent of +these deposits, considering their origin, is very striking; and they can be +compared in this respect only with the great coral-reefs of the Indian and +Pacific Oceans. In other parts of the world, for instance in South America, +there are SUPERFICIAL calcareous deposits of great extent, in which not a +trace of organic structure is discoverable; these observations would lead +to the inquiry, whether such deposits may not, also, have been formed from +disintegrated shells and corals. + +CAPE OF GOOD HOPE. + +After the accounts given by Barrow, Carmichael, Basil Hall, and W.B. Clarke +of the geology of this district, I shall confine myself to a few +observations on the junction of the three principal formations. The +fundamental rock is granite (In several places I observed in the granite, +small dark-coloured balls, composed of minute scales of black mica in a +tough basis. In another place, I found crystals of black schorl radiating +from a common centre. Dr. Andrew Smith found, in the interior parts of the +country, some beautiful specimens of granite, with silvery mica radiating +or rather branching, like moss, from central points. At the Geological +Society, there are specimens of granite with crystallised feldspar +branching and radiating in like manner.), overlaid by clay-slate: the +latter is generally hard, and glossy from containing minute scales of mica; +it alternates with, and passes into, beds of slightly crystalline, +feldspathic, slaty rock. This clay-slate is remarkable from being in some +places (as on the Lion's Rump) decomposed, even to the depth of twenty +feet, into a pale-coloured, sandstone-like rock, which has been mistaken, I +believe, by some observers, for a separate formation. I was guided by Dr. +Andrew Smith to a fine junction at Green Point between the granite and +clay-slate: the latter at the distance of a quarter of a mile from the +spot, where the granite appears on the beach (though, probably, the granite +is much nearer underground), becomes slightly more compact and crystalline. +At a less distance, some of the beds of clay-slate are of a homogeneous +texture, and obscurely striped with different zones of colour, whilst +others are obscurely spotted. Within a hundred yards of the first vein of +granite, the clay-slate consists of several varieties; some compact with a +tinge of purple, others glistening with numerous minute scales of mica and +imperfectly crystallised feldspar; some obscurely granular, others +porphyritic with small, elongated spots of a soft white mineral, which +being easily corroded, gives to this variety a vesicular appearance. Close +to the granite, the clay-slate is changed into a dark-coloured, laminated +rock, having a granular fracture, which is due to imperfect crystals of +feldspar, coated by minute, brilliant scales of mica. + +The actual junction between the granitic and clay-slate districts extends +over a width of about two hundred yards, and consists of irregular masses +and of numerous dikes of granite, entangled and surrounded by the clay- +slate: most of the dikes range in a N.W. and S.E. line, parallel to the +cleavage of the slate. As we leave the junction, thin beds, and lastly, +mere films of the altered clay-slate are seen, quite isolated, as if +floating, in the coarsely crystallised granite; but although completely +detached, they all retain traces of the uniform N.W. and S.E. cleavage. +This fact has been observed in other similar cases, and has been advanced +by some eminent geologists (See M. Keilhau "Theory on Granite" translated +in the "Edinburgh New Philosophical Journal" volume 24 page 402.), as a +great difficulty on the ordinary theory, of granite having been injected +whilst liquified; but if we reflect on the probable state of the lower +surface of a laminated mass, like clay-slate, after having been violently +arched by a body of molten granite, we may conclude that it would be full +of fissures parallel to the planes of cleavage; and that these would be +filled with granite, so that wherever the fissures were close to each +other, mere parting layers or wedges of the slate would depend into the +granite. Should, therefore, the whole body of rock afterwards become worn +down and denuded, the lower ends of these dependent masses or wedges of +slate would be left quite isolated in the granite; yet they would retain +their proper lines of cleavage, from having been united, whilst the granite +was fluid, with a continuous covering of clay-slate. + +Following, in company with Dr. A. Smith, the line of junction between the +granite and the slate, as it stretched inland, in a S.E. direction, we came +to a place, where the slate was converted into a fine-grained, perfectly +characterised gneiss, composed of yellow-brown granular feldspar, of +abundant black brilliant mica, and of few and thin laminae of quartz. From +the abundance of the mica in this gneiss, compared with the small quantity +and excessively minute scales, in which it exists in the glossy clay-slate, +we must conclude, that it has been here formed by the metamorphic action--a +circumstance doubted, under nearly similar circumstances, by some authors. +The laminae of the clay-slate are straight; and it was interesting to +observe, that as they assumed the character of gneiss, they became +undulatory with some of the smaller flexures angular, like the laminae of +many true metamorphic schists. + +SANDSTONE FORMATION. + +This formation makes the most imposing feature in the geology of Southern +Africa. The strata are in many parts horizontal, and attain a thickness of +about two thousand feet. The sandstone varies in character; it contains +little earthy matter, but is often stained with iron; some of the beds are +very fine-grained and quite white; others are as compact and homogeneous as +quartz rock. In some places I observed a breccia of quartz, with the +fragments almost dissolved in a siliceous paste. Broad veins of quartz, +often including large and perfect crystals, are very numerous; and it is +evident in nearly all the strata, that silica has been deposited from +solution in remarkable quantity. Many of the varieties of quartzite +appeared quite like metamorphic rocks; but from the upper strata being as +siliceous as the lower, and from the undisturbed junctions with the +granite, which in many places can be examined, I can hardly believe that +these sandstone-strata have been exposed to heat. (The Rev. W.B. Clarke, +however, states, to my surprise ("Geolog. Proceedings" volume 3 page 422), +that the sandstone in some parts is penetrated by granitic dikes: such +dikes must belong to an epoch altogether subsequent to that when the molten +granite acted on the clay-slate.) On the lines of junction between these +two great formations, I found in several places the granite decayed to the +depth of a few inches, and succeeded, either by a thin layer of ferruginous +shale, or by four or five inches in thickness of the re-cemented crystals +of the granite, on which the great pile of sandstone immediately rested. + +Mr. Schomburgk has described ("Geographical Journal" volume 10 page 246.) a +great sandstone formation in Northern Brazil, resting on granite, and +resembling to a remarkable degree, in composition and in the external form +of the land, this formation of the Cape of Good Hope. The sandstones of the +great platforms of Eastern Australia, which also rest on granite, differ in +containing more earthy and less siliceous matter. No fossil remains have +been discovered in these three vast deposits. Finally, I may add that I did +not see any boulders of far-transported rocks at the Cape of Good Hope, or +on the eastern and western shores of Australia, or at Van Diemen's Land. In +the northern island of New Zealand, I noticed some large blocks of +greenstone, but whether their parent rock was far distant, I had no +opportunity of determining. + + +INDEX TO VOLCANIC ISLANDS. + +Abel, M., on calcareous casts at the Cape of Good Hope. + +Abingdon island. + +Abrolhos islands, incrustation on. + +Aeriform explosions at Ascension. + +Albatross, driven from St. Helena. + +Albemarle island. + +Albite, at the Galapagos archipelago. + +Amygdaloidal cells, half filled. + +Amygdaloids, calcareous origin of. + +Ascension, arborescent incrustation on rocks of. +-absence of dikes, freedom from volcanic action, and state of lava-streams. + +Ascidia, extinction of. + +Atlantic Ocean, new volcanic focus in. + +Augite, fused. + +Australia. + +Azores. + +Bahia in Brazil, dikes at. + +Bailly, M., on the mountains of Mauritius. + +Bald Head. + +Banks' Cove. + +Barn, The, St. Helena. + +Basalt, specific gravity of. + +Basaltic coast-mountains at Mauritius. +-at St. Helena. +-at St. Jago. + +Beaumont, M. Elie de, on circular subsidences in lava. +-on dikes indicating elevation. +-on inclination of lava-streams. +-on laminated dikes. + +Bermuda, calcareous rocks of. + +Beudant, M., on bombs. +-on jasper. +-on laminated trachyte. +-on obsidian of Hungary. +-on silex in trachyte. + +Bole. + +Bombs, volcanic. + +Bory St. Vincent, on bombs. + +Boulders, absence in Australia and Cape of Good Hope. + +Brattle island. + +Brewster, Sir D., on a calcareo-animal substance. +-on decomposed glass. + +Brown, Mr. R., on extinct plants from Van Diemen's land. +-on sphaerulitic bodies in silicified wood. + +Buch, Von, on cavernous lava. +-on central volcanoes. +-on crystals sinking in obsidian. +-on laminated lava. +-on obsidian streams. +-on olivine in basalt. +-on superficial calcareous beds in the Canary islands. + +Calcareous deposit at St. Jago affected by heat. +-fibrous matter, entangled in streaks in scoriae. +-freestone at Ascension. +-incrustations at Ascension. +-sandstone at St. Helena. +-superficial beds at King George's sound. + +Cape of Good Hope. + +Carbonic acid, expulsion of, by heat. + +Carmichael, Capt., on glassy coatings to dikes. + +Casts, calcareous, of branches. + +Chalcedonic nodules. + +Chalcedony in basalt and in silicified wood. + +Chatham island. + +Chlorophaeite. + +Clarke, Rev. W., on the Cape of Good Hope. + +Clay-slate, its decomposition and junction with granite at the Cape of Good +Hope. + +Cleavage of clay-slate in Australia. + +Cleavage, cross, in sandstone. + +Coast denudation at St. Helena. + +Columnar basalt. + +"Comptes Rendus," account of volcanic phenomena in the Atlantic. + +Concepcion, earthquake of. + +Concretions in aqueous and igneous rocks compared. +-in tuff. +-of obsidian. + +Conglomerate, recent, at St. Jago. + +Coquimbo, curious rock of. + +Corals, fossil, from Van Diemen's Land. + +Crater, segment of, at the Galapagos. +-great central one at St. Helena. +-internal ledges round, and parapet on. + +Craters, basaltic, at Ascension. +-form of, affected by the trade wind. +-of elevation. +-of tuff at Terceira. +-of tuff at the Galapagos archipelago. +-their breached state. +-small basaltic at St. Jago. +--at the Galapagos archipelago. + +Crystallisation favoured by space. + +Dartigues, M., on sphaerulites. + +Daubeny, Dr., on a basin-formed island. +-on fragments in trachyte. + +D'Aubuisson on hills of phonolite. +-on the composition of obsidian. +-on the lamination of clay-slate. + +De la Beche, Sir H., on magnesia in erupted lime. +-on specific gravity of limestones. + +Denudation of coast at St. Helena. + +Diana's Peak, St. Helena. + +Dieffenbach, Dr., on the Chatham Islands. + +Dikes, truncated, on central crateriform ridge of St. Helena. +-at St. Helena; number of; coated by a glossy layer; uniform thickness of. +-great parallel ones at St. Helena. +-not observed at Ascension. +-of tuff. +-of trap in the plutonic series. +-remnants of, extending far into the sea round St. Helena. + +Dislocations at Ascension. +-at St. Helena. + +Distribution of volcanic islands. + +Dolomieu, on decomposed trachyte. +-on laminated lava. +-on obsidian. + +Dree, M., on crystals sinking in lava. + +Dufrenoy, M., on the composition of the surface of certain lava-streams. +-on the inclination of tuff-strata. + +Eggs of birds embedded at St. Helena. +-of turtle at Ascension. + +Ejected fragments at Ascension. +-at the Galapagos archipelago. + +Elevation of St. Helena. +-the Galapagos archipelago. +-Van Diemen's Land, Cape of Good Hope, New Zealand, Australia, and Chatham +island. +-of volcanic islands. + +Ellis, Rev. W., on ledges within the great crater at Hawaii. +-on marine remains at Otaheite. + +Eruption, fissures of. + +Extinction of land-shells at St. Helena. + +Faraday, Mr., on the expulsion of carbonic acid gas. + +Feldspar, fusibility of. +-in radiating crystals. +-Labrador, ejected. + +Feldspathic lavas. +-at St. Helena. +-rock, alternating with obsidian. +-lamination, and origin of. + +Fernando Noronha. + +Ferruginous superficial beds. + +Fibrous calcareous matter at St. Jago. + +Fissures of eruption. + +Fitton, Dr., on calcareous breccia. + +Flagstaff Hill, St. Helena. + +Fleurian de Bellevue on sphaerulites. + +Fluidity of lavas. + +Forbes, Professor, on the structure of glaciers. + +Fragments ejected at Ascension. +-at the Galapagos archipelago. + +Freshwater Bay. + +Fuerteventura (Feurteventura), calcareous beds of. + +Galapagos archipelago. +-parapets round craters. + +Gay Lussac, on the expulsion of carbonic acid gas. + +Glaciers, their structure. + +Glossiness of texture, origin of. + +Gneiss, derived from clay-slate. +-with a great embedded fragment. + +Gneiss-granite, form of hills of. + +Good Hope, Cape of. + +Gorges, narrow, at St. Helena. + +Granite, junction with clay-slate, at the Cape of Good Hope. + +Granitic ejected fragments. + +Gravity, specific, of lavas. + +Gypsum, at Ascension. +-in volcanic strata at St. Helena. +-on surface of the ground at ditto. + +Hall, Sir J., on the expulsion of carbonic acid gas. + +Heat, action of, on calcareous matter. + +Hennah, Mr., on ashes at Ascension. + +Henslow, Prof., on chalcedony. + +Hoffmann, on decomposed trachyte. + +Holland, Dr., on Iceland. + +Horner, Mr., on a calcareo-animal substance. +-on fusibility of feldspar. + +Hubbard, Dr., on dikes. + +Humboldt on ejected fragments. +-on obsidian formations. +-on parapets round craters. +-on sphaerulites. + +Hutton on amygdaloids. + +Hyalite in decomposed trachyte. + +Iceland, stratification of the circumferential hills. + +Islands, volcanic, distribution of. +-their elevation. + +Incrustation, on St. Paul's rocks. + +Incrustations, calcareous, at Ascension. + +Jago, St. + +James island. + +Jasper, origin of. + +Jonnes, M. Moreau de, on craters affected by wind. + +Juan Fernandez. + +Keilhau, M., on granite. + +Kicker Rock. + +King George's sound. + +Labrador feldspar, ejected. + +Lakes at bases of volcanoes. + +Lamination of volcanic rocks. + +Land-shells, extinct, at St. Helena. + +Lanzarote, calcareous beds of. + +Lava, adhesion to sides of a gorge. +-feldspathic. +-with cells semi-amygdaloidal. + +Lavas, specific gravity of. + +Lava-streams blending together at St. Jago. +-composition of surface of. +-differences in the state of their surfaces. +-extreme thinness of. +-heaved up into hillocks at the Galapagos archipelago. +-their fluidity. +-with irregular hummocks at Ascension. + +Lead, separation from silver. + +Lesson, M., on craters at Ascension. + +Leucite. + +Lime, sulphate of, at Ascension. + +Lonsdale, Mr., on fossil-corals from Van Diemen's land. + +Lot, St. Helena. + +Lyell, Mr., on craters of elevation. +-on embedded turtles' eggs. +-on glossy coating to dikes. + +Macaulay, Dr., on calcareous casts at Madeira. + +MacCulloch, Dr., on an amygdaloid. +-on chlorophaeite. +-on laminated pitchstone. + +Mackenzie, Sir G., on cavernous lava-streams. +-on glossy coatings to dikes. +-on obsidian streams. +-on stratification in Iceland. + +Madeira, calcareous casts at. + +"Magazine, Nautical," account of volcanic phenomena in the Atlantic. + +Marekanite. + +Mauritius, crater of elevation of. + +Mica, in rounded nodules. +-origin in metamorphic slate. +-radiating form of. + +Miller, Prof., on ejected Labrador feldspar. +-on quartz crystals in obsidian beds. + +Mitchell, Sir T., on bombs. +-on the Australian valleys. + +Mud streams at the Galapagos archipelago. + +Narborough island. + +Nelson, Lieut., on the Bermuda islands. + +New Caledonia. + +New Red sandstone, cross cleavage of. + +New South Wales. + +New Zealand. + +Nulliporae (fossil), resembling concretions. + +Obsidian, absent at the Galapagos archipelago. +-bombs of. +-composition and origin of. +-crystals of feldspar sink in. +-its irruption from lofty craters. +-passage of beds into. +-specific gravity of. +-streams of. + +Olivine decomposed at St. Jago. +-at Van Diemen's land. +-in the lavas at the Galapagos archipelago. + +Oolitic structure of recent calcareous beds at St. Helena. + +Otaheite. + +Oysters, extinction of. + +Panza islands, laminated trachyte of. + +Pattinson, Mr., on the separation of lead and silver. + +Paul's, St., rocks of. + +Pearlstone. + +Peperino. + +Peron, M., on calcareous rocks of Australia. + +Phonolite, hills of. +-laminated. +-with more fusible hornblende. + +Pitchstone. +-dikes of. + +Plants, extinct. + +Plutonic rocks, separation of constituent parts of, by gravity. + +Porto Praya. + +Prevost, M. C., on rarity of great dislocations in volcanic islands. + +Prosperous hill, St. Helena. + +Pumice, absent at the Galapagos archipelago. +-laminated. + +Puy de Dome, trachyte of. + +Quail island, St. Jago. + +Quartz, crystals of, in beds alternating with obsidian. +-crystallised in sandstone. +-fusibility of. +-rock, mottled from metamorphic action with earthy matter. + +Red hill. + +Resin-like altered scoriae. + +Rio de Janeiro, gneiss of. + +Robert, M., on strata of Iceland. + +Rogers, Professor, on curved lines of elevation. + +Salses, compared with tuff craters. + +Salt deposited by the sea. +-in volcanic strata. +-lakes of, in craters. + +Sandstone of Brazil. +-of the Cape of Good Hope. +-platforms of, in New South Wales. + +Schorl, radiating. + +Scrope, Mr. P., on laminated trachyte. +-on obsidian. +-on separation of trachyte and basalt. +-on silex in trachyte. +-on sphaerulites. + +Seale, Mr., geognosy of St. Helena. +-on dikes. +-on embedded birds' bones. + +Seale, on extinct shells of St. Helena. + +Sedgwick, Professor, on concretions. + +Septaria, in concretions in tuff. + +Serpulae on upraised rocks. + +Seychelles. + +Shells, colour of, affected by light. +-from Van Diemen's land. +-land, extinct, at St. Helena. +-particles of, drifted by the wind at St. Helena. + +Shelly matter deposited by the waves. + +Siau, M., on ripples. + +Signal Post Hill. + +Silica, deposited by steam. +-large proportion of, in obsidian. +-specific gravity of. + +Siliceous sinter. + +Smith, Dr. A., on junction of granite and clay-slate. + +Spallanzani on decomposed trachyte. + +Specific gravity of recent calcareous rocks and of limestone. +-of lavas. + +Sphaerulites in glass and in silicified wood. +-in obsidian. + +Sowerby, Mr. G.B., on fossil-shells from Van Diemen's land. +-from St. Jago. +-land-shells from St. Helena. + +St. Helena. +-crater of elevation of. + +St. Jago, crater of elevation of. +-effects of calcareous matter on lava. + +St. Paul's rocks. + +Stokes, Mr., collections of sphaerulites and of obsidians. + +Stony-top, Little. +-Great. + +Stratification of sandstone in New South Wales. + +Streams of obsidian. + +Stutchbury, Mr., on marine remains at Otaheite. + +Subsided space at Ascension. + +Tahiti. + +Talus, stratified, within tuff craters. + +Terceira. + +Tertiary deposit of St. Jago. + +Trachyte, absent at the Galapagos archipelago. +-at Ascension. +-at Terceira. +-decomposition of, by steam. +-its lamination. +-its separation from basalt. +-softened at Ascension. +-specific gravity of. +-with singular veins. + +Trap-dikes in the plutonic series. +-at King George's sound. + +Travertin at Van Diemen's land. + +Tropic-bird, now rare, at St. Helena. + +Tuff, craters of. +-their breached state. +-peculiar kind of. + +Turner, Mr., on the separation of molten metals. + +Tyerman and Bennett on marine remains at Huaheine. + +Valleys, gorge-like, at St. Helena. +-in New South Wales. +-in St. Jago. + +Van Diemen's land. + +Veins in trachyte. +-of jasper. + +Vincent, Bory St., on bombs. + +Volcanic bombs. +-island in process of formation in the Atlantic. +-islands, their distribution. + +Wacke, its passage into lava. + +Wackes, argillaceous. + +Webster, Dr., on a basin-formed island. +-on gypsum at Ascension. + +White, Martin, on soundings. + +Wind, effects of, on the form of craters. + + + + +End of this Project Gutenberg Etext of Volcanic Islands by Charles Darwin. + diff --git a/old/3054.zip b/old/3054.zip Binary files differnew file mode 100644 index 0000000..edcc370 --- /dev/null +++ b/old/3054.zip |