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+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.
+
+
+
+
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