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- VOLCANIC ISLANDS - 1/30 -


VOLCANIC ISLANDS

BY

CHARLES DARWIN

EDITORIAL NOTE.

Although in some respects more technical in their subjects and style than Darwin's "Journal," the books here reprinted will never lose their value and interest for the originality of the observations they contain. Many parts of them are admirably adapted for giving an insight into problems regarding the structure and changes of the earth's surface, and in fact they form a charming introduction to physical geology and physiography in their application to special domains. The books themselves cannot be obtained for many times the price of the present volume, and both the general reader, who desires to know more of Darwin's work, and the student of geology, who naturally wishes to know how a master mind reasoned on most important geological subjects, will be glad of the opportunity of possessing them in a convenient and cheap form.

The three introductions, which my friend Professor Judd has kindly furnished, give critical and historical information which makes this edition of special value.

G.T.B.

VOLCANIC ISLANDS.

TABLE OF CONTENTS.

CRITICAL INTRODUCTION.

CHAPTER I.--ST. JAGO, IN THE CAPE DE VERDE ARCHIPELAGO.

Rocks of the lowest series.--A calcareous sedimentary deposit, with recent shells, altered by the contact of superincumbent lava, its horizontality and extent.--Subsequent volcanic eruptions, associated with calcareous matter in an earthy and fibrous form, and often enclosed within the separate cells of the scoriae.--Ancient and obliterated orifices of eruption of small size.--Difficulty of tracing over a bare plain recent streams of lava.--Inland hills of more ancient volcanic rock.--Decomposed olivine in large masses.--Feldspathic rocks beneath the upper crystalline basaltic strata.--Uniform structure and form of the more ancient volcanic hills.--Form of the valleys near the coast.--Conglomerate now forming on the sea beach.

CHAPTER II.--FERNANDO NORONHA; TERCEIRA; TAHITI, ETC.

FERNANDO NORONHA.--Precipitous hill of phonolite.

TERCEIRA.--Trachytic rocks: their singular decomposition by steam of high temperature.

TAHITI.--Passage from wacke into trap; singular volcanic rock with the vesicles half-filled with mesotype.

MAURITIUS.--Proofs of its recent elevation.--Structure of its more ancient mountains; similarity with St. Jago.

ST. PAUL'S ROCKS.--Not of volcanic origin.--Their singular mineralogical composition.

CHAPTER III.--ASCENSION.

Basaltic lavas.--Numerous craters truncated on the same side.--Singular structure of volcanic bombs.--Aeriform explosions.--Ejected granite fragments.--Trachytic rocks.--Singular veins.--Jasper, its manner of formation.--Concretions in pumiceous tuff.--Calcareous deposits and frondescent incrustations on the coast.--Remarkable laminated beds, alternating with, and passing into obsidian.--Origin of obsidian.-- Lamination of volcanic rocks.

CHAPTER IV.--ST. HELENA.

Lavas of the feldspathic, basaltic, and submarine series.--Section of Flagstaff Hill and of the Barn.--Dikes.--Turk's Cap and Prosperous Bays.-- Basaltic ring.--Central crateriform ridge, with an internal ledge and a parapet.--Cones of phonolite.--Superficial beds of calcareous sandstone.-- Extinct land-shells.--Beds of detritus.--Elevation of the land.-- Denudation.--Craters of elevation.

CHAPTER V.--GALAPAGOS ARCHIPELAGO.

Chatham Island.--Craters composed of a peculiar kind of tuff.--Small basaltic craters, with hollows at their bases.--Albemarle Island; fluid lavas, their composition.--Craters of tuff; inclination of their exterior diverging strata, and structure of their interior converging strata.--James Island, segment of a small basaltic crater; fluidity and composition of its lava-streams, and of its ejected fragments.--Concluding remarks on the craters of tuff, and on the breached condition of their southern sides.-- Mineralogical composition of the rocks of the archipelago.--Elevation of the land.--Direction of the fissures of eruption.

CHAPTER VI.--TRACHYTE AND BASALT.--DISTRIBUTION OF VOLCANIC ISLES.

The sinking of crystals in fluid lava.--Specific gravity of the constituent parts of trachyte and of basalt, and their consequent separation.-- Obsidian.--Apparent non-separation of the elements of plutonic rocks.-- Origin of trap-dikes in the plutonic series.--Distribution of volcanic islands; their prevalence in the great oceans.--They are generally arranged in lines.--The central volcanoes of Von Buch doubtful.--Volcanic islands bordering continents.--Antiquity of volcanic islands, and their elevation in mass.--Eruptions on parallel lines of fissure within the same geological period.

CHAPTER VII.--AUSTRALIA; NEW ZEALAND; CAPE OF GOOD HOPE.

New South Wales.--Sandstone formation.--Embedded pseudo-fragments of shale.--Stratification.--Current-cleavage.--Great valleys.--Van Diemen's Land.--Palaeozoic formation.--Newer formation with volcanic rocks.-- Travertin with leaves of extinct plants.--Elevation of the land.--New Zealand.--King George's Sound.--Superficial ferruginous beds.--Superficial calcareous deposits, with casts of branches; its origin from drifted particles of shells and corals.--Their extent.--Cape of Good Hope.-- Junction of the granite and clay-slate.--Sandstone formation.

INDEX.

GEOLOGICAL OBSERVATIONS ON VOLCANIC ISLANDS.

CRITICAL INTRODUCTION.

The preparation of the series of works published under the general title "Geology of the Voyage of the 'Beagle'" occupied a great part of Darwin's time during the ten years that followed his return to England. The second volume of the series, entitled "Geological Observations on Volcanic Islands, with Brief Notices on the Geology of Australia and the Cape of Good Hope," made its appearance in 1844. The materials for this volume were collected in part during the outward voyage, when the "Beagle" called at St. Jago in the Cape de Verde Islands, and St. Paul's Rocks, and at Fernando Noronha, but mainly during the homeward cruise; then it was that the Galapagos Islands were surveyed, the Low Archipelago passed through, and Tahiti visited; after making calls at the Bay of Islands, in New Zealand, and also at Sydney, Hobart Town and King George's Sound in Australia, the "Beagle" sailed across the Indian Ocean to the little group of the Keeling or Cocos Islands, which Darwin has rendered famous by his observations, and thence to Mauritius; calling at the Cape of Good Hope on her way, the ship then proceeded successively to St. Helena and Ascension, and revisited the Cape de Verde Islands before finally reaching England.

Although Darwin was thus able to gratify his curiosity by visits to a great number of very interesting volcanic districts, the voyage opened for him with a bitter disappointment. He had been reading Humboldt's "Personal Narrative" during his last year's residence in Cambridge, and had copied out from it long passages about Teneriffe. He was actually making inquiries as to the best means of visiting that island, when the offer was made to him to accompany Captain Fitzroy in the "Beagle. " His friend Henslow too, on parting with him, had given him the advice to procure and read the recently published first volume of the "Principles of Geology," though he warned him against accepting the views advocated by its author. During the time the "Beagle" was beating backwards and forwards when the voyage commenced, Darwin, although hardly ever able to leave his berth, was employing all the opportunities which the terrible sea-sickness left him, in studying Humboldt and Lyell. We may therefore form an idea of his feelings when, on the ship reaching Santa Cruz, and the Peak of Teneriffe making its appearance among the clouds, they were suddenly informed that an outbreak of cholera would prevent any landing!

Ample compensation for this disappointment was found, however, when the ship reached Porta Praya in St. Jago, the largest of the Cape de Verde Islands. Here he spent three most delightful weeks, and really commenced his work as a geologist and naturalist. Writing to his father he says, "Geologising in a volcanic country is most delightful; besides the interest attached to itself, it leads you into most beautiful and retired spots. Nobody but a person fond of Natural History can imagine the pleasure of strolling under cocoa-nuts in a thicket of bananas and coffee-plants, and an endless number of wild flowers. And this island, that has given me so much instruction and delight, is reckoned the most uninteresting place that we perhaps shall touch at during our voyage. It certainly is generally very barren, but the valleys are more exquisitely beautiful, from the very contrast. It is utterly useless to say anything about the scenery; it would be as profitable to explain to a blind man colours, as to a person who has not been out of Europe, the total dissimilarity of a tropical view. Whenever I enjoy anything, I always look forward to writing it down, either in my log-book (which increases in bulk), or in a letter; so you must excuse raptures, and those raptures badly expressed. I find my collections are increasing wonderfully, and from Rio I think I shall be obliged to send a cargo home."


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