Geological Observations of Volcanic Islands | Page 4

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