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 ******r 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.