Narrative of the surveying voyages of his majesty's ships Adventure and Beagle (vol.3)

April, 1836. theory of lagoon island. 561 land at the foot of the mountain was very small in quantity, the channel extremely deep, and the islets on the reef itself, which result from the gradual accumulation of fragments, singularly few in number; all of which, together with the wall-like structure of the reef both inside as well as outside, indicated to my mind, that, without doubt, the movements of subsidence had lately been rapid. At the end of the chapter, it is stated that this island is shaken by earthquakes of extreme violence. I may here mention a circumstance, which to my mind had the same weight as positive evidence, though bearing on another part of the question. M. Quoy, when discussing in general terms the nature of coral reefs, gives a description which is applicable only to those which, skitting the shore, do not require a foundation at any greater depth than that from which the coral-building polypi can spring. I was at first astonished at this, as I knew he had crossed both the Pacific and Indian oceans, and must, as I thought, have seen the class of widely-encircUng reefs, which indicate a sub- siding land. He subsequently mentions several islands as instances of his description of the general structure; by a singular chance, the whole can be shown, by his own words, in different parts of his account, to have been recently ele- vated. Therefore, that which appeared so adverse to the theory, became as strong in its confirmation. Continental elevations, as observed in South America and other parts, seem to act over wide areas with a very uniform force ; we may therefore suj^pose that continental subsidences act in a nearly similar manner. On this assumption, and taking on the one hand, lagoon islands, encircling and barrier reefs, as indications of subsidence; and on the other, raised shells and corals, together with mere skirting reefs, as our proof of elevation, we may test the truth of the theory, — that their configuration has been determined by the kind of subterranean movement, — by observing whether any uniform results can be obtained. I think it can be shown that such is the case in a very VOL. III. 2 o