The facts stand thus;—there are many large tracts of ocean, without any high land, interspersed with reefs and islets, formed by the growth of those kinds of corals, which cannot live at great depths; and the existence of these reefs and low islets, in such numbers and at such distant points, is quite inexplicable, excepting on the theory, that the bases on which the reefs first became attached, slowly and successively sank beneath the level of the sea, whilst the corals continued to grow upwards. No positive facts are opposed to this view, and some general considerations render it probable. There is evidence of change in form, whether or not from subsidence, on some of these coral-islands; and there is evidence of subterranean disturbances beneath them. Will then the theory, to which we have thus been led, solve the curious problem,—what has given to each class of reef its peculiar form? [Illustration] AA—Outer edge of the reef at the level of the sea. BB—Shores of the island. A′A′—Outer edge of the reef, after its upward growth during a period of subsidence. CC—The lagoon-channel between the reef and the shores of the now encircled land. B′B′—The shores of the encircled island. N.B.—In this, and the following woodcut, the subsidence of the land could only be represented by an apparent rise in the level of the sea. [Illustration] A′A′—Outer edges of the barrier-reef at the level of the sea. The cocoa-nut trees represent coral-islets formed on the reef. CC—The lagoon-channel. B′B′—The shores of the island, generally formed of low alluvial land and of coral detritus from the lagoon-channel. A″A″—The outer edges of the reef now forming an atoll. C′—The lagoon of the newly formed atoll. According to the scale, the depth of the lagoon and of the lagoon-channel is exaggerated.