atoll

& a. [A Maldive word Anglicised. IMG:content-0860.png:[graphic] In Fr. a toll on .] A. As substantive: The name applied by geologists and others to any one of the lagoon islands or annular coral reefs found in the Pacific and the Indian Oceans, the Red Sea, 358 and some other parts of the tropics. An atoll is a ring of coral rock, oval rather than circular in form. One reaches eighty-eight miles in its longer, by twenty in its shorter, diameter; but in general they are of much more limited dimensions. On the top of the coral-rock, which rises but slightly above the sea-level, is vegetation of some luxuriance-the cocoa-nut ATOLL. being the most conspicuous plant. On the convex circumference of the ring is a beach of white sand, exterior to which is a line of breakers, and a few feet beyond them the unfathomable ocean. The ring of land, which is le than half a mile acro , encircles a lagoon of comparatively still water, which, from reflection, is of a bright but pale-green colour. In the view of Mr. Darwin, now almost universally adopted, there was once an island, po ibly even containing high land, in the place now occupied by the lagoon. It was surrounded by a "fringing reef" of living coral close to the shore. As, from geological causes, it slowly subsided into the deep and disappeared, the coral animals built up to the surface of the water, and formed the ring of rock constituting the modern island. In the larger atolls there are generally two or three breaks in the ring, affording ship-channels into the lagoon; these mark the spots where fresh water, discharged from the old subsiding land into the sea, prevented the coral animals, which are marine, from locating themselves or building. [CORAL.] "... hence I have invariably used in this volume the term ' atoll , which is the name given to these circular groups of coral islets by their inhabitants in the Indian Ocean, and synonymous with lagoon-island."Darwin: Coral Reefs (1842), p. 2.