REEFS, or CORAL REEFS, are a peculiar kind of rock which occurs abundantly in some parts of the ocean. They differ from other rocks in extending over a considerable space of the sea in length, with a very narrow width. Generally they are also only a few feet below water ; so that on the side where they are washed by the open sea, a heavy surf is continually running against them.
Reefs appear under various forms. In some places they are con tiguous to the shores, as in the Red Sea; in others they fringe the shores, being separated from them by a narrow channel of moderate depth. Sometimes they surround islands at a considerable distance from the shores, and the islands so encircled are almost without excep tion of volcanic origin. Very frequently they surround a portion of the sea, within which there are some small islands, which are often contiguous to the reefs, and seem to be a part of them, as both are composed of the same material. There are also coral reefs at a great distance from the land, which run nearly parallel to it, like a barrier. Among the latter are those of the Coral Sea, along the eastern coast of Australia.
The attention of navigators and naturalists was first attracted to those islands which go under the name of Lagoon Islands. A reef approaching in form to a circle surrounds a part of the sea and forms a lagoon, on the inner part of which there are usually several smaller islands, wooded and inhabited, which are contiguous to the reefs them selves, and frequently extend along them with little interruption.
The water on the outside is always nearly unfathomable, but within the reefs good anchorage is generally found, and the reefs themselves have one or more openings of deep water by which the largest vessels may enter the lagoon. Accurate observers soon discovered that the reefs were composed of coral, and were the production of an animal.
On these circumstances the first theory of reefs was founded. This theory received some addition from Forster, who accompanied Captain Cook on his second voyage. According to him, they are raised by these small animals perpendicularly like a wall, from a great depth to a very small distance from the surface of the sea. The waves after wards bring sand, muscles, tang, and pieces of coral, and deposit them on the reef, which, in this way, is gradually raised above the sea-level, and becomes fit for vegetation. Currents and sea-fowls bring the seeds of marine plants, which, being decomposed, form a mould on which the cocoa-nut palm, when carried thither by the waves, thrives luxuri antly. This theory of Forster was adopted and extended by Flinders,
who in surveying the coasts of Australia had abundant opportunities of observing the formation of these islands; and still more by Peron and Chamisso. The last-mentioned naturalist accompanied Kotzebue on his first voyage of discovery (1815-1818).
But facts have since been observed by Quoy and Gaimard, who ac companied Freycinet on his voyage, and published their observations in ythe Annales des Sciences Naturelles,' which are inconsistent with this theory. They found that the reefs did not entirely consist of coral, and that the layer of coral hardly exceeded a few fathoms in depth. Besides this the polypiaria which are able to form such a layer do not inhabit the sea to any great depth, and are not found where the water is more than 30 feet deep. They therefore supposed that these animals executed their work only in those parts of the sea where the bottom had been raised by some natural cause nearly to the surface of the water. They supported their theory by adducing the fact that the continuity of every lagoon.reef was broken at one or more places, so as to constitute a strait of very deep water, which could not be the case if these works were carried on at a depth of from 100 to 200 fathoms. This theory coincides with the views adopted by Von Buch, in his description of the Canary Islands, and has been adopted by Ehrenberg, in his pamphlet Ueber die Natur und Bil dung der Coralleninseln und Corallenbanke ire rothen Mecre ;' and by Darwin, in the third volume of the' Surveying Voyages of the Adven ture and Beagle.' The last-mentioned writer however does not doubt that the reefs which form the lagoon islands are composed of coral to a great depth, much greater indeed than those polypiaria which form them can live in. He explains this in a very ingenious way, by supposing that they formerly had an elevated island in the middle, but that in those portions of the sea where they now occur, the bottom of the waters was gradually lowered by eubaidence, by which the polypiaria were enabled to continue their work farther, and the mountainous islands in the middle disappeared. In other parts of the Pacific and Indian oceans the contrary is supposed to have taken place : the bottom of the sea was elevated, and in these tracts volcanic islands occur. He observes that in this respect these seas may be divided into broad bands, in which an elevation and submersion has alternately taken place ; and he draws from it some important inferences (p. 562-569).