CORAL ISLANDS exist most abundantly in the tropical and sub-tropical parts of the Pacific ocean. The formation of coral goes on, in favorable circumstances, with wonder ful rapidity, for masses of coral have been found to increase in height several feet in a few months; and a channel cut in the reef surrounding a coral island, to permit the pas sage of a schooner, has been choked up with coral in ten years. It was at one time supposed that the coral polvpes began their labors at the bottom of the ocean, and reared their pile from its greatest depths; but it has been ascertained that none of them live at depths of more than 20 or 30 fathoms, and most of them are inhabitants of much shal lower water. It appears, therefore, that the foundation of their still marvelous struc tures must be on rocks that do not reach the surface, probably in most cases volcanic rocks similar to those which, being further upheaved, form the volcanic and often moun tainous islands of Polynesia. Around these volcanic islands, which—although some of them are the largest islands of the Pacific ocean—are far fewer in number than its C. I., a fringing reef of coral is often found immediately attached to the land; whilst in many other cases, the reef surrounds the island, the intervening space—of irregular, but no where of great width—forming a lagoon or channel of still water, protected by the reef from winds and waves. According to a theory proposed by Mr. Darwin, and now very generally accepted, this latter kind of reef is formed from a reef of the former or merely fringing kind, by the gradual subsidence of the rocky basis carrying down the fringe of coral to a greater depth; whilst the greatest activity of life is displayed by pol•pes of the kinds most productive of large masses of coral in the outer parts which are most exposed to the waves. In this manner also he accounts for the formation of true C. I., or atolls, which consist merely of a narrow reef of coral surrounding a central lagoon; and very often of a narrow reef—perhaps half a mile in breadth—clothed with luxuriant vegetation, bordered by a narrow beach of snowy whiteness, and forming an arc, the convexity of which is towards the prevailing wind, whilst a straight line of reef not generally rising above the reach of the tide, forms the chord of the arc. There is gen erally a navigable passage through the reef into the inclosed lagoon, the waters of which are still and beautifully transparent, and the depth of water close to the precipitous sides of the reef is almost always very great. The passages through the reefs surround ing the larger volcanic islands are often opposite to the months of streams; but even where this Is not the case, there is a strong current in these channels from the flux and reflux of 'lie tides. Islets bearing a few cocoa-nut trees often appear at intervals in the
line of a low coral reef, and very generally mark the sides of passages through it. When a reef has reached the surface of the water, sand, shells, fragments of coral, and other substances, begin to accumulate, and cocoa-nut trees often grow where the waves still wash their roots. Further accumulations from the ocean, with decayed leaves, stems, etc., gradually convert the reef into fertile land. Many C. I., of considerable extent and population, arc nowhere more than a few feet above the level of the sea. Some• times a volcanic upheaval seems to have taken place after the coral was formed, and this is supposed to have been the origin of the islands—comparatively few in number— called crystal island.% composed of coral rock, more or less modified by the action of air, water, and other agents. Islands of this class sometimes rise to an devotion of 500 ft., and often exhibit precipitous cliffs, and contain extensive caverns. They do not exhibit, however, the picturesque beauty of the volcanic islands, nor the soft and gentle loveli ness which often characterizes the true C. L, and which has received the enthusiastic praise of all voyagers in the south seas.
Coral reefs sometimes include within their circuit more islands than one. Reefs also sometimes extend to a great length In a straight line, generally parallel to a coast, the submergence of which they are supposed to indicate. There is such a reef on the e. coast of Australia, extending not less than 350 m., without being broken by a channel.
Coral ina and Corallinacem, a genus and family of marine alga, of the sub-order cerammeem, remarkable for rigidity, which is mostly owing to a calcareous incrustation. When the calcareous matter is removed by a weak acid. the resemblance to other ceramiacem becomes very apparent. The common C. (C. eleinalis), extremely abundant on the British coasts, at first appears as a thin, round, shells, purplish patch, on a smooth rock, the shell ola mollu.sk,..or the frond of a sea-weed, gradually enlarges, and usually sends up a frond of jointed branching filaments, in a bushy tuft, an object of great beauty in the rock-pools. Although, as its name imports, this C. was once officinal, it has no medicival virtues. Seine of the corallines expand into leafy lobes, usually fan-shaped. Coral]iacs are most abundant in tropical seas, and there display their greatest beauty.
The name C. is often popularly given to zoophytes of the class anthozoa, and genera sertularia, thuiarea, antennularia, plumularia, laornedea, campanularia, etc., having branching polypidoms and hydraforn polypes of which the British coasts produce many small but extremely beautiful species.