CORAL ISLAND AND CORAL REEF. An island or marine ridge formed from the petrified skeletons of coral polyps. They are numerous in the warmer portions of the Pacific and in the Indian Ocean, where the growth of coral goes on with great rapidity, occurring to a lesser extent in the Gulf of Mexico and along the Atlantic shores of the West Indies. The coral islands and reefs may be classed, according to their general form, into fringing reefs, barrier reefs, and atolls. Fringing reefs are closely attached to the shore line of an island or land mass and extend outward as a submarine plat form. Barrier reefs lie at some distance from the land, the intermediate space being occupied by a shallow lagoon of salt water. Usually some parts of the barrier rise above the level of the ocean as islets which support a scanty vegetation, while the position of the submerged reef is in dicated by a line of breakers. An excellent illus tration of this type is the Great Britain Reef of Australia, 1250 miles long, lying off the east coast of Queensland. Atolls (q.v.) are of rude
circular form, inelosing a lagoon, but without any visible land to which the reefs are attached. Their presence is made known by a girdle of breakers and by wave-formed islets on which the cocoanut palm and a few other tropical plants grow. The central lagoon of placid, transparent water is usually less than 300 feet deep, and when there are passages through the reef it constitutes a safe harbor for ships. Soundings have shown that the slope of the bottom is gentle in the in terior, but very steep on the seaward side of the reef, indeed being sometimes almost perpendicular. The Pelew Islands of the Caroline Archipelago, the Low Archipelago in the Pacific, and the Laccadive and Maldive island groups in the Indian Ocean, exhibit many examples of atolls.