CORAL. 403 CORAL ISLAND AND CORAL REEF. others (Choetetidae, Monticuliporidie) are even placed by some authors, as Ulrirh, among the Polyzoa. All these very primitive groups of tabulate corals plajed important rOles as Pale- ozoic reef-builders. BiBUOGRAPHY. Milne-Edwards and Haime, "JHonographie des Polypiers fossiles des terrains PalOozoiques," Archives du Muscinii, vol. v. (Paris, 1851) ; Roemer and Freeh, Lethwa Palro-
- oica, vol. i. (Stuttgart, 1883) ; Ogilvie, "Struc-
ture and Classification of Corals," Philosophiciil Transaclioiis, vol. clxxxvii. (London, 1806) ; Ci. C. Bourne, The Anthozoa," in Lankester's Treatise on Zoology, part ii. (London, 1900). CORALBERRY. See S^'owBEREY. CORAL-'FISH. A name given in a general way to various tropical fishes of the families Chietodontidie and Pomacentridic, because they frequent submarine coral growths. All are much compressed, high-backed fishes of brilliant hues ; and most of them are marked with vertical black bars, and possess filamentous appendages upon the fins and tail. .They remain among the branching corals for safety, and are further protected, ap- parently, by the curious pattern of their colora- tion. Many species abound in the waters from Bermuda to Brazil, where several species are known as 'angel-fishes.' All are small, but ex- cellent eating. CORAL ISLAND AND CORAL REEF. An island or marine ridge formed from the petrified skeletons of coral potyps. They are numerous in the warmer portions of the Pacific and in the Indian Ocean, where the growth of coral goes on M'ith 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, accoi'ding 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 Beef of Australia. 1250 miles long, lying ofT the east coast of Queensland. AtoUs (q.v.) are of rude circular form, inclosing a lagoon, but without any visible land to which the reefs are attached. Their jiresence is made known by a girdle of breakers and by wave-formed islets on which the cocoanut- palm and a few other tropical plants gi'ow. The central lagoon of placid, transparent water is usually less than 300 feet dee]), 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. Formation of Reefs. The great masses of coral rock have been built up by the continuous growth of various genera and species of corals, which secrete carbonate of lime dissolved in the sea-water. The coral polyps flourish only under certain conditions; their growth re<iuires clear, warm, salt water, an abundance of food, and a water-depth of not more than twenty fathoms. They cannot live in muddy or brackish water, or in regions where the mean temperature for any month falls below fiS" F. The jxjlyp also does not grow above the level of the lowest tides. As to the rate of growth of coral many observations and estimates have been made. Some species build up a reef as rapidly as three inches per year, others increase less than one inch in the same time. Alexander Agassiz estimates that the corals on the Florida coasts would construct a. reef from the depth of seven fathoms to the sur- face in a period of from 1000 to 1200 years. L'nder the special conditions necessary' for the growth of coral it is a matter of interest to dis- cover by what means the reefs and atolls have been formed, that often rise from depths of sev- eral hundred and even of thousands of feet. Dar- win, who recognized the fact that shallow water was necessary for the living polyp, inferred that the sites of the reefs must have undergone a grad- ual subsidence, corresponding to the growth of the reef upward. According to his theory, the colo- nies of polyps first settled along the shores of an Fig. 1. island, where after a time a fringing reef would be formed ( Fig. 1 ) . Then if the ocean floor sub- sided at a sufficiently slow rate to permit the growth of the coral to be continued on the out- ward side of the reef, the water-channel would gradually widen and deepen and a barrier reef (Fig. 2)" would be formed, which, after a long Fig. 2. interval, upon the submergence of the entire is- land, would give way to an atoll ( Fig. 3 ) . This Fig. 3. simple explanation, first advanced by Darwin in 1835 and afterwards elaborated by J. D. Dana, found wide acceptance among geologists. It was soon discovered, however, that in certain cases the theory of submergence did not conform with the actual conditions. Semper, in 18(18, directed attention to the Pelew Islands, where the sea floor and the reefs built thereon have actually
