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 requires 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 68° F. The polyp 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 hi 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.
Under 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 island, where after a time a fringing reef would be formed (Fig. I). 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 interval, upon the submergence of the entire land, would give way to an atoll (Fig. 3). This simple explanation, first advanced by Darwin in 1S35 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 1808, directed attention to the Pelew Islands, where the sea floor and the reefs built thereon have actually undergone elevation. Later, Murray pursued the study of the question still further and was able to show that reefs do not necessarily require a sinking shore for their formation, but they may grow on a stable foundation such as a submarine Lank. raised to near sea-level by accumulation of fossil organisms, or the slopes of a volcanic island. _\fter becoming established in such localities the corals continue their growth outward, and at the same time wave-action washes down debris from the reef to the bottom, forming a platform for their further activity. In case the original foun dation was above sea-level the projecting portion may have been cut down by breakers. The solvent action of water would remove the dead coral from the interior of the reef and thus excavate a basin to be occupied by the lagoon. Both theories of reef-building are probably to be accepted as valid, and each must be tested by actual conditions before its application in any particular instance can be determined.
BIBLIOGRAPHY. Darwin, The Structure and Bibliography. Darwin, The Structure and Distribution of Coral Islands (New York, 1S91) ; Dana, Corals and Coral Islands (New York, 1S90) Agassiz, .1 Visit to the Great Barrier Reef of Australia in the Strainer Croydon, in 1896 (Cambridge. Mass.. 1898) ; Murray, "On Coral Reefs and Other Carbonate of Lime Formations in Modern Seas." in Proceedings of the Royal So ciety of Edinburgh, vol. xvii. (Edinburgh, 1891) ; :Murray, "'On the Structure and Origin of Coral Reefs and Islands," in Proceedings of Royal So ciety of Edinburgh, vol. s. (Edinburgh, 1850) ; Agassiz and Pourtales, Monograph of the Corals of Florida (Cambridge. Mass.. 1S71) : Heilprin, The Bermuda islands (Philadelphia, 1889). See ATOLL ; CORAL.