Pacific Ocean Coral Islands
reef water theory
PACIFIC OCEAN CORAL ISLANDS - the origin of coral islands was specially studied by Darwin during the voyage of the " Beagle " in 1831-36, and lie shortly afterwards published a theory on the subject which has been fully detailed in the article CORAL (vol. vi. r. 377). This theory was so simple, and it appeared so complete, that it acquired universal acceptance; and the continuous action of subsidence in promoting the development of fringing reefs into barriers, and of barriers into atolls, was long unquestioned. In 1851 L. Agassizl expressed the opinion that the theory of subsidence was insufficient to explain the formation of the coral reefs and keys of Florida. In 1863 Carl Seinper stated that an attentive study of the Pelew Islands showed the complete inadequacy of this theory, and in 1868 lie reiterated his convictions.2 Coral-reef builders starting on a bank, whether formed by elevation or subsidence, by erosion or the upward growth of deep-sea deposits composed largely of organic remains, tend ultimately to assume the atoll or barrier form. When the coral reef or colony approaches the sum-face, the central portions are gradually placed at a disadvantage as compared with the peripheral parts of the mass, in being farther removed from the food supply which is brought by the oceanic currents, and consequently dwindle and die. In proportion as the reef approaches the surface, the centre becomes cut oil from the food supply and the conditions become increasingly uncongenial. At last an outer ring of vigorously growing reef corals encloses a central lagoon. The windward side of the reef grows most vigorously, not because of a larger supply of oxygen and greater aeration of the water, hut because that is the direction in which the oceanic currents bring the food to the reef. As the atoll extends seawards from vigorous growth the lagoon becomes larger, chiefly from the removal of lime in solntion by the action of the carbonic acid in sea water which flows in and out at each tide. This solvent action of sea water on dead calcareous organisms was shown IT the "Challenger's " observations to be universal.
Mr Murray reverses the order of growth as given IT Darwin for the groups in the Indian Ocean. He regards the Laccadive, Caroline, and Chagos archipelagos as various stages in the growth of coral reefs towards the surface, and he explains the various appearances in the Maldive group of atolls without any necessity for disseverment by oceanic currents as argued by Darwin. Precisely the same explanation is applied to the case of a barrier reef. It commences in the shallow water near the shore, and afterwards extends seawards on a talus built up of lumps of coral broken off by the surf. A very careful examination of the barrier reef at Tahiti was made by Lieutenant Swire of H.M.S. "Challenger" and Mr Murray, and they found that such an explanation was completely justified by the form and nature of the reef. There was much dead coral on the inner side of the barrier, which in many places was perpendicular or even overhanging; while, on the contrary, the outer surface was all alive, and sloped gradually seawards. A section of it, drawn to a true scale, is given in fig. 10.
This section shows that a ledge, over which there is a depth of from 30 to 40 fathoms of water, runs out for 250 yards from the edge of the reef. This ledge is covered with luxuriant heads and bosses of coral. Beyond it there is a steep irregular slope at an angle of about 45°, the talus being formed apparently of coral masses broken off from the ledge, and piled up; this slope is covered with living Sponges, Alcyonarians, Hydroids, Polyzorr, Foramindfcra, and other forms of life. The angle of inclination then decreases to 30°, and the ground is covered with coral sand; while beyond 500 yards from the edge of the reef the declivity is