Coral Islands

Coral Islands. Scattered over a vast elliptical area of the Central Pacific measuring some 9,000 miles in extreme length by about 4,500 miles in width, there are a series of archipelagoes consisting of numerous small, low, ring-shaped islands, or atolls. Chamisso, a German poet and naturalist, first explained how the annular form could have been produced by the greater growth of the corals on the outside of the reef where food and air were more abundant: hence the outside would grow upward while the coral in the middle would languish and die. But this theory, though adequate for atolls in shallow seas, such as those of the Persian Gulf, or parts of the West Indies, would not account for cases where the central lagoon is deeper than the zone of coral growth. Hence to explain these deep lagoons the theory of the growth of the atolls on the rims of volcanic craters was proposed. But the form and size of the islands were quite fatal to this view, as no craters were known of such vast dimensions or remarkable shapes. Moreover the existence of groups of volcanoes, all rising to within a couple of hundred feet of the surface and none rising above it, was highly improbable. Darwin pointed out that coral reefs may be divided into three classes :(1) fringing reefs occurring in shallow water and closely skirting a shore line. (2) Barrier reefs, formed at some distance from the shore and separated from it by a broad and often deep channel: such reefs may either encircle an island or group of islands, or run for hundreds of miles at some distance from a continental shore. (3) The true oceanic atolls, ring-shaped reefs, enclosing a lagoon with or without islands in it. Darwin's theory was that the reefs were first formed always on shallow shores as fringing reefs; then during a period of subsidence the reefs were carried downward, the coral polypes building upward to keep within the zone of coral growth; this would usually only be possible to the outer portion of the reefs: for the corals there would take all the food and oxygen in the water, while those nearer shore would be further handicapped by occasional floods of mud and water. Hence the outer parts of the reefs would grow upward and keep pace with the subsidence, and become separated from the receding shore by a lagoon channel; they would thus pass to the form of barrier reefs. By a continuation of this process the islands would decrease in size, and the reefs contract into a ring broken only by channels on the leeward side. By a still further continuation of the subsidence the islands would all disappear and nothing but a simple atoll remain. This theory was so simple and complete that it gained almost universal acceptance for many years, and it was not till 1863 that serious doubt was thrown upon it by the work of Semper. Since then several authors. Kein, L. Agassiz, and others, have adduced cases of atolls in various shallow seas for which the subsidence theory was not required. But Darwin had already pointed out similar cases and that his theory was not intended to apply to them. The most important criticism was that of Dr. J. Murray of the Challenger Expedition, whose theory is the only one that seriously disputes the field with that of Darwin. This newer theory is based on the fact that all over the tropical seas skeletons of the organisms that live on the surface fall as a perpetual hail to the bottom; where the shells fall on submerged peaks they would in time, aided by the molluscs, deep-sea corals, etc., raise these to the zone of reef-coral growth. The ring-shaped form of the atolls would then be due (1) to the greater growth of the corals on the outside, (2) to the reefs spreading seaward on a talus of coral rock, and (3) to the removal of the inner limestone by solution : the last factor would also form lagoon channels.

Dr. Murray points out in favour of his theory that so far as is known the rocks on which coral reefs have been built are always volcanic. Great stress is also laid on the asserted absence of any reefs of the thickness that must result from the subsidence method of formation. Hence it is claimed that the new theory accounts for all the facts and therefore ought to be accepted as it does not involve so many important postulates as Darwin's theory does. Speaking generally, Dr. Murray's theory has been somewhat widely accepted by zoologists but rejected by geologists, Sir A. Geikie being the principal exception among the latter. The geologists urge in objection to the new theory that the formation and deepening of lagoons by solution will not work, as no such action is seen in places that would be most favourable to it, and that the lagoons really tend to silt up. But the principal objection to it is that it leaves unexplained many facts that seem to afford evidence for the subsidence theory: thus it is claimed that there are coral reefs very much thicker than any that could be formed without subsidence, as in the case of the Dolomite Reefs of Tyrol. Direct evidence of subsidence is afforded (1) by the existence of reefs and reef rocks at a greater depth than the lower limit of coral growth; (2) by borings on the Sandwich Islands, near Honolulu, which have proved the existence of masses of coral rock 500 feet in thickness at a depth of over 1,000 feet below the sea level; (3) the existence of lagoons and channels more than thirty fathoms deep. Indirect evidence for submergence is also given by the irregular forms of many coral-girt islands such as the Marequesas, where the "spiderleg-like ridges" and fiords between them are proof of subaerial erosion when the land was at a higher level than at present. The distribution of coral islands is also of much importance in this connection. Dana has pointed out that stationary reefs have broad reef grounds and are well wooded, while islands that are rapidly subsiding become smaller, are low, bare, and often tide-washed. The Pacific Archipelagoes generally have a central group or line of islands having the characters of stationary reefs, while in travelling away from these the atolls become smaller and smaller until they disappear in an area of especially deep sea; on the other side of such a line of depression the reefs reappear as small shoals and tide-washed atolls, and grow to larger and wooded islands as they approach a line of rest or elevation.

As many important conclusions rest on the theory of the formation of coral reefs, it is not surprising that it has occupied so much attention during the last few years. The few borings that have been made have done much to re-establish Darwin's theory with the limitations that he himself placed on it; but the question will probably not be finally settled until a considerable series of borings in atolls have been made. But the cost and the mechanical difficulties involved in such borings will probably long delay the application of this final test.