We proceed to the theoretic explanation of these curious mineral formations.
Naturalists and navigators have been much divided in opinion as to the true origin of these madreporic islands. Most of them have admitted that these enormous banks are composed of the mineral spoils and earthy detritus of the madrepores and corals, which, developing themselves in their midst, or upon the bed of the ocean, multiplying and superposing themselves, age after age, and generation after generation, have finally concluded by forming deposits of this immense extent. The growth of the vast madreporic column would be finally arrested by the want of water when its summit approached the level of the sea. It is thus that Forster, Péron, Flinders, and Chamisso have explained the formation of the atolls and madreporic reefs. This opinion has also found a supporter, in our times, in the French admiral, Du Petit Thouars. But he objects, with reason, that the corals cannot live at the prodigious depth of sea at which the base of these islets lie. It has therefore been found necessary to seek for another cause to satisfy the diverse conditions of the phenomena, and explain, at the same time, the strange circular arrangement of these islands, which is almost constant, and which it is essential to keep in view.
Sir Charles Lyell was of opinion that the base of an atoll was always the crater of an ancient submarine volcano, which, when crowned with corals and madrepores, would naturally reproduce this circular wall formed of heaped-up corals.
This theory supposes the existence of volcanic craters in the neighbourhood of all the coral islands. It is quite certain that these islands are often found not far from extinct volcanoes, and Sir Charles Lyell has published a very curious map in connection with the subject; nevertheless, the coincidence does not always exist. We have already remarked on the theory by which Mr. Darwin seeks to explain the complicated conditions of the phenomena. The explanation proposed accounts for the known facts, as well as the present appearance of the madreporic islands. The circular atolls and madreporic banks which are disposed as a sort of girdle, are principally formed of porites, millepora, and astrea, zoophytes which cannot exist at any great depth in the ocean, but which swarm on the rocks at some few fathoms only below the limits of the tide. These animals, by means of their accumulated débris, soon form a sort of coating round the island, which constitutes the littoral reefs: this marginal tongue or shoulder, according to Mr. Darwin, is the first stage in the existence of a madreporic island. At this point the author introduces a geological cause, namely, a great subsiding movement of the soil, in which the madreporic colony is sunk under the water. It is evident that after submersion the zoophyte will only continue to develop itself on the upper surface, and within the limits which its nature prescribes. The madrepores exhibiting their greatest vitality at the points most exposed to the fury of the waves, it will be near the outer edge of the reef that the development will be most rapid. If the subsidence of the island thus surrounded should still continue, as mountain after mountain and island after island slowly sink beneath the water, fresh bases would be successively afforded for the growth of the corals, and the outer edge elevated by their continual labour, thus transforming the space into a sort of circular lagune. The madreporic deposits would thus form an isolated girdle, and the lagune, which occupies the centre, would become deeper and deeper in proportion to the lowering of the soil. This is the second stage of the madreporic isle.
The existence of the atolls are thus subordinated to two principal conditions: the progressive subsidence of the shore washed by the sea, and the existence of coral formed of stony cells, the growth and multiplication of which are extremely rapid.
It follows from this that madreporic isles cannot exist in all seas; that they only have their birth in the Torrid zone, or at least near the Tropics, for it is only in these regions where the warmth exists, so necessary to their development, that the madrepores show themselves in greatest abundance.
The great field of madreporic formations, in short, is found in the warm parts of the Pacific Ocean. It is from this point, as from a common centre, round which are ranged the series of madreporic isles and islets, that it will be useful, in concluding this chapter, to trace their geographical distribution. We borrow the materials for this from Milne Edwards's tableaux of their distribution in the principal seas of the world.
It is, as we have said, only in the warm parts of the Pacific Ocean that the great mass of these islands are found. They give birth towards the south to the group of atolls known as the archipelago of the Bashee Islands, the extreme limit of the region being the Isle of Ducie. A multitude of other islands of the same nature are sparsely scattered over the sea, up to the east coast of Australia. There are enormous areas here, in which every single island is of coral formation, and is raised to the height at which the waves can throw up fragments. The Radack group is an angular square, four hundred miles long by two hundred and forty broad. Between this group and the Low Archipelago itself, eight hundred and forty miles by four hundred and twenty, there are groups and single islands covering a linear space of more than four thousand miles. To the north of the Equator, the archipelago of the Caroline Islands constitutes a very considerable group of madreporic formation, comprehending upwards of a thousand, extending in a broad belt over nearly forty degrees of longitude. On the other hand, all along the coast of the American continent, round the Galapagos and the Isle of Paques, we find no trace of them. The reason assigned is, that in these regions a great current of cold water, flowing from the Antarctic Pole, so much lowers the temperature of the sea, that the zoophytes no longer possess the requisite vigour.
We still meet with atolls in the Chinese Seas, and madreporic barrier reefs are abundant round the Marianne and Philippine Islands. These marginal reefs form also an immense tract, from the Isle of Timor, along the south coast of Sumatra, up to the island of Nicobar, in the Bay of Bengal.