We have said that madreporic or coralline formations affect three forms, to which the names of atolls, barrier reefs, and fringing reefs have been applied. We have spoken of atolls; we shall now say a few words on barrier and fringing reefs.
Barrier reefs are formations which surround the ordinary islands, or stretch along their banks. They have the form and general structure of atolls. Like atolls, the barrier reefs appear placed on the edge of a marine precipice. They rise on the edge of a plateau which looks down on a bottomless sea. On the coast of New Caledonia, only two lengths of his ship from the reef, Captain Kent found no bottom in a hundred and fifty fathoms. This was verified at Gambier Island in the Pacific Ocean, in Qualem Island, and at many others.
According to Mr. Darwin, the barrier reef situated on the western coast of New Caledonia is four hundred miles long; that along the eastern coast of Australia extends almost without interruption for a thousand miles, ranging from twenty or thirty to fifty or sixty miles from the coast. As to the elevation of the islands thus surrounded with reefs, it varies considerably. The Isle of Tahiti rises six thousand eight hundred feet above the level of the sea; the Isle of Maurua to six hundred; Aituaki to three hundred; and Manonai to about fifty feet only.
Around the Isle of Gambier the reef has a thickness of a thousand and sixty feet, at Tahiti of two hundred and thirty. Round the Fiji Islands it is from two to three thousand.
The fringing reefs immediately surrounding the island, or a portion of it, might be confounded with the barrier reefs we have been describing, if they only differed in their smaller breadth; but the circumstance that they abut immediately on the coast in place of being separated by a channel or lagoon more or less deep and continuous, proves that they are in direct communication with the slope of the submarine soil, and permits of their being distinguished from the barrier reefs. The dangerous breakers which surround the Mauritius are a striking example of the fringing reef. This island is almost entirely surrounded by a barrier of these rocks, the breadth of which varies from a hundred and fifty to three hundred and thirty feet; their rugged and abrupt surface is worn almost smooth, and is rarely uncovered at low water. Analogous reefs surround the Isle of Bourbon; all round this island the polyps construct on the volcanic bottom of the sea detached mammalons, which rise from a fathom to a fathom and a half above the water.
Madreporic coasting reefs present themselves also on the eastern coast of Africa and of Brazil. In the Red Sea, reefs of corals exist which may be ranked among the madreporic coasting reefs, in consequence of the limited breadth of the gulf. Ehrenberg and Hemprich examined a hundred and fifty stations in the Red Sea, all of which had outlying fringing reefs of this description.
It may be asked, With what rapidity are these coral and madreporic banks formed, so as to become atolls and fringing reefs? To answer this question even approximately is very difficult. On the coast of the Mauritius, according to M. d'Archaic,[6] the learned professor of the Jardin des Plantes, the edge of the reef is produced by Madrepora corymbosa, M. pocillifera, and two species of Astrea, which pursue their operations at the depth of from eight to fifteen fathoms. At the base is a bank of Seriatopora, from fifteen to twenty fathoms in height. At the bottom, the sand is covered with Seriatopora. At twenty fathoms we also meet with fragments of Madrepora. Between twenty and forty fathoms the bottom is sandy, and the sounding-rod brings up great fragments of Caryophylla. According to MM. Quoy and Gaimard, the Astreas, which, as these naturalists consider, constitute the greater part of the reefs, cannot live beyond four or five fathoms deep. Millepora alcicornis extends from the surface to the depth of twelve fathoms; the Madrepores and Seriatopores down to twenty fathoms. Considerable masses of Meandrina have been observed at sixteen fathoms; and a Caryophylla has been brought up from eighty fathoms in thirty-three degrees south latitude. Among the polyps which do not form solid reefs, Mr. Darwin mentions Cellaria, found at a hundred and ninety fathoms deep, Gorgonia at a hundred and sixty, Corallines at a hundred, Millepora at from thirty to forty-five, Sertularias at forty, and Tubulipora at ninety-five fathoms.
According to Dana, none of the species which form reefs—namely, Madrepora, Millepora, Porites, Astreas, and Meandrineas—can live at a greater depth than eighteen fathoms. It is only near the surface of the water that the zoophytes which produce minerals and form madreporic banks put forth their powers; the points most exposed to the beating of the waves is that which is most favourable to their growth; it is there that the Astreas, Porites, and Millepores most abound.
The proportionate increase of the structures, according to Mr. Darwin, depends at once upon the species which construct the reefs and upon various accessary circumstances. The ordinary rate of increase of the madrepores, according to Dana, is about an inch and a half annually; and, as their branches are much scattered, this will not exceed half an inch in thickness of the whole surface covered by the madrepore. Again, in consequence of their porosity, this quantity will be reduced to three-eighths of an inch of compact matter. It is, besides, to be noted that great spaces are wanting; the sands filling up the destroyed part of the polyp are washed out by the currents in the great depths where there are no living corals, and the surface occupied by them is reduced to a sixth of the whole coralline region, which reduces the preceding three-eighths to one-sixth. The shells and other organic débris will probably represent a fourth of the total produce in relation to corals. In this manner, taking everything into account, the mean increase of a reef cannot exceed the eighth of an inch annually. According to this calculation, some reefs which are not less than two thousand feet thick would require for their formation a hundred and ninety-two thousand years.
It is necessary to add, however, that in favourable circumstances the increase of the masses of coral may be much more rapid. Mr. Darwin speaks of a ship which, having been wrecked in the Persian Gulf, was found, after being submerged only twenty months, to be covered with a bed of coral two feet in thickness; he also mentions experiments made by Mr. Allen on the coast of Madagascar, which tend to prove that in the space of six months certain corals increased nearly three feet.