On examining the soundings of the seas in which they lie, and on comparing their positions, it appears probable that the various dispositions, as well as the places of the reefs, are in a great measure determined by the forms of 'he submarine land, and that they are placed on the tops of the hills, or on the most elevated parts of the bottom. When they form straight or curved lines, the side of the submarine structure to windward, or that which is ex posed to the breach of the sea, rises almost vertically in the manner of a wall ; while, to the leeward, they shelve gradually away, so as to deepen the water as they proceed in this direction, while at the other side they have reached its surface. It is supposed that there is here some design in effecting a purpose, which it is thought that accident can scarcely have determined; and that the intention of the animal, in thus building up to the windward, was to procure shelter for continuing its production to leeward. Whatever may be thought of that opinion, it is this abrupt rising from the bottom which renders them so dangerous to ships, as, from deep soundings, they may in a moment be on shore, and almost without warning.
When the groups are circular, there are some peculiari ties in them, as well as in the result, which are worthy of notice. A number of detached rocks and islands are first observed, forming a chain, which becomes gradually united in different places, so as to hold out the prospect of its some day becoming continuous. All round this, on the outside, the water is deep and the walls vertical, but within, it is found to shoal in different places, so as to con vey the idea of a large platform, surrounded by an elevat ed margin, with a depression in the middle. In the smaller circles, when this process is completed, the reefs repre sent a circular basin. This basin continues salt, and is a receptacle for sea-water for some time, during which it continues to grow shallower, as the animals within it con tinue their operations upwards. But as the water shoals, and the rains fall into it, it at length becomes freshened, so that the animals die, and the operation of filling it up ceases. Thus it becomes a fresh-water lake, and forms that receptacle which is so common a feature in all the flat islands of those seas.
Of whatever size the circle may be, but particularly if large, the islands begin first to collect on the outside, or ridge of the reef, while, within it, projecting parts or banks are scattered in different places. The reef, or dam, to wind ward, under the protection of which the whole mass ex tends, is produced by the fragments of the corals. When ever they have arrived at the surface of high water-mark, they cease to grow any longer, as the animal cannot live out of the water. But at low water, the reef is of course
above the sea. Thus its force breaks off the upper parts, and washes them onwards to leeward, where they collect ; while the animals, still working upwards on the windward side, keep up a constant supply of materials destined to the same end. Thus, a bank of dead matter, or of frag ments and sand produced by the wear of the corals, is formed on the top of the living rock, and cemented by the solvent power of the water, or the carbonate of lime. In this manner it is raised above the level of the high water mark, and kept smooth by the surf which continually sweeps over it, until it is raised even beyond its reach. The sand and fragments consolidate in time, so as to pro duce regular strata ; and fragments of these, forming large blocks of stone, are frequently piled up upon the reef, and farther onwards, till a large extent of surface thus becomes consolidated by the aid of more sand and fragments, and sometimes by that of other shells also, into a solid mass of land. The same process going on in the interior parts where the projecting banks are, all these at length extend and unite ; so that islands of any magnitude may, in this manner, at length be produced. Occasionally the lakes before mentioned are also filled up by the growth and de composition of vegetables, becoming at first marshy spots, and at length dry land.
The remainder of the operation is, to clothe these islands with soil and vegetation. This is the work of time, yet it is more rapid than would be expected. The first found ation of it is laid by the sand which the sea produces from the destruction of the corals, and by the sea-plants which take root and grow upon it. Sea birds, finding a place to settle in, add something ; and at length the seeds of vari ous plants floating about the ocean are arrested, and begin to grow, when a vegetable covering succeeds. Among these plants, the most conspicuous are the Smvola, Pan danus, Cerbera, Morinda, Hernandia, and others, which first begin to grow on the outer bank, where their seeds are first arrested, and at length spread over the whole. Last of all comes man,and the island forms a part of the in habited world.
It is evident that islands formed on this principle can have no great elevation above the water ; and accordingly those which are entirely flat are scarcely elevated more than five or six feet above the high water-mark. But as many of them are higher, it is necessary to resort to some new principles for effecting this purpose.