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Show 558 CORAL FORMATIONS. April, 1836. of ocean, of m~re than a thousand miles in one direction and several hundreds in another, scattered over with islands, none of which rise to a greater height than that to which waves can throw fragments, or the wind heap up sand. Now if we leave out of the question subsidence, the foundation on which these reefs are built, must in every case come to the surface within that small limit (we may say twenty fathoms) at which corals can live. This conclusion is so extremely improbable that it may at once be rejected: for in what country can there be found a broad and grand range of mountains of the same height within a hun- • dred and twenty feet? But on the idea of subsidence, the case is at once clear : as each point, one after the other according to its altitude, was submerged, the coral grew upwards, and formed the many islets now standing at one level. Having endeavoured on general grounds not only to remove any extreme degree of improbability in the belief of a general subsidence, but likewise to show that it is almost necessary to account for the existence of a vast number of reefs on one level, we will now see how far the same idea will apply to the peculiar configuration in the several classes. Let us imagine an island merely fringed by reefs extending to a short distance from the shore; in which case, as we have before remarked, there is no difficulty in understanding their structure. Now let this island subside by a series of movements of extreme slowness, the coral at each interval growing up to the surface. Without the aid of sections it is not very easy to follow out the result, but a little reflection will show that a reef encircling the shore at a greater or less distance, according to the amount of subsidence, would be produced. If we suppose the sinking to continue, the encircled island must, by the submergence of the central land but upward growth of the ring of coral, be converted into a lagoon island. If we take a section of some encircled island on a true scale, as for instance Gambier, which has been so well described by Captain Beechey, we shall not find April, 1836. THEORY OF LAGOON ISLANDS. 559 the amount of movement very great, which would be necessary to change a well-characterized encircling reef, into as characteristic a lagoon island. It will at once be evident that a coral reef, closely skirting the shore of a continent, would, in like manner after each subsidence, rise to the surface; the water, however, always encroaching on the land. Would not a barrier reef necessarily be produced, similar to the one extending parallel to the coast of Australia? It is indeed but uncoiling one of those reefs which encircle at a distance so many islands. Thus the three great classes of reef, lagoon, encircling, and barrier, are connected by one theory. It will perhaps be remarked, if this be true, there ought to exist every intermediate form between a closely-encircled and a lagoon island. Such forms actually occur in various parts of the ocean : we have one, two, or more islands encircled in one reef; and of these some are of small proportional size to the area enclosed by the coral formation ; so that a series of charts might be given, showing a gradation of character between the two classes. In New Caledonia, where the double line of reef projects 140 miles beyond the island, we may imagine we see this change in progress. At the northern extremity, reefs occur, some of which are of the encircling kind, and others almost with the character of true lagoon islands. The line of reef which fronts the whole west coast of this great island, has by some been called a barrier. It is four hundred miles long; and may be said thus to form a link between an ordinary encircling reef and the great Australian barrier. I should perhaps have entered before into the consideration of one apparent difficulty in the origin of lagoon islands. It may be said, granting the theory of subsidence, a mere circular disc of coral would be formed, and not a cup-shaped mass. In the first place, even in reefs closely fringing the land (as before remarked), the corals do not grow on the shore itself, but leave a shallow channel. Secondly, the strong and vigorous species which |