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Show 5G4 CORAL FORMA'l'IONS. April, 1836. of the coast of the mainland, it may be expected that any outlying islands would have formed lagoon islands. Now Bligh and others distinctly state that some of the islands there are precisely similar to the well-known lagoon islands in the Pacific ; there are also encircled islands, so that the three classes supposed to be produced by the same movement are there found in juxtaposition ; as likewise happens, but in a less evident manner, at New Caledonia and in the Society Archipelago. The New Hebrides line of islands, may be observed to bend abruptly at New Britain, thence to run nearly east and west; and, lastly, to resume its former north-west direction in Sumatra and the peninsula of Malacca. 'rhe figure may be compared to the letter S laid obliquely, but the line is often double. We have shown that the southern part, as far north as New Ireland, abounds with proofs of elevation, so is it with the rest. Since the time of Bougainville every voyager adduces some fresh instance of such changes throughout a great part of the East Indian archipelago. I may specify New Guinea, Wageeoo, Ceram, Timor, Java, and Sumatra. Coral reefs are abundant in the greater part of these seas, but they merely skirt the shores. In the same manner as we have followed the curved line of elevation, so may we that of subsidence. At Keeling Island, I have already mentioned that there exist proofs of the latter movement : and it is a very interesting circumstance, that during the last earthquake, by which that island was affected, Sumatra, though distant nearly 600 miles, was violently shaken. Bearing in mind that there is evidence of recent elevation on the coast of the latter, one is strongly tempted to believe that as one end of the lever goes up, the other goes down : that as the East Indian archipelago rises, the bottom of the neighbouring sea sinks and carries with it Keeling Island, which would have been submerged long ago in the depths of the ocean, had it not been for the wonderful labours of the reef-building polypi. As I have remarked, the islands in this great archipelago are only skirted with reefs; and it appears from the state- April, 1836. AREAS OF A L'l'EltNA'l'E MOVEMENTS. 565 ments of those who have visited them, as well as from an examination of the charts, that lagoon islands are not found there. 1,his in itself is remarkable, but it becomes far more s~ ':hen it is known, that according to all accounts (and dtstmctly stated by Mr. De la Beche*) they are likewise absent in the West Indian sea, where coral is most abundant: now every one is aware of the numerous proofs of recent elevation in most parts of that archipelago. Again, Ehrenberg has observed that lagoon islands do not occur in the Red Sea: in Lyell's Geology, and in the Geographical Journal, proofs are given of recent elevation on the shores of a large part of that sea. Excepting on the theory of the form of reefs being determined by the kind of movement to which they have been subjected ; it is a most anomalous circumstance, and which has never been attempted to be solved, that the lagoon structure being universal and considered as characteris~ ic in certain parts of the ocean, should be entirely absent m others of equal extent. I may h.ere also just recal to mind the cases of skirting reefs mentwned by M. Quoy (to which number several others might be added), where proofs of elevation occurred. Some general law must determine the marked difference between reefs ~erely skirting the shore, and others rising from a deep ocean m the form of distant rings. We have endeavoured to show that with a subsiding movement, the first and simple class must nacessarily pass into the second, and more remarkable structure. To proceed with our examination : to the westward of the prolongation of the line of subsidence, of which Keelina Island is the index, we have an area of elevation. For o~ the northern end of Ceylon and on the eastern shores of India, elevated shells and corals, such as now exist in the neighbouring sea, have been observed. Again in the middle ~f the Indian ocean, the Laccadive, Maldive, and Chagos hne of atolls or lagoons show a line of subsidence. The •» G t~o logical Manual, p . 141. |