Narrative of the surveying voyages of his majesty's ships Adventure and Beagle (vol.3)

April, 1836. AREAS of alternate 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. This in itself is remarkable, but it becomes far more so when it is known, that according to all accounts (and distinctly stated by Mr. De la Beche*) they are likewise ab- sent 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, Ehren- berg has observed that lagoon islands do not occur in the Red Sea : in LyelPs Geology, and in the Geographical Jour- nal, 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 circum- stance, and which has never been attempted to be solved, that the lagoon structure being universal and considered as cha- racteristic in certain parts of the ocean, should be entirely absent in others of equal extent. I may here also just recal to mind the cases of skirting reefs mentioned 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 merely skirting the shore, and others rising from a deep ocean in the form of distant rings. We have endeavoured to show that with a subsiding movement, the first and simple class must necessarily pass into the second, and more re- markable structure. To proceed with our examination : to the westward of the prolongation of the line of subsidence, of which Keeling Island is the index, we have an area of elevation. For on 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 of the Indian ocean, the Laccadive, Maldive, and Chagos line of atolls or lagoons show a line of subsidence. The * Geological Manual, p. 141.