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Show 560 CORAL FORMATIONS. April, 1836. alone build a solid reef, are never found within the lagoon . they only flourish amidst the foam of the nevertiring breakers. Nevertheless, the more delicate. corals, though checked by several causes, such as strong tides and deposits of sand, do constantly tend to fill up the lagoon ; but the process must become slower and sl~wer, as th~ water in the shallow expanse is rendered subJect to ace1de~tal impurities. A curious instance of this happ~ne~ at Keelmg Island, where a heavy tropical storm of ram killed nearly all the fish. When the coral at last has filled up ~he l.agoon to the height of lowest water at spring-tides, whiCh IS the extreme limit possible,-how, afterwards, is th~ work to be completed? There is no high land whence sediment ean be oured down ; and the dark-blue colour of the ocean ~espeaks its purity. The wind, carrying c~lcareous dust from the outer coast, is the only agent whiCh can finally convert the lagoon island into solid land, and how slow must this process be ! Subsidence of the land must always be most difficult to detect excepting in countries long civilized,-for the movement 'itself tends to conceal all evidence of it. N evertheless, at Keeling Island, tolerably conclusive evidence of such movement could be observed. On every side of the lagoon, in which the water is as tranquil as in the most sheltered lake, old cocoa-nut trees were undermined and falling. Captain FitzRoy likewise pointed out to me on. the ?each the foundation-posts of a storehouse, which the m~ab1tants said had stood, seven years before, just above h1gh-w~ter mark, but now was daily washed by the tide. Upon asking the people whether they ever experienced earthquakes, they said, that lately the island had been shaken b~ a very bad one; and that they remembered two others durmg the l~st ten years. I no longer doubted concerning the cause whiCh made the trees fall, and the storehouse to be washed by the daily tide. . At Vanikoro, the encircled island already mentwned, I gathered from Captain Dillon's account, that the alluvial April, 18.'36. 'l'HEORY OF LAGOON ISLA~D. 561 land at the foot of the mountain was very small in quantity, the channel extremely deep, and the islets on the reef itself, which result from the gradual accumulation of fragments, singularly few in number; all of which, together with the wall-like structure of the reef both inside as well as outside, indioated to my mind, that, without doubt, the movements of subsidence had lately been rapid. At the end · of the chapter, it is stated that this island is shaken by earthquakes of extreme violence. I may here mention a circumstance, which to my mind had the same weight as positive evidence, though bearing on another part of the question. M. Quoy, when discussing in general terms the nature of coral reefs, gives a description which is applicable only to those which, skirting the shore, do not require a foundation at any greater depth than that from which the coral-building polypi can spring. I was at first astonished at this, as I knew he had crossed both the Pacific and Indian oceans, and must, as I thought, have seen the class of widely-encircling reefs, which indicate a subsiding land. He subsequently mentions several islands as instances of his description of the general structure ; by a singular chance, the whole can be shown, by his own words, in different parts of his account, to have been recently elevated. Therefore, that which appeared so adverse to the theory, became as strong in its confirmation. Continental elevations, as observed in South America and other parts, seem to act over wide areas with a very uniform force ; we may therefore suppose that continental subsidences act in a nearly similar manner. On this assumption, and taking on the one hand, lagoon islands, encircling and barrier reefs, as indications of subsidence; and on the other, raised shells and corals, together with mere skirting reefs, as our proof of elevation, we may test the truth of the theory,-that their configuration has been determined by the kind of subterranean movement,-by observing whether any uniform results can be obtained. I think it can be shown that such is the case in a very VOL. III. 2 0 |