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Show 554 CORAL FORMATIONS. April, 1836. even from the smallest particle to large fragments of rock, bears the stamp of having been subjected to the power of organic arrangement. Captain FitzRoy, at the distance of but little more than a mile from the shore, sounded with a line, 7200 feet long, and found no bottom. This island is, therefore, a lofty submarine mountain, which has a greater inclination than even those of volcanic origin on the land. I will now give a sketch* of the general results at which I have arrived, respecting the origin of the various classes of reefs, which occur scattered over such large spaces of the intertropical seas. The first consideration to attend to, is, that every observation leads to the conclusion that those lamelliform corals, which are the efficient agents in forming a reef, cannot live at any considerable depth. As far as I have personally seen, I judge of this from carefully' examining the impressions on the soundings, which were taken by Captain FitzRoy at Keeling Island, close outside the breakers, and from some others which I obtained at the Mauritius. At a depth under ten fathoms, the arming came up as clean as if it had been dropped on a carpet of thick turf; but as the depth increased, the particles of sand brought up became more and more numerous, until, at last, it was evident the bottom consisted of a smooth layer of calcareous sand, interrupted only at intervals by shelves, composed probably of dead coral rock. To carry on the analogy, the blades of grass grew thinner and thinner: till, at last, the soil was so sterile, that nothing sprung from it. .. As long as no facts, beyond those relating to the structure of lagoon islands were known, so as to establish some more comprehensive theory, the belief that corals constructed their habitations, or, speaking more correctly, their skeletons, on the circular crests of submarine craters, was from Malacca and Java, and the small fragments of pumice, drifted here, together with the seeds of East Indian plants. The one block of greenstone, moreover, on the Northern Lagoon must be excepted. • This sketch was read before the Geological Society, May, 1837. April, 1836. ENCIRCLING REEFS, 555 both ingenious and very plausible. Yet the sinuous margin of some, as in the Radack Islands of Kotzebue, one of which is fifty-two miles long, by twenty broad, and the narrowness of others, as in Bow Island (of which there is a chart on a large scale, forming part of the admirable labours of Captain Beechey), must have startled every one who considered this subject. The very general surprise of all those who have beheld lagoon islands, has perhaps been one chief cause why other reefs, of an equally curious struc.ture have been almost overlooked:* I allude to the encircling reefs. We will take, as an instance, Vanikoro, celebrated on account of the shipwreck of La Peyrouse. The reef there runs at the distance of nearly two, and in some parts three miles from the shore, and is separated from it by a channel having a general depth between thirty and forty fathoms, and, in one part, no less than fifty, or three hundred feet. Externally, the reef rises from an ocean profoundly deep. Can any thing be more singular than this structure? It is analogous to that of a lagoon, but with an island standing, like a picture in its frame, in the middle. A fringe of low alluvial land in these cases generally surrounds the base of the mountains ; this, covered by the most beautiful productions of a tropical land, backed by the abrupt mountains and fronted by a lake of smooth water, only separated from the dark waves of the ocean by a line of breakers, form the elements of the beautiful scenery of Tahiti- so well called the Queen of Islands. We cannot suppose these encircling reefs are based on an external crater, for the central mass sometimes consists of primary rock, or on any accumulation of sedimentary deposits, for the reefs follow indifferently the island itself, or its submarine prolongation. Of this latter case there is a grand instance • Mr. De la Beebe, however, seems to have been fully aware of the difficulty. He says, "there are certain situations, where coral reefs run, as it were, in a line with the coast, but separated from it by deep water, which would seem to require a different explanation." - Geological Manual, p. 142. |