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

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 observa- tion leads to the conclusion that those lameUiforra 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 Keehng 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 struc- ture of lagoon islands were known, so as to establish some more comprehensive theory, the behef that corals con- structed 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 green- stone, moreover, on the Northern Lagoon must be excepted. * Tills sketch was read before the Geological Society, May, 1837.