The various theories which had been propounded failing to explain the existence of the coral islands, Mr. Darwin was led to reconsider the whole subject. Numerous soundings taken all round the Cocos atoll showed that at ten fathoms the prepared tallow in the hollow of the sounding rod came up perfectly clean, and marked with the impression of living polyps. As the depth increased, these impressions became less numerous, but adhering particles of sand succeed, until it was evident that the bottom consisted of smooth sand. From these observations, it was obvious to him that the utmost depth at which the coral polyps can construct reefs is between twenty and thirty fathoms. Now, there are enormous areas in the Indian Ocean in which every island is a coral formation raised to the height to which the waves can throw up fragments and the winds pile up sand; and the only theory which seems to account for all the circumstances embraced, is that of the subsidence of vast regions in this ocean. "As mountain after mountain and island after island slowly sunk beneath the water," he says, "fresh bases would be successively afforded for the growth of the corals. I venture to defy any one to explain in any other manner how it is possible that numerous islands should be distributed throughout vast areas, all the islands being low, all built of coral absolutely requiring a foundation within a limited depth below the surface." The _Porites_, according to Mr. Darwin, form the most elevated deposits of those which are situated nearer the level of the water: _Millepora complanata_ also enters into the formation of the upper banks. Various other branched corals present themselves in great numbers in the cavities left by the _Porites_ and _Millepora_ crossing each other. It is difficult to identify species occupying themselves in the deeper parts, but, according to Darwin, the lower parts of the reefs are occupied by polyps of the same species as in the upper parts; at the depth of eighteen fathoms and upwards, the bottom consists alternately of sand and corals. The total breadth of the circular reef or ring which constitutes the atoll of the Keeling or Cocos Island varies from two hundred to five hundred yards in breadth. Some little parasitic isles form themselves upon the reefs, at two or three hundred yards from their exterior edge, by the accumulation of the fragments thrown up here during great storms. They rise from two to three yards above the sea level, and consist of shells, corals, and sea urchins, the whole consolidated into hard and solid rock.