448 LAGOON-ISLANDS. [CHAP. xx. 



those massive kinds, to whose growth on the exposed outer shores 

 the very existence of the reef depends, cannot live within the 

 lagoon, where other delicately-branching kinds flourish. More- 

 over, on this view, many species of distinct genera and families 

 are supposed to combine for one end ; and of such a combination, 

 not a single instance can be found in the whole of nature. The 

 theory that has been most generally received is, that atolls are 

 based on submarine craters ; but when we consider the form and 

 size of some, the number, proximity, and relative positions of 

 others, this idea loses its plausible character: thus, Suadiva atoll 

 is 44 geographical miles in diameter in one line, by 34 miles in 

 another line; Kimsky is 54 by 20 miles across, and it has a 

 strangely sinuous margin ; Bow atoll is 30 miles long, and on an 

 average only 6 in width; Menchicoff atoll consists of three atolls 

 united or tied together. This theory, moreover, is totally inappli- 

 cable to the northern Haldiva atolls in the Indian Ocean (one of 

 which is 88 miles in length, and between 10 and 20 in breadth", 

 for they are not bounded like ordinary atolls by narrow reefs, but 

 by a vast number of separate little atolls ; other little atolls rising 

 out of the great central lagoon-like spaces. A third and better 

 theory was advanced by Chamisso, who thought that from the 

 corals growing more vigorously where exposed to the open sea, as 

 undoubtedly is the case, the outer edges would grow up from the 

 general foundation before any other part, and that this would 

 account for the ring or cup-shaped structure. But we shall 

 immediately see, that in this, as well as in the crater-theory, a 

 most important consideration has been overlooked, namely, on 

 what have the reef-building corals, which cannot live at a great 

 depth, based their massive structures ? 



Numerous soundings were carefully taken by Captain Fitz Eoy 

 on the steep outside of Keeling atoll, and it was found that within 

 ten fathoms, the prepared tallow at the bottom of the lead, invari- 

 ably came up marked with the impressions of living corals, but as 

 perfectly clean as if it had been dropped on a carpet of turf; as 

 the depth increased, the impressions became less numerous, but the 

 adhering particles of sand more and more numerous, until at last 

 it was evident that the bottom consisted of a smooth sandy layer : 

 to carry on the analogy of the turf, the blades of grass grew 

 thinner and thinner, till at last the soil was so sterile, that nothing 

 sprang from it. From those observations, confirmed by many 



