496 KEELING ISLAND chap. 



that those massive kinds, to whose growth on the exposed 

 outer shores the very existence of the reef depends, cannot Hve 

 within the lagoon, where other delicately branching kinds flourish. 

 Moreover, 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; Rimsky 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 ; Menchlcofif 

 atoll consists of three atolls united or tied together. This 

 theory, moreover, is totally inapplicable to the northern Maldiva 

 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 

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

 that within ten fathoms the prepared tallow at the bottom of 

 the lead Invariably 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 



