496 
KEELING ISLAND 
CHAP. 
that 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. 
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 ; Menchicoff 
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 
