Cn. V. 
OF CORAL-REEFS. 
149 
as sliip-channels. If the corals perished entirely, or 
on the greater part of the circumference of an atoll, 
the result would he an atoll-shaped bank of dead 
rock more or less entirely submerged ; and further 
subsidence, together with the accumulation of sedi- 
ment, would obliterate its atoll-like structure, and 
leave only a bank with a nearly level surface. 
We meet with all these cases hi the Cliagos group 
of atolls. Here within an area of ICO miles by GO, 
there are two atoll-formed hanks of dead rock (besides 
another very imperfect one) entirely submerged ; a 
third hank with merely two or three small pieces of 
living reef which rise to the surface ; and a fourth, 
namely, Peros Banlios (Plate I. fig. 9), with a por- 
tion nine miles in length dead and submerged. As 
by our theory this area has subsided, and as there is 
nothing improbable in the death of the corals on por- 
tions or over the whole surface of a reef, either from 
changes in the state of the surrounding sea or from the 
subsidence being great or sudden, these Cliagos banks 
present no difficulty. So far, indeed, are any of the above- 
mentioned cases of dead submerged reefs from offering 
any difficulty, that their occurrence might have been 
anticipated on our theory ; and as fresh atolls are sup- 
posed to be in progressive formation by the sub- 
sidence of encircling barrier-reefs, a weighty ob- 
jection might even have been raised, namely that 
atolls must increase indefinitely in number, unless 
proofs of their occasional destruction could have been 
adduced. 
