138 CORAL-REEFS. 



where the force of the breakers is less than to windward ; 

 and therefore the corals would be less vigorous on this side, 

 and be less able to resist any destroying agent It is like- 

 wise owing to this same cause, that reefs are more frequently 

 breached to leeward by narrow channels, serving as by ship- 

 channels, than to windward. If the corals perished entirely, 

 or on the greater part of the circumference of an atoll, 

 an atoll-shaped bank of dead rock, more or less entirely 

 submerged, would be produced; and further subsidence, 

 together with the accumulation of sediment, would often 

 obliterate its atoll-like structure, and leave only a bank with 

 a level surface. 



In the Chagos group of atolls, within an area of 160 

 miles by 60, there are two atoll-formed banks of dead rock 

 (besides another very imperfect one), entirely submerged; 

 a third, with merely two or three very small pieces of living 

 reef rising to the surface ; and a fourth, namely, Peros 

 Banhos (Plate IV., Fig. 3), with a portion nine miles in 

 length dead and submerged. As by our theory this area 

 has subsided, and as there is nothing improbable in the 

 death, either from changes in the state of the surrounding 

 sea or from the subsidence being great or sudden, of the 

 corals on the whole, or on portions of some of the atolls, 

 the case of the Chagos group presents no difficulty. So 

 far indeed are any of the above-mentioned cases of sub- 

 merged reefs from being inexplicable, that their occurrence 

 might have been anticipated on our theory; and as fresh 

 atolls are supposed to be in progressive formation by the 

 subsidence of encircling barrier-reefs, a weighty objection, 

 namely that the number of atolls must be increasing 

 infinitely, might even have been raised, if proofs of the 

 occasional destruction and loss of atolls could not have 

 been adduced. 



