146 THEORY OF THE FORMATION Ch. V} 



driven out from the lagoon over this side of the reef, 

 where the force of the breakers is less than to wind- 

 ward, and where the corals are, in consequence, less 

 vigorous and less able to resist any destroying agency. 

 It is owing to this same cause, that reefs are fre- 

 quently breached to leeward by channels which serve 

 as ship-channels. If the corals perished entirely, or 

 on the greater part of the circumference of an atoll, 

 the result would be 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 in the Chagos group 

 of atolls. Here 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 bank with merely two or three small pieces of 

 living reef which rise to the surface; and a fourth, 

 namely, Peros Banhos (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 



part would remain of the same height on both sides. I may here 

 observe that in most cases (for instance at Peros Banhos, the G-arnbier 

 group and the Great Chagos bank"), and I suspect in all cases, the dead 

 and submerged portions do not blend or slope into the living and perfect 

 parts, but are separated from them by an abrupt line. In some in- 

 stances small patches of living reef rise to the surface from the middle 

 of the submerged and dead parts. 



