470 DEAD OR SUBMERGED REEFS, [chap. xx. ' 



oval basins of coral-rock just lipping the surface of the 

 sea, sometimes clothed with vegetation, and each con- 

 taining a lake of clear water ! 



One more point in detail : as in two neighbouring 

 archipelagoes corals flourish in one and not in the other, 

 and as so many conditions before enumerated must affect 

 their existence, it would be an inexplicable fact if, during 

 the changes to which earth, air, and water are subjected, 

 the reef-building corals were to keep alive for perpetuity 

 on any one spot or area. And as by our theory the areas 

 including atolls and barrier-reefs are subsiding, we ought 

 occasionally to find reefs both dead and submerged. In 

 all reefs, owing to the sediment being washed out of the 

 lagoon or lagoon-channel to leeward, that side is least 

 favourable to the long-continued vigorous growth of the 

 corals ; hence, dead portions of reef not unfrequently occur 

 on the leeward side ; and these, though still retaining 

 their proper wall-like form, are now in several instances 

 sunk several fathoms beneath the surface. The Chagos 

 group appears from some cause, possibly from the sub- 

 sidence having been too rapid, at present to be much less 

 favourably circumstanced for the growth of reefs than 

 formerly : one atoll has a portion of its marginal reef, nine 

 miles in length, dead and submerged ; a second has only 

 a few quite small living points which rise to the surface ; 

 a third and fourth are entirely dead and submerged ; a 

 fifth is a mere wreck, with its structure almost obliterated. 

 It is remarkable that in all these cases, the dead reefs 

 and portions of reef lie at nearly the same depth, namely, 

 from six to eight fathoms beneath the surface, as if they 

 had been carried down by one uniform movement. One of 

 these "half-drowned atolls," so called by Captain Moresby 

 (to whom I am indebted for much invaluable information), 

 is of vast size, namely, ninety nautical miles across in one 

 direction, and seventy miles in another line ; and is in 

 many respects eminently curious. As by our theory it 

 follQWs that new atolls will generally be formed in each 

 new area of subsidence, two weighty objections might 

 have been raised, namely, that atolls must be increasing 

 indefinitely in number ; and secondly, that in old areas of 

 subsidence each separate atoll must be increasing indefinitely 

 in thickness, if proofs of their occasional destruction could 

 not have been adduced. Thus have we traced the history 

 of these great rings of coral-rock, from their first origin 



