1836.] DEAD OR SUNKEN REEFS. 349 



abruptly from the unfathomable ocean, with its central expanse studded, 

 and its edge symmetrically bordered with oval basins ot coral-rock just 

 lipping the surface of the sea, sometimes clothed with vegetation, and 

 each containing 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 inexplic- 

 able 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 follows 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 destruc- 

 tion could not have been adduced. Thus have we traced the history 

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

 normal changes, and through occasional accidents of their existence, 

 to their death and final obliteration. 



In my volume on "Coral Formations" I have published a map, in which 

 I have coloured all the atolls dark blue, the barrier-reefs pale blue, and 

 the fringing-reefs red. These latter reefs have been formed whilst the 

 land has been stationary, or, as appears from the frequent presence 

 of upraised organic remains, whilst it has been slowly rising: atolls 

 and barrier-reefs, on the other hand, have grown up during the directly 

 opposite movement ol subsidence, which movement must have been 

 very gradual, and in the case of atolls so vast in amount as to have 



