DEAD AND SUNKEN REEFS. 277 



washed out of the lagoon or lagoon-channel to lee- 

 ward, that side is least favourable to the long-con- 

 tinued vigorous growth of the corals ; hence dead 

 portions of reef not unfrequently occur on the lee- 

 ward 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 subsidence having been too rapid, at pres- 

 ent 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 be- 

 neath the surface, as if they had been carried 

 down by one uniform movement. One of these 

 " half-drowned atolls," so called by Captain Mores- 

 by (to whom I am indebted for much invaluable 

 information), is of vast size, namely, ninety nauti- 

 cal 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 mu«t 

 be increasing indefinitely in number ; and, second- 

 ly, 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 ti-aced the his- 

 toiy of these gi-eat rings of coral rock, from their 

 first origin through their normal changes, and 

 II.— A A 



