1836.] MALDIVA ATOLLS. 459 



in depth, in such a manner, that it is scarcely possible to say 

 whether it ought strictly to be called three separate atolls, or one 

 great atoll not yet finally divided. 



I will not enter on many more details ; but I must remark that 

 the curious structure of the northern Maldiva atolls receives (taking 

 into consideration the free entrance of the sea through their broken 

 margins) a simple explanation in the upward and outward growth 

 of the corals, originally based both on small detached reefs in their 

 lagoons, such as occur in common atolls, and on broken portions of 

 the linear marginal reef, such as bounds every atoll of the ordinary 

 form. I cannot refrain from once again remarking on the singu- 

 larity of these complex structures a great sandy and generally 

 concave disk rises abruptly from the unfathomable ocean, with its 

 central expanse studded, and its edge symmetrically bordered with 

 oval basins of coral-rock just lipping the surface of the sea, some- 

 times clothed with vegetation, and each containing a lake of clear 

 water ! 



One more point in detail : as in two neighbouring archipelagoes 

 corals nourish in one and not in the other, and as so many con- 

 ditions 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-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 subsidence 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 



