504 CHARLES DARWIN 



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 

 singularity 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 sym- 

 metrically bordered with oval basins of coral-rock just 

 lipping the surface of the sea, sometimes clothed with vege- 

 tation, and each containing a lake of clear water ! 



One more point in detail : as in the two neighbouring archi- 

 pelagoes corals flourish in one and not in the other, and as 

 so many conditions before enumerated must affect their exist- 

 ence, 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 unfre- 

 quently 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 for- 

 merly: 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 



v mere wreck, with its structure almost obliterated. It is 



