5o8 DEAD AND SUNKEN REEFS chap. 



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 symmetrically bordered with oval 

 basins of 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 archi- 

 pelagoes 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 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 remark- 

 able that in all these cases the dead reefs and portions of reef 



