508 
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 
