478 MALDIVA ARCHIPELAGO. [cHap. Xx. 
it is impossible to look at a map of them without believing that 
they were once more intimately related. And in this same archi- 
pelago, Mahlos-Mahdoo atoll is divided by a bifurcating channel 
from 100 to 182 fathoms 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 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 vege- 
tation, and each containing a lake of clear water! 
One more point in detail: as in two neighbouring archipe- 
Jagoes 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 subsid- 
ing, we ought occasionally to find reefs both dead and sub- 
merged. 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 lee- 
ward side; and these, though still retaining their proper wall- 
like form, are now in several instances sunk several fathoms be- 
neath 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 
