48 
ATOLLS. 
Ch. I. 
half destroyed with the dead corals. In the central and 
deepest part of the Maldiva lagoons, the bottom consists, 
as I am informed by Captain Moresby, of stiff clay 
(probably a calcareous mud) ; nearer the border it con- 
sists of sand, and in the channels through the reef, of 
hard sand-banks, sandstone, conglomerate rubble, and a 
little live coral. Close outside the reef the bottom is 
sandy, and slopes abruptly into unfathomable depths. 
In most lagoons the depth is considerably greater 
in the centre than in the channels ; but in Tilla- 
dou-Matte, where the marginal ring-formed reefs 
stand far apart, the same depth is carried across the 
entire atoll, from the deep-water line on one side to 
that on the other. I cannot refrain from once again 
remarking on the singular structure of these atolls, 
— a great sandy and generally concave disk rises 
abruptly from the unfathomable ocean, with the central 
expanse studded and the margins symmetrically fringed 
with oval basins of coral-rock, just lipping the surface 
of the sea, sometimes clothed with vegetation, and each 
containing a little lake of clear salt water. 
In the southern Maldiva atolls, of which there are 
nine large ones, all the small reefs within the lagoons 
come to the surface, and are dry at low-water spring- 
tides ; hence in navigating them there is no danger 
from submarine hanks. This circumstance is very 
remarkable, as within some atolls, for instance those of 
the neighbouring Chagos group, not a single reef comes 
to the surface, and in most other cases a few only do, 
and the rest lie at all intermediate depths from the 
