from ten to twenty fathoms deeper than those in the northern part. This is
well exemplified in the case of Addoo, the southernmost atoll in the group,
for although only nine miles in its longest diameter, it has a depth of
thirty-nine fathoms, whereas all the other small atolls have comparatively
shallow lagoons; I can assign no adequate cause for this difference in
depth. In the central and deepest part of the lagoons, the bottom
consists, as I am informed by Captain Moresby, of stiff clay (probably a
calcareous mud); nearer the border it consists 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 and the line joining its
detached portions (where intersected by many channels), the bottom is
sandy, and it 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 singularity of these atolls,--a great sandy and generally
concave disc rises abruptly from the unfathomable ocean, with its central
expanse studded and its border 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 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 banks. 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 bottom upwards. When treating
of the growth of coral I shall again refer to this subject.