ndings at immense
depths. But in the centre of each atoll there is a lagoon from fifteen
to forty-nine fathoms deep. In the channels between the atolls no
soundings can usually be obtained at the depth of 150 or even 250
fathoms, but during Captain Moresby's survey, soundings were struck at
150 and 200 fathoms, the only instances as yet known of the bottom
having been reached, either in the Indian or Pacific oceans, in a space
intervening between two separate and well characterized atolls.

The singularity in the form of the atolls of this archipelago consists
in their being made up, not of one continuous circular reef but of a
ring of small coral islets sometimes more than a hundred in number, each
of which is a miniature atoll in itself; in other words, a ring-shaped
strip of coral surrounding a lagoon of salt water. To account for the
origin of these, Mr. Darwin supposes the larger annular reef to have
been broken up into a number of fragments, each of which acquired its
peculiar configurations under the influence of causes similar to those
to which the structure of the parent atoll has been due. Many of the
minor rings are no less than three, and even five miles in diameter, and
some are situated in the midst of the principal lagoon; but this happens
only in cases where the sea can enter freely through breaches in the
outer or marginal reef.

The rocks of the Maldives are composed of sandstone formed of broken
shells and corals, such as may be obtained in a loose state from the
beach, and which is seen when exposed for a few days to the air to
become hardened. The sandstone is sometimes observed to be an aggregate
of broken shells, corals, pieces of wood, and shells of the
cocoa-nut.[1122]

The Laccadive islands run in the same line with the Maldives, on the
north, as do the isles of the Chagos Archipelago, on the south; so that
these may be continuations of the same chain of submerged mountains,
crested in a similar manner by coral limestones.

_Origin of the circular form--not volcanic._--The circular and oval
shape of so many reefs, each having a lagoon in the centre, and being
surrounded on all sides by a deep ocean, naturally suggested the idea
that they were nothing more than the crests of submarine volcanic
craters overgrown by coral; and this theory I myself advocated in the
earlier editions of this work. Although I am now about to show that it
must be abandoned, it may still be instructive to point out