Corals and coral islands.

190

CORALS AND CORAL ISLANDS.

This feature is well exhibited in the Mahlos Mahdoo atoll,
an enlarged map of which, from Darwin's work, is here in.
serted. The atoll consists of three main atoll-shaped portions;
but in each of these, the border is made up in part of atolls.
Many of the subordinate atolls of the border are "three, and
some even five miles in diameter, while those within the lagoon
are usually smaller, few being more than two miles across, and
the greater number less than one. The depth of the little
lagoons within these small annular reefs is generally from five
to seven fathoms, but occasionally more; and in Ari atoll,
many of the central ones are twelve, and some even more than
twelve fathoms deep. These subordinate atolls rise abruptly
from the platform or bank on which they stand, with their
outer margin bordered by living corals." "The small atolls
of the border, even where most perfect and standing farthest
apart, generally have their longest axis directed in the line
which the reef would have held if the atoll had been bounded
by an ordinary wall." (Darwin, on Coral Reefs, pp. 33, 34.)
The Maldives are among the largest atoll reefs known;
and they are intersected by many large open channels; and
Mr. Darwin observes, that the interior atolls occur only near
these channels, where the sea has free access. We may view
each large island in the archipelago as a sub-archipelago of
itself. Although thus singular in their features, they illustrate no new principles with regard to reef-formations.
Mr. Darwin thus remarks (Op. cit. pp. 33, 34),-" I can
in fact point out no essential difference between these little
ring-formed reefs (which, however, are larger, and contain
deeper lagoons than many true atolls that stand in the open
sea), and the most perfectly characterized atolls, excepting
that the ring-formed reefs are based on a shallow foundation
instead of on the floor of the open sea, and that instead of being