atolls. The ring-formed reefs on the sides of this bifurcating channel are
elongated, so that the northern and southern portions of Mahlos Mahdoo may
claim, as far as their external outline is concerned, to be considered as
distinct and perfect atolls. But the intermediate portion, lying in the
fork of the channel, is bordered by reefs less perfect than those which
surround any other atoll in the group of equally small dimensions. Mahlos
Mahdoo, therefore, is in every respect in so intermediate a condition, that
it may be considered either as a single atoll nearly dissevered into three
portions, or as three atolls almost perfect and intimately connected. This
is an instance of a very early stage of the apparent disseverment of an
atoll, but a still earlier one in many respects is exhibited at Tilla-dou-
Matte. In one part of this atoll, the ring-formed reefs stand so far apart
from each other, that the inhabitants have given different names to the
northern and southern halves; nearly all the rings, moreover, are so
perfect and stand so separate, and the space from which they rise is so
level and unlike a true lagoon, that we can easily imagine the conversion
of this one great atoll, not into two or three portions, but into a whole
group of miniature atolls. A perfect series such as we have here traced,
impresses the mind with an idea of actual change; and it will hereafter be
seen, that the theory of subsidence, with the upward growth of the coral,
modified by accidents of probable occurrence, will account for the
occasional disseverment of large atolls.

The Great Chagos bank alone remains to be described. In the Chagos group
there are some ordinary atolls, some annular reefs rising to the surface
but without any islets on them, and some atoll-formed banks, either quite
submerged, or nearly so. Of the latter, the Great Chagos Bank is much the
largest, and differs in its structure from the others: a plan of it is
given in Plate II., Figure 1, in which, for the sake of clearness, I have
had the parts under ten fathoms deep finely shaded: an east and west