and the most perfectly characterised 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 scattered irregularly, they are grouped
closely together on one large platform, with the marginal rings arranged in
a rudely formed circle.

The perfect series which can be traced from portions of simple linear reef,
to others including long linear lagoons, and from these again to oval or
almost circular rings, renders it probable that the latter are merely
modifications of the linear or normal state. It is conformable with this
view, that the ring-formed reefs on the margin, even where most perfect and
standing furthest apart, generally have their longest axes directed in the
line which the reef would have held, if the atoll had been bounded by an
ordinary wall. We may also infer that the central ring-formed reefs are
modifications of those irregular ones, which are found in the lagoons of
all common atolls. It appears from the charts on a large scale, that the
ring-like structure is contingent on the marginal channels or breaches
being wide; and, consequently, on the whole interior of the atoll being
freely exposed to the waters of the open sea. When the channels are narrow
or few in number, although the lagoon be of great size and depth (as in
Suadiva), there are no ring-formed reefs; where the channels are somewhat
broader, the marginal portions of reef, and especially those close to the
larger channels, are ring-formed, but the central ones are not so; where
they are broadest, almost every reef throughout the atoll is more or less
perfectly ring-formed. Although their presence is thus contingent on the
openness of the marginal channels, the theory of their formation, as we
shall hereafter see, is included in that of the parent atolls, of which
they form the separate portions.

The lagoons of all the atolls in the southern part of the Archipelago are