Sect. III. 
MALDIYA ATOLLS. 
47 
ring-like structure in these central reefs is con- 
tingent 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 mar- 
ginal 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 from 10 to 20 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 9 miles in its longest diameter, it has 
a depth of 39 fathoms, whereas all the other small atolls 
have comparatively shallow lagoons ; I can assign no 
adequate cause for this difference in depth, excepting 
that the southern part of the Archipelago has subsided 
to a greater degree or at a quicker rate than the 
northern part ; and this conclusion agrees well with the 
fact that, in the Chagos group, lying 280 miles still 
further southwards, most of the atolls are sunken and 
