STRUCTURE OF CORAL ISLANDS. 19] 
scattered irregularly, they are grouped closely together.” —‘ 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. A1- 
though their presence is thus contingent on the openness of 
the marginal channels, the theory of their formation, as we 

GREAT CHAGOS BANE, 
shall hereafter see, is included in that of the parent atolls, of 
which they form the separate portions.” 
