STRUCTURE OF CORAL ISLANDS. 191 



scattered irregularly, they are grouped closely together' 1 — " 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. Al- 

 though their presence is thus contingent on the openness of 

 the marginal channels, the theory of their formation, as we 



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shall hereafter see, is included in that of the parent atolls, of 

 which they form the separate portions." 



