46 ATOLLS. Ch. I, 



reef like that surrounding an ordinary atoll, to others 

 which are ring-formed and much elongated but con- 

 taining only a very narrow lagoon, to others which are 

 oval or almost circular, renders it probable that the 

 latter are merely modifications of a linear and normal 

 reef. The fact that the marginal annular reefs 

 generally have their longest axes directed in the line 

 which the exterior linear reef would have held, agrees 

 with this view. We may also infer that the central 

 annular 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 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. 



