220 CORAL AND ATOLLS 



one ; and, in an advanced case, is a bed of ever-thickening coral ; 

 for as the land continues to sink, the upward building of the 

 coral adds new layers to the accumulating coral bed. 



The Second Point that any theory attempting the expla- 

 nation of atoll formation must make clear is the regularity of 

 the tendency to a circular shape. So constant is this feature 

 that the explanation of it must be intimately bound up with 

 the proper history of the development of the structure. 

 Darwin's theory possessed this charm, that it carried with it, 

 in the picture of the disappearing peak with its outlying reef, 

 an idea of the development of a ring formation, which is very 

 easy to understand. 



The atoll shape comes about in the making of the structure 

 by subsidence, and is afterwards maintained by the corals 

 of the outer edge of the reef growing faster than those on the 

 inner edge. The corals of the central portion are those that 

 fare worst both as regards their supply of oxygen and of food, 

 and they also have to contend against the adverse influences 

 of sediment and impure water ; in this way the lagoon 

 becomes a stereotyped feature of the atoll. The vigorous 

 growing edge of the reef afterwards became the site of an 

 accumulation of wave-tossed debris, which gradually rose into 

 the dignity of island dry land ; and so a circular ring of 

 islands was formed around a central lagoon. The formation 

 of all the characteristic features of a typical atoll is, there- 

 fore, very naturally explained by Darwin's theory of the reef 

 foundation. 



As a side issue of this aspect of the question, a Third 

 Point arises, for it is obvious that the shape of the individual 

 islands demands some explanation. In Darwin's theory there 

 is no special provision for dealing with this problem. 



The Fourth Point is that any theory which deals with the 

 formation of atolls, and attempts to explain their formation, 

 must also take into account the other coral formations — the 

 fringing reef and the barrier reef. In respect of this detail 

 the theory of Subsidence was quite satisfactory, for the explana- 

 tion brought all these structures into an ordered sequence. 



