52 CORAL-REEFS. 



little lagoon within these small annular reefs is generally 

 from five to seven fathoms, but occasionally more ; and in 

 Ari atoll many of the central ones are twelve, and some 

 even more than twelve fathoms deep. These rings rise 

 abruptly from the platform or bank, on which they are 

 placed ; their outer margin is invariably bordered by living 

 coral 1 within which there is a flat surface of coral rock; on 

 this flat, sand and fragments have in many cases accumu- 

 lated and been converted into islets, clothed with vegetation. 

 I can, in fact, point out no essential difference between 

 these little ring-formed reefs (which, however, are larger, 

 and contain deeper lagoons than many true atolls that stand 

 in the open sea), and the most perfectly characterised atolls, 

 excepting that the ring-formed reefs are based on a shallow 

 foundation, instead of on the floor of the open sea, and that 

 instead of being scattered irregularly, they are grouped 

 closely together on one large platform, with the marginal 

 rings arranged in a rudely formed circle. 



The perfect series which can be traced from portions of 

 simple linear reef, to others including long linear lagoons, 

 and from these again to oval or almost circular rings, 

 renders it probable that the latter are merely modifications 

 of the linear or normal state. It is conformable with this 

 view, that the ring-formed reefs on the margin, even where 

 most perfect and standing furthest apart, generally have 

 their longest axes directed in the line which the reef would 

 have held, if the atoll had been bounded by an ordinary 

 wall. We may also infer that the central ring-formed 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 is 



1 Captain Moresby informs me that Millepora complanata is one of 

 the commonest kinds on the outer margin, as it is at Keeling atoll. 



