CORAL-REEFS. 145 



distance as the southern rim of the Great Chagos Bank is 

 from the northern rim, there are two other small banks with 

 from 10 to 20 fathoms on them; and not far eastward 

 soundings were struck on a sandy bottom, with between 

 no and 145 fathoms. The northern portion with its 

 ledge-like margin, closely resembles any one segment of 

 the Great Chagos Bank, between two of the deep-water 

 channels, and the scattered banks, southward appear to be 

 the last wrecks of less perfect portions. 



I have examined with care the charts of the Indian and 

 Pacific Oceans, and have now brought before the reader all 

 the examples, which I have met with, of reefs differing from 

 the type of the class to which they belong ; and I think it 

 has been satisfactorily shown, that they are all included in 

 our theory, modified by occasional accidents which might 

 have been anticipated as probable. In this course we have 

 seen, that in the lapse of ages encircling barrier-reefs are 

 occasionally converted into atolls, — the name of atoll being 

 properly applicable, at the moment when the last pinnacle 

 of encircled land sinks beneath the surface of the sea. We 

 have, also, seen that large atolls during the progressive 

 subsidence of the areas in which they stand, sometimes 

 become dissevered into smaller ones ; at other times, the 

 reef-building polypifers having entirely perished, atolls are 

 converted into atoll-formed banks of dead rock ; and these 

 again through further subsidence and the accumulation 

 of sediment modified by the force of the oceanic currents, 

 pass into level banks with scarcely any distinguishing 

 character. Thus may the history of an atoll be followed 

 from its first origin, through the occasional accidents of its 

 existence, to its destruction and final obliteration. 



Objections to the theory of the formation of Atolls and 

 Barrier-reefs. — The vast amount of subsidence, both 



87s 



