150 THEOET OP THE FOKMATION Ch. V. 



channels, and the scattered banks southward and east- 

 ward appear to be the last wreck of the less perfect 

 portions of one great and now ruined atoll. 



I have examined with care the charts of the Indian 

 and Pacific Oceans, and have now laid before the reader 

 all the cases which I have met with, of reefs differing 

 from the class to which they belong ; and I think it has 

 been shown, that they are all included in our theory, 

 modified by occasional accidents, such as might have 

 been anticipated. We have thus seen, that in the lapse 

 of ages encircling barrier-reefs are converted into atolls, 

 — the term atoll being applicable as soon as the last 

 pinnacle of encircled land sinks beneath the surface of 

 the sea. We have seen that large atolls, during the pro- 

 gressive subsidence of the areas in which they stand, 

 sometimes become dissevered into smaller ones. At other 

 times, when the reef-building polypifers perish, atolls 

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

 these again, through further subsidence and the accu- 

 mulation of sediment, pass into level banks with scarcely 

 any distinguishing character. Thus may the history of an 

 atoll be followed from its birth, through the occasional ac- 

 cidents of its existence, to its death and final obliteration. 



Objections to our theory of the formation of Atolls 

 and Barrier-reefs. — The vast amount of subsidence 

 both in area and depth, necessary to have submerged 

 every mountain, even the highest, throughout the 

 immense spaces of ocean now interspersed with atolls, 

 will probably strike most persons as a formidable objec- 

 tion to the theory. But as continents, as large as the 



