Ch. V. 
OF CORAL-REEFS. 
iso 
lOo 
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 
spaces supposed to have subsided, have been raised 
above the level of the sea, — as whole regions are now 
rising, for instance, in Scandinavia and South America, 
— and as no reason can be assigned why subsidence 
should not have occurred in some parts of the earth’s 
crust on as great a scale as elevation, this objection has 
little force. The remarkable point is, that a subsiding 
movement to such an extent and amount should have 
taken place within a period, during wdiich the corals 
have continued to add matter to the same reefs. An- 
other and less obvious objection to the theory may 
perhaps be advanced, namely, that, although atolls and 
