Sect. II. 
ATOLLS. 
39 
is quite insignificant. Lastly, in several parts of the 
Pacific and Indian Oceans there are banks, lying at 
greater depths than in the cases just mentioned, of the 
same form and size with the neighbouring atolls, but 
with their atoll-like structure wholly obliterated. It 
appears from the survey of Freycinet, that there are 
banks of this kind in the Caroline Archipelago, and, as 
is reported, in the Low Archipelago. When we discuss 
the origin of the different classes of coral formations, 
we shall see that the submerged state of the whole of 
some atoll-formed reefs, and of portions of others 
generally but not invariably on the leeward side, and 
the existence of more deeply submerged banks now 
possessing little or no signs of their original atoll-like 
structure, are probably the effects of a uniform cause, — 
namely, the death of the coral, during the subsidence of 
the area, in which the atolls or banks are situated. 
There are seldom (with the exception of the Maldiva 
atolls), more than two or three channels, and generally 
only one leading into the lagoon, of sufficient depth for 
a ship to enter. In small atolls, there is usually not 
even one. Where there is deep water, for instance 
above 20 fathoms, in the middle of the lagoon, the 
channels through the reef are seldom as deep as the 
centre, — it may be said that the rim only of the saucer- 
shaped hollow forming the lagoon is notched. Sir C. 
Lyell 1 has observed that the growth of the coral would 
tend to obstruct all the channels through a reef, except 
those kept open by discharging the water, which during 
1 Principles of Geology, vol. iii. p. 289. [Vol. ii. p. 609, ed. 1872.] 
