Sect. II. 
ATOLLS. 
33 
powerful enemy (the S.W. swell) to oppose, is carried 
out much further, and with less abruptness, than the 
other.’ In some cases, the less inclination of a certain 
part of the external slope, for instance of the northern 
extremities of the two Keeling atolls, is caused by a 
prevailing current which there accumulates a bed of 
sand. Where the water is perfectly tranquil, as within 
a lagoon, the reefs generally grow up perpendicularly, 
and sometimes even overhang their bases : on the 
other hand, on the leeward side of Mauritius, where 
the water is generally tranquil although not invariably 
so, the reef is very gently inclined. Hence it appears 
that the exterior angle is much varied. We can, 
however, discern the effects of uniform laws in the 
close similarity in form between the sections of Keel- 
ing atoll and of the atolls in the Low Archipelago — 
in the general steepness of the reefs of the Maldiva 
and Chagos atolls— and in the perpendicularity of 
those rising out of water always tranquil ; but from 
the complex action of the surf and currents on the 
growing powers of the coral and on the deposition of 
sediment, we can by no means follow out all the 
results. 
Where islets have been formed on the reef, that 
part which I have called the ‘ flat,’ and which is partly 
dry at low water, appears similar in every atoll. In 
the Marshall group in the N. Pacific, it may be 
inferred from Chamisso’s description, that the reef, 
where islets have not been formed on it, slopes gently 
from the external margin to the shores of the lagoon ; 
