sand is thick.

Captain Beechey has observed, that the submarine slope is much less at the
extremities of the more elongated atolls in the Low Archipelago, than at
their sides; in speaking of Ducie's Island he says (Beechey's "Voyage," 4to
edition, page 44.) the buttress, as it may be called, which "has the most
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 varies much; nevertheless in the close similarity in form
between the sections of Keeling 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, we may discern the effects of uniform laws; 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 sometimes
called the "flat" and which is partly dry at low water, appears similar in
every atoll. In the Marshall group in the North 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; Flinders states that the Australian barrier has a similar