entrances into the Maldiva and Chagos atolls. This, probably, would check
the growth of the coral in them, far more effectually than a mere current
of water. In the many small atolls without any channel, these causes have
not prevented the entire ring attaining the surface. The channels, like
the submerged and effaced parts of the reef, very generally though not
invariably occur on the leeward side of the atoll, or on that side,
according to Beechey (Beechey's "Voyage," 4to edition, volume i., page
189.), which, from running in the same direction with the prevalent wind,
is not fully exposed to it. Passages between the islets on the reef,
through which boats can pass at high water, must not be confounded with
ship-channels, by which the annular reef itself is breached. The passages
between the islets occur, of course, on the windward as well as on the
leeward side; but they are more frequent and broader to leeward, owing to
the lesser dimensions of the islets on that side.

At Keeling atoll the shores of the lagoon shelve gradually, where the
bottom is of sediment, and irregularly or abruptly where there are
coral-reefs; but this is by no means the universal structure in other atolls.
Chamisso (Kotzebue's "First Voyage," volume iii., page 142.), speaking in
general terms of the lagoons in the Marshall atolls, says the lead
generally sinks "from a depth of two or three fathoms to twenty or
twenty-four, and you may pursue a line in which on one side of the boat you
may see the bottom, and on the other the azure-blue deep water." The shores
of the lagoon-like channel within the barrier-reef at Vanikoro have a similar
structure. Captain Beechey has described a modification of this structure
(and he believes it is not uncommon) in two atolls in the Low Archipelago,
in which the shores of the lagoon descend by a few, broad, slightly
inclined ledges or steps: thus at Matilda atoll (Beechey's "Voyage," 4to
edition, volume i, page 160. At Whitsunday Island the bottom of the lagoon
slopes gradually towards the centre, and then deepens suddenly, the edge of