In fact the reefs extend only to that distance
from the shore at which a foundation within the requisite depth
from 20 to 30 fathoms is found. As far as the actual reef is
concerned, there is no essential difference between it and that
forming a barrier or an atoll: it is, however, generally of less
width, and consequently few islets have been formed on it. From the
corals growing more vigorously on the outside, and from the noxious
effect of the sediment washed inwards, the outer edge of the reef
is the highest part, and between it and the land there is generally
a shallow sandy channel a few feet in depth. Where banks of
sediment have accumulated near to the surface, as in parts of the
West Indies, they sometimes become fringed with corals, and hence
in some degree resemble lagoon-islands or atolls, in the same
manner as fringing-reefs, surrounding gently sloping islands, in
some degree resemble barrier-reefs.
(PLATE 96. SECTION OF CORAL-REEF.
AA, Outer edges of the fringing-reef, at the level of the sea. BB,
The shores of the fringed island. A'A', Outer edges of the reef,
after its upward growth during a period of subsidence, now
converted into a barrier, with islets on it.
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