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
or sediments 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.
No theory on the formation of coral-reefs can be considered
satisfactory which does not include the three great
[picture]
classes. We have seen that we are driven to believe in the subsidence
of those vast areas, interspersed with low islands, of which not one
rises above the height to which the wind and waves can throw up matter,
and yet are constructed by animals requiring a foundation, and that
foundation to lie at no great depth. Let us then take an island
surrounded by fringing-reefs, which offer no difficulty in their
structure; and let this island with its reefs, represented by the
unbroken lines in the woodcut, slowly subside. Now, as the island
sinks down, either a few feet at a time or quite insensibly, we may
safely infer, from what is known of the conditions favourable to the
growth of coral, that the living masses, bathed by the surf on the
margin of the reef, will soon regain the surface. The water, however,
will encroach little by little on the shore, the island becoming lower
and smaller, and the space between the inner edge of the reef and the
beach proportionately broader. A section of the reef and island in
this state, after a subsidence of several hundred feet, is given by the
dotted lines. Coral islets are supposed to have been formed on the
reef; and a ship is anchored in the lagoon-channel. This channel will
be more or less deep, according to the rate of subsidence, to the
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