B'B', The shores of
the now encircled islands. CC, Lagoon-channel. NB.--In this and
Plate 97, the subsidence of the land could be represented only by
an apparent rise in the level of the sea.)
No theory on the formation of coral-reefs can be considered
satisfactory which does not include the three great 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 reef, represented by the
unbroken lines in Plate 96, 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 proportionally broader.
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