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.
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