Now In This Map We See That The Reefs
Tinted Pale And Dark-Blue, Which Have Been Produced By The
Same Order Of Movement, As A General Rule Manifestly Stand
Near Each Other.
Again we see, that the areas with the two
blue tints are of wide extent; and that they lie
Separate from
extensive lines of coast coloured red, both of which
circumstances might naturally have been inferred, on the theory
of the nature of the reefs having been governed by the nature
of the earth's movement. It deserves notice that in more
than one instance where single red and blue circles approach
near each other, I can show that there have been oscillations
of level; for in such cases the red or fringed circles consist
of atolls, originally by our theory formed during subsidence,
but subsequently upheaved; and on the other hand, some of
the pale-blue or encircled islands are composed of coral-rock,
which must have been uplifted to its present height before that
subsidence took place, during which the existing barrier-reefs
grew upwards.
Authors have noticed with surprise, that although atolls
are the commonest coral-structures throughout some enormous
oceanic tracts, they are entirely absent in other seas,
as in the West Indies: we can now at once perceive the
cause, for where there has not been subsidence, atolls cannot
have been formed; and in the case of the West Indies and
parts of the East Indies, these tracts are known to have been
rising within the recent period.
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