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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