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. The larger areas, coloured
red and blue, are all elongated; and between the two colours
there is a degree of rude alternation, as if the rising of one
had balanced the sinking of the other. Taking into consideration
the proofs of recent elevation both on the fringed
coasts and on some others (for instance, in South America)
where there are no reefs, we are led to conclude that the
great continents are for the most part rising areas: and from
the nature of the coral-reefs, that the central parts of the
great oceans are sinking areas. The East Indian archipelago,
the most broken land in the world, is in most parts an area
of elevation, but surrounded and penetrated, probably in
more lines than one, by narrow areas of subsidence.
I have marked with vermilion spots all the many known
active volcanos within the limits of this same map. Their
entire absence from every one of the great subsiding areas,
coloured either pale or dark blue, is most striking and not
less so is the coincidence of the chief volcanic chains with
the parts coloured red, which we are led to conclude have
either long remained stationary, or more generally have been
recently upraised. Although a few of the vermilion spots
occur within no great distance of single circles tinted blue,
yet not one single active volcano is situated within several
hundred miles of an archipelago, or even small group of
atolls. It is, therefore, a striking fact that in the Friendly
archipelago, which consists of a group of atolls upheaved
and since partially worn down, two volcanos, and perhaps
more, are historically known to have been in action.
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