I Cannot Refrain From Once Again Remarking On The
Singularity Of These Complex Structures - A Great Sandy And
Generally Concave
Disk rises abruptly from the unfathomable
ocean, with its central expanse studded, and its edge
symmetrically bordered with oval basins
Of coral-rock just
lipping the surface of the sea, sometimes clothed with
vegetation, and each containing a lake of clear water!
One more point in detail: as in the two neighbouring
archipelagoes corals flourish in one and not in the other, and
as so many conditions before enumerated must affect their
existence, it would be an inexplicable fact if, during the
changes to which earth, air, and water are subjected, the
reef-building corals were to keep alive for perpetuity on any
one spot or area. And as by our theory the areas including
atolls and barrier-reefs are subsiding, we ought occasionally to
find reefs both dead and submerged. In all reefs, owing to the
sediment being washed out of the lagoon-channel to leeward,
that side is least favourable to the long-continued vigorous
growth of the corals; hence dead portions of reef not
unfrequently occur on the leeward side; and these, though still
retaining their proper wall-like form, are now in several
instances sunk several fathoms beneath the surface. The
Chagos group appears from some cause, possibly from the
subsidence having been too rapid, at present to be much less
favourably circumstanced for the growth of reefs than formerly:
one atoll has a portion of its marginal reef, nine miles
in length, dead and submerged; a second has only a few
quite small living points which rise to the surface, a third
and fourth are entirely dead and submerged; a fifth is a
mere wreck, with its structure almost obliterated. It is
remarkable that in all these cases, the dead reefs and portions
of reef lie at nearly the same depth, namely, from six to
eight fathoms beneath the surface, as if they had been carried
down by one uniform movement. One of these "half-drowned
atolls," so called by Capt. Moresby (to whom I
am indebted for much invaluable information), is of vast
size, namely, ninety nautical miles across in one direction,
and seventy miles in another line; and is in many respects
eminently curious. As by our theory it follows that new
atolls will generally be formed in each new area of subsidence,
two weighty objections might have been raised,
namely, that atolls must be increasing indefinitely in number;
and secondly, that in old areas of subsidence each separate
atoll must be increasing indefinitely in thickness, if proofs
of their occasional destruction could not have been adduced.
Thus have we traced the history of these great rings of
coral-rock, from their first origin through their normal
changes, and through the occasional accidents of their
existence, to their death and final obliteration.
In my volume on "Coral Formations" I have published a
map, in which I have coloured all the atolls dark-blue, the
barrier-reefs pale-blue, and the fringing reefs red. These
latter reefs have been formed whilst the land has been
stationary, or, as appears from the frequent presence of
upraised organic remains, whilst it has been slowly rising:
atolls and barrier-reefs, on the other hand, have grown up
during the directly opposite movement of subsidence, which
movement must have been very gradual, and in the case of atolls
so vast in amount as to have buried every mountain-summit over
wide ocean-spaces. 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.
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