This Theory, Moreover, Is
Totally Inapplicable To The Northern Maldiva Atolls In The
Indian Ocean (One Of Which Is 88
Miles in length, and between 10
and 20 in breadth), for they are not bounded like ordinary
atolls by narrow
Reefs, but by a vast number of separate
little atolls; other little atolls rising out of the great
central lagoon-like spaces. A third and better theory was
advanced by Chamisso, who thought that from the corals growing
more vigorously where exposed to the open sea, as undoubtedly is
the case, the outer edges would grow up from the general
foundation before any other part, and that this would account
for the ring or cup-shaped structure. But we shall
immediately see, that in this, as well as in the crater-theory,
a most important consideration has been overlooked, namely,
on what have the reef-building corals, which cannot live at
a great depth, based their massive structures?
Numerous soundings were carefully taken by Captain Fitz
Roy on the steep outside of Keeling atoll, and it was found
that within ten fathoms, the prepared tallow at the bottom
of the lead, invariably came up marked with the impression
of living corals, but as perfectly clean as if it had been
dropped on a carpet of turf; as the depth increased, the
impressions became less numerous, but the adhering particles
of sand more and more numerous, until at last it was evident
that the bottom consisted of a smooth sandy layer: to carry
on the analogy of the turf, the blades of grass grew thinner
and thinner, till at last the soil was so sterile, that nothing
sprang from it. From these observations, confirmed by many
others, it may be safely inferred that the utmost depth at
which corals can construct reefs is between 20 and 30 fathoms.
Now there are enormous areas in the Pacific and Indian
Ocean, in which every single island is of coral formation,
and is raised only to that height to which the waves can
throw up fragments, and the winds pile up sand. Thus
Radack group of atolls is an irregular square, 520 miles long
and 240 broad; the Low Archipelago is elliptic-formed, 840
miles in its longer, and 420 in its shorter axis: there are
other small groups and single low islands between these two
archipelagoes, making a linear space of ocean actually more
than 4000 miles in length, in which not one single island
rises above the specified height. Again, in the Indian Ocean
there is a space of ocean 1500 miles in length, including
three archipelagoes, in which every island is low and of
coral formation. From the fact of the reef-building corals
not living at great depths, it is absolutely certain that
throughout these vast areas, wherever there is now an atoll,
a foundation must have originally existed within a depth of
from 20 to 30 fathoms from the surface. It is improbable in
the highest degree that broad, lofty, isolated, steep-sided
banks of sediment, arranged in groups and lines hundreds of
leagues in length, could have been deposited in the central
and profoundest parts of the Pacific and Indian Oceans, at
an immense distance from any continent, and where the
water is perfectly limpid. It is equally improbable that the
elevatory forces should have uplifted throughout the above
vast areas, innumerable great rocky banks within 20 to 30
fathoms, or 120 to 180 feet, of the surface of the sea, and
not one single point above that level; for where on the whole
surface of the globe can we find a single chain of mountains,
even a few hundred miles in length, with their many summits
rising within a few feet of a given level, and not one
pinnacle above it? If then the foundations, whence the atoll-
building corals sprang, were not formed of sediment, and if
they were not lifted up to the required level, they must of
necessity have subsided into it; and this at once solves the
difficulty. For as mountain after mountain, and island after
island, slowly sank beneath the water, fresh bases would be
successively afforded for the growth of the corals. It is
impossible here to enter into all the necessary details, but I
venture to defy [12] any one to explain in any other manner
how it is possible that numerous islands should be distributed
throughout vast areas - all the islands being low - all being
built of corals, absolutely requiring a foundation within a
limited depth from the surface.
Before explaining how atoll-formed reefs acquire their
peculiar structure, we must turn to the second great class,
namely, Barrier-reefs. These either extend in straight lines
in front of the shores of a continent or of a large island, or
they encircle smaller islands; in both cases, being separated
from the land by a broad and rather deep channel of water,
analogous to the lagoon within an atoll. It is remarkable
how little attention has been paid to encircling barrier-reefs;
yet they are truly wonderful structures. The following sketch
represents part of the barrier encircling the island of Bolabola
in the Pacific, as seen from one of the central peaks.
In this instance the whole line of reef has been converted
into land; but usually a snow-white line of great breakers,
with only here and there a single low islet crowned with
cocoa-nut trees, divides the dark heaving waters of the ocean
from the light-green expanse of the lagoon-channel. And
the quiet waters of this channel generally bathe a fringe of
low alluvial soil, loaded with the most beautiful productions
of the tropics, and lying at the foot of the wild, abrupt,
central mountains.
Encircling barrier-reefs are of all sizes, from three miles
to no less than forty-four miles in diameter; and that which
fronts one side, and encircles both ends, of New Caledonia,
is 400 miles long. Each reef includes one, two, or several
rocky islands of various heights; and in one instance, even
as many as twelve separate islands.
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