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.
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