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.
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