As the branches are open, this would not be
equivalent to more than half an inch in height of solid coral for the
whole surface covered by the madrepore; and, as they are also porous,
to not over three-eighths of an inch of solid limestone. But a coral
plantation has large bare patches without corals, and the coral sands
are widely distributed by currents, part of them to depths over one
hundred feet where there are no living corals; not more than one-sixth
of the surface of a reef region is, in fact, covered with growing
species. This reduces the three-eighths to ONE-SIXTEENTH. Shells and
other organic relics may contribute one-fourth as much as corals. At the
outside, the average upward increase of the whole reef-ground per year
would not exceed ONE-EIGHTH of an inch.
"Now some reefs are at least two thousand feet thick, which at
one-eighth of an inch a year, corresponds to one hundred and ninety-two
thousand years."*
* Dana, Manual of Geology, p. 591.
Halve, or quarter, this estimate if you will, in order to be certain of
erring upon the right side, and still there remains a prodigious
period during which the ancestors of existing coral polypes have
been undisturbedly at work; and during which, therefore, the climatal
conditions over the coral area must have been much what they are now.
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