terms:--

"The rate of growth of the common branching madrepore is not
over one and a half inches a year. 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."[1]

[Footnote 1: 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 the 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.