as each generation takes a certain
time to grow to its full size, and can only separate its calcareous
skeleton from the water in which it lives at a certain rate, it is
clear that the reefs are records not only of changes in physical
geography, but of the lapse of time. It is by no means easy, however,
to estimate the exact value of reef-chronology, and the attempts which
have been made to determine the rate at which a reef grows vertically,
have yielded anything but precise results. A cautious writer, Mr.
Dana, whose extensive study of corals and coral reefs makes him an
eminently competent judge, states his conclusion in the following
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.

And all this lapse of time has occurred within the most recent period
of the history of the earth. The remains of reefs formed by coral
polypes of different kinds from those which exist now, enter largely
into the composition of the limestones of the Jurassic period; and
still more widely dif