tensive 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."*

* 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.

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;[126] and still
more widely different coral polypes have contributed their quota to the
vast thickness of the carboniferous and Devonian strata. Then as regards
the latter group of rocks in America, the high authority already quoted
tells us:--

"The Upper Helderberg period is eminently the coral reef period of the
palaeozoic ages. Many of the rocks abound in coral, and are as truly
coral reefs as the modern reefs of the Pacific. The corals are sometimes
standing on the rocks in the position they had when growing: others are
lying in fragments, as they were broken and heaped by the waves; and
others were reduced to a compact limestone by