“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 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 the finer trituration before consolidation into rock.  This compact variety is the most common kind among the coral reef rocks of the present seas; and it often contains but few distinct fossils, although formed in water that abounded in life.  At the fall of the Ohio, near Louisville, there is a magnificent display of the old reef.  Hemispherical Favosites, five or six feet in diameter, lie there nearly as perfect as when they were covered by their flower-like polypes; and besides these, there are various branching corals, and a profusion of Cyathophiyllia, or cup-corals."[1]

[Footnote 1:  Dana, “Manual of Geology,” p. 272.]