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.]

Thus, in all the great periods of the earth's history of which we