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 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 flowerlike polypes; and besides these, there are
various branching corals, and a profusion of Cyathophyllia, or
cup-corals.
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