But
the great cycles of the geologic periods are of a scope far too vast
for their changes to be perceptible to us except through their
influence upon the course of evolution.
_The Later Cycles of Geologic Time._ The Reptilian Era opens with a
period of extreme elevation, which rivalled that of the Glacial Epoch
and was similarly accompanied by extensive glaciation of which some
traces are preserved to our day in characteristic glacial boulders,
ice scratches, and till, imbedded or inter-stratified in the strata of
the Permian age. Between these two extremes of continental emergence,
the Permian and the Pleistocene, we can trace six cycles of alternate
submergence and elevation, as shown in the diagram (Fig. 5),
representing the proportion of North America which is known to have
been above water during the six geologic periods that intervene.
From this diagram it will appear that the six cycles or periods were
by no means equal in the amount of overflow or complete recovery of
the drowned lands. The Cretacic period was marked by a much more
extensive and long continued flooding; the great plains west of the
Mississippi were mostly under water from the Gulf of Mexico to the
Arctic Ocean. The earlier overflows were neither so extensive nor so
long continued. The great uplift of the close of the Cretacic regained
permanently the great central region and united East and West, and the
overflows of the Age of Mammals were mostly limited to the South
Atlantic and Gulf coasts.
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