Dinosaurs | Page 4

therefore, and ends with a time of
emergence, and includes a long era of submergence.
These epochs of elevation are accompanied by the development of cold
climates at the poles, and elsewhere of arid conditions in the interior of
the continents. The epochs of submergence are accompanied by a warm,
humid climate, more or less uniform from the equator to the poles.
The earth has very recently, in a geologic sense, passed through an
epoch of extreme continental elevation the maximum of which was
marked by the "Ice Age." The continents are still emerged for the most
part almost to the borders of the "continental shelf" which forms their
maximum limit. And in the icy covering of Greenland and Antarctica a
considerable portion still remains of the great ice-sheets which at their
maximum covered large parts of North America and Europe. We are
now at the beginning of a long period of slow erosion and subsidence
which, if this interpretation of the geologic record be correct, will in the
course of time reduce the mountains to plains and submerge great parts
of the lowlands beneath the ocean. As compensation for the lesser
extent of dry land we may look forward to a more genial and favorable
climate in the reduced areas that remain above water.
[Illustration: Fig. 3.--Relative Length of Ages of Reptiles, Mammals
and Man.]

Length of Geologic Cycles. But these vast cycles of geographic and
climatic change will take millions of years to accomplish their course.
The brief span of human life, or even the few centuries of recorded
civilization are far too short to show any perceptible change in climate
due to this cause. The utmost stretch of a man's life will cover perhaps
one-two hundred thousandth part of a geologic period. The time
elapsed since the dawn of civilization is less than a three-thousandth
part. Of the days and hours of this geologic year, our historic records
cover but two or three minutes, our individual lives but a fraction of a
second. We must not expect to find records of its changing seasons in
human history, still less to observe them personally.
[Illustration: Fig. 4.--Relative Length of Prehistoric and Historic Time.]
There are indeed minor cycles of climate within this great cycle. The
great Ice Age through which the earth has so recently passed was
marked by alternations of severity and mildness of climate, of advance
and recession of the glaciers, and within these smaller cycles are minor
alternations whose effect upon the course of human history has been
shown recently by Professor Huntington ("The Pulse of Asia"). 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.
Sedimentary Formations. During the epochs of greatest overflow great
marine formations were deposited over large areas of what is now dry
land. These were followed as the land rose to sea level by extensive
marsh and delta formations, and these in turn by scattered and
fragmentary dry land deposits spread by rivers over their flood plains.
In the marine formations are found the fossil remains of the sea-animals
of the period; in the coast and delta formations are the remains of those
which inhabited the marshes and forests of the coast regions; while the
animals of the dryland, of plains and upland, left their remains in the
river-plain formations.
[Illustration: Fig. 5.--Geologic Cycles and the Land Area of North