NORTH AMERICA IN THE AGE OF REPTILES.
ITS GEOGRAPHIC AND CLIMATIC CHANGES.
North America in the Age of Reptiles would have seemed almost as
strange to our eyes in its geography as in its animals and plants. The
present outlines of its coast, its mountains and valleys, its rivers
and lakes, have mostly arisen since that time. Even the more ancient
parts of the continent have been profoundly modified through the
incessant work of rain and rivers and of the waves, tending to wear
down the land surfaces, of volcanic outbursts building them up, and of
the more mysterious agencies which raise or depress vast stretches of
mountain chains or even the whole area of a continent, and which tend
on the whole so far as we can see, to restore or increase the relief
of the continents, as the action of the surface waters tends to bring
them down to or beneath the sea level.
_Alternate Overflow and Emergence of Continents._ In a broad way these
agencies of elevation and of erosion have caused in their age-long
struggle an alternation of periods of overflow and periods of
continental emergence during geologic time. During the periods of
overflow, great portions of the low-lying parts of the continents were
submerged, and formed extensive but comparatively shallow seas. The
mountains through long continued erosion were reduced to gentle and
uniform slopes of comparatively slight elevation. Their materials were
brought down by rivers to the sea-coast, and distributed as
sedimentary formations over the shallow interior seas or along the
margins of the continents. But this load of sediments, transferred
from the dry land to the ocean margins and shallow seas, disturbed the
balance of weight (isostasy) which normally keeps the continental
platforms above the level of the ocean basins (which as shown by
gravity measurement are underlain by materials of higher specific
gravity than the continents). In due course of time, when the strain
became sufficient, it was readjusted by earth movements of a slowness
proportioned to their vastness. These movements while tending upon the
whole to raise the continents to or sometimes beyond their former
relief, did not reverse the action of erosion agencies in detail, but
often produced new lines or areas of high elevation.
[Illustration: Fig. 2.--North America in the Later Cretacic
Period. Map outlines after Schuchert.]
_Geologic Periods._ A geologic period is the record of one of these
immense and long continued movements of alternate submergence and
elevation of the continents. It begins, 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
America (after Schuchert).]
These last, however, fragmentary and loose and overlying the rest,
were the first to be swept away by erosion during the periods of
elevation; and of such formations in the Age of Reptiles very little,
if anything, seems to have been preserved to our day. Consequently we
know very little about the upland animals of those times, if as seems
very probable, they were more or less different from the animals of
the coast-forests and swamps. The river-plain deposits of the Age of
Mammals on the other hand, are still quite extensive, especially those
of its later epochs, and afford a fairly complete record in some parts
of the continent of the upland fauna of those regions.
_Occurrence of Dinosaur Bones._ Dinosaur bones are found mostly in the
great delta formations, and since those were accumulated chiefly in
the early stages of great continental elevations, it follows that our
acquaintance with Dinosaurs is mostly limited to those living at
certain epochs during the Age of Reptiles. In point of fact so far as
explorations have yet gone in this country, the Dinosaur fauna of the
close of the Jurassic and beginning of the Comanchic and that of the
later Cretacic are the only ones we know much about. The immense
interval of time that preceded, and the no less vast stretch of time
that separated them, is represented in the record of Dinosaur history
by a multitude of tracks and a few imperfect skeletons assigned to the
close of the Triassic period, and by a few fragments from formations
which may be intermediate in age between the Jurassic-Comanchic and
the late Cretacic. Consequently we cannot expect to trace among the
Dinosaurs, the gradual evolution of different races, as we can do
among the quadrupeds of the Age of Mammals.
_Imperfection of the Geologic Record._ The Age of Mammals in North
America presents a moving picture of the successive stages in the
evolution of modern quadrupeds; the Age of Reptiles shows (broadly
considered) two photographs representing the land vertebrates of two
long distant periods, as remote in time from each other as the later
one is remote from the present day. Of the earlier stages in the
evolution of the Dinosaurs there are but a few imperfect sketches in
this country; in Europe the picture is more complete. In the course of
time, as exploration progresses, we shall no doubt recover more
complete records. But probably we shall never have so complete a
history of the terrestrial life of the Age of Reptiles as we have of
the Age of Mammals. The records are defective, a large part of them
destroyed or forever inaccessible.