DINOSAURS
Chapter 2:
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.
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.
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.
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.
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. |
