A first book in organic evolution

by D. Kerfoot Shute

These statements may be illustrated and emphasized by a brief reference to the development of the Continent of North America. This Continent has grown from comparatively a small beginning to its present great proportions. In doing so it has passed through eras of stupendous duration. These eras in the order of their occurrence are as follows:

(1) Archæan era; (2) Palæozoic era (subdivided into Cambrian, Silurian, Devonian and Carboniferous periods); (3) Mesozoic era (subdivided into Triassic, Jurassic and Cretaceous periods); (4) Cenozoic era (subdivided into Tertiary and Quaternary periods); the Tertiary is subdivided into Eocene, Miocene and Pliocene epochs; the Quaternary is subdivided into Glacial, Champlain and Terrace epochs; (5) Psychozoic era, or recent epoch.

The physical geography of the continent at the close of that early geologic era known as the Archæan is shown on the map ([Fig. 11]). At this time the vast portion of the continent, whose outlines nevertheless existed, was submerged under a shallow sea, as indicated by the dark shading on the figure. The white V-shaped mass, starting just above the site of the great lakes and extending on the one hand in a northeasterly direction to Labrador, and on the other in a northwesterly direction to the Arctic Ocean, is the emerged land of this Archæan time. Smaller masses of Archæan land are also seen at the site of the Blue Ridge Mountains in the east and at that of the Rocky Mountains in the west. Around these lands as a nucleus the North American Continent has been built. Therefore, at the close of the Archæan or beginning of the next, or Palæozoic era, the whole interior portion of the continent was covered by a shallow sea which beat against the Canadian Archæan land on the north, the Blue Ridge Archæan land on the east, and the Rocky Mountain Archæan land on the west. This shallow sea is known as the Palæozoic Sea. Throughout the vast ages of the Palæozoic era, immense sediments were being deposited along the marginal sea bottoms. The deposition of these sediments was simultaneous with a further sinking of the submerged continent, so that the shallowness of the Palæozoic Sea was maintained; finally, the uplifting forces predominated, and the submerged land along the margins of the Canadian Archæan appeared as dry land, and thus increased the area of the infant continent. During all this period there was a steady and slow growth of the land southward from the Canadian Archæan, so that towards its close the visible continent had increased nearly, though not exactly, to the proportions attained in a still later (Cretaceous) period ([Fig. 12]).

Fig. 12.—Cretaceous North America. The white portion of the figure indicates emerged land—the growing continent.

From Shaler’s First Book in Geology. By courtesy of the publishers, D. C. Heath & Co.

At the close of the Palæozoic era the slow, steady changes that had been going on were replaced by more rapid and comparatively revolutionary changes, which caused great alterations in the physical geography and climate. Hitherto the continent had been comparatively low. Now the vast sedimentary accumulations constituting the marginal sea bottom of the eastern portion of the Palæozoic Sea, which had been accumulating through all Palæozoic time, were uplifted into the great Appalachian chain of mountains.

During the earlier ages (Silurian and Devonian) of this Palæozoic era, the place of the Appalachian chain of mountains was marginal sea bottom; but during the later ages (Carboniferous) it was, through repeated oscillations, in an uncertain state, being sometimes swamp land, sometimes covered with river sediment, and sometimes covered by the sea. It was during this Carboniferous age that the great coal measures were formed; at this time also the climate was probably very uniform, warm and moist, loaded with carbonic acid gas and deficient in oxygen. This period was undoubtedly a paradise for the great coal-forming plants, but was very unsuitable for the hot-blooded air-breathing animals, such as mammals and birds, none of which existed at that period. Throughout all geological time the excessive amount of moisture in the air has been gradually removed by the growth of continents in size and height; also the superabundant carbon dioxide in the atmosphere has been removed in many ways, especially by the plants in the coal period appropriating the carbon. Many ages later, at the close of the Jurassic period, the Sierra Nevada range of mountains was uplifted. Up to this time the site of these mountains was a marginal sea bottom receiving vast amounts of sediment, and the Pacific coast-line was east of the site of the Sierra range. Naturally vast changes in physical geography and climate occurred in consequence. During these and the following Cretaceous ages that the continent was growing, the great interior Palæozoic sea and what may be called the Gulf of Mexico were more and more restricted, as shown in the map of North America in the Cretaceous period of its growth ([Fig. 12]). This great inland sea, separating the continent into an eastern and western portion, is now called Cretaceous Sea instead of Palæozoic. This Cretaceous Sea covered the whole plains and plateau region of the continent, and extended from the Gulf of Mexico to the Arctic Ocean. At the end of the Cretaceous period of the continent this sea was obliterated by the gradual upheaval of this region and replaced by great lakes. At the same time the western marginal bottom of the sea was uplifted into the Wahsatch range of mountains; also at this time a line of islands in the Cretaceous Sea was uplifted into the Colorado mountains. All these events were entailing tremendous changes in physical geography and climate. [Fig. 13] is a representation of the map of North America in the early Tertiary period, the time succeeding the Cretaceous period. In this period, the continent continuing to uplift, the lakes that occupied the site of the Cretaceous Sea are obliterated; the Coast Range mountains of California and Oregon are uplifted from marginal sea bottom ([Fig. 13], dark shading); the Atlantic and Gulf borders are extended (dark shading), so that at the close of the Tertiary period the North American continent had attained its present form, except the southern portion of Florida and its keys. Since then the latter have grown and are still growing.

Fig. 13.—Early Tertiary North America.