CHAPTER V

THE GEOGRAPHICAL DEVELOPMENT OF THE AMERICAS IN CENOZOIC TIMES

I. TERTIARY PERIOD

In the interior regions of western North America the transition from the Mesozoic to the Cenozoic was so gradual that there is great difficulty in drawing the line between them and therefore, as might be expected, there is much difference of opinion as to just where that line should be drawn. From one point of view, the matter is of no great consequence; but from another, it is of the utmost importance, for, unless the events in different continents can be approximately synchronized, it will often prove a hopeless undertaking to trace the course of migration of the various mammalian groups and determine their place of origin and primary home. Until a definitive answer can be given to the question as to when the Cenozoic era began, many significant points must be left in doubt, and much remains to be done in the geology of the Far West before that definitive solution can be reached.

1. _Paleocene Epoch_

So far as North America is concerned, the best available evidence points to the conclusion that we should regard the Fort Union, Puerco and Torrejon as the most ancient of the Cenozoic formations (see Table, p. 17), though retaining so many features of Mesozoic life that a separate division of the Tertiary, the Paleocene epoch, is made for them. Such a separation is not the common practice in this country, where it is more usual to employ the terms “Lowest” or “Basal” Eocene. In my judgment, however, the balance of advantage is in favour of giving to this so-called Basal Eocene a rank equivalent to that of the four other universally recognized and admitted epochs of the Tertiary period. No marine rocks of Paleocene date have yet been found in North America, which indicates that the continent was at least as extensive as it is now. The very scanty development of deposits representing this epoch in Europe renders the comparison with the fossils of the Old World unsatisfactory and hence leads to uncertainty, when it is attempted to determine the land-connections of the time. During the Mesozoic era the shallow Bering Sea had repeatedly been elevated into a land joining North America with Asia and had as often been depressed, so as to separate the continents and allow the waters of the Arctic Ocean to mingle with those of the Pacific. A like alternation of junction and separation went on during the Tertiary and Quaternary periods and, by a comparison of the fossil mammals of Europe and America for any particular division of geological time, it is almost always feasible to say whether the two continents were connected, or altogether separated. This statement does not imply that the proportion of common elements in the two faunas during epochs of continental connection was a constant one at all times, for that was by no means true. Mere land-connections or separations are not the only factors which limit the spread of terrestrial animals; if they were, the community of forms between North and South America would be much greater than it actually is. Climatic barriers are of almost equal importance in determining animal distribution, and changes of climate may greatly alter the conditions of migration between connected continents. As the connections between North America and the Old World were probably in high latitudes, where the seas are narrow, changes of climate produced a greater effect upon migration than they could have done had the land-bridges been in the tropical or warm temperate zones. That these vicissitudes of climate really did occur and are not mere guesses to bolster up a tottering hypothesis, there is abundant evidence to prove.

In the Paleocene, or most ancient epoch of the Tertiary period, the geographical condition of North America was approximately as follows: The continent had attained nearly its modern outlines and on the Atlantic and Pacific coasts probably extended farther seaward than it does to-day. Florida, however, and perhaps a narrow strip of the northern Gulf coast were still submerged, the Gulf of Mexico opening broadly into the Atlantic. It is very probable that the continent was connected with the Old World by a land occupying the site of Bering Sea and perhaps also by way of Greenland and the North Atlantic; and there is some evidence, though not altogether convincing, that it was also joined to South America. The great mountain ranges were largely what they now are, though subsequent upheavals greatly modified the Rocky Mountains, Sierra Nevada and the ranges of the Pacific coast, while the lofty St. Elias Alps of Alaska were not in existence. The region of high plateaus, between the Rockies and the Sierras, was much less elevated than it is now. The Appalachians, which were of far more ancient date than the western ranges, had been worn down by ages of weathering and stream-erosion into a low-lying, almost featureless plain, with some scattered peaks rising from it here and there, of which the mountains of western North Carolina were the highest. In general, it may be said that while the _average_ height of the continent above the sea-level may have been as great or greater than at present, yet the inequalities of surface appear to have been less marked, and both along the Atlantic coast and in the interior were vast stretches of plains.

The Paleocene formations of the western interior are of non-marine or continental origin. In northwestern New Mexico is the typical area of the _Puerco_ and _Torrejon_, a series of beds 800 to 1000 feet in thickness and for the most part quite barren of fossils, but there are two horizons, one near the top and the other near the bottom of the series, which have yielded a very considerable number of fossil mammals, and of these the lower is the Puerco, the upper the Torrejon. The _Fort Union_ is quite different in character and is composed of great areas of sandstone and clay rocks, with a maximum thickness of 2000 feet, in eastern Wyoming, South Dakota, Montana and the adjoining parts of Canada. The modes of formation of these beds have not yet been fully determined; that they may have been partly laid down in shallow lakes is indicated by the masses of fresh-water shells in certain localities. In others are preserved multitudes of leaves, which have given a very full conception of the plants of the time, and great swamps and bogs have left the traces of their presence in beds of lignite, or imperfectly formed coal. Deposits made on the flood-plains of rivers and wind accumulations are probably also represented. “Vast stretches of subtropical and more hardy trees were interspersed with swamps where the vegetation was rank and accumulated rapidly enough to form great beds of lignite. Here were bogs in which bog iron was formed. Amid the glades of these forests there wandered swamp turtles, alligators, and large lizards of the characteristic genus _Champsosaurus_” (Osborn, p. 100).

Fort Union mammals are relatively rare and most of those that have been found are very fragmentary; they are amply sufficient, however, to demonstrate the Paleocene date of the beds and to make it probable that they include both the Puerco and the Torrejon faunas.

The climate, as shown by the plants, was much milder and more uniform than that of the Recent epoch, though some indication of climatic zones may already be noted. The vegetation was essentially modern in character; nearly all our modern types of forest-trees, such as willows, poplars, sycamores, oaks, elms, maples, walnuts and many others, were abundantly represented in the vast forests which would seem to have covered nearly the entire continent from ocean to ocean and extended north into Alaska and Greenland, where no such vegetation is possible under present conditions. Numerous conifers were mingled with the deciduous trees, but we do not find exclusively coniferous forests. Palms, though not extending into Greenland, flourished magnificently far to the north of their present range. On the other hand, the Paleocene flora of England points to a merely temperate climate, while that of the succeeding Eocene was subtropical.

_South America._—Nothing is definitely known concerning the condition of Central America and the West Indies and very little as to South America. As no marine rocks of Paleocene date have been found in any of these regions, it may be inferred that all the existing land areas were then above the sea, and there is some evidence that South America was much more extended in certain directions than now. From the character and distribution of modern plants, fresh-water fishes, land and fresh-water shells, there is strong reason to believe that in late Mesozoic times a land-bridge connected Brazil with equatorial Africa and this connection may have continued into the Paleocene, though it is only fair to observe that some highly competent authorities deny the reality of this bridge. There is also evidence, though incomplete, of a connection between South America and Australia by way of the Antarctic continent, and it is clear that that polar region could not have had the rigorous climate of the present time. In the upper part of the Cretaceous, the last of the Mesozoic periods, there was a possibility of migration, however indirect, between every continent and every other, for the huge land reptiles called Dinosaurs have been found in the non-marine Cretaceous rocks of every continent, which could not have been the case, had any of the great land areas been isolated. There is no known reason to assume that the land-bridges were essentially different in the Paleocene.

2. _Eocene Epoch_

_North America._—The Eocene witnessed quite extensive geographical changes, though but little is known of it in Central or South America, or the West Indies. Along the Atlantic and Gulf coasts of the United States there was an extensive submergence of the coastal plain, the sea covering the southern half of New Jersey and extending thence to the southwestward in an ever broadening band, through the South Atlantic and Gulf states. Northern Florida was under water and the Gulf extended as a narrow sound, known as the “Mississippi Embayment,” up the valley of that river to southern Illinois and westward into Texas. The Embayment was present in the Cretaceous and again in the Eocene, but it is not known whether it persisted through the Paleocene; probably it did not, as the whole Atlantic coast region appears to have stood at a higher level then than now. While the condition of Mexico and Central America during the Eocene is not known in any save the vaguest manner, it is evident that there was then a broad communication between the Atlantic and the Pacific, completely severing North and South America, though the place of this transverse sea has not been fixed. On the Pacific side, a long, narrow arm of the sea occupied what is now the great valley of California, extending north into Oregon and Washington. It will be noted that in North America the Eocene sea was almost confined to the neighbourhood of the present coast-lines, nowhere penetrating very far inland, except in the Mississippi Embayment, and thus differing widely from the condition of Europe at that epoch, where much of what is now land was submerged. The greatly expanded Mediterranean covered most of southern Europe, where the great mountain ranges, the Pyrenees, Alps, etc., had not yet been formed. Very important, from the point of view of American geography, is the fact that Europe was completely separated from Asia by a narrow strait or sea, which ran down the eastern side of the Ural Mountains from the Arctic Ocean and joined the enlarged Mediterranean. During the existence of this Ural Sea any land connection of North America with Europe must necessarily have been by means of a North Atlantic bridge, or by one across the Arctic Sea, since communication with Asia by way of Alaska would not have reached eastern Europe.

[Illustration: FIG. 48.—Map of North America during the Eocene epoch. The present limits of the continent are shown in outline; white areas = land; horizontal lines = sea; dotted areas = non-marine deposits; black circles with white dots = active volcanoes. (After Schuchert.)]

Any such general statement of geographical conditions during the Eocene as the foregoing sketch, cannot but be to some extent misleading, because it brings together, as contemporary, arrangements which were, in some cases at least, separated by considerable intervals of time and which were subject to continual change. Along nearly all coasts the position of the sea was quite different in the latter part of the epoch from what it had been in the earlier portion. On the north side of the Gulf of Mexico, for example, the sea retreated from time to time, and the successive divisions of the Eocene rocks are so arranged that the later ones are farther to the south. Limitations of space, however, forbid the attempt to follow out these minor changes.

[Illustration: FIG. 49.—Bad Lands of the lower Eocene. Wasatch stage. Big Horn Basin, Wyo. (Photograph by Sinclair.)]

In the western interior are found extensive non-marine or continental deposits of Eocene date, which must be considered more in detail, because of the highly important bearing which they have upon mammalian history. With the exception of a few small areas in Colorado, these deposits are all situated in the plateau region west of the Rocky Mountains, and were made of the débris of older rocks washed down by rain and rivers and deposited in broad basins. Some of them are the sediments of shallow or temporary lakes, and one series, at least, is made up of volcanic ash and dust showered upon the land, or into water of no great depth. The oldest of these Eocene stages, known as the _Wasatch_ (see Table, p. 17) covers a very large region, though in a discontinuous manner; the principal area begins in New Mexico, where it lies over the Torrejon, of the Paleocene, and extends far to the north through western Colorado and eastern Utah to the Uinta Mountains, around the eastern end of which it passes in a narrow band and then expands again over southwestern Wyoming. A second area is in the Big Horn Basin of northwestern Wyoming and southern Montana, and probably two small areas in southern Colorado are of the same date. The Wasatch beds are richly fossiliferous and have yielded a most interesting and important series of mammals, which were far more advanced than those of the Paleocene; and, at first sight, the student is tempted to believe that they must be of very much later date. A more critical examination shows that this appearance of a great lapse of time between the Paleocene and the Wasatch is deceptive; the more advanced and characteristic of the Wasatch mammals were obviously not the descendants of ancestors in the North American Paleocene, but were altogether newcomers to this continent, immigrants from some region which cannot yet be identified. On the other hand, a considerable number of the old, indigenous types still persisted, and these, when compared with their Paleocene ancestors, are found not to have changed so much as to require a very great length of time, geologically speaking, for the degree of development involved. This is the earliest recorded one of the great waves of mammalian migration which invaded North America down almost to our own time.

The same wave of migration extended to Europe, and that there was a broad and easy way of communication between that continent and North America is plain, for the similarity between the Wasatch mammals and those of the corresponding formation in France, the _Sparnacian_, is remarkably close. At no subsequent time were the mammalian faunas of North America and Europe so nearly identical as during the Wasatch-Sparnacian age, which is especially remarkable when the discrepancy is noted between the vast stretches of the Wasatch (150,000 square miles) and the very limited areas in France.

If, as is probable, the Ural Sea was in existence at that time, the land-connection with Europe must have been across the North Atlantic, most likely from Greenland eastward. At the present time a land-bridge in such high latitudes would be of little service in bringing about a similarity of mammals in the two continents, for the severity of the Arctic climate would be as effective a barrier against the intermigration of all save the Arctic mammals as the ocean itself; but in the mild and genial Eocene climate the latitude of the bridge was of small consequence.

The second of the Eocene stages, the _Wind River—Green River_, is found in two very different phases. The Wind River phase occupies the basin of that stream, north of the Wind River Mountains in central Wyoming, and in the Big Horn Basin of the same state it very extensively overlies the Wasatch, and in this phase the sediments are very like those of the latter, flood-plain and wind accumulations. A widely distant area of this stage occurs in the Huerfano Cañon in Colorado. The Wind River beds contain numerous mammals which were clearly sequential to those of the Wasatch, of which they were the more or less modified descendants. With two possible exceptions, there were no new immigrants and the connection with the Old World may have been already severed, as it assuredly was in the succeeding age, the Bridger, though divergent development had not yet had time to produce the very striking differences in the mammals of North America from those of Europe, which characterized the Bridger.

The Green River phase is a thick body of finely laminated “paper shales,” which seem to have been deposited in a very shallow lake and occupy some 5000 square miles of the Green River valley in southern Wyoming and northern Utah, where they overlie the Wasatch, just as do the Wind River beds in the Big Horn Basin. These fine-grained and thinly laminated shales have preserved, often in beautiful perfection, countless remains of plants, insects and fishes, but no traces of mammals, other than footprints, have been found.

The third of the Eocene stages of the interior is the _Bridger_ of southern Wyoming and northeastern Utah, where it lies upon the Green River shales, but overlaps these shales both eastward and westward, extending out upon the Wasatch. The Bridger beds are largely made up of volcanic ash and dust deposited partly upon the land and partly in shallow or temporary lakes. The frequency with which the remains of fishes, crocodiles and fresh-water shells are found indicates deposition in water, and the large crystals of gypsum which are abundant in certain localities show that the water became salt, at least occasionally. From the immense mass of volcanic débris, it is evident that volcanic activity broke out at this time on a much greater scale than had been known in that region since the Cretaceous period. Two different horizons, or substages, are distinguishable in the Bridger, lower and upper, each of which has its distinct mammalian fauna, though the two are very closely allied. Their difference from the contemporary mammals of Europe is very great, hardly any genera being common to the two continents. So striking a difference indubitably points to a severance of the land-connection, a severance which, as was shown above, probably took place during the Wind River stage, for its effects would not be immediately apparent; time would be required for the operation of divergent evolution, the fauna of each continent developing along its own lines, to make itself so strongly felt. Had the connection never been renewed, North America, on the one hand, and Eurasia on the other, would to-day be utterly different from the zoölogical point of view, instead of containing, as they do, a great many identical or closely similar animals of all classes, a likeness due to subsequent migrations.

The fourth and last of the stages referred to the Eocene is the _Uinta_, the geological position of which is the subject of much debate; almost as good reasons can be brought forward for placing it in the Oligocene as in the Eocene, so nearly is it on the boundary line between those two epochs. The Uinta is found in the Green River valley of northeastern Utah and northwestern Colorado, where it lies upon the upper Bridger and is the latest of the important Tertiary formations to be found in the plateau region west of the Rocky Mountains. It is probable that the separation of North America from the Old World still continued, for, as a whole, the Uinta fauna is totally different from that of the upper Eocene of Europe. There were, however, a few doubtful forms, which may prove to be the outposts of a renewed invasion.

The Eocene climate was decidedly warmer than the present one, and subtropical conditions extended over the whole United States and perhaps far into Canada. On the other hand, signs of increasing aridity in the western part of the continent are not wanting, and that must have resulted in a great shrinkage of the forests and increase of the open plains. The vegetation was essentially the same as in the Paleocene, when it had already attained a modern character, the differences from the present being chiefly in regard to geographical distribution. Large palms were then flourishing in Wyoming and Idaho, and another indication of a warm climate is furnished by the large crocodiles which abounded in all of the Eocene stages.

So far as North America was concerned, the Eocene epoch was brought to a close by extensive movements of the earth’s crust, which more or less affected the entire continent and were registered both on the sea-coasts and in the mountain ranges of the interior. Upheaval added a narrow belt of land along the Atlantic and Gulf coasts and the Mississippi Embayment was nearly closed. On the Pacific side the sea withdrew from the great valley of California and Oregon, and in the interior the plateau region was elevated by a great disturbance, which also increased the height of the western mountains.

Our knowledge of Eocene land-mammals in North America is almost wholly derived from the formations of the western United States, but it may be inferred from the uniform climatic conditions that there were no very great geographical differences among the animals. This inference is confirmed by the discovery of a Bridger genus, very fragmentary but identifiable, in the marine Eocene of New Jersey.

_South America._—No Eocene rocks, marine or continental, are known in the West Indies or Central America, but the latter region has been so imperfectly explored that no great importance can be attached to this fact. North and South America were separated completely, as is proved by the entire dissimilarity of their mammalian faunas, but the position of the transverse sea or strait cannot be determined. There is much reason to believe that the Greater Antilles were connected into a single large land, which has been called “Antillia” and may have been joined to the mainland of Central America. Certain marine rocks in Patagonia and Chili have been referred to the Eocene by South American geologists, but the reference is almost certainly erroneous, the rocks in question being much more probably Miocene. The Andes, probably throughout their length and certainly in their southern half, stood at a much lower level than they do now, and, no doubt, were rising, either slowly and steadily, or periodically and more rapidly, throughout the whole Tertiary period. At all events, their present height in the south is due to movements in the Pliocene or later. Continental deposits of Eocene date have been discovered only in northern Patagonia (Casa Mayor) where they occupy depressions in the worn and eroded surfaces of the Cretaceous rocks; the mode of their formation has not been carefully studied.

There is great uncertainty as to the status of the land-bridge which, it is believed, in the Cretaceous period connected South America with Africa. Some of the evidence goes to show that the connection persisted throughout the Eocene epoch, but the testimony is that of fragmentary and therefore imperfectly understood fossils and is far from being unequivocal. The connection with Antarctica probably continued.

3. _Oligocene Epoch_

_North America._—The Oligocene, or third of the Tertiary epochs, was a time of great significance in the history of the American mammals and of great geographical changes in the West Indian and Central American regions, but in North America proper the changes were not so widespread. On the Atlantic coast the marine Oligocene is but scantily displayed except in the Florida peninsula, where it is found in a thickness of some 2000 feet, but it is well developed along the north shore of the Gulf of Mexico, where the coast-line followed that of the Eocene, only a little farther to the south, marking the retreat of the sea at the end of the Eocene. The Gulf Stream entered the Atlantic over the site of northern Florida and flowed northward nearer the coast than it does to-day, in consequence of which warm-water conditions extended far to the north and West Indian shells flourished on the New Jersey coast. In the middle Oligocene part of northern Florida was elevated into an island and the water over much of the remainder of the peninsula became shallower, but this did not greatly alter the course of the Gulf Stream. The Pacific encroached upon the western shore of Oregon and British Columbia and very extensively upon that of Alaska, where strata no less than 10,000 feet thick are assigned to this epoch.

In the western interior Oligocene formations are among the most important and widely spread of the continental Tertiaries and are divisible into two principal stages and each of these again into three substages. Of these, the older or _White River_ stage covers a vast region in northeastern Colorado, western Nebraska, eastern Wyoming and southern South Dakota, with separate areas in the Black Hills, North Dakota and the Northwest Territory of Canada. The deposits are believed to be chiefly of fluviatile origin, and many of the ancient stream-channels, some of great size, may still be traced, filled with the consolidated sands and gravels of the old rivers. The country was very flat and the divides between the streams very low, so that in seasons of flood great regions were converted into shallow, temporary lakes, in which were deposited the finer silt and mud, but were dry for most of the year. The volcanic activity which had gone on so impressively in the Bridger Eocene was renewed in White River times, as is proved by thick beds of pure volcanic ash, which must have been carried long distances by the wind, for they occur far from any volcanic vent.

[Illustration: FIG. 50.—Map of North America in the upper Oligocene. Explanation as in Fig. 48. (After Schuchert.)]

The White River fauna is more completely known than that of any other Tertiary formation of this continent. The first discovery of these fossils was made more than 70 years ago and since then oft-repeated expeditions have brought to light an astonishing number and variety of mammals. Not only are these beds remarkable for the immense quantity of material which they have yielded, but also for its completeness and beauty of preservation, a most unusual number of skeletons having been obtained. The mammals demonstrate that the land-connection with the Old World had been re-established, for many European genera, which could not have been derived from an American ancestry, are found in the White River beds. At the same time, there was no such proportion of forms common to both continents as there had been in the Wasatch-Sparnacian stage of the lower Eocene, each having many genera and even families which did not extend their range into the other. The reason for this remarkable and, at first sight, inexplicable difference between the lower Eocene and the lower Oligocene is probably to be found in climatic changes, in consequence of which relatively fewer genera were able to take advantage of the reopened connection, which lay far to the north. The White River mammals, like those of the Recent epoch, are thus divisible into two groups or elements, one set indigenous and descended from ancestors which are found in the American Eocene, and the other composed of late immigrants from the Old World. Migrants from North America likewise made their way to Europe.

The upper continental Oligocene of the interior has received the peculiar appellation of the _John Day_, from the river of that name in eastern Oregon, a large part of which was buried to a depth of 3000 or 4000 feet in stratified volcanic ash and tuff. This great mass of finely divided volcanic material was derived from the craters of the Cascade Mountains to the westward; a long-continued series of eruptions would be needed to form such thick accumulations at such a distance from the sources of supply. The John Day evidently succeeded the White River very closely in time, but is marked by the disappearance of almost all the European migrants. This fact, together with the absence of any new immigrant genera, is evidence that the connection had again been broken and it was not renewed until after a considerable lapse of time.

There are many reasons for believing that the Oligocene climate marked the beginning of the very long and gradual process of refrigeration which culminated in the glacial conditions of the Pleistocene epoch, but the change was slight and probably chiefly affected the far north. The climate, however, remained notably warmer than the present one of the same extra-tropical latitudes, as is abundantly proved by the fossils. The Atlantic coast, as noted above, was bathed in warm waters, the plants of the Alaskan Oligocene point to temperate conditions and the vegetation of Europe was subtropical, palms growing in the north of Germany. The change which was distinctly to be noted in the Great Plains region of North America was probably due rather to the elevation and increased altitude of the western interior than to general climatic alteration. Crocodiles are very rare indeed in the White River beds and those that have been found all belong to dwarf species, while none are known from the John Day. Unfortunately, hardly anything has been ascertained concerning the Oligocene vegetation of the region, but the reptiles indicate diminished warmth.

_South America._—Marine Oligocene strata have great extent around the Gulf of Mexico and the Caribbean Sea, and the distribution of these shows that Antillia was broken up by great submergences, the islands of the Greater Antilles being much smaller than they are to-day. The greater part of Central America and the Isthmus were under water, a broad sea, broken only by scattered islands, separating North and South America. Very little is known of the Oligocene in the latter continent save a non-marine formation in northern Patagonia, the _Deseado_ stage (or Pyrotherium Beds), which, like the Eocene of the same region, occupies depressions in the worn and irregular surface of the Cretaceous rocks. The attribution of the Deseado to the Oligocene is open to some doubt, because of the entire absence in its mammalian fauna of any elements which are also found in the northern hemisphere. Hence, there are no means of direct comparison.

4. _Miocene Epoch_

_North America._—The Atlantic and Gulf coasts, which had been raised in the Oligocene, were again depressed, almost restoring the Eocene coast-line, the chief differences being the presence of the Florida islands and the nearly complete closing of the Mississippi Embayment. There was a remarkable change in the marine fauna from that of Oligocene times; a cool current flowed southward along the coast and entered the Gulf of Mexico through the strait between the Florida island and the mainland, bringing the northern animals with it and driving out the tropical forms. This complete faunal change, which might fairly be called a revolution, was the most sudden and striking in the Tertiary history of the continent.

On the Pacific coast also there was a depression, which caused a renewed transgression of the sea. The Coast Range formed a chain of reefs and islands in the Miocene sea, which again filled the great valley of California, except in the northern part of what is now the Sacramento Valley, where there was an accumulation of continental deposits. The immense thickness (5000 to 7000 feet) of the California Miocene is largely made up of volcanic material, which testifies to the great activity of the vents. In the Sierras, the height of which was increased in the upper Miocene, there was also a great display of vulcanism, recorded in the lava-flows and tuffs of the time. In the region of Lower California and northwestern Mexico considerable changes of the coast-line took place during the Miocene; in the earlier half of the epoch the Gulf of California was much shorter and narrower than it is to-day and the peninsula was broadly united with the mainland to the east as well as to the north. A wide submergence marked the upper Miocene, reducing the peninsula to a long, narrow island and enlarging the gulf considerably beyond its present limits, flooding an extensive area in northwestern Mexico and sending a small bay into southeastern California. There were great disturbances in the course of the epoch, for in the Santa Cruz Mountains near San Francisco the lower Miocene strata were crumpled into folds, before those of the upper Miocene were deposited upon them. British Columbia, Washington and Oregon were invaded by the sea, which extended up the valley of the Columbia River and its southern tributary, the Willamette, though here the beds are far thinner than those of California. Much of Alaska, both on the north and west coasts and in the valley of the Yukon, was submerged, and the land-connection with Asia appears to have been broken. This is made probable not only by the submergence of the Alaskan coast, but also by the fact that the marine animals of the California coasts and shoal waters, which could not migrate across the ocean, were quite unlike the contemporary forms of the eastern Asiatic shore, which would hardly have been the case, had a continuous coast-line united the two continents. On the other hand, there was a renewed connection with Europe, as is shown by the appearance of Old World land-mammals, beginning scantily in the lower and becoming numerous in the middle Miocene. This connection, it will be remembered, had been interrupted during the upper Oligocene. Many students of the problem have maintained that the land-bridge was by way of the West Indies and the Mediterranean lands, but such a bridge would not account for the facts of mammalian distribution, which would seem to require its location in the far north.

[Illustration: FIG. 51.—Map of North America in the upper Miocene. Explanation as in Fig. 48. (Modified from Schuchert.)]

Several distinct lines of evidence go to prove that the junction of the Americas dates from the Miocene, possibly from the beginning of it. The absence of Atlantic species from the Pacific Miocene is an indication that the passage from ocean to ocean had been closed, and this is confirmed by the geology of the Central American and Isthmian region. In the middle Miocene of Oregon and Nebraska have been found remains, which are unfortunately too incomplete for altogether convincing identification, but which can be interpreted only as belonging to the extinct and most characteristically South American group of edentates, the †ground-sloths or †Gravigrada; if this reference is correct, the fact of the junction cannot be questioned.

Continental deposits of Miocene date, chiefly accumulations made by rivers and the wind, cover vast areas of the western interior, though but rarely to any considerable depth. These have been divided into several stages and have received various names; the lower Miocene, known as the _Arikaree_, _Harrison_ or _Rosebud_, overlies the White River in South Dakota, western Nebraska and eastern Wyoming, with smaller areas in Montana and Colorado. In the deposits of this stage there are no mammals of indisputably Old World type, though a few which I consider to be such are a probable indication of renewed connection with Europe. The middle Miocene is found typically in central Montana, where it is called the _Deep River_ (or _Smith River_) stage, but occurs also in numerous small, scattered and widely separated areas in Oregon, Wyoming, Colorado and Texas, with local names in these different states. It is most likely that these middle Miocene formations are not strictly contemporaneous in the geological sense, but rather form a closely connected and successive series. The mammals of the Deep River stage leave no doubt that the way of migration from the Old World was again open.

The _Loup Fork_, or upper Miocene, itself susceptible of further subdivision, is by far the most extensive of the Miocene formations and covers much of the Great Plains region, in separate areas, from South Dakota far into Mexico. Perhaps also referable to the upper Miocene is a small, but very interesting formation, the _Florissant_, which is in the South Park of Colorado; it was made by very fine volcanic material showered into a small and shallow lake. The finely laminated papery shales of the Florissant have preserved countless plants and insects and many fishes, and these throw very welcome light upon the vegetation and climatic conditions of the epoch and afford an interesting contrast to the fauna and flora of the Green River shales of the lower or middle Eocene. That the Florissant shales are Miocene, no one questions, but their isolated position and the fact that they have yielded no mammals make it somewhat doubtful whether they belong in the middle or later part of the epoch.

In the western portion of the continent vulcanism was displayed on a grand scale during the Miocene. Mention has already been made of the quantity of volcanic material in the marine Miocene of California and also in the lavas and tuffs of the Sierras. The magnificent cones, such as Mts. Hood and Tacoma, which are the glory of the Cascades, are believed to date from this time. In Idaho and eastern Oregon and Washington are the immense lava-fields of the Columbia River, which are, partly at least, of Miocene date and were chiefly extruded through great fissures, the lava flooding the valleys and plains in a fiery sea of molten rock. In Oregon these lavas rest upon the upper Oligocene (John Day stage) and middle Miocene beds are deposited upon them, which fixes their date sufficiently. In the Yellowstone Park was piled up a huge mass of volcanic products, lava-flows and beds of ash and tuff, to a thickness of several thousand feet. The ash-beds have preserved the petrified forests, with their tree-trunks still standing one above another; one locality in the Park shows seven such forests, each one killed and buried by a great discharge of ash and then a new forest established and growing upon the surface of the accumulation. In the tuffs are leaf-impressions which permit identification of the plants.

In the latter part of the Miocene and at its close there were important crustal movements, which affected all the Pacific coast mountain ranges, though this epoch was no such time of mountain making in America as it was in the Old World. The principal elevation of the Coast Range in California and Oregon was due to these movements, and the Sierras and the plateaus of Utah and Arizona were increased in height. On the Atlantic side the Florida island was joined to the mainland and thus the present shape of the continent was almost exactly gained.

The Miocene climate of North America, as indicated by the plants of Florissant, the Yellowstone Park and Oregon, was distinctly milder than at present, a southern vegetation of warm-temperate character extending to Montana and perhaps much farther north, but it was not so warm as it had been in the Eocene, and palms are not found in any of the localities mentioned, nor do crocodiles occur in any of the northern Miocene formations. In Europe the climate of the early Miocene was considerably warmer than in North America, the vegetation of central and western Europe being very much like that of modern India. This difference between the two sides of the Atlantic was probably due, in large part, to the manner in which Europe was broken and intersected by arms and gulfs of the warm southern sea. In the latter half of the epoch, however, the climate became colder, the subtropical flora giving way to a distinctly temperate one.

_South America._—In Central America, where marine Oligocene beds are of great extent, no Miocene is known, and on the Isthmus Oligocene is the latest marine formation, except a narrow fringe of Pleistocene on the Caribbean coast. These facts and others already cited lead to the conclusion that in the Miocene the connection of the Americas was complete and that the Isthmus was considerably broader than at present, extending nearly to Jamaica. The condition of the Greater Antilles is but vaguely understood, but they were involved in the general elevation of the Caribbean region and were at least as large as they are now and may have been considerably larger, and Cuba was perhaps joined to Central America, as Hayti probably was.

In South America proper nearly the whole of Patagonia was submerged by the transgression of a shallow, epicontinental sea, in which were accumulated the beds of the _Patagonian_ stage, containing an exceedingly rich and varied assemblage of marine fossils, an assemblage which has very little in common with the contemporary formations of the northern hemisphere. It is this lack of elements common to the northern faunas which has led to the long debate concerning the geological date of the Patagonian formation, the South American geologists very generally referring it to the Eocene. However, the occurrence of genera of Cetaceans (whales and dolphins), which are also found in the Miocene of Maryland and Virginia, is very strong evidence that the proper date of the Patagonian is Miocene. A continuous coast-line, or at least an unbroken continuity of shoal-water conditions, seems necessary to account for the similarity of the Patagonian fossils with those of New Zealand and Australia, and that this connection was by way of the Antarctic continent is indicated by the occurrence of similar fossils in the South Shetland Islands, an Antarctic group. On the Chilian coast the _Navidad_ formation, which is believed to be approximately contemporaneous with the Patagonian, has so different a fauna as to point to some kind of a barrier between the Atlantic and the Pacific, and this barrier, Dr. von Ihering holds, was the land-extension from South America to Antarctica.

After some oscillations of retreat and advance, the sea withdrew from Patagonia and the terrestrial accumulations of the _Santa Cruz_ stage were formed. These beds are partly composed of river-deposits, but chiefly of more or less consolidated volcanic ash or tuff, and have yielded a surprising number of beautifully preserved mammals. No other assemblage of South American Tertiary Mammalia is so well known and understood as the Santa Cruz fauna, and the very large number of all but complete skeletons which have been found strongly suggests that many of the animals were buried alive in the showers of volcanic ash. The Santa Cruz fauna is completely and radically different from any of the North American assemblages, and at that time no immigrant from the north had penetrated so far as Patagonia.

In the upper Miocene the Andes stood at a much lower level than they do now; fossil plants, some of them collected at a great height in the mountains, are the remains of a luxuriant and purely tropical flora nearly identical with the vegetation of the modern forests of Bolivia and Brazil. Such a vegetation could not exist at the altitudes where the fossils occur and these demonstrate a great elevation of the mountains since those leaves were embedded. The same mild climatic conditions which prevailed in the northern hemisphere during the Miocene must also have characterized Patagonia, subtropical shells extending far to the south of their present range.

Whatever may have been true of the land-bridge connecting South America with Africa during the early Tertiary epochs, it must have been submerged in the Miocene, otherwise there would not have been the open pathway for the Cetacea of Patagonia to reach the Atlantic coast of North America and _vice versa_.

5. _Pliocene Epoch_

_North America._—The Pliocene of North America is not nearly so well displayed or so satisfactorily known as the preceding Tertiary epochs, and only of comparatively late years has it been recognized at all upon the Atlantic coast. The Atlantic and Gulf shores had very nearly their present outlines, but with some notable differences. It would seem that the northeastern portion of the continent stood at a higher level than it does now, north Greenland being joined with the islands of the Arctic archipelago and Newfoundland with Labrador, the Gulf of St. Lawrence then being land. From Nova Scotia to southern New Jersey the coast-line was many miles to the east and south of its present position, but the sea encroached here and there upon the shores of Virginia, the Carolinas and Georgia, and southern Florida was mostly under water, as was also a narrow strip of the Gulf coast from Florida to Texas and along the east of Mexico. On the Pacific side of the continent the marine Pliocene is far thicker and more important than on the east coast and in California is largely made up of volcanic materials. Quite extensive disturbances in this region had marked the close of the Miocene, the strata of which in the Coast Range had been violently compressed and folded. An elevation of the land had caused the sea to withdraw from the central valley of California and had restored Lower California to its peninsular conditions, reducing the gulf to the narrow limits which it had had in the lower Miocene and extending southern Mexico to the west and south. British Columbia and southeastern Alaska stood at higher than their present levels and the countless islands of that region were part of the mainland. Bering Strait was closed, for at least a great part of the epoch, and, as a continuous shore-line was thus formed and a way of migration opened, the marine fauna of California and Japan became closely similar.

[Illustration: FIG. 52.—Map of North America during the Pliocene epoch, Bering Strait open. Explanation as in Fig. 48. (Modified from Schuchert.)]

In the interior, the Pliocene continental formations and faunas followed so gradually upon those of the Miocene, that there is great doubt as to where the line between them should be drawn. These interior formations are mostly of small extent and are very widely scattered, and much remains to be learned regarding the mammals of the epoch. In northern Kansas are the _Republican River_ beds, which are so doubtfully Pliocene, that they may almost equally well be called uppermost Miocene. Other lower Pliocene stages, representing various divisions of time, are the _Alachua_ of northern Florida, the _Snake Creek_ of western Nebraska, the _Thousand Creek_ and _Virgin Valley_ of northwestern Nevada and the _Rattlesnake_ of Oregon. Probably middle Pliocene is the _Blanco_ of northwestern Texas, a valley cut in the middle and lower Miocene rocks and filled in with Pliocene deposits. Possibly upper Pliocene, or, it may be, lowest Pleistocene, are the _Peace Creek_ of southwestern Florida and the so-called “_Loup River_” (not Loup Fork) of western Nebraska.

The volcanic activity of the Rocky Mountain and Pacific coast regions, which was so remarkable in the Miocene, continued into and perhaps through the Pliocene. The great outflow of light-coloured lava which built up the central plateau of the Yellowstone Park is referred to the Pliocene, and some of the enormous fissure-eruptions which formed the vast Columbia River fields of black basaltic lava were probably Pliocene, as some were demonstrably Miocene. Both of these epochs were remarkable for volcanic activity in the western part of the continent.

The Pliocene climate, as may be inferred from the plants and marine shells, was colder than that of the Miocene, and refrigeration was progressive, as is shown by the proportion of Arctic shells in the Pliocene beds of the east coast of England, rising from 5 per cent in the oldest to more than 60 per cent in the latest beds. In the Arctic regions the cold must have been severe, at least during the latter half of the epoch, for in the succeeding Pleistocene we find an Arctic fauna already fully adapted to the extreme severity of present day polar conditions and time was necessary for such an adaptation. In the western interior the climate was not only colder, but also drier than it had been in the Miocene, the desiccation which had begun in the latter epoch becoming progressively more and more marked.

_South America._—The Greater Antilles were larger than at present and Cuba was much extended, especially to the southeastward, and was probably connected with the mainland, not as one would naturally expect, with Yucatan, but with Central America; this island, it is most likely, was cut off from Hayti. The Isthmian region was considerably broader than it is now and afforded a more convenient highway of intermigration. Costa Rica was invaded by a Pliocene gulf, but it is not yet clear whether this persisted for the whole or only a part of the epoch. In the Argentine province of Entrerios is a formation, the _Paraná_, which is most probably Pliocene, though it may be upper Miocene. This formation is largely marine and shows that the present Rio de la Plata was then a gulf from the Atlantic. A few northern hemisphere mammals in the Paraná beds show that the migration had advanced far into South America. A large part of Patagonia was again submerged beneath the sea, which extended to the Andes in places, but just how general the submergence was, it is impossible to say, for the _Cape Fairweather_ formation has been largely carried away by erosion and only fragments of it remain. Along the foothills of the Andes these beds are upturned and raised several thousand feet above the sea-level, a proof that the final upheaval of the southern mountains took place at some time later than the early Pliocene. Continental formations of Pliocene date are largely developed in Argentina; the _Araucanian_ stage is in two substages, one in the province of Catamarca, where the beds are much indurated and were involved in the Andean uplift, the other, of unconsolidated materials, is at Monte Hermoso near Bahia Blanca on the Atlantic coast. The very small proportion of northern animals in the Araucanian beds is surprising, but not more so than the almost complete absence of South American types in the upper Miocene and lower Pliocene of the United States. Intermigration between the two Americas would seem to have been a much slower and more difficult process than between North America and the Old World, and the reason for the difference is probably the greater climatic barriers involved in a migration along the lines of longitude. Upper Pliocene is found in the Tarija Valley of Bolivia and probably also in Ecuador, in both of which areas the proportion of northern animals was very greatly increased.

II. QUATERNARY PERIOD

The Quaternary period was a time of remarkable geographical and climatic changes, which had the profoundest and most far-reaching effects, partly by migration and partly by extinction, upon the distribution of animals and plants, effects which are naturally more obvious than those of earlier geological events, just because they were the latest. It is customary to divide the period into two epochs, (1) the _Pleistocene_ or _Glacial_, and (2) the _Recent_, which continues to the present day.

1. _Pleistocene Epoch_

When Louis Agassiz first suggested (1840) the idea of a time, comparatively recent in the geological sense, when northern and central Europe was buried under immense sheets of slowly moving ice, like the “ice-cap” of modern Greenland, the conception was received with incredulity. Nearly thirty years passed before this startling theory gained the general acceptance of geologists, but now it is one of the commonplaces of the science, for no other hypothesis so well explains the complicated phenomena of Pleistocene geology. One great obstacle to the acceptance of the glacial theory was the supposed fact that the Pleistocene glaciation was something quite unique in the history of the earth, a violent aberration in the development of climates. Now, however, we have every reason to believe that at least three other and very ancient periods had witnessed similar climatic changes and that “ice-ages” were recurrent phenomena. This is not the place to discuss or even to summarize the evidence which has convinced nearly all geologists of the reality of Pleistocene glacial conditions on a vast scale in Asia, Europe and, above all, in North America. The reader who may wish to examine this evidence will find an admirable presentation of it in Vol. III of the “Geology” of Professors Chamberlin and Salisbury.

_North America._—There has long been a difference of opinion among students of the Pleistocene as to whether the glaciation was single, or several times renewed. That there were many advances and retreats of the ice, is not denied; the question is, whether there were truly interglacial stages, when the ice altogether disappeared from the continent and the climate was greatly ameliorated. The present tendency among American and European geologists is decidedly in favour of accepting several distinct glacial stages (Chamberlin and Salisbury admit six of these) separated by interglacial stages, and for this there are very strong reasons. While it is out of the question to present the evidence for this conclusion here, one or two significant facts may be noted. On the north shore of Lake Ontario, near Toronto, are certain water-made deposits, which rest upon one sheet of glacial drift and are overlaid by another. The fossils of the aqueous sediments are in two series, upper and lower, of which the older and lower contains plants and insects indicative of a climate considerably warmer than that of the same region to-day and corresponding to the temperature of modern Virginia. In the upper and newer beds the fossils show the return of cold conditions, much like those of southern Labrador, and this was followed by the reëstablishment of the ice, as recorded in the upper sheet of drift. Even far to the north, on the Hudson’s Bay slope, an interglacial forest is embedded between two glacial drift-sheets. In Iowa and South Dakota numerous mammals of temperate character occur in interglacial beds.

At the time of their greatest extension, the glaciers covered North America down to latitude 40° N., though the great terminal moraine, which marks the ice-front and has been traced across the continent from Nantucket to British Columbia, describes a very sinuous line. The ice was not a homogeneous sheet, moving southward as a whole, but flowed in all directions away from several, probably four, centres of accumulation and dispersal. At the same time, the western mountain ranges had a far greater snow-supply than at present, and great glaciers flowed down all the valleys of the Rocky Mountains as far south as New Mexico and in the Sierras to southern California, while the Wasatch, Uinta and Cascade ranges and those of British Columbia and Alaska were heavily glaciated, but, strange to say, the lowlands of Alaska were free from ice. During the periods of greatest cold the rain-belt was displaced far to the south of its normal position, bringing a heavy precipitation to regions which are now extremely arid. In the Great Basin were formed two very large lakes; on the east side, rising high upon the flanks of the Wasatch Mountains, was Lake Bonneville, the shrunken and pygmy remnant of which is the Salt Lake of Utah, and on the west side, in Nevada, was Lake Lahontan. Lake Bonneville, which was nearly two-thirds the size of Lake Superior, discharged northward into the Snake River, a tributary of the Columbia, but Lahontan had no outlet. Each of these lakes had two periods of expansion, with a time of complete desiccation between them.

Over the Great Plains the principal Pleistocene formation is that known as the _Sheridan_, or, from the abundance of horse-remains which are entombed in it, the _Equus Beds_. These beds extend as a mantle of wind-drifted and compacted dust from South Dakota to Texas and in places contain multitudes of fossil bones; they correspond to one of the early interglacial stages and in South Dakota pass underneath a glacial moraine.

The upheaval which came at or near the end of the Pliocene had raised the continent, or at least its northeastern portion, to a height considerably greater than it has at present, and this must have facilitated the gathering of great masses of snow; but before the end of the Pleistocene a subsidence of the same region brought about important geographical changes. The depression, which lowered the coast at the mouth of the Hudson about 70 feet below its present level, increased northward to 600 feet or more in the St. Lawrence Valley and allowed the sea to invade that valley and enter Lake Ontario. From this gulf ran two long, narrow bays, one far up the valley of the Ottawa and the other into the basin of Lake Champlain. The raised beaches, containing marine shells and the bones of whales, seals and walruses, give eloquent testimony of those vanished seas. The recovery from this depression and the rise of the continent to its present level inaugurated the Recent epoch.

When the ice had finally disappeared, it left behind it great sheets of drift, which completely changed the surface of the country and revolutionized the systems of drainage by filling up the old valleys, only the largest streams being able to regain their former courses. Hundreds of buried valleys have been disclosed by the borings for oil and gas in the Middle West, and these, when mapped, show a system of drainage very different from that of modern times. Innumerable lakes, large and small, were formed in depressions and rock-basins and behind morainic dams, the contrast between the glaciated and non-glaciated regions in regard to the number of lakes in each being very striking.

On the west coast events were quite different; marine Pleistocene beds in two stages are found in southern California. The upheavals late in the Pleistocene, or at its close, were far greater than on the Atlantic side, 4000 feet in southeastern Alaska, 200 feet on the coast of Oregon and rising again to 3000 feet in southern California; all the western mountain ranges and plateaus were increased in height by these movements. The volcanoes continued to be very active, as may be seen from the lava-sheets and streams in Alaska, all the Pacific states, Arizona and New Mexico.

_South America._—No such vast ice-sheets were formed in the southern hemisphere as in the northern. Patagonia was the only part of South America to be extensively covered with ice and there traces of three glaciations have been observed, of which the first was the greatest and reached to the Atlantic coast, and there were great ice-masses on the coast of southern Chili. Mountain glaciers existed throughout the length of the Andes across the Equator to 11° N. lat., the elevation increasing northward to the tropics. The surface of the great Argentine plain of the Pampas between 30° and 40° S. lat. is covered with a vast mantle, largely of wind-accumulated dust, the _Pampean_, which is the sepulchre of an astonishing number of great and strange beasts. The Pampean formation corresponds in a general way to the Sheridan or Equus Beds of North America, but involves a much greater lapse of time, beginning earlier, possibly in the late Pliocene, and apparently lasting through the entire Pleistocene. While largely of æolian origin, the Pampean seems to be in part made of delta deposits laid down by rivers. One striking difference between the Pampean, on the one hand, and the Sheridan and the loess of the Mississippi Valley and of Europe, on the other, is that the former is in many places much more consolidated and stony, which gives it a false appearance of antiquity. Another and very rich source of Pleistocene mammals is found in the limestone caves of eastern Brazil, which have yielded an incredible quantity of such material, but not in such a remarkably perfect state of preservation as the skeletons of the Pampean.

Very little is known of the Pleistocene in the West Indies, though probably to this date should be assigned the notable oscillations of level which are recorded in the raised sea-terraces of Cuba and other islands. The Windward groups were joined, at least in part, to the continent and large extinct rodents reached Antigua, which would not be possible under present conditions. The Isthmus of Panama was 200 feet or more higher than it is now and correspondingly wider, but was depressed to a lower than the present level, and finally raised to the height it now has. Marine beds, of presumably Pleistocene date and certainly not older, extend from the Caribbean shore to Gatun, some seven miles, and are nowhere more than a few feet above sea-level.

The question of Pleistocene climates is a very vexed one and is far from having received a definitive answer. Limitations of space forbid a discussion of the problem here and I shall therefore merely state the conclusions which seem best supported by the evidence so far available. Such immense accumulations of ice might be due either to greatly increased snow-fall, or to a general lowering of the temperature. The balance of testimony is in favour of the latter factor and no great refrigeration is required. Professor Penck has calculated that a reduction of 6° or 7° in the average yearly temperature would restore glacial conditions in Europe. Even the tropics were affected by the change, as is shown not only by the glaciation of the Andes, but also by Mt. Kenya, which is almost on the Equator in eastern Africa and still has glaciers. The presumably Pleistocene ice covered the whole mountain like a cap, descending 5400 feet below the present glacier limit. It was pointed out above that the interglacial stages had greatly ameliorated climatic conditions and that, in some of them at least, the climate was warmer than it is to-day in the same localities. The cause of these astonishing fluctuations and of the climatic changes in general, to which Geology bears witness, still remains an altogether insoluble mystery.