[AG] The former name is related to Palæozoic and Mesozoic, the latter to the older terms Primary and Secondary. For the sake of euphony we shall use both. The term Neozoic was proposed by Edward Forbes for the Mesozoic and Cainozoic combined; but I use it here as a more euphonious and accurate term for the Cainozoic alone.
It is further to be observed that the new things introduced in the later Mesozoic came in little by little in the progress of the period, and anticipated the great physical changes occurring at its close. On the other hand, while many family and even generic types pass over from the Mesozoic to the earlier Tertiary, very few species do so. It would seem, therefore, as if changes of species were more strictly subordinate to physical revolutions than were changes of genera and orders—these last overriding under different specific forms many minor vicissitudes, and only in part being overwhelmed in the grander revolutions of the earth.
Both in Europe and America there is evidence of great changes of level at the beginning of the Tertiary. In the west of Europe beds often of shallow-water or even fresh-water origin fill the hollows in the bent Cretaceous strata. This is manifestly the case with the formations of the London and Paris basins, contemporaneous but detached deposits of the Tertiary age, lying in depressions of the chalk. Still this does not imply much want of conformity, and according to the best explorers of those Alpine regions in which both the Mesozoic and Tertiary beds have been thrown up to great elevations, they are in the main conformable to one another. Something of the same kind occurs in America. On the Atlantic coast the marine beds of the Older Tertiary cover the Cretaceous, and little elevation seems to have occurred Farther west the elevation increases, and in the upper part of the valley of the Mississippi it amounts to 1700 feet. Still farther west, in the region of the Rocky Mountains, there is evidence of elevation to the extent of as much as 7000 feet. Throughout all these regions scarcely any disturbance of the old Cretaceous sea-bottom seems to have occurred until after the deposition of the older Tertiary, so that there was first a slow and general elevation of the Cretaceous ocean bottom, succeeded by gigantic folds and fractures, and extensive extravasations of the bowels of the earth in molten rocks, in the course of the succeeding Tertiary age. These great physical changes inaugurated the new and higher life of the Tertiary, just as the similar changes in the Permian did that of the Mesozoic.
The beginning of these movements consisted of a great and gradual elevation of the northern parts of both the Old and New Continents out of the sea, whereby a much greater land surface was produced, and such changes of depth and direction of currents in the ocean as must have very much modified the conditions of marine life. The effect of all these changes in the aggregate was to cause a more varied and variable climate, and to convert vast areas previously tenanted by marine animals into the abodes of animals and plants of the land, and of estuaries, lakes, and shallow waters. Still, however, very large areas now continental were under the sea. As the Tertiary period advanced, these latter areas were elevated, and in many cases were folded up into high mountains. This produced further changes of climate and habitat of animals, and finally brought our continents into all the variety of surface which they now present, and which fits them so well for the habitation of the higher animals and of man.
The thoughtful reader will observe that it follows from the above statements that the partial distribution and diversity in different localities which apply to the deposits of such ages as the Permian and the Trias apply also to the earlier Tertiary; and as the continents, notwithstanding some dips under water, have retained their present forms since the beginning of the Tertiary, it follows that these beds are more definitely related to existing geographical conditions than are those of the older periods, and that the more extensive marine deposits of the Tertiary are, to a great extent, unknown to us. This has naturally led to some difficulty in the classification of Neozoic deposits—those of some of the Tertiary ages being very patchy and irregular, while others spread very widely. In consequence of this, Sir Charles Lyell, to whom we owe very much of our definite knowledge of this period, has proposed a subdivision based on the percentage of recent and fossil animals. In other words, he takes it for granted that a deposit which contains more numerous species of animals still living than another, may be judged on that account to be more recent. Such a mode of estimation is, no doubt, to some extent arbitrary; but in the main, when it can be tested by the superposition of deposits, it has proved itself reliable. Further, it brings before us this remarkable fact, that while in the older periods all the animals whose remains we find are extinct as species, so soon as we enter on the Neozoic we find some which still continue to our time—at first only a very few, but in later and later beds in gradually increasing percentage, till the fossil and extinct wholly disappear in the recent and living.
The Lyellian classification of the Tertiary will therefore stand as in the following table, bearing in mind that the percentage of fossils is taken from marine forms, and mainly from mollusks, and that the system has in some cases been modified by stratigraphical evidence:—
| Tertiary, or Neozoic Time. | Post-pliocene, including that which immediately precedes the Modern. In this the shells, etc., are recent, the Mammalia in part extinct. Pliocene, or more recent age. In this the majority of shells found are recent in the upper beds. In the lower beds the extinct become predominant. Miocene, or less recent. In this the large majority of shells found are extinct. Eocene, the dawn of the recent. In this only a few recent shells occur. |
If we attempt to divide the Tertiary time into ages corresponding to those of the older times, we are met by the difficulty that as the continents have retained their present forms and characters to a great extent throughout this time, we fail to find those evidences of long-continued submergences of the whole continental plateaus, or very large portions of them, which we have found so very valuable in the Palæozoic and Mesozoic. In the Eocene, however, we shall discover one very instructive case in the great Nummulitic Limestone. In the Miocene and Pliocene the oscillations seem to have been slight and partial. In the Post-pliocene we have the great subsidence of the glacial drift; but that seems to have been a comparatively rapid dip, though of long duration when measured by human history; not allowing time for the formation of great limestones, but only of fossiliferous sands and clays, which require comparatively short time for their deposition If then we ask as to the duration of the Neozoic, I answer that we have not a definite measure of its ages, if it had any; and that it is possible that the Neozoic may have as yet had but one age, which closed with the great drift period, and that we are now only in the beginning of its second age. Some geologists, impressed with this comparative shortness of the Tertiary, connect it with Mesozoic, grouping both together. This, however, is obviously unnatural. The Mesozoic time certainly terminated with the Cretaceous, and what follows belongs to a distinct aeon.
But we must now try to paint the character of this new and peculiar time; and this may perhaps be best done in the following sketches: 1. The seas of the Eocene. 2. Mammals from the Eocene to the Modern. 3. Tertiary floras. 4. The Glacial period. 5. The Advent of Man.
The great elevation of the continents which closed the Cretaceous was followed by a partial and unequal subsidence, affecting principally the more southern parts of the land of the northern hemisphere. Thus, a wide sea area stretched across all the south of Europe and Asia, and separated the northern part of North America from what of land existed in the southern hemisphere. This is the age of the great Nummulitic Limestones of Europe, Africa, and Asia, and the Orbitoidal Limestones of North America. The names are derived from the prevalence of certain forms of those humble shell-bearing protozoa which we first met with in the Laurentian, and which we have found to be instrumental in building up the chalk, the Foraminifera of zoologists. (Fig. p. 243.) But in the Eocene the species of the chalk were replaced by certain broad flat forms, the appearance of which is expressed by the term nummulite, or money-stone; the rock appearing to be made up of fossils, somewhat resembling shillings, sixpences, or three-penny pieces, according to the size of the shells, each of which includes a vast number of small concentric chambers, which during life were filled with the soft jelly of the animal. The nummulite limestone was undoubtedly oceanic, and the other shells contained in it are marine species. After what we have already seen we do not need this limestone to convince us of the continent-building powers of the oceanic protozoa; but the distribution of these limestones, and the elevation which they attain, furnish the most striking proofs that we can imagine of the changes which the earth’s crust has undergone in times geologically modern, and also of the extreme newness of man and his works. Large portions of those countries which constitute the earliest seats of man in Southern Europe, Northern Africa, and Western and Southern Asia, are built upon the old nummulitic sea-bottom. The Egyptians and many other ancient nations quarried it for their oldest buildings. In some of these regions it attains a thickness of several thousand feet, evidencing a lapse of time in its accumulation equal to that implied in the chalk itself. In the Swiss Alps it reaches a height above the sea of 10,000 feet, and it enters largely into the structure of the Carpathians and Pyrenees. In Thibet it has been observed at an elevation of 16,500 feet above the sea. Thus we learn that at a time no more geologically remote than the Eocene Tertiary, lands now of this great elevation were in the bottom of the deep sea; and this not merely for a little time, but during a time sufficient for the slow accumulation of hundreds of feet of rock, made up of the shells of successive generations of animals. If geology presented to us no other revelation than this one fact, it would alone constitute one of the most stupendous pictures in physical geography which could be presented to the imagination. I beg leave here to present to the reader a little illustration of the limestone-making Foraminifera of the Cretaceous and Eocene seas. In the middle above is a nummulite of the natural size. Below is another, sliced to show its internal chambers. At one side is a magnified section of the common building stone of Paris, the milioline limestone of the Eocene, so called from its immense abundance of microscopic shells of the genus Miliolina. At the other side is a magnified section of one of the harder varieties of chalk, ground so thin as to become transparent,[AH] and mounted in Canada balsam. It shows many microscopic chambered shells of Foraminifera. These may serve as illustrations of the functions of these humble inhabitants of the sea as accumulators of calcareous matter. It is further interesting to remark that some of the beds of nummulitic limestone are so completely filled with these shells, that we might from detached specimens suppose that they belonged to sea-bottoms whereon no other form of life was present. Yet some beds of this age are remarkably rich in other fossils. Lyell states that as many as six hundred species of shells have been found in the principal limestone of the Paris basin alone; and the lower Eocene beds afford remains of fishes, of reptiles, of birds, and of mammals. Among the latter are the bones of gigantic whales, of which one of the most remarkable is the Zeuglodon of Alabama, a creature sometimes seventy feet in length, and which replaces in the Tertiary the great Elasmosaurs and Ichthyosaurs of the Mesozoic, marking the advent, even in the sea, of the age of Mammals as distinguished from the age of Reptiles.