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 then 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 then 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.