During the pleistocene period, peculiar conditions existed, by which a great amount of loose material, known by the name of _drift_, was spread over the northern portions of both hemispheres. In America it is found from Nova Scotia nearly to the Rocky Mountains, and extending as far south as Pennsylvania and the Ohio river. In Europe, it is found from the Atlantic to the Ural Mountains, and reaching south into Germany and Poland. It is also found in the colder portions of South America, and in the vicinity of several mountains, as the Alps. It consists of irregular accumulations of earthy substances of different degrees of fineness, but characterized by containing masses of rock of considerable size, often of many tons weight, called boulders. Rocks having the same lithological characters exist in situ north of where the boulders and other drift are now found, though at a distance often of one or two hundred miles. There can be no doubt but that the drift has been transported from these northern localities; and the polished, striated and grooved condition of the rocky surface, wherever the drift is distributed, has obviously been produced by the passage of the drift materials over it. Towards the close of this period, while the land was a few hundred feet below its present level, there were deposited in the valleys of the drift region beds of blue and gray clay, materials which are used in making bricks and coarse pottery; also beds of sand, sometimes evenly spread out, but often thrown into irregular mounds and ridges. In regions which are not covered with drift,--as the south of Europe and the United States,--the pleistocene deposits are succeeded, without apparent change of conditions, by those which are now taking place. The formations of the tertiary period are distinguished from those of the cretaceous period by the absence of deep-sea fossils, and from the oölite by the absence of its characteristic saurians. The mollusks are also very different, such genera as the cerethium (Fig. 37), murex (Fig. 38), and conus (Fig. 39), which abound in the present seas, first appearing in the tertiary period. The nummulite (Fig. 40), a peculiar form of chambered shell, is so abundant as to constitute in some places almost the entire rock. The period is however characterized by the existence of a large number of pachydermatous animals, of which the tapir, hog, horse and elephant, are examples of living species. [Illustration: Fig. 37.] [Illustration: Fig. 38.] [Illustration: Fig. 39.] [Illustration: Fig. 40.] The _Paleotherium_ (Fig. 41) resembled, in most respects, the tapir. It was furnished with a short proboscis, and the foot was divided into three toes. The length of the largest species was about that of the horse; but its body was larger, and it was of less height. [Illustration: Fig. 41.] [Illustration: Fig. 42.] The _Anoplotherium_ (Fig. 42) was a more slender animal, and resembled in size and general form the gazelle.