The chalk and greensand are largely developed in England; and the same formation, with different lithological characters, is found in great force flanking the principal mountain ranges of southern Europe, and extending into Asia. In this country the system commences with the greensand and friable limestones of New Jersey, and following the Alleghany range to its southern termination, it bends around into a north-western direction, and is continued into Missouri.
7. The Tertiary System.—The tertiary strata embrace the formations from the cretaceous to the human era. They consist of clay, sand, sandstone, marl and limestone, and are distinguished from the lower rocks by being less consolidated; though the limestones are in some instances solidified, and resemble the strata of earlier origin. The tertiary strata are generally of less thickness than the older formations, and less continuous, being local deposits formed in lakes and estuaries. In a few instances they have been thrown into inclined positions, though in most cases they have been but slightly disturbed, and raised but a few hundred feet above the present level of the sea.
The late tertiary strata seldom overlap the older, so as to indicate their relative ages by superposition. They have therefore been separated into groups according to the proportions of living and extinct species of shells which they are found to contain. The oldest tertiary or Eocene formation[A] contains only four per cent, of living species, the Miocene contains seventeen per cent., the Pleiocene forty per cent., and the Pleistocene ninety per cent.
[A] Eös, dawn, and kainos, recent. The formation which commenced at the dawn of the recent period, containing but a small number of living species. Miocene (meion, less), less recent than the Pleiocene (pleion, more). Pleistocene (pleistos, most), most recent.
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.