Page:Popular Science Monthly Volume 12.djvu/694

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fauna, the largest and most characteristic genus of which is the ungulate Coryphodon, and hence I have called these deposits the Coryphodon beds. The middle Eocene strata, which have been termed the Green River and Bridger Series, may be designated as the Dinoceras beds, as the gigantic animals of this order are only found here. The uppermost Eocene, or the Uintah group, is especially well characterized by large mammals of the genus Diplacodon, and hence may be termed the Diplacodon beds. The fauna of each of these three subdivisions was essentially distinct, and the fossil remains of each were entombed in different and successive ancient lakes. It is important to remember that these Eocene lake-basins all lie between the Rocky Mountains on the east and the Wahsatch Range on the west, or along the high central plateau of the continent. As these mountain chains were elevated, the inclosed Cretaceous sea, cut off from the ocean, gradually freshened, and formed these extensive lakes, while the surrounding land was covered with a luxuriant tropical vegetation, and with many strange forms of animal life. As the upward movement of this region continued, these lake-basins, which for ages had been filling up, preserving in their sediments a faithful record of Eocene life-history, were slowly drained by the constant deepening of the outflowing rivers, and they have since remained essentially dry land.

The Miocene lake-basins are on the flanks of this region, where only land had been since the close of the Cretaceous. These basins contain three faunas, nearly or quite distinct. The lowest Miocene, which is only found east of the Rocky Mountains, alone contains the peculiar mammals known as the Brontotheridæ, and these deposits may be called the Brontotherium beds. The strata next above, which represent the middle Miocene, have as their most characteristic fossil the genus Oreodon, and are known as the Oreodon beds. The upper Miocene, which occurs in Oregon, is of great thickness, and from one of its most important fossils, Miohippus, may be designated as the Miohippus Series. The climate here during this period was warm temperate.

Above the Miocene, east of the Rocky Mountains and on the Pacific coast, the Pliocene is well developed, and is rich in vertebrate remains. The strata rest unconformably on the Miocene, and there is a well-marked faunal change at this point, modern types now first making their appearance. For these reasons, we are justified in separating the Miocene from the Pliocene at this break; although in Europe, where no marked break exists, the line seems to have been drawn at a somewhat higher horizon. Our Pliocene forms essentially a continuous series, although the upper beds may be distinguished from the lower by the presence of a true Equus, and some other existing genera. The Pliocene climate was similar to that of the Miocene. The Post-Pliocene beds contain many extinct mammals, and may thus be separated from recent deposits.