674 THE POPULAR SCIENCE MONTHLY. 



fauna, the largest and most characteristic genus of which is the ungu- 

 late Coryphodon, and hence I have called these deposits the Corypho- 

 don 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 character- 

 ized by large mammals of the genus Diplacodon, and hence may be 

 termed the Diplacodon beds. The fauna of each of these three sub- 

 divisions 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 sur- 

 rounding 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 

 tilling 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 Brontotherida?, 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, 3fiohippus, 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 ex- 

 isting genera. The Pliocene climate was similar to that of the Mio- 

 cene. The Post-Pliocene beds contain many extinct mammals, and 

 may thus be separated from recent deposits. 



