26 



lowest Miocene, which is only found east of the Eocky Moun- 

 tains, alone contains the peculiar mammals known as the Bran- 

 totheridce, and these deposits may be called the Brontotherium 

 Beds. The strata next above, which represent the middle Mio- 

 cene, 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, Mwhippus, may be designated as the Miohip- 

 pus Series. The climate here during this period was warm 

 temperate. 



Above the Miocene, east of the Eocky 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 distin- 

 guished 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 ex- 

 tinct mammals, and may thus be separated from recent deposits. 



Eeturning now to our subject from this geological digression, 

 — which will hardly be deemed unprofitable, since I have given 

 you in few words the results of a great deal of hard mountain 

 work, — let us consider the Tertiary mammals, as we know 

 them from the remains already discovered, and attempt to 

 trace the history of each order down to the present time. We 

 have seen that a single small Marsupial, from the Trias, is 

 the only mammal found in all the American rocks below the 

 Eocene ; and yet in beds of this age, immediately over the 

 Chalk, fossil mammals of many different kinds abound. 



The Marsupials, strange to say, are here few in number, and 

 diminutive in size; and have as yet been identified only by frag- 



