THE TERTIARY PERIOD. 221 



Agriochoerus was a genus of hog-like ruminants, that flourished 

 during these times. It was related to the Oreodon of the Miocene. 

 It differed from the latter and from all known ruminants in having 

 the orbits open behind. — Leidy. 



This group closed the deposits of the Eocene period. This re- 

 gion of lakes had been rising during the latter portion of Eocene 

 times, and their final extinction closed this period. 



CHAPTER V 



THE TERTIARY PERIOD, CONTINUED.— MIOCENE 



EPOCH. 



Inauguration of the Miocene Epoch. — Formation of a Lake on the Plains. 

 — Boundaries. — Where the Miocene is Exposed in Nebraska. — Extent. — 

 Miocene Lakes farther West. — Basin Region. — Oregon Region. — An Age of 

 Lakes. — Name of the Eastern Lake. — Kinds of Rock. — Whence the Materials 

 were obtained. — Why the Miocene Beds are Thin on the Plains. — Length of 

 the Miocene Epoch. — Bad Lands.— Flora of the Miocene. — Animal Life. — 

 Insectivora. — Rodentia. — Horse Family. — Titanotheriums. — Symborodons. — 

 Mastodons and Elephants. — Rhinoceros'.— River Horse. — Hog Family. — 

 Camel Family. — Musk Deer. — Oreontidae. — Carnivora. — Hyaenodons. — Dre- 

 panodons. — Quadrumanna in the Miocene. — Mammals in the Miocene, not 

 Described nor Found. — Closing of the Miocene Epoch — Its Gradual Char- 

 acters. — Lava Floods at the Close.— Formation of the Coast Range. — Farther 

 Depression of the Plains. — Effect on life of these Changes. 



THE Miocene Epoch was gradually inaugurated. During the 

 Eocene Epoch the plains were an extended land surface, 

 made up of the eroded materials of the Cretaceous and the Per- 

 mian and Carboniferous rocks. There was free drainage to the 

 sea, but of the rivers and their tributaries of that time, we know 

 nothing. The upward movement of the plateau regions that event- 

 ually drained the old Eocene lakes was accompanied by a subsi- 

 dence of portions of the adjoining plains. The old mountain lakes 

 were shifted eastward, the depressions in the plains making room 

 for them. While the mountains went upward, the plains 

 went downward, like the changing waves of the sea. As this 

 movement was slowly in progress for ages before it was consum- 



