THE TERTIARY PERIOD. 231 



forests that covered the land gave shelter and food to countless 

 numbers of the mammalia that here enjoyed a happy existence. 

 The conditions were most favorable, not only to the perpetuation 

 and development of animal forms, but for the evolution of species 

 that were only to be developed completely during the following 

 epoch. 



Like the preceding epochs, the Miocene was destined to come to 

 a close. The changing conditions evidently were not sudden — they 

 were of such a gradual character as slowly to alter the environment 

 of the animal life of the times. With change of climate came 

 change of flora, which in turn changed or destroyed the rich and 

 wonderful Miocene forms of animal life. The final catastrophe 

 came at the close. It was one of the greatest revolutions that oc- 

 curred in the history of the globe. At the end of the Jurassic, "the 

 Sierras, which had been a marginal sea bottom, were crushed to- 

 gether and folded into a mountain ranee. This transferred the coast 

 farther westward, and the present coast range became the marginal 

 sea bottom, and received an abundance of sediment, until, in turn, 

 at the end of the Miocene, it also yielded to the lateral pressure 

 from the Pacific, and was raised up into the coast range." — (Le 

 Conte). Coincident with this movement, great fissures were 

 formed in the Cascade, and great floods of lava poured out, which 

 in north California covered in wide sheets a great extent of coun- 

 try, several hundred feet thick. The lava flood in Oregon, in 

 places, was 3,000 feet thick. It extended from Washington Terri- 

 tory to British Columbia. The area of this great flood of lava cov- 

 ered at least 80,000 square miles, a space much larger than the 

 whole of Nebraska. Richthofen has shown (Natural History of 

 Volcanic Rocks), that this great lava flood could not have proceeded 

 from the dozen extinct craters that cover this region, and that 

 therefore, as stated above, it must have proceeded from earth frac- 

 tures or fissures. At the same time the Plateau region was farther 

 elevated, the Miocene lakes were drained or shifted eastward, and 

 the region of the plains was still more depressed. This sinking of 

 the plains extended far to the south, almost to the gulf, and to the 

 east in its central portion about to where Columbus is located, on 

 the Union Pacific Railroad, and for an unknown distance to the 

 north. On the Niobrara its eastern line was near the mouth of 

 Keya Paha or Turtle Hill River. On the Republican, it was near 

 the center of Harlan County. It thus changed the whole aspect 



