222 GEOLOGY. 



mated, the probabilities are that the great Miocene lake of the plains- 

 commenjed to form before Uinta lake had terminated its history. 

 There probably were no great convulsive throbs of the earth's 

 crust, separating sharply the two epochs. The Eocene shaded into 

 the Miocene epoch. This lake of the plains extended from near 

 the north line of Kansas across Nebraska, a large part of Dakota 

 Territory, west of the Black Hills, and northward to Manitoba. 

 Its exact geographical extent has not been ascertained in Nebraska, 

 owing to the superincumbent Pliocene, which overlaps it, and 

 through which it only projects at intervals. The best exposures in 

 Nebraska commence on the Niobrara River, about 300 miles west 

 of the mouth of the Keya Paha or Turtle Hill River, and extend' 

 to the west line of the State, taking in the White Earth River re- 

 gion and the space between the latter and the north line of the 

 State. It is finely represented on and north of the latter river in 

 Dakota Territory, constituting there a portion of the famous Ma- 

 koo-si-tcha or Mauvais Terre of the French, which has been ren- 

 dered into English by the term Bad Lands, although in the Dakota 

 tongue it means simply a country hard to travel over. On the 

 west the Miocene abuts against the undulating surface of the Lar- 

 amie Group, and therefore did not extend quite to the foot-hills of 

 the Colorado Range. The extent of this great fresh water lake 

 has been variously estimated at from 100,000 to 130,000 and up- 

 wards of square miles. 



The local subsidence of the plains on the east, next to the moun- 

 tains, was accompanied by a somewhat similar depression between 

 the Wasatch and the Sierras, forming also a large Miocene lake in 

 that region. Another great Miocene lake extended from Wash- 

 ington Territory through Oregon to Nevada and Colorado. In 

 eastern Oregon, the deposits of this epoch are enormously thick, 

 the depth reaching 5,000 feet, overlaid, however, by the lava beds, 

 which were poured from fissures at the close of the Miocene. It 

 does not fall within the plan of this work to discuss any of these 

 old Miocene lake beds except the one covering a portion of Ne- 

 braska. 



From the above it is seen that the Miocene was pre-eminently an 

 age of great fresh water lakes. It is questionable whether on this 

 continent any other geological epoch was represented by such a 

 number and such large basins of fresh water. 



