THE TERTIARY PERIOD. 233 



plains into British America." (King). The Pliocene, therefore, 

 in eastern Nebraska, overlies the Cretaceous In south-west Ne- 

 braska it lies on the Fort Pierre Cretaceous. Further west, the 

 disturbance, as already stated, were much greater at the close of 

 the Miocene. There severe crumpling and fissuring of the earth's 

 crust had taken place. The basin region subsided to such an extent 

 that the Pliocene lake that was formed extended from the Wasatch 

 to the Sierras, and northward to the Columbia, while its southward 

 extension has not been ascertained. King believes that the ejection * 

 of trachytes occurred at the close of the Miocene, and that the ' 

 ejection of rhy elites marks the beginning, in this region, of the 

 Pliocene epoch. According to him, the basaltic eruptions occurred 

 wholly, within the Pliocene.* Still another Pliocene lake existed 

 in North Park, (North Park Group of Hayden.) It only comes 

 within the plan of this work to discuss the Pliocene lake deposits 

 of the plains, which cover so large a portion of Nebraska. These 

 deposits constitute the Loup Fork Group of Hayden, and the Nio- 

 brara of Marsh. 



On the plains the Pliocene beds, wherever their point of junction 

 could be observed, are conformable to the underlying Miocene. 

 King, however, remarks that they are in places unconformable, 

 which I have not observed. Often they shade so insensibly into 

 each other that the line of junction could only be ascertained by 

 the fossils which they entombed. 



Thickness of the Pliocene Beds. Along the foot-hills of the Col- 

 orado Range, the Pliocene beds average nearly 2,000 feet in thick- 

 ness. They thin out eastward, probably because the mass of ma- 

 terials was obtained from the mountains, the greater part of which 

 was precipitated along, or near its western shores. In Nebraska, 

 Kansas, and Dakota towards the east, the Pliocene beds become 

 thinner; until they run out entirely. It is certain, however, that 

 originally they were much thicker than at present. Owing to them 

 oeing the upper rocks at the time, they must have been subjected 

 to an enormous amount of erosion during the subsequent Quater- 

 nary age. The monuments of this erosion are still visible in 

 many places. In township 10 North and 26 West of 6th Meridian 

 there is a Pliocene peak, nearly 300 feet high, that represents the 

 original level of these deposits. In 13 North, 51 West, there are 

 limestone cliffs 75 feet high, and similar ones all over this region in 



*See King's Systematic Geology of the 40th Parallel. 



