THE TERTIARY PERIOD. 223 



Clarence King has suggested for the Miocene lake that extended 

 .through Nebraska the name of Sioux Lake. Hayden, who first 

 studied these beds in this region, called them the White Earth River 

 Group. 



Kinds of Rock. The materials of these Miocene beds vary a 

 great deal in character. This would naturally be expected in a lake 

 bed which received the drainage, through countless ages, of the 

 rivers that now have their outlet through the Missouri. Varying 

 currents and other conditions would naturally frequently change 

 the character of the sediments deposited on the bottom. The rocks- 

 that supplied the materials that were carried into this Miocene lake 

 evidently came from the Archaean nucleus of the Rocky Mountains- 

 and the Black Hills, the Palaeozoic, the Juro-Trias and the different 

 groups of the Cretaceous. The eroded materials going seaward 

 were stopped in these old lake beds. Erosion, however, through 

 the Miocene, was by no means as rapid as at present. The height 

 of the plateau . region was much less than at present; the atmos- 

 phere was moister, the rainfall much gentler and more constant, 

 and a warm, temperate climate obtained. The extreme cold of 

 winter, which is such a mighty agent in the disintegration of rock, 

 and which now characterizes these regions, did not then exist. Hill, 

 valley, plain, mountain and plateau, were also covered by dense 

 growths, in places, of grasses, and in places of mighty forests, 

 which protected the land from the denuding agencies which are 

 now constantly at work. As already stated, the extreme thickness 

 of the Miocene in the West reaches its maximum in Oregon, where 

 beds 5,000 feet in vertical thickness are found. Owing to the causes 

 alluded to above, on the plains the Miocene beds are comparatively 

 thin. Meek estimates their thickness at from 530 to 600 feet. 

 Where I measured them, on the Upper Niobrara, they rarely ex- 

 ceeded 400 feet. 



If we calculate the length of Miocene times on the same principle 

 as Eocene, this epoch was probably a quarter of a million years 

 long. It should be remembered, however, that there is no certainty 

 about the length of geological periods. k 



In Nebraska, on and north of the White Earth, and on the Upper 

 Niobrara, the rocks of the Miocene have the following character: 

 Indurated grit, of a reddish brown color, with occasional layers of 

 concretions of silicate of lime, often shading into, first, a coarse and 

 then a fine green sandstone. Above this occur, sometimes, an- 



