18 GEOLOGY OF THE TERTIARY FORMATIONS OF 



although the great moimtain range had been fully marked out prior to the deposition 

 of the sediments of the White River group, yet the elevation continued through that 

 period and long after, slightly disturbing some of the beds, and elevating others much 

 above their original position. 



It seems to be an important inquiry, from whence were derived the materials 

 which comjTOse the sediments of the great basin. In their external appearance and in 

 their mineral compositions, these beds differ from any other Tertiary formation 

 known in the west, — so much so, that if I were to see isolated remnants in any 

 portion of the Rocky Mountains I should be able to detect them at a glance. The 

 matrix attached to the bones of Tltanotherium has been analyzed by Dr. D. D. Owen, 

 and found to contain a very large proportion of silica, lime, and some alumina. It 

 seems to me quite possible that the numerous little streams that must have poured 

 into the lake, having their sources in the Rocky Mountains, and especially in the 

 Black Hills, which must at that time have begun to emerge from beneath the super- 

 incumbent fossiliferous formations, cut their way through the metamorphic rocks and 

 the granitoid nucleus, distributing the decomposed silica, lime and alumina over the 

 bottom of the lake. We know that feldspar is a predominant constituent of the 

 granitoid nucleus of the mountains, and, when decomposed, forms a rock of a whitish 

 or yellowish-white color, much like the mixed sediments of the Bad Lands. One of 

 the most interesting features in regard to this great fresh-water lake is the evidence 

 of its growth from a germ, as it were, until it spread over the great area on which it 

 has left its traces by the deposition of its peculiar sediments. It seems to have com- 

 menced its existence near the south-eastern base of the Black Hills, gradually 

 spreading southward and eastward from that point, as the surface, and especially the 

 Black Hills, arose above the surrounding country. The lowest stratum, or the 

 Tltanotherium bed, is found only in the valley of White River, and near the Shyenne, 

 occupying a comparatively small area ; and as we proceed southward higher and 

 more recent beds are seen, until the Pliocene beds appear, and then graduate into the 

 Post-pliocene inarls far southward toward the Arkansas. It seems to me that the law 

 of growth from small beginnings must apply equally to all the fresh-water lakes, 

 either of the past or present. 



The basin-like character of this formation is most admirably shown. Along the 

 northern border, extending for nearly one hundred miles, there is a high level 

 plateau covered with grass. This plateau is cut through by numerous affluents of 

 the Shyenne River, as Bear, Sage and Bull Creeks. It is underlaid by cretaceous 

 rocks, and from it we descend quite abruptly forty or fifty feet to the Titanotlierium 

 bed, the concave outline revealing the ancient lake shore perfectly. Sometimes there 

 were extensions of the upper beds across the Shyenne northward, overlapping the 

 lignite beds, as the colors on the nuip will show. 



