12 GEOLOGY OF THE TERTIARY FORMATIONS OF 



2d. The great Lignite Basin, which occupies by far the most extensive area of any 

 of the fresh-water lakes of this period. Its limits have not yet been explored, still 

 we believe that it extends fir southward, possibly even to California, westward far 

 over the mountains to Utah, and possibly to the Pacific, and northward probably to 

 the Arctic Sea, interrupted here and there by the upheaval of mountain ranges. 



The beds composing this great basin seem to have been deposited in waters that 

 were at first brackish, and then gradually becoming fresh. It is chiefly remarkable 

 for the beauty and extent of its fossil flora, and for the numerous beds of Lignite, 

 varying in thickness from a few inches to twelve or fifteen feet. The occurrence of 

 immense fan palms, and many other plants now found growing only in tropical cli- 

 mates, points directly to the conclusion that along the shores of this great lake there 

 grew most luxuriant forests, equalled only by those now existing in Central America 

 or Brazil. 



The characters of the mollusca all indicate a very mild temperature during this 

 portion of the Tertiary period. 



3d. The Wind River deposits are also quite remarkable, as holding a kind of inter- 

 mediate position between the Fort Union group and the White River group, and the 

 evidence seems to indicate that it was an independent fresh-water lake during the 

 Tertiary period. The beds are intermediate in color and composition, but very few 

 fossils have been found in them to give exactness to our conclusions. A few frag- 

 ments of remains of mammals and Testudo, with a few terrestrial and fresh-water 

 shells, allied to forms found in both the Lignite and White River groups. 



4th. The White River group is the formation from which most of the vertebrate 

 remains described in Dr. Leidy's memoir have been obtained. This formation is 

 mostly composed of a series of whitish indurated clays, marls and sands, which have 

 been worn and cut by the streams and other atmospheric agencies into myriads of 

 deep valleys or gorges, so as to form a most wonderful and almost interminable 

 labyrinth, which so im^^edes the course of the traveller that it has caused the native 

 Indian tribes to give to this region the name of the Bad Grounds, or, as the Canadian 

 voyageurs have translated it, Mauvaises Terres. That portion of the fourth Basin in 

 the region of White River is regarded of Miocene age. Along the Niobrara and the 

 Loup Fork, and extending southward, are a series of beds composed of incoherent 

 materials, mostly fine marls and sands, which seem to have been deposited after the 

 upper surHxce of the White River group had been worn into ravines and depressions. 

 This occurs along the Niobrara River and the Loup Fork. This group of beds seems 

 to have filled up the channels of the rivers after they had been worn out to nearly 

 their present width and depth. Tlie relations which these Tertiary Basins sustain to 

 each other iire pretty well shown by the following general section : 



