PLIOCENE LAKE EAST OF THE liOCKY MOUXTAIXS. 119 



the formation, and the close relations of its Fauna to that of the Niobrara 

 Group east of the Rocky Mountains. It may be taken for granted that in 

 all the vallej's of the Great Basin, where tliere are proofs of the existence of 

 water during Quaternary or Post-glacial times, there must have been lakes 

 during the earlier Tertiary periods, although the conditions might not have 

 been favorable to the development of organic life in such localities, for it is 

 easily seen that large lake-areas in broad valleys would be much more favor- 

 ably situated for the growth of animals and plants than narrow and deep 

 valleys, into which coarse sediment was continually being carried by streams 

 descending the steep slopes of the adjacent ranges. As being in harmony 

 with the.se considerations, it cannot fail to be noticed how comparatively 

 poor in fossil remains are the deposits accumulated, largely by rapid river- 

 action, on the west slope of the Sierra Nevada, and how rich, on the other 

 hand, are those Inroad lake-basins of the earlier Tertiary period, farther east 

 in the Cordilleras. 



The region east of the Rocky Mountains, included within the area of the 

 Plains, was undoubtedly extensively covered by water during the Pliocene 

 epoch. The precise limits of this immense lacustrine area cannot at present 

 be given. It is'known, however, that it bordered close upon the base of the 

 Rocky Mountains on the west, and extended into Kansas in the opposite 

 direction. North and south it stretched from Texas to fiir be\ond the United 

 States boundary-line. Of course over tlils vast area the thickness and char- 

 acter of tlie deposits are very variable. Near the elevated range on the 

 west the accumulated debris wouiil necessarily be greater in amount and 

 coarser in character. Fossils have been found in many localities, the re- 

 mains of the manmialia being of the most interesting character. This Plio- 

 cene lake-area was obliterated by the latest orographic disturbance which 

 gave their present form and altitude to the Rocky Mountain ranges. The 

 overlying Pliocene with all the underlying deposits have all been raised 

 bodily from the western edge, in a simple uplift without thrust or fold, so 

 that the net result, as respects drainnge, is quite unlike that which mani- 

 fested itself farther west. On the eastern side of the mountains there is an 

 extremely gradual and constantly diminishing slope down to the Mississippi 

 Valley, without any lakes whatever; while on the other side the original 

 corrugations in the older rocks have not been so entirely obliterated, by 

 filling with detrital materials, that there is not ample opportunity for the 

 accumulation of lar^e bodies of water, if only the climatic conditions were 



