372 PROCEEDINGS OP THE AMERICAN ACADEMY. 



istrine deposition: — U A considerable number of large lakes being 

 formed, the next process was 1 1 1 « - desiccation of these lakes and 

 the evolution of river systems. So long as the region occupied a 

 ] w altitude this process, we may infer, would be very protracted. 

 B ire a arge lake can !"• drained its outlet must be cut down. But 

 ral causes in the present instance would combine to render this 

 >n very slow and feeble. The elevation being small, the declivity 

 and consequent conasive power at the outlet must be correspondingly 

 small. .Moreover, the waters issuing from a large lake contain little or 

 no sediment . . . Corrasion by clear water is an exceedingly Blow pro- 

 cess" (Ibid., 218). On the other hand, fluviatile and subaerial depo 

 may accumulate at considerable altitudes above sea level in interior 

 basins. 



The preservation of numerous vertebrate fossils was explained by 

 Marsh as " probably, without exception, due to their entombment be- 

 neath the waters of the great fresh-water lakes which existed in this 

 [Colorado] region during Mesozoic and Cenozoic time" (D. S. ('<■ S., 

 Monogr. XX\'II.. 525). From the time when Warren first called 

 attention to the inclined position of the Pliocene strata of the Plains, 

 their attitude has been taken to prove a post-Pliocene elevation of the 

 Rocky mountain System by all writers who have considered the subject. 

 If the Pliocene of the Plains is fluviatile instead of lacustrine, a much 

 smaller elevation may be demanded. 



The Tertiary lakes of the Pocky mountain district have become 

 stock subjects of geological teaching, as the Bubject is represented bj 

 the text-books generally in use, and it is here that my own interest in 

 the mailer is especially aroused. Dana, Leconte, Scott, and Tarr all 

 assert the existence of Tertiary lakes without qualification ; they 

 give no indication that a large share of the BO-called lacustrine 

 formations may really 1"' of llu viatile or other subaerial origin. Similar 

 statements are naturally made by the standard European text-books, 



such as those by Geikie, I. apparent, and Credner, who naturally adopt 

 the lacustrine origin of our western Tertiaries without demur. The 

 older generation of geologists, who had a first-hand acquaintance with 

 the facts, may have interpreted "-lacustrine " very liberally, including 

 therein a considerable share of marginal subaerial deposits; but the 

 brief and direct statements of the text books leave the coming generation 

 of ideologists no option in the matter; they will accept the completely 

 laCUStri Hgin Of all the deposits so-called. The probability or possi- 

 bility iA' fluviatile origin is not given a chance to gain a hold in the 



