372 PROCEEDINGS OF THE AMERICAN ACADEMY. 



lacustrine deposition : — "A considerable number of large lakes being 

 'formed, the next process was the desiccation of these lakes and 

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

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

 Before a large lake can be drained its outlet must be cut down. But 

 several causes in the present instance would combine to render this 

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

 and consequent corrasive 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 slow pro- 

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

 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 " (U. S. G. S., 

 Monogr. XXVII., 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 jiost-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 Rocky mountain district have become 

 stock subjects of geological teaching, as the subject is represented by 

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

 the matter 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 so-called lacustrine 

 formations may really be of fluviatile or other subaerial origin. Similar 

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

 such as those by Geikie, Lapparent, 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 geologists no option in the matter ; they will accept the completely 

 lacustrine origin of all the deposits so-called. The probability or possi- 

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



