DAVIS. — ROCKY MOUNTAIN TERTIARIES. 347 



priate general name for the aggraded floors of interior continental basins 

 is also unfortunate ; for several of their features are peculiar to them- 

 selves and all are highly specialized and well correlated. 



2. The Various Origins of Stratified Deposits. — Let us look now at the 

 problem of fluviatile deposits from the other side. A century ago, it was 

 the habit of geologists to regard all stratified deposits as of marine origin. 

 About half a century ago, certain stratified deposits in which no marine 

 fossils were found came to be regarded as of lacustrine origin. In still 

 more recent years, the importance of fluviatile, teolian, and other strictly 

 subaerial agencies in the formation of extensive deposits has been recog- 

 nized ; yet it is still almost habitual to attribute extensive bodies of strati- 

 fied deposits to a source in bodies of standing water, without a sulficient 

 consideration of other possible origins. It is therefore the intention of 

 this paper to suggest a revision of certain so-called lacustrine deposits, 

 in view of the possibility that they may have been at least in part 

 formed by great rivers, by small streams, or even by the wash and 

 creep of a subarid climate, and that lakes may have had but a very 

 subordinate part to play in their accumulation. With this object in 

 mind, attention may be given first to some extracts from the accounts of 

 certain Tertiary formations in the Rocky Mountain region which have 

 been repeatedly described as fresh-water " lake deposits " in the reports 

 of our western surveys ; and further on it may be considered whether 

 many of these deposits are not in reality ancient analogues of the fluvia- 

 tile deposits in modern piedmont plains. 



3. Accounts of Tertian/ Lakes in the Rocky Mountain Region. — 

 Among the earliest references to Tertiary lakes in the Rocky mountain 

 region are those by Hayden. He wrote: — "I would infer that this 

 great fresh-water lake [White river] must have spread over 150,000 

 square miles" (Geol. Surv. Terr., 1st Ann. Rep. (1867), 58). " With the 

 commencement of the Tertiary was ushered in the dawn of the great lake 

 period of the West. The evidence seems to point to the conclusion that 

 from the dawn of the Tertiary period, even up to the commencement of 

 the present, there was a continuous series of fresh-water lakes all over 

 the continent west of the Mississippi river. . . . The earliest of these 

 great lakes marked the commencement of the Tertiary period, and seems 

 to have covered a very large portion of the American continent west of 

 the Mississippi, from the Arctic sea to the Isthmus of Darien. . . . Every 

 year, as the limits of my explorations are extended in any direction, I 

 find evidences of what appear to be separate lake basins, covering greater 

 or less areas" (Ibid., 2nd Ann. Rep. (1868), 114, 115). 



