366 PROCEEDINGS OF THE AMERICAN ACADEMY. 



tiary formations of the Green river and other Rocky mountain basins, 

 that a fiuviatile origin for many of the latter becomes probable ; but re- 

 examination in the field with special attention to discriminating struc- 

 tures will be necessary before definite conclusions can be announced. It 

 is also probable that the basins produced by the Pliocene deformation of 

 the previously denuded Rocky mountains of Montana may have been at 

 least during part of their existence occupied by fluviatile plains as well 

 as by lakes. Hayden referred to them only as lakes : — " These . . . 

 broad valleys [of the Missouri headwaters in Montana] have all been lake- 

 basins during the last portion of the Tertiary period," and on another 

 page he says: — "The great valleys . . . during the latter Tertiary period 

 were the basins of fresh-water lakes, so that we have everywhere the 

 white and yellowish-white sands, marls, clays, sandstones, and pudding- 

 stones of the Pliocene lake deposits passing up into the Quaternary or 

 local drift" (Geol. Surv. Teir., 1871 (1872)", 147, 141). 



The same comment may be made regarding deposits of the basins of 

 South park and San Luis valley, Colorado, which were described as 

 lacustrine by Stevenson (Wheeler's Survey, iii, 453, 461). 



14. Deposits in Arid Basins. — In regions of drier climate, such as 

 interior continental basins, calcareous, saline, and alkaline matter may 

 slowly accumulate along with detritus of finer or coarser texture in the 

 central depression, while conglomerates, gravels, and sands would gather 

 to greater thickness in laterally confluent fans around the mountain bor- 

 ders. The importance of marginal deposits of this kind, both recent and 

 Tertiary, is attested by the following quotation from Powell, who says: 

 "I think that many geologists would ascribe this [Bishop mountain] con- 

 glomerate to the action of ice, but throughout all that portion of the 

 Rocky mountain region which I have studied, I have so frequently found 

 gravels and conglomerates of subaerial origin, and have in so many cases 

 found reason to change my opinion concerning them, often having attrib- 

 uted a drift-like deposit to glacial action, and afterward on further study 

 abandoned the theory, being able to demonstrate its subaerial origin, and 

 witnessing on every hand the accumulation of such gravels in valleys, 

 and over plains where mountains rise to higher altitudes on either side, 

 and having in many cases actually seen cliffs breaking down and the 

 gravels rolling out on the floods of a storm, I am not willing to disregard 

 explanations so obvious and so certain for an extraordiuary and more 

 violent hypothesis. . . . Nor need the thickness and extent of this 

 Bishop mountain conglomerate serve to weaken this explanation, for the 

 sub-aerial gravels in the valleys between the ranges in the Basin province 



