CONTINENTAL DEPOSITS OF ROCKY MOUNTAIN REGION 603 



and on the western flanks of the Elk mountains, in the western part of the State. 

 The Denver and Arapahoe beds together are known to have a thickness of more 

 than 2,000 feet ; the Middle Park beds are more than 6,000 feet in thickness, accord- 

 ing to the reports of A. R. Marvine ; the South Park beds are also of considerable 

 thickness; the Animas River beds exceed 1,000 feet in thickness, and the Ruby- 

 beds, west of the Elk mountains, reach a thickness of 2,000 feet. If one of these 

 formations be supposed to be of fluviatile origin from anything in the texture or 

 composition of the strata, then all must be placed in the same category for the 

 same reason. 



It is further certain from a field examination of these formations that they had 

 very much greater lateral extent at the time of deposition than they now possess, 

 as well as a greater thickness in some cases. This seems to me a very important 

 factor in the problem, making it absolutely essential to give some plausible grounds 

 for the assumption that rivers once existed capable of the accumulation of such 

 enormous masses of finely stratified matter at so many points in the mountains of 

 Colorado. It is also to be taken into account that the Livingston formation of 

 Montana, identical in lithologic character and known to be of the same age as the 

 Denver beds, has a thickness of over 7,000 feet, according to Mr W. H. Weed. 



The present relation of the Great plains to the Rocky Mountain front near 

 Denver may suggest to a physiographer that formations of fluviatile origin are to 

 be looked for in the vicinity of the mountains, either in the valleys of the present 

 streams or as remnants of older deposits, but it can not be assumed that moun- 

 tain and plain occupied this relation to each other in the period of the Denver 

 formation. The Denver beds are upturned in vertical position at the base of the 

 foothills, and great orographic movements have taken place in several parts of 

 Colorado since the deposition of the equivalent formations. 



These formations have been described as lacustrine by those who have studied 

 them in the field, and it appears to me that the burden of proof rests on Professor 

 Davis in advocating a fluviatile origin for them. 



Remarks in discussion were also made by W. H. Weed, I. C. Russell, 

 and the President. In closing the discussion Professor Davis spoke as 

 follows : 



For evidence of the habitual reference of the fresh-water Tertiaries to a lacus- 

 trine origin I refer again to such a quotation as that from Dutton given in my 

 paper. As to the occurrence of coarse and variable strata among deposits that 

 have been classed as lacustrine, the most striking examples that I have found are 

 contained in the account of the Vermilion Creek beds in the Report of the For- 

 tieth Parallel Survey, and in the description of the Arapahoe and Denver forma- 

 tions in the Monograph on the Denver Basin, United States Geological Survey. 

 The foregoing discussion of the origin of these deposits seems to me appropriate, 

 in spite of my not having studied the localities where the formations occur. It is 

 not proposed to question the facts of observation, which are accepted as reported, 

 but to discuss the discussion by which the reported facts are interpreted. Every 

 geologist must carry on his own discussion of the methods by which others reach 

 their theoretical results, unless he merely accepts the results along with the facts. 

 It is the more necessary that each student of our western geology should discuss 

 for himself the lacustrine origin of the fresh- water Tertiary deposits of the Rocky 



