252 • Tertiary. 



To the eastward of the lino of the Denver and Rio Grande Raih-oad, 

 the surface is cut up into more or less rectangular masses, with rather 

 broad table- shaped summits, vaiying from 400 to 800 feet in height. 

 The sides are often ver^' steep, almost inaccessible. At a remote period 

 in the past, the erosion has been very great, carving out by an almost 

 inappreciably slow process, these broad valleys, leaving these buttes 

 here and there, comj)osed of horizontal beds, .to aid in forming some 

 conception of the amount of denudation which has taken place. It is 

 not possible at the present time to estimate the original thickness of 

 this group, but believe it to have been very much greater than the 

 highest beds now existing would indicate. The summits of many of 

 the buttes are capped with a greater or less thickness of a beautiful 

 purplish trachyte, which must have ascended in the form of dikes from 

 beneath, and flowed over the surface. Much of the trachyte is a sort 

 of breccia, composed of rather coarse sandstones, which must have been 

 caught in the melted material. It is quite evident that these outflows 

 occurred during the existence of the lake, though at a late period. Dr. 

 Hayden synchronized the age of this group with the upper portion of 

 the White River Group far to the northward, and probably with the 

 fresh water deposits in the South Park. 



Lake basins have occupied a large part of the country from the 

 Isthmus of Darien to the Arctic Circle. In many instances they were 

 merely expansions of river valleys, like the greater number of th-e lake 

 basins of the present time. During the later Cretaceous and early 

 Tertiary periods, the western portion of the continent was covered 

 with immense lakes, but during the Pliocene and the interval to 

 modern time, thqusands of small lakes, with a few of large size, were 

 distributed over the great area west of the Mississippi, and the basins 

 with their peculiar deposits are found in the parks, among the moun- 

 tains, and along every important vallej^ 



Dr. Hayden believed there are evidences of glacial action and 

 morainal deposits in the valley of the Upper Arkansas river, at eleva- 

 tions of 9,000 feet and upward, and along both flanks of the Sawatch 

 mountains; but, he said that he observed no proof of any wide ex- 

 tended drift-action, like that of the New England States, in the 

 Rocky mountains, as the superficial deposits are all of local origin, 

 and the source is limited to the drainage of the streams in which the 

 deposits are found. For example, all the marls and coarser deposits 

 in the valley of the Upper Arkansas, have the same origin, and the 

 forces that produced them were limited geographicallj^ to the drainage 



