110 Cincinnati Society of Natural History. 
To the eastward of the line of the Denver and Rio Grande Railroad, 
the surface is cut up into more or less rectangular masses, with rather 
broad table-shaped summits, varying from 400 to 800 feet in height. 
The sides are often very 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, composed 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 heautiful 
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. Itis quite evident that these outflows 
occurred during the existence of the lake, though ata 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 the 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, thousands 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 valley. 
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 geographically to the drainage 
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