duttox.] EROSION IN MIOCENE TIME. 67 



strata is found in immense volume. These facts lead OS to infer 1 hat 

 the great Eocene lake, soon after its waters became quite fresh, began 

 to shrink its area, and that its bottom became through a slow pro- 

 gression dry land. The southern and southwestern portions were the 

 first to emerge; then the middle portions; the lake gradually ret i acting 

 its boundary to the northward, until in the latter part of the Eocene it. 

 occupied a greatly diminished area In the vicinity of the San Rafael 

 country and the southern base of the Uinta Mountains. At the close 

 of the Eocene this remnant of the lake also disappeared. 



We now reach another turning point in the history of the region. 

 Hitherto and for an immense stretch of geological time it had been an 

 area of deposition and of subsidence. It now became an area of eleva- 

 tion and denudation, and these processes have been in operation ever 

 since. In the periods of deposition and subsidence, from the Carbo- 

 niferous to the Eocene, both inclusive, the thickness of the strata accu- 

 mulated varied from 14,000 to 20,000 feet, and the subsidence of the base 

 of the Carboniferous was of nearly equal extent. In the periods of ele- 

 vation and denudation these vast masses of strata rose bodily up again; 

 the amount of elevation varying according to locality from 0,000 to 18,000 

 feet. The havoc wrought by erosion has been, as already shown, stu- 

 pendous ; the thickness of strata removed exceeding 10,000 feet in 

 some considerable areas, and averaging probably 5,500 to (5,000 feet over 

 the entire province. 



The points which it is desirable to notice in this chapter concerning 

 the progress of the Tertiary and Quaternary erosion of the province, 

 are few and of the broadest nature. In truth it is necessary to speak 

 very guardedly. For while the most general features of the work have 

 left well-marked traces which can be interpreted, yet when we come to 

 details the vast erosion has swept away so much of its mass that a large 

 portion of the evidence of the details has vanished with the rocks. There 

 is reason to believe that the greater part of the denudation was accom- 

 plished in Miocene time. This was a period of slow but continuous up- 

 lifting, reaching a great amount in the aggregate, and it was most prob- 

 ably also a period of rapid erosion. The uplifting, however, was on- 

 equal in the different parts of the province. The comparatively even 

 floor of the old lake was deformed by broad swells and plateaus rising 

 above the surrounding country. As we shall see hereafter, the action 

 of the denuding agents is much more vigorous and efficient upon the 

 higher than upon the lower parts of a region; and consequently these 

 upswellings at once became the objects of special attention from the de- 

 stroying forces and were wasted more rapidly than the lower regions 

 around them. Here were formed centers or short limited axes, from 

 which erosion proceeded radially outwards, and the strata rising gently 

 towards them from all directions were beveled off. Thus were formed 

 those areas of maximum erosion, already spoken of, and of which the 

 San Rafael Swell is the most perfect and simplest type. 



