toe ye a SS Soe) ee TL ie ged ae = een ae Cee 
J. LeConte—Structure and Origin of Mountains. 97 
greatest, viz: on the side of the steeper slope. Such a fault is 
ideally represented in figure 2. 
But very few great mountain ranges belong to this simpler 
type. Perhaps the best illustration which can be found is the 
Uintah Mountains. The figure given above (fig. 1) may be 
structure of this mountain. This simple structure, however, is 
complicated in a portion of its extent by a prodigious fault of 
20,000 feet on the north or originally steeper side, just where 
the bend of the strata is greatest, as in the ideal figure (fig. 2). 
Figure 8 is a perspective view taken from Powell, showing in 
3. 
Uintah Mountains—Upper part restored, showing fault; lower part showing 
the present condition as produced by erosion (after Powell). 
its upper part the restored form and the amount of slip on the 
north side, and in the lower part the amount of subsequent 
erosion, in this case, 25,000 feet in thickness. 
f the crust in the uprising region be extremely rigid, then the 
vault instead of being forty or fifty miles across, as in the case 
of the Uintah Mountains, may be a hundred or several hundred 
miles across. In such case a great plateau is formed (geanticline 
of Dana). And since an arch of such extent, whether filled or 
unfilled beneath with fused or semi-fused matter, cannot sus- 
tain itself, such elevated plateaus are peculiarly liable to fis- 
sures by breaking down of the arch, and to slips by gravitative 
adjustment of the broken parts. If such faults be of compara- 
tively recent origin, or occur in a region where erosion is ex- 
ceptionally small, then they will form conspicuous escarpments 
or even conspicuous mountain-redges in the general direction of 
4. 
a tr) 
¢ 2 © 3 ss a7 
Kaibab plateau. 
Kanab plateau. 
East and west section, across plateau region north of Grand Cafion. 
Section east and west across a portion of Utah. (After Howell). . 
the axis of the uplift. Such is evidently the origin of the 
north and south escarpments of the plateau-region described by 
