256 J. E. SPURK ORIGIN AND STRUCTURE OF THE BASIN RANGES 



many of the most pronounced scarps are along no fault line, while many 

 heavy faults have absolutely no direct effect on the topography. The 

 theory appears, then, in general untenable. Indeed, the writer holds that 

 in the erosion of rocks of unequal resistance scarps are natural and in 

 many cases inevitable. Where not only the rocks but the distribution 

 of eroding agents is unequal the inequality of the resultant topography 

 is still greater. Thus in arid regions, where the general erosion is slight, 

 whatever temporary or permanent lakes and streams exist undercut the 

 bases of the otherwise comparatively stable mountains. Major Powell* 

 speaks of the cliffs of the Colorado i)lateau as being " carried back for 

 great distances by undermining, which is a process carried on only in 

 an arid region." In the Great basin, mountain springs of comparatively 

 insignificant volume have succeeded, by virtue of their monopoly of ero- 

 sion, in eroding deep canyons with perpendicular walls, wliile the adja- 

 cent channels of occasional drainage are shallow valleys. In past times, 

 especially during the Pliocene, great lakes have fretted the bases of the 

 numerous mountain ranges which rose above their waters, so that the 

 continual tendency has been to cut them back. 



Relative Ascendency of Ekosion and Deformation 



The greater i)ortion of the Basin ranges already described have been 

 found independently in each case to be ranges of erosion. Deformation 

 seems to have been slower than erosion, so that in the long run it has gen- 

 erally been outstri])ped. Yet it has often l^een more spasmodic, so that 

 for limited periods it has held its own. Thus there are folds and faults 

 which are directly expressed in the topogra])hy. These, however, are 

 exceptional, as tiie}^ are in most [)rovinces, and excite special attention. 

 One may find similar phenomena in other mountain regions, such as 

 the Rockies. 



Relative Ascendency of Folding and Euosion 



the synclinal ridge of erosion 



The most common mountains are those which possess synclinal ridges ; 

 in these erosion has overbalanced deformation ))y folding (see plate 24, 

 figure 5, and plate 25, figures 1 and 2). 



THE ANTICLINAL RIDGE OF EROSION 



The next commonest class comprises anticlinal ridges which offer evi- 



* Explorations of the Colorado River, pp. 69-72. 



See also C. E. Dutton : Monograph ii, U. S. Geol. Survey, pp. 222, 227. 



