36 GEOLOGY OF THE HIGH PLATEAUS. 



whether the period is closed or not, goes, I believe, only back into the late 

 Pliocene. These faults are so important not only to the history of the High 

 Plateaus, but also to the general history of the Plateau Province at large, 

 that it seems proper to enter at some length upon the considerations which 

 have led to this opinion concerning their age. 



Kecognizing- the great magnitude of the results accomplished in this 

 region by erosion since the Eocene, we are naturally led to inquire whether 

 we may not here and there gain some conception of the relative ages of cer- 

 tain events by ascertaining the amount of erosion which has been effected 

 since their occurrence. The laws of erosion, both generally and in their 

 somewhat abnormal application to this strange region, are sufficiently un- 

 derstood to enable us to decide where erosion ought to be most rapid and 

 where most sluggish. Of all portions of the Plateau Province the best 

 watered is the District of the High Plateaus. It is also the loftiest, and gives, 

 therefore, to its water-courses the swiftest descents and the greatest trans- 

 porting power. On the other hand, its rocks are the hardest and most dura- 

 ble. Thus the altitude and copious rainfall favor a rapid rate of erosion, 

 while the greater durability of the rocks retards it. Not all of the rocks, 

 however, are of this adamantine character. Indeed, some of the most 

 voluminous formations are conglomerates, some well consolidated, but most 

 of them only moderately so. Around the borders of the district are the 

 sedimentaries, differing lithologically in no material respect from those of 

 the province at large. By comparing the effects of erosion in rocks of dif- 

 ferent classes similarly situated we find great irregularities, but so far as can 

 be seen these irregularities are due chiefly to the relative durability of the 

 rocks. The sedimentaries are most powerfully eroded, and clearly disin- 

 tegrate far more rapidly than the volcanics, and considerably more so than 

 the conglomerates. There is seldom difficulty in distinguishing the erosion 

 which has occurred during or since the faulting from that which may have 

 occurred before it ; and when we first separate this erosion from the earlier 

 we find that in the sedimentaries it is very considerable. Vast ravines have 

 been scored and deep canons cut into the risen blocks. The fronts have been 

 battered and scoured by the storms of unknown millenniums and pared off 

 until they stand back of the fault-planes which mark the rifts where they 



