COMPARATIVE EECENCY OF FAULTS. 35 



portions of some of the faults did not occur simultaneously, or, perhaps 

 more properly, at the same rate of progress. There is evidence that some 

 portions of a fault progressed through intervals of alternate repose and 

 activity. But while the entire Tertiary history of this district, or at least 

 that portion of its history since the Eocene, was marked by the recurrence 

 of disturbing forces here and ihere, there is one period wliicli appears 

 to have been pre-eminently a period of faulting and uplifting, standing out 

 conspicuously as a culminating period in the movements. It was this period 

 which more than any other gave, not indeed birth, but certainly the maxi- 

 nuim growth and expansion to the structural features of the district. Tliis 

 period was a comparatively recent one. To name it in terms of the ordi- 

 nary geological calendar would probably convey the impression that the 

 means of determining and correlating the ages of events occuiTing within 

 the district with reference to those occurring outside of it are greater than 

 they really are. Since the middle Eocene all direct connection of the Ter- 

 tiary history of the Plateau Province with external regions ceases. Since 

 then everything is relative. The order of sequence is plain, but so far as 

 time is concerned we are out of sight of stars and landmarks, and run through 

 the succeeding periods only by dead reckoning. The next age which 

 we can fix after the Eocene is the Glacial period. We recognize high up 

 in the plateaus and mountains the traces of local glacial action, and it has 

 the same general traces of geological recency and historic or prehistoric 

 antiquity as elsewhere. But between these two ages we are conscious only 

 of a vast stretch of time, in which great results were accomplished in a 

 cei-tain definite order. Each individual feature in that progressive evolution 

 was one which by its very nature required long periods to accomplish, and 

 the last of them all was the great uplifting and fracturing of the rocks which 

 had previously accumulated, 



I place the age of the principal displacement in a period which had its 

 commencement in the latter part of Pliocene time, and extended down to an 

 epoch which, even in a historical sense, may not be extremely ancient, and 

 which certainly falls on this side of the Glacial period. Perhaps it is still 

 in progress. Perhaps the plateaus are to-day growing higher and the faults 

 increasing their shear. But the beginning of this last period of faulting. 



