| OCR Text |
Show COMPAEATIVE EECENOY 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 there, there is one period which 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 maximum growth and expansion to the structural features of the district. This period was a comparatively recent one. To name it in terms of the ordinary geological calendar would probably convey the impression that the means of determining and correlating the ages of events occurring 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 Tertiary 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 onhy, of a vast stretch of time, in which great results were accomplished in a certain 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, |
