40 GEOLOGY OF THE HIGH PLATEAUS. 



streams sink their channels and every additional yard of deposit will be 

 accumulated at a slower rate. 



It was the uplifting along great lines of dislocation which set this cone- 

 building process going. The .abrupt descents gave the creeks and brooks 

 their power to transport this coarse debris, and those slopes are now long 

 and steep. But as the work proceeds the mountains and tables are gradu- 

 ally rounded and smoothed down and the valley plains built up. As yet 

 comparatively little has been accomplished in this direction, but the work 

 is under full headway. In comparing what has been effected since the 

 beginning of the displacements with work of the same character which 

 has been accomplished in ages prior to the displacements, we shall be most 

 forcibly impressed with the littleness of the one and the greatness of the 

 other. It is a comparison of hundreds with thousands. More than that: 

 the hundreds of feet of modern valley cones represent the utmost activity 

 of a process which has worked without interruption and under conditions 

 the most favorable, while the thousands of feet of ancient accumulations 

 represent the same process in all degrees of activity, now intense, now fad- 

 ing and dying out, and then probably long intervals of cessation. 



Thus, whether we view the denudation of the High Plateaus or the 

 accumulations in the valleys at their bases, we reach the same conclusions. 

 The faults are very late occurrences in the history of the district. But when 

 we come to ask what is the age, in terms of the geological chronology, to 

 which they must be referred, we can give no further answer than this: they 

 belong to a very late one. There is no record of Miocene or Pliocene in 

 this disturbed region, and we have nothing to mark the lapse of time, except 

 relatively, since the close of the Eocene. But in other parts of the world, 

 where we have some knowledge of the strata, we infer that the Miocene 

 was a longer age than the Pliocene and the Pliocene longer than the Qua- 

 ternar}-, though these are impi'essions rather than conclusions, and to be 

 held lightly. Judging, however, by the magnitude of results accomplished 

 by erosion in the High Plateaus since the faults were started, and compar- 

 ing these results with similar work accomplished in other localities, and 

 taking into the account the conditions under which they were accomplished, 

 it seems perfectly safe to say that if we carry back the faulting to the mid- 



