PLIOCENE AGE OF THE KAIBAB FAULTS. 
191 
It will aow become apparent why we infer that at the commencement 
of the Pliocene by far the greatest part of that great denudation which 
swept away the Mesozoic beds had been accomplished. Before that epoch 
(early ^Pliocene) a moist climate prevailed for an immense stretch of geo- 
logical time — the whole of the Miocene and most of the Eocene — and the 
region was rising. Since that epoch an arid climate has for the most part 
prevailed, and the period of its prevalence has been much shorter. We 
cannot doubt that the longer duration of a more efficient cause must have 
produced a far greater erosion than the much shorter duration of a less 
efficient cause. The exact ratio of the results produced in the two dura- 
tions, respectively, cannot indeed be determined; but we cannot well avoid 
the conclusion that the disparity was very great. 
The reason for believing that at the beginning of Pliocene time the 
district was at a much lower level than at present is equally forcible. The 
East Kaibab monocline and the Grand Wash fault, which bound the Grand 
Canon district, cannot have originated at an older epoch than the one in 
question. They mark the difference between the amount of final elevation 
of the district and of the regions on either side of it. It is true that a fault 
may indicate either an absolute uplift on one side or a downthrow on the 
other; Which interpretation shall in any given case be adopted turns upon 
collateral facts. In the present instance thei’e is no reason for hesitation. 
The Grand Canon district is obviously an uplift between the two above- 
mentioned displacements. The other view raises insuperable difficulties at 
once. It would require us to believe that the whole country extending in- 
definitely east of the Kaibab had once been several thousand feet higher 
than at present, and subsequently subsided without leaving any evi- 
dence of such an event; and the same inference would be applicable to the 
region extending from the Grand Wash indefinitely westward. Such arbi- 
trary and needless assumptions are not worthy of consideration. The only 
rational conclusion left us is that the Grand Canon platform has been raised 
since the Miocene by an unknown amount, though a part of that amount is 
directly indicated in the displacements now observable on either side of it. 
The reduction of these displacements to their original condition would 
diminish the altitude of any given stratum from 2,500 to ." ,500 feet upon the 
