142 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



the retreating cliff face approaches such a fault, the crest becomes sharp 

 and serrated, and strong eastward slopes locally replace the ordinary 

 gentle eastward descent where faults are absent. Powell noted the 

 sharp crags and relates them to the " line of displacement," but he did 

 not specify the aid that local faults would give (a, p. 192). When the 

 cliff face retreats still further, so as to remove the sandstones from in 

 front of a fault plane, the crest line of the remaining cliff sags to a 

 lower level than usual. It would be necessary to follow the crest of the 

 escarpment for some distance in order to determine the measure of 

 verity in this suggestion. 



The Western Faults. — Attention may next be given to the fault 

 lines of the Grand canyon district west of the Kaibab. These fall into 

 the class to which Dutton gave a very late date : " None of them will go 

 back of the Pliocene in age, and I think it probable that none of them 

 will go behind the middle Pliocene " (a, p. 43). " Their formation 

 seems to have been incidental to the uplifting of the platform which 

 took place about the time the present Grand Canon began to cut. . . . 

 Before this epoch we know nothing of them" (c, p. 22G). Some of the 

 faults may now be considered in more detail. 



The West Kaibab Faults. — Of the three faults indicated in Dutton's 

 map along the western border of the Kaibab, we identified only the 

 eastern one. It forms a tine wall, of generally graded slope and much 

 scored by ravines. Its date is not easily determined, as it does not 

 cross the cliff-making formations on the north until, according to 

 Dutton, it joins the east Kaibab flexure. The recession of the cliff 

 from the fault line here, as elsewhere, affords an uncertain measure of 

 the antiquity of the faulting ; for if the faulting occurred before the 

 opening of the canyon cycle, much of the height of the present cliff may 

 then have been fronted by weak Permian strata, since removed. 



The Toroweap Fault. — The next displacement west of those which 

 limit the Kaibab arch is the Toroweap fault. This is regarded by 

 Dutton as for the most part of very recent date; at least part of 

 its movement being later than the erosion of the broad-open flat-floored 

 upper part of the canyon, called the esplanade, because the floor of the 

 esplanade drops down to the west on passing the fault line; and later 

 also than the eruption of a certain lava flow that was poured into the 

 esplanade, because the lava also is dislocated by the fault (c, p. 94). 

 On the latter point, I am unable to offer any evidence ; but as the prob- 

 lem now presents itself, the dislocation of the floor of the esplanade 

 does not seem to bear on the date of the fault ; for if we regard the 



