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. 
Tue WesterN Fautts. — 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 Cafion began to cut... . 
Before this epoch we know nothing of them” (c¢, p. 226). 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 fine wall, of generally graded slope and much 
scored by ravines. Its date is not easily determined, as it does not 
cross the cliffmaking 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 autiquity 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 (¢, 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 
