180 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
walls, and the spurs between these cirques should be acutely sharpened 
cusps. Cirque-like curves should occur in the rim of the plateau only 
where no significant area sheds drainage from it, and notched re-en- 
trants should occur in the high-level cliffs chiefly at the head of side 
canyons where back-country drainage is delivered to them. As a conse- 
quence of all this, a rather systematic relation should frequently be 
found between the two sets of forms; the curved re-entrants of the 
higher cliffs should frequently stand back of and above the sharp re-en- 
trant notches of the lower cliffs; while the sharp spurs of the upper 
cliffs should project forward along the axis of the rounded spurs of the 
lower cliffs. 
As far as this scheme was tested on the ground, it seemed to give 
reasonable explanation to a good number of examples; but unfortu- 
nately it was not reduced to formal statement until after leaving the 
canyon; hence, as so often happens, the observations made on the 
ground were less critical than they might have been if they had been 
immediately accompanied by analysis. 
The niches of the massive Red-wall limestone, described but left un- 
explained by Dutton (e¢, p. 260, Plate XLI.) seem to exemplify a spe- 
cial case of a problem that McGee has discussed in connection with the 
“origin and hade of normal faults” (a, p. 296). The niches all occur 
over the heads of subordinate ravines or gulches, and are, therefore, to 
be associated with the sapping by the underlying weaker strata and the 
falling away of the basal part of the massive limestone. In the absence 
of numerous planes of bedding and jointing, the upward breaking of the 
rock may be compared to the upward propagation of a fault ; and McGee 
shows that in such case the fracture must be a curved surface with 
decreasing hade upwards, so that the broken face may eventually be- 
come vertical or even overhanging. The overhanging arch by which the 
niche is covered seems to correspond to the upper part of such a fracture. 
Tue Espuanape. — Although the Kaibab and the Uinkaret are only 
forty miles apart, the canyon in these two plateaus exhibits very unlike 
cross-profiles. In both, the double cliffs of the upper Aubrey are re- 
peated with similar outlines. In the Kaibab, a platform of moderate 
width is worn on both the Red-wall group and the Tonto sandstone, the 
lower one being rather wider than the upper, and the two being sepa- 
rated by the huge Red-wall cliff and the long gray waste-covered slope 
of Tonto shales beneath it. In the Kanab and Uinkaret sections, as 
seen from Vulean’s throne at the mouth of the Toroweap valley, the 
Red-wall platform is greatly widened ; it becomes a broad floor, stretch- 
