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 i-ather 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 (c, 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. 290). 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. 



The Esplanade. — 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 ana Uinkaret sections, as 

 seen from Vulcan's throne at the mouth of the Toroweap valley, the 

 Red- wall platform is greatly widened ; it becomes a broad floor, stretch- 



