DAVIS: THE GRAND CANYON OF THE COLORADO. 191 



dred feet deeper than its present floor, and was subsequently built up 

 by many floods of basalt coming from the cones on the Uinkaret and by 

 considerable quantities of alluvium washed from its cliffs and overlook- 

 inw mesas" (c, p. 92). We may therefore conclude that in the early 

 stages of the can} r on cycle, when the intermittent side streams had cut 

 down their canyons into the Red-wall group and the canyons had al- 

 ready widened somewhat through the sapping of the upper Aubrey cliff- 

 makers by the weak red beds of the lower Aubrey, the Toroweap was 

 flooded with lava, while most of its neighbors remained free from vol- 

 canic interference. The streams in the latter continued to deepen their 

 courses, while deepening was practically stopped in the Toroweap. But 

 just as the main canyon has widened above the resistant floor of the 

 esplanade, so the Toroweap continued to widen, hence it is to-day nor- 

 mally broad but abnormally shallow. It is a " hanging valley " because 

 of local volcanic interruption of the normal work of canyon-cutting. 



Directly opposite the Toroweap is a similar high-floored valley 

 (c, p. 99), splendidly exhibited from Vulcan's throne, and already re- 

 ferred to as the South Toroweap. Like its northern fellow, it is floored 

 with lava near the main canyon, on which it opens close to the es- 

 planade level (Dutton, c, Plate XVIIL). Hence its deficiency of depth 

 again seems to be best accounted for by the difficulty of wearing away 

 its lava sill, an accidental and purely local detail, rather than by a 

 failure of rainfall, which must have been general. 



Dutton mentions several other high-floored valleys, which he 

 classes with the Toroweap as indicating a decrease of rainfall at the 

 beginning of or early in the Pliocene (canyon cycle). One of these is 

 the Queantoweap, which follows the Hurricane fault along the boundary 

 between the Uinkaret and Shivwits plateaus (c, pp. 99, 115). Not 

 having seen this valley, I shall not venture to express an opinion about 

 its origin, yet it may be noted that a small flow of lava near its mouth 

 is marked on the geological map (Dutton, c, Atlas, sheet VIII.) ; and 

 judging by the great lava cascades that plunge into the valley (ibid., c, 

 p. 116), some lava may lie concealed beneath the alluvium of the 

 valley floor. A third high-floored dry valley mentioned by Dutton is 

 that on the summit of the Kaibab uplift (c, pp. 194, 197, 223) already 

 discussed and explained as a series of independent anticlinal valleys, 

 and thus seeming to be within reach of explanation without recourse to 

 climatic change. A fourth example is House-rock valley (c, p. 201), 

 which has already been referred to as a normal subsequent valley worn 

 on weak monoclinal strata. Taken altogether, the lava-floored valleys, 



