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 clitfs and overlook- 
ing mesas” (c, p. 92). We may therefore conclude that in the early 
stages of the-canyon 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 ” becanse 
of local volcanic interruption of the normal work of canyon-cutting. 
Directly opposite the Toroweap is a similar high-floored valley 
(ec, 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 XVIII.). 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 detajl, 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 (2bzd., 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 (¢, 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, 
