DAVIS: THE GRAND CANYON OF THE COLORADO. 189 
the beginning of the Pliocene (canyon cycle) of certain branches of. the 
Colorado, whose previous existence implied the continuance of a good 
water supply, just as their later disappearance implied its cessation 
(c, pp. 99, 201, 223) ; the arid climate thus introduced continued to 
the present day, except for the occurrence of an inferred pluvial period 
corresponding to the glacial period elsewhere. 
Other interpretations seem to me possible for several of the facts here 
brought forward. I have elsewhere presented some considerations re- 
garding the possible explanation of at least some of the fresh-water Ter- 
tiary deposits of the Rocky mountain region as fluviatile rather than as 
lacustrine formations (6, p. 360), and in so far as this alternative explana- 
tion is found applicable, the evidence above quoted for the humidity of 
the plateau period will weaken. The argument fora change from humid 
to arid climate, based on the apparent disappearance of certain streams, 
seems to me open to serious question ; it is considered in the next sec- 
tion. The ravines that are adduced by Dutton as the evidence of a 
brief Pleistocene pluvial period appear to be open to explanation under 
as arid a climate as prevails to-day, as will be specially considered in 
the next section but one. Thus the climatic history of the district 
becomes extremely uncertain. Whatever conclusions are reached as to 
the climatic conditions of any period of past time — the glacial period, 
for example —in surrounding regions may be permitted for the Grand 
canyon district, but no special climatic conditions are demanded by 
local evidence. Yet among the several opinions quoted above, the long- 
enduring arid climate suggested by Powell seems to me the most prob- 
able, because for a long time the Grand canyon district has lain to 
leeward of a mountainous area, and during much of this time the dis- 
trict stood lower than it does now, while the mountains to windward 
were higher. 
Tue TorowEar.— The Toroweap (T, Figure 1) has already been 
referred to as a valley that has been eroded along or near the southern 
part of a long fault-line that comes from the northeast past Pipe spring ; 
it is part of the boundary between the Uinkaret and Kanab plateaus. 
The peculiar feature of the valley now in evidence is its shallowness ; 
for its broad floor leads forward to the level of the esplanade and shows 
in its surface form hardly any sign of incision to a deeper level. Dut- 
ton infers from this that the stream by which the valley was originally 
eroded became extinct about the time of the latest upheaval of the 
region, after which the inner canyon was cut down beneath the espla- 
nade by the main river, whose water supply in distant mountains had 
