24 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
the stream consequent on the faulting seems never to have been tempted 
from its original course. 
The high floor of the Toroweap must certainly be referred to the 
floods of lava that have been poured into it, as was suggested in my 
previous essay (b, p. 189) and not to weakening of stream action due to 
change of climate. The deep incision of several neighboring lateral 
canyons of no larger or even of smaller drainage area than the Toroweap, 
but unobstructed with lava, leaves no room for doubt on this point. 
The same explanation must apply to the high floor of the South 
Toroweap valley, for although the lava there is of relatively small area, 
it is precisely in the right situation —at the mouth of the valley — to 
be most effective in retarding erosion. It is the breadth of the two 
Toroweap valleys that at first seems abnormal ; but I am convinced that 
the difficulty here is more apparent than real. The breadth of these 
valleys, like that of the esplanade, is practically independent of the 
depth to which erosion has proceeded beneath the esplanade sandstone. 
The esplanade level once being reached by the valley stream, the widen- 
ing of the valley goes on rapidly because the weak lower Aubrey beds 
waste easily and sap the upper Aubrey cliff at the top of the valley walls. 
The upper walls of the main canyon would not be less widely separated 
than they are to-day if during the actual lapse of time in the canyon 
cycle, the river had never cut the inner canyon below the esplanade : 
the Toroweap valleys would be hardly wider than they are even if their 
streams had cut down to grade with the main river, without hindrance 
from lava floods; good warrant for the latter inference being found in 
the rough equality of width between the upper Aubrey cliffs in the 
deep Cataract canyon (a southern branch of the Colorado) and in the 
shallow Toroweap (see Dutton, b, Atlas sheet VIII). 
It may be noted in passing that at a distant future stage of erosion, 
the existing re-entrants in the Aubrey cliffs on the west side of the 
Toroweap, down which the great lava flows have cascaded from the 
cones on the Uinkaret, will be converted into salients ; for the Aubrey 
buttresses between the lava cascades will pretty surely weather back 
faster than the cascades themselves, inasmuch as the cascading lavas 
cover the weak lower Aubrey beds on whose weathering the retreat of 
the strong upper Aubrey so largely depends. In that distant future 
time, this district may therefore be expected to present, in horizontal 
plan, an exemple of inverted relief, — cliff re-entrants changed to cliff 
salients, — with which we are so familiar in vertical profile where 
ancient lava-flooded valleys have been transformed into existing lava- 
capped table mountains, 
