190 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
not. been cut off (c, p. 99). There are two arguments against this view. 
First, a number of side canyons in the neighborhood of the Toroweap, 
of similar or smaller drainage area and equally dry, are cut down to 
essentially accordant junction with the bottom of the main canyon. A 
small valley of this kind is seen to enter the main canyon from the 
south at a short distance west of the South Toroweap ;1 a view of its 
upper part is included in a drawing made by Holmes to illustrate the 
dikes in the canyon wall (Dutton, ¢, Plate XVIII.). Although the 
drainage area from which the wet-weather stream is here gathered is 
but a small fraction of that which supplies the floods of the Toroweap, 
this short side valley is cut deep below the esplanade level for several 
miles back from the inner canyon wall, and as its stream line descends 
with the steep grade appropriate to short lateral canyons, it makes an 
essentially accordant junction with the Colorado. A view of the inner 
canyon from near Vulcan’s throne (Dutton, c, Plate XVII.) has al- 
ready been referred to as showing the accordant entrance of two side 
canyons into the main canyon in the neighborhood of the Toroweap. 
The same relation obtains in the case of an unnamed side canyon, com- 
ing from the north twelve miles east of the Toroweap : its drainage area 
is similar to that of its high-floored neighbor, yet it is cut down so that 
its temporary floods may join the main river in perfectly accordant 
fashion, as far as one may judge from the topographic map of the local- 
ity. Many other examples of this kind might be given, as could be 
inferred from what has already been said in a previous section as to the 
generally accordant junction of side and main canyons. It, therefore, 
does not seem possible to ascribe the failure of erosion in the Toroweap 
to the desiccation of a once permanent stream ; for in that case all the 
neighboring streams in small side canyons must also have been desic- 
cated, and should have high floors ; yet as a matter of fact their streams 
have not ceased to erode effectively. 
Secondly, some local obstacle to erosion in the Toroweap would suffice 
to explain its peculiar form, and such an obstacle certainly occurs there, 
for the broad floor of the valley has been heavily and repeatedly sheeted 
over with floods of resistant lava, supplied by the magnificent lava cas- 
cades from the Uinkaret, as pictured in one of Holmes’s most effective 
drawings (Dutton, ce, Atlas, sheet V.). The occurrence of the lava 
flooring is fully recognized by Dutton, who wrote: “There is reason to 
believe that at some prior epoch it [the Toroweap] was cut a few hun- 
1 T have used this name for a valley corresponding to the Toroweap and appar- 
ently structurally continuous with it but south of the canyon. 
J 
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