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 

 Bmall 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 (Dixtton, c, 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, c, 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 I have used this name for a valley corresponding to the Toroweap and appar- 

 ently structurally continuous with it but south of the canyon. 



