COURSE OF THE COLORADO RIVER 71 



Sacramento valle}'', is deeply "undeiiain with recent gravels. My own 

 observations in the same valley north of the Colorado river immediately 

 lead me to raise the question as to how much of the soft deposits occupy- 

 ing the valley are those of Tertiary lake beds and how much those of later 

 wash materials. The solution of this problem might very greatly modify 

 the various views expressed. It is known that a few miles north of the 

 Colorado river, on the west side of the south-flowing Virgin, in the can- 

 yons of Muddy river and the Meadow wash, very thick Tertiary beds are 

 exposed. Were one to drill a well on the top of the mesa or smooth, even 

 surface of the interment plain, he might penetrate soft deposits consider- 

 ably more than a thousand feet, and yet less than a hundred feet would 

 be recent wash gravels. 



The Lower Colorado Eiver region has been regarded as lately covered 

 by the sea. According to this notion, all of the plains must have been 

 formed by depositions in a quiet sea that was thickly studded with 

 islands — the present isolated mountains. Siich an hypothesis must have 

 very much stronger evidences to support it than it now has before it can 

 be even considered as a possible explanation of the phenomena presented. 



The Colorado river is not a typical watercourse of the arid region. 

 Being a large, perennial, throiigh-flowing stream, its work and evolution 

 are those of a river of the humid regions. It rises in the highest 

 "Rockies" and reaches the sea. Of the same class of great streams are 

 the Rio Grande, the Rio Pecos, and the Canadian river of New Mexico. 

 Typical geographic development in an arid climate goes on without the 

 drainage regularly reaching the sea. A very clear distinction must there- 

 fore be made between the materials composing the waste mantle in and 

 near the valle3's of these streams and those commonly met with in the in- 

 termont or bolson plain. 



Lee* notes that the Colorado river "in selecting its course seems to 

 have shown little consideration for the easiest lines of erosion. It has 

 disregarded mountain and valley alike. From casual observation it 

 would seem to have chosen about the roughest course possible." If we 

 consider this river as having cut its channel down from the surface of an 

 old baseleveled plain, Lee's inferences might be sustained. If, however, 

 the stream has followed the lowest line of short intermont plains, an en- 

 tirely different interpretation must be held. If the intermont plains are 

 thinly floored with wash deposits and are mainly the product of general 

 leveling without baseleveling, the explanation of the great channelway 

 through short rock-bound canyons connecting one open plain with an- 

 other is not difficult. The case is identical with those of the vallevs of 



8 Loc. clt., p. 277. 



