72 C. K. KEYES INTERMONT PLAINS OF THE ARID REGION" 



the other great through-flowing streams of the region. The course of the 

 Colorado river needs to be considered in the light of its development 

 under conditions of an arid climate rather than that of a normal humid 

 one. 



SURFACE OF THE COLORADO PLATEAU 



For the most part the surface of the great Colorado plateau in north- 

 em Arizona is a stratum plain composed of hard Carbonic limestones. 

 Around the margins on all sides younger strata are found. It has been 

 generally regarded that this surface is essentially a plain of denudation 

 that was formed at baselevel. Whatever may have been the original 

 character of this surface, it appears now that it has been profoundly 

 wind-swept. It may be that it was almost entirely leveled by eolian 

 action. 



The thick resistant limestones of Carbonic age that usually constitute 

 the surface of the great dome are not broken by faults, as in the regions 

 surrounding. The limestone is the same which forms the foundation of 

 many of the tilted mountain blocks beyond the confines of the plateau. 

 The latter is thus not subject to the influences of differential erosion, 

 whether of wind or water. 



The rock-floor of this great plateau is in many areas scarcely covered 

 by sufficient soil to support a scant vegetation. The Santa Fe railway 

 west of Gallup traverses many miles of level country, into which soft 

 materials had to be brought for long distances to construct a grade that 

 would obviate rockwork. The canyon Diablo is excavated out of a rock 

 stratum plain as if graven with a gigantic chisel. 



The rock-floor of the plain of the Colorado plateau is often bared for 

 scores of miles. The consideration of its origin from the viewpoint of 

 leveling without baseleveling is fraught with unusual interest. 



BOLSONS OF NORTHERN MEXICO 



Many of the intermont plains of the northern part of the central table- 

 land of Mexico have very shallow soil coverings. In all observed cases 

 the rock-floors are an even plain beveling much disturbed strata. Many 

 of the bolsons through which the Mexican Central railway passes display 

 at many points the rock-floor under only a few inches or a few feet of 

 loam and debris. 



On the west side of the Sierra Madre, McGee^ has called attention to 

 certain peculiarities of the rock-floor of the vast plain of Sonora. He 

 says: 



i 



' Bulletin of the Geological Society of America, vol. 8, 1897, p. 90. 



