570 PROCEEDINGS OF THE ALBUQUERQUE MEETING 



The next paper was read by title. It was 



GEOLOGIC PROCESSES AND GEOGRAPHIC PRODUCTS OF THE ARID REGION 



BY CHARLES E. KEYES 



[Abstract] 



Contents 



Page 



Introductory ^^'^ 



Relation of New Mexico to adjacent arid regions 571 



Characteristic geographic features 572 



Vastness and evenness of intermont plains 572 



Isolation of the mountains 572 



Complete encirclement of mountains by plains 573 



Characteristic absence of foothills v 5'?3 



Resistant character of mountain rocks 573 



Soft substructure of the plains 573 



Beveled rocli-structure of plains 573 



Plains character of the rock-floor itself 573 



Representation of former plains-levels by the plateau plains 574 



Normal torrential action of water in the mountains 574 



Frequent occupation of plains by lakes 574 



Marked absence of rock- weathering 574 



Remarkable thinness of surface mantle : . . . 574 



Transported nature of the surface materials 574 



Gravelly character of surface deposits largely only apparent. 574 



Tendency of mantle to make the plains even 575 



Absence of distinct waterways on the plains 575 



R51e of sheet-flood action 575 



Lack of direct evidence of former humid climate 575 



Intkodxjctoey 



The Albuquerque meeting of the Geological Society is in its history the first, 

 I believe, ever held under conditions of an arid climate. Under such circum- 

 stances attention is appropriately directed to certain features of the landscape 

 about and to some of the peculiarities of the geologic operations producing 

 them. In the normal humid climate some of these relief features would be 

 ascribed to very different causes. Under conditions of aridity the geologic 

 agencies at work modifying the facial expression of our globe operate in a 

 manner so very different from what they do in a humid land that the effects 

 can only be fully appreciated after long residence in a dry country. 



It is a remarkable fact that the arid regions have given us our two most 

 comprehensive and important generalizations regarding the evolution of the 

 geographic features of our earth. The first of these is, of course, the theory 

 of the base-level of erosion, as first propounded by Powell, after his extensive 

 explorations of the Great Basin region of western America. The second is the 

 hypothesis of the complete leveling of elevated continental areas without base- 

 leveling, as urged by Passarge, for the great interior plateau of South Africa. 

 One principle develops the geographic cycle under conditions of normal humid- 

 ity ; the other, not less significant, is the foundation for the recognition of a 

 distinct geographic cycle in a dry climate. 



