558 C. R. KEYES THE GEOGRAPHIC CYCLE IN AN ARID CLIMATE 



ing topographic aspects of the landscape. A generalized cross-section, 

 based on the geologic structure of the Sierra de los Caballos, in Xew 

 Mexico, indicates the common relations of relief and tectonics (figure 

 1). The perfect independence of the two are fully discussed in another 



„ 10000 A.rr . 



Bedded Pywoastics 



AND RHYOUrt 



Flows 



Figure 1. — Passing of arid Youth: Rim of a Desert Basin 



place.®^ Another good example of the final mural ridge is that of the 

 Palomas range, in southwestern Arizona, standing above the main moun- 

 tain block more than 1,000 feet (figure 2). 



After the youthful stage here repre- 

 sented the upper remnantal portion of 

 the mountain block is rapidly removed 

 and reduced to a low, rounded mound 

 projecting but slightly above the level 

 of the general plains surface. Trans- 

 formation from youth to old age is 

 quick, decisive, complete. The appar- 

 ently graded plains on either side of 

 the old ridge gives it the aspect of a 

 worn-down mountain buried to its 

 shoulders in the waste of its own sub- 

 FiGURE 2.— Approach of arid Maturity: gfances. In the casc of the Caballos, 



Last of a Desert Range . 



already mentioned, the rock-floor of the 

 plains is not confined merely to the higher parts of the piedmont slopes, 

 as has been explained by Davis,®^ but extends for 30 miles across the 

 intermont plain of the Jornada del Muerto to the next desert range to 

 the east — the Sierra San Andreas.^^ 



^•^Bull. Geol. Soc. America, vol. 21, 1910. p. 543. 



«8 Journal of Geology, vol. xiii, 1905, p. 387. 



«" U. S. Geol. Survey Water Supply and Irrigation Papers, No. 123, 1905, p. 12. 



