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



arid cycle and the attainment of the mature stage into which that region 

 is just about to enter the broad belts of weak Cretaceous rocks have been 

 removed to depths of 5^000 feet and over. If at the beginning of the 

 cj^le of aridity the original surface were a plain, as there appear to be 

 strong reasons^* for believing, the present lofty mountain ranges must 

 have differentially developed through the more rapid deflation of the 

 belts of weak rock now forming the areas of intermont plains ; for, as is 

 well known, the stratigraphy of the region is remarkable in that the re- 

 sistant rocks are mainly segregated in the lower part of the geologic 

 column and the weak rocks are confined to the upper portion. 



As the mountains rear their forms more and more above the general 

 plains surface, while the latter is being gradually lowered through defla- 

 tion, they finally become local rain-provokers of some small influence. 

 During the period of arid youth the streams developed on the mountain 

 slopes become slowly larger and larger and longer and longer until now, 

 as the region is about to enter into its maturity, they attain their maxi- 

 mum size and efficiency. The mountains are now their loftiest, their 

 sides are steepest, into them the intermont plains are encroaching deep- 

 est. The moisture gathering about them is greater in amount than at 

 any time before or than will be afterwards. The mountain watercourses 

 reach their greatest extension notwithstanding the fact that they carrs'- 

 relatively little water, are intermittent in character, and their lower 

 reaches seldom pass beyond the foot of the ranges. Instead of being 

 headwater remnants of extensive stream systems which have long since 

 withered away under the influences of arid climate, as is a necessary 

 consequence of the adapted normal cycle hypothesis, they must be re- 

 garded as original streams coming into being as the differential relief 

 effects of regional deflation became more and more pronounced. With 

 the advancement of physiographic maturity these streams must begin to 

 wither, and as senile relief approaches they must with few exceptions 

 undergo complete obliteration. 



It is the custom to consider all water action upon the desert ranges as 

 normal stream erosion in the process of dissecting recently upraised 

 orographic blocks. This hypothesis seems to fall at once when it is con- 

 sidered that the major faulting of the mountain blocks is, as already 

 stated, mainly verv' ancient, and not modem, as it has been so long as- 

 sumed to be. 



Certain effects of general deflation have greatly contributed to impart- 

 ing to the mountain sides the infantile aspects of stream work. As re- 

 cently suggested,^^ the locus of maximum lateral deflation in the desert 



6*Proc. Iowa Acad. Scl., vol. xiii, 1908. p. 221. 

 *5 Science, n. s., vol. xxix, 1909, p. 753. 



