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



rounded aspect. In a dry climate, under ordinarily favorable conditions 

 for deflation, the same general effects hold true, only in a somewhat 

 different way. Of the jDositive relief features only the basinal rims can 

 be properly considered. In place of the floor of each basin being intri- 

 cately dissected by a ramifying system of more or less deep stream val- 

 leys, the effects produced in an arid country are as if all valleys of the 

 more familiar lowlands of a humid land were everywhere filled. In 

 order to picture more vividly the broader physiographic features of 

 moist lands, this very procedure is indeed fancied. 



With the ideal conception, facts ascertained for the northern Mexican 

 tableland, for instance, seem fully to accord. Hypsometric differences 

 of a mile are not unusual, and these are made possible by the great thick- 

 nesses of weak rocks brought by profound faulting into juxtaposition 

 with extensive hard masses. In this region after the removal of the 

 enormous thicknesses of soft materials from the broad belts of weak 

 rocks the sharp ranges of hard mountain rock appear to be just begin- 

 ning to have their summits notably worn off. 



The effect of unlike initial tectonics on the arrangement of the local 

 relief forms at successive stages is pointed out by Davis.** Contrasting 

 a relief of coarse pattern with that of finer type, the region of central 

 Asia is compared with that of western America. In the case of the first 

 the vast even plains of eastern and western Turkestan are separated by 

 a single broadly, uplifted mountain belt; in the arid region of south- 

 western United States many short lofty ranges stud the general plains 

 surface. 



IDEAL TYPE OF EARLY ARID STAGE 



The Great Basin, oftenest drawn on in illustration of the youthful 

 stage of relief under influences of aridity, appears to represent a devel- 

 opment of a considerable later type of desert topography. The facial 

 expression of the northern Mexican tableland and of the desert regions 

 of southern Arizona and of Sonora seem better to display what is to be 

 expected of the features of this stage after exposure to the protracted 

 arid conditions. With the deflative idea in mind, this region has re- 

 ceived more attention than any other in this country. Moreover, its 

 deformatioual periods are definitely fixed. 



Throughout this broad area the alternation of hard and weak rock- 

 belts is of the fine-pattern type. On this account features are presented 

 which enable critical determinations to be made. In one respect the 

 entire region is exceptionally peculiar; the resistant terranes are all 

 segregated at the bottom of the stratigraphic column and the weak rocks 



^ Journal of Geology, vol. xlil, 1905, p. 384. 



