CHARACTER OF THE OLDER TECTONICS 549 



existed many actually flowing rivers. Special stress is also placed on 

 warping, folding, and faulting that "went on continuously all through 

 the Tertiary into the Pleistocene, and is even now progressing." 



Toward the northern end of the Mexican tableland, in New Mexico, 

 the ancient tectonics is well displayed. Some of the details I have re- 

 cently described^ at some length, although it is not as yet at all certain 

 that all of the thrust-planes recognized can be regarded as belonging to 

 the earlier period of deformation. On the whole, the general character 

 of the ancient tectonics is that of gentle plication in contradistinction to 

 that of the more recent structures which are principally due to faulting. 



OLDER GEOLOGIC STRUCTURES OF THE GREAT BASIN REGION 



Of the four grand provinces of the arid region the Great Basin is 

 physiographically the oldest. Only locally is the topography at all youth- 

 ful. Over most of the area the mountains are worn down to compara- 

 tively inconspicuous eminences, so low that with some exceptions forests 

 no longer cover their summits. They are described as buried up to their 

 shoulders by detritus. Irrespective of structure, they everywhere appear 

 to be made up of the most resistant rocks. 



The rocks are mainly hardened limestones, indurated sandstones, and 

 eruptives. The contrasts of distinct and regular hard and soft rock- 

 belts, such as are found elsewhere, are not so strong as in some of the 

 other parts of the arid region. Viewed in the light of a distinct geo- 

 graphic cycle for an arid climate we have to look to mountain-making 

 agencies other than deformation and dislocation for an adequate and 

 satisfactory explanation of the present configuration of the individual 

 Basin ranges. 



Although mountain ranges of the Great Basin province display both 

 gentle folding and profound faulting, the main effects of these have been 

 manifestly long since mastered by vigorous desert-leveling, as Spurr has 

 well urged. The most pronounced flexing and most of the faulting is 

 relatively old — pre-Tertiary at least. Nowhere does there appear to be 

 any genetic relationship between the rearing of the mountains and these 

 foldings. The present mountains, therefore, seem to owe their existence 

 not so much to general flexing or to recent and profound faulting as they 

 do to the differential effects of true desert-leveling, in which the most 

 resistant rocks longest retain the highest elevations. 



Critical inspection of the various diagrams of the Basin ranges, as 



' Journal of Geology, vol. xlii, 1905, pp. 63-70. 



XXXIX— Bull. Geol. Soc. Am., Vol. 21, 1909 



