556 C. R. KEYES PROFILES AND STRUCTURES IN DESERT RANGES 



with thick lava-flows. The lavas are presumably early Tertiary in age. 

 Through frequent and extensive faulting, old continuous fields of erupted 

 rocks are now disposed as broken and apparently tilted blocks, as in the 

 case of the sedimentaries in other parts of the arid region. Little of the 

 Paleozoic and Mesozoic strata is now displayed anywhere within the prov- 

 ince. Where the old formations have been observed, as in the Prescott 

 Forest Eeserve, the Paleozoics show marked deformation. 



Although the mountain ranges of this province rise higher and more 

 sharply above the plains-surface than they do anywhere else in the desert 

 region, so very little of the older tectonics is open to inspection that from 

 the visible ancient structures alone it would be difficult to say just how 

 much flexing has had to do with mountain rearing. It is, however, man- 

 ifest that whatever flexing is observable, it is quite ancient, and that 

 there is little or no effect apparent from recent folding. The 3,000 to 

 5,000 feet of section which the mountains expose above the plains are 

 largely made up of the thick lava-flows and the interbedded tuffs. The 

 only geologic structures which can have any close relationship with the 

 rearing of the mountains is profound faulting. 



All evidences in this province seem to point conclusively to the fact that 

 the mountains for the most part at least are true residuals, fashioned 

 perhaps from faulted blocks, under conditions of general desert-leveling. 

 McGee's^^ observations in the Sonoran district of northern Mexico also 

 furnish incontestable testimony in support of this suggestion. 



OLDER GEOLOGIC STRUCTURES OF THE MEXICAN TABLELAND 



Several factors contribute to make the Mexican tableland a region 

 especially instructive in the present connection. In comparison with the 

 other three provinces mentioned there has been less volcanic action. The 

 topography is much newer. The mountains appear to be more strictly 

 of the ideal Basin-range type. There are clear and abundant evidences 

 of ancient and profound faulting by which the mountains were originally 

 l)locked out. There are positive proofs that late folding has not con- 

 tributed to any appreciable extent to the rearing of the ranges. The 

 flexing and most other features accompanying great compressive action 

 are clearly shown to have been already in existence in mid-Cretacic times. 

 The ancient tectonics are sharply set off from those more recent and are 

 of a very different character. 



Xear the north end of the province the Sandia and Manzano ranges 

 display typical features (figure 10). The throw of the main fault is at 



12 Bulletin of the Geological Society of America, vol, 8, 1897, pp. 87-112. 



