CHARACTER OF THE OLDER TECTONICS 555 



faults which might account for the average Basin range are old events, as 

 considered by Dutton and Spurr; that most of the present mountains 

 rise above the general plains-level largely for the reason that they are ero- 

 sional residuals, composed of the more resistant rock-masses in the midst 

 of weak rock areas, as early urged by King and later by Spurr ; that the 

 general shapes of the individual residuals are controlled to a very appre* 

 ciable extent by the ancient geologic structures, frequently accentuated, 

 perhaps, by late minor fault-movements; that it is possible to formulate 

 the salient Basin-range features by the aid of direct eolic denudation of 

 an anciently faulted and folded region, in accordance with the present 

 known workings of general desert-leveling under conditions of aridity, 

 as I have more fully set forth in another place, and that the present tilted 

 aspect of so many of the desert ranges is largely illusory, because in 

 anciently broken and inclined strata which had been planed o3 the soft 

 beds have been profoundly stripped from the resistant layers, leaving the 

 residual ridges with long, bare backslopes, as particularly noted in con- 

 nection with the descriptions of the so-called tilted block-mountains of 

 the Mexican Tableland province. 



OLDER GEOLOGIC STRUCTURES OF TEE COLORADO PLATEAU PROVINCE 



With a few minor exceptions, the mountainous elevations which rise 

 above the general plains-surface of this area are chiefly of volcanic origin. 

 It is a remarkable fact that almost the entire surface of this vast area is 

 essentially a stratum plane, or at least the hard layers are arranged en 

 echelon. Unlike in any other part of the country surrounding, there is, 

 on the whole, little evidence of marked folding of the Appalachian t3rpe. 

 In the main, the whole province is a simple, single, quaquaversal arch. 

 Typical Basin-range structure is, therefore, practically wanting. 



Only around the margins of the great dome is there noteworthy fault- 

 ing. It was on the Great Basin side of the dome that Gilbert gained 

 his first conceptions of the faulted and tilted blocks that afterwards were 

 made to stand for his idea of the Basin-range type of mountain struc- 

 ture. With the great simplicity of structure so apparent, there was 

 seemingly at that time no other alternative but to extend the profound 

 faulting of the Grand Canyon region indefinitely over the plains beyond. 



OLDER GEOLOGIC STRUCTURES OF THE CALIFORNIA^ GULF BASIN 



In the valley of the lower Colorado Eiver and the adjacent country on 

 both sides, in southern California and southern Arizona, the mountains 

 are composed largely of great successions of pyroclastic beds interspersed 



