758 ' H. H. ROBINSON 



factory, as there is no direct evidence to be derived from the district 

 itself. The only method of attack is by correlation with the neigh- 

 boring Basin Range country of Nevada, which in itself has not been 

 studied in any great detail. Yet, notwithstanding the tentative 

 nature of any conclusions that may be reached, it is desired to 

 present the results of such a correlation, not only because they 

 differ from those previously reached, but also because it is believed, 

 in the light of known geologic events of recent date — especially in 

 California — that they possess a very considerable suggestive value 

 and may be useful to those who may later have occasion to study 

 this general region. 



The folding and flexing (I), as represented by the Kaibab and 

 Echo Cliff monoclines, does not antedate the middle part of the 

 Eocene, since the disturbance involves strata of Lower Eocene age 

 in the high plateaus of Utah.^ It seems probable, also, from other 

 considerations, that it did not occur later than the end of the 

 Eocene. The folding in the Grand Canyon District, to all appear- 

 ances, is of the same age; it must be carefully distinguished, 

 however, from certain broad warpings of much later date. Up to 

 the present the folding and flexing movements have received much 

 less study than the later faulting, although they are fully equal 

 to the latter in importance, when their magnitude and extent, 

 and the geographic changes involved are considered. 



The correlation of greatest probability is that between the 

 peneplain developed at the close of the peneplain cycle of erosion 

 (IV), and the mature topography and local peneplains of the Basin 

 Range country of southern Nevada and of Arizona. The point 

 has not been so thoroughly studied as is desirable, but it seems 

 clear to the writer, as the result of reconnaissance work, that there 

 was originally a direct continuity between the maturely dissected 

 Basin Ranges with local peneplains extending from their footslopes 

 and the more distant highly developed peneplain of the present 

 plateau region ; they were the related parts of a single physiographic 

 province. There is little doubt that this transition may be actu- 

 ally traced in Arizona, with only minor breaks, from the Brad- 

 shaw Mountains through the Black Hills on the west side of the 



'^ Button, op. cit., 74. 



