762 H. H. ROBINSON 



recognizable in the ranges themselves is questionable. Judging 

 from the changes produced by erosion during the post-peneplain 

 cycle in the plateau region, they should be most easily fecognized 

 about the footslopes of the ranges and in the peneplained areas. 



A second point is that while the Basin Range country and 

 plateau region, as they are known today, were probably to some 

 extent differentiated at the close of the Miocene, the line of 

 demarkation between the two regions was more or less obliterated, 

 at some localities entirely, by the close of the peneplain cycle of 

 erosion, which has been placed at the close of the Pliocene. At that 

 time the Basin Range country in general constituted the uplands 

 whose lowlands were situated, in part at least, in the present Grand 

 Canyon District. At the close of the Pliocene the faulting appears 

 to have been of such a nature that the two districts were again 

 to some extent separated, but it was not until the pronounced 

 faulting which introduced the canyon cycle of erosion that the 

 Basin Range and Plateau provinces, as they are known today, 

 were given, or began to be given, what has since developed into 

 their maximum degree of demarkation. 



It may also be noted, as a climatic incident in the history of 

 the region, that a small glacier lived in the large interior valley of 

 San Francisco Mountain, situated on the plateau south of the Grand 

 Canyon, during the canyon cycle of erosion. An attempt has been 

 made to calculate the temperature on the mountain at the time of 

 the glaciation; the problem was approached by three different 

 methods and the results showed reasonable agreements. Without 

 going into the processes of calculation, which are reserved for a 

 future paper, it may be said that the average result gave a tempera- 

 ture of 15° F. less than that of today. This result is in close agree- 

 ment with determinations made elsewhere, and as it is a fair 

 assumption that the region has experienced only slight changes 

 in elevation since the glacier existed on the mountain, it may be 

 taken as an approximate measure of the difference in temperature 

 in this general region between what was certainly one of the latest 

 and perhaps the last stage of the Glacial period and the present 

 time. It is evident that if conditions were only sufficiently favor- 

 able for the existence of a small glacier on the mountain after the 

 region had been elevated possibly as much as 5,000 feet at or 



