Robinson — Tertiary Peneplain of the Plateau District. Ill 



western part of the plateau country. The folding presumably 

 gave rise to closed basins with the resultant formation of lakes, 

 and deposition of sediments in them, and also to open basins, 

 in which fluviatile deposits were laid down. The cycle of ero- 

 sion begun at this time has continued unbroken to the present. 



With the next period of uplift, known as the " First Period 

 of Faulting," it would appear that the plateau district was 

 rather clearly marked out, since the faulting on both the east 

 and west borders was such as to lower the country on either 

 side of the plateau, or to raise the plateau itself. The date of 

 this first period of faulting cannot be definitely fixed, but from 

 what is known of the history of the Basin Ranges, it may be 

 provisionally placed at the close of the Miocene. 



By far the greater part of the erosion of the southern portion 

 of the plateau country had been accomplished by the time the 

 "Second Period of Faulting" occurred near the close of the 

 Pliocene. At the close of this cycle of erosion, known as 

 "■ The Period of the Great Denudation," the relief produced 

 by the monoclinal folding and the faulting of the first period 

 had been obliterated to a large extent, and the surface of the 

 entire southern plateau country, and even a greater area, had 

 been reduced to a peneplain. 



Immediately after the development of the peneplain, and 

 before the second period of faulting, widespread eruptions of 

 basalt occurred over much of the southern portion of the 

 plateau, and it is to this capping of lava that the peneplain 

 owes its preservation. In the San Francisco Mountains, as 

 determined by the writer,"" a second period of volcanic activity 

 ensued in very late Pliocene, or early Quaternary time, during 

 which San Francisco Mountain and the neighboring large 

 cones were built up by flows of andesite, trachy-andesite, and 

 rhyolite. To this same general period it seems probable that 

 Mount Floyd, the Sierra Blanca and Mount Taylor should be 

 assigned. 



The second period of faulting near the close of the Pliocene 

 gave rise to the Plateau district essentially as it stands to-day, 

 and introduced the " Canyon Cycle of Erosion," during which, 

 in Pleistocene time, the Grand Canyon of the Colorado has 

 been cut, a considerable amount of the soft strata overlying 

 the present surface rock of the plateau — the resistant Upper 

 Aubrey cherty limestone — has been stripped off and cliff pro- 

 files refreshed. The amount of erosion accomplished during 

 this cycle is, however, insignificant when compared with that 

 of the period of the great denudation. 



During the canyon cycle of erosion a third period of volcanic 

 activity occurred in the San Francisco Mountains characterized 

 by eruptions of basalt from many small cones. These erup- 



* Unpublished report ; to be published by the U. 8. G. S. 



