120 Robinson — Tertiary Peneplain of th Plateau District. 



"Upon the San Jose, a few miles west of its junction with 

 the Pnerco of the East, there is exposed a complex disturbance, 

 consisting' of a number of faults and folds,, that were not suf- 

 ficiently studied to warrant an attempt at their representation. 

 The residual throw is to the east and the trend of the dipping 

 strata north and south. The basalt of the Lucera plateau, 

 and Mesa Red on da, rests on the disturbed strata, and is not 

 tilted with them." 



Nothing definite can be said in regard to the original extent 

 of the peneplain in this part of the plateau, since topographic 

 maps are lacking and much of it has never been geologically 

 studied. The opinion only may be stated that it must have 

 covered a very large area, at a minimum estimate possibly 5,000 

 square miles. 



The Mountain District of Arizona. — The peneplain in the 

 Black and Mogollon mesas is terminated on the west and 

 south by an erosion cliff overlooking the Mountain district, 

 which makes it highly probable, as noted, that it formerly 

 extended somewhat into that district. The facts on which 

 this conclusion is based are meager, and what is here presented 

 may be considered largely suggestive and as indicating a point 

 for future study. 



In the Black Hills and Bradshaw Mountains, west of the 

 Black Mesa, it seems possible to obtain a rather definite idea as 

 to the presence of the peneplain. The topography of the 

 Black Hills and of considerable areas about the Bradshaw 

 Mountains* is of the mesa type and has been produced by 

 the erosion of horizontal flows of basaltic lava. The eroded 

 eastern edges of the flows, capping the Black Hills from 

 Jerome for 40 or more miles southward, overlook the 

 Verde valley and face the eroded and lava-capped Aubrey 

 Cliff, from 10 to 20 miles distant. To explain the present 

 position and condition of the lava flows it is necessary to 

 assume that the surface on which the lava rests originally con- 

 tinued across the Verde valley, and has since been obliterated, 

 in connection with faulting, by the erosion of the valley. 



The lava in the Black Hillsf rests either upon a small 

 thickness of limestone or upon granite, while in the Bradshaws 

 it lies on granitic or metamorphic rocks. The formations are, 

 therefore, of a very much lower horizon than those upon 

 which the lava rests in the Aubrey Cliff. This difference 

 may be explained by the erosion, essentially to grade, of the 

 relief produced either by a monocline or fault. The first case 

 would duplicate the condition found at the Black Point mono- 

 cline (figure 1) ; the second, that near the point where the 



* Jerome and Bradshaw Mountain topographic sheets. U. S. GK S. 

 \ Reid, Sketch of the Geology and Ore Deposits of the Cherry Creek 

 District, Ariz. Econ. Geol., vol. i, No. 5, 1906. 



