1912] 



Baker: Western El Paso Range 



131 



major axis the western portion of the range plunges westward 

 like an anticlinal fold. 



The zone of northward tilting is limited to the region south 

 of the outcrop of the basalt flows because north of the flows the 

 bevelled surface of the tilted Rosamond beds is mantled by the 

 alluvial debris derived from the bedrock of the Sierra Nevada. 

 The surface of this alluvial debris slopes downward and south- 

 ward close to the base of the outcrop of the basalt flows (pi. 9, 

 fig. 1). We are accordingly able to distinguish two separate 

 epochs of tilting of the Rosamond, separated the earlier from the 

 later by a period of erosion long enough to develop a peneplain 

 of at least local extent. 



It is not certain whether the basalt flow of Black Mountain 

 has been deformed since its outpouring or whether the original 

 slopes of the surface over which it outflowed are preserved with 

 the attitudes which they had at the time of the lava outflow. 

 But the smooth even slope of bevelled Rosamond, covered by the 

 basalt of domical form, without any traces of the presence of 

 valleys at the time of the basalt eruption implies either that the 

 uplift of the El Paso Range came so soon before the basalt out- 

 flow that there was little time between these two events for 

 erosion of the surfaces of the newly uplifted range, or else that 

 the deformation which produced the range has been subsequent 

 to the basalt eruption. It is the opinion of the writer, based 

 partly upon his view of the antecedency of the canons crossing 

 the entire uplift, that the considerable erosion of the range was 

 necessarily contemporaneous with its uplift, and that therefore 

 the deformation was later than the eruption of basalt and caused 

 the warping of the basalt sheet. This view is in harmony with 

 the previously presented conclusions of the writer as to warping 

 and faulting of the late basalt flows in other portions of the 

 Mohave Desert. 



There possibly is a fault near the south base of Black Moun- 

 tain between the northern or Black Mountain range and the 

 southern or granite range. The southward-dipping surfaces of 

 the lava mesas on the south side of Black Mountain dip into the 

 northern flank of the granite range making a very appreciable 

 notch in the cross-section of the range as a whole. Unfortunately, 



