132 University of California Publications in Geology L"Vol. 7 



the structure was not examined in that locality. If this sug- 

 gested fault is not present this notch must be due to erosion 

 which would probably have taken place between the time of the 

 uplift and the outpouring of the Black Mountain basalt. The 

 fact that the basalt flowed upon an essentially even surface is 

 not favorable to the latter hypothesis. 



The western part of the northern or subsidiary range has 

 been much reduced by erosion folowing the great orogenic uplift, 

 whereas the eastern portion owes a part of its greater height and 

 a form more nearly approaching the original contour of the range, 

 unaffected by erosion, to the protective covering of basalt which 

 has greatly inhibited erosion. But there has nevertheless been a 

 greater amount of uplift in the vicinity of Black Mountain 

 than farther west, as indicated by the superior height of the 

 granite ridge south of Black Mountain, which height gradually 

 becomes less to the westward. 



Historical Geology 

 geologic history op the consolidated rock members 



The rocks, which are probably the oldest in the El Paso Range, 

 were originally sedimentary sandstones and quartz-conglomerates. 

 These rocks have undergone great metamorphism, probably of 

 the dynamo-regional type, which has obscured or destroyed the 

 original structure of the sediments and caused a recrystallization 

 of their constituents. The original conglomerate with its matrix 

 of sand grains and its oval or round pebbles of milky quartz has 

 now become a hard quartzite-conglomerate, which breaks quite as 

 often across the original pebbles as through the matrix or between 

 the matrix and the pebbles. The finer textured sandstones have 

 been changed to hard dense quartzite, and in some cases have 

 developed slaty or schistose structures. 



Intruded into the metamorphic series is a diorite-porphyry, 

 which probably consolidated at a considerable depth below the 

 then-existing surface, and which, like the metamorphics, has been 

 altered by hydrometamorphism with the formation of the sec- 

 ondary minerals chlorite, epidote, calcite, muscovite, quartz, and 



