1912] 



Baker: Western El Paso Range 133 



pyrite. Intruding the metamorphics is also a dike rock, probably 

 a quartz porphyry, which has also suffered considerable altera- 

 tion. The relations of the granite with the other rocks of the 

 basement complex was not ascertained. But it is coarse-grained 

 in texture and is evidently a plutonic rock which consolidated 

 deep beneath the surface and has since been exposed by erosion 

 of great amount. The granite, after its partial or entire con- 

 solidation, was intruded by pegmatite and aplite dikes. 



The only definite clue to the age of the rocks of the basement 

 complex is furnished by the unconformably overlying and hence 

 younger Rosamond series which is not older than Upper Miocene. 

 There was great erosion after the deposition, metamorphism, and 

 intrusion of the rocks of the basement complex before the time 

 of deposition of the overlying Rosamond series, for the deep- 

 seated rocks of the basement complex were laid bare at the sur- 

 face and covered by the Rosamond sediments laid down on the 

 surface of the land. Therefore the rocks of the basement complex 

 must be of considerably greater age than the Rosamond series. 



The materials of the Rosamond have been derived from three 

 sources: (1) by erosion from the rocks of the basement complex; 

 (2) by erosion from lava flows later in age than the rocks of 

 the basement complex; (3) from pyroclastic materials, compris- 

 ing ash and pumice, blown out from volcanoes and deposited 

 either by wind action, by the agency of water, or by settling 

 during and after volcanic eruptions. The agencies of deposition 

 were of the continental or terrestrial type and are believed to 

 have been the same as those forming the desert alluvial deposits 

 being laid down at the present day. All the fossils found in the 

 Rosamond series in the El Paso Range represent the remains of 

 terrestrial mammals and tortoises. There is little or no evidence 

 of lacustrine sedimentation in the lower half of the Rosamond of 

 the El Paso Range, and the finer sediments of the upper strata 

 may well be eolian deposits or deposited by any or all of the 

 subaerial processes with their materials derived from the erosion 

 of low-lying areas. Contemporaneous with the deposition of the 

 middle portion of the Rosamond were two outflows of basaltic 

 lava. 



