GEOLOGY OF THE CO SO RANGE 71 



Of the rocks themselves no detailed petrographic investigation 

 seems to have been made. The various writers have described them 

 as granitic, with basalt flows north and west. The granite of the 

 dome has the characteristics of the intrusive granodiorite of the Sierra 

 . Nevada, and ranges from a basic hornblende-biotite granite to a 

 diorite. Traces of an older basic diorite are included in the normal 

 fades. A few dark-colored basic dikes and a great number of 

 pegmatite dikes also occur. Dikes of diorite-porphyrite are found, 

 striking in general north and south. These are frequently andesitic 

 in character. The basalt flows are agglomeratic at the base, grading 

 upward to massive, often vescicular, at the top. Some of the earlier 

 mud flows picked up considerable sandy material from the lake beds. 

 The quartz veins are small and in two series, of different ages and 

 characteristics in the localities visited. The older veins strike north- 

 west and southeast and bear evidence of having been formed at great 

 depths, the present veins being but the roots of the original ones. 

 The gangue is quartz, with strong alteration of the granite walls. 

 This alteration is shown by the development of large plates of biotite 

 and the recrystallization of the constituents of the grano-diorite for 

 a short distance from the vein. The ores are pyrite and chalcopyrite, 

 carrying some gold. The later quartz veins strike northeast and 

 southwest. The granite near the veins is metasomatically replaced 

 by silica at times. The ores are pyrite, chalcopyrite, sphalerite, galena, 

 sulphantimonides of silver and lead, and finely divided free gold rich 

 in silver. These veins are connected genetically with the intrusions 

 of the diorite-porphyrite, as seen in exposures studied in the Inyo 

 Range to the north. 



The Coso Range is pecuhar in that its Kne of evolution did not 

 wholly follow those of the surrounding ranges. The first uphft was 

 simultaneous with the intrusion of the Sierra Nevada bathoHth, 

 after which it was probably a range of great height and geographic 

 importance. It must at that time have formed a southeast extension 

 of the older Sierra Nevada, though probably as a distinct range 

 underlaid by an intrusive granitic dome. The overlying sedimentaries 

 were removed almost entirely by erosion before Tertiary faulting broke 

 a long stable condition of the crust and formed lake basins along 

 the east flank of the Sierra. The hnes of this faulting were such 



