260 GEOLOGY OF THE EUREKA DISTRICT. 



niul with these changes occur more or less variation in both mineral and 

 chemical composition. 



Age of Pyroxene-andesites Elsewhere.— Similar Surface flowS of pyroxene- 



antlesites occur at numeroiis localities in the Great Basin, all the way from 

 the Sierra Nevada Range to the Salt Lake Desert, although not always in 

 as large bodies as Richmond Mountain, nor always associated with basalts. 

 They are best shown along the Truckee Canyon in the Virginia Range, and 

 in the Augusta, Cortez, and Wahweah ranges. In the Waliweah Range lavas 

 whicli were considered by Prof. Zirkel as augite-trachytes can not be dis- 

 tinguished from the Richmond Mountain rock in any of their petrograph- 

 ical features. In the opinion of the writer many bodies of lava which 

 formerly were classed as augite-trachytes, augite-andesites, and basalts, 

 properly belong to this group of pyroxene-andesites, and in some instances 

 rocks which liad been determined as rhyolite from the fact that they were 

 supposed to carry large amounts of sanidine have within recent years been 

 shown to belong- to this same natural group. 



The Eureka District offers no ])ositive direct evidence from super- 

 position of the relative age of the hornblende-andesite and pyroxene- 

 andesite, but this apparent l)reak in the chain of evidence is more 

 than made good elsewhere, inasnuich as pyroxene-andesites of the 

 Richmond Mountain type liave been observed breaking through 

 hoiiiblende-andesites not unlike those found along the line of the 

 Hoosac fault. Similar volcanic rocks, as regards porphyritic secre- 

 tions and groundmass structure, have been described by Mr. S. F. 

 Emmons' as cutting through and overlying the hornblende-andesites in the 

 Augusta, Mountains, V)oth in the i-egiou of Crescent Peak and Antimony 

 Canyon. In the Truckee Canyon, rocks which luxve been called augite- 

 andesites can not be distinguished from those of Richmond Mountain, 

 '^riie}' were observed by the geologists of the Fortieth Parallel Exploration 

 to break through sanidine-trachytes (hornblende-mica-andesites) and were 

 regarded by them at that time as an exception to the natural order of 

 succession, all andesites being supposed to be older than the so-called 

 trachytes. Along the walls of the same deep gorge and in its lateral 



'U.S. Geol. Explor. 40th Par., vol. ii, p. 654. 



