LA VAS IN THE GREAT BASIN REGION 627 



( Augite-biotite hornblende latite. 



4. Latite \ Ty . . .. , ., 



( Biotite-augite latite. 



5. Hornblende-pyroxene andesite breccia. 



6. Olivine-basalt. 



Tintic district. — Passing from the Sierra Nevada and the 

 western border of the Great Basin (from the Sonora region and 

 the Washoe district) eastward past Eureka, we will next cite 

 the record of Tertiary vulcanism in the Tintic Range, which lies 

 south of Salt Lake and southwest of Utah Lake, and is approxi- 

 mately the same distance from Eureka as Eureka is from 

 Washoe. This region has been studied by Messrs. Tower and 

 Smith. 1 The succession of lavas is as follows : 



1. Biotite rhyolite. 



2. Pyroxene-hornblende-biotite andesite — tuffs and breccias. 



3. Pyroxene-hornblende andesite (latite). 



4. Olivine-basalt. 



The rhyolitic flows of the first member of the succession 

 given above are known to be continuous and contemporaneous 

 with dikes of rhyolite which, on account of their somewhat dif- 

 ferent habit, were given the name of quartz-porphyry in the text. 

 This intrusive rhyolite appears to be susceptible of correlation 

 with the rhyolite or "quartz-porphyry," described by the writer, 2 

 from the northern end of the Oquirrh Range, which lies south of 

 the Tintic district (Eagle Hill porphyry). The rhyolite in the 

 Tintic district is known to be later than Upper Eocene. 



The pyroxene-hornblende andesites (or latites) described by 

 Messrs. Tower and Smith are regarded by them as belonging to 

 the same general body as certain large masses of granular raon- 

 .zonite with which they are connected by transitional phases ; 

 the monzonite representing portions of the magma which have 

 consolidated as intrusive masses under conditions favoring 

 better crystallization than those under which the extrusive sheets 

 ■of pyroxene andesite (or latite) have consolidated. These 



1 " Geology and Mining Industry of the Tintic District, Utah," Nineteenth Annual 

 Rept. U. S. Geol. Surv., Part III, Economic Geol., p. 632. 



2 J. E. Spurr, "Economic Geology of the Mercur Mining District," Sixteenth 

 Ann. Rept., Part II, p. 377. 



