634 J. E. SPURR 



varies in different portions of the petrographic province from 

 early to late Eocene. Strict contemporaneity is not to be 

 expected, but only broad correspondence. 



2. Siliceous intermediate. — In the region of the 40th Parallel 

 Survey, Mr. King 1 considered that the beginning of Miocene 

 time came between the main period of the hornblende andesites 

 (No. 2 of the succession here outlined) and that of the augite 

 andesites (No. 4 of this succession). Mr. King found his 

 Miocene beds (sediments of the Pah-Ute Lake) largely made up 

 of tuffs derived from what was at that time regarded as trachyte, 

 and he therefore considered the trachytic period as Miocene. 

 The trachytes of the 40th Parallel Survey have been shown to 

 be mainly andesites, in part hornblende-mica andesites and in 

 part pyroxene andesites. 2 



In the Virginia, Pine Nut and Sweetwater ranges, the horn- 

 blende-pyroxene-biotite andesites were erupted and eroded 

 previous to the formation of the Shoshone Lake, to which they 

 formed the shores. The Shoshone Lake probably existed in 

 late Pliocene and earliest Pleistocene time ; this puts back the 

 age of these andesites to at least early Pliocene. 



In the Silver Peak region andesites occur, together with 

 abundant andesitic tuffs, in portions of the Esmeralda formation, 

 which is probably late Eocene or early Miocene. 



In the El Paso Range, according to Fairbanks, andesite 

 occurs as interstratified sheets in the late sediments of the Upper 

 Eocene. 



Taken altogether, it may be said that the period of eruption 

 of the medium-siliceous andesitic lavas was chiefly in the Mio- 

 cene, although it probably ran back to the Upper Eocene. The 

 periods of eruption No. I and No. 2 therefore overlap, and they 

 are actually found close together in certain of the Upper Eocene 

 lake sediments. 



Acid. — Concerning the age of the third member of the sue 1 

 cession there are somewhat better data. In the 40th Parallel 

 'Explorations of the 40th Parallel, Vol. I, p. 692. 



2 Hague and Iddings : Volcanic Rocks of the Great Basin. Am. Jour. Sci., 3d 

 series, Vol. XXVII, January 1884, p. 456. 



