ANDESITE LATER THAN RHYOLITE. 261 



branches pyroxene-andesite is exposed overlying rhyolite 1 and for the same 

 reason was regarded as an anomalous occurrence, whereas it is now evident 

 that it belongs more properly to that group of pyroxene-andesite which 

 is found associated with and passing over into basalt. Inasmuch as 

 it distinctly overlies the adjoining rhyolite it was designated on the 

 geological maps of the Fortieth Parallel Exploration as basalt, although 

 in the text mention was made of its andesitic character. At Jacob's 

 Promontory, in the Shoshone Range, a body of lava which had been 

 determined as rhyolite has also proved on further examination to be 

 allied to pyroxene-andesite, and here, as at Eureka, it is found associated 

 with basaltic flows, although of earlier age but overlying typical rhyolite. 

 Numerous localities might be mentioned where similar pyroxene-andesites 

 occur, but their relationship with neighboring rhyolites is obscure. Nearly 

 similar pyroxene-andesites occur throughout California, according to the 

 descriptions given by Mr. George F. Becker,' who has also identified these 

 lavas from the west side of the Sierras with similar andesites in the neigh- 

 borhood of Steamboat Springs, Nevada, which closely resemble those of 

 Truckee Canyon. Quite recently Mr. H. W. Turner 3 has reported the 

 occurrence of basic andesite overlying rhyolite at a number of localities 

 along the western Sierra foothills. 



These instances suffice to show that this type of rock occurs over 

 widely separated areas, but it should, however, as regards its geological 

 position, in 110 way be confounded with an older body of pyroxene-andesite 

 of somewhat similar composition, such as is well represented in the Washoe 

 District on the slopes of Mount Davidson, in the Virginia Range. The 

 latter in general present a high degree of crystallization, carrying more 

 porphyritic secretions and consequently less glass. On the other hand, the 

 former present all those characters which ordinarily characterize surface 

 flows, and are for the most part darker in color, as they cany fewer well 

 developed feldspars. The hornblende and pyroxene-andesites of Washoe 

 have been well described elsewhere in numerous publications upon that 

 much discussed region. In the opinion of the writer the geologists of the 



1 U. S. Geol. Explor. 40th Par., vol. n, p. 830. 



2 Geologyof the quicksilver deposits of the I'arific Slope, Mon. IT. S. Oral. Surv. vol. XIII. 



'Mohawk Lake Beds. Phil. Soc. <{ Wash., Rull. xi, pp. 385-410. 



