ANDESITE LATER THAN IMIYOLITE. 261 



brandies pvroxoiic-audcsite is exposed overlying- rliyolite' and for tlie same 

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

 tliat it belongs more pro])erly tt) that gi'oup of pyroxene-andesite which 

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

 it distincth' overlies the adjoining rliyolite it was designated on the 

 giMdogical maps of the ^^>rtieth Parallel Exploration as basalt, althougli 

 in tlie 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 rliyolite has also proved on further examination to be 

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

 with basaltic Hows, although of earlier age )jut overlying typical rliyolite. 

 Numerous localities might 1k' mentioned where similar pyroxene-andesites 

 owiu', but their relationsliip with neighboring rhyolites is obscure. Nearly 

 similar pyroxene-andesites occur throughout California, according- to the 

 descriptions given by Mr. (reorge F. Becker," who has also identified these 

 lavas from the we.st side of the Sierras with similar andesites in the neigli- 

 l)orliood of Steaml)oat Springs, Nevada, which closely resemble those o£ 

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

 occurrence of basic andesite overlying- rliyolite at a number ()f localities 

 along- the western Sierra foothills. 



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

 wideh- sejiarated ari^as, l)ut it should, however, as regards its geological 

 position, in no way Ije c(»nfounded with an older bo(U' 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 Ivangc. The 

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

 porphyritic secretions and consequently less glass. ( )n the other hand, the 

 fornicr present all thost- characters which ordinarih" characterize surface 

 Hows, and are for the most part darker in color, as they (iarry fewer well 

 developed feldspars. The hornlilende and pyroxene-andesites of Washoe 

 have been well described elsewhere in numenms publications upon tliat 

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



'U. S. Geol. Explor. 40th Par., vol. ii, ji. 830. 



■^Geology of tin- quicksilver di'iiosits of the I';iiilic Slope, Moii. 1'. S. (ieol. Surv. vol. xni. 



^Mohawk Lake Beds. I'liil. Soc. . f Wash.. Hull. xi. pp. 385-'U0. 



