1921] Smiley: Flora of the Sierra Nevada of California 7 



appears to be a definite succession for the lavas which produced these 

 volcanics. Rhyolitic flows preceded the more abundant andesitic 

 extrusions, with basaltic lavas as the last member of the series. 

 Residual portions of these consolidated lavas are to be found in the 

 higher mountains capping the granite and sedimentaries. As a result 

 of the sequence of the extrusions, the older lava rocks were buried 

 beneath later products of eruption and become exposed as erosion 

 removes the younger rocks. The soils derived from the weathering 

 of these volcanics appear in bands along the sides of the valleys or 

 encircle the summits in bands of irregular width. 



Rhyolite, as it appears in the Sierra, is a light colored gray or 

 pinkish rock, usually of fine grain but occasionally becoming vesicular ; 

 it is most abiuidant as a surface soil-forming rock in the central part 

 of the range, as in northern Placer County about Soda Springs and 

 Summit Valley, where it forms the country-rock over considerable 

 areas. Andesite and andesitic breccias are present in greater amoiuit 

 than the rhyolite, which they overlie; they are present throughout 

 the range and fonn the country-rock west and southwest of Lake 

 Tahoe and also in the southern Sierra on the east slope of the range 

 near the headwaters of Owens River. Andesite is reported^^ to be 

 the surface rock of the crest between Owens River and the head of 

 the North Fork of the San Joaquin. Near Mineral King large bodies 

 of sheared andesite occur on Crystal Creek at 10,000 feet elevation.* 

 In color andesite varies from dark gray to reddish. Like rhyolite, the 

 andesite may be vesicular and in this condition weathers more readily 

 than when fine grained ; the breccias naturally weather more rapidly 

 than the massive rock. Basalt is widespread in the Sierra and ex- 

 posures are known in all sections of the range from Plumas County 

 (Mt. Ingalls) to Tulare Countj^, where the most recent extrusions 

 appear to have taken place. ^^ Basalt is generally darker and more 

 compact than either rhyolite or andesite but in places becomes vesicu- 

 lar or scoriaceous. 



In many places within the higher mountains there are surface rocks 

 derived from lava rocks or tuffs through changes subsequent to solidi- 

 fication or deposition. Turner^ believed that "it is now plain that 

 the chief part of the rocks laid down on the geologic map as porphyrite 

 and amphibolite schist are altered forms of original surface lavas and 

 tuffs corresponding to modern basalts and andesites." Other alter- 

 ation products of igneous rocks present in the Sierras include some 

 serpentine, which is found on the crest of the Grizzly Mountains in 

 Plumas County, and also on the Dardenelles in Alpine County,^- ^^ 



