6 University of California Puhlications in Botany [Vol.9 



the two most nearly allied rock tj^pes; though less silieious than true 

 granite, with respect to the silica content, it is distinctly nearer to 

 granite than to diorite and, of course, still less resembles gabbro. The 

 lime (CaO) content is high for a granitoid rock though here again 

 granodiorite more nearly approaches true granite than the more basic 

 plutonics. In color granodiorite is of a light gray tone varying accord- 

 ing to the changing per cent of hornblende and biotite. The structure 

 is medium to coarse-grained and crumbles easily to a coarse light 

 yellowish-gray sand. 



Associated locally in the north with the granodiorite is a rock 

 approaching normal granite, granitite or biotite granite. In the north- 

 ern section of the range, this rock is confined to the higher ridges 

 and summits ; in the southern Sierra it becomes more widely developed, 

 and granodiorite is reduced in amount. The structure of the rock 

 is coarse on account of the usually large alkali feldspar crj'stals. A 

 large part of the Pyramid Peak Range, west of Lake Tahoe, is com- 

 posed of this coarse granite. The immense sand slopes on the east 

 side of Angora Peak, in the same district, have been formed by its 

 decay. A large part of the crest about the headwaters of the 

 Tuolumne and northward from Mt. Conness to the ridges about High- 

 land Lakes and the Blue Lakes, in Alpine County, is composed of 

 this coarse granite. ^^ The same variety is found on the slopes of 

 Mt. Wliitney, the orthoclase prisms becoming 8 to 9 cm. long^- ; else- 

 where in the country between Kern Cafion and the crest there is 

 found a similar porphj^'itic granitite with pale flesh-colored crj-stals 

 of orthoclase averaging 4 cm. in length over wide areas.*' The fresh 

 rock is harder and firmer in texture than the granodiorite, and the 

 outcrops are more highl}' colored with iron stain. 



Other granitic rocks of the high mountains include diorite, a dark 

 green, medium to coarse-grained rock composed of green hornblende, 

 a little black mica, and white soda-lime feldspar. Dioritic areas are 

 not extensive but b^- no means uncommon, especially in the northern 

 half of the range. The most basic of the intrusive rocks of the Sierra 

 occurring in any quantity is gabbro; it is found in small patches, the 

 total area of the gabbro exposures being vanishingly small as compared 

 with the diorite, the least abundant of the rock types so far mentioned. 



Rocks derived from extruded magmas still cover large areas in 

 the north and on the western flank of the Sierra; in the southern 

 mountains such rocks are less abundant within the high mountain 

 region though present in certain districts. As is well known there 



