THE GRANITIC ROCKS OF THE SIERRA NEVADA 147 



metamorphosed by granodiorite ; also north of Genesee Valley x 

 there is a quartz-gabbro which is a basic facies of the great 

 batholith, and the adjoining Triassic slates have been metamor- 

 phosed into hornfels. There is also evidence that the rocks of 

 this batholith are later in age than the serpentines and associated 

 magnesian rocks. This is particularly evident in the Bidwell 

 Bar quadrangle, where the rocks of the magnesian series are 

 cut by dikes of granodiorite, and in general throughout this 

 quadrangle the rocks surrounding the granodiorite areas, 

 whether of sedimentary or of igneous origin, show contact 

 metamorphism. However, in the Bidwell Bar quadrangle the 

 age of the sedimentary rocks, so far as known, is Carboniferous, 

 and it can therefore only be said that in this region the grano- 

 diorite is post-Carboniferous. 



In a paper 2 published in 1893, Lindgren describes typical 

 granodiorite as follows: "The rock consists in typical develop- 

 ment of feldspar, quartz, biotite, and hornblende with medium- 

 grained hypidiomorphic structure. The soda-lime-feldspars are 

 usually considerably and to a variable extent in excess of the 

 alkali-feldspars. The silica varies between 60 and 73 per cent. ; 

 the amount of lime is variable, but it rarely exceeds, while it 

 usually falls somewhat short of, the sum of the alkalies. While 

 in some varieties which cannot be distinguished from the others 

 in the field, there is more potash than soda, a frequently occur- 

 ring relation is 2 per cent. Ka 2 to 4 per cent. Na s O. It will 

 be seen that the rock very closely approaches some quartz-mica- 

 diorites and often might be indicated by that name." 



In his later paper on the gold quartz veins of Nevada City 

 and Grass Valley, published in the Seventeenth Annual Report 

 of the U. S. Geological Survey, Lindgren gives the limits of 

 chemical variation and average composition of granodiorite as 

 follows. 



1 Dii.ler, Bull. 150 U. S. Geol. Surv., p. 338. 



2 The Auriferous Veins of Meadow Lake, California. Am. Jour. Sci., Vol. XLVI, 

 1893, pp. 202, 203. 



