THE GRANITIC ROCKS OF THE SIERRA NEVADA 145 



with the potash. As the biotite forms only about one twentieth 

 of the rock, the error in ignoring the soda in the biotite is small, 

 and would not sensibly alter the result. If, however, the biotite 

 forms a considerable part of the rock, its soda content should 

 be separately calculated and the amount of soda in the biotite 

 deducted from the total soda, the remainder being considered 

 as in albite. After deducting the lime (CaO) for the apatite, 

 titanite, and biotite, the remaining lime is calculated as in 

 anorthite. The oxides of strontium and barium are placed with 

 the lime. After deducting the iron-oxides (Fe 2 3 and FeO + 

 MnO) for the biotite, all the remaining iron oxides are calcu- 

 lated as in magnetite. After deducting the silica (Si0 3 ) for 

 the titanite, biotite, and feldspar, all the remaining silica is cal- 

 culated as quartz. 



GRANODIORITE 



The granitoid rocks of the Sierra Nevada that were intruded 

 at the close of Jurassic time, may be regarded as portions of one 

 great batholith that may be supposed to underlie the entire 

 range. The quartz-monzonites hereafter described are not 

 regarded as belonging to the granodiorite batholith. All of the 

 areas of this batholith are not connected at the surface, but it is 

 more than probable that these separated areas are merely extruded 

 tongues or gigantic apophyses of the main mass. Viewed from 

 this standpoint, the variation of the chemical and mineral com- 

 position of this batholith is extraordinary. The rocks range from 

 acid quartz-diorites containing over 70 per cent, of silica through 

 quartz-mica-diorites, quartz-hornblende-diorites and quartz- 

 pyroxene-diorites to gabbros and even olivine-gabbros. That 

 these different rocks may be regarded as facies of one magma, 

 appears to be indicated by the usual absence of sharp contacts 

 and the existence of transition forms between them. The table 

 of analyses given below probably represents very accurately the 

 chemical variation of the rocks of the batholith. While the 

 authors of the Gold Belt folios have always had in mind, as the 

 typical granodiorite, a rock intermediate between granite and 



(jUAl^ 





