H. W. TURNER 



In his investigation of Pyramid Peak quadrangle, Lindgren 

 found large areas of granitic rocks containing amphibole and bio- 

 tite and resembling granodiorite in a general way. Some of these 

 rocks are somewhat basic and probably correspond to the grano- 

 diorite of the Gold Belt region in chemical composition ; but other 

 areas contain more alkali and less lime than the typical grano- 

 diorite of this paper. Lindgren, however, concluded that this 

 alkali-rich rock should be called the typical granodiorite, inasmuch 

 as it occurs in very large areas, and also since it occupies a place 

 almost exactly intermediate between quartz-diorite and granite. 

 Since that time much field work has been done in the south central 

 Sierra Nevada where the granular complex is finely exposed, most 

 of the soil and loose rock having been removed by the glaciers 

 that formerly covered the region. The opportunities, therefore, of 

 studying the relations of the granites in this section are unex- 

 celled. The different granitic rocks resemble one another so much 

 in general aspect that the contacts between different kinds are 

 sometimes discovered only after careful search. As a rule a per- 

 son will pass from one kind of granite to another without having 

 observed any change in the rock until the difference is called to 

 his attention by some striking feature. It is not to be wondered 



LIMITS OF VARIATION AND AVERAGE COMPOSITION OF GRANO- 

 DIORITE 



SiO s ..... 

 A1.0,.... 

 Fe 3 .... 



FeO 



CaO 



MgO 



K 2 



Na s O 



Remainder 



Limits of varia- 

 tion 



Per cent. 

 59 to 68 



14 



to 

 to 

 to 

 to 

 to 

 to 

 to 



17 

 AX 



4H 



Average composi- 

 tion 



Per cent. 

 65 

 16 

 1.50 



3 



5 



2 



2.25 



3-50 



i-75 



1 Three and one half in one analysis. 



* Certain masses in the foothills go below 1 per cent. 



