THE GRANITIC ROCKS OF THE SIERRA NEVADA 



149 



at, therefore, that the relations of the different granites is not 

 yet clearly understood. The exposures at a number of points 

 clearly show that the granodiorite proper is intrusive in, and 

 therefore younger than, the biotite-granite. Other exposures 

 near Yosemite Valley show a sharp contact between the rock 

 here called quartz-monzonite and granodiorite proper. While 

 the evidence of these two rocks being distinct is not altogether 

 satisfactory at present, it is probable that the quartz-monzonite 

 is later in age than the granodiorite. In order to show the 

 chemical relations of granodiorite to related rocks, some partial 

 analyses are here given : 



The rock described by vom Rath and called tonalite, has 

 sometimes been regarded as a synonym for granodiorite. That 

 such is not the case, however, will appear from the analysis given 

 above. If this analysis is reliable it is clear that tonalite is a 

 typical quartz-diorite unusually rich in silica. The analysis of 

 the tonalite is the mean of two analyses by Kenngott. 1 The 

 specimen analyzed came from Avio See and vom Rath, 2 who 

 originated the name, analyzed the plagioclase of the tonalite of 

 Adamello, and found the feldspar to be basic andesine 



The quartz-diorite analysis may be regarded as a fair average 

 of the smaller granodiorite areas noted in the Gold Belt folios 

 and of the marginal portions of larger areas. This analysis is 

 the mean of five basic quartz-diorite analyses given in the large 

 table of granodiorite analyses. 



'Zeit. Geol. Ges., Vol. XVII, 1865, p. 572. 



2 Zeit. Geol. Ges., Vol. XVI, 1864, p. 249. 



