THE GRANITIC ROCKS OF THE SIERRA NEVADA 153 



diorite in mineral composition. One of the chief differences is 

 the more acid character of the plagioclase which is, so far as 

 known, always oligoclase. It also contains zircon, which is not 

 usually found, and certainly is not abundant, in granodiorite. It 

 is probably, moreover, of later age. The quartz-monzonite of the 

 Sierra Nevada would be called a hornblende-biotite-granite by 

 Rosenbusch. The three analyses given in the table below indi- 

 cate that the chemical composition of this rock is remarkably 

 uniform. The quartz-monzonite of the east wall of Yosemite 

 Valley appears to be in sharp contact with the granodiorite mass 

 that lies immediately west. At any rate the transition from the 

 even-grained monzonite with scattered amphiboles to the more 

 basic granodiorite, occurs within a very few feet at a number of 

 points. More investigation, however, is needed. 



The adoption of the term quartz-monzonite instead of grano- 

 diorite will perhaps be objected to by some petrographers on the 

 ground that granodiorite is the older term. As has been shown, 

 however, the latter rock does not occupy a strictly intermediate 

 position between granite and quartz-diorite unless we extend the 

 range of its chemical variation so as to include the quartz-mon- 

 zonite of the higher parts of the Sierra. If we should confine 

 the term quartz-monzonite, or granite-diorite, to quartz-feldspar 

 rocks in which the potash and soda-lime feldspar are present in 

 about equal amount, as Brogger has done, we would then have 

 to exclude from this type nearly all of the rocks called grano- 

 diorite (quartz-orthoclase-diorite) in the Gold Belt folios. It 

 would, therefore, seem better to let the term granodiorite stand 

 for the rocks for which it has been used, and use one of the per- 

 fectly definite terms quartz-monzonite or granite-diorite for the 

 rocks intermediate between granite and quartz-diorite. The term 

 monzonite has already been adopted by the United States Geolog- 

 ical Survey for folio use, and it seems, therefore, desirable for the 

 members of the survey likewise to use the term quartz-mon- 

 zonite for monzonites containing abundant quartz, in the same 

 way that we use quartz-diorite for diorites containing abundant 

 quartz. 



