358 Spurr — Quartz-muscovite rock from Belmont, Nevada. 



Arzruni found, however, that those beresites which consist 

 entirely of quartz and mnscovite, and which had been regarded 

 by Karpinsky as fresh feldspar-free rocks, actually contained 

 feldspar in their original condition, although the feldspar has 

 undergone a complete transition to mica and quartz. It 

 appears, also, that the alteration has affected only the ortho- 

 clase, while the plagioclase has remained unaltered. Since in 

 the different parts of the rock the relative proportion of ortho- 

 clase and plagioclase is' very variable, it results that the varieties 

 rich in plagioclase contain feldspar, even when the orthoclase 

 is altered to quartz and muscovite, while the varieties poor or 

 lacking in plagioclase can reach a transitional stage where they 

 can be regarded as feldspar-free beresites. As the result of 

 the study Arzruni concludes that beresite is a genuine musco- 

 vite granite with a tolerably constant mineralogic constitution.. 

 The rocks described from Belmont are identical in nearly 

 every particular with those of Beresovsk, except that in Bel- 

 mont the decomposition has not affected the rocks to any 

 extent, permitting a much more accurate study. 



Analysis of Belmont rock. 



A sample of the quartz-muscovite rock from Belmont proved 

 to have the following composition : 



Analyst, Dr. H. N. Stokes. 



Si0 2 84-15 



A1X) 967 



FeO 



-51 



FeO 



___■ -07 



MeO 



CaO 



. '04 



-53 



Na 



2-65 



K 



1-57 



H 2 0- 



H*0+ 



-21 



-74 



TiO 



_ trace 



F 



-02 



MnO. - 



trace 



SrO... 



trace 







100-14 



A rough computation of this analysis shows the proportion 

 by weight of albite to muscovite to be about 5 : 6. The speci- 

 men then is one which contains much more unaltered feldspar 

 than some of the other varieties. Those varieties which origi- 

 nally contained orthoclase, but no albite, would now be rocks 

 free from feldspar. 



