CHEMICAL COMPOSITION OF NEVADITE. 349 



measured iu the crystal figured was 72° 53'. Of course this canuot be regarded, uuder 

 the circuiustauces, as auytkiug more thau au approximate determination. 



Chemical composition. — The specimen Subjected to quantitative analysis is from 

 the northeastern part of Clialk Mountain [397] and is of the relatively finer-grained 

 modification. Tliis was choseu in order to obtain more easily an average sample of 

 the rock. The analysis is by W. F. Hillebrand. 



SiO, 74.45 



AI2O3 14. 72 



I-ViOt None 



I''eO 0.56 



MnO, 0.28 



CaO 0.83 



MkO 0.37 



KcO 4.53 



NajO . . 3. 97 



LiiO Trace 



H,0 0.6t! 



P2O. 0.01 



100. 38 



The rarity of biotite and of magnetite in this rock, which has already been 

 emphasized, is certainly confirmed by this analysis. In fact, it is evident tbat no 

 minerals, aside from quartz and feldspar, play any important role. The large amount 

 of soda shown by the analysis made it desirable to know how large a share of it was 

 contained in the sanidiue, and an analy^-is of a large clear crystal was therefore made. 

 The result was as follows : 



Si O:. 65. 04 



Al 2O3 20. 40 



CaO 0. 71» 



KjO 9.74 



Na.O ) 



Li,0 



4.11 



H,0 0.29 



100. 37 



From this it may be safely assumed that a large part of the soda found in the rock 

 belongs to the sanidine, for no visible impurities were present, such as plagioclase 

 grains. The same holds true for the lime. It is worthy of note that the silica per- 

 centage is the highest yet obtained in any rock of the region. 



BLACK niLL RHYOLITE. 



The rhyolite forming Black Hill, in the southeastern corner of the mapped dis- 

 trict, is like the Chalk Mountain Nevadite in being composed almost wholly of quartz 

 and feldspar, but the resemblance otherwise is not very marked, for the Black Hill 

 rock possesses a groundmass which is fully equal quantitatively to the small imbedded 



