CHEMICAL COMPOSITION OF NEVADITE. 349 



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

 the circumstances, as anything more than an approximate determination. 



Chemical composition. The specimen subjected to quantitative analysis is from 

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

 modification. This was chosen iu order to obtain more easily an average sample of 

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



SiO 2 74.45 



A1. 2 3 14.72 



KcoO 3 None 



FeO 0.56 



MnO 2 0.28 



CaO 0.83 



MgO 0.37 



K. 2 O 4.53 



Na 2 3.97 



Li 2 O Trace 



H 2 O 0.6C 



P,0. 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 that 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 sanidine, and an analysis of a large clear crystal was therefore made. 

 The result was as follows : 



100. 37 



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

 belongs to the sanidiue, 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 HILL RHYOLITK. 



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 



