EEGION SOUTH OF GRANITE MOUNTAIN. 701 



chemical analysis of this rock differs but little from the analyses of the 

 rhyolites of this range already recorded. It is interesting to observe the 

 detection of carbonic acid, which in a manner determines the presence of 

 calcite pointed out by the microscope. 



Mr. R. W. Woodward, who made the analysis, reported the following 

 composition: 



Silica - 75.07 75.15 



Alumina 11.40 11.27 



Ferric oxide 1.75 2.14 



Manganous oxide trace trace 



Lime 0.61 0.78 



Magnesia 0.11 0.11 



Soda 1.15 1.26 



Potassa . 8.83 8.28 



Lithia ^ trace trace 



Carbonic acid trace trace 



Water 1.74 1.67 



100.66 100.66 

 Specific gravity, 2.4, 2.3. 



Breaking through and overwhelming these trachytes is a series of suc- 

 cessive Thyolitic dikes, well exposed in Pine Nut Canon. The same wide 

 variety of texture is observable as at the Sou Spring Hills, dikes occur- 

 ring here in which defined crystals amount to one-third or one-half of 

 the mass. The sanidin crystals often form Carlsbad twins ; the quartz- 

 grains, which are abundant, contain good glass-inclusions. One of the 

 most recent of these dikes which penetrate the older rock presents some 

 peculiarities in the appearance of the feldspars that deserves special men- 

 tion. The rock has a reddish-gray groundmass formed of quartz, sanidin, 

 and biotite, containing free quartz in relatively large-sized grains. The 

 sanidin possesses a most remarkable compactness of structure and a pecu- 

 liar vitreous lustre, which give the fragments a very close resemblance to 

 the associated quartz-grains. The larger crystals give out in refracted light 

 the most beautiful blue colors, which are more brilliant than the most strik- 



