TRUCKEE OANOK 833 



nently throughout the light-gray groundmass, while the microscope reveals 

 augite, magnetite, and apatite. This occurrence of both sanidin and 

 augite is to be specially noticed. 



Directly north of the Truckee Ferry lie some low purple hills, which 

 consist also of sanidin-trachyte, with many small grains of magnetite, but 

 no augite. It is an unusually dense, dark-colored trachyte, and the long 

 sanidin crystals stand out very prominently. 



An analysis of this rock by Mr. R. W. Woodward yielded the fol- 

 lowing: 



Silica 56.51 56.45 



Alumina 19.61 19.85 



Ferric oxide 5.10 4.95 



Ferrous oxide 0.98 0.97 



Manganous oxide 0.11 0.11 



Lime 7.87 7.70 



Magnesia : 2.66 2.66 



Soda 3.12 3.15 



Potassa J 3.67 3.84 



Lithia trace trace 



Water - 0.40 0.38 



100.05 100.06 

 Specific gravity, 2.5, 2.6. 



Overlying these sanidin-trachytes are horizontal beds of trachytic 

 breccias of a very dark and basic appearance, which are in turn directly 

 overflowed by dark-gray augite-trachytes, consisting, as shown by the 

 microscope, of sanidin, a little plagioclase, some brown hornblende, and a 

 considerable portion of pale-green augite. Far richer in augite than the 

 trachyte which underlies it, it approximates much more closely in its mode 

 of jointing and weathering to the basalts, and presents a series of mural 

 faces in all respects like the latter rock. 



An analysis of this augite-trachyte was made by Mr. R W. Woodward, 

 who reported as follows: 



Silica 50.36 50.03 



Alumina 17.00 16.99 



53 D G ' . 



