EEGION WEST OF PYEAMID LAKE. 845 



the geological maps, although microscopical analysis indicates that it is 

 more closely allied to dacites, the quartz-bearing variety of the andesites. 

 The rock has the characteristic greenish-gray groundmass of the typical 

 Washoe propylite, with the same arrangement and structure of the plagio- 

 clase and hornblende, and the same general field aspect. Its groundmass 

 is 'exceedingly fine-grained, through which are scattered many well-pre- 

 served flesh-colored 'feldspars, occasionally one-half of an inch in length, and 

 dark-green fragments of fibrous hornblende. Mica is present as sparse dark 

 plates of biotite. Unlike the normal propylites of Washoe, the rock is 

 characterized by the presence of a very considerable amount of translucent 

 quartz-grains, all of them more or less cracked and fissured. Under the 

 microscope, in thin sections, the quartz is shown to carry liquid-inclusions, 

 which is a characteristic feature of quartz-propylites. 



A chemical analysis of this rock yielded Mr. R. W. Woodward the 

 following result : 



Silica - 68.45 68.46 



Alumina 16.97 16.85 



Ferrous oxide 1.36 1.43 



Manganous oxide : trace trace 



Lime 3.07 2.92 



Magnesia 0.73 0.75 



Soda-' 3.60 3.60 



Potassa. . 3.99 3.98 



Lithia trace trace 



Carbonic acid - 0.59 0.59 



Water 1.42 1.45 



100.18 100.03 

 Specific gravity, 2.38, 2.44. 



It is interesting to note the presence of carbonate of lime, as under the 

 microscope may be detected minute crystals of calcite, derived from the 

 decomposition of feldspars. 



As the area occupied by these quartz-propylites has not been carefully 

 determined, it is possible that they may be represented on the geological 



