WACHOE MOUNTAINS. 4.81 



dark steel-gray color, an oily resinous lustre, a compact, microcrystalline 

 texture, with a decidedly conclioidal fracture, characteristics of andesites. 

 It breaks readily under the hammer, with a fresh, bright surface, and with- 

 stands atmospheric agencies remarkably well. Mineralogically, that which 

 first strikes the attention in this rock is the great number of brilliant biotite 

 plates with which it is" studded, and which protrude from the groundmass 

 in a striking manner. The feldspar-crystals are all small, but show both 

 sanidin and finely-sti-iated triclinic forms, the latter especially brilliant. A 

 few small acicular hornblende individuals are visible to the naked eye. 



Under the microscope, Zirkel has shown that plagioclase is the pre- 

 vailing feldspar, that hornblende is exceedingly rare, no augite being pres- 

 ent, and that a few quartz-grains are scattered through the groundmass. 

 He also calls attention to this rock as being "the mica equivalent of the 

 hornblende-andesites ". 



This rock shows a strong family resemblance to the Spring Canon 

 group. Yet it is quite remarkable that it should be so rich in mica and 

 free from augite, and the others characterized by the presence of so much 

 augite as to deserve the classification of "augite-andesites". Geologically, 

 they both bear the same relation to the rhyolites. This andesite, from the 

 northern end of the mountains, has been carefully analyzed by Mr. R, W. 

 Woodward, who obtained: 



SiHca ----- 67.81 67.63 



Alumina ^ 17.60 18.08 



Ferrous oxide - 2.11 2.17 



Manganous oxide trace trace 



Lime .* 3.15 3.16 



Magnesia ..-- 1.08 1.14 



Soda - -.- 2.97 2.87 



Potassa- - - 3.85 3.86 



Ignition 1.57 1.49 



100.14 100.40 



This analysis is really quite remarkable, and varies widely from the 

 normal hornblende-andesites. It stands some 10 per cent, higher in silica 



31 D G 



