Vol. 4] Smith. — Upper Region of Main Walker River. 



chlorite, and it is idiomorphic toward the chlorite and thus ante- 

 dates it in formation. The necessary ingredients for these pro- 

 ducts doubtless were derived from the ferro-magnesian minerals 

 for the most part. Much of the quartz possesses inclusions along 

 cracks, and the latter preserve nearly a singleness of direction. 

 The feldspar also shows alteration to a fine fibrous or shredded 

 mineral giving interference colors of yellow and green of the 

 second order. It is thought to be sericite. The epidote shows 

 fracturing, so that it was formed previous to the general defor- 

 mation suffered by the rock. This rock evidently would be 

 termed a biotite granite. 



No. 77 is a representative sample of the granite south of 

 Mickey Pass. It is a nearly white rock, being sprinkled through- 

 out with hornblende 4 mm. in average length, the quartz is pres- 

 ent, fairly idiomorphic, and of a width of 3 mm. Feldspar forms 

 most of the rock, and twinning is apparent, though the rock is 

 somewhat altered. The rock is porphyritic. This rock is some- 

 what similar to Nos. 9-1, 95 and 157, and its only apparent dif- 

 ference is that it is more acidic. No. 109, which occurs near the 

 Ludwig Mine, differs only a little from it, and that in containing 

 a little epidote. The groundmass is very fine, and is resoluble 

 with difficulty. 



The Porphyritc. — This rock was not studied microscopically. 

 It occurs in the north end of the first ridge shown on PI. 1. and 

 also in a few dykes in the third ridge and Blue Jay Butte. It is 

 a porphyritic rock, apparently free from quartz, and has a dark 

 colored aphanitic groundmass in which are phenocrysts of feld- 

 spar averaging from 3 mm. to 4 mm. in length. 



The Hornblende Andesite. — No. 142 is from the dykes of 

 Mason Butte, and 146 is from near Mickey Pass. Phenocrysts of 

 dark hornblende are inclosed in a light blue or lavender colored 

 groundmass. The hornblende is abundant and from 2 mm. to 

 3 mm. in length. Occasionally the feldspar assumes a length of 

 1.5 mm. The rock weathers to a dull red color, and may easily 

 be confounded at a distance with the later andesite and with the 

 rhyolite. The hornblende is occasionally twinned. The feldspar 

 shows a growth by zones which have not been equally spaced, 

 and sections approximating parallelism to co P qo (010) have a 



