TERTIARY ROCKS 513 



yill. The rock is very light-colored, fine-grninecl, and is composed chiefly 

 of feldspar and quartz. The feldspars are orthoclase and albite; these occur 

 as phenocrysts. Quartz makes up most of the ground-mass. Pyrite is abun- 

 dant, and occurs in an unaltered and fresh condition. It appears to be pri- 

 mary. The ground-mass is hypocrystalline, with some glass. 



IX. This rock is a little coarser than VIII, and the ground-mass is not so 

 uniform, varying from coarse to fine in different parts. It contains glass. 

 The feldspars are the same, except that they have been somewhat altered, as 

 they contain flakes of sericite. 



Where partly decomposed the porphyritic character of the rock is not 

 so apparent and it resembles a granite without ferro-magnesian min- 

 erals; on this account I used the field term "acid granite." At one place 

 the border has a narrow zone of very fine-grained acid rock, suggesting 

 aplite. Pyrite is disseminated through it, practically at the surface. 

 The acid rock is of special interest because Lawson says that from its 

 microscopic appearance it is not to be considered a plutonic rock, yet it 

 certainly seems to have the field relations of a small granitic batholith 

 that has been exposed by deep erosion. The northeastern portion of the 

 area is crossed by a high ridge that includes the Mars, N'eptune, and 

 Venus peaks. By the time one has climbed the last in the warm humid 

 atmosphere it appears to be about 1,100 feet high, yet rhyolite extends 

 from base to summit. A short distance southeast of the peak the augite- 

 andesite described under V attains nearly as high an altitude. The con- 

 tact, though somewhat irregular, may be located approximately on both 

 slopes of the ridge and is evidently very steep, if not vertical. Neptune 

 Peak is in the rhyolite area, but the nearly equally as high Mars Peak is 

 composed of the extrusive andesite, containing large dikes of rhyolite, 

 suggesting apophyses from the main mass. Elsewhere near the Mars and 

 Bonanza mines there is evidence that the contact of the rhyolite with 

 other rocks is very steep to vertical and intrusive in nature. Mine 

 workings, residuals, and the soil indicate that the main area is free from 

 any other rock species, suggesting that the rhyolite is younger than the 

 andesites. 



The rhyolite forms the hanging wall of the Bonanza vein toward the 

 southwest. The Mars and Venus veins are partly in it and the Neptune 

 vein entirely so at the surface. They are richer in quartz and poorer in 

 sulphides than the veins in the andesites. The Lone Star vein, though 

 scarcely 2 miles distant, is much richer in sulphides, particularly galena. 

 The veins of the Siempre Viva, Constancia, Concordia, and Trinidad 

 mines are characterized by the presence of a large quantity of fine- 

 grained specularite, and are generally rather rich in sulphides. At the 



