164 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



rich ore, like those which occurred in the Fraction, were not encountered to any 

 extent. Metallic minerals, other than a limited amount of iron, are not often noted 

 in the veins. Some ruby silver and argentite, like that in the Fraction, have been 

 reported, but were not seen by the writer. 



VEINS OF GOLD HILL. 

 GOLD HILL A FAULT BLOCK. 



The Gold Hill block is of especial interest, as being the only outcropping 

 block of earlier andesite besides the Mizpah Hill block. It is, roughly speaking, 

 a triangular area. It is bounded on the north and south by faults and on the 

 east by the intrusive dacite of Golden Mountain. The fact that the contact of 

 this dacite, as shown on the map, is nearly a straight line, suggests strongly the 

 idea that it has been determined by a preexisting fault. This idea is strengthened 

 by an inspection of the boundary just northeast of the Tonopah and California 

 shaft, where the intrusive dacite contracts to a narrow dike, which separates the 

 block in which the Tonopah and California shaft is situated from the Gold Hill 

 block. The former block has at its surface the white tuffs (Siebert tuffs) of the 

 lake beds, under which the Tonopah and California encountered the earlier 

 andesite. This block is therefore depressed with reference to the Gold Hill block, 

 and the dacite dike has been intruded along the fault plane. 



NATURE OF GOLD HILL ANDKSITE. 



The character of the andesite of Gold Hill has been the subject of critical 

 study. On the western extremity of the block at a point south of Mizpah Hill, 

 the andesite has the same peculiar appearance as at Mizpah Hill. Farther east, 

 toward the top of the hill, the andesite takes on a different appearance, being 

 darker and showing somewhat larger feldspar phenocrysts and frequent pheno- 

 crysts of altered but easily recognizable biotite. The latter kind of andesite 

 resembles in some ways the later andesite, and at one time aroused in the mind 

 of the writer the same doubt as to its affiliation that the andcsites of the Fraction, 

 West End, and MacNamara did. Critical study, however, established the following 

 points: That there is no real boundary between the typical Mizpah Hill variety 

 of andesite and the biotite-bearing andesite of the eastern part of the Gold Hill 

 block; that under the microscope the last-named phase showed many other char- 

 acteristics of the earlier andesite, while it was seen to contain, as ferromagnesian 

 phenocrysts, biotite to the practical exclusion of hornblende or pyroxene; and that 

 the Gold Hill andesite contained small but typical quartz veins like those of Mizpah 

 Hill. One of these Gold Hill veins has produced rich ore, although in limited 

 quantity. Moreover, while the Gold Hill shaft shows in its upper portion the 



