WEST END ANDESITES. 187 



andesite, thus bringing up again the question of the exact age, which has just 

 been raised with respect to what is probably the corresponding rock in the 

 West End. It was found, however, that the peculiarities which suggested the 

 correlation of the Gold Hill andesite with the later andesite, namely, the fre- 

 quently large-sized feldspars and the presence of biotite, could be paralleled in 

 specimens found in Mizpah Hill, even in the workings of the Mizpah mine, and 

 again in the Montana Tonopah, where there was no question as to the andesite 

 being other than the earlier andesite. 



Moreover, in Gold Hill this andesite incloses veins having all the character- 

 istics of the veins found in Mizpah Hill, such as have not been found in the 

 undoubted later andesite. Therefore the evidence decidedly favors the conclusion 

 that the Gold Hill rock is the earlier andesite. If it is true, as has been con- 

 cluded, that the veins of the Wandering Boy and the Fraction were originally a 

 part of the Valley View system and that they were displaced by faulting, the 

 evidence grows still stronger. 



THE WEST END ANDESITE PROBABLY EARLIER ANDESITE. 

 l. ( 



The writer is forced to the conclusion that the andesite exposed on the 200- 

 foot level of the West End belongs to the earlier andesite. 



CONTACT BETWEEN EARLIER AND LATER ANDESITES. 



PLACE AND CHARACTER OF CONTACT. 



The conclusion that the rock on the 220-foot level is the earlier andesite 

 having been reached, the question comes up as to the line of demarcation between 

 the earlier andesite below and the later andesite above. Since the West End 

 fault probably dips southwestward and is normal, the shaft, after passing through 

 the fault and leaving the rhyolite, is in the block lying northeast of the fault, 

 which may be called the Midway block. This block is characterized at the sui'- 

 face everywhere by undoubted later andesite. It is, then, likely that the contact 

 between the later andesite and the earlier andesite occurs in the West End shaft 

 somewhere above 196 feet, and from considerations given it may be assumed, 

 temporarily at least, that it lies between 116 and 196 feet (see p. 185). 



This assumption is rendered somewhat doubtful by the fact that no contact 

 was observed, but, on the other hand, the rock is thoroughly decomposed and 

 much disturbed by faulting, so that the presence of a contact would be obscured. 



NATURE OF SIMILAR CONTACTS ELSEWHERE. 



At another point where the writer has seen the contact between the over- 

 lying later andesite and underlying earlier andesite, in the same fault block, at 

 the Tonopah Extension, the contact is by no means striking, and could not be 



