Vol. 4] 



Rcid. — The Comstock Lode. 



195 



At 3350 feet an entirely new rock, as far as macroscopica] ap- 

 pearances go, begins gradually to appear. This rock is of the 

 texture of the pinkish diorite so common on the surface, but is 

 almost black in color, with the peculiar luster of augitic rocks. 

 The microscope shows it to be a true augite diorite, precisely the 

 same rock as the surface stone except that its augite is fresh or 

 only slightly altered. The usual amount of free quartz is pres- 

 ent, as in all these rocks, and likewise some few flakes of biotite. 

 This phase of the diorite mass appears to be the core of Alt. 

 Davidson. 



Proceeding farther in the tunnel, the pinkish rock tends to 

 reappear gradually beyond 4500 feet, and at 4700 feet a peculiar 

 mottled facies shows itself. This has not yet been investigated, 

 but is merely a variety of the diorite. At 4908 feet in, the ' ' west 

 fault," as it will be called, occurs, beyond which appears the 

 "west diabase." The rocks of this section, when properly 

 studied, will complete the work of Hague and Iddings east of the 

 lode, and will serve to bring into greater relief the admirable 

 work of these men. 



CONCLUSIONS. 



Form of Lode. — The Comstock lode is divisible on structural 

 grounds into two main portions: (1) the Virginia City portion, 

 and (2) the Gold Hill portion. The Silver City fault and lode, or 

 "branch," as now called, is a distinct unit, probably of later age. 

 The grounds for this belief are, as seen, the facts of it being a dis- 

 tinct fault, and that the vein filling is different from that of the 

 Comstock lode proper. The only grounds for a belief in a later 

 age are those of structure ; it is well shown that much faulting 

 took place after the first formation of the main lode, and to com- 

 bine all the facts presented it is necessary to assume a period of 

 faulting not coincident with that which formed the bonanza 

 gashes, but later than the first faulting. In this country of great 

 and long continued faulting such an assumption is not without 

 a good basis. 



The Virginia City portion of the lode is bounded on both 

 north and south by a series of east-west faults. Also, to the north, 

 some forking of the lode occurs, with one strong branch bending 



