192 GEOLOGY OF THE COMSTOCK LODE. 



any now occupied by stratified rocks. Further indications of such a history 

 are observable in the Sierra Nevada, where a thin and not very extensive 

 body of highly crystalline stratified limestone is completely inclosed in 

 diorites, which are granular on one side and porphyritic on the other. I 

 am able to offer no better suggestion than that this mass was carried into 

 its present position by the granular diorite, and covered over sooner or later 

 by a porphyritic outflow. 



Eruptive diorite. — Bcsidcs tlic dioHtic mass forming Mount Davidson and 

 the adjoining hills, there is somewhat obscure surface evidence of a large 

 area of this rock beneath later eruptive masses. Near the Forman shaft are 

 several small patches of mica-diorite, which, however, might easily be passed 

 unnoticed; and in the Flowery district, about a mile and a half east of 

 Flowery Peak, dioritic porphyries again appear. Diorites occur in almost 

 all the CoMSTOCK mines from the Silver HiU north to the Utah, and are 

 also found in those of the Flowery region. The dump of the Lad// Bri/ai/, 

 for example, consists largely of fresh, coarsely granular, quartzose diorite 

 To the west and northwest of Mount Davidson it also appears to be covered 

 by but a thin cap of andesite, so that at least two islands of the older rock 

 are wholly surrounded by the younger. Diorite forms the foot wall of the 

 Lode throughout the Virginia mines and is replaced in this position by met- 

 amorphics in Gold Hill. On the hanging wall it is found in the Yellow 

 Jacket in masses apparently displaced, and in the Sierra Nevada and Utah it 

 forms both walls of the fissure which has been mainly explored. Frag- 

 mentary masses also appear embedded in diabase at intermediate points 

 l)ut not to an important extent. Before the eruption of the earlier diabase, 

 the diorite no doubt formed a continuous mass, partly overlying and partly 

 underlying the metamorphic strata, and probably extended over the coun- 

 try now occupied by later rocks along the line of the Sutro Tunnel. If so, 

 this area has sunk under the subsequent outflows, but how far it is as yet 

 impossible to say, though it is a matter of importance to the future of the 

 Lode. At the time of the faulting the whole west wall in "Virginia and 

 Gi-old Hill seems to have risen, the dislocating tendency having been adjusted 

 towards the ends of the fissure by diverging cracks. This action has moulded 

 the eastern face of the range opposite Virginia City and the northern por- 



