Vol. 4] 



Reid.—The Comstock Lode. 



197 



first period of vein-filling was due to the primary faulting, and 

 low-grade materials were placed in the open fissure. The second 

 and later period opened new fissures, rift veins in Virginia City, 

 and openings within the vein in Gold Hill, in which the rich con- 

 centrated ores of the bonanzas were deposited. This second 

 period probably continues in the depths, as it would surely do 

 above were the lode still intact from man's hand. The details 

 of this ore deposition have not yet been thoroughly studied out, 

 nor can they be until our knowledge of the physical chemistry 

 of the subject is more complete. 



Location of Bonanzas. — The deep ore bodies of Virginia City 

 have been, and will be, found within the hanging wall, in more or 

 less vertical fissures, of which the surface "east vein," the Great 

 Bonanza, and the vein now being worked, are examples. More 

 such bodies should be found by properly-driven cross-cuts and 

 drifts lower down and to the eastward. There is also a large 

 stretch of the lode above the 2150 feet level which has not been 

 thoroughly explored. The probable reason for the peculiar rifting 

 of the hanging wall block is that the cementing of the first frac- 

 ture by quartz, and the concomitant weakening of the hanging 

 wall by the leaching action of the ground waters, enabled the 

 later stresses to fracture the hanging wall block as it is found. 

 The reason for believing in the existence of still deeper similar 

 rifts filled with ore is that the surface for two miles eastward 

 from the lode shows the hanging wall block to be greatly altered 

 by the action of hot waters, and therefore weakened. The Sutro 

 tunnel section corroborates this, and the mine workings also show 

 the rocks east of the lode not to be solid nor unaltered. 



Also, there is considerable concentration of ore taking place 

 from above by the surface, or vadose, waters. These ores will 

 occur on or near the footwalls of the numerous branches of the 

 lode which outcrop on the surface, within a few hundred feet of 

 the outcrops. Such material is low-grade, however, and. in the 

 main, not yet available because of the high cost of mining and 

 milling. A body of future reserves is thus assured. The low- 

 grade of these ores has been proved by numerous assays. Fur- 

 ther, the west wall of the lode has never been thoroughly investi- 

 gated, and such work might prove very profitable. 



