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University of California Publications. 



[Geology 



to the east in the Sierra Nevada ground. The faults or slips to 

 the south are those approximately in and east of Bullion Ravine. 

 But few of these east-west fractures contain much secondary 

 mineral. The few which do become veins are largely calcite 

 bearing, and probably of different age from the others. Between 

 these two lines of east-west motion is located the Virginia City 

 portion, differing from the other part in having a greater rela- 

 tive movement of the foot and hanging walls. This motion has 

 been so great that an unequal movement of the hanging wall 

 block was produced, the bottom moving farther than the top, 

 with consequent rupture. These ruptures produced the second- 

 ary vertical gashes, or veins of rifting. This motion causing rup- 

 ture, however, was distinctly later in age than the first vein form- 

 ing movements. Hence, when the secondary openings were 

 formed, they were filled with concentrations from the previous 

 deposits as well as with original supplies from great depths. And 

 there is no good reason for assuming that either the movements 

 or the ore deposition have ceased, but rather all facts tend to 

 confirm the idea that ore is yet being moved from place to place 

 in the greater depths as well as fresh supplies from below being 

 brought up by the hot waters. 



In the Gold Hill portion the relative movement of the walls 

 of the lode has been less ; there have been no rift veins formed, 

 and the ore bodies are within the lode walls, near the hanging. 

 The same two periods of deposition of vein-filling were present 

 here, the bonanzas occupying later fissures near the hanging wall 

 of the earlier vein. No doubt deposition is still progressing in 

 depth here, though not enough mining work has been done to 

 allow a definite statement in this regard. The one exception to 

 form in this portion of the lode, in the Yellow Jacket mine, was 

 due to the fact that the vein, in its proper plane, did not reach 

 the surface, so that the relative movement of the hanging wall 

 block downward was taken up near the surface by a gash or rift. 



Deposition of Ores. — On account of the importance of the 

 subject, a reiterated statement is not out of place regarding the 

 two periods of ore tleposition. Had the second of these periods 

 not existed, there would be practically no ore on the Comstock, 

 hence the relations of these two must be of vital concern. The 



