THE LODE. 269 



metals, and are often surrounded by low grade ores in great quantities. 

 Though there are exceptions to the rule, large bodies of quartz connnonly 

 contain bonanzas. The occurrence of these bodies depends on very com- 

 plex conditions, and no attempt can be made to account for their position 

 until the sections of the Lode have been passed in review. With two very 

 important exceptions they have all been found in the secondary fissure, not 

 on that with a constant dip. Excepting the Justice body they have all 

 occurred in contact with the east-country diabase. 



Complex structure of the comstock. — Thc Ordinary conccption of a vein is a simple 

 crack in the earth's crust charged with ore and gangue. The Comstock does 

 not realize this conception even approximately. With the possible exception 

 of the east-and-west veins near Silver City, the whole fissure system of the 

 District is referable to a single mechanical cause and the charging of the 

 fissures is in all probability due to simultaneous lixiviation. The branches 

 of the Lode to the north and south are structurally integral portions of the 

 Comstock, but the Lode considered as a great ore deposit is limited to the 

 contact of the diabase with the underlying rocks. 



Cross-section through the c. & c. — The most interesting vertical cross-section of 

 the Lode is that through the C. & C, Consolidated Virginia, and Andes shafts; 

 and fortunately this was pretty thoroughly accessible at the time of examina- 

 tion. The foot wall is diorite, and the hanging wall substantially diabase, 

 while the surface is capped with earlier hornblende-andesite. The secondary 

 fissure at this point was not simple but multiform, splitting the wedge of 

 country rock into sheets or sharper wedges. The intervening space is filled 

 with quartz, none of which has been sloped on the plane of this section, 

 though remunerative ore has been extracted in the Andes at a short distance 

 from it, and a very important ore body occurred near the surface some 500 

 feet to the north. The quartz contains numerous fragments of country rock, 

 too small to be shown in the drawing; and some of the horse is so silicified 

 as to be regarded in mining as quartz. At 400 feet from the surface the 

 different fissures unite, and the main fissure is supposed to continue without 

 interruption to the bottom of the Consolidated Virginia shaft, where it is a mere 

 crack. Why the vein has not been prospected for an interval of about 

 1,200 feet I cannot say. The great bonanza which has yielded over one- 



