268 GEOLOGY OF THE COMSTOCK LODE. 



replaced by alteration products. The foot or west wall of the main fissure 

 is granular diorite for more than three-quarters of its length, but at the 

 southern end it is chiefly composed of metamorphic slates. The foot wall is 

 much less altered than the hanging. The northern branches, excepting the 

 most easterly one, are inclosed in porphyritic diorites, though stringers of 

 diabase also make their appeai-ance in one or two spots on the fissure which 

 extends toward the Utah shaft. The southern branches pass along a variety 

 of contacts. 



Black dike. — Accompanyiug the vein for about half its length is the nar- 

 row dike of younger diabase called "black dike." It is found only a little 

 north of the middle of the main Lode, extending thence southward and 

 following the southwest branch. It usually lies directly upon the foot wall, 

 but occasionall}^ passes a short distance behind it. In the higher levels it 

 was so decomposed as to be unrecognizable as diabase. 



Contents of the vein. — The contcnts of the vein are simple, on the whole, con- 

 sisting of country rock in fragments varying in size from that of a grain of 

 sand to horses thousands of feet in length, clay, quartz, and argentiferous 

 minerals. The quantity of calcite, except in the Justice, is wholl}^ insignif- 

 icant, and gypsum, zeolites, etc., are rare. Some of the quartz is said to 

 contain no silver or gold; but for the most part it carries both, though in 

 varying quantities. That which lies upon or is inclosed in diorite carries 

 gold, but little silver; very little of this, however, will pay the expense of 

 extraction and treatment. The qviartz associated with the hanging wall 

 carries more silver, accompanied by gold of a value nearly equal to that of 

 the silver.^ The variation in the tenor of the quartz is extreme, as it usually 

 is in silver veins ; and it is only in certain spots that the quartz assays above 

 the fifteen or twenty dollars necessary to warrant extraction at the present 

 prices of labor and supplies; while occasionally the value per ton of com- 

 paratively small masses runs up to several thousand dollars. Masses of ore 

 which will pay for extraction are called throughout the region west of the 

 Rocky Mountains bonanzas, a Mexican mining term which avoids the ambi- 

 guit}' of the English term ore. The bonanzas, therefore, do not represent by 

 any means all of the quartz which carries a perceptible amount of precious 



' See table of thp proportions of gold and silver in Comstock bullion, p. 9. 



