SOURCE OF THE ORE. 81 



it is difficult to conceive of a derivation from different sources without a 

 difference in the time of deposition, which would necessarily result in a 

 variation in the character of the ore. 



Metau contained in the country rock. — There is a sharp distinction between the com- 

 position of the ore and that of the inclosing rock. Iron oxide forms the 

 gangue of the ore bodies, and about one-half of the ore is composed of 

 that mineral, the other portion being made up of lead, arsenic, sulphur, 

 zinc, silver, carbonic acid, etc. The first four of these substances do not 

 seem to occur in the least metamorphosed limestone, and only appear in the 

 more altered limestone in small bunches and seams in the neighborhood of ore 

 bodies. There is a large block of stratified limestone on the sixth level of 

 the Richmond, which is very little altered, and it shows no evidence of ever 

 having contained any quantity of the metals enumerated above. It is hard, 

 compact, and crystalline-granular. It is distinctly stratified, and has been 

 comparatively little disturbed. The highly-altered limestone, on the other 

 hand, contains notable quantities of some of these metals. 



Ratio of the ore to the limestone. — As close a calculation as possible has been made 

 of the relative proportion of ore to limestone, in order that some idea may 

 be formed of what percentage of the metals that rock must have originally 

 contained if the ore had been uniformly distributed in small pai'ticles 

 throughout its mass. The ratio of the limestone to the ore extracted from 

 that portion of the mineral zone situated between the main fissure (the 

 Ruby Hill fault) and the contact fissure, between the quartzite and lime- 

 stone in the ground southeast of the Richmond shaft, is about 100 to 1. 

 In the mineral zone northwest of this shaft the ratio of limestone to ore is 

 somewhat greater. If a reduction of one-half is made for large bodies of 

 low-grade ore which up to the present time would not warrant extraction, 

 and for yet undiscovered masses, of which it is but reasonable to suppose 

 there are some in existence in the portion of the mineral belt southeast of 

 the Richmond shaft, the ratio of the limestone to the ore would be 50 to 1. 

 Putting the assay value of the ore at $40 in gold and silver to the ton of 2,000 

 pounds, and the percentage of lead at 10 per cent., which cannot be regarded 

 as too high when all the ore which has been removed from the mines is 

 taken into account, then each ton of limestone must originally have con- 

 tained 80 cents in value of the precious metals and 0.2 per cent. lead. 

 2054 L 6 



