142 SILVER LEAD DEPOSITS OF EUREKA, NEVADA. 



ing the ground below its level, and is only advantageous as a means of 

 ventilation and as an exit for the water. Ground below the tunnel level 

 can be worked and drained to advantage only by the help of a vertical 

 shaft from the surface, and such a shaft costs as much as if there was no 

 tunnel. The advantages of the tunnel system in Prospect Mountain, how- 

 ever, as a means of prospecting are numerous. In the first place, owing to 

 the topography of the country, it is possible in many places to gain a foot or 

 more in depth with every two feet of tunnel. Again, the deposits are found 

 throughout a belt of limestone over a mile in width, and are as likely to be 

 discovered by a tunnel as a shaft. Moreover, many of the claims on the 

 surface are owned by small companies, which cannot afford expensive hoist- 

 ing machinery, but which could pay their proportion of the outlay neces- 

 sary for the part of the tunnel developing their ground. 



The presence of ore cannot be predicted with certainty, but this much 

 at least can be said, that all the indications, and the results so far obtained, 

 point to the existence of numerous ore bodies in the heart of Prospect 

 Mountain, and although mining for the precious metals has not been 

 reduced to anything like as great a certainty as is the case with coal and 

 iron mining, a skillful use of the knowledge already obtainable will in 

 some measure reduce risks which invariably attend mining operations. The 

 air and ventilation are good in all the mines of the district where proper con- 

 nections have been made, even in the lower levels. 



Electrical observation and assays. — In describing the second list of assays, page 

 84, reference was made to some observations by Dr. Barus in the Rich- 

 mond mine in regard to the electrical activity of ore bodies. Mr. Becker, 

 in summarizing this investigation," says: "Of the different surveys made, 

 the one on the 600-foot level of the Richmond mine, west drift, presents 

 the greatest interest, because it was here that all the precautions neces- 

 sary could be satisfactorily applied. The line of survey, moreover, lay 

 completely outside of the ore body, and all the points tapped were in rock, 

 essentially of the same kind. The measurements were made in various 

 galvanometric ways, and the results were subsequently checked by a 'zero' 



"The methods employed and the results obtained are fully explained by Dr. Carl Barus in the 

 Geology of the Comstoek Lode, Chap. X. 



