324 GEOLOGY OF THE COMSTOCK LODE. 



EXPERIMENTS MADE AT THE RICHMOND MINE, EUREKA DISTRICT, 



NEVADA. 



Opportunities for investigation. — 111 determining to make the study of local cur- 

 rents a part of the work to be done under his charge, Mr. Becker^ had 

 selected both the Comstock Lode and the Eureka district as available local- 

 ities, in which to test the applicability of an electrical method as an aid to 

 prospecting. The former is a fissure vein, in which the ore, comparatively 

 free from base material, is scattered irregularly through a quartz gangue. 

 At Ruby Hill, Eureka, the ore is principally plumbic carbonate and sul- 

 phide and oxide of iron — the whole containing more or less silver and gold — 

 occurring, moreover, in huge, apparently isolated masses in limestone. In 

 most of the cases fissures containing vein matter and connecting the cham- 

 bers have been traced. The faciHties ofi'ered for the prosecution of the 

 investigation by the Eureka deposits were therefore, to all appearances, 

 unusually great. The immense ore bodies in sight were furthermore at a 

 mean distance of not more than 400 feet from the surface, and a series of 

 electric surveys could easily be carried out over, through, and under them. 

 Finally, it appeared not at all improbable, insomuch as the ore bodies in 

 places extend to within 100 feet from the surface, and are in fact to some 

 extent above the mean surface of the surrounding country,^ that local elec- 

 trical currents might actually be detected on the surface itself. In consid- 

 eration of this encouraging prospect due pains were taken to work up all 

 the experimental details with corresponding care. 



Arrangem'ent of terminals. — Abovc all tilings it was uccessaiy to devisc somc 

 method of obtaining electric contact between the ends of the metallic, cir- 

 cuit and the rocks, which would be free from the difficulties met with in 

 the Comstock. Metallic plates, etc., used alone, are objectionable (see page 

 358) ; but it is clear that through the intervention of a suitable liquid, eff'ects 

 of polarization, etc., can be avoided. The following contrivance, based on 



' Cf. First Annual Report U. S. Geolog. Surrey, p. 46, 1880. 



'Being in Ruby Hill, an elevation of some hundreds of feel above the extensive pUun pailially 

 surromKlini; it. 



