148 SILVER-LEAD DEPOSITS OF EUREKA, NEVADA. 



"The analogy between the electrical method and the method of assays 

 is clear, for in the latter advantage is also taken of the fact that the indica- 

 tions of ore occupy a greater space than the ore body itself. The differ- 

 ences are also clear, for while the assay method depends upon the solubility 

 of the ore, the permeability of the rock, the distribution of fissures, and the 

 like, the electrical method depends upon the distribution of electrical 

 activity in the body producing the effect, and upon the electrical conduc- 

 tivity of the surrounding rock. The two methods are, therefore, entirely 

 independent, and it is a particularly interesting fact that the results obtained 

 in the Richmond mine were accordant. Mr. Curtis and I have met with a 

 coincidence of two independent effects of the same cause, both of which 

 indicate in different ways the presence of ore in the vicinity of point XV. 

 of the 600-foot level of the Richmond mine. It is gratifying to find that 

 an ore body was actually discovered, subsequently to our experiments and 

 independently of them, precisely where we had most reason to look for it 

 I greatly regret not to have been able to be present to study the distribu- 

 tion of potential relative to the new body in detail. 



"There is one more remark with a bearing on these inferences which 

 I desire to make. The relation of the earth-potential encountered along 

 any line of electrical survey to distance, when expressed graphically, appears 

 as a broken line possessing certain distinct characteristics. I proved, how- 

 ever, that the progress in the values of earth-potential, observed on passing 

 from one point of a drift to another, is continuous, and that therefore the 

 potential line in our diagram, however sinuous, never suffers a break of 

 continuity; whence it follows that we may regard the curves obtained as 

 containing unknown disturbing effects superimposed on the decidedly larger 

 electrical effect attributable to the ore bodies. I infer that in any extended 

 line of electrical survey, besides the large field of electrical excitation due 

 to the ore bodies, very many smaller fields, distributed throughout the 

 mine, are constantly encountered and intersected." 



use of the mothod of prospecting by assays. — The method of prospecting by assays 

 has one important advantage over the electrical method; it can be carried 

 out with comparatively little expense and with little loss of time. It must 

 be remembered, however, that the assays will be useless unless made with 



