146 SILVER-LEAD DEPOSITS OF EUREKA, NEVADA. 



series of phenomena are to be regarded as two effects of one cause, and are- 

 not immediately dependent upon one another, each affords a remarkable 

 confirmation of the inferences which would be most naturally drawn from 

 the other. The facts were submitted to Dr. Barus for his opinion, which is 

 as follows: 



"It is entirely impossible that there should be any direct connection be- 

 tween the assay value of the rock at a given point and the value of earth poten- 

 tial for the same point, The nature of the distribution of electrical potential 

 can be made clear to those unfamiliar with the subject by the aid of an anal- 

 ogy. Instead of drawing inferences with reference to an ore body considered 

 as a source of electrical activity, I will suppose that we have to do with a hot 

 body, that is, one whose temperature is decidedly above that of the surround- 

 C b 









\ v Y'Ol--'' / / / 







FlG. 7. — Illustration of electrical activity. 



ing rock. Let c b, Fig. 7, represent the surface of the earth. Let A be an 

 area (in section) of constant high temperature beneath it, Suppose the 

 body has been in place for an indefinite length of time, so that the thermal 

 distribution has become stationary. Let the problem be that of finding the 

 body A from observations made on the surface of the earth. The first 

 step would be to take earth temperatures at convenient number of points 

 intermediate between c and some remote point, b. If the distances were 

 then plotted as abscissas and the earth temperatures as ordinates, a curve 

 would be obtained which in the simplest case would be characterized for 

 an abscissa corresponding to a point nearest the hot body A by a maximum^ 



