402 GEOLOGY OF THE COMSTOCK LODE. 



imum, or both, or a number of such characteristic variations. In short, the 

 earth-potential at any point may be regarded as a function of the distance of 

 this point from the assumed origin of the line. The assertion that this 

 function will pass through a characteristic change of the kind specified is 

 only another way of stating that the line may be chosen so long that, 

 in comparison with its extent, the field of sensible electrical action will be 

 local, or its linear dimensions in the direction in question small. Maxima 

 in a general sense are, therefore, to be regarded as criteria, and as indicat- 

 ing the part of the line nearest to the electrically active ore body. 



Practically, since we possess no means of measuring potential abso- 

 lutely, it is sufficient to assume a value (zero) for one of the points of the 

 series. The electromotive force between this and any of the other points 

 is then the potential of the latter. 



Methods employed. — In making thc actual measurements, the simple problem 

 above enunciated became quite complicated, because the small lode electi'o- 

 motive forces were afi'ected by a number of errors, which, in the aggregate, 

 might possibly produce an effect in the same order. On the Comstock, 

 where the mine workings were, without exception, in very barren or nearly 

 exhausted parts of the vein, no definite evidence of currents due to the Lode 

 itself was obtained. Even at Eureka, in spite of the enormous ore bodies in 

 sight, the range of variation of potential corresponding to a distance of 

 2,000 feet in the underground experiments very rarely I'eached 0.1 volt; 

 while usually this variation lay within a felv hundredths of a volt. These 

 limits, in a case where such disturbances exist as action between terminals, 

 polarization, earth currents (normal), bad insulation of circuit at any point, 

 difference of potential between liquids in contact, incidental effects due to 

 masses of metal distributed throughout the mine, etc., are to be considei'ed 

 as comprehending a rather dangerously small interval. This small varia- 

 tion of potential is to be attributed to the earthy character of the Eureka 

 ores. For the manner in which the effects of the disturbances were to a 

 large extent eliminated, the reader must be referred to Chapter X. 



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 necessary could be satisfactorily applied. The 



