404 GEOLOGY OF THE COMSTOOK LODE. 



probable that the interpretation of the results would furnish clews for 

 the economical exploitation of mines, comparable in value to those of a 

 purely geological character. Both should go hand in hand. Under ground, 

 this general method of research is not always feasible, as it presupposes that 

 the mine has been already widely exploited. On the surface of the earth, 

 however, it may to some extent be applied; and in this case the endeavor 

 would be to obtain the traces of the equipotential surfaces on that of the 

 earth. Suppose, for instance, that the potential at every point in several 

 square miles of the earth's surface were known. Then let this surface 

 be projected on a fixed horizontal ("XY") plane, and the value of earth- 

 potential corresponding to each of the points be constructed as "Z." In 

 this way a new (potential) surface would be obtained coextensive horizon- 

 tally with the former. Terrestrial electrical action would manifest itself 

 upon the new surface as a whole and would not affect its regularity. Local 

 action, on the other hand, would produce an effect circumsci'ibed in com- 

 parison with the horizontal extent of the area under consideration. We 

 should expect to find a hillock or depression, or both, or a number of these 

 inequalities in the imaginary potential surface. 



