366 GEOLOGY OF THE COMSTOCK LODE. 



character, even though dependence must be phiced on the latter for a CQm- 

 plete interj)retation of the i-esuks. 



Furthermore, it will be desirable to caiuy out Fox's original idea, namely, 

 of investigating the electrical properties of oi'es and those minerals of the 

 heavy metals which are usually found associated with them; not that the 

 results of sucli an investigation could ever furnish a clew as to the par- 

 ticular ore to which an observed electric effect is due (it is here that our 

 knowledge of the locality must aid us), but that the class of ores, in pros- 

 pecting for which an electric method would be peculiarly applicable, could 

 thus be defined. The knowledge Ave possess of the conductivity and the 

 position of ores in the electrical scale is largely the result of experiments 

 made a long time ago. Recent observers have made but few quantitative 

 additions, and even these — probably from improperly chosen methods — are 

 frequently discordant. 



The method which has been described seems to me especially worthy 

 of consideration, from the fact that by means of it an electric survey, made 

 on the surface, may detect not only the presence but also the approximate 

 position of ore bodies under ground. With such an end in view the exper- 

 iments should l)e extended over a large area, and the potential at all por- 

 tions of the siirface determined. Suppose, now, that at each point of the 

 projection of the latter on a fixed horizontal plane, a vertical line is erected, 

 of a length proportional to the earth-potential at this point. The ends of 

 all such lines together make up a second imaginary surface, coextensive 

 with the first, which will represent the electric state graphically at each 

 point of the territory over which the survey has been carried. 



The efi"ect of normal earth-currents would then express itself in the 

 progress and contour of the imaginary surface as a whole, and would not 

 destroy its regularity. If its extent is not too large, this (normal) surface 

 will be a more or less inclined plane. As it has been observed that 

 earth-currents are not constant, even for short periods of time, the latter is, 

 moreover, to be regarded as slowly oscillating, more or less parallel to itself, 

 about a certain temporarily fixed position of equilibrium. But it is prob- 

 able that the limiting positions of the plane are so near to one another that 

 for this purpose they may be regarded as coincident. 



