Chap. 10] 



ELECTRICAL METHODS 



677 



Anthracite coal is a good conductor of electricity and also shows strong 

 spontaneous polarization. A positive potential center, instead of a nega- 

 tive one, is found on the upper part of anthracite beds. 



It was found by Ostenneier^^ that metal concentrations in placer de- 

 posits may show spontaneous polarization. In the rivers of the bight of 



Sc*lt 



la Ftrritrt 



WO 

 777X PatentitJs between -30 i-Xm 



F^ " " -so'-ee- 



' less than 'tfO m¥. 



Fig. 10-26. Self-potential survey on mineral stringers in the Pallieres contact zone, 

 D6partement du Gard, France (after Poldini). 



Cienaga in Colombia, conditions appear to be favorable because the river 

 gravels contain boulders with impregnations of pyrrhotite which are prob- 

 ably in the immediate vicinity of the mother lodes partially eroded by 



" See S. F. Kelly, Eng. and Min. J., 114(16), 1922. 

 "Metall und Erz, 30(2), 21-24 (1933). 



