678 



ELECTRICAL METHODS 



[Chap. 10 



the river bed. A cane provided with two electrodes at its end and a 

 simple potentiometer consisting of a rheostat, galvanometer, and dry cell 

 was sufficient to locate the eroded vems and boulders in the river bed 

 (see Fig. 10-27). 



By accurate spontaneous polarization measurements, taken at short 

 intervals, small potential differences have been established on formation 

 boundaries." At five-meter intervals the potential differences are of the 

 order qf several tenths of a millivolt, whereas in undisturbed terrain their 

 value is generally but one-tenth of this magnitude. Boundary potentials 

 are probably caused by differences in the conductivity of solutions filUng 

 the pores, so that "concentration elements" are formed (see ii, a, page 631). 



Fig. 10-27. Spontaneous polarizations obtained on ore placers and eroded veins in 



Colombia (after Ostermeier). 



In Fig. 10-28, curve h shows the potential gradients; curve h' shows the 

 potentials, and the section below indicates a formation boundary between 

 granite and gneiss which was located on the basis of the potential measure- 

 ments by four auger holes. 



In areas of metamorphosed sediments it is sometimes possible not only 

 to locate formation boundaries but also to make more detailed studies of 

 geologic structure. Conditions showTi in Fig. 10-29 made the self-poten- 

 tial method applicable because graphite occurred in some of the k6y beds. 

 In the copper district of Katanga the ore occurs in anticlines of the so-called 

 mine series of the Kundelungu formation, made up largely of slates and 

 dolomites and containing a graphitic horizon. Another similar horizon 

 occurs in the so-called Muaslira series, immediately beneath the Great 



■^* H. Hunkel, Zeit. Prakt. Geol., 36(7 and 9) (July and Oct., 1928). 



