412 



MAGNETIC METHOD 



[Chap. 8 



Iron ores of contact-metamorphic origin sometimes show reverse polari- 

 zation, indicating effects of thermal and mechanical processes. An instance 

 of this sort was observed by the author and H. Seblatnigg on the magnetite 

 deposit of Berggiesshuebel in Saxony. ^^° Its western portion shows distinct 

 negative anomalies (max, —1500 7) while the eastern part, offset by a 



^X 



/JZ- 



■-0,fcgs 



Fig. 8-60. Magnetite anomalies above Kiiruna iron ore body (after Carlheim- 



Gyllenskold). 



fault, reveals positive anomalies of approximately the same magnitude. 

 Another definite instance of reverse polarization was observed by Bahne- 

 man (see below) in South Africa. Rapid changes in amplitude, frequently 

 leading to negative anomalies, are found near deposits formed by magmatic 

 differentiation, indicating irregular concentration of magnetite in the 

 process of plastic flow and magnetostriction in the process of cooling. 



i2» Beitr. angew. Geophys., 1(1) 110-116 (1930). 



