Chap. 8] 



MAGNETIC METHOD 



315 



Tab LBS 39 

 COERCIVE FORCE AND REMANENT MAGNETIZATION OF MINERALS" 



" With few exceptions, only data for magnetite crystals and magnetite ores are available. Fields in gauss. 



observed on rock powders mixed with known quantities of magnetite (see 

 Fig. 8-15). According to Slichter", the deviation from linearity is due to 

 the fact that with an increase in magnetite the demagnetizing influence 

 of the gaps between the magnetite particles decreases. 



From experiments made on powders of different fineness, Puzicha 

 concluded that fine-grained powders have a lesser susceptibiUty than 

 powders of coarse grain. Hence, plutonic rocks are generally more mag- 

 netic than volcanic rocks. 



2. In the course of the geologic history of a formation, a number of physical 

 forces are likely to affect its mag- 



netic properties. They are of a 

 thermal and mechanical nature and 

 occur in connection with igneous 

 intrusions, regional metamorphism, 

 tectonic movements, mechanical and 

 chemical concentration, disintegra- 

 tion, and lightning. 



The effect of lightning on rock 

 magnetization is greater than is 

 generally assumed. Current inten- 



Kf^ 



1000 



1200 



400 



2 4 



1* L. B. Slichter, A.I.M.E. Geophysi- 

 cal Prospecting, 247 (1929). 



6 » to 12 14 fg 



Weifht Mtfnelife per mi//t 

 Fig. 8-15. Relation between suscepti- 

 bility and magnetite content (granite, 

 magnetite powder) (after Puzicha). 

 (Susceptibilities in field of 15 gauss.) 



