Chap. 8] 



MAGNETIC METHOD 



309 



tion in the earth's field could be strongly magnetized and that specimens 

 whose magnetism was due to other causes (lightning discharges, and the 

 like) could not be magnetized further. Hence, magnetizing forces produc- 

 ing strong residual magnetism are great enough to magnetize a rock to 

 saturation; but rocks are not magnetized to saturation by induction in 

 the earth's magnetic field. 



C. Numerical Data on Magnetic Properties of Minerals 



AND ROCKS^^ 



Tables 35 to 40 contain data on susceptibilities, coercive force and 

 remanent magnetism of both minerals and rocks (see also Figs. 8-11 to 

 8-14). 



500\ 



m 



300. 

 200 

 100 \ 

 



•J 



5 10 15 20 Amp. 



Fig. 8-14. Hysteresis and susceptibility curves for Middle Devonian diabase 

 (containing pyrrhotite but no magnetite, Harz mountains, 1 amp. = 29.4 gauss). 

 (After Beyer.) 



" Largely after H. Reich, Handb. d. Geophys., VI(1), 58-71 (1931). 



