316 



MAGNETIC METHOD 



[Chap. 8 



sities in lightning flashes are of the order of 20,000 amperes and give 

 rise to considerable magnetic fields in their vicinity. This explains the 

 irregular magnetic polarities encountered on exposed mountains, hill 

 tops, and the like. Pockels showed by laboratory experiments that 



Table 40 

 COERCIVE FORCE AND REMANENT MAGNETIZATION OF ROCKS « 



" Fields in gauss. 



rocks actually may be magnetized by spark discharges from an induc- 

 tion coil. Columns of basalt erected near lightning rods will even- 

 tually show strong permanent magnetizations. The earlier geophysical 

 literature abounds in articles in which authors have attempted to recon- 

 struct the direction of the earth's magnetism in previous geologic periods 



