Chap. 8] 



MAGNETIC METHOD 



421 



of about 500 7. Placer channels may also be located structurally if the 

 bedrock is magnetic. Fig. 8-68 shows a negative magnetic anomaly above 

 a bedrock depression corresponding to an auriferous gravel channel in 

 California. 



Structural magnetic prospecting has been applied in various countries 

 in coal exploration. Edge and Laby^*^^ surveyed the brown coal field at 

 Gelliohdale, Victoria, with torsion balance and magnetometer. The 

 brown coal is about 400 feet thick and occurs 50 to 100 feet from the 



-^2800 

 > 



2600 

 Fig. 8-68. Magnetic lows over placer channels in California (after E. W. Ellsworth). 



surface, in troughs of Jurassic sandstone. Magnetic anomalies were 

 caused by this sandstone and basaltic intrusions. Since Carboniferous 

 deposits were formed in geosynclines, the regional depressions of the crystal- 

 line basement and therefore negative magnetic anomalies have been found 

 to be characteristic of productive Carboniferous in many areas in Europe 

 (productive belt through Mons, Namur, and Luettich;^^^ Ruhr Basin be- 

 tween Muenster, Duesseldorf, Elberfeld, and Soest;^^" Carboniferous areas 



»8 Ibid., 168. 



"» Reich, op. ciL, I, 20. 



"0 Ibid., I, 31. 



