MAGNETIC METHODS 



169 



beds may be intercalated with beds free from magnetite. In schists and 

 gneisses the tendency to banding may cause a marked tabular distribution 

 of ferromagnetic minerals. 



Attitude of Tabular Concentrations. — The attitude (dip and strike) 

 of tabular concentrations of magnetic materials is of importance, since it 

 affects the type of magnetic anomaly produced. Certain ferromagnetic 

 masses are sufficiently concentrated that the earth's magnetic field induces 

 polarity in them, as has been indicated. In the northern hemisphere and 

 under ordinary conditions, such induction in a tabular mass produces south 

 polarity along the outcropping edge. There are, however, certain very 

 limited attitudes which such a magnetic tabular concentration can assume 

 in which its relation to the earth's magnetic field is such that the induced 

 polarity in the outcropping edge will be north polarity. Specifically, if the 

 magnetic bed strikes magnetic east-west and dips against the earth's 

 magnetic field at an angle very slightly greater than the inclination of the 

 earth's field, its outcrop will show north polarity. (See Figure 71.) 



NORTHERN HEMISPHERE 



EARTH S FIELD 



MAGNETIC BODY WITH NORMAL OR 



S. - POLARITY NEARER THE SURFACE. 



MAGNETIC BODY WITH ABNORMAL 

 OR N.+ POLARITY NEARER THE 

 SURFACE. 



Fig. 71. — Showing the effect of the attitude of a tabular magnetic body on its 

 induced polarity. 



The actual occurrence of such induced north polarity is rare. In general, 

 sharper distortions of the earth's magnetic field are made by steeply dipping 

 magnetic formations, or those whose dip most closely approaches parallel- 

 ism with that of the lines of force of the earth's magnetic field. 



Topography. — The effects of topography are difficult to predict. It is 

 certain, however, that variations in bed rock topography in a placer area 

 would be a factor in determining the shape of the magnetic anomaly. Varia- 

 tion in bed rock depth alone would alter the magnetic effects arising from a 

 given set of magnetic conditions. 



The sub-outcrop of a magnetic orebody would show up in a considerably 

 different manner if it were on a steep slope, at the top of a hill or ridge, 

 or in a valley. In the case referred to, where the buried magnetic body lav on 

 a steep slope, a magnetic traverse up the slope across the body would have 

 quite different magnetic values for those stations below the body than for 

 those above it. 



