166 



EXPLORATION GEOPHYSICS 



Figure 70 is a graphic classification of 317 samples of rock as to type 

 and magnetic susceptibility.f A sufficient number of samples was tested 

 for each of the 5 major rock types which were considered to make the 

 susceptibility data representative and significant. The percentages of the 

 samples of a given type of rock falling in a particular range of magnetic 

 susceptibility are shown in the figure. The figure illustrates, for example, 



MAJOR ROCK TYPES 



Fig. 



70. — Graph showing the percentage of test samples of 5 major rock types having 

 magnetic susceptibility ranges from > 10-* to < 4x10-^ c.g.s. 



that of the 74 specimens of granite (and allied types of rock) whose mag- 

 netic susceptibility was measured, 60% had a susceptibility of less than 

 10~* c.g.s. The remaining 40% of the samples was divided as follows : 

 23% in the susceptibility range of from 10^"* to 10"^, 16% from 10~^ to 

 4xl0~^, and only 1% above this latter figure. The data from which this 

 graph was constructed did not indicate how much above or below the 4xl0~^ 

 and the 10~^ values respectively the susceptibilities measured. 



However, in planning magnetic surveys as applied to mining problems, 

 it must be remembered that a conclusion which may be valid in one locality, 

 on the basis of susceptibility contrasts, may not apply in another, although 



t Based on data in Table 20-3, p. 296, Handbook of Physical Constants; G.S.A. Special Paper 

 No. 36, section on Magnetic Properties of Rocks, by L. B. Slichter (January, 1942). 



