MAGNETIC METHODS 161 



(60,000 gammas) strength. The earth's magnetic field, nevertheless, 

 appears to be sufficient, coupled with the rather wide distribution of the 

 ferromagnetic minerals, to account for and give rise to measurable and 

 significant magnetic anomalies. These anomalies occur under quite a wide 

 variety of geologic settings, and have patterns that can usually be recognized 

 as having geologic causes and relationships. 



The magnetic susceptibility of rocks usually is directly proportional to 

 their content of ferromagnetic minerals. This rule applies to igneous, meta- 

 morphic, and sedimentary rocks. 



The Ferromagnetic Minerals.— The key ferromagnetic minerals are : 

 (1) magnetite, which is of the most importance; (2) ilmenite, of much 

 importance but to a lesser extent than magnetite; (3) pyrrhotite; and (4) 

 franklinite, which is definitely ferromagnetic but of least significance in 

 magnetic mapping. 



Typical values of magnetic susceptibility of these minerals, determined 

 at magnetic fields of the same order as that of the earth, are given in the 

 following table. 



Magnetic Susceptibilities of Ferromagnetic Minerals 

 Mineral Volume susceptibility k 



1 . Magnetite ( FegO^ ) k = 0.032 c.g.s. f 



2. Ilmenite (Fe Ti O3) k = 0.030 c.g.s.j 



3. Pyrrhotite (Fey So) k = 0.028 c.g.s.J 



4. Franklinite k = 0.003S c.g.s. t 



A study of the amount of magnetite and ilmenite present in rocks of 

 different types has been made by N. H. Stearn.§ In general, it was found 

 that the greater the percentage of these two minerals present in a particular 

 rock, the higher its susceptibility. Steam's work further indicates that the 

 average sedimentary rock (which is considered to be 82% shale, 12% sand- 

 stone and 6% limestone) has a combined magnetite-ilmenite content of 

 0.09%. The average granite carries 2.03% and the average basalt 6.53% 

 of these two ferromagnetic minerals. 



Although average rocks are not encountered in the field, such general- 

 izations are useful for an understanding of magnetic phenomena. Figures 

 for the average sandstone show 0.83% of combined magnetite and ilmenite, 

 and for the average igneous rock (composed of 65% granite and 35% 

 basalt) 4.6%. From this, it appears that if all the magnetite and ilmenite 

 in the earth's crust were distributed evenly, these two minerals would make 

 up 2.95% of all rocks. The average shale and limestone do not carry 

 sufficient magnetite or ilmenite to be magnetic. 



It should not be assumed from this study of hypothetical average 

 rocks of various kinds that shales may not be magnetic, for there are cases 



t The susceptibility of magnetite varies considerably for different samples, with a range from 

 0.04 to 0.097 for pulverized samples. For solid magnetite values from 1 to 20 c.g.s. are of record. 



t Pulverized samples. 



§ N. H. Stearn, "A Background for the Application of Geomagnetics to Exploration," A.I.M.E. 

 Geophysical Prospecting Volume, 1929, pp. 330-331. 



