162 EXPLORATION GEOPHYSICS 



of definitely magnetic shales. Likewise it should not be assumed that 

 because a rock is intrusive (and therefore would show considerable mag- 

 netite ilmenite content on averages) it is necessarily magnetic. An intrusion 

 covering over a square mile, which forms a considerable hill in Huerfano 

 County, Colorado, is no more magnetic than the shales surrounding the 

 area. 



The high values of magnetic susceptibility of the ferromagnetic min- 

 erals, especially magnetite, contrast sharply with the susceptibilities of the 

 more common minerals and rock types listed below. 



Magnetic Susceptibilities of Certain Minerals and Rocks 



Because of the range in the values of susceptibility for a given type of 

 rock as shown in available tables, an attempt to group rocks and minerals 

 strictly on a basis of their susceptibilities is beset with exceptions. This 

 is evident in the case of the granite as given above. It is also evident that 

 generally the ferromagnetic minerals are from 100 to 500 or more times 

 higher in susceptibility than the great majority of other minerals or rocks. 

 Even in the case of sedimentary rocks there usually exist sufficient contrasts 

 in susceptibilities between formations to cause detectable and mapable mag- 

 netic anomalies. 



These susceptibility contrasts are due to the content of the ferromagnetic 

 minerals in a given rock or formation. Further evidence of this is furnished 

 by the following examples. A specimen of sphalerite with an iron content 

 (Fe304 magnetite) of 0.21% showed a susceptibility of 0.000001 c.g.s. 

 units. Where the magnetite content was 14.69%, the susceptibility was 

 0.000519. 



The magnetite content of shale samples gives the same general relation- 

 ship. A shale with 0.4% of magnetite gave a susceptibility 4 times as 

 great as a shale with 0.075% of magnetite. A granite-magnetite powder 

 mixture tested in a magnetic field of 15 gauss showed that for a weight of 

 magnetite of 2 units per 1000 the susceptibility was about 0.0003 c.g.s. For 

 a similar mixture with 14 units of magnetite the susceptibility increased 

 to about 0.002. 



