314 



MAGNETIC METHOD 



[Chap. 8 



Table 38 — Concluded 

 SUSCEPTIBILITIES OF IGNEOUS ROCKS 



D. Factors Affecting Rock Magnetization 



Two factors affect rock magnetization: (1) mineral composition, (2) 

 geologic history. 



1. Mineral composition. At first sight it might be assumed that the 

 magnetization should be dependent upon the iron content. However, 

 the valency of the iron is of far greater importance, since the trivalent iron 

 is more magnetic than the bivalent iron. It is, therefore, more appropriate 

 to state that the magnetite content chiefly determines the magnetization of 

 rocks and formations. Next in order is pyrrhotite, then follows ilmenite 

 and titano-magnetite, although data found in the literature regarding the 

 last two are conflicting. Basic rocks (abundant in augite, hornblende, 

 olivine, and magnetite and lacking in quartz) are more magnetic than 

 acidic rocks (rocks abundant in quartz and acidic feldspars, such as ortho- 

 clase, with little or no magnetite). 



Magnetization depends not only on the tj^pe of magnetic mineral but 

 also on the quantity present. Approximate proportionality has been 



