MAGNETIC METHODS 167 



the same mineral may be present and conditions quite similar. Therefore 

 the best procedure is to test samples of the mineral deposit where work is in 

 view, covering not only the mineral or ore itself, but also the surrounding 

 rock. 



Geological Factors 



There are a number of geological factors meriting consideration which 

 will be discussed briefly. Any one of them might furnish a clue for an 

 interpretation of a magnetic survey. 



Thermal Effects. — A rigorous analysis of the effects of temperature 

 on the magnetization of rock i)i situ is difficult, due to the many unknown 

 variables. In general the permeability of magnetic materials increases con- 

 tinually with increase of temperature until a point known as the "temper- 

 ature of recalescence" is reached. f After passing this temperature, the 

 permeability decreases very rapidly, and within a few degrees of the recales- 

 cence temperature, ferromagnetic materials appear to lose their magnetic 

 properties completely. The temperature at which magnetic materials be- 

 come non-magnetic is also known as the Curie point. The Curie points for 

 some ferromagnetic minerals are: magnetite| 515° C, pyrrhotitej 300° C, 

 nickel§ 310° C, iron§ 690° C. to 870° C. These values for the Curie point 

 have an important bearing on the question of the depth of the earth's crust 

 at which magnetization of rocks can exist. It is estimated that due to 

 increase of temperature with depth, permanent magnetization of rocks dis- 

 appears below about 60,000 feet, or about 11.5 miles.ff 



It is not known if the same ferromagnetic body actually exists at tem- 

 peratures beyond the Curie point, or whether another body with different 

 structure and chemical properties has been formed. When a magnetic rock 

 is heated until its magnetism disappears, and is then cooled, its magnetic 

 property will reappear at a lower temperature than that at which it was lost. 



Although in all probability not a strictly comparable case, the sun has a 

 strong magnetic field notwithstanding its very high surface temperature 

 of some 5900° C. Also, analysis of the distribution of the earth's magnetic 

 field leads to the view that 52 per cent of its strength appears to originate 

 in the core. As previously mentioned, however, below a depth of 60,000 

 feet magnetization of rocks would no longer exist due to the high temper- 

 atures. For this reason, the magnetic properties which appear to exist 

 within the earth and the sun may be explained by assuming that they are 

 due to an actual flow of electric current of high amperage. 



Lightning. — Lightning may afifect the local magnetization of exposed 

 rocks. The large current intensities in lightning discharges (of the order of 



t J. H. Jeans, The Mathematical Theory of Electricity and Magnetism, (Cambr. Univ. Press, 

 5th Ed.) p. 412. 



t R. Ambronn, Elements of Geophysics, (McGraw-Hill, New York, 1928) p. 95. 



§ S. G. Starling, Electricity and Magnetism, (Longhams, Green and Co., Ltd., 5th Ed. 1929) 

 p. 287. 



tt See also J. A. Fleming, Terrestrial Magnetism and Electricity, (McGraw-Hill, 1939) p. 321. 



