Chap. 8] MAGNETIC METHOD 317 



from the magnetization of lavas and the like. Such deductions appear 

 hardly justified, since so many factors, such as lightning, contact and 

 dynamic-metamorphic effects, and mechanical stresses, can be of influence. 

 Magnetizations of rocks, taken by and large, offer no positive evidence 

 of changes in direction or intensity of the earth's magnetic field in previ- 

 ous geologic periods. 



The remanent magnetism of ferromagnetic substances decreases with 

 an increase in temperature. In fact, all magnetic parameters (coercive 

 force, remanent magnetization, susceptibility) are individually dependent 

 on temperature. A correct analysis of thermal relations is difiicult, since 

 the same ferromagnetic body may not exist after changes in temperature, 

 and another body may have been formed with different structural and 

 chemical properties (magnetization of pottery and bricks). The intensity 

 of magnetization decreases first slowly and then more rapidly with tem- 

 perature until the critical, or "Curie," point is reached (348° for pyrrhotite, 

 525° for magnetite, and 645° for hematite). According to these figures, 

 rocks could not be magnetized beyond a depth of about 20 kilometers. On 

 the other hand, it is quite possible that by such extraordinary pressures 

 and temperatures as occur in the earth's interior, unexpected results may 

 be produced. The sun, notwithstanding its high surface temperatures 

 (5900°C.) has a strong magnetic field. Further, an analysis of the surface 

 distribution of the earth's magnetism has led to the conclusion that 52 

 per cent of it originates in the core. When a magnetic rock is heated to 

 the Curie point and then cooled, its magnetism reappears at a much lower 

 temperature. This is known as temperature hysteresis of magnetization 

 and occurs particularly in pyrrhotite. 



Considerable forces are at work in mountain building, folding, faulting, 

 volcanic intrusions, epeirogenic movements, and earthquakes. They can- 

 not fail to affect the magnetization of rocks and are likely to bring about 

 changes in susceptibility and remanent magnetization. A reciprocal rela- 

 tion exists between deformations caused by magnetization (magnetostric- 

 tion) and magnetic effects due to deformations such as stretching (Villari 

 effect), bending (reciprocal Guillemin effect), and twisting (Wertheim 

 effect). Different materials have quite different magnetomechanical 

 characteristics. 



There is, further, a distinct magnetic hysteresis in mechanical cycles. 

 When the stresses have ceased, the magnetizations never return to their 

 original values. In connection with the possible relation of mechanical 

 stresses and abnormal magnetic polarization, it is of great interest that 

 negative magnetizations may be produced by simultaneous tension and 

 torsion (see Fig. 8-16). Occasional strong magnetizations of drill rods 

 and drill cores may be explained by this effect. 



