Miscellaneous Subsurface Methods 607 



so strong as to resist at 20° C. the effect of a magnetic field of opposite sign and 

 ten times larger than that of the earth. Such a field weakens the residual mag- 

 netism by only five to fifteen percent, according to the conditions of baking. 

 In bricks or pottery that had been well baked at about 500° C, an earth field of 

 0.45 oersted of direction opposite to the magnetism of the material has only a 

 slight effect when the temperature is raised to 150° C. 



(b) If any material has remained about twenty minutes in a magnetic 

 field, a prolongation of the time at the same temperature and in the same field 

 causes no change larger than ten percent. The time effect is asymtotic after a 

 day; the change, with exposure for weeks or months, is less than one percent. 



(c) Strong percussions change the magnetism slightly during the first 

 1,000 percussions. But lO'* or 10^ percussions cause no further appreciable 

 decrease. 



(d) The apparent coercive force of the thermoresidual magnetism of 

 highly baked bricks, etc., at 20° C. is about 70 oerstads — therefore higher than 

 any natural magnetic field on the earth has ever been, except in close prox- 

 imity to lightning. 



(e) The intensity of the residual magnetism is less if the cooling goes on 

 very rapidly — for example, from 600° C. to 200° C. in ten minutes — than if the 

 cooling is slower; that is in accord with the observations of Melloni. If the time 

 of cooling from 600° to 200° C. is more than an hour, there is no appreciable 

 change for the longer period. In the baking of pottery and in the natural cool- 

 ing of rocks the time was mostly longer than an hour. 



This all adds up to the fact that the residual magnetism of rocks is a 

 rather indestructible feature as affected by the normal forces of nature. 

 On the contrary, one other conclusion reached by Koenigsberger should 

 be mentioned : "The older the rock the lower the residual magnetism. It is 

 not known whether the magnetic field in former geologic periods was 

 weaker or whether some unknown effect has weakened the residual mag- 

 netism." 



It has been found that the residual magnetism of Tertiary granites 

 is somewhat greater than that of Cretaceous granites and considerably 

 greater than that of Paleozoic granites. This gradual weakening with age 

 has exceptions but is general for any particular kind of rock, such as 

 granite, diorite, or gabbro. Rock type, however, is predominate over age; 

 for example, a Paleozoic gabbro or basalt normally has more residual 

 magnetization than a Tertiary granite or rhyolite. Upon exposure to the 

 chemical-physical agents of weathering, residual magnetism is rather 

 rapidly destroyed. 



Conclusions 



Limestones, shales, and sandstones normally have sufficient residual 

 polarity to permit accurate magnetic orientation. However, cores should 

 be carefully selected in the field so as to eliminate as far as possible those 

 which possess little bedding as well as those which are strongly cross- 

 bedded. Little pertinent strike and dip information can be expected by 

 properly orienting such cores. Likewise, cores should not be stored near 

 strong magnetic fields nor be exposed to excessive heat. The top of the 

 core should always he so marked. 



