Miscellaneous Subsurface Methods 601 



particle magnetically with the earth's field during the process of sedi- 

 mentation. In the deposition of coarse clastic sediments the forces asso- 

 ciated with current action and gravitation normally far overbalance those 

 of magnetic attraction. As a result a minimum degree of alignment of 

 residual magnetic particles with the earth's magnetic field is accomplished. 

 In isolated cases good alignment may be attained, but normally it is not 

 to be expected in coarse sediments. In fine shales the ability of the earth's 

 magnetic field in regard to rotating and orienting small residual magnetic 

 particles may be substantial as compared to all opposing forces. As 

 the fine particles slowly settle in relatively quiet water the magnetic 

 forces have ample time to play an important role in sedimentary align- 

 ment. The relatively short time available in the more rapid deposition 

 of coarser sediments does not permit the magnetic forces to be so effective. 

 Thus, we may confidently expect the finer clastic sediments (shales and 

 sandstones) to show stronger and more consistent polarization than their 

 coarser equivalents (grits and conglomerates) . 



Secular Variation of the Earth's Magnetic Poles (Refer to figure 312). 



Information is available to show that in historic time the magnetic 

 poles of the earth have migrated an appreciable amount. Records of the 

 magnetic declination have been compiled from as early as 1540 in London, 

 England, at which time a 7° E. declination was registered there. In 1580 

 the magnetic declination at London had reached a maximum easterly 

 declination of 11° E. From that date the declination moved progressively 

 westward and reached an extreme of 24° W. in 1810, a movement of 35° 

 in 230 years. Since 1810 the declination has again gradually shifted back 

 eastward, and it was therefore assumed that the magnetic poles had a 

 period of rotation of approximately 460 years (1580 to 1810 being half 

 a cycle). Recent information elsewhere tends to disprove any simple and 

 regular period of rotation of the magnetic poles and instead indicates a 

 more complicated phenomenon. 



Declination records in the United States date back to 1620 at East- 

 port, Maine, where a 19° W. declination was measured then. A spread 

 of but 10° has been observed at Eastport, the extremes being 12° W. in 

 1760, when a reversal in trend occurred, to more than 22° W. at present. 

 At San Francisco the earliest documented information is a 12° E. declina- 

 tion in 1780. By 1940 the declination had increased to 18° E. and it has 

 since continued to move eastward. From the foregoing data the following 

 generalizations may be made: 



1. The declination of the magnetic poles changes appreciably with 

 time. 



2. Neither the amount nor rate of this change is universally uniform 

 but is dependent upon geographic location and time. 



