Miscellaneous Subsurface Methods 



603 



3. There is probably no simple and regular period of the rotation 

 of the magnetic poles, although measurements at London indicate a 460- 

 year cycle. Measurements of the magnetic declination of varved clays 

 and lava flows have been reported as demonstrating this cyclic action. 



We may thus expect considerable error to be introduced by assum- 

 ing coincidence between the present magnetic declination and that existent 

 at the time the sediment was deposited. 



Secondary Residual Magnetism 



Occasionally cements such as vivianite (iron phosphate) are deposited 

 in the pore space of sedimentary rocks. Vivianite, which normally con- 

 tains some Fe203, is ferromagnetic and has been found as the principal 

 ferromagnetic mineral in cores showing strong directional magnetism. 



Figure 313. Graph showing declination of residual magnetism in lava flows of 

 different ages from Mount Etna {A) and historically observed magnetic declina- 

 tions at same locality (5) . (After Koenigsberger, "Terrestrial Magnetism and 

 Atmospheric Electricity," 1938.) 



Lacking experimental data, we must assume that such secondary ferromag- 

 netic minerals succeed in aligning their magnetic moments with that of 

 the earth's field during their period of growth in the sediment. Thus if 

 the residual magnetic properties of a core are found to result substan- 

 tially from the ferromagnetic cementing material, it may be possible to 

 make rather reliable orientation determinations on (1) coarse clastic sedi- 

 ments that normally possess little directional alignment, (2) sediments in 

 which the matrix is nonmagnetic, and (3) sediments that were highly 

 tilted or folded prior to cementation. 



