SECT. 1] 



THE MAGNETIC FIELD OVEK THE OCEANS 



209 



Fig. 23. Observed and computed anomalies for four north-east Pacific seamounts, showing 

 how the predicted effects of a seamoLint are sometimes overshadowed by anomahes 

 of opposite sign and longer wave-length associated with its roots. Anomalies in 

 gammas. Contour interval 100 y. Areas less than 2000 fm deep are shaded. (From 

 R. G. Mason, unpublished.) 



anomalies of the correct sign do in fact occur where they should be, but are 

 masked by larger negative anomalies of greater wave-length. It is evident that 

 the latter anomalies represent a deficiency of magnetic material in the roots of 

 the seamounts as compared to the normal crust. There is no seamount for 

 which the magnetization has been conclusively shown to be reversed, though 

 Laughton et al. (1960) have suspected that the one shown in Fig. 20 may be. 



When a number of volcanic masses are joined together, as on the Mid- 

 Atlantic Ridge, a very complicated pattern is produced in which it is difficult 

 to correlate topography and magnetic field. Heezen, Tharp and Ewing (1959) 



