SECT. 1] 



THE MAGNETIC FIELD OVER THE OCEANS 



211 



EXPLANATION 

 162° 30' 



Contours showing 

 flighliine inter- 

 sections 



11° 40' 



11° 30' 



11° 20 



162° 20 



Contour interval 50 gommos 

 Uncorrected for regionol grodient 



Fig. 25. Magnetic map of Eniwetok. (After Keller et ah, 1954, fig. 7.) 



Keller et al. (1954) conclude that the anomalies observed across the Aleutian 

 Trench are produced by susceptibility contrasts within the rocks of the sea 

 floor and bear no direct relation to the trench. However, they could also arise 

 from variations in the depth of the basement rocks under the sea floor. In the 

 absence of information from other sources, for example seismic measurements, 

 it is impossible to decide between these two alternatives. 



In the case of the Tonga Trench the magnetic measurements were supported 

 by echo-sounder profiles and by several seismic sections. Fig. 29 shows a 

 possible interpretation of the magnetic profiles in terms of a magnetic basement, 

 taking into account these other sources of information. Although this approach 

 cannot be expected to lead to precise values for the thickness of the sediments, 

 it does give a qualitative picture of how the thickness might vary from place to 

 place. The discrepancy between the observed and comj)uted curves cannot be 



