210 



BtTLLARD AND MASON 



[chap. 10 



have found a sj'stematically positive anomaly over the central valley on the 

 crest of the ridge, as is shown in Fig. 26. When the field is varying so rapidly, it 

 is not easy to be sure that the anomaly is really associated with the valley and 

 Hill (1960) failed to find any anomaly associated with it. Girdler (1958), C. L. 

 Drake (unpublished) and L. T. D. Allan (unpublished) have found striking 



43°00 



42''20'N- 



1 2° 20' 



I2°00' 



Fig. 24. Magnetic map of Galicia Bank. Contour interval 20 y. 



anomalies over the central valley of the Red Sea ; some sections are shown in 

 Figs. 27a and b. It seems possible that the valley on the ridge and that in the 

 Red Sea are similar features and that the magnetic anomalies are associated 

 with dykes filling a widening crack. 



9. The Deep Trenches 



Magnetic profiles have been published for the Aleutian (Keller et al., 1954), 

 Tonga (Raitt et al., 1955) and Kuril-Kamchatka (Galperin et al., 1958) trenches. 

 Fig. 28 shows eight crossings of the Aleutian Trench and Fig. 29 two profiles 

 across the Tonga Trench. 



