206 



BtTLLAKD AND MASON 



[chap. 10 



The continental slope and the rise leading up to it frequently show no 

 significant anomaly. A profile taken while running south along longitude Sl^'W 

 into the Bay of Biscay is shown in Fig. 18 and sections taken by Miller and 

 Ewing (195G) in the Gulf of Mexico in Fig. 19. Such smooth profiles might be 

 expected in places where the outer part of the shelf is composed of sediments. 

 They are not, however, always found; for examjile, Keller et al. (1954) find a 

 positive anomaly of about 500 y at the outer edge of the shelf off the east coast 

 of the United States. 



I7 00W 



so 



I 



40 



I 



30 

 I 



20 



I 



I6 00W 



Legend 



-ve anomaly = 



= 1000 fm contour 



Scarp = -/"^^VuiAiiii 



2 4 6 8 10 



Fig. 20. Magnetic map of a seamount north of Madeira. Contour interval 50 y. (After 

 Laughton et al., 1960, fig. 14.) 



8. Seamounts and Mid- Ocean Ridges 



Typical seamounts show well marked magnetic anomalies which may reach 

 several thousand gammas if the seamount approaches the surface or emerges 

 as a volcanic island. Fig. 20 shows a plot of the field over a small seamount 

 north of Madeira and Fig. 21 sections over Bermuda. Figs. 22 and 23 show idiots 

 of two seamounts in the area covered by the survey of the north-east Pacific. 



Fig. 24 shows a plot of the magnetic field over Galicia Bank off the north- 

 west coast of Spain. There is no conspicuous anomaly over the bank and it is 

 clear that it is not a basaltic volcano of the kind shown in Fig. 20. Dredging on 



