214 



BULLARD AND MASON 



[chap. 10 



15" S 



COMPUTED 



20°S 



OBSERVED 



300 350 km 



INFERRED BASEMENT 



Fig. 29. Magnetic profiles across the Tonga Trench and their interpretation in terms of a 

 magnetic basement. The bars are seismic depths and the dots are estimated depths 

 based on the "half-width" of suitable magnetic anomalies. (After Raitt et al., 1955.) 



accounted for by any reasonable configuration of the basement. It could be 

 caused by structural differences between opposite sides of the trench ; it could 

 also be explained by susceptibility changes within the basement rocks, or by 

 their possessing a remanent magnetization aligned in some direction other than 

 that of the present field. 



References 



Abragam, A., 1961. The Principles oj Nuclear Magnetism. Clarendon Press, Oxford. 

 Abragam, A., J. Combrisson and I. Solomon, 1957. Polarisation nuclcaire par effet Over- 



hauser dans les solutions d'ion paramagnetiques. C.R. Acad. Sci. Paris, 245, 157-160. 

 Alldredge, L. R. and G. D. Van Voorhis, 1961. Depth to source of magnetic anomalies. 



J. Geophys. Fes., 66, 3793-3800. 

 Andrew, E. R., 1956. Nuclear Magnetic Resonance. Cambridge Univ. Press. 

 Bauer, L. A., W. J. Peters, J. A. Fleming, J. P. Ault and W. F. G. Swami, 1917. Ocean 



magnetic observations. Publ. Carnegie Inst. Wash., No. 175, Pt. 3. 

 Bemmelen, W. van, 1899. Die Abweichimg der Magnetnadel ... bis zur Mitte XVIII 



Jahrhunderts. Observations Batavia Roy. Mag. Met. Obs., Suppl. to vol. 21, 109 pp. 

 Birch, F., J. F. Schairer and H. C. Spicer (Eds.), 1942. Handbook of physical constants. 



Oeol. Soc. Amer. Spec. Paper 36, 296-297. 



