34 HISTORY OF THE OCEANS 



The general agreement in depth of the transition from inner 

 mantle to outer mantle (see modified Gutenberg velocity-depth 

 curve, Fig. 7) with the maximum depth of focus of earthquakes 

 suggests that the outer mantle may be a structural as well as a 

 compositional unit. 



Further work, both experimental and theoretical, is required to 

 resolve the discrepancy in velocity distributions deduced from 

 body waves, Rayleigh waves, and Love waves, and to use these 

 new velocities to calculate better distributions of density, tem- 

 perature, melting point, and other physical properties of the mantle. 



The contrast between continental and oceanic structure in 

 regard to seismic wave velocities are definitely established to 

 depths of about 100 km. A similar contrast for geothermal prop- 

 erties is being established. The other physical properties of the 

 upper mantle are not yet known in sufficient detail to permit their 

 use in comparison of continental and oceanic areas. Our present 

 limited knowledge about the upepr mantle, however, can lead to 

 decisive judgments of some theories of the origin of continents 

 and ocean basins. 



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information from seismology. Bull. Geol. Soc. Am., 65, 385-400. 

 Birch, F. 1952. Elasticity and constitution of the earth's interior. /. 



Geophys. Research, 57, 227-286. 

 . 1960. The velocity of compressional waves in rocks to 10 kilobars. 



I. /. Geophys. Research, 65, 1083-1102. 

 Bullard, E. C. 1954. The interior of the earth. The Earth as a Planet, 



G. P. Kuiper, Editor. University of Chicago Press, Chicago, 111. 

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Vening Meinesz, s' Gravenhage, Koninkl. Ned. Geol.-Mihnbivk. 



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 Coulomb, J. 1952. Love waves of the Queen Charlotte Islands earthquake 



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