In their investigations of the interior of the earth, geophysicists have generally regarded the mantle as composed of homogeneous shells and have considered the crust of the earth as varying laterally in composition and thickness. Evidence from measurements of the gravitational field and of the heat flow to the surface have suggested that differences exist in the composition of the upper mantle. Very recent information on seismic velocities and attenuations clearly indicates that substantial lateral variations in the physical properties of the mantle exist beneath both continents and oceans. The geological complexities evident at the earth's surface now appear to be reflected in the upper mantle.

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