Laboratory studies have been carried out on synthetic mineral systems representative of basalt, on natural basalts and eclogites, and on peridotites reconstructed from natural minerals. The transformation from basalt through pyroxenite to eclogite was found to take place over a broad pressure range, approximately 4 to 8 kb, depending on bulk composition and temperature. The mineralogical changes involve complex solid solutions of phases varying in velocity of transmission of compressional waves from about 5.5 to 8.5 km./sec. The series of assemblage changes do not appear to be accompanied by marked changes in velocity of compressional waves. High-pressure changes in hydrous sediments which produce assemblages of uniquely dense hydrous minerals or metamorphic assemblages w ll probably have

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similar characteristics. Therefore such assemblage changes are probably not responsible for a sharp continental Moho discontinuity but may contribute to a deeper than average, gradual, continental Moho. Assemblage changes are known for certain bulk compositions in the region of pressure and temperature believed to exist near the oceanic Moho; however, they do not pertain to the rocks believed to exist there. Although the Moho discontinuity is not sufficiently well resolved, the subtle high-pressure assemblage changes in a varying heat flow and geothermal gradient imposed on a complex structural configuration do not appear to be as critical as significant bulk compositional changes in determining the character of a discontinuity.

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