Contents: The Petrologic Nature of the Lower Oceanic Crust and Upper Mantle; Velocities of Compressional and Shear Waves in DSDP LEG 27 Basalts; Structure and Constitution of the Lower Oceanic Crust; Elastic Wave Velocities in Volcanic and Plutonic Rocks Recovered on DSDP Leg 31; and Constitution of the Lower Continental Crust Based on Experimental Studies of Seismic Velocities in Granulite.