Shear and longitudinal velocities were measured by the ultrasonic phase comparison method as a function of pressure to 8 kbar on synthetic glasses of basalt, andesite, rhyolite, and quartz composition and on natural obsidian. Velocities of most of the glasses decrease anomalously with pressure, but increasingly more-normal behavior occurs with decrease in SiO₂ content. The pressure derivatives of rigidity and bulk modulus increase linearly, from −3.39 to −0.26 and from −5.91 to +2.09, respectively, with decrease in SiO₂ content from 100 to 49%. The change from negative to positive in the pressure derivatives of both moduli and observed at Poisson's ratio of about 0.25 is consistent with the Smyth model for the anomalous elastic behavior of glass. If the temperature in the upper mantle is about 1500°C, tholeiitic basalt would be molten in accordance with the partial melt explanation for the low-velocity zone; at 1300°C and below, basalt would be in the glassy state, especially if more felsic than tholeiite. At a temperature of 1370°C and at 30 kbar, reasonable values for the upper mantle at 100 km depth, the basalt glass of this study would have a viscosity of about 10¹³ P. On the basis of the theory of viscoelasticity the glass would support shear wave propagation at frequencies above 0.1 Hz. Under such conditions of PT, 10 to 30% basalt glass in a matrix either of eclogite or olivine would reduce the seismic velocities by 3 to 9% and could also account for the values observed in the low-velocity zone.