The lithospheric velocity structure beneath the eastern Snake River Plain (ESRP) is inferred from data on Rayleigh wave dispersion (periods 6 s to 40 s) and teleseismic shear wave delays, as well as other data from refraction profiling, heat flow, volcanic history, and geology. The unusual velocity structure indicates a 12-km thick high-velocity layer (depth 8 to 20 km), with P and S velocities of 6.54 and 3.87 km/s, respectively; this overlies a 21-km thick lower crust, with P and S velocities of 6.82 and 3.61 km/s, respectively. P velocities and layer thicknesses are based on our interpretation of refraction data (Braile et al., 1979) and are similar to the results of Sparlin et al. (1979). Upper-mantle S velocity is 4.1--4.2 km/s, and a lid appears to be absent or weakly developed. Although the mild shear velocity inversion in the lower crust might result from viscous effects (possibly due to partial melt), an anomaly in mineral composition is the preferred explanation. This may be a major depletion of quartz, as a result of its fusuon into the voluminous rhyolitic magmas which erupted onto the ESRP between 10 and 6 m.y. ago. The overlying high-velocity layer is thought to comprise acidic and basic intrusions of late Cenozoic age, and Archean granitoid rocks whose velocity and density may have been increased by thermal metamorphism related to plutonism.