Seismic anomalies which are interpreted as midcrustal magma bodies are present in several areas in the Basin and Range province of the western United States. In general, the upper boundaries of these inferred magma bodies coincide in depth with prominent midcrustal increases in P-wave velocity. Crustal density profiles calculated from measured P-wave velocity profiles indicate that near- liquidus basalts should be less dense than the crust below the midcrustal velocity increases, and more dense above them. This implies that rising mafic magmas are trapped where they lose buoyancy at midcrustal density discontinuities. Buoyant trapping may generate large silicic magma systems by focussing ascending mafic magmas into a small depth interval in the crust. ¿ American Geophysical Union 1988