Receiver functions constructed from teleseisms arriving from the northwest and recorded at station WTX in Socorro, NM show strong P-to-S conversions we interpret as coming from a mid-crustal sill-like magma body. Analysis of these conversions suggest that both the upper and lower surfaces of the magma body are distinct and provide sharp velocity contrasts with the surrounding host rock. Models that provide acceptable fits to these conversions all have magma body thicknesses less than 200 m. The absence of similar phases in data from the south and southeast suggests that the magma body is discontinuous near its edges. ¿ American Geophysical Union 1992