Geophysical logs from the Toa Baja drillhole on the north central coast of Puerto Rico are used to distinguish the variation in physical properties with depth in this non-volcanic island arc massif. A marked unconformity separates shallow Oligocene to Miocene limestones with considerable permeability contrast from a deeper sequence of volcaniclastic rocks with decreasing shaliness. A volcanic sill or flow overlying a strongly mineralized zone is found to cause the impedance contrast responsible for the strong seismic reflection anomaly (bright spot) that was the exploration target. ¿ American Geophysical Union 1991