THE igneous oceanic crust is typically thought of as comprising two layers : an upper crust (’seismic layer 2’) characterized by a rapid increase in seismic velocity with depth, and a thicker lower crust (’seismic layer 3’) which is distinguished from layer 2 by both a higher P-wave velocity (6.69 +/- 0.26 km s(-1)) and a much smaller vertical velocity gradient (<1 km s(-1) km(-1))(1-3). A direct correlation has never been established between this seismic layering and the in situ lithological and physical properties of oceanic crust. The transition between seismic layers 2 and 3 has been variously interpreted as a change in igneous rock texture from doleritic sheeted dykes to gabbro(4,5), an increase in metamorphic grade from greenschist- to amphibolite-facies rocks(2,6-9), or a change in bulk crustal porosity with depth(2,10). We have re-examined available seismic refraction data from around Hole 504B, the deepest (>1.8 km) continuous hole drilled into the oceanic crust(11-13), and find that at this location the seismic layer 2/3 boundary lies within the sheeted-dyke complex, where it is associated with gradual downhole changes in crustal porosity and alteration, not a lithological transition from sheeted dykes to gabbro.