A 0.4 to 0.5 s-1 in situ vertical velocity gradient in the diabase dike section of DSDP/ODP Hole 504B has been attributed to the effects of increasing pressure and downhole changes of mineralogy caused by hydrothermal alteration. An analysis of velocities measured in diabase samples as a function of pressure shows that increasing pressure accounts for a gradient of at most 0.1 s-1. Neither laboratory data nor a Voigt-Reuss-Hill average velocity model in which Vppyx>Vp amph shows a significant variation of velocity with depth; composition and pressure combined account for a gradient not greater than 0.2 s-1. These results indicate that either the elastic moduli of the amphibole minerals in these rocks are higher than those of the pyroxene phase (Vpamph>Vppyx,) or the seismic properties of the dikes are affected by a network of thin, large-scale cracks. ¿ 2001 American Geophysical Union