Drilling and logging of Integrated Ocean Drilling Program (IODP) Hole 1256D have provided a unique opportunity for systematically studying a fundamental problem in marine geophysics: What influences the seismic structure of oceanic crust, porosity or composition? Compressional wave velocities (V (sub p) ) logged in open hole or from regional refraction measurements integrate both the host rock and cracks in the crust. To determine the influence of cracks on V (sub p) at several scales, we first need an accurate ground truth in the form of laboratory V (sub p) on crack-free, or nearly crack-free samples. We measured V (sub p) on 46 water-saturated samples at in situ pressures to determine the baseline velocities of the host rock. These new results match or exceed V (sub p) logs throughout most of the hole, especially in the lower dikes and gabbros, where porosities are low. In contrast, samples measured at sea under ambient laboratory conditions, had consistently lower V (sub p) than the V (sub p) logs, even after correction to in situ pressures. Crack-free V (sub p) calculated from simple models of logging and laboratory porosity data for different lithologies and facies suggest that crustal velocities in the lavas and upper dikes are controlled by porosity. In particular, the models demonstrate significant large-scale porosity in the lavas, especially in the sections identified as fractured flows and breccias. However, crustal velocities in the lower dikes and gabbros are increasingly controlled by petrology as the layer 2-3 boundary is approached.