Compressional wave velocities at pressures to 1000 kg/cm2 and densities are given for a representative suite of rocks selected from 42 dredge hauls on the Mid-Atlantic Ridge near 45° N. The spectrum of rocks studied includes vesicular and massive basalts, metabasalts, meta-gabbros, and serpentinites. Evidence is presented for block faulting of an originally continuously layered crust of vesicular basalt and massive basalt underlain by a metamorphosed basalt and gabbro sequence. A study of the velocities of these layers at in situ pressures shows that they correlate well with the seismic layering of the oceanic crust. The velocity of the massive basalt layer is in agreement with that reported for layer 2. The underlying layer, consisting of low-to-medium grade metamorphosed basalts and gabbros (greenstones and greenschists) exhibits higher velocities. None of these exceed 6 km/s but it is suspected that these rocks at greater burial depths will exhibit velocities comparable to those of layer 3. The occurrence of serpentinites at all elevations on the slopes of seamounts implies that they do not form a continuous layer and because of their relatively low velocity, it is unlikely that layer 3 is composed of these rocks. Their presence as hydrated diapiric intrusives is more plausible.