Compressional wave velocities, measured in 114 samples of Keweenawan volcanics and clastics at hydrostatic pressures of up to 2 kb, are used to compute Keweenawan formation velocities. In certain instances, formation velocities are sufficiently distinct from one another to permit a reliable geological identification of seismic refractors in the Lake Superior basin. It is further concluded that slightly metamorphosed volcanics, such as the Portage Lake Series of the Michigan Copper district, cannot be responsible for 'Upper Refractor' velocities of 6.6–6.9 km/s observed at depths of about 10 km beneath Lake Superior. Amphibole-bearing volcanics, hitherto unreported, were sampled in the basal parts of the Keweenawan extrusive sequence, and represent a higher grade of regional metamorphism than commonly found in the Portage Lake Series. Since metamorphic grade appears to increase with depth, amphibole-bearing volcanics with sample velocities as high as 6.7 km/s at 2 kb, must seriously be considered as the cause of the Upper Refractor.