The results of seismic refraction profiles in the Gulf of St. Lawrence and on the northwest and northeast coasts of Newfoundland are presented. The thickness of the crust is about 45 km in the region of the Gulf of St. Lawrence southwest of the Cabot Strait Trough, and off the northeast coast of Newfoundland east of the Long Range Mountains. One interpretation of the data suggests that the compressional wave velocities through the underlying mantle are 8.50 and 8.69 km s−1 respectively. An "intermediate" layer about 20 km thick is identified with compressional wave velocities of 7.35 and 7.52 km s−1 beneath these areas. A thinner crust, 33 km thick approximately, underlies the west coast of Newfoundland, and a crustal thickness of 35 km is postulated near Anticosti Island. The compressional wave velocity in the upper part of the mantle beneath this thinner crust is close to 8 km s−1. The intermediate layer thins and, possibly, pinches out in the vicinity of Anticosti Island and northwest Newfoundland. The results lead to the suggestion that we see within the crust and upper mantle the subsurface expression of the two-sided nature of the Appalachian system. The system shows no sign of quietly dying away beneath the northeastern coast of Newfoundland.