A seismic crustal refraction experiment was carried out in the Gulf of St. Lawrence region in the early summer of 1964. The underlying mantle was found to lie at a depth of approximately 45 km. in the area south of the Cabot Strait

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trough; the associated compressional wave velocity in the mantle, determined from reverse profiles is 8.5 km./sec. A thinner crust, in the order of 35 km. is postulated on the north side of the trough near Anticosti Island.

A considerable thickness (15 to 20 km.) of the so-called "Intermediate Layer" with a compressional wave velocity of 7.35 km./sec. is deduced from the reversed travel-time curves. Thinner crustal layers with velocities of 6.3 km./sec. and 5.9 km./sec. respectively, overlie the intermediate layer. Travel-time curves were adjusted to reduce the delay time effect, associated with variations in sedimentary thicknesses and velocities, underlying the shot points and receiving stations.

It is suggested from seismic crustal studies within the Appalachian system that variations in the depth to the crust mantle interface find their surface expression in the two-sided symmetrical system proposed by Williams (1964).

Evidence is presented indicating that the mantle velocity of 8.5 km./sec. is real and that the intermediate layer is a discrete unit; however, alternate interpretations are discussed.

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