The structure of the Earth's crust and upper mantle under the Canadian Shield as determined by the Project Early Rise seismic experiment of 1966 is presented in this paper. The 39 Lake Superior shots were recorded (1) along a 1500 km line running from the shot location in a direction NNW across the Canadian shield, and (2) in a region north and northeast of Lake Superior at distances of 300 to 600 km from the shot. The seismic line crossed the geological boundary between the rocks of the Superior structural province and the younger Churchill structural province.
The travel-time graph had 3 branches showing apparent velocities of (1) 6.3 km/sec up to 2.4° (2) 8.10 km/sec from 2.4° to 5.8° and (3) 8.54 km/sec from 5.8° to 13.4°. When corrected for curvature the latter two velocities were found to be 8.05 and 8.43 km/sec respectively with the higher velocity occurring at a depth of approximately 84 km. A detailed analysis of the residuals showed that the Mohorovičić discontinuity is relatively horizontal at a depth of 30 to 35 km under the Superior province while under the younger Churchill province it is deeper than 40 km. Since normal values of 35 km are reached in the regions of Ontario just north and east of the lake, the unusually thick 50 km crust under Lake Superior is not extensive.
Very weak amplitudes in the distance range 10° to 12.4° followed by a large amplitude indicate that there may be a slight decrease in P-wave velocity at a depth of 95 km.