Seismic waves generated by large chemical explosions at Greenbush Lake, British Columbia, were recorded at 15 stations along two profiles which extend from the shot point across the North Cascade Mountains of British Columbia and Washington into western Washington. T = Δ/6.13 expresses the time-distance curve for Pg. The equation T = 6.21 + Δ/7.91 expresses the Pn arrival time along a line of azimuth of 210.4° out to a distance of 519 km and the equation T = 6.85 + Δ/8.06 expresses the Pn arrival time along a line of azimuth of 223.0° out to a distance of 540 km. Calculations, which assume a mantle velocity of 7.96 km/sec, indicate that crustal thickness increases toward the south and decreases toward the west. The average crustal thickness along azimuth 210.4° is 32 km and along 223.0° is 29 km. Amplitude data have large scatter but suggest a Q value near 384 for rocks of the upper mantle beneath the North Cascade Mountains.