We use the ratios between P and S wave velocities (V_P/V_S), derived from seismic refraction data, to infer the composition of the crust in the Grenville and the Appalachian Provinces of North America. The crust exhibits V_P/V_S increasing with depth from 1.64 to 1.84; there is a clear distinction between the Grenville Province (average V_P/V_S=1.81) and the Appalachian Province (average V_P/V_S=1.73) which persists at all depths. The boundary between these provinces is east dipping extending for 100 km east of the Champlain thrust. In the Appalachian Province the increase in V_P/V_S ratios with depth from 1.67 to 1.74±0.02 may reflect a normal decrease of silica content in the continental crust. In the Grenville Province beneath the Central Granulite Terrane, an anomalous V_P/V_S ratio of 1.82±0.02 is observed extending to a depth of 10 km; this correlates with the abundance of Ca‐plagioclase in the Marcy Anorthosite. At greater depth (15–20 km), where seismic lamination and high electrical conductivity is observed, V_P/V_S is 1.84±0.02 and correlates with the Tahawus Complex, a layered mafic intrusion. Within the 25‐km‐thick lower crust of the Grenville Province the V_P/V_S is 1.84±0.02 and P‐velocity is 7.0±0.1 km/s, which are typical for plagioclase‐bearing rocks (gabbro‐norite). The high V_P/V_S ratio in the Grenville Province has not been reported in crust of any other age. Since the Grenville Province contains 75% of the world's known anorthosites, high V_P/V_S ratio is related to high plagioclase. We suggest that the composition of the Grenville lower crust was significantly modified by the emplacement of the anorthosites in the mid‐Proterozoic.