Estimating the determining composition of plagioclase by a rapid extinction method using albite twin lamellae is successful, when treated statistically and cautiously, in establishing provenance area of a sediment. The method involves recording all symmetrical extinction angles of albite twin lamellae. It differs from the Michel-Levy method which records only the maximum value for each thin section. Recording all symmetrical extinction angles allows the detection of a polymodal population of plagioclase compositions. The apparent anorthite content of plagioclase ("An") in two conglomerate horizons and three granite bodies is used: (1) to relate conglomerate horizons separated by a body of water, (2) to relate both conglomerate horizons to a common provenance area (two of the granite bodies), and (3) to show assimilation of the conglomerate into the third granite body, allowing areal extension of the conglomerate palaeo-environment. The usefulness of this approach is limited.