Forty-two thin sections of the Naknek Formation from seven localities on the Alaska Peninsula have a mean composition of Q 37 F 52 L 10 . Samples from localities between Paulc Bay and the Black Hills, a distance of 500 km, are remarkably uniform in texture, composition, and degree of alteration. The Nakaek is derived from erosion of an Early Jurassic magmatic arc. The Naknek Formation on the Alaska Peninsula is similar in composition to the Naknek Formation in the Kamishak Bay-Cape Douglas area of lower Cook Inlet and to Upper Jurassic sandstone dredged from the continental shelf near the Pribilof Islands. Fossiliferous Upper Jurassic sandstone dredged from the Bering continental slope is derived almost exclusively from a volcanic source and is essentially unaltered.