P acquired land pursuant to the Migratory Bird Conservation Act. The title to one parcel was acquired by purchase on July 23, 1937. The other was acquired by a judgment of condemnation entered August 30, 1939. Both the 1937 sale and the 1939 judgment of condemnation reserved to D oil, gas, sulfur, and other minerals for a period of 10 years from the date of vesting of title in P. The reservation was to continue 'as long as oil, gas, sulfur or other mineral is produced . . . or so long thereafter as D shall conduct drilling or reworking operations thereon with no cessation of more than sixty (60) days consecutively until production results; and, if production results, so long as such mineral is produced. The deed recited that at the end of 10 years or at the end of any period after 10 years during which the above conditions had not been met, 'the right to mine, produce and market said oil, gas, sulfur or other mineral shall terminate . . . and the complete fee title to said lands shall thereby become vested in the United States. No drilling, reworking, or other operations were conducted and no minerals were obtained for a period of more than 10 years following the sale and condemnation. In 1955, P issued oil and gas leases applicable to the lands in question. D continued to claim the mineral rights and entered various transactions purporting to dispose of those rights. D relied upon Louisiana Act 315 of 1940 which provided that rights such as P acquired shall be imprescriptible. P filed this quiet title action. The District Court held for D and the Court of Appeals affirmed. P appealed. The essential premise of the Court of Appeals' decision was that state law governs the interpretation of a federal land acquisition authorized by the Migratory Bird Conservation Act.