Treaties and other international acts series. 2700. Oil shale study in Brazil, agreement between United States and Brazil, modifying and extending agreement of Aug. 16, 1950, effected by exchanges of notes signed Rio de Janeiro Mar. 31, May 14, June 27, and Aug. 20, 1952; entered into force Aug. 20, 1952, operative retroactively from Feb. 16, 1952.