1. P. W. de Lange, W. J. de Wet, J. Turkstra, andI. H. Venter, Analyt. Chemistry40, 451 (1968).
2. H. J. M. Bowen, Report on the 5th Radiochemical Conference, Zvikov-Castle, Czechoslovakia, May 19?24, 1969.
3. M. M. Vole, D. J. M. Provan, andY. S. Tooms, Institution of Mining and Metallurgy Trans., Vol. 77, B. Bulletin 741, Aug. 1968, p. B 81?104.
4. E. H. W. Hornbrook, Nature223, 888 (1969).
5. H. V. Warren andR. Delevault, Bull. Geolog. Soc. America61, 123 (1950).