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2. Tilton, G. R., and Reed, G. W., in Earth Science and Meteorites (North Holland Pub. Co., 1963).
3. Hamaguchi, H., Reed, G. W., and Turkevich, A., Geochim. Cosmochim. Acta, 12, 337 (1957).
4. Tilton, G. R., Carnegie Inst. Wash. Year Book 55, 167 (1956).
5. Morgan, J. W., and Lovering, J. F. (in the press).