1. Craig, H., Geochim. Cosmochim. Acta, 3, 53 (1953).
2. Wasserburg, G. J., Mazor, E., Zartman, R. E., in Earth Science and Meteoritics, edited by Geiss, J., and Goldberg, E. D., 219 (North-Holland Publ. Co., Amsterdam, 1963).
3. Silverman, S. R., in Isotopic and Cosmic Chemistry, edited by Craig, H., Miller, S. L., and Wasserburg, G. J., 92 (North-Holland Publ. Co., Amsterdam, 1964).
4. Nakai, N., J. Earth Sci., Nagoya Univ., 8, 174 (1960).
5. Colombo, U., Gazzarrini, F., Gonflantini, R., Sironi, G., Tongiorgi, E., paper presented at Intern. Meeting Organic Geochem., Paris, Sept. 28–30 (1964) (in the press).