1. Oparin, A. I. The Origin of Life (Macmillan, London, 1936).
2. Urey, H. C. The Planets (Yale University Press, New Haven 1952).
3. Rubey, W. W. Geol. Soc. Am. Spec. Pap. 62, 631 (1955).
4. Cloud, P. E. Jr, Science 160, 729 (1968).
5. Abelson, P. H. Proc. natn. Acad. Sci. U.S.A. 55, 1365 (1966).