1. Urey, H. C., The Planets (Yale Univ. Press, New Haven, 1952).
2. Runcorn, S. K., Nature, 193, 311 (1962).
3. Runcorn, S. K., Nature, 195, 1248 (1962).
4. Runcorn, S. K., Phil. Trans. Roy. Soc. (in the press).
5. Munk, W. H., and Davies, D., Isotopic and Cosmic Chemistry, edit. by Craig, H., Miller, S. L., and Wasserburg, G. J. (North Holland Pub. Co., Amsterdam, 1964).