1. Curtis, C. D. Phil. Trans. R. Soc. (in the press).
2. Abelson, P. & Hoering, T. C. Proc. natn. Acad. Sci. U.S.A. 47, 623 (1961).
3. Rosenfeld, W. & Silverman, S. Science 130, 1658 (1959).
4. Claypool, G., Presley, B. J. & Kaplan, I. R. in Initial Reports of the Deep Sea Drilling Project 19, 879 (US Government Printing Office, Washington, D.C., 1973).
5. Hallam, A. & Sellwood, B. J. Geol. 84, 301 (1976).