1. Gass, I. G., and Gibson, I. L., Nature, 221, 926 (1969).
2. Le Pichon, X., and Heirtzler, J. R., J. Geophys. Res., 73, 6, 2101 (1968).
3. Mohr, P. A., Bull. Geophys. Obsv. Addis Ababa, 11 (1967).
4. Mohr, P. A., Nature, 213, 664 (1967).
5. Gibson, I. L., IUGG Upper Mantle Symposium, Zurich, 8, 38 (1967).