1. R. A. Daly, Amer. Journ. Sci. (5), vol. v, 1923, p. 35a.
2. J. W. Gregory, Report Brit. Assoc., Leicester, 1907, p. 494.
3. T. Crook, Nature, 19 Aug., 1922, pp. 253–5.
4. Mm. Mag., vol. xvii, 1914, pp. 55–86; also see A. Holmes, Petrographic Methods and Calculations, 1921, pp. 9–11.