1. A. C. Lane, Report of the Committee on the Measurement of Geologic Time, Washington 1933.
2. O. B. Muench, American Mineralogist,10, 20 (1935).
3. C. N. Fenner, American J. Sci. (Silliman)16, 369 (1928).
4. W. F. Hillebrand, Bull. U. S. Geological Survey78, 43 (1891).
5. E. Kroupa, Mikrochem.27, 165 (1939);E. Kroupa undF. Hecht, Z. anorg. Chem.236, 181 (1938).