1. Spencer, L. J., Min. Mag., 25, 425 (1939).
2. Spencer, L. J., Min. Mag., 24, 503 (1937).
3. Clayton, P. A., and Spencer, L. J., Min. Mag., 23, 501 (1934).
4. Arkell, A. J., “Early Khartoum” (Oxf. Univ. Press, 1949).
5. Caton-Thompson, G., J. Roy. Anthrop. Inst., 76 (1946), 87 (1949).