1. Derr, J. Bull. seis. Soc. Am. 63, 2177–2187 (1973).
2. Derr, J. & Persinger, M. A. Experientia (in the press).
3. Dmowska, R. Geophys. Surv. 3, 157–174 (1977).
4. Paul, W. Mining Lore (Morris Printing Co., Portland, 1970).
5. Hoenig, S. A. & Cole, M. A. preprint, Univ. Arizona (1980).