The authors wish to thank Ralph K. Iler for his helpful 
comments and suggestions, particularly on topics relating to 
the theory of silicification, and Umesh Banerjee for his time 
and effort in preparing the SEM micrographs appearing in the 
plates of this manuscript. We also wish to thank D.O. Bar- 
ghoorn, T.D. Brock, E. Dorf, J. Ito, W.H. Lyford, P.L. 
Parker, R.A. Scott, R. Siever, and E. Wheeler for their con- 
tinued interest and assistance. 
This work was supported in part by NASA Grant NGL 
22-007-069 and by NSF Grants GA 13821 and GA 37140. 
33 
