1911-12, 545 



by chemical means, viz., saline water containing organic matter in 

 solution. This might be described as the gathering of silica from 

 the mother-sediment ; and (b) the replacement of the calcium 

 carbonate of the fossil by silica ; solutions of the latter diffusing 

 through the colloid organic matter of the shell-material and attacking 

 the carbonate at various points. These points became centres of 

 chemical action around which the periodic movement spreads as 

 recorded by the rings of chalcedony and by alternations of 

 compact and porous rings of chalcedony. 



(3.) Mr. Wickes has drawn my attention to descriptions of 

 silicified bore-holes by Hudleston (in " Yorkshire Oolites," Proc. 

 Geol. Assoc, 1878, p. 443). The silicification of such bore-holes 

 I regard in the same light as the silicification of the edges of 

 broken shells. The rings of beekite at each end of the bore-holes 

 shew that the shell was completely surrounded, inside and outside, 

 by the silicifying medium. My own conclusion is that the 

 majority of the systems of beekite rings on fossils cannot be 

 referred to a bored origin, and where such can be definitely 

 ascribed to bore-holes the periodic nature of the diffusion rings is 

 still evident. 



(4.) Mr. Wickes has asked me for a chemical explanation of 

 the absence of beekite from the following fossils, which I now 

 give :— 



(a) Vertebrata. — Phosphate of lime is the chief constituent 

 of bones. According to Liebig the phosphate of bones 

 requires 1,500 times its own weight of water saturated 

 with carbonic acid to dissolve it, while according to 

 Bischof carbonate of lime requires only 1,000 parts for 

 its dissolution. The latter is, therefore, a more easily 

 replaceable substance. In fossil bones much of the 

 phosphate still persists, although in marine remains it is 

 often replaced by a basic silicate of alumina — a still 

 more insoluble substance. 



