PETRIFACTION OF MOLLUSC SHELL 1085 



(KicsclriiKjchcn), which have l)een named heckite rings, after Dr. 

 Beek, sometime Dean of Bristol, who first called attention to them. 

 These rings often form a regular ornamentation of the surface of 

 shells and have been mistaken for original features. According to 

 T. M. McKenny Hughes (I5:i'd5 ct scq.) , these rings form in a 

 layer }i to >^-inch thick, just beneath the outside film of lime. 

 Blum (4:790) records cases where the calcium carbonate of the 

 shell is still largely retained, while at many places single tubercles 

 of silica project, surrounded each by one or two rings, but seldom 

 more. This was especially noted in brachiopods. In some cases 

 the shell was changed to chert with only scattered rings. A shell of 

 OrtJiis rectangularis from the Carbonic limestone was wholly silici- 

 fied, the silica appearing in the form of small spheres which are 

 arranged in place of the former radial striations of the shell. A 

 shell of Gryphffia contained several layers of silica in the form of 

 beekite rings. A Plicatula aniiata had its inner surface preserved 

 in compact yellowish-brown chert ; Avhile its outer surface, with 

 all its original roughnesses, was composed of beekite rings. Pecten 

 vagans showed the reverse, with a layer of stalactitic quartz be- 

 tween the two layers. Area had l^oth outer and iimer surfaces made 

 up of beekite rings, while between these layers appeared a porous 

 mass of chert. A specimen of llxogyra rcnifonnis from the Oxford 

 Oolite was replaced by beekite rings, while Trigonia costata, to 

 which it adhered, was replaced by chert only. Ijelemnite guards 

 had their surfaces covered with beekite rings, while the interior 

 was still fibrous calcite. Others had been changed entirely to 

 beekite. The rings here had l:)ecome concentric cylinders, the axes 

 of which coincided with the original calcite fibers. In other cases 

 the guard was composed of a number of concentric layers or fun- 

 nels, each of which was composed of beekite rings. 



So far as present observation goes, there seems to be no in- 

 herent character within the organism or the formation within which 

 it is embedded which determines whether silicification is to be ac- 

 companied by the formation of beekite rings or not. Both cases 

 have been found within the same formation at the same locality and 

 within the same genus. 



Silicified shells are among the most acceptable fossils, for they 

 will readily weather out in relief or even become entirely free, or 

 they may also be easily separated from the enclosing matrix by 

 the use of weak acid. . 



A great variety of minerals besides silica replaces the calcium 

 salts of mollusc shells. The chambers of ammonites often con- 

 tain ankerite (Ouenstedt), others again are chiefly filled by stron- 



