io84 PRINCIPLES OF STRATIGRAPHY 



(e) Shells of molluscs. These are composed of calcareous salts, 

 either carbonate of lime or mixed carbonate and phosphate of 

 lime, penetrated and bound together by an oiganic network of con- 

 chiolin. In the Pelecypoda the shell consists of three layers: (i) 

 the outer or periostracum, a horny integument without lime; (2) 

 the middle prismatic or porcelaneous layer, consisting of slender 

 prisms perpendicular to the surface and closely crowded; and (3) 

 the inner or nacreous layer, which has a finely lamellate structure 

 parallel to the shell surface. Many pelecypod shells consist en- 

 tirely of aragonite. In Ostrea and Pecten the whole shell is cal- 

 cite, while in some others (Pinna, Mytilus, Sjiondylus, etc.) the 

 nacreous layer is aragonite, while the prismatic layer is calcite. 

 In the gastropod and cephalopod shell the inner or nacreous layer is 

 often wanting, while the periostracum is generally present. The 

 structure of the middle layer differs much from that of the pele- 

 cypods. The shells are mostly aragonite, except those of a few 

 gastropods (Scalaria and some species of h'usus) and a few 

 cephalopods (e. g.. the guard of Belemnites), which are of calcite. 

 The first process of alteration in the shells is the removal by 

 decay of the horny periostracum covering the shell and of the 

 conchiolin which penetrates the calcareous mass. As a result the 

 shell is rendered porous, which can be proved by applying it to the 

 tongue, when it will l)e found to be adhesive. This porous condition 

 may be observed in many jMiocenic and later shells. The aspect 

 of a shell which has thus undergone the first change is more or less 

 chalky, instead of firm and often shiny, as in the fresh shell. Fre- 

 quently shells composed of aragonite are entirely destroyed, while 

 in those in which both calcite and aragonite occur the latter is dis- 

 solved away while the calcite remains unimpaired. Water carrying 

 salts in solution will enter the pores and there deposit its milieral 

 matter, until the pores are filled. If the matter in solution is car- 

 bonate of lime, this process of infiltration will result in the complete 

 calcification of the shell, whereby the finest structural details will 

 be fully preserved. Those portions of the shell which originally 

 were aragonite may be changed to calcite. In some cases, however, 

 the whole shell is converted into crystalline calcite, and then the 

 finer structure is destroyed. At other times granular limestone re- 

 places the shells. If, however, the infiltrating substance is silica, 

 the process of fossilization does not stop with the filling of the 

 pores, but from the greater insolubility of the silica it becomes the 

 dominating substance and gradually replaces the more soluble lime. 

 This process is frequently most active around certain centers, and 

 is then indicated by the formation of concentric rings of silica 



