sTi;ATi(,i;.\riiic.\i. 



in the action of percolating water, with the result that many shell* 

 are .iis-solved, leaving only the hollows in which they lay. These 

 hollow spaces may remain empty or they may be filled subse- 

 quently with infiltrated mineral matter, 6r if disturbances take 

 place before the final consolidation of the rock, the internal cast 

 may be squeezed on to the external impression of tin- .-hell and 

 receive an impress of its ornamentation. In any of these cases 

 Mini.- record of the original organism remains, but if the rock 

 material is a loose sand or other very soft kind of sediment all 

 trace of the organism may be destroyed. 



With regard to the durability of the original shell much 

 depends. on its original mineral composition. In some shells and 

 tests the carbonate of lime consists of the variety known as calcite, 

 while in others it has the structure of arragonite, and it is a well- 

 ascertained fact that arragonite is much less stable than calcite. 

 It was pointed out by Mr. Sorby l that shells composed of 

 arragonite disappeared much faster than the calcite shells wlu-n 

 both were exposed to carbonated waters, and he pointed out that 

 the creatures which possess hard calcareous structures can be 

 separated into two sets according to the nature of the calcareous 

 substance. These two sets may be tabulated as follows : 



Cali-it ii- Structures. Arragonite Structures. 



Foraminifera. Calcareous sponges. 



Kchinodermata. Actinozoa (corals, etc.). 



Auuelida. Lamellibranchia (except Oysters, 

 Most Bryozoa. Pec to us, and the outer layer of 



Brachiopoda. Spondylus, Pinna, and Mytilus). 



Oysters and Pectens. Gastropoda (with a few exceptions). 



Crustacea. Gephalopooa. 



l'n> lessor P. F. Kendall has also made observations and experiment* 

 which confirm those of Mr. Sorby.- He has pointed out that in the 

 Coralline Crag of Aldeburgh some fossils remain perfect, while of 

 others there are only casts, and that the casts are without exo 

 of arragonite shells, while the former are calcitic structures, such 

 as Bryozoa, Echinoderms, Serpulae, Brachiopoda, and the l>; 

 Molluscan genera, Anomia, Ostrea, Pecten, Lima, and Pinna ; with 

 the latter, however, occurred the Gastropod shell Scalaria, which he 

 thought must be calcitic, an inference which Mil^ecjuent exi*Tiuu-nt 

 proved to be correct 



3. Destruction of Fossiliferous Dtporitt, There is another set of 

 aLji-n.-ji- which contribute to render the geological record a very 

 imperfect one. When any mass of marine deposit is raised into 



1 (jiutrt. Journ. Oeol. Soc. voL XXXT. (1879), Pre. Addraa. 

 9 Oeol. Mag. 2nd Dec. vol. x. p. 497. 



