54 . III. DIST. CHAR. Mode. 



testaceous rcliquia; the exterior parts of which 

 often consist of very thin layers of earthy stone, 

 while the interior and more general mass is crystalline 

 Petrifactions of this kind are sometimes hollow, 

 and have the surface of the cavity set with regular 

 crystals. In other instances, crystalline stones, as 

 spars, &c. occupy the whole bulk of the petrifac- 

 tion in a solid mass. Sometimes, however, spars, 

 or other stones the result of crystallization, form only 

 those parts of the specimen, which have immedi- 

 ately taken the external figure of the original, the 

 interior part, if the petrifaction represent a hollow 

 body, as a shell, &c., being filled with a nucleus f" 

 of earthy matter. 



t Nuclei, or kernels, as they are sometimes called, are bodies 

 of mineral matter, moulded while in a soft and plastic state, in 

 the cavilies of fossil shells, &c. 



A Nucleus is said to be invested, when it occurs covered, either 

 \vith the original body in which it was formed, or with mineral 



matter which ha* taken the form and place of such body 



bare, when its covering has been destroyed by various natural oper- 

 ations. 



Some authors regard nuclei as a proper and complete mode of 

 petrifaction; and, taking the mode for the foundation of their ar- 

 rangements, class and describe these bodies, separately from the 

 fossils in which they have been moulded. If, however, the prin- 

 ciple, which we shall endeavour to establish in the following pages, 

 be admitted that one species of organic body can give but 

 one real or permanent species of rellquium~it will be evident, 

 miclei are not to be considered as distinct petrifactions. Indeed, 

 were the separation now objected to followed up, each individual 



