64 . III. DIST. CHAR. Mode. 



a bituminous stain, on the substance of the matrix : 

 scarcely any visible impression from the body, as 

 originally inclosed^ remaining. f 



The animal bodies,, which have undergone the 

 mode of petrifaction \ve are now considering., are 

 chiefly shells and corals. The shells are always 

 filled with a nucleus of earthy stone^f of the same 

 kind as that in which they are included : the sub- 

 stance of the petrifaction itself is mostly calcareous 

 spar, evidently the production of the original body, 

 the organic texture of which has been wholly obli- 

 terated during its mineralization, fff Petrifactions 



f We have a beautiful example of this mode, in a specimen 

 now in our hands, in which hardly any impression of the leaf (a 

 fern) is discernible even with the help of a glass, although its ge- 

 neral form and structure are most minutely marked, by a dark, 

 staining,bituminous matter, incorporated, as it were, in the substance 

 of the stone. Our specimen is in an indurated marl: Mr. Par- 

 kinson has described one of the same kind in gritstone, (v. Organic 

 remains, pi. III. f. 5.) 



f j- The nucleus distinguishes shells which have undergone trans- 

 mutation, from those casts, in which the whole mass of the petri- 

 faction is calcareous spar ; but, still, as the change effected by redin- 

 tegration also admits of nuclei, it is sometimes difficult to deter- 

 jnine the mode, in which the sparry petrifaction of a shell, with an 

 earthy nucleus, has been formed. In general, we conceive, that 

 petrifactions of shells by transmutation will be found to have the 

 calcareous spar, of which they consist, covered, more or less, with 

 the chalky matter, above noticed.(p. 60.). -In testaceous petrifactions 

 by redintegration, nothing of the kind, we believe,is ever observable. 



tft Professor Playfair has remarked, that the petrified shells and 

 corals of limestone strata, though sparry, are often foliated and 



