. III. DIST. CHAR. Mode. 63 



dies.f carbon or bitumen,, the product of their 

 change,, generally constitutes the whole of the pe- 

 trifaction, and entirely fills up the moulded impres- 

 sion of their form,, on the inclosing stone. In some 

 instances,, in which we may suppose the leaves, 

 &c. to have been of a particularly thin and delicate 

 structure, the only vestige of the transmutation is 

 the figure of the vegetable,, delineated., as it were, by 



passing through the bituminous change, it would Jill its own 

 mouldy with its own altered substance, forming such a surface, 

 as the surrounding stony matter would adhere to but slightly ; and 

 would therefore dispose to that separation by which its form is 

 displayed." (Organic Remains p. 433. vide, also, p. 431.) 



f Frequently, but improperly, considered as mere impressions 

 ef leaves, &c. since an accurate investigation will generally disco- 

 ver the matter of the petrifactions adherent to what is really tlieir 

 impressions in the matrix. In petrifactions of fern, the no- 

 dules of ironstone, in which these fossils are often hiclosed, mostly 

 split so as to exhibit, on one part, the plant itself petrified, on the 

 other, its impression. 



It may not be unnecessary to observe, in this place, that petri- 

 factions of leaves are not always the production of a transmutation 

 of the original vegetable matter. We possess several specimens 

 of petrified fern, in which not the least trace of a bituminous, or 

 carbonaceous substance, is apparent. The matter of the sub- 

 stitution being the same as that of the matrix, an argillaceous 

 iron-ore ; but separable from it, in form of a thin flake, which 

 would possess, if it coyld be detached in a perfect state, the 

 exact thickness, as well as the figure, of the original. Such 

 specimens are evidently casts, and have been formed by redinte- 

 gration (v. III. 6.), in the impressions on the matrix, left emptv, 

 for a time, bv the decay and removal of the vegetable bodies. 



