MODE OF PRESERVATION OF FOSSIL PLANTS. 9 



greater or less induration of the rocks in which they are en- 

 tombed. Such casts are occasionally hollow, but more 

 frequently they consist of the amorphous substance of the 

 rock which has filled up the cavity, and which exhibits, 

 often with remarkable minuteness, the external aspects of 

 the original specimen. 2. Carbonisation ; in which the 

 original substance of the plant has been chemically altered 

 and converted into lignite or coal. All trace of the form of 

 the original plant is generally lost, as is the case with the 

 extensive beds of coal; but frequently, when the organism 

 has been buried in a bed of clay, the external appearance is 

 faithfully preserved, as in the ferns and other foliage found 

 in the shales of the coal-measures. 3. Infiltration ; in which 

 the vegetable tissues, though carbonised, retain their original 

 form from the infiltration of some mineral in solution, chiefly 

 lime or silex, which has filled the empty cells and vessels, 

 and so preserved their original form. This mode of pre- 

 servation occurs in the calcareous nodules in coal-beds, in the 

 remarkable ash-beds discovered by Mr. WUnsch in Arran, and 

 generally in the secondary rocks. 4. Petrifaction ; in w^hicli 

 the structure is preserved, but the whole of the original 

 substance has been replaced, atom for atom, by an inorganic 

 substance, generally lime, silex, or some ore of iron. This 

 is the condition of the beautiful fossils from Antigua, and of 

 many stems and fruits from rocks of all ages in Britain. 



Carbonised vegetables, or those w^hich have passed into 

 the state of Lignites, often undergo modifications which 

 render it difiicult to understand them rightly. Sometimes 

 a portion of the organs of vegetables which have passed into 

 the state of lignite is transformed into pyrites, or else pyrites 

 of a globular shape is found in the middle of the tissue, and 

 may be taken for a character of organisation. The section of 

 certain Dicotyledonous fossil woods, in that case, may resemble 

 Monocotyledons. Petrifaction, as in the case of silicified 



