140 I>- H. Scott. 



ficial features of the specimen. In petrifaction, on tlie other hand, 

 the tissues of the plant have been permeated by some mineral sub- 

 stance in solution, which, on subsequently setting- hard, has fixed 

 and preserved the internal structure, often with astonishing per- 

 fection. 



The following forms of preservation fall under the head of in- 

 crustations. In Casts, the whole mass of the specimen may be 

 replaced by mineral matter, or, as is more usually the case, a thin 

 film of carbonaceous substance may remain on the surface, the mark- 

 ings on which are shown in their natural form. The surrounding 

 matrix shows a mould of the cast, with the elevations and depressions 

 of its surface reversed. We frequently meet with internal casts, 

 where some cavity in the specimen, such as a fistular pith, has 

 become filled with mineral matter, which has taken the impress of 

 the surrounding structures, such as the wood. The medullary casts 

 of Calamitean and Cordaitean stems are familiar examples of this 

 type of fossil, which has often proved misleading owing to confusion 

 between casts of the pith-cavity and those representing the whole 

 stem. In the case of thin, flat organs such as leaves, we commonly 

 speak of impressions, the organ having left its impress on the matrix 

 both above and below. The substance of the leaf itself may still be 

 present, in a carbonized condition, so that in addition to external 

 features such as the venation, something of the actual structure may 

 be preserved. The cuticularized epidermis, as well as sporangia and 

 spores, if present, are often perfectly recognizable on microscopic 

 examination. But though some useful indications of minute structure 

 may thus be obtained from carbonaceous impressions, our knowledge 

 of the internal organization of fossil plants depends essentially on 

 the investigation of petrified specimens. The chemical bodies which 

 have played the most important part as agents of petrifaction are 

 Silicic acid and Calcium carbonate, often accompanied by other sub- 

 stances. The well-known Permo-carboniferous petrifactions of France 

 and parts of Germany, are silicified, while in the British carboniferous 

 deposits the structural material is usually calcified. On the whole 

 the preservation of the calcareous petrifactions is the better, but in 

 favourable cases silicified specimens may show a marvellous perfection 

 of detail. 



In the Coal-Measures of England and certain parts of Germany 

 (e. g. Langendreer in Westphalia) calcareous nodules , crowded with 

 vegetable fragments, occur, either in the Coal-seam itself, or in the 

 roof-bed above it. Both in calcified and silicified material the most 

 delicate tissues, such as cambium and phloem, the endosperm of 

 seeds, and apical meristems are frequently preserved cell for cell; 

 occasionally even the nuclei of the cells have been clearly recognized. 



