the Process of Petrifaction. 75 



carbon, contained in such a plant, can blacken the layers of 

 clay to a considerable distance ; from which we may conclude, 

 that the black colour of the slate-clay covering the coal, pro- 

 ceeds, not perhaps from the disintegrated coal mixed with it, 

 but from the plants which it includes. The experiment succeeds 

 still better when we mix powdered coal or asphaltum with the 

 clay. But the impression is always distinguished by a sur- 

 rounding portion of a different, and for the most part of a 

 darker colour ; whence it appears that the carbonaceous matter 

 of the clay, if it is not, as we have supposed, derived from the 

 plant, exercises at least no influence in the conversion of the 

 plant. It is, therefore, by no means the coaly mass, which, as is 

 generally supposed, fills up the space formerly occupied by the 

 plant ; but it is the more or less preserved substance of the plant 

 itself, converted into coal, which we find in the impressions. Hence 

 we understand, also, why different species occurring on one and 

 the same slaty layer present different tints of colour and different 

 degrees of lustre ; for this is to be attributed to the individuality 

 of the plant. These experiments have succeeded not only with 

 Jems, but also with numerous dicotyledonous plants. As I 

 have remarked nothing in the plants of the coal-formation, at 

 least in those which I have had an opportunity of examining in 

 Silesia, and in the museums of Berlin and Prague, that would 

 lead to the belief of their having been destroyed by putrefac- 

 tion, I think we may be entitled to draw the conclusion, that 

 every thing wfiich we find in this Jbrviation presents a true pic- 

 ture of the then existing vegetation^ and that nothing has been lost. 



Lindley placed in water a number of plants, amounting to 

 173 species, from the most diversified genera, and allowed them 

 to decompose for two years ; and he certainly found that the 

 species whose analogues we usually find, or think we find, in 

 the flora of the coal-formation, were the best preserved.* But it 

 is next to be proved if actual traces of destruction have been 

 found, and then it may be time to venture to form an opinion. 



W'hen, as already described, we heat the plants included be- 

 tween the plates of clay, until the organic matter is destroyed, 

 we obtain a perfect impression, of the upper as well as of the 

 under side, a condition which may be compared to that in 



• The FossU Flora of Great Britain. 



