502 ON THE COAL-FORMATION 



ness of the same individual strata, we shall treat 



afterwards. 



With respect to the organic remains generally 

 found in coal strata, abundance of them are found 

 in the Clackmananshire coal-field. Those which 

 occur in the earth cover or alluvial soil have al- 

 ready been noticed. 



In the strata a great many varieties of the im - 

 pressions of vegetables and of shells are found. 

 The slate-clay abounds with delicate impressions 

 of moss plants, ferns, and reeds. The indurated 

 clay, with impressions of large plants, the form of 

 which can only be indistinctly traced. The iron- 

 stone abounds chiefly w 7 ith petrified mussels, with 

 a few of the jointed reed species. The sandstone, 

 and in particular what is near a bed of coal, has 

 in general great numbers of the jointed reeds in 

 it, fully formed, that is, of their natural shape, 

 only a little compressed ; and the interior of these 

 is commonly covered with coal. 



It is remarkable, that in no instance have we 

 found petrified shells in the sandstone strata of 

 Clackmananshire ; and in the slate-clay, petrified 

 shells are always composed of ironstone, of whicli 

 the slate-clay is the matrix or bed. Many of the 

 ironstones are very full of petrified mussels, and 

 some of them seem to be a complete congeries of 

 shells. The immediate roof of the coal is fre- 



