164 ON THE ORIGIN OF COAL. 



quiet and gentle flow of water, than those com- 

 posed of bind, and show every appearance of 

 having been a long time in formation, as the 

 nearly total disappearance of plants by decora- 

 position, and the dispersion of their carbon 

 throughout the strata, as well as the abundance 

 of shells of the genus Unio in the middle, and 

 of the Pecten, Goniatites, &c. in the lower parts of 

 the Coal-field prove. 



The black bass roofs, in the upper Coal-field, 

 afibrd an evidence of the very long periods of time 

 which must have elapsed during their formation, 

 as many of them are entire masses of bituminous 

 casts of Cyprides, Microconchi, shells and fish 

 bones, and teeth, mingled with decomposed 

 vegetable and animal matter. 



In the lower Coal-field, coarse gritstones and 

 black shales abound, but the seams of Coal are 

 few and thin. 



In the middle Coal-field, fine grained white 

 sandstones, and light coloured argillaceous 

 deposits are plentiful, and the most numerous 

 and valuable seams are there met with. In the 

 upper field, so long as the rock deposits resemble 

 those of the middle one, the seams are pretty 



