ON THE ORIGIN OF COAL. 1G3 



They are of four kinds, namely, Sandstone, Bind 

 (hardened silty clay), Black Shales (fine clay 

 coloured with bitumen), and Black Bass (bitumi- 

 nous clay approaching to cannel). 



Sandstone roofs present exactly such an appear- 

 ance as a strong current of water flowing over a 

 tract of luxuriant vegetation would now produce, 

 namely, prostrate trees lying in all directions, 

 mingled with sand. The tender and fragile parts 

 of plants are broken and dispersed by the currents 

 that prostrated them, or have since disappeared 

 on the subsequent percolation of water, which first 

 decomposed, and then removed them. 



Blue bind roofs exhibit every appearance of a 

 moderate current of water, sufiicient to bring the 

 clay, which on ceasing to be suspended in water, 

 although sufficient to weigh down, and bury in 

 fine grained mud, the delicate and small plants 

 found in them, was not able to overthrow the 

 Sigillaria, Ulodendra, Lepidodendra, and other 

 large trees. For it must be remembered, that 

 nearly all the upright specimens of the stems of 

 fossil trees, found in our Coal measures, are 

 large ones. 



The black shale roofs indicate even a more 



