CARBONIFEROUS. 1 07 



hard dull coke like an imperfectly coking coal. 



When cellulose is burnt, it forms a coke which 



ohere, like the substance known as 



mother of coal. Hence differences which coals 



w in burning may depend upon the original 

 substance of the plants. 



The first important variety of coal Anthracite, 



which contains the largest percentage of carbon, 



ich, and burns with little smoke. 



In son.- ities anthracite appears to result 



from the distilling action of the heat which is 



generated underground by the folding of the rocks 



a coal-held. This separates the mineral oil 



al, SO that the petroleum escapes into 



CS like water, and may rise to the sur- 



i in sprinj 



Other coals are often termed bituminous, but 

 no substance at all like bitumen exists in coal. 



h coal is insoluble in any of the solvents which 

 • bitumen ; but it softens at a low tempera- 

 ture. Cakil - partly melt, and make a com- 

 pact coke. 



The non-caking coal, like the steam coal of 



Ith Wale-, does not change its form in burning. 

 The properties of tl S vary with the different 



beds, suggesting differences in the species of plants 

 which formed them. 



The group i)\ rocks termed Coal Measures is 

 commonly divided into three parts. The lower 

 coal m< or Ganister beds, are usually bar- 



ren, or only contain thin coals, which are not often 

 valuable. Secondly, the middle coal measures 

 contain most of the thick workable beds of coal. 

 They correspond to the Pennant group of South 

 Wales and the Bristol coal-fields. Thirdly, the 

 upper coal measures yield a good deal of coal in 



