2l6 THE SEDIMENTARY ROCKS 



limestones. The different kinds of coal vary much in hardness and 

 chemical composition, but they are all connected by intermediate 

 gradations. Bituminous Coal has (neglecting the ash) 70 to 75% 

 of carbon and 25 to 30% of volatile matters, chiefly hydrocarbons, 

 which are driven off on destructive distillation. Under the term 

 bituminous are included many varieties of coal, which differ 

 much in their behaviour and in their value for different purposes. 

 Anthracite is a hard, lustrous coal, that is nearly pure carbon 

 (aside from the ash) and has little or no volatile matter ; it burns 

 without smoke or flame and gives an intense heat. Semibitumi- 

 nous or Steam Coal is intermediate in character and composition 

 between the bituminous and anthracite varieties. 



Cannel Coal does not belong in the series of coals above enu- 

 merated, but forms a very distinct variety. It occurs in lenticular 

 patches, not in beds, and is very compact, though not very hard or 

 heavy. This coal has from 70 to 85 % of carbon and the high propor- 

 tion of 6 to 7% of hydrogen, giving off large quantities of gas when 

 heated, and burning with a white, candle-like flame. Even with 

 the microscope, it is difficult to detect the vegetable structure of 

 cannel, so thoroughly has the material been macerated. Evidently, 

 cannel is an exceptional coal "and has been formed in a somewhat 

 peculiar way. While the ordinary coals evidently represent ancient 

 peat bogs, which by subsidence allowed the sea, or other body of 

 water, to overflow them and were thus sealed up and buried under 

 sedimentary deposits, cannel was formed in pools of clear water, 

 in which vegetable matter was accumulated and very completely 

 disintegrated. This- is shown not only by the shape of the coal 

 patches, but also by the fossil fish not infrequently found in cannel. 



The following table (from Kemp) displays the composition of 

 the typical varieties of coal, not including the ash : — 



c. 



Wood 50 



Peat 59 



Lignite 69 



Bituminous Coal 82 



Anthracite ....... 95 



H. 



0. 



N. 



6 



43 



I 



6 



33 



2 



5-5 



25 



O.8 



5 



J 3 



O.8 



2.5 



2-5 



trace 





