GEOLOGY OF THE BRISTOL COAL-FIELD, 347 



A good gas Coal has a large proportion of volatile Hydrocarbon, 

 while a good steam Coal has a large proportion of solid Carbon. 



For example, the following are the natural arrangement in some 

 of the samples mentioned in the foregoing table, and copied from 

 Messrs. Cossham and Co.'s paper read at the British Association 



Great Yein (Kingswood) a good Steam Coal — 

 Fixed Carbon ... 70-84 

 Volatile Matter ...21-62 

 Ash ... ... 5-84 



Water 1-50 



100-00 



Hollybush Vein (a good Gas Coal)- 



Fixed Carbon ... 59-10 

 Volatile Matter .. 34-97 

 Ash and Water ... 5-93 



100-00 



"When pure Coal is burnt the asb only contains a minute portion 

 of Alkaline salts. The greatest part consists of Calcium, 

 Magnesium, Aluminium, Iron, and Silica, most likely introduced 

 with the coal by infiltration of water. As may easily be supposed 

 the Coal in different parts of the same seam varies considerably, in 

 one place burning freely and leaving little ash, while in another the 

 percentage of mineral matter is so great that " slates " are produced 

 which, when placed in an ordinary fire, explode and fly all over the 

 room. These originate from the mud present in the oiiginal water 

 in which the plants grew. 



The formation of the Coal measures must have taken an 

 enormous time, as the transition from wood to coal was necessarily 

 very gradual and slow. An immense number of ages must have 

 elapsed since the life and growth of the tree, and the product we 

 now use, and upon which we so greatly depend. 



