FORMATION OF COAL. 20^ 



which to combine ; when this point is reached the process stops. 



Kinds of Coal. — We may class coals under the heads of Cannel, 

 Anthracitic, and Bituminous 5 all the varieties contain the same 

 constituents but differ in the proportions, and yet occur not only 

 intermixed, but a seam may be Bituminous coal in one place, and 

 Anthracitic in another. An instance of this is shown in the 

 Welsh nine feet 3 near Cardiff it is semi-Bituminous, and then 

 changes to a pure Anthracite in Carmarthenshire. 



Again the Low Main Seam of Northumberland and Durham is 

 met with as a steam coal in parts of Northumberland, and on the 

 Tyne is a gas coal.'^ Many theories have been invented to account 

 for these varieties of coal ; the most notable of these refer the 

 cause to pressure, or heat, or the two combined, the force being 

 derived from faults or disturbance of strata and the heat from 

 trap-rocks. 



With regard to the pressure, it is well known that when a 

 Bituminous coal nears a fault, it deteriorates and becomes soft, 

 whereas if the above theory is correct, it should assume more the 

 form of Anthracite. 



As to the heat from trap-rocks, the occurrence of such even if 

 not in immediate contact with a seam of coal, would, no doubt, 

 cause an alteration, as distillation would result, but it is difficult to 

 conceive of a large area being so affected • and it is well known 

 that there are no igneous rocks near the Anthracite seams of 

 South Wales. 



But both the theories in question are set aside by Cannel and 

 other varieties of coal occurring in close proximity to one another, 

 a remarkable instance of which is seen in Daviesse County, 

 Indiana, where a seam of Cannel lies close over a Bituminous one, 

 so that it would have been impossible for one to be affected without 

 the other. 



May not Chemistry be consulted as to the secret. Most writers 

 on this question appear to have lost sight of the effect the dehris 



* N. England Institute Mining Engineers, Vol. xxvi., p. 44—46- 



o 



