56 OP MINERAL COAL. 
There are two principal kinds of coal, 1. anthracite, which ia 
found in Kilkenny county, (Ireland), in some parts of Wales, and 
in immense quantities in the State of Pennsylvania. It is com- 
posed principally of carbon with a portion of earthy matter and 
water. It burns with very little of either smoke or flame. 2. 
bituminous coal, which is the kind raised from most of the British 
coal fields. This, besides the carbon which is its principal con- 
stituent, contains both hydrogen and nitrogen. Of the manner 
in which these elements are combined or associated, we have little 
accurate knowledge. It has been supposed that the hydrogen is 
united to a part of the carbon, forming bitumen, which exists in 
the mass of the coal. In consequence of the volatility of the 
bitumen, this kind yields a flame when it is burnt, and some sorts 
more than others. Newcastle coal undergoes a kind of fusion, by 
which the separate pieces are united into one mass, when it feels 
the influence of the heat, from which it is called caking coal. — 
Other varieties from other localities have this quality imperfectly 
or not at all. 
All the beds of coal, slate clay, and sandstone, taken together 
forma stratum, which in Northumberland, is three thousand feet, 
or more than half a mile in thickness. The thickness, however, 
varies greatly in different places, and this is probably the greatest 
that occurs throughout the whole extent of the formation. About 
twenty feet of it only are made up of seams of workable coal, but 
these seams extend over an area of one hundred and eighty square 
miles. It is from this quarter that London and all the eastern and 
the southern part of England, as far west as Plymouth, are sup- 
plied with fuel, and a demand is created for an annual excavation 
of about three millions of cubic yards. The beds of this field are 
eighty-two in number. Twenty-five are beds of coal, generally 
too thin to be worked, and fifty-seven, sandstone, and slate clay. 
The two most important coal seams are the high main and low 
main; the former four hundred and fifty feet below the surface, 
and six feet thick; the latter three hundred and sixty feet deeper, 
and six feet three inches thick. The main seam in the Dudley 
and Birmingham coal field is thirty feet in thickness. 
The coal formation has been spoken of hitherto as a single 
stratum, separating the more ancient from the more recent beds, 
and it might be supposed that it extends quite through the island. 
But this is by no means the case. It consists of a number 
of separate deposits, which are probably of about the same age, 
though they do not very intimately resemble each other; occupy- 
ing irregular, basin-shaped cavities, scattered through the central 
and western parts of the kingdom. The most extensive and 
important, are those of Northumberland, of the West Riding of 
Yorkshire, of Lancashire, of Stafford and Warwickshires, of 
South Wales, and in Scotland, that which extends across the is- 
land from the opening of the Frith of Forth, in a south-westerly 
direction, and within which the cities of Edinburg and Glasgow 
are included. 
