COAL FIELDS. 283 



inch to three feet, but over wide areas this parting is found about ten to 

 twelve inches in thickness. The roof coal generally consists of several 

 thin sheets separated from each other by layers of shale. The combined 

 layers of coal and shale in the roof stratum have a maximum thickness 

 of eight feet, and an average of three to four feet can be found continu- 

 ously for hundreds of square miles. The roof coal is never mined, as 

 the fuel is too impure to find any place in the market. 



The lower division of the Pittsburgh coal, which in reality consti- 

 tutes all that we know by this name, ranges in thickness from three and 

 one half feet to nine feet. It shows three persistent shale partings which 

 divide the coal into four benches. These benches are universally known 

 in the Pittsburgh district, as the Upper, or Breast coal, the Bearing-in 

 coal, the Brick coal and the Bottom coal. The Upper Bench, or the 

 Breast coal, is the main reliance of the miner. The Bearing-in bench is 

 from two to four inches thick and is constant in its occurence. Its name 

 indicates the use to which it is put by the miner, and its main contribu- 

 tion to the output of a mine is in the form of slack. 



The third bench, or the Brick coal, consists of bright, well-jointed 

 coal, equal, or nearly equal in quality, to the coal of the main bench. It 

 holds a thickness of eight to fourteen inches, through large areas. 



The bottom bench is so slaty and sulphurous, or, in a word, so nearly 

 worthless, that it is seldom mined. It generally has a thickness of about 

 four inches. 



The roof coal thickens to the northward at the expense of the lower 

 or main division; conversely the latter thickens to the southward and 

 reaches its maximum of nine feet in some of the townships of Fayette 

 county. In the vicinity of Pittsburgh and in Allegheny county generally, 

 it is about five and a half feet thick. In northern Washington, or the 

 Pan Handle district, it falls to its lowest mark, or three and one half feet; 

 and at the same time its quality is less approved than at any other points. 



In composition the Pittsburg coal ranges as follows : 



Fixed carbon 58 to 64 percent. 



Volatile matter 30 to 35 per cent. 



Ash 4 to 14 per cent. 



Sulphur 1 to 3 percent. 



Moisture 1 to 4 per cent. 



These figures are the result of a very large number of analyses, con- 

 tinued through many years. They show that the best phase of the 

 Pittsburg seam is a coal of great excellence, while in its poorer condi- 

 tion, it may show a large amount of ash and sulphur, and take a com- 

 paratively low place among our fossil fuels. There are large districts, 

 however, in western Pennsylvania, in which the better phase predom- 

 inates. 



The Pittsburgh Coal Seam in Ohio. 



The statements now given reveal the general structure and char- 

 acter of the Pittsburgh coal in the district from which it takes its name. 



