THE LOWER COAL MEASURES. 141 
often constant over wide areas in thickness, in composition and in their 
place in the coal seam. They divide the seam into well-marked 
benches, which are often sharply distinguished from each other in both 
chemical and physical properties. ‘These benches often constitute, in 
fact, quite distinct coal seams. But even in the undivided seams there 
are in many instances considerable differences between the different 
portions of the seam. 
Sometimes, however, the parting will gradually change its position 
in the seam as the latter is followed through a considerable territory, 
leaving much more or much less of the coal below it than where first 
observed. The thickness of these partings varies from a fraction of an 
inch to 2 feet, but in the large majority of our worked coals, they range 
trom 4 inch to 3 inches. ‘They sometimes furnish the ‘bearing in” for 
the miner, and in most cases they facilitate his work in removing the 
coal so as perhaps to compensate for the extra labor that he is obliged 
to expend in separating them from the coal that is sent out. They 
furnish a considerable part of the “slate” that is so frequent a subject 
of dispute between operator and miner. They are often so characteristic 
of the seam in position and in composition that they go far to settle 
questions of geological identity. It will be found that considerable use 
has been made of these “ partings” in the subsequent pages of this and 
other chapters. They are generally shown under the title Structure of 
the Coal Seam. In addition to these regular partings there are many 
inconstant layers of shale or pyrites that occur anywhere in the seam. 
The roof and the floor of the coal are subjects of great economic 
interest in the working of any seam. In the Ohio field the normal 
cover of the seams is shale, and the normal floor or seat of the coal is 
fire-clay, but in the practical development of the field numerous excep- 
tions are found to occur to both these statements. While shale is the 
cover of every well-formed seam, less frequently, but still in the case | 
of several quite wide-extended though not very valuable seams, a stratum 
of limestone makes the roof of the coal. 
The sandstone seldom descends to the coal without doing it damage. 
A deterioration of quality as well as of quantity in the seam is univer- 
sally associated in the minds of all who are practically acquainted with 
the subject with a sandstone roof. The coal is generally thinner, more 
impure, and of shorter grain, and thus harder to mine, wherever the 
sandstone comes down upon the coal. This descent of the sandstone 
