54 ^/'^' A)ncrican Geologist. January, isat- 
much, or they may often exceed the coal in thickness by two 
or tln'ee times. In practical riiining- oi)erations all of this 
"roof" coal is wasted, because the coal layers make a good 
support for the overlying strata, and are, therefore, left as 
the roof of the mine. In this way about 2,000 tons per acre 
of the Pittsburg coal is always lost without any attempt to 
recover it. This waste is so large that some of the mining 
companies are considering the question of putting in crush- 
ing and washing machinery with a view to taking down these 
roof coals, and thus preventing the great loss of fuel which 
their abandonment entails upon any mine. There is no 
doubt that the time will come, many generations hence, when 
at great cost, the Pittsburg bed will be re-mined to secure 
the coal which is now rejected, both in its roof and bottom 
members, since all of it would be valuable fuel if freed from 
the included slates and clays. 
The "over-clay" is an impure fire clay, and varies much 
in thickness, sometimes almost disappearing, and again 
thickening up to two or even five feet. The clay is usually 
mottled and much slickensided, so that it becomes a danger- 
ous trap when left as a mine support, since large pieces of it 
will drop from the roof without any warning sound. Hence 
it is generally taken down at once, and the miner, has, there- 
fore, given it the name of "draw-slate" in many regions. 
It often contains what appear to be stems and rootlets of 
plants. 
The next succeeding (downward) division of this seam, 
the "breast coal" of the miners, also often termed the "main 
bench," is the most important and valuable division of the 
whole bed. Its thickness gradually increases from the Pitts- 
burg region (where it is usually about three feet) up the 
Monongahela, attaining a maximum of six feet at Browns- 
ville, while to the eastward in the Georges creek and North 
Potomac basin of Maryland and West Virginia, it increases 
still more to seven and one-half or even ten feet. The top 
of this member is nearly always of a bony nature for a thick- 
ness of one to four inches, and frequently this must be sep- 
arated and rejected in mining, but even where this is not 
required, the top of the "breast" coal is distinctly harder than 
the rest of it, and inclined to a cannelly structure. Westward 
