142 GEOLOGY OF OHIO. 
gives rise to one of the most serious interruptions and irregularities 
which the miner meets. The coal seam has often been carried away in 
large part or entirely by the intrusive currents or streams that brought 
in the sandstone, and the miner is either obliged to cut long passages 
through the solid rock to reach the coal that lies beyond, or else to 
make expensive detours for the same purpose. These intrusive sand-- 
stones are termed horsebacks in mining parlance. ‘There are several 
seams that are especially liable to trouble from this source. The Upper 
Freeport coal, for example, has suffered great violence and loss at the 
hands of the Mahoning sandstone. 
A firm sandstone, separated by a few feet of shale from the coal, 
gives the most advantageous roof. 
The character of the intervening shale is a matter of great conse- 
quence as regards both the economy and the safety of the mine. Some 
shales slake in such a way when the air reaches them that they cannot 
be held up by posts. They give way and crumble from around the 
post. Others can be held up without difficulty as long as is desired. 
It sometimes happens that it is most economical to leave the uppermost 
bench of the coal as a roof. A black slate often makes an excellent 
roof. Where there is no sandstone or other firm rock above the coal, 
we find the “bad roof” that complicates the task of the mine engineer 
in many of our fields. The cover of the Pittsburgh coal, and also of 
the Bellaire coal in Eastern Ohio, are examples of this sort. The 
entries of mines worked in these seams are kept up largely by timbering 
while the rooms “ fall shut” as soon as they are left. 
The floor of a coal seam is, as has been already stated, normally 
fire-clay, but there are numerous exceptions to this order among our 
Ohio seams. The Sharon coal in some portions of the Massillon field 
is often underlain by a foot or two of a hard rock, comparable to the 
ganister that makes the seat of some of the English coals. The pick 
brings fire with every blow. The Pittburgh coal of Belmont county 
has for the most part a black slate floor, and no fire-clay. The Diamond 
seam at Linton has no fire-clay. <A layer of cannel makes the floor of 
the main coal here, and cannels, as Newberry and others have shown, 
are, as a rule, without underlying clays. | 
The amount and quality of the fire-clay floor is connected in im- 
portant ways with the economical working of a coal seam. Some clays 
will slake as the air reaches them, “throwing the track” by their 
