THE LOWER COAL MEASURES. 143 
expansion, and interrupting the drainage. The whole working of the 
seam may be affected by this one condition. If the clay has this 
tendency to ‘‘creep,” the coal must be mined in small rooms. When 
workings are abandoned, they are soon closed by an apparent rise of 
the floor. The size of the pillars, left to maintain the entries, depends 
on the bottom almost as much as upon the cover. When the bottom is 
solid, from the clay being thin or wanting, the pillars and ribs are 
greatly reduced in section. Two or three yards square are now adequate 
to the work for which 8 or 10 yards are required under other conditions. 
These facts have a very important bearing on the amount of coal that 
can be taken out of the mine. 
‘“‘Clay veins,” one of the most serious drawbacks to mining in many 
fields, are connected with, and proceed from the fire-clay floor. They 
seem to have been formed in the earlier stages of the history of the coal 
seam, by some inequality of pressure or resistance, whereby the bottom 
clay was forced in thin sheets through the hardening coal, destroying 
its continuity, and contaminating it with foreign matter. The “grain” 
of the coal is often affected for a number of yards on both sides of these 
“spars,” as they are termed. ‘The miner is thus made aware of their 
proximity some time before they are reached, by the unkind working of 
his coal. None of the seams that are extensively mined are without 
clay veins, but some are confused and interfered with by them much 
more than others. The disturbance will often be localized in a small 
part of a single field. 
The “grain” of the coal is but another name for the dividing 
planes or joints that intersect it. Coal, as is well known, furnishes an 
excellent illustration of joints, a phenomenon common to all rocks. The 
system is particularly clear in our bituminous coals. 
The leading or master joints make the “face” of the coal; the 
secondary joints are called the “ends” or “butts” of the coal. These 
joints the engineer expects to follow in opening the mine. The entries 
are either “face” or “butt” entries, as a rule, and the coal is then said 
to be worked ‘on the square,” but sometimes, owing to the topography 
or boundaries, the entries are run “ quartering.” The miner’s work is 
greatly facilitated by following the faces and ends of the coal. The 
directions of these joints are quite regular for large areas. In Ohio the 
main joints or faces of the coal, as indeed of all the other rocks of the 
scale, bear a few degrees east of north. The “ends” are app-roximately 
at right angles to the face. 
