314 GEOLOGY OF OHIO. 
Meanwhile, the roof stratum begins to break by the sides of the pillars 
and fall down in immense pieces, while the workmen still persevere, 
boldly drawing and retreating, till every prop is removed. Should any 
props be so firmly fixed by the top pressure that they will not give way 
to the blows of heavy mauls, they are cut through with axes, the work- 
men making it a point of honor to leave not a single prop in the goaf. 
The miners next proceed to cut away the pillars nearest to the sides of 
the goaf, setting prop-work, then driving it and returning as before, 
until every panel is removed, excepting small portions of pillars which 
require to be left under dangerous stones to protect the retreat of the 
workmen. While this operation is going forward and the goaf extend- 
ing, the superincumbent strata being exposed without support over a 
large area, break progressively higher up, and where strong beds of 
sandstone are thus giving way, the noise of the rending rocks is very 
peculiar, at one time loud and sharp, at another, hollow and deep. 
The fourth or long-wall system, by which all the coal is mined 
out as the workings progress, is the best method of working coal 
wherever the conditions are adapted to this manner of mining. Seams 
from 4 feet downward are best suited for long-wall working, but in ad- 
dition the coal requires to be tolerably hard and firm, so as to bear the 
pressure of the overlying strata. The roof requires to be comparatively 
free from water, and an ordinary amount of refuse or waste material 
must be at command in the mine to make the system a success. Beds 
of coal which have bands of shale running through the body of the 
coal, or have a falling or draw shale for a roof, are best adapted for 
long-wall, this material serving for building packwalls or filling up 
the gob. | 7 
In laying out the workings of a mine on the long-wall system, the’ 
coal left underground is a strong square pillar surrounding the bottom 
of the pit, tu preserve the shaft from becoming involved in the breaking: 
of the overlying strata which follows the removal of the coal. The 
main galleries of the mine are then advanced along the line of strike of 
the seam, and the walls or working faces are opened out from the entry, 
one working-place following another, like troops marching in echelon. 
Where the conditions are suitable, the walls are opened out in all direc- 
tions and the workings advanced in the form of an ellipse. The wall- 
faces, which vary in width according to the surrounding conditions, 
are undermined across their whole width by the workmen before leaving 
