356 GEOLOGY OF OHIO. 
necessary quantity of pillar coal was sacrificed by this plan, and it was 
changed to the present method, which consists in working rooms for- 
ward 80 yards, and removing all the pillar coal after the rooms are 
finished or worked out. 
As the mines are now worked, double parallel entries are driven 
in the face and butt slips of the coal. The butt entries are 160 feet 
between the blocks. Rooms are opened on the north and south, tollow- 
ing the line of the face entries. The rooms are broken off the entry 
7 feet wide, and are advanced 15 feet before being opened to their full 
width of 7 yards. The thickness of pillars between rooms is 6 yards, 
which are cut through when the rooms are worked forward 40 yards, 
for the purpose of getting better air. 
The pillars of a range of workings are attacked as soon as the rooms 
are finished up. The pillars first cut away are those in the first range 
of rooms. As soon as a butt entry has been driven forward to its 
boundary, and all the working rooms finished up, the pillars of the four 
interior rooms are worked away. They are brought back in echelon 
with the purpose of throwing the weight of the superincumbent strata 
away from the miners. The roof of the excavated area is held up by a 
free use of prop-wood, and seldom falls in alarming masses until the air 
crossing 40 yards back has been reached. The slate forming the im- 
mediate cover of the coal now begins to fall, but the massive sandrock 
above remains firm. When the range of pillars has been cut away 
within 15 yards of the entry, the miners change front, and attack the 
entry stumps on the butts of the coal, and work them off. 
The first range of pillars having thus been withdrawn, a new range 
adjoining those just removed is attacked, and worked away as in the 
first case, the slate falling down in the excavated space. When sufh- 
cient space has been made to allow the sandrock above to break, a week 
or ten days occurs from the time it begins to rend and crack before it 
finally gives way and falls. ‘The rending of the rocks is terrific; loud 
cracking and rumbling goes on; sometimes the noise is loud and sharp, 
at other times it resembles the sound of distant thunder. Fragments 
of the pillar coal break and fly off, and finally the massive strata, several 
hundred. feet in thickness, falls with tremendous noise and force. All 
this time the workmen continue robbing the pillars, secure from danger, 
under cover of the undisturbed portion of the mine. ) 
In some of the mines of the Hocking Valley, as well as in other 
