Coal.]

In the working of thick seams inclined at a high angle, such as those in
the south of France, and in the lignite mines of Styria and Bohemia, the
method of working in horizontal slices, about 12 or 15 ft. thick, and
filling up the excavation with broken rock and earth from the surface,
is now generally adopted in preference to the systems formerly used. At
Monceaux les Mines, in France, a seam 40 ft. thick, and dipping at an
angle of 20 deg., is worked in the following manner. A level is driven
in a sandstone forming the floor, along the course of the coal, into
which communications are made by cross cuts at intervals of 16 yds.,
which are driven across to the roof, dividing up the area to be worked
into panels. These are worked backwards, the coal being taken to a
height of 20 ft., the opening being packed up with stone sent down from
the surface. As each stage is worked out, the floor level is connected
with that next below it by means of an incline, which facilitates the
introduction of the packing material. Stuff containing a considerable
amount of clay is found to be the best suited for the purpose of
filling, as it consolidates readily under pressure.

In France and Germany the method of filling the space left by the
removal of the coal with waste rock, quarried underground or sent down
from the surface, which was originally used in connexion with the
working of thick inclined seams by the method of horizontal slices, is
now largely extended to long-wall workings on thin seams, and in
Westphalia is made compulsory where workings extend below surface
buildings, and safety pillars of unwrought coal are found to be
insufficient. With careful packing it is estimated that the surface
subsidence will not exceed 40% of the thickness of the seam removed, and
will usually be considerably less. The material for filling may be the
waste from earlier workings stored in the spoil banks at the surface;
where there are blast furnaces in the neighbourhood, granulated slag
mixed with earth affords excellent packing. In thick seams packing adds
about 5d. per ton to the cost of the coal, but in thinner seams the
advantage is on the other side.

In some anthracite collieries in America the small coal or culm and
other waste are washed into the exhausted workings by water which gives
a compact mass filling the excavation when the water has drained away. A
modification of this method, which originated in Silesia, is now
bec