,
these constructions being pillar-shaped (Fig. 44), the intervals
between them provide outlets for broken ore, and specially built
passes are unnecessary. The method has two disadvantages as against
the square-set or filling process, in that more staging must be
provided from which to work, and in stopes over six feet the erection
of machine-drill columns is tedious and costly in time and wages.

[Illustration: Fig. 44.--Longitudinal section of stull and waste
pillars.]

In wide deposits of markedly flat, irregular ore-bodies, where a
definite system is difficult and where timber is expensive, cribs
of cord-wood or logs filled with waste after the order shown in
Figure 31, often make fairly sound pillars. They will not last
indefinitely and are best adapted to the temporary support of the
ore-roof pending filling. The increased difficulty in setting up
machine drills in such stopes adds to the breaking costs,--often
enough to warrant another method of support.

[Illustration: Fig. 45.--Sublevel caving system.]

CAVING SYSTEMS.--This method, with variations, has been applied
to large iron deposits, to the Kimberley diamond mines, to some
copper mines, but in general it has little application to the metal
mines under consideration, as few ore-bodies are of sufficiently
large horizontal area. The system is dependent upon a large area of
loose or "heavy" ground pressing directly on the ore with weight,
such that if the ore be cut into pillars, these will crush. The
details of the system vary, but in general the _modus operandi_
is to prepare roadways through the ore, and from the roadways to
put rises, from which sublevels are driven close under the floating
mass of waste and ore,--sometimes called the "matte" (Fig. 45).
The pillars between these sublevels are then cut away until the
weight above crushes them down. When all the crushed ore which
can be safely reached is extracted, retreat is made and another
series of subopenings is then driven close under the "matte." The
pillar is reduced until it crushes and the operation is repeated.
Eventually the bottom strata of the "matte" become largely ore,
and a sort of equilibrium is reached when there is not much loss
in this direction. "Top slicing" is a variation of the above method
by carrying a horizontal stope from the rises immediately under the
matte, supporting the floating material with timber. At Kimberley
the system is varied in that galleries are run out to t