valueless material. In such instances,
sampling after each stoping-cut is essential, the unprofitable ore
being broken down and used as waste. Where values fade into the
walls, as in impregnation deposits, the width of stopes depends
upon the limit of payability. In these cases, drill-holes are put
into the walls and the drillings assayed. If the ore is found
profitable, the holes are blasted out. The gauge of what is profitable
in such situations is not dependent simply upon the average total
working costs of the mine, for ore in that position can be said to
cost nothing for development work and administration; moreover,
it is usually more cheaply broken than the average breaking cost,
men and machines being already on the spot.

CHAPTER XI.

Methods of Supporting Excavation.

TIMBERING; FILLING WITH WASTE; FILLING WITH BROKEN ORE; PILLARS
OF ORE; ARTIFICIAL PILLARS; CAVING SYSTEM.

Most stopes require support to be given to the walls and often to
the ore itself. Where they do require support there are five principal
methods of accomplishing it. The application of any particular method
depends upon the dip, width of ore-body, character of the ore and
walls, and cost of materials. The various systems are by:--

1. Timbering.
2. Filling with waste.
3. Filling with broken ore subsequently withdrawn.
4. Pillars of ore.
5. Artificial pillars built of timbers and waste.
6. Caving.

TIMBERING.--At one time timbering was the almost universal means of
support in such excavations, but gradually various methods for the
economical application of waste and ore itself have come forward,
until timbering is fast becoming a secondary device. Aside from
economy in working without it, the dangers of creeps, or crushing,
and of fires are sufficient incentives to do away with wood as
far as possible.

There are three principal systems of timber support to excavations,--by
stulls, square-sets, and cribs.

Stulls are serviceable only where the deposit is so narrow that
the opening can be bridged by single timbers between wall and wall
(Figs. 28 and 43). This system can be applied to any dip and is most
useful in narrow deposits where the walls are not too heavy. Stulls
in inclined deposits are usually set at a slightly higher angle than
that perpendicular to the walls, in order that the vertical pressure
of the hanging wall will serve to tighten them in position. The
"stull" system can, in inclined deposits,