more evaded by the ingenuity of engineers in charge. The
author believes it soon will be recognized that the situation is
rare indeed where complete square-setting is necessarily without an
economical alternative. An objection is sometimes raised to filling
in favor of timber, in that if it become desirable to restope the
walls for low-grade ore left behind, such stopes could only be
entered by drawing the filling, with consequent danger of total
collapse. Such a contingency can be provided for in large ore-bodies
by installing an outer shell of sets of timber around the periphery
of the stope and filling the inside with waste. If the crushing
possibilities are too great for this method then, the subsequent
recovery of ore is hopeless in any event. In narrow ore-bodies
with crushing walls recovery of ore once left behind is not often
possible.

The third sort of timber constructions are cribs, a "log-house" sort
of structure usually filled with waste, and more fully discussed
under artificial pillars (Fig. 31). The further comparative merits
of timbering with other methods will be analyzed as the different
systems are described.

FILLING WITH WASTE.--The system of filling stope-excavations completely
with waste in alternating progress with ore-breaking is of wide
and increasingly general application (Figs. 32, 33, 34, 35).

Although a certain amount of waste is ordinarily available in the
stopes themselves, or from development work in the mine, such a
supply must usually be supplemented from other directions. Treatment
residues afford the easiest and cheapest handled material. Quarried
rock ranks next, and in default of any other easy supply, materials
from crosscuts driven into the stope-walls are sometimes resorted
to.

In working the system to the best advantage, the winzes through
the block of ore under attack are kept in alignment with similar
openings above, in order that filling may be poured through the
mine from the surface or any intermediate point. Winzes to be used
for filling should be put on the hanging-wall side of the area to
be filled, for the filling poured down will then reach the foot-wall
side of the stopes with a minimum of handling. In some instances,
one special winze is arranged for passing all filling from the
surface to a level above the principal stoping operations; and
it is then distributed along the levels into the winzes, and thus
to the operating stopes, by belt-conveyors.

[Ill