inzes and to the
ore-passes, by starting them from crosscuts in the wall, and by
carrying them at greater angles than the pitch of the ore (Fig.
36). These artifices combined have worked out most economically
on several mines within the writer's experience, with the dip as
flat as 30 deg. For very flat dips, where filling is to be employed,
rill-stoping has no advantage over flat-backed cuts, and in such
cases it is often advisable to assist stope transport by temporary
tracks and cars which obviously could not be worked on the tortuous
contour of a rill-stope, so that for dips under 30 deg. advantage lies
with "flat-backed" ore-breaking.

[Illustration: Fig. 36.--Cross-section showing method of steepening
winzes and ore passes.]

On very wide ore-bodies where the support of the standing ore itself
becomes a great problem, the filling system can be applied by combining
it with square-setting. In this case the stopes are carried in
panels laid out transversally to the strike as wide as the standing
strength of the ore permits. On both sides of each panel a fence
of lagged square-sets is carried up and the area between is filled
with waste. The panels are stoped out alternately. The application
of this method at Broken Hill will be described later. (See pages
120 and Figs. 41 and 42.) The same type of wide ore-body can be
managed also on the filling system by the use of frequent "bulkheads"
to support the ore (Fig. 31).

Compared with timbering methods, filling has the great advantage
of more effective support to the mine, less danger of creeps, and
absolute freedom from the peril of fire. The relative expense of
the two systems is determined by the cost of materials and labor.
Two extreme cases illustrate the result of these economic factors
with sufficient clearness. It is stated that the cost of timbering
stopes on the Le Roi Mine by square-sets is about 21 cents per
ton of ore excavated. In the Ivanhoe mine of West Australia the
cost of filling stopes with tailings is about 22 cents per ton
of ore excavated. At the former mine the average cost of timber
is under $10 per M board-measure, while at the latter its price
would be $50 per M board-measure; although labor is about of the
same efficiency and wage, the cost in the Ivanhoe by square-setting
would be about 65 cents per ton of ore broken. In the Le Roi, on the
other hand, no residues are available for filling. To quarry rock
or drive crosscuts into the walls