might make this system cost 65
cents per ton of ore broken if applied to that mine. The comparative
value of the filling method with other systems will be discussed
later.

FILLING WITH BROKEN ORE SUBSEQUENTLY WITHDRAWN.--This order of support
is called by various names, the favorite being "shrinkage-stoping."
The method is to break the ore on to the roof of the level, and by
thus filling the stope with broken ore, provide temporary support
to the walls and furnish standing floor upon which to work in making
the next cut (Figs. 37, 38, and 39.) As broken material occupies 30
to 40% more space than rock _in situ_, in order to provide working
space at the face, the broken ore must be drawn from along the level
after each cut. When the area attacked is completely broken through
from level to level, the stope will be full of loose broken ore,
which is then entirely drawn off.

A block to be attacked by this method requires preliminary winzes
only at the extremities of the stope,--for entry and for ventilation.
Where it is desired to maintain the winzes after stoping, they
must either be strongly timbered and lagged on the stope side,
be driven in the walls, or be protected by a pillar of ore (Fig.
37). The settling ore and the crushing after the stope is empty
make it difficult to maintain timbered winzes.

[Illustration: Fig. 37.--Longitudinal section of stope filled with
broken ore.]

Where it can be done without danger to the mine, the empty stopes
are allowed to cave. If such crushing would be dangerous, either
the walls must be held up by pillars of unbroken ore, as in the
Alaska Treadwell, where large "rib" pillars are left, or the open
spaces must be filled with waste. Filling the empty stope is usually
done by opening frequent passes along the base of the filled stope
above, and allowing the material of the upper stope to flood the
lower one. This program continued upwards through the mine allows
the whole filling of the mine to descend gradually and thus requires
replenishment only into the top. The old stopes in the less critical
and usually exhausted territory nearer the surface are sometimes
left without replenishing their filling.

The weight of broken ore standing at such a high angle as to settle
rapidly is very considerable upon the level; moreover, at the moment
when the stope is entirely drawn off, the pressure of the walls
as well is likely to be very great. The roadways in this system
therefore r