ch, at 5%, means
an annual loss of interest amounting to 7 cents per ton of ore
treated.

_E_. A mine once started on the system is most difficult to alter,
owing to the lack of frequent winzes or passes. Especially is this
so if the only alternative is filling, for an alteration to the
system of filling coincident with breaking finds the mine short
of filling winzes. As the conditions of walls and ore often alter
with depth, change of system may be necessary and the situation
may become very embarrassing.

_F_. The restoping of the walls for lower-grade ore at a later
period is impossible, for the walls of the stope will be crushed,
or, if filled with waste, will usually crush when it is drawn off
to send to a lower stope.

The system has much to recommend it where conditions are favorable.
Like all other alternative methods of mining, it requires the most
careful study in the light of the special conditions involved. In many
mines it can be used for some stopes where not adaptable generally.
It often solves the problem of blind ore-bodies, for they can by
this means be frequently worked with an opening underneath only.
Thus the cost of driving a roadway overhead is avoided, which would
be required if timber or coincident filling were the alternatives.
In such cases ventilation can be managed without an opening above,
by so directing the current of air that it will rise through a
winze from the level below, flow along the stope and into the level
again at the further end of the stope through another winze.

[Illustration: Fig. 40.--Longitudinal section. Ore-pillar support
in narrow stopes.]

SUPPORT BY PILLARS OF ORE.--As a method of mining metals of the
sort under discussion, the use of ore-pillars except in conjunction
with some other means of support has no general application. To
use them without assistance implies walls sufficiently strong to
hold between pillars; to leave them permanently anywhere implies
that the ore abandoned would not repay the labor and the material
of a substitute. There are cases of large, very low-grade mines
where to abandon one-half the ore as pillars is more profitable
than total extraction, but the margin of payability in such ore must
be very, very narrow. Unpayable spots are always left as pillars,
for obvious reasons. Permanent ore-pillars as an adjunct to other
methods of support are in use. Such are the rib-pillars in the
Alaska Treadwell, the form of which is indicated by