with troughs the distance can be expanded up to 150
or 175 feet.

[Footnote *: Page 136.]

[Footnote **: Intervals given are measured on the dip.]

In dips of over 40 deg. to 50 deg., depending on the smoothness of the
foot wall, the distance can again be increased, as stope-transport is
greatly simplified, since the stope materials fall out by gravity.
In timbered stopes, in dips over about 45 deg., intervals of 150 to
200 feet are possible. In filled stopes intervals of over 150 feet
present difficulties in the maintenance of ore-passes, for the wear
and tear of longer use often breaks the timbers. In shrinkage-stopes,
where no passes are to be maintained and few winzes put through, the
interval is sometimes raised to 250 feet. The subject is further
discussed under "Stoping."

Another factor bearing on level intervals is the needed insurance
of sufficient points of stoping attack to keep up a certain output.
This must particularly influence the manager whose mine has but
little ore in reserve.

[Illustration: Fig. 19.]

PROTECTION OF LEVELS.--Until recent years, timbering and occasional
walling was the only method for the support of the roof, and for
forming a platform for a stoping base. Where the rock requires no
support sublevels can be used as a stoping base, and timbering
for such purpose avoided altogether (Figs. 38, 39, 42). In such
cases the main roadway can then be driven on straight lines, either
in the walls or in the ore, and used entirely for haulage. The
subheading for a stoping base is driven far enough above or below
the roadway (depending on whether overhand or underhand stoping
is to be used) to leave a supporting pillar which is penetrated
by short passes for ore. In overhand stopes, the ore is broken
directly on the floor of an upper sublevel; and in underhand stopes,
broken directly from the bottom of the sublevel. The method entails
leaving a pillar of ore which can be recovered only with difficulty
in mines where stope-support is necessary. The question of its
adoption is then largely one of the comparative cost of timbering,
the extra cost of the sublevel, and the net value of the ore left.
In bad swelling veins, or badly crushing walls, where constant
repair to timbers would be necessary, the use of a sublevel is a
most useful alternative. It is especially useful with stopes to
be left open or worked by shrinkage-stoping methods.

If the haulage level, however, is to be the stoping