mining camp on earth are thousands
of mines opened on this empirical figure, without consideration
of the reasons for it or for any other distance.

The governing factors in determining the vertical interval between
levels are the following:--

_a_. The regularity of the deposit.
_b_. The effect of the method of excavation of winzes and rises.
_c_. The dip and the method of stoping.

REGULARITY OF THE DEPOSIT.--From a prospecting point of view the
more levels the better, and the interval therefore must be determined
somewhat by the character of the deposit. In erratic deposits there
is less risk of missing ore with frequent levels, but it does not
follow that every level need be a through roadway to the shaft or
even a stoping base. In such deposits, intermediate levels for
prospecting alone are better than complete levels, each a roadway.
Nor is it essential, even where frequent levels are required for
a stoping base, that each should be a main haulage outlet to the
shaft. In some mines every third level is used as a main roadway,
the ore being poured from the intermediate ones down to the haulage
line. Thus tramming and shaft work, as stated before, can be
concentrated.

EFFECT OF METHOD OF EXCAVATING WINZES AND RISES.--With hand drilling
and hoisting, winzes beyond a limited depth become very costly to
pull spoil out of, and rises too high become difficult to ventilate,
so that there is in such cases a limit to the interval desirable
between levels, but these difficulties largely disappear where
air-winches and air-drills are used.

THE DIP AND METHOD OF STOPING.--The method of stoping is largely
dependent upon the dip, and indirectly thus affects level intervals.
In dips under that at which material will "flow" in the stopes--about
45 deg. to 50 deg.--the interval is greatly dependent on the method of
stope-transport. Where ore is to be shoveled from stopes to the
roadway, the levels must be comparatively close together. Where
deposits are very flat, under 20 deg., and walls fairly sound, it is
often possible to use a sort of long wall system of stoping and to
lay tracks in the stopes with self-acting inclines to the levels.
In such instances, the interval can be expanded to 250 or even 400
feet. In dips between 20 deg. and 45 deg., tracks are not often possible,
and either shoveling or "bumping troughs"[*] are the only help
to transport. With shoveling, intervals of 100 feet[**] are most
common, and