s must greatly
expand, to provide for extended ventilation, pumping, and winding
necessities. Moreover inclined shafts of a degree of flatness possible
for moderate depths become too long to be used economically from
the surface. The vast majority of metal-mining shafts fall into
the first class, those of moderate depths. Yet, as time goes on
and ore-deposits are exhausted to lower planes, problems of depth
will become more common. One thing, however, cannot be too much
emphasized, especially on mines to be worked from the outcrop, and
that is, that no engineer is warranted, owing to the speculation
incidental to extension in depth, in initiating early in the mine's
career shafts of such size or equipment as would be available for
great depths. Moreover, the proper location of a shaft so as to
work economically extension of the ore-bodies is a matter of no
certainty, and therefore shafts of speculative mines are tentative
in any event.

Another line of division from an engineering view is brought about
by a combination of three of the factors mentioned. This is the
classification into "outcrop" and "deep-level" mines. The former
are those founded upon ore-deposits to be worked from or close
to the surface. The latter are mines based upon the extension in
depth of ore-bodies from outcrop mines. Such projects are not so
common in America, where the law in most districts gives the outcrop
owner the right to follow ore beyond his side-lines, as in countries
where the boundaries are vertical on all sides. They do, however,
arise not alone in the few American sections where the side-lines
are vertical boundaries, but in other parts owing to the pitch of
ore-bodies through the end lines (Fig. 3). More especially do such
problems arise in America in effect, where the ingress questions
have to be revised for mines worked out in the upper levels (Fig.
7).

[Illustration: Fig. 3.--Longitudinal section showing "deep level"
project arising from dip of ore-body through end-line.]

If from a standpoint of entrance questions, mines are first classified
into those whose works are contemplated for moderate depths, and those
in which work is contemplated for great depth, further clarity in
discussion can be gained by subdivision into the possible cases arising
out of the factors of location, dip, topography, and boundaries.

MINES OF MODERATE DEPTHS.

Case I. Deposits where topographic conditions permit the
alternative