es, while a large mill eating
away rapidly on the same mine under the same scale of development
would leave small reserves. On the above scheme of valuation the
extension in depth would be worth very different sums, even when the
deepest level might be at the same horizon in both cases. Moreover,
no mine starts at the surface with a large amount of ore in sight.
Yet as a general rule this is the period when its extension is most
valuable, for when the deposit is exhausted to 2000 feet, it is
not likely to have such extension in depth as when opened one hundred
feet, no matter what the ore-reserves may be. Further, such bases
of valuation fail to take into account the widely varying geologic
character of different mines, and they disregard any collateral
evidence either of continuity from neighboring development, or from
experience in the district. Logically, the prospective value can
be simply a factor of how _far_ the ore in the individual mine
may be expected to extend, and not a factor of the remnant of ore
that may still be unworked above the lowest level.

An estimation of the chances of this extension should be based
solely on the local factors which bear on such extension, and these
are almost wholly dependent upon the character of the deposit.
These various geological factors from a mining engineer's point
of view are:--

1. The origin and structural character of the ore-deposit.
2. The position of openings in relation to secondary alteration.
3. The size of the deposit.
4. The depth to which the mine has already been exhausted.
5. The general experience of the district for continuity and
the development of adjoining mines.

THE ORIGIN AND STRUCTURAL CHARACTER OF THE DEPOSIT.--In a general
way, the ore-deposits of the order under discussion originate primarily
through the deposition of metals from gases or solutions circulating
along avenues in the earth's crust.[*] The original source of metals
is a matter of great disagreement, and does not much concern the
miner. To him, however, the origin and character of the avenue
of circulation, the enclosing rock, the influence of the rocks
on the solution, and of the solutions on the rocks, have a great
bearing on the probable continuity of the volume and value of the
ore.

[Footnote *: The class of magmatic segregations is omitted, as
not being of sufficiently frequent occurrence in payable mines to
warrant troubling with it here.]

All ore-deposits vary i