be called "proved" varies naturally with the type
of deposit,--in fact with each mine. In a general way, a fair rule
in gold quartz veins below influence of secondary alteration is
that no point in the block shall be over fifty feet from the points
sampled. In limestone or andesite replacements, as by gold or lead
or copper, the radius must be less. In defined lead and copper
lodes, or in large lenticular bodies such as the Tennessee copper
mines, the radius may often be considerably greater,--say one hundred
feet. In gold deposits of such extraordinary regularity of values
as the Witwatersrand bankets, it can well be two hundred or two
hundred and fifty feet.

"Probable ore" should be ore which entails continuity of values
through a greater distance than the above, and such distance must
depend upon the collateral evidence from the character of the deposit,
the position of openings, etc.

Ore beyond the range of the "probable" zone is dependent upon the
extension of the deposit beyond the realm of development and will
be discussed separately.

Although the expression "ore in sight" may be deprecated, owing to
its abuse, some general term to cover both "positive" and "probable"
ore is desirable; and where a general term is required, it is the
intention herein to hold to the phrase "ore in sight" under the
limitations specified.

CHAPTER III.

Mine Valuation (_Continued_).

PROSPECTIVE VALUE.[*] EXTENSION IN DEPTH; ORIGIN AND STRUCTURAL
CHARACTER OF THE DEPOSIT; SECONDARY ENRICHMENT; DEVELOPMENT IN
NEIGHBORING MINES; DEPTH OF EXHAUSTION.

[Footnote *: The term "extension in depth" is preferred by many
to the phrase "prospective value." The former is not entirely
satisfactory, as it has a more specific than general application.
It is, however, a current miner's phrase, and is more expressive.
In this discussion "extension in depth" is used synonymously, and
it may be taken to include not alone the downward prolongation of
the ore below workings, but also the occasional cases of lateral
extension beyond the range of development work. The commonest instance
is continuance below the bottom level. In any event, to the majority
of cases of different extension the same reasoning applies.]

It is a knotty problem to value the extension of a deposit beyond
a short distance from the last opening. A short distance beyond
it is "proved ore," and for a further short distance is "probable
ore." Mines are very seldom