"blocked out" so far
as it is necessary for stoping purposes with modern wide intervals
between levels, and still be in blocks too large to warrant an
assumption of continuity of values to their centers (Fig. 1). As
to the third class of "possible" ore, it conveys an impression
of tangibility to a nebulous hazard, and should never be used in
connection with positive tonnages. This part of the mine's value
comes under extension of the deposit a long distance beyond openings,
which is a speculation and cannot be defined in absolute tons without
exhaustive explanation of the risks attached, in which case any
phrase intended to shorten description is likely to be misleading.

[Illustration: Fig. 1.--Longitudinal section of a mine, showing
classification of the exposed ore. Scale, 400 feet = 1 inch.]

Therefore empirical expressions in terms of development openings
cannot be made to cover a geologic factor such as the distribution
of metals through a rock mass. The only logical basis of ore
classification for estimation purposes is one which is founded
on the chances of the values penetrating from the surface of the
exposures for each particular mine. Ore that may be calculated
upon to a certainty is that which, taking into consideration the
character of the deposit, can be said to be so sufficiently surrounded
by sampled faces that the distance into the mass to which values
are assumed to extend is reduced to a minimum risk. Ore so far
removed from the sampled face as to leave some doubt, yet affording
great reason for expectation of continuity, is "probable" ore.
The third class of ore mentioned, which is that depending upon
extension of the deposit and in which, as said above, there is great
risk, should be treated separately as the speculative value of the
mine. Some expressions are desirable for these classifications, and
the writer's own preference is for the following, with a definition
based upon the controlling factor itself.

They are:--

Proved Ore Ore where there is practically no risk of
failure of continuity.

Probable Ore Ore where there is some risk, yet warrantable
justification for assumption of continuity.

Prospective Ore Ore which cannot be included in the above
classes, nor definitely known or stated in
any terms of tonnage.

What extent of openings, and therefore of sample faces, is required
for the ore to