entally with the sampling. A doubt can then always be settled
by resampling at once, and much knowledge can be gained which may
relieve so exhaustive a program as might be necessary were results
not known until after leaving the mine.

ASSAY OF SAMPLES.--Two assays, or as the case may be, analyses,
are usually made of every sample and their average taken. In the
case of erratic differences a third determination is necessary.

ASSAY PLANS.--An assay plan is a plan of the workings, with the
location, assay value, and width of the sample entered upon it. In
a mine with a narrow vein or ore-body, a longitudinal section is
sufficient base for such entries, but with a greater width than one
sample span it is desirable to make preliminary plans of separate
levels, winzes, etc., and to average the value of the whole payable
widths on such plans before entry upon a longitudinal section. Such
a longitudinal section will, through the indicated distribution
of values, show the shape of the ore-body--a step necessary in
estimating quantities and of the most fundamental importance in
estimating the probabilities of ore extension beyond the range of
the openings. The final assay plan should show the average value
of the several blocks of ore, and it is from these averages that
estimates of quantities must be made up.

CALCULATIONS OF AVERAGES.--The first step in arriving at average
values is to reduce erratic high assays to the general tenor of
other adjacent samples. This point has been disputed at some length,
more often by promoters than by engineers, but the custom is very
generally and rightly adopted. Erratically high samples may indicate
presence of undue metal in the assay attributable to unconscious
salting, for if the value be confined to a few large particles
they may find their way through all the quartering into the assay.
Or the sample may actually indicate rich spots of ore; but in any
event experience teaches that no dependence can be put upon regular
recurrence of such abnormally rich spots. As will be discussed
under percentage of error in sampling, samples usually indicate
higher than the true value, even where erratic assays have been
eliminated. There are cases of profitable mines where the values
were all in spots, and an assay plan would show 80% of the assays
_nil_, yet these pockets were so rich as to give value to the whole.
Pocket mines, as stated before, are beyond valuation by sampling,
and aside from