TH OF EXHAUSTION.--All mines become completely exhausted at
some point in depth. Therefore the actual distance to which ore
can be expected to extend below the lowest level grows less with
every deeper working horizon. The really superficial character of
ore-deposits, even outside of the region of secondary enrichment
is becoming every year better recognized. The prospector's idea
that "she gets richer deeper down," may have some basis near the
surface in some metals, but it is not an idea which prevails in
the minds of engineers who have to work in depth. The writer, with
some others, prepared a list of several hundred dividend-paying
metal mines of all sorts, extending over North and South America,
Australasia, England, and Africa. Notes were made as far as possible
of the depths at which values gave out, and also at which dividends
ceased. Although by no means a complete census, the list indicated
that not 6% of mines (outside banket) that have yielded profits,
ever made them from ore won below 2000 feet. Of mines that paid
dividends, 80% did not show profitable value below 1500 feet, and
a sad majority died above 500. Failures at short depths may be
blamed upon secondary enrichment, but the majority that reached
below this influence also gave out. The geological reason for such
general unseemly conduct is not so evident.

CONCLUSION.--As a practical problem, the assessment of prospective
value is usually a case of "cut and try." The portion of the capital
to be invested, which depends upon extension, will require so many
tons of ore of the same value as that indicated by the standing
ore, in order to justify the price. To produce this tonnage at
the continued average size of the ore-bodies will require their
extension in depth so many feet--or the discovery of new ore-bodies
of a certain size. The five geological weights mentioned above
may then be put into the scale and a basis of judgment reached.

CHAPTER IV.

Mine Valuation (_Continued_).

RECOVERABLE PERCENTAGE OF THE GROSS ASSAY VALUE; PRICE OF METALS;
COST OF PRODUCTION.

The method of treatment for the ore must be known before a mine
can be valued, because a knowledge of the recoverable percentage
is as important as that of the gross value of the ore itself. The
recoverable percentage is usually a factor of working costs. Practically
every ore can be treated and all the metal contents recovered, but
the real problem is to know the method and per