THE VALUATION OF MINES AND OIL PROPERTIES 257
ductive. Subsequent experience indicates that this safety
factor was not sufficient.
A method of oil land valuation, which has been applied more
recently, is that adopted by the engineers in valuing the properties
 which market their product through the Independent Oil
Producers Agency, which valuation had in view the merging of
a number of companies in one large organization or corporation.
The method used was based on the determination of a production
 curve for each property. A study of the history of
producing wells has given basis for the assumption that the
wells in a field constantly decrease in production at a rate the
changes of which can be plotted in curves and, using these curves
as a basis, old wells, new wells and wells not yet drilled can all
be taken into account in estimating the future production of the
oil lands. The total operating expenses have been estimated
from existing records and the prospective future net receipts
have been discounted in obtaining the present value of the properties.
 Amortization of all capital except the salvage of surface
 improvements has been assumed to take place in ten
years. ‘ History” is also prerequisite to this method and
attention should be called to the fact that, because of the
inaccessibility of the reservoirs of oil, the more extensive the
record of previous operations, the more satisfactory will be
the valuation no matter what method may be adopted.
PRODUCTION AND PRICE OF COPPER, SILVER,
LEAD AND ZINC IN THE UNITED STATES
Plates and Tables
Accompanying Chapter on Valuation of Mines and Oil Properties
Figures 4 to 7, and Tables 15 to 18 which directly follow this
page deal only with four of the principal metals, copper, silver,
lead and zinc. The statistics given are based on the best
available data and the figures are believed to be sufficiently
accurate to justify their use by valuating engineers who have in
hand problems in which these metals have a part.
The figures submitted for the United States prices and production
 are those of the United States Geological Survey.