s do not yet
indicate definite limits to the possibilities of exploration for most
mineral resources, and of the consequent fact that for a long time in
the future, as in the past, discoveries of new mineral deposits will be
roughly proportional to the effort and money spent in finding
them,--which means, also, proportional to the demand,--makes it
impossible, for most of the mineral resources, to set any definite
limits on reserves. It is comparatively easy to measure known reserves;
but a quantitative appraisal of the probable and possible reserves for
the future is extremely difficult. Successive revisions of estimates
have, with but few exceptions, progressively increased the total mineral
supplies available. The result is that the time of exhaustion has been
pushed far into the future for most of the important minerals, thus
minimizing the urge for immediate and drastic conservational action,
which followed naturally from early estimates of very limited supplies.
For both coal and iron, supplies are now known for hundreds or even
thousands of years. For oil and lead, on the other hand, the reserves
now known have a life of comparatively few years, but the possibilities
for successful exploration make it probable that their life will be
greatly extended. Notwithstanding this tendency to lengthen the
exhaustion period, the limits of mineral resource life are still small
as compared with the life of the nation or of civilization,--and the
fundamental desirability of conservation is not materially affected.

It is not easy to predict the rate of production for the future. At the
present rate of coal production in the United States, the supplies to a
depth of 6,000 feet might last 6,000 years; but if it be assumed that
the recent _acceleration_ of production will be continued indefinitely
into the future, the result would be exhaustion of these supplies in
less than 200 years. It is generally agreed that exhaustion will come
sooner than 6,000 years, but will require more time than 200 years. The
range between these figures offers wide opportunity for guessing. It is
supposed that per capita consumption may not increase as fast in the
future as in the past, that possibly an absorption point will be
reached, and that there will be limits to transportation and
distribution; but how to evaluate these factors no one knows. In the
case of some of the metallic resources, such as iron, the fact that the
world's stock on hand is