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 constantly increasing—losses due to rusting, ship-wrecks, etc., being only a small fraction of the annual output—suggests that a point will be reached where new production will cease to accelerate at the present rate and may even decline. But again, the factors are so complex and many of them so little known, that no one can say how soon this point will be reached.
For the immediate future, there is little to be feared from shortage of mineral supplies in the ground. The difficulties are more likely to arise from the failure of means to extract and distribute these supplies fast enough to keep up with the startling acceleration in future demand indicated by the figures of recent years. The speed and magnitude of recent material developments in many lines cannot but raise question as to whether we have the ability to understand and coödinate the many huge, variable, and accelerating factors we have to deal with, or whether some of the lines of development may not get so far ahead of others as to cause serious disturbance of the whole material structure of civilization. Coal alone, which now constitutes a third of our railway tonnage, may with increased rate of production require two-thirds of present railway capacity. Will railway development keep up? It may be noted that national crises and failures in the past history of the world have seldom, if ever, been due to shortage of raw materials, or in fact to any failure of the material environment.
In its early stages the conservation movement in this country concerned itself principally with the raw material. Later there came the recognition of the fact that conservation of raw materials is closely bound up with the question of conservation of human energy. The two elements in the problem are much like the two major elements in mineral resource valuation (see pages 329-330). If in saving a dollar's worth of raw material, we spend two dollars worth of energy, it naturally raises question as to the wisdom of our