§ 7. Rapid consumption of coal. With care, the use of agricultural and of forest lands may be durative; but the extraction of coal is a purely consumptive use of the mine. Every ton used to-day is subtracted from the supplies for future generations. The coal deposits in the earth have only recently been drawn upon. A modern town with a few thousand inhabitants probably uses to-day a greater quantity of coal than was used in all Europe two centuries ago. The large deposits of coal in England and their early development long gave to English industry a great advantage over other countries. In England, however, has first been felt the fear of the exhaustion of the coal supply. Professor Jevons, in 1865, sounded the note of alarm; he prophesied that because the coal deposits in America were many times as great as those of England, industrial supremacy must inevitably pass to America. Already the supremacy in coal and iron production has passed to America, and that in many other industries where fuel is an important element in cost soon will come. In England the accessible supply of coal is limited, deeper shafts must be sunk, and tunnels extended far under the ocean bed, and the coal got with greater difficulty and at greater expense. Coal has risen in price in England within the last few years, and will continue to rise in the future. The coal deposits of America have been estimated to be thirty-seven times as great as those of England, but many of the best American mines show signs of diminution. The best anthracite beds will be gone in less than three quarters of a century. And yet there is in America little thought of the future in this regard.

§ 8. Disappearance of mineral stores. There are many other natural materials which, now that the exploration of the earth's surface is pretty well completed, appear to form a limited and unincreasable stock. Their gradual consumption is making and will make great changes in the economic world. Natural gas is a wonderful substitute for coal, and when first found in a locality it brings a brief prosperity, but it is soon exhausted. Petroleum, little used before 1865, will help light the world for but a few decades, for it is drawn from natural reservoirs slowly, if at all, replenishing. The recent increase in the use of gasoline for motor-vehicles has directed thought to the limits of possible supply. Iron ore, the most essential single mineral resource, has been taken from the earth in greater quantities within the last fifty years than altogether before in the history of the globe, and the limits of rich accessible supplies in the United States are already in sight. China may be the next great center of iron and steel production. Copper, tin, lead, gold, silver, and potter's clay are a limited stock, inadequate to increasing needs. When any deposit has been worked out, the abandoned quarry, mine, or claybank is most often useless for any other purpose.

Some of these materials are made available more than once through the useful services of the junk man. New processes are devised for extracting metals from lower grade ores which before were worthless. Sometimes a good substitute is found, such as aluminum, which gives many of the same uses as iron and copper and which can be extracted from clay by the use of electricity generated by a waterfall. This would promise an almost inexhaustible quantity, but as yet obtainable only at high cost. Many other substitutes will doubtless be discovered, but the outlook in some directions has little promise.

§ 9. Civilization's consumption of the earth's stores. There is a striking contrast between the modes in which the earth's surface is utilized by modern man and by his ancestors. The savage uses the fruits that he finds, and those fruits are, almost without exception, renewed the next year. The earlier civilizations did not go deep enough into natural resources to use up permanently the world in which they lived. The only mines that were worked out under the great ancient empires were gold and silver mines, while the mines of heavier, useful metals were touched but lightly. But from the eighteenth century the earth's crust has been exploited at an ever-accelerating rate. Scientific knowledge and mechanical improvement have combined to unlock the storehouses of the Geologic Ages. If this movement continues, many important materials must be exhausted in the not far distant future.

§ 10. Land as a site for residence, commerce, and manufactures. Probably the most durative of all economic agents is solid land-surface used solely for standing room. Yet geology reveals that every part of the earth's crust has been under the ocean, some of it many times. Every part of the world's surface is more or less rising or falling, changes within historic times having been enough to depress and again elevate large stretches of sea coast. Slight earthquake shocks are felt in nearly every part of the habitable globe. Before the end of man's tenancy on the globe great changes will take place in the land surface. Not only San Francisco, but New York, may some day sink into the sea, beneath which may now lie the building sites of the future metropolis. But these catastrophic changes are rare, and the slow secular changes hardly enter into the calculations of men. "The solid earth" is the synonym of the everlasting and unchangeable. Building sites for residence and business purposes - factories, offices, stores - are the purest type of durative agents known to us, despite the occurrence of volcanic eruption, and of earthquakes in limited districts, and of intruding waters and crumbling walls almost everywhere.

The covering of the ground with dwellings does something to protect it from the natural wear of rain and winds, as do also the erection of stone and cement walks, the planting of trees, the diversion of streams, and many other safeguards. The space needed for existence is small. With a density of population equal to that of the most crowded districts in the East Side in New York, all the people of the world could be housed in the State of Delaware. The problem of residence land is to get ample space for health and a happy life conveniently near to places of work, where man can earn a livelihood. The scarcity appears in very high rents for the miserable tenements of the poor, and in fabulous prices for residence sites in the fashionable neighborhoods.

Sites for manufacturing, commerce, banking, and trade that are conveniently located in relation to workers, to consumers, and to transportation facilities for raw materials and finished products, are few in any community. Their uses are highly valued. Rapid transit by producing a larger supply of accessible sites does something to relieve the pressure for limited residence land, but it makes possible still greater pressure for the central business locations.