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 dis covered, 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 an cestors. 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 me chanical improvement have combined to unlock the store houses 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 manu factures. Probably the most durative of all economic agents is solid land-surface used solely for standing room. Yet geol ogy 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 his toric 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. Build ing sites for residence and business purposes—factories, offi ces, stores—are the purest type of durative agents known to us, despite the occurrence of volcanic eruption, and of earth quakes 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 con veniently near to places of work, where man can earn a live lihood. 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 acces sible sites does something to relieve the pressure for limited residence land, but it makes possible still greater pressure for the central business locations.
§ 11. Production of usable land surface in cities. The work of man is doing much by form changes to increase the area suitable for residence and business. Large districts on the river fronts of New York are filled land. The larger part of the most valuable lands within the city of Boston were once tidewater swamps, which have been filled and made usable by great outlays. Great hills have been dumped into the Bay of San Francisco to convert mud fiats into solid earth, for railroad terminals and warehouses. In almost every city much has been done to level hillsides, to fill val leys, or to drain swamps. Along many picturesque lakes the steep banks for miles are dug with pick and shovel or blasted with dynamite, and dumped over into the water to make level sites for cottages. Wooden, stone, or cement retaining walls are built so that the debris from the streams and the sands washed up by waves may be ietained to widen the solid land. Suitable places for docks, warehouses, and factories, and other needs of commerce and industry, are created on the shores of navigable waters. The engineer in tunneling mountains and building roadbeds over marshes or along swampy riversides, or in digging canals between rivers or between oceans, is mak ing the kind of land surface suitable to the uses of trans portation and trade. It is characteristic of nearly all these artificially altered spaces that they are as solid and enduring as natural formations of level land, and are subject only to the slow action of rain, streams, waves, winds, or to rare up heavals of nature. Man's works are in these cases as enduring as nature's.