In the past they have not been used freely because wooden buildings have been so much cheaper; but cement, concrete and brick are now manufactured much more cheaply, on account of improved methods, while the price of lumber has been increasing rapidly. Within the last ten years, the value of cement manufactures has increased nearly six times. In 1900 we used seventy pounds of cement for each person; in 1907, two hundred and twenty-eight pounds. The value of brick and other products made from clay has doubled in the same period and is now $160,000,000, while the value of building-stone quarries is three times as great as it was ten years ago. There are many reasons why these materials should take the place of wood; as they are stronger, more durable, do not require paint, and are so much less liable to loss by fire. The waste of minerals used in building is due to improper and reckless methods of taking them from the ground and preparing them for market and in careless methods in manufacturing. Of such minerals as quartz, grindstone, millstone, emery stone, mineral paints, talc and salt, there seems to be enough to meet the needs of the future as well as the present. Such supplies as sulphur, asphalt, magnesia, borax, and asbestos, as well as coal and iron, are not very plentiful. If used carelessly, they will be exhausted in a few years; if wisely, they may be expected to last beyond the limits of the present century. Our supplies of quicksilver, antimony, graphite, mica, tin, nickel, platinum, and many minerals less well known, as well as our petroleum, natural gas, copper, gold, silver, lead, zinc, and phosphate rock will be almost exhausted well within the present century unless large new deposits are discovered. REFERENCES Report of National Conservation Commission. The Conservation of Mineral Resources. U. S. Government Reports. Report of the U. S. Geological Survey. Production of Gold in 1908. U. S. Government Reports. Production of Silver in 1908. Production of Lead in 1908. Production of Zinc in 1908. Production of Structural Materials. About twenty pamphlets on other minerals. CHAPTER IX ANIMAL FOODS GRAZING Food is of two classes: vegetable, which comes directly from the earth, and animal, which has fed on vegetable life. This is, of course, a more concentrated form of food, and much less of it is needed to sustain life. For the plentiful supply of vegetable food we must depend upon the fertility of the soil, as we have seen. Our animal food can not be classed among our natural resources, but as a product of them, and requires the same care and wise use.