The most important other sources of raw materials for industry are the mineral deposits in the earth's surface.[8] This country is stored more bountifully, probably, than is any other country, with the metal ores of iron, copper, lead, zinc, gold, and silver. Aluminum is the most abundant metal, composing about 8 per cent of the crust of the earth, but by present methods it can be extracted only at considerable cost from certain compounds that are limited in amount. The details as to our metal stores are too complex for fuller treatment here, and may be found in treatises on economic geology or on industrial geography. The determination of wise policies as to the use of these stores involves many economic problems, private and public. Another great class of material wealth is in the form of tools, machinery, and other agencies for carrying on the industrial processes of farming and of manufacturing. These are sometimes called instrumental goods, or the industrial equipment. Still another class consists of the great mass of completed direct goods, such as houses to live in, libraries, museums, school buildings, theaters, all kinds of buildings and equipment for pleasure and entertainment, parks, and pleasure resorts in mountains, at lakes or sea shore. The possession and use of these forms of wealth give rise to some economic problems of public ownership and to others connected with the institution of private property in general, as sketched in the following chapter. [Footnote 1: It is to be observed that these figures appear under the general title of Part I, "Estimated valuation of national wealth: 1850-1912," and the tables are spoken of (volume on Wealth, Debt, and Taxation, p. 20) as "estimates of the aggregate wealth of the nation as prepared by the United States censuses," but the tables themselves are described (pp. 23-25) as the "estimated true valuation of all property," this phrase being used as equivalent to "wealth." For the definitions of wealth and property see Vol. I, pp. 264-265.] [Footnote 2: This change will be described below in ch. 6, in treating of the standard of deferred payments.] [Footnote 3: See Vol. I, pp. 265, 278, 508 for the distinction between wealth and capital.] [Footnote 4: See Vol. I, p. 25, for the definition of utility.] [Footnote 5: See Vol. I, p. 510 on the paradox of value.] [Footnote 6: That is, "the amount which can be developed upon the basis of the flowage of the streams for a period of two weeks in which the flow is the least," all the rest being allowed to escape unused. Van Hise, "Conservation of Natural Resources," p. 119.] [Footnote 7: These and other figures in this section relate to the year 1913.] [Footnote 8: Coal has been mentioned above, sec. 9.] CHAPTER 2 THE PRESENT ECONOMIC SYSTEM