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.