In this chapter it is our purpose to briefly consider geology in its direct relations to the arts and industries. When we realize that the value of strictly geologic products taken from the earth each year in the United States alone amounts to billions of dollars, we can better appreciate the practical application of geological science. Such products include coal, petroleum, natural gas, many valuable metal-bearing minerals, and many nonmetalliferous minerals and rocks. In most cases these valuable products of nature have been slowly accumulated or concentrated at many times and under widely varying conditions throughout the millions of years of known geological time. To trace the extent of, and most advantageously remove, such deposits for the use of man is always invariably impossible unless geological knowledge is brought to bear. In many cases the problems involved are intricate, and only the trained geologist is able to at all successfully cope with them. In such cases it is necessary not only to have a thorough knowledge of minerals and rocks as such, but also of their origin and structure. Much of the practical application of geology is carried out by the mining engineer who should have, above all, a thorough knowledge of the great principles of geology. Our plan of discussion is to consider, first, coal, petroleum, and natural gas; then the most important metalliferous deposits of ores; and finally nonmetalliferous minerals and rocks of exceptional commercial importance. Underground waters have already been discussed from the practical standpoint in the chapter on "Waters Within the Earth." Certain minerals have already been sufficiently considered from the economic standpoint in the chapter on "Mineralogy." COAL, PETROLEUM, AND NATURAL GAS Coal. Most valuable of all geological products is coal. Although it is not, strictly speaking, a mineral, both because of its organic origin and lack of definite chemical composition, coal is generally classed among our mineral resources. Some idea of the national importance of coal in the United States may be gained when we realize that the energy derived from a single year's output is equivalent to that of hundreds of millions of men working full time through the year. The uses of coal are too well known to need mention here. Coal is, beyond question, of organic (plant) origin as shown by its very composition; perfect gradations between plant deposits like peat and true coal; and the presence of microscopic plant remains and spores in the coal. An excellent summary of just what happens during the transition of ordinary vegetable matter into coal has been given by D. White as follows: "All coal was laid down in beds analogous to the peat beds of to-day. All kinds of plants, especially such species as were adapted to the particular region where the deposit was located, in whole or in part went into the deposit.