In the United States the deposits at Searles Lake, California, have been produced by the same processes on a smaller scale. In this case evaporation has not been carried to completion, but the crystallization and separation out of other salts has concentrated the potassium (with the magnesium) in the residual brine or "mother liquor." The deposits of this lake or marsh also contain borax (see p. 276), and differ in proportions of salts from the Stassfurt deposits. This is due to the fact that they were probably derived, not from ocean waters, but from the leaching of materials from the rocks of surrounding uplands, transportation of these materials in solution by rivers and ground waters, and concentration in the desert basin by evaporation. The alkali lakes of Nebraska are believed to be of very recent geologic origin. They lie in depressions in a former sand dune area, and contain large quantities of potash supposedly accumulated by leaching of the ashes resulting from repeated burnings of the grass in the adjacent country. Of other natural mineral sources, alunite is the most important. The principal deposits worked are at Marysville, Utah, but the mineral is a rather common one in the western part of the United States, associated with gold deposits, as at Goldfield, Nevada. Alunite occurs as veins and replacement deposits, often in igneous associations, and is supposed to be of igneous source. Its origin is referred to in connection with the Goldfield ores (p. 230). FOOTNOTES: [15] Gale, Hoyt S., The potash deposits of Alsace: _Bull. 715-B, U. S. Geol. Survey_, 1920, pp. 17-55. [16] Gale, Hoyt S., Potash deposits in Spain: _Bull. 715-A, U. S. Geol. Survey_, 1920, pp. 1-16. CHAPTER VIII THE ENERGY RESOURCES--COAL, OIL, GAS (AND ASPHALT) COAL ECONOMIC FEATURES Coal overshadows all other mineral resources, except water, in production, value, and demand. It is the greatest of the energy sources--coal, petroleum, gas, and water power. Roughly two-thirds of the world's coal is used for power, one-sixth for smelting and metallurgical industries, and one-sixth for heating purposes. Coal constitutes over one-third of the railroad tonnage of the United States and is the largest single tonnage factor in international trade; 70 per cent of the pre-war tonnage of outgoing cargoes from England was coal.