The Tertiary formations, which comprehend the greater part of the Cenozoic group, are spread in discontinuous patches over the west ern United States and in a long belt that stretches along the southern Atlantic Coast and the Gulf region, reaching up the Mississippi Valley to the Ohio River. Some important coal beds of lignitic character were deposited in the interior during the early Tertiary. The strata otherwise include sandstones, shales, soft lime stones and unconsolidated detrital materials. The later Quaternary deposits are largely rep resented by glacial accumulations from the ice sheets that extended from the highlands of Can ada into the northern part of the county; on melting the ice left behind, a mantle of sands, gravels and clays, here and there heaped up into hillocks (kames, drumlins, eskers, etc.), that lie over all of the other formations. To the south of the limits of ice advance, which did not reach below the Ohio River, stream and es tuary deposits were formed at this time and on the uplands the finely divided, probably wind blown material called "loess." At the outset of the ice invasion the land surface was consider ably higher than at present and the northeastern coast line no doubt extended 100 miles or more further out into the Atlantic, to the limits of the continental shelf. This elevation and in crease of land area thus indicated were influ ential in producing the glacial climate, although not the only factors concerned with it. At the close of the Glacial epoch a subsidence took place, causing the sea to advance upon the land and producing fiords and drowned valleys along the coast, as now in evidence.
Mineral Wealth.— The United States is well provided with mineral resources, particularly of the more essential materials like coal, petro leum, salt and the ores of iron, copper, lead and zinc. Clays of various kinds and building stones are widespread, while the precious metals and many of the rarer minerals occur in important quantities. Practically all of the mineral re quirements for industrial needs and home con sumption are supplied from domestic sources; the principal exceptions that may be noted are tin, platinum, manganese ores, nitrates and pot ash salts, of which the native production is usually supplemented by imports from other countries. The growth of mining has been one of the important elements in the remarkable in dustrial development that has taken place in the last four or five decades and given the country its prominent position in the world's industry and trade.
Coal.— The coal fields of the continental United States underlie an estimated area of 310,296 square miles, not including certain pos sible fields about which little is now known or fields in which the coal is too deep to be avail able. The coal content is put at 3,076,204 mil lions of tons, and is now being consumed at the rate of 600 million tons a year. The principal fields are the Appalachian, which produces both bituminous and anthracite the interior bitumi nous field, the Rocky Mountain and Great Plains fields of bituminous and lignite and the Pacific Coast fields, inclusive of Alaska, in which the coal is chiefly bituminous. Thirty
States are represented in the mining industry, the chiefproducers being Pennsylvania, West Virginia, Ohio, Illinois, Kentucky, Indiana, Ala bama, Colorado, Virginia, Iowa, Kansas and Wyoming.
Petroleum.— The exploitation of the oil re sources began about 1860 in western Pennsyl vania. In the following years the field was en larged by drilling so as to cover parts of New York, Ohio and West Virginia, constituting with Kentucky and Tennessee the Appalachian field, which is still an active producer. The oil from this field contains a 'high percentage of the lighter distillates and has a paraffin base. Next in aggregate yield is California which has been a producer for over 40 years, mainly of heavy asphaltic oils. Its product comes mainly from the southern part of the State and has grown rapidly in the last 15 years. Even more recent and phenomenal has been the growth of oil pro duction in Texas, Oklahoma and Kansas, or the Mid-Continent field, which now accounts for fully one-half of the entire yield of the country. Other important States in the industry are Louisiana, Illinois, Indiana and Wyoming. The total output in 1918 was 355,927,716 barrels, val ued at $703,943,961.
Iron.— The production of iron ore is carried on in 21 States, while there are unworked de posits in several others. The principal ore is hematite, which is mined most extensively in the Lake Superior district of Minnesota, Wisconsin and Michigan, with Duluth as the chief shipping centre. Alabama has a large mining industry, based on deposits of hematite that occur in the vicinity of Birmingham. New York State con tributes mainly magnetite ores that are found in the Adirondacks and the Highlands of the Hudson. Thepossession of important iron ore supplies and of coking coal• gives a foundation for large iron and steel manufactures, in which the United States ranks pre-eminent. The out put of pig-iron in 1918 was 38,230,440 long tons, valued at $1,180,759,565.
Gold and Silver.-- The precious metals are produced mainly in the West. The opening of the California placer mines in 1849 marked the beginning of large-scale operations which soon gave the country leadership in the industry, cnly reliquished in late years. All of the Rocky Mountain States yield more or less gold and silver, but California, Colorado, Montana, Nevada, Idaho and Utah are the leading pro ducers. South Dakota should be added to the list as having important gold resources. The gold placers which at first supplied most of the metal, are of secondary importance at present; quartz veins and lodes have largely superseded them for mining purposes. The output of gold in 1918 was 3,313,313 fine ounces and of silver 67,879,206 fine ounces.