THE UNITED STATES, the short title usually given to the great federal republic which had its origin in the revolt of the British colonies in North America, when, in the Declaration of Independence, they described themselves as " The Thirteen United States of America." Officially the name is " The United States of America," but " The United States " (used as a singular and not a plural) has become accepted as the name of the country; and pre-eminent usage has now made its citizens " Americans," • in distinction from the other inhabitants of North and South America. The area of the United States, as here considered, exclusive of Alaska and outlying possessions, occupies a belt nearly twenty degrees of middle latitude in width, and crosses Boundaries North America from the Atlantic to the Pacific. sad Area. The southern boundary is naturally defined on the east by the Gulf of Mexico; its western extension crosses obliquely over the western highlands, along an irregular line determined by aggressive Americans of Anglo-Saxon stock against Americans of Spanish stock. The northern boundary, after an arbitrary beginning, finds a natural extension along the Great Lakes, and thence continues along the 49th parallel of north latitude to the Pacific (see Bulletin 171, U.S. Geological Survey). The area thus included is 3,026,789 sq. m.1 I.—PHYSICAL GEOGRAPHY Coast.—The Atlantic coast of the United States is, with minor exceptions, low; the Pacific coast is, with as few exceptions, 1 The following are the states of the Union (recognized abbreviations being given in brackets): Alabama (Ala.), Arizona (Ariz.), Arkansas (Ark.), California (Cal.), Colorado (Col.), Connecticut (Conn.), Delaware (Del.), Florida (Fla.), Georgia (Ga.), Idaho, Illinois (Ill.), Indiana (Ind.), Iowa (Ia.), Kansas (Kan.), Kentucky (Ky.), Louisiana (La.), Maine (Me.), Maryland (Md.), Massachusetts (Mass.), Michigan (Mich.), Minnesota (Minn.), Mississippi (Miss.), Missouri (Mo.), Montana (Mont.), Nebraska (Neb.), Nevada (Nev.), New Hampshire (N.H.), New Jersey (N.J.), New Mexico (N. Mex.), New York (N.Y.), North Carolina (N.C,), North Dakota (N. Dak.), Ohio (O.), Oklahoma (Okla.), Oregon (Oreg.), Pennsylvania (Pa.), Rhode Island (R.I.), South Carolina (S.C.), South Dakota (S. Dak.), Tennessee (Tenn.), Texas (Tex.), Utah, Vermont (Vt.), Virginia (Va.), West Virginia (W.Va.), Washington (Wash.), Wisconsin (Wis.), Wyoming (Wyo.) ; together with the District of Columbia (D.C.).hilly or mountainous. The Atlantic coast owes its oblique N.E.–S.W. trend to crustal deformations which in very early geological time gave a beginning to what later came to be the Appalachian mountain system; but this system had its climax of deformation so long ago (probably in Permian time) that it has since then been very generally reduced to moderate or low relief, and owes its present altitude either to renewed elevations along the earlier lines or to the survival of the most resistant rocks as residual mountains. The oblique trend of the coast would be even more pronounced but for a comparatively modern crustal movement, causing a depression in the north-east, with a resulting encroachment of the sea upon the land, and an elevation in the south-west, with a resulting advance of the land upon the sea. The Pacific coast has been defined chiefly by relatively recent crustal deformations, and hence still preserves a greater relief than that of the Atlantic. The minor features of each coast will be mentioned in connexion with the land districts of which the coast-line is only the border. General Topography and Drainage.—The low Atlantic coast and the hilly or mountainous Pacific coast foreshadow the leading features in the distribution of mountains within the United States. The Appalachian system, originally forest-covered, on the eastern side of the continent, is relatively low and narrow; it is bordered on the south-east and south by an important coastal plain. The Cordilleran system on the western side of the continent is lofty, broad and complicated, with heavy forests near the north-west coast, but elsewhere with trees only on the higher ranges below the Alpine region, and with treeless or desert intermont valleys, plateaus and basins, very arid in the south-west. Between the two mountain systems extends a great central area of plains, stretching from the Gulf of Mexico northward, far beyond the national boundary, to the Arctic Ocean. The rivers that drain the Atlantic slope of the Appalachians are comparatively short; those that drain the Pacific slope include only two, the Columbia and the Colorado, which rise far inland, near the easternmost members of the Cordilleran system, and flow through plateaus and intermont basins to the ocean. The central plains are divided by a hardly perceptible height of land into a Canadian and a United States portion; from the latter the great Mississippi system discharges southward to the Gulf of Mexico. The upper Mississippi and some of the Ohio basin is the prairie region, with trees originally only along the watercourses; the uplands towards the Appalachians were included in the great eastern forested area; the western part of the plains has so dry a climate that its herbage is scanty, and in the south it is barren. The lacustrine system of the St Lawrence flows eastward from a relatively narrow drainage area. Relation of General Topography to Settlement.—The aboriginal occupants of the greater part of North America were comparatively few in number, and except in Mexico were not advanced beyond the savage state, The geological processes that placed a much narrower ocean between North America and western Europe than between North America and eastern Asia secured to the New World the good fortune of being colonized by the leading peoples of the occidental Old World, instead of by the less developed races of the Orient. The transoceanic invasion progressed slowly through the 17th and 18th centuries, delayed by the head winds of a rough ocean which was crossed only in slow sailing vessels, and by the rough " backwoods " of the Appalachians, which retarded the penetration of wagon roads and canals into the interior. The invasion was wonderfully accelerated through the 19th century, when the vast area of the treeless prairies beyond the Appalachians was offered to the settler, and when steam transportation on sea and land replaced sailing vessels and wagons. The frontier was then swiftly carried across the eastern half of the central plains, but found a second delay in its advance occasioned by the dry climate of the western plains. It was chiefly the mineral wealth of the Cordilleran region, first developed on the far Pacific slope, and later in many parts of the inner mountain ranges, that urged pioneers across the dry plains into the apparently inhospitable mountain region; there the adventurous new-corners rapidly worked out one mining district after another, exhausting and abandoning the smaller " camps " to early decay and rushing in feverish excitement to new-found river fields, but establishing important centres of varied industries in the more important mining districts. It was not until the settlers learned to adapt themselves to the methods of wide-range cattle raising and of farming by irrigation that the greater value of the far western interior was recognized as a permanent home for an agricultural population. The purchase of " Louisiana "—a great area west of the Mississippi river —from the French in 1803 has sometimes been said to be the cause of the westward expansion of the United States, but the Louisiana purchase has been better interpreted as the occasion for the expansion rather than its cause; for, as Lewis Evans of Philadelphia long ago recognized (1949), whoever gained possession of the Ohio Valley—the chief eastern part of the central plains—would inevitably become the masters of the continent. Physiographic Subdivisions.—The area of the United States may be roughly divided into the Appalachian belt, the Cordilleras and the central plains, as already indicated. These large divisions need physiographic subdivision, which will now be made, following the guide of " structure, process and stage "; that is, each subdivision or province will be defined as part of the earth's cruet in which some similarity of geological structure prevails, and upon which some process or processes of surface sculpture have worked long enough to reach a certain stage in the cycle of physiographic development. The Appalachians.—The physiographic description of the Appalachian mountain system offers an especially good opportunity for the application of the genetic method based on " structure, process and stage." This mountain system consists essentially of two belts: one on the south-east, chiefly of ancient and greatly deformed crystalline rocks, the other on the north-west, a heavy series of folded Palaeozoic strata; and with these it will be convenient to associate a third belt, farther north-west, consisting of the same Palaeozoic strata lying essentially horizontal and constituting the Appalachian plateau. The crystalline belt represents, at least in part, the ancient highlands from whose ruins the sandstones, shales and limestones of the stratified series were formed, partly as narine, partly as fluviatile deposits.