deposits material instead of cutting its bed. The hills of maturity have melted away to gentler slopes, and increasingly finer waste is being supplied.

59. Valleys and man. Valleys are natural lines of communication by boat, by roads along the banks, and by canals and railroads paralleling the streams. This is particularly the case in plateaus and mountains, where, in order to reach the passes, valleys must be followed up. From the standpoint of man valleys in different stages of development have various advantages and disadvantages. A plain cut by new valleys with wide interstream spaces affords the largest acreage of arable land. In old age, where valley bottoms and gentle slopes abound, the amount of agricultural land is again increased, while in early maturity, because of steep slopes and easily washed soil, agricultural conditions are most unfavorable. Cities in such locations are limited to a narrow belt of land between the stream and the base of the hills, as at Ems, Prussia, and Charleston, West Virginia. For land routes the canyon bottoms of young streams are not feasible, but in a mature state the topography forces roads to follow the valleys or the crests of ridges. For water navigation young rivers are unavailable because of their rapids and falls, and streams that have reached old age are subject to blocking by silt. Water power is at its best on young rivers, for while older and larger streams may possess greater energy, they are less easily controlled, and may be set aside for use in navigation. Floods are more abundant on mature land surfaces and more disastrous where wide-bottomed valleys have been developed by old-age streams.

An illustration of the relation between highland and valley is presented in the history of Connecticut, where the uplands were settled at an early date and are still occupied by decaying villages. The growth of population has, however, been in the valleys, where manufacturing cities have sprung up rapidly and drawn the men from the hill towns. At one time the residents of Waterbury climbed the hills to Wolcott to purchase supplies. Waterbury is now the chief brassmanufacturing city of the world, and Wolcott has scarcely enough population to support a post office. The concentration of population in valleys is shown by an examination of maps and census reports. Eighteen of the twenty-one principal cities and towns in Indiana are founded on alluvial land, and half of the population of Iowa is confined to alluvial plains. The same condition holds true in Missouri and Ohio, while two thirds of the cities and towns of Pennsylvania are located on river banks. Brigham calls attention to the fact that,

of 41 communities of the rank of city in New York, 11 are on the Hudson and 6 on the Mohawk, and that four fifths of the population and nine tenths of the wealth of that state are contained in the counties bordering the Hudson River and Erie Canal. From statistics of the eleventh census, from which these facts were taken, McGee estimated that "fully 25 per cent or 30 per cent of the population of the eastern United States is crowded upon the 14 per cent of alluvial lowland.1

SAND DUNES

60. Wind-formed hills of any shape whatever are termed dunes. They rarely occur singly, but usually in groups, forming a small collection of ridges, mounds, or hummocks, or an undulating surface covering hundreds of square miles. They may lie in long parallel ridges like the waves of the sea, or be arranged in irregular order, and they vary in height from a few inches to over 400 feet. The most favorable conditions for their formation are where unobstructed winds blow over a plain covered by fine material and unprotected by plant covering; accordingly they are characteristic features of shore lines and of arid regions. Dunes travel in the direction of prevailing winds and form and re-form as time goes on, and it is to the wind that they owe their present position and their details of structure. The sweep of wind across the water is unobstructed, and in the construction of beaches a great amount of finely ground material is furnished in a form suitable for aërial transportation. The sands of the beaches are driven inland, and in their march landward they may completely obliterate previous topography, burying the original soil and destroying forests and the works of man. On the west coast of France a belt of dunes 50 to 80 feet high and 3 miles in width is advancing inland at a rate of 30 to 60 feet per year. Streams tributary to the sea are blocked and forced to run parallel with the coast line, forming large inland lakes. For a distance of 100 miles, between the mouth of the Gironde and the Mimizan, there are only two places where the water has an outlet to the ocean. The sand dunes on this coast extend for 150 miles, from Biarritz to Point de Grave, and include 500 square miles of desert. In Holland the shifting dunes of the coast, which range from 40 to 50 feet in height, form a belt 1 to 3 miles wide, which stands as a wall to protect the Netherlands from the sea. The entire island group of Bermuda consists of shell sand, which has been molded into hills by the wind.

1 The Forum, April, 1891, p. 225.

2 Wheeler, The Sea Coast, p. 54.

The migration of dunes may result in completely destroying forests and buildings, passing entirely over them and leaving the ruins behind. At Dune Park, Indiana, the shifting sand hills bordering Lake Michigan are now exposing dead trees which were overwhelmed, buried, and again resurrected. On the coast of Prussia a pine forest of several hundred acres was completely destroyed between 1804 and 1827, and a church on the Kurische Nehrung was completely buried in drifting sand in 1839, to be revealed again only after a lapse of thirty years. At certain points in New Jersey, within the lifetime of the present inhabitants, orchards have been buried so completely that only the tops of trees are now exposed.

It is important to man that the shifting sands, represented in dunes, should be controlled, and a large amount of money has been spent in devising means to this end. Forests and certain selected types of grass have been planted in the sand, rendering it more difficult for the wind to take hold. On the coast of the Bay of Biscay over 200,000 acres of sand dunes have been planted with pine. Near the mouth of the Vistula, Prussia has planted 14,000 acres in forests, and parts of the sand hills of Nebraska have been treated in a similar way. On low-lying coasts dunes act as a wall to ward off the attack of the sea, and in such regions effort is made to keep them in position and to increase their height and length by means of artificial works.

PHYSIOGRAPHY OF THE UNITED STATES1

61. The United States as a whole. The United States as a whole, with the exception of Florida and southern Texas, is included between parallels 30° and 50° of north latitude, a position which accounts for its climate and weather, the character of its products, and, in a general way, for the commercial activities of its population. Topographically, it consists of a great central lowland area bordered on the east by narrow highlands and on the west by plateaus and mountains which make up one of the largest and most diversified highland regions of the world. In fact, the western uplands are so prominent and occupy such a large area that the United States might be viewed as divided into two parts by the 100th meridian, the eastern part being a lowland plain with slight elevations,-the Appalachian Mountains rising above it, and the western part occupying nearly half of the area of

1 This treatment of the physiographic provinces of the United States is made very general, since the purpose is to bring out clearly the broad geographic controls in American commerce.

the country, standing at an elevation of 4000 to 6000 feet, with large areas over 10,000 feet, and about 50 peaks, including volcanoes, reaching an elevation of 14,000 feet. The shore line which bounds the country has on the Pacific and the northern Atlantic border the irregularity characteristic of depressed coasts, while a typical coastal plain stretches from New Jersey to the Mexican border.

The climate of the United States is characteristic of the north temperate zone, being controlled largely by the prevailing westerlies with their cyclonic storms of great variation in speed and direction. The character of these winds, together with latitude and topography, determines the amount of heat received during the year by the different parts of the country. They are also responsible for the fact that, with the exception of California, the United States has four seasons, based on temperature. The warm states, that is, those in which the mean annual temperature is 60° or above, include South Carolina, Georgia, Florida, Alabama, Mississippi, Louisiana, Texas, northern New Mexico, Arizona, and a large part of California. The temperate states (mean annual temperature between 50° and 60°) include all of the middle Atlantic section, the Ohio valley, Tennessee, Missouri, Kansas, Oklahoma, the southern parts of Illinois and Indiana, and southeast Colorado. The cold states (mean annual temperature between 40° and 50°) consist of New England, the region bordering the Great Lakes, Minnesota, South Dakota, Nebraska, Iowa, and all the states of the western-highland area. The lowest actual temperature has been recorded at Miles City, Montana, -65° (January, 1888), and the highest, 130°, at Mammoth Tank, California (August 17, 1885). North Dakota has an absolute range of 150°, Key West of 59°. With the exception of the higher mountain tops, no part of the United States is permanently too cold for vegetation; on the other hand, there is no state which has not been visited by frost.

So far as agriculture is concerned, it is important to know the time of the first and last killing frosts of the year, for this gives the length of the growing season; and since every plant species requires a certain sum total of heat in order to complete its growth, the length of the season will determine the type of product for each section. A study of the map1 (fig. 14) will show that the length of the growing season varies considerably in different parts of the country. Thus the last frost to be expected at Phoenix, Arizona, is February 23, and the first frost of the fall is on December 3, making a growing season of almost

1 This map, as well as other climatic data used in this chapter, is based upon Climatology of the United States, issued by the Weather Bureau.