indefinite wavy line at the 2000-foot contour. At the south the Staked Plains form one of the most level areas of its size in the world. Through Kansas, Nebraska, and Dakota the plains are cut into sections by the streams draining the Rockies. Within this area are special departures from the plains type, such as the Nebraska sand hills, the Bad Lands, the Black Hills, and the volcanic knobs of Montana. Owing to deficient rainfall, agriculture has been only partially successful in this treeless district, but it includes the choicest grazing lands of the United States (fig. 16, i, and § 239). 66. Rocky Mountains. Near the eastern border of the great western highlands are the ranges of the Rocky Mountains, extending entirely across the country and forming the water parting between the Pacific and Atlantic oceans. Some of the valleys and "parks" of this region are centers of agricultural activity, especially of fruit growing, but level stretches of arable land are rare, the rainfall is scanty (fig. 16, h), and the activities of man are chiefly concerned with mineral wealth. 67. Western plateaus. Between the Rockies and the Sierra Nevada Mountains is a region of high plateaus with an area about one fifth of the entire United States. At the north is the Columbia plateau, built up of successive lava flows (§ 49). In the center, occupied by the states of Utah and Nevada, is a great basin of interior drainage, 150,000 square miles in area (§ 91); farther south are the Colorado plateaus, in southwestern Colorado, southern Utah, northern Arizona, and New Mexico, an elevated area made up of a great series of broken blocks deeply trenched by streams. The entire region is arid, and accordingly the population, everywhere sparse (1-6 per square mile), is concentrated on oases and in narrow valleys where, with the aid of irrigation, crops may be produced (fig. 13). Mining, however, is the chief commercial pursuit. The rainfall of the Columbia plateau varies from 10 to 15 inches, the Great Basin has less than 10 inches, and the Colorado plateau has a rainfall varying from 24 inches at Flagstaff to 5 inches in southern Utah. Arizona and New Mexico are rarely visited by cyclonic storms, and what little rain falls comes largely in July and August, or is irregularly distributed (fig. 16, j, k). 68. Pacific slope. Bordering the plateau districts on the west are the high narrow ranges of the Cascades and the Sierra Nevadas. These mountains have a well-watered windward (westward) side, with a consequent growth of timber (§ 229), and an arid eastern exposure. With the exception of those engaged in mining, the population of the the Pacific slope is concerned with the cultivation of two valleys, northern one extending south from Seattle and Vancouver, and the southern one occupying central California. The northern valley has a precipitation of 50 to 60 inches, and rain may be expected any day from October to May. The California valley has less than half as much rain, and the lowlands in the southern part of the state are practically rainless. The precipitation is distributed so as to make two seasons (fig. 16, 1). Wheat, fruit, and wine are the products of this region, the former going largely to Europe by way of Cape Horn, and the fruit going eastward over the highlands to the continental interior and the Atlantic slope. CHAPTER III SOIL 69. Introductory. In order to occupy the earth, man must be supplied with food, and the evident source of his food is plants, or animals, which, in turn, subsist upon plants. It is therefore essential that conditions should be right for the growth of a variety of vegetable forms; or, expressed in another way, it is of great importance to man that plants have learned to support themselves and to increase in number by taking advantage of their physical environment, earth, water, air. Some low forms of vegetation, such as lichens and "rock tripe," are able to support life on bare rock surfaces, but most plants require loose earth in which to spread their roots. Soil, therefore, becomes a fundamental geographic control in determining the extent and character of man's food supply. From it come the principal articles of commerce, and its tillage is the basis of civilization.1 Man may subsist on uncultivated foods such as nuts, fruits, and roots (cf. $233) or on wild animals, but an increase of population and development of civilization imply an improvement of natural species (§ 158) and an intelligent cultivation of the ground. Soil is the covering of the earth, overlying solid rock as a sheet or mantle of waste; it is a surface layer of decomposed rock which varies in depth from a few inches to several hundred feet, depending upon the climate, the character of the rock, and the length of time the soil has been forming. The moon has no soil, for it has no atmosphere, and the solid, fresh rock from the quarry is likewise without a soil mantle. The hilltops of New England have a very thin covering of soil, because most of that which had been forming for ages was carried southward and into the sea by the continental glacier. In Georgia, on the other hand, the soil attains a depth of nearly a hundred feet; in the tropics its depth is even greater; and on some alluvial plains the decomposed rock extends for a depth of over a thousand feet. All of the earth's surface above the sea is subject to soil formation because it is exposed to the atmosphere, whose 1 In France 50 per cent of the population are engaged in agriculture, while statistics show 60 per cent in Spain, 90 per cent in Russia, 95 per cent in India, and 35 per cent in the United States. chemicals are effective in rock decomposition, and also to the mechanical action of wind, water, and ice. 70. Physical processes. Rocks are shattered by sudden changes of heat and cold, by roots which force their way into crevices, and, in greater degree, by water which enters joints and cracks, visible and invisible, and which, upon freezing, breaks the rocks into innumerable fragments, continually reducing their size and, at the same time, increasing the surface exposed to chemical action. Rocks are also eroded by glaciers which, because of their great weight and their slow but irresistible motion, are competent to crush rock fragments, grind bowlders to powder, transport ancient soil and new soil from place to place, and to cut grooves into the solid rock itself. The presence of glacial "rock flour" is responsible for the great fertility of the soil of southern Sweden, and a large part of the north temperate zone owes its soil to glacial agencies. Rivers likewise aid in soil-making. They wear the bottoms and sides of valleys, transport material and deposit it in the form of flood plains, fans, and deltas, making the richest, most easily tilled lands of the world. Winds also, when armed with sand grains and driven against ledges, are effective in reducing rocks to dust and may serve to transport loose material of all sorts, depositing it as dunes (§ 60) or as wide plains of fertile loess (§ 72). 71. Chemical processes. The action of chemicals in rock decomposition is much more widespread and effective than all other forces combined, for the atmosphere and ground water are everywhere at work, producing chemical changes, and no part of the earth's surface is beyond their reach. Water, when it falls as rain, and, later, when it enters the ground or runs off in streams, is charged with certain acids which are able to decompose the firmest rock. The most effective chemicals are carbon dioxide (CO), oxygen from the air, and various acids furnished by certain low forms of life or by decaying vegetation. Armed with these weapons, the water enters the ground through joints and crevices along channels furnished by roots, and even through rock itself; and since no part of a ledge is able to withstand these chemically charged waters, all rocks gradually decay. Working in connection with physical processes, chemical agents tend to loosen the bonds which hold rocks together, to reduce rock masses to smaller and smaller fragments, until, finally, firm rock is converted into the unconsolidated, impure material recognized as soil. 72. Classification of soils. Soils are of two main types: (1) residual, those which have originated in the place where we now find them, and (2) transported, those which have been carried to their present resting place. Residual soils are the result of the decomposition of rock in place by the continued process of weathering, and they contain the minerals of the original rock, with the exception of those which have been removed by acid-laden waters. If the rock underneath is granite, the soil will be decomposed granite; if the bed rock is limestone, the soil will be weathered limestone. Limestone is so readily dissolved by ground water that its residual soil may consist partly or entirely of the impurities in the rock, and its fertility may be accordingly affected. The "red earth" of Bermuda and the red clays of the southern Appalachians are limestone soils of marked. fertility, but in parts of Maryland it is necessary to supply lime to "limestone soils," so complete has been the removal of this easily dissolved substance. A quarry section through residual soil, extending from the surface to the unchanged rock below, shows the following divisions: (1) soil, usually gray or brown, rarely black or yellow, consisting of minute fragments of rock together with broken roots, leaves, etc.; (2) subsoil, much like the soil in texture, but usually different in color and containing less organic matter; (3) a considerable amount of partly decomposed rock, increasing in solidity with the depth; and, finally, (4) the firm rock itself, fresh enough to be used for building. The different parts of the section record the amount of chemical and physical decomposition. Examples of residual soils are the irrigated lava plains. of Idaho and Washington, the Black Prairies of the cotton states, the laterite of India, and the famous "black earth" of Russia, which extends from the valley of the Dnieper to the base of the Urals. Transported soils have been carried from their place of origin and deposited elsewhere, and are therefore much less uniform in texture and in composition than residual soil. They may consist of material collected from a wide area and may be totally unlike the bed rock on which they rest; furthermore, there is no gradual transition from soil to ledge, but rather an abrupt change from loose soil to firm rock. Sometimes the transition is so sudden that rock suitable for building is in immediate contact with soil fit for agriculture. In accordance with their origin, transported soils are divided into three classes, alluvial, glacial, and eolian. Alluvial soils are carried and deposited by streams, and consist, accordingly, of gravels, sands, and the finest silts. They constitute the soil of the flood plains, deltas, and alluvial plains and valleys of the world (§§ 44, 45, 48). They are the most easily cultivated of all types of soil because of the absence of stones and because of their horizontal position, and |