firm sod results. If there isn't a firm sod, the water that runs off will cut the terrace and wash parts of it away. The sod prevents this by holding the ground and keeping a smooth, regular surface. How does the grass hold the dirt? Great forests act in the same way on a hillside. Conservation of the forests is thus very important. The great

(Courtesy of A. M. Lythgoe, Metropolitan Museum of Art, New York City.)

The tiny particles of desert sand, blown by the winds of centuries, wear away the hardest rock.

floods of China are said to be partly due to the cutting away of her forests. Why?

151. How Does the Air Aid Weathering? -- Even the invisible air helps to destroy rocks, for it furnishes oxygen which unites with rock substances to soften and break them up. What happens to a piece of iron if it is left out of doors for some time? We say it rusts; that is,

it unites with oxygen from the air and forms iron oxide, which is the red powder, rust. In the same way other hard, firm substances are changed to fine particles after uniting with oxygen. The carbon dioxide in the air unites with water and forms an acid (carbonic acid); this breaks up certain rocks.

Air in motion, as wind, assists in erosion, especially in dry climates and desert regions. Great windstorms carry fine, loose particles of sand and throw them violently against any object, rock, or plant which the wind passes over. Huge mounds of sand are moved by the wind, and each grain hitting a rock surface chips it a little. The great stone monuments which were built in Egypt thousands of years ago show well the power of the wind and sand to break off a great deal of stone if given time enough (Fig. 94).

152. How Do Water and Ice Cause Weathering? The water of streams, lakes, and oceans aids in the weathering of rocks in ways which we have already studied. Frost and ice are powerful agents in breaking up rocks. If you leave water in your pipes on a cold winter's night, and do not have a fire in the house, the next morning you may find that one of the pipes has frozen and burst. Why does it burst?

Perform this experiment: Fill an old medicine bottle entirely full of water and stopper it tightly. Leave it outside on a freezing night. The next morning you will probably find that the stopper has either been forced out, or the bottle is broken. Water expands when it freezes (cf. § 124). In the same way water seeps into cracks and joints in the rocks; there it freezes, expands, and forces even great rocks apart. After a cold winter in which there has been a great deal of freezing weather, chips of rock and even large slabs are found at the foot of cliffs. These are forced off by the ice and frost.

Great sheets of ice assisted erosion in past ages, and ice streams (glaciers) are at work in the mountains now. An immense ice sheet once covered the Great Lakes region of the United States. Huge deposits of mud and rock (Fig. 95) were left after the melting of the ice. With the

FIG. 95.

Rocks brought by the great ice sheets and left when the ice melted. rocks frozen in its under side, the ice sheet was able to scour and scrape out lakes and to grind surface rocks to powder.

153. How Is Our Soil Formed? Did you ever wonder how "hard heads," or granite bowlders, came to our fields, when the bed rock below may not be granite at all? Is it possible that the soil of our gardens was

not always where we find it, but was brought from other regions? There are two ways in which soil is formed on any particular spot. First, we have the soil which was formed from the rocks beneath it by the weathering of those rocks. It is composed of the same substance as the rocks below. The only difference we find in the character of such soil as we dig down is that the particles become larger all the time, until we strike bed rock.

The other class of soil is that which is brought, or deposited there, from a distance. Glacial soil belongs to this class; it is that brought by glaciers and spread over immense areas; these may have a totally different sort of bed rock. Another soil is formed on the slopes of hillsides or mountains. The action of gravity and of the rain causes great masses of it to move slowly down into the valleys. Alluvial soil is formed by the deposits of lakes and streams. A fourth kind of soil is fine and loose and deposited by the wind. It is found chiefly in desert regions. In general, the transported soils are made up of particles weathered from rocks of many kinds and are a richer combination for the growing of crops than the soils which were formed where we find them. Why?

Which has the smallest also decayed vegetable particle is surrounded by Water is also held be

154. What is the Structure of Soil? We can see, as we look at it carefully, that soil is made up of particles of different sizes. We call them, according to size, gravel, sand, silt, and clay. particles? Usually there is matter, or humus. Each tiny a film, or covering, of water. tween the particles by capillary action (cf. § 108), so that the plant can draw moisture from deep down in the ground, even in dry weather. The water holds in solution those substances which are needed by the plant and

which must be taken from the ground. Thus water is absolutely necessary to the life of the plant. Plants die if their leaves are not in the air, so that they can breathe; there must be air in the soil, too, so that the roots also can breathe. A soil that is very solid and airless chokes the roots so that the health of the plant suffers.

155. Why Must Soil Be Tilled? — Did you ever notice how much labor a farmer or gardener spends in stirring up, or tilling, the soil in which he plants his seeds? Soil is tilled so as to improve its structure, in order that it may hold air and water better and may permit the roots to gather the plant's food from larger amounts of ground. Tillage also turns under rubbish, manure, and other fertilizers, so that they become a part of the soil and enrich it.

In ancient times men had very few tools, or implements, to help them in tilling the soil, but the modern farmer has many of them. A plow is really a slanting knife, or shovel, that is forced into the ground, so that it turns a slice of soil over. Cultivating is shallow plowing; it stirs up the top layer of the ground, so that it is in a powdery condition. Disk plows have a revolving cutting instrument called a disk, instead of a knife plowshare. A gang plow consists of many plowshares or disks attached to one frame. It is drawn by many horses or by an engine, or tractor (Fig. 96).

156. What is Irrigation? If you go to the Far West you will be shown great mountains, cañons, and cataracts as its wonders; but these are not the only great wonders of the west. As great a marvel as any is the transformation of land once a desert waste into fertile orchards and farms. How has this apparent miracle come about?