. Conservation. Forests and forestry. WATER CIRCULATION AND ITS CONTROL 345 banks of sward or above the stone wall are carefully leveled and the water which falls upon them is retained. Agriculture is thus extended not only over far-spreading plains and lowlands, Init up the steep banks of river gorges and ravines. The famous vineyards of the Rhine are planted on rock wall ter- races, on steep slopes such as we aban- don to erosion. As our population in- creases and the demand for agricultural land becomes more urgent we, too, will build terraces, and high river banks throughout the hnmid regi
. Conservation. Forests and forestry. WATER CIRCULATION AND ITS CONTROL 345 banks of sward or above the stone wall are carefully leveled and the water which falls upon them is retained. Agriculture is thus extended not only over far-spreading plains and lowlands, Init up the steep banks of river gorges and ravines. The famous vineyards of the Rhine are planted on rock wall ter- races, on steep slopes such as we aban- don to erosion. As our population in- creases and the demand for agricultural land becomes more urgent we, too, will build terraces, and high river banks throughout the hnmid regions will grow special products. Land hunger will eventually force us to it—not soon, however—and in the meantime our steep, cleared lands cannot be left a prey to erosion. They must be recov- ered where gullied and must be pro- tected from gullying by watchful care, such as that which guards Holland from the sea. Our enemy is no less dangerous, not a whit less insidious and persistent. Since terracing is a possi- bility of the future only, we must re- store the forest trees. They have held the soil during ages and on them we can confidently rely. We are wont to think that reforesta- tion is a need of high mountains only, but the need is not thus limited. Tt ex- tends to all steep cleared slopes. Thou- sands of miles of these stretch along our rivers, from Georgia to Maine, from Mississippi to Minnesota. Throughout the humid region they bor- der every river. Along all their length, in all their ramifications that penetrate the richest agricultural districts, they must necessarily be covered by trees. From the plains and gentle slopes ap- propriate to farming there rise in all hill and mountain districts steeper slopes that are not api)ropriate to farm- ing. When farmed thev are quickly erode 1 and destroyed. It may be set down as an obvious economic principle that no land is appropriate to a use which destroys its usefulness. This principle is net affected by the fact that i
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