Abstract

                                It is a commonplace of geology that the surface of the earth is constantly in motion; that our mountain ranges have taken shape and our river courses and valleys have been excavated by rain, frost and wind. In many parts of the world this movement of the soil surface is taking place rapidly, with serious consequen- ces to the agriculture of those countries, owing to soil erosion. 1 The soil is dependent for its stability on its normal covering of vegetation, but as a rule the soil proper is not more than 6 inches to a foot deep, and it is in the surface layer that the fertility resides. There is thus always a danger that if the cover of vegetation is removed, the soil itself may shift under the action of either wind or rain. The earliest cases of erosion caused by such movement of the soil are those which follow deforestation in regions where the mountains in which the rivers take their rise are below the permanent snow line-e.g., in the eastern Mediterranean. There is no evidence in support of the belief that forests increase the rainfall of a country; indeed, by the transpiration from their leaves, they must reduce the total amount of the rainfall retained by the soil, but they serve as its regulator; as the rain falls, it is absorbed by the spongy soil below the trees, rich in humus, and reappears later in the springs and rivers when the rainy season is over. Too commonly, however, the forests have been cut down without regard to their regeneration, both for the value of their timber and for u desire to extend the grazing land.

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