THE FOREST AS A SOIL-FIXER 57 
covered with forests of pine. When the wind is light 
the drift of sand is unnoticeable, but during times of 
gale it becomes a biting sand-blast. In desert regions 
there is nothing to interrupt the force of the wind, 
and nothing to hold the soil in place. The sand is 
shifted here and there with every caprice of the wind, 
often in the form of dangerous sand-storms. 
Geologists reckon that the erosive action of water 
has been and still is one of the most potent of all 
agencies in modifying the nature of the earth’s sur- 
face. In mountainous districts of many parts of the 
world huge gorges and gulches have been cut by the 
rush of water, and soil and débris in huge quantities 
have been dumped into the valleys. By maintaining 
the proper kind of forest cover, the soil, even on steep 
slopes, will remain in place. Roots grasp and hold 
the particles of soil, so that the whole mountainside is 
knitted together. The forest is the agent by means 
of which mankind may harness and control these un- 
ruly forces. Not only erosion but, in several parts 
of the world, owing to forest destruction, great land- 
slides have occurred, which have buried villages. 
Rocks, earth, and snow are held in place by a forest 
cover, and landslides and avalanches are thus pre- 
vented. In the Alps and Pyrenees, with the expen- 
diture of much labor, the loose rocks are cemented 
to the mountain-side, and the earth is held in place by 
