54 OUTLINES OF FORESTRY. 



or pebbles. These act as planes, cutters, or chisels 

 that gnaw, cut, or wear away even the hardest 

 rock. This process is technically called erosion, 

 and is of great aid in the formation of soil. 



There collects on the sides of mountains above 

 the limit of perpetual snow an immense accumula- 

 tion of snow, which, through gradual pressure, is 

 converted into hard ice, and forms masses called 

 glaciers. The glaciers slowly move or slip down 

 the sides of the mountain. They receive the 

 drainage of snow from the slopes of the valleys 

 through which they move, just as rivers receive the 

 drainage of liquid water. The glaciers carry with 

 them considerable mineral matter, both in the 

 shape of small rocks and large boulders. As the 

 mass moves down the mountains, this mineral 

 matter is pressed against the sides of the val- 

 leys, or along the bottom of the bed through 

 which it is moving, and cuts, grooves, or grinds 

 the hard rocks, and thus aids in the production 

 of soil. 



From its great solvent power, water is able to 

 finally sink into what were originally impervious 

 rocks, by gradually dissolving out the soluble 

 portions of the rocks. In this way the rock is ren- 

 dered rotten by the removal of the materials which 



