THE CONTENTS OF THE SOIL 17 



2. How Soil Is Made 

 2a. The inorganic dements 



25. The basis of soil is fragments of rock. 

 To this base is added the remains of plants and 

 animals (or organic matter). When in condition 

 to grow plants, it also contains water. The 

 character of any soil, therefore, is primarily 

 determined by the kind of rock from which it 

 has come, and the amount of organic matter 

 and water which it contains. 



26. As the surface of the earth cooled, it 

 became rock -bound. Wrinkles and ridges ap- 

 peared, forming mountains and valleys. The 

 tendency is for the elevations to be lessened and 

 the depressions to be filled. That is, the surface 

 of the earth is being leveled. The chief agency 

 in this leveling process is weathering. The hills 

 and mountains are worn down by alternations of 

 temperature, by frost, ice, snow, rain and wind. 

 They are worn away by the loss of small par- 

 ticles : these particles, when gathered on the 

 hillsides or deposited on lower levels, form soil. 



27. The weathering agencies which reduce 

 the mountains operate also on level areas ; 

 but since the soil then remains where it is 

 formed, and thereby affords a protection to 

 the underlying rock, the reduction of the rock 



