CHAPTER XXI 

 ROCKS AND SOIL 



In a handful of soil taken from the garden, we have 

 the most useful manufactured product on this earth. It 

 is not a factory product, taking shape before our eyes in 

 a few minutes or hours, but is something that has taken 

 ages to produce. 



Plants cannot grow roots in solid rock nor can they 

 flourish on ledge or cliff. Nature has broken up the solid 

 rock, with air and water and cold, ground it piece by 

 piece to powder, and moved it by flowing stream and 

 glacier. Now it forms the basis of all soil, on which grow 

 great forests and fertile crops the plant life that in the 

 end supports all living things. 



Still, rock does not have to be reduced to powder to 

 serve us. It gives us building materials and the minerals 

 for our modern life and, by blocking streams, enables us 

 to use the energy of falling water. In its seams and folds 

 and fossils is written the history of much that has taken 

 place upon and within the earth itself in past ages. The 

 study of rock and soil carries us back to the beginning 

 of things on this earth. 



Rock is the inorganic material which forms the solid part of 

 the earth. It extends from just under a thin layer of soil to un- 

 known depths. Many of our best geologists believe that it extends 

 to the center of the earth in a solid form. The loose material on 

 the surface of the earth is sometimes called mantle rock or rock 

 waste. This breaks up and so forms the soil in which grows most 

 of our plant life. The soil, however, is not a pure rock waste or 

 powder but is mixed with more or less organic matter which has 

 come from the decay of both animal and plant bodies. 



Classes of Rocks. Rocks are divided into various classes. 

 They are named according to the method of their formation, 

 and are usually classified as igneous, sedimentary and meta- 

 morphic. 



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