CHAPTER IV 



HOW ROCKS AND CLIMATE AEFECT SOILS 



Among the factors which determine soil types was mentioned 

 '^kind of material" which refers largely to the kind of rock from 

 which the mineral particles came. Any traveler interested in 

 soils can easily observe the marked effects that rocks and climate 

 have upon soils in different sections of the country. In this chapter 

 we shall consider some of these effects both from a physical and 

 chemical point of view in order that we may understand certain 

 facts and soil conditions that may come within our observation, 

 experience, or reading. 



ROCKS IN THEIR RELATION TO SOILS 



Kinds of Rocks and Their Changes. — ^There are many kinds of 

 rocks, all of which may be divided into three main groups, according 

 to the manner in which they were formed. 



Igneous rocks were the first to form on the earth. They are 

 also formed by the solidification of molten material from within 

 the earth. Lava rocks formed through volcanic eruptions are, 

 therefore, igneous rocks. Other igneous rocks are granite, basalt, 

 and syenite. 



Sedimentary rocks are bedded rocks formed from sediments 

 such as sand, shells, mud, etc., deposited in sheets through the 

 action of water and wind — ^mostly through water action. Through 

 pressure and cementation these sediments gradually change into 

 rocks; such as sandstone, limestone and shale. Most sedimentary 

 rocks have been formed under sea water. All materials forming 

 these rocks came originally from igneous rocks. Sedimentary and 

 igneous rocks may change into metamorphic rocks. 



Metamorphic rocks are so called because they are rocks which, 

 through long periods of time, have changed their structure as a 

 result of great pressure, heat, and water solutions. Slate, marble, 

 quartzite, and schist are examples. A limestone may change into 

 a marble, which, under proper conditions may further change its 

 structure and become a schist. Thus a sedimentary rock may 

 change into a metamorphic rock, and, in turn, may undergo a 

 further change sufficient to be designated as another kind of meta- 

 morphic rock; namely, a schist. 



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