32 SOILS: PROPEBTIES AND MANAGEMENT 



residual soil is formed in situ, the rocks that underlie it, 

 if sound, show the character and composition of the rocks 

 from which the soil was actually a product. In such 

 soils the changes that a rock undergoes in forming a 

 residual clay are to be studied to the best advantage. 

 An examination of the various grades of material that are 

 found overlying the country rock (Fig. 3) in an area where 



Fig. 3. — The gradual transition of country rock into residual soil by 



weathering in situ. 



this residual mantle exists, reveals more or less accurately 

 the gradations from rock to soil. Residual soils, besides 

 being old soils, are usually nonstratified and present a 

 heterogeneous mass of material. Since they have been 

 subjected to leaching over vast periods, a very large 

 amount of the soluble materials have been washed out, 

 tending to leave high percentages of the persistent ma- 

 terials, such as silica, iron, and aluminium. An analysis 

 of an Arkansas limestone, its residual clay and the cal- 

 culated percentage loss of the various constituents pres- 

 ent in the fresh rock, illustrates this point : — 



