14 ENGINEERING FOR LAND DRAINAGE. 



climatic changes, and the result is a belt of red clay soil 

 on a bench of the hillside below. In such cases all 

 stages of soil formation may be seen from the rock-like 

 shale to the plastic clay. The lava-covered slopes of 

 volcanic mountains become covered v^ith soil in the 

 same manner. Time and the changes incident to a 

 humid climate will convert lava, at one time a seething 

 mass, into a fruitful soil. 



One peculiarity of these natural changes is that the 

 rocks by being brought to such a state of comminution 

 lose their identity. The minute particles of different 

 rocks become so thoroughly blended that their original 

 condition can only be inferred by fragments of un- 

 changed rock which may chance to be found in the 

 mixture. The hundreds of forms known to the geol- 

 ogist and mineralogist under distinctive names, each 

 one of which when chemically analyzed is found to 

 contain from six to twelve elements, will never be 

 known after being changed to soil. Hence it is only 

 the general processes which relate to the origin of soils 

 that are of particular interest in the discussion of this 

 subject. The composition of soils can be known only 

 through chemical analyses, and, since the same element 

 is found in a variety of rocks, it is impossible to deter- 

 mine from such analyses what the original rocks were. 

 A physical analysis will determine the comparative 

 fineness of the particles constituting the soil, a matter 

 of much importance in its treatment. 



Sedentary Soils are those which remain where they 

 are formed and constitute a covering for the rock from 

 which they originated. They have usually little depth 

 and comprise but a small part of useful soils* 



