COMPOSITION OF ROCK 51 



duced in one locality and transported to another. 

 Soils are either sedentary or transported. Sedentary 

 soils are those which occupy the original position 

 where they were formed. They usually have but 

 little depth before rock surface is reached. The stones 

 in such soils, except where modified by weathering, 

 have sharp angles because they have not been ground 

 by transportation. In the southern part of the United 

 States, east of the Mississippi River, there are large 

 areas of sedentary soils as ferrogenous clays in an ad- 

 vanced state of decay. 



Transported soils are those which have been formed 

 in one locality and carried by various agents as glaciers, 

 rivers and winds to other localities, the angles of stones 

 in these soils having been ground off during transpor- 

 tation. Transported soils are divided into classes ac- 

 cording to the ways in which they have been formed; 

 as, drift soils produced by glaciers, alluvial soils formed 

 by rivers and deposited by lakes, aeolian soils formed 

 by winds, and colluvial soils formed of rocks and 

 debris from mountain sides. 



In some localities volcanic soils are found. They 

 are extremely varied in texture and composition ; 

 some are very fertile and contain liberal amounts of 

 alkaline salts and phosphates, while others contain so 

 little plant food that they are sterile. 



ROCKS AND MINERALS FROM WHICH SOILS ARE FORMED 



58, Composition of Rocks. Rocks are composed of 

 either a single mineral or of a combination of minerals. 

 Most of the common minerals are definite chemical 



