ROCKS AND MINERALS 55 



various agents as glaciers and rivers, to other locali- 

 ties, the angles of stones in these soils being ground off 

 during transportation. Transported soils are divided 

 into classes according to the ways in which they have 

 been formed ; as, drift soils produced by glaciers, and 

 alluvial soils formed by rivers and deposited by lakes. 

 Other agents which have taken a part in soil 

 transportation are wind and volcanic action. Many 

 soils have been either formed or modified by the wind. 

 The denuding action of heavy wind storms upon many 

 prairie soils is well known. Soil particles are carried 

 long distances and then deposited, forming wind-drifted 

 soils. In some localities volcanic soils are found ; they 

 are extremely varied in texture and composition — some 

 are very fertile and contain liberal amounts of alka- 

 line 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 Rock. — Rocks are composed of 

 either a single mineral or of a combination of minerals. 

 Most of the common minerals have a variable range of 

 composition, due to the fact that one element or com- 

 pound may be partially or entirely replaced by another. 

 Most of the common rocks from which soils have been 

 produced are composed of feldspar, mica, hornblende, 

 and quartz. 



59. Quartz and Feldspar. — Quartz is the principal 

 constituent of many rock formations. Pure quartz is 



