8 SOIL PHYSICS AND MANAGEMENT 



animals. It is quite impossible to draw any distinct lines between 

 the groups. 



(a) Chemical Precipitates. Rocks formed in this way are not 

 of a great deal of importance as soil formers, but have great eco- 

 nomic value. These include the precious stones, the ores both of 

 useful and precious metals and deposits of plant food, especially 

 potassium and phosphorus. 



(b) Sedimentary or Fragmental. This division includes 

 sandstones and shales. Sandstones may be divided into classes ac- 

 cording to the material that cements the particles together, as 

 siliceous, ferruginous, or calcareous. Siliceous sandstones break 

 down largely through physical or mechanical agencies, forming a 

 sandy soil of unusually low agricultural value. A good example of 

 this is the St. Peter's sandstone of northern Illinois. Ferruginous 

 sandstones are broken down in a way similar to the siliceous, except 

 that chemical agents are more apt to affect the cementing material. 

 The resulting soil is a sand, colored by compounds of iron and does 

 not possess a high degree of fertility. In the breaking down of 

 calcareous sandstones, the lime is dissolved out by the action of 

 carbonated water, thus freeing the particles, and forming a rather 

 poor sandy soil. The decomposition of felspathic sandstones may 

 give rise to soils of fair fertility because- of the potassium and lime 

 present, but on the other hand micaceous sandstones produce soils 

 of low value. 



Shales vary largely in physical composition. Some are composed 

 of clay while others contain much coarser material, such as silt or 

 even sand. The indurated character of shales is principally due to 

 pressure and they are consequently easily broken down into soil 

 material. The stratification also aids this process. The soils formed 

 from shales vary from very heavy clay to silty or sandy ones, and 

 may be extremiely difficult to work. In general shale soils are not 

 of high agricultural value. 



(c) Organic. This includes those deposits that have heen 

 formed through the agency of organisms. They consist of coal, 

 chalk, marl, and limestone. 



Calcareous rocks include chalk, marl, and the various limestones 

 (Fig. 1.) Soils are formed from these through the solvent power of 

 carbonated water which removes the lime and magnesia as the 

 bicarbonate, leaving the insoluble impurities as soil material. This 

 may consist of particles of sand or quartz or some of the finest soil 

 constituents as silt or clay (Fig. 2). Limestones frequently con- 



