8 SOILS: PROPERTIES AND MANAGEMENT 



plication that may arise, the mineral composition of some 

 common rocks is given below : — 



Granite — Quartz, orthoclase, and plagioclase with mica 

 and hornblende. 



Syenite — Orthoclase and mica with hornblende and 

 augite. 



Basalt — Plagioclase and hornblende or augite with 

 apatite, pyrite, and mica. 



Peridotite — Olivine with augite, pyrite, mica, and horn- 

 blende. 



Limestone — Calcium or magnesium carbonate with traces 

 of silica and iron. 



Sandstone — Sand cemented with iron, silica, or calcium 

 carbonate. 



This complex character of rocks has an important 

 bearing on the question of soil formation, since the presence 

 or absence of certain minerals may have considerable in- 

 fluence on the physical or chemical characteristics of the 

 resultant soil. It is the minerals, therefore, rather than 

 the rocks themselves, that must be looked to in a study 

 of the great mass of inorganic matter, some active and 

 some inactive, which makes up the bulk of ordinary 

 soils. The question of the composition of a soil thus 

 becomes more intensely geologic as we proceed. 



6. Soil-forming minerals. — A great many minerals 

 have been discovered, studied, and classified, but only a 

 comparatively few occur in any abundance in the normal 

 soil. Nevertheless, it may be said that practically all 

 soils contain all the common rock-forming minerals. 

 This is to be expected, as fragments of practically all the 

 common rocks go to make up an ordinary soil. The 



