THE OEIGIN OF SOILS 19 



Before proceeding to discuss the properties of soils, 

 therefore, it is necessary to briefly consider the nature 

 of the rocks, the minerals of which the latter chiefly con- 

 sist, and the changes by which alluvium, drift material 

 and sedimentary soils are formed from them. 



MINERALS. 



Minerals are found in a pure state in small crystals or 

 fragments, sometimes in larger lumps or boulders, and 

 occasionally in enormous masses which are then called 

 rocks. Bocks, however, are usually composed of a mixture 

 of minerals, and it is in that form that minerals are most 

 widely distributed. Quartz and silicates of potash, soda, 

 magnesia, lime and alumina are amongst the commonest 

 rock-forming minerals, but oxides, sulphides, fluorides, 

 carbonates, sulphates, phosphates and other compounds are 

 also present in larger or smaller quantities. The following 

 may be taken as typical examples of the different groups. 



Magnetite. Magnetic oxide of iron, Fe 3 04, is a heavy 

 black substance, easily converted by oxidation into the 

 common red oxide, Fe20 3 . It occurs in small grains in 

 volcanic rocks and also massive. 



Haematite. Ferric oxide, Fe20 3 , is very widely dis- 

 tributed. It occurs massive in two varieties, known as 

 the red and brown haematite respectively, and is the com- 

 mon iron ore. It also enters into the composition of rocks, 

 to which it imparts its colour. The red colour of soils is 

 generally attributable to the presence of ferric oxide. This 

 does not necessarily consist of haematite, but may be 

 derived from magnetite and other iron-bearing minerals 

 originally present in the rocks. 



Limonite is a hydrated ferric oxide ; it occurs in many 

 sedimentary rocks and often serves to cement the particles 

 together. It is formed by oxidation of ferrous salts result- 



c 2 



