THE OEIGIN OF SOILS 



Limestone. Limestones are chiefly of organic origin, as 

 has been previously pointed out. They consist essentially 

 of carbonate of lime, and, in many cases, are nearly pure. 

 On the other hand, some of the deposits are associated with 

 a large amount of impurity consisting of amorphous silica, 

 sand, clay or ferruginous matter. These are called siliceous 

 limestones, sandy limestones, argillaceous limestones, and 

 so on. The purer varieties are usually white or greyish in 

 colour, crystalline and hard enough to be ultilised as build- 

 ing stones. Chalk, which is of the same chemical nature, 

 is also white, but is dull, soft and friable. The oolitic 

 limestone is of different origin and structure. It consists 

 of rounded grains of carbonate of lime embedded in a more 

 or less calcareous matrix. 



Dolomite, which Consists of the carbonates of calcium 

 and magnesium, is regarded as a mineral rather than a 

 rock, but it occurs in large rock-like masses and is known 

 as magnesian limestone. The following table shows the 

 average composition of some of the well-known varieties 

 of limestone: 



AVERAGE CHEMICAL COMPOSITION OF LIMESTONES. 



Peat. Peat differs widely from all other rocks and soil- 

 forming materials both in nature and properties. It has 



