THE ORIGIN OF SOILS 



33 



sand, calcareous sand and marly sand, all of which are 

 valuable for the lime they contain. 



Sandstones are formed of particles of sand cemented 

 together. The matrix, or cementing material, is described 

 as calcareous, ferruginous, siliceous or argillaceous, ac- 

 cording as it consists of lime, oxide of iron, silica or clay. 

 Phenomena of this kind sometimes occur in cultivated soils 

 and are called "pan formations." They may be due to 

 other causes, but are generally an indication of a tendency 

 of soils to revert to rock. 



Clay. Clay beds, such as the Oxford clay, London clay, 

 etc., are widely distributed, and cover large tracts on the 

 geological map. It is often said that clay consists of kaolin, 

 but the term is applied indiscriminately to quantities of 

 any mineral or mixture of minerals reduced to an impalp- 

 able powder. In this condition the particles, when wet> 

 cohere together and give the mass that vague quality of 

 plasticity which is the characteristic property of clay. The 

 difference between clay and sand is, in the main, a ques- 

 tion of size of the particles. The most finely pulverised 

 minerals are, however, more readily decomposed than those 

 of larger size, and, as might be expected, have a somewhat 

 different chemical composition. 



Analysis l of the particles of different sizes separated 

 from the same sample of soil gave the following results : 



S.M. 



1 Hall, " J C. S. Trans., 1904.' 



