THE ORIGIN OF SOILS 35 



owing to its state of fine division, it weighs much less than 

 an equal volume of sand (vide ante). In the agricultural 

 sense it is "heavy " or difficult to cultivate owing to the 

 coherence of the particles. 



Corresponding to the minute size of the particles, clay 

 exhibits a great internal surface and a finely porous char- 

 acter. It can, therefore, absorb a larger quantity of water 

 and retain it more strongly than material of coarser texture. 

 The presence of the water hinders the entrance of air and 

 so retards oxidation. 



Clays are derived chiefly from granites and similar rocks. 

 They consist of the most finely pulverised sediment, and 

 some of the minerals are usually in an advanced state of 

 decomposition. Clay, therefore, usually has associated 

 with it a certain amount of plant food in a more or less 

 readily assimilable condition in addition to that contained 

 in the undecomposed minerals. It does not, however, as 

 a rule, yield very fertile soils because, in addition to the 

 difficulties of cultivation, they are ^apt to be cold, wet and 

 poorly oxidised. 



It will be noticed that the unproductiveness of clay soils 

 is attributed not to chemical but to physical causes which 

 are ultimately dependent upon the size of the particles. 



Analyses of clays, showing the proportion of the 

 principal ingredients, are given in the table on p. 36. 



Shales. Shale is simply clay which has become har- 

 dened and laminated. It is easily split up into layers along 

 the planes of bedding. This characteristic property is 

 greatly promoted by the presence of certain minerals such 

 as mica, sand, etc. ; calcareous and bituminous matters 

 produce a similar effect. The composition of shales is 

 very variable. They "merge into clays on the one hand and 

 into slates on the other. The slates are of similar character 

 but harder. They both yield a very poor class of soil. 



Marl. Marl consists of clay mixed with carbonate of 



D 2 



