36 SOILS AND MANUKES 



AVERAGE CHEMICAL COMPOSITION OF CLAYS. 



lime, but retaining its plasticity. When partly indurated 

 it is called marl shale or marl slate, being easily split into 

 layers. The harder varieties are called marl stone. They 

 crumble very readily on exposure to weather. In the 

 neighbourhood of marl beds, .marl is extensively used as a 

 source of lime and produces good effects, especially on the 

 lighter soils. The proportion of carbonate of lime varies 

 considerably. As it increases the plasticity gradually 

 diminishes, and when it becomes predominant the material 

 ceases to be regarded as marl and is called argillaceous 

 limestone. The following table shows the average 

 composition of certain marls : 



AVERAGE CHEMICAL COMPOSITION OF MARLS. 



