FORMATION OF ARABLE LANDS. 29 



The mixture of alumina and lime constitutes another 

 species of soil, which by itself is but little productive, 

 especially if alumina constitutes more than one half of it ; 

 but it is used with great advantage in improving some 

 other kinds of land. The soil formed from this mixture 

 is called marl, or a marly soil ; the nature of it varies 

 much, according to the difference in the proportions of its 

 constituent principles ; it is called clayey, or fat, when 

 the qualities of alumina predominate in it, and calcareous, 

 or poor, when the calcareous sub-carbonate gives it its 

 character. Marl often contains shells, whole beds of it 

 being sometimes formed almost entirely of their ruins ; the 

 ^'fahluns " * are of this species ; this is the poorest kind, 

 and the most suitable for improving argillaceous soils. 

 The fat marl is often mingled with siliceous sand, which 

 serves to enhance its value when used in amending light 

 and calcareous earths. I have seen marl containing f"^^ 

 of sand, -^j^jy *of alumina, and yi^% of carbonate of lime, 

 used with advantage upon soils purely calcareous. 



Marl is usually found in beds, buried at a slight depth 

 in the earth ; when taken out and exposed to the air it 

 presents appearances which vary according to its quality. 

 Under the combined influence of air and water, it is 

 generally reduced to powder ; but the decomposition is 

 much more speedy and complete, when the two earths are 

 in their proper proportions, than when either of them pre- 

 dominates. 



The action of water dissolves, and carries off the alu- 

 mina gradually, the carbonic acid of the atmosphere com- 

 bines with the lime, which remains unsaturated, whilst 

 the oxygen acts upon the iron, increasing its oxidation, 

 till an entire change is produced in the nature of the 

 earth, which acquires properties before strangers to it ; it 

 becomes pulverulent, and it is in this state that it is used 

 to fertilize other earths. 



When marl is very argillaceous, it may be hardened by 

 the action of fire, and it then becomes sonorous, like well 

 baked potter's ware ; when it is very calcareous, fire con- 

 verts it into lime ; and I have seen it in Cevennes forming 

 an excellent mortar when combined with a sufficient quan- 

 tity of sand. 



There is an immense difference in the proportions ia 



[* Probably " muschelkalk," or variegated marls. — ^Tb.] 



