I.] CLASSIFICATION OF SOILS 35 



all grades, better working than the clays and more 

 fertile than the pure sands ; sometimes the clay forma- 

 tion itself contains sand, as in the upper beds of the 

 London- Clay, or we may have a fine-grained sandstone 

 mixed with clay, as in some of the carboniferous rocks. 

 In all these cases, when chalk is absent, and 'drainage 

 incomplete, there will be an accumulation of humus, 

 resulting in a peaty formation. 



Some argillaceous limestones give rise to typical 

 ;< marls," mixtures of chalk and clay ; e.g., some of the 

 beds of the Lias and of the Keuper. 



Other limestones with a sandy basis, and fine- 

 grained sandstones cemented by carbonate of lime,. give 

 rise to " loams," which are free-working soils, mainly 

 :omposed of fine sand with some clay and a little 

 :alcium carbonate. The alluvial soils in the valleys 

 are loams, passing in places into gravels ; these are 

 generally the richest soils; as a rule they are mixtures 

 derived from many formations, and so are well supplied 

 with humus and the mineral elements of plant food ; 

 they are deep, and not over consolidated, thus admitting 

 Df the percolation of water and the descent of roots ; yet 

 they are fine-grained enough to prevent them drying 

 out too rapidly. But though these terms, sands, clays, 

 marls, loams, and peaty soils, serve for rough descriptive 

 purposes, a more exact determination of the constituent 

 particles is necessary to properly characterise a soil, 

 ind for this we must resort to what is termed the 

 'mechanical analysis" of a soil. 



