CHAPTER I 



THE ORIGIN OF SOILS 



Sedentary Soils, and Soils of Transport Weathering The Com- 

 position of Rock-forming Minerals and their Weathered 

 Products Distinction between Soil and Subsoil General 

 Classification of Soils. 



THE study of soils must begin with some knowledge 

 of their origin and their relationship to the rocks that 

 underlie them, out of which, in most cases, they have 

 , been formed. 



Perhaps the best way of arriving at an idea of the 

 natural processes which result in soil, is to visit a 

 river valley and examine, first a quarry on the flanks 

 of the hills, and then one of the cuttings for gravel or 

 brick earth, which often lie a little above the river level. 



The face of the quarry shows at a depth of 10 feet 

 or so from the surface the massive rock, unaltered as 

 yet by any action of the weather. Closer examination, 

 however, shows that even at this depth the rock is 

 not quite solid ; if it be a stratified rock the planes of 

 bedding are apparent, along which the rock can be 

 split. Joints again traverse the rock at right angles 

 to the bedding planes, and along both joints and bed- 

 ding planes it is evident that water makes its way, for 

 the edges of the cracks are slightly altered and dis- 

 coloured. Nearer the surface, the cracks and lines of 



6 



