THE WORK OF THE SOIL 



BY A. D. HALL, M.A., F.R.S., 



Director of the Experimental Station at Rothamsted. 



CHAPTER XIII 



THE ORIGIN OF SOILS 



IN order to understand how the soil has come into being it is 

 desirable to find some comparatively fresh excavation, like a stone 

 quarry, preferably one situated a little way up the sloping side 

 of a valley, so that the layer of soil covering the rock may not be 

 too deep. The rock exposed in the quarry may be road stone, 

 building stone, chalk, or even nothing more than hard clay or 

 sand, in any case, much the same sequence will be visible. At 

 the bottom of the quarry will be seen the native rock, showing, 

 except in such cases as basalt or granite, a distinct bedded struc- 

 ture of layer superimposed upon layer, often varying in colour or 

 hardness from band to band. Nearer the surface the structure 

 of the rock, whether massive or bedded, begins to fail ; fractures 

 become common until the layers consist of loose stones, though 

 they are still lying in the positions they occupied before the 

 breaking up took place. A little higher the stones become smaller 

 and are separated by intervals filled with loose disintegrated 

 rock, often a mere coarse sand; higher still, the loose material 

 is greater in amount than the stones, which now lie scattered 

 about in what fairly may be termed soil, since all the structure of 

 the underlying rock has disappeared. Nearer the surface, again, 

 the soil gains still more on the stones, which in some cases dis- 

 appear entirely ; finally, at the top, there is a darker layer, six 

 inches to a foot or more in thickness, which constitutes the soil 

 proper as distinct from the subsoil, this name being given to the 

 fine material below. 



