CHAP, v.] SEDENTARY SOILS 79 



are discoloured, as though water has been oozing along 

 them and had caused a certain amount of rusting and 

 rotting. Higher still, the rock is obviously disintegrated ; 

 the large fragments, which still are arranged in keeping 

 with the structure of the unaltered rock below, are 

 separated by layers of loose sand and stones, or by clay, 

 and the proportion of this loose material increases the 

 nearer one gets to the surface. Finally the rock either 

 wholly disappears, or becomes represented only by 

 stones in the loose material, which we may now call 

 the subsoil. A few inches below the surface, this 

 subsoil gives place to the soil proper, distinguished as a 

 rule by its darker colour and its admixture with 

 vegetable matter from the plants that have been grow- 

 ing upon the surface. 



Such a soil passing by insensible degrees into the 

 rock below is called a " sedentary soil," because it has 

 grown upon the spot ; but in some quarries and pits we 

 shall not meet with the sequence described above, but 

 one in which the surface material turning into soil is 

 obviously quite distinct from the fundamental rock below, 

 a sharp line of distinction marking the change from one 

 to the other. Such cases will be considered later ; it now 

 remains to examine into the causes which have brought 

 about the degradation of rock into soil. Water obvi- 

 ously plays a considerable part in the process ; even the 

 most solid rock is traversed by certain planes of weak- 

 ness — bedding planes or joints, along which minute 

 fissures water slowly percolates, and, as we have observed 

 in the quarry, begins to degrade and discolour the edges 

 of the cracks. In pure water itself few of the minerals 

 constituting rocks are soluble ; but the rain, as it soaks 

 down through the upper layer of soil pervaded by plants' 

 roots, dissolves some of the carbon dioxide that is there 

 being produced, and then becomes a much more effective 



