THE ORIGIN OF SOILS 189 



As a rule certain changes of colour accompany the passage 

 from rock into soil that has just been described ; not infrequently 

 the rock possesses some shade of dark olive green or grey or black, 

 and only begins to show yellow or brown stains and rusty marks 

 along the fractures at a higher level ; the fine broken-down rock, 

 which gradually becomes the subsoil, is nearly always of an ochre 

 colour yellow, brown, or red which becomes darker or duller as 

 it passes into the soil. According to the nature and situation of 

 the rock, the transition just described may extend over as much as 

 twenty feet, or may be complete in two or three ; indeed, on the high 

 downs hard unbroken chalk may be found only a foot below the 

 surface, and soil and subsoil together may not be more than six 

 inches thick. The important feature is that the rock passes by in- 

 sensible gradations into the soil, and that no sharp line of separa- 

 tion can be drawn at any point in the passage. Not infrequently, 

 however, a different sequence may be observed : the rock surface 

 ends abruptly without any of the breaking up described above ; 

 instead, it is overlaid by a bed of clay or sand or gravel of entirely 

 different character, which in its turn passes by insensible degrees 

 into the soil. Leaving such cases alone for the present, the 

 quarry merits further examination to ascertain by what agencies 

 the change from rock into subsoil and soil has been effected. 



If the weather has not been dry for too long a period it will be 

 seen that even the hardest and most uniform rock near the base 

 of the quarry is still traversed by a number of up-and-down cracks, 

 the " joints " of the stone, and that water works along these 

 joints, as their discoloured edges demonstrate. Higher up the 

 joint cracks become more numerous and a little wider, the edges 

 are also somewhat rounded, as though the water oozing along 

 them had softened and removed a little of the sides ; moreover, 

 down some of them fine roots of trees and other strong growing 

 plants will be found to have wandered. As these roots are traced 

 upwards they become thicker, and evidently exert a certain amount 

 of pressure outwards, thus widening the crack and bursting the 

 stone. 



The mechanical effect of the roots is obvious enough, that of 

 the water is rather more subtle ; in some cases the water will 

 appear to have dissolved away the cement which binds together 



