38 THE PRINCIPLES OF 



chips to big land-fast boulders. Stones are useful because they 

 are constantly being broken down; even tillage implements 

 assist in this. Small pieces are continually detached from 

 them. In addition, there is the constant action of the atmo- 

 spheric forces completing the passage from the insoluble to the 

 soluble state. Mineral fragments are no doubt a potential 

 source of plant food, and a guarantee against ultimate exhaus- 

 tion, especially where the stones are parts of the original 

 parent rocks as, for instance, where there are fragments of 

 chalk in a chalk soil fragments of the lias clay in a clay 

 soil fragments of oolitic limestone in an oolitic soil. They 

 are guarantees for the future keeping up of the fertility of 

 the soil. It was the late Dr. Voelcker's opinion that the 

 ultimate exhaustion of soils was a " bugbear." He considered 

 that it was an impossibility, and he was right. Ultimate 

 exhaustion of the soil is an impossibility on account of these 

 fragments. To effect the temporary exhaustion of a soil is very 

 easy. It is done by a quick removal of the soluble portions ; 

 but soluble portions are replaced by fresh materials brought 

 gradually out of these stones through attrition, and the very 

 slow action of disintegrating forces, especially upon the minuter 

 fragments. Neither must we forget that stones have a 

 wonderful effect upon the mechanical texture of soils. Of 

 that I could give many examples. Upon the oolitic hills of 

 Gloucestershire, where I resided for many years, we should 

 have had the soil utterly unfit for sheep but for the presence 

 of large numbers of fragments of the underlying great oolite. 

 Bits of the rock break up in flat, plate-like fragments as the 

 ground is ploughed, and the intermixture of these portions 

 of oolitic rock so temper the soil that the farmers are able 

 to treat it as a light soil, and grow barley and fold sheep upon 

 it in the winter. 



I have met with many similar cases. Take, for example, 

 the soils round Dartford in North Kent, which is naturally a 



