48 PHYSICAL CONSTITUTION OF SOIL 



These weights supply excellent examples of the consolida- 

 tion of a soil below the surface. 



The distribution of the stones in the Rothamsted soils 

 deserves some attention. Their distribution in the subsoil 

 is extremely irregular in different places, but in the surface 

 soil two points are invariably noticed, namely the deficiency 

 of stones at the surface of the old pasture land, and their 

 marked accumulation at the surface of the arable land. These 

 facts admit of explanation. The sod of a pasture is to a 

 considerable extent a new formation, consisting of living 

 and dead vegetable matter which has been added to and has 

 covered the previous soil. On pastures also the burying action 

 of worms is carried on to its fullest extent, the worm-casts 

 of fine earth, brought from the subsoil, covering the stones and 

 causing them apparently to sink beneath the surface. In the 

 case of arable land the circumstances are quite different. 



When a stony piece of land is dug or ploughed in the 

 autumn, and left exposed to the weather, it will be found 

 in spring time covered with stones. During the winter frosts 

 the surface soil has swollen and become disintegrated, the 

 stones have partially protruded, and have been left surrounded 

 by pulverulent matter. When rain comes, the fine soil is 

 washed away from the stones, which are left exposed on the 

 land, often very much to the farmer's astonishment, who 

 not unfrequently asserts his belief that ' stones grow.' The 

 stones thus accumulated on the surface may be buried again 

 at the next ploughing, but the kind of action here noticed, 

 continued for centuries, tends to a gradual removal of the 

 finest particles from the surface soil, and consequently to 

 an increase in the proportion of stones, where these, as at 

 Rothamsted, consist of flint or other minerals little affected 



