No. 6. DEPARTMENT OF AGRICULTURE. 309 



integrate first, the hornblende next, while the biotite mica remains 

 for a long time unaffected. Thus there results a clay, the final pro- 

 duct of the disintegration, mixed with quartz particles, or sand, to- 

 gether with fragments of undecomposed rock of greater or less size, 

 giving the soil its open, porous or even stony character, so common in 

 regions underlaid by the ancient crj'stalline rocks. 



Besides the direct chemical actions already enumerated, other 

 factors in soil formation of a physical nature might be mentioned. 

 These are the expansion and contraction of rock masses; frost and 

 freezing water; plant roots forcing their way into rocky crevices; 

 beating and scouring rain; all tending to disintegrate the rocky cov- 

 ering of the earth and to open it more thoroughly to the subtle action 

 of meteoric waters. 



Another type of soils are those formed from the disintegration of 

 limestones. Limestones are impure mixtures of carbonate of lime 

 Vvith various proportions of sand and clay. In the disintegration 

 the great bulk of the carbonate of lime is leached out, and the insolu- 

 ble sand and clay are left as the final product. Thus a limestone 

 composed of 75 per cent, of carbonate of lime, may when converted 

 into soil contain only a trace of the original carbonate. This resi- 

 dual soil is however more or less rich in the mineral elements of plant 

 food while the as yet mndecomposed particles in the residual sand, 

 by continued disintegration, add new food materials to growing 

 plants. 



Sandstones undergo disintegration by the solution in meteoric 

 waters of the materials which bind together individual grains. In 

 this way the component sand particles are loosened, together with 

 clay, which is generally an important constituent of most sand- 

 stones. 



Whatever may be the character of the rock or of its contained 

 minerals the process is the same, i. e., the dissolving out by means of 

 percolating waters of the elements of plant food contained within the 

 minerals. These percolating waters are furthermore made active 

 solvents in the disintegration of rock through the acid products 

 which they contain, which in turn are produced by the decay of or- 

 ganic matter through the agency of micro-organisms. Of the acid 

 products the most active in this regard is carbon dioxide, which is 

 the final product of the decomposition by bacteria of organic mat- 

 ter. 



The chemical union between the carbon dioxide in percolating 

 waters and the potash soda lime and magnesia in the minerals, re- 

 sults in the formation of carbonates and bi-carbonates of these bases, 

 which being soluble, are in a large measure carried away in solution 

 so that the residual soil contains but a certain proportion of these 

 original stores of agricultural wealth. This loss of mineral plant 



