40 SOILS AND MANUEES 



tion ; the older rocks are destroyed and new ones are 

 continually being formed. With the exception of the 

 organic rocks, which consist of the remains of animal and 

 vegetable organisms, they are composed of minerals. The 

 igneous rocks generally consist of crystalline minerals 

 fused together, and are very hard. In the sedimentary 

 deposits the minerals may be crystalline as in sandstones, 

 or amorphous as in slates and shales. They are often 

 cemented together by a matrix of some kind, or consoli- 

 dated and hardened by pressure. In other cases they are 

 comparatively soft. 



Formation of Soils. Kocks of all kinds whatever their 

 origin, composition or physical properties undergo 

 change at the surface where they are exposed to the action 

 of the weather. The harder rocks are disintegrated and 

 pulverised, and the minerals are decomposed chemically as 

 above described. The softer materials also are loosened, 

 oxidised and chemically altered. The pulverent material 

 thus produced is, at least physically, capable of acting as a 

 medium for the growth of plants and usually contains some 

 plant foods derived from the minerals. In addition to the 

 effects produced by the mechanical and chemical agents 

 frost, water, carbonic acid, etc. vegetable and animal 

 organisms play a not inconsiderable part especially in the 

 final stages in which the pulverised material is converted 

 into soil properly so called. 



The formation of organic matter is probably due, in 

 the first instance, to the action of certain bacteria and 

 alge which possess the power of assimilating the free 

 nitrogen of the air (pp. 187, 142) and which are normally 

 present in the soil. Bacteria are found on the surface of 

 bare rocks, on exposed mountains, and even the hardest 

 stones and rock masses soon become covered over with a 

 lichenous growth. The quantity of organic matter so 

 formed may be, at first, small, but its presence in the soil 



