FORMATION OF SOIL. 25 



The rate of chemical decomposition depends on the tempera- 

 ture ; the higher the latter, the quicker the decomposition. 

 Hence it is far more rapid in tropical than in cold climates. 



r. Organic Afienriefi. 



When mechanical and chemical action have commenced the 

 decomposition of the rock, organic action sets in. Lichens, 

 followed by mosses, appear on the surface of the rock, which 

 further accelerate disintegration by keeping the surface moist. 

 Next, the roots of these plants penetrate into the finest fissures 

 and assist mechanical action. In this manner a soil is 

 gradually formed, which consists of fragments of rock and 

 remnants of dead plants, suitable for the support of more 

 highly organised plants, such as grasses and herbs ; these are 

 followed by shrubs and trees, which penetrate with their roots 

 deeper and deeper into the rents and fissures of the rock, and 

 further accelerate disintegration. 



When this process has gone on for a sufficient length of 

 time, the outer part of the earth's crust consists of a layer of 

 varying depth, which, commencing from below, changes 

 gradually from the solid rock into broken rock, or brash, then 

 into smaller pieces of rock, or subsoil, and ultimately into the 

 formed or surface soil. 



At first sight it would appear, that the composition and 

 quality of the soil depend solely on the composition of the 

 original rock, out of which it has been formed. This is, on 

 closer investigation, found to be tlie case only to a limited 

 extent, because, in the first place, certain important substances, 

 such as potash, magnesia, lime, may be carried away and lost 

 during the process of disintegration ; secondly, organic sub- 

 stances are added ; and thirdly, the fertility of the soil depends 

 not only on its chemical composition, but also, and often 

 chiefly so, on its physical properties. All that can be said is, 

 that certain kinds of rock yield ordinarily a soil of a certain 

 description, but subject to modifications, which depend on the 

 peculiarities of each case. On the whole, the attempt of 



