NOTES ON THE SOIL. 



481 



classes: (1) sedentary soils, formed from the underlying 

 rock ; (2) transported soils, which after their formation have 

 been carried into their present position by water, ice, or 

 wind. A sedentary soil has many of the characters of the 

 rock below it, but is usually darker in colour, owing to the 

 presence of organic matter or to exposure to the air, o? to 

 both causes. Between soil and rock there is an intermediate 

 layer, the subsoil, consisting of partly broken-down rock and 

 differing from the soil in texture, composition, and colour. 



Most rocks consist of fragments or crystals held together 

 by cementing material or by interlocking. The texture is 

 seldom so close as to make the rock impervious to water; 

 even the most close-grained granite or the finest marble will 

 absorb water. The chief agents that cause the " weathering " 

 or breaking-up of rocks and the formation of soil are rain, 

 changes of temperature, frost, the air, earthworms, and the 

 roots of plants. 



Rain-water in falling through the air dissolves a certain 

 amount of oxygen and carbon dioxide, which help it in dis- 

 solving minerals in rocks. The oxygen oxidises some of the 

 rock substances e.g. rocks containing iron have a red colour 

 due to iron oxide. The carbon dioxide gives the water an 

 acid action so that it can dissolve carbonates and some other 

 substances insoluble in water alone. The mechanical effect of 

 water in wearing down rocks is well known ; soft rocks like 

 clays and sandstones are often broken up merely by the 

 beating of rain on their surface, and even the hardest rocks 

 may be worn down by running water, especially when it 

 carries sand or mud in suspension. Besides the direct action 

 of water, there is the action of water under the influence of 

 frost. If water is in the crevices of a rock when freezing 

 occurs, its expansion pushes asunder masses of rock, and the 

 same action breaks up rock-fragments and further pulverises 

 the soil itself. 



Apart from freezing, mere change in temperature tends to 

 break up rocks, especially when the change is large and rapid. 

 The sun's heat during the day expands rocks, and the rapid 

 radiation of heat after sunset causes them to cool and con- 

 tract. The effect of these alternate expansions and contrac- 

 tions is to crack the rock and loosen the particles on the 

 surface, and the effect is increased by the fact that the 



S.B. 31 



