482 NOTES ON THE BOIL. 



different minerals in rocks do not expand and contract 

 equally. Even in this country one often notices a difference 

 of about 30 C., and sometimes more, between the tempera- 

 ture during the day and the minimum temperature at night. 

 The air helps in breaking up rocks in two ways : (1) chemi- 

 cally, by its oxygen uniting with other substances and pro- 

 ducing softer materials ; (2) mechanically, by the wind 

 carrying dust and wearing down rocks, and also causing 

 rain to beat on rock surfaces. 



Sedentary and Transported Soils. As a rule the rocks from 

 which a soil is derived are found on digging down, but sometimes the 

 soil-materials have been derived from distant rocks by the action of 

 running water, wind, and ice-sheets. 



(1) Sedentary soils, in which the products of weathering remain 

 where they are formed, only occur where the surface is fairly horizontal 

 or where plants are growing. The latter prevent the broken -up material 

 from being quickly carried away by rain, and also by their growth 

 assist in the further breaking-up of the rock. Sedentary soils are 

 chalky, sandy, or clayey, according as the underlying rock was lime- 

 stone, sandstone, or clay. These form the three chief types of soil, but 

 there are of course gradations from one to the other. In the case of 

 limestone, water containing carbon dioxide often dissolves away the 

 lime and magnesia, leaving the less soluble portions of the rock to form 

 the soil. These portions are usually clay and fine sand, so that a soil 

 formed in this way is often clayey, and may contain less lime than 

 other soils not derived from limestone. 



(2) The mud carried by every stream after rain is an example of soil 

 in the process of transportation. Another is seen in the way rain 

 washes soil from hill-sides, or from sloping fields (the cultivation of soil 

 helps the action of water in this case, since the surface is repeatedly 

 disturbed by ploughing, etc.). Notice the difference in level of the 

 soil surface when a hedge runs across a slope ; the soil carried down 

 the slope is stopped by the hedge or fence, and is gradually piled up 

 against its upper side, while at the lower side the soil is usually carried 

 away, leaving the hedge or fence higher above the surface. In books 

 on Physical Geography you will find accounts of the formation of 

 deltas, alluvial plains, etc., by rivers, also of the action of glaciers and 

 ice- sheets. 



The Nature of Soil. Dry soil consists chiefly of small frag- 

 ments of rock of various kinds, usually with varying amounts of 

 organic matter derived from the breaking down of plant and animal 

 tissue. This organic matter, or humus, can be removed from the soil 

 by burning it. The soil -grains are coated with substances which have 

 been dissolved in the soil-water, and have been deposited when the 

 water evaporated. 



