CHAPTER V 



THE ORIGIN AND NATURE OP^ SOILS 



The Weathering of Rocks to Soil. Solution of Rock Materials in 

 Water containing Carbon Dioxide. Action of Frost. Trans- 

 port of Soil by Rain and Running Water. Action of Worms. 

 Approximate Analysis of Soils. Properties of Clay and Sand. 

 Chemical Constituents of Soils. Soils and Subsoils. 



Before we take up the question of the composition 

 of the soil, it will be well to learn something of its origin, 

 and for this purpose nothing is so instructive as a visit 

 to a quarry where a clean face of rock can be seen pass- 

 ing into soil, on the surface of which vegetation may 

 still be growing. At some distance below the surface 

 the rock is solid and massive ; in some cases, as in lime- 

 stone or sandstone, it will be stratified or divided into 

 parallel layers, which can be distinguished by changes 

 of colour or texture, even when there is no actual 

 division along the planes of bedding ; in other cases, as 

 in granite or basalt, the rock will be massive and show 

 no sign of splitting into layers. The colour of the rock 

 will be pretty uniform, and at depth it will often be 

 some shade of dark green or blue or grey, though of 

 course there are numerous exceptions, such as red or 

 yellow sandstones, white or yellow limestones, reddish 

 granites, etc. Following the rock upwards to the surface, 

 there comes a point when the rock begins to show signs 

 of decay ; the divisions that traverse the rock, whether 

 bedding planes or joints, become somewhat larger and 



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