22 SOILS: PROPERTIES AND MANAGEMENT 



by showing the various stages that orthoclase may undergo 

 in producing a residual clay : — 



KAlSisOg + HOH = HAlSi 3 8 + KOH 



2 KOH + C0 2 = K 2 C0 3 + H 2 

 HAlSi 3 8 - 2 Si0 2 = HAlSi0 4 (kaolinite) 



The silica in this case may become quartz or colloidal 

 silica, or, what is more probable, may unite with certain 

 elements to produce complex hydrated silicates. 



20. A general statement of weathering. — The question 

 of rock weathering is complicated because no one action 

 can be considered alone. All forces are acting together, 

 tending to produce a great complex of reaction and inter- 

 action. No amount of explanation or speculation can 

 ever fully clarify the question as to the formation of a 

 soil from a parent rock. Nevertheless, knowing in general 

 the separate forces and reactions produced, we may formu- 

 late the phenomenon in a general and superficial way. 

 The change that a rock undergoes in the formation of a 

 residual clay is first a physical breaking-down accom- 

 panied by chemical changes, which consist in the hydra- 

 tion of the feldspars, the oxidation of the iron, and the 

 solution and carbonation of the soluble bases. 



21. Factors affecting weathering. — It is readily to be 

 seen that the activity of the various agencies of weather- 

 ing will be modified by certain factors which determine 

 not only the kind of rock decay but also its rate. Of 

 these, climate is probably of the greatest importance. 

 The difference in the weathering in an arid region as 

 compared to that in a humid region will illustrate this 

 point. Under arid conditions the physical forces will 

 dominate and the resulting soil will be coarse. Freezing 

 and thawing, heat and cold, the action of the wind, and 



