26 SOILS AND MANtJBES 



of 4 C., i.e., just before it freezes, and the force of expan- 

 sion produced when it solidifies on further cooling is prac- 

 tically irresistible. Water lodges in the cracks and inter- 

 stices of rocks and is absorbed into the mass of those which 

 are porous, and produces tremendous disintegrating effects 

 when it freezes. Gigantic masses are split and rent, 

 smaller fragments crumble to powder, and tiny particles 

 are reduced to dust. It is not the duration or severity of 

 the frost but the alternation of periods of frost and thaw 

 that produces the greatest effects. Frost is probably the most 

 powerful of all the weathering agents, and, in cold, moist 

 climates, is the principal cause of the crumbling of rocks. 



Water acts both mechanically and chemically. It is 

 proverbial that constant dropping wears away stone. The 

 friction of the water and of stones, one against the 

 other in streams, rubs down all angularities and leaves 

 the stones smooth and rounded. Few minerals are abso- 

 lutely insoluble in water, and though the process of solu- 

 tion may be slow it is constantly going on, and considerable 

 quantities of material are removed in this way. 



Carbonic acid greatly increases the dissolving power of 

 water. Many substances, e.g., carbonate of lime, which 

 are practically insoluble in pure water, are readily dis- 

 solved in water containing carbonic acid, and even hard 

 intractable substances like felspar are slowly attacked. 



Oxygen combines with all oxidisable minerals, e.g., mag- 

 netite, exposed to its action. The products are generally 

 softer and more friable or more soluble than the original 

 substance. Oxidation of sulphur-bearing minerals pro- 

 duces sulphates, and in some cases sulphuric acid, a 

 powerful solvent. 



The ultimate effect of all these forces, acting jointly or 

 separately for long periods of time, is to produce physical 

 and chemical disintegration of the hardest and most durable 

 rocks. They operate on large masses and small fragments, 



