46 THE SOIL. III. 



heaps of debris, known as moraines, mark the suc- 

 cessive positions of the termination of the glaciers. 

 The fine portions of the glacial deposits possess a 

 composition similar to that of the rock from which 

 they were formed, comparatively little chemical 

 change having taken place in their production. 



(iv.) Chemically. As already stated, many minerals, e.g., 

 felspar, exposed to the action of water, undergo 

 chemical changes leading to their disintegration. In 

 many cases soluble compounds are produced and 

 carried away in solution by the water. This is pre- 

 eminently the case with calcium carbonate, which, 

 though almost insoluble in pure water, dissolves 

 readily in water containing carbon dioxide, probably 

 because of the formation of calcium bicarbonate, 

 Ca(HCO 8 ) f . Hence it is found that all river and 

 spring waters contain dissolved mineral matter, and 

 in many cases calcium carbonate is the largest con- 

 stituent. The action of water on felspar has already 

 been described. 



In these and in other ways water, by its solvent properties, 

 aids greatly in rock disintegration. 



2. Air. This also acts in several ways : 



(i.) Mechanically. In mountainous districts high winds 

 undoubtedly act destructively upon rocks both by the 

 actual pressure exerted on projecting portions and 

 also by hurling pebbles and smaller fragments of rock 

 against them. The same action is sometimes seen in 

 the production of fantastic forms in sandstone, pro- 

 duced by the erosion due to sand blown by the wind 

 against the lower portion of a projecting rock, giving 

 rise to an undercutting. This action, well shown at 

 Brimham Rocks, in Yorkshire, is probably not a very 

 important one. 



(ii.) Chemically. Many rocks contain the lower oxides of 

 metals, especially of iron. On exposure to air such 

 oxides combine with an additional quantity of oxygen, 

 in so doing altering their volume and changing their 



