III. THE SOIL 41 



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(HC0 3 ) 2 . Hence 

 it is found that all river and spring waters contain dissolved mineral 

 matter, and in many cases calcium carbonate is the largest constituent. 

 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 un- 

 doubtedly 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 .results are sometimes seen 

 in the production of fantastic forms in sandstone, produced 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 Eocks, in Yorkshire, is probably not a very im- 

 portant 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 colour. The change in volume, accompanied very often 

 by falling to powder, aids in breaking up the rock. Air in the presence 

 of water also oxidises metallic sulphides, e.g., iron sulphide, and so 

 produces disintegration in rocks containing such compounds. The 

 carbon dioxide of the air, acting with water, is necessary for many of 

 the chemical changes described as being due to water. 



3. Earthworms play an important part in the formation and 

 modification of soil. Darwin l has shown that they bring portions of 

 the subsoil to the surface, render the soil more porous and pulverulent, 

 and aid greatly in the conversion of vegetable refuse, leaves, etc., into 

 humus. This they do partly by drawing dead leaves, etc., into their 

 holes, and partly by actually passing the vegetable matter and the soil 

 containing it through their bodies. This matter is ejected and de- 

 posited on the surface of the ground at the rate of about 10 tons to 

 the acre per annum, burying all small objects, like stones, fragments 

 of bones, etc., to a depth which increases at an average rate of about 

 ^y of an inch per year. The number of earthworms to the acre is 

 estimated to average over 25,000, so that the part played by them must 

 be most important. According to Russell, 2 however, the effect of earth- 

 worms in promoting decomposition of organic matter and formation of 

 nitrates is very small, though their effect in loosening the soil is valuable. 

 In tropical countries white ants probably perform much the same kind 

 of work as that done by earthworms in temperate climates. In South 



1 " Vegetable Mould and Earthworms," 1881. 

 2 Jour. Agric. Sci., 1910', 3, 246. 



