14 THE ORIGIN OF SOILS [chap. 



lime, magnesia, and iron protoxide also form soluble 

 bicarbonates, all minerals containing these bases are 

 liable to attack. Probably some of the organic acids 

 produced by the decay of vegetable matter in the sur- 

 face soil aid in the solvent power of soil water ; yet, 

 undoubtedly, water containing carbonic acid is the great 

 natural solvent, and some of the more striking cases of 

 its action in breaking down rocks will be discussed later 

 under the heads of felspar, augite, and calcium carbonate. 



The attack of frost and water upon rocks is much 

 assisted by the roots of plants and trees ; if we examine 

 a fresh section of the soil over a quarry or brick pit, the 

 roots of ordinary field plants can be traced downwards 

 for 4 feet or more, while the roots of a tree may be seen 

 working far into tiny fissures of the almost unaltered 

 rock. The roots follow the water in the fissures : at first 

 they can enter very minute cracks ; as they grow, the 

 pressure they exert widens the cracks ; finally, the roots 

 decay and leave a channel down which water can per- 

 colate freely. The fine roots themselves cause a certain 

 solvent action ; after plants had been grown in a pot 

 filled with powdered granite rock, which had been freed 

 from all fine particles by washing, an appreciable quantity 

 of mud and clay was found to have been formed. 



The opening up of the subsoil to weathering by the 

 action of roots is also carried out by worms, which have 

 been observed making their burrows to the depth of 5 

 feet, thus introducing both air and water into the lower 

 strata. But the great work of worms in regard to soil 

 lies rather in the production of the fine surface layer of 

 mould rich in vegetable matter : Darwin calculated that 

 on an ordinary chalky pasture the whole of the fine 

 surface soil to a depth of 10 inches was passed through 

 worms and cast up on the surface in the course of fifty 

 years. During their passage through the gizzard of the 



