8o THE ORIGIN AND NATURE OF SOILS [chap 



solvent Such water, for example, will dissolve carbonate 

 of lime, hence limestone and chalk rocks waste away 

 under the weather, and leave nothing behind but the 

 2 or 3 per cent, of fine red clay, which is the only 

 other material making up the rock. Wherever a railway 

 cutting traverses chalk or limestone, the surface of the 

 rock will be seen to be let down in places into deep and 

 tapering pipes ; these represent lines of soakage, along 

 which the surface water has been continually travelling 

 to a fissure below, until the rock within the influence of 

 this gradual current has all been dissolved away. Not 

 only does carbonate of lime occur in chalk and limestone 

 m a more or less pure state, but it is found in other 

 rocks acting as a cement to grains of sand, etc., and as it 

 dissolves in the percolating rain-water the rock disinte- 

 grates. Many of the complex minerals which make up 

 the bulk of the older primitive and volcanic rocks, are 

 also decomposed by water charged with carbon dioxide ; 

 for instance, the felspar, which is one of the fundamental 

 minerals in granite and in all the basaltic rocks, breaks 

 down to clay under its influence. In Cornwall the 

 surface of the granite is covered with a white clay thus 

 formed, in which the still undecomposed grains of quartz 

 and mica, the other minerals making up the granite, 

 are lying unaltered ; the basalts and trap rocks similarly 

 yield a clay, that is red because of the oxides of iron 

 which are simultaneously formed as the felspar decays 

 into clay. Iron compounds are very generally diffused 

 among the rocks ; the greens and browns and blacks of 

 the unweathered rocks are due to compounds of iron, 

 which all break up and go into solution under the 

 action of water and carbon dioxide, eventually becoming 

 oxidised into something akin to iron rust, and this 

 oxidised rusty substance is the cause of the yellows, 

 browns, and reds which predominate in soils. Perco- 



