THE SOIL S9 



winter's day the crevices of a rock become filled with water. 

 If the temperature falls, the water begins to freeze, at first on the 

 surface, so that every crevice becomes plugged with ice. As 

 the liquid wibhin continues to lose heat it tends to solidify. 

 This it can only do if it be allowed to expand, and in order to 

 do this it must either widen or lengthen the crevice which con- 

 tains it. When the next thaw comes the enlarged crevice again 

 fills with water ; the next frost repeats the action, and so the 

 process goes on, and in this way a rock, even the hardest, is soon 

 broken up into fragmentg Continuous frost appears rather to 

 hinder denudation by cementing the parts of a rock more firmly 

 together. In addition to the effect of expansion just described, 

 ice formation probably acts as a disintegrating agent upon soils 

 and rocks by virtue of crystallisation ; the mere growth of 

 crystals of ice, apart altogether from the expansion which 

 accompanies their formation, exerts a disruptive effect upon 

 the material in which they are formed. 



(c) Chemically, — Many minerals when exposed to the action 

 of water are acted upon in such a way as to lead to their dis- 

 integration. A portion is often carried away in solution, while 

 the remainder crumbles away and is removed by rain or 

 running water. In many rocks the cementing material which 

 holds the grains of silica together is thus acted upon, when the 

 silica, though perhaps not appreciably dissolved by water, 

 becomes loosened and easily removed. Calcium, ferrous and 

 magnesium carbonates are particularly liable to be thus removed, 

 for though they are only very slightly soluble in pure water 

 they readily dissolve if carbon dioxide be present, owing to 

 the formation of bicarbonates ; thus — 



CaCO, + H,C03 = Ca(HC03),. 



2. Air. — This also acts in various ways. 



(a) Mechanically. — Wind actually detaches large projecting 

 pieces of rock in mountainous districts^ and sends them crashing 

 down on to the rocks or screes below. In addition, by hurling 

 sand and small pebbles against the surface of rocks it brings 



