i.J FORMATION OF SOILS. 9 



combinations with some of the materials in the rock. 

 When carbonic acid is also present it helps to dissolve 

 in the water, portions of the rock which would not 

 have been soluble in pure water. Thus the solvent 

 action of water and its associated gases dissolves 

 out certain portions of the rock, and thereby the rock 

 has holes made in it, which gradually increase in size, 

 and thus expose a larger surface to be subsequently 

 acted upon by further supplies of water. 



9. There is a thjrcLagency which exerts its influence, 

 and often does so with great force, that is, frost. 

 When the surface of a rock has been penetrated by 

 water, and the temperature of the air falls below the 

 freezing point, the water becomes frozen. As water 

 freezes it gets bigger, and the particles of a wet rock 

 are pushed apart so as to make room for the water 

 which is freezing. When the frost has ceased and a 

 thaw takes place, portions of the surface, being thereby 

 released from the solid bands of ice, are thrown off 

 from the rock. The extent to which this takes place 

 depends in a great measure upon the size of the holes 

 which the water and gases may have made in the 

 rock. Sometimes the openings scarcely penetrate 

 below the surface, and in such cases the surface of 

 the rock only is affected ; at other times large masses 

 of rock are thrown off. 



10. These three agents wear away our hardest 

 rocks, and thus they are broken down and pulverized 

 into soil. Softer^ rocks are of course acted upon more 

 rapidly than hard rocks, but every rocky surface is 

 thus made to yield its contribution to the soil. The 

 lower forms of vegetation then establish themselves 

 on this newly-made soil, and their rootlets penetrate 

 and obtain their food from it. In due course these 

 plants die, and add decaying matter to the soil, which 

 thereby becomes nttedlbr the support of higher forms 

 of vegetation, and these prepare the way for those of 

 still higher organisation, 



