II SOILS 7 



may also be well seen in churchyards, for inscriptions on 

 tombstones are, after a time, completely obliterated by the 

 destructive effect of air and moisture, whereas similar 

 inscriptions on tablets protected in churches are unaffected 

 after a number of years. 



Variations of Temperature have also a powerful effect Rocks worn 



a.wuy by 



in the formation of soils. Rocks expand by heat and con- alterations 

 tract by cold. Thus in the hot tropical sunshine the sur- ture.""^^"^^" 

 faces of the rocks expand and are again contracted by the 

 cool air of the nights ; this causes them to crack, and then 

 pieces flake off, thereby giving rise to a constant small, but 

 certain, addition to the soil. In colder countries, frost has a 

 very destructive action on all rocks. Water contracts by 

 cold until it is near to the freezing point, but as it becomes 

 frozen it expands. In cold climates this sudden expansion Frost. 

 at the freezing point often gives rise to great trouble by 

 causing water pipes to burst, for the force exerted by the 

 expansion is so great as to burst strong iron vessels. It is 

 easily understood, therefore, why rocks are split and crum- 

 bled by frost. Rain water soaking into the holes and 

 crevices of the rocks, suddenly expanding with great force 

 on freezing, splits them up into fragments, and thus allows 

 fresh surfaces to be acted on by the atmosphere — which 

 easily finds its way into all the open spaces. 



Vegetation.' — Plant life assists in the formation of soils in Soils formed 

 two ways. When the roots of a plant reach a rock, the acid pfants!°" ° 

 sap eats into the surface dissolving out certain elements which 

 are taken up by the plant as food, and liberating sandy part- 

 icles which are added to the soil. Again, the roots enter into 

 the crevices of the rock and by their gradual increase in bulk 

 force the two sides apart, acting in this way like a wedge. The 

 force thus exerted is very great, and even such delicate and 

 soft plants as mushrooms can in their growth exert so power- 

 ful a pressure as to lift heavy weights. Some years ago, 



