THE HARDENING OF ROCKS 



rial is being removed by the rains and the rivers ; the 

 scale that is empty is the sea, in which the eroded 

 material is laid down to form beds and strata. These 

 two scales are never quite balanced. But suppose a time 

 comes when we have taken all the material we can from 

 the weighted scale, so as to make the hitherto unweighted 

 one the heavier what will happen ? The newly weighted 

 scale will inevitably fall, and we shall have to begin to 

 reverse our system of taking from, and adding to, the 

 scales. Similarly there will always come a time when 

 there will be a flow of the earth-mass from the areas 

 which have been receiving great loads of sediments, 

 towards the areas which have been robbed to supply 

 them. Think for a moment how the weight of a mountain 

 set up in a plain might act if we can imagine some giant 

 force piling up the mountain higher and higher. The 

 mere weight of the mountain would tend to make it 

 settle, and begin to press outwards all round its base. 

 If you find a difficulty in seeing how this could be, 

 imagine the mountain to be made of pitch. In such 

 a case we can quite easily realise how it would spread. 

 Similarly mountains, or even great plains and plateaux, 

 of sediment built up for millions of years in the oceans, 

 would tend to spread ; and they would spread towards the 

 land which in the first place had supplied them with materials. 

 At first, of course, the stiffness or rigidity of the land 

 would resist this spreading. But the masses thus built 

 up would become so great and so heavy in the course of 

 millions of years that no stiffness of the land could resist 

 their spread. They would begin to roll or slide towards 



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