2O TALKS ABOUT THE SOIL. 



Rub your hand over the face of the cliff. It is dusty. 

 The air attacks every part of the surface, and it slowly 

 decays and turns to dust. In the case of the pudding- 

 stone, the process is comparatively rapid, because the 

 iron cement that binds the pebbles together rusts away 

 and lets them free, just as beads are let loose when 

 the string breaks. The fallen tree and the vast heap 

 of shattered rocks at the base of the cliff plainly show 

 that the destruction must be comparatively rapid. 

 Could the air alone do this ? From the appearance 

 of the cliff, there must be other causes at work. 



We notice that the cliff is full of cracks. When it 

 rains, the water must flow down through all these 

 cracks, and lodge in countless minute fissures in the 

 face of the rock. After a heavy rain, when the rock 

 is filled with water, it may clear away, and a sharp, cold 

 wind come out of the north-west. Every drop of 

 water freezes and expands, and bursts open the rock, 

 splitting off minute specks and scales, or throwing 

 down great lumps that crash through the trees, and 

 destroy every thing before them. Here is another 

 and more powerful cause at work breaking down the 

 rock. 



In the summer there is no frost ; and yet the rain 

 may be at work washing moss and dust into cracks 

 already opened, and forming a sponge ready to hold 

 water that freezing next winter will act with still greater 

 force. \ The dry dust sifted into the cracks and open- 

 ings formed in the rock will also expand when wet, and 

 push off small pieces, or start a great mass that last 



