CKUMBLING OF ROCKS. 63 



Professor Stockliardt says of the weathering and 

 decay of rocks, that the crumbling of rock into 

 earth still continues, and renders it capable of be- 

 coming and remaining a supporter and sustainer of 

 vegetable growth. The principle on which it acts 

 may be thus described : 



" Through the alternations of heat and cold^ cracks 

 and fissures are formed in the most solid rocks ; 

 with the assistance of air and water, these are deep- 

 ened and widened, and separate fragments are de- 

 tached from their connection with the great mass. 

 Through the same alternations of temperature a 

 daily circulation of air is produced in the porous 

 soil. Moreover, all chemical processes go on more 

 rapidly and energetically at a high temperature 

 than at a low ; therefore, the warmer a soil is, the 

 more rapidly will weathering and decomposition 

 proceed in it. 



" Air, in union with the watery vapor always 

 present in it, affords oxygen, or water, to all bodies 

 which have a tendency to combine with these sub- 

 stances. The particles of iron (protoxide), which 

 scarcely any rock is without, make use of it, and 

 become converted into iron-rust, which does not 

 possess a fixing or binding power, like the protoxide 

 of iron, and therefore no longer offers any obstacle 

 to its disintegration. The fact that all rocks which 

 are traversed by a yellow vein of iron-rust may be 

 split or broken readily, demonstrates this clearly. 



