WHERE THE ROCK PARTICLES COME FROM 



99 



expands ; and as it expands it exerts enormous power in 

 order to find room for itself. Go to a quarry and notice 

 the numberless cracks, joints, and fissures in the rocks. 

 These openings were formed mainly by freezing water 

 during the winter. Many rock masses are rent asunder 

 by this power. Usually, at the foot of a rocky ledge we 



How ROOTS HOLD THE SOIL. 



The soil has been removed from these roots to show how they penetrate 

 and prevent its washing away. 



may see large masses of fragments which are the results 

 of this kind of disintegration. 



In our science studies we have learned, no doubt, about 

 many other physical forces that break up the solid rocks. 

 We know that glaciers have acted as gigantic plows in 

 breaking loose and carrying away and crushing up enor- 

 mous quantities of rocks ; and we know, too, that erosion 

 is still working, more gently but unceasingly, to the same 

 end. 



