32 



THE PRINCIPLES OP AGRICULTURE 



are growing, he will readily be able to satisfy himself that roots 

 force open cracks and thereby split and sever the stone. Fig. 2 

 is an example, showing how a black cherry tree, gaining a 

 foothold in a crevice, has gradually forced the parts of the rock 



Fig. 3. Lichens have obtained a foothold. 



asunder. This particular example is the "half-way stone" be- 

 tween the Michigan Agricultural College and the city of Lansing. 

 Fig. 3 shows a stone upon which lichens have obtained a foot- 

 hold. Any person who has worked much in a garden will have 

 seen how roots often surround a bone, taking their food from its 

 surface and insinuating themselves into the cracks. Roots will 

 corrode or eat out the surface of marble. The grinding up of stones 

 is well illustrated on any lake shore, where the pebbles represent 

 what is left at the present time of the stones and fragments. The 

 rolling stones in brooks represent a similar action. 



30c. By chemical action is meant the change from which results 

 a new chemical combination. It produces a rearrangement of 

 molecules. For example, the change which takes place when, by 



