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 



'5^^- 





Fig. 3. Lichens have ol)taiiiecl 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. Koots will 

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

 is well illustrated on any h-dce sliore, where tlie pebbles represent 

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

 rolling stones in brooks repi'esrnt a similar action. 



3u<'. By chemical action is meant tlie change from which results 

 a new chemical combination. It produces a rearrangement of 

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



