2 AQRTCULTURE FOR COMMON SCHOOLS 



weathering of rocks has been the source of most of what we 

 call soil. This process is still going on. The small pieces of 

 gravel in the soil (in our fields) are being made smaller each 

 year by the wear and tear of the winds and rain and by the 

 freezing and thawing of winter. Wherever a gravel pit or stone 

 quarry is opened many boulders and pieces of rock are thrown 

 aside. The next spring after these have been exposed many 

 rocks will be seen to be crumbling, while others do not seem 

 to be affected. Some rocks are more easily broken up than 

 others, but all will yield in time. 



2. Plants. — Growing plants have had much to do in mak- 

 ing soil and are still exerting an influence on the breaking 

 down of rocks. We have all seen moss and lichens growing 

 upon large boulders. In some cases there are two or three 

 inches of material on the boulder. These plants are attacking 

 the rock by means of the acids excreted from their root-like 

 parts. The carbonic acid produced by the decay of the 

 dead parts also tends to break up the rock. Sometimes 

 the amount of material on the boulder is deep enough for 

 large plants to grow in it. Then the decomposing action is 

 all the greater, because of the greater activity of the real 

 roots. Of course, all the action is very small for any one 

 year, but when carried on for many years it amounts to a 

 great deal. The material on the boulder is real soil, but 

 in most instances it has been derived from the rock and the 

 decayed vegetation. 



We must not forget also that if there is ever so small a crack 

 in the rock the tiny roots of plants will work their way into h 

 and make it larger and so let in the water which we shall see 

 helps to tear the rock apart. The writer has seen a large 

 boulder weighing several tons split in two pieces and a tree 



