THE ORIGIN OF THE SOIL 3 



growing up between them. When this tree was young the 

 crack was small, but as the tree grew the roots spread the 

 crack wider and wider. Most of us have seen brick and ce- 

 ment sidewalks made uneven by the roots of trees growing 

 under them. 



Sometimes plants grow year after year in swampy places or 

 in shallow ponds. At the end of summer the plants die down 

 and fall to the ground or into the water. The next year a 

 new growth of plants comes up and dies down again in the 

 autumn. After this process has continued for many years 

 there is quite an accumulation of vegetable matter on the 

 ground or in the pond. This rots more or less and comes to 

 be a kind of soil. If the rotting takes place under water, in 

 which case it will not be very complete, peat is formed. Peat 

 is generally rather solid and shows the original shape of the 

 plants. It is on the road to form coal. But if the vegetable 

 matter is exposed to the air and is sometimes water-soaked, 

 and sometimes dry, it decays completely and forms muck. 

 Muck is generally soft and spongy and does not show any 

 trace of what it was formerly. Frequently muck has a good 

 deal of sand and clay mixed with it by water which washes 

 sand and clay particles over the vegetable matter. 



3. Water. — Water has a great deal to do with the making 

 of soil. Some rocks are porous and let a good deal of water 

 soak into them. When the water freezes it bursts the rock, 

 which gradually crumbles. Water will also dissolve some of 

 the material of which the rock is made and in this way 

 weakens its structure. 



Doubtless we have all seen the water in a shallow stream 

 rolling the gravel stones along on the bottom. As these peb- 

 bles roll along and strike against each other they knock off 



