THE NEW AGRICULTURE. 93 



" Now you will readily see how it is that rain does so much in 

 the destruction of rocks. It not only dissolves out some parts of 

 them, and leaves a crumbling crust on the surface, but it washes 

 away this crust, and thereby exposes a fresh surface to decay. 

 There is in this way a continual pushing along of powdered stone 

 over the earth's surface. Part of this material accumulates in hol- 

 lows, and on sloping or level ground ; part is swept into the rivers 

 and carried away into the sea. As the mouldering of the surface 

 of the land is always going on, there is a constant formation of 

 soil. Indeed, if this were not the case, if after a layer of soil had 

 been formed upon the ground, it were to remain there unmoved 

 and unrenewed, the plants would by degrees take out of it all. the 

 earthy materials they could, and leave it in a barren or exhausted 

 state. But some of it is being slowly carried away by rain, fresh 

 particles from mouldering rocks are being washed over it by the 

 same agent, while the rock or subsoil underneath is all the while 

 decaying into soil. The loose stones, too, are continually crum- 

 bling down and making new earth. And thus, day by day, the 

 soil is slowly renewed. 



" Plants, also, help to form and renew the soil. They send their 

 roots among the grains and joints of the stones, and loosen theai 

 Their decaying fibers supply most of the carbonic acid by which 

 these stones are attacked, and furnish also most of the organic 

 matter in the soil. Even the common worms, which you see when 

 you dig up a spadeful of earth, are of great service in mixing the 

 soil and bringing what lies underneath up to the surface. 



" One part of the rain sinks under the ground, and you have 

 traced its progress there until it comes to the surface again. You 

 have now to trace, in a similar way, the other portion of the rain- 

 fall which flows along the surface in brooks and rivers. 



" You cannot readily meet with a better illustration of this sub- 



