THE NEW AGRICULTURE. 89 



clear and limpid. In countries where chalk or limestone is an 

 abundant rock, this action of water is sometimes singularly 

 shown in the way in which the surface of the ground is worn 

 into hollows. In such districts, too, the springs are always hard; . 

 that is. they contain much mineral matter in solution, whereas 

 rainwater and springs which contain little impurity are termed 

 soft 



" When a stone building has stood for a few hundred years, the 

 smoothly-dressed face which its walls received from the mason is 

 usually gone. Again, in the burying-ground surrounding a ven- 

 erable church you see the tombstones more and more mouldered 

 the older they are. This crumbling away of hard stone with the 

 lapse of time is a common familiar fact to you. But have you ever 

 wondered why it should be so ? What makes the stone decay, and 

 what purpose is served by the process ? 



" If it seem strange to you to be told that the surface of the 

 earth is crumbling away, you should take every opportunity of 

 verifying the statement. Examine your own district. You will 

 find proofs that, in spite of their apparent steadfastness, even the 

 hardest stones are really crumbling down. In short, Wherever 

 rocks are exposed to the air they are liable to decay. Now let us 

 see how this change is brought about. 



" First of all we must return for a moment to the action of car- 

 bonic acid, which has been already described. You remember 

 that rainwater abstracts a little carbonic acid from the air, and 

 that, when it sinks under the earth, it is enabled by means of the 

 acid to eat away some parts of the rocks beneath. The same action 

 takes place with the rain, which rests upon or flows over the sur- 

 face of the ground. The rainwater dissolves out little by little 

 such portions of the rocks as it can remove. In the case of some 

 rocks, such as limestone, the whole or almost the whole, of the 



