ON THE EARTH'S HISTORY 



harder ones are left sticking out. Gradually the surface 

 is carved out into heights and hollows. The harder 

 rocks become the hills and ridges; the softer rocks are 

 worn into valleys and plains. If there were no water 

 left on the earth's surface a great deal of this process 

 would still go on. In some respects it might become 

 more violent, for owing to the absence of moisture the 

 winds of the earth would always be laden with fine 

 particles ; and every one who has seen a " sand - blast " 

 at work, or even the modified sand-blast which is some- 

 times used for cleaning the stonework of some of our 

 cities, will appreciate what a tornado laden with sand 

 grains might do in the way of destroying the surface 

 of any rock on which it was playing. But, as a matter 

 of fact, the action of water in carving the surface of the 

 earth is the most important of all the factors we have at 

 present to consider. 



As rain falls from the clouds it absorbs the gases of the 

 air, including oxygen and carbonic acid. Now both these 

 are what we call corroding agents. If water is allowed to 

 fall on a steel knife the knife rusts; but it has been shown 

 by Dr. Gerald Moody, during the last few years, that if 

 there were no acid gas present, the rusting would not 

 take place. Oxygen and carbonic acid will rust other 

 things beside metal; they will rust stone. Moreover, 

 when the rain reaches the earth it absorbs any other 

 acids of the soil which rotting vegetation may afford, 

 and reinforced by these it goes on to attack the stones 

 over which it flows. When it rolls along as a brook or 

 a river it is no doubt attacking in this way the rocks 



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