Pioneer Laborers '23 



which have fallen upon it from the cliffs above, dislodged 

 by the frost, we should be disposed to think it a very 

 rough laborer indeed, merely engaged in carting away the 

 wreckage made by others. But this would be a great mis- 

 take; for the glacier is a giant mill-stone, pressing upon the 

 rocks beneath with a power which is simply irresistible. 



Glaciers move on in solemn silence, it may be at the rate 

 of perhaps only an inch or two in the twenty-four hours, 

 but they go on steadily and noiselessly, and as they go, 

 they grind the rocks beneath to a powder so fine, that 

 when at last it escapes from the glacier-mill in the stream, 

 which flows out from beneath, it has been reduced to 

 nothing but mud. 



One other grinder, equally mighty and thorough, but 

 by no means silent, must be mentioned in conclusion. 

 This is the volcano, which, besides pouring forth streams 

 of lava, often buries the surrounding country many feet 

 deep in the finest dust and ashes, or in mud, if the erup- 

 tion be accompanied, as it often is, by rain. 



But the* laborers which we have been thus briefly con- 

 sidering are only pioneers. They accomplish only the 

 rougher work of preparation, and very much remains to 

 be done before anything that can properly be called **soil" 

 is ready for the crops. 



QUESTIONS FOR REVIEW 



1. Illustrate the fact that all plants require other food than 

 air and water. 



2. Describe the action of oxygen upon iron. 



3. What effect has water upon minerals? 



4. Show how carbon dioxide works upon the rocks. 



5. By what different methods does water break the rocks 

 apart? 



6. How are the rocks of the Sahara crumbled? 



7. By what agencies are the rocks ground and polished? 



