THE SURFACE OF THE EARTH 171 



5. Name and locate the principal mountain sys- 

 tems in the United States. 



6. What change takes place in the earth's crust 

 to form mountains? 



7. Which are older, the Rocky or Appalachian 

 Mountains ? How do they differ ? 



8. Name the principal industries in mountain 

 regions. 



9. What is a plateau? 



10. Of what economic importance are the plains of 

 the United States? 



11. What are the principal sources of dust ? 



12. If sand is blown by a strong wind, what are some 

 of the effects which it has on the earth 's surface ? 



13. How are sand dunes formed? Why are they 

 destructive to plant life? How can their migration be 

 prevented ? 



14. Explain how rocks are broken off by changes 

 in the temperature of the atmosphere. 



15. Where is this effect most evident? 



16. How is the earth's surface changed as a result 

 of the chemical action of the atmosphere? 



17. What is meant by erosion? 



18. Explain fully why erosion is of great 

 importance. 



