THE WORK OF THE ATMOSPHERE 105 



makes possible the work of streams and of glaciers, and the 

 existence of land life. 



QUESTIONS 



1. On which side of Lake Michigan should dunes be best de- 

 veloped ? Why ? 



2. Why are the most extensive dune areas of the Sahara in its 

 western portion ? 



3. On which side of an east and west mountain range in the 

 northern hemisphere should rock splitting due to temperature 

 changes be most important ? Why ? 



4. Compare and contrast the importance of rock splitting by 

 temperature changes at Chicago and Denver, making clear the 

 reasons for the differences. 



5. Describe the sequence of events recorded by Figure 99. 



FIG. 99. View on South Manitou Island, Lake Michigan. (Russell, 

 U.S. Geol. Surv.) 



6. Why would it be of value to coat with tar the bottoms of 

 telegraph poles in arid regions ? Why coat only the bottoms ? 



7. How would a considerable increase in moisture change the 

 geological processes in operation in the Sahara ? 



8. How might depressions made by the wind be distinguished 

 from depressions made by rivers ? 



9. Why are some wind-swept rock surfaces smooth, while others 

 are minutely pitted ? 



