14 PHYSICAL GEOLOGY 



The air when in motion performs mechanical work of great 

 importance, especially in dry regions, transporting dust and 

 sand, often for great distances, and wearing exposed rock 

 surfaces (pp. 86-91). Wind-formed waves bring about 

 important changes along ocean coasts and lake shores. The 



FIG. 1. Diagram showing the general relations of the lithosphere, hydro- 

 sphere, and lower atmosphere. 



atmosphere also acts as a blanket, protecting the rest of the 

 earth from the fierce heat of the sun and preventing it from 

 cooling off rapidly by radiation of heat. Winds distribute heat 

 and tend to equalize temperatures. 



Although the atmosphere is known to extend more than one 

 hundred miles above sea level and probably continues very 

 much higher, yet three fourths of the air lies below the tops of 

 the highest mountains, and its geological activity is confined 

 largely to its bottom portion, where it is in contact with the 

 land and the water. 



