280 THE EARTH'S SURFACE AND PLANT LIFE 



Now if we heat the water, thus decomposing the acid and driving 

 out the gas, the white substance again appears. 



Oxygen, carbon dioxide, and moisture are the chief weath- 

 ering agents of the atmosphere. Rocks which are exposed 

 to the atmosphere, especially in moist climates, undergo de- 

 composition. If the climate is warm and dry, rocks may 





ROCKS WEATHERING AND FORMING STEEP SLOPES 



stand for hundreds of years without apparent change, whereas 

 the same rock in another locality, where the weather condi- 

 tions are different, will crumble rapidly. A striking example 

 of this is found in the great stone obelisk, called Cleopatra's 

 Needle, which was brought from Egypt to Central Park, New 

 York, some time ago. Although it had stood for 3000 years 

 in Egypt without losing the distinctness of the carving upon 



