168 A YEAR IN SCIENCE 



change of temperature causes rocks to scale off. In 

 mountainous regions great piles of this broken off 

 debris collect at the base of mountains and cliffs. Such 

 deposits are called talus. The total effect of this shell- 



Permission of U. 8. Geological Survey. 



FIG. 64. Great piles of debris, called talus, have collected at the 

 base of these mountain peaks in Colorado. 



ing off of exposed rock surfaces through the ages has 

 been very great. If you examine the surface of a 

 boulder or rock, you can notice this shelling off effect. 

 Chemical. The oxygen of the air is its most active 

 chemical element. If a piece of iron is exposed to 

 the air in the presence of moisture, it rusts. The 

 oxygen of the air unites with the iron and forms a 

 red rust; in time the whole of the iron is converted 

 into the oxide. Oxygen attacks many rocks in a simi- 

 lar manner. As the oxidizing process is continually 

 going on, the total effect is very considerable. Other 



