164 A YEAR IN SCIENCE 



large part of the agricultural lands. They differ 

 greatly in height, origin, fertility, position, and shape 

 of surface. Plateaus and plains, like mountains, fre- 

 quently have their origin in the folding of the earth's 

 crust. 



Minor land forms. Hills, cliffs, mesas, buttes, ridges, 

 flats, and other minor land forms constitute the third 

 group of relief features. They, unlike the other 

 groups, are caused by erosion and weathering brought 

 about by the atmosphere and water. 



Effect of the atmosphere on the earth's surface. The 

 atmosphere has both a mechanical and a chemical 

 effect upon the earth's surface. 



Mechanical. Small particles can be blown about 

 from place to place by the wind. This is a mechanical 

 change. Dust is the easiest material for the wind to 

 move. The sources of dust are highways, fields, 

 streets, volcanoes, meteoric smoke, pollen, and spores 

 of plants. During some volcanic eruptions much dust 

 is thrown into the air and carried long distances by 

 the wind. Meteors or "shooting stars" when they 

 come into the atmosphere from outer space are prob- 

 ably traveling eighteen or twenty miles per second. 

 The friction of the atmosphere is sufficient to heat 

 them to such a temperature that they are consumed. 

 The dust resulting settles to the earth. It is said that 

 the bottom of the deep sea is covered to a considerable 

 depth with this meteoric dust free from other accumu- 

 lations. Loess beds are deposits of dust which occur 



