CHAPTER XLII 

 THE EARTH AND ITS NEIGHBORS 



281. The Earth is Very Old. During the past hun- 

 dred years men have studied the rock formations of the 

 earth, the causes of volcanoes, mountain-making, and the 

 rapidity with which the mountains are worn away by 

 the process of erosion. The facts which have been thus 

 collected show that the earth is many millions of years 

 old. Mountains have been made by a gradual upheaval 

 of the earth's surface, caused by expansion due to heat. 

 Mountains have also been made by the flow of lava 

 from volcanoes and cracks in the earth or sides of moun- 

 tains. 



The Appalachian Mountains in the eastern part of the 

 United States are very old and were once much higher than 

 they are at present. The Potomac, Susquehanna, and 

 Juniata rivers seem to have cut across the mountain ridges 

 as fast as the ridges were elevated; this is shown by the 

 gaps in the mountains through which these streams flow. 

 These gaps make the construction of roads and railroads 

 very easy. They save the trouble of going through the 

 mountains or over them. 



The Rocky Mountains, although about three million 

 years old, are young when compared to the Appalachians. 

 The Rockies are still in the process of formation. Earth- 

 quakes are frequent along the western coast and the vol- 

 cano in northern California which has recently been active 



