CHAPTER XIX 

 EARTHQUAKES; VOLCANOES 



245. Causes of Earthquakes. In the process of moun- 

 tain making, the thick, rigid layers of rock bend and break 

 here and there, and when a fracture occurs, there is a mighty 

 jarring of rocks, sometimes perceptible for many miles. The 

 earthquake motion on 

 the surface of the earth 

 is really a side to side 

 movement, as is shown 

 by the swinging of 

 chandeliers and of pic- 

 tures upon walls, and 

 even by the swaying 

 of chimneys. The sen- 

 sation which people 

 experience is one of 

 giddiness, such as is pro- 

 duced by unexpected 

 swaying of the body. 



It is the surface motion 



FIG. 119. ONE RESULT OF THE 

 SAN FRANCISCO EARTHQUAKE 



that is called an earth- 

 quake. 



The fracture below 

 may cause the broken 

 rock to separate and the 

 separation may extend 

 upward to the surface, or 

 the fracture may make the broken edges slip up or down. 

 In the latter case, the displacement is called a fault. 



225 



1. If this crack had been made under 

 a building, what would probably have 

 been the effect? 2. Measure the widest 

 part of the crack and compare it with the 

 length of a block. 3. An ordinary paving 

 block of this form is about 10 in. long; 

 what is the width of the crack? 



