CRUSTAL MOVEMENTS 191 



of such movements. The bold fronts of some of these mountains 

 are fault-scarps (Fig. 189). 



EIERRA 

 NEVADA 



Fig. 189. A type of mountain structure seen in the ranges of the Great 

 Basin. The dotted lines at the top indicate the part removed by erosion. 

 Length of the section, 120 miles. (Gilbert.) 



There is sometimes horizontal as well as vertical movement 

 along a fault plane. This is the case in the fault which caused 

 the earthquake of California, in the spring of 1906. 



Earthquakes 



Definition. Earthquakes are tremblings or quakings of the 

 earth's surface, due to causes with which man has nothing to do. 

 The passing of a railway-train causes the surface of the ground to 

 vibrate, and this vibration is often felt in buildings near the track. 

 Such vibrations are not called earthquakes; but an equal amount 

 of quaking, due to happenings beneath the surface not caused by 

 man, would be called an earthquake. 



Strength and destructiveness. Earthquakes vary much in 

 strength. Some are so gentle that they are barely felt, while others 

 are so violent that buildings are overthrown, crevasses opened in 

 the surface of the land, and masses of rock loosened from cliffs. 

 Others, sometimes called earth tremors, are so slight that they are 

 known only by means of delicate instruments set up for the purpose 

 of recording all vibrations of the surface, however slight. The 

 number of tremors too slight to be felt is far greater than the num- 

 ber of strong earthquakes. 



Although violent earthquakes are in some cases destructive 

 to buildings and to life, the actual movement of the land surface 

 is usually very little. It is commonly measured in millimeters (a 

 millimeter is about -fa of an inch) rather than in inches or feet. 

 Bodies on the surface often move much more than the ground does. 

 The relation between the two is illustrated by the fact that a blow 

 on the floor may cause a hard ball (glass or ivory) which rests upon 



