I 



CHAPTER VIII 



THE MOVEMENTS AND DEFORMATIONS OF THE EARTH'S 

 BODY (DIASTROPHISM) 



The outer parts of the lithosphere are subject to a variety of 

 movements, some rapid and some slow, some slight and some great, 

 some limited to small areas, some affecting extensive tracts, and 

 some involving the whole earth. For present purposes, they may 

 be classed as (i) small and rapid, and (2) great and slow. Sudden 

 movements of local masses, such as avalanches and landslides, are 

 put in the first class. 



MINUTE AND RAPID (SEISMIC x ) MOVEMENTS 



The crust of the earth is in a state of perpetual tremor. For 

 the most part, these tremors are too slight to be sensible, though 

 detected by delicate instruments. Some of them precede or follow 

 earthquake vibrations, but more of them have no connection with 

 violent movements. Many spring from the ordinary incidents of 

 the surface, such as waves, waterfalls, winds, tides, the tread of 

 animals, the rumble of traffic, and the blasting in mines. Move- 

 ments due to such causes demonstrate the elastic nature of the crust, 

 but are not considered here. 



Earthquakes 2 



Earthquakes are tremors of appreciable violence springing from 

 sources within the earth. The causes are various. The most 

 common is probably the slipping of rock masses on each other in 

 the process of faulting (Chapter X). To the same class belong 

 movements due to slumping, which is superficial faulting. Tremors 



1 The science of earthquakes is Seismology. Earthquakes and other similar 

 movements are seismic movements. The instruments which record seismic move- 

 ments are seismographs', etc. 



2 Recent and instructive books on Earthquakes are Button's Earthquakes; 

 Hobbs's Earthquakes. An Introduction to Seismic Geology; Milne's Earthquakes 

 (4th ed.), and the same author's Seismology; and Knott's Physics of Earthquake 

 Phenomena. 



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