CHAPTER IX 



THE MOVEMENTS AND DEFORMATIONS OF THE EARTH'S 

 BODY (DIASTROPHISM) 



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

 movements; some very rapid and some very slow, some very 

 slight and some very great, some limited to small areas, some 

 affecting extensive tracts, and some involving the whole earth- 

 body. For practical treatment, they fall mainly into two groups, 

 (1) the small and rapid, and (2) the great and slow. Sudden move- 

 ments of local masses, such as avalanches and landslides, are put 

 in the first class. Minute and slow movements, unless they rise 

 to importance by long continuance, are here neglected. 



MINUTE AND RAPID 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 

 detectable by delicate instruments. Some of them precede or 

 follow sensible vibrations, such as earthquakes, but more of them 

 have no connection with violent movements. Many spring from 

 the ordinary incidents of the surface; waves, waterfalls, winds, 

 tides, rainfall, the rise and fall of atmospheric pressure, the tivud 

 of animals, the rumble of traffic, the blasts of mines, and many 

 other processes. These are serviceable for demonstrating the 

 elastic nature of the crust, but are here neglected. 



Earthquakes l 



When tremors of appreciable violence spring from sources within 

 the earth, they are classed as earthquakes. The causes are various; 



1 Recent and instructive books on Earthquakes are Button's Earthquake*', 

 Hobbs's Earthquakes; An Introduction to Seismic Geology; Milne's Earth-^ 

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

 Earthquake Phenomena. 



338 



