THE PHYSICS OF THE EARTH. 



193 



except ivhen set in motion by the dreadful paroxysms of an earth- 

 quake. 



In tidal and other observations the earth therefore behaves as a 

 soHd, and the rigidity of the earth inferred by Kelvin and Darwin 

 is confirmed. Yet a layer of plastic matter or quasi-viscous fluid 

 exists just beneath the crust, and when disturbed by earthquakes 

 gives rise to the development of ridges in the crust called moun- 

 tains, chiefly by the expulsion of lava from under the sea. 



Fig. 2. Curve of Rigidity for the Earth, showing the plastic layer 

 just beneath the crust. 



§ i8. Wiechert's Researches on the Interior Constitution of the 

 Earth and on the Plastic or Viscous Layer ivhich he Infers to Exist 

 Just Beneath the Crust from Oscillations of Long Period Noticed 

 in Seismic Vibrations. — Professor E. Wiechert, of Gottingen, has 

 devoted much attention to the problem of the constitution of the 

 earth's interior. He long ago reached the conclusion that the great 

 interior nucleus probably is a mass of iron covered with a thick 



