296 EARTHQUAKES. 



admission of water through fissures to the heated rocks 

 beneath. A smaller number of earthquakes originate at 

 actual volcanoes. Some earthquakes are produced by the 

 su^dden fracture of rocky strata or the production of 

 faults. This may be attributable to stresses brought 

 about by elevatory pressure. Lastly, we have earth- 

 quakes due to the collapse of underground excavations. 



