CHAPTER XIV 



VOLCANOES 



209. Subsurface Earth. Many excavations and borings 

 have been made deep into the earth's surface, and it has 

 been found that the temperature increases with the depth. 

 The rate of increase is not the same in different places, nor 



MOUNT SHASTA. 

 An extinct volcano. 



is the increase always uniform in the same place. The 

 average of a number of deep excavations in different parts 

 of the earth gives a rise of 1 F. for each 70 or 80 feet of 

 descent. 



The greater the pressure to which rocks are subjected the 

 more difficult it is to melt them. If it were not for this, the 



440 



