CHAPTER XVII 



WITHIN THE EAKTH'S OKUST 



Beneath the Earth's Surface. Many excavations and 

 borings have been made deep into the earth's crust and it 

 has been found that the temperature increases with the 

 depth. The rate of increase is not the same in different 

 places, nor is the increase always uniform in the same 



SAN MIGUEL HARBOR IN THE AZORES 

 Notice the volcanic cone in the distance. 



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 

 solid part of the earth could not be more than 40 or 50 miles 



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