218 



GEOLOGY, 



Initial Temperature. 

 50° 



50° 



50° 



80° 



80° 



80° 



Rate of Increase of 

 Temperature. 



.° for 50 feet 



for 60 " 



for 70 " 



for 50 " 



L° for 60 " 

 for 70 " 



Temperature at a Depth 

 of 6,800 Feet. 



186° 



Fahr. 



163° 





147° 





216° 





193° 





177° 





Only one of these temperatures reaches the boiling-point of water 

 at sea-level. It is therefore clear that at this depth water has not even 

 closely approached the boiling temperature for this depth, and since 

 this is the depth of the critical pressure, it is clear that it cannot boil 

 at any greater depth. The descent of water is therefore not stopped, 

 under normal conditions of crust al temperature, because it reaches 

 its boiling-point. Locally, as in the vicinity of active or recently ex- 

 tinct volcanoes, the case may be different. 



It is conceivable that water may descend until it reaches its critical 

 temperature (somewhere between 610° and 635° Fahr.). The depth 

 at which the critical temperature would be reached, under various 

 assumptions, is shown in the following table : 



Initial Temperature. 



50° 



50° 



50° 



80° 1° for 50 



80° 1° for 60 



80° 1° for 70 



Rate of Increase of Temperature. 



1° for 50 feet 



1° for 60 



1° for 70 



Depth of Critical Temperature. 

 28,000 to 29,250 feet 

 33,600 to 35,100 

 39,200 to 40,950 

 26,500 to 27,750 

 31,800 to 33,300 

 37,100 to 38,850 



There is good reason, in the increasing density beneath the sur- 

 face, for beheving that the rate of increase of temperature decreases 

 with depth, and therefore that the rate of 1° for 50 feet for the dep'hs 

 concerned is too high. The greater depths of the table above are there- 

 fore believed to more nearly represent the truth than the lesser ones. 

 (See discussion of underground temperature in Chapter XI.) 



If descending water attained its critical temperature, the extent 

 to which the resulting water-gas might be absorbed is not known. So 

 for as limited hy temperature, therefore, it is not possible to assign a 

 limit to the descent of water under average conditions of crustal tem- 

 perature. 



Other considerations seem to place a limit to the descent of water. 

 Rock, soHd and unyielding as it seems, is yet plastic when under suffi- 



