198 GEOLOGY 



would be reached, supposing the upper rock to be full of water, is 

 about 6,800 feet. The temperature of the water at this depth, 

 under various assumptions, is shown in the following table: 



Initial Temperature 



6,800 feet 



50 .............................. - .............. 1 for 50 feet 186 Fahr. 



50 ....................................... 1 for 75 " 141 " 



50 ...................................... 1 for 100 " 118 " 



None of these temperatures is so high as 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 cannot 

 boil at any greater depth, where the pressure would be greater. 

 The descent of water is therefore not stopped, under normal condi- 

 tions of crustal temperature, because it reaches its boiling-point. 

 Locally, as in the vicinity. of active or recently extinct volcanoes, 

 the case may be different. 



It is conceivable that water may descend until it reaches il 

 critical temperature, that is, the temperature which, if increj 

 will cause the water to become water-gas in spite of pressure. Tl 

 critical temperature is somewhere between 610 and 635. Tl 

 depth at which the critical temperature would be reached, unc 

 various assumptions, is shown in the following table: 



There is good reason, in the increasing density beneath 

 surface, and on other grounds as well, for believing that the rate of 

 increase of temperature decreases with depth, and therefore that 

 the rate of 1 for 50 feet for the depths concerned is far too high. 

 The greater depths of the table above are therefore believed to 

 more nearly represent the truth than the lesser ones. If tlii 

 correct, the depth at which the critical temperature would be reached 

 is probably not less than 50,000 feet. 



