THE WORK OF GROUND-WATER. 219 



ciently great pressure. The cracks and cavities affecting it are believed 

 to descend a distance which is but sUght in comparison \\ith the radius 

 of the earth. Even if openings were once formed at greater depths, 

 they could not persist, for the adjacent rock, under the pressure which 

 there exists, would ''flow" in, in effect (though perhaps not in principle) 

 much as stiff Hquid might, and close them. The outer zone of the 

 earth where ca^dties may exist is known as the zone of fracture} The 

 depth of the zone of fracture differs for different rocks, but is not be- 

 lieved to extend below some such depth as five or six miles, even for 

 the most resistant.' It is to be noted that these depths are less than 

 those at which the critical temperature of water would be reached 

 under most of the conditions, including all the more probable ones, 

 specified in the above table. 



Let it be assumed that water descends through openings in the 

 rock to a depth of six miles. At this depth it would, under the various 

 assumptions specified in the first and second columns of the following 

 table, have the temperature indicated in the third column: 



Initial Temperature. ^^'^T^i^^^^^^Z '' 



50° 1° for 50 feet 



50° 1° for 60 '' 



50° 1° for 70 '' 



80° 1° for 50 " 



80° 1° for 60 " 



80° 1° for 70 '* 



In two of these cases, namely, those in which the assumed rate of 

 increase of temperature is highest, the temperature of the water at the 

 assumed lower Hmit of the zone of fracture is above the critical tempera- 

 ture of water. If the assumptions involved in these two cases be cor- 

 rect, water might descend to the point where it would be converted 

 into water-gas, and in this condition it might be occluded by the hot 

 rock. In the other cases, involving the more probable assumptions, 

 the critical temperature is not reached at a depth of six miles. If 

 pores and cracks do not extend to greater depths, Hquid water could 

 not; and since the water at this depth has probably not reached its 

 critical temperature, it cannot exist as water-gas. If it does not exist 



iHoskins. 16th Ann. Rept., U. S. Geol. Surv., p. 853. 



'Van Hise. Principles of North American Pre-Cambrian Geclogy, 16th Ann. Rept., 

 U. S. Geol. Surv. 



Temperature at Depth 



of Six 



Miles. 



683° 



Fahr. 



578° 



a 



502° 



(t 



713° 



(I 



608° 



(C 



532° 



a 



