GROUND-WATER 199 



If descending water reached its critical temperature by going 

 down to these great depths, the extent to which the resulting 

 water-gas might be absorbed into the interior of the earth is not 

 known. So far as limited by temperature, therefore, it is not possible 

 to assign a limit to the descent of water under average conditions 

 of crustal temperature. 



But there is reason to think that water does not go down to 

 the depths necessary for the critical temperature. Rock, solid 

 and unyielding as it seems, is yet mobile under sufficiently great 

 pressure. Cracks and cavities are believed to affect it to depths 

 which are slight in comparison with the radius of the earth. If 

 openings were formed at sufficient depths below the surface they 

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

 there exits, would "flow" in and close them. The flow is, in effect 

 (though not in principle) much like the flow of a stiff liquid. The 

 outer zone of the earth where cracks and cavities may exist has 

 been called the zone of fracture, 1 and the thickness of the zone of 

 fracture is not believed to exceed six miles, even for the most 

 resistant rock. 2 This depth is probably much less than that at 

 which the critical temperature of water would be reached, as indi- 

 cated in the last table. 



If water descends through openings in the rock to a depth of 

 six miles, it will, under the assumptions specified in the first and 

 second columns of the following table, have the temperature indi- 

 cated in the third column: 



Initial Tpmnpratiirp ^ ate * Increase Temperature at 



of Temperature Depth of Six Miles 



50 .................................... 1 for 50 feet 683 Fahr. 



50 .................................... 1 for 75 " 472 " 



50 .................................... 1 for 100 " 367 " 



In the first of these cases, the temperature of the water at the 

 assumed lower limit of the zone of fracture, is above the critical 

 temperature of water. If the assumptions involved in this case 

 are correct, water might descend to the point where it would 

 be converted into water-gas, and in this condition it might be 



1 Hoskins. 16th Ann. Kept., U. S. Geol. Surv., p. 853. 



2 Van Hise. Principles of North American Pre-Cambrian Geology. 16th 

 Ann. Kept., U. S. Geol. Surv. 



