142 PRINCIPLES OF STRATIGRAPHY 



is so enormous as to close effectively all cavities and pores. This 

 is the limit of the zone of rock fracture, the zone below it being 

 that of rock flowage. This is, then, the depth to which ground 

 water can penetrate, though by far the largest quantity exists con- 

 siderably above this. More recently the experiments of Adams and 

 King have shown that pore spaces may exist at greater depths, even 

 down to eleven miles. (Kemp-22.) 



Slichter, assuming that the geologic limit of the existence of 

 ground water is at an average depth of six miles below the surface 

 of the land, and five miles below the floor of the sea, estimates the 

 entire amount of ground water to be about 565,000 million million 

 cubic yards, or about 430,000 million million cubic meters. This is 

 nearly one-third the amount of the water of the sea as a whole. Ac- 



FiG. 24. Diagrammatic section illustrating the position of the water table. 

 (After Slichter.) 



cording to these estimates the total amount of ground water is suffi- 

 cient to cover the entire surface of the earth to a uniform depth of 

 from 3,000 to 3,500 feet. (Slichter-37.) These estimates are 

 based on the selection of an average pore space for all rocks of 

 10 per cent., which Slichter himself regards as too large rather 

 than too small. 



It has, however, been shown by Kemp that, according to the ex- 

 perience in deep mines, ground water of meteoric origin is limited 

 to the upper 1,000 feet of the earth's crust. This makes the total 

 amount of ground water much less than that given by the estimates 

 above cited. Kemp concludes that this ground water is sufficient 

 to cover the earth's surface with a layer of water from only 50 to 

 100 feet deep. Fuller subsequently made more close calculations, 

 concluding therefrom that the layer of water would be 96 feet deep. 

 (Fuller-14: jp-Zi".) See the preliminary discussion on pp. 4 and 5 

 and further under thermal springs, their possible relation to mag- 

 matic waters, etc. (pp. 200-203). Finally movements of ground 

 water, pp. 257-261. 



