200 GEOLOGY 



occluded by the hot rock. In the other cases, which involve the 

 more probable assumptions, the critical temperature is not ap- 

 proached closely at a depth of six miles. If pores and cracks do 

 not extend to greater depths, liquid water could not; and since 

 the water at this depth has probably not reached its critical temper- 

 ature, it must still be in the form of liquid. It would seem, there- 

 fore, that the descent of water under ordinary conditions, is limited 

 by the zone of fracture, rather than by temperature. 



Some recent experiments suggest that, at high temperatures 

 and under great pressures, water may enter into combination with 

 rock material, with contraction of volume. 1 If this is correct, 

 water (in some form of combination) may perhaps go down below 

 the zone of fracture. 



Movement of ground-water. 2 Ground-water is in more or less 

 continual movement. If all the water is pumped out of a well, it 

 soon fills up again by inflow from the sides. Springs and flowing 

 wells also demonstrate the movement of ground-water. Near 

 the surface the movement is primarily downward if the rock 

 through which it passes is equally permeable in all directions; but 

 so soon as the descending water reaches the water-surface, its 

 downward flow is checked, and its movement is partly lateral. 



Ground-water moves chiefly by slow percolation, for most of it 

 is not organized into definite streams. Small streams are seen 

 in some caves, and subterranean streams issue as springs in some 

 places; but most streams which issue as springs probably have 

 definite channels for short distances only, before they appear at the 

 surface. It is probable that ground-water frequently flows in 

 considerable quantity along somewhat definite planes, without 

 having open channels. This is the case especially along the sun 

 of an impervious layer overlain by a porous one. The "reser- 

 voirs" from which artesian wells draw their supply are not usually 

 streams or lakes, but porous beds of rock, containing abundant 

 water. As the supply is drawn off at one point, it is renewed by 



p 



1 Barus. Bull. 92, U. S. Geol. Surv. 



2 For a full discussion of this subject see Kin-;, l!)tli Ann. Kept., IT. S. 

 Geol. Surv., Pt. II, and Slichter, Water Supply and Irrigation Paper No. r>7. 

 U. S. Geol. Surv. 



