I 



MOVEMENT OF GROUND-WATER 33 



and it doubtless goes down as far as they do. But it is probable 

 that cracks do not go down more than a few miles, and that pores 

 are limited to similar depths. The reason for this is that rock, solid 

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

 pressure. If cracks or openings were formed in it at great depths, 

 it is calculated that they could not persist, for the rock, under the 

 pressure which exists there, would "flow" in and close them. The 

 flow is, in effect, much like the flow of a stiff liquid. The outer zone 

 of the earth, where cracks and cavities may persist, is the zone of 

 fracture, and it is probable that the descent of water under ordinary 

 Conditions, is limited to this zone, variously estimated to have a depth 

 of six to eleven miles. 1 



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. The "reservoirs" from which artesian wells draw their 

 supply are porous beds of rock, containing abundant water. As 

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

 elsewhere. Since the freedom of movement of ground-water is 

 influenced greatly by the porosity of the rock, and since the rock is, 

 on the average, most porous near the surface, the movement of 

 ground-water is gr.eatest near the surface, and less and less with 

 increasing depth. Movement in the lower part of the subterranean 

 hydrosphere doubtless is extremely slow. 



Amount of ground-water. The porosity of surface rocks varies 



1 Some recent experiments suggest that, at high temperatures and under great 

 pressures, water may enter into combination with rock material, with contraction 

 of volume. If so, water in combination (not free) may perhaps go below the zone 

 of fracture. Barus. Bull 92, U. S. Geol. Surv. 



2 For a full discussion of this subject see Kin^, ioth Ann. Rept., U. S. Geol. 

 Surv., Pt. II, and Slichter, Water Supply and Irrigation Paper 67, U. S. Geol. Surv. 



