220 GEOLOGY. 



in the form of water-gas, its occlusion by the hot rock substance 

 would not be probable. It would seem, therefore, that the descent of 

 water under ordinary conditions is much more Hkely to be Hmited 

 by the zone of fracture, than by temperature. 



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

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

 soon fills up again to its normal level by inflow from all sides. Springs 

 and flowing wells also demonstrate the movement of ground-water. 

 Near the surface the movement of ground-water is primarily down- 

 ward if the medium through which it passes is equally permeable in 

 all directions; but so soon as the descending water reaches the water 

 surface, its descent is checked and its movement is partly lateral. 



The commonest sort of movement of ground-water is that exem- 

 plified as the water sinks beneath the surface, namely, slow percola- 

 tion through the pores and cracks of the soil and rock. Ground-water 

 is not generally organized into definite streams, though underground 

 streams, mostly small, are sometimes seen in caves and cre^dces, and 

 sometimes issue as springs. Most underground streams which issue as 

 springs probably have definite channels for short distances only before 

 they issue. It is probable that ground-water frequently flows in con- 

 siderable quantity along somewhat definite planes, without having 

 open channels. Thus every porous bed of rock is likely to serve as 

 the pathway along which subterranean drainage passes. This is espe- 

 cially true where the porous bed is underlain by an impervious one. 

 The ^^reservoirs'' from which artesian wells draw their supply are not 

 usually streams or lakes, but porous beds of rock through which abun- 

 dant water passes. 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 notably influenced by the porosity of the rock, and since the 

 rock is, on the average, most porous and the pores largest near the sur- 

 face, the movement of ground-water is, on the average, greatest near 

 the surface, and least at its lower limit. In general the decrease of 

 movement is much more rapid than the decrease in the size of the pores. 

 It follows that while the upper part of the ground-water, especially that 

 above ground-water level, moves somewhat freely, the lower part moves 



* For a full discussion of this subject see King, 19th Ann. Rept., U. S. Geol. Surv., 

 Pt. II, and Slichter, Water Supply and Irrigation, Paper No. 67, U. S. Geol. Surv. 



