MOVEMENTS OF GROUND WATER 257 



Comparing this with the sea sands, it is at once apparent that 

 the rounding is much more pronounced, though the sands were car- 

 ried only a few miles. These sands have a common origin and are 

 of about the same degree of fineness. 



Movement of Underground Waters. 



The movement of underground water depends on the nature of 

 the material through which it flows and the space available for this 

 flow. Underground channels or tunnels may allow the water to 

 flow as readily as it does upon the surface. If the channel is filled 

 completely, the water is under hydrostatic pressure, and it may be 

 forced up a rising slope. Such a stream may act as an aspirator, 

 drawing w^ater or air through fissures past which it flows, and caus- 

 ing currents to flow toward it by suction, and even causing water 

 to rise from a lower to a higher level. This has been regarded by 

 Penck and others as the cause of the currents flowing from the 

 ocean into the land at the island of Cephalonia, Greece, the water 

 replacing that which is drawn up by suction of a strong under- 

 ground stream, which completely fills its channel. For these cur- 

 rents, which formerly drove the "sea mills of Cephalonia," Crosby 

 and Crosby (14) and Fuller (25) have ofifered other explanations. 

 Underground streams of this kind are further illustrated by some 

 subglacial streams, which, being confined in a tunnel-like tube, often 

 flow uphill, as shown by the eskers which mark their former course. 

 (Grabau-28.) 



Water flowing through a porous soil has of course a vastly lower 

 rate of motion under the same gravitational influence on account 

 of friction. The rate of movement of water through such a porous 

 medium depends on the following factors : 



1. Size of pores in the water-bearing medium. 



2. Porosity of material, i. e., the relative abundance of pores. 



3. Pressure gradient, or change in pressure or head, per unit of 

 length, measured in the direction of the motion. 



4. Temperature of water. 



In general the rate of flow varies with the variation in all of 

 these factors, the law governing this rate of flow or velocity {v) 



P 

 being expressed by the formula of Darcy, v = k j, where p is the 



difference in pressure at the ends of the column of soil (measured 

 by height of water column), /; the length of the column and k a 

 constant depending upon the determinable characters of the soil. 



