68 DYNAMICAL GEOLOGY. 



versal rain-erosion ; another part sinks into the earth, and 

 after a longer or shorter subterranean course, comes up 

 again as springs, and joins the surface-waters to form the 

 streams. Still a third part does not come up on the land- 

 surface at all, but by subterranean passages finds its way 

 to the sea. In some countries this third part is large. 

 This is especially so if the country-rock be limestone or 

 lava, for these rocks are affected with subterranean gal- 

 leries and caves. In Florida, for example, rivers often 

 disappear, ingulfed in the earth, and continue to the sea 

 by subterranean passages. In shallow seas off such coasts 

 places are known where fresh water comes up in large 

 quantities, and ships may be supplied with drinking-water. 

 Such submarine springs are known off the coast of Florida, 

 the West Indies, the Hawaiian Isles, and the shores of 

 the Mediterranean. There is still another part of sub- 

 terranean water which, perhaps, is not rain-water, or, if 

 so, is not now circulating like the others. As far down 

 as the earth has been penetrated, perhaps below even the 

 sea-bed, there is found rock-water, but not in flowing 

 streams. This may be rightly called volcanic water, as it 

 is probably concerned in volcanic eruptions. 



Perpetual Ground-Water. In passing downward 

 from the surface we find the ground contains more and 

 more water, until it becomes saturated and the water mov- 

 able. The highest level at which the water is always 

 movable is called the level of perpetual ground-water. It 



FIG. &5. -Diagram of perpetual ground-water. S, surface of ground ; WW, ground- 

 water ; sp, ./>, hillside springs. 



is deepest on the hilltops, comes nearer the surface on 

 the slopes, and under favorable conditions may reach the 



