AQUEOUS AGENCIES. G9 



surface nojiv the bottom or in the valleys and issue as 

 springs (.Fig. 35). 



Spring's. Springs are the issuance of underground 

 waters, and wells are artificial springs. Often water is 

 observed to ooze out on hillsides or at hill-bottoms, mak- 

 ing a marshy spot. In such cases, if we examine, we shall 

 usually find a reason for it in the fact that a water-bear- 

 ing stratum of sand or gravel, underlaid by an impervious 

 stratum of clay, outcrops at this point. Water falling on 

 the hill sinks down until it reaches the impervious clay, 

 and then flows out laterally (Fig. 36). These may be 

 called seepage-springs. 



Again : in other places, especially mountain-regions, 

 we find strong or bold springs. Usually, in such cases, we 



PIG. 36. Hillside spring. 6, sand ; c, clay ; a, spring. 



may find a fissure through which the water comes up to 

 the surface (Fig. 37). 



In still other places, but only in countries where rocks 

 of cavernous structure, such as limestone and lava, prevail, 



FIG. 37. -Fissure spring. 



we sometimes find great springs, from which issue rivers 

 of considerable size. Perhaps the most remarkable ex- 

 ample is the "Silver Spring" of Florida. The river 

 which flows from this celebrated spring is so considerable 



