70 DYNAMICAL GEOLOGY. 



that small steamers go up the river and into the spring, 

 and the cotton of the region is shipped at the Silver 

 Spring landing. For fifty to sixty miles around, there 

 are no surface-streams. The country-rock being here a 

 very soft and cavernous limestone, the rain-water is all 

 absorbed and finds its way by underground streams to the 

 surface at Silver Spring. This spring is also celebrated 

 as having probably the clearest water in the world. 



Artesian Wells. Ordinary wells are artificial seepage- 

 springs. Artesian wells are artifical great springs, i. e., 

 they are the tappings of underground streams which other- 

 wise would have reached the sea without coming to the 

 surface. In a level country, underlaid by regular strata 

 which turn up and outcrop on mountains or hills (Fig. 

 38), we are almost certain to reach artesian water. In 



FIG. 38. Artesian well. 



such cases, the pressure of water from the hills will cause 

 the water to rise, not only to the surface, but often to 

 spout as a fountain. Fig. 38 shows the conditions under 

 which this will probably occur. There are few places 

 where artesian water may not be reached by deep boring, 

 though the conditions of abundant supply are by no means 

 universal. The deepest artesian wells are those near 

 Leipzig, 5,735 feet deep ; near Pittsburg, Pa., 4,625 feet ; 

 at St. Louis, nearly 4,000 feet. Water from such great 

 depths is always warm. 



Thus, then, rain-water falling upon the land returns to 

 sea, whence it came, partly by surface drainage, partly by 



