74 AQUEOUS AGENCIES. 



Section 4. — Chemical Agencies of Watee. 

 Subterranean Waters, Springs, etc. 



As we have already seen (page 9), of the rain which falls on any 

 hydrographical basin, a part runs from the surface, producing universal 

 erosion. A second part sinks into the earth, and, after a longer or 

 shorter subterranean course, comes up as springs, and unites with the 

 surface-water to form rivers ; while a third portion never comes up at 

 all, but continues by subterranean passages to the sea. This last por- 

 tion is removed from observation, and our knowledge concerning it is 

 very limited. But there are numerous facts which lead to the convic- 

 tion that it is often very considerable in amount. In many portions of 

 the sea near shore, springs, and even large rivers, of fresh water, are 

 known to well up. Thus, in the Mediterranean Sea, " a body of fresh 

 water fifty feet in diameter rises with such force as to cause a visible 

 convexity of the sea-surface."* Similar phenomena have been ob- 

 served in many other places in the same sea, and also in the Gulf of 

 Mexico near the coast of Florida, among the West India Isles, and near 

 the Sandwich Islands. Besides the last mentioned, there is still another 

 portion of subterranean water existing permanently in every part of 

 the earth far beneath the sea-level, filling fissures and saturating sedi- 

 ments to great depths, and only brought to the surface by volcanic 

 forces. This, in contradistinction from the constantly-circulating me- 

 teoric water, may be called volcanic water. 



Springs. — The appearance of subterranean waters upon the surface 

 constitutes springs. They occur in two principal positions, viz. : 1. 



Fig. 68.— Hill-side Spring. 



Upon Mil-sides, just where porous, water-bearing strata such as sand 

 outcrop, underlaid by impervious strata like clay ; 2. On. fissures pene- 

 trating the country rock to great depth. 



Most of the small springs occurring everywhere belong to the first 

 class. The figure (Fig. 68) represents a section of a hill composed 

 mostly of porous strata, b, but underlaid by impervious clay stratum, c. 

 Water falling upon the surface sinks through b until it comes in contact 



* Herschel's Physical Geography. 



