SUBTERRANEAN WATERS. 653 



ing subjects of study. But it is not necessary to enter into details respecting them in 

 this place, as they illustrate no new principles. 



As the forms and stratification of delta-deposits depend partly upon wave-action, this 

 subject comes up again, under the head of The Ocean. 



B. SUBTERRANEAN WATERS. 



It is an obvious fact that a considerable part of the water which 

 reaches the earth's surface descends into the soil, and becomes in a 

 sense subterranean. There are also subterranean streams, which have 

 their rise in hills and mountains, and are fed, like ordinary rivers, 

 by the rains and snows, and especially those that fall about elevated 

 regions. These waters become under-ground streams, by following the 

 dip of tilted strata, or by infiltrating through pervious or loosely ag- 

 gregated deposits ; and they flow over some impervious layer. The 

 layers of stratified rocks are often so porous that water easily perco- 

 lates through them, down to a stratum that will hold it ; and seldom 

 fit so closely, together that it cannot find its way between them. 



Again, there is a small percentage of moisture in all or nearly all 

 strata, which is mechanically inert, that may properly be included 

 under the head of subterranean waters. 



1. Subterranean Streams. 



The large size of some of the under-ground rivers is proved by 

 direct observation in caverns, where they have the variety of cascades 

 and quiet waters which characterizes the streams of the surface. The 

 Mammoth Cave, of Kentucky, and the Adelsberg, twenty-two miles 

 northeast of Trieste, in Austria, are examples. And again, as in the 

 Appalachians, and the Jura Mountains, they sometimes come out of 

 the hills with sufficient force and volume to turn the wheel of a large 

 mill. AH wells and springs are tappings of these subterranean waters. 

 Some small lakes receive their supply of water mainly from springs, 

 or subterranean flows. 



The outward flow of the under-ground waters of a continent pre- 

 vents, on sea-shores, the in-flow of the salt water. Springs are common 

 on shores ; occasionally, their waters rise in large volume in a harbor, 

 or out at sea, some miles distant from a coast. 



If subterranean streams have their rise in elevated regions, their 

 inferior portions, beneath the plains of a country, must be under hydro- 

 static jDressure ; and this should appear, whenever a boring is made to 

 the waters, by their rising toward the surface, or, if the pressure is 

 great, above it, in a jet. Borings of this kind have been made in 

 many parts of Europe and America, with this effect. They were first 

 attempted in France, and are called Artesian wells, from the district 

 of Artois, in France, where they were early used. In Fig. 1094, a b 

 represents an argillaceous stratum, on which the water descends, and 



