WATER^S A MECHANICAL AGENT. 



207 



187. 



hence that the pressure is rarely, if ever, due, as has been supposed, to the 

 pressure of confined gas. The facts exhibit on a grand scale the influence of 

 a large elevated lake on the conditions of subterranean pressure. 



Wherever subterranean water flows between nearly impervious sloping 

 layers, so that it is confined to a given channel, it is like the water in a long 

 inclined tube ; and on opening a hole through the overlying material it will 

 rise in a jet, owing to the hydrostatic pressure. The height of the jet so 

 produced is that of the source, diminished by the loss from friction and the 

 resistance of the air ; it may be hundreds of feet. 



In the annexed cut (Fig. 187), ab represents a water-supporting layer; 

 be, the boring ; and cd, the jet of water. Such wells are called Artesian 

 wells, as they were first made in the district of Artois, in France. They 

 are now an important means of securing water for irrigation and other 

 purposes in various parts of the world. By this means abundant water is 

 now obtained even on the seacoast region of New Jersey, from Cretaceous 

 and Tertiary strata, and over various parts of the dry regions of Montana, 

 Colorado, and Xevada, where arid sands have been covered thereby with 

 foliage. But if the rocks are porous 

 throughout, with no impervious layers, 

 boring is of no avail. Borings in regions 

 of metamorphic or crystalline rocks gen- 

 erally prove failures unless a chance bed 

 of decomposed rock extending down from 

 the surface should be reached ; for such 

 rocks have been consolidated and crystal- 

 lized while under heavy pressure. Where 

 slates are vertical, a horizontal boring 

 across the bedding may give a constant 

 stream ; but such a source is a small one. 



3. Denudation ; Transportation. — Subterranean rivers have sometimes 

 large size, especially in limestone regions, where excavation is easy, as ex- 

 plained on page 130, under Chemical Geology. Those of the caverns of 

 Kentucky and Indiana have their cascades, like ordinary rivers, and may 

 be navigated for long distances. Into such caverns rivers sometimes 

 enter and become "lost rivers;" while from others issue great streams, 

 whose source is unknown. The cave of Adelsberg, 22 miles northeast of 

 Trieste, has its river ; and the Jura Mountains send forth streams to day- 

 light full grown. The work of denudation and transportation is like 

 that above ground, although less supplied with materials for transporta- 

 tion and wear. 



Subterranean waters do much efficient work in a quiet way by the trans- 

 portation of sand along the course of streamlets that have their outlet at 

 the base of bluffs. The undermining of centuries in this way may make 

 chambers that lead to the sinking of masses of the land, and determine 

 lines of surface drainage. 



Section illustrating the origin of Artesian 

 wells. 



