654 



DYNAMICAL GEOLOGY. 



Fig. 1094. 



b c the boring ; bed is the jet of water. The rise of the jet falls 



far short of the height of the 

 source, because of the great amount 

 of friction along the irregular 

 rocky bed of the stream, and also 

 the resistance of the air. 



In some cases, subterranean 

 waters are under pressure, from a 

 stratum of gas over them, which 

 is sufficient to send them to the 



Section illustrating tbc origin of Artesian wells. Surface without Other aid. 



The Artesian well of Grenelle, near the Hotel des Invalides, in Paris, is 2,000 feet 

 deep. At 1,798 feet, water was struck; and it darted out t» a height above the surface 

 of 112 feet, and at the rate of nearly one million of gallons a day. The pressure indi- 

 cated by the jet was equal to that of a column of water 2,612 feet high, or 1,1G0 pounds 

 to the square inch. Another, in the north of Paris, has been carried down to a depth 

 exceeding 2,000 feet, with a diameter of more than four feet to the bottom. All but 

 157 feet of it is below the sea-level. 



Another well, in Westphalia in Germany, is 2,385 feet deep. 



An Artesian boring at St. Louis has been carried to a depth of 3,843| feet, but with- 

 out obtaining a flow of water to the surface ; the last 250 feet were in granyte of the 

 Archaean, so that the whole of the Paleozoic of the region, from the Carboniferous 

 downward, was passed through. (Broadhead.) A well at Louisville, Kentucky, 2,086 

 feet deep, supplies an abundance of water, though a little brackish. In California, 

 Artesian wells have been resorted to successfully, for agricultural purposes. 



Borings are often successful in alluvial regions, fifty or one hundred miles from any 

 high land. A second boring in the same region sometimes seriously lessens the amount 

 of water afforded by the first, by giving the same subterranean stream a new place of 

 exit. The layer from which the boring and jet rise may be gradually worn through by 

 the flow, and the water, or part of it, become lost by being thus let off to a lower level. 



The sti-atitied sands and gravel of a region have often, at some depth below the sur- 

 face, a half-consolidated layer called hard-pan (often consolidated by oxyd of iron, 

 through the aid of percolating waters), which holds the water above it, and thereby 

 makes an underground stream or basin for the supply of wells. The same result comes 

 also from the presence of a clayey layer. Artesian borings to this water-layer some- 

 times secure a flow to the surface, and a jet of moderate height. 



Subterranean streams produce erosion, like running water above 

 ground, and may excavate a channel in the same way. Caverns are 

 made partly by erosion and partly by the dissolving action of water. 

 A common effect of such excavations is the production of subsidences 

 of the soil and overlying rocks, and the formation of sink-holes. Small 

 shakings of the earth may be a consequence of the fractures of under- 

 mined strata. 



2. Mechanical Effects, from the Softening or Loosening 



of Beds. 



Subterranean waters act mechanically, also, by softening or loosen- 

 ing permeable beds of rock -material, and adding to their weight. The 

 following are among the consequent results : — 



