ARTESIAN WELLS. 



405 



The essential conditions of an artesian supply are: 

 1. A permeable layer of rock confined between impermeable layers. 

 '2. An inclined position of the beds and their outcropping in a manner 

 to present an extensive area of absorption to rains and stream waters. 



3. An effective barrier t<> the outlet of the contained water at a lower 

 level than the surface of the well area, whether a barrier of texture, struc- 

 ture, or position. 



4. A difference in altitude between the area of absorption and the 

 surface 'it' the well area such that the resulting increase in hydrostatic 

 pressure shall he sufficient to overcome the frictional resistance encountered 

 in the passage of the water from the source to the point of delivery. 



5. An adequate rainfall. 



The first tour of these conditions ore illustrated in the following 

 figures, taken from a paper by Prof. Thomas ( '. ( 'hamlierlin: 1 



Fig. 17.— Ideal section illustrating the chief requisite conditions of artesian wells A. a porous strata 

 impervious beds below and above A. acting as confining strata: F, the height of the water level in the porons Weil A, or, 

 in other words, the height of the reservoir or fountain head; D and E, flowing wells springing from the porous water-filled 

 bed, A . 



i 



Fig. 18.— Section illustrating the thinning out of a porons water-bearing bed. A inclosed between impervious beds, 

 i and C, thus famishing r he necessary conditions for an artesian fountain, 1 >. 



Being in. losed b. 



in IX bed, A. inlo a clo 

 i beds. B and C, it tarnishes the conditions lor an artesh 



■textured impervious one. 



fountain, D. 



From what follows it will lie seen that the artesian conditions of 

 composition, stratigraphy, and structure are well fulfilled in the Denver 

 field. The formations affording the water supply — the Denver, Arapahoe, 

 and Laramie — are a succession of sands and shales, and the structure of 

 the field is that of a shallow basin, its mountain edge sharply upturned. 



'The requisite and qualifying conditions of artesian wells: Fifth Ann. Rept. U. S. Geo!. Sur- 

 vey, 1885, pp. H'5-173. 



