ARTESIAN FLOWS 57 



escape of the water, which has passed downward through the joints 

 from some elevated source, is prevented from escaping by over- 

 lying clays or other obstructions to circulation. Several such wells 

 are found at Portsmouth, N.H., and at other points in the gran- 

 ite areas of New England and the Piedmont plateau of the South, 

 and are occasionally found in the crystalline areas of the West. 



The flows from granite are generally small, for the reason that 

 the openings along the joint planes in rocks of this type are very 

 small, being usually under T ^Q of an inch in width, although when 

 several such passages are intersected by a single well the volume 

 is occasionally higher, amounting sometimes to from 20 to 50 

 gallons a minute. 



Flows from Traps and Lavas. Certain traps, like those of 

 the Connecticut valley, are often quite porous in their upper por- 



FIG. 31. Section illustrating conditions of flow from vesicular trap. A, Vesicular 



zone feeding well I. 



tions, and, when overlain by impervious retaining beds, not in- 

 frequently give rise to flowing wells. In the thicker lavas, such 

 as those covering large portions of eastern Washington and Oregon, 

 certain strata are sometimes almost as porous as sandstones and 

 afford large volumes of water to wells, many of which flow. 



Location of Flowing Wells. In order that water may have 

 sufficient head to flow out upon the surface, it must be confined 

 under some impervious or relatively impervious clay or other bed. 

 This effectually shuts out pollution from the overlying material, 

 and any contamination that reaches the well must be transmitted 

 laterally for relatively long distances. As pollution rarely ex- 

 tends through the ground to any great lateral distance from its 

 source, it follows that artesian waters are almost never polluted. 



In artesian wells, the water, being under greater head than that 



