632 PROCEEDINGS OF THE NEW YORK MEETING 



PERVIOUS STRATUM 



A pervious stratum, although a commou form of reservoir, Is seldom essen- 

 tial to artesian flows. In addition to the porous stratum postulated by the 

 first requisite, flows may be obtained from lamination, bedding, cleavage, and 

 shearing planes, from solution passages and mechanically eroded reservoirs, 

 from vesicular zones in igneous rocks, from irregulax", joint, and fault fractures, 

 and from vein and igneous contacts. Metamorphic and igneous as well as 

 stratified roclvs not only may, but actually do, yield flows in a large number 

 of instances ; in many others the water falls only a few feet short of the 

 surface, while in practically all the wells the waters rise materially when 

 encountered-^that is, are truly artesian. 



With the exception of the vesicular lavas, the sources of water mentioned 

 are not in the nature of beds, but are in the form of actual openings — a type 

 of passage not recognized among the older requisites. 



IMPERVIOUS BED BELOW 



From the nature of the passages enumerated above and given in more detail 

 in the list on page 628 it is apparent that the second requisite — the impervious 

 underlying stratum — loses much of its force. Such impervious beds are an 

 adjunct to many flows in stratified rocks, but numerous other agents may 

 serve the same purpose. In bedded rocks the following may be mentioned: 

 Stratification, friction, mineral crusts, frost zones, confined air and gas, fresh 

 or salt water, cementation, heat and pressure. In jointed and fractured rocks 

 impervious footwalls, vein fillings, converging walls, interrupted joints, fresh 

 and salt water, heat and pressure, are the most important. 



IMPERVIOUS BED ABOVE 



The objections to the postulation of an upper impervious bed are similar to 

 those of the lower confining bed just enumerated. In the case of the bedded 

 rocks the following, in addition to the postulated impervious stratum, may 

 serve as confining agents : Stratification, friction, mineral crusts, frost zones, 

 confined air and gas, fresh water and sea water. In the case of jointed and 

 fractured rocks the more common additional agents are impervious hanging 

 wall, impervious surface coverings, frost and vein fillings, weathering products, 

 converging walls, interrupted joints, and sea water. 



IXCLIXATIOX OF BEDS 



Inclination of tlie water-bearing bed, while a common factor in ai'tesian 

 flow, is by no means essential. Water appears to penetrate in many cases into 

 lenses of sandstone in rocks like those of the Carboniferous of Pennsylvania 

 through joints and similar openings, and flows ai-e obtained independent of any 

 inclination of the bed affording the water. The same is true in the case of the 

 horizontal beds yielding flows by virtue of the opposition of the stratification 

 planes. Joint and solution passages also afford artesian flows independent of 

 any inclination at the point penetrated. 



In both bedded and crystalline rocks the pressure must, of course, be trans- 

 mitted from connecting passages or other water reservoirs at higher levels, but 



