EARLE, INTERBEDDED IRON ORE DEPOSITS 137 



an excellent field for progressive replacement, whereas we find more or 

 less clean-cut contacts and imderlying rich iron ore beds. A second 

 point of disagreement is in relation to the direction of movement of the 

 iron-bearing solutions under these theories, which, in the various papers 

 examined, seems to be by vertical descent of ground water, often limited 

 in depth to a few hundred feet. Thus I. C. Eussell (292) refers to a 

 case in Attalla, Alabama, in which the ore changed to ferruginous lime- 

 stone within a few hundred feet of the surface. 



PAET II. ARTESIAN REPLACEMENT THEORY 



Artesian Conditions 



controlling factors 



Artesian conditions result from a natural arrangement of strata in 

 such manner that they act as a retaining basin or catch basin in porous 

 strata in which water is or may be confined under hydrostatic pressure 

 sufficient to cause the water to rise when the reservoir is tapped. 



The conditions requisite for the existence of artesian wells, as set 

 forth by Chamberlin (54a), are the following: 



1) A porous stratum for the penetration of water; 



2) An impervious underlying layer to prevent the downward escape 

 of water; 



3 ) An impervious overlying layer to retain the water under pressure ; 



4) An inclination of the layers, at least in part, so that the point of 

 entrance is higher than any other portion of the retaining layers ; 



5) A reasonably large exposure of the porous layer, in order that free 

 entrance may be provided for the penetrating waters; 



G ) Sufficient rainfall for water supply ; 



7) Absence of any place of escape for the retained water. 



This summar\' of the usually quoted factors may be taken to indicate 

 ideal artesian conditions, but many variations may exist and still allow 

 artesian flow, although these requisites or adequate substitutes for them 

 must be present. 



The pervious medium may be any crystalline or sedimentary non- 

 crystalline rock or stratum which contains enough pore space to permit 

 a circulation of the penetrating water. Sandstones (particularly of 

 coarse texture), fossiliferous limestones and even in some cases coarsely 

 crystalline limestone may serve as a carrier and saturation medium for 

 artesian waters. In some instances, even bedding planes, laminations 

 or fracture systems may be adequate. 



