138 ANNALS NEW YORK ACADEMY OF SCIENCES 



The impervious floor is not absolutely necessary. In some cases, arte- 

 sian flow might be expected if the underlying layer, although penetrable, 

 was less porous than the middle layer, so that the incoming water would 

 accumulate with greater rapidity than it could escape downward. This 

 would occur in the case of the two sandstones of marked difference in 

 texture, the upper coarser and the lower finer. 



The impervious layer above is more essential, but even here extreme 

 differences of texture may give some results, even though the overlying 

 layer is somewhat porous. It must be remembered, moreover, that poros- 

 ity is relative and that absolutely impervious strata are unknown. 



Another very important consideration with reference to these porous 

 and impervious layers is the possibility of accumulations by precipitation 

 •or by the mechanical filling of voids at or near the line of contact be- 

 tween the strata. If, for example, two sandstones, one coarser than the 

 •other, are in contact, precipitation would first occur near the contact in 

 the pore spaces of the finer textured rock, as there the penetrating solu- 

 tions would move with less freedom and rapidity. 



Mechanical sediments as well as mineral crustvS might be expected to 

 play some part toward establishing more complete artesian conditions. 



Circulating ground waters, following lines of least resistance, tend to 

 establish more or less definite channels, and if these channels are fairly 

 well retained, nature itself will attempt to improve conditions by steadily 

 increasing the density of the carrying medium through cementation and 

 other processes of filling the minute channels of escape. 



Inclination of artesian beds is necessary only for the purpose of allow- 

 ing gravity to establish hydrostatic pressure. If the water head is suffi- 

 ciently high to develop enough pressure to overcome the friction and 

 other causes of retarding a free flow of water, it is fair to assume that 

 artesian waterways may exist in practically horizontal layers, the move- 

 ment of the water depending upon the amount of pressure exerted by tlie 

 water column. 



The absence of an avenue of escape may be accounted for in many 

 ways; for example, a marine slope may be terminated at the lower end 

 by a change of texture from coarse to very fine, as sand to mud ; or pre- 

 cipitation may take place at the lower end of a runway, thus filling the 

 voids in the previously porous layers. 



CIRCULATION OF ARTESIAN Vi^ATERS 



It has been suggested that artesian water is stagnant until tapped; 

 but the writer is inclined to believe that absolutely stagnant artesian 

 waters would be unusual if not impossible. 



