CONTROLLING FACTORS OF ARTESIAN FLOWS 633 



the supply itself does not necessarily come from a higher level, since besides 

 the downward moving meteoric water supplies may be furnished by the sea, 

 by waters chemically or physically excluded from the crust, or by direct or 

 magmatic exclusion from the centrosphere. 



OUTCROP OF rOROUS STRATUM 



The postulated suitable exposure of the edge of the porous stratum so that 

 it may take in a sufficient supply of water, though a common, is ,far from an 

 essential factor of artesian flows. Some of the horizontal sandy beds from 

 ■which the flows ai'e obtained in Long island and Michigan never outcrop, the 

 water penetrating directly downward through the overlying layers. Moreover, 

 throughout extensive areas of the Silurian, Devonian, and Carboniferous rocks 

 in Pennsylvania. West Virginia, and Ohio, in the areas underlain by Cretaceous 

 beds in the Fort Monroe district of Virginia, and the Wilmington region of 

 North Carolina, and in many lesser areas elsewhere, the deep artesian waters 

 represent originally included sea waters and not waters entering through the 

 outcrop. Other sources, including waters chemically or physically excluded 

 from the crust, or by direct or magmatic exclusion from the centrosphere, may 

 furnish artesian supplies independent of the condition of outcrop. 



ADEQUATE RAINFALL 



From the fact that water may be. and in the case of the salt waters of the 

 Carboniferous and Coastal Plain rocks just described, as well as in the case of 

 magmatic waters actually is, derived over extensive areas from sources other 

 than rainfall, it is clear that the latter should not be included as an absolute 

 requisite. 



POINTS OF ESCAPE 



There are very few artesian systems in which there is not more or less 

 leakage. In the thicker and more pei'sistent beds the leakage is often sufficient 

 to insure circulation for long distances from the outcrop. Thus, in the Cre- 

 taceous beds beneath Charleston, South Carolina, and Savannah, Georgia, fresh 

 water has replaced the salt at least as far as the seacoast, or a distance of 

 over 100 miles from their outcrop. At Fort Monroe. Virginia, and Wilmington, 

 North Carolina, on the other hand, there appears to be but little leakage and 

 the fresh water circulation reaches to a much less distance, only salt water 

 being obtained at the localities mentioned. The absence of leakage appears to 

 have determined the presence of salt waters in the oil-bearing rocks of Penn- 

 sylvania and elsewhere. 



In order that leakage may prevent flows, it must take place near the point 

 at which the water horizon is tapped. It has been shown many times by 

 Chamberlin and others that its influence is limited. The requisites should 

 therefore postulate the absence of near-by leakage rather than the non-occur- 

 rence of leakage. 



Essentials of Artesian Flows 

 The essentials of artesian flows, as recognized by the writer, are as follows : 



