LIMITS OF CIRCULATION OF WATER. 577 



small circulations The principle of the distribution of the flowage of water 

 over the entire available area is well illustrated hy the case of water flowing 

 horizontally into a beaker from one side and overflowing the beaker on the 

 other side. The movement of the water is not confined to the liquid near 

 the surface, but all portions of the water in the beaker, from the top to the 

 very bottom, take part in the flowage, although, of course, the rate of 

 movement is much more rapid at the top than at the bottom." 



While the circulation may extend to the very bottom of the zone of 

 fracture, in many cases before that depth is reached a stratum is met all 

 the openings of which are subcapillary.' As shown on pages 143-146, such 

 a stratum is practically impervious, and therefore becomes a practical limit 

 of circulation in that direction. Such impervious strata may be onl} x a few 

 meters below the level of ground water, and thus the circulation be exceed- 

 ingly shallow. From such a position an impervious stratum may be at 

 various depths to the bottom of the zone of fracture. 



While one system of circulation may thus be of very limited depth, it 



Fig. 12.— Ideal section illustrating the chief requisite conditions of artesian wells. A, a porous stratum; B and C, 

 impervious beds below and above A, acting as confining strata; F. the height of the water level in the porous bed A, or, in 

 other words, the height of the reservoir or fountain head; D and E, flowing wells springing from the porous water-filled 

 bed A. After Chamberlin. 



does not follow that other systems of circulation are not utilizing' the other 

 parts of the zone of fracture below the first circulation ; for veiy frecpiently 

 a belt below, which contains openings and is available for flowage, may 

 elsewhere reach the surface and have a feeding area. Indeed, such are the 

 circulations of many systems which are limited above by impervious strata. 

 One of the simplest of such systems is represented by fig. 12. There are 

 an indefinite number of variations of the systems of underground circula- 

 tions, dependent upon various combinations of pervious and impervious 

 strata. In some instances there may be several superimposed or adjacent 

 systems, the relations of which are exceedingly complex. Some of the 

 cases are of general geological interest. A number have been discussed by 



"Slichter, C. S., Theoretical investigation of the motion of ground waters: Nineteenth Ann. Rept. 

 U. S. Geol. Survey, pt. 2, 1899, p. 331. 

 MON XLVII — 04— — 37 



