576 A TREATISE ON METAMORPHISM. 



imposed upon the vertical movement carries it sooner or later to some point 

 where upward movement is taking place. Thus the amount which con- 

 tinues downward is an ever-decreasing fraction of the amount of precipita- 

 tion which joins the sea of ground water. 



Thus far the discussion of the flowage of ground water has been 

 carried on as if it were through a homogeneous porous medium extending 

 indefinitely in all directions in which the pressure and the temperature are 

 the same throughout. It is needless to. say that such are not the conditions 

 of natural systems of underground flowage. Under natural conditions there 

 are many other factors which very greatly modify the nature of the flowage. 

 Among these are limiting formations, gravity, increase in temperature with 

 depth, the relative lengths of the vertical and horizontal components, and 

 preferential use of large and continuous channels. 



LIMITING FORMATIONS. 



It has been shown (pp. 190-191) that the bottom of the belt of cemen- 

 tation corresponds with the bottom of the zone of fracture. It has been 

 explained that to the bottom of the zone of fracture supercapillary or 

 capillary openings may exist. Below the bottom of the zone of fracture the 

 openings in the rocks are subcapillary and therefore practically impervious 

 to a rapid circulation. The bottom of the zone of fracture is the lowest pos- 

 sible boundary of efficient circulation. But there is no theoretical reason 

 why a ground-water system may not utilize the entire zone of fracture to its 

 lower boundary. Indeed, the well-known hydrodynamical principle that 

 tliQr entire available cross section is utilized by flowing currents demands 

 that the circulation extend to the bottom of the zone of fracture. This 

 generalized statement conforms well with the lines of flow in a homo- 

 geneous medium as determined by Professor Slichter. (See figs. 7 and 9.) 

 It therefore appears highly probable that, in any system of ground -water 

 circulation, where there is no impervious rock nearer the surface than the 

 bottom of the zone of fracture the entire zone of fracture is searched, 

 although waters joining and departing from the underground sea disappear 

 and appear at its surface. While this is true, other things being equal, the 

 more direct route is utilized to a greater degree than the more indirect 

 route, and therefore the remoter corners of available space have relatively 



