580 A TREATISE ON METAMORPHISM. 



comparatively short, not many times longer than the vertical component, 

 there is a strong tendency for the circulation to be relatively shallow. 

 Where, however, the horizontal component is very great, the relatively short 

 distance between the bottom and top of the zone of fracture of a porous 

 stratum is of comparatively little consequence. In such cases the circulation 

 through the deeper channels, provided they are equal in area and size to 

 the shallow channels and the water is of the same temperature, might be 

 practically the same as through the shallow ones. For instance, in the case 

 already cited of the Dakota sandstone of the James River Valley, its thick- 

 ness, about 100 meters, is insignificant compared with the horizontal journey 

 of hundreds of kilometers. Therefore, in such a case, if the sandstone were 

 equally porous throughout and of the same temperature, for the greater 

 part of the journey there would be no appreciable difference between the 

 amount of flow in the upper and that in the lower part of the sandstone, 

 although in the very early and very late stages of the journey the upper part 

 of the sandstone would have a greater flow. Where the horizontal journey 

 is long, it is possible, on account of increase of temperature, giving decreased 

 viscosity, that the circulation is more rapid through the lower part of a 

 formation than through the upper part. This would almost certainly be 

 true if all parts were equally porous. But if this were so, sooner or later 

 the more rapid flowage would carry the process of cementation of the lower 

 part of the formation farther than the higher part; and as the openings 

 became partially closed, this would tend to lessen the amount of the deeper 

 circulation. 



PREFERENTIAL USE OF LARGE CHANNELS. 



A very important factor in the flowage of ground water is the great 

 variation, in any natural system of underground flow, of the area of avail- 

 able space and the rate of movement, In nature the points of entrance for 

 ground water are indefinitely numerous, and the places of exit compara- 

 tively few. The water falls upon the ground everywhere and enters the 

 innumerable pores. After a longer or shorter underground course, perhaps 

 passing under many subordinate hills and valleys, it escapes to the surface 

 as a spring or by seepage, nearer the drainage level than where it entered 

 the ground. The water began its journey through an almost infinite num- 

 ber of openings; it issues at many openings, but these are few compared 

 with the vast number of those at which it entered. 



