6 BULLETIN 502, U. S. DEPARTMENT OF AGRICULTURE. 



tinuity of the individual joints be short, the minor intersecting fis- 

 sures may cause long continuous openings. A knowledge of this 

 condition is very important in determining the nature of the circula- 

 tion of the underground water. Naturally the circulation is greatest 

 where the vertical joints and horizontal fractures are most open and 

 numerous. 



MOVEMENT OF WATER. 



The joints and cleavage planes serve as the principal channels for 

 the free circulation of the water, and it is apparent that a well must 

 strike one or more open fissures in order to obtain water. The evi- 

 dence for this statement consists (1) of observations on the corre- 

 spondence of the direction of the major joints observable in the rock 

 at the surface with the appearance of seepage in the lower areas; 

 (2) of the fact that many wells have been drilled within a few feet 

 of each other without encountering water at the same depth; (3) of 

 the dissimilar pressures in adjacent wells of the same depth. 



Often when water is struck in a relief well the rise is very sudden. 

 Such a rise means that the water is under pressure and that enough 

 of it is in the larger openings that extend back up the slope to cause 

 the initial rise. Small sustained flows come probably from areas of 

 close jointing which are more or less continuous. While the move- 

 ment of water in the close joints is slow, yet the aggregate capacity 

 for storage is many times that of the larger fissures. These areas of 

 close jointing collect water during the irrigation season and gradu- 

 ally feed it out to such larger and freer channels as may cut across 

 them. 



While the greatest movement of the water is along the direction of 

 the systems of joints, yet mechanical and other agencies have left 

 the ridges and knolls of the shale so badly broken and shattered that 

 often they carry water quite freely in other directions and may dis- 

 charge water at their points regardless of whether they run parallel 

 with or perpendicular to the system of jointing. This condition, 

 together with the closing of joints and the rapid decrease in number 

 and greater spacing with depth, makes it evident that there will be a 

 much freer circulation of water in the upper portions of the shale, 

 or rather, in the remainder of the upper portions which now form 

 the ridges. This accounts partly for the phenomenon of water fol- 

 lowing the shale ridges and leaking from them rather than from 

 the other portions. 



This condition is illustrated on the contour maps in figures 4 (p. 28) 

 and 7 (p. 33) , where the surface of the ground, the ground water, and 

 the shale have been represented by distinctive lines. The most inter- 

 esting feature of the maps is the general resemblance of the water 

 contours to those of the shale, showing the influence of the shale 

 topography on the movement of the underground water. In figure 4 



