SYSTEM OF CIRCULATION SIMILAR TO A TREE. 583 



factors is the more important is uncertain, but probably the existence of 

 larger fractures near the surface is more important than the effect of gravity 

 and temperature. 



It has been seen that during the first part of the underground journey 

 of water the vertical component is downward, and that during the latter 

 part of its journey the vertical component of much of it is upward. It 

 follows that, on the average, the downward movements of water are through 

 the smaller, and the upward movements through the larger, openings in the 

 rocks. Of course, where large openings are available for the downward- 

 moving water they are fully utilized, and therefore to a greater extent than 

 a similar area of smaller openings. But even if this be true, the statement 

 still holds that, on the average, the larger openings are more extensively 

 used by the upward-moving water than by the downward-moving water. 

 From the foregoing it appears that a system of circulation of ground water 

 has a very close analogy to a tree of peculiar character. 



The points of entrance of water are the ends of the indefinite number 

 of twigs; these twigs unite into a branch; the branches unite to produce a 

 larger branch; the larger branches unite into a trunk; and at the end of a 

 trunk is a spring. The analogy of an underground drainage system to a 

 tree is even closer than that of a surface system; for in a system of under- 

 ground water circulation three dimensions are concerned to an important 

 extent, while in a surface system of drainage the movement of the water is 

 approximately confined to a plane. But, from what has gone before, it is 

 clear that the tree of ground water has a peculiar shape. The twigs and 

 branches have an important downward component; the larger branches 

 of the tree may be considered as approximately horizontal; and the trunk 

 usually has an upward component, which may be slight but is often 

 important. Thus twig's, branches, and trunks together ordinarily make a 

 great U. Where the water issues near the places of entrance the sides 

 of the U are rather close together. Where there is great lateral move- 

 ment of the water the sides of the U are very far apart. Such a system 

 of ground water is somewhat similar to that of a surface system of drainage. 



Ordinarily where the water first enters the underground sea the area 

 of available space is exceedingly great and the rate of movement is very 

 slow. As the water gathers in the larger openings the speed increases, and 

 in the final trunk channels the water may have a very rapid movement, as 



