588 A TREATISE ON METAMORPHISM. 



many supercapillary channels of much greater cross section than the well. 

 The rapid movement is restricted to the section of water in the well, and 

 there the friction is great. This Wisconsin illustration is typical of natural 

 systems of ground water flow; for, as shown on page 583, a natural system 

 of flowage simnlates a tree. At the outset there are an indefinite number 

 of minute openings through which the water slowly moves to the branch 

 openings. In the branch openings the movement is faster. The branches 

 unite into the trunk channels, and in these the movement of the water is 

 rapid. 



The analogy to a tree has been suggested in order to get definitely 

 in mind the general character of the circulation of ground water. But the 

 analogy must not be pushed too far. A tree commonly has but a single, 

 continuous, solid trunk, although willows and other trees have many trunks. 

 Very frequently, indeed commonly, the trunk channels of ground water 

 circulation are veiy complex. While a main watercourse may exceptionally 

 occupy a single open passage, ordinarily it occupies a number of inter- 

 locking passag-es. These may be the parallel openings of a complex fault, 

 the smaller numerous openings of a zone of fissility, or the more open places 

 of sandstones or conglomerates. In short, a trunk channel of ground water 

 differs from other channels only in that because the openings are larger 

 than the average, they are places where there is more circulation. 



Thus far the discussion has not taken into account the geological 

 work of the circulation itself. But the character of the circulation is being 

 continually affected by the material deposited. 



Later in this chapter it will be explained that under quiescent condi- 

 tions the process of deposition of mineral material in the openings of the 

 belt of cementation continues until the openings are practically closed. In 

 proportion as this process is advanced the openings become smaller and the 

 circulation slower. In many great, almost perfectly cemented formations, 

 as the process neared completion, the movement of the ground water must 

 have become exceedingly slow, and finally practically ceased. After a 

 formation has been once cemented, orogenic movements may again produce 

 fractures within it, and thus form new openings through which an under- 

 ground circulation may again be set up. 



