1028 A TREATISE ON METAMORPHISM. 



Black Hills to the James River Valley, about 400 kilometers, whereas 

 the vertical circulation is limited to a depth less than 1,000 meters. In this 

 case the vertical journey is insignificant in magnitude as compared with the 

 horizontal journey. While this is an extreme case, in general the horizontal 

 journey is several to many times longer than the vertical journey. 



Large channels are utilized to a much greater extent than small chan- 

 nels. This preference for large channels frequently amounts to dominance, 

 particularly for the latter part of the journey. Large channels may be 

 called the trunk channels of circulation. The importance of trunk channels 

 is due to the fact that (1) large channels are likely to be more direct 

 than smaller channels, and (2) the friction along the walls per unit of 

 water is very much less than in capillary and in subcapillary channels. 

 In the belt of cementation, as a rule the channels are larger and more 

 numerous in its upper part, and steadily diminish in number and in size 

 with depth. Therefore large channels tend to produce a shallow circulation, 

 and thus counteract the influence of lessened viscosity with depth and 

 the effect of gravity, which tends to produce a deep circulation. (See pp. 

 578-582.) The preferential use of trunk-channels leads to the conclusion 

 that fault, joint, and bedding-parting openings, and those of conglomerates, 

 all of which are likely to be of supercapillary size, are largely utilized by 

 the ground-water circulation. 



As a matter of observation, it is well known that the water which joins 

 the belt of cementation is distributed throughout the belt, and that it enters 

 at an infinite number of points. It is further known that at least a large 

 part of the water issuing from the belt of cementation emerges from trunk 

 channels. Combining these facts with the principles controlling the circula- 

 tion, it follows that the circulation of the belt of cementation is analogous 

 to that of a tree, the twigs and branches of which have an important down- 

 ward component, the medium branches of which are approximately hori- 

 zontal, and the trunk channel of which usually has an upward component. 



CIRCULATION IN ZONE OF COMBINED FRACTURE AND FLOWAGE. 



Circulation in the zone of combined fracture and flowage is intermediate 

 in its nature between that of the zone of fracture and the zone of flowage. 

 It has been pointed out (pp. 766-768) that as the bottom of the zone 

 of fracture is approached probably the openings decrease in size, until 



