582 A TREATISE ON METAMORPHISM. 



Slichter, that the pressure a short distance from the bottom of a well is com- 

 monly as great while the well is flowing as it is at the mouth of the well 

 when the flow is stopped." If the flowage was mainly, or even largely, 

 through supercapillary openings, this would not be the case. 



Large openings are favorable to a somewhat direct course; small 

 openings are favorable to a circuitous route.. As the openings decrease in 

 size, a more circuitous route must be taken, for to pass a given volume 

 of water from one point to another it is necessary that a wide range of open- 

 ings be utilized. It has been pointed out (pp. 569-570) that the openings 

 of the belt of cementation are relatively large in its upper part and probably 

 diminish in magnitude as depth increases. So far as this is true, therefore, 

 large channels for trunk circulation are, on the average, most numerous near 

 the level of ground water, and diminish in number and size as depth increases. 

 From this, and the principle that the ground water tends to utilize the 

 larger openings to the greatest extent, it follows that the abundant cir- 

 culation tends to follow a relatively shallow course. 



The direct course of water in large openings is illustrated by limestone 

 regions where there are numerous large joints and sometimes caves within 

 which the water is quickly concentrated. In such instances the flowage 

 of water is very largely in the upper part of the zone of fracture. 



RESULTANT CIRCULATION. 



Summarizing, it has been seen that the movement of water in a 

 homogeneous medium is similar to the moA^ement of a current of electricity 

 through a homogeneous medium, in that it utilizes the entire available 

 cross section. But by the water the more direct course is utilized to a 

 much greater extent than the less direct course.- For the actual underground 

 circulation this general statement must be modified in various ways. The 

 circulation is limited wherever a formation is met in which the openings 

 are subcapillary. Gravity and increase of temperature with depth tend to 

 cause the water to take a deeper course than it otherwise would. On the 

 other hand, the preference of water for the larger openings, and the greater 

 abundance of opening's of large size near the surface, tend to give the water 

 a shallow course. Therefore the influence of gravity and temperature and 

 that of large openings oppose each other. Which of these two opposing 



"Slichter, G. S., Theoretical investigation of the motion of ground waters: Nineteenth Ann. Rept. 

 U. S. Geol. Survey, pt. 2, 1899, pp. 363-364. 



