PRINCIPLES CONTROLLING ORE DEPOSITS 743 



tubes, if the temperature be considerably increased — and but a 

 moderate depth is required to give considerable increase — the 

 water moves several times as fast as it would at the surface under 

 conditions similar in all respects save temperature. Therefore, 

 because of these three factors, long journey, high temperature, 

 and low viscosity, we cannot exclude the deep circulation from 

 consideration. This circulation is indeed believed to be very 

 important in the deposition of ores. 



We are now prepared to consider the actual journey of under- 

 ground water. Where water falls upon porous ground it finds 

 innumerable openings through which it enters and begins its 

 underground journey. This circulating water, as far as prac- 

 ticable, under the law of the minimum expenditure of energy, 

 follows the paths of easiest resistance. But these are the 

 larger openings, because resistance due to friction along the 

 walls and within the current is very much less per unit circula- 

 tion in large than in small openings. While therefore water 

 enters the ground at innumerable small openings, as it goes down 

 it more and more seeks the larger openings. Once found, it 

 holds to them. The farther it continues its journey, the greater 

 the proportion of the water which follows the larger openings. 

 But if this be true, the water in its descending course is more 

 likely to be widely dispersed and in the smaller openings ; and 

 in its upward course more likely to be concentrated and in the 

 larger openings. 



We can now follow the course of underground water in detail, 

 but in doing this it is necessary to consider the elements of the 

 problem separately. It is only by passing from a simple case 

 to the very complex one of nature that we can understand the 

 latter. Here is a chart (Fig. 3) which shows the surface of a 

 slope, the level of groundwater, and the flowage of water in the 

 simplest imaginable case. Below the level of groundwater all 

 the openings in the rocks, great and small, are filled with water. 

 The rocks are saturated. In the case represented I have supposed 

 that all of the water enters at a single point, A ; and that all of 

 it issues at a single point, B. The curved lines represent the 



