NOVEMBEE 15, 1901.] 



SCIENCE. 



755 



there shall be a difference of elevation of 

 thousands of feet between where the water 

 enters and where it issues, in order that the 

 rocks shall be searched for depths of thou- 

 sands of feet. A few hundred feet is suf- 

 ficient. Therefore, the underground waters, 

 falling on the slopes, passing through the 

 areas where they may gather material, and 

 issuing at various places in the valleys, have 

 an opportunity to pick up the ores, provided 

 the metals exist in the rocks traversed. By 

 the water the metals are carried to the 

 places where they now are. 



Thus far it has been supposed that the 

 ground is uniformly porous, like an evenly 

 grained sandstone without joint or fracture 

 of any kind, in which the water can go in 

 all directions with equal ease. But abso- 

 lute uniformity does not exist in nature. 

 The openings in rocks are never of uniform 

 size; they are never equally distributed. 



It is now necess^^ry to take up the final 

 important point in the circulation of under- 

 ground water. So far as it can, it passes 

 from small openings to large openings. 

 Where the openings ai'e small the resistance 

 per unit area is very large. Where the 

 openings are exceedingly small, the resist- 

 ance is very great. Where the openings 

 are large, the resistance is slight ; and the 

 water, following the lines of least resistance, 

 travels to the extent that space permits in 

 the trunk channels. To illustrate, every 

 engineer knows that if water be carried in 

 pipes from the hills to the mines, one pipe 

 will carry vastly more water in a given time 

 with a given head than many smaller pipes 

 of the same aggregate cross section. This 

 is because of friction, which increases 

 rapidly as the cross section decreases. And 

 water does the same thing in natural as in 

 artificial pipes. Therefore, the underground 

 water more and more follows the larger 

 courses. It falls everywhere on the slopes 

 of the hills ; it enters the ground every- 

 where. Therefore, in its earlier course it is 



widely disseminated, and thus dispersed 

 can most effectively pick up the valuable 

 metals with which it comes into contact. 

 But as the journey is continued, it collects 

 more and more into the larger channels. 

 But in the early part of the journey of 

 ground water its vertical component is apt 

 to be downward. But as it must sooner or 

 later reach the surface, in the later part of 

 its journey its vertical component is apt to 

 be upward. However, it has just been seen 

 that the early part of its course is apt to be 

 in small openings, and the later part in 

 larger openings. As it collects in the larger 

 channels, it is more likely to be ascending. 

 Therefore, upon the average — I say upon 

 the average — descending water is mainly in 

 the small openings, and ascending water 

 mainly in the large channels. 



It is now advisable to consider the circu- 

 lation in definite vertical large, or trunk 

 channels. Suppose half way down the slope 

 there is a vertical opening of unusual size 



Fig. 6. 



transverse to the plane of the chart (Fig. 6), 

 and another similar opening below the val- 

 ley. If you please, we will call them 

 fissures. These fissures, because large 

 openings, will be fully utilized by the un- 

 derground water. We readily see that 

 ground water will enter the higher fissure 

 at many points and from various directions. 



