752 



SCIENCE. 



[N. S. Vol. XIV. No. 359. 



porous strata of the Fox Hills, Laramie 

 and Arapahoe formations have more or less 

 independent circulations. If a liihiting 

 stratum be supposed to be half way down 

 on the chart (Fig. 2) the lines of flow above 



Fig. 2. 



this stratum would not be as they are now, 

 but would be flatter and would be limited 

 by the impervious rock. 



Under natural conditions, wherever there 

 is an impervious rock there is a limit of 

 some particular circulation in that direc- 

 tion. A limiting stratum may, therefore, be 

 very near the surface, at the bottom of the 

 zone of fracture or at any intermediate 

 depth ; and theoretically a moderate head 

 is sufficient to do the work of driving the 

 water to any of these depths. Indeed, there 

 is no escape from the conclusion that at 

 least some circulation does occur in the 

 deeper parts of the zone of fracture with a 

 very moderate head. Of course, in propor- 

 tion as the head is great the circulation at 

 depth is likely to be vigorous. But it may 

 be objected that a deep circulation, while 

 theoretically possible, must be exceedingly 

 small in quantity, and consequently of 



comparatively little account in the depo- 

 sition of ores. But the consideration of 

 the underground circulation in reference 

 to the Denver artesian wells shows that 

 this objection has little weight. More- 

 over, the deeply circulating water, if less 

 in quantity than that near the surface, 

 takes a longer journey and is longer in con- 

 tact with the rocks through which it is 

 searching for the metals. Kot only so, but 

 it is at a higher temperature than the water 

 at higher levels ; and this also is favorable 

 to taking mineral material in solution. 

 And, finally, because it has a higher tem- 

 perature, it has less viscosity. While the 

 variable viscosity of water is not so very 

 important in reference to circulation in su- 

 percapillary tubes, in capillary tubes, which 

 constitute a very large fraction of under- 

 ground openings, and especiall}^ those at 

 considerable depth, the viscosity is im- 

 portant — the flowage increasing directly as 

 the viscosity decreases. The viscosity of 

 water at 90° C. is only one-fifth as much 

 as it is at 0° C. ; and, therefore, with a given 

 head of water in capillary tubes, if the tem- 

 perature be considerably increased^and 

 but a moderate depth is required to give 

 considerable increase — the water moves sev- 

 eral 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 tempera- 

 ture 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 underground 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 

 practicable, under the law of the minimum 

 expenditure of energy, follows the paths of 

 easiest resistance. But these are the larger 



