742 C. R. VAN HISE 



nearly independent circulations. To illustrate, at Chicago the 

 St. Peter's sandstone, the Potsdam sandstone, and even different 

 parts of the Potsdam sandstone have more or less independent 

 circulations. If a limiting stratum be supposed to be half-way 

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

 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 mod- 

 erate head. Of course in proportion 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 deposition of ores. But the 

 consideration of the underground circulation in reference to the 

 Chicago artesian wells, shows that this objection has little weight. 

 (See p. 737.) Moreover, the deeply circulating water, if less in 

 quantity than that near the surface, takes a longer journey and 

 is longer in contact with the rocks through which it is searching 

 for the metals. Not 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 temperature it has less viscosity. While the variable 

 viscosity of water is not so very important in reference to circu- 

 culation in super-capillary tubes, in capillary tubes, which con- 

 stitute a very large fraction of underground openings, and 

 especially those at considerable depth, the viscosity is important 

 — 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 



