PRINCIPLES CONTROLLING ORE DEPOSITS 7$y 



is believed to be the result of a single concentration by ascending 

 waters. Such ore deposits may continue without any appreci- 

 able diminution in richness to the lowest limits to which man 

 may expect to penetrate the earth ; but these_are exceptional 

 cases. Even ore deposits which are the result of a single con- 

 centration by ascending water may diminish in richness at con- 

 siderable depth. It has been seen that in the fissure at the 

 bottom of the valley on this chart (Fig. 6) that the water 

 ascends to the surface. It is evident that the upper part of the 

 fissure receives the greatest supply of water, and this water to a 

 large extent does not penetrate any great depth ; while the 

 lower part of the fissure receives less water, but this water 

 penetrates to a considerable depth. It may happen that the 

 water relatively near the surface traverses the rocks containing 

 the main supply of metals and therefore brings the chief con- 

 tributions of valuable material, or such waters may carry the 

 precipitating agent. In such instances the ore deposits pro- 

 duced by ascending water alone, would diminish in richness 

 with depth ; but such decrease would not be likely to be very 

 rapid. Upon the other hand, if the above conditions be reversed, 

 a deposit may increase in richness for a considerable depth ; but 

 as a matter of fact this appears to be a very infrequent case. 



As illustrations of the ore deposits of the class produced 

 by ascending waters alone are the copper deposits of Lake 

 Superior. These deposits, while very bunchy and extremely 

 irregular in the distribution of copper, are wonderfully persist- 

 ent in depth. The copper of the ore was deposited in the 

 metallic form. As compared with sulphides, this material is not 

 readily oxidized. In this district the rocks above the level of 

 groundwater are not appreciably weathered. Doubtless there was 

 a belt of weathered material before the glacial epochs, but if so, 

 it has been swept away by ice erosion ; and since the glacial 

 period sufficient time has not elapsed to weather appreciably 

 the rocks which now lie within the theoretical belt of weather- 

 ing. If there once were in this district an upper belt of weather- 

 ing in which there were deposits of exceptional richness, this 



