SOUDAN FORMATION. 233 



partly instrumental in fracturing' tlie brittle iron formation and rendering 

 it thereby more permeable to percolating water, which was the agent 

 which effected the accumulation. A consideration of the above fact 

 further strengthens the theory that the folding and fracturing preceded 

 the accumulation of the ore. That this accumulation clearly took place 

 subsequent to this fracturing is furthermore proved by the fact that the 

 fractures which traverse the iron formation have been frequently cemented 

 by infiltrated iron ore. 



In the case of the Tower and Soudan deposits there appears no evi- 

 dence of folding subsequent to the accumulation of the ore deposits, for the 

 ore is uniformly fairly massive; although such folding occurred. Moreover, 

 we do not find in these deposits the micaceous hematites or schist ores which 

 are fovmd occasionally in the Marquette district of Michigan, and which owe 

 their origin to the shearing to which they were subjected while they were 

 so deeply buried that they were essentially in the zone of flowage and did 

 not undergo fracturing, such as is produced under ordinary conditions. 



The case is somewhat different, however, at Ely. There, it is certain, 

 more or less extensive earth movements took place after the ore was depos- 

 ited, for the ore and the overlying jasper are fractured through and 

 through, so that they resemble in places a breccia; and since these ores are 

 very thoroughly fractured, we conclude that the movement to which they 

 owe this fracturing took place while the ores were relatively near the 

 surface, or, in other words, were in the zone of fracture for the ore and the 

 associated jaspers Had they been more deeply buried, micaceous hematites 

 would have been produced, and the cost of exploitation would have been 

 very much greater than it is at present. As it is now the ore is almost a 

 rubble, and can be mined much more economically than can the massive 

 ore at Tower and Soudan. It is interesting to note that subsequent to the 

 formation of this rubble the ore, at least at the extreme east end of the Ely 

 trough, has been cemented together by infiltrated material — iron ore to a 

 certain extent, but also calcite and siderite to a still greater extent. Where 

 this cementation of the brecciated ore has taken place, as, for instance, in 

 the Savoy mine, the ore is almost as hard as that obtained from the Soudan 

 mines. 



From the above statements the impossibility of fixing the time of the 

 formation of the ore deposits very definitely will be recognized. The 



