42 Van Ilise — Iron Ores of the Penokee- Gogebic Series. 



steadily work tlieir way to a greater and greater depth along 

 the foot-wall, until auch depths were reached as to prevent its 

 farther penetration. 



Now, suppose erosion to gradually sweep away the rocks which 

 are between the old surface of the country and the present 

 surface. Beginning at the base of the formation the rocks at. 

 the present surface would be more and more exposed to the 

 action of percolating waters, which would in turn affect the 

 middle, and finally the higher layers until its whole width was 

 subject to the agencies of alteration There is, then, a gradual 

 increase in the time that percolating waters have acted upon 

 the various layers of the formation in passing from south to 

 north. The difference in time to which the highest and 

 lowest layers have been subjected to such action, is at least 

 the length of time that it has taken erosion to remove the 

 thickness of rock between the old surface of the country and 

 the present surface ; therefore the slower the erosion is taken 

 to have been, the greater the difference in time. 



Next suppose that erosion had continued until the surface 

 of the land is at some intermediate point. In tracing the per- 

 colating waters, it is necessary to take into account the deflec- 

 tion to which they would be subjected by its layers, and the 

 impenetrable character of the underlying slates and intersect- 

 ing dykes. The' relative position of the ore-bodies, quartzites 

 and dykes has already been given. The water which fell 

 upon the layers of the iron formation near its base, would 

 readily pass through the rock, it being here already much 

 altered and broken by the long action of water. Passing 

 through these ferruginous cherts, the water would quickly 

 reach a dyke or the fragmental quartzite, and would follow 

 along this barrier, deflected to the north if upon the quartz- 

 ite, and to the south if upon a dyke, until it reached a 

 trough made by the dyke and quartzite, along which it would 

 follow, traveling toward the east as it penetrated deeper. 

 Such water would be likely to contain oxygen in solution, and 

 would be capable, if it contained alkalies, which might be 

 readily obtained from the alteration of the basic dykes, of tak- 

 ing up a small amount of silica. Other water falling upon 

 higher lavers would make its way slowly and with difficulty 

 through these less altered parts, and would oxidize iron carbo- 

 nate until all oxygen had been extracted from it. This oxida- 

 tion of a part of the iron carbonate would liberate carbon 

 dioxide, which would be taken into solution and added to the 

 carbon dioxide which the water already contained.* Such 



* In this discussion carbonated water is taken as the agent of solution. It is 

 likely enough that organic acids have helped to take the iron carbonate in solution 

 and bear it to the points of precipitation. 



