72 THE CRYSTAL FALLS IKON-BEAEIXG DISTRICT. 



were formed after tlie beds were tilted, and tlie iron derived from the 

 upward extension of tlie rocks, which has been removed by erosion. 



CONDITIONS FAVORABLE FOR ORE CONCENTRATION. 



The conditions favorable for the accumulation of ore deposits have 

 been ascertained hj Van Hise from studies in the other ii-on-beariug dis- 

 tricts of the Lake Superior region. He summarizes these results as follows:^ 



[1] The iroa ore is confined to certain definite horizons, known as the iron-bearing 

 formations. . . . \a\ All ore bodies have been found to be distributed very irregu- 

 larly in these iron-bearing formations. This is due to the fact that they are secondary 

 concentrations produced by downward percolating waters, and the ore bodies therefore 

 occur at the places where water is concentrated, in accordance with the laws of the 

 underground circulation of waters, [b] These places are just above an impervious 

 formation, at the contact of the Upper Huronian and Lower Huronian and where the 

 rocks are shattered. [c\ The impervious basement formation may be a surface 

 volcanic, a subsequent intrusive, an argillaceous stratum, or any other impermeable 

 formation, [d] These impervious basements are most effective when they are in the 

 form of pitching troughs, thus concentrating the waters from tbe sides along a well- 

 defined channel. These pitching troughs may be formed by a single one of the above 

 rocks or by a combination of two or more of them. The horizon marked by the uncon- 

 formity between the Upper and Lower Huronian is a great natural zone of percolating 

 waters. Here oftentimes the basement formation of the Upper Huronian is itself a 

 lean ore, having derived its material from the Lower Huronian, but in this case a 

 secondary concentration has occurred in order to produce the present ore bodies. 

 [e] Finally, as a result of folding, the iron-bearing formations have been shattered, 

 thus producing natural water-courses. More frequently than not, more than one of 

 these classes of phenomena are found together where the great ore bodies occur, and 

 in many cases all are combined. The original source of the iron ores has been ascer- 

 tained to be in many cases a lean carbonate of iron, often with a good deal of 

 carbonate of calcium and magnesium, formed as an ocean deposit. 



Van Hise adds to the above statement that generally the ore bodies, 

 as a result of their methods of concentration, somewhere reach the rock 

 surface. 



The Mansfield ore body has well-defined foot and hanging walls of 

 normally imper^aous rock. The iron-bearing foiTnation is much fractured. 

 "We thus have certain of the conditions favorable to the concentration of an 

 ore body. Whether a trough is completed by a slight cross fold in the 

 formation, or possibly by an intersecting dolerite dike, has not been 

 determined. 



' Fourteeuth Anu. Kept. U. S. Geol. Survey, Part 1, 1893, pp. 107-108. 



