OKIGIN OF IRON ORES OF NEGAUNEE FORMATION. 403 



at the beginning of the concentnitiou of tlie ore deposits is uncertain, 

 but since it is still found in places sheltered from i)ercolating waters, such 

 as the deeper horizons of the iron formation, adjacent to and probably- 

 protected by greenstone masses, it is probable that the quantity was very 

 considerable. The oxides or carbonates of iron may also have been taken 

 into solution through the agency of organic acids. The downward- 

 moving waters passed along and through the beds of the iron-bearing 

 formation until they came in contact with an impervious substance. Here 

 were also converged oxygen-bearing waters more directly from the 

 surface. The union of these two currents precipitated the iron oxide. 

 The abundant waters traversing these ore-bearing localities slowly dis- 

 solved the silica, its place being taken by the ore. That this interchange 

 actually did occur is known of the localities in wliich a detailed exami- 

 nation has been made, as, for instance, at Republic. It is probable that 

 in the ore deposits associated with the soaprocks the removal of silica 

 was due in part to them. Originally diabases, they must have contained 

 alkalis. The alkaline waters produced by their alteration thus furnished 

 a menstruum capable of taking the silica into solution. This desilicificatiou 

 of the iron-bearing formation by alkaline waters was many years ago 

 suggested by Brooks^ for a part of the Marqviette district. Romiuger- not 

 only made the same suggestion in reference to the Jackson mine, but 

 further held that the siliceous matter removed was replaced by oxide of 

 iron carried by water solutions. 



The percolating waters which carried material along the readiest paths 

 to form the ore bodies, and which removed the silica, also helped to jas- 

 perize the upper part of the Negaunee formation, although this may have 

 been 2:)artly done before Upper Marquette time. Whatever the time at 

 which the work was done, the process seems to have been as follows : The 

 quartz grains of the ferniginous chert were separated by mashing. The 

 upper part of the ore formation was more extensively traversed by solutions 

 than the deeper-lying portions. It naturally followed that the ferruginous 

 material was in part deposited about and through the minute particles of 



' Geol. of Michigan, Vol. I, p. 134. ^Ibid., Vol. IV, p. 75. 



