225 
The Iron Ores of the Lake Superior Beg ion. 
a 
of this transition zone and even when so minute as not to be visible in the 
hand specimen are discoverable by a microscopical examination. How¬ 
ever, before all of the silica is removed, iron oxide begins to be intro¬ 
duced, and finally when the interchange is complete, in the places of the 
siliceous bands is a solid body of iron ore. 
It is usually found that the eruptive rocks underlying the ore-bodies 
are greatly altered, the alkalies having been removed. It. is probable 
that these alkalies have been an important agent in the solution of the 
adjacent silica. It is evident that the pitching troughs are places along 
which abundant water must travel because the surface waters are not 
able to penetrate the underlying formation, and the overlying formation 
is a porous one. It is equally evident that the contact plane between 
the Upper and Lower Huronian, where there is a coarse conglomerate at 
the base of the Upper Huronian, is also a horizon along which under¬ 
ground waters travel, and this is particularly true where violent folding 
has shattered the underlying ore-formation, and here it will be remem¬ 
bered the ore-bodies of this horizon usually occur. 
Along these channels of percolation, as in the case of fissures, waters 
from various sources meet. A portion of these waters will have traveled 
for a considerable distance through the iron carbonates. Such waters 
will have oxidized this iron carbonate in part and thus become carbon¬ 
ated and take other iron carbonate into solution. Such iron-bearing 
waters will meet along the impervious formations other waters which 
have reached these positions by shorter paths, traveling perhaps wholly 
through already altered and brecciated ore-formation material contain¬ 
ing no iron carbonate. Such waters carrying no iron, but containing 
oxygen, will precipitate the iron from the carbonated solutions. 
Those ore-bodies which underlie one or more impervious formations, 
but rest upon other impervious formations have derived their material 
from the areas of iron formation material between the dike or other 
basement formations of the ore-bodies in question and those of the next 
overlying deposit. In a given instance this part of the ore-formation 
will have a considerable surface area for the entrance of percolating 
waters between the outcrops of the underlying and overlying impervious 
formations. (See Plate VII, fig. 3.) The waters have here carried the iron 
oxide along the impervious formation upon which the ore rests and have 
precipitated it under the overlying impervious body. Whether the ore- 
body thus produced fills the entire space between the two impervious 
formations depends upon the supply of material which was available 
and upon the perfection with which the process of concentration has 
been carried out. 
In the production of the magnetic ores it appears that there was not a 
sufficient amount of oxygen to peroxidize the iron, although there was 
15—A. & L. 
