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Wisconsin Academy of Sciences , Arts and Letters. 
enough to precipitate it or a part of it at least. Pyrite associated with 
the magnetite indicates the presence of actual reducing agents and 
these may have changed some of the original hematite of this horizon 
to the form of magnetite. This lack of oxygen at the plane separating 
the Lower and Upper Hnronian may be due to the fact that immediately 
above is the impervious slate of the latter; consequently from the sur¬ 
face there w T as no direct path for percolating waters. 
From what has been said as to the transition zone between the ore- 
bodies and the iron formation, it will be seen that the process of the 
solution of the silica often runs in advance of the introduction of the 
iron oxide. It is wholly possible that the silica has been sometimes 
removed so far ahead as to cause a considerable sagging of the ore-for¬ 
mation. This suggestion is made because the brecciated character of 
the rocks adjacent to the ore-bodies frequently indicates that local 
fractures have occurred. 
Many of the intrusives which cut the ore-formations are probably of 
Keweenawan age. If this be true it is evident that the concentration of 
these ore-deposits has occurred since Keweenawan time. It is also 
manifest that the final concentration did not occur until the folding 
and erosion subsequent to both the Lower and Upper Huronian series,, 
and for a part of the districts at least these were post-Keweenawan. It 
is almost certain that the Lower Huronian iron formation was exten¬ 
sively modified before the Upper Huronian series was deposited, but it 
is also probable that the ore-bodies now mined have been produced 
simultaneously with those of the Upper Huronian, otherwise the Lower 
Huronian ore-deposits would not invariably be found above the eruptive 
rocks with which they are in contact. While the final concentration 
did not begin until the later foldings to which these series have been 
subjected, there is no evidence that the process has ceased at the pres¬ 
ent time. 
The ore-bodies at the base of the Upper Huronian were concentrated 
in the lean detritus of the Lower Huronian iron formation at the 
same time and in the same manner as the ore-deposits just considered. 
The remarkable likeness of the Upper and Lower Huronian ore- 
formations is then explained to be due to the likeness of the original 
iron-bearing formations of the Upper and Lower Huronian, and to the 
fact that the concentration of both was due to the same causes operat¬ 
ing at the same time. 
The Huronian rocks of Lake Superior are often spoken of as the iron¬ 
bearing series. The foregoing discussion shows that ore-bodies occur 
only in certain definite formations which constitute but a small percent¬ 
age of the entire Huronian series. Moreover it is evident that valuable 
ores are only found within these formations where a combination of 
peculiar conditions occur causing local concentrations of iron oxide. A 
