224 
Wisconsin Academy of Sciences , Arts and Letters. 
lies unconformably upon impervious green crystalline schists of the 
Archean complex. They are overlain by a slate or shale of great thick¬ 
ness. They have a gentle dip to the southward. In certain respects they 
are like the Penokee deposits. Whether the bodies will be found in 
pitching troughs, it is yet too early to say. 
Genesis of the ores .— The peculiar forms and relations of the Lake 
Superior iron ores exclude a large number of explanations which have 
been advanced for the genesis of these deposits. It is evident that in 
their present position they are not eruptives. Even if it be argued that 
the iron-bearing formations are igneous it would hardly be held that 
these peculiar ore-bodies are of direct intrusive origin. The forms 
which these bodies have are wholly unlike those of intrusive rocks. .In¬ 
stead of being continuous downward as such rocks should be they usu¬ 
ally terminate below upon igneous rocks. It is equally plain that the 
ore-deposits are not of direct sedimentary origin, although it is believed 
that the formations containing them, and from which they are derived 
are sedimentary. We know of no way by which sediments could be de¬ 
posited in such irregular forms as these. Also their frequent connection 
with subsequent intrusive rocks shows that between the ore-deposits 
and the latter there is some genetic connection. Although by the miners 
the ores are often spoken of as veins, these deposits have never been 
seriously regarded as fissures, nor can they be regarded as deposits 
which have filled caves. 
All of the evidence plainly points in one direction, that is, that they 
are concentrations produced by downward percolating water. These 
waters removed a part of the original material of the iron-bearing for¬ 
mations at the places where the ore-bodies occur and introduced iron 
oxide nearly simultaneously. This explains the forms, positions and re¬ 
lations of the ore-deposits. They rest upon tilted walls or troughs of 
impervious formations because water has here been converged. They 
occupy places once taken by a part of the ore-formation because this is 
readily penetrated by water, because it was rich in iron carbonate, and 
because the constituents other than iron oxide are readily soluble. 
The original condition of the ore-formation, as has been said, is a lean 
sideritic and cherty slate. In order that the ore-bodies should be formed, 
silica must have been removed and iron oxide introduced. That this inter¬ 
change has actually occurred is shown by an examination of the iron- 
formation rocks associated with the ore-bodies. It has been noted that 
the change from the ore-bodies to the rocks above is a transition rather 
than abrupt. Along this transition zone it is a common thing to see sil¬ 
ica ban(Is die out by gradual removal. In the iron formation proper the 
silica is frequently in nearly solid bands, alternating with bands richer 
in iron. In passing toward the ore, cavities appear in the rock, the silica 
being removed so that the stratum is here a porous one. The cellular 
or geodal cavities formed by the removal of silica are very characteristic 
