The Iron Ores of the Lake Superior Region. 
223 
the latter case this may be sometimes due to the fact that erosion has 
removed all or nearly all of the Lower Hnronian iron-bearing formation 
The ores above this contact plane, like those already considered, are 
frequently adjacent to or underlain by subsequent intrusives which serve 
as impervious basement formations. 
The ores adjacent to the contact plane include the magnetites of the 
Marquette district. Those belonging to the Lower Huronian have a some¬ 
what different aspect from those of the Upper Huronian. This is due 
to the extraneous mechanical detritus of the latter. In thin section it 
is often found that more or less mica has developed. In examining the 
structure of the ore-deposits, it is seen that much of the magnetite is in 
veins or cavities in finer grained and partly or wholly original material. 
An examination of thin sections of the overlying magnetic quartzites 
and conglomerates shows conclusively that much of the magnetite is a 
secondary infiltration. 
Ores of other districts .— It is unnecessary to give the details of the oc¬ 
currences of ores in other districts, but it may be said, while the 
character of the bounding formations may be somewhat different, that 
the ore-deposits as in the Penokee and in the Marquette districts rest 
upon formations which are impervious. These may be fragmental 
slates, contemporaneous surface volcanics, or subsequent intrusives. 
Also any one of these, or two of them, may combine to form a pitching 
trough. And even in the cases in which there is but a single impervious 
wall upon which the ore rests, it is usually found that the deposit has a 
pitch. The ore-bodies usually grade above into the other phases of 
rock of the iron-bearing formation, as in the Penokee and Marquette 
districts. 
In the Western Menominee district the ore-bodies very extensively 
rest upon surface volcanics which appear to be here the inferior forma¬ 
tion of the Upper Huronian series. In both the Upper Menominee and 
the Upper Marquette ores are also found resting upon the slates. In 
some cases these underlying slates have a monoclinal dip as in the Pen¬ 
okee district, and in others are folded into pitching synclinals. The 
iron-bearing horizons of these districts do not appear to be continuous 
belts of pure iron formation materials as is that of the Penokee. They 
are often but sideritic phases of the great slate formation of the Upper 
Huronian. Where this slate becomes usually sideritic and the other 
conditions explained as requisite occur together the concentration 
of ore-bodies has taken place. 
In the Vermilion district the ore-bodies commonly rest upon im¬ 
pervious schists believed to be greatly modified volcanics or upon intru¬ 
sive massive greenstones of later age. Not infrequently one or both of 
these combined form pitching troughs. 
In the newly developed Mesabi range the ores, according to Mr. Mer- 
riam, are near the base of the series resting upon a quartzite which 
