220 Wisconsin Academy of Sciences , Arts ancl Letters. 
In each there is one main iron-bearing formation both of which were 
originally of nearly the same character, ancl their subsequent trans¬ 
formations have been much alike. The chief differences are that hard 
specular hematite, magnetite, and the actinolite-magnetite-schists are 
more common in the older formation; while the soft hematites, limon- 
ites and cherts are more common in the newer formation. However, 
most phases of rocks and ores are found in both newer and older forma¬ 
tions, the main difference being that of relative proportions. 
As a consequence of the likeness of original characters and subse¬ 
quent transformations it is possible to treat together the genesis of the 
ores of these two series. 
A third horizon at which ore-bodies occur is at or near the base of the 
Upper Huronian. This formation is not continuous, it being present 
only when the base of the Upper Huronian chances to rest upon the iron¬ 
bearing formation of the Lower Huronian. Here the detritus has been 
largely derived from the immediately subjacent formation and is conse¬ 
quently rich in iron. As will be seen, a farther concentration of the iron 
oxides at this horizon has occurred at the same time as the concentra¬ 
tion of the iron ores of the two main formations. At this third horizon 
are to be placed several of the important mines and certain of the ore- 
bodies of others of the mines of the Marquette district. The history of 
these deposits explains why they frequently occur adjacent to the ore- 
deposits of the Lower Huronian. 
The Lower Huronian includes the Eastern Menominee, Felch Moun¬ 
tain, Lower Marquette, Vermilion, and Kaministiquia districts; and the 
Upper Huronian includes the Western Menominee, Upper Marquette, 
Penokee, Mesabi, and Animikie districts. It thus appears that in the 
Felch Mountain and Vermilion districts only the iron-bearing formation 
of the Lower Huronian is known, and in the Penokee and Mesabi dis¬ 
tricts only the iron-bearing formation of the Upper Huronian is known. 
In the Marquette and Menominee districts the iron formations of both 
series are represented, but the relations of the two are much more easily 
made out in the former than in the latter. The Kaministiquia and 
Animikie series also come together in a single district and the uncon- 
formable relations which here obtain are as clear as in the Marquette 
district. 
As areas to serve as types we will first consider the Penokee and Lower 
Marquette series, the first belonging to the Upper Huronian and the 
second to the Lower Huronian. These are chosen because here the in¬ 
vestigations have gone farther. 
The Penokee ores .-— In the Penokee district the iron-bearing formation 
comprising all the varieties of rocks above mentioned as belonging to 
this member, is on an average about 800 to 850 feet thick, but the iron 
ores are mainly confined to the lower 400 feet. The formation rests upon 
an argillaceous quartz-slate the uppermost horizon of which is a per- 
