742 Qn VG JEJE Tah 
throughout, and contains the greater iron ore deposits of the district. 
The formation comprises sideritic slate, which may be griineritic, mag- 
netitic, hematitic, or limonitic; griinerite-magnetite-schist ; ferrugin- 
ous slate; ferruginous chert; jaspilite, and iron ore. Large quanti- 
ties of intrusive greenstones are associated with the formation, the 
masses of which vary in magnitude from great bosses two miles or 
more long anda half mile wide to small dikes. The largest area of 
the Negaunee formation is in the east-central part of the district. 
From this area two belts extend west to the western end of the district. 
Upon the whole the formation is soft, and occupies lowlands between 
the more resistantsgreenstones and the Ajibik quartzites. The formation 
is underlain by the Siamo slate or Ajibik quartzite, into which it 
grades, and is overlain unconformably by the Upper Marquette series. 
The sideritic slate is the original form from which the other varieties 
of rock have developed. The griinerite-magnetite-schists were formed 
by partial recrystallization of the silica, by oxidation of the iron oxide 
in part to magnetite, by a union of a part of the silica and iron pro- 
toxide, producing griinerite, and with the loss of carbon dioxide. 
The ferruginous slates are the direct result of the decomposition of 
the iron carbonate and the peroxidation of the iron, with partial or 
complete recrystallization of the silica. The ferruginous cherts differ 
from the ferruginous slates in that the iron oxide and the chert are 
largely concentrated into alternate bands. The jaspilites differ from 
the ferruginous cherts in that each of the quartz grains of the chert 
bands is stained red by included hematite. ‘The iron ores resulted 
from the concentration of the iron oxides through the agency of 
downward-percolating waters. These concentration-bodies usually 
occur upon impervious basements in pitching troughs. The pitching 
troughs are formed by the Siamo slate, the Ajibik quartzite, a mass or 
dike of greenstone, or by some combination of these. The ore deposits 
are likely to be of large size where, as a result of the folding, the iron- 
bearing formation is much fractured, thus permitting the ready access 
of percolating waters. The ore deposits occur at the bottom of the 
Negaunee formation, within the Negaunee formation, and at the 
contact horizon between the Negaunee formation and the overlying 
Ishpeming formation. From the position of the ore deposits 
above the impervious formations, it is concluded that their concentra- 
tion occurred during or subsequent to the folding which took place 
later than Upper Marquette time. 
