990 The Crystalline Rocks of the Northwest. (October, 
dislocation the sediments of one formation were enabled to pene- 
trate transversely into the strata of another. 
This mica schist formation has an aggregate thickness of 
about 5000 feet, and sometimes is hornblendic rather than mica- 
ceous. 
- III. The next lower grand division, which is the third, might 
be styled the Zack mica slate group. This group contains much 
carbon, causing it to take the form of graphitic schists, in which 
the carbon sometimes amounts to over forty per cent? These 
schists are frequently quartzose, and also ferruginous, even com- 
posing valuable ore-deposits, as at the Commonwealth mine in 
Wisconsin. Associated with these black mica slates, which often 
appear also as dark clay-slates, are actinolitic schists, the whole 
being, in some places, interstratified with diorite. Their estimated 
thickness is 2600 feet. 
IV. Underneath this is a very thick series of obscure, iydrom- 
caceous and greenish magnesian schists, in which, along with beds 
of gray quartzite, and clay slates, occur the most important de- 
posits of hzmatitic iron ore. The lower portion of this series, 
which at Marquette is represented rather by hornblende and chlo- 
ritic quartz-schists, and more rarely is mined asa magnetic qu 
schist, at Penokee is known as “ the magnetic belt.” This divs- 
ion of the crystalline rocks has numerous heavy beds of diorite. 
V. Below this series of soft schists, which terminate downward 
with the magnetic iron ores, is the great quartzite and marble 
group. The marble lies above the quartzite, and in the Mane 
nee region has a minimum thickness of at least one thousand feet; 
while at Marquette it graduates into a dolomitic quartzite of in- 
definite extent, the whole group there being essentially 4 quay 
ite. This isa most persistent and well-marked horizon. The 
quartzite sometimes holds feldspar, thus having an appearance 
granulite. In northern Minnesota, the great slate-conglomerat® 
of Ogishke Muncie lake seems to represent the lower portion 
the great quartzite of this group, and to be the equivalent of 
lower slate-conglomerate of the “ typical Huronian” in Ca with 
In both places this conglomerate is sometimes speckled 
masses of red jasper. The marble of this group appears mane 
to the conglomerate south of Ogishke Muncie lake, and in SY 
£ * À 
< eens analysis of a specimen from near Aitkin, Minnesota, showed be" 
“two and forty-three per cent of carbon. 
