LAKE    SUPERIOR  REGION.  195 
In  the  north-central  Wisconsin  area  the  pre-Cambrian  a  was  worn 
down  to  base-level  by  subaerial  erosion  before  the  much  later  Pots- 
dam sandstone  (upper  Cambrian)  was  deposited  upon  it. 
Nothing  definite  can  be  stated  concerning  the  correlation  of  these 
various  groups  and  formations  with  pre-Cambrian  formations  in 
other  districts.  It  is  believed,  however,  that  the  upper  sedimentary 
series  may  be  the  stratigraphic  equivalent  of  the  Baraboo  quartzite, 
which  has  been  previously  correlated  h  with  the  Ajibik  quartzite  of 
the  Marquette  and  which  belongs  with  the  Middle  Marquette  (or 
Middle  Huronian)  series.  This  belief  is  based  merely  upon  similar- 
ity to  the  Baraboo  quartzite  in  the  general  character  of  the  rock  and 
upon  similar  relations  to  older  igneous  and  sedimentary  formations. 
The  intrusive  group  of  igneous  rocks,  consisting  of  rhyolite,  dio- 
rite-gabbro,  and  granite-syenites,  contains  phases  similar  to  those 
occurring  in  the  igneous  basement  in  the  Baraboo  district.  The 
igneous  intrusives  are  also  similar  to  those  bordering  the  southern 
side  of  the  Menominee  district.  The  lower  sedimentary  series  of  the 
area,  consisting  of  much  metamorphosed  quartzites  and  slates  in  the 
form  of  fragmentary  masses  in  the  later  intrusives,  may  be  of  Lower 
Huronian  age,  while  the  basal  group  of  the  area  probably  belongs 
with  the  Archean,  since  it  shows  the  usual  complex  mixture  and  meta- 
morphism  of  the  igneous  rocks  below  the  Huronian  sedimentaries. 
Weidman,129  in  1908,  reports  that  in  northwestern  Wisconsin  the 
pre-Cambrian  is  largely  of  igneous  origin.  The  most  prominent 
sedimentary  areas  are  the  prominent  ridge  of  quartzite  at  the  junc- 
tion of  Flambeau  and  Chippewa  rivers  and  the  numerous  quartzite 
ridges  along  the  divide  of  Chippewa  and  Red  Cedar  rivers,  in  Rusk, 
Sawyer,  and  Barron  counties.  In  general  these  quartzites  dip  west- 
erly away  from  the  crystalline  and  schistose  area,  with  strongly 
marked  eastward  escarpments  overlooking  the  nearly  flat  plane  of 
the  older  rocks.  While  no  final  conclusion  has  been  reached  con- 
cerning the  relative  age  of  these  quartzites,  the  opinion  is  expressed 
that  there  are  here  represented  at  least  two  and  probably  three  series. 
The  quartzites,  such  as  the  small  outcrops  along  the  railroad  about  3 
miles  east  of  Weyerhauser,  are  greatly  metamorphosed  and  are  cor- 
related with  the  Rib  Hill  quartzite  at  Wausau.  The  prominent  ridge 
of  quartzite  at  the  junction  of  the  Flambeau  and  the  Chippewa  is 
correlated  with  the  upper  sedimentary  series  in  north-central  Wis- 
consin and  the  Baraboo  quartzite.  The  prominent  ridges  of  quartz- 
ite in  eastern  Barron  County  and  in  the  adjacent  parts  of  Rusk  and 
Sawyer  counties  are  but  slightly  metamorphosed,  the  bedding  is  in 
"Weidman,  Samuel,  The  pre-Potsdam  peneplain  of  the  pre-Cambrian  of  north-central 
Wisconsin  :  Jour.  Geology,  vol.  11,  1903,  pp.  289-313. 
b  Weidman,  Samuel,  The  Baraboo  iron-bearing  district  :  Bull.  Wisconsin  Geol.  and  Nat 
Hist.   Survey  No.   13,   1904,   p.   170. 
