338  PKE-CAMBRIAN    GEOLOGY   OF    NORTH   AMERICA. 
diabase  dikes,  probably  of  Keweenawan  age.  The  principal  rocks 
of  the  district,  the  upper  Huronian,  strike  in  general  east-northeast 
and  dip  from  35°  to  80°,  averaging  perhaps  65°,  N. 
Iron  River  and  vicinity. — In  the  broad  triangular  area  between 
the  east  end  of  the  Penokee-Gogebic  district  and  the  Marquette,  Crys- 
tal Falls,  and  Menominee  districts  is  a  great  slate  formation  consti- 
tuting the  upper  part  of  the  upper  Huronian.  In  isolated  areas 
within  this  slate  appear  iron  ores,  productive  at  Iron  River  and 
Florence.  Locally  there  appear  also  older  Huronian  sediments,  the 
correlation  of  which  is  doubtful.  Finally,  at  various  places  within 
this  district  are  areas  of  basic  volcanics,  the  structural  relations  of 
which  are  for  the  most  part  obscure.  The  volcanic  rocks  of  Brule 
River  in  the  southern  part  of  the  Iron  River  district  are  probably 
Keewatin,  while  the  remainder  are  upper  Huronian,  with  the  excep- 
tion of  a  short  extension  of  the  Hemlock  formation  (lower  Huro- 
nian) into  the  east  side  of  the  district. 
Northeastern  Wisconsin. — Extending  southwest  from  the  Menomi- 
nee district  into  north-central  Wisconsin  is  an  area  containing  Algon- 
kian  rocks  with  indefinite  limits.  This  area  contains  considerable 
masses  of  quartzite  and  greenstone  schists,  the  latter  probably  be- 
longing to  the  basement  complex. 
North-central  Wisconsin. — In  north-central  Wisconsin  are  two  un- 
conformable sedimentary  groups,  tentatively  correlated  by  Weidman 
with  the  middle  Huronian  and  lower  Huronian.  These  rest  uncon- 
formably  upon  a  Laurentian  complex  of  granite,  gneiss,  syenite,  and 
diorite.  The  lower  of  the  two  Huronian  groups  is  intruded  by 
great  masses  of  rhyolite,  diorite,  gabbro,  peridotite,  granite,  syenite, 
and  related  rocks,  these  making  up  75  per  cent  of  the  entire  area. 
Chippewa  River  and  Barron  County. — In  Chippewa  and  Barron 
counties  are  quartzites  apparently  of  two  ages.  These  quartzites  are 
similar  to  those  of  the  Huronian  of  Michigan.  The  slight  metamor- 
phism  and  the  geographical  relations  of  the  upper  quartzite  with  the 
Gogebic  phase  of  the  Penokee  group  suggest  its  upper  Huronian  age. 
Central  Minnesota. — In  the  vicinity  of  Carlton,  Cloquet,  and  Little 
Falls,  Minn.,  and  extending  to  the  southwest,  is  a  considerable  area  of 
well-banded  graywacke  and  slate  similar  in  all  respects  to  the  upper 
Huronian  of  the  Mesabi  district  to  the  north.  Where  they  become 
hornblendic  and  micaceous  Hall  believes  them  to  be  the  result  of 
metamorphism  of  the  graywacke  and  slate  by  intrusive  granite. 
Near  Dam  Lake,  south  of  Kimberly,  is  a  quartzite  exposure  which 
has  been  correlated  with  the  upper  Huronian  of  the  Mesabi  district. 
In  the  Cuyuna  district  to  the  west,  in  the  vicinity  of  Deerwood 
and  Brainerd,  extensive  drilling  through  a  thick  drift  covering  has 
shown  the  presence  of  an  iron  formation  containing  iron  carbonate, 
iron  ore,  and  ferruginous  chert  phases,  underlain  by  magnetic  quartz 
