LAKE    SUPERIOR   REGION.  171 
many  reverse  dips  to  the  north.  Its  thickness  is  placed  at  from  1,000 
to  1,250  feet. 
Above,  the  Sturgeon  quartzite  grades  into  the  Eandville  formation, 
which  is  mainly  a  homogeneous  dolomite  interstratified  with  siliceous 
or  argillaceous  layers.  This  formation  appears  in  three  belts.  The 
northern  one  is  just  south  of  the  belt  of  the  Sturgeon  quartzite.  The 
central  belt  is  on  the  north  side  of  Lake  Antoine  for  a  portion  of  its 
length,  passes  eastward  between  the  Cuff  and  the  Indiana  mines,  and 
ends  at  the  bluff  known  as  Iron  Hill,  in  the  east  half  of  sec.  32,  T. 
40  N.,  E.  29  W.  The  southern  belt  of  dolomite  extends  all  the  way 
from  the  western  side  of  the  sandstone  bluff  west  of  Iron  Mountain  to 
the  village  of  Waucedah,  at  the  eastern  end  of  the  mapped  area. 
Structurally  the  northern  belt  of  dolomite  is  a  southward-dipping 
monocline,  while  the  two  southern  belts  are  anticlines.  The  thick- 
ness is  not  determined  on  satisfactory  evidence,  but  is  probably  1,000 
feet  or  more.  The  Eandville  formation  is  found,  in  a  number  of 
mines,  in  contact  with  the  basal  formation  of  the  Upper  Menominee 
series.  .  Here  there  is  a  coarse  conglomerate  in  the  basal  part  of  the 
overlying   formation,  indicating  unconformity. 
The  Negaunee  formation,  overlying  the  Eandville  dolomite,  is 
represented  in  the  district  by  so  few  and  so  small  outcrops  that  it  is 
mapped  with  the  Vulcan  formation.  Its  presence  is  inferred  mainly 
from  the  occurrence  of  abundant  iron-formation  debris  in  the  basal 
conglomerate  of  the  Upper  Menominee  formation,  showing  that  the 
Lower  Menominee  iron-bearing  series  must  have  been  present.  In 
the  Marquette  district  an  iron-bearing  formation  (the  Negaunee) 
occupies  an  exactly  similar  stratigraphical  position. 
Upper  Menominee  series. — The  formations  between  the  uncon- 
formity at  the  top  of  the  Lower  Menominee  and  the  unconformity  at 
the  base  of  the  Lake  Superior  sandstone  are  placed  in  the  Upper 
Menominee  series.  These  occur  in  two  great  series,  the  Vulcan  and 
the  Hanbury. 
The  Vulcan  formation  is  unconformable  above  the  upper  part  of 
the  Lower  Huronian,  which  for  the  most  of  the  district  is  the  Eand- 
ville formation,  and  unconformable  beloAV  the  Hanbury  slate.  For 
parts  of  the  district  the  Vulcan  iron-bearing  formation  does  not  ap- 
pear at  all  between  the  dolomite  and  the  slate,  and  its  absence  is 
explained  by  the  unconformity  between  the  Vulcan  formation  and 
the  Hanbury  slate.  The  Vulcan  formation  embraces  three  members. 
These  are,  from  the  base  up,  the  Traders  i  ron-bearing  member,  the 
Brier  slate,  and  the  Curry  iron-bearing  member.  They  are  mapped 
as  a  single  formation.  The  principal  area  of  the  Vulcan  iron  forma- 
tion is  in  the  belt,  900  to  1,300  feet  wide,  following  the  sinuosities  of 
the  southern  border  of  the  southern  belt  of  Eandville  dolomite.  It  is 
generally  absent  north  of  the  southern  belt,  except  at  the  east  end, 
