LAKE    SUPERIOR   REGION.  337 
The  series  have  been  folded  into  complex  folds,  the  trend  of  the 
closer  folds  varying  from  north-south  to  east-west.  The  closer  folds 
in  the  northeastern  part  of  the  district  are  nearly  north-south;  in 
the  central  part,  northwest-southeast ;  in  the  eastern  and  southeastern 
parts,  nearly  east- west.     All  of  these  folds  have  steep  pitches. 
Menominee  district. — The  Huronian  rocks  of  this  district  have  been 
divided  into  the  "  Lower  Menominee  "  and  "  Upper  Menominee/'' 
These  terms  are  now  discarded  in  favor  of  lower  Huronian  and 
upper  Huronian.  The  lower  Huronian  comprises  the  Sturgeon 
quartzite  and  Randville  dolomite,  in  all  respects  similar  to  forma- 
tions of  this  name  in  the  Crystal  Falls  district  and  similar  respec- 
tively to  the  Mesnard  quartzite  and  the  Kona  dolomite  of  the  lower 
Huronian  of  the  Marquette  district.  A  middle  Huronian  has  not 
been  found,  although  iron-formation  pebbles,  supposed  to  represent 
it,  are  found  in  the  conglomerate  at  the  base  of  the  next  overlying 
group.  The  upper  Huronian  comprises  (1)  the  Vulcan  formation, 
consisting  of  three  members,  in  ascending  order  (a)  Traders  iron- 
bearing  member,  (b)  Brier  slate  member,  and  (c)  Curry  iron-bear- 
ing member,  and  above  this  (2)  the  Hanbury  slate,  bearing  in  lower 
portions  calcareous  slates,  siderite,  and  iron  oxide.  The  thickness 
of  the  Vulcan  formation  averages  650  feet.  The  thickness  of  the 
Hanbury  slate  is  uncertain,  but  is  thought  to  be  2,000  to  3,000  feet. 
There  seems  to  be  an  unconformity  between  the  upper  and  lower 
Huronian,  marked  by  a  basal  conglomerate  but  not  by  structural 
discordance  or  evidence  of  extensive  intervening  erosion. 
In  the  Menominee  district  the  structure  has  been  described  as  a 
northwest-southeast  trending  synclinorium  with  a  minor  anticlinal 
roll  which  brings  up  the  older  formations  in  the  central  portion  of 
the  district.  However,  attention  is  directed  to  the  fact  that  the 
lower  Huronian,  which  ought  to  appear  on  the  south  side  of  the 
trough,  is  here  missing,  and  the  synclinal  structure  is  inferred  rather 
than  actually  observed. 
Penol-ee- Gogebic  district. — In  the  Penokee-Gogebic  district  are 
found  representatives  of  the  lower  Huronian  and  upper  Huronian, 
with  intervening  unconformity.  The  middle  Huronian  seems  to  be 
lacking.  The  lower  Huronian  comprises  a  lower  quartzite  overlain 
by  a  cherty  limestone  (the  latter  well  exposed  along  Bad  River), 
similar  respectively  to  the  Mesnard  quartzite  and  the  Kona  dolomite 
of  the  Marquette  district.  The  upper  Huronian  (also  called 
"Penokee-Gogebic  series")  comprises,  from  the  base  up,  the  Palms 
formation,  the  iron-bearing  Ironwood  formation,  and  the  Tyler  slate, 
possibly  13,300  feet  in  thickness.  Much  the  largest  portion  of  the 
district  is  occupied  by  the  upper  Huronian  rocks,  principal  in  volume 
among  which  is  the  slate.     Intrusive  into  the  upper  Huronian  are 
55721— Bull.  360—09 22 
