LAKE    SUPERIOR   REGION.  311 
Superior  is  regarded  in  part  as  having  been  once  continuous  with  the 
Animikie  series  and  is  now  separated  merely  because  of  erosion  on 
the  crowns  of  the  folds,  the  close  folding  of  the  Vermilion  schists 
being  produced  concomitantly  with  the  broad  simple  trough  of  Lake 
Superior.  In  support  of  this  position  is  the  fact  that  the  great  con- 
glomerate of  Ogishki  Manissi,  with  the  alternating  quartzites  and 
slates  of  Knife  Lake,  is  strikingly  like  the  Huronian  rocks.  More- 
over, in  the  vicinity  of  Agamok  Lake  the  Animikie  quartzites  appear 
gradually  to  take  on  a  folded  condition. 
In  these  various  Huronian  areas  quartzites,  graywackes,  and  clay 
slates,  with  intermediate  phases,  make  up  the  most  of  the  clastic 
series.  As  has  been  seen,  these  are  rocks  which  have  been  indurated 
by  metasomatic  changes,  and  it  follows  that  the  bulk  of  the  rocks 
which  form  the  Huronian  do  not  properly  fall  under  the  head  of 
metamorphic  rocks.  The  various  augitic  and  hornblendic  green- 
stones, peridotites,  and  felsitic  porphyries  are  regarded  as  eruptives, 
while  many  of  the  schists  are  modified  rocks  of  the  same  character. 
The  cherty  and  jaspery  rocks  are  supposed  to  be  some  sort  of  original 
chemical  sediment,  certainly  not  the  result  of  metamorphism  of  sedi- 
mentary material.  The  limestones  are  in  no  essential  respect  differ- 
ent from  many  met  with  in  the  formations  of  later  date. 
Irving,321  in  1886,  discusses  the  origin  of  the  ferruginous  schists  and 
iron  ores  of  the  Lake  Superior  region.  An  examination  of  the  Animi- 
kie, Penokee,  Marquette,  Menominee,  and  Vermilion  districts  reveals 
the  fact  that  in  all  of  them  is  found  abundant  carbonate  of  iron, 
which  oftentimes  grades  into  the  other  forms  of  the  iron-bearing 
formation.  The  silica  of  the  jasper,  actinolite,  magnetite  schists,  and 
other  forms  of  the  iron  belt  never  shows  any  evidence  of  fragmental 
texture,  so  easily  discovered  in  the  case  of  the  ordinary  quartzites  and 
graywackes,  and  is  therefore  of  chemical  origin.  Associated  with  the 
iron-bearing  beds  is  often  a  considerable  quantity  of  carbonaceous  or 
graphitic  schists.  It  is  concluded :  (1)  That  the  original  form  of  the 
iron-bearing  beds  of  the  Lake  Superior  region  was  that  of  a  series 
of  thinly  bedded  carbonates,  interstratified  with  carbonaceous  shaly 
layers  in  places,  which  were  more  or  less  highly  ferriferous.  ('2) 
That  by  a  process  of  silicification  these  carbonate-bearing  layers  were 
transformed  into  the  various  kinds  of  ferruginous  rocks  now7  met 
with.  (3)  That  the  iron  thus  removed  from  the  rock  at  the  time  of 
silicification  passed  into  solution  in  the  percolating  waters,  was  re- 
deposited  in  various  places,  and  thus  formed  the  ore  bodies  and  bands 
of  pure  oxide  of  iron.  (4)  That  in  other  places,  instead  of  leaching 
out,  the  iron  has  united  with  the  silicifying  waters  to  form  the  sili- 
cates now  found,  such  as  actinolite.  (5)  That  some  of  the  silicifying 
process  went  on  before  the  folding,  but  some  afterward,  and  to  the 
latter  period  belong  probably  the  larger  bodies  of  crystalline  ore. 
