LAKE    SUPERIOR   REGION.  335 
rentian  granite  and  gneiss.  Lawson  holds  that  the  Laurentian 
granite  represents  fused  Keewatin  rocks.  The  writers  believe  that 
the  relations  clearly  exhibited  at  the  contacts  of  the  Laurentian  and 
Keewatin  are  those  of  normal  igneous  intrusion,  and  that  there  is  no 
evidence  presented  to  warrant  an  appeal  to  subcrustal  fusion  to 
explain  the  relations.  (See  observations  of  Van  Hise  and  Lane, 
pp.  284,  294-295,  324-327.) 
Michipicoten  district. — The  Archean  of  the  Michipicoten  district 
presents  close  similarities  to  the  Archean  of  other  parts  of  the  Lake 
Superior  region,  especially  of  the  Vermilion  district.  It  consists  of 
Keewatin  greenstones  Avith  ellipsoidal  and  other  surface  structures, 
of  tuffs,  both  basic  and  acidic,  of  an  iron-bearing  formation,  and  of 
slate,  all  intruded  by  Laurentian  granites  and  gneisses.  The  iron- 
bearing  formation  is  believed  to  be  the  counterpart  of  that  seen  in 
the  Keewatin  in  the  Vermilion  and  Marquette  districts  and  elsewhere 
in  the  Lake  Superior  region. 
Other  points  on  the  north  shore. — Greenstones  and  green  schists 
with  associated  slates  and  iron  formation,  similar  in  many  respects 
to  those  of  the  Michipicoten  and  Vermilion  districts,  are  known  in 
many  areas  in  Ontario.  They  are  typically  developed  in  the  Matta- 
wan,  Atikokan,  and  Steep  Rock  Lake  iron  ranges,  northwest  of  Lake 
Superior. 
ALGONKIAN  SYSTEM. 
The  full  succession  of  the  Algonkian  system  for  the  Lake  Superior 
region  comprises  four  unconformable  sedimentary  divisions,  all  of 
them  being  associated  with  igneous  rocks.  The  three  lower  divisions 
are  collectively  referred  to  the  Huronian  series;  the  upper  is  the 
Keweenawan  series.  All  are  unconformably  above  the  basement  com- 
plex and  unconformably  below  the  Paleozoic  for  the  region.  In  no 
district  are  all  of  these  series  represented.  While  there  are  similari- 
ties between  the  Huronian  series  of  the  different  districts  which  are  be- 
lieved to  warrant  their  correlation,  the  differences  are  such  as  to  make 
it  desirable  to  discuss  the  Huronian  successions  of  the  individual 
districts  separately,  in  order  to  avoid  the  impression  of  finality  in 
correlation,  which  would  inevitably  tend  to  obscure  or  perhaps  dis- 
tort the  facts.  The  Keweenawan  series  is  essentially  a  unit  for  the 
region  and  it  is  so  described  on  a  subsequent  page. 
HURONIAN    SERIES. 
Marquette  district. — In  the  Marquette  district  all  three  groups  of 
the  Huronian  series  are  present.  They  have  been  locally  called  the 
"Lower  Marquette,"  "Middle  Marquette,"  and  "Upper  Marquette," 
but  these  terms  are  uoav  superseded  by  lower  Huronian,  middle 
Huronian,  and  upper  Huronian.  Prior  to  1904  the  middle  Huronian 
and  lower  Huronian  had  been  mapped  together  as  "Lower  Huronian.'" 
