LAKE    SUPERIOR   REGION.  333 
Marquette  district  has  been  discriminated  under  the  name  Palmer 
gneiss.  The  characters  of  this  rock  are  peculiar  and  it  is  possible 
that  it  may  represent  in  part  a  much  altered  acidic  sediment.  It  is 
certain  that  some  of  the  rocks  mapped  as  Palmer  gneiss  are  sedi- 
mentary, but  such  sedimentary  rocks  really  belong  to  the  Algonkian 
and  have  been  included  in  the  Palmer  gneiss  because  of  errors  in 
mapping.  How  far  into  Wisconsin  these  basement  complex  granites 
and  gneisses  extend  is  not  known.  In  north-central  Wisconsin  gran- 
ites of  apparently  similar  characters  have  been  found  to  be  later  than 
the  basement  complex,  and  it  is  not  known  where  in  the  great 
granitic  area  extending  from  central  Wisconsin  to  the  State  boundary 
the  line  between  the  granites  of  the  basement  complex  and  the  later 
granites  should  be  drawn. 
Central  and  eastern  Minnesota. — In  this  region,  along  Minnesota, 
Mississippi,  and  Snake  rivers,  are  areas  of  granite,  gneiss,  massive 
greenstone,  gabbro,  hornblendic  schist,  micaceous  schist,  etc.,  which 
have  been  in  the  past  referred  to  the  Archean.  Hall,  however,  con- 
cludes that  certain  of  the  schists  have  resulted  from  the  intrusion 
of  granite  into  the  Huronian  sediments,  as  typically  developed  at 
Carlton  and  Cloquet,  and  thus  that  most  of  the  acidic  igneous  rocks 
are  of  Algonkian  age.  Certain  of  the  massive  greenstones  running 
eastward  from  Mississippi  River  are  older  than  the  granites  and  litho- 
logically  resemble  the  Keewatin  greenstones  in  the  Mesabi  and  Ver- 
milion districts.  These  are  accordingly  mapped  as  Keewatin,  while 
the  granites  are  mapped  as  Algonkian. 
Mesabi  district. — In  the  Mesabi  district  the  Archean  is  represented 
by  several  comparatively  small  areas  of  Keewatin  greenstone  and 
green  schist,  comprising  ellipsoidal  basalt  and  hornblendic,  chloride, 
and  micaceous  schists,  intrusive  into  which  are  granite  porphyries 
in  small  quantity. 
Vermilion  district. — In  the  Vermilion  district  the  Keewatin  series 
of  the  Archean  is  mainly  represented  by  greenstone,  much  of 
which  is  ellipsoidal  basalt,  in  some  cases  showing  surface  phases. 
It  occurs  in  great  oval-shaped  areas  wholly  or  partly  surrounded  by 
sediments.  Closely  associated  with  the  greenstones  in  relatively 
small  lenses  and  bands  is  the  other  chief  rock  of  the  Keewatin,  the 
iron-bearing  Soudan  formation,  which  includes  jaspilite,  iron  ore,  and 
slate.  The  relations  of  the  iron-bearing  formation  and  greenstone 
have  been  much  studied,  but  with  only  partially  positive  results.  The 
iron-bearing  formation  is  almost  certainly  sedimentary,  because  of 
its  association  and  mterstratincation  with  slates  and  because  of  the 
similarity  to  other  known  sedimentary  iron-bearing  formations  of 
the  Lake  Superior  region;  and  yet,  with  the  exception  of  a  trivial 
amount  of  slate  and  conglomerate  at  one  or  two  places,  the  jaspilites, 
making  up  the  great  mass  of  the  formation,  rest  with  sharp  contact 
