236  PRE-CAMBEIAN    GEOLOGY    OF    NOETH    AMERICA. 
Winchell  (N.  II.),202  in  1899,  discusses  the  general  structural  geol- 
ogy of  northeastern  Minnesota.  The  ancient  rocks  of  this  area  he  places 
in  two  main  systems,  the  Archean  and  the  Taconic.  The  former  is  fur- 
ther subdivided  into  the  Upper  and  Lower  Keewatin,  separated  from 
each  other  by  an  unconformity.  The  Pewabic  quartzite  also  is  placed 
with  the  Keewatin,  but  is  not  assigned  to  either  of  the  main  divisions. 
Overlying  the  Archean  with  strong  unconformity  is  the- Taconic,  rep- 
resented by  Animikie  and  Keweenawan  rocks,  these  divisions  being 
supposed  to  represent  respectively  the  Lower  and  Middle  Cambrian 
of  other  parts  of  the  country.  The  Coutchiching  and  Laurentian 
rocks  before  mapped  as  separate  formations  are  now  included  within 
the  Keewatin. 
The  Lower  Keewatin  comprises  greenstone,  with  associated  sur- 
face volcanics  which  are  both  subaerial  and  subaqueous,  argyllitic 
slates,  siliceous  schists,  quartzites,  arkoses,  "  greenwackes,"  iron  ores, 
and  marble. 
The  greenstone,  designated  the  Kawishiwin,  is  the  oldest  known 
rock  in  the  State,  and  is  supposed  to  represent  a  portion  of  the  orig- 
inal crust  of  the  earth.  With  its  associated  volcanic  rocks  it  occurs  in 
two  main  belts.  The  southern  belt  begins  in  the  vicinity  of  Gunflint 
Lake  and  extends  westward  by  way  of  Gobbemichigamma  Lake,  the 
Kawishiwi  River,  and  White  Iron  Lake,  to  Tower,  and  indefinitely 
westward.  The  northern  belt  of  greenstone  enters  the  State  from 
Hunters  Island,  appearing  conspicuously  at  the  south  side  of  Bass- 
wood  Lake.  At  Pipestone  Rapids  and  Fall  Lake  it  widens  southward 
and  apparently  unites  at  the  surface  with  the  southern  belt,  the  over- 
lying Upper  Keewatin  being  absent  for  a  distince  of  a  few  miles.  But 
farther  west  it  is  again  divided  by  the  Stuntz  conglomerate,  the 
northern  arm  running  to  the  north  of  Vermilion  Lake,  west  of  which 
its  extension  is  unknown,  and  the  southern  one  running  south  of  the 
lake. 
The  fragmental  stratified  rocks  of  the  Lower  Keewatin  are  most 
important  toward  the  western  part  of  the  area  of  exposure  of  crystal- 
line rocks.  They  occupy  a  wide  area  south,  west,  and  north  of  Tower. 
The  iron  ores  of  Tower  and  Ely,  on  the  Vermilion  iron  range,  occur 
in  the  upper  part  of  the  Lower  Keewatin.  It  is  probable  that  the  im- 
mediately inclosing  rock  is  a  sedimentary  one,  although  composed  of 
the  elements  of  a  basic  eruptive.  %  The  sediments  extend  south  to  the 
Giants  range  of  granite,  where  they  are  metamorphosed  to  mica  schists 
by  the  granite.  Toward  the  west  they  extend  as  far  as  the  Mississippi 
River  and  its  northern  tributaries  and  across  the  Bowstring,  although 
the  drift  prevents  the  delimitation  of  the  belt.  To  the  northwest  they 
extend  toward  Rainy  Lake,  in  this  direction  being  converted  into  mica 
schists  and  gneisses  by  the  intrusion  of  granite;  in  unmodified  form 
they  are  found  at  one  point  only  on  Rainy  Lake,     These  fragmental 
