LAKE    SUPERIOR   REGION.  203 
lar  and  several  hundred  feet  long;  the  ends  of  the  bands  come  out 
squarely  to  the  contact  with  the  schist  as  to  a  fault. 
Winchell  (Alexander),1"'5  in  1887,  gives  detailed  observations 
made  on  an  extended  trip  in  northeastern  Minnesota.  The  region  pre- 
sents a  series  of  schists  flanked  on  the  north  and  south  by  massive 
crystalline  rocks.  In  the  western  part  of  the  district  these  rocks  are 
gneissic  on  both  sides,  but  to  the  east  the  gneissic  rocks  on  the  south 
are  replaced  by  gabbro  and  greenstone.  The  schists  and  bedded 
crystallines  stand  in  a  nearly  vertical  attitude,  having  a  persistent  and 
uniform  strike  and  dip,  the  latter  oscillating  from  80°  N.  to  80°  S. 
The  rocks  are  sericitic,  chloritic,  micaceous,  and  hornblendic  schists 
and  argillites  and  graywackes.  The  schists  grade  into  the  gneissic 
rocks,  there  being  nowhere  an  abrupt  passage  from  one  class  to  the 
other.  In  the  passage  from  the  schists  to  the  gneisses  there  is  first  an 
increase  in  frequency  of  ramifying  veins,  then  lumps  of  gneiss  or 
granite  in  the  schists,  and  next  interstratification  of  the  schists  and 
gneisses.  The  conglomerate  at  Ogishki  Manissi  Lake,  which  attains 
an  enormous  development  and  contains  varieties  of  granitic  and 
quartzose  bowlders,  as  well  as  flint,  jasper,  and  other  substances,  is 
regarded  as  a  local  phase  of  the  schists,  as  the  bowlders  are  inter- 
bedded  with  the  flinty  argillites  and  sericite  schists.  The  entire  sys- 
tem of  gneisses  and  schists  is  regarded  as  belonging  to  one  structural 
system,  as  they  all  possess  a  common  dip  and  pass  by  gradations  into 
one  another.  The  iron-bearing  rocks  are  interlaminated  with  the 
country  schists,  and  while  they  exhibit  much  persistence,  they  do  not 
persist  without  interruption.  In  structure  the  region  is  a  simple  syn- 
clinal fold,  the  strata  of  which  have  a  thickness  of  106,204  feet.  The 
succession  from  the  bottom  upward  is  granite,  gneiss,  micaceous  and 
hornblende  schists,  graywacke,  argillite  schist  bearing  conglomerates, 
and  sericitic  and  chloritic  schists  bearing  iron  ores.  As  the  plainly 
fragmental  rocks  grade  by  imperceptible  stages  into  the  gneiss  and 
granite,  the  whole  is  regarded  as  a  sedimentary  series.  While  granite 
pebbles  are  found  in  the  conglomerates,  this  is  not  the  underlying 
granite,  as  many  of  the  fragments  differ  in  character  from  the  in- 
ferior granite. 
Winchell  (N.  H.),150  in  1887,  gives  very  numerous  details  as  to 
the  geology  of  northeastern  Minnesota.  At  several  places  there  are 
transitions  between  the  granite  gneiss  and  a  fine-grained  mica  schist. 
In  the  syenite  are  sometimes  found  angular  fragments  of  mica  schist. 
The  Vermilion  group  is  defined  as  including  the  lower  portion  of  the 
complex  series  of  schists  designated  Keewatin  by  Lawson.  It  em- 
braces the  mica  schists  and  hornblende  schists  of  Vermilion  Lake  and 
their  equivalents,  and  lies  between  the  graywackes  on  the  one  side  and 
the  basal  syenites  and  granites  on  the  other. 
