238  PRE-CAM  BRIAN    GEOLOGY    OF    NORTH    AMERICA. 
Associated  with  the  Keewatin  rocks  are  granites  of  at  least  two 
periods  of  intrusion,  one  later  than  the  Lower  Keewatin  and  one  later 
than  the  Upper  Keewatin.  The  later  granite  is  believed  to  be  repre- 
sented by  the  higher  parts  of  the  Giants  range  and  the  Snowbank 
Lake  granite.  The  earlier  granite  is  represented  by  the  granites  at 
Kekequabic  Lake,  Saganaga  Lake,  Basswood  Lake,  Burntside  Lake, 
Vermilion  Lake,  Lac  la  Croix,  and  Kabetogoma  Lake.  The  origin  of 
the  granite  is  discussed. 
The  Taconic  is  unconformably  above  the  Keewatin  rocks.  It  com- 
prises the  Animikie  and  Keweenawan  divisions. 
The  Animikie  rocks  enter  the  State  at  Pigeon  Point  and  run  west- 
ward  along  the  international  boundary  to  the  eastern  part  of  sees.  22 
and  27,  T.  65  N.,  R.  4  W.  They  reappear  again  southwestward  from 
Birch  Lake,  on  the  northwest  side  of  the  gabbro  mass,  and  thence 
continue  along  the  south  side  of  the  Giants  range,  constituting  the 
Mesabi  iron  series,  to  Pokegama  Falls.  The  higher  parts  of  the 
Animikie  are  best  developed  toward  the  east,  while  the  lower  parts 
are  best  developed  toward  the  west. 
The  Animikie  rocks  comprise  the  Pokegama  quartzite,  Mesabi  iron- 
bearing  formation,  and  some  limestone  and  slate,  all  strictly  con- 
formable with  one  another.  The  thickness  is  several  hundred  feet, 
sometimes  reaching  nearly  1,000  feet.  The  dip  of  the  series  is  uni- 
formly 8°  to  12°. 
The  iron-bearing  formation  and  the  Pokegama  quartzite  constitute 
the  base  of  the  formation.  The  quartzite  in  places  is  beneath  the  iron 
formation ;  in  other  places  it  is  in  the  same  horizon ;  and  in  still  others 
is  above  the  iron  formation.  Commonly  the  base  of  the  Animikie  is 
marked  by  a  conglomerate,  containing  debris  from  the  underlying 
Keewatin  rocks.  This  is  a  narrow  horizon  which  soon  graduates 
upward  into  a  quartzite  known  as  the  Pokegama  quartzite,  from  its 
typical  development  near  Pokegama  Falls  on  Mississippi  River. 
The  thickness  of  the  quartzite  is  not  known  to  exceed  50  feet,  and  is 
sometimes  less  than  25  feet. 
Above  the  quartzite,  or  in  alternating  beds  with  it  or  below  it, 
appears  the  iron-bearing  or  taconite  member  of  the  Animikie,  which 
contains  the  iron-ore  deposits  of  the  Mesabi  iron  range.  The  ore  is 
usually  hematite  in  the  western  part  of  the  range  and  magnetite  in 
the  eastern  part.  It  was  previously  supposed  to  have  been  derived 
from  the  alteration  of  a  greenish  glauconitic  sand  rock ;  but  later  work 
has  seemed  to  show  that  the  greensand  is  a  volcanic  sand,  and  that 
the  so-called  taconitic  rock  itself  has  resulted  from  igneous  forces. 
This  is  accounted  for  by  supposing  a  chain  of  active  volcanoes  to  have 
existed  where  the  Mesabi  iron  range  is  now  found.  These  volcanoes 
yielded  flows  and  ejectamenta  to  the  adjacent  waters,  which  have 
been  modified  into  the  various  phases  of  the  iron  formation  now  seen. 
