248  PRE-CAMBRIAN    GEOLOGY    OF    NORTH   AMERICA. 
more  directly  from  the  surface.  Pari  passu  with  this  precipitation 
silica  was  removed,  affording  space  for  the  accumulation  of  the  iron, 
to  form  the  ore  deposits  as  now  known.  The  Tower  and  Soudan 
deposits  differ  only  in  detail  from  the  Ely  deposit. 
Granites  intrusive  into  the  Archean  occupy  a  wide  area  and  are 
named  from  the  topographic  features  with  which  they  are  conspicu- 
ously associated.  That  these  intrusives  are  older  than  the  Ogishke 
conglomerate  (Lower  Huronian),  which  succeeds  in  age  the  Soudan 
formation,  is  shown  conclusively  by  the  fact  that  pebbles  derived  from 
them  occur  in  this  conglomerate.  The  general  period  of  intrusion  of 
all  of  these  acidic  igneous  rocks  is  placed  between  the  time  of  the 
deposition  of  the  latest  sediments  of  the  Archean  and  that  of  the 
deposition  of  the  earliest  sediments  of  the  Lower  Huronian  series. 
The  Lower  Huronian  occurs  in  two  detached  areas,  one  of  which, 
known  as  the  Vermilion  Lake  area,  extends  from  the  western  limit  of 
the  area  mapped,  in  the  vicinity  of  Tower,  to  within  about  11  miles 
of  Ely  on  the  east,  and  the  second  of  which,  known  as  the  Knife  Lake 
area,  begins  about  7  miles  east  of  Ely  and  extends  eastward  to  the 
eastern  limit  of  the  area  mapped.  At  the  base  of  the  series  there  lies 
a  great  conglomerate,  known  as  the  Ogishke  conglomerate,  contain- 
ing pebbles  and  finer  detritus  from  all  of  the  rocks  of  the  Archean. 
Above  this  conglomerate  in  the  eastern  portion  of  the  district  there 
are  found  in  a  few  localities  small  masses  of  the  iron-bearing  Agawa 
formation.  This  formation  is  petrographically  the  same  as  the  Soudan 
formation.  In  it,  however,  there  is  in  places  a  development  of  the 
carbonate-bearing  facies.  No  iron  ores  have  been  found  in  it.  Over- 
lying the  Ogishke  conglomerate  in  the  western  portion  of  the  district 
and  the  intervening  iron-bearing  Agawa  formation  where  present  in 
the  eastern  portion  of  the  district,  there  occurs  a  thick  series  of  slates 
of  varying  character,  to  which  the  name  Knife  Lake  slates  has  been 
given.  These  slates  have  been  closely  folded,  and  have  been  more 
metamorphosed  where  intruded  by  granites  of  the  Giants  range, 
Snowbank  Lake,  and  Cacaquabic  Lake  and  by  the  Duluth  gabbro. 
These  igneous  rocks  occupy  a  considerable  area,  and  their  intrusive 
relations  with  the  Lower  Huronian  are  unquestionable.  The  Lower 
Huronian  sediments  now  stand  nearly  vertical. 
The  Upper  Huronian  or  Animikie  series  is  found  in  the  extreme 
eastern  portion  of  the  district,  where  it  is  continuous  with  the  Anim- 
ikie of  the  Mesabi  district  to  the  west  and  Thunder  Bay  to  the  east. 
At  the  bottom  of  the  series  occurs  an  iron-bearing  formation  known  as 
the  Gunflint  formation.  Above  this  is  a  great  slate-graywacke  forma- 
tion to  which  the  name  Rove  slate  has  been  given.  The  Gunflint  forma- 
tion is  correlated  with  the  Biwabik  formation  of  the  Mesabi  district.  It 
has  a  very  limited  development  in  the  Vermilion  district,  and  its  most 
