LAKE    SUPERIOR   REGION.  247 
rocks  have  thus  been  produced,  which  in  most  cases  show  the  original 
textures  of  the  greenstones,  but  contain  also  a  development  of  fresh 
biotite,  hypersthene,  brown-green  hornblende,  and  magnetite. 
The  Soudan  iron  formation  is  widely  distributed  in  the  western 
part  of  the  district,  but  is  practically  wanting  in  the  eastern  half.  It 
is  found  mostly  in  narrow  belts,  which  consist  largely  of  greenstone 
so  intimately  associated  with  the  iron  formation  that  it  has  been  im- 
possible to  separate  them  on  the  map.  The  formation  consists  of 
(1)  a  very  subordinate  fragmental  portion,  made  up  of  some  con- 
glomerate, clearly  recognizable  as  having  been  derived  from  the 
underlying  greenstones,  grading  up  into  sediments  of  finer  character ; 
and  (2)  lying  above  this  fragmental  portion,  the  iron-bearing  forma- 
tion proper,  which  consists  of  siliceous  rocks,  largely  white  cherts — 
though  varying  in  color  from  white,  green,  yellow,  and  purplish  to 
black — with  red  jasper  and  carbonate-bearing  chert,  griinerite-mag- 
netite  schist,  hematite,  magnetite,  and  small  quantities  of  pj^rite. 
These  iron-bearing  rocks  are  clearly  of  sedimentary  origin.  They 
do  not  now  present  their  original  characters,  but  are  presumed  to 
have  been  derived  from  rocks  that  were  largely  carbonate-bearing  fer- 
ruginous cherts.  The  relation  of  the  iron  formation  to  the  adjacent 
greenstones  is  clearly  that  of  a  sedimentary  overlying  an  igneous 
series.  The  few  basal  conglomerates  of  the  iron  formation  that  have 
been  found  consist  of  pebbles  derived  from  the  underlying  greenstone, 
showing  conclusively  their  relationship.  This  relationship  is  ob- 
scured, however,  in  most  places,  by  the  absence  of  the  conglomerates, 
and  by  the  fact  that  the  iron  formation  has  been  very  closely  infolded 
in  the  greenstone.  In  consequence  of  the  extreme  folding  and  of  the 
impossibility  of  determining  different  horizons  in  the  iron  formation, 
it  has  been  impracticable  to  ascertain  its  thickness. 
The  iron-ore  deposits  of  the  Vermilion  district  show  a  striking 
analogy  with  those  of  the  Marquette  district.  Like  them,  they  may 
occur  in  two  positions  with  respect  to  the  iron-bearing  formation. 
They  are  found,  first,  at  the  bottom  of  this  formation,  and,  second, 
within  it,  the  ores  in  both  cases  being  the  same  in  character.  The  Ely 
deposits  are  typical  of  the  deposits  occurring  at  the  base  of  the  forma- 
tion. They  are  found  at  the  bottom  of  a  closely  compressed  synciine 
of  the  iron  formation,  where  it  lies  in  the  relatively  impervious  green- 
stone. The  source  of  the  iron  was,  in  the  first  instance,  the  Ely  green- 
stone. From  this  it  was  removed  through  the  action  of  water  and 
collected  in  the  Archean  sea  to  form  the  sedimentary  deposits  of  the 
Soudan  formation.  After  the  folding  of  the  formation  this  dissem- 
inated iron  was  carried  by  downward-percolating  waters  into  places 
favorable  for  its  accumulation,  such  as  the  bottom  of  this  synclinal 
trough,  where  it  was  precipitated  by  oxygen-bearing  waters  coming 
