LAKE    SUPERIOR   REGION.  321 
by  the  United  Stales  Geological  Survey,  and  thus  the  Stuntz  con- 
glomerate is  correlated  with  the  Lower  Iluronian  series  instead  of 
with  the  Upper  Huronian,  as  in  former  reports.  The  underlying 
greenstones,  green  schists,  and  iron  formation,  the  latter  of  sedimen- 
tary origin,  are  thus  thrown  into  the  Archean. 
In  the  Mesabi  district  the  Keewatin  of  the  Minnesota  Survey, 
which  was  in  large  part  classed  as  Archean  by  the  United  Stales 
Geological  Survey  and  recognized  by  all  as  being  unconformable 
below  the  Animikie,  is  subdivided  on  the  basis  of  recent  work  in  the 
district  by  C.  K.  Leith  into  an  upper  series  of  graywackes,  slates, 
and  conglomerates,  correlated  with  the  Lower  Huronian,  and  a  lower 
basement  complex,  consisting  mainly  of  greenstones  and  green  schists, 
correlated  with  the  Archean.  This  correlation  is  based  on  the  equiva- 
lence of  the  Animikie  with  the  Upper  Huronian,  long  maintained  by 
geologists  of  the  United  States  Geological  Survey. 
In  the  Marquette  district  certain  jaspers  and  associated  cherty  and 
slaty  rocks  found  intimately  associated  with  the  basement  rocks  of 
the  Archean  and  previously  supposed  to  be  Huronian  rocks  infolded 
with  the  Archean,  are  now  themselves  called  Archean. 
In  the  Michipicoten  district  of  Canada  the  iron  formation  and 
associated  greenstones  and  green  schists  are  correlated,  respectively, 
with  the  iron  formation  and  associated  greenstones  and  green  schists 
of  the  Vermilion  district,  and  are  therefore  classed  as  Archean. 
The  iron  ores  of  the  Lake  Superior  region  are  supposed  to  have 
originated  from  iron  carbonate  in  all  districts;  outside  of  the  Mesabi. 
In  the  Mesabi  district  the  ores  have  mainly  resulted  from  the  altera- 
tion of  a  green  ferrous*  silicate  in  small  granules,  as  first  shown  by 
Spurr.  but  the  ores  have  come  also  in  small  part  from  the  alteration 
of  iron  carbonate,  which  is  correlative  in  origin  with  the  green  gran- 
ules. The  ferrous  silicate  granules  are  believed  not  to  be  glauconite 
as  they  were  named  by  Spurr. 
Van  Hise,333  in  1902,  briefly  sketches  the  history  of  geological 
mapping  in  the  Lake  Superior  region,  calling  attention  to  the  diffi- 
culty of  preparing  accurate  maps,  and  concludes  that  the  maps  which 
have  been  published  from  time  to  time  since  the  earliest  map  of  Foster 
and  Whitney  represent  reasonably  close  approximations  to  the  facts 
as  then  known,  and  that  notwithstanding  their  many  imperfections 
they  have  been  of  service  at  the  time  of  publication.     • 
Spurr,334  in  1902,  discusses  the  origin  of  the  pre-Cambrian  iron 
ores  of  the  Lake  Superior  region.  He  repeats  his  conclusion  that 
the  Mesabi  ores  have  resulted  from  the  alteration  of  a  green  ferrous 
silicate  of  the  class  of  glauconite,  and  further  states  that  his  conclu- 
sion in  reference  to  the  Mesabi  iron  formation  may  be  probably  ap- 
plied to  most  of  the  other  Lake  Superior  iron  ores. 
55721— Bull.  360—09 21 
