LAKE   SUPERIOR   REGION.  379 
district,  in  turn  mi  con  form  ably  below  this  series,  are  Archean. 
Prior  to  that  time  the  greenstones,  with  intrusive  granites,  had  been 
referred  to  the  Archean  by  •  the  United  States  geologists,  the  iron 
formation  to  the  lower  Huronian,  and  the  Knife  Lake  slate  had  been 
correlated  with  the  Animikie  as  upper  Huronian.  The  Minnesota 
Survey  had  held  the  Knife  Lake  formation  to  be  unconformably  be- 
low the  Animikie,  but  had  made  no  satisfactory  discrimination  of  the 
lower-middle  Huronian  sediments  from  the  Archean  greenstones  and 
iron  formation.  Since  this  conclusion  concerning  the  Archean  age 
of  the  iron  formation  associated  with  greenstones  and  green  schists  was 
reached,  increasing  quantities  of  iron  formation,  slate,  and  occasion- 
ally carbonate  considered  to  be  of  this  age,  have  been  discovered, 
particularly  in  Ontario.  In  fact,  outside  of  the  Animikie  district, 
on  the  northwest  shore  of  Lake  Superior,  practically  all  the  iron 
formation  of  Ontario  in  the  vicinity  of  Lake  Superior  is  believed 
to  belong  to  the  Archean.  The  iron  formation  is  in  layers  and  lenses 
which  have  such  relations  to  the  greenstones  and  green  schists,  con- 
sisting largely  of  fragmental  volcanic  material,  as  to  suggest  that  the 
igneous  and  sedimentary  rocks  have  been  interbedded  and  then  in- 
folded in  an  intricate  fashion.  It  is  impossible  to  separate  them  on 
any  structural  basis.  An  area  of  greenstone  a  mile  square  may  con- 
tain a  hundred  small  lenses  of  iron  formation.  In  recognition  of  the 
close  relation,  both  structural  and  chronological,  of  the  iron  forma- 
tion, slate,  and  carbonate,  with  the  rocks  of  the  Archean  the  term 
Archean  is  retained  for  the  entire  group.  While  this  term  had  pre- 
viously been  used  for  rocks  believed  to  be  essentially  of  igneous  origin, 
forming  the  basement  complex  of  the  region,  it  had  been  recognized 
that  rocks  of  possible  sedimentary  origin  were  doubtless  included  in 
this  basement  complex,  as  for  instance  the  Palmer  gneiss  and  the  con- 
glomerate at  Deer  Lake  in  the  Marquette  district  and  the  Quinnisec 
schists  of  the  Menominee  district.  So  general  had  become  the  use 
of  the  term  Archean,  however,  to  cover  strictly  igneous  rocks  basal 
to  the  Algonkian  elsewhere  in  the  United  States  that  the  formal  ref- 
erence to  the  Archean  in  the  Lake  Superior  region  of  small  bodies 
of  recognized  sediments  has  led  to  confusion  and  much  opposition. 
The  definition  and  use  of  the  term  Archean  are  discussed  on  pages 
26,  361. 
Huronian. — The  Lake  Superior  "  Huronian  "  of  the  larger  number 
of  the  Canadian  geologists  includes  the  quartzites,  slates,  fine-grained 
green  schists,  all  of  which  are  sometimes  conglomeratic,  and  the  peb- 
bles, which  are  often  distorted  and  metamorphosed.  It  also  includes 
the  mica  schists,  hornblende  schists,  and  fine-grained  gneisses  bearing 
calcite,  with  certain  ferruginous  schists  and  basic  and  acidic  volcanic 
products.  The  attitude  of  the  "  Huronian  "  schists  is  either  vertical 
or  steeply  inclined.  The  Animikie  is  not  included  in  the  "  Huronian." 
On  the  south  shore  Brooks  and  Pumpelly  in  their  earlier  work  in- 
