378  PRE-CAMBKIAN    GEOLOGY   OF    NORTH    AMERICA. 
includes  rocks  of  the  basement  complex  and  of  the  Huronian.  Van 
Hise,  believing  that  the  term  Keewatin  was  not  originally  applied  to 
a  structural  unit,  and  believing  also  that  its  usefulness  had  been  im- 
paired by  its  promiscuous  application  to  green  rocks  of  different  ages 
elsewhere,  proposed  the  term  "  Mareniscan  "  to  cover  the  greenstone, 
green  schist,  and  iron  formation  portion  of  the  basement  complex. 
When  Lawson's  original  Keewatin  area  of  the  Lake  of  the  Woods  was 
examined  by  the  joint  committee  of  Canadian  and  United  States 
geologists  it  was  ascertained  that  the  Keewatin  of  this  area  is  a 
structural  unit,  and  the  term  Keewatin  was  recommended  to  cover 
all  of  the  greenstone,  green  schist,  and  iron  formation  portion  of  the 
basement  complex  in  place  of  "  Mareniscan."  This  is  the  sense  in 
which  Keewatin  is  now  generally  used. 
"  C outchiching  " — This  name  was  used  by  Lawson  for  a  series  of 
mica  schists  in  the  Rainy  Lake  district  which  he  believed  to  underlie 
the  Keewatin.  These  rocks  were  subsequently  found  by  Van  Hise  to  be 
metamorphosed  Huronian  sediments  above  the  Keewatin.  Winchell, 
in  the  belief  that  he  was  following  Lawson,  applied  the  term  "  Cou- 
tchiching"  to  hornblende  schists  resulting  from  the  metamorphism 
of  Keewatin  greenstones  by  the  intrusion  of  granites  in  northeast- 
ern Minnesota.  At  the  present  time  the  term  is  practically  aban- 
doned, except  perhaps  by  Lawson. 
Archean. — With  the  Canadian  geologists  Archean  has  always 
meant  pre-Cambrian,  and  in  this  they  have  followed  Dana.  As 
some  of  them  have  included  Keweenawan  and  Animikie  rocks  in  the 
Cambrian,  their  "Archean "  means  pre- Animikie.  Archean  with 
Van  Hise  and  others  of  the  United  States  Geological  Survey  covers 
the  basement  complex  beneath  the  Huronian. 
By  the  United  States  geologists  the  Archean  was  formerly  re- 
garded as  essentially  of  igneous  origin  and  as  comprising  both  acidic 
and  basic  rocks.  It  was  not  until  1899  that  the  geologists  of  the 
United  States  Geological  Survey  recognized  the  presence  in  the 
Archean  of  chemically  deposited  iron  formation  and  fragmental 
slates.  The  iron  formation  and  associated  slates  had  been  regarded 
as  ferruginous  vein  material  or  had  been  assigned  to  the  Huronian. 
Reexamination  of  the  relations  of  the  iron  formation  and  slate  of 
the  Michipicoten  district  and  of  Pine  street,  Marquette,  to  the  as- 
sociated greenstones  and  green  schists,  led  Van  Hise,  in  1899,  to  sus- 
pect that  they  belong  to  the  same  series,  as  had  been  previously 
thought  by  Seaman  from  studies  in  the  Marquette  district.  In  the  same 
year  Leith,  by  the  discovery  of  lower-middle  Huronian  rocks  in  the 
Mesabi  district  identical  in  structure  and  lithology  with  those  of  the 
Vermilion  district,  was  able  to  show  that  the  slates,  graywackes,  and 
conglomerates  of  the  Knife  Lake  slate  of  the  Vermilion  district  are 
unconformably  below  the  upper  Huronian.  It  followed  that  the 
iron  formation,  slate,  and  associated  greenstone  of  the  Vermilion 
