GENERAL  ACCOUNT    OE   THE    PRE-CAMBRIAN    ROCKS.  27 
Lake  of    the  Woods  district,  but    only  later  defined  by  the    above- 
mentioned  committee  in  the  broad  sense  used  in  this  volume. 
The  second  division  of  the  Archean,  the  Laurentian,  consists  of 
granite,  granitoid  gneiss,  gneiss,  and  acid  schists,  characterized  by 
red  or  light  colors.  Wherever  this  series  comes  into  contact  with 
the  Keewatin  the  rocks  of  the  Laurentian  are  found  to  be  intrusive 
into  the  Keewatin.  In  general  the  intrusions  occurred  under  deep- 
seated  conditions. 
Thus  the  separation  between  the  two  great  divisions  of  the  Archean 
is  upon  the  basis  of  age,  but  with  this  go  further  contrasts  as  to  vol- 
canic and  plutonic  characters  and  contrasts  in  lithology  and  color, 
in  general  the  Keewatin  being  composed  of  intermediate  or  basic 
rocks,  while  the  Laurentian  is  mainly  composed  of  acidic  rocks. 
In  the  Lake  Superior  part  of  the  pre-Cambrian  shield  the  closest 
studies  have  been  made  of  the  relations  of  the  Laurentian  and 
Keewatin,  and  in  a  number  of  areas  there  has  been  an  attempt 
to  separate  the  sedimentary  portion  of  the  Keewatin  from  its 
volcanic  portion.  For  instance,  in  the  Keewatin  of  the  Ver- 
milion district  the  larger  masses  of  the  iron  formation  have  been 
separated  from  the  metabasalts  and  schists  in  the  mapping,  and  evi- 
dence is  offered  to  show  that  the  iron  formation  is,  at  least  in  part, 
stratigraphically  above  the  associated  greenstones.  In  the  Lauren- 
tian of  the  same  district  several  igneous  masses  have  been  discrim- 
inated and  their  relative  ages  determined.  In  the  Marquette  district 
the  Keewatin  has  been  subdivided  into  the  Mona  schist  and  the 
Kitchi  schist,  and  the  Laurentian  is  also  subdivided.  Notwithstand- 
ing the  success  in  subdividing  the  Archean  into  formations  upon  a 
lithological  basis  in  certain  of  the  Lake  Superior  districts,  the  fact 
remains  that  for  the  Lake  Superior  region  as  a  whole  vastly  more  re- 
mains to  be  done  than  has  been  accomplished. 
This  same  division  of  the  Archean  into  Keewatin  and  Laurentian, 
which  prevails. in  the  Lake  Superior  region,  has  been  found  by  us  to 
apply  to  the  great  region  north  of  Lake  Huron. 
Miller  has  also  found  this  broad  subdivision  into  Keewatin  and 
Laurentian  to  apply  in  eastern  Ontario  and  western  Quebec.  Miller 
and  Knight  have  recently  concluded  that  part  of  the  Grenville  rocks 
of  the  Hastings  district  are  similar  to  the  Keewatin  in  lithology  and 
relations  and  are  to  be  correlated  with  it.  The  problem  of  correlation 
of  the  Grenville  as  a  whole  is  still  unsolved. 
We  believe,  from  field  observation  and  from  a  study  of  the  litera- 
ture, that  the  classification  may  apply  somewhat  generally  in  eastern 
Canada  and  Newfoundland. 
It  thus  appears  that  the  division  of  the  Archean  into  Kee- 
watin and  Laurentian  applies  for  the  greater  area  extending  from 
the    west   end    of    the    Lake    of    the    Woods    at    least    to    the    east- 
