28  PRE-CAMBRIAN    GEOLOGY   OF    NORTH    AMERICA. 
ern  part  of  Ontario,  a  distance  of  700  miles.  While  the  great 
northern  extension  of  this  shield  has  not  been  studied  in  such  detail 
as  to  enable  the  geologists  who  have  traversed  it  at  intervals  wide 
apart  to  map  the  Keewatin  and  Laurentian  separately,  those  who 
are  familiar  with  the  southern  part  of  the  area  of  the  pre-Cambrian 
shield  do  not  doubt  that  this  discrimination  can  be  carried  out  when 
detailed  mapping  is  undertaken  for  this  vast  region. 
In  this  pre-Cambrian  shield,  in  addition  to  the  granites,  gneisses, 
and  schists  which  antedate  the  Algonkian,  there  are  later  igneous 
rocks  which  intrude  the  Algonkian.  For  great  areas  where  recon- 
naissance work  has  been  done  the  granites  and  gneisses  of  the  Archean 
have  not  been  discriminated  from  those  intrusive  into  the  Algonkian, 
but  both  have  been  mapped  under  the  designation  "  Laurentian." 
This  practice  has  been  necessary  because  of  the  difficulty  of  making 
discriminations  for  the  immense  areas  of  crystalline  rocks  accessible 
only  by  arduous  travel  by  canoe.  In  recognition  of  this  situation,  the 
international  geological  committee  in  1905,  while  defining  Lauren- 
tian as  above  stated,  recommended  that  its  use  be  also  permitted,  pref- 
erably with  an  explanatory  clause,  for  broad  areas  of  granites  and 
gneisses  where  the  masses  of  different  ages  have  not  been  discriminated. 
This  recognizes  the  present  usage  of  "  Laurentian  "  on  many  of  the 
Canadian  maps,  but  should  not  be  construed  as  an  approval  of  the  view 
that  this  "  Laurentian  "  as  a  whole  is  intrusive  into  the  Algonkian. 
A  considerable  part  of  the  rocks  mapped  as  "  Laurentian  "  is  intrusive 
into  the  Algonkian,  but  another  large  part  is  intrusive  only  into  the 
Keewatin.  "  Laurentian,"  as  used  in  the  broad  sense,  becomes  a  catch- 
all for  pre-Cambrian  gneisses  and  granites  of  any  area  where  struc- 
tural relations  have  not  been  discriminated.  There  is  danger  that 
unless  the  narrower  and  much  more  desirable  application  of  the  term 
recommended  by  the  committee  be  emphasized  the  discrimination  of 
Archean  and  Algonkian  granites,  so  important  for  structural  pur- 
poses, will  be  overlooked.  This  has  been  one  of  the  most  fruitful 
sources  of  controversy  in  the  mapping  of  the  pre-Cambrian  to  the 
present  time.  It  is  obvious  that  statements  concerning  the  relations 
of  the  "  Laurentian  "  in  the  broader  sense  can  not  have  the  same  sig- 
nificance as  those  relating  to  the  Laurentian  as  limited  by  the  inter- 
national committee. 
A  similar  dual  classification  of  the  Archean  has  been  made  by 
Keith  in  the  southern  Appalachians  and  Piedmont,  where  an  ancient 
gneiss  series  is  intruded  by  granites.  There  is  little  evidence  upon 
which  to  base  a  correlation  of  this  Archean  or  its  subdivisions  with 
that  of  the  pre-Cambrian  shield. 
For  other  areas  of  pre-Cambrian  little  has  been  done  toward  the 
subdivision  of  the  Archean. 
