LAKE    SUPERIOR   REGION.  377 
grained  gneisses,  both  older  and  younger  than  the  associated  rocks, 
but  in  general  regarded  as  the  basement  upon  which  associated  rocks 
were  formed.  It  was  recognized  that  these  rocks  are  cut  by  basic  erup- 
tives.  This  usage  was  followed  by  many  of  the  American  geologists  up 
to  the  time  of  Brooks,  who  excluded  from  the  "  Laurentian  "  the  large 
part  of  the  granite  and  gneiss  which  the  Canadians  believed  to  be  of 
later  age  and  intrusive.  Dawson  made  the  same  discrimination  in  the 
region  about  the  Lake  of  the  Woods.  Irving,  in  his  later  work,  dif- 
fered from  those  who  preceded  him  in  that  he  included  in  the  "Lau- 
rentian "  all  the  thoroughly  crystalline  schists,  with  some  of  the 
obscure  green  schist  conglomerates;  that  is,  he  placed  as  a  part  of  the 
"  Laurentian  "  a  large  group  of  finely  schistose  rocks  cut  by  granite 
veins  which  had  heretofore  been  regarded  as  greatly  metamorphosed 
detrital  material  and  had  been  placed  in  the  Huronian.  He  also  re- 
garded as  belonging  here  many  of  the  fine-grained  crystalline  schists 
of  similar  character  on  the  north  shore,  placed  by  the  earlier  Cana- 
dian geologists  with  the  Huronian.  For  this  "  Laurentian,"  in- 
creased in  magnitude,  he  used  the  term  Archean.  Lawson,  Mclnnes, 
W.  H.  C.  Smith,  Coleman,  Willmott,  and  Alexander  and  N.  H. 
Winchell  use  the  term  "  Laurentian  "  to  cover  practically  the  same 
class  of  rocks  as  the  earlier  Canadian  geologists,  although  Lawson 
differs  from  them  in  regard  to  their  origin  and  age.  Coleman  and 
Willmot  emphasize  strongly  the  intrusive  contact  of  the  "  Lauren- 
tian "  with  the  sedimentary  Huronian  and  maintain  that  it  includes 
no  rocks  older  than  the  Huronian  sediments. 
It  was  early  remarked  by  Macfarlane,  and  later  by  Whittlesey, 
Brooks,  and  Rominger,  that  the  Laurentian  of  Lake  Superior  differs 
from  that  system  in.  eastern  Canada  in  containing  no  limestones, 
quartzites,  iron  ores,  or  other  rocks  of  the  plainly  detrital  class. 
Brooks  and  Chamberlin  remark  that  it  is  doubtful  whether  the  Lake 
Superior  Laurentian  is  the  equivalent  of  the  eastern  "  Laurentian  " 
of  Canada.  Van  Hise  and  others  of  the  LTnited  States  Geological 
Survey  have  used  the  term  Laurentian  for  granites  and  gneisses  of 
the  basement  complex  or  Archean  age.  This  definition  was  recom- 
mended also  by  the  joint  committee  of  Canadian  and  United  States 
geologists,  but  as  a  concession  to  the  difficulties  of  mapping  in  Cana- 
dian territory  it  was  agreed  that  the  term  should  also  be  used,  prefer- 
ably with  an  explanatory  note,  for  large  areas  of  granite  and  gneiss  in 
which  the  parts  of  Archean  or  basement  complex  age  have  not  been 
separated  from  those  of  later  age.  Still  later,  Lane  and  Seaman  have 
used  "Laurentian  "  as  equivalent  to  Archean. 
Keewatin. — This  name,  first  applied  by  Lawson  to  the  greenstones, 
green  schists,  and  iron  formation  of  the  Lake  of  the  Woods  area,  has 
been  by  other  writers  applied  to  green  rocks  of  different  ages.  In 
northern  Minnesota  the  term  "  Keewatin,"  as  used  by  X.  H.  Winchell, 
