544  PRE-CAMBRIAN    GEOLOGY   OF   NORTH   AMERICA. 
intrusive  into  the  crystalline  schists  referred  to  the  Huronian.  Huro- 
nian  rocks  occur  principally  in  a  large  area  near  the  center  of  the 
region  and  in  small  areas  north  of  the  center  of  the  region  and  south 
of  Lake  Mistassini  in  the  eastern  part.  The  large  tract  of  Huronian 
rocks  forms  a  part  of  the  great  belt  of  Huronian  rocks  extending 
continuously  from  the  eastern  side  of  Lake  Superior  to  Lake  Mis- 
tassini, a  distance  of  700  miles.  The  Huronian  may  be  grouped  in 
three  classes,  namely,  (1)  crystalline  schists,  together  with  some  other 
rocks  forming  a  comparatively  small  proportion  of  the  same  series, 
(2)  massive  greenstones,  and  (3)  granites.  The  schists  embrace  a 
considerable  variety,  but  the  greater  part  of  them  are  dark-green 
hornblendic  or  dioritic,  and  they  often  pass  into  more  or  less  massive 
greenstones,  so  that  it  becomes  difficult  to  map  the  two  varieties  sepa- 
rately. Dolomite,  quartzite,  arkose,  conglomerate,  and  agglomerate 
are  exceptional  occurrences. 
Tyrrell  and  Dowling,64  in  1903,  report  on  the  northeastern  portion 
of  the  district  of  Saskatchewan  and  adjacent  parts  of  the  districts 
of  Athabaska  and  Keewatin,  comprising  an  area  adjacent  to  the 
north  end  of  Lake  Winnipeg.  The  east,  northeast,  and  northern 
portions  of  the  area  mapped  are  occupied  by  Laurentian  and  Huron- 
ian rocks,  the  Laurentian  rocks  being  in  the  larger  areas.  They 
consist  of  granites  and  gneisses,  some  of  which  are  intrusive  into 
the  Huronian  and  some  of  which  are  probably  basal  to  it.  Huronian 
rocks  are  found  in  small  areas  at  Cross  Lake  and  at  Pipe  Lake  and 
in  a  large  area  extending  from  Wekusko  Lake  to  Athapapuskow 
Lake.  They  consist  of  conglomerates,  quartzites,  basic  eruptives  and 
greenstones,  and  altered  schists,  similar  to  rocks  of  Lawson's  Kee- 
watin and  Coutchiching  series. 
Bell,05  in  1904,  describes  the  Laurentian  and  Huronian  rocks  of 
the  Moose  River  basin.  The  former  include  acidic  igneous  rocks;  the 
latter,  greenstones,  green  schists,  and  certain  sediments,  with  doubtful 
relations  to  one  another  and  to  the  Laurentian. 
Wilson,66  in  1904,  reports  on  Nagagami  River  and  other  branches 
of  the  Kenogami.  Numerous  exposures  of  Archean  rocks  are  de- 
scribed, but  the  general  geology  is  not  summarized.  An  interesting 
feature  is  the  occurrence  in  this  area  of  Silurian  limestones. 
McInnes,67  in  1904,  reports  on  a  geological  reconnaissance  along 
Winisk  River,  Keewatin  district,  The  nature  and  distribution  of 
the  Laurentian  and  Huronian  rocks  crossed  by  the  river  are  described. 
At  a  point  26  miles  from  Hudson  Bay  on  the  river  an  anticline  brings 
up  a  series  of  quartzites  and  slates  underlying  unconformably  the 
Silurian  rocks  of  this  area.  The  trend  of  the  anticline  would  carry 
it  eastward  to  Sutton  Mill  Lake,  where  rocks  of  the  Nastapoka  series 
were  noted  by  Dowling  in  1901,  and  it  seems  not  unlikely  that  these 
