AREA   NORTH   AND   NORTHEAST   OF   LAKE    HURON.   '  433 
schists,  as  in  the  Vermilion  district  of  Minnesota.  Following  the 
nomenclature  of  the  joint  geological  committee,  he  finds  the  suc- 
cession from  the  base  up :  Laurentian  granites,  Keewatin  greenstones, 
Lower  Huronian  sediments,  trap  dikes.  The  iron  formation  con- 
sists of  banded  jasper  and  siliceous  material.  Its  length  is  approxi- 
mately G  or  8  miles  and  its  width  90  to  300  feet.  Intrusions  have  cut 
it  to  a  considerable  extent.  Miller's  report  adds  to  the  literature 
the  description  of  another  area  in  Ontario  in  which  Lake  Superior 
succession  and  conditions  of  iron  formation  are  repeated. 
Kerr,6G  in  1906,  reports  on  the  explorations  of  the  Mattagami 
Valley,  in  Ontario,  northeast  of  Lake  Huron.  Most  of  the  rock  ex- 
posures are  in  the  southern  part  of  the  area,  south  of  Niven's  second 
base  line.  The  Huronian  belt  extends  from  the  foot  of  Kenogamisse 
northward  as  far  as  Loon  portage  on  the  Mattagami,  where  the  first 
Laurentian  rocks  were  seen.  The  contact  between  the  two  forma- 
tions was  also  located  south  of  this,  on  Muskego  River.  The  Lau- 
rentian stretches  from  here  to  Poplar  Rapids,  as  far  north  as  his 
exploration  extended. 
Brock,67  in  1907,  maps  and  describes  the  geology  of  the  Larder 
Lake  district,  in  eastern  Ontario.  West  and  north  of  the  lake  is  a 
complex  consisting  of  phyllites,  schists,  cherts,  ferruginous  dolomites, 
and  greenstones,  cut  by  igneous  rocks.  These  rocks  as  a  rule  are  lying 
on  edge  and  are  characterized  by  the  disturbances  and  metamorphism 
to  which  they  have  been  subjected.  Cutting  them  at  various  points 
about  the  lake  are  pegmatite  and  quartz  porphyry  dikes,  evidently 
connected  with  a  granite  intrusion.  From  the  number  of  granite 
bowlders  scattered  over  the  surface  it  is  evident  that  not  very  far 
away  the  granite  is  exposed.  While  later  than  the  rocks  of  the  com- 
plex, the  rocks  of  the  granite  family  are  undoubtedly  older  than 
the  sedimentary  rocks  mentioned  below  as  overlying  the  old  com- 
plex, since  these  sedimentaries  contain  fragments  of  the  granites. 
Lying  unconformably  upon  the  preceding  complex  is  a  series  of 
sedimentary  rocks  consisting  of  slates,  quartzites,  and  conglomerates. 
These  are  for  the  most  part  undisturbed,  with  gentle  dips,  except  in 
the  immediate  vicinity  of  a  later  igneous  intrusion,  where  they  may 
show  considerable  local  metamorphism.  In  such  cases  differen- 
tiation from  the  earlier  complex  may  be  somewhat  difficult,  but  the 
undisturbed  condition  of  these  rocks  affords  the  readiest  criterion  for 
their  recognition.  This  series  is  exposed  on  most  of  the  islands  of  the 
main  body  of  the  lake,  on  the  north  shore  near  the  narrows,  and  on 
the  east  shore  of  the  lake. 
Later  than  and  intrusive  in  the  rocks  above  mentioned  is  an  igneous 
rock  which  in  places  presents  a  gabbro  facies  and  in  other  places  a 
diabase,  and  also  a  series  of  usually  small,  basic,  mica  dikes. 
55721— Bull.  360—09 28 
