282  PRE-CAMBRIAN   GEOLOGY   OF    NORTH    AMERICA. 
Lake  Minnetakie  area,  and  the  Wabigoon  Lake  area.  They  consist  of 
altered  traps,  hornblende  schists  and  other  green  schists,  altered  por- 
phyrites,  quartz  porphyries,  phyllites,  and  conglomerates.  In  general 
they  resemble  Lawson's  Keewatin  series  to  the  south.  The  Laurentian 
consists  of  hornblende  syenite,  hornblende  granite  gneiss,  mica  sye- 
nite, biotite  granite  gneiss,  and  various  granitic  rocks. 
Coleman,271  in  1898,  gives  an  interesting  general  account  of  the 
clastic  Huron ian  rocks  of  western  Ontario,  in  the  region  extending 
from  the  Lake  of  the  Woods  on  the  west  to  Mille  Lacs  on  the  east, 
a  distance  of  200  miles,  with  a  width  north  of  Rainy  Lake  of  120 
miles.  The  Huronian,  including  the  Keewatin  and  Coutchiching 
rocks,  is  in  general  an  immense  series  of  waterworn  sediments,  in  the 
upper  part  mixed  with  eruptives,  perhaps  largely  later  injections,  but 
partly  pyroclastic.  The  Keewatin  is  largely  of  eruptive  origin,  though 
it  contains  important  sedimentary  members;  the  Coutchiching  is  en- 
tirely sedimentary. 
The  Keewatin,  and  in  the  southern  part  of  the  region  the  under- 
lying Coutchiching,  form  sharp  synclines,  curving  as  wide  meshes 
around  the  areas  of  Laurentian,  which  vary  from  less  than  a  mile  to 
50  miles  in  diameter. 
Diabase  and  porphyry  eruptives  form  an  important  part  of  the 
Keewatin.  These  are  in  large  part  surface  flows,  represented  by  ash 
rocks,  agglomerates,  etc.,  but  many  of  them  are  probably  laccolithic 
sills.  The  water-formed  elastics  of  the  Keewatin  include  limestones, 
slates,  quartzites,  grits,  graywackes,  breccias,  and  pebble  and  bowlder 
conglomerates.  The  limestones  are  of  limited  extent,  being  found  in 
any  thickness  only  at  Steep  Rock  Lake.  The  slate  on  analysis  yields 
7.44  per  cent  of  carbon,  pointing  perhaps  to  the  presence  of  life.  The 
conglomerates  are  in  places  schistose.  Near  Shoal  Lake  the  most  com- 
mon pebbles  are  quartz  porphyry  and  porphyrite,  felsite,  and  green 
schists  indistinguishable  from  the  adjoining  Keewatin  schists;  black 
and  red  quartzite,  white  pulverulent  sandstone,  vein»quartz,  and  anor- 
thosite.  No  gneiss  or  granite  pebbles  have  been  found.  Most  of  these 
pebbles  are  easily  matched  by  Keewatin  rocks,  sometimes,  however, 
many  miles  distant;  a  few  are  evidently  Coutchiching,  and  none  are 
Laurentian. 
The  break  represented  by  this  conglomerate  comes  high  up  in  the 
Keewatin,  instead  of  at  its  base,  just  above  the  Coutchiching,  as  held 
by  Lawson.  Striking  evidence  that  the  break  is  not  at  the  base  of  the 
Keewatin  is  found  at  Shoal  Lake,  where  a  few  bowlders  of  the  coarse- 
grained anorthosite  found  in  the  schist  conglomerate  are  exactly  like 
portions  of  a  boss  of  anorthosite  2  miles  away.  As  this  anorthosite 
area  contains  masses  and  strips  of  characteristic  Keewatin  schist, 
swept  off  during  its  eruption,  it  is  evident  that  an  immense  lapse  of 
time  separates  the  conglomerate  and  the  underlying  Keewatin.    It  is 
