276  PKE-CAMBRIAN    GEOLOGY   OF    NOKTH    AMERICA. 
unconformity  or  a  fault.  The  mica  schist  is  cut  by  granite  apophyses 
at  various  places.  If  foliation  is  taken  as  thickness  the  series  would 
vary  from  1.386  to  5.548  miles.  However,  the  mica  schists  are  re- 
garded as  repeated  by  folding.  Although  having  isoclinal  dips,  there 
is  a  considerable  variation  which  may  indicate  such  folding. 
Keewatin  rocks  are  confined  to  the  southeastern  part  of  Hunters 
Island.  They  consist  of  quartzites,  soft  gray  schists,  quartz  porphy- 
ries, felsite,  sericite  schists,  conglomerates,  altered  traps,  hornblende 
schists,  and  other  green  schists.  Occasionally  contained  in  them  are 
areas  of  banded  jasper  and  hematite.  On  the  north  side  of  Cache 
Bay  is  a  felsitic  conglomerate  in  contact  with  a  coarse-grained  horn- 
blende granite.  Beds  of  dolomite  are  associated  with  this  conglom- 
erate. The  breadth  of  the  Keewatin  series  gives  no  certain  criterion 
by  which  to  estimate  its  thickness.  The  dip  shows  an  apparently 
simple  synclinal  structure. 
As  to  the  relations  of  the  Laurentian  to  the  Huronian  series,  they 
have  a  general  parallel  schistosity,  and  there  are  many  phenomena 
suggestive  that  the  granitic  and  syenitic  type  is  of  igneous  eruptive 
origin,  later  than  the  Huronian,  but  the  hornblendic  and  micaceous 
phases  of  these  granites  may  be  rocks  of  different  determinable  ages, 
the  discovery  of  which  may  throw  light  not  only  on  the  genesis  of  the 
Laurentian  but  on  its  relations  to  the  overlying  Coutchiching  and 
Keewatin.  Cutting  both  Laurentian  and  Coutchiching  rocks  are 
diabase  dikes  in  such  attitudes  as  to  leave  little  doubt  that  they  were 
intruded  since  the  last  folding,  on  which  assumption  their  geological 
age  is  post-Keewatin. 
Lawson,  in  1893,  gives  a  petrographical  and  structural  account  of 
the  anorthosites  of  the  northwest  shore  of  Lake  Superior.  See  sum- 
mary in  section  3,  Minnesota,  pages  215-216. 
Smith,258  in  1893,  gives  a  general  description  of  the  Archean  rocks 
in  the  southern  half  of  the  Rainy  Lake  district  in  the  Province  of 
Ontario,  between  the  Thunder  Bay  district  and  the  Lake  of  the 
Woods.  The  rocks  here  found  are  divided  into  Lower  Archean  and 
Upper  Archean,  the  term  Archean  being  defined  to  include  all  pre- 
Cambrian  rocks.  The  Lower  Archean  series,  or  Laurentian,  com- 
prises a  lower  granitic  and  syenitic  division  and  an  upper  micaceous, 
hornblendic,  and  trappean  division,  for  the  most  part  schistose.  The 
first  usually  occurs  in  rounded  or  ovoid  areas,  between  which  are  the 
rocks  of  the  Upper  Archean  or  Ontarian. 
The  Ontarian  system  includes  the  Coutchiching  and  Keewatin 
series.  The  Coutchiching  rocks  are  mainly  mica  schists,  and  have  an 
estimated  thickness  of  9,000  feet,  the  apparent  thickness  of  24,000  to 
29,000  feet,  given  by  Lawson,  being  believed  to  be  due  to  multiple 
folds.  These  mica  schists  are  regarded  as  clastic  in  origin,  because 
of  their  fine  and  even  lamination.     The  Keewatin  consists  for  the 
