LAKE   SUPERIOR   REGION.  371 
Lawson,  N.  H.  Winchell,  W.'H.  C.  Smith,  Mclnnes,  Coleman,  Will- 
mott,  and  others,  working  principally  on  the  north  shore  of  Lake 
Superior,  have  reached  conclusions  somewhat  different  from  those  of 
the  United  States  Geological  Survey  above  outlined. 
Lawson  recognized  a  physical  break  at  the  base  of  the  Kewee- 
nawan  and  a  great  break  at  the  base  of  the  Animikie.  The  latter  he 
calls  the  "  Eparchean  interval,"  and  believes  it  to  be  the  greatest  of 
the  unconformities  in  the  Lake  Superior  region.  It  is,  indeed,  in  the 
region  north  and  northwest  of  Lake  Superior  familiar  to  him,  the 
most  conspicuous  of  the  unconformities,  and  where  the  Animikie  rests 
upon  deeply  eroded  Archean  rocks  it  is  the  greatest  of  the  uncon- 
formities ;  at  such  localities  this  unconformity  represents  two  or  more 
unconformities  and  their  intervening  groups,  just  as  under  the  Creta- 
ceous, where  the  series  rests  upon  the  Archean,  there  is  a  still  greater 
but  not  more  conspicuous  unconformity.  He  divided  the  underly- 
ing complex  about  the  Lake  of  the  Woods  and  Rainy  Lake,  Ontario, 
into  Keewatin,  "  Coutchiching,"  and  Laurentian,  this  being  the  order 
of  occurrence  downward,  but  in  age  the  granitic  and  gneissic  rocks  are 
later  than  and  intrusive  in  the  schistose  rocks.  In  this  matter  Law- 
son  agrees  with  the  earlier  work  of  Bigsby  upon  Rainy  Lake  and  that 
of  Dawson  upon  the  Lake  of  the  Woods,  except  that  Dawson  did  not 
regard  all  of  the  granite  gneiss  of  the  Lake  of  the  Woods  as  later 
igneous  material.  These  relations  are  the  same  as  those  described  by 
Foster  and  Whitney  and  by  Wadsworth  between  the  granite  gneisses 
and  the  "Azoic  "  slates  on  the  south  shore.  With  Dawson,  Lawson 
agrees  that  the  Laurentian  gneiss  and  granite  and  the  overlying 
schists  are  conformable.  By  both  of  these  writers  the  schistose  rocks 
of  the  Lake  of  the  Woods  are  regarded  as  sedimentary  and  largely  of 
volcanic  origin.  There  is  the  further  agreement  between  Lawson  and 
Dawson  for  the  north  shore  and  Foster  and  Whitney,  Wadsworth, 
Irving,  and  Williams  for  the  south  shore  that  they  regard  the  green- 
stone slates  as  largely  in  the  nature  of  volcanic  ash.  Lawson  gives 
the  schistose  rocks  about  Rainy  Lake  a  twofold  division,  both  series 
being  regarded  as  sedimentary  and  in  apparent  conformity,  but  there 
are  great  differences  in  the  materials  of  which  the  series  are  composed 
as  well  as  in  degree  of  crystallization,  and  basal  conglomerates  are 
found  at  the  bottom  of  the  upper  series.  Between  the  two  he  believes 
there  is  a  considerable  geological  break.  The  upper  he  regards  as 
the  equivalent  of  the  schistose  series  of  the  Lake  of  the  Woods.  To 
cover  the  two  series  he  proposed  the  system  name  "  Ontarian."  Law- 
son's  succession  is,  therefore :  Keweenawan ;  unconformity :  Animikie ; 
unconformity ;  Keewatin ;  unconformity ;  Coutchiching ;  irruptive  un- 
conformity; Laurentian,  cutting  both  Keewatin  and  Coutchiching. 
It  is  of  interest  to  note  that  Rominger's  early  conclusions  as  to  the 
general  relations  of  the  rock  series  on  the  south  shore  were  almost 
