LAKE   SUPERIOR   REGION.  3o7 
successions  of  the  groups  of  the  different  districts  and  their  relative 
positions  with  reference  to  the  unconformities.  Indeed,  likenesses 
of  lithology,  successions,  metamorphism,  structure,  and  other  rela- 
tions of  the  groups  are  often  the  basis  for  the  correlation  of  the  uncon- 
formities. Consideration  of  all  the  known  factors  taken  together 
affords  a  satisfactory  basis  for  correlation. 
The  Keweenawan  series  is  recognized  by  all  as  a  great  structural 
unit  occupying  considerable  parts  of  the  north,  northwest,  and  south- 
west shores  of  Lake  Superior.  The  same  may  be  said  of  the  Animikie 
or  upper  Huronian  group.  In  both  we  have  well-marked,  easily 
ascertainable  horizons,  extending  well  over  the  Lake  Superior  region. 
They  are  separated  from  each  other  by  an  unconformity,  and  uncon- 
formities of  great  magnitude  separate  them  from  series  both  above 
and  below.  The  unconformity  beneath  the  Animikie  is  so  striking 
and  widespread  that  it  can  scarcely  be  mistaken.  The  extent,  litho- 
logical  unity,  and  relations  to  conspicuous  unconformities  make  the 
Keweenawran  and  Animikie  rocks  satisfactory  horizons  from  which 
to  carry  down  correlation  of  the  other  less  extensive,  separated,  and 
lithologically  diverse  groups  of  the  Lake  Superior  region.  The  Kee- 
watin  and  Laurentian  rocks  making  up  the  basement  complex  also 
present  sufficient  lithological  and  structural  uniformity  to  make  them 
useful  in  correlation  in  connection  with  the  overlying  unconformity. 
It  is  not  always  possible  to  use  them  alone,  however,  because  of  the 
difficulty  of  distinguishing  Laurentian  granites  and  gneisses  from 
Huronian  granites  and  gneisses,  or  of  distinguishing  some  of  the 
Keewatin  volcanics  from  those  of  the  higher  horizons. 
Beginning  northeast  of  Lake  Superior  we  shall  consider  in  turn 
the  regions  north,  west,  and  south  of  the  lake. 
The  rocks  of  the  Michipicoten  district  are  believed  by  all  to  be 
unconformably  beneath  the  Animikie,  although  the  Animikie  is  not 
found  near  the  lake  shore  in  this  area.  The  reasons  for  this  are  the 
steep  attitude  and  metamorphosed  character  of  the  Michipicoten 
rocks  as  compared  with  the  flat-lying,  little-metamorphosed  rocks 
both  to  the  west  of  Nipigon  Bay  and  in  the  Sudbury  basin  to  the 
east.  The  lower  rocks  of  the  Michipicoten  district  consist  of  green- 
stones, green  schists,  iron  formation,  and  slate,  typical  in  all  their 
aspects  to  the  Keewatin  of  the  rest  of  the  region.  The  upper  forma- 
tion, the  Dore  conglomerate,  rests  upon  the  Keewatin  and,  being  be- 
neath the  Animikie,  is  called  lower-middle  Huronian.  The  granite 
is  regarded  by  Coleman  and  Willmott  as  intrusive  into  the  Huronian. 
It  is  certain  that  some  of  it  is  intrusive  into  the  lower-middle  Huro- 
nian sediments,  but  that  other  parts  of  it  antedate  the  Huronian  sedi- 
ments is  extremely  probably  because  of  the  great  quantities  of  granite 
fragments  similar  to  those  of  the  Laurentian  in  the'Dore  conglomer- 
ate at  the  base  of  the  Huronian  series. 
