LAKE*SUPERIOR   REGION.  361 
the  Lake  Superior  region  to  which  stratigraphic  methods  may  be 
applied,  and  to  discriminate  them  broadly  from  the  intricate  complex 
of  crystalline  rocks  of  the  Keewatin  and  Laurentian.  The  term 
Algonkian  meets  this  need.  Its  use  further  implies  emphasis  on  the 
unconformity  between  the  sedimentary  series  and  the  basement  com- 
plex, an  unconformity  which  in  the  Lake  Superior  country  is  of 
fundamental  importance.  Finally,  it  makes  possible  a  broad  correla- 
tion of  the  Lake  Superior  pre-Cambrian  sedimentary  series  above  a 
crystalline  basement  complex  with  pre-Cambrian  sedimentary  series 
elsewhere  in  North  America  and  other  parts  of  the  world  without 
commitment  as  to  correlation  of  individual  formations  or  groups. 
The  term  Archean  is  used  to  cover  the  Keewatin  and  Laurentian 
rocks  of  the  basement  complex.  The  same  need  is  felt  here  for  a 
collective  term  to  express  local  Lake  Superior  conditions  as  in  the 
case  of  the  Algonkian ;  and  it  is  even  more  necessary  for  purposes  of 
correlation  with  other  districts,  for  nowhere  is  it  possible  to  correlate 
single  formations  or  groups  belonging  to  the  basement  complex  for 
widely  separated  areas. 
The  correlation  of  the  subdivisions  of  the  Algonkian  in  different 
parts  of  the  Lake  Superior  region  is  more  difficult  than  the  correla- 
tion of  the  Algonkian  system  as  a  whole.  It  involves  consideration 
of  varying  conditions  of  deposition  in  the  different  parts  of  the  dis- 
trict. The  Keweenawan  series  constitutes  essentially  a  continuous 
unit  involving  no  difficulty  in  correlation  as  a  whole.  The  Huronian 
rocks  were  deposited  under  different  conditions  in  the  following  three 
subprovinces :  (1)  On  the  north  shore  the  so-called  lower-middle  Hu- 
ronian is  thought  to  be  probably  in  part  a  subaerial  sediment.  Cole- 
man finds  what  he  regards  as  adequate  evidence  of  glacial  origin  in 
the  Cobalt  district.  The  lower  Huronian  occupies  a  position  uncon- 
formably  beneath  the  upper  Huronian  and  unconformably  above  the 
Archean,  a  position  occupied  jointly  by  the  lower  and  middle  Hu- 
ronian of  the  south  shore.  Our  exact  knowledge  would  be  better  ex- 
pressed by  calling  this  group  on  the  north  shore  the  lower-middle 
Huronian,  but  as  the  term  lower  Huronian  is  used  so  generally  on  the 
maps  of  this  territory,  it  is  thought  that  the  use  of  the  term  lower 
Huronian  will  perhaps  make  less  confusion  than  the  introduction  of 
the  term  lower-middle  Huronian. 
(2)  In  the  belt  running  from  the  Gogebic  district  eastward  to  the 
north  shore  of  Lake  Huron  there  are  two  mutually  unconformable 
sedimentary  groups,  both  unconformably  beneath  the  upper  Huronian 
and  unconformably  above  the  Archean.  These  have  been  called  the 
middle  Huronian  and  lower  Huronian.  These  rocks  are  well- 
assorted,  water-deposited  sediments,  indicating  proximity  of  shore 
lines. 
