360  PRE-CAMBRIAN    GEOLOGY    OF    NORTH   AMERICA. 
Correlation  of  Lake  Superior  rocks  with  original  Huronian  rocks 
on  north  shore  of  Lake  Huron. — In  the  Huronian  of  the  original 
Huronian  district  there  are  two  unconformable  sedimentary  groups. 
These  two  sedimentary  groups  are  believed  to  underlie  unconformably 
the  Animikie  group.  Nowhere  are  Animikie  rocks  in  direct  contact 
with  original  Huronian  rocks  as  mapped  in  detail  by  Logan  and 
Murray,  but  north  of  Sudbury  such  rocks,  showing  far  less  folding 
and  metamorphism  than  the  original  Huronian  series,  do  rest  uncon- 
formably upon  rocks  closely  similar  lithologically  to  the  original 
Huronian  rocks. 
The  Sudbury  rocks  are  a  part  of  Logan's  Huronian  series.  There- 
fore the  Huronian  series  of  the  north  shore  of  Lake  Huron  is  divided 
into  three  unconformable  units,  upper  Huronian,  middle  Huronian, 
and  lower  Huronian,  the  two  lower  groups  appearing  in  the  area  on 
the  north  shore  mapped  in  detail  by  Logan  and  the  upper  one  a  little 
to  the  northeast  in  the  Sudbury  district.  The  original  Huronian 
rocks  are  known,  moreover,  to  rest  unconformably  upon  Keewatin 
green  schists  and  Laurentian  granites.  In  their  unconformable  posi- 
tion beneath  the  Animikie  and  above  the  Keewatin  and  Laurentian, 
in  their  divisibility  into  two  groups  by  an  unconformity,  in  lithology, 
and  in  succession  they  are  similar  to  the  lower  and  middle  Huronian 
of  the  Marquette  district. 
General  correlation. — The  Keweenawan  and  all  the  underlying 
rocks  are  assigned  to  the  pre-Cambrian  because  of  the  magnitude  of 
the  unconformity  between  the  Potsdam  sandstone  and  middle  Kewee- 
nawan, because  of  the  importance  of  volcanism  in  the  Keweenawan 
and  its  complete  absence  in  the  Cambrian,  because  of  the  absence  of 
fossils  in  the  Keweenawan,  and  finally  because  the  broad  relations  of 
the  Paleozoic  to  the  underlying  rocks  over  North  America  point  to  a 
continuous  overlap  from  easterly,  westerly,  and  southwesterly  direc- 
tions, with  the  result  that  over  a  large  area  in  the  north-central  por- 
tion of  the  continent  upper  Cambrian  or  Ordovician  rocks  were 
deposited  directly  upon  the  pre-Cambrian.  If  the  Keweenawan  is 
really  Cambrian  we  should  have  here  a  middle  or  lower  Cambrian 
series  of  enormous  thickness  but  limited  area,  indicating  radical 
departure  from  the  general  conditions  of  overlap  shown  by  the  conti- 
nent as  a  whole.  The  joint  committee  report  placed  the  Keweenawan 
in  the  pre-Cambrian,  but  Lane,  one  of  the  signers,  dissented  on  this 
point. 
The  Keweenawan  and  Huronian  rocks  are  together  placed  in  the 
Algonkian,  using  this  term  to  cover  the  sedimentary  and  related 
igneous  rocks  beneath  the  Cambrian  and  above  a  crystalline  basal 
complex.  The  joint  committee  made  no  recommendation  on  this 
point.  It  is  constantly  necessary,  both  in  description  and  in  map- 
ping, to  refer  collectively  to  the  pre-Cambrian  sedimentary  rocks  of 
