356  PRE-CAMBRTAN    GEOLOGY   OF    NORTH   AMERICA. 
In  the  region  about  Michipicoten  Harbor  the  Dore  conglomerate, 
believed  to  represent  the  base  of  the  Huronian  series,  may  be  found 
resting  against  Keewatin  green  schists,  with  differences  in  lithology, 
deformation,  metamorphism,  and  amount  of  igneous  intrusion. 
Notwithstanding  the  local  obliteration  of  the  pre-Huronian  uncon- 
formity, the  evidence  is  conclusive  that  here  is  a  break  of  the  first 
magnitude  at  the  base  of  the  lowest  Huronian  group.  Below  it  is 
a  series  of  Laurentian  and  Keewatin  rocks  of  great  lithological  com- 
plexity, very  largely  igneous,  but  in  small  part  sedimentary,  highly 
crystalline  throughout,  with  exceedingly  common  schistose  structures. 
Above  the  unconformity  the  Huronian  rocks  present  comparatively 
simple  lithology,  clear-cut  sedimentary  origin,  and  usually  less  defor- 
mation and  metamorphism  than  in  the  Laurentian  and  Keewatin 
rocks.  Of  greatest  significance  is  the  great  predominance  of  igneous 
rocks  below  the  unconformity  and  of  sedimentary  rocks  above,  rep- 
resenting fundamental  changes  of  physical  conditions.  It  is  not  at 
all  unlikely  that  the  sedimentary  portion  of  the  Archean,  now  known 
in  small  patches,  will  be  found  to  be  more  extensive,  but  the  Archean 
as  a  whole  is  sufficiently  well  known  to  preclude  the  possibility  that 
anything  more  than  a  small  percentage  of  it  will  be  shown  to  be  of 
sedimentary  origin.  For  the  rocks  below  the  unconformity  the  term 
basement  complex  is  highly  appropriate.  It  is  really  the  undivided 
complex  below  the  stratified  rocks  of  the  geological  column  for  Avhich 
the  term  Archean  was  designed. 
CORRELATION. 
Correlation  of  Lake  Superior  rocks  with  one  another. — A  detailed 
correlation  of  the  formations  of  the  various  districts  of  the  Lake  Su- 
perior region  has  been  summarized  in  the  table  on  pages  328-329. 
The  terms  used  are  the  ones  adopted  by  the  joint  committee  of  Can- 
adian and  United  States  geologists,  with  the  addition  of  the  terms 
Archean  and  Algonkian,  expressing  the  writers'  view  of  the  grouping 
of  the  series  recognized  by  the  committee.  The  principal  lines  of 
evidence  upon  which  this  correlation  is  based  are  sketched  below. 
To  avoid  confusion,  the  various  views  which  have  been  presented  in 
the  past  and  the  variety  of  names  used  will  not  be  discussed.  These 
are  taken  up  on  pages  367  et  seq. 
The  four  great  unconformities — at  the  base  of  the  Cambrian,  at  the 
base  of  the  upper  Huronian,  at  the  base  of  the  Ajibik  quartzite  of  the 
middle  Huronian,  and  at  the  base  of  the  Huronian  series — serve  as  a 
basis  for  the  major  grouping  of  the  pre-Cambrian  rocks  and  their 
correlation  throughout  the  region.  If  all  of  these  unconformities 
could  be  discovered  in  all  parts  of  the  region  they  would  be  sufficient 
for  a  complete  correlation.  As  they  have  not  been  recognized  so 
widely,  it  is  necessary  to  rely  on  a  comparison  of  the  lithology  and 
