LAKE    SUPERIOR   REGION.  351 
If  it  be  granted  that  the  Keweenawan  and  Cambrian  are  uncon- 
formable throughout,  there  may  still  arise  the  question  whether  the 
Keweenawan  deposition  was  not  going  on  in  the  isolated  Lake 
Superior  basin  both  before  and  during  lower  and  middle  Cambrian 
time,  while  the  Paleozoic  sea  was  transgressing  from  the  south — in 
other  words,  whether  the  unconformity  in  question  may  not  represent 
an  inter-Cambrian  interval  rather  than  a  pre-Cambrian  interval.  It 
has  seemed  to  us,  as  it  has  to  Irving,  Chamberlin,  and  others  who 
have  had  the  facts  in  hand,  that  the  unconformity  is  probably  too 
great  for  an  inter-Cambrian  unconformity,  but  no  decisive  evidence 
is  yet  at  hand  to  close  the  question  finally. 
We  would  emphasize  the  facts  that  the  Keweenawan  represents 
conditions  of  deposition  radically  different  from  those  of  the  Cam- 
brian and  that  evidence  of  unconformity  is  strong  for  part  of  the 
series,  but  we  recognize  the  possibility  that  the  deposition  of  the 
upper  part  of  the  Keweenawan  may  have  continued  into  lower  and 
middle  Cambrian  time. 
UNCONFORMITY   BENEATH   THE  KEWEENAWAN. 
Evidence  of  the  existence  of  an  unconformity  at  the  base  of  the 
Keweenawan  is  for  the  most  part  not  conspicuous,  because  of  the 
slight  divergence  in  strike  and  dip  between  the  Keweenawan  beds  and 
the  underlying  upper  Huronian  rocks,  because  of  the  faulting  which 
has  obscured  the  relations  in  Michigan,  Wisconsin,  and  part  of  Min- 
nesota, and  because  of  the  fact  that  the  bottom  member  of  the  series 
in  parts  of  Minnesota  and  Wisconsin  is  intrusive  gabbro.  The  broad 
field  distribution  of  the  Keweenawan  series  affords  excellent  evidence 
of  its  unconformity  on  the  underlying  series,  for  it  comes  in  contact 
now  Avith  one  and  now  with  another  of  the  underlying  series.  The 
most  direct  evidence  of  unconformity  may  be  seen  in  the  Port  Arthur 
district  on  the  northwest  shore  of  Lake  Superior,  where  the  base  of 
the  Keweenawan,  consisting  of  coarse  conglomerates,  sandstones,  and 
limestones,  rests  against  the  eroded  edges  of  the  gently  tilted  slates  or 
the  iron  formation  of  the  Animikie  group  and  highly  tilted  lower- 
middle  Huronian  graywackes  and  granite.  In  the  central  part  of  the 
Gogebic  district  the  basal  member  of  the  series,  usually  a  gabbro  but 
locally  conglomerate,  rests  against  the  upper  Huronian  iron  forma- 
tion, while  in  the  east  and  west  ends  of  the  district  it  rests  against 
upper  Huronian  slate.  In  the  Mesabi  district  also  the  gabbro  at  the 
base  of  the  Keweenawan  laps  diagonally  across  the  three  members  of 
the  upper  Huronian  onto  the  Giants  Range  granite.  It  might  be 
argued,  because  of  the  intrusive  nature  of  the  gabbro,  that  its  rela- 
tions to  the  underlying  series  would  afford  no  sound  evidence  of 
unconformity  at  the  base  of  the  Keweenawan.  It  is  thought,  how- 
ever, that  the  surface  of  contact  is  essentially  an  erosion  surface 
