352  PRE-CAMBRIAN    GEOLOGY   OF    NORTH   AMERICA 
because  of  its  low  angle  to  the  bedding  of  the  upper  Huronian  rocks, 
otherwise  it  would  be  necessary  to  assume  that  the  gabbro  had  in- 
truded itself  along  a  plane  gently  inclined  to  the  Huronian  series,  in 
which  case  there  ought  to  be  Animikie  rocks  both  above  and  below 
the  gabbro. 
UNCONFORMITY    BENEATH    THE    UPPER    HURONIAN,    OR    ANIMIKIE. 
The  unconformity  beneath  the  upper  Huronian,  or  Animikie,  has 
been  recognized  generally  throughout  the  Lake  Superior  region  where 
Animikie  rocks  are  exposed.  The  evidence  is  especially  striking  in 
the  Thunder  Bay  and  adjacent  districts  on  the  northwest  shore  of 
Lake  Superior,  where  gently  tilted  Animikie  sediments  rest  with  basal 
conglomerates  upon  highly  tilted  and  eroded  rocks  of  lower-middle 
Huronian,  Keewatin,  and  Laurentian  age.  The  difference  in  amount 
of  deformation  and  consequent  metamorphism  between  the  Animikie 
and  the  underlying  rocks  is  striking.  A  characteristic  feature  of  the 
underlying  sediments  is  a  steeply  inclined  secondary  cleavage,  which 
is  absent  in  the  Animikie.  Scarcely  less  obvious  are  the  relations  of 
the  Animikie  to  the  underlying  rocks  in  northeastern  Minnesota, 
where  many  actual  contacts  as  well  as  the  broad  field  relations  point 
to  the  existence  of  the  unconformity. 
On  the  south  shore  of  Lake  Superior  the  upper  Huronian  is  more 
highly  inclined,  but  otherwise  its  relations  to  the  underlying  lower 
and  middle  Huronian  and  Archean  rocks  are  fully  as  clear  as  on  the 
north  shore.  These  relations  are  best  seen  in  the  Marquette  district. 
Here  at  many  points  at  the  bottom  of  the  upper  Huronian  is  a  basal 
conglomerate,  the  detritus  of  which  is  mainly  derived  from  the  under- 
lying formations  in  direct  contact  with  it.  Also  at  some  of  these 
points  there  is  a  marked  discordance  between  the  bedding  of  the  two 
formations.  This  unconformable  relation  is  especially  clear  between 
the  Negaunee  formation,  constituting  the  upper  member  of  the  middle 
Huronian,  and  the  Goodrich  quartzite,  the  lowest  formation  of  the 
upper  Huronian.  In  the  Gogebic  district  the  Animikie  is  locally  in 
contact  with  the  cherty  limestones  of  the  lower  Huronian,  and  in 
other  places  with  the  Archean.  The  dominant  fragments  of  the  basal 
conglomerate  vary  accordingly.  While  the  unconformity  at  the  base 
of  the  upper  Huronian  is  clear,  at  various  places  in  Michigan  and 
Wisconsin  the  upper  Huronian  has  been  so  closely  folded  that  its 
general  distribution  does  not  give  decisive  evidence  of  its  uncon- 
formity on  the  underlying  rocks. 
The  evenness  of  the  contact  of  the  Animikie  group  on  the  underly- 
ing group  on  the  north  shore  and  in  the  Gogebic  district  on  the  south 
shore  indicates  a  truncation  of  the  underlying  group  for  at  least  the 
western  half  of  the  Lake  Superior  region  amounting  almost  to  base- 
leveling.     The  frequent  thinness  of  the  conglomerates  at  the  base  of 
