LAKE   SUPERIOR   REGION.  381 
beds.  That  so  few  acidic  dikes  are  found  in  the  upper  Huronian  of 
the  south  shore  can  be  explained  only  by  the  fact  that  the  acidic 
eruptives  of  the  Keweenawan  are  mostly  remote  from  the  Marquette, 
Menominee,  and  Penokee  districts.  A  closer  study  will  probably  show 
in  these  districts  a  greater  abundance  of  acidic  eruptives  than  has 
been  supposed.  The  deep-seated  pipes  and  bosses  formed  by  the 
eruptions  of  the  Keweenawan  felsites  and  porphyries  perhaps  crys- 
tallized in  the  form  of  granite.  It  may  well  be  that  large  masses  of 
intrusive  granite  are  of  Keweenawan  or  Animikie  age. 
Keweenawan  ("  Keweenian,"  "  Nipigon,"  "Copper-bearing"). — 
Keweenawan  has  been  used  from  the  beginning  for  the  series  of 
interbedded  eruptives  and  sediments  with  basic  intrusives  uncon- 
formably  overlying  the  Animikie  and  underlying  the  Potsdam. 
Logan  included  the  copper-bearing  series  and  the  Animikie  rocks 
in  his  "  Upper  Copper-bearing  group,"  and  thought  it  to  be  the 
downward  extension  of  the  Cambrian. 
The  Keweenawan  is  thought  to  be  of  middle  or  lower  Cambrian 
age  by  most  of  the  Canadian  geologists  and  by  Winchell,  Seaman, 
and  Lane.  The  geologists  of  the  United  States  Geological  Survey 
believe  it  to  be  pre-Cambrian. 
BASIC    ERUPTIVES    AND    STRATIGRAPHY. 
At  one  other  point  the  problem  of  Lake  Superior  stratigraphy  has 
been  made  more  difficult  by  certain  of  the  geologists  than  was  neces- 
sary. The  diabases,  diorites,  and  gabbros  were  by  several  writers  in 
early  days,  and  recently  by  N.  H.  Winchell,  regarded  as  metamorphic 
sedimentary  rocks.  Logan,  Murray,  and  Foster  and  Whitney  are 
notable  exceptions.  Partly  as  a  consequence  of  this  fact  came  the 
minute  subdivision  of  the  rocks  of  the  Marquette,  Menominee, 
Penokee,  and  other  districts.  As  all  now  regard  these  rocks  as 
intrusives,  the  facts  that  they  are  often  in  bosslike  masses  and  that 
when  interleaved  they  do  not  necessarily  continue  for  any  great 
distance  present  no  difficulty,  while  a  great  formation  like  the 
"  Upper  Slate "  of  the  Penokee  and  Marquette  districts  is  left  as 
a  whole  rather  than  divided  into  a  number  of  members  separated 
by  layers  of  greenstone.  Also  it  is  now  known  that  the  "  dioritic 
schists  "  are  ancient  eruptives,  in  part  contemporaneous  and  inter- 
bedded with  the  sedimentary  rocks.  The  volcanic  character  of  these 
rocks  was  suggested  by  Foster  and  Whitney,  and  their  igneous  origin 
was  appreciated  by  Eominger.  Later  investigations  by  Wadsworth 
and  Williams,  with  the  microscope,  led  to  the  same  conclusion. 
These  groups  of  surface  volcanics  and  associated  fragmental  rocks 
have  now  been  found  in  the  Archean,  the  lower  Huronian,  the  middle 
Huronian,  and  the  upper  Huronian,  and  those  of  the  tour  horizons 
present  astonishing  lithological  similarities.     This  makes  the  correla- 
