298  PRE-CAMBRIAN    GEOLOGY   OF    NORTH   AMERICA. 
associated  with  the  green  schists,  make  it  seem  wiser  to  place  them  in 
the  Keewatin. 
The  conglomerate  forms  a  fixed  horizon  separating  the  Keewatin 
and  Lower  Huronian.  It  contains  pebbles  of  practically  all  the  types 
of  rock  in  the  region,  and  some  which  are  not  now  found  there.  The 
absence  of  arkose  pebbles  is  significant,  but  these  rocks  seem  to  be 
rather  local,  and  since  the  material  for  the  conglomerate  has  probably 
been  transported  some  distance  this  might  account  for  the  absence 
of  these  pebbles. 
One  finds  no  objection  to  placing  the  basic  eruptive  in  the  Keweena- 
wan,  since  it  is  composed  of  a  large  sheet  of  olivine  diabase  cutting 
the  other  rocks  of  the  region. 
SECTION  5.     LAKE  SUPERIOIl  REGION  (GENERAL). 
SUMMARY    OF   LITERATURE. 
Schoolcraft,298  in  1821,  makes  various  observations  on  the  crys- 
talline rocks.  At  Granite  Point  is  found  a  bluff  of  granite  which  is 
traversed  by  irregular  veins  of  greenstone  trap.  The  sandstone  laps 
upon  the  granite  and  fits  into  its  irregular  indentations  in  a  manner 
that  shows  it  to  have  assumed  that  position  subsequently  to  the  up- 
heaving of  the  granite.  Its  horizontality  is  preserved  even  to  the  im- 
mediate point  of  contact.  All  the  rock  along  the  south  shore  of  Lake 
Superior  is  either  red  or  gray  variegated  sandstone,  which  appears 
to  be  referable  to  one  formation.  On  passing  by  the  Porcupine  Moun- 
tains the  red  sandstone  is  visible  along  the  shore  in  a  position  nearly 
vertical,  dipping  under  the  lake  toward  the  north.  Red  sandstone 
in  a  vertical  position  is  found  at  the  mouth  of  Montreal  River  and 
for  a  few  miles  beyond  it  toward  Chequamegon  Bay.  On  St.  Louis 
River,  after  passing  red  sand  rock  in  a  horizontal  position,  is  found 
on  the  banks  of  the  river  a  slate  (argillite)  in  a  vertical  position 
traversed  by  greenstone  and  milky  quartz.  At  the  grand  portage  of 
the  St.  Louis  the  country  is  rough,  consisting  of  slate  in  a  vertical 
position.  This  continues  for  a  long  way  and  is  succeeded  by  horn- 
blende, which  continues  to  the  head  of  Grand  Rapids. 
Cunningham,299  in  1845,  Sanders,300  in  1845,  Campbell,301 
Sanders,302  Gray,303  in  1845,  and  Gray,304  in  1846,  give  various 
detailed  observations  as  to  the  mineral  regions  of  Lake  Superior,  but 
little  or  nothing  of  structural  interest. 
Rogers,305  in  1848,  remarks  that  the  south  shore  of  Lake  Superior 
is  outlined  by  a  series  of  east-west  dikes. 
Agassiz,306  in  1850,  describes  the  outlines  of  the  shore  of  Lake 
Superior  as  largely  due  to  six  different  sets  of  dikes  of  different  min- 
eralogical  character,  and  each  system  running  parallel  to  one  of  the 
main  shore  lines,  although  it  would  be  a  mistake  to  ascribe  the  form 
