LAKE    SUPERIOR   REGION.  301 
from  a  ferriferous  solution  at  the  time  of  formation  of  the  stratified 
rocks. 
Whitney,314  in  1857,  again  maintains  the  Potsdam  age  of  the  sand- 
stones of  the  Cupriferous  series.  Underlying  this  series  unconform- 
ably  on  the  south  shore  is  the  Azoic  series,  which  is  identical  in  char- 
acter with  the  rocks  of  Thunder  and  Black  bays.  The  rocks  on  the 
north  shore  of  Lake  Huron  and  in  the  north  and  east  of  Canada  are 
identical  in  position  and  lithological  character  with  the  Azoic  system. 
Whittlesey,315  in  1876,  finds  nowhere  on  the  American  side  of  the 
boundary,  except  at  Vermilion  Lake,  rocks  which  are  like  the  Lau- 
rentian  of  Canada.  The  great  masses  of  granite  and  syenite  around 
which  the  Huronian  is  formed  do  not  resemble  the  Laurentian  of  the 
Canadian  geologists.  Between  the  Canadian  and  American  Huronian 
there  is  a  very  close  resemblance.  The  conclusion  of  Foster  and 
Whitney  that  the  traps  o»f  Lake  Superior  are  of  Potsdam  age  is 
adopted.  The  Bohemian  range  resembles  more  nearly  the  Huronian 
than  it  does  the  trap  series.  .  In  this  range  are  bands  of  friction 
conglomerates  with  the  evidences  of  metamorphic  sandstone,  passing 
into  jasper,  vesicular  trap,  and  breccia.  A  friction  conglomerate  also 
occurs  at  Aminicon,  Douglas  County,  Wis. 
Pumpelly,  in  1878,  describes  Lake  Superior  as  divided  into  two 
basins  by  Keweenaw  Point.  See  summary  in  Chapter  III,  section  1, 
Michigan,  page  128. 
Irving,316  in  1883,  gives  a  systematic  account  of  the  Copper-bearing 
rocks  of  Lake  Superior.  From  this  group  is  excluded  the.  so-called 
lower  group  of  Logan,  the  Animikie  group  of  Hunt,  and  also  the  hori- 
zontal sandstones  known  as  the  Eastern  and  Western  sandstones;  al- 
though it  includes  the  dolomitic  sandstones  with  accompanying 
crystalline  rocks  between  Black  and  Thunder  bays,  and  occupies  the 
valley  of  Black  Sturgeon  and  Nipigon  rivers,  as  well  as  Lake  Nipigon. 
The  Keweenaw  or  Copper-bearing  series  then  includes  the  succession 
of  interbedded  traps,  amygdaloids,  felsitic  porphyries,  porphyry  con- 
glomerates, and  sandstones,  and  the  conformable  overlying  sandstone 
typically  developed  in  the  region  of  Keweenaw  Point  and  Portage 
Lake.  These  rocks  have  their  most  widespread  extent  about  the  west 
half  of  Lake  Superior,  but  also  occur  in  the  eastern  part  of  the  lake. 
The  entire  geographical  extent  in  the  immediate  basin  of  Lake  Su- 
perior is  about  41,000  square  miles. 
The  eruptive  rocks  include  basic,  intermediate,  and  acidic  kinds, 
but  there  is  no  such  chronological  relation  between  these  three  kinds 
as  is  found  to  be  the  rule  in  Tertiary  and  post-Tertiary  times.  In 
the  Palisades  of  the  Minnesota  coast  quartz  porphyries  are  found  both 
overlain  and  underlain  by  basic  rocks,  with  abundant  evidence  that 
the  porphyry  is  a  surface  flow.     The  same  phenomena  are  seen  at 
