LAKE    SUPERIOR   REGION.  307 
horizon  of  600  or  700  feet  of  the  Keweenawan  sandstone.  Also  in 
northeastern  Minnesota  and  in  the  Penokee  district  the  overlying 
Keweenawan  is  now  in  contact  with  one  member  of  the  underlying 
series  and  now  with  another.  Further,  in  the  Keweenawan  sand- 
stones of  Thunder  Bay  are  found  chert  and  jasper  pebbles  from  the 
Animikie,  while  in  the  Wisconsin  Keweenawan  are  quartzite  pebbles 
apparently  from  the  underlying  Huronian.  More  abundant  than 
these  in  the  Keweenawan  conglomerates  are  pebbles  of  older  gneiss 
and  granite.  Lithologically  the  Keweenawan  rocks  are  also  unlike 
the  Huronian.  The  bedded  and  sedimentary  series  of  the  two  groups 
are  in  strong  contrast.  The  shales  and  sandstones  of  the  Kewee- 
nawan have  nothing  in  common  with  the  quartz  slates  and  quartz 
schists  of  the  Huronian.  Also  in  the  Huronian  there  is  nothing  like  the 
acidic  eruptives  of  the  Keweenawan.  They  have  the  common  feature 
only  of  basic  eruptive  rocks,  and  of  these  in  the  Huronian  there  are 
no  amygdaloidal  or  vesicular  layers.  A  further  difference  between 
the  Huronian  and  the  Keweenawan  is  in  the  degree  of  metamorphism. 
The  Keweenawan  sediments  are  unaltered,  while  the  Huronian  sedi- 
ments are  metamorphic.  Whether  this  metamorphism  took  place 
before  or  during  the  period  of  Keweenawan  eruptions  and  deposition 
is  uncertain. 
That  the  closely  plicated  Huronian  rocks  were  folded  before  Kewee- 
naAvan  time  is  indicated  by  the  fact  that  the  troughs  of  Huronian 
schists  adjacent  to  Lake  Nipigon  lie  directly  athwart  the  flat-lying 
Keweenawan  beds.  If  these  schists  are  truly  Huronian  and  equiva- 
lent to  the  unfolded  rocks,  as  supposed  by  Bell,  there  can  be  no  doubt 
of  the  existence  of  a  genuine  unconformity  between  the  two  systems. 
The  Keweenawan  syncline  forming  the  bed  of  Lake  Superior  is 
found  to  comprise  the  whole  basin,  as  well  as  a  considerable  area  in 
northern  Wisconsin  and  Minnesota.  Not  only  the  form  of  the  lake 
as  a  whole,  but  its  chief  bays  are  due  to  subordinate  folding  or  fault- 
ing of  the  Keweenawan  series.  In  the  great  synclinal  movement  the 
underlying  Huronian  has  partaken. 
Chamberlin,317  in  1883,  gives  a  summary  of  the  arguments  for  re- 
garding the  copper-bearing  series  of  Lake  Superior  as  pre-Potsdam: 
(1)  The  general  stratigraphical  relations,  the  weakest  argument  of 
all,  indicate  this.  The  Potsdam  sandstone  throughout  the  entire 
basin  of  Lake  Superior  is  always  horizontal,  or  nearly  so,  while  the 
Keweenawan  series  at  many  points  immediately  adjacent  have  suf- 
fered extensive  disturbance.  (2)  The  difference  in  thickness  is 
enormous;  the  Potsdam  is  rarely  1,000  feet  thick,  while  the  Kewee- 
nawan series  has,  in  addition  to  a  vast  thickness  of  interstratified 
eruptives  and  detrital  rocks,  an  upper  portion  free  from  igneous  mat- 
ter 15,000  feet  in  thickness.  (3)  The  sandstones  of  the  Keweenawan 
series  are  largely  composed  of  silicates,  while  the  Potsdam  sandstone 
