'a*  his*:.]  LAKE  SUPERIOR  REGION.  139 
ike  the  Huronian  that  they  are  regarded  as  a  folded  continuation  of  the 
Anirnikie  beds. 
That  the  Animikie  Huronian  is  beneath  the  Keweenawan  rocks  is 
hown  by  the  fact  that  the  Keweenawan  beds  along  the  Minnesota 
oastarepassed  in  descending  order  until  the  Animikie  slates  are  reached 
it  Grand  portage  bay,  but  there  is  not  a  direct  downward  continuation 
)f  the  Keweenawan  into  the  Animikie,  for  between  the  two  there  has 
Deen  an  intervening  period  of  erosion.  This  is  shown  by  the  fact  that 
it  Grand  portage  bay  where  the  two  formations  come  together  the  un- 
lerlying  slates  suddenly  rise  entirely  across  the  horizon  of  600  or  700 
?eet  of  the  Keweenawan  sandstone.  Also  in  northeastern  Minnesota 
md  in  the  Penokee  district  the  overlying  Keweenawan  now  is  in  con- 
act  with  one  member  of  the  underlying  series  and  now  with  another. 
Further,  in  the  Keweenawan  sandstones  of  Thunder  bay  are  found  chert 
md  jasper  pebbles  from  the  Animikie,  while  in  the  Wisconsin  Kewee- 
lawan  are  quartzite  pebbles  apparently  from  the  underlying  Huronian. 
VIore  abundant  than  these,  in  the  Keweenawan  conglomerates  are  peb- 
bles of  older  gneiss  and  granite.  Lithologically  the  Keweenawan  rocks 
ire  also  unlike  the  Huronian.  The  bedded  and  sedimentary  series  of 
3he  two  groups  are  in  strong  contrast.  The  shales  and  sandstones  of 
he  Keweenawan  have  nothing  in  common  with  the  quartz-slates  and 
piartz-schists  of  the  Huronian.  Also  in  the  Huronian  there  is  nothing 
ike  the  acid  eruptives  of  the  Keweenawan.  They  have  the  common 
feature  only  of  basic  eruptive  rocks,  and  of  these  in  the  Huronian  there 
ire  no  amygdaloidal  or  vesicular  layers.  A  further  difference  between 
the  Huronian  and  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  uncer- 
tain. 
That  the  closely  plicated  Huronian  rocks  were  folded  before  Kewee- 
nawan time  is  indicated  by  the  fact  that  the  troughs  of  Huronian  schists 
adjacent  to  lake  Mpigon  lie  directly  athwart  the  flat-lying  Keweenawan 
beds.  If  these  schists  are  truly  Huronian  and  equivalent  to  the  un- 
folded rocks,  as  supposed  by  Bell,  there  can  be  no  doubt  of  the  exist- 
ence of  a  genuine  unconformity  between  the  two  systems.  The  Ke- 
weenawan synclinal  forming  the  bed  of  lake  Superior  is  found  to  com- 
prise the  whole  basin,  as  well  as  a  considerable  area  in  northern  Wis- 
consin and  Minnesota.  Not  only  the  form  of  the  lake  as  a  whole,  but 
its  chief  bays  are  due  to  subordinate  folding  or  faulting  of  the  Kewee- 
nawan series.  In  the  great  synclinal  movement  the  underlying  Huro- 
nian has  partaken. 
Chamberlin,192  in  1883,  gives  a  summary  of  the  arguments  for  re- 
garding the  copper-bearing  series  of  lake  Superior  as  pre-Potsdam: 
(1)  The  weakest  argument  of  all,  the  general  stratigraphical  relations, 
indicate  this.     The  Potsdam  sandstone  throughout  the  entire  basin  of 
