LAKE   SUPERIOR   REGION.  367 
RELATIONS    OF   KEWEENAWAN   AND    UNDERLYING    SERIES. 
Concerning  the  relations  of  the  Keweenawan  and  next  underlying 
series  opinion  is  nearly  unanimous.  As  these  two  series  were  folded 
together  to  form  the  basin  of  Lake  Superior,  the  earlier  writers  re- 
garded them  as  conformable.  That  they  are  really  discordant  was 
first  recognized  by  Brooks  and  Pumpelly,  who  found  that  the  base 
of  the  Keweenawan  is  now  in  contact  with  one  formation  of  the 
underlying  series  and  now  with  another,  and  from  this  general  rela- 
tion they  argued  an  erosion  interval.  Brooks  also  brought  forward 
as  evidence  of  this  conclusion  the  wholly  unaltered  character  of  the 
Keweenawan  detritals  and  the  simplicity  of  its  folding  as  compared 
with  the  Huronian.  The  Wisconsin  geologists  corroborated  Brooks's 
and  Pumpelly 's  results.  When  the  relations  of  the  series  on  the 
north  shore  were  closely  examined  actual  evidence  of  the  erosion 
interval,  consisting  of  basal  conglomerates,  was  found  by  Irving  at 
Thunder  Bay.  The  same  was  seen  by  McKellar,  and  upon  mapping 
the  two  series  in  northeastern  Minnesota  Irving  and  Merriam  found 
that  the  same  discordance  which  was  found  on  the  south  shore  ap- 
peared— that  is,  the  base  of  the  Keweenawan  is  now  in  contact  with 
one  member  of  the  underlying  series  and  now  with  another.  Van 
Hise  and  Clements  made  similar  observations  about  Thunder  Bay. 
The  recent  detailed  mapping  of  the  district  north  and  east  of  Thun- 
der Bay  by  W.  N.  Smith  brings  out  the  clearest  evidence  of  the 
magnitude  of  the  unconformity  yet  presented.  The  coarse  con- 
glomeratic base  of  the  Keweenawan  here  rests  in  turn  on  the  tilted 
and  eroded  edges  of  the  slates  or  the  iron  formation  of  the  Animikie 
group  and  loAver-middle  Huronian  graywackes,  His  observations 
were  confirmed  by  the  joint  committee  of  the  Canadian  and  the 
United  States  geological  surveys  in  1904.  Selwyn  found  at  Thunder 
Bay  no  evidence  of  this  erosion  interval ;  but  this  testimony  is  nega- 
tive and  stands  alone.  The  belief  that  there  is  a  physical  break  at  the 
base  of  the  Keweenawan  is  supported  by  a  great  mass  of  evidence 
from  many  localities. 
GENERAL  SUCCESSION  ACCORDING  TO  DIFFERENT  WRITERS. 
In  taking  the  next  step  dowmward  we  come  to  the  complex  about 
which  there  has  been  so  great  diversity  of  opinion,  and  it  is  difficult 
even  yet  to  see  clearly  all  the  results  which  legitimately  follow  from 
the  work  done.  The  crude  notion  that  the  sandstones,  traps,  jaspers, 
gneisses,  granites,  and  all  other  rocks  of  Lake  Superior  represent  one 
great  formation,  the  crystalline  phases  being  more  metamorphosed 
materials,  as  maintained  by  some  of  the  earlier  geologists,  would  now 
hardly  be  held  by  anyone.  Also  it  is  doubtful  if  anyone  would  deny 
that  the  rocks  below  the  Keweenawan  series  are  divisible  on  a  struc- 
