LAKE    SUPERIOR  REGION.  309 
member  of  the  Penokee  series.  These  relations  are  taken  to  imply 
that  between  the  Keweenawan  and  Penokee  series  there  was  a  long 
period  of  erosion.  There  is  also  an  unconformity  between  the  granite 
gneiss  schist  complex  and  the  Penokee-Gogebic  series.  This  is  shown 
in  the  manner  in  which  the  regularly  succeeding  belts  of  the  iron 
series  traverse  the  courses  of  the  lower ;  in  the  strong  contrast  between 
the  two  series  in  degree  of  crystallization,  the  lower  series  being 
nearly  completely  crystalline,  while  the  higher  is  little  altered;  in  the 
highly  folded  and  contorted  condition  of  the  lower  series  as  con- 
trasted with  the  unfolded  condition  of  the  higher;  in  the  contrast 
between  the  contacts  of  the  granite  with  the  lower  schists  and  with 
the  higher  slates,  the  former  being  invaded  by  it  in  an  intricate  man- 
ner, the  latter  never ;  in  the  discordant  lamination  of  the  two  sets  of 
rocks  when  in  contact  or  close  proximity;  and  in  the  occurrence  in 
the  upper  series,  not  only  at  horizons  above  the  base,  but  also  at  points 
on  the  contact  line,  of  abundant  detrital  material  from  the  lower 
series. 
In  the  Marquette  district  is  found  a  slaty  iron-bearing  series  which, 
by  common  consent,  is  regarded  as  the  equivalent  of  the  Penokee- 
Gogebic  series ;  but  the  two  have  one  point  of  contrast — the  Marquette 
is  highly  folded.  Here  intervenes  between  the  iron-bearing  slates  and 
the  granites  and  gneisses  a  set  of  greenish  hornblendic  rocks,  called 
by  Rominger  a  dioritic  group,  which  at  their  contact  with  the  bound- 
ing granite  are  penetrated  by  them  in  the  most  intricate  manner,  so 
that  one  can  not  resist  the  conclusion  that  the  granite  is  the  more 
recently  formed  rock.  These  green  schists  are  regarded  as  the  equiva- 
lent of  those  cut  by  granite  in  the  Penokee-Gogebic  district.  On  this 
view  the  slate  series  of  the  Marquette  district,  consisting  in  the  main 
of  little  altered  rocks,  was  built  up  on  a  basement  composed  of  gran- 
ite and  gneiss  and  greenish  schist,  and  subsequently  pushed  into 
troughlike  forms.  In  support  of  this  view  is  cited  the  failure  of  the 
granite  to  penetrate  the  slates  and  quartzites  associated  with  the  iron, 
and  the  occurrence  in  the  higher  series  of  fragments  from  the  lower, 
recomposed  rocks  at  points  where  the  quartzites  come  into  contact 
with  the  basement  rocks. 
The  Archean  in  these  regions  is  then  divisible  unless  the  upper 
series  are  called  Cambrian,  for  which  there  will  be  no  ground  until 
fossils  have  been  discovered  in  them.  These  upper  series  are  com- 
pared with  the  Huronian  of  Lake  Huron,  and  are  found  to  be  litho- 
logically  like  them,  and  to  bear  the  same  relations  to  the  underlying 
rocks,  and  to  them  the  term  Huronian  is  applied,  while  the  under- 
lying complex  is  regarded  as  Laurentian. 
Irving,320  in  1885,  gives  a  preliminary  account  of  an  investigation 
of  the  Archean  formations  of  the  Northwestern  States.  The  prob- 
lems to  be  solved  are  discussed.     An  examination  of  the  Original 
