LAKE    SUPERIOR   REGION.  185 
by  beds  of  white  and  red  quartzite,  and  these  graduate  into  alternat- 
ing layers  of  quartzite  and  iron  ore.  The  iron-ore  horizon  is  here 
hematitic  and  soft,  but  is  the  equivalent  of  the  hard  magnetic  horizon 
to  the  west.  In  this  part  of  the  belt  are  presented  the  greatest  proba- 
bilities of  the  existence  of  workable  ore. 
Wright,  in  1879,  discusses  the  Laurentian  series  south  of  Lake 
Superior,  including  those  of  Penokee  Gap.  See  summary  in  Chapter 
III,  section  1,  Michigan,  page  128. 
Irving,112  in  1880,  gives  a  comprehensive  account  of  the  general 
structure  of  northern  Wisconsin.  Here  are  found  four  great  sys- 
tems— the  Laurentian,  Huronian,  Keweenawan,  and  Lower  Silu- 
rian— which  are  unconformable  with  one  another.  The  rocks  of  the 
crystalline  nucleus  are  correlated  with  the  Laurentian  of  Canada 
because  they  sustain  the  same  structural  relations  to  the  Huronian, 
Keweenawan,  and  Lower  Silurian  as  does  the  typical  Laurentian  of 
Canada,  and  because  they  have  the  same  general  lithological  pecul- 
iarities. There  can  be  no  reasonable  doubt  that  they  are  directly 
continuous  with  the  Canada  Laurentian.  The  prevailing  rocks  are 
granite  and  gneiss.  These  rocks  are  greatly  folded  and  certainly 
have  an  enormous  thickness.  It  is  evident  that  they  are  of  true  sedi- 
mentary origin. 
The  granites  are  generally  without  distinct  bedding,  but  no  erup- 
tive granite  recognizable  as  such  has  been  observed. 
Lying  immediately  against  the  Laurentian,  and  sharply  defined 
from  it,  extending  from  Montreal  River  to  Lake  Numakagon,  is  a 
belt  of  schistose  rocks  which  are  beyond  question  the  westward  exten- 
sion of  the  iron-bearing  series  of  the  Upper  Peninsula  of  Michigan. 
This  belt  has  an  aggregate  thickness  of  strata  of  13,000  feet.  The 
subdivisions,  beginning  below,  are  (1)  crystalline  limestone;  (2) 
quartz  schist  and  argillitic  mica  schist;  (3)  tremolitic  magnetite 
schists,  magnetic  and  specular  quartzites,  and  lean  magnetic  and  spec- 
ular ores;  (4)  alternations  of  black  mica  slate  with  diorite  and  schis- 
tose quartzites;  (5)  mica  schists  with  coarse  intrusive  granite.  These 
major  divisions  are  again  subdivided  at  Penokee  Gap  and  vicinity. 
The  system  always  dips  north,  usually  at  a  high  angle,  and  the  strikes 
are  oblique  to  the  underlying  Laurentian  gneiss,  proving  the  uncon- 
formity of  the  two  systems,  the  actual  contact  of  which  can  be  seen 
in  several  places.  These  rocks  are  regarded  as  the  equivalent  of  the 
Huronian  of  Canada,  because  they  are  the  direct  continuation  of  the 
iron-bearing  system  of  Marquette,  because  the  grand  divisions  of  the 
Bad  River  and  Marquette  system  are  similar,  because  they  show  the 
same  relations  to  the  Laurentian  and  Keweenawan  systems  as  are 
found  in  the  Huronian  of  Canada,  i.  e.,  newer  than  the  former  and 
older  than  the  latter,  and  because  the  Marquette  and  Menominee  sedi- 
