144  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  (bull.  86. 
Huronian  of  the  Marquette  district.  The  slate  belt  of  the  St.  Louis  anc 
Mississippi  rivers  is  undoubtedly  the  equivalent  of  the  Animikie  series 
and  of  the  Huronian.  Equivalent  with  those  are  also  the  quartzites  o; 
Chippewa  and  Barron  counties,  the  ferruginous  schists  of  the  Black 
Biver,  the  Baraboo  quartzites,  and  the  quartzite  series  of  southerr 
Minnesota  and  southeastern  Dakota. 
At  .New  Ulin  and  Bedstone  in  Minnesota  the  quartzites  and  conglom 
erates  plainly  uncomformably  overlie  the  gneiss.  The  thickness  of  this 
formation  here  exx>osed  is  probably  about  5,000  or  6,000  feet,  a  contin 
uous  section  being  found  by  Merriam  at  Sioux  Falls,  South  Dakota 
having  a  thickness  of  not  less  than  3,000  to  4,000  feet.  The  tilted  post 
tion  of  these  quartzites,  their  great  thickness,  their  lithological  contrast  j 
with  the  Potsdam  sandstone,  make  it  evident  that  between  these  series 
and  the  overlying  Potsdam  sandstone  is  a  great  unconformity. 
In  the  Animikie  series  is  a  strongly  marked  continuous  horizon  on 
cherty  and  jaspery  magnetitic  schists  and  quartzites.  The  series  as  a 
whole  is  quite  flat-lying,  although  having  subordinate  irregularities 
The  series  having  ferruginous  schists  north  and  west  of  lake  Superior 
are  regarded  in  part  as  having  been  once  continuous  with  the  Animikie 
series  and  are  now  separated  merely  because  of  erosion  on  the  crowns 
of  the  folds,  the  close  folding  of  the  Vermilion  schists  being  produced 
concomitantly  with  the  broad  simple  trough  of  lake  Superior.  In  sup -i 
port  of  this  position  is  the  fact  that  the  great  conglomerate  of  Ogishki 
Manissi,  with  the  alternating  quartzites  and  slates  of  Knife  lake,  is 
strikingly  like  the  Huronian  rocks.  It  is  also  the  case  that  in  the 
vicinity  of  Agamok  lake  the  Animikie  quartzites  appear  gradually  tt 
take  on  a  folded  condition. 
In  these  various  Huronian  areas  quartzites,  graywackes,  and  clay 
slates,  with  intermediate  phases,  make  up  the  most  of  the  clastic  series- 
As  has  been  seen,  these  are  rocks  which  have  been  indurated  by  meta 
somatic  changes,  and  it  follows  that  the  bulk  of  the  rocks  which  form 
the  Huronian  do  not  properly  fall  under  the  head  of  metamorphic  rocks. 
The  various  augitic  and  hornblendic  greenstones,  peridotites,  and  fels- 
itic  porphyries  are  regarded  as  eruptives,  while  many  of  the  schists  are 
modified  rocks  of  the  same  character.  The  cherty  and  jaspery  rocks 
are  supposed  to  be  some  sort  of  original  chemical  sediment,  certainly  | 
not  the  result  of  metamorphism  of  sedimentary  material.  The  lime- 
stones are  in  no  essential  respect  different  from  many  met  with  in  the 
formations  of  later  date. 
Irving-,197  in  1886,  discusses  the  origin  of  the  ferruginous  schists  and  | 
iron  ores  of  the  lake  Superior  region.    An  examination  of  the  Animi- 
kie, Penokee,  Marquette,  Menominee,  and  Vermilion  districts  reveals  the 
fact  that  in  all  of  them  is  found  abundant  carbonate  of  iron,  which  often- 
times grades  into  the  other  forms  of  the  iron-bearing  formation.     The  || 
silica  of  the  jasper,  actinolite,  magnetite  schists,  and  other  forms  oil 
the  iron  belt  never  shows  any  evidence  of  fragmental  texture,  so  easily  | 
