310  PRE-CAMBRIAN   GEOLOGY    OF    NORTH   AMERTCA. 
Huronian  area  of  Murray  and  Logan  shows  that  it  is  a  series  of  rocks 
which  is  bent  into  gentle  folds,  and  which  is  composed  for  the  most 
part,  excluding  eruptive  material,  of  quartzites  and  graywackes,  with 
a  subordinate  proportion  of  limestone  and  chert.  The  rocks  as  a 
whole  are  very  little  altered  and  resemble  more  the  fossiliferous 
formations  than  the  crystalline  schists. 
The  Marquette  and  Menominee  iron-bearing  series  are  highly  folded 
and  the  metasomatic  changes  which  the  crystalline  members  of  the 
series  have  undergone  are  often  extreme.  Excluding  the  greenish 
schists  of  the  lower  part  of  the  series,  which  may  belong  to  an  older 
formation,  the  rocks  are  mainly  fragmental  slates  and  quartzites,  in- 
cluding a  large  proportion  of  basic  eruptives,  and  also  iron  ores,  lime- 
stones, etc.,  the  whole  having  a  distinctly  Huronian  aspect.  The 
various  greenstone  layers  of  Brooks's  scheme  are  regarded  as  eruptive, 
either  contemporaneous  or  subsequent,  as  are  also  many  of  the  green- 
ish schists  which  by  gradation  pass  into  the  massive  greenstones. 
The  iron  ore  and  jasper  are  regarded  as  of  sedimentary  origin,  being 
remarkably  like  similar  material  in  the  Penokee-Gogebic  and  Vermil- 
ion formations,  where  there  can  be  no  doubt  of  their  water-deposited 
character.  In  the  Marquette  district,  as  well  as  in  the  Vermilion 
Lake  district  in  Minnesota,  are  conglomerates  overlying  the-  iron  belt 
which  sometimes  contain  fragments  of  the  underlying  formation  sev- 
eral feet  in  length.  These  fragments  prove  the  existence  of  the  jas- 
pery  and  chalcedonic  material  in  its  present  condition  before  the 
formation  of  the  overlying  quartzite. 
The  Penokee-Gogebic  iron  belt  is  regarded  as  continuous  with  the 
Huronian  of  the  Marquette  district.  The  slate  belt  of  St.  Louis  and 
Mississippi  rivers  is  undoubtedly  the  equivalent  of  the  Animikie 
series  and  of  the  Huronian.  Equivalent  with  those  are  also  the 
quartzites  of  Chippewa  and  Barron  counties,  the  ferruginous  schists 
of  Black  River,  the  Baraboo  quartzites,  and  the  quartzite  series  of 
southern  Minnesota  and  southeastern  Dakota. 
At  New  Ulm  and  Redstone  in  Minnesota  the  quartzites  and  con- 
glomerates plainly  overlie  the  gneiss  unconformable.  The  thickness 
of  this  formation  here  exposed  is  probably  about  5,000  or  G,000  feet,  a 
continuous  section  being  found  by  Merriam  at  Sioux  Falls,  S.  Dak., 
having  a  thickness  of  not  less  than  3,000  to  4,000  feet.  The  tilted 
position  of  these  quartzites,  their  great  thickness  and  their  lithologi- 
cal  contrast  with  the  Potsdam  sandstone  make  it  evident  that  be- 
tween these  series  and  the  overlying  Potsdam  sandstone  is  a  great 
unconformity. 
In  the  Animikie  series  is  a  strongly  marked  continuous  horizon  of 
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 
