LAKE    SUPERIOR   REGION.  107 
breccia.  The  time  of  the  Ajibik  quartzite  marks  a  rapid  advance  to 
the  west  of  the  Lower  Marquette  sea,  and  therefore  the  formation 
extends  to  the  western  end  of  the  district.  In  the  eastern  part  of  the 
area  the  Ajibik  quartzite  grades  down  into  the  Wewe  slate,  but  for  the 
major  part  of  the  district  it  rests  unconformably  upon  the  Basement 
Complex.  At  many  localities  contacts  and  basal  conglomerates  are 
known.  The  Ajibik  quartzite  grades  above  either  into  the  Siamo 
slate  or  into  the  Negaunee  iron  formation.  The  thickness  is  from  700 
to  900  feet. 
The  Siamo  slate  is  chiefly  an  altered  mudstone,  although  locally  it 
is  a  graywacke  or  quartzite,  The  larger  area  of  exposure  of  the 
formation  is  confined  to  the  eastern  part  of  the  district,  although  a 
belt  of  the  formation  runs  near  the  north  side  of  the  Marquette  series 
to  the  west  end  of  the  district.  The  Siamo  slate  grades  into  the 
Ajibik  quartzite  below  and  into  the  Negaunee  iron  formation  above. 
The  thickness  is  from  600  to  1,200  feet. 
The  Negaunee  iron  formation  is  nonfragmental,  heavily  ferrugi- 
nous throughout,  and  contains  the  greater  iron-ore  deposits  of  the 
district.  The  formation  comprises  sideritic  slate,  which  may  be 
griineritic,  magnetitic,  hematitic,  or  limonitic;  grunerite-magnetite 
schist;  ferruginous  slate;  ferruginous  chert;  jaspilite,  and  iron  ore. 
Large  quantities  of  intrusive  greenstones  are  associated  with  the 
formation,  the  masses  of  which  vary  in  magnitude  from  great  bosses 
2  miles  or  more  long  and  half  a  mile  wide  to  small  dikes.  The  largest 
area  of  the  Negaunee  formation  is  in  the  east-central  part  of  the  dis- 
trict. From  this  area  two  belts  extend  west  to  the  western  end  of 
the  district.  On  the  whole  the  formation  is  soft,  and  occupies  low- 
lands between  the  more  resistant  greenstones  and  the  Ajibik  quartz- 
ite. The  formation  is  underlain  by  the  Siamo  slate  or  Ajibik 
quartzite,  into  which  it  grades,  and  is  overlain  unconformably  by 
the  Upper  Marquette  series. 
The  sideritic  slate  is  the  original  form  from  which  the  other  varie- 
ties of  rock  have  developed.  The  grunerite-magnetite  schists  were 
formed  by  partial  recrvstallization  of  the  silica,  by  oxidation  of  the 
iron  oxide  in  part  to  magnetite,  by  union  of  a  part  of  the  silica  and 
iron  protoxide,  producing  griinerite,  and  with  the  loss  of  carbon  diox- 
ide. The  ferruginous  slates  are  the  direct  result  of  the  decomposition 
of  the  iron  carbonate  and  the  peroxidation  of  (he  iron,  with  partial 
or  complete  recrvstallization  of  the  silica.  The  ferruginous  cherts 
differ  from  the  ferruginous  slates  in  that  the  iron  oxide  and  the  chert 
are  largely  concentrated  into  alternate  bands.  The  jaspilites  differ 
from  the  ferruginous  cherts  in  that  each  of  the  quartz  grains  of  the 
chert  bands  is  stained  red  by  included  hematite.  The  iron  ores  re- 
sulted from  the  concentration  of  the  iron  oxides  through  the  agency 
of  downward-percolating  waters.     These  concentration  bodies  usually 
