158  PRE-CAMBRIAN    GEOLOGY    OF    NORTH   AMERICA. 
occur  upon  impervious  basements  in  pitching  troughs.  The  pitching 
troughs  are  formed  by  the  Siamo  slate,  the  Ajibik  quartzite,  a  mass  or 
dike  of  greenstone,  or  by  some  combination  of  these.  The  ore  de- 
posits are  likely  to  be  of  large  size  where,  as  a  result  of  the  folding, 
the  iron-bearing  formation  is  much  fractured,  thus  permitting  the 
ready  access  of  percolating  waters.  The  ore  deposits  occur  at  the 
bottom  of  the  Negaunee  formation,  within  the  Negaunee  formation, 
and  at  the  contact  horizon  between  the  Negaunee  formation  and  the 
overlying  Ishpeming  formation.  From  the  position  of  the  ore  de- 
posits above  the  impervious  formations  it  is  concluded  that  their 
concentration  occurred  during  or  subsequent  to  the  folding  which 
took  place  later  than  Upper  Marquette  time. 
The  Upper  Marquette  series  is  composed  of  the  following  forma- 
tions, in  conformable  succession  from  the  base  upward:  Ishpeming 
formation,  Michigamme  formation,  Clarksburg  formation. 
The  Ishpeming  formation  includes  two  classes  of  rocks,  which  are 
called  the  Goodrich  quartzite  and  the  Bijiki  schist.  These  rocks  are 
sufficiently  different  to  have  different  formation  names,  but  the  Bijiki 
schist  in  the  west  end  of  the  district  occupies  a  part  of  the  horizon  of 
the  Goodrich  quartzite  in  the  central  part. 
The  Goodrich  quartzite  includes  quartzites,  micaceous  quartz 
schists,  mica  schists,  mica  gneisses,  and  at  the  base  a  basal  conglom- 
erate. For  the  major  part  of  the  district  this  conglomerate  rests  upon 
the  Negaunee  formation,  and  the  rock  is  an  ore,  chert,  jasper,  and 
quartz  conglomerate.  At  a  few  places  the  Archean  rocks  are  sub- 
jacent, and  here  their  materials  predominate  in  the  conglomerate. 
The  Goodrich  quartzite  is  confined  to  the  central  and  western  parts 
of  the  district.  For  the  major  part  of  the  district  it  rests  unconform- 
ably  upon  the  Negaunee  formation.  In  places  erosion  has  cut 
through  the  Negaunee  formation  into  the  Ajibik  quartzite,  and  in  a 
few  cases  even  to  the  Archean,  and  here  the  Goodrich  quartzite  may 
be  found  resting  on  the  lower  formations.  For  the  greater  part  of 
the  area  the  Goodrich  quartzite  grades  up  into  the  Michigamme  or 
Clarksburg  formation,  but  in  the  northwestern  part  of  the  district  it 
passes  up  into  the  Bijiki  schist.  The  thickness  is  from  600  to  1,550 
feet. 
The  Bijiki  schist  is  a  banded  grunerite-magnetite  schist,  which  has 
been  derived  by  metasomatic  and  dynamic  processes  from  an  impure 
siderite.  It  is  confined  to  the  western  part  of  the  district.  The 
Bijiki  schist  grades  into  the  Goodrich  quartzite  below  and  into  the 
Michigamme  formation  above.  The  thickness  is  from  zero  to  520 
feet. 
The  Michigamme  formation  includes  slates,  graywackes,  mica 
schists,  and  mica  gneisses.  The  formation  is  exposed  in  a  single 
large  belt,  running  from  the  center  to  the  western  end  of  the  district. 
