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PRE-CAMBRIAN    GEOLOGY    OF    NORTH   AMERICA. 
Marquette  and  Penokee  elastics  is  a  crystalline  basement  complex. 
Within  the  pre-Keweenawan  elastics  in  each  district  is  a  second  phys- 
ical break.  Below  this  break,  in  the  Penokee  district,  the  formations 
of  the  Lower  Marquette  are  now  represented  only  by  the  Cherty  lime- 
stone. That  other  members  once  existed  is  indicated  by  the  presence 
of  fragments  of  jasper  and  quartzite  in  the  lowest  horizon  of  the 
Quartz  slate.  Formations  composed  of  these  rocks  and  a  cherty 
limestone  are  the  characteristic  members  of  the  Lower  Marquette. 
The  correspondence  of  the  members  of  the  Penokee  series  proper 
with  those  of  the  Upper  Marquette  is  complete.  The  Upper  Mar- 
quette and  Penokee  series,  looked  at  broadly,  are  great  slate  forma- 
tions, both  of  which  contain,  near  the  base,  an  iron-bearing  horizon. 
In  the  Penokee  series  that  portion  of  the  slate  overlying  the  ore 
formation  has  been  called  the  Upper  slate  member,  and  that  below  it 
the  Quartz  slate  member.  The  lower  part  of  the  Quartz  slate  is  a 
quartzite  and  conglomerate,  which  corresponds  to  the  quartzite  and 
conglomerate  forming  the  base  of  the  Upper  Marquette  series.  The 
uppermost  horizon  of  the  Penokee  Quartz  slate  is  a  narrow  layer  of 
persistent  quartzite,  which  does  not  appear  to  be  represented  in  the 
Marquette  district.  The  ore-bearing  member  is  identical  in  character 
in  both  districts,  being  unquestionably  derived  from  a  lean,  cherty 
carbonate  of  iron.  The  characteristic  rocks  of  both  are  now  the  iron 
carbonates,  ores,  and  cherts  containing  bands  and  shots  of  ore.  The 
chief  difference  between  the  two  is  that  in  the  Penokee  district  the 
actinolite-magnetite  schists  are  more  prevalent  and  the  iron-bearing 
formation  is  more  persistent.  Connected  with  this  fact  is  perhaps 
the  presence  of  the  upper  horizon  of  quartzite,  which  shows  that  a 
clearing  up  of  the  waters  occurred  before  the  beginning  of  deposition 
of  the  iron-bearing  sediments.  A  still  further  analogy  between  the 
Penokee  and  the  Upper  Marquette  series  is  the  presence  in  both  of 
abundant  surface  volcanics.  We  have  then  in  the  two  districts  the 
following  parallel  descending  pre-Keweenawan  succession : 
PENOKEE. 
Upper  slate,  locally  mica  schist. 
Iron-bearing  formation. 
Quartz  slate;  upper  horizon  persist- 
ent quartzite;  central  mass  a  slate; 
lower  part  often  conglomeratic,  bear- 
ing fragments  of  lower  series,  and 
locally  a  quartzite. 
Unconformity. 
Eroded  away. 
Limestone. 
Unconformity. 
Basement  complex. 
MARQUETTE. 
Upper  slate,  rather  extensively  mica 
schist. 
Iron-bearing  formation. 
Lower  slate;  lower  part  quartzite  or 
quartzite  conglomerate,  bearing  frag- 
ments of  lower  series,  either  Lower 
Marquette  or  Archean. 
Unconformity. 
Iron-bearing  formation. 
Limestone  and  lower  quartzite. 
Unconformity. 
Basement  complex. 
