154  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull. 86. 
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  qnartz-slate  is  a  quart zite  and  con- 
glomerate, which  corresponds  to  the  qnartzite  and  conglomerate  form- 
ing 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  character  of  the  ore-bearing  member  is  identical  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-magiietite-schists 
are  more  prevalent,  and  that  the  iron-bearing  formation  is  more  persist- 
ent. Connected  with  this  fact  is  perhaps  the  presence  of  the  upper  hori- 
zon 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  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-hearing  formation. 
Qnartz-slate;  upper  horizon  persistent 
qnartzite;  central  mass  a  slate;  lower 
part  often  conglomeratic,  bearing  frag- 
ments of  lower  series,  and  locally  a 
qnartzite. 
Unconformity. 
Eroded  away. 
Limestone. 
Unconformity. 
Basement  complex. 
MARQUETTE. 
Upper  slate,  rather  extensively  mica- 
schist. 
Iron-hearing  formation. 
Lower  slate;  lower  part  qnartzite  or 
quartzite-conglomerate,  hearing  frag- 
ments of  lower  series,  either  lower  Mar- 
quette or  Archean. 
Unconformity. 
Iron -hearing  formation. 
Limestone  and  lower  qnartzite. 
Unconformity. 
Basement  complex. 
Van  Hise,206  in  1891,  describes  the  physical  break  between  a  Lower 
and  an  Upper  Huronian  series.  In  the  Marquette  district  the  Lower 
series  includes  the  lower  quartzite  and  novaculite  of  Brooks,  the  lime- 
stone formation  as  well  as  the  chief  iron-beariu  g  formation  containing 
the  hard  ores,  which  is  composed  chiefly  of  jasper  and  actinolitic  and 
magnetitic  slates.  The  Upper  series  has  at  its  base  a  vitreous  quart- 
zite, but  is  chiefly  composed  of  black  slates  sometimes  carbonaceous, 
graywackes,  and  mica-schists,  together  of  great  thickness,  and  locally 
contains  belts  of  ferruginous  cherts  and  slates,  including  ore-bodies, 
which  are,  however,  of  a  different  character  from  the  ores  of  the  Lower 
series.  The  area  occupied  by  the  Upper  series  is  equal  to  or  greater 
than  that  of  the  Lower  series.     That  the  two  series  are  separated  by 
