LAKE    SUPERIOR   REGION. 
The  Huronian  series  is  divided  into  lower,  middle,  and  upper  divi- 
sions. The  lower  division  comprises  the  lower  quartzite,  of  great 
thickness,  the  great  marble  formation,  and  the  great  iron-ore  horizon 
containing  magnetitic,  hematitic,  and  jaspery  schists,  as  well  as  de- 
posits of  iron.  The  Middle  Huronian  comprises  quartzites,  clay 
slates,  and  obscure  soft  schists.  The  Upper  Huronian  includes  mica 
schists,  gneisses,  and  granite,  the  last  of  which  may  possibly  be  erup- 
tive, but  is  the  topmost  member  of  the  Huronian  succession.  Inter- 
stratified  with  the  Huronian  are  diorites,  diabases,  gabbros  and  green- 
stones, and  greenstone  schists,  which  are  believed  to  be  conformable 
beds  of  metamorphosed  sediments.  They  are  never  found  in  the  form 
of  dikes.  The  thickness  of  the  Huronian  in  the  Menominee  is  not 
far  from  10,000  to  15,000  feet.  There  is  great  difficulty  in  ascertain- 
ing exactly  the  thickness  on  account  of  the  sharp  folds,  where  thick 
beds  double  back  upon  themselves.  This  especially  affects  the  quartz- 
ites, clay  slates,  and  greenstones.  The  relative  proportions  of  the 
different  kinds  of  rocks  of  the  Huronian  and  a  correlation  of  the 
successions  in  various  districts  of  the  Menominee  are  given,  and  the 
twenty  members  (including  the  upper  granite)  are  correlated  in  de- 
tail with  successions  north  of  Lake  Huron  and  in  the  Marquette, 
Gogebic,  Penokee,  and  central  Wisconsin  regions.  The  resemblances 
between  the  Marquette,  Menominee,  Sunday  Lake,  and  Penokee 
series  are  so  numerous  as  to  point  unmistakably  to  their  having  been 
formed  in  one  basin  under  essentially  like  conditions.  Detailed  sec- 
tions and  maps  of  the  rock  exposures  are  given  at  numerous  points. 
The  youngest  Huronian  member,  the  granite,  has  a  wide  extent. 
While  this  is  true,  granite  dikes  are  rare  in  the  Menominee  Huronian 
and  have  never  been  observed  in  the  Marquette  series.  No  rocks  af- 
fording the  slightest  suggestion  of  conglomeratic  structure  have  been 
found  in  the  Laurentian,  its  rocks  everywhere  being  much  metamor- 
phosed, and  in  many  places  so  much  as  to  destroy  all  traces  of  bed- 
ding. Underlying  the  quartzite  at  Sturgeon  River  Falls  is  a  schist 
conglomerate  which  has  numerous  pebbles  of  what  appear  to  be 
granite  and  gneiss  from  the  adjacent  Laurentian.  In  the  Pine  and 
Poplar  River  regions  is  found  conglomeratic  quartz  schist  containing 
micaceous  iron  and  magnetite.  In  the  Commonwealth  section  are 
included  quartz  schists  which  are  conglomeratic,  containing  pebbles 
of  white  quartz  and  jasper.  There  are  also  found  in  the  upper  beds 
of  the  Huronian  conglomeratic  micaceous  quartz  schists.  At  various 
places  granites  and  gneisses  overlie  conformably  the  younger  Huro- 
nian schists,  into  which  they  send  dikes.  As  the  evidence  of  bedding 
is  rare,  it  is  possible  that  toward  the  end  of  the  Huronian  period 
there  was  a  great  eruptive  overflow  of  these  rocks.  Cutting  the 
Laurentian  rocks  in  all  directions  are  dikelike  masses  of  granite  and 
greenstone.     The  abundant  greenstone  dikes  of  the  Laurentian  are 
