LAKE    SUPERIOR   REGION. 
schists,  quartz  rock,  and  black  slates,  magnetic  and  specular  schists 
and  slates,  metamorphic  diorite,  and  diorite  schists.  Its  lowest  por- 
tion is  simple  siliceous  schist  with  some  granular  white  quartz,  gray 
quartzite,  and  black  slate;  the  central  portion  consists  of  magnetic 
and  specular  slates  and  schists  in  which  all  the  ores  are  found,  and  the 
highest  and  northernmost  portion  consists  of  diorites,  diorite  slates, 
diorite  schists,  and  quartz  slates.  The  Copper-bearing  series  is  next 
north  of  and  immediately  overlies  the  Huronian,  and  is  of  enormous 
thickness,  never  less  than  4  miles.  The  lower  portions  of  the  group 
are  probably  in  part  of  igneous  origin,  but  the  upper  portions  are  be- 
yond all  doubt  exclusively  the  results  of  sedimentation.  The  group 
consists  of  shales,  sandstones,  conglomerates,  amygdaloids,  and  traps. 
The  sedimentary  series  do  not  altogether  overlie  the  trappean  beds, 
but  near  their  junction  are  directly  and  unmistakably  interstratified 
with  them.  The  Silurian  rocks  in  Ashland  County  are  in  a  horizontal 
position  in  a  trough  between  two  lines  of  highly  tilted  beds  of  the 
Copper-bearing  series.  At  one  place  the  horizontal  sandstone  is  found 
within  a  few  hundred  feet  of  the  copper-bearing  trap  and  within  2 
miles  of  vertical  sandstones  of  the  same  group. 
In  Douglas  County  the  horizontal  sandstone  is  traceable  to  within 
a  short  distance  of  the  trap,  and  sometimes  to  actual  contact,  the  trap, 
wherever  observable,  dipping  southward  and  having  no  tilted  sand- 
stones and  conglomerates  associated  with  it.  As  the  Huronian  and 
Copper-bearing  series  are  in  apparent  conformity  it  is  concluded  that 
they  were  once  spread  out  horizontally  one  over  the  other,  and  owe 
their  present  highly  tilted  position  to  one  and  the  same  disturbance; 
and  that  subsequently,  after  a  long  period  of  erosion,  the  horizontal 
Silurian  sandstones  were  laid  down  over  and  against  the  upturned 
edges  of  the  Copper-bearing  series;  and  that  hence  the  Copper-bear- 
ing series  is  more  nearly  allied  to  the  Archean  than  to  the  Silurian 
rocks.  One  fact  is,  however,  difficult  of  explanation  on  this  hypothe- 
sis. In  Douglas  County  at  several  places  the  horizontal  sandstones, 
when  traced  to  their  junction  with  the  southward-dipping  trap,  pre- 
sent a  remarkable  change;  the  horizontal  layers  are  suddenly  seen  to 
change  from  their  ordinary  position  to  a  confused  mass  of  broken 
layers,  dipping  in  every  conceivable  direction  and  increasing  in  con- 
fusion as  the  trap  is  approached,  until  finally  the  whole  changes  to  a 
confused  breccia  of  mingled  trap  and  sandstone  fragments.  It  is  sug- 
gested that  this  appearance  is  due  to  the  movement  of  the  solid  trap 
northward  against  the  sandstone  since  the  deposition  of  the  latter 
rock.  The  great  Lake  Superior  syncline  of  Copper-bearing  rocks  is 
found  to  extend  west  into  northern  Wisconsin. 
Brooks  (T.  B.),  in  187C>,  discusses  the  Huronian  rocks  south  of  Lake 
Superior,  including  the  Penokee  series.  See  summary  in  Chapter  III, 
section  1,  Michigan,  pages  126-128. 
