LAKE    SUPERIOR   REGION.  121 
In  the  iron  group  is  a  granite  dike  on  Sturgeon  River  12  feet  wide, 
which  breaks  through  the  iron  ore  and  jasper  at  a  right  angle  to  the 
schist.  Over  the  iron  formation,  at  the  Michigamme  mine,  is  found  a 
conglomerate  of  jasper  and  fragments  of  quartz  in  an  iron  and  quartz 
base.  There  is  a  discordance  between  the  Laurentian  and  the  Huro- 
nian.  The  Potsdam  sandstone  rests  upon  the  Huronian  and  Lauren- 
tian unconformable. 
Brooks  and  Pumpelly,32  in  1872,  maintain  that  the  copper-bearing 
rocks  of  Lake  Superior  are  unconformably  below  the  Lower  Silurian 
sandstone.  This  is  shown  by  the  fact  that  the  horizontal  strata  abut 
against  the  steep  faces  of  the  cupriferous  series  on  Keweenaw  Point, 
the  latter  dipping  away  from  the  sandstone  at  an  angle  of  60°  to  40.°. 
Also  for  a  long  distance  between  Montreal  Biver  and  Lake  Gogebic 
the  cupriferous  series  conform  in  strike  and  dip  to  the  Huronian 
schists,  dipping  steeply  to  the  north  at  an  angle  of  50°  to  70°,  while 
the  Silurian  sandstone  to  the  north,  in  a  flat-lying  condition,  covers 
an  extensive  country.  In  sec.  13,  T.  40  N.,  R.  41  W.,  the  Silurian 
sandstone  is  found  in  a  nearly  horizontal  position,  while  4  miles  dis- 
tant the  cupriferous  series  dip  to  the  north  at  an  angle  of  50°. 
It  is  concluded  that  the  cupriferous  series  was  formed  before  the 
tilting  of  the  Huronian  beds  upon  which  it  rests  conformably,  and 
consequently  before  the  elevation  of  the  great  Azoic  area.  After  the 
elevation  of  these  rocks,  and  after  they  had  assumed  their  essential 
lithological  characters,  came  the  deposition  of  the  Lake  Superior 
sandstone  and  its  accompanying  shales  as  a  product  of  the  erosion  of 
the  older  rocks,  and  containing  fossils  which  show  them  to  belong  to 
the  Lower  Silurian.  At  several  places  in  the  Upper  Peninsula  of 
Michigan  a  lack  of  conformity  between  the  Laurentian  and  the 
Huronian  has  been  detected,  but  when  the  Huronian  and  the 
Cupriferous  are  seen  in  contact  there  seems  to  be  a  well-marked  con- 
cordance between  them. 
Brooks,33  in  1873,  divides  the  rocks  of  the  Upper  Peninsula,  in 
descending  order,  into  Lower  Silurian,  Copper-bearing  rocks,  Iron- 
bearing  rocks,  and  Granitic  rocks,  and  gives  a  systematic  account  of 
the  last  two,  and  especially  of  the  economic  geology  of  the  iron- 
bearing  series.  The  copper-bearing  rocks  correspond  witli  the  Upper 
Copper-bearing  rocks  of  the  Canadian  geologists  and  occupy  a  nar- 
row belt  on  the  northwestern  edge  of  the  Upper  Peninsula.  This 
series  includes  sandstones  which  are  nearly  or  quite  identical  with 
the  Silurian  in  appearance,  but  their  great  mass  is  made  up  of  dif- 
ferent varieties  of  trap,  often  amygdaloidal,  interstratified  with  beds 
of  peculiar  conglomerates.  The  layers  of  these  rocks  are  inclined, 
dipping  northwest  and  north  toward  Lake  Superior,  from  vertical  to 
as  low  as  23°  on  Keweenaw  Point.  The  iron-bearing  rocks  are 
assumed  to  correspond  with  the  Huronian  system  of  Canada.     They 
