134  PKE-CAMBKIAN    GEOLOGY    OF    NORTH   AMERICA. 
previous  report  as  occurring  in  the  north  part  of  T.  47  N.,  R.  25  W., 
is  now  considered  as  a  recemented  mixture  of  granite  fragments 
mingled  with  arenaceous  material,  although  it  is  singular  that  the 
orthoclase  crystals  copiously  found  in  the  mass  have  all  sharp  outlines 
and  are  rather  fresh. 
The  upheaval  of  the  granite  and  its  intrusion  into  the  overlying 
strata  occurred  in  all  probability  near  the  termination  of  the  Huronian 
period,  as  we  find  the  granite  in  contact  with  all  the  Huronian  strata 
up  to  the  youngest,  and  these  always  in  a  dislocated  position.  In- 
trusive belts  of  granite  are  usually  never  found  to  intersect  beds 
higher  than  the  Iron  group,  except  in  the  country  north  of  the 
Penokee  range  in  Wisconsin  and  in  the  vicinity  of  Duluth,  Minn., 
where  granite  or  granite-like  rocks  cut  across  eruptive  belts  of 
gabbro  which  are  themselves  more  recent  than  any  of  the  sedimentary 
strata  of  the  Huronian.  These  granites  differ  from  the  ordinary 
granites  at  the  base  of  the  Huronian,  and  are  most  likely  younger. 
The  dislocation  of  the  Huronian  beds  is  not  exclusively  due  to  the 
upheaval  and  intrusion  of  the  granite,  but  has  been  caused  in  part 
by  diorite  and  diabase  intrusives  which  intersect  the  granite  as  well 
as  the  incumbent  beds.  The  diorites  intersecting  the  granite  are 
identical  with  similar  rocks  interstratified  with  the  schists  of  the 
Huronian  group  conformably  or  transversely  intersecting  them,  and 
they  therefore  represent  one  and  the  same  volcanic  injection.  From 
the  massive  forms  of  diorite  a  gradation  exists  into  a  schistose  con- 
dition. This  led  to  the  conclusion  in  the  former  report  that  the 
massive  diorites  had  suffered  secondary  fusion;  but  as  the  author  is 
now  convinced  that  schistose  structure  is  not  necessarily  the  result  of 
aqueous  sedimentation,  it  is  concluded  that  the  Dioritic  group  does 
not  belong  in  the  sedimentary  succession.  Dolerite  or  diabase  rocks 
intersect  in  dikelike  form  all  the  Huronian  rocks,  as  well  as  the 
granites.  As  they  are  like  those  of  the  copper-bearing  series,  these 
rocks,  as  well  as  the  contemporaneous  flows,  are  regarded  as  belonging 
to  the  same  geological  period.  In  the  Felch  Mountain  region  one 
dike  (15  or  1G  feet  in  thickness)  of  noncrystalline  granite  cuts  across 
the  iron-bearing  series.  In  sec.  33,  T.  42  N.,  R.  28  W.,  another  granite 
dike  cuts  through  the  iron-bearing  rocks. 
Above  the  Iron  group  of  the  Marquette  and  Menominee  districts, 
before  described,  are  found  at  many  localities  in  both  these  regions 
important  deposits  of  iron  which  belong  in  the  Arenaceous  Slate 
group.  There  are,  therefore,  two  iron  horizons  instead  of  one,  as 
before  supposed.  The  Mica  schist  formation,  supposed  to  belong- 
above  the  Arenaceous  Slate  group,  is  found  to  dip  conformably  below 
it  in  some  places,  and  therefore  is  really  a  part  of  the  Arenaceous  Slate 
group,  and  is  believed  to  represent  its  middle  horizon.  The  slate 
group  about  L'Anse  and  Huron  Bay  is  black  and  often  graphitic. 
