104  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
The  upheaval  of  the  granite  and  its  intrusion  into  the  overlying  strata 
occurred  in  all  probability  near  the  termination  of  the  Huroni an  period, 
as  we  find  the  granite  in  contact  with  all  the  Huronian  strata  up  to  the 
youngest,  and  these  always  in  a  dislocated  position.  Intrusive  belts  of 
granite  are  usually  never  found  to  intersect  beds  higher  than  the  iron- 
bearing  group,  except  in  the  country  north  of  the  Penokee  range  in 
Wisconsin,  and  in  the  vicinity  of  Duluth,  Minnesota,  where  granite  or 
granite-like  rocks  cut  across  eruptive  belts  of  gabbro  which  are  them- 
selves 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  inter- 
secting the  granite  are  identical  with  similar  rocks  interstratified  with 
the  schists  of  the  Huronian  group  conformably  or  transversely  inter- 
secting them,  and  they  therefore  represent  one  and  the  same  volcanic 
injection.  From  the  massive  forms  of  diorite  a.  gradation  exists  into  a 
schistose  condition.  This  led  to  the  conclusion  in  the  former  report 
that  the  massive  diorites  had  suffered  secondary  fusion  $  but  as  the  1 
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  dike-like-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  16  feet  in  thickness)  of  holocrystalline  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-ore  group  of  the  Marquette  and  Menominee  districts 
before  described  is  found  at  many  localities  important  deposits  of  iron 
in  both  these  regions  which  belong  in  the  Arenaceous  Slate  group. 
There  are,  therefore,  two  iron  horizons  instead  of  one,  as  before  sup- 
posed. The  mica-schist  formation,  supposed  to  belong  above  the  Are- 
naceous 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.  The  slate  beds  at  Plum- 
bago creek  are  succeeded  by  schistose  beds  richer  in  red  feldspar  and 
containing  little  quartz,  which  might  by  superficial  examination  be  mis- 
taken for  granite,  but  which  is  evidently  a  fragmental  rock  formed  by 
the  detritus  of  the  granite  which  near  by  forms  large  mountain  masses, 
and  the  granite  of  which  is  very  rich  in  red  feldsj)ar  and  contains  com- 
paratively little  quartz. 
