132  PRE-CAMBKIAN    GEOLOGY    OF    NORTH   AMERICA. 
which  are  evidently  corrugated  by  the  intrusion  of  this  mass.  In 
places  the  ore-bearing  formation  is  not  found  incumbent  on  the  Dio- 
litic  group.  At  Teal  Lake  the  quartzite  is  found  under  the  ore  and 
the  diorite  over  the  ore,  which  leads  to  the  conclusion  that  these  strata 
are  in  an  overturned  position. 
The  Arenaceous  slate  group,  of  great  thickness,  is  so  designated 
because  a  large  portion  of  the  rocks  here  included  consist  of  sandy 
siliceous  layers,  alternating  with  slaty  argillitic  rock  beds  and  occa- 
sionally with  compact  quartzite.  The  strata  in  different  localities 
often  differ  considerably.  The  rocks  of  this  group  are  incumbent 
on  the  quartzite  formation,  but  also  sometimes  rest  upon  the  ore- 
bearing  rocks,  and  often  are  found  in  direct  contact  with  the  dioritic 
series.  The  rocks  are  sometimes  conglomeratic  or  brecciated,  the 
fragments  consisting  of  different  kinds  of  rocks.  In  the  black  roof- 
ing slates  of  Huron  Bay  the  cleavage  is  discordant  with  the  bedding. 
Occupying  a  position  above  the  black  slates  are  the  ore  deposits  of 
the  Taylor  mine,  near  L'Anse,  and  of  the  Northampton  and  D'Alaby, 
north  of  Champion ;  also  the  S.  C.  Smith  and  other  mines.  These  ores 
are  contemporaneous  and  equivalent  to  that  of  the  Commonwealth 
in  the  Menominee  district. 
The  Mica  schist  group  is  found  exposed  for  the  most  part  about 
Michigamme  village.  The  Serpentine  group  includes  the  rocks  of 
Presque  Isle  and  those  of  similar  class.  The  Silurian  sand  rock  re- 
poses unconformably  upon  the  Serpentine  formation  at  Presque  Isle. 
Besides  the  serpentine  and  other  magnesian  silicates,  limestone  com- 
prises an  important  share  of  this  group. 
The  seven  previous  groups,  considered  to  be  a  succession  of  sedi- 
mentary strata,  are  intersected  by  various  dikes,  among  which  are  a 
dioritic  rock  and  dolerite  dikes,  the  latter  of  which  are  later  in  age 
than  the  former.  No  proof  has  been  found  of  any  discordance  be- 
tween the  granites  of  Marquette  and  the  adjoining  Huronian  beds. 
On  the  contrary,  outcrops  of  the  two  kinds  of  rocks  exhibit  a  remark- 
able parallelism  in  strike  and  dip,  and  in  many  localities  the  Hu- 
ronian schists  and  belts  of  granite  are  interlaminated  in  perfect  con- 
formity. The  granite  is,  however,  regarded  as  intrusive  masses. 
The  granites  are  therefore,  with  reference  to  the  stratified  sedimen- 
tary rocks,  actually  the  younger  rock. 
Menominee  region. — Many  localities  and  sections  in  the  Menomi- 
nee iron  region  are  described  in  detail.  The  Silurian  sandstone  is 
found  to  rest  unconformably  upon  the  nearly  vertical  Huronian 
strata.  Near  Sturgeon  River  Falls,  in  the  river,  the  quartzite  forma- 
tion reposes  unconformably  on  the  granite.  Thick  layers  of  lime- 
stone are  found  in  the  series,  and  this  is  sometimes  conglomeratic. 
The  fissile  phyllite  schists  are  found  in  discordance  with  the  dioritic 
schists  at  Lake  Hanbury.     The  granitic  and  gneissic  rocks  south  and 
