100  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
The  Arenaceous  Slate  group  of  great  thickness  is  so  designated 
because  a  large  portion  of  the  rocks  here  included  consist  of  sandy  sili- 
ceous layers,  alternating  with  slaty  argillitic  rock  beds  and  occasion- 
ally with  compact  quartzite.  The  strata  in  different  localities  quite 
often  differ  considerably.  The  rocks  of  this  group  are  incumbent  on 
the  quartzite  formation,  but  also  sometimes  rest  upon  the  ore-bearing 
rocks,  and  quite  often  are  found  in  direct  contact  with  the  dioritic 
series.  The  rocks  are  sometimes  conglomeratic  or  brecciated,  the  frag- 
ments consisting  of  different  kinds  of  rocks.  In  the  black  roofing 
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.  0.  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  oi 
Presque  isle  and  those  of  similar  class.  The  Silurian  sandrock  reposes 
unconformably  upon  the  Serpentine  formation  at  Presque  isle.  Besides 
the  serpentine  and  other  magnesian  silicates,  limestone  comprises  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  between  the 
granites  of  Marquette  and  the  adjoining  Huronian  beds.     On  the  con- 
trary, outcrops  of  the  two  kinds  of  rocks  exhibit  a  remarkable  parallel- 
ism in  strike  and  dip,  and  in  many  localities  the  Huronian  schists  and 
belts  of  granite  are  in  terlam  mated  in  perfect  conformity.    The  granite  J 
is,  however,  regarded  as  intrusive  masses.     The  granites  are  therefore,  1 
with  reference  to  the  stratified  sedimentary  rocks,  actually  the  younger  ] 
rock. 
The  Menominee  region. — Many  localities  and  sections  in  the  Menomil 
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  formation  reposes  un-j 
conformably  on  the  granite.  Thick  layers  of  limestone  are  found  inj 
the  series,  and  this  is  sometimes  congloTiieratic.  The  fissile  phyllitel 
schists  are  found  in  discordance  with  the  dioritic  schists  at  lake  IJan-j 
bury.  The  granitic  and  gneissic  rocks  south  and  north  of  the  Fetch ' 
mountain  ore  formation  are  found  to  be  absolutely  identical.  The  dio-, 
ritic  rocks  are  found  generally  and  play  the  part  of  an  intrusive  with 
regard  to  the  strictly  sedimentary  rock  beds  of  the  Huronian  series.] 
The  dioritic  group  is  held  to  be  older  than  the  iron -bearing  group  be- 
cause it  exhibits  a  greater  degree  of  metamorphism  and  on  the  ground  \ 
that  it  is  lithologically  like  the  equivalent  dioritic  group  of  the  Mar- 1 
