170 
PRE-CAMBRIAN    GEOLOGY    OF    NORTH   AMERICA. 
The  pre-Cambrian  succession  is  as  follows : 
Algonkian 
Upper  Menominee. 
Unconformity. 
Lower  Menominee  . 
Unconformity. 
Archean 
'Hanbury  slate. 
Vulcan  formation,  subdi- 
vided into  the  Curry  ore- 
bearing  member,  Brier 
slate,  and  Traders  ore- 
bearing  member. 
{Negaunee  formation. 
Randville  dolomite. 
Sturgeon  quartzite. 
Granites  and  gneisses,  cut 
by  granite  and  diabase 
dikes. 
Quinnesec  schists,  cut  by 
acidic  and  basic  dikes 
and  veins. 
In  general  the  Algonkian  rocks  constitute  a  trough  bounded  on  the 
north  by  the  Archean  rocks. 
Archean. — The  Quinnesec  schists  are  dark-green  or  black  basic 
schists  and  spheroidal  greenstones,  cut  by  large  dikes  of  gabbro,  dia- 
base, and  granite,  and  by  smaller  dikes  of  a  schistose  quartz  por- 
phyry. These  occur  in  two  areas,  one  along  Menominee  River  to  the 
south  of  the  Huronian  rocks,  and  another  in  the  west-central  end  of 
the  district. 
Bordering  the  Algonkian  trough  on  the  north  is  a  complex  of 
granites,  gneisses,  hornblende  schists,  and  a  few  greenstone  schists,  all 
cut  by  dikes  of  diabase  and  granite.  This  complex  is  called  the 
Northern  Complex.  Most  of  the  Archean  rocks  are  igneous.  Al- 
though there  is  no  evidence  of  this,  some  of  the  fragmental  tuffs  may 
have  been  water  deposited.  The  Quinnesec  schists  and  the  Northern 
Complex  are  called  Archean  because  they  resemble  lithologically 
other  areas  of  Archean  rocks  in  the  Lake  Superior  country,  and  they 
both  underlie  the  Algonkian  series.  The  Northern  Complex  under- 
lies the  series  with  unconformity.  The  Quinnesec  schists  have 
not  been  observed  in  contact,  and  hence  the  presence  or  absence  of  a 
normal  erosion  unconformity  can  not  be  inferred. 
Lower  Menominee  series. — The  formations  of  the  Lower  Menomi- 
nee series  are  observed  only  in  the  center  and  on  the  northern  side 
of  the  Menominee  trough.  The  Sturgeon  formation  is  composed 
mainly  of  a  hard,  white  vitreous  quartzite  forming  a  continuous 
border  of  bare  hills  bordering  the  Archean  complex.  At  its  base  is 
a  coarse  conglomerate  made  up  of  debris  from  the  underlying  Arch- 
ean complex.  The  belt  is  in  general  a  southward-dipping  monocline 
with  dips  varying  from  25°  to  perpendicularity,  although  there  are 
