332  PRE-CAMBRIAN   GEOLOGY   OF   NORTH   AMERICA. 
The  nature  and  distribution  of  the  Archean  rocks  for  different 
parts  of  the  region  are  discussed  in  subsequent  paragraphs. 
Michigan  and  Wisconsin. — In  Michigan  and  Wisconsin  the  Keewa- 
tin  rocks  cover  extensive  areas,  although  subordinate  in  amount  to 
the  Laurentian.  A  considerable  area  of  Keewatin  rocks  is  found  in 
the  northern  part  of  the  Marquette  district.  This  area  includes  vari- 
ous green  schists,  among  which  are  chloritic  and  micaceous  schists, 
resulting  from  the  alteration  of  basic  and  intermediate  rocks  which 
present  evidences  of  surface  volcanic  origin  and  perhaps  of  deposi- 
tion under  water.  Where  strongly  schistose  or  gneissic  the  rocks 
have  been  called  the  Mona  schist,  and  where  coarser  and  giving  evi- 
dence of  derivation  from  surface  volcanics  they  have  been  called  the 
Kitchi  schist.  Associated  with  the  green  schists  of  the  Marquette 
district  are  various  nearly  massive  igneous  rocks,  among  which  peri- 
dotite  is  important.  There  are  also  small  areas  of  jasper  and  car- 
bonate rocks,  formerly  supposed  to  be  of  the  nature  of  ferruginous 
vein  material,  but  now  believed  to  represent  a  sedimentary  iron  forma- 
tion infolded  or  interbedded  with  the  other  rocks  of  the  Keewatin. 
Green  schists  similar  to  those  of  the  Marquette  district  occur  south 
of  the  Menominee  district  and  have  been  known  as  the  Quinnesec 
schist.  They  contain  tuffaceous  and  fragmental  phases  similar  to 
those  of  the  Kitchi  schist  of  the  Marquette  district.  The  excellent 
description  of  them  by  G.  H.  Williams  in  Bulletin  62  of  the  United 
States  Geological  Survey  has  become  classic.  Their  relations  to  the 
Huronian  sediments  can  not  be  seen  in  the  Menominee  district  proper, 
but  to  the  west  along  Pine  River  they  have  been  found  to  be  uncon- 
formably  beneath  upper  Huronian  sediments.  Because  of  this  and 
of  their  occurrence  on  the  south  side  of  the  Menominee  syncline,  con- 
taining lower  Huronian  rocks,  they  are  assigned  to  the  Keewatin. 
Similar  rocks  on  the  north  side  of  the  Menominee  district  near  Twin 
Falls,  mapped  by  Bayley  and  Van  Hise  as  equivalent  to  the  Quinnisec 
schist  (Keewatin),  have  been  subsequently  found  to  be  intrusive  into 
the  upper  Huronian.  The  same  is  true  of  greenstones  along  Brule 
River  to  the  west. 
South  of  the  Penokee-Gogebic  district  are  two  areas  of  Keewatin 
schists,  including  micaceous,  chloritic,  and  hornblendic  varieties, 
which  have  been  known  respectively  as  the  eastern  and  western  green 
schist  areas. 
In  Michigan  and  Wisconsin  the  predominating  rocks  of  the  base- 
ment complex  are  Laurentian  granites  and  gneisses,  which  appear 
both  north  and  south  of  the  Marquette  district,  within  the  Crystal 
Falls  district  and  east  of  it,  north  and  south  of  the  Menominee  dis- 
trict, and  south  of  the  Penokee-Gogebic  district.  These  granites  and 
gneisses  show  a  variety  of  characters  and  are  certainly  not  all  of  the 
same  age,  although  with- minor  exceptions  they  antedate  the  Algon- 
kian  rocks.     One  of  the  areas  of  acidic  rocks  on  the  south  side  of  the 
