26  PBE-CAMBEIAJST   GEOLOGY   OP   NORTH   AMERICA. 
ARCHEAN   SYSTEM. 
The  Archean  system  is  a  crystalline  complex  beneath  the  base  of 
the  determined  sedimentary  succession.  Dana,  followed  by  the  Cana- 
dian and  other  geologists,  used  the  term  "  Archean  "  in  the  sense  of 
pre-Cambrian.  It  is  still  so  used  by  the  geologists  of  the  Canadian 
and  some  other  surveys.  The  United  States  Geological  Survey  has 
restricted  the  term  to  a  complex  of  basic  and  acidic  surface  and  deep- 
seated  igneous  rocks,  of  schists  and  gneisses  in  part  derived  from 
them  and  in  part  of  unknown  origin,  and  of  shreds  and  small  masses 
of  metamorphosed  sediments,  all  unconformably  below  and  older 
than  the  Algonkian  sedimentary  rocks,  which  are  the  lowest  series 
in  which  ordinary  stratigraphic  methods  have  been  applied.  Their 
lithological  variations  are  many.  The  common  kinds  are  granites, 
micaceous,  hornblendic,  chloritic,  and  quartzose  schists  and  gneisses, 
metabasalt,  metadiabase,  and  metagabbro.  The  sedimentary  rocks 
include  slate,  graywacke,  conglomerate,  tuffs,  iron  formation,  and 
marble.  The  Archean  as  a  whole  is  homogeneous  in  its  hetero- 
geneity. Its  characteristic,  though  not  universal,  structure  is  cleav- 
age or  schistosity,  often  much  folded  and  contorted.  It  frequently 
possesses  also  a  banded  structure,  sometimes  developed  with  the  cleav- 
age during  rock  flowage  and  sometimes  due  to  minute  parallel  igneous 
intrusions  parallel  to  the  schistose  layers. 
SUBDIVISION    OF    ARCHEAN    SYSTEM. 
To  the  present  time  the  subdivisions  of  the  Archean  system  are 
mainly  those  of  igneous  succession  rather  than  those  of  sedimentary 
stratigraphy,  although  in  two  or  three  districts  there  has  been  an 
attempt  to  apply  sedimentary  stratigraphy  to  relatively  small  areas. 
In  the  Lake  Superior-Lake  Huron  part  of  the  pre-Cambrian  shield 
of  the  Archean  it  has  been  possible  to  divide  the  rocks  into 
two  series.  The  older  series  of  the  Archean  has  been  called  the 
Keewatin.  This  division  of  the  Archean  consists  largely  of  various 
greenstones  and  green  schists,  which,  where  not  greatly  metamor- 
phosed, show  the  structures  and  textures  of  volcanics.  In  other 
words,  the  most  characteristic  and  extensive  class  of  rocks  of  the 
Keewatin  is  a  surface  material.  A  very  common  feature  of  these 
volcanics  is  a  peculiar  ellipsoidal  structure  which  has  been  fully 
described  by  Clements  and  others.  With  these  igneous  rocks  are  sub- 
ordinate amounts  of  iron-bearing  formation  and  slate.  Prior  to  1905 
the  Keewatin  had  been  called  the  "  Mareniscan  "  by  the  senior  author, 
but  he  now  follows  the  recommendation  of  the  joint  committee  of 
Canadian  and  United  States  geologists  that  Keewatin  be  used  because 
of  its  prior  use  by  Lawson  for  the  series  as  typically  exposed  in  the 
