22  PRE-CAMBRIAN    GEOLOGY   OF    NORTH    AMERICA. 
in  amount  to  the  sediments.  The  Archean  is  characterized  mainly 
by  igneous  rocks,  with  the  sediments  in  very  small  quantity.  The 
Archean  sediments,  moreover,  are  frequently  of  "  wacke  "  type,  and, 
so  far  as  known,  are  not  largely  of  the  cleanly  assorted  kinds  result- 
ing from  complete  decomposition,  as  in  the  Algonkian.  The  differ- 
ences between  the  Archean  and  the  Algonkian  may  even  express  them- 
selves in  topographic  contrasts.  For  instance,  in  the  Lake  Superior 
region  a  peneplain  on  the  Archean  has  most  irregular  minor  varia- 
tions on  the  surface,  while  the  same  peneplain  crossing  the  Algonkian 
rocks  is  characterized  by  linear  and  somewhat  regular  features.  In 
the  preparation  of  the  final  Lake  Superior  report  a  physiographic 
study  of  the  region  resulted  in  subdivision  into  physiographic  prov- 
inces which  were  found  to  correspond  essentially  with  the  division 
into  Archean  and  Algonkian. 
The  Archean  rocks  are  highly  folded,  contorted,  mashed,  and  meta- 
morphosed and  usually  contain  secondary  structures.  Also  the  older 
of  them  have  been  intruded  again  and  again  by  plutonic  igneous 
rocks  on  a  scale  not  equaled  in  the  rocks  of  any  other  period.  Thus 
the  Archean  presents  intricacy  of  structure  and  metamorphism  not 
paralleled  by  any  other  system  of  rocks.  The  Algonkian  rocks  also 
have  been  extensively  folded,  metamorphosed,  and  intruded  by  igne- 
ous rocks,  but  in  these  respects  are  less  intricate  in  their  structures 
than  are  the  Archean  rocks.  It  follows  from  these  differences  that 
to  the  Algonkian  rocks  ordinary  stratigraphic  methods  can  be  usually 
made  to  apply,  as  in  the  Paleozoic,  whereas  in  the  Archean  there  has 
been  little  progress  in  stratigraphy.  In  addition  to  the  above  points 
of  difference  it  has  been  found  that  in  general  there  is  a  great  uncon- 
formity at  the  top  of  the  Archean.  This  immense  unconformity, 
strongly  marked  by  basal  conglomerates,  explains  the  contrasting 
points  in  lithological  character,  amount  of  deformation,  intricacy  of 
intrusion,  and  degree  of  metamorphism  of  the  two  systems  of  rocks. 
So  striking  are  the  differences  between  the  two  systems  in  lithology, 
structure,  and  metamorphism  that  in  field  study  and  mapping  a  broad 
dual  classification  is  naturally  made,  regardless  of  the  significance 
of  such  classification  or  theories  of  origin.  This  practical  distinction 
has  been  recognized  by  many  workers  who  have  given  much  time  to 
mapping  the  pre-Cambrian  formations,  from  the  days  of  Logan  to 
the  present  time.  In  the  Marquette  district  of  Michigan,  for  illus- 
tration, this  classification  was  made  from  the  first  by  a  number  of 
geologists,  although  under  different  names  and  with  different  inter- 
pretations. We  believe  it  is  not  going  too  far  to  state  that  usually 
where  the  pre-Cambrian  succession  has  been  fully  worked  out,  in  an 
area  where  it  is  reasonably  full  and  well  exposed,  enough  criteria  for 
the  separation  have  been  found  to  make  the  dual  classification  into 
