32  PRE-CAMBRIAN   GEOLOGY   OF    NORTH   AMERICA. 
cleavage  of  the  Archean  represent  a  considerable  amount  of  crustal 
shortening  which  took  place  before  the  deposition  of  the  Algonkian 
and  later  rocks,  there  is  not  sufficient  evidence  to  show  that  this  short- 
ening before  Algonkian  time  was  greater  than  that  during  later 
periods  of  equal  length. 
Conclusion  concerning  origin  of  Archean. — The  rocks  of  the  Arch- 
ean include  (1)  dominantly  plutonic  and  surface  igneous  rocks,  (2) 
small  masses  of  sediments,  (3)  considerable  masses  of  schists  and 
gneisses  clearly  derived  from  secondary  alteration  of  the  igneous  and 
sedimentary  rocks,  (4)  very  subordinate  amounts  of  gneisses  result- 
ing from  subcrustal  fusion  of  sediments,  and  (5)  other  schists  and 
gneisses  of  unknown  origin. 
In  the  Lake  Superior  region  and  over  large  areas  in  Canada  the 
oldest  determinable  rocks  of  the  Archean  are  dominantly  surface  vol- 
canics,  with  which  are  very  subordinate  amounts  of  sediments.  In 
these  ancient  rocks  there  have  been  intruded  plutonic  igneous  rocks 
upon  a  great  scale. 
ALGONKIAN  SYSTEM. 
The  Algonkian  system,  as  this  term  is  used  by  the  United  States 
Geological  Survey,  includes  sedimentary  formations  and  their  meta- 
morphosed equivalents  with  associated  igneous  rocks  beneath  the 
Cambrian  and  resting  upon  the  Archean  complex.  It  includes  the 
greater  part  of  the  sedimentary  rocks  of  pre-Cambrian  age  and  prac- 
tically all  to  which  present  stratigraphic  methods  have  been  found  to 
apply,  though  it  contains  also  sedimentary  rocks  so  deformed  and 
metamorphosed  that  their  stratigraphy  is  obscure.  The  sedimentary 
rocks  not  included  are  those  in  small  quantities  so  intimately  asso- 
ciated with  the  rocks  of  the  Archean  unconformable  below  that  their 
stratigraphic  relations  to  the  adjacent  rocks  are  not  fully  known. 
The  Algonkian  sediments  are  known  to  contain  a  few  fossils,  repre- 
senting the  earliest  forms  of  life  yet  found.  No  indisputable  fossils 
have  been  found  in  the  sediments  of  the  Archean  below. 
EXTENT   AND    SUBDIVISION    OF    THE    ALGONKIAN. 
In  the  Lake  Superior  region,  where  the  Algonkian  system  is  de- 
veloped in  fullest  succession,  it  may  be  subdivided  on  the  basis  of 
unconformity  into  four  groups,  each  consisting  of  several  forma- 
tions. North  of  Lake  Huron  three  Algonkian  groups  are  known. 
In  other  areas  the  divisions  are  fewer.  Part  of  the  Hastings  and 
Grenville  rocks  of  eastern  Ontario  and  western  Quebec  are  probably 
Algonkian,  but  their  stratigraphy  and  correlation  are  not  yet  solved. 
The  largest  area  of  Algonkian  known  is  that  of  the  Belt  series 
