GENERAL   ACCOUNT   OF   THE   PRE-CAMBRIAN    ROCKS.  29 
It  is  possible — indeed  probable — that  some  of  the  rocks  which  in 
the  eastern  part  of  the  United  States  and  in  the  Cordilleras  have  been 
referred  to  the  Archean  will  in  the  future  be  ascertained  to  be  Algon- 
kian.  It  has  already  been  pointed  out  that  in  a  given  area,  until  it 
can  be  ascertained  whether  a  group  of  pre-Cambrian  rocks  is  Archean 
or  Algonkian,  the  general  term  pre-Cambrian  may  wisely  be  used. 
ORIGIN   OF   THE   ARCHEAN. 
The  lithological  complexity  of  the  Archean  is  such  that  no  general- 
ized statement  can  be  made  to  cover  the  origin  of  all  its  rocks. 
Rocks  in  accord  with  Mutton's  law  that  the  present  is  the  hey  to  the 
past. — The  abundant  igneous  rocks  and  the  schists  and  gneisses  clearly 
recognized  as  their  altered  equivalents  are  not  different  from  igneous 
rocks  of  later  age,  except  perhaps  in  amount  of  metamorphism  shown 
in  many  localities,  and  hence  so  far  as  these  rocks  are  concerned  a  dis- 
cussion of  the  origin  of  the  Archean  would  involve  a  discussion 
of  the  origin  of  igneous  rocks  in  general.  The  small  amounts 
of  sedimentary  rocks  and  schists  and  gneisses  known  to  be  their  altered 
equivalents  are  also  not  different  in  kind,  though  differing  in  relative 
proportions  of  kinds,  in  total  amount,  and  in  metamorphism,  from 
sedimentary  rocks  of  Algonkian  and  later  eras.  The  oldest  known 
Archean  rocks,  the  Keewatin  of  Lake  Superior  and  eastern  Canada, 
come  under  the  classes  of  rocks  definitely  recognizable  as  of  igneous 
and  sedimentary  origins.  Some  of  the  oldest  pre-Cambrian  rocks 
have  been  regarded  as  sedimentary  in  the  Piedmont  area  from  Penn- 
sylvania southward  into  the  Carolinas,  and  in  the  Georgetown  dis- 
trict of  Colorado.  It  is  of  interest  to  note  also  that  the  earliest  sedi- 
ments and  gneisses  of  the  Lewisian  in  Scotland  are  in  places  so 
intimately  associated  as  to  suggest  that  the  gneisses  may  be  intrusive 
into  the  sediments,  which  would  make  the  sedimentary  rocks  the  old- 
est in  the  region.  Sederholm  finds  sedimentary  rocks  well  toward 
the  base  of  the  pre-Cambrian  of  Finland  and  believes  that  certain 
earlier  gneisses  will  prove  to  be  sediments. 
But  the  Archean  contains  in  addition  large  masses  of  gneisses  and 
schists  whose  origin  has  not  been  proved.  They  are  essentially  the 
same  as  gneisses  and  schists  which  have  been  shown  to  result  from  the 
metamorphism  either  of  igneous  or  of  sedimentary  rocks,  and  it  is 
probable  that  a  part  of  them,  an  unknown  proportion,  are  so  derived. 
That  any  of  them  represent  rocks  of  different  origin,  possibly  the  so- 
called  original  crust  of  the  earth  or  its  downward  crystallization,  nec- 
essary to  the  nebular  hypothesis  of  the  origin  of  the  earth,  there  is  yet 
no  positive  evidence  to  show,  but  the  existence  of  such  rocks  is  possi- 
ble. It  has  been  held  that  the  granites  and  gneisses  of  the  Laurent  i an 
have  resulted  from  the  subcrustal  fusion  of  igneous  or  sedimenhirv 
