GENERAL  ACCOUNT   OF   THE   PRE-CAMBRIAN   ROCKS.  28 
systems  corresponding  to  Archean  and  Algonkian  a  natural  and  con- 
venient one  for  purposes  of  description  and  mapping. 
The  above  statements  in  reference  to  the  Archean  differ  from  those 
in  the  previous  correlation  bulletin  (No.  86)  in  that  the  presence  of 
sedimentary  rocks  in  the  Archean  is  now  definitely  recognized.  It 
was  never  held  that  sedimentary  rocks  did  not  exist  in  the  Archean, 
for  the  possibility  of  sedimentary  origin  of  some  of  the  doubtful 
Archean  rocks  was  repeatedly  mentioned,  but  emphasis  was  placed 
on  the  fact  that  the  rocks  then  assigned  to  the  Archean  were  igneous 
so  far  as  their  origin  was  recognized.  This  became  then  one  of  the 
common  criteria  for  the  separation  of  the  Archean  from  the  Algon- 
kian. But  it  should  be  remembered  that  this  was  only  one  of  the 
criteria  used  and  that  the  general  contrasts  in  lithology,  structure, 
and  metamorphism,  together  with  the  magnitude  of  the  unconform- 
ity separating  them,  were  given  equal  weight.  The  discovery  of 
definitely  recognizable  sediments  in  the  Archean  now  requires  that 
less  emphasis  be  laid  upon  sedimentary  origin  as  a  criterion  for  sep- 
arating the  Algonkian  from  the  i^rchean,  although  the  criterion  still 
remains  an  important  one;  for  the  Algonkian  rocks,  as  already  in- 
dicated, are  characterized  by  well-assorted  sediments,  to  many  of 
which  ordinary  stratigraphic  methods  apply,  while  the  Archean  is 
not  so  characterized.  The  assignment  of  sedimentary  rocks  to  the 
Archean  does  not  in  the  least  affect  the  other  important  criteria  for 
the  discrimination  of  the  Algonkian  and  the  Archean. 
The  recognition  of  sediments  in  the  Archean  has  naturally  led  to 
some  confusion  where  undue  emphasis  has  been  placed  on  sedimen- 
tary origin  as  a  criterion  for  the  separation  of  the  Algonkian  from 
the  Archean.  This  is  one  of  the  easiest  criteria  to  apply  and  has  been 
seized  upon  and  made  the  one  criterion  in  districts  where  structural 
and  metamorphic  discrimination  has  not  been  made  or  is  not  possible. 
Our  development  of  knowledge  may  be  summed  up  in  the  state- 
ment that  the  broad  dual  classification  into  Archean  and  Algonkian 
was  based  on  a  considerable  number  of  factors;  that  one  of  these 
factors  has  been  found  to  be  less  conclusive  as  a  means  of  discrimina- 
nation  than  previously  thought,  although  still  important;  that  the 
remaining  factors  have  the  same  weight  as  formerly;  and  that  the 
classification  into  Archean  and  Algonkian  is  as  fundamental  as  before 
thought.  The  classification  was  proposed  as  a  working,  not  as  a 
theoretical,  classification.  With  advance  of  knowledge,  modification 
of  our  understanding  of  its  significance  was  to  have  been  expected, 
and  may  be  expected  in  the  future.  Sederholm  a  aptly  expresses  the 
situation  when  he  writes: 
I  think  we  have  warnings  enough  from  the  prior  history  of  the  geology  of  the 
oldest  sediments.     Many  times  when  geologists  have  succeeded  in  subdividing 
a  Personal  letter. 
