24  PRE-CAMBRIAN    GEOLOGY   OF    NORTH   AMERICA. 
the  basement  of  the  oldest  clastic  formations,  then  recognized  as  such,  they 
have  begun  with  separating  a  younger  clastic  series  from  a  new  basement  com- 
plex, older  than  that  which  bore  that  name  before.  In  the  beginning,  no 
opinion  has  been  expressed  as  to  the  question  whether  this  complex  contains 
elastics  or  not.  But  after  a  time  this  practical  division  made  in  the  field  has 
been  confounded  with  the  theoretical  one,  the  "  basal  plane  "  of  life  is  thought 
to  lie  at  the  bottom  of  the  clastic  series,  and  the  basal  complex  is  unduly 
regarded  as  Azoic. 
Chamberlin  and  Salisbury  a  state  that  the  mode  of  derivation  is 
perhaps  the  most  important  distinction  between  the  Algonkian  as  a 
whole  and  the  Archean. 
The  Algonkian  sediments  were  produced  by  mature  decomposition  of  older 
rocks,  and  this  implies  that  they  were  not  derived  by  rapid  mechanical  abrasion, 
such  as  that  which  accompanies  and  follows  great  elevation  and  excessive 
precipitation.  The  great  series  of  quartzites  were  derived  from  the  complete 
decomposition  of  quartz-bearing  rocks,  and  involved  the  almost  complete  separa- 
tion of  the  quartz  grains  from  other  constituents,  while  the  thick  beds  of  shale 
arose  from  the  complementary  clayey  products  of  decomposition,  from  which 
most  of  the  basic  oxides  had  been  removed  by  carbonation.  It  is  scarcely  too 
much  to  say  that  the  material  of  the  larger  part  of  these  great  series  first  be- 
came soils  on  the  surface  of  tbe  parent  areas,  and  were  only  removed  at  a  rate 
that  permitted  the  renewal  of  the  soil  beneath  as  fast  as  it  was  washed  away 
above. 
They  believe  that  the  Archean  sediments  contrast  with  Algonkian 
sediments  in  being  the  results  of  immature  decomposition.  This  sug- 
gestion is  a  most  interesting  one  and  may  afford  a  new  line  of  attack 
on  the  problems  of  the  subdivision  of  the  pre-Cambrian. 
DIFFICULTIES  IN  LOCAL  USE  OF  CLASSIFICATION. 
It  has  been  urged  that  even  if  it  be  desirable  to  use  general  terms 
like  Archean  and  Algonkian  for  certain  regions,  there  are  other 
regions  where  such  classification  is  unnecessary,  undesirable,  and  mis- 
leading in  that  it  implies  a  correlation  which  is  not  warranted ;  that 
the  terms  Algonkian  and  Archean  are  really  local  terms  and  should 
not  be  used  in  general  classification.  It  is  agreed  that  there  are  pre- 
Cambrian  successions  in  which  the  known  facts  do  not  now  seem  to 
warrant  this  dual  subdivision  of  the  pre-Cambrian.  But  the  diffi- 
culties of  applying  this  classification  locally  should  not  stand  in  the 
way  of  classifying  our  knowledge  of  the  districts  in  which  it  does 
apply,  which,  as  a  matter  of  fact,  include  the  vast  regions  of  pre- 
Cambrian  in  which  are  the  fullest  and  best-known  pre-Cambrian 
successions  in  the  world.  Even  in  post-Algonkian  fossil-bearing 
rocks  it  is  frequently  impossible  to  carry  the  same  classification,  even 
as  to  systems,  from  one  region  to  another,  and  when  the  classification 
is  carried  over  it  seldom  has  precisely  the  same  significance  litho- 
logically,  stratigraphically,  and  chronologically.     Yet  if  the  classi- 
a  Text-book  of  geology,  vol.  2,  1906,  pp.  199-200. 
