van  msis.i  DISCUSSIONS    OF    PRINCIPLES.  L9l 
NECESSITY  FOR  A  GROUP  BETWEEN  CAMBRIAN  AND  AJtCHEAN. 
The  Olenellus  fauna  is  taken  as  the  base  of  the  Cambrian.  The  rea 
sons  for  thus  delimiting  the  Cambrian  below  are  fully  considered  by 
Walcott  in  one  of  this  series  of  correlation  papers  and  will  be  sum- 
marized on  a  subsequent  page.  His  results  are  accepted.  The  Cam 
brian  fauna  in  development  is  tar,  some  biologists  say  nine-tenths  of 
the  way,  up  the  life  column.  This  statement,  if  accepted,  implies  a 
prior  life  of  vast  duration. 
Just  as  another  period  of  life  has  succeeded  tin4  Cambrian,  another 
lias  preceded  it.  The  progress  of  paleontologic  knowledge  has  of  late 
been  downward.  Before  there  was  a  recognized  Cambrian  there  was 
a  well  known  Silurian,  and  it  is  probable  that  when  all  parts  of  the 
world  become  geologically  known  other  faunas  wiil  be  discovered  beiow 
the  Cambrian  as  distinctive  in  character  as  the  Cambrian  is  from  flic 
Silurian.  If  this  be  done,  definite  information  will  be  available  to  cor- 
relate rock  series  of  different  parts  of  the  world  in  the  time  place  be- 
tween the  Cambrian  and  Archean. 
If  the  condition  of  the  globe  was  such  that  life  existed  in  pre-Cam- 
brian  time,  it  also  was  such  that  stratified  rocks  could  be  deposited 
not  unlike  those  of  later  times,  so  that  the  only  question  which  arises 
is  whether  any  of  these  stratified  rocks  now  remain  in  such  a  condition 
as  to  be  recognizable.  The  foregoing  pages  and  the  literature  sum- 
marized give  a  mass  of  evidence  upon  this  point  which  is  overwhelming. 
Such  intervening  clastic  series  do  exist  below  the  Olenellus  fauna  in 
many  regions  in  North  America,  and  in  some  cases  the  volumes  of  rock 
and  great  intervening  erosions  represent  a  lapse  of  time  which  may 
be  not  inaptly  compared  with  all  subsequent  time.  If  geological  his 
tory  were  to  be  divided  into  three  approximately  equal  divisions, 
these  divisions  would  not  improbably  be  the  time  of  the  Archean,  the 
time  of  the  clastic  series  between  the  Archean  and  the  Cambrian,  and 
the  time  of  the  Cambrian  and  post  Cambrian.  In  this  connection  it 
is  well  to  recall  that  many  years  ago  Logan  suggested  that  the  thick- 
ness of  the  Laurentian  and  Huronian  may  surpass  that  of  all  suc- 
ceeding formations,  and  the  appearance  of  the  so-called  Primordial 
fauna  may  be  considered  a  comparatively  modern  event. 
It  is  imperative  that  some  term  shall  be  available  to  cover  the  great 
mass  of  rocks  between  the  Cambrian  and  Archean.  Irving  was  the 
first  to  realize  and  urge  the  necessity  for  such  a  term  and  proposed 
for  it  Agnotozoic.  This  term  implies  the  existence  of  life  in  this  sys- 
tem, and  the  evidence  upon  this  point  is  conclusive.  Life  is  indicated 
by  the  presence  of  thick  beds  of  graphitic  limestones,  beds  of  iron  car- 
bonate, and  by  great  thicknesses  of  carbonaceous  shales,  which  are  rep- 
resented by  graphitic  schists  in  the  more  metamorphosed  phases  of 
the  rocks.  It  has  been  urged  by  Whitney,  Wadsworth.  and  others  that 
the  limestone  and  graphitic  schists  may  have  an  origin  other  than 
