34  PRE-CAMBR1AN    GEOLOGY    OF    NORTH    AMERICA. 
Additional  evidence  of  life  in  the  Algonkian  is  the  existence  of 
carbonaceous  shales,  slates,  and  schists.  Limestone  and  iron-bearing 
formations  may  point  in  the  same  direction.  Still  another  doubtful 
bit  of  evidence  bearing  toward  the  existence  of  life  is  the  fact  that 
the  Algonkian  sediments  derived  from  the  Archean  are  the  result 
of  mature  decomposition,  which  is  held  by  Chamberlin  and  Salis- 
bury a  to  imply  the  agency  of  vegetation. 
The  progress  of  paleontological  knowledge  has  been  downward. 
Before  there  was  a  recognized  Cambrian  there  was  a  well-defined 
Silurian,  and  it  is  possible  that  when  all  parts  of  the  world  become 
geologically  known  other  faunas  will  be  discovered  below  the  Cam- 
brian as  distinctive  in  character  as  the  Cambrian  is  from  the  Silurian, 
though  this  is  no  longer  held  probable  by  geologists  best  qualified 
to  judge.  The  first  traces  of  such  life  have  already  been  found,  as 
above  noted,  and  the  great  areas  of  Algonkian  sediments  not  differing 
essentially  from  post-Cambrian  sediments  afford  an  inviting  field 
for  further  search  for  fossils.  It  is  widely  agreed  that  beds  contain- 
ing the  Olenellus  fauna  shall  be  taken  as  the  base  of  the  Cambrian. 
This  fauna  is  abundant  and  varied.  It  includes  all  the  steins  of  the 
animal  kingdom  except  the  vertebrate.  Some  biologists  say  that  nine- 
tenths  of  the  differentiation  had  taken  place  at  the  beginning  of 
Cambrian  time,  implying  a  prior  life  of  vast  duration. 
Beneath  the  Olenellus  horizon  the  lack  of  fossils  is  striking,  even 
where  there  is  no  unconformity  separating  the  Olenellus  horizon  from 
the  underlying  sediments,  and  these  sediments  are  of  such  a  nature  as 
to  indicate  that  the  waters  in  which  they  were  laid  down  were  as 
satisfactory  for  supporting  life  at  the  beginning  as  at  the  end  of  the 
enormous  lapse  of  time  which  they  represent,  and  all  the  conditions 
have  since  been  favorable  for  the  preservation  and  discovery  of  fos- 
sils. This  great  contrast  in  fossil  content  signifies  an  important 
change  of  conditions,  the  precise  nature  of  which  does  not  yet  appear. 
W.  K.  Brooks  developed  the  hypothesis  that  the  late  Algonkian 
oceans  contained  all  the  fundamental  types  of  animals,  but  with  soft 
bodies,  and  that  these  did  not  begin  to  secrete  hard  parts  until  early 
Cambrian  time.  Upon  reaching  or  "  discovering  "  the  shore,  there 
began  a  rapid  development  and  struggle  for  existence,  which  required 
the  development  of  hard  parts,  giving  rise  to  the  fossil  forms  now 
found  in  the  Cambrian.  Daly  varies  this  hypothesis  by  suggesting 
that  the  amount  of  lime  and  magnesia  necessary  for  the  hard  parts 
of  animals  was  not  present  in  the  ocean  until  late  pre-Cambrian 
time;  that  prior  to  this  time  all  the  lime  and  magnesia  was  de- 
posited through  the  medium  of  organic  ammonium  carbonate,  leav- 
«  Text-book  of  geology,  vol.  2,  1900,  p.  139, 
