492  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
organic.  Whitney  and  AV ads  worth  have  gone  so  far  as  to  say  that 
there  is  no  valid  evidence  of  life  in  any  pre-Potsdam  rocks.  This  was, 
however,  before  it  was  generally  recognized  that  the  Potsdam  is  Upper 
Cambrian  and  that  an  abundant  Cambrian  life  extends  far  below.  If 
it  were  true  that  these  limestones  and  ore  beds  are  no  evidence  of  life 
(and  it  may  be  admitted  that  another  origin  is  possible  without  imply- 
ing that  it  is  probable),  it  will  hardly  be  maintained  that  the  hydro- 
carbons which  occur  so  abundantly  in  the  little  metamorphosed  shales 
of  the  Huronian  about  lake  Superior  are  other  than  of  organic  origin, 
and,  if  so,  the  graphitic  schists  which  stand  in  the  same  great  system 
in  the  geological  column  are  in  all  probability  only  these  hydrocarbo- 
naceous  shales  in  a  more  altered  con dition .  However,  we  are  not  obliged 
to  depend  upon  the  presence  of  these  varieties  of  rocks  as  the  only 
evidence  of  life.  Whether  the  llozoon  eanadense  found  in  the  original 
Laurentian  of  Canada  is  of  organic  origin  will  not  be  discussed  here. 
Its  literature  is  voluminous  and  it  is  a  question  which  concerns  the 
paleontologists.  It  is  doubtless  true  that  many  of  the  specimens  which 
have  been  called  Eozoon  are  results  of  the  forces  of  crystallization; 
but,  admitting  this,  it  does  not  follow  that  all  of  the  material  called 
Eozoon  is  of  this  character.  Passing  by  this  question ,  the  pre-Cambrian 
fossils  described  by  Walcott  in  the  Grand  canyon  of  the  Colorado 
include:  "A  minute  Discinoid  or  Patelloid  shell,  a  small  Lingula-like 
shell,  a  species  of  Hyolithes,  and  a  fragment  of  what  appears  to  have 
been  the  pleural  lobe  of  the  segment  of  a  trilobite  belonging  to  a  genus 
allied  to  the  genera  Olenellus,  Olenoides,  or  Paradoxides.  There  is 
also  an  obscure  Stromatopora-like  form  that  may  or  may  not  be  or- 
ganic." 
A  Lingula-like  shell  has  been  found  by  Winchell  in  the  pjpestones  of 
Minnesota.  Selwyn  has  described  tracks  of  organic  origin  in  the  Ani- 
mikie  (Upper  Huronian)  series  of  lake  Superior.  Murray,  Howley,  and 
Walcott  found  several  low  types  of  fossils  in  the  pre  Olenellus  elastics- 
of  Newfoundland. 
That  these  fossils  are  of  organic  origin  can  not  be  doubted.  But  while 
many  will  admit  the  clastic  character  of  the  great  groups  of  rocks  con- 
sidered and  the  organic  origin  of  the  forms  mentioned  as  well  as  the 
carbon  of  the  carbonaceous  shales  and  schists,  they  will  say  that  these 
are  merely  evidences  that  the  rocks  in  which  they  lie  are  Cambrian.  } 
The  reply  to  this  is  that  it  is  a  question  of  nomenclature.  If  it  be  pre- 
mised that  all  clastic  and  fossiliferous  rocks  more  ancient  than  the 
Olenellus  horizon  are  Cambrian  it  is  useless  to  try  to  prove  that  there 
are  pre-Cambrian  clastic  rocks  which  bear  life.  It  is,  however,  neces- 
sary to  recognize  that  the  carrying  downward  of  the  term  Cambrian 
to  cover  not  only  the  great  thicknesses  of  rocks  which  are  now  in- 
cluded within  it,  but  all  pre- Olenellus  elastics,  will  probably  make  the 
Cambrian  as  great  as  or  greater  than  all  the  subsequent  periods  put 
together.     That  this  is  inadvisable  is  plain,  and  the  clastic  rock  masses 
