86  PRE-CAMBRIAN   GEOLOGY   OF    NORTH   AMERICA. 
groups,  seem  to  have  accorded  to  such  groups  the  taxonomical  rank  to 
which  they  are  entitled.  Certainly  there  has  been  no  general  recogni- 
tion of  these  groups,  such  as  would  lead  to  the  provision  for  them  of 
a  proper  place  in  the  general  geological  column. 
If  it  is  agreed  that  all  clastic  formations  which  unconformably 
underlie  the  Cambrian  are  to  be  thrown  out  of  the  Cambrian  group, 
it  is  necessary  to  inquire  whether  the  new  groups  are  to  be  regarded  as 
Paleozoic.  All  may  be  regarded  as  Archean ;  Paleozoic  may  be  carried 
down  to  the  break  between  the  Keweenawan  series  and  the  Huronian ; 
Archean  may  be  restricted  to  the  gneissic  basic  series;  and,  finally, 
some  entirely  new  term  of  equal  rank  with  Paleozoic  and  Archean 
may  be  introduced  to  cover  the  formations  between  the  gneissic  series 
and  the  Cambrian.  The  apparent  relative  extent  of  the  time  intervals 
between  these  several  groups  and  the  indications  presented  by  them 
of  the  existence  of  life  during  their  deposition  lead  to  the  conclusion 
that  there  should  be  introduced  a  system  term  equivalent  to  Paleozoic 
and  Archean.  In  favor  of  restricting  Archean  to  the  gneissic  base- 
ment terrane  are  the  facts  that  this  is  essentially  a  crystalline  schist 
series,  having  rarely  any  traces  of  fragmental  constitution,  because 
it  shows  an  amount  of  disturbance  prior  to  the  deposition  of  the 
Huronian  which  entirely  outweighs  that  received  by  the  Huronian, 
while  the  amount  of  denudation  of  the  pre-Huronian  land  surface,  as 
compared  with  that  which  followed  the  Huronian,  was  immensely 
greater;  and  because  many  believe  that  the  exact  conditions  which 
gave  rise  to  the  pre-Huronian  basement  formation  have  never  been 
repeated  in  later  geological  times. 
There  is  no  satisfactory  evidence  of  the  existence  of  life  previous  to 
the  deposition  of  the  Huronian.  That  it  existed  plentifully  in  the 
Huronian  is  indicated  by  the  high  development  of  life  at  the  begin- 
ning of  the  Cambrian  and  its  consequent  necessary  existence  for  great 
periods  prior  to  that  time,  by  the  occasional  discovery  of  obscure 
fossil  remains,  by  the  abundant  occurrence  of  shales  and  slates  filled 
with  organic  matter,  and  by  extended  ferruginous  strata  whose  orig- 
inal accumulation  in  the  form  of  carbonate  was  certainly  dependent 
upgn  the  existence  of  organic  matter.  That  the  carbon  of  the  shales 
is  of  organic  origin  is  shown  by  residual  traces  of  hydrocarbons  and 
by  the  fact  that  the  carbonaceous  substance  in  character  and  occur- 
rence is  entirely  similar  to  that  contained  in  the  carbonaceous  shales 
of  later  formations.  If  the  term  Paleozoic  is  to  be  used  to  cover  all 
formations  accumulated  after  the  beginning  of  life,  it  should  extend 
downward  over  the  groups  in  question;  but  such  is  not  its  ordinary 
use,  and  to  extend  it  downward  over  the  Keweenawan  and  Huronian 
strata  and  the  intervals  indicated  by  the  unconformities  between  the 
groups  already  discovered,  and  over  such  groups  as  shall  be  dis- 
covered in  the  vast  area  of  the  earth's  surface  not  yet  geologically 
