460  PRE-CAMBRIAN    ROCKS    OF    NORTH   AMERICA.  [bull.  86. 
(3)  To  aid  in  the  establishment  of  correlations  between  the  groups 
of  regions  distantly  removed  from  one  another ;  but  caution  is  needec 
in  attempting  such  correlations  in  proportion  as  the  distances  betweei 
the  regions  compared  grow  greater. 
They  are  improperly  ignored — 
(1)  When  the  evidence  they  offer  as  to  separateness  is  allowed  to  b( 
overborne  by  anything  but  the  most  complete  and  weighty  of  paleon 
tological  evidence. 
As  here  used  the  terms  system,  group,  and  formation  are  the  thre 
orders  of  magnitude  in  stratigraphical  subdivisions.  Cenozoic,  Meso 
zoic,  and  Paleozoic  are  systems;  Carboniferous,  Devonian,  etc.,  groups 
and  the  subordinate  members  of  these  groups  are  formations. 
Applying  these  principles,  it  is  concluded  that  such  series  as  th 
Keweenawan  and  Huronian  are  entitled  to  the  rank  of  groups  (1)  be 
cause,  notwithstanding  they  include  a  considerable  content  of  volcanic 
crystallines,  they  are  nevertheless  in  the  main  made  up  of  genuine  sed- 
imentary strata,  whose  formation  by  the  same  processes  which  have, 
been  at  work  in  the  accumulation  of  later  sedimentaries  is  easily, 
demonstrable;  (2)  because  they  have  accumulated  during  the  existence 
of  life  on  the  globe,  as  hereafter  maintained ;  (3)  because  of  their  great 
volumes,  which  are  not  only  comparable  with,  but  very  considerably 
exceed  those  of  the  ordinary  rock  groups;  (4)  because  they  are  divisible 
into  subordinate  members  which  are  in  turn  fully  entitled  to  the  rank! 
of  formations;  (5)  because  of  their  entire  structural  separateness  from 
the  oldest  of  the  groups  above  them,  from  each  other  and  from  thd 
crystalline  basement  rocks  below  them;  and,  finally,  (0)  because  oij 
their  presumptively  wide  extent. 
Conditions  similar  to  those  of  the  lake  Superior  region  recur  in  the] 
Grand  Canyon  of  the  Colorado  and  probably  also  in  central  Texas.  In] 
Newfoundland,  again,  we  have  unconformably  placed  beneath  the  Cam-j 
brian,  here  developed  with  an  enormous  thickness,  two  mutually  disj 
cordant  series,  the  upper  one  of  which  is  entitled  on  the  principles 
advocated  in  this  paper  to  full  recognition  as  a  clastic  group,  while  thej 
lower  one  is  crystalline  and  gneissic.  In  numerous  other  regions  sim-j 
ilar  conditions  have  been  more  or  less  distinctly  made  out;  but  thd 
geological  column,  as  it  is  now  ordinarily  presented,  provides  be| 
neath  the  Cambrian  for  one  great  division  only — the  Archean.  Byj 
some  authors  this  Archean  is  recognized  as  divisible  into  Huroniain 
and  Lauren tian;  but  very  few  writers,  even  when  they  have  recognized] 
the  independent  existence  of  pre-Cambrian  and  post-Laurentian  groups 
seem  to  have  accorded  to  such  groups  the  taxonomical  rank  to  which] 
they  are  entitled.  Certainly  there  has  been  no  general  recognition  oi 
these  groups,  such  as  would  lead  to  the  provision  for  them  of  a  propen 
place  in  the  general  geological  column. 
If  it  is  agreed  that  all  clastic  formations  which  unconformably  under^j 
lie  the  Cambrian  are  to  be  thrown  out  of  the  Cambrian  group,  it  is  nec« 
