SUMMARY   OF   GENERAL  LITERATURE.  85 
The  structural  breaks  called  unconformities  are  properly  used  in 
classification  — 
(1)  To  mark  the  boundaries  of  the  rock  groups  of  a  given  region. 
(2)  To  aid  in  establishing  correlations  between  the  formations  of 
different  parts  of  a  single  geological  basin. 
(3)  To  aid  in  the  establishment  of  correlations  between  the  groups 
of  regions  distantly  removed  from  one  another ;  but  caution  is  needed 
in  attempting  such  correlations  in  proportion  as  the  distances  between 
the  regions  compared  grow  greater. 
They  are  improperly  ignored — 
(1)  When  the  evidence  they  offer  as  to  separateness  is  allowed  to 
be  overborne  by  anything  but  the  most  complete  and  weighty  of 
paleontological  evidence. 
As  here  used  the  terms  system,  group,  and  formation  are  the  three 
orders  of  magnitude  in  stratigraphical  subdivisions.  Cenozoic,  Mes- 
ozoic,  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  the 
Keweenawan  and  Huronian  are  entitled  to  the  rank  of  groups  (1)  be- 
cause, notwithstanding  the  fact  that  they  include  a  considerable  con- 
tent of  volcanic  crystallines,  they  are  in  the  main  made  up  of  genuine 
sedimentary  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  exist- 
ence of  life  on  the  globe,  as  hereafter  maintained;  (3)  because  of 
their  great  volumes,  which  not  only  are  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  struc- 
tural separateness  from  the  oldest  of  the  groups  above  them,  from 
each  other,  and  from  the  crystalline  basement  rocks  below  them ;  and, 
finally,  (6)  because  of  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  find  unconformably  beneath  the 
Cambrian,  here  developed  to  an  enormous  thickness,  two  mutually 
discordant  series,  the  upper  one  of  which  is  entitled  on  the  principles 
advocated  in  this  paper  to  full  recognition  as  a  clastic  group,  while 
the  lower  one  is  crystalline  and  gneissic.  In  numerous  other  regions 
similar  conditions  have  been  more  or  less  distinctly  made  out:  but  the 
geological  column,  as  it  is  now  ordinarily  presented,  provides  be- 
neath the  Cambrian  for  one  great  division  only — the  Archean.  By 
some  authors  this  Archean  is  recognized  as  divisible  into  Huronian 
and  Laurentian;  but  very  few  writers,  even  when  they  have  recog- 
nized the  independent  existence  of  pre-Cambrian  and  post-Laurentian 
