84  PRE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
group  may  perhaps  be  of  later  age  than  the  Huronian  proper, 
although  pre-Cambrian. 
As  in  Newfoundland,  the  typical  Huronian  of  New  Brunswick  is 
overlain  by  conglomerates,  sandstones,  and  shales.  The  Huronian 
rocks  of  Huron,  Newfoundland,  and  New  Brunswick  are  also  com- 
pared with  the  Pebidian  of  Wales.  The  Huronian  marks  a  period  of 
igneous  disturbance  and  coarse  mechanical  deposition  succeeding  to 
the  Laurentian  foldings. 
Irving,18  in  1888,  after  a  detailed  consideration  of  the  principles 
applicable  to  the  classification  of  the  early  Cambrian  and  pre-Cam- 
brian formations,  reaches  the  following  general  conclusions  as  to  the 
use  of  lithological  characters  and  structural  breaks  in  correlation. 
Lithological  characters  are  properly  used  in  classification: 
(1)  To  place  adjacent  formations  in  different  groups,  on  account 
of  their  lithological  dissimilarities,  when  such  dissimilarities  are 
plainly  the  result  of  great  alteration  in  the  lower  one  of  the  two 
formations  and  are  not  contradicted  by  structural  evidence,  or  if  used 
as  confirmatory  evidence  only,  when  such  dissimilarities  are  the  result 
of  original  depositional  conditions. 
(2)  To  collect  in  a  single  group  adjacent  formations  because  of 
lithological  similarities  when  such  similarities  are  used  as  confirma- 
tory evidence  only. 
(3)  To  correlate  groups  and  formations  of  different  parts  of  a 
single  geological  basin  when  such  correlations  are  checked  by  stratig- 
raphy, and  particularly  by  observations  made  at  numerous  points 
between  the  successions  correlated. 
They  are  improperly  used : 
(1)  To  place  adjacent  formations  in  different  groups,  on  account 
of  lithological  dissimilarities,  when  such  dissimilarities  are  merely 
the  result  of  differences  in  original  depositional  conditions,  and  when 
such  evidence  of  distinction  is  not  confirmed  by  or  is  contradicted  by 
structural  and  paleontological  evidence. 
(2)  To  collect  in  a  single  group  adjacent  formations  because  of 
lithological  similarities  when  such  similarities  are  not  confirmed  by 
or  are  contradicted  by  other  evidence. 
(3)  To  establish  general  correlations  between  the  clastic  groups  of 
different  geological  basins,  except  possibly  when  the  gneissic  and 
true  crystalline  schist  basement  formation  of  one  region  is  compared 
with  the  .similar  basement  formation  of  another. 
(4)  To  establish  and  determine  any  world-wide  subdivisions  of  the 
noneruptive  basement  crystallines — that  is,  those  which  underlie  the 
clastic  groups  here  called  Huronian — at  least  until  very  much  more 
definite  evidence  of  the  existence  of  such  subdivisions  be  gathered 
than  has  hitherto  been  done. 
