vanhisk.]  DISCUSSIONS    OF    PRINCIPLES.  459 
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  compared  with 
the  Pebidian  of  Wales.  The  Huronian  marks  a  period  of  igneous  dis- 
turbance and  coarse  mechanical  deposition  succeeding  to  the  Lauren - 
tian  foldings. 
Irving,18  in  1888,  after  a  detailed  consideration  of  the  principles  ap- 
plicable to  the  classification  of  the  early  Cambrian  and  pre-Oambrian 
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  confirm- 
atory evidence  only,  when  such  dissimilarities  are  the  result  of  original 
depositional  conditions. 
(2)  To  collect  in  a  single  group  adjacent  formations  because  of  litho- 
logical similarities  when  such  similarities  are  used  as  confirmatory  evi- 
dence only. 
(3)  To  correlate  groups  and  formations  of  different  parts  of  a  single 
geological  basin  when  such  correlations  are  checked  by  stratigraphy, 
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  re- 
sult of  differences  in  original  depositional  conditions,  and  when  such 
evidence  of  distinction  is  not  confirmed  by  or  is  contradicted  by  struc- 
tural and  paleontological  evidence. 
(2)  To  collect  in  a  single  group  adjacent  formations  because  of  litho- 
logical 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,  i.  e.,  those  which  underlie  the  clas- 
tic 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. 
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  dif- 
ferent parts  of  a  single  geological  basin. 
