518  PRE-CAMBRIAN    EOCKS    OF    NORTH    AMERICA*.  [bull.  8^ 
The  principles  applicable  to  correlation  by  unconformities  are  given 
by  Irving  as  follows: 
The  structural  breaks  called  unconformities  are  properly  used  in  classification: 
(1)  To  mark  the  boundaries  of  the  rock  series  of  a  given  region. 
(2)  To  aid  in  establishing  correlations  between  the  formations  of  different  parts  of 
a  siugle  geological  basin. 
(3)  To  aid  in  the  establishment  of  correlations  between  the  series  of  regions  dis- 
tantly removed  from  one  another;  but  caution  is  needed  in  attempting  such  correla- 
tions 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. 
Irving's  discnssion  leading  to  these  principles  shows  that  oftentimes 
nnconformities  are  the  most  widespread  and  important  of  any  of  the 
means  available  to  obtain  starting  planes  for  comparisons  and  that  they 
have  the  place  of  first  importance  in  making  the  major  subdivisions  for 
the  origin  of  the  pre-Cambrian  clastic  rocks.  An  erosion  interval  can 
only  occur  as  a  result  of  the  raising  of  a  district  above  the  sea,  a  time 
of  degradation,  and  then  a  depression  below  the  sea;  and  if  there  is  a' 
true  unconformity  there  must  also  have  been  an  orographic  movement 
and  erosion  long  enough  continued  to  truncate  the  folds.  The  erosion 
interval,  if  extended  over  a  large  area,  implies  a  considerable  time  break  j 
while  the  unconformity,  if  it  is  marked,  can  hardly  be  less  than  regional 
in  extent.  When  the  newer  series  is  undisturbed,  an  unconformity  i^ 
one  of  the  easiest  of  phenomena  to  detect,  but  more  frequently  than  not 
among  the  pre-Cambrian  rocks  the  older  and  newer  series  have  again 
been  folded,  and  this  folding  has  oftentimes  gone  so  far  as  to  produce! 
a  cleavage  or  foliation,  which  cuts  across  both  older  and  newer  seriea 
and  makes  their  most  prominent  structure  in  absolute  conformity  J 
Even  if  this  degree  of  folding  has  not  occurred  and  the  process  has  not 
gone  far  enough  to  produce  prominent  secondary  structures,  the  discord- 
ance in  angle  of  inclination  is  more  likely  to  be  overlooked  than  when] 
the  series  are  in  an  undisturbed  condition. 
Since  unconformities  are  so  valuable  in  structural  work,  it  is  impor- 
tant that  the  principles  be  clearly  recognized  upon  which  they  may  ba 
established  in  disturbed  regions.  This  subject  has  been  discussed  at 
length  by  Irving,  and  from  his  paper  the  substance  of  much  which  fol- 
lows is  taken.  An  unconformity  between  series  implies  a  difference  in 
number  of  orographic  movements  with  intervening  erosion.  This  diw 
ference  in  number  may  be  one  or  more  than  one.  Even  when  the  dif j 
ference  of  orographic  movements  to  which  the  series  have  been  subjected 
is  but  one  the  time  gap  between  the  two  must  have  been  very  consider- 
able, and  it  may  have  been  of  vast  duration.  Consequently  discordant 
series  may  differ  in  degree  of  consolidation,  in  the  development  o^ 
cleavage  and  foliation,  and  in  their  relations  to  eruptives.  At  the  be- 
ginning of  the  deposition  of  the  newer  series  basal  conglomerates  ares 
often  formed. 
