vanhise.]  DISCUSSIONS    OF    PRINCIPLES.  523 
in  simplicity  of  structure  of  the  two  series  would  increase  in  proportion 
to  the  number  and  intensity  of  the  earlier  movements.  However,  as 
the  movements  which  have  affected  the  newer  series  increase,  the  diffi- 
culty of  discovering  discordances  by  general  field  relations  is  increased. 
These  and  other  cases  of  general  field  relations  which  show  unconformity 
may  not  appear  to  the  observer  Avhile  doing  detailed  work,  since  no 
contacts  or  other  ordinary  indication  of  unconformity  is  found,  but 
strongly  appear  when  the  work  is  platted.  To  the  mind  of  the  writer 
general  field  relations  of  the  kinds  above  cited  are  sometimes  more  de- 
cisive evidence  of  unconformity  than  almost  any  kind  of  local  rela- 
tions. When  the  local  proofs  above  considered,  combined  with  general 
field  relations,  unite  as  evidences  of  unconformity,  as  is  generally  the 
case  if  the  worker  takes  advantage  of  all  the  facts  available,  the  ac- 
cumulated evidence  for  discordant  relations,  even  in  difficult  and  folded 
regions,  is  often  decisive. 
Unconformities  have  been  frequently  inferred  on  insufficient  ground. 
This  has  sometimes  resulted  from  regarding  surface  igneous  rocks  as 
sedimentary,  and  the  basal  conglomerate  overlying  such  a  formation  as 
evidence  of  unconformity.  More  frequently  the  misinterpreted  eviden ce 
of  unconformity  is  a  discordance  in  foliation  j  sometimes  cited  as  occur- 
ring in  actual  contact,  but  at  other  times  being  a  discordance  only  in 
the  strike  and  dip  of  foliation  at  some  distance.  A  contact  discordance 
of  foliation  or  bedding  may  occur  as  a  result  of  faulting.  The  strikes 
and  dips  of  banded  and  contorted  schists  and  gneisses  often  vary  so 
greatly  within  short  intervals  that  a  difference  of  this  kind  can  not  be 
taken  as  an  indication  of  discordances.  This  error  has  occurred  because 
it  has  been  assumed  that  cleavage  foliation  accords  with  sedimentation. 
When  it  is  practically,  not  theoretically,  recognized  that  this  structure 
is  secondary — may  be  produced  in  either  sedimentary  or  igneous  rocks — 
and  that  it  generally  does  not  correspond  over  large  areas  with  bed- 
ding, such  evidence  will  cease  to  be  used  as  indications  of  structural 
discordance. 
The  application  of  the  foregoing  principles  demands  that  in  working 
out  the  structure  of  the  crystalline  formations  the  ground  must  be  gone 
over  in  detail.  No  single  section  will  be  adequate  to  give  a  proper  idea 
of  the  structure,  nor  will  it  do  to  consider  that  as  a  result  of  several  or 
a  dozen  sections  the  structure  of  a  large  district  may  be  worked  out 
and  the  formations  mapped,  as  has  been  too  frequently  done.  Forma- 
tions which  outcrop  in  one  section  may  not  be  exposed  in  another,  or  a 
formation  between  one  section  and  the  next  may  entirely  change  its 
character  or  disappear  altogether.  If  the  district  is  a  difficult  one  the 
only  safe  way  is  to  take  advantage  in  the  field  of  all  available  over- 
ground and  underground  facts,  and  to  collect  abundant  material  for 
supplementary  office  work.  When  only  one  structure  is  present  and 
the  character  of  that  the  least  doubtful,  it  must  not  be  assumed  to  be 
bedding.    In  regions  in  which  the  exposures  are  infrequent  it  may  be 
