522  PRE-CAMBRTAN    ROCKS    OF    NORTH    AMERICA.  [bull.  pg. 
that  an  unusually  conglomeratic  layer  is  basal.  A  conglomerate  is 
likely  to  be  basal  when  the  major  portion  of  the  debris  is  derived  from 
the  immediately  subjacent  member ;  but  even  here  the  exception  must  be 
made  that  in  case  this  subjacent  member  is  a  surface  igneous  rock  the 
presence  of  the  conglomerate  is  no  evidence  of  a  time  break.  If,  how- 
ever, the  igneous  formation  is  of  such  a  character  as  does  not  originate 
except  as  a  deep-seated  rock,  the  fact  that  it  is  at  surface  and  yields 
fragments  to  the  overlying  formation  is  evidence  of  a  time  gap.  Also 
evidence  of  a  break  is  just  as  decisive  when  the  underlying  rock  has  a 
foliation  which  has  been  produced  prior  to  the  deposition  of  the  con- 
glomerate. This  may  be  determined  from  the  fact  that  fragments 
broken  from  a  foliated  rock  are  apt  to  be  longer  in  the  direction  of  lami- 
nation, and  when  deposited  in  the  overlying  series  they  naturally  lie 
with  their  foliation  at  an  angle  to  that  of  the  underlying  series.  It 
matters  not  whether  the  foliated  rock  of  the  inferior  series  be  of  sedi- 
mentary or  of  igneous  origin.  If  sedimentary,  a  long  time  has  been 
required  to  obliterate  evidence  of  its  fragmental  character;  if  igneous, 
its  foliation  shows  the  effect  of  long-acting  forces.  While  basal  con- 
glomerates are  often  found  they  are  also  often  absent  where  other  evi- 
dence shows  that  there  are  discordant  relations  between  two  series. 
This  absence  is  explained,  at  least  in  some  cases,  by  Pumpelly's  disin- 
tegration theory,  the  encroaching  shore  line  finding  a  set  of  disinte- 
grated rocks  in  which  the  mass  is  ready  to  yield  particles  of  the  con- 
stituent minerals  rather  than  pebbles. 
(7)  General  field  relations  are  often  sufficient  to  establish  discordant 
relations  between  series  when  all  other  lines  of  evidence  are  lacking. 
When  in  a  region  immense  stretches  of  rocks  of  one  series  are  always 
found  in  an  undisturbed  condition,  Avhile  an  adjacent  series  is  always 
disturbed,  discordant,  relations  may  be  inferred.  This  is  particularly 
evident  when  the  horizontal  series  fills  bays  in  the  older  rocks,  or  is 
found  as  inliers  surrounded  by  the  other  rocks.  Again,  the  general 
field  relations  may  establish  an  unconformity  even  if  both  series  are 
disturbed.  One  case  of  this  is  the  occurrence  of  a  uniform  belt  of 
stratified  rocks  which,  rjerhaps  witb  a  monoclinal  structure  and  a 
somewhat  uniform  strike  and  dip,  runs  for  great  distances,  the  rocks 
of  the  adjacent  unconformable  series  being  here  of  one  kind  and  there 
of  another  kind.  The  evidence  for  the  unconformity  in  this  case  is 
still  further  emphasized  if  the  lower  series,  instead  of  having  a  simple 
structure,  is  folded  in  a  complex  manner.  General  field  relations  may 
betray  unconformity  even  when  the  newer  series  has  been  folded  in  a 
more  complex  manner,  as,  for  instance,  having  been  subjected  to  two 
orographic  movements,  the  first  of  which  placed  it  in  a  monoclinal 
attitude,  and  the  second  of  which,  at  right  angles  to  this  first  force, 
gave  it  a  fluted  structure.  The  lower  series,  instead  of  having  this 
regular  structure,  being  subjected  to  still  earlier  orographic  movements, 
would  be  more  irregular  in  its  foldings  and  faultings,  and  the  difference 
