VANiiisE.1  DISCUSSIONS    OF    PRINCIPLES.  519 
Hence,  as  guiding  phenomena  in  the  discovery  of  unconformities,  we 
have  (1)  ordinary  discordance  of  bedding;  (2)  difference  in  the  number 
of  dynamic  movements  to  which  the  series  have  been  subjected;  (3)  dis- 
cordance of  bedding  of  upper  series  and  foliation  of  lower  ;  (4)  relations 
$nth  eruptives;  (5)  difference  in  degree  of  crystallization;  (6)  basal  con- 
glomerates; (7)  general  field  relations. 
(1)  In  cases  of  ordinary  discordance  of  bedding  nothing  need  be  said, 
except  to  state  that  unconformities  should  hot  be  inferred  from  a  single 
small  contact  where  the  apparent  discordance  may  be  due  to  false  bed- 
ding or  to  local  currents  or  very  local  minor  disturbances.  Also,  the 
discordances  caused  by  faulting  may  be  mistaken  for  an  unconformity 
if  care  is  not  taken.  The  amount  of  evidence  for  the  unconformity 
should  be  sufficient  to  show  a  real  discrepancy  of  bedding  for  a  consid- 
erable area.  Within  a  short  distance  the  amount  of  discordance  of 
bedding  between  two  series  may  vary  greatly.  Frequently  a  dynamic 
movement  mainly  relieves  itself  along  a  comparatively  narrow  zone. 
This  zone  is  one  of  uplift  and  consequently  of  great  denudation.  The 
adjacent  plain  may  be  little  folded  and  not  deeply  eroded.  When  a 
new  series  is  deposited  upon  this  older  series  the  former  lies  approxi- 
mately parallel  to  the  bedding  of  the  older  upon  the  plain,  but  along 
the  zone  of  disturbance  the  first  may  lie  directly  athwart  the  bedding 
of  the  second. 
(2)  Difference  in  the  number  of  dynamic  movements  to  which  the 
series  have  been  subjected  is  often  an  important  means  of  determining 
unconformities.  In  order  that  an  unconformity  shall  occur  the  older 
series  must  have  been  subjected  to  at  least  one  more  orographic  move- 
ment than  the  newer.  In  the  most  favorable  case  the  older  series  has 
undergone  two  or  more  orographic  movements  while  the  newer  series 
has  undergone  but  a  single  one.  When  the  lines  of  these  movements 
are  in  the  same  direction  and  result  in  folding  the  only  difference  be- 
tween the  two  series  consists  in  steepness  of  inclinations;  but  in  case 
the  earlier  movements  were  in  a  different  direction  from  the  last  the 
older  series  will  show  a  compound  series  of  folds  due  to  the  resultant 
effect  of  the  two  or  more  movements,  while  the  newer  series  will  be 
sinrply  folded.  As  a  matter  of  course,  in  this  discrimination,  bedding 
must  be  used  rather  than  foliation.  Oftentimes  it  will  happen  that  the 
latest  movement  has  produced  a  prominent  cleavage  or  foliation  which 
is  common  to  both  older  and  newer  series;  and  under  these  circum- 
stances the  real  discordance  which  may  exist  between  the  two  series  is 
particularly  apt  to  be  overlooked,  and  a  district  will  be  described  as 
having  a  simple  monoclinal  structure,  or  one  in  which  the  series  is  re- 
produced by  faulting,  when  evidence  is  at  hand  for  two  or  more  dis- 
cordant series. 
The  orographic  movements,  instead  of  producing  folding,  may  cause 
pointing  or  faulting,  these  results,  as  suggested  by  Willis,  being  per- 
haps due  to  insufficient   load.    These  phenomena  are,  however,  ser- 
