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  Williams 
  — 
  Devonian 
  Interval 
  in 
  northern 
  Arkansas. 
  

  

  which 
  here 
  and 
  there 
  contains 
  large 
  numbers 
  of 
  small 
  quartz 
  

   pebbles, 
  varying 
  between 
  1 
  and 
  10™°" 
  in 
  diameter. 
  This 
  sand- 
  

   stone 
  we 
  assume 
  to 
  belong 
  to 
  the 
  Oriskany 
  of 
  New 
  York 
  " 
  (p. 
  

   cv). 
  The 
  evidence 
  of 
  elevation 
  and 
  erosion, 
  prior 
  to 
  the 
  

   deposit 
  of 
  this 
  so-called 
  Oriskany, 
  is 
  found 
  in 
  the 
  absence 
  of 
  

   the 
  formations 
  immediately 
  preceding 
  the 
  Devonian 
  formation 
  

   in 
  complete 
  sections. 
  The 
  beginning 
  of 
  permanent 
  sedimen- 
  

   tation 
  with 
  well-worn 
  pebbles 
  is 
  what 
  should 
  be 
  expected, 
  as 
  

   the 
  land 
  was 
  depressed, 
  whatever 
  the 
  time 
  at 
  which 
  it 
  took 
  

   place. 
  A 
  similar 
  thin 
  layer 
  of 
  much 
  worn 
  pebbles 
  and 
  sand, 
  

   separates 
  the 
  magnesian 
  limestones 
  (Ordovician) 
  from 
  the 
  

   underlying 
  gneiss, 
  at 
  a 
  section 
  on 
  the 
  southern 
  slopes 
  of 
  the 
  

   Adirondack 
  at 
  Little 
  Falls, 
  New 
  York. 
  It 
  is 
  followed 
  by 
  

   black 
  mud 
  shale 
  with 
  Lingulas, 
  and 
  then 
  the 
  regular 
  magnesian 
  

   limestone 
  of 
  considerable 
  thickness. 
  So 
  other 
  cases 
  might 
  be 
  

   cited, 
  the 
  age 
  of 
  the 
  beginning 
  of 
  the 
  new 
  sedimentation 
  being 
  

   determined 
  by 
  the 
  first 
  fossils 
  above 
  the 
  abrupt 
  change, 
  and 
  

   the 
  unconformity 
  may 
  not 
  be 
  indicated 
  by 
  conspicuous 
  modifi- 
  

   cation 
  of 
  the 
  plane 
  of 
  sedimentation. 
  We 
  should 
  less 
  expect 
  

   real 
  unconformity 
  in 
  the 
  central 
  part 
  of 
  a 
  continental 
  mass, 
  as 
  

   in 
  the 
  Mississippi 
  valley 
  region, 
  than 
  on 
  the 
  borders 
  where 
  the 
  

   folding 
  and 
  faulting 
  has 
  been 
  chiefly 
  concentrated. 
  

  

  