﻿438 
  J. 
  Barrell 
  — 
  Upper 
  Devonian 
  Delta 
  of 
  the 
  

  

  deposits 
  ; 
  or, 
  on 
  the 
  other 
  hand, 
  where 
  dead 
  land 
  animals 
  or 
  

   plants 
  are 
  floated 
  out 
  to 
  sea. 
  In 
  delta 
  deposits 
  some 
  marginal 
  

   intermingling 
  wider 
  than 
  the 
  tidal 
  zone 
  may 
  be 
  expected 
  from 
  

   these 
  actions, 
  and 
  the 
  wide 
  shifting 
  of 
  the 
  shores 
  with 
  variable 
  

   river 
  building 
  will 
  be 
  expected 
  to 
  add 
  to 
  the 
  marginal 
  grada- 
  

   tion. 
  Shore 
  effects 
  will, 
  however, 
  not 
  be 
  recorded 
  at 
  great 
  

   distances, 
  so 
  that 
  the 
  nature 
  of 
  the 
  subaerial 
  delta 
  phase 
  should 
  

   be 
  established 
  on 
  its 
  landward 
  side, 
  the 
  marine 
  phase 
  where 
  it 
  

   also 
  is 
  homogeneous 
  and 
  at 
  a 
  distance 
  from 
  the 
  land. 
  With 
  

   the 
  character 
  of 
  these 
  separated 
  zones 
  clearly 
  determined, 
  the 
  

   debatable 
  ground 
  between 
  will 
  usually 
  resolve 
  itself 
  into 
  that 
  

   which 
  belongs 
  to 
  the 
  sea 
  and 
  that 
  which 
  belongs 
  to 
  the 
  land. 
  

   The 
  Catskill 
  formation 
  was 
  consequently 
  most 
  studied 
  on 
  its 
  

   eastern 
  side, 
  farthest 
  from 
  the 
  marine 
  fossils 
  of 
  the 
  Chemung. 
  

  

  Barrenness 
  of 
  a 
  formation 
  in 
  fossils 
  should 
  not 
  be 
  urged 
  as 
  a 
  

   positive 
  criterion 
  of 
  origin, 
  but 
  scanty 
  fossils 
  in 
  an 
  otherwise 
  

   barren 
  formation 
  are 
  of 
  the 
  highest 
  importance 
  ; 
  for 
  under 
  

   conditions 
  making 
  fossilization 
  difficult 
  it 
  is 
  highly 
  improbable 
  

   that 
  organisms 
  not 
  native 
  to 
  the 
  habitat 
  and 
  introduced 
  by 
  the 
  

   accidents 
  of 
  nature 
  should 
  be 
  the 
  ones 
  preserved, 
  unless 
  it 
  be 
  

   shown 
  that 
  they 
  have 
  a 
  unique 
  suitability 
  for 
  such 
  preserva- 
  

   tion. 
  

  

  There 
  are, 
  however, 
  two 
  criteria 
  whose 
  use 
  seems 
  especially 
  

   applicable 
  to 
  the 
  Catskill 
  and 
  associated 
  formations. 
  First 
  of 
  

   these 
  are 
  marks 
  of 
  subaerial 
  exposure 
  developed 
  both 
  broadly 
  

   and 
  vertically 
  through 
  mechanical 
  sediments. 
  These 
  consist 
  

   chiefly 
  of 
  mud-cracks, 
  rainprints, 
  and 
  rootmarks. 
  Where 
  

   these 
  occur 
  in 
  such 
  relations 
  they 
  seem 
  to 
  show 
  conclusively 
  

   a 
  terrestrial 
  origin 
  and 
  not 
  to 
  represent 
  the 
  littoral 
  facies 
  of 
  a 
  

   marine 
  or 
  lacustrine 
  formation. 
  ISTot 
  only, 
  as 
  stated 
  previously, 
  

   does 
  the 
  shore 
  form 
  at 
  any 
  one 
  time 
  but 
  a 
  narrow 
  border 
  to 
  

   the 
  accumulating 
  mantle, 
  but 
  except 
  on 
  the 
  front 
  of 
  an 
  advanc- 
  

   ing 
  delta 
  it 
  is 
  commonly 
  a 
  region 
  of 
  erosion 
  rather 
  than 
  of 
  

   accumulation. 
  Tidal 
  flats 
  are 
  furthermore 
  flooded 
  and 
  drained 
  

   twice 
  per 
  day, 
  with 
  the 
  result 
  they 
  are 
  always 
  limited 
  in 
  width 
  

   and 
  are 
  cut 
  by 
  deep 
  channels. 
  These 
  conditions 
  are 
  quite 
  dis- 
  

   tinctive 
  from 
  those 
  which 
  are 
  found 
  inland. 
  For 
  over 
  deltas 
  

   and 
  play 
  a 
  basins 
  on 
  the 
  contrary 
  every 
  part 
  is 
  alternately 
  cov- 
  

   ered 
  by 
  water 
  and 
  by 
  air 
  for 
  considerable 
  periods 
  of 
  time. 
  

   Where 
  the 
  climate 
  is 
  suitable 
  mud 
  cracking 
  is 
  developed 
  

   habitually 
  and 
  on 
  a 
  broad 
  scale. 
  

  

  Mud-cracking 
  in 
  chemical 
  sediments, 
  that 
  is 
  in 
  limestones, 
  

   must, 
  however, 
  be 
  distinguished 
  in 
  significance 
  from 
  the 
  crack- 
  

   ing 
  in 
  claystones. 
  Limestones 
  are 
  carried 
  in 
  solution 
  and 
  their 
  

   development 
  requires 
  a 
  comparative 
  absence 
  of 
  sand 
  and 
  clay, 
  

   the 
  mechanical 
  deposits 
  carried 
  by 
  rivers 
  and 
  by 
  waves. 
  The 
  

   solutions 
  to 
  have 
  sufficient 
  concentration 
  may 
  come 
  from 
  per- 
  

  

  