﻿4:66 
  J. 
  Barrell 
  — 
  Upper* 
  Devonian 
  Delta 
  of 
  the 
  

  

  bers 
  of 
  gray 
  sandstone 
  and 
  red 
  shale. 
  In 
  certain 
  regions 
  and 
  

   at 
  certain 
  horizons 
  the 
  one 
  becomes 
  the 
  dominant 
  facies, 
  in 
  

   other 
  regions 
  and 
  times 
  the 
  other 
  dominates 
  the 
  formation. 
  

   Under 
  the 
  view 
  that 
  the 
  whole 
  constitutes 
  a 
  delta 
  the 
  alterna- 
  

   tions 
  of 
  minor 
  shale 
  and 
  sandstone 
  members 
  is 
  to 
  be 
  ascribed 
  

   to 
  the 
  lateral 
  shifting 
  of 
  distributaries, 
  the 
  red 
  shales 
  represent- 
  

   ing 
  flood 
  plain 
  areas 
  temporarily 
  removed 
  from 
  the 
  presence 
  of 
  

   currents. 
  The 
  delta 
  was 
  far 
  from 
  the 
  regions 
  of 
  erosion, 
  with 
  

   the 
  result 
  that 
  the 
  same 
  kind 
  of 
  material 
  was 
  brought 
  year 
  

   after 
  year 
  to 
  each 
  locality 
  subject 
  to 
  change 
  only 
  as 
  the 
  distrib- 
  

   utaries 
  shifted. 
  The 
  oxidation 
  of 
  the 
  shales 
  shows 
  that 
  the 
  soil 
  

   was 
  drained 
  and 
  aerated 
  for 
  considerable 
  periods 
  yearly. 
  The 
  

   stratification 
  planes 
  in 
  the 
  muds, 
  initially 
  poor 
  because 
  of 
  uni- 
  

   formity 
  of 
  material, 
  were 
  customarily 
  more 
  or 
  less 
  obliterated. 
  

   There 
  are 
  several 
  subaerial 
  agencies 
  which 
  result 
  in 
  this 
  where 
  

   the 
  rate 
  of 
  sedimentation 
  is 
  sufficiently 
  slow. 
  — 
  Worms 
  and 
  other 
  

   burrowing 
  forms 
  carry 
  on 
  a 
  vertical 
  mixing 
  of 
  the 
  soil. 
  The 
  

   growth 
  and 
  decay 
  of 
  roots 
  tends 
  in 
  some 
  degree 
  to 
  the 
  same 
  end. 
  

   But 
  a 
  very 
  effective 
  agent 
  for 
  vertical 
  mixing 
  in 
  climates 
  sub- 
  

   ject 
  to 
  periodic 
  dryness 
  is 
  mud-cracking, 
  where 
  the 
  binding 
  

   action 
  of 
  roots 
  does 
  not 
  prevent 
  it. 
  Each 
  wet 
  season 
  tills 
  the 
  

   cracks 
  largely 
  by 
  slumping 
  in 
  from 
  the 
  top, 
  and 
  each 
  dry 
  season 
  

   reopens 
  them. 
  The 
  evidence 
  seems 
  to 
  show 
  that 
  in 
  Catskill 
  

   times 
  all 
  of 
  these 
  processes 
  were 
  in 
  operation, 
  but 
  the 
  writer, 
  

   from 
  observations 
  on 
  present 
  mud 
  flats, 
  is 
  inclined 
  to 
  ascribe 
  a 
  

   chief 
  place 
  to 
  the 
  effect 
  of 
  mud-cracking. 
  

  

  The 
  uniformity 
  in 
  the 
  character 
  of 
  the 
  delta 
  from 
  northeast 
  

   to 
  southwest, 
  its 
  development 
  marginal 
  to 
  the 
  uplands, 
  and 
  the 
  

   somewhat 
  rapid 
  gradation 
  from 
  gravel 
  to 
  sand 
  and 
  clay 
  on 
  leav- 
  

   ing 
  the 
  mountains 
  suggests 
  the 
  presence 
  of 
  a 
  number 
  of 
  com- 
  

   paratively 
  short 
  streams 
  which 
  built 
  flat 
  coalescing 
  fans 
  rather 
  

   than 
  the 
  debouchement 
  of 
  one 
  or 
  two 
  great 
  continental 
  rivers. 
  

   The 
  form 
  of 
  the 
  plain 
  was 
  somewhat 
  similar 
  to 
  that 
  plain 
  of 
  

   Tertiary 
  alluvium 
  which 
  faces 
  the 
  Rocky 
  Mountains, 
  and 
  was 
  

   built 
  by 
  overloaded 
  rivers 
  in 
  a 
  region 
  of 
  semi-arid 
  climate. 
  

   Here 
  the 
  rivers 
  flow 
  out 
  at 
  gradients 
  of 
  near 
  10 
  feet 
  per 
  mile 
  

   and 
  fall 
  to 
  7 
  to 
  5 
  feet 
  per 
  mile 
  at 
  a 
  distance 
  of 
  some 
  hundreds 
  

   of 
  miles. 
  

  

  An 
  average 
  gradient 
  of 
  5 
  feet 
  per 
  mile 
  for 
  the 
  Catskill 
  plain 
  

   over 
  a 
  width, 
  at 
  the 
  close 
  of 
  the 
  Devonian, 
  of 
  from 
  80 
  to 
  100 
  

   miles 
  would 
  appear 
  therefore 
  to 
  be 
  a 
  reasonable 
  estimate. 
  

   Toward 
  the 
  mountains 
  the 
  slope 
  of 
  the 
  piedmont 
  gravel 
  plain 
  

   doubtless 
  steepened, 
  perhaps 
  to 
  15 
  or 
  20 
  feet 
  per 
  mile 
  in 
  the 
  

   case 
  of 
  large 
  streams, 
  even 
  higher 
  for 
  streams 
  of 
  lesser 
  volume. 
  

  

  Subsidence 
  of 
  the 
  geosyncline 
  under 
  this 
  plain, 
  depressing 
  

   the 
  grade 
  of 
  the 
  rivers, 
  would 
  result 
  in 
  alluvial 
  sedimentation 
  

   at 
  elevations 
  of 
  several 
  hundred 
  feet 
  above 
  the 
  sea 
  and 
  at 
  indefi- 
  

  

  