﻿Appalachian 
  Geosyncline. 
  465 
  

  

  the 
  many 
  distinctions 
  between 
  the 
  subaerial 
  and 
  subaqueous 
  

   deposits. 
  The 
  conclusion 
  may 
  be 
  accepted, 
  therefore, 
  without 
  

   further 
  argument, 
  that 
  delta 
  conditions 
  prevailed 
  and 
  that 
  the 
  

   Catskill 
  formation 
  is 
  made 
  of 
  the 
  deposits 
  on 
  the 
  subaerial 
  plain. 
  

   This 
  section 
  consequently 
  will 
  be 
  devoted 
  to 
  an 
  analysis 
  of 
  

   special 
  features 
  of 
  the 
  delta 
  as 
  derived 
  from 
  the 
  study 
  of 
  the 
  

   formation. 
  

  

  The 
  Portage 
  and 
  Chemung 
  are 
  seen 
  to 
  be 
  the 
  shallow 
  sea 
  

   equivalents 
  of 
  the 
  Oneonta 
  and 
  Catskill, 
  a 
  subaqueous 
  topset 
  

   plain. 
  The 
  Skunnemunk 
  conglomerate 
  is 
  a 
  downfolded 
  rem- 
  

   nant 
  of 
  a 
  piedmont 
  alluvial 
  gravel 
  plain 
  which 
  lay 
  between 
  the 
  

   flat 
  delta 
  surface 
  and 
  the 
  mountains. 
  The 
  Pocono 
  sandstone, 
  

   into 
  which 
  the 
  Catskill 
  passes 
  by 
  transition, 
  is 
  seen 
  to 
  be 
  divided 
  

   into 
  two 
  phases,— 
  a 
  marine 
  phase 
  in 
  western 
  Pennsylvania 
  and 
  

   Ohio, 
  a 
  fluviatile 
  phase 
  in 
  eastern 
  Pennsylvania. 
  In 
  the 
  Pocono, 
  

   the 
  sharp 
  delimitation 
  of 
  the 
  two 
  phases 
  is 
  obscure 
  but 
  between 
  

   the 
  Catskill 
  and 
  Chemung 
  the 
  color 
  contrast 
  draws 
  the 
  dividing 
  

   line 
  separating 
  the 
  subaerial 
  and 
  subaqueous 
  topset 
  beds. 
  The 
  

   margin 
  of 
  the 
  delta 
  no 
  doubt 
  held 
  lagoons, 
  varying 
  from 
  brack- 
  

   ish 
  to 
  fresh 
  water 
  ; 
  so 
  that 
  marine 
  fossils 
  should 
  be 
  somewhat 
  

   more 
  restricted 
  in 
  their 
  range 
  than 
  the 
  gray 
  and 
  olive 
  shales. 
  

  

  The 
  delta 
  began 
  its 
  existence 
  when 
  the 
  rivers 
  were 
  given 
  a 
  

   sufficient 
  load 
  of 
  detritus 
  to 
  stem 
  back 
  the 
  planing 
  erosion 
  of 
  

   the 
  sea. 
  So 
  far 
  as 
  known, 
  this 
  was 
  near 
  the 
  close 
  of 
  the 
  middle 
  

   Devonian. 
  For 
  a 
  time 
  the 
  rivers 
  gained 
  ground 
  and 
  the 
  

   Oneonta 
  formation 
  was 
  built 
  out. 
  But 
  at 
  the 
  close 
  of 
  Portage 
  

   time 
  the 
  sea 
  gained 
  a 
  temporary 
  ascendency 
  and 
  advanced 
  

   nearly 
  to 
  the 
  eastern 
  limit 
  of 
  the 
  present 
  Catskill 
  Mountains. 
  

   During 
  the 
  Chemung, 
  however, 
  it 
  was 
  again 
  gradually 
  dammed 
  

   back 
  and 
  retreated 
  to 
  western 
  Pennsylvania, 
  the 
  oscillating 
  

   shore 
  line 
  being 
  marked 
  by 
  the 
  transition 
  zone 
  of 
  sediments. 
  

   Back 
  from 
  the 
  margin 
  the 
  uniformity 
  of 
  the 
  red 
  shale 
  shows 
  

   that 
  the 
  delta 
  surface 
  was 
  sufficiently 
  well 
  drained 
  to 
  prevent 
  

   the 
  formation 
  of 
  extensive 
  swamps, 
  such 
  as 
  exist 
  over 
  the 
  lower 
  

   and 
  flatter 
  lands 
  of 
  the 
  Mississippi 
  and 
  the 
  Ganges 
  delta. 
  This 
  

   was 
  doubtless 
  in 
  part 
  due 
  to 
  the 
  climatic 
  factor, 
  but 
  also 
  in 
  

   part 
  owing 
  to 
  the 
  relatively 
  short 
  courses 
  of 
  the 
  rivers 
  and 
  

   consequently 
  a 
  steeper 
  gradient 
  to 
  the 
  delta 
  plain. 
  The 
  axis 
  

   of 
  the 
  geosyncline, 
  the 
  line 
  of 
  maximum 
  subsidence, 
  was 
  near 
  

   the 
  ancient 
  mountains. 
  The 
  streams 
  had 
  to 
  deposit 
  most 
  of 
  

   their 
  burden 
  over 
  this 
  zone 
  and 
  had 
  but 
  little 
  left 
  with 
  which 
  

   to 
  push 
  the 
  ocean 
  waters 
  farther 
  back. 
  The 
  western 
  part 
  of 
  

   the 
  geosyncline 
  was 
  in 
  consequence 
  mostly 
  marine, 
  passing 
  out 
  

   into 
  the 
  open 
  shallow 
  sea 
  ; 
  the 
  eastern 
  part 
  was 
  mostly 
  main- 
  

   tained 
  as 
  land, 
  passing 
  through 
  a 
  piedmont 
  plain 
  to 
  the 
  moun- 
  

   tains. 
  

  

  The 
  Catskill 
  formation 
  consists 
  typically 
  of 
  alternating 
  mem- 
  

  

  