﻿Appalachian 
  Geosyncline. 
  467 
  

  

  nite 
  distances 
  from 
  it. 
  This 
  suggests 
  a 
  very 
  different 
  concep- 
  

   tion 
  from 
  that 
  of 
  brackish 
  water 
  sedimentation 
  or 
  deposits 
  in 
  

   partially 
  enclosed 
  standing 
  waters 
  such 
  as 
  has 
  usually 
  been 
  

   assigned 
  as 
  the 
  cause 
  of 
  the 
  Catskill 
  facies. 
  

  

  The 
  interstratification 
  and 
  intermixture 
  of 
  sand 
  and 
  clay 
  

   together 
  with 
  minor 
  quantities 
  of 
  muscovite 
  and 
  feldspar 
  indi- 
  

   cates 
  that 
  the 
  material 
  was 
  derived 
  directly 
  from 
  the 
  rocks 
  and 
  

   was 
  not 
  the 
  reworked 
  sands 
  from 
  a 
  coastal 
  plain. 
  The 
  great 
  

   thickness 
  and 
  coarseness 
  of 
  the 
  Skunnemunk 
  conglomerates 
  

   implies 
  land 
  of 
  high 
  relief, 
  and 
  rivers 
  of 
  strong 
  grade. 
  The 
  

   large 
  volume 
  of 
  the 
  liner 
  materials 
  deposited 
  farther 
  west 
  is 
  

   equally 
  testimony 
  of 
  an 
  even 
  larger 
  volume 
  of 
  the 
  sources 
  of 
  

   upply. 
  The 
  Skunnemunk 
  conglomerate 
  has 
  been 
  partly 
  de- 
  

   stroyed 
  by 
  erosion, 
  but 
  2,500 
  feet 
  of 
  coarse 
  gravel 
  still 
  remain. 
  

   Only 
  rarely, 
  however, 
  during 
  Catskill 
  times, 
  did 
  gravel 
  reach 
  

   the 
  main 
  area 
  in 
  New 
  York 
  and 
  Pennsylvania, 
  25 
  miles 
  farther 
  

   west. 
  A 
  condition 
  existed, 
  therefore, 
  which 
  prevented 
  the 
  

   sweeping 
  outwards 
  of 
  gravels. 
  When 
  a 
  river 
  reaches 
  a 
  part 
  of 
  

   its 
  course 
  which 
  is 
  below 
  grade 
  it 
  must 
  deposit 
  its 
  coarser 
  bur- 
  

   den 
  in 
  order 
  to 
  carry 
  the 
  finer 
  material 
  through. 
  This 
  suggests 
  

   that 
  subsidence 
  was 
  rapid, 
  and 
  continued 
  not 
  only 
  in 
  the 
  zone 
  

   where 
  the 
  thick 
  Catskill 
  beds 
  still 
  exist, 
  but 
  over 
  an 
  eastern 
  border 
  

   which 
  in 
  the 
  later 
  folding 
  has 
  been 
  uplifted 
  and 
  subjected 
  to 
  

   erosion, 
  save 
  for 
  the 
  single 
  narrow 
  syncline 
  within 
  the 
  pre- 
  

   Cambrian 
  gneisses 
  which 
  still 
  holds 
  the 
  Skunnemunk 
  conglom- 
  

   erate. 
  Another 
  factor 
  which 
  may 
  have 
  a 
  bearing 
  is, 
  — 
  that 
  in 
  

   a 
  dry 
  climate 
  rivers 
  which 
  flow 
  from 
  high 
  mountains 
  tend 
  to 
  

   shrink 
  in 
  volume 
  on 
  reaching 
  the 
  plains 
  and 
  therefore 
  quickly 
  

   drop 
  their 
  coarser 
  waste. 
  This 
  steepens 
  the 
  gradient 
  but 
  can- 
  

   not 
  continue 
  indefinitely 
  without 
  progressive 
  elevation 
  of 
  the 
  

   mountains, 
  or 
  subsidence 
  of 
  the 
  plain. 
  

  

  Beyond 
  the 
  shore 
  of 
  the 
  delta 
  plain 
  were 
  the 
  waters 
  of 
  a 
  

   shallow 
  sea, 
  whose 
  bottom 
  was 
  probably 
  affected 
  by 
  waves 
  

   throughout. 
  The 
  upbuilding 
  then 
  was 
  wholly 
  by 
  the 
  deposit 
  

   of 
  topset 
  beds,* 
  the 
  sediment 
  being 
  spread 
  by 
  rivers 
  on 
  the 
  

   landward 
  side 
  ; 
  by 
  waves 
  on 
  the 
  side 
  of 
  the 
  sea. 
  The 
  delta 
  

   w 
  T 
  as 
  not 
  built 
  out 
  by 
  means 
  of 
  foreset 
  beds 
  such 
  as 
  are 
  seen 
  

   where 
  small 
  deltas 
  grow 
  out 
  into 
  relatively 
  deep 
  water 
  of 
  con- 
  

   stant 
  level. 
  

  

  The 
  several 
  formations 
  are 
  thus 
  seen 
  to 
  be 
  the 
  members 
  of 
  

   a 
  larger 
  structure 
  — 
  a 
  delta 
  system, 
  which 
  is 
  marked 
  by 
  the 
  

   existence 
  of 
  subaerial 
  aggradation 
  but 
  which 
  is 
  linked 
  to 
  and 
  

   embraces 
  the 
  synchronous 
  marine 
  formations, 
  the 
  latter 
  form- 
  

   ing 
  the 
  foundation 
  over 
  which 
  the 
  subaerial 
  deposits 
  are 
  com- 
  

   monly 
  extended. 
  

  

  * 
  Following 
  the 
  classification 
  given 
  on 
  pp. 
  385-387 
  in 
  the 
  paper 
  by 
  the 
  

   writer 
  " 
  Criteria 
  for 
  the 
  recognition 
  of 
  ancient 
  delta 
  deposits." 
  Geol. 
  Soc. 
  

   Am., 
  Bull, 
  xxiii, 
  pp. 
  377-446, 
  1912. 
  

  

  