﻿of 
  the 
  Northern 
  Appalachians. 
  425 
  

  

  interpretation 
  of 
  the 
  physiographic 
  history 
  of 
  the 
  region, 
  

   but 
  it 
  should 
  be 
  noted 
  that 
  the 
  time 
  when 
  superimposi- 
  

   tion 
  took 
  place 
  must 
  be 
  set 
  much 
  later, 
  i. 
  e., 
  post-Miocene. 
  

  

  The 
  sequence 
  of 
  events 
  outlined 
  by 
  Professor 
  Barrell 
  

   is 
  notably 
  different 
  from 
  older 
  schemes 
  ; 
  in 
  fact 
  it 
  ap- 
  

   pears 
  to 
  have 
  but 
  two 
  points 
  in 
  common 
  with 
  them, 
  

   namely, 
  the 
  initial 
  and 
  the 
  final 
  dates. 
  The 
  divergencies 
  

   are 
  plainly 
  enough 
  brought 
  about 
  by 
  the 
  recognition 
  of 
  

   many 
  marine 
  baselevels 
  and 
  by 
  other 
  new 
  interpretations 
  

   of 
  the 
  physiographic 
  evidence. 
  There 
  appears 
  no 
  reason 
  

   to 
  doubt 
  the 
  correctness 
  of 
  Professor 
  Barrell's 
  view 
  that 
  

   the 
  physiographic 
  history 
  of 
  the 
  Appalachian 
  region 
  is 
  

   much 
  more 
  complex 
  than 
  has 
  been 
  generally 
  supposed, 
  

   but 
  whether 
  all 
  the 
  erosion 
  surfaces 
  in 
  the 
  regions 
  speci- 
  

   fied 
  were 
  originally 
  the 
  result 
  of 
  marine 
  denudation 
  is 
  a 
  

   question 
  for 
  future 
  workers 
  to 
  determine. 
  The 
  opinion 
  

   may 
  be 
  ventured 
  that 
  future 
  study 
  will 
  not 
  result 
  in 
  much 
  

   increasing 
  the 
  number 
  of 
  normal 
  erosion 
  cycles 
  that 
  can 
  

   be 
  recognized; 
  it 
  is 
  to 
  be 
  expected, 
  rather, 
  that 
  the 
  in- 
  

   crease 
  will 
  be 
  in 
  the 
  number 
  of 
  partial 
  cycles. 
  And 
  

   also 
  it 
  would 
  seem 
  as 
  though 
  the 
  number 
  of 
  marine 
  cycles 
  

   should 
  distinctly 
  exceed 
  the 
  number 
  of 
  normal 
  cycles, 
  

   although 
  this 
  point 
  depends 
  somewhat 
  upon 
  the 
  question 
  

   whether 
  such 
  crustal 
  oscillations 
  as 
  are 
  indicated 
  by 
  the 
  

   Connecticut 
  terraces 
  should 
  be 
  considered 
  as 
  producing 
  

   interruptions 
  in 
  the 
  normal 
  cycle 
  or 
  as 
  inaugurating 
  new 
  

   cycles. 
  In 
  any 
  case 
  the 
  two 
  cycles 
  are 
  commensurable 
  

   only 
  under 
  the 
  single 
  condition 
  of 
  prolonged 
  crustal 
  rest 
  

   and 
  as 
  Professor 
  Barrell 
  has 
  pointed 
  out, 
  it 
  is 
  becoming 
  

   increasingly 
  evident 
  that 
  this 
  condition 
  is 
  the 
  exception 
  

   rather 
  than 
  the 
  rule. 
  

  

  Very 
  likely 
  the 
  conception 
  of 
  the 
  marine 
  origin 
  of 
  the 
  

   erosion 
  planes 
  will 
  not 
  gain 
  quick 
  acceptance 
  because 
  

   the 
  physiographic 
  history 
  of 
  the 
  Appalachian 
  region 
  is 
  

   so 
  well 
  organized 
  on 
  the 
  basis 
  of 
  the 
  fluvial 
  origin 
  of 
  the 
  

   erosion 
  surfaces, 
  and 
  for 
  the 
  more 
  general 
  reason 
  that 
  

   fluvial 
  denudation 
  has 
  come 
  to 
  be 
  looked 
  upon 
  as 
  much 
  

   more 
  competent 
  than 
  marine 
  denudation 
  to 
  develop 
  ex- 
  

   tensive 
  erosion 
  planes. 
  The 
  prevailing 
  opinion 
  would 
  

   appear 
  to 
  be 
  that 
  it 
  is 
  difficult 
  to 
  recognize 
  marine 
  plains, 
  

   on 
  the 
  basis 
  of 
  physical 
  characteristics, 
  after 
  they 
  have 
  

   been 
  uplifted 
  and 
  modified 
  to 
  any 
  extent 
  by 
  subaerial 
  

   erosion, 
  and 
  presumably 
  it 
  should 
  be 
  more 
  difficult 
  to 
  

   detect 
  the 
  effect 
  of 
  partial 
  marine 
  planation, 
  as 
  on 
  an 
  

  

  