﻿of 
  the 
  Northern 
  Appalachians. 
  423 
  

  

  did 
  not 
  describe 
  these 
  surfaces 
  in 
  his 
  early 
  articles, 
  the 
  

   explanation 
  being, 
  no 
  doubt, 
  that 
  his 
  attention 
  was 
  then 
  

   concentrated 
  on 
  the 
  marine 
  phase 
  of 
  the 
  problem. 
  Later, 
  

   however, 
  when 
  he 
  began 
  to 
  study 
  in 
  detail 
  the 
  process 
  of 
  

   marine 
  denudation 
  in 
  order 
  to 
  formulate 
  criteria, 
  the 
  

   character 
  of 
  the 
  land 
  surfaces 
  assumed 
  a 
  place 
  of 
  impor- 
  

   tance, 
  as 
  appears 
  in 
  this 
  article. 
  The 
  topographic 
  fea- 
  

   tures 
  due 
  to 
  fluvial 
  erosion 
  proper 
  were 
  also 
  being 
  

   searched 
  for 
  and 
  preliminary 
  former 
  grades 
  for 
  several 
  

   of 
  the 
  large 
  rivers 
  had 
  been 
  worked 
  out. 
  For 
  the 
  Con- 
  

   necticut, 
  for 
  example, 
  Professor 
  Barrell 
  recognized 
  four 
  

   such 
  grades 
  which 
  appear 
  to 
  be 
  correlated 
  with 
  the 
  

   Litchfield, 
  Prospect, 
  Towantic 
  or 
  Appomattox, 
  and 
  Sund- 
  

   erland 
  marine 
  terraces. 
  The 
  description 
  of 
  the 
  marine 
  

   and 
  subaerial 
  elements 
  in 
  the 
  topography 
  of 
  western 
  

   Massachusetts 
  and 
  Connecticut 
  is, 
  however, 
  one 
  of 
  the 
  

   unwritten 
  parts 
  of 
  his 
  article 
  so 
  that 
  what 
  follows 
  is 
  an 
  

   interpretation 
  rather 
  than 
  a 
  statement 
  of 
  Professor 
  

   Barrell 
  's 
  ideas. 
  

  

  The 
  surface 
  over 
  which 
  the 
  Cretaceous 
  sea 
  advanced 
  

   was 
  one 
  characteristic 
  of 
  old 
  age 
  in 
  a 
  normal 
  cycle, 
  the 
  

   softer 
  rocks 
  being 
  rather 
  perfectly 
  reduced 
  and 
  the 
  re- 
  

   sistant 
  ones 
  having 
  moderate 
  relief. 
  This 
  surface 
  may 
  

   be 
  considered 
  equivalent 
  to 
  the 
  fundamental 
  plane 
  of 
  

   reference, 
  the 
  " 
  Jurassic' 
  ' 
  or 
  " 
  Cretaceous 
  ' 
  ' 
  peneplain, 
  

   recognized 
  by 
  earlier 
  investigators. 
  Whether 
  this 
  sur- 
  

   face 
  was 
  reduced 
  to 
  a 
  true 
  marine 
  plain 
  is 
  purely 
  con- 
  

   jectural, 
  for 
  no 
  trace 
  of 
  it 
  now 
  remains 
  ; 
  Professor 
  Bar- 
  

   rell 
  stated 
  in 
  his 
  original 
  address 
  that 
  "the 
  [original 
  

   surface 
  of 
  the] 
  Cretaceous 
  peneplain 
  of 
  subaerial 
  denu- 
  

   dation, 
  as 
  modified 
  by 
  marine 
  planation, 
  was 
  destroyed 
  

   by 
  subaerial 
  erosion 
  in 
  Tertiary 
  time." 
  

  

  Likewise 
  the 
  character 
  of 
  only 
  a 
  part 
  of 
  the 
  surfaces 
  

   over 
  which 
  the 
  sea 
  advanced 
  in 
  the 
  Eocene 
  and 
  again 
  in 
  

   the 
  Miocene 
  can 
  be 
  determined, 
  for 
  these 
  surfaces 
  to 
  a 
  

   large 
  extent 
  were 
  later 
  destroyed. 
  However, 
  as 
  the 
  

   shore-lines 
  during 
  these 
  two 
  periods 
  were 
  supposed 
  to 
  

   have 
  been 
  located 
  not 
  far 
  from 
  the 
  present 
  shore 
  of 
  

   Long 
  Island 
  Sound, 
  it 
  is 
  evident 
  that 
  the 
  greater 
  part 
  

   and 
  perhaps 
  the 
  whole 
  of 
  the 
  present 
  region 
  was 
  then 
  

   exposed 
  to 
  subaerial 
  erosion. 
  It 
  thus 
  seems 
  reasonable 
  

   to 
  infer 
  that 
  the 
  main 
  outlines 
  of 
  the 
  present 
  topography 
  

   were 
  etched 
  by 
  erosion 
  during 
  the 
  Eocene 
  and 
  that 
  in 
  a 
  

   general 
  way 
  they 
  have 
  persisted 
  in 
  that 
  part 
  of 
  the 
  region 
  

   not 
  later 
  covered 
  by 
  the 
  sea. 
  

  

  Am. 
  Jour. 
  Sci.— 
  Fourth 
  Series, 
  Vol. 
  XLIX, 
  Xo. 
  294.— 
  June. 
  1920. 
  

   30 
  

  

  