﻿of 
  the 
  Northern 
  Appalachians. 
  331 
  

  

  were 
  regarded 
  as 
  forming 
  land, 
  an 
  emerged 
  ancient 
  

   coastal 
  plain, 
  since 
  the 
  close 
  of 
  the 
  Cambrian. 
  

  

  At 
  the 
  present 
  time 
  it 
  is 
  well 
  recognized 
  that 
  even 
  in 
  a 
  

   single 
  geologic 
  period 
  subaerial 
  denudation 
  will 
  go 
  far 
  

   toward 
  baseleveling 
  a 
  land 
  mass, 
  that 
  superficial 
  deposits 
  

   may 
  be 
  removed 
  no 
  matter 
  how 
  wide 
  their 
  extent, 
  and 
  

   that 
  a 
  slight 
  submergence 
  may 
  cause 
  a 
  wide 
  advance 
  of 
  

   the 
  sea. 
  There 
  is 
  no 
  reason, 
  therefore, 
  for 
  restricting 
  

   shore-lines 
  to 
  the 
  limits 
  of 
  outcrops 
  as 
  has 
  previously 
  

   been 
  done. 
  The 
  position 
  of 
  the 
  shore 
  for 
  every 
  period 
  is 
  

   a 
  problem 
  open 
  to 
  special 
  investigation 
  and 
  to 
  be 
  decided 
  

   by 
  a 
  variety 
  of 
  evidence. 
  

  

  Suess 
  had 
  early 
  called 
  attention 
  to 
  the 
  feature 
  of 
  posi- 
  

   tive 
  and 
  negative 
  movements 
  of 
  the 
  strand-line, 
  but 
  the 
  

   earliest 
  expression 
  of 
  these 
  views 
  in 
  American 
  geology 
  

   appears 
  to 
  have 
  been 
  by 
  Gilbert. 
  Over 
  twenty 
  years 
  ago 
  

   he 
  gave 
  a 
  presidential 
  address 
  before 
  the 
  Geological 
  

   Society 
  of 
  America 
  on 
  "Continental 
  problems.' 
  ' 
  Under 
  

   the 
  topic 
  Bo 
  continents 
  groivf 
  11 
  he 
  ventured 
  to 
  doubt 
  if 
  

   the 
  lands 
  had 
  progressively 
  extended 
  through 
  geologic 
  

   time. 
  He 
  said 
  that 
  possibly 
  there 
  should 
  be 
  no 
  dissent, 
  

   but 
  the 
  evidence 
  on 
  which 
  the 
  doctrine 
  was 
  founded 
  ap- 
  

   peared 
  to 
  him 
  so 
  far 
  from 
  conclusive 
  that 
  he 
  ventured 
  

   to 
  doubt. 
  He 
  then 
  called 
  attention 
  to 
  the 
  fact 
  that 
  the 
  

   farthest 
  transgressions 
  of 
  the 
  sea 
  must 
  have 
  left 
  thin 
  

   overlapping 
  deposits 
  which 
  were 
  subject 
  to 
  erosion 
  

   through 
  later 
  time 
  and 
  that 
  the 
  progress 
  of 
  denudation 
  

   was 
  chiefly 
  dependent 
  upon 
  elevation 
  above 
  baselevel. 
  

   On 
  the 
  other 
  hand, 
  unconformities 
  are 
  difficult 
  to 
  detect 
  

   and 
  impossible 
  to 
  follow 
  seaward 
  under 
  overlying 
  for- 
  

   mations. 
  It 
  was 
  pointed 
  out 
  that 
  as 
  a 
  consequence 
  we 
  

   are 
  unaware 
  of 
  the 
  maximum 
  limits 
  of 
  the 
  oscillations 
  of 
  

   land 
  and 
  sea 
  in 
  both 
  directions 
  and 
  consequently 
  inclined 
  

   to 
  ignore 
  their 
  existence. 
  His 
  remarks 
  were 
  in 
  the 
  

   nature 
  of 
  keen 
  philosophical 
  analysis, 
  but 
  as 
  they 
  were 
  

   not 
  strongly 
  backed 
  by 
  his 
  own 
  convictions 
  nor 
  by 
  illus- 
  

   trative 
  demonstrations, 
  they 
  did 
  not 
  impress 
  geologists 
  

   as 
  much 
  as 
  their 
  vital 
  importance 
  warranted. 
  The 
  rapid 
  

   change 
  of 
  view 
  during 
  the 
  opening 
  decade 
  of 
  the 
  twen- 
  

   tieth 
  century 
  was 
  in 
  reality 
  due 
  to 
  the 
  intensive 
  study 
  of 
  

   faunal 
  horizons 
  in 
  the 
  field 
  and 
  laboratory, 
  a 
  method 
  

   largely 
  due 
  to 
  H. 
  S. 
  Williams, 
  and 
  to 
  the 
  recognition 
  of 
  

  

  17 
  Bull. 
  Geol. 
  Soc. 
  Am., 
  vol. 
  4, 
  pp. 
  187-190, 
  1893. 
  

  

  