﻿AGE 
  OF 
  THE 
  EARTH— 
  CHAMBEKLHST 
  AND 
  OTHERS. 
  247 
  

  

  ciably 
  to 
  the 
  total 
  effect. 
  It 
  seems 
  clear, 
  therefore, 
  that 
  a 
  large 
  cor- 
  

   rection 
  is 
  to 
  be 
  made 
  to 
  the 
  present 
  rate 
  of 
  denudation 
  because 
  of 
  the 
  

   relatively 
  high 
  elevation 
  of 
  the 
  continents. 
  

  

  4. 
  Correction 
  for 
  area. 
  — 
  This 
  is 
  to 
  a 
  large 
  degree, 
  but 
  not 
  wholly, 
  

   an 
  effect 
  of 
  the 
  elevation 
  of 
  the 
  continents, 
  but 
  none 
  the 
  less 
  it 
  

   deserves 
  separate 
  recognition. 
  When 
  elevation 
  increases 
  the 
  land 
  

   area, 
  base 
  leveling 
  and 
  sea 
  transgression 
  at 
  once 
  set 
  in 
  and 
  combine 
  

   their 
  forces 
  to 
  reduce 
  the 
  exposed 
  area. 
  The 
  result 
  is 
  very 
  large 
  

   variations 
  in 
  the 
  areas 
  of 
  the 
  ancient 
  lands. 
  The 
  estimates 
  of 
  

   Schuchert 
  and 
  others 
  for 
  North 
  America 
  show 
  variations 
  that 
  range 
  

   from 
  the 
  full 
  surface 
  of 
  the 
  continental 
  platform 
  down 
  to 
  half 
  that 
  

   surface. 
  As 
  a 
  rule, 
  of 
  course, 
  the 
  lesser 
  surfaces 
  were 
  also 
  low 
  sur- 
  

   faces, 
  and 
  the 
  two 
  influences 
  were 
  cumulative. 
  At 
  stages 
  of 
  low 
  

   elevation 
  and 
  slack 
  drainage 
  deep 
  soils 
  were 
  likely 
  to 
  accumulate, 
  

   and 
  these 
  favored 
  thick 
  vegetation, 
  which 
  helped 
  to 
  hold 
  the 
  soils. 
  

   Thus 
  in 
  several 
  ways 
  small 
  area 
  and 
  low 
  elevation 
  united 
  their 
  influ- 
  

   ence 
  in 
  a 
  cumulative 
  effect 
  which 
  could 
  not 
  have 
  been 
  other 
  than 
  

   large. 
  

  

  Partial 
  summary. 
  — 
  Summarizing 
  at 
  this 
  point, 
  it 
  appears 
  that 
  

   four 
  important 
  corrections 
  quite 
  certainly 
  must 
  be 
  applied 
  to 
  the 
  

   present 
  rate 
  of 
  geologic 
  action 
  to 
  reduce 
  it 
  to 
  a 
  mean 
  rate 
  for 
  the 
  

   whole 
  of 
  geologic 
  time. 
  These 
  corrections 
  are 
  cumulative. 
  There 
  

   seems 
  to 
  be 
  no 
  way 
  at 
  present 
  to 
  evaluate 
  them 
  rigorously 
  or 
  perhaps 
  

   even 
  very 
  closely. 
  The 
  weighing 
  of 
  their 
  value 
  is 
  greatly 
  affected 
  by 
  

   the 
  individual 
  judgment 
  and 
  that, 
  in 
  turn, 
  by 
  individual 
  experiences 
  

   and 
  opportunities 
  of 
  observation. 
  Speaking 
  for 
  myself 
  alone, 
  it 
  

   does 
  not 
  seem 
  to 
  be 
  overstraining 
  the 
  importance 
  of 
  these 
  corrections 
  

   to 
  suppose 
  that 
  their 
  cumulative 
  value 
  will 
  be 
  found 
  great 
  enough 
  to 
  

   bring 
  the 
  old-time 
  estimates 
  up 
  to 
  figures 
  of 
  about 
  the 
  same 
  order 
  as 
  

   those 
  of 
  the 
  current 
  radioactive 
  estimates. 
  

  

  5. 
  The 
  lower 
  end 
  of 
  the 
  geologic 
  column. 
  — 
  Below 
  the 
  base 
  of 
  

   the 
  Paleozoic 
  series 
  the 
  geologic 
  terranes 
  are 
  much 
  obscured 
  by 
  

   diastrophism 
  and 
  metamorphism. 
  It 
  remains 
  to 
  inquire 
  what 
  is 
  the 
  

   testimony 
  of 
  this 
  obscured 
  portion 
  as 
  to 
  the 
  horizon 
  at 
  which 
  the 
  

   sediments 
  began 
  to 
  be 
  deposited, 
  for 
  that 
  is 
  essential 
  to 
  measuring 
  

   the 
  whole 
  period 
  of 
  deposition. 
  It 
  was 
  once 
  thought 
  that 
  the 
  Cam- 
  

   brian 
  beds 
  lay 
  close 
  upon 
  " 
  the 
  original 
  crust," 
  and 
  that 
  they 
  either 
  

   represented 
  the 
  real 
  beginning 
  of 
  the 
  sedimentary 
  series 
  or 
  else 
  an 
  

   early 
  stage 
  close 
  to 
  the 
  beginning. 
  But 
  as 
  field 
  work 
  progressed 
  it 
  

   was 
  found 
  that 
  first 
  one 
  and 
  then 
  another 
  thick 
  series 
  of 
  sediments 
  

   lay 
  below 
  the 
  Cambrian. 
  It 
  was 
  further 
  found 
  that 
  there 
  were 
  

   marked 
  unconformities 
  between 
  these 
  great 
  terranes, 
  and 
  that 
  these 
  

   were 
  of 
  such 
  a 
  nature 
  as 
  to 
  imply 
  long 
  intervals 
  of 
  time 
  unrepre- 
  

   sented 
  by 
  deposits 
  ; 
  that 
  is, 
  times 
  when 
  the 
  deposition 
  took 
  place 
  else- 
  

   where. 
  The 
  number 
  of 
  such 
  strongly 
  unconformable 
  terranes 
  has 
  

   been 
  notably 
  increasing 
  as 
  investigation 
  proceeds. 
  The 
  correlation 
  

  

  