﻿246 
  ANNUAL, 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1922. 
  

  

  cut 
  down 
  the 
  continents 
  and 
  developed 
  peneplains. 
  10 
  The 
  periods 
  oc- 
  

   cupied 
  in 
  the 
  process 
  of 
  lowering 
  the 
  surface 
  by 
  denudation 
  are 
  held 
  

   with 
  good 
  reason 
  to 
  be 
  greater 
  than 
  those 
  occupied 
  in 
  its 
  elevation 
  

   by 
  deformative 
  action. 
  It 
  is 
  needless 
  to 
  say 
  that 
  elevation 
  increases 
  

   the 
  velocity 
  of 
  the 
  run-off, 
  and 
  that 
  this 
  velocity 
  greatly 
  increases 
  

   the 
  transporting 
  power. 
  " 
  It 
  is 
  generally 
  agreed 
  that 
  the 
  present 
  

   altitude 
  of 
  the 
  continents 
  is 
  greater 
  than 
  their 
  mean 
  elevation 
  during 
  

   geologic 
  history. 
  Geologists 
  recognize 
  at 
  least 
  two 
  stages 
  in 
  which 
  

   the 
  continents 
  were 
  exceptionally 
  high 
  and 
  broad: 
  That 
  which 
  at- 
  

   tended 
  the 
  transition 
  from 
  the 
  Paleozic 
  to 
  the 
  Mesozoic 
  Era, 
  and 
  that 
  

   which 
  attended 
  the 
  transition 
  of 
  the 
  Tertiary 
  to 
  the 
  present 
  epoch. 
  

   The 
  existing 
  stage 
  thus 
  falls 
  in 
  one 
  of 
  the 
  most 
  notable 
  stages 
  when 
  

   continental 
  elevation 
  and 
  breadth 
  were 
  greatest, 
  though 
  perhaps 
  

   not 
  at 
  its 
  climax. 
  Geikie 
  estimates 
  the 
  present 
  mean 
  elevation 
  of 
  the 
  

   land 
  at 
  2,441 
  feet. 
  12 
  The 
  mean 
  elevation 
  of 
  the 
  great 
  peneplains 
  

   is 
  a 
  matter 
  of 
  judgment 
  rather 
  than 
  of 
  knowledge, 
  but 
  no 
  one 
  would 
  

   probably 
  put 
  the 
  elevation 
  at 
  much 
  more 
  than 
  a 
  third 
  of 
  this. 
  

   Probably 
  a 
  third 
  is 
  too 
  high. 
  The 
  mean 
  elevation 
  for 
  all 
  the 
  ages, 
  

   high 
  and 
  low, 
  quite 
  surely 
  falls 
  somewhere 
  between 
  2,400 
  and 
  800 
  

   feet, 
  and 
  probably 
  nearer 
  the 
  800 
  than 
  the 
  2,400. 
  There 
  can 
  be 
  little 
  

   doubt, 
  then, 
  that 
  the 
  present 
  rate 
  of 
  denudation 
  and 
  deposition 
  is 
  

   much 
  above 
  the 
  mean 
  rate. 
  

  

  There 
  are 
  incidental 
  conditions 
  attending 
  high 
  relief 
  which 
  add 
  

   appreciably 
  to 
  the 
  immediate 
  effects 
  of 
  the 
  steep 
  declivities 
  to 
  which 
  

   it 
  gives 
  rise. 
  Eelief 
  of 
  the 
  surface 
  increases 
  the 
  vertical 
  air 
  currents, 
  

   and 
  these 
  favor 
  precipitation 
  ; 
  they 
  also 
  tend 
  to 
  concentrate 
  the 
  pre- 
  

   cipitation 
  and 
  give 
  it 
  enhanced 
  effect. 
  High 
  relief 
  often 
  induces 
  

   sharp 
  showers 
  and 
  distinctly 
  rapid 
  run-off. 
  The 
  smooth 
  surfaces 
  of 
  

   the 
  stages 
  of 
  lower 
  elevation, 
  on 
  the 
  other 
  hand, 
  favor 
  a 
  more 
  even 
  

   distribution 
  of 
  rain, 
  a 
  larger 
  absorption 
  into 
  the 
  soil, 
  and 
  a 
  slower 
  

   run-off 
  of 
  the 
  remainder. 
  So, 
  too, 
  accidented 
  surfaces 
  are 
  likely 
  to 
  

   be 
  ineffectively 
  protected 
  by 
  vegetation, 
  for 
  lack 
  of 
  soil, 
  or 
  of 
  ade- 
  

   quate 
  moisture. 
  These 
  and 
  other 
  incidental 
  influences 
  add 
  appre- 
  

  

  10 
  The 
  following 
  group 
  of 
  papers 
  emphasizes 
  the 
  rhythmical 
  nature 
  of 
  elevation 
  and 
  

   stability 
  and 
  of 
  the 
  action 
  of 
  the 
  atmosphere 
  and 
  hydrosphere 
  upon 
  the 
  periodic 
  deforma- 
  

   tions 
  of 
  the 
  earth 
  body 
  and 
  thus 
  form 
  the 
  basis 
  for 
  the 
  arguments 
  in 
  this 
  and 
  the 
  next 
  

   section 
  : 
  "A 
  group 
  of 
  hypotheses 
  bearing 
  on 
  climatic 
  changes," 
  T. 
  C. 
  Chamberlin, 
  Jour. 
  

   Geol., 
  Vol. 
  V 
  (1897), 
  pp. 
  681-683; 
  " 
  The 
  ultimate 
  basis 
  for 
  time 
  divisions 
  and 
  the 
  classi- 
  

   fication 
  of 
  geologic 
  history," 
  ibid.. 
  Vol. 
  VI 
  (1898), 
  pp. 
  449-463; 
  " 
  A 
  systematic 
  source 
  of 
  

   provincial 
  faunas," 
  ibid., 
  Vol. 
  VI 
  (1898), 
  pp. 
  597-609; 
  "The 
  influence 
  of 
  great 
  epochs 
  

   of 
  limestone 
  formation 
  upon 
  the 
  constitution 
  of 
  the 
  atmosphere," 
  Ibid., 
  Vol. 
  VI 
  (1898). 
  

   pp. 
  609-622 
  ; 
  and 
  specifically 
  as 
  applied 
  to 
  the 
  question 
  of 
  age 
  : 
  " 
  Rhythm 
  and 
  the 
  meas- 
  

   urement 
  of 
  time," 
  J. 
  Barrell, 
  Bull. 
  Geol. 
  Soc. 
  Am., 
  Vol. 
  28 
  (1917), 
  pp. 
  745-904. 
  The 
  argu- 
  

   ment 
  used 
  in 
  the 
  present 
  paper 
  'will 
  be 
  found 
  stated 
  as 
  a 
  quotation 
  from 
  Chamberlin 
  in 
  

   Holmes's 
  "The 
  age 
  of 
  the 
  earth," 
  cited 
  above 
  (1913), 
  pp. 
  79-81. 
  See 
  also 
  T. 
  C. 
  Chamber- 
  

   lin, 
  " 
  Diastrophism 
  as 
  the 
  ultimate 
  basis 
  of 
  correlation," 
  in 
  " 
  Outlines 
  of 
  geologic 
  his- 
  

   tory," 
  compiled 
  by 
  B. 
  Willis 
  and 
  R. 
  D. 
  Salisbury 
  (1910), 
  The 
  University 
  of 
  Chicago 
  

   Press, 
  pp. 
  298-306. 
  

  

  11 
  See 
  the 
  special 
  investigation 
  of 
  G. 
  K. 
  Gilbert, 
  " 
  The 
  transportation 
  of 
  debris 
  by 
  run- 
  

   ning 
  water," 
  Prof. 
  Paper, 
  86, 
  U. 
  S. 
  Geol. 
  Surv. 
  (1914). 
  

  

  ""Textbook 
  of 
  geology," 
  4th 
  ed.. 
  Vol. 
  I 
  (1903), 
  p. 
  49. 
  

  

  