﻿296 
  TV. 
  H. 
  Twenhofel 
  — 
  The 
  Comanchean 
  and 
  

  

  The 
  theory 
  of 
  universal 
  unconformities 
  grew 
  up 
  dur- 
  

   ing 
  the 
  past 
  century 
  on 
  the 
  assumption 
  that 
  either 
  all 
  the 
  

   continents 
  or 
  all 
  the 
  oceans 
  move 
  as 
  a 
  whole, 
  and 
  it 
  

   received 
  tremendous 
  support 
  from 
  the 
  postulate 
  of 
  Suess 
  

   that 
  the 
  lowering 
  of 
  the 
  strand-line 
  and 
  the 
  rising 
  of 
  the 
  

   land 
  came 
  about 
  through 
  earth 
  shrinkage 
  and 
  greater 
  

   depression 
  of 
  the 
  ocean 
  basins. 
  Chamberlin 
  and 
  Salis- 
  

   bury 
  gave 
  it 
  further 
  support 
  when 
  they 
  stated 
  their 
  view 
  

   that 
  the 
  " 
  def 
  ormative 
  movements 
  begin 
  . 
  . 
  . 
  with 
  a 
  

   depression 
  of 
  the 
  bottom 
  of 
  the 
  ocean 
  basins 
  by 
  which 
  

   their 
  capacity 
  is 
  increased" 
  and 
  "The 
  epicontinental 
  

   waters 
  are 
  correspondingly 
  withdrawn 
  into 
  them." 
  25 
  

   The 
  writer 
  wishes 
  that 
  the 
  theory 
  of 
  universal 
  depres- 
  

   sion 
  or 
  elevation 
  of 
  the 
  strand-line 
  could 
  be 
  established, 
  

   as 
  the 
  problems 
  of 
  intercontinental 
  correlation 
  and 
  those 
  

   of 
  the 
  opposite 
  sides 
  of 
  the 
  same 
  continent 
  would 
  thus 
  

   be 
  immensely 
  simplified. 
  That 
  some 
  withdrawals 
  of 
  the 
  

   sea 
  may 
  have 
  been 
  of 
  this 
  general 
  character 
  is 
  quite 
  prob- 
  

   able, 
  and 
  if 
  such 
  could 
  be 
  determined 
  they 
  might 
  be 
  made 
  

   to 
  constitute 
  the 
  high 
  places 
  of 
  the 
  geologic 
  time-scale 
  — 
  

   perhaps 
  the 
  divisions 
  between 
  eras. 
  That 
  most 
  of 
  the 
  

   movements 
  were 
  in 
  the 
  nature 
  of 
  local 
  regional 
  elevation 
  

   and 
  depression, 
  taking 
  place 
  irrespective 
  of 
  what 
  the 
  

   oceans 
  and 
  the 
  other 
  continents 
  were 
  doing, 
  seems 
  to 
  the 
  

   writer 
  to 
  be 
  far 
  more 
  probable 
  and 
  to 
  rest 
  on 
  a 
  firmer 
  

   basis 
  of 
  fact 
  ; 
  and 
  that 
  the 
  separation 
  of 
  the 
  geologic 
  

   time-scale 
  into 
  periods 
  is 
  based 
  on 
  the 
  planes 
  of 
  division 
  

   thus 
  created 
  seems 
  far 
  more 
  probable 
  than 
  that 
  they 
  are 
  

   due 
  to 
  general 
  withdrawals 
  of 
  the 
  sea. 
  

  

  If 
  the 
  bounding 
  planes 
  between 
  period 
  divisions 
  of 
  the 
  

   geologic 
  column 
  are 
  determined 
  by 
  local 
  regional 
  with- 
  

   drawals 
  of 
  the 
  sea, 
  how 
  is 
  it 
  to 
  be 
  decided 
  for 
  each 
  con- 
  

   tinent 
  what 
  constitutes 
  a 
  period? 
  For 
  nomenclature 
  

   relating 
  to 
  the 
  geologic 
  time-scale 
  to 
  be 
  of 
  real 
  value 
  to 
  

   geologists 
  and 
  students 
  in 
  general, 
  the 
  larger 
  time 
  units 
  

   should 
  have, 
  so 
  far 
  as 
  possible, 
  the 
  same 
  limitations 
  on 
  

   one 
  continent 
  as 
  another. 
  If 
  the 
  postulates 
  considered 
  

   above 
  are 
  valid, 
  it 
  would 
  mean 
  that 
  after 
  the 
  boundaries 
  

   of 
  a 
  period 
  or 
  system 
  have 
  been 
  determined 
  on 
  one 
  conti- 
  

   nent, 
  they 
  will 
  have 
  to 
  be 
  determined 
  for 
  other 
  continents 
  

   on 
  a 
  basis 
  of 
  paleontologic 
  criteria 
  irrespective 
  of 
  where 
  

   the 
  diastrophic 
  breaks 
  may 
  be. 
  Otherwise 
  a 
  term 
  given 
  

  

  25 
  Chamberlin 
  and 
  Salisbury, 
  Geology, 
  vol. 
  3, 
  p. 
  192, 
  1907. 
  

  

  