﻿GEOLOGICAL 
  CLIMATES 
  SCOTT 
  279 
  

  

  Though 
  the 
  historical 
  method 
  of 
  approach 
  is 
  the 
  more 
  interesting 
  

   and, 
  perhaps, 
  the 
  more 
  instructive, 
  it 
  is 
  in 
  the 
  interests 
  oi' 
  brevity 
  

   and 
  lucidity 
  to 
  deal 
  with 
  the 
  various 
  hypotheses 
  which 
  have 
  been 
  

   propounded 
  to 
  explain 
  the 
  climatic 
  changes 
  which 
  have 
  occurred 
  in 
  

   the 
  recorded 
  history 
  of 
  the 
  earth 
  in 
  a 
  more 
  systematic 
  manner. 
  The 
  

   subjoined 
  table 
  presents 
  a 
  classified 
  arrangement 
  of 
  the 
  principal 
  

   hypotheses 
  which 
  have 
  been 
  offered 
  in 
  explanation 
  of 
  the 
  problem 
  of 
  

   climate. 
  Obviously, 
  it 
  will 
  not 
  be 
  practicable 
  to 
  devote 
  more 
  than 
  a 
  

   very 
  brief 
  time 
  to 
  the 
  discussion 
  of 
  the 
  various 
  hypotheses. 
  

  

  I. 
  Terrestrial 
  causes. 
  

  

  A. 
  The 
  earth 
  as 
  a 
  whole. 
  

  

  (1) 
  Chanues 
  in 
  the 
  eccentricity 
  of 
  the 
  orbit. 
  

  

  (2) 
  Shifting 
  of 
  the 
  earth's 
  axis. 
  

  

  (3) 
  Shifting 
  of 
  the 
  earth's 
  exterior 
  on 
  the 
  interior. 
  

  

  (4) 
  The 
  internal 
  heat 
  of 
  the 
  earth. 
  

  

  B. 
  Atmospheric 
  factors. 
  

  

  (1) 
  Variation 
  in 
  the 
  proportion 
  of 
  carbon 
  dioxide. 
  

  

  (2) 
  In 
  the 
  amount 
  of 
  suspended 
  volcanic 
  dust. 
  

  

  C. 
  Oceanic 
  factors. 
  

  

  Variations 
  in 
  salinity. 
  

  

  D. 
  Topographical 
  factors. 
  

  

  Changes 
  in 
  the 
  area, 
  altitude, 
  and 
  disposition 
  of 
  the 
  land 
  

   masses. 
  

   II. 
  Cosniical 
  causes. 
  

  

  A. 
  Passage 
  through 
  cold 
  regions 
  in 
  space. 
  

  

  B. 
  Variations 
  of 
  the 
  sun's 
  activity. 
  

  

  As 
  the 
  table 
  indicates, 
  the 
  methods 
  of 
  explanation 
  fall 
  into 
  two 
  

   principal 
  categories: 
  (1) 
  The 
  terrestrial, 
  in 
  which 
  the 
  source 
  of 
  

   change 
  arose 
  on 
  the 
  earth 
  itself; 
  and 
  (2) 
  the 
  cosmical, 
  in 
  which 
  the 
  

   cause 
  lies 
  outside 
  of 
  the 
  earth 
  or 
  even 
  outside 
  of 
  the 
  solar 
  system. 
  

   Of 
  the 
  terrestrial 
  agencies 
  of 
  change 
  we 
  may 
  make 
  two 
  groups 
  : 
  {a) 
  

   Those 
  which 
  affect 
  the 
  earth 
  as 
  a 
  whole, 
  and 
  (&) 
  those 
  which 
  are 
  more 
  

   or 
  less 
  local 
  and 
  partial 
  in 
  their 
  operation. 
  

  

  A. 
  (1) 
  Croll's 
  hypothesis 
  was 
  published 
  when 
  I 
  was 
  an 
  under- 
  

   graduate 
  and 
  just 
  beginning 
  the 
  study 
  of 
  geology, 
  and 
  I 
  can 
  well 
  

   remember 
  the 
  enthusiasm 
  with 
  which 
  it 
  was 
  received 
  in 
  this 
  country. 
  

   "At 
  last," 
  we 
  said, 
  " 
  the 
  climatic 
  mystery, 
  which 
  has 
  been 
  troubling 
  

   us 
  for 
  so 
  long, 
  has 
  found 
  a 
  solution." 
  But 
  the 
  enthusiasm 
  was 
  short- 
  

   lived, 
  for 
  its 
  requirement 
  of 
  an 
  ice 
  age 
  at 
  intervals 
  for 
  each 
  hemi- 
  

   sphere 
  of 
  25,000 
  years 
  was 
  found 
  to 
  be 
  incompatible 
  with 
  the 
  ascer- 
  

   tained 
  facts 
  of 
  geological 
  history. 
  Probably 
  Doctor 
  Croll 
  himself 
  

   would 
  have 
  been 
  aghast 
  at 
  the 
  number 
  of 
  glacial 
  periods 
  which 
  the 
  

   earth 
  must 
  have 
  passed 
  through, 
  according 
  to 
  his 
  supposition. 
  The 
  

   ten 
  or 
  at 
  most 
  twenty 
  million 
  years 
  which 
  in 
  1875 
  were 
  allowed 
  for 
  

   the 
  age 
  of 
  the 
  sun 
  have 
  been 
  almost 
  indefinitely 
  extended 
  by 
  the 
  

   later 
  physical 
  discoveries. 
  Had 
  ice 
  ages 
  occurred 
  with 
  the 
  rhythmical 
  

  

  