﻿GEOLOGICAL 
  CLIMATES 
  SCOTT 
  281 
  

  

  modifications 
  of 
  the 
  atmosphere. 
  It 
  is 
  universally 
  understood 
  that 
  

   the 
  atmosphere 
  has 
  a 
  blanketing 
  effect, 
  permitting 
  the 
  direct 
  rays 
  

   of 
  the 
  sun 
  to 
  pass 
  through 
  it 
  freely, 
  but 
  opaque 
  to 
  the 
  dark 
  heat 
  

   reflected 
  from 
  the 
  ground. 
  In 
  this 
  way 
  the 
  air 
  acts 
  like 
  the 
  glass 
  

   in 
  a 
  cold 
  frame, 
  which 
  Tyndall 
  poetically 
  called 
  " 
  a 
  trap 
  to 
  catch 
  a 
  

   sunbeam." 
  This 
  blanketing 
  effect 
  is 
  least 
  in 
  thin, 
  dry, 
  and 
  pure 
  air 
  

   and 
  is 
  greatly 
  increased 
  by 
  the 
  presence 
  of 
  vapor 
  of 
  water 
  and 
  

   carbon 
  dioxide 
  in 
  the 
  atmosphere. 
  It 
  has 
  seemed 
  a 
  natural 
  inference 
  

   that 
  a 
  large 
  augmentation 
  of 
  the 
  amount 
  of 
  carbon 
  dioxide 
  present 
  

   in 
  the 
  air 
  would 
  so 
  raise 
  the 
  blanketing 
  effect 
  that 
  genial 
  climatic 
  con- 
  

   ditions 
  would 
  be 
  produced, 
  even 
  in 
  the 
  polar 
  regions. 
  There 
  is, 
  

   however, 
  a 
  fatal 
  objection 
  to 
  this 
  inference 
  revealed 
  by 
  experiment, 
  

   namely, 
  that 
  the 
  amount 
  of 
  carbon 
  dioxide 
  already 
  normally 
  present 
  

   in 
  the 
  atmosphere 
  exerts 
  nearly 
  the 
  maximum 
  blanketing 
  effect, 
  and 
  

   a 
  large 
  increase 
  in 
  the 
  amount 
  of 
  gas 
  would 
  not 
  produce 
  a 
  corre- 
  

   sponding 
  rise 
  of 
  temperature. 
  

  

  B. 
  (2) 
  A 
  screen 
  of 
  volcanic 
  dust 
  remaining 
  long 
  suspended 
  in 
  

   the 
  upper 
  atmosphere, 
  as 
  did 
  the 
  fine 
  dust 
  after 
  the 
  great 
  eruption 
  

   of 
  Krakatoa 
  in 
  1883, 
  might 
  so 
  cut 
  off 
  the 
  sun's 
  heat 
  as 
  to 
  cause 
  a 
  

   refrigeration 
  of 
  the 
  earth's 
  surface 
  temperature, 
  and 
  such 
  a 
  screen 
  

   has 
  actually 
  been 
  appealed 
  to 
  as 
  a 
  cause 
  of 
  glacial 
  climates; 
  but 
  the 
  

   supposed 
  cause 
  seems 
  neither 
  actual 
  nor 
  adequate. 
  There 
  is 
  nothing 
  

   in 
  known 
  geological 
  history 
  which 
  would 
  justify 
  us 
  in 
  supposing 
  that 
  

   such 
  masses 
  of 
  volcanic 
  material, 
  diffused 
  over 
  the 
  whole 
  earth, 
  

   were 
  ever 
  maintained 
  for 
  tens 
  or 
  hundreds 
  of 
  thousand 
  years. 
  The 
  

   geological 
  periods 
  in 
  which 
  vulcanism 
  was 
  most 
  active, 
  such 
  as 
  the 
  

   Ordovician 
  and 
  the 
  Devonian, 
  were 
  not 
  those 
  of 
  widespread 
  glacia- 
  

   tion. 
  Indeed, 
  the 
  opposite 
  conception 
  is 
  held 
  by 
  those 
  who 
  find 
  the 
  

   explanation 
  of 
  higher 
  air 
  temperatures 
  in 
  the 
  greatly 
  increased 
  con- 
  

   tent 
  of 
  carbon 
  dioxide 
  ; 
  they 
  maintain 
  that 
  the 
  most 
  actively 
  volcanic 
  

   periods 
  were 
  the 
  warmer 
  ones, 
  the 
  carbon 
  dioxide 
  being 
  supplied 
  

   by 
  the 
  volcanoes. 
  

  

  C. 
  Ocean 
  currents 
  are 
  familiar 
  means 
  of 
  modifying 
  climates, 
  the 
  

   warm 
  and 
  cold 
  currents 
  having 
  a 
  marked 
  effect 
  on 
  the 
  air 
  vv'ith 
  which 
  

   they 
  come 
  into 
  contact. 
  It 
  has 
  been 
  maintained 
  that 
  variations 
  in 
  

   the 
  density 
  of 
  sea 
  water, 
  due 
  to 
  changes 
  of 
  salinity, 
  would 
  greatly 
  

   modify 
  the 
  system 
  of 
  oceanic 
  circulation. 
  To 
  this 
  it 
  may 
  be 
  objected 
  

   that 
  the 
  great 
  ocean 
  currents 
  are 
  due 
  to 
  prevailing 
  winds, 
  and 
  so 
  long 
  

   as 
  the 
  system 
  of 
  winds 
  remained 
  unchanged 
  variations 
  in 
  the 
  density 
  

   of 
  the 
  water 
  could 
  have 
  little 
  effect 
  on 
  the 
  currents. 
  

  

  D. 
  Changes 
  in 
  the 
  size, 
  distribution, 
  and 
  altitude 
  of 
  the 
  land 
  

   masses 
  were, 
  as 
  we 
  have 
  already 
  seen, 
  first 
  invoked 
  by 
  Lyell 
  to 
  

   explain 
  the 
  earth's 
  vicissitudes 
  of 
  climate, 
  and 
  of 
  late 
  there 
  has 
  been 
  

   a 
  strong 
  revival 
  of 
  interest 
  in 
  causes 
  of 
  this 
  nature, 
  and 
  three 
  quite 
  

  

  