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  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1927 
  

  

  Until 
  Louis 
  Agassiz 
  propounded 
  his 
  glacial 
  theory 
  in 
  1840 
  it 
  was 
  

   assumed 
  that 
  the 
  Recent 
  epoch, 
  or 
  present 
  time, 
  was, 
  climatically 
  

   speaking, 
  something 
  altogether 
  exceptional 
  in 
  the 
  history 
  of 
  the 
  earth, 
  

   as 
  before 
  that, 
  according 
  to 
  the 
  universally 
  accepted 
  belief 
  of 
  the 
  

   time, 
  there 
  had 
  been 
  throughout 
  the 
  ages 
  an 
  unbroken 
  succession 
  of 
  

   mild 
  and 
  genial 
  climates, 
  without 
  polar 
  accumulations 
  of 
  ice 
  and 
  

   snow 
  and 
  with 
  no 
  well-marked 
  distinctions 
  of 
  latitude. 
  Agassiz's 
  

   conception 
  introduced 
  an 
  entirely 
  new 
  factor 
  into 
  the 
  problem, 
  and 
  

   was, 
  indeed, 
  so 
  novel 
  and 
  revolutionary 
  in 
  character 
  that 
  it 
  was 
  long 
  

   rejected 
  by 
  most 
  geologists; 
  and 
  even 
  so 
  late 
  as 
  1895 
  Sir 
  Henry 
  

   Howarth, 
  a 
  trustee 
  of 
  the 
  British 
  Museum, 
  stigmatized 
  it 
  as 
  "the 
  

   glacial 
  nightmare." 
  The 
  theory 
  made 
  but 
  slow 
  progress 
  toward 
  

   general 
  acceptance, 
  until 
  eventually 
  the 
  evidence 
  became 
  so 
  cogent 
  

   that 
  nearly 
  all 
  geologists 
  were 
  converted 
  to 
  it. 
  Nowadays 
  it 
  is 
  taken 
  

   UP 
  a 
  matter 
  of 
  course 
  and 
  is 
  taught 
  in 
  all 
  the 
  elementary 
  textbooks. 
  

   Thus 
  it 
  became 
  necessary 
  to 
  account 
  for 
  a 
  time 
  of 
  exceptional 
  cold, 
  

   tliough 
  this 
  Avas 
  a 
  matter 
  of 
  debate 
  among 
  those 
  who 
  accepted 
  the 
  

   glacial 
  hypothesis. 
  

  

  Some 
  of 
  the 
  most 
  eminent 
  geographers 
  maintained 
  that 
  the 
  glacial 
  

   epoch 
  had 
  been 
  due 
  to 
  a 
  greatly 
  increased 
  snowfall, 
  bringing 
  about 
  

   accumulations 
  in 
  the 
  winter 
  which 
  could 
  not 
  be 
  melted 
  in 
  the 
  summer, 
  

   rather 
  than 
  to 
  any 
  great 
  decrease 
  of 
  temperature. 
  Thus 
  it 
  was 
  uncer- 
  

   tain 
  just 
  what 
  the 
  problem 
  consisted 
  of 
  and 
  just 
  what 
  it 
  was 
  that 
  

   called 
  for 
  explanation. 
  Gradually, 
  however, 
  the 
  proofs 
  of 
  lowered 
  

   temperature 
  rather 
  than 
  of 
  increased 
  precipitation 
  seemed 
  irresistible, 
  

   and 
  now 
  everyone 
  accepts 
  that 
  view 
  of 
  the 
  problem. 
  It 
  is 
  not 
  neces- 
  

   sary 
  to 
  assume 
  any 
  increase 
  of 
  snowfall 
  to 
  account 
  for 
  glacial 
  condi- 
  

   tions; 
  but, 
  on 
  the 
  other 
  hand, 
  temperature 
  changes 
  would 
  of 
  them- 
  

   selves 
  necessarily 
  have 
  caused 
  great 
  alterations 
  in 
  the 
  distribution 
  of 
  

   the 
  rainfall. 
  For 
  example, 
  the 
  cold 
  of 
  Pleistocene 
  times 
  extended 
  the 
  

   rain 
  belt 
  so 
  far 
  southward 
  as 
  to 
  make 
  the 
  now 
  arid 
  Great 
  Basin 
  a 
  

   region 
  of 
  moist 
  climate, 
  supporting 
  immense 
  fresh-water 
  lakes, 
  while 
  

   the 
  rise 
  in 
  annual 
  temperature 
  which 
  caused, 
  or 
  at 
  least 
  accompanied, 
  

   the 
  disappearance 
  of 
  the 
  continental 
  ice 
  sheets 
  restricted 
  the 
  rain 
  belt 
  

   to 
  its 
  present 
  limits. 
  

  

  After 
  the 
  general 
  acceptance 
  of 
  Agassiz's 
  hypothesis, 
  it 
  was 
  thus 
  

   believed 
  that, 
  through 
  much 
  the 
  greater 
  part 
  of 
  its 
  recorded 
  history, 
  

   the 
  earth 
  had 
  had 
  a 
  mild, 
  genial, 
  and 
  almost 
  uniform 
  climate, 
  with- 
  

   out 
  definite 
  climatic 
  zones; 
  that 
  this 
  condition 
  had 
  been 
  broken 
  by 
  

   the 
  cold 
  of 
  the 
  Pleistocene, 
  the 
  partial 
  recovery 
  from 
  which 
  had 
  

   led 
  to 
  the 
  present 
  order 
  of 
  things. 
  

  

  CLIMATES 
  OF 
  THE 
  EARTH'S 
  PAST 
  

  

  We 
  now 
  know, 
  however, 
  that 
  the 
  problem 
  is 
  much 
  more 
  com- 
  

   plicated 
  than 
  would 
  appear 
  from 
  this 
  brief 
  statement. 
  In 
  1879 
  

  

  