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  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1927 
  

  

  the 
  Mediterranean 
  Sea, 
  The 
  whole 
  trans-Mississippian 
  region 
  of 
  

   North 
  America 
  was 
  a 
  desert 
  in 
  Permian-Triassic 
  times, 
  though 
  per- 
  

   haps 
  not 
  so 
  extremely 
  arid 
  as 
  central 
  Europe; 
  and 
  even 
  the 
  eastern 
  

   part 
  of 
  the 
  continent, 
  north 
  of 
  Virginia, 
  was 
  subarid 
  ; 
  at 
  least 
  most 
  

   students 
  of 
  the 
  Newark 
  formation 
  are 
  led 
  to 
  that 
  opinion 
  by 
  the 
  

   character 
  of 
  the 
  sediments. 
  

  

  If, 
  now, 
  we 
  plot 
  on 
  a 
  Mercator's 
  chart 
  the 
  known 
  arid 
  regions 
  of 
  the 
  

   Permian 
  and 
  Triassic 
  periods, 
  we 
  are 
  immediately 
  reminded 
  of 
  the 
  

   desert 
  zones 
  which 
  in 
  both 
  hemispheres 
  encircle 
  the 
  earth 
  at 
  the 
  pres- 
  

   ent 
  time. 
  These 
  zones 
  of 
  desert 
  are 
  the 
  trade-wind 
  belts, 
  where 
  the 
  

   equatorial 
  outflow 
  of 
  heated 
  and 
  expanded 
  air 
  descends 
  once 
  more 
  to 
  

   the 
  earth's 
  surface. 
  Such 
  descending 
  currents, 
  being 
  adiabatically 
  

   warmed 
  through 
  condensation, 
  are 
  always 
  dry, 
  and 
  hence 
  the 
  zone 
  of 
  

   desert 
  where 
  they 
  impinge 
  on 
  the 
  land. 
  In 
  both 
  Northern 
  and 
  

   Southern 
  hemispheres 
  the 
  Temperate 
  Zones 
  are 
  areas 
  in 
  which 
  the 
  

   prevailing 
  winds 
  are 
  westerlies, 
  blowing 
  some 
  80 
  per 
  cent 
  of 
  the 
  

   time 
  from 
  some 
  westerly 
  quarter, 
  Equatorward 
  from 
  the 
  westerlies 
  

   in 
  both 
  hemispheres, 
  lie 
  the 
  " 
  horse 
  latitudes," 
  belts 
  of 
  light, 
  variable 
  

   winds, 
  and 
  beyond 
  these 
  again 
  are 
  the 
  trade-wind 
  belts. 
  Between 
  

   the 
  northeast 
  and 
  the 
  southeast 
  trades 
  is 
  the 
  equatorial 
  belt 
  of 
  calms. 
  

   The 
  whole 
  tropical 
  wind 
  system, 
  with 
  its 
  five 
  zones, 
  swings 
  north 
  and 
  

   south 
  with 
  the 
  sun 
  in 
  its 
  apparent 
  path 
  between 
  the 
  solstices, 
  so 
  that 
  

   certain 
  regions 
  have 
  the 
  trade 
  winds 
  in 
  summer 
  but 
  not 
  in 
  winter. 
  

   Such 
  areas 
  are 
  southern 
  California 
  and 
  the 
  southern 
  parts 
  of 
  the 
  

   great 
  Mediterranean 
  peninsulas 
  of 
  Europe 
  — 
  the 
  Iberian, 
  the 
  Italian, 
  

   and 
  the 
  Balkan 
  — 
  and 
  these 
  all 
  have 
  nearly 
  rainless 
  summers, 
  the 
  

   precipitation 
  of 
  the 
  year 
  being 
  concentrated 
  in 
  the 
  winter. 
  

  

  If 
  we 
  may 
  assume 
  that 
  during 
  most 
  of 
  the 
  Permian 
  and 
  the 
  whole 
  

   of 
  the 
  Triassic 
  periods 
  there 
  was 
  such 
  an 
  increase 
  of 
  solar 
  activity 
  

   as 
  to 
  raise 
  the 
  surface 
  atmospheric 
  temperature 
  some 
  8° 
  or 
  10° 
  F, 
  

   above 
  the 
  present 
  annual 
  averages, 
  this 
  would 
  have 
  the 
  effect 
  of 
  

   displacing 
  or 
  extending 
  the 
  trade- 
  wind 
  belt 
  some 
  hundreds 
  of 
  miles 
  

   north 
  of 
  its 
  present 
  position 
  (note 
  well, 
  confining 
  our 
  attention 
  to 
  the 
  

   Northern 
  Hemisphere), 
  and 
  with 
  it 
  the 
  desert 
  zone 
  of 
  the 
  northern 
  

   continents. 
  That 
  such 
  would 
  be 
  the 
  actual 
  effect 
  of 
  a 
  moderate 
  rise 
  of 
  

   atmospheric 
  temperature 
  is 
  made 
  very 
  probable 
  by 
  the 
  converse 
  effect 
  

   of 
  lowered 
  temperature 
  in 
  the 
  Pleistocene, 
  to 
  which 
  reference 
  has 
  

   already 
  been 
  made. 
  During 
  the 
  times 
  of 
  principal 
  ice 
  extension 
  

   precipitation 
  was 
  so 
  increased 
  in 
  what 
  is 
  now 
  the 
  arid 
  and 
  semiarid 
  

   West 
  that 
  immense 
  fresh-water 
  lakes 
  were 
  established 
  in 
  the 
  Great 
  

   Basin, 
  and 
  almost 
  every 
  valley 
  in 
  the 
  Colorado 
  Rockies 
  and 
  in 
  the 
  

   Sierra 
  Nevada 
  down 
  to 
  middle 
  California 
  was 
  occupied 
  by 
  a 
  great 
  

   stream 
  of 
  ice. 
  

  

  