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  ANNUAL 
  EEPOKT 
  SMITHSONIAN 
  INSTITUTION, 
  19 
  2 
  9 
  

  

  But 
  the 
  surface 
  waves 
  inform 
  us 
  also 
  about 
  the 
  kind 
  of 
  rock 
  imme- 
  

   diately 
  beneath 
  the 
  deep 
  oceans, 
  whose 
  waters 
  hide 
  from 
  view 
  about 
  

   two-thirds 
  of 
  the 
  solid 
  surface 
  of 
  the 
  whole 
  earth. 
  The 
  measured 
  

   velocities 
  of 
  the 
  surface 
  waves 
  show 
  that 
  the 
  earth's 
  skin 
  beneath 
  

   the 
  deep 
  oceans 
  is 
  crystallized 
  basalt. 
  Thus 
  the 
  material 
  forming 
  

   an 
  earth 
  shell 
  directly 
  beneath 
  the 
  continents 
  is 
  continuous 
  with, 
  

   and 
  chemically 
  identical 
  with, 
  the 
  surface 
  rock 
  under 
  the 
  deep 
  sea. 
  

  

  Granite, 
  the 
  principal 
  rock 
  of 
  the 
  continents, 
  is 
  a 
  relatively 
  light 
  

   rock. 
  Basalt, 
  the 
  essential 
  rock 
  beneath 
  the 
  oceans, 
  is 
  relatively 
  

   heavy. 
  It 
  is 
  for 
  this 
  reason 
  that 
  the 
  continents 
  float 
  high 
  on 
  the 
  

   earth's 
  body; 
  they 
  are 
  pressed 
  up 
  by 
  the 
  surrounding, 
  heavy, 
  sohd 
  

   basalt, 
  much 
  as 
  icebergs 
  are 
  pressed 
  up 
  by 
  the 
  denser 
  water 
  of 
  the 
  sea. 
  

   This 
  is 
  why 
  we 
  have 
  dry 
  land, 
  with 
  its 
  endless 
  importance 
  for 
  man 
  and 
  

   organic 
  life 
  in 
  general. 
  

  

  Seismology 
  tells 
  us 
  why 
  our 
  home 
  is 
  stable, 
  in 
  spite 
  of 
  mighty 
  

   forces 
  which 
  tend 
  to 
  level 
  the 
  earth's 
  crust 
  and 
  drown 
  us 
  all. 
  We 
  

   may 
  confidently 
  expect 
  also 
  that 
  this 
  continued 
  "X-raying" 
  of 
  the 
  

   outer 
  earth 
  will 
  furnish 
  new 
  information 
  as 
  to 
  the 
  reason 
  why 
  moun- 
  

   tains 
  stand 
  so 
  high 
  and 
  are 
  able 
  to 
  keep 
  their 
  heads 
  in 
  the 
  clouds, 
  far 
  

   above 
  the 
  general 
  level 
  of 
  the 
  continents. 
  And 
  to 
  geophysics, 
  espe- 
  

   cially 
  to 
  seismology, 
  we 
  look 
  for 
  new 
  help 
  in 
  finding 
  out 
  the 
  conditions 
  

   for 
  the 
  earth's 
  periodic 
  revolutions 
  when 
  mountain 
  chains 
  were 
  born 
  

   and 
  sea-bottoms 
  became 
  the 
  pinnacles 
  of 
  the 
  world. 
  

  

  