﻿X-EAYING 
  THE 
  EARTH 
  DALY 
  267 
  

  

  thicL 
  this 
  more 
  rigid 
  " 
  iron 
  " 
  may 
  be. 
  The 
  published 
  conchisions 
  as 
  to 
  

   the 
  composition 
  and 
  precise 
  thicknesses 
  of 
  the 
  still 
  higher 
  shells 
  are 
  un- 
  

   certain 
  and 
  demand 
  further 
  testing. 
  Yet 
  the 
  principle 
  that 
  the 
  earth 
  is 
  

   layered 
  seems 
  proved 
  once 
  for 
  all 
  and 
  leads 
  to 
  an 
  apparently 
  inescap- 
  

   able 
  and 
  highly 
  significant 
  conclusion 
  : 
  The 
  shell 
  structure 
  of 
  the 
  earth 
  

   seems 
  to 
  defy 
  explanation 
  unless 
  it 
  be 
  assumed 
  that 
  our 
  planet 
  was 
  

   formerly 
  molten. 
  It 
  must 
  have 
  been 
  fluid 
  enough 
  to 
  stratify 
  itself 
  by 
  

   gravity. 
  The 
  ''heavier" 
  materials 
  sank 
  toward 
  the 
  center, 
  the 
  

   "lighter" 
  materials 
  rose 
  toward 
  the 
  surface, 
  and 
  the 
  whole 
  mass 
  

   finally 
  arranged 
  itself 
  as 
  layers 
  or 
  shells, 
  with 
  the 
  very 
  dense 
  iron 
  in 
  

   the 
  central 
  region. 
  It 
  seems 
  necessary 
  to 
  assume 
  primitive 
  fluidity 
  

   right 
  to 
  the 
  surface, 
  and, 
  further, 
  to 
  assume 
  that 
  the 
  earth 
  was 
  thus 
  

   fluid 
  after 
  practically 
  all 
  of 
  its 
  substance 
  had 
  been 
  collected 
  in 
  the 
  

   planet-making 
  process. 
  This 
  general 
  deduction 
  must 
  control 
  future 
  

   research 
  on 
  the 
  cosmogonic 
  problem 
  — 
  the 
  origin 
  of 
  the 
  earth 
  and 
  its 
  

   brothers 
  and 
  sisters 
  of 
  the 
  solar 
  system. 
  The 
  earth 
  was 
  born 
  in 
  

   fervent 
  heat 
  and 
  in 
  the 
  beginning 
  was 
  fervently 
  hot, 
  even 
  at 
  the 
  

   surface. 
  

  

  While 
  telling 
  us 
  much 
  about 
  the 
  heart 
  of 
  the 
  earth, 
  the 
  seismogram 
  is 
  

   still 
  more 
  authoritative 
  and 
  eloquent 
  concerning 
  the 
  uppermost 
  

   layers 
  of 
  the 
  globe. 
  By 
  studying 
  the 
  instrumental 
  records 
  of 
  the 
  

   reflections, 
  refractions, 
  accelerations, 
  and 
  retardations 
  of 
  earthquake 
  

   waves, 
  seismologists 
  have 
  found 
  that 
  the 
  continental 
  rocks 
  reach 
  

   downwards 
  about 
  30 
  miles. 
  At 
  that 
  level 
  there 
  is 
  a 
  rather 
  abrupt 
  

   change 
  to 
  a 
  world-circling 
  shell 
  of 
  a 
  quite 
  difl'erent 
  nature. 
  The 
  

   dominant 
  rock 
  of 
  the 
  continents 
  is 
  granite 
  . 
  According 
  to 
  the 
  facts 
  of 
  

   geology, 
  as 
  of 
  seismology, 
  the 
  underlying 
  shell 
  or 
  substratum 
  is 
  the 
  

   heavier, 
  dark-colored 
  basalt, 
  and 
  is 
  apparently 
  the 
  source 
  of 
  this 
  

   commonest 
  of 
  lavas 
  and 
  the 
  primary 
  seat 
  of 
  all 
  volcanic 
  energies. 
  

  

  The 
  depth 
  of 
  the 
  continental 
  rock, 
  so 
  determined 
  from 
  the 
  writing 
  

   of 
  the 
  longitudinal 
  and 
  transverse 
  waves 
  on 
  seismographs, 
  is 
  con- 
  

   firmed 
  by 
  study 
  of 
  a 
  different 
  kind 
  of 
  vibrations 
  which 
  come 
  pouring 
  

   into 
  the 
  station 
  still 
  later 
  than 
  the 
  transverse 
  wave. 
  This 
  third 
  

   division 
  of 
  a 
  typical 
  seismogram 
  is 
  written 
  by 
  a 
  long 
  train 
  of 
  oscilla- 
  

   tions, 
  corresponding 
  to 
  what 
  are 
  called 
  surface 
  waves, 
  because 
  they 
  

   faithfully 
  follow 
  the 
  great 
  curve 
  of 
  the 
  earth's 
  rocky 
  skin. 
  Surface 
  

   waves 
  are 
  the 
  strongest 
  of 
  all 
  the 
  vibrations 
  recorded 
  by 
  distant 
  earth- 
  

   quakes. 
  They 
  are 
  caused 
  by 
  the 
  reflection 
  of 
  the 
  longitudinal 
  and 
  

   transverse 
  waves 
  as 
  these, 
  coming 
  from 
  the 
  interior, 
  impinge 
  at 
  low 
  

   angles 
  upon 
  the 
  contact 
  of 
  rock 
  with 
  ocean 
  water 
  and 
  of 
  rock 
  with 
  the 
  

   air. 
  That 
  contact 
  acts 
  like 
  the 
  wall 
  of 
  a 
  gigantic 
  whispering 
  gallery. 
  

   From 
  the 
  character 
  and 
  velocities 
  of 
  the 
  surface 
  waves, 
  expert 
  seis- 
  

   mologists 
  have 
  corroborated 
  the 
  evidence, 
  won 
  from 
  the 
  longitudinal 
  

   and 
  transverse 
  waves, 
  concerning 
  the 
  nature 
  and 
  depth 
  of 
  the 
  conti- 
  

   nental 
  rock. 
  

  

  