﻿THE 
  SEISMIC 
  FOCUS. 
  165 
  

  

  thought 
  of 
  the 
  theory 
  of 
  the 
  constitution 
  of 
  the 
  earth 
  outlined 
  above, 
  

   it 
  is 
  certain 
  that 
  the 
  actual 
  conditions 
  and 
  processes 
  of 
  nature 
  

   are 
  more 
  complicated 
  than 
  indicated, 
  but 
  there 
  also 
  can 
  be 
  no 
  doubt 
  

   of 
  the 
  fact 
  that 
  the 
  surface 
  rocks 
  can 
  be, 
  and 
  are, 
  thrown 
  into 
  a 
  state 
  

   of 
  strain. 
  Instances 
  are 
  given 
  in 
  almost 
  every 
  text-book, 
  and 
  

   it 
  is 
  easy 
  to 
  understand 
  that, 
  if 
  this 
  strain 
  increases 
  beyond 
  the 
  

   power 
  of 
  the 
  rock 
  to 
  resist 
  it, 
  there 
  will 
  be 
  a 
  sudden 
  fracture, 
  

   giving 
  rise 
  to 
  an 
  earthquake-wave 
  ; 
  and 
  also 
  that 
  the 
  longer 
  

   the 
  strain 
  accumulates, 
  and 
  the 
  greater 
  the 
  power 
  of 
  resistance, 
  

   the 
  greater 
  will 
  be 
  the 
  ultimate 
  shock 
  when 
  yielding 
  does 
  take 
  

   place. 
  

  

  Turning 
  now 
  to 
  the 
  consideration 
  of 
  the 
  facts 
  which 
  have 
  been 
  

   recorded 
  in 
  the 
  foregoing 
  chapters, 
  the 
  first 
  point 
  to 
  notice 
  is 
  that 
  

   this 
  earthquake 
  was 
  not 
  the 
  result 
  of 
  a 
  mere 
  explosion 
  or 
  of 
  the 
  

   rending 
  open 
  of 
  a 
  fissure. 
  The 
  evidences 
  of 
  permanent 
  displace- 
  

   ment 
  are 
  too 
  numerous, 
  and 
  it 
  may 
  safely 
  be 
  said 
  that, 
  whatever 
  the 
  

   nature 
  of 
  the 
  origin 
  of 
  the 
  earthquake 
  may 
  have 
  been, 
  it 
  was 
  in- 
  

   timately 
  bound 
  up 
  with 
  a 
  very 
  considerable 
  shifting 
  of 
  one 
  part 
  of 
  

   the 
  earth's 
  crust 
  relative 
  to 
  another. 
  

  

  The 
  next 
  point 
  to 
  be 
  noticed 
  is 
  that 
  these 
  indications 
  of 
  dis. 
  

   placement 
  are 
  not 
  confined 
  to 
  one 
  spot 
  or 
  one 
  line, 
  but 
  are 
  scattered 
  

   over 
  a 
  large 
  area. 
  We 
  may 
  consequently 
  regard 
  the 
  earthquake 
  

   either 
  as 
  a 
  compound 
  one, 
  having 
  a 
  number 
  of 
  separate 
  foci, 
  

   that 
  is 
  to 
  say, 
  as 
  being 
  not 
  one 
  earthquake, 
  but 
  a 
  number 
  of 
  

   earthquakes 
  which 
  happened 
  to 
  take 
  place 
  at 
  the 
  same 
  time. 
  Or, 
  

   as 
  a 
  more 
  probable 
  alternative, 
  we 
  may 
  regard 
  what 
  appear 
  to 
  be 
  foci 
  

   as 
  offshoots 
  from 
  the 
  true 
  focus, 
  which 
  was 
  deep 
  seated, 
  and 
  from 
  

   which, 
  what 
  may 
  be 
  called, 
  offshoots 
  ran 
  up 
  to 
  the 
  surface. 
  

  

  There 
  is 
  one 
  form 
  of 
  structural 
  disturbance, 
  consistent 
  with 
  the 
  

   latter 
  hypothesis, 
  which 
  will 
  afford 
  an 
  explanation 
  of 
  the 
  facts 
  observed, 
  

   and 
  that 
  is 
  what 
  is 
  known 
  as 
  an 
  overthrust. 
  An 
  overthiust 
  is 
  in 
  

   effect 
  a 
  reversed 
  fault, 
  bufe»differs 
  in 
  the 
  fact 
  that 
  the 
  fissure 
  along 
  

  

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