﻿88 
  OLDHAM; 
  GREAT 
  EARTHQUAKE 
  OF 
  1897 
  

  

  of 
  vertical 
  movement 
  is 
  necessary. 
  In 
  fig. 
  9 
  suppose 
  the 
  wave 
  to 
  be 
  

  

  4lllli||llgiilP^ 
  

  

  Fig. 
  9. 
  Diagram 
  to 
  illustrate 
  the 
  formation 
  of 
  sand 
  vents. 
  

  

  emerging 
  in 
  the 
  direction 
  of 
  the 
  parallel 
  lines 
  in 
  s 
  s. 
  The 
  upper 
  sur- 
  

   face 
  of 
  this 
  layer 
  will 
  move 
  upwards 
  against 
  the 
  stratum 
  o, 
  of 
  loose 
  

   sand 
  or 
  mud 
  and 
  water, 
  and 
  the 
  wave 
  motion 
  will 
  be 
  transmitted 
  

   through 
  it 
  to 
  the 
  upper 
  layer 
  g 
  g. 
  But 
  the 
  inertia 
  of 
  this 
  will 
  cause 
  

   to 
  be 
  strongly 
  compressed, 
  and 
  if 
  there 
  is 
  an 
  opening 
  through 
  to 
  the 
  

   surface, 
  whether 
  pre-existent 
  or 
  a 
  fissure 
  formed 
  by 
  the 
  earthquake, 
  

   this 
  pressure 
  may 
  be 
  sufficient 
  to 
  force 
  sand 
  and 
  water 
  to 
  the 
  surface. 
  

   By 
  the 
  repetition 
  of 
  the 
  pressure 
  a 
  succession 
  of 
  spurts 
  may 
  be 
  forced 
  

   up 
  ; 
  but 
  a 
  large 
  body 
  of 
  semiliquid 
  matter, 
  once 
  set 
  in 
  motion, 
  does 
  

   not 
  come 
  to 
  rest 
  at 
  once, 
  and 
  so, 
  by 
  the 
  momentum 
  of 
  the 
  moving 
  

   water, 
  its 
  flow 
  would 
  be 
  made 
  more 
  or 
  less 
  continuous, 
  and 
  the 
  

   spurting, 
  which 
  has 
  been 
  described 
  as 
  taking 
  place, 
  is 
  to 
  be 
  

   attributed 
  to 
  a 
  closing 
  in 
  of 
  the 
  sides 
  of 
  the 
  fissure, 
  and 
  consequent 
  

   forcing 
  out 
  of 
  the 
  water 
  between 
  its 
  walls, 
  rather 
  than 
  to 
  the 
  direct 
  

   action 
  of 
  the 
  earthquake 
  wave 
  on 
  the 
  underlying 
  stratum. 
  

  

  After 
  the 
  passage 
  of 
  the 
  earthquake 
  the 
  pressure 
  on 
  the 
  soft 
  

   water 
  bearing 
  stratum 
  will 
  be 
  relieved, 
  and 
  the 
  water 
  which 
  had 
  been 
  

   forced 
  out 
  may 
  be 
  sucked 
  back 
  with 
  sufficient 
  force 
  to 
  wash 
  down 
  

   the 
  upper 
  edges 
  of 
  the 
  fissure 
  and 
  form 
  a 
  crateriform 
  hollow 
  with 
  

   deeply 
  scored 
  sides. 
  

  

  Turning 
  now 
  to 
  a 
  consideration 
  of 
  the 
  special 
  facts 
  revealed 
  by 
  this 
  

  

  earthquake, 
  which 
  lead 
  to 
  a 
  modification 
  or 
  addition 
  to 
  the 
  explanation 
  

  

  given 
  above, 
  there 
  is 
  one 
  which 
  stands 
  out 
  prominently, 
  that 
  the 
  

  

  existence 
  of 
  a 
  free 
  su 
  rface, 
  such 
  as 
  that 
  formed 
  by 
  the 
  river 
  channel 
  

  

  ( 
  88 
  ) 
  

  

  