﻿FISSURES 
  AND 
  VENTS. 
  

  

  87 
  

  

  The 
  mode 
  in 
  which 
  the 
  fissures, 
  with 
  which 
  we 
  are 
  at 
  present 
  con- 
  

   cerned, 
  are 
  formed 
  is 
  fully 
  dealt 
  with 
  in 
  Vol. 
  XXVIII 
  of 
  the 
  Quarterly 
  

   Journal 
  of 
  the 
  Geological 
  Society 
  of 
  London, 
  and 
  at 
  page 
  52 
  of 
  

   Volume 
  XIX 
  of 
  these 
  Memoirs. 
  It 
  will 
  consequently 
  be 
  unnecessary 
  

   to 
  go 
  over 
  the 
  same 
  ground 
  in 
  detail, 
  but 
  an 
  explanation 
  of 
  the 
  

   process 
  involved 
  may 
  be 
  given 
  in 
  as 
  few 
  words 
  as 
  seem 
  compatible 
  

  

  Fig. 
  8. 
  Diagram 
  to 
  illustrate 
  the 
  formation 
  of 
  earth 
  fissures. 
  

  

  with 
  clearness. 
  In 
  fig. 
  8 
  let 
  g 
  g 
  represent 
  an 
  upper 
  stratum 
  of 
  the 
  

   alluvium, 
  cut 
  by 
  a 
  river 
  channel, 
  and 
  a 
  sandy 
  or 
  muddy 
  bed 
  of 
  less 
  

   cohesion, 
  along 
  which 
  the 
  upper 
  stratum 
  g 
  can 
  slide 
  over 
  the 
  lower 
  s 
  s. 
  

   When 
  g 
  g 
  is 
  thrown 
  into 
  movement 
  by 
  the 
  earthquake 
  wave 
  there 
  will 
  

   occur 
  a 
  time 
  when 
  all 
  the 
  particles 
  between 
  the 
  lines 
  a 
  and 
  b 
  will 
  be 
  

   moving 
  towards 
  the 
  river, 
  and 
  all 
  to 
  the 
  left 
  will 
  be 
  moving 
  in 
  the 
  

   opposite 
  direction. 
  In 
  these 
  circumstances 
  the 
  momentum 
  of 
  the 
  

   mass 
  a 
  b, 
  which 
  is 
  partially 
  free 
  to 
  move 
  over 
  the 
  layer 
  o 
  } 
  

   sets 
  up 
  a 
  strain 
  which 
  tends 
  to 
  part 
  the 
  surface 
  layer 
  at 
  a, 
  the 
  

   plane 
  of 
  separation 
  between 
  the 
  portions 
  of 
  the 
  surface 
  layer 
  

   which 
  are 
  moving 
  in 
  opposite 
  directions, 
  that 
  is 
  to 
  say, 
  in 
  opposite 
  

   semiphases 
  of 
  the 
  wave 
  ; 
  if 
  this 
  strain 
  is 
  sufficiently 
  great, 
  a 
  fissure 
  

   will 
  be 
  formed 
  and 
  the 
  piece 
  a 
  b 
  will 
  move 
  forward 
  towards 
  the 
  

   river 
  channel, 
  and 
  this 
  movement 
  may 
  well 
  give 
  rise 
  to 
  fresh 
  fissures 
  by 
  

   the 
  strains 
  set 
  up 
  in 
  the 
  moving 
  mass, 
  

  

  I 
  have 
  spoken 
  of 
  the 
  fissures 
  as 
  entirely 
  due 
  to 
  the 
  horizontal 
  

   movement 
  of 
  the 
  wave 
  particle, 
  and 
  this 
  horizontal 
  movement 
  would 
  

   of 
  itself 
  be 
  sufficient 
  to 
  account 
  for, 
  and 
  is 
  probably 
  the 
  main 
  factor 
  in 
  

   their 
  formation, 
  but 
  for 
  the 
  ejection 
  of 
  sand 
  and 
  water 
  a 
  certain 
  amount 
  

  

  ( 
  87 
  ) 
  

  

  