﻿FISSURES 
  AND 
  VENTS. 
  89 
  

  

  in 
  fig. 
  8, 
  is 
  not 
  essential 
  to 
  the 
  formation 
  of 
  fissures 
  in 
  a 
  compressible 
  

   material, 
  such 
  as 
  forms 
  on 
  alluvial 
  plain. 
  Throughout 
  the 
  region 
  where 
  

   fissuring 
  was 
  common 
  the 
  fissures 
  are 
  certainly 
  more 
  numerous 
  and 
  

   conspicuous 
  near 
  the 
  edges 
  of 
  river 
  channels, 
  or 
  of 
  the 
  artificial 
  

   reservoirs 
  known 
  in 
  India 
  as 
  tanks, 
  but 
  they 
  are 
  not 
  confined 
  to 
  such 
  

   positions 
  and 
  may 
  be 
  met 
  with 
  in 
  positions 
  where 
  they 
  cannot 
  be 
  

   accounted 
  for 
  by 
  the 
  movement 
  of 
  a 
  portion 
  of 
  the 
  alluvium 
  bodily 
  

   towards 
  an 
  excavation, 
  whether 
  natural 
  or 
  artificial. 
  

  

  The 
  possibility 
  of 
  the 
  formation 
  of 
  such 
  fissures 
  is 
  a 
  direct 
  conse- 
  

   quence 
  of 
  the 
  soft 
  and 
  unconsolidated 
  nature 
  of 
  the 
  soil, 
  but 
  the 
  

   mechanism 
  is 
  not 
  so 
  clear. 
  It 
  seems 
  improbable 
  that 
  the 
  inertia 
  

   of 
  the 
  stretches 
  of 
  alluvium 
  thrown 
  into 
  movement 
  in 
  opposite 
  di- 
  

   rections 
  could 
  cause 
  this 
  compression 
  and 
  subsequent 
  separation 
  

   along 
  the 
  lines 
  of 
  fissure 
  ; 
  the 
  late 
  Mr. 
  R. 
  Mallet 
  showed 
  that 
  they 
  

   could 
  not 
  be 
  the 
  direct 
  result 
  of 
  a 
  wave 
  of 
  elastic 
  compression 
  1 
  ; 
  and 
  

   the 
  most 
  probable 
  explanation 
  is 
  that 
  their 
  formation 
  was 
  in 
  some 
  way 
  

   connected 
  with 
  the 
  visible 
  undulations 
  of 
  the 
  ground, 
  induced 
  by 
  the 
  

   earthquake, 
  which 
  travelled 
  as 
  independent 
  quasielastic 
  or 
  purely 
  

   gravitational 
  waves. 
  Such 
  waves 
  traversing 
  the 
  alluvium 
  would 
  

   throw 
  each 
  part 
  into 
  alternate 
  compression 
  and 
  extension 
  as 
  the 
  

   surface 
  was 
  bent 
  into 
  a 
  concave 
  or 
  convex 
  curve, 
  and 
  in 
  a 
  compres- 
  

   sible 
  material, 
  decidedly 
  lacking 
  in 
  elasticity, 
  this 
  might 
  easily 
  lead 
  

   to 
  the 
  formation 
  of 
  fissures. 
  

  

  A 
  very 
  noteworthy 
  point 
  about 
  these 
  fissures, 
  formed 
  away 
  from 
  

   and 
  independent 
  of, 
  the 
  river 
  courses, 
  is 
  the 
  manner 
  in 
  which 
  they 
  

   usually 
  run 
  parallel 
  to, 
  and 
  along 
  either 
  side 
  of, 
  any 
  road 
  or 
  embank- 
  

   ment. 
  It 
  is 
  a 
  well 
  known 
  mechanical 
  principle 
  that 
  where 
  there 
  is 
  

   a 
  sudden 
  change 
  in 
  section, 
  in 
  other 
  words 
  a 
  sudden 
  change 
  in 
  

   strength, 
  the 
  point 
  of 
  junction 
  is 
  a 
  point 
  of 
  weakness, 
  and 
  

   the 
  strength 
  at 
  that 
  point 
  is 
  less 
  than 
  it 
  would 
  have 
  been 
  if 
  one 
  

   part 
  had 
  not 
  been 
  stronger 
  than 
  the 
  other. 
  For 
  instance, 
  in 
  a 
  

   screw 
  bolt, 
  as 
  ordinarily 
  made, 
  the 
  portion 
  on 
  which 
  the 
  screw 
  thread 
  

  

  ' 
  Brit. 
  Aps. 
  Rep., 
  1850, 
  p. 
  53. 
  

  

  ( 
  89 
  ) 
  

  

  