﻿FISSURES 
  AND 
  VENTS. 
  I05 
  

  

  forcing 
  up 
  of 
  the 
  central 
  piers 
  of 
  the 
  bridges 
  shows 
  that 
  the 
  filling 
  

   up 
  of 
  the 
  channels 
  was 
  due 
  to 
  an 
  actual 
  raising 
  of 
  their 
  beds, 
  and 
  

   not 
  to 
  the 
  outpouring 
  of 
  sand 
  through 
  vents 
  and 
  fissures. 
  

  

  The 
  mechanism 
  of 
  this 
  effect 
  is 
  easy 
  to 
  understand. 
  If, 
  instead 
  

   of 
  the 
  continuous 
  stratum 
  represented 
  in 
  figs. 
  8 
  and 
  9, 
  we 
  suppose 
  it 
  

   partially 
  cut 
  through, 
  as 
  in 
  fig. 
  12, 
  by 
  the 
  tank 
  T, 
  the 
  well 
  W 
  or 
  

  

  w 
  

  

  ^Ossr- 
  — 
  1 
  w 
  

  

  '/A 
  

  

  Fig. 
  12. 
  Diagram 
  to 
  illustrate 
  the 
  filling 
  up 
  of 
  stream 
  channals 
  and 
  excavations. 
  

  

  the 
  stream 
  bed 
  R, 
  it 
  is 
  obvious 
  that 
  these 
  introduce 
  areas 
  of 
  special 
  

   weakness 
  and 
  that 
  when 
  pressure 
  is 
  brought 
  to 
  bear 
  on 
  the 
  bed 
  of 
  

   loose 
  sand 
  it 
  is 
  at 
  these 
  spots 
  that 
  the 
  overlying 
  layers 
  of 
  alluvium 
  will 
  

   most 
  readily 
  yield. 
  Owing, 
  however, 
  to 
  the 
  area 
  over 
  which 
  yielding 
  

   takes 
  place, 
  it 
  will 
  often 
  happen 
  that 
  pressure 
  is 
  completely 
  

   relieved 
  without 
  any 
  actual 
  outpouring 
  at 
  the 
  surface, 
  and 
  as 
  the 
  

   same 
  amount 
  of 
  relief 
  can 
  be 
  given 
  by 
  a 
  lesser 
  actual 
  rise 
  where 
  the 
  

   area 
  is 
  large 
  than 
  where 
  it 
  is 
  small, 
  the 
  raising 
  of 
  the 
  floors 
  of 
  

   depressions 
  will, 
  ceteris 
  paribus^ 
  be 
  inversely 
  proportional 
  to 
  the 
  

   area. 
  This 
  is 
  in 
  accordance 
  with 
  experience 
  ; 
  the 
  greatest 
  raising 
  

   is 
  usually 
  observed 
  in 
  wells, 
  and 
  these 
  have 
  often 
  been 
  affected 
  when 
  

   there 
  has 
  been 
  no 
  noticeable 
  change 
  in 
  tanks 
  or 
  watercourses. 
  

   Next 
  to 
  wells 
  narrow 
  watercourses 
  have 
  been 
  most 
  affected, 
  while 
  

   in 
  large 
  tanks 
  and 
  natural 
  pools 
  the 
  raising 
  of 
  the 
  bottom 
  has 
  been 
  

   less 
  than 
  in 
  water 
  channels 
  of 
  equal 
  depth 
  but 
  smaller 
  width 
  in 
  the 
  

   neighbourhood. 
  

  

  This 
  filling 
  up 
  of 
  river 
  channels 
  took 
  place 
  over 
  a 
  large 
  area, 
  but 
  

  

  probably 
  nowhere 
  so 
  conspicuously 
  as 
  in 
  the 
  tract 
  of 
  lowland 
  which 
  

  

  lies 
  between 
  the 
  foot 
  of 
  the 
  Garo 
  Hills 
  and 
  the 
  Brahmaputra. 
  This 
  

  

  • 
  tract 
  is 
  intersected 
  by 
  numerous 
  channels, 
  which 
  carry 
  a 
  limited 
  

  

  drainage 
  in 
  the 
  dry 
  weather, 
  but, 
  when 
  the 
  Brahmaputra 
  is 
  in 
  flood* 
  

  

  ( 
  105 
  ) 
  

  

  