﻿OLDER 
  ALLUVIUM. 
  51 
  

  

  of 
  littoral 
  origin, 
  strictly 
  cotemporary 
  with 
  the 
  deposit 
  in 
  the 
  Irrawadi 
  

   valley 
  on 
  the 
  opposite 
  side 
  of 
  the 
  Yomah, 
  and 
  like 
  it 
  deposited 
  in 
  the 
  

   sea 
  when 
  the 
  land 
  stood 
  at 
  a 
  lower 
  level 
  than 
  at 
  present. 
  During 
  

   the 
  South-west 
  monsoon 
  the 
  whole 
  of 
  the 
  Gulf 
  of 
  Martaban 
  opposite 
  

   the 
  mouths 
  of 
  the 
  Irrawadi, 
  is 
  a 
  sea 
  of 
  muddy 
  water, 
  from 
  which 
  at 
  the 
  

   subsidence 
  of 
  the 
  monsoon 
  a 
  considerable, 
  wide-spread, 
  and 
  homogenous 
  

   deposit 
  must 
  every 
  year 
  be 
  taking 
  place 
  ; 
  and 
  the 
  conditions 
  over 
  the 
  

   whole 
  area 
  are 
  so 
  similar 
  and 
  constant 
  that 
  I 
  see 
  no 
  difficulty 
  in 
  suppos- 
  

   ing 
  that 
  the 
  clay 
  thrown 
  down 
  over 
  that 
  area 
  should 
  prove 
  identical, 
  if 
  

   elevated, 
  with 
  the 
  older 
  clay 
  of 
  the 
  Irrawadi 
  valley. 
  All 
  then 
  that 
  my 
  

   supposition 
  of 
  the 
  marine 
  origin 
  of 
  this 
  older 
  clay 
  requires 
  is, 
  that 
  a 
  

   gradual 
  elevation 
  should 
  have 
  taken 
  place 
  whereby 
  the 
  sea 
  was 
  forced 
  

   to 
  recede 
  to 
  its 
  present 
  limits, 
  and 
  the 
  estuary 
  to 
  yield 
  to 
  the 
  encroach- 
  

   ments 
  of 
  the 
  land. 
  

  

  In 
  the 
  Records 
  of 
  the 
  Geological 
  Survey, 
  No. 
  1 
  (1870), 
  I 
  have 
  

   already 
  contrasted 
  the 
  present 
  conditions 
  of 
  the 
  two 
  areas 
  within 
  which 
  

   the 
  Ganges 
  and 
  Irrawadi 
  respectively 
  debouch. 
  I 
  will 
  therefore 
  very 
  

   briefly 
  touch 
  on 
  the 
  arguments 
  whereon 
  I 
  rely 
  to 
  establish 
  the 
  fact 
  

   that, 
  whereas 
  the 
  area 
  of 
  the 
  Bengal 
  Sundribuns 
  is 
  one 
  of 
  depression, 
  

   that 
  of 
  the 
  Gulf 
  of 
  Martaban 
  and 
  the 
  Delta 
  of 
  the 
  Irrawadi 
  is 
  one 
  

   undergoing 
  elevation. 
  That 
  the 
  area 
  covered 
  by 
  the 
  Delta 
  of 
  the 
  Ganges 
  

   is 
  one 
  of 
  subsidence, 
  is 
  accepted 
  as 
  established 
  by 
  the 
  sections 
  exposed 
  

   in 
  wells 
  and 
  borings 
  near 
  Calcutta, 
  whereby 
  we 
  find 
  that 
  Calcutta 
  is 
  

   built 
  on 
  a 
  thickness 
  of 
  some 
  60 
  to 
  80 
  feet 
  of 
  Gangetic 
  deposits 
  of 
  partly 
  

   fluviatile 
  and 
  partly 
  marsh 
  origin. 
  Let 
  us 
  now 
  endeavour 
  to 
  estimate 
  the 
  

   condition 
  of 
  the 
  Delta 
  of 
  the 
  Irrawadi, 
  as 
  to 
  whether 
  it 
  is 
  rising 
  or 
  subsi- 
  

   ding. 
  First 
  of 
  all 
  there 
  is 
  the 
  striking 
  absence 
  over 
  the 
  entire 
  area 
  (with 
  

   the 
  exception 
  previously 
  described 
  near 
  Pantanau) 
  of 
  any 
  deposit 
  homo- 
  

   logous 
  with 
  the 
  upper 
  alluvium 
  so 
  widely 
  spread 
  in 
  Bengal. 
  The 
  Delta 
  is 
  

   everywhere 
  composed 
  of 
  the 
  older 
  clay 
  ; 
  and 
  the 
  entire 
  absence 
  in 
  the 
  

  

  actual 
  river 
  channel 
  of 
  any 
  newer 
  fluviatile 
  group 
  of 
  beds, 
  as 
  met 
  with 
  in 
  

  

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