﻿MARINE 
  AND 
  ESTUARINE 
  ALLUVIUM. 
  19 
  

  

  is 
  in 
  great 
  part 
  due 
  to 
  the 
  drainage 
  effected 
  by 
  the 
  canal 
  which 
  runs 
  

   right 
  through 
  the 
  length 
  of 
  the 
  backwater, 
  and 
  for 
  which, 
  excepting 
  for 
  

   a 
  couple 
  of 
  miles 
  near 
  Covelong, 
  the 
  channel 
  had 
  to 
  be 
  excavated 
  to 
  a 
  

   depth 
  of 
  several 
  feet. 
  From 
  enquiries 
  made 
  of 
  the 
  natives 
  in 
  various 
  

   villages 
  along 
  the 
  backwater, 
  it 
  was 
  ascertained 
  that 
  it 
  is 
  only 
  after 
  very 
  

   heavy 
  monsoons 
  that 
  the 
  landwater 
  accumulates 
  sufficiently 
  to 
  fill 
  the 
  

   bed 
  of 
  this 
  remarkable 
  lagoon. 
  

  

  The 
  following 
  list 
  shows 
  the 
  shells 
  occurring 
  most 
  frequently 
  in 
  

   the 
  marine 
  and 
  estuarine 
  beds 
  at 
  Madras 
  : 
  — 
  

  

  Pelecypoda. 
  Gasteropoda. 
  

  

  Ostrea, 
  several 
  species. 
  

  

  Placuna. 
  

  

  Area. 
  

  

  Cy 
  titer 
  ea 
  castanea. 
  

  

  Tellina. 
  

  

  Potamides 
  telescopium. 
  

  

  „ 
  Jluviatile. 
  

   Oliva, 
  sp. 
  

   Natica. 
  

   Murex. 
  

  

  These 
  shell 
  beds 
  are 
  the 
  principal 
  source 
  of 
  the 
  finer 
  sort 
  of 
  chunam 
  

   for 
  which 
  Madras 
  is 
  celebrated. 
  

  

  That 
  the 
  elevatory 
  action 
  which 
  upraised 
  the 
  marine 
  and 
  estuarine 
  

  

  Marine 
  and 
  estuarine 
  beds 
  SOutH 
  ° 
  f 
  MadraS 
  als0 
  affected 
  tll0se 
  to 
  the 
  

   alluvia 
  north 
  of 
  Madras. 
  north 
  canno 
  t 
  be 
  doubted, 
  although 
  no 
  positive 
  

  

  proofs 
  in 
  the 
  shape 
  of 
  fossil 
  remains 
  were 
  met 
  with 
  anywhere 
  north 
  of 
  

  

  Madras. 
  The 
  alluvium, 
  as 
  seen 
  on 
  the 
  surface 
  and 
  in 
  the 
  few 
  river 
  

  

  sections, 
  is 
  almost 
  entirely 
  sandy, 
  rarely 
  clayey. 
  But 
  sections 
  are 
  so 
  rare 
  

  

  and 
  of 
  such 
  slight 
  depth 
  that 
  it 
  is 
  impossible 
  to 
  pretend 
  to 
  anything 
  

  

  like 
  a 
  satisfactory 
  acquaintance 
  with 
  the 
  formations. 
  Even 
  well-sections, 
  

  

  which 
  in 
  other 
  parts 
  of 
  the 
  country 
  afford 
  some 
  little 
  help, 
  are 
  here 
  

  

  useless, 
  as, 
  from 
  the 
  mode 
  of 
  sinking, 
  the 
  sides 
  cannot 
  be 
  seen 
  to 
  a 
  depth 
  

  

  of 
  more 
  than 
  2 
  feet 
  or 
  a 
  yard, 
  because, 
  owing 
  to 
  the 
  friable 
  nature 
  of 
  

  

  the 
  strata 
  sunk 
  through, 
  the 
  wells 
  have 
  to 
  be 
  built 
  up 
  at 
  the 
  top 
  as 
  the 
  

  

  lower 
  part 
  sinks 
  with 
  the 
  progress 
  of 
  excavation. 
  In 
  smaller 
  wells 
  the 
  

  

  sides 
  are 
  kept 
  from 
  falling 
  in 
  by 
  means 
  of 
  earthen-ware 
  tubbing, 
  i. 
  e., 
  

  

  large 
  cylinders 
  of 
  earthen-ware 
  from 
  3 
  to 
  4^ 
  feet 
  in 
  diameter, 
  but 
  only 
  

  

  10 
  inches 
  to 
  a 
  foot 
  deep, 
  are 
  allowed 
  to 
  sink 
  in 
  the 
  excavation 
  as 
  it 
  

  

  proceeds, 
  and 
  others 
  added 
  from 
  above 
  till 
  the 
  requisite 
  depth 
  be 
  attained. 
  

  

  ( 
  19 
  ) 
  

  

  