﻿LOBSTER 
  AND 
  CLAM 
  INVESTIGATIONS. 
  197 
  

  

  ditions 
  change, 
  so 
  that 
  tracts 
  once 
  favorable 
  for 
  clams 
  may 
  no 
  longer 
  

   support 
  them. 
  The 
  nature 
  of 
  some 
  of 
  these 
  changes 
  will 
  be 
  mentioned 
  

   later. 
  

  

  Clams 
  are 
  sometimes 
  found 
  in 
  beds 
  of 
  almost 
  pure 
  sand, 
  but 
  in 
  such 
  

   cases 
  the 
  water 
  currents 
  disturb 
  the 
  bottom 
  ver}- 
  little. 
  Even 
  when 
  

   established 
  in 
  such 
  localities, 
  however, 
  their 
  condition 
  is 
  precarious, 
  

   for 
  a 
  gale 
  or 
  an 
  unusually 
  strong 
  tide 
  may 
  at 
  any 
  time 
  overwhelm 
  them. 
  

   Such 
  a 
  destruction 
  has 
  often 
  been 
  noticed. 
  In 
  this 
  connection 
  it 
  may 
  

   be 
  stated 
  that 
  very 
  slow 
  currents 
  sometimes 
  prevent 
  the 
  existence 
  of 
  

   clams 
  by 
  depositing 
  fine 
  silt 
  which 
  they 
  hold 
  in 
  suspension; 
  this 
  on 
  

   settling 
  has 
  the 
  same 
  effect 
  as 
  shifting 
  sand. 
  Of 
  course 
  not 
  all 
  slow 
  

   currents 
  deposit, 
  for 
  many 
  times 
  they 
  carry 
  little 
  or 
  no 
  sediment. 
  At 
  

   the 
  heads 
  of 
  estuary-like 
  arms 
  of 
  the 
  sea 
  which 
  receive 
  fresh 
  water 
  

   streams, 
  one 
  often 
  finds 
  such 
  a 
  deposit 
  of 
  mud, 
  in 
  which 
  clams 
  are 
  not 
  

   living, 
  although 
  clam 
  food 
  may 
  be 
  present 
  in 
  large 
  quantities. 
  

  

  As 
  in 
  pure 
  sand, 
  so 
  also 
  in 
  nearly 
  pure 
  mud, 
  clams 
  are 
  sometimes 
  

   present; 
  but 
  here 
  also 
  the 
  action 
  of 
  currents 
  and 
  waves 
  disturbs 
  the 
  

   bottom 
  very 
  little, 
  and 
  there 
  is 
  no 
  very 
  active 
  deposition 
  of 
  silt. 
  

  

  Cementing 
  suhstances. 
  — 
  (a) 
  Fine 
  sediments. 
  The 
  surfaces 
  of 
  clam 
  

   beds 
  are 
  tenacious 
  from 
  several 
  causes. 
  When 
  sand 
  is 
  mixed 
  with 
  

   tine 
  sediments, 
  its 
  grains 
  are 
  held 
  by 
  this 
  cementing 
  substance. 
  Clay, 
  

   the 
  finest 
  of 
  .sediments, 
  resists 
  the 
  erosive 
  action 
  of 
  water 
  to 
  a 
  con- 
  

   siderable 
  degree, 
  and 
  it 
  is 
  often 
  found 
  in 
  more 
  or 
  less 
  abundance 
  on 
  

   clam 
  fiats. 
  Much 
  of 
  the 
  best 
  ground 
  on 
  a 
  clam 
  flat 
  is 
  often 
  a 
  tena- 
  

   cious 
  mixture 
  of 
  sand 
  and 
  fine 
  sediment. 
  

  

  (b) 
  Growth 
  of 
  algfe. 
  Another 
  very 
  important 
  agency 
  in 
  rendering 
  

   the 
  surface 
  tenacious, 
  and 
  thus 
  preventing 
  the 
  shifting 
  of 
  its 
  particles, 
  

   is 
  the 
  growth 
  of 
  algje, 
  which 
  forms 
  a 
  close, 
  thin 
  mat 
  over 
  certain 
  areas, 
  

   which 
  are 
  sometimes 
  very 
  extensive. 
  The 
  presence 
  of 
  the 
  algse 
  gives 
  a 
  

   flaky 
  or 
  cake-like 
  appearance 
  to 
  the 
  bottom. 
  It 
  does 
  not 
  extend 
  deep 
  

   into 
  the 
  sand, 
  but 
  binds 
  the 
  surface 
  grains 
  closely 
  enough 
  to 
  prevent 
  

   their 
  movement 
  even 
  by 
  strong 
  currents. 
  This 
  firm 
  dark 
  -green 
  crust 
  

   has 
  been 
  seen 
  on 
  beds 
  in 
  many 
  localities, 
  but 
  the 
  plants 
  composing 
  it 
  

   have 
  not 
  been 
  identified. 
  Whether 
  the 
  growth 
  occurs 
  only 
  where 
  cur- 
  

   rents 
  are 
  swift 
  has 
  not 
  been 
  determined, 
  but 
  certain 
  localities 
  have 
  been 
  

   noticed 
  to 
  have 
  this 
  covering 
  where 
  the 
  tide 
  rushes 
  with 
  much 
  force. 
  

   This 
  combination 
  of 
  firm 
  bottom, 
  which 
  prevents 
  erosion, 
  and 
  swift 
  

   current, 
  bearing 
  abundant 
  food, 
  seems 
  to 
  afl'ord 
  the 
  best 
  conditions 
  

   for 
  clam 
  growth. 
  

  

  (c) 
  The 
  growth 
  of 
  thatch 
  and 
  eelgrass. 
  The 
  growth 
  of 
  thatch 
  plants 
  

   or 
  eelgrass 
  may 
  convert 
  a 
  waste 
  of 
  sand 
  into 
  a 
  clam 
  bed. 
  Thatch 
  is 
  

   found 
  on 
  many 
  beaches 
  between 
  the 
  tide 
  lines, 
  and 
  also 
  covers 
  large 
  

   parts 
  of 
  clam 
  flats. 
  The 
  plants 
  grow 
  close 
  together. 
  Their 
  blades 
  

   rise 
  to 
  a 
  height 
  of 
  2 
  or 
  3 
  feet, 
  and 
  their 
  roots 
  form 
  a 
  feltwork 
  beneath 
  

  

  