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  Geological 
  Survey 
  of 
  Louisiana 
  [Sect. 
  

  

  mineral 
  fragments. 
  Sedimentary 
  clays 
  on 
  the 
  other 
  hand 
  show 
  

   no 
  relation 
  to 
  the 
  underlying 
  parent 
  rock. 
  They 
  are 
  usually 
  

   distinctly 
  stratified, 
  and 
  there 
  may 
  be 
  a 
  number 
  of 
  distinctly 
  

   similar, 
  or 
  markedly 
  different 
  layers 
  one 
  on 
  top 
  of 
  the 
  other. 
  

  

  Composition 
  

  

  Either 
  residual 
  or 
  sedimentary 
  clays 
  may 
  be 
  composed 
  entirely 
  

   of 
  very 
  fine 
  grains 
  of 
  clay 
  substance 
  as 
  it 
  is 
  called, 
  or 
  they 
  may 
  

   be 
  made 
  up 
  of 
  a 
  mixture 
  of 
  both 
  coarse 
  and 
  fine 
  ones. 
  

  

  The 
  relative 
  amount 
  of 
  these 
  present 
  exercises 
  an 
  important 
  

   effect 
  on 
  the 
  behavior 
  of 
  the 
  clay 
  as 
  will 
  be 
  explained 
  later. 
  

  

  In 
  nature 
  it 
  is 
  possible 
  to 
  find 
  all 
  grades 
  of 
  .clay 
  varying 
  from 
  

   the 
  nearly 
  pure 
  ones 
  to 
  those 
  which 
  are 
  most 
  impure. 
  

  

  A 
  clay 
  which 
  is 
  nearly 
  pure 
  such 
  as 
  the 
  higher 
  grades 
  of 
  kaolin 
  

   would 
  have 
  only 
  silica, 
  alumina 
  and 
  combined 
  water 
  in 
  its 
  com- 
  

   position, 
  while 
  the 
  impure 
  clays 
  would 
  have 
  not 
  only 
  the 
  above 
  

   mentioned 
  ingredients, 
  but 
  in 
  addition 
  lime, 
  magnesia, 
  iron 
  oxide 
  

   and 
  alkalies, 
  and 
  the 
  less 
  pure 
  the 
  clay 
  the 
  greater 
  the 
  quantity 
  

   of 
  these 
  substances 
  which 
  are 
  found 
  in 
  it. 
  The 
  most 
  important 
  

   effect 
  of 
  these 
  impurities 
  as 
  a 
  whole 
  is 
  to 
  alter 
  the 
  fusibility 
  of 
  

   the 
  clay, 
  and 
  the 
  greater 
  the 
  percentage 
  of 
  them 
  that 
  the 
  clay 
  

   contains 
  the 
  more 
  easily 
  will 
  it 
  melt. 
  

  

  A 
  second 
  effect 
  of 
  these 
  impurities, 
  especially 
  the 
  iron, 
  is 
  their 
  

   influence 
  on 
  the 
  color 
  of 
  the 
  clay 
  in 
  burning. 
  

   Physicai. 
  Properties 
  of 
  Clay 
  

  

  The 
  physical 
  properties 
  of 
  clays 
  are 
  extremely 
  important, 
  and 
  

   on 
  them 
  depend 
  many 
  possibilities. 
  The 
  most 
  important 
  of 
  

   these 
  properties 
  are 
  plasticity, 
  tensile 
  strength, 
  fusibility, 
  color 
  

   on 
  burning, 
  shrinkage, 
  and 
  slaking. 
  

  

  Plasticity. 
  — 
  This 
  property 
  has 
  already 
  been 
  explained 
  as 
  that 
  

   by 
  virtue 
  of 
  which 
  the 
  clay 
  can 
  be 
  molded 
  into 
  any 
  desired 
  shape, 
  

   which 
  shape 
  it 
  retains 
  when 
  dry. 
  The 
  plasticity 
  of 
  a 
  clay 
  

   depends 
  almost 
  entirely 
  on 
  its 
  physical 
  condition, 
  that 
  is 
  on 
  the 
  

   size 
  and 
  shape 
  of 
  the 
  clay 
  grains, 
  and 
  stands 
  in 
  absolutely 
  no 
  

   direct 
  relation 
  to 
  the 
  amount 
  of 
  kaolinite 
  which 
  the 
  clay 
  contains, 
  

   although 
  this 
  fact 
  is 
  often 
  erroneously 
  stated 
  in 
  many 
  books. 
  

   The 
  cause 
  of 
  plasticity 
  being 
  thus 
  partly 
  understood 
  it 
  is 
  easily 
  

   conceivable 
  that 
  clays 
  will 
  vary 
  widely 
  in 
  the 
  degree 
  of 
  plasticity 
  

  

  