﻿CLAYS 
  OF 
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  YOEK 
  507 
  

  

  As 
  calcite 
  effervesces 
  when 
  moistened 
  with 
  muriatic 
  acid, 
  its 
  pres- 
  

   ence 
  in 
  the 
  clay 
  may 
  often 
  be 
  detected 
  by 
  the 
  addition 
  of 
  this 
  

   chemical 
  to 
  it. 
  Calcite 
  may 
  be 
  scattered 
  through 
  the 
  clay 
  in 
  

   the 
  form 
  of 
  small 
  gi^ains 
  or 
  be 
  present 
  as 
  concretions 
  (commonly 
  

   called 
  '' 
  clay 
  dogs"). 
  It 
  not 
  infrequently 
  happens 
  that 
  some 
  lay-- 
  

   ers 
  of 
  the 
  clay 
  contain 
  a 
  much 
  larger 
  percentage 
  of 
  carbonate 
  

   of 
  lime 
  than 
  others, 
  and 
  indeed, 
  with 
  a 
  very 
  great 
  increase 
  in 
  the 
  

   amount 
  of 
  carbonate 
  of 
  lime, 
  the 
  clay 
  might 
  pass 
  into 
  a 
  marl. 
  

   AVhere 
  a 
  deposit 
  of 
  clay 
  rests 
  on 
  a 
  bed 
  of 
  limestone, 
  the 
  lower 
  

   layers 
  of 
  the 
  material 
  may 
  be 
  more 
  calcareous 
  than 
  the 
  upper 
  

   ones. 
  The 
  carbonate 
  of 
  lime 
  found 
  in 
  clays 
  is 
  at 
  times 
  derived 
  

   from 
  particles 
  of 
  limestone 
  if 
  the 
  clay 
  is 
  a 
  sedimentary 
  one, 
  or 
  in 
  

   the 
  case 
  of 
  either 
  sedimentary 
  or 
  residual 
  clays 
  it 
  may 
  come 
  froin 
  

   the 
  de'Composition 
  of 
  lime 
  soda 
  feldspars, 
  or 
  again 
  it 
  may 
  be 
  in- 
  

   troduced 
  by 
  percolating 
  waters. 
  

  

  Gypsum 
  may 
  be 
  present 
  in 
  the 
  clay 
  as 
  grains, 
  needles, 
  or 
  well 
  

   formed 
  crystals, 
  or 
  lamellar 
  masses. 
  It 
  is 
  much 
  softer 
  than 
  calcite, 
  

   being 
  scratched 
  by 
  the 
  linger 
  nail, 
  often 
  has 
  a 
  pearly 
  luster, 
  is 
  

   transparent, 
  and 
  does 
  not 
  efferA'^esce 
  with 
  acid. 
  In 
  hard 
  burned 
  

   bricks 
  gypsum 
  simply 
  acts 
  as 
  a 
  flux, 
  but 
  in 
  lightly 
  burned 
  ones, 
  it 
  

   gives 
  rise 
  to 
  soluble 
  sulfates, 
  which 
  cause 
  efflorescence. 
  In 
  the 
  

   salina 
  shales 
  it 
  often 
  forms 
  large 
  transparent 
  plates. 
  

  

  Mica. 
  This 
  can 
  frequently 
  be 
  easily 
  detected 
  by 
  the 
  naked 
  eye, 
  

   even 
  i:hough 
  it 
  may 
  be 
  present 
  in 
  a 
  very 
  finely 
  divided 
  condition, 
  for 
  

   the 
  small 
  scales 
  of 
  it 
  have 
  a 
  high 
  luster. 
  Mica 
  is 
  seldom 
  absent 
  

   in 
  .clays 
  and 
  is 
  usually 
  present 
  to 
  a 
  greater 
  or 
  less 
  extent 
  in 
  the 
  

   best 
  known 
  kaolins. 
  Owing 
  to^ 
  its 
  nature 
  it 
  floats 
  very 
  easily, 
  

   and 
  is 
  consequently 
  very 
  hard 
  to 
  eliminate 
  by 
  washing. 
  As 
  white 
  

   mica 
  is 
  verv^ 
  refractory, 
  and 
  when 
  finely 
  ground 
  possesses 
  a 
  certain 
  

   amount 
  of 
  plasticity, 
  its 
  presence 
  in 
  small 
  amounts 
  is 
  not 
  very 
  

   injurious. 
  

  

  The 
  mica 
  found 
  in 
  clays 
  is 
  generally 
  derived 
  from 
  igneous 
  or 
  

   metamorphic 
  rocks, 
  such 
  as 
  granites, 
  gneisses, 
  or 
  schists. 
  Two 
  

   kinds 
  of 
  mica 
  are 
  commonly 
  found 
  in 
  clay, 
  namely 
  biotite 
  and 
  

   muscovite. 
  The 
  biotite 
  mica 
  is 
  a 
  silicate 
  of 
  iron, 
  magnesia 
  and 
  

  

  