﻿Ill] 
  Special 
  Report 
  No. 
  4 
  : 
  Clays 
  265 
  

  

  Origin 
  of 
  Clays 
  

  

  Resid^ml. 
  — 
  Clay 
  is 
  a 
  secondar}- 
  substance 
  ; 
  that 
  is 
  it 
  is 
  formed 
  

   by 
  the 
  decay 
  of 
  other 
  rocks, 
  especially 
  those 
  containing 
  the 
  min- 
  

   eral 
  feldspar. 
  When 
  feldspar 
  decays 
  it 
  yields 
  the 
  mineral 
  

   kaolinite, 
  \Yhich 
  is 
  a 
  hydrated 
  silicate 
  of 
  alumina. 
  A 
  mass 
  of 
  

   kaolinite 
  would 
  be 
  called 
  kaolin, 
  and 
  this 
  latter 
  is 
  the 
  purest 
  

   form 
  of 
  clay 
  known, 
  but 
  thus 
  far 
  no 
  absolutely 
  pure 
  clay 
  has 
  

   been 
  found 
  in 
  nature. 
  The 
  feldspar 
  often 
  occurs 
  in 
  the 
  forms 
  

   of 
  veins, 
  and 
  its 
  decomposition 
  gives 
  rise 
  to 
  veins 
  of 
  kaolin. 
  As 
  

   a 
  rule 
  the 
  feldspar 
  is 
  associated 
  with 
  other 
  minerals, 
  especially 
  

   quartz 
  and 
  mica, 
  so 
  that 
  the 
  kaolin 
  thus 
  formed 
  is 
  a 
  mixture 
  of 
  

   kaolinite, 
  quartz, 
  mica 
  and 
  even 
  some 
  undecomposed 
  feldspar. 
  

   A 
  deposit 
  of 
  clay 
  formed 
  under 
  these 
  conditions 
  and 
  containing 
  

   only 
  the 
  minerals 
  mentioned 
  would 
  be 
  pure 
  white, 
  and 
  would 
  

   also 
  be 
  called 
  a 
  residual 
  clay, 
  because 
  it 
  represents 
  the 
  residuum 
  

   of 
  rock 
  decay 
  and 
  is 
  found 
  at 
  the 
  localit}^ 
  where 
  it 
  was 
  formed. 
  

   Very 
  often 
  the 
  feldspar 
  is 
  intermixed 
  with 
  minerals 
  which 
  con- 
  

   tain 
  iron 
  in 
  some 
  form, 
  and 
  in 
  the 
  decay 
  of 
  such 
  feldspathic 
  

   rocks 
  the 
  iron 
  is 
  set 
  free 
  in 
  the 
  form 
  of 
  iron 
  oxide 
  and 
  colors 
  

   the 
  clay 
  red. 
  Residual 
  clays 
  of 
  this 
  type 
  are 
  very 
  common 
  all 
  

   through 
  the 
  south 
  in 
  those 
  regions 
  which 
  are 
  underlaid 
  by 
  

   gneisses 
  and 
  other 
  crystaline 
  rocks, 
  and 
  they 
  form 
  the 
  great 
  

   brick-making 
  material 
  of 
  many 
  of 
  the 
  Southern 
  states. 
  

  

  Sedimentary. 
  — 
  As 
  the 
  land 
  surface 
  is 
  gradually 
  worn 
  down 
  by 
  

   weathering, 
  the 
  particles 
  of 
  residual 
  material 
  are 
  washed 
  down 
  

   into 
  the 
  lakes 
  or 
  seas 
  and 
  there 
  spread 
  out 
  over 
  the 
  bottom 
  in 
  the 
  

   form 
  of 
  sediment. 
  Beds 
  of 
  clay 
  formed 
  in 
  this 
  manner 
  are 
  

   known 
  as 
  sedimentary 
  clays. 
  

  

  Structure 
  

  

  Two 
  types. 
  — 
  The 
  structure 
  of 
  these 
  two 
  types 
  of 
  cla}' 
  deposit 
  

   is 
  very 
  different. 
  In 
  residual 
  clays 
  w^e 
  find 
  that 
  there 
  is 
  a 
  

   gradual 
  passage 
  from 
  the 
  fine 
  grained 
  clay 
  at 
  the 
  surface 
  into 
  

   that 
  which 
  contains 
  a 
  mixture 
  of 
  fine 
  particles 
  and 
  angular 
  frag- 
  

   ments, 
  and 
  this 
  in 
  turn 
  passes 
  by 
  stages 
  into 
  the 
  undecomposed 
  

   rock 
  beneath. 
  Indeed 
  the 
  structure 
  of 
  the 
  parent 
  rock 
  is 
  often 
  

   observable 
  for 
  several 
  feet 
  up 
  into 
  the 
  clay 
  mass, 
  as 
  in 
  the 
  

   iDrocess 
  of 
  rock 
  decay 
  there 
  is 
  often 
  very 
  little 
  movement 
  of 
  the 
  

  

  