﻿CLAYS 
  OF 
  ]S'EW 
  YOEK 
  497 
  

  

  To 
  define 
  clay 
  from 
  the 
  physical 
  standpoint, 
  vre 
  may 
  say 
  

   that 
  it 
  is 
  a 
  fine-grained 
  mixture 
  of 
  mineral 
  fragments 
  of 
  variable 
  

   composition, 
  possessing 
  when 
  wet 
  plasticity 
  which 
  permits 
  it 
  to 
  be 
  

   molded 
  into 
  any 
  desired 
  form 
  and 
  retaining 
  that 
  form 
  when 
  dry.^ 
  

   That 
  furthermore, 
  when 
  heated 
  above 
  a 
  certain 
  temperature 
  it 
  loses 
  

   its 
  chemically 
  combined 
  water 
  and 
  becomes 
  converted 
  into 
  a 
  rock- 
  

   like 
  masSj^ 
  which 
  if 
  reground 
  and 
  mixed 
  with 
  water 
  no 
  longer 
  shows 
  

   plasticity. 
  * 
  

  

  All 
  clays 
  found 
  in 
  nature 
  contain, 
  so 
  far 
  as 
  known, 
  a 
  variable 
  

   amount 
  of 
  kaolinite, 
  the 
  hydrated 
  silicate 
  of 
  alumina, 
  which 
  is 
  

   commonly 
  spoken 
  of 
  as 
  the 
  clay 
  base 
  or 
  clay 
  substance. 
  (It 
  should 
  

   be 
  stated 
  that 
  it 
  is 
  sometimes 
  the 
  custom, 
  specially 
  abroad 
  in 
  the 
  

   case 
  of 
  many 
  impure 
  clays 
  to 
  call 
  the 
  finest 
  particles 
  of 
  the 
  clay 
  

   irrespective 
  of 
  their 
  composition 
  the 
  '' 
  clay-substance.'') 
  

  

  A 
  mass 
  of 
  kaolinite 
  would 
  be 
  termed 
  kaolin. 
  These 
  two 
  

   terms 
  are 
  often 
  used 
  interchangeably, 
  though 
  the 
  former 
  is 
  simply 
  

   the 
  mineralogic 
  name 
  while 
  the 
  latter 
  is 
  a 
  rock 
  term. 
  Pure 
  

   kaolin 
  has 
  not 
  thus 
  far 
  been 
  found 
  in 
  nature, 
  though 
  some 
  very 
  

   nearly 
  pure 
  is 
  known. 
  Properly 
  speaking 
  the 
  term 
  kaolin 
  should 
  

   be 
  restricted 
  to 
  white 
  burning 
  residual 
  clays, 
  a 
  usage 
  which 
  iB 
  

   widespread 
  but 
  has 
  not 
  become 
  universal. 
  -' 
  The 
  name 
  kaolin 
  is 
  a 
  

   corruption 
  of 
  the 
  Chinese 
  Kauling, 
  which 
  means 
  high 
  ridge, 
  and 
  is 
  

   the 
  name 
  of 
  a 
  hill 
  near 
  Jauchau 
  Fu, 
  where 
  the 
  mineral 
  is 
  obtained.'' 
  

   (Dana, 
  System 
  of 
  min., 
  p. 
  687) 
  

  

  Kaolinite 
  is 
  a 
  secondary 
  mineral 
  formed 
  by 
  the 
  decomposition 
  

   of 
  feldspar. 
  This 
  is 
  commonly 
  caused 
  by 
  percolating 
  waters 
  aided 
  

   by 
  disintegrating 
  causes, 
  the 
  result 
  being 
  that 
  the 
  alkalies 
  and 
  alka- 
  

   line 
  earths 
  of 
  the 
  feldspar 
  are 
  caiTied 
  off 
  in 
  solution, 
  while 
  the 
  

   alumina 
  and 
  silica, 
  left 
  behind, 
  unite 
  with 
  water 
  to 
  form 
  hydrated 
  

   silicate 
  of 
  alumina. 
  The 
  feldspars 
  are 
  essentially 
  anhydrous 
  sili^ 
  

   cates 
  of 
  alumina, 
  containing 
  in 
  addition 
  varying 
  amounts 
  of 
  lime, 
  

   potash 
  or 
  soda, 
  and 
  depending 
  partly 
  on 
  their 
  chemical 
  composition 
  

   and 
  partly 
  on 
  their 
  physical 
  characters. 
  Xine 
  varieties 
  are 
  usually 
  

   recogTiized, 
  which 
  fall 
  into 
  two 
  groups 
  known 
  as 
  the 
  orthoclase 
  

   and 
  plagioclase 
  groups. 
  

  

  