﻿496 
  I^EW 
  YOEK 
  STATE 
  MUSEUM 
  

  

  OEIGm 
  AND 
  NATUEE 
  OF 
  CLAY 
  

  

  The 
  teiTii 
  " 
  clay 
  '' 
  is 
  difficult 
  to 
  defin.e, 
  being 
  often 
  "used 
  in 
  a 
  

   rather 
  loose 
  manner. 
  Recent 
  investigations 
  on 
  the 
  part 
  of 
  the 
  

   writer 
  and 
  others 
  lead 
  to 
  the 
  conclusion 
  that 
  the 
  term 
  " 
  clay 
  " 
  

   indicates 
  a 
  substance 
  of 
  peculiar 
  physical 
  characters, 
  but 
  hav- 
  

   ing 
  absolutely 
  no 
  constancy 
  of 
  either 
  chemical 
  or 
  mineralogic 
  

   composition. 
  Two 
  substances 
  may 
  appear 
  to 
  be 
  entirely 
  unlike 
  from 
  

   a 
  chemical 
  standpoint, 
  and 
  yet 
  their 
  physical 
  characters 
  may 
  be 
  

   such 
  that 
  both 
  would 
  be 
  classed 
  under 
  the 
  head 
  of 
  clay. 
  

  

  Probably 
  the 
  best 
  mineralogic 
  definition 
  of 
  clay 
  is 
  that 
  given 
  by 
  

   Dr 
  G. 
  P. 
  Merrill 
  in 
  his 
  book, 
  Bochs_, 
  rock-weathering 
  and 
  soils, 
  in 
  

   which 
  he 
  defines 
  it 
  ''as 
  an 
  indefinite 
  mixture 
  of 
  more 
  or 
  less 
  hy- 
  

   drated 
  aluminous 
  silicates, 
  free 
  silica, 
  iron 
  oxid, 
  carbonates 
  of 
  lime, 
  

   and 
  various 
  silicate 
  minerals 
  which, 
  in 
  a 
  more 
  or 
  less 
  decomposed 
  

   and 
  fragTuental 
  condition, 
  have 
  survived 
  the 
  destructive 
  agencies 
  to 
  

   which 
  they 
  have 
  been 
  subjected 
  ". 
  

  

  The 
  only 
  feature 
  characteristic 
  of 
  all 
  clays 
  is 
  that 
  they 
  are 
  

   plastic 
  when 
  wet 
  and 
  when 
  burned 
  harden 
  to 
  a 
  rock-like 
  mass. 
  

   This 
  degree 
  of 
  plasticity 
  has 
  little 
  probably 
  to 
  do 
  with 
  the 
  chemical 
  

   or 
  mineralogic 
  composition, 
  for 
  clays 
  of 
  either 
  high 
  or 
  low 
  

   plasticity 
  may 
  vary 
  widely 
  in 
  their 
  make-up. 
  It 
  seems 
  to 
  depend, 
  

   and 
  this 
  point 
  will 
  be 
  discussed 
  in 
  more 
  detail 
  later, 
  wholly 
  on 
  

   texture 
  and 
  structure, 
  that 
  is 
  on 
  the 
  shape 
  and 
  size 
  of 
  the 
  particles. 
  

   As 
  Dr 
  Merrill 
  points 
  out, 
  pure 
  quartz, 
  chalcedony, 
  flint, 
  feldspar 
  or 
  

   other 
  silicates, 
  will 
  when 
  reduced 
  to 
  an 
  impalpable 
  powder 
  possess 
  

   a 
  pastiness 
  and 
  even 
  an 
  odor 
  similar 
  to 
  that 
  of 
  clay. 
  Most 
  of 
  

   these 
  simple 
  mineral 
  mixtures 
  of 
  extreme 
  fineness 
  do 
  not 
  seem 
  to 
  

   hold 
  together 
  like 
  clay 
  mixed 
  by 
  nature, 
  probably 
  because 
  they 
  

   lack 
  the 
  plastic 
  particles 
  which 
  true 
  clay 
  contains. 
  Clay 
  may 
  

   show 
  all 
  degrees 
  of 
  plasticity, 
  and 
  by 
  the 
  increase 
  of 
  one 
  or 
  another 
  

   of 
  its 
  component 
  minerals 
  may 
  pass 
  into 
  other 
  rock 
  types, 
  as 
  into 
  

   limestone 
  on 
  the 
  one 
  hand 
  by 
  the 
  increase 
  of 
  carbonate 
  of 
  lime, 
  or 
  

   into 
  sandstone 
  by 
  the 
  addition 
  of 
  sand. 
  

  

  