﻿CLAYS 
  OF 
  NEW 
  YOKK 
  841 
  

  

  run 
  of 
  paving 
  brick 
  clays, 
  but 
  this 
  is 
  no 
  serious 
  objection. 
  The 
  lean 
  

   character 
  of 
  many 
  can 
  be 
  overcome 
  by 
  the 
  addition 
  of 
  plastic 
  clay, 
  

   as 
  in 
  the 
  case 
  of 
  the 
  Cairo 
  shale, 
  in 
  which 
  instance 
  the 
  mixture, 
  as 
  

   already 
  stated, 
  had 
  a 
  tensile 
  strength 
  of 
  100 
  pounds 
  a 
  square 
  inch. 
  

   The 
  amount 
  of 
  fluxes 
  present 
  permits 
  their 
  vitrifying 
  at 
  com- 
  

   paratively 
  low 
  temperature. 
  But 
  if 
  necessary 
  their 
  refractoriness 
  

   could 
  be 
  easily 
  increased 
  by 
  the 
  addition 
  of 
  a 
  certain 
  amount 
  of 
  

   fire 
  clay. 
  

  

  Feldspar 
  and 
  quartz 
  

  

  Mineralogic 
  characters. 
  Feldspar, 
  or 
  ^' 
  spar 
  '' 
  as 
  it 
  is 
  com- 
  

   mercially 
  called, 
  is 
  one 
  of 
  the 
  commonest 
  of 
  rock-forming 
  min- 
  

   erals, 
  and 
  yet, 
  owing 
  to 
  its 
  usual 
  intimate 
  association 
  with 
  other 
  

   mineral 
  species, 
  commercially 
  valuable 
  deposits 
  of 
  it 
  are 
  compara- 
  

   tively 
  rare. 
  The 
  deposit 
  must 
  be 
  large 
  and 
  of 
  snfiicient 
  piirity. 
  

   Its 
  most 
  common 
  associate 
  is 
  quartz, 
  but 
  the 
  two 
  possess 
  properties 
  

   which 
  render 
  them 
  easily 
  distinguishable. 
  

  

  Feldspar 
  is 
  usually 
  of 
  a 
  cream 
  or 
  red 
  color, 
  but 
  at 
  times 
  may 
  be 
  

   white. 
  It 
  cleaves 
  readily 
  in 
  two 
  directions 
  nearly 
  at 
  right 
  angles 
  

   to 
  each 
  other, 
  so 
  that 
  fragments 
  often 
  show 
  two 
  smooth 
  cleavage 
  

   surfaces, 
  the 
  result 
  of 
  this 
  property 
  of 
  splitting. 
  Chemically, 
  feld- 
  

   spar 
  is 
  a 
  complex 
  silicate 
  of 
  alumina 
  and 
  potash, 
  soda 
  or 
  lime. 
  

  

  Quartz 
  differs 
  from 
  feldspar 
  in 
  lacking 
  cleavage, 
  and 
  being 
  

   harder. 
  Its 
  hardness 
  is 
  7 
  in 
  the 
  scale, 
  and 
  it 
  easily 
  scratches 
  glass. 
  

   It 
  has 
  also 
  a 
  bright 
  glassy 
  luster, 
  and 
  breaks 
  with 
  a 
  conchoidal 
  or 
  

   shell-like 
  fracture. 
  

  

  There 
  are 
  two 
  well 
  marked 
  groups 
  of 
  feldspar 
  minerals, 
  the 
  

   potash 
  feldspars, 
  of 
  which 
  orthoclase 
  is 
  the 
  type, 
  and 
  the 
  lime 
  soda 
  

   feldspars, 
  or 
  plagioclases. 
  

  

  It 
  is 
  the 
  orthoclase 
  feldspar 
  that 
  is 
  usually 
  mined, 
  though 
  there 
  

   is 
  undoubtedly 
  some 
  plagioclase 
  in 
  some 
  of 
  our 
  commercial 
  feld- 
  

   spars, 
  but 
  a 
  systematic 
  chemical 
  investigation 
  of 
  the 
  American 
  

   materials 
  has, 
  however, 
  never 
  been 
  carried 
  out. 
  

  

  While 
  there 
  is 
  but 
  slight 
  variation 
  in 
  the 
  hardness 
  of 
  these 
  feld- 
  

   spars, 
  there 
  is 
  a 
  variation 
  in 
  the 
  chemical 
  composition 
  and 
  fusibility. 
  

  

  