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  3SrEW 
  TOKK 
  STATE 
  MUSEUM 
  

  

  Besides 
  the 
  above 
  species 
  which 
  are 
  monoclinic, 
  there 
  is 
  an 
  

   important 
  group 
  of 
  orthorhombic 
  pyroxenes. 
  

  

  These 
  are 
  hypersthene, 
  bronzite 
  and 
  enstatite. 
  

  

  Of 
  the 
  micas 
  there 
  are 
  many 
  species. 
  The 
  most 
  important 
  

   rock-making 
  mica 
  is 
  Motite, 
  a 
  silicate 
  of 
  alumina, 
  potash, 
  iron 
  and 
  

   magnesia. 
  It 
  is 
  brownish 
  black 
  in 
  color 
  and 
  is 
  abundant 
  in 
  the 
  

   granites 
  and 
  gneisses. 
  

  

  Muscovite, 
  sl 
  silicate 
  of 
  alumina 
  and 
  potash, 
  is 
  less 
  important 
  

   as 
  a 
  rock 
  mineral 
  but 
  is 
  valuable 
  commercially 
  for 
  its 
  thin 
  trans- 
  

   parent 
  plates 
  used 
  in 
  stove 
  doors, 
  etc. 
  

  

  The 
  hydro-micas, 
  margarodite 
  and 
  damourite, 
  are 
  similar 
  to 
  

   the 
  true 
  micas 
  in 
  composition 
  but 
  contain 
  water. 
  

  

  Olivine, 
  or 
  chrysolite, 
  is 
  a 
  silicate 
  of 
  iron 
  and 
  magnesia 
  which 
  

   occurs 
  usually 
  in 
  small 
  crystals 
  or 
  grains 
  in 
  igneous 
  rocks. 
  It 
  

   is 
  pale 
  green 
  in 
  color. 
  

  

  Olivine 
  is 
  of 
  special 
  importance 
  because 
  from 
  it, 
  by 
  decomposi- 
  

   tion, 
  is 
  derived 
  a 
  large 
  proportion 
  of 
  the 
  serpentine 
  rocks. 
  

  

  Besides 
  these 
  few 
  minerals 
  which 
  are 
  essential 
  components 
  of 
  

   rocks 
  and 
  usually 
  by 
  their 
  presence 
  or 
  absence 
  determine 
  the 
  

   rock 
  species, 
  there 
  are 
  others 
  which 
  are 
  only 
  accessory 
  and 
  while 
  

   of 
  frequent 
  occurrence 
  do 
  not 
  so 
  invariably 
  affect 
  the 
  name 
  of 
  

   the 
  rock 
  in 
  which 
  they 
  occur. 
  Such 
  are 
  garnet, 
  zircon 
  and 
  

   staurolite. 
  

  

  In 
  addition 
  to 
  the 
  rock-making 
  minerals 
  are 
  those 
  which 
  occur 
  

   in 
  large 
  masses 
  in 
  other 
  rocks 
  and 
  have 
  a 
  commercial 
  value. 
  

   Such 
  are 
  corundum, 
  or 
  emery, 
  the 
  ores 
  of 
  iron, 
  e. 
  g. 
  magnetite, 
  

   hematite, 
  spathic 
  ore; 
  coal, 
  asphalt, 
  halite 
  or 
  rock 
  salt, 
  gypsum, 
  

   the 
  ore 
  of 
  lead 
  and 
  silver, 
  galenite; 
  the 
  ore 
  of 
  copper 
  and 
  gold, 
  

   chalcopyrite 
  and 
  graphite 
  or 
  black 
  lead. 
  

  

  Of 
  rarer 
  occurrence 
  and 
  great 
  commercial 
  value 
  are 
  the 
  gems 
  

   diamond, 
  ruby, 
  sapphire, 
  emerald, 
  etc. 
  None 
  of 
  these 
  are 
  found 
  

   in 
  New 
  York. 
  . 
  

  

  