﻿compone:n^ts 
  or 
  the 
  earth 
  s 
  crust, 
  minerals 
  and 
  rocks 
  121 
  

  

  The 
  feldspars 
  are 
  silicates 
  of 
  alumina 
  combined 
  with 
  potash, 
  

   soda 
  or 
  lime. 
  The 
  more 
  common 
  species 
  are: 
  orthoclase 
  and 
  mi- 
  

   crocline, 
  silicates 
  of 
  alumina 
  and 
  potash. 
  

  

  Albite, 
  silicate 
  of 
  alumina 
  and 
  soda. 
  

  

  Anorthitey 
  silicate 
  of 
  alumina 
  and 
  lime. 
  

  

  Oligoclase, 
  andesite 
  and 
  labradorite, 
  which 
  contain 
  both 
  lime 
  and 
  

   soda, 
  and 
  are 
  intermediate 
  between 
  albite 
  and 
  anorthite. 
  

  

  In 
  crystallization 
  orthoclase 
  is 
  monoclinic, 
  the 
  others 
  named 
  

   are 
  triclinic. 
  

  

  The 
  triclinic 
  feldspars 
  are 
  usually 
  called 
  plagioclase 
  in 
  techni- 
  

   cal 
  rock 
  nomenclature, 
  and 
  are 
  referred 
  to 
  collectively 
  by 
  this 
  

   term. 
  ' 
  

  

  The 
  magnesia-iron 
  silicates 
  are 
  classified 
  in 
  three 
  principal 
  

   groups, 
  the 
  amphiboles, 
  pyroxenes 
  and 
  micas. 
  

  

  The 
  amphiboles 
  are 
  monoclinic 
  and 
  comprise 
  hornblende, 
  actino- 
  

   lite 
  and 
  tremolite. 
  

  

  Hornblende 
  is 
  a 
  silicate 
  of 
  alumina, 
  iron, 
  lime 
  and 
  magnesia; 
  

   it 
  is 
  very 
  tough 
  and 
  somewhat 
  fibrous 
  in 
  fracture, 
  its 
  color 
  

   varies 
  from 
  dark 
  green 
  to 
  blackish 
  green. 
  This 
  is 
  a 
  very 
  import- 
  

   ant 
  constituent 
  of 
  granites 
  and 
  other 
  crystalline 
  rocks. 
  

  

  Actinolite 
  is 
  a 
  fibrous 
  variety, 
  generally 
  light 
  green 
  in 
  color 
  and 
  

   containing 
  less 
  alumina. 
  

  

  Tremolite 
  is 
  usually 
  white 
  and 
  contains 
  but 
  little 
  iron 
  and 
  no 
  

   alumina. 
  It 
  occurs 
  generally 
  in 
  crystals 
  scattered 
  through 
  crys- 
  

   talline 
  limestone. 
  

  

  Asbestus 
  is 
  a 
  finely 
  fibrous 
  tremolite. 
  

  

  The 
  pyroxenes 
  have 
  very 
  nearly 
  the 
  same 
  chemical 
  composition 
  

   as 
  the 
  amphiboles 
  and 
  are 
  also 
  monoclinic 
  but 
  crystallize 
  with 
  a 
  

   different 
  prismatic 
  angle. 
  

  

  Augite, 
  which 
  corresponds 
  closely 
  to 
  hornblende 
  in 
  composition 
  

   and 
  resembles 
  it 
  in 
  many 
  ways, 
  is 
  an 
  important 
  constituent 
  of 
  

   many 
  eruptive 
  rocks 
  such 
  as 
  diabase, 
  basalt, 
  etc. 
  

  

  Pyroxene 
  is 
  lighter 
  in 
  color 
  than 
  augite 
  and 
  similar 
  to 
  actinolite 
  

   in 
  composition. 
  

  

  Diopside 
  corresponds 
  closely 
  to 
  tremolite 
  in 
  composition 
  and 
  

   like 
  it, 
  occurs 
  in 
  limestones. 
  

  

  