﻿J. 
  P. 
  Iddings 
  — 
  Densities 
  of 
  Igneous 
  Rocks, 
  363 
  

  

  Art. 
  XXVI. 
  — 
  Relative 
  Densities 
  of 
  Igneous 
  Rocks 
  Calcu- 
  

   lated 
  from 
  their 
  Norms; 
  by 
  J. 
  P. 
  Iddings. 
  

  

  In 
  discussing 
  the 
  relative 
  densities 
  of 
  large 
  masses 
  of 
  

   igneous 
  rocks 
  in 
  connection 
  with 
  the 
  problem 
  of 
  isostasy 
  

   in 
  lectures 
  on 
  Volcanism 
  1 
  delivered 
  at 
  Yale 
  University 
  in 
  

   1914, 
  it 
  was 
  stated 
  that 
  an 
  estimate 
  of 
  the 
  relative 
  densi- 
  

   ties 
  of 
  holocrystalline 
  igneous 
  rocks 
  could 
  be 
  made 
  from 
  

   the 
  calculated 
  norm 
  with 
  reasonable 
  accuracy, 
  when 
  the 
  

   actual 
  mineral 
  composition 
  of 
  the 
  rock, 
  or 
  the 
  mode, 
  was 
  

   normative, 
  or 
  nearly 
  so 
  .; 
  that 
  is, 
  when 
  the 
  rock 
  does 
  not 
  

   contain 
  considerable 
  hornblende 
  or 
  mica, 
  or 
  is 
  not 
  other- 
  

   wise 
  abnormative. 
  In 
  case 
  these 
  minerals 
  are 
  abundant 
  

   the 
  density 
  of 
  the 
  rock 
  must 
  be 
  slightly 
  more 
  than 
  the 
  

   estimated 
  density, 
  since 
  hornblende 
  and 
  mica 
  have 
  higher 
  

   specific 
  gravities 
  than 
  their 
  normative 
  components. 
  

  

  The 
  data 
  on 
  which 
  the 
  statement 
  was 
  based 
  were 
  not 
  

   published 
  at 
  the 
  time, 
  but 
  may 
  be 
  found 
  useful 
  in 
  other 
  

   calculations. 
  The 
  relative 
  densities 
  of 
  certain 
  minerals 
  

   that 
  occur 
  pure 
  in 
  nature, 
  or 
  have 
  been 
  formed 
  in 
  labora- 
  

   tories, 
  are 
  definite 
  quantities. 
  Those 
  of 
  minerals 
  with 
  

   variable 
  compositions, 
  that 
  is, 
  isomorphous 
  mixtures, 
  

   may 
  be 
  estimated 
  from 
  the 
  proportions 
  of 
  their 
  hypo- 
  

   thetical 
  component 
  molecules, 
  such 
  as 
  the 
  proportions 
  of 
  

   forsterite 
  and 
  of 
  fayalite 
  in 
  an 
  olivine. 
  The 
  specific 
  

   gravities 
  used 
  in 
  the 
  calculations 
  referred 
  to 
  were 
  as 
  

   follows 
  : 
  

  

  Quartz 
  = 
  2-65 
  

   Orthoclase 
  = 
  2-54 
  

   Albite 
  = 
  2-61 
  

   Anorthite 
  =2-77 
  

   Leucite 
  = 
  2-48 
  

   Nephelite 
  =2-62 
  

   Corundum 
  = 
  400 
  

  

  aFeO, 
  14; 
  MgO, 
  28. 
  

   bFeO, 
  27; 
  MgO, 
  14. 
  

  

  Specific 
  Gravities. 
  

  

  Diopside 
  = 
  3-28 
  

  

  Enstatite 
  = 
  3-18 
  

  

  Hypersthene 
  a 
  = 
  3-33 
  

  

  Hypersthene 
  b 
  =3-53 
  

  

  Forsterite 
  = 
  3-21 
  

   Olivine 
  = 
  3-27-3-37 
  

  

  Fayalite 
  = 
  4-00 
  

  

  Magnetite 
  

   Ilmenite 
  

  

  = 
  517 
  

   = 
  4-73 
  

  

  Hematite 
  

  

  = 
  5-22 
  

  

  Apatite 
  

  

  Pyrite 
  

  

  Fluorite 
  

  

  = 
  3-20 
  

   = 
  5-03 
  

   = 
  318 
  

  

  By 
  calculating 
  the 
  mass 
  of 
  each 
  component 
  of 
  the 
  norm 
  

   of 
  any 
  analyzed 
  holocrystalline 
  rock, 
  the 
  sum 
  will 
  be 
  the 
  

   mass 
  of 
  the 
  whole, 
  which 
  may 
  be 
  corrected 
  for 
  100 
  per- 
  

  

  1 
  Iddings, 
  J. 
  P.: 
  The 
  Problem 
  of 
  Volcanism, 
  p. 
  123, 
  1914. 
  Yale 
  "Uni- 
  

   versity 
  Press, 
  New 
  Haven, 
  Conn. 
  

  

  