﻿364 
  J, 
  P. 
  Iddings 
  — 
  Densities 
  of 
  Igneous 
  Rocks. 
  

  

  cent. 
  The 
  correspondence 
  between 
  the 
  specific 
  gravities 
  

   of 
  certain 
  rocks, 
  which 
  have 
  been 
  determined 
  with 
  a 
  bal- 
  

   ance, 
  and 
  those 
  estimated 
  from 
  the 
  norms 
  derived 
  from 
  

   the 
  chemical 
  analyses 
  of 
  the 
  same 
  rocks 
  has 
  been 
  found 
  

   to 
  be 
  as 
  follows 
  : 
  

  

  1. 
  Khyolite, 
  liparose, 
  Four 
  Mile 
  Creek, 
  Castle 
  Mt., 
  Montana,. 
  

  

  H. 
  S. 
  Washington's 
  Tables 
  of 
  Rock 
  Analyses 
  (1903), 
  p. 
  

   146, 
  No. 
  28. 
  Sp. 
  gr. 
  from 
  norm, 
  2-62; 
  corrected 
  for 
  

   H,0, 
  2-61; 
  from 
  rock, 
  2-61. 
  

  

  2. 
  Quartz-tourmaline-porpnyry, 
  liparose, 
  same 
  locality, 
  H. 
  S. 
  

  

  W. 
  Tables, 
  p. 
  146, 
  No. 
  29. 
  Sp. 
  gr. 
  from 
  norm, 
  2-62; 
  

   corrected 
  for 
  H 
  2 
  0, 
  2-59 
  ; 
  from 
  rock, 
  2-59. 
  

  

  3. 
  Granite, 
  lassenose, 
  Muhlberg, 
  Odenwald, 
  Hesse; 
  H. 
  S. 
  "W. 
  

  

  Tables, 
  p. 
  180, 
  No. 
  68. 
  Sp. 
  gr. 
  from 
  norm, 
  2-721; 
  cor- 
  

   rected 
  for 
  H 
  2 
  0, 
  2-688; 
  from 
  rock, 
  2-665. 
  

  

  4. 
  Gabbro, 
  anvergnose, 
  Minnesota; 
  H. 
  S. 
  "W. 
  Tables, 
  p. 
  332, 
  

  

  No. 
  26. 
  Sp. 
  gr. 
  from 
  norm, 
  2-989; 
  corrected 
  for 
  H 
  2 
  0, 
  

   2-970; 
  from 
  rock, 
  2-967. 
  

  

  5. 
  Essexite, 
  essexose, 
  Salem 
  Neck, 
  Mass.; 
  H. 
  S. 
  W. 
  Tables, 
  

  

  p. 
  298, 
  No. 
  3. 
  Sp. 
  gr. 
  from 
  norm, 
  2-917; 
  from 
  rock, 
  

   2-919. 
  

  

  6. 
  Gabbro-diorite, 
  anvergnose, 
  "Windsor 
  Eoad, 
  Baltimore, 
  Md., 
  

  

  H. 
  S. 
  W. 
  Tables, 
  p. 
  330, 
  No. 
  14. 
  Sp. 
  gr. 
  from 
  norm, 
  

   3-006; 
  from 
  rock, 
  3069. 
  

  

  7. 
  Hornblende-picrite, 
  wehrlose, 
  Northmeadow 
  Creek, 
  Mont., 
  

  

  H. 
  S. 
  W. 
  Tables, 
  p. 
  354, 
  No. 
  2. 
  Sp. 
  gr. 
  from 
  norm, 
  3-25 
  ; 
  

   from 
  rock, 
  3-35. 
  

  

  From 
  these 
  examples 
  it 
  is 
  seen 
  that 
  the 
  hornblende- 
  

   bearing 
  gabbro, 
  No. 
  6, 
  and 
  the 
  picrite, 
  No. 
  7, 
  have 
  higher 
  

   specific 
  gravities 
  than 
  those 
  calculated 
  from 
  the 
  norms, 
  

   as 
  already 
  pointed 
  out. 
  When 
  the 
  possible 
  sources 
  of 
  

   error 
  in 
  determining 
  the 
  specific 
  gravities 
  of 
  rocks 
  by 
  

   ordinary 
  methods 
  are 
  taken 
  into 
  account, 
  the 
  correspond- 
  

   ence 
  between 
  the 
  observed 
  and 
  calculated 
  specific 
  grav- 
  

   ities 
  in 
  the 
  cases 
  cited 
  is 
  striking, 
  and 
  indicates 
  that 
  for 
  

   purposes 
  of 
  generalization 
  and 
  for 
  the 
  comparison 
  of 
  the 
  

   relative 
  densities 
  of 
  large 
  volumes 
  of 
  igneous 
  rocks 
  of 
  

   hypothetical 
  average 
  compositions 
  this 
  method 
  of 
  esti- 
  

   mating 
  specific 
  gravities 
  is 
  sufficiently 
  accurate. 
  

  

  In 
  the 
  course 
  of 
  Silliman 
  Lectures, 
  already 
  referred 
  to, 
  

   this 
  method 
  of 
  estimating 
  relative 
  densities 
  was 
  applied 
  

   to 
  the 
  discussion 
  of 
  isostatic 
  conditions 
  with 
  respect 
  to 
  

   contrasted 
  petrographical 
  provinces, 
  the 
  average 
  of 
  all 
  

   analyses 
  of 
  igneous 
  rocks 
  in 
  each 
  region 
  being 
  taken 
  as. 
  

   an 
  exponent 
  of 
  the 
  mass 
  of 
  igneous 
  rocks 
  of 
  each 
  prov- 
  

  

  