﻿J. 
  P. 
  Iddings 
  — 
  Densities 
  of 
  Igneous 
  Rocks. 
  365 
  

  

  ince, 
  an 
  assumption 
  varying 
  greatly 
  in 
  valne 
  in 
  different 
  

   cases, 
  according 
  to 
  the 
  number 
  and 
  range 
  of 
  the 
  chemical 
  

   analyses 
  and 
  the 
  relative 
  quantities 
  of 
  the 
  various 
  rocks 
  

   in 
  each 
  region. 
  From 
  averages 
  of 
  chemical 
  analyses 
  

   estimated 
  by 
  F. 
  W. 
  Clarke 
  for 
  some 
  large 
  groups 
  of 
  

   rocks, 
  the 
  following 
  specific 
  gravities 
  were 
  calculated 
  : 
  

  

  California 
  2-85 
  Great 
  Britain 
  2-91 
  

  

  Colorado 
  2-80 
  Scandinavia 
  and 
  Finland 
  .... 
  2-85 
  

  

  Yellowstone 
  Park. 
  . 
  2-85 
  Germany 
  and 
  Austro-Hungary 
  2-81 
  

  

  Atlantic 
  Coast 
  .. 
  . 
  . 
  2-89 
  Italy 
  2-86 
  

  

  The 
  lowest 
  value 
  represents 
  the 
  normative 
  density 
  for 
  

   the 
  average 
  of 
  the 
  igneous 
  rocks 
  analyzed 
  from 
  the 
  High 
  

   plateau 
  of 
  Southwest 
  Colorado 
  ; 
  the 
  highest 
  value, 
  that 
  

   for 
  the 
  igneous 
  rocks 
  of 
  Great 
  Britain. 
  

  

  The 
  average 
  of 
  43 
  chemical 
  analyses 
  of 
  Hawaiian 
  lavas, 
  

   discussed 
  by 
  Whitman 
  Cross, 
  yielded 
  a 
  specific 
  gravity 
  

   of 
  3-12, 
  and 
  the 
  average 
  of 
  25 
  analyses 
  of 
  lavas 
  from 
  the 
  

   Island 
  of 
  Eeunion, 
  prepared 
  for 
  Professor 
  A. 
  Lacroix, 
  

   gave 
  3-12 
  also. 
  Whereas 
  the 
  analyses 
  of 
  rocks 
  from 
  

   Tahiti 
  and 
  the 
  neighboring 
  Society 
  Islands, 
  published 
  by 
  

   Lacroix, 
  represented 
  such 
  a 
  large 
  percentage 
  of 
  the 
  rela- 
  

   tively 
  scarce 
  trachytic 
  and 
  syenitic 
  rocks 
  that 
  the 
  average 
  

   of 
  the 
  15 
  analyses 
  by 
  no 
  means 
  represented 
  the 
  average 
  

   composition 
  of 
  the 
  rocks 
  of 
  the 
  islands, 
  which 
  according 
  

   to 
  Lacroix 
  are 
  largely 
  basalts 
  rich 
  in 
  mafic 
  minerals. 
  

   The 
  normative 
  specific 
  gravity 
  for 
  the 
  average 
  of 
  these 
  

   analyses 
  is 
  2-96, 
  which 
  is 
  too 
  low 
  to 
  represent 
  the 
  relative 
  

   density 
  of 
  the 
  igneous 
  rocks 
  of 
  the 
  Petrographical 
  Prov- 
  

   ince 
  of 
  this 
  part 
  of 
  the 
  South 
  Pacific 
  Ocean. 
  

  

  A 
  study 
  of 
  the 
  islands 
  by 
  the 
  writer 
  in 
  1915 
  showed 
  the 
  

   great 
  preponderance 
  of 
  basalts 
  rich 
  in 
  olivine 
  and 
  pyrox- 
  

   ene, 
  with 
  comparatively 
  low 
  content 
  of 
  feldspar. 
  It 
  also 
  

   showed 
  the 
  relatively 
  small 
  amount 
  of 
  trachytic 
  and 
  

   syenitic 
  rocks, 
  which 
  in 
  an 
  estimate 
  of 
  the 
  average 
  com- 
  

   position 
  of 
  the 
  volcanic 
  lavas 
  of 
  the 
  islands 
  may 
  be 
  con- 
  

   sidered 
  a 
  negligible 
  quantity. 
  An 
  average 
  of 
  17 
  analyses 
  

   of 
  representative 
  rocks 
  from 
  Tahiti 
  and 
  the 
  neighboring 
  

   Society 
  Islands, 
  omitting 
  those 
  of 
  trachytic 
  lavas, 
  fur- 
  

   nishes 
  a 
  norm 
  from 
  which 
  a 
  specific 
  gravity 
  of 
  3-23 
  was 
  

   calculated 
  for 
  an 
  anhydrous 
  mass, 
  and 
  of 
  3-20 
  for 
  one 
  

   containing 
  the 
  H 
  2 
  of 
  the 
  average 
  analysis. 
  Specimens 
  

   of 
  coarse-grained 
  rocks 
  from 
  the 
  cores 
  of 
  the 
  dissected 
  

   volcanoes 
  of 
  Tahiti 
  proper 
  and 
  from 
  the 
  peninsula 
  of 
  

  

  