﻿Composition 
  of 
  the 
  Trajp 
  of 
  Rooky 
  Hill, 
  W. 
  J, 
  281 
  

  

  of 
  the 
  dike, 
  i. 
  e. 
  at 
  different 
  steps 
  in 
  the 
  crystallization 
  of 
  the 
  

   magma. 
  

  

  In 
  the 
  analysis 
  of 
  the 
  diallage-like 
  augite 
  the 
  sample 
  used 
  in 
  

   No. 
  2 
  was 
  separated 
  at 
  the 
  highest 
  specific 
  gravity 
  of 
  the 
  solu- 
  

   tion. 
  That 
  used 
  in 
  ]^o. 
  3 
  was 
  selected 
  by 
  hand, 
  as 
  the 
  crystals 
  

   were 
  quite 
  large. 
  

  

  Analyses 
  of 
  Diallagic 
  Augite. 
  

  

  

  No. 
  2. 
  

  

  Eatio. 
  

  

  No. 
  3. 
  

  

  Ratio. 
  

  

  lY.* 
  

  

  SiO, 
  

  

  . 
  . 
  47-72 
  

  

  •795 
  

  

  48-54 
  

  

  •809 
  

  

  50^71 
  

  

  A1A-- 
  

  

  3-44 
  

  

  -014 
  

  

  5-50 
  

  

  •053 
  

  

  3^55 
  

   (MnO) 
  

  

  FeA---- 
  

  

  5-93 
  

  

  •037 
  

  

  2-77 
  

  

  -017 
  

  

  •81 
  

  

  FeO 
  

  

  ._ 
  18-34 
  

  

  •254 
  

  

  21^25 
  

  

  •295 
  

  

  15^30 
  

  

  CaO 
  

  

  . 
  11-40 
  

  

  •205 
  

  

  10-97 
  

  

  •196 
  

  

  13-35 
  

  

  MgO 
  

  

  .. 
  12-89 
  

  

  •320 
  

  

  7-67 
  

  

  •191 
  

  

  13-63 
  

  

  Na,0.... 
  

  

  •86 
  

   -37 
  

  

  -013 
  

   -003 
  

  

  3-10 
  

  

  •039 
  

  

  1-48 
  

  

  Ignition 
  . 
  . 
  

  

  -00 
  

  

  

  

  -82 
  

  

  

  1-17 
  

  

  

  100-95 
  

  

  100-62 
  

  

  100-00 
  

  

  

  No. 
  2. 
  

  

  R,0 
  : 
  RO 
  

  

  : 
  R.O3 
  

  

  : 
  SiO, 
  

  

  

  

  

  -016 
  : 
  779 
  

  

  : 
  -051 
  

  

  : 
  795 
  

  

  

  

  No. 
  3. 
  

  

  R,0 
  : 
  RO 
  

  

  : 
  K.O3 
  

  

  : 
  SiO, 
  

  

  

  

  

  •039 
  : 
  682 
  

  

  : 
  -07 
  

  

  : 
  809 
  

  

  

  From 
  the 
  chemical 
  composition 
  these 
  two 
  pyroxenes 
  lie 
  

   between 
  the 
  molecule 
  of 
  hedenbergite 
  (CaFeSi^Og) 
  on 
  the 
  one 
  

   side 
  and 
  diopside 
  (CaMgSi^Oe) 
  on 
  the 
  other, 
  of 
  which 
  they 
  are 
  

   isomorphons 
  mixtures'' 
  ; 
  combined 
  with 
  these 
  is 
  a 
  small 
  pro- 
  

   portion 
  of 
  the 
  alkali-bearing 
  pyroxene 
  (NaFeSi^Og). 
  The 
  mag- 
  

   nesia-bearing 
  molecule 
  separates 
  out 
  iirst 
  at 
  higher 
  tempera- 
  

   tures, 
  decreasing 
  the 
  percentage 
  of 
  Mg 
  in 
  the 
  rock 
  as 
  a 
  whole 
  ; 
  

   this 
  has 
  been 
  carried 
  to 
  such 
  an 
  extent, 
  that 
  when 
  No. 
  3 
  is 
  

   reached, 
  this 
  locality 
  being 
  the 
  last 
  to 
  consolidate, 
  the 
  per- 
  

   centage 
  of 
  MgO 
  has 
  been 
  reduced 
  to 
  1-58. 
  The 
  magnesia 
  

   content 
  has 
  been 
  a 
  decreasing 
  factor 
  from 
  the 
  beginning 
  of 
  

   crystallization 
  ; 
  decreasing 
  from 
  7-59 
  per 
  cent 
  in 
  the 
  contact, 
  

   to 
  1^58 
  per 
  cent 
  in 
  No. 
  3. 
  As 
  the 
  pyroxene 
  is 
  the 
  only 
  mag- 
  

   nesia-bearing 
  mineral 
  present, 
  with 
  the 
  exception 
  of 
  olivine 
  in 
  

   the 
  contact 
  region, 
  the 
  greater 
  portion 
  of 
  this 
  change 
  in 
  per 
  

   cent 
  of 
  magnesia 
  present 
  is 
  caused 
  by 
  the 
  variation 
  in 
  the 
  

   pyroxene 
  molecule. 
  The 
  change 
  in 
  the 
  iron-bearing 
  molecule 
  

   is 
  not 
  so 
  clear, 
  it 
  being 
  obscured 
  by 
  the 
  magnetic 
  oxides, 
  which 
  

   increase 
  along 
  the 
  same 
  direction 
  as 
  the 
  ferrous 
  component 
  of 
  

   the 
  pyroxene 
  ; 
  what 
  has 
  been 
  lost 
  by 
  the 
  lack 
  of 
  magnesia 
  in 
  

  

  * 
  Augite 
  from 
  West 
  Rock, 
  New 
  Haven, 
  Proc. 
  U. 
  S. 
  National 
  Museum, 
  vol. 
  iv, 
  

   p. 
  132. 
  

  

  